Classification and Characteristics of a good Refractory

Introduction

Refractories are materials which can withstand very high temperatures. They are quite hard and chemically stable.

Thus, refractories are materials which are used for the construction of furnaces, kilns, ovens, crucibles and retorts etc, on account of their high resistance to heat.
Refractories are sold in the form of fire bricks silica, magnesite, chromite bricks, silicon carbide and zirconium refractories etc.

Characteristics of a Good Refractory

A good refractory possesses the following characteristics:

It should not fuse at high temperature.
It should be chemically inert towards corrosive gases, metals and slags, produced in the furnaces.
It should have high refractoriness.
It should withstand the applied load of structure at operating temperatures.
It should have uniform expansion and contraction.
It should not get cracked at operating temperatures.

Uses of refractory

They are used in making crucible for processing materials at high temperature.
They are mostly used for the construction of lining of furnaces, kilns etc.
They are used in manufacturing cement, glass, ceramics, paper, metals etc.

Classification of Refractories

Refractories can be classified into two major types based on their physical and chemical nature. They are,
(i) Classification based on the temperature range of fusion.
(ii) Classification based on the chemical composition.

Classification based on the temperature range of fusion	Classification based on the chemical composition
(a) Refractory (1580-1780°C)	(a) Acid refractory
(b) Highly refractory(1780-2000°C )	(b) Basic refractory
(c) Super-refractory (>2000°C)	(c) Neutral refractory

Table (1) Comparison of acid, basic and neutral refractories

Acid refractory	Basic refractory	Neutral refractory
1. Acid refractories react readily with basic slags. But they are not attacked by acidic materials.	1. Basic refractories react readily with acidic slags. But they are not attacked by basic materials.	1. Neutral refractories are inert to both acidic and basic slags.
2.They consist of acidic oxides like silica (SiO₂) and alumina (Al₂O₃)	2.They consist of basic oxides like CaO, MgO etc.	2.They are made from weakly acidic/ basic materials like carbon, Zirconia(ZrO₂) and chromite (Feo CrO₂)
3.Examples: alumina, silica, and fire clay refractories.	3.Examples: magnesite, dolomite refractories.	3.Examples: graphite, zirconia, chromite and carborundum (SiC) refractories.