Classification of Refractories

Refractories can be classified on the basis of chemical composition and use and methods of manufacture as shown below:

Classification based on Chemical composition	Examples
ACID which readily combines with bases.	Silica, Semisilica, Aluminosilicate.
BASIC which consists mainly of metallic oxides which resist the action of bases.	Magnesite, chromemagnesite, Dolomite.
NEUTRAL which doesn't combine; neither with acids nor bases.	Chrome, Pure. Alumina
Special	Carbon, Silicon Carbide, Zirconia.
Classification based on end use	Blast furnace Casting Pit
Classification based on method of manufacture	•Dry Press Process •Fused Cast •Hand Moulded •Formed Normal, fired or Chemically bonded.) •Unformed (Monolithics – plastics, Ramming Mass, Gunning Castable, Spraying.)

Mineral-based refractories are classified according to their chemical composition:

i. Acid bricks contain at least 92%~ silicon oxide (SiO₂);

ii. Semi-basic bricks contain at least 65% silicon oxide. but less than 30% alumina (A1₂O₃);

iii. Neutral bricks contain at least 30% alumina;

iv. Basic bricks contain at least 60% magnesium oxide (MgO).

v. Synthetic refractories e.g. silicon carbide are produced by melting and casting processes.

The structure of the furnace consists mainly of refractory bricks and cement, which must be able to withstand the high furnace temperatures and must be carefully selected and constructed. The furnace structure may contain monolithic refractories, which can be shaped in situ, e.g. those used for burner quarls. There are three basic types of monolithic refractories:

· Castables;

· Mouldables;

· Ramming mixtures

Different furnace zones normally operate at different temperatures. The correct selection

of refractory materials for the various parts of the furnace and for various components e.g. hearths, walls, etc, is important. This process is governed not only by properties like thermal conductivity, expansion, etc, but also by the experience of the furnace designer or builder.

The hearth is the most important and the most severely treated region of a furnace. It should be able to bear the required load and withstand chemical attack and mechanical wear. The selection of hearth refractories is less critical for top and bottom fired furnaces, than for top fired only pusher types.

For optimum strength and thermal insulation, the walls, roof and hearth of most furnaces are constructed using layers of refractory materials. Thermal insulation is determined by the thermal properties of the refractory, and these properties are important in minimising transmission and storage heat losses. Table 5.5 compares the thermal properties of typical high density and low density refractory materials. Structural heat losses can be reduced by using low thermal mass refractory materials in the construction of the furnace.

TABLE 5.5 TYPICAL REFRACTORY PROPERTIES
Property	High Thermal Mass(High Density Refractories)	Low Thermal Mass (Ceramic Fibre)
Thermal Conductivity, W/m K	1.2	0.3
Specific Heat, J/kg K	1000	1000
Density ,kg/m3	2300	130