Gold Dissolution Rate Research Articles

Features affecting the cupric chloride leaching of gold

The current study investigated gold dissolution in cupric chloride solution, which is one of the most promising alternatives to replace cyanide, although not yet in operation on industrial scale. In this paper, the gold dissolution reaction rate and mechanism were studied by varying the process variables of temperature (28–95 °C), cupric concentration (0.02–1.0 M), chloride concentration (1–5 M), rotational speed (100–2500 RPM), and pH (0.5–2.0). The parameters affecting either the anodic or cathodic reaction, or both, were identified for the first time to the best of the authors’ knowledge in this wide range and with these parameter intervals. Tafel and linear polarization methods as well as cyclic voltammetry were used for pure gold with both rotating disk and stationary gold electrodes. An increase in the gold dissolution rate was observed with an increase in temperature, chloride concentration, and rotational speed. Additionally, an increase in cupric concentration ([Cu2+] = 0.02–0.75 M) promoted the gold dissolution rate, whereas the gold dissolution rate decreased with [Cu2+] from 0.75 to 1.0 M. The conditions for maximizing the gold dissolution rate in cupric chloride solution were concluded to be T > 55 °C, [Cu2+] = 0.5–0.75 M, [Cl−] = 5 M, and pH = 1.0 and the highest gold dissolution rate (2.9 · 10−4 mol m−2 s−1) was achieved at 95 °C with [Cu2+] = 0.5 M, [Cl−] = 5 M, pH = 1.0, and ωcyc = 2500 RPM. The pH was shown not to affect the gold dissolution rate at all, but only to affect the solubility of the oxidant. It was suggested that gold dissolved as aurous species in the conditions of this study, although the increase in chloride concentration promoted the dissolution of gold as both, auric and aurous, species.The reaction mechanism was interpreted using mixed potential theory. An increase in temperature was shown to promote only the cathodic reduction of cupric ion to cuprous at lower temperatures (28–55 °C); however, both the anodic gold dissolution reaction and cathodic cupric reduction reaction were enhanced at higher temperatures (65–95 °C). The cathodic reaction was also enhanced with an increase in cupric concentration (0.02–0.5 M), whereas the anodic reaction was promoted when the cupric concentration was increased from 0.5 to 0.75 M. When the cupric concentration was increased from 0.75 to 1.0 M, the cathodic reaction rate decreased. However, the reason for the decrease in the cathodic reaction rate was not clear. An increase in chloride concentration enhanced the cathodic reaction in the investigated range (1–5 M), whereas an increase in rotational speed (i.e., improved mass transfer) increased the anodic gold dissolution rate, specifically at low rotational speeds.

DISSOLUTION KINETICS OF GOLD IN SYSTEM THIOUREA–THIOSULFATE WITH OXIDANT OF Fe(III)EDTA

Cyanidation remains the predominant technology for the dissolution of gold from mineral raw materials. Environmental and technological disadvantages in the use of cyanides are the cause of the development of alternative systems, which include reagent-oxidant and reagent-complexant. A special place is occupied by the solvents, in which the formation of heteroligand complexes of gold is possible. Thus due to the formation of more durable compounds the rate of dissolution of the metal increases. In this work, as the ligands the thiourea and the ions of thiosulfate in the form of sodium salt were used, as the oxidant – complex of the iron (III) and EDTA. The influence of the mixed solvent composition on the kinetic characteristics and the reaction mechanism was studied by the rotating disc method. To confirm the role of different ligand complexes, the dependence of the dissolution rate of gold by the mixed system – thiosulphate and thiourea in the isomolar series, was determined. With a constant total ligands concentration, the molar ratio of reagents was changing. On the basis of the data obtained, a mathematical model of the process of oxidative dissolution of gold is proposed. A characteristic feature of the calculated results is a substantial increase in the reaction constants for the formation of heteroligand complexes, which confirms their role in the process of gold oxidation. The influence of pH and concentration of oxidant and ligand-forming substances on the kinetics of gold dissolution is established. To elucidate the mechanism of the limiting stage of the heterophase process, the experimental activation energy is calculated and the effect of the disk rotation frequency on the dissolution rate is studied. The experimental activation energy is 28.1 kJ/mol. The order by the disk rotation frequency is 0.35. Evaluation of the contribution of the diffusion and kinetic components of rate is estimated on the basis of the modified Levich equation for the mixed regime. Diffusion component of rate is almost an order of magnitude smaller than the kinetic one. Calculation of the diffusion flux of the supplied reagents showed that the only process that inhibits mass transfer is the diffusion of the reaction products from the surface of the rotating disc. The formation of more durable heteroligand complexes and a greater equilibrium constant of the oxidation reaction lead to an increase in the concentration of products at the surface and, consequently, to an increase in the rate of diffusion of the metal into the solution. An important factor affecting the oxidation of gold in the system studied is the formation of intermediate solid reaction products on the metal surface. To identify solid products, IR spectra of reflection of polished gold surface after etching in the system studied were obtained. A strong absorption band in the 808-762 cm-1 region is observed on the spectra. Absorption in this region is associated with valence symmetrical vibrations of the bond of the –C = S group adsorbed or chemically bound to the surface of the molecules of thiourea and its oxidation products. Depending on the composition of the solution and the temperature, the maximum of the peak shifts, and its intensity also changes. This can be explained by a change in the composition of the adsorbed compounds. The efficiency of regeneration of the oxidant Fe(III)EDTA by oxygen in thiourea-thiosulfate solution has been showed.

Process mineralogy characteristics of acid leaching residue produced in low-temperature roasting-acid leaching pretreatment process of refractory gold concentrates

To provide a theoretical basis for a suitable process to extract gold from refractory gold concentrates, process mineralogy on the acid leaching residue of gold calcine was studied by chemical composition, X-ray diffraction, scanning electron microscopy-energy spectrum, and mineral dissociation analysis. The results showed that the acid leaching residue contained Au 68.22 g/t, Ag 92.71 g/t, Fe 0.44%, As 0.10%, and S 0.55%. Gold and silver minerals existed as native gold, argentite, and proustite. Quartz, the main gangue mineral, accounted for 78.33wt/%. The dissociation degree analysis showed that the proportions of monomer and exposed gold in acid leaching residue were 96.66wt%. The cyanidation results showed that the cyanide gold leaching rate of acid leaching residues was close to 100wt%. However, the maximum cyanide gold leaching rate of gold calcine was only 85.31wt%. This suggests that acid leaching can increase the gold dissolution rate in the cyanide process.

Optimizing the alkaline oxidation pretreatment of a refractory gold ore using taguchi orthogonal array method

This study investigates the alkaline oxidation pretreatment process of Zarshuran refractory gold ore. Taguchi method-orthogonal array design (OAD) is applied to evaluate and optimize the influence of five main factors including solution pH, aeration rate, agitation speed, temperature, and oxidation time. In order to establish relationship between the gold dissolution rate and influential factors, a linear regression model with R2 of 0.9706 is fitted to experimental data. The results indicated that in the range studied, the agitation speed does not have any significant impact on the gold recovery, while oxidation time and temperature were the most significant factors. Au recovery increased with increasing the aeration rate, pretreatment time, and temperature as well as decreasing the solution pH. Using Taguchi orthogonal array optimization technique (OAOT), the maximum gold recovery was about 96.72%. Also, the optimal conditions were 11 for solution pH, 150 ml min−1 for aeration rate, 240 rpm for agitation rate, 50 °C for temperature, and 24 h for oxidation time.

INFLUENCE OF FORMATION OF THE COMPLEXES WITH DIFFERENT KIND OF LIGANDS ON KINETICS OF OXIDATIVE DISSOLUTION OF METALS

Rotating disk method has been used to study the dissolution of gold in the presence of an oxidizer and a mixture of dissimilar ligandforming reagents. Kinetic regularities of gold dissolution were investigated in aqueous systems thiourea-thiocyanate with Fe (III) and Cu (II) ions as the oxidizer; thiosulfate-thiocyanatewithFe(III)EDTA oxidizing complex and thiosulfate-thiocyanate with ammonia complex [Cu(NH3)4]2+ asoxidizing agent. Peculiarities of these mixed systems is a possibility to form the monoligandas as well as geteroligand complexes of gold. The influence of gold complexes formation with different sorts of ligands on the metal dissolution rate have been considered. The dependence of the dissolution rate on the ratio of ligands concentrations and on the oxidizing agent nature was investigated. Interrelation was established: the use of dissimilar ligandforming reagents - the higher thermodynamic stability of the geteroligand complexes of gold (relative to the homoligand ones) - synergistic increase of the gold dissolution rate. It was found that the heterophase reaction makromechanism for the thiourea-thiocyanate-Fe(III) system is determined by the proximity of the rates of chemical and diffusion stages (experimental activation energy - 21.6 kJ/mol; the reaction order in respect to the disk rotation frequency – 0.23). Diffusive limitation is due to the slow withdrawal of the reaction products -Au (I) compounds - from the solid phase surface. The formation of stronger heteroligand complexes and the corresponding increase in the reaction products concentration at the surface increases a rate of the metal diffusion into the solution. For the thiourea-thiosulfate-Fe(III)EDTA system a film formation of the reaction solid products on the gold surface was observed. To identify it the infrared reflection spectra of the polished gold surface after the etching were obtained. There is a strong absorption band on the spectra at 804 cm-1. Absorption in this area has been associated with the stretching vibrations of C=S bond of the thiourea molecules joined to the surface and the products of their oxidation.

Intensive leaching of gold from gravity concentrate at low concentration of sodium cyanide

Studies on the intensive leaching of gold gravity concentrate in the drum and conical types at low concentration of sodium cyanide laced with reagent-activator. As a reagent-used organic acid activator with 1.5 and 3.0 kg/t. List the parameters of both devices intensive cyanidation. Tests conducted in comparison with well-known methods of intensive leaching of gold in similar devices with a high concentration of sodium cyanide, in which gold extraction reaches high results. However, there is significant consumption of sodium cyanide, which is an expensive reagent. Similar results can be achieved with a low concentration of sodium cyanide using a reagent-activator, with significant reduction in the consumption of sodium cyanide. It is shown that the use of reagent-activator on the basis of an aliphatic acid significantly intensifies the leaching process. Found that the gold content in the tails of the intensive cyanidation by using reagent-activator on the basis of an aliphatic acids in both vehicles lower than without the use of this reagent. As a result of study of dissolution rate of gold in rolling the unit higher than that of the conical, but for intensive cyanidation gravity concentrates may use both types of machines, drum and conical.

Kinetics and mechanisms of gold dissolution by ferric chloride leaching

Gold dissolution was investigated in ferric chloride solution, being one alternative cyanide-free leaching media of increasing interest. The effect of process variables ([Fe3+]=0.02–1.0M, [Cl−]=2–5M, pH=0–1.0, T=25–95°C) on reaction mechanism and kinetics were studied electrochemically using rotating disk electrode with ωcyc=100–2500 RPM and Tafel method. The highest gold dissolution rate (7.3·10−4molm−2s−1) was achieved at 95°C with [Fe3+]=0.5M, [Cl−]=4M, pH =1.0 and ωcyc=2500 RPM. Increase in gold dissolution rate was observed with increase in temperature, ferric ion concentration and chloride concentration, but gold dissolution rate did not have a clear dependency on pH. Redox potential was found to vary between 636 and 741mV vs. SCE during experiments. According to the calculated equilibrium and measured open circuit potentials, gold was suggested to dissolve as aurous ion Au+ and form AuCl2−, rather than auric ion Au3+ and form AuCl4−. Further, it is suggested that AuCl2− does not oxidize to AuCl4− under the investigated conditions. Levich plot and the calculated activation energies suggested that gold dissolution was limited by mass and electron transfer. According to a mechanistic kinetic model developed in the current work, intrinsic surface reaction mainly controls gold dissolution, especially at higher rotational speeds (>1000 RPM). Uncertainties in the model parameters of the mechanistic kinetic model were studied with Markov chain Monte Carlo methods.

A review on electrochemical dissolution and passivation of gold during cyanidation in presence of sulphides and oxides

With the rapid depletion of free-milling types of gold ores, sulphidic gold ores are often oxidized prior to cyanidation due to the refractoriness. This results in the increase in the processing of oxidized gold ores. Thus, evaluation of the electrochemical dissolution of more complex and oxidized gold ores become ever important for the mining industry and for R&D. In this study, past achievements and recent developments in terms of electrochemical methods used in gold dissolution and passivation studies of pure gold, sulphidic, and oxidized gold ores are presented.At potentials close to open circuit potential (OCP) as in practice, slowdown in the dissolution rate of gold could be due to passivation by either adsorbed layers or surface products, or combinations thereof. Different terms such as passivation, inhibition, retarding effect, and slowdown used in gold dissolution studies have been discussed. The developments in electrochemical approaches, such as two separate containers, have been summarized. Conventional electrochemical as well as recent methods, such as electrochemical noise measurement with its in-situ detection of corrosion rate, and scanning reference electrode technique are evaluated.In the majority of the previous polarization studies, anodic reaction was examined in absence of oxygen whereas cathodic reaction in absence of cyanide, separately, and the dissolution was considered as the intersection of both curves. However, some recent findings report that this does not represent the actual gold dissolution rate as in practice. Additionally, the direction of scanning in cathodic polarization tests, either OCP to cathodic potentials or cathodic potential to OCP, is also reported.

Gold and copper leaching from gold-copper ores and concentrates using a synergistic lixiviant mixture of glycine and cyanide

The presence of cyanide soluble copper in the cyanidation of gold-copper ores and concentrates significantly increases the cyanide consumption in order to achieve sufficient gold recovery. In addition, cyanide recovery or cyanide detoxification/destruction processes are also required which adds extra cost to the process. This research introduces a leaching approach to extract gold, silver and copper from gold-copper ores and concentrates using a synergistic lixiviant leaching process using glycine in the presence of low concentrations of cyanide. The effects of glycine and cyanide concentrations on gold, silver and copper leaching kinetics and recovery were studied. It is shown that, in the presence of glycine, gold, silver and copper extraction increase significantly in solutions containing copper-cyanide species at a very low, or zero, free cyanide concentration. It has also been shown that the gold dissolution rate in glycine-cyanide system is almost three times higher than the gold dissolution rate in the conventional cyanidation. Kinetic studies were conducted to evaluate the effects of glycine concentration, pH, and CN/Cu ratio on the dissolution of gold and silver. It has also been shown that the gold dissolution rate increases by increasing the glycine concentration up to 2.0g/L (Gly:Cu molar ratio of 2.2:1) and any further glycine addition has no significant effect on the dissolution of gold. It is shown that the presence of glycine in solutions containing copper (I)-cyanide species can significantly enhance the dissolution of precious and base metals. The proposed leaching approach significantly reduces the cyanide consumption by at least 75% and most of the copper is present as cupric glycinate in the final leach solution. In addition, gold, silver and copper extraction was higher than the conventional cyanidation process utilising similar cyanide dosages for all the gold-copper sources studied.

Mechanism and kinetics of gold leaching by cupric chloride

A non-cyanide lixiviant for gold dissolution, cupric chloride, was studied. The work used a rotating disk electrode technique to investigate the dissolution reaction mechanism and kinetics of gold dissolution in concentrated chloride (3M) solution with cupric ion concentration in the range 0.02–1.0M and temperature 65–95°C. A reaction mechanism including a disproportionation step of Au(I) is proposed. Increase in the gold dissolution rate was found to be proportional to temperature and cupric ion concentration up to 0.5M. The dissolution rate was observed to decrease with cupric ion concentration of 1.0M. A mechanistic kinetic model was developed to study the rate-controlling steps in the dissolution process. Uncertainties in the model parameters of the mechanistic kinetic model were studied with Markov chain Monte Carlo (MCMC) methods. The modeling results showed that the gold dissolution is mainly under mixed control, where both the intrinsic surface reaction and mass transfer have an effect on the overall dissolution rate.

Gold Recovery from PCBs with Thiosulfate as Complexing Agent

Waste of electrical and electronic equipment (WEEE) is the fastest growing advanced type of solid waste streams in the urban environment worldwide and contains interesting amounts of precious metals. Hydrometallurgical technique is fast emerging as preferred process for the recovery of a variety of metals due to its lower energy consume and lower smelter emissions than conventional pyrometallurgical processes. In this work, a hydrometallurgical process for the recovery of gold and silver from electronic scraps was studied. In place of cyanide, thiosulfate was chosen as complexing agent for gold. Thiosulfate leaching can be considered a non-toxic process and the gold dissolution rates can be faster than conventional cyanidation. The electronic scraps, obtained from “end of life” mobile phones, were crushed and pre-treated with nitric acid before the leaching. Different parameters were studied: concentration of thiosulfate, temperature and reaction time. Moreover, the use of ultrasound to assist the hydrometallurgical gold extraction was investigated, as its application in ores leaching shows a greater metals release in shorter time and the advantage of working at lower concentration of reagents and at lower temperature. In this work, the use of ultrasound allowed a higher recovery of the precious metals than conventional leaching in all the conditions studied (different concentrations of reagents, temperature and reaction time). Moreover, the studied process allowed also the recovery of the other metals present in the waste (Cu, Sn and Ag).

A study of the electrochemical dissolution and passivation phenomenon of roasted gold ore in cyanide solutions

This study investigates the electrochemical interactions between gold and its associated oxide minerals either in one (OC) or two separate containers (TC) in the presence or absence of slurry. Magnetite showed negative effect on gold leaching, while maghemite and hematite demonstrated positive effect, relatively. The presence of slurry generally resulted in lower current densities. However, when the tailings of slurry was used again as a new feed in two separate containers (TC), the dissolution rate of gold was increased by 3 times (30 to 90μAcm−2), suggesting that the removal of detrimental ions in the first stage. Moreover, the current density of gold was enhanced up to 10 times by magnetic pre-treatment of roasted gold ore. It was found that in cyanide solutions saturated with atmospheric oxygen, cathodic Tafel slope only (8.60×10−7±4.56%molm−2s−1) provides close corrosion rates of gold to that in practical cyanidation (10.57×10−7±1.33%molm−2s−1), suggesting that cathodic polarization is the rate controlling one. Furthermore, a new electrode consisted of equal quantities of magnetite and hematite in one electrode was developed to examine the influence of these two major iron oxides on gold leaching, concurrently 0.04M NaCN at 100rpm agitation was found to be optimal. SEM analysis indicated the presence of Fe-oxide and Au–C compound products that could be responsible for partial passivation of gold surface.

Gold dissolution and copper suppression during leaching of copper–gold gravity concentrates in caustic soda-low free cyanide solutions

In the cyanidation process, copper–gold deposits containing significant amounts of cyanide soluble copper can lead to high cyanide consumption with low gold extraction. The significant levels of weak acid dissociable (WAD) cyanide must then be destroyed prior to tailings disposal. This increases the cost to the gold mining companies to cover both the extra cyanide consumption in the leaching stage and the additional cost for cyanide recovery/destruction. Therefore, in this study, a selective cyanide leaching process of gold over copper from copper–gold concentrate (also see Oraby and Eksteen, 2014) containing 490g/t Au and 0.97% Cu present as metallic copper, oxides and sulfides has been investigated. To decrease the cyanide consumption, cyanide is added into the leach solutions to maintain a ratio of cyanide to total reactive copper (CN/Cu) below 2. At low CN/Cu ratio, increasing the pH of solution can lead to the precipitation of solubilized copper as CuO/Cu(OH)2, releasing cyanide ions for further both gold and copper dissolution. A comparison of leaching gold in cyanide–caustic, cyanide–ammonia and conventional cyanidation processes has been made. Higher gold extraction and lower copper concentration in the final leach was achieved in the cyanide–caustic system than in either the cyanide–ammonia or conventional cyanidation processes. The effects of the caustic soda (pH) concentration or cyanide concentration on gold extraction and copper suppression have also been studied. The results show that at high pH (>12), the gold dissolution rate increases significantly in solutions containing caustic soda and cyanide at zero, or very low free cyanide concentration.

Gold leaching in cyanide-starved copper solutions in the presence of glycine

In the cyanidation of copper–gold ores, the presence of copper minerals can lead to soluble gold losses, the production of weak acid dissociable (WAD) cyanide, as well as a number of operational challenges in CIP/CIL circuits with regard to competitive adsorption, and subsequent difficulties associated with elution, electrowinning and smelting. In addition, copper minerals are significant cyanide consumers, leading to higher cost in ore treatment. This paper presents a process to enhance the dissolution of gold using copper–cyanide solutions in the presence of glycine where the solution is cyanide starved. The effect of glycine addition on gold leaching kinetics in copper–cyanide solutions under different leaching conditions was studied. The results show that, in the presence of glycine, gold dissolution rate increases significantly in solutions containing copper–cyanide species at a very low, or zero, free cyanide concentration. In the presence and absence of glycine, gold dissolution rates in solutions containing 10mM Cu(CN)32− were 11.1μmol/m2·s and 0.65μmol/m2·s, respectively. It is shown that the average gold dissolution rate in a Cu–CN−-glycine system is about 6.5 times higher than the gold dissolution rate in the conventional cyanidation, the presence of cyanide being similar in each system. Kinetic and electrochemical studies were conducted to evaluate the effects of glycine concentration, pH, CN/Cu ratio, and initial copper concentrations on the dissolution of gold. It has also been shown that the gold dissolution rate increases by increasing the glycine concentration up to 1g/L and any further glycine addition has a negative effect on the dissolution of gold. Increasing leaching pH up to 12 enhances gold dissolution in the copper–cyanide–glycine solutions. However, increasing leaching pH to 13 has an adverse effect on the dissolution of gold.

The copper–ethanediamine–thiosulphate leaching of gold ore containing limonite with cetyltrimethyl ammonium bromide as the synergist

The leaching of thiosulphate gold for a gold ore containing limonite was investigated. The results indicate that the copper ion–ethanediamine(en)–thiosulphate system was more suitable than the copper ion–ammonia–thiosulphate system for gold extraction of the gold ore. The use of cetyltrimethyl ammonium bromide (CTAB) as an effective synergistic additive in the gold extraction process was demonstrated to increase the gold extraction rate and to reduce the thiosulphate consumption for the gold ore. One possible reason for this improvement may be that CTAB hydrolyses to [CTA+] and subsequently attracts the negative charged slime, thereby causing a change in the slurry rheology, which results in the gold surface's being exposed and thus easily leached in the thiosulphate leaching system. Another reason may be the formation of ion pairs, [CTA+]3[Au(S2O3)23−] •nH2O and [CTA+][AuBr2−]•nH2O, which leads to the stabilisation of [ Au(S2O3)23−] in the leaching system. The two effects of CTAB increase the gold extraction rate remarkably and sharply reduce the thiosulphate consumption. The gold dissolution rate of the gold ore could reach 94.3% and the thiosulphate consumption could be reduced to 1.12kg/t ore in the thiosulphate gold leaching system containing 0.1mol/L of sodium thiosulphate, 0.06M ethanediamine, 0.005M copper ion and 1.5kg/t of CTAB and operated at a rate of agitation of 150rpm.

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The leaching of thiosulphate gold for a gold ore containing limonite was investigated. The results indicate that the copper ion–ethanediamine(en)–thiosulphate system was more suitable than the copper ion–ammonia–thiosulphate system for gold extraction of the gold ore. The use of cetyltrimethyl ammonium bromide (CTAB) as an effective synergistic additive in the gold extraction process was demonstrated to increase the gold extraction rate and to reduce the thiosulphate consumption for the gold ore. One possible reason for this improvement may be that CTAB hydrolyses to [CTA+] and subsequently attracts the negative charged slime, thereby causing a change in the slurry rheology, which results in the gold surface's being exposed and thus easily leached in the thiosulphate leaching system. Another reason may be the formation of ion pairs, [CTA+]3[Au(S2O3)23−] •nH2O and [CTA+][AuBr2−]•nH2O, which leads to the stabilisation of [ Au(S2O3)23 −] in the leaching system. The two effects of CTAB increase the gold extraction rate remarkably and sharply reduce the thiosulphate consumption. The gold dissolution rate of the gold ore could reach 94.3% and the thiosulphate consumption could be reduced to 1.12 kg/t ore in the thiosulphate gold leaching system containing 0.1 mol/L of sodium thiosulphate, 0.06 M ethanediamine, 0.005 M copper ion and 1.5 kg/t of CTAB and operated at a rate of agitation of 150 rpm.

Novel process for recycling gold from secondary sources: Leaching of gold by dimethyl sulfoxide solutions containing copper bromide and precipitation with water

A novel method for recovering gold from secondary sources, the leaching of gold using dimethyl sulfoxide (DMSO) solutions containing copper bromide followed by the precipitation of gold with water, is presented. Gold dissolution was conducted in a DMSO solution with 0.1–0.2M of CuBr2 and 0–0.2M of KBr at 333–348K. The mechanism of gold dissolution was investigated by electrochemical measurements. The precipitation of dissolved gold was performed by the addition of water, during which the effects of the amount of water and the pH on the recovery rate were investigated. It was found that the initial gold dissolution rate in the DMSO solutions with CuBr2 was up to 37mgcm−2h−1, which is larger than those obtained with other leaching methods. The results of the electrochemical measurements indicate that the anodic dissolution of gold in DMSO containing CuBr2 occurs at relatively negative potentials and is paired with the cathodic reduction of Cu2+ to Cu+. A recovery rate of gold up to 87% was obtained by precipitation with water. A small amount of copper was precipitated with gold, which can be avoided by using water of lower pH, as is expected by the Eh-pH diagram. Our results demonstrate that a circulating system for gold leaching and recovery, which would offer a number of advantages, including eco-friendliness, easy operation, low costs, and minimization of chemical sludge production, can be developed.

Effect of Temperature on Gold Dissolution in Acidic Media

Temperature has a tremendous effect on the rate and amount of gold dissolution. In potentiodynamic experiments the total amount of dissolved gold increases by a factor of 4.5 when the temperature is increased from 25 to 75°C. Moreover, the previously reported correlation between the onset of oxide formation and the commencement of the dissolution process is found to be correct for all studied temperatures. Negative shift in the advent of oxidation is accompanied by a similar shift in the dissolution onset potential. Similarly, the dissolution rate rises significantly with temperature during potentiostatic and galvanostatic polarization steps. A correlation between the temperature-dependent oxygen evolution reaction (OER) and gold dissolution is found. Arrhenius plots for the OER and gold dissolution are used to obtain information on the apparent activation energy EA for both processes. It is found that for OER and Au dissolution EA = 57 kJ mol−1. The later finding suggests that there is a common intermediate for both reactions, the formation of which constitutes the rate-determining step.

Effect of thiourea on the rate of anodic processes at gold and graphite electrodes in thiosulfate solutions

Dependencies of the anode current on the potential were measured by means of voltammetry on the fresh surface of gold and graphite electrodes in the solutions of 0.01M sodium thiosulfate with the addition of thiourea (1×10−5–1×10−3M). Using EQCM measurements, we studied the partial currents of gold dissolution in the indicated solutions. The results give us ground to state that small amounts of thiourea (Tu) added cause a substantial increase both in the rate of thiosulfate oxidation and in the rate of gold dissolution. On graphite, the addition of Tu causes a decrease in the electrode polarization and is likely to lead to the change of the mechanism of the electrode reaction of thiosulfate oxidation. On the gold electrode, the addition of Tu has a multivariable effect on the anode process in thiosulfate solutions. On one hand, Tu functions as a depassivating agent with respect to the reaction of thiosulfate oxidation. On the other hand, it acts as an accelerating agent for the reaction of gold dissolution. After the addition of a small amount of Tu, the rate of gold dissolution becomes comparable with the rate of thiosulfate oxidation. Results show that no indices of gold passivation by polysulfides are observed with the solutions with Tu addition. At the same time, gold depassivation by Tu additive is seems not to be the reason of an increase in the rate of gold dissolution.

Effect of Silver in Thiosulfate Leaching of Gold–Silver Alloys in the Presence of Copper and Ammonia Relative to Pure Gold and Silver

One promising alternative to cyanide for the leaching of gold is thiosulfate due to its low toxicity and efficiency in leaching gold from low-grade and carbonaceous ores and concentrates. Much of the published research focuses to optimizing the reagent concentrations to develop a suitable process for industrial applications. However, there are few studies on the effect of silver content on gold dissolution rate in the thiosulfate leaching system. In this paper, the dissolution rate of pure gold, pure silver, and gold–silver alloys of silver compositions, 4, 8, 20, and 50 wt% are reported. The effects of various parameters on the rate of gold and silver dissolution have been determined. The results show that the pure silver dissolution rate is around 6.89 times greater than pure gold. The results show that silver enhances gold dissolution in thiosulfate solutions. The dissolution rate of gold from pure gold is higher than that obtained from 4% and 8% silver alloys, but less than that obtained from 20% and 50% silver alloys.