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. 0.04 per cent ). The slag and softened metal At Port Pirie , a specially pure lead has been are removed continuously, the slag being produced; the material used as a basis for retreated to produce antimonial lead as well the process is not, as would be expected, as to be used in the dezincing process men- refined lead as bismuth, an impurity difficult tioned later. to remove, is often present. To overcome From the softened metal, steps must be this, the. antimonial slag from the softening taken to remove the dissolved oxygen, as furnace is used as a starting-point owing to this could upset the desilverizing process the fact that bismuth remains with the lead and increase zinc consumption. The molten at this stage and does not enter the slag. metal is cooled and then re-heated to de- After reducing the slag, the arsenic and anti- silverizing temperature ( 650°C.) before feed- mony are removed by combination with ing to the desilverizing kettles through a caustic soda; copper and silver by using zinc; launder under a charcoal cover. and the zinc and final traces of antimony by A desilverizing kettle is egg-shaped, being using caustic soda and nitre, the resultant made in three sections bolted together, metal giving lead by a difference of 99.9992 23 ft. deep with a maximum diameter of per cent, which clearly illustrates the 10 ft. These kettles are set in brickwork extent to which refining can be taken if so with six independent heating zones. A required. layer of zinc is kept on top of the charge and Zinc - Turning now to zinc, the refining molten metal from the deoxidizing section of which involves distillation as distinct from flows in passing through the zinc layer and the direct production of zinc by electrolysis BERRY - REFINING OF NON-FERROUS METALS 105 which cannot be strictly classified as a re- forming a crust which can be skimmed off. fining process. This crust and impure tin from other sources However, a certain amount of refining is are treated in a liquating furnace. accomplished by liquation or segregation, The process of liquating or, as it is some- where the `end product' is spelter, an im- times called, draining merely consists of pure class of zinc containing a small quantity holding the charge at the top of the sloping of lead, This is carried out in a simple fur- hearth of a liquating furnace at a temperature nace consisting of two sections. In the first, just above the melting point of the metal con- the metal is melted under reducing conditions cerned. The lowest melting point and, there- at a temperature slightly above freezing, the fore, purest metal which melts out, drains excess lead separates out and collects at the away down to the slope, where it can be bottom, whence it can be tapped off periodi- collected for subsequent treatment. The cally. The refined metal, i.e. the alloy which residues which remain behind in the liquating has the lowest melting point and which con- furnace are returned to the reverberatory tains about 1 per cent of lead, overflows into furnace. the second section, where an adequate tem- The metal in the kettle is 'poled' or, perature is maintained to permit pouring perhaps, we should say, boiled, because poles into ingots. are no longer used and air or steam is now the The New Jersey Zinc Company has patent- medium. for agitation of the metal producing ed a process for the refining of zinc by distilla- the oxidation of impurities. Copper, as in tion, the first stage of which is to distil off the case of lead, is removed by the use of the volatiles, namely zinc and cadmium, and sulphur. Antimony and arsenic, if not then to distil off the cadmium from the removed as already described, can be removed zinc. by the addition of aluminium, the compounds Tin - The refining of tin is another inter- of aluminium with these two metals being esting study, as it involves pre-smelting insoluble. The aluminium additions are treatment to remove certain impurities in followed by further boiling to remove the addition to the post-smelting refining pro- dross and the excess aluminium. cess. For example, at the ore-dressing stage, Other metals - Let us now turn to some of sulphides of iron and arsenic are removed by the lesser known refining processes. Take, flotation, or, alternatively, for the removal for example, the platinum metals which are of arsenic, the concentrate may be roasted. to be recovered from sweepings etc. The Acid leaching is often used to remove sweepings are treated in a reverberatory other impurities, including copper, while furnace with litharge and the products are magnetic separation is used to remove slag, matte and lead, but owing to the differ- wolfram. ence in affinity for matte and lead, the The smelting is in two stages, a primary metals of this group are divided between smelt producing relatively pure metal and a the products. Platinum and palladium high tin-content slag, and a secondary smelt- will be found in the lead, while osmium, ing • of the slag, producing an 'irony' tin iridium and ruthenium will be found in the and a low tin-content slag, the 'irony' tin matte. being returned. to the primary smelting The mechanics of the refining of magnesium charge. are apparently still under discussion, but the The tin, depending upon its purity, is sent process used is one of fluxing. There are either to a liquating furnace, or to a refining several fluxes under various trade names, kettle which has a capacity of some 60 tons but it would appear that all contain magne- of metal. In the kettle the temperature is sium chloride which reacts with the oxide allowed to drop and impurities. to solidify, suspended in the melt, and as it absorbs the 106 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA oxide the flux thickens and drops to the Another point, especially in the case of bottom of the crucible. precious metals, is the time factor and the In bismuth refining, an alloy of bismuth ' lock-up ' of the metal in process, and this, and lead is formed, and to remove the lead, of course, is most important where prices in bismuth sulphide is added to the melt. The the bullion markets may rise or fall within reaction between the bismuth sulphide and the time the refiner has his customer's metal the lead gives bismuth and lead sulphide in hand. which is removed-another example of utiliz- ing the affinity of certain metals for sulphur. Discussions Aluminium, which has a strong affinity for oxygen, cannot be refined by normal methods, MR. S. SARKAR ( Metal Corporation of India and any refining process must be based on Limited) electrolysis. Even so, there are still limits in The different processes leading to the production of refined lead as practised in the Tundoo Smelter respect of the purity of the material to be of Metal Corpn. of India Ltd. produce lead as refined if this process is to be carried out high as 99.993 per cent which is perhaps the economically. The electrolytic refining, or same or better than the Broken-Hill smelter in ` three-layer ' process, as it is called, is Australia. used for this particular metal. A lower MR. J. G. BERRY ( Copper Corporation, layer of impure aluminium, which has been Ghatsila) alloyed with copper, rests on the carbon block In clarification of a point raised by Mr. Sarkar, furnace bottom of the cell; the electrolyte it is pointed out that the assay quoted in the paper is a mixture of sodium fluoride, aluminium of 99.9992 per cent lead was a metal refined to a fluoride and barium fluoride, and the pure super purity and not to the standard ` Broken-Hill ' aluminium is ladled off from the surface of brand. the bath. MR. S. VISWANATHAN (Tata Iron & Steel The latest proposal for the refining of Co. Ltd.) aluminium scrap is to use the property of the solubility of aluminium in mercury. (a) The definition of refining should not include pre-smelting work; (b) as impurities are known to However, it can be well imagined that the modify the physical properties, one would, there- mechanical difficulties of such a method are fore, be interested to know the effect of impurities extremely great, especially when it is re- on some non-ferrous metals; and (c) what are the membered that the solubility of aluminium difficulties for the utilization of SOa obtained from in mercury is best at temperatures of the the copper smelter at Ghatsila ? order of 650°C, MR. J. G. BERRY ( Author's Reply) In concluding this rather general survey In reply to ( a) it is my opinion, as stated in the of non-ferrous metal refining, there is one paper, that pre-smelting work is necessary in some point which must be stressed and which cases to remove certain undesirable elements and must be borne in mind when one is devising as such should be included. (b) In non-ferrous methods for the refining of any particular work the processes as described are for the most metal, namely the economics of the process part common commercial practices. The effect of impurities on non-ferrous metals is a different which must be carefully assessed. Scrap subject and would not come within the purview of metal is cheap, and the more impure it is, refining. (c) Regarding the utilization of SO2 the the cheaper, but even so it has a value, and problem has been under examination for some time, to that value must be added the treatment bat there are two points which are the stumbling charges, so that it is quite obvious that blocks - (i) lack of market in the near vicinity and (ii) technical difficulties arising out of the use of expensive and complicated procedures can- one converter at a time. Cost of transport of not be considered for the lower-valued acid precludes the Calcutta market. Jamshedpur metals. users have their own plants and have not shown I BERRY -REFINING OF NON-FERROUS METALS 107 any enthusiasm for the acid if produced at MR. A. K. GUPTA { Sepulchre Brothers Ghatsila. (India) Ltd.] The large overseas producers have several con- Mr, J, G. Berry has pointed out various diffi- verters in operation at the same time and they do culties experienced at the Copper Corpn. for carry- not attempt to collect all the sulphur dioxide from ing out the recovery of sulphur front the smelter a single converter as would be necessary in our gases, one of which is its low strength. ' case. They can pick and choose and take the gas It may be mentioned that at the Copper Smelter from the second blow and ignore the slagging blow Works at Hariavalta, Sweden, two sulphuric acid which produces a low gas strength. After some plants are working starting with smelter gases con- experimentation we think we can overcome this taining varying percentages of SO5 of 1-7 to 7 per difficulty. cent, with a conversion efficiency of 98 per cent.