Advances in Mineral Processing and Hydrometallurgy—3rd Edition

1. Introduction and Scope

After the success of the two prior editions, a third edition of a Special Issue of Metals has been commissioned, specifically devoted to aspects of mineral processing and hydrometallurgy. This is a unique, focused Special Issue, as the specific combination of hydrometallurgy and mineral processing is not common. Both hydrometallurgy and mineral processing will be relied upon to produce the critical minerals and metals needed for renewable energy.

As Free [1] states, water makes aqueous processing of metals possible. Water is an unusual substance from a chemical perspective. Most substances with similar molecular weights, such as methane and ammonia, are gases at room temperature. However, water is a liquid at room temperature. Water’s unusual properties are primarily due to hydrogen bonding effects. These are related to the tendency of hydrogen to donate electrons and the tendency of oxygen to accept electrons. As such, hydrometallurgy offers superior separations, as well as the capability to treat low-grade materials to produce fungible compounds and metals.

Wills and Napier Munn [2] recently published a seminal distinct book on mineral processing. They stated “As-mined” or “run-of-mine” ore consists of valuable minerals and gangue. Mineral processing—sometimes called ore dressing, mineral dressing, or milling—follows mining and helps prepare the ore for extraction of the valuable metal in the case of metallic ores, and it produces a commercial product of products, such as iron ore and coal. Apart from regulating the size of the ore, it is a process of physically separating the grains of valuable minerals from the gangue minerals to produce an enriched portion, or concentrate, containing most of the valuable minerals, as well as a discard, or tailing, containing predominantly the gangue minerals. The importance of mineral processing is taken for granted today, but it is interesting to reflect upon the fact that less than a century ago, ore concentration was often a crude operation, involving simple gravity and hand-sorting techniques performed by the mining engineers. The twentieth century saw the development of mineral processing as a serious and important professional discipline, and without physical separation, the concentration of many ores, and particularly the metalliferous ores, would be hopelessly uneconomic.

Fuerstenau and Han [3] published a book devoted exclusively to the combination of mineral processing and hydrometallurgy. In this unique publication, they stated that to advance technology in the production of material resources, nations look towards practicing and future engineers. Current and future mineral processing engineers must obtain sound and rigorous training in the sciences and technologies that are essential for effective resource development. Many industrial and academic leaders have recognized the need for more textbooks and references in this critical area. This was the driving force for writing a comprehensive reference book that covers mineral processing and hydrometallurgical extraction.

Hence, this third-edition Special Issue was designed to highlight research papers on characterization, recycling and waste minimization, mineralogy, geometallurgy, thermodynamics, kinetics, comminution, classification, physical separation, liquid–solid separation, leaching, solvent extraction, ion exchange, activated carbon, precipitation, reduction, process economics, and process control. Specific metals of interest include gold, silver, PGMs, aluminum, copper, zinc, lead, nickel, and titanium. Critical Metals articles on topics such as lithium, antimony, tellurium, gallium, germanium, cobalt, graphite, indium, and rare earth metals were welcomed. Eleven high-quality, peer-reviewed articles from around the globe were selected.

2. Overview of the Published Articles

Wang et al. [Contribution 1] provided an article on the in situ leaching of ionic rare earths. The ion exchange reaction between rare earth ions and leaching agent ions was carried out, allowing the rare earth ions to be leached from the ore body as the leaching solution flows through the pores. This indicates that the leaching process of rare earth ions is closely related to the seepage field, ion exchange field, and ion migration process of the leaching solution. In this study, an ionic rare earth mine located in Longnan of Jiangxi Province was taken as the research object. By conducting nuclear magnetic resonance scanning on the ore samples of this mine and vectorizing the nuclear magnetic resonance images, a two-dimensional geometric model of pores was obtained. Then, COMSOL Multiphysics software was used to establish a coupled numerical model of seepage–exchange–migration of the ionic rare earth mine during the leaching process at the pore scale to study the seepage situation of leaching solution with different injection strengths and concentrations, as well as the exchange and migration process. The results show that increasing the concentration of magnesium ions can increase the difference in ion diffusion concentration, accelerate the forward exchange rate of ions, promote the forward exchange reaction, and improve the concentration gradient of rare earth ions in the leaching solution. The more significant the diffusion effect, the higher the ion migration rate, simultaneously inhibiting the reverse adsorption of rare earth ions and accelerating the leaching efficiency of rare earth ions. In addition, increasing the strength of the injection solution allows rare earth ions to leach out of the ore body earlier; shortens the leaching cycle; and, thus, reduces the peak concentration of leached rare earth ions. By analyzing the effects of the strength of the injection solution and leaching concentration on ionic rare earth leaching, the influence of those two factors on engineering economy can be briefly evaluated, which can be provided as a reference for the optimization of ionic rare earth mining technology

Qin et al. [Contribution 2] investigated the effects of nickel content in nickel-bearing pyrite on photocatalytic properties, light absorption properties, and oxidative decomposition of thiosulfate were studied. The leaching experiments show that the consumption of thiosulfate in the Cu²⁺-ethylenediamine (en)-S₂O_3²⁻ system increases with an increase in nickel content in nickel-bearing pyrite. The consumption of Cu(en)_2²⁺ initially increases and then decreases with an increase in leaching time. There is a clear correlation between the change trend in its consumption and the doping amount of nickel in pyrite. The XPS results show that, in the Cu²⁺-ethylenediamine (en)-S₂O_3²⁻ gold leaching system (temperature 25 °C, time 35 h, solution: 0.1 mol/L S₂O_3²⁻, 5 mmol/L Cu(en)_2²⁺, 200 mL solution), the nickel of pyrite-containing nickel can be transferred to the leaching solution and becomes nickel ion. In this leaching system, Cu(II), which was originally complexed with en, is reduced to Cu(I) in a brief time. The consumption of Cu(en)_2²⁺ increased rapidly in the 5 h period and then decreased gradually after 5 h. The results showed that the presence of free Ni²⁺ in the solution facilitated the conversion of bivalent copper ions to monovalent copper ions. Free Ni²⁺ ions can compete with Cu²⁺ ions for en ligands. When ethylenediamine complexes with Ni²⁺, the decomposition of Cu(en)_2²⁺ into Cu(en)+ and en occurs more rapidly. And the en, which was originally to be oxidized with Cu(en)+ to form Cu(en)_2²⁺, forms Ni(en)_2²⁺. As a result, the concentration of Cu(en)_2²⁺ continues to decrease in a brief period

Alguacil [Contribution 3] reported that though indium has been removed from the fifth list (2023) of critical raw materials for the European Union list of critical metals, its recovery is still of paramount importance due to its wide use in a series of high-tech industries. As its recovery is strongly associated with zinc mining, the recycling of In-bearing wastes is also of interest, for both profitable and environmental reasons. With unit operations (in hydrometallurgy and pyrometallurgy or extractive metallurgy) playing a key role in the recycling of indium, the present work reviewed the most recent innovations (2024) regarding the use of these operations in the recovery from this valuable metal from different solid or liquid wastes

Wang et al. [Contribution 4] provided another article noting that the vast seabed holds tremendous resource potential that can provide necessary materials for future human societal development. This study focuses on the mineralogy of seafloor manganese nodules off the coast of China in the Western Pacific and the primary techniques for extracting valuable metal elements from manganese nodules. The research indicates that the main valuable metal elements in the manganese nodules from this region include Cu, Co, Ni, Mn, Fe, etc. The key to extracting these valuable metals lies in reducing Mn(IV) to Mn(II) to disrupt the structure of the nodules, thereby releasing the valuable elements. The extraction processes for the main valuable metal elements of manganese nodules are mainly divided into two categories: pyrometallurgical–hydrometallurgical and solely hydrometallurgical. To cope with the challenges of environmental change and improve utilization efficiency, bioleaching, hydrogen metallurgy, and co-extraction are gaining increasing attention. For promoting commercialization, the future development of manganese nodule resources can refer to the technical route of efficient short-process extraction technology, the comprehensive recovery of associated resources, and tail-free utilization.

Menad et al. [Contribution 5] noted that currently Electric Arc Furnace Slag (EAFS) is undervalued and only used in road construction, while blast furnace slag (BFS) is used as an interesting alternative in construction materials to replace natural aggregates in the manufacture of concrete. Steel slag (SS) represents a promising secondary resource due to its high content of critical metals, such as chromium (Cr) and vanadium (V). These metals are essential for various strategic industries, making it crucial to consider slag as a resource rather than waste. However, the primary challenge lies in selectively recovering these valuable metals. In this work, we explore the development of a hydrometallurgical process aimed at efficiently extracting Cr and V from Electric Arc Furnace Slag (EAFS). The characterization of the investigated EAFS shows that the main crystalline phases contained in this heterogeneous material are srebrodolskite, larnite, hematite, and spinel (magnesio-chromite). The targeted metals seem to be dispersed in various mineral species contained in the SS. An innovative hydrometallurgical method has been explored, involving physical preparation by co-grinding slag with alkaline reagents followed by treatment in a microwave furnace to modify the metal-bearing species to facilitate metal processing dissolution. The results obtained showed that the leaching rates of Cr and V were, respectively, 100% and 65% after 15 min of treatment in the microwave furnace, while after 2 h of conventional heat treatment, as explored in a previous study, 98% and 63% of the Cr and V were leached, respectively.

Redrovan [Contribution 6] noted that the thiosulfate–glycine–copper system has emerged as a promising alternative for gold recovery, offering significant advantages over cyanidation and ammoniacal thiosulfate leaching. Recognizing the limitations of thiosulfate degradation in ammoniacal systems, this study focused on optimizing the thiosulfate–glycine–copper system for gold recovery using an auriferous ore with (10 g t⁻¹) of Au. The ore was associated with aluminosilicates such as grossular (64%) and clinochlore (12%). The leaching conditions were systematically varied, including thiosulfate (0.5–1 M); glycine (0.3–1.75 M); copper sulfate (2–10 mM); pH (9.3–10.5); temperature (20–60 °C), 6 h; and potassium permanganate concentrations (0.004–0.04 M). Dosing intervals were also optimized. Thus, the best conditions were thiosulfate (0.7 M), glycine (1.75 M), copper sulfate (5 mM), pH 9.3, 60 °C, and permanganate addition every 2 h. This system achieved 89.3% gold recovery in just 6 h, comparable to cyanidation (89.8% in 24 h) and ammoniacal thiosulfate (58% in 6 h), but without generating toxic effluents, such as in the cyanidation process. Additionally, a gold dissolution mechanism was proposed, highlighting glycine’s role in stabilizing cupric ions and enhancing thiosulfate efficiency. This study underscores the thiosulfate–glycine–copper system as a sustainable and effective method for gold recovery.

Kenzhaliyev et al. [Contribution 7] studied an innovative approach to processing refractory zinc-bearing clinker through the synergistic application of microwave thermal treatment and ultrasonic-assisted leaching. Microwave irradiation induces phase transformations in the clinker, improving its reactivity and facilitating subsequent zinc dissolution, while ultrasonic cavitation enhances mass transfer by disrupting passivation layers. Key process parameters, including acid concentration, temperature, pulp density, and leaching time, were systematically investigated using response surface methodology (RSM) and central composite design (CCD). The results demonstrate that the optimized process conditions led to a significant increase in zinc recovery from refractory materials.

Hirata-Miyasaki and Anderson [Contribution 8] noted that demand and prices for antimony have increased over the last few years. Recycling supplied 15% of domestic consumption in the US, while the remaining 85% was imported. Hydrometallurgical processes have long used alkaline sulfide systems and hydrochloric acid, closing doors on innovative approaches. Bromine compounds have been recently used to recover PGMs and REEs successfully; thus, antimony leaching with bromine compounds is theoretically feasible. This research was conducted to develop a viable technology for hydrobromic acid between 50 °C and 70 °C as a leaching reagent on dross through single- and two-stage leaching using design of experiment (DoE) and adding sustainability to current industrial processes while minimizing waste products in recycling processes. The preliminary results showed that bromine, specifically hydrobromic acid, can be used as a leaching reagent for antimony dissolution. By decreasing the lead content in the solids and increasing the concentration, temperature, and reaction time, antimony leaching from the dross increased from 20% to 50%. The findings, coupled with acid regeneration, can be implemented as an alternative to other reagents in industrial plants.

Safiulina et al. [Contribution 9] stated that N,O-donor hybrid heterocyclic extractants have immense potential for separation of actinides from lanthanides in spent nuclear fuel reprocessing processes. We demonstrate that this type of reagent can be used for primary concentration of actinides contained in eudialyte, a promising mineral containing a heavy group of lanthanides. With respect to lanthanide ions, the efficiency of their extraction decreases in the series L3 >> L1 > L2, and the extraction of actinides decreases in the series L1 ≈ L3 >> L2. For the extractant, L2 based on 2,2′-bipyridine-6,6′-dicarboxylic acid diamide, the efficiency of lanthanide purification from U, Th exceeds 50. The structure and stereochemical features of the ligands do not have a significant effect on the composition of the formed complexes. The solvation numbers are close to 1 for all range f-elements studied, except for thorium, which indicates the predominant formation of complexes with the composition ratio of 1:1. Solvation numbers 1.4–1.5 are observed for thorium(IV), and the established values indicate the formation of a mixture of complexes with the composition ratios of 1:1 and 2:1.

Seo et al. [Contribution 10] addressed the recycling of spent lithium ion batteries that generates Na₂SO₄-containing wastewater, resulting in environmental problems and resource losses. This study investigates a treatment method using bipolar membrane electrodialysis (BMED) to recover H₂SO₄ and NaOH from Na₂SO₄ solutions. The acid and/or base recovery efficiency, energy consumption, operational stability, and economic feasibility of two BMED configurations, i.e., two- and three-compartment systems, were systematically compared. The current density, initial concentrations, and initial concentrations and volumes of the acid and base were optimized under constant current conditions. The three-compartment system exhibited higher acid purity and stability, whereas the two-compartment system offered lower energy consumption and membrane degradation. Under optimal conditions, both systems successfully recovered H₂SO₄ and NaOH from Na₂SO₄-containing wastewater. A techno-economic analysis based on a lab-scale process demonstrated the cost advantages of the two-compartment system versus the long-term operational stability of the three-compartment system. These findings suggest that BMED is a viable and effective solution for the treatment of Na₂SO₄-containing wastewater from battery recycling processes.

Jovanović et al. [Contribution 11] examined the leaching behavior of copper and iron from a sphalerite concentrate in sulfuric acid utilizing an ensemble MnO₂–KI oxidation system. Temperature was shown to significantly influence leaching kinetics, with efficiency notably improving between 40 °C and 80 °C. The introduction of KI affected the balance between sulfur passivation and oxidant availability, facilitating increased leaching efficiencies. At 3 wt.% KI, maximum recoveries of 82.1% Cu and 85.3% Fe were achieved, indicating a notable decrease in surface passivation. Kinetic study analysis revealed low activation energies of 28.90 kJ mol⁻¹ for copper and 18.94 kJ mol⁻¹ for iron, indicating that both processes proceed readily at moderate temperature regimes. Despite being diffusion-controlled, the mechanisms of dissolution are different—iron leaching is more complicated, involving pyrite oxidation, sulfur layer formation, transformation to marcasite, and, ultimately, iron(III) release, whereas copper leaching involves direct interaction of chalcopyrite with the oxidants, like the behavior of sphalerite. The MnO₂–KI-H₂SO₄ system’s promise for more sustainable polymetallic concentrate processing is demonstrated by its reduced energy requirements and enhanced leaching efficiency.

3. Summary and Outlook

This third Special Issue of Metals was again well supported by diverse submissions and a final book publication of eleven high-quality peer-reviewed articles. Due to this continued success, another Special Issue (“Advances in Mineral Processing and Hydrometallurgy—4th Edition” https://www.mdpi.com/journal/metals/special_issues/M92897U5GK (accessed on 7 August 2025)) has been commissioned as a follow-up to highlight contributions from the global hydrometallurgy and mineral processing community.