EP0476862A1 - Production électrolytique de brome et son utilisation dans la récupération de métaux précieux et le traitement des eaux - Google Patents

Production électrolytique de brome et son utilisation dans la récupération de métaux précieux et le traitement des eaux Download PDF

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Publication number
EP0476862A1
EP0476862A1 EP91307894A EP91307894A EP0476862A1 EP 0476862 A1 EP0476862 A1 EP 0476862A1 EP 91307894 A EP91307894 A EP 91307894A EP 91307894 A EP91307894 A EP 91307894A EP 0476862 A1 EP0476862 A1 EP 0476862A1
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Prior art keywords
solution
bromine
gold
bromide
leaching
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German (de)
English (en)
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Ahmad Dadgar
Jonathan Howarth
Rodney H. Sergent
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Great Lakes Chemical Corp
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Great Lakes Chemical Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B11/00—Obtaining noble metals
    • C22B11/04—Obtaining noble metals by wet processes
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/24—Halogens or compounds thereof
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C1/00—Electrolytic production, recovery or refining of metals by electrolysis of solutions
    • C25C1/20—Electrolytic production, recovery or refining of metals by electrolysis of solutions of noble metals

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  • 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.
  • precious metals such as gold and silver have been recovered from ores by leaching with alkaline cyanide solution.
  • 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: 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.
  • cyanide leaching suffers from well-known disadvantages.
  • 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.
  • 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: Gold may then be recovered by reduction with zinc metal or raising of the leaching solution pH.
  • aqua regia is expensive, and highly corrosive and emits toxic fumes.
  • 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: 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.
  • bromine 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 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 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 98% 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.
  • bromine nonetheless presents its own toxicity problems.
  • 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.
  • 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.
  • 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.
  • 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.
  • the present invention is directed to 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 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 solution is recycled to the electrogeneration system to produce further bromine-containing cell discharge solution for use in the gold leaching stage.
  • 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 from 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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/l, preferably between about 0.5 and about 5 moles/l, bromide ion.
  • the feed solution may be prepared by dissolving an alkali metal bromide in water and acidifying with an acid such as HBr, sulfuric acid, or HCl to the desired pH.
  • the solution may contain between about 0.5 and about 8.8 moles/l 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.
  • 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.
  • conditions are controlled to avoid increase of the cell discharge solution temperature to greater than about 50°C.
  • the product solution 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/l of equivalent bromine, between about 0.1 and about 4.0 moles/l unreacted bromide ion, and between about 0.1 and about 4.0 moles/l alkali metal ion.
  • the product solution containing between about 0.03 and about 2.5 moles/l equivalent bromine, between about 0.4 and about 3.0 moles/l bromide ion, and between about 0.4 and about 3.0 moles/l alkali metal ion.
  • Equivalent bromine is defined as the sum of the molar concentrations of molecular bromine, perbromide ion (Br3 ⁇ ), 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.
  • 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.
  • the electrogeneration system may comprise one or more banks of cells rather than the single cell that is illustrated in Fig. 1.
  • 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 cell(s) a and reservoir such as the bromide solution makeup tank until the desired conversion has been realized.
  • the cell(s) preferably operate on a flow basis, but in the latter (batch) case, recirculation is required to reach the desired conversion.
  • 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 electrogeneration 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.
  • the ratio of anode surface to cell compartment volume is preferably at least about 80 cm ⁇ 1, more preferably 100-150 cm ⁇ 1.
  • 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 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 AuBr4 ⁇ 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 precipitation, carbon adsorption, solvent extraction, electrowinning, or ion exchange.
  • the process of Fig. 2 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.
  • Fig. 3 An especially preferred embodiment of the process of the invention is illustrated in Fig. 3.
  • 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 comprising 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 transferred to the next thickener nearer the leaching system and a solids fraction which is transferred to the next thickener more remote from the leaching system.
  • 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 solids stream which moves with progressively decreasing gold content from thickener 55 to thickener 61.
  • Solid tailings are withdrawn from the bottom of thickener 61.
  • 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.
  • 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 the neighborhood of 80-120 oz./cubic foot, while that of XAD-7 is in range of about 10-20 oz./cubic foot.
  • 80% loading is typically achieved in 1-2 hr. and maximum loading is reached in about 3-6 hr.
  • 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.
  • gold may be recovered from the auriferous 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 which is expressly incorporated herein by reference.
  • acid washed sphagnum peat moss having a particle size typically in the range of -10 to +200 mesh, is contacted with the auriferous solution in a suitable contacting apparatus.
  • 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.
  • 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.
  • 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 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.
  • Fig. 5 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.
  • 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.
  • Fig. 6 shows how the principle of the process of Fig. 5 can be implemented in a continuous operation.
  • 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 cell(s) 67, 75, and 97 respectively by means of pumps 69, 79 and 81, while leaching slurry moves forward progressively 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.
  • electrogeneration 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 membrane 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.
  • 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.
  • 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.
  • the conversion of bromide ion in the electrogeneration system is typically between about 4% and about 50%, preferably between 20% and 40%.
  • the equivalent bromine content of the product solution is between about 0.01 and about 3.66 moles/l, preferably between about 0.4 and about 3.0 moles/l.
  • the product solution has a pH of between about 0 and about 6, preferably between about 0 and about 3, and an alkali metal ion content of between about 0.1 and about 4.0 moles/l, preferably between about 0.4 and about 3.0 moles/l.
  • 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.
  • 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.
  • 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 illustrated 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 pregnant 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.
  • 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.
  • 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.
  • the integrated process described above provides unique advantages in process design, operation, and economics, and is thus highly preferred.
  • a biocidally effective amount of the aqueous bromine solution produced in the electrogeneration process is introduced into the water to be treated.
  • a treatment solution comprising the aqueous bromine solution may be injected via a brominating apparatus into a stream that is circulated 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 flow of water circulated through the tower, or into a side stream circulated through a brominating apparatus.
  • the frequency, duration and dosage of aqueous bromine solution is sufficient to suppress the growth of microorganisms.
  • the bromine is preferably supplied at a rate which kills bacteria.
  • 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.
  • 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.
  • 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.
  • 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 appropriate bromine level may be selected, with the final level being optimized on a step-by-step basis in the described manner.
  • 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 Ib/hr for each 1000 gpm.
  • 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.
  • 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 microorganisms are in circulation in the system (i.e., not adhering to the walls or other structural surfaces of the system), there is no noticeable detrimental effect on the heat-exchange capacity of the system.
  • 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.
  • biocidally effective 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 substantial elimination of bulk water phase microorganisms.
  • aqueous system as used herein encompass all such systems.
  • 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.
  • 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.
  • this solution was circulated at a flow rate of 125 L/sec. between a 300 gal. pilot scale reservoir for the solution and a Chloropac cell operated at a constant amperage of 100A. At this amperage, the Chloropac cell is rated to produce 1/2 Ib. Cl2 per hour. Velocity through the annular portion of the Chloropac cell between the electrodes was about 1.83 m/sec.
  • Example 2 Further electrolysis runs were conducted in the manner described in Example 1, except that the simulated barren solution was buffered with 6 mol dm ⁇ 1 sulfuric acid instead of 48% HBr. 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.
  • Example 4 Runs were made according to the general procedure 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 Na2SO4 concentration was 0.25 mol dm ⁇ 3 and in Example 5 it was 0.33 mol dm ⁇ 3.
  • 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 efficiency 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 from 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.
  • 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 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.
  • the residue tails
  • Fresh leaching solution restores the ORP to the 800-900 mV range in which effective removal of gold from the source is realized.
  • DOWEX-21K ion exchange resin was used for recovery of gold from the leaching solution.
  • leaching solution 100 mL
  • particulate ion exchange resin 1.0 g
  • Loadings of 125-150 kg/tonne were realized after about 4 hours of contact.
  • gold was eluted from the loaded resin using an acetone/HCl solution prepared from three volumes of acetone and one volume of lM HCl.
  • gold was eluted using a thiourea/HCl solution prepared from equal volumes of lM thiourea and lM HCl. After each elution, the resin was regenerated by contacting it for two hours with lM HCl solution.
  • 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 (12 dm3) containing 5% Br ⁇ ion and residual Br2 (not determined) was the catholyte, and a 5% H2SO4 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 (PbO2) was the anode.
  • 0.51 ppm Au was determined in the catholyte which indicates a 99.7% recovery of the gold which plates on the nickel surface.
  • the counter reaction is the oxidation of water to oxygen.
  • anodic oxidation of Br ⁇ to Br2 at, for example, graphite anodes could also have been the reaction of choice.

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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0580780A1 (fr) * 1991-04-12 1994-02-02 Great Lakes Chemical Corporation Compositions de perbromures inorganiques et leurs procedes d'utilisation
EP0652979A1 (fr) * 1992-07-29 1995-05-17 Great Lakes Chemical Corporation Compositions de perbromure inorganique et procedes d'utilisation
US5868911A (en) * 1995-03-27 1999-02-09 Elcat, Inc. Apparatus for generating bromine
WO2001083835A2 (fr) * 2000-04-28 2001-11-08 Mintek Procede de recuperation d'or
US6660307B2 (en) 2001-04-16 2003-12-09 United States Filter Corporation Process for generating stabilized bromine compounds
WO2017199254A1 (fr) 2016-05-19 2017-11-23 Bromine Compounds Ltd. Procédé de récupération d'or à partir de minerais
CN111106004A (zh) * 2018-10-29 2020-05-05 东泰高科装备科技有限公司 一种砷化镓刻蚀方法
WO2020183469A1 (fr) 2019-03-13 2020-09-17 Bromine Compounds Ltd. Procédé de récupération d'or à partir de minerais
CN113789525A (zh) * 2021-09-29 2021-12-14 山东海王化工股份有限公司 电解酸化溴化钠生产溴素的工艺方法
CN114933283A (zh) * 2022-04-24 2022-08-23 天津长芦汉沽盐场有限责任公司 一种淡化浓海水溴素生产工艺
CN114956264A (zh) * 2022-06-09 2022-08-30 南京华工创新环境研究院有限公司 一种利用电化学使液体中的溴分离的方法

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9382140B2 (en) 2011-12-01 2016-07-05 Metafix Inc. Method for reducing chromium

Citations (5)

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US3873438A (en) * 1971-09-08 1975-03-25 Engelhard Min & Chem Electrolytic cell assembly including bipolar concentric electrodes
FR2349664A1 (fr) * 1976-04-28 1977-11-25 Diamond Shamrock Techn Electrodes a tension de claquage elevee pour des electrolytes contenant un bromure
EP0016290A1 (fr) * 1979-03-23 1980-10-01 European Atomic Energy Community (Euratom) Procédé continu d'élimination de l'anhydride sulfureux des gaz de combustion, et hydrogène et acide sulfurique ainsi produits
EP0168022A2 (fr) * 1984-07-10 1986-01-15 Europäische Atomgemeinschaft (Euratom) Procédé de production d'hydrogène et de soufre au départ d'hydrogène sulfuré
US4904358A (en) * 1988-02-16 1990-02-27 Inland Aqua-Tech Co., Inc. Gold and silver recovery processes by electrolytic generation of active bromine

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3873438A (en) * 1971-09-08 1975-03-25 Engelhard Min & Chem Electrolytic cell assembly including bipolar concentric electrodes
FR2349664A1 (fr) * 1976-04-28 1977-11-25 Diamond Shamrock Techn Electrodes a tension de claquage elevee pour des electrolytes contenant un bromure
EP0016290A1 (fr) * 1979-03-23 1980-10-01 European Atomic Energy Community (Euratom) Procédé continu d'élimination de l'anhydride sulfureux des gaz de combustion, et hydrogène et acide sulfurique ainsi produits
EP0168022A2 (fr) * 1984-07-10 1986-01-15 Europäische Atomgemeinschaft (Euratom) Procédé de production d'hydrogène et de soufre au départ d'hydrogène sulfuré
US4904358A (en) * 1988-02-16 1990-02-27 Inland Aqua-Tech Co., Inc. Gold and silver recovery processes by electrolytic generation of active bromine

Cited By (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0580780A1 (fr) * 1991-04-12 1994-02-02 Great Lakes Chemical Corporation Compositions de perbromures inorganiques et leurs procedes d'utilisation
EP0580780A4 (fr) * 1991-04-12 1994-03-23 Great Lakes Chemical Corporation
EP0652979A1 (fr) * 1992-07-29 1995-05-17 Great Lakes Chemical Corporation Compositions de perbromure inorganique et procedes d'utilisation
EP0652979A4 (fr) * 1992-07-29 1995-09-27 Great Lakes Chemical Corp Compositions de perbromure inorganique et procedes d'utilisation.
US5868911A (en) * 1995-03-27 1999-02-09 Elcat, Inc. Apparatus for generating bromine
WO2001083835A2 (fr) * 2000-04-28 2001-11-08 Mintek Procede de recuperation d'or
WO2001083835A3 (fr) * 2000-04-28 2002-03-07 Mintek Procede de recuperation d'or
US6660307B2 (en) 2001-04-16 2003-12-09 United States Filter Corporation Process for generating stabilized bromine compounds
WO2017199254A1 (fr) 2016-05-19 2017-11-23 Bromine Compounds Ltd. Procédé de récupération d'or à partir de minerais
US11041227B2 (en) 2016-05-19 2021-06-22 Bromine Compounds Ltd Process for recovering gold from ores
AU2017265940B2 (en) * 2016-05-19 2021-12-16 Bromine Compounds Ltd. A process for recovering gold from ores
CN111106004A (zh) * 2018-10-29 2020-05-05 东泰高科装备科技有限公司 一种砷化镓刻蚀方法
WO2020183469A1 (fr) 2019-03-13 2020-09-17 Bromine Compounds Ltd. Procédé de récupération d'or à partir de minerais
US11827952B2 (en) 2019-03-13 2023-11-28 Bromine Compounds Ltd. Process for recovering gold from ores
CN113789525A (zh) * 2021-09-29 2021-12-14 山东海王化工股份有限公司 电解酸化溴化钠生产溴素的工艺方法
CN114933283A (zh) * 2022-04-24 2022-08-23 天津长芦汉沽盐场有限责任公司 一种淡化浓海水溴素生产工艺
CN114956264A (zh) * 2022-06-09 2022-08-30 南京华工创新环境研究院有限公司 一种利用电化学使液体中的溴分离的方法
CN114956264B (zh) * 2022-06-09 2023-06-30 南京华工创新环境研究院有限公司 一种利用电化学使液体中的溴分离的方法

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AU8346191A (en) 1992-03-12
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CA2050201A1 (fr) 1992-03-05

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