Ammoniacal thiosulfate leaching of refractory oxide gold ore

Accepted Manuscript Ammoniacal thiosulfate leaching of refractory oxide gold ore

Esmaeil Mohammadi, Mehdi Pourabdoli, Mehdi Ghobeiti-Hasab, Akbar Heidarpour PII: DOI: Reference:

S0301-7516(17)30093-5 doi: 10.1016/j.minpro.2017.05.003 MINPRO 3045

To appear in:

International Journal of Mineral Processing

Received date: Revised date: Accepted date:

21 November 2015 11 September 2016 11 May 2017

Please cite this article as: Esmaeil Mohammadi, Mehdi Pourabdoli, Mehdi GhobeitiHasab, Akbar Heidarpour , Ammoniacal thiosulfate leaching of refractory oxide gold ore, International Journal of Mineral Processing (2017), doi: 10.1016/j.minpro.2017.05.003

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ACCEPTED MANUSCRIPT Ammoniacal Thiosulfate Leaching of Refractory Oxide Gold Ore *

Esmaeil Mohammadia, Mehdi Pourabdolia, , Mehdi Ghobeiti-Hasabb, Akbar Heidarpoura a

Department of Metallurgy and Materials Engineering, Hamedan University of Technology, Hamedan, Iran

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Department of Metallurgy and Materials Engineering, Dezful Branch, Islamic Azad university, Dezful, Iran

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Abstract Ammoniacal thiosulfate leaching of refractory oxide gold ore was investigated. According to X-ray fluorescence and fire assay analyses, the ore contained about 33.01 wt.% Si, 8.53 wt.% Al, 7.26 wt.% K, 3.00 wt.% Fe, and 2.80 ppm Au. Moreover, X-ray diffraction analysis and mineralogical studies using polished thin sections showed that the ore was composed of quartz, epidote, muscovite, and orthoclase as major minerals and goethite and jarosite as minor minerals. The effects of temperature, leaching time, and the concentrations of thiosulfate, copper ion, and ammonia on the gold extraction were studied. Maximum gold extraction of 55% was obtained using thiosulfate, ammonia, and copper sulfate concentrations of 0.1M, 3M, and 0.0125M, respectively. This extraction value was achieved after room temperature leaching of a pulp with a density of 20% for 16 h. Stirring speed and the pH of the aqueous solution were 400 rpm and 10, respectively. Leaching rate using the preceding optimum conditions showed a transition after a leaching time of 2 h indicating a change in the process mechanism. Keywords: Thiosulfate leaching, Gold ore, Non-cyanide leaching, Sarigunay mine

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1. Introduction Sarigunay is an epithermal gold deposit in Iran with an estimated source of 52 million metric tons of oxide ore at an average grade of 1.77 g/t Au. It is located about 40 km northeast of Qorveh town in Kurdistan province (Bartram, 2005; Richards et al., 2006). For more than one hundred years, cyanidation has been used as the predominant process for gold extraction from mineral sources. The major reason for adopting cyanide rather than other lixiviants is its higher chemical stability and lower cost (Keskinen, 2013; Oferi-Sarpon and Osseo-Asare, 2013). However, the toxic nature of cyanide compounds, problems associated with cyanidation of ores containing carbonaceous materials (carbon in the ore *

Corresponding author. E-mail address: [email protected] (M. Pourabdoli)

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removes gold from the cyanide solution), and the high consumption of cyanide due to the formation of metal cyanide species (e.g. copper cyanide) in the presence of impurities such as Cu, As, Sb, Zn, and Ni have favored the application of non-cyanide processes. On this basis, several non-cyanide leaching methods have been proposed for the gold extraction. For example, chloride, thiourea, and thiosulfate methods have been widely studied in the past few decades. Chloride method relies on using hazardous and corrosive chlorine; thus requires appropriate corrosion resistant equipments. Furthermore, its reaction selectivity for impurities is poor (Marsden and House, 2009; Ghobeiti Hasab et al., 2014, 2013; Hashemzadehfini et al., 2011; Gobeiti Hasab et al., 2014). In comparison with the cyanidation process, thiourea method is more costly. This is due to the high price of the leachant and its high consumption (Örgül and Atalay, 2002). Thiosulfate leaching, on the other hand, is an attractive method which involves reduced environmental risks, high reaction selectivity, and moderate investment (Jeffrey et al., 2003). Because of its non-toxic nature, acceptable gold leaching rates, and high recovery of gold from sulfide ores containing copper and carbonaceous materials, thiosulfate gold leaching is receiving everincreasing attention (Feng and van Deventer, 2010). Recently, thiosulfate leaching has been used for leaching of the refractory gold concentrate calcine (Xu et al., 2015). One of the main reagents for the thiosulfate leaching of gold is copper (II) ion. The major positive role of cupric ion during thiosulfate leaching is to catalyze the gold dissolution. Nevertheless, copper ion act as oxidant, accelerates the degradation of the thiosulfate, and increases its consumption during the process. This makes it necessary to introduce additives to stabilize copper in thisulfate solutions. Ammonia has been used as an additive to stabilize copper ion in thisulfate solutions (Rath et al., 2003). Besides, ammonia is an integral part of thiosulfate leaching of gold as it prevents the thiosulfate decomposition in aqueous solutions. In the absence of ammonia, thiosulfate decomposes and forms a sulfur coating on the surface of ore particles which leads to the gold passivation. Furthermore, ammonia prevents the dissolution of iron oxides, silica, silicates, and carbonates which are the most common gangue minerals in gold ores (Orbay, 2009). A possible mechanism for the thiosulfate leaching reactions is as follows (Yu et al., 2015): Au+5S2O32-+Cu(NH3)42+=Au(S2O3)23-+4NH3+Cu(S2O3)354Cu(S2O3)35-+16NH3+O2+2H2O=4Cu(NH3)42++12S2O32-+4OH-

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ACCEPTED MANUSCRIPT To the best knowledge of the authors, the ammoniacal thiosulfate leaching of Sarigunay refractory gold ore has not been studied yet. In this study, the effects of thiosulfate, ammonia, and copper ion concentrations as well as leaching parameters such as temperature and time on the extraction are studied.

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2. Material and methods The gold-bearing ore sample was obtained from the Sarigunay mine. The ore was crushed to -10 mm by a jaw crusher (Retsch BB 200). Subsequently, the ore was milled by a planetary ball mill (Retsch PM 100). Screen analysis by sieve shaker (Retsch AS 400) showed that the d80 value of the sample after ball milling was 100 µm, namely 80 wt.% of the sample particles were smaller than 100 µm. Cyanide leaching test (Marsden and House, 2009) was performed to determine the refractory nature of the ore. For this purpose, 10 g of ball milled ore was dissolved for 2 h in a solution with a sodium cyanide (≥95%, Merck) concentration of 3 g/L, a pH of 10.5, a stirring speed of 400 rpm, and a liquid to solid ratio of 5 at 50°C. For the thiosulfate leaching tests, a weighed amount of sodium thiosulfate pentahydrate (≥ 97%, Merck) was dissolved in distilled water. Subsequently, the determined volume of ammonia solution (25%, Merck) and anhydrous copper sulfate (≥99%, Merck) were added to the solution. Experiments were carried out in a 200 mL glass reactor located in a water bath equipped with a thermometer. A magnetic stirrer-heater (IKA, RH Basic 2) was used for heating and stirring the solution. The initial pH of leaching solutions was adjusted at 10 according to Aylmore (2001) findings. Also, pulp density of 20% and stirring speed of 400 rpm were used in experiments. Sodium hydroxide (≥99%, Merck), hydrochloric acid (37%, Merck), and a digital pH meter (HANNA, HI-2210) were used for adjusting and measuring the pH. The gold concentration in solutions was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Varian Vista-PRO). X-ray powder diffraction (XRD, Philips X’pert pro diffractometer) with Cu-Kα radiation, X-ray fluorescence (XRF, Philips PW1480), scanning electron microscopy (SEM, CamScan MV2300), and fire assay analyses were utilized for gold ore characterizing. Table 1 shows the chemical analysis of Sarigunay refractory oxide gold ore. The gold content of the ore was determined as 2.8 ppm by fire assay method. The XRD pattern of the ore is seen in Fig.1. As it is seen, the gold ore has an oxide nature and quartz, epidote, muscovite and orthoclase are major minerals in the ore. Moreover, the mineralogical studies using polished and thin sections under an optical microscopy confirmed that quartz and epidote were main phases, while oxides such as goethite and jarosite were minor phases.

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Table 1 Fig.1

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Fig.2 shows the SEM image of polished section of the ore. Early brassy pyrite and arsenical pyrite (probable host to gold) located in the quartz background are seen in the image.

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3. Results and discussion 3.1. Cyanide leaching test Cyanide leaching test was performed to determine the leachable portion or the refractory nature of the ore. The result showed that 74% of gold could be readily leached by cyanidation. Therefore, it was concluded that the ore is refractory because the gold extraction by cyanidation process is less than 80% (Marsden and House, 2009).

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3.2. Effect of thiosulfate concentration The effect of thiosulfate concentration on gold extraction is shown in Fig.3. The gold extraction value was increased to about 40% using 0.1M thiosulfate after 2 h leaching. Increase of the thiosulphate concentration in the solution accelerates the reaction between copper (II) and thiosulphate. In higher thiosulfate concentrations, i.e. 0.1-0.4M, the gold extraction value approximately was fixed. This is due to formation of some unfavorable products such as tetra-thionate, penta-thionate, tri-thionate, sulfite, and sulfate which consume thiosulphate and decline the gold dissolution (Rath et al., 2003; Yu et al., 2015; Zhang et al., 2004). Above all, the 0.1M tiosulfate was considered as a proper concentration of tiosulfate for next experiments. Fig. 3

3.3. Effect of copper ion concentration The effect of copper ion concentration on the gold extraction was investigated. The results are depicted in Fig.4. It is evident that gold extraction increases significantly with increase in copper ion concentration up to 0.0125M. Equation 3 shows the role of copper ion in the form of tetra amine complex on gold oxidation (Feng and van Deventer, 2010, 2007; Rath et al., 2003):

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Au+5S2O32-+Cu (NH3)42+=Au)S2O3)23-+4NH3+Cu (S2O3)35-

(3)

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Cu2++S2O32-+H2O=Cu+SO42-+2H+

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With increase in the copper ion concentration from 0.0125 to 0.05M, the gold extraction increases slightly because of less stability of Cu(NH3)42+species and more stability of solid copper compounds such as CuO, Cu 2O, and Cu2S in this range of copper ion concentration (Zhang et al., 2004; Abbruzzese et al., 1995; Webster, 1984). The formation of these compounds reduces the copper ion concentration in the leaching solution and thus declines the gold extraction. On the other hand, copper ion is an oxidant which elevates the solution potential and subsequently accelerates the thiosulfate oxidation (degradation) according to equation 4 (Orbay, 2009). It seems that the optimum value of copper ion concentration is 0.0125M. This value is close to the findings by other researchers (Yu et al., 2015; Zhang et al., 2004; Feng and van Deventer, 2007; Aylmore et al., 2007; Abbruzzese et al., 1995). (4)

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3.4. Effect of ammonia concentration The effect of ammonia concentration on gold extraction is shown in Fig.5. It is seen that gold extraction increases to about 40% with increase in ammonia concentration up to 3M. This is due to the stability of copper ion in the form of Cu(NH3)42+ according to equation 5. Cu2++ 4NH3 =Cu(NH3)42+

(5)

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With increase in the ammonia concentration from 3 to 4M, the pH increases to high levels (more than 12). In the high pH values, stability regions of Cu(NH3)42+and Cu(S2O3)35- are narrowed, while stability regions of solid copper compounds such as CuO,Cu2O,Cu2S are extended (Webster, 1984; Zipperian and Raghavan, 1988; Tripathi et al., 2012; Jeffrey, 2001).Therefore, the gold extraction value approximately was fixed in the range of 3-4M ammonia. Fig.5 3.5. Effect of temperature Effect of temperature on gold extraction is shown in Fig.6. The gold extraction increases when the temperature increases up to 40°C, while in higher temperatures the gold extraction value decreases. In enough amounts

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ACCEPTED MANUSCRIPT of Cu (II) ion, the leaching reaction is chemically controlled (Jeffrey, 2001). Therefore, with raising the temperature up to 40°C, the gold extraction was improved and reached to about 57%. The diminished gold extraction at temperatures higher than 40°C may be ascribed to the promotion of thiosulfate ion passivation by copper ion and their consumption during equation 6 (Webster, 1984). Cu2++S2O32-+H2O=CuS+SO42-+2OH-

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Also, higher temperature causes to increasing the ammonia volatilization and loss. According to some research works (Keskinen, 2013; Orbay, 2009), a temperature between 25°C and 55°C in the thiosulfate system is suitable to achieve the best result of gold dissolution and prevention of both ammonia losses and copper reduction.

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3.6. Effect of time The ammoniacal thiosulfate leaching of the ore was investigated as a function of time. The results are given in Fig.7. As it is seen, the gold extraction increases with increase in time and attains 40% after 2 h leaching. The effective dissolution is achieved by a sufficient amount of thiosulfate concentration and none of ash layer around the particles. For these reasons, high dissolution rate was achieved in the first 2 hours. Then, from 2-16 h, dissolution rate declined due to consumption of thiosulfate concentration and probably the formation of ash layer around particles and thus controlling the reaction by product layer diffusion mechanism (Aazami et al., 2014; Navarro et al., 2002; Aylmore, 2001). Therefore, it can be said that a transition after a leaching time of 2 h indicates a change in the process mechanism. According to Fig.7, the gold extraction was reached to about 55% after 16 h leaching.

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4. Conclusions According to the results of the present work, the following conclusions were obtained: 1-Gold extraction was increased with increase in thiosulfate concentration up to 0.1 M after 2 h leaching. Improvement in gold extraction was not observed using more than 0.1M thisulfate concentration.

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2-Gold extraction was increased with increase in copper ion concentration up to 0.0125M. Excess amounts of copper ion caused the gold extraction to increase slightly. 3-Gold extraction was increased with increase in ammonia concentration from 0 to 3M. Beyond that, increasing the gold extraction value was negligible. 4-Gold extraction increased with increase in temperature up to 40°C. Excess increasing the temperature up to 70°C caused the gold extraction to decline. 5-Leaching rate using the optimum conditions showed a transition after a leaching time of 2 h indicating a change in the process mechanism. 6-Maximum gold extraction of 55% was obtained after 16 h using thiosulfate, ammonia, and copper sulfate concentrations of 0.1M, 3M, and 0.0125M, respectively.

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Acknowledgments The authors would like to thank A. Nasri, the late F. Anvari Farah, and H. Noor Baghayifor kindly helping in experimental works. This research was supported by research grants obtained by Hamedan University of Technology.

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Ghobeiti Hasab, M., Rashchi, F., Raygan, Sh., 2014.Chloride-hypochloride leaching and hydrochloric acid washing in multistage for extraction of gold from a refractory concentrate. Hydrometallurgy 142, 56-59. Hashemzadehfini, M., Ficeriova, J., Abkhoshk, E., Karimi, B., 2011.Effect of mechanical activation on thiosulfate leaching of gold from complex sulfide concentrate.Trans. Nonferrous Met. Soc. China 21 (12), 2744-2751. Jeffrey, M.I., 2001.Kinetic aspects of gold and silver leaching in ammonia– thiosulfate solutions. Hydrometallurgy 60 (1), 7-16. Jeffrey, M.I., Breuer, P.L., Chu, C.K., 2003.The importance of controlling oxygen addition during the thiosulfate leaching of gold ores. Int. J. Miner. Process. 72 (1–4), 323-330. Keskinen, S., 2013.Comparison of cyanide and thiosulfate leaching for gold production. BSc Thesis, Faculty of technology, Lappeenranta University of Technology. Marsden, J.O., House, C.I., 2009.The Chemistry of Gold Extraction, second ed. SME, USA. Navarro, P., Vargas, C., Villaroel, A., Alguacil, F.J., 2002.On the use of ammoniacal/ammonia-thiosulfate for gold extraction from a concentrate. Hydrometallurgy 65 (1), 37-42. Oferi-Sarpong, G., Oseo-Asare, K., 2013.Preg-robbing of gold from cyanide and non-cyanide complex: effect of fungi pretreatment of carbonaceous matter. Int. J.of Miner.Process. 119, 27-33. Oraby, E.A., 2009.Gold leaching in thiosulfate Solution and its environmental effect compared with cyanide. PhD Thesis, Department of Civil Engineering, Curtin University of Technology. Orgul, S., Atalay, U., 2002.Reaction chemistry of gold leaching in thiourea solution for a Turkish gold ore. Hydrometallurgy 67 (1–3), 71–77. Rath, R.K., Hiroyoshi, N., Tsunekawa, M., Hirajima, T., 2003.Ammoniacal thiosulfate leaching of gold ore. Eur. J. Mine. Process. Environ. Prot. 3 (3), 344-352. Richards, J.P., Wilkinson, D., Ulrich, T., 2006.Geology of the Sari Gunay epithermal gold deposit-northwest Iran.Econ.Geol. 101 (8), 1455–1496. Tripathi, A., Kumar, M., Sao, D.C., Agrawal, A., Chakaravarty, S., Mankhand, T.R., 2012.Leaching of gold from the waste mobile printed circuit boards (PCBs) with ammonium thiosuifate.Int. J. Metall. Eng. 1 (2), 17-21. Webster, J.G., 1984. Thiosulfate complexing of gold and silver during the oxidation of a sulfide–bearing carbonate lode system, upper ridges mine, PNG.Australian Institute of Mining and Metallurgy.Gold Mining, Metallurgy and Geology, Symposium (Kalgoorlie). Xu, B., Yang, Y., Jiang, T., Li, Q., Zhang, X., Wang, D., 2015.Improved thiosulfate leaching of a refractory gold concentrate calcine with additives. Hydrometallurgy 152, 214–222.

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Yu, H., Zi, F., Hu, X., Nie, Y., Xiang, P., Xu, J., Chi, H., 2015.Adsorption of the gold-thiosulfate complex ion onto cupric ferrocyanide (CuFC)impregnated activated carbon in aqueous solutions, Hydrometallurgy154, 111-117. Zhang, X.M., Senanayake, G., Nicol, M.J., 2004. A study of the gold colloid dissolution kinetics in oxygenated ammoniacal thiosulfate solutions. Hydrometallurgy 2004, 74(3-4), 243-257. Zippeian, D., Raghavan, S., 1988.Gold and Silver extraction by ammoniacal thiosulfate leaching from a rhyolite ore. Hydrometallurgy 19(3), 361-375.

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List of Tables Table 1- Chemical analysis of the refractory oxide gold ore

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List of Figures Fig.1-XRD pattern of the refractory oxide gold ore Fig.2-SEM image of polished section of the sample Fig.3-Effect of thiosulfate concentration on gold extraction Fig.4-Effect of copper ions concentration on gold extraction Fig.5-Effect of ammonia concentration on gold extraction Fig.6-Effect of temperature on gold extraction Fig.7-Effect of leaching time on gold extraction

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Table 1- Chemical analysis of the refractory oxide gold ore Si

Al

Fe

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K

Mg

Ti

Mn

P

S

Au

Wt. %

33.01

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3.04

0.13

0.02

7.26

0.24

0.27

0.22

0.07

0.04

2.8 ppm

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Element

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Graphical abstract

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Highlights: 1- Various parameters of ammoniacal thiosulfate leaching of refractory oxide gold ore were studied on gold extraction. 2- The maximum gold extraction of 55% was achieved.

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3- A transition in the process mechanism after a leaching time of 2 h was observed.

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