Module: 6
Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
Module: 6
Lecture: 28
CLAY PRODUCTS AND REFRACTORIES
2. STRUCTURAL CLAY PRODUCTS
Grinding
Mixing
Screening
Maturation
Raw
Clay
Soaking
Water
Firing
Moulding
Clay
Product
Dryer
Figure: Manufacturing of Clay Products
Block diagram of manufacturing process
Diagram with process equipment
Animation
Unglazed pipes, tiles, terracotta and building bricks are manufactured from
locally clays without glaze, which contain the fluxes needed for binding. For building
and face bricks, red burning clay is used, which is cheap and durable.
Bricks can be manufactured from
 Soft mud process
 Stiff mud process
 Dry press process
Soft mud process
The clay mixture contain 30% water is moulded in moulds. This mould bricks
are dried in tunnel drier. While common bricks are burnt in scove kiln, in which coal is
used as fuel. The kiln is built from the green bricks with the outside walls daubed or
scoved with clay. It is a variation of undraft type of kiln. After burning the kiln is
completely dismantled.
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Lecture: 28 Clay products and Refractories
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Stiff and mud process
Clay mixture contains 12-15% water, which is just enough to stick together. The
clay is forced out through a die in a screw or auger machine. The extruded clay bar
passes long or short belt conveyor on to a cutting table on which a frame with
number of wires which automatically cuts the bar into appropriate lengths. These
bricks may be repressed to make face bricks.
Dry process
The water content of the clay is reduced to 4-7% which make the clay nonplastic. The brick unit is moulded at high temperature.
PROPERTIES
 Depending on the content of the soil, clay can appear in various colours,
from a dull gray to a deep orange-red.
 Clays exhibit plasticity when mixed with water in certain proportions, when
dry, it becomes firm and when fired in a kiln, permanent physical and
chemical changes occur which converted clay into a ceramic material.
Because of these properties, clay is used for making pottery items, both
utilitarian and decorative.
 Different types of clay, when used with different minerals and firing conditions,
producing earthenware, stoneware, and porcelain.
USES
 Clay is the starting raw material for manufacturing bricks, tiles, terracotta,
pottery, earthenwares, sewer, drain pipes, and covers for electrical cables
 Clay is one of the oldest building materials on Earth, among other ancient,
naturally-occurring geologic materials such as stone and organic materials
like wood.
 Between one-half and two-thirds of the world's population, in traditional
societies as well as developed countries, still live or work in a building made
with clay as an essential part of its load-bearing structure. Also a primary
ingredient in many natural building techniques
 Used to create adobe, cob, cordwood, and rammed earth structures and
building elements such as wattle and daub, clay plaster, clay render case,
clay floors and clay paints
 A traditional use of clay as medicine goes back to prehistoric times. Kaolin
clay and attapulgite have been used as anti-diarrheal medicines
 Used where natural seals are needed, such as in the cores of dams, or as a
barrier in landfills against toxic seepage
 Clay tablets were used as the first known writing medium, inscribed
with cuneiform script through the use of a blunt reed called a stylus.
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Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
 Clays sintered in fire were the first form of ceramic, Bricks, cooking pots, art
objects, dishware, and even musical instruments such as the ocarina can all
be shaped from clay before being fired.
 Used in many industrial processes, such as paper making, cement production,
and chemical filtering.
 Used in the manufacture of pipes for smoking tobacco.
3. REFRACTORY MATERIALS
Refractories are ceramic materials that can withstand unusually high heat as
well as abrasion and the corrosive effects of acids and alkalis. They are used in
furnaces, stills for the cracking of petroleum, ceramic kilns, boilers, incinerators,
electrolytic cells for aluminum production etc. Refractories are an important
constituent of nuclear reactors. Jet engines would not last very long without
refractory parts.
Fire clay is an important raw material for refractories. In recent years non-clay
refractories of alumina, zirconia, silicon carbide, chromia, magnesite, graphite and
other less common materials are developed. The cost of these refractories is much
higher than that of fireclay. However, their use in critical parts of a furnace will keep
it in operating condition for longer periods, with less time taken out for repairs.
CLASSIFICATION
 Acid refractories
 Basic refractories
 Neutral refractories or special refractories
Acid refractories
The prime ingredient for acid refractory is silica. Acid refractory have high
temperature loadbearing capacity and are used in the arched roofs of steel and
glass making furnaces at temperatures as high as 16500C. At this temperature small
portion of the brick will actually exist as a liquid. Alumina should be kept at 0.2 to
1.0% by weight because it adversely effect on the performance of these refractories.
These refractory materials are resistant to acid slags that are rich in silica. But they
are readily attacked by basic slags composed of CaO and / or MgO, there so
contact with these oxides should be avoided.
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Lecture: 28 Clay products and Refractories
Sr.
No.
1.
2.
Dr. N. K. Patel
Acid refractories
Silica(SiO2)
Aluminium silica
(46% Al2O3 + 54% SiO2)
Alumina (Al2O3)
Silmanite
(63%Al2O3 + 37%SiO2)
3.
4.
Fusion
temperature
17000C
17800C
20500C
19000C
Basic refractories
The main constituent of basic refractories is magnesia (MgO), along with
calcium, chromium and iron compounds. The presence of silica is harmful for use in
high temperature performance; Basic refractories are especially resistant to attack
by slags containing high concentrations of MgO and CaO, and find extensive use in
some steel making open hearth furnaces.
Sr. No.
1.
2.
3.
Basic refractories
Magnesia
Bauxite
Dolomite
Fusion temperature
22000C
18000C
15000C
Special refractories or neutral refractories
The special refractories are relatively expensive due to use of high-purity oxide
materials and very little porosity. Alumina, silica, magnesia, beryllia (BeO), zirconia
(ZrO2) and mullite (3AI2O3.2SiO2), as well as carbide compounds, in addition to
carbon and graphite are used for manufacture of special refractories. Silicon
carbide (SiC2) has been used for electrical resistance elements, as a crucible
material, and in internal furnace components. As carbon and graphite are
susceptible to oxidation at temperatures in excess of about 8000C, they find limited
application in spite of refractory characteristics.
Sr. No.
1.
2.
3.
4.
Neutral refractories
Chromite
Graphite
Silicon carbide
Zirconia
Fusion temperature
21800C
30000C
27000C
22000C
Silica and high alumina refractories
Refractories are obtained from ores of silica or those of silica and alumina.
After mining or chemical production and calcining, refractory materials are crushed,
ground and prepared to size. They are then mixed with other materials and shaped
as bricks. Bricks are used for lining-melting and other applications. Bricks of acid,
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Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
basic and neutral refractories having different shapes are available for furnace
construction. Bricks during construction work are boned and cushioned with the help
of a mortar consisting of chrome, silicon carbide, silica and alumina. Brick work may
be coated with thin mortar for further protecting the same during furnace operation.
MANUFACTURE
Composition of silica and basic refractory bricks
Sr.
No.
1.
2.
Type of Brick
Silica
(SiO2)
Alumina
(Al2O3)
Lime
(CaO)
Magnesia
(MgO)
Silica
Basic
Chrome
95-97
0.2-1.2
1.8-3.5
3-6
15.33
3-6
33-39
Magnesite
Forsterite
----
Iron
oxide
(Fe2O3)
0.3-0.9
Chromic
oxide
(Cr2O3)
----
Other
Oxides
0.05-0.3
----
14-19
11-17
30-45
1-2
0.4-2.0
1-5
85-95
0.5-4.0
----
0.5-1.0
----
----
47-55
9-11
----
3-4
Composition of fireclays and high alumina bricks
Sr.
Type of Brick
No.
1.
Fireclay
Super duty
Semi-Silica
Medium duty
Low duty
2.
High Alumina
60% Alumina
80% Alumina
90% Alumina
99% Alumina
Silica
(SiO2)
Alumina
(Al2O3)
Titania
TiO2
Other
oxides
49-53
72-80
57-70
60-70
40-44
18-24
25-36
21-32
2.0-2.5
1.0-1.5
1.3-2.1
1.0-2.0
3-4
1.5-2.5
4-7
5-8
31-37
11-15
8-9
0.5-1.0
57.5-62.5
77.5-82.5
89-91
98-99
2-3.3
3-4
0.4-0.8
Trace
3-4
3-4
1-2
0.6
Water
Clay
Hot air out
Rotary kiln
Dry
Press
Grinding
Screening
Mixing
Refractory
Bricks
Dryer
Hot air in
Figure: Manufacturing of Refractory
Block diagram of manufacturing process
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Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
Diagram with process equipment
Animation
It involves the following steps
Crushing
The clay is crushed to 25 mm in size.
Grinding
The crushed materials are ground in suitable grinding machine down to 200
mesh size.
Screening
Screening is carried out to separate fine and coarse materials. The desired
size material is used for brick making and oversize is recycled to grinding machine.
Mineral-dressing
Mineral dressing is used to purify the raw materials and producing better
refractories. Purification is carried out by settling, magnetic separation and by
chemical methods.
Storage
Pure materials are stored in storage bins using bucket elevators,.
Mixing
Proper distribution of plastic material throughout the mass is necessary for
easy moulding.
Moulding
It can be accomplished either manually or mechanically at high pressure.
Refractories of low density and low strength can be produce by hand-moulding.
While mechanical-moulding produces refractories of high density and strength.
Deairing is essential in order to increase the density and strength of refractory by
mechanical moulding.
Deairing
It is carried out by
 Allowing air inside the void space in the refractory to go out by decreasing
the rate of pressure application and release
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Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
 Double-pressing the material viz. first pressed and allowed to crack. Then it is
repressed again so as to close the voids.
 Applying vacuum through vents in the moulds
Drying
Drying is carried out to remove the moisture from refractories. Drying is carried
out in tunnel dryer usually very slowly and under well-set conditions of humidity and
temperature, depending upon the type of refractory. Rack cars are placed in a
long tunnel and heat is supplied by steam, which passes through the tunnel.
Firing
To stabilize and strengthen the structures of refractories firing is carried out in
tunnel kilns or shaft kilns or rotary kilns. The bricks are generally fired at a temperature
as high as or higher than their arc temperature.
The firing temperature of different bricks is as follows
 High-fired super duty bricks : 14800C
 Kaolin bricks
: 17000C
 Some basic bricks
: 18700C
PROPERTIES
The following properties need to be considered while selecting a refractory
material.
Refractoriness
Refractoriness is indicated in terms of pyrometric cone equivalent (PCE) value
which indicates the temperature of softening the refractory. PCE should be well
above the operating temperature. As inner end of the refractory wall is at higher
temperature compare to the outer end, therefore, unless the brick melts away
completely, it can often be used to withstand a temperature higher than its
softening temperature. The outer end will be at a lower temperature and still in a
solid state, thus giving strength.
Strength
Strength under the combined effect of temperature and load is an important
factor, particularly in taller furnaces, the refractory has to support a heavy load.
Refractory must be strong enough to resist physical wetting away and to take load.
Refractoriness under load is an important consideration because usually a refractory
fails at a lower temperature when subjected to load because some crystals which
have become fluidic act as a lubricant and deformation becomes easier when
subjected to load.
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Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
Specific gravity
Specific gravity reflects true porosity of refractory. Porosity increases with
increase in bulk density. It is also a criterion while buying the raw material because
raw materials of higher specific gravity will give fewer bricks or article per unit weight.
Eventually increases the production cost. Refractory material in powdered form is
used for making furnace lining or for repairs. Lesser weight of a material with lower
specific gravity will be required for this purpose. Materials of lower specific gravity
are also preferable because bricks in the lower part of a tall structure will not be
subjected to a heavy load
Porosity and slag permeability
If the refractory has open pores the gases and slags will enter the material
more easily and to a greater depth and may react and reduce the life of the
refractory.
Thermal expansion
In furnace design, allowance has to be made for thermal expansion. This
becomes a very important factor if the refractory is subjected to rapid changes in
temperature as a furnace door which is, on opening, is suddenly exposed to air at
room temperature. Within a single brick in the wall there may be a temperature drop
of a few hundred degrees from the hot face to the cold face. This causes
differences in thermal expansion in different sections of the brick. There may be
changes in the internal structures of the material with the result that there is
expansion in volume. All these result in the development of high internal stresses.
Thermal conductivity
It is important factor if the refractory is subjected to rapid changes in
temperature. Thermal conductivity should be low so that the heat of the furnace
may not be lost. Exceptions to this rule are coke over walls, mulle furnaces and
retorts which is heated form outside. They should be made of refractories of high
thermal conductivity. To prevent heat loss sometimes a refractory is backed by an
insulate material like asbestos. The life of refractories is shorter which are constantly
maintained at high temperature.
Electrical conductivity
It should be low for electrical furnaces. Except graphite, all other refractories
are bad conductors of electricity. The electrical resistivity of the refractories should
drop rapidly with increase in temperature.
Chemical composition
The chemical composition of the refractory should be such that the
surrounding do not chemically attack the refractory and corrode it.
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Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
Characteristics of refractories
 Can withstand high temperatures without being fused
 Crucibles and furnace sides and bottoms containing molten metal are made
up of refractories
 Use as ladles for pouring metal into the mould
 Constitute furnace walls and roof and thus minimize heat losses
Properties of various refractory bricks
Sr.
Type
No.
1.
High Alumina
Properties
50% Alumina to
90% Alumina
 High refractoriness which is increasing with
alumina content
 High mechanical strength at high temperatures
 Excellent to fair resistance to spilling
 Greater resistance to corrosion
 Good spilling resistance and thermal insulation
value
 Fair resistance to fluxes and acid slags
 Lower resistance to basic slags and fluxes
 Rigidity under load at high temperatures
 Resistance to structural spilling
 Volume stability
 Resistance to volatile alkalis or fumes
 High refractoriness and resistance to abrasion
 High mechanical strength at high temperatures
 Greater thermal conductivity as compared to
high duty fireclay brick, at high temperatures
 High resistance to corrosion by acid slags
 Fair resistance to attack by oxides of lime,
magnesia and iron
 Readily attacked by basic slags and fluorine
 Poor resistance to spilling at low temperatures
 Better resistance to spilling
 High resistance to corrosion by basic slags
 Mechanical strength and stability of volume at
high temperatures
 High-resistance to corrosion by basic and
moderately acid slags and fluxes
 Basic slags do not adhere to chrome bricks
 Absorbed iron oxide may damage expansion
 Possess thermal conductivity lower than that of
magnesite brick but higher than fireclay brick
2.
Fireclay
Low, medium
and high duty
3.
Fireclay
Semi-Silica
4.
Silica
Super duty
Conventional
5.
Chrome fired
magnesite
Chemically
bonded
Chrome fired
6.
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Lecture: 28 Clay products and Refractories
7.
Magnesite fired
8.
Forsterite-Fired
-Magnesia
88-90%
-above 90%
-Chemically
bonded
metal encased
Dr. N. K. Patel
 Extremely high refractoriness and high thermal
conductivity
 Poor resistance to slags containing high % silica
 Great resistance to corrosion by basic slags
 Chemically boned and metal encased bricks
have marked resistance to spilling
 Excellent strength at high temperature
 High refractoriness
 Attacked by acid slags
 Fair resistance to most basic slags
 Good corrosion resistance to alkali compounds
USES
Application and selection of different refractories
NPTEL
Sr.
No.
1.
Type of refractory
Fire bricks
2.
Silica brick
3.
Magnesite-chrome brick
4.
Dolomite
5.
Chrome magnesite brick
6.
7.
High Alumina fireclay
Magnesite
Name of the furnace used for melting ferrous metals
/alloys
 Bottom of induction furnace
 Sub-hearth of open hearth furnace
 Doors of open hearth furnace
 Spout of open hearth furnace
 Spout of direct arc furnace (acidic)
 Side walls and roof of direct arc furnace
 Roof of open hearth furnace
 Melting zone of basic cupola
 Direct arc furnace roof (for Ni and Cu melting)
 Backing of open hearth furnace
 Side walls of direct arc furnace(basic)
 Side walls of direct arc furnace (basic)
 Side walls of open hearth furnace
 Blocks and ends of open hearth furnace
 Ladle refractories - as lining nozzle, stopper etc.
 Hearth and side walls of direct arc furnaces for
melting nickel and copper
 Sub - hearth of direct arc furnace for melting steel
(basic)
191
Module: 6
Lecture: 28 Clay products and Refractories
Dr. N. K. Patel
Refractories used in various furnaces
Sr.
No.
1.
2.
3.
4.
5.
6.
7.
8.
NPTEL
Furnace
Iron blast furnace
Hearth and Bosh
Shaft or stock
Top
Hot blast stoves
Hot blast main
Bustle pipe
Acid open-hearth Furnace
Portions above the working floor
Regenerative walls
Basic Open-hearth Furnace
Roof
Side walls
Hearth
Acid Bessemer Converter
Body
Tyres
Basic Bessemer Converter
Basic Electric Furnace
Roof
Bottom and sides
Cupola
Reheating Furnace
Roof
Hearth
Refractory material
Entire lining consists of fireclay but of different
qualities
Coarsely ground fireclay
Medium ground fireclay
Finely ground fireclay
Porous fireclay bricks
Fireclay
Fireclay
Silica bricks
Fireclay bricks
Silica bricks
Silica bricks
Dolomite or magnesite
Ganister
Fireclay
Calcined dolomite or magnesite
Silica bricks
A layer of fireclay bricks next to shell and upon
this a layer of magnesite
Fireclay lining
Silica bricks
Chromite or magnesite bricks
192