METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, DECEMBER 1996—889

Application of Centrifugal Fields in Fused Salt Electrowinningwith a View to Reducing Electrolytic Energy Consumption

ANTONY COX and DEREK J. FRAY

A high-temperature, laboratory scale electrochemical cell was designed, constructed, and commis-sioned to investigate the use of centrifugal fields in fused salt electrolysis production of light metals.Fused salt electrowinning of zinc was initially investigated due to the simpler physical and chemicalnature of the Zn(l)|ZnCl2(l):KCl(l)|Cl2(g) system. Current efficiencies of 93 pct were obtained for anelectrode spacing of 8 mm using 70-mm-diameter plane disc electrodes rotating at 100 rpm, signif-icantly reducing the resistive contribution to the cell voltage. By reducing the immersion depth ofthe electrodes from 25 to 10 mm, current efficiencies of 88 pct were obtained for an electrode spacingof only 4 mm for the same operating conditions, further decreasing the resistive contribution to thecell voltage.

I. INTRODUCTION

FUSED salt electrowinning techniques are a rapid wayof producing high-purity light metals,[1,2,3] such as lithium,sodium, and magnesium, with a minimum waste of startingmaterials but are often criticized for their large energy con-sumption and low space-time yields relative to pyrometal-lurgical reduction techniques.[4] Resolving these twoproblems will result in a major advancement in the lightmetals industry, with the demand for expensive metals,such as lithium, predicted to increase. This article is con-cerned with reducing the energy consumption in fused saltelectrowinning processes using a novel centrifugal separa-tion technique.

Factors influencing the cell energy consumption are thepresence of parasitic side reactions, nonuniform current dis-tribution, poor heat insulation, and large reduction poten-tials. However, one of the main problems that has proveddifficult to resolve is the current use of a large electrodespacing. Reducing this spacing from about 100 mm (typicalof commercial cells) to less than 10 mm significantly re-duces the resistance between the electrodes. However, thisenhances recombination of electrolysis products, resultingin poor current efficiencies.

The current efficiency f may be calculated from the ex-pression

zFwf 5 [1]

Mit

where z is the number of electrons transferred, F is Fara-day’s constant, w is the mass of recovered metal, M is therelative atomic mass of the metal, i is the constant elec-trolysis current applied, and t is the time of the electrolysis.Apart from direct fluid mechanical factors discussed so far,other factors may also affect the current efficiency. Thepresence of electrochemical side reactions at the electrodesurfaces may reduce the current that would otherwise drivethe primary electrode reactions. The electrolytic decompo-

ANTONY COX, Research Fellow, and DEREK J. FRAY, Professor,are with the Department of Materials Science and Metallurgy, Universityof Cambridge, Cambridge CB2 3QZ, UK.

Manuscript submitted February 1, 1995.

sition potentials for potassium chloride and zinc chloride at500 7C are 3.75 and 1.60 V, respectively.[5] Consequently,potassium deposition is not feasible in the zinc chloride /potassium chloride molten salt system. Nonuniform currentdensities may cause uneven accumulation of zinc on thecathode which could lead to recombination that may notoccur under conditions of uniform deposition of zinc. How-ever, the parallel equisized plate geometry adopted in theapparatus significantly reduces the risk of current distor-tions developing on the electrodes.

The principal factor affecting the current efficiency in thenovel cell used in this work is thought to be the fluid dy-namics established within the volume bound by the elec-trode spacing. Segregation of products by diaphragms[6] hasbeen attempted but imposes a resistive overpotential whichcounters the original intention of using a small electrodespacing. The factor to be addressed in this work is the re-duction of the large electrode spacing which is currentlyused. Separation may be achieved by the use of rotating,plane disc electrodes. The technique has already been ap-plied to laboratory-scale fused salt electrowinning of zincby Copham and Fray.[7] An electrode spacing of 4 mm anda rotational velocity of 100 rpm yielded a maximum currentefficiency of only 75 pct using plane disc electrodes. Thecause of the low current efficiency is thought to be due toelectrolyte turbulence and downflow. The existence of thelatter has been demonstrated using computational fluid dy-namics (CFD) software,[8] becoming significant at rotationspeeds in excess of 70 rpm. The CFD simulation confirmedthe existence of recirculation zones directing some of theelectrolyte back toward the perforated anode creating elec-trolyte down-flow. Furthermore, gas retention at rotationspeeds in excess of 70 rpm was observed by Cox et al.[8]

and Wiemer[9] in a purpose built water analogue to simulatethe high-temperature system. It is likely that the gas dis-persing from the anode is being swept back through theventilation holes by these electrolyte recirculation zones.

Copham and Fray[7] had more success using conical-shaped electrodes which gravitationally assisted zinc sepa-ration. Current efficiencies of 86 pct were achieved.However, a conical electrode arrangement is not amenableto the separation of metals less dense than the electrolyte,which is the ultimate aim of this research. The work pre-

890—VOLUME 27B, DECEMBER 1996 METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 1—Schematic diagram of cell and associated equipment.

Fig. 2—Variation of current efficiency with rotation speed for 8-mmspacing.

sented in this article seeks to address the problems of elec-trolyte turbulence and down-flow with a view to eventuallyapplying the technique to light metal electrowinning, con-centrating on rotating coplanar electrodes which would beappropriate to the electrowinning of light metals.

II. EXPERIMENTAL

A. Apparatus and Setup

The apparatus used is shown in Figure 1. Electrical con-tact to the 70-mm diameter, equisized graphite electrodes(Electro-Carb 5) was achieved through mountedcopper-impregnated graphite brushes (Electro-Carb 15C)via an electrically insulated concentric steel tube and rod.The outer steel shaft was bonded to the anode using ahighly conducting graphite-loaded cement (Graphibond551R). The electrodes were rotated by direct drive throughthe rod and shaft system from an overhead electric motor(Parvalux 260 rpm 96 N m). The upper anode plate wasuniformly perforated with 5-mm-diameter holes to allow

the chlorine gas to disperse from the gap. The adjustablespacing between the electrodes was made possible by theuse of alumina spacer rings.

The alumina crucible was supported in a sand-packed,custom built steel container on a retractable arm. This al-lowed the crucible to be placed in and removed from thefurnace using an air-hydraulic system.

Whereas the crucible entered through the base of the fur-nace, the electrode assembly was supported and enteredthrough the top of the furnace by an adjustable arm whichcould move in a horizontal and vertical plane.

The chlorine gas produced from the electrolysis was re-moved from the furnace using an extractor fan which drewthe gas into a fume cupboard for subsequent chemical re-action and removal with calcium carbide (preheated to 2007C) located in a tube furnace.

B. Procedures

The salt used for zinc electrowinning consisted of a 50wt pct zinc chloride/potassium chloride eutectic with den-sity 2.02 3 103 kg m23, absolute viscosity 2 3 1023 kgm21 s21, and specific conductivity 84.1 s m21 at its meltingpoint of 450 7C.[10]

The cell was operated at 500 7C. The general procedurewas that the electrodes were immersed in the melt and ro-tated for 20 minutes for the fluid to reach mechanical equi-librium. A current of 30 A (7800 A m22) was applied for1 hour. After electrolysis, the electrode assembly was liftedclear of the cell and the salt allowed to cool. The salt wasdissolved in 4 L of hot water, leaving zinc metal to sink tothe bottom of the vessel. The majority of the aqueous saltliquor was decanted and the zinc residue removed andwashed by Buchner filtration. The metal was then dried andweighed.

The cell voltage was measured at two different points inthe circuit. The total cell voltage was measured by placinga voltmeter across the terminal outputs of the power supply.The best estimate of the voltage across the electrodes wasmade by placing a voltmeter across the top of the anodeand cathode busbars. Access to the electrodes during elec-trolysis was not possible. Consequently, the total resistancebetween the busbars and the electrodes was measured toallow for any voltage drop in the latter.

III. RESULTS AND DISCUSSION

The graph in Figure 2 indicates a maximum current ef-ficiency of 93 pct at a rotation speed of about 100 rpm foran 8-mm electrode spacing. The centrifugal force at thisspeed was optimal in fulfilling the following important sep-aration criteria:

(1) adequate physical separation of gas and liquid metalphases;

(2) removal of metal from the gap faster than its rate offormation;

(3) production of insignificant electrolyte turbulence; and(4) production of insignificant electrolyte down-flow.

All the preceding factors determine the extent of the recom-bination reaction:

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, DECEMBER 1996—891

Fig. 3—Variation of Taylor number with rotation speed for 8-mm spacing.

Zn 1 Cl 5 ZnCl(l) 1 2(g) (l)

and therefore the current efficiency of the cell.

A. Adequate Interelectrode Physical Separation of Gasand Liquid Metal Phases

Wall jets have been observed inside the electrode spacingusing a CFD simulation of the electrode geometry.[8] Thesephenomena are very fast moving zones of fluid relative tothe bulk, located close to the surface of a solid, rotatingbody within a liquid environment. These wall jets probablyassisted in the separation by forcing the zinc droplets to thecathode edge. The efficiency of the interelectrode separa-tion is a function of rotational velocity and electrode spac-ing and is quantified as a dimensionless value by theTaylor’s number:[11,12]

2va ra 5 [2]=

h

where r is electrolyte density (kg m23), h is dynamic vis-cosity of the electrolyte (kg m21 s21), v is angular velocityof the electrodes (rads21), and a is semielectrode spacing(m). Jansson et al.[12] states the condition a . 7 for ade-quate separation of gas and liquid product phases. When a5 16, separation in the gap would practically be a maxi-mum, with the wall jets occupying only about 10 pct of thespacing bounded by the ion-conducting faces of the elec-trodes.

The graph in Figure 3 shows how the separation (ex-pressed through the Taylor number), and therefore the cur-rent efficiency, would be affected by the rotation speedusing an 8-mm electrode spacing. The threshold rotationalvelocity for adequate separation is approximately 50 rpm,giving a Taylor number of about 7, whereas the rotation at100 rpm yields a Taylor number of 13. The graph indicatesthere is still scope to improve the separation by spinning atfaster speeds, but the current efficiency of 93 pct measuredat 100 rpm for an 8-mm spacing (Figure 2) suggests thatproduct phase separation at this rotation speed and with aTaylor number of 13 is sufficient. For a 4-mm spacing, theresultant Taylor number for a rotation speed of 100 rpm is6.5, which will still probably confer adequate separation,as demonstrated in Section D.

With no rotation of the electrodes, a current efficiencyof 43 pct was obtained. This may be explained by the for-mation of a continually forming sheet of liquid zinc of finite

thickness determined by the extent of the chlorine gas flumeextending from the face of the anode plate. A zone of re-combination would result from the head of the zinc metalsheet contacting the gas flume.

B. Removal of Metal from the Gap Faster Than Its Rateof Formation

For a given current density, the rate at which metal drop-lets exit the gap is influenced mainly by the centrifugalforce which in turn depends on the rotational velocity, ra-dius of the electrodes, and mass of the zinc droplets. Usinga modification of Stokes’ law applied to a centrifugal field,

2 2Drd v rV 5 [3]c 18h

where Vc is the settling velocity of a particle in a centrifugalfield, Dr is the difference in density between the liquiddroplet and the bulk liquid phase, d is the measured meandiameter of a liquid droplet, and r is the radius of the elec-trode. The maximum retention time of a droplet of liquidzinc within the gap may be calculated by integrating thepreceding expression with respect to distance, since

2 2Drd v r drV 5 5c 18h dt

r t

18h 1∴ * dr 5 * dt

2 2Drd v rr 00

where r0 is the closest position from the center of the cen-trifuge wall (i.e., radius of central support shaft). It is as-sumed the droplet is in the most unfavorable location (i.e.,near the center of the interelectrode gap) and that the initialdroplet acceleration time is negligible. Hence,

18h rt 5 ln [4]r 2 2Drd v r0

whereh 5 0.002 kg m21s21;Dr 5 (rZn 2 rZnCl2/KCl) 5 (6570 2 2020) 5 4550 kg m23;d 5 7 3 1024 m (mean diameter estimated from entrainedsolid droplets of zinc in the salt);r 5 3.5 3 1022 m; andr0 5 7 3 1023 m.

The graph in Figure 4 illustrates the inverse square rela-tionship of zinc droplet retention time with rotational ve-locity using Eq. [4]. The graph indicates that the retentiontime increases rapidly at rotation speeds less than about 40rpm. The depreciation in current efficiency with the de-crease in rotation speed, particularly at speeds less thanapproximately 40 rpm, also observed in the graph of Figure2, is probably due to retention of liquid zinc and poor in-terelectrode segregation of gas and zinc phases (as de-scribed in Section A), resulting in an accumulation of themetal within the electrode spacing causing subsequentcross-gap mixing.

The separation process is probably a two-step processconsisting of interelectrode segregation of the gas and metalphases as described in Section A and removal of the gasand liquid phases from the gap. The longer the liquid metal

892—VOLUME 27B, DECEMBER 1996 METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 4—Variation of droplet retention time with rotation speed.

Fig. 5—Variation of current efficiency with electrode spacing for arotation speed of 100 rpm.

Fig. 6—Variation of current efficiency with anode immersion depth foran electrode spacing of 4 mm and a rotation speed of 100 rpm.

phase resides in the interelectrode gap, the greater thechance of cross-gap mixing despite the phase segregationsuggested by Jansson et al.[12] Consequently, as the rotationspeed is gradually reduced from 100 to 40 rpm, the currentefficiency is determined to a large extent by the progres-sively larger zinc droplet residence times within the gap.At rotation speeds less than 40 rpm, the poor current effi-ciency is due to both progressively inadequate separationof gas and metal phases (Figure 3) and progressively largerretention times.

The graph in Figure 2 suggests reasonable current effi-ciencies (.80 pct) at rotation speeds greater than approx-imately 70 rpm, which, from the graph in Figure 4, suggestsretention times less than 0.5 seconds. However, it shouldbe noted that the latter is calculated for droplets near thecenter of the interelectrode gap. Moving to the edge of theelectrode, the retention times of droplets that form closerto the electrode edge will be progressively smaller.

The gradual decrease in current efficiency above 100 rpmis explained in subsequent sections.

C. Production of Insignificant Electrolyte Turbulence

Szeri and Adams[13] and Jansson[14] have correlated trans-lational and rotational Reynolds numbers with mass transferbehavior between rotating parallel discs. The rotationalReynolds number, Re, may still be used to distinguish be-

tween laminar and turbulent throughflow of fluid within theinterelectrode gap,[14] because the fluid flow is essentiallyradial with respect to the plates (i.e., across and parallel tothe plates as opposed to ahead of the liquid hitting theplates at an acute angle).[15] Consequently, the presence orabsence of an anode (the upper plate) has little effect onthe onset of fluid turbulence in the vicinity of the plate orplates. From the expression

2rr vRe 5 [5]

h

where r is the radius of electrodes (m), a rotational Reyn-olds number of 1.4 3 104 was calculated. The thresholdvalue for significant turbulence to occur in a parallel con-centric disc geometry is given as 1.8 3 105.[15] Conse-quently, electrolyte turbulence was probably not significantup to rotational velocities of 100 rpm for this system. How-ever, beyond approximately 100 rpm, the rotational Reyn-olds number began to exceed the critical value for the onsetof turbulence, which probably contributed to the gradualdecrease in current efficiency illustrated in Figure 2.

Figure 5 shows that reducing the electrode spacing belowabout 7 mm has a deleterious effect on current efficiency.Since the Reynolds number is independent of the plate toplate distance, the depreciation in yield was unlikely to becaused by electrolyte turbulence. The probable cause is in-creased recombination due to closer proximity of electrol-ysis products within the cavity. Even with a laminarflowing electrolyte, there exists the possibility of turbulentgas flow in such a confined space, resulting in cross-gapmixing. The observation compares favorably to Cophamand Fray’s work[7] when allowance for the size differencein electrodes is made.

D. Production of Insignificant Electrolyte Down-Flow

Figure 6 implies a limiting immersion depth of about 50mm for a rotation speed of 100 rpm. At depths greater thanthis value, most of the gas is probably forced back into thespacing through the vent holes due to electrolyte recircu-lation eddies and consequent fluid down-flow. As explainedin Section I, the latter has been demonstrated using a math-ematical simulation with the aid of computational fluid dy-namics software. Retention of the gas was also observed ina physical model of the high-temperature cell at rotationspeeds in excess of 70 rpm and an immersion depth of 200

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, DECEMBER 1996—893

mm. The gas probably disperses much more slowly, partlyfrom the periphery of the anode and partly through the ventholes, but with some accumulation and recombination oc-curring, as implied in Figure 6. A maximum current effi-ciency of 88 pct was obtained for the smallest possibleelectrode spacing of 4 mm.

At immersion depths less than 50 mm, the mass of bulkliquid above the anode is reduced, with the effect that thedownward force of the recirculating fluid is also reducedand progressively more gas is dispersed through the ventholes with minimal recombination occurring and conse-quently improved current efficiencies.

E. Electrical Energy Consumption

Applying a current density of 104 A m22 at a rotationalvelocity of 100 rpm for an electrode spacing of 4 mm re-sulted in an average current efficiency of 88 pct. The ob-served total cell voltage was 8.5 V 5 0.5. The observedvoltage across the busbars was 4.7 V 5 0.1, which impliesa loss of 3.8 V due to component contributions from poorcontacts in the brush housing and between the brushes andthe busbars. The measured total resistance between the bus-bars and the electrodes was 0.03 V, which corresponds toa voltage of 0.9 V. (It was impracticable in this study todirectly measure the latter voltage during electrolysis.) Thelatter is not really important, since it is the internal cellvoltage that is being addressed in this work. Consequently,the best estimate of the true internal voltage of the cell (i.e.,the voltage across the electrodes) is (4.7 2 0.9) V or 3.8V 5 0.1. Incorporated within this voltage is the contribu-tion of the electrolyte, which can be calculated from

idV 5 [6]iR kS

where ViR is the resistive contribution of the cell voltage(V) for an electrolyte with specific conductivity k (S m21)bound by two coplanar discs of area S (m2) spaced apartby d (m) for an electrolysis current of i (A). Using thevalues k 5 84.1 S m21, d 5 4 3 1023 m, S 5 3.85 3 1023

m2, and i 5 30 A, an electrolyte resistive component of0.37 V to the overall cell voltage was calculated.

The electrical energy consumption of the cell due to elec-trolysis may be calculated from

zFVE 5 [7]

6(3.6 3 10 )fM

where

E 5 the electrolytic energy consumption (kW h kg21);z 5 the number of electrons transferred;F 5 Faraday’s constant (C mol21);V 5 the internal cell voltage (V);f 5 the current efficiency (expressed as a fraction of

unity); andM 5 the relative atomic mass of the deposited metal (kg).

Using the value 3.8 V 5 0.1 for the cell voltage and acurrent efficiency of 88 pct, an electrolytic energy con-sumption of 3.54 kW h kg21 50.09 was obtained.

The power consumption of the electric motor rotating theelectrodes at 100 rpm was 6 W. This is equivalent to(6/1000) 3 60 kW h or 0.36 kW h. Consequently, the en-

ergy required to rotate the electrodes is insignificant com-pared to the overall energy savings. The total electricalenergy consumption of the process is therefore (3.54 kW hkg21 1 0.36 kW h) or 3.90 kW h kg21 50.09.

IV. CONCLUSIONS

The centrifugal separation of a heavy metal from its gas-eous by-product in a novel fused salt electrolysis cell in-corporating subcentimeter interelectrode gaps wassuccessfully accomplished. This resulted in a significant re-duction in the resistive contribution to the cell voltage inthe laboratory-scale fused salt electrowinning of zinc, re-sulting in an electrolytic energy consumption of 3.54 kWh kg21 50.09 and a total electrical energy consumption of3.90 kW h kg21 50.09, taking into account the energyrequired to rotate the electrodes. The latter could be furtherreduced since the plate diameter on a pilot scale version ofthe cell would be much larger, allowing the electrodes tobe spun at lower speeds to attain a centrifugal field of thesame magnitude in the laboratory-scale cell.

Although there are zinc electrowinning cells with anelectrolytic energy consumption as low as 2.22 kW hkg21,[16] this was mainly achieved by modification of theelectrode geometry. The separation technique studied in thisarticle represents a significant contribution to reducing theenergy consumption in fused salt electrowinning cells.Moreover, the technique has great potential in the light met-als industry in which fused salt electrowinning is often theonly economic means of production. Furthermore, suchtechniques are notoriously expensive due to the large elec-trode spacing deployed. Consequently, if a similar degreeof success could be achieved with light metals, this wouldrepresent a major contribution to the light metals industry.This comes at a time when the demand for expensive met-als, such as lithium, is predicted to increase due to the ad-vent of super-lightweight lithium-aluminum alloys.

The next stage will be to assess the feasibility of thetechnique to fused salt electrowinning of light metals byconstructing a physical and mathematical model of the pro-cess.[8]

ACKNOWLEDGMENTS

The authors wish to thank the Engineering and PhysicalSciences Research Council (EPSRC) who provided finan-cial support for this work.

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1. D.J. Fray: Electrochemical Technology, Society of the ChemicalIndustry, London, 1980, pp. 99-111.

2. B. Lott: Electrochemical Technology, Society of the ChemicalIndustry, London, 1980, pp. 113-21.

3. B.B. Clow: Light Metals, TMS-AIME, Warrendale, PA, 1987, pp.847-50.

4. E. Gimzewski: The British Petroleum Company, Sunbury-on-Thames,Middlesex, England, personal communication, 1985.

5. W.J. Hamer, M.S. Malmberg, and B. Rubin: J. Electrochem. Soc.,1956, vol. 103 (1), pp. 8-16.

6. William H. Krusei and Derek J. Fray: Metall. Trans. B, 1993, vol.24B, pp. 605-15.

7. P.M. Copham and D.J. Fray: Metall. Trans. B, 1990 vol. 21B, pp.977-85.

894—VOLUME 27B, DECEMBER 1996 METALLURGICAL AND MATERIALS TRANSACTIONS B

8. A. Cox, J. Morris, and D.J. Fray: University of Leeds, Leeds,England, unpublished research, 1995.

9. K. Wiemer: Master’s Thesis, University of Cambridge, Cambridge,United Kingdom, 1989.

10. P.M. Copham: Ph.D. Thesis, University of Cambridge, Cambridge,United Kingdom, 1987.

11. F. Kreith: Int. J. Heat Mass Transfer, 1966, vol. 9, p. 265-282.

12. R.E.W. Jansson, R.J. Marshall, and J.E. Rizzo: J. Appl. Electrochem.,1978 vol. 8, pp. 281-85.

13. A.Z. Szeri and M.L. Adams: J. Fluid Mech., 1978, vol. 86 (1), pp.1-14.

14. R.E.W. Jansson: Electrochimica Acta, 1978, vol. 23, pp. 1345-50.15. R.E.W. Jansson and R.J. Marshall: J. Appl. Electrochem., 1978, vol.

8, pp. 287-91.16. D.J. Fray: J. Appl. Electrochem., 1973, vol. 3, pp. 103-12.