The rapid increase in the production and consumption of electronic and electrical equipment has resulted in a significant global increase in electronic waste (e-waste). This complex waste stream has valuable rare earth elements, precious metals, and base metals, positioning it as an important secondary resource for the recovery of critical metals. With proper extraction and recovery technologies, e-waste presents a strategic opportunity to supplement primary mineral resources and support supply chain security. However, as demand for critical metals continues to rise, conventional extraction methods often face limitations related to environmental impact, energy intensity, and chemical consumption. This review examines innovative and sustainable extraction techniques for recovering critical metals from e-waste, with particular emphasis on advancements in biohydrometallurgy. It explores the integration of biological processes, including the use of microorganisms and bioleaching, to enhance metal recovery while reducing ecological footprints. In addition, this review considers the role of advanced hydrometallurgical processing in improving efficiency and selectivity. Although substantial research has proved the technical feasibility of biomining for base metals, precious metals, and rare earth elements at laboratory scale, key challenges remain. Therefore, this paper moves beyond the question of whether metals can be bioprocessed and instead critically evaluates issues related to scalability, process integration, techno-economic viability, and sustainability.
Recycling large-tonnage copper slag as a secondary source of iron in the form of magnetite is limited by the liberation of finely disseminated phases, whereas particle shape has so far been regarded as a consequence of breakage rather than a controllable feed parameter. In the finite element model, particle contours were taken from optical micrographs, while the third dimension and the texture with explicitly resolved phases and interphase boundaries were built using rules adopted from a published mineralogical description. Of 93 impact simulations, 68 formed the main sample covering seven particle shapes, impact angles of 30–90° and velocities of 22.5–80 m/s, the rest being control, tuning or rejected runs. Three slag samples were crushed in a disintegrator at rotor speeds of 1000–3000 rpm. For the series of particles of equal length, liberation is governed by the ratio of the smallest particle dimension to the grain size; the transition is threshold-like, and, for the adopted strength of interphase boundaries, the critical thickness increases with impact energy from about two to four grain sizes. With no parameters fitted to the experimental data, the calculated critical thickness is close to the threshold size of about 315 µm and of the same order as the median product size, while the concentration of liberated fayalite in the fine class and the rotor speed giving the greatest effect agree qualitatively with the simulations. Classification of the feed by size and shape is proposed as a strategy for further testing.
The acid durability of recycled-aggregate concrete (RAC) remains largely undocumented in sub-Saharan Africa, and the small, collinear datasets of such campaigns are rarely released. Three concretes (natural, partially recycled and fully recycled) were cast with quarry aggregates or aggregates from two demolition-waste sources in Yaoundé, Cameroon, characterized to French and European standards. After 90 days of water curing (25.4 to 25.9 MPa), specimens were immersed in nominally 5% sulfuric or hydrochloric acid, and residual compressive strength and mass loss were measured on one specimen per age after 7, 14, 21, 28 and 90 days of immersion. After 90 days in sulfuric acid, the natural concrete retained 64% of its strength and the recycled concretes 31% and 27%; in hydrochloric acid, retention was 44%, 44% and 55%. At nearly equal mass loss, the recycled concretes retained 7.4 MPa in sulfuric acid against 12.6 MPa in hydrochloric acid, consistent with an expansive mechanism but also with the higher acid dose of the sulfuric bath; reaction products were not analysed. Mix-descriptor collinearity is quantified, degradation kinetics are described by monotonic trends and simple empirical laws, and aggregate sources are pre-qualified by an anchored multi-criteria screening index. The dataset is released with the article.
Mild pyrolysis of high-density polyethylene (HDPE) can produce wax-rich intermediates, but selectivity is governed by cracking severity. This study examined the effects of in situ low-pressure steam and wood biochar on HDPE product distribution in a stirred batch reactor. Screening at 350–410 °C identified 390 °C as a transition between heavy condensed material and lighter products; maximum total reactor pressures during the focused experiments were 8.8–20.5 bar. At 390 °C, neat HDPE gave 83.0 wt.% condensed product and a 17.0 wt.% apparent gas fraction calculated by difference. Water (3.2 or 9.1 wt.%) and biochar (HDPE:biochar = 2:1) applied separately reduced the apparent gas fraction and increased condensed-product recovery to 91.3–93.9 wt.%. The higher water loading favored the >C40 TGA volatility interval, whereas biochar did not selectively enrich C20–C40. Their combination showed an apparent non-additive redistribution: the apparent >C40 fraction decreased to 22.3 wt.%, while C20–C40,
Spent lithium-ion battery (LIB) cathodes were investigated as a multimetallic resource for catalyst precursor synthesis. Cathode material was leached under ultrasound using citric or sulfuric acid, with and without H2O2, to compare metal recovery, stability, and thermal transformation. Sulfuric acid with H2O2 achieved the highest extraction efficiencies, with 93.10% Co and 97.15% Mn, while Li extraction exceeded 95%. Maximum sulfuric concentrations were 12,822 mg L−1 Co, 4588 mg L−1 Mn, 185 mg L−1 Ni, and 659 mg L−1 Li. Initial citrate solution without H2O2 contained 25,870.90 mg L−1 Co, 15,121.95 mg L−1 Mn, 4118.14 mg L−1 Li, 965.79 mg L−1 Al, 162.74 mg L−1 Cu, and 12.69 mg L−1 Ni. Both liquors precipitated. The citrate system exhibited metal–citrate coordination, whereas the sulfuric system retained hydrated sulfate and aquo/sulfate species. After drying, the citrate precursor formed a carbon- and oxygen-rich matrix, while the sulfate precursor retained sulfur and oxygen at 400 °C. Citrate decomposition occurred at 200–400 °C, while sulfate transformation extended to approximately 612 °C. Reduction occurred at 400–900 °C for citrate and predominantly at 800–900 °C for sulfate. Both routes provide complementary precursor platforms for catalyst preparation; however, catalytic performance was not evaluated in this study and remains to be demonstrated.
