In-situ microwave-assisted leaching and selective separation of Au(III) from waste printed circuit boards in biphasic aqua regia-ionic liquid systems (original) (raw)
Related papers
Recycling
Constant growth in waste electrical and electronic equipment (WEEE) levels necessitates the development of new, commercially viable recycling processes. Waste printed circuit boards (WPCBs) are a subgroup of WEEE that are of increasing interest due to their relatively high level of valuable metal content including Au, Ag, and platinum group metals (PGMs). Currently, precious metals like gold are mainly recycled from WEEE streams through copper smelting/refining; however, the possibility to peel gold from WPCBs prior to smelting, could offer advantages for recycling. In this study, the suitability of aqua regia for selective or partially selective gold leaching from un-crushed WPCBs was investigated. The redox potential of aqua regia solutions and the dissolution efficiencies of Au, Cu, and Fe from WPCBs were investigated at different temperatures (40-80 • C) and concentrations (2-32%) in batch leaching tests. The redox potential of aqua regia solution was found to depend on concentration and temperature. It is suggested that Au dissolution in aqua regia requires dissolved Cu 2+ ions originating from the WPCB material to work. Au extraction (>50%) was shown to require a redox potential >700 mV with [Cu 2+ ] > 2500 ppm, as a potential >850 mV alone was insufficient without cupric ions. Significant amounts of Au and Cu could be dissolved with only minor Fe dissolution at ≥8% aqua regia at 80 • C. Results suggest that leaching of uncrushed WPCBs in 8% aqua regia (T = 80 • C) can provide the opportunity for partial Au recovery prior to further processing.
Current Science
The main aim of the present study is to develop a new method to recover gold and other precious metals such as copper, silver, etc. with the judicious combination of microwave heating and acid leaching of e-waste samples like printed circuit board of computers, mobiles, electronic devices, etc. In the present study, microwave heating is much required for segregation of melted plastics and metals from e-waste and it also generates high temperature in much lesser time. The investigation basically consists of seven stages, i.e. heating the e-waste using microwave energy, grinding, physical separation of molten plastics and metal slag followed by leaching in nitric acid, leaching in aqua regia for unreacted materials, removing with concentrated nitric acid and washing and purifying the metal deposits. The results obtained from XRD, FESEM-EDAX and TEM studies confirm that precious metals like gold and silver can be obtained with the new method, which is also an environmental friendly approach.
Microwave-assisted leaching—a review
Hydrometallurgy, 2004
Microwave applications in mining and process metallurgy have been the subject of many research studies over the past two decades. This paper reviews microwave-assisted leaching of copper, gold, nickel, cobalt, and manganese, lead and zinc and also coal desulphurisation. It has been recognised that microwave technology has great potential to improve the extraction efficiency of metals in terms of both reduction in required leaching time and increased recovery of valuable metal. Despite a significant number of research studies in this area and potential for achieving highly attractive benefits, there is no agreement to the mechanism of interaction of microwaves with hydrometallurgical systems. D
Resources, Conservation and Recycling, 2020
Waste printed circuit boards (PCBs) were pretreated by either microwave or conventional pyrolysis prior to acid leaching processes. Microwave pyrolysis can provide higher weight loss of waste PCBs and better metal recycling than conventional pyrolysis. After microwave and conventional pyrolysis of waste PCBs, the overall copper recovery rates of two-stage acid leaching can be up to approximately 96 and 75%, respectively. The gold recovery rates of thiourea leaching from microwave-and conventional-pyrolyzed PCBs were approximately 80 and 69%, respectively. Therefore, size-reduction processes for the pretreatment of waste PCBs would not be necessary, as long as microwave pyrolysis is applied prior to hydrometallurgical processes. The gold recovery rates of this study should be credible, since the sum of recovery and non-recovery rates was close to 100%. The initial acid leaching of copper may be governed by the copper content in the PCBs as well as the concentration of oxidizing agent in the leaching solution. Besides, the reaction rate of thiourea leaching of microwave-pyrolyzed PCBs may be largely influenced by the thickness of product layer. Both acid and thiourea leaching experimental results were better fitted by the pseudo-second-order reaction model. However, the maximum recovery rate determined by using the pseudo-first-order reaction model should be more reasonable.
Resources Conservation and Recycling, 2018
Printed circuit boards (PCBs) represent one of the most complicated and valuable components in electric and electronic equipment (EEE). Waste PCBs (WPCBs) contain more than 40 kinds of metals with a wide and variable range of concentrations, such as environmentally harmful metals (e.g. Pb, Cr, As, Cd and Hg) and others of economic value (e.g. Cu, Sn, Au, Ag and Pd). Recovery of metals from WPCBs is of great importance for both environmental protection and resource re-utilization. In contrast to metal recovery from natural resources, these secondary resources have to be essentially stripped completely of its harmful metallic content before the remaining plastic substrate can be disposed, charred or incinerated. Hydrometallurgy has been successfully used for metals recovery from primary and secondary resources around the world, owing to its easily controlled process and high recovery rates at relatively low costs, and a number of researches using various hydrometallurgical methods for metals recovery from WPCBs has been published each year since 2002. This study provides an up-to-date review of the hydrometallurgical recovery of metals from WPCBs and gives perspectives of this particular area, which is expected to provide an insight for the selection of suitable hydrometallurgical leaching and purification methods and to point out the novel and potential technologies that would be the future focuses of this area. Meanwhile, 50-80% of the WEEE finally ends up in developing countries, where relevant regulation is lacking. In 2012, about 70% of WEEE in the world was disposed in China, and the rest went to India, Pakistan, and some countries in Southeast Asia and Africa (Zhang et al., 2012a). However, most of the recycling activities in these developing countries are of a primitive and limited nature. They are often conducted in labour-intensive workshops, mainly focusing on the profitable metals, Cu and Au in particular. The remaining toxic metals (e.g. As, Cd, Cr, Pb, Hg) and hazardous organic substances are often simply disposed by landfill or incineration, which are harmful to human bodies, animals and environment (Awasthi et al., 2016; Li et al., 2017; Nnorom and Osibanjo, 2009). According to the latest statistics conducted by United Nations University (UNU), International Telecommunication Union (ITU) and International Solid Waste Association (ISWA), only 20% WEEE generated in the world was documented to be collected and recycled in 2016, and the recycle rates in different countries vary greatly (Baldé et al., 2017), which implies our global society is still far from a closed-loop system for the manufacture-use-recycle of EEE. For example, Reuter et al. (2018) has shown that, for the few mobile companies (such as Fairphone) that are willing to make their bill of
Microwave-Leaching of Copper Smelting Dust for Cu and Zn Extraction
Materials
Industrial wastes may contain high concentrations of valuable metals. Extraction and recovery of these metals have several economic and environmental advantages. Various studies showed positive effects of microwaves as a pretreatment method before the leaching of minerals. However, there are empty rooms for exploring simultaneous microwave and leaching (microwave-leaching) of industrial waste material for the production of valuable metals. This investigation examined the microwave-leaching method to extract copper and zinc from a copper-smelter dust (CSD). The results of microwave-leaching mechanism were compared with conventional heating leaching based on kinetics modelling. The final Cu recovery in the conventional heating and microwave irradiation was 80.88% and 69.83%, respectively. Kinetic studies indicated that the leaching reactions follow diffusion across the product layer. Based on X-ray powder diffraction (XRD) analyses, during conventional experiments sulfate; components ...
Hydrometallurgical Process for Selective Metals Recovery from Waste-Printed Circuit Boards
Metals
This paper presents an experimentally-proved hydrometallurgical process for selective metals recovery from the waste-printed circuit boards (WPCBs) using a combination of conventional and time-saving methods: leaching, cementation, precipitation, reduction and electrowinning. According to the results obtained in the laboratory tests, 92.4% Cu, 98.5% Pb, 96.8% Ag and over 99% Au could be selectively leached and recovered using mineral acids: sulfuric, nitric and aqua regia. Problematic tin recovery was addressed with comprehensive theoretical and experimental work, so 55.4% of Sn could be recovered through the novel physical method, which consists of two-step phase separation. Based on the results, an integral hydrometallurgical route for selective base and precious metals recovery though consecutive steps, (i) Cu, (ii) Sn, (iii) Pb and Ag, and (iv) Au, was developed. The route was tested at scaled-up laboratory level, confirming feasibility of the process and efficiencies of metals recovery. According to the obtained results, the proposed hydrometallurgical route represents an innovative and promising method for selective metals recovery from WPCBs, particularly applicable in small scale hydrometallurgical environments, focused on medium and high grade WPCBs recycling.
Sādhanā
The whole world understands about the crisis of harmful electronic waste as it is increasing the usage and its disposal. The Government of India asked researchers to come out with innovative alternative solutions, apart from existing conventional methods for safe reuse, recycling and proper disposal particularly for electronic solid waste system. The solution is found which consists of microwave heat treatment followed by acid leaching. The e-waste was first crushed and then the sample was melted in microwave heat treatment to recover the valuable metal in the form of metallic mixture. This mixture was further subjected to acid leaching process in the presence of hydrogen peroxide to form leached liquor. The analysis with X-ray diffraction, image mapping and energy-dispersive X-ray spectroscopy shows that the leached liquor sample mainly contain iron, aluminum and copper, mostly in the form of alloys. The results with field-emission scanning electron microscope analysis, also shows that approximately ninety percent leaching efficiency is observed for nickel, cobalt and copper with hydrochloric acid as solvent, whereas iron and aluminum produced less than forty percent. Further, these results are also compared with the existing methods based on the response surface method through thermal plasma process.
Gold Recovery from Waste Printed Circuit Boards by Advanced Hydrometallurgical Processing
MATERIALS TRANSACTIONS, 2019
The scope of this study was to improve the hydrometallurgical processes involving iodine-iodide leaching and precipitation for recovery of gold from waste printed circuit boards. Firstly, the influence of different precipitating agents, namely ascorbic acid, trisodium citrate and sodium hydroxide on the recovery of gold from gold-iodide leach liquor were investigated in order to define the most effective precipitating agent. The leach liquor was prepared by dissolving pure gold chips in 1:6 molar ratio of iodine-iodide solution at 40°C, 550 rpm for 12 h. The variables, which affect the efficiency of gold precipitation from the leach liquor, were the molar ratio of precipitating agents to gold, pH and redox potential of the solutions. The attained high gold precipitation efficiency from the leach liquor was more than 99% under the highly acidic (pH < 1.6) and alkaline conditions (pH > 13) induced by 0.1 M ascorbic acid and 0.1 M sodium hydroxide respectively, but 64.5% of gold at a weak alkaline condition (pH 8) with 0.1 M trisodium citrate. Secondly, physico-chemical properties of resultant colloidal solutions and prepared gold particles were examined. Finally, recycling of waste printed circuit boards (WPCBs) via iodine-iodide leaching followed by the ascorbic acid reduction was discussed. Results indicate that over 95% of gold extracted from WPCBs by two-step iodine-iodide leaching under the defined conditions, while the dissolution efficiencies of other precious metals (Ag, Pd) and metal impurities (Cu, Al, Fe, Ni, Pb and Zn) were less than 1% and 3%, respectively. The vast majority of Au (99.8%), Cu (95.6%) and Ag (76.8%) were precipitated from the pregnant leach solution by ascorbic acid reduction at ambient conditions.