Glass Industry
Definition
• Glass may be defined, physically as a rigid undercooled liquid
having no definite melting point and sufficiently (high
viscosity greater than 1012 Pa. s) to prevent crystallization,
and chemically, as the union of the nonvolatile inorganic
oxides resulting from the decomposition and fusion of alkali
and alkaline earth compounds, sand, and other glass
constituents, ending in a product with random atomic
structure.
• When glass is cooled from the hot molten state, it gradually
increases in viscosity without crystallization over a wide
temperature range, until it assumes its characteristic hard,
brittle form. Cooling is controlled to prevent crystallization,
or high strain.
Glass
Glass is actually more like a liquid, but at room
temperature it is so viscous or 'sticky' it looks and
feels like a solid. At higher temperatures glass
gradually becomes softer and more like a liquid. It
is this latter property which allows glass to be
poured, blown, pressed and molded into such a
variety of shapes.
Glass
Glass is distinguished by its amorphous (noncrystalline) structure.
Molecules that link up with each other to form long chains and networks.
Hot glass cools, chains unable to organize into a pattern. Solidification
has short-range order only.
Amorphous structure occurs by adding impurities (Na+,Mg2+,Ca2+, Al3+).
Impurities: interfere with formation of crystalline structure
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History
• Glass technology has evolved for 6,000 years
• A most important development in th
glass technology
was the use of a blow pipe in 12 century
• The first glass was coloured because of the
presence of various impurities such as oxides of
iron and chromium. Virtually colourless glass was
first made some 1,500 years ago.
• Today many products of glass are made in fully
automatic processing lines
• Venice (Murano), glass industry started in 8th
century and it enjoyed the monopoly of glass
industry
• Although glass is one of the oldest materials, its
properties are unique and not yet fully understood.
Applications
• Chemistry
• Pharmacy
• Electrical and electronics industries
• Optics industries
• Construction and lighting industries
Major constituents
• Silica sand
• Soda ash
• Lime stone
• Dolomite
• Feldspathic materials
• Lead oxide
• Boric acid
• Crushed glass
Minor constituents
• The remainder of the batch consists of several
additional ingredients, chosen from a group of
some 15 to 20 materials commonly referred to
as minor ingredients.
• These latter additions are added with a view to
providing some specific function or quality, such
as colour, which is to be realized during the
glass preparation process.
Types of glass
• Fused Silica
• Alkali silicates
• Soda-lime glass.
• Lead glass
• Borosilicate glass
• Special glasses
• Glass fibers
Fused silica
• Fused silica, or vitreous silica, is made by the high-
temperature pyrolysis of silicon tetrachloride or by
fusion of quartz or pure sand. It is sometimes
erroneously referred to as quartz glass and is
characterized by low expansion and a high
softening point which impart high thermal
resistance and permit it to be used beyond the
temperature ranges of other glasses. This glass is
also extraordinarily transparent to ultraviolet
radiation.
Alkali silicates
Alkali silicates are the only two-component glasses of
commercial importance. Sand and soda ash are simply
melted together and the products designated sodium
silicates, having a range of composition of
composition of Na2O, SiO2 to Na2O, 4SiO2.
Silicate of soda (Sodium silicate) solution also known
as water (soluble) glass, is widely' consumed as an
adhesion for paper in the manufacture of corrugated-
paper boxes.
Soda-lime-silica glass, window glass
Soda-lime-silica glass, window glass: silica + sodium
oxide (Na2O) + lime (CaO) + magnesia (MgO) +
alumina (Al2O3). Is transparent, easily formed and
most suitable for window glass. It has a high thermal
expansion and poor resistance to heat (500–600 °C). It
is used for windows, some low-temperature
incandescent light bulbs, and tableware. Container
glass is a soda-lime glass that is a slight variation on
flat glass, which uses more alumina and calcium, and
less sodium and magnesium, which are more water-
soluble. This makes it less susceptible to water
erosion.
Lead glass
• By substituting Lead oxide for calcium oxide in
the glass melt, lead glass is obtained . These
glasses are of of great importance in optical
work because of their high index of refraction
and dispersion.
• Used for the construction of electric light bulbs,
neon-sign tubing
• It is also suitable for shielding from nuclear
radiation.
Sodium borosilicate
Sodium borosilicate glass, Pyrex: silica + boron
trioxide (B2O3) + soda (Na2O) + alumina (Al2O3). Stands
heat expansion much better than window glass. Used
for chemical glassware, cooking glass, car head lamps
etc. The lower coefficient of thermal expansion (CTE)
also makes them less subject to stress caused
by thermal expansion, thus less vulnerable
to cracking from thermal schock. They are commonly
used for reagent bottles, optical components and
household cookware.
Special glasses
Special glasses, Colored and coated, translucent,
safety, optical. Photochromic glasses, and glass
ceramics are special glass.
Glass fibers
• Glass fibers are produced from special glass
compositions that are resistant to weather
conditions, The very large surface area of the
fibers makes them vulnerable to attack by
moisture in the air. This glass is low in silica,
about 55%, and low in alkali. Used in
construction and civil works.
Raw Materials
• In production of glass, large tonnages of glass sand are
used in USA every year. Soda ash, salt cake, and
limestone or lime are required are also required in huge
quantities. In addition, there is heavy consumption of
lead oxide, pearl ash (potassium carbonate), saltpeter
(Potassium nitrate), borax (sodium borate), boric acid,
arsenic trioxide, feldspar, and fluorspar, together with a
great variety of metallic oxides, carbonates, and the
other salts required for colored glass.
• In finishing operations, such diverse products as
abrasives and hydro-floric acid are consumed.
Sand
• Sand for glass manufacture should be almost
pure quartz (a mineral composed of silicon and
oxygen atoms in a continuous framework of
SiO4 silicon–oxygen tetrahedra), A glass sand
deposit has, in many cases, determine the
location of glass factory (If a glass factory. Its
iron content should not exceed O.45 % for table
ware or 0.15% for optical glass, as iron affects
the color of most glass adversely.
Soda
Soda ( Na2O) is principally supplied by dense Soda
Ash (Na2CO3). Other sources are sodium
bicarbonate, salt cake (Sodium sulfate), and
sodium nitrate. The latter is useful in oxidizing iron
and in accelerating the melting. The important
sources of lime (CaO) are limestone and burnt
lime from dolomite (CaC03 ' MgCO3), the latter
introducing MgO into the batch.
Feldspars
• Feldspars have the general formula R2O, Al2O3,
6Si02, where R2O represents Na2O or K2O or a
mixture of these two. They have many advantages
over most other materials as a source of Al2O3
because they are cheap, pure, and fusible and are
composed entirely of glass-forming oxides. Al2O3 is
used only when cost is secondary item. Feldspars
also supply Na2O , K2O and SiO2. The Alumina
content serves to lower the melting point of the
glass and to retard devitrification.
Borax
Borax, as a minor ingredient, supplies glass with both
Na2O and boric oxide. Though seldom employed in
window or plate glass, borax is now in common use in
certain types of container glass. There is also a high-
index borate glass that has a lower dispersion value
and a higher refractive index than any glass previously
known and is valuable as an optical glass. Boric acid is
used in batches where only a small amount of alkali is
wanted because its price is about twice that of borax.
Salt Cake
Salt Cake, ling accepted as a minor ingredient of glass
and also other sulfates such as ammonium and
barium sulfate are encountered frequently in all types
of glass. Salt cake is said to remove the troublesome
sum from tank furnaces. Carbon should be used with
sulfates to reduce them to sulfites. Arsenic trioxide
may be added to facilitate the removal of bubbles.
Nitrates of either sodium of potassium serve to
oxidize iron and make it less noticeable in the finished
glass. Potassium nitrate or carbonate is employed in
many better grades of table, decorative and optical
glass.
Cullet
Cullet is crushed glass from iperfect articles, trim,
and other waste glass. It facilitates melting and
utilizes waste materials. It may be as low as 10 %
of the charge or as high as 80%.
Refractory blocks
Refractory blocks for the glass industry have been
developed especially because of the severe
conditions encountered. Sintered zircon alumina,
mullite, mullite alumina and electrocast zirconia –
alumina-silica, alumina and chrome-alumina are
typical of those for glass tanks.
Chemical Reactions
Steps involved in Glass Manufacturing
• Transportation of raw materials to the plant
• Sizing of some raw materials
• Storage of raw materials
• Conveying, weighing and mixing raw materials, and feeding
them into the glass furnace
• Burning of the fuel to secure temperature needed for glass
formation
• Reaction in the furnace to form glass
• Saving of heat by regeneration or recuperation
• Shaping of glass products
• Annealing of glass Products
• Finishing of glass products
Methods of feed charging
• Continuous and automatic method
• Shovel and wheelbarrow methods
In spite of the modernization of plants, however, the
manual charging of small furnace is still carried on, though
a dusty atmosphere is created. The trend, however, is
toward mechanical batch transporting and mixing systems
so completely enclosed that practically no dust is emitted
at any stage of the handling of glass or raw materials.
Method of Manufacture
The manufacturing procedure may be divided into
four major phase:
1. Melting
2. Shaping or forming
3. Annealing
4. Finishing
Melting
Glass furnaces may be classified as either pot or
tank furnaces
Pot furnaces may have an approximate capacity of
2 ton or less and are used in small production of
special glasses or where it is essential to protect
the melting batch from products of combustion.
They are employed principally in the
manufacturing of optical glass and art glass by the
casting process.
The pots are crucibles made of selected clay or
platinum.
It is very difficult to melt glass in these vessels
without contaminating the product or partly
melting the container itself , except when
platinum is used.
Tank Furnace
In a tank furnace, batch materials are charged into one end of a large
“tank built of refractory blocks, some of which measure 38 × 9 × 1.5m
and have a capacity of 1350 ton of molten glass.
The glass forms a pool in the hearth of the furnace, across which the
flames play alternately from one side and the other. The molten glass
is worked out of the opposite end of the tank, the operation being
continuous.
In this type of furnace , as in the pot of furnace the walls gradually
corrode under the action of the hot glass. The quality of the glass and
the life of the tank are dependent upon the quality of the construction
blocks. For this reason , much attention has been given to glass
furnace refractories. Small tank of furnaces are called day tanks and
supply a day’s demand of 1 to 10 t molten glass. They are heated
either electro thermally or by gas.
