Refractory

They are typically composed of oxides, carbides or nitrides of the following elements: silicon, aluminium, magnesium, calcium, boron, chromium and zirconium.

Refractories are defined by ASTM C71 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)".

The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories.

[7] 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 would oxidize and burn.

In the metallurgy industry, refractories are used for lining furnaces, kilns, reactors, and other vessels which hold and transport hot media such as metal and slag.

Refractories have other high temperature applications such as fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, utility boilers, catalytic cracking units, air heaters, and sulfur furnaces.

They are stable to alkaline materials but can react to acids, which is important e. g. when removing phosphorus from pig iron (see Gilchrist–Thomas process).

The main raw materials belong to the RO group, of which magnesia (MgO) is a common example.

For the first half of the twentieth century, the steel making process used artificial periclase (roasted magnesite) as a furnace lining material.

Standard shapes have dimensions that conform to conventions used by refractory manufacturers and are generally applicable to kilns or furnaces of the same types.

There are ranges of standard shapes of different sizes manufactured to produce walls, roofs, arches, tubes and circular apertures etc.