Zircon
Zircon containing 64% ZrO is used generally for foundry applications.
Zircon's exceptional qualities of hardness and durability makes it a must-use for the manufacture of ceramics and refractory tiles and also for a range of other high-tech applications such as armour plating on military aircraft, heat shield in space shuttles and potentially as solid oxide fuel cells in hydrogen powered vehicles and in many industrial and chemical applications. Owing to its chemical inertness, very low heat conductivity, high specific gravity, low expansion, good resistance to abrasion, high melting point and no shrinkage on being heated up to 17500C, zircon is found to be an outstanding refractory material. In foundry industry, zircon is used as facing for foundry moulds as it increases the resistance to metal penetration and affords a uniform finish to castings. Zircon sand is preferred to silica sand because of its uniform size, higher melting point, low thermal expansion and resistance to molten metal, acidic chemicals, slag, etc.
Industrial Applications
Zircon, baddeleyite or manufactured zirconia are all useful refractory materials. They are valued for the preparation of special moulds and refractory bricks. High-grade zircon melts at about 2190ºC, softens between 1600ºC and 1800ºC and shows little shrinkage upto 1750ºC. Zircon exhibits many characteristics that make it very suitable for super refractory purposes. In addition to a high melting point it was very low thermal expansion and good resistance to abrasion. Zircon is used when acidic refractory is required while zirconia refractories are considered to be basic. A cheaper type of zircon-refractory is prepared with mixing zircon with alumina. It is reported to have refractory properties as good as those of zircon. At high temperatures the following reaction takes place:
2ZrSiZO4 + 3Al2O3 = 2ZrO2 + 3Al2O3. 2SiO2
The dissociation of zircon starts at 1400ºC and at about 1600ºC a complete reaction takes place forming a stable product of zirconia and mullite. The high grade zircon sand is largely used in foundry as mould. It does not wet the molten metal, thus giving a better and smoother surface to the casting. It is estimated that about 75% of the total world zircon production is used as moulding sand and in the manufacture of refractory bricks, zirconium compounds and ceramics. Only 25% is used for the extraction of metal or in the manufacture of alloys.
Prepared zirconia and also zircon are extensively used in the glass and ceramic industry for opacifying enamels, porcelain and glazes.
Zircon sands bonded into bricks are also used in large quantities as refractory bricks in glass and aluminium industries. Zircon has special properties of resistance to spalling and wetting by glass and aluminium which makes it suitable for the above purpose. Finely powdered zircon and zirconia are used as an abrasive in polishing optical glasses.
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