Refractory bricks are also known as firebrick or fire brick.
Refractory bricks or fire bricks are the type of bricks which can withstand high temperature as a construction material. The color of fire bricks is usually white, or yellowish white, and they weigh about 3 kg each. They must have a high fusion point.
Fire clay is used for making this type of bricks. This clay can resist high temperature without undergoing any appreciable change in structural properties like melting or softening.
Composition of Fire Clay
Fire clay should contain a higher percentage of ‘non-detrimental’ silica and alumina. They should have a very small quantity of detrimental lime, magnesia, iron oxide, and alkalis.
- Silica: Silica (SiO2) becomes soft at about 2800℉ and finally fuses and becomes a glassy substance at about 3200℉. It melts around 3300℉. This high softening and melting point has established it as the principal material for the production of refractory bricks.
- Alumina: Alumina (Al2O3) has even higher softening and fusion temperature than silica. It melts at about 3800℉. It is, therefore, used in combination with silica.
- Lime, Magnesia, Iron oxide and Alkalis: The presence of these detrimental components acts as fluxes to lower the softening and fusion temperature.
Table 1. Approximate Composition of Fire Clays
| Components | Percentage |
|---|---|
| Silica | 60-96 |
| Alumina | 2-36 |
| Lime, Magnesia, Iron oxide and Alkalis | 2-5 |
Manufacturing Process of Fire Brick
The manufacturing process of fire brick is similar to ordinary bricks. The usual steps are
- Digging
- Weathering
- Tempering
- Moulding
- Burning
Fire bricks are produced from fire clay. The burnings are carried out in a superior type of kiln under carefully graduated temperature control. Processes of burning and cooling are kept rather slow.
Types of Fire Bricks
There are mainly three varieties of refractory bricks.
- Acid Refractories
- Ordinary Fire Bricks
- Silica Bricks
- Ganister Bricks
- Basic Refractories
- Magnesite Bricks
- Dolomite Bricks
- Bauxite Bricks
- Neutral Refractories
- Chromite Bricks
- Carborundum
- Chrome Magnesite Bricks
- Spinal Bricks
- Forsterite Bricks
Brief descriptions of these refractory brick types are given below.
Acid Refractories
i) Ordinary Fire Bricks
- Made of natural fire clay with a mixture of crushed clay and sand
- Contains 50-75% silica and 20-40% alumina with fluxing matter below 10%
- Used as good quality acid refractory lining to furnaces
- Resists upto 1600℃
ii) Silica Bricks
- Contains 95% silica
- Manufactured from sandstones or quartzite with an addition of 2-3% of lime to act as a bind material and flux
- Suitable for
the acid lining of furnaces for producing glass
for the walls and arches of basic open-hearth steel furnaces
copper melting furnaces
- Resists upto 2000℃
iii) Ganister Bricks
-Ganister Bricks are made from the siliceous variety of ganister rock. This rock containing 85% of silica and 10% clay with an addition of 2% of lime
- Very hard, compact and rigid at a high temperature of 2100℃
Basic Refractories
i) Magnesite Bricks
- Manufactured from fire clay containing 85% of magnesium oxide,3-5% iron oxide, and the balance lime and alumina
- Used as the basic lining of furnaces
- Resists upto 1800℃-2100℃
ii) Dolomite Bricks
- Manufactured from the fire clay containing a high percentage of dolomite
- Cheap substitutes for magnesia bricks in furnace lining. Serpentine is used to improve their strength
- Resists temperature upto 1400℃-1600℃
iii) Bauxite Bricks
- Manufactured from fire clay containing 86% Bauxite
- Resists upto 1600℃
Neutral Refractories
They are used to separate the acid and basic linings of a furnace so that they cannot react with each other. They are not very important as they are not used for fire resisting purpose.
i) Chromite Bricks
- Made from fire clay containing 50% of chrome iron ore, 30% iron oxide (Ferrous oxide) and 20% bauxite with little silica
- Resists temperature upto 2000℃
ii) Carborundum
- A special type of natural refractory bricks
- Made from fire clay containing a higher percentage of silicon carbide
- Used in electric furnaces
- Resists temperature upto 1900℃
iii) Chrome Magnesite Bricks
They are made from a mixture of chrome and magnesite.
iv) Spinal Bricks
These bricks are manufactured from a mixture of alumina and magnesia taken in equal proportions.
v) Forsterite Bricks
These bricks are manufactured from magnesium silicone oxide mineral ( 2 MgO SiO2)
Laying of fire bricks
Fire clay mortar is used for laying of fire bricks, not lime or cement mortar. Generally, 1.5 to 2 cft of fire clay mortar is required for laying 100 nos of standard ordinary size fire bricks.