Tin Fact Sheet

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Introduction

Tin (Sn) is one of the few metals which has been used and traded by humans for more than 5000 years. One of its oldest uses is in combination with copper to make bronze. Copper was first coated with tin in the first century AD and tin-plated iron was manufactured in the 16th century.

It has the advantageous combinations of a low melting point, malleability, resistance to corrosion and fatigue, and the ability to alloy with other metals. It also is non-toxic and easy to recycle.

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Occurrence

Cassiterite (SnO2) is by far the most important tin ore, although small amounts of tin are recovered from sulphide minerals such as stannite (Cu2FeSnS4). Tin occurs in both primary and secondary deposits.

Primary deposits are typically associated with granite intrusive rocks which form when magma bodies are embodied into rock beneath the earth's surface, rather than on it as in the case of volcanic rock.

Primary deposits can occur within the granite or within pegmatities or aplites (dyke like rocks) associated with the granite. They occur also in rocks surrounding the margins of the intrusive rocks as veins, disseminations, skarns or carbonate replacements generated by tin bearing fluids derived from the granite magmas.

Secondary deposits (placers) derive from the weathering and erosion of primary tin deposits. Cassiterite is chemically resistant, heavy and readily forms residual concentrations. These concentrations may develop over a primary deposit (eluvial) and on slopes below the deposit (colluvial). When the cassiterite reaches a drainage system, it may be transported to a river channel and concentrated into an alluvial placer deposit. A placer deposit buried by younger sediments or lava is known as a deep lead. Deposits in oceanic submerged river channels are important sources of tin. More than half of the world's tin production is from deposits such as these, mainly in Malaysia, Indonesia and Thailand.

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Australian Resources and Deposits

Estimates of Australia's tin resources are shown in the Mineral Statistics Fact Sheet.

More than 85% of Australia's economic tin resources are located at the Renison Bell deposit in Tasmania, a primary carbonate replacement deposit which supports one of the world's largest underground tin mines. Australia's next most important mine is at Greenbushes in Western Australia, where tin is recovered along with tantalite (a tantalum mineral) from a weathered primary deposit.

Other areas where tin production has been important include Ardlethan and Cleveland, which are no longer operating. Gibsonvale, Moolyella, Mount Bischoff, the Herberton/Mount Garnet and Emmaville districts have also been important producers of tin and remain prospective areas. Areas with minor production and/or possible important future sources of tin include Collingwood, Doradilla and the Bynoe Harbour district.

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Australia in the World

The world's economic resources of tin total approximately seven million tonnes, of which Australia has approximately 1.3%. Eastern Asian countries, including China (27%), Malaysia (15%), Thailand (12%) and Indonesia (10%) possess the majority of the world's economic tin resources. Other countries with large tin resources include Brazil (16%), Bolivia (6%), Peru (4%) and Russia (4%).

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Mining

The main method of mining large placer tin deposits is by bucket-line dredging. The alluvium containing the tin is excavated and transported by a continuous chain of buckets to the interior of the dredge where it is washed and roughly concentrated. In South-East Asia particularly, smaller deposits, or those unsuitable for dredging (e.g. because the bedrock is very rough) are worked by gravel pumping. The alluvium is broken up by a high pressure jet of water and the resulting slurry is pumped to the concentrating plant.

The impure cassiterite concentrate is further concentrated by gravity methods which involve passing the concentrate in a stream of water over equipment such as jigs, spirals, or shaking tables. This separates the heavy cassiterite from the lighter minerals such as quartz. Magnetic or electrostatic separation removes the heavy mineral impurities. The end product is a cassiterite concentrate containing about 70% tin.

Vein and disseminated tin deposits are mined by the same methods used in hard-rock mining of other non-ferrous ores such as zinc. The ore is broken by drilling and blasting, transported to a concentrator where it is crushed and ground and then concentrated by gravity methods. The concentrate is usually of a lower grade (about 50% tin) than placer concentrate because of the fine grain size of the cassiterite and the difficulty of removing all the associated sulphide minerals.

Although flotation is not as efficient for tin ores as it is for sulphide ores, it is used increasingly to improve the amount of tin recovered and to recover tin from the residues of earlier treatment.

Smelting

Cassiterite is reduced to tin by heating with carbon at 1200°C to 1300°C. Reverberatory furnaces are used to smelt tin concentrate and, for additional tin recovery, to re-smelt slag, which is left after the ore has been smelted. A furnace charge consists of cassiterite, a carbon-reducing agent, and limestone and silica fluxes. Smelting takes 10 to 12 hours. The molten batch is tapped into a settler from which the slag overflows into pots. The molten tin from the bottom of the settler is cast into slabs or pigs (of about 34 kg) for refining, and the cooled slag, which contains 10 to 25% tin, is crushed and re-smelted.

Refining

Tin produced by smelting concentrate or slag commonly contains metallic impurities which must be removed by refining before the tin is marketed. Refining may be by heat treatment or by electrolytic processes. Heat treatment is the more widely used method and involves heating the tin from smelters on an inclined hearth to a temperature just above the melting point of pure tin, but below that of the melting point of the impurities. The relatively pure molten tin flows into a kettle and impurities remain behind in a residue which is re-treated to recover more tin. As there is not a great demand for tin of extremely high-purity, the more costly electrolytic method is rarely used.

Tin concentrate produced at Greenbushes also contains tantalum and niobium. The concentrate is smelted in an electric furnace and tantalum and niobium are recovered from the slag. The tin produced here contains a small amount of antimony and is used for alloys.

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Uses

There are many important uses for tin. Most is used to produce tinplate, or steel coated with tin which is used for food packaging. Tin and tin alloys are used also for solder, especially in the electronics industry. It is commonly used as an alloy for bearing metal and as an alloy in metallic coatings.

Inorganic compounds of tin are used in ceramics and glazes. Organic compounds of tin are used in plastics, wood preservatives, pesticides and in fire retardants.

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Suggestions for Further Reading

  • Australia's Identified Mineral Resources 2007, Geoscience Australia, Canberra.
  • Hughes, F.E. (Ed) 1990 Geology of the Mineral Deposits of Australia and Papua New Guinea, Monograph 14, Australasian Institute of Mining and Metallurgy, Melbourne.
  • Knight, C.L. (Ed) 1975 Economic Geology of Australia and Papua New Guinea, 1. Metals, Monograph 5, Australasian Institute of Mining and Metallurgy, Melbourne.
  • Raggat, H.G. 1968 Mountains of Ore, Lansdowne, Melbourne.
  • Woodcock, J.T. and Hamilton, J.K. (Eds) 1993 Australasian Mining and Metallurgy : The Sir Maurice Mawby Memorial Volume, Volume 2, The Australasian Institute of Mining and Metallurgy, Melbourne.
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