SILICA

What is 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. Silica is found commonly in the crystalline state and rarely in an amorphous state. It is composed of one atom of silicon and two atoms of oxygen resulting in the chemical formula SiO2.

The first industrial uses of crystalline silica were probably related to metallurgical and glass making activities in three to five thousand years BC. It has continued to support human progress throughout history, being a key raw material in the industrial development of the world especially in the glass, foundry and ceramics industries. Silica contributes to today’s information technology revolution being used in the plastics of computer mouses and providing the raw material for silicon chips.

Geology and occurrence of industrial silica

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 sub-rounded 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.

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 to remove 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.

Silica has played a continuous part in man’s development and been one of the basic raw materials supporting the industrial revolution (as refractory, flux, and moulding sand) and today’s information technology revolution (providing the raw material for silicon chips).


Industrial silica is used in a vast array of industries, the main ones being the glass, foundries, construction, ceramics, and the chemical industry.


Silica in its finest form is also used as functional filler for paints, plastics, rubber, and silica sand is used in water filtration and agriculture.

Other examples of everyday uses include the construction and maintenance of an extensive range of sports and leisure facilities.

Crystalline silica is also irreplaceable in a series of high-tech applications, for example in optical data transmission fibres and precision casting. It is also used in the metallurgical industry as the raw material for silicon metal and ferrosilicon production. Another specialized application is in the oil production.

Altogether there are several hundreds of applications of industrial silica in our daily life. Silica products have become so obvious to us that we don’t even know they are being applied. Reading this page, you will be surprised to find out how many times per day you see, touch and use products containing crystalline silica.

For more information on the socio-economic aspects related to industrial silica uses, please have a look into the Socio - Economic Review of Crystalline Silica Usage, Brian Coope, September 1997, whose conclusion is that if man wishes to live in silica free environment he must move to another planet.

The glass products containing silica include containers (bottles, jars, drinking vessels), flat glass (for windows, automotive glass, mirrors, etc.), decorative glass (glasses, decanters, bowls, figurines), fibreglass (reinforcing and insulating), technical glass (screens), and optical glass (spectacles and binoculars).
Quartz sand is a basic material for the production of moulds and cores in metal casting. It is also used for precision casting, dental applications and jewellery casting.

The construction industry is by far the largest volume consumer of silica minerals. Industrial silica is used in construction aggregates, in concrete, dimension stone, masonry mortars, tile glues, floor screeds, cement manufacture, road line markings, asphalt, in bridge and sewer refurbishment, in decorative bricks, not to mention in glass and steel structures.

Industrial silica is a structural ingredient of clay bodies and a major constitutent of ceramic glazes, ranging from refractory bricks to wall bricks, and from sanitaryware to tableware and tiles.

Quartz derivatives are used in many areas, such as pesticides, fertilisers and pharmaceuticals preparations. Another derivative from industrial silica is silicon carbide, which is the raw materials for abrasives, anti-slip and polishing products.

Silica in its finest forms find important usage as reinforcing filler for use in paint, plastics, rubber, and sealants.
In paints, silica is used to render the paint more resistant to chemicals and for enhancing hardness and wear resistance.

Ultrafine silica displays strong reinforcing properties in rubberplastics to impart flexural and compressive strenght. formulations and is thus a major ingredient in car tyres. Silica is also used in
Silica sand is the principal filtration medium used by the water industry to extract solid impurities from waste water.

Silica sand is used in farming, market gardening, horticulture, aquaculture, and forestry, in applications ranging from soil additive, surfacing material, and animal feed material.

Silica sand with soil is used in the manufacture of football and other sports pitches and golf courses. It is also used, often with polypropylene fibre or with rubber, for all-weather horse racing tracks, show jumping rings, dog racing tracks and equestrian training areas.

Quartz and the carbon reducing agents (wood, coal, coke, charcoal, electrodes) are put in an oven heated at a very high temperature (> 2000°C) thanks to an electric arc created through the electrodes. The metal is then cast, cooled and adapted according to the refining, granulometric and packaging specifications required by the customer.

In our daily lives, Silicon is the raw material for the following applications:

Silicones and silanes used for their waterproofness, for glues and mastics’ adhesion, for their insulating properties and for moulds’ production.

Iron and steel metallurgy: silicon is used to produce special up-market steels.

High-performance concrete: reinforcing concrete’s mechanical characteristics (e.g. resistance to compression).

Electronics: highly purified silicon gives birth to micro chips through high-tech and ultra automated processes.

Aluminium alloys: silicon increases the cast flow and the mechanical properties of aluminium alloys.

Closely sized grades of silica sand, with rounded to sub rounded particles, are used to stimulate oil well production. The sand is pumped into the oil bearing strata and increases its permeability thereby promoting the flow of oil into the well.