In 1891, Mr. Acheson had discovered silicon carbide(Carborundum). It is a man-made artificial abrasive. It is formed from two main raw materials,
- Silica obtained in sand
- Carbon in the form of coke.
Its manufacturing is done by subjecting silica and carbon to high temperatures in sophisticated furnaces designed by thermal processing technology. The word carborundum was named to the fusing process of corundum that occurs between carbon and silica.
Silicon Carbide or Carborundum manufacturing description:
Chemical formula for silicon carbide is SiC. Sand is selected as raw material because it is rich source of silica, having about 98-99.5% composition. Sand is fed to a special type of resistance furnace that works on the principle of indirect heating. It is designed in such way that cast iron pieces are lined with the supportive firebricks. The solid materials of sand and coke are placed on its permanent bed. Electrodes made of carbon are used for producing the heat in 30 to 50 ft height furnace. Movable side stream help to charge the material and to remove the formed product after the process.
The raw material composition charged to the resistance furnace would be around 53.5% SiO2, 5% sawdust, 40% coke (amorph) and finally some traces of salts. Active volume of the furnace is filled completely with the raw material. The purpose of adding sawdust is that the product porosity is based on the sawdust composition. Similarly, salt has a function of flux formation. The temperature about 2200 oC is maintained at the core of the furnace by the iron and carbon electrodes with 6000 amps at 250 volts. The operation continued about 36 hours. After completion of the reaction, the product drawn out and then cooled for 24 hours gradually to form the silicon carbide crystal in a structured manner. The, so formed product of silicon carbide treated to use it as abrasive. Sulphuric acid treatment and than sodium hydroxide treatment are done for purification. The crystal material dried in kiln. By the method of screening and sieving different grades are named based on the uniformity of silicon carbide crystals.
Process bottlenecks in carborundum manufacturing:
The temperatures of the process should not exceed 2200oC. If so, this may cause graphite formation due to decomposition of silicon carbide (by the process of volatilization of silicon).
The chemical reaction for carborundum manufacturing:
- SiO2 + 2C → Si +2CO + 144.8 KCal
- Si + C → SiC – 30.5KCal
The final reaction obtained by adding above to simultaneous reaction that occurs in the core of the furnace.
- SiO2 + 3C → SiC + 2CO +114.3 KCal
Properties and uses of carborundum:
It is semiconductor, nonreactive to chemicals at normal temperature but reacts at higher temperatures. It has high thermal conductivity. It has the value of hardness property near to diamond. It is used to make grinding stones, cutting edge tools, crushers, grinding machines. It is widely used for making crucibles. It is used in high technology gadget that participates in fabricating automobile and heavy machinery parts, aeroplanes and engine parts.