Tin (Sn)
Tin (Sn) is a silver-white metal recognized for its versatility and historical significance. It belongs to Group VIA of the periodic table with an atomic number of 50 and is primarily sourced from the mineral cassiterite (SnO₂). While tin is dispersed throughout the Earth’s crust, major deposits are found in regions like China, Indonesia, and Peru. Traditionally, tin has been utilized in food containers, solders, and alloys, such as bronze and pewter.
Tin exists in two allotropes: gray tin and white tin, with distinct physical properties and melting points. It has a relatively low melting point of 231.97°C and is known for its corrosion resistance, making it ideal for coating other metals. With ten stable isotopes, tin has the highest number for any element, adding to its unique characteristics. In addition to its historical use in bronze, tin is essential in modern applications, including lead-free soldering and various industrial processes. Its non-toxic nature has led to its adoption in food and beverage packaging, ensuring safety for consumers. Continued research into tin’s properties and uses highlights its ongoing relevance in various fields.
Tin (Sn)
Where Found
Although tin is widely distributed throughout the Earth’s crust, the average concentration is very low, less than 0.001 percent. The primary oremineral is cassiterite (SnO2) which generally occurs in granitic igneous rocks, associated hydrothermal veins near igneous rocks, or the weathered and eroded debris of granitic rocks. The major producers of tin are China, Indonesia, and Peru.
Primary Uses
Traditionally, tin has been used in the production of tin plate, used in making food containers. Tin is also alloyed with lead to make solders; with copper to make bronze; and with lead, brass, or copper tomake pewter.
Technical Definition
Tin (chemical symbol Sn) is a silver-white metal that belongs to Group VIA of the periodic table. Tin has an atomic number of 50 and an atomic weight of 118.65. Tin comes in two forms (allotropes): gray (alpha) tin and white (beta) tin. Gray tin, a face-centered crystalline structure with a density of 5.75 grams per cubic centimeter, changes into white tin at 13.2° Celsius. White tin has a body-centered tetragonal structure with a density of 7.28 grams per cubic centimeter. The melting point of tin is 231.97° Celsius, and the boiling point is 2,270° Celsius, giving this element one of the largest temperature ranges for a liquid metal. Tin has ten stable isotopes, the highest number of any element.
Description, Distribution, and Forms
Tin is a widely distributed element in the Earth’s crust and can form both inorganic and organic compounds. Tin generally forms two series of inorganic compounds. Since tin has two valence states, II and IV, the inorganic compounds are built using these two states of tin. Some of the more commercially important compounds of tin (II) include stannous chloride (SnCl2), stannous oxide (SnO), and stannous fluoride (SnF2). The most common tin (IV) compounds are stannic oxide (SnO2) and stannic chloride (SnCl4). Tin can also form compounds with carbon; more than five hundred organotin compounds are known. Some of these compounds are nontoxic and are used extensively as stabilizers for polyvinyl chloride. Other organotin compounds are toxic and are used as biocides in fungicides and disinfectants.
Although tin is widely distributed throughout the crust of the Earth, it is concentrated in ore deposits in three main regions. The primary tin regions are from the Korean Peninsula through China to Southeast Asia; Thailand to Indonesia; and Peru, Bolivia, and Brazil. The tin deposits found in these regions are associated with granitic igneous rocks, related hydrothermal veins, or stream deposits (placers) that contain the eroded tin minerals. More than 99 percent of the tin produced through history has been derived directly or indirectly from granitic rocks. The tin-bearing granitic rocks were formed near convergent plate boundaries in or near subduction zones. Some of the granitic rocks produced in the subduction zones contain the tin ore mineral cassiterite; commonly the associated hydrothermal veins also carry cassiterite. These hydrothermal veins tend to be rich in fluorine and boron and often contain minerals such as tourmaline, apatite, fluorite, and topaz. Cassiterite is also found associated with the minerals arsenopyrite, molybdenite, and wolframite. Lode deposits of tin are found primarily in Bolivia and in England. The Bolivian lode tin ores also contain the rare tin minerals stannite (CuFeSnS4) and cylindrite (PbSn4FeSb2S14). These vein deposits are worked by underground mining techniques used in mining many other base metals.
History
Early civilizations used tin as an alloy with copper to make bronze. The earliest bronzes appear to have been an alloy of copper and arsenic, but the later discovery of tin/copper bronzes yielded a safer and better material. The earliest tin bronze items were probably produced by the peoples of the Middle East as long ago as 3500 b.c.e., and Egyptian bronze artifacts date back to at least 3000 b.c.e. Although these items contain about 10 percent tin, it is unlikely that these early people knew of tin as a distinct and separate metal. No evidence of smelting of pure tin from ore or use of the metal by itself has been discovered. Rather, tin ores may have been added to the copper ores when smelting the copper, and the resultant liquid contained both copper and tin.
The Romans learned how to “roast” the tin from ore deposits, and they used tin as a coating on iron objects. Because tin is easy to apply to other metals, and because it does not corrode under normal conditions, this use of tin has continued to the present. Food canning was developed in 1812, and much of the tin mined today is used to coat steel food containers. Tin was also used to produce pewter as far back as 1500 b.c.e., and this alloy was used extensively by the Romans. In the early 1800’s, tin was first mixed with copper and antimony to make a babbitt metal that reduced friction of bearings in machinery. This alloy, created by Isaac Babbitt, was an important contribution to the Industrial Revolution.
Obtaining Tin
Deposits of cassiterite are concentrated in the upper levels of the granitic intrusions and associated hydrothermal veins, and they are commonly exposed to weathering and erosional forces. As a result, most of the currently operating mines are working tin deposits less than 300 million years old. The older deposits have generally been eroded away. The older tin ores that have escaped erosion are found primarily in the central parts of continental masses. The oldest known tin ores are found in South Africa and are more than 2.5 billion years old.
Because cassiterite is both heavy (with a specific gravity of 6.8 to 7.1) and hard (with a Mohs scale hardness of 6 to 7), it is commonly found as a placer mineral in streams and rivers that drain tin-bearing igneous regions. As the rocks of the tin region are weathered and eroded, the softer minerals are broken down into small clasts, and the lighter materials are carried downstream to the oceans. The heavy minerals such as cassiterite and wolframite (a tungsten-bearing mineral) are left behind in the alluvial deposits. Most of the Southeast Asian deposits of tin are alluvial accumulations that formed in ancient rivers that are now exposed on dry land.
A small percentage of tin has been produced from base-metal sulfide deposits. These deposits occasionally contain cassiterite concentrated in volcanic-sedimentary deposits that were formed in or near high-temperature submarine vents. The major deposits of this type are found in Canada and Portugal. The Neves Corvo, Portugal, deposit is a massive sulfide ore containing copper, zinc, lead, silver, and tin.
Although many of the tin deposits first exploited were located in Europe, the majority of the tin mined now comes from Asia and South America. World production of tin is approximately 280,000 metric tons. The largest producer is China, which mines about 40 percent of the world total. Second in mine production is Indonesia (30 percent of the world total), followed by Peru (16 percent), Bolivia (6 percent), and Brazil (4 percent). Additional countries—including Australia, Malaysia, Vietnam, and Russia—also mine tin. The last operational tin mine in the United States closed in 1990. World tin reserves have been estimated at about 6 million metric tons, most of which is in Southeast Asia and South America. World consumption is about 350,000 metric tons annually.
Pure tin can also be obtained by recycling tin-plate scrap and tin-plated steel cans. Other secondary tin can be extracted from other scrap materials and from solutions commonly involved in the manufacturing of electronic equipment. Total production of secondary tin from recycling averages about 15,000 metric tons per year, and the United States is the largest producer.
The vein deposits of South America, England, and Australia are mined by the same techniques used in hard-rock base-metal mining throughout the world. Alluvial sands are generally mined by surface mining methods. The sands can be cleared of any barren overburden and then excavated by directing high-pressure water jets that disaggregate the sands. These sands are then washed over a series of baffled sluices that will retain the heavy minerals such as cassiterite. The cassiterite and other heavy minerals are periodically removed from behind the baffles. This ore is then sent to smelters for refining. Some alluvial cassiterite has been washed out to the oceans, and in some Southeast Asian areas the shallow ocean floor is dredged to recover the ore. Stream tin accounts for about 80 percent of the tin recovered each year.
Although cassiterite is almost 79 percent tin, the tin ores and concentrates vary in the amounts of impurities they contain. Generally the ores are first roasted to drive off any sulfur in the associated minerals, and then the tin minerals are heated in the presence of carbon (coke) to reduce the cassiterite to liquid tin with the release of oxygen, which reacts with the carbon to form carbon dioxide. Commonly, limestone is used as a flux to reduce the temperature of the reaction to approximately 1,400° Celsius. The liquid tin is extracted and the floating slag removed and reprocessed. The smelted tin is then refined either electrolytically or by fire. Electrolytically refined tin can reach a purity as great as 99.999 percent.
Uses of Tin
Tin and its alloys have many important commercial and industrial uses. In the past, primary use was in the production of tin plate, which is used in the production of food containers. Tin plate is made by coating steel with a thin (1 micrometer thick) layer of tin. Until the middle of the twentieth century, tin plate was manufactured by immersing the steel in a hot bath of molten tin, but most tin plate is now made by electrolytically plating the tin onto the steel. The tin plate is used primarily to make cans for food and beverages. However, aluminum has become the metal of choice in the production of food cans.
Tin alloys are also used in coating steel and other metals. Zinc, nickel, copper, and lead are each alloyed with tin to produce coatings with specific properties. Terneplate is a tin/lead coating for steel that is commonly used to retard corrosion. Many gasoline tanks are also made of terneplate.
Because of its low melting temperature and its ability to alloy with many metals at different concentrations, tin has long been a major component of solders used to join metal parts. The most common solder is an alloy of tin and lead. Tin and lead can be alloyed at all possible relative concentrations, but most commonly the tin in solder ranges from 30 percent to 70 percent. Tin/lead solders soften over a range of temperatures, and this allows for use in a variety of different soldering techniques. Because of the toxic effects of lead, much research has been concentrated on producing lead-free solders. Tin/silver solders and tin/zinc solders that melt at low temperatures have been developed. In addition, silver and indium are usable alloy metals for lead-free solders.
Babbitt metals are important tin, antimony, and copper alloys that are commonly manufactured into bearings that run against steel shafts. This white metal alloy is soft enough to allow for irregularity in the steel shafts. Babbitt metal can also embed any loose metal particles that arise from the running of the machine, which helps to reduce scratching or scoring of the other machine parts, thus prolonging the life of the machines. Babbitt’s discovery of these alloys was an important ingredient in the Industrial Revolution. Although Babbitt and others experimented with various concentrations of tin, antimony, and copper, the best white metals contain about 7 percent antimony and 3 percent copper. These tin alloys do not bear heavy loads well, however, and are replaced by a tin/aluminum alloy when necessary. This alloy, composed of about 80 percent aluminum, is used in diesel engines and some automobiles. Heavy-duty bearings are composed of leaded bronzes.
Two traditional uses of tin are in the production of bronze and pewter. Tin bronzes today are composed of tin and copper or tin, copper, and lead. The bronze used in the making of bells and musical instruments contains up to 20 percent tin to give it the correct tonal qualities. The pewter of ancient Roman times was an alloy of tin and lead, but the recognition of the toxic nature of lead has caused a shift in the composition of modern pewter. Most pewter today is an alloy containing mostly tin, with only minor amounts of copper and antimony. The copper and antimony give strength to the weak tin and allow it to be used in plate ware as well as in jewelry.
Tin is utilized in many other ways. It is used in small amounts to soften some metal alloys, making the metals easier to machine. It is also alloyed with aluminum and titanium for use in the aerospace industry. Silver and tin are alloyed to make one of the most commonly used dental fillings.
Tin has also replaced the lead and tin/lead capsules that surround corks on wine bottles. Research has shown that the lead from the older types of capsules contaminated some of the wines and that corks posed a potential health hazard. The lead-based capsules were banned in Europe in 1993 and in the United States in 1996. Tin capsules are nontoxic. Between 50 and 60 percent of wine capsules are made from tin.
Research into uses for tin continues. Because of its unusual physical properties, and because it is nontoxic, tin will continue to be an important metal.
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