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Paper

Introduction

In large-scale gold mining operations the finelycrushed ore is leached in sodium cyanidesolution to yield a leach solution containing ~6ppm gold. This solution is then treated withactivated carbon to adsorb the gold. Theloaded carbon is screened off and the adsorbedgold is then eluted with a hot caustic solutionto yield a gold eluate containing ~600 ppmgold. The carbon is heated periodically toregenerate it for reuse later. The eluate thenpasses through electrowinning cells where thegold is deposited on to cathodes of knittedstainless steel wool. The loaded cathodes areremoved from the electrowinning cells andmanually washed using high-pressure waterjets to dislodge a gold sludge. This is filtered,after which the filter cake is smelted and castinto impure* gold bars. These bars are sent toa refinery, generally at a remote location, forfurther purification. In recent times the

electrowinning has also been carried out in so-called sludge reactors1, which allow theelectrowinning and dislodgment steps to becarried out in situ and automatically. Theproduct remains a gold sludge that is filteredand smelted as before. There is thus anincentive, not only to convert the eluate and/orthe sludge to a value-added product, directly atthe mine itself, but also to explore thepossibilities of eliminating the carbonactivation step. If a degree of gold refiningcould be achieved at the same time, this wouldbe an added bonus.

In small-scale gold mining operations theproduct is often a gold gravity concentrate,which may be treated with mercury to recoverthe gold using amalgamation and mercuryvolatilization. In any event, the value of theproduct that can be sold is generally very low.Again, there is an incentive to add value to theproduct and possibly to refine it as well.

The aim of the present investigation wastherefore to develop apparatus and proceduresto try to meet the requirements outlined above.Patents2,3 have been granted covering thework reported here.

Experimental

EquipmentA flow diagram and the general arrangementof the equipment used are shown in Figure 1.The primary electrowinning (EW) tank and theelectrowinning/electroforming (EWEF) cell,together with the electromagnetic pump andrelevant pipe-work comprise the primaryelectrowinning circuit for gold eluate solutionsfrom two South African gold mines. These

Innovations in precious metal recoveryby C. van Tonder* and M.J. Sole

Paper written on project work carried out in partial fulfilment of B Tech (EngineeringMetallurgy) degree

Synopsis

This investigation relates to a multi-purpose electrolytic system. Itsprimary purpose is to electrowin gold from high or low concen-tration gold cyanide solutions and/or from gold sludge, and toconvert the recovered gold into a gold (alloy) electroform orelectrode at the gold mine itself. These operations are carried outsequentially and in situ in the same electrolytic cell. Electrorefiningof the gold recovered may also be done in the cell. The systemapplies to silver as well, and may be adaptable to base and platinumgroup metals. It could also be used in environmental applications, toremove undesirable metals present in aqueous solution at very lowconcentration. The work to be discussed forms part of recently filedpatent applications. Subsequent and future developments of thesystem are briefly outlined.

This paper, in part or in full, has been published by theMetallurgical Mine Managers Association (MMMA), with permissionfrom the authors.

* University of Johannesburg Zenzele Technology Demonstration Centre,

Randburg, South Africa. The South African Institute of Mining and

Metallurgy, 2006. SA ISSN 0038223X/3.00 +0.00. Paper received Feb. 2006; revised paperreceived Mar. 2006.

297The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 s

*The main impurity in South African gold ores is silver,with a small amount of copper also present.

Innovations in precious metal recovery

eluates typically contain >600 ppm gold. Inside the primarytank is located one or more primary cylindrical EW cells (onlyone is shown), which plug into receptor discharge cone(s), asshown in Figure 1. Various designs of interchangeable EWcells have been investigated, but for this investigation thesimplest one was chosen, the main consideration being togenerate gold sludge easily and to transfer it to the EWEFcell. A thermostatically controlled heater provides fortemperature control of the circulating solution.

The primary EW cell essentially consists of two coaxialstainless steel cylinders, where the outer one is the anodeand the polished inner one is the cathode. The latter may berotated periodically so that electrodeposited gold particles,which are loosely adherent to the cathode surface, arescraped off by means of a fixed rubber scraper, and dropdownwards under gravity. The action of eluate circulationcauses them to be further transported down into the EWEFcell. Here they are caught up on the high surface area (HSA)cathode of that cell, which acts as a filter (see Figures 2 and3). Thus, the gold particles are in electrical contact with thatelectrode. Since secondary electrowinning may also be carriedout simultaneously in that cell, the gold from both sources iseffectively combined at the cathode.

However, initially the EWEF cell was used on its own toelectrowin gold from original and diluted eluate solutions.These solutions were contained in the smaller electroforming(EF) solution tank (prior to filling the tank with the electro-

forming solution required for the electroforming tests). Byappropriate valve manipulation, the solution could be re-circulated through that system using the same magnetic drivepump as before (see Figure 1). A thermostatically controlledheater enables the temperature of the circulating solution tobe kept at the required temperature.

The EWEF cell is shown in more detail in Figure 2. It isconstructed of polypropylene and fitted with a large inlet portfor gold eluate and/or sludge, and a smaller one for goldeluate (or electroforming) solution. It contains threeelectrodes. The axial one A is a polished, tapered, stainlesssteel or titanium rod, which acts as a mandrel during electro-forming operations. It is fitted with jack-screws, as shown, sothat after removal from the cell, the gold electroform may beremoved mechanically. Clearly, other mandrel designs andmaterials could also be used. During electrowinningoperations mandrel A may be made anodic or left uncharged.

Surrounding the mandrel is a cylindrical coaxial HSAelectrode B. As shown in Figure 3, this comprises a porouspolypropylene tube around which is wound, in the form of aSwiss roll, a combination of oxide-coated titanium orstainless steel wire-cloth and graphite felt. Two oxide-coatedtitanium or stainless steel strips provide the currentconnections, and cable ties serve to hold the system tightlytogether. During electrowinning this electrode is madecathodic and during electroforming (or electroplating) it ismade anodic. Surrounding electrodes A and B is a third

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298 APRIL 2006 VOLUME 106 NON-REFEREED PAPER The Journal of The South African Institute of Mining and Metallurgy

Figure 1Flow diagram and general arrangement of equipment used to electrowin and electroform gold metal from gold eluate and gold sludge

HEATER

PRIMARY EW TANK

POLISHED ST/STEEL CYLINDER

ST/STEEL PIPE

RUBBER SCRAPER

EF SOLN. TANKHEATER

BEARING

VALVE

EWEF CELL

PUMP

TO DRAINS

PRIMARY EW CELL

electrode C, which acts as an anode during electrowinningand is uncharged during electroforming operations. It is a co-axial cylinder of expanded oxide-coated titanium or stainlesssteel mesh.

As indicated above, the EF tank may be used for botheluate solutions during electrowinning, or for an electro-forming solution during electroforming. It is drained, ofcourse between these operations. The electroforming solutionused contained, typically, potassium cyanide (550g/l),potassium hydroxide (520 g/l), di-potassium hydrogenphosphate (520 g/l), and sometimes a brightening agent.

A 10 volt, 50 amp DC power supply was used duringelectrowinning, and a 30 volt, 3 amp (ave.) pulse powersupply for the electroforming operations.

Procedures

Undiluted gold eluate/sludge

The (200 l) primary EW tank and the (2, 5 l) EWEF cell werefilled with gold eluate solution, which was then recirculatedthrough the system. The primary heater was switched on andthe solution temperature was controlled at the desired

Innovations in precious metal recoveryJournal

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299The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 s

Figure 2Schematic diagram of the EWEF Mark I Cell used to electrowin and electroform gold/silver alloy in situ

Figure 3Detail cross-section of the high surface area (HSA) electrode used in the EWEF Mark I cell

JACK SCREWS

Ti THREADED RODS

POLYPRO PLUG

ST/STEEL INSERTWELD

INLET

GOLD ELECTROFORMEXPANDED Ti MESH

WELD

Ti MANDREL

O-RING SEALS

POLYPRO TOP PLATE

POLYPRO CELL

EF SOLN INLET

POROUS POLYPRO TUBE

Ti WIRE CLOTH/GRAPHITE FELTSWISS ROLL

POLYPRO BOTTOM PLATE

Ti STRIP CONNECTORS

RECIRCULATION/DRAIN OUTLET

POLYPRO COVER PLATE

A B C

POLISHEDST/STEELCONE

EW SOLN./SLUDGE

ST/STEELDISC.

GRAPHITE FELT

Ti CONNECTORS

CABLE TIES

POROUS POLYPRO TUBE

COATED Ti WIRE CLOTH


temperature. DC power was supplied to both the primary EWcell and to the secondary EWEF cell, at cathode currentdensities of typically 0.10.3 amps/square decimeter, forperiods of up to six hours. During this time solution sampleswere taken as a function of time and these were sub-sequently analysed for gold content. At the end of theelectrowinning operation, the current and heater wereswitched off, and, with the solution still being circulated, theprimary EW cathode was manually rotated, such that the goldadhering to its surface was scraped off by the stationaryrubber scraper. The gold sludge produced was then washeddownwards to be caught up on the outer surface of the HSAelectrode. The EWEF cell was isolated and the barren solutionin it was drained and returned to the primary EW tank.

The EF tank and the EWEF cell were then filled withelectroforming solution. Using the same pump, this solutioncould be recirculated and heated for this second-stageelectroforming operation, in the same manner as describedabove. Solution circulation was continued at temperature fora while, to allow some of the finely divided gold on the HSAelectrode to dissolve in the high cyanide solution. (In somecases gold solution was added to bring the gold content ofthe overall electroforming solution up to about 0.5 g/l). TheDC or pulsed current was then switched on, with electrode Bnow anodic, electrode C uncharged, and the central mandrelA cathodic. Electroforming was continued for up to threedays, taking solution samples for analysis, as before. At suchlow gold concentrations electroforming kinetics is necessarilyslow, but, as will be discussed later, it is believed that thisproblem can be overcome in large-scale (continuous)operations.

At the conclusion of electroforming the EWEF cell wasdrained, the mandrel A was removed from the cell, and thegold (alloy) electroform separated from the mandrel. Themetallurgical characteristics of this electroform varied,depending upon the electrodeposition parameters used, butintegral tapered cylinders or thimbles could be producedunder optimum conditions.

Original and diluted eluate solutions

Important objectives in this case were to establish whether ornot very dilute gold solutions, such as those obtaining ingold leach solutions (i.e. prior to the activated carbon goldconcentration step) could, in principle, be treated directly,using EWEF cells of this type, as well as to compare thekinetics of gold extraction under different conditions.

Eluate solutions, at various dilutions, were treated in thesmaller volume secondary system alone, using proceduresanalogous to those discussed above. Both electrowinning andelectroforming of gold were found to be possible in the EWEFMark I cell.

Gold electrorefining

Various approaches were tried to see whether the silver , themain impurity, could also be removed in situ, so that themain end product, gold metal, would be relatively pure. Thesimplest and best approach was to plate out the silver first,using direct current and a very low cathode current density.

In cyanide solution silver (and copper), having lowerelectrodeposition potentials than gold, are the more noblemetals and will plate out preferentially. Thus, theseimpurities could be deposited on the mandrel A initially.When this process was complete, the mandrel could beremoved from the cell and the silver removed from it, beforereplacing it in the cell. The electrodeposition parameterswould then be optimized for gold electroforming.

An alternative approach would be first to electroplate thegold alloy on to a titanium rod, which would later be used asan anode in a conventional gold (chloride) electrorefiningcell.

Results and discussion

Electrowinning of gold eluates

The results of using the EWEF Mark I cell to extract goldfrom various gold eluates from two South African gold mines(A and B) are given in Table I. The volume of solution usedin each run was 18 litres. These results are also plottedgraphically in Figures 4 and 5. Six successive runs werecarried out, the first four (A) from eluate solutions from goldmine A, and the last two (B) from eluate solutions from goldmine B. Thus, the initial gold concentrations varied from ca.100 ppm to ca. 900 ppm. Various other parameters werevaried, including temperature, flow rate, and the cathodecurrent density (expressed in terms of the geometric area ofthe HSA cathode, which was ca. 5 dm2.)

Referring to Table I and Figure 4, it is evident that in allcases gold extraction is very effective, the residual goldconcentration being reduced to less than 1 ppm , i.e. goldextractions of >99%, in times ranging from 1.5 to 4 hours.

In Figure 5 the logarithm of the residual gold concen-tration is plotted as a function of time. It is clear that in theregion down to about 1 ppm gold a linear relationshipapplies. The times required to reach this value of goldconcentration under the different conditions are readilydefined by the intersections of the graphs with the abscissa.

Some other trends were observed: comparing runs 1 and 2, higher operational temper-

atures improved the kinetics of gold extraction and alsomeant a lower voltage requirement

comparing runs 3 and 4, a higher cathode currentdensity improved the kinetics of gold extraction, butwith a higher voltage requirement, although the lowerflow rate and temperature of run 4 would also havehad some effect

comparing runs 2,6,and 5, it is evident that at the sametemperature (ca. 44 C), flow-rate (32 l/min) andcathode current density (2A/dm2), the times to reducethe gold concentration to ca. 1 ppm gold are fairlysimilar, irrespective of the initial gold concentration

in some cases (Runs 4 and 5) very long extractiontimes resulted in a small amount of gold going backinto solution

it may also be noted that the slopes of the graphs inFigure 5 are dependent upon the kinetics; the steeperthe gradient, the faster the kinetics.

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In later testwork, when using the larger 10 litre EWEFMark III cell of similar design, the results shown in Figure 4,were confirmed, as is illustrated in Figure 6. These resultsshow quite dramatically that gold solutions ranging inconcentration from over 2 500 ppm, down to 20 ppm gold,can be treated in EWEF-type electrowinning cells.

It is evident, therefore, that high surface area graphitefelt, which may have a typical specific surface area of > 260m2/kg 4, is extremely effective in extracting gold from eluatesolutions. Furthermore, according to Stavart et al. 4, 2000 kgof gold per m3 of felt can be loaded at 400 A/m2 from dilutegold solutions (30 mg/l Au), with classic Faraday yields of612%. Thus, the potential for employing such electrodes ingold hydrometallugy is very significant.

In addition to the above, electrowinning tests runs werecarried out in the primary electrowinning circuit (see Figure 1), where the intention was primarily to produce goldsludge. There was a transparent section in the discharge pipeleading to the EWEF Mark I cell, and so when the gold-loaded primary EW cathode was rotated, as described above,the gold sludge could be seen to be falling into the cell. Thissludge would later be converted into gold electroforms (seebelow).


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The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 301 s

Table I

Gold extraction from gold eluates from two South African gold mines using the EWEF Mark I cell

Run Elapsed time (min) Residual gold (ppm) Ave. temp. deg. C Voltage (volts) Current density (A/dm2) Flow Rate (l/min)

1 A 0 103 21 6.5 2 325 91.510 82.215 78.530 56.660 36.7120 14.8240 1.05360 0.08

2 A 0 103 43 4.2 2 325 98.515 56.030 26.860 6.7120 0.96240 0.34

3 A 0 349 43 4.8 4 3230 -95 1.4180 0.68280 0.35

4 A 0 342 34 3.8 2 1630 13390 14.5150 0.99315 1.25

5 B 0 895 44 3.8 2 325 79719 51030 25190 0.16190 0.30305 1.06

6 B 0 164 45 4.5 2 325 15416 10640 45.772 12.9110 2.09240


Electroforming

Following the completion of the six electrowinning runs,calculation showed that there should have been about 30grammes of gold on the HSA electrode. To this was added 2grammes of particulate gold metal by entraining it in the re-

circulating solution, so that it would be trapped, as describedabove. As mentioned above, the aim was now to strip all thegold from electrode B, by making it anodic with respect toelectrode A in Figure 2. Because of the limited amount ofgold present, only part of electrode A was used to produceelectroforms.

Ideally, the type of electroforming solution that should beused would contain a high concentration of gold (typically840 g/l), and might contain proprietary additives to achievea good integral fine-grained electrodeposit at relatively fastdeposition rates. Because of the impracticability of usingsuch a solution in the presence of such a small amount ofelectrowon goldany effects would be swampedtheelectroforming solution used was 18 litres of a gold eluatesolution from mine B. At the start this contained ca. 200 ppmgold. To this solution was added sodium cyanide (10 g/l), di-potassium hydrogen phosphate (5 g/l), sodium hydroxide (5 g/l) and a proprietary brightener (5 ml/l).

Electroforming was carried out at 4045 C, initially at ca.0.15 A/dm2, and later this was decreased to 0.1 A/dm2.Finally, the current was changed from DC to a pulsed output,still at 0.1 A/dm2. These changes were made in an effort toimprove the quality of the electroform produced. This initiallyhad appeared dark and rough, and possibly unsuitable to beremoved from the mandrel as an integral electroform.

As the gold concentration of the electroforming solutionwas very low and very low currents were needed, this impliedvery long deposition times, and a really good deposit washardly to be expected. Nevertheless, a good electroform wasobtained at an average current density of 0.3 A/dm2 undergood pulse condition. The gold alloy electroform produced isshown in Figure 7.

Finally, when nearly all the gold had been removed fromthe total of 50 litres of gold eluate treated, it was found thatthe mass of gold recovered in the form of electroformedtapered tubes represented a gold recovery of ca. 95%.

In commercial practice, where gold is continually beingextracted, and a gold mass of a kilogram, or more, is presenton the HSA electrode, there would be no problem abouthaving a high gold concentration electroforming electrolyte in

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Figure 5Plots of log (residual gold concentration) as a function of time, which provide an indication of gold extraction kinetics

Figure 6Graphs showing gold extraction from various solutions as afunction of time, using the EWEF Mark III cell for electrowinning

RUN 5

EG. #1EG. #4

EG. #2

EG. #5

EG. #3EG. #6

RUN 4RUN 6

RUN 1

RUN 3

RUN 2

50 100 150 200 250

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0ELAPSED TIME (MINUTES)

LOG

(GOL

D CO

NCEN

TRAT

ION

IN pp

m)34C

45C

43C 43C

44C21C

circulation. In that case the cost of the circulating load of goldin the electroforming solution would not be prohibitiveinrelation to the value of the gold recovered. Essentially, duringthe electroforming operation gold on the anode woulddissolve, while gold in solution would not be consumed andwould merely serve as a high-speed transport medium.

Recent developments

The experiments reported above were conducted in alaboratory-scale batch unit. This has subsequently been up-scaled to become a small pilot plant, having two EWEF (nowdesignated EWEREF) cells of differing design, and withvolumes of 50 and 100 litres, respectively (see Figure 8).Initial tests in the laboratory demonstrated that separatesilver and gold electroforms could be produced in theEWEREF Mark IV cell. These are shown in Figure 9.

However, before all the intended tests could becompleted, the opportunity arose to install this plant on alarge gold mine. Here gold eluate (and sludge) would bereadily available, initially on a batch basis, but eventually ona continuous basis, and there would be the opportunity tobuild up experience under realistic plant conditions.

Test work at the Kloof gold plant of Gold Fields Limited iscurrently in progress, and, as a result, improved cell andsystem designs, for which patent protection5 has beensought, are in the developmental stage. In addition, certainlaboratory tests have shown that the kinetics of gold electro-forming can be increased by an order of magnitude, or more,


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The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER APRIL 2006 303 s

Figure 7Electroformed tapered tube of gold/silver alloy produced inthe EWEF Mark I cell

Figure 8Small-scale gold recovery pilot plant, showing the 400 l reservoir and the EWEREF Mark IV and Mark VI cells

yet still yielding good integral electroforms of hard gold.These unusual synergistic techniques have also beenincorporated into the latest cell designs, and patentapplications.

It is the intention to design cathode mandrels, such thatvarious up-graded pure gold (~99.5%) products can beproduced from gold eluate/sludge in situ and at the minelocation. These products might include gold plates, gold foil6and hard (~140 VPN) gold jewellery7. It is perhaps too earlyto speculate, but we believe that it will eventually be possibleto do the same, using gold leach solution (~6 ppm gold) asthe starting material. If this proves to be viable, it couldrevolutionize the gold mining industry.

Conclusions

The following conclusions may be drawn from the investi-gations reported here:

Gold, silver, and gold/silver alloy electroforms can beproduced in situ in specially designed electrochemicalcells, directly from gold cyanide eluates and goldsludge originating from South African (and other) goldmines. There is the further possibility of doing thesame with the corresponding low concentration goldcyanide leach solutions

The potential exists to utilize this type of in situelectroforming to manufacture value-added preciousmetal products in the form of plates, tubes, foil,jewellery, etc. at the mine itself, either as a nicheoperation on a large gold mine, or as a completesystem on a small gold mine

Electrorefining of gold and silver can be included in theoperational sequence at the mine, thereby eliminatingfiltration of sludge, smelting, and gold refining else-where

If the process features outlined can be fully commer-cialized, it will be possible to eliminate a number ofsteps in the production chain from gold ore to end-products, thereby reducing costs and enhancingprofitability

In principle, operations could be fully automated inorder to minimize the possibility of gold theft.

AcknowledgementsThis work was carried out under the auspices of ZenzeleTechnology Demonstration Centre, and the authors wish tothank most sincerely the centre manager, Dr Colin Logan,without whose enthusiastic support this project would nothave been possible. Thanks are also due to AngloGoldAshanti for providing gold eluate solution, and to AllenPhilips, consulting metallurgist to Gold Fields, who not onlyarranged for the supply of similar solutions, but also madepossible the installation of our pilot plant at their Kloof goldplant. The helpful cooperation of staff at the Kloof plant isalso gratefully acknowledged. Phil Vokes of C. I. Engineeringmade valuable contributions equipment design, manufactureand maintenance, and his services are most appreciated. Thispaper was first published in Circular No. 2, November 2005,Mine Metallurgical Managers Association. Permission wasgranted for this paper to be published in this edition of theJournal.

About the authorsMichael J. Sole, C.E.F., has been a member of the AmericanElectroplaters and Surface Finishers Society (AESF) for over25 years, and has specialised in the electroforming of goldand base metals. He holds a masters degree in PhysicalChemistry from Rhodes University, Grahamstown, SouthAfrica, and a Ph.D. in Chemical Physics from the Universityof Cambridge, Cambridge, England, where he also held anInternational Atomic Energy Agency Fellowship. He is theauthor of numerous scientific papers and several patents. Hewas the keynote speaker at the AESF Symposium onElectroforming, Las Vegas, USA, and guest speaker at theWorld Gold Councils 1st Symposium on Gold JewelleryTechnology in Vicenza, Italy. He is semi-retired and works aselectrochemical consultant at Zenzele TechnologyDemonstration Centre, in Randburg, South Africa.

Carl van Tonder holds a diploma in Physical Metallurgyfrom the Pretoria Technikon, South Africa, and is currentlyworking towards a bachelors degree in Physical Metallurgy atthe University of Johannesburg. His previous experienceincludes gold jewellery electroforming under Dr Sole atHuddy Diamond, and process development in precious metalsat Rand Refinery and at Johnson Matthey in Johannesburg.He is currently employed as a technician at ZenzeleTechnology Demonstration Centre.

References1. CARTNER, W.N. South African Patent Application No. 9811751,

Electrowinning Cell, lodged 22 December 1998.

2. SOLE, M.J. South African Patent No. 2002/6170, Removal of Metals fromSolution, granted 31 December 2003.

3. SOLE, M.J. South African Patent No. 2005/0298, Metals Recovery SystemEMD Mark III, filed 12 January 2005.

4. STAVART, A., LEROY, C., and VAN LIERDE, A. Potential Use of Carbon Felt inGold Hydrometallurgy. Minerals Engineering, vol. 12, no. 5, 1999. pp. 545558.

5. SOLE, M.J. South African Patent Application No. 2005/02020ElectroWinning, ElectroRefining and ElectroForming System for GoldRecovery, filed 10 March 2005.

6. SOLE, M.J. South African Patent Application No. 2004/3697, Gold FoilManufacture, filed 14 May 2004.

7. SOLE, M.J. South African Patent No. 2005/01154, Electro Deposition filed9 February 2005. u


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Figure 9Gold, silver and gold/silver alloy electroforms produced in theEWEF Mark IV cell

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