Silica

Silica is the name given to a group of minerals composed of silicon and oxygen, the two most abundant elements in the earth’s crust. In spite of its simple chemical formula, SiO2 , silica exists in many different forms. Silica is found commonly in the crystalline state but occurs also in an amorphous state resulting from weathering or plankton fossilisation. Silica exists in ten different crystalline forms or polymorphs, quartz being by far the most common. Quartz is the second most common mineral on the earth’s surface and it is found in almost every type of rock, i.e. igneous, metamorphic and sedimentary. Since it is so abundant, quartz is present in nearly all mining operations. It occurs in the host rock, in the ore being mined, as well as in the soil and surface materials above the bedrock, which are called the overburden. Industry mainly uses the crystalline forms of silica, i.e. quartz and cristobalite. Both are sold as sand, which is a granular material with particles greater than 0.063 millimetres, or as flours that consist of finer particles. Silica is hard, chemically inert and has a high melting point because of the strength of the bonds between the atoms. These are prized qualities in various industrial uses. Quartz is usually colourless or white but is frequently coloured by impurities such as iron. Quartz may be transparent to translucent, hence its use in glassmaking, and has a vitreous lustre. Depending on how the silica sand was formed, quartz grains may be sharp and angular or rounded. For industrial use, pure deposits of silica capable of yielding products of at least 98% SiO2 are required. Silica sand may be produced from sandstone, quartzite and loosely cemented or unconsolidated sand deposits. High grade silica is normally found in unconsolidated deposits below thin layers of overburden. It is also found as ‘veins’ of quartz within other rocks and these veins can be many metres thick. Silica sand deposits are normally exploited by quarrying and the material extracted may undergo considerable processing before sale. The objectives of processing are to reduce impurities and increase the grade of silica present and to produce the optimum size distribution of the product depending upon end use. After processing the sand may be sold in the moist state or it may be dried. Dry grinding in rotary mills, using beach pebbles or alumina balls as grinding media is the most common way to produce silica and cristobalite flour. Since the natural resources of cristobalite are not sufficient for industrial use, it has to be synthetised by the conversion of quartz in a rotary kiln at high temperature (>1500°C) with the assistance of a catalyst.