What are Refractories?

Refractories are high temperature resistant materials. Refractory materials can retain their strength at high temperatures. They are used to make crucibles and are used in linings for high-temperature industrial furnaces, kilns and reactors and other processing units.

In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also resist abrasion wears and erosion by chemical agents. Refractory materials must be strong at high temperatures, resistant to thermal shock, chemically inert, and have low thermal conductivities and coefficients of expansion. Refractories are more heat resistant than metals and are required for heating applications above1000F(538C).

Refractories are produced from natural and synthetic materials, usually nonmetallic, or combinations of compounds and minerals such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, zirconia, and others. Aluminium oxide (Al2O3), Magnesium oxide (MgO) and Silicon oxide (SiO2) are the most important refractory materials, though fireclay is widely used as well. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide is another refractory material. It is very strong at high temperatures, but will burn in the presence of oxygen, if the protective silica coating comes off.

Refractories must be chosen according to the temperature inside the unit and the chemical nature of the material being processed. For example, carbon cannot be used when it will be in contact with oxygen, as it will burn. Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in acidic environment because they will be eroded.

Zircon, fireclay and silica are acidic. They are generally not attacked or affected by acidic materials, but easily affected by basic materials. At high temperatures, acidic refractories may also react with limes and basic oxides. Dolomite and magnesite are basic. These are used on areas where slags and atmosphere are basic; they are stable to alkaline materials but could react with acids. Alumina, chromite, silicon carbide, carbon and mullite are neutral. These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases.

Refractories are made in varying combinations and shapes depending on their applications. Refractories are widely used in the blast furnaces, hot stoves, and open-hearth furnaces, cement kilns, glass furnaces, nonferrous metallurgical furnaces, ceramic kilns, steam boilers, and paper plants. Special types of refractories are also used in rockets, jets, and nuclear power plants.