FIRE OR REFRACTORY BRICKS

These bricks are made from fire-clay. Fire-clay is a refractory clay which is capable of resisting a high temperature without being melted or softened. It is used in the manufacture of fire-bricks, crucibles, lining materials for furnace, hollow tiles etc. Refractory material is able to stand a high temperature without losing its shape. The constituents of good fire-clay are-

             Alumina-    25-35%

             Silica-         75-65%

Impurities such as lime, magnesia, iron oxide and alkalies should not exceed 5 percent.

Fire-clay are classified into the following categories-

                                                   Temperature Range
1. High Duty fire-clay:               1482°C to 1648°C

2. Medium duty fire-clay:          1315°C to 1482°C

3. Low duty fire-clay:                 upto 870°C

The manufacturing process of fire-bricks is same as the ordinary clay bricks. The burning and cooling of these bricks are done gradually.  Fire-bricks are usually white or yellowish in colour and its weight is about 30 to 35 N. These bricks can resist high temperature without softening or melting. The compressive strength of fire-bricks varies from 200 to 220 N/mm2 and percentage of water absorption varies from 5 to 10%. Fire-bricks are used for lining of interior surface of furnaces, chimney, kilns, ovens, fireplaces etc. Following are the three varieties of fire or refractory bricks-

1. Acidic Bricks

2. Basic Bricks

3. Neutral Bricks 

1. ACIDIC BRICKS: These bricks are used for acidic lining.

(a) Ordinary Fire- Bricks: These bricks are prepared from natural fire-clay and they provide a good material for acidic refractory lining. 

(b) Silica Bricks: They contain very high percentage of silica about 95 to 97 percent. A small quantity of lime about 1 to 2 percent is added to work as binding material. Silica bricks are moulded under pressure and burnt at very high temperature. The compressive strength of silica bricks is about 15 N/mm2. These bricks can stand a temperature upto 2000°C.

2. BASIC BRICKS: These bricks are used for basic lining. Refractory material are used for manufacture of these bricks.  

Ex. Magnessia bricks, Bauxite bricks

3. NEUTRAL BRICKS: These bricks are used for neutral lining. They offers resistance to corrosive action of slag and acid slags.

(a) Chromite Bricks: These bricks are prepared from a mixture of chrome, iron ore, ferrous oxide, bauxite and silica. These bricks are unaffected by acidic and basic actions.

(b) High Alumina Bricks: These bricks contain higher percentage of alumina and they are found to be more inert to the slags.