What are Refractory and Refractory Materials?

Refractories are inorganic non-metallic material which can withstand high temperature without undergoing chemical changes while remaining in contact with molten slag, metal and gases. They are exposed to environments above 1,000 °F. Here are some classifications of refractory materials. There as follows: ----

• Refractories can be classified on the basis of:-

(I) Chemical composition (their chemical Behavior )

2) The methods of manufacture

3 )Physical form(shaped and unshaped

Refractories)

Classification Based on Chemical

Composition

(I) Acid Refractories: Acid refractories are those which are reacted by alkalis (basic slags). These are used in areas where slag and atmosphere are acidic. Examples of acid refractories are:

Silica (SiO2), Zirconia (ZrO2).

(II) Basic Refractories:- Basic refractories are those which are attacked by acid slags but stable to alkaline slags, dusts and fumes at elevated temperatures. Since they do not react with alkaline slags, these refractories are of considerable importance for furnace linings where the environment is alkaline. for example Dolomite (CaO*MgO)

(III) Neutral Refractories: Neutral Refractories are chemically stable to both acids and bases and are used in areas where slag and atmosphere are either acidic or basic. The common examples of these materials are: Carbon graphite, Chromites (Cr2O3),Alumina.

Classification Based on Physical Form

• Shaped Refractories: Shaped refractories are those which have fixed shaped when delivered to the user. These are what we call bricks. Brick shapes maybe divided into two: standard shapes and special shapes. Standards shapes have dimension that are conformed to by most refractory manufacturers and are generally applicable to kilns and furnaces of the same type. Special shapes are specifically made for particular kilns and furnaces.

Characteristics of Good Insulating Refractory

• Low thermal conductivity

• High degree of porosity

The high porosity of the brick is created during manufacturing by adding a fine organic material to the mix, such as sawdust. During firing, the organic addition burns out, creating internal pores. Other ways to accomplish high porosity involves:

a) by using materials which expand and open up on heating;

b) by using volatile compounds like naphthalene;

c) By aeration

d) by using substances which by themselves have open texture e.g. insulating brick grog etc

e) by using foaming agents to slip

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