A fire brick or refractory brick is a block of refractory ceramic material used in lining kilns, fireboxes, furnaces, and fireplaces. Fire brick is designed mainly to withstand high heat, but should also usually have a very low thermal conductivity to save energy. Usually, dense bricks are used in applications with extreme thermal, mechanical, and chemical stresses, such as the inside of a furnace, wood-fired kiln, which is subject to high temperatures, fluxing from ash, or slag, abrasion from wood.
Dense fire bricks have a very high thermal mass which is sometimes a useful property in that they will retain heat e.g. in a pizza oven after the fire has been taken out. Kiln bricks are made by firing a base composition of clay in the kiln until it is partly vitrified and for special purposes may also be glazed.
Refractory or fire bricks usually the primary raw material is usually chamotte with other materials and contain 30-40% alumina. The maximum service temperature increases with kiln bricks can be obtained with an alumina content of 80% and above and increasing alumina content.
Less dense fire or refractory bricks
In other less strong situations such as more porous bricks are a better choice, natural gas-fired kilns, usually referred to as Insulation Bricks. They are weaker, easier to form, and insulate far better than dense bricks but they are much lighter. In this case, they have a low thermal mass and we can’t be used to store heat. Insulation bricks have a better thermal shock resistance than dense refractory bricks but the main disadvantage is their very low strength.
The insulation property of these bricks usually comes from vermiculite or perlite. As with the dense refractory or fire bricks, there is a range of grades corresponding to different maximum service temperatures. The most common used Grade 23 – 1260oC.
Composition of Fire Clay
Fire clay should contain a higher percentage of alumina and ‘non-detrimental’ silica. They should have a very small quantity of detrimental magnesia, iron oxide, lime, and alkalis.
becomes a glassy substance at about 3200℉ and Silica (SiO2) becomes soft at about 2800℉ and finally fuses. silica melts around 3300℉. This high melting point and softening have established it as the principal material for the production of refractory bricks.
Alumina (Al2O3) has even fusion temperature and higher softening than silica. alumina melts at about 3800℉. alumina, therefore used in combination with silica.
- Lime, Magnesia, Iron oxide, and Alkalis:
The presence of these detrimental components acts as fluxes to fusion temperature and lower the softening.
Table 1. Approximate Composition of Fire Clays
|Lime, Magnesia, Iron oxide, and Alkalis
Manufacturing Process of refractory Brick
The manufacturing process of a refractory brick is similar to ordinary bricks. The usual steps are
Refractory bricks are produced from fire clay. The brick burnings are carried out in a superior type of kiln under the carefully graduated temperature control. Processes of cooling and burning are kept rather slow.
Types of Fire Bricks
There are mainly three types of refractory bricks.
1. Acid Refractories
- Ganister Bricks
- Ordinary Fire Bricks
- Silica Bricks
2. Basic Refractories
- Dolomite Bricks
- Bauxite Bricks
- Magnesite Bricks
3. Neutral Refractories
- Chrome Magnesite Bricks
- Spinal Bricks
- Forsterite Bricks
- Chromite Bricks
Briefly of these refractory brick types are given below.
i) Ordinary Fire Bricks
1. Made of natural fire clay with a mixture of crushed sand and clay
2.Contains 20-40% alumina and 50-75% silica with fluxing matter below 10%
3. Used as good quality acid fire lining to furnaces
4. Resists up to 1610℃
ii) Silica Bricks
- silica Contains 95%.
- Manufactured from quartzite or sandstones with an addition of 2-3% of lime to act as a flux and bind material.
- Suitable for the acid lining of furnaces for producing glass for the arches and walls of basic open-hearth steel furnaces copper melting furnaces.
- Resists up to 2010℃.
iii) Ganister Bricks
- Ganister Bricks are made from the siliceous types of ganister rock. This rock 10% clay with an addition of 2% of lime and containing 85% of silica.
- Ganister brick Very hard, rigid and compact at a high temperature of 2100℃
i) Magnesite Bricks
- Manufactured from fire clay containing 85% of magnesium oxide, the balance lime and alumina, and 3-5% iron oxide.
- Used as the basic lining of furnaces
- Basic refractories of Resists up to 1800℃-2100℃
ii) Dolomite Bricks
- Manufactured from the fire clay containing is a very high percentage of dolomite
- Dolomite bricks Cheap substitutes for magnesia bricks in furnace lining. Serpentine is used to improve the strength of dolomite brick.
- Dolomite brick of Resists temperature up to 1400℃-1600℃
iii) Bauxite Bricks
- Manufactured from fire clay containing 86% Bauxite bricks.
- Bauxite bricks Resist up to 1600℃.
They are used to separate the basic linings and acid of a furnace so that they cannot react with each other. Neutral refractories are not very important as they are not used for fire-resisting purposes.
i) Chromite Bricks
- Made from 30% iron oxide (Ferrous oxide), 20% bauxite with little silica, and fire clay containing 50% of chrome iron ore.
- Chromite Bricks Resists temperature up to 2000℃
- A special type of natural fire bricks
- Used in electric furnaces for the carborundum
- Carborundum Resists temperature up to 1900℃
iii) Chrome Magnesite Bricks
They are made from a mixture of magnesite and chrome.
iv) Spinal Bricks
Spinal bricks are manufactured from a mixture of magnesia and alumina taken in equal proportions.
v) Forsterite Bricks
forsterite bricks are manufactured from magnesium silicon oxide mineral ( 2 MgO SiO2).