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ISQ ipegysi
These materials are capable of withstanding high temperature in different industrial processes.
In addition to the thermal endurance, a refractory should be able to resist the chemical action
of the material being heated (molten metal slag in the case of metallurgical furnaces and the
gases generated in the furnace) and withstand the mechanical load. They have high dimensional
and chemical stability and do not lose their physical shape and chemical composition.
Refractories confine the heat and prevent the heat loss to the atmosphere from the outside walls
of furnaces. Refractories are not pure compounds and hence do not possess a sharp fusion
point. Therefore, softening temperature is determined rather than fusion point. The ability of
a material to withstand prolonged action of high temperature (1580° C and onwards) without
appreciable softening under service condition is known as refractoriness expressed in degrees
C. It is generally measured by the softening or the melting point of the material and is determined
by the pyromeric cone method, with the aid of tetrahedral cones or elongated pyramids, made
of the same material to be tested, the size being 20 mm base and 40 mm height. They are heated
until they soften and bend to touch the base. These cones are then compared with the standard
Segar cone of known refractoriness, kept under same thermal load. The value is known as
Pyromeric Cone Equivalent (PCE). Fire-clay and high alumina clay soften gradually over a
range of temperature, whereas, others silica softens over a relatively narrow range.
g
y22f
2 2g
2f
e
combine readily with bases. Their chief constituent is silica: quartz, sand,
ganister and silica bricks.
f
consist mainly of basic oxides: magnesite and dolomite.
x
consist of materials which do not combine with either basic or acidic
oxides: silicon carbide, chromite and carbon.
y
2
2f2 2
containing not less than 93 per cent SiO 2 are used for roof, metallurgical
furnaces and glass tanks. This is an acid refractory made from hard, dense quartzite and sand
stone.
e
E 2
may be semi-acid type containing silica (over 65 per cent),
chamotte with 30–45 % alumina are used in brickwork, lining of furnances; alumina (less than
35 per cent) or high-alumina variety with more than 45 per cent alumina are used in glass
industry for furnace brickwork. Aluminosilicate items are used to line cupolas, coke ovens etc.
w
consist chiefly of MgO (80–85%) and their refractoriness may be as
high as 2000° C. In is an important basic refractory. It finds application in extractive metallurgy.
g
are obtained from chromous iron ore blended with magnesia and
alumina. Their refractoriness is 1800–2000° C and they withstand attack by iron ore slags. They
are used in steel making furnaces.