0), ',. Operation of the bottom blown oxygen cupel at Britannia Refined Metals, ltd.

K. R.Barrett ..;!j MIM Technology Marketingl Ltd'l Northfleetl England

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the surface of the bath and is removed by a mechanical skimming SY!lQQsis machine. The rich crust contains excessive lead, which must be The cupellation of silver/lead removed by pressing in H~ward bullion to recover precious presses or by liquation. At metals is one of the oldest Britannia Refined Metals Ltd the metallurgical processes known to crust is liquated under a man. In recent times this has chloride flux cover to produce a been carried out in small rich silver/zinc alloy (triple " alloy) . The pressed crust or ., reverberatory furnaces but the process suffers from a number of triple alloy is then retorted to ~ recover zinc leaving a rich ie' disadvantages. A new process has been developed at the Northfleet silver/lead bullion for recovery refinery of Britannia Refined of silver. Pressed crust is Metals Ltd which gives much usually retorted in Faber du improved performance. The Faur retorts but the triple process depends upon the allay is retorted in electric injection of oxygen through a furnaces under vacuum. At nitrogen shrouded lance at the Britannia Refined Metals the bottom of the furnace. The lance Hoboken Vacuum Induction retort is used. The silver/lead bullion is consumable and of unique design. Reaction rates are high produced would typically contain giving short process time, fuel 15% silver from pressed crust or consumption is low, oxygen 70% silver from liquated triple efficiency is high and silver alloy. Bath types of bullion can recirculation is low. The new be effectively cupelled in the process has resulted in Bottom Blown Oxygen Cupel. substantially lower operation costs. CUQ.ellation III Introduction » The final stage in the I :1 The recovery of silver and recovery of silver is the gold from lead bullion is an cupellation of the retort important part of the lead bullion. Cupellation is one of II the oldest metallurgical refining process absorbing a processes known to man and ,; major part of the total cast. Considerable work has been records of its use to recover carried out at Britclnnia Refined silver from silver/lead bullion Metals Ltd (BRM), to improve the go back to at least 500 B.C. The efficiences of processes used process is described by Agricola (1). In the first stage silver in De Re Metallica (A.D. 1613), is removed from the lead bullion was used and well known to the by the Parkes Process. Zinc is Romans and was described by added to the molten bullion Pliny(A.D.33). Cupellation contained in hemispherical steel depends upon on the selective kettles. A rich zinc/silver oxidation of lead, when air is crust is f.ormedwhich float5 on

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i I others was reviwed and test work blown across the surface of the commissioned. s ,on, molten bullion allowing silver Signifi~ant improvement in to concentrate in the bath. rate of rea~tion was a~hieved J In modern times cupellation f but acute refractory wear was .I has been carried out in experienced due to the washing .I reverberatory furnaces of up to a~tion of the slag caused by 15-20 tonnes capacity. The rotation of the furna~e. Oxygen furnaces are fired by oil or gas utilisation was found to be low 1 burners and air blown across the . at 607. during the trial period. i surfa~e of the bath through A further disadvantage was the t tuyeres. The furna~e ~an be inabilty to ~ast the silver tilted to allow litharge to flow produced directly into bars or I out over the water ~ooled anodes and the need to provide a breast. Three furna~es of this holding furna~e with the type were used at BRM to produ~e resulting added capital and up to 450 tonnes of silver per operating ~osts from that. I ~ annum. The pro~ess suffers from The BRM resear~h was a number of serious therefore directed at the bottom f disadvantages. blowing of oxygen in a On melting, an infusible . non-rotating refractory lined zin~ oxide ri~h dross is formed, furnace. Small s~ale tests were 'I whi~h must be raked off and ~arried out at Imperial College ~arries with it entrained silver and these showed su~h promise ri~h bullion. This dross must be :j II , fed ba~k to the furna~e and that a pilot plant was built at

i,' BRM to prove the pro~ess. After

~I fluxed with litharge but even so l three years of development and ,Ii! a vis~ous slag is produ~ed p' pilot plant work a plant scale ~arrying ex~essive silver. furna~e was built and ii! In order to allow ~onta~t of ~ommissioned in Oct 1986. H the air with the bath it is '-'n: Patents have now been granted necessary to run off the thin for the Bottom Blown Oxygen ,," layer of litharge more or less Cupel (BBOC). ~. ~ontinuously as it is formed. A i~ skilled operator ~an minimise The Bottom Blown OXYQen Cupel the amount of silver bullion run off with the litharge but even The furna~e at BRM is a so the re~ir~ulation of silver refractory lined ~ylindrical is high at about 87.. furnace with a ~apa~ity of 3.0 Fuel effi~iency is low due tonnes and is hydrauli~ally to the need to maintain high tilted. It is equipped with a excess air in the furnace charge door and hygiene hood at ij atmosphere and heat transfer is the top. The furna~e is fired poor. Pro~ess gas volume is high with a natural gas/air burner. A resulting in high gas handling key feature of the furna~e is ~osts .nd in ex~essive the lance, designed by BRM ~ ~arry-over of precious metals in en.gineers. The lan~e is nitrogen fume. Rea~tion rate is low due shrouded and located at the to poor conta~t between oxygen bottom of the furna~e as in the tuyere air and the lead illustrated in Fig 1. The lance in the bath. Pre~ious metals in is ~onsumable and as wear takes pro~ess is high due to the pla~e it is automati~ally relatively high furna~e volume. advan~ed to maintain a constant When fully ~harged the furnace position in relation to the silver inventory ~an be as high furna~e. This is a~hieved by as 7000 kg. Interest ~harges on means of a temperature sensing such sto~ks are substantial Because of these thermo~cuple at the tip of the lan~e. When the temperature disadvantages a research rea~hes - a predetermined point programme was undertaken at BRM the lance is advanced by means to find a more effe~tive way of of a small hydrauli~ motor. Due cupelling silver lead bullion. A to the nitrogen shroud, the rate more intense rea~tion using of ~onsumption is low and one oxygen rather than air was lan~e lasts for several charges. required and the available The lan~e drive assembly is technology was reviewed. The Top shown in fig.2. The design of Blown Rotary Converter (TBRC) the furna~e ~llows for tilting was an obvious choice and the to the optimum positions for work of Volker Jung(2) and blowing and slag removal. For blowing, the furnace is tilted

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~~..~~'" ,:~, , , ,I' :,~ ~ to position 'A' Fig 3, so that L!; rM;.:~" ,~ the free energy of oxide , there is no washing of the formation. This data shows that , \,j1 furnace lining with slag. Oxygen the sequence of oxidation is :'~ IiI; i is virtually totally absorbed zinc-lead-copper. ! ' and there is little turbulence I:;' ~."i ' , I.;j of the surface of the charge. The free i r: energy of formation of ':I ' Slag removal, or casting silver, silver oxide is positive at the .,1 l ':1 : ,]~ is carried out in the 'B' oper~ing te~r~~ , ll i position, where slag pots or an indicating that it is unstable I' r "l ingot casting machine can be and that silver cannot be "I"

located. On the BRM furnace ' ~. oxidised in the process. 1,: it oxygen and nitrogen are stored The selectivity of the ','", ,[ in liquid tanks and fed directly oxidation reactions is indicated '1: to the furnace via evaporators. 1 ~, ,_c by the equilibrium oonstants for 11" ~, The use of oxygen in place of the reactions as follows. E:r' air results in substantially ~H I reduced process gas volume, l)PbO + Zn = Pb + ZnO reduced precious metal [, carry-over ,in fume and reduced The equilibrium constant k at gas handling costs. Due to the 1000<=>C(Where Ink = - AG) i perfect contact between oxygen RT and the bath, the oxidation rate t. is very high and the lead in the 3.7 X 105 indicating that charge is eliminated in three zinc removal will be highly i hours. The rate of elimination selective with respect to lead. is illustrated in fig.4.Zinc oxide is readily fluxed and 2)Cu20 + Pb = 2Cu + PbO J entrainment of metal in the slag I' is low. due to the minimal f~ turbulence and the ability of k = 2.3 indicating that copper ~". ,~ metal lies to settle from the removal will accompany lead slag prior to tapping.' oxidation over a wide range of ' ~ i , Recirculation of silver has been lead content. F'~ shown to be as low as 37. in ~ by-products. The intense These predictions are reaction allows a much smaller observed in practice, where the f furnace to be used making !- majority of the residual zinc in ""~ possible a substantial reduction the input bullion is removed in in inventory. Energy consumption the early stages of oxidation ~ 1 - is low as, during the oxidation with relatively little lead. l ;ft'j. period due to the exothermic Only when the zinc content haa oxidation of lead, the process ,i: ~ been reduced to a very low level is autogenous and the burner does the rate of lead oxidation .. ;;- is not required during this increase. Zinc oxide has a :1'~ .-,..,. period. Copper is removed at BRM melting point in excess of "- by means of additions of lead in 1BOO<=>C and will therefore "'"- the final stages. The quality of remain solid until fluxed with ,£.i silver produced is >997 and can lead oxide. I,.~. be cast directly into ingots or Copper is removed together 1 anodes for electrolytic with lead throughout cupellation refining. At BRM due to the " but the rate of removal I

absence of gold or platinium increases as the lead content is :1 group metals, market 999 silver reduced to low levels at the end I

is produced by f,¥e refining in of the process. At BRM copper is i a further stage, Fine removed in the final stages by Cupellation. means of small additions of lead. Each of the oxidation Process chemistry. reactions is exothermic and the The chemistry of the heat of reaction is given as cupellation process is easily follows:- understood and relies on the relative, oxidation potentials of the impurity metals, with respect to silver. The i thermodynamic stability of the oxides within the operating I temperature regime is given by I I i ,I t Iii 591 ,:,f'r """

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content from 0.5'l.to <0.04'l..The procedure is to maintain the K.Cal/KQ Therms/l000 KQ bath temperature close to the melting point at 960cC during Copper 281 11.2 this treatment. The lead ingots are added and blowing continued Lead 235 9.3 until all the lead has been oxidised. T~e lead oxide carries Zinc 1041 41.3 copper oxi~e with it and the bath is sampled after the For typical lead oxidation litharge from each shot is rates of 400 - 500 Kg/hr the tapped. When the copper content heat release is approx 100,000 is reduced to the target level K.Cal (or 4 Therms)/hr which is the oxygen is turned off the greater than heat losses from lance and the bath purged with the furnace and an increase in nitrogen to reduce the oxygen bath temperature results. content of the silver before casting. operatinQ procedure. Casting of silver at BRM is carried out by tilting the By means of an electric furnace to the pouring position . hoist, 3 tonnes of Vacuum Retort and pouring the 1.8-2.0 tonnes bullion blocks are added to the directly to 450 kg blocks for furnace in the vertical position final treatment in the Fine and melted by means of the Cupel. For plants where natural gas/air burner. The electrolytic refining is furnace is then tilted to the necessary the anodes can be "A" position and blowing conveniently cast in the same commenced. Blowing is continued way. A materials balance is with oxygen at up to 9.0 l/sec given in Table 1 and typical (nitrogen 1.0 l/sec) and the furnace performance in Table 2. burner turned off. Zinc contained in the retort bullion Other aQglications of the BBoC. is first oxidised and forms a viscous slag but as blowing is The use of the bottom continued the litharge formed blowing technology, using the dilutes the zinc content and consumable nitrogen shrouded produces a fluid liquid slag. lance has led to investigations This allows prills of metal to into other applications, for settle and separate from the which such a furnace is slag. After 1 1/2 hours the suitable. Test work has been ~ furnace temperature will have carried out at the Warren '.. stabilised at about 900cC and Springs Labororatory of the the first slag can be tapped." United Kingdom Department of J The furnace is tilted Trade and Industry on the ~~ hydraulically and the liquid treatment of black copper (3). i slag po~~ed to moulds. . J:n,a,ck copper typicallycontains Oxygen blowing is continued Cu 75 - 85'l., Fe 5'l., Zn 3'l., Sn at the same rate for a similar 1.5 - 4.5'l., Pb 4'l.and Ni 1.5 - period to remove the remainder 4.5'l.. The impurities must be of the lead. During this period removed by oxidation to produce it is necessary to turn on the fire, refined copper. This is burner to a low fire position, traditionally carried out in a as the lead concentration is reverberatory furnace or more reduced and the heat of reaction recently in the TBRC. The pilot .1 is insufficient to maintain the scale tests carried out gave bath temperature. Litharge is very encouraging results, when again tapped and the bath compared with the TBRC. The sampled to determine the conclusions of this work were as I residual lead and copper follows. contents. Blowing is continued until the silver concentration l)The consumable lance j is >990, when copper removal is assembly, developed for the commenced. cupellation of lead silver Copper removal is carried bullion can operate at the out by the addition of lead higher temperature experienced "shots". Each shot is 150 kg of in the refining of black copper. lead and three or four are 2)oxygenefficiency is as required to reduce copper much as three times greater when using the BBoC, than when using

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I Table 1. BBOC Materials Balance "..\ Weight Ag Pb Zn Ag Pb Zn Percent Content I Charge: Kg Retort Bullion 3000 76 17 2 2280 510 60 159 30 35 30 48 56 48 I Retort Slag 1 Lead 450 100 450 I

Percent Content I Product: 1 Silver 2249 99.7 2249 i 939 74 1:..'Qi tharge 1235 6 76 6 74 Fume 154 ...> 50 30 5 77 46

Tabl e 2. BBOC Operating Performance Classical BBOC Cupel* Ave PbO production rate kg/hr/tonne furnace capacity 21.5 42.3 Peak PbO prod rate kg/hour 375 850 Silver production/ day kg 919 4106 Oxygen used M::!S/tonne Ag 30 Nitrogen used M::!S/tonne Ag 5 Fuel used Therms/ tonne Ag 992 61 Silver Recirculation 7- 8 3.4 Ag in process kg (max) 7000 2328

i * Based on cupellation of 157- silver retort bullion at Britannia I Refined Metals Ltd.

It I,;,~ 1~ "5' I~ '~ io i I I ~ I I i I I ~.. LANCE I I "" ,~ ~UID" IVB" I I ;- lANCESE.oI.""' I ) ~ ""' I // 'W/ ~ z; ft:r .;; ! J~"" ~,'" oXYcm i i' .;[. .-];. J,' ~ ~ ~ "iT ~ Fig.1 BBoe Shrouded Lance '!'

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top blowing. 3)Because of the greater efficiency, refining times can be reduced when bottom blowing. However the heat release on bottom blowing is more evenly distributed and slag temperature is lower. 4)Both top and bottom blowing are low fuming processes. 5)TMe high efficiency of oxygen use in in bottom blowing offers greater potential for process control.

The successful application of the BBOC to other processes will be extended to other materials. Test work is planned for the treatment. of copper matte, rich in precious metals and for the treatment, including cupellation, of slimes from copper and lead electrolytic plants. Conclusions

The BBOC at Britannia Refined Metals has proved to be an outstanding success and the process is now an established part of the flow sheet. The process has resulted in considerable interest in applications other than cupellation of silver/lead bullion. As a result research is currently in hand or planned for the use of the BBOC for the treatment of copper and lead electrolytic refinery slimes and for the treatment of black copper and matte rich in pr;ecious metals. It is clear that the process has many potential applications where an intense reactor using bottom blown oxygen can be used.

References. 1. Barrett.K.R. & Knight.R.P. Improvements in Silver Recovery from Lead Bullion at Britannia Refined Metals, England. .Proceedinas Silver Conference Mexico City Nov 1988 2. Jung.V. Recovery of Precious Metals from Lead Bullion. AIME Journal of Metals Oct 1981

3. G.Holt, A.W.Piper, R.P.Knight & J.H.Bird. Comparative study of the oxygen refining of black copper by top and bottom blowing. IMM Extraction Metallurav JUL 1989 Conference.

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