ren wei 201406 masc thesis

8/19/2019 Ren Wei 201406 MASc Thesis

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DEVELOPMENT OF A FREE LIME MONITORINGSYSTEM FOR THE KRAFT RECOVERY PROCESS

USING ZETA POTENTIAL

by

Wei Ren

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Department of Chemical Engineering and Applied ChemistryUniversity of Toronto

© Copyright by Wei Ren 2014


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Development of a Free Lime Monitoring System for the Kraft

Recovery Process Using Zeta Potential

Wei Ren

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2014

ABSTRACT

The presence of Ca(OH)2 (or free lime) in lime mud can cause many problems in the recovery

process of kraft pulp mills. Conventional free lime analyses require extensive laboratory work

and give inconsistent results. A systematic study was performed to determine if zeta potential, a

quickly measurable dispersion characteristic, can be used to indicate the presence of free lime in

the recovery process. Measurements were made on synthetic wet lime mud to simulate samples

collected after the white liquor clarifier in pulp mills. The results show that zeta potential

increases from negative to positive when the free lime content in the lime mud exceeds a critical

threshold. This change from negative to positive zeta potential of the lime mud slurry can be

used as a basis for developing an on-line monitoring system that effectively detects free lime in

the lime mud in order to avoid problems associated with overliming in the recovery process.


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ACKNOWLEDGMENTS

I wish to express my utmost gratitude to my thesis supervisor Professor Honghi Tran for his

excellent advice and guidance throughout the course of this study. His enthusiasm, motivation

and encouragement were of extreme importance for the completion of my research.

I want to thank my committee members, Professor Edgar Acosta, Professor Donald Kirk and

Professor Will Cluett for their insightful comments and valuable pieces of advice.

I also want to thank Fariba Azgomi and Sue Mao for their valuable suggestions, and providing

me with essential literatures relevant to my research. My gratitude goes to Anna Ho for her help

with administrative activities. I am grateful to all my friends, especially Eric, Liming, Anton,

Michael, Masoumeh and Naz for their support and friendship. I also want to extend my

appreciation to Carolyn Winsborough for proofreading my thesis and other technical reports.

I wish to acknowledge members of the research program on “ Increasing Energy and Chemical

Recovery Efficiency in the Kraft Pulping Process” for their discussions and advice during

consortium meetings and their financial support. I would like to thank Domtar Espanola Mill and

DMI Peace River Mill for giving me extensive information regarding kraft pulp mill operations

and providing samples used in my research.

I want to give my sincere gratitude to my parents Jiyi Ren and Hong Liu for their technical

advices during my study, but more importantly for giving me their unconditional support and

love my whole life.

Last but not least, I want to thank my fiancée Nan Ma for all she has done for me over the past

ten years. Only with her inspiration, motivation, encouragement and love I am able to finish my

research. I want to dedicate this thesis to her.


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TABLE OF CONTENTS

ACKNOWLEDGMENTS ........................................................................................................... iii

TABLE OF CONTENTS ............................................................................................................ iv

LIST OF TABLES ....................................................................................................................... vi

LIST OF FIGURES .................................................................................................................... vii

1 INTRODUCTION .................................................................................................................... 1

1.1 General Background ........................................................................................................... 1

1.2 Causticizing Reaction and Free Lime ................................................................................. 3

1.3 Zeta Potential Overview ..................................................................................................... 5

1.4 Objectives ........................................................................................................................... 6

2 LITERATURE REVIEW ........................................................................................................ 7

2.1 Current Methods of Free Lime Analysis ............................................................................ 7

2.2 Zeta Potential (ζ) ................................................................................................................. 8

2.2.1 Zeta Potential Theory .............................................................................................. 8

2.2.2 Previous Studies and Applications ........................................................................ 10

2.3 Sound Theories and Applications ..................................................................................... 13

2.4 Factors Affecting Free Lime Concentration ..................................................................... 14

2.4.1 Lime Properties ..................................................................................................... 14

2.4.2 Operating Conditions in Causticizing Plant .......................................................... 15

3 METHODOLOGY ................................................................................................................. 18

3.1 Material ............................................................................................................................. 18

3.2 Experimental Setup ........................................................................................................... 19

3.3 Experimental Procedures .................................................................................................. 21

3.3.1 Particle Size Determination .................................................................................. 21

3.3.2 Initial Experimental Procedure ............................................................................. 24


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3.3.3 Procedure Modification ........................................................................................ 25

3.3.4 Modified Experimental Procedure ........................................................................ 30

4 RESULTS AND DISCUSSION ............................................................................................ 32

4.1 Effect of Free Lime on Zeta Potential for Analytical Samples ......................................... 32

4.1.1 Analytical Grade Chemicals ................................................................................. 32

4.1.2 Analytical Grade Lime and Mill Green Liquor .................................................... 36

4.2 Effect of Free Lime on Zeta Potential for Mill Samples .................................................. 39

4.2.1 Effect of Lime Properties ...................................................................................... 43

4.2.2 Effect of Causticizing Reaction Time ................................................................... 45

4.2.3 Effect of Lime Mud Storage Time ........................................................................ 46

5 CONCLUSIONS .................................................................................................................... 50

6 IMPLICATIONS.................................................................................................................... 52

REFERENCES ............................................................................................................................ 54

APPENDICES ............................................................................................................................. 58

Appendix A Ammonium Chloride Test ................................................................................... 58

Appendix B Additional Sound Theories .................................................................................. 60

Appendix C ABC Titration (with Automatic Titrator) ............................................................ 61

Appendix D Thermogravimetric Analysis (TGA) / Differential Scanning Calorimetry

(DSC) ................................................................................................................................ 62

Appendix E Zeta Potential Data for Wet and Dry Lime Mud ................................................. 63

Appendix F Zeta Potential Data for Stored Sample ................................................................. 64


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LIST OF TABLES

Table 2-1. Correlation between Zeta Potential and Dispersion System ....................................... 10

Table 2-2. Properties of Quicklime .............................................................................................. 14

Table 2-3. Factors Affecting Free Lime Concentration ................................................................ 17

Table 3-1. Green Liquor Composition .......................................................................................... 19


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LIST OF FIGURES

Figure 1-1. Typical Kraft Recovery Process .................................................................................. 2

Figure 1-2. Overall Causticizing Plant Process ............................................................................ 2

Figure 1-3. Goodwin’s Curve - Effect of TTA on Equilibrium CE at Different Sulphidity [4] ...... 4

Figure 1-4. Acoustic and Electroacoustic Spectrometer ................................................................ 5

Figure 2-1. Double Layer Model for Zeta Potential ....................................................................... 9

Figure 2-2. Potential Charge as a Function of Distance from Particle Surface ......................... 10

Figure 2-3. Effect of Liming Ratio on Zeta Potential ................................................................... 11

Figure 2-4. Zeta Potential as a Function of Free Lime Content .................................................. 12

Figure 3-1. Zeta Potential Measurement Probe ........................................................................... 20

Figure 3-2. Particle Size Distribution for CaCO3 ......................................................................... 22

Figure 3-3. Particle Size Distribution for Ca(OH)2 (from Calcination of Lime Mud) ................. 22

Figure 3-4. Average Particle Sizes for Lime and Lime Mud Samples .......................................... 23

Figure 3-5. Average Zeta Potentials for Lime and Lime Mud Samples ........................................ 24

Figure 3-6. Effect of Liming Ratio on Zeta Potential at Different Causticizing Reaction

Temperatures (Pure Chemicals, 1 Hour Reaction) ...................................................................... 25

Figure 3-7. Effect of Measurement Temperature on Zeta Potential (Pure Chemicals, 1 Hour

Reaction, 90°C Reaction Temperature) ........................................................................................ 26

Figure 3-8. Effect of Liming Ratio on Zeta Potential when Measuring White Liquor Slurries

Synthesized with Pure Chemicals and Mill Green Liquor Samples (1 Hour Reaction, 90°C

Reaction Temperature) ................................................................................................................. 27


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Diluted with Distilled Water at Different Concentrations (Pure Chemicals, 1 Hour Reaction,

90°C Reaction Temperature) ........................................................................................................ 27

Figure 3-10. Effect of Liming Ratio on Zeta Potential when Measuring Lime Mud Diluted with

Distilled Water at Different Concentrations (Pure Chemicals, 1 Hour Reaction, 90°C Reaction

Temperature) ................................................................................................................................. 28


Distilled Water at 1wt% (Mill Green Liquor, 1 Hour Reaction, 90°C Reaction Temperature) ... 28

Figure 3-12. Sample Compartments for Zeta Potential Measurement ......................................... 29

Figure 3-13. Effect of Liming Ratio on Zeta Potential when Measuring 10mL of Lime Mud

Diluted in Distilled Water (Pure Chemicals, 1 Hour Reaction, 90°C Reaction Temperature) .... 30

Figure 3-14. Experimental Procedure Flow Chart ....................................................................... 31

Figure 4-1. Measurement Repeatability with 10mL Samples (Pure Chemicals, 1 Hour Reaction,


Figure 4-2. Weight Loss Profile for Lime Mud (0.6 Liming Ratio and Pure Chemicals) ............. 34

Figure 4-3. Weight Loss Profile for Lime Mud (1.2 Liming Ratio and Pure Chemicals) ............. 35

Figure 4-4. Zeta Potential and Free Lime as a Function of Liming Ratios (Pure Chemicals, 1

Hour Reaction, 90°C Reaction Temperature) .............................................................................. 35

Figure 4-5. Zeta Potential as a Function of Free Lime Content (Pure Chemicals, 1 Hour


Figure 4-6. Zeta Potential and Free Lime as a Function of Liming Ratios (Mill Green Liquor, 1


Figure 4-7. Zeta Potential as a Function of Free Lime Content (Mill Green Liquor, 1 Hour



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Figure 4-8. Effect of Liming Ratio on Zeta Potential – Dry and Wet Samples (Pure Lime, 1 Hour


Figure 4-9. Effect of Liming Ratio on Zeta Potential using Pure Lime and Reburned Lime (1


Figure 4-10. Effect of Liming Ratio on Zeta Potential – Dry and Wet Samples (Reburned Lime, 1


Figure 4-11. Lime Reactivity Test Setup ....................................................................................... 43

Figure 4-12. Wet Slaking Curve for Powdered and Pelleted Reburned Lime .............................. 44

Figure 4-13. Effect of Liming Ratio on Zeta Potential – Powdered and Pelleted Reburned Lime

(1 Hour Reaction, 90°C Reaction Temperature) .......................................................................... 45

Figure 4-14. Effect of Reaction Time on Zeta Potential (Mill Green Liquor, 90°C Reaction

Temperature) ................................................................................................................................. 46

Figure 4-15. Effect of Storage Time on Zeta Potential (Mill Green Liquor, 1 Hour Reaction,


Figure 4-16. Effect of Extended Storage Time on Zeta Potential (Mill Green Liquor, 1 Hour


Figure 4-17. Effect of Reaction Time on Zeta Potential – 1 Month Storage (Mill Green Liquor,


Figure 6-1. Potential Online Free Lime Monitoring System ........................................................ 53


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1 INTRODUCTION

1.1 General Background

The kraft process is the dominant pulping process, netting approximately 130 million tons/year

of pulp globally and accounting for over 90% of the world’s chemical pulp production [1]. The

main advantages of kraft pulping are its ability to handle almost all softwood and hardwood

species, the high strength and quality of kraft pulp and its high chemical efficiency (about 97%).

In this process, wood chips (which consist of fibre and lignin) are treated in a digester at an

elevated temperature and pressure in an aqueous solution consisting mainly of sodium hydroxide

(NaOH) and sodium sulphide (Na2S), known as “white liquor ”. The fibre in the wood chips is

separated and washed to make the pulp. The residue, weak black liquor (including lignin, spent

chemicals and water), is sent to the kraft recovery system where the organic chemicals are used

as fuels to provide steam and power for the pulp mill while the inorganic chemicals are recycled

back into the pulping process.

A schematic of the kraft recovery process is shown in Figure 1-1. Weak black liquor is

concentrated through a series of evaporators to increase its solids content. The resulting heavy

(concentrated) black liquor is burned in the recovery boiler to produce steam and power. The

inorganic portion of the black liquor forms molten smelt, consisting mainly of sodium carbonate

(Na2CO3) and Na2S. The smelt is dissolved in water in the dissolving tank to produce green

liquor, which is subsequently causticized in the causticizing plant with calcium hydroxide

(Ca(OH)2) to produce NaOH according to the slaking and causticizing reaction.

Slaking: CaO + H2O Ca(OH)2 (1)

Causticizing: Na2CO3 + Ca(OH)2 2NaOH + CaCO3 (2)

The resulting slurry which consist of NaOH, Na2S, unreacted Na2CO3, unreacted Ca(OH)2 and

lime mud (CaCO3) is sent through a clarifier to filter out the precipitates (lime mud). The

recovered white liquor consisting mainly of NaOH and Na2S is recycled back to the digester to

be reused in the pulping process while the washed water (weak wash) is recycled to the

dissolving tank for dissolving smelt.


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Figure 1-1. Typical Kraft Recovery Process [1]

The lime mud slurry is dewatered and fed into a lime kiln where it is calcined to produce

reburned lime, which is reused in the slaker. The calcination reaction is endothermic and occurs

at temperatures over 800C. The conventional causticizing process from the dissolving tank to

the lime kiln can be seen in Figure 1-2.

Figure 1-2. Overall Causticizing Plant Process [2]

Slaker and causticizer operations are instrumental for maintaining high process efficiency and

minimizing problems in the causticizing plant. Hence it is important to understand how the

causticizing reaction proceeds and the problems associated with free lime.


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1.2 Causticizing Reaction and Free Lime

In this study, free lime is defined as unreacted calcium hydroxide (Ca(OH)2) in the lime mud. In

addition, the causticizing efficiency (CE) is defined as the percentage of NaOH concentration in

the white liquor [3] ([NaOH] and [Na2CO3] are concentrations expressed in g/L Na2O):

Causticizing Efficiency (%) =

× 100 (3)

The causticizing reaction has a slow reaction rate and does not go to completion but reaches

equilibrium. The slaker and several causticizers are used in series to maximize the retention time

and reaction efficiency. Theoretically, the equilibrium or maximum attainable CE is the same as

the maximum CaO/Na2CO3 ratio (liming ratio). When targeting an equilibrium CE of 80%, for

example, the amount of lime added into the slaker should be adjusted so that the liming ratio is

equal to 0.80. Exceeding this ratio would cause the system to contain free lime; and the system is

said to be “overlimed”. Similarly, when an insufficient amount of lime is added into the slaker,

causing the liming ratio to be much lower than 0.80, the system is said to be “underlimed”.

The theoretical CE is dependent on the total titratable alkali (TTA) and the sulphidity of the

white liquor as defined in Equation 4 and Equation 5, respectively.

Total Titratable Alkali = [NaOH] + [Na2S] + [Na2CO3] (4)

Sulphidity =

× 100 (5)

The Na2S concentration ([Na2S]) is expressed in g/L Na2O. The relationship between CE, TTA

and sulphidity is represented in the Goodwin curve, which is used to describe the highest

achievable CE at a given condition (Figure 1-3). In the causticizing plant, a CE closely equal to

the theoretical value is desired as underlimed systems containing unreacted Na2CO3 or dead load

in recovered white liquor reduces equipment efficiencies in downstream processes. Furthermore,

additional make-up white liquor is required at the digester as NaOH yield decreases. On the other

hand, at any given TTA, there will be an upper CE limit to avoid overlimed systems. Overliming

can also cause problems including poor mud settling rate, low mud solids content, high soda

content, poor kiln thermal performance, high TRS emissions and ring formations [5].


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Figure 1-3. Goodwin’s Curve - Effect of TTA on Equilibrium CE at Different Sulphidity [4]

Therefore, the ability to maintain an optimum CE is crucial to operations in the causticizing

plant. Conventionally, the amount of lime fed into the slaker is adjusted to achieve the targeted

CE at a certain causticizing stage, such as at one of the causticizers. This means CE is used as the

indicator for detecting overlimed systems. Even though the targeted CE at mills is usually lower

than the equilibrium CE under normal operating conditions to avoid overliming, if the lime

reactivity is low or the residence time for the causticizing reaction is too short, free lime may

exist in the white liquor and subsequently in the lime mud.

Lime addition at the slaker is based on CE, which is calculated based on TTA and sulphidity of

the liquor sample collected at sampling ports in the causticizing plant (e.g. one of the

causticizers) [6]. The conventional method used to measure liquor composition is based on

titration, commonly referred to as the ABC test. It is a standard method with TAPPI (Technical

Association of the Pulp and Paper Industry) [7] which provides accurate results but involveslengthy analytical procedures. However, the ABC test gives inconsistent measurements when the

test is performed by different operators. On the other hand, free lime is not measured regularly,

unless problems associated with overliming occur frequently.

Contrary to the conventional titration method, this study focuses on developing an online

monitoring tool for the kraft recovery process that gives fast indication on the presence of free

White Liquor TTA (g/L)

stiizi

ffii

(

)


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lime, which can be used to adjust the liming ratio at the slaker. Based on a recent study, zeta

potential () – a variable that is used to characterize stability of dispersion systems – is found to

be suitable for this purpose. Possible correlation was found to exist between zeta potential and

free lime based on Azgomi’s research [8]. A systematic study is carried out to verify the

relationship between zeta potential and free lime using an acoustic and electroacoustic

spectrometer (Figure 1-4).

Figure 1-4. Acoustic and Electroacoustic Spectrometer [9]

1.3 Zeta Potential Overview

Zeta potential is a characterization of colloidal surface charge in solutions. The electrostatic

interaction forces between particles vary depending on the dispersion composition. The stability

of the colloidal system can be determined based on the magnitude of zeta potential. Particles

with higher absolute values of zeta potential will repel each other (stable system) while particles

with lower absolute values will cause agglomerations (unstable system) [10]. The settling rate of

lime mud particles in white liquor can be affected by free lime content, where an increase in free

lime content causes a decrease in settling rate [8]. Slow settling indicates a stable system and


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should theoretically have a higher zeta potential value. Hence, a positive correlation should exist

between zeta potential and free lime content when other conditions are unchanged.

The acoustic and electroacoustic spectrometer based on sound theory has not been used

previously for measurements of zeta potential of liquor slurries as an indication of free lime in

the kraft recovery process. However, due to its capability to measure dispersion systems at

higher concentration, volume and temperature compared to the conventional light scattering

techniques, it presents more opportunities to be incorporated as an online monitoring system in

the causticizing plant.

1.4 Objectives

The objective of this work is to establish a well-defined relationship between free lime and zeta

potential. This study will serve as a foundation for the implementation of an online monitoring

system in the kraft recovery process. The work consists of the following stages:

Validating previous findings of correlation between zeta potential and liming ratio;

Comparing zeta potential measurements of liquor samples produced using analytical

chemicals and using samples collected at pulp mills;

Analyzing the effects of different parameters (e.g. reaction time) on zeta potential.

This thesis contains six chapters. Chapter 1 gave a summary of the motivations and objectives

for this study. Chapter 2 is a literature review introducing current methods of free lime analysis

and their shortcomings; the advantages of using zeta potential to measure free lime; the validity

and application of acoustic and electroacoustic principles when making these measurements; and

the effect of lime properties and causticizing plant operating conditions on the presence of free

lime. Chapter 3 discusses the methodology used in this study including materials and instruments

as well as establishing a procedure for efficient measurements using the spectrometer. Chapter 4

presents and discusses the results. Finally, Chapter 5 and Chapter 6 summarize the key findings

and indicate the implications of this study.


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2 LITERATURE REVIEW

2.1 Current Methods of Free Lime Analysis

The free lime content in lime mud is often only analyzed when overliming problems occur in the

causticizing plant. There are no standard free lime testing method currently adapted by TAPPI

and PAPTAC (Pulp and Paper Technical Association of Canada). Two conventional free lime

measurement methods used in kraft pulp mills are the ammonium chloride (NH4Cl) method and

the muffle furnace method.

The exact procedure of the ammonium chloride method can be found in Appendix A. Even

though the procedure varies from mill to mill, the fundamental concept remains the same. The

origin of this analytical method was found to be based on ASTM Standard C-25 [11] and ASTM

Standard C-114 [12], which are standard methods used in the limestone and cement industries.

However, the transition to the pulp and paper industry remains unclear. The principle of the

NH4Cl analysis is to dissolve white liquor in a weak acid solution, where the free lime (Ca(OH)2)

will form soluble calcium chloride (CaCl2). CaCl2 is used in a series of reactions until titration is

performed at the end with potassium permanganate (KMnO4).

Previous studies have indicated that the NH4Cl analysis might not be able to reliably measure the

free lime content [7] since the reaction time between Ca(OH)2 and NH4Cl took much longer toreach completion than the suggested reaction time in the procedures (30 minutes). In addition,

the residual sodium carbonate (Na2CO3) in the sample could neutralize the weak acid solution

required to produce CaCl2 which disrupts the overall reaction.

Another method for free lime determination is based on sample weight loss due to chemical

decomposition at different temperatures, commonly used in thermogravimetric analysis (TGA).

Since Ca(OH)2 decomposes at 380°C, a muffle furnace is used to heat up the sample and the

weight is recorded at 300°C and 500°C [13]. The difference in weight between these two

temperatures is used to determine the amount of free lime in the sample.

A problem with using this method is that other chemicals with decomposition temperatures in the

range between 300°C and 500°C would contribute to the total amount of free lime. For example,

magnesium hydroxide (Mg(OH)2) decomposes at approximately 350°C. Thus, the amount of free


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lime calculated using the muffle furnace analysis would often be higher than the actual amount,

as the total weight loss is also caused by the decomposition of Mg(OH)2.

Measuring the free lime content using either the NH4Cl method or the thermal decomposition

method has some disadvantages that make them unsuitable for integration into an online

monitoring system. The most important disadvantage is that the required laboratory work is

extensive (e.g. long reaction or heating time). There is a need for a new method that can be used

to accurately and reliably determine the free lime content. Zeta potential is a characteristic of

slurry systems that is affected by the free lime content and can be measured efficiently.

2.2 Zeta Potential (ζ)

Many theories were developed to explain interactions in aqueous systems between particles and

the surrounding solution. Zeta potential is mainly based on the magnitude of electrical potential

difference that exists between a particle’s surface and the continuous medium. The potential is

affected by chemical composition and concentration in the dispersion, such as the amount of free

lime content. Previous studies were able to identify how some chemicals (e.g. Na 2CO3) that exist

in the slurry systems will affect the zeta potential.

2.2.1 Zeta Potential Theory

Zeta potential is a measurement of the colloidal surface charge or the electrochemical

equilibrium on particle interfaces in an aqueous solution. It depends on the properties of the

particle surface and the solution. The electrostatic interaction forces among particles have an

important impact on the properties of the solution. The stability of the colloidal system can be

determined based on the magnitude of zeta potential. In general, dispersion systems with higher

absolute zeta potential values will cause particles to repel one another while systems with lower

absolute zeta potential values will lead to particle agglomeration resulting in faster settling [10].

Zeta potential is part of the DLVO theory, named after Derjaguin, Landau, Verwey andOverbeek [14, 15]. It is used to quantitatively explain the collection of fine dispersions in an

aqueous solution and the interactions between charged particles in a liquid medium. DLVO

theory is based on both the effect of van der Waals and double layer forces [16]. Van der Waals

force is the sum of attractive and repulsive forces between molecules excluding covalent bonds,

hydrogen bonds and electrostatic interactions. The double layer force is based on electrostatic


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charges accumulated at the surface of the particle when it is in a dispersed phase [17]. Zeta

potential is used to characterize the double layer force.

Zeta potential can be explained using the distribution of electrical charge in the double layer as

shown in Figure 2-1. Charges build up at the surface of the colloid, forming an electrostatic field

that affects other ions in the solution. Two forces are experienced by the negatively charged

colloid: the attraction forces from the oppositely charged ions (counter-ions) and the repulsion

forces from the same charged ions (co-ions) in the solution [17].

Figure 2-1. Double Layer Model for Zeta Potential

When counter-ions are absorbed to the surface of the colloid, an inner layer is formed called the

“Stern layer”. The counter -ions on the outer surface are also attracted to the colloid but encounter

the repulsive force from the Stern layer. This dynamic equilibrium results in the formation of the

“diffuse layer”. The slipping plane is the imaginary boundary that sets apart the stationary layer

of fluid attached to the colloid and the free flowing dispersion medium [10].

The potential difference between the particle surface and the continuous phase is a function of

the distance from the particle surface (Figure 2-2). The absolute value of the potential charge

decreases linearly within the Stern layer and then exponentially within the diffuse layer, where it

+

+

++

+

+

+

+

+

+

-

-

--

-

-

-

-

-

Colloid Surface

Slipping Plane+ ++

+

+

+

+++

+

+

+

+

+

+

-

+

+

+

+

++

+

+

+ +

+

+

+

-

-

-

-

-

-

-

+

-

+

-

+

-

+

-

Stern Layer

Diffuse Layer


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approaches zero at the imaginary boundary of the double layer. Theoretically, zeta potential is

the potential difference at the slipping or shear plane.

Figure 2-2. Potential Charge as a Function of Distance from Particle Surface [18]

The type of ions in the double layer structure has a strong correlation to the sign and magnitude

of zeta potential. Table 2-1 summarizes the correlation between typical values of zeta potential

and the dispersion system [19].

Table 2-1. Correlation between Zeta Potential and Dispersion System

Zeta Potential (mV) Dispersion System

0 to ±10 Unstable, rapid agglomeration and fastest settling rates

from ±10 to 50 Moderate stability, decreased agglomeration and slower settling rates

greater than 50 Excellent stability, minimum agglomeration and slowest settling rates

Azgomi’s research indicated a decrease in settling rate of precipitated lime mud and an increase

in zeta potential when liming ratio exceeds a critical level [8].

2.2.2 Previous Studies and Applications

Since lime mud and white liquor slurries contain mainly CaCO3, Ca(OH)2, Na2CO3, NaOH and

Na2S, a literature review was conducted to summarize previous findings of correlation between

zeta potential and these chemicals. It was found that the results varied greatly between different

research groups. The main discrepancy was on the potential-determining ions (PDI) or chemicals

that had the most effect on the zeta potential of the system. Some research groups suggested that


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in pure aqueous suspensions, Ca2+

and CO32-

were acting as the PDI [20, 21]. Other group

suggested that OH-, HCO3

- and H

+ were acting as the PDI [22, 23]. Aside from these chemicals,

other impurities that might exist in the slurry system could also change the surface charge based

on the intensity of their concentration [24].

In Azgomi’s study, zeta potential values were measured using a micro-electrophoretic apparatus

based on laser light-scattering theories [8]. White liquor was prepared using both pure lime and

reburned limes from various pulp mills and pure Na2CO3 as a substitute for green liquor.

Measurements were made on 0.1g of lime mud (separated from the white liquor) dissolved in

500mL of distilled water. At liming ratios below 1.0, zeta potentials were relatively constant and

negative between -15mV to -10mV. However, when liming ratios exceeded 1.0, zeta potentials

became positive and reached a value above 20mV (Figure 2-3). In addition, free lime (Ca(OH)2)

concentration was found to be a function of liming ratio using the thermal decomposition

method. More specifically, free lime content increased as liming ratios increased. When zeta

potential was plotted against the free lime content, a linear relationship could be observed, as

shown in Figure 2-4. Another study also demonstrated that increasing Ca2+

ions in suspension

increased the zeta potential values until Ca2+

ions saturated the particle surface [25].

Figure 2-3. Effect of Liming Ratio on Zeta Potential [8]

-20

0

20

40

60

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Z e t a P o t e n t i a l ( m V )

Liming Ratio ([CaO]/[Na2CO3])

Lime A Lime B

Lime C Lime DPure Lime


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Figure 2-4. Zeta Potential as a Function of Free Lime Content [8]

As discussed in the previous section, due to the disadvantages associated with current methods

for measuring free lime content, the possibility for their implementation as an online monitoring

system in the causticizing plant is limited. Based on Azgomi’s research, measuring the zeta

potential of dispersion systems and correlating the results to the amount of free lime in the

system were much faster than the traditional laboratory approach. The results were also

consistent within the scope of her study. This provides great motivation for further investigation

into the relationship between free lime and zeta potential, as well as the exploration of more

efficient control strategies in the causticizing plant. There are several aspects of Azgomi’s work

that require additional research and understanding:

- White liquor used for zeta potential measurement should be synthesized with green liquor

samples obtained from kraft pulp mills instead of Na2CO3 solution; and

- Zeta potential measurement should be made on more concentrated lime mud solutions to

simulate normal conditions in causticizing plants.

In order to measure zeta potential of slurry samples at higher concentrations and with higher

degrees of inconsistency in chemical composition due to the presence of impurities in actual

green liquor, a spectrometer based on sound theories is used which differs from the laser

diffraction instrument used in Azgomi’s study that imposed the above restrictions.

-20

0

20

40

60

0 5 10 15 20 25


Free Lime (%)

Lime A Lime B

Lime C Lime D

Pure Lime


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2.3 Sound Theories and Applications

The spectrometer for particle size and zeta potential measurement used in this study relies on the

concept of acoustic and electroacoustic theories, respectively. A basic understanding of these

theories will give insights into the reasoning and advantages behind the use of sound technology

for zeta potential measurements as oppose to the more conventional light scattering instruments.

A more detailed explanation of these theories can be found in Appendix B.

The acoustic theory for dispersion systems is based on a combined relationship between different

acoustic properties (attenuation, sound speed and angle of sound scattering) and system

properties (composition, particle size distribution and surface charge) [26].

The main acoustic interaction within a colloidal system can be divided into two groups:

attenuation caused by either absorption (conversion of acoustic energy into thermal energy) or

scattering (re-direction of acoustic energy). The absorption of ultrasound for dispersion systems

can be calculated based on density using acoustic measurement systems [26]. On the other hand,

scattering of ultrasound is harder to measure due to its nonlinear effects. Measuring both

absorption and scattering mechanisms are difficult for actual dispersion systems due to the

necessity of an extended set of input parameters, which are often unavailable. For the

spectrometer used in this study, only the absorption effect – intensity of the incident sound beam

after transmission – is measured, which is not affected by the non-linear scattering effects [27].

In addition to acoustic theories, electroacoustic mechanisms are utilized by this spectrometer in

making zeta potential measurements. Interactions between sound beams and electric fields in

colloidal systems result in electroacoustic effects. In the presence of sound waves, anions and

cations with different mass or friction coefficients will generate an alternating electric potential

within the solution [28]. It is often referred to as “Ion Vibration Potential” (IVP ) and

measurement techniques based on electroacoustic theories are developed for porous bodies [29],

colloidal systems [30, 31] and colloid vibration potential [32, 33].

Since zeta potential cannot be directly measured, it is calculated from other experimentally

measurable characteristics of the system such as the IVP. The fundamental capability for all zeta

potential analyzers is that they require a means of moving liquid relative to the particle surface

that causes disturbance within the double layer; and a means of monitoring and recording the


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generated signals. Motion induction could be accomplished by an external electric field or a

mechanical pressure field while monitoring could be accomplished by using electrical,

mechanical or optical technologies. The spectrometer used in this study accomplishes both

motion induction and signal monitoring through electrical means.

In addition to understanding why a spectrometer based on sound theories is suitable for zeta

potential measurements of concentrated slurry samples, it is also important to acknowledge the

factors that affect the amount of free lime in the slurry. The correlation between zeta potential

and free lime can then be used to monitor and/or control these factors to minimize free lime

problems in the causticizing plant.

2.4 Factors Affecting Free Lime Concentration

Zeta potential is affected by the type of ions and their concentration in the dispersion. Many

factors can affect the free lime concentration during the chemical recovery process including

properties of lime used during the causticizing reaction and the operating conditions in the

causticizing plant.

2.4.1 Lime Properties

In the lime industry, calcium oxide (CaO) is referred to as quicklime, while calcium hydroxide

(Ca(OH)2) is referred to as hydrated lime or slaked lime. Lime with high magnesium content is

termed dolomitic lime.

Pure quicklime is produced by calcination of natural limestone (CaCO3) which is similar to

calcining lime mud to produce reburned lime in the causticizing plant. During the calcination

process, carbon dioxide is released from CaCO3 producing CaO with a porous structure. This

allows water to penetrate the CaO particles during the slaking reaction. The properties of the

main calcium-containing compounds in the kraft recovery process are shown in Table 2-2.Table 2-2. Properties of Quicklime [34]

Calcium OxideCalcium

Hydroxide

Calcium

Carbonate

Molecular Weight (g/mol) 56.1 74.1 100

Specific Gravity 3.2 – 3.4 2.3 – 2.4 2.3 – 2.7

Bulk Density (g/cm ) 0.88 – 0.96 0.40 – 0.56 0.96 – 1.1

Specific Energy (kJ/kg) 0.44 0.67 0.92


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Olsen and Direnga [35] reported that the properties of lime (CaO) and lime mud (CaCO 3)

influenced the properties of white liquor and the overall efficiency in the causticizing plant. Lime

kiln produces reburned lime by calcining lime mud at high temperatures. The quality of lime is

often based on its residual calcium carbonate (CaCO3) content, reactivity and availability

(fraction of CaO in the reburned lime available for reaction). Slaking and causticizing reactions

at the slaker mainly use reburned lime fed from the lime kiln, and only use fresh lime for makeup

due to losses during the causticizing cycle. Therefore when the quality of the reburned lime is

poor, the reactivity of lime will also be lower which reduces the causticizing efficiency. This will

cause the produced white liquor slurry to contain an excess amount of free lime even when the

liming ratio is lower than the equilibrium ratio.

The particle size of lime could adversely affect the kinetics of the causticizing reaction [36].

Ideal reburned lime should consist of soft pebbles with a diameter of approximately 2cm [37].

Pure lime was found to be much more reactive than reburned lime due to having lime availability

of nearly 100% and a large specific surface area of approximately 2m2/g [37]. On the other hand,

reburned lime had a lime availability in the range of 87% – 92% and a specific surface area of

less than 0.5m2/g [37].

The overall causticizing efficiency is often found to be correlated to lime reactivity [38, 39]. Due

to conditions in the causticizing plant, the residence time of lime in the slaker is strictlycontrolled and a highly reactive lime shall slake within 5 minutes upon contact with water [37].

Many studies concluded that the rate of the causticizing reaction is affected by the properties of

the lime [40, 41]. Therefore, in addition to using a high liming ratio, the presence of free lime in

the causticizing plant may be caused by low lime quality due to non-optimized operating

conditions at the lime kiln.

2.4.2 Operating Conditions in Causticizing Plant

Due to the importance of reburned lime properties and its continued use in the lime cycle, the

calcination process needs to be controlled carefully. At low calcining temperatures, lime

structure was found to consist of microscopic particles with a high specific surface area (high

porosity) and are termed “soft- burned” limes. When temperature and/or duration of the

calcinations were increased, the particles grew and lime porosity decreased, lowering the specific

surface area and reactivity of the lime [42]. These limes are termed “hard- burned” or sintered


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lime. It was found that when the calcination temperature exceeded 1100°C, a decrease in

causticizing efficiency occurred due to the use of sintered reburned lime at the slaker [43]. When

testing and comparing pure lime and reburned lime collected from 10 Canadian kraft pulp mills,

Dorris [42] found that lime sintering in the kiln contributed to the poor reactivity of reburned

lime. Long residence time (i.e. greater than 1 hour) in the lime kiln also causes sintering of

calcium oxide particles while short residence time results in incomplete calcination process [44].

Chemical content in the slaking water also affects the lime property. Potgieter [45] found that

slaking water containing a chloride content higher than 250mg/L enhances the solubility of lime

which leads to an increase in lime reactivity. On the other hand, slaking water containing

carbonate and sulphate contents higher than 250mg/L reduces slaking rate as CaCO3 and CaSO4

can partially or completely coat the lime particles [45]. Carbonate content has a more severe

impact on hydration rates as it is less soluble than CaSO4.

Causticizing efficiency (CE), as discussed in Section 1.2, is dependent on the amount of CaO

added in the slaker to convert Na2CO3 in the green liquor to NaOH. The ratio between CaO and

Na2CO3 is also known as the liming ratio. When trying to maximize the CE at the slaker and the

causticizers, using a liming ratio equal or higher than the stoichiometric liming ratio can induce a

higher conversion rate of NaCO3 to NaOH in a shorter period of time [43, 46]. However, due to

the adverse effect of free lime content in the causticizing cycle, targeted CE in the causticizing plant is often 3% to 11% below the equilibrium CE [47]. In order to maintain the optimum CE,

liming ratio should be adjusted as soon as possible after changes occur in green liquor

composition (i.e. changes in Na2CO3 concentration) [48].

Non-process elements (NPE) in pulp mills are defined as elements that do not have any function

during the production of pulp or during the chemical and energy recovery process. NPE presents

challenges when monitoring and minimizing the free lime content in the causticizing plant.

Typical NPE in the causticizing cycle can be characterized by their accumulation tendencies

[49]. Some NPE, such as calcium (Ca), magnesium (Mg) and manganese (Mn) could be easily

removed from the system due to their low-solubility in alkaline solution. Other NPE were found

to have a higher tendency to remain in the system such as iron (Fe), aluminum (Al) and silicon

(Si) [49]. Potassium (K) and chloride (Cl) were found to be the most difficult to remove due to


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their high solubility. The variability in NPE concentration in liquor systems was found to be

related to the difference in chemicals used in kraft pulp mills [50].

Among other NPE, Mg can accumulate in the causticizing cycle. Due to its similarity to CaCO 3,

magnesium carbonate (MgCO3) can be calcined to magnesium oxide (MgO) in the lime kiln at

temperatures lower than lime calcination [51]. Excess MgCO3 can cause lime to become hard-

burned at normal operating conditions [52]. In addition, MgO is slaked by water to magnesium

hydroxide (Mg(OH)2), similar to Ca(OH)2, but it does not react with Na2CO3 in the green liquor.

High Mg content can cause increased grit production [45]. This is a typical example of how NPE

can be carried and recycled through the causticizing plant, acting as dead weight in various

chemical processes. More importantly, NPE can alter the value of zeta potential non-linearly, due

to changes in particle surface potential caused by polyvalent ions within the dispersion system.

Even though the majority of sodium content is removed during the white liquor clarifier, an

unusual form of sodium termed “guarded” sodium is present in the lime mud which cannot be

reduced by washing [53]. Guarded sodium will always form in CaCO3 and is not significantly

affected by the liming ratio. Since zeta potential is highly correlated to pH, an excess amount of

guarded sodium can reduce the effect of free lime on zeta potential. Table 2-3 summarizes how

lime quality or the slaking reaction is affected by different causticizing plant operating

conditions.Table 2-3. Factors Affecting Free Lime Concentration

Operating Condition Effect on Lime Quality or Slaking Reaction

Kiln temperature > 1100°C Production of sintered lime

Kiln residence time > 1 hour Production of sintered lime

Kiln magnesium content increases Production of sintered lime

Slaking water [Cl] increases Slaking rate increases

Slaking water [SO4] and [CO3] increase Slaking rate decreases


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3 METHODOLOGY

The relationship between liming ratio and zeta potential was studied using a combination of

materials including analytical chemicals and samples collected from kraft pulp mills. White

liquor was synthesized in the laboratory using the following combinations.

1) Analytical grade sodium carbonate (Na2CO3) and calcium oxide (CaO);

2) Mill green liquor (mainly Na2CO3 and Na2S) and analytical grade CaO;

3) Mill green liquor and reburned lime (mainly CaO).

The zeta potential of white liquor and lime mud produced using different liming ratios and other

variable experimental or measurement conditions (e.g. sample concentration) were measured

using the acoustic and electroacoustic spectrometer, model number DT-1202 from Dispersion

Technology. The instrument is referred to as DT-1202 in this thesis. Thermogravimetric analysis

(TGA) was used to measure the free lime content of white liquor or lime mud. The experimental

parameters and procedures were initially based on Azgomi’s research [8], and then modified to

match the specific requirements of the DT-1202 and improve the overall efficiency of the

method for free lime determination.

3.1 Material

Analytical grade chemicals with a purity of >99.9% were used. For mill green liquor samples,

the standard TAPPI ABC test discussed in Section 1.2 was used to determine their properties. In

addition to TTA, effective alkali (EA) and active alkali (AA), defined below respectively in

Equation 6 and Equation 7 ([NaOH] and [Na2S] are expressed in g/L Na2O), are used to calculate

the Na2CO3, NaOH and Na2S concentration in the green liquor samples. Detailed procedures for

the ABC test can be found in Appendix C. The result of the ABC test is shown in Table 3-1.

Effective Alkali = [NaOH] + ½[Na2S] (6)

Active Alkali = [NaOH] + [Na2S] (7)

Other materials used in this study included chemicals used for calibrating DT-1202’s zeta

potential measurement probe. Since the DT-1202 makes measurements by inducing electric


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current through an electrode, instrument coating and aging would require frequent calibration

under extensive use. This was done daily to ensure the instrument gave accurate results within

the instrument’s limits. The calibration solution for zeta potential is 10wt% silica in 0.01M

potassium chloride (KCl) which gives a standard zeta potential value of -38.0mV. This solution

is obtained by diluting the 50wt% colloidal silica provided by Dispersion Technology with

0.01M KCl solution prepared using analytical chemicals.

Table 3-1. Green Liquor Composition

g/L Na2O

Effective Alkali (EA) 29.1

Active Alkali (AA) 47.7

Total Titratable Alkali (TTA) 121

3.2 Experimental SetupZeta potential measurements were measured with the DT-1202. The instrument was also used to

determine the particle size for CaO and CaCO3, which was used as initial input parameters for

calculating zeta potential. The DT-1202 determines zeta potential of dispersions based on

measurements of colloid vibration current (CVI) [54 – 56]. In addition, the DT-1202 uses the

macroscopic fitting method for particle size distribution (PSD) measurements, similar to light

scattering, x-ray scattering and neutron scattering [57].

Particle size measurements are mainly based on sample weight. Discussed in Section 2.3, the

attenuation of ultrasound caused by individual particles is proportional to the particle’s weight or

volume. The DT-1202 and the acoustic spectroscopy technique are conventionally used by other

research groups for development and quality control in a wide range of industries such as cement

and ceramics industries [58, 59]. The sampling chamber in the DT-1202 has an ultrasound

generation end and a receiving end. At the generation end, a probe sends ultrasound at different

frequencies while the probe at the receiving end catches the incident beam at various positions.

The combination of frequencies and measurement positions are used to develop an experimentalPSD model, which is fitted to either a unimodal (single peak) or a bimodal (double peak) curve.

The theoretical model that matches the experimental curve with minimum deviation was used to

calculate the particle size of the sample.


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The DT-1202 uses electroacoustic technology to measure zeta potential. The piezo-crystal at the

bottom of the zeta potential probe receives an input pulse, producing ultrasound waves that

eventually reach the dispersion in the sample chamber through the gold electrode (Figure 3-1).

Figure 3-1. Zeta Potential Measurement Probe

The transducer and amplifier help with the adjustment and transmission of ultrasound waves

based on the user inputted concentration and composition of the dispersion. The ultrasound

pulses, when transmitted within the dispersion, generate particle motions relative to the liquid

when the densities between the two phases are different. These movements generate dipole

moments which cause oscillating potential on the gold electrode only. The stainless steel shell on

the outside of the zeta potential probe, separated by a non-conducting ceramic spacer, has an

electric potential of zero. The difference in potential between the two reference points equals the

colloid vibration current (CVI) which is used in combination with the concentration and the

particle size to calculate the sample’s zeta potential.

Lime reactivity tests were performed in an isotherm cylindrical Dewar container with a digital

laboratory stirrer. The SDT Q600 from TA Instruments was used for thermogravimetric analysis.


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A detailed procedure for TGA can be found in Appendix D. The Ca(OH)2 and CaCO3

concentration are determined based on weight losses caused by their decomposition to CaO and

water or carbon dioxide, respectively. The decomposition reactions proceed as shown below:

Ca(OH)2 CaO + H2O (8)

CaCO3 CaO + CO2 (9)

3.3 Experimental Procedures

The initial procedure was based on Azgomi’s research [8], but due to the differences between the

DT-1202 and the laser diffraction instrument used in Azgomi’s study, modifications to sample

preparation method (e.g. concentration) were made.

3.3.1 Particle Size Determination

Zeta potential measurements using electroacoustic techniques are primarily based on knowing

the concentration, density and particle size of the sample, using theories outlined in Section 2.3.

More specifically, the DT-1202 used in this study gave the most accurate and consistent results

when measuring dispersion systems with colloids having a similar particle size. Hence, the

particle size distributions (PSD) of Ca(OH)2 and CaCO3 were compared.

Lime calcinations were performed with 15g of either pure CaO or lime mud. The sample was

heated to 850C in an oven for 50 minutes to avoid soft-burned or hard-burned lime. Particle size

and zeta potential measurements were made after samples cooled down to room temperature.

For particle size measurements, calibrations were not required for the DT-1202. Dispersion

samples consisted of Ca(OH)2 (CaO dissolved in water) and/or CaCO3 and were diluted in

distilled water at 8wt% concentration. Lime mud samples were used as the basis for CaCO3

particle size measurements. For determining the particle size of Ca(OH)2, three different sampleswere used: pure CaO, reburned lime from calcination of pure CaCO 3 and reburned lime from

calcination of lime mud. The PSD of lime mud and reburned lime (from calcination of lime mud)

can be seen in Figure 3-2 and Figure 3-3 as produced by the DT-1202, respectively.

Each point on the particle size distribution curve is equal to the total weight percentage of all

particles with that same diameter. The average particle size of the sample is at the peak of the


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distribution curve. When more than one peak exists for a particular sample, either two chemicals

with distinct size are present, or a high degree of agglomeration exists in the sample.

Figure 3-2. Particle Size Distribution for CaCO3

Figure 3-3. Particle Size Distribution for Ca(OH)2 (from Calcination of Lime Mud)

It can be observed that for lime mud, two peaks exist and a bimodal distribution is used to

characterize the particle size distribution. However, for reburned lime, a single peak exists and a

unimodal distribution is sufficient to characterize the particle size distribution. For lime mud, the

0.0

1.0

2.0

3.0

4.0

0.1 1 10 100 1000

P S D ( W e i g h t B a s i s )

Diameter (μm)

0.0

0.5

1.0

1.5

2.0

0.1 1 10 100 1000

P S D ( W e i g h t B a s i s )

Diameter (μm)


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average particle size is around 12μm at the first peak and approximately 200μm at the second

peak. On the other hand, the average particle size for reburned lime is approximately 10μm.

The necessity for using a bimodal distribution to model the PSD of lime mud can be caused by

the presence of both fine CaCO3 particles and more coarse particles due to agglomeration

between CaCO3, free lime and other impurities in the lime mud. The calcination process for

converting CaCO3 into CaO caused the reburned lime to have a more uniform chemical

composition and reduced the amount of agglomeration when compared to the original lime mud.

Therefore, only a unimodal distribution was required to characterize the PSD for reburned lime.

Comparison is made between the average particle sizes of the four samples, as shown in Figure

3-4. It can be concluded that the average value for each sample is within the same order of

magnitude. Lime mud has the largest diameter of 12.7μm while the three Ca(OH)2 samples have

an average diameter of 10.7μm. These findings are consistent with previous results where

Ca(OH)2 and CaCO3 particles were found to be in the range of 5μm to 20μm in diameter [8].

Since the main colloids (Ca(OH)2 and CaCO3) in the sample measured in this study have similar

sizes, the DT-1202 can be used with confidence for producing accurate and consistent results.

Figure 3-4. Average Particle Sizes for Lime and Lime Mud Samples

Zeta potentials for these samples were measured and the results are shown in Figure 3-5. The

zeta potential for all Ca(OH)2 samples is similar at an average value of 44mV. On the other hand,

0

3

6

9

12

15

Calcined CaCO3 Calcined Mud Pure Ca(OH)2 Pure Mud

P a r t i c l e S i z e ( u m )

Sample Type


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lime mud has a zeta potential of approximately 12mV. These measurements indicate that free

lime does have an impact on zeta potential.

Figure 3-5. Average Zeta Potentials for Lime and Lime Mud Samples

3.3.2 Initial Experimental Procedure

The following procedure was adapted from Azgomi’s research [8] to be used as a guideline for

making zeta potential measurements. Validation and modifications for the following procedural

steps will be discussed in the next section.

A sealed plastic bottle containing 100mL of pure Na2CO3 solution or mill green liquor was

heated up to 90C in a hot water bath with constant mixing. Analytical CaO or mill reburned

lime was transferred quickly into the bottle, while maintaining constant agitation. The reaction

was carried out for 1 hour, allowing the slaking and causticizing reactions to reach equilibrium.

The resulting white liquor slurry was filtered with a vacuum filtration unit at a constant vacuum

of 0.5bar, monitored with a pressure gauge. This was done to separate the white liquor and the

precipitated lime mud cake. The produced white liquor contained dissolved sodium content and

other impurities, which might adversely affect zeta potential measurements as the overall charge

in the system could be changed, as discussed in Section 2.2.2. The wet lime mud was dried in an

oven at 120C over a period of 90 minutes. The dried lime mud was diluted to 1wt%

concentration in 150mL distilled water and zeta potential was measured at room temperature.

0

10

20

30

40

50

Calcined CaCO3 Calcined Mud Pure Ca(OH)2 Pure Mud


Sample Type


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3.3.3 Procedure Modification

Due to the differences between electroacoustic and light-scattering techniques and efforts to

minimize the time required for making sample measurements, the initial procedure was modified

to better suit the specific scope of this study. A series of experiments were performed todetermine the validity of each parameter used in the initial procedure (e.g. dilution

concentration) by changing its value within a reasonable range. In addition, considerations were

given to other variables that could potentially change during normal operations in causticizing

plants (e.g. lime particle size). Ultimately, a value for each parameter was chosen based on its

tendency to give consistent results with the least amount of measurement time.

The causticizing reaction temperature was changed between room temperature (approximately

25°C) and 90°C. The result shown in Figure 3-6 indicates that when carrying out the causticizing

reaction at room temperature, there is minimum variation on zeta potential for liming ratios

between the range of 0.8 and 1.2. This is expected as causticizing reactions at low temperature

will have slow kinetics, and the degree of completion for the causticizing reaction will be

relatively low regardless of the liming ratio. On the other hand, the causticizing reaction

completed at 90°C shows a strong correlation between zeta potential and liming ratio.

Figure 3-6. Effect of Liming Ratio on Zeta Potential at Different Causticizing Reaction

Temperatures (Pure Chemicals, 1 Hour Reaction)

0

10

20

30

40

50

0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2



9…

S…

90°C

25°C


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The measurement temperature was investigated using samples produced at a liming ratio equal to

1.1. After mixing the lime mud sample with distilled water at 1wt%, the lime mud was heated to

50°C – the upper limit for the DT-1202 without damaging the zeta potential measurement probe.

The sample was cooled by air while the temperature was monitored using the DT-1202’s

temperature probe. A measurement was made with every 5°C decrease in sample temperature

until the lime mud reached room temperature (approximately 25°C). The results are shown in

Figure 3-7. It can be concluded that the measurement temperature has an insignificant impact on

zeta potential, and that measurements can be made at room temperature to reduce sample

preparation time in this study.

Figure 3-7. Effect of Measurement Temperature on Zeta Potential (Pure Chemicals, 1 Hour

Reaction, 90°C Reaction Temperature)

Instead of measuring zeta potential of lime mud, measurements could be made directly on the

synthesized white liquor. If correlations existed between zeta potential of white liquor and liming

ratio, it would decrease the overall analysis time. Figure 3-8 and Figure 3-9 show the effect of

liming ratio on zeta potential of white liquor with immediate measurement or after sample

dilution, respectively. It can be concluded that liming ratio has no effect on zeta potential when

measuring white liquor samples. This may be due to impurities and high sodium content in white

liquor obscuring the effect of Ca(OH)2, causing free lime to become a less determinant factor on

the zeta potential of the sample, even at very low concentrations (e.g. 0.7wt%). The direct

dilution of white liquor is therefore not desirable for accurate measurement of zeta potential.

0

10

20

30

40

50

25 30 35 40 45 50


Temperature (°C)


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Figure 3-8. Effect of Liming Ratio on Zeta Potential when Measuring White Liquor SlurriesSynthesized with Pure Chemicals and Mill Green Liquor Samples (1 Hour Reaction, 90°C

Reaction Temperature)


Diluted with Distilled Water at Different Concentrations (Pure Chemicals, 1 Hour Reaction,

90°C Reaction Temperature)

Since the spectrometer measures zeta potential by monitoring particle movements under an

electric field, the sample concentration plays an important role. Highly concentrated samples

might hinder particle movement due to increased chemical bonds between particles and

0

20

40

60

80

100

0.2 0.4 0.6 0.8 1 1.2



Pure Chemicals

Mill Samples

0

10

20

30

40

50

60

70

80

0.6 0.7 0.8 0.9 1 1.1 1.2



0.7wt% 1.0wt%

1.5wt% 3.0wt%

Pure Chemicals

Mill Samples

0.7wt%

1.0wt%

1.5wt%

3.0wt%


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decreased free space in the dispersion. Figure 3-10 and Figure 3-11 show the zeta potential of

lime mud produced using pure Na2CO3 and mill green liquor samples.

Figure 3-10. Effect of Liming Ratio on Zeta Potential when Measuring Lime Mud Diluted with Distilled Water at Different Concentrations (Pure Chemicals, 1 Hour Reaction, 90°C Reaction

Temperature)


Distilled Water at 1wt% (Mill Green Liquor, 1 Hour Reaction, 90°C Reaction Temperature)

0

10

20

30

40

50

60

0.4 0.6 0.8 1 1.2 1.4



1wt% 3wt%

0

15

30

45

60

75

0.4 0.6 0.8 1 1.2 1.4



Run 1 Run 2

Run 3 Run 4

1wt%

3wt%


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Using pure chemicals, at 3wt% concentration, the zeta potential only increases slightly with an

increase in liming ratio. At 1wt% concentration, there is a much higher increase in zeta potential

with an increase in liming ratio. With a lower concentration, the particle movement is more

readily measurable by the zeta potential probe. When substituting Na2CO3 with mill green

liquors, similar results are obtained. However, the results between different experimental runs

are not consistent with each other. More importantly, there are no negative zeta potentials for

underlimed systems as shown in Azgomi’s study [8] (Figure 2-3).

In order to obtain more consistent results, the sample volume was varied by switching from the

large sample chamber with an attached magnetic stirrer (holds up to 130mL of sample) to the

small sample cup (holds up to 20mL of sample), both shown in Figure 3-12. The zeta potential

probe is inserted into the sample chamber or sits under the sample cup. The magnetic stirrer

cannot be attached to the sample cup, so when making zeta potential measurements in this

configuration, the sample will settle on top of the zeta potential probe.

Figure 3-12. Sample Compartments for Zeta Potential Measurement

To reduce the sample volume for the sample cup, only 20mL of synthesized white liquor was

prepared. After filtration and drying, lime mud was added to 20mL of distilled water to achieve

1wt% in concentration. 10mL of this solution was transferred into the sample cup for zeta

potential measurements and the result is summarized in Figure 3-13. It can be seen that the

correlation between zeta potential and liming ratio is much closer to Azgomi’s research [8]. The

reason for the differences in results may be due to the effect of constant mixing associated with

using the large sample chamber. Due to the low concentration of lime mud samples, the current

induced by mixing may disrupt particle movements within the dispersion. The angle of sound

reflection, the main variable used to determine the sign of zeta potential, is also affected by

Location of Zeta

Potential Probe

Sample

Chamber

Sample Cup

Zeta Potential

Probe


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mixing. By allowing the sample to settle onto the surface of the probe, a more accurate

representation of sound induced particle movements and sound reflection angle can be captured.

Figure 3-13. Effect of Liming Ratio on Zeta Potential when Measuring 10mL of Lime Mud Diluted in Distilled Water (Pure Chemicals, 1 Hour Reaction, 90°C Reaction Temperature)

3.3.4 Modified Experimental Procedure

Based on the investigations in the previous section, a modified experimental procedure was used

for zeta potential measurements of lime mud that was separated from white liquor produced

using different liming ratios.

Tightly sealed plastic bottles containing 20mL of pure Na2CO3 or mill green liquor were heated

in a hot water bath at 90C with continued agitation. Different amounts of pure CaO or mill

reburned lime were transferred into the bottles to achieve a range of liming ratios. The slaking

and causticizing reactions were allowed to be carried out for 1 hour so the system had enough

time to reach equilibrium.

The resulting white liquor slurry was filtered in a vacuum filtration unit under a 0.5bar vacuum,

and the separated wet lime mud slurry was then dried for 90 minutes in an oven at 120°C. The

lime mud was diluted in 20mL of distilled water to 1wt% concentration. Zeta potential

measurements were made with 10mL of diluted lime mud sample at room temperature. The

sample chamber sat on top of the zeta potential probe and the sample was allowed to settle

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before making measurements. A simplified process diagram for the modified experimental

procedure is shown below in Figure 3-14.

Figure 3-14. Experimental Procedure Flow Chart

Causticizing Reaction in

Water Bath

Vacuum

Filtration

Oven DryingDilution with

Distilled Water

Zeta Potential

Measurement

Lime

Green Liquor

White

Liquor SlurryWhite Liquor

Solids (Lime Mud Slurry)


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4 RESULTS AND DISCUSSION

Zeta potential (ζ) of lime mud was found to be dependent on the amount of free lime in the lime

mud, based on Azgomi’s research [8]. To validate this relationship, zeta potential of samples

using analytical chemicals and mill samples will be compared and discussed. To further

investigate this relationship, zeta potential measurements were correlated to free lime

measurements for samples prepared under different experimental conditions (e.g. reaction time)

and measurement conditions (e.g. storage time).

Zeta potential values will be expressed in millivolts (mV), liming ratios in concentration ratio of

calcium oxide to sodium carbonate or [CaO]/[Na2CO3], and free lime content as weight

percentage in either white liquor or lime mud samples.

4.1 Effect of Free Lime on Zeta Potential for Analytical Samples

The experimental results from modifications of the initial procedure in Section 3.3.3 gave great

insights on how the acoustic and electroacoustic spectrometer can be used effectively to measure

zeta potential. It was found that the measurements were sensitive to different parameters due to

both chemical properties such as increased molecular interactions at high concentrations and

physical properties such as mixing induced currents causing chaotic particle movements. This

section will focus on the discussion of differences and similarities between using pure CaO andeither analytical Na2CO3 or mill green liquor to produce white liquor. Zeta potential of diluted

lime mud (at 1wt% concentration) will be measured with the 10mL sample cup at room

temperature (25°C).

4.1.1 Analytical Grade Chemicals

Using analytical chemicals, causticizing reactions were carried out at 90°C over a period of 1

hour. Experiments were repeated up to 20 times (and each experiment was measured 3 times) to

determine the reproducibility of results when using the modified experimental procedures as

established in the previous section. The result is shown in Figure 4-1, where it can be seen that

when the liming ratio is equal to or below 0.8, the zeta potential average between repeated

experiments is negative with a very small error bar. In addition, when liming ratio increases

above 1.0, the zeta potential value becomes less consistent, with slightly larger error bars, but

results of repeated experiments are all positive values.


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Figure 4-1. Measurement Repeatability with 10mL Samples (Pure Chemicals, 1 Hour Reaction,

90°C Reaction Temperature)

According to Azgomi’s work , underliming and overliming conditions should have negative and

positive zeta potential values, respectively [8]. The result in Figure 4-1 shows a similar trend,

except when liming ratio is equal to 1.0, where the measured zeta potentials are in both positive

and negative regions, represented by the larger error bar. This is expected and will be referred to

as the “transition zone” where the system is either slightly underlimed or overlimed. Using pure

chemicals, the lime mud samples prepared at a liming ratio of 1.0 only have a minimum amount

of excess free lime. Therefore, each sample used for dilution may not contain enough free lime

for it to become a dominant factor when determining the zeta potential, in which case the zeta

potential will remain negative.

Using zeta potential theory outlined in Section 2.2.1, the negatively charged colloid in the lime

mud slurry can be the dissociated carbonate (CO32-) ions. Since the Stern layer consists of

calcium (Ca2+

) and sodium (Na+) ions, the surface charge at the particle interface is caused by

adsorption of these charged ions. In general, dispersions at high pH will have negative zeta

potentials. Theoretically, in the lime mud slurry, the presence of sodium hydroxide (NaOH) will

cause the dispersion to have a negative zeta potential value. However, at overliming conditions,

positive zeta potentials are observed.

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The reason for consistent measurements of positive zeta potentials when the sample contains an

excess amount of free lime might be due to changes in the double layer thickness. Since the Stern

layer contains positively charged ions, additional Ca2+

ions (dissociated from excess amount of

Ca(OH)2) can further compress the double layer when compared to having only Na+ ions. In

addition, zeta potential measurements are obtained based on electric charge generated by particle

movements. Hence, the ability of particles to move through the dispersion (known as electrical

mobility) plays an important role in determining the magnitude and the sign of zeta potential.

Combination of a thinner double layer thickness and the change in electrical mobility caused by

excess free lime can potentially change the sign of zeta potential from negative to positive.

For samples from the above experiments, the free lime content was measured using TGA. The

result for weight losses at different temperatures is shown in Figure 4-2 and Figure 4-3, for

liming ratios of 0.6 and 1.4, respectively. The initial decrease in weight at 100°C corresponds to

evaporation of water. Decrease in weight at approximately 400°C is due to the decomposition of

Ca(OH)2 to CaO and H2O. The final decrease in weight at approximately 800°C is due to the

decomposition of CaCO3 to CaO and CO2. Comparing the two plots, the sample weight loss

caused by Ca(OH)2 decomposition is insignificant when liming ratio is 0.6. On the other hand,

there is a clear decrease in sample weight when the liming ratio is 1.2. This is expected as the

amount of free lime in underlimed systems should be much lower than in overlimed systems.

Figure 4-2. Weight Loss Profile for Lime Mud (0.6 Liming Ratio and Pure Chemicals)

50

70

90

110

0 150 300 450 600 750 900

W e i g h t ( % )

Temperature (°C)

96

97

98

99

100

350 375 400 425 450


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Figure 4-3. Weight Loss Profile for Lime Mud (1.2 Liming Ratio and Pure Chemicals)

The zeta potential and free lime content for lime mud prepared at different liming ratios is shown

in Figure 4-4. Since pure chemicals are used, the free lime contents are expected to be

consistently low for liming ratios below 1.0. When overliming occurs, the free lime content

increases to almost 6wt%. The result also shows that zeta potential increases with liming ratios.

Figure 4-4. Zeta Potential and Free Lime as a Function of Liming Ratios (Pure Chemicals, 1

Hour Reaction, 90°C

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