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Refining of Non-Ferrous Metals

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Refining of Non-Ferrous Metals REFINING OF NON-FERROUS METALS J. G. BERRY- Abstract Fundamentals and Examples The fundamentals of refining of non - ferrous How, then, is the objective achieved ? It metals have been outlined . Examples of removal may be achieved by using the affinity of an electrolysis, of impurities by selective oxidation , undesirable element for another element, thus distillation , volatilization , etc., have been given. Refining of copper, lead, zinc, tin, and some other forming a compound, which is insoluble in the metals have been described . The importance of metal to be refined and which can easily be economics and time involved in refining process removed. Selective oxidation, electrolysis, has been stressed, distillation or volatilization may be used, while flotation, magnetic separation or chemi- cal reaction may be used to remove undesir- Introduction able elements prior to the reduction stage. T will be my endeavour in this paper to One of the earliest attempts at refining I indicate the fundamentals of refining would be the ` purification' of gold by as. they are applied to the more common thermal methods, while the most recent members of this group, with a description of development in this field is the solution and some of the processes, and, in doing so, I precipitation of metals under controlled hope to promote discussion which, after all, pressures from ores and concentrates or is the objective of this symposium. scrap, such as brass. In the fire-refining of copper; many im- purities are oxidized and removed in the Definition slag, these impurities being oxidized in pre- What, then, is the definition of ` refining ' ? ference to the copper itself. On the other Strictly speaking, - it is the metallurgical hand, in the refining of metals in which process for the removal of impurities which copper is itself an impurity its affinity for follows the reduction of an ore or concentrate sulphur may be used to bring about its to metal. It can also be applied to any elimination. process used for the purification of scrap. Zinc is refined by distillation, while im- The term `refining', for the purposes of purities, such as arsenic, can be removed by this discussion, must, however, be expanded volatilization. Electrolysis, as a method to include the removal of impurities from of refining, is used for copper and, of course, an ore or concentrate before the reduction the treatment of bauxite is a well-known stage; as, in some cases, if such impurities example of ` pre-extraction ' refining, assum- are allowed to enter the product of reduction, ing that the proposed expansion of the they are difficult to remove in the subsequent original definition of refining is permitted. refining process. Whether before or after smelting, the Outlines of Refining Procedures objective of the metallurgist is the removal of undesirable elements from a metal or Copper-By far the greater tonnage of alloy. copper is produced from sulphide ores which, *J. G. BERRY, B.Sc. (Land.), M.I.M.M., M.A.I.M.E., rounder Member, I.I.M., Works Manager, Indian Copper Corporation Ltd., Ghatsila ( Bihar). 102 BERRY - REFINING OF NON-FERROUS METALS 103 after concentration, are smelted to produce Care must be taken in fire-refining and a a ` matte' which, in turn, is 'blown' to blister balance struck between the amount of im- copper in a converter. The blister copper, purities removed and the time taken, as the which is sent to the refinery may contain longer the oxidation , the more copper goes varying amounts of sulphur, iron, arsenic, into the slag which must be re-treated to aluminium, selenium, tellurium, bismuth, recover that copper . This recovery is nickel, cobalt, gold and silver. achieved by utilizing the affinity of copper The actual refining process starts in the for sulphur and the slags , resulting from the converter where, if bismuth is present, this converting and refining operations, when impurity can be reduced to 0001 per cent or added to the reverberatory furnace, react less by prolonging the blowing at the slagging with and enter the matte. stage, and a concentrate, with a percentage Depending on the residual impurities, of iron higher than normal, is produced to 'oxidation ' is followed by 'poling' with enable this to be done. Bismuth, if allowed green poles to reduce the dissolved copper to remain in the blister copper, cannot be oxide, after. which the metal can be ingotted removed by fire-refining. and sold for many purposes. If, however, Fire-refining of the blister copper is carried high conductivity copper is required, and if out either to produce an ingot of relatively the copper is suitable from the point of view high purity, or as a preliminary to electrolytic of impurities, the fire -refining process can refining, and even after electrolytic refining be continued, and it is interesting to note the cathode copper may be again fire-refined that some 6 or 7 per cent of the world's to give the correct pitch for the production production is high conductivity fire-refined of copper-wire bars. In the fire-refining copper. process, air is used for oxidation and the Electrolytic methods of refining remove ` end point ' is noted by studying the ` set ' the difficult elements , such as nickel , cobalt, of samples taken at intervals from the bath. selenium and tellurium, together with the It is a feature of this process that the oxida- gold and silver; and, as already stated, after tion of the impurities is more rapid than that the electrolytic process, the cathodes are of the copper, thus enabling the impurities melted and the copper subjected to a short to be removed with comparative ease. fire-refining process by which the correct Of the impurities which have been noted, pitch or oxygen content is obtained before sulphur is eliminated almost at once. Iron casting into wire-bars. is readily removed in the slag, as also is Lead- Lead, which may contain any of aluminium. Arsenic and antimony, if pre- the following impurities , viz, ' copper, anti- sent, are removed by using soda-ash and lime, mony, arsenic , bismuth and silver , has a very the lime being added to reduce the wear and interesting course to follow from the blast tear on the furnace lining. These latter ele- furnace to the refined product , and at Port ments are, of course, removed after the nor- Pirie, South Australia , Dr. G. K. Williams, mal slagging process has been completed. the then Chief Metallurgist at the Broken Selenium and tellurium cannot be removed Hill Associated Smelter ( Proprietary) Ltd., by fire-refining, although there are possibil- worked out a scheme for the continuous flow ities of some success using soda-ash under of metal from the bullion storage kettles to reducing conditions. However, these ele- the finished product. ments are normally removed by electrolysis, Copper is the first impurity to be removed, together with the precious metals which, if and this is done by drossing and by the present, as would be expected, do not oxidize utilization of the affinity of copper for sul- and, therefore, cannot be removed by fire- phur. The blast furnace product is received refining. in kettles, where it is allowed to cool, As the 104 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA temperature drops, the dross formation on into the main bulk of lead below. As the reduces the copper to under 0.1 per cent. lead passes through the zinc layer, the zinc At this stage, sulphur is added and the final takes up the gold and silver, while the lead drossing carried out at as near to the freezing dissolves some of the zinc. When the zinc point as possible. is sufficiently enriched, it is removed by I remember, as a young man, watching ladles and fresh zinc added. The lead is this process being carried out on a very small discharged from the kettle by a syphon, the scale in a solder pot, and the operator, quite a mouth of which is set some 18 in. from the character, called the process ` cold burnin '. bottom. Thus the incoming lead having I have never heard the term before or since, passed through the zinc layer is displaced but it aptly describes the process. The dross downward into the cooler zones, where the from this section is smelted to recover the zinc alloy crystallizes out and rises back into copper, while the lead is pumped to the the upper sections. The temperature of the bullion storage kettles. lead, as it is displaced downward, falls nearly The lead is now ready for the continuous to the freezing point and the desilverized refining process, The first stage is ` soften- lead, rising up in the syphon, is re-heated ing ', in which, as is indicated by the name, by the metal in the upper sections. the hardening elements, namely arsenic and The desilverized lead still contains a little antimony, are removed. The molten metal zinc, and the final refining takes place in a, is agitated with compressed air and the oxi- furnace similar to that in which the ` soften- dation of antimony and arsenic takes place ing ' was carried out, the same principle, together with a proportion of the lead. The namely oxidation, being involved. Here concentration of antimony in the bath has a trouble was experienced owing to lack of pronounced effect on the rate of oxidation fluidity of the slag, but by adding some anti- and the blowers are controlled so that the monial slag from the softening furnace, this antimony is oxidized quickly and the bath trouble was overcome without upsetting the maintained within the critical range (0.02- refining process.
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