Eco-friendly and economical gold extraction by nickel catalyzed ammoniacal thiosulfate leaching-resin adsorption recovery
Introduction
Cyanidation has been the predominant process used for gold leaching from its ores/concentrates because of its simple process and mature technology (Hilson and Monhemius., 2006). However, at present more attention has been paid to non-cyanide processes, such as thiosulfate, thiourea, chloride and thiocyanate because cyanide is highly toxic and can pose potential threats to the environment and human health in the actual production (Jeffrey et al., 2001, Xu et al., 2016a). Among them, thiosulfate is widely recognized as the most hopeful replacement to cyanide (Alzate et al., 2017, Bisceglie et al., 2017, Lu et al., 2017, Muir and Aylmore, 2004, Zhang and Xu, 2016). Actually, the rate of thiosulfate leaching of gold is extremely slow without adding catalyst (Ter-Arakelyan et al., 1984). Cupric ion was found to have a strong catalysis for gold leaching with the addition of ammonia which helped to stabilize copper (II) in the form of [Cu(NH3)4]2+ in alkaline media (Abbruzzese et al., 1995, Breuer and Jeffrey, 2000, Chen et al., 1996, Jeffrey, 2001, Zipperian et al., 1988).
However, thiosulfate is a metastable anion that is prone to break down in ammoniacal thiosulfate solution, which can be considerably accelerated by [Cu(NH3)4]2+ (Jiang et al., 1993, Yang et al., 2019). Undoubtedly, this will cause a high thiosulfate consumption, which is one of the obstacles for the industrial application of thiosulfate leaching (Xu et al., 2017a). Moreover, large quantities of sulfur-containing species, such as sulfite, sulfate, sulfide, polythionates (SxO62−) and polysulfides (Sx2−) will be generated (Breuer and Jeffrey, 2003, Feng and van Deventer, 2002, Grosse et al., 2003, Senanayake, 2004, Xu et al., 2018). The presence of them in the leach solutions is harmful to gold leaching because they can form a compact passivation coating on gold surface, thus resulting in impeding the gold leaching (Jeffrey et al., 2008, Mirza et al., 2015).
For traditional copper-ammonia catalyzed leaching, gold recovery from leach solution is also difficult due to the presence of a huge amount of disturbing ions (Dong et al., 2017). Over the past decades, several recovery methods, including resin adsorption, activated carbon adsorption, mesoporous silica adsorption, solvent extraction, precipitation and electrowinning, have been reported in many literature (Jeffrey et al., 2010, Navarro et al., 2007, Fotoohi and Mercier, 2014, Zhao et al., 1999, Sullivan and Kohl, 1997, Choo and Jeffrey, 2004). Among them, resin adsorption has been widely considered to be the most suitable (Arima et al., 2003, Dong et al., 2019, Nicol and O'Malley, 2002, Jeffrey et al., 2010). Its advantages include high gold loading capacity, fast adsorption speed, low requirements for solution clarity and easy regeneration of resin at ambient temperature (Grosse et al., 2003). However, the competitive adsorption of undesirable anions predominantly including [Cu(S2O3)3]5- and SxO62− with [Au(S2O3)2]3- on resin surface is inevitable (Dong et al., 2017, Zhang and Dreisinger, 2004). As a result of this, gold adsorption on the resin is disturbed and its desorption from the loaded resin also becomes complicated. Therefore, a high cost for gold recovery is required, which is another problem that limits the industrial application of thiosulfate leaching.
In order to solve the abovementioned problems, a potential countermeasure is to displace copper-ammonia catalysis with other catalyses. Copper-ethanediamine catalysis assisted with synergist (cetyltrimethyl ammonium bromide) was used for gold leaching from its ore, where 94.3% of gold could be leached, while thiosulfate consumption was only 1.12 kg/t-ore (Yu et al., 2014). Nevertheless, the toxicity of ethanediamine has restricted the application of this new catalysis. Ferric-oxalate and ferric-EDTA have also been adopted to replace traditional copper-ammonia catalysis (Heath et al., 2008). A key advantage of these two new catalyses is that thiosulfate consumption was negligible, but the gold leaching rate was very low if thiourea was not added. Moreover, the gold leaching was remarkably hampered in the presence of pyrite and pyrrhotite in actual leach slurry, and the possible reason is that these sulfide minerals catalyzed the oxidation of thiosulfate by iron(III), leading to the lack of iron(III) oxidant. A novel multi-staged process for extracting valuable metals from a complex polymetallic sulfide concentrate has been proposed in our previous study (Xu et al., 2016b). After oxygen pressure leaching with sulfuric acid of copper and zinc, and silicofluoric acid leaching of lead, the remained gold and silver in the residue were extract with nickel and copper catalyzed ammoniacal thiosulfate solutions. The thiosulfate consumption with nickel-ammonia catalysis was significantly lower than that with copper-ammonia catalysis, and the gold leaching percentages under these two catalyses were comparable. Nickel-ammonia was also applied to catalyze the gold leaching from a silicate-type gold ore by thiosulfate. Gold leaching percentage could reach 95%, whilst thiosulfate consumption was as low as 1.2 kg/t-ore (Arima et al., 2004). Furthermore, nickel barely loaded on resin surface in the simulated leach solution, and thus the loaded gold could be desorbed by a simple one-step process (Arima et al., 2003). From the above, an evident decrease of thiosulfate consumption can be realized by the displacement of traditional copper-ammonia with nickel-ammonia, and furthermore gold recovery by using resin adsorption method from synthetic leach solution is also facilitated. However, to the best of our knowledge, study on the kinetics of thiosulfate leaching of gold with nickel-ammonia catalysis and the recovery of gold from actual leach solution by ion-exchange resin have not been reported.
In this work, firstly, the catalytic mechanism of nickel-ammonia was presented based on the thermodynamic analysis, and the feasibility of this catalysis was demonstrated by comparing with traditional copper-ammonia catalysis for thiosulfate leaching of gold from a gold concentrate calcine. Then, the kinetics study of gold leaching by thiosulfate with nickel-ammonia catalysis was performed, where the reaction orders of reagents and apparent activation energy were calculated. Finally, gold recovery from the obtained actual leach solutions using ion-exchange resin was carried out to further demonstrate the advantages of nickel-ammonia catalysis.
Section snippets
Materials
Prior to gold leaching from its ores or concentrates, oxidative pretreatment is generally required to eliminate the adverse effect of harmful elements in the ores (Xu et al., 2016a). From all the pretreatment methods, oxidative roasting has received the most attention owing to its mature technology, high efficiency and moderate investment, and has been extensively used in China to pretreat gold ores in the past few decades (Michelis et al., 2013). Thus, gold concentrate calcine is a
Thermodynamic analysis
Thermochemical software HSC Chemistry 6.0 was adopted to draw the Eh-pH diagram of Ni–NH3–S2O32–H2O system as presented in Fig. 1, where the used thermodynamic data were from the literature (Smith et al., 1998). It is well-known that thiosulfate leaching of gold is generally conducted at pH 9–11. From the figure, it can be inferred that at this pH range the redox couple of Ni3O4/[Ni(NH3)6]2+ may act as the catalytic role during gold leaching, which is similar to the role of the [Cu(NH3)4]2+
Conclusions
In this work, a clean and economical hydrometallurgical process was developed to leach and recover gold from a gold concentrate calcine. Our major findings can be summarized as follows:
- (1)
Based on the thermodynamic analysis, the catalytic mechanism of nickel-ammonia for gold leaching in thiosulfate solution was put forward: the anodic dissolution of gold was catalyzed by ammonia, and the cathodic reduction of oxygen was catalyzed by nickelous nickelic oxide. Nickel catalyzed ammoniacal thiosulfate
Acknowledgments
The authors wish to express their thanks to the National Natural Science Foundation of China (Nos. 51504293 and 51574284), the China Postdoctoral Science Foundation (No. 2014M550422), Qinghai Provincial Major Scientific and Technological Special Project of China (No. 2018-GX-A7), Hunan Provincial Natural Science Foundation of China (Nos. 2015JJ3149 and 2018JJ4038), the Fundamental Research Funds for the Central Universities of Central South University (No. 150110003) and the Open-End Fund for
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