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Saiya Li, Yujie Zhao

The comprehensive recycling of aluminum electrolysis cell waste barrier material is urgent. This study focuses on the sulfuric acid leaching of waste barrier material, systematically examining the effects of factors such as reaction temperature, liquid-to-solid ratio, sulfuric acid concentration, and reaction time on the leaching of elements like lithium, aluminum, sodium, and silicon. The experimental results show that under the conditions of 0.9 mol/L sulfuric acid concentration, a liquid-to-solid ratio of 20:1, a reaction temperature of 90 ◦C, and a reaction time of 1.5 h, the leaching rates were 84.5% for lithium, 85.6% for aluminum, 98.5% for sodium, and 4.8% for silicon. The sulfuric acid leaching process of the waste barrier material follows a shrinking core model and is controlled by internal diffusion. The apparent activation energies for the leaching reactions of lithium, aluminum, and sodium were 4.29 kJ/mol, 8.99 kJ/mol, and 9.11 kJ/mol, respectively. The selective leaching of lithium, sodium, and aluminum from silicon was successfully achieved in the sulfuric acid leaching of the waste barrier material.

Kıvanç Korkmaz

Bu tez çalışmasında, fraktör nikel cevherinden elde edilen karışımın karakterizasyonu ve hibrit malzemenin karşılaştırılması amacıyla, farklı süreçler ve sıcaklık gibi kritik parametreler kullanılarak yapıldığı bir çalışma olarak tanımlanmaktadır. Çalışmada, optimum şartlarda verilen örneklerin yüksek sıcaklıkta 324 kg asit/ton cinsinden kobalt verimleri elde edildiği gözlemlenmiştir. Deneyler, önceki benzer uygulamalara ve tekniklere dayanarak gerçekleştirildi. İki ayrı cevher kullanılarak yapılan deneyler sonrasında, hidrometalurji yönteminin etkili olduğu ve çözünme kinetiğinin değiştiği belirlenmiştir. Bu sonuçlar, yüksek basınçlı asit yöntemlerinin, nikel ve kobaltın etkin geri kazanımı için önemli olduğunu göstermektedir.

Yuanmin Zou, Alexander Chernyaev

This study focuses on the effect of an emerging source of waste, lithium iron phosphate (LFP) cathode materials, on the hydrometallurgical recycling of the currently dominant industrial battery waste that is rich in transition metals (Ni, Co, Mn, and Li). The effects of the dosage of LFP, initial acidity, and timing of LFP reductant addition were investigated in sulfuric acid (H2SO4) leaching using conditions of time at 3 hours, temperature at 60 °C, and rotation at 300 rpm. The results demonstrated that the addition of LFP increased both transition metal extraction and acid consumption. Furthermore, the redox potential was lowered due to the increased presence of Fe2+. An initial acidity of 2.0 mol/L H2SO4 with an acid consumption level of 1.3 kg H2SO4/kg black mass provided optimal conditions for achieving a high leaching yield of 100% for Co, 87.6% for Ni, 91.1% for Mn, and 100% for Li, while also creating process solutions with concentrations of Co at 8.8 g/L, Ni at 13.8 g/L, Li at 6.7 g/L, Mn at 7.6 g/L, and P at 12.1 g/L.

Takumi Ichikawa, Yuki Nagase

The sustainable recovery of rare earth elements (REE-Y) from electronic waste is critical for clean-energy technologies. Yet, the commercial viability of recovering REE-Y from shredder residue char (SR-char) remains unexplored. This study assesses a hybrid physical–chemical process utilizing SR-char, which integrates particle size classification and dry magnetic separation with optimized hydrochloric acid leaching. A first-order gross-profit screening model was developed to evaluate direct reagent economics of the proposed process, calculating revenue minus acid and neutralization costs, while excluding capital expenditures, labor, utilities, and separation losses. Results indicate that magnetic separation at 8000 G pre-concentrated REE-Y to >1800 g/t, and subsequent hydrochloric acid leaching yielded extractions of ~2000 g/t in the 500–1000 µm fraction. However, the profit model highlighted that maximized extraction in the presence of high concentrations of metals like Fe, Ca, and Al could lead to financial losses due to excessive reagent and neutralization expenses. Thus, physical pre-concentration to minimize non-target metal content is essential for economic viability. This targeted approach shifts optimization from metallurgical yield to economic feasibility, offering a transferable framework for evaluating other complex secondary REE-Y resources where impurity-driven costs significantly impact process economics.

Tamara Azevedo Schueler, Luka N. Mettke

Mine tailings, a substantial global waste stream, often contain recyclable metals with economic and environmental potential. This study focuses on the Rammelsberg mine’s Bollrich tailings pond, where residues from more than 50 years of mining and flotation processes pose contamination risks due to heavy metals. Building on prior projects aimed at recovering valuable metals from these tailings, this study optimizes the leaching of base metals, copper (Cu), zinc (Zn), and lead (Pb), using both inorganic and organic acid approaches. Leaching experiments conducted with sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) demonstrated up to 52% Cu and 77% Zn extraction, with optimal conditions involving controlled acid and solid concentrations and moderate temperatures. The limited solubility of lead (Pb) in H2SO4 prompted additional tests with organic acids (citric, acetic, and gluconic acids), where citric acid achieved the highest Pb leaching efficiency, reaching up to 86% with pH adjustment and H2O2 addition. Subsequent tests with barite fractions showed that citric acid is an effective lixiviant for Pb across different tailing types, achieving high co-extraction of Cu and Zn. A final combined leaching test with H2SO4 followed by citric acid indicated that sulfate compounds from the initial H2SO4 treatment inhibited Pb recovery, underscoring the need for optimized sequential processes. This work demonstrates a practical and sustainable approach to recovering valuable metals from mine tailings by optimizing leaching processes and highlighting the importance of sequential treatments to maximize metal recovery and minimize environmental impact.

Joseph J. Lee, Luke Plante

Ultramafic rocks represent a significant source of cations for carbon dioxide mineralization, which is essential for mitigating climate change. This study investigates an innovative method for enhancing the dissolution of ultramafic minerals to facilitate cation release for CO2 sequestration and critical element extraction. We employed the mineral-dissolving microbe Gluconobacter oxydans, which produces gluconic acid-based lixiviants, to accelerate the leaching process. Our results demonstrate that Mg2+ leaching is accelerated by a factor of 20 compared to deionized water, with further improvements of 73% observed when comparing leaching dynamics over 24 to 96 hours. Notably, at a 1% pulp density, G. oxydans shows only a modest improvement over gluconic acid alone; however, at 60% pulp density, it exceeds gluconic acid's effectiveness by 3.2 times. Furthermore, we found that biolixiviants derived from cellulosic hydrolysate matched the performance of those from glucose, thus widening the potential feedstock options for bioleaching. We also managed to decrease the carbon atom requirement in the biolixiviant feedstock for effective Mg2+ ion release and CO2 mineralization from 525 to 1, marking a significant advancement in carbon capture technology.

Ahmad Heydarian, Farzane Vakilchap

Accumulating used lithium-ion battery cathodes and associated environmental concerns necessitate efficient recycling strategies. This study introduces a breakthrough spent-medium bioleaching approach optimized for high-pulp-density conditions to address key challenges related to bacterial inhibition and sulfur availability for bacterial acidic agent production. Using response surface methodology, we optimized key variables including sulfur dosage, inoculum size, and initial pH, resulting in an optimized sulfate concentration of 40.3 g/l and a ΔpH of 1.87. We evaluated metal removal efficiency at pulp densities of 10–50 g/l, achieving significant extraction rates of lithium (92%), nickel (88%), and cobalt (78%) at the highest density of 50 g/l after 7 days of processing. A comparative analysis with traditional chemical leaching methods confirmed the effectiveness and sustainability of our green bioleaching strategy. Furthermore, a kinetic study utilizing the Avrami equation and the shrinking core model established that diffusion through the product layer was the controlling factor for the leaching rate. This research presents a comprehensive and sustainable strategy for the recycling of used lithium-ion batteries at high pulp densities by integrating process optimization, spent-medium bioleaching, and kinetic modeling for critical metal extraction.

Hoda Fereydouni, Tannaz Naseri

Mining activities produce significant amounts of gold mine tailings (GMT) that are rich in rare earth elements (REEs), yet effective extraction methods are limited. This study presents a novel approach combining bioleaching with oxalic acid pretreatment to recover Pr, Ce, and Eu from GMT. The pretreatment involved exposing the GMT to a 2 M oxalic acid solution at 90 °C for 6 hours, which enhanced the bioavailability of REEs by selectively removing iron. Following this, the pretreated GMT was introduced to the Acidithiobacillus thiooxidans bacterium during its logarithmic growth phase to maximize bacterial activity and subsequent acid production during the bioleaching process. Structural analyses confirmed surface modifications during bioleaching, while kinetic modeling highlighted that the rate of chemical reactions was the limiting factor. The results demonstrated a marked improvement in the recovery rates of REEs, with Pr, Ce, and Eu increasing by 24.4%, 14.4%, and 9.1%, respectively, after iron removal. This integrated approach of pretreatment and bioleaching presents a promising strategy for enhancing the recovery of REEs from GMT, potentially offering a sustainable resource recovery method in mining operations.

Alexa M. Schmitz, Brooke Pian

Biological methods are a promising route for the environmentally-friendly production of rare earth elements (REE), which are essential for sustainable energy and defense technologies. In this study, we aimed to enhance the REE-bioleaching capability of Gluconobacter oxydans by targeting key genetic mechanisms identified in previous research. The methodology involved the clean deletion of the phosphate-specific transport system gene pstS, which activated the phosphate starvation response and resulted in a more acidic biolixiviant, increasing bioleaching efficiency by up to 30%. Additionally, the knockout of pstS was coupled with the over-expression of the mgdh membrane-bound glucose dehydrogenase gene using the P112 promoter, leading to a significant reduction in biolixiviant pH by 0.39 units. Our results revealed that this engineered strain, G. oxydansΔpstS,P112:mgdh, produced a 53% increase in REE-bioleaching at a pulp density of 10% and an impressive 73% increase at a pulp density of 1%. This research illustrates the potential of systems biology-guided engineering to create high-efficiency microbial strains for the sustainable extraction of critical materials like rare earth elements.

Shaoliang Zhang, Qin Chen

In this study, we investigated the ability of Acidithiobacillus ferrooxidans to oxidize Fe2+ to Fe3+ and recover battery black powder, establishing a leaching system for decommissioned lithium iron phosphate battery black powder. Using ultrasonic waves, we aimed to enhance the leaching effect by removing impurities and promoting microbial activation through a cavitation reaction. A filter bag experiment was designed to explore the leaching mechanism of A. ferrooxidans, focusing on whether it was contact or non-contact based. Our findings showed that under optimal leaching conditions, the lithium leaching rate achieved 99.7%, reducing the leaching time from 7 to 5 days, thus demonstrating efficient leaching of lithium. The results concluded that the leaching mechanism of A. ferrooxidans for lithium iron phosphate primarily operated through a contact leaching approach, signifying an effective method for lithium recovery from waste lithium-ion power batteries.

Mabel Keke, Joseph Okechukwu Ezeugo

Certain hydrometallurgy processes are used in the majority of the main metal manufacturing processes that yield a final metal product. Tin is an essential strategic metal that is widely used in novel energy components, aircraft, and other cutting-edge industries. Yet, with decreasing availability of high-grade tin ores, the use of low-grade tin metals for metal tin processing is emerging as a significant development. However, there are intrinsic difficulties with low-grade tin ores that prevent them from being used directly in tin extraction. Although hydrometallurgy is still a useful technique for improving ore grade, problems such as mineral complexity and fine particle dispersal still prevail. This study explores hydrometallurgical processing in terms of essentials and their applications to tin treatment. It offers a thorough analysis of the properties of cassiterite resources, oxidative reagents, and pertinent case studies. Through an exploration of cassiterite's several dissolution techniques, the study emphasizes its leaching efficiency and distinctions between different oxidative reagents. In addition, it highlights significant barriers to cassiterite dissolution and suggests determined, practical approaches to facilitate effective metal recovery.

Massimiliana Pietrantonio, Stefano Pucciarmati

The global amount of end-of-life mobile phones is increasing over the years and their proper valorization is nowadays of strategic importance. Mobile phones can be considered as an important source of valuable materials, such as metals, plastics, and glass, which can be recovered and reintroduced into new production cycles. In this paper, a recovery process based on selective materials’ separation and hydrometallurgy was proposed. After manual dismantling followed by the separation of the different fractions (plastic, metallic fraction, printed circuit boards, batteries, displays and glass), a hydrometallurgical process based on leaching and precipitation/reduction was applied on printed circuit boards with the aim of recovering the metals of interests. Tin was precipitated from an aqua regia leachate with gaseous ammonia and gold was afterward recovered as metallic gold by reduction with sodium borohydride; silver was first precipitated as silver chloride from a nitric acid leachate and then reduced to metallic silver. Copper was selectively precipitated with oxalic acid from the solution coming from silver recovery and then recovered as metallic copper by means of a mild thermal treatment, without chemicals addition. The developed process allowed the recovery of gold, silver and copper in metallic form with yield and purity grade ≥ 98%.

L. Sandig‑Predzymirska, T. Veiga Barreiros

The recovery of platinum group metals (PGMs) from secondary sources is becoming increasingly relevant from an economic and environmental perspective. This study aims to explore a hydrometallurgical approach for recycling platinum (Pt) and ruthenium (Ru) from proton exchange membrane (PEM) electrodes. The methodology includes a comprehensive recycling scheme comprising pre-treatment, leaching, volatilization, and precipitation processes. A parametric study was conducted to examine various factors influencing the recovery yields, such as reagent concentration, leaching temperature, and solid-to-liquid ratio. The results showed high recovery rates, achieving 90% for Pt and 82% for Ru after 4 hours of leaching at 75 °C in 4 M HCl with the addition of 1.5 M AlCl3. Two methods were implemented for Pt/Ru separation: Ru volatilization and selective Pt precipitation. The use of 1 M Na2S2O8 for Ru volatilization resulted in a complete recovery (up to 100%) in just 1.5 hours. Moreover, selective precipitation of Pt was accomplished with 5 M NH4Cl, resulting in up to 96% Pt recovery within 30 minutes while only minimal Ru precipitation occurred. This efficient recycling procedure allows for the reuse of obtained PGM salts as precursors for manufacturing new electrocatalysts.

SALIH AYDOGAN, MOHAMED TAHA OSMAN ABDELRAHEEM

This article describes the dissolution kinetics of metallic silver (Ag) in ammonium carbonate and hydrogen peroxide (H2O2) solution. The influences of temperature, rotation speed, H2O2 concentration, and ammonium carbonate concentration were investigated. The results indicate that ammonium carbonate concentrations between 0.025 and 0.1 M have a significant impact on the dissolution rate. The dissolution rate is positively impacted by H2O2 concentrations between 0.025 M and 0.10 M. Furthermore, there is a positive relationship between the dissolution rate of Ag and the rotation speed. Silver dissolves more readily at temperatures between 20 °C and 55 °C. However, a temperature greater than 40 °C led to the formation of a silver carbonate layer on the disc when using high concentrations of hydrogen peroxide and ammonium carbonate. The activation energy of 11.10 kJ/mol was calculated, supporting the validity of the Levich equation, which is predicated on the suggestion that mass transfer control describes the extraction rate.

MAHMOUD MOTASIM, MUSTAFA BOYRAZLI

Although cyanidation remains widely employed for silver recovery due to its cost-effectiveness, it presents serious environmental and health hazards. In this study, the dissolution kinetics of silver were investigated in an environmentally benign leaching system composed of DL-malic acid and hydrogen peroxide, employing the rotating disc method. High-purity silver discs (99.99 pct) were used to examine the effects of rotation speed, disc surface area, temperature, and the concentrations of DL-malic acid and hydrogen peroxide on the leaching rate. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were conducted to characterize the silver surface before and after leaching. Additionally, contact angle measurements were performed to evaluate the interfacial interaction between the leaching solution and the silver surface. The results demonstrated that the dissolution rate increased with temperature, rotation speed, hydrogen peroxide concentration, and surface area. However, elevated concentrations of DL-malic acid resulted in the formation of a passivating layer on the silver surface, likely due to chelation effects, which significantly impeded the access of protons (H+) to the silver surface, thereby reducing the dissolution rate. Contact angle analysis further indicated reduced wettability at higher DL-malic acid concentrations, with values reaching up to 87.80 deg. XRD analysis confirmed the partial oxidation of the silver surface, forming Ag2O. The reaction was determined to be chemically controlled, with an activation energy of 44.14 kJ/mol.

Mustapha A. Raji, Derik J. van der Westhuizen

Today, the rate of energy consumption is increasing tremendously. Uranium, a critical energy metal, plays a vital role in the advancement of the nuclear industry. In this investigation, the extent of uranium ore dissolution was examined under various experimental conditions, including leachant concentration, reaction temperature, and particle size. At optimal conditions (2.5 mol/L HNO3, 75 °C, -90 + 75 µm), approximately 92.1% dissolution rate was achieved within 120 min. The activation energy (Ea), estimated to be 19.67 kJ/mol, indicates that the reaction is a diffusion-controlled mechanism and more economical than other studies. However, the thermodynamic parameters, including ΔH, ΔG, and ΔS, respectively, were estimated. The positive value of ΔH affirms that the uranium dissolution by nitric acid solution is an endothermic process. Hence, this investigation provides a valuable reference for future studies on Bra uranium deposits (Nigeria) cum other uranium mines with similar mineralogical indices.

Tugba Deniz Tombal, Ilgin Kursun

The extraction of copper (Cu) from chalcopyrite (CuFeS2) using hydrometallurgical methods has garnered increased attention globally, particularly utilizing hydrochloric (HCl) and nitric acids (HNO3) as key lixiviants. This study aims to determine optimal leaching conditions for chalcopyrite, finding that dissolution efficiencies (DEs) of 95% for HCl and 96% for HNO3 were achievable under optimal parameters. Kinetic modeling revealed that the reaction rates were diffusion-controlled, with correlation coefficients of 0.9955 for HCl and 0.9956 for HNO3. Additionally, particle shape analysis post-leaching demonstrated a shift towards more circular morphologies, with HCl yielding less concave and more rounded particles, while HNO3 produced more angular and concave shapes. The findings indicated that optimal leaching can be achieved at a particle size of 63-75 µm, 15% solids ratio, 120 minutes of leaching time, and 250 rpm stirring speed. Variations in morphological properties were further influenced by lixiviant concentration, temperature, and type of acid used. This comprehensive analysis underscores the importance of suitable leaching conditions in enhancing chalcopyrite beneficiation efficiency.

Ayşegül Bilen Özkan, Burak Birol

The growing demand for rare-earth elements (REEs) has spurred interest in their recovery from waste materials, offering environmental and economic benefits. This study investigated the selective leaching of Y, Eu, and other rare-earth elements from phosphor-rich powder derived from fluorescent lamp waste, resulting in the recovery of Y–Eu mixed oxides and mixed oxides of Tb, La, and Ce. Conventional leaching examined the effects of temperature (25–80°C), time (30–360 min), acid type (HCl, HNO3, H2SO4), concentration (0.5–6 M), and solid-to-liquid ratio (1:100–5:100) on Y, Eu, and Ca leaching. Using 3.25 M HNO3 at 80°C for 90 min with a 3:100 S/L ratio, leaching efficiencies were 67.12% (La), 46.14% (Ce), 95.82% (Gd), and 40.04% (Tb). Incomplete dissolution of La, Ce, and Tb occurred due to stable phosphates and aluminates in blue and green phosphors. Conventional leaching residues were further processed via microwave-assisted leaching, optimizing temperature (80–160°C), acid type (HCl, HNO3), concentration (0.5–6 M), S/L ratio (1:100–5:100), and time (5–90 min). Optimal conditions (3.25 M HCl, 3:100 S/L ratio, 120°C, and 47.5 min) achieved recovery efficiencies of 96.81% La, 98.45% Ce, 97.16% Gd, and 97.83% Tb. Finally, Y-Eu oxides and Tb-La-Ce mixed oxides were obtained through calcination and oxalic acid precipitation from conventional and microwave-assisted leaching solutions, respectively.

FERIDE N. TU¨ RK, HASAN ARSLANOG˘LU

The extraction of cobalt (Co), nickel (Ni), molybdenum (Mo), and aluminum (Al) from an alumina-supported hydrodesulfurization (HDS) spent catalyst was examined using diethylene triamine pentaacetic acid (DTPA) as a chelating agent. To assess the impact of different metals on leaching efficiency, a roasting pretreatment was performed on powdered catalyst samples at various temperatures (300–700 °C) and durations (15–360 min). The morphological and textural modifications before and after roasting were characterized using scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis. The optimal roasting parameters were determined to be 600 °C for 180 min, under which the maximum metal extraction efficiencies were obtained: 71.05% for Mo, 80.06% for Co, 73.86% for Ni, and 15.33% for Al. Leaching experiments were conducted with a particle size range of +75 to -30 μm, a liquid-to-solid ratio of 15 mL/g, a DTPA concentration of 0.2 M, a leaching temperature of 60 °C, a duration of 180 min, and a stirring rate of 200 rpm. The findings highlight that both roasting temperature and time play a crucial role in enhancing metal dissolution from the spent catalyst.

D. Calla‑Choque, N. A. Trejo‑Hernández

Chalcopyrite is one of the most abundant copper minerals found in the Earth’s crust, typically processed through pyrometallurgical methods. This study investigates the effect of pH on copper leaching in a PO4 3−–H2O2 system, addressing the challenges posed by the high chemical stability of chalcopyrite. The results demonstrate that copper recovery is strongly dependent on both pH and H2O2 concentration. Specifically, at a pH of 3, increasing the H2O2 concentration to 0.5 M significantly enhances the kinetics of chalcopyrite oxidation at low solid-to-liquid (S/L) ratios (20–50 g/L), resulting in rapid and efficient copper recovery exceeding 90%. However, at higher S/L ratios (100–200 g/L), both the kinetics and overall efficiency decrease. Additionally, iron dissolution remained below 3%, with phosphate effectively enhancing copper recovery while minimizing iron solubility. These findings indicate a promising avenue for improving copper extraction processes from chalcopyrite using hydrometallurgical methods.

Faizan Muneer, Ida Strandkvist

The increasing availability of spent lithium-ion batteries (LIBs), which are rich in valuable metals such as nickel (Ni), cobalt (Co), and lithium (Li), makes them important secondary sources for metal extraction. This study focused on the recovery of Li from a flue dust generated during pyrometallurgical processing of NMC 622 battery material, where Li was present as Li2CO3 and LiF along with various impurities. To selectively extract Li, the flue dust was subjected to leaching with carbonated and limewater under varying temperature and S/L (solid/liquid) ratio. The leachates and leach residues were analyzed to determine Li recovery, co-leached impurities, and to identify possible factors limiting recovery. Leaching with carbonated water at an S/L ratio of 0.05 g/ml, 50 °C and pH of 7.5 resulted in a 70% recovery of Li over 60 min of leaching time, which reduced at higher S/L ratio and temperature. In contrast, when leaching with limewater at 75 °C and S/L ratio of 0.05 g/ml, 77% Li recovery was achieved within 10 min. During these conditions, the pH reached 10. SEM and XRD analysis revealed a CaCO3 precipitate such as calcite, aragonite, and vaterite in varying proportions, causing surface passivation and inhibition of the leaching reaction. Leaching in limewater not only yielded higher Li recovery but also resulted in lower concentration of co-leached impurities (sodium, potassium, aluminum, and zinc), as compared to carbonated water leaching. However, the impurity levels remained high, requiring further purification and separation to produce battery-grade LiOH·H2O.

Jere Vänskä, Mari Lundström

As the first generations of lithium iron phosphate (LFP) EV batteries face their end-of-life, increasing amounts of LFP-containing battery waste will enter the existing and development stages of hydrometallurgical recycling processes. This research investigates the oxidative and mild acid (0.05–0.1 M) leaching behavior of synthetic LFP black mass, i.e., LFP (LiFePO4) powder, in the presence of the typical impurities–Cu and Al–present in battery waste. Dissolved Fe, Dissolved LFP, Dissolved Cu, and Dissolved Al were observed as leaching responses using a Design of Experiments (DoE). Additionally, a Reference experiment at a higher acidity (1 M) was conducted. In the investigated conditions (T = 25 °C, t = 120 min, O2 feed), Li leaching resulted in similar final yields (t = 120 min) in all experiments (70–88 wt.%). Conversely, the leaching behavior of Fe varied substantially between experiments, with yields ranging from 5 to 76 wt.%. It was found that dissolved Fe2+ started to oxidize to Fe3+ and precipitate as phosphate almost immediately after dissolution. The presence of Al did not have any significant impact on the system, due to the passivation of the Al surface by Al2O3, whereas a high amount of Cu in the system was found to have a positive correlation with increased Fe2+ oxidation to Fe3+. This is assumed to be due to the related pH increase favoring Fe2+ oxidation and hindering Cu dissolution. Based on the results, predictive leaching models were built and an increased concentration of sulfuric acid was found to have a substantial effect, increasing leaching yield in all created models (Dissolved Fe, LFP, Cu, and Al).

Aylin Nur Erkmen, Roland Ulber

This study systematically concerns the leaching behavior and dissolution kinetics of gallium (Ga) with the objective of identifying sustainable leaching agents by incorporating organic acid reagents. A novel approach involving pH-adjusted experiments was utilized, elucidating the dissolution behavior and delineating acidolysis and complexolysis. The findings demonstrated the efficacy of oxalic acid (10 mM), which extracted 1105.2 ± 61.4 mg/L of Ga at pH 1.2. Dissolution kinetics based on the shrinking core model revealed a synergistic mechanism governed by film diffusion and surface chemical reaction control, with the latter dominating at higher temperatures. One-factor-at-a-time experiments clarified the influence of experimental parameters on Ga leaching yield, while the Box-Behnken design was employed to evaluate parameter interactions, confirming the significance of process parameters and interaction terms. Under optimized conditions—710.55 mM acid concentration, a reaction temperature of 84.5◦C, and a solid loading of 50 g/L—37% of Ga was effectively extracted within 3.2 hours. These findings underscore the selectivity and operational compatibility of oxalic acid compared to conventional leaching agents, highlighting its promising integration into biogenic production pathways and sustainable closed-loop gallium recovery processes.

Pit Völs, Thaís Veiga Barreiros

Solid oxide electrolyzer cells (SOECs) contain many critical and/or strategic elements such as Co, Mn, Ni, Sr and rare earth elements (Ce, Gd, La). To achieve a circular economy, an efficient recycling process needs to be developed. For electrolyte-supported cells, no mechanical separation processes come into question because of their brittle construction. Hence, a hydro-metallurgical approach was chosen in this work, starting with a leaching process of grinded membrane electrode assemblies from spent SOECs. Different leaching reagents (H2SO4, HNO3, HCl, citric acid, ammonia solution) with and without additives (Na2SO3, H2O2), which potentially support the reduction and thus the dissolution of higher oxides present, were tested. The best conditions were further investigated, resulting in an optimal leaching process using 4.5 M hydrochloric acid without the addition of any reducing agent at 60 °C for 2.5 h. The elements of the electrolyte (Hf, Zr, Y) were not leached under these conditions. Leaching efficiencies over 89% could be reached for all leached elements except Gd (leaching efficiency of at least 81%). Hence, an electrolyte separation is possible through this leaching process and both material flows (electrolyte residue and leaching solution) can be handled separately in further recycling steps.

Pavel Grudinsky, Ekaterina Vasileva

Copper-bearing Waelz slag (CBWS) is a solid by-product of the Waelz process, the disposal of which faces significant environmental challenges. In this study, oxidative sulfuric acid leaching was applied for the recovery of valuable elements from a CBWS sample containing 26.23% Fe, 0.82% Cu, and 0.81% Zn. Experimental leaching was conducted at temperature ranges, durations, and solid-to-liquid (S/L) ratios of 25–90 °C, 5–240 min, and 0.05–0.5 g/cm³, respectively. The consumption rates of H₂SO₄ and H₂O₂ ranged within 9.18–15.29 mmol/g and 0–7.35 mmol/g, which, at a 1:4:1 g/cm³/cm³ ratio, were equal to 225–375 g/dm³ H₂SO₄ and 0–250 g/dm³ H₂O₂, respectively. Various oxidants such as H₂O₂, MnO₂, air, oxygen, and Fe³⁺ ions were tested in the leaching experiments. The optimal leaching conditions were proven to be a temperature of 70 °C, duration of 180 min, S/L ratio of 0.2 g/cm³, and consumption rate of 13.4 mmol H₂SO₄/g. These leaching conditions led to the recovery of 96.1% Fe, 87.0% Cu, and 86.9% Zn with the addition of 2.94 mmol H₂O₂/g and 95.2% Fe, 84.7% Cu, and 67.5% Zn with the addition of 0.095 g MnO₂/g. These results suggest that metallic iron particles contained in a CBWS sample complicate copper dissolution.

Pavel Grudinsky, Ekaterina Vasileva

The limited availability of high-quality ore deposits and the environmental hazards of metallurgical wastes highlight the importance of developing resource-efficient metal recovery technologies. Zinc kiln slag (ZKS), also known as Waelz slag, a by-product material enriched in non-ferrous metals, was processed through oxidative HCl leaching with H2O2 as an oxidant. Thermodynamic simulation and laboratory experiments were applied to determine optimal leaching conditions to dissolve copper, zinc, and iron. Optimal leaching efficiency was achieved with consumptions of 0.8 g HCl and 0.1 g H2O2 per gram of ZKS, a liquid-to-solid (L/S) ratio of 5 mL/g, a temperature of 70 °C, and a duration of 180 min, which resulted in recoveries of 96.3% Cu, 93.6% Fe, and 76.8% Zn. The solid residue with 43.5 wt.% C is promising for reuse as a reductant material in pyrometallurgical processes. Copper and arsenic were separated from the leachate via cementation with iron powder, achieving recovery rates of 98.9% and 91.2%, respectively. A subsequent two-step iron precipitation produced ferric hydroxide with 52.2 wt.% Fe and low levels of impurities. As a result, the developed novel hydrochloric acid oxidative leaching and metal precipitation route for ZKS recycling provides an efficient and sustainable alternative to conventional treatment methods.

Martina Bruno, Carlotta Francia

Lithium recovery from Lithium-ion batteries requires hydrometallurgy but up-to-date technologies aren’t economically viable for Lithium-Iron-Phosphate (LFP) batteries. Selective leaching (specifically targeting Lithium and based on mild organic acids and low temperatures) is attracting attention because of decreased environmental impacts compared to conventional hydrometallurgy. This study analysed the technical and economic performances of selective leaching with 6%vv. H2O2 and citric acid (0.25-1 M, 25 °C, 1 h, 70 g/l) compared with conventional leaching with an inorganic acid (H2SO4 1 M, 40 °C, 2 h, 50 g/l) and an organic acid (citric acid 1 M, 25 °C, 1 h, 70 g/l) to recycle end of life LFP cathodes. After conventional leaching, chemical precipitation allowed to recover in multiple steps Li, Fe and P salts, while selective leaching allowed to recover Fe and P, in the leaching residues and required chemical precipitation only for lithium recovery. Conventional leaching with 1 M acids achieved leaching efficiencies equal to 95 ± 2% for Li, 98 ± 8% for Fe, 96 ± 3% for P with sulfuric acid and 83 ± 0.8% for Li, 8 ± 1% for Fe, 12 ± 5% for P with citric acid. Decreasing citric acid’s concentration from 1 to 0.25 M didn’t substantially change leaching efficiency. Selective leaching with citric acid has higher recovery efficiency (82 ± 6% for Fe, 74 ± 8% for P, 29 ± 5% for Li) than conventional leaching with sulfuric acid (69 ± 15% for Fe, 70 ± 18% for P, and 21 ± 2% for Li). Also, impurities’ amounts were lower with citric acid (335 ± 19 mg/kg of S) than with sulfuric acid (8104 ± 2403 mg/kg of S). In overall, the operative costs associated to 0.25 M citric acid route (3.17€/kg) were lower compared to 1 M sulfuric acid (3.52€/kg). In conclusion, citric acid could be a viable option to lower LFP batteries’ recycling costs, and it should be further explored prioritizing Lithium recovery and purity of recovered materials.

Abel Thekeparampil Varghese, Anjana Das

The critical role of NdFeB permanent magnets in the digital electronics and renewable energy sectors underscores the necessity for sustainable recovery methods for Neodymium (Nd). Traditional recovery techniques, predominantly reliant on solvent extraction, face drawbacks including high energy demands, extensive chemical use, and significant secondary organic waste generation. This study presents an innovative methodology employing Thioglycolic Acid and Choline chloride (TGA-ChCl) to synthesize deep eutectic solvents (DESs) for the leaching of Nd from spent NdFeB magnets. Vermiculite is introduced as an adsorbent in the subsequent recovery process. Key analyses, such as FTIR, confirm the formation of DES and its effectiveness in leaching, while NMR spectroscopy highlights the aliphatic hydrocarbon presence within DES. XRD and SEM analyses detail the crystallinity and structural characteristics of the materials involved.

Yusufujiang Mubula, Mingming Yu

Driven by the practical needs of reducing mining costs and protecting the environment, and with the growing focus on the green and efficient recovery of metal elements (Cu, Mn, Ni, Co, Li, V, Al, Fe, REEs) from mineral raw materials and secondary resources, ultrasonic-enhanced leaching has emerged as an effective method for achieving the resource recovery of the aforementioned metals. As the ultrasonic-enhanced leaching process can effectively recover metal elements from mineral resources and secondary resources, it can effectively reduce the energy consumption, shorten the recycling time, and effectively improve the efficiency of the recovery of metal elements in the recycling process. This paper provides a comprehensive overview of the latest references and scientific knowledge in the field of ultrasonic-enhanced leaching, classifies and summarizes the application of ultrasonic-enhanced leaching in the recovery of metal elements from mineral resources and secondary resources, and discusses the mechanisms of ultrasonic-enhanced leaching in detail.

Sara Saffaj, Diego Mantovani

Electronic waste (e-waste) is becoming a serious problem that impacts the environment due to its fast-growing volume. This rise is linked to high electronic and electrical equipment production to meet the increasing demand for high-end electronic devices. Conventional e-waste recycling approaches, including hydrometallurgy and pyrometallurgy, often involve substantial water and energy consumption and the generation of by-products, such as the emission of toxic gases or hazardous effluents. Within this context, solvometallurgy has emerged as a compelling alternative, whereby green non-toxic non-aqueous solvents, namely deep eutectic solvents (DESs), are used to extract and recover the metals with minimal water and harsh acid/base chemical use. The current study presents the solvo-leaching results of critical and strategic metals, i.e., copper (Cu) and nickel (Ni), and precious metals, i.e., gold (Au), from waste printed circuit boards (PCBs). Five different DESs were tested at mild conditions, namely at a temperature of 65 ◦C, a stirring speed of 300 rpm, a solid/liquid ratio of 10 g/L, and in the presence of iodine (I2) for 96 h. Among the different solvents tested, the one consisting of choline chloride (ChCl), acetic acid (AA), and I2 emerged as the optimal solvent, leading to the selective extraction of 99% of Cu, 92% of Ni, and 90% of Au from the PCB powder.

Fernando Sitorus, Elin Stromberg

The recycling of lithium-ion batteries (LIBs) is of critical importance due to the increasing demand for electric vehicles and portable electronic devices. It also helps address supply risks of critical raw materials and reduces the environmental impact of mining. Traditional recycling methods are often inefficient and environmentally damaging. This study investigates the use of deep eutectic solvents (DES), made from citric acid and choline chloride, for leaching valuable metals from black mass derived from spent LIBs. The research focuses on optimizing the leaching parameters, such as temperature, DES ratio, and solid-to-liquid ratio, to maximize the recovery of critical metals while minimizing environmental impact. The findings reveal that the optimized leaching process can efficiently recover significant amounts of critical metals, showcasing the potential for using DES as a sustainable alternative in the recycling of LIBs. These results contribute to the development of more eco-friendly recycling methods that support the circular economy and reduce reliance on virgin metals.

LINDA MAINA, KATARZYNA KIEGIEL

Phosphogypsum (PG) is a by-product of phosphate fertilizer production, consisting mainly of calcium sulfate dihydrate that retains toxic trace elements and radionuclides, leading to environmental contamination concerns due to the 200 million tons of PG produced annually. This study focuses on the hydrometallurgical recovery of critical rare-earth elements (REEs) from PG sourced from a former chemical plant in Wizo´w, Poland. Leaching experiments were conducted using various solvents to assess REE extraction efficiency, with elemental analysis performed via inductively coupled plasma–mass spectrometry (ICP-MS). Total REE concentrations (RREEs) were found to range from 0.04 to 1.57% w/w, demonstrating the potential of PG waste as a secondary resource for REE recovery. The findings support sustainable hydrometallurgical processing of PG, which offers environmental benefits by reducing waste and contributes to raw material security in the European Union (EU) context, where REEs are listed as critical raw materials due to their economic significance and growing demand in high-performance technologies.

Darwin Michell Cheje Machaca, Rosario Belen Juyo Salazar

Tin slag is a problematic residue with high economic value that is produced during the refining process of crude tin. Due to the highly refractory nature of these materials, the industrial extraction of the metals contained in the matrix depends on leaching with hydrofluoric acid, which poses problems in the handling and safety of materials. To minimize the effects related to the processing of these materials, a theoretical and experimental investigation was carried out to develop a process through thermal treatment, followed by aqueous or oxidative leaching to obtain a Pregnant Leach Solution (PLS) with the target metals. Here, an approach based on a thermodynamic simulation model is proposed to evaluate the optimal conditions for the maximum extraction of the Nb–Ta and Zr–Hf systems. The experimental results revealed that thermodynamic simulations allowed the identification of the ideal conditions for the formation of sulfates at 200°C, with a slag–H2SO4 ratio (g/mL) of 1:4 in a treatment time of 6 h, followed by two leaching routes. Aqueous leaching with a solid–liquid (S/L) ratio (g/mL) of 1:10 at 90°C for 2 h resulted in a recovery of 97% Nb, 61% Ta, 62% Zr, and 75% Hf. Meanwhile, oxidative leaching with a 0.5 M concentration, S/L ratio of 1:10 at 90°C for 2 h, resulted in a recovery of 81% Nb, 99% Ta, 98% Zr, and 99% Hf, confirming the efficiency of the process. This study highlights the importance of thermal treatment, thermodynamic modeling, water content, and the addition of oxidants in effective extraction, providing a pathway to optimize the recovery of economically valuable metals from tin slags.

Stefanie A. Hetz, Axel Schippers

Biohydrometallurgical laterite processing was explored at laboratory scale in this study. The extraction of nickel, cobalt, and other metals from six different laterite ores from three Brazilian deposits was tested via bioleaching in 2 L stirred tank reactors under aerobic conditions with acidophilic, sulfur-oxidizing bacteria and archaea. The experiments were run with three different mixed cultures, either a consortium of six different Acidithiobacillus (At.) thiooxidans strains at 30 °C, of five Sulfobacillus species at 45 °C, or a consortium of Archaea at 60 °C. The At. thiooxidans consortium resulted in the far best bioleaching performance in case the pH was not kept constant at 1.5 by NaOH addition. This led to low pH values of even < 1 in some bioreactor runs at the end of the experiments, and a maximal extraction of 83% Co and 83% Ni for 10% (w/v) pulp density of a laterite sample from the Barro Alto mine after 15 days was achieved. Another sample achieved a similar high Co extraction but much lower Ni extraction with bioleaching by At. thiooxidans, whereas the third sample from Barro Alto did not show increased Ni and Co extraction in comparison to the chemical control with the same pH. Overall, for laterite samples very rich in iron(hydr)oxides or silicates, the metal dissolution was only related to acid leaching because of a similar bioleaching and chemical leaching performance at the same pH each.

Reza Shokoohi, Roya Najafi‑Vosough

Bioleaching technology is being developed as a cost-beneficial and environmentally friendly strategy for the removal of metal ions from sewage sludge. Therefore, this study aims to evaluate the feasibility of bioleaching process for the removal of heavy metals (Pb, Al, Zn, and Cu) from sewage sludge using indigenous iron-oxidizing microorganisms. For this purpose, 10 mL of a sludge-enriched inoculum of iron-oxidizing bacteria with 2 g FeSO4.7H2O was added to a 250-mL Erlenmeyer flask containing 100 mL of the sludge mixture from the secondary and primary sedimentation unit (34.78 g L−1 solids). The samples were stirred in a shaking incubator at 28 °C and 120 rpm for 15 days and analyzed for pH, oxidation–reduction potential (ORP), Fe3+ and heavy metal concentration. In parallel, control samples were performed with 100 mL of sludge sample without the addition of the sludge-enriched inoculum and energy source. It was observed that the concentration of heavy metals decreased continuously over the period of 15 days and the removal efficiency reached 62.7%, 80.7%, 43%, and 75.5% for Al, Cu, Pb, and Zn, respectively. On the other hand, the changes in selective heavy metal removal in the control experiments are very negligible. The results also showed that the specific filtration resistance (SRF) of the sludge decreased by 66.87%. In addition, the results of the SEM, EDX − mapping and XRD analyses before and after the bioleaching experiments confirm the activities of the microorganisms. The results clearly showed that the bioleaching process is an effective approach for removing heavy metals from sewage sludge as well as improving sludge dewatering and reducing the subsequent costs for the dewatered sludge disposal.

Ryusei Takimoto, Cindy

Double refractory gold ores (DRGOs) present significant challenges in gold recovery due to the encasing of gold grains by sulfides and carbonaceous matter, leading to a phenomenon known as “gold robbing.” Traditional methods involving sequential pretreatment of DRGOs have shown challenges, prompting the need for more efficient approaches. This study aims to explore a novel one-step bio-oxidation treatment designed to liberate gold grains by employing a mixed culture comprising an iron-oxidizing bacterium (Acidimicrobium ferrooxidans), a sulfur-oxidizing bacterium (Acidithiobacillus caldus), and an acidophilic heterotroph (Alicyclobacillus acidoterrestris) under strongly acidic conditions. The methodology allowed for the complete removal of sulfide minerals at low pH, leading to the successful exposure and liberation of gold grains. This one-step process represents a significant advancement in treating DRGOs, simplifying what is typically a complex multistep procedure and demonstrating substantial improvements in processing efficiency. The findings suggest that utilizing a mixed culture in bio-oxidation can effectively enhance the recovery of gold from refractory ores, potentially transforming the approach to processing DRGOs in metallurgical practices.

Tariq M. Bhatti, Olli H. Tuovinen

The purpose of this study is to review the role of filamentous fungi in the bioleaching of uranium from ores and rocks. Specifically, the paper evaluates how fungi produce short-chain fatty acids through fermentation and partial oxidation of organic compounds, which facilitate the dissolution and sequestration of uranium. This review highlights the efficacy of various ore types, including oxide, carbonate, and silicate ores containing hexavalent uranium, suitable for zinc leaching processes. Two primary methods of uranium leaching employing fungi are explored: the one-step process, where biomass is not separated prior to contacting the minerals, and the two-step method, which utilizes spent culture media after fungal biomass removal. The significance of carboxylic acid production, particularly by genera such as Aspergillus and Penicillium, in forming soluble U(VI)-carboxylate complexes is discussed. Key results indicate that maintaining a low pH level (between 1.5 and 3.5) in leach solutions enhances the leaching efficiency by promoting proton attack on minerals and minimizing metal precipitation. Notably, this article is the first comprehensive review of the application of filamentous fungi in uranium bioleaching from ores and rocks, identifying the gaps in process optimization efforts in existing literature.

Song Wen He, Felipe Alejandro Garcia Paz

The present study explores the development and evaluation of precious metals (PM) recovery processes designed to extract silver (Ag), gold (Au), and palladium (Pd) from copper anode slimes (CAS) generated via the black copper route from waste printed circuit boards (PCBs). Two conceptual process flowsheets—Route 1 (conventional) and Route 2 (polar aprotic solvent)—were designed and simulated adopting established and novel metallurgical methods using FactSage™ and HSC Chemistry 10 software. Both routes employed leaching-precipitation techniques; however, Route 1 utilized conventional acid-salt precipitation methods, whereas Route 2 incorporated with a novel approach based on polar aprotic solvent leaching. The results showed that the simulated recovery rates for Ag, Au, and Pd were high across both processes, achieving 94.8%, 99.1–99.3%, and 99–99.2%, respectively. To evaluate the environmental impacts, a life cycle assessment (LCA) was conducted using openLCA, with the treatment of CAS in kg as the functional unit and a gate-to-gate system boundary. It was found that Route 1 had lower environmental impact potentials, yielding acidification potential (AP) of 0.52–0.93 kg SO₂ eq, eutrophication potential (EP) of 0.67–1.21 kg PO₄ 3⁻ eq, and global warming potential (GWP) of 61.83–110.4 kg CO₂ eq. Conversely, Route 2 exhibited higher AP (0.59–1.26 kg SO₂ eq), EP (0.7–1.33 kg PO₄ 3⁻ eq), and GWP (75.1–134.9 kg CO₂ eq), primarily due to the use of CuCl₂, DMF, and ethanol in the Au and Pd recovery steps.

İlker Acar, Özlem Gülmez

Nickel’s versatile properties and growing demand, particularly in stainless steel and electric vehicle industries, highlight the need for environmentally sustainable recovery methods amid depleting high-grade reserves. This study explores the bioleaching potential of Trametes versicolor OG2 for nickel extraction from chromite tailings, olivine, and serpentine under varying pH (4.5–6.3) and retention times (3 and 7 days). Optimal recovery from olivine (41.82%) and serpentine (30.97%) was achieved at pH 5 after 7 days, underscoring the influence of extended processing. The fungus demonstrated high tolerance to heavy metals, including nickel and chromium. Organic acids, particularly oxalic and citric acids, produced by the fungus, were identified as key leaching agents, enhancing the extraction of nickel and iron. However, high chromite levels in serpentine reduced metal dissolution due to toxic hexavalent chromium. These findings position T. versicolor as a sustainable and effective agent for nickel recovery, offering a promising pathway toward environmentally friendly mining and heavy metal remediation.

Amr M. Mowafy, Sulochana M

This study aims to characterize and optimize the production of siderophores produced by Pseudomonas monteilii strain MN759447 and investigate its antagonism toward fungi associated with mortality in Dalbergia sissoo plantation forests. Siderophores, which are iron-chelating compounds, play a crucial role in plant growth under iron-limiting conditions. The research involved assessing the production conditions of siderophores, which include parameters such as temperature, pH, and iron concentration. Furthermore, the biological efficacy of the produced siderophores against specific fungal pathogens was evaluated. The results indicated that the optimized conditions for maximal siderophore production significantly enhanced the compound's effectiveness in inhibiting fungal growth, suggesting a potential application in biocontrol strategies for managing phytopathogenic fungi. The findings contribute to understanding the role of microbial siderophores in agricultural practices and suggest new avenues for their application in sustainable pest management. Overall, this study highlights the importance of microbial interactions in ecology and agriculture, addressing a gap in research on the functional properties of siderophores in relation to plant health and productivity.

Anil Kumar, Alan Shemi

The present study investigated and optimized biogenic cyanide (bio-CN) production by using mine indigenous bacterial strains with the aim of extracting precious metals from Upper Group Two (UG-2) platinum group metals (PGM) ore. Among all the bacterial strains tested, under optimized conditions, Pseudomonas brassicacearum produced the highest bio-CN (14.1 ± 1 mg/L) and was therefore used in subsequent experiments. The study examined glycine consumption during cyano-genesis, as well as the stability and speciation of bio-CN. The findings confirmed that while glycine was consumed during cyanogenesis, the produced bio-CN was not stable and decreased over time, transforming into different cyanide species. The dose–response analysis for P. brassicacearum showed EC50 of 1785 g/L and 107.9 g/L against untreated and pre-treated PGM concentrate, respectively, suggesting higher toxicity for the latter. Two-step bioleaching of the pre-treated PGM concentrate showed extractions of 75.7, 18.7, 9.4, and 0.3% for Au, Pd, Rh, and Pt, respectively, at 10 g/L pulp density. The findings on bio-cyanide stability and bio-CN speciation presented in this paper are novel contributions. This study has established a foundation for developing a complete bio-based approach to PGM bioprocessing from UG-2 ores.

Daniel Kupka, Lenka Hagarová

This study describes the bioleaching of a flotation concentrate by acidophilic iron- and sulfur-oxidizing bacteria at ambient temperature in shake flasks. The concentrate was prepared from polymetallic Ag–Cu–Sb–siderite ores from Rožňava (Slovakia) and consisted of tetrahedrite and siderite, with small amounts of chalcopyrite, pyrite, and quartz. Biologically driven Cu and S extraction was significantly greater than the extraction in the abiotic control. The oxidation of 1 mol of tetrahedrite (nominally Cu10+Cu22+Sb43+S132−) to produce sulfate involved the transfer of 122 mol of electrons. The stoichiometry of the reaction was confirmed by gas analyses and the concentration profiles of the oxidation products and can be described by the equation Cu12Sb4S13 + 122Fe3+ + 52H2O → 12Cu2+ + 4Sb5+ + 13SO42− + 122Fe2+ + 104H+. The rate of bacterial ferrous iron oxidation highly exceeded that of the chemical reduction of Fe3+ at the tetrahedrite surface, as indicated by the high Fe3+-to-Fe2+ ratio in the leaching liquors. The sulfur entity (S2−) of tetrahedrite was oxidized ultimately to sulfate. After 120 days of tetrahedrite bioleaching, the concentration of Cu in the leaching liquor exceeded 4 g L−1, which is equivalent to > 80% metal yield. While the recovery of Cu and S increased proportionally with time, the Sb concentration approached 40 mg L−1 and subsequently decreased, which indicates the formation of secondary antimony minerals. Electron microprobe analyses of the solid residues showed that the alteration products are distinctly depleted in Cu and S and enriched with Sb and Fe when compared to the primary tetrahedrite.

Bienvenu Mbuya, Elvis Fosso‑Kankeu

The mineralogical characteristics of Copperbelt ores undergo continuous stratigraphic changes due to ongoing mining activities and weathering changes. These variabilities in mineralogy lead to drastically different behaviors in hydrometallurgical processing during the recovery of Cu and Co. Consequently, the technologies currently used in several hydrometallurgical industries have demonstrated their limitation in the effective processing of these ores. This paper focuses on the thermodynamic approach to assess the possibility of extracting Co and Cu from a mixture of ores characterized by Cu–Co oxide and Cu sulfide streams. To achieve the purpose of this investigation, the first phase consisted of thermodynamic predictions, while the second focused on experimental analysis. The thermodynamic feasibility of simultaneous Co and Cu extraction was assessed using potential–pH predominance and speciation diagrams of an actual leaching system. Thermodynamic calculations were used to construct Eh–pH and species distribution diagrams for the Co–Cu–Fe–SO4^2−–H2O system under standard leaching conditions. Software tools such as Hydra-Medusa, HSC Chemistry, Phreeqc, and Geochemist’s Workbench were employed to calculate thermodynamic equilibria. The redox behavior of Fe species at pH 1.5 was also investigated to account for the chemistry and potential mechanisms of minerals during the leaching of the mixed system. Results revealed that experiments conducted under a free and average redox potential of 440 mV, pH of 1.5, solid percentage of 10%, stirring speed of 600 rpm, and temperature of 50 °C for 2 h led to a leaching efficiency of 86 and 70% Co and Cu, respectively. The dissolution of Co and Cu in the mixed system was promoted by the impact of chemical reactions involving the contribution of H+, HSO4^−, and H2SO4, as well as redox reactions involving the Fe2+/Fe3+ redox couple and the galvanic interactions between oxidized and sulfidic minerals.

Kota Takatori, Hidekazu Kato

Chalcopyrite (CuFeS2) is recognized as the most refractory copper mineral when processed in conventional sulfate media leaching systems. This study investigates the efficacy of using a dimethyl sulfoxide (DMSO) solution combined with copper chloride (CuCl2) for the leaching of chalcopyrite. Leaching experiments were conducted on a copper concentrate predominantly composed of chalcopyrite under ambient pressure, with temperatures ranging from 313 to 413 K. The resulting leaching fractions of copper (Cu), iron (Fe), sulfur (S), gold (Au), and arsenic (As) were examined. Findings indicate that 90% of copper was extracted within 2 hours, and 94% within 4 hours at 373 K, demonstrating competitive efficiency compared to traditional processes. The DMSO solution with CuCl2 selectively dissolved the valuable metals Cu and Au from chalcopyrite, while leaving economically less valuable minerals, such as pyrite (FeS2) and As, in the residue. Additionally, it was observed that chalcopyrite oxidation proceeds in direct proportion to the stoichiometric ratio of Cu in the concentrate to the initial cupric ions in the DMSO solution, which is further enhanced by the presence of oxygen at temperatures below 373 K.

Patrícia Leitão, Aurora Futuro

The average global recovery rates for processing scheelite ores by physical methods typically range from 50 to 70%, and achieving a minimum grade of 60% WO3 in concentrates poses significant challenges. This research aimed to explore direct hydrometallurgical processing as a viable economic alternative for low-grade concentrates and medium- to high-grade ores, thereby enhancing recovery rates while minimizing penalties associated with concentrate grades. Specifically, the study focused on sodium carbonate leaching, which offers advantages like selective dissolution of problematic metals while facing challenges related to high temperature and pressure requirements. To assess feasibility, various operational parameters—including temperature, pressure, and reagent concentration—were rigorously tested using samples from both skarn and chalcopyrite-scheelite ore bodies. Results indicated that optimized operating conditions, despite being milder than industry norms, resulted in higher carbonate consumption but allowed for effective leaching. This research paves the way for potential industrial application by facilitating straightforward scale-up to pilot-plant design and optimization, making alkaline pressure leaching a promising method for scheelite processing.

Selma Fabiana Bazan, Hélio Anderson Duarte

Context Chalcopyrite is the most abundant copper sulfide mineral and is becoming increasingly important for meeting the growing demand for copper. However, its slow dissolution during the leaching process remains a technological challenge, in which the key step is the oxidation of chalcopyrite. The initial stages of chalcopyrite oxidation by Fe3⁺(aq) were investigated on the sulfur-terminated (001) and (112) surfaces. Adsorption of the [Fe(OH)3(H2O)2] complex is stronger on the (001)-S surface (−18.8 kcal mol⁻1) than on (112)-S (−13.4 kcal mol⁻1), forming bidentate bonds that promote electron transfer from the surface to the oxidizing agent. The subsequent redox step proceeds through a hydrogen-transfer mechanism, in which water undergoes homolytic dissociation to yield Fe2⁺–OH2 and an ∙OH radical that attacks surface sulfur atoms, forming S–OH species. This process is both thermodynamically and kinetically favorable on (001)-S (ΔE = −6 kcal mol⁻1; Ea = 11 kcal mol⁻1) but strongly hindered on (112)-S (Ea ≈ 80 kcal mol⁻1). The high stabilization of reaction products on (112)-S likely promotes surface passivation, which may explain the kinetic limitations observed experimentally during chalcopyrite leaching. Comparatively, although O2 is a stronger oxidant, its low solubility in aqueous media limits its effectiveness relative to Fe3⁺.

Lin Li, Aaron King

Ultrasound-assisted chalcopyrite leaching has been reported previously, showing a noticeable improvement in leaching kinetics. However, the effect of ultrasound on the chalcopyrite oxidation kinetics from an electrochemical perspective has not been addressed. This study examines the ultrasonic enhancement of chalcopyrite oxidation kinetics in sulfuric acid solution from both conventional leaching and electrochemistry aspects. Electrochemical techniques, including linear sweep voltammetry (LSV) and chronoamperometry (CA), were used to illustrate the kinetics of chalcopyrite ultrasound-assisted leaching. Tafel analysis by LSV showed that 20% amplitude ultrasound power had increased the chalcopyrite electrochemical dissolution rate by about 20% in both Fe3+-free and 10 mM Fe3+-containing 0.5 M sulfuric acid solution. The CA tests indicated a drastic increase in the Fe3+ reduction reaction when ultrasound was applied (20% amplitude). At 0.5 V, the Fe3+ to Fe2+ reduction current density at 30 min drastically increased from -65.54 without ultrasound to -1165.84 µA cm−2 with ultrasound.

Hiva Farhadi, Faramarz Doulati Ardejani

In this study, copper extraction from low-grade oxide-sulfide ores was investigated using a leaching method combined with response surface methodology (RSM) to optimize operational conditions and assess leaching kinetics. Given copper’s extensive industrial applications, sustainable recovery from low-grade ores is critical. Five key parameters—acid concentration, leaching time, particle size, temperature, and solids percentage—were identified as major influences on copper recovery. The results revealed that leaching time and solids percentage, along with interactions between temperature-time and temperature-solids percentage, had the most significant effects. Optimal conditions for 80% copper recovery while minimizing iron recovery below 3% included an acid concentration of 1.21 mol L−1, a leaching time of 108 min, a particle size of 438 μm, a temperature of 45 °C, and a solids percentage of 18.2%. Leaching kinetics were analyzed using shrinking core models, with the Dickinson model best describing the process, showing an activation energy of 32.63 kJ mol−1, indicative of mixed diffusion and chemical reaction control. The final kinetic model effectively predicted the influence of key parameters. These findings highlight the importance of optimizing process variables and selecting suitable kinetic models to enhance extraction efficiency, reduce costs, and improve sustainability in copper recovery.

Mikail Başyiğit, Elif Uzun

Hydrometallurgical copper production from chalcopyrite presents a significant challenge: the passive layer. This layer renders the dissolution behavior of chalcopyrite refractory, creating resistance to dissolution in acidic or alkaline media. To surmount this obstacle, various activation methods can be employed, with heat treatment being one of the most prominent for eliminating the passive surface of chalcopyrite. Depending on the applied temperature, heat treatment converts chalcopyrite into different mineral phases. Since the aim is to produce water-soluble phases, temperature monitoring and optimization are crucial. To achieve this, a novel methodology based on the optical properties of the heat-treated samples is proposed. In this context, chalcopyrite concentrates were subjected to heat treatment for 3 h at 10 different temperatures varying between 300 °C and 700 °C, in 50 °C increments. The resulting mineralogical transformations were subsequently determined by XRD analysis and quantified by their respective peak intensities. Image analysis was conducted to evaluate the relationship between mineralogical composition and visual appearance. The acquired images were converted into numerical data using image analysis and subsequently characterized as chromaticity. The findings indicate strong correlations between chromaticity and mineral peak intensities, demonstrating a successful modeling of the mineral transformation process.

Sameera Mohamed, Elizabet M. van der Merwe

This study compared the reactivity of a plagioclase concentrate subjected to two processes: (1) direct acid leaching and (2) thermochemical treatment with ammonium sulfate followed by leaching. The sample was prepared from coarse-grained pyroxenite rock retrieved from the Bushveld Igneous Complex, South Africa. It contained 78% plagioclase (labradorite), 9% orthopyroxene (enstatite) and 13% quartz. The elements contained in the concentrate were categorized into three groups based on their susceptibility to direct acid extraction after 6 h of leaching. Group 1 consisted of the highly reactive main elements of plagioclase (Al, Ca and Na, with extraction efficiencies of 95%, 89% and 81%, respectively). Group 2 included elements predominantly present in enstatite (Mg and Fe with extraction efficiencies of 41% and 55%, respectively). Group 3 was composed of slowly extractable Si (25%) from mostly plagioclase. Increasing the duration of direct acid leaching to 24 h had no effect on the extraction of Group 1 elements, whereas the extraction of Mg and Fe (Group 2) increased to >60%, and that of Si (Group 3) increased from 25 to 80%. The latter correlated with the nearly complete disappearance of the plagioclase blueprint in the XRD pattern of the residues generated after 24 h of leaching. In contrast, plagioclase had limited reactivity with ammonium sulfate during thermochemical treatment. Direct acid leaching of plagioclase-rich tailings can therefore generate leachates to be used as precursors for the preparation of value-added products, such as silica nanoparticles via a sol–gel route and calcium aluminate nanoparticles via solution combustion.

Krzysztof Gargul, Bożena Boryczko

Coppermaking from sulfide concentrates entails two major steps: smelting and converting. In continuous direct-to-copper smelting process these two steps are combined into one. The principal advantages of this process are: isolation of SO2 emission to a single, continuous, SO2-rich gas stream, minimization of energy consumption and minimization of capital and operating costs. Disadvantages of the process are that about 25% of the Cu entering a direct-to-copper smelting furnace ends up dissolved in the slag (when compared with < 10% in traditional Peirce–Smith converting) and the cost of recovering this Cu is significant. Decopperization process is based on the reduction of cuprous oxide and other metals, mainly lead and iron, in the liquid state in an electric furnace in the presence of coke and technological additives. This paper presents the results of laboratory tests on flash smelting slag leaching with sulfuric acid solutions. Hydrometallurgical treatment of the slag could be an alternative route to the presently used way of processing. The influence of a number of leaching parameters such as sulfuric acid concentration, amount of H2O2 added, liquid to solid phase (l/s) parameter and process temperature on the copper leaching efficiency was investigated. Under optimized process conditions, 95.6% of the copper contained in the original sample of slag was transferred into a solution. The experimental results obtained in the study were supplemented with the analysis of the kinetics of the copper leaching process from the flash smelting slag. The commonly known from the literature diffusion model and chemical reaction model were used. The activation energy of copper leaching from flash smelting slag was estimated in the range from 12.77 to 17.34 kJ/mol.

Zhiming Yan, Zushu Li

Global concerns about climate change are driving increased demand of electric vehicles for sustainable transportation and turbines in emerging energy solutions, where permanent magnets (PMs) and rare earth elements (REEs) play a critical role. However, global REEs recycling rates are only 3% and 8% for light and heavy REEs, respectively. This work proposes an effective approach to separate the REEs and iron via high-pressure selective leaching by low-concentrated nitric acid from the end-of-life NdFeB magnet and investigates the impurities behavior during the leaching and precipitation steps. The results from the optimized leaching conditions demonstrated over 95% REEs leaching efficiency with less than 0.3% Fe dissolution. Approximately 70% of Al and B were leached as well, while other elements (Co, Ni, Cu) had leaching efficiencies below 40%, leaving a hematite rich residue. Adjusting the pH removes Al and Fe in leachate but minimally affects Cu, Co, and Ni. Na2S addition is more effective against transition metals, but both methods result in around 10% REEs loss. Direct oxalate precipitation is suggested for the obtained leachate, which can yield over 97.5% REEs oxides with approximately 1.0% alumina, which is acceptable for magnet remanufacturing due to the aluminum content commonly found in magnets. The technology developed in this study offers opportunities for closed-loop recycling and remanufacturing of PMs, benefiting the environment, economy, and supply chain security.

Edzhe Soylu, Elif Emil‑Kaya

The sustainable management of municipal solid waste incineration (MSWI) fly ash (FA) and air pollution control (APC) residues remains challenging due to their hazardous nature and underutilized valuable metal content. This study evaluates sulfuric acid digestion for selective metal extraction method that minimizes calcium dissolution, supporting ash valorization. The process parameters (temperature, solid-to-liquid ratio, H2SO4 concentration, digestion time) were investigated for five different ashes (A–E) resulting from different MSWI technologies (rotary kiln, grate furnace and fluidized bed furnace). At 40 °C, a liquid-to-solid ratio of 5 and 98% H2SO4, > 99% copper extraction was achieved for ashes B and C, while other samples showed 45–75% extraction. For ash E, highest copper recovery was obtained at 70–80% acid concentrations. Zn extraction exceeded 90% in all ashes except for Ash D, where the extraction was 42.5%. Calcium dissolution remained negligible (< 1%) across all samples, with XRD confirming anhydrite (CaSO4) as the main phase in residues, which could be further processed into gypsum. The process also prevented silica gel formation and yielded amorphous aluminum sulfate, offering potential for conversion to valuable products like gibbsite (Al(OH)3) or boehmite (AlOOH). These findings demonstrate the feasibility of a tailored acid digestion approach for recovering metals and valorizing FA/APC residues.

T. Punt, A. P. van Wyk

This study investigated the performance of citric acid as lixiviant for cathode material from end-of-life lithium-ion batteries (LIBs). Black mass containing 84.2 wt% MNC (LiNi0.45Mn0.4Co0.15O2) and 15.8 wt% LCO (LiCoO2) material was leached at solid-to-liquid ratios of 20, 50, and 100 g/L. Leaching with 1.5 M citric acid, 2 vol.% H2O2, and a solid-to-liquid ratio of 20 g/L at 95 °C extracted 90% Co, 95% Li, 94% Mn, and 94% Ni within 20 min. At the highest solid/liquid ratio of 100 g/L with 1.5 M citric acid, 10 vol.% H2O2, and 95 °C, 84% Al, 84% Co, 87% Li, 86% Mn, and 96% Ni were leached after 40 min, producing a leach solution containing 0.68 g/L Al, 20.0 g/L Co, 6.0 g/L Li, 13.6 g/L Mn, and 19.3 g/L Ni. Using stepwise addition of H2O2, instead of initial bulk addition, did not significantly improve the leaching efficiency but did reduce the required leaching time. Temperature control of the reactor was also more manageable with stepwise addition of H2O2 and gas evolution was less vigorous. It was observed that the leaching efficiencies of Co, Li, and Mn decreased slightly at solid/liquid ratios of 50 g/L and 100 g/L, while the leaching of Ni increased slightly. The solution leached at 20 g/L could be stored for 6 weeks without any spontaneous precipitation, however, solutions leached at 50 g/L and 100 g/L showed changes in concentration after 1 month’s storage, suggesting that processes with high solid-to-liquid ratios must ensure that solutions are not stored for extended periods of time.

Päivi Kinnunen, Teppo Riihimäki

The clean energy transition has increased the global demand of nickel sulfate used in Li-ion batteries. A short-term solution is to refine the nickel sulfate product from nickel intermediates. In the long term, new direct nickel sulfate production technologies are needed. This research focused on the modeling-based concept development of a novel direct hydrometallurgical nickel sulfate process consisting of chemical leaching, impurity removal by precipitation, solvent extraction, and crystallization as an alternative to the conventional nickel sulfate production route via a nickel matte intermediate. The conventional process route with the studied nickel concentrate had lower chemical consumption and waste production compared to direct hydrometallurgical process where approximately 60% of iron was leached consuming oxygen, and the following iron precipitation step consuming calcium carbonate resulted in a high amount of iron precipitate together with gypsum. However, hydrometallurgical alternatives are often suitable for lower ore grades or volumes and can recover copper as by-product metal. The biggest impacts on carbon footprint from chemical consumption in the direct hydrometallurgical process were generated in iron precipitation and oxygen use in leaching. With the studied nickel concentrate, pyrrhotite played a key role in both oxygen use and iron precipitation. In the leaching step, 68% of total oxygen consumption was related to pyrrhotite leaching, while in iron removal 73% of total iron originated from pyrrhotite. Thus, especially pyrrhotite removal prior to leaching needs to be developed to reduce the carbon dioxide footprint, when the pyrrhotite content in the material is high.

Pərvanə Babayeva, Arif Heydərov

With the advancement of technology, the depletion of primary resources is accelerating. While approximately 90% of global bauxite reserves are utilized for aluminum production, alunite ore has gained attention as a potential alternative due to its aluminum content and the relatively easily removable impurities it contains. However, for alunite to be considered an economically viable source, undesirable components such as phosphorus must be effectively removed. In this study, the selective extraction of aluminum from alunite was investigated through leaching with a sodium hydroxide (NaOH) solution. The effects of leaching time, temperature, and liquid-to-solid ratio were examined using the Box–Behnken design within the framework of Response Surface Methodology (RSM). The optimization criterion was defined as maximizing aluminum solubility while minimizing phosphorus dissolution. The optimal process conditions were determined as follows: NaOH concentration of 7%, leaching duration of 30 min, leaching temperature of 69 °C, and a liquid-to-solid ratio of 56 mL/g. Under these conditions, aluminum extraction efficiency was measured at 89%, whereas phosphorus extraction was limited to 18%. X-ray diffraction (XRD) analysis of the raw alunite ore and the leach residue obtained under optimal conditions indicated a reduction in the intensity of aluminum-associated peaks. Furthermore, scanning electron microscopy (SEM) analysis revealed the formation of an irregular morphological structure in the residue.

Oluwayimika Olasunkanmi Oluokun, Iyiola Olatunji Otunniyi

Sustainable resource utilization ultimately depends on total recovery during recycling operations. PCB leach pulp for target metal recovery contains ceramic and polymeric particles resistant to reagent attack. The investigation herein shows the physicochemical behavior of five polymeric particles namely green, red, black, white, and blue in gold chloride complex [AuCl4]− solution. Gold concentration change was monitored over time to study how the gold solution interacted with the polymeric particles and how the interaction conforms to known adsorption isotherm and kinetic models. The black and green polymeric particles have the highest adsorption per unit mass to about 1.81 mg/g and 2.18 mg/g, respectively. Four adsorption kinetic models evaluated for insight on the adsorption mechanism observed show Elovich and pseudo-second-order (PSO) models fit the kinetic data best compared to other models with correlation coefficients of 0.9867 and 0.9863, respectively. Of the seven adsorption isotherm models evaluated, Sips model shows best fit for the isotherm data with correlation coefficients of 0.9727 and 0.9328 for green and black particles, respectively. Isotherm and kinetic model analyses of the experimental data indicated physisorption and chemisorption as driving mechanism of the interaction between PCB polymeric particles and gold chloride complex aliquot.

Antti Porvali, Tuomo Mäkelä

In this work, previously produced black mass was treated in sequence by milling, water leaching, and sulfuric acid leaching. Two goals were set: first, whether milling, with reductive additives, could impart reductive phase changes which would allow Li extraction to water in the water leaching; second, co-leaching of the black mass with the additive, CoS, was explored as to ascertain whether synergistic effects could be detected in a leaching system composed of reductive (CoS) and oxidative (NMC oxides) materials in the presence of redox mediator (Fe2+/Fe3+ redox pair). It was found out that in all experiments, similar Li concentration was obtained despite milling. Fluoride analysis indicated high F concentrations in the water solution and implicated formation of insoluble products such as LiF(s). Water-soluble fluoride ended up in all fractions: water leaching filtrates (ca. 500 mg/L), acid leaching filtrates (ca. 800–1700 mg/L), and leach residues (ca. 68–382 mg/kg). In acidic leaching, CoS appeared to enhance extraction of elements from cathode-active materials, improving extraction from 78 to 92% for Ni under relatively mild leaching conditions (T = 30 °C, solid conc. = 100 g/L, [H2SO4] = 1.4 M).

Tamara A. Schueler, Paula F. de Aguiar

The imbalance between raw materials of high economic importance and their supply has increased the search for new approaches to obtain valuable elements from mining tailings. In this study, the extraction of copper, zinc, and lead from sulfidic tailing in sulfate–chloride media was investigated. A 33 Box–Behnken design was applied to evaluate three variables over a 4-h testing period: sulfuric acid concentration (0.01–1.0 mol/L H2SO4), sodium chloride (10–60 g/L NaCl), and temperature (20–70 °C). The design showed two optimum working regions: a combination of a high NaCl level, low H2SO4 level, and medium temperature level for lead leaching, while for copper and zinc, a combination of a medium–high H2SO4 level and a high temperature level. The concentration of NaCl had only a slight impact on their leaching. Based on these results, two-stage leaching was performed. The first stage was carried out under an experimental condition that favored the leaching of lead (60 g/L NaCl, 0.01 mol/L H2SO4, 45 °C, 1 h, 10:1 liquid-to-solid ratio), whereas the second stage maximized the leaching of copper and zinc (60 g/L NaCl, 0.5 mol/L H2SO4, 70 °C, 24 h, 10:1 liquid-to-solid ratio). The global leaching rate was 66.8 ± 3.0% copper, 84.1 ± 5.2% zinc, and 93.9 ± 3.2% lead. The iron and arsenic content were also leached by about 20 and 50% at the end of the second stage. The study demonstrated that the use of sulfate–chloride media in a two-stage leaching considerably improved the extraction of the desired metals and was, therefore, suitable for their recovery.

Zeynel Abidin Sarı

Zinc plant residue (ZPR) contains significant amounts of valuable metal (Zn, Pb, Ag, etc.) compounds, as well as various heavy metals and harmful compounds that pollute the environment. Processing such residues allows for the recovery and reuse of valuable metals, which is crucial for sustainable resource management. This study investigated a two-stage leaching process of Zn, Pb, and Ag recovery from ZPR. The first stage of ultrasonic-assisted leaching of ZPR was applied to produce an industrial selective zinc acetate solution. Leaching experiments were carried out with an ultrasonic device in the presence of acetic acid, known as organic acid. Under optimum leaching conditions, the extraction of Zn and Fe metals was obtained as 76.13% and 1.32% Fe, respectively. According to the Brunauer–Emmett–Teller (BET) analysis results on the original sample and ultrasonic leaching residue (ULR), the BET surface area and micropore area increased. However, the mean adsorption pore width decreased. In the second stage, conventional sodium chloride leaching was applied to recover lead and silver from the remaining solid after the first stage. Under the optimum conditions in this stage, 80.12% of Pb and 96.2% of Ag were extracted. The presence of coordination between Zn2+/AcO− (acetate) and Pb2+/Cl− complexes in the leaching solution was revealed by Raman spectroscopy. Finally, according to the characterization analysis of the final leaching residue, it was determined that iron oxides and silicate species accumulated in the solid. In conclusion, a significant reduction in the rate of pollution and toxic metals in ZPR was noticed.

Kimmo Kärenlampi, Emilia Väänänen

Allanite is a common REE-bearing silicate mineral, which is found in potentially mineable abundances in many areas but has not been economically exploited for the REEs in an industrial scale. Conventional methods, such as sulfuric acid baking at high temperatures (200 to 650 °C) followed by extended water leaching near boiling temperature, have proven effective in decomposing allanite but come with substantial economic and environmental drawbacks. This study presents an alternative approach, investigating the dissolution behavior of allanite at low temperatures through direct treatment of an allanite concentrate with sulfuric acid. Through parametric optimization, the study reveals the practical and selective dissolution of allanite from a relatively coarse concentrate sample (particle size distribution D50 = 327 µm) after a 3-h leaching period with a mild dilute (2 M) H2SO4 solution at 22 °C, yielding overall recoveries of approximately 80% for light REEs and 60% for heavy REEs. The dissolved gangue elements, predominantly derived from allanite (e.g., Th, Fe, Al, Si, and Ca), exhibit contents in the leach solution comparable to or slightly higher than those reported in other studies utilizing acid bake–water leach processes. The notable efficiency of low-temperature sulfuric acid extraction is likely attributed to the heightened chemical reactivity of metamict allanite-(Ce) within the investigated concentrate. While these results are encouraging and demonstrate the potential of low-temperature leaching for allanite concentrates, additional research is necessary before testing the procedure on a larger scale.

Rajiv Ranjan Srivastava, Nirma Ilyas

Large quantities of spent catalysts containing strategic metals such as molybdenum, nickel, cobalt, and vanadium, are lost after hydrodesulfurization of petroleum. Here, we review the recycling of those metals using bacteria and fungi. We analyze bioleaching approaches, utilizing both chemoautotrophic and heterotrophic microorganisms, and examine how various operational parameters influence the extraction process. The formation of soluble species in the metabolic lixiviant derived from high-sulfur feedstocks creates optimal conditions for the activity of sulfur-oxidizing microorganisms, such as Acidithiobacillus thioparus. In contact, bioleaching with Penicillium simplicissimum at a pH range of 4–7 promotes the formation of stable anionic molybdate, which is advantageous for the subsequent recovery process.

Mingbing Hu, Yajian Shao

Magnesium sulfate leaching of ionic rare earth ores is generally characterized by a smooth outflow curve, a long leaching time, and a high impurity content in the leach liquor. To reveal the leaching law of rare earth cations and impurity aluminum ions in the leaching process of ionic rare earth ores in magnesium sulfate, equilibrium leaching and leaching kinetics experiments were carried out using ore samples of five particle sizes. Furthermore, prediction models of equilibrium constants and rate constants were constructed based on ion-exchange theory. The results show that the equilibrium constants of the rare earth and aluminum ion-exchange reactions decrease gradually with the increase in the magnesium ion concentration, the decrease in the temperature, and the increase in the surface area of the particles. Moreover, the equilibrium constant prediction models of rare earth and aluminum with magnesium sulfate were constructed using data fitting. From the leaching kinetics experiment, there is a significant relationship between the reaction rate constant of ion exchange and the surface area of the particles: the larger the particle size, the smaller the reaction rate constant.

Lusa Lwa Vidie Kishiko, Willie Nheta

It has been demonstrated that the traditional hydrometallurgical method is still economically viable in several industrial applications such as Bayer, Boix, Platsol, Sherrit-Gordon, and so on. The conventional extraction technique of valuable metals from their ores using an aqua medium has several challenges. The following can be listed for the illustration of this: (1) Inorganic acids used during the leaching process have been proven to be non-environmentally friendly and ready to lead to non-selective processes in general, except in rare cases used in alkaline environments. (2) Special linings are required in the reactors used due to the corrosive impact of acids such as HCl and H2SO4, especially when leaching at high temperatures, rendering all processes costly. (3) Practically, using inorganic acids while leaching samples containing amorphous silicate phases leads to gel formation. Solvometallurgy overcomes these challenges by substituting the aqueous phase for other polar solvents, such as polar molecular organic or ionic solvents. The advantage of this substitution lies in the ability to manipulate metal ion distribution using solvents with varying solvation properties. This review examines the potential of solvometallurgical processes (solvoleaching) over conventional hydrometallurgy as viable alternatives for metal extraction from sulfide ores.

Mingbing Hu, Yajian Shao

Magnesium sulfate leaching of ionic rare earth ores is generally characterized by a smooth outflow curve, a long leaching time, and a high impurity content in the leach liquor. To reveal the leaching law of rare earth cations and impurity aluminum ions in the leaching process of ionic rare earth ores in magnesium sulfate, equilibrium leaching and leaching kinetics experiments were carried out using ore samples of five particle sizes (<0.10, 0.10–0.25, 0.25–0.50, 0.50–1.00, and >1.00 mm). Furthermore, prediction models of equilibrium constants and rate constants were constructed based on ion-exchange theory. The results show that the equilibrium constants of the rare earth and aluminum ion-exchange reactions decrease gradually with the increase in the magnesium ion concentration, the decrease in the temperature, and the increase in the surface area of the particles. Moreover, the equilibrium constant prediction models of rare earth and aluminum with magnesium sulfate were constructed using data fitting. From the leaching kinetics experiment, there is a significant relationship between the reaction rate constant of ion exchange and the surface area of the particles: the larger the particle size, the smaller the reaction rate constant.

Jovana Djoki´ c, Nataša Gaji´ c

Copper anode slime (CAS), obtained from non-standard anodes by pyro-hydrometallurgical electronic waste (e-waste) processing, contains high concentrations of lead, tin (as metas-tannic acid), and base (Cu, Fe, Zn), precious (Au, Ag), and technological metals (In, Ga, Ge), which limit the efficiency of conventional valorization methods. In this study, an integrated alkali fusion–leaching process was applied to non-standard CAS. Thermodynamic modeling defined the key parameters for selective phase transformations and efficient metal separation. These parameters were experimentally investigated, and the optimized fusion conditions (CAS:NaOH = 40:60, 600 °C, 60 min), followed by water leaching (200 g/dm3, 80 °C, 60 min, 250 rpm), resulted in >97% Sn removal efficiency. Simultaneously, Au and Ag losses were negligible, resulting in solid residue enrichment. Oxidant addition (NaNO3) further enhanced the recovery of precious metals.

Johannes J. M. M. van de Ven, Patrick J. Teeuwisse

The growing demand for lithium-ion batteries (LiBs) for energy storage has intensified the need for the critical raw materials (CRMs) they contain, including Li, Co, Ni, and Mn. Consequently, the incentive to recycle LIBs is increasing. However, the commonly used hydrometallurgical processes often have a significant environmental footprint. Moreover, the relatively low value of certain battery materials (e.g., LiFePO4, LFP) results in a limited incentive for their recycling. This study explores the simultaneous recycling of LFP with various types of LiNixMnyCozO2-containing Black Mass (BM). Leaching studies over time were conducted using stepwise additions of LFP and H2O2 solution (1 vol%) to a mild lixiviant of 0.63 mol/L H2SO4 at 50°C. For pristine NMC 532, ± 95% leaching of Li, Ni, Co, and Mn was achieved. The Fe(II) present in LFP, as well as H2O2, acts as a reductant for the dissolution of Ni, Co, and Mn, later precipitating as FePO4 to the leaching residue. The Al and Cu present in industrially treated BM further enhanced the dissolution of the transition metals via a catalyzed reaction with the iron from LFP. This resulted in complete leaching of Li, Ni, Co, and Mn for mechanically pre-treated industrial black mass samples. However, the leaching residues acquired from these samples were highly contaminated with graphite. Also, while pyrolysis of the black mass benefits the leaching of Co and Mn, it results in difficulties in subsequent removal of Fe from the pregnant leach solution. The chemical processes and their performance are described in this work.

Manish Kumar Kar, Mengyi Zhu

This study introduces a novel approach in sustainable metallurgy for the efficient utilization and valorization of bauxite residue, aimed at producing sustainable alumina and green steel. The integrated process combines hydrogen reduction, alkaline leaching, and smelting of the leaching residue. Initially, the bauxite residue was pelletized with calcite and quicklime to create self-hardened pellets, leveraging the cementing effect of quicklime with water. These pellets underwent hydrogen reduction, achieving over 95% reduction, resulting in the formation of metallic iron and a leachable calcium aluminate phase for alumina recovery. The reduced pellets were then subjected to alkaline leaching, extracting 62% alumina. Subsequently, smelting at 1550 °C facilitated the near-complete separation of iron and calcium-rich slag. The process was analyzed using various analytical techniques, including X-ray diffraction, electron probe microanalysis, and inductively coupled plasma mass spectroscopy, complemented by thermodynamic calculations using FactSage 8.1 software. Iron oxide reduction to metallic iron was achieved at 1000 °C for 120 min, while sodium carbonate leaching effectively extracted alumina from the calcium aluminate slag. However, residual alumina was attributed to the formation of indissoluble gehlenite and a dense calcium carbonate layer that impeded leaching kinetics. Successful iron separation during smelting required temperatures above 1500 °C, though this process was challenged by the high viscosity of the oxide matrix and the purity of the iron.

Sait Kursunoglu

Over the past two decades, risk management in the mining sector has advanced significantly, increasingly prioritizing sustainable practices throughout the lifecycle of mining operations. A key component of this shift is the recovery of valuable materials from mining and mineral processing tailings, particularly critical and secondary raw materials, through the application of advanced reprocessing technologies. This review provides a comprehensive analysis of recent developments in tailings reprocessing, with a focus on leaching, and innovative hydrometallurgical techniques. Drawing on case studies from copper, silver, tungsten, antimony, and gold mining industries, the review highlights the potential for secondary resource recovery in response to rising global demand for critical metals such as rare earth elements, lithium, cobalt, and nickel, which are essential for clean energy and modern technologies. It also discusses the challenges and limitations associated with current recovery practices and outlines future trends aimed at improving metal extraction efficiency, while mitigating environmental impacts. The integration of circular economy principles into tailings management not only enhances resource efficiency but also supports decarbonization, strengthens supply chains, and contributes to the achievement of sustainable development goals. Ultimately, the transition to sustainable waste management and critical metal recovery is essential for minimizing environmental risks and promoting long-term economic and ecological resilience in the mining sector.

Manish Kumar Kar, Mengyi Zhu

This study introduces a novel approach in sustainable metallurgy for the efficient utilization and valorization of bauxite residue, aimed at producing sustainable alumina and green steel. The integrated process combines hydrogen reduction, alkaline leaching, and smelting of the leaching residue. Initially, the bauxite residue was pelletized with calcite and quicklime to create self-hardened pellets, leveraging the cementing effect of quicklime with water. These pellets underwent hydrogen reduction, achieving over 95% reduction, resulting in the formation of metallic iron and a leachable calcium aluminate phase for alumina recovery. The reduced pellets were then subjected to alkaline leaching, extracting 62% alumina. Subsequently, smelting at 1550 °C facilitated the near-complete separation of iron and calcium-rich slag. The process was analyzed using various analytical techniques, including X-ray diffraction, electron probe microanalysis, and inductively coupled plasma mass spectroscopy, complemented by thermodynamic calculations using FactSage 8.1 software. Iron oxide reduction to metallic iron was achieved at 1000 °C for 120 min, while sodium carbonate leaching effectively extracted alumina from the calcium aluminate slag. However, residual alumina was attributed to the formation of indissoluble gehlenite and a dense calcium carbonate layer that impeded leaching kinetics. Successful iron separation during smelting required temperatures above 1500 °C, though this process was challenged by the high viscosity of the oxide matrix and the purity of the iron.

Lidia Garcia, Joan Morell

The rising demand for lithium-ion batteries (LIBs), driven by the growing consumption of electronic devices and the expansion of electric vehicles, is leading to a concerning depletion of primary metal resources and a significant accumulation of electronic waste. This urgent challenge highlights the need for sustainable recovery methods to extract valuable metals from spent LIBs, aligning with circular economy principles. In this study, the preparation of spent batteries for the bioleaching process was achieved with minimal manipulation. This included a preliminary discharge to ensure safety in subsequent processes and a brief crushing to facilitate the access of leaching agents to valuable metals. Unlike most studies that grind batteries to obtain powders between 70 and 200 microns, our approach works with particles sized around 5 mm. Additionally, our preparation process avoids any thermal or chemical treatments. This straightforward pre-treatment process marks a significant advancement by reducing the complexity and cost of processing. A systematic study was conducted on various fractions of the large particle sizes, using Fe (III) produced through bio-oxidation by A. ferrooxidans and biogenically obtained H2SO4 from A. thiooxidans. The highest metal extraction rates we.

Mengjie Hu, Deqing Zhu

Acid leaching is an effective method for dephosphorization; however, it is time-consuming and requires a high amount of acid consumption, resulting in increased production costs and environmental risks. This work aims to remove silicon, aluminum, and phosphorus from high-phosphorus oolitic magnetite concentrate through high-pressure alkaline leaching and ultrasonic acid leaching. Compared with traditional acid leaching processes, the sulfuric acid dosage can be significantly reduced from 200 kg/t to 100 kg/t, and the pickling time is shortened from 60 min to 10 min. Thermodynamic and kinetic studies have demonstrated that acid leaching facilitates apatite dissolution at low temperatures, whereas the dephosphorization reaction is controlled mainly by diffusion. The application of ultrasonic waves leads to finer particle sizes and greatly increased specific surface areas, thereby accelerating the diffusion rate of the leaching agent. Furthermore, microscopic analysis revealed that under the influence of ultrasonic waves, numerous micro-fragments are formed, enhancing the overall efficiency of the leaching process.

Esmaeil Jorjani, Jessica Sauvageau

Conventional lithium extraction from spodumene via sulfuric acid roasting can achieve up to 98% recovery but suffers from high energy use, acidic residues, and purification complexity. This review evaluates alternative methods for both α- and β-spodumene, aiming for improved sustainability. For α-spodumene, Na2SO4–CaO salt roasting achieved >95% recovery at 900 °C via water leaching. Sodium carbonate roasting–NaOH leaching and mechanical activation–Na2SO4 roasting reached 95.9% and ~92%, respectively. High-pressure NaOH leaching reached 95.8%, while alkaline decomposition–acid leaching yielded 53.2–84.1%. Microwave-assisted calcination achieved up to 97% recovery, and fluoride-based roasting gave 93–98% but raised environmental concerns. Bioleaching is eco-friendly but slow, with <3.5% recovery. For β-spodumene, high-pressure leaching using sodium carbonate (>94%), sodium sulfate (90.7%–93.3%), sodium chloride (~93%), and nitric acid (~95%) provided high recoveries. Atmospheric leaching with HF and H3PO4 yielded ~90% and ~40%, respectively, while carbonic acid leaching reached 75% after multiple stages. Chlorination roasting achieved near-complete recovery. The Metso-Outotec high-pressure sodium carbonate leaching process is set for industrial-scale implementation at the Keliber project in 2025, confirming its.