Tin is a soft, silvery-white metal that carries a bluish cast. It was used in earlier times alloyed with copper to make bronze. Tin's softness makes it usable only when alloyed with other elements, except in the production of glass where molten glass is floated on top of molten tin. There are several ways to refine tin; of these ways, four use fire and one uses electricity.
Refiners heat impure tin in kettles and pump compressed air into the vessel from the bottom. As the air bubbles up, it oxidises the impurities, which float to the surface and can be skimmed off. In boiling and the other methods of refining tin -- except the electrolytic process -- the slag, or impurities, is processed again to recover any tin that the first process didn't get.
Refiners heat the impure tin until the temperature is just above the melting point of tin (232 degrees Celsius) on a sloping hearth. The tin melts and flows down the sloping hearth. The impurities are left behind and can be removed.
Molten tin is heated in closed vessels made of dense graphite at temperatures of 1100 to 1300 degrees Celsius. When the tin is hot, the air in the vessel is pumped out to produce a vacuum. The impurities in the tin have different boiling points. The vacuum is manipulated to boil away the impurities, leaving refined tin.
The impure tin is cast into long blocks that are put into an acidic electrolyte. There are thin sheets of high purity tin submerged in the acid. The refiners apply an electric current to both the pure and impure tin. The acid dissolves the tin in the impure ingot. The dissolved tin is drawn through the acid by the electric current and attaches itself to the pure tin. It takes approximately one week to complete this process.
The Final Product
Tin refined by the first three methods, boiling, liquation and vacuum distillation, are 99.85 per cent pure. Electrolytic refining produces tin that is up to 99.999 per cent pure. The resulting tin is cast into pigs, or ingots, and sold.
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