Most glass articles are manufactured by a process in which raw materials are concerted at high temperature to a homogeneous melt that is then formed into articles. The manufacturing process starts with:
Preparation of raw material
The efficiency of the melting operation and the uniformity and quality of the glass product are very often determined in the mix house. The method by which the batch is mixed depends on the types of the glass rather than on the size of the tank. High SiO2 glasses (soda lime, borosilicate, aluminosilicate) tend to be batch mixed in pan type mixer. The mixture is first dry blended and the small amount of liquid are coming into vogue for several reasons;
- A wet batch prevents dusting, controls air pollution, and ensures homogeneity, and therefore, increase melting efficiency and glass quality. (Chemical Technology, volume 11)
The manufacturing procedures may be divided into four major phases: (Shreve's, 1980)
- Melting
- Shaping or forming
- Annealing, and
- Finishing
Melting: Ideally, when the intimately mixed batch is charged into hot furnace, a series of melting, dissolution, volatilization, and redox reactions take place between the materials in a particular order and at the appropriate temperature. Preheating raises rapidly as possible to the melting temperature where significant reactions occur generally in a distinct change in the flow characteristics of the batch.
Dissolution of the more refractory (high melting material) grains, such as sand, is accelerated by fluxes (lower melting material), e.g. NaCO3.
Example NaCO3+SiO2 Na2Si2O3 occur first at about 550c and a layer is deposited on the silica grains. As the reaction continuous,
NaSiO3+SiO2 Na2Si2O3 occurs at 7000c. Finally at about 7800c, forms a mixture approaching eutectic liquid.
Fining: is the physical and chemical process of removing gas bubbles (seeds, blisters) from the molten glass melt. Gas is evolved during the first stage of melting because of:-
- The decomposition of the carbonates or sulfates
- Air trapped between the grain of the fine grained batch materials
- Water evolved from the hydrated batch materials
- The change in oxidation state of some of the batch materials e.g. red lead.
2Pb3O4 6PbO + O2
Melt homogenization followed by cooling to working temperatures completes the melting process.
Melting Units
Melting units range from small pots furnaces for manual production to large, continuous tanks for rapid machines forming. The largest furnaces are continuous regenerative furnaces that recover waste heat from burned glass. They produce large quantities of quality glass and are either cross or end-fired. Production ranges from less than 150 to 700ton/day. The two ports of an wend port furnace produce a u-shaped flame over the glass that enters the exhausts from the back well, where as cross- firing from side to side allows more even heating across a larger surface area. Each type of furnace has a melting portion and a conditioning portion which are separated by a refractory bridge well. Furnace designs range from shallow rectangular types to round and vertical.
Feeding system
The two common feeding designs used today are the screw feeder and the reciprocating pusher.
Screw feeder: - delivers the batch from a hopper to the furnace by tube and helical gear drive.
Reciprocating pusher: - forces a layer of the batch from the feed chutes onto the molten glass.
Fuel and efficiency:-natural gas, oil, and electricity are the primary source of energy; propane is as backup reserves in emergencies. Natural gas is the least expensive and most frequently used fuel with heat content ranging from 34-45mJ/m3 (Chemical technology, volume 11)
Glass production process video
Forming: There are for main methods of forming glass:
- Blowing
- Pressing
- Drawing
- Casting
Blowing: -
Deep items such as bottles, jars, or light-bulb envelopes are formed by the use of air pressure either from a pair lunges or an air compressor.
Pressing:- flat item such as dinner ware, optical and sealed beam lenses, filter glasses, and television tube panels are pressed between plunger and a mold most commonly with an automatic rotary press; cost iron, bronze, steel and some special alloys are common mold materials.
A gob of glass is fed into the mold at the first station. The table rotates and a retaining ring and plunger press the article at the next station.
Casting: - is the process of shaping glass by pouring it into a mold.
Annealing:
To reduce strain, it is necessary to anneal all glass objects, whether they are formed by machines or hand-molding methods. Annealing involves two operations.
- Holding a mass of glass above a certain critical temperature long enough to reduce internal strain by plastic flow to less than predetermined maximum.
- Cooling the mass to room temperature slowly enough to hold the strain blow this maximum.
With the forgoing data as basis, engineers are produced continuous annealing equipment, with automatic temperature regulation and controlled circulation, which permits better annealing at a lower cost and with less loss of product.
Finishing:
All types of annealed glass must undergo certain finishing operations which, though relatively simple, are very important. These include cleaning, grinding, polishing, cutting, sand blasting, enameling, grading, and gagging. Although all these are required for every glass object, one or more are almost always necessary. (Shreve's, 1980)