Synthetic abrasives

Synthetic abrasives

1. Silicon carbide (or) Carborundum (SiC)
Manufacture
Silicon carbide is manufactured by heating sand (60%) and coke (40%) with some saw-dust and a little salt in an electric furnace to about 1500 deg C.
Saw- dust evolves gases during burning which on circulation, increases the porosity of the charge. Salt reacts with iron and other similar impurities present in the raw materials, forming volatile chlorides. This also increases the porosity of the final product.
1500oC
SiO2 + 3C --------> SiC + 2CO

The silicon carbide, removed from the furnace, is then mixed with bonding agent (like clay, silicon nitride) and than shaped, dried and fired.

Properties
1. Silicon carbide possess a high thermal conductivity, low expansion and resistance to abrasion and spalling.
2. They are mechanically strong and with stand loads in furnaces upto 1650oC.
3. Heat conductivity of SiC is intermediate between metals and ceramic materials.
4. They are electrivity intermediate between conductors ans insulators.
5. The strength, density, abrasion-resistance, chemical resistance softening temperature of the various bonded refractories will be in the following order.

Self – bonded product > silicon nitride product > clay – bonbed product.

Uses
1. Silicon carbides are used as heating elements in furnace in the from of rods and bars.
2. They are also used for partition walls of chamber kilns, coke ovens muffle furnace and floors of heat-treatment funaces.
3. SiC bonded with tar are excellent for making high conductivity crucibles.

2. Norbide (or) Boron Carbide (B4C)

Manufacture
It is prepared by heating a mixture of boron oxide (B2O3) and coke (carbon ) in an electric furnace to about 2700oC.
2700oC
2 B2O3 + 7C ------> B4C + 6CO

Properties
1. Its hardness is 9 on moh’s scale.
2. It is light weigt and blackcoloured compound.
3. It is highly resistant to chemical attack and erosion.
4. It resists oxidation much better than diamond.

Uses
(i) It is used as hard materials for cutting and sharpening hard high- speed tools.
(ii) It is used to prepare scratch and wear resistant coatings.

Alundum (Al2O3)
Manufacture
It is prepared by heating a mixture of calcined bauxite, coke and iron in an electric furnace to about 4000oC. It is an artifical corundum. It is hard as carborundum but is less brittle tougher.

Uses
(i) It is used in grinding of hard steels and other materials of high tensile-strengths.
(ii) It is also used in the manufacture of abrasive wheels.