Silica Continue ...
Silica sand may be produced from both unconsolidated sands and crushed sandstones.The sand is a product of mechanical and chemical weathering of quartz-bearing igneous and metamorphic rocks such as granites and some gneiss. Erosion and chemical weathering break down the less stable minerals such as feldspars and release the more stable ones such as quartz and zircon. The stable mineral fragments are transported and redeposited in water. Wave and stream action may further modify the deposits by sorting and washing until a relatively pure deposit of silica sand remains.
Silica exists in nine different crystalline forms or polymorphs with the three main forms being quartz, which is by far the most common, tridymite and cristobalite. It also occurs in a number of cryptocrystalline forms. Fibrous forms have the general name chalcedony and include semi-precious stone versions such as agate, onyx and carnelian. Granular varieties include jasper and flint. There are also anhydrous forms - diatomite and opal.
Quartz is the second most common mineral in the earth's crust. It is found in all three of the earths rock types - igneous, metamorphic and sedimentary. It is particularly prevalent in sedimentary rocks since it is extremely resistant to physical and chemical breakdown by the weathering process. Since it is so abundant, quartz is present in nearly all mining operations. It is present 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. Most of the products sold for industrial use are termed silica sand. The word "sand" denotes a material whose grain size distribution falls within the range 0.06-2.00 millimetres. The silica in the sand will normally be in the
crystalline form of quartz. For industrial use, pure deposits of silica capable of yielding products of at least 95% SiO2
are required. Often much higher purity values are needed. Silica sand may be produced from sandstones, 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. On occasions, extremely high purity quartz in lump form is required and this is produced from quartzite rock. Silica is usually
exploited by quarrying and it is rare for it to be extracted by underground mining.
Physical and chemical properties
The three major forms of crystalline silica -quartz, tridymite and cristobalite- are stable at different temperatures and have subdivisions. For instance, geologists distinguish between alpha and beta quartz. When low temperature alpha quartz is heated at atmospheric pressure it changes to beta quartz at 573oC. At 870oC tridymite is formed and cristobalite is formed at 1470oC. The melting point of silica is 1610oC, which is higher than iron, copper and aluminium, and is one reason why it is used to produce moulds and cores for the production of metal castings.
The crystalline structure of quartz is based on four oxygen atoms linked together to form a three-dimensional shape called a tetrahedron with one silicon atom at its centre. Myriads of these tetrahedrons are joined together by sharing one another's corner oxygen atoms to form a quartz crystal.
Quartz is usually colourless or white but is frequently coloured by impurities, such as iron, and may then be any colour. Quartz may be transparent to translucent, hence its use in glassmaking, and have a vitreous lustre. Quartz is a hard mineral owing to the strength of the bonds between the atoms and it will scratch glass. It is also relatively inert and does not react with dilute acid. These are prized qualities in various industrial uses.Depending on how the silica deposit was formed, quartz grains may be sharp and angular, sub-angular, sub-rounded or rounded. Foundry and filtration applications require subrounded or rounded grains for best performance.
Processing technologies
Silica deposits are normally exploited by quarrying and the material extracted may undergo considerable processing before sale. The objectives of processing are to clean the quartz grains and increase the percentage of silica present, to produce the optimum size distribution of product depending upon end use and to reduce the amount of impurities, especially iron and chromium, which colour glass.
To meet these tight specifications, the sand often has to be subjected to extensive physical and chemical processing. This involves crushing, screening and further adjusting the grain-size distribution, together with removing contaminating impurities in the sand and from the surface of the individual quartz grains. The presence of metallic oxides in glassmaking sands usually results in coloured glass. If iron is present, the resulting glass is coloured green or brown. The iron level is consequently the most critical parameter in determining whether a particular sand can be used to make clear glass. Sands used to manufacture
colourless glass are therefore likely to be processed further by methods such as acid leaching, froth flotation or gravity separation. Figure 1 illustrates the range of iron level permitted in each of the grades of silica sand.
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