Refractory materials must be chemically and
physically stable at high temperatures. Depending on the operating environment,
they need to be resistant to thermal shock, be chemically inert, and have
specific ranges of thermal
conductivity and of the coefficient of thermal expansion.
The oxides
of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most
important materials used in the manufacturing of refractories. Another oxide
usually found in refractories is the oxide of calcium (lime). Fire clays are also widely
used in the manufacture of refractories.
Refractories must be chosen according to the
conditions they will face. Some applications require special refractory materials. Zirconia
is used when the material must withstand extremely high temperatures. Silicon carbide and carbon (graphite) are two other
refractory materials used in some very severe temperature conditions, but they
cannot be used in contact with oxygen,
as they will oxidize
and burn.
Binary compounds such as tungsten carbide or boron nitride can be very
refractory. Hafnium
carbide is the most refractory binary compound known, with a melting point of 3890 °C. The ternary compound tantalum
hafnium carbide has one of the highest melting points of all known
compounds (4215 °C).
Refractories can be classified on the basis of
chemical composition.
They consist of mostly acidic materials like alumina
(Al2O3) and silica (SiO2). They are not
attacked or affected by acidic materials, but easily affected by basic
materials. They include substances such as silica, alumina, and fire clay refractories.
These are used in areas where slags and atmosphere
are either acidic or basic and are chemically stable to both acids and bases.
The main raw materials belongs to, but not confined to, R2O3
group. The common examples of these materials are alumina (Al2O3),
chromia (Cr2O3) and carbon.
These are used on areas where slags and atmosphere
are basic; they are stable to alkaline materials but could react with acids.
The main raw materials belong to the RO group to which magnesia (MgO) is a very
common example. Other examples include dolomite and chrome-magnesia.
