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 made them applicable for structures, or as components of systems, that are exposed to environments above 1000 °F (800K, 500 °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 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.
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, 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 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 sintered before use. Castable refractory cement is also commonly used.
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. The anchorage used for refractory on roofs and vertical walls are more critical as they are to support the weight of refractory even at the elevated temperatures and operating conditions. The commonly used anchorages have circular or rectangular cross-section. Circular cross-section are used for low thickness refractory and they support relatively less weight per unit area; whereas the rectangular cross-section is used for high thickness refractory and can support higher weight of refractory per unit area. The number of anchors to be used depend on the operating conditions and the refractory materials. The choice of anchors shape, numbers and size has significant impact on the useful life of the refractory.
Refractory heat-up
Usually, refractories require special heat-up techniques to ensure that their performance will be attained as designed, and to avoid thermal shock and drying stresses until the operational status is achieved.
References
- ^ ASTM Volume 15.01 Refractories; Activated Carbon, Advanced Ceramics
