Tin

Physical properties

Tin is a soft, malleable, ductile and highly crystalline silvery-white metal. When a bar of tin is bent, a crackling sound known as the "tin cry" can be heard from the twinning of the crystals. Tin melts at the low temperature of about 232 °C (450 °F), the lowest in group 14. The melting point is further lowered to 177.3 °C (351.1 °F) for 11 nm particles.

β-tin (the metallic form, or white tin), which is stable at and above room temperature, is malleable. In contrast, α-tin (nonmetallic form, or gray tin), which is stable below 13.2 °C (55.8 °F), is brittle. α-tin has a diamond cubic crystal structure, similar to diamond, silicon or germanium. α-tin has no metallic properties at all because its atoms form a covalent structure in which electrons cannot move freely. It is a dull-gray powdery material with no common uses other than a few specialized semiconductor applications. These two allotropes, α-tin and β-tin, are more commonly known as gray tin and white tin, respectively. Two more allotropes, γ and σ, exist at temperatures above 161 °C (322 °F)  and pressures above several GPa. In cold conditions, β-tin tends to transform spontaneously into α-tin, a phenomenon known as "tin pest". Although the α-β transformation temperature is nominally 13.2 °C (55.8 °F), impurities (e.g. Al, Zn, etc.) lower the transition temperature well below 0 °C (32 °F) and, on the addition of Sb or Bi, the transformation might not occur at all, increasing the durability of the tin.

Commercial grades of tin (99.8%) resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony, lead, and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase its hardness. Tin tends rather easily to form hard, brittle intermetallic phases, which are often undesirable. It does not form wide solid solution ranges in other metals in general, and few elements have appreciable solid solubility in tin. Simple eutectic systems, however, occur with bismuth, gallium, lead, thallium and zinc.

Tin becomes a superconductor below 3.72 K and was one of the first superconductors to be studied; the Meissner effect, one of the characteristic features of superconductors, was first discovered in superconducting tin crystals.

Chemical properties

Tin resists corrosion from water, but can be attacked by acids and alkalis. Tin can be highly polished and is used as a protective coat for other metals. A protective oxide (passivation) layer prevents further oxidation, the same that forms on pewter and other tin alloys. Tin acts as a catalyst when oxygen is in solution and helps accelerate chemical attack.

Isotopes

Tin has ten stable isotopes, with atomic masses of 112, 114 through 120, 122 and 124, the greatest number of any element. Of these, the most abundant are ¹²⁰Sn (almost a third of all tin), ¹¹⁸Sn, and ¹¹⁶Sn, while the least abundant is ¹¹⁵Sn. The isotopes with even mass numbers have no nuclear spin, while those with odd have a spin of +1/2. Tin, with its three common isotopes ¹¹⁶Sn, ¹¹⁸Sn and ¹²⁰Sn, is among the easiest elements to detect and analyze by NMR spectroscopy, and its chemical shifts are referenced against SnMe
4.

This large number of stable isotopes is thought to be a direct result of the atomic number 50, a "magic number" in nuclear physics. Tin also occurs in 29 unstable isotopes, encompassing all the remaining atomic masses from 99 to 137. Apart from ¹²⁶Sn, with a half-life of 230,000 years, all the radioisotopes have a half-life of less than a year. The radioactive ¹⁰⁰Sn, discovered in 1994, and ¹³²Sn are one of the few nuclides with a "doubly magic" nucleus: despite being unstable, having very lopsided proton-neutron ratios, they represent endpoints beyond which stability drops off rapidly. Another 30 metastable isomers have been characterized for isotopes between 111 and 131, the most stable being ^121mSn with a half-life of 43.9 years.

The relative differences in the abundances of tin's stable isotopes can be explained by their different modes of formation in stellar nucleosynthesis. ¹¹⁶Sn through ¹²⁰Sn inclusive are formed in the s-process (slow neutron capture) in most stars and hence they are the most common isotopes, while ¹²²Sn and ¹²⁴Sn are only formed in the r-process (rapid neutron capture) in supernovae and are less common. (The isotopes ¹¹⁷Sn through ¹²⁰Sn also receive contributions from the r-process.) Finally, the rarest proton-rich isotopes, ¹¹²Sn, ¹¹⁴Sn, and ¹¹⁵Sn, cannot be made in significant amounts in the s- or r-processes and are considered among the p-nuclei, whose origins are not well-understood yet. Some speculated mechanisms for their formation include proton capture as well as photodisintegration, although ¹¹⁵Sn might also be partially produced in the s-process, both directly, and as the daughter of long-lived ¹¹⁵In.

Etymology

The word tin is shared among Germanic languages and can be traced back to reconstructed Proto-Germanic *tin-om; cognates include German Zinn, Swedish tenn and Dutch tin. It is not found in other branches of Indo-European, except by borrowing from Germanic (e.g. Irish tinne from English).

The Latin name stannum originally meant an alloy of silver and lead, and came to mean 'tin' in the 4th century BCE—the earlier Latin word for it was plumbum candidum, or "white lead". Stannum apparently came from an earlier stāgnum (meaning the same substance), the origin of the Romance and Celtic terms for 'tin'. The origin of stannum/stāgnum is unknown; it might be pre-Indo-European.

The Meyers Konversationslexikon speculates on the contrary that stannum is derived from (the ancestor of) Cornish stean, and is proof that Cornwall in the first centuries AD was the main source of tin.

History

Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BC, when it was observed that copper objects formed of polymetallic ores with different metal contents had different physical properties. The earliest bronze objects had a tin or arsenic content of less than 2% and are therefore believed to be the result of unintentional alloying due to trace metal content in the copper ore. The addition of a second metal to copper increases its hardness, lowers the melting temperature, and improves the casting process by producing a more fluid melt that cools to a denser, less spongy metal. This was an important innovation that allowed for the much more complex shapes cast in closed moulds of the Bronze Age. Arsenical bronze objects appear first in the Near East where arsenic is commonly found in association with copper ore, but the health risks were quickly realized and the quest for sources of the much less hazardous tin ores began early in the Bronze Age. This created the demand for rare tin metal and formed a trade network that linked the distant sources of tin to the markets of Bronze Age cultures.

Cassiterite (SnO₂), the tin oxide form of tin, was most likely the original source of tin in ancient times. Other forms of tin ores are less abundant sulfides such as stannite that require a more involved smelting process. Cassiterite often accumulates in alluvial channels as placer deposits because it is harder, heavier, and more chemically resistant than the accompanying granite. Cassiterite is usually black, purple or otherwise dark in color, and these deposits can be easily seen in river banks. Alluvial (placer) deposits could be easily collected and separated by methods similar to gold panning.

Compounds and chemistry

In the great majority of its compounds, tin has the oxidation state II or IV.

Inorganic compounds

Halide compounds are known for both oxidation states. For Sn(IV), all four halides are well known: SnF₄, SnCl₄, SnBr₄, and SnI₄. The three heavier members are volatile molecular compounds, whereas the tetrafluoride is polymeric. All four halides are known for Sn(II) also: SnF₂, SnCl₂, SnBr₂, and SnI₂. All are polymeric solids. Of these eight compounds, only the iodides are colored.

Tin(II) chloride (also known as stannous chloride) is the most important tin halide in a commercial sense. Illustrating the routes to such compounds, chlorine reacts with tin metal to give SnCl₄ whereas the reaction of hydrochloric acid and tin produces SnCl₂ and hydrogen gas. Alternatively SnCl₄ and Sn combine to stannous chloride by a process called comproportionation:

Tin can form many oxides, sulfides, and other chalcogenide derivatives. The dioxide SnO₂ (cassiterite) forms when tin is heated in the presence of air. SnO₂ is amphoteric, which means that it dissolves in both acidic and basic solutions. Stannates with the structure [Sn(OH)₆]²⁻, like K₂[Sn(OH)₆], are also known, though the free stannic acid H₂[Sn(OH)₆] is unknown.

sulfides of tin exist in both the +2 and +4 oxidation states: tin(II) sulfide and tin(IV) sulfide (mosaic gold).

Hydrides

Stannane (SnH₄), with tin in the +4 oxidation state, is unstable. Organotin hydrides are however well known, e.g. tributyltin hydride (Sn(C₄H₉)₃H). These compound release transient tributyl tin radicals, which are rare examples of compounds of tin(III).

Organotin compounds

Organotin compounds, sometimes called stannanes, are chemical compounds with tin–carbon bonds. Of the compounds of tin, the organic derivatives are the most useful commercially. Some organotin compounds are highly toxic and have been used as biocides. The first organotin compound to be reported was diethyltin diiodide ((C₂H₅)₂SnI₂), reported by Edward Frankland in 1849.

Most organotin compounds are colorless liquids or solids that are stable to air and water. They adopt tetrahedral geometry. Tetraalkyl- and tetraaryltin compounds can be prepared using Grignard reagents:

The mixed halide-alkyls, which are more common and more important commercially than the tetraorgano derivatives, are prepared by redistribution reactions:

Divalent organotin compounds are uncommon, although more common than related divalent organogermanium and organosilicon compounds. The greater stabilization enjoyed by Sn(II) is attributed to the "inert pair effect". Organotin(II) compounds include both stannylenes (formula: R₂Sn, as seen for singlet carbenes) and distannylenes (R₄Sn₂), which are roughly equivalent to alkenes. Both classes exhibit unusual reactions.

Occurrence

Tin is generated via the long S-process in low-to-medium mass stars (with masses of 0.6 to 10 times that of Sun), and finally by beta decay of the heavy isotopes of indium.

Tin is the 49th most abundant element in Earth's crust, representing 2 ppm compared with 75 ppm for zinc, 50 ppm for copper, and 14 ppm for lead.

Tin does not occur as the native element but must be extracted from various ores. Cassiterite (SnO₂) is the only commercially important source of tin, although small quantities of tin are recovered from complex sulfides such as stannite, cylindrite, franckeite, canfieldite, and teallite. Minerals with tin are almost always associated with granite rock, usually at a level of 1% tin oxide content.

Because of the higher specific gravity of tin dioxide, about 80% of mined tin is from secondary deposits found downstream from the primary lodes. Tin is often recovered from granules washed downstream in the past and deposited in valleys or the sea. The most economical ways of mining tin are by dredging, hydraulicking, or open pits. Most of the world's tin is produced from placer deposits, which can contain as little as 0.015% tin.

About 253,000 tonnes of tin have been mined in 2011, mostly in China (110,000 t), Indonesia (51,000 t), Peru (34,600 t), Bolivia (20,700 t) and Brazil (12,000 t). Estimates of tin production have historically varied with the dynamics of economic feasibility and the development of mining technologies, but it is estimated that, at current consumption rates and technologies, the Earth will run out of mine-able tin in 40 years. Lester Brown has suggested tin could run out within 20 years based on an extremely conservative extrapolation of 2% growth per year.

Secondary, or scrap, tin is also an important source of the metal. Recovery of tin through secondary production, or recycling of scrap tin, is increasing rapidly. Whereas the United States has neither mined since 1993 nor smelted tin since 1989, it was the largest secondary producer, recycling nearly 14,000 tonnes in 2006.

New deposits are reported in southern Mongolia, and in 2009, new deposits of tin were discovered in Colombia by the Seminole Group Colombia CI, SAS.

Production

Tin is produced by carbothermic reduction of the oxide ore with carbon or coke. Both reverberatory furnace and electric furnace can be used.

Industry

The ten largest companies produced most of the world's tin in 2007. It is not clear which of these companies include tin smelted from the mine at Bisie, Democratic Republic of the Congo, which is controlled by a renegade militia and produces 15,000 tonnes. Most of the world's tin is traded on the London Metal Exchange (LME), from 8 countries, under 17 brands.

Price and exchanges

Tin is unique among other mineral commodities by the complex agreements between producer countries and consumer countries dating back to 1921. The earlier agreements tended to be somewhat informal and sporadic and led to the "First International Tin Agreement" in 1956, the first of a continuously numbered series that effectively collapsed in 1985. Through this series of agreements, the International Tin Council (ITC) had a considerable effect on tin prices. The ITC supported the price of tin during periods of low prices by buying tin for its buffer stockpile and was able to restrain the price during periods of high prices by selling tin from the stockpile. This was an anti-free-market approach, designed to assure a sufficient flow of tin to consumer countries and a profit for producer countries. However, the buffer stockpile was not sufficiently large, and during most of those 29 years tin prices rose, sometimes sharply, especially from 1973 through 1980 when rampant inflation plagued many world economies.

During the late 1970s and early 1980s, the U.S. Government tin stockpile was in an aggressive selling mode, partly to take advantage of the historically high tin prices. The sharp recession of 1981–82 proved to be quite harsh on the tin industry. Tin consumption declined dramatically. The ITC was able to avoid truly steep declines through accelerated buying for its buffer stockpile; this activity required the ITC to borrow extensively from banks and metal trading firms to augment its resources. The ITC continued to borrow until late 1985 when it reached its credit limit. Immediately, a major "tin crisis" followed — tin was delisted from trading on the London Metal Exchange for about three years, the ITC dissolved soon afterward, and the price of tin, now in a free-market environment, plummeted sharply to $4 per pound and remained at that level through the 1990s. The price increased again by 2010 with a rebound in consumption following the 2008–09 world economic crisis, accompanying restocking and continued growth in consumption by the world's developing economies.

London Metal Exchange (LME) is the principal trading site for tin. Other tin contract markets are Kuala Lumpur Tin Market (KLTM) and Indonesia Tin Exchange (INATIN).

Price of tin in USD dollars per kg

Source:Helgi Library

Applications

In 2006, about half of all tin produced was used in solder. The rest was divided between tin plating, tin chemicals, brass and bronze alloys, and niche uses.

Solder

Tin has long been used in alloys with lead as solder, in amounts 5 to 70% w/w. Tin forms a eutectic mixture with lead in the proportion 63% tin and 37% lead. Such solders are primarily used for joining pipes or electric circuits. Since the European Union Waste Electrical and Electronic Equipment Directive (WEEE Directive) and Restriction of Hazardous Substances Directive came into effect on 1 July 2006, the lead content in such alloys has decreased. Replacing lead has many problems, including a higher melting point, and the formation of tin whiskers causing electrical problems. Tin pest can occur in lead-free solders, leading to loss of the soldered joint. Replacement alloys are rapidly being found, although problems of joint integrity remain.

Tin plating

Tin bonds readily to iron and is used for coating lead, zinc and steel to prevent corrosion. Tin-plated steel containers are widely used for food preservation, and this forms a large part of the market for metallic tin. A tinplate canister for preserving food was first manufactured in London in 1812. Speakers of British English call them "tins", while speakers of American English call them "cans" or "tin cans". One derivation of such use is the slang term "tinnie" or "tinny", meaning "can of beer". The tin whistle is so called because it was first mass-produced in tin-plated steel.

Specialized alloys

Tin in combination with other elements forms a wide variety of useful alloys. Tin is most commonly alloyed with copper. Pewter is 85–99% tin; Bearing metal has a high percentage of tin as well. Bronze is mostly copper (12% tin), while addition of phosphorus gives phosphor bronze. Bell metal is also a copper-tin alloy, containing 22% tin. Tin has sometimes been used in coinage; for example, it once formed a single-digit percentage (usually five percent or less) of American and Canadian pennies. Because copper is often the major metal in such coins, sometimes including zinc, these could be called bronze and/or brass alloys.

The niobium-tin compound Nb₃Sn is commercially used in coils of superconducting magnets for its high critical temperature (18 K) and critical magnetic field (25 T). A superconducting magnet weighing as little as two kilograms is capable of the magnetic field of a conventional electromagnet weighing tons.

A small percentage of tin is added to zirconium alloys for the cladding of nuclear fuel.

Most metal pipes in a pipe organ are of varying amounts of a tin/lead alloy, with 50/50 being the most common. The amount of tin in the pipe defines the pipe's tone, since tin has a desirable tonal resonance. When a tin/lead alloy cools, the lead cools slightly faster and produces a mottled or spotted effect. This metal alloy is referred to as spotted metal. Major advantages of using tin for pipes include its appearance, its workability, and resistance to corrosion.

Other applications

Punched tin-plated steel, also called pierced tin, is an artisan technique originating in central Europe for creating housewares that are both functional and decorative. Decorative piercing designs exist in a wide variety, based on local tradition and the artisan's personal creations. Punched tin lanterns are the most common application of this artisan technique. The light of a candle shining through the pierced design creates a decorative light pattern in the room where it sits. Lanterns and other punched tin articles were created in the New World from the earliest European settlement. A well-known example is the Revere lantern, named after Paul Revere.

Before the modern era, in some areas of the Alps, a goat or sheep's horn would be sharpened and a tin panel would be punched out using the alphabet and numbers from one to nine. This learning tool was known appropriately as "the horn". Modern reproductions are decorated with such motifs as hearts and tulips.

In America, pie safes and food safes were in use in the days before refrigeration. These were wooden cupboards of various styles and sizes – either floor standing or hanging cupboards meant to discourage vermin and insects and to keep dust from perishable foodstuffs. These cabinets had tinplate inserts in the doors and sometimes in the sides, punched out by the homeowner, cabinetmaker or a tinsmith in varying designs to allow for air circulation while excluding flies. Modern reproductions of these articles remain popular in North America.

Window glass is most often made by floating molten glass on molten tin (float glass), resulting in a flat and flawless surface. This is also called the "Pilkington process".

Tin is also used as a negative electrode in advanced Li-ion batteries. Its application is somewhat limited by the fact that some tin surfaces catalyze decomposition of carbonate-based electrolytes used in Li-ion batteries.

Tin(II) fluoride is added to some dental care products as stannous fluoride (SnF₂). Tin(II) fluoride can be mixed with calcium abrasives while the more common sodium fluoride gradually becomes biologically inactive in the presence of calcium compounds. It has also been shown to be more effective than sodium fluoride in controlling gingivitis.

Organotin compounds

Of all the chemical compounds of tin, the organotin compounds are most heavily used. Worldwide industrial production probably exceeds 50,000 tonnes.

PVC stabilizers

The major commercial application of organotin compounds is in the stabilization of PVC plastics. In the absence of such stabilizers, PVC would otherwise rapidly degrade under heat, light, and atmospheric oxygen, resulting in discolored, brittle products. Tin scavenges labile chloride ions (Cl⁻), which would otherwise initiate loss of HCl from the plastic material. Typical tin compounds are carboxylic acid derivatives of dibutyltin dichloride, such as the dilaurate.

Biocides

Some organotin compounds are relatively toxic, with both advantages and problems. They are used for biocidal properties as fungicides, pesticides, algaecides, wood preservatives, and antifouling agents. Tributyltin oxide is used as a wood preservative. Tributyltin was used as additive for ship paint to prevent growth of marine organisms on ships, with use declining after organotin compounds were recognized as persistent organic pollutants with an extremely high toxicity for some marine organisms (the dog whelk, for example). The EU banned the use of organotin compounds in 2003, while concerns over the toxicity of these compounds to marine life and damage to the reproduction and growth of some marine species (some reports describe biological effects to marine life at a concentration of 1 nanogram per liter) have led to a worldwide ban by the International Maritime Organization. Many nations now restrict the use of organotin compounds to vessels greater than 25 m (82 ft) long.

Organic chemistry

Some tin reagents are useful in organic chemistry. In the largest application, stannous chloride is a common reducing agent for the conversion of nitro and oxime groups to amines. The Stille reaction couples organotin compounds with organic halides or pseudohalides.

Li-ion batteries

Tin forms several inter-metallic phases with lithium metal, making it a potentially attractive material for battery applications. Large volumetric expansion of tin upon alloying with lithium and instability of the tin-organic electrolyte interface at low electrochemical potentials are the greatest challenges to employment in commercial cells. The problem was partially solved by Sony. Tin inter-metallic compound with cobalt and carbon, has been implemented by Sony in its Nexelion cells released in late 2000's. The composition of the active material is approximately Sn_0.3Co_0.4C_0.3. Recent research showed that only some crystalline facets of tetragonal (beta) Sn are responsible for undesirable electrochemical activity.

Precautions

Cases of poisoning from tin metal, its oxides, and its salts are almost unknown. On the other hand, certain organotin compounds are almost as toxic as cyanide.

Exposure to tin in the workplace can occur by inhalation, skin contact, and eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (Permissible exposure limit) for tin exposure in the workplace as 2 mg/m³ over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has determined a recommended exposure limit (REL) of 2 mg/m³ over an 8-hour workday. At levels of 100 mg/m³, tin is immediately dangerous to life and health.