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Non-Cyanide Leaching Processes in Gold Hydrometallurgy and Iodine-Iodide Applications: A review
Abstract
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.
... These are formation waters of oil fields under development containing 0.001-0.005% iodine [4]. ...
... Interest in iodine sorption was caused by the possibility of using natural iodine-containing raw materials for processing, as well as the prospect of using iodide solutions in the technology of gold recovery [ [4], [5]]. ...
... Sorption characteristics of synthesized ion exchangers in relation to iodine 1,4 dioxybenzene saturated with water (1) and iodine (2); 1,2 dioxybenzene saturated with water (3) and iodine(4) ...
Iodine is an essential micronutrient for humans and animals due to its important role as a component of thyroid hormones. Kazakhstan is a country, most regions of which suffer from a natural deficiency of iodine. At the same time, the country has a rich resource base for obtaining iodine and its compounds. These are formation waters of oil fields under development containing 0.001–0.005% iodine. To extract iodine from natural oil-field brines, we have proposed ion-exchange materials obtained from the waste of the oil refining industry. The article presents the results of studying the characteristics of iodine sorption in the synthesized ion exchangers. The synthesis of ion exchangers was carried out by polycondensation of dihydric phenols (1,3 dioxybenzene, 1,2 dioxybenzene, 1,4 dioxybenzene), hexamethylenediamine, and formaldehyde. The following sorption characteristics of iodine by the synthesized ion exchangers were studied: the influence of the pH of a solution on iodine sorption; kinetic curves of iodine sorption; iodine sorption isotherms; mechanism of iodine sorption. It has been established that the sorption of iodine on the synthesized samples is essentially independent of the pH of a medium, and the degree of its extraction in the entire area under study is 94–100%. Studies have been carried out on the dependence of the degree of extraction and the logarithm of the iodine distribution coefficient on the duration of the process. The sorption capacity of the synthesized ion exchangers with respect to iodine was estimated depending on the structure of the ion exchanger and sorption conditions. It has been established that sorbents based on 1,4 dioxybenzene, which are distinguished by high kinetic abilities and static exchange capacity (SEC = 2283.88 mg/g), are most preferable for iodine extraction. The mechanism of sorption of iodine by synthesized ion exchangers has been determined.
... Due to the limited resources of noble metals in Earth's crust, recycling [2][3][4][5][6][7][8][13][14][15] will have to accompany the traditional mineral extractions. [16,17] Circular economy is imperative for their sustainable use. ...
... Main established Au recovery processes include environmentally challenging and toxic methods, such as cyanidation [4,6,[16][17][18][19] (Figure 1, a), and in less developed regions, even the amalgam process. [20] Other dissolution procedures, [4][5][6][16][17][18][19] including aqueous iodine-iodide leaching [5-8, 16-19, 21-31] (Figure 1, b), come with marked improvements in safety and environment issues. ...
... Main established Au recovery processes include environmentally challenging and toxic methods, such as cyanidation [4,6,[16][17][18][19] (Figure 1, a), and in less developed regions, even the amalgam process. [20] Other dissolution procedures, [4][5][6][16][17][18][19] including aqueous iodine-iodide leaching [5-8, 16-19, 21-31] (Figure 1, b), come with marked improvements in safety and environment issues. As previously reported, elemental halogens can oxidize Au 0 to Au I or Au III cations and stabilize the formed Au cations in the solution as Lewis bases. ...
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.
... Also, the disposal of cyanide tailings is a great challenge, and it is mainly discharged to tailing dams at present [5]. Due to the above challenges, it is necessary to develop a more benign alternative reagent for cyanide in gold industries, such as thiourea [6][7][8], halogen [9,10], thiocyanate [11,12] and thiosulfate [13][14][15]. Among of them, thiourea (SC(NH 2 ) 2 ) is regarded as a suitable substitute to cyanide for gold leaching due to its low toxicity and fast kinetics [16]. ...
... As reported in the literatures [46,47], Cu 2+ , Ag + , Pb 2+ and Zn 2+ can form stable complexes with thiourea, and the complexation reactions between metal ions and thiourea are shown in Eqs. (9)(10)(11)(12). In our recent study, it was found that Zn mainly exists in the form of free Zn 2+ in thiourea solution at pH 2.0 due to its weak complexing ability with thiourea [48]. ...
... On an industrial scale, sulfide ore primarily undergoes beneficiation through flotation to obtain sulfide flotation concentrates, which are initially sent to pyrometallurgical plants (Gorain, Kondos, and Lakshmanan 2016;Konyratbekova, Baikonurova, and Akcil 2015;Lakshmanan, Roy, and Gorain 2019;Lunt and Briggs 2016). The processing of complex sulfide ores, poor in the content of basic metals does not lead to high-quality flotation concentrates that can be cost-effectively processed at pyrometallurgical plants. ...
... Recently, significant efforts have been made to develop new and improved reagents for leaching 'difficult-to-process' concentrates. Much of this effort is focused on finding alternative leaching reagents that could compete with conventional cyanide (Konyratbekova, Baikonurova, and Akcil 2015;Surimbayev et al. 2018Surimbayev et al. , 2019Yessengarayev, Baimbetov, and Surimbayev 2020). In addition, current studies are directed towards the development of nontoxic environmental friendly cyanide substitutes. ...
This review presents an updated data on various technologies for the processing of refractory gold-bearing sulfide concentrates. It primarily focuses on the hydrometallurgical treatment for sulfide mineral oxidation and gold extraction, including the use of bacteria, pressure oxidation, ultrafine grinding, and the Albion ProcessTM. The main operating plants that use hydrometallurgical technologies and their key characteristics are listed, along with schematics of primary processing routes. Besides, advantages and disadvantages of each technology are discussed, along with their cost comparisons.
... Cyanidation is the leading industrial process for extracting gold from different origins because it provides high selectivity for gold over the other elements. However, there are some disadvantages associated with the use of cyanide as a leaching agent, such as prolonged leaching time (commonly 24-72 h for gold ores in tank leaching systems), the high cost of the cyanide reagent, not being efficient for specific gold ores or minerals (i.e., primarily sulfide ores, arsenical, carbonaceous, mangeniferous, pyritic, copper oxides or silicates), lack of a possible solution for cyanide regeneration, and, most importantly, environmental problems, such as cyanide leakage into groundwater and the health risks that result from the extremely toxic nature of the cyanide (Konyratbekova et al., 2015;Angelidis et al., 1993;Olson, 1994). Therefore, it is essential to develop new hydrometallurgical methods for recovering gold from ores. ...
... In a commercial-scale gold tank cyanidation, a sodium or potassium cyanide solution usually is used with a concentration of 0.01-0.05% (100-500 mg/L) (Logsdon et al., 1999;Kanayev et al., 2016) and over a leaching period of 24-72 h (Konyratbekova et al., 2015;Angelidis et al., 1993;Olson, 1994). The reviewed research shows that biocyanidation works with a lower cyanide concentration (typically well below 100 mg/L) and takes a longer period, i.e., 4.5-20 days, for the shaking or stirring approach. ...
The recovery of gold from different sources, such as ores and concentrates, commonly is carried out in industry using cyanidation because it has the advantage of high selectivity of gold over other elements. However, cyanidation has some technical, economic, and environmental limitations. Therefore, investigations have been conducted of new hydrometallurgical approaches for recovering gold from gold-bearing ores by using cheap, non-toxic, and effective reagents, such as biogenic lixiviants. In this paper, we summarize the research that has been conducted on applying biogenic reagents, such as biogenic cyanide, iodine, and triiodide, and organic compounds, such as amino acids, for the extraction of gold from ores. The use of cyanogenic bacteria, such as Chromobacterium violaceum (C. violaceum), Pseudomonas aeruginosa (P. aeruginosa), Bacillus megaterium (B. megaterium), and Micrococcus species (sp.), for extracting gold is reviewed and discussed. Also, this work reviews the ability of iodide-oxidizing bacteria (IOB), such as Roseovarius tolerans, on oxidizing iodide (I⁻) to iodine (I2) and triiodide (I3⁻) and the solubilization of gold as gold diiodide and gold tetraiodide. The capabilities of organic compounds of microbial origin, such as amino acids, on the dissolution of gold also are reviewed and discussed. The advantages, disadvantages, and limitations of microbial leaching of gold over traditional cyanide-based gold leaching are presented. Recommendations for further studies are provided.
... The biological oxidation process [1,2] and fluidized roasting-cyanide leaching process are always employed to treat complex gold concentrates for the gold smelting industries [3,4]. However, these two technologies, although relatively mature, exhibit poor adaptability to raw materials and are ineffective in processing complex concentrates [5]. With the intensifying competition in the gold industry, there are more and more complex and difficult-to-process raw materials, such as As-containing, Sb-containing, or high-copper gold concentrates [6,7]. ...
The technique of gold collection in matte can effectively improve the trapping efficiency of precious metals such as gold, silver, and platinum. However, the underlying mechanism of gold collection from high-temperature molten matte is complex and not well understood. In this work, the first-principle calculations were utilized to investigate the adsorption behavior of gold atoms on a Cu2S surface. The effects of vacancies and As and Sb doping on the gold-trapping ability of Cu2S were also explored, and the electronic properties of each adsorption system, including the charge density difference, density of states, and charge transfer, were systematically analyzed. The results show that the Cu-terminated Cu2S(111) surface has the lowest surface energy, and the Au atom is chemically adsorbed on the Cu2S(111) with an adsorption energy of −1.99 eV. The large adsorption strength is primarily ascribed to the strong hybridizations between Au-5d and Cu-3d orbitals. Additionally, the Cu vacancy can significantly weaken the adsorption strength of Cu2S(111) towards Au atoms, while the S vacancy can notably enhance it. Moreover, due to the formation of strong covalent As–Au/Sb–Au bonds, doping As and Sb into Cu2S(111) can enhance the gold-trapping capability of Cu2S, and the Sb doping exhibits superior effectiveness. Our studied results can provide theoretical guidance for improving the gold collection efficiency of Cu2S.
... Cyanide leaching process utilises metal cyanide solutions to leach the targeted metal (Shams, Beiggy et al. 2004, Chen and Huang 2006, Jha, Lee et al. 2013). This process is used for recovery of gold within the industry despite the issue of toxicity involved with cyanide (Konyratbekova, Baikonurova et al. 2015). During gold recovery through cyanide leaching, the major factors that need to be taken into consideration are the presence of air/oxygen, pH level, temperature, concentration of cyanide, the presence of anions/cations between the solid to liquid phase, rate of stirring, and area of surface contact. ...
Electronic waste (e-waste is one of the fastest growing waste streams which contains valuable materials such as plastic, glass, and precious metals. This study investigates different recycling methods such as pyrometallurgy, hydrometallurgy, and bio-metallurgy process used to recover precious metals such as Au and Ag from e-waste based on the information available in the literature. Mechanisms, improvements, and suitability of each method were critically analysed and corelated to have a complete understanding in this area. It was found that recovered materials can be reused as a secondary input material source. e-waste also contains hazardous materials and chemicals which requires proper care during recycling to prevent contamination. Besides the development of proper experimental techniques, it also required to raise the awareness among the people to deal with this issue sustainably.
... The choice of solvent used by these techniques categorizes the method into either green or conventional (Kumari et al. 2015). Cyanide leaching may be used for gold recovery; however, cyanide is a very toxic compound that requires intensive care and treatment during usage and disposal (Konyratbekova et al. 2015). In this process, methods such as adsorption using activated carbon, cementation, solvent extraction, and ion exchange may be used as recovery technology. ...
The loss of highly sought-after metals such as gold, silver, and platinum during extraction processes not only constitutes a significant waste of valuable resources but also contributes to alarming environmental pollution. The ever-growing adverse impacts of these highly valued metals significantly increase the contamination of water bodies on discharge, while reducing the reusability potential of their corresponding processed wastewater. It is, therefore, of great interest to identify pragmatic solutions for the recovery of precious materials from processed water. In this review, pollution from targeted precious metals such as gold, silver, platinum, palladium, iridium, ruthenium, and rhodium was reviewed and analyzed. Also, the hazardous effects are elicited, and detection techniques are enumerated. An insightful approach to more recent treatment techniques was also discussed. The study reveals nano- and bio-sorption techniques as adoptable pragmatic alternatives, among other techniques, especially for industrial applications with merits of cost, time, waste management, and eco-friendliness. The results indicate that gold (46.2%), palladium (23.1%), platinum (19.2%), and silver (11.5%) are of utmost interest when considering recent recovery techniques. High yield and cost analysis reduction are reasons for the observed preference of this recovery process when considering groups of precious metals. The challenges and prospects of nanomaterials are highlighted.
HIGHLIGHTS
Precious metals are present in processed wastewater as pollutants.;
Conventional recovery methods are associated with limitations.;
Bio- and nano-sorption provide pragmatic alternatives to other existing techniques with cost-effectiveness, simplicity of design, and eco-friendly disposal methods.;
The technological approach via trapping and impregnation of precious metals in sorbents is effective in the recovery of precious metals.;
... Thiourea, thiosulfate (Zhang and Senanayake 2016), halides, malononitrile, acetonitrile, and polysulfides are a few of the lixiviants that have been investigated recently for the leaching of precious metal (Brent Hiskey and Atluri 1988). The most stable gold complexes are produced by the halide lixiviant systems using the nontoxic I − and I 3 − lixiviant ions (Konyratbekova, Baikonurova, and Akcil 2015;Udupa, Kawatra, and Prasad 1990). Therefore, a mixture of iodide and triiodide ions was chosen as lixiviant solution. ...
In-situ recovery can be challenging for application in low-permeability deposits, such as gold- and copper-bearing rocks because of the weak interaction between the mineral and the lixiviant solution. To increase mass transfer in such deposits, we propose the use of ultrasound. Ultrasonic wave emission into a porous solid immersed in liquid can create intense pressure increases and increases in temperature. The high temperature results in increased mass transfer between solid and solution, which for in-situ leaching, can increase the penetration of leaching solution into the solid. A series of experiments have been conducted to monitor the propagation of lixiviant solution through synthetic rock samples. The parameters investigated were ultrasound power, running time and synthetic core sample permeability. The optimum conditions for the maximum migration of lixiviant ions (in these experiments an iodide/tri-iodide model solution) through a synthetic core sample were determined. The migration of lixiviant ions through a synthetic core sample was found to be an order of magnitude higher with the application of ultrasound of 360 W for a running time of 24 h/day (continuous application of ultrasound). Under these conditions and for a test duration of 8 days, the greatest number of ions moved from the source reservoir through a core sample with a permeability of 177 mD to the target reservoir.
... Leaching with cyanide solutions is the main method of the hydrometallurgical recycling of noble metal ores [7]. However, the exceptional toxicity of cyanides has led to the long-term search for alternative leaching agents that can be used to recover gold from both different ores [8,9] and secondary raw materials, i.e., e-waste [10][11][12]. ...
This paper presents the results of the leaching of metals from computer PCBs by electrochemical hydrochlorination using alternating current (AC) with an industrial frequency (50 Hz). Leaching was carried out with a disintegrator-crushed computer motherboard with a particle size (d) of <90 μm. In the course of the research, the leaching efficiency of metals including Fe, Sn, Mn, Al, Cu, Zn, Pb, Ni, Ti, Sb, Cr, Co and V was evaluated depending on process parameters, such as AC density, experiment duration, hydrochloric acid concentration in the electrolyte solution, solid/liquid ratio, electrolyte temperature, and the loading option of raw material (loading option 1 involving loading into the electrolyte solution, and loading option 2 involving loading into the filter containers attached to electrodes). The research results showed that AC superimposition significantly intensifies the leaching of metals. It was established that the complete leaching of metals including Al, Mn, Sn, Ti and Zn, under experimental conditions (loading option 2, CHCl = 6 mol·L−1, i = 0.80 A·cm−2, S/L = 8.6 g·L−1), is reached after 1.5 h, and that of Cu and Ni is reached after 2 h from the beginning of the experiment. At the same time, the degree of leaching of other metals after 2 h is Co-78.8%, Cr-84.4%, Sb-91.7%, Fe-98.9%, V-98.1% and Pb-5.1%. The paper also reports the results on the leaching of all abovementioned metals, as well as Ag and Pd, with disintegrator-crushed mixed computer PCBs with d < 90 μm and loading option 1.
... Chalcopyrite grains observed in microscopy showed some percentage of iodine due to the formation and precipitation of CuI. The formation of CuI complexes and the absence of copper chloride results in better stability of the iodide ions [49,84,85]. This assertion was demonstrated with the EDS analysis of chalcopyrite grains and X-ray diffractograms shown in Figure 13. ...
One of the main problems in processing chalcopyrite ore with hydrometallurgical methods is its refractoriness, which is due to the formation of a layer that inhibits the contact of the ore with the leaching solution, thus reducing the dissolution rate. The main objective of this paper is to evaluate the leaching potential of iodide ions in copper extraction from chalcopyrite concentrate in an acidic seawater medium. Leaching tests were carried out in glass reactors stirred at 45 °C. Parameters such as iodide salt concentration and acidity were evaluated in ranges of 0–5000 ppm and 0–1.0 M, respectively. According to the results obtained, adding iodide ions to a medium acid enhances the leaching kinetics in the chalcopyrite concentrate, observing that it improves copper extraction at low concentrations of 100 ppm KI compared to high concentrations of 5000 ppm KI. As a result, part of the iodide required to oxidize copper tends to sublimate or is associated with other ions producing iodinated compounds such as CuI. Copper extraction reached 45% within the first 96 h, while at 216 h, it reached an extraction of close to 70% copper. The recovery rate improves at potentials between 600 and 650 mV, while at lower potentials, the copper extraction decreases. The mineral surface was analyzed using SEM/EDS and XRD analyses for the identification of precipitates on the surface, finding porous elemental sulfur and precipitated jarosite. An increase in iodide ions improves the leaching kinetics in the chalcopyrite concentrate, observing that it improves copper extraction at low concentrations of 100 ppm KI compared to high concentrations of 5000 ppm KI. As a result, part of the iodide required to oxidize copper tends to sublimate or is associated with other ions producing iodinated compounds such as CuI. Copper extraction reached 45% within the first 96 h, while at 216 h, it reached an extraction of close to 70% copper. The recovery rate improves at potentials between 600 and 650 mV, while at lower potentials, the copper extraction decreases. The mineral surface was analyzed using SEM/EDS and XRD analyses for the identification of precipitates on the surface, finding porous elemental sulfur and precipitated jarosite.
... Thiosulfate is one of the most promising substitutes with the characteristics of nontoxicity, high efficiency, and low cost. Moreover, thiosulfate exhibits excellent performance in dealing with carbonaceous or copper-bearing gold ores or concentrates where gold recovery by cyanidation is poor or cyanide consumption is high [7][8][9][10][11]. Accordingly, numerous studies have been carried out on the kinetics and mechanism of gold and silver leaching in thiosulfate-based solutions [12][13][14][15][16][17]. ...
The leaching behaviors of gold and silver from a complex sulfide concentrate in copper-tartrate-thiosulfate solutions were investigated in this paper. Experimental parameters, including temperature, initial pulp pH, and concentration of copper, tartrate, and thiosulfate, were systematically studied. The copper-tartrate-thiosulfate leaching system exhibits promising performance in dealing with the complex sulfide concentrate. Thiosulfate consumption could be greatly reduced due to the in-situ generation of thiosulfate derived from sulfur or disulfide ions. Increasing the temperature and concentrations of copper, tartrate, and thiosulfate can promote gold leaching. A low tartrate content and a moderate increase in temperature and copper and thiosulfate concentrations can accelerate silver leaching, but a higher temperature or copper content may depress silver extraction. A suitable pulp pH is critical for gold and silver leaching in copper-tartrate-thiosulfate solutions. An extraction of 74.50% Au and 36.33% Ag was obtained with the direct leaching of the concentrate, while the percentages can be significantly increased up to 82.60% and 70.38%, respectively, when leaching the calcine following the oxidative roasting pretreatment. The recycling of leaching solutions demonstrates that a suitable free tartrate content is of great significance in maintaining the stability of the novel system. As an alternative to the traditional cyanide or copper-ammonia-thiosulfate leaching processes, the copper-tartrate-thiosulfate leaching system provides an environmentally friendly, nontoxic, and relatively low-cost method for gold and silver leaching from ores or concentrates.
... A large number of research on non-cyanide gold lixiviants has been undertaken over the past four decades (Cui and Zhang, 2008;Konyratbekova et al., 2015;Liu et al., 2017;Liu et al., 2020;Syed, 2012;Zhang et al., 2021). The most widely researched non-cyanide lixiviants include thiosulfate, thiourea, halide, thiocyanate, and lime sulfur synthetic solution (LSSS), but none of them have achieved large-scale industrial applications due to their limitations (Aylmore, 2016;Hilson and Monhemius, 2006;Zhang and Senanayake, 2016). ...
Gold extraction from ores using non-cyanide lixiviants is currently a significant topic worldwide. Herein, we report the synthesis of an eco-friendly lixiviant that not only is much less toxic than sodium cyanide (NaCN) but also shows excellent effect in the leaching of gold. The lixiviant can be synthesized from roasting of the well-mixed potassium hexacyanoferrate(II) trihydrate (K4Fe(CN)6·3H2O), urea (CO(NH2)2) and sodium carbonate (Na2CO3) with a mass ratio of 1:6:2 under the conditions of heating rate 10–15 °C/min, roasting temperature 700 °C, and temperature holding time 1 h. X-ray diffraction (XRD) analysis suggested that the synthetic lixiviant contained a new phase that is efficient in leaching gold playing the role of the gold lixiviant. Gold leaching results from a gold concentrate showed that the synthetic lixiviant not only has a gold leaching efficacy comparable to NaCN but also achieves faster leaching kinetics than NaCN. Under the optimal leaching conditions of lixiviant concentration 0.3 wt%, initial pH 10–11, liquid to solid ratio 2.5:1, and agitation speed 600 rpm, the synthetic lixiviant could achieve a gold extraction of 87.0% in 8 h and 97.5% in 24 h on the gold concentrate, reducing the leaching time by 12 h compared with cyanidation (97.1% gold extraction in 36 h). In addition, the method of activated carbon adsorption was shown to efficiently adsorb almost all the gold (> 99.0%) from the leach solutions, and the barren leach solution could be reused back to the leaching stage to extract gold.
... In this process, the metal nanoparticles are selectively eluted, with the metal cations remaining intact. This method was rstly adopted for extracting Au from gold mine 154,155 and further developed to prepare metal oxides supported metal cation catalysts. 156,157 Via the leaching method, a series of singly dispersed Au catalysts could be prepared via a two-step process (depositionprecipitation followed by leaching approach), including Au/ CeO 2 , [158][159][160][161][162][163][164][165][166] 156 Not only limited to Au, the leaching method has successfully extended its application to the synthesis of several other SACs, with singly dispersed Pt/CeO 2 catalyst as a typical example. ...
Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.
... The main advantage of cyanide leaching is the high selectivity toward gold. However, long process duration, susceptibility to impurities, and high toxicity of cyanide leaching have prompted researchers to find a green alternative for gold extraction (Konyratbekova et al., 2015). In recent years, thiocyanate as one of the most promising alternatives to cyanide, with numerous advantages like lower toxicity, higher chemical stability, and lower chemical consumption, has attracted the attention of many researchers (Wu et al., 2019). ...
This work evaluated a green route for developing an eco-friendly flowsheet to regenerate base and precious metals from waste printed circuits boards (WPCBs). Copper (as nanoparticles with an average diameter of 50 nm) and other base metals were extracted via oxidative acid leaching with high efficiency. Thiocyanate was employed for the first time as a green and economical reagent for the extraction of gold from pretreated WPCB. The effect of various parameters, including reagent dosage and temperature, was evaluated on the gold leaching rate, and 100% gold dissolution was achieved at the optimal condition. It was found that ferric iron concentration as the gold leaching oxidant has a notable effect on gold extraction. Also, at temperatures above room temperature, the recovery rate increases in a short period and then decreases continuously. The activation energy of the optimum gold thiocyanate leaching was found to be 42.84 kJ/mol, indicating chemical reaction to be the rate-controlling step. Gold extraction from the thiocyanate medium was carried out by employing activated carbon, where 100% gold adsorption was achieved in 2 h. Toxicity assessment of final residue revealed that it could be categorized as an environmentally safe waste with negligible risk.
... Over the past three decades, considerable literature has examined the alternatives to cyanide for extracting gold from different ores/concentrates (Abbruzzese et al., 1995;Aylmore, 2016a;Chen et al., 2021;Cui and Zhang, 2008;Hiskey and Atluri, 1988;Jiang, 1990;Ke et al., 2020;Konyratbekova et al., 2015;Li et al., 2020a;Lin et al., 2010;Liu et al., 2017aLiu et al., , 2017bLiu et al., , 2018aLopes et al., 2018;Song et al., 2016;Syed, 2012;Wang et al., 2021). As outlined in Table 1, more than 25 possible gold leach agents have been investigated as substitutes for cyanide. ...
Cyanide is currently the predominant lixiviant for gold extraction. However, cyanide is highly toxic, causing potential environmental hazards and high detoxification costs. This has initiated extensive research aimed at seeking and developing eco-friendly substitutes for cyanide. Although significant research has been undertaken, most, if not all, of the traditional non-cyanide lixiviants such as thiosulfate, thiourea, halide and thiocyanate have proven to be difficult to achieve widespread adoption at gold mines because of their inherent limitations. In recent years, a range of novel eco-friendly synthetic gold lixiviants (NESGLs) such as “Jinchan”, one of the most representative products, have been developed successfully. Compared with cyanide, these NESGLs are much less or non-toxic, and have achieved comparable gold recoveries from ores without changing the original cyanidation process and equipment. The successful development of NESGLs appears to be making significant inroads into the dominance of cyanide, as evidenced by a growing number of their industrial applications at gold mines worldwide. This paper reviews the current status of gold extraction from its ores using the NESGLs with details focused on their leaching efficacy, synthesis, and composition as well as possible leaching mechanisms. Their future perspectives and developing trends of considerable concern are also discussed.
... However, concerns about the environmental impact of cyanidation (i.e., its toxicity) and the rising silver price have generated significant interest in establishing alternative processes (Sun et al., 2020). For this purpose, different leaching media have been developed, including halide (Sun et al., 2020;Fleming et al., 2003;Li and Miller, 2006), thiourea (Jing-Ying et al., 2012;Konyratbekova et al., 2015;Ahtiainen and Lundström, 2019), and other agents-based (amino acids, alkaline glycine lixiviant, etc.) leaching systems (Perea and Restrepo, 2018;Oraby et al., 2019;Hilson and Monhemius, 2006). Among them, halide leaching which has low selectivity during the extraction is a corrosive process and can cause hazardous working conditions (Hilson and Monhemius, 2006;Hasab et al., 2014). ...
The leaching of silver sulfide was performed for the first time from a novel copper-thiosulfate-metabisulfite solution. The effects of all leaching conditions, e.g., the concentrations of copper cations, thiosulfate, and metabisulfite, as well as temperature, pH, and leaching time on silver extraction from this novel leaching system were systematically investigated. Liquid samples were analyzed by atomic absorption spectrophotometry (AAS) to determine their silver content, and the remaining solid powder was then characterized by field-emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). Based on the results of leaching experiments, it was found that silver sulfide extraction efficiency could be enhanced to 97% with the addition of metabisulfite to the thiosulfate-copper leaching medium. It was revealed that silver extraction is kinetically controlled by diffusion from the product layer. The activation energies (Ea) of silver sulfide leaching and Cu²⁺ reactions were calculated to be 22.2 and 41.2 kJ/mol, respectively. Thus, the thiosulfate-copper-metabisulfite medium is introduced as an efficient leaching system for silver extraction.
... Metallic gold is stable both chemically and electrochemically [1,2]. Therefore, search for methods of gold extraction from various sources is constantly ongoing. ...
The kinetics and mechanism of corrosion of gold anode in a weakly basic aqueous solution of N,N-dimethylpropane-1,3-diamine have been studied by cyclic voltammetry and gravimetry. According to the scanning and transmission electron microscopy data, the corrosion products of gold anode are reduced at both steel and platinum cathodes under galvanostatic conditions to give not only electrolytic gold deposit but also colloidal gold nanoparticles in the electrolyte.
... In addition to sulfidic gold encapsulation, native carbonaceous matter, which adsorbs dissolved gold, may cause gold losses due to the phenomenon called preg-robbing (Helm et al. 2009). In the future, chloride solutions may provide an alternative to toxic cyanide, which is dominantly used in industrial gold plants despite the extensive search for safer substitutes, such as thiosulfate, thiourea, and halides (Hilson and Monhemius 2006;Konyratbekova, Baikonurova and Akcil 2014;Melashvili et al. 2016). ...
A simulation-based life cycle assessment was conducted to compare the impacts of different cyanide-free chloride processing routes for refractory ores to determine whether one-stage leaching of gold from the ores could be advantageous over chloride leaching after conventional pressure oxidation from an environmental perspective. It was shown that both leaching conditions and the refractoriness of the ore strongly affect the environmental impacts of the process. The global warming potential in the direct leaching route was determined to vary between 7.7–17.0 t CO2-eq for 1 kg gold/Au. Prerequisites for the sustainable application of the direct leaching process are suggested.
... Iodide is easier to handle than chloride and bromide because of its lower vapor pressure, weaker corrosive nature and low toxicity. The application of aqueous iodine or lugol's iodine as medication and disinfectant for different purposes since 1835 proves its innocuousness [84,86]. ...
Gold plays a central role in the recycling of electronic scrap and especially of printed circuit boards. Established recycling processes show several significant disadvantages in this respect, such as losses during the complex mechanical pre-treatment, a very demanding and lengthy metallurgical processing and the associated enormous investment and capital commitment costs. A promising alternative approach is the selective gold recovery of printed circuit boards by means of thiosulfate leaching. While the leaching agent for gold recovery from ores has been extensively investigated, there are very few reports on the leaching of electronic scrap. To fill this research gap, in this work the influence of PCB composition and morphology was investigated using innovative conditioning techniques. Furthermore, an extensive parameter study was conducted to identify suitable process conditions, critical effects and substances and to derive the reaction mechanisms. Copper and oxygen were found to be critical components of the leaching system. Although no significant gold extraction could be achieved without these substances, an excess of them causes an enormous reagent consumption and thus prevents efficient leaching. By adjusting the solvent composition, the leaching temperature and by modification of the experimental setup, an almost complete and reproducible selective gold extraction could be achieved in less than 24 hours.
... However, non-cyanide lixiviants mostly stay in the stage of laboratory research due to the low extraction rate (mostly less than 92% [11,12] and less than 60% for refractory ore [13]). ...
A sonochemical reactor was developed to study the ultrasound-assisted cyanide extraction of gold from gold ore at low temperature. The effects of ultrasound on gold leaching in low temperature and conventional conditions were investigated. At the low temperature of 10 °C, ultrasound-assisted extraction increased extraction rate of gold by 0.6%-0.8% and reduced the gold content of cyanide tailings to 0.28 g/t in the leaching of gold concentrate and cyanide tailings, respectively. At the conventional temperature of 25 °C, ultrasound-assisted extraction obtained a 0.1% higher extraction rate of gold compared with conventional extraction, with the unit consumption of NaCN reduction of 15%. The analysis of kinetic model also demonstrated that sonication indeed improved the reaction of gold leaching greatly. The mineralogy and morphology of ore were further analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and particle size analyzer to explore the strengthening mechanism of gold leaching. The results showed that the ore particles were smashed, the ore particle surface was peeled, the passive film was destroyed and the reaction resistance decreased under ultrasonic processing. Therefore, the extraction rate of gold was improved and the extraction time was shortened significantly in ultrasound-assisted cyanide extraction.
... Another method to treat refractory ores is through halide systems (fluoride, chloride, bromide, iodide, and astatine), which have environmental advantages over the traditional system since they act as a substitute for cyanide in the leaching of gold and platinum. Leaching ores with halides is characterized by high kinetic rates, complex stability, and the regeneration of leaching agents, among other characteristics [37][38][39][40][41][42][43][44]. The excellent results reported with the addition of halides include the use of iodine in leaching copper and gold [45][46][47]. ...
The unique properties of ionic liquids (ILs) drive the growing number of novel applications in different industries. The main features of ILs are high thermal stability, recyclability, low flash point, and low vapor pressure. This study investigated pure chalcopyrite dissolution in the presence of the ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate, [BMIm]HSO4, and a bromide-like complexing agent. The proposed system was compared with acid leaching in sulfate media with the addition of chloride and bromide ions. The results demonstrated that the use of ionic liquid and bromide ions improved the chalcopyrite leaching performance. The best operational conditions were at a temperature of 90 °C, with an ionic liquid concentration of 20% and 100 g/L of bromide.
... The best-known gold solvent is aqua regia. In presence of the oxidant nitric acid, hydrochloric acid forms a soluble tetrachloroaurate complex 5,[26][27][28][29][30][31] . In further notations aqua regia will be counted as halogenic reagent, even though strictly seen it is not. ...
Raising social awareness and environmental specifications on cyanide application force gold industry to search for alternative leaching reagents. therefore, researchers worldwide investigate cyanide alternatives for gold recovery since several decades. often the research activities cannot be compared directly, since different input materials and experimental conditions are used. Over the course of this study, different promising cyanide alternative reagents were investigated in terms of their capability of pure gold dissolution at different temperatures. All experiments took place under identical conditions by using uniform samples of 99.99% gold disks, to enable a comparability. Thiosulfate as one of the most promising reagent thiosulfate according to literature revealed an insufficient leaching behavior. The gold dissolution was hindered due to the formation of a sulfide passive layer. Also in the thiourea trials, a surface precipitation took place, though gold dissolution did not stop. the halogens iodine, bromine and the well-known gold solvent aqua regia dissolved gold very fast (up to ~1,000 mg·/(h ·cm²)). Methanosulfonic acid (MsA) was not capable to extract any gold. the experiments were compared with cyanide trials at identical conditions. The average dissolution rate of investigated reagents at 25 °C shows following order: aqua regia > iodine > bromine > cyanide > thiourea > thiosulfate > MSA.
... The gold exploitation depends on the way it is present in minerals, and its extraction can be done in the acid phase (pH < 3) with thiourea, thiocyanate, chlorine, aqua regia, ferric chloride; in neutral Processes 2019, 7, 225 2 of 24 phase with thiosulfate, halogens, sulfuric acids, bacteria; and in alkaline phase (pH > 10) with cyanide, ammonium cyanide, ammonium, sulfur, and nitriles [2,3]. However, the practical application of these processes is limited to extraction in the alkaline phase using cyanide because of its high selectivity with respect to gold [4][5][6]. ...
For gold extraction, the most used extraction technique is the Merrill-Crow process, which uses lixiviants as sodium or potassium cyanide for gold leaching at alkaline conditions. The cyanide ion has an affinity not only for gold and silver, but for other metals in the ores, such as Al, Fe, Cu, Ni, Zn, and other toxic metals like Hg, As, Cr, Co, Pb, Sn, and Mn. After the extraction stage, the resulting wastewater is concentrated at alkaline conditions with concentrations up to 1000 ppm of metals. Photocatalysis is an advanced oxidation process (AOP) able to generate a photoreaction in the solid surface of a semiconductor activated by light. Although it is well known that photocatalytic processes can remove metals in solution, there are no compilations about the researches on photocatalytic removal of metals in wastewater with cyanide. Hence, this review comprises the existing applications of photocatalytic processes to remove metal and in some cases recover cyanide from recalcitrant wastewater from gold extraction. The use of this process, in general, requires the addition of several scavengers in order to force the mechanism to a pathway where the electrons can be transferred to the metal-cyanide matrices, or elsewhere the entire metallic cyanocomplex can be degraded by an oxidative pathway.
Aqueous biphasic systems (ABSs) that are based on deep eutectic solvents (DESs) are environmentally benign systems to use for metal ion separation. In this work, a series of DESs was synthesized for the first time with PEG 400 as hydrogen bond donors and tetrabutylphonium bromide (P4Br), tetrabutylammonium bromide (N4Br), or tetrabutylammonium chloride (N4Cl) as hydrogen bond acceptors, and then they were combined with citrate (Na3C6H5O7), which is eco-friendly, to construct an ABS for use in the separation of Au(I) from an aurocyanide solution. Phase diagrams of DESs + Na3C6H5O7 + H2O systems were constructed using the experimentally measured data. Multiple factors that affect the efficiency of the gold extraction were studied; these factors were the species of salt or DES and their content, the equilibrium pH, the oscillation time, and the initial gold concentration. Gold(I) is preferentially retained in the DES-rich phase, and the P4Br:PEG 1:2 + Na3C6H5O7 + H2O system has a high extraction efficiency of 100.0% under optimized conditions. FT-IR, NMR, and TEM characterizations and DFT calculations show that the migration of Au(I) from the salt-rich to the DES-rich phase follows an ion exchange mechanism. Specifically, Au(CN)2- replaces Br- in the original P4Br and generates a stable ion pair with the quaternary phosphonium salt cation, P+, and this replacement is driven by electrostatic attractions. A new strong hydrogen bond network simultaneously forms between the anionic Au(CN)2- and the -OH group in the PEG 400 component. Finally, the gold of Au(I)-loaded P4Br:PEG 1:2 can be successfully reduced by sodium borohydride with an efficiency of 100.0%. The strategy to extract gold(I) from alkaline cyanide solutions using an ABS based on DESs as proposed in this work provides a potential platform for developing green technology for recovering gold.
The physical and chemical properties of gold promote its application, such as in the high-tech, electronic products, and aerospace industries. The easily leachable ore is gradually depleted. Thus, it becomes necessary to extract gold from other resources such as refractory ore and electrical and electronic equipment. The normal method of leaching for gold is cyanide leaching, but it is very dangerous for both environment and operator. Non-cyanide leaching methods, including thiourea leaching, halide leaching, and sulfate leaching have been developed to substitute cyanide leaching. A variety of methods to enrich gold from leaching solutions are described in this paper, including solvent extraction, electrowinning, activated carbon adsorption, and ion exchange resins. Among those methods, ion exchange resins can adsorb gold with high adsorption efficiency and regenerate easily as well. This paper focuses on the research progress of the recovery of gold from non-cyanide leachates by ion exchange resins, summarizes the existing resin types and elution processes, points out the limitations in the application of current ion exchange resins, and discusses possible solutions.
The iodination method is a cyanide-free and efficient gold-leaching process, but a large amount of electric energy is consumed for hydrogen evolution during the process of gold recovery by electrodeposition in the iodide/H2O system. Herein, [C2MIM][N(CN)2] ionic liquids were developed as electrolytes to overcome the limitations of water-based solvents for higher current efficiency by extending the electrochemical window. And the three-dimensional carbon felt with a high surface area and abundant active sites was selected as the cathode material to enhance the deposition rate. Cyclic voltammetry test demonstrated that both Au⁺ reduction and I⁻ oxidation are irreversible processes controlled by diffusion with the diffusion coefficients of 1.48 × 10⁻⁷ cm²/s and 2.91 × 10⁻⁷ cm²/s at 20 °C, respectively. The optimal current efficiency (88.03%), specific energy consumption (216.37 kWh/t) and recovery of gold (95.77%), and iodine (85.75%) were obtained by regulating the electrodeposition parameters, including applied potential, temperature, and stirring speed. Moreover, the gold leaching and deposition efficiency were stable at approximately 93% and 95% with 0.2% iodine supplement for four cyclic leaching-electrodeposition. SEM-EDS, XRD, and XPS analysis showed the deposits on the carbon fibers had good uniformity, and the deposit composition contained Au⁰, Cu⁰, and CuO. [C2MIM][N(CN)2] ionic liquid is an attractive alternative to non-concentrated aqueous electrolytes for high-efficiency and low-power electrodeposition applications.
Iodine has been shown to act as a good electrocatalyst for metal digestion in deep eutectic solvents (DESs) but little is known about its speciation or reactivity in these high chloride containing media. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were made at the iodine K-edge in a range of DESs with different glycolic or acidic hydrogen bond donors (HBDs), along with examining the effect of iodine concentration between 0.01 and 0.5 mol dm-3. Three groups of speciation were detected: mixed I2Cl-/I3- (glycol and lactic acid systems), mixed I3-/I2 (oxalic acid and urea systems), and singular I3- (levulinic acid system). UV-vis spectroscopy was used to confirm the speciation. Electrochemistry showed that iodine redox behaviour was unaffected by the changing speciation. Leaching data showed that metal oxidation was related not only to changing iodine speciation, but also the reactivity and coordination ability of the HBD.
BACKGROUD
The large and growing amount of electronic waste (e‐waste) caused by the rise of the electronics industry has negatively impacted the environment and secondary resource utilization. Thus, research on the recovery, recycling, and reuse of valuable metals from e‐waste has attracted considerable attention. In this study, a novel leaching system based on dicyandiamide ionic liquids (ILs): 1‐ethyl‐3‐methylimidazolium dicyandiamide, 1‐butyl‐3‐methylimidazolium dicyandiamide, and 1‐hexyl‐3‐methylimidazolium dicyandiamide were used to evaluate the effectiveness of leaching gold from typical e‐waste CPUs by iodination.
RESULTS
The response surface experiment and analysis of variance (ANOVA) for the results showed that the optimal gold leaching efficiency reached 93.81% after 170 min under a rotational speed of 300 rpm, temperature of 32 °C, and iodine dosage of 1.15%. The mechanism of reaction was determined and the main reaction products were Au[IN(CN) 2 ] ⁻ and Au[N(CN) 2 ] 2 ⁻ . Moreover, the kinetic behavior of gold leaching showed a good correlation with the Avrami model and the activation energy value was 14.01 kJ·mol ⁻¹ . The comparison of iodine consumption between aqueous medium and ILs showed that the required amount of iodine (calculated by ionic iodine) decreased by a factor of six times in ILs.
CONCLUSION
The iodine dosage, leaching temperature, and time had significant effects on the gold leaching efficiency in the IL leaching system. The reaction of gold leaching was jointly controlled by diffusion and chemical reactions. The coordination groups [N(CN) 2 ] 2 ²⁻ and [IN(CN) 2 ] ²⁻ in the pregnant solution formed stable complexes with Au(I). The consumption of iodine in ILs decreased remarkably, showing the significant practical value of ILs. © 2022 Society of Chemical Industry (SCI).
Overcoming the electrocatalytic energy barrier mediated by Gerischer-Mauerer's mechanism has always been a major challenge in ammonia detection. The previous report [1] that ammonia can electro-dissolve Au gives us inspiration to establish a more efficient and reusable electrochemical transmission technique for ammonia detection and, above all, remove the ammonia electrocatalytic barrier. In this work, the latent electro-dissolution activity of ammonia towards Au was employed to construct the sensing interface. The sensor applied the potentiostatic electrodeposition method to grow Au crystals on the surface of carbon cloth, offering a simple and low-cost alternative for practical production.
Additionally, the electro-dissolution sensing method effectively addresses the limitation of high energy barrier and toxic products (N-ads) in catalytic oxidation of ammonia. Cyclic Voltammetry and Tafel curve demonstrate the electrochemical properties of Au/CC and help to predict the relationship between the ammonia concentration and current through theoretical analysis, which were further confirmed by an i-t curve. We proved that the sensor has excellent reproducibility under high-intensity work in theory and it exhibits high stability over 7 weeks in practice. The economical and facile constructing electrode method and the outstanding performance in long-term reuse greatly reduce the cost. Generally, the sensor is suitable for continuous ammonia monitoring in wastewater and quantitative analysis of products by chemical companies.
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.
In this study, the synergistic extraction system of trioctylmethyl ammonium chloride (N263)-tributyl phosphate (TBP)-n-octanol-sulfonated kerosene was used to enrich and recover valuable metals from cyanide gold extraction of wastewater. The effects of the proportion of N263 and TBP, contact time, initial pH of water phase, temperature, and phase ratio (O/A) on the extraction percentage of metal cyanide complex ions and the synergistic extraction reaction mechanism were mainly investigated. Results showed that the single-stage extraction percentages of Cu, Zn, and Fe ions in wastewater were 99.87%, 99.8%, and 96.2%, respectively, and the saturated extraction capacity was 4356.4 mg/L under the conditions of N263 (20 vol.%)-TBP (20 vol.%)-n-octanol (10 vol.%)-sulfonated kerosene system at 25 °C, O/A of 1:1, pH of 10, and contact time of 5 min. Fourier transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS) analyses showed that metals entered the organic phase in the form of metal cyanide complex ions during extraction. Metal cyanide complex ions preferentially combined with TBP to lose hydrophilicity and then reacted with N263 cations to enter the organic phase. The saturated organic phase was stripped by NaOH and NaSCN mixed solution, and the total concentration of metal ions in the stripping solution reached 11996.6 mg/L.
The current paper introduces for the very first time recently invented electrodeposition-redox replacement (EDRR) method in continuous leaching-recovery-solution purification process. The mini-pilot study reveals that longer duration of the EDRR process is beneficial as it increases gold recovery per stage and decreases the specific energy consumption of the EDRR. This extraordinary behavior is postulated to be an inherent to an EDRR process, as during the EDRR the cathode surface is gradually transformed from stainless steel surface to gold(±copper) surface, thus increasing process performance with time. In addition, gold concentration of 1 mg/L in the PLS was established as the minimum concentration required for efficient operation of the EDRR process.
Gold extraction from ore to solution throughout the pilot test reached up to ca. 90%, whereas in reference cyanidation test only 64% of gold was dissolved. Overall, after 150 h of pilot experiment, 83% of dissolved gold was recovered from solution on the cathode, resulting in 68.5% holistic recovery of gold from ore. Furthermore, process simulation model was built, and it suggested that closed-loop operation of cupric chloride leaching – EDRR recovery – copper precipitation process can recover in total over 84% of gold from refractory telluride ore to the cathode providing further evidence of true potential of EDRR in sustainable gold extraction.
The use of citrate as an alternative reagent to ammonia in thiosulfate solutions offers an environmentally friendly, nontoxic, and cost-effective leaching method for gold extraction from ore/concentrate. In this work, the behavior of gold disc with copper-citrate-thiosulfate solutions has been investigated to identify the leaching mechanism of gold in the complex system of citrate-thiosulfate. The experimental parameters include rotating speed of gold disc, temperature, solution pH, and composition of lixiviant. The addition of copper and citrate in thiosulfate solutions can accelerate gold dissolution rate by about an order of magnitude. For the copper-citrate-thiosulfate system, thiosulfate ion works as the complexing agent for aurous ion, and it is extremely critical to the gold leaching process. The results of kinetic analysis of the leaching data indicate a chemical reaction-controlled process under the experimental conditions, giving an activation energy of 49.74 kJ/mol. The reaction orders of gold leaching with respect to copper, citrate, and thiosulfate concentration in copper-citrate-thiosulfate solutions are 0.95, −1.49, and 1.18, respectively. An acceptable gold leaching rate of 0.18 to 0.27 μmol·m⁻²·s⁻¹ could be obtained under the conditions of above 0.1 mol/L copper and thiosulfate concentrations.
The present chapter offers an overview of polyanions of the Group 17 as examples of catenation. The main knowledge obtained from the rich literature on polyiodides is summarized, and comparisons are made with other polyhalide systems in order to gain insights into similarities and differences. A strong emerging field is represented by polybromides in the liquid and solid state, as well as novel or rejuvenated areas of application. A detailed analysis of chemical bonding is given, and the special properties of halogen-halogen bonding are highlighted.
This research focuses on the hydrometallurgical processing of auriferous ores and their processing products, namely, flotation and gravity concentrates. The main valuable component of an ore sample of any deposit is gold. The gold content should be in the range of 11.11–12.87 g/ton. The main rock-forming minerals of the original ore are quartz (60.1%), quartz–chlorite–micaceous aggregates (3.8%), and carbonates (7.1%). In this study, original ores of various sizes were treated by direct and sorption cyanidation under various leaching modes, and the results obtained were presented. The original ore was leached with various concentrations of sodium cyanide (NaCN) in solution to study the effect of the complexing agent concentration on gold recovery. Data on the dynamics of leaching revealed that a decrease in the concentration of NaCN in solution from 0.2% to 0.03% leads to a decrease in gold recovery in solution by 26.81%. Original ores could easily be processed using hydrometallurgical methods. The recovery of gold with a coarseness of 95% − 0.045 mm from the original ore averaged 97.77%. This work features a full range of studies on the hydrometallurgical processing of concentrates and gravity tailings, as well as the effects of flotation concentration of gold recovery. The gravity concentrate is resistant to intensive cyanidation (i.e., only 67.07% gold recovery into the solution). The use of 1% lead nitrate (PbNO3) during intensive cyanidation could reduce the refractoriness of the concentrate and increase the recovery of gold into the solution by up to 94.35%. The total gold recovery from gravity concentrate is 98.71%, and the recovery of gold by cyanidation of gravity tailings with a cyanide concentration of 0.2% averages 96.57%. The recovery of gold during leaching of the flotation concentrate at the original size (95.5% − 0.074 mm) and a cyanide concentration of 0.2% is 96.64%. A decrease in the size of the flotation concentrate from 95.5% − 0.074 mm to 95% − 0.02 mm leads to a decrease in gold recovery by 35.43% because of the strong chemical activation of the material during grinding.
The kinetics and mechanism of corrosion of an Au anode in a weakly basic aqueous solution of 2,2-dimethyl-1,3-diaminopropane (2,2-DM-1,3-DAP) were studied by gravimetry and cyclic voltammetry. Scanning and transmission electron microscopy was used to determine that under galvanostatic conditions the products of anode corrosion are reduced on a steel cathode with the formation of not only an electrolytic Au deposition on the cathode, but also colloidal gold nanoparticles in the electrolyte medium. The product of the interaction of 2,2-DM-1,3-DAP with atmospheric CO2, namely, the carbamic acid internal salt 3-ammonio-2,2-dimethyl-propylcarbamate, was isolated from the reaction solution.
This chapter considers the ingredients in what we have called the toxic alchemy of African gold production. Over large parts of the continent gold producers have damaged the ‘mineral of life’, namely water and adversely affected the health of millions of Africans who have worked in gold mines or who live and work in close proximity to these mines. What is considered are the various toxic elements in the processing and refining of gold and the effects of their application on the environment and health in Africa. What African gold producers have done is to in effect reverse medieval alchemy and produce gold and leave mercury, lead and a range of other toxic substances. There is nothing uniquely African about either the metallurgy or technology applied in gold processing on the continent but what makes Africa unique is the extent and growth of the unregulated ASGM production of gold. This makes African gold production different from that of other continents. It is the absence of proper governance in the gold mining industry which stems not from an ‘African governance failure’, but from the extent of the increasingly important but seemingly ungovernable ASGM sector.
Hypochlorite leaching system is a strong oxidizing solution for metal sulfides, which are naturally associated with gold extraction. This study evaluated the hypochlorite leaching of refractory gold ore by using the response surface method–central composite design (RSM-CCD) as a statistical approach in determining the optimum condition of gold extraction from refractory gold ore. The studied parameters for the gold recovery were pH (4 to 6), concentrations of Ca(OCl)2 (0.5 to 1.5 M) and NaCl (2.5 to 3.5 M) in the solution. It was found that the maximum gold extraction of 82.46% (13.85 g/t) was achieved at a pH of 4.0 with NaCl and Ca(OCl)2 concentrations of 2.93 M and 1.08 M, respectively. The statistical analysis also revealed that the amount of gold recovery in chloride solution was significantly depended on pH as the main factor, followed by concentrations of Ca(OCl)2 and NaCl.
Gold-bearing dust generated during the roasting process of refractory gold concentrates is valuable as secondary gold resources. In this study, the characterization of gold-bearing dust from carbon-bearing gold concentrates was determined by direct cyanidation and mineralogy research. Moreover, identification of refractoriness in the dust was determined by selective removal of minerals. Results showed that the dust belonged to refractory ores. Gold was observed to be presented as native and electrum which main associated with iron oxides. After preleaching with a 6 mol/L NaOH solution, arsenic, and carbon removal rates were 99.66% and 60.63%, respectively, and gold extraction was 58.90%, only 4.60% higher than that of direct cyanidation. After preleaching with a 15 wt.% H2SO4 solution, iron, arsenic, and carbon removal rates were 33.65%, 80.38%, and 12.60%, respectively, and gold extraction achieved 80.40%. After roasting, carbon and arsenic removal rates were 95.00% and 54.65%, respectively, and gold extraction achieved 84.52% under the optimum condition. Removal of carbonaceous matter and iron oxides could efficiently improve gold extraction, and the adverse effects of carbonaceous matter and iron oxides were the main reasons for refractoriness. Based on these results, a proper process for gold extraction from the dust was developed, and gold extraction achieved 95.50%.
Rapid urbanization, advancements in science and technology, and the increase in tech-savviness of consumers have led to an exponential production of a variety of electronic equipment. The global annual growth rate of e-waste volume exceeds the growth rate of the human population. Electronic waste has now become a point of concern globally (53.6 million metric tons, 2019). However, merely 17.4% of all global e-waste is properly collected and recycled. China is the largest contributor to the global production of e-waste (~19%), the second being the United States. Indeed, only 14 countries generated over 65% of global e-waste production in 2019. E-wastes contain a wide range of organic, and inorganic compounds including various metals. Emerging contaminants like plastics are amongst the fastest growing constituents of electronic waste. The current challenges include the lack of reliable data, inadequate identification and quantification of new emerging materials, limited effectiveness of current recycling technologies, need for cutting-edge detection and recycling technologies, and the lack of e-waste management policies and international collaboration. In this review, we strive to integrate the existing data on production rates at different spatial scales, composition, as well as health, economical, and environmental challenges, existing recycling technologies; explore tangible solutions; and encourage further sustainable technology and regulatory policies.
An iodine-iodide system was investigated as an alternative lixiviant for HNO3 for leaching precious metals from the end-of-life c-Si photovoltaic (PV) cell. A series of batch experiments were conducted for the optimization of leaching kinetics and thermodynamic equilibrium followed by a life cycle assessment (LCA) using data from the experiments. The results showed that more than 95% of Ag and Al leached out within the first 5 min. The optimum conditions for equilibrium leaching were as follows: solid to liquid ratio of 1:10 for Ag (1:9 ml for Al), and I2 concentration of 0.35 M for Ag (0.3 M for Al), with I- concentration of 0.7 M. In addition, selective leaching of Ag could also be accomplished by adjusting the reaction pH to 9.6%, and 93% of reproducibility was achieved via the rejuvenation of the exhausted leaching solution, which can benefit the subsequent recovery process. The leaching efficiency of iodine-iodide system was nearly comparable to that of HNO3, and the environmental impacts of the two cycle of continuous process with rejuvenation of the iodine leaching solution can be effectively reduced especially in the acidification & eutrophication, respiratory effect, and mineral extraction categories with subsequent exclusion of the additional neutralization process.
As the mining industry moves toward the extraction of increasingly complex ores to meet global demands, one of the challenges in gold processing is the treatment of preg-robbing ores. Preg-robbing occurs when gold that can be otherwise recovered is lost through adsorption, reduction, or absorption by carbonaceous compounds or other clays and minerals in the ore. This review discusses the sources and mechanisms of preg-robbing and examines several operating factors that can exacerbate losses. Historical and current techniques to mitigate preg-robbing will be critically reviewed, along with promising new approaches that are in development or have recently been commercialized, such as high-temperature caustic conditioning, with special consideration to costs and sustainability. A secondary section details the interactions between chloride and gold in minerals processing, focusing on the negative impact of chloride ions during the pressure oxidation of double refractory ores, and examines the techniques undertaken by industry to approach this issue.
Employing activated carbon in the thiosulfate process has high potential applications for extracting gold. However, formidable challenges must be overcome to achieve such an application in an efficient and eco-friendly manner. Accordingly, this study aimed to develop a simple and environmentally friendly method for gold extraction. To this end, activated carbon with good gold capacity (15.9 kg/t) was synthesised for the first time by impregnating green reagent 1-methyl-5-mercapto-1,2,3,4-tetrazole (MMT), which can thermally decompose into a gas completely, and the mechanisms of adsorption and desorption process were studied. Acceptable desorption efficiency (nearly 100% in 12 h) was achieved for gold on activated carbon using environmentally safe and low-cost thiosulfate salt. More importantly, after gold desorption, the good gold adsorption performance (gold capacity of 13.8 kg/t) of activated carbon could be restored by treating it again with MMT. Additionally, green thiosulfate could be used not only as a leaching agent but as a desorbent for gold. Moreover, the results suggest that gold adsorption on MMT-AC takes place through a ligand exchange mechanism. Overall, the method reported herein exhibits good gold adsorption-desorption performance, while offering environmental friendliness and simplicity. With these achievements, this study is expected to accelerate the development of the expected potentially more environmentally friendly technology.
Alkaline sulfur-containing lixiviants, including thiosulfate, polysulfides, and alkaline sulfide solutions, stand out as a promising class of alternatives to cyanide because of their low toxicity, high efficiency, and strong adaptability. In this paper, we summarized the research progress and remaining challenges in gold extraction using these noncyanide reagents. After a brief introduction to the preparation method, the transformation process of various sulfur-containing species in alkaline solutions was discussed. Thereafter, some insights into the mechanism of gold leaching in alkaline sulfur-containing solutions were presented from different aspects, including thermodynamics analysis, electrochemical dissolution, and leaching kinetics. Moreover, recent progress in in-situ generation of sulfur-containing anions from gold-bearing sulfide minerals was outlined as well. Gold passivation caused by sulfur species was discussed in particular because it is considered the greatest challenge facing sulfur-containing leaching systems. Alkaline sulfur-containing lixiviants are expected to serve as alternatives in industrial applications of gold extraction, particularly for refractory gold ores containing copper and carbonaceous matter.
Abstract-The interaction of gold hydroxo-chloro complexes with iron oxides (ferrihydrites, goethites) during coprecipitation experiments is investigated. Chemical analyses of solids and solutions are coupled with a detailed characterization of the iron oxides with various methods, including X-ray diffraction, High Resolution Transmission Electron Microscopy (HRTEM), and Mössbauer spectroscopy. HCl solutions containing varying amounts of AuCl4− and ferric nitrate were titrated to neutral or alkaline pH, resulting in the coprecipitation of gold and iron oxide phases (ferrihydrite or goethite). Reference titrations were performed in the absence of iron. Most of the gold was removed from solution in the presence of iron oxides whereas gold remained dissolved in the reference samples. In association with iron oxides two forms of gold have been identified by HRTEM and 197Au Mössbauer spectroscopy: metallic gold as well as chloro and/or hydroxo combined gold. This combined gold is in a trivalent state as the primary product which means that a reduction process is not a necessary step for the adsorption of gold species on iron oxides. Metallic gold characterized in these products by means of HRTEM consists mainly of colloids ranging from 3–60 nm in diameter embedded in the ferrihydrite matrix, as isolated particles or as particles associated with goethite laths. The smallest metallic gold particles detected would be almost invisible to classical observation techniques used for solid phases. In our experiments, oxidation-reduction reactions between Fe2+ and Au3+ are responsible for the presence of metallic gold observed on the iron phases. Photochemical reactions may also take part in the reduction process. Colloidal gold and gold complexes could be associated to the iron oxides by strong electrostatic interactions. Our results suggest that poorly ordered iron oxides are highly efficient in trapping gold from solutions thanks to their high surface area, and favor gold precipitation during the first stage of the low temperature genesis of ironrich Au-containing systems.
This paper presents the experimental results for the leaching of printed circuit boards (PCB) from obsolete computers for extracting and recovering tin and copper by means of leaching followed by precipitation. Printed circuit boards were dismantled, cut into small pieces, and fed into a cylinder mill. The powder obtained was leached by using the aqueous solutions 2.18N H2SO4, 2.18N H2SO4 + 3.0N HCl, 3.0N HCl, and 3.0N HCl + 1.0N HNO3. The lowest values for the percentage of metal extraction were obtained with 2.18N H2SO4 (2.7% for Sn and lower than 0.01% for Cu), while the 3.0N HCl + 1.0N HNO3 leach system exhibited an extraction of 98% for Sn and 93% for Cu. Precipitates were obtained at different pH values by neutralizing the leach liquors using NaOH. The 3.0N HCl + 1.0N HNO3 leach system presented the highest recovery values from the powder feed (84.1% for Sn and 31.9% for Cu), as well as from the leach liquor (85.8% for Sn and 34.3% for Cu).
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.
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.
In this study, physical, chemical and hazardous characteristics of computer printed circuit boards (PCBs) were studied. PCBs manually obtained from end-of-life computers of various brands were subjected to size reduction down to -3.35 mm in two stages of crushing. Size fractions were obtained by dry-sieving and used to determine the liberation size of metals. Furthermore, hazardous characteristics of PCBs were investigated using standard protocols including TCLP (Toxicity Characteristic Leaching Procedure) and SPLP (Synthetic Precipitation Leaching Procedure) tests of US Environmental Agency (USEPA), ASTM D-3987 and EN 12457-2 tests to simulate different environmental scenario. The tests have shown that printed circuit boards can be classified as hazardous wastes. Copper content of PCBs was found to be 15.5 per cent. © (2010) by the Australasian Institute for Mining and Metallurgy (AusIMM).
Before attempting to deal with other aspects of organic chemistry, it is necessary that the nomenclature (naming system) of chemistry be understood. Many organic chemicals have more than one name, an example being a particular type of alcohol containing three carbon atoms, which can be called any of the following five names:
“isopropyl alcohol,” or “isopropanol,” or
“secondary propyl alcohol,” or “sec-propanol,” or
“2-propanol.”
It is shown that the dissolution of gold in the presence of a bioorganic product obtained by acid treatment of collagen-containing food-industry wastes is much more effective than in the presence of a bioorganic product obtained from the wastes of antibiotics production. In particular, when this new product is used, the dissolution of gold is, on the average, twice as great as in control experiments with the other product (a hydrolysate of nutrient-yeast biomass). The new product does not require a prior hydrolysis operation and may be recommended for use in heap leaching of finely dispersed gold.
The behaviour of metallic gold in acid solutions of thiocyanates in the presence of an oxidizing agent is studied. By the method of rotating disk, the kinetic characteristics of the dissolution process are calculated. An analysis of intermediate solid products is performed, and the limiting stage of the heterogeneous reaction is determined. The conclusion is made on the feasibility of using thiocyanates in the hydrometallurgy of gold.
The rates of solution of some nonferrous and noble metals in thiocyanate electrolytes under the action of alternating current are determined. Gold has the maximum rate of solution, and lead has the minimum rate. The rate of solution falls over time.
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.
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.
A review of the literature indicated that several studies have been reported on the dissolution of gold using iodide/iodine, bromide/bromine leaching systems. Although the reagent costs are much cheaper for chloride/chlorine (or hypochlorite) this system has not been fully evaluated. This study was conducted to evaluate the fundamental chemistry of the chloride/hypochlorite leaching system. The CHEMIX program was used to model the stability of various gold species existing in the solution. The gold dissolution rate at different conditions was determined using gold strips immersed in various mixtures of chloride/hypochlorite. Leaching experiments were conducted to test the extraction of gold from various ores (oxidised and copper-gold ores), showing a promising prospect for this technique in various applications.
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.
The chemistry of the leaching of silver sulfide by thiosulfate solutions that contain added copper sulfate has been examined. Reaction mechanisms have been identified and leaching results have been interpreted in the light of these. Competing reactions that lead to solution instability have been noted when ammonium thiosulfate is used, excess thiosulfate being required to stabilize the solutions: limitation of the leaching reactions through the formation of an impervious coating of copper sulfide has been inferred. Addition of sodium sulfite to the lixiviant helps to stabilize the thiosulfate and prevents sulfide precipitation, but, for reasons that are as yet unclear, it also results in a lowering of silver extraction.
The cleanest kinds of precious metal scrap are old jewelry, metal parts of old dentures, and the dust and bits produced in the manufacture of jewelry or dentures. Old jewelry may be made of sterling1 silver, carat2 gold (alloyed gold), or of base metals plated with gold. Dentists often use high-quality gold, platinum, or alloys of gold with platinum, silver, and/or copper.
Material and energy resource consumption is on the rise in both the industrialized and developing world (e.g., countries like India and China). In order to sustain this growth and provide resources for future generations, there is a need to design products that are easy to recover and recondition, thus enabling multiple use cycles. Processes are needed that can achieve this multi-use while producing zero (or very near zero) waste. There exist a number of barriers and challenges to achieving this vision of multi-use with zero waste; one such challenge is the development of a product recovery infrastructure that will minimize short-term impacts due to existing products and will be robust enough to recover products of the future. This paper identifies the barriers to developing such a recovery and reuse infrastructure. The aim is to achieve product multi-use and zero waste.
Some preliminary results on the secondary leaching of previously heap leached gold ore are presented.
Electronic waste, or e-waste, is an emerging problem as well as a business opportunity of increasing significance, given the volumes of e-waste being generated and the content of both toxic and valuable materials in them. The fraction including iron, copper, aluminium, gold and other metals in e-waste is over 60%, while pollutants comprise 2.70%. Given the high toxicity of these pollutants especially when burned or recycled in uncontrolled environments, the Basel Convention has identified e-waste as hazardous, and developed a framework for controls on transboundary movement of such waste. The Basel Ban, an amendment to the Basel Convention that has not yet come into force, would go one step further by prohibiting the export of e-waste from developed to industrializing countries. Section 1 of this paper gives readers an overview on the e-waste topic—how e-waste is defined, what it is composed of and which methods can be applied to estimate the quantity of e-waste generated. Considering only PCs in use, by one estimate, at least 100 million PCs became obsolete in 2004. Not surprisingly, waste electrical and electronic equipment (WEEE) today already constitutes 8% of municipal waste and is one of the fastest growing waste fractions. Section 2 provides insight into the legislation and initiatives intended to help manage these growing quantities of e-waste. Extended Producer Responsibility (EPR) is being propagated as a new
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.
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, since gold–iodide complexes 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 h. 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 h. Based on the obtained experimental data, a power law rate equation was applied to model the gold extraction kinetics in iodide/iodine solutions. The reaction was found to be first order with respect to tri-iodide concentration.
An alternative non-cyanide lixiviant, namely sodium hypochlorite, has been used for the selective extraction of gold and silver from a copper concentrate. Hypochlorite leaching of the as-received, as well as aqueous pressure-oxidized copper concentrate was carried out. Direct hypochlorite leaching yielded gold and silver recoveries of 42.7% and 45.0%, respectively. With aqueous pressure oxidation followed by hypochlorite leaching, it was possible to selectively recover 90.0% of gold and 92.5% of silver from the copper concentrate.
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.
The thermodynamics of halide-water and gold-halide-water systems are summarized in the form of potential-pH and activity-pH diagrams, calculated from recently published standard Gibbs energies of formation of the various species and phases considered. The stability of the gold-halide complexes increases in the order Cl < Br < I, which is reflected in the pH above which Au(OH)3 (c) is predicted to form, though even the chloride complexes are stable at pH < 7. These diagrams enable predictions to be made of the behaviour of gold in halide electrolytes; chloride is used in industrial gold electrorefining, and interest is increasing in coupled leach-electrowin processes involving electrogenerated chlorine, bromine and iodine.
A novel energy-saving hydrometallurgical recovery process for copper from electronic scrap employing the Cu(I)-ammine complex has been presented on the basis of a thermodynamic consideration. In order to experimentally explore the feasibility of the leaching stage in this process, the copper leaching behavior from a printed circuit board (PCB) in ammoniacal alkaline solutions has been investigated under a nitrogen atmosphere. Copper in PCB was oxidized by Cu(II) to form Cu(I)-ammine complex ions. The leaching reaction can be expressed as: Cu + Cu(NH3)(4)(2+) = 2Cu(NH3)(2+). The Cu(II)-ammine complex significantly enhanced the leaching rate, while the Cu(I)-ammine complex slightly depressed it. Crushing of the PCB effectively enhanced the leaching rate, because the exposed metallic copper area is increased by the crushing. The effect of temperature on the leaching rate was insignificant. Consequently, the feasibility of the leaching stage in the proposed copper recovery process has been experimentally confirmed.
An oxide gold ore was subjected to chloride/hypochlorite leaching at room temperature. The effects of three factors, including Ca(OCl)2 vs. NaOCl, OCl− concentration, and HCl concentration on gold leaching performance were investigated. Due to formation of CaOCl+ complex in solution and hence less reactivity, calcium hypochlorite produces a sluggish gold leaching kinetics, taking twice the time (46 h) to achieve maximum gold recovery of 58% compared to sodium hypochlorite. 10 g/L of total initial hypochlorite species in solution produces reasonable gold recoveries. The amount of added HCl and hence the initial pH was found to have a major effect on gold leaching kinetics and maximum gold recovery. A high level of 9 g/L of added HCl causes HClO to be very reactive, producing very fast kinetics, reaching 67% gold extraction in 4 h. It also causes a faster consumption of hypochlorous acid, through catalytic decomposition (by NiO and CuO) and disproportionation. Hypochlorous acid reactions with sulfide and ferrous content of ore proceed very slowly in the pH range of 4–11. Gold–chloro complexes are strongly adsorbed on quartz component of ore. To minimize this undesirable adsorption of gold–chloro species, the aging time must be limited to a few hours only.
Of the various mechanical metal recycling techniques employed in electronic scrap processing, air table separation, magnetic separation and eddy current separation technolo gies have proved to be the most commercially successful. In addition, it is very important, even indispensable, that, prior to the physical processing of electronic scrap, selective dis mantling and identification (if necessary) be employed. It is, however, recognized that problems such as process optimiza tion and organics handling remain and that in-depth charac terization of electronic scrap will be essential in this context.
Purpose
This paper aims to present a review carried out under DEFRA‐funded project WRT208, describing: the composition of WEEE, current treatment technologies, emerging technologies and research.
Design/methodology/approach
This paper summarises the output from the first part of the project. It provides information on the composition of WEEE and an extensive survey of technologies relevant to materials recycling from WEEE. A series of further papers will be published from this research project.
Findings
WEEE has been identified as one of the fastest growing sources of waste in the EU, and is estimated to be increasing by 16‐28 per cent every five years. Within each sector a complex set of heterogeneous secondary wastes is created. Although treatment requirements are complicated, the sources from any one sector possess many common characteristics. However, there exist huge variations in the nature of electronic wastes between sectors, and treatment regimes appropriate for one cannot be readily transferred to another.
Research limitations/implications
A very large number of treatment technologies are available, both established and emerging, that singly and in combination could address the specific needs of each sector. However, no single set of treatment methods can be applied universally.
Originality/value
This paper is the first part of work leading to the development of technical strategies and methodologies for reprocessing WEEE into primary and secondary products, and where possible the recovery of higher added‐value components and materials.
The electrochemical reduction of gold thiosulfate has been studied and compared to the reduction of gold cyanide. Gold thiosulfate is a potential replacement for gold cyanide in electro and electroless plating baths. Gold thiosulfate has a more positive reduction potential than gold cyanide and eliminates the use of cyanide. The standard heterogeneous rate constant, transfer coefficient, and diffusion coefficient for gold thiosulfate reduction were found to be 1.58 >< 1O cm/s, 0.23 and 7 x 10 cm2/s, respectively. The effect of sulfite as an additive to gold thiosulfate solutions was examined.
Abstract-A new hydrometallurgical recovery technology of leaching gold and silver from E-waste has been presented by Lime Sulfur Synthetic Solution (LSSS) method. The influences of sodium sulfite concentration, divalent copper ions concentration, aqueous ammonia concentration, reaction temperature and leaching time were investigated experimentally. The results indicated that it was favorable for gold and silver leaching when solid-liquid ratio was 1:3 , leaching time 2.5h, at temperature of 318K, under the condition of Na2SO3 0.1mol/I, Cu2+ 0.03 mol/I, NH4+ 0.5 mol/I solution, pH=10, for 5g e-waste power. The best leaching rate of gold and silver reached about 92% and 90% in this favorably experimental condition, which suggested this technological viability for gold and silver recovery. On the basis of practice, LSSS method has the advantages of non-toxic, low-cost, simple process, easy to operate and was expected to be a great potential method in recycling gold and silver from e-waste.
Gold dissolution from a roasted auriferous pyrite-arsenopyrite concentrate was studied using an iodine-iodide solution as lixiviant. The influence of iodine-iodide concentrations and pH on dissolution yield and side reactions with other ore constituents was examined. Available gold dissolution was fast and almost selective. Silver, lead and copper were found to cause excess iodide consumption, through side reactions, and may be considered as ‘iodidecides’. Thermodynamic considerations gave a satisfactory explanation of the experimental findings.
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.
Selection of a leaching system for gold involves consideration of ore texture and mineralogy, chemical requirements, leaching techniques, the development of flowsheets, and environmental management. Aqueous dissolution chemistry for alkaline, neutral, and acid systems is mainly considered here. All systems require an oxidant to oxidise gold and a ligand to complex with gold in solution. Adjustment of pH is usually necessary.Alkaline lixiviant systems (pH > 10)include cyanide, ammonia-cyanide, ammonia, sulphide, nitriles, and a few other minor possibilities. Oxygen is the main oxidant. Cyanide, which is the main ligand in these systems, forms an anionic complex, “Au(CN)2”, with Au(I). Gold dissolution rates are controlled by oxygen solubility in solution.Neutral lixiviant systems (pH 5-9)include thiosulphate, halogens, sulphurous acid, and bacteria plus natural organic acids as the ligand. Oxygen is the normal oxidant and either Au(I) or Au(III)complexes are formed.Acid leaching systems (pH ⩾ 3)may contain thiourea, thiocyanate, chlorine, aqua regia, or ferric chloride. Chloride is the ligand in the last three systems and the oxidants include chlorine, ferric chloride, hydrogen peroxide, and nitric acid which produce Au(III) anionic complexes, e.g. [AuClJ". Fast gold dissolution is possible but reagent consumptions are high. Thiourea is unusual in producing a cationic Au(I)complex, “Au(NH2CSNH2)2” and gold dissolution is slower.For treating simple auriferous oxide-silicate-carbonate ores, and many otfier materials, cyanide remains the preferred lixiviant.Most non-cyanide leaching systems appear to have little wide-spread practical application. Possible niche applications include the use of chlorine or aqua regia to dissolve coarse gold from gravity concentrates, oxidising acid chloride solutions for die treatment of auriferous base metal sulphide concentrates, thiosulphate for dissolving gold from gold-copper ores, and thiourea for auriferous hydrometallurgical intermediates.
Present paper focuses on the selective recovery of copper from the enriched ground printed circuit boards (PCBs) using leaching and solvent extraction. The metal-enriched ground sample obtained from the beneficiation of the sized PCBs in a laboratory scale column type air separator contained mainly 49.3% Cu, 3.83% Fe, 1.51% Ni, 5.45% Sn, 4.71% Pb, and 1.85% Zn. The leaching of the enriched sample with 3.5 mol/L nitric acid dissolved 99% copper along with other metals at 323 K temperature and 120 g/L pulp density in 1 h time. The composition of the leach liquor with wash solution was found to be 42.11 g/L Cu, 2.12 g/L Fe, 4.02 g/L Pb, 1.58 g/L Zn, and 0.4 g/L Ni. The McCabe–Thiele plot indicated the requirements of three counter-current stages for maximum extraction of copper from the leach liquor at pH 1.5 using 30, 40, and 50% (v/v) LIX 984 N at the phase ratios (A/O) of 1:3, 1:2, and 1:1.5, respectively. The counter-current simulation studies show the selective extraction of 99.7% copper from the leach liquor feed of 1.5 pH in three stages with 50% LIX 984 N at A/O phase ratio of 1:1.5. The stripping of copper from the loaded organic with sulfuric acid produced copper sulfate solution from which copper metal/powder could be recovered by electrolysis/ hydrogen reduction.
A hydrometallurgical method for the recovery of silver from electronic scrap materials is suggested. Electronic scrap materials, containing silver with the accompanying brass and beryllium bronze, are leached at 80°C with ferric chloride. The leaching should be carried out with the concentration of ferric chloride and phase ratio chosen so that in the final phase of leaching the concentration ratio of Fe(III) to Fe(II) is not lower than 1. Under such conditions silver is found to be only slightly solubilized. This was confirmed by electrochemical studies.Potentiodynamic studies of the behaviour of silver in hydrochloric acid demonstrated that at the electrode potential value of 0.77 V, corresponding to the redox potential at 25°C of the solution where the concentration ratio [Fe(III)]/[Fe(II)] = 1, the silver surface is passivated not only by silver chloride but also by silver oxide. This prevents solubilization of silver in chloride solutions owing to the formation of AgCl2− and AgCl32− complexes. The lowering of the solution oxidation potential resulting from consumption of Fe3+ ions during the leaching process causes a sudden increase in silver concentration in solution.
The recovery of copper, lead and tin from scrap printed circuit boards (PCBs) has been achieved using a combination of leaching, electrochemical ion exchange and electrodeposition. A simple aqueous nitric acid stripping solution, with the concentration range of 1–6 mol dm ⁻³ , has demonstrated the potential for selective extraction of copper and lead from the PCBs. Precipitation of tin as H 2 SnO 3 (metastannic acid) occurred at acid concentrations above 4 mol dm ⁻³ . Preliminary galvanostatic electrolysis from simulated leaching solutions has investigated the feasibility of electrodeposition of copper and lead at different concentrations of HNO 3 . Cathodic lead deposition, particularly at high electrolyte conditions, resulted in poor current efficiency. This was mainly due to dentritic metal formation and subsequent re‐dissolution. An alternative method investigated for recovering the metal values was the simultaneous electrodeposition of copper at the cathode and lead dioxide at the anode. Electrohydrolysis for acid and base regeneration from the spent nitric acid electrolyte has also been investigated.
© 2002 Society of Chemical Industry
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.
The potential for leaching gold directly from a refractory AuCu sulfide concentrate, to replace the current treatment of roasting and cyanidation, was investigated using ammoniacal thiosulfate with copper. Leaching the concentrate with a solution containing 0.8M S2O32−, 0.05M Cu2+ and 4M NH3 resulted in over 90% gold and 60% silver recovery.Evidence of dissolution of gold tellurides was observed and contributed to the high gold extraction from the ore. However the leaching of gold tellurides was slow and high concentrations in a concentrate would be expected to reduce recovery. There was no evidence of partial alteration of pyrite in the leach which would suggest that gold inclusions present in pyrite grains will not be recovered until some pretreatment process is carried out to liberate these inclusions.Pre-treating ore by ultra fine milling followed by thiosulfate leaching resulted in 94% gold and 60% silver recovery after 48 hours. Thiosulfate leaching of roasted concentrate resulted in 98% gold and 93% silver recovery after 48 hours.Under the conditions used in this study, reagent consumption was high ranging from 36 to 70 kg/t for thiosulfate, ∼ 5 kg/t for ammonia and 1 to 2 kg/t for copper after leaching for 24 hours. Extended leaching resulted in higher reagent consumption and loss of gold and silver from solution.Preliminary investigation into generating thiosulfate in-situ were undertaken using sulfur dioxide, together with sulfur and ammonia to leach gold directly from concentrate or roasted products. Up to 1.2 M of thiosulfate was generated with gold extraction of 80% for highly refractory concentrate and >98% for roasted samples. The cost of high reagent consumption observed could be overcome by using sulfur dioxide gas produced from the roaster, sulfur and ammonia to leach gold directly from ore or roasted products.
