GLASS INDUSTRY

CHEMICAL PROCESS TECHNOLOGY-I

DEPARTMENT OF CHEMICAL ENGINEERING
UNIVERSITY OF KARACHI
 INTRODUCTION
• When silica or quartz is heated up to 1650’C it
melts to a colorless liquid which on cooling
gives glass.
• This fused mass is highly sensitive to
temperature change therefore it requires
special heat treatment.
• The glass of various commercial qualities is
prepared by heating sand or quartz along with
metal oxide or carbonates.
 INTRODUCTION
• Morey defined glass as:
“ An inorganic substance in a condition that is
continuous with liquid state, but which, as a
result of reversible change in viscosity during
cooling, has attained so high a degree of
viscosity as to be, for all practical purposes,
rigid.”
 INTRODUCTION
• According to ASTM:
“An inorganic product of fusion that has been
cooled to a rigid condition without crystallizing.”

• Shelby’s definition is as:

“Glass is a solid that possesses no long-range
atomic order and, upon heating, gradually
softens to molten state”
Fig:

V-T relationships for glasses,

liquids, super-cooled liquids
and crystals.
 Glass Transition Temperature, Tg
• Tg is a point below which the viscoelastic melt
looses its liquid properties and behaves as a
solid.
• At temperature below Tg, material behaves as
solid glass.
• Faster cooling leads to a greater Tg and less
dense glass (Line C).
COMPOSITION OF GLASS
 COMPOSITION
• Glasses begin as mixtures of oxides.
• Their compositions can be represented by listing
the weight percentages of their components.
• Compare the percentages for
• 1, a typical, modern soda-lime-silica glass (used
to make bottles and windows)
• 2, laboratory and some baking ware
• 3, optical, high lead crystal
• 4, 96% silica glass (can withstand very high
temperatures)
• 5, a typical, ancient Roman soda-lime-silica glass.
MANUFACTURING PROCESS OF
GLASS
 RAW MATERIAL
• Sand • Manganese Oxide
• Soda Ash • Selenium Metal
• Calcium Oxide • Broken Glass
• Fled Spar • Fluxes
• Borax • Colouring Agent
• Magnesia • Reducing Agent
• Zinc • Oxidizing Agent Etc.
• Alumina
• Lead Oxide
 RAW MATERIAL
The oxides used for glass manufacture are
classified into following groups
a) Network former
b) Network modifier
c) Intermediate glass formers
d) Oxidizing agent
e) Refining agent
f) Cullet
g) Colouring agent
 RAW MATERIAL
Network former
• These are oxides of elements which are surrounded by
four oxygen atoms in the tetrahedral chain forming
glass.

Network modifier
• These are large diameter elements having higher co-
ordination number. On simple melting they do not give
glass but in presence of other network forming oxides
they can give glassy products easily. The important
network modifiers are oxides of alkali metal, alkaline
earth metals, lead, zinc etc.
 RAW MATERIAL
Intermediate glass formers
• They do not give glass on melting but in presence of some
network formers using their co-ordination number they
start giving glass. E.g., Oxides of aluminum, silica

Oxidizing agent
• Material like sodium nitrate or certain peroxides are used
to reduce the colour of impurities like iron oxides and
manganese oxide

Refining agent
• To reduce or to eliminate quantity of air bubbles from
molten glass refining agents like arsenic oxide or small
amount of feldspar is added to glass.
 RAW MATERIAL
Cullet
• Waste or broken glass species are called cullet. In
normal glass production 33% of charge is broken
glasses. Recycling of cullet increases the rate of
production.

Colouring agent
• Metal oxide is added as colourant during
manufacture of colour glasses e. g. oxides of
chromium and iron give green glass while copper
and cobalt give blue glass.
 MANUFACTURE
The manufacturing process of glass consisting of
the following four steps:
1. Melting the charge
2. Fabrication of the article
3. Annealing the article formed
4. Finishing treatments
 MANUFACTURE
Melting of the charge
• Amount of raw materials for the batch are calculated
from the chemical composition of individual
components.
• Minor ingredients are weighed accurately and given a
preliminary mixing with one of the dry batch
ingredients before adding to the main charge then to
the batch mixer which is a revolving drum provided
with blades to lift and spread the material uniformly.
• After proper mixing of ingredient it is charged into the
furnace.
• Two types furnaces are used for glass melting
a) Pot furnace
b) Tank furnace
 MANUFACTURE
Pot furnace
• In pot furnace, glass is melted in open or covered
pots (closed pots) of fire clay placed inside the
combustion chambers of the furnace fired
directly with coal (used in bangle industry) or
producer gas (used tableware manufacture).
• Pot furnaces are generally used for small scale
melting and fabrication by hand, for the
production of glass bangles, table wares, lamp
wares, thermos-flask etc.
 MANUFACTURE
Tank furnace
• In this process, cross flame regenerative type of
gas or oil used. The port is arranged along the
side of the tank above the glass level those on
any one side is alternatively incoming and
outgoing ports.
• Manufacturing large quantities of a particular
type of glass tank furnace is used. E.g.
manufacture of sheet glass container ware, lamp
shells and resistance glasses, continuous tank
furnaces are generally used.
 MANUFACTURE
• The regeneration system consists of chambers filled with
open brickwork through which hot waste gases and air
required for combustion pass alternatively at regular
intervals of about 30 minutes.
• The flame acts directly upon the raw batch and molten
glass.
• The temperature inside the furnace is generally kept at
870-985’C.
• The molten glass kept at a constant level by continuous
charging rate which is equal to the rate of withdrawal from
the furnace.
• After withdrawal from the furnace, slow cooling of molten
glass is carried to minimize permanent strain.
• The higher the temperature used for reheating, lesser will
be time to remove the strain.
 MANUFACTURE
Fabrication of an article
• The molten glass is fabricated to require size and
shape by either by hand or by machine.
• Following are the different ways fabrication
a) Blowing
b) Casting
c) Drawing
d) Pressing
e) Rolling
f) Spinning
 MANUFACTURE
Annealing
• Glass articles, after being manufactured, are to be
cooled down slowly and gradually.
• This process of slow and homogeneous cooling of glass
articles is known as annealing of glass.
• Annealing of glass is a very important process.
• If glass articles are allowed to cool down rapidly, the
superficial layer of glass cools down first, as glass is a
bad conductor of heat.
• The inter portion remains comparatively hot and it is,
therefore in a state of strain.
• Hence such glass article breaks to pieces under very
slight shocks or disturbances
 MANUFACTURE
• Following are the methods of annealing
a) Flue Treatment
b) Oven treatment
 MANUFACTURE
Flue treatment
• This method is useful for large scale production.
• In the process long flue is used which is
constructed in such a way that there is gradual
decrease in temperature from one end of it to the
other.
• The red-hot articles of glass are allowed to enter
at the hot end of flue and they are slowly moved
on travelling bands.
• They become cool when they reach the cool end
of flue.
 MANUFACTURE
Oven treatment
• This method is useful for small scale
production.
• The red-hot glass articles are placed in ovens,
in which arrangement is made to control the
temperature.
• After articles are placed in ovens, the
temperature is slowly brought down.
 MANUFACTURE
Finishing treatments
Finishing treatments involve following steps:
a) Bending
b) Cutting
c) Opaque making
d) Silvering
 MANUFACTURE
Bending
• Glass may be bent into desired shape by
placing it in ovens in which temperature can
be regulated.
• Glass in the form of rods, sheets or tubes is
placed in such ovens and heated.
• It is then bent when it is suitable heated.
 MANUFACTURE
Cutting
• Glass is cut in required sizes with the help of diamond
or rough glasses or small wheels of hardened steel

Opaque making
• Glass can also be made opaque or impervious to light.
• It is done by grinding the glass surface with emery.
• It can also be achieved chemically by the application of
hydrofluoric acid.
 MANUFACTURE
Silvering
• This process consists in applying a very thin
coat of tin on the surface of glass.
• Silver is deposited on this layer of tin.
• A suitable paint is then applied to give
protection, against atmospheric effects.
TYPES OF GLASS
 TYPES OF GLASS
Soda-lime or soft glasses
• The raw materials are silica (sand), calcium
carbonate and soda ash.
• Their approximate composition is
Na2O.CaO.6SiO2.
• About 90% of all glasses produced belong to soda
lime glass.
• The low cost, low melting point soda-lime glass
has resistant to de-vitrification and to water.
• They have poor resistance to common reagents
like acids.
 TYPES OF GLASS
USES
• They are used as window glass, electric bulbs,
plate glass, bottles, jars, building blocks and
cheaper tablewares, where high temperature
resistance and chemical stability are required.
 TYPES OF GLASS
Potash-lime or hard glasses
• Silica (sand), calcium carbonate and potassium
carbonate are the basic raw material for potash
lime glass.
• Their approximate composition is K2O.CaO.6SiO2.
• They possess high melting point, fuse with
difficulty and have good resistance to acids,
alkalis and other solvents compare to ordinary
glasses.
 TYPES OF GLASS
USES
• These glasses are costlier than soda-lime
glasses and are used for chemical apparatus,
combustion tubes, etc., which are to be used
for heating operations.
 TYPES OF GLASS
Lead glass or Flint glass
• Instead of calcium oxide, lead oxide is fused with
silica.
• As much as 80% of lead oxide is incorporated for
dense optical glasses.
• In addition, K2O is used instead of sodium oxide.
• Its approximate composition is K2O.PbO.6SiO2.
• Lead glass is more expensive than ordinary lime-
soda glass, but it is much easier to shape and to
work with.
 TYPES OF GLASS
• Lead glass has a lower softening temperature
and higher refractive index than soda glass.
• It has excellent electrical properties.
• It is bright, lustrous and possesses high
specific gravity (3 to 3.3).
 TYPES OF GLASS
USES
• Lead glasses are used for high quantity table
wares, optical lenses, neon sign tubing, cathode
ray tubes, electrical insulators and in the art
objects because of their high luster.
• High lead content glasses are used for extra
dense optical glasses, for windows and shields to
protect personnel from X-rays and gamma rays in
medical and atomic energy fields respectively.
 TYPES OF GLASS
Borosilicate glass or Pyrex glass or Jena glass
• It is the most common of the hard glasses of commerce
which contain virtually only silica and borax with a
small amount of alumina and still less alkaline oxides.
• Borosilicate glass has the following composition.
Component: SiO2 B2O3 Al2O3 K2O Na2O
Percentage: 80.5 13 3 3 0.5
• Boron and aluminium oxides substitutes Na2O and CaO
used the lime-soda glasses which results in a glass of
low thermal coefficient of expansion, and high
chemical resistance.
• Borosilicate glasses have a very much higher softening
point and excellent resistivity to shock.
 TYPES OF GLASS
USES
• They are used in pipelines for corrosive
liquids, gauge glasses, superior laboratory
apparatus, kitchenwares, chemical plants,
television tubes, electrical insulators etc.
 TYPES OF GLASS
99.5% silica glass or Vitreosil
• It is produced by heating SiO2 to its melting point
(1,750’C).
• Because of absence of fluxing agents, it is
extremely difficult to get rid of the bubbles.
• Shaping of the glass is difficult due to high
viscosity at its working temperature.
• The final product is translucent.
• It has high softening temperature about 1650’C,
compare to 96% silica glass.
 TYPES OF GLASS
• Its thermal expansion is very low. Due to their
opaque nature, they tend to be mistaken for pipe
when dirty and are, therefore, often broken
accidentally.
• If vitreosil glass is heated for long periods above
its melting point, it finally becomes transparent
and is then known as ―clear silica glass.
• It has considerable transmission properties e.g.
1mm of this material allows no less than 93% of
light to pass corresponding figure for good optical
glass is only 6%.
 TYPES OF GLASS
USES
• It is exposed for the construction of pipelines
for hot concentrated acid.
• Clear silica glass is used mainly for plant ware,
chemical laboratory wares, electrical
insulating materials, and in electrical heat
furnaces.
 TYPES OF GLASS
Alumino-silicate glass
• They possess exceptionally high softening
temperature and having the typical
constituent as follows:
Component
SiO2 Al2O3 B2O3 MgO CaO Na2OandK2O
Percentage
55 23 7 9 5 1
 TYPES OF GLASS
USES
• It is used for high pressure mercury discharge
tubes chemical combustion tube, certain
domestic equipment etc.
 TYPES OF GLASS
Safety glass
• Thin layer of vinyl plastic is introduced between
two or three flat sheets of glass and the whole is
subjected to slight pressure.
• It is then heated till the glass layers and plastic
layers merge into one another to give a sandwich.
• On cooling the glass becomes quite tough.
• When such a glass breaks it does not fly into
pieces as the inner plastic layer tends to hold
back the broken pieces of the glass.
 TYPES OF GLASS
USES
• It is mostly used in automobile and aero plane
industries as wind shield.
 TYPES OF GLASS
Optical or Crookes glasses
• They contain phosphorus and lead silicate,
together with a little cerium oxide, is capable of
absorbing harmful UV light.
• Very careful manufacturing process of heating
the molten mass for prolonged time secured the
homogeneity of the glass.
• In general optical glasses have low melting points
and are relatively soft.
• Their chemical resistant and durability are
appreciably lower than those of ordinary glasses.
 TYPES OF GLASS
USES
• Used for manufacturing of lenses.
 TYPES OF GLASS
Polycrystalline glass or Pyroceram
• It is the most recent development of producing glass by
adding one or more nucleating agents to a special or
convectional glass batch.
• Then it is shaped into desired form and subjected to
controlled heat treatment.
• The nucleating agents induced the formation of a large
number of sub-microscopic crystalline which act as
centers for further crystal growth.
• Crystalline glass is not ductile, but it has much greater
impact strength than ordinary glass.
• It exhibits high strength and considerable hardness and
can be formed and shaped into articles by any methods
of manufacturing.
 TYPES OF GLASS
Toughened glass
• It is made by dipping articles still hot in an oil
bath, so that certain chilling takes place.
• There so, outer layers of the articles shrink
and acquire a state of compression; while the
inner layers are in a state of tension.
• Such a glass is more elastic and capable of
withstanding mechanical and thermal shocks.
• When such a glass breaks, it does not fly but is
reduced to fine powder.
 TYPES OF GLASS
Insulating glass
• It is a transparent unit prepared by using two
or more plates of glass separated by 6-13 mm
thick gap, field up with dehydrated air and
then thematically sealing around the edges.
• This provides a high insulation against heat.
• Thus, if such a glass is used for separating
apartments, it does not transmit heat and
consequently the apartments will remain cool
during summer and warm during winter.
 TYPES OF GLASS
Wired glass
• It is formed by embedding a wire mesh at the
center of the glass sheet during casting due to
this when glass breaks it do not fall into
splinters.
• Additionally, it is more fire resistant than
ordinary glass.
 TYPES OF GLASS
Laminated glass
• It is made by pressing or bonding together two or more
sheets /plates of glass with one or more alternating layer of
bonding material like plastic resin, asphalt or synthetic
rubber.
• The essential qualities of the laminated glass are
1. It is shatter-proof, i.e. its pieces do not fly off when
suddenly broken.
2. It is shock-proof, i.e. it can with stand sudden changes of
temperature and pressure without breaking.
• A bullet-resistant laminated glass is manufactured by
pressing together several layers of glass with vinyl resins in
alternate layers.
• Ordinary, thickness of such glass varies from 12.7 mm - 76.5
mm. Even thicker types are made for specific uses.
 TYPES OF GLASS
USES
As safety glass in aircrafts, automobiles,
helicopters, submarines. Bullet resistant
lamination glass finds application in making
automobile wind screens, looking windows etc.
 TYPES OF GLASS
Glass wool
• It is a fibrous wool-like material composed of
intermingled fine threads or filaments of glass
which is completely free from alkali.
• Glass filaments are obtained by forcing molten
mass of glass through small orifice of average
diameter of 0.005 - 0.007mm continuously which
is sent to rapidly revolving drum resulting in wool
like form.
• It has low electrical conductivity and eight times
higher tensile strength than steel.
• It does not absorb moisture and it is completely
heat proof.
 TYPES OF GLASS
USES
• It is employed for heat insulation purpose, e.g.,
insulation of metal pipe lines, motors, vacuum
cleaners, walls and roofs of houses.
• Being resistant to chemicals, glass wool is used
for filtration of corrosive liquids like acids and
acidic solution.
• It is used for electrical and sound insulation.
• It is also employed in air filter as dust filtering
material.
• It is also used for manufacturing fiber-glass, by
blending with plastic resins.
 TYPES OF GLASS
Photosensitive Glass
• It is UV sensitive high alumina soda lime glass.
• The positive in UV region on glass is developed by thermal
treatment only at 540-550°C.
• The desired photo activity in UV region can be obtained by
admixture of high alumina soda lime glass with small
amounts of Cu2O NaCN.SnO2 in appropriate amounts.
• A blue colour is promoted by NaCN absence of tin oxide.
• In presence of tin oxide an impression in red is observed.
• By manipulation the ingredients in glass, brown and yellow
images can also be possible.
• A potash alumina glass mixed with LiSiO3, cerium and
silver, salts in appropriate proportions have also been used
as photosensitive glass.
 TYPES OF GLASS
Photochromic glass
• Large number of microscopic particles of silver
halides trapped in the three dimensional silicate
networks in fixed concentration.
• On exposure light, temporary colour centers
consisting of silver particles only are produced
and these add quickly producing total darkness.
• The intensity of darkness depends upon the
concentration of silver.
• Because reversible darkening is controlled by the
radiations in the UV region quite abundant in day
light, the photo blackening does not occurs
markedly in the lamp light night.
 TYPES OF GLASS
Fiber glass
• Fiber glass is nothing but molten glass process
mechanically to a flexible thread of filament.
• A hot platinum nozzle filled with molten glass
forces out the fluid in the form of a thin
continuous thread.
• when caught by a rapidly moving disc gets
converted into fiber through elongation and twist
given by the disc.
• fabrics made of glass are bad conductors of heat
and electricity and are noninflammable. Hence
articles made of fiber glass are fire proof.
 TYPES OF GLASS
USES
Such type of glass is used in textiles and
reinforcing and can be spun into yarn, gathered
into a mat, and made into insulation and a great
variety of other products may be with it.
PROPERTIES OF GLASS
 PROPERTIES
• Glass has excellent properties amongst
various material of construction.
• The good properties of glass can be
considered with respect to hardness,
transparency, refractive index, dispersion of
light, low expansion of coefficient, insulation,
thermal conductivity, chemical inertness etc.
 PROPERTIES
• Absorbs, refracts or transmits light
• Can take up a high polish and may be used as
substitute for very costly gem
• Has no definite crystalline structure
• Has no sharp melting point
• Affected by alkalis
• Excellent electrical insulator at elevated
temperatures due to the fact the glass can be
considered as an ionic liquid.
• Available in beautiful colours.
 PROPERTIES
• Behaves as more solid than most solids in the sense that it
is elastic. But when that elastic limit is exceeded, it
fractures instead of deforming.
• It can be blown, drawn or pressed. But it is difficult to cast
in large pieces.
• Extremely brittle.
• Usually not affected by air or water.
• It is not easily attacked by ordinary chemical reagents.
• Possible to intentionally alter some of its properties such as
fusibility, hardness refractive power etc.
• The glasses may be cleaned colourless, diffused and
stained.
• It is possible to weld pieces of glass by fusion.
 PROPERTIES
• It is transparent and translucent. The transparency is the most
used characteristic of glass and it is due to the absence of free
electrons. For the same reason. It is works as a good insulator.
• When heated, it becomes soft and transformed into a mobile
liquid, which on cooling formed into articles of desired shape.
• It is possible to manufacture glass lighter than cork or softer
than cotton or stronger than steel.
• The strength glass however is considerably affected by foreign
inclusions, internal defect are cords or chemically
heterogeneous areas.
• Not easily affected by air/oxidizing agent
• Highly stable against acid but affected easily by alkalis.
• When fused with excess of fusion mixture, glass decomposes
into silicates and carbonates of metal.