Metallic antimony rarely occurs native and as an ingredient in the sulphides of other metals. Its chief commercial source is antimony sulphide (grey antimony), containing 74 Per cent. antimony and 26 sulphur. The ready fusibility of this ore enables the usual dressing operationsto be dispensed with; it is simply-roasted in a furnace, generally of the kind shown in Fig. 6.
Here 2 chambers f are arranged between 3 fireplaces a, b, c, and covered with cast-iron lids perforated at spots corresponding with the centres of 2 fireclay cylinders e, fitted above them in the arch, through which the fire reaches them, while the smoke escapes by flues at the back. Boles in the sides near the bottom of the cylinders correspond with openings into the arch, and are closed with fire-clay stoppers, and luted. Crucibles of are placed on low carriages in the chambers f, to receive the fused metal as it runs from the cylinders. When the furnace is heated to bright redness, a charge of 4 cwt. of broken-down crude sulphide is put into the cylinders, which are then covered up;. the metal fuses out and flows into the crucible beneath, leaving the gangue in the cylinder. The charge is renewed every 3 hours, and the residue is drawn out through the holes in the sides of the cylinder. The yield per hour is about 90 lb. of pure sulphide, which has various uses in medicine and pyrotechny.
To obtain the metal from this sulphide, the latter, in coarse powder, is roasted at a low heat in an ordinary re-verberatory furnace. Much care is needed to regulate the furnace so that oxidation of the sulphur shall proceed uninterruptedly without risk of fusing the sulphide. The calcination should be complete in 12 to 15 hours, 100 lb. of the sulphide affording only 60 to 65 lb. of antimony oxide, though theoretically the yield would be 86 lb. This oxide (antimony glass) is used in the manufacture of enamels, and coloured pastes for artificial gems. To reduce the metal from the oxide, the latter is mixed with charcoal powder moistened with a solution of soda ash, and put into a number of crucibles in the same furnace, when a bright red heat is maintained till fusion is complete. The product is crude metal and scoria, the latter consisting of antimony and sodium sulphides in combination. The crude metal is re-melted with some of the scoria, which further purifies it; and again melted in large quantities with careful skimming, and slowly cooled.
The process adopted by the Costerfield Co., in Australia, for treating their ores, consisting of the sulphide and brown and white oxides of antimony (containing 33 to 65 Per cent. antimony, and 15 dwt. to 4 oz. gold per ton), is as follows. The antimonial tailings from the stamp. batteries are classified by huddling, and . the rich antimonial portion thus separated from quartz detritus is collected in bags, dried in the boiler-house, put into a smelting furnace with equal portions of uncrushed ore, and reduced to crude antimony (sulphide); the resulting slag and ciader are further treated by roasting or calcining in a reverbe-ratory furnace, to liberate the oxide, which passes off in fumes from both furnaces into the oxide flue; this may be 1000 ft. long, and 4 to 6 ft. wide and high. As the fumes cool on their passage to the smoke-stack, the oxide is. deposited in chambers constructed in the flue to receive it. The residue from the reverberatory furnace is afterwards crushed for the extraction of any gold it may contain.
Another process based on fusing the antimony sulphide with a portion of metallic antimony, using the same metal with fresh charges of ore till it becomes rich in gold, and then separating the two metals by oxidation of the antimony, while suitable for ores rich in antimony, will not answer for those containing less than 30 Per cent. of sulphide, as they are too silicious to fuse. Prof. Cosmo Newberv has intro-duced another method for treating such ores. They are placed uncrushed in a kiln, with sufficient salt to produce an amount of chlorine that will get rid of the sulphur, antimony, and arsenic. As soon as calcination commences, steam or watery vapour is introduced through the bottom of the kiln in such quantity as to keep the mass saturated, until all vapours of sulphur, antimony, and arsenic have disappeared. The charge is then drawn, and submitted to gold-extracting operations. The liberated metallic vapours are condensed by causing them to impinge against a series of discs. Antimony oxide collected in this manner is well suited for the manufacture of paint, if the temperature of the fumes has been lowered from that of the furnace till the vapour has been all condensed into solid particles of oxide in suspension. (See Lock's ' Gold,' pp. 1106-10.)