Materials Transactions, Vol. 45, No. 2 (2004) pp. 516 to 526 #2004 The Mining and Materials Processing Institute of Japan
Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction
Harunobu Arima1, Toyohisa Fujita2 and Wan-Tai Yen3
1Graduate Student, Queen’s University. Present address: Akita Zinc Co., Ltd., Akita 011-8555, Japan 2RACE, (Department of Geosystem Engineering), The University of Tokyo, Tokyo 113-8656, Japan 3Department of Mining Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
The use of copper as a catalyst for gold leaching in ammonium thiosulfate solution might cause the high consumption of thiosulfate. Also, the high copper consumption is resulted in the zinc precipitation process for recovering the gold from the pregnant solution. In this investigation, nickel was used as a catalyst to minimize the reagent consumption. On a 100 mass%-75 mm of silicate type gold ore containing 16 g/t Au and 0.2 mass% of Fe and C, the nickel catalyzed ammonium thiosulfate solution could extract 95% of gold with the 1.2 kg/t-ore of ammonium 3 3 thiosulfate consumption in 24 hours at the most favorable reagent combination of 0.0001 mol/dm NiSO4, 0.05 mol/dm (NH4)2S2O3 and 3 3 3 0.5 mol/dm NH4OH at pH9.5, while the standard cyanidation at 0.02 mol/dm (1.0 g/dm ) NaCN consumed around 1.5 kg/t-ore NaCN. In the concentration range of 0:0001 0:005 mol/dm3 Ni2þ, the ammonium thiosulfate consumption was 1 5 kg/t-ore, while the ammonium thiosulfate consumption of copper catalyzed lixiviant was greatly increased from 3 kg/t-ore to 21 kg/t-ore as the increase of Cu2þ concentration from 0.0001 mol/dm3 to 0.001 mol/dm3. The feasibility of recycling barren solution was confirmed with zinc precipitation at nearly 100% of gold recovery. Nickel consumption on the cementation process was less than 50%. For extracting gold from the copper bearing sulfide ore, a higher ammonia and thiosulfate concentrations were required with 0.0001 mol/dm3 of Ni2þ. The ammonium thiuosulfate consumption with nickel as catalyst on the copper bearing sulfide ore was about 1 5 kg/t-ore less than that using copper as catalyst.
(Received March 17, 2003; Accepted December 1, 2003) Keywords: ammonium thiosulfate leaching, cementation, nickel, gold
1. Introduction to 12 g thiosulfate can be taken daily by mouth with no ill effects.5) The current market price for ammonium thiosulfate The solubility of gold in a diluted solution of potassium is only US$0.30/kg.33) In addition to thiosulfate as a major cyanide was demonstrated by Elsner in 1846, as shown in eq. reagent, both ammonia and cupric ion are required in the gold (1):1) leaching process. Ammonia is required to stabilize the catalyst of cupric ion and modify the pH. The time weighted 4Au þ 8KCN þ O þ 2H O ! 4KAu(CN) þ 4KOH ð1Þ 2 2 2 exposure average limit of ammonia is 35 mg/dm3.5) In 1887, cyanidation was patented by MacArthur and the There has been extensive research on thiosulfate leaching Forrester brothers, and further developed to the practical use for gold extraction since the 1970s.29–31,34–41) Ammonium for gold recovery. Since then, cyanidation has been used as a thiosulfate leaching process is also applicable for the heap standard gold extraction process. The gold pregnant solution leaching process.28,31) However, there are problems in has been processed by cementaion, CIP/CIL (carbon in pulp/ commercializing the thiosulfate process in the gold milling carbon in leach) or ion exchange for gold recovery.2–4) industry, the problems are the most notably high reagent However, cyanide has a high toxicity with the time consumption and varying gold extraction results for the weighted exposure average limit of 5 mg/m3.5) Some different ores under the same leaching conditions. Further- incidents have occurred with spilled cyanide and a resulting more, the conventional gold recovery process of CIP (carbon environmental pollution.6,7) Several countries such as the in pulp) on cyanidation can not be applicable in non cyanide Czech Republic, Turkey and the USA in states, such as process.28,41,42) The alternative gold recovery process by Colorado, Wisconsin and Montana, have campaigned to ban cementation using zinc, copper and aluminium powders as cyanide applications.8–12) Also, when processing refractory well as ion exchange has not been systematically studied, ore containing sulfide and carbonaceous minerals, it is hence a favourable process is not still established.41,43–51) difficult to extract gold by cyanidation without pre-treatment, In our previous studies on the copper catalyzed ammonium such as roasting, pressure oxidation and bio-oxidation, thiosulfate leaching system, the most favorable reagent etc.13–15) combination to give high gold recovery with small thiosulfate Due to above environmental, political and technical consumption was extensively researched and evaluated by concerns, the development of non-cyanide process has been both thermodynamic and experimental approaches.52,53) The focused on an alternative in the study of gold extraction. best experimental result showed that 94% of gold extraction There are various non-cyanide lixiviants being proposed and 3 kg/t-ore of ammonium thiosulfate consumption could 3 3 by many investigators for gold extraction: thiourea, bromide, be achieved using 0.3 mol/dm NH4OH, 0.0001 mol/dm 16–24) 3 iodine, thiocyanate and thiosulfate. Among these lix- CuSO4, 0.05 mol/dm (NH4)2S2O3 at pH 9.5 for a iviants, thiosulfate has been suggested as the best cyanide 200 mesh ( 75 mm) silicate type ore containing 16 g/t substitute.25–32) Au. Using zinc powder, nearly 100% of gold could be 2 Thiosulfate (S2O3 ) is the least toxic and cheapest recovered from the pregnant solution. However, thiosulfate chemical which can stabilize gold in aqueous solution. Up gold leaching using copper as a catalyst had the following Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction 517 problems: various sulfide minerals were also conducted and evaluated (1) Since the stability of the cuprous thiosulfate complex is for their feasibility. higher than that of the cuprous ammine complex thermodynamically, the cupric ammine complex is 2. Thermodynamic Discussion on Gold Leaching and reduced to a cuprous thiosulfate complex, causing an Recovery oxidation of thiosulfate to tetrathionate through the intermediate product of a cuprous ammine complex as Figure 1 shows the Eh-pH diagram of the Ni-NH4OH- 2 3 shown in eqs. (2) and (3). S2O3 system for the reagent combination of 0.5 mol/dm NH OH, 0.5 mol/dm3 S O 2 and 0.005 mol/dm3 Ni2þ. The 2[Cu(NH ) ]2þ þ 2S O 2 4 2 3 3 4 2 3 concentrations of reagent except Ni2þ are within the range þ 2 ! 2[Cu(NH3)2] þ S4O6 þ 4NH3 ð2Þ applied on the previous experimental project of the copper þ 2 52) [Cu(NH3)4] þ 2S2O3 catalyzed gold leaching. Free energies of formation used in this calculation are listed in Table 1.54–64) Nernst Equations ! [Cu(S O ) ]3 þ 4NH ð3Þ 2 3 2 3 applied for this Eh-pH diagram is shown in Table 2. The (2) Although either the increase of ammonia concentration result of Fig. 1 suggests that the electrode potential in the 2þ or the decrease of thiosulfate concentration can make couple of {Ni3O4/[Ni(NH3)6] } may work for gold oxida- the cupric ion stable as a cupric ammine complex, the tion in ammonium thiosulfate solution, while that in the 2þ þ cupric ion is the important electron acceptor to oxidize couple of {[Cu(NH3)4] /[Cu(NH3)2] } works as an oxidant gold. And, a substantial amount of cupric ion is required for gold.53) in the ammonium thiosulfate leaching of gold extrac- In the gold leaching by nickel as a catalyst, nickelous oxide tion. Also, gold extraction kinetics and thiosulfate consumption are nearly influenced by the level of 52) copper concentration. 1.5 (3) In the study of gold cementation by zinc, copper and
aluminium powders, zinc powder was found to be the ↓N7 ↓N10 NiO2 best metal powder to recover gold at lower ammonia 1.0 ↓E2 ↓N6 (0.3 mol/dm3) and thiosulfate (0.05 mol/dm3) concen- Ni2O3 trations.52) But almost all of the copper in the solution ↓N9 Ni O was co-precipitated along with the gold since the 0.5 ↓N5 3 4 ↓N8
stability of the copper ions is lower than that of zinc V ↓ 52,53) 2- N5 [Ni(S2O3)2] ions thermodynamically. Thus, the addition of Eh, 2+ cupric ions is required for recycling the barren solution 0.0 NiO [Ni(NH3)6] NiO to the leaching circuit, increasing the total reagent ↓E1 consumption. ↑N2 ↑N4 -0.5 In order to reduce the reagent consumption in the ↑N3 thiosulfate leaching of gold, nickel ion was used instead of ↑N4 copper ions in the present investigation. Effects of oxygen, Ni ammonia, copper and thiosulfate on both gold extraction and -1.0 thiosulfate consumption were investigated using the same ore 468101214 studied on the conventional copper catalyzed system.52) The pH gold in the pregnant solution was cemented by zinc powder. 2 Fig. 1 Eh-pH diagram for the Ni-NH3-S2O3 -H2O system at the 3 3 2þ The optimal reagent composition found in this investigation condition of 0.5 mol/dm NH4OH, 0.005 mol/dm Ni and 0.5 mol/ 3 2 was evaluated thermodynamically. Gold leaching tests for dm S2O3 . Nernst Equations (N1) (N11) are shown in Table 2.
Table 1 Free energy of formation for various species54–61)
species G (kJ/mol) species G (kJ/mol) species G (kJ/mol) 2 H2O 237:18 NiO 215:93 Ni(S2O3)2 1098:02 3 OH 157:29 Ni3O4 711:91 Au(S2O3)2 1030:15 3 NH3 26:57 Ni2O3 469:94 Cu(S2O3)2 1065:01 2 5 S2O3 522:50 NiO2 215:94 Cu(S2O3)3 1595:73 2þ Ni 48:24 Cu2O 146:00 þ 2þ Au 163.20 CuO 129:70 Ni(NH3)6 256:66 3þ þ Au 434.18 Au(NH3)2 41:4 þ 3þ Cu 50.00 HNiO2 349:22 Au(NH3)4 65.43 2þ 2 þ Cu 65.52 NiO4 — Cu(NH3)2 63:51 2þ AuO3 24:27 Cu(NH3)4 112:65 2 Ni(OH)2 447:3 CuO2 182:00 Au(OH) 134.86
Au(OH)3 289:15 1 .Aia .Fjt n .T Yen W.-T. and Fujita T. Arima, H. 518
Table 2 Nernst equations applied in Eh-pH diagram at 298 K on Metal-Ammonia-Thiosulfate-Water System (The presence of other polythionate species was not considered.)
Half cell reaction Nernst equation 2 3 3 2 B1 Au + 2S2O3 = Au(S2O3)2 +e B1 E ¼ 0:15 þ 0:059 log½Au(S2O3)2 0:118 log½S2O3 þ þ B2 Au + 2NH3 = Au(NH3)2 +e B2 E ¼ 0:12 þ 0:059 log½Au(NH3)2 0:118 log½NH3 þ 3þ 3þ þ B3 Au(NH3)2 + 2NH3 = Au(NH3)4 +2e B3 E ¼ 0:89 þ 0:059 logf[Au(NH3)4 ]/[Au(NH3)2 ]g 0:059 log½NH3 3 3þ 2 3þ 3 2 B4 Au(S2O3)2 + 4NH3 = Au(NH3)4 +2S2O3 +2e B4 E ¼ 0:81 þ 0:059 logf[Au(NH3)4 ]/[Au(S2O3)2 ] þ 0:059 log½S2O3 0:118 log½NH3 þ 3 þ 3 2þ B5 Au(NH3)2 +3H2O= AuO3 + 2NH3 +6H +2e B5 E ¼ 3:50 þ 0:030 logf[AuO3 ]/[Au(NH3) ]gþ0:059 logðNH3Þ 0:177pH 3 3 2 þ 3 3 2 B6 Au(S2O3)2 +3H2O= AuO3 +2S2O3 +6H +2e B6 E ¼ 3:48 þ 0:030 logf[AuO3 ]/[Au(S2O3)2 ]gþ0:059 logðS2O3 Þ 0:177pH þ þ þ B7 Au(NH3)2 +3H2O = Au(OH)3 + 2NH3 +3H +2e B7 E ¼ 2:29 þ 0:059 log½NH3 0:089pH 0:030 log½Au(NH3)2 3 2 þ 2 3 B8 Au(S2O3)2 +3H2O = Au(OH)3 +2S2O3 +3H +2e B8 E ¼ 2:27 þ 0:059 log½S2O3 0:089pH 0:030 log½Au(S2O3)2 þ B9 Au + H2O = Au(OH) + 2H +e B9 E ¼ 3:03 0:059pH N1 Ni = Ni2þ +2e N1 E ¼ 0:25 þ 0:030 log½Ni2þ 2 2 2 2 N2 Ni + 2S2O3 = Ni(S2O3)2 +2e N2 E ¼ 0:27 þ 0:030 log½Ni(S2O3)2 0:059 log½S2O3 2þ 2þ N3 Ni + 6NH3 = Ni(NH3)6 +2e N3 E ¼ 0:50 þ 0:030 log½Ni(NH3)6 0:177 log½NH3 þ N4 Ni + H2O = NiO + 2H +2e N4 E ¼ 0:13 0:059pH þ N5 3NiO + H2O= Ni3O4 +2H +2e N5 E ¼ 0:897 0:059pH þ N6 2Ni3O4 +H2O = 3Ni2O3 +2H +2e N6 E ¼ 1:31 0:059pH þ N7 Ni2O3 +H2O = 2NiO2 +2H +2e N7 E ¼ 1:43 0:059pH 2þ þ 2þ N8 3Ni(NH3)6 +4H2O= Ni3O4 + 18NH3 +8H +2e N8 E ¼ 2:74 0:089 log½Ni(NH3)6 0:236pH þ 0:531 log½NH3 2 2 þ 2 2 N9 3Ni(S2O3)2 +4H2O= Ni3O4 +6S2O3 +8H +2e N9 E ¼ 2:05 þ 0:177 log½S2O3 0:236pH 0:089 log½Ni(S2O3)2 2 2 þ 2 2 N10 2Ni(S2O3)2 +3H2O= Ni2O3 +4S2O3 +6H +2e N10 E ¼ 1:80 þ 0:118 log½S2O3 0:177pH 0:059 log½Ni(S2O3)2 2 2 þ 2 2 N11 Ni(S2O3)2 +2H2O = NiO2 +2S2O3 +4H +2e N11 E ¼ 1:61 þ 0:059 log½S2O3 0:118pH 0:030 log½Ni(S2O3)2 þ E1 2H =H2 +2e E1 E ¼ 0:059pH E2 2H2O+O2 = 4OH +4e E2 E ¼ 1:23 0:059pH þ þ 9.25-pH þ F1 NH4 +H2O= NH4OH + H F1 ½NH3 ¼½NH3 T =ð1 þ 10 Þ; ½NH3 ¼½NH4OH , ½NH3 T ¼½NH4 þ½NH3 Equations B2, B3, B4, B5, B7, N3 and N8 are calculated with eq. F1. Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction 519
(Ni3O4), formed from nickel ammine complex, act as an 1.5 oxidant for gold. The oxidation reaction of nickel ammine complex is shown as the combination of anodic reaction (4) 3+ [Au(NH3)4] ↑ and cathodic reaction (5): 1.0 B4 ↑E2 2þ 3Ni(NH3)6 þ 8OH ↑B3 ! Ni O þ 18NH þ 4H O þ 2e ð4Þ V 3 4 3 2 0.5 + 3- [Au(NH3)2]
Eh, [Au(S2O3)2] 1/2O2 þ H2O þ 2e ! 2OH ð5Þ
The nickelous oxide (Ni3O4) in eq. (4) is reduced back to a 0.0 nickel ammine complex with the oxidation reaction of gold ↑B1 as shown in the combination of cathodic reaction (6) and ↑B2 Au anodic reaction (7): ↑E1 -0.5 Ni3O4 þ 18NH3 þ 4H2O þ 2e 468101214 2þ ! 3Ni(NH3)6 þ 8OH ð6Þ pH þ 2Au þ 4NH ! 2[Au(NH ) ] þ 2e ð7Þ 2 3 3 2 Fig. 2 Eh-pH diagram for the Au-NH3-S2O3 -H2O system at the 3 3 2 The overall reaction from (4) to (7) is expressed as the condition of 0.5 mol/dm NH4OH, 0.5 mol/dm S2O3 and 5 10 5 mol/dm3 Au Nernst Equations (B1) (B4) are shown in Table 2. following reaction (8): 2þ Catalyzed by [Ni(NH3)6] # Gold surface Ammonium Thiosulfate Solution
4Au þ 8NH3 þ O2 þ 2H2O þ ! 4[Au(NH3)2] þ 4OH ð8Þ 2+ - 3Ni(NH3)6 , 8OH Since the aurous ammine complex is not stable, the aurous ammine complex in eq. (8) is immediately converted to a 1/2O2, H2O stable aurous thiosulfate complex according to reaction (9), e- as discussed in the previous study on copper catalyzed Ni3O4 (hydr), 18NH3, H2O system.52) + Au(NH3)2 [Au(NH ) ]þ þ 2S O 2 ! [Au(S O ) ]3 þ 2NH ð9Þ 3 2 2 3 2 3 2 3 Au 2- 2S2O3 The sum of reactions (8) and (9) gives reaction (10), which is
3- the overall gold oxidation reaction in a nickel catalyzed 2NH3, Au(S2O3)2 system. 4Au þ 8S O 2 þ O þ 2H O 2 3 2 2 ð Þ 3 10 ! 4[Au(S2O3)2] þ 4OH Fig. 3 Schematic gold dissolution mechanisms in ammonium thiosulfated Figure 2 shows the Eh-pH diagram of the Au-NH4OH- solution on gold surface using Ni. 2 3 S2O3 system for the reagent combination of 0.5 mol/dm 3 2 5 3 NH4OH, 0.5 mol/dm S2O3 and 5 10 mol/dm Au. as shown in eq. (12): Compared to Fig. 1, the electrode potentials of aurous 2Zn þ [Ni(NH ) ]2þ þ 2S O 2 þ 2H O ammine complex at pH9.5 is located in higher regions than 3 6 2 3 2 2þ 2 those of nickel ammine complex. Thus, it is considered that ! [Zn(NH3)4] þ [Zn(S2O3)2] ð12Þ þ 3 the stable formula of Au is [Au(S2O3)2] rather than þ 2NH3 þ 2OH þ H2 "þNi # [Au(NH ) ]þ in the nickel catalyzed ammonium thiosulfate 3 2 Figure 3 summarizes the schematic electrochemical solution. As well, the higher stability of nickel ion against mechanism of ammonium thiosulfate leaching of gold, which aurous ion suggests minimizing the loss of the catalyst of corresponds to eqs. (4) to (10). nickel ion in the gold cementation by zinc. It means that the reagent cost is reduced to recycle the barren solution to the 3. Experimental gold leaching stage, while the copper catalyzed system co- precipitated the entire catalyst of cupric ion along with gold. 3.1 Gold leaching The chemistry of gold cementation by zinc powder is 3.1.1 Ore sample expressed as eq. (11). Gold ore sample was supplied from Hishikari Mine, 3 2Zn þ 2[Au(S2O3)2] þ 4NH3 þ 2H2O Kagoshima Japan by Sumitomo Metal Mining, Co., Ltd. The 2 2þ total amount of ore sample was 150 kg, and the ore size was ! [Zn(S2O3)2] þ [Zn(NH3)4] ð11Þ inch 15 20 cm (0 8 inch). Those ore was crushed to þ 2S O 2 þ 2OH þ H "þ2Au # 2 3 2 1:70 mm ( 10 mesh) by a jaw crusher (Denver H5902CB), Also, some nickel precipitation by zinc powder could occur a gyratory crusher (Allis-Chalmers) and a crushing roll 520 H. Arima, T. Fujita and W.-T. Yen
Table 3 Assay results of Hishikari ore. 3.1.3 Thiosulfate analysis Au (g/t) Fe (mass%) Cu (mass%) S (mass%) C (mass%) The consumption of thiosulfate ion in a gold pregnant solution was assayed by the iodimetric titration method after 16.26 0.18 0.00 0.00 0.19 the leaching.67) Procedure of the iodimetric titration was as Ore Matrix: Silicate Type follows: (1) After a 10 cm3 of thiosulfate solution was taken into a 3 (Sturtevant Laboratory Roll). The crushed ore was divided 50 cm beaker, pH of the solution is measured. If the pH into 2 kg each using a rotation vibratory feeder. Finally, the is higher than 8, the solution pH was neutralized to be 2 kg crushed ores were passed through a riffle 3 times, and around pH 7 7:5 by 10 vol% H2SO4 solution (The total then 0.3 kg samples were packed into paper bags for leaching added volume of 10 vol% H2SO4 solution should be samples. noted). This neutralized thiosulfate solution was trans- 3 Table 3 shows the results of chemical analysis of the ore. ferred into a 25 cm burette for the iodimetric titration. The ore matrix was silicate. The gold grade was 16.26 g/t by (2) The thiosulfate solution prepared in procedure 1 was 3 the fire assay method.65,66) From a 0.5 g of pulverized sample filtered against 5 cm of I2 solution, containing a 3 3 (100 mass%-75 mm), 0.18 mass% of iron was detected by 2 3 cm of 0.5 g/dm starch solution as indicator, in 3 aqua regia leaching. Copper was not detected. Sulfur and a50cm beaker for iodimetiric titration. The end point carbon contents determined by a sulfur and carbon combus- is reached, when the colour of I2 solution changed from tion analyzer (Leco1 SC-444DR) were 0.00 mass% and deep brown to colour less. The total volume of 0.19 mass%, respectively. thiosulfate solution required for titration was applied 3.1.2 Leaching procedure to evaluate the thiosulfate concentration. 3 For thiosulfate leaching, ammonium thiosulfate, ammo- To analyze a 0:2 0:5 mol/dm thiosulfate solution, a 3 nium hydroxide, copper sulfate and nickel sulfate were used. 0.05 mol/dm iodine solution was used. In the range of a 3 3 For cyanidation, sodium cyanide and calcium hydroxide 0:01 0:2 mol/dm thiosulfate solution, a 0.02 mol/dm were used. All chemicals were reagent grade supplied from iodine solution was used. The iodine solution was prepared 3 3 Aldrich Chemical, Alfa Aesar and EM science. by dissolving 0.05 mol/dm I2 with 40 g of KI in 1 dm flask. A 0.3 kg-crushed sample was ground by a rod mill (Denver The starch solution was made by dissolving a 1.0 g of soluble 3 Sala) with 666 cm3 of water at certain grind time, and the starch in a 500 cm water by heating. The accuracy of the ground sample was filtered by a filter press. The filtered cake iodimietric titration was confirmed using standard ammoni- was cut into quarter, and then the half of sample, composed um thiosulfate solutions. of two opposite quadrants, was used for leaching. The 150 g From the data obtained from the iodimetric titration ground ore was put into a 2 dm3 bottle, and the calculated presented above, thiosulfate concentration was evaluated to amounts of ammonia, ammonium thiosulfate and copper the following equation: sulphate or nickel sulfate were fed into a bottle, and then a 2 MI L1 water was added to make a 33 solids% of slurry. The bottle MT ¼ ð14Þ was rolled on a bottle roller for 24 hours at an ambient V1 L2 temperature. Rotation speed of the bottle was about 60 rpm V2 measured by a VMR digital taco meter. Where, M is thiosulfate concentration [mol/dm3]; M is After the leaching test, the gold pregnant solution was T I iodine concentration [mol/dm3]; L is initial iodine volume filtered using a vacuum pump with a buchner funnel and 1 (In this procedure, 5 cm3 at constant); L is thiosulfate collected for gold assay. The filtered cake (residue) was 2 volume used to titrate iodine solution [cm3]; V is initial washed with a 100 cm3 of water 4 times to wash out all gold 1 thiosulfate solution volume (In this procedure, 10 cm3 at pregnant solution trapped in the residue. The total volumes of constant); V is thiosulfate solution volume [cm3] after pH pregnant and washed solutions were measured by a cylinder. 2 adjustment. The gold concentrations were measured by an atomic Based on the thiosulfate concentration obtained from eq. adsorption spectrophotometer. The gold leached residue (14), the total ammonium thiosulfate consumption during was dried in an oven, and assayed for gold by the fire assay leaching was evaluated to the following: method.65) The percentage of gold extraction was calculated from the total amount of gold in the leached residue and the Ammonium Thiosulfate Consumption [kg/t-ore] pregnant solution as following: 2 148 Vlixviant ½S2O3 consumed ð15Þ Gold Extraction [%] ¼ Wsample ð½Au preg.sol. þ½Au washed.sol.Þ ¼ 100 where, 148 is the molecular weight of ammonium thiosulfate ð½Au preg.sol. þ½Au washed.sol. þ½Au residueÞ at 1 mol [g/mol]; Vlixiviant is total volume of ammonium ð12Þ thiosulfate lixviant utilized in leaching [dm3]; 2 where [S2O3 ]consumed. is total amount of thiosulfate consumed 3 ½Au]preg.sol.: total amount of gold in pregnant solution [mg] [mol/dm ]; Wsample is ore sample weight utilized for leaching ½Au]washed.sol.: total amount of gold in washed solution [mg] [kg]. ½Au]residue: total amount of gold in leached residue [mg] Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction 521
3.2 Gold cementation from ammonium thiosulfate solu- 4.2 Effect of ammonia, nickel and thiosulfate on gold tion extraction and ammonium thiosulfate consumption Zinc powders (supplied from Alfa Aesar) were used for The effects of ammonia, nickel and thiosulfate concen- gold cementation. The average diameter of the powder was trations on gold extraction and thiosulfate consumption were 45 mm and purity was more than 98%. A known amount of investigated at the standard reagents composition of 0.5 mol/ 3 3 3 3 the metal powder was added to a 50 cm pregnant solution of dm NH4OH, 0.005 mol/dm NiSO4 and 0.5 mol/dm the leaching experiments described in the section 3.1, and the (NH4)2S2O3. The reagent concentrations was varied in the 3 suspension was gently agitated by a magnetic stirrer for 30 range of 0:1 0:5 mol/dm NH4OH, 0:0001 0:005 mol/ 3 3 minutes at an ambient temperature under atmosphere. dm NiSO4 and 0:01 0:5 mol/dm (NH4)2S2O3. After the gold cementation, the pulp was filtered imme- Figure 5 shows the effect of ammonia on gold extraction diately with a micro filter. The filtrate was assayed for gold, and ammonium thiosulfate consumption with 0.005 mol/dm3 3 nickel, thiosulfate and ammonia. Gold, nickel and thiosulfate NiSO4 and 0.5 mol/dm (NH4)2S2O3. The gold extraction concentrations were evaluated in the same manner described decreased from 95% to 60% as the ammonia concentration in section 3.1. The ammonia concentration was measured by decreased from 0.5 to 0.1 mol/dm3, while the ammonium Nessler’s method (JIS 0102). Nessler’s method is a semi- thiosulfate consumption increased from 5 to 9 kg/t-ore. This quantitative method, and the standard deviation is around result concludes that the increase of the nickel thiosulfate 3 þ 0:5 mol/dm NH4 /NH3. stability at the lower ammonia concentrations reduced the To investigate the feasibility of recycling the ammonium catalytic effectiveness for gold oxidation. Thus, the best 3 thiosulfate solution, the concentration of ammonia, nickel concentration for gold extraction is 0.5 mol/dm NH4OH. and thiosulfate before and after the gold cementation process Figure 6 shows the effect of nickel concentration on gold were evaluated. extraction and ammonium thiosulfate consumption with 3 3 0.5 mol/dm NH4OH and 0.5 mol/dm (NH4)2S2O3. The 4. Results and Discussion nickel concentration was varied in the range of 0:000 0:005 mol/dm3. The result shows that both gold 4.1 Preliminary gold leaching test extraction and ammonium thiosulfate consumption were Figure 4 shows the gold extraction result from a silicate respectively constant at 95% and 5 kg/t-ore in the range of type ore by using the reagent combination of 0.005 mol/dm3 0:0001 0:005 mol/dm3. At the nickel concentration of 3 3 3 NiSO4, 0.5 mol/dm (NH4)2S2O3 and 0.5 mol/dm NH4OH 0 mol/dm , the gold dissolution was not observed. The at pH 9.5. The gold extraction at 24 hours was 95% and the evidence suggests that nickel may act as a catalyst for gold ammonium thiosulfate consumption was about 5 kg/t-ore, extraction with small thiosulfate consumption. Also, the while 21 kg/t-ore of ammonium thiosulfate was consumed result, in which the gold oxidation was not initiated without 3 52) when 0.001 mol/dm CuSO4 was used as a catalyst. The the presence of a catalyst, confirms the reports from other ammonium thiosulfate consumption with copper catalyst was researchers.37,39) The main cause of thiosulfate consumption about 4 times higher than that with a nickel catalyst. It was in the nickel catalyzed system could be attributed to the obvious that the nickel ion significantly decreased the natural degradation of thiosulfate in the gold leaching thiosulfate consumption. process. The minimum nickel concentration can be selected
100 15
100 80 12
80 60 9
60 40 6 Consumption (kg/t-ore) 3 O Au Extraction (%) 2 S 2 )
40 4 20 3 Au Extraction (%) (NH
20 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0 . -3 NH4OH, M/mol dm 0 5 10 15 20 25 30 35
Retention Time, t /hour Fig. 5 Effect of ammonia concentration on gold extraction and ammonium 3 3 thiosulfate consumption with 0.005 mol/dm NiSO4 and 0.5 mol/dm 3 Fig. 4 Gold extraction kinetics at the condition of 0.5 mol/dm NH4OH, (NH4)2S2O3 on a 100 mass%-75 mm ground ore. Pulp density: 33 mass%, 3 3 0.005 mol/dm NiSO4 and 0.5 mol/dm (NH4)2S2O3 at pH 9.5. Pulp Ambient temperature, Atmospheric condition, Retention Time: 24 hours, density: 33 mass%, Ambient temperature, Atmospheric condition, Rota- Rotation speed: 60 rpm, pH 9.5. : Au extraction, : (NH4)2S2O3 tion speed: 60 rpm. consumption. 522 H. Arima, T. Fujita and W.-T. Yen
100 15 100
80 12 80
minimum Ni2+ conc. 60 9 60 0.0001mol/dm3
40 6 40 Consumption (kg/t-ore) 3 Au Extraction (%) O Au Extraction (%) 2 S 2 ) 4 20 3 20 (NH
0 0 0 0.000 0.002 0.004 0.006 0 5 10 15 20 25 30 35
. -3 Retention Time, t /hour NiSO4-6H2O, M/mol dm
Fig. 6 Effect of nickel concentration on gold extraction and ammonium Fig. 8 Gold extraction kinetics of the nickel catalyzed ammonium 3 3 3 thiosulfate leaching at the condition of 0.5 mol/dm NH4OH, thiosulfate consumption with 0.5 mol/dm NH4OH and 0.5 mol/dm 3 3 0.0001 mol/dm NiSO4 and 0.05 mol/dm (NH4)2S2O3 at pH 9.5. Pulp (NH4)2S2O3 on a 100 mass%-75 mm ground ore. Pulp density: 33 mass%, Ambient temperature, Atmospheric condition, Retention Time: 24 hours, density: 33 mass%, Ambient temperature, Atmospheric condition, Rota- tion speed: 60 rpm. Rotation speed: 60 rpm, pH 9.5. : Au extraction, : (NH4)2S2O3 consumption. consumption remained at 5 kg/t-ore. The 0.05 mol/dm3 100 15 (NH4)2S2O3 is the minimum concentration for achieving higher Au extraction under this reagent composition. From the above results, the optimum reagent combination 80 12 3 3 can be proposed as 0.0001 mol/dm NiSO4, 0.05 mol/dm 3 (NH4)2S2O3 and 0.5 mol/dm NH4OH at pH 9.5. Figure 8 shows the result of gold extraction with this reagent 60 9 composition, which resulted in 95% recovery in 24 hours. The ammonium thiosulfate consumption was 1.2 kg/t-ore,
40 6 while the reagent consumption of copper catalyzed lixiviant Consumption (kg/t-ore) 3 3 O was 3 5 kg/t-ore under 0.0001 mol/dm CuSO , 0.05 mol/ Au Extraction (%) 4 2
S 3 3
2 dm (NH ) S O and 0.5 mol/dm NH OH at pH 9.5 and the ) 4 2 2 3 4 4 20 3 sodium cyanide consumption was about 1.8 kg/t-ore with the (NH 0.02 mol/dm3 (1.0 g/dm3) NaCN at pH 10.5. This exper- imental result demonstrates the advantage of nickel catalyzed 0 0 ammonium thiosulfate leaching.52,53) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 For further confirmation, ammonia and thiosulfate con- (NH ) S O , M/mol . dm-3 3 4 2 2 3 centrations were varied with 0.0001 mol/dm NiSO4. Figure 9 shows that the decrease in the ammonia concen- Fig. 7 Effect of thiosulfate concentration on gold extraction and ammo- 3 3 tration from 0.5 to 0.1 mol/dm causes a decrease in the gold nium thiosulfate consumption with 0.005 mol/dm NiSO4 and 0.5 mol/ 3 dm NH4OH on a 100 mass%-75 mm ground ore. Pulp density: 33 mass%, extraction from 95 to 75% and an increase in the ammonium Ambient temperature, Atmospheric condition, Retention Time: 24 hours, thiosulfate consumption from 1.2 to 3.5 kg/t-ore. The result Rotation speed: 60 rpm, pH 9.5. : Au extraction, : (NH ) S O 3 4 2 2 3 supports that 0.5 mol/dm NH4OH is an optimum concen- consumption. 3 tration with the combination of 0.0001 mol/dm NiSO4 and 3 0.05 mol/dm (NH4)2S2O3. as 0.0001 mol/dm3. Figure 10 shows that the decrease of thiosulfate concen- Figure 7 shows the effect of thiosulfate concentration on tration from 0.05 mol/dm3 to 0.01 mol/dm3 decreased the gold extraction and reagent consumption in the range of gold extraction from 95% to 30%. The result also supports 3 3 0:01 0:5 mol/dm (NH4)2S2O3. Nickel and ammonia con- that 0.05 mol/dm (NH4)2S2O3 is an optimum concentration 3 3 centrations were maintained at 0.005 mol/dm NiSO4 and with the combination of 0.0001 mol/dm NiSO4 and 3 3 0.5 mol/dm NH4OH. The results show that the maximum 0.5 mol/dm NH4OH. Thus, the results of Figs. 9 and 10 gold extraction of 95% with the lowest ammonium thio- confirmed that an optimum reagent combination is 3 3 sulfate consumption of 2 kg/t-ore was obtained at 0.05 mol/ 0.0001 mol/dm NiSO4, 0.05 mol/dm (NH4)2S2O3 and 3 3 dm (NH4)2S2O3. The increase of thiosulfate concentration 0.5 mol/dm NH4OH, which resulted in the 95% of gold 3 from 0.05 to 0.1 mol/dm increased the ammonium thio- extraction with 1.2 kg/t-ore of (NH4)2S2O3 consumption, sulfate consumption from 2 to 5 kg/t-ore. In the range of experimentally. This optimum reagent combination was used 3 0:1 0:5 mol/dm (NH4)2S2O3, the ammonium thiosulfate to construct the Eh-pH diagram as shown in Fig. 11. At the Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction 523
100 6 1.5
↓N11 ↓N7 NiO 80 ↓E2 ↓N10 2 1.0 ↓N6 Ni O 4 ↓N9 2 3 Ni O 60 3 4 0.5 ↓N8 V 2- ↓N5 [Ni(S2O3)2] 40 Eh, Consumption (kg/t-ore) 3 0.0 2+ O [Ni(NH ) ] Au Extraction (%) 2 3 6 2
S NiO 2
) ↓E1 4 20
(NH ↑N2 -0.5 ↑N3 ↑N4 0 0 Ni 0 0.1 0.2 0.3 0.4 0.5 0.6 -1.0 . -3 NH4OH, M/mol dm 468101214 pH Fig. 9 Effect of ammonia concentration on gold extraction and ammonium 3 3 thiosulfate consumption with 0.0001 mol/dm NiSO4 and 0.05 mol/dm 2 Fig. 11 Eh-pH diagram for the Ni-NH3-S2O3 -H2O system at the (NH4)2S2O3 on a 100 mass%-75 mm ground ore. Pulp density: 33 mass%, 3 3 2þ condition of 0.5 mol/dm NH4OH, 0.0001 mol/dm Ni and 0.05 mol/ Ambient temperature, Atmospheric condition, Retention Time: 24 hours, 3 2 dm S2O3 . Nernst Equation (N1) (N11) are shown in Table 2. Rotation speed: 60 rpm, pH 9.5. : Au extraction, : (NH4)2S2O3 consumption.
100 100 6
80 80
60 4 60
40 Au Recovery (%) 40 Consumption (kg/t-ore) 3 O
Au Extraction (%) 2 2 20 S 2 ) 20 4 (NH 0 010203040 0 0 Time, t /min 0.00 0.01 0.02 0.03 0.04 0.05 0.06
. -3 þ (NH4)2S2O3, M/mol dm Fig. 12 Gold cementation by zinc powders at Metal/Au mass ratio of 30 without de-aeration from the pregnant solution contained 0.0001 mol/dm3 2þ 3 2 3 5 Fig. 10 Effect of thiosulfate concentration on gold extraction and Ni , 0.05 mol/dm S2O3 , 0.5 mol/dm NH4OH and 5 10 mol/ 3 3 þ ammonium thiosulfate consumption with 0.0001 mol/dm NiSO4 and dm Au . 3 0.5 mol/dm NH4OH on a 100 mass%-75 mm ground ore. Pulp density: 33 mass%, Ambient temperature, Atmospheric condition, Retention Time: 24 hours, Rotation speed: 60 rpm, pH 9.5. : Au extraction, : 4.3 Gold cementation and solution recycling (NH4)2S2O3 consumption. Figure 12 shows the result of gold recovery from a nickel catalyzed thiosulfate pregnant solution, which was obtained pH of 9.5, which was the gold leaching condition in Fig. 8, under an optimum leach condition of 0.0001 mol/dm3 3 3 the stable region of the nickel ammine complex was wider NiSO4, 0.05 mol/dm (NH4)2S2O3 and 0.5 mol/dm than that of Fig. 1. This is due to the decrease of nickel and NH4OH, by zinc powder cementation. The 97% of the gold thiosulfate concentrations from 0.005 to 0.0001 mol/dm3 and was recovered in 10 minutes with a Zn/Auþ mass ratio of 30. from 0.5 to 0.05 mol/dm3, respectively. The equilibrium The final nickel concentration did not change significantly electrode potential for gold to form an aurous ammine and only 0:00001 0:00005 mol/dm3 of Ni2þ was lost, complex was higher than that of nickel as compared to in while the entire copper ion was co-precipitated along with Figs. 2 and 11. The result supports that metallic gold was gold in the zinc precipitation from a copper catalyzed 3 2) oxidized to thiosulfate complex ([Au(S2O3)2] ), while the ammonium thiosulfate solution. The stability of nickel in nickel ammine complex was stable enough to act as a catalyst the ammonium thiosulfate solution during zinc precipitation in eqs. (4) (10). suggests that the nickel catalyzed ammonium thiosulfate leaching has an advantage against the copper catalyzed ammonium thiosulfate leaching process for minimizing the 524 H. Arima, T. Fujita and W.-T. Yen cost of reagent loss. maintained at 0.0001 mol/dm3. The feasibility of recycling the nickel catalyzed thiosulfate Table 5 shows that the optimum reagent combinations for 3 barren solution to the leaching stage followed by zinc the ores were 0:1 0:3 mol/dm (NH4)2S2O3 and 3 4 mol/ 3 3 precipitation was also evaluated. When the barren solution dm NH4OH with 0.0001 mol/dm NiSO4. Overall, a higher was recycled to the leaching stage, the solution composition ammonia concentration was required for the ores containing was readjusted based on the analytical results of nickel and copper bearing sulfide minerals as compared to the silicate thiosulfate consumption. After four stages of recycling test, ore because the dissolved copper content from the sulfide around 95% of gold was successively extracted with only mineral caused the thiosulfate consumption and retarded the 1 3 kg/t-ore of ammonium thiosulfate consumption. About gold extraction at the lower ammonia concentration. The blue 30 50% of nickel ion was replenished in each stage. color leach solution was the obvious evidence of copper Ammonia concentration was kept at the same value during dissolution. The gold extractions in thiosulfate leaching were the gold cementation process. This confirms that recycling larger than the results of cynidation at 1.0 g/dm3 (0.02 mol/ the barren solution followed by zinc precipitation would not dm3) NaCN, but the reagent consumptions in thiosulfate affect the gold extraction and the thiosulfate consumption. leaching were obviously higher than that for cyanidation. For In terms of the cementation using copper or aluminum the copper bearing ore of sample A and the massive sulfide powders, it was confirmed that a dissolved copper ion caused ore of sample D, 20 and 24 kg/t-ore of ammonium thiosulfate the thiosulfate consumption, and that aluminum was insolu- consumption occurred respectively, while 3 4 kg/t-ore ble at low ammonia (0.3 mol/dm3) and thiosulfate (0.05 mol/ NaCN was consumed with cyanidation. When copper was dm3). The stability of copper and aluminum in an ammonium used as a catalyst, thiosulfate consumption was 25 kg/t-ore thiosulfate solution has been discussed in our previous and 30 kg/t-ore respectively for samples A and D. For study.52,53) Thus, those processes were not feasible. samples B and C, the thiosulfate consumption was 8 and 9 kg/t-ore respectively with nickel as a catalyst, while 13 and 4.4 Extraction of gold ore containing sulfide minerals 10 kg/t-ore (NH4)2S2O3 was consumed respectively with In this section, the optimum reagent combination of nickel copper as a catalyst. The gold extraction by thiosulfate with catalyzed ammonium thiosulfate leaching was investigated either copper or nickel as catalyst was relatively the same. on four gold ores containing iron and copper sulfide minerals. However, the gold extraction by cyanidation was 8 21% The 24 hour standard cyanidation process at 1.0 g/dm3 lower than that by thiosulfate leaching. (0.02 mol/dm3) NaCN was also conducted to compare with the thiosulfate leaching. Table 4 shows the assay result of 5. Conclusion four gold ore samples A, B, C and D. Three of the ores (A, B and C) contained 1:5 2:5 mass% S, 0:01 0:30 mass% Cu Nickel catalyzed ammonium thiosulfate leaching test was and 4 6 mass% Fe. Ore D contained 50 g/t Au, 30 mass% S, carried out to investigate the optimum reagent combination 28 mass% Fe and 0.1 mass% Cu. In the thiosulfate leaching on the variable types of ore at 100 mass%-200 mesh (75 mm). test, the thiosulfate concentration was varied in the range of The optimum reagent combination was found as 0.0001 mol/ 3 3 3 3 0:05 0:4 mol/dm . The ammonia concentration was varied dm NiSO4, 0.5 mol/dm NH4OH and 0.05 mol/dm 3 in the range of 0:5 4 mol/dm . The nickel concentration was (NH4)2S2O3 for silicate type ore studied. Gold extraction was 95% with a 1.2 kg/t-ore of ammonium thiosulfate consumption. At an optimum reagent composition, zinc Table 4 Assay results of gold ores contained iron and copper sulfide powder was the best media for gold cementation at nearly minerals. 100% of gold recovery. About 30 50% of nickel was co- precipitated along with gold by the cementation of zinc Au Cu Fe S C Sample power. In the barren solution recycling test followed by (g/t) (mass%) (mass%) (mass%) (mass%) cementation, the 4 stages of the solution recycling with nickel A 25 0.3 5.2 2.5 0.8 addition could give 95% gold extraction with 1 3 kg/t-ore of B 7 0.01 6.4 1.7 0.3 ammonium thiosulfate consumption successively. For the C 3 0.1 4.3 1.8 0.5 thiosulfate leaching of gold ores contained sulfide minerals, D 50 0.1 28.2 30 0.5
Table 5 Comparison of the gold extraction and the reagent consumption between the ammonium thiosulfate leaching and the standard cynidation at 1.0 g/dm3 (0.02 mol/dm3) NaCN for 24 hours gold leaching test.
Optimum reagent combination Au recovery (%) Reagent consumption (kg/t-ore)
Cyanidation (NH4)2S2O3 (NH4)2S2O3 (NH4)2S2O3 NH4OH NiSO4/CuSO4 Thiosulfate at Sample Ni Cu NaCN (mol/dm3) (mol/dm3) (mol/dm3) Leaching 1.0 g/dm3 catalyzed catalyzed NaCN A 0.2 3 0.0001 98 90 20 25 3.0 B 0.2 3 0.0001 96 88 8 13 2.0 C 0.1 3 0.0001 95 81 9 10 3.4 D 0.3 4 0.0001 77 56 25 30 4.0 Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction 525 the requirement of higher ammonia concentration and the (1997) 1–9. result of higher thiosulfate consumption were unavoidable. 23) R. Mentha-Biney, K. J. Reid and M. T. Hepworth: Minerals & Metallurgical Processing 14(1) (1997) 7–13. The ammonium thisulfate consumption in nickel catalyzed 24) L. Chai, M. Okido and W. Wei: Hydrometallurgy 53 (1999) 255–266. lixiviant was 1 5 kg/t-ore less than that in copper catalyzed 25) T. Jiang: PhD Thesis, (Central South Institute of Technology, China, lixiviant. 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