Silica is the true glass-forming compound. It has a high melting point of 1700-centigrade creating difficulty in work. Thus in the manufacture of glass certain fluxes, chiefly soda ash and lime are added. The composition of quartz varies from the type of glass produced. The silica sand of purity 99.9% and iron F2O3 should be less then 0.008% in optical glass and 0.013% in the case of colorless glass. At low concentration iron gives green color whereas at high concentration it gives brown color.
In the Ceramic industry pure silica sand in fine mesh is used in the preparation of glazing material along with china clay, feldspar etc. It is used to increase the whiteness and to prevent distortion during firing. It is invariably used after calcinations at about 900 centigrade when it turns white from black.
Glass sand, quartz free from organic and clay impurities is used in the manufacture of sand paper, abrasive cloth, etc.
It is important gumming material produced by fusing silica sand with sodium carbonate at temperature ranging from 1200 to 1400 centigrade. The quartz should be crushed to size of 20 – 100 mesh, SiO2 should be 99%, Al2O3 less than 0.25%, Fe2O3 less than 0.03 and CaO + MgO less than 0.5%.
It is prepared by the chlorination of silicon, Ferro silicon or silicon carbide. It is used as a modifying agent in plastics and as a drying material in oil paint.
Activated Silicon or Silica Gel
It is precipitated silica prepared from a dilute solution of neutral sodium silicate. It is used as moisture absorbent or desiccants particularly in gas drying plants. It is also used as food preservatives.
It is used in the manufacture of high silicon steels. Ferro silicon is prepared from high purity quartz and iron scraps in the electrical furnaces. For one ton of Ferro silicon containing 75% silicon requires 2 tons of q, 1.1 tons of coke, 0.25% of iron in addition to 55 kgs. of carbon electrodes