The Glass Industry

-8-

The Glass Industry

8.1 The glass industry in Egypt

Excluding a number of hand production shops concentrating on tableware, there are
at present more than 20 industrial scale glass manufacturing plants in Egypt (2007). A
wide range of products are made there ranging from glass packaging to sodium silicate,
high quality float glass, tableware including drinks tumblers, and sophisticated lead crystal
products including chandeliers. An industry that 30 years ago was effectively government-
owned is now mostly in private hands, but foreign participation is minimal at present.
Glass container production in Egypt is dominated by several main players among
which we cite Misr Glass Manufacturing, Middle East Glass Manufacturing, Kandil glass
factory, National Glass Company and Arab Pharmaceutical Glass. At present, it is
estimated that the Egyptian production of glass containers tons exceeds 1800 Mt per year.
The local market production for container glass has increased from about 0.2Mt in 1997 to
1.8 Mt in 2015.
Egypt flat glass manufacturing, on the other hand, has seen a rapid expansion from
2005 till 2015. There are currently three producers of flat glass in Egypt: Sphinx
Company, Saint Gobain Company and the Egyptian glass Company. Their total annual
capacity exceeds 550000 ton per year.

8.2 Raw materials

The raw materials for glass production differ in nature according to the type of
glass product. For example, flat glass and container glass utilize soda lime glass
essentially manufactured out of quartz, limestone and soda ash. On the other hand, fiber
glass is based on quartz and borax.
8.2.1 Raw material quality
(a) Quartz
This is the major glass former oxide used in commercial glass. For use in the glass
industry it should contain at least 99% silica. Iron oxides impurities impart a reddish color
to the glass produced. In Egypt, pure sand is present in South of Sinai and in Maadi, south
of Cairo.
(b) Limestone
This is a network intermediate, that is, Ca++ ions insert themselves between Si –
O tetrahedra. Its purity should exceed 95%. In Egypt, the purest brand is located near
Minia, in Upper Egypt.
(c) Soda ash
Sodium carbonate is the third major constituent and is the main source of sodium
oxide. It acts as a flux, reducing the temperature required to melt silica. It is locally
manufactured in Alexandria by the Solvay process. The main impurity is sodium chloride.
Because of its volatility, its vapors attack the upper refractory lining of the furnace. Its
level should not exceed 0.5%.

123
 The Glass Industry

(d) Cullet
This consists of crushed broken glass, usually originating from the same plant. Its
use represents many advantages. First, it utilizes a waste; second, since this is a glass, it
will readily melt in the furnace. Its percentage in the raw batch usually ranges from 30%
to 60%.

8.2.2 Raw materials preparation

Sand, used in glass production, is ground to have 85% of the particles passing from
60 mesh screen (0.246 mm) and be retained on 150 mesh screen (0.104 mm). Ideally,
other raw materials, except cullet, should have the same particle size distribution to
minimize segregation problems on storage or mixing. The different raw materials are
stored in separate silos. They are then withdrawn from these silos and weighed to the
required values. This is currently performed using automatically controlled dozers. A
typical soda lime glass batch contains, on a cullet free basis, 60% quartz, 18% limestone,
22% soda ash. After weighing, they are wetted with a few percent of 50% caustic soda
solution before admitting to the mixer. The reason is to have a thin film built around the
particles to prevent segregation on mixing. After mixing, the charge is conveyed to the
melting furnace and stored at its entrance, known as the dog house. The rate of feeding to
the furnace is controlled by a gate which in turns is regulated by a feedback control system
through measuring the liquid melt level in the furnace.

8.3 Glass melting

8.3.1 Physico – chemical changes on melting
As the raw batch is delivered to the furnace, the raw materials are first dehydrated;
then, low melting materials begin to fuse, usually liberating gases (such as CO2). The
molten fluxes then dissolve silica and other refractory oxides. Reactions in presence of a
liquid are rapid, particularly as the melt viscosity is initially low (~ 100 cP). However, as
silica dissolves, the viscosity rises by several thousand folds. That is why; early melting of
silica causes the fluidity of the melt to drop considerably, reducing the melting rate. This
can be avoided by eliminating very fine quartz particles from the raw batch.
The burners in gas fired (or oil fired) furnaces are located laterally above the glass
line, so that the solid batch gradually melts. The unmelted materials then float over the
melt and heat transfer is reduced since these are bad conductors. However, decreased
viscosity causes strong free convection currents to develop that ultimately cause the batch
to melt completely.
8.3.2 Melting problems
(a) Refractory corrosion
The presence of large amounts of fluxing alkalis in the batch can cause severe
corrosion particularly as they are in the molten state. That is why, the following provisions
are necessary:
 The use of high refractory non – acidic materials such as alumina and stabilized
zirconia or AZS.
 Decreasing the points of contact between refractory elements by using large blocks
rather than bricks.
 Minimizing the porosity of refractories by using fused cast types.

124
 The Glass Industry

(b) Devitrification
It is known that glass is an undercooled non – crystalline liquid. The presence of
any crystals is a source of stones in the final glass product. Besides, if these stones form in
flow channels, they will restrict flow. The process of crystal formation is known as
devitrification. This can be averted by proper batch composition and rapid cooling of the
molten glass.
(c) Volatilization
Loss of material from the batch by evaporation from the batch pile or melt surface
has to be minimized for the following reasons:
 It results in stone defects when surface layers are not completely remixed and
homogenized in the melt.
 It alters the melt composition by increasing the level on non – volatile materials.
 The vapors produced will attack the superstructure refractories besides requiring
expensive gas cleaning equipment to reduce pollution.
This can be minimized by decreasing the glass melt temperature and reducing the
melt – free surface area in the furnace.
(d) Bubble formation
They often affect the appearance of glassware more than its function. Bubbles are
formed when gases get trapped inside the melt, for not having the opportunity to reach the
surface or for being insoluble in the melt. They are usually eliminated in the "refining"
section of the furnace. Also, the addition of some volatile oxides helps increasing the
bubble size and hence their rising velocity.

8.3.3 The tank furnace

With few exceptions, the tank furnace is the most common type used for melting
glass. Figure (8.1) shows different views of this furnace. This is a continuous furnace of
rectangular shape, divided into two compartments by a "bridge" wall. The larger
compartment into which the batch is introduced is the melting end; while the other
compartment where glass is cooled and distributed for use is known as the refining end.
Tank depths vary from 0.5 to 1.5 m. depending on how rapidly glass is drawn from the
furnace.
The lower part (the bath) which contains the molten glass is constructed from fused
zirconia or AZS refractory blocks. Above the walls a crown covers the furnace. It consists
of an arched roof of silica bricks resting on side walls of fused cast high alumina
refractories.
Burners are located on the two sides of the tank and are situated above the free
liquid surface. The flue gases resulting from fuel combustion along with gases emanating
from the decomposition of the raw batch leave the tank from lateral flue ducts to the
regenerators. The refining end is separated from the melting zone by a bridge through
which passes a cooling duct. This helps to remove foam or scum from the liquid admitted
to the refining chamber. This latter zone being at a lower temperature than the melting
section, the refractory lining is made of high alumina or sillimanite blocks. The shape of
this zone is roughly semi – circular to allow for the distribution of the melt discharge ports
along its periphery. In this zone, bubbles are finally eliminated and the cooled melt
discharged.

125
 The Glass Industry

Fig 8.1 Glass tank furnace with regenerators

The regenerators, located at each side of the tank are chambers built with fire clay
refractories and containing a checker work of magnesite bricks. They act as heat
exchangers between hot flue gases and cold air. They are run intermittently, so that, at one
time, hot flue gases are made to pass through one of the two exchangers by having one
side of the burners operative, while burners on the other side are closed. These gases heat
up its refractory checker work. After sufficient time, the flow of gases is shifted to the
second regenerator by reversing the direction of firing, and cold air allowed passing
through the first. While its internal checker bricks cool down, air is heated up and is used
as secondary air for combustion. When the checker work is cold enough, the cycle is
reversed and hot flue gases allowed to flow through the second regenerator.

Fig 8.2 Details of Glass Tank Furnace

126
 The Glass Industry

Fig 8.3 Temperature distribution along the furnace

Another design of tank furnaces does not use a bridge wall between the two
compartments, but rather contains floating refractory arms located at both sides of the tank
that narrow its cross section and allow for the smooth flow of molten glass to the refining
section. The details of the glass tank furnace as well as the temperature distribution along
the furnace are shown in Figures (8.2) and (8.3).

8.4 Glass forming techniques

8.4.1 Sheet glass
The oldest method of sheet glass forming is the Fourcault process. It utilizes a
slotted refractory block, known as the debiteuse that is forced into the glass melt. Molten
glass is then made to flow through the slot, usually by drawing it with a chain introduced
in the slot. As the glass is withdrawn, it cools down and acquires a paste like consistency.
Itis then made to pass through water cooled steel rollers before being admitted to the
annealing furnace after which it is surface polished then cut into definite lengths.
Nowadays, 90% of sheet glass is produced by the float process. In this process,
soft glass (at about 1050oC) overflows from the tank furnace to be drawn into a ribbon by
cooled steel rollers (Figure 8.4). It is then admitted on the surface of molten tin where it
spreads to an equilibrium thickness of 7.1 mm. The atmosphere inside this section is
reducing to prevent oxidation of tin. This is usually secured by a H2:N2 mixture. The
upper surface of the glass is polished by the overhead flame (Figure 8.5).

127
 The Glass Industry

By careful control of glass drawing speed, the sheet thickness can be increased to reach
more than 25 mm.

Fig 8.4 Float furnace for flat glass production

1050oC Direction of glass flow 600oC

Electrically heated roof`

Tin bath

Supporting steel structure

Fig 8.5 Tin bath for flat glass production

8.4.2 Container manufacture

The softened glass from a delivery port is cut into individual lumps (gobs) by two
parallel sheets exerting a shearing action. These gobs then move downwards on a gravity
chute to be admitted into molds arranged on a belt. The glass gob is then blown by an
upward puff of air that shapes the piece into the required shape. The mold is then opened
and the piece removed for annealing (Figure 8.6). This process has been used for several
decades to produce bottles at a rate of about 10 bottles per minute. Modern modifications
have enabled this process to produce up to 600 bottles per minute.
8.4.4 Pressed glass ware
Pressing molten glass into an exact shape with reasonable dimensional tolerances is
a difficult task. This is since, as the glass cools inside the mold, its viscosity can increase
several thousand times in a few seconds. That is why this operation has to be done very
rapidly. This method is used for the mass production of cups and saucers.

128
 The Glass Industry

8.4.5 Shaping of glass tubes

Plunger

Gob sliding
on Chute

Shears

Molds
Glass gob

Compressed air

Gob entering Gob flowing Air puff Mold is opened and

mold into mold bottle taken out

Fig 8.6 Shaping of glass bottles

The most common process for manufacturing glass tubing is the Danner process. In this
method, molten glass flows out of the tank on a rotating refractory mandrel that is air
cooled through a central pipe. Glass then coats its external surface. This mandrel is
inclined by about 20o, which makes it possible for glass to flow to its lower end. As air is
continuously blown, the glass, on cooling, will form a tube (Figure 8.7). The pipe
diameter can be regulated by controlling air flow rate and the speed at which glass is
drawn from the mandrel

129
 The Glass Industry

Air Drawing
glass tubing

Fig 8.7 Shaping of glass tubing by the Danner process

8.4.6 Fiber glass

The composition of fiber glass is different from standard soda lime in that it
contains much more silica (52 – 72%). There are several types of fiber glass types: A –
type, a multipurpose brand, E – type which has elevated electrical resistance, C – type
which is chemical resistant and T- type of high thermal resistance. The melting tank does
not radically differ from the conventional type, except that it involves the use of
recuperators instead of regenerators. In this type of exchangers, cold air is flown in a
special channel passing into a duct where hot flue gases flow. This is because the reversal
of firing direction, used in regenerative furnaces, causes a disturbance in steady flow of
molten glass to the forming equipment. Forming is usually done by introducing molten
glass into a spinning cup containing small holes in its walls. This is rotated at high speed
resulting in the formation of a multitude of glass streams. Air is blown continuously
against these streams and converts them into fibers.

8.5 Annealing
Because of the relatively rapid cooling of glass, internal tensile stresses form. As
the viscosity highly increases these stresses cannot be relieved except by crack formation.
That is why it is necessary to reheat the glass product near its transition temperature. This
will allow for these stresses to be removed by plastic motion of glass. Annealing
necessitates cooling down glass following some definite schedule so as to eliminate all
stresses without causing the appearance of new ones. This is commonly done in a
continuous furnace known as the annealing lehr. Typical annealing temperatures range
from 400 to 700oC for time periods of a few hours. Glass articles move inside the furnace
on a metallic belt or on rollers.

Suggested additional reading

1. W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to

Ceramics, 2nd ed., J. Wiley & Sons, N.Y., 1976
2. P.J. Doyle, Glassmaking Today, Portcullis Press, Redhill, England,
1979
3. F.V. Tooley, Handbook of Glass Manufacture, Ashleye Pub., 1984

130
 The Glass Industry

4. I. Bulavin, I. Makarov and V. Kholkhov, Heat Processes in Glass and Silicate

Technology, Mir Pub., Moscow, 1986
5. W. Trier, Glass Furnaces, Trans. Soc. For Glass Tech., England, 1987
6. R.D. Smith, Refractory Ceramic Fibers, Chemical Science and
Technology Handbook, American Ceramic Soc., 1990
7. Glass_manufacturing_book. pdf file, 2000

131