nikel in bindura

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E l e c t r o w i n n i n g of nickel at the BinduraSmelting and Refining Companyby H. T. BROWN*, B.Sc. (M em ber) and P. G . MASON*, B.Sc. (M ember)

SYNOPSISA fter a brief discussion of theoretical considerations in the electrow inning of nickel, an account is given of thee lectrow in nin g proc ess an d p lan t u sed a t th e B ind ura Sme ltin g a nd R efin in g C ompan y.SAMEVATTINGNa 'n kort bespreking van die teoretiese oorw egings in verband m et die elektrow inning van nikkel, w ord d1arverslag gedoen oor die elektrow inningsproses en -aanleg w at deur die B indura Sm elting and R efining C om plnyg eb ru ik wor d.

IntroductionThe refinery at the Bindura Smelting and RefiningCom pany (B.S.R), Rhodesia, treats the nickel-copper-

sulphide alloy that is produced in its pyrom etallurgicalsmelting plant. The design of the refinery circuit wasformulated by Outokumpu Oy, and the nickel andcopper products are electrowon from solution. Thecontrol of the refinery circuit is based on the rate at whichnickel is plated out of solution. New nickel-copper alloymust be added to match that rate, and allowances mustbe made for the sulphide content, or 'non-leaching'fra ctio n, in th e ca lc ulation .Copper is essential to the process, and care has to betaken with the addition rate of the alloy, as well as withthe copper-plating rate, ifthe circuit is not to be denudedo f c op pe r.This paper examines the theoretical aspects of theelectrowinning of nickel, and describes the equipmentused at B.S.R and some of the problems that havebeen encount er ed .

T he oretica l C on side ra tio nsThe nickel is electrowon with insoluble anodes and

nickel cathodes in a diaphragmed compartment from apurified nickel sulphate solution containing boric acid.The cell potential can be considered to be made up of

three po ten ti al s:Ecell, th e c ell p ote ntia l,EA, the anode electrode potential, andEo, the cathode electrode potential.E cell= EA -E o+ ER , whereER =potential across the electrolyte.The anode and cathode electrode potentials, EA an d

Eo, are m ade up of further potentials:KP . ( el ec tr od e pot en tia l) =RP .+C .P .+A .D .KP. is the driving force required for a reaction to

occur at a given electrod~lectrolyte interface. It isthe voltage between the electrode and the adjacentelectrolyte and depends on composition of electrodeand electrolyte, temperature, and current density.RP. (reversible potential) is the voltage that can be

theoretically measured across the interface when nocurrent is flowing. It is independent of whether the"'B indura Sm elting and R efining C om pany, B indurs, R hodesia.

electrode is an anode or cathode. Standard referencere versib le p oten tia ls a re a vailab le .C.P. (concentration polarization) is the voltage neces-

sary to overcom e the effects of gradients in the concentra-tion of reacting ions. These gradients are produced inthe electrolyte layer adjacent to the electrode by opera-ting at practical current densities. The concentrationpolarization increases w ith current density.

A.O. (activation overvoltage) is necessitated by som eslow step in the electrode reaction. The transfer of anelectron in the initial ion discharge at the cathode andthe incorporation of the nickel atom in the crystallattice are slow steps and require an overvoltage. Theactivation overvoltage increases w ith current density.In electrowinning, the electrons are discharged fromthe cathode and collected onto the anode. The electrode

potential for the various reactions that take place atthe anode or cathode are best represented by the electro-m otive series of Table 1. The m ore negative the potential,the more readily the reaction takes place at the anode;and the more positive the potential, the more readilythe reverse reaction takes place at the cathode.

TABLE I

ElementZincIronCobaltNickelLeadHydrogenCopperOxygen

STANDARDELECTRODEPOTENTIALAT 25 0Rev er sib le e le ctr od epotential, V-0,76-0,44-0,28

-0,25-0,130,00+0,34+1,23

Anod ic r ea ct io nZn=Zn'+ +2eFe=Fe'+ +2eC o=C o'+ +2eN i=N i'+ +2ePb=Pb'+ +2eH.=2H+ +2eCu=Cu'+ +2e2H.O=O.+4H+ +4e

The Cathodic ReactionComparison of the reversible electrode potentialsindicates that hydrogen would be evolved at the cathode,its potential being more positive than that of nickel.This does not occur because the hydrogen electrodepotential is made substantially more negative than thenickel electrode potential by the follow ing.(1) The concentration of hydrogen ions in the cathode

compartment is kept low by the use of a diaphragmbag, thus inducing a high concentration polariza-tion.(2) The deposition of hydrogen on nickel requires ah ig h lib era tio n o ve rv olta ge .

JOURNA L O F THE SOU TH A FR IC AN IN STIT UT E O F M IN ING AND META LL URGY FEBRUARY 1977 14 3


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It can be mentioned here that certain organic chemi.cals, e.g., amines from boiler-water chemicals andlubricating oils, are considered to low er the overvoltageto such an extent that hydrogen is evolved at the cathodein place of nickel deposition. Thus, at the cathode thereaction can be taken asNi2++2e=Ni. (1 )At 225 A/m2 and 65 C, an electrode potential of

-0,56 V would be made up of the following voltages,which are estimates from rough measurement to serveonly as a guide:Reve rs ib le potenti al - 0,25 VDeposition overvoltage -0,15 VConcentration overvoltage -0,16 V.

Some hydrogen is evolved, but this is probably due toth e re ac tio n

2H2O+2e=H2+20H-. . . . . . . . . . . . (2 )The hydroxyl ion could raise the catholyte pH and causethe nickel to precipitate. To prevent this, boric acid i3introduced as a buffer. Boric acid, which is a weakacid, norm ally form s com plexes w ith nickel ions, prevent-ing the precipitation of the nickel hydroxide.Because of the high electrode potential, it is necessary

to purify the catholyte of metal ions such as copper,cobalt, lead, iron, and zinc that would otherwise platew ith the nickel.The Anodic ReactionBecause insoluble anodes are used, the reaction must

involve either the decomposition of water or the oxida-tion of the anion if current is to pass. The electrodepotential for the oxidation of the sulphate anion isgreater than that for the decomposition of water, andoxidation therefore becomes the principle anodicreaction:

2H2O=O2+4H++4e. (3)At 225 A /m2 and 65 C, an electrode potential of

+1,99 V was made up ofReversible potential +1,23 VOxygen overvoltage +0,76 V.The potential is high enough for hydroxyl ions to beoxidized at the anode but, because their concentrationin acid solution is low, they consume very little anodecurrent.The O verall Cell ReactionReactions (1) and (3) combine to give the overallcell reaction2Ni2++2H2O=2Ni+4H++O2or2N iSO4+2H2O=2N i+2H2SO4+02' (5)The c ombin ed p ote ntia ls a reEA=+1,99 VEc= -0,56 VER=+1,05 V.E cell=EA -E c+ER

=1,9 9- (-0 ,5 6)+ 1,0 5=3,60 V.It should be noted that ER includes a vo ltage dropacross the diaphragm bag of 0,25 V.

. . . (4)

144 FEBRUARY 1977 JO UR NA L O F T HE SOU TH A FR IC AN IN STIT UTE O F M IN IN G A ND META LLU RG Y

The PlantCellsThe cells are constructed in pairs, and are of reinforced

concrete w ith an acid-resistant lining.Cell measurements6,6 by 1,15 by 1,2 m internal

Ce ll l in ing sOriginal: 4 % antimonial lead. Testing: pure lead,fibreglass, Telcovin (thermoplastic), neoprene. (It hasbeen necessary to replace antimonial lead linings every2 to 3 years.)Cathodes39 trimmed nickel starting sheets or stainless-steelblanks.

Anodes40 6 mm pure-lead plates (originally 4 % antimonial

lead).Spacing

160 mm anode to anode.Cathode compartmentFibreglass cathode bag fram e w ith terylene diaphragmbag. Catholyte distributed to each com partm ent.Unscree ne d p la tin g su rfO eStarting sheets: 1000 mm by 855 mm. Cathodes:

990 mm by 805 mm.Fume

Anodes are bagged and the cloth effectively filtersth e g ase s.Power SourceThe d.c. power to the cells is obtained from siliconrec ti fi er s connect ed to a 24 phase 4800 kV.A transformerarrangement. On load tap-changing and transductors

are used to provide constant current output up to amaximum of 15 000 A. The voltage range is 40 to 280 V.The Process

Nickel is first plated at a current density of 210 A/m2onto stainless-steel blanks to form three-day startingsheets, which are trimmed and have nickel loops weldedto them. Thinner sheets may be used but would requirerigidizing. The stainless-steel blanks are sanded with a270 grit emery cloth, and need only a thin coating ofshellac.The trimmed and looped starting sheets are placedin an etching tank of 20 g/l sulphuric acid, and the currentis reversed for about 30 minutes. The etched sheets areremoved and, without being dried, are placed in a10 to 15 g/l boric acid solution in a holding cell to awaittransfer to the comm ercial cells. In this manner, passiva-tion of the etched starting sheets is avoided, and aclear wettable sheet is made available for plating, thusreducing the tendency for split or pitted cathodes.Plating at a current density of 225 A/m2 continues

in the commercial cells for a further six days to providecathodes w ith in a specified thickn ess. T he tan khou se iscurrently operating at 14,5 kA, with current efficiencyin the region of 95 per cent.The cathodes are pulled, washed, sheared into squares,packed, and despatch ed. S ome w ho le cath odes are so ld.


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Process ControlC urrent density, catholyte flow , nickel concentration,pH, purity, and temperature are the most important

con tr ol p ar ame te rs .Depletion of nickel ions in the liquid film adjacent

to the cathode increases the concentration polarization,and thus the electrode potential. If this is taken to theextreme, excessive evolution of hydrogen may occur.Replenishment of the nickel ions in this liquid filmdepends on electromigration and diffusion. The rate ofdiffusion is controlled by the concentration of nickel inthe bulk of the electrolyte, which in turn is dependenton the concentration in the catholyte, the catholyteflow, and the plating rate (curre~t density).It is evident therefore that the current density, the

nickel concentration in the catholyte, and the drop innickel concentration (on which the flow is based) areinterdependent. It is current practice at B .S.R. tooperate with a drop in nickel concentration of 28 gjl(from 96 gjl in the catholyte) at a current density of2 25 A jm2.Liberation of hydrogen at the cathode is inhibited by

the high concentration overvoltage. However, if thepH is allowed to drop by the back diffusion of hydrogenions, excessive liberation of hydrogen may occur. Toprevent this, a suitable diaphragm m em brane is requiredso that a hydraulic head, preferably 25 mm, is main-tained in the cathode compartment. On the other hand,if the pH is too high, nickel can precipitate at the cathodesurface, affecting the quality of the deposit.The pH of the catholyte is controlled to 3,5 by the

addition of acid, and boric acid is added to preventnickel precipitation. An indication of the pH of theelectrolyte can be gained by inspection of the froth layeron the solution surface. Suspect bags are checked andc ha nge d a s ne ce ssa ry .As most metal ions will be deposited at the cathode

at the high potentials employed, cathode purity isdirectly related to the efficiency of catholyte purification.T he follow ing are typical analyses.

Catholyte Cathodesp.p.m . p.p.m .

Cu 1 10Co 1-2 40Pb 1 10Fe . . . . .. 1 5Zn 1 2Owin g to the lack of sensitivity of ca tho ly te a nalyses,

no correlations have been possible between catholyteand cathode impurity at this level.The temperature of the solution affects the stresses

in the cathode deposit, the stresses being relieved oncethe temperature is above about 55 cC . As PVC piping inthe circuit lim its the upper temperature to 65 cC, thecatholyte temperature is normally controlled at 62 cC .A fter long shutdow ns, it is essential that the tem peratureshould be raised before plating commences if the cathodesare not to split.The plating current can be interrupted for several

hours without adverse effect to the cathodes. It is notthe practice at B.S.R . to hold with a trickle currEnt.After a long shutdown, the current is reversed for a sLortperiod before normal plating is commenced, but it isuncertain whether this achieves anything other thanheating of the electrolyte. However, cells that areshorted out and left loaded become passive, and currentreversal is essential prior to norm al plating.

ProblemsPitting of the C athodeSodium lauryl sulphate is added to the catholyte toprevent pitting of the cathode surface. This additivemay act as a wetting agent; or it may increase the

hydrogen overvoltage, or form complexes with organicm aterial, particularly oil, present in the catholyte.N odules on the C athodeParticulate matter adhering to the cathode forms

nodules. For this reason, the catholyte is clarified inStellar filters, and a turbidimeter has been installed tocause the filtrate to recycle if breakthrough occurs.Because precipitation of nickel at the cathode can alsoresult in the growth of nodules, the pH of the catholytei s r educed .Curren t Di st ri bu ti onUneven distribution of current between the cathodes

can result in wide variation of current density andcathode quality. Cells are regularly steam-cleaned, anda close watch is kept on the busbar contacts.Catholyte Distr ibutionThe flow of catholyte to each bag is adjusted by the

action of a screw clamp on flexible PVC pipe, and ismeasured by rotameter. The system is far from ideal,and there are large variations between the flows. Thisnaturally affects the drop in nickel concentration, andthus the quality of the nickel cathode. An improvedm ethod of flow distribution is being sought.

JOURNA L O F TH E SOU TH A FR IC AN IN STIT UT E O F M IN ING AND MET ALLURGY FEBRUARY 1977 14 5

nikel in bindura

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