Gold Leaching Research Articles

Carbon-in-Leach Gold Recovery from Fungi-treated Carbonaceous Ore: Effect of Entrained Biomass on Activity of Activated Carbon

The ability of the fungus, <i>Phanerochaete chrysosporium</i> to reduce preg-robbing of carbonaceous matter (CM) in gold ores has been confirmed by many researchers, and studies are ongoing to minimize the effect of entrained biomass on subsequent gold leaching and adsorption processes. This paper presents a study on gold extraction from surrogate carbonaceous gold ore (CGO), and fungal-treated CGO to ascertain the influence of entrained biomass on the downstream carbon-in-leach (CIL) process. The surrogate CGO was prepared by adding 3% anthracite-grade CM to free-milling gold ore (FGO). The main minerals in the FGO were quartz, feldspar and sericite, with 8.4 g/t gold, 0.18% sulphide sulphur and 0.06% organic carbon. Aside biotreatment of the CGO with cell-free liquor of <i>P. chrysosporium</i>, anthracite and activated carbon (AC) were also contacted with <i>P. chrysosporium</i> to confirm the direct effect of biomass on the CMs’ ability to preg-rob aurocyanide. Preg-robbing effect of the as-received anthracite and AC were 95% and 80% respectively, and these reduced to 81% and 13% respectively. Water-washing, acid-washing and alkaline-washing of the treated CMs returned respective preg-robbing effects of 88%, 92% and 85% for AC and 32%, 38% and 28% for anthracite. Scanning electron microscopy and Raman spectroscopy of anthracite revealed a decrease in the degree of orderliness in the structure required for gold adsorption. The FGO and the prepared CGO gave cyanidation gold recoveries of 94.5% and 54.8% respectively. Following cell-free treatment of the CGO, direct cyanidation recorded 88.3% solution recovery, which was increased to 91.3% in CIL. The overall recovery onto activated carbon in CIL was 81%, which improved to 82% and 85% respectively after washing the fungal-treated CGO with water and NaOH. The results here affirm that thorough washing of fungal-treated CGO assists in removing some entrained biomass. However, the results also call for additional studies on purification of the cell-free liquor to further minimize the biomass effect, and sustainability of the enzyme activity during pretreatment to enhance the overall gold recovery.

Recycling of gold from waste electronic components of devices

Tremendous generation of e-waste and its illegal recycling are causing immense threat to environment as well as the loss of precious metals. The present research reports a novel hybrid process for the total recovery of gold from small depopulated components of e-waste. Connectors and integrated circuits (ICs) liberated from printed circuit boards (PCBs) were pulverized and processed for gold leaching using 10 g/L sodium cyanide solution at 40 °C and mixing time 15 min, where more than 95% gold was found to be leached out in single stage. From the obtained leach liquor, gold metal was recovered by charcoal adsorption followed by heat treatment. The raffinate left after adsorption of gold was found to contain ~10 mg/L gold, which was also recovered using Amberlite IRA 400Cl at equilibrium pH 9.6 in 30 min maintaining aqueous/resin (A/R) ratio 25 mL/g. The raffinate solution was enriched to 882.41 mg/L and the solution was further processed to get metal/salt using cementation/ evaporation. Obtained leached residue is processed for non-ferrous metals recovery and finally disposed-off after treatment and TCLP test. The effluent left after leaching could be easily decomposed and treated in ETP using standard environmental procedure.

Selective Leaching of Gold from Copper Anode Slime Using Concentrated Hydrochloric Acid and Hydrogen Peroxide

Selective Leaching of Gold from Copper Anode Slime Using Concentrated Hydrochloric Acid and Hydrogen Peroxide

Selective Leaching for Uranium and Gold from their Bearing Sedimentary Lower Carbonaceous Sandstone Rocks, Southwestern Sinai, Egypt

Uranium as a nuclear element with strategic importance and gold as one of the important economic precious metals made their selective leaching from the sedimentary lower carbonaceous sandstone rocks, Southwestern Sinai, Egypt with economic concentrations (5ppm gold and 160 ppm uranium), an important issue for facilitating either their accurate determination or highest percentage recovery. A sequential leaching of uranium followed by gold was proposed in this work after studying individually their optimized leaching conditions. From the latter; a lixiviant mixture of sample to sodium carbonate ratio 1:1, sample to sodium bicarbonate ratio 1:1 together with sodium thiosulphate, as oxidant, 0.5g was used for the selective leaching of uranium. On the other hand, a leaching reagent mixture of 5% (w/v) from each of potassium cyanide, sodium thiosulphate and 10 ml from 30% hydrogen peroxide was used for the selective leaching of gold where several factors were optimized; concentration of individual component in each lixiviant, agitation time, solid to liquid ratio, temperature and pH. A percentage leaching of 97% and 96% for uranium and gold respectively were reached. Uranium was selectively separated using Amberlite IRA 402 Cl resin ion exchange resin with 99.5 % percentage recovery. The recovery of gold needs a large bulk weight from the sample due to its relatively low concentration.

A fundamental study of gold leaching in a thiosulfate‑oxygen‑copper system in the presence of activated carbon

Activated carbon has been found to significantly enhance the dissolution of gold in a thiosulfate‑oxygen‑copper system. Leaching tests using gold powder showed that the gold dissolution in a thiosulfate‑oxygen‑copper system in the absence of activated carbon was very slow, with only 2% gold dissolved over 24 h. In contrast, in the presence of activated carbon as an additive the gold leach recovery was dramatically improved to over 95% in the same period. Leaching and electrochemical studies using a gold rotating disc electrode have identified two impacts of activated carbon on the dissolution of gold: 1) enhancing the oxygen reduction half reaction via galvanic interaction and; 2) enhancing the gold oxidation half reaction via removal of the surface passivation. A combination of the two effects can potentially increase the gold leach rate to the same order of magnitude as a typical cyanidation rate (10−5 mol m−2 s−1). It has also been found that the presence of copper and the use of an elevated temperature are beneficial for the gold leaching; whilst the use of oxygen instead of air and change in pH between pH 7 and pH 10 have little impact.

Insights into the Chemistry and Structural Features of the Copper(II) 2,2′-Bipyridyl–Thiosulfate System

In this paper, we present our findings on a series of copper(ii) 2,2′-bipyridyl (bipy) complexes that inhibit the oxidation of thiosulfate, a current problem in the gold leaching process. The formation of six complexes, five of which have been structurally characterized by X-ray crystallography, illustrate a thermally induced, controllable switching between oxidation states, which in turn inhibits the oxidation of thiosulfate. These findings give further insight and understanding into the rich chemistry of the coinage metals and the hydrolytic processes involved with gold leaching.

Silver, gold and palladium leaching from electronic scrap using bromine- bromide solution

This study investigated the thermodynamics and kinetics of gold and accompanying noble metals, such as silver, palladium and platinum, leached from the electronic scrap by bromine-bromide solutions. Theoretical and practical analysis of the behavior of gold in the traditional Br2 - Br- - H2O system confirmed the fact that in a neutral and slightly acidic medium, when bromine hydrolysis proceeds slightly, a bromine in bromide solution can be used to leach gold. In this case, bromine-bromide leaching compared with cyanidation is more favorable in terms of kinetics and selectivity. In particular, when brominated in a neutral medium as a result of the formation of a passivating copper monovalent copper (Cu2O) film on the surface of copper particles, copper does not go into solution, while copper forms stable soluble anionic complexes with cyanide. For the first time in order to stabilize the pH in the alkaline area the conditions for gold leaching in the presence of a phosphate buffer solution (NaH2PO4) were investigated. The effect of the pH of the leach solution, the concentration of active bromine, bromide ions and the concentration of the buffer solution on the leaching kinetics of gold was studied. It was established experimentally that a noticeable dissolution of gold begins at pH ≤ 8, and at pH 6, almost all gold passes into the solution. A sufficient concentration of active bromine under these conditions can be considered 6.6 g · dm-3 Br2 at a bromide concentration of 20 g · dm-3 NaBr. Leaching with the productive solution turnover made it possible to reduce the consumption of bromine from 89-95 to 20-32 kg Br2 per ton of scrap and raise the concentration of gold from 76 to 195 mg · dm-3. In this paper, it was first shown that silver and palladium in the system under study (pH = 5-6) begin to dissolve noticeably only at high concentrations of bromide ions. Complete dissolution of palladium is achieved with an excess of bromide ions and pH values of 1.5-2.0. Platinum turned out to be more resistant to bromine-bromide leaching because of the formation of a sparingly soluble platinum dibromide on its surface.

Formation Process of the Passivating Products from Arsenopyrite Bioleaching by Acidithiobacillus ferrooxidans in 9K Culture Medium

Arsenopyrite is a common sulphide mineral occurring in deposits of gold ore that makes the extraction of gold difficult and, thus, pre-treatment is necessary prior to gold leaching. Bioleaching pre-treatment of arsenopyrite has drawn significant attention owing to its environmental friendliness, low cost and simple operation. A critical impedance of bioleaching to its large-scale industrial application is the slow leaching kinetics. Various passivating products on the surface of arsenopyrite have been found to limit the bioleaching process. This paper reports results from an in-depth investigation into the formation process of passivating products from arsenopyrite bioleaching by Acidithiobacillus ferrooxidans in 9K culture medium including bioleaching experiments and physicochemical analyses of the materials as well as thermodynamic analyses of the leaching system. The results of phase transformation and morphological change of the solid products suggest that the passivation occurring in the bioleaching of arsenopyrite is largely attributed to an initially formed passivating film consisting mainly of realgar (As2S2), orpiment (As2S3) and elemental sulphur (S0) on the arsenopyrite surface. Based on the results, the paper also proposes possible passivation mechanisms to allow for a better understanding on the passivation behaviour of the bioleaching of arsenopyrite.

Effect of an additive TEA on thiosulfate leaching of low sulfur gold concentrate

The environmentally friendly and non-cyanide thiosulfate gold leaching technology was used to extract gold from a low-sulfur gold concentrate. The effect of TEA on leaching of pure gold under the condition of adding quartz indicated that TEA could significant increase the dissolution rates of gold and the mixed solution potentials and decrease the consumption of thiosulfate. Quartz has little effect on the gold leaching process. Moreover, the effect of additive Triethanolamine (TEA) on the gold extraction was further studied. The chemical analysis and process mineralogy analysis of the concentrate indicated that the concentrate contained 59 g/t Au, 64.49% SiO2, 19.55% Al and 5.76% Fe. The concentrate was composed of quartz, muscovite and chlorite as major minerals. Quartz was the main gold-bearing mineral in concentrate. The gold particles of less than 20 μm in the concentrate reached 50.56%. Traditional thiosulfate leaching technology was used to extract gold from the concentrate, and the gold leaching rate was only 44.49% after 48 hours. When 0.024 M (MTEA:Mcopper=2:1) TEA was added in the thiosulfate leaching gold system, the thiosulfate consumption significantly reduced by about 32% and the leaching rate of gold increased to 73.40%.

SIMULATION AND EXPERIMENTAL INVESTIGATION OF POWER CONSUMPTION, GAS DISPERSION AND MASS TRANSFER COEFFICIENT IN A MULTI-PHASE STIRRED BIOREACTOR

Abstract Bio-oxidation leaching of refractory gold concentrate was carried out in a typical gas-liquid-solid three-phase stirred bioreactor. The bacteria, which adhered to solid particle surfaces and spread over the liquid phase, require sufficient oxygen and nutrient of carbon and nitrogen, and then the mass transfer rate should be considered. However, the mass transfer coefficient, which could be used to evaluate the mass transfer rate, was determined by gas holdup, bubble diameter and power consumption. Therefore, a three-phase Eulerian equation with k-e turbulence model and additional models of two bubble diameter, a torque and a mass transfer coefficient were applied in the simulation. The simulation results were validated with experimental data and used to analyze the spatial distribution of the mass transfer coefficient. The results demonstrated that the simulation and experimental data of Sauter mean bubble diameter and power number were in good agreement and the simulation results of the mass transfer coefficient were agree with the values calculated by an empirical correlation. The Davoody model can be strongly recommended for further simulation of the bubble diameter, and the gas holdup and mass transfer coefficient were very unevenly distributed in the lower zone. Hence, the lower impeller clearance should be considered.

Electrochemical influence of quartz on cyanide leaching of gold

Quartz is a common gangue mineral in the gold ore. The electrochemical behavior of gold electrodes was investigated in a quartz system with different particle sizes using cyanide leaching. Electrochemical experiment results show that the peak current density varied with the quartz particle size, the dissolution speed of gold increased and the adsorption rate of gold on quartz increased with quartz particle size decreased. It was suggested that the quartz could improve the dissolution speed of gold in cyanide leaching, while the leaching rate of gold reduced, due to the decreasing of the passivation film formation on its surface.

Ammonium thiosulfate extraction of gold from printed circuit boards (PCBs) of end-of-life mobile phones and its recovery from pregnant leach solution by cementation

In this study, the ammonium thiosulfate leaching of gold (Au) from printed circuit boards (PCBs) of end-of-life mobile phones and the subsequent recovery of dissolved metals from pregnant leach solution via cementation were elucidated under various conditions. Over 99% of Au was extracted from crushed PCBs (D50 = 85 μm) under the following conditions: 1 M of Na2S2O3, 10 mM of CuSO4, 1 M of ammonia/ammonium concentration, 0.1 g/10 ml of solid-to-liquid ratio, and a 24-h of leaching time in the presence of oxygen. In contrast, the recovery of Au ions via cementation was more favorable in the absence of oxygen. Among the various cementation agents evaluated, copper (Cu) was the most selective and recovered around 95% of extracted Au from the pregnant leach solution even in the presence of various coexisting metal ions. The cementation of Au exhibited two distinct kinetic regions: (1) an initially rapid rate, and (2) a more gradual and constant rate. Finally, the use of Cu plates to recover Au ions from ammonium thiosulfate pregnant solution of crushed PCBs is proposed, which could selectively recover about 80% of dissolved Au and is easier to handle and reuse compared with Cu powder.

Eco-friendly leaching of gold from a carbonaceous gold concentrate in copper-citrate-thiosulfate solutions

Thiosulfate is considered to be one of the most promising alternative reagents to cyanide due to its non-toxicity and excellent leaching efficiency. However, the use of ammonia as the additive and high reagent consumption have severely limited the development and commercial application of the traditional copper-ammonia-thiosulfate leaching system. In this work, the potential use of an environmentally friendly copper-citrate-thiosulfate leaching system for gold leaching from a refractory carbonaceous gold concentrate has been examined. The experimental results showed that when citrate served as the stabilizer, the copper-citrate-thiosulfate system exhibited similar extraction capability for gold with a significant decrease in thiosulfate consumption compared to the traditional copper-ammonia-thiosulfate system. Thiosulfate consumption increased with Cu2+ and S2O32− concentration, leaching time and temperature, but decreased with citrate concentration and initial pH. The mechanism of the catalysis of gold dissolution in copper-citrate-thiosulfate solutions by the binuclear cupric-citrate complex (Cu2Cit2H−24−) is presented based on the thermodynamic analysis. The stability of the Cu2Cit2H−24− complex decreased with increasing Cu2+ and S2O32− concentration, increased with an increase of citrate concentration. Moreover, the microwave roasting process applied to concentrate facilitated the subsequent gold leaching process probably by exposing encapsulated gold to thiosulfate solutions.

Effect of Oxidative Pretreatment and Lead Nitrate Addition on the Cyanidation of Refractory Gold Ore

Cyanidation is the main process in gold leaching. There are several parameters that affect gold recovery and cyanide consumption. In this paper, the effect of the oxidative pretreatment and lead nitrate addition on the cyanidation of Zarshuran refractory gold ore containing 3.32 ppm Au, 1.09% As, and 0.5% sulfur were investigated. Au recovery was increased from 79.4% to 89.4% in the presence of 500 g/t lead nitrate. Results of pretreatment experiments showed that lead nitrate had little effect on the Au recovery in comparison to the direct leaching condition. Cyanide consumption in the direct leaching tests was about 2.5 times higher than that of pretreatment-leaching tests. The amount of arsenic in the pretreatment solution was dramatically decreased and hence the cyanide consumption was decreased. Thermodynamic calculations showed that the precipitation of arsenic as $$ {\text{Ca}}_{3} \left( {{\text{AsO}}_{4} } \right)_{2} \cdot 6{\text{H}}_{2} {\text{O}} $$ is the main reason for lower cyanide consumption in the pretreatment-leaching tests. The results showed that cyanide consumption is still high (about 2 kg/t sodium cyanide), and that copper ions could be considered as one of the cyanide consumers that needs further investigation.

Enhancing gold recovery from refractory bio-oxidised gold concentrates through high intensity milling

ABSTRACTThis paper aims at investigating the gold leaching behaviour of refractory, bio-oxidized flotation gold concentrates, before and after high intensity milling, for enhancing gold extraction. Specifically, the role of factors such as milling time and ceramic balls-to-pulp mass ratio, coupled with corresponding physico-chemical changes on gold extraction yield and leaching kinetics was studied. From the results, high intensity milling of the bio-oxidised flotation concentrate reduced the average particle size by increasing the fraction of fines mainly through abrasion/attrition, and increased surface area by both fine particle and crevice production. Furthermore, notable increase in sodium cyanide consumption was observed with an increase in the mechanical stress deployed, consistent generally with degree of mineral relative amorphisation and reactive gangue mineral exposure. About 1.6–1.8 times variable increase in gold yield was observed after 24 h cyanide leaching of activated samples, irrespective of the impacted mechanical stress and degree of mineral amorphisation.

Silver, gold and palladium leaching from pre-prepared electronic scrap using bromine-bromide solution

The leaching of silver, gold, and palladium from printed circuit boards in the bromine - bromide system after preliminary removal of the associated metals and copper has been investigated. It was established experimentally that the dissolution of gold with a turnover of leaching solution from a previously prepared PCB concentrate in comparison with the leaching of the original electronic scrap proceeds at a higher rate and much more completely, so the average degree of gold extraction rose from 87.2 to 89.39%. The consumption of bromine decreased from 20-32 to 12 kg Br2 per ton of scrap. So by seven leaching stages, it was possible to obtain a productive solution with the content, g · dm-3: 0.0652 Au, 0.015 Pb, 0.00052 Cu. Pretreatment PCB concentrate also increases the rate and completeness of silver and palladium leaching. At the same time, preliminary removal of the main part of copper allows reducing its content in the productive solution by almost 10 times. Under the conditions of collective leaching of silver, gold and palladium with a slightly acidic (pH = 1.98) bromine-bromide solution, with a content of 100.8 g · dm-3 of bromide ions, it is possible to convert 98.64% Au, 97.23% Ag and 98.33% Pd to the solution. The kinetics of precipitation of gold, silver and palladium from productive solutions using zinc dust, copper powder and hydrazine solution has been studied. As a results it was to precipitate gold on zinc dust in the form of cement powder with a purity of 99.4% and collective (Ag-Pd) and (Au-Ag-Pd) products using a 10% solution of hydrazine with content, mass. % : 1.18 Cu, 90.6 Ag, 7.2Pd and 1.2 Cu, 83.31Ag, 7.15 Pd, 7.95 Au, respectively.

Effect of Triethanolamine as a New and Efficient Additive on Thiosulfate-Copper-Ammonia System Leaching of Gold

A new and efficient additive, triethanolamine (TEA), was used on pure gold leaching in a thiosulfate-copper-ammonia system. An appropriate TEA concentration enhanced the gold dissolution rate and reduced thiosulfate consumption. The beneficial effect became most pronounced in the case of MTEA:Mcopper = 1:1, where the gold dissolution rate increased by approximately 50% and thiosulfate consumption decreased by approximately 10%. The possible mechanisms of TEA are as follows: TEA combines with cupric and ammonia, which enhances the catalytic oxidation ability of Cu(II). Meanwhile, TEA can reduce the consumption of thiosulfate caused by the oxidation of Cu(II) and hence decreases the formation of passivation layers on the gold surface. Furthermore, the deprotonated ethanolic oxygen atoms of [Cu(NH3)x(TEA)y] act as bridging ligands, which could form an “electronic bridge” that benefits the electron transfer from Au0 to Cu2+. In addition, the actual main reaction complex in the cupric-ammonia-TEA complex ([Cu(NH3)x(TEA)y]) was [Cu(NH3)3TEA]2+.

Chemical oxidation of arsenopyrite using a novel oxidant—Chlorine dioxide

Arsenopyrite is a typical arsenic-containing mineral, and gold is generally hosted by arsenopyrite mineral which adversely affects gold leaching. In this study, a novel environmentally friendly oxidant, chlorine dioxide (ClO2), was used for the first time to efficiently oxidize arsenopyrite under acidic conditions, preventing the adverse effect of arsenopyrite on gold leaching. The addition of solid NaClO2 to acidic arsenopyrite slurry made the oxidation of arsenopyrite feasible for practical purposes, as NaClO2 generates ClO2 under acidic conditions, which then oxidizes arsenopyrite. For a 400 mL sample of arsenopyrite pulp with the initial concentration of 4.07 g/L (0.025 M), the oxidation efficiencies of Fe and As in arsenopyrite reached 94.25% and 85.39%, respectively for NaClO2 and H2SO4 concentrations of 0.125 M each, under magnetic stirring for 1 h at 30 °C in a constant-temperature water bath. At the end of the reaction, both the acidity and oxidation-reduction potential of the system increased, and the ClO2 concentration correspondingly decreased. Chemical composition analysis, X-ray diffraction analysis, particle size distribution measurements, scanning electron microscopy combined with energy dispersive spectrometer analysis, and X-ray photoelectron spectroscopy of arsenopyrite mineral ore and its oxidized residues were performed under different reaction conditions. Analysis results indicated that tiny amorphous precipitates of the oxidation by-products, such as FeAsO4·7H2O, H3AsO3, Fe2(SO4)3·7H2O, and hydrated ferric oxides would cover the surface of unreacted arsenopyrite at low H2SO4 concentration, which prevented the oxidation-reduction reaction from proceeding. As the concentrations of H2SO4 and NaClO2 increased, fine arsenopyrite particles were oxidized first, and ClO2 gradually eroded the surface of the unreacted arsenopyrite minerals to form cracks, defects, and pores, until arsenopyrite was fully oxidized. ClO2 presents several advantages: it can be easily prepared, it is a strong oxidant, and is non-polluting. Therefore, the procedure in this study is expected to be used as a novel method for preventing the effect of arsenopyrite enwrapping gold in refractory gold ores and for directly leaching gold.

Gold Leaching with Humic Substances

The study results of gold leaching using the products of humic acids, as well as stage-by-stage fractionation of parent and gold-containing substances of these acids are presented. The gold content in the pregnant solution was up to 14–30 mg/l. Chemical analysis of the supernatant obtained by a stepwise change in pH of the solution and subsequent centrifugation determined that gold-bearing acids contain both organic and dissolved gold, which is stable to precipitation at pH 2. Fractionation of the original humic acids and subsequent leaching of gold by its individual fractions slows down the dissolution kinetics, and the fractions distinguished at different pH vary significantly in their activity. The most active is the fraction obtained by centrifugation at pH value of 4.6. The spectrum is presented, and the kinetics of gold dissolution by these acids modified by the cyanide complex is shown.

Recovery of Precious Metals from Discarded Mobile Phones by Thiourea Leaching

The objective of this work was to recover gold and silver from the printed circuit board (PCB) of discarded mobile phones by thiourea leaching. Effects of thiourea concentration, leaching temperature, leaching time, and ferric ion (Fe3+) concentration on the recovery of gold and silver were investigated. The PCB was pretreated physically to reduce the thiourea consumption and enhance the leaching process. It was found that the percentage of gold leaching was higher than that of silver at different conditions studied. The highest percentages of gold (96%) and silver (90%) leachings were achieved with 20 g/L of thiourea and 8 vol% of Fe3+ at 4 h of leaching time and 25°C of leaching temperature.