8- Glass manufacturing processes:
Introduction:
Commercially produced glass can be classified as soda-lime, lead, fused silica, borosilicate, or96 percent silica. Soda-lime glass, since it constitutes 77 percent of total glass production, is discussedhere. Soda-lime glass consists of sand, limestone, soda ash, and cullet (broken glass).
Themanufacture of such glass is in four phases:
(1) preparation of raw material,
(2) melting in a furnace,
(3) forming and
(4) finishing.
Figure 8.1 is a diagram for typical glass manufacturing.
The products of this industry are flat glass, container glass, and pressed and blown glass. Theprocedures for manufacturing glass are the same for all products except forming and finishing.
Container glass and pressed and blown glass, 51 and 25 percent respectively of total soda-lime glassproduction, use pressing, blowing or pressing and blowing to form the desired product. Flat glass,which is the remainder, is formed by float, drawing, or rolling processes.
As the sand, limestone, and soda ash raw materials are received, they are crushed and stored inseparate elevated bins. These materials are then transferred through a gravity feed system to a weigherand mixer, where the material is mixed with cullet to ensure homogeneous melting. The mixture isconveyed to a batch storage bin where it is held until dropped into the feeder to the melting furnace.
All equipment used in handling and preparing the raw material is housed separately from the furnaceand is usually referred to as the batch plant. Figure 8.2 is a flow diagram of a typical batch plant.
The furnace most commonly used is a continuous regenerative furnace capable of producingbetween (50 and 300 tons) of glass per day. A furnace may have eitherside or end ports that connect brick checkers to the inside of the melter. The purpose of brickcheckers is to conserve fuel by collecting furnace exhaust gas heatthat, when the air flow is reversed, is used to preheat the furnace combustion air. As material entersthe melting furnace through the feeder, it floats on the top of the molten glass already in the furnace.
As it melts, it passes to the front of the melter and eventually flows through a throat leading to therefiner. In the refiner, the molten glass is heat conditioned for delivery to the forming process.
After refining, the molten glass leaves the furnace through forehearths (except in the floatprocess, with molten glass moving directly to the tin bath) and goes to be shaped by pressing,blowing, pressing and blowing, drawing, rolling, or floating to produce the desired product. Pressingand blowing are performed mechanically, using blank molds and glass cut into sections (gobs) by a setof shears. In the drawing process, molten glass is drawn upward in a sheet through rollers, withthickness of the sheet determined by the speed of the draw and the configuration of the draw bar. Therolling process is similar to the drawing process except that the glass is drawn horizontally on plain orpatterned rollers and, for plate glass, requires grinding and polishing. The float process is different,having a molten tin bath over which the glass is drawn and formed into a finely finished surfacerequiring no grinding or polishing. The end product undergoes finishing (decorating or coating) andannealing (removing unwanted stress areas in the glass) as required, and is then inspected and preparedfor shipment to market. Any damaged or undesirable glass is transferred back to the batch plant to beused as cullet.
Figure 8.1. Typical glass manufacturing process.
Figure 8.2. General diagram of a batch plant.
1- Materials Handling:
The diversity of glass Industry results in the use of a wide range of raw materials. Themajority of these materials are solid inorganic compounds, either naturally occurring mineralsor man-made products. They vary from very coarse materials to finely divided powders. Liquidsand, to a lesser extent, gases are also used within most sectors.
The gases used include hydrogen, nitrogen, oxygen, sulphur dioxide, propane, butane andnatural gas. These are stored and handled in conventional ways for example, direct pipelines,dedicated bulk storage, and cylinders. A wide range of liquid materials are used, including somewhich require careful handling such as phenol and strong mineral acids. All standard forms ofstorage and handling are used within the industry e.g. bulk storage, intermediate bulk containers(IBCs), drums and smaller containers.
Very coarse materials (i.e. with particle diameter > 50 mm) are only used in stone woolproduction. These materials are delivered by rail or road haulage and conveyed either directly tosilos or stockpiled in bays. Storage bays can be open, partially enclosed or fully enclosed. Where course material is stored in silos they are usuallyopen and are filled by a conveyor system. The materials are then transferred to the furnace byenclosed conveyor systems. Materials are mixed simply by laying them on the feeder conveyorsimultaneously.
Granular and powdered raw materials are delivered by rail or road tanker and are transferredeither pneumatically or mechanically to bulk storage silos. Pneumatic transfer of the materialsrequires them to be essentially dry. Displaced air from the silos is usually filtered. Lowervolume materials can be delivered in bags or kegs and are usually gravity fed to the mixingvessels.
In large continuous processes the raw materials are transferred to smaller intermediate silosfrom where they are weighed out, often automatically, to give a precisely formulated "batch".
The batch is then mixed and conveyed to the furnace area, where it is fed to the furnace fromone or more hoppers. Various feeder mechanisms are found in the industry ranging fromcompletely open systems to fully enclosed screw fed systems. To reduce dust during conveyingand "carry-over" of fine particles out of the furnace, a percentage of water can be maintained inthe batch, usually 0 - 4 % (some processes e.g. borosilicate glass production use dry batchmaterials). The water content can be introduced as steam at the end of the mixing operation butthe raw materials may have an inherent water content. In soda-lime glass, steam is used to keepthe temperature above 37°C and so prevent the batch being dried by the hydration of the sodaash.
Due to its abrasive nature and larger particle size, cullet is usually handled separately from theprimary batch materials and may be fed to the furnace in measured quantities by a separatesystem.
In discontinuous processes the batch plant is much smaller and is often manually operated.Following mixing, the batch can be stored in small mobile hoppers each containing one chargefor the melter. Several charges will be made up, sometimes of different formulation, and storedclose to the melter for use during a specific melting period. Common with large scale meltingthe mixed batch cannot be stored for too long before use, because the different components cansettle-out, which makes it difficult to obtain an homogenous melt. The presence of water in thebatch helps to mitigate this tendency.
2- Glass Melting:
Glass melting can be divided into four phases:
a) Heating
b) Primary melting
c) Fining and Homogenization
d) Conditioning
A) Heating
The conventional and most common way of providing heat to melt glass is by burning fossilfuels above a bath of batch material, which is continuously fed into, and then withdrawn fromthe furnace in a molten condition. The temperature necessary for melting and refining the glassdepends on the precise formulation, but is between 1300°C and 1550°C. At these temperaturesheat transfer is dominated by radiative transmission, in particular from the furnace crown, whichis heated by the flames to up to 1650 °C, but also from the flames themselves. In each furnacedesign heat input is arranged to induce recirculating convective currents within the melted batchmaterials to ensure consistent homogeneity of the finished glass fed to the forming process. Themass of molten glass contained in the furnace is held constant, and the mean residence time is ofthe order of 24 hours of production for container furnaces and 72 hours for float glass furnaces.
b) Primary melting
Due to the low thermal conductivity of the batch materials the melting process is initially quiteslow allowing time for the numerous chemical and physical processes to occur. As the materialsheat up the moisture evaporates, some of the raw materials decompose and the gases trapped inthe raw materials escape. The first reactions (decarbonisation) occur around 500°C. The rawmaterials begin to melt between 750°C and 1200°C. First the sand begins to dissolve under theinfluence of the fluxing agents. The silica from the sand combines with the sodium oxide fromthe soda ash and with other batch materials to form silicates. At the same time large amounts ofgases escape through the decomposition of the hydrates, carbonates, nitrates and sulphates;giving off water, carbon dioxide, oxides of nitrogen, and oxides of sulphur. The glass meltfinally becomes transparent and the melting phase is completed. The volume of the melt is about35 - 50 % of the volume of the virgin batch materials due to the loss of gases and theelimination of interstitial spaces.
c) Fining and Homogenisation
The glass melt must be completely homogenised and free of bubbles before it can be formedinto the products. This involves the complete dissolution and even distribution of allcomponents and the elimination of all bubbles by refining.
During the melting process gas bubbles are formed mainly from carbon dioxide given off by thedecomposition of the carbonate materials (principally soda ash and limestone) and to a muchlesser extent from air trapped in the raw materials. These bubbles must be eliminated from theglass melt as they potentially cause defects in the finished product affecting mechanical strengthand appearance. The upward movement of bubbles contributes to the physical mixing of themelt necessary to obtain a homogenous material with optimal physical properties. The bubblesrise at speeds determined by their size and the viscosity of the glass. Large bubbles rise quicklyand contribute to mixing, while small bubbles move slowly, at speeds that may be small withrespect to the larger scale convection currents in the furnace and are thus more difficult toeliminate. Small bubbles remaining in the finished glass are termed "seeds".
d) Conditioning
A conditioning phase at lower temperatures follows the primary melting and fining stages.During this process, all remaining soluble bubbles are reabsorbed into the melt. At the sametime, the melt cools slowly to a working temperature between 900°C and 1350°C.In batch melting, these steps occur in sequence, but in continuous furnaces the melting phasesoccur simultaneously in different locations within the tank. The batch is fed at one end of thetank and flows through different zones in the tank and forehearth where primary melting, fining,and conditioning occur. The refining process in a continuous furnace is far more delicate.
Glass does not flow through the tank in a straight line from the batch feeder to the throat wherethe glass reaches the working temperature for processing. It is diverted following thermalcurrents. The batch pile, or the cold mixture of raw materials, is not only melted at the surface,but also from the underside by the molten glass bath. Relatively cold, bubbly glass forms belowthe bottom layer of batch material and sinks to the bottom of the tank. Appropriate convectioncurrents must bring this material to the surface, since fining occurs in tank furnaces primarily atthe surface of the melt, where bubbles need to rise only a short distance to escape. If thermal
currents flow too fast, they inhibit fining by bringing the glass to the conditioning zone toosoon. Guiding walls or weirs can be built into the inner tank structure to create ideal glass flowpaths.