Thiosulfate Consumption Research Articles

Effect of Nickel Impurities in Pyrite on Catalytic Degradation of Thiosulfate

The effects of nickel content in nickel-bearing pyrite on photocatalytic properties, light absorption properties, and oxidative decomposition of thiosulfate were studied. The leaching experiments show that the consumption of thiosulfate in the Cu2+-ethylenediamine (en)-S2O32− system increases with an increase in nickel content in nickel-bearing pyrite. The consumption of Cu(en)22+ initially increases and then decreases with an increase in leaching time. There is a clear correlation between the change trend in its consumption and the doping amount of nickel in pyrite. The XPS results show that in the Cu2+-ethylenediamine (en)-S2O32− leaching gold system (temperature 25 °C, time 35 h, solution: 0.1 mol/L S2O32−, 5 mmol/L Cu(en)22+, 200 mL solution), the nickel of pyrite-containing nickel can be transferred to the leaching solution and becomes nickel ion. In this leaching system, Cu(II), which was originally complexed with en, is reduced to Cu(I) in a short time. The consumption of Cu(en)22+ increased rapidly in the 5 h period and then decreased gradually after 5 h. The results showed that the presence of free Ni2+ in the solution facilitated the conversion of bivalent copper ions to monovalent copper ions. Free Ni2+ ions can compete with Cu2+ ions for en ligands. When ethylenediamine complexes with Ni2+, the decomposition of Cu(en)22+ into Cu(en)+ and en occurs more rapidly. And the en, which was originally to be oxidized with Cu(en)+ to form Cu(en)22+, forms Ni(en)22+. As a result, the concentration of Cu(en)22+ continues to decrease in a short period of time.

Solution circulation for green and sustainable gold extraction with an integrated low potential thiosulfate leaching-resin adsorption recovery process

A systematic research of solution circulation for gold extraction from a gold ore with an integrated low potential thiosulfate leaching-resin adsorption recovery process was performed by using the analysis and detection methods of ICP-OES, SEM-EDS, HPLC, XPS, etc. The pulp potential was controlled at about 200 mV under low reagent concentration and closed condition where the gold leaching percentage achieved 89.02 % while the thiosulfate consumption was only 19.22 kg/t. 100 % of the gold and 32.61 % of the copper in the leaching solution were adsorbed by the resin, and their desorption percentages separately reached 98.59 % and 98.86 % after selective two-stage desorption. The leaching and adsorption percentages of gold first decreased and separately kept at about 91 % and 96 % after 10 solution circulations while the gold desorption percentage and thiosulfate consumption were basically unchanged with solution circulation. The sulfur-oxygen anions in the solution including S2O32–, S3O62–, S4O62–, SO32– and SO42–, and only SO42– was gradually accumulated with solution circulation. SO42–, S2O32– and S3O62– were the main sulfur-oxygen anions adsorbed on the resin, and they did not accumulate obviously on the resin with solution circulation. The novelty of this study is that gold was recovered as elemental gold product based on the the proposed process, and the effect of solution circulation on gold extraction and the accumulation and conversion rules of sulfur-oxygen ions during solution circulation were revealed for the first time.

Thiocyanate facilitating thiosulfate extraction of gold via inhibiting formation of passive layer

Thiosulfate leaching is a feasible alternative for cyanidation. However, thiosulfate is easily decomposed and forms the passive layer on the ore surface, thus hindering gold extraction. In this paper, by adding thiocyanate to reduce the undesirable effect, copper-ammonia-thiosulfate leaching system is utilized to extract gold from the calcine. This research investigated the factors affecting gold extraction and kinetics of the leaching system, and the potential reason and reaction mechanism of the thiocyanate for improving gold leaching efficiency were discussed and analyzed. Gold leaching percentage is increased from 62.99% to 88.45% by adding thiocyanate that is close to that of cyanidation. By using X-ray photoelectron spectroscopy, gold in the leachate may be in the form of gold(I) thiosulfate complex. The leaching mechanism and the reason for improving gold extraction by adding thiocyanate were investigated via characterization and the leaching thermodynamic analysis. The possible reason for improving gold leaching percentage lied in that the thiocyanate reduced the decomposition of thiosulfate, thus inhibiting the form of passivation layer. According to the leaching mechanism, thiocyanate is used as an additive in the copper-ammonia-thiosulfate leaching system, which is of great significance for facilitating gold recovery, lessening thiosulfate consumption, and reducing ammonia dosage.

Study of gold leaching from pre-treated waste printed circuit boards by thiosulfate‑cobalt-glycine system and separation by solvent extraction

The recovery of gold (Au) from waste printed circuit boards (WPCBs) derived from discarded mobile phones is crucial for the resource preservation and environmental protection. In this context, thiosulfate has emerged as a viable alternative to the toxic compound cyanide for Au leaching. However, the conventional thiosulfate leaching system has a main problem that is large thiosulfate consumption, which limits its commercial application. In this study, we developed a novel thiosulfate leaching system that utilizes a cobalt (Co(II))-glycine (gly) complex as the catalyst. Our findings demonstrate that the Co(II)-glycine-thiosulfate system exhibits excellent Au leaching performance coupled with low thiosulfate consumption. The effects of several parameters on Au leaching efficiencies and thiosulfate consumption were investigated. Increasing the concentrations of Co(II), thiosulfate, or glycine was beneficial to the Au leaching. However, increasing temperature promoted the oxidative decomposition of thiosulfate and reduced cobalt stability, negatively impacting Au leaching. Additionally, small amounts of base metals remained after pretreatment of WPCBs using HCl system and H2SO4-H2O2 system. Therefore, the roasting-assisted leaching process eliminated the adverse effects of these impurity metals and further improved Au leaching efficiency to 97.8% while also decreased thiosulfate consumption. The recovery of Au from the pregnant leach solution was also studied, and our findings demonstrated that N1923 can efficiently extract Au from the solution through anion exchange. Our study developed a novel and efficient Au thiosulfate leaching system catalyzed by the Co(gly)3/Co(S2O3)34− redox couple, and put forward a promising approach to enhance Au recyclability from WPCBs while significantly reducing thiosulfate consumption.

A Pretreatment of Refractory Gold Ores Containing Sulfide Minerals to Improve Gold Leaching by Ammonium Thiosulfate: A Model Experiment Using Gold Powder and Arsenic-Bearing Sulfide Minerals

The use of thiosulfate to extract gold from refractory ores is promising because of its non-toxicity and high selectivity. Sulfide minerals (i.e., pyrite, arsenopyrite, chalcopyrite), major gold carriers in refractory gold ores, however, hinder gold extraction due to the high consumption of a lixiviant. In this study, a new method to improve gold extraction from sulfide bearing gold ores is proposed based on the model experiments using a mixture of gold powder and arsenopyrite-bearing sulfide (HAsBS) ore. The effects of HAsBS ore on gold leaching in ammonium thiosulfate solutions were investigated, and it was found that gold extraction in the presence of HAsBS ore was suppressed because of the unwanted decomposition of thiosulfate on the surface of sulfide minerals. To improve gold extraction in the presence of the sulfide minerals, this study investigated the effects of the pretreatment of HAsBS ore using ammonium solutions containing cupric ions and confirmed that HAsBS ore was oxidized in the pretreatment and its surface was covered by the oxidation products. As a result, thiosulfate consumption was minimized in the subsequent gold leaching step using ammonium thiosulfate, resulting in an improvement in gold extraction from 10% to 79%.

A clean and efficient innovative technology for refractory sulfide gold ore: In situ gold extraction via self-generated thiosulfate

Green non-toxic extracting gold from refractory sulfide gold ore using thiosulfate is an essential guarantee for the sustainable development of the gold industry. However, there are many bottlenecks, including environmental pollution from traditional pretreatment of sulfide gold ore and high consumption of chemical thiosulfate. Herein, a clean and efficient innovative technology for the comprehensive utilization of refractory sulfide gold ore (high sulfur and high arsenic) was first proposed. For high arsenic sulfide gold ore, a sufficient sulfur source (addition of pyrite or sulfur) could be supplemented, and then thermal activation pretreatment for thiosulfate generation. This alternative technology can prevent the volatilization of hazardous gases (SO2 and As2O3) and instead collect them as less harmful solids (S8 and As2S3). Moreover, thermal activation pretreatment of this sulfide gold ore significantly enhances the oxidation capacity and results in the generation of sufficient thiosulfate (286 mM). The pretreatment also breaks down the sulfide inclusions to expose the encapsulated gold, which can be in situ extracted via self-generated thiosulfate. As a result, a gold extraction rate of 80.2% was achieved for this refractory sulfide gold ore without the addition of chemical thiosulfate. This innovative technology is of high academic and industrial interest, addressing the high thiosulfate consumption and promoting the industrialization of green gold extraction from refractory sulfide gold ore.

Eco-friendly and low-energy innovative scheme of self-generated thiosulfate by atmospheric oxidation for green gold extraction

Thiosulfate is a promising alternative lixiviant to poisonous cyanide for gold extraction due to undesirable environmental and safety aspects of the cyanidation techniques. However, high thiosulfate consumption has restricted its widespread commercialization as a technological bottleneck, which must be solved urgently. In this study, an eco-friendly and low-energy innovative scheme of self-generated thiosulfate by atmospheric oxidation was first proposed. Specifically, pyrrhotite, a thermal decomposition product of pyrite (700 °C, 40min), has higher chemical reactivity and can self-generate abundantly thiosulfate (0.30M). Furthermore, compared with hexagonal pyrrhotite, monoclinic pyrrhotite is a better raw material to self-generate thiosulfate because of its higher sulfur content and a more well-developed pore structure by Scanning electron microscopy (SEM). Finally, a preliminary exploration of gold extraction on different types of gold ores has verified that the self-generated thiosulfate can replace conventional sodium thiosulfate. The results we obtained from this study provide significant insight for the commercialization of green gold leaching.

Role of Thiosulfate in NO2 Absorption in Aqueous Sulfite Solutions

NOx emissions continues to be a major challenge in order to reduce the environmental impact of thermal conversion of non-recyclable waste and fuels with high nitrogen contents. Oxidation of NO to NO2 followed by absorption of NO2 in aqueous solutions with sulfur compounds as additives has recently gained more attention in order to reduce NOx emissions from flue gases. One major challenge in NO2 absorption is the high consumption of sulfite, requiring sulfite oxidation inhibitors such as thiosulfate. The present study clarifies the chemistry and consumption of thiosulfate in wet scrubbing of NO2 with buffered solutions under well-controlled conditions at neutral pH at 25 °C. Absorption rates for the reaction of thiosulfate and NO2 in the scrubber were determined, and the ability of thiosulfate to inhibit the sulfite oxidation was investigated and quantified. With 1 mM thiosulfate (no sulfite) in the scrubber solution, the NO2 absorption rate increased by 30% as compared to pure water and by 160% with 100 mM thiosulfate. The absorption rate increased with time in the presence of oxygen. With 1 mM sulfite (no thiosulfate), the initial NO2 absorption rate increased by 200% as compared to pure water. However, the absorption rate decreases significantly after short periods of time due to the high consumption of sulfite through undesired oxidation by O2. In tests with sulfite and thiosulfate as an oxidation inhibitor, high removal efficiencies could be maintained over extended periods of time. With a 10 mM sulfite solution, 1 mM thiosulfate decreased the sulfite oxidation rate from 130 to 31 mg·L–1·min–1, and 2 mM thiosulfate decreased the rate to 16 mg·L–1·min–1. For the investigated conditions, the consumption of thiosulfate did not change significantly when higher sulfite concentrations were used. The results of the study demonstrate high NO2 absorption rates for low concentrations of sulfite and thiosulfate at neutral pH.

A metabolic puzzle: Consumption of C1 compounds and thiosulfate in Hyphomicrobium denitrificans XT

Many obligately heterotrophic methylotrophs oxidize thiosulfate as an additional electron source during growth on C1 compounds. Although two different pathways of thiosulfate oxidation are implemented in Hyphomicrobium denitrificans XT, a pronounced negative effect on growth rate is observed when it is cultured in the simultaneous presence of methanol and thiosulfate. In this model organism, periplasmic thiosulfate dehydrogenase TsdA catalyzes formation of the dead-end product tetrathionate. By reverse genetics we verified the second pathway that also starts in the periplasm where SoxXA catalyzes the oxidative fusion of thiosulfate to SoxYZ, from which sulfate is released by SoxB. Sulfane sulfur is then further oxidized in the cytoplasm by the sulfur-oxidizing heterodisulfide reductase-like system (sHdr) which is produced constitutively in a strain lacking the transcriptional repressor sHdrR. When exposed to thiosulfate, the ΔshdrR strain exhibited a strongly reduced growth rate even without thiosulfate in the pre-cultures. When grown on methanol, cells exhibit significantly increased NAD+/NADH ratios in the presence of thiosulfate. In contrast, thiosulfate did not exert any negative effect on growth rate or increase NAD+ levels during growth on formate. On both C1 substrates, excretion of up to 0.5 mM sulfite as an intermediate of thiosulfate (2 mM) oxidation was recorded. Sulfite is known to form adducts with pyrroloquinoline quinone, the cofactor of periplasmic methanol dehydrogenase. We rationalize that this causes specific inhibition of methanol degradation in the presence of thiosulfate while formate metabolism in the cytoplasm remains unaffected.

Systematic study of critical parameters on magnesium saturated thiosulfate gold leaching process; part A: Effect of ammonia and lime for pH adjustments and concentration of magnesium and copper(II)

Thiosulfate gold leaching is an alternative gold leaching method, especially for leaching of preg-robbing gold ores. Here we report a thiosulfate process that has been developed for gold leaching, with emphasis on lowering thiosulfate consumption in the process. Ammonium thiosulfate has been used as the primary lixiviant along with magnesium hydroxide to saturate the solution with magnesium in the presence of copper(II) as an oxidizing agent. The resulting magnesium thiosulfate solution (Mg-TS) can potentially leach gold from the ore at considerably high efficiency. The potential for substantial decrease in the thiosulfate consumption, while achieving high gold recovery, gives the Mg-TS process a cutting-edge advantage over other processes. The study demonstrates the effect of pH, magnesium, and initial Cu(II) concentration on gold leaching kinetics in the Mg-TS process. As a comparison, pH of the tests was controlled with Ca(OH)2 and NH4OH; it was concluded that high pH, if controlled by NH4OH, is beneficial for gold leaching due to increase in ligand (NH3) availability and favorable pH range. Increasing pH using Ca(OH)2 was only effective up to a pH threshold of 9.8, after which the efficiency of the process decreases due to the precipitation of Cu(II) catalyst/oxidant. Increasing copper(II) concentration to 8 mM leached 87.8% of gold which was very close to the gold recovery with cyanidation (87.3%). The Mg-TS process identified is an operationally green and sustainable, efficient and low-cost thiosulfate process, which is feasible for in-situ/heap leaching of low-grade gold ores and is believed to present a competitive advantage over cyanidation process.

Leaching of Gold and Silver from a Complex Sulfide Concentrate in Copper-Tartrate-Thiosulfate Solutions

The leaching behaviors of gold and silver from a complex sulfide concentrate in copper-tartrate-thiosulfate solutions were investigated in this paper. Experimental parameters, including temperature, initial pulp pH, and concentration of copper, tartrate, and thiosulfate, were systematically studied. The copper-tartrate-thiosulfate leaching system exhibits promising performance in dealing with the complex sulfide concentrate. Thiosulfate consumption could be greatly reduced due to the in-situ generation of thiosulfate derived from sulfur or disulfide ions. Increasing the temperature and concentrations of copper, tartrate, and thiosulfate can promote gold leaching. A low tartrate content and a moderate increase in temperature and copper and thiosulfate concentrations can accelerate silver leaching, but a higher temperature or copper content may depress silver extraction. A suitable pulp pH is critical for gold and silver leaching in copper-tartrate-thiosulfate solutions. An extraction of 74.50% Au and 36.33% Ag was obtained with the direct leaching of the concentrate, while the percentages can be significantly increased up to 82.60% and 70.38%, respectively, when leaching the calcine following the oxidative roasting pretreatment. The recycling of leaching solutions demonstrates that a suitable free tartrate content is of great significance in maintaining the stability of the novel system. As an alternative to the traditional cyanide or copper-ammonia-thiosulfate leaching processes, the copper-tartrate-thiosulfate leaching system provides an environmentally friendly, nontoxic, and relatively low-cost method for gold and silver leaching from ores or concentrates.

Leaching of a carbonaceous gold concentrate in copper-tartrate-thiosulfate solutions

The leaching behavior of gold from a carbonaceous gold concentrate in copper-tartrate-thiosulfate solutions was studied in this paper. Experimental parameters, including temperature, initial pH, copper, tartrate, and thiosulfate concentration, were systematically investigated. The capability of leaching gold from the carbonaceous gold concentrate by copper-tartrate-thiosulfate solutions is comparable to that in cyanidation or traditional copper-ammonia-thiosulfate systems. The consumption of thiosulfate can be significantly reduced in the presence of tartrate. The mechanism of gold leaching in copper-tartrate-thiosulfate solutions is catalyzed by the mononuclear CuL2H-46− complex when the solution pH is beyond 10.5 based on thermodynamic analysis. The stability of the CuL2H-46− complex increases with an increase in the tartrate content and decreases with increasing copper and thiosulfate concentrations. A moderate increase in solution pH and temperature exhibits less effect on the consumption of thiosulfate or copper-tartrate complex, but a higher temperature may cause a sharp decline in the content of copper-tartrate complex. The recycling of the leaching solutions demonstrates that maintaining a suitable concentration of copper ions and thiosulfate and a suitable solution pH are crucial to obtain a considerable gold leaching rate.

A thermodynamic analysis on thiosulfate leaching of gold under the catalysis of Fe3+/Fe2+ complexes

• Thermodynamic analysis of Fe-catalyzed thiosulfate leaching of gold is performed. • Catalysis of EDTA 4– , ox 2– , CN – and cit 3– complexes of Fe 3+ /Fe 2+ are involved. • Possible mechanisms for Fe-catalyzed thiosulfate leaching of gold are elucidated. • Reasons for low reactivity of Fe complexes towards thiosulfate are proposed. Thiosulfate is an eco-friendly gold lixiviant that is promising to replace the highly toxic cyanide. Copper is a conventional catalyst in accelerating the rate of thiosulfate leaching of gold, but it causes high thiosulfate consumption and complex gold recovery process. Attempts on the replacement of copper by other transitional metals such as nickel, cobalt and iron have been made to address the two issues. Among the catalysts, iron is cheap with great potential of development and industrial application. Few research has focused on the thermodynamics for iron-based thiosulfate systems. In this paper, a systematic thermodynamic analysis on the thiosulfate leaching of gold catalyzed by Fe 3+ /Fe 2+ and the ligand of EDTA 4– , oxalate (ox 2– ), cyanide (CN – ) or citrate (cit 3– ) is conducted by constructing a series of predominance area and speciation diagrams. Information on the stability, speciation, and redox behavior of iron species under various solution conditions are provided to better understand the complex solution chemistry and possible mechanisms for the iron-catalyzed gold thiosulfate leaching. The likely reasons for the high stability and low consumption of thiosulfate under the catalysis of Fe 3+ /Fe 2+ complexes are also proposed.

Thermodynamic analysis of gold leaching by copper-glycine-thiosulfate solutions using Eh-pH and species distribution diagrams

Glycine and thiosulfate, as alternatives to cyanide, are all considered to be one of the most promising lixiviants for gold. In this paper, a novel copper-catalytic glycine thiosulfate collaborative system of gold leaching was proposed. Based on the available thermodynamic data, a series of Eh-pH and species distribution diagrams for gold/copper-glycine-thiosulfate system under various conditions were constructed using thermodynamic calculations. The analysis results for thermodynamic diagrams indicated that the copper-glycine-thiosulfate system had many potential advantages over the copper-ammonia/citrate-thiosulfate system, such as wider pH range and lower thiosulfate consumption. The electrochemical models for the copper-catalytic mechanism of gold leaching with glycine-thiosulfate were also proposed in this study. Moreover, the relevant results obtained in this study provide a theoretical basis for the optimization of gold leaching with copper-catalytic glycine thiosulfate system.

Thiosulfate as external electron donor accelerating denitrification at low temperature condition in S0–based autotrophic denitrification biofilter

This study was carried out to determine the inhibition of low temperature on the performance of S0–based autotrophic denitrification (S0–SAD) biofilter, and proposed to enhance the nitrate removal efficiency with thiosulfate as external electron donor. With the decline of temperature from 30 °C to 10 °C at 0.25 h of empty bed contact time (EBCT), the nitrate removal rate presented a logarithmical drop, and the effluent nitrate dramatically increased from 9.19 mg L−1 to 15.13 mg L−1. EBCT was prolonged until 0.33 h for 20 °C, 0.66 h for 15 °C and 1.5 h for 10 °C, respectively, to maintain the effluent nitrate below 10 mg L−1. Such excessive variation of EBCT for different temperature is undoubtedly incredible for practical engineering. Thiosulfate, as the external electron donor, was adopted to compensate the efficiency loss during temperature decrease, which significantly prompted nitrate removal rate to 0.59, 0.53 and 0.31 kg N m−3 d−1 at 20 °C, 15 °C and 10 °C conditions, respectively, even at a short EBCT of 0.25 h. It not only acted as compensatory electron donor for nitrate removal, but also promoted the contribution of elemental sulfur via accelerating the DO consumption and extended larger effective volume of S0–layer for denitrification. Meanwhile, the significant enrichment of Sulfurimonas and Ferritrophicum provided biological evidences to the enhancement process. However, the incomplete consumption of thiosulfate was observed especially at EBCT of 0.25 h and 10 °C, and the thiosulfate runoff needs to be concerned in case of contaminating the effluent. Herein, approximately extending EBCT to 0.66 h and decreasing thiosulfate dosage were conducted simultaneously, thereby achieving 100% thiosulfate utilization efficiency and expected nitrate removal. This study provided a fundamental guidance to design and operate S0–SAD biofilter in response to seasonal temperature variation for practical engineering.

Review of gold leaching in thiosulfate-based solutions

Numerous non-cyanide leaching lixiviants have been developed, among which thiosulfate is considered the most promising alternative to cyanide due to its non-toxicity, low price, high leaching rate and excellent characteristics in dealing with carbonaceous and copper-bearing gold ores. The traditional copper−ammonia−thiosulfate system has been studied extensively. However, with many years of process development, there are still some problems and challenges with this gold leaching system. A series of studies using nickel-, cobalt- and ferric-based catalyst to substitute copper have been conducted with the purpose of reducing the consumption of thiosulfate. A variety of non-ammonia thiosulfate leaching systems including oxygen−thiosulfate, copper−thiosulfate, copper−EDA−thiosulfate, ferric− EDTA−thiosulfate, and ferric−oxalate−thiosulfate leaching systems have been also developed to eliminate the potential side-effect of ammonia. In this review, the basic theory and process development of some main gold leaching systems based on thiosulfate solutions were systematically summarized to illustrate the research status on thiosulfate leaching process. The potential effects of various additives such as organic ligands containing amino, carboxyl or hydroxy functional groups on gold thiosulfate leaching were described in detail. The potential opportunity and challenge for promoting the industrial development of thiosulfate-based gold leaching systems were also discussed.

Thiosulfate leaching of gold catalyzed by hexaamminecobalt(III): Electrochemical behavior and mechanisms

Thiosulfate is a green gold lixiviant that has promise to replace cyanide at the gold mining industry. Hexaamminecobalt(III) (Co(NH3)63+) is effective in catalyzing the thiosulfate leaching of gold from ores, but the electrochemical behavior and catalytic leaching mechanisms remain unclear. This paper reports an in-depth investigation into the electrochemical behavior of gold leaching in ammoniacal thiosulfate solutions catalyzed by Co(NH3)63+. The electrochemical results suggested that Co(NH3)63+ is more efficient than Cu(NH3)42+ in catalyzing the ammoniacal thiosulfate leaching of gold. Compared with Cu(NH3)42+, the coulombic efficiency of gold dissolution (QAu/QT) under the Co(NH3)63+ catalysis is much higher. In the presence of Co(NH3)63+, the QAu/QT was nearly 100% at 150–250 mV and higher than 78.9% at 250–350 mV; at 250 mV, the QAu/QT was approximately 100% in the initial electrolysis of 300 s and remained basically unchanged at around 75% after 1200 s. Leaching results further showed that the catalysis of Co(NH3)63+ not only achieved a continuous leaching of gold with time, but also significantly decreased the consumption of thiosulfate from 36.5% (for Cu(NH3)42+) to 20.9% after leaching 24 h. In the ammoniacal thiosulfate solution, Co(NH3)63+ is likely reduced to mixed ligand Co(II) complexes of Co(NH3)x(S2O3)0 (x = 4, 5), which are readily oxidized back to Co(NH3)63+. Thus, an improved catalytic mechanism of gold leaching is proposed that the redox cycle between Co(NH3)63+ and Co(NH3)x(S2O3)0 (x = 4, 5) catalyzes the leaching of gold with ammoniacal thiosulfate.

Hexaamminecobalt(III) catalyzed thiosulfate leaching of gold from a concentrate calcine and gold recovery from its pregnant leach solution via resin adsorption

Co(II) is effective in catalyzing gold thiosulfate leaching, but Co(III) has seldom been adopted directly as the catalyst. This paper examined the effect of hexaamminecobalt(III) on the thiosulfate leaching of gold from a concentrate calcine and the gold recovery via resin adsorption from the leach solution. Under the leaching solution conditions, the Co(NH3)63+/Co(NH3)x2+ (x = 4, 5) redox couple is thermodynamically formed playing the catalytic role. Experimental results verify the catalysis of Co(NH3)63+, which is better than that of Co(II) and Cu(II) in thiosulfate leaching of gold. Replacing Cu(II) by Co(NH3)63+ or Co(II), the thiosulfate consumption was significantly reduced and the complex two-stage gold elution process was simplified to a low-cost one-stage process. Compared with Co(II), Co(NH3)63+ that is already the oxidant could shorten the leaching time from 8 h to 6 h without affecting the gold extraction (~80%). In addition, the thiosulfate consumption was further lowered from 18.8 kg·t-1ore to 15.0 kg·t-1ore under the catalysis of Co(NH3)63+, which was confirmed by the X-ray photoelectron spectroscopy (XPS) analysis. Based on the thermodynamic analysis, the decrease of thiosulfate consumption may result from the formation of a lower concentration of the mixed-ligand Co(III) complexes such as Co(NH3)x(S2O3)+ (x = 4, 5). Gold recovery results suggested that, comparing Co(NH3)63+ with Co(II), the gold in the Co(NH3)63+ catalyzed leaching solution could be recovered more efficiently by resin adsorption. Therefore, Co(NH3)63+ is a catalyst more efficient than Co(II) and has promise to replace Cu(II) in the extraction and recovery of gold from its ores by thiosulfate leaching and resin adsorption.

A systematic and comparative study of copper, nickel and cobalt-ammonia catalyzed thiosulfate processes for eco-friendly and efficient gold extraction from an oxide gold concentrate

A systematic and comparative research was performed using copper, nickel and cobalt-ammonia catalyzed thiosulfate processes for environmentally friendly and efficient gold extraction from an oxide gold concentrate. 88.3% of gold was leached by thiosulfate with conventional copper-ammonia catalysis while its thiosulfate consumption was as high as 53.6 kg/t-concentrate. Thiosulfate consumption was reduced to 26.5 and 30.6 kg/t-concentrate, and 81.2% and 90.2% of gold leaching percentages were obtained with nickel-ammonia and cobalt-ammonia catalyses. The electrochemical leaching mechanism combined with XPS and SEM-EDS analyses illustrated the essential reason for the differences of three catalyses in gold leaching percentage and thiosulfate consumption. For copper-ammonia catalysis, two-stage desorption was required to desorb gold from the gold-loaded resin due to the co-adsorption of copper with gold while one-stage desorption was feasible for nickel-ammonia and cobalt-ammonia catalyses because nickel and cobalt did not co-adsorb with gold, and their gold recovery percentages were 82.4%, 98.5%, 97.7%, respectively. After 6-time cycles of barren leachate, the gold leaching and recovery percentages of copper-ammonia catalysis considerably decreased while those of nickel-ammonia and cobalt-ammonia catalyses only declined slightly. Thiosulfate leaching with cobalt-ammonia catalysis has the advantages of high gold leaching percentage, low thiosulfate consumption, simple desorption process, good leachate stability and recycle performance of barren leachate, and therefore it has a bright industrial application prospect.

A phenomenological study of the silver sulfide passivation and oxidative degradation of thiosulfate in the thiosulfate-ammonia‑copper-citrate leaching system

The oxidative decomposition of thiosulfate is one the main problems found in the thiosulfate leaching of precious metals. This work presents a detailed analysis of the effect of different dissolved components on the silver sulfide dissolution kinetics and efficiency faced by the thiosulfate-citrate‑copper-ammonia system. The analysis comprises the quantification of thiosulfate concentration; furthermore, it is discussed the effect of copper sulfides and copper hydroxides -formed in the leaching system- on the thiosulfate decomposition and silver dissolution efficiency. The results revealed that citrate promotes the complexation of cupric ions decreasing the thiosulfate consumption in the leaching system. The individual and combined effects of copper solid species (Cu(OH)2, CuS, Cu2S and solid residues obtained from silver sulfide leaching) on the thiosulfate stability and silver extraction efficiency, showed that the thiosulfate decomposition that occurs in the system is not significantly related to these copper species, while the silver extraction (~80%) does not show any substantial change. However, the presence of Cu2S and a small amount of dissolved silver have certain effect on the the partial in-situ regeneration of thiosulfate in the leaching system. Finally, it was also found that the silver sulfide dissolution is passivated obtaining an 80% of silver extraction. Such passivation is mainly attributed to: a) the precipitation of copper-sulfide and copper-hydroxide species on the unreacted silver sulfide sites; these copper phases act as a physical barrier which inhibits the direct contact of fluid reactants with the silver sulfide, and b) the silver sulfide dissolution reaches an equilibrium state.