Editorial for Special Issue “Sustainable Production of Metals for Low-Carbon Technologies”

1. Introduction and Scope

In 2015, 193 governments agreed to act on climate change by drastically reducing carbon dioxide (CO₂) emissions as envisaged in the sustainable development goal (SDG) number 13 [1]. The high consumption of metals during the period of industrialization (i.e., since 1900) and the global call for cutting down on CO₂ emissions have led to high demand for metals such as cobalt (Co), copper (Cu), gold (Au), zinc (Zn), lead (Pb), lithium (Li), nickel (Ni), tin (Sn), vanadium (V), rare earth metals, etc. [2]. These metals are critical as the world moves to low-carbon technologies based on renewable energy sources (RESs) and electric vehicles (EVs), and away from the heavy use of fossil fuels. To meet the high forecasted demand, unconventional sources of metals such as low-grade complex ores, seafloor massive sulphides (SMSs), and wastes (e.g., tailings, metallurgical residues, and electronic wastes (e-wastes)) have become very important sources of metals [3,4]. Moreover, these metals should be extracted in a sustainable manner without negatively impacting the environment.

The sustainable extraction of these critical metals begins with studies on the advancements in the beneficiation (i.e., gravity separation, magnetic separation, flotation, etc.) of valuable minerals from unconventional sources. The concentrated minerals are then subjected to advanced extractive metallurgy where unconventional methods (e.g., unconventional hydrometallurgical processes, pyrometallurgical processes, or the combination of the two) are used to obtain metal with lesser or no negative impacts on the environment.

2. Contributions

Eleven research articles and one review article were submitted, peer-reviewed, and have been published in this Special Issue of Minerals entitled “Sustainable production of metals for low-carbon technologies”. The articles cover mineral concentration techniques (flotation and magnetic separation) and hydrometallurgy processes (leaching and recovery of extracted metal). All the articles published have high scientific rigour and will be of great interest to wider readers in the scientific community. The contents of the published articles are briefly summarized as follows.

2.1. Concentration of Valuable Minerals and Metals

2.1.1. Flotation

One unconventional source of critical metals for low-carbon technologies is the mineral tailings that were produced before the advancement of flotation techniques. These tailings are not only important as a secondary resource, but pose serious environmental threats due to the formation of acid mine drainage (AMD) from the oxidation of sulphide minerals and their subsequent dissolution [5,6]. Therefore, the reprocessing of these tailings to recover valuable sulphide minerals solves both the secondary resource concern and the negative impact on the environment. Flotation is one of the methods studied to recover valuable minerals containing critical metals from tailing. The challenges encountered in the use of flotation in the recovery of valuable minerals are that the particle sizes of these minerals sent to tailing dams are too fine for conventional flotation. In a review paper by Bilal et al. [7], the authors was found the challenges of fine particles’ flotation from tailings can be addressed by decreasing the bubble size in the flotation vessel (e.g., the use of column flotation, micro- or nano-bubble flotation) and increasing the particle size approaches (e.g., polymer flocculation, shear flocculation, oil agglomeration, and carrier flotation).

The depletion of high-grade and easy-to-process ore has led to researchers exploring complex ores, such as Cu–Zn–Pb sulphide ore, for the recovery of valuable minerals. The challenges encountered when Cu–Zn–Pb sulphide ore is treated for selective flotation (i.e., recovery of each sulphide mineral) include: (1) the unwanted activation of Zn sulphide minerals during Cu sulphide flotation by Pb²⁺ released from anglesite (PbSO₄) and (2) the activation of Zn sulphide minerals by dissolved Cu²⁺ as the result of galvanic interaction between Cu sulphide minerals (e.g., chalcopyrite (CuFeS₂)) and pyrite (FeS₂) [8,9]. To eliminate the unwanted activation of Zn sulphide by dissolved Pb²⁺ from PbSO₄, Aikawa et al. [8] suggested pretreating the Cu–Zn–Pb ore by washing it in ethylene diamine tetra acetic acid (EDTA) to dissolve PbSO₄ and the subsequent cementation of dissolved Pb by zero-valent iron (ZVI). The cemented Pb on the surface of ZVI is easily harvested from the pulp by the magnetic separation method [10]. Meanwhile, to control galvanic interaction between CuFeS₂ and FeS₂ in flotation cells leading to unwanted activation, Aikawa et al. [9] suggested purging nitrogen, hence reducing the dissolved oxygen and resulting in limiting the anodic dissolution of CuFeS₂ when in contact with FeS₂.

Existing flotation plants need simple but effective tools for diagnosis and optimization as the mined sulphide ore becomes more complex and lean of valuable minerals. In this Special Issue, a study by Han et al. [11] showed that linear circuit analysis (LCA) and mass balance (MB) simulation are simple yet powerful tools that can be used to diagnose and optimize the gold ore flotation circuit. From their simulation results, they showed that the current process flotation circuit can be changed to an alternative circuit to obtain the current recovery while improving the gold grade of the concentrate by 128%. The suggested approach of diagnosis and optimization has the potential of being applied to other plants that concentrate various minerals.

The combustion of high-fluorine (F) coal generates large amounts of F-bearing compounds (gases: HF_(g), SiF_4(g), and CF_4(g); solid particulates: SiF_6(s) and CaF_2(s)), which enter into the atmosphere and cause detrimental environmental and ecological consequences. In this Special Issue, Ni et al. [12] devoted their attention to the reduction in the F content in Chinese coal by the combination of gravity separation for coarse coal (>0.5 mm) and flotation for fine coal (<0.5 mm). The initial F content in the studied coal was 347.74 μg/g, which could be reduced to 90.14 μg/g after the proposed combination of gravity-flotation separation process.

2.1.2. Magnetic Separation and Thermal Treatment

In recent years, there has been an astronomical generation of electronic waste (e-waste), and the current outlook points to more generation of this waste. This e-waste is vital in ‘urban mining’, as it contains substantial amounts of critical metals that should be extracted. However, the liberation and later concentration of metals from e-waste, particularly from printed circuit boards (PCBs), is difficult because of the layered structure of non-metal (i.e., epoxy resin, glass fibre) and valuable metals (e.g., Cu). In this Special Issue, research by Kim et al. [13] showed that pretreatment of PCBs by thermal treatment under a 300 °C air atmosphere reduced the mechanical strength of PCB from 386.36 to 24.26 MPa, and thus copper liberation improved from 9.3 to 100% in the size range of a coarser size fraction (>1400 μm). After thermal treatment, the concentration of Cu was undertaken via electrostatic separation that produced a high-grade Cu concentrate (>90%) with a high recovery (>90%).

Rare earth metals are critical to low-carbon technologies as they are mainly used in the manufacturing of critical components used in generators for wind turbines, traction motors for electric vehicles, and strong permanent magnets. Although these metals are critical for low-carbon technologies, rare earth deposits are found in only a few countries, and they are mostly of very low grades. Therefore, it is vital to beneficiate mined rare earth ores before the extraction of these metals. In this Special Issue, Park et al. [14] studied the beneficiation of low-grade rare earth ore from the Khalzan Buregtei deposit (Mongolia) by magnetic separation. They demonstrated that for fine particle sizes (i.e., −0.5 + 0.1 mm and −106 + 75 µm), dry high-intensity magnetic separation (DHIMS) and wet high-intensity magnetic separation (WHIMS) produced total rare earth oxide (TREO) concentrates with the enrichment ratios of 2.8 and 5.5 and recoveries of 70% and 80%, respectively.

2.2. Extraction of Critical Metals by Hydrometallurgy

2.2.1. Alkaline Leaching of Materials and Critical Metal Recovery

Alkaline leaching of metals from concentrate, ores, or waste (mine waste and e-waste) achieves the selective solubilization of amphoteric elements—Al, Pb, Au, and Zn, among others—leaving the iron (Fe), calcium (Ca), and magnesium (Mg) host minerals that may constitute a large percentage of the processed materials. The challenge, however, is that solid–liquid separation by filtration, especially for strong alkaline, is difficult [15]. Silwamba et al. [15] investigated the concurrent recovery of dissolved Pb and Zn by cementation on aluminium metal powder from alkaline pulp (i.e., when zinc leach plant residues were leached using NaOH solution) before filtration. The results showed that around 100% of the dissolved Pb was cemented in the leaching pulp when Al metal powder was added. Meanwhile, less than 2% of dissolved Zn was cemented in leaching pulp. This meant that Pb could be recovered in alkaline leach pulp, and this could eliminate the challenges of the thorough filtration of difficult alkaline pulp.

Another alkaline leaching and metal recovery paper of this Special Issue was by Jeon et al. [16]. They investigated the kinetics of the recovery of Au from ammonium thiosulfate solution by the enhanced cementation technique by galvanic interaction of aluminium (Al; electron donor) and activated carbon (AC; electron mediator) to recover gold (Au). The addition of AC in the Au cementation on Al metal eliminates the challenges of poor Au recovery in the ammonium thiosulfate solution [17]. The reported kinetics results showed that the recovery followed first-order kinetics (i.e., the cementation rate increased with a higher initial concentration of Au, smaller electron donor size, greater both electron donor and mediator quantity, decrease in temperature, and higher shaking speed in the system).

2.2.2. Acid Leaching of Materials and Critical Metals Recovery

Sulphuric acid is one of the most widely used lixiviants used in hydrometallurgical processes for the dissolution of metals from their host materials (minerals) because it is cheap (often produced as a by-product) and can be regenerated. In this Special Issue, Jha et al. [18] used the sulphuric acid leaching of rare earth metals (REMs) from end-of-life nickel metal hydride (NiMH) batteries, which are now generated as waste due to their extensive use in the manufacturing of portable electronic devices as well as electric vehicles. The obtained results showed that more than 90% of REMs (Nd, Ce, and La) dissolved when 2 M H₂SO₄ was used at 75 °C and leached for 60 min in the presence of 10% H₂O₂ (v/v). The dissolved REMs were concentrated by solvent extraction and selective precipitation.

In a similar study, Ahn et al. [19] reported the leaching of Li, Ni, Co, and Mn in sulphuric acid from cathode active materials that have been used in lithium-ion batteries after their carbothermic pretreatment. The obtained results indicated carbothermic treatment enhanced the conventional leaching process using hydrogen peroxide because the leaching efficiencies of Li, Ni, Co, and Mn increased to over 99.9% within 120 min in all samples roasted at 600–900 °C.

The study by Yang et al. [20] was about the recovery of magnesium from ferronickel slag (FNS) to prepare magnesium oxide by leaching in sulfuric acid. The FNS was leached in two stages (i.e., first leached in 2.4 M sulphuric acid and regenerated residues subjected to 2.4 M sulphuric acid leaching again), and the overall leaching efficiency was 95.82%. The dissolved Mg was precipitated as carbonates and later on calcinated to produce MgO of 94.85% purity.

3. Conclusions and Outlook

The papers published under this Special Issue cover a variety of topics of mineral beneficiation and critical metals extraction as well as recovery from a wide variety of materials and ores. The papers reported the advancement made in the beneficiation and extraction of these metals in a sustainable manner. However, there are still many challenges that need to be overcome, especially the cost–benefit study of extracting these critical metals using non-conventional processes and from secondary resources. It is believed that this Special Issue will help to arouse future scientific discussions and debates on the challenges of the production of metals critical to low-carbon technologies.