|Lead concentrates which are obtained from lead, lead-zinc and complex ores are the main source of bismuth. The common bismuth concentration in them is several hundredths of a percent, sometimes they reach 0.2%. Bismuth gets into the crude lead during the processing from which it is separated after refining. Bismuth is extracted using magnesium and calcium which transfer it as CaMg2Bi2 in dross, the surface layer. Another way of separation also is utilized using potassium and magnesium. Electrolytic refining during which bismuth passes into slimes is also applied. |
Bismuth concentrates which contain approximately 3-5% of bismith (rarely the concentration may reach 30-60%) may be obtained by flotation on bismuth ores and are processed by either reducing fusion or precipitating smelting adding metal iron.
Refining consists of several steps of fusion processing: by sulphur adding coal for iron and copper removing; by alkali adding oxidizer or by air blasting for removing arsenic, antimony and tin; zinc used for removing gold and silver, and chlorine applied for removing lead and zinc. Electrolytic refining of water solutions [BiCl3, Bi2(SiF6)3], as well as fusions is also used. Combination of various methods is used for extracting high-pure bismuth.
|Formerly bismuth was extracted principally from native ores by the process of liquation. The ore was heated in inclined cylindrical furnaces and the molten bismuth allowed to flow away from the gangue. This crude bismuth was afterwards refined by an oxidising fusion, sometimes followed by poling. As native ores are, however, always associated with oxide or sulphide minerals, or both, neither of which can be treated satisfactorily by liquation,the process is now obsolete. |
Bismuth ores do not occur commonly in sufficient quantities to justify direct treatment, but in such cases the main principles underlying the extraction are, firstly, the maintaining of a low temperature on account of the volatility of the metal, and secondly, the use of fluxes suitable for the formation of a fusible slag of a sufficiently low density to enable the metal to separate. The operation may be carried out either in crucibles or in reverberatory furnaces. (1) In the case of oxide ores—which are comparatively unimportant—charcoal or other form of carbon is used as the reducing agent, the fluxes being sodium carbonate, lime and oxide of iron or oxide of manganese. (2) In the case of sulphide ores, particularly if associated with sulphides of arsenic and antimony, the ore is first roasted; arsenic and antimony are thus converted into oxides, partly volatilised and partly removed as scum, while bismuth is converted into a mixture of oxide and sulphate. Reduction to metal is carried out as for oxide ores with the addition of a little iron to remove sulphur. In Bolivia, where the sulphide ore is sufficiently rich and plentiful to be of economic value, smelting with iron is practised, either with or without a preliminary roasting. If there is a preliminary roasting, some bismuth is liable to be converted into bismuth sulphate, which ultimately will tend to pass into a matte. The fusion is carried out in crucibles and reverberatory furnaces, the fused products consisting of crude metallic bismuth, a matte containing copper with 5 to 8 per cent, bismuth, and slag. The matte is subsequently treated in a similar manner until its bismuth content is reduced, to 2 to 3 per cent. Both metal and final matte are then refined. The roasting of bismuth sulphide ores presents some difficulty, as the elimination of sulphur is usually incomplete unless either an excess of oxide is present, or special precautions are taken.
The crude metal obtained in this manner may be refined by an oxidising fusion which removes lead and other easily oxidised metals; the addition of bismuth sulphide assists in the removal of copper as sulphide. The resulting metal is then poled. Frequently bismuthyl chloride is used as a flux in this refining process. A considerable quantity of crude bismuth, however, is converted directly into pharmaceutical products, and for this purpose wet methods are usually employed. Electrolytic refining processes are now more general for the purpose of obtaining pure metallic bismuth.
As has been mentioned, in most localities bismuth ores are associated with ores of other metals, notably lead, tin, copper, nickel, cobalt, antimony, arsenic, gold and silver. From these ores the bulk of the world's supply is now obtained, and the method adopted for the extraction of the bismuth depends to a large extent upon the nature of the associated minerals, although in some cases the ores may be concentrated by the usual mechanical processes and the bismuth concentrates treated separately. Lead-bismuth ores are usually treated as for the extraction of lead, and the crude product subsequently desilverised by the Parkes process; much of the bismuth is retained in the desilverised lead. This is then subjected to the Tredinnick-Pattinson process—a modification of the Pattinson process for the desilverising of lead—whereby the bismuth is concentrated in a small quantity of lead. The bismuth-lead alloy is then treated by the Betts electrolytic process. In this process the electrolyte is composed of an acid solution of lead fluosilicate containing free hydrofluosilicic acid; the cells are of concrete with a lining of asphalt and are arranged in cascade, there being usually seven cells in cascade. The general arrangement is similar to that employed for the electrolytic refining of copper. The current density is 1.8 to 1.9 amperes per square decimetre, and the potential drop between electrodes is 0.35 to 0.4 volt at the beginning, rising to 0.65 to 0.70 volt at the eighth day. The electrolyte is made to circulate through the cells. The anodes are composed of crude lead containing 2 per cent, of impurities, including bismuth. Antimony is usually present, and is an advantage, as it adheres to the anode, and during the process forms with lead a protective network which remains in position after the bulk of the lead has been dissolved, thus holding the slime, which can be withdrawn with the used anode. Bismuth is found in these slimes, the treatment of which depends very largely upon their composition. This treatment has not been fully published. In one process the slimes are fused with alkalies under oxidising conditions, with the addition of sodium sulphide if copper is present. Arsenic, lead and copper pass into the slag and the bismuth, with gold and silver, is cast into anodes. In a second process if gold and silver are present these are removed first, followed by antimony, which is partly converted into lead antimonite slag. By a further oxidation fusion copper is oxidised, and copper oxide and bismuth collect as a slag, from which the bismuth can be obtained electrolytically. In the electrolytic refining of the slimes, the electrolyte is either a solution of bismuth methyl sulphate (4 per cent.) containing methyl hydrogen sulphate (10 per cent.), or a solution of bismuth chloride with sufficient hydrochloric acid (10 per cent.) to prevent hydrolysis. In the latter case the current density is 1.5 to 3.3 amperes per square decimetre and the potential drop 0.5 to 1 volt.
Many other processes have also been suggested for the recovery of bismuth both from ores and alloy residues.
Analysis of anodes of crude Bithmuth (As used for Electrolytic Refining)
|Arsenic||. . .||0.92||0.26||Trace||. . .|
|Lead||. . .||. . .||1.32||0.87||2.20|
|Sulphur||. . .||0.43||0.99||0.21||. . .|
|Iron||. . .||. . .||1.31||0.45||. . .|
|Silver||. . .||. . .||186.1 oz. per ton||156.1 oz. per ton||3.20|
An earlier method for the extraction of bismuth from bismuth-lead ores involved the process of cupellation. Bismuth will not oxidise until all the lead is oxidised; part of it, therefore, during cupellation, will pass into the silver, and part into the final litharge produced. Bismuth may be recovered from the latter by dissolving in hydrochloric acid and precipitating as bismuthyl chloride, which, in turn, may either be used as such, or may be reduced by heating with charcoal and sodium carbonate. Gold and silver may be removed from bismuth by the addition of a little zinc to the molten metal and treating as in the Parkes desilverising process.
Wet processes are seldom used for the extraction of the metal, but are mainly employed for the preparation of medicinal and pharmaceutical products. In general, the ore is dissolved in hydrochloric acid, aqua regia or sulphuric acid, and from the solution bismuth is precipitated by iron. If hydrochloric acid is used as the solvent, bismuthyl chloride is precipitated, and from this the metal is obtained as previously described. The impure bismuth may be refined by liquation, dissolved in nitric acid, precipitated as basic nitrate, redissolved in nitric acid and precipitated as hydroxide by ammonia; this bismuth hydroxide, after washing and drying, may be reduced by hydrogen. Most pharmaceutical preparations are to-day made from refined bismuth.
The method employed in Norway by the Norsk Hydro-Electrisk Kvaelstof Aktieselskab involves the treatment of bismuth ores with crude nitric acid obtained by the solution of oxides of nitrogen in water. If the ore contains bismuth trioxide the action is simple neutralisation according to the equation
Bi2O3 + 6HNO3 = 2Bi(NO3)3 + 3H2O
The action is more complicated when sulphide ores are treated. The ore is first partially roasted, ground finely, and added to the crude nitric acid. The main reaction may perhaps be represented by the equation
Bi2S3 + 8HNO3 = 2Bi(NO3)3 + 2NO + 4H2O + 3S
When the acid is almost completely neutralised, the lye is removed, concentrated and poured into milk of lime. Bismuth trioxide is thus precipitated and calcium nitrate remains in solution
2Bi(NO3)3 + 3Ca(OH)2 = Bi2O3 + 3Ca(NO3)2 + 3H2O
This process is used mainly for the production of salts of bismuth.
Pure bismuth may also be obtained by converting the nitrate into oxide and reducing the oxide by fusion with potassium cyanide. A further purification may be effected by liquation. Bismuth containing less than 0.01 per cent, of impurities (the chief of which is copper) has been obtained. Below are given the upper and lower limits of the impurities present in six samples of bismuth examined spectrographically.
|Copper||0.001 to 0.006 per cent.|
|Silver||0.006 to 0.046 per cent.|
|Tellurium||0.000 to 0.004 per cent.|
|Thallium||0.000 to 0.005 per cent.|
|Lead||0.002 to 0.061 per cent.|
|Total||0.009 to 0.122 per cent.|