A malleable, fairly soft, silvery-white, metallic element in group IVA of the periodic table;
it occurs mainly as cassiterite (tin
dioxide, TnO2), from which it is obtained by smelting with coal.
Tin exhibits allotropy: white (β)
tin, the normal form, changes below 13.2°C to gray (α) tin, a
powdery metalloid form resembling germanium,
and known as "in pest."
|Cassiterite (SnO2) is the sole source of commercial tin. It is extracted by the ore being crushed (1) and passed through a sink-float separation system (2). The washed ore is roasted in an oxidizing atmosphere (3) to remove arsenic and sulfur. Tungsten is removed by electromagnetic separation (4). The tin oxide is then roasted in a blast furnace (5) with coke (6). The tin produced is refined in a reverbatory furnace (7). The slag produced from the blast furnace is reworked and the tin obtained is also refined in the reverbatory furnace. Further refining takes place to remove any remaining impurities (8), before the tin is pressed and rolled (9). Tin is mainly used for electroplating or in combination with other metals as an alloy. Pure tin is used principally for coating steel to prevent corrosion. The tin is applied in a layer c.6x10-6mm (2.5x10-6in) thick by either dipping the steel into molten tin and rolling, or more commonly by electrolysis. Tin-coated steel is used extensively in the preparation of cans used for food storage because it is not poisonous. As an alloy, tin is used in solders, where its low melting point is an advantage, and in bronze, pewter and various other alloys used in machine bearings. Because of its low melting point and resistance to atmospheric corrosion, tin is used in modern glass-making processes where the molten glass is floated on a layer of molten tin in a hermostatically controlled bath and allowed to congeal. The resulting glass is so smooth that it needs no polishing or grinding.
Tin is unreactive, but dissolves in concentrated acids and alkalis, and
is attacked by halogens. It is used to coat
other metals to prevent corrosion, and
forms part of numerous alloys, such as soft
solder, pewter, type metal, and bronze.
|Cassiterite. Credit: Mineral Information
|relative atomic mass
Compounds of tin
Tin forms organotin compounds, used in biocides, and also inorganic compounds:
tin (II) and tin (IV) salts. Tin (IV) oxide (SnO2)
is a white powder (sublimes at 1,800°C) prepared by calcining cassiterite or burning finely divided tin; it is used in glazes and as an abrasive. Tin (II) chloride (SnCl2) is a white crystalline solid, prepared by dissolving
tin in hydrochloric acid and used
as a reducing agent, in tin-plating,
and as a mordant. Melting point 246°C,
boiling point 652°C.