Toward Achieving a Carbon-Neutral Society: Beneficiation and Extractive Metallurgy for Producing Critical Metals from Ores/Wastes

1. Introduction and Scope

The global commitment to achieving a carbon-neutral society has accelerated the transition toward renewable energy, electric mobility, and advanced electronic systems [1]. This transition, however, has led to an unprecedented increase in the demand for a wide range of critical metals, including copper (Cu), lithium (Li), nickel (Ni), cobalt (Co), and rare-earth elements (REEs) [2]. According to the International Energy Agency (IEA), the demand for key energy-transition minerals in 2040 is projected to increase by approximately 1.2 times for Cu, 4.5 times for Li, 1.7 times for Ni, 1.5 times for Co, and 1.6 times for REEs compared with their respective demand levels in 2024 [3].

To meet the future demand for these metals, technological advancements in beneficiation and extractive metallurgical processes for both primary ores and secondary resources—such as mine tailings, metallurgical slags, industrial residues, electronic waste, and wastewaters—are essential. Beneficiation and extractive metallurgy play a central role in enabling such low-carbon metal supply chains by integrating physical separation, chemical extraction, and environmentally benign process design.

This Special Issue, entitled “Toward Achieving a Carbon-Neutral Society: Beneficiation and Extractive Metallurgy for Producing Critical Metals from Ores/Wastes”, aims to highlight recent scientific and technological advances in the sustainable production of critical metals from both primary and secondary resources. The scope covers a wide range of processes, including flotation, magnetic and gravity separation, thermal reduction, leaching, solvent extraction, adsorption, and emerging hybrid and low-energy technologies.

2. Contributions

This Special Issue comprises nine articles that collectively demonstrate how modern beneficiation and extractive metallurgy can contribute to carbon-neutral and circular production of metals, including Li, gold (Au), Cu, Ni, Co, and REEs.

Eom et al. (Contribution 1) provided a comprehensive review of Li extraction from primary hard-rock resources, with a particular focus on mechanochemical treatment as a promising strategy for improving Li recovery. The review highlights recent developments in mechanochemical activation as an energy-efficient pretreatment to enhance Li leachability from spodumene- and lepidolite-type minerals.

Mhandu et al. (Contribution 2) studied a thiosulfate-based green hydrometallurgical process as an environmentally benign alternative to conventional cyanide-based gold extraction. It has been reported that the efficiency of thiosulfate leaching of gold from sulfidic refractory ores is low due to the passivation of the gold surface and/or the decomposition of thiosulfate [4,5,6]. The mechanistic investigations of Mhandu et al. (Contribution 2) to elucidate the detrimental effects of arsenopyrite/pyrite during thiosulfate leaching of gold revealed that thiosulfate decomposition on the arsenopyrite/pyrite surface—rather than passivation of the gold surface—is the primary cause of the drastic decrease in gold extraction efficiency. Furthermore, Mhandu and coworkers (Contribution 2) proposed a new flowsheet incorporating a pre-oxidation step using cupric ammine complexes prior to thiosulfate leaching, which significantly improved gold extraction efficiency from 10% to 79%.

Although thiosulfate is a promising lixiviant, recovering gold from thiosulfate media remains challenging. Cementation employing aluminum (Al) as the electron donor and activated carbon (AC) as the electron mediator has been reported to enable effective recovery of gold ions from thiosulfate media [7]; however, Cu ions present as tetraammine complexes—acting as the oxidant for gold—were simultaneously recovered, resulting in increased reagent consumption upon reuse of the lixiviant. Zoleta et al. (Contribution 3) demonstrated that substituting AC with iron oxides, such as hematite (Fe₂O₃) and magnetite (Fe₃O₄), enables the Al-based cementation process to become selective for gold ions, contributing to the practical implementation of cyanide-free gold processing.

Fang et al. (Contribution 4) investigated the applicability of microwave treatment for enhancing the grinding and flotation performance of copper–nickel sulfide ore. It was found that the grindability of the ore improved with increasing microwave treatment duration, as evidenced by reductions in D₈₀ and relative work index (RWI) values, along with an increase in the degree of liberation. However, excessively prolonged microwave treatment reduced flotation efficiency due to surface oxidation of the minerals. Under optimal conditions, microwave treatment was demonstrated to effectively improve both grinding and flotation performance.

Simonič et al. (Contribution 5) investigated the potential of bio-based secondary materials (i.e., torrefied wood-waste biomass) for the adsorption of Cu(II) and Ni(II) ions, demonstrating that thermally treated biomass can serve as an efficient and low-cost sorbent for metal removal and recovery from aqueous systems. In addition, the torrefied biomass exhibited enhanced hydrophobicity and improved energy properties, highlighting its potential as an improved solid fuel.

Gong et al. (Contribution 6) studied the reprocessing of Co-bearing Cu slag via carbothermic reduction followed by magnetic separation to recover Co, which is an indispensable metal for the energy transition, particularly for the production of Li-ion batteries. Carbothermic reduction treatment altered the distribution of Co, which was mainly present in iron silicate minerals in the pristine slag but predominantly existed as cobalt–iron alloy in the roasted slag. As a result, the magnetic separation concentrate exhibited an increase in Co grade from ~1.3% to 4.0% with a recovery of 94.2%, demonstrating that carbothermic reduction followed by magnetic separation is a promising approach for transforming slags into secondary cobalt resources.

Similarly, Lim et al. (Contribution 7) emphasized the importance of reprocessing non-ferrous slags (e.g., tin, copper, nickel, vanadium, and titanium slags) for the recovery of critical and strategic metals. The review article by Lim and coworkers (Contribution 7) systematically analyzes pyrometallurgical and hydrometallurgical routes for metal recovery from industrial slags and highlights their role as long-term urban and industrial metal reservoirs.

Rare-earth elements are also critical for transitioning toward a low-carbon future. Jürjo et al. (Contribution 8) focus on the leaching and extraction of REEs and associated rare metals from Estonian graptolite-argillite and phosphorite ores, demonstrating the feasibility of valorizing non-traditional geological resources as alternative REE sources.

Finally, Litvinova et al. (Contribution 9) provided fundamental insights into REE behavior through a thermodynamic study of rare-earth ion associations in the Ln³⁺—CO₃²⁻—H₂O system, elucidating carbonate complexation and ion-pair formation mechanisms relevant to alkaline and carbonate-based extraction processes.

Together, these contributions span a wide spectrum of resources—ranging from primary ores to slags, industrial wastes, and biomass—and employ diverse physical, chemical, and hybrid process routes, clearly illustrating the multifaceted nature of modern sustainable metallurgy.

3. Conclusions and Outlook

This Special Issue has presented recent progress in beneficiation and extractive metallurgical technologies for the sustainable production of critical metals from both primary ores and secondary resources. The collected papers clearly demonstrate that advanced physical separation, innovative hydrometallurgical and pyrometallurgical processes, and bio-based and low-carbon approaches can significantly contribute to resource efficiency and carbon-neutral metal production. Despite the significant progress made in this field, numerous challenges remain unresolved. In this context, this Special Issue is intended to serve as a springboard for future scientific discussion and debate on emerging and challenging topics related to the sustainable supply of critical metals.