Silica 

Silica Sand

Its consists of small grains or particles of mineral and rock fragments. Although these grains may be of any mineral composition, the dominant component of sand is the mineral quartz, which is composed of silica (silicon dioxide). Other components may include aluminium, feldspar and iron-bearing minerals. Sand with particularly high silica levels that is used for purposes other than construction is referred to as silica sand or industrial sand. For a particular source of sand to be suitable for glassmaking, it must not only contain a very high proportion of silica but also should not contain more than strictly limited amounts of certain metallic elements. Silica sand is also normally required to be well-sorted, i.e. to have grains of an approximately uniform size. Most sources of sand used by the construction industry do not satisfy these requirements and are not, therefore, suitable for glassmaking. Industrial uses of silica sand depend on its purity and physical characteristics. Some of more important physical properties are: grain size and distribution, grain shape, sphericity, grain strength and refractoriness.

What is Industrial Sand (silica sand)

Industrial sand is a term normally applied to high purity silica sand products with closely controlled sizing. It is a more precise product than common concrete and asphalt gravels. Silica is the name given to a group of minerals composed solely 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 shapes and crystalline structures. Found most commonly in the crystalline state, it also occurs in an amorphous form resulting from weathering or plankton fossilization.

For industrial and manufacturing applications, deposits of silica yielding products of at least 95% SiO2 are preferred. Silica is hard, chemically inert and has a high melting point, attributable to the strength of the bonds between the atoms. These are prized qualities in applications like foundries and filtration systems. Quartz may be transparent to translucent and has a vitreous lustre, hence its use in glassmaking and ceramics. Industrial sand’s strength, silicon dioxide contribution and non-reactive properties make it an indispensable ingredient in the production of thousands of everyday products. Some silica sand deposits may cater for the used primarily as metallurgical sand. The copper and zinc at some smelter uses the sand as a fluxing agent which, in the molten state, reacts with various impurities in the ore and produces a slag. The slag is drawn off with the impurities, leaving a more refined metal behind.

Silica sands have a large number of other industrial uses depending on their characteristics.

• production of glass
• foundry sand
• ceramics
• sandblasting and other abrasives
• building products
• filler and extender
• production of silicon and silicon carbide
• pigments
• hydraulic fracturing and propping in the oil industry
• ultra high silica products in the electronic and fibre optic industries, fused
• silica, silicone products
• water filtration

Geology and occurrence of industrial Silica

Silica sand is an industrial term used for sand or easily disaggregated sandstone with a veryhigh percentage of quartz (silica) grains. Quartz is the most common silica crystal and thesecond most common mineral on the earth's surface. It is found in almost every type ofrock; igneous, metamorphic and sedimentary. While quartz deposits are abundant, and quartz is present in some form in nearly all mining operations, high purity and commerciallyviable deposits occur less frequently. Silica sand deposits are most commonly surfaceminedin open pit operations, but dredging and underground mining are also employed. Extracted ore undergoes considerable processing to increase the silica content by reducingimpurities. It is then dried and sized to produce the optimum particle size distribution for theintended application.

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 morestable 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 differenttemperatures 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 573C.At 870C tridymite is formed and cristobalite is formed at 1470C. The melting point of silica is 1610C, 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.

Percentage of iron (as ferric oxide)

Source: BS 2975.

Ranges of acceptable iron content in silica sand

In ascending order of permitted iron content, the three most commonly produced categories of glass are:

(a) colourless container glass (or `flint' glass);

(b) clear flat glass (or `float' glass); and

(c) coloured container glass.

These are also the most significant of the various applications for sand from the quarries relating to this merger.

Industrial Sand Applications (Glassmaking)

Silica sand is the primary component of all types of standard and specialty glass. It provides the essential SiO2 component of glass formulation and its chemical purity is the primary determinant of colour, clarity and strength. Industrial sand is used to produce flat glass for building and automotive use, container glass for foods and beverages, and tableware. In its pulverized form, ground silica is required for production of fibreglass insulation and reinforcing glass fibres. Specialty glass applications include test tubes and other scientific tools, incandescent and fluorescent lamps, television and computer CRT monitors.

Grades of silica sand for glassmaking

The glass industry has established different standard specifications for the silica sand intended for seven types of glass. The requirements for these grades of silica sand are set out in BS 2975:1988, British standard methods for sampling and analysis of glassmaking sand (BS 2975) and cover the following applications:

Figure 2.

BS 2975 gives detailed chemical and physical specifications for each of these grades of silica/glass sand. These specify parameters such as:

(a) minimum silica levels;

(b) maximum levels of aluminium, iron, chromium, copper, cobalt, nickel and vanadium;

(c) maximum alkali levels;

(d) maximum losses on heating; and(e) particle size distributions.

The amounts by which many of these parameters are permitted to vary between deliveries are also specified. Normally ranging between 0.1 and 0.5 mm in diameter, that in the case of glassmaking sands.BS 2975:1988, British standard methods for sampling and analysis of glass-making sands, British Standards, 1988.

Scope for substituting different grades of silica glass

Sand suitable for one use can normally be used for any other application with a higher permitted level of iron. Thus, flint glass sand can be used to make float glass or colouredcontainer glass (since iron can be added to the batch if needed to produce the required colour). As sands with lower levels of iron usually command a premium price, this may not be economically viable unless there are compensating savings, say, in the form of lower transport costs than an alternative quarry.On the other hand, if a glassmaker wished to use sand with a higher level of iron for a lower-iron-level application, the quarry would have to introduce additional processing. This might, for example, take the form of acid leaching to remove deposits of iron from the surface of the grains of sand. The extent of the capital investment and operating costs needed to do this would determine whether it was an economic proposition. In some locations, it might not be possible for the quarry operator to secure planning permission for the additional plant. Where the iron content occurs as inclusions within the grains, it is often difficult for a quarry operator to reduce the iron specification significantly. Consequently, much sand suitable for coloured-glass containers is not capable of being processed to meet the requirements for float glass or flint glass.It is sometimes possible to produce a number of different grades of silica sand from a single quarry by selective quarrying of different parts of the deposit.Cleaning the quartz grains and increasing silica content is achieved by washing to remove clay minerals and scrubbing by attrition between particles. Production of the optimum size distribution is achieved by screening to remove unwanted coarse particles and classification in an upward current of water toremove unwanted fine material. Quartz grains are often iron stained and the staining may be removed or reduced by chemical reaction involving sulphuric acid at different temperatures.Impurities present as separate mineral particles may be removed by various processes including gravity separation, froth flotation and magnetic separation. For the highest purity, for electronics applications, extra cleaning with aggressive acids such as hydrofluoric acid combined with thermal shock may be necessary.After processing, the sand may be dried and some applications require it to be ground in ball mills to produce a very fine material, called silica flour. Also, quartz may be converted to cristobalite in a rotary kiln at high temperature, with the assistance of a catalyst. Some specialist applications require the quartz to be melted in electric arc furnaces followed by cooling and grinding to produce fused silica.