The growing global demand for metals stands in contrast to the current scenario of decreasing ore grades, more complex mineralogy, and more intense environmental concerns related to the extractive sector. In parallel, increasing amounts of waste are being generated as the automotive and electronic devices industries expand their activities. Expected advances in sustainable energy technologies and the boom of automation leveraged by the industry 4.0 transition should increase even more both the demand for metals and the generation of their respective waste products. In this context, the term “urban mining” [1
] has emerged as a concept that goes beyond recycling as a mere ecological activity, but rather as a necessity aimed at maximizing the use of non-renewable resources such as metals through technologies that minimize environmental impacts.
Because they include non-ferrous and precious metals such as gold, copper, cobalt, and rare earths in their composition, wastes from electrical and electronic equipment (WEEE) are among the main targets of urban mining. WEEEs are also notable for being the waste stream with the highest growth rate worldwide, up to 5% per year. However, of the approximately 50 million tons generated annually, it is estimated that only 13% is effectively recycled [2
]. Developing nations have shown increasing per capita generation of electronic scrap, although still presenting poor recycling rates. Brazil, for instance, recycles only about 4% of its total solid waste generated [4
Metals such as copper and precious (Au, Ag, etc.) are mainly embedded in printed circuit boards (PCBs), making them a fundamental constituent of WEEEs. PCB weight contributions range from about 12% in liquid crystal displays (LCDs) to more than 21% in mobile phones and are typically composed of about 40% metals, 30% plastics, and 30% ceramic compounds [5
]. The content of different metals in PCBs varies according to device type and age. For instance, PCBs from older devices, such as cathode ray tube (CRT) monitors, contain considerable amounts of Cu, Fe, Sn, and Pb used in soldering and lead frames [3
]. PCBs currently make up most of the electronic waste globally recycled since they are relatively rich in copper and precious metals, especially gold.
Recycling of PCBs involves the initial stages of physical separation (dismantlement, shredding, size classification, gravity concentration, magnetic separation, etc.), whose primary focus is to separate the metallic from the non-metallic fraction [8
]. The metal-rich concentrate thus generated is then treated through hydrometallurgical or pyrometallurgical processing routes [10
]. Nowadays, pyrometallurgical plants are the main recycling centers for most WEEE, involving high operational and energy costs [3
]. Thus, they require a constant supply of material with stable composition and preferably containing high contents of precious metals. These restrictions favor limiting the recycling of electronic wastes to only a few large metallurgical companies, whose plants concentrate in specific regions of the globe. Among these, one can cite Aurubis (Hamburg, Germany) [11
], Glencore (Toronto, Ontario, Canada) [12
], and Umicore (Brussels, Belgium) [13
]. Although each of them displays specific features in their extractive routes, the processing basically involves adapting conventional pyrometallurgical operations of non-ferrous metals, particularly copper, nickel, and lead, to the context of electronic wastes. Because they are easier to control, operate at lower temperatures, and generate fewer emissions, hydrometallurgical routes have been pointed out as a trend in future WEEE processing [3
]. However, the generation of large quantities of wastewaters contributes to their still comparatively lower use.
Current research on recycling of PCBs and, more generally, electronic wastes have focused on the use of refined processing operations, such as microwave pyrolysis [14
], slurry electrolysis [15
], and vacuum-gasification-condensation [16
], to cite a few. Bio-metallurgical techniques such as bioleaching are receiving special attention in recent years, as they have the potential to overcome typical limitations of pyro- and hydrometallurgical processes, such as high operational costs and production of toxic gases and wastewaters [10
]. Their long-scale applications for recycling, however, are still germinal. On the other hand, fewer studies have been dedicated to investigating improvements in physical separation technologies capable of generating high purity concentrates (or even pure metals) at minimal economic and environmental costs. Nevertheless, some physical separation techniques, such as gravity [18
] and electrostatic [21
] separation and froth flotation [23
], have received some attention. Among these, gravity separation processes stand out for their relatively low costs and technical versatility, favored by the fact that electronic wastes have individual components of varied densities, from about 0.9 g/cm³ for plastic substrates to 11.3 g/cm³ for lead frames.
The improvement of physical separation routes for recycling electronic wastes is particularly interesting for processing old and low value scraps whose precious metal content is relatively low, despite still having significant amounts of valuable metals, especially non-ferrous. TVs, monitors, and printer boards fall into this category, which despite usually having less than 20 ppm of gold (versus 200 ppm in PC-boards), might contain up to 12%, 15%, and 30% of Cu, Al, and Fe by weight, respectively [3
]. Old, discarded PCBs can also have up to 5% Pb in solders [26
]. Still, when all the printed circuit board assembly (PCBA,) consisting of resistors, capacitors, transformers, and wires, is considered, the content and variety of metals can be considerable higher. That said, it is worth mentioning that old and low value-added electronic scraps still make up a large fraction of the WEEE generated in developing countries, which in some cases also receive old WEEE as a donation from developed countries [27
]. In this sense, the implementation of low capital cost and pollution-free recycling methods focusing on physical separation of electronic wastes is not only technically possible, as recently showed by Zhu et al. [28
], but can be decisive for the income and subsistence of small- and medium-scale recyclers.
Within the context above, and as part of an ongoing project concerned with the development and improvement of physical separation operations for WEEE recycling, this paper aims to examine the feasibility of recovering metals and generating metal concentrates through the application of physical separation techniques in wasted printed circuit boards assemblies (PCBAs). Special focus is given to gravity separation processes.
Electronic wastes from old devices, such as TV boards and stereos, generally have fewer valuable metals when compared to new ones, so that its recycling has been little addressed in the literature. Nevertheless, these wastes represent a significant part of the electronic scrap generated in developing countries, being commonly sold by collectors as low-value material to recycling hubs abroad. Increasing the content of metals in this waste stream could be potentially beneficial for recyclers’ income and, consequently, for the recycling market itself. On this basis, this study investigated the possibility of concentrating metals from old, wasted PCBs through a physical separation-based route.
It was found that a combination of fragmentation, size classification, density, and magnetic separation operations allow obtaining four distinct products: (a) a non-metallic fraction, basically composed of plastics and resins; (b) a product enriched with aluminum; (c) a magnetic material stream, containing mainly iron; (d) a final concentrate containing more than 50% in mass of copper and enriched with Sn and Pb. A particular advantage of the proposed route is the possibility of selective recovery of aluminum, which is usually slagged in smelting processes and, thus, discarded or under-reused.
After fragmentation and classification of PCBs into three particle size fractions, it was verified that most of the material was situated in the +5 mm coarser fraction (55.73%), followed by the −5 + 2 mm and −2 mm particle size fractions (22.22% and 18.19%, respectively). Cu, Al, Fe, and Sn constituted the major metals encountered in all particle sizes, including some peak concentrations of Zn, Sb, Pb, Ba, and Mn in specific size ranges.
Results obtained from the float-and-sink analysis showed that aluminum was mostly concentrated in the density range of +2.1–2.8 g/cm³, especially in the +5 mm fraction (about 55% in mass of Al), whereas the highest concentration of other metals occurred in the +2.8 g/cm³ density, the −5 + 2 mm size having the greatest proportion of material in this range. After a detailed analysis of the composition of the densest fraction (+2.8 g/cm³), a tendency for the formation of large individual tangles composed of copper and polymer fibers was observed. Although detrimental to the separation, preliminary tests have indicated the possibility of exploring this entanglement effect for selective recovery of copper through drumming. The analysis of the macro constitution, especially in larger sizes, also showed the possibility for recovery of other products such as ferrite and graphite insulators.
Future studies should include follow-up work to evaluate metals extractive routes from the obtained concentrates, the possibility of recovering specific materials (such as ferrite transformer cores), and whether the inclusion of other physical separation techniques (such as drumming and electrostatic separation) can upgrade the content of metals in concentrates.