fabrication of water glass adhesive silicate from
TRANSCRIPT
FABRICATION OF WATER GLASS ADHESIVE SILICATE
FROM AMORPHOUS SILICA OF RICE HUSK ASH
by
Samia Sharif
MASTER OF SCIENCE IN GLASS AND CERAMIC ENGINEERING
Department of Glass and Ceramic Engineering
BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
DHAKA, BANGLADESH
June, 2018
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CANDIDATE’S DECLARATION
It is hereby declared that except where specified by reference to other works, the studies
embodied in this thesis are the results of investigation carried out by the author under the
supervision of Dr. Muhammad Hasanuzzaman, Assistant professor, Department of Glass
and Ceramic Engineering, BUET. Neither the thesis nor any part of it has been submitted
elsewhere for any other purposes.
(Samia Sharif)
Student ID: 0412172003
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ACKNOWLEDGEMENT
In the name of Allah, the Most Gracious and the Most Merciful
I am grateful to the Allah for the good health and well-being that were necessary to
complete this research work and thesis paper. I would like to express my deep gratitude
to some people who have been supporting me during this thesis accomplishment.
I would like to express my heartfelt acknowledgment to my thesis supervisor Assistant
Professor Dr. Muhammad Hasanuzzaman from the department of Glass and Ceramic
Engineering of BUET for his thoughtful supervision, resourceful advice, and patient
guidance throughout the course of this research work and for his thorough understanding
that helps me balancing my professional and academic life. Some important aspects that
I have learned from him- quantitative approaches of scientific problems, critical
evaluation of results, and proper guidance to write thesis paper, will be the invaluable
legacy that I will carry on for the rest of my career. I admire his profound knowledge,
experience, and enthusiasm in research.
I would like to gratefully convey my gratitude to my course work advisor Dr. Md. Abdul
Matin for his guidance, advice and invaluable support throughout the course work. I
would like to express my special gratitude to all committee members Dr. Md. Abdullah
Zubair, Dr. Md. Saiful Islam, Dr. Fahmida Gulshan for their time and efforts for
reviewing and commenting this dissertation. I would also like to thank Dr. A.K.M.
Hakim for his valuable suggestions and competent guidance and also would like to thank
all the members of GCE department as well for their timely technical and administrative
support throughout the course.
I want to express my heartfelt appreciation to my father A.B.M. Shahid Sharif for his
encouragement for research work and endless support throughout my study life. The
unconditional love of my mother Asura Khondaker is the source of my faith to a better
life. I especially thankful to my parent-in-laws for their support and motivation to
complete my studies after marriage. A special thanks to my younger brother for
supporting me throughout my career.
I am overwhelming in thanking to my spouse Mahmudul Islam Tapu for his technical
support in laboratory work and moral support during my study period. In the last but not
the least, I dedicated my research work to my 2 years old son Shehzad Islam.
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ABSTRACT
Producing water glass or sodium silicate adhesive from locally available rice husk ash
(RHA) is an innovative thinking to create a new industrial sector in Bangladesh. New
understandings and advances in water-glass adhesive production technologies and
modification of water glass as adhesive are highlighted in this research paper. The purpose
of this research is to produce high performance water glass adhesive having outstanding
binding properties and fast drying time for corrugated carton packaging industries from
amorphous silica rich agricultural biomass waste- RHA through hydrothermal treatment.
Water glass is basically a compound containing silica (silicon dioxide, SiO2) and sodium
oxide (Na2O) combined in various ratios (1.6 to 3.9 weight ratio of SiO2/Na2O) to show
very useful properties of being soluble in water. Soluble silicates with higher weight ratios
of silica and alkali (above 3.5 weight ratio of SiO2/Na2O) are comparatively better
adhesives than low ratio silicates due to their higher content of polymeric silica. When
used as binders and adhesives, which depend on the presence of polysilicate ions, sodium
silicates having a SiO2:Na2O weight ratio of 2.5 to 3.8 is required. For maximum adhesive
strength the lower ratio (2.5-2.8) types are used because they can be obtained with a
higher concentration of solids. For more water-resistant bonds, the higher weight ratios
(3.2-3.8) are preferred. High ratio water glass has high binding strength and faster drying
properties. Producing higher mole ratio (above 3.5) water glass at lower temperature and
pressure is still a challenging issue.
In reviewing the literature, no data was found on fabrication of water glass adhesive
having higher mole ratio (above 4.0) from amorphous silica of RHA using lower
temperature and lower consumption of sodium hydroxide. Based on the new
understandings and experimental results reported in this thesis, thus an economically
viable route has been proposed to fabricate higher mole ratio water glass adhesive from
rice husk instead of quartz sand. Ratio modification was made by increasing polymeric
silica content of water glass to increase the binding strength and faster drying properties.
The reaction parameters such as strength of alkaline solution, reaction period, agitation
and temperature were investigated to establish optimum conditions under which
maximum silica conversion could be possible at 88%. RHA was characterized by XRF
for its chemical analysis and purity. Other properties of water glass were also
characterized.
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A method has been proposed in this research for producing a stable water glass solution
having a stabilized density in the range from 38o Bé° to 40o Bé° and a SiO2:Na2O ratio in
the range from 1.8:1 to 4.2:1 with the proper proportion of silica (SiO2) content in the
range from 23.7% to 29.9% by weight and alkali (Na2O) content in the range from 7.1%
to 13.3% by weight. Aqueous water glass solution is produced by reacting the amorphous
silica rich biomass ash preferably RHA (which is produced in the generation of heat and
electricity by firing solid biomass fuel at 600-750ºC) with 0.6-1.1 M aqueous alkali metal
hydroxide solutions (sodium hydroxide), in a high pressurized closed vessel under
elevated pressure (2-8 bar) and at temperatures above 120-170°C; while the ratio by
weight of sodium hydroxide to rice husk ash were being maintained by 3:1 to 9:1, and the
aqueous water glass solution obtained is subsequently filtered and concentrated at or
above 120ºC temperature and under 2-3 bar pressure in an evaporator to get specific
density and viscosity which possess proper adhesive properties along with quick drying
time. The reaction is carried out at a temperature in the range from 120 to 170°C and
under a pressure in the range from 2 to 8 bar. The ratio of biomass ash having greater than
90% amorphous silica to sodium hydroxide by weight between 3:1 and 9:1. The diluted
aqueous water glass solution is concentrated at a temperature in the range from 110ºC to
130ºC and under pressure in the range from 1.5 bars to 2.8 bar in order to stabilize the
solution against gelation and particle growth.
Physical analyses such as viscosity, pH, specific gravity, were carried out on the extracted
sodium silicate which was compared with the reference sodium silicate sample. The
results of the XRF analysis carried out on the RHA shows that the inorganic content of
this ash contains a good percentage amount of SiO2 (>96%). The high silica content
therefore justifies the use of the RHA as a silica source for this research work. Under this
research, water glass binder having unique adhesive bonding and fast-curing
characteristics with high ratio above 3.5 was successfully produced from rice husk ash at
lower temperature compare to that of conventional method.
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TABLE OF CONTENTS
ACKNOWLEDGEMENT............................................................................................. iv
ABSTRACT ................................................................................................................. v
TABLE OF CONTENTS ............................................................................................. vii
LIST OF FIGURES ...................................................................................................... xii
LIST OF TABLES ....................................................................................................... xvi
LIST OF ABBREVIATIONS ................................................................................... xviii
CHAPTER 1. INTRODUCTION ................................................................................. 1
1.1 Background of the Research .............................................................................. 1
1.2 Statement of the Problem ................................................................................... 3
1.3 Aim and Objectives of the Research ................................................................. 4
1.4 Research Questions and Basic Assumptions .................................................... 5
1.5 Justification of the Research .............................................................................. 6
1.6 Significance of the Research .............................................................................. 6
1.7 Methodology ........................................................................................................ 7
1.8 Layout of the Report........................................................................................... 7
CHAPTER 2. LITERATURE REVIEW ................................................................... 10
2.1 Definition of Adhesive ...................................................................................... 10
2.2 Water Glass or Sodium Silicate ....................................................................... 11
2.2.1 Atomic structure of water glass ................................................................ 12
2.2.2 History of water glass ............................................................................... 14
2.2.3 Physical properties of water glass ............................................................. 14
2.2.4 Manufacturing process of water glass ...................................................... 15
2.3 Why it is Called Water Glass? ......................................................................... 17
2.4 Demand of Water Glass in Various Market ................................................... 18
2.5 Applications of Water Glass ............................................................................ 19
2.5.1 Adhesive with staying power .................................................................... 19
2.5.2 Strong bonding cement ............................................................................. 20
2.5.3 More durable concrete with water glass ................................................... 20
2.5.4 Pulp bleaching and de-inking paper for recycling .................................... 21
2.5.5 Building better detergents and soaps with water glass ............................. 22
2.5.6 Silica based products ................................................................................ 25
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2.5.7 Better textiles ............................................................................................ 25
2.5.8 Environmentally friendly foundry binder ................................................. 27
2.5.9 Soil stabilization ....................................................................................... 27
2.5.10 Silica sols and water treatment ................................................................. 28
2.5.11 Coatings .................................................................................................... 28
2.6 Understanding of Physico-Chemical Properties of Water Glass.................. 31
2.6.1 Ratio of SiO2/Na2O ................................................................................... 31
2.6.2 Alkali content (Na2O) ............................................................................... 33
2.6.3 Silica content (SiO2) ................................................................................. 33
2.6.4 Total solid content .................................................................................... 33
2.6.5 Density and specific gravity ..................................................................... 35
2.6.6 Viscosity ................................................................................................... 35
2.6.7 pH.............................................................................................................. 36
2.7 Interconnecting Relations Between the Properties of Water Glass ............. 36
2.7.1 Modulus and concentration of water glass ............................................... 36
2.7.2 Viscosity/specific gravity/density of water glass change with temperature
.................................................................................................................. 37
2.7.3 Viscosity as a function of weight ratio and solid content of water glass .. 38
2.7.4 Relationship between specific gravity and density of water glass ............ 40
2.7.5 Viscosity as a function of density of water glass ...................................... 40
2.7.6 Relationship between density and solid content of water glass ................ 41
2.7.7 Interrelationship between ratio, solid content, viscosity and specific
gravity ....................................................................................................... 42
2.7.8 Relationship between pH and ratio of water glass .................................... 44
2.8 Reaction Mechanisms of Water Glass ............................................................ 44
2.8.1 Hydration/ Dehydration ............................................................................ 44
2.8.2 Surface charge modification ..................................................................... 45
2.8.3 Metal ion reaction ..................................................................................... 45
2.8.4 Sol and gel Formation ............................................................................... 45
2.8.5 Precipitation reactions............................................................................... 46
2.8.6 Interaction with organic compounds......................................................... 46
2.9 Mechanisms of Water Glass Film Formation and its Characteristics ......... 46
2.9.1 The binding strength of the dried water glass film ................................... 46
2.9.2 The progress of silicate gel formation ...................................................... 47
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2.9.3 Evaporation drying ................................................................................... 48
2.9.4 Chemical setting ....................................................................................... 49
2.10 Beneficial Properties of Bond and Films Made by Water Glass .................. 50
2.10.1 Moisture resistance ................................................................................... 50
2.10.2 Heat Resistance ......................................................................................... 50
2.10.3 Electrical properties .................................................................................. 50
2.10.4 UV transmission ....................................................................................... 51
2.10.5 Opacity and color ...................................................................................... 51
2.10.6 Flexibility .................................................................................................. 51
2.11 Modification of Water Glass to Enhance its Properties ................................ 52
2.11.1 Enhancement of waterproofing properties of water glass ........................ 52
2.11.2 Physical modification of water glass ........................................................ 52
2.11.3 Surface characteristics .............................................................................. 52
2.12 Research Focus: Water Glass as a Versatile-Economical Adhesive ............ 53
2.12.1 Characteristics of water glass adhesive for corrugated carton .................. 54
2.12.2 Advantages of using water glass as adhesive ........................................... 55
2.12.3 Broad range of application of water glass as adhesive / binder ................ 56
2.12.4 Proper application rules of water glass adhesive ...................................... 58
2.13 Research Focus: Water Glass in Corrugated Paperboard ........................... 59
2.13.1 Manufacturing of corrugated board .......................................................... 59
2.13.2 Effect of moisture on the quality of corrugated paperboard ..................... 60
2.14 Finding an Economical Silica Source: RHA................................................... 62
2.14.1 High silica content with amorphous characteristic ................................... 64
2.14.2 Abundant and cheap source of silica ........................................................ 65
2.14.3 Quality of silica comparable with other expensive sources of silica ........ 65
2.14.4 Disposal problem ...................................................................................... 66
2.15 Evaluation on Available Technologies for Production of Water Glass from RHA.................................................................................................................... 66
2.15.1 Statement of the problems of existing studies .......................................... 69
2.15.2 Advantages of alkaline extraction method over conventional furnace
method ...................................................................................................... 69
CHAPTER 3. EXPERIMENTAL METHODOLOGY ............................................ 72
3.1 Introduction ....................................................................................................... 72
3.2 Research Methodology ..................................................................................... 74
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3.2.1 Characterization of RHA .......................................................................... 74
3.2.2 Digestion - alkaline extraction .................................................................. 74
3.2.3 Filtration and product evaluation .............................................................. 75
3.2.4 Evaporation ............................................................................................... 75
3.3 Research Materials ........................................................................................... 76
3.3.1 Rice Husk Ash (RHA) .............................................................................. 76
3.3.2 Sodium hydroxide ..................................................................................... 77
3.4 Laboratory Reagent, Equipment and Instruments ....................................... 77
3.4.1 Laboratory reagents .................................................................................. 77
3.4.2 Equipment and instruments ...................................................................... 78
3.5 Characterization of Research Materials and Products ................................. 79
3.5.1 Characterization of research material – RHA ........................................... 79
3.5.2 Characterization of water glass and residue obtained as carbon cake ...... 80
3.6 Characterization Instruments ......................................................................... 81
3.6.1 X-ray fluorescence (XRF) technique ........................................................ 81
3.6.2 X- ray diffraction (XRD) technique for phase analysis ............................ 82
3.6.3 Field Emission Scanning Electron Microscope (FESEM) ....................... 85
3.7 Analytical Methods ........................................................................................... 86
3.7.1 Reactive amorphous silica content of RHA .............................................. 87
3.7.2 Moisture content of RHA by hot air oven drying method ........................ 89
3.7.3 Specific gravity and density of water glass adhesive ............................... 89
3.7.4 Total soluble silicate of water glass adhesive ........................................... 90
3.7.5 Total alkalinity of water glass adhesive .................................................... 91
3.7.6 Total soluble silica content of water glass solution .................................. 92
3.7.7 Weight ratio of total soluble silica to total alkalinity ................................ 92
3.8 Experimental Procedure .................................................................................. 92
CHAPTER 4. RESULTS AND DISCUSSION .......................................................... 94
4.1 Characterization of RHA ................................................................................. 94
4.1.1 Optical image of RHA (as-received) ........................................................ 94
4.1.2 Elemental analysis of RHA....................................................................... 96
4.1.3 Reactive amorphous silica content and moisture content of RHA ........... 98
4.1.4 Phase analysis of RHA ............................................................................. 99
4.1.5 Morphological properties of RHA .......................................................... 101
4.2 Quantitative Results of the Experiments Conducted in this Research ...... 104
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4.3 Characterization of Products Obtained from Different Stages of Experiments ..................................................................................................... 111
4.4 Optimization of Various Reaction Parameters to Produce Water Glass Adhesive from RHA in an Economical Point of View ................................. 112
4.4.1 Optimum ratio of RHA and sodium hydroxide and concentration of NaOH
................................................................................................................ 113
4.4.2 Optimum ratio of RHA and water .......................................................... 120
4.4.3 Optimum reaction temperature ............................................................... 122
4.4.4 Optimum reaction period ........................................................................ 125
4.5 Characteristics of Water Glass ...................................................................... 127
4.5.1 Visual appearance of the water glass solution ........................................ 130
4.5.2 Relation between viscosity and ratio of silica and sodium oxide ........... 132
4.6 Characterization of Water Glass ................................................................... 134
4.6.1 Elemental analysis of water glass adhesive ............................................ 134
4.6.2 Morphological properties of water glass adhesive ................................. 134
4.7 Characterization of Extracted Amorphous Silica........................................ 135
CHAPTER 5. CONCLUSIONS AND FUTURE WORK ...................................... 139
5.1 Conclusions ...................................................................................................... 139
5.2 Future Work .................................................................................................... 140
REFERENCES............................................................................................................ 141
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LIST OF FIGURES
Figure 1.1: Experimental procedure. ................................................................................ 7
Figure 2.1: (a) Liquid water glass; (b) Powder water glass; (c) Solid water glass. ....... 12
Figure 2.2: The irregular arrangement of ions in sodium silicate glass .......................... 13
Figure 2.3: Basic building block of a silica glass network.. ........................................... 13
Figure 2.4: Molecular structure of crystalline silica, quartz glass and sodium silicate glass… ......................................................................................................... 14
Figure 2.5: Basic flow diagram of producing water glass by Furnace route ................. 15
Figure 2.6: Basic flow diagram of producing water glass by hydrothermal route ......... 16
Figure 2.7: Sodium silicate crystals liquefy readily in water ......................................... 17
Figure 2.8: Sodium silicate crystals magnified 100x. Comstock Images ...................... 17
Figure 2.9: Geographic structure of production capacities of water glass...................... 18
Figure 2.10: Application segments of water glass in Europe ........................................ 19
Figure 2.11: Corrugated paper board ............................................................................. 20
Figure 2.12: Sodium silicate pasting in red mark area .................................................. 20
Figure 2.13: Sodium silicate acts as water proofing sealer on concrete surface ........... 21
Figure 2.14: Sodium silicate acts as penetrating and waterproofing sealer on concrete surface . ....................................................................................................... 21
Figure 2.15: Builders binding to calcium and magnesium ions in water. Sodium ions from sodium silicate help the builder .................................................................. 23
Figure 2.16: Buffering capacity of various alkali solutions at 0.02% Na2O concentration ..................................................................................................................... 24
Figure 2.17: Basic flow diagram of precipitated silica and silica gel from sodium silicate ..................................................................................................................... 26
Figure 2.18: Sodium silicate is used as binder in welding rod ...................................... 29
Figure 2.19: Sodium Silicate having 1:3.22 ratio of Na2O/SiO2 .............................. 32
Figure 2.20: Sodium Metasilicate having 1:1 ratio of Na2O/SiO2. ................................. 32
Figure 2.21: The component phase diagram of Na2O- SiO2 -H2O ................................. 37
Figure 2.22: Viscosities of water glass solutions as a function of temperature .............. 37
Figure 2.23: Viscosities of water glass solutions as a function of ratio at constant solid contents ........................................................................................................ 39
Figure 2.24: Viscosity of various weight ratio sodium silicate solutions ....................... 39
Figure 2.25: Viscosity of water glass solutions as a function of density ........................ 41
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Figure 2.26: Densities of sodium silicate solutions as a function of solid contents. ...... 41
Figure 2.27: Relationship between solid content and density at various ratio of water glass ..................................................................................................................... 43
Figure 2.28: pH values of sodium silicate solutions of various ratios ........................... 44
Figure 2.29: Gel times of 3.22 ratio sodium silicate–sulphuric acid mixtures at 25ºC. . 45
Figure 2.30: The tensile strength and moisture resistance vs. modulus of the water glass ..................................................................................................................... 47
Figure 2.31: Viscosity-specific gravity-water content relations of sodium silicate adhesive ......................................................................................................................... ..................................................................................................................... 55
Figure 2.32: Effect of moisture content after manufacturing of corrugated board (after 4 h) .................................................................................................................. 61
Figure 2.33: Effect of moisture content after printing of paperboard ............................ 62
Figure 2.34: Rice husk (RH) .......................................................................................... 64
Figure 2.35: Various types of RHA produced by burning RHA ................................... 64
Figure 2.36: (a) Landed disposal waste material- RHA; (b) Expensive quartz sand from beaches. ....................................................................................................... 70
Figure 3.1: The schematic representation of work. ........................................................ 73
Figure 3.2: Basic Process Flow Diagram. ...................................................................... 76
Figure 3.3: RHA as-received collected from various rice mills. .................................... 77
Figure 3.4: Caustic soda flakes or Sodium hydroxide from local manufacturer. ........... 77
Figure 3.5: NaOH, HCl, Na2CO3, methyl orange. .......................................................... 78
Figure 3.6: (a) Analytical Balance; (b) Muffle Furnace; (c) Hot-Plate Magnetic Stirrer; and (d) Hot Air Oven. ................................................................................. 79
Figure 3.7: XRF machine................................................................................................ 82
Figure 3.8: X-ray diffraction (XRD)............................................................................... 83
Figure 3.9: Field Emission Scanning Electron Microscope (FESEM). .......................... 85
Figure 3.10: Experimental flow diagram of determination of reactive amorphous silica content in RHA. ........................................................................................... 88
Figure 3.11: Hydrometer................................................................................................. 89
Figure 3.12: Density was tested by deeping the hydrometer in the liquid...................... 90
Figure 3.13: Nickel crucible in muffle furnace. .............................................................. 90
Figure 3.14: (a) Before end point and (b) after end point. .............................................. 91
Figure 4.1: Optical images of RHA. ............................................................................... 95
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Figure 4.2: Macroscopy of RHA as received ................................................................. 95
Figure 4.3: Extractable amorphous silica content of various rice husk ash collected from different location. ........................................................................................ 99
Figure 4.4: XRD pattern of typical RHA ..................................................................... 100
Figure 4.5: XRD pattern of RHA (as-received-Sample 7). .......................................... 101
Figure 4.6: Microstructure of RHA (as-received Sample-7) analyzed by FESEM. The honey-comb and flaky structure is the proof of silica structure. (a) SEM image of RHA (1 µm; x 20,000); (b) SEM image of RHA (1 µm; x 10,000); (c) SEM image of RHA (100 nm; x 50,000); (d) SEM image of RHA (1 µm; x 20,000). ................................................................................................... 102
Figure 4.7: Typical micrograph of RHA analyzed by SEM ........................................ 103
Figure 4.8: FE-SEM micrograph showing the presence of silica just underneath the outer epidermal cell surface fractured (a); SEM micrograph showing silica grains in RHA (b) ................................................................................................ 103
Figure 4.9: Effect of experimental variables on the quantity of produced final water glass solution. (a) Constant Parameters- Ratio of Water/RHA- 7:1, Temperature: 150ºC, Reaction Period: 3 h; (b) Constant Parameters- Ratio of RHA/NaOH- 5.26:1, Temperature: 150ºC, Reaction Period: 3 h; (c) Constant Parameters- Ratio of RHA/NaOH- 5.26:1, Ratio of Water/RHA- 7:1, Reaction Period: 3 h; (d) Constant Parameters- Ratio of RHA/NaOH- 5.26:1, Ratio of Water/RHA- 7:1, Temperature: 150ºC. .................................................... 110
Figure 4.10: Percent yield of the rice husk silica (RHS), extracted from the rice husk ash (RHA) as a function of the NaOH concentration in the reactant mixture. Volume of the reactant mixture, 60 ml; Concentration of RHA, 166.66 g/L; Extraction time, 90 min ............................................................................ 116
Figure 4.11: Percent yield of the rice husk silica (RHS), extracted from the rice husk ash (RHA) as a function of the ratio (Moles NaOH/grams RHA) in the reactant mixture. Volume of the reactant mixture, 60 ml; Extraction time, 90 min ................................................................................................................... 117
Figure 4.12: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Ratio of RHA and Sodium Hydroxide in the reactant mixture solution; (b) Influence of Ratio of RHA/NaOH on the cost of 1 kg water glass adhesive. ................................................................................................................... 118
Figure 4.13: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Molarity of Sodium Hydroxide solution in the reactant mixture solution; (b) Influence of Molarity of NaOH on the cost of 1 kg water glass adhesive. ................................................................................................................... 119
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Figure 4.14: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Ratio of RHA and Water in the reactant mixture solution; (b) Influence of Ratio of RHA/Water on the cost of 1 kg water glass adhesive. ............ 121
Figure 4.15: Silica conversion as function of reaction time for different temperatures (H2O/SiO2 molar ratio = 22; NaOH/SiO2 ratio = 2; closed system) . ....... 122
Figure 4.16: (a) Yield of silica as silicate solution extraction from RHA as a function of the reaction temperature; (b) Influence of reaction temperature on the cost of 1 kg water glass adhesive. ......................................................................... 124
Figure 4.17: Percent yield of the rice husk silica (RHS), extracted from the rice husk ash (RHA) as a function of the extraction time. Volume of the reactant mixture, 60 ml; Concentration of NaOH, 1 mol/L; Concentration of RHA, 166.66 g/L ................................................................................................................... 126
Figure 4.18: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Reaction period; (b) Influence of Reaction period on the cost of 1 kg water glass adhesive. ........................................................................................... 127
Figure 4.19: (a) Sodium silicate before addition of acid; (b) Sodium silicate after addition of acid. ...................................................................................................... 130
Figure 4.20: Optical image of water glass solution prepared from RHA collected from different rice mills. .................................................................................... 131
Figure 4.21 Relation between weight ratio of water glass and it's viscosity. .............. 133
Figure 4.22: FESEM micrograph of water glass powder (Sample 4). ......................... 135
Figure 4.23: As produced precipitated silica from RHA (a) White free flow powder; (b) Slightly greyish crystal. ............................................................................. 135
Figure 4.24: XRD patterns of the as-produced extracted amorphous silica-1 from RHA. ................................................................................................................... 136
Figure 4.25: XRD patterns of the as-produced extracted amorphous silica-2 from RHA. ................................................................................................................... 136
Figure 4.26: FE-SEM images of precipitated silica at different magnifications (a) FESEM images of precipitated silica (1 µm; X 10000); (b) FESEM images of precipitated silica (100 nm; X 30000). ...................................................... 137
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LIST OF TABLES
Table 2.1: Principle use of water glass in various industries ......................................... 30
Table 2.2: Properties of Silicates as a function of ratio ................................................ 32
Table 2.3: Typical properties of PQ sodium silicate solutions ...................................... 34
Table 2.4: Typical properties of Oxychem sodium silicate solutions ............................ 34
Table 2.5: pH values of selected water glass solution ................................................... 36
Table 2.6: Correction of density for temperatures ........................................................ 38
Table 2.7: Densities of selected PQ silicates at various temperatures ........................... 38
Table 2.8: Density and specific gravity equivalents ...................................................... 40
Table 2.9: Effects of evaporation on viscosities of PQ water glass solution ................. 48
Table 2.10: High temperature properties of soluble silicates in anhydrous state ........... 50
Table 2.11: Characteristics of water glass as adhesive ................................................... 54
Table 2.12: Moisture % vary during manufacturing of corrugated board ..................... 61
Table 2.13: Effect of moisture content after manufacturing of corrugated board (after 4 h) ..................................................................................................................... 61
Table 2.14: Effect of moisture content after printing of paperboard ............................. 62
Table 2.15: Studies that investigated methods used to extract silica from RHA............ 67
Table 2.16: Comparison of alkaline extraction of RHA with furnace method of silica sand ..................................................................................................................... 70
Table 3.1: Laboratory Reagents ..................................................................................... 78
Table 3.2: List of the equipment and instruments used in this research ........................ 78
Table 4.1: Elemental composition of rice husk ash from different countries ................. 97
Table 4.2: Elemental analysis of RHA by XRF.............................................................. 97
Table 4.3: Determination of extractable amorphous silica content of RHA using analytical method. ........................................................................................................ 99
Table 4.4: Experimental Parameters used in this research (Exp. No. 1-24) ................ 108
Table 4.5: Quantitative analysis of the Experiment (experiment no-1 to 6) ................ 109
Table 4.6: Characterization of products obtained from different stages of experiments by analytical methods (Exp. No. 1-24) .......................................................... 111
Table 4.7: Effect of NaOH concentration on the yield of silica .................................. 114
Table 4.8: Influence of Ratio of RHA/NaOH and Molarity of NaOH on the yield of Silica as well as the cost of raw materials per kg product (Exp. 1-6) ................. 115
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Table 4.9: Influence of ratio of water/RHA on the yield of silica as well as on the feasibility of the production of water glass adhesive (Exp. 7-12) ............. 120
Table 4.10: Influence of reaction temperature on the yield of silica as well as on the feasibility (Exp. 13-18) ............................................................................. 123
Table 4.11: Influence of reaction period on the yield of silica (Exp. 19-24) ............... 125
Table 4.12: Typical properties of PQ sodium silicate solutions ................................... 128
Table 4.13: Characterization of properties of diluted water glass solution after filtration and before evaporation .............................................................................. 129
Table 4.14: Observation of viscosity and binding properties ....................................... 132
Table 4.15: Elemental analysis of water glass powder by XRF ................................... 134
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LIST OF ABBREVIATIONS
RH Rice Husk
RHA Rice Husk Ash
MR Molar Ratio
WR Weight Ratio
oBe’ Degree Baume (Unit of Density specially used for water glass solution)
SEM Scanning Electron Microscope
FESEM Field Emission Scanning Electron Microscope
XRF X-ray Fluorescence
XRD X-ray Diffraction
CHAPTER ONE
INTRODUCTION
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CHAPTER 1. INTRODUCTION
1.1 Background of the Research
Sodium silicate (Na2SiO3), also known as water glass or soluble glass, which is a
combination of sodium (Na), oxygen (O), and silicon (Si) in the form of sodium oxide
(Na2O) and silicon-di-oxide or silica (SiO2) [1-4] that combined in various ratios which
forms a glassy solid with the very useful property of being soluble in water [5-7]. Solid
sodium silicate glass is then further dissolved in water to make aqueous solutions having
SiO2:Na2O weight ratios of 1.6-3.9. The properties of water glass, such as viscosity,
concentration, adhesive strength, rate of solidification, solubility in water, and stability of
the solution are closely related and are largely controlled by the variables of the alkali
metal used, the ratio of SiO2 to alkali metal oxide expressed by either weight or molar
ratio, and the concentration of solids in the solution [8, 9]. Water glass is sold as solid
lumps or powders or as a clear, concentrated syrupy liquid [2, 8-12].
Soluble silicates with higher weight/mole ratios of silica and alkali (above 3.5) are
comparatively better adhesives than low ratio silicates due to their higher content of
polymeric silica [8, 9, 11-14]. When it is used as binders and adhesives, which depend on
the presence of polysilicate ions, sodium silicates having a SiO2:Na2O molar ratio of 2.5
to 3.8 is required [11]. For maximum adhesive strength, sodium silicate having lower
ratio (2.5-2.8) have used due to higher concentration of solids. For more water-resistant
bonds, the higher ratios (3.2-3.8) are preferred due to higher polymeric silica content.
High ratio water glass has higher binding strength and faster drying properties [14]. But
producing higher mole ratio (above 3.5) water glass at lower temperature and pressure is
still a challenging issue.
The viscosity and density of this compound varies according to the ratios of silica and
sodium oxide used [8, 15]. This alkaline substance is available as a nearly colorless, glass-
like powder, or as a syrup-like liquid when mixed with water and heated under pressure
[9] . Sodium silicate has a variety of industrial, agricultural and manufacturing uses
including packaging, soap and detergent, ceramic, food and healthcare, pulp and paper,
elastomers, drilling fluids, coating, textiles, silica and silica gels, pottery, paints, foundry,
wood processing, and refractories etc. [16-18].
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The binding and coating applications of water glass as binder and adhesive are the main
focus of this research. Water glass in aqueous solutions has physical and chemical
properties that are useful in bonding and coating applications. When applied as a thin
layer on or between surfaces of other materials, the silicate solution dries to form a tough,
tightly adhering inorganic bond or film which can exhibit the following characteristics
[13, 14]:
▪ Resistant to temperatures up to 870ºC
▪ Non-flammable
▪ Odorless and non-toxic
▪ Moisture resistant
▪ Bondable to metals, particles (e.g., refractory materials), fibrous materials
(e.g., paper, fiberglass), glass, ceramics
▪ Strong and rigid
▪ Low cost
Technologies related to production method and applications of water glass adhesive
silicate have been studied and developed throughout the world in great variety of ways.
Commercially available water glass having mole ratio of silica and alkali up to 3.3 is
manufactured by the fusion of high purity soda ash and silica sand in furnaces at
temperatures of above 1300ºC which results in high production cost [19, 20]. The water
glass can also be manufactured by hydrothermal treatment of caustic soda and quartz sand
at temperatures of 200ºC [21, 22]. But the limitation of this route is that it can only
produce alkaline silicate with mole ratio less than 2.5 [23]. Due to energy intensive high
cost production process and limitation of producing high ratio water glass at low
temperature, it is necessary to develop a new process. In this research, an economically
cost effective way of producing water glass adhesive was described which uses a non-
conventional raw material RHA for extraction of silica in the form of sodium silicate [19,
24, 25].
RHA contains among the highest amount of biogenic silica in its amorphous form (>95%
SiO2) [26-28] compared to other biomass materials, such as ash from sugarcane bagasse
(57-73% SiO2) [22, 29-31]. In addition, the percentage of ash in RH is many times higher
(at 13-25 wt. %, dry basis) [32, 33] compared to that of sugarcane bagasse (at only 1.9-
6.8 wt. %, dry basis) [34]. Further, it was reported that such a high percentage of silica is
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very unusual in nature and that no other plant waste even approaches the amount of silica
that is found in rice husk [34, 35]. The recovery of amorphous silica from rice husk is
deemed the most economical source of silica due to the presence of abundant source of
RH around the country. Bangladesh produces on average 338.90 lac MT (2011-2012)
Paddy per year, giving approximately 677.8 lac MT rice husk. Generally, RH constitutes
about one fifth or 23% (by weight) of the dried paddy rice. Bangladesh has 53423 rice
mills (Large, Medium, and Small). Approximately 95% RH is produced by milling
process of rice mills. Rice husk can be converted into different types of fuel through a
variety thermo-chemical conversion process. Therefore, rice husk is used as a potential
source of heat energy and one of the largest sources of biomass because it contributes
26% of the total biomass about 42390 thousand MT biomass produced in Bangladesh. At
present in Bangladesh about 70-75% of husk is consumed for steam production in mills.
The average calorific value of RH in natural and dry state is 14700 kJ/kg. After burning
in boiler, Approximately 25% of the consumed RH is converted into RHA which is
considered as a completely waste material till date in Bangladesh [36].
The advantage of using rice husk ash as an economical raw material for sodium silicate
for its superior quality and cost effectiveness compared to present technology of
producing silica from quartz. The process of conversion starts from a raw material which
has little or no cost, on the contrary this waste RHA would cause environmental pollution.
The demand for RHA is growing in the construction industry and infrastructure projects
owing to its multiple benefits, such as comparatively low cost, environment sustainability,
and simplicity in production process. For example, the limited supply of silica fumes has
increased its rice across South Asian countries. In such context, RHA acts as a sustainable
and low-cost alternative for silica fumes. The impact of aforementioned factor is
propelling the growth of the global RHA market [37, 38]. Proposed process is energy
efficient and also consumes much lower energy compared to an alternative process
involving fusion of selected quality of sand [39].
1.2 Statement of the Problem
The production of high ratio water glass adhesive requires high temperature furnace
process or hydrothermal process. The raw materials requirement for existing furnace
process are high quality quartz sand and sodium carbonate or soda ash which is 100%
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import based, therefore, the inadequacy of the materials pose a problem to the water glass
adhesive manufacturing industries. Moreover, crystalline structure of quartz sand requires
high temperature (1000-1400ºC) which made the process expensive [12].
The main challenge of the existing process is to produce water glass adhesive at lower
temperature, while keeping silica to alkali ratio high (> 3.5). The hydrothermal process
based on quartz cannot produce water glass adhesive having more than 2.5 ratio of silica
and alkali. The furnace process has limitation of producing up to 3.3-3.4 ratio water glass
adhesive [40, 41].
RHA, on the other hand, is produced in abundance as an unutilized waste biomass product
and their use will be a good source of silica for water glass adhesive production in
Bangladesh. Alkaline extraction of RHA for high ratio water glass adhesive requires
much lower temperature consumption compared to existing furnace process, as
amorphous structure requires lesser energy (less than 200ºC) than crystalline structure
[41]. Hence, it will be an efficient and economically viable method of producing water
glass adhesive through alkaline extraction of RHA. Therefore, this study explores the
utilization of the abundant biomass (RHA) as raw materials for high ratio (> 3.5) water
glass adhesive production. This process would reduce the cost of production in addition
to preventing the environmental pollution associated with the RHA disposal.
1.3 Aim and Objectives of the Research
The aim of the research is to produce high ratio water glass adhesive by alkaline extraction
of amorphous silica rich locally available RHA by hydrothermal method at lower temperature
compared to conventional method.
The objectives of this research work are:
▪ To design and develop an economical production route for producing high ratio
water glass adhesive from RHA
▪ To determine the optimum parameters including ratio of RHA and sodium
hydroxide, molarity of sodium hydroxide, ratio of RHA and water, and reaction
period for extracting silica from RHA as water glass solution
▪ To synthesis water glass having higher weight ratio of silica and alkali (more than
3.5)
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▪ To determine the optimum RHA quality by analyzing the chemical composition
and crystal phases of the RHA for the production of water glass adhesive
▪ To characterize the RHA and water glass adhesive produced from RHA by XRF,
XRD, FESEM and other analytical method in order to the elemental composition,
crystal phases, morphological properties and the physico-chemical properties
respectively
▪ To overcome existing problems of producing high ratio water glass by replacing
amorphous silica source (RHA) in place of crystalline silica (quartz sand) in
hydrothermal process.
1.4 Research Questions and Basic Assumptions
The basic assumptions initially based on available data and literature are:
▪ Has RHA a suitable or economical silica source for water glass adhesive
production?
Assumption: RHA as silica source will be a suitable and economical raw material
for the production of water glass adhesive.
▪ Will it possible to extract the amorphous silica from RHA as silicate solution at
lower temperature?
Assumption: The amorphous silica from RHA will be highly reactive at lower
temperature.
▪ Will it possible to produce high ratio water glass solution from RHA using
hydrothermal method?
Assumption: Water glass adhesive having high ratio of silica and alkali more than
3.5 from RHA can be possible by hydrothermal method.
▪ Can the water glass solution be evaporated to get optimum density and viscosity?
Assumption: Water glass solution can be evaporated to get optimum density about
40 ºBe`.
▪ Do the physical and chemical properties of water glass adhesive conform to
standard water glass adhesive?
Assumption: The properties of water glass adhesive will match the standard
commercial water glass adhesive.
▪ Will the production cost reduce compared to existing method?
Assumption: The production cost will be reduced.
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1.5 Justification of the Research
Although various uses for RHA have been recommended in literatures, their disposal or
utilization remains a major concern especially in the context of Bangladesh. Soluble
silicates produced from silica are widely used in the field of packaging, ceramic, soap and
detergent, welding electrode, paint, and construction chemical as a major component.
RHA is rich in amorphous silica (about 70-80%) and can be an economically feasible raw
material for production of water glass. RHA has high content of silica and being readily
available, with an estimated 3.4 million ton of rice produce yearly in Bangladesh, can be
used as a substitute for the relatively expensive and imported raw material employed
without compromising the quality.
The reasons which are responsible for agricultural waste to be considered as good
economical silica source and have potential for the large scale production are [42, 43]:
▪ Low cost of the raw material
▪ High silica content
▪ Comparable silica quality
▪ Fine sized amorphous silica source
▪ Less energy requirement to extract the amorphous silica compared to
crystalline silica of quartz sand
1.6 Significance of the Research
The production of water glass adhesive depends on high temperature process and import
of some raw materials. Due to these facts there is a need to utilize the available abundant
biomass, (e.g. RHA) for the production of water glass adhesive using the non-
conventional methods. This study proposes production of highly pure end product with
homogenously and lower temperature that is useful in various applications. Moreover,
utilizing the biomass (RHA), which is otherwise landfilled and wasted, is an
environmentally friendly and energy wise efficient approach that will enhance the
technologically advancement of the society and knowledge.
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1.7 Methodology
The basic flow diagram of experimental methodology is shown in Figure 1.1.
Figure 1.1: Experimental procedure.
1.8 Layout of the Report
RHA could be a suitable candidate as feedstock for producing water glass adhesive
because of their high silica content (>95%) and large availability. In recent years
environmental demand and sustainable development have become increasingly
important. It is important to study and utilize RHA biowaste, and convert RHA into value
added materials. This report describes the research work done to produce water glass
adhesive from locally available RHA. This work includes the investigation of digestion
conditions such as digestion temperature, ratio of RHA and sodium hydroxide, ratio of
RHA (RHA)
Characterization: 1. Silica content (%) 2. Moisture content (%) 3. Loss on ignition (%)
Slurry of Carbon-Silica Solution
Step-1: Alkaline Extraction
Diluted Sodium Silicate Solution
Step-2: Filtration Characterization: 1. Density (Baume) 2. Specific gravity 3. Solid content (%) 4. Alkali Content (%) 5. Silica content (%) 6. Weight ratio of silica: soda
Step-3: Evaporation
Concentrated Water Glass Adhesive
Characterization: 1. Density (Baume) 2. Specific gravity 3. Solid content (%) 4. Alkali Content (%) 5. Silica content (%) 6. Weight ratio of silica: soda
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RHA and water and digestion period to extract silica from RHA as water glass. The work
is reported in six chapters.
Chapter 1 introduce the research topic and the overview of the research. It also includes
background of research, statement of the problem, aim and objective, justification and
significance of the research.
In Chapter 2, a review of literature on conventional process of producing water glass and
its physico-chemical properties and applications are presented. A critical review on prior
arts on production methodologies using RHA are also described in this chapter. From this
literature review, hypotheses related to the research were developed to serve as a guide
for the experimental works.
Chapter 3 presents the experimentations and methodologies used in this research for the
production of water glass from RHA.
Chapter 4 comprises the results and discussion. The effect of experimental variables to
produce water glass adhesive from amorphous, carbon-free siliceous RHA are also
described and analyzed here. The analysis converged to overall understanding, and thus
leads to optimization of reaction/digestion temperature, reaction period, consumptions of
raw materials in order to control the ratio of silica and alkali content in final adhesive
solution.
Chapter 5 sums up the findings and provides recommendations for future work in this
field.
CHAPTER TWO
LITERATURE REVIEW
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CHAPTER 2. LITERATURE REVIEW
2.1 Definition of Adhesive
An adhesive is a material used for holding two surfaces together. An adhesive must wet
the surfaces, adhere to the surfaces, develop strength after it has been applied, and remain
stable [44]. Adhesion is a specific interfacial phenomenon. There are three main theories
of adhesion, adsorption and diffusion. All probably apply to most adhesives. Surface
preparation before applying the adhesive is of prime importance. For a material, to
perform as an adhesive, it must have four main requirements [45]:
▪ It must "wet" the surfaces - that is it must flow out over the surfaces that are being
bonded, displacing all air and other contaminates that are present.
▪ It must adhere to the surfaces - that is after flowing over the whole surface area it
must start to adhere and stay in position and become "tacky".
▪ It must develop strength - the material must now change its structure to become
strong or non-tacky but still adherent.
▪ It must remain stable - the material must remain unaffected by age, environmental
conditions and other factors as long as the bond is required.
There are different types of adhesive in market based on their application. The raw
materials for adhesives are mainly organic/inorganic polymeric materials, both naturally
occurring and synthetic. Water glass adhesive is the most commonly used inorganic
polymeric adhesive for corrugated carton [45]. This research is mainly focused on water
glass or sodium silicate-based adhesive.
Commonly known as "water-glass", soluble sodium silicates are colorless low-cost
inorganic materials. When used as adhesives, liquid silicates offer significantly low cost,
versatility, and ease of handling. Their main use is to bond porous substrates such as paper
and cardboard where an inexpensive, fast processing adhesive is required. Other adhesive
applications for sodium silicates include bonding of wood, metal foils or glass to porous
substrates, glass fiber insulation bonding, as well as the fabrication of foundry molds and
abrasive wheels. Because of its inorganic nature sodium silicates can be formulated into
industrial cements with exceptionally high resistance to temperatures and chemicals.
Sodium silicate adhesives are usually supplied as a viscous water solution. The adhesive
bond forms either by (1) the evaporation of water or (2) chemical reaction. Sodium silicate
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water solutions are often used directly as an unmodified adhesive, but they can also be
blended with polymeric additives to improve specific properties such as toughness [15].
2.2 Water Glass or Sodium Silicate
Sodium Silicate, also known as water glass is a translucent-opaque compound of oxides
of sodium and silica. It has a range of chemical formula varying in sodium oxide (Na2O)
and silicon dioxide or silica (SiO2) contents or ratios. It is soluble in water and it is
prepared by reacting silica and caustic soda under specified pressure and temperature [8].
The general formula for sodium silicate is M2O. x SiO2
Where M is Na, and x is the ratio of silica to alkali in the product. Conventionally this is
expressed as a molar ratio (MR) or weight ratio (WR), defining either the number of
moles silica (SiO2) per mole of alkali metal oxide (M2O), or the ratio of weight percent
silica to weight percent alkali, respectively. The use of weight ratio is more common
within industrial applications [8, 9, 46].
All of the above alkali silicates are alkaline substances (pH values of the concentrated
products being usually between 10 and 13). The alkalinity of the products increases as the
MR or WR is reduced [46].
Alkali silicates are produced and marketed as glass lumps, ground glass, aqueous
solutions or dried powders. The physical, chemical, toxicological and eco-toxicological
behavior of these products is strongly dependent on the mole ratio or weight ratio of
SiO2:M2O, as this controls the degree of alkalinity of individual products [46].
Sodium silicate are available in aqueous solution and in solid form including powder form
or glass form shown in Figure 2.1. The pure compositions are colorless or white, but
commercial samples are often greenish or blue owing to the presence of iron-containing
impurities [8-10].
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(a) (b) (c)
Figure 2.1: (a) Liquid water glass [47]; (b) Powder water glass [48]; (c) Solid water glass [49].
In industry, the various grades of sodium silicate are characterized by their SiO2:Na2O
weight ratio (weight ratios can be converted to molar ratios by multiplication with 1.032),
which can vary between 2:1 and 3.75:1. Grades with the ratio below 2.85:1 are termed
alkaline. Those with a higher SiO2:Na2O ratio are described as neutral [2, 8, 9].
2.2.1 Atomic structure of water glass
A random atomic arrangement as would appear in a sodium silicate glass is shown
schematically in Figure 2.2. Here the building blocks of the glass network are polyhedra
formed around what is known as a network-forming (NWF) cation that is, a positively
charged ion such as, in this case, silicon (Si4+). The four positive charges of the silicon
ion lead it to form bonds with four oxygen atoms, forming SiO4 tetrahedra, or four-sided
pyramidal shapes, connected to each other at the corners. An oxygen atom that connects
two tetrahedra is known as bridging oxygen. An oxygen atom joined to only one silicon
atom is a nonbridging oxygen; it is one remaining negative charge is satisfied by bonding
to a network-modifying (NWM) cation in this case, a univalent sodium ion (Na+) which
occupies an interstice adjacent to the SiO4 tetrahedron. This corner-sharing tetrahedral
structure achieves a liquid-like randomness, rather than a crystalline regularity, because
there is a bending of the Si-O-Si bond at the bridging oxygen. In addition, there are twist
angles arising between two connecting tetrahedra, as shown in Figure 2.3 [50].
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Figure 2.2: The irregular arrangement of ions in sodium silicate glass [50].
Figure 2.3: Basic building block of a silica glass network. [50].
In Figure 2.3, Silicon ions bond to oxygen atoms, forming tetrahedral structures that are
connected by a bridging oxygen atom. The tetrahedra revolve around the oxygen-silicon
bond, while the angle at which the two tetrahedra are connected also varies.
Figure 2.4 shows the molecular structure and arrangement of ions in crystalline silica
(sand), quartz glass and sodium silicate glass.
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Figure 2.4: Molecular structure of crystalline silica, quartz glass and sodium silicate glass [6].
2.2.2 History of water glass
Water glass was defined in Von Wagner's Manual of Chemical Technology (1892
translation) as any of the soluble alkaline silicates, first observed by Jean Baptist van
Helmont around 1640 as a fluid substance made by melting sand with excess alkali.
Glauber made what he termed "liquor silicum" in 1646 from potash and silica. Johann
Nepomuk von Fuchs, in 1818, obtained what is now known as water glass by treating
silicic acid with an alkali, the result being soluble in water, "but not affected by
atmospheric changes". Von Wagner distinguished soda, potash, double (soda and
potash), and fixing (i.e., stabilizing) as types of water glass. The fixing type was "a
mixture of silica well saturated with potash water glass and a sodium silicate" used to
stabilize inorganic water color pigments on cement work for outdoor signs and murals
[2].
2.2.3 Physical properties of water glass
Sodium silicates are translucent-opaque liquids that feel slippery to the touch. These
products do not have a distinguishing odor. Sodium silicates are stable at normal
temperatures and pressures and are not combustible. Spills of sodium silicate can be very
slippery. Spilled material dries to form a glass film that can cut skin.
Sodium silicate is stable in neutral and alkaline solutions. In acidic solutions, the silicate
ion reacts with hydrogen ions to form silicic acid, which when heated and roasted forms
silica gel, a hard, glassy substance.
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2.2.4 Manufacturing process of water glass
Sodium silicate is commonly manufactured using a reaction in liquid phase or in solid
phase. Both processes use an alkaline source and a silica source as raw materials [23].
▪ Solid phase (furnace route)
Water glass has been manufactured since the 19th century, and the basic principles of
making “silicate of soda” have not changed since that time. It is commonly produced by
roasting various quantities of soda ash (sodium carbonate, Na2CO3) and silica sand (a
ubiquitous source of SiO2) in a furnace at temperatures between about 1000 and 1400°C,
a process that gives off carbon dioxide (CO2) and produces sodium silicate (Na2SiO3;
usually represented by its two constituents, sodium oxide (Na2O) and silica (SiO2)) [2, 8,
51]:
Na2CO3 + SiO2 → Na2O.SiO2 + CO2
This roasting produce fused glassy lumps called cullet, which can be cooled and sold in
that form or ground up and sold as powders. Lump or ground water glass in turn can be
fed into pressurized reactors for dissolving in hot water. The solution is cooled to a
viscous liquid and sold in containers ranging in size from small jars to large drums or
tanks. Figure 2.5 represents the flow diagram of producing water glass by furnace method
[8, 51].
Figure 2.5: Basic flow diagram of producing water glass by Furnace route [51].
Sodium sulfate can be also used as alkali source which is melt at temperatures far below
that of silica (<1000°C vs >1400°C) in the presence of carbon. Either is melted, and silica
dissolved into the molten material, where it reacts to form sodium silicate [51]:
2 Na2SO4 + C + 2 SiO2 → 2 Na2SiO3 + 2 SO2 + CO2
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▪ Liquid phase (hydrothermal route)
Sodium silicate liquid can also be prepared directly by dissolving silica sand under
pressure in a heated aqueous solution of caustic soda (sodium hydroxide, NaOH) shown
in Figure 2.6 [2, 51]:
SiO2 + 2NaOH → Na2O·SiO2 + H2O
Figure 2.6: Basic flow diagram of producing water glass by hydrothermal route [51].
In general, solutions are subsequently filtered to remove any residual turbidity and
adjusted to yield products to a particular specification.
▪ Raw materials
The main raw materials for the production of soluble silicates are:
▪ Silica source: Quartz sand [23], RHA [19], Sugarcane bagasse ash [30], Fly ash
[52] etc.
▪ Alkali source: Alkali carbonates (e.g. soda ash Na2CO3 [9]), alkali hydroxides
(e.g. NaOH [19]),
▪ Utilities: Process water, fuels / energy, (e.g. oil, gas, electricity).
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2.3 Why it is Called Water Glass ?
It comes in several forms, including powders, gels and most commonly as clear, rocky
pebbles that resemble glass. Sodium silicate is nicknamed as "water glass" because it is
water soluble and turns into a viscous liquid when dissolved, but resembles small shards
of glass when dried out [2, 10, 53].
Sodium silicate is an extremely water-soluble substance that turns water highly viscous
if enough is added. However, the substance will return to its normal, solid form when it
is dried out. The solid form has a glassy, angular look (like Figure 2.7 and Figure 2.8) to
it, which is why ancient alchemists nicknamed the substance "water glass" [53].
Figure 2.7: Sodium silicate crystals liquefy readily in water [53].
Figure 2.8: Sodium silicate crystals magnified 100x. Comstock Images [53, 54].
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2.4 Demand of Water Glass in Various Market
Sodium Silicate market size is forecast to exceed USD 10 billion by 2024; according to a
new research report by Global Market Insights [1]. This global rising demand of water
glass makes it an interesting research material.
Sodium silicates are one of the most popular non-organic chemicals, and are
manufactured all over the world. However, due to their relatively low rice and high
importance of the liquid form in trade (silicates dissolved in water), international
turnovers are performed usually on a local level, i.e. only on the European continent. The
largest production capacities are located in China (about 40%). The most developed
regions of the world (Europe, North America, Japan) are responsible for approx. 40% of
global production capacities in total (Figure 2.9) [55].
Figure 2.9: Geographic structure of production capacities of water glass [55].
Sodium silicates are manufactured in solid form (glassy sodium silicate) and in liquid
form (sodium water glass usually obtained by dissolving glassy sodium silicate in water).
Sodium silicates are used for production of precipitated silica (about 35% of consumption
in Europe, used mainly in the tyre and beauty product industries), detergents (more than
15%), paper, zeolites and in other industries (Figure 2.10) [9, 18, 55].
North America 13%
Europe 19%
Japan 8%
Other 18%
China 42%
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Figure 2.10: Application segments of water glass in Europe [55].
2.5 Applications of Water Glass
2.5.1 Adhesive with staying power
Liquid sodium silicates of suitable concentration, usually in the weight ratio range of 2.8
to 3.3 are widely used as adhesives in making fiber drums, paper tubes, and miscellaneous
materials [8, 9].
Sodium silicate is an excellent adhesive for sealing fiberboard boxes because it sets
quickly and firmly holds the flaps together. The advantages of soluble silicate adhesive
include the easy wetting of the surfaces to be joined, controlled penetration, suitable
viscosity, good setting characteristics, and high strength [13, 14].
Sodium silicates are especially valuable as adhesives because they can change from a
liquid to a semi-solid condition upon the loss of a small amount of water. This property
results in the adhesive taking a quick, initial set - so important in the modern, highspeed
machine manufacture of corrugated paper products (Figure 2.11 and Figure 2.12) [13,
56].
Paper 12% Zeolite 11%
Detergent 17% Other 25%
Silica 35%
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Figure 2.11: Corrugated paper board [57].
Figure 2.12: Sodium silicate pasting in red mark area [56].
2.5.2 Strong bonding cement
Sodium silicates are important in air-setting refractory specialties and chemical-resistant
mortars. Cements made with soluble silicate binders offer resistance to high temperatures,
acids, slumping, and redissolving after set. Sodium silicates are also used to modify the
physical properties of hydraulic materials such as portland cement. Adding silicate to
cement can reduce permeability by increasing the total number of bonds formed between
aggregate particles. Silicates can also be used to modify the set time in cold weather or
adverse conditions [9].
2.5.3 More durable concrete with water glass
Sodium silicate solutions have a place as a concrete treatment; however, their role are as
concrete surface hardeners. Sodium silicate solutions perform effectively as
waterproofing sealers since they have properties of depth of penetration. Sodium silicate
is also used as penetrating sealer on concrete surface (Figure 2.13 and Figure 2.14) [8, 9,
58].
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Figure 2.13: Sodium silicate acts as water proofing sealer on concrete surface [58].
Figure 2.14: Sodium silicate acts as penetrating and waterproofing sealer on concrete surface [59].
There are a number of outstanding advantages of sodium silicates as binders in the cement
mixtures. These include resistance of the set cements to acid, to high temperature, and to
water. Silicate cements may be set by the reaction of an added substance, causing the
formation of a silica gel or heavy metal silicate [8].
Other advantages of sodium silicates as binders in cements are ease of application, low
costs, and a strong bonding action for many types of surfaces [9].
2.5.4 Pulp bleaching and de-inking paper for recycling
Sodium silicates are used for de-inking, sizing, coating, and bleaching of recycled paper
products in the pulp and paper industry. Sodium silicates have long been the stabilizer of
choice for hydrogen peroxide bleaching of cellulose. Hydrogen peroxide is an efficient
and economical bleaching agent when properly stabilized against heavy metal ions,
enzymes and other process impurities [8, 9, 18].
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Optimal efficiency is achieved when the solution is correctly buffered in the alkaline pH
range. Sodium silicates are effective and economical stabilizers and buffering agents.
Sodium silicates and hydrogen peroxide are used together for the bleaching of cellulose,
in the following industries:
The Waste Paper or Paper Recycling Industry, where sodium silicates aid in repulping
and de-inking operations. Silicates stabilize and buffer peroxide bleaching steps to
provide a whiter recycled pulp [8].
The de-inking process solubilizes and removes nonfibrous materials, such as inks, from
the fiber stock. This is usually accomplished by:
▪ Mechanical pulping of paper to be recycled
▪ Alkaline cook solution to suspend nonfibrous materials (primarily inks)
▪ Washing and/or flotation to separate de-inked fibers from impurities
Sodium silicates perform many of the roles in the waste paper reclaiming process. They
act as buffering agents, regulating the intensity of alkaline conditions. They also prevent
unnecessary degradation of the fiber stock during the alkaline cook. Silicates also act as
dispersing agents, suspending inks and other undesirable materials, preventing their
redeposition onto the fiber [8].
2.5.5 Building better detergents and soaps with water glass
The detergent industry is one of the largest consumers of soluble silicates incorporating
them in liquid and powder formulations for fabric washing and machine dishwashing in
both the domestic and industrial markets [8]. Builders binding to calcium and magnesium
ions in water. Sodium ions from sodium silicate help the builder (Figure 2.15).
Key roles of sodium silicate in detergent [9]:
▪ Wetting. Sodium silicates reduce the surface tension of liquids to improve soil
removal.
▪ Emulsification. Sodium silicates disperse oily soil into fine droplets suspended in the
wash solution, keeping the particles separate and preventing them from recombining.
▪ Deflocculation. Sodium silicates help break up inorganic or particulate soils into fine
particles, making them easier to remove from surfaces and easier to suspend in
solutions.
23
Figure 2.15: Builders binding to calcium and magnesium ions in water. Sodium ions from sodium silicate help the builder [60].
▪ Prevention of soil redeposition. Sodium silicates help prevent suspended soils from
reattaching to cleaned surfaces.
▪ Alkalinity and buffering. The alkalinity of sodium silicates enables them to
neutralize acidic soils, emulsify fats and oils, and disperse or dissolve proteins. Their
buffering capacity – stronger than most alkaline salts maintains the desired pH in the
presence of acidic compounds or on dilution.
▪ Corrosion inhibition. The polysilicate ion acts to form a physical barrier to prevent
alkali attack and protect sensitive glazed dishware, glass, and metallic surfaces,
including metal buttons, zippers, and washing machine components.
▪ Stabilization. Sodium silicates stabilize chlorine and oxygen bleaches during the
cleaning cycle.
▪ Phosphate replacement. Sodium silicates are widely used as a partial phosphate
replacement in phosphate-free formulations. They also can be used in conjunction
with synthetic zeolites to completely replace phosphates, while maintaining
detergency performance.
24
Figure 2.16 compares the excellent buffering capacity of sodium silicate to other alkalis
commonly used in detergent formulations.
Figure 2.16: Buffering capacity of various alkali solutions at 0.02% Na2O concentration [9].
Many detergent operations are performed using both the liquid and dry granular forms of
sodium silicates. Such operations range from metal cleaning, textile processing,
laundering and de-inking paper, to washing dishes, dairy equipment, bottles, floors, and
locomotives. For instance, in the textile industry, sodium silicates are used with bleaches,
soaps, wetting agents, synthetic detergents and other alkalis in operations such as cleaning
and finishing, kier boiling, wool scouring, bleaching and degumming. A large amount of
research has been done on the principles of cleaning and detergent processes, helping to
establish the value of sodium silicates as detergents [9].
25
2.5.6 Silica based products
Precipitated silica and silica gels may be prepared by treating sodium silicate solutions
with acids (Figure 2.17), washing the precipitated silicic acid to remove soluble salts,
drying, and reducing the resultant product to suitable particle size range. Siliceous ratio
silicates are generally used for making silica gels. The end product, a granular glassy
material, has an immense internal pore area, giving it the capacity to absorb large
quantities of moisture as well as other substances. This property makes gels useful as
dehumidifying agents for air and other gases, and as filtering agents to clarify juices and
beers. Specially prepared silica gels are used for making thermal insulation materials [8].
2.5.7 Better textiles
Sodium silicates are used in many textile mill applications [9]:
▪ Bleaching. They deactivate metals that catalyze the decomposition of hydrogen
peroxide. They also buffer pH at the optimum level for peroxide bleaching.
▪ Yarn and fabric pre-treatment. Silicates are used to remove wax, oil, and motes
from cotton. Choice of the proper silicate ratio and solids promotes better cleaning
and prevents redeposition of soil.
▪ Pad-batch dyeing processes. These processes are preferred over other dyeing
processes because they use less water, exhibit better dye bonding, can be operated at
lower temperatures, and have batch-to-batch uniformity. Sodium silicates buffer the
pH of the dye liquor and can remove oils and dirt that interfere with dye bonding.
26
Figure 2.17: Basic flow diagram of precipitated silica and silica gel from sodium silicate [61].
27
2.5.8 Environmentally friendly foundry binder
Another application of the gel-forming properties of sodium silicate is in the foundry
industry. Mixtures of sand and silicate for making both molds and cores in the foundry
are given an initial set by forcing CO2 gas under pressure through compacted forms. The
initial hardening of the silicate-sand mix in the CO2 process is caused by a chemical
reaction between the carbon dioxide gas and sodium silicate. The alkali of the latter is
partially neutralized, forming a gelatinous silicic acid which binds the sand particles
together in a stiff mass. Later, as the mix loses moisture, further bond strength is provided.
Molds and cores made by this process can be used immediately and need not be dried or
baked, as required in the case of oil or resin bonded forms. Sodium silicate solutions in
the weight ratio range from 3.22 to 2.00 are generally recommended for foundry use.
Often certain organic materials, such as sugars, are first mixed in the silicate to impart
special properties [8].
2.5.9 Soil stabilization
The gel-forming property of sodium silicate is used to advantage in soil stabilization.
Sodium silicate solutions, along with reacting chemicals, have been used for the
consolidation of porous soil structures for many years. Soils are solidified and stabilized
to increase their load-bearing capacity, to arrest settlement and lateral movement of
foundations, and to control the flow of water in earthwork engineering projects such as
dams, mines, tunnels, and excavations.
The stabilization of porous soils by sodium silicate is brought about by an induced gel
formation of a silicate solution after introduction into the soil. Gelling of the silicate
solution results in a modified soil structure of increased strength and reduced
permeability. The gelling may be brought about by either immediate or slow reaction. In
the first case, or immediate reaction, separate solutions of untreated sodium silicate and
of a reacting material are alternately introduced into the same soil formation. Upon
contact of the two solutions in the subsurface soil, immediate gelling occurs. In the second
case, which is the preferred method, reacting chemicals are mixed with the silicate
solution, causing a delayed self-hardening action. The delayed gelling solution, or
chemical grout, solidifies at a pre-determined time after being pumped into a pervious
soil formation. This grout solution may be varied in concentration, viscosity, and gel time
28
to meet various mixing and injection requirements. Hardening of the grout solution in the
soil is uniform and complete. This method of single injection has the additional advantage
of using only one final solution [8].
2.5.10 Silica sols and water treatment
Silica sols of colloidal silica may be prepared in different ways. Methods of preparation
include dialysis, electro dialysis, neutralization of a sodium silicate solution by an acid or
acid substance, peptizing a silica hydrogel, and passage of a sodium silicate solution
through an ion exchanger for sodium removal.
The theory of formation of colloidal silica in the activated form as used in water treatment
is generally regarded to be the growth of silica particles from low molecular weight silicic
acid. Freshly liberated from the sodium silicate solution by neutralization, these acids
polymerize, increasing in molecular size, and form micelles of polymerized silicic acid
with a strong negative charge. After a controlled aging period, the silica sol is diluted to
prevent further polymerization and to stabilize for a relatively short time the activated
silica.
Activated silica sols are used in water purification as coagulant aids, flocculating
impurities with aluminum and iron salts. In lime softening of water, activated silica acts
as a primary coagulant. Activated silica is used in the treatment of both raw and waste
waters.
Corrosion of iron in water systems may be controlled by the addition of small amounts of
sodium silicate, usually of siliceous ratio, which deposits a thin protective film of silica
on the metal. In alkaline cleaning operations, the presence of sodium silicate in the
detergent solution inhibits attack by the alkali on aluminum and will greatly retard the
attack on zinc. Again, a protective film on the metals derived from the sodium silicate
inhibits the action of the alkaline solution [8, 9, 18].
2.5.11 Coatings
Sodium silicate solutions, either unaltered or suitably modified, are used in making
various paints and coatings. Upon losing small amounts of water, thin layers of liquid
silicates first became tacky and then change to hard films.
29
Considerable loss of water occurs at ordinary room temperatures, but to render the film
more water-resistant, elevated temperatures are necessary for drying. If heat is to be used
in the drying operation, it is important that silicate films and coatings not be exposed to
an initial temperature that is too high. The temperature should first be slowly raised to
around 100ºC and held there until the majority of the water is removed. Then, the
temperature may be increased to the final level desired, such as in a 150- 370ºC range.
Sudden heating of the wet film to a high temperature is not desirable. Such treatment in
many instances would cause steam formation, resulting in blistering and loss of integrity
in the dried film. Infra-red heat is suitable for drying silicate films.
Sodium silicates used in coating and paint formulations vary in ratio from about 2.0 to
3.3. The alkaline ratio silicates dry somewhat more slowly than the siliceous liquids. The
alkaline ratio solutions dry to form films that are slightly more flexible than those of
siliceous ratios. If high temperatures are not practical, the water resistance of a sodium
silicate film may be enhanced by reaction with an acid or certain salt, such as boric acid,
phosphoric acid, sodium silicofluoride, and aluminum phosphate. Such materials may be
incorporated in the paint formula or applied as a solution in the form of a second coating
or curing treatment.
Two other well-established applications of sodium silicate coatings are for welding rods
(Figure 2.18) and roofing granules [8, 16].
Figure 2.18: Sodium silicate is used as binder in welding rod [16].
Sodium silicates have many useful properties not shared by other alkaline salts. This fact,
combined with their low cost, results in their use in a wide range of applications in
different industries. As described in previous section, sodium silicates are used in industry
as adhesives, detergents, ingredients in cleaning compounds, cements, binders, and
30
unique coatings, as well as coagulant aids, anticorrosives, catalyst bases, deflocculants,
chemicals, zeolites, etc. The different properties and functional characteristics of soluble
silicates can be used to efficiently and economically solve many problems arising in
industrial and chemical processes. The principle uses of water glass along with their
functions in specific field are summarized on below Table 2.1.
Table 2.1:Principle use of water glass in various industries [9]
Industry and Typical Applications
Silicate Function Principal Benefit
Ceramics Refractory cements Binder Air set, green strength Slip casting Deflocculant High solids Slurry thinners Deflocculant Reduces water Clay refining Deflocculant Improves fluidity Soap and Detergent Industries Household laundry powders
Binder, corrosion inhibitor, deflocculant
Processing aid for spray-dried and agglomerated products, corrosion protection, detergency
Liquid detergents and cleaners Deflocculant, buffer Detergency, corrosion protection Pulp and Paper industries Peroxide bleaching of pulp Chemical reaction Conserves peroxide, produces
whiter pulp De-inking Detergency Ink removal Raw water treatment Flocculation Clearer effluent Head box additive Flocculation Retains fines and fillers on the
wire Coating Film formation Grease-proofs, moisture resistant Adhesives for Laminating/Labeling
Adhesion Economical, strong bonds
White water treatment Flocculation Increases size of floc, improves clarification
Paper Board Spiral-wound tubes Adhesion Adds rigidity, economical Fiber drums Adhesion Adds rigidity, economical Water Treatment Lead and copper control Chemical reaction Reduces levels of toxic metals, Raw and wastewater treatment Flocculant Increases size and speeds
formation of floc Water line corrosion prevention Film formation Protective film inhibits corrosive
attack on metal Iron and manganese stabilization
Chemical reaction Improves taste, eliminates "red water"
31
Building Products/Construction
Hardening concrete Chemical reaction, sealant Hardens, acid resistant Acid-proof cements Binder Ease of use, economical Refractory cements Binder Strong bond, excellent refractory
and acid resistance. Thermal insulation Adhesion, film formation Fireproof bond. Soil solidification/grouting Gel reaction Economical binder Textiles Peroxide bleaching Chemical reaction Conserves peroxide, whiter whites Pad-batch dyeing Buffering Dye fixation, lower processing
costs Petroleum Processing Drilling muds Chemical reaction Controls heaving shale Corrosion prevention Chemical reaction Efficient, economical Emulsion breaking Chemical reaction Breaks emulsion Metals Porous castings Impregnation Seals leaks, fills voids Coating welding rods Binder Good bond and fluxing action Ore beneficiation Deflocculant Separation aid, corrosion control Foundry molds and core Binders Binder High strength, eliminates fumes Smelting dusts Agglomeration Eliminates dust, improves
environmental conditions
Pelletizing Binder Aids balling, increases strength of formed pellets
Briquetting Binder Improves flow characteristics and cohesive properties
2.6 Understanding of Physico-Chemical Properties of Water Glass
Liquid sodium silicates are solutions of glasses which are made by fusing varying
proportions of sand (SiO2) and soda ash (Na2CO3). These proportions are usually defined
by the specific product’s SiO2/Na2O weight ratio.
Sodium silicates have a wide range of characteristics to meet various application needs.
This section briefly reviews the major characteristics of sodium silicates.
2.6.1 Ratio of SiO2/Na2O
Weight ratio, it is defined by the ratio of SiO2/Na2O. Weight ratio is the most important
physical property of silicate. It determines the reactivity of silicate and the physical
32
properties such as viscosity. As the ratio of silicate increases, the alkalinity of the solution
decreases while the desired strength increases.
Sodium silicate composition can be designated as: Na2O • (SiO2) x – where x is the ratio
of the components and falls in the practical range from 0.4 to 4.0. Since a molecule of
Na2O weighs nearly the same as a molecule of SiO2, molecular and weight ratios are
nearly equal. Consequently, silica-to-alkali weight ratios are customarily used in the U.S.
for sodium silicates more siliceous than the metasilicates (which have a 1:1 mole ratio).
It is important to identify the sodium silicate required by specifying both the weight ratio
of silica to alkali and the concentration [9]. Mole ratio of 2 types sodium silicate are
shown in Figure 2.19 and Figure 2.20.
Figure 2.19: Sodium Silicate having
1:3.22 ratio of Na2O/SiO2 [8].
Figure 2.20: Sodium Metasilicate having
1:1 ratio of Na2O/SiO2 [8].
Properties of sodium silicate as a function of ratio are summarized in Table 2.2.
Table 2.2: Properties of Silicates as a function of ratio [8, 62]
High Alkalinity Low
High Buffering Low
High Solubility Low
High Bound moisture Low
High Drying time Low
Low Desired strength High
1.6 2.0 2.4 2.8 3.2
Ratio of SiO2 and Na2O
33
Method of analysis: It may be calculated directly by dividing the %SiO2 (determined
from either one of the above described procedures) by the %Na2O that has been
chemically determined. A more rapid, but still quite accurate, method consists of carefully
measuring the gravity and viscosity at 20ºC. The ratio of the SiO2/Na2O ratio can be
determined by dividing the silica content by alkali content [8, 9, 63].
2.6.2 Alkali content (Na2O)
The sodium oxide content of sodium silicate is determined by a chemical titration of a
sample with standard hydrochloric acid to pH 4.3 using either a pH meter or a suitable
indicator such as methyl orange. The usual procedure is to accurately weigh a 25-gram
sample of the silicate and dilute to 500 ml with deionized water. A 50 ml aliquot is then
titrated with 0.2 N HCI. For highly alkaline grades, a 15 gram sample is suggested [8, 9,
63].
Calculations:
% Na2O = (ml HCI x Normality x 3.1) / Sample Weight
2.6.3 Silica content (SiO2)
For all but the most critical situations, the SiO2 content may be calculated by subtracting
the %sodium oxide from the % total solids as determined by the previously described
procedures. For more precise purposes, a weighed sample is diluted with deionized water
and acidified with dilute hydrochloric acid. After evaporating to dryness on a hot plate,
the resultant silica gel is rinsed free of chlorides. The residue is ignited in a muffle furnace
in a carefully weighed crucible. After cooling and weighing, the residue is calculated
directly as SiO2.
2.6.4 Total solid content
The total solid of liquid silicates is the residue remaining after careful ignition of a small
sample in a muffle furnace. The procedure consists of accurately weighing a 1 to 2 gm
sample into a tared, previously fired, porcelain crucible and igniting to 1050ºC for one
half h. A few drops of 30% hydrogen peroxide should be added to prevent spattering.
34
After cooling in a desiccator, reweigh the crucible. The weight of the residue is the %
solids and is reported to the nearest 0.01%.
PQ, one of the largest manufacturers of water glass in world, offers liquid sodium silicates
ranging in weight ratio from 3.25 to 1.60 and in densities from 40Bé° (degrees Baume)
(unit of density) to 52Bé° at 20°C (Table 2.3). The properties of the liquid soluble silicates
are summarized in Table 2.3 [9].
Table 2.3: Typical properties of PQ sodium silicate solutions [9]
Product Name
WT. Ratio (SiO2/Na2O)
Properties Viscosity centipoise
Characteristics %
Na2O %
SiO2
Bé° lb/gal g/cm3 pH
STIXSOTM 3.25 9.22 30.0 42.7 11.8 1.42 11.3 830 Syrupy liquid
N® and N® 3.22 8.90 28.7 41.0 11.6 1.39 11.3 180 Syrupy liquid
O® 3.22 9.15 29.5 42.2 11.8 1.41 11.3 400 More concentrated
K® 2.88 11.00 31.7 47.0 12.3 1.48 11.5 960 Sticky, heavy
M® 2.58 12.45 32.1 49.3 12.6 1.52 11.8 780 Syrupy liquid
STARTM 2.50 10.60 26.5 42.0 11.7 1.41 11.9 60 Brilliant clear
RUTM 2.40 13.85 33.2 52.0 13.0 1.56 12.0 2100 Heavy syrup
DTM 2.00 14.70 29.4 50.5 12.8 1.53 12.7 400 Syrupy, alkaline
B-WTM 50 1.60 16.35 26.2 50.3 12.8 1.53 13.4 280 High alkalinity, syrupy
Occidental Chemical (Oxychem), another largest manufacturer of water glass, offers
liquid sodium silicates ranging in weight ratio from 3.25 to 1.80 and in densities from
41Bé° to 52Bé° at 20°C (Table 2.4). The properties of the liquid soluble silicates are
summarized in Table 2.4 [8].
Table 2.4: Typical properties of Oxychem sodium silicate solutions [8]
Product Name
WT. Ratio (SiO2/Na2O)
Properties Viscosity centipoise
Characteristics %
Na2O %
SiO2
Bé° pH
40 3.22 9.1 29.3 41.5 11.3 200 Syrupy liquid 40 clear 3.22 9.1 29.3 41.5 11.3 200 Specially clarified 42 3.22 9.3 30.0 42.5 11.3 400 Syrupy liquid 20 clear 3.25 8.9 28.9 41.0 11.0 175 Specially clarified
47 2.84 11.2 31.8 47.0 11.5 650 Sticky, heavy JW clear 2.54 10.6 26.9 42.0 11.7 65 Clarified Syrupy liquid JW-25 2.54 10.6 26.9 42.0 11.7 65 Syrupy liquid 52 2.40 13.9 33.4 52.0 11.8 1800 Heavy syrup 50 2.00 14.7 29.4 50.0 12.3 340 Syrupy, alkaline WD-43 1.80 13.3 23.9 43.8 12.5 60 High alkalinity, syrupy
35
Liquid silicates range in viscosity from very fluid, slightly sticky consistencies to thick
substances that barely flow. As a general rule, the higher SiO2/Na2O ratio products (2.8
to 3.22) are used for adhesive and coating applications [8, 9].
2.6.5 Density and specific gravity
The silicate industry expresses density in degrees Baume (Bé°), which can be converted
to specific gravity as follows [8]:
Specific gravity = 145 / (145 – degrees Baume`)
Method of analysis: The specific gravity of liquid sodium silicates is usually measured
at 20ºC using a Baume hydrometer standardized against a standards certified hydrometer.
Measurements may be taken at temperatures other than 20ºC and extrapolated to 20ºC for
reporting purposes by referring to Table 2.
2.6.6 Viscosity
Viscosity is a function of weight ratio, solids and temperature. The viscosity of the silicate
binder must be low enough so that good mixing and wetting of particles can be achieved.
The viscosity of silicates increases as the content of Na2O by weight increases. Silicate
viscosity can be reduced by diluting with water or by heating.
Method of analysis: The viscosity of liquid sodium silicate solutions may be determined
by either a Stormer or a Brookfield Viscometer. The instruments must be standardized
against Bureau of Standard oils in the viscosity range of the material being measured. The
standard temperature for reporting viscosity data is at 20ºC. Readings may be taken at
other temperatures and extrapolated to 20ºC by referring to Chart 1. Data may be reported
as either Stormer Seconds or Centipoises. The conversion factor for converting Stormer
Seconds to Centipoises is 2.75 (Stormer Seconds x 2.75 = Centipoises.) The single most
important factor in obtaining satisfactory results, besides careful instrument calibration,
is a precise determination of the temperature of the silicate solution [8, 9].
36
2.6.7 pH
pH of silicates is also a function of ratio; as the weight ratio increases, the pH decreases.
All silicate products are in the pH range from 11 to 13. Table 2.5 gives pH values of
commercial concentrations of selected grades of liquid sodium silicate [9].
Table 2.5: pH values of selected water glass solution [9]
WT. Ratio
(SiO2/Na2O)
Density at 20ºC
% Na2O % SiO2 Bé° pH
3.22 9.1 29.3 41.5 11.3
3.22 9.1 29.3 41.5 11.3
3.22 9.3 30.0 42.5 11.3
3.25 8.9 28.9 41.0 11.0
2.84 11.2 31.8 47.0 11.5
2.54 10.6 26.9 42.0 11.7
2.54 10.6 26.9 42.0 11.7
2.40 13.9 33.4 52.0 11.8
2.00 14.7 29.4 50.0 12.3
1.80 13.3 23.9 43.8 12.5
2.7 Interconnecting Relations Between the Properties of Water Glass
2.7.1 Modulus and concentration of water glass
The zone of water glass in Na2O-SiO2-H2O three component phase diagram is only the
shadowed area in Figure 2.21. In this zone, the sodium silicate dissolves in water existing
in both true solution state and colloid state. The mol ratio (m) of SiO2/Na2O is defined
“modulus”. When m = 2, only the true solution or crystalline of the silicate can be
obtained; when m = 4 all the silicate transfer to suspended colloid micelle and lost its
viscosity. Therefore, the modulus “m” of sodium water glass must be greater than 2 and
less than 4, and the concentration of water glass must be greater than 35% and less than
42% [43, 64].
37
Figure 2.21: The component phase diagram of Na2O- SiO2 -H2O [43].
2.7.2 Viscosity/specific gravity/density of water glass change with temperature
The viscosity of sodium silicates (at commercial concentrations) can be reduced to less
than 1 poise if they are heated sufficiently and if evaporation is prevented (Figure 2.22).
There is a rather large change in viscosity values with change in temperature for the
relatively viscous sodium silicate solutions. Figure 2.22 gives curves showing the change
in viscosity with temperature change for the most common grades of sodium silicate made
by PQ.
Figure 2.22: Viscosities of water glass solutions as a function of temperature [9].
38
Gravity correction for temperature is shown in Table 2.6. The standard temperature used
for measuring gravity and viscosity values of sodium silicate solutions is 20°C. However,
it is sometimes desired to convert gravity Figures obtained at temperatures other than
20°C to approximate values at 20°C.
Table 2.6: Correction of density for temperatures [8]
Gravity correction for temperature changes to convert baume to approx. values at 20°C for Oxychem Silicate 40 grade
Temperature (ºC)
Add/subtract Correction Baume
Temperature (ºC)
Add/subtract Correction Baume
4.4 Subtract 0.7 54.4 Add 1.3
10.0 Subtract 0.5 60.0 Add 1.6
15.6 Subtract 0.2 65.6 Add 1.8
20.0 No correction 0.0 71.1 Add 2.0
26.7 Add 0.2 76.7 Add 2.3
32.2 Add 0.4 82.2 Add 2.5
37.8 Add 0.6 87.8 Add 2.7
43.3 Add 0.9 93.3 Add 3.0
48.9 Add 1.1 100 Add 3.2
Silicate density decreases as temperature increases, as shown in Table 2.7.
Table 2.7: Densities of selected PQ silicates at various temperatures [9]
Densities of Selected PQ Water Glass at Various Temperatures Temperature
(oC) N® density
(Bé°) O® density
(Bé°) STAR® density (Bé°)
RU® density (Bé°)
10 41.5 42.6 42.4 52.3 21 41.0 42.2 42.0 52.0 32 40.6 41.7 41.5 51.7 38 40.3 41.4 41.3 51.3 49 39.9 41.0 40.8 51.0 60 39.4 40.5 40.4 50.6
2.7.3 Viscosity as a function of weight ratio and solid content of water glass
Figure 2.23 compares viscosity at constant solids content but varying ratios. Silicate
solutions have minimum viscosity at a 2.0 weight ratio. Viscosity increases as the weight
ratio becomes either more siliceous or more alkaline.
39
Figure 2.23: Viscosities of water glass solutions as a function of ratio at constant solid contents [9].
The viscosities of the more siliceous (higher ratio) silicates raise more rapidly with
increasing concentration than do the viscosities of the more alkaline products (Figure
2.24).
Figure 2.24: Viscosity of various weight ratio sodium silicate solutions [9].
40
2.7.4 Relationship between specific gravity and density of water glass
Table 2.8 shows the relationship between specific gravity and density (degrees Baume).
The density is measured with specially designed narrow-range hydrometers at a standard
temperature of 20°C.
Table 2.8: Density and specific gravity equivalents [8]
Density (Bé°) and specific gravity equivalents Degrees Baume
Specific Gravity
Degrees Baume
Specific Gravity
35.0 1.3182 48.0 1.4948 36.0 1.3303 49.0 1.5104 37.0 1.3426 50.0 1.5268 38.0 1.3551 51.0 1.5426 39.0 1.3679 52.0 1.5591 40.0 1.3810 53.0 1.5761 41.0 1.3942 54.0 1.5934 42.0 1.4078 55.0 1.6111 43.0 1.4216 56.0 1.6292 44.0 1.4356 57.0 1.6477 45.0 1.4500 58.0 1.6667 46.0 1.4646 59.0 1.6860 47.0 1.4796 60.0 1.7059
2.7.5 Viscosity as a function of density of water glass
Sodium silicate solutions exhibit increased viscosity when water is removed (Figure
2.25).
41
Figure 2.25: Viscosity of water glass solutions as a function of density [9].
2.7.6 Relationship between density and solid content of water glass
Density increases linearly with solids content, as shown in Figure 2.26 for selected
sodium silicates.
Figure 2.26: Densities of sodium silicate solutions as a function of solid contents [9].
42
2.7.7 Interrelationship between ratio, solid content, viscosity and specific gravity
Figure 2.27 shows the interrelationships of the ratio, gravity, and the solids content for
the various liquid sodium silicates, where the solids content for different Baume values
at 20ºC were made by Oxychem.
43
Figure 2.27: Relationship between solid content and density at various ratio of water glass [8].
Solids content of various weight ratio sodium silicate Solutions
Gravity – Degrees Baume @ 20o
Solid
Con
tent
– P
erce
nt (%
)
3.30 Ratio
3.22 Ratio
3.00 Ratio
2.54 Ratio
2.40 Ratio
2.00 Ratio
1.87 Ratio
1.58 Ratio
44
2.7.8 Relationship between pH and ratio of water glass
The pH of silicate solutions is closely related to concentration and ratio. The pH decreases
as the ratio increases (Figure 2.28). The high pH of silicate solutions is maintained until
the alkali is almost completely neutralized. While this buffering action – the ability to
resist changes in pH – increases with increasing proportions of soluble silicate, even dilute
silicate solutions will maintain a relatively constant pH despite the addition of acid.
Figure 2.28: pH values of sodium silicate solutions of various ratios [9].
2.8 Reaction Mechanisms of Water Glass
Depending on how they are modified, silicates may be involved in four basic types of
chemical reactions. Hydration/dehydration, surface charge modification, metal ion
reaction and precipitation/gelation.
2.8.1 Hydration/ Dehydration
Glassy nature of silicates imparts strong and rigid physical properties to a dried film
coating. Bonds formed by dehydration can be dissolved in water unless silicate is post
treated to temperature of more than 250ºC.
45
2.8.2 Surface charge modification
Dissolved silica donates its ionic charge of 2 to other materials and cause them to repel
one another causes dispersion, and on the other hand flocculation effect of silicates only
occurs with liquid silicates
2.8.3 Metal ion reaction
Soluble silica reacts with multivalent metal ions such as Calcium, Magnesium, Cadmium,
Iron, Lead etc. and precipitates these metals out of solution and renders them in to
insoluble and non-reactive product that displays long term stability.
2.8.4 Sol and gel Formation
Sodium silicates react with acidic compounds. When solutions of relatively high
concentration are acidified, the silicate anions polymerize to form a "gel". Activated
"sols" can be formed when relatively dilute concentrations of dissolved silica are
acidified. The degree of polymerization of silicate anions depends on solution
concentration, temperature, pH, and other factors. Gelation occurs most rapidly at pH
ranging from 5 to 8 (Figure 2.29). Time-delayed gelation – unstable sols – can occur in
pH ranges of 8 to 10 and 2 to 5. Colloidal silica sols can be prepared from sodium silicates
through ion exchange, dialysis, and other means.
Figure 2.29: Gel times of 3.22 ratio sodium silicate–sulphuric acid mixtures at 25ºC
[9].
Kg scale
46
Gelation occurs when the pH of a silicate solution drops below 10.7, such as due to
addition of acid. It is another method of insolubilizing silicates other than heating dried
silicates above 250ºC. Bonds formed are weaker but less likely to be destroyed by water.
2.8.5 Precipitation reactions
Sodium silicate solutions react with dissolved polyvalent cations to form precipitates.
Depending on conditions – pH, concentration, temperature, and so forth – the result is
either insoluble metal silicates or hydrated silica with adsorbed metal oxides or
hydroxides. This type of reaction can be used to form pigments or compounds that can be
used as extenders or fillers, ion exchange media, catalysts, adsorbents, and other products.
Calcium chloride reacts instantly with silicate solutions to provide an effective
mechanism for insolubilizing a silicate bond or coating. Sodium aluminosilicates, formed
by reactions with aluminum compounds, serve as ion exchange media for water softening
and synthetic zeolite molecular sieves. The extent and rate of reaction of silicates with
various metallic salts depends on the nature of the salt and its physical and molecular
structure. Such calcium carbonates as calcite, for example, exhibit limited interaction with
soluble silicate, while precipitated calcium carbonate shows high reactivity.
2.8.6 Interaction with organic compounds
Relatively few organic compounds are compatible with soluble silicate solutions. Simple
polar solvents can cause phase separation or dehydration. In mixtures with water-
immiscible oleophilic substances, the silicate separates into the aqueous phase; although,
for liquid detergent formulations, this can be overcome by adding a suitable hydrotrope
or emulsion stabilizer. A few miscible compounds, such as glycerin, sugar sorbitol, and
ethylene glycol, are sometimes used as humectants or to help plasticize a silicate film.
Organic ester setting agents are used to produce time delayed gelation of silicate solutions.
The hydrolysis of the esters consumes the alkalinity of the silicate solution over an
extended period of time.
2.9 Mechanisms of Water Glass Film Formation and its Characteristics
2.9.1 The binding strength of the dried water glass film
The tensile strength and moisture resistance vs modulus of the water glass is demonstrated
in Figure 2.30. As shown in Figure 2.30:, the tensile strength peaks at m=2.2-2.3, and
47
then decreases quickly until the modulus near 4. When the modulus m >4, the strength
drops to an unacceptable level. But the moisture resistance constantly increases with
modulus. Hence the optimum hardening condition of water glass lies between m=3.5-4.0.
But, at this value, no gel could be formed, unless the water content is reduced to less than
20% to complete the hardening process [43].
Figure 2.30: The tensile strength and moisture resistance vs. modulus of the water glass [43].
2.9.2 The progress of silicate gel formation
The formation of silicate gel may be divided into three steps:
(a) The enlargement of sol particle. The smaller the sol particle size is, the stronger the
binding strength of the gel
(b) The enlarged particles are linked together into a straight or branch chain
(c) Form a frame network with steric skeleton like following:
If a particle is linked with one or two other particles, its coordinate number is 1 or 2, and
symbol by “O”. With three or more other colloid particles, its coordinate number is 3 or
Modulas
Ten
sile
stre
ngth
or
moi
stur
e re
sist
ance
48
4, and symbol by “ ” . The larger a gel has average coordinate number, the higher
the strength and rigidity, but the worse the collapsibility and reclaim ability.
Silicates are converted to solid films or bonds by two methods: (1) evaporation of water
(dehydration) or (2) chemical setting mechanism. These can be used separately or in
combination. Chemical setting is often used to improve film moisture resistance, to
reduce setting time, and to increase ultimate bond strength as needed.
2.9.3 Evaporation drying
As water evaporates, liquid silicates become progressively tackier and more viscous. As
shown in Table 2.9. PQ’s N® sodium silicate, for example, has an initial water content
of 62.4% and a viscosity (at 20ºC) of 1.8 poises. With a weight loss of 6% by evaporation
the viscosity increases to 20 poises, and after 12% weight loss, to 2300 poises. When
evaporation approaches 14%, the viscosity is approximately 40000 poises, at which point
the silicate has in effect set. Further dehydration brings it to a final hardened condition.
Table 2.9: Effects of evaporation on viscosities of PQ water glass solution [14]
Product Name
Wt. Ratio SiO2/Na2O
Viscosities at 20ºC Approx. wt. loss for initial set. % Initial 6% Wt.
Loss 12% Wt.
loss N® 3.22 1.8 20 2300 13.6
O® 3.22 4.0 120 20000 11.2
K® 2.88 9.6 150 10000 12.0
Soluble silicates with higher SiO2/Na2O ratio are more desirable for applications
employing evaporation drying than are more alkaline, lower ratio grades. The higher ratio
silicates change from an almost water like condition to a semi-solid state when only a
small amount of water is evaporated.
Lower ratio silicates (less than 2.5) dehydrate more slowly, because their higher alkali
content creates a greater affinity for water. They may require heat for drying or treatment
with chemical setting agents to achieve the desired set. Flexibility increases in lower-ratio
silicates because of their tendency to hold onto water more tenaciously than higher ratio
silicates and thus have some degree of internal plasticization of the film by the residual
water. Because the low-ratio silicates tend to retain more water, they are less brittle than
the higher ratio silicates.
49
Silicate films are subject to moisture pick-up and degradation. However, this process can
be slowed if water is completely removed from the silicate. Air drying alone usually is
not adequate for films or bonds that will be exposed to weather or high moisture
conditions. For such applications, heat is usually recommended. Initially, the temperature
should be increased gradually to 90-100ºC to slowly remove excess water. Then final
curing can be done at least 175-370ºC. Heating too quickly may cause steam to form
within the film, resulting in blistering or puffing when the steam is released. For some
applications, where an insulated coating is desired, this intumescent property can be
useful. Infrared and microwave heating have been used successfully for hardening silicate
systems.
2.9.4 Chemical setting
For relatively insoluble bonds or films, liquid sodium and potassium silicates can be
reacted with a variety of acidic or soluble metal compounds. Neutralizing an alkali silicate
with acidic materials polymerizes the silica and forms a gel. Chemical setting agents that
can be used in this manner include: mineral and organic acids, carbon dioxide (CO2) gas,
and acid salts such as sodium bicarbonate (NaHCO3) and monosodium phosphate
(NaH2PO4). Multivalent metal compounds react with silicate solutions to form coatings
or bonds by precipitation of insoluble metal silicate compounds.
Chemical setting reactions generally occur rapidly, and these materials frequently are
applied as an after-treatment. Calcium chloride, magnesium sulfate, aluminum sulfate,
borax, and sodium metaborate used in this manner are generally applied as 5 to 10%
solutions. Chemical setting agents that dissolve slowly in water, such as finely divided
zinc oxide or sodium silica fluoride, can be used for silicate binders or coatings that
exhibit longer working lives. These agents usually are used at a level of approximately
7% by weight based on the weight of liquid silicate. Silica fluoride is particularly effective
for ambient temperature curing procedures. Some chemical setting agents will only react
with the silicate at elevated temperatures. Kaolinite clays and minerals, which decompose
at 200-260ºC into acidic compounds, are examples of this type of setting agent.
50
2.10 Beneficial Properties of Bond and Films Made by Water Glass
2.10.1 Moisture resistance
Silicate films are usually moisture sensitive. They can be made somewhat impermeable
to moisture and weathering when proper drying or setting procedures are used. The use
of clay and zinc oxide as chemical setting agents at high temperatures are especially
desirable since, after curing at 700ºC, it produces films capable of actually shedding
water.
2.10.2 Heat Resistance
When silicate films are completely dehydrated, they provide excellent resistance to high
temperature. Most silicates used for coatings or binders have softening points of
approximately 550-650ºC and flow points of 720-840ºC (Table 2.10).
Table 2.10: High temperature properties of soluble silicates in anhydrous state [14]
High temperature properties of water glass
PQ products Approx. softening point (ºC) Approx. flow point (ºC)
N, O 649 840
K 640 827
RU 615 790
D 590 760
B-W-50 565 729
Resistance to higher temperatures can be achieved by adding clay to the formulation.
Depending on the aluminum or magnesium content of the clay, service temperatures up
to 1760 to 1870ºC are possible due to the formation of a ceramic bond. Mixtures of
copper, nickel, chromium and stainless-steel powders in the silicate provide high
temperature-resistant coatings for metals.
2.10.3 Electrical properties
When completely dehydrated, sodium and potassium silicates exhibit good dielectric
properties. Sodium silicate (3.22 SiO2/Na2O ratio), when dehydrated, has a specific
resistance of approximately 3x1010 ohm-centimeters - about the same as common plate
51
glass. Electrical resistance is lower when more alkaline (lower SiO2/Na2O ratio) sodium
silicates are used.
2.10.4 UV transmission
Silicate films are generally stable to UV light exposure. Specially clarified sodium silicate
solutions, transmit 92 to 98% of light in wave lengths ranging from 430 to 700 nm. Below
400 nm transmission drops off rapidly, exhibiting a value of approximately 40% at 325
millimicrons. UV absorption is usually a function of the type of filler or pigment used in
a silicate film or adhesive.
2.10.5 Opacity and color
Silicate films can be made opaque by the use of titanium dioxide or aluminum pigments.
Fillers such as clay are used for semi-opaque films. Alkali-resistant pigments are
necessary for use with silicate vehicles. The following are suggested: white- titanium
dioxide; red- lime-free iron oxide; blue- ultramarine; green- chrome oxide; yellow- ochre;
brown- umbers or sienna’s; black- grease-free carbon black. When silicate coatings are
used on paper or other materials, the coating's alkalinity may darken the material's color.
The addition of 0.5 to 5.0% hydrogen peroxide, based on the weight of the silicate, may
be helpful.
2.10.6 Flexibility
Silicate coatings or adhesives alone generally are not suitable where a high degree of
flexibility is required. However, a moderate degree of flexibility is obtained by the
addition of plasticizers to the silicate solution. Typically, 1 to 5% by weight of sugar,
glycerin or other polyhydric alcohols are used. Up to 30% of sorbitol can be used,
provided the silicate solution is diluted to avoid excessive thickening. Rubber lattices can
also be employed as plasticizers. Incorporation of finely ground clays and similar fillers
will improve flexibility to some extent.
52
2.11 Modification of Water Glass to Enhance its Properties
2.11.1 Enhancement of waterproofing properties of water glass
Although not completely waterproof, silicate bonds show considerable resistance to
moisture, which increases with age and ratio of silica to alkali. The water resistance may
be improved by the addition of fine powders such as zinc oxide, a silica gel-coated
anhydrite, or whiting or other amorphous form of calcium carbonate, which react slowly
with the silicate to form an insoluble mass. Up to 1.0% of zinc oxide may be dispersed in
the silicate at 150 to 180oC. to form an apparently homogeneous product. The addition to
silicates of complex ammine salts formed by adding aqueous ammonia or amines to a
zinc or copper salt solution also improves their water resistance.
2.11.2 Physical modification of water glass
The fresh (newly prepared) water glass has a comparatively homogeneous polymerization
degree (molecular weight). On storage, the disproportion of molecular weight happens
with time. The higher polymerized particles go on further polymerization to form colloid
particles and grow to large particles slowly. The excess Na+ attacks the lower
polymerized molecules to depolymerize to small molecules, at last to sodium
orthosilicate. Hence the extremely aged water glass becomes a mixture of orthosilicate
and large colloid particles, thus reducing its binding strength by 20%-30% in about a
month.
The aging of the water glass can be eliminated with strong magnetic field, ultra-sound
vibration, AC electric charge, boiling under refluxing, or heat in autoclave, to provide the
energy to the water glass system and to re-homogenize the polymerization degree of the
silicates. This is called ‘physical modification’ and through it the lost binding strength
can be recovered.
2.11.3 Surface characteristics
Since silicate coatings and adhesives are inorganic aqueous polymers they perform most
effectively on hydrophilic, non-oily surfaces, where they achieve proper wetting and,
hence, maximum adhesion. Generally, a thin continuous silicate film between the surfaces
to be bonded provides optimum adhesion.
53
2.12 Research Focus: Water Glass as a Versatile-Economical Adhesive
From various applications, the adhesive application of water glass is mainly focused in
this research paper. Liquid sodium silicates are solutions of water-soluble glasses made
from varied proportions of sand (SiO2) and soda ash (Na2CO3). They range in viscosity
from slightly sticky fluids to thick substances that resist flow. The more siliceous silicates
(2.8 to 3.2 ratio) have long been used as dependable, low cost adhesives for corrugated
paper board.
Sodium silicate adhesives are used for bonding a variety of porous surfaces and materials
such as paper, mineral wool (used in insulation), perlite, mica and wood. Table 1 lists
properties and typical applications of the four sodium silicates commonly used as
adhesives. As silicates lose water, their viscosities increase and strong bonds are formed.
The viscosities of the more siliceous silicates rise more rapidly than the more alkaline
sodium silicate. Hence, alkaline silicate has a longer setting time.
One of the largest uses for water glass silicates having silica to alkali ratio of 2.8 to 3.9 is
as adhesives for many types of materials, particularly paperboard used in the manufacture
of corrugated board. The present annual consumption for this purpose is about 15,000
tons of the commercial silicate solutions containing 32 to 47% solids.
Although the largest amounts of silicate used as adhesive are for bonding paper, they are
also used for wood, metals, and other materials. Aluminum sheets coated with silicate
may be bonded to cellulose. Acid-proof tanks are made by using silicate to hold fabric
with a poly-vinyl halide coating (which does not itself adhere well to metal) to a metal
tank. Characteristics and a few applications of commercial grades of silicates at present
most commonly used as adhesives are summarized in Table 2.11.
54
Table 2.11: Characteristics of water glass as adhesive [18]
Characteristics of water glass as adhesive
% Na2O
% SiO2
Wt. ratio
Density at 20oC (oBe´)
Application
11.0 31.9 2.9 47 Sealing cartons, shipping containers, metal foils, wall boards, and trunk making
8.9 28.7 3.2 41 Corrugated paper board, flooring, paper tubes
9.2 29.5 3.2 42 Combined board. Plywood
8.3 28.2 3.4 39.7 Corrugated paper board
6.7 25.3 3.7 35.0 Special issues
2.12.1 Characteristics of water glass adhesive for corrugated carton
Viscosity - concentration -specific gravity relationships of three ratios used as adhesives
are shown in Figure 2.31. Comparison with Figure 2.31 indicates that the loss of 1 or 2%
of water from commercial adhesive silicates converts them from a liquid to a solid
material. This loss of water, either by evaporation or by sorption through the porous
materials to be bonded, is the mechanism by which silicate adhesives "set". The rate of
set at an equivalent viscosity increases with ratio of silica to alkali. Tackiness is greater
for the more alkaline ratios. This fast setting is of particular value on corrugating
machines, which have been successfully operated at speeds as high as 150 feet per minute
with a silicate adhesive and best modern techniques [65].
The strengthening and stiffening action of silicate adhesives in corrugated containers is
due to the formation of a strong reinforcing shoulder at the point of contact between the
corrugating medium and liners. The film at the point of contact is very thin and little
penetration of the silicate into the paper occurs in a well-made corrugated fiberboard box.
The performance of sodium silicate adhesives in the manufacture of corrugated fiberboard
is now fairly satisfactorily understood. From 7 to 14 kg of silicate are commonly used per
90 square meter of fiberboard.
55
Figure 2.31: Viscosity-specific gravity-water content relations of sodium silicate adhesive [18].
Setting may occur in less than 15 seconds. This is frequently avoided by spreading a
viscous silicate in a thin layer and drying rapidly. Another method is to pretreat the paper
or other material with ammonium salts or metallic salts such as zinc sulfate or magnesium
chloride. Pretreatment with aluminum chloride not only prevents staining and desizing
but also gives a waterproof bond [18].
2.12.2 Advantages of using water glass as adhesive
Liquid silicates are inexpensive, readily available, and convenient. Silicates are
nonflammable adhesives. They are odorless and will not impart any taste to the contents
of packages in which they are used as adhesives. Sodium silicate is “generally recognized
as safe” (GRAS Status) by the Food and Drug Administration for use as an adhesive in
Viscosity, Centipoises
Perc
ent w
ater
Deg
rees
Bau
me
56
the manufacture of food packaging materials made from paper, paperboard or other
materials.
In addition to providing strength, silicates are resistant to grease and oil. There is no
attraction for rodents or vermin to destroy stored containers since these pests do not ingest
silicates. When used as adhesives, liquid silicates offer versatility and ease of handling.
Some additional advantages are:
▪ Available in selective viscosities and ratios (SiO2:Na2O) to provide control of
properties and performance
▪ Adaptable for air-drying, heat setting, or chemical setting mechanisms
▪ Easy to dilute to the required viscosity
▪ Spread easily and provide good substrate-to-substrate contact
▪ Quick set with relatively little water loss
▪ Cleanup with water. When allowed to harden, silicates may need hot water to
remove
The advantages of a sodium silicate adhesive include good spreading and contact, a rate
of controllable over wide limits up to very fast, and formation of a permanent strong,
rigid, water- and heat-resistant bond. The quality of a silicate bond is not ordinarily
affected by storage and its strength is greater than that of paperboard and other materials
with which it is frequently used. The low Price, uniform quality, controllable
characteristics, convenience, and unattractiveness to animal life are also advantages.
2.12.3 Broad range of application of water glass as adhesive / binder
Because of the low cost and versatile properties of silicates solution as adhesive, they are
useful in a broad range of applications. Typical applications include:
▪ Tube winding
Silicate adhesives are widely used in the production of both spiral and convolute wound
tubes and cores. The silicate binds by anchoring to the individual fibers and imparts
greater strength to the tube. Neutral silicates are generally the most economical and
efficient adhesives for tube winding. Alkaline silicate is suggested for first-time use since
added tack and greater latitude in setting time are helpful until experience is gained in
57
working with silicate adhesives. Paper tubes and cores gain rigidity from silicate. It is
economical to use silicates because they cost less per kg than the equivalent amount of
paper fiber necessary for a comparable improvement in strength.
▪ Corner-board
Corner board is composed of layers of paper bonded with sodium silicate and bent into a
V. In addition to economical bonding the silicate provides strength and rigidity to the
corner board. The bonding occurs as the silicate penetrates the paper and dehydrates.
▪ Laminating metal foil
Sodium silicates are well-adapted for bonding aluminum and other metal foils to paper.
Alkaline silicate, which is commonly used for this application, forms an adhesive bond
by losing moisture into the paper laminate and to the air. To obtain good adhesion, the
foil should be clean and the silicate must wet the foil. Silicates give high bond strength,
greater than that of the laminated materials. The foil or paper will tear while the silicate
bond remains intact. Other lamination applications are wood to felt, mica sheets, cork,
and other fibers.
▪ Paper board coating
Sodium silicate adds stiffness and strength to paperboard and increases its resistance to
fire, grease and vermin. For some requirements, silicate is used alone as a grease proofing
agent.
▪ Case sealing
For a number of years, alkaline silicates have been used for sealing boxes. The silicate is
picked up by the glue rolls of an automatic case-sealing machine and spread on the flaps
of the cases. As in other adhesives applications, the silicate sets by losing moisture to the
paper and the air, thus forming a strong bond.
▪ Coating metals
In the coating or bonding of metals and similar rigid materials, the difference in
coefficient of thermal expansion between silicate and the bonded surfaces may be a
limiting factor. However, where temperatures are relatively constant and there is no
mechanical strain, ultrathin silicate films which have been dehydrated by baking can hold
permanently. A thin silicate film has greater elasticity and is more serviceable than a thick
58
one for coating or bonding metal. A compatible surfactant in the amount of 0.05 - 0.1%
by weight relative to the silicate will aid surface wetting. Good adhesion to metal often
can be obtained after the surface has been thoroughly cleaned with alkali or solvent or is
degreased or sandblasted.
Typical metal coating applications are:
▪ Thin silicate films on lithographic plates to render the surface area hydrophilic
and acceptable for further processing
▪ Steel ingots coated with thin films of silicate to protect against corrosion
▪ Silicate films on silver flatware to protect against scratches during manufacture
▪ A thin coating of silicate - protects stainless steel colored by controlled oxidation,
rendering it resistant to atmospheric conditions, abrasion and fumes as well as to
cleaning agents commonly used on architectural or decorative finishes. The
silicate film does not change the color of the coated steel but rather adds a depth
of color to the steel sheet. Baking at about 250ºC cures the silicate and
considerably delays any moisture uptake. The silicate coating is smooth, glossy
and resistant to scratching
▪ A silicate coating to impregnate aluminum for ferrous castings by special vacuum
and pressure techniques to reduce their porosity
▪ Anodized aluminum surfaces sealed with sodium silicate solutions
▪ Sodium or potassium silicates used as a binder for zinc dust in the application of
zinc-rich primers on metal surfaces exposed to moisture or weather
2.12.4 Proper application rules of water glass adhesive
▪ Slightly elevated temperature improves the penetration properties: When properly
applied, sodium silicates penetrate the surfaces to be joined. In addition to bonding, the
silicates harden and strengthen the substrates. Penetration and consistency of the bonding
can be enhanced by using sodium silicate at slightly elevated temperatures; i.e., 24-30oC.
The elevated temperature should be maintained with minimal variation in the glue pot.
This keeps the viscosity at a constant level. Since drying will occur more quickly, line
speeds may be increased. In addition to maintaining a constant temperature, control over
% solids should be regulated.
59
▪ Addition of water is a wise decision or wrong? Any addition of water to decrease
viscosity should be done only by measuring and checking the density with a hydrometer.
The viscosity decreases as water is added and the solids decrease. Many adhesive users
experience adhesion problems in the winter. Cooler silicate temperature results in a more
viscous silicate so they add water to reduce viscosity and do not realize the solids
reduction slows the set time.
▪ Addition of surfactant to enhance adhesive penetration: Silicates can be used as an
adhesive alone or in combination with an anionic or non-ionic surfactant. In some cases,
it is desirable to modify the surface tension of a silicate adhesive to improve penetration
into the substrates being bonded. The addition of a small amount of a surface-active agent
can enhance adhesive penetration and improve the final bond. This may be needed when
a large amount of recycled fiber in the board makes the surface more hydrophobic. Solids
concentration, ratio and temperature play an integral part in finding the best conditions
for adhesion to occur.
2.13 Research Focus: Water Glass in Corrugated Paperboard
Corrugated board has become quite versatile material in today’s modern packaging world.
The global packaging industry is valued at 540 billion USD and is expected to touch the
1000 billion mark by 2025 [66]. Sodium silicate adds stiffness to paperboard and
increases its resistance to moisture, fire, grease and vermin. Silicate may also be used on
its own as a grease proofing agent. Sodium silicate serves as a priming coat for the
application of secondary materials such as paraffin, chlorinated naphthalene, waxes,
gums, nitrocellulose, lacquers and other paperboard coatings [67].
2.13.1 Manufacturing of corrugated board
Corrugated board is manufactured from a number of special layers, called fluting medium
and linerboard. In classical corrugators, the paper is humidified by means of high-pressure
steam. The humidity aims to soften the paper fiber so that the formation of flute and
consequently gluing would go smoothly. The process adds a considerable amount of
water to the papers. After formation of the board, this humidity has to be removed by
drying. For this newly formed corrugated board is heated from the bottom by hot plates.
It arrives to the corrugators on large rolls. At the single facer, it is heated, moistened and
60
formed into a fluted pattern on geared wheels. This is joined to a flat linerboard with a
water glass-based adhesive to form single face board. At the double backer, a second flat
liner board is adhered to the other side of the fluted medium to form single wall corrugated
board [66].
2.13.2 Effect of moisture on the quality of corrugated paperboard
The paper is hygroscopic in nature, so adsorption occurs due to humidity, temperature
and adhesive level. However, humidity influences the strength and stability of the
material which is a disadvantage of corrugated board. There is a no. of converting process
i.e. printing, slotting, folding and gluing. Quality is controlled at each and every step in
in-factory by checking the moisture % of paper/board to be used [66]. Moisture content
of corrugated paperboard depends on the ratio of silica/alkali and density of sodium
silicate. The advantages of sodium silicate as an adhesive for corrugated paperboard
include [67]:
▪ Even spreading and contact
▪ A set-rate controllable over wide limits
▪ Formation of a permanent, strong and rigid bond, resistant to pilferage, vermin, heat and moderately moisture
The more siliceous silicates (2.8-3.2 ratio) are particularly effective as adhesives or
binders due to the higher content of polymeric silica. These products set quickly by
removal of very small amounts of water, which converts them from a liquid to a solid
[14, 67].
▪ How does moisture % vary during manufacturing of corrugated board
As it is clear from Table 2.12, the moisture content of paper rises during manufacturing
from Infeed paper machine (2-7 %) to online sheet pasting (15-18%). It might be due to
the contact of corrugated board with adhesive causing moisture to become high. But after
4 h the moisture content of corrugated board is found decreased due to its high absorbency
and formation of equilibrium with the ambient conditions. The similar trend was found
in all four experiments [66]
61
Table 2.12: Moisture % vary during manufacturing of corrugated board [66]
Sl. No.
Infeed paper moisture content
Jobs Outfeed (in flute making)
Online sheet
pasting
After 4 h
Room Temp.
Relative Humidity
1 5-6% 10-16% 14-18% 11-17% 21ºC 50%
2 6-8% 8-15% 16-18% 22% 22ºC 45%
3 5-7% 8-12% 10-17% 9-14% 22ºC 45%
▪ Effect of moisture content after manufacturing of corrugated board (after 4 h)
As shown in Figure 2.32 and Table 2.13, after 4 h of its manufacturing, if moisture content
is found less than 8%, the corrugated board starts crisping. Also, moisture content beyond
12 % causes the corrugated board to have curling in it. The moisture content from 8% to
12% give maximum good results after 4 h. Hence it is considered standard for corrugated
board manufacturing.
Table 2.13: Effect of moisture content after manufacturing of corrugated board (after 4 h) [66].
Moisture % variation Effect of moisture %
Below 8% Crisp, easy to breakdown 11-13% Less curl occurs, package fails. 8-12% Ok standard without release of ink and coating. 14-17%
More curl, so creasing is not done. Ink Peeling off during creasing, Scuff test fails at 130 rounds.
Figure 2.32: Effect of moisture content after manufacturing of corrugated board (after 4 h) [66].
18 16 14 12 10 08 06 04 02 00
Moi
stur
e %
62
▪ Effect of moisture content after printing of paperboard
In Table 2.14 and Figure 2.33, during printing, if moisture content is less than 9%, it
causes picking or plucking in the corrugated board. 9-12% moisture content is found most
suitable for corrugated board, causing no problems during printing. Beyond this, the
moisture causes scuming, ink peeling off line, and delamination in various job, depending
upon the finishing and other specifications to be done etc.
Table 2.14: Effect of moisture content after printing of paperboard [66]
Moisture %variation Effect / Defect
Less than 9-12 Pinking and Plunking
Equal to 9-12 Standard
More than 9-12 Scuming, Ink peeling off at crease line
Figure 2.33: Effect of moisture content after printing of paperboard [66].
2.14 Finding an Economical Silica Source: RHA
Conventional methods for preparation of amorphous silica requires the use of high
temperature (in the excess of 1500ºC) and pressure for extracting silicon in pure form
from natural deposits of quartzite rock or quartz sand, such as through the thermal
decomposition technique and vapor-phase reaction [68-72]. Quartz sand is used as the
raw material as it is the second most common mineral on earth, therefore making it the
most common form of crystalline silica. Another preparation method is the sol-gel process
but it involves high raw materials cost [73]. Such preparation methods result in extremely
high production costs, which is subsequently reflected in its high market price.
18 16 14 12 10 08 06 04 02 00
63
Recovery of silica from RHA [19, 24, 74-80], sugarcane leaf ash , bagasse ash and silica
fumes, in the form of sodium silicate has many industrial applications. Agricultural waste
such as leaves, are rich in essential compounds.
▪ Sugarcane ash: Sugarcane, one of the major crops, has a high amount of silica in the
leaves as well as a sugarcane industry waste called as bagasse (fibrous dry matter). The
silica content in these resources is an essential raw material for sodium silicate
production. Generally, the bagasse is used as a fuel in boilers. Burning of bagasse
produces ash, which is rich in silica content. Sugar cane leaves are another important
agricultural waste that is rich in silica content and economically viable source. The
usage of these wastes provides a better solution for waste disposal from sugarcane
industries.
▪ Silica fumes: The smelting process, that is part of the silicon and ferrosilicon industry
results in a by-product called Silica Fumes. Highly pure quartz when reduced to silicon
at a temperature range of 2000°C produces vapors of SiO2. These vapors oxidize and
condense when exposed to lower temperature zone and results in tiny particles made
up of non-crystalline silica. Silica fume is also referred to as condensed silica fume,
micro silica or silica dust. The size of silica fume particle is extremely small. Silica
fume has a very high content of amorphous silicon dioxide and consists of very fine
spherical particles. The major component of Silica fume is SiO2, that forms around
90%. Minor quantities of iron, magnesium, and alkali oxides are also found.
▪ Rice husk ash: RH (Figure 2.34) is a form of waste from the Rice milling industries
and is produced in abundance around the country. RH is found to contain amorphous
silica in the range of 20 – 25 wt% [81-84], which upon thermal degradation yields an
ash product-RHA (Figure 2.35) with an excess of 95 wt% silica. The amorphous nature
of silica in RHA makes it extractable at a lower temperature range [5] and hence,
thermal treatment of RHA to produce water glass is viewed as a more economical
process having the potential to replace the conventional high temperature processes.
This is because thermal treatment of RH actually produces energy instead of
consuming energy. The energy produced could be recovered in the form of heat or
electricity.
64
Figure 2.34: Rice husk (RH) [85].
Figure 2.35: Various types of RHA produced by burning RHA [86].
The presence of silica in RH has been discovered far back in 1938 [87, 88] while its
recovery potential had been realized since 1984 [27]. It is considered a good source of
silica having the potential for large-scale production due to the following reasons:
2.14.1 High silica content with amorphous characteristic
The silica (SiO2) in RH exists in the hydrated amorphous form like silica gel. Thermal
degradation and pyrolysis of RH, followed by combustion of the char, result in a highly
porous and amorphous silica particulate mass with a varying percentage of unburnt
carbon [26]. Combusted at moderate temperature, the white ash obtained from RH
65
contains approximately 92-97 wt. % amorphous silica [89, 90] and some amount of
metallic impurities that can be further removed by a simple acid-leaching treatment. Other
studies consistently reported that RHA contains very high silica content such as 87.7 wt.
% as SiO2 [91], >90 wt. % silica [84], >95 wt. % silica [26, 92] and 87 – 97 wt. % silica
[92].
2.14.2 Abundant and cheap source of silica
Rice is cultivated in more than 75 countries [35] and over 97% of RH are generated in
developing countries [91]. RH accounted for 14-35 wt. % of the paddy harvested,
depending on the variety, with an average of 20 wt. % [29]. Thus, worldwide annual Husk
output is estimated at 80 million tons [26].
RHA is considered as a form of waste after burning of RH from Rice milling processes
and is often left to the open field. The fact that it is a cheap and abundant source of silica
remains largely unrealized. To some extent, RH has been utilized as fuel for cooking and
parboiling of paddy Rice in some developing country, but it is neither fully nor efficiently
utilized. Such underutilization clearly shows the wastage and loss of resources which in
reality could generate revenue through the recovery of silica via methods such as
combustion.
2.14.3 Quality of silica comparable with other expensive sources of silica
As reviewed by Real et al., a number of published literatures [89] had concluded that
RHA are an excellent source of high-grade amorphous silica. The silica obtained from
RHA is a good material for synthesis of very pure silicon, silicon nitride silicon carbide
(and magnesium silicide. In addition, this silica has been claimed to be an excellent source
of very pure silicon, useful for manufacturing solar cells for photovoltaic power
generation and semiconductors. In the manufacture of silicon carbide from RH silica, the
processing temperature could be lowered to 1500ºC due to the high surface area and
intimate contact available from carbon and silica in RH. This is considered to be less
energy-intensive compared to conventional methods using coal and quartz sand in electric
furnaces, whereby the processing temperatures are in the order of 2500ºC. With silica
content in the excess of 95 wt. %, RHA can also be used as a substitute for silica in cement
manufacture.
66
2.14.4 Disposal problem
The current practices to dispose of the large quantities of RH through open burning or
rotting in field are not environmental-friendly. Open burning results in air pollution with
the formation of smoke and particulate matters in the form of char and ash. Rotting in
field, on the other hand, results in formation of methane (CH4), which is a potent
greenhouse gas. Combustion of biomass such as RH can actually reduce the greenhouse
effect by converting emissions that would have been methane into the less potent
greenhouse gas carbon dioxide. Since CH4 is some 25 times more potent as a greenhouse
gas than carbon dioxide (CO2) and that the two gases have similar atmospheric residence
times, trading off CH4 emissions for CO2 emissions from combustion leads to a large net
reduction of the greenhouse effect associated with the disposal of RH. Rotting in the field
leads to a slow decay of the material, with eventual emissions of approximately equal
amount of CH4 and CO2 from the carbon that is released during the decay.
2.15 Evaluation on Available Technologies for Production of Water Glass from
RHA
Screening for the available methods or technologies showed that sodium silicate could be
produced from RH through alkaline extraction or heat treatment in various thermal
treatment technologies, as shown in Table 2.15.
67
Table 2.15: Studies that investigated methods used to extract silica from RHA
Ref Objective Summary of method used Main results
[93] Investigate the efficacy of acid leaching of RHA before alkaline extraction and washing of silica obtained with water.
• Acid leaching • Extraction of silica with NaOH • Precipitation with HCl • washing with water
Initial acid leaching did not improve purity, the washing with water reduced Na and K levels, and silica yield was excellent when extraction was carried out with NaOH 1 N.
[94] To produce nanosilicon powder from three kinds of RH.
• Combustion of RH in the open. • Acid leaching •Incineration at 700ºC
Spherical, completely amorphous silica particles with large specific surface area and composed only by Si and O from all kinds of RH.
[95] To investigate the ideal conditions to obtain high purity silica.
• washing of RH with water • Acid leaching • Incineration at 500, 600, 700, 800,
and 900ºC for 2 h under atmospheric conditions
All silicas had amorphous particles, and RH leached with HCl produced the highest content of silica (99.582%) at 600ºC.
[96] To develop a new, recycling-based technique to produce silica from RHA.
• Acid leaching of RHA • Extraction in reactor with NH4F • Acid precipitation of silica
Spherical, completely amorphous silica particles measuring 50 to 60 nm in diameter and containing only Si and O, with yield of 94.6%.
[97] To investigate the effect of experimental conditions on the characteristics of the nanosilica obtained.
• Leaching of RH with water and then HCl
• Incineration • Extraction with NaOH, forming
silicate • Precipitation
Best results with pH 3, silicate 0.15 M, aging time of 12 h at 50ºC, with 99.48% purity silica.
68
[98] To evaluate the leaching of RHA with NaOH to easily obtain highly reactive colloidal silica.
• Extraction of silica with NaOH 1M • Precipitation using H2SO4 drop by
drop
The best Si:Na mass ratio was 4:1, and the highly reactive colloidal silica was obtained at low energy investment.
[99] To obtain high purity silica with large specific surface area and to evaluate the competitiveness of silica from RH.
• washing of RH with water • Chemical treatments (acid leaching,
alkaline leaching) • Incineration at 600ºC under different
conditions (static oven, argon flow, oxygen flow, air flow)
Amorphous silica particles with maximum specific surface area of 321 m2/g 99.66% purity, depending on the treatment. The highest purity silica was obtained with prior acid treatment and incineration in oxygen atmosphere.
[100] To obtain high purity silica with large specific surface area and to evaluate the competitiveness of silica from RH.
• Washing RH with water • Different chemical treatments (acid
leaching, alkaline leaching, and enzymatic digestion)
• Incineration at 600ºC (static oven, air flow, steam)
Except for leaching with NaOH, all other treatments produced amorphous silica with large specific surface area, and high purity silica was obtained with acid leaching with HCl.
[32] To optimize the production of silica from RH. • Washing RH with water • Acid leaching • Incineration • Leaching of RHA
Silica with approximate purity of 99.98% with three washing cycles, 6-h acid leaching, 6 leaching cycles with HCl 3% at 90ºC, and the main contaminant was Ca (100 ppm).
69
2.15.1 Statement of the problems of existing studies
The problems of existing studies are briefly described as follows:
▪ Higher consumption of sodium hydroxide which causes high production cost:
Most of studies of alkaline extraction process of RHA use the ratio of RHA and sodium
hydroxide 1:1 to 4:1. The ratio of SiO2/Na2O of produced sodium silicate solution is 1:1
to 1:2 which has no commercial value in the field of liquid sodium silicate market. The
production cost is also very high because of using high consumption of sodium hydroxide.
▪ Using atmospheric temperature to extract amorphous silica of RHA which
increases the requirement of sodium hydroxide:
They use atmospheric temperature and pressure to extract amorphous silica from RHA.
It can be possible to extract high percentage of silica (almost 90%) at atmospheric
temperature but it requires same quantity of RHA i.e. 1 kg sodium hydroxide is required
to extract 1 kg of silica from RHA. But if temperature is increased from 100ºC to 150-
170ºC, the requirement of sodium hydroxide will be decreased by 70-80% to produce
commercial grade high ratio sodium silicate. To reduce the production cost, the
requirement of sodium hydroxide should be reduced.
▪ No evidence of researches is found to produce water glass having high mole ratio
used in packaging industries:
Most of the researches highlighted the synthesis process of silica from RHA. There is no
evidence of research on synthesis of sodium silicate having higher mole ratio from RHA.
Therefore, this work is unique in doing such kind of work.
2.15.2 Advantages of alkaline extraction method over conventional furnace
method
Table 2.16 describes the advantages of alkaline extraction process to produce sodium
silicate from RHA over the furnace route using sand and sodium carbonate.
70
Table 2.16: Comparison of alkaline extraction of RHA with furnace method of silica sand
Sl. No
Description New technology Conventional Technology
(a) Technology Alkaline extraction of RHA at lower temperature (less than 200ºC)
Conventional sand process using high temperature (above 1000-1400ºC) furnace method
(b) Silica source RHA – a waste material and almost have negligible value
Quartz sand – expensive than RHA (about 3 Tk/kg)
(c) Alkali source Caustic soda flakes- locally manufactured
Sodium carbonate – imported material
(d) Temperature Around 150-170ºC temperature require for digestion
Almost 1400ºC temperature require
(e) Source of heat
Low temperature steam boiler High temperature and expensive furnace
(f) Fuel Biomass like RH, sawdust, coal Natural Gas/diesel/furnace oil
(g) CO2 emission No CO2 emits from the reaction Huge amount of greenhouse gas-CO2 is emitted from reaction and as well as furnace
(h) Environmental issues
Solving the disposal problem of RHA in open field Figure 2.36 (a)
Conventional process using sand from beautiful beaches Figure 2.36 (b)
(a) (b)
Figure 2.36: (a) Landed disposal waste material- RHA; (b) Expensive quartz sand from beaches.
CHAPTER THREE
EXPERIMENTAL METHODOLOGY
72
CHAPTER 3. EXPERIMENTAL METHODOLOGY
3.1 Introduction
In this chapter, the methodologies related to the production of water glass adhesive from
amorphous silica rich RHA are presented. This chapter is divided into following
categories:
▪ Research Methodology: Synthesis process of water glass adhesive from RHA
▪ Research Materials
▪ Laboratory Equipment and Instruments
▪ Experimental Procedure
▪ Analytical Methods
▪ Characterization Techniques
The experimentation has also been programmed to identify optimum circumstances that
prevail in the synthesis process of water glass adhesive from RHA. Most of the testing
methods adopted in this work confirm to BSTI. However, where BSTI methods are not
available, other related international standards like BIS, ASTM has been followed. Total
24 no of experiments were performed.
The present study has been undertaken to evaluate systematically and quantitatively the
influence of following process parameters on the extraction of silica as water glass from
RHA, an abundant bio-waste.
▪ Ratio of RHA and caustic soda
▪ Molarity of caustic soda solution
▪ Ratio of RHA and water
▪ Digestion temperature
▪ Reaction period
73
The schematic of experimental work is presented in Figure 3.1 and Figure 3.2:
Figure 3.1: The schematic representation of work.
Rice husk ash
Characterization of rice husk ash
Alkaline extraction
Water glass adhesive
Characterization of water glass
Amorphous silica rich rice husk ash
1. XRF (X-ray fluorescence) to determine elemental composition and purity of rice husk ash, water glass and carbon residue
2. FESEM to understand morphological characteristics of rice husk ash, water glass and carbon residue.
3. XRD analysis to identify phase structure of RHA and carbon residue.
4. Chemical extraction method to determine amorphous silica content present in rice husk ash
5. Chemical titration method to determine alkali content and silica content in water glass
6. Measurement of solid content in sodium silicate by furnace drying method
74
3.2 Research Methodology
Synthesis process is mainly divided into major four stages as follows: Characterization,
Digestion, Filtration and Evaporation.
3.2.1 Characterization of RHA
Characterization studies of RHA were carried out in order to determine the following
properties:
▪ Moisture content of RHA
▪ Amorphous silica content in RHA
▪ Water insoluble impurities in RHA
This characterization stage is very important to find out the amorphous silica rich RHA.
3.2.2 Digestion - alkaline extraction
Digestion refers to extraction of the insoluble silica present in the ash to soluble salt in
the form of sodium silicate.
For this purpose, known volumes of aqueous dispersions, containing 1 kg of the RHA
and sodium hydroxide solution having specific Molarity, were heated to boiling under
pressurized conditions for various periods of time (1-6 h).
As a result of this treatment, silica content of the RHA leached out to the aqueous phase
of the dispersion in the form of soluble sodium silicate according to the following
reaction:
SiO2 + NaOH Na2SiO3 + H2O
Digestion studies were carried out with following process conditions in order to optimize
the parameters:
▪ Ratio of RHA and sodium hydroxide,
▪ Molarity of sodium hydroxide solution,
▪ Temperature,
▪ Ratio of RHA and water and
▪ Reaction period
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3.2.3 Filtration and product evaluation
The reaction step resulted in a sodium silicate solution and solid impurities containing
carbon, unreacted-silica and other insoluble water residues. The diluted sodium silicate
solution was attained by a vacuum filtration system.
▪ The filtrate diluted sodium silicate was analyzed to determine the following physic-
chemical properties:
▪ Specific Gravity
▪ Density
▪ Total soluble silicate
▪ Total alkalinity
▪ Total soluble silica
▪ Ratio of soluble silica and total alkalinity
▪ The residue obtained was oven dried to constant weight at 105oC. This residue is
called activated carbon. The residue was thoroughly washed with water till it is alkali
free as indicated by pH paper (pH 6-7). The washed water containing small quantity
sodium silicate can be reused to reduce the cost. Samples of the dry residue were
analyzed to determine the following properties:
▪ Unreacted Amorphous Silica
▪ Recoverable Sodium Silicate.
3.2.4 Evaporation
The diluted sodium silicate solution was then concentrated to desired final specific gravity
using heat-exchanger evaporation system and transferred to storage tank for curing.
Curing was done at room temperature for 7 days to get optimum adhesive properties.
A viscous, transparent, slightly brownish water glass adhesive was obtained after
evaporation of diluted silicate solution.
76
F
Figure 3.2: Basic Process Flow Diagram.
3.3 Research Materials
The most important two materials required for this research were RHA and sodium
hydroxide.
3.3.1 Rice Husk Ash (RHA)
As silica source, RHA as the raw material for production of water-glass adhesive was
collected from various rice mill. The rice mills currently dispose the RHA in open field.
By economic reasons, the ash was used as received, i.e., without being submitted to
purifying process such as any thermal treatment or acid leaching etc. in order to increase
silica content. Quality of RHA is completely depending on the design of boiler. By
reviewing literatures, high purity amorphous silica rich RHA can be obtained from
fluidized bed combustion system at 700-850ºC. The following 8 samples have selected
for this research shown in Figure 3.3.
RHA
NaOH
Digestion
Filtration
Evaluation
Evaporation
Water Glass Adhesive
77
Figure 3.3: RHA as-received collected from various rice mills.
3.3.2 Sodium hydroxide
Caustic soda flakes or sodium hydroxide was used as sodium source. It was
collected from local suppliers (Figure 3.4).
Figure 3.4: Caustic soda flakes or Sodium hydroxide from local manufacturer.
3.4 Laboratory Reagent, Equipment and Instruments
3.4.1 Laboratory reagents
The following laboratory reagents were used in this research shown in Table 3.1 and
Figure 3.5.
78
Table 3.1: Laboratory Reagents
Sl. No.
Name of the reagents
Specification Source Function
1. Caustic soda pearl 99% purity Merck, India Determination of silica content of RHA
2. Hydrochloric acid (HCl)
37% purity Merck India Determination of alkali content of water glass solution
3. Sodium carbonate anhydrous
99% purity Merck India Standardization of HCl
4. Methyl orange - Merck India Used as indicator in titration
Figure 3.5: NaOH, HCl, Na2CO3, methyl orange.
3.4.2 Equipment and instruments
The following equipment and instruments were used in these experiments shown in
Table 3.2 and Figure 3.6.
Table 3.2: List of the equipment and instruments used in this research
Sl. No. Name of the apparatus 1. Analytical balance 2. Hotplate magnetic stirrer 3. Muffle furnace 4. Hot air oven 5. Desiccator 6. Laboratory filtration unit 7. Nickel crucible 8. Vacuum pump 9. Laboratory glass wares
79
(a) (b)
(c) (d)
Figure 3.6: (a) Analytical balance; (b) Muffle furnace; (c) Hot-plate magnetic stirrer; and (d) Hot air oven.
3.5 Characterization of Research Materials and Products
The characterization section is mainly divided into two categories as follows:
▪ Characterization of research material – RHA
▪ Characterization of research output – water glass adhesive and residue
obtained as filter cake
3.5.1 Characterization of research material – RHA
Different methods and conditions of production of RHA result in silica with different
quality, structure, and surface morphology. Therefore, in order to identify the most
suitable production condition, it is imperative to determine the amorphous silica content
in RHA samples.
The collected raw material RHA as- received was characterized and analyzed for
investigating the following properties:
80
▪ Reactive amorphous silica content of RHA using alkaline extraction method
▪ Elemental analysis of RHA using XRF
▪ Crystal phases of RHA using XRD
▪ Morphological properties of RHA using FESEM
▪ Moisture content of RHA by hot air oven drying method
3.5.2 Characterization of water glass and residue obtained as carbon cake
The following samples were collected from different stages of the experiments:
▪ Diluted water glass solution from filtration stage as filtrate
▪ Residue (containing unreacted silica, un-burnt carbon and water insoluble
impurities) from filtration stage as filter cake
▪ Evaporated water glass solution from evaporation stage
▪ water glass adhesive from curing stage
The collected above products from different stages of experiments were characterized and
analyzed for investigating the following properties:
▪ Water glass adhesive collected from filtration, evaporation and curing stage: The
following properties are determined by analytical methods:
▪ Solid content
▪ Alkali content
▪ Silica content
The following properties are characterized using various characterizations
Instruments:
▪ Density and specific gravity by Hydrometer
▪ Silica and alkali content by XRF
▪ Morphological characteristics by FESEM
▪ Viscosity by viscosity meter
▪ Residue (containing unreacted silica, un-burnt carbon and water insoluble
impurities) from filtration stage as filter cake
The following properties are characterized using various characterization
instruments:
81
▪ The structure of unreacted silica by XRD
▪ Elemental composition of residue by XRF
▪ Morphological characteristics by FESEM
3.6 Characterization Instruments
The following instrumental techniques were employed to characterize the research
materials and products obtained from different stages of experiments.
▪ X-ray fluorescence (XRF) for Elemental analysis
▪ X-ray diffraction (XRD) for phase analysis
▪ Field Emission Scanning Electron Microscope (FESEM) for morphological analysis
3.6.1 X-ray fluorescence (XRF) technique
X-ray fluorescence (XRF) shown in Figure 3.7, was used to analyze the following
characteristics:
▪ Elemental composition of RHA
▪ Elemental composition of residue obtained from filter cake containing unreacted
silica, un-burnt carbon, and water insoluble impurities
▪ Elemental composition of water glass
Principle
XRF spectroscopy is widely used for the quantitative elemental analysis of samples. XRF has
the advantage of generally being non-destructive, multi elemental detection, fast analysis and
cost effective. The main advantage is, analyses are generally restricted to elements with
atomic number greater than 9.
The principle of X-ray fluorescence is as follows; the sample excited by X-rays emits a short
wavelength radiation (fluorescence), characteristic of each alloying element. A parallel beam
of the secondary radiation is directed by means of a collimator onto the analyzing crystal; it
is separated according to wavelengths and then reflected into a radiation detector mounted on
a high precision goniometer. The angular position of crystal and goniometer is a function of
the wavelength and allows the identification of the element in the matrix.
82
Figure 3.7: XRF machine.
3.6.2 X- ray diffraction (XRD) technique for phase analysis
X-ray diffraction (XRD) shown in Figure 3.8 is used to analyze the following
characteristics:
▪ Determination of silica structure of RHA
▪ Determination of unreacted silica structure of carbon cake residue obtained
from filter cake containing unreacted silica, un-burnt carbon, and water
insoluble impurities
Principle
X-ray diffraction (XRD) is a nondestructive analytical technique mainly used for the
phase identification and structural characterization of crystalline materials.
83
Figure 3.8: X-ray diffraction (XRD).
X-rays are a type of electromagnetic radiation. The wavelength of x-rays is approximately
1 Å which has the same size equivalent to that of an atom. To detect the diffracted X-
rays, an electronic detector is placed on the other side of the sample from the X-ray tube
and rotated through different Bragg’s angles. The goniometer keeps track of the angle (θ),
and the detector records the detected X-rays in units of counts/sec and sends this
information to the computer.
In order to identify the unknown substance, the powder diffraction pattern is recorded
with the help of a camera or a diffractometer and a list of d-values and the relative
intensities of the diffraction lines is prepared. These data are compared with the standard
line patterns available for various compounds in the Powder Diffraction File (PDF)
database. It contains line pattern of more than 60,000 different crystallographic phases.
In practice, for any unknown sample, the appearance of three most intense characteristic
lines from the standard PDF line pattern is a sufficiently convincing evidence of the
existence of a crystalline phase in either a homogeneous substance or even in a multi
component mixture. It may be relevant to mention here that while the peak positions and
intensities associated with an X-ray diffraction pattern enable qualitative analysis, these
parameters are also useful for lattice constant determination and stress determination of
the sample.
84
Distinct peaks in the result indicate the presence of crystalline substance in the sample.
Sometimes extra peaks might appear due to the impurity or structural transformation after
thermal treatment. In fact, the intensity, width and position of peaks in the patterns reflect
information about structure, quantity, texture etc. of crystallites. The amorphous structure
is indicated by a background hump at peak position of approximately 22o on the
diffractogram.
The relationship between the wavelengths of the X-rays λ, the incidence angle θ, and
spacing between two crystal lattice planes (hkl) d, is shown in the Bragg's Law expressed
as:
2dhkl sinθ = nλ
After characterization by X-ray diffraction, a number of peaks are seen in the diffraction
patterns. These patterns are identified by comparing their positions and relative intensities
with the known structures in the database.
The average crystallite size (d) of the synthesized powders was calculated from the XRD
patterns using the Scherrer formula,
D = kλ/βcosθ
Where k is the dimensionless shape factor with a typical value of about 0.9, λ is the
wavelength of Cu Kα radiation with the value of 1.5418 Å, θ is the Bragg angle for the
diffraction peak and β is the full width at half maximum intensity (FWHM) of the
corresponding diffraction peak [101].
Experimental procedure
▪ Philips diffractometer (model 3040-XPert PRO) facility was used in order to
determine the phases present in the samples
▪ Powder sample were pack on a sample stage so that it can be irradiated by the X-ray
▪ The high-intensity CuKα) with X’PERT graphics software package was used to
analyze the structures of the sample
▪ An aluminum-glass composite sample holder with a rectangular slot measuring 2 cm
× 1 cm × 0.2 cm was filled with the powdered sample using the front-loading method
▪ The samples were analyzed using a CuKα radiation (λ = 1.54056Å) with a nickel
85
filter. Bragg’s angle (2theta) range of 10 -50o and a scan speed of 0.72 degree per
minute with a step-size of 0.015° was used
▪ The Philips diffractometer was operated at 40 kV and 40 mA
▪ The structural pattern was recorded and analyzed with X’PERT High Score™
software
3.6.3 Field Emission Scanning Electron Microscope (FESEM)
FESEM, shown in Figure 3.9, is used to observe the morphology of following products:
▪ RHA collected from various rice mills
▪ Water glass adhesive
▪ Residue obtained from filtration stage containing small quantity water glass,
unreacted silica, unburnt carbon with water insoluble impurities
FESEM is a microscope that works with electrons (particles with a negative charge)
instead of light. These electrons are liberated by a field emission source. The object is
scanned by electrons according to a zig-zag pattern. A FESEM is used to visualize very
small topographic details on the surface or entire or fractioned objects. Researchers in
biology, chemistry and physics apply this technique to observe structures that may be as
small as 1 nanometer (= billion of a millimeter).
Figure 3.9: Field Emission Scanning Electron Microscope (FESEM).
86
Principle
Electrons are liberated from a field emission source and accelerated in a high electrical
field gradient. Within the high vacuum column these so-called primary electrons are
focused and deflected by electronic lenses to produce a narrow scan beam that bombards
the object. As a result, secondary electrons are emitted from each spot on the object. The
angle and velocity of these secondary electrons relates to the surface structure of the
object. A detector catches the secondary electrons and produces an electronic signal. This
signal is amplified and transformed to a video scan-image that can be seen on a monitor
or to a digital image that can be saved and processed further.
Experimental procedure
▪ The FESEM-(JEOL JSM 7600F) field emission scanning electron microscope was used
to characterize the morphologies of the samples
▪ Sample specimens with diameter of 12 mm each were gold coated in a gold sputter coater
for 90 seconds at 15 mA current output. The gold coating was necessary to ensure a
conducting surface was obtained for electron bombardment and characterization
▪ Powder samples were adhered with carbon tape that was attached to a Copper stub. Then
sample holder was mounted on a holder and inserted in FESEM
▪ The micrographs obtained from FESEM analysis was used for microstructural study
▪ The FESEM was operated at 20 kV and a working distance of 15 mm
▪ Selected areas of interest were focused and micrographs were taken
3.7 Analytical Methods
The following analytical methods were required to analyze the raw materials and
products:
▪ Reactive amorphous silica content of RHA by alkaline extraction method
▪ Loss on drying (LOD) by hot air oven drying method
▪ Specific gravity and density of water glass solution by hydrometer
▪ Solid content of water glass solution by furnace method
▪ Alkali content of water glass solution by chemical titration method
▪ Silica content of water glass solution by acid neutralization method
87
3.7.1 Reactive amorphous silica content of RHA
There are some specific analytical methods are available to evaluate amorphous silica
content in RHA. One of them is that the degree of amorphousness of silica is estimated
by calculating the percentage of available silica that is dissolved in an excess of boiling
2.5M sodium hydroxide in a two-three h extraction period.
In this research, reactive amorphous silica content of RHA was determined by mass
quantification using alkaline extraction method of Silica from RHA followed by
neutralization.
Experimental procedure:
The following procedure was adopted for estimating the reactive amorphous silica content
of RHA (Figure 3.10):
▪ A known volume (~10 gm) of RHA and sodium hydroxide pellet having the similar
quantity of RHA were weighted carefully using a 4-digit analytical weighing balance
▪ Under atmospheric pressure and boiling temperature, the mixture of raw materials
was placed into the glass beaker. Temperature and agitation were maintained through
Hot-plate magnetic stirrer. Reaction period was 2-3 h
▪ Demineralized water as added time to time to keep constant volume of the reaction
mixture
▪ The reaction step resulted in a sodium silicate solution and solid impurities,
predominantly constituted of unreacted silica and water insoluble residues. The
sodium silicate solution was attained by a vacuum filtration system
▪ The solid residues obtained in the stage of filtration had been washed with distilled
water and later dried at 120°C, for 8 h. The conversion was determined from of known
silica mass in the RHA and from the residue content gotten in the filtration
▪ Quantification of silica conversion (wt.%)
The silica conversion by alkaline extraction method was determined by mass balance
using following formula: -
SiO2 conversion (wt.%) = (wt. of RHA – wt. of Residue / wt. of RHA) x 100
88
1 2 3
RHA
Mixture of RHA and sodium
hydroxide solution
Temperature and agitation controlled by hot-plate magnetic stirrer for 4 h
4 5 6
Filtration completed by sintered
crucible Residue- Carbon cake
Filtrate: water glass solution
Figure 3.10: Experimental flow diagram of determination of reactive amorphous silica content in RHA.
89
3.7.2 Moisture content of RHA by hot air oven drying method
Typically, RHA contains significant amount of moisture content. Determination of
moisture content is important for the raw material charge calculation. The removal of
moisture was made by drying using hot air oven.
▪ A known mass (~5-10 gm) of RHA sample was taken in a pre-weighted dry watch
glass.
▪ The watch glass was then placed in hot air oven to dry at 120oC for 2 h
▪ After 2 h, the watch glass with dry RHA sample was weighted and quantified for
the determination of moisture content in RHA
▪ The moisture content of RHA was determined by mass balance using following
formula: -
Moisture content (wt. %) = (wt. of raw RHA – wt. of dry RHA) / wt. of raw RHA x 100
3.7.3 Specific gravity and density of water glass adhesive
Specific gravity and density can be determined by the same hydrometer (used for liquid
which is Heavier than water).
▪ The material to be tested was poured into a clean hydrometer jar, the diameter of
which should be at least 2.5 cm greater than the diameter of the hydrometer used
(Figure 3.11and Figure 3.12)
▪ All the air bubbles were removed that formed in the liquid
▪ The jar was kept in a vertical position and lower down the hydrometer gently into
the material
▪ The point was read to which the sample rises, with the eye-placed at the principle
surface of the material
▪ This reading was the specific gravity as well as density of the material under test
Specific Gravity (SG) = 145 / (145 – Density in Baume degree)
Figure 3.11: Hydrometer.
90
Figure 3.12: Density was tested by deeping the hydrometer in the liquid.
3.7.4 Total soluble silicate of water glass adhesive
The soluble silicate content of water glass solution was determined by furnace method.
▪ A sample of known mass was taken in a pre-weighted dry nickel crucible (Figure
3.13)
▪ The nickel crucible with sample was then placed in pre-heated muffle furnace at
500oC and kept it in the furnace for 1 h
▪ The hot nickel crucible with dry sample was placed in desiccator for cooling
▪ The cooled nickel crucible was weighted and quantified for the determination of
the solid content of the sample
▪ The solid content was determined by mass balance using following formula: -
Total soluble silicate (wt.%) = (wt. of dry sample / wt. of liquid sample taken) x 100
Figure 3.13: Nickel crucible in muffle furnace.
91
3.7.5 Total alkalinity of water glass adhesive
The principle of the method is based on titration of the total alkalinity of a test portion
with a standard volumetric solution of hydrochloric acid in the presence of methyl orange
as indicator.
The following reagents were required for this method:
▪ 1 N standard volumetric solution of hydrochloric acid
▪ Methyl orange, having 0.5 g/l solution
Experimental procedure
▪ A liquid sample of water glass of known mass was weighted and placed the
test portion in a 250-ml volumetric flask and dissolved with water
▪ Few drops of methyl orange solution were added to the test sample
▪ Titration was made with the standard volumetric hydrochloric acid solution
until the indicator changes from yellow to orange-pink (Figure 3.14)
▪ The total alkalinity, expressed as a percentage by mass of sodium oxide was
determined by the given formula:
Alkali content = 3.1 x (volume of HCl used in the titration / wt. of sample taken)
(a) (b)
Figure 3.14: (a) Before end point and (b) after end point.
92
3.7.6 Total soluble silica content of water glass solution
From water glass solution obtained in the filtration stage as filtrate, a sample of known mass was taken and was added hydrochloric acid until it reaching pH 7.0. In this pH the silica precipitates, also resulting a salt, the sodium sulfate (Na2SO4). The removal of sodium sulfate was made using boiling distilled water by vacuum filtration. The process resultant silica gel was placed to dry at 120°C for 8 h, weighed and quantified for the determination of the silica content converted to silicate, in function of the amount of silica contained in the RHA;
The silica conversion by acid neutralization method was determined by mass balance using following formula: -
SiO2 precipitation (wt. %) = (wt. of dry precipitated silica / wt. of water glass) x 100
3.7.7 Weight ratio of total soluble silica to total alkalinity
The ratio of total soluble silica to total alkalinity was calculated by following given formula:
Molar Ratio = total soluble silica content (wt. %) / total alkalinity (wt. %)
3.8 Experimental Procedure
The experimental procedure of the synthesis process of water glass adhesive by alkaline extraction of RHA was described below:
▪ At first, known volume of RHA and water was charged into 10 L stainless steel mixing tank to prepare RHA-water slurry under agitation. Mixing procedure was run for 10 minutes. Sodium hydroxide solution was prepared in another mixing tank.
▪ After preparing the raw materials, RHA-slurry and caustic soda solution was charged into pressure vessel. The reaction was carried out at specific temperature and pressure for specific periods under agitation.
▪ After completing the reaction, laboratory vacuum filtration unit was used to separate the residue and diluted sodium silicate as filtrate. The filtrate – diluted sodium silicate solution was further evaporated to get desired specific gravity by boiling in burner.
There are total 24 nos. of experiment conducted in this research work. The quantity of raw materials and process parameters used in those experiments are tabulated in Table 4.4.
CHAPTER 4
RESULTS AND DISCUSSION
94
CHAPTER 4. RESULTS AND DISCUSSION
In this chapter the results of the experimental works will be presented and discussed
systematically. The discussion will be segmented into five parts namely, (i)
Characterization of as-received RHA, (ii) Quantitative result of the experiments, (iii)
Characterization of water glass, (iv) Influence of different reaction variables on silica
extraction (v) Characterization of as - produced silicate material.
4.1 Characterization of RHA
To understand the characteristics of each waste is fundamental to selecting, designing and
implementing waste management solutions in the industry. High silica content and low
levels of elemental impurities are necessary pre-requisites for producing water glass from
RHA. The elemental analysis of RHA using XRF and chemical composition analysis of
the as-received RHA using analytical technique were discussed in this section. For this
research, total 8 no’s of RHA samples were collected from various local Rice mills. Rice
mills are located in Comilla, Mymensingh, and Brahmanbaria zone.
4.1.1 Optical image of RHA (as-received)
The following Figure 4.1 presents the photographs of the various origin rice husk ash
(RHA) samples obtained from the local rice mills (as-received). Two types of boilers -
fluidized bed combustion boiler (FBC) and inclined grate boiler, had been found in those
rice mills. The fluidized bed technology is capable of overcoming all the disadvantages
associated with the use of other combustion process like muffle furnace, rotary kiln and
inclined grate furnace for the production of amorphous silica. It offers a continuous and
self-sustaining process without the need for auxiliary fuel, except during the brief start-
up period. In fact, the combustion of rice husk in fluidized bed offers an added bonus of
heat recovery. The reaction time is also very rapid (in the order of minutes), thus
increasing the production rate significantly. The turbulent bubbling action in the sand bed
provides a high degree of mixing between the reactants and more importantly aids in
breaking down the rigid skeleton-like structure of the ash [95]. From below Figure 4.1, it
is observed that the some of the ashes are black with gray particles with visual impurities
like sand, un-brunt RH, and un-burnt carbon as result of incomplete combustion during
the RH burning in boiler. And others had a charcoaled-black appearance with some brown
color and a gritty feel in between fingers.
95
As depicted Figure 4.2, it was observed by other researchers that the ashes are black with
gray particles as result of different steps of carbon combustion during the rice hull
burning. The burning of rice hull in air always leads to the formation of silica ash, which
varies from the gray to black depending on inorganic impurities and unburned amounts.
The literature has classified RHA into high-carbon char, low-carbon gray ash, and carbon-
free pink or white ash [19, 102].
Sample-1 Sample-2 Sample-3 Sample-4
Source: Comilla zone-1
Type of boiler: Inclined grate Source: Comilla zone-3
Type of boiler: FBC Source: Ashugonj zone
Type of boiler: Inclined grate Source: Brahmanbaria Type of boiler: FBC
Sample-5 Sample-6 Sample-7 Sample-8
Source: Comilla zone-2
Type of boiler: Inclined grate Source: Ashugonj zone-3
Type of boiler: Inclined grate Source: Ashugonj zone-2
Type of boiler: FBC Source: Brahmanbaria-2
Type of boiler: FBC
Figure 4.1: Optical images of RHA.
Figure 4.2: Macroscopy of RHA as received [19, 103].
96
4.1.2 Elemental analysis of RHA
Research findings have revealed that there are two distinct stages in the burning of rice
husks to produce rice husk ash-carbonization and decarbonization [104]. Carbonization
is the first stage during which decomposition of volatile matter in rice husks occurs at a
temperature above 300ºC, releasing combustible gas and tar to yield rice husk char, which
is black in color. Decarbonization involves the combustion of fixed carbon in the presence
of oxygen to yield rice husk ash. The conversion of raw (untreated) rice husks to rice husk
ash (RHA) by incineration is accompanied by color changes, which include brown, grey,
ivory, milky-white [106]. These color changes are associated with the completeness of
the combustion process as well as the structural transformation of silica in the ash from
the amorphous to the crystalline state which is dependent on temperature and duration of
incineration [105].
White RHA has been found suitable for most studies since its whiteness is indicative of
complete oxidation of the carbon in rice husk char and thus, availability of a large portion
of silica in the ash. In several research investigations, the ash was obtained at combustion
temperatures in the range of 500 to 800ºC which produces amorphous silica, while a
temperature greater than 800ºC, gives crystalline silica [95, 106]. The transition
temperature from amorphous to crystalline silica has been reported to be about 800ºC.
Others have reported that 650-700ºC is the optimum temperature for producing reactive
(amorphous) RHA for obtaining the maximum amount of silica. An earlier study has also
showed that white ash is obtainable at 500-800ºC and that the ‘whiteness’ of the ash
improved with increased combustion time of 5-6 h [106, 107].
The chemical composition of RHA varies as it is dependent on several factors such as soil
chemistry, climatic conditions, paddy variety, use of fertilizer, type of fertilizer, year of
harvest, sample preparation and methods of analysis. The elemental composition of RHA
samples from different locations in the world is presented in Table 4.1 in which the variation
in values is evident.
97
Table 4.1: Elemental composition of rice husk ash from different countries
Element
(as oxide)
Concentration (in wt %) of elements in RHA from different
locations
Brazil [108] Indonesia [109] India [110] Nigeria [106]
SiO2 94.4 89.08 80-95 94-98
Al2O3 0.61 1.75 1.0-2.5
Fe2O3 0.03 0.78 0.5 0.1-1.0
CaO 0.83 1.29 1.0-2.0 0.03-0.3
MgO 1.21 1.17 0.5-2.0 0.2-0.3
Na2O 0.77 0.85 0.2-0.5
K2O 1.06 1.38 0.2 0.5-1.9
MnO 0.59 0.14 - 0.04-0.2
The chemical composition of the collected RHA was determined by X-ray fluorescence spectroscopy (XRF). The results of the tests are tabulated below in Table 4.2:
Table 4.2: Elemental analysis of RHA by XRF
No. of
Sample
Chemical Composition, wt%
1 2 3 4 5 6 7 8 9 10 11
SiO2 Al2O3 K2O MgO Fe2O3 MnO SO3 ZnO P2O5 CaO Na2O
Sample-1 90.83 1.42 1.06 0.57 0.42 0.24 0.47 0.01 2.25 - -
Sample-2 96.38 1.11 - 0.75 0.67 0.19 0 0.02 - - -
Sample-3 90.17 0.59 2.35 0.31 0.42 0.24 0.47 0.01 2.67 - -
Sample-4 91.87 0.17 1.03 1.25 0.54 0.34 0.16 0.02 2.36 1.08 1.27
Sample-5 88.38 1.07 0.89 1.12 0.68 - 2.03 0.02 1.15 1.02 2.95
Sample-6 90.82 0.54 1.89 1.33 0.69 0.24 1.67 0.01 2.31 - -
Sample-7 92.79 0.42 2.05 0.64 0.52 0.07 0.2 0.01 1.16 1.89 0.2
Sample-8 97.24 0.66 - 0.63 0.38 0.2 - 0.01 - - -
Silicon dioxide is the predominant compound, and other oxides are present in
insignificant quantities. According to XRF test report, RHA contains 88% - 97% SiO2
and the rest contains following elements (in oxide form) in traceable amount: 0.17-1.42%
Al2O3, 0.00-2.35% K2O, 0.31-1.33% MgO, 0.38-0.69% Fe2O3, 0.07-0.34 MnO, 0.00-
2.03% SO3, 0.01-0.02 ZnO, 0.00-2.67% P2O5, 0.00-1.89% CaO and 0.00-2.95% Na2O.
98
It is evident that the ash produced from the rice hull burning contains a great amount of
silica and small amounts of other elements considered as impurities. The most common
trace elements in RHA are aluminium, potassium, magnesium, iron, manganese, sulfur,
zinc, phosphorous, calcium and sodium in oxide form. Differences in composition may
occur due to geographical factors, type of ground, year of harvest, types of boiler used in
rice production, etc. The analysis carried out on the collected RHA shows that the
inorganic content of this ash contains a good percentage amount of SiO2 (> 90%). The
high silica content therefore justifies the use of the RHA as a silica source for this
research.
RHA may also pick-up impurity from the combustion reactor. Another possibility can be
due to adherent soil particles on the rice husks since no pre-cleaning treatment was done
before combustion. It is showed that pre-treating the husks in dilute hydrochloric acid
before combustion significantly reduces the total impurity content of the as-produced
RHA [111, 112]. However, the logistics and cost of pre-treating the husks has to be
compared to the combusted husks without pre-cleaning treatment.
4.1.3 Reactive amorphous silica content and moisture content of RHA
The reactive amorphous silica was determined by alkaline extraction method and
moisture content was checked by hot air oven drying method. The obtained results are
tabulated in Table 4.3.
The range of percentage of reactive amorphous silica was 67% to 92% (Table 4.3 and
Figure 4.3). The moisture content of RHA was varied from 0.1% to 6%. Differences in
percentage of reactive amorphous silica may occur due to the control parameters of boiler
in Rice mills. Most of the water glass solutions prepared from tested RHA was looked
like brownish to greyish color due to presence of un-burnt RH in ash as impurities.
This difference in the amount of the extracted silica suggested the fact that the regional
variation had obvious effect on the silica content of the RHA and should be taken into
consideration before using RHA for the production of water glass adhesive on the
commercial production.
99
Table 4.3: Determination of extractable amorphous silica content of RHA using analytical method.
Sample no.
Location Type of boiler used in Rice mill
Reactive amorphous silica content (%)
Moisture content
1 Comilla zone Chain-grate 76% 4% 2 Lakhsam zone FBC 88% 0.5% 3 Ashugonj zone Chain-grate 74% 4% 4 Brahmanbaria FBC 77% 3% 5 Comilla Chain-grate 67% 5% 6 Ashugonj Chain-grate 76% 6% 7 Ashugonj FBC 85% 0.1% 8 Brahmanbaria FBC 92% 1%
Figure 4.3: Extractable amorphous silica content of various rice husk ash collected from different location.
4.1.4 Phase analysis of RHA
The crystal structure of silica determines its reactivity and degree of health danger. Whilst
crystalline silica is less reactive and deemed carcinogenic, the amorphous silica is rather
reactive and with no harmful effect. From literature, the XRD pattern of the typical RHA
is collected and shown in Figure 4.4. It can be seen that no defined peaks corresponding
to these Bragg 2θ angles are found in Figure 4.4. A rather broad peak spanning 2θ angle
range of 18-30⁰ which is characteristic of amorphous structures is observed [113].
76%88%
74% 77%67%
76%85% 92%
0%20%40%60%80%
100%Extractable amorphous silica content (%)
100
Position(2Theta)
Figure 4.4: XRD pattern of typical RHA [113].
The X rays diffractograms of the collected RHA (sample-7) is presented in Figure 4.5.
The RHA diffractogram indicates the presence of silica in the both form, crystalline and
amorphous. The peak at 2θ = 21.9 indicates the presence of crystalline phase, whereas
broad hump at 2θ = 18-25 identifies the presence of amorphous silica. The crystalline,
amorphous silica or presence of both is related directly to the temperature of burning or
the method of ash attainment. When the burning temperature of the RH is high, the silica
contained in the ash is predominantly crystalline. Presence of crystalline silica was
detected by other researchers when burning temperatures was around 1000°C. It gets ash
with predominant crystalline silica, getting 83% of crystalline silica, at temperature
1350°C; whereas in the range of temperature of 450-700°C, the silica was detected as
amorphous phase [113]. Therefore, it can be concluded that the RHA (as-received)
collected from local Rice mills contain a considerable amount of amorphous silica with
traceable amount of crystalline silica.
101
Figure 4.5: XRD pattern of RHA (as-received-Sample 7).
4.1.5 Morphological properties of RHA
SEM analysis was carried out on the rice husk ash at various magnifications with a high-
resolution field emission scanning electron microscope. The morphological features of
the as-received RHA observed by scanning electron microscopy (SEM) is shown in
Figure 4.6. The micrograph was taken under the secondary electron imaging mode (SEI)
at a magnification of 1000X. The as-received RHA shows a porous and multifaceted
particle shape and size. The hydrated silica subsequently polymerizes to form a skeletal
silica network which may explain the flaky and honey comb-like structure in the SEM
micrograph of Figure 4.6.
0
500
1000
1500
2000
2500
0 10 20 30 40 50 60 70 80 90
Inte
nsity
(Cou
ns)
Position( 2Theta)
As-received RHA sample-7
102
Figure 4.6: Microstructure of RHA (as-received Sample-7) analyzed by FESEM. The honey-comb and flaky structure is the proof of silica structure. (a) SEM image of RHA (1 µm; x 20,000); (b) SEM image of RHA (1 µm; x 10,000); (c) SEM image of RHA (100 nm; x 50,000); (d) SEM image of RHA (1 µm; x 20,000).
The morphological features of typical RHA observed by scanning electron microscopy
(SEM) is shown in Figure 4.7. The micrograph was taken under the secondary electron
imaging mode (SEI) at a magnification of 1000X. The main components of rice husk
include hydrated silica, cellulose and hemi cellulose component totaling a 55-60% and
lignin component of approximately 22%. The porous and honeycomb morphology was
also observed by other research group, shown in Figure 4.8, and can be attributed to the
burning out of the organic component in the rice husk during combustion [113].
(a) (b)
(c) (d)
103
Figure 4.7: Typical micrograph of RHA analyzed by SEM [113].
Figure 4.8: FE-SEM micrograph showing the presence of silica just underneath the outer epidermal cell surface fractured (a); SEM micrograph showing silica grains in RHA (b) [114].
It is worth to note here that abundant silica was found to be present as small particles
(Figure 4.8). Size of these particles varied from less than 100 nm to 1μm. Large size
particles were clearly observed and it was further confirmed as a result of the
accumulation of smaller particles [114].
(a)
(b)
104
4.2 Quantitative Results of the Experiments Conducted in this Research
It is found from the various studies that the reaction between RHA and sodium hydroxide
solution can be carried through in two types of systems: open and closed. In the open
system, the reaction is carried out under atmospheric pressure. The system consisted of a
glass reaction vessel, heating source, temperature measuring device and reflux condenser,
to keep constant volume of the reaction mixture. In the closed system, under pressure and
temperature (100-200°C), the mixture is placed in the high-pressure autoclave [76, 103].
In this work, the closed system process was chosen to produce water glass from rice husk
ash. The reaction step resulted in a sodium silicate solution and solid impurities,
predominantly constituted of unreacted silica and non-soluble residues. The sodium
silicate solution is attained by a vacuum filtration system.
As a result of this treatment, silica content of the RHA leached out in the form of soluble
sodium silicate according to the following reaction:
SiO2+NaOH Na2SiO3 + H2O
The dispersion was filtered and the liquid was recovered. The latter was then acidified
with concentrated sulfuric acid, which precipitated the dissolved silicate in the form of
white gelatinous solid (SiO2) according to the following reaction [115, 116]:
Na2SiO3+H2SO4 SiO2+Na2SO4+H2O
The conversion silica to silicate, expressed in weight percentage, can be determined by
mass balance, as described procedures to follow:
a) the solid residues obtained in the stage of filtration had been washed with distilled
water and later dried at 110°C, for 24 h. The conversion is determined from of known
silica mass in the RHA and from the residue content collected from the filtration.
This conversion can also be calculated by the following procedure
b) from sodium silicate solution, a sample of known mass was collected and sulfuric
acid was added until it reached pH 7.0. In this pH the silica precipitated, resulting a
salt, the sodium sulfate (Na2SO4). The removal of sodium sulfate was made using
distilled water (50°C) by vacuum filtration. The resultant silica gel was placed to dry
at 110°C for 24 h, weighed and quantified for the determination of the silica content
converted to silicate, in function of the amount of silica contained in the RHA;
105
To get a bigger trustworthiness in the result of the conversion, the procedures “a” and
“b”, previously described, had been both realized. Thus, the sum of the residues attained
mass (procedure “a”) and the silica mass collected for procedure “b” will have to be equal
to the used ash mass in the reaction. The percent yield of the extracted silica from the
RHA was estimated according to the following formula:
(W1 x 100) % yield of silica =--------------------
W2 where,
W1 = Weight (g) of the extracted silica and
W2= Weight (g) of the rice husk ash
106
Experimental parameters used in this study are listed in Table 4.4 and Figure 4.9 shows
the results obtained from various analysis carried out on the produced sodium silicate by
alkali extraction of rice husk ash.
In a clean stainless-steel high-pressure reaction vessel, a specific amount of rice husk ash
(sample-7) and sodium hydroxide was mixed with water by maintaining specific ratio of
RHA and sodium hydroxide (varies from 3.33:1 to 9.09:1) and ratio of RHA/Water
(varies from 1:4 to 1:9) tabulated in Table 4.4. In this stage, the mixture was heated for
specific period (varies from 1 to 6 h) at specific temperature (120-170ºC). After that, the
resultant slurry was filtered through press-filter and a clear diluted slightly reddish water
glass solution was obtained. The diluted solution was allowed to settle for at least 24 h
without any interruption. After setting the diluted solution containing water glass was
separated from suspended solid by decantation. The coagulated mass was visible when
temperature of the solution was lower down to room temperature. After sufficient time
the coagulated mass was then separated by cloth filtration. The filtrate was heated at
100oC and stirring was running at 60 to 70 rpm. After evaporation, a clear viscous water
glass silicate was formed having desired density and viscosity which can be used as
adhesive.
The quantitative analysis of final product was tabulated in Table 4.5. Results in the Figure
4.9 (a) showed that the ratio of RHA and sodium hydroxide has significance effect on the
quantity of produced final product (water glass). The quantity of produced final product-
water glass solution was decreased from 2.21 kg to 0.98 kg. However, the extracted
amount of silica is significantly increased to 61.63% when the ratio of RHA/NaOH was
decreased to 3.33. But according to Figure 4.12 (b), the lowest cost of raw materials for
producing 1 kg water glass adhesive is 9.11 Tk in the experiment no-4, when the ratio of
RHA/NaOH is 1:5.26.
Results in the Figure 4.9 (b) showed that the ratio of RHA and water has effect on the
quantity of produced final product water glass. The quantity of produced water glass
solution was increased from 1.34 kg to 1.62 kg. The amount of the extracted silica was
highest as 47.6% when the ratio of RHA/Water was 7; however, the extracted amount of
silica is significantly decreased to 38% when the ratio of RHA/water was decreased to 4.
According to Figure 4.14 (b), the lowest cost of raw materials for producing 1 kg water
107
glass adhesive is 9.11 Tk in the experiment no-10 (4), when the ratio of RHA/Water is 1:
7.
Results in the Figure 4.9 (c) showed that the reaction temperature has significant influence
on the quantity of produced final product water glass as well as on silica conversion. The
quantity of produced final product-water glass solution was increased from 0.91 kg to
1.66 kg. It is observed that the conversion increases with the increase of temperature,
reaching from 22% at 120°C to 49% at 170oC. According to Figure 4.16 (b), the lowest
cost of raw materials for producing 1 kg water glass adhesive is 8.895 Tk in the
experiment no-18, when the temperature is 170oC. With decreasing temperature from
170oC to 150oC, the yield of silica was not significantly changed.
Results in the Figure 4.9 (d) showed that the reaction period also has significant effect on
the quantity of produced final product water glass as well as on silica conversion. The
quantity of produced final product-water glass solution was increased from 0.95 kg to
1.65 kg. Results in the Figure 4.18 (a) showed that the amount of the extracted silica was
highest (almost 48%) when the reaction period was 6 h; however, the extracted amount
of silica was not significantly changed after 3 h of reaction periods. According to Figure
4.18 (b), the lowest cost of raw materials for producing 1 kg water glass adhesive is 8.908
Tk in the experiment no-24, when the reaction period is 6 h. But with increasing reaction
period from 3 h to 5 h, the yield of silica was almost constant.
108
There are total 24 nos of experiment conducted in this research work. The quantity of raw materials and process parameters of those
experiments are tabulated as below in Table 4.4:
Table 4.4: Experimental Parameters used in this research (Exp. No. 1-24)
Variable Parameters
Ratio of RHA and Caustic Soda Ratio of RHA and Water Digestion temperature Reaction period
Sl. No. Parameters
Experiment No 1 2 3 4* 5 6 7 8 9 10
(4)* 11 12 13 14 15 16
(4)* 17 18 19 20 21 22
(4)* 23 24
Consumption of raw materials
1 RHA, kg 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
2 Caustic Soda flakes, kg
0.3 0.27 0.23 0.19 0.15 0.11 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19
3 Water, L 7 7 7 7 7 7 4 5 6 7 8 9 7 7 7 7 7 7 7 7 7 7 7 7
Reaction parameters
1 Ratio of RHA and Caustic soda
3.33 3.70 4.35 5.26 6.67 9.09 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.26
2 Molarity of NaOH, M
1.07 0.96 0.82 0.68 0.54 0.39 1.19 0.95 0.79 0.68 0.59 0.53 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68
3 Temperature, oC 150 150 150 150 150 150 150 150 150 150 150 150 120 130 140 150 160 170 150 150 150 150 150 150
4 Reaction time, h
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 2 4 3 5 6
* Be noted that experiment no 10, 16 and 22 are similar as experiment no. 4.
109
Table 4.5: Quantitative analysis of the Experiment (experiment no-1 to 6)
Sl. No. Parameters
Result of Experiment 1-24 1 2 3 4/10/
16/21* 5 6 7 8 9 11 12 13 14 15 17 18 19 20 22 23 24
Output of the experiment
1 Water glass solution after filtration, kg
6.94 6.81 6.66 6.50 6.22 5.97 3.51 4.62 5.51 7.42 8.27 5.96 6.08 6.21 6.50 6.51 6.11 6.27 6.50 6.46 6.57
2 Water glass solution after evaporation, kg
2.15 1.99 1.80 1.57 1.22 0.96 1.30 1.46 1.49 1.57 1.57 0.89 1.01 1.17 1.59 1.61 0.91 1.15 1.57 1.58 1.61
3 Final product after curing, kg
2.21 2.05 1.85 1.62 1.25 0.98 1.34 1.50 1.54 1.62 1.62 0.91 1.04 1.21 1.64 1.66 0.94 1.18 1.62 1.63 1.65
Yield of Silica Extraction 1 Dry silica,
kg 0.59 0.56 0.51 0.46 0.36 0.29 0.37 0.42 0.43 0.46 0.46 0.21 0.25 0.31 0.46 0.47 0.22 0.30 0.46 0.46 0.47
2 Yield of Silica from RHA, %
61.63 57.87 53.43 47.60 37.03 29.75 38.52 43.75 45.01 47.41 47.41 21.96 26.54 32.67 48.34 49.09 22.51 31.60 47.71 48.06 48.86
*Experiment no 10/16/21 are similar as experiment no. 4
110
Figure 4.9: Effect of experimental variables on the quantity of produced final water glass solution. (a) Constant Parameters- Ratio of Water/RHA- 7:1, Temperature: 150ºC, Reaction Period: 3 h; (b) Constant Parameters- Ratio of RHA/NaOH- 5.26:1, Temperature: 150ºC, Reaction Period: 3 h; (c) Constant Parameters- Ratio of RHA/NaOH- 5.26:1, Ratio of Water/RHA- 7:1, Reaction Period: 3 h; (d) Constant Parameters- Ratio of RHA/NaOH- 5.26:1, Ratio of Water/RHA- 7:1, Temperature: 150ºC.
2.21 2.05
1.85
1.62
1.25
0.98
1.34 1.50 1.54 1.62 1.62 1.62
0.91 1.04
1.21
1.62 1.64 1.66
0.94
1.18
1.62 1.62 1.63 1.65
-
0.50
1.00
1.50
2.00
2.50
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Qua
ntity
of W
ater
Gla
ss o
btai
ned,
kg
Exp. No
Effect of experimental variables on the quantity of produced final water glass solution
(a) Variable-1: Ratio of RHA/NaOH
(b) Variable-2: Ratio of Water/RHA
(c) Variable-3: Temperature (oC)
(d) Variable-4: Reaction Period (h)
3.03 3.70 4.35 5.26 6.67 9.09 4:1 5:1 6:1 7:1 8:1 9:1 120 130 140 150 160 170 1 2 3 4 5 6
111
4.3 Characterization of Products Obtained from Different Stages of Experiments
Density of the product was chosen according to the use of the product. Finally, the prepared samples were characterized. The analytical characterization of the final product
obtained from different stages and various experimental procedures was tabulated in Table 4.6.
Table 4.6: Characterization of products obtained from different stages of experiments by analytical methods (Exp. No. 1-24)
Sl. No Parameters Result of Experiment 1-24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Raw material characterization 1 Silica Content in
RHA by XRF, % 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92% 92%
2 Extractable amorphous silica in RHA by analytical method, %
85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85% 85%
3 Molarity of NaOH 1.07 0.96 0.82 0.68 0.54 0.39 1.19 0.95 0.79 0.68 0.59 0.53 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 Product characterization after filtration 1 Density at 70ºC,
Baume 12 11 10 9 8 6 14 12 10 9 8 7 6 7 7 9 9 9 6 7 9 9 9 9
2 Solid content in solution, % 11.43 10.80 9.98 8.94 7.25 5.92 13.67 11.66 10.04 8.94 7.82 7.02 5.51 6.15 7.00 8.94 9.04 9.14 5.51 6.79 8.94 8.94 9.04 9.04
3 Alkali content, % 2.91 2.64 2.28 1.90 1.53 1.14 3.14 2.58 2.19 1.90 1.69 1.51 1.98 1.96 1.94 1.90 1.90 1.90 1.98 1.95 1.90 1.90 1.90 1.90 4 silica content, % 8.52 8.16 7.70 7.03 5.72 4.78 10.53 9.08 7.85 7.03 6.14 5.51 3.54 4.19 5.05 7.03 7.14 7.24 3.54 4.84 7.03 7.03 7.14 7.14 5 Ratio of silica and
alkali 2.92 3.09 3.39 3.69 3.73 4.19 3.35 3.52 3.58 3.69 3.64 3.64 1.79 2.14 2.60 3.69 3.75 3.81 1.79 2.48 3.69 3.69 3.75 3.75
Product characterization after evaporation 1 Density at 70ºC
Baume 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37
Product characterization after 48 h curing 1 Density at 25⁰C,
Baume 39 39 39 39 39 38 39 39 39 39 39 39 40 40 40 39 39 39 40 40 39 39 39 39
2 Solid content, % 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 37% 3 Alkali content, % 9.4% 9.1% 8.4% 7.9% 7.8% 7.1% 8.5% 8.2% 8.1% 7.9% 8.0% 8.0% 13.3% 11.8% 10.3% 7.9% 7.8% 7.7% 13.3% 10.6% 7.9% 7.9% 7.8% 7.8% 4 silica content, % 27.6% 27.9% 28.6% 29.1% 29.2% 29.9% 28.5% 28.8% 28.9% 29.1% 29.0% 29.0% 23.7% 25.2% 26.7% 29.1% 29.2% 29.3% 23.7% 26.4% 29.1% 29.1% 29.2% 29.2%
112
4.4 Optimization of Various Reaction Parameters to Produce Water Glass Adhesive from RHA in an Economical Point of View
Rice husk is a hard material and is composed of lignin, cellulose, traces metals, and
abundant of amorphous silica [117, 118]. Since silica is used in many technological
applications; therefore, rice husk is considered a cheap natural source of silica [117]. For
this purpose, rice husk is converted into ash by the controlled burning process and the ash
is then employed as the starting material for the extraction of silica. It was reported earlier
that ash of the rice husk contains over 80 % of the amorphous silica; however, the exact
amount of the extracted silica depends on the regional climates, history of the ash
formation from the rice husk [119], and the extraction methods.
As such, researchers have been working on establishing experimental strategies for the
economic recovery of larger amount of silica from the rice husk ash. For example,
Kalapathy et al. [120] worked on the extraction of silica from the rice husk ash (RHA).
The silica was extracted in alkaline media as silicate which was precipitated in the form
of silica gel by the acid addition. They heated silica gel at 80oC for twelve h and obtained
amorphous silica xerogels.
In another paper [93] the same group of researchers employed a slightly different method
for the extraction of silica from the RHA. In this method they acidified solutions of the
RHA-extracted silicate with different acids. It was found that the elemental composition
of the gelatinous precipitated silica was dependent upon the composition of the acid used
and the subsequent washing process.
Della et al. [117] made attempts to extract silica from the RHA. Before the extraction
process, the authors burnt out the RHA derived carbonaceous material at various
temperatures for different periods of time. It was noted that this treatment significantly
affected the content of silica in the ash samples. They claimed the extraction of 95% of
the silica from the RHA sample, produced at 700oC.
YalcË et al. [115] worked on the extraction of relatively pure silica from the rice husk.
The authors washed rice husk with acidic as well as alkaline solutions and then incinerated
at 600oC in the static air atmosphere for converting them into ash. The ash was then
employed for the extraction of the silica. The silica obtained through this method was
amorphous in nature and had a purity > 99%.
113
Muchidzuki et al. [121] studied the morphological properties of silica derived from the
rice husk in hydrothermal and steam explosion processes. XRF and XRD techniques were
employed for the analysis of their products. The authors reported that hot water treatment
dissolved a small quantity of silica, affecting the particle morphology but did not affect
the amorphous nature of silica obtained from the rice husk.
Witoon et al. [122] prepared bimodal porous silica from the rice husk ash. Porosity in the
finished product was found to depend upon the precipitation pH.
Following the above-mentioned work, it may be noted that the extraction process and the
yield of the extracted silica dependent upon various factors, including variations in the
region, where rice was grown. As such, it was of interest to carry out a systematic study
of the extraction of silica from the rice husk ash to produce water glass, prepared from
the husk of the locally grown rice crop.
After studying various literature, in this study optimization of the process parameters were
considered based on economic point of view. It was noted from the state of the arts that
the recovery of silica was dependent upon the applied processing parameters, such as
concentration of sodium hydroxide, extraction time, solid/ solution ratio, etc. As such,
these parameters were systematically varied in order to explore the optimum conditions
for maximum recovery of SiO2 from the RHA. The process parameters which were varied
to optimize the most suitable reaction parameters in an economical point of view, were
as follows (details result can be found in Table 4.5, Table 4.6 and Figure 4.9:
▪ The ratio of RHA and Sodium Hydroxide (varying from 3.33:1 to 9.09:1)
▪ The Molarity of Sodium Hydroxide solution (varying from 0.39 M to
1.07 M)
▪ The Ratio of RHA and Water (varying from 4:1 to 9:1)
▪ The Reaction Temperature (varying from 120oC to 170oC)
▪ The Reaction periods (varying from 1 h to 6 h)
4.4.1 Optimum ratio of RHA and sodium hydroxide and concentration of NaOH
It was claimed that conversion of about 90% of silica contained in RHA into sodium
silicate was achieved in an open system at temperatures of about 100°C [80]. The initial
step is extraction of silica from ash as sodium silicate using aqueous sodium hydroxide.
This reaction was carried out in an open stainless-steel reactor for about 60 minutes at a
114
temperature of 99oC and at atmospheric pressure. RHA contains mostly amorphous silica
which reacts at around 90-100oC with NaOH solution to yield sodium silicate. The RHA
also contains some unburnt carbon from the coal as also some ash from the burnt coal. A
viscous, transparent, colorless sodium silicate solution is obtained after filtration of the
reacted slurry. Temperature of digestion was kept constant at 99oC. The process
parameters which were varied were the quantities of RHA, NaOH, water and time for
digestion shown in Table 4.7.
Table 4.7: Effect of NaOH concentration on the yield of silica [80]
Parameters Effect of concentration of NaOH on the yield of silica Run No.
1 Run No.
2 Run No.
3 Run
No. 4 Run
No. 5 Run
No. 6 Run
No. 7 Run
No. 8 RHA, g 10 10 10 10 10 10 10 10 NaOH, g 12 12 12 10 11 12 13 14 Ratio of RHA/NaOH
1:0.83 1:0.83 1:0.83 1:1 1:0.91 1:0.83 1:0.77 1:0.71
Water, ml 30 40 60 50 50 50 50 50 Temperature, oC 99 99 99 99 99 99 99 99 Time, min 60 60 60 60 60 60 60 60 Dry Silica, g 5.95 8.33 6.36 8.03 6.98 8.76 8.62 8.68 Yield of silica, % 59.50% 83.30% 63.60% 80.30% 69.80% 87.60% 86.20% 86.80%
But, in the above research, they didn’t show any feasibility study of the production of
silica extraction process from rice husk ash. The percentage of yield of silica was
significantly high but the consumption of sodium hydroxide is also very high which
makes the process expensive.
To reduce the consumption of sodium hydroxide as well as the production cost, attempts
were made in this study to explore the effect of higher ratio of RHA/NaOH varies from
3.33:1 to 9.09:1 considering the extent of silica extraction.
Table 4.8 presents how the ratio of RHA and Sodium hydroxide and Molarity of Sodium
hydroxide solution influenced the yield of silica as well as on the feasibility of the
investigated experimental conditions. Experiments (Experiment no. 1-6) were carried out
in pressure vessel at temperatures of 150°C, with 3 h of reaction period, and the behavior
of the silica extraction was observed. 1 kg of RHA was mixed with 7 L of water having
different concentration (0.3 kg, 0.27 kg, 0.23 kg, 0.19 kg, 0.15 kg and 0.11 kg) of sodium
115
hydroxide keeping all other parameters constant including 150oC temperature and 3 h
reaction period. Experiments were carried out to study the effect of the ratio of
RHA/NaOH and the molarity of NaOH solution on yield of silica (Table 4.8).
Table 4.8: Influence of Ratio of RHA/NaOH and Molarity of NaOH on the yield of Silica
as well as the cost of raw materials per kg product (Exp. 1-6)
Sl.
No.
Parameters QTY
Exp-1 Exp-2 Exp-3 Exp-4 Exp-5 Exp-6
Raw Materials
1 RHA, kg 1 1 1 1 1 1
2 Caustic Soda flakes, kg 0.3 0.27 0.23 0.19 0.15 0.11
3 Water, L 7 7 7 7 7 7
Reaction Parameters
1 Ratio of RHA and Caustic soda 3.33 3.70 4.35 5.26 6.67 9.09
2 Molarity of NaOH, M 1.07 0.96 0.82 0.68 0.54 0.39
3 Temperature, oC 150 150 150 150 150 150
4 Reaction time, h 3 3 3 3 3 3
Yield of Silica Extraction
1 Dry silica, kg 0.59 0.56 0.51 0.46 0.36 0.29
2 Yield of Silica from RHA, % 61.63 57.87 53.43 47.60 37.03 29.75
Yield of water glass
1 Final Product obtained, kg 2.21 2.05 1.85 1.62 1.25 0.98
Feasibility Check
1 Price of RHA, Tk/kg 2.00 2.00 2.00 2.00 2.00 2.00
2 Price of Caustic Soda Flakes, Tk/kg 67.00 67.00 67.00 67.00 67.00 67.00
3 Cost of Raw materials per kg water
glass, Tk/kg of water glass adhesive
10.00 9.81 9.41 9.11 9.60 9.52
It is observed from the above Table 4.8 that extraction of silica was significantly high
(67%) when ratio of RHA/NaOH was 3.33:1 and molarity of NaOH was 1.07 M; whereas
extracted silica was negligibly low (29.75%) when ratio of RHA/NaOH was 9.09:1 and
molarity of NaOH was 0.39 M. considering the economic feasibility of production, from
the analysis of experiment no 1-6, it can be concluded that the optimum ratio of rice husk
116
ash and sodium hydroxide should be 5.26:1 and optimum molarity of sodium hydroxide
solution should be 0.68 M.
In reviewing previous literature, it was found that for RHA, the range of sodium
hydroxide concentration should preferably be 0.8-1.0 mol/L (M) for extraction of
considerable amount of silica (shown in Figure 4.10) [19, 120].
Figure 4.10: Percent yield of the rice husk silica (RHS), extracted from the rice husk ash
(RHA) as a function of the NaOH concentration in the reactant mixture. Volume of the
reactant mixture, 60 ml; Concentration of RHA, 166.66 g/L; Extraction time, 90 min
[115].
Extraction of silica was at its maximum, when the concentration of sodium hydroxide and
RHA were 1.0 mol/L and 166.66 g/L, respectively; while the aging time was kept to 90
minutes [115]. Foletto et al. [103] claimed that maximum extraction of silica took place
when the mentioned ratio was in the range of 0.01-0.06 shown in Figure 4.11. We believe
that these results would prove to be useful for the industry people involved in the
extraction of silica from the RHA at the commercial level.
Not
Det
ecte
d
Not
Det
ecte
d RH
S Y
ield
, %
NaOH concentration, (mol/L) 0.4 0.6 0.8 1.0 1.2
0
20
4
0
60
80
117
▪ Effect of ratio of RHA/NaOH on silica extraction and economic-feasibility of water glass adhesive production
Figure 4.11: Percent yield of the rice husk silica (RHS), extracted from the rice husk ash (RHA) as a function of the ratio (Moles NaOH/grams RHA) in the reactant mixture. Volume of the reactant mixture, 60 ml; Extraction time, 90 min [103].
Attempts were made to explore the effect of the ratio of RHA/NaOH and concentration
of NaOH on the silica extraction and economic feasibility. Experiments (1-6) were
performed in which ratio of RHA/NaOH was varied in the range of 3.33 to 9.09, while
keeping other parameters constant. Results are demonstrated in Figure 4.12 and Figure
4.13. Figure presents the influence of Ratio of RHA/NaOH variation on the silica
extraction as well as on the feasibility study.
RH
S Y
ield
, %
Ratio (Moles NaOH/grams RHA) 0.005 0.010 0.015 0.020 0.025 0.030 1.0 1.2
0
20
4
0
60
80
118
Figure 4.12: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Ratio of RHA and Sodium Hydroxide in the reactant mixture solution; (b) Influence of Ratio of RHA/NaOH on the cost of 1 kg water glass adhesive.
(Constant reaction parameters: Ratio of RHA and Water: 7, Temperature: 150oC, Reaction period: 3 h)
Results in the Figure 4.12 (a) showed that the amount of the extracted silica was 29.75%
when the ratio of RHA/NaOH was 9.09; however, the extracted amount of silica is
significantly increased to 61.63% when the ratio of RHA/NaOH was decreased to 3.33.
But according to Figure 4.12 (b), the lowest cost of raw materials for producing 1 kg
water glass adhesive is 9.11 Tk in the experiment no-4, when the ratio of RHA/NaOH is
1:5.26.
61.63 57.8753.43
47.60
37.0329.75
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
3.33 3.70 4.35 5.26 6.67 9.09
Yie
ld o
f Sili
ca, %
Ratio of RHA and NaOH
(a) Effect of ratio of RHA/NaOH on silica extraction
10.009.81
9.41
9.11
9.609.52
8.68.89.09.29.49.69.8
10.010.2
3.33 3.70 4.35 5.26 6.67 9.09
Cos
t of R
aw M
ater
ials
per
kg
prod
uct,
tk
Ratio of RHA/NaOH
(b) Influence of the ratio of RHA/NaOH on Feasibility
119
▪ Effect of influence of molarity of NaOH solution on silica extraction and economic-feasibility
Figure 4.13 presents the influence of Molarity of Sodium Hydroxide solution on the silica extraction as soluble silicate.
Figure 4.13: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Molarity of Sodium Hydroxide solution in the reactant mixture solution; (b) Influence of Molarity of NaOH on the cost of 1 kg water glass adhesive.
(Reaction Parameters: Ratio of RHA and Water: 7, Temperature: 150oC, Reaction period: 3 h)
Results depicted in Figure 4.13 (a) correlates concentration of sodium hydroxide with the
amount of silica extraction. Increasing concentration of sodium hydroxide from 0.4 M to
1.07 M, the extracted amount of silica increased from 29.75% to 61.63%. But according
61.63 57.87 53.4347.60
37.0329.75
0.0010.0020.0030.0040.0050.0060.0070.00
1.07 0.96 0.82 0.68 0.54 0.39
Yie
ld o
f Sili
ca, %
Molarity of NaOH solution, M
(a) Effect of molarity of NaOH solution on silica extraction
10.009.81
9.41
9.11
9.60 9.51
8.68.89.09.29.49.69.8
10.010.2
1.07 0.96 0.82 0.68 0.54 0.39Cos
t of R
aw M
ater
ials
per
kg
prod
uct,
tk
Ratio of RHA/NaOH
(b) Influence of molarity of NaOH on feasibility
120
to Figure 4.13 (b), the lowest cost of raw materials for producing 1 kg water glass adhesive
is 9.11 Tk (experiment no-4) when the molarity of NaOH solution is 0.68 M.
However, from the economical point of view, for the synthesis of water glass adhesive
from RHA employed in the present investigation, the ratio of the RHA to sodium
hydroxide should preferably be 1: 5.26 and the molarity of sodium hydroxide solution
should be 0.68 M based on the parameters set for experiment no. 4.
4.4.2 Optimum ratio of RHA and water
Table 4.9 presents how the ratio of RHA and water influenced the yield of silica as well
as on the feasibility of the investigated experimental conditions. The experiments
(Experiment no. 7-12) were carried out in pressure vessel at temperature 150°C, with 3 h
reaction period, and the behavior for the silica extraction was observed. 1 kg of RHA was
mixed in water having different composition including 4 L, 5L, 6L, 7L, 8L and 9L
keeping all other parameters constant including 150oC temperature and 3 h reaction
period. Experiments were carried out to study the effect of the ratio of RHA/water on the
yield of silica extraction (Table 4.9).
Table 4.9: Influence of ratio of water/RHA on the yield of silica as well as on the feasibility of the production of water glass adhesive (Exp. 7-12)
Sl. No.
Parameters Result of Experiment Exp-7 Exp-8 Exp-9 Exp-
10(4) Exp-11 Exp-12
Raw Materials 1 RHA, kg 1 1 1 1 1 1 2 Caustic Soda flakes, kg 0.19 0.19 0.19 0.19 0.19 0.19 3 Water, L 4 5 6 7 8 9
Reaction Parameters 1 Ratio of RHA and Caustic soda 5.26 5.26 5.26 5.26 5.26 5.26 2 Molarity of NaOH 1.19 0.95 0.79 0.68 0.59 0.53 3 Temperature, oC 150 150 150 150 150 150 4 Reaction time, h 3 3 3 3 3 3
Yield of Silica Extraction 1 Dry silica, kg 0.37 0.42 0.43 0.46 0.46 0.46 2 Yield of Silica from RHA, % 38.52 43.75 45.01 47.60 47.41 47.41
Feasibility Check 1 Price of RHA 2.00 2.00 2.00 2.00 2.00 2.00 2 Price of Caustic Soda Flakes 67.00 67.00 67.00 67.00 67.00 67.00 3 Cost of Raw materials per kg water glass,
Tk 11.02 9.81 9.57 9.11 9.12 9.12
121
Figure 4.14 presents the influence of ratio of RHA/water variation on the silica extraction
as well as on the feasibility study.
Figure 4.14: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Ratio of RHA and Water in the reactant mixture solution; (b) Influence of Ratio of RHA/Water on the cost of 1 kg water glass adhesive.
(Constant Reaction Parameters: Ratio of RHA and NaOH: 5.26, Temperature: 150oc, Reaction period: 3 h)
Results in the Figure 4.14 (a) showed that the amount of the extracted silica was highest
(47.6%) when the ratio of RHA/Water was 7; however, the extracted amount of silica is
decreased to 38% when the ratio of RHA/water was decreased to 4. According to Figure
38.5243.75 45.01 47.60 47.41 47.41
0.00
10.00
20.00
30.00
40.00
50.00
4.00 5.00 6.00 7.00 8.00 9.00
Yie
ld o
f Sili
ca, %
Ratio of RHA and Water
(a) Effect of the ratio of RHA and water on silica extraction
11.029.81 9.57 9.11 9.12 9.12
0.000
2.000
4.000
6.000
8.000
10.000
12.000
4.00 5.00 6.00 7.00 8.00 9.00Raw
Mat
eria
ls C
ost t
o pr
oduc
e 1
kg w
ater
gla
ss
Ratio of Water/RHA
(b) Effect of the ratio of water/RHA on production cost
122
4.14 (b), the lowest cost of raw materials for producing 1 kg water glass adhesive is 9.11
Tk (experiment no-10(4)) when the ratio of RHA/Water is 1:7.
Low ratio of RHA and water such as 1:4 to 1:5 causes filtration problem as it forms a
gelatinous solution due to higher polymeric silica content which is quite difficult to filter.
On the other hand, higher ratio increases the cost of evaporation. Therefore, for the
easiness of filtration and from the economical point of view, it can be concluded that for
the synthesis of water glass adhesive from RHA, employed in the present investigation,
the ratio of the RHA to Water should preferably be 1:7 based on the parameters set for
experiment no. 10(4).
4.4.3 Optimum reaction temperature
The pioneer work carried out by Foletto et al. [103], showed in Figure 4.15, indicate the
influence of reaction temperature on silica conversion. They observed that the conversion
increases with the temperature rise, reaching 92% at 200°C, in only 20 minutes of reac-
tion. The conversion reaches a constant value after a certain reaction time.
Figure 4.15: Silica conversion as function of reaction time for different temperatures (H2O/SiO2 molar ratio = 22; NaOH/SiO2 ratio = 2; closed system) [103].
To reduce the consumption of sodium hydroxide, temperature and pressure need to be
optimized. Therefore, attempts were made to explore the effect of higher temperature
varies from 120oC to 170oC on the silica extraction.
123
Table 4.10 presents how the reaction temperature influenced the yield of silica as well as
on the feasibility of the investigated experimental conditions. These experiments
(Experiment no. 13-18) were carried out in the pressure vessel at controlling variable
temperatures of 120-170°C, with 3 h of reaction period, and the behavior of silica
extraction was obseved keeping all other parameters constant.
Table 4.10: Influence of reaction temperature on the yield of silica as well as on the feasibility (Exp. 13-18)
Sl. No.
Parameters Result of Experiment Exp-13
Exp-14
Exp-15
Exp-16(4)
Exp-17
Exp-18
Raw Materials 1 RHA, kg 1 1 1 1 1 1 2 Caustic Soda flakes, kg 0.19 0.19 0.19 0.19 0.19 0.19 3 Water, L 7 7 7 7 7 7
Reaction Parameters 1 Ratio of RHA and Caustic soda 5.26 5.26 5.26 5.26 5.26 5.26 2 Molarity of NaOH 1.19 0.95 0.79 0.68 0.59 0.53 3 Temperature, oC 120 130 140 150 160 170 4 Reaction time, h 3 3 3 3 3 3
Yield of Silica Extraction 1 Dry silica, kg 0.21 0.25 0.31 0.46 0.46 0.47 2 Yield of Silica from RHA, % 21.96 26.54 32.67 47.60 48.34 49.09
Feasibility Check 1 Price of RHA 2.00 2.00 2.00 2.00 2.00 2.00 2 Price of Caustic Soda Flakes 67.00 67.00 67.00 67.00 67.00 67.00 3 Cost of Raw materials per kg water glass,
Tk 16.11 14.15 12.18 9.11 9.00 8.90
Figure 4.16 presents the influence of reaction temperature on the silica extraction as well
as on the feasibility study.
124
Figure 4.16: (a) Yield of silica as silicate solution extraction from RHA as a function of the reaction temperature; (b) Influence of reaction temperature on the cost of 1 kg water glass adhesive.
(Constant Reaction Parameters: Ratio of RHA and NaOH: 5.26, Ratio of RHA/Water:
1:7, Reaction period: 3 h)
The influence of reaction temperature on silica conversion is illustrated in Figure 4.16 (a).
It is observed that the conversion increases with the temperature rise, reaching from 22%
at 120 °C to 49% at 170oC. According to Figure 4.16 (b), the lowest cost of raw materials
for producing 1 kg water glass adhesive is 8.90 Tk (experiment no-18) when the
temperature is 170oC. With decreasing temperature from 170oC to 150oC, the yield of
silica was not significantly changed.
2227
33
48 48 49
0102030405060
120 130 140 150 160 170
Yie
ld o
f sili
ca, %
Temperature, ºC
(a) Effect of Temperature on silica extraction
16.1114.15
12.18
9.11 9.00 8.90
0.002.004.006.008.00
10.0012.0014.0016.0018.00
120 130 140 150 160 170Raw
mat
eria
l cos
t to
prod
uce
1 kg
w
ater
gla
ss, B
DT
Temperature, oC
(b) Effect of Temperature on feasibility of production
125
It can be the temperature that increases the steam generation cost and therefore from
economical point of view, we concluded the fact that for the synthesis of water glass
adhesive from RHA, employed in the present investigation, the reaction temperature
should preferably be 150oC based on the parameter set for experiment no. 16(4).
4.4.4 Optimum reaction period
The results presented in Table 4.11 was of interest to study the effect of the reaction period
on the extraction process of silica from the RHA. For this purpose, the experiments
(Experiment no. 19-24) of alkaline extraction of RHA were performed in which the
reaction period was varied from 1 to 6 h, whereas the molarity of sodium hydroxide
(0.68M), the ratio of RHA and NaOH (1:5.26), the ratio of RHA and water (1:7), and the
temperature at 150oC were kept constant in all the reactant mixtures. Time was counted
from the moment the temperature reached its investigating temperature.
Table 4.11: Influence of reaction period on the yield of silica (Exp. 19-24)
Sl. No.
Parameters Result of Experiment Exp-19
Exp-20
Exp-21
Exp-22(4)
Exp-23
Exp-24
Raw Materials 1 RHA, kg 1 1 1 1 1 1 2 Caustic Soda flakes, kg 0.19 0.19 0.19 0.19 0.19 0.19 3 Water, L 7 7 7 7 7 7
Reaction Parameters 1 Ratio of RHA and Caustic soda 5.26 5.26 5.26 5.26 5.26 5.26 2 Molarity of NaOH 1.19 0.95 0.79 0.68 0.59 0.53 3 Temperature, oC 150 150 150 150 150 150 4 Reaction time, h 1 2 4 3 5 6
Yield of Silica Extraction 1 Dry silica, kg 0.22 0.30 0.46 0.46 0.46 0.47 2 Yield of Silica from RHA, % 22.51 31.60 47.71 47.60 48.06 48.86
Feasibility Check 1 Price of RHA 2.00 2.00 2.00 2.00 2.00 2.00 2 Price of Caustic Soda Flakes 67.00 67.00 67.00 67.00 67.00 67.00 3 Cost of Raw materials per kg water glass,
Tk 15.71 12.44 9.09 9.11 9.06 8.91
Haq et al. [115] performed a similar series of experiments in which the extraction time
was varied from 30 to 120 min, whereas the concentration of sodium hydroxide (1 mol/L)
and the amount of the dispersed RHA (166.66 g/L) were kept constant. They found that
126
the percent yield of the extracted silica increased with the increase in the extraction time
up to 90 min; however, beyond this time the silica yield became almost constant (See
Figure 4.17).
Figure 4.17: Percent yield of the rice husk silica (RHS), extracted from the rice husk ash (RHA) as a function of the extraction time. Volume of the reactant mixture, 60 ml; Concentration of NaOH, 1 mol/L; Concentration of RHA, 166.66 g/L [115].
Figure 4.18 presents the influence of reaction period on the silica extraction as well as on
the feasibility study.
22.5131.60
47.60 47.71 48.06 48.86
0.0010.0020.0030.0040.0050.0060.00
1.00 2.00 3.00 4.00 5.00 6.00
Yie
ld o
f sili
ca, %
Reaction time, h
(a) Effect of reaction period on silica extraction
Extraction time (min)
RH
S Y
ield
(%)
20 40 60 80 100 120
80
40 20 0
60
127
Figure 4.18: (a) Yield of Silica as silicate solution extraction from RHA as a function of the Reaction period; (b) Influence of Reaction period on the cost of 1 kg water glass adhesive.
(Constant Reaction Parameters: Ratio of RHA and NaOH: 5.26, Temperature: 150oC,
Ratio of RHA/Water: 1:7)
Results in the Figure 4.18 (a) showed that the amount of the extracted silica was highest
(48%) when the reaction period was 6 h; however, the extracted amount of silica was not
significantly changed after 3 h of reaction period.
The lowest cost of raw materials for producing 1 kg water glass adhesive is 8.91 Tk
(experiment no-24) when the reaction period is 6 h. But with increasing reaction period
from 3 h to 5 h, the yield of silica was almost constant. Higher reaction period increases
the utility cost including steam generation, electricity etc. Therefore, from economical
point of view, the observation clearly showed that for the synthesis of water glass
adhesive from RHA, employed in the present investigation, the optimum reaction period
should preferably be 3 h based on the experiment no. 22(4).
4.5 Characteristics of Water Glass
Sodium silicates have a wide range of characteristics to meet various application needs.
This section briefly discusses the major characteristics of sodium silicates.
PQ, largest manufacturer of water glass, offers liquid sodium silicates ranging in weight
ratio from 3.25 to 1.60 and in densities from 40Bé° (degrees Baume) to 52Bé° at 20°C
(Table 4.12). The properties of the liquid soluble silicates manufactured by PQ are
summarized in Table 4.12 [9].
15.7112.44
9.11 9.09 9.06 8.91
0.000
5.000
10.000
15.000
20.000
1.00 2.00 3.00 4.00 5.00 6.00
Raw
mat
eria
l cos
t to
prod
uce
1 kg
wat
er g
lass
Reaction time, h
(b) Effect of reaction period on the production cost of water glass
128
Table 4.12: Typical properties of PQ sodium silicate solutions [9]
Product Name
Wt. Ratio (SiO2/Na2O)
Properties Viscosity centipoise
Characteristics % Na2O % SiO2 Bé° g/cm3 pH
STIXSOTM 3.25 9.22 30.0 42.7 1.42 11.3 830 Syrupy liquid N® and N® 3.22 8.90 28.7 41.0 1.39 11.3 180 Syrupy liquid E® 3.22 8.60 27.7 40.0 1.38 11.3 100 Specially clarified O® 3.22 9.15 29.5 42.2 1.41 11.3 400 More concentrated
K® 2.88 11.00 31.7 47.0 1.48 11.5 960 Sticky, heavy M® 2.58 12.45 32.1 49.3 1.52 11.8 780 Syrupy liquid STARTM 2.50 10.60 26.5 42.0 1.41 11.9 60 Brilliant clear RUTM 2.40 13.85 33.2 52.0 1.56 12.0 2100 Heavy syrup DTM 2.00 14.70 29.4 50.5 1.53 12.7 400 Syrupy, alkaline B-WTM 50 1.60 16.35 26.2 50.3 1.53 13.4 280 High alkalinity,
syrupy
Viscosity of liquid silicates vary from fluid, slightly sticky consistencies to thick
substances that barely flow. As a general rule, higher SiO2/Na2O ratio products (2.8 to
3.22) are used for adhesive and coating applications [8, 9].
Table 4.13 described the analytical properties of water glass obtained after evaporation
and curing.
129
Table 4.13: Characterization of properties of diluted water glass solution after filtration and before evaporation
Experiment No.
WT. Ratio (SiO2/Na2O)
Total Solid content, %
Total alkali content, % Na2O
Total Silica Content, % SiO2
Density at 20oC , Bé°
pH Viscosity, flowing times in second
1 2.92 37 9.40 27.60 39 11.4 43 2 3.09 37 9.10 27.90 39 11.3 47 3 3.39 37 8.40 28.60 39 11.0 58 4 3.69 37 7.90 29.10 39 10.7 83 5 3.73 37 7.80 29.20 39 10.6 86 6 4.19 37 7.10 29.90 38 9.9 586 7 3.35 37 8.50 28.50 39 11.0 50 8 3.52 37 8.20 28.80 39 10.9 70 9 3.58 37 8.10 28.90 39 10.8 75 10 3.69 37 7.90 29.10 39 10.7 83 11 3.64 37 8.00 29.00 39 10.7 79 12 3.64 37 8.00 29.00 39 10.7 80 13 1.79 37 13.30 23.70 40 12.7 33 14 2.14 37 11.80 25.20 40 12.5 37 15 2.6 37 10.30 26.70 40 11.6 41 16 3.69 37 7.90 29.10 39 10.7 83 17 3.75 37 7.80 29.20 39 10.6 86 18 3.81 37 7.70 29.30 39 10.6 179 19 1.79 37 13.30 23.70 40 12.8 33 20 2.48 37 10.60 26.40 40 11.8 60 21 3.69 37 7.90 29.10 39 10.7 83 22 3.69 37 7.90 29.10 39 10.7 83 23 3.75 37 7.80 29.20 39 10.6 86 24 3.75 37 7.80 29.20 39 10.6 86
130
4.5.1 Visual appearance of the water glass solution
The sodium silicate obtained have slightly yellowish color (Figure 4.19 and Figure 4.20).
After adding acid, the gel formation of silica occurred which indicates that silica was
extracted properly from synthesized sodium silicate. Addition of sulphuric acid to sodium
silicate solution results in immediate white precipitation, which is believed to be caused
by the formation of sodium sulphate and silica. A slight reduction in pH (by adding acid)
results in an immediate gelation in addition to precipitation.
Figure 4.19: (a) Sodium silicate before addition of acid; (b) Sodium silicate after
addition of acid.
(a) Sodium Silicate before addition of acid
(b) Sodium Silicate after addition of acid
131
Sample-1 Sample-2 Sample-3 Sample-4
Sample-5 Sample-6 Sample-7 Sample-8
Figure 4.20: Optical image of water glass solution prepared from RHA collected from different rice mills.
132
4.5.2 Relation between viscosity and ratio of silica and sodium oxide
The binding property of water glass solution was investigated by making corrugated
board using the water glass adhesive prepared from RHA (Table 4.14 and Figure 4.21).
Viscosity was manually checked by viscosity cup and stop watch.
Table 4.14: Observation of viscosity and binding properties
No. of
exp.
Ratio of silica/alkali
Density after
evaporation (oBe’)
Density after 48 hr. curing
Viscosity, flow time
(sec)
Observation on binding property
1. 2.92 37 39 43 Viscosity- very low; Binding property - poor. Slow drying.
2. 3.09 37 39 47 Viscosity- very low; Binding property - poor. Slow drying.
3. 3.39 37 39 58 Viscosity- low; Binding property - good. Moderate drying.
4. 3.69 37 39 83 Viscosity- medium; Binding property - good. Fast drying.
5. 3.73 37 39 86 Viscosity- medium; Binding property - good. Very Fast drying.
6. 4.19 37 38 586 Viscosity- very high; cannot apply as it forms gelatinous solution.
7. 3.35 37 39 50 Viscosity- low; Binding property - good. Moderate drying.
8. 3.52 37 39 70 Viscosity- medium; Binding property - good. Fast drying.
9. 3.58 37 39 75 Viscosity- medium; Binding property - good. Fast drying.
10. 3.69 37 39 83 Viscosity- medium; Binding property - good. Fast drying.
11. 3.64 37 39 79 Viscosity- medium; Binding property - good. Fast drying.
12. 3.64 37 39 80 Viscosity- medium; Binding property - good. Fast drying.
13. 1.79 37 40 33 Viscosity- very low; Binding property – very poor. Slow drying.
14. 2.14 37 40 37 Viscosity- very low; Binding property – very poor. Slow drying.
15. 2.60 37 40 41 Viscosity- very low; Binding property - poor. Slow drying.
16. 3.69 37 39 83 Viscosity- medium; Binding property - good. Fast drying.
133
17. 3.75 37 39 86 Viscosity- medium; Binding property - good. Very Fast drying.
18. 3.81 37 39 179 Viscosity- medium; Binding property - good. Very Fast drying.
19. 1.79 37 40 33 Viscosity- very low; Binding property - poor. Slow drying.
20. 2.48 37 40 60 Viscosity- low; Binding property - poor. Slow drying.
21. 3.69 37 39 83 Viscosity- medium; Binding property - good. Fast drying.
22. 3.69 37 39 83 Viscosity- medium; Binding property - good. Fast drying.
23. 3.75 37 39 86 Viscosity- medium; Binding property - good. Very Fast drying.
24. 3.75 37 39 86 Viscosity- medium; Binding property - good. Very Fast drying.
Figure 4.21 Relation between weight ratio of water glass and it's viscosity.
From this investigation, it is evident that the viscosity and density have effect on the
binding properties as well as drying time of corrugated board. With increasing ratio of
silica and sodium oxide, viscosity also increased at ratio 4.19, the resultant product
formed a gelatinous solution which cannot be used on corrugated board due to high
viscosity. Drying time was increase with decreasing ratio of silica/alkali. Therefore, from
economical point of view, the observation clearly showed that for the synthesis of water
glass adhesive from RHA, the optimum ratio of silica to alkali and density at room
temperature should preferably be 3.69 and 39 Bé°, respectively.
0
100
200
300
400
500
600
700
2.9
2
3.0
9
3.3
9
3.6
9
3.7
3
4.1
9
3.3
5
3.5
2
3.5
8
3.6
9
3.6
4
3.6
4
1.7
9
2.1
4
2.6
3.6
9
3.7
5
3.8
1
1.7
9
2.4
8
3.6
9
3.6
9
3.7
5
3.7
5
Viscosity, (seconds)
Relation between weight ratio of water glass and it's viscosity
Series1Ratio of Silica and Sodium Oxide
134
4.6 Characterization of Water Glass
4.6.1 Elemental analysis of water glass adhesive
The chemical composition of the water glass was determined by X-ray fluorescence
spectroscopy (XRF). The results of the tests are tabulated below in Table 4.15. Four
samples of water glass were analyzed by XRF to check the weight ratio of SiO2:Na2O and
the presence of impurities in water glass solution. It was observed that weight ratio
obtained from analytical methods are slightly varied from XRF analysis. Because during
analytical process, it was assumed the silica content including all impurities. However,
XRF analysis shows the exact value of silica along with other impurities separately.
Table 4.15: Elemental analysis of water glass powder by XRF
No. of Exp.
Chemical Composition, Wt% Weight ratio obtained by analytical method
1 2 3 4 5 6 7 8 Weight Ratio
SiO2 Na2O Al2O3 K2O Fe2O3 SO3 P2O5 CaO SiO2 / Na2O
Exp-1 70.72 25.52 0.20 2.31 0.07 0.41 0.67 0.09 2.77 2.92 Exp-4 73.69 21.29 0.35 2.59 0.11 0.65 1.13 0.15 3.46 3.70 Exp-6 76.67 19.35 0.26 2.42 0.08 0.56 0.56 0.07 3.96 4.17 Exp-9 73.68 21.81 0.24 2.86 0.12 0.52 0.65 0.10 3.38 3.59
4.6.2 Morphological properties of water glass adhesive
The morphological features of the water glass adhesive was observed by scanning
electron microscopy (SEM), which are shown in Figure 4.22. The micrograph was taken
under the secondary electron imaging mode (SEI) at a magnification of 1000X. As can
be seen from Figure 4.22 that the as-received RHA has a porous and multifaceted particle
shape and size. The main components of water glass include silica and alkali with
traceable amount of impurities. The glassy film particles in Figure 4.22, is a prove of
producing water glass. The produced sodium silicate subsequently polymerizes to form a
glassy film after drying.
135
Figure 4.22: FESEM micrograph of water glass powder (Sample 4).
4.7 Characterization of Extracted Amorphous Silica
A photograph of the as-produced precipitated silica powder after drying and milling is
shown in Figure 4.23. The material looked like white free flow powder and slightly
greyish.
(a)
(b) Figure 4.23: As produced precipitated silica from RHA (a) White free flow powder;
(b) Slightly greyish crystal.
136
XRD patterns of the as-produced amorphous precipitated silica from RHA are shown in
Figure 4.24 and Figure 4.25. An amorphous broad peak with an equivalent Bragg angle
at 2Ɵ = 21.8º was recorded.
Figure 4.24: XRD patterns of the as-produced extracted amorphous silica-1 from RHA.
Figure 4.25: XRD patterns of the as-produced extracted amorphous silica-2 from RHA.
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100
Inte
nsity
(Cou
nts)
Position (º2Theta)
As-Produced Extracted Amorphous Silica-1 from Experiment-4
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100
Inte
nsity
(Cou
nts)
Position (º2Theta)
As-Produced Extracted Amorphous Silica-2 from Experiment-7
137
Figure 4.26 shows the FE-SEM image of the amorphous SiO2 particles at different optical
magnifications. The majority of primary silica particles were 0.3 – 0.8 µm in size. These
primary particles showed a tendency to form bigger particles (aggregates).
(a)
(b)
Figure 4.26: FE-SEM images of precipitated silica at different magnifications (a) FESEM images of precipitated silica (1 µm; X 10000); (b) FESEM images of precipitated silica (100 nm; X 30000).
CHAPTER 5
CONCLUSIONS AND FUTURE WORK
139
CHAPTER 5. CONCLUSIONS AND FUTURE WORK
5.1 Conclusions
In this study, RHA, which has negative impact on our environment, was used to produce
a value-added product sodium silicate solution. The principal objective of this research
work is to develop a simple and cost-effective process for the production of sodium
silicate. Based on the results obtained in this study, the following conclusions can be
drawn:
▪ It was found that RHA contains 88% - 97% SiO2 and the rest contains the
following elements (in oxide form) in traceable amount: 0.17-1.42% Al2O3, 0.89-
2.35% K2O, 0.31-1.33% MgO, 0.38-0.69% Fe2O3, 0.07-0.34% MnO, 0.16-2.03%
SO3, 0.01-0.02 ZnO, 1.15-2.67% P2O5, 1.02-1.89% CaO and 0.2-2.95% Na2O.
Therefore, sourced RHA can be used as silica for the production of water glass.
▪ XRD patterns confirms the presence of crystalline and amorphous structure of
silica in RHA. The morphological image of SEM also depicted amorphous nature
of SiO2 in RHA with crystalline impurities.
▪ A method was developed for producing a stable water glass solution having a
stabilized density of 39o Be’ (1.38 specific gravity) and a SiO2: Na2O ratio of
3.69:1 with the proper portion of SiO2 (29.2% by weight) and Na2O content (7.8%
by weight) which meets all the criteria of standard adhesive. The following
reaction parameters were optimized:
o Digestion Temperature: 150oC
o Reaction Period: 3 h
o Ratio of RHA and Sodium Hydroxide: 5.26:1
o Ratio of RHA and Water: 7:1
▪ By analyzing the water glass using XRF, it was found that it contains 70.72% -
76.67% SiO2 and 19.35% - 25.52% Na2O having weight ratio of silica/alkali 2.92
to 3.70. The rest contains some metal impurities (in oxide form) in traceable
amount. XRF data had shown that the produced water glass had optimum weight
ratio of silica and alkali which was 3.69~ 3.70.
▪ The produced water glass is amorphous in nature which was confirmed by XRD
analysis.
▪ Sodium silicate can be extracted from RHA as a silica source.
140
▪ The extracted soluble sodium silicate displayed lesser viscosity as compared with
standard sample which may be attributed to the low temperature of extraction.
▪ The extracted soluble sodium silicate has displayed similar physical
characteristics as compared to the standard sample.
5.2 Future Work
Many new areas can be explored for further development of this work. Therefore, the
developed cost-effective low temperature process for extracting sodium silicate from
RHA can be treated as a promising technique having multifaceted advantages. Some ideas
along these lines are presented below.
▪ Effect of burning temperature of RH to produce above 96% amorphous silica
rich RHA (RHA).
▪ Removal of color from produced water glass adhesive by controlling the quality
of RHA.
▪ Finding the suitable storage temperature to store water glass adhesive having
higher weight ratio of silica and alkali.
▪ Synthesis of activated carbon obtained from filter cake during filtration stage of
producing water glass.
141
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