Liberation characteristics after cryogenic modification and air table separation of discarded printed circuit boards

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.

Assessment of Pre-Treatment Techniques for Coarse Printed Circuit Boards (PCBs) Recycling

Waste electrical and electronic equipment or e-waste generation has been skyrocketing over the last decades. This poses waste management and value recovery challenges, especially in developing countries. Printed circuit boards (PCBs) are mainly employed in value recovery operations. Despite the high energy costs of generating crushed and milled particles of the order of several microns, those are employed in conventional hydrometallurgical techniques. Coarse PCB pieces (of order a few centimetres) based value recovery operations are not reported at the industrial scale as the complexities of the internal structure of PCBs limit efficient metal and non-metal separation. Since coarse PCB particles’ pre-treatment is of paramount importance to enhance metal and non-metal separations, thermal, mechanical, chemical and electrical pre-treatment techniques were extensively studied. It is quite evident that a single pre-treatment technique does not result in complete metal liberation and therefore several pre-treatment flowsheets were formulated for coarse PCB particles. Thermal, mechanical and chemical pre-treatments integrated flowsheets were derived and such flowsheets are seldom reported in the e-waste literature. The potential flowsheets need to be assessed considering socio-techno-economic considerations to yield the best available technologies (BAT). In the wider context, the results of this work could be useful for achieving the United Nations sustainable development goals.

Analysis of rejects from waste printed circuit board processing as an alternative fuel for the cement industry

Waste Printed circuit boards (PCBs) are one of the most valuable and recycled components of electronic waste due to the presence of precious metals such as copper, silver, gold and palladium. The rejects of the PCB recycling process, named non-metal fraction (NMF) have continuously been sent to landfills. Several researchers have proposed alternative use of NMF as secondary materials such as fillers in composites or as adsorbent. This study is focused on the potential application of the PCB recycling rejects as waste-derived fuel or alternative fuel in the cement industry. Approximately 2 million metric tonnes (Mt) of this waste was produced in 2014 globally and estimated to reach 6.5 million Mt in 2050. The presence of high organic matter in the NMF renders it useful as an alternative fuel. The organic content of the NMF could also potentially be increased using gravity separation and thus increasing its net calorific value. The study showed that the NMF could provide up to 21 MJ kg^-1 of heating value with low heavy metal and ash concentration. A comparison with other waste-derived fuel sources is also presented in the paper.

Comparative study of pyrolytic carbons prepared from printed circuit boards by magnetic and electrostatic separation

To discover the influence of separation technologies for PCBs on the preparation, characterization and application of pyrolytic carbon, two kinds of nonmetal fraction from magnetic and electrostatic separation were chosen as the precursors.

Sampling waste printed circuit boards: Achieving the right combination between particle size and sample mass to measure metal content

The current worldwide expansion of waste PCB (WPCB) deposits represents both a pressing environmental issue and an economic opportunity, fostering the development of numerous recycling processes across the world. An important input for designing such processes is the metallic content of WPCBs, which is assayed by grinding and leaching samples taken from the stack of WPCBs to be recycled. The content values come with substantial uncertainties, arising mainly from the uneven distribution of the metals within the structure of WPCBs. This study aims to quantify the effects on these uncertainties of the particle size, the mass of the sample digested and the number of digestion replicates. It focused on the abundance of six metals in WPCBs: Cu, Fe, Zn, Pb and Ni, and also Co, which is a critical element for the EU. A batch of 485 kg of WPCBs was put through several shredding and splitting steps to produce three fractions: one shredded to 2 mm, and two ground to 750 μm and 200 μm. From each sample, 16 samples of 0.5 g, 2 g or 5 g were digested in hot aqua regia. Bootstrapping of the results allowed the error around the mean content to be estimated, for each metal and for all the experimental conditions. Considering the largest sample masses and three replicated digestions, the uncertainties for Zn (resp. Ni) were reduced from 35% to 10% (resp. from 70% to 10%) when the particle size was reduced from 2 mm to 200 μm.

Inconsistencies of e-waste management in developing nations - Facts and plausible solutions

Electronic and electrical equipment (EEE) became an integral part of daily life and had an immense influence on the economy. The skyrocketing demand, progressive technologies, and high dependency resulted in inconceivable utilization of EEE. However, these scientific expansions shortened the life span of EEE, thereby generating massive volumes of waste electronic and electrical equipment (WEEE). On a global perspective, Oceania generates a per capita of 17.3 kg/inh (inhabitants), followed by Europe 16.6 kg/inh, America 11.6 kg/inh, Asia 4.2 kg/inh and the least contribution by Africa 1.9 kg/inh. As known, EEE comprises complex metallic and non-metallic fractions causing severe discrepancies within the ecosystem, endangering the living species; if not dealt with properly. Thus, there is a pressing need of immediate addressal on the effective e-waste management strategies both from developed and developing countries. On the spin side, the separation of the precious fractions from the EEE on the end-of-life may be a twin dimensional strategy of economic addition, and plummeting the alarming level threats to ecology. However, these menaces are well tackled by the developed countries to some extent by the stringent law enactments, establishing proper recycling facilities, and trading to the underdeveloped and developing nations. But, the majority of the developing and under developed nations lacks the statutes, gaps in policy making, socio-economic-cultural barriers, technology, and the appropriate treatment facilities. In addition, the review identified ten major shortfalls (10L's) refraining the effective e-waste management, especially in the developing and under developed nations. Among which, integration of the formal and informal sectors, mandated network registry, stringent law enforcements, regulated transboundary movements, manufacturers responsibility, consumer awareness and improved eco designs, investing on effective recycling facilities, and improved disposal facilities holds the key. Further, replacing the traditional and conventional procedures with the futuristic and eco-friendly approaches such as chelation, inducing ionic liquids, integrated processes or hybrid technologies, micro factories, photo catalysis, and green adsorption will substantially harness the current barriers of the e-waste management. Finally, the present review will be a thorough glancing for the future research of e-waste management of meso-micro-macro scales.

Effect of thermal shock process on the microstructure and peel resistance of single-sided copper clad laminates used in waste printed circuit boards

Efficient pre-processing is essential to the mechanical recovery of waste printed circuit boards (WPCBs). In this work, a thermal shock pretreatment was utilized to damage the interface between metals and nonmetals of single-sided copper clad laminates (SSCCLs), which are usually employed as the base material of printed circuit boards (PCBs). The effects of three thermal shock treatment parameters-i.e., peak temperature, holding time, and thermal shock cycle times-on the adhesion strength of SSCCLs were evaluated by orthogonal experiments. Microstructures and peel resistance of SSCCLs before and after thermal shock were characterized by scanning electron microscopy (SEM) and 90° peel test, respectively. Our results showed that the impact of three major factors that influence liberation efficiency was in the sequence of peak temperature > shock cycle times > holding time. Furthermore, the optimal thermal shock level could be achieved when the peak temperature was 300°C with the soaking time of 30 min and three cycle times. In the meantime, the corresponding peel strength of the SSCCLs (0.065 N/mm) was sharply decreased by 94% in comparison with those without thermal shock treatment. The manual dismantling experimental data verified the good feasibility of the optimal thermal shock process, suggesting that the copper foil could be readily dismantled from the substrate by hand after pretreatment, with a successful separation rate of 100% and a peeling efficiency of ~ 30 seconds per piece. Therefore, the optimal thermal shock process could notably improve liberation of metals and nonmetals, which would be helpful for efficient recycling of WPCBs.Implications: The interface between copper foil and laminate dielectric in a PCB can be weakened significantly via efficient thermal shock method. Thus, a good liberation could be achieved after thermal shock. In this work, a manual peeling of copper foil from the SSCCL substrates was achieved efficiently after optimal thermal shock pretreatment, confirming the feasibility of a shorter process of metal recovery from scrap SSCCLs without pulverization. The results will be useful for the pretreatment of recovery of the WPCBs.

Progressive utilisation prospects of coal fly ash: A review

The rapid surge in global energy needs has paved way for the development of various alternatives to natural resources every now and then. However, dependence on coal-based energy has not reduced greatly. Thus, massive quantities of coal fly ash (CFA) are generated worldwide, which is a serious threat to ecology owing to constraints associated with its storage and disposal. There exists a pressing and ongoing need to develop new, and green product streams from CFA to reduce the threat to the environment. The present review begins with an emphasis on the generation, physicochemical properties, and potential dangers of CFA. Then, it focuses on impending applications such as synthesis of geopolymers (alternative to cement), silica aerogels (insulating materials), carbon nanotubes (carbon allotropes) for electronic devices, and the separation of radioactive isotopes as well as rare earth elements from CFA. Furthermore, the review analyses factors restraining the motive for effective management strategies that drives utilisation of CFA (either in raw and processed state) for new product streams. Finally, the review elucidates the role of CFA as an emerging input in delivering eco-friendly amenities and future derivatives.

Innovating e-waste management: From macroscopic to microscopic scales

Waste electrical and electronic equipment (WEEE or e-waste) has become a global problem, due to its potential environmental pollution and human health risk, and its containing valuable resources (e.g., metals, plastics). Recycling for e-waste will be a necessity, not only to address the shortage of mineral resources for electronics industry, but also to decline environmental pollution and human health risk. To systematically solve the e-waste problem, more attention of e-waste management should transfer from macroscopic to microscopic scales. E-waste processing technology should be significantly improved to diminish and even avoid toxic substance entering into downstream of material. The regulation or policy related to new production of hazardous substances in recycled materials should also be carried out on the agenda. All the findings can hopefully improve WEEE legislation for regulated countries and non-regulated countries.