Eco-friendly recovery of metals from waste mobile printed circuit boards using low temperature roasting

Can e-waste recycling provide a solution to the scarcity of rare earth metals? An overview of e-waste recycling methods

Recycling e-waste is seen as a sustainable alternative to compensate for the limited natural rare earth elements (REEs) resources and the difficulty of accessing these resources. Recycling facilitates the recovery of valuable products and minimizes emissions during their transportation. Numerous studies have been reported on e-waste recycling using various techniques, including thermo-, hydro- and biometallurgical approaches. However, each approach still has technical, economic, social, or environmental limitations. This review highlights the potential of recycling e-waste, including outlining the current unutilized potential of REE recycling from different e-waste components. An in-depth analysis of e-waste generation on a global scale and Australian scenario, along with various hazardous impacts on ecosystem and human health, is reported. In addition, a comprehensive summary of various metal recovery processes and their merits and demerits is also presented. Lifecycle analysis for recovering REEs from e-waste indicate a positive environmental impact when compared to REEs produced from virgin sources. In addition, recovering REEs form secondary sources eliminated ca. 1.5 times radioactive waste, as seen in production from primary sources scenario. The review outcome demonstrates the increasing potential of REE recycling to overcome critical challenges, including issues over supply security and localized dependency.

Green Deep Eutectic Solvents (DESs) for Sustainable Metal Recovery from Thermally Treated PCBs: A Greener Alternative to Conventional Methods

Waste PCBs the core of e-waste is rich in copper, tin, zinc, iron, and nickel. Leaching base metals from PCB used to be done in toxic, corrosive acidic/alkali mediums. In this work, an environmentally friendly method for leaching metals from thermally treated PCBs (TPCBs) of mobile phones was proposed using choline chloride based deep eutectic solvents (DES). DES selectivity and solubility of metals from metal oxides were the main screening criteria. FA-ChCl had the maximum solubility of Cu, Fe, and Ni, while Urea-ChCl had high Zn selectivity and solubility. Oxalic acid has high selectivity for Sn. FA-ChCl extracted Cu and Fe best at 16 h, 100 °C, and 1/30 g/mL. Urea-ChCl extracted Zn (90.4±2.9 %) from TPCBs at 100 °C, 21 h, 1/20 g/mL, and 400 rpm. Oxalic acid (1 M) removed 92.3±2.1 % Sn from TPCBs in 1 h at 80 °C and 1/20 g/mL. The shrinking core model-based kinetic investigation of FA-ChCl for Cu extraction showed a diffusion-controlled process. The proposed method is greener than mineral acids utilized for metal extraction.

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.

A sustainable route for the recovery of metals from waste printed circuit boards using methanesulfonic acid

The rapid increase in electronic waste (e-waste) generation and its unsustainable management pose a threat to the environment and human well-being. However, various valuable metals are present in e-waste, which makes it a potential secondary source to recover metals. Therefore, in the present study, efforts were made to recover valuable metals (Cu, Zn, and Ni) from waste printed circuit boards (WPCB) of computers using methanesulfonic acid (MSA). MSA is contemplated as a biodegradable green solvent and has a high solubility for various metals. The effect of various process parameters (MSA concentration, H₂O₂ concentration, stirring speed, liquid to solid ratio, time, and temperature) was investigated on metal extraction to optimize the process. At the optimized process conditions, 100% extraction of Cu and Zn was achieved, while Ni extraction was around 90%. The kinetic study for metal extraction was performed using a shrinking core model and findings showed that MSA-aided metal extraction is a diffusion-controlled process. Activation energies were found to be 9.35, 10.89, and 18.86 kJ/mol for Cu, Zn, and Ni extraction, respectively. Furthermore, the individual recovery of Cu and Zn was achieved using the combination of cementation and electrowinning, which resulted in 99.9% purity of Cu and Zn. The current study proposes a sustainable solution for the selective recovery of Cu and Zn from WPCB.

Integration of electronic waste management: a review of current global generation, health impact, and technologies for value recovery and its pertinent management technique

The fast evolution of waste electrical and electronic equipment (WEEE) has developed into a prime environmental perturb in recent days. Today, electrical and electronic products merely become a needed part of people life's and professional lives. The whole process of e-waste contains an organized collection system, appropriate dismantling, and its treatment of recycling. The unparalleled hastening of e-waste and unceremonious discarding lead to an adverse impact on a country's development. Currently, challenges in e-waste have a lack of practical aid, poor structure, and insufficient economic support. Several legislations have been imposed which aim to enhance the handling of e-waste. Operative management of e-waste is now essential for the protective atmosphere and human beings as well. This article provides the systemic flow of the e-waste definition, global information, and generation of e-waste and composition of e-waste which were discussed. The study categorized the hazardous effect of e-waste on human beings, and the content analysis of e-waste in recent LCA applications was highlighted. Further different metal extraction and recovery techniques from e-waste have been reviewed. A few sets of current practices and some recommendations on a global scale level were provided. Finally, based on analysis, some approaches to e-waste was accomplished, and equitable environmental management was taken into account to identify the future outlook areas.

Metal bioleaching from printed circuit boards by bio-Fenton process: Optimization and prediction by response surface methodology and artificial intelligence models

Recycling printed circuit boards (PCBs) in the e-waste stream is essential for ecological protection and metal recycling for a circular economy. Efficient metal recovery from PCBs is highly dependent on the determination of the optimum combination of inputs in the recycling process. In this study, optimization and predictive modelling of the bio-Fenton process were performed employing the response surface methodology (RSM) and the artificial intelligence (AI) models for efficient enzymatic metal bioleaching from discarded cellphone PCBs. The Box-Behnken design (BBD) of RSM was chosen as the design matrix. Further, two AI models, i.e., support vector machine (SVM) and artificial neural network (ANN) were employed to predict complex metal bioleaching process. Experiments were performed based on variations of four input process parameters, namely, glucose oxidase (GOx) content (100-1000 U/L), Fe²⁺ content (10-50 mM), PCB pulp density (1-10 g/L), and shaking speed (150-450 rpm). Results revealed that the maximum simultaneous enzymatic metal extraction of 100% Cu, 70% Ni, 40% Pb, and 100% Zn was attained at the optimized conditions: GOx content: 300 U/L, Fe²⁺ content: 10 mM, pulp density: 1 g/L, and shaking speed: 335 rpm. A comparative analysis of the AI models suggested that the ANN-based model predicting more accurate results than the SVM-based model with coefficient of determination values > 0.99 for all the targeted metals. The FTIR analysis confirmed the partial disintegration of PCB polymeric base by OH radicals (OH•), which helped in liberating and exposing the embedded metals to the bio-Fenton solution. Further, the oxidation of metals by ferric ions produced from GOx-mediated oxidation of ferrous ions ensued efficient enzymatic metal bioleaching. Selective metal recovery of >99% was obtained by the chemical precipitation of bioleachate.

Applications of crushing and grinding-based treatments for typical metal-containing solid wastes: Detoxification and resource recovery potentials

Metal-containing solid wastes can induce serious environmental pollution if managed improperly, but contain considerable resources. The detoxification and resource recoveries of these wastes are of both environmental and economic significances, being indispensable for circular economy. In the past decades, attempts have been made worldwide to treat these wastes. Crushing and grinding-based treatments have been increasingly applied, the operating apparatus and parameters of which depend on the waste type and treatment purpose. Based on the relevant studies, the applications of crushing and grinding on four major types of solid wastes, namely spent lithium-ion batteries (LIBs) cathode, waste printed circuit boards (WPCBs), incineration bottom ash (IBA), and incineration fly ash (IFA) are here systematically reviewed. These types of solid wastes are generated in increasing amounts, and have the potentials to release various organic and inorganic pollutants. Despite of the widely different texture, composition, and other physicochemical properties of the solid wastes, crushing and grinding have been demonstrated to be universally applicable. For each of the four wastes, the technical route that involving crushing and grinding is described with the advantages highlighted. The crushing and grinding serve either mainstream or auxiliary role in the processing of the solid wastes. This review summarizes and highlights the developments and future directions of crushing and grinding-based treatments.

A critical review of the pre-processing and metals recovery methods from e-wastes

E-wastes being potential sources of numerous valuable metals are promoted to undergo recycling and recovery under the umbrella of urban mining and circular economy. Thus, the present study provides a critical review of the technological details of different metal recycling processes, pre-treatment methods, and the advancements made in these techniques. Critical evaluation of different metal recovery techniques has also been presented based on the available life cycle assessment (LCA), techno-economic, and industrial-scale studies. The study revealed that the integrated metal recovery techniques serve better in terms of recovery efficiency and environmental performance than any single recovery technique. Also, scaling up of biometallurgical, electrochemical, and super critical fluid extraction methods needs to be promoted due to their better environmental performances.

Copper recovery through biohydrometallurgy route: chemical and physical characterization of magnetic (m), non-magnetic (nm) and mix samples from obsolete smartphones

The more modern electronics are, the smaller and complex printed circuit boards are. Thus, these materials are continually changed (physicochemically), increasing the copper concentrations in smartphones. In this sense, it is challenging to set standardized recycling processes to improve metal recovery. In addition, biohydrometallurgy is a clean and cheap process to obtain critical metals from low-grade sources and waste electronic equipment. Therefore, the aim of this work was to characterize, physicochemically, 21 PCBs from smartphones manufactured from 2010 to 2015, and then to recover the copper by Acidithiobacillus ferrooxidans (biohydrometallurgy). The PCBs were comminuted and separated into Magnetic (M), Nonmagnetic (NM) and without magnetic separation (MIX) samples. It was identified 217.8; 560.3 and 401.3 mg Cu/g of PCBs for M, NM and MIX samples, respectively. Regarding biohydrometallurgy, the culture media iron-supplemented (NM + Fe and MIX + Fe) increased the copper content by 2.6 and 7.2%, respectively, and the magnetic separation step was insignificant.

Efficient recovery of Cu and Ni from WPCB via alkali leaching approach

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.

Urban mining of obsolete computers by manual dismantling and waste printed circuit boards by chemical leaching and toxicity assessment of its waste residues

Waste residues and acidic effluents (post-processing of E-waste) released into the local surroundings cause perilous environmental threats and potential risks to human health. Only limited research and information are available toward the sustainable management of waste residues generated post resource recovery of E-waste components. In the present study, the manual processing of obsolete computer (keyboard, monitor, CPU, and mouse) and chemical leaching of waste printed circuit boards (WPCBs) (motherboard, hard drive, DVD drive, and power supply) were performed for urban mining. The toxicity characteristics of typical pollutants in the residues of the WPCBs (post chemical leaching) were studied by toxicity characteristics leaching procedure (TCLP) test. Manual dismantling techniques resulted in an efficient urban mining concept with an overall average profit estimation of INR 2513.73/US$ 34.59. The chemical leaching of WPCBs showed a high concentration of metal leaching like Cu (229662 ± 575.3 mg/kg) and Pb (36785.67 ± 13.07 mg/kg) in the motherboard after stripping epoxy coating. The toxicity test revealed that the concentration of Cu (245.746 ± 0.016 mg/l) in the treated waste residue and Cu (430.746 ± 0.0015 mg/l) and Pb (182.09 ± 0.0035 mg/l) in the non-treated waste residue exceeded the threshold limit. The concentrations of other elements As, Cd, Co, Cr, Ag, Mn, Zn, Ni, Fe, Se, and In were within the permissible limit. Hence, the waste residue stands non-hazardous except Cu and Pb. Stripping out the epoxy coating of WPCBs enhances the metal leaching concentrations. The study highlighted that efficient and appropriate E-waste urban mining has immense potential in tracing the waste scrap into secondary resources. This study also emphasized that the final processed waste residue (left unattended or discarded due to lack of appropriate skill and technology) can be taken into consideration and exploited for value-added materials.

Recovery of Metals from Heat-Treated Printed Circuit Boards via an Enhanced Gravity Concentrator and High-Gradient Magnetic Separator

The recovery and reuse of waste printed circuit boards (PCBs) has attracted more and more attention from global researchers, as recycling of waste PCB metals is of great significance to the rational utilization of metal material resources. This study puts forward a clean and economical method in which enhanced gravity separation and wet high-gradient magnetic separation were combined to recover waste PCBs with heat treatment at a temperature of 240 °C. The heat treatment could improve the metal liberation effect of the PCBs, and the thermal behavior was measured by thermogravimetric analysis (TGA). The pyrolysis of the non-metal fraction (NMF) began around 300 °C, and the glass transition temperature of epoxy resin was 135.17 °C. The enhanced gravity separation technique was used for the separation of metals and NMF under the compound force field. The metals grade of the gravity concentrates fraction (GRF) was 82.97% under the optimal conditions, and the metals recovery reached 90.55%. A wet high-gradient magnetic separator was applied to classify the GRF into magnetic (MA) and non-magnetic (NMA) fractions, which could achieve iron and copper enrichment. After the three stages combined process, the copper and iron grades of the NMA and MA fractions were 70.17% and 73.42%, and the recovery reached 74.02% and 78.11%, respectively.

Sustainable and high-efficiency recycling of valuable metals from oily honing ferroalloy scrap via de-oiling and smelting separation

Oily ferroalloy scraps generated from machinery honing enterprises are typical hazardous municipal materials that release benzene-series volatile organic compounds (VOC), which endanger human physical and mental health. Therefore, harmless treatment and resource reuse for these hazardous materials is urgent. In this study, the VOC emission, and pyrolysis and de-oiling behaviors of oily honing scrap was first characterized to evaluate the environmental risks. Smelting separation was then proposed to economically and eco-friendly recover valuable metals from the de-oiled ferroalloy scraps. The thermodynamics of Al₂O₃-SiO₂ and CaO-Al₂O₃-SiO₂ systems was calculated to optimize the slag formation. Results showed that after de-oiling and smelting with CaO addition, the hazardous VOC are removed, and the valuable metals are recovered in ferroalloy state. Under optimum conditions, a crude Fe-Mo-Cu alloy with Fe, Mo and Cu recoveries of 98.5 wt%, 97.9 wt%, and 98.4 wt% were obtained. In addition, the slag containing few toxic elements and VOC can be used for silicate cement production. Pyrolysis, de-oiling behaviors and mechanism for slagging and growth of Fe-Mo-Cu alloy during smelting were discussed via various testing techniques, and leaching toxicity of the cleaned slag was also characterized in this work. This process is also applicative to recover metals from various honing ferroalloy scraps.

Inorganic-organic mesoporous hybrid segregators for selective and sensitive extraction of precious elements from urban mining

This study aimed to develop a highly selective extraction protocol for gold (Au^III) ions from electronic urban waste (EUW) using simple, low-cost Inorganic-organic mesoporous hybrid segregators. The unique features of mesoporous hybrid segregator architectures are of particular to ensure effective adsorption system in terms of selective and sensitive recovery of Au^III ions from EUW. The segregator platform featured 3D micrometric, mesocage double-serrated plant-leaf-like γ-Al₂O₃ sheets with hierarchy surfaces containing tri-modal mesopores interiorly and uniformly arranged toothed edges of ~20-40 and ~15 nm groove width and depth at the exterior surfaces, respectively. Rational incorporation of actively organic chelates into hierarchical γ-Al₂O₃ sheet platforms leads to the production of a couple of selective segregators 1 and 2 (namely, SC1 and SC2) for Au^III ions at specific conditions by applying batch and fixed-bed columnar techniques. The mesocage SC segregators offer a selective extraction approach of Au^III ions from mixed element contents released from a computer motherboard (CMB). Our finding indicated that the textural and hierarchal features of the mesocage SC segregators played key roles in the selective adsorption/recovery of Au^III ions at pH 2-2.5 with high capacity (136-141 mg/g range) and effective reusability ≫10 consecutive cycles. In general, the developed SCs could be utilized as a real extractor of Au^III recovery from spent CMBs.

Separation of metals from Ni-Cu-Ag-Pd-Bi-Sn multi-metal system of e-waste by leaching and stepwise potential-controlled electrodeposition

Electronic waste, as hazardous waste, contains a large amount of metals, which is of great recovery value. However, they are difficult to separate due to wide variety and complex distribution. Most of current recycling methods are environmentally-unfriendly or complicated. In this study, a simple, efficient and green approach for metals separation from Ni-Cu-Ag-Pd-Bi-Sn multi-metal system of e-waste was proposed combining mild leaching and stepwise potential-controlled electrodeposition. The leaching efficiencies of Ni, Ag, Pd, Cu and Bi were 99.16%, 99.09%, 94.91%, 99.61% and 23.76% with 1 mol/L HNO₃ at 80 °C. The leaching process was analyzed. It showed that the existence of Ag-Pd continuous solid solution in the alloy lowered the oxidation potential of Pd, which facilitated its leaching. Sn precipitated as SnO₂. Then Ag-Pd alloy and Cu-Bi alloy were separately extracted from the leaching solution by stepwise electrodeposition. 97.72% of Ag and 98.05% of Pd were recycled after 5 h with potential of 0.35 V. The recovery efficiencies of Cu and Bi were 97.87% and 97.33% after 7 h with potential of 0.05 V. The EDS results showed high purity property of Ag-Pd and Cu-Bi alloy. This process can achieve cleaner and efficient extraction of metals from multi-metal system in e-waste.

Study of reaction characteristics and controlling mechanism of chlorinating conversion of cathode materials from spent lithium-ion batteries

Spent lithium-ion batteries (LIBs) recycling has attracted much attention because it is highly favorable to environment protection and sustainable development. Developing a cleaner method for metals extraction can greatly reduce risk of secondary pollution. Chlorinating technology has been proved as an efficient method for metals extraction instead of traditional hydrometallurgy. In this paper, cathode materials from spent LIBs could be rapidly converted into metal chlorides by NH₄Cl roasting at 623 K for 20 min. The results indicated nearly 100% metal leaching rates were achieved. Further, in-depth study is performed to obtain the mechanism function of chlorinating conversion based on roasting and TGA experiments. The apparent activation energy as 73.40 kJ/mol was firstly obtained, and then the reaction model of chlorination reaction was determined by model fitting and verifying. Herein, sub-reactions of chlorination reaction were figured out and their contributions were used to determinate reaction controlling mechanisms of chlorination reaction. The results indicated that nucleation reaction played a leading role in the initial stage (0.05 <α < 0.43) while phase boundary reaction took the control in next stage (0.43 <α < 0.95), which gave a good explanation to activation energy change. Finally, our findings provided inspirations for studying the controlling mechanism of gas-solid reaction.

Environmentally friendly approach for the recovery of metallic fraction from waste printed circuit boards using pyrolysis and ultrasonication

Electronic waste (e-waste) with an annual growth rate of 3-5% is one of the fastest-growing waste streams. The unregulated accumulation and improper recycling can cause grave hazards to human beings and the environment. On the contrary, e-waste can be considered as a secondary source of metals and energy due to its high metal content and polymeric material. Thus, the present study demonstrates technology for the metallic fraction recovery and the production of valuable gases from e-waste. The process involves pyrolysis at a temperature range of 200 °C-600 °C in a fixed bed setup for 10-60 min. Under optimized operating conditions, 35 wt% combustible gases and 60 wt% solid product were obtained at a pyrolysis temperature of 400 °C in 20 min. The gaseous product consisted of CH₄, H₂, CO and CO₂, having the heating value 28 MJ/kg whereas, the solid product is a mixture of metals and other solid residue material. Treatment of the solid product using an ultrasonication process resulted in around 90 wt% recovery of metallic fraction, thereby leaving behind solid residue. Moreover, the transfer of precious metals (Au, Ag, Pd and Pt) was nearly 100% to the metallic fraction. This process combines mild temperature pyrolysis and ultrasonication process to provide a solution for efficient management of e-waste, metallic fraction recovery and valuable gases production.