COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

STUDY OF GOLD LEACHING WITH BROMINE AND BROMIDE AND THE INFLUENCE OF SULPHIDE MINERALS ON THIS REACTION *Mariam Melashvili1, Chris Fleming1, Inna Dymov1, Mani Manimaran2, and Joe O’Day2 1

SGS Canada Inc. 185 Concession Street Lakefield, Ontario K0L 2H0 (*Corresponding author: [email protected]) 2

Albemarle Corporation 451 Florida Street Baton Rouge, LA 70821 ABSTRACT

This paper presents experimental evidence that supports the use of bromine/bromide as a potential alternative to conventional cyanidation for gold leaching. The study evaluated a brominebased lixiviant, Stabilized Bromine, with considerably lower vapour pressure than liquid bromine. Using a rotating device, the rate of gold dissolution was determined at various concentrations of this reagent. The reactivity of pure pyrite, arsenopyrite and chalcopyrite minerals was also evaluated in stabilized bromine reagent. Furthermore, bromine leaching of gold in a number of gold bearing ores was undertaken. Results showed that significantly higher gold recovery can be achieved with bromine than with cyanide when leaching gold encapsulated in sulphides, but bromine consumption was high owing to simultaneous oxidation of the sulphides. The greatest promise for the bromine leach process was with oxidized gold ores. The recovery of gold from oxide ores with bromine was comparable to that achievable with cyanide and bromine consumption was reasonable when leaching was conducted at a near neutral pH of ~6. In the case of oxidized gold ores containing copper mineralization, it is possible that bromine consumption may be lower than cyanide consumption, since there is evidence to suggest that bromine is less reactive than cyanide with copper minerals.

KEYWORDS bromine/bromide leaching, gold dissolution rate, sulphide minerals, copper

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COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

INTRODUCTION The chemistry of bromine/bromide leaching of gold is relatively well understood. It has been known as a lixiviant since the 19th century when it was first described by Duflos and Lange according to Rose (1894) after Michaelis (1987). Extensive research was carried out in the 1990’s. The reaction mechanism was presented by Pesic and Sergent (1992) using a rotating disk techniques. They found that the dominant mechanism and consequently dissolution rate was dependent on the brominebromide ratio. The electrochemical technique was also used by Meersberger et al. (1993) to show the affect of various minerals on gold dissolution rates. Alternatives to bromine oxidant were tested by Trindade et al. (1994), with limited success. The general characteristics of bromine/bromide leaching systems are: rapid leaching, nontoxicity and applicability over a wide range of pH values (from acidic to neutral). Such properties can be advantageous when treating oxidised ores and concentrates that are naturally acidic, since the cost of neutralization is eliminated. However, when treating a sulphide ore, it is important to recognize the reaction between bromine and sulphide as this reaction consumes a significant amount of bromine and likely compromises the economics of the process. To eliminate problems associated with the high vapour pressure and corrosive nature of bromine, several organic based bromine carriers (e.g. N-halo hydantoins such as Geobrom 3400) have been developed to stabilize bromine, and are well documented as lixiviants for gold. This paper focuses on a novel bromine/bromide based organic reagent called Stabilised Bromine, produced by Albemarle Corporation. The Stabilised Bromine used in the current study contained 11-15% active bromine and exhibited excellent stability compared to neat bromine. It was a clear, light-coloured solution even at the highest concentration tested. Materials and Methods A rotating disk experimental method was used in this study. The disk was fabricated using a pure gold plate mounted in epoxy and attached to a Teflon rod, which was screwed into a rotating shaft. The surface of the gold was polished with grid 2000 sand paper, washed with acetone and rinsed with deionised water. The rotating disk was then inserted in the central port of a 500 mL glass reactor with 4 ports on the top. The other three peripheral ports were used for ORP and pH probes and for pumping pH mediator into the reactor or for sample removal for analysis. The rotation speed of disk was fixed at 500 RPM. The average leaching rates were determined by removing small volumes of solution at specified time intervals and measuring the dissolved metal content. Leaching studies on sulphide minerals were conducted using pure minerals purchased from Ward’s Natural Science collection and gold bearing mineral samples obtained from SGS Minerals. Mineral samples of pyrite (Peru, Huanzala), arsenopyrite (Huanggang Mine, Ulanhad League, Inner Mongoloa, China) and chalcopyrite (Durango, Mexico), received from Ward’s Natural Science collection, were ground and washed with deoxygenated water prior to testing. The pyrite sample was subjected to additional treatment in order to remove surface oxides. The treatment included washing with dilute hydrochloric acid followed by deoxygenated water and finally acetone. The bench scale leaching set-up consisted of a 1000 mL baffled reactor and an overhead stirrer with a flat-bladed impeller. At the start of each test, 5 g of mineral was added to 250 mL of 10 g/L NaBr solution and placed in the reactor. The stirring speed was maintained at 375 RPM. The rate of addition of stabilized bromine was generally synchronised with the redox controlled pumping system. The rate of oxidation of sulphide was estimated by analysing the kinetic solution samples for their sulphate content. The unreacted sulphide in the solids at the end of each test was also determined and the amount of sulphide oxidized was compared to the calculated amount based on the increase in sulphate concentration in solution. There was good agreement between the two methods of determining the extent of sulphide oxidation. In the gold ore leaching experiments, predetermined amounts of solid feed and deionised water were added to the reactor and the pH was adjusted with dilute sulphuric acid. The solution potential was then controlled during leaching by the addition of bromine reagent, and samples were taken at intervals for analysis. The gold analyses were performed using the fire assay method and the bromide content was quantitatively determined by Ion Chromatography. The copper, iron and arsenic concentrations were

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COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

determined X-ray Fluorescence analysis. The sulphides were determined by Leco analysis. The concentration of bromine was estimated by titrating against thiosulphate in the presence of starch indicator. RESULTS AND DISCUSSION Rate of Gold Dissolution The rate of gold dissolution was estimated at various Stabilized Bromine dosages and active bromine concentrations between 0.65 g/L and 38.4 g/L. The sodium bromide was added at a concentration of 9 to 10 g/L except for one test, where the addition of large volumes of acid, necessary to reach the target pH 4, diluted the sodium bromide concentration to ~5 g/L. The test conditions and experimental results are summarized in Table 1 and the rate of gold dissolution is plotted in Figure 1. Table 1 - Varying Bromine Concentrations in Rotating Disk Gold Leaching Tests Reagents 2

cm Pure Br2 Stab. Br2 Stab. Br2 Stab. Br2 Stab. Br2 Stab. Br2

Initial Rate

Final

pH

EH

Au

Br

mg cm h

4.2 4.6 4.2 4.1 4.1 4.0

V 1.11 1.06 1.07 1.07 1.04 1.04

Test Conditions Au 0.60 0.41 0.62 0.62 0.62 0.62 10

NaBr

g/L 0.75 0.65 1.47 6.93 12.9 38.4

g/L 10.0 9.83 9.82 9.25 8.60 5.12

-2

6.53 1.39 1.59 2.48 3.45 9.22

-

-1

g/L 8.3 8.8 9.3 13 18 35

Stab. Br2 (38.4 g/L Br2) Pure Br2 (0.75 g/L Br2) Stab. Br2 (12.9 g/L Br2) Stab. Br2 (6.93 g/L Br2) Stab. Br2 (1.47 g/L Br2) Stab. Br2 (0.65 g/L Br2)

8 Au (mg cm -2 )

Br2

6

4

2

0 0

10

20

30 40 Time (min)

50

60

Figure 1 – Rates of Gold Dissolution at Various Bromine Concentrations

The trend to a higher gold leaching rate with increasing Stabilized Bromine is evident. It is also evident that pure bromine achieved higher gold dissolution rates at lower reagent concentrations than Stabilized Bromine, presumably because the active bromine concentration was higher. The experimental technique developed in this project was validated by the observed gold dissolution rate of 6.5 mg/cm2/h with 0.75 g/L of pure bromine, which is close to published rates of 5.07 and 7.6 mg/cm2/h for bromine concentrations of 0.62 g/L and 0.93 g/L, respectively (Pesic and Sergent, 1991). It is well known that the activity of bromine increases with the acidity of solution. Table 2 summarizes the results of tests run under similar conditions, but at different pH. In addition, a single test was run with cyanide under the same experimental conditions and at a similar reagent concentration, so that rates of gold leaching with bromine could be compared with the conventional gold leaching process. The kinetic plots are presented in Figure 2 and Figure 3.

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COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

Table 2 - Varying pH of Bromine Solution in Rotating Disk Gold Leaching Tests Test Conditions

Reagents Au 2

Initial Rate

Final

Br2

NaBr

NaCN

pH

EH

Au

Br

mg cm h 2.48 3.45 0.06

Stab. Br2 Stab. Br2 Stab. Br2 Stab. Br2

cm 0.62 0.62 0.62 0.60

g/L 6.93 12.9 12.8 14.1

g/L 9.25 8.60 8.45 9.40

g/L -

4.1 4.1 6.0 3.3

V 1.07 1.04 1.02 1.14

NaCN

0.62

-

-

5.0

11

0.17

4.0

18 pH 3, 14.1 g/L Br2, Au pH 4, 12.9 g/L Br2, Au*4 pH 6, 12.8 g/L Br2, Au*40

16

-1

17.1 3.03

g/L 13 18 17 -

Stab. Br2 (12.9 g/L Br2) NaCN (5 g/L NaCN) Stab. Br2 (6.93 g/L Br2)

3.5 3.0

12

Au (mg cm -2 )

Au (mg cm -2 )

14

-2

-

10 8 6

2.5 2.0 1.5

4

1.0

2

0.5

0

0.0 0

10

20

30

40

50

60

Time (min)

Figure 2 – The effect of pH on the Dissolution of Gold in a Bromine/Bromide Lixiviant

0

10

20 30 40 Time (min)

50

60

Figure 3 – Rates of Gold Leaching with Bromine/Bromide and Cyanide

In order to show all three rates in a single graph (Figure 2), the gold dissolution values corresponding to pH 6 and pH 4 for illustration purposes were multiplied by 40 and 4, respectively. The test conducted at pH 6 produced very little gold dissolution, and a leach rate of only about 0.06 mg/cm2/h. The rates of gold dissolution increased to 3.45 mg/cm2/h when the leach solution was acidified to pH 4. The highest rate of gold dissolution (17.1 mg/cm2/h) was achieved with Stabilized Bromine at pH 3. Rates of gold dissolution in cyanide and stabilized bromine are compared in Figure 3. The dissolution rate in cyanide was 3.03 mg/cm2/h which is in the same range as rate of gold dissolution achieved with Stabilized Bromine. However, whereas gold dissolution rates with cyanide are limited by the low solubility of oxygen in aqueous solution, the rate of gold leaching with bromine can be increased by increasing the concentration of active bromine in solution, as illustrated in Figure 3 (concentration of 12.9 g/L Stabilised Bromine). This study has determined a gold dissolution rate in cyanide of 3.03 mg/cm2/h, which is close to a rate of 3.25 mg/cm2/h under ideal conditions reported by Fleming (2012) and 2.88 mg/cm2/h reported by Marsden and House (2006). In bromine/bromide medium, the gold is oxidised by bromine and stabilized by bromide as the gold bromide complex, in an electrochemical reaction represented by the half cell reactions below: AuBr4- + 3e- = Au + 4Br Br2 (aq) + 2e- = 2Br-

E= 0.87 V vs. SHE E= 1.07 V vs. SHE

(1) (2)

The thermodynamics of this system dictate that the stability region for gold bromide extends from acid to neutral pH at potentials between ~0.9-1.5 V vs. SHE, as depicted in Figure 4. This EH-pH diagram was constructed for gold and bromide concentrations of 10-4 M and 10-1 M, respectively. At lower gold concentrations, the stability region of gold bromide shifts towards more acidic region (not shown here).

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COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

Figure 4- EH-pH Diagram by HSC According to the half cell reactions, the product of bromine reduction is the bromide ion. The data in Table 1 and Table 2 show the final bromide concentration is higher than the concentration of bromide added as the sodium salt at the start of the tests. If the difference is plotted against the amount of bromine consumed in the test, the production of bromide from added bromine may be correlated. This relationship has been plotted in Figure 5. Based on this approach, it appears that about 1.7 mol of bromide was produced from every mol of bromine added as Stabilized Bromine, which is slightly less than the theoretical ratio of 2. The balance presumably represents bromine that was lost as unreacted vapour. The plot of gold dissolution rates versus final bromide concentrations is shown in Figure 6. The data points lie closely to a single straight line with the slope of ~1.3, which is therefore the reaction order with respect to bromide concentration. By comparison, a reaction order of 1.4 for gold dissolution was estimated using a similar bromine/ bromide carrier, Geobrom 3400, in acidified solution (Pesic and Sergent, 1991). 0.45

-1.2 y = 1.6889x R² = 0.9962

0.40

-1.3 Log [Au] (mmol cm -2 h -1 )

Bromide (mol)

0.35 0.30 0.25 0.20 0.15 0.10 0.05

-1.4

y = 1.3356x - 0.8618 R² = 0.9963

-1.5 -1.6 -1.7 -1.8 -1.9 -2.0 -2.1

0.00 0.00

-2.2 0.05

0.10

0.15

0.20

0.25

Bromine (mol)

Figure 5 - The Relationship between Bromine addition and Bromide Production

-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 Log [Bromide] (M)

Figure 6 - The Rate of Gold Dissolution as a function of Total Bromide Concentration

In order to evaluate the effect of minerals on gold dissolution, tests were designed in such a way that both pure gold and the relevant mineral were rotating on separate shafts in a single reactor. Three minerals were independently mounted in epoxy disks of the same dimensions as the disks containing the pure gold sample. The test results are summarised in Table 3. The calculation of initial dissolution rates was based on the concentration of gold in the leach solution after 0.5 hour.

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COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

Table 3 - Rates of Gold Dissolution in the Presence of Sulphide and Oxide Minerals Test Conditions

Au & Minerals Au 2

cm Pure Gold Pyrite Chalcopyrite Copper Oxide Arsenopyrite

0.62 0.62 0.62 0.62 0.62

NaBr

Br2 g/L 12.9 14.2 14.2 14.2 13.8

Initial Rate

pH

g/L 8.6 9.4 9.5 9.5 9.2

EH V

4.1 3.9 3.3 4.3 4.2

1.0 1.0 1.1 1.1 1.1

Au -2

mg cm h

-1

3.66 5.37 1.47 2.67 2.97

Final Solution Composition Au

Fe

Cu

As

mg/L

mg/L

mg/L

mg/L

30.8 13.5 6.57 15.2 18.2

2.32 131 0.13

319 12.5 -

500 kg/t) renders this option for processing this type of refractory gold ores very unattractive. The data are presented in Table 9 and leaching kinetics in Figure 16. Table 9 - Summary of Leaching Tests on Refractory Ore Conditions

Reagent addition

pH

EH

Br2

NaBr H2 SO4

V

kg/t

kg/t

1.12 -

869 -

54 -

% 17.0 17.0

3.7 11.0

Reagent Consumption CaO

Br2

NaCN

kg/t

kg/t

kg/t

kg/t

kg/t

kg/t

%

g/t

g/t

618 -

11.3

1.8

530 -

2.97

1.61

70.8 23.8

1.30 3.32

4.46 4.18

100

80

90

70

Au extraction (%)

80

Au

NaCN

Free milling gold

Au Extraction (%)

Solids

70 60 50 40 30

CaO Extraction Residue Head

Refractory gold

60 50 40 30 20

20

10

pH 6 (bromine/bromide) pH 11(cyanide)

10 0 0

5

10 15 Time (h)

20

pH 3.7 (bromine/bromide) pH 11(cyanide)

0 0

25

Figure 15 - Gold Extraction by Bromine/Bromide and Cyanide from Oxidised Material

5

10 15 Time (h)

20

25

Figure 16 - Gold Extraction by Bromine/Bromide and Cyanide from Refractory Ore

One possible application of bromine/bromide leaching would be in the treatment of oxidized copper/gold ores. The copper in oxidized copper/gold ores reacts readily with cyanide and consumes about 4 mole of cyanide for every mole of copper leached (~3 kg NaCN per kg Cu). This can render the processing of such ores with cyanide uneconomic. By contrast, the copper in oxidized copper/gold ores is fairly inert in a bromine/bromide leaching medium, and there is a possibility that this process will enjoy lower reagent consumption than cyanide. To test this, an ore was prepared by mixing a copper oxide ore and an oxidized gold ore, and samples of the blended feed were treated with both bromine/bromide and cyanide. The leaching of both copper and gold were monitored, as well as reagent consumptions, and the data are presented in Table 10. Table 10 - Summary of Leaching Tests on Copper Oxide Ore Conditions Solids % 17 17

Reagent Consumption

Reagent addition

pH

EH

Br2

NaBr

NaCN

CaO

Br2

NaCN

CaO

kg/t 36.9 -

kg/t 50.0 -

kg/t 34.0

kg/t 4.3

kg/t 20.3

kg/t

5.6 11

V 0.98 -

kg/t 23.9

23.9

Au

Cu

Extraction Residue Head Extraction Residue % 85.4 85.3

g/t 2.31 1.93

g/t 15.7 14.6

% 20.4 76.6

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% 0.86 0.21

Head % 1.06 0.97

COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

The results were promising in that only ~20% of the copper leached in the bromine/bromide lixiviant, whereas ~77% of the copper leached in cyanide, with a very high cyanide consumption of 24 kg NaCN/t. Gold extraction was similar in both tests (~85%).

Future Process Considerations Even when relatively clean oxidized gold ores are processed, bromine consumptions of 10 kg/t or more will not be uncommon. The economic viability of this process will therefore require that the bromide ions that are generated by bromine reduction are re-oxidized to free bromine for internal recycling in the process. This will most likely be achieved electrolytically, in a divided cell of some sort. A simplified block diagram of a conceptual integrated process is shown in Figure 17.

Conversion by electrolysis Br2

-

Br

Leachant Makeup Stabilized Bromine

Gold Recovery CIP/CIL or RIP/RIL

Leaching Process Formation of AuBr4

-

Figure 17- Process Flowsheet

CONCLUSIONS In the rotating disk gold leaching experiments, the gold dissolution rate was linear as long as the composition of the leach solution remained fairly constant. The rate increased with increasing bromine concentration and decreasing pH (in the pH range 6 to 3). The dissolution rate using Stabilised Bromine under fairly mild conditions was in the same range as gold dissolution rates with cyanide under practical operating conditions. However, gold dissolution with cyanide is limited by the concentration of oxygen in the leach solution, whereas much faster rates of leaching can be achieved with bromine by lowering the pH (to around 3) and/or increasing the concentration of bromine. A typical leach residence time in a cyanidation circuit would be 24 hours or more, whereas in a bromine/bromide leach process, the residence time would be no more than 4 to 6 hours. The rate of gold dissolution in Stabilised Bromine was also examined in the presence of sulphide minerals. In all cases, the rate of gold leaching was adversely affected by the presence of sulphide minerals. The lowest gold leaching rate occurred in the presence of chalcopyrite, while the least effect on the gold dissolution rate was seen with arsenopyrite. When pyrite was present, the rate of gold leaching initially increased, but then decreased after the first few hours, and the final leach efficiency was significantly lower than without pyrite. Leaching tests with bromine were carried out with pure pyrite, arsenopyrite and chalcopyrite mineral samples. Pyrite was found to be very reactive in bromine, even at low potential and high pH (where the free bromine concentration would be very low). However, the reaction produces acid, reducing the pH rapidly and increasing the bromine activity. The oxidation of the sulphide in pyrite theoretically consumes 3.75 moles of bromine per mole of sulphide, and this was confirmed experimentally. Arsenopyrite on the other hand was found to be quite unreactive in bromine/bromide solution. The reaction was very slow at pH 4 and no acid was produced, nor any increase in sulphate concentration in solution observed. It is possible that sulphide was oxidised only as far as elemental sulphur under these experimental conditions. Arsenopyrite was more reactive in alkaline bromine solution, and exhibited increased extraction of arsenic and better oxidation of sulphides, even at a very low solution potential of 650mV (which would be too low for the simultaneous dissolution of gold). Chalcopyrite was relatively inert at pH 11-12, and no reaction was detectable until sulphuric acid had been added to lower the pH of the leach solution to the acidic region. Published by the Canadian Institute of Mining, Metallurgy and Petroleum

COM 2014 - Conference of Metallurgists Proceedings ISBN: 978-1-926872-24-7

The extraction of gold from two oxidized gold ores with Stabilised Bromine was satisfactory. Consumption of bromine was also reasonable. However, the rate of gold leaching slowed and bromine consumption increased significantly when sulphide was present, owing to the oxidation of sulphides by bromine and the resultant low free bromine concentration in solution. In tests carried out with a refractory gold ore, gold recovery by bromine leaching (~70%) was significantly higher than with cyanide (~24%) because the gold encapsulated in the sulphides was liberated when the sulphides were oxidized. The greatest promise for the bromine/bromide leach process is with oxidized gold ores, where gold leach recoveries with bromine (88 - 95%) are comparable to those achievable with cyanide (90 – 95%), and where bromine consumption is reasonable, (provided the leaching is conducted at a near neutral pH of ~6). In the case of oxidized gold ores containing copper mineralization, it is possible that bromine consumption and cost may be lower than cyanide because of the more selective nature of the bromine leach process in the presence of oxidized copper minerals. This may present the best opportunity for developing a bromine leaching process that is competitive with cyanide.

ACKNOWLEDGEMENTS The authors wish to thank Dave Matthews and Nadia Pavlovskaya for their assistance in laboratory testing. Thanks extend to Kaven Stogran for providing pure minerals and Vivien Delaney and Cheryl Mina for editorial work on the paper. REFERENCES Trindade, R.B.E., Rocha, P.C.P., Barbosa, J.P., 1994. Dissolution of gold in oxidized bromide solutions. Hydrometallurgy ’94, pp 527-540 Pesic, B., Sergent, R.H., 1991. A rotating disk study of gold dissolution by bromine. JOM 1991, Volume 43, issue 12, pp 35-37 Pesic, B., Sergent, R.H., 1993. Reaction mechanism of gold dissolution with bromine. Metallurgical Transactions B. Volume 24B, pp 419-431 Wan, R.Y., Le Vier, M. Miller, J.D., 1993. Research and development activities for the recovery of gold from noncyanide solutions. Hydrometallurgy, Fundamentals, Technology and Innovations. SME. Pp 415-436 Van Meersbergen., M.T., Lorenzen., L and Van Deventer., J.S.J. 1993. The Electrochemical Dissolution of Gold in Bromine Medium. Mineral Engineering, Vol. 6, 1067-1079. Fleming., C.A.”. Gold processing Workshop” Lakefield 2012 Kozin., L.F., Melekhin,V.T. 2004. Extraction of gold from ores and concentrates by leaching with the use of cyanides and alternative reagents.Russian Journal of Allpied Chemistry, Vol 77, No 10, 1573-1592. Marsden, J.O., Houes, C.I. 2006. The chemistry of gold extraction. Society for Mining, Metallurgy, and Exploration, Inc. Second Edition. Meersbergen, M.T., Lorenzen, L. van Deventer, J.S.J., 1993. The electrochemical dissolution of gold in bromine medium. Mineral Engineering, Vol 6, Nos. 8-10, 1067-1079. Lorenzen, L. van Deventer, J.S.J., Meersbergen, M.T., 1994. Interrelationship between lixiviants and galvanic interaction during dissolution of gold. Institution of Mining and Metallurgy and the Society of Chemical Industry, Hydrometallurgy ‘94. 484-499. von Michaelis, H., 1987. The prospects for alternative leach reagents - Can Precious metals producers get along without cyanide? Eng. Min. J., 188 (1987): 42-44 Freiberg, M. 1993. Bromine as a gold extractant. Reviews in Chemical Engineering , Volume 9, Nos 34 . 333-344. Aylmore, M.G. 2010. Alternative lixiviants to cyanide. Alta 2010 Gold ore processing symposium. Rose, T. 1894. Metallurgy of Gold. Charles Griffin & Co., London

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