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Gold Extraction from a Copper Bearing Ore by Thiosulphate Leaching 85

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Gold Extraction from a Copper Bearing Ore by Thiosulphate Leaching 85 83 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 ex- traction 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.
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