July 2016    Print this article

What is silica?

Dominic Fragasso, Eng.
Denis Blackburn, Eng.
With the collaboration of N’Golo Togola, P.Geo.
Ministère de l'Énergie et des Ressources naturelles

Silica, or silicon dioxide (SiO2), is a term used for a family of minerals that includes quartz, cristobalite, tridymite and tripoli. Silicon (Si) is the most abundant element in the Earth’s crust after oxygen. It is does not exist in its pure form in nature; instead, it occurs as silicon dioxide (SiO2) in all the minerals belonging to the silicate class. Silicate minerals constitute 97% of the Earth’s crust. They have a basic structure composed of silica tetrahedra (SiO4).

The most common silica mineral in nature is quartz. Quartz has a hardness of 7 on the Mohs scale compared to a hardness of 10 for diamond. It is colourless in its pure state, or white to grey. Impurities impart hues of pink, purple, green or red. Commercially, the term silica is used to refer to deposits rich in quartz or other silicate minerals.

Natural sources of silica

There are four main sources of natural silica:

Massive quartz: Massive quartz occurs as veins, lenses and masses of hydrothermal or magmatic origin. It is milky white and contains only trace impurities.

Quartzite: This metamorphic rock of sedimentary (sandstone) origin is essentially composed of quartz grains. It has a glassy appearance and a medium- to coarse-grained granoblastic texture (that is, interlocking equidimensional crystals). Its colour ranges from white to grey-pink or grey-green depending on the impurities (iron, feldspar, mica).

Sandstone: This sedimentary rock is essentially composed of sand-sized detrital grains. The grains may consist entirely of quartz (quartz sandstone) or a mixture of quartz and other minerals (feldspar, amphibole, iron oxide, etc.). Sandstone ranges in colour from white to greyish with stains of pink to brick red, or pale to dark green.

Silica sand: Sands composed essentially of quartz grains are used in the industry as silica sand. Silica sands are of marine, fluvial (river) or eolian (wind) origin. The most sought-after deposits have high silica contents and well graded quartz grains (rounded to subrounded).

Physical properties and applications

The hardness of quartz makes it more resistant to abrasion than other minerals in a rock. When rock formations are subjected to erosion by water, wind or frost action, quartz will successfully resist destruction. It accumulates along seashores to form sand beaches or in the meanders of rivers. After becoming compressed and naturally cemented, these sands form sandstones, quartz sandstones or orthoquartzites.

Natural sand, once sorted, is a highly sought-after abrasive material. Sand can also be produced by grinding and sieving sandstones and quartzites. Sieved sand is the material of choice for filtering drinking water and for purifying wastewater. Sands with high uniformity coefficients1 are good choices for hydraulic fracturing. They are used during drilling to increase rock permeability and facilitate the withdrawal of petroleum or natural gas trapped in sedimentary rocks (shale).

Silica sand is also used in the manufacture of commercial glass (glass bottles, white glass, flat glass). Ground into a flour, it can be used as a filler in paints, plastics, rubbers, adhesives, grouts, sealants and caulking products. Silica also has applications in the manufacture of glass fibres used for reinforcing structures, for electrical applications and in low-density but resistant constructions. Silica sand is also used to make ceramics (dishes, porcelain, electrical insulators, pottery, tiles, bathroom accessories) and Chinese porcelain.

Very pure (lascas)2 quartz crystals obtained from quartz veins are used in piezoelectric3 (quartz watches), electronic (circuits) and optical (glass lenses) applications. High-purity silica (>99% SiO2) can be micronized and purified for several purposes: optical fibres, optical lenses, mirrors, halogen lamp bulbs, epoxies, artistic glass, etc.

Silica also has a number of other applications after undergoing a variety of specialized treatments. Fused silica is used in sealants, polyester resins, pharmaceutical products, precision casting and refractory ceramics, and as a bonding material to fix semiconductors in electronics. Precipitated silica is used in the manufacture of food and pharmaceutical products, chemical fertilizers and silicon-based rubber. Silica gel is used as an abrasive in toothpaste, a clarifying agent in beer, an absorbent agent in cosmetics and an anti-caking agent in food.

Silica can also be used to make silicon carbide by heating high-purity silica sand with petroleum coke to temperatures between 2,200° C and 2,400° C for 36 hours. The silicon carbide is then crushed and ground to the desired size. Silicon carbide is used as an abrasive, as a refractory, as moulds and as a component of metal-matrix composites.

Finally, silica has applications in metallurgy in the manufacture of ferrosilicon and metallurgical silicon. Ferrosilicon is a deoxidizing agent in the manufacture of stainless steel and as an alloying component in the manufacture of cast iron. Purified by chlorine gas, it is used to make semiconductors and photovoltaic cells for solar panels. The following diagram shows the main uses for silica.

World production

In 2013, the world production of silica was 142 Mt (USGS 2013). The United States is by far the leading producer, with 62 Mt. Europe is in second place, with a production of 38 Mt. Other producing countries are, in order: Asia (13,600 kt), Russia (3,554 kt), India (3,432 kt), Australia and New Zealand (3,132 kt), Africa (2,659 kt), South America (2,097 kt), Canada (1,690 kt) and Mexico (1,244 kt).

Silica in Québec

Active mines

Aside from sand pits, there are two operating mines in Québec:

Silica projects

Four projects are at the deposit appraisal or advanced exploration stage. In the mineral development process, a great deal of exploration work is carried out during the advanced exploration stage to accurately determine the deposit’s position; once mineral resources have been defined, it can be described as an ore deposit with estimated tonnage. The four projects are as follows:

Several silica showings in Québec are of interest for their high degree of purity. The following link leads to an inventory of the main prospects (in French only):
http://mern.gouv.qc.ca/mines/industrie/mineraux/mineraux-exploration-silice.jsp

Processing plants

To process silica, a mixture of silica (SiO2), carbon (C) and, in the case of ferrosilicon production, iron (Fe) is heated to obtain metallurgical silicon (Si + CO2) or ferrosilicon (SiFe + CO2). The poster published on the MERN’s website provides more information on the processing of high-purity silica.

Elkem is a Norwegian company that owns a ferrosilicon factory in the Saguenay region. The company has just been bought by the Chinese company National Blustar. The annual production capacity is on the order of 40,000 tonnes. The plant started operating in 1966 and employs about 85 people.

Globe Speciality Metals is an American company that owns a metallurgical silicon plant in Bécancour. The annual production capacity is on the order of 50,000 tonnes. The plant started operating in 1976 and employs about 182 people.

Market outlook in Québec

The market for high-purity silica should grow in the coming years due to the appearance of new technological applications that are becoming increasingly common. One such example is solar panels, which are manufactured using silicon semiconductors.

Furthermore, Québec, renowned for its supply of hydroelectric power, can provide electricity at a competitive price to metallurgical silicon plants looking to set up in the province. There are several high-purity silica showings in the province that have not yet been mined but benefit from their position near existing transportation routes (road, rail, sea).

As evident from the above, factors conducive to mining silica in the province will ultimately contribute to the development of Québec’s silica industry.

References

Harben, P. W., 2002. The Industrial Minerals HandyBook. A guide to markets, specifications & prices, 4th Edition. New York, pages 310–321.

Jacob, H. L., 2000. Les ressources québécoises en silice, Ministère des Ressources naturelles du Québec; ET 99-04, 34 pages.

Jébrak, M. and Marcoux, É., 2008. Géologie des ressources minérales, Ministère des Ressources naturelles du Québec, MM 2008-01.

Dolley, T. P., 2013. U.S. Geological Survey Minerals Yearbook, Silica, pages 66.1–66.15.

Schnebele, E. K., 2013. U.S. Geological Survey Minerals Yearbook, Silicon, pages 67.1–67.16.

1. Uniformity coefficient: a uniformity coefficient of 1.0 characterizes sand consisting of equal-sized grains. Below 1.0, uniformity decreases.

2. Lascas: very high-purity quartz with minor amounts of aluminum, iron and titanium.

3. Piezoelectric: electrical vibration induced by applied mechanical stress.

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