Refractory
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The term refractory refers to the quality of a material to retain its strength at high temperatures. ASTM C71 defines refractories as "non metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to enviroments above 1000 °F (538 °C)"1.
Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucibles.
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[edit] Refractory materials
Refractory materials must be chemically and physically stable at high temperatures. Depending of the operating environment they need to be resistant to thermal shock, be chemically inert, and/or have specific values of thermal conductivity and 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 required some special refractory materials. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon 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.
[edit] Types of refractories
Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in acidic environment because they will be corroded. Zircon, fireclay and silica are acidic, dolomite and magnesite are basic and alumina, chromite, silicon carbide, carbon and mullite are neutral. Refractory metals are also frequently used.
Refractory materials are used extensively in the metals industries, along with glass melting and other heat treatment operations.
There are two common forms of refractories, bricks or monolithics. Bricks are pre-sintered forms which can hold their shape. Monolithics are loose material which can be formed into complex shapes, or sprayed into place, and have to be sintered before use.
[edit] Refractory Anchorage
Anti-abrasion refractory attached to pipes, chambers, etc, will require anchorage systems such as wire formed anchors or hexmetal to support the refractory linings.
[edit] Refractory heat-up
Usually, refractories require special heat-up techniques to insure that their performance will be attained as designed, and to avoid thermal shock and drying stresses until the operational status is achieved.
[edit] References
1 ASTM Volume 15.01 Refractories; Activated Carbon, Advanced Ceramics
