Article

A Potential Alternative for Precious Metal Recovery from E-waste: Iodine Leaching

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Abstract

Printed circuit board (PCB) is the essential part of electronic devices and has an important source of base and precious metals with high economic potential. In this study, selective leaching of gold from PCB was performed in an iodine-hydrogen peroxide (I2-H2O2) solution system. The effects of different parameters such as iodine concentrations, H2O2 concentrations and solids-% on the gold leaching dynamics were investigated. The results show that increasing solids-% has a negative influence on the gold leaching. However, gold recovery from the e-wastes in a solution containing 3% iodine, 1% H2O2 with solids-% of 15% resulted with 100% recovery among all leaching tests.

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... Research is centred around optimising the use of leaching agents that meet these criteria [147]. Numerous studies have focused on the application of hydrometallurgy for the recovery of base and precious metals from e-wastes [148][149][150][151][152][153][154][155][156][157][158]. ...
... As such, several substitutes have been investigated. These include thiourea, thiosulfate and halides [148,149,[151][152][153][154]. ...
... Halide leaching offers an effective alternative to cyanide leaching [148,149,176,188]. The most common halide leaching agents are iodide, bromide and chloride [8]. ...
Article
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Electronic e-waste (e-waste) is a growing problem worldwide. In 2019, total global production reached 53.6 million tons, and is estimated to increase to 74.7 million tons by 2030. This rapid increase is largely fuelled by higher consumption rates of electrical and electronic goods, shorter life cycles and fewer repair options. E-waste is classed as a hazardous substance, and if not collected and recycled properly, can have adverse environmental impacts. The recoverable material in e-waste represents significant economic value, with the total value of e-waste generated in 2019 estimated to be US $57 billion. Despite the inherent value of this waste, only 17.4% of e-waste was recycled globally in 2019, which highlights the need to establish proper recycling processes at a regional level. This review provides an overview of global e-waste production and current technologies for recycling e-waste and recovery of valuable material such as glass, plastic and metals. The paper also discusses the barriers and enablers influencing e-waste recycling with a specific focus on Oceania.
... It has been researched extensively in the past few years, in an attempt to improve gold recovery from the pregnant solution and decrease the costs related to the reagent by trying to reuse it. As a part of the halides leaching techniques, iodine solutions have fast leaching rates similar to that of the chlorine solutions (aqua regia) [16]. It is also worth noting that among halogens, the gold-iodine complexes are the most stable in aqueous solutions [12]. ...
... Regarding the chemistry of the solution, it is understood that the slightly water-soluble iodine reacts with the present iodide ion (that may come from dissolved potassium iodide) to form a soluble tri-iodide ion that serves as an oxidant for the elemental gold during the process [16] with reactions shown below [17]: ...
... The studies conducted in iodine solutions are mainly performed in the presence of hydrogen peroxide. Through experimental trials, the great importance of adding hydrogen peroxide to the system was shown, with one study [16] reporting a very low percentage of gold dissolution (35%) when no hydrogen peroxide was added. According to Chen et al. [18], almost all present gold (97.5%) in the used PCBs was dissolved, being leached under their concluded optimum conditions. ...
Article
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The importance of gold recycling from waste electronic and electrical equipment (WEEE) is continuously increasing due to raising gold demand and the need for new recycling methods for this complex waste. In state-of-the art processes gold is recovered in the copper route. In necessary pre-treatment steps, a significant loss of gold occurs. Furthermore, in this process the gold is bound for a longer time period in a copper phase and is separated in a mixture with other noble metals, which needs further treatment. A direct gold extraction from WEEE by selective leaching and precipitation would have many advantages. Due to raising social awareness of the ecological impacts of cyanidic gold extraction and environmental specifications, cyanide cannot be used as gold extractant in modern processes. Therefore, over the course of this study, two experimental sets have been conducted on each of six non-cyanide leaching reagents to test their feasibility and compare them against each other. Aqua regia and iodine/iodide leaching systems were the most successful reagents, yielding 100% of gold while showing very fast leaching rates. A comparison between the two successful reagents was carried out, with the results favoring iodine leaching due to its high selectivity for precious metals and reduced environmental impact.
... Halide leaching method includes chloride, bromide and iodide leaching. Several studies reported significant metal extraction from electronic scrap using various halide lixiviant [99][100][101][102][103]. Effect of various process parameters such as halide concentration, oxidant concentration, and pulp density were investigated for Au extraction from electronic scrap using iodine leaching method while keeping other parameters constant i.e. pH = 7, ambient temperature = 35°C, leaching time = 240 min, stirring speed = 170 rpm [102]. ...
... Several studies reported significant metal extraction from electronic scrap using various halide lixiviant [99][100][101][102][103]. Effect of various process parameters such as halide concentration, oxidant concentration, and pulp density were investigated for Au extraction from electronic scrap using iodine leaching method while keeping other parameters constant i.e. pH = 7, ambient temperature = 35°C, leaching time = 240 min, stirring speed = 170 rpm [102]. Approximately 90% Au recovery was obtained using 3% iodine concentration in presence of 2% hydrogen peroxide as an oxidant whereas, no significant gold recovery was observed in absence of oxidant. ...
... In the third stage, residue from acid leaching was further processed for Au recovery using thiourea. The Au recovery of 80% was reported using 20 g/L thiourea, 28.25 g/L ferric sulfate, 10 g/L sulfuric acid, process time of 3 hr, stirring speed of 400 rpm at 25 • C. Summary of other research works for metal recovery using thiourea are mentioned in Table 7. [125,174,175,176,177,178,179,180]. The extraction of Au can be done using halide leaching as it forms Au + and Au 3+ complexes with chlorine and iodine. ...
... Sahin et al. studied the recovery of Au from e-waste using iodine leaching [176]. The study of the effect of iodine concentration on the leaching of Au suggests that change in the iodine concentration does not change the extraction of Au significantly in the absence of oxidant. ...
Article
The technological advancement in the field of electrical and electronic equipment leads to the rapid increase in the obsolescence rate of these devices and the generation of electronic waste (e-waste). The problem is further exacerbated due to the limited awareness about disposal methods of e-waste coupled with limited viable recycling avenues. Rising demand and limited recycling capacity further lead to increased resource extraction and mining activities having a degenerative impact on biota. The improper recycling and unregulated accumulation of e-waste too has its ramifications on human health, economy, and environment at large. However, e-waste is also an asset as metals and plastics account for around 75 wt.% of e-waste and provide an opportunity for resource recovery. Therefore, there is a need to find a sustainable solution for resource recovery and the sound management of e-waste. Hence, this review focuses on the current technologies from the aspect of e-waste management and resource recovery i.e. chemical conversion of e-waste plastic to valuable products as well as energy generation and metal recovery from e-waste using hydrometallurgical and pyrometallurgical approaches. This review covers various technologies such as pyrolysis, catalytic pyrolysis, gasification, and supercritical fluids to recycle e-waste plastic. In addition, the recovery of metals using hydrometallurgical technologies such as cyanide, thiosulfate, thiourea, halide, and acid leaching has also been critically discussed. The pyrometallurgical approach for the recovery of metals has been also discussed. This review considers the environment-friendly approach, economic value, and recycling efficiency of the process as important parameters for e-waste recycling. Novel and greener technologies for metal recovery are suggested for the future. The review recommends an integrated sustainable solution for e-waste recycling which includes e-waste plastic recycling and metal recovery which is expected to provide guidance for future research and technology development for industrial practice.
... E-waste growth presents equally a problem, due to the toxicity of its parts, and an opportunity, as valuable minerals contained in the waste are recoverable [11]. Several studies have shown that The EPA drafted the Plug-In to eCycling Guidelines for Materials Management in 2004. ...
... As previously noted, advances in e-waste recovery and recycling differ from country to country. Some countries such as the United States have progressed more than others, especially when compared to developing countries [11,22,[70][71][72][73]. However, EEE production and the challenges it brings are a worldwide phenomenon. ...
Article
Full-text available
Households and businesses produce high levels of electrical and electronic waste (ewaste), fueled by modernization and rapid obsolescence. While the challenges imposed by e-waste are similar everywhere in the world, disparities in progress to deal with it exist, with developing nations lagging. The increase in e-waste generation highlights the need to develop ways to manage it. This paper reviews global and South African e-waste management practices with a specific case study on Cathode Ray Tube (CRT) waste. CRTs present the biggest problem for recyclers and policy makers because they contain lead and antimony. Common disposal practices have been either landfilling or incineration. Research into South African CRT waste management practices showed there is still more to do to manage this waste stream effectively. However, recent developments have placed e-waste into a priority waste stream, which should lead to intensified efforts in dealing with it. Overall, these efforts aim to increase diversion from landfill and create value-adding opportunities, leading to social and environmental benefits.
... Several studies reported significant metal extraction from WEEE using various halide lixiviants Altansukh et al. 2016;Sahin et al. 2015). Iodination of electronic scrap was investigated for the recovery of Au at the ambient temperature of 35 ∘ C and leaching time of four hours. ...
... Approximately 90% Au recovery was obtained using 3% iodine concentration in the presence of 2% hydrogen peroxide as oxidant, whereas no significant gold recovery was observed in the absence of the oxidant (Sahin et al. 2015). In the iodide leaching process, concentration ratio of iodine to iodide (I − ) is an important parameter to recover precious metals from waste. ...
Chapter
This chapter presents a range of conventional technologies for metal extraction from contaminated sites. The positive and negative aspects of each process have been reviewed in order to understand the applicability of each process for metal recovery from industrial/electronic waste in a sustainable way. A detailed literature survey on various mineral processing methods indicates that pyrometallurgical operations, despite showing significant extraction of metals, are not preferred due to the high energy requirement and release of toxic gases into the ecosystem. Hydrometallurgical processes are easy to implement in the laboratory and offer better reaction control and high extraction efficiency. However, the reagents are, in general, hazardous to handle; therefore, some of them such as cyanide leaching have already been phased out. In addition, the secondary pollution possibility, the need for pretreatment, and electrochemical method based post processing increase the process cost and limit the applicability of hydrometallurgical processes at large scales. An urgent need is felt to work on a sustainable framework to develop an eco‐efficient process for metal recovery.
... Several studies reported significant metal extraction from WEEE using various halide lixiviants Altansukh et al. 2016;Sahin et al. 2015). Iodination of electronic scrap was investigated for the recovery of Au at the ambient temperature of 35 ∘ C and leaching time of four hours. ...
... Approximately 90% Au recovery was obtained using 3% iodine concentration in the presence of 2% hydrogen peroxide as oxidant, whereas no significant gold recovery was observed in the absence of the oxidant (Sahin et al. 2015). In the iodide leaching process, concentration ratio of iodine to iodide (I − ) is an important parameter to recover precious metals from waste. ...
Chapter
Adsorption is a surface phenomenon based on scientific and engineering principles and is widely explored for water treatment and purification. The low cost and eco‐friendly nature of the process makes it suitable to treat various kinds of waste for the extraction and recovery of precious metals from it. Factors governing the adsorption and kinetics of the process are elucidated in this chapter using adsorption kinetics models. Researchers are exploring new and green adsorbents specifically for metal recovery from waste streams. Clay, biopolymers, zeolites, and agricultural and industrial sludge have the advantage of low economics and high adsorption capacity. The chapter explores the potential of green adsorbents for the extraction of metals from industrial waste and waste electrical and electronic equipment (WEEE). In addition, a brief review on the scope of the adsorption process in other research areas is included at the end of the chapter. A case study is provided to understand the reaction conditions and efficiency of the process. Technical feasibility and challenges to the industrial scale application of the process are discussed, which may provide further research goals in this emerging field for solid waste management.
... Upon the introduction of potassium iodide (iodide ion) into a solution, the solubility of AuI increases because of the formation of a complex ion, AuI 2 [30]. The relevant reactions are shown below along with their thermodynamic data [31,32]: ...
Article
The thermodynamic equilibria and kinetic aspect of gold dissolution in iodine–iodide leaching were studied with emphasis on the effects of pH value and temperature on the system. The results of thermodynamic analysis of iodine in aqueous solution were given and numerous forms of iodine exist mainly in the acid region of pH values. An increase of the potential of the system results in an increase of iodine speciation. The oxidizing potential of the system will increase by the addition of element iodine. The IO3– anions are stable in the potential range from –2.0 to –0.75 V and at pH value greater than 12.1. An increase of the temperature shifts boundaries of existence of various iodine species in the acid region of pH values. Some of them become unstable. The determined values of the diffusion coefficients and the thickness of the diffusion boundary layer, as well as the solvent concentration on the disc surface (14 mg/L) indicate that the process proceeds in the external diffusion region. Thus, while choosing the conditions of leaching from gold-containing materials of different origins of iodide solvents, it is necessary to carry out the process within the acidic region of pH values, where I−, I3− and IO4− ions are capable to form complex compounds with metals.
... The most common reagents used for base metals extraction are ammonia, sulfuric acid and chloride [17][18][19]. The most common reagents for precious metals extraction from PCBs are halides, thiosulfate, cyanide, thiourea and aqua regia [16,[20][21][22][23]. However, most of these reagents have some environmental and technical concerns due to either high toxicity or high reagent consumptions. ...
Article
Full-text available
Electronic waste (E-waste) is accumulating rapidly globally and pose a significant environmental challenge. One of the ways to cover the cost of waste processing (in addition to reducing the costs associated with landfill) is through recovery of metals. In addition, toxic and dangerous metals can and must be removed prior to repurposing, incineration or pyrolysis of the plastic substrates. E-waste is usually either transported to landfills or processed by pyrometallurgical and hydrometallurgical processes. Recently, a number of hydrometallurgical approaches have been considered in metals recovery from different electronic components. In this study, glycine (amino acetic acid) or its salts is considered as a lixiviant in an alkaline environment for base and precious metals recovery from shredded and ground printed circuit boards (PCBs). It was found that alkaline glycine solutions selectively dissolve copper, zinc, and lead over precious metals. Gold and silver were then recovered in a subsequent leaching step using glycine and small amounts of cyanide (at starvation levels, implying no free cyanide is present). The leach system remains alkaline throughout both stages of processing. In the two-stage glycine leaching system, gold, silver, zinc, lead and copper recoveries were 92.1%, 85.3%, 98.5%, 89.8%, and 99.1% respectively. The recoveries of precious and base metals by direct cyanidation, single stage glycine–cyanide leaching, and ammonia leaching were lower than the recoveries of these metals using the two-stage glycine and glycine–cyanide systems. Graphic Abstract Open image in new window Flowsheet of a two-stage glycine leaching method for metal extractions from waste PCBs proposed in this study
... The recycling of Ti and Ti-based materials from e-waste can be performed through numerous techniques, such as Ti smelting and Kroll methods [105]. Unfortunately, despite the high efficiency (even up to 100%) of the strong acids in the leaching process [106], their extensive use is a critical issue in TiO2 recycling. Other challenges in recycling the wastederived photocatalytic materials are the dissatisfactory control over the resulting structure and poor reproducibility that is a significant problem since a few variations lead to significant changes in photocatalytic performance [107,108]. ...
Article
Full-text available
Waste-derived materials obtained from the recovery and recycling of electronic waste (e-waste) such as batteries and printed circuit boards have attracted enormous attention from aca-demia and industry in recent years, especially due to their eco-friendly nature and the massive increment in e-waste due to technological development. Several investigations in the literature have covered the advances achieved so far. Meanwhile, photocatalytic applications are especially of interest since they maintain mutual benefits and can be used for H2 production from solar water splitting based on semiconductor processing as a proper environmentally friendly technique for solar energy conversion. In addition, they can be utilized to degrade a variety of organic and non-organic contaminations. Nonetheless, to the best of the authors' knowledge, there has not been any comprehensive review that has specifically been focused on e-waste-derived photocatalytic materials. In this regard, the present work is dedicated to thoroughly discussing the related mechanisms, strategies , and methods, as well as the various possible photocatalysts synthesized from e-wastes with some critiques in this field. This brief overview can introduce modern technologies and promising possibilities for e-waste valorization, photocatalytic processes, and new photocatalytic degradation methods of eco-friendly nature. This paper discusses various e-waste-obtained photocatalytic materials , synthesis procedures, and applications, as well as several types of e-waste, derived materials such as TiO2, ZnO, indium tin oxide, and a variety of sulfide-and ferrite-based photocatalytic materials. Citation: Bahadoran, A.; De Lile, J.R.; Masudy-Panah, S.; Sadeghi, B.; Li, J.; Sabzalian, M.H.; Ramakrishna, S.; Liu, Q.; Cavaliere, P.; Gopinathan, A. Photocatalytic Materials Obtained from E-Waste Recycling: Review, Techniques, Critique, and Update.
... Xu et al. reported that the gold extraction reached approximately 95% under the optimum conditions: an iodide concentration of 1.0-1.2%, H 2 O 2 concentration of 1-2%, leaching time of 4 h, solid/liquid ratio of 1/10, pH 7 and 25 • C [27]. Batnasan et al. [28] demonstrated an iodine-iodide leaching process to recover valuable metals from waste printed circuit boards. ...
Article
Full-text available
This paper demonstrates the recovery of valuable metals from shredded Waste Printed Circuit Boards (WPCBs) by bromine leaching. Effects of sodium bromide concentration, bromine concentration, leaching time and inorganic acids were investigated. The most critical factors are sodium concentration and bromine concentration. It was found that more than 95% of copper, silver, lead, gold and nickel could be dissolved simultaneously under the optimal conditions: 50 g/L solid/liquid ratio, 1.17 M NaBr, 0.77 M Br 2 , 2 M HCl, 400 RPM agitation speed and 23.5 • C for 10 hours. The study shows that the dissolution of gold from waste printed circuit boards in a Br 2-NaBr system is controlled by film diffusion and chemical reaction.
... Hydrometallurgy and pyrometallurgy are most conventional technologies used for recovery of metals from e-waste Chauhan et al., 2018;Zhang and Xu, 2016). Most of these studies were made for the recovery of base metals like Cu, Ni, Zn, Pb and precious metals like Au and Ag using solvents like HCl, H 2 SO 4 , HNO 3, cyanide, halides, thiourea and thiosulphate, which possess serious threat to environment (Sahin et al., 2015;Yang et al., 2011;Kim et al., 2011;Tuncuk et al., 2012). Cyanide which is an excellent solvent for recovery of Au and Ag is highly toxic in nature and is hazardous to human and other living being. ...
Article
The substantial growth of electronic waste (e-waste) in recent years has become a serious threat to environment. However, there is an excellent opportunity to recover and reuse metals present in e-waste, which eventually leads to conservation of natural resources for future generation. A greener and sustainable approach for the recovery of metals from electronic waste is the need of the hour. In this study, thermal decomposition of printed circuit boards (PCBs) was carried out in presence of nitrogen for conversion of polymers into oil and combustible gases. The metal rich pyrolysis residue was roasted in presence of ammonia chloride as chlorinating agent to recover metals. The effect of roasting parameters on the metal recovery investigated in temperature range of 200 °C to 325 °C for 1 h to 5 h while the NH4Cl dosage varied from 1 g/g to 4 g/g. Under the optimized roasting conditions, around 93% Cu, 100% Ni, 100% Zn, and 100% Pb were recovered at temperature of 300 °C, time of 4 h and NH4Cl dose of 3 g/g. The present process provides an eco-friendly solution for the recovery of metals from e-waste, which are valuable and avoid pollution.
... However, cyanide is extremely toxic to the human body and environment [5,13]. Therefore, new metal leaching systems based on environmental benign reagents, such as thiourea, thiosulphate, and KI/I 2 have been extensively studied [14][15][16][17][18]. However, problems still exist, such as inefficiency, high cost and the requirements of strict operation conditions have limited application of these systems in real circumstances. ...
Article
Full-text available
The extraction of precious metals is of significant importance for recycling valuable precious metal resources, however, most of the current methods for extraction are inefficient, energy-intensive, or environmentally unfriendly. Herein, we report a new chemical dissolution system for precious metals employing the trichloroisocyanuric acid (TCCA) as an oxidant and tributyl monomethyl ammonium chloride (N4441Cl) as a complexant. The leaching yield was achieving 100 wt% for the metals of Au, Pd, Cu, and Ag at 25 °C, and the average dissolution rate of gold was reaching 375 mg/h, 107.1 times higher than that of traditional cyanide method. Based on the analysis of UV-Vis and XPS results, a possible reaction mechanism was proposed: the precious metals were probably oxidized by TCCA, then the formed precious metal ions coordinate with N4441Cl strongly to promote the dissolution process. Applied to gold dissolution from ores, waste electronic and electrical equipment (WEEE) and waste catalysts, the leaching yield of the TCCA-N4441Cl coordinative composite reached 97, 100, and 100 wt%, respectively, demonstrating that this method is not only efficient and environmentally friendly, but also with great adaptability and high potential for real applications.
... In that study, 2.0 mol/L HCl solution was used at a current density of 714 A/m 2 , for 2 h, at 323 K and 400 rpm. Au leaching using an iodine hydrogen peroxide (I 2 -H 2 O 2 ) system (Sahin et al., 2015) resulted in 100% leaching efficiency, in a solution that contained 3% iodine (v/v), 1% H 2 O 2 (v/v), and 15% (w/v) solids. In another study, Au was removed and recovered with 94-100% efficiency using I − /I 2 in a threestep leaching system (Serpe et al., 2015). ...
Article
The wealth of the society depends on several metals, including base metals, precious metals and increasingly rare earth elements (REE). They are collectively termed as technology metals. Numerous applications stimulated the use of technology metals, and their supply is at stake, owing to the high demand and uneven geographical distribution of these metals. Their stable supply is crucial for the transition to a sustainable and circular economy. There is an increasing interest in secondary sources of these metals. This article outlines the global state of electronic waste, its management and the latest technological developments in metal recovery from various streams of electronic waste. An emphasis is given to printed circuit boards (PCB), hard disc drives (HDD) and displays regarding their critical metal content. Physical, pyrometallurgical and (bio)hydrometallurgical metal recovery technologies are overviewed. In addition, perspectives on electronic waste as a secondary source of critical metals are given.
... It was observed that 0.2% iodine resulted in low Au leaching rate but increasing the iodine concentration to 1-2% with 1% of H 2 O 2 , 95% of Au was leached. Sahin et al. (2015) investigated a two-step leaching of Au from waste PCBs. Firstly, Cu and other base metals were removed (2 M H 2 SO 4 , 0.2 M H 2 O 2 for 120 min, at 80 C). ...
Article
Full-text available
Waste electrical and electronic equipment (WEEE) contains economically significant levels of precious, critical metals and rare earth elements, apart from base metals and other toxic compounds. Recycling and recovery of critical elements from WEEEs using a cost-effective technology are now one of the top priorities in metallurgy due to the rapid depletion of their natural resources. More than 150 publications on WEEE management , leaching and recovery of metals from WEEE were reviewed in this work, with special emphasize on the recent research (2015-2018). This paper summarizes the recent progress regarding various hydrometallurgical processes for the leaching of critical elements from WEEEs. Various methodolo-gies and techniques for critical elements selective recovery (using ionic liquids, solvent extraction, electrowinning, adsorp-tion, and precipitation) from the WEEEs leachates are discussed. Future prospects regarding the use of WEEEs as secondary resources for critical raw materials and its techno-economical and commercial beneficiaries are discussed.
... (formula)[33,34]and iodine[35,36]were investigated for their effectiveness to recover valuable metals from waste PCB. Sulphuric Acid leaching, often in combination with an oxidant is the most common method to extract base metals from their primary and secondary sources[37], including discarded PCB. ...
Article
Full-text available
Waste electrical and electronic equipment (WEEE) is an important secondary source of valuable metals. Particularly discarded printed circuit boards (PCB) contain high concentrations of valuable metals, varying greatly among the type of board, the manufacture year, source device, and utilized PCB production technology. Chemical hydrometallurgical processing is an efficient method to selectively extract and subsequently recover metals from discarded polymetallic PCB. In this work, we propose a two-step process to extract copper (Cu) and gold (Au) from a discarded high-grade telecom server PCB. The boards contained 262.4 and 0.320 mg/g Cu and Au, respectively, which constituted the 98.1% of the total value of metals. The metal extraction process was optimized using Response Surface Methodology (RSM) by central composite design (CCD). The optimized process parameters showed that 3.92 M sulphuric acid, 3.93 M Hydrogen peroxide, 6.98% (w/v) pulp density and 3.7 hours contact time, and 0.038 M Copper sulphate, 0.3 M ܵ2ܱ3 2 െ 0.38 M Ammonium Hydroxide, 10.76% pulp density (w/v) 6.73 hours were optimal for the maximal extraction of Cu and Au, respectively. At optimal conditions, 99.2% and 92.2% of Cu and Au, respectively, were extracted from the discarded PCB.
... They believed that the supercritical water oxidation pre-treatment combined with iodine-iodide leaching process is environment friendly for the recovery of Au, Ag, and Pd from waste mobile phones. Iodide is relatively safe, reliable for leaching with high chemical stability; however, very little success in last few years was achieved by several researchers (Sahin et al. 2015;Tuncuk et al. 2012). Another challenge for leaching gold by halide is very high rate of reagent consumption and requirement of special reactor. ...
Article
Full-text available
A large number of waste mobile phones have already been generated and are being generated. Various countries around the world have all been positively exploring the way of recycling and reuse when facing such a large amount of waste mobile phones. In some countries, processing waste mobile phones has been forming a complete industrial chain, which can not only recycle waste mobile phones to reduce their negative influence on the environment but also turn waste into treasure to acquire economic benefits dramatically. However, the situation of recycling waste mobile phones in China is not going well. Waste mobile phones are not formally covered by existing regulations and policies for the waste electric and electronic equipment in China. In order to explore an appropriate system to recover waste mobile phones, the mobile phone production and the amount of waste mobile phones are introduced in this paper, and status of waste mobile phones recycling is described; then, the disposal technology of electronic waste that would be most likely to be used for processing of electronic waste in industrial applications in the near future is reviewed. Finally, rationalization proposals are put forward based on the current recovery status of waste mobile phones for the purpose of promoting the development of recycling waste mobile phones in developing countries with a special emphasis on China.
... So, in recent years, more attention has been paid to several non-cyanide leaching processes like thiourea leaching, thiosulfate leaching, and halide leaching (Zhang et al. 2012). Sahin et al. (2015) have developed a process for 100% gold recovery with 3% iodine, 1% H 2 O 2 , and 15% solid concentration. ...
Article
Full-text available
This article details the electronic waste (e-waste) generation, their composition, health, and environment hazards, and legal rules for disposal as well as their significance as a potential secondary source of metals and other components. Moreover, valuable metal extraction technologies from the e-waste are reviewed in general and waste cell phones in particular. E-waste is nowadays preferentially used for recovery of metals mainly from printed circuit boards (PCBs). Different techniques, namely pyrometallurgy, hydrometallurgy, and biohydrometallurgy used for metal extraction from e-waste are swotted. The economic and environmental valuation features of these technologies are also included. Compared to other methods, biohydrometallurgy is the method of choice, as in it natural components like air and water are used, has low operating and maintenance cost, and operate at ambient temperature and pressure. Microbial aspects of metal extraction from e-waste are summarized.
... This is also not a sustainable way to solve the existing problem as it may cause a significant loss of precious metals. Henceforth, several environmentally sound technologies are being developed to address the precious metals recycling from WPCBs mainly using thiourea and thiosulfate (Gu et al., 2019), HCleH 2 O 2 (Imre-Lucaci et al., 2017), (NH 4 ) 2 S 2 O 8 (Alzate et al., 2016), and I 2 eH 2 O 2 mixture (Sahin et al., 2015) followed by efficient separation techniques like solvent extraction and polymer inclusion membrane (Jha et al., 2020). Nevertheless, the simpler operation, low costs reagent, and high leaching efficacy with fast kinetics obtained in cyanide and aqua-regia solutions have kept it unmatched for gold recycling (Zia, 2017). ...
Article
In the present study, sustainable recycling of gold from the secondary stream of waste printed circuit boards using brine leaching with electro-Cl2 and solvo-chemical strategies was investigated. More than 99% of gold was leached in 2.0 mol.L–1 NaCl solution at pH, 1.0; electro-Cl2 rate, 0.62 mmol.min–1; temperature, 50 °C; and time, 75 min. The kinetic data best fitted to the logarithmic model, while activation energy (16.3 kJ.mol–1) indicated an intermediate-controlled leaching process. Results corroborated with the solution chemistry and reaction thermodynamics of the Au-NaCl-(electro)Cl2 system could reveal the stepwise reaction mechanism. In-situ generated Cl2(g) first dissolved in a brine solution that forming Cl2(aq), which subsequently converted to NaClO and diffused on gold particles to liberate the gold-chloro complex. Gold from the brine leach liquor was quantitatively separated using 0.5 mol.L–1 TBP at pH(eq) = 0.5 and O:A = 1:1. The formation of [HAuCl4·2TBP.H2O]¯ complex through the ion-pair solvation into the organic phase was supported by advanced spectral analysis. Finally, a high-purity gold with >99% efficiency was stripped by contacting the Au-loaded organic phase with 0.2 mol.L–1 thiosulfate solution; however, using the same concentration of thiourea yielded only a 94% efficacy. This study successfully eliminates the use of highly toxic lixiviants (like aqua-regia and cyanide) in gold metallurgy along with the regeneration and reusable benefits of the organic solvent (TBP).
... However, cyanide is toxic and harmful to the environment [12,13]. Other alternative methods using as thiourea, thiosulphate, and KI/I2 also cannot achieve both environmental friendliness and high efficiency [14][15][16][17][18]. Therefore, it is of great significance to develop a novel leaching method for gold. ...
Article
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Gold as a precious metal resource has high recycling significance. However, the current extraction methods cannot achieve the both efficiency and environmental friendliness. In this paper, we propose a new gold leaching agent, which can leach gold under light condition by mixing iodoform (CHI3) with 1-butyl-3-methylimidazolium dicyanamide (BmimN(CN)2) ionic liquid. Under 25 °C and 13 W incandescent lamp irradiation, the leaching yield of gold can achieve 100 wt%, and the average leaching rate is 945 mg Au/(h·mol·CHI3) (18.9 times of that of the cyanidation method). Through the analysis of the results of radical inhibition experiment, UV-Vis and XPS, a possible leaching mechanism is proposed: the iodine radical generated by light oxidizes Au0 to Au+, and then forms AuN(CN)2 by coordinating with N(CN)2−. Subsequently, the ionic liquid and Au N(CN)2 form a stable [Bmim]·[Au(N(CN)2)2] ion pair structure, further promoting the dissolution reaction. The leaching yield of gold can reach 81.9 wt% and 100 wt%, respectively, when applied to ore and waste electrical and electronic equipment (WEEE); the leaching yield of gold can also reach 100 wt% when applied to a waste catalyst by adding a Soxhlet extraction. The results show that this method is not only efficient, mild, and environmentally friendly, but also has strong adaptability and wide application prospects.
... Spent lamps were calculated 1.7% of total electrical and electronic wastes as the 5B lighting equipment e-waste group by European Union in 2005 (Erust et al., 2013;EU Directive 2002/96). E-waste stream is very fast growing in the modern world (Huang et al., 2009;Behnamfard et al., 2013) and these wastes should to be designed considering their recycle and reuse potential (Petter et al., 2014;Sahin et al., 2015). Though the lighting industry has achieved significant reductions in mercury content, the mercury is still an important component for the working of fluorescent lamps (NEMA, 2002;Tunsu et al., 2015). ...
... Spent lamps were calculated 1.7% of total electrical and electronic wastes as the 5B lighting equipment e-waste group by European Union in 2005 (Erust et al., 2013;EU Directive 2002/96). E-waste stream is very fast growing in the modern world (Huang et al., 2009;Behnamfard et al., 2013) and these wastes should to be designed considering their recycle and reuse potential (Petter et al., 2014;Sahin et al., 2015). Though the lighting industry has achieved significant reductions in mercury content, the mercury is still an important component for the working of fluorescent lamps (NEMA, 2002;Tunsu et al., 2015). ...
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In this paper, oxidative leaching and electrowinnig processes were performed to recovery of mercury from spent tubular fluorescent lamps. Hypochlorite was found to be effectively used for the leaching of mercury to the solution. Mercury could be leached with an efficiency of 96% using 0.5 M/0.2 M NaOCl/NaCl reagents at 50 �C and pH 7.5 for 2-h. Electrowinning process was conducted on the filtered leaching solutions and over the 81% of mercury was recovered at the graphite electrode using citric acid as a reducing agent. The optimal process conditions were observed as a 6 A current intensity, 30 g/L of reducing agent concentration, 120 min. electrolysis time and pH of 7 at the room temperature. It was found that current intensity and citric acid amount had positive effect for mercury reduction. Recovery of mercury in its elemental form was confirmed by SEM/EDX. Oxidative leaching with NaOCl/NaCl reagent was followed by electrowinning process can be effectively used for the recovery of mercury from spent fluorescent lamps.
Thesis
The well-being of the society depends on a number of metals, including base metals, precious metals and increasingly rare earth elements (REE). The usage of these metals increased in numerous applications, including electrical and electronic equipment (EEE), and their interrupted supply is at stake. There is an increasing interest in the secondary sources of these metals, particularly waste electrical and electronic equipment (WEEE) in order to compensate their potential supply deficit. This PhD thesis demonstrates the advantages and bottlenecks of biological and chemical approaches, as well as the advances and perspectives in the development of sustainable processes for metal recovery from WEEE. Furthermore, a novel process for the recovery of metals from WEEE is described, and a techno-economic assessment is given. Discarded printed circuit boards (PCB) from personal computers (PC), laptops, mobile phones and telecom servers were studied. Following an extensive literature review, a novel characterization and total metal assay method is introduced and applied to waste board materials. Discarded PCB contained metals in the range of (%, by weight): copper (Cu) 17.6 - 39.0, iron (Fe) 0.7 - 7.5, aluminum (Al) 1.0 - 5.5, nickel (Ni) 0.2 - 1.1, zinc (Zn) 0.3 - 1.2, as well as gold (Au) (in ppm) 21 - 320. In addition, multi-criteria analysis (MCA) using the analytical hierarchical process (AHP) methodology is applied for selection of the best-suited technology. A proof-of-concept for a two-step bioleaching extraction was given, in which 98.4% and 44.0% of the Cu and Au, respectively, were extracted. The two-step extraction concept was applied to the chemical leaching of metals from PCB. Cu leaching was carried in an acidic oxidative mixture of H2SO4 and H2O2, whereas Au leaching for carried out by S_2 O_3^(2-) in a NH_4^+ medium, catalyzed by CuSO4. Under the optimized parameters, 99.2% and 96.6% of Cu and Au, respectively, were extracted from the board material. Selective recovery of Cu from the bioleaching leachate using sulfidic precipitation and electrowinning was studied. Cu was selectively recovered on the cathode electrode at a 50 mA current density in 50 minutes, with a 97.8% efficiency and 65.0% purity. The techno-economic analysis and environmental sustainability assessment of the new technology at an early stage of development was investigated
Article
Nowadays, rapid economic growth, continuous technological innovation and the improvement of living standards have result in large amounts of waste electric and electronic equipment (WEEE). Amongst all these WEEE, waste printed circuit boards (WPCBs) are considered as the most valuable components due to precious metals contained. Previous studies found that the presence of precious metals are richer in WPCBs than in typical metal mines, which are driven recycling precious metals from WPCBs to a profitable business without proper pollution controls in developing countries. However, recovering precious metals from WPCBs is a challenge because WPCBs are both valuable and harmful simultaneously, which are caused by their complex materials makeup. Hence, the proper technologies to recycle metals from WPCBs without negative effects to the environment and human health are urgent and essential. In this article, the current metals recycling technologies from WPCBs are reviewed. Then, an integrated technological route, including metals enrichment and precious metals recovery, is proposed. Finally, in order to promote the development of metals (precious metals) recovery from WPCBs, some improvements and recommendations in techniques and the future trend are also put forward.
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Gold leaching in iodine-iodide solution has been intensely researched in 1980s; while due to the high cost, it has not been applied in industry. Currently, as the drawbacks of other hydrometallurgies appearing such as aqua regia and cyanidation, mining and exploration companies are shifting their attention to the iodide leaching, because which has low electrode potential, fast leaching kinetics, high leaching rate, mild reaction conditions and the lixiviant is easy to recover. The common gold leaching agents, although considerable researches have been undertaken, most, if not all, of which have been proven to have limitations that hinder their widespread adoption in the gold mining industry. Iodide leaching could serve as an effective mean for recovering gold through the dynamic, thermodynamic and electrochemical assessment. And a series of parameters, such as oxidants selection, the iodine/iodide ratio, solid-to-liquid ratio, acidity (pH) and leaching time, affect the gold leaching efficiency. Through a comparison with several other hydrometallurgical processes, the presenting merits promote iodine-iodide system to be a promising method for gold recovery. The key challenge of industrialization is how to reduce the cost and to recover gold from the lixiviant.
Article
Hydrometallurgical processes such as sulphuric acid leaching for recovering metal from waste from electrical and electronic equipment (WEEE), especially metal-rich printed circuit board (PCB), have provided a route for WEEE recycling. This study investigated the effect of the rate of oxidants (hydrogen peroxide) addition in volume per hour and temperature on the extraction of copper and other valuable metals from waste printed circuit board (WPCB). The optimum condition for the extraction of copper and valuable metal in sulphuric acid leaching were determined. The crushed and ground WPSB were leached in the sulphuric acid solution using hydrogen peroxide as an oxidant. The influence of two parameters; temperature (30 °C and 75 °C) and rate of oxidant addition (20 ml/hr and 60 ml/hr) were investigated against the extraction of copper (Cu, 30 wt%), aluminium (Al, 10.5 wt%), iron (Fe, 2.4 wt%), and zinc (Zn, 0.2 wt%). Scanning electron microscope-backscattered electron (SEM-BSE) image shows that shredding PCB into sizes smaller than 2.00 mm is sufficient to expose laminated copper components for an efficient sulphuric acid leaching process. The optimal condition for high extraction of copper was 1.5 M sulphuric acid, with 30 % hydrogen peroxide at rate 60 ml/hr, for ∼ 10 g of PCBs powder with a solid–liquid ratio of 1:10 for 1 h at temperature 75 °C.High percentage of extraction has been achieved in sulphuric acid media within 1 h (Cu (98 %), Al (52 %), Fe (52 %), Zn (92 %).
Article
Precious metals are widely applied in many industry fields due to their excellent corrosion resistance, good electrical conductivity and high catalytic activity. However, the reserves of precious metals falls short of the production globally. The rapid generation of end-of-life products has become the significant resources of precious metals. Among these products, electronic waste (e-waste) and spent catalysts are more concentrated since they account for over 90% of precious metals in industry. This article provides an overview of various technologies on the recovery of precious metals from e-waste and spent catalysts. It shows that recycling technologies have been significantly improved in recent years. The recycling processes have transferred from leaching by aqua regia, cyanide and chlorine in acid solution to less pollution agents leaching. Environment-oriented technologies have been raised great attention in precious metals recycling. The advantages and environmental impacts of these recycling technologies have been discussed in detail. However, there are still some challenges for future promotion. In order to achieve the environment-friendly and sustainable recycling for precious metals with high recovery rate, several considerations have been proposed.
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The demand for gold in the field of national storage, jewelry, and electronics has never been higher. However, gold extraction is not always welcomed by local residents due to the caused environmental contamination. For more than a century, cyanide and aqua reiga were dominantly used for gold mining due to their low costs, high efficiencies, and easy operations. In this article, an improved chlorination process, a traditional cyanidation process, and a traditional aqua regia process were quantitatively analyzed and compared from the perspective of environmental impact. The Biwer-Heinzle method is employed. The results showed that the improved chlorination process had the lowest environmental hazard. The values of General Environmental Indices computed for the Inputs and Outputs were quite small, closing to the minimum values of 0 and 1, respectively, depending on the calculation approach used, via arithmetic average or via multiplication. The results also revealed that the aqua regia process had greater negative impact on the environment than the cyanidation process, because aqua regia requires much higher concentrations and larger amounts. The findings provide a reference for the selection of gold extraction method and promote the gold extraction to be more environmentally friendly.
Article
Regulations force to Waste Electrical and Electronic Equipment (WEEE) management by recycling the materials by safe and suitable methods, due to generating massive amounts of WEEE. This research aims towards extract metals from waste random-access memory (RAM) devices in different solutions. In addition, the effect of different parameters such as reagent concentration, oxidant concentration and solid/liquid ratio were investigated with full factorial experimental design tests and analysis of variance (ANOVA). The results showed that the extraction of gold and silver was 96.81% and 99.02% respectively under the following conditions: concentration of 2% iodine and 3% hydrogen peroxide as oxidizing agent, 5% solid/liquid ratio and leaching period of 2 h. An increase of the hydrogen peroxide concentration increased gold and silver extraction. While about 79.30% silver was found to be extracted using 2 M sulfuric acid, 1.5 M ammonium persulfate, 5% solid/liquid ratio and leaching period of 5 h, 79.43% copper was extracted by using ammonia instead of sulfuric acid under the same conditions. Ammonium persulfate was found to be a good oxidizing agent for sulfuric acid and ammonia leaching, since it provided selective extraction of silver and copper respectively. Two-step sequential bench scale reactor leaching tests were conducted to extract copper (98.73%), gold (99.98%) and silver (96.90%) selectively with high extraction. Two-step leaching approach was concluded as the most appropriate method for selective extraction of targeted metals from waste RAM devices.
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For the rapid growth of population electrical and electronics equipment waste are generated 20 to 50 million tones in world-wide. Half a tonne of e-waste creates by the resident of advanced country in every year. E-waste contains different toxic substances including metals, plastics and refractory oxides which are hazardous or risky for our environment and human wellbeing, thus e-waste management is an essential. Hence, this review outlined the global status of e-waste and its current progress on management worldwide. An exhaustive survey of literature was made on the latest technological approaches in noble and base metals recovery from waste printed circuit boards (PCBs) of electrical and electronic equipment. An emphasis was given to review the most important features of existing industrial routes associated with the metal recovery systems from PCBs. The discussions of green technologies as alternatives of conventional approaches to obtain precious metals from e-waste were overviewed. The application of microbial bioleaching approaches in the extraction metals from e-waste was highlighted. Finally, the concern for the challenges and barriers associated with the e-waste management process in Bangladesh was outlined.
Article
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The dissolution of the main metals (Cu, Zn, Sn, Pb and Fe) found in waste printed circuit boards (WPCBs) was investigated by electrochemical corrosion measurements (potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)) in different bromide-based systems that could be used as lixiviants in hydrometallurgical route of metals recovery. The analysis of the corrosion products was carried out by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. All measurements showed that the addition of bromine in the electrolyte favors to great extents the dissolution process of all studied metals as compared to bromine-free electrolytes. In the investigated experimental conditions, the highest dissolution rates of the metals were obtained in acidic KBr solution containing 0.01 mol/L bromine and they decreased in the following order: Zn >> Sn > Pb > Fe > Cu. The XRD and XPS chemical assessment allowed the identification of the dissolution products formed on the metallic surfaces after exposure to the electrolytes. They consisted mainly of oxides in the case of Cu, Zn, Sn and Fe, while the presence of PbBr2 was also noticed on the lead surface. Based on the results of EIS and surface investigations, several models explaining the corrosion behavior of the metals were proposed and discussed. The obtained results demonstrate that all studied metals could be successfully leached using brominated solutions, providing a viable alternative for the selective and efficient recovery of the base metals from WPCBs through a multi-step hydrometallurgical processing route.
Article
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.
Article
Waste will become the major resource in the future circular economy. In particular, E-waste is a major sector growing at an annual rate of about 2 million tonnes (Mt) with rising users of electrical and electronic items worldwide. This is a consequence of versatility and affordability of technological innovation, thus resulting in massive sales and e-waste increases. Most end-users lack knowledge on proper recycling or reuse, often disposing of e-waste as domestic waste. Such improper disposals are threatening life and ecosystems because e-waste is rich in toxic metals and other pollutants. Here we review e-waste generation, policies and recycling methods. In 2019, the world e-waste production reached 53.6 Mt, including 24.9 Mt in Asia, 13.1 Mt in USA, 12 Mt in Europe. In Asia, China (10.1 Mt), India (3.23 Mt), Japan (2.57 Mt) and Indonesia (1.62 Mt) are the largest producers contributing to about 70% of the total world e-waste generated. Only 17.4% (9.3 Mt) of the world e-waste was recycled by formal means, and the remaining 82.6% (44.3 Mt) was left untreated or processed informally. As a consequence, most countries have framed policies to provide regulatory guidelines to producers, end-users and recyclers. Yet the efficiency of these local policies are limited by the transfer of products across borders in a globalized world. Among formal recycling techniques, biohydrometallurgy appears most promising compared to pyrometallurgy and hydrometallurgy, because biohydrometallurgy overcomes limitations such as poor yield, high capital cost, toxic chemicals, release of toxic gases and secondary waste generation. Challenges include consumer’s contempt on e-waste disposal, the deficit of recycling firms and technology barriers.
Chapter
Managing waste generated from electrical and electronic equipment is recognized as one of the major environmental challenges for the twenty-first century around the world. Every year e-waste is growing at a rate of 3–5% and is estimated to reach 52.2 million tons by the year 2021. Most of the research technologies developed for the recycling of e-waste in pilot or laboratory scale require validation in terms of design, operation, and cost. Proper management can not only protect the environment from pollution but also provide alternate secondary sources for these metals.
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The large generation of electronic waste (e-waste) is posing a serious threat to society. It is important to develop sustainable technology for the effective management of e-waste and the recovery of valuable metals from it. The present study employed hydrometallurgical approach for Cu and Ni extraction from waste printed circuit boards (WPCB) of mobile phones. This study demonstrates the application of ammonia-ammonium sulfate leaching for the maximum recovery of Cu and Ni. Investigations revealed that the most favourable reaction parameters for efficient metal extraction are - ammonia concentration - 90 g/L, ammonium sulfate concentration - 180 g/L, H2O2 concentration - 0.4 M, time - 4 h, liquid to solid ratio - 20 mL/g, temperature - 80 °C and agitation speed - 700 rpm. Under these conditions, 100% Cu and 90% Ni were extracted. Furthermore, the kinetic study was performed using the shrinking core model which revealed that the internal diffusion is the rate-controlling step for Cu and Ni extraction. The activation energies for Cu and Ni extraction were found out to be 4.5 and 5.7 kJ/mol, respectively. Finally, Cu was recovered with 98.38% purity using electrowinning at a constant DC voltage of 2.0 V at Al cathode. The present study provides a solution for concurrent extraction of Cu and Ni from the raw WPCB of mobile phones and selective recovery of Cu from metal leached solution. The process has the potential to recover the resources from WPCB while minimising the pollution caused by mismanagement of WPCB.
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Decrease in life span of electronic devices and consumer’s urge to use advanced technology leads to obsolescence of such devices, resulting in electronic waste generation. The technology for e-waste components recycling has made great progress. However, due to growing environmental concern, there is a need to find alternatives for conventional e-waste recycling methods to achieve a safer environment. Rapid surge in e-waste generation is a matter of concern due to elevated levels of heavy metals and persistent organic pollutants (POPs) in air, soil and water caused by informal recycling practices. Moreover, recycling of printed circuit board (PCB), a major part of electronic waste, in order to recover metals, results in release of waste acid leach water, if untreated, which contain heavy metals like Pb, Cr, Ni, etc. This has given rise to the development of several remediation techniques for soil- and water-like phytoremediation, soil washing, bioremediation, and application of nanoparticles, which have been compared and reviewed in this paper along with their limitations for application at larger scale for real-contaminated soil. This review focuses on the existing remediation techniques and their limitations to decrease environmental hazards caused by the release of various pollutants through e-waste recycling. Graphic abstract
Article
Although iodination gold leaching is a clean, non-toxic and efficient non-cyanide gold leaching process, it still has the problems of high reagents cost and difficult leaching solution treatment. To solve these problems, a process of iodination leaching-electrodeposition recovery was proposed. Thermodynamic analysis confirmed the feasibility of iodination gold leaching, and the influence of different factors on gold leaching was studied. The result shows that roasting could effectively improve the gold leaching effect, the concentration of initial iodine and initial iodide ion had a significant effect on the gold leaching, and the gold leaching could be achieved under normal temperature and neutral conditions. The kinetic analysis shows that the gold leaching process conforms to the shrinkage kinetics model, and the control step was the solid film diffusion control. The apparent activation energy of this reaction was 8.642 kJ/mol, and the reaction order of initial iodine concentration and initial iodide ion concentration were 0.72 and 0.52, respectively. Gold was recovered from the leaching solution by electrodeposition with a recovery percentage of more than 99%. More importantly, the iodine was formed in the anode area, in other words, the reagents used for gold leaching were enriched in the anode area during the gold recovery process. The recovery percentage of iodine in the anode area reached 83%, and the anolyte could be recycled. This process fundamentally solves the problem of high cost of iodination leaching.
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Electronic waste is a dominant global issue with over 50 million tons generated annually. Still, as an amalgamation of precious and rare raw materials, electronic waste is a considerable economic resource with the most valuable components located on the printed circuit boards. Gold is widely used in electronics in numerous applications, although principally for contact points and external connectors. The recovery of gold, due to its high value, is one of the main motivations for recycling e-waste. Although pyrometallurgy and hydrometallurgy processing are still the preferred modes of recovery for gold, the use of high-energy consuming pyro-methods, and the use of gold cyanidation that uses harmful lixiviants are increasingly discouraged. Thiourea has received attention as an alternative lixiviant for gold leaching due to its fast reaction kinetics and less harmful nature. This review aims to provide an up-to-date evaluation of thiourea-gold leaching studies from electronic waste, with emphasis on the recent progression from the classic chemical method to a more sustainable hybrid bioleaching-based system, while its challenges are highlighted. The complementary methods applied for gold retrieval from the pregnant solution are also described with a focus on sustainable methods that have the potential to provide a closed-loop system, the key objective for material recovery in a circular economy. Graphical Abstract
Chapter
This chapter basically covers solder strip leaching, base metal leaching, and precious metal leaching techniques. Previously performed laboratory and industrial leach test results, conditions, reagents, extraction ratios, leach reactions, and flowsheets are reviewed and discussed in detail. Alternative leaching reagents are compared from chemistry, research level, and commercial extend points of view. Analytic hierarchy process (AHP) is used to compare various leaching processes. Brominated epoxy resin leaching, purification of metals from leachates by solvent extraction, and industrial scale refining solutions are also covered in this chapter. Occupational, health, and safety hazardous characteristics determination tests were briefly mentioned.
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With the new legislation for Waste Electrical and Electronic Equipment (WEEE) coming up in Europe a substantial increase of end-of-life electronic equipment to be treated will take place. In this context, often much attention is placed on logistical issues, dismantling and shredding/pre-processing of electronic-scrap, whereas the final, physical metals recovery step in a smelter is often just taken for granted. However, a state-of-the-art smelter and refinery process has a major impact on recycling efficiency, in terms of elements and value that are recovered as well as in terms of toxic control and overall environmental performance. Umicore has recently completed major investments at its Hoboken plant, where besides precious metals and copper a large variety of base and special metals are recovered. Equipped with state-of the art off-gas and waste water purification installations, the plant has been developed to the globally most advanced full-scale processor of various precious metals containing secondary materials such as automotive catalysts and electronic-scrap, generating optimum metal yields at increased productivity. To utilise this potential to its full extend for WEEE fractions like circuit boards or mobile phones, especially the interface between pre-processing (shredding/sorting) and smelting/refining is of importance. Here, a mutual optimization of sorting depth as well as of destination of the various fractions produced can lead to a substantial increase in overall yields, especially for precious and special metals.
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In recent years, copper-bearing gold ores have proved to be difficult to treat since copper minerals present severely interfere with cyanide leaching leading to prohibitively high consumption of cya-nide and low gold extractions. This study was undertaken to assess sulphuric acid leaching in the presence/absence of hydrogen peroxide for the removal of copper as a pre-treatment process prior to cyanide leaching of a copper sulphide bearing gold ore. Direct cyanide leaching of the ore with air resulted in a low extraction of gold (i.e. 6.7% at 1.5 g/L NaCN over 24 h). High concentration of cyanide (i.e. 5 g/L NaCN) appeared to be required to achieve acceptable gold extractions. Further tests were also carried out to evaluate the addition of peroxide (8.8 kg H2O2 per tonne ore) as a source of oxygen in cyanide leaching which also resulted in low extraction (i.e. 40% Au). In the pre-treatment tests, acid leaching with/without the addition of hydrogen peroxide (4.5-45 kg H2O2 per tonne ore) was carried out to eliminate the copper interference. A separate test using distilled-deionised water with the addition of hydrogen peroxide (45 kg H2O2 per tonne ore) was also con-ducted. Partial removal of copper by 42-48% in the pre-leaching stage was achieved. The pre-treatment of the ore substantially improved the extraction of gold in subsequent cyanide leaching with large recovery (i.e. 97-98%) over 8 h, while, cyanide consumption decreased by up to 1.87-fold. It can be inferred from these findings that the removal of copper from the ore by sulphuric acid-hydrogen peroxide pre-treatment can be appropriately employed as an alternative to intensive cyanide leaching for sulphide-rich copper-bearing gold ores.
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This paper presents an overview of the various methodologies used in the recovery of gold from secondary sources. Gold recovery is interesting due to its vast industrial applications, high market prices and extensively used precious metal, the sanctuary value attributed to gold during international political and economical crises, and the limited resource of this metal may explain the recent increasing gold share value. The state of art in recovery of gold from spent sources by pyrometallurgy; hydrometallurgy; bio-hydrometallurgy techniques is highlighted in this paper. This article also provides an overview of past achievements and present scenario of recovery studies carried out on the use of some promising methods which could serve as an economical means for recovering gold. The present review also highlights the used varieties of leaching, cementing, reducing agents, peeling, coagulants, adsorbents, agglomeration solvents, ion exchange resins and bio-sorbents in real situations and hopes to provide insights into recovery of gold from spent sources. Evaluation of lucrative and environmentally friendly technologies to recover gold from primary and secondary spent sources was made in this study.
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Copper minerals present difficulties during the cyanide leaching of gold ores leading to excessively high consumption of cyanide (and oxygen) coupled with low extraction of gold. Copper cyanide species, if present at high levels, adversely affects the downstream processes such as the activated carbon adsorption and the effluent treatment. In this study, the extraction of gold from a copper-bearing gold ore by ammoniacal cyanide leaching was studied. Furthermore, ammonia leaching as a pretreatment process ahead of cyanide leaching was also examined. Only ~12% of gold was extracted by direct cyanide leaching of the ore. The addition of lead nitrate did not affect the leaching of gold. When ammoniacal cyanide leaching system was used, the extraction of gold was significantly improved to >90%. Similarly, the pretreatment of the ore by ammonia leaching was shown to lead to high gold extractions (98%), in subsequent cyanide leaching with significant reductions in the consumption of cyanide. This appeared to be linked with the ready dissolution/removal of copper during ammonia leaching.
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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).
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Rapid economic growth in Asia and the increasing transboundary movement of secondary resources will increasingly require both 3R endeavors (reduce, reuse, recycle) in each country and appropriate control of international material cycles. Recently, managing electrical and electronic waste (E-waste) has become an important target for domestic and international material cycles from the viewpoints of environmental preservation and resource utilization efficiency. To understand the current status of E-waste issues in the context of international material cycles and to discuss the future tasks related to achieving 3R in the region, we organized the National Institute for Environmental Studies (NIES) E-waste Workshop in December 2004. This article reviews past studies on E-waste and briefly describes the topics presented and discussions held at the workshop. The topics at the workshop included E-waste generation, recycling systems, international trade, and environmental impacts. In addition, we discussed various issues such as terminology, current environmental concerns, and possible solutions. Transboundary shipments of E-waste should be conducted taking into consideration the concept of sustainable development. The direction of future research and possible collaborations are also discussed.
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The kinetics of the oxidation of iodine by hydrogen peroxide, a part of the Bray-Liebhafsky reaction, has been studied between 298 and 323 K using a spectrophotometric method. The rate of reaction passes through a maximum when the concentration of hydrogen peroxide increases. At low hydrogen peroxide concentration, the order of reaction with respect to [I2] is roughly one, whereas at high hydrogen peroxide concentration the apparent order is higher and there is no simple kinetic law. This reaction can be observed only with sufficient acid and iodate concentrations. However, its rate does not change much with perchloric acid concentration in the range 0.04 to 0.2 mol dm~3 and iodate concentration in the range 0.005 to 0.05 mol dm~3. The maximum first order rate constant is 8E-03 s~1 at 298 K with activation energy of 80 kJ mol~1. The results are discussed considering a model proposed previously for the Bray-Liebhafsky reaction.
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Non-cyanide lixiviants are not widely used for gold and silver recovery at this time, and cyanide remains unchallenged as the best all-around lixiviant. However, the non-cyanides may find potential applications in future treatment of gravity concentrates or in situations where environmental constraints prevent the use of cyanide. Bromine is used in at least one western US geochemical assay laboratory as a means of obtaining rapid analyses of leachable gold from samples. In some cases where gold recoveries can be improved by leaching with non-cyanide reagents, an incentive exists to examine the use of such reagents as long as the increased costs of reagent consumption do not offset the benefits. In cases where platinum group metals are present, an added incentive may exist for a further look at the non-cyanides.
Article
Using the iodine-iodide leaching system, the effects of different iodides (ammonium iodide, potassium iodide, hydrogen iodide)on gold concentrates leaching process were discussed from the influence factors, such as initial iodine content, iodine and iodide ratio and solution pH value. The results show that, when ammonium iodide or potassium iodide is used as complex agent, under the conditions of initial iodine content of 1%, iodine and iodide molar ratio of 1:8, pH value of 7, liquid-solid ratio of 4:1, stirring speed of 600 r/min, leaching time of 4 h and temperature of 25°C, the gold leaching rates are around 90%; whereas the gold leaching effect is poorer when hydrogen iodide(aqueous solution is hydroiodic acid) is used as complex agent, and the gold leaching rate is only 75%. Considering the difference of leaching effect and availability of industry and so on, potassium iodide is the suitable complex reagent of gold concentrate leaching in iodine-iodide solution.
Article
The technical process of electrodepositing gold of iodine leaching solution from waste printed circuit board (PCB) was investigated. The results show that the most optimized conditions of electrodepositing gold of iodine leaching solution from waste PCB are elicited that the cell carbon rods(cathode) and titanium plate (anode) are used as electrode, the cell is partitioned by anion exchange membrane, the iodine concentration in anode solution is 0.1%-0.8%, n(I2):n(I-)=1:10, the cell voltage is 10-14 V, the gold concentration in cathode solution is 15-50 mg/L, the electrolyzing time is 1-4 h, the electrodepositing rate of gold is more than 95%. After electrolytic experiment, the anode iodine solution can be recycled for electrolyte or leaching gold from waste PCB.
Article
A proposalfor a modified in situ leaching method for extracting gold from oxidized gold ores using a non-cyanide lixiviant is described. A non-cyanide lixiviant is suggested because of the obvious concerns posed by injecting cyanide-bearing solutions into the subsurface. Oxidized gold ores were chosen as a focus because earlier research on the use of sodium thiosulfate as a lixiviant under anaerobic conditions indicated that (lie presence of pyrite led to rapid thiosulfate breakdown. A reconnaissance research program involving ore characterization and hydrometallurgical test work on samples from four Australian ore deposits and preliminary reactive transport modeling studies was carried out. This work showed that lixiviant-oxidant combinations of sodium thiosulfate and ferric ED TA and iodide and iodine are both capable of extracting high percentages of accessible gold from the selected samples in bottle roll tests under anaerobic conditions. The ore characterization and reactive transport studies suggested that both physical and chemical methods of permeability enhancement may be required to lift bulk permeability and the availability of gold for dissolution to sufficiently high levels to obtain adequate gold recoveries. Assuming that such methods prove to be both necessary and economically viable, the mining method would no longer be regarded as simple in situ leaching. Therefore, the term "in-place leacliig" has been adopted for the proposed gold extraction system.
Chapter
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.
Chapter
The history of gold begins in antiquity. Bits of gold were found in Spanish caves that were used by Paleolithic people around 40,000 B.C. Gold is the "child of Zeus," wrote the Greek poet Pindar. The Romans called the yellow metal aurum ("shining dawn"). Gold is the first element and first metal mentioned in the Bible, where it appears in more than 400 references. This book provides the most thorough and up-to-date information available on the extraction of gold from its ores, starting with the miner­ alogy of gold ores and ending with details of refining. Each chapter con­ cludes with a list of references including full publication information for all works cited. Sources preceded by an asterisk (*) are especially recom­ mended for more in-depth study. Nine appendices, helpful to both students and operators, complement the text. I have made every attempt to keep abreast of recent technical literature on the extraction of gold. Original publications through the spring of 1989 have been reviewed and cited where appropriate. This book is intended as a reference for operators, managers, and designers of gold mills and for professional prospectors. It is also designed as a textbook for extractive metallurgy courses. I am indebted to the Library of Engineering Societies in New York, which was the main source of the references in the book. The assistance of my son, Panos, in typing the manuscript is gratefully acknowledged.
Article
The thermodynamic equilibria and kinetic aspect of gold dissolution in iodine–iodide leaching were studied with emphasis on the effects of pH value and temperature on the system. The results of thermodynamic analysis of iodine in aqueous solution were given and numerous forms of iodine exist mainly in the acid region of pH values. An increase of the potential of the system results in an increase of iodine speciation. The oxidizing potential of the system will increase by the addition of element iodine. The IO3– anions are stable in the potential range from –2.0 to –0.75 V and at pH value greater than 12.1. An increase of the temperature shifts boundaries of existence of various iodine species in the acid region of pH values. Some of them become unstable. The determined values of the diffusion coefficients and the thickness of the diffusion boundary layer, as well as the solvent concentration on the disc surface (14 mg/L) indicate that the process proceeds in the external diffusion region. Thus, while choosing the conditions of leaching from gold-containing materials of different origins of iodide solvents, it is necessary to carry out the process within the acidic region of pH values, where I−, I3− and IO4− ions are capable to form complex compounds with metals.
Article
Waste generated by the electrical and electronic devices is huge concern worldwide. With decreasing life cycle of most electronic devices and unavailability of the suitable recycling technologies it is expected to have huge electronic and electrical wastes to be generated in the coming years. The environmental threats caused by the disposal and incineration of electronic waste starting from the atmosphere to the aquatic and terrestrial living system have raised high alerts and concerns on the gases produced (dioxins, furans, polybrominated organic pollutants, and polycyclic aromatic hydrocarbons) by thermal treatments and can cause serious health problems if the flue gas cleaning systems are not developed and implemented. Apart from that there can be also dissolution of heavy metals released to the ground water from the landfill sites. As all these electronic and electrical waste do posses richness in the metal values it would be worth recovering the metal content and protect the environmental from the pollution. Cyanide leaching has been a successful technology worldwide for the recovery of precious metals (especially Au and Ag) from ores/concentrates/waste materials. Nevertheless, cyanide is always preferred over others because of its potential to deliver high recovery with a cheaper cost. Cyanidation process also increases the additional work of effluent treatment prior to disposal. Several non-cyanide leaching processes have been developed considering toxic nature and handling problems of cyanide with non-toxic lixiviants such as thiourea, thiosulphate, aqua regia and iodine. Therefore, several recycling technologies have been developed using cyanide or non-cyanide leaching methods to recover precious and valuable metals. Copyright © 2015 Elsevier Ltd. All rights reserved.
Article
Gold is one of most important sources of currency entry in the economics of any state. Besides mining raw materials, fit for effective gold extraction, residues of gold extraction factories are the source of this precious metal. Moreover, the role of industrial wastes, which has a rich source of non-ferrous and noble metals, increases. Processing expediency of complicated wastes is determined by their harmful ecological influence on the environment. Involving to processing of gold the enumerated sources demands the use of high leading technologies, taking into account the variety of their composition. One of the ways of increasing effectiveness of processing the gold-containing materials may be introduction of direct hydrometallurgy processes. Hydrometallurgical methods of processing differ in low power capacity, simplicity of implement process, decreasing pollution of the environment. It is necessary for this not only to sort out selective solvents, but also to elaborate methods of regeneration and recycling of the used solutions and solvents. At present leaching of gold-containing raw materials by cyanide is the main hydrometallurgical process and the main advantage of cyanide is high selectivity with regard to gold. However, leaching by cyanide solutions has a set of substantial drawbacks: process duration, high cost of the reagent, non-adaptability of “stubborn” ores and concentrates, absence of possible regeneration of cyanide solutions and the main thing is high toxicity of cyanide compounds. Therefore elaboration of new hydrometallurgical methods of extracting gold using selective non-toxic reagents, in particular, iodine solutions is important scientific and practical task.
Article
Electronic waste has been increasing proportionally with the technology. So, nowadays, it is necessary to consider the useful life, recycling, and final disposal of these equipment. Metals, such as Au, Ag, Cu, Sn and Ni can be found in the printed circuit boards (PCB). According to this, the aims of this work is to characterize the PCBs of mobile phones with aqua regia; obtaining "reference" values of leaching, to gold and silver, with cyanide and nitric acid, respectively; and study the process of leaching of these metals in alternative leaching with sodium thiosulfate and ammonium thiosulfate. The metals were characterized by digesting the sample with aqua regia for 1 and 2h at 60°C and 80°C. The leaching of Au with a commercial reagent (cyanide) and the Ag with HNO3were made. The leaching of Au and Ag with alternative reagents: Na2S2O3, and (NH4)2S2O3 in 0.1M concentration with the addition of CuSO4, NH4OH, and H2O2, was also studied. The results show that the digestion with aqua regia was efficient to characterize the metals present in the PCBs of mobile phones. However, the best method to solubilize silver was by digesting the sample with nitric acid. The leaching process using sodium thiosulfate was more efficient when an additional concentration of 0.015 and 0.030M of the CuSO4 was added.
Article
The recovery of gold from iodine–iodide solutions using strong base anion exchange resin has been investigated. The gold iodide complex can be effectively loaded on the resin provided the resin is not heavily loaded with triiodide. The loading of triiodide is found to be extremely strong due to the dissociation of the loaded triiodide to iodide and iodine, the latter being deposited on the resin by physiosorption thereby fouling the surface. This may be potentially detrimental to the gold recovery process but can be overcome by pre-reduction to remove most of the triiodide before loading. To strip the iodine from the resin it requires a reductive elution process. A sodium chloride based eluant solution containing sulfite is found to be highly effective for the elution of both gold and iodine.
Article
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.
Article
The electrochemistry of gold in different halide solutions, with special emphasis on iodide is presented. The electrochemical techniques used during this investigation included cyclic and linear sweep voltammetry. A glassy carbon rotating disk electrode was used to investigate the electrochemistry of the iodide and a gold rotating disk electrode to explore the oxidation behavior of gold in iodide solutions. The effects of iodide concentration, electrode rotation and sweep rate on the electrochemical behavior of gold were examined. In addition, reduction of iodine species at the gold electrode was also investigated.Iodide is shown to be a powerful complexing agent for gold. Cyclic voltammograms of gold in the presence of 10−2M chloride, bromide and iodide, respectively, show that the anodic currents for the oxidation of gold in iodide solution are much greater than that in either bromide or chloride. Two oxidation peaks, which represent the oxidations of Au to Au(I) and to Au(III), were observed. It is confirmed that iodide is oxidized sequentially to tri-iodide and then to iodine and both of these reactions are reversible. At high concentrations of iodide and/or a slow scan rate, passivation, which is caused by the formation of solid iodine at the gold electrode surface, was found. The cathodic reduction curves show that reduction of iodide species on gold is a function of iodine concentrations but it is insensitive to iodide concentration.
Article
A novel hydrometallurgical process was proposed for selective recovery of Cu, Ag, Au and Pd from waste printed circuit boards (PCBs). More than 99% of copper content was dissolved by using two consecutive sulfuric acid leaching steps in the presence of H2O2 as oxidizing agents. The solid residue of 2nd leaching step was treated by acidic thiourea in the presence of ferric iron as oxidizing agent and 85.76% Au and 71.36% Ag dissolution was achieved. The precipitation of Au and Ag from acidic thiourea leachate was investigated by using different amounts of sodium borohydride (SBH) as a reducing agent. The leaching of Pd and remained gold from the solid reside of 3rd leaching step was performed in NaClO-HCl-H2O2 leaching system and the effect of different parameters was investigated. The leaching of Pd and specially Au increased by increasing the NaClO concentration up to 10V% and any further increasing the NaClO concentration has a negligible effect. The leaching of Pd and Au increased by increasing the HCl concentration from 2.5 to 5M. The leaching of Pd and Au were endothermic and raising the temperature had a positive effect on leaching efficiency. The kinetics of Pd leaching was quite fast and after 30min complete leaching of Pd was achieved, while the leaching of Au need a longer contact time. The best conditions for leaching of Pd and Au in NaClO-HCl-H2O2 leaching system were determined to be 5M HCl, 1V% H2O2, 10V% NaClO at 336K for 3h with a solid/liquid ratio of 1/10. 100% of Pd and Au of what was in the chloride leachate were precipitated by using 2g/L SBH. Finally, a process flow sheet for the recovery of Cu, Ag, Au and Pd from PCB was proposed.
Article
The extraction of metals from waste printed circuit boards (WPCBs) in H2SO4-CuSO4-NaCl leaching system was studied. The effect of initial concentration of cupric (0.5-7.5 g/L Cu2+), chloride (4.7-46.6 g/L Cl-) and temperature (20-80 degrees C) on the leaching of copper was investigated using response surface methodology, i.e., three-level Box-Behnken design. Extraction of other metals including Fe, Ni, Ag, Pd and Au was also determined. The importance of the main effects of the leaching parameters on the extraction of copper was found to be in the decreasing order of temperature, the initial concentration of Cu2+ and Cl-. The findings have shown that the initial Cl-/Cu2+ molar ratio should be maintained sufficiently high to maximise extraction of copper. However, an excessively high Cl-/Cu2+ ratio can exert a detrimental effect on the process due to a decrease in the activity of Cu2+ as oxidant. It was found that the highest levels of all the parameters should be selected to achieve high leaching recoveries (>= 91%) for Cu, Fe, Ni and Ag. Under these conditions, the dissolution of palladium was limited to 58%. The effect of solids ratio (1-15% w/v) and air/oxygen (2-4 L/min) on the rate and extent of leaching were also tested. Increasing the solids ratio (1-15 w/v) was observed to adversely affect the leaching of metals with no copper extraction at 10 w/v in the absence of air/oxygen. Air/oxygen was confirmed to be a suitable oxidant to regenerate Cu2+ and hence, maintain high Cu2+/Cu+ ratios, i.e., redox potentials during the leaching process. The presence of air/oxygen led to a significant improvement in the leaching of metals, e.g., %14 Cu (no air/oxygen) cf. complete copper extraction at >= 2 L/min air/oxygen over 120 min. The current findings suggested that, particularly at high solids ratios (>= 5% w/v), the regeneration of Cu2+ by the introduction of air/oxygen is essential for high extraction of metals, Pd in particular.
Book
Each of the 24 chapters of this book on standard electrode potentials in aqueous solutions was prepared by knowledgeable specialized experts and reviewed by referees who are credited herein. For the sake of space, discretion was exercised as to which half-reactions would be included for a given element, data have been limited to a single temperature, 25/sup 0/C, and no attempt was made to give complete citation to all pertinent publications. Separate abstracts have been prepared for three chapters.
Article
The thermodynamic equilibria and kinetic aspects of gold dissolution in iodide electrolytes have been studied with emphasis on the effect of different oxidants on the system. In conjunction with kinetic measurements, the chemix computer program was used to predict the concentration profiles of the predominant species at equilibrium in different solution conditions for the systems Au-I−-I2-H2O and Au-I−-OCl−-H2O.The thermodynamic study showed that I3− is the predominant oxidants species in both systems. However, if the concentrations of OCl− and I− are equal, solid iodine is formed. In these systems iodide (I−)_is used to form I3− (responsible for the gold oxidation) and more free iodide needed for the gold complexation is destroyed in the I−-OCl− system than the I−-I2 system. The formation of solid AuI also explains the lower rate of gold dissolution determined for certain conditions in the kinetic study.The thermodynamic modelling supports the kinetic measurements which show that, although the I−-OCl− system has a higher oxidation capacity, it does not extract gold as well as the I−-I2 system. In all cases there exist optimum oxidant/iodide ratios for achieving maximum gold extraction rates. A mixture which has the highest I3− and free I− concentration will attain the best gold extraction rate.
Article
a b s t r a c t Waste of electric–electronic equipment (WEEE) with an annual growth rate of about 3–5% is the fastest growing waste stream in municipal wastes. Notwithstanding their environmental pollution potential, waste of electrical and electronic equipment (WEEE) with their high content of base and precious metals, in particular, are regarded as a potential secondary resource when compared with ores. For the recovery of metals from WEEE, various treatment options based on conventional physical, hydrometallurgical and pyrometallurgical processes are available. These process options with particular reference to hydromet-allurgical processes were reviewed in this study. With their relatively low capital cost, reduced environ-mental impact (e.g. no hazardous gases/dusts), potential for high metal recoveries and suitability for small scale applications, hydrometallurgical processes are promising options for the treatment of WEEE. Since the metals are present in native form and/or as alloys, an oxidative leaching process is required for the effective extraction of base and precious metals of interest. A two-stage process based on oxidative acid leaching of base metals (Cu in particular) followed by leaching of precious metals using cyanide, thiosulfate, thiourea or halide as lixiviant(s) can be suitably developed for the hydrometallurgical treat-ment of WEEE. However, further research is required to develop new, cost effective and environmentally friendly processes and/or refine existing ones for leaching and, in particular, downstream processes.
Article
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.
Article
The dissolution of gold in iodine-iodide solutions was studied using the rotating disc technique. The effect of changes on disc rotational speed, temperature, iodine and iodide concentrations, solution pH and various additives were investigated. Kinetic data and thermodynamic considerations were used to explain the fundamental aspects of the process. Gold can be leached at a greater rate by iodine-iodide solution than is possible by thiourea and by the conventional cyanidation techniques.
Article
Of the halogens, the gold iodide complexes are the most stable in aqueous solutions. A series of experiments were performed to investigate the kinetics and mechanism of the leaching reaction between gold and iodide. Using a rotating disk technique, the effects of rotation speed, iodide and iodine concentration, temperature, pH and the presence of different electrolytes were measured. Oxygen and hydrogen peroxide were also examined as oxidants in the iodide system. A first order reaction rate was found with respect to I3− and half order reaction rate with respect to I−. A comparison of gold leaching between iodide and cyanide is also presented, in which a rate of about 2.6 × 10−9 mol/cm2 sec for 10−2M Nal and 5 × 10−3M I2 was obtained. This value is close to that for typical cyanidation.
Article
E-waste comprises discarded electronic appliances, of which computers and mobile telephones are disproportionately abundant because of their short lifespan. The current global production of E-waste is estimated to be 20–25 million tonnes per year, with most E-waste being produced in Europe, the United States and Australasia. China, Eastern Europe and Latin America will become major E-waste producers in the next ten years. Miniaturisation and the development of more efficient cloud computing networks, where computing services are delivered over the internet from remote locations, may offset the increase in E-waste production from global economic growth and the development of pervasive new technologies. E-waste contains valuable metals (Cu, platinum group) as well as potential environmental contaminants, especially Pb, Sb, Hg, Cd, Ni, polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs). Burning E-waste may generate dioxins, furans, polycyclic aromatic hydrocarbons (PAHs), polyhalogenated aromatic hydrocarbons (PHAHs), and hydrogen chloride. The chemical composition of E-waste changes with the development of new technologies and pressure from environmental organisations on electronics companies to find alternatives to environmentally damaging materials. Most E-waste is disposed in landfills. Effective reprocessing technology, which recovers the valuable materials with minimal environmental impact, is expensive. Consequently, although illegal under the Basel Convention, rich countries export an unknown quantity of E-waste to poor countries, where recycling techniques include burning and dissolution in strong acids with few measures to protect human health and the environment. Such reprocessing initially results in extreme localised contamination followed by migration of the contaminants into receiving waters and food chains. E-waste workers suffer negative health effects through skin contact and inhalation, while the wider community are exposed to the contaminants through smoke, dust, drinking water and food. There is evidence that E-waste associated contaminants may be present in some agricultural or manufactured products for export.
Article
The hydrometallurgical route of copper and tin extraction from printed circuit boards (PCBs) of used personal computers after thermal pretreatment is discussed. The samples were thermally pretreated within temperature range of 300-900°C during 15, 30 and 60 min. Two methods of the thermal pretreatment were studied: burning and pyrolysis. The leaching solution of 1 M HCl at 80°C was used. The original as well as thermally pretreated samples were leached. The weight losses within the range from 5 to 35% were achieved. The increase of burning temperature causes the copper extraction into solution up to 98%, while copper extraction into solution from non-burned samples was up to 6%. In the case of the tin leaching the highest extraction was achieved when the original sample was leached. The increase of the burning temperature caused the lowering of the copper extraction. The increase of the pyrolysis temperature enhances copper and tin extraction.
Article
From the use of renewable resources and environmental protection viewpoints, recycling of waste printed circuit boards (PCBs) receives wide concerns as the amounts of scrap PCBs increases dramatically. However, treatment for waste PCBs is a challenge due to the fact that PCBs are diverse and complex in terms of materials and components makeup as well as the original equipment's manufacturing processes. Recycle technology for waste PCBs in China is still immature. Previous studies focused on metals recovery, but resource utilization for nonmetals and further separation of the mixed metals are relatively fewer. Therefore, it is urgent to develop a proper recycle technology for waste PCBs. In this paper, current status of waste PCBs treatment in China was introduced, and several recycle technologies were analyzed. Some advices against the existing problems during recycling process were presented. Based on circular economy concept in China and complete recycling and resource utilization for all materials, a new environmental-friendly integrated recycling process with no pollution and high efficiency for waste PCBs was provided and discussed in detail.
Article
Waste electric and electronic equipment, or electronic waste, has been taken into consideration not only by the government but also by the public due to their hazardous material contents. In the detailed literature survey, value distributions for different electronic waste samples were calculated. It is showed that the major economic driver for recycling of electronic waste is from the recovery of precious metals. The state of the art in recovery of precious metals from electronic waste by pyrometallurgical processing, hydrometallurgical processing, and biometallurgical processing are highlighted in the paper.
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