DOLOMITE BRICKS
Dolomite is a magnesium limestone (CaMgCO3).
It occurs in nature.
Calcined dolomite brick particles are united with the
help of silicate binders and water.
Mixture is then molded to bricks which are air dried
and burnt to temperature ranging between 1200 to
1560oC for about one day which gives stabilized
dolomite bricks. Such bricks are quite stable towards
basic slag.
Properties
 Dolomite bricks are more porous, more
shrinkage softness and less stronger than
magnesia bricks.
 They can be used up to 2300oC without load
and up to 1650oC with load.
Uses
Dolomite is generally used as a repairing
material rather than as a direct refractory because of
its defects like porosity, shrinkage and softness.
However stabilized dolomites are used in electric
furnaces, Bessemer converters, open hearth furnaces
etc, cheap substitute for magnesia bricks.
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NEUTRAL REFRACTORIES
CARBON REFRACTORIES
Carbon refractories are again divided into two
types depending on the nature of raw material.
a) Carbon Refractories From Coke
Carbon bricks are made from blast furnace
coke by crushing and grinding in edge runner mill
mixed with tar (as binding material) then the
mixture is passed through pug mill, heated with a
steam jacket for few hours.
After this, the mixture is molded into the
bricks which are first dried in cool room and then
fired in kilns from 11oo to 1300oC.
b)Graphite Bricks
Graphite is the refractory material while
fireclay is the binding material. Pure graphite
is crushed, finely powdered and mixed with
fireclay. It is then processed for the
manufacturing of the bricks as the fireclay
bricks.
Properties
 They are light grey in color.
 They possess high crushing strength.
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 If not exposed to air, they can withstand high
temperature. However, if heated in presence of
air, they burn.
 They are not attacked by either acid or basic
slags.
 They can only be used under neutral
conditions.
 They are practically infusible, insensitive to
spalling.
 They are close textured and can withstand
under fluctuation of temperature.
 They are also not attacked by corrosive slags
etc.
Uses
Carbon and graphite bricks are widely used as
material of construction of electrodes, linings of
highly chemical resistant equipments, reactors,
electric furnaces, copper, aluminum and lead
smelting furnaces.
SILICON CARBIDE BRICKS
Silicon carbide is made in an electric furnace at
temperature of 1500oC from sand and coke with the
addition of some sawdust and a little amount of a salt.
When sawdust is burnt, the gases are produced which
increases the porosity of the charge.
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Salt reacts with iron and similar impurities present in
raw material forming volatile chlorides. This
increases the purity of the final product.
The silicon carbide is removed from the furnace then
the product is mixed with bonding agent like clay or
silicon nitride then shaped by molding and dried at
approximately 1400 to 2000oC in electric furnace.
Recently, self bond type silicon carbide bricks have
been prepared; in this silicon carbide particles are
mixed with a temporary binding agent such as glue
then pressed and fired at 2000oC, at this temperature
the inter-crystalline bond of silicon carbide develops.
Properties
I. Silicon Carbide Bricks are dark grey and blue
black in color.
II. They have high thermal conductivity and very
low co-efficient of expansion.
III. Clay bonded bricks can be used up to 1750oC.
IV. Nitride bonded bricks have a high strength and
superior thermal shock resistance than clay
bonded bricks.
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V. Self bonded products have high refractoriness,
superior strength, high density, high abrasion
resistance and high chemical resistance.
VI. However silicon carbide bricks tend to oxidize to
silica when heated in air at temp of 900 to 1000O
C, but this drawback is overcome by coating
them with thin layer of zirconium.
Uses
 They are used for partition walls of chamber
kilns, Coke ovens and muffle furnaces due to
their hardness, great strength and toughness.
 They are also used as heating elements in
furnaces in the form of rocks and bars.
ZIRCONIA BRICKS
Zirconia bricks are obtained by heating zirconite
mineral (ZrO2) using colloidal zirconia (prepared by
drying zirconium nitrate solution and taking residue
into solution and again drying) or alumina or binding
material at 1700oC.
Since mineral zirconite under goes volume change on
heating and cooling, so it is stabilized by adding
MgO2 or CaO.
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Properties
I. They can be used up to 2000oC but specially
prepared bricks can be used up to 2600oC.
II. Their refractoriness is about 1900oC under load.
III. They are quiet resistant to thermal shock.
Uses
Zirconia bricks are very costly and hence are
used only where high temperature is maintained e.g.
high frequency electric furnace.
CHROMITE BRICKS
These are made by firing at 1500 to 1700oC
crushed chromite (FeO.Cr2O3) mixed with a little
clay as binding material.
Properties
I. Chromite bricks are neutral in character.
II. They posses high density, resistance to acidic
and basic slags and moderate resistance to
spalling.
III. They can be used up to 1800oC without load and
1430oC with load.
IV. They posses high crushing strength.
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V. They have moderate thermal conductivity.
Uses
 Their most important use is in separating acid
and basic refractory linings so as to prevent their
interaction even at high temperature, for example
between acid roofs of basic open hearth furnaces
and the basic bricks of side walls.
 Their application is in bottom of soaking pits,
Sodium Carbonate recovery furnaces etc.
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