tin processing

Written by B.T.K. Barry

Fact-checked by The Editors of Encyclopaedia Britannica

Related Topics:: tin; materials processing

tin processing, preparation of the ore for use in various products.

Tin (Sn) is a relatively soft and ductile metal with a silvery white colour. It has a density of 7.29 grams per cubic centimetre, a low melting point of 231.88 °C (449.38 °F), and a high boiling point of 2,625 °C (4,757 °F). Tin is allotropic; that is, it takes on more than one form. The normal form is white tin, or beta tin, which has a body-centred tetragonal crystal structure. The second allotrope, gray or alpha tin, has a face-centred cubic structure. Gray tin is theoretically stable below 13 °C (55 °F), but in practice it is readily formed only at about −40 °C (−40 °F). This transformation is difficult to initiate and is severely retarded by the presence of alloying elements or trace impurities. Nonetheless, it has given rise to the extremely rare laboratory curiosity known as tin pest.

Tin finds industrial application both as a metal and in chemical compounds. As a metal it is used in a very wide variety of industrial applications—but almost always in combination with other elements as an alloy or coating, since its intrinsic softness precludes its use as a structural material. Although tin is usually a minor constituent in alloys, it is an essential one on account of the way in which its special properties confer improvements to the matrix metal.

The major commercial applications of tin are in tinplate, solder alloys, bearing metals, tin and alloy coatings (both plated and hot-coated), pewter, bronzes, and fusible alloys. In its chemical reactions, tin exists in two valence states (II and IV) and is amphoteric (able to react as both an acid and a base). In addition, it can link directly with carbon to form organometallic compounds. These properties have given rise to many important uses for tin chemicals—for example, in electroplating, agricultural and pharmaceutical products, and plastics and ceramics.

History

There is evidence from both archaeology and literature that tin was one of the earliest metals to be known and used. Its earliest application was as an alloy with copper to form bronze, which was fashioned into tools and weapons. Bronze articles (typically containing about 10 percent tin) have been found in the Middle East dating from about 3500 bce and in Egypt from 3000 bce. Other ancient civilizations also used bronze articles; for example, Chinese bronzes have been dated to about 2250 bce.

Tin was obviously an important item of trade from early times, as it is mentioned in at least three books of the Bible (Numbers, Isaiah, and Ezekiel) dating from as long ago as 700 bce.

Pewter is a tin alloy that also has a long history. Probably the oldest known piece, dating from about 1500 bce, was found in Egypt, but it was the Roman civilization that developed pewter ware for household vessels and ornamental use. These applications have continued to the present day, although the alloy compositions have changed markedly.

The use of tin as a coating for other metals also has ancient historical roots, with tinned copper vessels for cooking tracing back to Roman times. Most important was the development of tinning iron sheet in order to form tinplate. This began in central Europe in the 14th and 15th centuries and gradually spread throughout the continent. The original uses of tinplate were for household articles, including lanterns, plates, and drinking vessels; however, with the introduction of food canning in 1812, packaging became the major use of tinplate.

An important date from more recent history is 1839, when the American metalsmith Isaac Babbitt first used tin-based alloys in bearings for machinery. Babbitt metal considerably aided the development of the industrial society. Further developments in tin alloys, coatings, and chemicals have contributed to advances in transport, telecommunication, aerospace, packaging, agriculture, and environmental protection.

Ores

The principal tin mineral is cassiterite, or tinstone (SnO₂), a naturally occurring oxide of tin containing about 78.8 percent tin. Of less importance are two complex sulfide minerals, stannite (Cu₂FeSnS₄), a copper-iron-tin sulfide, and cylindrite (PbSn₄FeSb₂S₁₄), a lead-tin-iron-antimony sulfide. These two minerals occur chiefly in lode deposits in Bolivia, often in association with other metals such as silver.

Unlike most base metals, economically viable deposits of cassiterite are restricted to a few geographic areas. The most important of these is in Southeast Asia and includes the tin-mining areas of China—which accounted for nearly half of all tin production in the early 21st century. Myanmar (Burma), Thailand, Malaysia, Indonesia, Brazil, Australia, Nigeria, and Congo (Kinshasa) are other major tin contributors. Minor producers are Peru, South Africa, the United Kingdom, and Zimbabwe. There is no significant tin deposit in the United States and only relatively small production in Canada.

About 80 percent of the world’s tin comes from alluvial or secondary deposits. Most of these occur on land, but in certain areas, notably in Indonesia and Thailand, the deposits are mined offshore by dredging the seabed.

Even in the richest tin fields, the concentration of tin is very low. This means that up to seven or eight tons of ore may have to be mined in order to recover one kilogram of cassiterite.

Mining and concentrating

Vein deposits, such as those in Bolivia and the United Kingdom, usually occur in granite formations and are recovered by conventional underground hard-rock mining techniques. In deep mines, primary crushing equipment is usually located underground in order to reduce the ore to a manageable size before transportation to the surface.

The more productive alluvial fields are relatively shallow deposits of fine-grained minerals that have accumulated in ancient riverbeds or valleys. They are mined by one of several surface-mining methods, principally gravel pumping, dredging, and, to a smaller extent, open-pit mining. A large proportion of tin ore is mined by gravel pumping. In this method, the barren overburden is removed, often by draglines or shovels, and high-pressure water jets are used to break up and dislodge the tin-bearing sand. A submerged gravel pump then sucks up the slurry of mud and water and raises it to a series of sluice boxes, or palongs, which slope downward and have baffles placed at intervals along their length. As the slurry flows along, the heavier minerals, including cassiterite, fall to the bottom, while the lighter waste material flows over the end of the boxes to tailings dumps. Periodically the flow is stopped and the crude concentrate removed.

In places where water is plentiful, an area above an alluvial deposit is flooded, often by diverting a river, and a mining dredge floated on it. Dredges have endless bucket chains at one end that dig and lift the tin-bearing ore to the primary processing plant, which is usually located on board. Ores are concentrated by gravity separation methods, including jigs and shaking tables. The concentrate is then collected for further treatment onshore, while the barren material is discharged over the stern of the dredge.

Tin concentrates from the alluvial mining areas of Southeast Asia are relatively free of impurities, although there may be small quantities of related minerals such as wolframite, scheelite, and columbite. Concentrates shipped to the smelter usually contain 70 to 75 percent tin metal. On the other hand, the complex sulfide ores found in underground deposits, such as those of Bolivia, require more complicated mineral processing, often involving froth flotation, in order to produce a clean tin concentrate. Even then, Bolivian concentrates may average only 50 to 60 percent tin.

Extraction and refining

Smelting

Before being smelted, low-grade concentrates from complex ores are first roasted in a reverberatory or multiple-hearth furnace at temperatures between 550 and 650 °C (1,025 and 1,200 °F) to drive off the sulfur. Depending on the type and quantity of impurities, oxidizing, reducing, or chlorinating reactions take place. Roasting is frequently followed by leaching with water or acid solutions to remove impurities made soluble by roasting.

After appropriate preparation, the furnace feed for smelting comprises tin oxide and some impurities, including iron oxides, that were not removed in mineral processing or roasting.

Tin smelting furnaces are one of three basic types: reverberatory furnaces, blast furnaces, or electric furnaces. Usually the operation is carried out as a batch process.

The principle of tin smelting is the chemical reduction of tin oxide by heating with carbon to produce tin metal and carbon dioxide gas. In practice, the furnace feed contains the tin oxide concentrate, carbon in the form of anthracite coal or coke, and limestone to act as a flux and a slag-producing agent.

In a typical reverberatory process (the most commonly used), the furnace is heated to 1,300–1,400 °C (2,375–2,550 °F) for a period of some 15 hours, during which it is stirred frequently, especially during the later stages. This process produces a pool of molten tin, on top of which floats a slag containing most of the unwanted impurities.

At the completion of smelting, the impure tin is tapped off and cast into large slabs, while the slag is solidified into granules by being poured into water tanks. The impure tin slabs go for further refining, and the granulated slag, which may still contain some tin, is retreated.

Refining

There are two methods of refining impure tin. Fire refining is most commonly used and produces tin (up to 99.85 percent) suitable for general commercial use. Electrolytic refining is used on the products of complex ores and to produce a very high grade of tin (up to 99.999 percent).

One fire-refining method is called boiling. In this, impure tin from the smelter, or tin from the liquation furnace (see below), is heated in vessels or kettles that are agitated by compressed air. The effect is to oxidize the impurities, which rise to the surface and form a dross.

Another fire-refining method is liquation. Used to treat both impure tin and dross from smelting, it removes those impurities that have a higher melting temperature than tin. The materials to be treated are placed on a sloping hearth in a reverberatory furnace and heated to a temperature just above the melting point of tin. The tin melts slowly and runs down the slope, to be collected in a vessel, leaving the unmelted residues on the hearth. These are subsequently removed and treated.

Vacuum distillation is sometimes used in fire refining. In this process, molten tin is heated in a dense graphite vessel at high temperatures (1,100 to 1,300 °C, or 2,000 to 2,375 °F). A vacuum is applied, and impurities are removed by selective distillation at their respective boiling temperatures.

In electrolytic refining, impure tin is cast into anodes. These are placed into an acidic electrolyte with starting cathodes made of thin sheets cast from high-purity tin. Special agents are required in the electrolyte in order to obtain dense, compact cathode deposits. After a period of about a week, the cathodes are removed.

Tin is normally sold in the form of ingots, or pigs, which are cast from refined tin. Most metallic tin is produced at smelters and refineries located near mining areas.

Secondary tin

Important sources of tin scrap are used bearings, solder alloys, or bronzes. Frequently, it is economic not to recover high-grade tin from these but to use the tin-containing scrap to produce alloys directly.

Tin residues may be treated like tin concentrates and smelted and refined as described above. Electrolytic refining is frequently employed for secondary-tin production.

Tinplate, whether as clean can-makers’ scrap or from used cans, is another source of secondary tin. The tinplate is detinned electrolytically to produce a high grade of tin and a clean steel scrap, which is returned to steelmakers.