A refractory is a material that retains 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 environments above 1000 °F (538 °C)". [ASTM Volume 15.01 "Refractories; Activated Carbon, Advanced Ceramics"]
Refractory materials are used in linings for
furnaces, kilns, incinerators and reactors. They are also used to make crucibles.
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 values of thermal conductivityand of the coefficient of thermal expansion.
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.
Zirconiais used when the material must withstand extremely high temperatures. Silicon carbideand carbonare 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.
Types of refractories
Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in an acidic environment because they will be corroded. Zircon, fireclay and silica are acidic, dolomiteand magnesite are basic and alumina, chromite, silicon carbide, carbon and mulliteare neutral. Refractory metalsare also frequently used.
Refractory materials are used extensively in the metal industries, along with glass melting and other heat treatment operations.
There are two common forms of refractories, bricks and monolithics. Bricks (also known as
firebrick) 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 sinteredbefore use. Castable refractory cementis also commonly used.
Anti-abrasion refractory attached to pipes, chambers, etc, will require anchorage systems such as wire formed anchors or
hexmetalto support the refractory linings.
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.
* [http://www.refractoriesinstitute.org/ The Refractories Institute]
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