Write a Note on Synthetic Abrasives

SYNTHETIC ABRASIVEs

(i)Silicon carbide (SiC) or Carborundum

Manufacture:

Silicon carbide is manufactured by heating together a mixture of 52%-54% of sand (SiO₂) + 35% of Coke (C) + 7%-11% of sawdust + 1.5%-4% of Salt, in an electric furnace at around 1300^°C- 220^°C

SiO₂ + 3C → SiC +2CO

The carborundum thus formed has the following composition:

Silicon = 65% ; Carbon = 30% ;Impurities =5%

Properties of SiC

1. Hardness of silicon carbide lies in between corundum and diamond in Mohs scale.
2. It is very hard and chemistry inactive.
3. It can withstand high temperatures without any damage.
4. It is brittle in nature.
5. It has high thermal conductivity.
6. It is electrically intermediate between conductors and insulators.

Uses:

1. In the manufacture of grinding machines, grinding stones, crushers since SiC is as hard as diamond.
2. For giving edge to cutting tools (razors, knives, swords etc. ).
3. In the manufacture of carbon rods in resistance heaters.
4. It is used in electric heater.
5. In the manufacture of abrasive papers and cloths.

(ii)Boron carbide (B₄C ) or norbide:

Manufacture:

It is manufactured by heating boric oxide with coke in an electric furnace to about 2700^°C.

2B₂O₃ +7C → B₄C + 6CO

It is inert and one of the hardest (Mohs scale = 9.5-9.7) of all artificial abrasives.

Properties:

1. It is a light- weight and black-coloured compound.
2. Its hardness is 9 on Mohs scale.
3. It is highly resistance to chemical and erosion.
4. It resists oxidation much better than diamond.

Uses:

1. It is used for cutting steel.
2. It is used in the nozzles for sand blasting.
3. On the hard materials for making grinding dies.
4. For cutting and sharpening of hard high speed tools.

(iii)Alundum(Al₂O₃)

Manufacture:

It is prepared by heating a mixture of calcined bauxite , coke and iron in an electronic furnace at about 4000^°C.

4Al + 3O₂ → 2Al₂O₃

It is considered as artificial corundum(Al₂O₃).

Properties:

1. It is not as hard as carborundum but is less brittle and tougher.
2. It is stable at high temperatures.
3. It is very hard (9 in Mohs scale).
4. It is resistant to hydration and to attack by acid.

Uses:

1. It is used in the grinding of hard steels.
2. It is used in the manufacture of abrasive wheels.

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