Thiosulfate leaching of gold from waste mobile phones

Augmentation in bioleaching potential of indigenous Bacillus sp. ISO1 for metals recovery from waste computer-printed circuit boards

The bio-cyanidation process of various cyanogenic microorganisms is found to be a sustainable and effective method for metals recovery from primary and secondary sources. This process has surpassed the limitations of the chemical cyanide treatment process; thus, prioritized as a promising approach for e-waste "urban mining" strategies. The main focus of the study was to enhance the bioleaching capacity of indigenous Bacillus sp. ISO1 and to implement optimized parameters in large-scale bioleaching operations. The assessment of various amino acids unveiled that like other cyanogenic microorganisms Bacillus sp. ISO1 also preferred glycine as a prime precursor for cyanide synthesis, as maximum metal solubilization was achieved with glycine amino acid. Other amino acids influenced the bacterial growth but not significantly affected the biocyanidation process. The evaluation and optimization of methionine as a lixiviant stimulator demonstrated that the addition of 1 mg/L methionine effectively enhance the production of glycine-utilizing cyanide lixiviant, that led to a significant solubilization of Cu (86%), Au (75%), and Ag (63%) metals. Furthermore, the kinetics of metal solubilization and operating conditions were explored at increased volume (i.e., 3 L working volume) of bioleaching medium to assess the industrial scale potential of this potent bacterial strain with optimized parameters such as temperature, pH, pulp density, and inoculum size. The significant recovery of Cu (˃ 60%) and other metals at this substantial volume suggested the implementation of a bioleaching process with this potent bacterial strain at industrial scale operations.

Process and systematic study of gold recovery from flexible printed circuit boards (FPCBs) using a choline chloride-ethylene glycol system

The traditional hydrometallurgy technology has been widely used to recover precious metals from electronic waste. However, such aqueous recycling systems often employ toxic/harsh chemicals, which may cause serious environmental problems. Herein, an efficient and environment-friendly method using a deep eutectic solvent (DES) mixed system of choline chloride-ethylene glycol-CuCl2·2H2O is developed for gold (Au) recovery from flexible printed circuit boards (FPCBs). The Au leaching and precipitation efficiency can reach approximately 100 % and 95.3 %, respectively, under optimized conditions. Kinetic results show that the Au leaching process follows a nucleation model, which is controlled by chemical surface reactions with an apparent activation energy of 80.29 kJ/mol. The present recycling system has a much higher selectivity for Au than for other base metals; the two-step recovery rate of Au can reach over 95 %, whereas those of copper and nickel are < 2 %. Hydrogen nuclear magnetic resonance spectroscopy (HNMR) and density functional theory (DFT) analyses confirm the formation of intermolecular hydrogen bonds in the DES mixed system, which increase the system melting and boiling points and facilitate the Au leaching process. The Au leaching system can be reused for several times, with the leaching efficiency remaining > 97 % after five cycles. Moreover, ethylene glycol (EG) and choline chloride (ChCl) act as aprotic solvents as well as coordinate with metals, decreasing the redox potential to shift the equilibrium to the leaching side. Overall, this research provides a theoretical and a practical basis for the recovery of metals from FPCBs.

Spatial gradients and molecular transformations of DOM, DON and DOS in human-impacted estuarine sediments

Dissolved organic matter (DOM) constitutes the most active fraction in global carbon pools, with estuarine sediments serving as significant repositories, where DOM is susceptible to dynamic transformations. Anthropogenic nitrogen (N) and sulfur (S) inputs further complicate DOM by creating N-bearing DOM (DON) and S-bearing DOM (DOS). This study delves into the spatial gradients and transformation mechanisms of DOM, DON, and DOS in Pearl River Estuary (PRE) sediments, China, using combined techniques of UV-visible spectroscopy, Excitation-emission matrix (EEM) fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial high-throughput sequencing. Results uncovered a distinct spatial gradient in DOM concentration, aromaticity (SUVA254), hydrophobicity (SUVA260), the content of substituent groups including carboxyl, carbonyl, hydroxyl and ester groups (A253/A203) of chromophoric DOM (CDOM), and the abundances of tyrosine/tryptophan-like protein and humic-like substances in fluorophoric DOM (FDOM). These all decreased from upper to lower PRE, accompanied by a decrease in O3S and O5S components, indicating seaward reduction in the contribution of terrestrial OM, especially anthropogenic inputs. Additionally, sediments exhibited a reduction in molecular diversity (number of formulas) of DOM, DON, and DOS from upper to lower PRE, with molecules tending towards a lower nominal oxidation state of carbon (NOSC) and higher bio-reactivity (MLBL), molecular weight (m/z) and saturation (H/C). While molecular composition of DOM remained similar in PRE sediments, the relative abundance of lignin-like substances decreased, with a concurrent increase in protein-like and lipid-like substances in DON and DOS from upper to lower PRE. Mechanistic analysis identified the joint influence of terrestrial OM, anthropogenic N/S inputs, and microbial processes in shaping the spatial gradients of DOM, DON, and DOS in PRE estuarine sediments. This study contributes valuable insights into the intricate spatial gradients and transformations of DOM, DON, and DOS within human-impacted estuarine sediments.

Review on the gentle hydrometallurgical treatment of WPCBs: Sustainable and selective gradient process for multiple valuable metals recovery

The metal resource crisis and the inherent need for a low-carbon circular economy have driven the rapid development of e-waste recycling technology. High-value waste printed circuit boards (WPCBs) are an essential component of e-waste. However, WPCBs are considered hazardous to the ecosystem due to the presence of heavy metals and brominated organic polymers. Therefore, achieving the recycling of metals in WPCBs is not only a strategic requirement for building a green ecological civilization but also an essential guarantee for achieving a safe supply of mineral resources. This review systematically analyzes the hydrometallurgical technology of metals in WPCBs in recent years. Firstly, the different unit operations of pretreatment in the hydrometallurgical process, which contain disassembly, crushing, and pre-enrichment, were analyzed. Secondly, environmentally friendly hydrometallurgical leaching systems and high-value product regeneration technologies used in recent years to recover metals from WPCBs were evaluated. The leaching techniques, including cyanidation, halide, thiourea, and thiosulfate for precious metals, and inorganic acid, organic acid, and other leaching methods for base metals such as copper and nickel in WPCBs, were outlined, and the leaching performance and greenness of each leaching system were summarized and analyzed. Eventually, based on the advantages of each leaching system and the differences in chemical properties of metals in WPCBs, an integrated and multi-gradient green process for the recovery of WPCBs was proposed, which provides a sustainable pathway for the recovery of metals in WPCBs. This paper provides a reference for realizing the gradient hydrometallurgical recovery of metals from WPCBs to promote the recycling metal resources.

Investigation of the Bimodal Leaching Response of RAM Chip Gold Fingers in Ammonia Thiosulfate Solution

Oxidative thiosulfate leaching using Cu(II)-NH₃ has been explored for both mining and recycling applications as a promising method for Au extraction. This study seeks to understand the dissolution behavior of Au from waste RAM chips using a Cu(II)-NH₃-S₂O₃ solution. In the course of this work, bimodal leaching and Au loss were observed in a manner that we have not identified in the literature. Identification of the existence of a specific Au-Ni-Cu lamellar structure in the gold fingers from RAM chips by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) revealed the possibility of interference between Au recovery and the existence of Cu and Ni. During leaching, the co-extraction of Ni was found to predict a negative impact on the Au recovery, as a result of chemical interactions from the Au-Ni-Cu interlayer. Decopperization as a pretreatment was found necessary to remove the pre-existing Cu and promote Au leaching. As part of the study parameters, such as Cu(II) concentration, aeration rates, thiosulfate and ammonia concentrations, particle sizes, and temperatures, were investigated. A satisfactory Au recovery of 98% was achieved using 50 mM Cu(II), 120 mL/min aeration rate, 0.5 M (NH₃)₂S₂O₃, and 0.75 M NH₄OH (i.e., AT/AH ratio of 0.67) for 4 h residence time at room temperature (25 °C). However, there were several high recoveries prior to Au loss from the lixiviant. It was revealed that the main cause of lower Au recovery was due to a precipitation or cementation reaction that included a sulfur species formation. Because of the bimodal leaching, a composite response comprised of the time to Au loss and maximum recovery was developed, termed leaching proclivity, to facilitate statistical analysis. Furthermore, this study explores the interactions between Au-Ni-Cu and provides suggestions for improving Au thiosulfate leaching under the interference of co-existing metals from waste PCB materials.

Improvement of gold bioleaching extraction from waste telecommunication printed circuit boards using biogenic thiosulfate by Acidithiobacillus thiooxidans

Cyanide usage in gold processing techniques has become increasingly challenging due to its toxicity and environmental impact. It is possible to develop environmentally friendly technology using thiosulfate because of its nontoxic characteristics. Thiosulfate production requires high temperatures, resulting in high greenhouse gas emissions and energy consumption. The biogenesized thiosulfate is an unstable intermediate product of Acidithiobacillus thiooxidans sulfur oxidation pathway to sulfate. A novel eco-friendly method was presented in this study to treat spent printed circuit boards (STPCBs) using biogenesized thiosulfate (Bio-Thio) obtained from Acidithiobacillus thiooxidans cultured medium. To obtain a preferable concentration of thiosulfate among other metabolites by limiting thiosulfate oxidation, optimal concentrations of inhibitor (NaN₃: 3.25 mg/L) and pH adjustments (pH= 6-7) were found to be effective. Selection of the optimal conditions has led to the highest bio-production of thiosulfate (500 mg/L). The impact of STPCBs content, ammonia, ethylenediaminetetraacetic acid (EDTA), and leaching time on Cu bio-dissolution and gold bio-extraction were investigated using enriched-thiosulfate spent medium. The suitable conditions were a pulp density of 5 g/L, an ammonia concentration of 1 M, and a leaching time of 36 h, which led to the highest selective extraction of gold (65 ± 0.78%).

A sequential leaching procedure for efficient recovery of gold and silver from waste mobile phone printed circuit boards

The present study reports a sequential, non-acid process for effective recovery of copper and precious metals from mobile phone printed circuit boards. In this process, gold and silver were first enriched during the synthesis process of cuprous chloride and then leached by thiosulfate method. Results indicated that the distribution of gold and silver in the liquid and solid phases during the synthesis of cuprous chloride process was affected by the [Cu]/[Cu²⁺] ratio. Enrichment of gold and silver in the residue after the cuprous chloride synthesis could be achieved by the adjusting the [Cu]/[Cu²⁺] ratio. The silver and gold leaching rates of the residue after cuprous chloride synthesis (93.8 % silver and 99 % gold) were much higher than those of the raw PCB sample (27.0 % silver and 14.2 % gold) under the same conditions. This process has the advantages of high leaching efficiency, high leaching rate and avoiding the use of HNO₃ or aqua regia commonly used for copper, gold and silver recovery. Thus, this study offers a promising and environmentally friendly method for recovering valuable metals from e-waste.

Selective extraction of precious metals in the polar aprotic solvent system: Experiment and simulation

The traditional hydrometallurgical process is the mainstream technology to recover precious metals from e-waste, which usually adopts strong acid/base and strong oxide with high environmental cost and energy consumption. In the present study, the selective extraction of precious metals was simulated and experimented with DMF as the solvent and Cl^- ions provided by CaCl₂ and CuCl₂ (oxidizing agent). The leaching and precipitation rates of precious metals (Au, Ag, Pd) can reach more than 98% under optimization conditions. Kinetic data shows that the control model of the leaching process on precious metals was determined by linear fitting of the shrinkage model. The complex trace precious metals were extracted selectively using dimethylglyoxime and deionized water as precipitators by the leaching-precipitation-cycle method. Meanwhile, the waste liquid produced by this reaction process could be cyclically utilized. Furthermore, the leaching mechanism of precious metals was proposed. DMF could be complexed with the metals as well as coordination ions (Cl^-), which can reduce the redox potentials. Cu(II) could be easily reduced to Cu(I) in the DMF system due to the higher second ionization energy of copper, which is not influenced by the hydration effect, thus shifting the equilibrium to the metal leach side. Oppositely, the addition of water promoted the conversion of Cu(I) to Cu(II) since the higher hydration energy of Cu(II) compensates for the second ionization energy. This research opens up a new path of sustainable development and provides basic theory and practical experience for environmentally friendly recovery of precious metals from e-waste.

Development of quaternized polyethylenimine-cellulose fibers for fast recovery of Au(CN)2- in alkaline wastewater: Kinetics, isotherm, and thermodynamic study

The purpose of this study was to fabricate quaternized polyethylenimine-cellulose fibers (QPCFs) for the fast recovery of Au(I) from alkaline e-waste leachate. QPCFs were prepared by quaternizing PEI-modified cellulose fibers using a (3-chloro-2-hydroxypropyl)trimethylammonium chloride solution. The maximum Au(I) adsorption capacity of QPCFs was estimated to be 109.87 ± 3.67 mg/g at pH 9.5 using the Langmuir model. The values of k₁ and k₂ calculated by the pseudo-first and pseudo-second-order models were 1.79 ± 0.15 min^-1 and 0.045 ± 0.003 g/mg min, respectively. Adsorption equilibrium was reached within 5 min. Thermodynamic studies revealed that the Au(I) adsorption process by the QPCFs was spontaneous (ΔG° < 0) and exothermic (ΔH° < 0). The characterization and adsorption mechanism of QPCFs were investigated by Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectrometry. Quaternary amine sites were well developed in the QPCFs. Oxidation or reduction of adsorbed Au(I) was not observed. When QPCFs were applied to the solution obtained by bioleaching of e-waste, the recovery efficiencies of Au and Cu were 61.7 ± 3.1% and 11.1 ± 2.9%, respectively, indicating that QPCFs have Au selectivity. Therefore, QPCFs are suitable for actual wastewater applications because of their high adsorption performance and fast adsorption rate.

Renewable redox couple system for sustainable precious metal recycling from e-waste via halide-regulated potential inversion

Precious metal (PM) retrievement from e-waste is of great significance for reducing virgin mining activity and promoting rare resource sustainability. However, current PM recycling methods rely mainly on caustic aqua regia or unstable sulfur-based ligand, which has caused severe environmental damage and process inefficiency. Here, we propose an environmentally friendly halide-regulated strategy, utilizing milder and renewable oxidant-cupric/ferric ion for facile PM dissolution. This is realized by the synergistic effect of enhanced oxidizing ability of Cu(II) and reduced oxidation potential of PM with halide addition. Electrochemical tests and leaching experiment results show that Cu(II)/Cu(I) redox potential experiences great change with bromide, increasing from 0.4 to 0.75 V. Fast corrosion feature was observed for Au in Cu(II)/Fe(III)-Br^- and Pd in Cu(II)/Fe(III)-Cl^-, and it can be accelerated by increasing oxidant and halide concentration. Our proposed strategy outperforms traditional methods with stable and fast dissolution, where 2.5 mol/L Br^- is appropriate for Au dissolution. Moreover, selective dissolution of base metal, Pd/Ag, and Au can be achieved via ligand alteration and be further combined with electrodeposition technique for multi metal recovery and oxidant regeneration. This halide-regulated strategy can lead PM recycling from pollutive status towards environmentally friendly road.

E-Waste Recycling and Resource Recovery: A Review on Technologies, Barriers and Enablers with a Focus on Oceania

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.

Electrochemical approaches for selective recovery of critical elements in hydrometallurgical processes of complex feedstocks

Critical minerals are essential for the ever-increasing urban and industrial activities in modern society. The shift to cost-efficient and ecofriendly urban mining can be an avenue to replace the traditional linear flow of virgin-mined materials. Electrochemical separation technologies provide a sustainable approach to metal recovery, through possible integration with renewable energy, the minimization of external chemical input, as well as reducing secondary pollution. In this review, recent advances in electrochemically mediated technologies for metal recovery are discussed, with a focus on rare earth elements and other key critical materials for the modern circular economy. Given the extreme heterogeneity of hydrometallurgically-derived media of complex feedstocks, we focus on the nature of molecular selectivity in various electrochemically assisted recovery techniques. Finally, we provide a perspective on the challenges and opportunities for process intensification in critical materials recycling, especially through combining electrochemical and hydrometallurgical separation steps.

Studies on leaching characteristics of electronic waste for metal recovery using inorganic and organic acids and base

In this paper, we report leaching of precious and scattered metals such as gold (Au), copper (Cu), nickel (Ni), zinc (Zn), iron (Fe), and lead (Pb) from printed circuit boards of scrap mobile phones by hydrometallurgical process using inorganic acid, organic acid and base. The amount of metals leached by different leachants are quantified using atomic absorption spectroscopy. Among various inorganic acids, aqua regia (mixture of nitric acid (HNO₃) and hydrochloric acid) is found to be the strongest leachant for most of the metals such as Zn (2.04 wt %), Fe (17.90 wt %), Ni (0.66 wt %), Pb (5.86 wt %) and Au (0.04 wt %). The basic leachant, ammonium thiosulphate is found to be very effective in leaching of Au (0.03125 wt %). The dissolution of Cu in HNO₃ gives the highest amount of Cu in the solvent, that is, ∼ 7.52 wt %. The metallic phases present in the electronic waste before and after leaching are identified by X-ray diffraction analysis. The microscopic structure has been studied using a scanning electron microscope which depicts erosion of the structure after leaching.

Dissolution of Metals in Different Bromide-Based Systems: Electrochemical Measurements and Spectroscopic Investigations

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.

Economic and environmental feasibility of hydrometallurgical process for recycling waste mobile phones

Waste mobile phones contain significant amounts of valuable metals and non-metallic materials. Consequently, the extraction of valuable materials from discarded phones, which is a more cost-effective method compared with primary mining, is an essential step for maximizing the recovery of secondary resources and minimizing e-waste pollution. We designed a green and efficient path for recovering valuable metals from waste mobile phones and explored its technical feasibility from both environmental and economic perspectives through life cycle assessment and revenue expenditure model. The results showed that the hydrometallurgical process had three characteristics of high recovery efficiency, significant environmental friendliness and economic feasibility. The recovery efficiencies of valuable metals were higher than 90%. Simultaneously, the return on investment was 29%, indicating that the recycling enterprises can achieve self-sufficiency. Thirdly, the environmental benefits were more significant compared to environmental damage released by hydrometallurgical process, representing a significant environmental friendliness. Within the overall recycling process, the core process made the greatest contribution to the environmental burden (45.38-65.68%), followed by manual disassembly process. A comparison of sub-processes in core process revealed that the mechanical crushing and sorting phase had the greatest environmental impacts that were primarily attributed to power consumption. Consequently, future research should focus on the development of energy-efficient pretreatment techniques and energy-saving equipment. The industrial practice of recycling waste mobile phones is still in its infancy in China. Future studies should also focus on the comparing different treatment processes, with the aim of providing technical support for the advancement of industry.

Challenges and opportunities in the recovery of gold from electronic waste

Rapid global technological development has led to the rising production of electronic waste that presents both challenges and opportunities in its recycling. In this review, we highlight the value of metal resources in the printed circuit boards (PCBs) commonly found in end-of-life electronics, the differences between primary (ore) mining applications and secondary ('urban') mining, and the variety of metallurgical separations, in particular those that have the potential to selectively and sustainably recover gold from waste PCBs.

Behavior, mechanism and equilibrium studies of Au(iii) extraction with an ionic liquid [C4-6-C4BIm]Br2

The first gemini-type benzimidazole ionic liquid ([C4-6-C4BIm]Br2) with two sites of action was synthesized and applied for gold extraction. The effects of [C4-6-C4BIm]Br2 concentration, initial gold(iii) concentration, acidity, and the loading capacity of [C4-6-C4BIm]Br2 were examined in detail. It was found that [C4-6-C4BIm]Br2 has excellent extraction ability for obtaining high purity gold. The anion exchange mechanism between [C4-6-C4BIm]Br2 and Au(iii) was proved by Job's method and 1H NMR and FTIR spectroscopy. Quantum chemical calculations were carried out to prove the mechanism theoretically. The extraction equilibrium process was modeled using Langmuir and Freundlich isotherms. Thermodynamic parameters ΔH, ΔG and ΔS indicated that the extraction process was exothermic and spontaneous. The pseudo-second-order kinetic model well fitted the experimental data (R = 0.99). In addition, [C4-6-C4BIm]Br2 has high selectivity for Au(iii) compared to other base metals. In summary, [C4-6-C4BIm]Br2 has great application prospects in industry due to its excellent characteristics such as low cost, easy availability, and high extraction capacity.

Synthesis of N-(3-aminopropyl)imidazole-based poly(ionic liquid) as an adsorbent for the selective recovery of Au(iii) ions from aqueous solutions

A novel poly(ionic liquid) adsorbent (PIL-APIBCl) exhibits a high adsorption capacity of 236.68 mg g⁻¹ for Au(iii).

Gold (III) adsorption from dilute waste solutions onto Amberlite XAD7 resin modified with L-glutamic acid

The main purpose of this paper was to obtain a material with efficient adsorbing properties and selectivity, to recover the gold (III) from residual diluted solutions resulted from the electroplating process. In this regard, a material was obtained by physico-chemical functionalization of a chemically inert support with functional groups of nitrogen and carboxyl. As a source of functional groups glutamic acid was used, and Amberlite XAD7 type acrylic resin was used as solid support. In order to establish the mechanism of the adsorption process, kinetic, thermodynamic and equilibrium studies were performed. The maximum adsorption capacity of the material has been established, and a gold (III) recovery process has been proposed using thermal decomposition of the exhausted adsorbed material. Main objective of this study was to evaluate an environmental friendly adsorbent material to recover gold from secondary industrial sources.

Kinetic Investigation and Dissolution Behavior of Cyanide Alternative Gold Leaching Reagents

Raising social awareness and environmental specifications on cyanide application force gold industry to search for alternative leaching reagents. Therefore, researchers worldwide investigate cyanide alternatives for gold recovery since several decades. Often the research activities cannot be compared directly, since different input materials and experimental conditions are used. Over the course of this study, different promising cyanide alternative reagents were investigated in terms of their capability of pure gold dissolution at different temperatures. All experiments took place under identical conditions by using uniform samples of 99.99% gold disks, to enable a comparability. Thiosulfate as one of the most promising reagent thiosulfate according to literature revealed an insufficient leaching behavior. The gold dissolution was hindered due to the formation of a sulfide passive layer. Also in the thiourea trials, a surface precipitation took place, though gold dissolution did not stop. The halogens iodine, bromine and the well-known gold solvent aqua regia dissolved gold very fast (up to ~1,000 mg·h⁻¹·cm⁻²). Methanosulfonic acid (MSA) was not capable to extract any gold. The experiments were compared with cyanide trials at identical conditions. The average dissolution rate of investigated reagents at 25 °C shows following order: aqua regia > iodine > bromine > cyanide > thiourea > thiosulfate > MSA.

Gold Bioleaching from Printed Circuit Boards of Mobile Phones by Aspergillus niger in a Culture without Agitation and with Glucose as a Carbon Source

Hydrometallurgical and pyrometallurgical processes to recover gold (Au) from cell-phone printed circuit boards (PCBs) have the disadvantage of generating corrosive residues and consuming a large amount of energy. Therefore, it is necessary to look for biological processes that have low energy consumption and are friendly to the environment. Among the biological alternatives for the recovery of Au from PCB is the use of cyanogenic bacteria and filamentous fungi in cultures with agitation. Considering that it is important to explore the response of microorganisms in cultures without agitation to reduce energy expenditure in the recovery of metals from PCB, the present investigation evaluated the capacity of Aspergillus niger MXPE6 and a fungal consortium to induce Au bioleaching from PCB in a culture medium with glucose as a carbon source and without agitation (pH 4.5). The results indicate that the treatments with PCB inoculated with the fungal consortium showed a considerable decrease in pH (2.8) in comparison with the treatments inoculated with A. niger MXPE6 (4.0). The fungal consortium showed a significantly higher Au bioleaching (56%) than A. niger MXPE6 (17%). Finally, the use of fungal consortia grown without agitation could be an alternative to recover metals from PCB, saving energy and material resources.

Interference of coexisting copper and aluminum on the ammonium thiosulfate leaching of gold from printed circuit boards of waste mobile phones

Ammonium thiosulfate solution is an ideal lixiviant to extract gold (Au) from electronic wastes (E-wastes) because it is non-toxic, less corrosive, and more selective than conventional cyanide or halide solutions. It was reported recently, however, that Au leaching efficiency in ammonium thiosulfate medium dramatically decreased at high solid-to-liquid ratios (S/L), even though the amounts of reagents used were in excess. To understand how this occurred, leaching experiments were conducted using printed circuit boards (PCBs) from waste mobile phones, and Au distribution in the leaching residues was examined by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Significant amounts of Au were found together with copper (Cu) and aluminum (Al), implying that extracted Au ions were likely re-deposited during leaching onto Cu and Al found in PCBs via cementation (i.e., reductive deposition). A more detailed elucidation of this phenomenon by cementation experiments using pure Cu and/or Al powders indicates that these metals could only recover Au ions alone via cementation at very high amounts, however, this process became more extensive when Cu and Al powders were suspended together in solution even though the amounts of the individual metals were much lower. Electrochemical experiments (chronoamperometry) in ammonium thiosulfate solutions containing Au ions using an Al working electrode also showed that Au ion cementation was dramatically enhanced when Cu powder was present in solution, and the bulk of Au was cemented on Cu powder rather than on the Al electrode. These results suggest that coexistence of Cu and Al interfered with the extraction of Au in ammonium thiosulfate medium at high S/L because of the enhanced re-deposition of extracted Au via galvanic interaction.

Optimization of HNO3 leaching of copper from old AMD Athlon processors using response surface methodology

The present study investigates the optimization of HNO₃ leaching of Cu from old AMD Athlon processors under the effect of nitric acid concentration (%), temperature (°C) and ultrasonic power (W). The optimization study is carried out using response surface methodology with central composite rotatable design (CCRD). The ANOVA study concludes that the second degree polynomial model is fitted well to the fifteen experimental runs based on p-value (0.003), R² (0.97) and Adj-R² (0.914). The study shows that the temperature is the most significant process variable to the leaching concentration of Cu followed by nitric acid concentration. However, ultrasound power shows no significant impact on the leaching concentration. The optimum conditions were found to be 20% nitric acid concentration, 48.89 °C temperature and 5.52 W ultrasound power for attaining maximum concentration of 97.916 mg/l for Cu leaching in solution.

Evaluating waste printed circuit boards recycling: Opportunities and challenges, a mini review

Rapid generation of waste printed circuit boards has become a very serious issue worldwide. Numerous techniques have been developed in the last decade to resolve the pollution from waste printed circuit boards, and also recover valuable metals from the waste printed circuit boards stream on a large-scale. However, these techniques have their own certain specific drawbacks that need to be rectified properly. In this review article, these recycling technologies are evaluated based on a strength, weaknesses, opportunities and threats analysis. Furthermore, it is warranted that, the substantial research is required to improve the current technologies for waste printed circuit boards recycling in the outlook of large-scale applications.

Waste Printed Circuit Board (PCB) Recycling Techniques

With the development of technologies and the change of consumer attitudes, the amount of waste electrical and electronic equipment (WEEE) is increasing annually. As the core part of WEEE, the waste printed circuit board (WPCB) is a dangerous waste but at the same time a rich resource for various kinds of materials. In this work, various WPCB treatment methods as well as WPCB recycling techniques divided into direct treatment (landfill and incineration), primitive recycling technology (pyrometallurgy, hydrometallurgy, biometallurgy and primitive full recovery of NMF-non metallic fraction), and advanced recycling technology (mechanical separation, direct use and modification of NMF) are reviewed and analyzed based on their advantages and disadvantages. Also, the evaluation criteria are discussed including economic, environmental, and gate-to-market ability. This review indicates the future research direction of WPCB recycling should focus on a combination of several techniques or in series recycling to maximize the benefits of process.

Fungal Biorecovery of Gold From E-waste

Waste electric and electronic devices (e-waste) represent a source of valuable raw materials of great interest, and in the case of metals, e-waste might become a prized alternative source. Regarding gold, natural ores are difficult to mine due to their refractory nature and the richest ores have almost all been exploited. Additionally, some gold mining areas are present in geopolitically unstable regions. Finally, the gold mining industry produces toxic compounds, such as cyanides. As a result, the gold present in e-waste represents a nonnegligible resource (urban mining). Extraction methods of gold from natural ores (pyro- and hydrometallurgy) have been adapted to this particular type of matrix. However, to propose novel approaches with a lower environmental footprint, biotechnological methods using microorganisms are being developed (biometallurgy). These processes use the extensive metabolic potential of microbes (algae, bacteria, and fungi) to mobilize and immobilize gold from urban and industrial sources. In this review, we focus on the use of fungi for gold biomining. Fungi interact with gold by mobilizing it through mechanical attack as well as through biochemical leaching by the production of cyanides. Moreover, fungi are also able to release Au through the degradation of cyanide from aurocyanide complexes. Finally, fungi immobilize gold through biosorption, bioaccumulation, and biomineralization, in particular, as gold nanoparticles. Overall, the diversity of mechanisms of gold recycling using fungi combined with their filamentous lifestyle, which allows them to thrive in heterogeneous and solid environments such as e-waste, makes fungi an important bioresource to be harnessed for the biorecovery of gold.

Recovery of metals and nonmetals from electronic waste by physical and chemical recycling processes

This paper reviews the existing and state of art knowledge for electronic waste (e-waste) recycling. Electrical and/or electronic devices which are unwanted, broken or discarded by their original users are known as e-waste. The main purpose of this article is to provide a comprehensive review of e-waste problem, strategies of e-waste management and various physical, chemical and metallurgical e-waste recycling processes, their advantages and disadvantages towards achieving a cleaner process of waste utilization, with special attention towards extraction of both metallic values and nonmetallic substances. The hazards arise from the presence of heavy metals Hg, Cd, Pb, etc., brominated flame retardants (BFRs) and other potentially harmful substances in e-waste. Due to the presence of these substances, e-waste is generally considered as hazardous waste and, if improperly managed, may pose significant human and environmental health risks. This review describes the potential hazards and economic opportunities of e-waste. Firstly, an overview of e-waste/printed circuit board (PCB) components is given. Current status and future perspectives of e-waste/PCB recycling are described. E-waste characterization, dismantling methods, liberation and classification processes are also covered. Manual selective dismantling after desoldering and metal-nonmetal liberation at -150μm with two step crushing are seen to be the best techniques. After size reduction, mainly physical separation processes employing gravity, electrostatic, magnetic separators, froth floatation, etc. have been critically reviewed here for separation of metals and nonmetals, along with useful utilizations of the nonmetallic materials. The recovery of metals from e-waste material after physical separation through pyrometallurgical, hydrometallurgical or biohydrometallurgical routes is also discussed along with purification and refining. Suitable PCB recycling flowsheets for industrial applications are also given. It seems that hydrometallurgical route will be a key player in the base and precious metals recoveries from e-waste. E-waste recycling will be a very important sector in the near future from economic and environmental perspectives. Recycling technology aims to take today's waste and turn it into conflict-free, sustainable polymetallic secondary resources (i.e. Urban Mining) for tomorrow. Recycling technology must ensure that e-waste is processed in an environmentally friendly manner, with high efficiency and lowered carbon footprint, at a fraction of the costs involved with setting multibillion dollar smelting facilities. Taking into consideration our depleting natural resources, this Urban Mining approach offers quite a few benefits. This results in increased energy efficiency and lowers demand for mining of new raw materials.

Determination of the potential gold electrowinning from an ammoniacal thiosulphate solution applied to recycling of printed circuit board scraps

The use of electrochemical techniques in the selective recovery of gold from a solution containing thiosulphate, ammonia, and copper, obtained from the leaching of printed circuit boards from mobile phones using ammoniacal thiosulphate, are shown in this work. First, cyclic voltammetry tests were performed to determine the potential of electrodeposition of gold and copper, and then, electrowinning tests at different potentials for checking the rates of recovery of these metals were performed. The results of the cyclic voltammetry show that copper deposition occurs at potentials more negative than -600 mV (Ag/AgCl), whereas the gold deposition can be performed at potentials more positives than -600 mV (Ag/AgCl). The results of electrowinning show that 99% of the gold present in solutions containing thiosulphate and copper can be selectively recovered in a potential range between -400 mV (vs Ag/AgCl) and -500 mV (vs Ag/AgCl). Furthermore, 99% of copper can be recovered in potentials more negative than -700 mV (vs Ag/AgCl).

Selective enrichment of low concentration Au(iii) from acidic chloride media by poly ionic liquid sorbent

Hydrophilic ionic liquid (imidazolium chloride, imCl)–polyvinyl chloride ionomer (imCl–PVC) as a green sorbent to recover precious gold from acidic chloride solution was characterized by SEM, FTIR, XPS and NMR.

Mobile phone waste management and recycling: Views and trends

There is an enormous growth in mobile phone consumption worldwide which leads to generation of a large volume of mobile phone waste every year. The aim of this review is to give an insight on the articles on mobile phone waste management and recycling, published in scientific journals, major proceedings and books from 1999 to 2015. The major areas of research have been identified and discussed based on available literature in each research topic. It was observed that most of these articles were published during the recent years, with the number of articles increasing yearly. Material recovery and review on management options of waste are found to be the leading topics in this area. Researchers have proved that economically viable refurbishing or recycling of such waste is possible in an environmentally friendly manner. However, the literatures indicate that without proper consumer awareness, a recycling system cannot perform to its maximum efficiency. The methodologies followed and analytical techniques employed by the researchers to attain their objectives have been discussed. The graphical representations of available literature on current topic with respect to year of publication, topics and location have also been explored.

Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants--A review

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.

Hydrometallurgical Recovery of Metals from Large Printed Circuit Board Pieces

The recovery of precious metals from waste printed circuit boards (PCBs) is an effective recycling process. This paper presents a promising hydrometallurgical process to recover precious metals from waste PCBs. To simplify the metal leaching process, large pieces of PCBs were used instead of a pulverized sample. The chemical coating present on the PCBs was removed by sodium hydroxide (NaOH) treatment prior to the hydrometallurgical treatment. Among the leaching reagents examined, hydrochloric acid (HCl) showed great potential for the recovery of metals. The HCl-mediated leaching of waste PCBs was investigated over a range of conditions. Increasing the acid concentration decreased the time required for complete metal recovery. The shaking speed showed a pronounced positive effect on metal recovery, but the temperature showed an insignificant effect. The results showed that 1 M HCl recovered all of the metals from 4 cm × 4 cm PCBs at room temperature and 150 rpm shaking speed in 22 h.

Selective adsorption and recovery of Au(III) from three kinds of acidic systems by persimmon residual based bio-sorbent: a method for gold recycling from e-wastes

A low cost bio-sorbent, named "PPF resin", was prepared by crosslinking the persimmon residual with formaldehyde. The adsorption behavior of PPF resin towards Au(III) from varied HCl and HNO3 concentration solutions was studied. PPF resin could adsorb almost complete Au(III) from high acidic systems. The influence of dilution ratio, solid-liquid ratio and time towards Au(III) from aqua regia leached PCBs liquor was censored in detail by batch and continuous adsorption methods. The PPF resin before and after adsorption was characterized by FT-IR, XRD and XPS spectra which provided evidences for the reduction of Au(III) to Au(0) with a proposed mechanism of Au(III) adsorption-reduction process. After saturated column adsorption of 0.1g PPF resin, 0.0506 g gold (purity: 99.9%) was obtained by the method of incineration. The present results provide a new approach for gold recovery from the secondary resources.

Comparison of Leaching Processes of Gold and Copper from Printed Circuit Boards of Waste Mobile Phone

Waste mobile phone has become the largest number of electronic waste, and recycling of metals from mobile phone would ensure resource recycling and reduce environmental degradation. Based on the contents of metals analyzed by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), containing copper and precious metals such as gold, the paper compared the extraction processes of gold, copper from waste mobile phone printed circuit board (PCB). In this study, two processes, nitric acid and thiourea (NT), sulfuric acid-hydrogen peroxide and iodine (SAHPI) were used to leach copper and gold, respectively. The recovery rate of copper was found to be 96.42%, and 94.3% of the gold was leached in the former process. Similar trends were obtained for the leaching of copper and gold in the latter process, but it was lower that about 95.27% of copper was recycled, while 93.4% of gold were leached. Both the two processes were nontoxic and non-cyanide system. However, thiourea is not stable and easy to decompose in alkaline solution, and the technology has disadvantage of instability. Iodine leaching on the other hand is comparatively a environmental process. Therefore, the optimal choice is the combined process of SAHPI method, while further research is required to develop cost effective and environmentally friendly processes.

Evaluation of gold and silver leaching from printed circuit board of cellphones

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 0.030 M of the CuSO4 was added.