Gold Leaching Kinetics Research Articles

Prospects for refractory gold-sulfide ore processing

Cyanide-refractory ores constitute 30 % of the world’s gold mineral resource base. With the global decrease in the availability of high-grade and free-milling ores, low-quality ores, including those rich in sulfur and arsenic, are increasingly being processed. The authors have conducted an assessment of the primary factors complicating the leaching process of refractory gold. These factors include the influence of gold distribution within the ore, the presence of preg-robbing effects, and the impact of cyanicidal minerals, notably pyrrhotite, on the leaching process. Sulfide minerals significantly affect the kinetics of gold leaching and associated reagent costs. The behavior of Fe5S6 is elucidated through the concept of “chemical depression”. Under cyanide leaching conditions, pyrrhotite actively and directly reacts with NaCN/KCN, undergoing surface oxidation by dissolved oxygen in the pulp. This leads to the formation of ferrocyanide complexes and rhodanides, which are unable to leach gold. Presently, there are two approaches to enhance the process parameters of refractory ore processing technology. The first approach involves the inclusion of preparation operations for cyanidation, aimed at liberating gold from the sulfide matrix (including hydrometallurgical and pyrometallurgical oxidation technologies and mechanical activation). An alternative approach is to use alternative reagents as leaching agents (notably thiourea, sodium and ammonium thiosulfates, and halides). The article explores means of modifying the technological process for gold extraction when ores contain substantial amounts of pyrrhotite or concentrates.

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.

Extraction of Gold and Copper from Flotation Tailings Using Glycine-Ammonia Solutions in the Presence of Permanganate

This study presents the novel idea of a cyanide-free leaching method, i.e., glycine-ammonia leaching in the presence of permanganate, to treat a low-grade and copper-bearing gold tailing. Ammonia played a key role as a pH modifier, lixiviant and potential catalyst (as cupric ammine) in this study. Replacing ammonia with other pH modifiers (i.e., sodium hydroxide or lime) made the extractions infeasibly low (<30%). The increased additions of glycine (23–93 kg/t), ammonia (30–157 kg/t) and permanganate (5–20 kg/t) enhanced gold and copper extractions considerably. Increasing the solids content from 20 to 40% did not make any obvious changes to copper extraction. However, gold leaching kinetics was slightly better at lower solids content. It was indicated that the staged addition of permanganate was unnecessary under the leaching conditions. Recovery of gold by CIL was shown to be feasible, and it improved gold extraction by 15%, but no effect was observed for copper extraction. Percentages of 76.5% gold and 64.5% copper were extracted in 48 h at 20 g/L glycine, 10 kg/t permanganate, 20 g/L carbon, pH 10.5 and 30% solids. Higher extractions could be potentially achieved by further optimization, such as by increasing permanganate addition, extending leaching time and ultra-fine grinding.

Recovery of Gold from Ore with Potassium Ferrocyanide Solution under UV Light

In this study, potassium ferrocyanide, a nontoxic cyanide precursor in dark and diffuse reflection environment, was applied as reagent for the leaching of gold. The free cyanide ions could gradually release from potassium ferrocyanide solution under the ultraviolet light. Orthogonal leaching experiments were performed in gold ore to analyze the effect of solution pH, potassium ferrocyanide dosage, and temperature in a potassium ferrocyanide solution system under UV light. Response surface methodology (RSM) was applied to explore the role of potassium ferrocyanide in gold leaching; optimized results showed that the gold recovery reached 67.74% in a high-alkaline environment at a 12.6 pH, 3.8 kg/t potassium ferrocyanide dosage, 62 °C, and irradiance of 10 mW·cm−2. The gold leaching kinetics were monitored by quartz crystal microbalance with dissipation (QCM-D) of potassium ferrocyanide solution. The results indicate that the gold extraction process could be divided into two stages: adsorption and leaching, and a rigid adsorption layer formed on the reaction surface. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis of the gold sensor surface after leaching reaction showed that –C≡N appears on the gold sensor surface, and the gold is oxidized to form AuCN complexes.

Gold deportment and leaching study from a pressure oxidation residue of chalcopyrite concentrate

Pressure oxidation (POX) is an effective method for base metal extraction and a pretreatment of refractory gold ores. A residue from pressure leaching of chalcopyrite concentrate was collected and investigated for gold recovery and deportment of metals. A comprehensive mineralogical analysis and gold deportment study were conducted using techniques including X-ray diffraction (XRD), optical microscope, scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX), and dynamic secondary ion mass spectrometry (D-SIMS). Gold leaching tests were performed using cyanide and glycine as lixiviants. The gold grade is 3.1 g/t, with 0.31% residual copper in the sample. Hematite was a dominant iron oxide (31.5%), and iron sulfate salts (45.4%) existed in various minerals phases. The gold deportment study revealed that 32.3% of the total gold was present as a visible gold, and the remainder was sub-microscopic gold. The visible gold was native gold with an average particle size of less than 20 μm. Colloidal gold, the dominant existence of invisible gold, was found in hematite, jarosite, and iron sulfate. Hematite was the major sub-microscopic gold carrier, forming a nonporous rimmed structure and locking gangue minerals randomly. Gold in arsenopyrite was found as solid-solution, accounting for 7.2%. Gold extraction of 84% was achieved by 500 mg/L NaCN, while only 35% of gold was extracted by 100 mg/L NaCN. A significant synergistic effect of cyanide and glycine was observed. The addition of glycine in cyanide leaching solution boosted the gold leaching kinetics and recovery. Gold recovery of 85% was observed with 100 mg/L NaCN and 0.3 M glycine in 24 h. In the hybrid leaching system, copper was stabilized by glycine and further utilized as a catalyst for gold leaching. To achieve higher gold extraction and lower lime consumption, a product with high sulfur oxidation degree and low iron sulfate content is preferable in pressure oxidation operation. Hematite is a stable product and does not impair gold leaching.

Leaching of a polymetal gold ore and reducing cyanide consumption using cyanide-glycine solutions

Many aspects of gold ores leaching by glycine (C2H5NO2) and glycine-cyanide solutions are still not well understood. The base-metal components have been shown to have a strong impact on gold leaching kinetics. In this research, with the aim of reducing cyanide consumption, leaching of a polymetal (Pb-Cu-Zn) gold ore using a synergistic lixiviant mixture of cyanide and glycine has been studied. The results showed that by increasing cyanide concentration from 500 to 1500 ppm, the gold dissolution increased from 46% to about 80%. The gold dissolution of 90% could be achieved by adding 0.5–1 M/L glycine to 500 ppm cyanide while the copper dissolution increases to more than 30%. Interestingly, 80% gold dissolution could be obtained by 0.5 M/L glycine even using 200 ppm cyanide over 72 h. At the optimal approach, cyanide consumption can be reduced by about 80% compared with 1500 ppm cyanide (without glycine). In the studied sample, besides Cu, Pb, Zn and Fe minerals are the other cyanide consumers. Their dissolutions increased as glycine concentration increased. This can be considered as a negative aspect of the cyanide-glycine leaching and indicates its poor selectivity for gold. Pb, Zn, Fe, Ag, and Cu are more sensitive to glycine concentration rather than cyanide concentration.

The ultrasound leaching kinetics of gold in the thiosulfate leaching process catalysed by cobalt ammonia

The effect of ultrasound on gold leaching rate and kinetics was investigated in the Co-NH3-S2O32− system. First, the effects of ultrasound power, temperature, the concentration of thiosulfate, cobalt and ammonia on gold leaching rate were studied under ultrasound. The optimized leaching parameters are 0.03 M Co2+, 0.2 M S2O32−, 1 M NH3, 323 K, and 750 W ultrasonic power. Moreover, the leaching rate of gold is 8 times faster, and the extraction percentage is approximately 25% higher, under ultrasound than under conventional conditions. The experimental results show that ultrasound can effectively enhance the leaching rate and the extraction percentage. The kinetic analysis indicates that the internal diffusion step controlled the ultrasound leaching process. The increase in the leaching rate under ultrasound is due to the decrease of activation energy from 22.65 kJ/mol to 13.86 kJ/mol. Therefore, ultrasound efficiently improved the leaching rate of gold in the Co-NH3-S2O32− leaching system.

The effect of flotation collectors on the electrochemical dissolution of gold during cyanidation

Gold-bearing ores, especially the sulphidic ones, are often treated by flotation prior to cyanidation. Following flotation process, the flotation concentrate or tailings are redirected to the cyanidation circuit for gold extraction that may contain appreciable amounts of collector used in flotation, and also remained sulfide minerals, i.e., predominantly pyrite. This current research examines the effect of collector on gold leaching by conditioning gold prior to cyanidation and direct addition in cyanidation in a packed bed leach reactor. Effects of pyrite, and solid-liquid separation on gold leaching and its electrochemical behaviour in presence of collector were also investigated. It was found that conditioning of gold with Aerophine 3418A in absence of pyrite has a detrimental effect especially on leach kinetics of gold since it adsorbs onto gold and decreases its further dissolution. In presence of pyrite, a significant improvement was obtained in gold leaching due to the positive galvanic interactions. These findings suggest that although collector adsorption adversely affects gold leaching, i.e., partial passivation, yet galvanic interactions could continue to enhance its extraction. Following conditioning, solid-liquid separation prior to cyanidation is shown to moderately increasing gold leaching by up to 10%. When Aerophine 3418A was added directly during cyanidation, gold leaching showed a slight increase in absence of pyrite. Conversely, in presence of pyrite, a significant increase in gold leaching was observed. Moreover, if collector was added after the first one hour of cyanidation, gold leaching kinetics was enhanced. Cyclic voltammetry tests within open circuit potential findings support the cyanide leaching results. SEM-EDS analysis of leach residues has demonstrated that there is less amount of gold in the residue both when pyrite is present and collector is added directly. XPS analyses have confirmed the presence of metallic gold in leach residues. These results have revealed the effect of Aerophine 3418A on gold leaching is different when it is directly added than that of conditioning and the positive galvanic effect of pyrite occurs even in the presence of collector.

Study on the properties and gold leaching kinetics of a new organic leaching agent

A new organic leaching agent (3YL) was applied the process of leaching gold in our recent research project. Compared to conventional cyanidation method, studies show that 3YL can serve as an alternative and non-toxic leaching agent, which has several advantages over cyanides including leaching speed, environmental effect, and other aspects. In this paper, the temperature and the acidity of the dissociation process for 3YL were investigated. The results show that within a certain range, the temperature and acidity have significantly positive effect on the dissociation. Meanwhile, the dynamic process of gold leaching was also studied. The experimental data are consistent with the existing model, and the results show that the process is controlled by external diffusion.

A review of factors affecting gold leaching in non-ammoniacal thiosulfate solutions including degradation and in-situ generation of thiosulfate

Cyanide has been used for more than a century as a preferred lixiviant for extracting gold from ores and concentrates. While cyanide is very effective in treating certain ores and concentrates, it is not suitable for carbonaceous and high copper-bearing ores. Research activities over many decades have indicated that thiosulfate has potential as an alternative lixiviant for extracting gold from problematic ores/concentrates which are not amenable for cyanidation. A number of studies have shown faster gold leaching kinetics with thiosulfate in the presence of ammonia and copper(II) in the form of a cupric tetraamine complex that acts as an oxidant for gold. However, ammonia is toxic and the lixiviant system is highly complex due to the presence of several ligands, which particularly affects speciation of the copper ions and their reactions in the system. For this reason, recent studies have been focusing on alternative oxidants that do not use ammonia. In this article, a comprehensive review of fundamental studies of leaching of gold from different ores in ammonia-free thiosulfate solutions is presented with special emphasis on factors that affect gold leaching kinetics including mineralogical composition of ores and concentrates. Important aspects of thiosulfate gold leaching such as thiosulfate stability, in-situ generation and consumption of thiosulfate and passivation of gold are reviewed.

Effect of Pyrite on Thiosulfate Leaching of Gold and the Role of Ammonium Alcohol Polyvinyl Phosphate (AAPP)

The effect of pyrite and the role of ammonium alcohol polyvinyl phosphate (AAPP) during gold leaching in ammoniacal thiosulfate solutions were investigated using pure gold foils. The results showed that pyrite catalyzed the decomposition and also significantly increased the consumption of thiosulfate. This detrimental effect became more severe with increasing pyrite content. Further, the presence of pyrite also substantially slowed the gold leaching kinetics and reduced the overall gold dissolution. The reduction in gold dissolution was found to be caused primarily by the surface passivation of the gold. The negative effects of pyrite, however, can be alleviated by the addition of AAPP. Comparison of zeta potentials of pyrite with and without AAPP suggests that AAPP had adsorbed on the surface of the pyrite and weakened the catalytic effect of pyrite on the thiosulfate decomposition by blocking the contact between the pyrite and thiosulfate anions. AAPP also competed with thiosulfate anions to complex with the cupric ion at the axial coordinate sites, and thus abated the oxidation of thiosulfate by cupric ions. Moreover, the indiscriminate adsorption of AAPP on the surfaces of gold and passivation species prevented the passivation of the gold surface by surface charge and electrostatic repulsion. Therefore, AAPP effectively stabilized the thiosulfate in the solution and facilitated the gold leaching in the presence of pyrite.

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.

Effect of silver on gold cyanidation in mixed and segregated sulphidic minerals

AbstractGold is mostly found in nature in metallic form and is associated with sulphide minerals and other precious metals, most often silver. Pyrite, chalcopyrite, sphalerite, and stibnite are the sulphidic minerals investigated in the present gold‐silver cyanidation study by adopting the metal‐sulphide multi‐layer packed‐bed reactor approach. Gold leaching kinetics was enhanced remarkably for the pyrite and chalcopyrite sulphide minerals, with 94.5 % and 85 % recovery respectively. The influence of a bilayer sulphidic mineral system on the dissolution of gold was also investigated with a maximum gold dissolution of 87 % for the pyrite‐chalcopyrite bilayer system. The effect of silver on the dissolution of gold was investigated with the gold associated with sulphidic minerals in an arrangement of mixed as well as segregated mineral layers. Three sets of mineral systems were established: pyrite‐chalcopyrite‐silica, pyrite‐sphalerite‐silica, and pyrite‐stibnite‐silica systems. The addition of silver enhanced the gold dissolution for the pyrite‐sphalerite‐silica system, slightly lessened the gold dissolution with the pyrite‐stibnite‐silica system, and retarded gold dissolution severely in the case of the pyrite‐chalcopyrite‐silica system, irrespective of the dispersion of gold and silver in the same as well as in the segregated mineral layers.

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.

Optimizing the thiosulfate leaching of gold from printed circuit boards of discarded mobile phone

An environmentally benign process involving thiosulfate leaching was developed in order to recover gold from the printed circuit boards (PCBs) of discarded mobile phone. The effect of concentration of the reagents such as thiosulfate, copper(II) and ammonia on the leaching of gold was investigated in the temperature range 20–50°C. Parameters were optimized through modeling of the leaching process using response surface methodology (RSM) based on central composite design (CCD). The optimum conditions for leaching of gold from PCBs were identified to be 72.71mM thiosulfate, 10.0mM copper(II) and 0.266M ammonia. The initial rate of gold leaching was found to be 2.395×10−5mol∙m−2∙s−1 under the optimum conditions. As regards the kinetics of gold leaching, the pseudo-second order kinetic model with chemical control was found to be applicable in the low concentration range (40–60mM thiosulfate, 5–7mM copper(II) and 0.22–0.247M ammonia), compared to that of pseudo-first order kinetic model at mid concentration range of the reactants viz., 60–70mM thiosulfate, 7–9mM copper(II) and 0.247–0.263M ammonia. The apparent activation energy of the reaction in the temperature range 20–50°C was found to be 78.6kJ∙mol−1. The samples were characterized before and after leaching using scanning electron microscopy (SEM) which corroborated the chemical controlled leaching mechanism.

Untangling galvanic and passivation phenomena induced by sulfide minerals on precious metal leaching using a new packed-bed electrochemical cyanidation reactor

Because permanent galvanic contacts between gold rotating disc electrodes and slurried sulfide-rich ores are uneasy to achieve, the standard gold rotating disc electrode/slurry cyanidation arrangement has a tendency to inflate overly the importance of passivation phenomena over the corrective trend of galvanic interactions inherently present within the ore grains. A new packed-bed electrochemical reactor (PBER) was thus developed and tested to decouple and quantify the individual contributions of passivation phenomena (PP) and galvanic interactions (GI) on precious metal (gold and silver, PM) leaching rates during the cyanidation of sulfide-rich ores. The PBER was filled with homogenized mixtures of minerals (pyrite, chalcopyrite, sphalerite, chalcocite, galena, stibnite, and industrial ore), gold and silver powders, which were arranged as electrically isolated sulfide-quartz segregated bilayer(s) with permanent (inter)particle–particle electrical contacts among all constituents in each layer. Implantation of PM powders successively in the quartz layer and then in the sulfide layer, enabled PP and GI to be singled out and quantified separately for each sulfide. Galvanic interactions were found to ameliorate, to various degrees, the leaching of PM. Locating the PM powders in the quartz layer emphasized the negative impact of PP of the sulfide layer on Au and Ag dissolution, except for galena which enhanced gold leaching kinetics. Galvanic interactions due to pyrite, chalcopyrite and an industrial ore were so positive that they largely outweighed the negative impact of PM passivation. The galvanic current measured between Au/Ag electrode and each of the above minerals confirmed the positive effect of GI on gold leaching rate. In close agreement with PM recoveries, stronger galvanic currents were measured for Au-pyrite, Au-chalcopyrite and Au-industrial ore galvanic couples in comparison to those for Au-sphalerite, Au-chalcocite and Au-stibnite.

Use of ferricyanide for gold and silver cyanidation

Low gold and silver leaching kinetics has been commonly observed in traditional gold-silver cyanidation process, especially in heap leaching and in situ leaching operations. The different mineralogy of gold and silver in the ores is suspected to be the main reason, e.g., the occurrence of low solubility acanthite (Ag 2S) typically results in low overall silver extraction. Due to the low solubility of oxygen in cyanide solution, the reactivity and availability of oxidant is believed to be the critical limitation for gold and silver dissolution. The use of ferricyanide as the auxiliary oxidant for gold and silver cyanidation has been examined. The rotating disc test results prove the assistant effect of ferricyanide on increasing the dissolution rate of gold and silver. The potential use of ferricyanide in gold/silver cyanidation process is proposed based on the leaching results of actual ores.

Towards a more environmentally friendly process for gold: models on gold adsorption onto activated carbon from ammoniacal thiosulfate solutions

In the context of gold extraction, leaching is the dissolution of the metal or even a mineral in a liquid phase. Thus, it is of a primary concern the dissolution of gold in an aqueous solution, operation which requires both a complexant and an oxidant to achieve acceptable leaching rates and yields. A limited number of ligands form complexes of sufficient stability and at a suitable rate for their use in gold leaching operations. Cyanide is mostly used because of its relatively low cost and great efficiency for gold dissolution. The main disadvantage associated with its use is the extremely toxic character of the reagent. Thus, several other ligands had been considered for gold extraction from ores, regarding to environmental pressures and even restrictions to the use of cyanide, potential of having faster gold leaching kinetics than cyanide, possibility of their use in acidic media, which is suitable in the treatment of refractory ores, and a greater degree of selectivity than cyanide for gold over other metals. Nevertheless, some of them presented some disadvantages which limited their practical use. Ammoniacal thiosulfate leaching has a considerable potential as an effective and less hazardous procedure (than cyanide) for gold extraction from auriferous ores. Once gold is dissolved, separation of the precious metal from the solution can be achieved using different procedures (i.e. adsorption onto activated carbon). In the present investigation, up to fivel models were used to correlate the adsorption of gold onto the carbon, being the Fleming model the one which best fit the experimental results on gold adsorption.

Development of an energy-balance model to predict the economic impact of installing a tailings wash thickener at the Fort Knox Mine, Fairbanks, Alaska

The Fort Knox Mine is located in Alaska’s interior, where the average ambient air temperatures range from −24°C (−11°F) in January to 16°C (61°F) in July. The mill processes a free-milling gold ore utilizing both a gravity recovery circuit and conventional cyanide leach/carbon-in-pulp circuits. Mathematical models have been developed to accurately predict the impact of leach circuit slurry temperature on gold leaching, carbon adsorption and cyanide destruction kinetics. Additionally, an energy-balance approach has been used to model the seasonal variations in slurry temperatures throughout the Fort Knox mill. The energy-balance model, combined with the kinetics models previously developed to predict plant performance, was used to justify a tailing thickener installation at the Fort Knox Mine. This paper describes the development of the energy-balance model and its prediction of plant performance assuming a tailings wash thickener to reduce heat loss from the mill. Also presented is an analysis of the post-project plant performance that validates the model and shows that the mine produced an additional 596 kg (19,155 oz) of gold while reducing mill reagent costs by $6,302,000 during the first 38 months of operation.