What is tin. How and where is tin used? Industrial types of tin deposits

Tin is a chemical element with the symbol Sn (from Latin: stannum) and atomic number 50. It is a post-transition metal in group 14 of the periodic table of elements. Tin is obtained primarily from the mineral tin ore containing tin dioxide SnO2. Tin has chemical similarities to its two neighbors in group 14, germanium and lead, and has two main oxidation states, +2 and the slightly more stable +4. Tin is the 49th most abundant element and has the highest number of stable isotopes on the periodic table (with 10 stable isotopes), thanks to its "magic" number of protons. Tin has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal, but at low temperatures, tin changes to the less dense gray α-tin, which has a diamond-like cubic structure. Tin metal is not easily oxidized in air. The first alloy used on a large scale was bronze, made from tin and copper, beginning in 3000 BC. e. After 600 BC e. pure metallic tin was produced. A tin-lead alloy of 85-90% tin, usually consisting of copper, antimony and lead, was used to make tableware from the Bronze Age until the 20th century. Nowadays, tin is used in many alloys, most commonly in soft tin/lead alloys, which typically contain 60% or more tin. Another common use for tin is as a corrosion-resistant coating on steel. Inorganic tin compounds are rather non-toxic. Because of its low toxicity, tinned metal has been used to package food using tin cans, which are actually made primarily of steel or aluminum. However, overexposure to tin can cause problems with the metabolism of essential trace elements such as copper and zinc, and some organotin compounds can be almost as toxic as cyanide.

Characteristics

Physical

Tin is a soft, malleable, ductile and highly crystalline silvery-white metal. When a tin plate is bent, a cracking sound known as "tin crack" can be heard from the twinning of the crystals. Tin melts at a low temperature, around 232 °C, the lowest in group 14. The melting point drops further to 177.3 °C for 11 nm particles. β-tin (metallic form, or white tin, BCT structure), which is stabilized at room temperature and above, is malleable. In contrast, α-tin (the non-metallic form, or gray tin), which is stabilized at temperatures up to 13.2 °C, is brittle. α-tin has a 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 that does not have any widespread use beyond a few specialized semiconductor applications. These two allotropes, α-tin and β-tin, are better known as gray tin and white tin, respectively. Two more allotropes, γ and σ, exist at temperatures above 161 °C and pressures above several gigapascals. Under cold conditions, β-tin spontaneously transforms into α-tin. This phenomenon is known as the "tin plague". Although the α-β transformation temperature is nominally 13.2 °C and impurities (eg Al, Zn, etc.) below the transition temperature are below 0 °C and, with the addition of Sb or Bi, the transformation may not occur at all, increasing the durability of tin. Commercial grades of tin (99.8%) resist transformation due to the inhibitory effect of small amounts of bismuth, antimony, lead and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, silver increase the hardness of the substance. Tin quite easily forms hard, brittle intermetallic phases, which are often undesirable. Tin does not form many solid solutions in other metals in general, and several elements have appreciable solid solubility in tin. Simple eutectic systems, however, are observed with bismuth, gallium, lead, thallium and zinc. Tin becomes a superconductor below 3.72 K and is 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 coating for other metals. A protective oxide (passive) layer prevents further oxidation, the same as that formed on tin-lead and other tin alloys. Tin acts as a catalyst when oxygen is in solution and helps accelerate chemical corrosion.

Isotopes

Tin has ten stable isotopes with atomic masses 112, 114, 120, 122 and 124, the largest number of any element. The most common of these are 120Sn (almost a third of all tin), 118Sn and 116Sn, while the least common are 115Sn. Isotopes with even mass numbers have no nuclear spin, while isotopes with odd numbers have a spin of +1/2. Tin, with three common isotopes 116Sn, 118Sn and 120Sn, is one of the easiest elements to detect and analyze using NMR spectroscopy. This large number of stable isotopes is believed to be a direct result of atomic number 50, the "magic number" in nuclear physics. Tin also occurs in 29 unstable isotopes, covering all other atomic masses from 99 to 137. Apart from 126Sn, with a half-life of 230,000 years, all radioisotopes have half-lives of less than a year. Radioactive 100Sn, discovered in 1994, and 132Sn are among the few nuclides with a “double magic” nucleus: although unstable, having a very uneven proton-neutron ratio, they represent endpoints beyond which stability declines rapidly. Another 30 metastable isomers were characteristic of isotopes between 111 and 131, the most stable being 121mCH with a half-life of 43.9 years. Relative differences in the abundance of stable tin isotopes can be explained by their different modes of formation in stellar nucleosynthesis. 116Sn through 120Sn inclusive are formed by the s-process (slow neutrons) in most stars and are therefore the most common isotopes, while 122Sn and 124Sn are not only formed by the R-process (fast neutrons) in supernovae and less commonly. (The isotopes 117Sn through 120Sn also benefit from the r-process.) Finally, the rarest proton-rich isotopes, 112Sn, 114Sn, and 115Sn, cannot be produced in significant quantities in the s- and r-processes and are considered to be among the p-processes. nuclei, the origin of which is not fully understood. Some proposed mechanisms for their formation include proton capture as well as photodisintegration, although 115Sn can also be partially produced in the s-process, both at once, and as a “daughter” of long-lived 115In.

Etymology

The English word tin (tin) is common to the Germanic languages ​​and can be traced to reconstructed Proto-Germanic *tin-om; cognates include German Zinn, Swedish tenn and Dutch tin. The word is not found in other branches of Indo-European languages, except as a borrowing from Germanic (for example, the Irish word tinne came from English tin). The Latin name stannum originally meant an alloy of silver and lead, and in the 4th century BC. e. it came to mean "tin" - the earlier Latin word for it was plumbum quandum, or "white lead". The word stannum appears to have been derived from the earlier stāgnum (same substance), the origin of the Romanesque and Celtic designation for tin. The origin of stannum/stāgnum is unknown; it may be pre-Indo-European. According to Meyer's Encyclopedic Dictionary, on the contrary, stannum is considered to be a derivative of Cornish stean and is evidence that Cornwall was the main source of tin in the first centuries AD.

Story

The extraction and use of tin began in the Bronze Age, around 3000 BC. BC, when it was noted that copper objects formed from polymetallic ores with different metal contents have different physical properties. The earliest bronze objects contained less than 2% tin or arsenic and are therefore believed to be the result of unintentional alloying by tracing the metal content of the copper ore. Adding a second metal to copper increases its strength, lowers its melting point, and improves the casting process by creating a thinner melt that is denser and less spongy when cooled. This made it possible to create much more complex forms of closed bronze objects. Bronze objects with arsenic appeared primarily in the Middle East, where arsenic is often found in association with copper ore, however, the health risks associated with the use of such objects soon became clear, and the search for sources of much less dangerous tin ores began early Bronze Age. This created a demand for the rare metal tin and formed a trade network linking distant sources of tin to the markets of Bronze Age cultures. Cassiterite, or tin ore (SnO2), an oxide of tin, was most likely the original source of tin in ancient times. Other forms of tin ores are less common sulfides such as stannite, which require a more active smelting process. Cassiterite often accumulates in alluvial channels as placer deposits because it is heavier, tougher, and more chemically resistant than granite. Cassiterite is usually black or generally dark in color, and its deposits are easily visible in river banks. Alluvial (placer) deposits can be easily collected and separated by methods similar to gold panning.

Compounds and chemistry

In the vast majority, tin has an oxidation state of 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 heaviest elements are volatile molecular compounds, while tetrafluoride is polymeric. All four halides for Sn(II) are also known: SnF2, SnCl2, SnBr2 and SnI2. These are all polymeric solids. Of these eight compounds, only iodides are colored. Tin(II) chloride (also known as stannous chloride) is the most important tin halide commercially. Chlorine reacts with tin metal to create SnCl4 while the reaction of hydrochloric acid and tin produces SnCl2 and hydrogen gas. In addition, SnCl4 and Sn combine with tin chloride through a process called co-proportionation: SnCl4 + CH → 2 Sncl2 Tin can form many oxides, sulfides and other chalcogenide derivatives. SnO2 dioxide (cassiterite) is formed when tin is heated in the presence of air. SnO2 is amphoteric in nature, which means it dissolves in acidic and basic solutions. Stannates with the structure Sn(OH)6]2, like K2, are also known, although free stannous acid H2[CH(on)6] is unknown. Tin sulfides exist in both +2 and +4 oxidation states: tin(II) sulfide and tin(IV) sulfide (mosaic gold).

Hydrides

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

Organotin compounds

Organotin compounds, sometimes called stannanes, are chemical compounds with tin-carbon bonds. Of the tin compounds, the organic derivatives are the most commercially useful. Some organotin compounds are very toxic and are used as biocides. The first known organotin compound was diethyltin diodide (C2H5)2SnI2), which was discovered by Edward Frankland in 1849. Most organic tin compounds are colorless liquids or solids that are resistant to air and water. They adopt tetrahedral geometry. Tetraalkyl and tetraaryltine compounds can be prepared using Grignard's reagents:

4 + 4 RMgBr → R

Mixed alkyl halides, which are more common and have greater commercial value than tetraorganic derivatives, are prepared by rearrangement reactions:

4Sn → 2 SnCl2R2

Divalent organotin compounds are rare, although more common than divalent organogermanium and organosilicon compounds. The greater stabilization that Sn(II) has 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.

Emergence

Tin is formed in the long-term s-process in low- and medium-mass stars (with masses from 0.6 to 10 times the mass of the Sun) and, finally, during the beta decay of heavy indium isotopes. Tin is the most abundant 49th element in the earth's crust, at 2 ppm compared to 75 mg/L for zinc, 50 ppm for copper, and 14 ppm for lead. Tin does not occur as a 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, cypindrite, frankeite, canfieldite and tillite. Tin minerals are almost always associated with granite rock, usually at the 1% tin oxide level. Due to the high specific gravity of tin dioxide, about 80% of mined tin comes from secondary deposits recovered from primary deposits. Tin is often recovered from granules washed downstream in the past and deposited in valleys or the sea. The most economical methods of mining tin are scooping, hydraulics or open pits. Most of the world's tin is produced from placer deposits, which may contain as little as 0.015% tin. World tin mine reserves (tons, 2011)

China 1500000

Malaysia 250000

• Indonesia 800000

  Brazil 590000

  Bolivia 400000

  Russia 350000

  Australia 180000

  Thailand 170000

  Others 180000

  Total 4800000

Approximately 253,000 tonnes of tin were mined in 2011, mainly from China (110,000 tonnes), Indonesia (51,000 tonnes), Peru (34,600 tonnes), Bolivia (20,700 tonnes) and Brazil (12,000 tonnes). Estimates of tin production have historically varied depending on economic viability dynamics and developments in mining technology, but at current rates of consumption and technology, it is estimated that the Earth will run out of tin mining within 40 years. Lester Brown suggested that tin could run out within 20 years based on an extremely conservative extrapolation of 2% growth per year. Economically recoverable tin reserves: Million. tons per year

Recycled or scrap tin is also an important source of this metal. Tin recovery through secondary production or recycling of scrap tin is growing at a rapid pace. While the United States has not mined tin since 1993 nor smelted tin since 1989, it has been the largest secondary producer of tin, processing nearly 14,000 tons in 2006. New deposits are found in southern Mongolia, and in 2009 new tin deposits were discovered in Colombia by Seminole Group Colombia CI, SAS.

Production

Tin is produced by carbothermic reduction of oxide ore using carbon or coke. Reverberatory furnaces and electric furnaces can be used.

Price and exchange

Tin is unique among other mineral commodities due to complex agreements between producing and consuming countries dating back to 1921. Earlier agreements tended to be somewhat informal and sporadic and led to the "First International Tin Agreement" in 1956, the first of a permanent series of agreements that effectively ceased to exist in 1985. Through this series of agreements, the International Tin Council (ITC) had a significant influence on tin prices. MCO supported the price of tin during periods of low prices by purchasing tin for its buffer stock and was able to contain the price during periods of high prices by selling tin from this stock. This was an anti-market approach designed to ensure a sufficient flow of tin to consuming countries and profits for producing countries. However, the buffer stock was not large enough, and for most of those 29 years, tin prices rose, sometimes sharply, especially from 1973 to 1980, when rampant inflation plagued many of the world's economies. In the late 1970s and early 1980s, US government tin inventories were in an aggressive sales mode, in part to take advantage of historically high tin prices. The slump of 1981-82 was quite harsh for the tin industry. Tin consumption dropped sharply. MCO was able to avoid a truly drastic reduction by accelerating purchases for its buffer stock; these activities required MCOs to borrow on a large scale from banks and metal trading firms to increase their resources. MCO continued to borrow funds until the end of 1985, when it reached its credit limit. Immediately after this came the great “tin crisis”, and then tin was excluded from trading on the London Metal Exchange for a period of three years, the MCO soon collapsed, and tin prices, already in a free market, plummeted to $4 per pound (453 g) , and remained at this level until the 1990s. The price increased again by 2010 with a rebound in consumption following the 2008–09 World Economic Crisis, accompanying renewed and continued growth in consumption in the developing world. The London Metal Exchange (LME) is the main trading platform for tin. Other tin markets are Kuala Lumpur Tin Market (KLTM) and Indonesia Tin Exchange (INATIN).

Applications

In 2006, about half of all tin produced was used in solders. The remaining uses were 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 quantities ranging from 5 to 70%. Tin forms a eutectic mixture with lead in the proportion of 63% tin and 37% lead. Such solders are used to join pipes or electrical circuits. On 1 July 2006, the European Union's Waste Electrical and Electronic Equipment Directive (WEEE Directive) and the RoHS Directive came into force. The lead content in such alloys has decreased. Replacing lead comes with many problems, including higher melting points and the formation of tin whiskers. Tin plague can occur in lead-free solders.

Tinning

Tin bonds take well to ironing and are used to coat lead, zinc and steel to prevent corrosion. Tinned steel containers are widely used for food preservation, and this forms a large part of the tin metal market. In London in 1812, the first tin canister was made for preserving food. In British English these are called "tins", but in America they are called "cans" or "tin cans". The slang name for a can of beer is "tinnie" or "tinny". Copper cooking vessels such as pots and pans are often lined with a thin layer of tin, since the combination of acidic foods with copper can be toxic.

Specialized alloys

Tin combines with other elements to form many useful alloys. Tin is most often alloyed with copper. Tin-lead alloy has 85-99% tin; Bearing metal also contains a high percentage of tin. Bronze is primarily copper (12% tin), while the addition of phosphorus produces phosphor bronze. Bell bronze is also a copper-tin alloy containing 22% tin. Tin was sometimes used in coins to create American and Canadian pennies. Because copper was often the base metal in these coins, sometimes including zinc, they may be called bronze and/or brass alloys. The niobium-tin compound Nb3Sn has been commercially used in superconducting magnet coils due to its high critical temperature (18 K) and critical magnetic field (25 T). A superconducting magnet weighing just two kilograms can create the same magnetic field as electromagnets with normal weight. A small proportion of tin is added to zirconium alloys for cladding nuclear fuel. Most metal pipes on an organ have varying amounts of tin/lead, with 50/50 alloys being the most common. The amount of tin in the pipe determines the tone of the pipe, as tin gives the instrument the desired resonance. When a tin/lead alloy cools, the lead cools slightly faster and produces a mottled or mottled effect. This metal alloy is called spotted metal. The main benefits of using tin for pipes are its appearance, performance and corrosion resistance.

Other Applications

Perforated tinned steel is a craft technique that originated in Central Europe to create household items that were both functional and decorative. Perforated tin lanterns are the most common application of this technique. Candle light passing through the perforations creates a decorative light pattern. Lanterns and other perforated tin items have been created in the New World since the earliest European settlements. A famous example is the Revere lantern, named after Paul Revere. Before the modern era, in some areas of the Alps, goat or ram horns were sharpened and metal was punched through it in the shape of the alphabet and numbers from one to nine. This teaching tool was known simply as the "horn". Modern reproductions feature motifs such as hearts and tulips. In America, wooden cabinets of various styles and sizes were used for cakes and food before refrigeration, designed to repel pests and insects and keep perishable foods from dust. These were either floor or hanging cabinets. These cabinets had tin inserts in the doors and sometimes on the sides. Window glass is most often made by placing molten glass on molten tin (float glass - sheet glass produced from molten metal), resulting in a perfectly smooth surface. This is also called the Pilkington process. Tin is also used as the negative electrode in modern lithium-ion batteries. Its use is somewhat limited by the fact that some tin surfaces catalyze the decomposition of carbonate electrolytes used in lithium-ion batteries. Stann(II) fluoride is added to some dental care products (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

Among all the chemical compounds of tin, organic tin compounds are the most commonly used. Their global industrial production probably exceeds 50,000 tons.

PVC stabilizers

The main commercial use of organotin compounds is in the stabilization of PVC plastic. In the absence of such stabilizers, PVC would otherwise rapidly degrade when exposed to heat, light and atmospheric oxygen, resulting in a discolored and brittle product. Tin scavenges labile chloride ions (Cl−), which would otherwise cause HCl to be lost from plastic. Typical tin compounds are carboxylic acid derivatives of dibutyltin dichloride, such as dibutyltin dilaurate.

Biocides

Some organotin compounds are relatively toxic, which has its advantages and disadvantages. They are used for their biocidal properties as fungicides, pesticides, algaecides, wood preservatives and anti-rot agents. Tributyltin oxide is used as a wood preservative. Tributyltin was used as a marine paint additive to prevent the growth of marine organisms on ships, although use decreased after organotin compounds were recognized as persistent organic pollutants with extremely high toxicity to some marine organisms (eg, scarlet grass). The EU banned the use of organotin compounds in 2003, while concerns about the toxicity of these compounds to marine life and damage to the reproduction and growth of some marine species (some reports describe biological effects on marine life at concentrations of 1 nm per liter) led to a worldwide prohibited by the International Maritime Organization. Currently, many states restrict the use of organotin compounds to vessels longer than 25 m.

Organic chemistry

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

Lithium-ion batteries

Tin forms several intermetallic phases with lithium metal, making it a potentially attractive material for battery applications. The large volumetric expansion of tin upon lithium doping and the instability of the organotin electrolyte interface at low electrochemical potentials are the greatest challenges for use in commercial cells. The problem was partially resolved by Sony. Tin intermetallic compounds with cobalt and carbon are marketed by Sony in its Nexelion cells released in the late 2000s. The composition of the active substance is approximately Sn0.3Co0.4C0.3. Recent studies have shown that only certain crystalline facets of tetragonal (beta)Sn are responsible for undesirable electrochemical activity.

Tin is one of the few metals known to man since prehistoric times. Tin and copper were discovered before iron, and their alloy, bronze, is, apparently, the very first “artificial” material, the first material prepared by man.
The results of archaeological excavations suggest that even five millennia BC people knew how to smelt tin itself. It is known that the ancient Egyptians brought tin for the production of bronze from Persia.
This metal is described under the name “trapu” in ancient Indian literature. The Latin name for tin, stannum, comes from the Sanskrit "sta", meaning "solid".

Mention of tin is also found in Homer. Almost ten centuries BC, the Phoenicians delivered tin ore from the British Isles, then called the Cassiterides. Hence the name cassiterite, the most important of the tin minerals; its composition is Sn0 2. Another important mineral is stannin, or tin pyrite, Cu 2 FeSnS 4 . The remaining 14 minerals of element No. 50 are much less common and have no industrial significance.
By the way, our ancestors had richer tin ores than we do. It was possible to smelt metal directly from ores located on the surface of the Earth and enriched during the natural processes of weathering and leaching. Nowadays, such ores no longer exist. In modern conditions, the process of obtaining tin is multi-stage and labor-intensive. Ores from which tin is smelted now, they are complex in composition: in addition to element No. 50 (in the form of oxide or sulfide), they usually contain silicon, iron, lead, copper, zinc, arsenic, aluminum, calcium, tungsten and other elements. Today's tin ores rarely contain more than 1% Sn, and placers contain even less: 0.01-0.02% Sn. This means that to obtain a kilogram of tin, at least a hundredweight of ore must be mined and processed.

How is tin obtained from ores?

The production of element No. 50 from ores and placers always begins with enrichment. Methods for enriching tin ores are quite varied. In particular, the gravity method is used, based on the difference in density of the main and accompanying minerals. At the same time, we must not forget that those who accompany them are not always empty breeds. They often contain valuable metals, such as tungsten, titanium, and lanthanides. In such cases, they try to extract all the valuable components from the tin ore.
The composition of the resulting tin concentrate depends on the raw materials, and also on the method by which this concentrate was obtained. The tin content in it ranges from 40 to 70%. The concentrate is sent to kilns (at 600-700° C), where relatively volatile impurities of arsenic and sulfur are removed from it. And most of the iron, antimony, bismuth and some other metals are leached with hydrochloric acid after firing. After this is done, all that remains is to separate the tin from the oxygen and silicon. Therefore, the last stage of rough tin production is smelting with coal and fluxes in reverberatory or electric furnaces. From a physicochemical point of view, this process is similar to the blast furnace process: carbon “takes away” oxygen from tin, and fluxes transform silicon dioxide into slag, which is light compared to metal.
There are still quite a lot of impurities in rough tin: 5-8%. To obtain grade metal (96.5-99.9% Sn), fire or, less commonly, electrolytic refining is used. And the tin needed by the semiconductor industry with a purity of almost six nines - 99.99985% Sn - is obtained mainly by the method of zone melting.

Another source

In order to get a kilogram of tin, it is not necessary to process a hundredweight of ore. You can do it differently: “rip off” 2000 old tin cans.
There is only half a gram of tin per jar. But multiplied by the scale of production, these half-grams turn into tens of tons... The share of “secondary” tin in the industry of capitalist countries is approximately a third of total production. There are about one hundred industrial tin recovery plants operating in our country.
How do you remove tin from tinplate? It is almost impossible to do this by mechanical means, so they use the difference in the chemical properties of iron and tin. Most often, tin is treated with chlorine gas. Iron does not react with it in the absence of moisture. It combines with chlorine very easily. A fuming liquid is formed - tin chloride SnCl 4, which is used in the chemical and textile industries or sent to an electrolyzer to obtain metal tin from it. And the “whirlwind” will begin again: they will cover steel sheets with this tin and get tinplate. It will be made into jars, the jars will be filled with food and sealed. Then they will open them, eat the cans, and throw away the cans. And then they (not all, unfortunately) will again end up in “secondary” tin factories.
Other elements cycle in nature with the participation of plants, microorganisms, etc. The tin cycle is the work of human hands.

Tin in alloys

About half of the world's tin production goes into cans. The other half goes to metallurgy, to produce various alloys. We will not talk in detail about the most famous of the tin alloys - bronze, referring readers to the article about copper - another important component of bronzes. This is all the more justified since there are tin-free bronzes, but there are no “copper-free” bronzes. One of the main reasons for the creation of tin-free bronzes is the scarcity of element No. 50. Nevertheless, bronze containing tin still remains an important material for both mechanical engineering and art.
Equipment also requires other tin alloys. However, they are almost never used as structural materials: they are not strong enough and are too expensive. But they have other properties that make it possible to solve important technical problems with relatively low material costs.
Most often, tin alloys are used as antifriction materials or solders. The former allow you to preserve machines and mechanisms, reducing friction losses; the latter connect metal parts.
Of all antifriction alloys, tin babbits, which contain up to 90% tin, have the best properties. Soft and low-melting lead-tin solders well wet the surface of most metals and have high ductility and fatigue resistance. However, their scope of application is limited due to the insufficient mechanical strength of the solders themselves.
Tin is also included in the typographic alloy garta. Finally, tin-based alloys are very necessary for electrical engineering. The most important material for electrical capacitors is staniol; this is almost pure tin, turned into thin sheets (the share of other metals in staniol does not exceed 5%).
By the way, many tin alloys are true chemical compounds of element No. 50 with other metals. When fused, tin interacts with calcium, magnesium, zirconium, titanium, and many rare earth elements. The compounds formed in this case are quite refractory. Thus, zirconium stannide Zr 3 Sn 2 melts only at 1985° C. And not only the refractoriness of zirconium is to blame here, but also the nature of the alloy, the chemical bond between the substances that form it. Or another example. Magnesium cannot be classified as a refractory metal; 651° C is far from a record melting point. Tin melts at an even lower temperature - 232° C. And their alloy - the Mg2Sn compound - has a melting point of 778° C.
The fact that element No. 50 forms quite numerous alloys of this kind makes us critical of the statement that only 7% of the tin produced in the world is consumed in the form of chemical compounds. Apparently, we are talking here only about compounds with non-metals.

Compounds with non-metals

Of these substances, chlorides are the most important. Iodine, phosphorus, sulfur, and many organic substances dissolve in tin tetrachloride SnCl 4. Therefore, it is used mainly as a very specific solvent. Tin dichloride SnCl 2 is used as a mordant for dyeing and as a reducing agent in the synthesis of organic dyes. Another compound of element No. 50, sodium stannate Na 2 Sn0 3, has the same functions in textile production. In addition, it makes silk heavier.
Industry uses tin oxides to a limited extent. SnO is used to produce ruby ​​glass, and Sn0 2 - white glaze. Golden-yellow crystals of olive disulfide SnS 2 are often called gold leaf, which is used to “gild” wood and gypsum. This, so to speak, is the most “anti-modern” use of tin compounds. What about the most modern?
If we keep in mind only tin compounds, then this is the use of barium stannate BaSn0 3 in radio engineering as an excellent dielectric. And one of the tin isotopes, il9Sn, played a significant role in the study of the Mössbauer effect - a phenomenon that led to the creation of a new research method - gamma resonance spectroscopy. And this is not the only case where an ancient metal has served modern science.
Using the example of gray tin - one of the modifications of element No. 50 - a connection was revealed between the properties and the chemical nature of the semiconductor material. And this, apparently, is the only thing for which gray tin can be remembered with a kind word: it did more harm than good. We will return to this variety of element No. 50 after talking about another large and important group of tin compounds.

About organotin

There are a great variety of organoelement compounds that include tin. The first of them was received back in 1852.
At first, substances of this class were obtained in only one way - in an exchange reaction between inorganic tin compounds and Grignard reagents. Here is an example of such a reaction:
SnCl 4 + 4RMgX → SnR 4 + 4MgXCl (R here is a hydrocarbon radical, X is a halogen).
Compounds of the SnR4 composition have not found wide practical application. But it is from them that other organotin substances are obtained, the benefits of which are undoubted.

Interest in organotin first arose during the First World War. Almost all organic tin compounds obtained by that time were toxic. These compounds were not used as toxic substances; their toxicity to insects, molds, and harmful microbes was used later. Based on triphenyltin acetate (C 6 H 5) 3 SnOOCCH 3, an effective drug was created to combat fungal diseases of potatoes and sugar beets. This drug turned out to have another useful property: it stimulated the growth and development of plants.
To combat fungi that develop in the apparatus of the pulp and paper industry, another substance is used - tributyltin hydroxide (C 4 H 9) 3 SnOH. This greatly improves the performance of the equipment.
Dibutyltin dilaurate (C 4 H 9) 2 Sn (OCOC 11 H 23) 2 has many “professions”. It is used in veterinary practice as a remedy against helminths (worms). The same substance is widely used in the chemical industry as a stabilizer for polyvinyl chloride and other polymer materials and as a catalyst. Speed
the reaction of formation of urethanes (polyurethane rubber monomers) in the presence of such a catalyst increases by 37 thousand times.
Effective insecticides have been created based on organotin compounds; organotin glasses reliably protect against x-rays, polymer lead and organotin paints are used to cover the underwater parts of ships to prevent mollusks from growing on them.
All these are compounds of tetravalent tin. The limited scope of the article does not allow us to talk about many other useful substances of this class.
Organic compounds of divalent tin, on the contrary, are few in number and have so far found almost no practical use.

About gray tin

In the frosty winter of 1916, a shipment of tin was sent by rail from the Far East to the European part of Russia. But what arrived at the scene was not silver-white ingots, but mostly fine gray powder.
Four years earlier, a disaster occurred with the expedition of polar explorer Robert Scott. The expedition heading to the South Pole was left without fuel: it leaked from iron vessels through seams soldered with tin.
Around the same years, the famous Russian chemist V.V. Markovnikov was approached by the commissariat with a request to explain what was happening with the tinned teapots that were supplied to the Russian army. The teapot, which was brought into the laboratory as an illustrative example, was covered with gray spots and growths that crumbled even when lightly tapped with a hand. The analysis showed that both the dust and the growths consisted only of tin, without any impurities.

What happened to the metal in all these cases?
Like many other elements, tin has several allotropic modifications, several states. (The word “allotropy” is translated from Greek as “another property,” “another turn.”) At normal above-zero temperatures, tin looks so that no one can doubt that it belongs to the class of metals.
White metal, ductile, malleable. White tin crystals (also called beta tin) are tetragonal. The length of the edges of the elementary crystal lattice is 5.82 and 3.18 A. But at temperatures below 13.2 ° C, the “normal” state of tin is different. As soon as this temperature threshold is reached, a restructuring begins in the crystal structure of the tin ingot. White tin is converted into powdered gray or alpha tin, and the lower the temperature, the greater the rate of this conversion. It reaches its maximum at minus 39° C.
Gray tin crystals of cubic configuration; the dimensions of their elementary cells are larger - the edge length is 6.49 A. Therefore, the density of gray tin is noticeably lower than white tin: 5.76 and 7.3 g/cm3, respectively.
The result of white tin turning to gray is sometimes called "tin plague". Stains and growths on army teapots, carriages with tin dust, seams that have become permeable to liquid are the consequences of this “disease”.
Why don't similar stories happen now? For only one reason: they learned to “treat” the tin plague. Its physicochemical nature has been clarified, and it has been established how certain additives affect the susceptibility of the metal to the “plague”. It turned out that aluminum and zinc promote this process, while bismuth, lead and antimony, on the contrary, counteract it.
In addition to white and gray tin, another allotropic modification of element No. 50 was discovered - gamma tin, stable at temperatures above 161 ° C. A distinctive feature of such tin is fragility. Like all metals, tin becomes more ductile with increasing temperature, but only at temperatures below 161 ° C. Then it completely loses its ductility, turning into gamma tin, and becomes so brittle that it can be crushed into powder.

Once again about the broom shortage

Often articles about elements end with the author’s speculations about the future of his “hero.” As a rule, it is drawn in pink light. The author of the article on tin is deprived of this opportunity: the future of tin - a metal undoubtedly the most useful - is unclear. It's unclear for one reason only.
Several years ago, the American Bureau of Mines published calculations from which it followed that proven reserves of element No. 50 would last the world for at most 35 years. True, after this, several new deposits were found, including the largest in Europe, located on the territory of the Polish People's Republic. And yet, the shortage of tin continues to worry experts.
Therefore, finishing the story about element No. 50, we want to once again remind you of the need to save and protect tin.
The shortage of this metal worried even the classics of literature. Remember Andersen? “Twenty-four soldiers were exactly the same, and the twenty-fifth soldier was one-legged. It was the last to be cast, and there wasn’t enough tin.” Now the tin is missing quite a bit. It is not for nothing that even two-legged tin soldiers have become rare - plastic ones are more common. But with all due respect to polymers, they cannot always replace tin.
ISOTOPES. Tin is one of the most “multi-isotopic” elements: natural tin consists of ten isotopes with mass numbers 112, 114-120, 122 n 124. The most common of them is i20Sn, accounting for about 33% of all terrestrial tin. Almost 100 times less than tin-115, the rarest isotope of element No. 50.
Another 15 isotopes of tin with mass numbers 108-111, 113, 121, 123, 125-132 were obtained artificially. The lifetime of these isotopes is far from the same. Thus, tin-123 has a half-life of 136 days, and tin-132 only 2.2 minutes.

The chemical element tin is one of the seven ancient metals known to mankind. This metal is part of bronze, which is of great importance. Currently, the chemical element tin has lost its popularity, but its properties deserve detailed consideration and study.

What is an element

It is located in the fifth period, in the fourth group (the main subgroup). This arrangement indicates that the chemical element tin is an amphoteric compound capable of exhibiting both basic and acidic properties. The relative atomic mass is 50, so it is considered a light element.

Peculiarities

The chemical element tin is a plastic, malleable, light substance of silvery white color. As it is used, it loses its shine, which is considered a disadvantage of its characteristics. Tin is a dispersed metal, so there are difficulties with its extraction. The element has a high boiling point (2600 degrees), a low melting point (231.9 C), high electrical conductivity, and excellent malleability. It has high tear resistance.

Tin is an element that does not have toxic properties and does not have a negative effect on the human body, therefore it is in demand in food production.

What other properties does tin have? When choosing this element for making dishes and water pipelines, you will not have to fear for your safety.

Finding in the body

What else is tin (a chemical element) characterized by? How is its formula read? These issues are discussed in the school curriculum. In our body, this element is located in the bones, promoting the process of bone tissue regeneration. It is classified as a macronutrient, therefore, for full life, a person needs from two to ten mg of tin per day.

This element enters the body in larger quantities with food, but the intestines absorb no more than five percent of the intake, so the likelihood of poisoning is minimal.

With a lack of this metal, growth slows down, hearing loss occurs, the composition of bone tissue changes, and baldness occurs. Poisoning is caused by the absorption of dust or vapors of this metal, as well as its compounds.

Basic properties

The density of tin is average. The metal is highly corrosion resistant, which is why it is used in the national economy. For example, tin is in demand in the manufacture of tin cans.

What else is tin characterized by? The use of this metal is also based on its ability to combine various metals, creating an external environment resistant to aggressive environments. For example, the metal itself is necessary for tinning household items and utensils, and its solders are needed for radio engineering and electricity.

Characteristics

In terms of its external characteristics, this metal is similar to aluminum. In reality, the similarity between them is insignificant, limited only by lightness and metallic luster, resistance to chemical corrosion. Aluminum exhibits amphoteric properties, so it easily reacts with alkalis and acids.

For example, if aluminum is exposed to acetic acid, a chemical reaction is observed. Tin, on the other hand, can only react with strong concentrated acids.

Advantages and disadvantages of tin

This metal is practically not used in construction because it does not have high mechanical strength. Basically, nowadays it is not pure metal that is used, but its alloys.

Let us highlight the main advantages of this metal. Malleability is of particular importance; it is used in the process of making household items. For example, stands and lamps made of this metal look aesthetically pleasing.

The tin coating significantly reduces friction, thereby protecting the product from premature wear.

Among the main disadvantages of this metal, one can mention its low strength. Tin is unsuitable for the manufacture of parts and components that involve significant loads.

Metal mining

Melting of tin is carried out at a low temperature, but due to the difficulty of its extraction, the metal is considered an expensive substance. Due to the low melting point, when applying tin to the surface of a metal, significant savings in electrical energy can be achieved.

Structure

The metal has a homogeneous structure, but depending on the temperature, its different phases are possible, differing in characteristics. Among the most common modifications of this metal, we note the β-variant, which exists at a temperature of 20 degrees. Thermal conductivity and its boiling point are the main characteristics given for tin. When the temperature decreases from 13.2 C, an α-modification called gray tin is formed. This form does not have plasticity and malleability, and has a lower density because it has a different crystal lattice.

When moving from one form to another, a change in volume is observed, since there is a difference in density, resulting in the destruction of the tin product. This phenomenon is called the “tin plague.” This feature leads to the fact that the area of ​​use of the metal is significantly reduced.

Under natural conditions, tin can be found in rocks in the form of a trace element, and its mineral forms are also known. For example, cassiterite contains its oxide, and tin pyrite contains its sulfide.

Production

Tin ores with a metal content of at least 0.1 percent are considered promising for industrial processing. But at present, deposits in which the metal content is only 0.01 percent are also being exploited. Various methods are used to extract the mineral, taking into account the specifics of the deposit, as well as its variety.

Tin ores are mainly presented in the form of sands. Extraction comes down to its constant washing, as well as the concentration of the ore mineral. It is much more difficult to develop a primary deposit, since additional structures, construction and operation of mines are required.

The mineral concentrate is transported to a plant specializing in smelting non-ferrous metals. Next, the ore is repeatedly enriched, crushed, and then washed. Ore concentrate is restored using special furnaces. To completely restore tin, this process is carried out several times. At the final stage, the process of cleaning rough tin from impurities is carried out using a thermal or electrolytic method.

Usage

The main characteristic that allows the use of tin is its high corrosion resistance. This metal, as well as its alloys, are among the most resistant compounds to aggressive chemicals. More than half of all tin produced in the world is used to make tinplate. This technology, associated with applying a thin layer of tin to steel, began to be used to protect cans from chemical corrosion.

The rolling ability of tin is used to produce thin-walled pipes from it. Due to the instability of this metal to low temperatures, its domestic use is quite limited.

Tin alloys have a significantly lower thermal conductivity value than steel, so they can be used for the production of washbasins and bathtubs, as well as for the manufacture of various sanitary fittings.

Tin is suitable for the production of minor decorative and household items, making tableware, and creating original jewelry. This dull and malleable metal, when combined with copper, has long become one of the most favorite materials of sculptors. Bronze combines high strength and resistance to chemical and natural corrosion. This alloy is in demand as a decorative and building material.

Tin is a tonally resonant metal. For example, when it is combined with lead, an alloy is obtained that is used to make modern musical instruments. Bronze bells have been known since ancient times. An alloy of tin and lead is used to create organ pipes.

Conclusion

The increasing attention of modern manufacturing to issues related to environmental protection, as well as to problems related to maintaining public health, has influenced the composition of materials used in the manufacture of electronics. For example, there has been increased interest in lead-free soldering process technology. Lead is a material that causes significant harm to human health, which is why it is no longer used in electrical engineering. Soldering requirements became more stringent, and tin alloys began to be used instead of dangerous lead.

Pure tin is practically not used in industry, since problems arise with the development of the “tin plague”. Among the main areas of application of this rare scattered element, we highlight the production of superconducting wires.

Coating contact surfaces with pure tin allows you to increase the soldering process and protect the metal from corrosion.

As a result of the transition to lead-free technology by many steel manufacturers, they began to use natural tin to cover contact surfaces and leads. This option allows you to obtain high-quality protective coating at an affordable cost. Due to the absence of impurities, the new technology is not only considered environmentally friendly, but also makes it possible to obtain excellent results at an affordable cost. Manufacturers consider tin to be a promising and modern metal in electrical engineering and radio electronics.

Tin is a light metal with atomic number 50, which is in group 14 of the periodic table of elements. This element was known in ancient times and was considered one of the rarest and most expensive metals, so tin products could be afforded by the richest inhabitants of the Roman Empire and Ancient Greece. Special bronze was made from tin, which was used back in the third millennium BC. At that time, bronze was the most durable and popular alloy, and tin served as one of the impurities and was used for more than two thousand years.

In Latin, this metal was called the word “stannum”, which means durability and strength, but this name previously denoted an alloy of lead and silver. It was only in the 4th century that this word began to be used to refer to tin itself. The very name “tin” has many versions of its origin. In ancient Rome, wine vessels were made of lead. It can be assumed that tin was the name given to the material from which vessels were made for storing the drink tin, consumed by the ancient Slavs.

In nature, this metal is rare; in terms of prevalence in the earth’s crust, tin occupies only 47th place and is mined from cassiterite, the so-called tin stone, which contains about 80 percent of this metal.

Industrial Applications

Since tin is a non-toxic and very durable metal, it is used in alloys with other metals. For the most part, it is used to make tin plate, which is used in the production of cans, solders in electronics, and also for the production of bronze.

Physical properties of tin

This element is a white metal with a silvery sheen.

If you heat the tin, you can hear a crackling sound. This sound is caused by the friction of the crystals against each other. Also, a characteristic crunch will appear if a piece of tin is simply bent.

Tin is very ductile and malleable. Under classical conditions, this element exists in the form of “white tin”, which can be modified depending on temperature. For example, in the cold, white tin will turn gray and have a structure similar to that of diamond. By the way, gray tin is very fragile and literally crumbles into powder before your eyes. In this regard, history has the terminology “tin plague”.

Previously, people did not know about this property of tin, so buttons and mugs for soldiers, as well as other useful things, which after a short time in the cold turned into powder, were made from it. Some historians believe that it was precisely because of this property of tin that the combat effectiveness of Napoleon’s army decreased.

Obtaining tin

The main method of obtaining tin is the recovery of the metal from ore containing tin(IV) oxide using coal, aluminum or.

SnO₂ + C = Sn + CO₂

Particularly pure tin is obtained by electrochemical refining or by zone melting.

Chemical properties of tin

At room temperature, tin is quite resistant to exposure to air or. This is explained by the fact that a thin oxide film appears on the metal surface.

In air, tin begins to oxidize only at temperatures above 150 °C:

Sn + O₂ → SnO₂

If tin is heated, this element will react with most non-metals, forming compounds with the +4 oxidation state (which is more characteristic of this element):

Sn + 2Cl₂ → SnCl₄

The interaction of tin and concentrated hydrochloric acid proceeds rather slowly:

Sn + 4HCl → H₂ + H₂

Tin reacts very slowly with concentrated sulfuric acid, while it does not react at all with dilute sulfuric acid.

The reaction of tin with nitric acid, which depends on the concentration of the solution, is very interesting. The reaction proceeds to form stannous acid, H₂SnO₃, which is a white amorphous powder:

3Sn + 4HNO₃ + nH₂O = 3H₂SnO₃ nH₂O + 4NO

If mixed with dilute nitric acid, this element will exhibit metallic properties with the formation of tin nitrate:

4Sn + 10HNO₃ = 4Sn(NO₃)₂ + NH₄NO₃ + 3H₂O

Heated tin can react with alkalis to release hydrogen:

Sn + 2KOH + 4H₂O = K₂ + 2H₂

you will find safe and very beautiful experiments with tin.

Oxidation states of tin

In its simple state, the oxidation state of tin is zero. Sn can also have an oxidation state of +2: tin(II) oxide SnO, SnCl₂, tin(II) hydroxide Sn(OH)₂. The oxidation state +4 is most typical for tin(IV) oxide SnO₂, halides(IV), for example, SnCl₄ chloride, tin(IV) sulfide SnS₂, tin(IV) nitride Sn₃N₄.

Tin or Stannum (lat.) is a low-melting, ductile metal with a silver-white color (see photo). The Latin name means “durable, resistant” and was originally the name given to the alloy with lead and silver. And the Slavic name, which has Baltic roots, simply means the color of the metal – white.

This element belongs to the seven most ancient metals. Already 6000 years ago, humanity was familiar with it. It was most widespread as part of bronze and was strategically important during the “Bronze Age” about 4,000 years ago. Money was printed from this composition until the 16th century, dishes and jewelry were made, and it was used as an anti-corrosion coating. Mentions of metal were found even on the pages of the Bible.

Occurs in nature in the form of minerals. The most common are cassiterite (river tin) and stanine (tin pyrite). Tin is extracted from them for industrial purposes: electronics, batteries, glass processing (it becomes impenetrable to the rays of an X-ray machine). Also, compounds of this element are used to make cans and substances that repel insects.

Tin has another remarkable ability - its presence in the composition of the materials of a musical instrument, which will distinguish this instrument with excellent purity of sound and melody.

The element was discovered in living organisms in 1923. When studying the remains of ancient people, it turned out that the tin content in the bones was 1000 times less than that of modern humans. This may be due to the fact that we can absorb it from the air. And the development of industry has led to the fact that about a quarter of a million tons end up in the atmosphere in the form of exhaust gases.

Action of tin

The effect of a macroelement on a living organism can hardly be called toxic; it is often used in the food industry. Its role is not fully understood. The element is found mainly in bones, and some of it is also found in the lungs, heart, kidneys, and intestines. And with age, the content in the lungs may increase, this is due to environmental influences.

To date, the following facts of biological effects are known:

• participation in growth processes;
• is part of the stomach enzyme gastrin;
• actively participates in redox reactions;
• Due to its concentration in bone tissue, it promotes their proper development and the development of the musculoskeletal system.

It can have a beneficial effect on the body only when it is contained in fatty acids. Mineral compounds can have a toxic effect.

Relatively recently, tin was used by doctors to treat many diseases - epilepsy, neuroses, helminthiasis, eczema, clouding of the cornea. The external use of tin chloride was mainly practiced. Fortunately, today progress has brought more effective and less toxic drugs without metal content.

Tin is a fairly chemically inactive element, so from this point of view it will not bring any particular benefit or harm. The only interactions observed are with copper and zinc. They mutually neutralize each other's action.

Daily norm

The daily macronutrient requirement ranges from 2 to 10 mg, depending on age and gender. Although our body enters about 50 mg per day only with food (and a dosage of 20 mg is considered toxic), poisoning will not occur. This is explained by the fact that our gastrointestinal tract is able to absorb only 3-5% of the total incoming amount. All other metal is simply excreted naturally through urine.

Lack of tin in the human body

A deficiency of a macroelement in the body occurs with a chronic intake of less than 1 mg per day. This process may be accompanied by hearing loss, weight loss due to loss of appetite, slowed growth, mineral imbalance, and hair loss (partial or complete pathology).

Such processes are quite rare, because Usually, the macronutrient intake from food is sufficient and is most often caused by digestive problems and difficulties with absorption.

Harm from excess tin intake

Employees of enterprises that use tin salts are mainly at risk of getting an excess of macronutrients: the production of plastics, pesticides, linoleum, etc. Due to the regular absorption of vapors and dust, lung diseases develop. Also at risk are people living dangerously close to highways (within half a kilometer) - they receive a high dose from exhaust gases. Tin in large quantities suppresses the content of magnesium, which can protect cells from tumors.

There is another source of high doses of the element - cans. During long-term storage, they begin to deteriorate, especially if the contents are rich in nitrates. Therefore, after opening such a jar, it is recommended to immediately transfer the products into glass. It is strictly prohibited to store canned food in open form.

The bodies of older people and children cannot quickly remove tin from the body, so it begins to accumulate. A very tiny dose is enough to cause poisoning.

There is an interesting theory from the story of the fall of the Roman Empire. Tin got into the wine, abundantly consumed by the ancient Romans, from dishes and caused health problems. Only in the seventh century were doctors able to determine the cause of the disease, but it was too late - the empire fell.

The complications that arise from excess tin are quite unpleasant. A dose of 2 grams of a macroelement is considered dangerous, but it is not lethal (this norm has not yet been determined). It can cause anemia, liver disease, respiratory tract disease, and nervous system disorders. A disease such as stannosis may develop - a severe cough accompanied by sputum production and shortness of breath.

But that’s not all – there are a lot of main symptoms of poisoning:

If tin is consumed in large doses over a long period of time, there is a risk of structural changes in chromosomes, which can lead to serious consequences at the genetic level.

When exposed to the central nervous system, this macroelement can cause depression. And children can be aggressive, lack interest in studying, playing, and reading.

Treatment is usually prescribed based on symptoms - diets, hepatoprotectors (liver protection), drugs containing copper and zinc. In case of critical poisoning, medications are administered that can bind and remove toxins - chelating substances.

What foods does it contain?

Products containing tin can be found of both animal and plant origin. The bulk comes from pig meat, beef, poultry, milk and its derivatives. Peas, sunflower seeds, potatoes, and beets can also provide some amount of the element. Other vegetables contain very small doses of tin.

In addition, we receive macronutrients from water and air every day. And don’t forget that frequent consumption of canned food can also supply the body with excess amounts of tin.

Some plants are capable of absorbing large amounts of the element from the environment. Therefore, you should be careful about products grown near highways and industrial zones.

Indications for use

Indications for prescribing a macronutrient are mainly used by homeopaths. They treat diseases such as:

• bronchitis, lung diseases;
• migraine;
• pancreatitis;
• small height and weight;
• and also used as an anthelmintic.

It has been noticed that when taking small doses of medications containing tin, the patient’s mental state often changes - a good mood is replaced by irritability, melancholicity, and tearfulness. Therefore, such appointments are used in extremely rare cases.