Gold Electrowinning Research Articles

Towards continuous selective electrowinning of gold from waste printed circuit boards in acidic chloride medium: a parametric study

Towards continuous selective electrowinning of gold from waste printed circuit boards in acidic chloride medium: a parametric study

The role of thiosulfate and sulfite in gold thiosulfate electrowinning process: An electrochemical view

In this study, electrochemical and analytical techniques are employed to explore the effect of thiosulfate, sulfite, and a mixture of thiosulfate-sulfite in the electrolyte on the gold electrowinning process. The open circuit potential (OCP) analysis showed negative potential shifts from − 0.02 V (in the electrolyte containing Au-TS (E1)) to − 0.21 V, − 0.33 V, and − 0.42 V in the presence of thiosulfate oversupply (electrolyte E2), sulfite (electrolyte E3), and thiosulfate-sulfite mixture (electrolyte E4). The linear sweep voltammetry (LSV) results exhibited − 0.4 V potential shift by the addition of sulfite to the gold-thiosulfate electrolyte, attributing to the formation of Au(S2O3)(SO3)25- as the most stable gold complex. Based on chronoamperometry (CA) and atomic absorption spectrometer (AAS) results, sulfite could control the kinetics and thermodynamics of gold reduction reactions by minimizing undesired cathodic side reactions such as thiosulfate decomposition or water reduction, resulting in purer gold deposition, elimination of colloidal sulfur, and a more efficient EW process. The presence of thiosulfate accelerated the kinetics of the gold reduction reaction. However, adding sulfite to the electrolyte stabilized the kinetic about 10 times faster. SEM and EDS analysis depicted the morphological changes in the deposited gold layer from different electrolytes during CA, showing that thiosulfate oversupply results in the formation of a greater number of smaller grains by accelerating the nucleation process, while sulfite addition led to the formation of a smoother and more uniform gold layer on the cathode surface.

Electrowinning of gold from a dilute solution using hydrocyclone-type cell

ABSTRACT The redox reaction mechanism of gold−cyanide [Au(CN)2 −] during hydrocyclone electrowinning was investigated in this work by conducting Hull cell test, potentiodynamic polarization, and computational fluid dynamics (CFD) simulations. A leaching solution of waste printed circuit boards (WPCBs) was prepared as an electrolyte, and the effects of flow rate and applied voltage on the recovery of gold and destruction of cyanide through hydrocyclone electrowinning were investigated. A numerical simulation of the flow field in the cell was performed, and the results were visualized by conducting a finite element analysis in FLUENT. The CFD simulations and Hull cell test showed that the cylindrical cathode region had a flow velocity of ~11.05 m·s−1, which was higher than that at the center of the cylindrical part (<6.45 m·s−1). The optimum conditions for electrowinning were found to be as: an electrode distance of 4.9 mm and a current density of 73.78 mA·cm−2. The anodic polarization results showed that an Ir-based mixed metal oxide (MMO) anode could more efficiently oxidize the free cyanide because of its high current density than other types of anodes (Ir-based MMO: 299.3 mA·cm−2 > SS304: 3.5 mA·cm−2 > Pt-coated Ti: 1.3 mA·cm−2). Through the demonstrated hydrocyclone electrowinning test, the gold concentration could be decreased from 100 to 0.24 mg·L−1 within 12 h while destroying almost total amount of free cyanide in a day.

Carbon Re-Adsorption of Gold from Eluate as an Alternative to Gold Electrowinning

Carbon Re-Adsorption of Gold from Eluate as an Alternative to Gold Electrowinning

Failure investigation of stainless steel anodes used in gold electrowinning

Failure of austenitic stainless steel anodes in a gold electrowinning process (EW) was reported after a short period of service (3–6 weeks). Different analytical and characterization techniques were employed to conduct root cause failure analysis (RCFA) of the failed anodes. Electrochemical behaviour of the stainless steel alloy in the industrially supplied and synthesized electrowinning solutions were evaluated using different electrochemical test methods. The main failure mechanism of these anodes was found to be transpassive dissolution. Local solution acidification in the pores and crevices also contributed to the high corrosion rate of the stainless steel anodes as the transpassive dissolution rate increases when the pH of the electrowinning solution decreases. Gold and silver were found in the anodic layer along with typical stainless steel corrosion products (iron and chromium oxides), contributing to gold loss in the electrowinning process. Materials selection, design change, solution and process modifications are recommended to prevent similar failures or maximize the lifetime of the anodes.

Non-Precious Group Metal Electrocatalysis of Sulfite Electro-Oxidation Reaction for Gold Electrowinning Processes

The electro-oxidation of sulfite and its radicals has a wide range of applications from gold electrowinning to wastewater treatment, hydrogen production and in the food industry (1-4). While efforts have been made to study the electro-oxidation of sulfite ions on noble metals (e.g. Au) as well as carbon surfaces, metal oxides in general have not been thoroughly investigated for this electrochemical reaction (1, 5). During gold recovery from thiosulfate solutions, Au+ ions are reduced to Au from the thiosulphate gold complex on the cathode while oxygen evolution occurs on the anode via the following reactions (6): Au(S2O3)2 3- + e- = Au + 2S2O3 -2, E0= 0.15 V (1) 4OH- = 2H2O + O2 + 4e-, E0= 0.4 V (2) To drive the cathodic reaction, high current densities are used, which result in high anodic potentials on the stainless-steel anodes. Such high anodic potentials may cause transpassive corrosion of the anode as well as significant, and undesirable, changes in solution chemistry (7). This study systematically investigated the effect of operating factors on the corrosion behavior of stainless steel anodes in thiosulfate solutions during gold electrowinning. Further, novel non-precious group materials such as manganese oxides and perovskites were used as electrocatalysts for sulfite electro-oxidation to ultimately enhance the activity and long-term durability of the anodes in the gold electrowinning process (8). A 2n half-fraction factorial design study was performed to find the effect of five important parameters involved in the gold electrowinning process (i.e. thiosulfate concentration, sulfite concentration, sulfate concentration, temperature and applied anodic current density), on the anodic potential of the stainless steel anode. As shown in the Fig. 1A, increasing the sulfite concentration and temperature of the gold electrowinning solution lead to a low anodic potential of 550 mV which is about 250 mV lower than the transpassive corrosion potential for the 316 stainless steel anodes. To further reduce the anodic potential and employ the high sulfite concentration (i.e. 3000-32000 ppm) in the gold electrowinning solution, non-precious metal oxides were tested to facilitate the electrowinning process by catalyzing the following sulfite electro-oxidation reaction instead of the oxygen evolution reaction (eq. 2): SO3 2- + 2OH- = SO4 2- + H2O + 2e-, E0= -0.93 V (3) Fig. 1B shows that a combination of manganese oxide and perovskite electrocatalysts provides promising electrocatalytic activity and durability for sulfite electro-oxidation (i.e. up to 500 mV lower onset-potential over 7 hours of testing at 3.5 mAcm-2) compared to the Ni, commercial Ag/AgO, Pt, carbon, commercial Pb-2 wt% Ag and stainless steel electrodes tested here. Fig. 1 A) Surface plot produced from the 2n half-fraction factorial design study on the effect of five electrowinning parameters on the performance of 316 stainless steel gold electrowinning anode. B) A comparison between the electrocatalytic activity of various electrocatalyst materials for sulfite electro-oxidation in a solution of 100 ppm NaCl + 3000 ppm Na2SO4 + 32000 ppm Na2SO3 at 3.5 mA cm-2 and 45 °C. All MnO2-based catalysts are under the license with Catalyst Square Materials Ltd (8).

Ion-exchange membranes in gold electowinning processes on flow-through carbon fiber electrodes

The change in the properties of ion-exchange membranes preventing the negative effect of anodic reaction products on the electrowinning of gold on carbon fiber cathodes from (1) sulfuric acid thiourea eluates formed during the extraction of gold from ores and (2) citrate–phosphate washing solutions from gold plating of electronic components has been studied. By using electron microscopy, microanalysis, and atomic absorption spectroscopy, it has been shown that the main cause of the change in the properties of the membranes during their operation is the inclusion of components of the solutions subjected to treatment and the products of electrode reactions in the membrane. The service life of the membranes in said processes has been determined.

Recovery of silver from cyanide leach solutions by precipitation using Trimercapto-s-triazine (TMT)

Cyanide leaching is the most common method used for production of silver from gold/silver ores. Silver (and gold) can be recovered from pregnant leach solutions (PLSs) using Merrill-Crowe process (zinc cementation) and activated carbon adsorption. Due to their inherent limitations and paucity of selectivity, development of alternative processes for selective recovery of silver from PLSs is of great interest. In the current study, selective recovery of silver by precipitation from cyanide solutions using an environmentally friendly organic reagent i.e. Trimercapto-s-triazine (C3N3S3; TMT) was investigated. The results have shown that precipitation of silver by TMT is a rapid process in that 87.9% of silver could be precipitated at the initial period of 5min. Response surface methodology (i.e. central composite design) was adopted to examine the main and interaction effects of [TMT]/[Ag] ratio (0.09–36.52), [CN]/[Ag] ratio (3−10) and initial concentration of silver (1–40mg/L Ag) on the precipitation of silver in five levels. The statistical analysis of the experimental data revealed that ratio of [TMT]/[Ag] (i.e. concentration of TMT) was the key parameter exerting a profound effect on the precipitation of silver. However, effects of [CN]/[Ag] ratio and Ag concentration appeared to be statistically insignificant. Complete precipitation of silver was found to be readily achieved under suitable conditions. The precipitation process was proved to be highly selective for silver over gold and copper as also confirmed by the tests on real pregnant leach solutions. It was found that the presence of TMT had no discernible effect on adsorption of gold by activated carbon, which was observed to even improve from copper containing solutions. Silver-TMT precipitates were characterised using SEM/EDS, FT-IR and other techniques. The current findings demonstrate that this precipitation process can be exploited as a potent alternative for selective recovery of silver from cyanide leach solutions of silver and gold/silver ores or purified/concentrated solutions ahead of gold electro-winning.

Optimization of Influential Factors in Gold Electrowinning using Response Surface Methodology

Optimization of Influential Factors in Gold Electrowinning using Response Surface Methodology

Selective Electrochemical Gold Recovery from Leaching Solutions of Thiosulfate

This work presents a study of the selective electrowinning of gold from thiosulfate electrolyte, taking into account difficulties related with low gold concentration and metal impurities that usually limit gold recovery. The cathodic and anodic processes were evaluated in a potential window where gold and copper reduction and the oxidation of thiosulfate and Cu (I) species occurs. Three materials with different reactivity were evaluated as cathode: Pt, Ti and DSA. It was possible to define the effect of each material in the electrochemical reactions involved and to identify three potential regions where selective electrodeposition happens: the first one involves selective deposition of gold, the second accounts for the co-deposit formation of copper and gold and the last one for the massive copper electrodeposition. The use of DSA as cathode electrode showed an intermediate electrochemical behavior between Pt and Ti and good selectivity. For these reasons DSA is proposed as a good cost/benefit option for gold electrowinning process from thiosulfate solutions.

The deportment of mercury during thiosulfate leaching and resin-in-pulp recovery of gold from ores

This paper reports on the behavior of mercury and its compounds during the thiosulfate leaching and recovery of gold. Thiosulfate solutions are shown to dissolve Hg0 and Hg2Cl2 to a similar extent as cyanide, but the leaching of HgS is significantly slower in thiosulfate solutions compared to cyanide. Hence, the use of thiosulfate may be an option for treating ores containing high levels of cinnabar, especially since the legislation concerning the discharge or sale of mercury is becoming more restrictive. The deportment of mercury during gold thiosulfate loading and recovery using a strongly basic anion-exchange resin was also studied. Although the mercury thiosulfate complex loads on the resin, the adsorption is weaker than for gold, with an equilibrium constant between gold and mercury of 51. Resin loaded with gold, mercury and copper was pre-eluted with thiosulfate (to remove copper) and eluted with a chloride and sulfite eluant. The majority of the mercury is eluted by the eluant and, hence, its deportment closely follows gold and the eluate would need to be treated to remove either mercury during or prior to gold electrowinning.

Investigation of deceleration causes in gold electrowinning from aqueous cyanide solutions on porous carbon electrodes

Deceleration of gold electrowinning from model cyanide solutions on porous cathode of graphitized carbon felt was investigated. It was established that calcium ions present in the solution did not negatively affect the electrolysis rate. The main reason of the termination of gold electrowinning on the graphitized cathode was the corrosion of the stainless steel anode resulting in penetration into the cathode chamber of the electrolyzer of CrO 4 2− anions further reduced to Cr(III) and probably Cr(II) compounds. X-ray Photoelectron Spectroscopy investigation of the passivated carbon cathode showed that the film formed thereon consisted of Cr(III) compounds containing cyanide and hydroxy ligands. This film covers the active sites on the surface of the porous carbon cathode preventing the deposition thereon of ad-ions of gold(I). Inert aqua and hydroxo complexes of chromium(III) stronger impede the rate of gold(0) deposition on the porous carbon cathode than chromium(III) cyanide compounds. This fact originates presumably from the requirement of lower cathode potentials for the reduction of the former to labile chromium(II) complexes than those necessary for the thermodynamically stable anions [Cr(CN)6]3−.

Biogenic sulphide for cyanide recycle and copper recovery in gold–copper ore processing

Many complex-ore gold deposits with cyanide-soluble copper minerals such as chalcocite, covellite, bornite, cuprite, malachite and azurite are now being considered for processing. In some cases the ore is not currently being mined largely due to the various issues associated with the cyanide-soluble copper minerals in the ore, which results in low gold recoveries and high cyanide consumption and detoxification costs. Moreover, the presence of copper in tailings supernatant tends to stabilize weak-acid dissociable cyanide in a form toxic to wildlife. In the SART process (sulphidization–acidification–recycle–thickening), the cyanide associated with the copper cyanide complexes is released by NaHS dosing to precipitate copper and convert cyanide to HCN gas under weakly acidic conditions, allowing it to be recycled back to the leach process as free cyanide. Copper is also recovered as a valuable, high-grade (∼70% Cu) Cu 2S by-product. Cyanide recycling allows the leach circuit to be operated at higher cyanide levels, maximizing leach efficiency and minimizing copper deportment to gold electrowinning. The first commercial plant is at Telfer in Western Australia. Biogenically-produced hydrogen sulphide can be used to replace chemical sulphide at significantly lower cost and with the added advantage of lowering the acid demand substantially. While the capital cost of a biogenic SART plant will be higher than a cyanide detoxification plant, this will be more than offset by the revenues generated by copper recovery and the savings realized by cyanide recycle, allowing for a short payback time on the incremental capital. BioteQ is currently working with Columbia Metals Corporation Ltd., Canada, to apply its sulphide generating and precipitation technology in a SART circuit at both of Columbia’s gold projects (La Jojoba and Lluvia de Oro) in Northern Mexico. Processing of ores from both mines has presented challenges resulting from the presence of cyanide-soluble copper. This paper reviews the BioteQ biogenic SART process technology, economics and commercial status as well as environmental and operating outcomes.

Gold electrowinning from diluted cyanide liquors: performance evaluation of different reaction systems

This work deals with the performance of porous electrodes by using electrolytic cells with similar characteristics to the Zadra's one and with mechanical devices to increase the mass transport to the cathode. The flow-by configuration was the experimental arrangements used. Gold electrowinning was carried out with distinct cyanide solutions, namely: pure synthetic solution and synthetic solution with metallic impurities. This study aimed at investigating the effect of some parameters on the direct electrowinning of gold from cyanide solutions and a comparative study of two kinds of three-dimensional electrodes (stainless steel mesh and mild still wool). The current efficiency and the energy consumption were also analysed considering three competing cathodic reactions: gold deposition, oxygen reduction and hydrogen evolution. A mathematical model was also evaluated aiming at determining, experimentally, the mass transport coefficients considering the reaction systems under study.

Electrowinning of gold from anode slimes

An electrowinning process using air agitation in an acid electrolyte is described for the recovery of gold at 25 °C from anode slimes of the Wohlwill electrorefming cell. The degree of purification of gold improved from 93.4% to over 99%. The rate of gold dissolution is estimated to be 2.2 g A −1 per h at 80 mA cm −2 and 3 V. The corresponding current efficiency is calculated to be 90%, which appears to be the limit of the process because of an adverse effect from the formation of an anode skin during electrowinning. The energy efficiency of the process is estimated to be about 46%. The feasibility of operating a prototype cell is described.

Elution and electrowinning of gold from gold-selective strong-base resins

The recovery of gold was investigated using the new gold-selective resin, Minix, which was developed by Mintek. The most suitable elution technique for this resin was found to be an acidic thiourea eluant. The gold-selective adsorption characteristics of Minix, particularly its high selectivity against cobalt and iron cyanides, make it possible to use the advantages of the thiourea method (fast kinetics and circuit simplicity) without fear of poisoning the resin, as is the case with ordinary commercial resins. Electrowinning is compatible with the elution technique and provides a simple and effective method of recovering the gold from the eluate. The composition of the eluate does not affect the electrowinning efficiency much and it is therefore possible to operate both the elution and the electrowinning at their optimum efficiency. The feasibility of elution with simultaneous electrowinning was demonstrated in the laboratory. Complete elution and gold recovery were achieved in less than 5 h, with an average current efficiency of about 50%.

The direct electrowinning of gold from dilute cyanide leach liquors

A cylindrical electrowinning cell employing a three-dimensional steel wool cathode has been used to investigate the effect of free cyanide concentration, pH, solution conductivity and base metal impurities on the electrowinning of gold from alkaline cyanide leach liquors containing up to 5 mg/l gold. The recovery of gold in a given time decreased: (a) as the pH decreased below 13, (b) if more than 10 mg/l of Fe, Cu, Zn or Hg were present in solution, (c) as the free NaCN concentration was increased, (d) with decreasing conductivity of the solution. High gold recoveries can be obtained with relatively low electrical energy consumption; greater than 96% recovery in one hour from 5 mg/l gold solutions with an electrical energy consumption of 112 kWh/kg at a current efficiency of 0.33% is possible. Experimentally derived graphs of solution concentration versus time of electrolysis were analysed by means of a simple reactor model and approximate mass transfer coefficients were calculated. These were compared with coefficients calculated from data on the performance of Mintek cells in South African gold recovery operations. The application of these data to the prediction of required electrode area and hence cost for a commercial dilute electrowinning installation is discussed.

Carbon-in-pulp processes in South Africa

The carbon-in-pulp (CIP) process is now well established in the South African gold industry, with a total of over 1 million metric tons of material being treated each month in six large plants. This tonnage is derived from ground ore, from filtered pulp, from calcine, and from reclaimed sand-tailings dumps. With this new process and the new large plants, those concerned with CIP are experiencing a learning process, since each new plant presents a different set of problems to be overcome. The knowledge and experience gained from these plants is being shared, and will be of benefit in the design and operation of new plants. The CIP process can be subdivided into the following unit operations: • prescreening at a fine size, • adsorption on carbon, • carbon acid-washing and elution, • carbon reactivation, and • gold electrowinning. Results concerning each of these unit operations, which were obtained from pilot plants and from large-scale CIP plants in South Africa, are presented. The latest developments in each of these areas are summarized.