Optimization of process parameters and kinetics of fluoride extraction from spent potlining using response surface methodology

Over the years, spent potlining (SPL) treatment has only focused on the extraction of its hazardous compounds, especially fluorides and cyanides. The literature has not sufficiently addressed the optimization and kinetics of fluoride extraction using statistical modeling to determine relevant factors for efficient, cost-effective, and sustainable SPL treatment. Hence, this study is focused on response surface methodology (RSM) combined with central composite design (CCD) to statistically model fluoride extraction of SPL behaviour in acidic environments. Shrinkage core model (SCM) was used to investigate the kinetics of fluoride extraction. The RSM analyses suggested a second-order quadratic model with outstanding accuracy, statistically supported by R2 and adjusted R2 values of 0.986 and 0.973, respectively. The quadratic model indicates the main factors influencing fluoride extraction, showing the complex interactions of temperature, particle size, acid concentration, and leaching time. These main factors were observed to have significant effects on fluoride extraction, except for particle sizes of the SPL. The optimization process, a key success of this study, achieved fluoride extraction of 87.49% at specific factor levels of 48.43 °C, 0.752 mm, 1.2 M, and 10 min. Subsequently, the SCM investigations suggested that diffusion through a liquid film mechanism best approximates the fluoride extraction kinetic behaviour with R2 > 0.80 across varying temperatures. Investigations into temperature dependence with the Arrhenius plot further validated that the reaction kinetics were principally controlled by diffusion through liquid film, with an activation energy of 36.26 kJ/mol. Integrating these kinetic frameworks provides a novel approach to analyzing and optimizing SPL fluoride extraction. Overall, adopting the present study in the industrial settings with the optimized parameters will ensure efficient, sustainable, and cost-effective treatment of SPL.

Fly ash treatment via conventional and microwave-assisted organic acid leaching: kinetics and life cycle assessment

Heedless disposal of oil-based fly ash contributes to the contamination of the air, water, and soil. Acid leaching of industrial solid wastes is recognized as a versatile, cost-effective, and environmentally friendly solid waste treatment approach. The present study investigated the viability of conventional leaching (CL) and microwave-assisted leaching (MAL) of predominant heavy metals from Mazut-burnt fly ash. For this purpose, the practicality of four organic acids with various specifications (ascorbic, gluconic, citric, and oxalic acids) on the dissolution efficiency of fly ash components was examined. Utilization of oxalic acid led to achieving full V recovery, complete Fe removal, and Ni enrichment in the residue in both CL and MAL setups. The Ni content of the sample was enriched from 6% in the calcinated sample to 23.7% in the oxalic acid leaching residue. Using citric acid resulted in the co-extraction of V, Ni, and Fe with nearly 70% V, 50% Ni, and 89% Fe dissolved in CL. The dissolution efficiencies were slightly lower in MAL. Oxalic acid was selected as the most promising organic acid reagent for fly ash treatment, so its CL kinetics was studied and defined by the shrinking particle model. The model showed that the controlling steps in the leaching of V differ over time, changing from a chemical reaction before 60 min to fluid film diffusion or mixing afterward. The kinetic study proved MAL as an effective technique in overcoming the leaching kinetic barriers. A life cycle assessment study was conducted to determine the environmental impacts of the proposed process. Accordingly, the MAL using oxalic acid was the most environmentally friendly process among the studied ones, and the utilization of microwaves leads to the reduction of the leaching processes' environmental impacts by decreasing the processing time.

Water as a Sustainable Leaching Agent for the Selective Leaching of Lithium from Spent Lithium-Ion Batteries

The development of a sustainable recycling process for lithium from spent lithium-ion batteries is an essential step to reduce the environmental impact of batteries. So far, the industrial implementation of a recycling process for lithium has been hindered by low recycling efficiencies and impurities in the recycled material. The aim of this study is thus to develop an easy-to-implement recycling concept for the selective leaching of lithium from spent lithium-ion batteries with water as a sustainable leaching reagent. With this highly selective process, the quantity of chemicals used can be substantially decreased. The influence of the leaching temperature, the solid/liquid-ratio, the mixing rate, and the number of stages in multistage operation were investigated utilizing NCM-material. High leaching efficiencies and a high selectivity were achieved at moderate temperatures of 40 °C and a solid/liquid-ratio of 100 g L-1. In multistage operation, a selectivity for lithium higher than 98% was achieved with 57% leaching performance of lithium. XRD-measurements showed that lithium carbonate was quantitatively leached, while lithium metal oxides remained in the black mass. Finally, the leaching kinetics were determined, proving that the first leaching period is diffusion controlled and, in the second period, the leaching rate is rate controlling. This work confirms the concept of a green leaching process by which lithium can be recycled with a high degree of purity.

Selective recovery of gold and silver from electronic wastes through a sequential process of Qalkari and room-temperature hydrometallurgy

This work was focused on the selective recovery of gold and silver from electronic wastes using a sequential process of pyrometallurgy (Qalkari) and room-temperature hydrometallurgy. In the first step, electronic wastes underwent Qalkari recycling, yielding tablets containing precious elements (Qalkari furnace product) and melting slag (Qalkari furnace waste). In the subsequent hydrometallurgy phase, the nitric acid concentration and the input solid amount were optimized for the effective room-temperature recovery of gold. Due to the successful separation of precision elements and disturbing substances in Qalkari, the gold recovery efficiency of 99.99% was obtained at the acid concentration of 50% (v/v) and the solid input of 15% (w/v). Afterwards, HCl, NH4Cl, and NaCl were used for silver recovery from the Qalkari-processed gold-recovered leaching solution, leading to the efficiency of 99.99%. But NH4Cl was recognized as the most effective precipitant as it promises the most enhanced potential for the possible subsequent recovery of palladium. In conclusion, this study draws the effectiveness of Qalkari in recycling electronic wastes, with a significant impact on the efficiency of succeeding room-temperature hydrometallurgical processes for gold and silver recovery within a reasonable leaching time.

The effects of electrochemical pretreatment and curing environment on strength and leaching of stabilized/solidified contaminated sediment

Stabilization and solidification (S/S) is known to improve the structural properties of sediment and reduce contaminant mobility, enabling the utilization of dredged contaminated sediment. Further reduction of contaminants (e.g., tributyltin (TBT) and metals) can be done using electrochemical treatment prior to S/S and could potentially minimize contaminant leaching. This is the first study on how electrochemical pretreatment affects the strength and leaching properties of stabilized sediments. It also investigates how salinity and organic carbon in the curing liquid affect the stabilized sediment.The results showed that the electrolysis reduced the content of TBT by 22% and zinc by 44% in the sediment. The electrolyzed stabilized samples met the requirements for compression strength and had a reduced surface leaching of zinc. Curing in saline water was beneficial for strength development and reduced the leaching of TBT compared to curing in fresh water. The results indicate that pretreatment prior to stabilization could be beneficial in reducing contaminant leaching and recovering metals from the sediment. The conclusion is that a better understanding of the changes in the sediment caused by electrochemical treatment and how these changes interact with stabilization reactions is needed. In addition, it is recommended to investigate the strength and leaching behavior in environments similar to the intended in situ conditions.

Enhanced Dissolution of Metal Oxides in Hydroxylated Solvents - Towards Application in Lithium-Ion Battery Leaching

The recovery of critical metals from spent lithium-ion batteries (LIBs) is rapidly growing. Current methods are energy-intensive and hazardous, while alternative solvent-based strategies require more studies on their 'green' character, metal dissolution mechanism and industrial applicability. Herein, we bridged this gap by studying the effect of dilute HCl solutions in hydroxylated solvents to dissolve Co, Ni and Mn oxides. Ethylene glycol emerged consistently as the most effective solvent, dissolving up to four times more Co and Ni oxides than using aqueous acidic media, attributed to improved chloro-complex formation and solvent effects. These effects had a significant contribution compared to acid type and concentration. The highest Co dissolution (0.27 M) was achieved in 0.5 M HCl in 25 % (v/v) glycerol in water, using less acid and a significant amount of water compared to other solvent systems, as well as mild temperatures (40 °C). This solvent was applied to dissolve battery cathode material, achieving 100 % dissolution of Co and Mn and 94 % dissolution of Ni, following what was concluded to be a mixed mechanism. These results offer a simple alternative to current leaching processes, reducing acid consumption, enhancing atomic efficiency, and paving the way for optimized industrial hydrometallurgical processes leaning to 'greener' strategies.

Environmentally-friendly biorecovery of manganese from electrolytic manganese residue using a novel Penicillium oxalicum strain Z6-5-1: Kinetics and mechanism

Bioleaching is a promising route for electrolytic manganese (Mn) residue (EMR) reutilization due to being eco-friendly and cost-effective. However, microbes with high bioleaching efficiency are scarce. This work aimed to isolate, screen, and characterize a novel fungal strain with high Mn-bioleaching efficiency from EMR, and study the kinetics and mechanism. The novel Penicillium oxalicum strain Z6-5-1 was found to selectively bioleach Mn from EMR. A maximum Mn²⁺ recovery of 93.3 % was achieved after 7 days and was mainly dependent upon acidolysis of the bio-organic acids, specifically gluconic acid and oxalic acid, as well as mycelial biosorption. This efficiency was the highest reported in the literature for a fungus over such a short time. EMR strongly induced P. oxalicum to produce gluconic acid and oxalic acid. The novel transcription factor PoxCxrE of P. oxalicum controlled the production of bio-organic acids by regulating the expression of rate-limiting enzyme genes involved in the biosynthesis of bio-organic acids. Scanning electron microscopy, laser particle size analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy were employed to analyze EMR changes after bioleaching. This study provides an alternative fungal resource for Mn-bioleaching of EMR, and a novel target for metabiotic engineering to improve bio-organic acid production.

f-block MOFs: A Pathway to Heterometallic Transuranics

A novel series of heterometallic f-block-frameworks including the first examples of transuranic heterometallic ²³⁸ U/²³⁹ Pu-metal-organic frameworks (MOFs) and a novel monometallic ²³⁹ Pu-analog are reported. In combination with theoretical calculations, we probed the kinetics and thermodynamics of heterometallic actinide(An)-MOF formation and reported the first value of a U-to-Th transmetallation rate. We concluded that formation of uranyl species could be a driving force for solid-state metathesis. Density of states near the Fermi edge, enthalpy of formation, band gap, proton affinity, and thermal/chemical stability were probed as a function of metal ratios. Furthermore, we achieved 97 % of the theoretical maximum capacity for An-integration. These studies shed light on fundamental aspects of actinide chemistry and also foreshadow avenues for the development of emerging classes of An-containing materials, including radioisotope thermoelectric generators or metalloradiopharmaceuticals.

A New Hydrometallurgical Process for Metal Extraction from Electric Arc Furnace Dust Using Ionic Liquids

This research proposes a new hydrometallurgical method for Zn, In, and Ga extraction, along with Fe as a common impurity, from electric arc furnace dust (EAFD), using ionic liquids. EAFD is a metal-containing waste fraction generated in significant amounts during the process of steelmaking from scrap material in an electric arc furnace. With valuable metal recovery as the main goal, two ionic liquids, [Bmim⁺HSO₄^-] and [Bmim⁺Cl^-], were studied in conjunction with three oxidants: Fe₂(SO₄)₃, KMnO₄, and H₂O₂. The results indicated that the best combination was [Bmim⁺HSO₄^-] with [Fe₂(SO₄)₃]. An experimental series subsequently demonstrated that the combination of 30% v/v [Bmim⁺HSO₄^-], 1 g of [Fe₂(SO₄)₃], S/L ratio = 1/20, a 240 min leaching time, and a temperature of 85 °C was optimal, resulting in maximum extractions of 92.7% Zn, 97.4% In, and 17.03% Ga. In addition, 80.2% of the impurity metal Fe was dissolved. The dissolution kinetics of these four elements over a temperature range of 55-85 °C was found to be diffusion controlled. The remaining phases present in the leached residue were low amounts of ZnO, Fe₃O₄, ZnFe₂O₄, and traces of Ca(OH)₂ and MnO₂, and additional sharp peaks indicative of PbSO₄ and CaSO₄ appeared within the XRD pattern. The intensity of the peaks related to ZnO and Fe₃O₄ were observed to have decreased considerably during leaching, whereas some of the refractory ZnFe₂O₄ phase remained. SEM-EDS analysis revealed that the initial EAFD morphology was composed of spherical-shaped fine-grained particle agglomerates, whereas the leached residue was dominated by calcium sulphate (Ca(SO₄))-rich needle-shaped crystals. The results clearly demonstrate that [Bmim⁺HSO₄^-] is able to extract the target metals due to its acidic properties.

Application of Response Surface Methodology (RSM) for Simultaneous Optimization of Kinetic Parameters Affecting Gold Leaching in Thiosulfate Based Media: A Statistical Approach

Over the years, the use of new alternative lixiviants for gold extraction has been investigated to overcome the environmental concerns resulting from the cyanidation process. Moreover, with global economic factors causing a decline in gold prices, it is crucial that novel hydrometallurgical methods of extracting gold minimise operational costs by using low-priced reagents such as thiosulfate. In the current study, the response surface methodology (RSM) approach is used to optimize the kinetic factors (temperature and copper, ammonia, and thiosulphate concentration) affecting gold leaching. Gold ore assayed at 16 g/t was characterized through X-ray fluorescence and X-ray diffraction spectrometric analysis as well as scanning electron microscope-energy dispersive spectrometric technique. Gold ore was predominantly siliceous with minor pyritic content. The results indicate a strong relationship between the actual gold leaching recovery data and the RSM model. Correlation coefficients R2 and adjusted R2 are equivalent to 0.9869 and 0.9817. Gold leaching in copper-ammonia-thiosulfate media is best described as a surface chemical reaction-controlled process, suggesting that gold dissolution in thiosulfate is considerably affected by the increase in temperature. The effect of temperature is mostly significant, contributing up to 64.65% of the gold recovery response model. The contribution percentages of the effects of time, thiosulfate [S2O3], ammonia [NH3], and copper [Cu] concentrations were calculated as 12.81%, 5.88%, 5.19%, and 4.65%, respectively. All investigated kinetic parameters were found statistically significant with $p$ value <0.05. The optimal concentrations of gold leaching media to achieve potentially complete dissolution of gold from its ore in copper-ammoniacal thiosulphate media based on the effect of the investigated parameters were 0.5 M S2O3, 3 M NH3, and 0.003 M Cu2+ with a desirability value equivalent to unity (d = 1.000).

Application of Mixed Potential Theory to Leaching of Mineral Phases

Leaching is a central unit operation in the hydrometallurgical processing of minerals, which often occurs by means of electrochemical reactions. Application of mixed potential theory to explain the kinetics of oxidative and reductive leaching processes is a useful concept in explaining observed results. Native metals, selected oxides, and most base metal sulfides are electron-conducting phases. For these minerals, leaching may take place by normal corrosion, passivation or galvanic couple mechanisms, which provide individual electrode kinetics enabling the calculation of mixed potentials and overall reaction kinetics. Examples of the electrochemical nature of selected leaching processes are presented and include the effect of mixed potentials, geometry, and associated kinetic reactions.

The Kinetics of Pyrite Dissolution in Nitric Acid Solution

Refractory sulphidic ore with gold captured in pyrite has motivated researchers to find efficient means to break down pyrite to make gold accessible and, ultimately, improve gold extraction. Thus, the dissolution of pyrite was investigated to understand the mechanism and find the corresponding kinetics in a nitric acid solution. To carry this out, the temperature (25 to 85 °C), nitric acid concentration (1 to 4 M), the particle size of pyrite from 53 to 212 µm, and different stirring speeds were examined to observe their effect on pyrite dissolution. An increase in temperature and nitric acid concentration were influential parameters to obtaining a substantial improvement in pyrite dissolution (95% Fe extraction achieved). The new shrinking core equation (1/3ln (1 − X) + [(1 − X)^−1/3 − 1)]) = kt) fit the measured rates of dissolution well. Thus, the mixed–controlled kinetics model describing the interfacial transfer and diffusion governed the reaction kinetics of pyrite. The activation energies (E_a) were 145.2 kJ/mol at 25–45 °C and 44.3 kJ/mol at higher temperatures (55–85 °C). A semiempirical expression describing the reaction of pyrite dissolution under the conditions studied was proposed: 1/3ln(1 − X) + [(1 − X)^−1/3 − 1)] = 88.3 [HNO₃]^2.6 r₀^−1.3 e^−44280/RT t. The solid residue was analysed using SEM, XRD, and Raman spectrometry, which all identified sulphur formation as the pyrite dissolved. Interestingly, two sulphur species, i.e., S₈ and S₆, formed during the dissolution process, which were detected using XRD Rietveld refinement.

Effect of Pyrite on the Leaching Kinetics of Pitchblende in the Process of Acid In Situ Leaching of Uranium

In the process of acid in situ leaching of sandstone uranium ore, pyrite, which is a common associated mineral of pitchblende, would inevitably participate in the reaction. Therefore, it is important to study the influence of pyrite on the leaching kinetics of pitchblende. In this study, we compared the difference leaching rates of pitchblende in the systems of sulfuric acid–hydrogen peroxide, sulfuric acid–hydrogen peroxide–pyrite and sulfuric acid–pyrite and studied the influence of temperature and pyrite quantity on the leaching rate of pitchblende. The results show that the leaching process of pitchblende follows the shrinking particle model controlled by a chemical reaction, and the apparent activation energy Ea of the leaching reaction is (3.74 ± 0.40) × 10 kJ/mol. Pyrite itself cannot promote the dissolution of pitchblende; however, it can promote the leaching of pitchblende in the presence of an oxidizer. Increasing the quantity of pyrite in a certain range can increase the leaching rate of pitchblende, and the reaction order of pyrite is 0.36.

Electrochemical Approaches for the Recovery of Metals from Electronic Waste: A Critical Review

Electronic waste (e-waste) management and recycling are gaining significant attention due to the presence of precious, critical, or strategic metals combined with the associated environmental burden of recovering metals from natural mines. Metal recovery from e-waste is being prioritized in metallurgical extraction owing to the fast depletion of natural mineral ores and the limited geographical availability of critical and/or strategic metals. Following collection, sorting, and physical pre-treatment of e-waste, electrochemical processes-based metal recovery involves leaching metals in an ionic form in a suitable electrolyte. Electrochemical metal recovery from e-waste uses much less solvent (minimal reagent) and shows convenient and precise control, reduced energy consumption, and low environmental impact. This critical review article covers recent progress in such electrochemical metal recovery from e-waste, emphasizing the comparative significance of electrochemical methods over other methods in the context of an industrial perspective.