Process for recovering gold from ores

Abstract

A process for recovering gold from a gold-containing raw material, comprising leaching the gold-containing material with an aqueous solution comprising elemental bromine and bromide source to form a pregnant leach solution with the gold dissolved therein; separating said pregnant leach solution from the gold-depleted raw material, removing elemental bromine from said pregnant leach solution, extracting the gold from the pregnant leach solution in an acidic environment into an organic extractant, to form a gold-loaded extract and bromide-containing raffinate, stripping the extract with an alkaline aqueous solution to form a gold-bearing aqueous solution, generating gold (Au.sup.0) and treating bromide-containing stream(s) to produce recyclable elemental bromine.

Claims

1. A process for recovering gold from a gold-containing raw material, comprising leaching the gold-containing material with an aqueous solution comprising elemental bromine and bromide source to form a pregnant leach solution with the gold dissolved therein and a gold-depleted raw material; separating said pregnant leach solution from the gold-depleted raw material, removing elemental bromine from said pregnant leach solution, extracting the gold from the pregnant leach solution in an acidic environment into an organic extractant, to form a gold-loaded extract and bromide-containing raffinate, stripping the extract with an alkaline aqueous solution to form a gold-bearing aqueous solution, generating gold and treating one or more bromide-containing streams to produce recyclable elemental bromine.

2. The process according to claim 1, wherein the gold-containing raw material is an ore which generates a pH of 6.0 to 8.0 when added to water, such that the leaching takes place at a pH in the range from 6.0 to 8.0.

3. The process according to claim 1, wherein after the separation of the leach solution from the gold-depleted raw material, the process further comprises a step of washing the solid gold-depleted raw material with an aqueous Br.sub.2/Br.sup.− reagent, separating the solid from the aqueous washing liquid to obtain an aqueous solution with gold constituent, and combining the aqueous solution with gold constituent with the pregnant leach solution.

4. The process according to claim 2, wherein before the extraction step, the process further comprises lowering the pH of the pregnant leach solution to below 6.0 with an addition of a mineral acid.

5. The process according to claim 1, wherein the removal of elemental bromine from the pregnant leach solution before the extraction step is achieved by washing said pregnant leach solution with a water immiscible organic solvent, to obtain bromine-loaded organic solvent.

6. The process according to claim 5, wherein the pregnant leach solution is washed with a water immiscible organic solvent comprising aliphatic hydrocarbon.

7. The process according to claim 5, wherein bromine is released from the bromine-loaded organic solvent by treating the bromine-loaded organic solvent with an aqueous base solution, to obtain an aqueous mixture of BrO.sub.3.sup.− and Br.sup.−.

8. The process according to claim 1, wherein the removal of elemental bromine from the pregnant leach solution before the extraction step is achieved by injecting air into the pregnant leach solution and collecting vapor by way of a scrubber with an alkaline solution.

9. The process according to claim 1, wherein the step of extracting the gold from the pregnant leach solution is achieved with an organic extractant selected from the group consisting of organophosphorous compounds and amine compounds.

10. The process according to claim 9, wherein the organophosphorous compound is alkyl phosphate ester.

11. The process according to claim 10, wherein the alkyl phosphate ester is tri n-butyl phosphate.

12. The process according to claim 1, wherein the step of stripping the extract with an alkaline aqueous solution is carried out while maintaining in a stripping vessel an alkaline environment with pH of not less than 9.

13. The process according to claim 12, wherein the alkaline aqueous solution comprises alkali hydroxide or alkali carbonate.

14. The process according to claim 1, wherein gold is recovered from the gold-bearing aqueous solution by electrowinning.

15. The process according to claim 1, wherein each of the aqueous bromide containing solutions produced throughout the process is delivered to a regeneration unit comprising one or more electrolytic cells, to produce elemental bromine.

16. The process according to claim 15, comprising forcing the aqueous bromide containing solutions to flow across a reverse osmosis membrane and delivering a concentrate obtained to the one or more electrolytic cells.

17. The process according to claim 1, wherein the gold-containing raw material is selected from the group consisting of oxidized ore, oxidized concentrate and oxide ore.

Description

EXAMPLES

(1) Methods

(2) Bromine in aqueous phase was measured by titration with potassium iodide sodium thiosulphate solution and starch indicator. The chemistry of the titration is such that potassium iodide is oxidized to iodine in stoichiometric ratio with bromine. The resultant iodine is blue in the presence of starch. Thiosulfate reduces the iodine to iodide, with the end-point at the disappearance of the blue color.

(3) Bromide in aqueous phase was measured by potentiometric titration with silver nitrate.

(4) Bromate was measured using ion chromatography.

(5) Gold in aqueous phase was measured by atomic absorption spectroscopy (AAS).

Examples 1 to 9

Leaching Gold from an Ore with the Aid of Aqueous Br.SUB.2./Br− Reagent

(6) The experiments were carried out at ambient temperature in 500 ml jacketed glass reaction vessel hermetically sealed with FEP-Silicone O-Ring and a 5-neck reaction vessel lid (ChemGlass), equipped with a cooling condenser (to prevent bromine vapor escape), pH electrode for on-line measurements and Reference Redox electrode for ORP (oxidation reduction potential) measurements.

(7) The general procedure used in experiments reported below consists of the following steps. Br.sub.2/Br aqueous leaching solution is introduced into the reaction vessel. The concentration of elemental bromine and sodium bromide in the aqueous solution is 5 g/l and 5 g/l, respectively. The homogenized ground ore [Roaster calcine, 80% of the particles less than 75 micron)] is added to the solution. The weight ratio liquid:solid is 2:1. The reaction slurry is stirred by means of a mechanical Teflon coated stirrer rotating at a range of 300 rpm. On completion, the reaction slurry was filtered using closed 500 mL filter funnel with a 90 mm Microfiber Glass filter, to obtain the PLS.

(8) The filter cake was washed and vacuum filtered through the same filter paper with a fresh leach solution. The filter cake was washed a second time with water, dried at 110° C. and sent for gold analyses by fire assay (F.A.). The first filtrate was combined with the PLS.

(9) The effect of two variables was investigated separately in the set of experiments reported below: leach time and pH of the reaction mixture. In Examples 1 to 4, the experiments were run under the resulting natural pH of the slurried material in the leach solution, with leach time being varied in the range from 2 minutes to 30 minutes. The experimental conditions and results are set out in Table 1.

(10) TABLE-US-00002 TABLE 1 Leach time ORP Au recovery Example (min) pH (mV) (%) 1 2 8.0 910 82 2 12 7.9 910 84 3 15 7.9 910 86 4 30 7.9 910 86

(11) In Examples 5 to 9, the leach time was constant (15 minutes), but the pH of the reaction mixture was adjusted within the acidic and basic ranges, with the aid of H.sub.2SO.sub.4 (10% w/w solution) and NaOH (10% w/w solution), respectively. The experimental conditions and results are set out in Table 2.

(12) TABLE-US-00003 TABLE 2 Leach time ORP Au recovery Example (min) pH (mV) (%) 5 15 3.0 903 70 6 15 6.0 910 79 7 15 7.9 (natural) 910 86 8 15 8.0 870 84 9 15 10.0  770 74

(13) The results shown in Tables 1 and 2 indicate that leaching of gold from the ore could be achieved at a fairly short period of time under the natural pH of the slurried material. In fact, longer leach times may even lead to inferior results.

Example 10 (Comparative) and 11-12 (of the Invention)

Leaching Gold from an Ore with the Aid of Aqueous Br.SUB.2./Br− Reagent Followed by Different Washings of the Filter Cake

(14) A set of leaching experiments were run according to the general procedure outlined above. Roaster calcine ore was used, with [20 g/l Br.sub.2+20 g/l Br-] as the leaching reagent at liquid:solid ratio of 2:1. Following the solid/liquid filtration, the filter cake was washed twice as described above, and the filtrates were collected and combined. In Example 10, the wash liquid was water; in Examples 11 and 12, a fresh leach solution was used to wash the filter cake. The experimental conditions and results are tabulated in Table 3; Au percentage recovery reported includes the contribution of the filtrates collected.

(15) TABLE-US-00004 TABLE 3 Au recovery Example pH time Wash liquid (%) 10 7.48 15 water 80.6 11 6.90 15 Fresh Br.sub.2/Br− reagent 91.0 (20 g/l Br2, 20 g/l Br−) 12 7.48 15 Fresh Br.sub.2/Br− reagent 89.5 (20 g/l Br2, 20 g/l Br−)

(16) It is seen that Au percentage recovery increases as a result of washing of the gold-depleted solid cake with the Br.sub.2/Br-reagent of the invention.

Examples 13 to 23

Extraction of Gold from Br.SUB.2./Br− Pregnant Leach Solution with Various Extractants

(17) A test for the suitability of different organic compounds for extracting gold from aqueous solutions was made by preparing gold solutions (NaAuBr.sub.4) in deionized (DI) water, or adding such gold solutions to bromine/bromide solutions, and then vigorously shaking the tested aqueous solution with the organic liquid extractant at O:A ratio of 1:1 for three minutes at a separating funnel and allowing the mixture to stand. In the qualitative test, phases' clarity and separability were observed and in the quantitative test, percentage of extraction was determined by measuring gold in the aqueous raffinate. The experimental conditions (e.g., the composition of the test solution and organic extractant used) and results are tabulated in Table 4.

(18) TABLE-US-00005 TABLE 4 PLS Ext [Au] [Br2] Br− % Ex. Extractant pH ppm gpL gpL Rec Observation 13A 10% Tripropyl- 2.1 2.9 0 0 71 Pink raffinate 13B ene glycol 1.6 9.5 0 0 88 Blue/grey n-butyl Ether precipitate (Dowanol) pink raffinate 14A Ester alcohol 3.7 3.0 0 0 98 black precipitate 14B (Texanol) 3.1 10.7 0 0 100 black precipitate 14C 4.7 3.7 0 0 65 Clear, heated for phase separation 15A Tri n-butyl 3.7 3.0 0 0 98 Clear phases 15B phosphate 3.2 10.7 0 0 100 Clear phases 16  Trialkyl 4.2 3.3 0 0 98 Clear phases phosphine oxide (Cyanex 923) 17A Guanadine 3.8 3.3 0 0 49 Clear, heated (LIX 79) for phase separation 17B 3.3 10.7 0 0 36 Aqueous phase turbid, heated for phase separation 18  Quaternary 4.3 3.7 0 0 49 Aq. Phase amine & turbid. Clear, nonyl phenol no ppte, heated (LIX 7820) for phase sep. Persistent white emulsion 19  Tri-octyl/ 4.3 3.7 0 0 99 Clear phases dodecyl amine (Alamine 336) 20  Acetic acid 3.5 3.0 30 3 88 Odourous, n-amyl ester clear phases 21  Diethylene 3.5 3.0 30 3 80 Odourous, glycol di butyl clear phases ether 22  Diethylene 3.5 3.0 30 3 n.d. Phases glycol di completely methyl ether miscible 23A Tri isobutyl 6.0 3.4 0 0 98 Clear phases 23B phosphine 0.6 5.5 33.7 1.8 99 Clear phases 23C sulfide 3.4 5.2 31.1 1.0 99 Clear phases

(19) The results presented in Table 4 indicate that phosphorous and amine compounds can be used to extract gold from aqueous solutions. In all subsequent experiments reported below, tri n-butyl phosphate was employed.

Examples 24-44

Extracting Gold from Br.SUB.2./Br− Pregnant Leach Solution with the Aid of Tri n-Butyl Phosphate

(20) In the following set of experiments, tri n-butyl phosphate in a low aromatic diluent (Cleansol 63L) was used to extract gold from various gold bromide (AuBr.sub.4.sup.−)—containing aqueous solutions. The concentration of the tri n-butyl phosphate extractant in the organic solution varied in the range from 5% to 50% (v/v).

(21) In Examples 24 to 26, the aqueous solutions were prepared by adding sodium aurobromide (NaAuBr.sub.4) to distilled water. Therefore, these solutions are devoid of elemental bromine and bromide. The concentration of AuBr.sub.4 in the solutions is set out in Table 5.

(22) In Examples 27 to 36, the aqueous solutions were prepared by combining sodium bromide (NaBr), hydrobromic acid (HBr) and commercial hydrogen peroxide (H.sub.2O.sub.2) in a closed vessel (whereby elemental bromine is generated in situ due to the oxidation of bromide with H.sub.2O.sub.2) followed by the addition of aurobromide (NaAuBr.sub.4). In this way, AuBr.sub.4.sup.−, Br.sub.2 and bromide-containing aqueous solutions were formed, having the compositions tabulated in Table 5.

(23) In Examples 37 to 44, aqueous solutions consisting of actual PLS generated by leaching of an oxide ore under the conditions set out in Examples 1-9 were used. Therefore, these solutions contain AuBr.sub.4.sup.−, Br.sub.2 and bromide; the exact compositions are set out in Table 5.

(24) All tests were carried out at by combining the organic solution and the aqueous solution at O:A ratio of 1:1. The phases were shaken vigorously for 3 minutes in a separating funnel and allowed to separate before sampling. The exact experimental conditions are found in Table 5. The concentrations of the various species were measured in the raffinate and are presented in Table 5.

(25) TABLE-US-00006 TABLE 5 % O:A PLS RAFFINATE extract- RA- [Au] [Br−] [Br2] [Au] [Br−] [Br2] Ex. ant TIO pH mg/L g/L g/L mg/L g/L g/L 24 50 1:1 3.7 3.0 0 0 <0.05 0 0 25 50 1:1 3.2 10.0 0 0 <0.05 0 0 26 50 1:1 2.4 110.5 0 0 0.295 0 0 27 25 1:1 4.4 3.3 ~30 n.m <0.1 ~30 3.7 28 25 1:1 3.5 4.7 28.8 ~2.5 <0.1 28.6 n.m 29 20 1:1 3.5 4.7 28.8 ~2.5 <0.1 29.2 n.m 30 15 1:1 3.5 4.7 28.8 ~2.5 <0.1 28.9 n.m 31 10 1:1 3.5 4.7 28.8 ~2.5 0.6 29.0 n.m 32 5 1:1 3.5 4.7 28.8 ~2.5 3.1 29.0 n.m 33 25 1:1 1.4 3.3 ~30 6.8 <0.1 ~30 1.4 34 25 1:1 1.1 4.4 22.4 8.2 <0.1 19.9 1.1 35 25 1:1 3.6 4.1 21.7 0.6 <0.1 21.3 3.1 36 25 1:1 3.7 5.8 22.6 21.0 0.15 18.7 0.6 37 25 1:1 8.5 3.9 8.9 0.3 3.1 9.0 0.1 38 25 1:1 7.4 3.1 8.7 0.5 0.3 8.5 <0.1 39 25 1:1 1.6 1.8 9.7 1.6 <0.1 7.5 0.3 40 25 1:1 4.0 2.0 6.7 1.1 <0.1 6.6 0 41 25 1:1 6.0 1.9 6.8 1.2 <0.1 6.6 0 42 25 1:1 7.2 1.9 6.5 1.3 <0.1 6.4 0.1 43 25 1:1 8.1 1.9 7.0 0.7 1.0 7.3 0.3 44 25 1:1 10.6 1.9 6.8 0.8 2.0 6.9 0.9

(26) The results shown in Table 5 indicate that bromine/bromide (Br.sub.2/Br.sup.−) gold solutions could be readily extracted with an organo phosphorous extractant dissolved in an organic solvent at a concentration in the range from 15% to 50% (v/v).

Examples 45 to 57

Extracting Gold from Br.SUB.2./Br− Pregnant Leach Solution with the Aid of Tri n-Butyl Phosphate and Stripping Gold from the Organic Extract

(27) In the next set of Examples, extraction experiments were run according to the general procedure described in the previous set of Examples, to extract gold from pregnant leach solution and obtain gold-bearing organic extract. Extraction experiments were carried out at by combining the extraction medium, consisting of tri n-butyl phosphate dissolved in Cleansol 63L at 25% (v/v), and the aqueous PLS at O:A ratio of 1:1. The composition of the PLS and the aqueous raffinate are set forth in Table 6, indicating that extraction was successfully achieved.

(28) Next, various reagents were tested under different conditions to recover the gold from the gold-bearing organic extract. The reagents which were considered for this purpose include hot and cold deionized (DI) water; sodium sulfite (Na.sub.2SO.sub.3) solution at a concentration of 1 g/liter; Na.sub.2SO.sub.3 solution at a concentration of 1 g/liter acidified with sulfuric acid; Na.sub.2SO.sub.3 solution at a concentration of 1 g/liter basified with dilute sodium hydroxide; 1M sodium hydroxide solution; ambient DI water with continuous adjustment to pH>10 by adding either dilute solutions of sodium hydroxide (1M) or sodium carbonate solution (20 g Na.sub.2CO.sub.3 per liter). All stripping experiments were carried out at O:A ratio of 1:1, either by shaking the phases vigorously in separating funnels and allowing the phases to separate before sampling (Examples 45-55), or in stirred beakers (Examples 56-57). A summary of stripping experiments is shown in Table 6.

(29) TABLE-US-00007 TABLE 6 PLS Raffinate Stripping: Strip solution Ex. [Au] [Br−] [Br2] [Au] [Br−] [Br2] Reagent [Au] [Br−] [Br2] 45 pH mg/l g/l g/l pH mg/l g/l g/l Temperature pH mg/l g/l g/l 46 4.2 3.7 0 0 3.0 0.6 0 0 DI water 40° C. 5.8 <0.5 0 0 47 4.2 3.7 0 0 3.0 0.6 0 0 1 g/l Na.sub.2SO.sub.3 8.1 2.8 0 0 40° C. 48 1.4 3.3 ~30 6.8 1.4 <0.2 ~33 2.9 1 g/l Na.sub.2SO.sub.3 8.8 0 nm 0.5 ambient 49 4.4 3.3 ~30 6.8 3.7 <0.2 ~33 1.1 1 g/l Na.sub.2SO.sub.3 8.8 2.9 nm 0 ambient 50 0.8 5.2 34.0 5.1 1.0 <0.1 32.3 3.2 1 g/l Na.sub.2SO.sub.3 1.7 <0.1 2.1 0 ambient 51 0.8 5.5 34.0 26.4 0.9 <0.1 32.7 26.4 1 g/l Na.sub.2SO.sub.3 2.2 <0.1 0.8 0 ambient 52 4.3 3.5 31.4 8.5 3.0 <0.1 31.1 nm 1 g/l Na.sub.2SO.sub.3 2.0 0.2 1.1 nm ambient 53 4.3 3.5 31.4 8.5 3.0 <0.1 31.1 nm 1 g/l Na.sub.2SO.sub.3 1.6 <0.1 1 nm acidified with H2SO4 ambient 54 4.3 3.5 31.4 8.5 3.0 <0.1 31.1 nm 1 g/l Na.sub.2SO.sub.3 11 0.6 1 nm basified with NaOH ambient 55 3.5 4.8 30 1.2 3 0.2 29.6 0.2 1M NaOH <13 2.9 0.4 0.5 ambient 56 3.5 4.8 30 1.2 3 0.2 29.6 0.2 DI, pH kept ~10 3.1 2.6 nm at 10 with 1M NaOH ambient 57 3.7 4.4 33.9 nm 3.6 <0.1 29.7 2.2 DI, pH kept ~10 4.4 2.3 nm at 10 with Na.sub.2CO.sub.3 ambient

(30) It is seen from the results in Table 6 that stripping of gold from the gold-loaded organic phase is achieved with the aid of aqueous base, preferably at pH above 10.

Examples 58-61

Extracting Gold from a Washed Pregnant Leach Solution with the Aid of Tri n-Butyl Phosphate and Stripping Gold from the Organic Extract

(31) In the next set of Examples, the effect of pre-washing of a pregnant leach solution with an organic solvent to remove bromine from the PLS prior to gold recovery was studied. The PLS samples were produced by leaching of ore samples from a gold mine in the USA, using a leach solution having concentration of 5 g/l Br.sub.2 and 5 g/l Br.sup.− according to the experimental procedure described above, followed by the following steps:

(32) Acidification: the PLS tested was acidified with HBr (10% solution) to pH=5.7 (this is because the natural pH was ˜6.8). Washing: the acidified PLS and the organic solvent (Cleansol) used for bromine removal were vigorously shaken at O:A ratio of 1:1 for three minutes in a separating funnel, following which the aqueous and organic phases were separated. Bromine is removed from PLS and loaded onto the Cleansol; loaded Cleansol is stripped at 20:1 with 4% NaOH to recover the bromine in the form of bromate and bromide in liquor solution. Extraction: gold was then extracted from the PLS into tri n-butyl phosphate dissolved in Cleansol (25% v/v) according to the general procedure illustrated in previous examples, at O:A ratio of 1:5, and the gold content of the raffinate was measured.

(33) Stripping: the gold-loaded organic extract was then subjected to stripping through five stages in a cross-current fashion. The strip solution in each stage consists of fresh DI water (pH adjusted to <10 with NaOH 4.2% added to maintain pH above in all stripping stages). In each of the five stripping stages the O:A ratio was 20:1, which is combined to give 4:1.

(34) It should be noted that Example 58 is a reference Example, with no washing taking place in the experiment. In Examples 59 and 60, one cycle of washing was done whereas in Example 61, the washing was repeated two times.

(35) Table 7 shows the results of the washing step, that is, bromine concentration (g/l) in the PLS before and after Cleansol wash (different batches of PLS were used, which account for the different bromine content in the PLS).

(36) TABLE-US-00008 TABLE 7 58 59 60 61 PLS 1.16 0.56 1.08 0.66 Washed PLS 0.16 0.24 0.10

(37) Table 8 shows the gold concentration measured in the four PLS samples prior to extraction (ppm), gold concentration in the raffinate following extraction (ppm), and gold concentration in each the gold-bearing aqueous solution generated in each of the stripping stages (ppm).

(38) TABLE-US-00009 TABLE 8 Example 58 59 60 61 Gold concentration (ppm) in PLS (before extraction) PLS 2.4 1.94 2.04 1.27 Gold concentration (ppm) in raffinate generated after extraction Raffinate >0.1 >0.1 >0.1 >0.1 Gold concentration (ppm) in strip solutions after each stage Stripping Stage 1 9.2 55.50 11.60 78.50 Stripping Stage 2 30 111.00 102.00 55.50 Stripping Stage 3 47.8 21.90 73.00 8.00 Stripping Stage 4 72.5 2.05 23.20 0.55 Stripping Stage 5 48.2 0.22 6.10 0.00 Strip recovery (%) 86.5 98.3 100 100

(39) It is seen that in Examples 59-61, where Cleansol wash was applied, the major portion of gold was stripped in stages 1-3 and overall recovery was close to 100%, whereas in Example 58, devoid of Cleansol wash, the major portion of gold was stripped in stages 3-5 and the recovery was ˜86%.

Example 62

Gold Electro-Winning

(40) A simulated strip solution at approximately 50 ppm Au concentration was used for this experiment. A primitive cell was constructed with a stainless steel wool (kitchen scrubby) cathode contained in a membrane chamber and lead anodes. A rectifier supplied 4 V. The current slowly fell from 1.4 A to 0.6 amps. Solution was continuously recycled to the cell for a total of 12 hours. From FIG. 4 it can be seen that the solution was depleted in 12 hours. About half of the physical gold was recovered by vigorous water washing of the cathode and the remainder by complete digestion of the steel wool by hydrochloric acid. Both residues were digested in aqua-regia for analysis and the gold accounted for.

Examples 63-65

Electrochemical Oxidation of Bromide to Generate Bromine

(41) The experimental set-up used consists of a flow cell. Graphite electrodes with active area of 10 cm.sup.2 were used. The half cells are separated by a membrane (polyethylene) that prevents the migration of bromine from the anode compartment to the cathode compartment (in the cathode compartment, hydrogen evolves while bromine is generated in the anode compartment). Each of the catholyte and anolyte solutions consists of 150 mL of bromide-containing aqueous solution (sodium bromide). The solutions are caused to circulate with the aid of peristaltic pump producing a rate of flow of 210 ml/minute.

(42) In Example 63, the initial concentration of bromide in each of the circulated solutions was 10% by weight. For bromine generation, the mode of operation of the cell was set to galvanostatic with 90 mA/cm.sup.2 of current flowing through the cell with a potential limit of 4.9 V. pH was kept at 4 by titrating HCl (20% solution) into the cathode compartment. After 3.56 hours, the concentration of bromine measured at the anodic side was 5.56 w/w.

(43) In Example 64, the initial concentration of bromide in each of the circulated solutions was 3.84% by weight. For bromine generation, the mode of operation of the cell was set to galvanostatic with 50 mA/cm.sup.2 of current flowing through the cell with a potential limit of 4.9 V. pH was kept at 6 by titrating HBr (52% solution) into the cathode compartment. After 3.0 hours, the concentration of bromine measured at the anodic side was 2.06 w/w.

(44) In Example 65, the initial concentration of bromide in each of the circulated solutions was 2.3% by weight. In addition, the solutions contain 4.1% by weight bromate. The pH of the solutions is 13.31. For bromine generation, the mode of operation of the cell was set to potentiostatic at 5.0 V applied to the cell. pH was not controlled. After 3.0 hours, the experiment was terminated and following acidification, the bromine concentration measured at the anodic side was 5.73 w/w. The experimental conditions and results are tabulated in Table 9.

(45) TABLE-US-00010 TABLE 9 Initial Final Bromide Bromine Conc. Current conc. Ex (w %) pH pH control density (w %) 63 10.0 w % Br.sup.− 4 Yes; with HCl 90 mA/cm.sup.2 5.6 w % 64  3.8 w % Br.sup.− 6 Yes; with HBr 50 mA/cm.sup.2 2.1 w % 65 2.3 w % Br.sup.− + 13.3 No  5 mA/cm.sup.2 5.7 w % 4.1 w % Bromate