Article

Thermodynamic and kinetic of iodine−iodide leaching in gold hydrometallurgy

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Abstract

The thermodynamic equilibria and kinetic aspect of gold dissolution in iodine–iodide leaching were studied with emphasis on the effects of pH value and temperature on the system. The results of thermodynamic analysis of iodine in aqueous solution were given and numerous forms of iodine exist mainly in the acid region of pH values. An increase of the potential of the system results in an increase of iodine speciation. The oxidizing potential of the system will increase by the addition of element iodine. The IO3– anions are stable in the potential range from –2.0 to –0.75 V and at pH value greater than 12.1. An increase of the temperature shifts boundaries of existence of various iodine species in the acid region of pH values. Some of them become unstable. The determined values of the diffusion coefficients and the thickness of the diffusion boundary layer, as well as the solvent concentration on the disc surface (14 mg/L) indicate that the process proceeds in the external diffusion region. Thus, while choosing the conditions of leaching from gold-containing materials of different origins of iodide solvents, it is necessary to carry out the process within the acidic region of pH values, where I−, I3− and IO4− ions are capable to form complex compounds with metals.

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... Other problem is the formation of gas that must be controlled to avoid any health risk. Bromide leaching offers a number of advantages, such as rapid extraction, non-toxicity, and adaptability to a wide range of pH values [20]: 5Au + 20KBr + 3KMnO 4 + 29HCl → 5H[AuBr 4 ] + 3MnCl 2 + 23KCl + 12H 2 O 5Au + 5KBr + KMnO 4 + 8HCl → 5AuBr + MnCl 2 + 6KCl + 4H 2 O . Dissolution rates of Au in bromide are higher compared to CN -, however the reagent consumption is much higher. ...
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Chapter
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... There are number of methods can be used in extracting the residual gold from the tailings such as gravity concentration, flotation, panning, pyrometallurgy, and hydromettalurgy. In this study, residual gold are extracted from the tailings by leaching, a hydrometallurgical process 3,4,5 . The hydrometallurgical method is regarded to be more rigorous and predictable, and more easily controlled 6 . ...
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Iodine–iodide leaching is an effective and environmentally friendly gold extraction method that is expected to replace cyanidation. However, as the price of iodine is relatively high, for the industrial application of iodine–iodide leaching, decreasing the required amount of iodine is important to reduce production costs. In this study, persulfate was used to replace iodine as the oxidant for the extraction of gold from refractory gold ore. Using single-factor tests, the optimal leaching conditions were determined as a solid–liquid ratio of 1:3, a stirring speed of 350 rpm, 0.02 mol/L ammonium persulfate, and 0.1 mol/L potassium iodide. At pH 5, the gold extraction rate reached 83.69% after stirring for 1.5 h at room temperature. Even under neutral conditions, a gold extraction rate of more than 80% could be achieved. Using the activated carbon adsorption method to recover the gold from the leaching solution, a high gold recovery rate was obtained without solid–liquid separation. Compared with cyanidation, the S2O8²⁻ – I⁻ – H2O system has the advantages of nontoxicity, eco-friendly, a fast leaching speed, and a high gold extraction rate. Thus, this system was a promising alternative method for gold extraction from refractory ore.
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The extraction of gold from refractory gold ores (RGOs) without side reactions is an extremely promising endeavor. However, most RGOs contain large amounts of sulfide, such as pyrite. Thus, investigation of the influence of sulfide on the gold leaching process is important to maximize the utilization of RGOs. In this work, the effects of pyrite on the stability of the thiourea system were systematically investigated under different conditions. Results showed that the decomposition rate of thiourea was accelerated sharply in the presence of pyrite. The effect of pyrite on gold recovery in thiourea leaching systems was then confirmed via a series of experiments. The decomposition efficiency of thiourea decreased by 40% and the recovery efficiency of gold increased by 56% after the removal of sulfide by roasting. Under optimal conditions, the efficiency of the gold recovery system increased to 83.69% and only 57.92% of thiourea decomposition was observed. The high consumption of thiourea by the leaching system may be attributed to not only adsorption by mineral particles but also catalytic decomposition by some impurities in the ores, such as pyrite and soluble ferric oxide.
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In Chile, the hydrometallurgical plants are operating below their capacity due to a depletion of copper oxide ores. To obtain suitable pregnant leach solutions (PLSs) for hydrometallurgical plants, leaching solutions combining iodine-based oxidants and hydrogen peroxide in a chloride–acid medium, at room temperature and pressure were studied. Factorial experiments were conducted to evaluate the effects of the different leaching solution reagents (KI, NaIO3, NaCl, H2O2, and H2SO4). The results showed that the most influential variable is the H2O2 concentration; increasing the PLS concentration from 3 g/L to 15 g/L increased the copper extraction percentage by ~25%. In decreasing order of importance, the factorial experimental results showed that the H2O2, H2SO4, NaCl, NaIO3, and KI concentrations affect the copper extraction percentage. The highest copper extraction percentage (i.e., 60.6%) was obtained using a leaching solution containing the highest reagent concentrations. At these conditions, the copper concentration in the PLS was 16.9 g/L. An economic evaluation of the laboratory-scale leaching experiments showed an increase in the unit cost (USD/t Cu) for experiments involving leaching solutions without H2O2 because of poor copper concentration in the PLS. As the concentrations of the reagents NaIO3 and KI, increase, the unit cost increases, because the reagents are relatively expensive and have a limited effect on the copper extraction percentage.
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Gold is a scarce element in the Earth’s crust but indispensable in modern electronic devices. New, sustainable methods of gold recycling are essential to meet the growing eco‐social demand of gold. Here, we describe a simple, inexpensive, and environmentally benign dissolution of gold under mild conditions. Gold dissolves quantitatively in ethanol using 2‐mercaptobenzimidazole as a ligand in the presence of a catalytic amount of iodine. Mechanistically, the dissolution of gold begins when I 2 oxidizes Au 0 and forms [Au I I 2 ] ‐ species, which undergoes subsequent ligand‐exchange reactions and forms a stable bis‐ligand Au(I) complex. H 2 O 2 oxidizes free iodide and regenerated I 2 returns back to the catalytic cycle. Addition of a reductant to the reaction mixture precipitates gold quantitatively and partially regenerates the ligand. We anticipate our work will open new pathway to more sustainable metal recycling with utilization of just catalytic amounts of reagents and green solvents.
Article
Gold is a scarce element in the Earth’s crust but indispensable in modern electronic devices. New, sustainable methods of gold recycling are essential to meet the growing eco‐social demand of gold. Here, we describe a simple, inexpensive, and environmentally benign dissolution of gold under mild conditions. Gold dissolves quantitatively in ethanol using 2‐mercaptobenzimidazole as a ligand in the presence of a catalytic amount of iodine. Mechanistically, the dissolution of gold begins when I 2 oxidizes Au 0 and forms [Au I I 2 ] ‐ species, which undergoes subsequent ligand‐exchange reactions and forms a stable bis‐ligand Au(I) complex. H 2 O 2 oxidizes free iodide and regenerated I 2 returns back to the catalytic cycle. Addition of a reductant to the reaction mixture precipitates gold quantitatively and partially regenerates the ligand. We anticipate our work will open new pathway to more sustainable metal recycling with utilization of just catalytic amounts of reagents and green solvents.
Article
Oxygen pressure alkaline self-leaching of gold is an environmentally friendly and smokeless leaching process. But the insoluble iron oxide generated will precipitate on the surface of parent minerals, thus reducing the oxidation reaction and gold leaching efficiency. Simultaneously, due to the different pH requirements of pyrite oxidation and gold leaching, the pH must be strictly controlled to achieve the maximum efficiency. In this work, dissolution effect and buffering effect of sodium carbonate in the gold self-leaching process were carried out. As the addition of sodium carbonate, soluble iron-carbonate complexes would be formed to remove passivation layer and expose fresh surface of pyrite, resulting in improvement pyrite oxidation rate. Meanwhile, the buffering capacity of sodium carbonate can avoid the thiosulfate decomposition and improve the concentration of thiosulfate. The maximum gold extraction efficiency of gold-bearing flotation concentrate could achieve 91.56% under the following condition: a concentration of sodium hydroxide, sodium carbonate and sodium carboxymethyl cellulose at 0.6 M, 0.3 M and 100 mg/dm³, respectively; stirring speed at 800 rpm; temperature at 308.15 K; oxygen pressure at 0.5 Mpa; leaching time of 18 h and a liquid-solid ratio of 5:1.
Article
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This review presents developments and applications in bioleaching and mineral biooxidation since publication of a previous mini review in 2003 (Olson et al. Appl Microbiol Biotechnol 63:249-257, 2003). There have been discoveries of newly identified acidophilic microorganisms that have unique characteristics for effective bioleaching of sulfidic ores and concentrates. Progress has been made in understanding and developing bioleaching of copper from primary copper sulfide minerals, chalcopyrite, covellite, and enargite. These developments point to low oxidation-reduction potential in concert with thermophilic bacteria and archaea as a potential key to the leaching of these minerals. On the commercial front, heap bioleaching of nickel has been commissioned, and the mineral biooxidation pretreatment of sulfidic-refractory gold concentrates is increasingly used on a global scale to enhance precious metal recovery. New and larger stirred-tank reactors have been constructed since the 2003 review article. One biooxidation-heap process for pretreatment of sulfidic-refractory gold ores was also commercialized. A novel reductive approach to bioleaching nickel laterite minerals has been proposed.
Article
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Cyanide is the reagent of choice for gold and silver extraction, but also a toxic chemical that may cause severe environmental pollution problems. Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. The phytotoxicity of cyanide is indirectly connected to the efficiency of this enzyme system: Plants only survive cyanide exposure up to a dosage they can metabolize. Cyanide phyto-toxicity was measured for the subtropical grass Sorghum bicolor. Potassium cyanide was not toxic when added to the irrigation water at up to 125mg KCN/l (50mg CN/l). In a degradation test, cyanide was efficiently degraded by sorghum roots and leaves. Cyanide elimination using plants seems to be a feasible option for gold and silver mine waste and wastewater. Theoretical estimates indicate that a large area of land is needed. But the process is cost effective, sustainable, and has less critical emissions than any competing technology. Until now, phytotreatment of gold mining wastewater has only been tested on a lab scale. With the current knowledge, a pilot-scale demonstration could be implemented immediately
Article
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This paper presents an overview of the various methodologies used in the recovery of gold from secondary sources. Gold recovery is interesting due to its vast industrial applications, high market prices and extensively used precious metal, the sanctuary value attributed to gold during international political and economical crises, and the limited resource of this metal may explain the recent increasing gold share value. The state of art in recovery of gold from spent sources by pyrometallurgy; hydrometallurgy; bio-hydrometallurgy techniques is highlighted in this paper. This article also provides an overview of past achievements and present scenario of recovery studies carried out on the use of some promising methods which could serve as an economical means for recovering gold. The present review also highlights the used varieties of leaching, cementing, reducing agents, peeling, coagulants, adsorbents, agglomeration solvents, ion exchange resins and bio-sorbents in real situations and hopes to provide insights into recovery of gold from spent sources. Evaluation of lucrative and environmentally friendly technologies to recover gold from primary and secondary spent sources was made in this study.
Article
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Cyanide has been recognized for a long time as a powerful lixiviant for gold and silver, forming very stable cyano complexes with both metals. While cyanide is very effective in leaching free milling ores, there are certain classes of gold and silver ores (i.e., carbonaceous, pyritic. arsenical, manganiferous, cuperferous) that are considered refractory to conventional cyanidation dissolution. Recently there has been considerable effort directed towards new and improved reagents for leaching these difficult-to-treat ores and concentrates. A large portion of this effort has been devoted to finding alternative lixiviants that might compete with conventional cyanidation. Furthermore, there is a general interest in developing non-toxic environmentally safe substitutes for cyanide.There are a number of reagents that form stable complexes with gold and silver e.g., thiourea, thiosulfate, halides, malononitrile, acetonitrile and polysulfides. The chemistry of gold and silver dissolution using alternative lixiviants is discussed in this paper. Special emphasis is given to the application of Eh-pH diagrams to interpret the dissolution behavior.
Article
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This review describes the historical development and current state of metals leaching and sulfide mineral biooxidation by the minerals industries. During the past 20 years commercial processes employing microorganisms for mineral recovery have progressed from rather uncontrolled copper dump leaching to mineral oxidation and leaching in designed bioheaps for oxidation of refractory gold ores and for copper recovery. Also during this period of time, stirred tank bioleaching has been commercialized for cobalt recovery and for biooxidation of refractory gold ores. Chalcopyrite bioleaching in stirred tanks is on the verge of commercialization. Commercial applications of biohydrometallurgy have advanced due to favorable process economics and, in some cases, reduced environmental problems compared to conventional metal recovery processes such as smelting. Process development has included recognition of the importance of aeration of bioheaps, and improvements in stirred tank reactor design and operation. Concurrently, knowledge of the key microorganisms involved in these processes has advanced, aided by advances in molecular biology to characterize microbial populations.
Article
Non-cyanide lixiviants are not widely used for gold and silver recovery at this time, and cyanide remains unchallenged as the best all-around lixiviant. However, the non-cyanides may find potential applications in future treatment of gravity concentrates or in situations where environmental constraints prevent the use of cyanide. Bromine is used in at least one western US geochemical assay laboratory as a means of obtaining rapid analyses of leachable gold from samples. In some cases where gold recoveries can be improved by leaching with non-cyanide reagents, an incentive exists to examine the use of such reagents as long as the increased costs of reagent consumption do not offset the benefits. In cases where platinum group metals are present, an added incentive may exist for a further look at the non-cyanides.
Article
Using the iodine-iodide leaching system, the effects of different iodides (ammonium iodide, potassium iodide, hydrogen iodide)on gold concentrates leaching process were discussed from the influence factors, such as initial iodine content, iodine and iodide ratio and solution pH value. The results show that, when ammonium iodide or potassium iodide is used as complex agent, under the conditions of initial iodine content of 1%, iodine and iodide molar ratio of 1:8, pH value of 7, liquid-solid ratio of 4:1, stirring speed of 600 r/min, leaching time of 4 h and temperature of 25°C, the gold leaching rates are around 90%; whereas the gold leaching effect is poorer when hydrogen iodide(aqueous solution is hydroiodic acid) is used as complex agent, and the gold leaching rate is only 75%. Considering the difference of leaching effect and availability of industry and so on, potassium iodide is the suitable complex reagent of gold concentrate leaching in iodine-iodide solution.
Article
The technical process of electrodepositing gold of iodine leaching solution from waste printed circuit board (PCB) was investigated. The results show that the most optimized conditions of electrodepositing gold of iodine leaching solution from waste PCB are elicited that the cell carbon rods(cathode) and titanium plate (anode) are used as electrode, the cell is partitioned by anion exchange membrane, the iodine concentration in anode solution is 0.1%-0.8%, n(I2):n(I-)=1:10, the cell voltage is 10-14 V, the gold concentration in cathode solution is 15-50 mg/L, the electrolyzing time is 1-4 h, the electrodepositing rate of gold is more than 95%. After electrolytic experiment, the anode iodine solution can be recycled for electrolyte or leaching gold from waste PCB.
Article
Two types of gold ores were subjected to iodide/iodine leaching at room temperature. The effects of three factors, including ore type (a carbonaceous and an oxide gold ores), iodide/iodine concentration, and the presence of oxygen in solution on gold leaching performance were investigated. The carbonaceous ore showed only 20% gold extraction into the solution; since gold-iodide complexes would readily adsorb on organic matter. In contrast, gold extraction from the oxide ore in a solution containing 20 g/L iodide and 4 g/L iodine reached 77% in 6 h and 89% in 24 h. With iodate as the oxidant, no gold could be leached in 48 hours. When the iodate was partially converted to iodine by adding HCl to the solution, gold leaching was then started. Iodine reactions with the sulfide and ferrous minerals of the oxide ore slowly consumed iodine within 48 hours.
Article
A proposalfor a modified in situ leaching method for extracting gold from oxidized gold ores using a non-cyanide lixiviant is described. A non-cyanide lixiviant is suggested because of the obvious concerns posed by injecting cyanide-bearing solutions into the subsurface. Oxidized gold ores were chosen as a focus because earlier research on the use of sodium thiosulfate as a lixiviant under anaerobic conditions indicated that (lie presence of pyrite led to rapid thiosulfate breakdown. A reconnaissance research program involving ore characterization and hydrometallurgical test work on samples from four Australian ore deposits and preliminary reactive transport modeling studies was carried out. This work showed that lixiviant-oxidant combinations of sodium thiosulfate and ferric ED TA and iodide and iodine are both capable of extracting high percentages of accessible gold from the selected samples in bottle roll tests under anaerobic conditions. The ore characterization and reactive transport studies suggested that both physical and chemical methods of permeability enhancement may be required to lift bulk permeability and the availability of gold for dissolution to sufficiently high levels to obtain adequate gold recoveries. Assuming that such methods prove to be both necessary and economically viable, the mining method would no longer be regarded as simple in situ leaching. Therefore, the term "in-place leacliig" has been adopted for the proposed gold extraction system.
Article
Waste mobile phone has become the largest number of electronic waste, and recycling of metals from mobile phone would ensure resource recycling and reduce environmental degradation. Based on the contents of metals analyzed by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), containing copper and precious metals such as gold, the paper compared the extraction processes of gold, copper from waste mobile phone printed circuit board (PCB). In this study, two processes, nitric acid and thiourea (NT), sulfuric acid-hydrogen peroxide and iodine (SAHPI) were used to leach copper and gold, respectively. The recovery rate of copper was found to be 96.42%, and 94.3% of the gold was leached in the former process. Similar trends were obtained for the leaching of copper and gold in the latter process, but it was lower that about 95.27% of copper was recycled, while 93.4% of gold were leached. Both the two processes were nontoxic and non-cyanide system. However, thiourea is not stable and easy to decompose in alkaline solution, and the technology has disadvantage of instability. Iodine leaching on the other hand is comparatively a environmental process. Therefore, the optimal choice is the combined process of SAHPI method, while further research is required to develop cost effective and environmentally friendly processes.
Article
The refractoriness of the gold in many pyritic ores from the eastern Transvaal is due mainly to the association between the gold and the sulfides, but it can be due to the presence of graphitic carbonaceous material, which complicates the recovery of the gold by virtue of its adsorption activity. In an examination of the roasting characteristics of three flotation concentrates from the Barberton area, it was found that the gold in the concentrates occurred mostly in two forms: in very fine gold particles distributed within the arsenopyrite, and in a slightly coarser form loosely associated with the pyrite. Direct cyanidation of the concentrates gave gold extractions of from 16 to 30 percent. After the concentrates had been roasted at temperatures of 650 to 700 degree C for 20 minutes with rabbling, the extractions varied from 86 to over 90 percent. Examination of the residues after cyanidation indicated that gold was present in the porous calcines or in the calcined sulfide product. This observation, together with porosity measurements, suggests that physical encapsulation of the gold particles is the main reason for the refractory behavior of the gold in the calcines.
Article
Printed circuit board (PCB) is the essential part of electronic devices and has an important source of base and precious metals with high economic potential. In this study, selective leaching of gold from PCB was performed in an iodine-hydrogen peroxide (I2-H2O2) solution system. The effects of different parameters such as iodine concentrations, H2O2 concentrations and solids-% on the gold leaching dynamics were investigated. The results show that increasing solids-% has a negative influence on the gold leaching. However, gold recovery from the e-wastes in a solution containing 3% iodine, 1% H2O2 with solids-% of 15% resulted with 100% recovery among all leaching tests.
Article
Waste generated by the electrical and electronic devices is huge concern worldwide. With decreasing life cycle of most electronic devices and unavailability of the suitable recycling technologies it is expected to have huge electronic and electrical wastes to be generated in the coming years. The environmental threats caused by the disposal and incineration of electronic waste starting from the atmosphere to the aquatic and terrestrial living system have raised high alerts and concerns on the gases produced (dioxins, furans, polybrominated organic pollutants, and polycyclic aromatic hydrocarbons) by thermal treatments and can cause serious health problems if the flue gas cleaning systems are not developed and implemented. Apart from that there can be also dissolution of heavy metals released to the ground water from the landfill sites. As all these electronic and electrical waste do posses richness in the metal values it would be worth recovering the metal content and protect the environmental from the pollution. Cyanide leaching has been a successful technology worldwide for the recovery of precious metals (especially Au and Ag) from ores/concentrates/waste materials. Nevertheless, cyanide is always preferred over others because of its potential to deliver high recovery with a cheaper cost. Cyanidation process also increases the additional work of effluent treatment prior to disposal. Several non-cyanide leaching processes have been developed considering toxic nature and handling problems of cyanide with non-toxic lixiviants such as thiourea, thiosulphate, aqua regia and iodine. Therefore, several recycling technologies have been developed using cyanide or non-cyanide leaching methods to recover precious and valuable metals. Copyright © 2015 Elsevier Ltd. All rights reserved.
Article
Chemical replacements for cyanide have been investigated for decades; however cyanide remains the exclusive lixiviant of choice in the mining industry due to a combination of its availability, effectiveness, economics and ability to use it with acceptable risk to humans and the environment. About 90% of the significant gold producing operations worldwide currently utilize cyanide for gold and silver extraction. Despite the number of cyanide-related mining operations, there have been no documented accounts during the previous three decades of the death of humans due to cyanide as a direct consequence of major mining-related environmental incidents. Major mining-related environmental incidents have not been concentrated in any geographic location, may occur regardless of the size of the company and do not occur more frequently with a specific type of mining activity. The main aspects of cyanide management that should be addressed at mining operations include transportation of cyanide to site, process solution conveyance, worker health and safety training, water management and treatment, emergency response and preparedness, workplace and environmental monitoring, and community relations. If these aspects of cyanide management are integrated into an overall cyanide management plan, dramatic reductions in risk and potential incidents at mine sites will be realized.
Article
Gold is one of most important sources of currency entry in the economics of any state. Besides mining raw materials, fit for effective gold extraction, residues of gold extraction factories are the source of this precious metal. Moreover, the role of industrial wastes, which has a rich source of non-ferrous and noble metals, increases. Processing expediency of complicated wastes is determined by their harmful ecological influence on the environment. Involving to processing of gold the enumerated sources demands the use of high leading technologies, taking into account the variety of their composition. One of the ways of increasing effectiveness of processing the gold-containing materials may be introduction of direct hydrometallurgy processes. Hydrometallurgical methods of processing differ in low power capacity, simplicity of implement process, decreasing pollution of the environment. It is necessary for this not only to sort out selective solvents, but also to elaborate methods of regeneration and recycling of the used solutions and solvents. At present leaching of gold-containing raw materials by cyanide is the main hydrometallurgical process and the main advantage of cyanide is high selectivity with regard to gold. However, leaching by cyanide solutions has a set of substantial drawbacks: process duration, high cost of the reagent, non-adaptability of “stubborn” ores and concentrates, absence of possible regeneration of cyanide solutions and the main thing is high toxicity of cyanide compounds. Therefore elaboration of new hydrometallurgical methods of extracting gold using selective non-toxic reagents, in particular, iodine solutions is important scientific and practical task.
Article
Gold extraction by iodine-iodide solution is an effective and environment-friendly method. In this study, the method using iodine-iodide for gold leaching is proved feasible through thermodynamic calculation. At the same time, experiments on flotation gold concentrates were carried out and encouraging results were obtained. Through optimizing the technological conditions, the attained high gold leaching rate is more than 85%. The optimum process conditions at 25°C are shown as follows: the initial iodine concentration is 1.0%, the iodine-to-iodide mole ratio is 1:8, the solution pH value is 7, the liquid-to-solid mass ratio is 4:1, the leaching time is 4 h, the stirring intensity is 200 r/mim, and the hydrogen peroxide consumption is 1%.
Article
The recovery of gold from iodine–iodide solutions using strong base anion exchange resin has been investigated. The gold iodide complex can be effectively loaded on the resin provided the resin is not heavily loaded with triiodide. The loading of triiodide is found to be extremely strong due to the dissociation of the loaded triiodide to iodide and iodine, the latter being deposited on the resin by physiosorption thereby fouling the surface. This may be potentially detrimental to the gold recovery process but can be overcome by pre-reduction to remove most of the triiodide before loading. To strip the iodine from the resin it requires a reductive elution process. A sodium chloride based eluant solution containing sulfite is found to be highly effective for the elution of both gold and iodine.
Article
The electrochemistry of gold in different halide solutions, with special emphasis on iodide is presented. The electrochemical techniques used during this investigation included cyclic and linear sweep voltammetry. A glassy carbon rotating disk electrode was used to investigate the electrochemistry of the iodide and a gold rotating disk electrode to explore the oxidation behavior of gold in iodide solutions. The effects of iodide concentration, electrode rotation and sweep rate on the electrochemical behavior of gold were examined. In addition, reduction of iodine species at the gold electrode was also investigated.Iodide is shown to be a powerful complexing agent for gold. Cyclic voltammograms of gold in the presence of 10−2M chloride, bromide and iodide, respectively, show that the anodic currents for the oxidation of gold in iodide solution are much greater than that in either bromide or chloride. Two oxidation peaks, which represent the oxidations of Au to Au(I) and to Au(III), were observed. It is confirmed that iodide is oxidized sequentially to tri-iodide and then to iodine and both of these reactions are reversible. At high concentrations of iodide and/or a slow scan rate, passivation, which is caused by the formation of solid iodine at the gold electrode surface, was found. The cathodic reduction curves show that reduction of iodide species on gold is a function of iodine concentrations but it is insensitive to iodide concentration.
Article
My perceptions of the biohydrometallurgical field span four decades and stem from being a professional microbiologist conducting academic research and research for process development and applications. My experiences have given me an appreciation for knowledge gained through fundamental research and the transfer of this knowledge to development of commercial scale applications of microbial processes. The symposia series for international activities in biohydrometallurgy has been a major factor in advancing knowledge and applications for microbial bioleach systems. The first international biohydrometallurgy meeting was held in Braunschweig, Germany in 1977. This was the predecessor for the International Biohydrometallurgy Symposia. As evident from the Symposia, advances in development and applications of biohydrometallurgy technologies follow an evolutionary, rather than revolutionary progression from demonstration of knowledge at the laboratory scale to engineering commercial plants.
Article
The application of bacterial oxidation as a pretreatment step for the extraction of gold from arsenical gold sulphide concentrates offers potentially significant economic advantages over oxidative pretreatment alternatives. In this study the bacterial oxidation of an extremely refractory gold sulphide concentrate, from Olympias, Greece, is examined.Leaching tests were conducted in air-stirred pachuca reactors in order to determine the effect of pulp density on the degree of pyrite and arsenopyrite oxidation.Cyanidation tests were conducted in order to determine the degree of gold and silver liberation in relation to the degree of each sulphide phase oxidised and to the cyanide consumed.Mathematical analysis of the leaching data allowed the gold distribution in each sulphide mineral phase to be estimated and the gold recoveries according to the degree of each mineral oxidised predicted.
Article
This study investigates the biooxidation of a refractory gold concentrate using a mixed culture of acidophilic mesophiles, moderate thermophiles and extreme thermophiles and their effect on the subsequent cyanidation and gold recovery. The experiments with high % solids using mixed mesophiles showed better oxidation potential compared to moderate thermophiles and extreme thermophiles. However, the extreme thermophiles performed better than mesophiles and moderate thermophiles during the biooxidation with
Article
Sodium cyanide is used in the extraction of gold from its ores. It, and many of its derivatives, are acutely toxic to humans and aquatic life. The awareness of environmental impacts has been heightened in the last decade, the effects of which are appearing in greater restrictions on discharges and emissions. A company has been recovering residual cyanide from its tailings in a process not dissimilar to one practised over 60 years ago. This admirable development, however, is an ‘end of pipe’ solution. Cyanide recovery has been investigated through the concept of cleaner production. There would be a 20% increase in the recovery of cyanide by removing it as soon as the gold has dissolved and before carbon adsorption. The presence of a cyanide recovery circuit would enable leaching to be conducted at a pH of around 9 instead of the conventional 10.5, with a consequential halving in consumption of pH modifying reagents. There are several other economic and environmental advantages.
Article
Cyanidation is the most widely used industrial process for the recovery of gold and silver from ores and concentrates. But, difficulties encountered in the dissolution of refractory ores, public paranoia associated with the toxicity of cyanide and the environmental limitations imposed on cyanide disposal are the basic disadvantages of the process. Consequently, general research interest is concentrated in developing less toxic or preferably non-toxic substitutes for cyanide. For this purpose, thiourea and its derivatives, thiosulphate, iodine-iodide, chlorine and ammonia have been tested as potential alternatives to cyanide. In this article, comparative information with cyanide is given regarding the dissolution characteristics of gold and silver in cyanide alternative lixiviants with emphasis on the leaching tests conducted for ores and concentrates.
Article
This chapter illustrates commercial bacterial-oxidation practice, and considers some of the design and operational characteristics of bacterial-oxidation plants for the treatment of refractory gold concentrates. New process advancements that improve the cost effectiveness of bacterial oxidation for the treatment of concentrates by improving the rate of oxidation and reducing cyanide consumption in downstream gold recovery are discussed in the chapter. These advancements have arisen as a result of the laboratory investigations combined with the observations made on existing plants. Refinements to the bacterial-oxidation process, which alter the sulfur chemistry of the final residue, have been found beneficial in reducing cyanide consumption.
Article
Cyanide leaching is the traditional process for the extraction of gold from primary raw materials. When the gold host rock contains high concentrations of sulphides, an oxidative thermal or a chemical pretreatment is often needed to access precious metal particles. Biooxidative leaching may be an interesting alternative, if compared with the environmental effects and costs of the conventional pretreatment processes, to liberate gold from the sulphide matrix and then to make it amenable to cyanidation. In this work, bioleaching with Thiobacillus ferrooxidans and Thiobacillus thiooxidans has been investigated at a bench scale on a refractory gold-bearing arsenopyrite (2 g/t Au) ore coming from the Gölcück mine (Turkey). The factors influencing the biooxidative pretreatment, in order to enhance the gold recovery in a conventional cyanidation process, were tested using a factorial plan of experiments. Direct cyanide leaching of the arsenopyrite ground to −74 μm showed no gold dissolution at all, but with fine grinding to −30 μm, gold recovery reached about 55.3% after 48 h of cyanidation. On the contrary, cyanidation accomplished on a 72 h bioleached arsenopyrite has allowed 84.3% of gold to be solubilized in 2 h, using the following bioleaching conditions: 20% pulp density, pH 2, stirring conditions 200 rpm, temperature 30°C, time 3 days. Under these conditions, the preliminary tests in a semi-continuous lab-scale microfermentor have been performed to evaluate the scale-up of the biooxidative process. It was possible to solubilise 95.2% Au after 48 h cyanidation for the samples bioleached during 3 days, and 96.8% Au when the time of bioleaching was 7 days.
Book
Each of the 24 chapters of this book on standard electrode potentials in aqueous solutions was prepared by knowledgeable specialized experts and reviewed by referees who are credited herein. For the sake of space, discretion was exercised as to which half-reactions would be included for a given element, data have been limited to a single temperature, 25/sup 0/C, and no attempt was made to give complete citation to all pertinent publications. Separate abstracts have been prepared for three chapters.
Chapter
There are several water and tailings treatment processes that have been successfully used worldwide for cyanide removal at mining operations. The key to successful implementation of these processes involves consideration of the following:•Site water and cyanide balances under both average and extreme climate conditions.•Goals to be adopted for cyanide levels in treated effluent, including the form of cyanide to be regulated (free vs. WAD vs. total cyanide).•The range of cyanide treatment processes available and their ability to be used individually or in combination to achieve treatment objectives.•Proper treatability testing, design, construction, maintenance and monitoring of both water- and cyanide-management facilities.By carefully considering these aspects of water and cyanide management before, during and after mine operation, operators can reduce the potential for environmental impacts associated with the use of cyanide. Another aspect of cyanide treatment to be considered is the potential environmental impact of the cyanide-related compounds - cyanate, thiocyanate, ammonia, nitrate and nitrite. These compounds may be present in mining solutions to varying extents and may require treatment if water is to be discharged. Each of these cyanide-related compounds is affected differently in the treatment processes discussed, and this should be considered when evaluating cyanide-treatment alternatives for a given site. Table 13 provides a simplified summary of the general applications of various treatment technologies for the removal of iron cyanide and WAD cyanide. This table represents a very simplified summary, but can be used as a conceptual screening tool when evaluating cyanide-treatment processes.
Article
The thermodynamic equilibria and kinetic aspects of gold dissolution in iodide electrolytes have been studied with emphasis on the effect of different oxidants on the system. In conjunction with kinetic measurements, the chemix computer program was used to predict the concentration profiles of the predominant species at equilibrium in different solution conditions for the systems Au-I−-I2-H2O and Au-I−-OCl−-H2O.The thermodynamic study showed that I3− is the predominant oxidants species in both systems. However, if the concentrations of OCl− and I− are equal, solid iodine is formed. In these systems iodide (I−)_is used to form I3− (responsible for the gold oxidation) and more free iodide needed for the gold complexation is destroyed in the I−-OCl− system than the I−-I2 system. The formation of solid AuI also explains the lower rate of gold dissolution determined for certain conditions in the kinetic study.The thermodynamic modelling supports the kinetic measurements which show that, although the I−-OCl− system has a higher oxidation capacity, it does not extract gold as well as the I−-I2 system. In all cases there exist optimum oxidant/iodide ratios for achieving maximum gold extraction rates. A mixture which has the highest I3− and free I− concentration will attain the best gold extraction rate.
Article
This paper reviews the current status of commercial biomining operations around the world, identifies factors that drive the selection of biomining as a processing technology, describes challenges to exploiting these innovations, and concludes with a discussion of biomining's future. Biomining is commercially applied using engineered dumps, heaps and stirred tanks. Overcoming the technical challenges of lowering costs, processing low-grade, low-quality and complex ores and utilizing existing capital investments at mines requires better understanding of microbial activities and innovative engineering. Surmounting biomining commercial challenges entails improved mining company/biomining innovator cooperation and intellectual property control.
Article
The problems associated with gold mining are considered from the standpoint of application of heap leaching, a method for gold recovery. Data on the kinetics and mechanism of gold dissolution in cyanide and alternative solvents [thiocarbamide solutions containing Fe(III) ions and hypochlorite-chloride bromine-bromide, iodine-iodide, and copper-thiosulfate solutions], which make it possible not only to improve the working efficiency in gold mining, but also to lessen the contamination of the environment with highly toxic compounds.
Article
The application of bacterial oxidation as a pretreatment step for the extraction of gold from arsenical gold sulphide concentrates offers potentially significant economic advantages over oxidative pretreatment alternatives. In this study the bacterial oxidation of an extremely refractory gold sulphide concentrate, from Olympias, Greece, is examined.Leaching tests were conducted in air-stirred pachuca reactors in order to determine the effect of pulp density on the degree of pyrite and arsenopyrite oxidation.Cyanidation tests were conducted in order to determine the degree of gold and silver liberation in relation to the degree of each sulphide phase oxidised and to the cyanide consumed.Mathematical analysis of the leaching data allowed the gold distribution in each sulphide mineral phase to be estimated and the gold recoveries according to the degree of each mineral oxidised predicted.
Article
The dissolution of gold in iodine-iodide solutions was studied using the rotating disc technique. The effect of changes on disc rotational speed, temperature, iodine and iodide concentrations, solution pH and various additives were investigated. Kinetic data and thermodynamic considerations were used to explain the fundamental aspects of the process. Gold can be leached at a greater rate by iodine-iodide solution than is possible by thiourea and by the conventional cyanidation techniques.
Article
All life forms have absolute requirements for a wide range of metals. For some prokaryotic microorganisms, metals have not only a nutritional requirement but also pivotal roles in energy generation, by acting as electron donors or electron acceptors. In the last half of the twentieth century, scientific technologies emerged that harnessed the abilities of microorganisms to transform metals, and many of these bioprocesses are increasing in scope in the new millenium. This review considers the impact that biohydrometallurgy has on the environment, and vice-versa. The various ways in which microorganisms transform metals, and the continued importance of natural environments as sources of biological systems that have potential in optimising existing operations or for developing new biotechnologies, are described. Abandoned mines are, from a biological perspective, dynamic ecosystems and often develop their own unique complex food webs, in which metals (and sulfur) often play central roles. Finally, the major issue of pollution associated with past and present mining activities is considered, in particular the problem of acid mine drainage. New biologically based approaches that are currently being developed not only to remediate affected water courses, but also to recover metals, are discussed.
Article
Advances in hydrometallurgy are providing increasingly simple means for controlling the entire lead chain from concentrate to recycled lead. Used in parallel with pyrometallurgy, these processes allow furnace temperatures to be reduced to the minimum, which is essential for casting or alloying. Fumes and atmospheric pollution are minimized, furnace slags are digested, and most residues (other than purification cements) are non-toxic and convertible into marketable products. These new processes provide the cleanest and healthiest practicable means for recycling lead from batteries. By substituting melting for smelting, the heat requirement and cycle time per charge are reduced by more than half. A new hydrometallurgical plant could be installed alongside an existing pyrometallurgical plant without interference, doubling its potential capacity when operational (and more, if electrowinning is used). Over 99.5% of the lead originally present is recovered in tests of a combined PLACID–pyro plant. The average purity of electrowon PLACID lead is 99.995%. Results from the PLINT process should be similar. The purity of the lead chain can thereby be sustained through recycling. Perfect solid/paste separation is not mandatory, and PLINT-type plant units can be of any size. Such processes constitute a good basis for development of clean processes, which are suitable for use in Asian societies.
Article
Of the halogens, the gold iodide complexes are the most stable in aqueous solutions. A series of experiments were performed to investigate the kinetics and mechanism of the leaching reaction between gold and iodide. Using a rotating disk technique, the effects of rotation speed, iodide and iodine concentration, temperature, pH and the presence of different electrolytes were measured. Oxygen and hydrogen peroxide were also examined as oxidants in the iodide system. A first order reaction rate was found with respect to I3− and half order reaction rate with respect to I−. A comparison of gold leaching between iodide and cyanide is also presented, in which a rate of about 2.6 × 10−9 mol/cm2 sec for 10−2M Nal and 5 × 10−3M I2 was obtained. This value is close to that for typical cyanidation.
Article
The application of ammonium thiosulfate for the treatment of copper–gold ores has been investigated. Copper minerals and copper–gold samples were leached in an ammonium thiosulfate solution under varying experimental conditions, i.e. aeration, temperature and reagent addition. The behaviour of thiosulfate, tetrathionate and sulfate in solution was studied using ion chromatography. Experiments showed that both gold extraction and thiosulfate stability are affected by a combination of aeration and cupric ions in solution. It is important to establish a balance between providing sufficient air and cupric ions for fast gold dissolution, and at the same time minimize the amount of air in the presence of cupric ions to prevent excessive thiosulfate degradation. Promising results, i.e. high gold extractions and low thiosulfate consumption, were obtained during a 24-h leach without forced aeration.
Article
Several oxidants (hypochlorite, iodine and hydrogen peroxide) were used to evaluate the dissolution of gold in iodide electrolytes at ambient temperatures. Evans diagrams constructed for the two half cells involved in the dissolution process show that hydrogen peroxide is not a suitable oxidant for the iodide system. The gold dissolution rate in an iodine-iodide mixture is dependent upon the concentration of iodide, iodine and the solution pH. An optimum iodine-iodide mole ratio of 0.35–0.4 was found for 0.1 M KI solutions, with pH ranging from 2.7 to 11.5; these are capable of dissolving gold at a maximum rate of 17 mg/cm2h. If a small addition of hypochlorite is made (< mM for 0.1 M KI solutions), gold will be dissolved faster than with iodine as oxidant. However, the gold dissolution rate in a hypochlorite-iodide mixture is strongly dependent upon the solution pH. The optimum hypochlorite-iodide mole ratio is 0.25 for KI solutions of 0.02 to 0.1 M at pH 2.7. Cyanidation, using the same concentration of cyanide yielded gold dissolution rates within the range of 1.3 (at pH 12.5) and 3.5 mg/cm2h (at pH 8.5).
Article
Biomining is the use of microorganisms to extract metals from sulfide and/or iron-containing ores and mineral concentrates. The iron and sulfide is microbially oxidized to produce ferric iron and sulfuric acid, and these chemicals convert the insoluble sulfides of metals such as copper, nickel and zinc to soluble metal sulfates that can be readily recovered from solution. Although gold is inert to microbial action, microbes can be used to recover gold from certain types of minerals because as they oxidize the ore, they open its structure, thereby allowing gold-solubilizing chemicals such as cyanide to penetrate the mineral. Here, we review a strongly growing microbially-based metal extraction industry, which uses either rapid stirred-tank or slower irrigation technology to recover metals from an increasing range of minerals using a diversity of microbes that grow at a variety of temperatures.
Article
Waste electric and electronic equipment, or electronic waste, has been taken into consideration not only by the government but also by the public due to their hazardous material contents. In the detailed literature survey, value distributions for different electronic waste samples were calculated. It is showed that the major economic driver for recycling of electronic waste is from the recovery of precious metals. The state of the art in recovery of precious metals from electronic waste by pyrometallurgical processing, hydrometallurgical processing, and biometallurgical processing are highlighted in the paper.
Gold processing update. The prospects for alternative leach reagents, can precious metals producers get along without cyanide?
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