Leaching Mechanism Research Articles

Kinetics of leaching: a review

Abstract Kinetics, as a fundamental requirement of nearly all industrial activities and engineering researches, plays a great role in leaching processes. Although there are many pieces of research on its application, there is not a clear pathway for investigating the kinetics of leaching and researchers usually follow different strategies in their studies. The conventional investigation techniques, which usually do not consider the mixed mechanisms and possibility of any change in the mechanism, normally include many calculations, plots, and inadequate capabilities to detect changes in the controlling mechanism of leaching. In this review, the main mathematical models of leaching and all possible scenarios are presented and discussed. The effect of various leaching parameters (including leaching agent, temperature, particle size, agitation, and solid to liquid ratio) on the rate of dissolution is summarized. Besides, two main approaches of rate determination step (single controlling mechanism and combined resistances method) are described and compared by reporting related equations and suitable examples. A technique to detect any changes in the leaching controlling mechanism is introduced and the alternatives to confirm the results are described. Additional models and equations were suggested for the cases that there is no agreement between data and the conventional models. Also, situations which are ignored in simple models (e.g., reversibility of the leaching reactions, adsorption and desorption of leached species, influence of charge and surface potential, existence of multiple reactants in the solid, galvanic effect, wide particle size distribution, etc.) to develop more legalistic models are discussed. Considering various possible mechanisms in the kinetics of leaching, equations are derived for industrial leaching reactors.

Reductive acid leaching of valuable metals from spent lithium-ion batteries using hydrazine sulfate as reductant

Abstract Hydrazine sulfate was used as a reducing agent for the leaching of Li, Ni, Co and Mn from spent lithium-ion batteries. The effects of the reaction conditions on the leaching mechanism and kinetics were characterized and examined. 97% of the available Li, 96% of the available Ni, 95% of the available Co, and 86% of the available Mn are extracted under the following optimized conditions: sulfuric acid concentration of 2.0 mol/L, hydrazine sulfate dosage of 30 g/L, solid-to-liquid ratio of 50 g/L, temperature of 80 °C, and leaching time of 60 min. The activation energies of the leaching are determined to be 44.32, 59.37 and 55.62 kJ/mol for Li, Ni and Co, respectively. By performing X-ray diffraction and scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy, it is confirmed that the main phase in the leaching residue is MnO2. The results show that hydrazine sulfate is an effective reducing agent in the acid leaching process for spent lithium-ion batteries.

Erosion of aerial lime and sticky rice mortars by cyclic wetting–drying and dilute sulfate acid actions

Lime mortar with organic additives is one of the most widely used building materials in ancient China and continues to play an important role in historic structure restoration. This study investigated the erosion of ordinary aerial lime and sticky-rice mortars under the cyclic actions of wetting–drying (WD) and dilute sulfate acid (DSA). Comprehensive experimental tests were conducted to study the mechanical properties, quality, microstructure and pore structure of the mortars according to cyclic WD and DSA actions. Results showed that both the WD and DSA actions can significantly degrade the compressive and shear strengths of the mortars. The DSA softens the mortar matrices more severely than the WD by a leaching mechanism. Apparently, sticky rice does not change the micro morphology, pore structure and packing pattern of the mortar matrices, but it inhibits the carbonation of the lime. Both the WD and DSA actions can erode the ordinary aerial lime and sticky-rice mortars, causing more porous matrices and coarser particles. The results of this study make it possible to clarify how the WD and DSA actions affect the durability of partially matured aerial lime mortars with or without sticky rice, with potential uses in the restoration of ancient buildings.

Modification of the structure of the stabilized basalt fiber

The study of thermophysical, physical-mechanical and acoustic characteristics of coarse continuous basalt fiber in the conditions of influence of operational factors and corrosion environment is carried out. It is shown that obtaining barrier materials with high thermophysical and acoustic properties is possible by creating a porous system in the material with a predetermined textural characteristic of pores of different class, purpose and combination. The influence of the porous system of coarse continuous basalt fiber on the operational properties of flexible heat and sound insulation products and materials has been studied. It is determined that the performance characteristics of flexible thermal insulation materials are in direct proportion to the degree of porosity of the constituent materials and their structural characteristics. Shown that the increase in the operating temperature of these materials, sound and thermal resistance is determined by the chemical and mineralogical composition of the source materials and their structural characteristics. The influence of the mechanism of selective leaching of coarse continuous basalt fibers on their textural characteristics and the zone of operating temperatures is studied.

Correlation between surface chemistry and morphology of PtCu and Pt nanoparticles during oxidation-reduction cycle

Process conditions during catalytic reactions induce significant changes in surface chemistry and structure of bi- (mono) metallic nanoparticles leading to their deactivation, and this can ultimately affect the reactions long-term performance. Here PtCu and Pt model nanoparticles are prepared by microwave synthesis and characterized by X-ray diffraction (XRD). Surface chemical and morphological changes of the nanoparticles during high-temperature oxidation and reduction treatments cycle are correlated by near in situ X-ray photoelectron spectroscopy (XPS) and ex situ transmission electron microscopy (TEM) - energy-dispersive X-ray spectroscopy (EDS) studies. At 300 °C the surface atomic composition of the PtCu nanoparticles switches reversibly upon the cycle and at the same time their morphology and composition are maintained. At 400 °C, the surface atomic composition does not fully restore and, while the shape is maintained, the size and composition are not. This occurs by a mechanism of Cu leaching out from the nanoparticles. These data delineate potential operating conditions for stable PtCu nanocatalysts.

Sustainable Metal Recovery from Secondary Resources: Screening and Kinetic Studies Using Analogue Heterotrophic Metabolites

This study presents a sustainable approach for metal extraction from a range of secondary resources. Organic acids analogue to those commonly produced by heterotrophic microbes are applied via a two-step heterotrophic bioleaching approach. New insights into leaching mechanisms are developed and the potential use of the leaching residues as cementitious component is evaluated. Tested materials include a jarosite sludge from Zn refining, four ferrous metallurgy residue sludges and two dry automotive recycling shredder residues. Screening experiments tested three organic acids (i.e. citric, gluconic, oxalic acid) and their combinations. Four physicochemical factors were varied, i.e. the solid/liquid ratio between 5 and 15%, total organic acid concentration between 20 and 200 mM, leaching times of 2 to 4 h and horizontal stirring reactor mixing rate. The highest extraction capacity was achieved for a Cr/Ni rich sludges from steel treatment with up to 96% Ni and 91% Cr extraction at a 5% S/L and a mixture of 100 mM citric and 100 mM oxalic acid. Further scale-up kinetic studies on this Cr/Ni sludge validated the optimal leaching conditions. Modelling of kinetic leaching data indicated pore diffusion rate as a bottleneck of the overall leaching process. Screening tests of the leached residue indicated potential use as sulfate-rich addition to cement, or as part of low-performance binders for large scale civil works.

Kinetics and mechanism of radium-isotopes dissolution in TENORM scale waste associated with petroleum production using certain organic carbon source: lactic acid solution-case study.

The present study is conducted to explore the dissolution as inferred from the kinetic mechanism for radium-isotopes (228Ra, 226Ra, and 224Ra) in the TENORM scale waste deposited in oilfield pipes and equipment, Gulf of Suez, Egypt. The main efficiency factors for Ra2+-compound dissolution by lactic acid (LA) solution, e.g., reactive organic carbon (i.e., electron-donor source), have been investigated, and optimum chemical conditions have been determined. The obtained data were also employed to predict the leaching kinetics and mechanism of the Ra2+-isotopes removal by three shrinking core models (SCM, liquid film process-chemical controlled process-diffusion controlled process) and Arrhenius model. The maximum leaching percentage of Ra2+-isotopes reached to 55-60% at the optimal leaching conditions (0.3M LA, 5h, 25°C, ϕ < 1mm, S/L ratio 10/50gmL-1). The Ra-isotopes removal proceeds kinetically by diffusion-controlled process. Activation energy (Ea) of the leaching process was 10.51kJmol-1. This value conforms that the leaching process for removal of Ra2+-isotopes in the TENORM scale waste by LA solution is controlled by a diffusion process. Values of thermodynamic parameters (∆Go,∆Ho,∆So) were determined and indicate that dissolution of Ra2+-isotopes in the studied waste is non-spontaneous and temperature dependent. Moreover, the leaching mechanism may be attributed to the dissolution of soluble exchangeable and acidic species of Ra2+-species and/or these due conversions of insoluble Ra-sulfate to more soluble Ra-sulfide and/or Ra-hydrogen sulfide by LA solutions.

Reuse of copper slag as a supplementary cementitious material: Reactivity and safety

Copper slag (CS) is a potential supplementary cementitious material (SCM). However, unified reactivity indices of different types of CS do not yet exist. In addition, little is known about the safety of reusing CS as a SCM. Here, we collected 10 CS samples, including 5 air-cooled and 5 water-cooled samples, to establish reactivity indices and assess the safety (i.e., the soundness, radioactivity and leaching of heavy metals). Geochemical modeling was subsequently used to reveal the leaching mechanisms. The results show that the 28-day reactivity indices range from 66.6–68.2% for the air-cooled samples and 67.3–74.3% for the water-cooled samples; by reducing the water content, they can be increased to approximately 90% and above 95%, respectively. Neither the air-cooled nor the water-cooled CS causes soundness problems, but at a 50% replacement level, they both cause radiological hazards. Moreover, reuse of CS as a partial cement replacement poses leaching risks of Cu, Pb, Zn, Ni and As. The geochemical simulation indicates the important role of the Fe element in stabilizing the heavy metals, since Cu, Pb, As, Cr and Mn heavy metals are all controlled by Fe-related phases (specific adsorption and co-precipitation). In addition, some heavy metals can also be controlled by oxides, silicates and chromate precipitates. Our results extend the knowledge of using CS as a SCM and call attention to its potential hazards.

B2O3-assisted low-temperature crystallization of pollucite structures and their potential applications in Cs+ immobilization

Pollucite structures present remarkable chemical stability and good corrosion resistance against the leaching medium, which renders them promising for the permanent disposal of radioactive 137Cs. However, it is still challenging to prepare pollucite with dense structures and high immobilization performance at low temperature so far. Additionally, the fundamental understanding of the mechanism underlying the leaching behaviors of Cs+ from pollucite remains largely unexplored. In this paper, we realize the low-temperature production of pollucite by tailoring the nature of geopolymer precursors and employing B2O3 additives. It has been shown that the introduction of B2O3 not only significantly reduces the crystallization temperature of pollucite, but also improves the immobilization performance of Cs through encapsulation effect. Differential scanning calorimetry and X-ray diffraction confirm the formation of crystalline pollucite from amorphous geopolymer at ∼700 °C in the presence of 7.5 wt% B2O3. By further increasing the treatment temperature to 1000 °C, pollucite grains can be well-encapsulated by glass phase, exhibiting leaching rates ∼2 order lower than their unencapsulated counterparts. Moreover, distinct leaching mechanisms that govern the leaching behaviors of Cs+ from specimens being treated at different temperatures are unveiled. We anticipate that the proposed scheme facilitates the low-temperature production of high-quality pollucite structures with promising applications in the immobilization of hazardous wastes.

Liquid spread mechanisms in high-temperature underground stope leaching

Liquid flow behavior is a key factor that affects the leaching rate and metal extraction of stope leaching (SL), but related experimental studies are very limited. The liquid spread mechanism of SL is complicated because of the high-temperature conditions, the structural characteristics of the stope, and the pore characteristics of the ore heap. In this study, a device was developed to investigate the liquid spread mechanisms of the upper wall and foot wall of the stope. The influence of temperature, dip angle, and flow rate on the liquid spread was investigated. It is shown that the most significant feature of liquid spread in a stope is the liquid flow towards the bottom of the stope vertically rather than horizontal liquid spread. The increase of temperature accelerates the liquid spread, leading to the emergence of preferential flow paths. When the dip angle decreased from 90° to 60°, the liquid spread along the foot wall increased, including the change of the preferential spread direction, though the initial wetting time of the upper wall decreased. Increasing the flow rate within a certain range does not result in a more obvious liquid preferential flow path. Although the liquid spread of stope leaching still requires more studies in detail, the results of this work could be helpful to understand industrial stope leaching mechanisms.

Comparative Study of Various Kinetic Models on Leaching of NCA Cathode Material

The kinetics study of NCA leaching in the HCl system was proposed. Various kinetic models such as shrinking core, logarithmic rate law, and Avrami equation were used to find out the most appropriate kinetic models for this process. The effect of HCl concentrations, leaching temperatures, solid to liquid (S/L) ratio, and leaching duration were observed. The optimum conditions of NCA leaching were at HCl concentration of 4 M, temperature of 80 °C, S/L ratio of 100 g/L, and leaching time of 1 h. The result shows that shrinking core model with diffusion control process of residue layer describes well the leaching mechanism in this research, which is indicated by the good fitting of coefficient values of correlation (R2) and confirmed by the activation energy values of Ni, Co, Al that were less than 40 kJ/mol.

Oxygen pressure acid leaching of artificial sphalerite catalyzed by Fe3+/Fe2+ self-precipitation

The mechanism of oxygen pressure acid leaching of sphalerite catalyzed by Fe3+/Fe2+ self-precipitation was investigated in this study. Artificial sphalerite was fabricated with varying amounts of iron content via the sintering of ZnS and FeS and used for the pressure acid leaching experiment. The variations in the potential of the pressure leaching system were investigated by using a self-designed potential autoclave. The results showed that compared to the non-iron sphalerite, there was a violent redox reaction between the 25.70% Fe-artificial sphalerite and dissolved oxygen during the process of pressure leaching; and the catalytic mechanism was attributed to the redox couple Fe3+/Fe2+, where Fe3+oxidizes the H2S gas film and the reduced Fe2+ state is subsequently oxidized by the dissolved oxygen. Furthermore, the effect of temperature, H2SO4 concentration, and oxygen partial pressure on the artificial sphalerite with different iron contents was studied. The sphalerite samples with iron content were observed to dissolve more easily in sulfuric acid compared to the non-iron samples. Moreover, the activation energy of artificial sphalerite was observed to be lower in the sample with 25.70% iron content (22.26 kJ/mol) compared to that with no iron (32.31 kJ/mol); and the apparent reaction orders were obtained with respect to H2SO4 concentration (1.10 and 1.36) and oxygen partial pressure (1.29 and 1.41), respectively. A comprehensive kinetic model was developed on the basis of the experimental data and the fitted leaching ratio plot; and the kinetic equations for the leaching of sphalerite catalyzed by Fe3+/Fe2+ self-precipitation were determined.

Influence of pH and Cations Contained in Rainwater on Leaching of Cd(II) from Artificially Contaminated Montmorillonite

Abstract The objective of this study was to investigate the influence of pH and ions contained in rainwater on leaching of Cd(II) from artificially-contaminated montmorillonite, with the final objective of deepening the understanding of the leaching mechanism of Cd(II) from soils. Batch Cd(II) sorption experiments were performed before performing leaching experiments, and the obtained results were modeled using a surface complexation model coupled with ion exchange reactions. The results showed that at a pH below 6.0, the sorption of Cd(II) onto montmorillonite was not significantly influenced by pH but was strongly hindered by NaNO3, showing that the dominant Cd(II) sorption mechanism was ion exchange at the permanent charge. These results indicate that for the typical pH range for rainwater (

Leaching Kinetics of Rare Earth Elements from Fire Clay Seam Coal

Recovery of rare earth elements (REEs) from coal samples collected from the Fire Clay coal seam using diluted mineral acid solutions was investigated. The initial processing step was coal recovery using conventional froth flotation which concentrated the REEs in tailing material resulting in an upgrade to values around 700 ppm on a dry whole mass basis. Leaching experiments were performed on the flotation tailings material using a 1.2 M sulfuric acid solution adjusted to a temperature of 75 °C to study the extractability of REEs from coal material. The effect of particle size, leaching time, leaching temperature, and solid concentration on REE leaching recovery were evaluated. The kinetic data obtained from leaching over a range of temperatures suggested that the leaching process follows the shrinking core model with possibly a mixed control mechanism that may be a result of several heterogenous materials leaching simultaneously. Leaching recovery increased rapidly at the beginning of the reaction then slowed as the system reached equilibrium. The apparent activation energy determined from test data obtained over a range of temperatures using 1 M sulfuric acid was 36 kJ/mol for the first 20 min of reaction time and 27 kJ/mol for the leaching period between 20 and 120 min. The leaching of light REEs during the initial stage was determined to be driven by a chemical reaction, followed by the formation of a product layer, which required lower activation energy in the later stage of leaching. In regards to the heavy REEs, the major mechanism for leaching is desorption and the product layer formation does not affect the heavy REEs significantly.

Curing temperature dependency of the release of arsenic from cemented paste backfill made with Portland cement

In this research, the effect of curing temperature on the metalloid (As) leachability of cemented paste backfill (CPB; a mix of tailings, cement and water) is studied. ASTM C 1308 leaching protocol is used to determine the leachability of CPB samples subjected to different curing temperatures (2, 20 and 35 °C). In addition, the effect of curing temperature on the microstructure of CPB is assessed to determine if the temperature dependence of the leaching characteristics of CPB is related to variations in the microstructure of the cement matrix. The microstructural techniques used include, powder x-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy techniques. The results obtained indicate that the curing temperature has a significant effect on the leachability of CPB. It is found that as curing temperature increased from 2 °C to 35 °C the performance of the CPB for arsenic immobilization decreased. The magnitude of this temperature-induced change in As-leachability of CPB depends on the curing temperature range (low (≤20 °C) or elevated (35 °C) temperatures). This curing temperature dependency of the leachability of CPB is attributable to the temperature-induced changes in the pore structure of CPB, formation/development of hydration products and pH in the CPBs during the curing process. The results also indicate that diffusion dominates as the leaching mechanism through the studied temperature range and is independent of curing temperature. However, curing temperature does control the availability for arsenic to leach. The new findings presented in this paper will contribute to design more environmental-friendly cemented paste backfill materials and structures, which is vital for sustainable mining.

Potential Use of Incineration Bottom Ash in Construction: Evaluation of the Environmental Impact

Knowledge of the long-term leaching behavior of potentially harmful substances is crucial for the assessment of the environmental compatibility of reusing municipal solid-waste incineration bottom ash (MSWI BA) in construction, i.e., as a road base layer. BA fractions obtained from wet-processing aiming at the improvement of environmental quality were used to investigate the mobility of relevant substances. Eluates from laboratory-scaled leaching procedures (column percolation and lysimeters) were analyzed to learn about the long-term release of substances. Unsaturated conditions and artificial rainwater (AR) were used in the lysimeter tests to simulate field conditions. In addition, batch test eluates were generated at usual liquid-to-solid ratios (L/S) for compliance testing purposes. A variety of cations and anions was measured in the eluates. The wet treatment reduces the leaching of chloride and particularly sulfate by more than 60%. The release of typical contaminants for the treated MSWI BA such as the heavy metals Cu and Cr was well below 1% in the conducted leaching tests. An increase in the Sb concentration observed in the lysimeter experiments starting at L/S 0.75 L/kg and in the column experiment at L/S 4 L/kg is assumed to be related to decreasing concentrations of Ca and thus to the dissolution of sparingly soluble calcium antimonate. The same leaching mechanism applies with V, but the concentration levels observed are less critical regarding relevant limit values. However, on the long term the behavior of Sb could be problematic for the application of MSWI BA in construction.Graphic

In Situ Study on Ni-Mo Stability in a Water-Splitting Device: Effect of Catalyst Substrate and Electric Potential.

Nickel–molybdenum (Ni–Mo) alloys are well studied as highly effective electrocatalyst cathodes for water splitting. Understanding deactivation pathways is a key to improving the performance of these catalysts. In this study, in situ characterization by UV/Vis spectroscopy and AFM of the morphology and Mo leaching of an Ni–Mo electrocatalyst was performed with the goal of understanding the stability and related Mo leaching mechanism. Switching the potential towards higher overpotentials results in a nonlinear change in Mo leaching. Multiple processes are proposed to take place, such as a decrease in the extent of Mo oxidation at the cathode induced by more strongly reducing potentials, while simultaneously the increase in the local pH at the cathode due to the hydrogen evolution reaction causes more Mo leaching. The change in capacitance of these materials depends strongly on the change in surface composition and not only on the surface area. In situ UV/Vis spectroscopy showed that Mo leaching is a continuous process over the course of 4 h of operation. Finally, the material was deposited on different substrates and the effect on Ni–Mo stability was studied. The substrate has a significant, albeit complex, influence on the stability and activity of Ni–Mo cathodes. In terms of stability in 1 m KOH, Ni–Mo was found to be best deposited on stainless steel substrates operated at low overpotentials, on which it showed nearly no change in capacitance and exhibited low Mo leaching.

Effects and mechanisms of ternary solution of NaOH-Na2CO3-Na2SO4 on the recovery of molybdenum from residues containing multiple molybdates

The recovery of molybdenum from secondary resources containing multiple molybdates is limited by low recovery efficiency. In this work, detailed investigations have been carried out to deeply and selectively recover Mo from volatilization residues of molybdenite concentrate. Characterizations show that the molybdenum is present as MoO3 and mixed molybdates of La, Ce, Nd, Fe, Ba, Ca, Mg, Sr etc. Leaching experiments involving various conditions were then performed to dissolve Mo from the residues. The leaching efficiency of Mo could not attain a high level until a ternary solution was used. Specifically, 98.4% of Mo and 13.1% of Si could be dissolved with 160 g/L NaOH, 20 g/L Na2CO3 and 30 g/L Na2SO4 at 95 °C for 120 min at liquid to solid ratio of 6:1 mL/g, while rare earths were enriched and totally kept in leach residues. Furtherly, thermodynamic calculations about the dissolution reactions involving various molybdates and leaching agents were conducted. The leaching mechanisms of Mo with ternary solution were revealed based on the calculations and SEM-EDS analysis. Generally, this work may provide not only a new strategy for the recovery of Mo from complex secondary resources, but also new insights into the leaching mechanisms of Mo in the aspect of thermodynamics.

Factors affecting the leaching mechanism of Mg bearing oxide impurities in Hydrogen Assisted Magnesiothermic Reduction (HAMR) of TiO2

Titanium production by a direct TiO2 reduction route which is highly energy-efficient and environment-friendly compared to commercial Kroll's process, known as HAMR (Hydrogen Assisted Magnesiothermic Reduction) process has been developed in recent times to meet the growing global demands of low-cost pure Ti. Removal of the Mg compounds after each reduction and de-oxygenation step in the HAMR process by leaching, is essential to ensure good purity and mechanical properties of the final Ti powder. In this study, we investigate the effects of temperature, particle size, and pH on the dissolution kinetics of Mg from Mg-bearing compounds such as MgO and MgCl2 that remain after the HAMR Process. We also report the underlying mechanism and rate-controlling steps of Mg removal using HCl in connection with the HAMR process. The dissolution kinetics followed a logarithmic rate kinetic model for diffusion through a porous medium that is commonly used for battery research. Kinetics of leaching were significantly improved using finer particles with 99.96 wt.% Mg removal and corresponding Ti loss of only 1.8 wt. %. The rate of dissolution exhibited a weak dependence with the pH and did not abide by the rate-controlling steps of dissolution of ionic oxides like MgO proposed by previous kinetic models.

Theoretical-molar Fe3+ recovering lithium from spent LiFePO4 batteries: an acid-free, efficient, and selective process

In spent lithium iron phosphate batteries, lithium has a considerable recovery value but its content is quite low, thus a low-cost and efficient recycling process has become a challenging research topic. In this paper, two methods about using the non-oxidizing inorganic iron salt - Fe2(SO4)3 to recover lithium from LiFePO4 are proposed. The method-1 is theoretical-molar Fe2(SO4)3 (Fe2(SO4)3 : LiFePO4 =1:2) dosage is added and more than 97% of lithium can be leached in just 30 min even under a pretty high solid-liquid ratio of 500 g/L. Spectrophotometry provides the evidence of Fe2+/Fe3+ substitution in the leaching process. In the method-2, the generated Fe2+ originating from LiFePO4 is fully utilized with the addition of H2O2, and the dosage of Fe2(SO4)3 is decreased by two thirds (Fe2(SO4)3 : LiFePO4 =1:6). Several sulphates (CuSO4, NiSO4, MgSO4) are employed to explore the leaching mechanism. All the results reveal that the reaction of Fe3+ substituting Fe2+ has a powerful driving force. In addition, these two leaching processes simultaneously present superior selectivity for the impurities. The Fe2(SO4)3 in two methods does not cause pollution and is easily regenerated by adding H2SO4. The proposed rapid, efficient and selective leaching thought would be a competitive candidate for recycling spent LiFePO4 batteries.