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Over the last few decades, a substantial amount of e-waste including waste printed circuit boards (WPCBs) has been produced and is accumulating worldwide. More recently, the rate of production has increased significantly, and this trend has raised some serious concerns regarding the need to develop viable recycling methods. The presence of other materials in the WPCBs, such as ceramics and polymers, and the multi-metal nature of WPCBs all contribute to the increased complexity of any recycling process. Among the viable techniques, pyrometallurgy, with the inherent ability to process the waste independent of its composition, is a promising candidate for both rapid and large-scale treatment. In the present study, firstly, the principles of the pyrometallurgical methods for WPCB recycling are discussed. Secondly, the different unit operations of thermochemical pretreatment including incineration, pyrolysis, and molten salt processing are reviewed. Thirdly, the smelting processes for the recovery of metals from WPCBs, as well as the issues surrounding slag formation and subsequent treatment are explained. Fourthly, alternative methods for the recovery of polymers and ceramics, in addition to metal recycling, are elucidated. Fifthly, emission control techniques and the potential for energy recovery are evaluated.

Despite attempts to enhance the recycling of waste printed circuit boards (WPCBs), the simultaneous recovery of major metals of WPCBs using an efficient approach is still a great challenge. This study mainly concerned with applying an effective statistical tool to optimize the recovery of metal content (i.e., Cu, Fe, Zn, Pb, Ni, Sn, and Al) embedded in WPCBs using a leaching agent without any additive or oxidative agent. Another target was to optimize a multi-response recovery process by minimizing time, energy, and acid consumption during the leaching. Effective parameters and their levels, including leaching time (20-60 min), temperature (25-45 °C), solid to liquid (S/L) ratio (1/8-1/20 g/ml), and acid molarity (1-2.7 M), were optimized. A well-established statistical approach (i.e., response surface methodology (RSM)) was applied to precisely quantify and interpret the effects. General optimum conditions for nine responses were introduced with the desirability of ≈ 85%. Finally, the solid residue of leaching was characterized and results showed the morphology, structure, and composition of the residue content (i.e., polymers and ceramics) remained the same after the leaching, indicating the neutral behavior of the leaching process on these two materials. Also, thermal behavior and phase analysis of the original WPCBs and leaching residue were compared and analyzed. Graphical abstract.

Environmental restrictions and economic benefits have obliged countries to promote recycling processes from secondary resources like spent lithium ion batteries (LIBs) instead of using primary ones. In spite of the developments have been made on industrial scale for the technologies involved in recycling processes, most of these technologies suffer from lack of efficiency and echo-friendliness. To reduce the footprints of the recycling processes, several efforts are made. A major development area is the use of organic acids which are considered as promising agents for leaching of valuable metals from spent LIBs. In this review paper, we provided an overview of the recent status of the recycling technologies of spent LIBs using organic acids. For this purpose, necessity of green processes and advantage of organic acids in recycling of spent LIBs is discussed. To fully understand the effect of these agents, production, origin, application, and structure of organic acids that have been used in recovery of metals from spent LIBs are also addressed. Afterwards, recycling processes using organic acids, and benefits and drawbacks of using them are summarized and possible complexes formed by these agents are proposed. Eventually, development of different reducing agents, ultrasonic agitation, and evolution and future prospect of green processes in recycling of spent LIBs is reviewed.

In this study, Aspergillus niger (A. niger) as an environmentally friendly agent for fungal bioleaching of waste printed circuit boards (WPCBs) was employed. D-optimal response surface methodology (RSM) was utilized for optimization of the bioleaching parameters including bioleaching method (one step, two step and spent medium) and pulp densities (0.5 g L−1 to 20 g L−1) to maximize the recovery of Zn, Ni and Cu from WPCBs. According to the high performance liquid chromatography analysis, citric, oxalic, malic and gluconic acids were the most abundant organic acids produced by A.niger in 21 days experiments. Maximum recoveries of 98.57% of Zn, 43.95% of Ni and 64.03% of Cu were achieved based on acidolysis and complexolysis dissolution mechanisms of organic acids. Based on the kinetic studies, the rate controlling mechanism for Zn dissolution at one step approach was found to be diffusion through liquid film, while it was found to be mixed control for both two step and spent medium. Furthermore, rate of Cu dissolution which is controlled by diffusion in one step and two step approaches, detected to be controlled by chemical reaction at spent medium. It was shown that for Ni, the rate is controlled by chemical reaction for all the methods studied. Eventually, it was understood that A. niger is capable of leaching 100% of Zn, 80.39% of Ni and 85.88% of Cu in 30 days.

Printed circuit boards (PCBs), one of the most complex components of e-waste, contain different metallic and non-metallic components. Recycling of waste PCBs is an important issue, from both aspects of hazardous waste management and recovery of valuable materials. In this study, for the first time, a mechanical-physical separation method for recovery of metallic elements of waste PCBs without any chemical or/and thermal processes was introduced. Two milling stages were applied to enhance the liberation degree, followed by a physical flotation process for enrichment. Based on the elemental analysis, the total metal concentration increased by 75% after the second milling in coarser particle size classifications and ceramic components has decreased significantly. Phase identification techniques confirmed that the ceramic portion is mainly CaO+SiO2+Al2O3, which generally exist inside the boards as glass fibres. Using Fourier transform infrared spectroscopy (FTIR) spectra 3 nominees of phenoxy resin, poly vinyl acetate and vinyl chloride were suggested as the major polymer content of the crushed waste PCBs. Scanning electron microscopy (SEM) demonstrated higher degree of liberation in the higher meshes for metallic and non-metallic components but it was lower for fibre glasses. The thermal pyrolysis using infrared gas analyser proved that the most volatile substances are not simple greenhouse gases such as CO, CO2 and CH4. Eventually, beneficiation using bromoform + acetone resulted in two phases of high density (sunk) and low density (float), in which the former one was enriched in metals excluding Al and Sn, and the latter one was almost depleted from metals.

Identifying a solution to directly transform waste printed circuit boards (PCBs) to useful alloys will save energy, conserve our valuable resources, and contribute to preservation of our environment. In this study a solid state mechanical alloying was used to directly convert waste PCBs to a homogenous nanostructured alloy (Cu79-Zn13-Fe3-Sn3-Ni1). Electron microscopy analysis showed 10 h milling time after a short period of cryomilling was enough to obtain a homogenous alloy and SAED ring pattern confirmed the nano-structure, although just a portion of particles size of the power was in nano-scale (

An environmentally-friendly route based on hydrometallurgy was investigated for the recovery of cobalt and lithium from spent lithium ion batteries (LIBs) using different organic acids (citric acid, Dl-malic acid, oxalic acid and acetic acid). In this investigation, response surface methodology (RSM) was utilized to optimize leaching parameters including solid to liquid ratio (S/L), temperature, acid concentration, type of organic acid and hydrogen peroxide concentration. Based on the results obtained from optimizing procedure, temperature was recognized as the most influential parameter. In addition, while 81% of cobalt was recovered, the maximum lithium recovery of 92% was achieved at the optimum leaching condition of 60°C, S/L: 30gL-1, citric acid concentration: 2M, hydrogen peroxide concentration: 1.25Vol.% and leaching time: 2h. Furthermore, results displayed that ultrasonic agitation will enhance the recovery of lithium and cobalt. It was found that the kinetics of cobalt leaching is controlled by surface chemical reaction at temperatures lower than 45°C. However, diffusion through the product layer at temperatures higher than 45°C controls the rate of cobalt leaching. Rate of lithium reaction is controlled by diffusion through the product layer at all the temperatures studied.

The waste of lithium ion batteries (LIBs) is the major secondary source of valuable elements such as lithium and cobalt. Accumulation of spent LIBs can lead to environmental pollution and also the loss of valuable metals. In this study, recovery of lithium and cobalt form LIBs was investigated using a hydrometallurgy process. To optimize the leaching parameters (type of organic acids, acid concentration, solid to liquid ratio (S/L), temperature, and hydrogen peroxide concentration), response surface methodology (RSM) was applied. Gluconic and lactic acids were used for the leaching experiments and results showed that in both the cases of lithium and cobalt, lactic acid is more efficient than gluconic acid. While the leaching parameters were set at the temperature of 79 °C, lactic acid concentration of 1.52 M, S/L ratio of 16.3 g.L−1, and hydrogen peroxide concentration of 4.84 Vol.%, 100% of lithium and 97.36% of cobalt were recovered. The head sample and the leaching residues were analyzed by inductivity coupled plasma optimal emission of spectrometry, X-ray diffraction, and high resolution scanning electron microscopy.