— Various metallic impurities such as copper, lead, iron, arsenic etc., are contained in most of the tin produced in smelting concentrate and slag. These impurities, which are reduced with the tin in the course of smelting, must be removed before the tin is marketed.
— Necessary refining is accomplished by the methods discussed below:
Refining process Impurities removed
Liquation Fe, As, Cu.
Blowing or tossing Fe, Al, Zn, Ni.
Treatment with sulphur Cu
Treatment with aluminium Sb, As, Cu, Ni.
Electrolysis Pb, Sb, As, Cu, Bi, Ag.
Liquation or sweating
— The impure tin is placed on the upper side of a sloping hearth and heated to a temperature slightly above the melting point of pure tin but below the melting points of the impurities.
The comparatively pure tin melts, forming so called first sweat tin, which flows down the hearth and into a kettle.
— Left on the hearth is a dross residue that contains impurities such as iron and copper, besides some tin. The temperature of the residue is then raised and so called second sweat tin is produced and collected separately for retreatment.
Boiling or tossing
— In boiling, the molten first-sweat tin is stirred with poles of green wood, producing a strong bubbling or boiling action. The metal impurities and some of the tin are oxidised during this poling process and are skimmed off in the form of an oxide dross.
— When tossing is used instead of poling, hand ladles of molten tin are poured from a height back into the kettle, thus exposing the tin to the oxidising action of the air.
— Boiling or tossing is continued until the operation yields the desired grade of refined tin, which is then cast into pigs for marketing.
— A purer tin can be obtained by electrolytic refining than by other treatments. The more costly electrolytic process, however, has seldom been used to refine primary tin because of the small demand for extremely high purity tin.
— Tin stands in a favourable position as regards electrolytic refining of the metal impurities it contains, only lead is close enough in the electrochemical series to be dissolved along with it, the other contaminating metals such as Cu, As, Sb and Bi, remaining undissolved. To take care of lead, sulphuric acid forms part of the electrolyte, precipitating the lead as insoluble lead sulphate as soon as released from the tin anode, thus preventing deposition and contamination of cathodes.
— An approximate composition of the electrolyte is
Sulphuric acid……………….. 8%
Hydrofluosilicic acid………. 5%
Refining is carried out in lead-lined tanks with cathode starting sheets and anodes weighing up to 160 kg each. After a week’s operation the cathodes weigh 45 kg or so and are removed from the tank and melted down into pigs assaying 99.9% tin.