Gold Extraction from a Copper Bearing Ore by Thiosulphate Leaching 85

特別 GOLD EXTRACTION FROM A COPPER BEARING 講演 ORE BY THIOSULPHATE LEACHING

W. T. Yen, K. Stogran and T. Fujita

Gold extraction by cyanidation is reduced to less than 30% as the copper content in the ore is higher than 0.3% Cu. Thiosulphate leaching process could overcome this problem. The effect of the variables on the gold extraction have been investigated. Under the optimun leaching conditions: 0.4 M thiosulphate, 0.2 M ammonia, 0.03 M cupric ion and pH 11, 90% of gold was extracted. The possible processes for gold recovery from the pregnant solution was also discussed.

Depending on the method of investigation and the ore type 1. INTRODUCTION used, the optimum concentration of thiosulphate re-

In a gold bearing copper ore, both copper and gold quired for the gold extraction was ranged from 0.2 M to 2.0 values are conventionally recovered by flotation followed by M S2O33-. The most common range of optimum concen- pyrometallurginal treatment. On the other hand, in a tration fell within 0.2 M-0.4 M. The optimum level of copper bearing gold ore, the gold value is much higher than NH3 was from 0.09 M to 4.1 M. Tozawa, et al. Suggested the copper and the priority is to use a hydrometallurgical that the excess OH ions in solution with a concentration method to extract the gold. Unfortunately, cyanide will be of greater than 0.5 M NH3 suppressed the dissolution of totally consumed by copper in the cyanidation and does not gold. The neccessity of cupric ions in the leach solution is have chance to react with gold. Therefore, a non-cyanide practically undisputed. The cupric ion serves as a catalyst lixiviant should be used for leaching this type of ore. in the gold dissolution. Without cupric ion, the dissolution

Thiourea has been used to extract gold from the copper of gold by thiosulphate is slow and incomplete. The opti- bearing gold ore. This paper is going to discuss the mum solution temperature was found by most of investi- possibility of using thiosulphate to extract the gold from the gators as 50•Ž-60•Ž. However, there were many suc- copper bearing ore. cessful performances being conducted in the ambient tem-

It have been reported by many investigators that perature gold is soluble in an ammoniacal thiosulphate solution. 2. CHEMISTRY OF GOLD DISSOLUTION Due to the environmental restriction, a growing interest has emerged on leaching gold with thiosulphate solution ac- The dissolution of gold in thiosulphate solution is complished high gold recovery, non-toxicity and fast usually explained as an electrochemical redox reaction leaching rate among those non-cyanide processes such as catalyzed by the presence of cupric ions in an alkaline so- thiourea, chlorination, etc. The early successful applica- lution. In anodic reaction, gold is oxidized, given up an tion of the thiosulphate have required the use of oxygen electron and complexed with ammonia, followed by com- under pressure and high thiosulphate concentration plexing with thiosulphate. In cathodic reaction, the copper together with copper as a catalyst in ammonia solution. ammonia complex receives an electron and reduces the

oxygen. The reactions are shown as follows: *平成8年6月13日本会第96回例会において発表 **Mining Enginee ring, Queen's University, Kingston, Anodic area:

Ontario, Canada. Oxidation: Au•¨Au++e- ***Mining College , Akita University. Complexing: Au++2NH3•¨Au(NH3)2+ 平成8年3月19日受理

Vol.43, No.2('96-夏) (43) 84 W. T. Yen¥K. Stogran¥T. Fujita

Au(NH3)2+ + 252O32 4. RESULTS •¨ 2NH3+Au(52O3)23-

Cathodic area: 1. Effect of Copper on the Gold Extraction by Cyani-

Reduction: Cu(NH3)42+ + e- +Cu(NH3)2+ + 2NH3 dation

Complexing: 4Cu(NH3)2+ +O2+ 2H2O+ 8NH3 The copper content of the ore was reduced by flotation

•¨ 4Cu(NH3)42++4OH- technique and increased by artificial mixing to a desired

The oveall reaction is: value. Figure 1 shows that gold extraction was 84% from

4Au+852O32 +O2+2H2O•¨4Au(S2O3)23 +4OH the ore contained no copper value. Low ultimate gold extraction may be due to the effect of stibnite. The gold ex- The presence of copper and ammonia in the thiosul- traction was greatly reduced to 36% as the copper content phate solution facilitate the reactions result in the produc- was increased to 0.2%. When the copper content reached tion of Cu(52O3)35- and cupric tetraimine ions 0.3%, the gold extraction was less than 30%. It is obiously Cu(NH3)42+, which act as a catalyst for the gold oxidation that the avialable free cyanide was rapidly consumed by reaction. copper and not enough chances to react with the gold. According to the Eh-pH diagram, both Au(NH3)2+ 2. Effect of Thiosulphate Concentration: and Au(52O3)23+ complexes are formed in the reactions. The thiosulphate concentration was varied from 0.1 M The Au(NH3)2+ complex is much easier to form but less to 0.8 M S2O32-. All other conditions were maintained at stable and most of them are converted to the more stable pH 11, 0.01 M Cue, and 0.10 M NH3. final product Au(52O3)23- Figure 2 shows that the gold extraction increased 3. EXPERIMENTAL rapidly with increasing thiosulphate concentration in the low S2O32- concentration range. At the concentration of The gold ore sample used in this study contains 7.26 0.4 M S2O32-, the gold extraction was 80%. The increase g/t Au, 1.5 g/t Ag and 0.3% Cu, which is similar to some of gold extraction was minimal for further increasing of the ore deposits in northwest of United States and South thiosulphate concentration. Thus, the standard thiosul- America. The copper minerals are 98% chacocite and 2% phate concentration of subsequent tests was fixed at 0.4 M bornite. Other sulphide minerals are 10% pyrite, 0.12% S2O32-. stibnite, 0.03% realgar, 0.12% molybdenite and 12 g/t 3. Effect of Ammonia Concentration: cinnabar. The non-sulphide minerals are quartz, micas, Figure 3 shows the relationship of gold extraction and calcite and feldspar. ammonia concentration at pH 11 with 0.01 Cu2+ and 0.4 All chemicals used in the test work are reagent grade. M S2O32-. In a solution of 0.1 M NH3, the gold extraction A standard cyanidation was conducted to demonstrate the effect of copper content on the gold extraction. The ore sample was leached in a bottle on rolls at 33% solids with 1.0 g/L NaCN at pH 10 for 24 hours. For all thiosulphate leach test, 250 grams ore sample was added into a 500 mL of ammonium thiosulphate leach solution, which was prepared by mixing ammonium hydroxide in the sodium thiosulphate solution. The copper sulphate solution was then added and the pH was adjusted by NaOH or H25O4 solution. The leaching was also car- ried out on rolls at ambient temperature for 24 hours. The main variables studied in this investigation were the concentrations of thiosulphate, ammonia and cupric ions, and solution pH.

Fig. 1 Effect of Copper Content on Gold Extraction by Cyanidation pH 10, 1 g/L NaCN, 24 hrs. leach

(44) 資源処理技術 GOLD EXTRACTION FROM A COPPER BEARING ORE BY THIOSULPHATE LEACHING 85

Fig. 4 Effect of Copper Ion on gold Extraction Fig. 2 Effect of Thiosulphate on Gold Extraction pH 11, 0.4 M 52O3-2, 0.2 M NH3 pH 11, 0.01 M Cu++, 0.1M NH3 is 72.5%. Increasing the ammonia concentration to 0.2 M The effect of cupric ion concentration on the gold ex- NH3, the gold extraction reaches a maximum of 82%. traction was examined using copper sulphate as the source Further increasing the ammonia concentration to higher of cupric ions. Under the leaching conditions of pH 11, than 0.4 M NH3, lower gold extractions are observed. The 0.40 M 52O32- and 0.2 M NH3, Figure 4 shows that gold reason for this retarding effect could be due to two causes. extraction is increased with increasing cupric ions concen- Firstly, the high ammonia concentration reduces the area tration. In a solution of 0.005 M Cue, about 69% of gold thermodynamic stability of Cu(NH3)42+ and Cu(S2O3)35 was extracted. As the cupric ion concentration doubled up while widening the regions of thermodynamic stability of to 0.01 M, the gold extraction increased to 80%. A max- other solid copper species, such as CuO and Cu2O. Se- imum gold extraction of 90% was obtained at the concen- condly, it has been found that a precipitate, (NH4)5 tration of 0.03 M Cue. Cu(52O3)3, was formed and reduces the oxidant activity of 5. Effect of Solution pH: the cuprotetraammine complex and covers the mineral The effect of pH on the gold extraction was investi- surfaces, hindering the further thiosulphate attack. gated from pH 8.0 to pH 11.0. Since the calcium ion 4. Effect of Cupric Ion Concentration:

Fig. 3 Effect of Ammonia on gold Extraction. pH 11, Fig. 5 Effect of pH on Gold Extraction, 0.4 M S2O3-2, 0.01 M Cu2+, 0.1 S2O3-2 0.03 M Cu2+, 0.2 M NH3

Vol. 43, No. 2 ('96-‰Ä) (45) 86 W. T. Yen¥K. Stogran¥T. Fujita would interfer the thiosulphate leaching process, the sodi- thiosulphate, (NH4)2S2O3,or sodium thiosulphate, Na2S2O3. um hydroxide solution was used to adjust the pH to 11. It was found that sodium thiosulphate alone could not dis- Other standard conditions were maintained constant at solve the gold. The gold dissolution does occur in the 0.40 M S2O3-2, 0.03 M Cu2+ and 0.20 M NH3. ammonium thiosulphate solution. Figure 5 shows that the gold extraction was 38% at pH The ammonium thiosulphate could be commercially 8.0 and 71% at pH 9.0. When the solution pH was raised obtained in three forms: solid crystal, 70% liquid and the up to 10 and 11, the gold extraction was increased to 85% mixture of Na2S2O3 and NH4OH. Most of researchers and 90% respectively. This value is slightly higher than found that the commercially available 70% ammonium that of other investigators, who reported that the opti- thiosulphate solution could not properly dissolve the gold mum pH value is 9.2 to 10. Since it is too expensive to and was not used for gold leaching study. The best gold adjust the pH to a higher value, we did not conducted any extraction results were obtained by using crystal solid am- tests with a pH higher than 11. monium thiosulphate or the mixture of sodium thiosulphate and ammonium hydroxide. In this study, the mixture was 5. DISCUSSION used in all tests.

1. Type of Minerals: 4. Effect of Leaching Temperature:

Chalcocite, bornite, malachite and other secondary Most of previous investigators reported that the opti- copper minerals are very soluble in cyanide solution (more mum leaching temperature was in the range of 50•Ž and than 70%'-90%). They consumed a great deal of cyanide 60•Ž. A recent study claimed that the bes gold extrac- leaving virtually no cyanide for gold leaching. The solution was obtained at 25•Ž in a solution containing 2 M bility of chalcopyrite in the cyanide is only 5% and its effect 52O32-, 4 M NH3 and 0.1 M CuSO4. The increase of on the gold extraction is much less severe. However, there temperature from 25•Ž to 60•Ž, the gold extraction is no pure chalcopyrite, which always associated with a decrease from 79% to 56% in a 3 hours leaching. The certain amount of secondary copper minerals and causes diminished gold extraction at the higher temperature was incapable of gold extraction by cyanidation. Thus, thisul- explained as: the formation of passivated cupric sulphide by phate is one of the alternative methods to extract gold from the reaction of cupric ions and thiosulphate: and the the copper bearing gold ore. decomposition of 52O32- into other sulphur compounds,

2. Pre-leaching of the Ore: S4O62- SO32-, S2-. The reactions can be described by The copper bearing ore sample has been pre-leached following equations: with NH4OH solution for 6 hours, followed by adding thiosulphate. The gold extraction results of pre-leaching Cu2+ + S2O32- + H2O•¨CuS + SO42- +2OH - has demonstrated that there were no differences between 452O32 - +2H2O+O2•¨2S4O62- +4OH the subsequent copper ions addition and no copper ions - 352O32-+3H2O•¨4SO32- +252- +6H+ addtion. The reason for this consequency is attributed to the fornation of Cu(NH3)42+ ions during the pre-leaching All tests in this investigation were conducted in ambient stage. The additional copper ions is not necessary. temperature. (23•Ž•`25•Ž).

Normally, the maximum dissolution of copper from 5. Gold Recovery From the Leach Solution: the minerals by NH4OH solution is 25%•`30%. The op- There are many possible methods to recover the gold timum concentration required is 5 M•`10 M NH3. The from thiosulphate leach pregnant solution, such as precipi- concentration of Cu(NH3)42+ required to produce an ul- tations and adsorptions. timate gold extraction is 60 ppm. Also, it was found that Zinc dust precipitation has been used to recover gold the thiosulphate concentration required under these condi- from the solution. In the pH range of 9.5•`11.0, more tions was slightly reduced. than 99% of gold was recovered by 10•`20 g/L zinc powder

Similarly, the addition of Cu(NH3)42+ to leach an ore in 60 minutes. Also more than 99% of silver and copper contained no copper minerals would enhance the gold ex- were recovered under the same conditions. If the retention traction as compared with that of CuSO4 addition. was reduced to 30 minutes, the gold recovery was 96%.

3. Thiosulphate Compounds: Gold and silver also could be recovered from the solu- Thiosulphate could be in the form of ammonium tion by iron powder. More than 99% of gold and silver

(46) 資源処理技術 GOLD EXTRACTION FROM A COPPER BEARING ORE BY THIOSULPHATE LEACHING 87 were recovered from the leach solution at pH9.5•`11.0 with thiosulphate leaching and less than 30% by cyanidation.

4 g/L iron powder at 20•Ž. If the solution temperature is The optimum conditions was: 0.40 M 52032-, 0.20 M increased to 30•Ž, same result was obtained at pH 9.5 and NH3, 0.03 M Cu+2 ans pH 11. The pre-leaching the ore pH 6.7. But at pH 8.1, both gold and silver recovery were with NH4 OH or ading Cu(NH3)42+ would enhance the only 28%. Other difference between zinc precipitation and gold extraction. The gold recovery from the leach solution iron precipitation is the low copper recovery in the iron is better processing by zinc or iron precipitation than by precipitation process. In the optimum gold recovery con- carbon adsorption or ion exchanger. ditions, the copper recovery was 10%•`25%. Both activated carbon and ion exchange resin were REFERENCES tried to recover the gold from the leach solution. In the 1) G. Deschenes, Proc. of Int. Symp. On Gold Metalurgy, carbon adsorption test, the gold recovery was depending on PergamonPress, pp. 359-377 (1987). the carbon concentration, retention time and pH. It was 2) Y. Umetsu & K. Tozawa, TohokuDaigaku SenkoSeiren KenkyushoTho, 28, 1, pp. 97-104 (1972). found the optimum conditions was: 60 g/L carbon, pH 3) R. M. G. S. Berezowsky, V. B. Sefton & L. S. Gorm 10.6-11.0 and 6 hours. The optimum gold recovery un- erly, Can Patent 246, 274 (1976). der these conditions was less than 95%. At the retention 4) B.J. Kerley, U.S. Parent 4, 269, 622 (1981). time of 2 hours and 4 hours with the same other conditions, 5) A. E. Perez, U.S. Parent 4, 654, 078 (1987). 6) D. Zipperian, S. Raghavan, & J. P. Wilson, the gold recovery was only 30% and 60% respectively. If Hydrometallurgy,19, pp. 361-375 (1988). the carbon concentration was reduced to 20 g/L or 40 g/L, 7) K. Tozawa, Y. Inui & Y. Umetsu, 110th AIMEAnnual the gold recovery was reduced to less than 20%. Similar Meeting, TMS-AIME Paper A-81-25 (1981). results were also obtained for the silver recovery. 8) Cao Changlin, Hu Jiexue & Gong Qian, Randol '92 Symposium,Vancouver, pp. 213-218 (1992). Several ion exchange resins have been selected for the 9) M. Hemmanti, J. L. Hendrix, J. H. Nelson & E. B. gold recovery tests. Results were not better than the car- Milosavljevic, Extractive Metallurgy '89 Symposium, bon adorption process. IMM, London, pp. 665-678 (1989). 10) C. Abbruzzese, P. Fornari, R. Massidda, F. Veglio & 6. CONCLUSION S. Ubaldini, Hydrometallurgy,39, pp. 265-276 (1995). 11) K. Y. Wan, K. M. Levier & R. B. Clayton, Patent ZA Gold extraction from a copper bearing ore was 90% by 93/7288, Gold Bull. 27, 4, p. 108 (1994).

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