CA2050201C - Electrogeneration of bromine and use thereof in recovery of precious metals and water treatment - Google Patents

Abstract

A process for generating bromine in an aqueous solution containing bromide ion. An aqueous solution containing bromide ions is caused to flow through an electrogeneration system that comprises paired anode means and cathode means. The system has an inlet and an outlet for the flow of the solution, the solution at the inlet having a pH of between about 0 and about 6 and a bromide ion concentration of between about 0.5 and about 8.8 moles/l. A direct electric potential is applied via the anode and cathode means to cause an electric current to pass through the flowing solution in the electrogeneration system and to generate bromine at the anode means by electrolytic oxidation of bromide ions. The relationship between the electric current and the throughput of the solution through the system is such that between about 4%
and about 50% of the bromide in the inlet solution is converted to bromine at the anode means. The pH of the discharged solution is between about 0 and about 6.
The aqueous bromine solution discharged from the electrogeneration system may be used in the recovery of precious metals, such as silver and gold, from sources thereof, and in the commercial and industrial treatment of water.

Description

ELECTROGENERATION OF BROMINE AND USE THEREOF
IN RECOVERY OF PRECIOUS I~iETALS AND WATER TREATMENT
Background of the Invention This invention relates to the field of the electrogeneration of bromine, and more particularly to an improved process for producing a bromine-containing solution that may be used for recovery of gold from sources thereof, or in other applications such as water treatment.
Conventionally, precious metals such as gold and silver have been recovered from ores by leaching with alkaline cyanide solution. By reaction with cyanide ion and oxygen the precious metal is converted to a cyanide complex (gold cyanide anion) which is taken up in the leaching solution. The dissolution of gold, for example, is illustrated by the following reaction:
pH-11 4Au+8CN +02+2H20. > 4Au(CN)2 t40H (1) Because of the high stability of the gold cyanide complex anion, even oxygen of the air is sufficient to oxidize gold in the presence of cyanide ion. Recovery of gold from the cyanide solution by precipitation may be illustrated by the following reaction equation:
Pb(OAc)2 _ Au(CN)2+2NaCN+Zn+2H20 ~ Zn(CN)4 +Au +H2+2NaOH (2) Alternatively, gold may be recovered from cyanide solution by adsorption of the gold cyanide complex onto activated carbon, desorption with a hot alkaline solution, and ~e~~~~ i~~.
recovery by electrowinning or by raising the pH. A
typical scheme for recovery via activated carbon is illustrated by the reaction equations set forth below:
Au(CN)2 o~> Au(CN)Z adsorbed (3) Hot alkaline Oti _ Au ~- Au(CN)2 desorbed While widely practiced on a commercial scale cyanide leaching suffers from well-known disadvantages.
Thus, leaching rates with alkaline cyanide solutions are slow, contact times in the range of 24-72 hours being common in the case of gold ores. Because of the toxicity of cyanide, care must be exercised to maintain cyanide solutions on the alkaline side in order to prevent the release of hydrogen cyanide gas. Severe environmental restrictions must be observed, requiring careful monitoring and control of all process purge streams.
Spent cyanide leaching solutions must be subjected to waste treatment operations before discharge to the environment.
Gold has also been leached commercially by use of aqua regia. a mixture of concentrated hydrochloric and concentrated nitric acid, according to the following reaction equation:
Au+4H++4C1 +N03 ~ AuC14tN0+2H20 (4) Gold inay then be recovered by reduction with zinc metal or raising of the leaching solution pH. However, this method is relatively unattractive because aqua regia is expensive, and highly corrosive and emits toxic fumes.
Moreover, it readily dissolves base metals and dissolves gold only relatively slowly in aqueous solution.

Thiourea has also been used as a lixiviant for the dissolution of gold from ores according to the following reaction equation:
Au+Fe3+ +2CS(NH2)2 ~ Au(CS(NH2)2]+ +Fe2+ (5) Although thiourea is effective, it is subject to oxidative degradation and is, thus, prone to high consumption levels in extracting gold from its ore.
South African published patent application number 88/8537 describes a process in which gold or silver is leached from a source thereof by contact with a leaching solution having a pH of between about 2 and about 10 and containing between about 0.01% and about 20% by weight equivalent molecular bromine, between about. 0.005%
and about 20% by weight bromide ion, and between about 0.005% and about 30% by weight total halide ion. The equivalent bromine concentration of the leaching solution in moles per liter is defined as equal to the sum of the actual molar concentration of molecular bromine, the molar concentration of perbromide ion, three times the molar concentration of bromate ion, and the molar concentration of hypobromite ion and hypobromous acid in the solution.
The leaching solution is produced by dilution and acidification of a concentrate that comprises bromide ion, perbromide ion, bromate ion, molecular bromine and an alkali metal or alkaline earth ion. The ratio of the molar concentration of bromate ion to the sum of the molar concentrations of molecular bromine and perbromide ion in the.composition is between about 0.05 and about 0.8. A
typical concentrate contains 2.14% bromine, 31.82% sodium perbromide, 14.80% by weight sodium bromide, 3.94% by weight sodium bromate and 47.30% by weight water.

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Other references which describe the general use of halogens, halides or other halide bearing compounds for the recovery of precious metals from sources thereof include Kolacsai and Zoltan PCT publication W085/00384, Shaeffer U.S. patent 267,723, Fink et al. U.S. patent 2,283,198, Harrison et al. U.S. patent 2,304,823, Jacobs U.S. patent 3,625,674, Wilson U.S. patent 3,709,681, Homick et al. U.S. patent 3,957,505, McGrew et al. U.S.
patent 4,557,759, Bazilevsky U.S. patent 3,495,976, Bahl et al. U.S. patent 4,190,489. Bahl et al. U.S. patent 4,375,989, Sergent et al. U.S. patent 4,637,865, Simpson U.S. patent 4,439,235, Falanga et al. U.S. patent 4,319,923, Jolles, "Bromine and its Compounds," Academic Press, New York, 1966, page 173, and Belohlav et al. U.S.
patent 3,222,276.
Although the use of bromine for leaching offers major advantages over the use of cyanide both with regard to safety and cost of handling leaching materials and waste streams, the cyanide process has remained competitive from an overall cost standpoint. Most gold processors have not yet found bromine leaching to be economically attractive enough to invest in making the transition from cyanide leaching.
Electrogeneration of bromine at the site of a gold recovery operation allows a lower consumption of bromine source material than can be attained in processes in which the leaching solution is prepared strictly by chemical mixing. Leaching of gold with a bromine leaching solution and separation of gold from the leachate produces a depleted bromide solution that can be recycled to the electrogeneration facility to produce fresh leaching solution.

Hess U.S. patent 4,904,358 describes a process for leaching of gold from ore by contacting a particulate ore in a leaching tank containing a 5-50% by weight NaBr solution and passing an electrolytic current through the solution in the 5 leaching tank via carbon electrodes suspended in the upper portion of the tank. The pH of the leaching solution is maintained at between about 4 and about 8 in the leaching tank.
Hess states that the quantity of current is controlled to avoid generation of hypobromous acid. Pregnant leach solution flows out the bottom of the tank through a permeable membrane for separation of solids and thence to another tank where the solution is contacted with zinc for precipitation of gold.
Israeli patent 10,390 which issued on June 19, 1958 describes a cell for generating bromine from sea water containing bromide ion. The cell has a vertical cylindrical cathode surrounding a horizontal disc shaped anode, with a vertical cylindrical baffle of impermeable non-conducting material between the electrodes and concentric with the cathode. The cell is operated at a temperature in the range of 70°C to 100°C, a bromide ion conversion of up to about 98o and a pH of 6-7. The anode is perforated to allow separation of bromine gas from the liquid phase, and the cell is apparently intended for the commercial production of bromine. The reference makes no mention of particular uses of the bromine.
A need has remained in the art for a bromine leaching process which can be operated at lower costs than the processes previously available to the art, and especially for such a process whose operating costs are low enough to provide economic justification sufficient to move gold processors to convert to such process from the industry standard cyanide process.

Not nearly so toxic as cyanide, bromine nonetheless presents its own toxicity problems. Thus, to be attractive to the industry, a bromine or bromide based process should not use solutions of such high bromine vapor pressure as to cause significant release of bromine vapors into the surrounding atmosphere, or to require substantial capital or operating costs to suppress such release.
Bromine and bromine releasing compositions have also been demonstrated to be advantageous for other applications, prominently water treatment. The toxicity concerns which affect the use of bromine in the leaching of precious metals must also be dealt with in the treatment of water. In fact, these concerns may be more serious in non-industrial applications such as swimming pool treatment where the experience of personnel responsible for treatment may be limited. and the potential for casual exposure may be greater than in an industrial setting. The safety and efficacy requirements for water treatment have largely been met by the use of organic sources of bromine such as 1-bromo-3-chloro-5,5-dimethylhydantoin or 1,3-dibromo-5,5-dimethylhydantoin.
As described in South African patent application number 88/8537, treatment of water can also be carried out using an activated bromine solution containing 21.56% by weight equivalent bromine, 19.06% by weight sodium bromide, 9.47%
by weight HBr, and 49.91% by weight water. However, it would be desirable to provide safe and effective bromine treatment at even lower costs than are made possible by the use of such sources.

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S~~r~ of the. Inver_~t '~cLn_ Among the several objects of the present invention may be noted the provision of an improved process for the electrogeneration of bromine in aqueous solution; the provision of such a process which generates an aqueous bromine solution that may be used for the recovery of precious metals such as gold and silver from sources thereof; the provision of such a process which generates bromine to produce an aqueous bromine solution at relatively low cost; the provision of such a process which may be utilized for regeneration of bromine from depleted solutions of bromide ions derived from the leaching of gold; the provision of such a process which may be used in a gold recovery process that may be -operated at relatively low cost; the provision of such a process which generates a bromine solution that is effective in water treatment and other applications; the provision of such a process whose operation involves minimal risk of exposure of attendant personnel to bromine toxicity; and. in particular, the provision of such a process which generates an aqueous bromine solution of low bromine vapor pressure that is useful and effective in the recovery of gold and the treatment of water.
It is a further particular object of the invention to provide an improved process for the recovery of gold from sources thereof, and to provide such a process which can be implemented to produce gold at low cost.
Briefly, therefore. the present invention is directed to a process for generating bromine in an aqueous solution containing bromide ion. In the process, an aqueous solution containing bromide ions is caused to flow through an electrogeneration system that comprises paired G4: i_J'Lie ~
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anode means and cathode means. The system has an inlet and an outlet for the flow of said solution, and the solution at the inlet of the system has a pH of between about 0 and about 6 and a bromide ion concentration of between about 0.5 and about 8.8 moles per liter. A direct electric potential is applied via the anode and cathode means to cause an electric current to pass through the flowing solution in the electrogeneration system and to generate bromine at the anode means by electrolytic oxidation of bromide ions. The relationship between the electric current and the throughput of the solution through the system is such that between about 4% and about 50%. preferably between about 5% and about 40%, of the bromide in said inlet solution is converted to bromine at said anode means, and the pH of the solution discharged from the outlet of said system is between about 0 and about 6, preferably between about 0 and about 3.
The invention is further directed to a process for the: leaching of gold from a source thereof. An aqueous bromine solution is prepared in accordance with the electrogeneration process described above. A solid particulate source of gold is contacted with the discharge solution from the electrogeneration system in a gold leaching stage, thereby causing the gold contained in said source to react with bromine, hypobromous acid, and bromide ions contained in the discharge solution, and producing a slurry comprising a pregnant leach solution containing AuBr4' ions and a particulate residue. The particulate residue is separated from the pregnant leach solution. Gold is recovered from an auriferous solution comprising the pregnant leach solution, thereby producing a depleted bromide solution. The depleted bromide solution is mixed with a source of bromide ion to produce a replenished bromide solution, and the replenished bromide Ro ~ v'9 ~.i, ::, ~.~'~.
solution is recycled to the electrogeneration system to produce further bromine-containing cell discharge solution for use in the gold leaching stage.
Other objects and features will be in part apparent and in part pointed out hereinafter.
Brief Description of the Drawings Fig. 1 is a schematic illustrating the electrogeneration process of the invention;
Fig. 2 is a general schematic showing the application of electrogeneration of bromine to the recovery of gold from a source material;
Fig. 3 is a slightly more detailed schematic showing the application of the process of the invention to recovery of gold from ore;
Fig. 4 is an illustration of a cell assembly that is especially preferred for use in the practice of the process of the invention;
Fig. 5 is a schematic flow sheet of an alternative embodiment of the process for recovery of gold in which an aqueous bromine leaching solution is circulated between a leaching tank and an electrogeneration system;
Fig. 6 is a schematic flow sheet showing the application of the principles of the process of Fig. 5 to a continuous cascade leaching reactor system; and Fig. 7 illustrates an especially preferred embodiment of the invention in which an aqueous leaching solution containing bromine is produced at the anode of a div-ided electrolytic cell and gold is recovered from a pregnant leach solution by electrowinning at the cathode of the same cell.

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Corresponding reference characters indicate corresponding parts in the several drawings.
Descrir~tion of ti3~ Preferred Embodimey In accordance with the present invention, it has been discovered that bromine can be generated in aqueous solution to produce an aqueous bromine solution, and that the bromine solution generated can be used in an economically advantageous process for the leaching of gold and silver frorn sources thereof. This solution has been demonstrated to be effective for recovery of gold from ores in high yield and at commercially acceptable leaching rates, and is also effective for the treatment of water and in other disinfectant applications. In particular, the solution is effective for industrial water treatment applications, such as the treatment of cooling tower water, and in other water treatment applications such as the treatment of swimming pool water. Although the oxidizing potential of the solution is more than adequate for such purposes, the free bromine content is limited so that the vapor pressure of the solution is relatively low. Thus, the solution may be used without creating hazards to operating personnel in a gold recovery plant or water treatment facility, and without the necessity of expensive facilities for the protection of personnel from bromine release.
By controlling the relationship between current and the flow of electrolytic solution through the electro-generation system, high current efficiencies can be realized in the process of the invention. By controlling the composition of the solution entering the electrogener-ation system and creating sufficient turbulence in the system to minimize overvoltages, the power~consumption per » r-'s~.
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unit weight of bromine produced is maintained within acceptable limits. Where the aqueous bromine solution is used for leaching of gold, separation of gold from the leaching solution produces a depleted bromide solution which can be recycled to the electrogeneration step.
Unreacted bromide ion is thus reclaimed for conversion to bromine, thereby limiting the consumption of reagents and making it possible to operate a gold recovery process at lower reagent cost than a conventional cyanide process.
Toxicity risks and waste treatment costs are both significantly lower in the process of the invention than in the cyanide process. Thus, the process can be used in the recovery of gold from ores and other sources at operating costs that are quite competitive with the cyanide process.
Fig. 1 is a schematic flow sheet of the electrogeneration process. A bromide solution prepared in a makeup tank 1 is transferred by a pump 3 to an electrolytic cell 5. Power is applied to the cell by a direct current power source 7 via an anode 9 and a cathode 11. The cell shown in Fig. 1 is an undivided cell, i.e., it contains no diaphragm or other impediment or obstruction to flow of electrolytic solution sufficient to cause a discontinuity in the concentration gradient between the anode and the cathode. Bromine is generated at the anode by the reaction:
2Br- ~ Br2 + 2e-Hydrogen is generated at the cathode by the reaction:
2H+ + 2e- ~ H2 I~n. ~ ~a:f'Li:a~d.
Although a single cell is illustrated in Fig. 1, it will be understood that the electrogeneration system may comprise a cell bank containing a plurality of cells. The cells of such a system may be arranged in a variety of ways, but are preferably connected electrically in series. Depending on production requirements, the desired equivalent bromine concentration of the product solution and electrical design considerations, several banks of cells may be used with the cells of each bank electrically in series, and the banks arranged either in series or in parallel with respect to each other. Depending on production requirements, the desired equivalent bromine concentration of the product solution, and the relationship of electrode area to flow of electrolytic solution, the cells may be hydraulically in series or hydraulically in parallel.
The feed solution entering the cell (or cell bank) from tank 1 has a pH of between about 0 and about 6, preferably between about 0 and about 3, and contains between about 0.5 and about 8.8 moles/1, preferably between about 0.5 and about 5 moles/1, bromide ion. The feed solution may be prepared by dissolving an alkali metal bromide in water and acidifying with an acid such as HHr, sulfuric acid, or HC1 to the desired pH. Thus. the solution may contain between about 0.5 and about 8.8 moles/1 of sodium ion. Turbulent flow velocity and/or mechanical agitation in the electrode region is established at a level sufficient to minimize overvoltages and maintain the individual cell voltage in the range of between about 4 and about 5 volts at a current density in the range of between about 2.0 and about 4.0, preferably between about 2.5 and about 3.0, kA/m2. Preferably, feed solution is introduced into the cell at essentially ambient temperature. Temperature rise in the cell (or bank of cells) is in the range of between about 4°C and about 20°C. Preferably, conditions are controlled to avoid increase of the cell discharge solution temperature to greater than about 50°C.
High current efficiency is maintained by controlling the relationship between current and the throughput of electrolytic solution through the system so that the conversion of bromide ion during passage through the cell bank is between about 4% and about 50%, preferably between about 5% and about 40%. For satisfactory produc-tivity, the current density should be in the range of between about 2.0 and about 4.0 kA/m2. The product solu-tion has a pH of between about 0 and about 6, preferably between about 0 and about 3, and contains between about 0.01 and about 3.66 moles/1 of equivalent bromine,. between about 0.1 and about 4.0 moles/1 unreacted bromide ion, and between about 0.1 and about 4.0 moles/1 alkali metal ion.
Preferably, the product solution containing between about 0.03 and about 2.5 moles/1 equivalent bromine, between about 0.4 and about 3.0 moles/1 bromide ion, and between about 0.4 and about 3.0 moles/1 alkali metal ion.
Equivalent bromine is defined as the sum of the molar concentrations of molecular bromine, perbromide ion (Hr3-), hypobromite ion, and hypobromous acid. It also includes any bromate ion present in the solution, but at the prevailing pH, no substantial bromate ion concentration would be anticipated. The molar ratio of equivalent bromine to bromide ion in the product solution is between about 0.05 and 0.6, preferably between 0.2 and 0.6. In this range, the solution has substantial oxidizing power, but does not have a substantial bromine vapor pressure.

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Where the solution leaving cell 5 is used in such applications as leaching of gold ore, a depleted bromide solution is produced which may optionally be recycled to tank 1 where it is replenished by addition of fresh alkali metal bromide, and adjusted with acid or base as necessary to provide a feed solution of the proper pH
for electrolysis in cell 5.
As noted above, the electrogeneration system may comprise one or more banks of cells rather than the single cell that is illustrated in Fig. 1. Moreover, the electrogeneration system may operate on a continuous basis as shown in Fig. 1 or on a batch basis in which the electrolytic solution is circulated between the cells) a and reservoir such as the bromide solution makeup tank until the desired conversion has been realized. Iii either case the cells) preferably operate on a flow basis, but in the latter (batch) case. recirculation is required to reach the desired conversion. Whether operation is continuous or batch, the relationship between electric current and throughput is such that the conversion of bromide ion is in the desired range described herein. It will be understood that, in a fully continuous operation, the throughput is the flow rate through the electrogenera-tion system, while in a recirculation or other batch operation the throughput is determined from the batch volume and time of application of power to recirculating solution.
In order to produce an aqueous bromine leaching solution at competitive cost. it is important that the cells of the electrogeneration system operate with high productivity and high electrical efficiency. High current efficiency is promoted in an undivided cell by operation at low bromide conversions, thereby minimizing the back reaction by which bromine is reduced to bromide ions at 15 °'"t'~r'° ,'~ ~~
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the cell cathode. Electrical efficiency is further promoted by the use of cells which are arranged to provide high rates of mass transfer between the bulk solution and the anode, thereby minimizing half cell overvoltage. High productivity is attained through high electrical effi-ciency, adequate current density, and a high ratio of electrode surface area to solution volume. Preferably, mass transport coefficient (km) for transfer of bromide ions from the bulk solution to the anode surface is at least about 5x10-'~ cm/sec. typically 5x10-4 to about 5x10-3 cm/sec. for the relationship:
IL ~ Fkm CR
where IL is the mass transport limited current density, F
is Faraday's constant, and CR is the bulk concentration of the bromide ion. The ratio of anode surface to cell compartment volume is preferably at least about 80 cm'l, more ;,preferably 100-150 cm-1. Hy operation within these parameters, productivities of between about 1x10-3 and about 5x10-3 moles Br2 per hour per cm3 of working volume in the cell can be achieved.
Fig. 4 is a schematic illustration of a type of undivided cell that can be utilized effectively to provide the desired electrical efficiency and productivity discussed above. A cell of the type illustrated is available from Electrocatalytic, Inc., of Union New Jersey under the trade designation "Chloropac". This cell, which was originally developed for generation of hypochlorite in shipboard seawater systems, is described in detail in literature available from Electrocatalytic. Inc. The apparatus depicted in Fig. 4 is a bipolar dual cell assembly which comprises an outer electrode subassembly 13 that includes two outer cylindrical electrodes 15 and 17 rw ~p T"<v'Jf,~~W
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that are substantially axially aligned and mechanically attached to each other through an insulating spacer 19.
The cell assembly further comprises an inner cylindrical electrode 21 that is of smaller diameter than either of electrodes 15 and 17, is concentric therewith, and is substantially coextensive longitudinally with subassembly 13. The annular space 23 between subassembly 13 and electrode 21 provides the path along which electrolytic solution may be caused to flow through the cell. As illustrated in the drawing, outer electrode 15 serves as an anode to which current is supplied to the bipolar dual cell assembly and outer electrode 17 serves as a cathode from which current is withdrawn. Accordingly, the portion 25 of inner electrode 21 facing anode 15 serves as a cathode and the portion 27 of the inner electrode :facing cathode 17 serves as an anode.
In a particularly preferred embodiment of the invention, each of electrodes 15, 17 and 21 is constructed of titanium, and both anode 15 and anodic portion 27 of electrode 21 are coated with platinum. The platinized surface catalyzes the anodic reaction and promotes generation of bromine at high current efficiency and minimum overvoltage.
In operation of the cell of Fig. 4, an electrolytic feed solution containing bromide ions is caused to flow through annular path 23 between the electrodes and a direct current is applied to the flowing solution. Bromide ions are oxidized to bromine at anodes 15 and 27, while hydrogen is generated in the solution at cathodes 17 and 25. To provide the desired rate of mass transfer from the bulk solution to the anode surface, the velocity through the cell is preferably about 1.22 to 2.44 m/sec., more preferably between about 1.52 and about 2.13 m/sec. Although the cells illustrated in Fig. 4 are ~-a' ~, r-P ~ ~~
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particularly preferred, a variety of different cell designs may provide the high rates of mass transfer, even potential and current distribution and high ratio of electrode area to working volume that characterize the Chloropac type unit.
As noted, the bromine solution produced in the electrogeneration system is advantageously used for leaching of gold from sources thereof. In particular, it is effective for economical leaching of gold from gold ores. As illustrated in Fig. 2, a process for recovery of gold includes a barren or makeup tank 1 in which electrolytic solution is prepared for delivery by a pump 3 to an electrogeneration system 5. Electrogeneration system 5 may consist of a single electrolysis cell or comprise a plurality of banks of cells, but in any case comprises paired anode and cathode means which may be either monopolar or bipolar, and which may be arranged in a variety of electrical and hydraulic configurations as discussed above. Aqueous bromine solution produced in system 5 is transferred by discharge pump 29 to a leaching tank 31 where it contacts a solid particulate source of gold, such as crushed gold ore. This causes the gold contained in the source to react with elemental bromine, perbromide ions, hypobromite ions and bromide ions to produce an aqueous auriferous solution containing Auer4-ions and a particulate residue. The resulting slurry is transferred from tank 31 by a pump 33 through a filter or other solid/liquid separation means 35 for separation of the solid residue from the pregnant leach solution, and thence to a pregnant leach solution tank 37.
Gold may be recovered from the pregnant leach solution by a variety of means. including zinc precip-itation, carbon adsorption, solvent extraction, elec-trowinning, or ion exchange. The process of Fig. 2 18 s-a t'~'': ,~"".
causes the gold to be removed by ion exchange. Pregnant leach solution is transferred by a pump 39 to a pair of ion exchange columns 41 loaded with an ion exchange resin. AuBr4- ions are removed from the solution and collected on the column. Residual bromine in the pregnant leach solution is reduced to bromide ion in the columns.
Depleted bromide solution is returned to the barren tank 1, where it is replenished by addition of fresh alkali metal bromide.
An especially preferred embodiment of the process of the invention is illustrated in Fig. 3. In this process, which operates on a continuous basis, gold ore is loaded into an ore bin 43 from which it is transferred by a conveyor 45 to a ball mill 47. Milled ore passes to a classifier 49. A fines fraction from the classifier is subjected to leaching for recovery of gold while a coarse fraction is recycled to ball mill 47. The fines fraction is delivered to the first of two cascade agitated leaching tanks 51 and 53 where it is contacted with an aqueous bromine solution. The resultant leaching slurry overflows tank 51 to tank 53 and overflows tank 53 to solids/liquid separation means comprising a thickener 55. Solids residue drawn from the bottom of thickener 55 is passed through a countercurrent washing system com-prising thickeners 57, 59, and 61. An aqueous washing medium is fed to the last of the series of thickeners, thickener 61. Solids/liquid contact and separation in each thickener yields a liquid fraction that is trans-ferred to the next thickener nearer the leaching system and a solids fraction which is transferred to the nest thickener more remote from the leaching system. Thus, operation of the countercurrent washing system provides a liquid stream which moves with progressively increasing gold content from thickener 61 to thickener 55 and a ~~ i~.;r~~.
solids stream Which moves with progressively decreasing gild content from thickener 55 to thickener 61. Solid tailings are withdrawn from the bottom of thickener 61.
In thickener 55, the Wash liquor containing soluble gold recovered from the residue mixes with the pregnant leach solution from leaching tank 53 to produce an auriferous solution that is transferred to ion exchange columns 41. Removal of gold by ion exchange produces a depleted bromide solution which is recycled for use in generating additional aqueous bromine solution. To maintain the water balance of the plant, the depleted bromide solution is concentrated by passing all or part of the solution through a reverse osmosis unit 62. Water removed by the reverse osmosis unit is used in the circuit or purged from the process. The concentrated bromide solution is transferred to the electrogeneration system 63. Electrogeneration system 63 includes a makeup tank (not shown) and one or a plurality of cells in which bromide is converted to bromine as discussed above. The spent bromide solution is replenished by addition of alkali metal bromide and acid in the makeup tank, thus producing fresh feed solution for the cells of the electrogeneration system. The aqueous bromine solution leaving system 63 has the composition described hereinabove and is effective for the removal of gold from ore. This solution is recycled to leaching tank 51 for further recovery of gold from ore.
Ion exchange columns 41 contain a commercial anion exchange resin such as the resin comprising secondary amine functional groups combined with a phenol-formaldehyde matrix sold under the trade designations "PAZ-4" by Sela, Inc., the resin comprising trimethylamine functional groups combined with a Styrene/divinylbenzene matrix sold under the trade designation "DOWEX-21K" by Dow Chemical Company, and the polyester resin sold under the trade designation "Amberlite XAD-7" by Rohm and Haas. The gold loading capacity of PAZ-4 and DOWEX-21K is in 5 the neighborhood of 80-120 oz./cubic foot, while that of XAD-7 is in the range of about 10-20 oz./cubic foot. In batch tests, 80s loading is typically achieved in 1-2 hr. and maximum loading is reached in about 3-6 hr. These data allow specification of ion exchange column height and resin 10 requirements in accordance with conventional design criteria.
An acidic ketone solution, for example an acetone/HCl solution, is preferably used for elution of the column. Other eluents such as thiourea/HCl may also be used.
As noted, gold may be recovered from the auriferous 15 solution by other means, such as carbon adsorption, zinc precipitation or solvent extraction. A particularly preferred method of recovery is by adsorption on sphagnum moss. This process is described in U.S. patent 4,936,910. In this process, acid washed sphagnum peat moss, having a particle size 20 typically in the range of -10 to +200 mesh, is contacted with the auriferous solution in a suitable contacting apparatus.
Conveniently, the auriferous solution may be passed through an ion exchange column that is packed with sphagnum moss in lieu of a conventional ion exchange resin. Alternatively, the moss may be slurried in the auriferous solution and thereafter separated from the aqueous phase by filtration after transfer of gold from the solution to the moss. For contact with sphagnum moss, it is preferred that the pH of the auriferous solution be less than about 7, preferably between about 2 and about 5. The moss has a capacity for adsorbing approximately 32 mg. Au per gram. After ry r~s.. r-;-~~l inn ~ a:9'li 5.., ~g ~,..
adsorption and removal of the aqueous phase by filtration, the gold bearing sphagnum moss is burned to an ash which is smelted to recover the gold.
Illustrated in Fig. 5 is an alternative embodiment of the invention in which a slurry of leaching solution and particulate gold-bearing material is circulated between a leaching zone (contained within leaching tank 65) and an electrogeneration system 67 by operation of a high volumetric capacity circulating pump 69. In this process, the driving force for gold leaching may be enhanced by maintaining (or restoring) a high bromine content in the leaching solution. Conditions for operation of the cell or cells of the electrogeneration system are comparable to those for the processes of Figs.
2 and 3, except that back mixing in the leaching. tank causes the feed solution to the cells to have a somewhat lower bromine content than in the other processes. The latter effect can be minimized by baffling the leaching tank or using a pipe reactor to approach plug flow conditions. As illustrated in Fig. 5, this process operates on a batch basis. However, Fig. 6 shows how the principle of the process of Fig. 5 can be implemented in a continuous operation. In Fig. 6, each of a series of cascaded leaching tanks 65, 71, and 73 is associated with an electrogeneration system, and leaching slurry is circulated between each leaching tank and its associated cells) 67, 75, and 97 respectively by means of pumps 69, 79 and, 81, while leaching slurry moves forward progres-sively from tank to tank. Such a scheme may be integrated into the process of Fig. 3, with or without an electrolytic system for regeneration of depleted bromide solution passing from the ion exchange column to the first leaching tank.

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In accordance with the invention, electrogener-ation of bromine to produce an aqueous bromine solution can also be conducted in divided cells. Such process may be carried out in a conventional plate and frame cell construction, using a diaphragm that preferably comprises a cation exchange membrane such as the perfluorosulfonic acid mernbrane sold under the trade designation "Nafion" by E.I, du Pont de Nemours & Co. The anode is preferably constructed of graphite, vitreous carbon, or the ceramic sold under the trade designation Ebonex by Ebonex Technology, Inc., or platinum, ruthenium dioxide, or iridium dioxide on a titanium substrate. The bromide ion content of the feed solution to the anode compartment of the cell is substantially the same as that of the solution described above for feed to an undivided cell. However, bromide ion can be supplied either in the form of an alkali metal bromide, in which case the pH of the feed solution is between about 0 and about 6, preferably about 0 to about 3, or hydrobromic acid, in which case the pH of the feed solution is approximately 0 or less. A proton source such as sulfuric acid or hydrochloric acid is fed to the cathode side of the cell.
Operating conditions are generally the same as described above for undivided cells, except that somewhat higher conversions can be tolerated without loss of cur rent efficiency. Using a divided cell, the conversion of bromide ion in the electrogeneration system is typically between about 4% and about 50%, preferably between 20% and 40%. Thus, the equivalent bromine content of the product solution is between about 0.01 and about 3.66 moles/1, preferably between about 0.4 and about 3.0 moles/1. Where an alkali metal bromide is used as the source of bromide ion, the product solution has a pH of between about 0 and about 6, preferably between about 0 and about 3. and an r.,, ~ r ~fi alkali metal ion content of between about 0.1 and about 4.0 moles/1, preferably between about 0.4 and about 3.0 moles/1. The product of a divided cell is particularly advantageous in such applications as industrial water treatment, such as cooling tower water, where the higher equivalent bromine concentration facilitates treatment of substantial volumes of water with modest volumes of aqueous bromine solution.
Where the product solution is used in leaching gold, it is generally preferred that the feed solution to the anode compartment comprise an alkali metal bromide.
This is particularly so in application of bromine leaching to the process in which sphagnum moss is used in recovery of gold from the leaching solution in accordance with the method described in U.S. Patent No. 4,936,910.
Further in~accordance with the invention, it has been discovered that an auriferous solution comprising the pregnant leach solution can be introduced into the cathode compartment of a divided cell, and gold directly recovered at the cathode. A schematic flow sheet illustrating this unique and advantageous electrowinning process is illus-trated in Fig. 7. The system includes a container 83 containing an anode 85 and a cathode 87 separated by a hydraulically impermeable membrane 89 comprising a cation exchange resin which divides the cell into an anode chamber 91 and cathode chamber 93. Direct current power is applied to the cell by a power source 95. Anolyte from chamber 91 is transferred to a leaching tank 97 where it contacts a particulate source of gold to produce a preg-pant leaching solution containing AuBr4- ions. A slurry of the pregnant leaching solution and solid residue is transferred to a solid/liquid separation means such as a filter 99 where the solid residue is removed and washed with an aqueous washing medium to produce an auriferous solution from which gold may be recovered.

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The auriferous solution from filter 99 is introduced into the cathode chamber 93 of the cell, where AuBr4- is cathodically reduced to deposit gold on the cathode. The cathode is preferably constructed of nickel foam, nickel mesh, or steel wool. The gold bearing cathodes are periodically removed from the cell and the gold recovered therefrom. Catholyte leaving the cell is recycled to a bromide solution makeup tank 101 where it is replenished by addition of alkali metal bromide prior to introduction into the anode chamber of the cell.
The feed solution introduced into anode chamber 91 from makeup tank 101 has the composition described hereinabove in connection with Figs. 1-4, and the anolyte transferred from cathode chamber 93 to leaching tank 97 comprises an aqueous bromine solution also having a composition as described above. Conditions in the leaching tank 97 are essentially the same as those of the processes of Figs. 1 to 4.
The auriferous solution introduced into cathode chamber 93 contains between about 6x10-6 and about 1.2x10-2, preferably about 1.2x10'5 to about 1.2x10-3, moles per liter AuBr4-, between about 0.1 and about 4.0, preferably between about 0.4 and about 3.0, moles per liter bromide ion, and between about 0.1 and about 4.0, preferably between about 0.4 and about 3.0, moles per liter alkali metal. The pH of the cathode feed solution is typically in the range of between about 0 and about 6, preferably between about 0 and about 3. The temperature of the catholyte in the cathode chamber is in the range of between about 10°C and about 50°C. The overall cell voltage is typically in the range of about 3V and about 6V.
A substantial amount of hydrogen is released together with gold at the cathode, so the cathodic current efficiency of the cell is relatively low, in the range of between about 0.1 and about 1%. Nonetheless, because of the value of the gold and the complications of other recovery methods, the cell operation is cost efficient compared to other methods of gold recovery. Moreover, recovery of gold at the cathode is essentially quanti-tative, so that, under most conditions, the catholyte discharged from the cell is completely devoid of AuBr4- or other Au species. However, any residual gold in the catholyte is recovered since the catholyte is recycled to the bromide solution makeup tank and thence through the anode chamber of the cell to the leach tank.
It will be understood that the process for recovery of gold from leach solution may be carried out at the cathode of a divided cell in which the anode reaction is other than the electrogeneration of bromine. However, the integrated process described above provides unique advantages in process design, operation, and economics, and is thus highly preferred.
For commercial or industrial treatment of water, a biocidally effective amount of the aqueous bromine solu-tion produced in the electrogeneration process is intro-duced into the water to be treated. For example, in treatment of swimming pool water, a treatment solution comprising the aqueous bromine solution may be injected via a brominating apparatus into a stream that is circu-lated between the pool and the apparatus. Cooling tower water may be treated by injection of the treating solution into the sump of the tower, into the main f low of water circulated through the tower, or into a side stream circulated through a brominating apparatus. In either case, the frequency, duration and dosage of aqueous bromine solution is sufficient to suppress the growth of microorganisms. In swimming pool treatment, the bromine is preferably supplied at a rate which kills bacteria. In r~5 T's-the case of cooling tower water, the dosage need not necessarily kill bacteria, but only limit bacterial growth to control biofouling.
The amount of aqueous bromine solution required to meet these criteria is dependent on a number of factors, among which include the volume of the recirculating system, the temperature and pH of the water therein, the location of the system (i.e., whether the system is located in an area where bacterial nutrients may easily enter the system), the quality of makeup water, and the amount of bacterial growth present at the time treatment is begun.
In a new recirculating system, bacterial growth may be easily controlled by simply adding an amount of aqueous bromine solution to the water and observing the results. If, after a period of time there is an observed build up of algae. bacteria, etc.. the amount of aqueous bromine solution should be increased. If no build up occurs, the quantity of bromine solution may be reduced until an accumulation of bacteria is noted, at which time the rate of addition of bromine solution may be increased.
Through such "trial and error" tests, the preferred quantity of bromine solution needed for biomass control for any system can be easily established.
Generally, aqueous bromine solution is provided in sufficient proportion that at least about 0.10 pound of bromine is provided daily per thousand gallons of water in the system. In determining the proper amount of bromine solution to be used. system volume is first ascertained.
In the case of an open recirculating water system, system volume is normally calculated based on the amount of contained water plus daily makeup for evaporation losses and blowdown. Once the total volume is determined, the 27 ~r~~a,9f..-.~;a f~ ~~.%Y rr.~7..i.
appropriate bromine level may be selected, with the final level being optimized on a step-by-step basis in the described manner.
Preferably, bromine is provided at a rate of between about 0.05 and about 0.15 pounds per thousand gallons per day. The benefits of treatment are achieved with larger amounts of bromine (e.g., at rates of 0.5 pounds per 1000 gallons of water or higher) although such higher quantities are typically only required where the system is quite dirty and then only for a relatively short period of time (e. g., a few days to a few weeks).
Aqueous bromine water can also be applied very efficiently on a shock basis. Typical recommendations are to feed bromine solution for one hour intervals, two to three times per day. The main purpose of shock feeding is to use less chemical while maintaining an ever decreasing biocount. Bromine solution can be introduced at a rate sufficient to provide about 1 to about 5 pounds per hour for every 1000 gpm of flowing water. As needed, the rate of introduction can be as high as 15 lb/hr for each 1000 9Pm~
Ordinarily, biofouling is controlled by retaining a measurable halogen residual in the recirculating water (all day or for shocking interval) and without complete destruction of all microorganisms in the bulk water phase.
As noted, biocidal effectiveness in cooling tower and water recirculating systems is not dependent upon complete biological kill of all microorganisms existing within the recirculating water. Rather, in cooling tower and water recirculating systems, it has been found that it is only necessary to substantially kill the microorganisms which adhere to the walls and other film forming structural surfaces of the system. Once such localized organisms are killed, the total microorganism count in the recirculating water is essentially irrelevant to the efficacy of the water treatment method; that is, as long as the mir.roorganisms are in circulation in the system (i.e., not adhering to the walls or other structural sur-faces of the system), there is no noticeable detrimental effect on the heat-exchange capacity of the system:
As a result, the novel method of the present invention does not have as its objective the complete eradication of all microorganisms from the recirculating water but, instead, is intended to remove microorganism growth and biofilm from the surfaces of the recirculating water system. ThuS~ the term "biocidally effective" as used herein should be understood to refer to the selective attack on biofilm forming organisms located at system surfaces but should not be understood to mean the substan-tial elimination of bulk water phase microorganisms.
Other applications of the process of this inven tion include disinfection and other biological control of aqueous systems in the industrial and consumer home use, as follows:
TndLStrial Applications Recirculating cooling water Once-through cooling water waste water Brewery pasteurizer water Air washer water Evaporative cooling water Air scrubber systems ' Humidifier systems Oilfield injection water Pond and lagoon water Degreaser disinfectants a 2 9 ~ ~.~.:i'': .~.
Closed cooling system water Irrigation system disinfection Metal working system disinfection Food plant disinfection Bleaching - pulp & paper Textile Metal etching Metal Extraction consumer Appl~cat~ons Toilet bowl cleaners/disinfectants Hard surface cleaners/disinfectants Air conditioning pan water Decorative fountain water Tile & grout cleaners Bleaching agent compositions Dishwashing formulation Laundry formulation Pool biocontrol/disinfection Spas & hot tub biocontrol/disinfection Thus, the term "aqueous system" as used herein encompass all such systems.
The following examples illustrate the invention.
A simulated barren solution was prepared having a composition typical of that which would be obtained after recovery of gold by ion exchange from a pregnant leach solution produced by bromine leaching. To this end, sodium bromide and 48% hydrobromic acid were mixed with water to produce a solution containing 5% by weight bromide ion and having a pH of 3. In a series of runs this solution was circulated at a flow rate of 125 L/sec.
between a 300 gal. pilot scale reservoir for the solution 3 0 .-" ' ,yl o~ ~~ a.u siJ;, ,~ ~o..
and a Chloropac cell operated at a constant amperage of 100A. At this amperage, the Chloropac cell is rated to produce 1/2 lb. C12 per hour. Velocity through the annular portion of the Chloropac cell between the electrodes was about 1.83 m/sec.
Measurements were made of current efficiency as a function of the conversion of bromide and the bromine content generated in the solution. The current efficiency decreased with conversion and bromine content, but the cumulative efficiency was still close to 80% at a bromine concentration of 56 mmol dm-3 and a conversion of 11.5%.
Further electrolysis runs were conducted in the manner described in Example 1, except that the simulated barren solution was buffered with 6 mol dm l sulfuric acid instead of 48% HHr. The results were essentially identical to those of Example 1. These results indicate that the depletion of Br- from the system has a negligible effect on current efficiency at low conversion. Loss in current efficiency with conversion in this low range can be substantially attributed to reduction of Br2 to Br- at the cathode.
Runs were made according to the general proce-dure of Example 2 except that the concentration of Br' was varied. In Example 3 the concentration was 4%, in Example 4 it was 3%, and in Example 5 it was 2.5%. To maintain conductivity, the solutions of Examples 4 and 5 further contained sodium sulfate as an auxiliary electrolyte. In Example 4, the Na2S04 concentration was 0:25 mol dm-3 and in Example 5 it was 0.33 mol dm-3.

~~~~Y~A~' In Example 3, the electrolysis was carried out to a conversion of 15.1% and bromine content of about 58 mmol dm'3. At this point the cumulative current effi-ciency was about 83-85%. In Example 4, the conversion was 18%, the bromine content about 48 mmol dm'3. and the cumulative current efficiency about 79%, while in Example 5, the conversion was 12.3%, the bromine content about 24 mmol dm-3, and the cumulative current efficiency about 84%.
A black sand concentrate (100 g) containing 6 kg/tonne Au was contacted in an agitation bottle with a bromine leaching solution (8.0 g) having a composition typical of a solution that may be prepared fr~om.the electrolysis of a sodium bromide solution as described hereinabove. The leaching solution had a pH of about 2 and contained about 0.68% by weight equivalent molecular bromine, about 0.43% by weight bromide ion, and about 0.43% by weight sodium ion. The resultant leaching slurry was agitated in the capped bottle using an overhead mixer at slurry temperature of about 22°C for 24 hours. During leaching the pH and oxidation-reduction potential (ORP) of the slurry were monitored but no adjustment was made while the run was in progress. Measurements indicated that the pH of the slurry was about 1.7 and the oxidation-reduction potential of the system was initially about 900 mV, falling off to about 800 mV. To establish the kinetics of extrac-tion, samples were withdrawn from the leaching bottle at 2, 4, 6, 12, 18, and 24 hours. Fresh water was added to the bottle to compensate for the sampling loss.
At the end of the run, the leaching slurry was filtered and the cake was repulped for 10 minutes in a volume of Water equal to twice the solids weight. The 32 ~~~~' ~ '~-r ~,/ V N7 Y~..L.
repulped slurry was then filtered and the cake was washed with a volume of water equal to the solids weight. The gold values in the leaching samples, filtrate, wash, and residue were determined by inductively coupled plasma spectrometry (ICP) and fire assay. The results indicated that 90% of the gold was dissolved during the first two hours, and that dissolution reached a maximum in about 4 hours. To optimize gold recovery, the residue ("tails") was releached twice with fresh leaching solution under conditions comparable to the initial leaching operation.
Fresh leaching solution restores the ORP to the 800-900 mV
range in which effective removal of gold from the source is realized.
To maintain a recovery of 95% of the gold, a total of 14.0 g of leaching solution was consumed; 8.0 g in the initial leach and a total of 6.0 g in the two stage leaching of the residue.
DOWEX-21K ion exchange resin was used for recovery of gold from the leaching solution. In the ion exchanger operation, leaching solution (100 mL) containing 300 mg/L Au and having a pH of 2-3 was mixed with particu-late ion exchange resin (1.0 g). Loadings of 125-150 kg/tonne were realized after about 4 hours of contact. In certain runs, gold was eluted from the loaded resin using an acetone/HC1 solution prepared from three volumes of acetone and one volume of 1M HC1. In other runs, gold was eluted using a thiourea/HC1 solution prepared from equal volumes of 1M thiourea and 1M HC1. After each elution, the resin was regenerated by contacting it for two hours with 1M HC1 solution.

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Example 7 Electrowinning of gold was carried out in the cathode compartment of a divided electrolytic cell. A
simulated pregnant gold bromide solution (146.6 ppm Au) (12 dm3) containing 5% Br' ion and residual Br2 (not determined) was the catholyte, and a 5% H2S04 solution served as the anolyte. The streams were recirculated (140 dm3hr-1) through a plate and frame-type cell equipped with a cation exchange membrane. Nickel foams (30 pores per inch) served as the cathode, and anodized lead shot (Pb02) was the anode. A cell current of 5A (Cell voltage =4.1V) was imposed for 1.5 hours. This was reduced to 2A for an additional 2.3 hours (Cell voltage =2.9V). On termination, 0.51 ppm Au was determined in the catholyte which indicates a 99.7% recovery of the gold which plates on the nickel surface.
At the cathode, three electrode reactions take place:
AuBr4 + 3e- > Au + 4Br-Br2 + 2e- > 2Br-2H20 + 2e- > H2 + 20H-At the anode in this example, the counter reac-tion is the oxidation of water to oxygen. However, it should be recognized that anodic oxidation of Br- to Br2 at, for eaample, graphite anodes, could also have been the reaction of choice.

Example 8 Four units of a plate and frame-type cell were used to process a 5% HBr solution. Particulate graphite anodes were separated from Pb cathodes by a cation exchange membrane. A 10% sulfuric acid solution was the catholyte.
Flow rates of 300 and 260 dm3hr"1 were established for the a nolyte and catholyte, respectively so that there was no differential fluid pressure across the membrane. A cell voltage of 14.3V was imposed across bipolar electrical connectors to force a cell current of l0A (individual cell voltage = 3.75V). A Faradaic current efficiency of 96.5%
was measured at 9.8% Br- conversion.

Claims (11)

1. A process for generating bromine in an aqueous solution containing bromide ion, comprising the steps of:
causing an aqueous solution containing bromide ions to flow through an electrogeneration system that comprises paired anode means and cathode means, said system having an inlet and an outlet for the flow of said solution, said solution at the inlet of said system having a pH of between 0 and 6 and a bromide ion concentration of between 0.5 and 8.8 moles per liter;
applying a direct electric potential via said anode means and said cathode means to cause an electric current to pass through said flowing solution in said system and to generate bromine at said anode means by electrolytic oxidation of bromide ions, the relationship between said electric current and the throughput of said solution through said system being such that between 4% and 50% of the bromide in said inlet solution is converted to bromine at said anode means, and the pH of the solution discharged from the outlet of said system is between 0 and 6.

2. A process as set forth in claim 1 wherein said electrogeneration system contains no impediment to flow of electrolytic solution that would be sufficient to cause a discontinuity in the concentration gradient between said anode means and cathode means, the relationship between said electric current and the flow rate of said solution through said system being such that not more than 15% of the bromide in said inlet solution is converted to bromine at said anode means.

3. A process as set forth in claim 2 wherein said electrogeneration system comprises one or more undivided cells each having an annular path for flow of said solution between substantially concentric cylindrical electrodes.

4. A process as set forth in claim 3 wherein said electrogeneration system comprises at least one bipolar dual cell assembly, said at least one bipolar dual cell assembly comprising an outer electrode subassembly comprising two axially aligned outer cylindrical electrodes mechanically attached to each other through an electrically insulating attachment means, said at least one bipolar dual cell assembly further comprising an inner cylindrical electrode of smaller diameter than said outside electrodes, said inner electrode being substantially concentric with said outer electrodes, whereby one of said outer electrodes serves as an anode when the other serves as a cathode, the portion of said inner electrode facing the anodic outer electrode thus functioning as a cathode and the portion of said inner electrode facing said cathodic outer electrode thus functioning as an anode.

5. A process as set forth in claim 4 wherein all of said electrodes are constructed of titanium, and wherein said anodic outer electrode and the anodic portion of said inner electrode are coated with platinum.

6. A process as set forth in claim 1 wherein said electrogeneration system comprises one or more cells in which the ratio of anode surface to the working cell volume is at least 80 cm-1.

7. A process as set forth in claim 1 wherein said electrogeneration system comprises one or more divided cells having an ion permeable membrane separating said anode means and said cathode means.

8. A process as set forth in claim 1 comprising the additional steps of:
contacting a solid particulate source of gold with said discharge solution in a gold leaching stage, thereby causing the gold contained in said source to react with bromine and hypobromous acid contained in said discharge solution, and producing a slurry comprising a pregnant leach solution containing AuBr4- ions and a particulate residue;
separating said particulate residue from said pregnant leach solution;
recovering gold from an auriferous solution comprising said pregnant leach solution, thereby producing a depleted bromide solution;

mixing said depleted bromide solution with a source of bromide ion to produce a replenished bromide solution; and recycling said replenished bromide solution to said electrogeneration system to produce further bromine-containing cell discharge solution for use in said gold leaching stage.

9. A process as set forth in claim 8 further comprising:
separating the solid particulate residue of said gold source from the slurry exiting said gold leaching stage;
washing said residue by contact with an aqueous washing medium in a countercurrent washing system comprising a series of thickeners to produce a wash liquor containing soluble gold recovered from said residue; and mixing said wash liquor with said pregnant leach solution to produce said auriferous solution.

10. A process as set forth in claim 9 wherein said particulate source of gold is contacted with an aqueous bromine solution in a leaching zone to produce a leaching slurry, and said leaching slurry is cascaded through a plurality of leaching tanks, the slurry in each leaching tank being circulated between said tank and an electrogeneration system associated therewith.

11. A process as set forth in claim 8 wherein said electrogeneration system comprises one or more divided cells having an ion permeable membrane separating said anode means and said cathode means, the process further comprising the steps of:

introducing said auriferous solution into a cathode chamber of a divided electrolytic cell adapted for gold recovery; and applying a direct current for deposition of gold from said auriferous solution at said cathode means.