WHAT IS Refractory?

A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000 °F (811 K; 538 °C)".

Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucibles.

Refractory Materials

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/or 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). Fireclays 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 degrees C. The ternary compound tantalum hafnium carbide has with 4488 K the highest melting point of all known compounds.

Classification of Refractory Materials
Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications.

Based on chemical composition
Acidic refractories

These are used in areas where slag and atmosphere are acidic. They are stable to acids but attacked by alkalis. The main raw materials belongs to the RO2 group, ex- silica (SiO2), zirconia (ZrO2) etc.

Neutral 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.

Basic refractories

These are used on areas where slags and atmosphere are basic, stable to alkaline materials but reacts with acids. The main raw materials belong to the RO group to which magnesia (MgO) is a very common example. Other examples includes dolomite and chrome-magnesia