CA1120423A - Electrowinning of metals - Google Patents

Abstract

Abstract of the Disclosure The invention disclosed is an improved process for the electrowinning of metals, particulary copper and zinc, from an acid solution of a sulphate of the metal in the presence of SO2, the improvement comprising introducing SO2 adjacent the anode/electrolyte interface at an appropriate rate. A novel porous electrode construction for use in the process is also described, includ-ing a hollow core through which SO2 may be sparged to provide SO2 at the electrode/electrolyte interface.

Description

In recent years t~lere has beerl an upsurge of interest in hydro-me~lllul-gicll l)roeesses maillly due to the impact of environmelltal consider-ations. There are probably other factors which have also contributed towarcls this marked trend away from pyrometallurgical processes and towards hydro-metallurgical operations, such as the increasing percentage o new metal production coming from low grade non-sulfide sources and mi~ed sulfide ores (eg. copper, zinc and lead). For a variety of tec~mical and economic reasons, these ores often cannot be processed efficiently by the normal pyrometallur-gical techniques. The successful industrial application af the selective solvent extraction of metals has provided the most significant breakthrough ;~ in hydrometallurgy. Accordingly, in view of environmental, technical and economical considerations, hydrometallurgical recovery of metals is gaining importance over the more traditional pyrometallurgical processes. ~owever, the hydrometallurgical operations necessitate the use of electrowimling processes as a means of recovering the metal value.
In most metal electrowinning operations, alloyed lead is the preferred anode material and the anodic reaction is the evolution of oxygen.
This reaction is well known for its irreversibility. Of all the possible known anode materials, lead-based insoluble anddes exhibit the largest oxygen overvoltage. The overvoltage for the anodic evolution of oxygen on lead-based anodes is generally greater than one volt. This high over-voltage may correspond to electrical energy losses of as much as 30 to 40%
of the total electrical energy consumption in metal electrowinning applica-tions The esca]ating electric power costs make the economic feasibility o. metal electrowinning increasingly sensitive to energy costs and to the inefficiency of energy utili7ation~ Additionally, over a prolonged period of operation, the conventional lead-based anodes tend to exfoliate and dissolve in the electrolyte, resulting in significant contamination of the cathode deposits by lead. This lead contarnination in metal electrowinning operations, together with the considerations of high energy consumption required per unit weight of metal are important drawbacks that vitiate ~ai~t more widespread use of electrowinning operations.

Several means o~ reducing the ;nerficiencies associated w;th high e;nls~ los~es due to ~he anodic reactions during rne~al electl^owillnillg have been developed. Cobalt saLts can be added to copper electrowinning eLectro-lyte to reduce corrosion of lead anocles. Gendron, A.S., EtteL, V.A. and Abe, S., Canadian Metallurgical Quar~erly 14, (l), 1975, p. 59 and Andersen, T.~., Adamson, D.L., Richards, K.J., Metallurgical Transactions, 5, June 1975, p. 1345O Cobalt addition catalyzes the oxygen discharge reaction and depolarizes the anode by about 100 mV, which results in a small saving of energy. However, the cost of cobalt salts is prohibitive and, therefore, its use will probably be Limited to operation where cobalt exists originally in the concentrate. This approach cannot be used in zinc electrowinning where cobalt causes severe interference with the zinc deposition process.
Another attractive approach is the use of dimensionally stable anodes of the oxygen discharge type, which reduce the oxygen evolution over-voltage by about 500 to 600 mV. The remarkable depolarization activity ofdimensionally stable anodes, which are now quickly replacing ~he traditional graphite anodes in the chlorine industry, is chiefly due to the transi~ion metal oxides such as ruthenium dioxide (RuOz) contained in their active coat-ing. The use of these dimensionally stable a~odes is particularly suited for metal recovery from electrolyte produced by solvent extraction process because of the low level of impurities in such electrolyte. Under the presently existing conventional copper electrowinning operations with lead lined cells these anodes would probably passivate after a short period of electrolysis due to lead o*ide deposition on the active coating. Also, under the conventional zinc electrowinning operation from electrolyte con-taining relatively high concentrations of manganese, the dimensionally stable anodes would be covered by deposited manganese dioxide after a short period of electrolysis overcoming the advantages of using dimensionally stable anodes.

Anode depolarization using sulfur dioxide has been investigated and a process has been developed by Continental Oil Company as described in U.S.
Patent ~o. 3,876,516 issued 8 April 1975. The process involves electro~inning ot coF)per rrom pregnclrlt sulEatc leach solutions follo-~inp, the injection of o tlle ~ uor to Lorm a SO2 sat~ at~d solution. This ar)l~roach l)resents a great potential for substantial energy saving as the anoclic reaction is the oxidation of suifur dioxide (E = 0.17 volt) instead of the high energy demand-ing anodic evolution of oxygen (E = 1.23 volt). However, the oxidation of sulfur dio~ide is kinetically sluggish, particularly at high current densities.
Only about 150 to 400 mV reduction of anode overvoltage could be achieved using the conventional current densities used in the industrial electrowinning practice (20-30 mA/cm or about 20-30 a/ft ).
Although the operatlon of carbonl ~r graphite anodes in saturated sulfur dioxide solution provides very significant decreases in anode potentials, the system presents a major drawback in that, at the saturation level, the vapor pressure of sulfur dioxide above the solution is large. This fact ls critical when an industrial metal electrowinning operation is envisaged as very complex and expensive engineering is required to avoid environmental problems, more particularly in the working area near the electrowinning cells. Decreas-ing the concentration of sulfur dioxide in solution to levels for which the sulfur dioxide vapor pressure reaches sufficiently low levels to eliminate the environmental problems did not appear -to be feasible.
Accordingly, it is an object of the invention to provide a novel method of reducing the anode overvoltage during metal electrowinning by improv-ing the kinetics of the sulfur dioxide oxidation reaction by introducing the sulfur dioxide reagent at the interface between the electrolyte and the anode.
According to one aspect of the invention, an improved method of metal electrowinning is contemplated in which the metal is electrowon from an acid solution containing a sulfate of the metal in the presence of SO2 and in which an anode of a material which is electroactive for the oxidation of SO2 is employed, the improvement comprising introducing S02 adjacent the anode/electro-lyte interface at a sufficient rate to reduce the anode overvoltage while 3Q providing a concentration of SO2 dissolved in the electrolyte at levels for w]lich the SO2 vapour pressure is sufficiently low to minimize atmospheric pollntiorl.
According ~o another as~ect of the invelltioll, a novel electrode for use in a metal e].ectrowinning cell :is contemplated, said electrode comprising a hollow core tubular casing of a porous material which is electroactive for the o~idation of SO2, said casing having a closed end and an o-pen end, said open end being adapted to be connected to a source of SO2 containing gas; and ..
means disposed within said hollow core for evenly distributing said gas along the length of said cas;ng, whereby said gas may be sparged through said casing from said hollow core and through said porous material.
The combination of the utili~ation of electro-active anode materials with the introduction of the reagent at the anode/electrolyte interface results in a substantial reduction in the electrical energy requirement for metal electrowinning.
In the drawings which illustrate the embodiments of the invention, .
Figure 1 is a graph illustrating variations of current densities versus anode voltage for different anode materials in copper electrowinning ; solution in presence and absence of sulfur dioxide;
: Figure 2 is a graph illustrating variations of current densities -.
versus anode voltage for different anode materials in copper electrowinning ...
solution in presence or absence of sulfur dioxide;
Figure 3 is a block diagram illustrating a laboratory scale unit for testing of anodes during copper electrowinning using different percentages of S2 in air;
Figure 4 is a side elevation in section of a porous carbon anode according to the invention;
Figure 5 is a graph illustrating comparisons in variations of current densities versus anode voltage for porous graphite anode with SO2 passing through the anode, with saturated SO2 in solution and for Pb - 6% Sb in absence of SO2; and Figure 6 is a graph illustrating comparisons in variations of cell .
voltages with respect to percentage of SO2 in air at different stoichiometric weigllts of SO2 through porous graphite anode in electrolytes of 60 gpl Cu, lO0 g~ 12S04 at 50 C and 21.5 mA/cm (20 A/ft ).
TWO m~a jor applications of the proposed tec~lolog~ have been identi-fied electl-owillning of copper and electrowinning of ~inc. Both metals are commonly electrowon from acid sulfate electrolyte solutions, using either lead-silver alloy anodes and aluminum cathode blanks for zinc and lead-antimony or lead-calcium anodes and copper or titanium cathode blanks for copper electro-winning. It was decided to carry out the initial screening of various anode materials to be used with sulfur dioxide for copper electrowinning.
The copper electrowinning conditions which were chosen for testing the effect of the combined utili~ation of electroactive anode materials with the oxidation of sulfur dioxide are typical of industrial copper electrowinning.
The electrolyte containing 30 gpl copper and 100 gpl H2S04 was prepared using reagent grade copper sulfate and sulfuric acid and distilled water. The tem-perature of the electrolyte was maintained for all tests at 50QC~ For the tests carried out in presence of sulfur dioxide, the concentration was maintained at the saturation level by sparging commercial grade sulfur dioxide gas directly in the solution. For the electrolyte composition as given above, the suifur dioxide concentration was found to be of lo -2 gpl at 50 C.
The initial screening of various materials with regard to their behaviour as anodes in the presence of sulfur dioxide at saturation concentra-tion in the electrolyte has been carried out in a H-shaped glass cell. The electrolyte is pumped to the anode compartment and returned through a bridge where samples are taken for analysis of sulfur dioxide concentration by iodo-metric titration~ A copper cathode is used in the ca~thode compartment, separa-ted from the anode compartment by a fritted glass disc, A water bath is used to maintain a constant temperature of 50 C. Constant current is being supplied from the power source at anodic current densities of 10, 20, 30, 40, 50 and 75 mA/cm . (Corresponding to 9.3, 18.6, 27.9, 37~2, 46.5 and 69.7 A/ft ) The corresponding anode voltages with respect to the Hg/Hg2S04 re~erence electrode 30 (Eo = +0.658 volts) were recorded after intervals of 0.5 hours. The surface area of anode material exposed to the electrolyte was 4 cm in all cases.

Examples of anodlc polarization curves obtained using this technique are presented in Eigure 1 for tests carried out in stllfur dioxide saturated solutions for the fo]lowing materials:
(1) Leacl-6% Sb

(2) Linseed oil impregnated graphite (Speer Carbon of Canada Ltd.)

(3) Porous carbon (POCO Graphite, Inc.) ~4) Porous graphite (POCO Graphite, Inc.) (5) 30% RuO2/70% TiO2 (Electrode Corporation) (6) MnO2 impregnated titanium (TIEAB Corp~) (7) 30% Ru02/70% TiO2 coating on titanium (8) 58.5% ~nW~4/6% ~u02/35~5% Ta2O5 coating on titanium The numbers, in brackets, preceding the anode materials refer to the number used in Figure 1. In ~igure 1, the curve marked 11, is the reference curve obtained for lead-6% antimony in absence of sulfur dioxide while curve 4a was obtained by sparging sulEur dioxide directly through a porous graphite material which will be discussed in further detail hereinaEter.
Since superior results were obtained with carbon based anodes, these materials were investigated further. Galvanodynamic polarization measurements have been carried out either in presence or in absence of sulfur dioxide using various carbon based anode materials.
The results of the anode polarization measurements on the different materials are summarized in Tables I and II for current densities of respec-tively 25 and 40 mA¦cm2. (Respectively, 23.2 and 37 A/ft2) The anode potential values obtained for the lead-6% antimony alloy in absence of sulfur dioxide have been chosen as reference values for comparison purposes as this material is conventionally used in industrial copper electrowinning practice.
With the exception of the commercial carbon cloth and the carbon-sulfur coatings on either carbon cloth or Ni mesh, testing of ~hese materials in presence of sulfur dioxide, has indi~ated large reductions in a~node potentials when compared with the conventionally used lead-~% antimony anode ., . ~

~z~

in ahsence o:[ sulfur dio~icle. The decrease :in anode potential was found to var~ bet~;een about 43% for the li.nseed oil impregnated graphite and about 85%
for the porolls graphi~e. Moreover, the carbon type electrodes tested in absence of sulfur dioxide disintegrated relatively rapidly upon application of the current, while in presence of sulfur dioxide no such degradation was observed.
TABL~ I
RESULTS OBTAINED FROM GALVANODYNAMIC TESTING OF ANODE ~TERIALS IN 30 GPL Cu, 100 GPL H2S04 AT 50C AT 25 mA/cm2, WITH AND WITHOUT SULFUR DIOXIDE

. _ Anode Materials Electrode Potential vs Anode Voltage Reduction andlor Hg~llg2S04 Ref~ Electrode (V In Presence of SO2 With Compositions . . Respect to Pb In : f S2 In Presence Absence of SO2 (Vo:Lts) _ __ __ _ ~_ _ead Pb-6% Sb (Pb Oxide pre- .
: conditioned) 1~298 1~500 -0.202 Pb-6% Sb (Etched, no reconditionin~) _ 1O050 .248 ~, , ~, . _ ~ _ arbon ._ .
inseed oil impO graphite 1.200 0~725 .573 orous carbon (Polo 0.950 0.340 .958 raphite Inc.~
orous graphite (Polo 1.060 00132 1.166 raphite Inc.) raphite (Ringsdorff _ 0O540 .758 Werke GMBH, Germany) GSR graphite (Union _ 0.595 O703 arbide) GSX graphite (Union _ 0.382 .916 arbide) 73-5 carbon (Speer _ 0.350 .948 arbon of Canada Ltd) raphite po~der in Teflo~ ~ _ 0.550 .748 0% Lamp black~50% acti-~ _ 0~250 1.048 vated charcoa~ in Teflon~
"arbon cloth _ 1.000 .298 _~. Carbon-sulfur on carbon _ 1.000 .298 cloth~
Carbon-sulfur on Ni mesh~ - . _ 1.100 ~198 ~laterials prepared at the Noranda Research CentreO

z~
TABLE II
RI:S!.'I.rS OB`l`AII~D FRO~I (.`~I.VANODYNA~IIC T~STINCS OF ANODE ~TERTALS TN 30 (;PL Cu, 100 (,PL H2SO~ AT 50 C AT A CURRENT DENSITY OF 40 m~/cm ~ITH.A~D ~IT~IOUT
SULFUR DIOXIDE
~ ' ~ ~ ~~
¦ ¦ Electrode Potential ~s I Anode Vo]tage Reduction I
j Anode Material.s ,Hg/Hg2SO4 Ref. Electrode (v)l in Presence of SO2 With ¦
I and/or ~ l Respect to Pb Tn Coating Compositions I nf bOen e ~ fPSO en e j ~bsence of SO2 (Volts) ,I,ead I ¦
Pb-6% Sb (Pb o~ide preconditioned 1 1.32910580 ' -0.260 Pb-6% Sb (Etched, no preconditioning) _ 1.125 .195 _ . _ _ ¦Carbon __ I .
Linseed oil impregO
graphite 1.2100.755 .565 Porous carbon (Polo Graphite) 1O0300.470 .850 Porous graphite (Polo Craphite) 100860.265 1.055 Graphite (from Rings- .
dorffWerke GMBH) . - 0.575 .745 ASGR graphite (Union Carbide~ _ 0.606 1 .714 AGSX graphite (Union . I
Carbide) _ 0.525 1 .795 873-5 carbon (Speer ~
Carbon Df Canada Ltd) _ 0.410 1 .910 Graphi ~ powder in I
Teflon~:~ _ 0.640 ¦ .680 50% Lamp black/50% l activated charcoal~ _ 0.440 ~ .880 Carbon cloth _ 1.16 l .160 Carbon-sulfur on carbon I
cloth - . 1.14 .180 Carbon-sulfur on Ni-mesh~ 1.46 -.140 _ ~Materials prepared at Noranda Research Centre, The term "carbon," is us~cl herein to mean those proclucts msde -Erom mi~tulsi of s~litable cal-l)ollaceous mat~rials (eg, petrol~um coke, carbon blacks, anchracite coal) wi~h binde~s such as coal tar pitcil, which have been formed to the desired shape by v~rious means) then baked to a rigid, hard structure by heat treatment to temperatures in the range of 900 to 1800 C.
The term "~raphite" refers herein to those products in ~hich the "baked carbon" is further heat treated, generally in an electric furnace (Acheson process) at temperatures of 2200 C or higher. The result of ~his process, is a change in the crystallographic structure of the carbon to the "graphitict' form. This is accompanied by marked changes in nearly all physical, electrical, and chemical properties as exemplified in Table IIa.
TABLE IIa TYPICAI. PROPERTIES OF POROUS CARBON AND POROUS GRAPHITE OBTAINED FROM POLO
GRAPHITE INC.

_ I _ _. _ . ~
POROUS CARBON POROUS GRAPHITE
PARAMETER UNIT GRADE AC GRADE AX
_ _ _ _ _ _ . _ . . .

APP~RENT DENSITY ¦ g/CC .~0 .88 COMPRESSIVE STREN&TH ~ psi 4000 2100 FLEXURAL STRENGTH ~ psi 1200 800 ELECTRICAL RESISTIVITY ohm-in x 10 58.0 16.5 COEFFICIENT OF THERMAL in/in/C x 10 7.0 ~ 6.5 EXPANSION

POROSITY ~ % 65 60 AVERAGE PORE SIZE microns l.5 ~ 1.5 - 8a -2~

The screening of var:io~ls materials wi~h regard to ~heir behaviour as ~n~l~i, in Lhe pre~nce o:~ sulful- dioxide a~ saturation concentration in the electrolyte, has been continued following the method described pre-ViOllsly~
Examples of anodic polarization curveg are presented in Figure 2 for tests carried out in absence of sulfur dioxide using a lead-5% antimony anode and in sulfur dioxide saturated solutions using an e].ectrode made of 100% graphite powder binded with Teflon~ and a hot-pressed anode made of 50%
lead/50% graphite powder.
Effect_of Sparging Sulfur Dioxide Directly Through a Porous ~ n~ A~
Measurements of anode potential variations as a function of current density have been carried out using the experimentaL set-up described in Figure 3.
The results, as shown in Figure 1, (curve 4a), have indicated that a further reduction in anode voltage could be obtained using this technique.
Comparison of these results with those obtained using the same material in sulfur dioxide saturated solution (curve 4) indicates that the concentration polarization is of about 40 mV at 20 mA/cm and of about 130 mV at 40 mA/cm 7 This is attributed to the depletion of the sulfur dioxide at the interface of the anode and the electrolyte under conditions of very slow agitation of the solution used during the tests. In view of these results, the possibility of introducing sulfur dioxide into the electrolyte either through the anodes or very close to the surface of the anode was envisaged. Such approach would be beneficial from the point of view of decreasing the concentration polariza-tion, and, therefore, allowing the operation of commercial electrowinning circuits using sulfur dioxide concentration in solution below the saturation limit without significantly increasing the cell operation voltageO
A large laboratory scale copper electrowinning cell has been designed and constructed for long-term testing of the behaviour ~f electroactive anode matcrials used for oxidation of sulfur dioxide.

3~2~

The experimental ~set-up i9 presented schema~ically in Figure 3. A
mi.~UL~:' of s~lltur clioxide gas and air in dlf:fererlt volurne ratios is sparged througll the porous carbonaceous anode 10 or through a glass-fritted gas disperser into the electrolyte in the case of the non-porous linseed oil impregnated graphite anode. The initial composi~ion of the acid copper sul~ate electrolyte 12 is 60 gpl Cu and 100 gpl H2SO4. This composition changes gradually to reach values of 25 gpl Cu and 200 gpl H2SO4 after a ten day test period. The solution is pumped to the electrolytic ce]l 14 and returned to a reservoir 16 by overflow by circulating means; conveniently a pump 17. The temperature of the electrolyte is maintained at 50C by an in-line heater 18 located in the feed line to the electrolysis cell 14. An anode current density of 21.5 mAlcm (20 A/ft2) is applied and the cell voltages are recorded.
The anodes are prepared as follows:
Lead-6% Antimony Anode . .
A conventional lead-6% antimony anode was used. The dimensions of the electrode are 31-114" x 2-1/2'l x 1/4". The anode surface was prepared by simple washing without removing the natural lead oxide layer.
Linseed Oil Impregnated Graphite ~node . .
A linseed oil impregnated graphite anode obtained from Spear Carbon of Canada Ltd. was used without any prior preparation. The dimensions of the anode were 33" x 3" x 1~
~ ~ A ~=s As seen in Figure 3, tubular hollow core porous carbon anodes 10 (pore grade 60) obtained from Union Carbide Ltd. (illustrated in figure 4) are e~
located in an electrolytic cell 14 between a pair of copper cathodes 13. The anodes 10 with dimensions of 36" x 1-l/4" outside diameter, 5/8" inside diameter were closed at one end 20. Even gas distribution over the entire length of the anode was obtained by inserting tubing 22 extending over the entire length of the central hollow core 24 of ~ carbon tube. The glass tubing includes a plurality of circumferentially located uniformly spaced openings 23 for gas distribution. The current was introduced to the anode through three contact ~)aths distributed at equal distance to induce proper current distribu~ion. This was achieved using a stainless steel rod 26 wel(led onto hose clamps 28 which er~ l`ulrLher masl~ed using a su:i.La~le coating rnaterial such as epoxy gl.ue and an acid resistant pain-; to prevent dissolution of the iron in acid.
Porous_Gra~hite Anodes Co~nercial grade porous graphi.te anodes (not shown) were obtalned from P0C0 Graphite Ltd. These anodes were only available in smal.l sections of 6" length and 2-1/2" diameter solid blocks~
A full length anode was constructed by fitting 5 blocks over a stainless steel tubing which served as gas and current distributor. Two copper cathodes of 3" x 30" active area each were employed. ~he entire length of the steel tubing was threaded to ensure good electrical contact and 4 slots were provided for gas distribution. Continuous copper electro~inning tests for several days, using an electrolyte containing 60 gpl copper and 100 gpl sulfuric acid at 50 C and for a current density of 21.5 mA¦cm (20 A/ft2), while .
sparging sulfur dioxide at a rate equivalent to its theoretical consumption .
rate, have resulted in a cell voltage 750 mV lower than that obtained during a ~.
comparative test using a lead-6% antimony anode in absence of sulfur dioxide.
The sulfur dioxide concentration in the solution could be maintained consistently below 1 gpl during this test. ..
To overcome the necessity of operation at high sulfur dioxide concentration in solution, a method has been Eound which consists of intro- .
ducing the sulfur dloxide into the system by sparging the gas through the l:
wa].ls of porous carbon or porous graphite electrodes from the inside of the electrode towards the electrode/electrolyte interface. Such a system would prvvide a means of supplying the sulfur dioxide directly at the anode elec-trolyte interface where the oxidation reaction takes place and avoid.the presence of high concentration of sulfur dioxide gas in -the solution. .
An example of the results obtained by this method is presented in --:
Figure 5 in form of a polarization curve (curve ~') obtained on a porous graphite anode with sulfur dioxide supplied through the body of the porous electrode. It is clear from this figure that sparging the sulfur dioxide througll the electrode, results in a decrease in anode potential, ~Yhen com-~2~Z3 pclred wit:h that o a conventi.onal le~ad al.loy anode in absence of sulfurdi.o~ , by more tll.m 1 volL for current densities as high as 75 mA/cm .
This substantial reduction in potential is obtained for sulfur dioxide concentration in solution which is always lower than 2 gpl and more impor-tantly of the order of l gpl or less for current densities normally applied in copper electrowinning (20 to 25 mA/cm ).
The resu].ts of the cell voltage measurements with respect to different volume ratios of sulfur dioxide and air at different stoichio-metrically required weights of sulfur dioxide are summari~ed in Table III

and presented graphically in Figure 6.
TABLE III

RESULTS OBTAINED WITH.A POROUS GRAPHITE ANODE OF SI~E 6-3/4l' LENGTH BY 2" DIA~ETER DURING COPPER ELECTRO~INNING TEST WITH .:
S2 SP~RGING~
. ~ _ ~ . .
S2 Feed Rate .
(Multiple of the Cell Voltage . ..
Stoichiometric Weight, W ~) % (vJv) SO2 In Air (volts) 0.63 - ` - 5.3 - 1.758 - 0.97 - - - ]00.0 _ I~ 651 8.1 1.334 36 - - - - 14 8- ` 1 4004 -8.4 1.240 ..
9.8 1.197 _ _ _ 100.0 10039 1.67 6.1 1.378 8.1 10289 11.1 1 ~ 219 _ _100.0 10243 _ 2.67 _____w_9.7 1.276 `
30.0 1.074 100 0 1 061 _ _'~

W = stoichiometric weight of SO2 required for the applied current~ ;
99.55 mg/min. ..
Test Conditions = 60 gpl Cu, 100 gpl H2S04 at 50 C at 21.5 mA/cm .-(20 A/ft ) for 2 hours period for each cell voltage reading.

The results indicate that a significant reduction in cell voltage can be obtained by sparging sulfur dioxide/air mixture similar in composition to that of off-gases produced industriaïly during roasting or con~erting operatiolls (S ~o 15,' sulfur dioxi(le in air). Within this range, optimal sul~ur dio~ e conct~ntrdt;ons appear to be oE the order of 10 to l5V/. ~or which a reduc-tion in the anode to catllode voltage oE about 700 to 750 mV i.9 obtained at sparging rates of the gas mi~ture corresponding to sulfur dioxide additions of 1.3 to 2.7 times the stoichiometric requirements. This reduction in cell voltage is only marginally less (about 100 mV) than that which rnay be obtained by sparging pure sulfur dioxide. The corresponding power saving can be expected to approach 0.6 to 0O7 Mwh per ton of copper produced when compared with the present industrial practice. The results of Figure 6 indicate cleariy that for sulfur dioxide concentration within the range of 5 to 100%, a significant reduction in the voltage of the copper electrowinning cell can be expected.
It was also found that for sulfur dioxide sparging rates larger than 1~36 times the stoichiometric requirement, only little further decrease in cell ;
voltage is to be expected.
It will be apprec;ated that the invention has been specifically ; described in respect of its use in a copper electrowinning process. It will be obvious to those skilled in the art that the invention is also applicable to the electrowinning of other metals such as zinc. In zinc electrowinning, the electrolyte generally contains soluble manganese ions~ Accordingly, the oxidation at the anode of these ions at sufficiently high potential must be taken into account in view of the formation of insoluble manganese dioxide which could alter the behaviour of the anodes. Also, zinc exhibits a very low standard electrode potential (E = -0.763 volt) which does not favour thermodynamically the deposition of zinc, the deposition taking place because of- the very large overpotential of hydrogen on zinc. The deposition of zinc is therefore only possible if the solution is substantially free of any metal ion impurities with low hydrogen overvoltage e.g. copper, cobalt and nickel.
Accordingly, -the application of the invention to copper electrowinning is to be considered as illustrative and by no means restrictive.

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method of electrowinning a metal in which the metal is electro-won from an acid solution containing a sulfate of the metal, in the presence of SO2 and in which an anode of a material which is electroactive for the oxidation of SO2 is employed, the improvement comprising introducing SO2 adjacent the anode/electrolyte interface at a rate sufficient to reduce the anode over-voltage, while providing a concentration of SO2 dissolved in the electrolyte at levels for which the SO2 vapour pressure is sufficiently low to minimize atmospheric pollution.

2. A method according to claim 1, wherein the SO2 is introduced in proportions of about 5-100%/v SO2 and 95-0%/v air.

3. A method according to claim 2, wherein the amount of SO2 is about 10-15%/v and the amount of air is about 90-85%/v.

4. A method according to claim 1, 2 or 3, wherein the anode is in the form of a hollow-core tubular casing of a porous carbonaceous material, includ-ing the step of sparging the SO2 from said hollow core through the porous anode.

5. A method according to claim 1, wherein the anode is made of a carbonaceous material.

6. A method according to claim 5, wherein the carbonaceous material is porous graphite.

7. A method according to claim 6, wherein the metal is selected from the group consisting of copper and zinc.

8. A method according to claim 7, wherein the acid is sulphuric acid.

9. A method according to claim 8, wherein the metal is copper.

10. A method according to claim g, wherein the anode is in the form of a hollow-core tubular casing, including the step of sparging the SO2 from said hollow core through the porous anode.

11. A method according to claim 10, wherein the SO2 is fed into said hollow core in admixture with air in proportions of about 10-15%/v SO2 and 90-85%/v air.

12. An electrode for use in a metal electrowinning cell, said electrode comprising a hollow core tubular casing of a porous material which is electro-active for the oxidation of SO2, said casing having a closed end and an open end, said open end being adapted to be connected to a source of SO2 containing gas; and means disposed within said hollow core for evenly distributing said gas along the length of said casing, whereby said gas may be sparged through said casing from said hollow core and through said porous material.

13. An electrode according to claim 12, wherein the electrode is made of a porous carbonaceous material.

14. An electrode according to claim 13, wherein the carbonaceous material is porous graphite.

15. An electrode according to claim 12, 13 or 14 wherein said gas distributing means comprises a hollow tube, said hollow tube adapted to be connected to the source of SO2 containing gas, and a plurality of circumferen-tial spaced openings uniformly spaced along the length of said tube.

16. In a method of electrowinning copper in which copper is electrowon from an acid solution containing a sulfate of copper, in the presence of SO2 and in which an anode of a material which is electroactive for the oxidation of SO2 is employed, the improvement comprising introducing a gas mixture consisting essentially of SO2 in admixture with air in proportions of about 10 to 15%/v SO2 and 90 to 85%/v air adjacent the anode/electrolyte interface along the length of the anode/electrolyte interface to provide even distribution of the gas mixture along said interface, at a rate sufficient to reduce the anode over-voltage, while providing a concentration of SO2 dissolved in the electro-lyte at levels for which the SO2 vapour pressure is sufficiently low to mini-mize atmospheric pollution.