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1 Introduction

The middle of the nineteenth century saw Cornwall in England as the centre of the world’s tin industry. Before the Second World War, tin smelting was entirely in the hands of the British and Dutch, but when the Japanese overran the Far Eastern deposits, the USA came into the picture by constructing a large tin recovery plant to treat the available ores from Bolivia in South America.

Cans made of steel that are used for storing edibles are invariably coated with tin. This is due to tin has good resistance properties of corrosion and toxicity. This particular application is universal that a can is often known as tin. Apart from plating, tin finds many other applications like as alloying element in bronze, white metal etc., and as soldering element.

2 Sources

An important mineral of tin is cassiterite (SnO2). It is found mainly in Bolivia, Malaysia and Indonesia. Cassiterite is usually associated with gangue minerals and metallic sulphides (PbS, ZnS, CuFeS2 and FeS2). Theoretical Sn content in cassiterite (SnO2) is 78.6%, but practically its content is about 1% Sn or even less.

3 Extraction of Tin

The following steps are involved for extraction of Sn from tin ore (Fig. 11.1):

Fig. 11.1
figure 1

Flow diagram of extraction of tin

Full size image
  1. (1)

    Concentration.

  2. (2)

    Reduction of concentrate.

  3. (3)

    Treatments of slags for recovery of metals.

  4. (4)

    Refining of impure metal.

3.1 Concentration

Tin ore, the content of which is about 1% Sn, is concentrated up to 65% Sn by the water gravity concentration process. At 600°C, SnO2 and Fe3O4 are stable (from Ellingham diagram, Fig. 11.2), at higher temperatures (1200–1300°C) FeO is stable. The slag has a higher affinity for SnO2 compound with iron oxide and because of the tendency of Fe for entering the tin phase. To overcome these difficulties and to attain a high degree of separation of tin from iron, leading to a high level of tin recovery, smelting of tin concentrate is done in three stages (reduction of concentrate and treatments of slags).

Fig. 11.2
figure 2

Ellingham diagram for SnO2, FeO, Fe3O4 etc

Full size image

3.2 Reduction of Concentrate

Then smelting reduction of concentrate is done by addition of coal (as reducer) and limestone (as flux) at 1300°C for 10 h. High purity metal (99% Sn) is produce initially and deliberately maintaining a high level of SnO2 in the slag (which contains 30% Sn), and all iron goes to slag (35% SiO2, 30% CaO, 15% FeO, 20% SnO2).

$$ {\text{SnO}}_{2} + 2{\text{C}} = {\text{Sn}} + 2{\text{CO}} $$
(11.1)

3.3 Treatments of Slags for Recovery of Metals

This first slag is cooled, crushed and then again smelting reduction is done by addition of iron scrap, coke, limestone and rough dross at 1300°C for 16 h. Rough metal (95% Sn, 5% Fe) and second slag (4% Sn) are formed. The second slag is again cooled, crushed and further smelted at 1300°C with the addition of iron scrap, coke, and limestone. Hard metal (80% Sn, 20% Fe) and third slag (1–2% Sn) are produced.

Sn–Fe alloy produced in the second and third smelting stages can be refined by liquation. Alloy is heated at higher temperature than melting point of tin (232°C). Tin (first run metal) flows out leaving behind Fe–Sn intermetallic. As temperature increases iron content in tin metal is also increased (second run metal).

Liquation drossing of hard metal (80% Sn) is done with the addition of rough metal (95% Sn). Rough dross, first run metal and second run metal are produced by liquation drossing. By smelting of rough dross, alloy (65% Sn, 25% Fe and some Cu, Pb) is again produced. Tin (> 99.5%) is acquired from first run metal. Second run metal again goes for second liquation.

3.4 Refining

Flow diagram of tin refining is shown in Fig. 11.3.

Fig. 11.3
figure 3

Flow diagram of tin refining

Full size image
  1. (a)

    Fe removal: Intermetallic compound is formed from common impurities, such as: Ca3Sn (melting point 675°C), Sn3As2 (596°C), FeAs (1030°C), Fe2As (919°C), Cu3As (825°C), Cu2Sb (585°C), FeSb2 (726°C), FeSN2 (very high temperature). Cooled to a temperature above melting point of tin (232°C), intermetallic compound is separated out and floats as tiny crystal. Coal helps to float as well as maintain reducing atmosphere. At 550°C, 99.8% Fe is removed from tin metal. Dry dross (content 25% Sn) is formed.

  2. (b)

    Cu removal: Copper is removed by addition of sulphur (2Cu + S = Cu2S), remaining iron is also removed in this stage.

  3. (c)

    As and Sb removal: Arsenic and antimony are removed by addition of alluminium. Alluminium forms intermetallic with arsenic and antimony, such as AlAs (1720°C) and AlSb (1070°C).

  4. (d)

    Al removal: Alluminium is removed by addition of NH4Cl:

$$ {\text{Al}} + 3{\text{NH}}_{4} {\text{Cl}} = {\text{AlCl}}_{3} + 3{\text{NH}}_{3} + 3/2{\text{H}}_{2} . $$
(11.2)
  1. (e)

    Pb removal: Lead is removed by addition of SnCl2:

$$ {\text{Pb}} + {\text{SnCl}}_{2} = {\text{PbCl}}_{2} + {\text{Sn}}. $$
(11.3)

Dross forms [PbCl2 + SnCl2 (excess)], which is reduced by zinc to form Pb–Sn alloy:

$$ {\text{PbCl}}_{2} + {\text{SnCl}}_{2} + 2{\text{Zn}} = 2{\text{ZnCl}}_{2} + {\text{Pb}} - {\text{Sn}}. $$
(11.4)
  1. (f)

    Bi removal: Bismuth is removed by addition of calcium and magnesium to form intermetallic compound (Bi2Mg5, Bi2Ca3 and Bi3Ca), from them Bi can be recovered.

  2. (g)

    Ca and Mg removal: Excess calcium and magnesium are removed by addition of NH4Cl:

$$ 2{\text{NH}}_{4} {\text{Cl}} + {\text{Ca}}\,\left( {\text{Mg}} \right) = {\text{Ca}}\left( {\text{Mg}} \right){\text{Cl}}_{2} + 2{\text{NH}}_{3} + {\text{H}}_{2} . $$
(11.5)
  1. (h)

    Casting: Casting of refined tin metal is done as per requirement.

4 Properties

The atomic weight of tin is 118.7 and specific gravity of tin is 7.29. Tin has a very low melting point (232°C), but a very high boiling point (2270°C). It is toxic in nature. It can resist corrosion. Cans are made of steels with coated tin on them to prevent corrosion of steels. As tin is also resistant to attack by the common organic acids, it is a safe and attractive medium for use in contact with foods. In point of fact only about 1.4% of the weight of a can consists of tin as coating.

Pure tin is a soft metal with high malleability and low tensile strength; small amounts of impurities, however, tend to modify these characteristics. Iron renders the metal hard; copper makes it both harder and stronger. The softness and plasticity of pure tin make it adaptable to all types of cold working, where high tensile strengths are not required. It may be rolled with heavy reduction without the necessity for annealing, and rolling can be continued down to the thinnest foil.

In many of its properties tin resembles lead, hence the use of the cheaper metal in such materials as roofing, sheets and collapsible tubes for toilet preparations. Tin exists in two allotropic forms, the normal white lustrous metal and a grey brittle modification. When exposed for a sufficient time to low temperatures the white modification changes to the grey powdery form.

Tin in the cast form is strongly crystalline, which gives rise to another peculiarity. When a bar of the metal is bent, a crackling noise is heard (known as tin cry) due to the friction of the crystals as they ride over one another.

5 Applications

Tin coatings are most commonly produced by immersing the prepared work in a bath of molten tin, but they may also be producing by applying the tin in molten, stick or powdered form to the prepared and heated surface. The hot dipping process may be used to effect tinning and soldering simultaneously, as for example in the manufacture of motor car radiator blocks and refrigerator cooling units.

Tin coating may be applied to cast iron, steel, copper and its alloys such as brass and bronze. The production of a tin coating involves the formation of a thin layer of alloy or intermetallic compound between the outer layer of pure tin and the base metal. The alloy layer formed on iron or steel consists of the compound FeSn2, while that on copper and its alloys consists of the compounds Cu6Sn5 and Cu3Sn.

The most important outlet of tin is to provide hygienic protective coatings and give a pleasing appearance to articles made from stronger constructional basis metals. Tin-lead alloy coatings applied in a similar manner to that in use for pure tin find employment where non-toxicity is not essential and where the cheaper coating can safely be used.

A second important application of hot tinning is for soldering. Soldering has special interest to the electrical and electronic industries where reliable joints must be achieved by spot soldering at high speeds. A third purpose is to assist binding of another metal to the basis metal as in the tinning of bearing shells prior to lining with white metal.