8- Glass manufacturing processes:
Introduction:
Commercially produced glass can be classified as soda-lime, lead, fused silica,
borosilicate, or 96 percent silica. Soda-lime glass, since it constitutes 77 percent of
total glass production, is discussed here. Soda-lime glass consists of sand, limestone,
soda ash, and cullet (broken glass).
The manufacture 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. The procedures 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 glass production, 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 in separate elevated bins. These materials are then transferred through a
gravity feed system to a weigher and mixer, where the material is mixed with cullet
to ensure homogeneous melting. The mixture is conveyed 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 furnace and 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
producing between (50 and 300 tons) of glass per day. A furnace may have either
side or end ports that connect brick checkers to the inside of the melter. The purpose
of brick checkers is to conserve fuel by collecting furnace exhaust gas heat that,
when the air flow is reversed, is used to preheat the furnace combustion air. As
material enters the 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 the refiner. In the refiner, the molten glass is heat conditioned for delivery
to the forming process.
After refining, the molten glass leaves the furnace through fore hearths (except in the
float process, 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. Pressing and blowing are performed mechanically,
using blank molds and glass cut into sections (gobs) by a set of shears. In the
drawing process, molten glass is drawn upward in a sheet through rollers, with
thickness of the sheet determined by the speed of the draw and the configuration of
the draw bar. The rolling process is similar to the drawing process except that the
glass is drawn horizontally on plain or patterned 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 surface requiring no
grinding or polishing. The end product undergoes finishing (decorating or coating)
and annealing (removing unwanted stress areas in the glass) as required, and is then
inspected and prepared for shipment to market. Any damaged or undesirable glass is
transferred back to the batch plant to be used 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.
The majority of these materials are solid inorganic compounds, either naturally
occurring minerals or man-made products. They vary from very coarse materials to
finely divided powders. Liquids and, to a lesser extent, gases are also used within
most sectors.
The gases used include hydrogen, nitrogen, oxygen, sulphur dioxide, propane,
butane and natural 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 some which require careful handling such as
phenol and strong mineral acids. All standard forms of storage 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
wool production. These materials are delivered by rail or road haulage and conveyed
either directly to silos or stockpiled in bays. Storage bays can be open, partially
enclosed or fully enclosed. Where course material is stored in silos they are usually
open and are filled by a conveyor system. The materials are then transferred to the
furnace by enclosed conveyor systems. Materials are mixed simply by laying them
on the feeder conveyor simultaneously.
Granular and powdered raw materials are delivered by rail or road tanker and are
transferred either pneumatically or mechanically to bulk storage silos. Pneumatic
transfer of the materials requires them to be essentially dry. Displaced air from the
silos is usually filtered. Lower volume materials can be delivered in bags or kegs
and are usually gravity fed to the mixing vessels.
In large continuous processes the raw materials are transferred to smaller
intermediate silos from 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 from one or more hoppers. Various feeder mechanisms are found in the
industry ranging from completely open systems to fully enclosed screw fed systems.
To reduce dust during conveying and "carry-over" of fine particles out of the
furnace, a percentage of water can be maintained in the batch, usually 0 - 4 % (some
processes e.g. borosilicate glass production use dry batch materials). The water
content can be introduced as steam at the end of the mixing operation but the raw
materials may have an inherent water content. In soda-lime glass, steam is used to
keep the temperature above 37°C and so prevent the batch being dried by the
hydration of the soda ash.
Due to its abrasive nature and larger particle size, cullet is usually handled separately
from the primary batch materials and may be fed to the furnace in measured
quantities by a separate system.
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 charge for the melter. Several charges will be made up, sometimes of
different formulation, and stored close to the melter for use during a specific melting
period. Common with large scale melting the mixed batch cannot be stored for too
long before use, because the different components can settle-out, which makes it
difficult to obtain an homogenous melt. The presence of water in the batch 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 fossil fuels above a bath of batch material, which is continuously fed into,
and then withdrawn from the furnace in a molten condition. The temperature
necessary for melting and refining the glass depends on the precise formulation, but
is between 1300°C and 1550°C. At these temperatures heat transfer is dominated by
radiative transmission, in particular from the furnace crown, which is heated by the
flames to up to 1650 °C, but also from the flames themselves. In each furnace design
heat input is arranged to induce recirculating convective currents within the melted
batch materials to ensure consistent homogeneity of the finished glass fed to the
forming process. The mass of molten glass contained in the furnace is held constant,
and the mean residence time is of the 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 quite slow allowing time for the numerous chemical and physical processes
to occur. As the materials heat up the moisture evaporates, some of the raw materials
decompose and the gases trapped in the raw materials escape. The first reactions
(decarbonisation) occur around 500°C. The raw materials begin to melt between
750°C and 1200°C. First the sand begins to dissolve under the influence of the
fluxing agents. The silica from the sand combines with the sodium oxide from the
soda ash and with other batch materials to form silicates. At the same time large
amounts of gases 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 melt finally becomes transparent and the melting phase
is completed. The volume of the melt is about 35 - 50 % of the volume of the virgin
batch materials due to the loss of gases and the elimination of interstitial spaces.
c) Fining and Homogenisation
The glass melt must be completely homogenised and free of bubbles before it can be
formed into the products. This involves the complete dissolution and even
distribution of all components and the elimination of all bubbles by refining.
During the melting process gas bubbles are formed mainly from carbon dioxide
given off by the decomposition of the carbonate materials (principally soda ash and
limestone) and to a much lesser extent from air trapped in the raw materials. These
bubbles must be eliminated from the glass melt as they potentially cause defects in
the finished product affecting mechanical strength and appearance. The upward
movement of bubbles contributes to the physical mixing of the melt necessary to
obtain a homogenous material with optimal physical properties. The bubbles rise at
speeds determined by their size and the viscosity of the glass. Large bubbles rise
quickly and contribute to mixing, while small bubbles move slowly, at speeds that
may be small with respect to the larger scale convection currents in the furnace and
are thus more difficult to eliminate. 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 same time, 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 phases occur simultaneously in different locations
within the tank. The batch is fed at one end of the tank and flows through different
zones in the tank and fore hearth 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 where the glass reaches the working temperature for processing. It is diverted
following thermal currents. 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 below the bottom layer of batch material and
sinks to the bottom of the tank. Appropriate convection currents must bring this
material to the surface, since fining occurs in tank furnaces primarily at the 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 too soon. Guiding walls or weirs can be built into the inner tank structure to
create ideal glass flow paths.