Govt. College of Engineering & Ceramic Technology

Technical Report

SHEET GLASS MANUFACTURING

Submitted to

Mrs. Debarchona Dutta

This report is prepared by the following students of Ceramic Technology, 2nd year, 4th sem.

Roll No. 081130200__

Milton Biswas[05]Kaushik Sarkar [10]Avisekh Barua [18]Ananya Saha [20]

Somnath Mandal [24]Santanu Mondal [33]Nilanjana Roy [35]

Dated 01/04/2010.

Aknowledgement

This report may not have been prepared without the help of Mrs. Debarchona Dutta who guided us in writing our first technical report. We also express our sincere thanks to our seniors Arin Dutta and Nandan Saha of 3rd yr IT and our classmate Tithi Bhakta of 2nd yr CSE.

Introduction Glass itself is ā large industry and sheet glass is the oldest but at the same time most abundantly used and manufactured glass product in the world. Though sheet glass manufacturing is nothing new to glass technologists but it has been ā trend in our college that students don’t get placed in the renowned glass companies and the recruiter’s say that its unfortunate that students cannot tell the few basic concepts of glass and its manufacturing. The reason may be may be they don’t get enough exposure to glass manufacturing processes or it may be lack of interest generated. So here an attempt has been made to report how sheet glass is manufactured in industries.

Another event which triggered us in selecting this topic is our participance in ā highly informative seminar on soda-lime-silica glass by Mr. T.K. Chakraborthy of St. Gobain at our college on the 11th and 13th of March, 2010. The facts and figures presented in this report have been taken from the above mentioned seminar, the internet and ā few of books from library.

What is glass?

Glass is an inorganic substance in ā condition which is continuous with and analogous to the liquid state of that substance, but which as the result of having been cooled from ā fused condition has attained so high ā degree of viscosity as to be for all practical purposes rigid.

10 Characteristics of glass1. Amorphous in nature2. Fragile 3. Lustrous4. Thermal durability5. Transparence6. Non-reactive with most of the chemicals7. Good insulator8. Can be shaped9. Moderate mechanical strength10. Used for aesthetics

Types of glass1. Soda-lime-silica glass2. Borosilicate glass3. Opthalmic glass4. Lead glass, etc.

Here soda-lime-silica glass is discussed.

Basic introduction of Float glass & its Manufacturing

Float glass Float glass is a term for perfectly flat, clear glass (basic product). The term "float" glass derives from the production method, introduced in the UK by Sir Alastair Pilkington in the late 1950's, by which 90% of today's flat glass is manufactured.

Production: The raw materials (silica sand, calcium, oxide, soda and magnesium) are properly weighted and mixed and then introduced into a furnace where they are melted at 1500° C. The molten glass then flows from the glass furnace into a bath of molten tin in a continuous ribbon. The glass, which is highly viscous, and the tin, which is very fluid, do not mix and the contact surface between these two materials is perfectly flat. When it leaves the bath of molten tin the glass has cooled down sufficiently to pass to an annealing chamber called a lehr. Here it is cooled at controlled temperatures, until it is essentially at room temperature.

Batch components

1. Glass forming oxides: The glass-forming oxygen polyhedra are triangles and tetrahedra, and cations forming such coordination polyhedra have been termed network formers. They form the basic 3-dimensional structure of the glass. Eg. SiO2, P2O5, B2O3.

2. Network modifiers or fluxes: These are alkali oxides. Alkali ions occupy random positions distributed throughout the structure to provide local charge neutrality. Since their major function is providing additional oxygen ions which modify the network structure, they are called network modifiers. Another function which they do is that they reduce the melting temperature of the batch by forming low melting phases for which they are also called fluxes. Eg. Na2O, CaO, PbO, K2O, Li2O.

3. Intermediates: Cations of higher valency and lower coordination number than the alkalis and alkaline earths may contribute in part to the network structure and are referred to as intermediates. It affects chemical durability, expansion and viscosity. Eg. Al2O3, Sb2O5, As2O3.

4. Colourants: They affect the visual aspect i.e. light transmission. They are transition metal oxides and are added along with soda ash to the glass batch. Eg. CoO, NiO, Fe2O3.

5. Fining agents: They help in removal of bubbles. Eg. Na2SO4.

Glass Formation

Glasses are usually formed by solidification from the melt. The structure of glasses can be clearly distinguished from that of liquids, since glass structure is effectively independent of temperature. This can best be seen by ā plot of the specific volume of the crystal, liquid, and glass as ā

function of temperature. On cooling the liquid, there is ā discontinuous change in volume at the melting point if the liquid crystallizes. However, if no crystallization occurs, the volume of the liquid decreases at about the same rate as above the melting point until there is ā decrease in the expansion coefficient at ā range of temperature called the glass transformation range. Below this temperature range the glass structure does not relax at the cooling rate used.

Role of Viscosity in Glass Manufacture

Viscosity is ā measure of the resistance of ā liquid to shear deformation

In the melt condition if viscosity be less then bubbles can be easily removed and ā bubble free homogeneous melt can be produced.

It affects annealing temperature where internal stress is removed. It plays ā vital role in forming temperature for commercial products.

Temperature Viscosity in poisesMelting range 50-500Working point 104

Softening point 107.6 Working range 104-108

Annealing point 1013.4

Annealing range 1012.5-1013.5

Strain point 1014.5

Room temperature > 1020

Major Raw Materials for Soda-Lime-Silica glass

1. Silica sand: Kurnool(AP): Chirtala(KN), etc2. Soda ash: Tuticorin(TN), TATA(GJ), etc3. Dolomite: Dehradun, etc4. Calcite: Salem(TN), Tirunivelli(TN), etc5. Sodium sulphate: 6. Cullet (Broken glass):

Raw Materials and Costs of Flat Glass

_ The mix of raw materials used in the production of flat glass is known as the batch, which is mainly composed of three components: silica sand, soda ash and dolomite/limestone.

_ Recycled glass (cullet) is used in the fabrication of

flat glass and represents on average 15 per cent of the materials used. Its addition helps reduce the energy required in the process.

_ Silica sand, soda ash, dolomite and

dolomite/limestone represent together 99 per cent of all raw materials used in the production of glass, excluding recycled glass. The remaining ingredients aid the melting and refining (bubble removal) reactions and impart colour and there is water addition during batch mixing to prevent subsequent segregation.

_ Silica sand is the main component of the batch as

it constitutes about 62 per cent of the batch weight excluding recycled glass.

Soda ash is one of the most expensive raw materials used in glass manufacturing and represents about 16 per cent of the batch weight but about 60 per cent of the batch cost.

In terms of costs, raw materials and energy are the single largest elements, followed by overheads and prime labour.

Basic Manufacturing

There are three main flat glass manufacturing methods for producing the basic glass from which all processed glass products are made.

Float: Over 85 per cent of the world’s flat glass is made by the float process (see below). This is the way all of the world’s high quality, optically clear glass is made.

_ Sheet: Approximately 8 per cent of the world’s flat glass is

made by the sheet process. This process predates, competes with and is gradually being replaced by float. The majority of the world’s sheet production is in China with the remaining pockets in the less developed markets of Africa, Eastern Europe and Asia.

Rolled: The rolling process makes patterned, figured and wired glass products. Semi-molten glass is squeezed between metal rollers to produce a ribbon with controlled thickness and surface pattern.

1

Chemical and phase changes occurring inside the furnace

1. Releaseof gases:

From Carbonates – CO2 , H2O From Nitrates(NOx ) – NO2 , N2O.

From Sulphates(SOx ) – SO2 , SO3

2. Formation of liquid phase: Batch components are directly melted in the Float furnace. Cullet helps in staring melting.

Alkali oxide components form various eutectics which result in lowering of melting temperature of the batch.

3. Volatilization of melt components: Many molten

components vapourise out at such high temperature inside the furnace. Tin vapours, Na2S , H2S etc volatilize.

4. Fining reactions: Gases are created by batch decomposition reactions, mainly of soda ash (Na2CO3.H2O), dolomite or calcite (CaCO3) which are trapped in interstices between particles. These are removed by fining or refining agents like sulphates (Na2SO4) and homogenization of the whole mass occurs.

Cross-section of ā float furnace showing glass flows

There are many simultaneous solid state reactions occurring between the various components.

Type of furnaces used

Pot furnaces Day Tank furnaces Continuous furnaces ( mostly used)

Float Glass Production

Introduction

Float glass is produced by floating a continuous stream of molten glass onto a bath of molten tin. The molten glass spreads onto the surface of the metal and produces a high quality, consistently level sheet of glass that is later heat polished. The glass has no wave or distortion and is now the standard method for glass production and over 90% of the world production of flat glass is float glass.

The float glass process was developed by Sir Alastair Pilkington and patented by Pilkington in 1959 and the detailed history of the development is process is described by Sir Alastair Pilkington in his review lecture to the Royal Society of London in 1969

The float glass processThe basic science

If molten glass is poured onto a bath of clean molten tin, the glass will spread out in the same way that oil will spread out if poured onto a bath of water. In this situation, gravity and surface tension will result in the top and bottom surfaces of the glass becoming approximately flat and parallel.

The molten glass does not spread out indefinitely over the surface of the molten tin. Despite the influence of gravity, it is restrained by surface tension effects between the glass and the tin. The resulting equilibrium between the gravity and the surface tensions defines the equilibrium thickness of the molten glass (T).

The resulting pool of molten glass has the shape shown below:

Vertical section through a pool of molten glass floating on molten tin

The equilibrium thickness (T) is given by the relation:

where Sg, Sgt, and St are the values of surface tension at the three interfaces shown in the diagram.

For standard soda-lime-silica glass under a protective atmosphere and on clean tin the equilibrium thickness is approximately 7 mm.

The best detailed explanation of the physics involved in the float glass process is described by Charnock (Charnock, H. Physics Bulletin 1970, 21, 153-156).

The raw materials

 The basic raw material composition for standard soda-lime-silica float glass are:

 

Raw Material %

Sand 72.6

Soda Ash 13.0

Limestone 8.4

Dolomite 4.0

Alumina 1.0

Others 1.0

The production process

This basic science was developed over a long period by Pilkington into the full scale continuous process that is illustrated below:

 

The basic float glass process

The batch of raw materials is automatically weighed and mixed and then continuously added to the melting furnace where it is taken to around 1050oC using gas fired burners. The mix then flows over a ‘dam’ where the continuous stream of molten glass flows onto the bath of molten tin. The stream of glass is pulled along the top of the molten tin by haul-off conveyors at the end of the float area which transport the glass into the annealing lehr.

At the start of the float area the molten glass spreads outwards with flat top and bottom surfaces and the thickness decreases towards the equilibrium thickness (T). The thickness can then be further controlled by the stretching effect of the conveyors as it cools until it reaches 600oC when it exits the float area and enters the annealing lehr.

Whilst the equilibrium thickness is approximately 7 mm the process has been developed to allow the thickness to be controlled between 0.4 mm and 25 mm.

For thin sheets, the exit conveyor speed can be increased to draw the glass down to thinner thicknesses. This drawing down will also result in a decrease in the sheet width and to prevent unacceptable sheet width decreases edge rolls are used. Edge rolls grip the outer top edge of the glass and not only reduce decrease in width but also help to reduce the thickness even further.

Using edge rolls to reduce the thickness of the glass produced

For thick sheets, the spread of the molten glass is limited by using non-wetted longitudinal guides. The glass temperature allows the spread to remain uniform and is reduced until the ribbon can leave the guides without changing dimensions

Low-e coatings

Much of the architectural glass produced is now coated with low-e (for low emissivity) coatings to enable the production of more energy efficient windows. As with any advanced technology, there are several different production methods and the products have different properties.

The two basic methods of producing low-e coatings are sputtering and pyrolytic deposition:

Sputtering - Soft coat and off-line coating

Sputtering uses a vacuum chamber to put several layers of coating on the basic glass and the total thickness of the coatings is around ten thousand times thinner than a human hair. Sputtered coatings are referred to as 'soft coats' and must be protected from humidity and contact. The sputtered coatings are very soft but inside a sealed unit, they will easily last for the life of the unit.

These sputtered 'soft coat' products can have emissivities ranging from 0.05 to 0.1 compared to uncoated glass that has a typical emissivity of 0.89. This means that 'soft coat' products will reflect between 95 and 90% of the long-wavelength radiant energy from the surface where uncoated glass will only reflect 11% of the radiant energy received by the surface.

Pyrolytic Deposition - Hard coat and on-line coating

Pyrolytic coating deposits a metallic oxide directly onto the glass surface whilst it is still hot. The low-e coating is effectively 'baked-on' to the surface and the resulting low-e coating is very hard and durable. The pyrolytic coatings are often referred to as 'hard coats'. Pyrolytic coatings can be up to 20 times thicker than sputtered coatings (they are still 500 times thinner than a human hair) and the baking process makes them much harder and resistant to wear.

Pyrolytic 'hard coats' have a low emissivity but this is higher than those achieved for soft coats. Hard coat products have emissivities ranging from 0.15 to 0.2.

The ability to apply 'hard coats' whilst the glass is still hot means that hard coated products are cheaper than soft coated products.

How Float Glass is Manufactured

Batching of raw materials

The main components of Soda Lime glass, Silica sand (73%), Calcium oxide (9%), Soda (13%) and Magnesium (4%), are weighed and mixed into batches to which recycled glass

(cullet) is added. The use of ‘cullet’ reduces the consumption of natural gas. The materials are tested and stored for later mixing under computer control.

Melting of raw materials in the furnace

The batched raw materials pass from a mixing silo to a five-chambered furnace where they become molten at a temperature of approximately 1500°C. Every operation is carefully monitored.

Float batch is melted using the heat generated through the combustion of fossil fuels

Drawing the molten glass onto the tin bath

The molten glass is "floated" onto a bath of molten tin at a temperature of about 1000°C. It forms a ribbon with a working width of 3210mm which is normally between 3 and 25mm thick. The glass which is highly viscous and the tin which is very fluid do not mix and the

contact surface between these two materials is perfectly flat.

Cooling the molten glass in the annealing lehr

On leaving the bath of molten tin, the glass - now at a temperature of 600°C - has cooled down sufficiently to pass to an annealing chamber called a lehr. The glass is now hard enough to pass over rollers and is annealed, which modifies the internal stresses enabling it to be cut and worked in a predictable way and ensuring flatness of the glass. As both surfaces are fire finished, they need no grinding or polishing.

Quality checks, automatic cutting, storage

After cooling, the glass undergoes rigorous quality checks and is washed. It is then cut into sheets up to 6000mm x 3210mm which are in turn stacked and stored ready for transport. An automatic stacker takes plates of glass directly from the end of the production line. This is approximately half a kilometre from the beginning of the float line. The entire production process from the batching of raw materials to cutting and stocking is fully automatic and computer-controlled.

On-line optical sensor

Different parts of Float Furnace

Dog house: Through which batch enters. Melting zone Refining zone Regenerators & flue line Neck: combines main furnace to working end Working end Feeders: Final shape is given Canal

Inside view of ā typical float glass melting furnace

Float Glass Plant

A float glass plant consists of the following main sections:1. The storage area for the treated raw materials,  2. The batch processing plant,3. The melting furnace section 4. The air pollution control5. the tin bath section6. the annealing lehr section7. the protective atmosphere generation plant,8. the glass panes storage building.

Modified Basic Manufacture

There are three main forms of modification to the basic manufacturing processes.

TintedExtra ingredients are added to the raw materials of glass at the melting stage to produce tinted products. Cobalt and nickel tint glass grey; ferrous oxide tints glass blue, while ferric iron generates a yellow tint both together tint glass green. Tinted glass is used in buildings and vehicles to control heat and light transmission.

Coated (On-line)Modified properties are produced from the basic glass by means of surface coatings. Glass can be coated on-line in the float process as the ribbon of glass is being formed in the float glass bath. The technology uses chemical vapour deposition to apply a microscopically thin coating on the glass at a temperature of about 600ºC. Pilkington K Glass™, Pilkington

Energy Advantage™and Pilkington Activ™are produced by this process.

Building Products Processing

Coating (Off-line)Off-line processes use a vacuum coating technology called sputteringLaminatingPlies of glass are bonded or laminated together with a layer of polymer film in betweenSilveringFloat glass is made into mirrors in a process which deposits a thin film of high purity silver on one surface of the glass.

Automotive Products Processing

Automotive TougheningToughened glass, or tempered glass as it is sometimes called, is most frequently used in the rear and side windows of vehicles. It is designed to be much stronger than non-safety glass. However, in the case of a breakage, it shatters into very small pieces rather than sharp shards of glass, therebysignificantly reducing the risk of injury.

Automotive LaminatingLamination is a form of safety glazing where normally two thin glass plies create a sandwich arounda polyvinylbutyral (PVB) interlayer. Normally used for a vehicle’s windshield, in the case ofbreakage, the glass is held in place by the interlayer, retaining emergency visibility for the driver

The Flat Glass Industry and Global Market Structure_

The global market for flat glass in 2008 was approximately 53 million tonnes. At normal price levels, this represents a value of around euro 22 billion at primary manufacture level. Of this tonnage, around 70 per cent is consumed in windows for buildings, 10 per cent in glazing products for automotive applications and 20 per cent used in furniture and other interior applications.

_ Over the long term, the market is growing in volume terms at around 4-5 per cent a year._ Europe, China and North America together account for over 70 per cent of global demandfor glass.

_ Europe is the most mature glass market and has the highest proportion of value-added products._ Four companies; NSG Group, Saint-Gobain, Asahi and Guardian, produce over 60 per cent of the world's high quality float glass. Much of the world’s lower quality float and sheet glass production is being replaced by high quality float._ There are only three glass groups with global automotive glazing capability and presence.NSG Group (as Pilkington Automotive), Asahi, Saint-Gobain/Central, together with their respective associates, supply around 70 per cent of the world’s Original Equipment (OE) glazing requirements.

Conclusion

At the heart of the world’s glass industry is the float glass process. Invented by Sir Alastair Pilkington and announced in 1959, the process manufactures clear, tinted and coated glass for buildings, and clear and tinted glass for vehicles. The process, originally able to make only 6mm thick glass, nowmakes it as thin as 0.4 mm and as thick as 25 mm. Molten glass, at approximately 1000ºC, is poured continuously from a furnace onto a shallow bath of molten tin. It floats on the tin, spreads out and forms a level surface. Thickness is controlled by the speed at which solidifying glass ribbon is drawn off from the bath. After annealing (controlled cooling) the glass emerges as a ‘fire’ polished product with virtually parallel surfaces

A float plant, which operates non-stop for between 10-15 years, makes around 6000 kilometres of glass a year in thicknesses of 0.4 mm to 25 mm and in widths up to 3 metres. The float process hasbeen licensed to more than 40 manufacturers in 30 countries. Over 380 float lines are in operation, under construction or planned worldwide with a combined output of about 1,000,000 tonnes of glass a week. The NSG Group operates or has interests in 49 float lines worldwide.

Recommendations & Suggestions1. Increasing the quantity of cullet helps in reducing the

entire production cost.

2. 100 kg of batch must produce 80 kg of glass.3. In areas like Dehradun mining of dolomite is restricted. So

it is imported from China. More mines of dolomite have to be explored.

4. If the bridge region of the furnace is well observed then losses due to refractory corrosion can be well handled.

5. This being ā comprehensive report suggestions from industry experts will be more beneficial.

Bibliography

1. Seminar on float glass manufacture by Mr. T.K. Chakraborthy of St. Gobain glass.

2. Official site of Pilkington glass Co.3. Official site of St. Gobain Glass4. Wikipedia5. Elements of Ceramics, F.H. Norton6. Introduction to Ceramics, Kingery, Bowen, Uhlmann.7. Elements of fuels, furnaces & refractories, O.P. Gupta.