Extraction of Antimony

Antimony is extracted mainly from ores containing antimony trisulphide in the form of stibnite. Rich ores are first subjected to liquation, whereby the fusible sulphide is removed from the gangue and concentrated. The concentrate is then reduced to metal by smelting with wrought iron in pots, in reverberatory furnaces or sometimes in blast furnaces,

Sb₂S₃ + 3Fe =2Sb + 3FeS

The reduction in pots is carried out in three stages: (1) A mixture of liquated ore, wrought iron and common salt is fused together and a crude product obtained containing about 90 per cent, antimony and 7 to 8 per cent, iron; (2) this is again remelted with more liquated ore and common salt, the bulk of the iron being thereby removed; (3) finally this second product is remelted with a special flux made by fusing together three parts of commercial potassium carbonate with two parts of liquated ore.

The reduction in reverberatory furnaces is carried out in two stages:

(1) Fusion of liquated ore with scrap iron to produce crude metal, and

(2) refining the crude metal. In the first stage the liquated ore is smelted with a flux containing soda ash, salt cake and common salt. This results in the production of a slag of low density containing sulphide of iron and sulphide of sodium, from which the crude antimony can settle fairly readily. The crude product contains 94 to 95 per cent, antimony mixed with iron, sulphur, lead and arsenic. This is refined by melting carefully under a flux of sodium carbonate. After a time the impurities can be skimmed off and then a mixture containing antimony oxide, antimonyl sulphide, sodium carbonate and a little coal is added. The refined metal is then ladled out and cast as " star antimony," the surface of the cakes showing a well-defined crystal structure. A method has also been suggested for refining electrolytically using an electrolyte containing antimony trifluoride with about 100 grams of free sulphuric acid and 20 grams of free hydrofluoric acid per litre.

Low-grade ores, which generally contain oxidised minerals in addition to sulphides, are first roasted to produce either the non-volatile tetroxide or the volatile trioxide. With oxidised ores recovery of antimony is incomplete unless anthracite is added to the charge. Thus, treated at 900° C. without anthracite, only the antimony present as sulphide is recovered, while with anthracite almost complete recovery has been effected. Using an ore containing 3.3 per cent, antimony, partly sulphidic and partly oxidised, the average anthracite consumption was 15 to 20 per cent, of the charge, of which 5 to 8 per cent, was used as reducer and 10 to 12 per cent, as fuel. The oxides obtained are reduced to metal by carbon in crucibles or reverberatory furnaces, using an alkaline flux containing soda ash, salt cake and common salt.

For roasting sulphide ores two methods are used, viz., simple roasting yielding mainly antimony tetroxide, and volatilisation roasting yielding mainly antimony trioxide. In the former, reverberatory furnaces are employed, the flue gases passing through a condensing plant for the recovery of any antimony trioxide that may be formed. For volatilisation roasting either the Herrenschmidt or the Chatillon process is used. In the Herrenschmidt process, the furnace is a rectangular shaft of firebrick with fire-bars arranged in steps. The gases from this furnace are cooled by passing them through a series of iron pipes into a wooden tower packed with coke down which water trickles. The water is collected at the bottom of the tower in tanks in which the oxide is allowed to settle. In the Chatillon process a double cupola type of furnace is employed. The flue gases containing volatilised oxide pass through a condensing plant (which consists of sheet iron tanks so disposed that the flue gases can pass all round them), and are then forced into a filtration chamber which is fitted with bags of coarse cotton canvas or of woollen material.

The volatilisation and oxidation of antimony trisulphide have been studied in detail. From the results it is suggested that rich or concentrated ores which do not contain precious metals, arsenic or lead should be roasted to antimony tetroxide, while poorer ores, and ores containing precious metals, should be roasted to antimony trioxide. It is further suggested that it is inadvisable to extract antimony from rich ores by fusing the sulphide with iron on account of the high fusion temperature required; if this process is adopted a reducing atmosphere should be employed and the dust in the flue gases recovered.

The reduction of oxides of antimony may also be effected by the action of arsenic on fused sodium hydroxide in the presence of the oxide.

Various wet methods for the extraction of antimony have been suggested but do not appear to have been successful. Electrolytic methods have, however, been developed and it is claimed that compact antimony of great purity can be obtained by these methods.

Chemically pure antimony has been prepared by converting antimony trichloride, purified by distillation, into chlorantimonic acid, which, after purification by recrystallisation, is hydrolysed to antimonic acid. The antimonic acid is finally reduced to metal by fusion with potassium cyanide.

The production of "secondary antimony" (i.e. the recovery of antimony from alloys and residues) forms an important means of obtaining the metal. In the United States of America in 1928, between 170,000 and 180,000 tons of old lead accumulators containing 3 per cent, antimony were available. For the treatment of lead-antimony alloys the Harris process is largely employed in England and some other countries. In this process the scrap metal is melted with an oxidising alkaline flux. Arsenic, antimony and tin are then removed by skimming as sodium arsenate, antimonate and stannate. The sodium antimonate is separated from these by lixiviation, and is reduced to antimony by treatment with carbonaceous material. In another method that has been suggested, the alloy is treated with metallic sodium in order to form an alloy of sodium and antimony. This is dissolved in molten caustic soda, and the antimony separated by treatment with water.

More than three-quarters of the world's supply of antimony ore are obtained from the province of Hunan, China. In France, the ore deposits at La Lucette yield an annual production of 3,000 tons of antimony. About 1,000 tons of antimonial lead, containing 21 per cent, antimony, are also produced annually in Burma. The total world production of antimony ore, returned as tons of metal content, is given in the following table:

	1926	1927	1928	1929	1930	1931
Long Tons	31,400	30,000	30,000	31,000	23,000	27,000