Refractories

Refractories are a type of ceramic material that can withstand high temperatures under various environmental conditions. Some refractories such as carbides, nitrides, and oxides (SiO2, Al2O3, SiC, and MgO) having a high melting point and wear resistance are used in extractive metallurgy, cement kilns and glass manufacturing industries as protected lining materials.

Characteristics of refractories

The salient characteristics of refractories are as follows:­

• Capacity­ to­ withstand­ high­ temperatures­ without­ melting­ or decomposing.
• Remains­ unreactive ­and ­inert ­when ­exposed­ to ­severe ­environments.
• Provide ­thermal ­insulation.
• Low­ thermal ­expansion ­or ­contraction.
• Low­ electrical­ conductivity.
• Long ­life ­without­ cracking ­or ­spalling­.
• Impermeable ­to ­liquids­ and­ gases.­

Classification of Refractories

The classification of refractories is discussed as follows:

Based on Fusing/Melting Temperature

Low-temperature refractories	The refractories with melting temperatures between 1,500°C and 1,700°C are called as low-temperature refractory materials.
High-temperature refractories	The refractories with melting temperatures between 1,700°C and 2,000°C are considered as high-temperature refractory materials.
Super-temperature refractories	The refractories with melting temperatures more than 2,000°C are considered as super-melting refractory materials.

Based on Nature of Reaction in lining for a furnace

Acidic refractories	Acidic refractories are those which are not attacked or affected by acidic materials but easily attacked by alkalis (basic slags). The acidic refractories contain acidic oxide (Ex: SiO2 and Al2O3) in nature.
Basic refractories	Basic refractories contain basic oxides (Ex: MgO, CaO, etc) in nature. These refractories have high refractoriness (about 1,800°C) and poor resistance and a high coefficient of thermal expansion.
Natural refractories	Neutral refractories are inert to both acid and basic materials. Examples of neutral refractories are carbon, carborundum, and chromate.

Based on Chemical Compositions

Aluminous refractories	Fire clay refractories such as fire bricks, chamotte, and aluminous clay are all aluminosilicates. Fire clays contain more silica (50%–80%) and have less alumina.
Siliceous refractories	The refractory materials of this class are made up of fine-grained quartz and contain more than 93% of SiO2. They are relatively lighter (density 1.8 g/cm2) and are also not deformed under load at high temperatures.
Insulating refractories	Insulating refractories prevent the flow of electric current through them and are used to prevent heat loss from the furnace by conduction.

Based on the Nature of Materials

Special refractories	Special refractories are referred as technical ceramics, high-temperature ceramics, and high-performance ceramics. These may be oxide refractories or non-oxide refractories or graphite.
Pure oxide refractories	Pure oxide refractories, referred to as the pure alumina (corundum) refractories, are usually fabricated by fusion in an electric arc furnace and the melt is poured into suitable moulds.
Non-oxide refractories	Non-oxide refractories include zircon, carbides, nitrides, and borides. Zircon has a high melting point of 2,200°C, low thermal expansion, good spalling resistance, and good resistance to abrasion.
Glass	Glass can be drawn into tubes, rods, blowing, rolling, etc. Optical properties determine the suitability for making glass for the optical system. Similarly, conductivity, dielectric strength, and dielectric losses determine the suitability of glass for manufacturing incandescent lamps, radio valves, x-ray tubes, etc.

• Jindal, U. C. (2012). Material Science and Metallurgy. Pearson Education India.