Refractory bricks.

Refractory bricks
Some important refractory bricks

Alumina bricks (or) Fire clay bricks (Acidic refractories)
Alumina bricks contain 50% or more of Al2O3. They are generally manufactured by mixing calcined bauxite (Al2O3 ) with clay binder.

Manufacture
1. Grinding and mixing
The raw materials (calcined bauxite & SiO2 ) and grog (calcined fire clay ) are ground to fine poeder and mixed and mixed with required amount of water to convert it into pasty material.

2. Moulding
The pasty material is converted into bricks by the general moulding technique like machine pressing or slip casting.

3.Drying and Firing
The bricks after moulding is dried slowly to remove the moisture and then fired in continuous kiln or tunnel kiln to about 1200-1400 for 6-8 days.

Properties
(i) Alumina bricks are acidic refractories.
(ii) They posses very low coefficient of expansion.
(iii) They also posses high porosity, and high temperature load-bearing capacity.
(iv) They are also very stable to both in oxidizing and reducing conditions.
(v) Then posses better resistance to thermal to thermal spalling than silica bricks.

Uses
1. Medium-duty bricks (containing 50 to 60% Al2O3)
It is used in lining of cement rotary kilns soaking pita, reheating furnaces, hearts and walls, etc., which are subjected to high abrasion.

2. High-dury bricts (containing 75% Al2O3)
It is used in hottest zone of cement rotary kilns, lower parts of soaking pits, brass melting reverberatories, aluminium melting furnaces, etc.,

3. Fire clay refractories are larely using in steel industries
Magnesite bricks (Basic refractories )
Mangesite bricks contain mainly MgO. They are generally manufactured by mixing calcined magnesite with caustic magnesia or iron oxide as binding material.

Manufacture
1. Grinding and mixing
The raw materials (calcined magnesite) and binding materials (caustic magnesia) are ground to fine powder and mixed with water to a pasty material.

2. Moulding
Moulding is usually done by machine pressing to a required shape.

3. Drying and firing
Drying is carried out at ordinary temperature to remove the moisture. Firing is done in a kiln at 1500oC for 8 hours and then cooled slowly.

Properties
1. Magnesite bricks are basic refractories.
2. They can be used upto 2000oC without load and upto 1500oC Under a load of 3.5 kg/cm2.
3. They have good resistance to basic slags, but combine with H2O and CO2
4 . They possess good strength, little shrinkage and have lot of spalling.
5. They have poor resistance to abrasions.

Uses
1. They are used where high temperature is required to be maintained, together with great resistance to basic materials.
2. They are used in steel industry for the lining of basic converters and and open –hearth furnaces.
3. They are also used in hot mixer linings, copper converters and reverberatory furnaces.

Zirconia bricks ( Neutral refractories)
Manufacture
They are prepared by mixing zirconite mineral (ZrO2 ) with colloidal zirconia or alumina as binder and finally heated to 1700oC. Small amount of MgO or Cao is added as stabilizer because mineral zirconite undergoes volume changes on hearting and cooling.

Properties
1. Zirconia bricks are neutral refractories.
2. Though zirconia bricks are neutral, they are affected by acidic slags.
3. They can be used upto 2000oC and upto 1500oC under a load of 3.5 kg/cm2 .
4. They are also quie resistant to thermal shocks.
5. Their thermal expansion is low.

Uses
They are used only where very high temperature is maintained, e.g., high – frequency electric furnaces.