The fundamentals of refining of non - ferrous metals have been outlined . Examples of removalof impurities by selective oxidation , electrolysis,distillation , volatilization , etc., have been given.Refining of copper, lead, zinc, tin, and some other metals have been described . The importance of economics and time involved in refining process has been stressed

Berry, J G

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REFINING OF NON-FERROUS METALSJ. G. BERRY-AbstractThe fundamentals of refining of non - ferrousmetals have been outlined . Examples of removalof impurities by selective oxidation , electrolysis,distillation , volatilization , etc., have been given.Refining of copper, lead, zinc , tin, and some othermetals have been described . The importance ofeconomics and time involved in refining processhas been stressed,IntroductionI T will be my endeavour in this paper toindicate the fundamentals of refiningas. they are applied to the more commonmembers of this group, with a description ofsome of the processes, and, in doing so, Ihope to promote discussion which, after all,is the objective of this symposium.DefinitionWhat, then, is the definition of ` refining ' ?Strictly speaking, - it is the metallurgicalprocess for the removal of impurities whichfollows the reduction of an ore or concentrateto metal. It can also be applied to anyprocess used for the purification of scrap.The term `refining', for the purposes ofthis discussion, must, however, be expandedto include the removal of impurities froman ore or concentrate before the reductionstage; as, in some cases, if such impuritiesare allowed to enter the product of reduction,they are difficult to remove in the subsequentrefining process.Whether before or after smelting, theobjective of the metallurgist is the removalof undesirable elements from a metal oralloy.Fundamentals and ExamplesHow, then, is the objective achieved ? Itmay be achieved by using the affinity of anundesirable element for another element, thusforming a compound, which is insoluble in themetal to be refined and which can easily beremoved. Selective oxidation, electrolysis,distillation or volatilization may be used,while flotation, magnetic separation or chemi-cal reaction may be used to remove undesir-able elements prior to the reduction stage.One of the earliest attempts at refiningwould be the ` purification' of gold bythermal methods, while the most recentdevelopment in this field is the solution andprecipitation of metals under controlledpressures from ores and concentrates orscrap, such as brass.In the fire-refining of copper; many im-purities are oxidized and removed in theslag, these impurities being oxidized in pre-ference to the copper itself. On the otherhand, in the refining of metals in whichcopper is itself an impurity its affinity forsulphur may be used to bring about itselimination.Zinc is refined by distillation, while im-purities, such as arsenic, can be removed byvolatilization. Electrolysis, as a methodof refining, is used for copper and, of course,the treatment of bauxite is a well-knownexample of ` pre-extraction ' refining, assum-ing that the proposed expansion of theoriginal definition of refining is permitted.Outlines of Refining ProceduresCopper-By far the greater tonnage ofcopper 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).102BERRY - REFINING OF NON-FERROUS METALS 103after concentration, are smelted to producea ` matte' which, in turn, is 'blown' to blistercopper in a converter. The blister copper,which is sent to the refinery may containvarying amounts of sulphur, iron, arsenic,aluminium, selenium, tellurium, bismuth,nickel, cobalt, gold and silver.The actual refining process starts in theconverter where, if bismuth is present, thisimpurity can be reduced to 0001 per cent orless by prolonging the blowing at the slaggingstage, and a concentrate, with a percentageof iron higher than normal, is produced toenable this to be done. Bismuth, if allowedto remain in the blister copper, cannot beremoved by fire-refining.Fire-refining of the blister copper is carriedout either to produce an ingot of relativelyhigh purity, or as a preliminary to electrolyticrefining, and even after electrolytic refiningthe cathode copper may be again fire-refinedto give the correct pitch for the productionof copper-wire bars. In the fire-refiningprocess, air is used for oxidation and the` end point ' is noted by studying the ` set 'of samples taken at intervals from the bath.It is a feature of this process that the oxida-tion of the impurities is more rapid than thatof the copper, thus enabling the impuritiesto be removed with comparative ease.Of the impurities which have been noted,sulphur is eliminated almost at once. Ironis readily removed in the slag, as also isaluminium. Arsenic and antimony, if pre-sent, are removed by using soda-ash and lime,the lime being added to reduce the wear andtear on the furnace lining. These latter ele-ments are, of course, removed after the nor-mal slagging process has been completed.Selenium and tellurium cannot be removedby fire-refining, although there are possibil-ities of some success using soda-ash underreducing conditions. However, these ele-ments are normally removed by electrolysis,together with the precious metals which, ifpresent, as would be expected, do not oxidizeand, therefore, cannot be removed by fire-refining.Care must be taken in fire-refining and abalance struck between the amount of im-purities removed and the time taken, as thelonger the oxidation , the more copper goesinto the slag which must be re-treated torecover that copper . This recovery isachieved by utilizing the affinity of copperfor sulphur and the slags , resulting from theconverting and refining operations, whenadded to the reverberatory furnace, reactwith and enter the matte.Depending on the residual impurities,'oxidation ' is followed by 'poling' withgreen poles to reduce the dissolved copperoxide, after . which the metal can be ingottedand sold for many purposes. If, however,high conductivity copper is required, and ifthe copper is suitable from the point of viewof impurities, the fire -refining process canbe continued , and it is interesting to notethat some 6 or 7 per cent of the world'sproduction is high conductivity fire-refinedcopper.Electrolytic methods of refining removethe difficult elements , such as nickel , cobalt,selenium and tellurium , together with thegold and silver; and, as already stated, afterthe electrolytic process, the cathodes aremelted and the copper subjected to a shortfire-refining process by which the correctpitch or oxygen content is obtained beforecasting into wire-bars.Lead- Lead , which may contain any ofthe following impurities , viz, ' copper, anti-mony, arsenic , bismuth and silver , has a veryinteresting course to follow from the blastfurnace to the refined product , and at PortPirie , South Australia , Dr. G. K . Williams,the then Chief Metallurgist at the BrokenHill Associated Smelter ( Proprietary) Ltd.,worked out a scheme for the continuous flowof metal from the bullion storage kettles tothe finished product.Copper is the first impurity to be removed,and this is done by drossing and by theutilization of the affinity of copper for sul-phur. The blast furnace product is receivedin kettles, where it is allowed to cool, As the104 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIAtemperature drops, the dross formationreduces the copper to under 0.1 per cent.At this stage, sulphur is added and the finaldrossing carried out at as near to the freezingpoint as possible.I remember, as a young man, watchingthis process being carried out on a very smallscale in a solder pot, and the operator, quite acharacter, called the process ` cold burnin '.I have never heard the term before or since,but it aptly describes the process. The drossfrom this section is smelted to recover thecopper, while the lead is pumped to thebullion storage kettles.The lead is now ready for the continuousrefining process , The first stage is ` soften-ing ', in which, as is indicated by the name,the hardening elements, namely arsenic andantimony, are removed. The molten metalis agitated with compressed air and the oxi-dation of antimony and arsenic takes placetogether with a proportion of the lead. Theconcentration of antimony in the bath has apronounced effect on the rate of oxidationand the blowers are controlled so that theantimony is oxidized quickly and the bathmaintained within the critical range (0.02-0.04 per cent ). The slag and softened metalare removed continuously, the slag beingretreated to produce antimonial lead as wellas to be used in the dezincing process men-tioned later.From the softened metal, steps must betaken to remove the dissolved oxygen, asthis could upset the desilverizing processand increase zinc consumption. The moltenmetal is cooled and then re-heated to de-silverizing temperature ( 650°C.) before feed-ing to the desilverizing kettles through alaunder under a charcoal cover.A desilverizing kettle is egg-shaped, beingmade in three sections bolted together,23 ft. deep with a maximum diameter of10 ft. These kettles are set in brickworkwith six independent heating zones. Alayer of zinc is kept on top of the charge andmolten metal from the deoxidizing sectionflows in passing through the zinc layer andon into the main bulk of lead below. As thelead passes through the zinc layer, the zinctakes up the gold and silver, while the leaddissolves some of the zinc. When the zincis sufficiently enriched, it is removed byladles and fresh zinc added. The lead isdischarged from the kettle by a syphon, themouth of which is set some 18 in. from thebottom. Thus the incoming lead havingpassed through the zinc layer is displaceddownward into the cooler zones, where thezinc alloy crystallizes out and rises back intothe upper sections. The temperature of thelead, as it is displaced downward, falls nearlyto the freezing point and the desilverizedlead, rising up in the syphon, is re-heatedby the metal in the upper sections.The desilverized lead still contains a littlezinc, and the final refining takes place in a,furnace similar to that in which the ` soften-ing ' was carried out, the same principle,namely oxidation, being involved. Heretrouble was experienced owing to lack offluidity of the slag, but by adding some anti-monial slag from the softening furnace, thistrouble was overcome without upsetting therefining process.At Port Pirie , a specially pure lead has beenproduced; the material used as a basis forthe process is not, as would be expected,refined lead as bismuth, an impurity difficultto remove, is often present. To overcomethis, the. antimonial slag from the softeningfurnace is used as a starting-point owing tothe fact that bismuth remains with the leadat this stage and does not enter the slag.After reducing the slag, the arsenic and anti-mony are removed by combination withcaustic soda; copper and silver by using zinc;and the zinc and final traces of antimony byusing caustic soda and nitre, the resultantmetal giving lead by a difference of 99.9992per cent, which clearly illustrates theextent to which refining can be taken if sorequired.Zinc - Turning now to zinc, the refiningof which involves distillation as distinct fromthe direct production of zinc by electrolysisBERRY - REFINING OF NON-FERROUS METALS 105which cannot be strictly classified as a re-fining process.However, a certain amount of refining isaccomplished by liquation or segregation,where the `end product' is spelter, an im-pure class of zinc containing a small quantityof lead, This is carried out in a simple fur-nace consisting of two sections. In the first,the metal is melted under reducing conditionsat a temperature slightly above freezing, theexcess lead separates out and collects at thebottom, whence it can be tapped off periodi-cally. The refined metal, i.e. the alloy whichhas the lowest melting point and which con-tains about 1 per cent of lead, overflows intothe second section, where an adequate tem-perature is maintained to permit pouringinto ingots.The New Jersey Zinc Company has patent-ed a process for the refining of zinc by distilla-tion, the first stage of which is to distil offthe volatiles, namely zinc and cadmium, andthen to distil off the cadmium from thezinc.Tin - The refining of tin is another inter-esting study, as it involves pre-smeltingtreatment to remove certain impurities inaddition to the post-smelting refining pro-cess. For example, at the ore-dressing stage,sulphides of iron and arsenic are removed byflotation, or, alternatively, for the removalof arsenic, the concentrate may be roasted.Acid leaching is often used to removeother impurities, including copper, whilemagnetic separation is used to removewolfram.The smelting is in two stages, a primarysmelt producing relatively pure metal and ahigh tin-content slag, and a secondary smelt-ing • of the slag, producing an 'irony' tinand a low tin-content slag, the 'irony' tinbeing returned. to the primary smeltingcharge.The tin, depending upon its purity, is senteither to a liquating furnace, or to a refiningkettle which has a capacity of some 60 tonsof metal. In the kettle the temperature isallowed to drop and impurities. to solidify,forming a crust which can be skimmed off.This crust and impure tin from other sourcesare treated in a liquating furnace.The process of liquating or, as it is some-times called, draining merely consists ofholding the charge at the top of the slopinghearth of a liquating furnace at a temperaturejust above the melting point of the metal con-cerned. The lowest melting point and, there-fore, purest metal which melts out, drainsaway down to the slope, where it can becollected for subsequent treatment. Theresidues which remain behind in the liquatingfurnace are returned to the reverberatoryfurnace.The metal in the kettle is 'poled' or,perhaps, we should say, boiled, because polesare no longer used and air or steam is now themedium. for agitation of the metal producingthe oxidation of impurities. Copper, as inthe case of lead, is removed by the use ofsulphur. Antimony and arsenic, if notremoved as already described, can be removedby the addition of aluminium, the compoundsof aluminium with these two metals beinginsoluble. The aluminium additions arefollowed by further boiling to remove thedross and the excess aluminium.Other metals - Let us now turn to some ofthe lesser known refining processes. Take,for example, the platinum metals which areto be recovered from sweepings etc. Thesweepings are treated in a reverberatoryfurnace with litharge and the products areslag, matte and lead, but owing to the differ-ence in affinity for matte and lead, themetals of this group are divided betweenthe products. Platinum and palladiumwill be found in the lead, while osmium,iridium and ruthenium will be found in thematte.The mechanics of the refining of magnesiumare apparently still under discussion, but theprocess used is one of fluxing. There areseveral fluxes under various trade names,but it would appear that all contain magne-sium chloride which reacts with the oxidesuspended in the melt, and as it absorbs the106 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIAoxide the flux thickens and drops to thebottom of the crucible.In bismuth refining, an alloy of bismuthand lead is formed, and to remove the lead,bismuth sulphide is added to the melt. Thereaction between the bismuth sulphide andthe lead gives bismuth and lead sulphidewhich is removed-another example of utiliz-ing the affinity of certain metals for sulphur.Aluminium, which has a strong affinity foroxygen, cannot be refined by normal methods,and any refining process must be based onelectrolysis. Even so, there are still limits inrespect of the purity of the material to berefined if this process is to be carried outeconomically. The electrolytic refining, or` three-layer ' process, as it is called, isused for this particular metal. A lowerlayer of impure aluminium, which has beenalloyed with copper, rests on the carbon blockfurnace bottom of the cell; the electrolyteis a mixture of sodium fluoride, aluminiumfluoride and barium fluoride, and the purealuminium is ladled off from the surface ofthe bath.The latest proposal for the refining ofaluminium scrap is to use the property ofthe solubility of aluminium in mercury.However, it can be well imagined that themechanical difficulties of such a method areextremely great, especially when it is re-membered that the solubility of aluminiumin mercury is best at temperatures of theorder of 650°C,In concluding this rather general surveyof non-ferrous metal refining, there is onepoint which must be stressed and whichmust be borne in mind when one is devisingmethods for the refining of any particularmetal, namely the economics of the processwhich must be carefully assessed. Scrapmetal is cheap, and the more impure it is,the cheaper, but even so it has a value, andto that value must be added the treatmentcharges, so that it is quite obvious thatexpensive and complicated procedures can-not be considered for the lower-valuedmetals.Another point, especially in the case ofprecious metals, is the time factor and the' lock-up ' of the metal in process, and this,of course, is most important where prices inthe bullion markets may rise or fall withinthe time the refiner has his customer's metalin hand.DiscussionsMR. S. SARKAR ( Metal Corporation of IndiaLimited)The different processes leading to the productionof refined lead as practised in the Tundoo Smelterof Metal Corpn. of India Ltd. produce lead ashigh as 99.993 per cent which is perhaps thesame or better than the Broken-Hill smelter inAustralia.MR. J. G. BERRY ( Copper Corporation,Ghatsila)In clarification of a point raised by Mr. Sarkar,it is pointed out that the assay quoted in the paperof 99.9992 per cent lead was a metal refined to asuper purity and not to the standard ` Broken-Hill 'brand.MR. S. VISWANATHAN (Tata Iron & SteelCo. Ltd.)(a) The definition of refining should not includepre-smelting work; (b) as impurities are known tomodify the physical properties, one would, there-fore, be interested to know the effect of impuritieson some non-ferrous metals; and (c) what are thedifficulties for the utilization of SOa obtained fromthe copper smelter at Ghatsila ?MR. J. G . BERRY ( Author's Reply)In reply to (a) it is my opinion, as stated in thepaper, that pre-smelting work is necessary in somecases to remove certain undesirable elements andas such should be included. (b) In non-ferrouswork the processes as described are for the mostpart common commercial practices. The effect ofimpurities on non-ferrous metals is a differentsubject and would not come within the purview ofrefining. (c) Regarding the utilization of SO2 theproblem has been under examination for some time,bat there are two points which are the stumblingblocks - (i) lack of market in the near vicinity and(ii) technical difficulties arising out of the use ofone converter at a time. Cost of transport ofacid precludes the Calcutta market. Jamshedpurusers have their own plants and have not shown IBERRY -REFINING OF NON-FERROUS METALS 107any enthusiasm for the acid if produced atGhatsila.The large overseas producers have several con-verters in operation at the same time and they donot attempt to collect all the sulphur dioxide froma single converter as would be necessary in ourcase. They can pick and choose and take the gasfrom the second blow and ignore the slagging blowwhich produces a low gas strength. After someexperimentation we think we can overcome thisdifficulty.MR. A. K. GUPTA { Sepulchre Brothers(India) Ltd.]Mr, J, G. Berry has pointed out various diffi-culties experienced at the Copper Corpn. for carry-ing out the recovery of sulphur front the smeltergases , one of which is its low strength. 'It may be mentioned that at the Copper SmelterWorks at Hariavalta, Sweden, two sulphuric acidplants are working starting with smelter gases con-taining varying percentages of SO5 of 1-7 to 7 percent, with a conversion efficiency of 98 per cent.

Refining of Non-Ferrous Metals

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Refining of Non-Ferrous Metals

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