Leaching Reagent Research Articles

A new process for the efficient decomposition of low-grade scheelite based on the formation of insoluble calcium fluorophosphate

With the exhaustion of wolframite and scheelite concentrates, low-grade scheelite has gradually become the main raw material of the tungsten industry. The current processes used for leaching low-grade scheelite with sodium hydroxide or sodium carbonate in an autoclave have the disadvantages of low tungsten leaching efficiency and the requirement of a large leaching reagent dosage. In this study, a new process for leaching low-grade scheelite by using a mixture of sodium phosphate, sodium hydroxide and calcium fluoride is proposed. In the leaching process, scheelite is converted into calcium fluorophosphate while transferring WO42− from the mineral to the solution. To explore the conditions favourable for the formation of calcium fluorophosphate during leaching, thermodynamic analysis of the Ca-W-P-F-H2O system was carried out. The results show that calcium fluorophosphate can be formed over a wide range of pH values. Increasing the concentration of total phosphorus and total fluorine in the solution is beneficial to the decomposition of scheelite. The effects of various parameters on the leaching of low-grade scheelite were studied, and the optimum process parameters were determined. A total of 98.21% of the tungsten can be leached from low-grade scheelite using 2 times the theoretical amount of sodium phosphate, 1.5 times the theoretical amount of calcium fluoride, a sodium hydroxide to ore mass ratio of 14%, and an L/S ratio of 4:1 at 453 K for 3 h, with a residual phosphorus concentration of only 0.12 g/L in the leaching solution. Mineralogical analysis of the leaching residue shows that most of the associated mineral calcite in the low-grade scheelite is not decomposed, which indicates that selective and efficient decomposition of scheelite is achieved by using this novel process.

Simultaneous preparation of antioxidant peptides and lipids from microalgae by pretreatment with bacterial proteases

Chlorella can produce large amounts of lipids and therefore has great potential for biodiesel production. In this study, Chlorella protothecoides was hydrolyzed by several kinds of extracellular bacterial proteases produced by Pseudoalteromonas sp. ZB23-2, B27-3 and JS4-1 before lipid extraction. Hydrolysates with high antioxidant activity were obtained. The scavenging activities of 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydrogen peroxide, and hydroxyl free radicals reached 33.47 ± 0.68%, 46.81 ± 2.38%, and 7.35 ± 0.37 µmol·TE/µmol, respectively. Likewise, proteolysis reduced biomass, which resulted in a reduction in lipid leaching reagents by 35.34–45.49%. Compared to the commonly used Kates and Paradis method (171.77 ± 2.50 mg/g), the modified ethanol lipid extraction combined with JS4-1 enzyme pretreatment (291.06 ± 1.70 mg/g) and acetone-ethanol lipid extraction combined with B27-3 protease pretreatment (277.20 ± 3.30 mg/g) resulted in a larger and more diverse lipid extraction. Protease pretreatment combined with less toxic solvents for lipid extraction improved microalgal biorefinery and reduced environmental pollution.

Two phase leaching for metal recovery from waste printed circuit boards: Statistical optimization

The rapid growth of technology is inevitable in humankind?s life and has a significant stint in electronic waste (e-waste) generation. Electronic waste possesses tremendous environmental and health effects, and one such major contributor to it is printed circuit boards (PCBs). The present work deals with the recovery of heavy metals from PCBs by using aqua regia as a leaching reagent in two stages (first stage HCl and HNO3 and second stage HCl and H2SO4). The response surface methodology was used to determine the optimal recovery conditions for the heavy metal ions: the recovery time of 5 h, the pulp density of 25 g/L, and the temperature of 90.1 ?C with desirability 0.761. These optimized values provide a maximum recovery rate of Cu (97.06%), Sn (94.66%), Zn (96.64%), and Pb (96.89%), respectively. EDXs are used to analyze the metal concentrations of the sample before and after treatment.

Optimization of Pyrite Bio-Oxidation to Produce Ferric Reagent for Sphalerite Leaching

Leaching of zinc from sulfide minerals is a subject of considerable interest over the last few years. Ferric solutions were commonly reported to leach base metals sulfide concentrates. The current study investigates the potential of biological ferric solutions for Zn and Pb extractions from a sphalerite concentrate at 65°C. Comparative leaching experiments with ferric sulfate and ferric chloride are also performed. Actually, biological ferric ions have resulted from pyrite bio-oxidation. The aim of this work is to introduce a potential application of this metabolite as a mineral oxidizing agent to recover zinc from the sphalerite concentrate. To produce a biogenic reagent with the highest ferric, the bio-oxidation process is optimized by investigating the effects of different factors including pH, pulp density, and inoculum percent. At the optimum conditions, a metabolite with 7.87 g/L of ferric ions and a pH = 0.98 is produced. The results indicate that pyrite bio-oxidation includes three phases: chemical dissolution, lag phase, and biological dissolution. The scanning electron microscope (SEM) images showed that a layer of crystals was present on the surface of pyrite. Based on energy-dispersive X-ray spectroscopy (EDS) and Raman spectroscopy, this layer is related to the presence of potassium jarosite. The metabolite was then used for ZnS direct leaching and results were compared with sulfide and chloride leaching. The final zinc recoveries with biological ferric ions, ferric sulfate, and ferric chloride were 78.0%, 88.9%, and 85.4%, respectively.

Ozone Ice as an Oxygen Release Reagent for Heap Leaching of Gold Ore

The issue of poor aeration efficiency and low oxygen transfer in the heap leaching of gold has gained considerable attention. In this study, ozone ice was studied as an oxygen release reagent in the cyanide heap leaching of gold at a low temperature of approximately 5 °C, owing to its effective oxidation and clean and green properties. Quartz Crystal Microbalance with Dissipation (QCM-D) was used to monitor the effect of different ratios of cyanide and oxygen concentrations on the gold leaching rate. The results showed that the leaching rate doubled when the dissolved oxygen (DO) was increased from 8.2 mg/L to 12 mg/L at a relatively high cyanide concentration of 60 mg/L. The release of oxygen during the process of ozone ice melting was analyzed by simulating the oxygen-deficient condition of the ore heap in column leaching. In the first stage of ice melting, the DO in the solution increased dramatically, and the rate of increase improved with increased initial ozone concentration in the ice. In the second stage of ice melting, the rate of increase in the DO of the solution was not significantly affected by the initial ozone concentration in the ice; this was consistent with the decomposition rate of ozone. The addition of ozone ice containing 300 mg/L ozone increased the gold extraction by 4.1% in the ore column leaching experiment, compared to a column with no ozone ice. However, continuously increasing the ozone concentration up to 600 mg/L had no further significant effect, because the dissolved oxygen in the leaching solution reached saturation. The results facilitate a better understanding of the decomposition law of ozone in the melting process of ozone ice and help to improve the oxygen deficit state in gold leaching heaps.

Leaching behavior of zinc and lead from electric arc furnace dust – Poly(vinyl) chloride residues after oxidative thermal treatment

Oxidative thermal treatment of electric arc furnace dust (EAFD) mixed with poly(vinyl) chloride (PVC) was carried out to study the impact of oxidizing environment on the extraction of valuable metals. The effect of oxygen partial pressure, temperature and holding time of the thermal treatment, as well as the pH of the leaching solution were considered as operational variables in this study. HCl evolved during PVC decomposition was found to react with the metal oxides present in EAFD producing metal chlorides. It was found that thermal treatment above 350 °C caused significant vaporization of lead and zinc chlorides. After thermal treatment, the residue was leached in water, the cheapest available leaching reagent. Zinc and lead were extracted with recoveries up to ∼100%, while iron recovery was as low as 12.5% being suppressed by oxidation of ferrous iron. Operating under oxidative conditions supresses the post dissolution of iron in water by oxidizing Fe3O4 to Fe2O3 and by oxidizing any formed FeCl2 back to Fe2O3. The solution pH was also found to significantly affect the extraction of both lead and iron, especially at pH values above 4.0 where a sharp drop in metals recovery was observed. The obtained results present base for development of sustainable solution for treatment of PVC and EAFD wastes.

Leaching kinetics of copper and valuable metal extraction from copper-cadmium residues of zinc hydrometallurgy by oxidation acid leaching

Abstract Multiple purification of zinc sulfate solution is an important process for zinc hydrometallurgy, and large quantities of copper-cadmium residues are generated as byproducts in this process. Copper-cadmium residues contain a large number of valuable metals that must be recovered. A comprehensive extraction process has been proposed using sulfuric acid as the leaching reagent and hydrogen peroxide as the oxidizing reagent. The effects of acid concentration, leaching temperature, leaching time, liquid-to-solid ratio, hydrogen peroxide dosage and stirring speed on the leaching efficiency were investigated. The optimum conditions were determined as an acid concentration of 150 g/L, liquid-to-solid ratio of 4:1, hydrogen peroxide amount of 20 mL, time of 60 min, temperature of 30 °C, particle size of −d75 μm, and agitation rate of 300 r/min. It was concluded that the leaching efficiency of copper and cadmium reached 97%, but because of the existence of zinc sulfide in the residues, a lower leaching efficiency of zinc was obtained. Furthermore, the leaching kinetics of copper was also studied based on the shrinking core model. The activation energy for copper leaching was 5.06 kJ/mol, and the leaching process was controlled by the diffusion through the product layer.

Effect of Microwave Pretreatment on Leaching of Tetrahedrite

In mineral processing, the use of microwave radiation is important especially in pre-treatment processes. At present, there is an acceleration of processes as well as an increase in the efficiency of metal recovery. One of the main problems in copper recovery from complex sulphide ores is the removal of impurities such as antimony, arsenic, mercury. In the hydrometallurgical processing scheme, the key step is the leaching. The extraction process can be influenced by the selection of suitable leaching reagents or by suitable pre-treatment of the ore. The article describes the effect of microwave radiation on the leaching Sb, As and Hg of tetrahedrite and tetrahedrite concentrate. The samples were irradiated at the power 900 W for 30 seconds. The leaching of irradiated and non-irradiated samples was realized in an alkaline sodium sulphide. The positive effect of microwave radiation was confirmed by an increase in the recovery of Sb and As already after 15 min of extraction. After microwave leaching of irradiated tetrahedrite samples, the yield of Sb was 43.2 %, in irradiated tetrahedrite concentrate, the yield of Sb was 81.3 %.

Optimization of process parameters for the selective leaching of copper, nickel and isolation of gold from obsolete mobile phone PCBs

The sequential separation of base and precious metals from the end-of-life mobile phone printed circuit boards (PCBs) is a significant challenge for the development of recycling process that exhibit material circularity. In this contribution, a simple, eco-friendly, and efficient leaching process is developed for the dissolution of copper and nickel from obsolete mobile phone PCBs to facilitate the isolation of a gold-rich residue. Chemical pre-treatment of downsized PCBs produced a metallic portion for which the leaching parameters for copper and nickel were optimised, including the type of leaching reagent, temperature, time, pulp density and agitation speed. It was found that quantitative dissolution of base metals occurred using 3.0 M nitric acid at 30 °C with a 50 g/L pulp density, 2 h residence time, and 500 rpm stirring speed; under these conditions, no gold was dissolved. Design of Experiment analysis using Response Surface Methodology was also undertaken to validate the process. Finally, the kinetics of the leaching process were studied and shown to conform to the chemically controlled surface reaction model, with activation energies of 39.7 and 18.4 kJ/mol for copper and nickel. Importantly, the leaching process optimised in this work avoids the need for high temperatures and reduces energy consumption and effluent generation, leading to the cleaner processing of obsolete mobile phone PCBs for the separation of gold from the dominant base metals.

Evaluation of physiochemical, thermal and kinetic properties of wheat straw by demineralising with leaching reagents for energy applications

The augmented concerns towards greenhouse emissions have stimulated the innovative technologies to reduce the carbon footprints and lessen the usage of fossil fuel by replacing them with biomass. Wheat straw is a biowaste and has a great potential for energy-production. However, the inorganic contents in the wheat straw cause operational problems such as low heat transfer and ash-deposition in combustion chamber leading to high maintenance requirements. Demineralization of wheat straw with various basic and acidic leaching reagents (NaOH, HCl, HNO3) was investigated in this study. The concentrations of leachants was varied from 0.6, 0.4 and 0.2M to study their effect on physical, chemical, thermal and kinetic behavior of the biomass. The 0.6 M HCl solution reduced the maximum ash content up to 92.71 % and raised the heating value to 3.98 % than raw wheat straw. The kinetic study of the samples treated with 0.6M HCl demonstrated the superior ignition and the activation-energy than other samples. On other hand NaOH damaged the structure of the wheat straw and had adverse effects on their physical and chemical properties. The results of the current study suggested that HCl treated wheat straw can provide a cost effective and eco-friendly solution for energy generation.

Cadmium isotope fractionation during leaching with nitrilotriacetic acid

This study aimed to determine the leaching mechanism of cadmium (Cd) from contaminated soil by nitrilotriacetic acid (NTA). 114Cd/110Cd isotope technology was adopted as the main characterization method, and experiments were conducted with a typical high-Cd soil to determine the speciation distribution of Cd, isotopic composition, and changes in bonding environment before and after leaching. The speciation distribution of Cd in the soil before leaching in order from most to least was exchangeable Cd, iron‑manganese oxide Cd, residual Cd, carbonate-bound Cd, and organic fraction Cd. The removal efficiency of exchangeable Cd was the highest, followed by iron‑manganese oxide Cd and carbonate-bound Cd. Residual Cd and organic fraction Cd content did not significantly change. Cd(NTA)− and Cd(NTA)24− were the major species of Cd in the leaching reagent, which was enriched in the heavy isotope 114Cd and had a strong bonding environment. The isotope fractionation coefficient α decreased as the proportion of Cd remaining in the soil decreased. Cd speciation distribution had an important influence on the 114Cd/110Cd fractionation degree in the leaching process. The Cd isotope composition in soil (from −0.12‰ ~ 0.04‰ to −0.35‰ ~ −0.18‰ without buffer and − 0.28‰ ~ −0.09‰ with buffer) can reflect the change of Cd speciation distribution, which determines the characteristics of the bonding environment and, in turn, determines the efficiency of NTA leaching to remove soil Cd. As such, isotope fractionation characteristics can be a significant theoretical support related to NTA leaching Cd contaminated soil.

Evaluation of ozone as an efficient and sustainable reagent for chalcopyrite leaching: Process optimization and oxidative mechanism

Chalcopyrite is the main copper-bearing mineral and is refractory to oxidative leaching. There are many investigations to optimize copper extraction from this mineral but most of the processes are energy intensive and environmental threatening. Ozone at low concentration is an efficient reagent for high extraction of copper from chalcopyrite, which is also ecofriendly. This paper presents a comprehensive investigation on the process optimization for both copper and iron extraction from chalcopyrite using ozone. The following variables were tested during 48-h leaching periods: temperature, ferric sulfate, ozone and sulfuric acid concentrations, solid/liquid ratio, and leaching time. Ozone leaching recovered 100% of the copper from chalcopyrite at 25 °C without addition of ferric. The overall reaction kinetics followed the shrinking core model by mixed control of diffusion through the product layer and chemical reactions. The proposed leaching mechanism was verified via various characterization techniques. Green chemistry metrics were evaluated, and the optimized process was demonstrated to be environmentally attractive. Ozone leaching appears to have strong potential as a “green” and technically feasible method to leach copper from chalcopyrite.

Mechanism of pyrite oxidation in copper(II)-ethylenediamine-thiosulphate gold leaching system

Although the Cu2+–ethylenediamine–S2O32− system is a promising alternative to cyanide-based gold leaching systems because of its low cost, nontoxicity, and environmental friendliness, its performance is strongly affected by the presence of pyrite. Therefore, a deep understanding of the mechanism of pyrite oxidation in this system is of high practical importance. Herein, this mechanism was probed to demonstrate the occurrence of rapid reactions on the pyrite surface at high S2O32− concentrations and reveal that the dissolution of bulk pyrite was most favoured at a certain optimal S2O32− concentration, i.e., S2O32− ions strongly affected the surface properties of pyrite. Anodic processes occurring in active, passive, and transpassive dissolution regions were studied by linear sweep voltammetry, with additional insights into the properties of pyrite electrodes and the related electrochemical reactions obtained by electrochemical impedance spectroscopy and equivalent circuit fitting. With increasing potential at range of 0.20 to 0.80 V, the reactions on the pyrite surface accelerated, and its roughness significantly increased. The products of pyrite surface oxidation were identified by Raman spectroscopy and X-ray photoelectron spectroscopy, and a corresponding mechanism was proposed. Thus, this work provides a theoretical basis for the regulation and control of gold leaching reagents in the Cu2+–ethylenediamine–S2O32− system.

Gold Leaching by Various Solutions, Alternative of Cyanide and Their Prospects in the Future

The article discusses four alternative cyanide substitutes for the recovery of gold from ores and concentrates. Cyanide has been the premier gold leach reagent for over 100 years due to its high recovery, low cost and reliability. However, environmental damage from improper management has led to widespread research aimed at identifying and developing less toxic leaching agents. The article discusses the most widely researched cyanide substitutes, but it is clear that none of them has yet made any significant incursion into the dominance of cyanide as the preferred reagent in the vast majority of gold mines around the world

Electrolysis designed for clean production of selective iron products from coal fly ash leachate

With the development of society, solid waste, such as coal fly ash has caused serious environmental problems. Extraction of high value metals from coal fly ash is vital for sustainable development and maintenance of social economy. This work aimed to present a nonhazardous and efficient electrolysis method to extract selective iron products from coal fly ash leachate. Compared with current precipitation, biological, and organic extraction processes, this electrolysis method can selectively prepare different iron products. No chemicals or expensive membranes were used to prevent the introduction of pollutant and membrane limitations. Moreover, no waste was produced enabling realization of zero pollution discharge. Batch experiments indicated that the degree of reduction of iron can be controlled during the electrolysis process, thereby allowing for the production of different types of iron products. The purity of iron electrolytic product was 98.30 wt% Fe2O3, 1.35 wt% SO3, and 0.09 wt% Al2O3, and current efficiency reached 60.3% under the optimal conditions: c (Fe2+) = 0.1 M, t = 4.0 h, and J = 2500 A/m2. Different iron products like Fe3O4, FeOOH and Fe can be prepared sequentially during electrolysis process. Moreover, O2 is generated on anode, and H2 is generated on cathode as clean energy, which increases economic benefits. The wastewater (containing H2SO4) is widely used as a leaching reagent during leaching of coal fly ash, which can return to leaching process enabling zero hazardous substance discharge. This study would provide an alternative clean and efficient method for extracting selective iron products from coal fly ash leachate.

Efficient extraction and separation of indium from waste indium–tin oxide (ITO) targets by enhanced ammonium bisulfate leaching

In this paper, a closed-loop hydrometallurgical process was proposed for indium recovery from waste indium–tin oxide (ITO) targets. Both high indium recovery and recirculation of leaching reagent have been achieved. This process included ammonium bisulfate leaching, selective precipitation of tin by adding ammonia, indium hydroxide precipitation and roasting to prepare indium oxide product. Moreover, a shrinking core model was implemented to depict indium leaching process with an activation energy of 52.64 kJ/mol calculated, which showed that the rate was controlled by the surface chemical reaction. And the reaction orders were determined as 0.60, 0.34 and −0.69 with respect to initial ammonium bisulfate concentration, oxygen partial pressure and particle size, respectively. Since the tin was more sensitive to temperature, acidity and oxygen, the leaching solution finally contained 14.31 g/L indium and 0.05 g/L tin. Subsequently, the application of two-step precipitation realized selective precipitation of tin for purifying solution and preparation of precursor. Indium oxide product was obtained by calcination of the precursor at 600°C for 1 h. Finally, the supernatant after indium precipitation was concentrated, evaporated and thermally decomposed, and then returned to the leaching process. This study provided a feasible process for high-value use of waste ITO targets.

Heap Leach Pad Surface Moisture Monitoring Using Drone-Based Aerial Images and Convolutional Neural Networks: A Case Study at the El Gallo Mine, Mexico

An efficient metal recovery in heap leach operations relies on uniform distribution of leaching reagent solution over the heap leach pad surface. However, the current practices for heap leach pad (HLP) surface moisture monitoring often rely on manual inspection, which is labor-intensive, time-consuming, discontinuous, and intermittent. In order to complement the manual monitoring process and reduce the frequency of exposing technical manpower to the hazardous leaching reagent (e.g., dilute cyanide solution in gold leaching), this manuscript describes a case study of implementing an HLP surface moisture monitoring method based on drone-based aerial images and convolutional neural networks (CNNs). Field data collection was conducted on a gold HLP at the El Gallo mine, Mexico. A commercially available hexa-copter drone was equipped with one visible-light (RGB) camera and one thermal infrared sensor to acquire RGB and thermal images from the HLP surface. The collected data had high spatial and temporal resolutions. The high-quality aerial images were used to generate surface moisture maps of the HLP based on two CNN approaches. The generated maps provide direct visualization of the different moisture zones across the HLP surface, and such information can be used to detect potential operational issues related to distribution of reagent solution and to facilitate timely decision making in heap leach operations.

Gold-leaching performance and mechanism of sodium dicyanamide

In this work, sodium dicyanamide (SD) was used as a leaching reagent for gold recovery, and the effects of the SD dosage and solution pH on the gold-leaching performance were investigated. A gold recovery of 34.8% was obtained when SD was used as the sole leaching reagent at a dosage of 15 kg/t. In the presence of a certain amount of potassium ferrocyanide (PF) in the SD solution, the gold recovery was found to increase from 34.8% to 57.08%. Using the quartz crystal microbalance with dissipation (QCM-D) technique, the leaching kinetics of SD with and without PF were studied. The QCM-D results indicate that the gold-leaching rate increased from 4.03 to 39.99 ng·cm−2·min−1 when the SD concentration was increased from 0 to 0.17 mol/L, and increased from 39.99 to 272.62 ng·cm−2·min−1 when 0.1 mol/L of PF was used in combination with SD. The pregnant solution in the leaching tests was characterized by X-ray photoelectron spectroscopy and electrospray mass spectrometry, which indicated that Au and (N(CN)2)− in the SD solution formed a series of metal complex ions, [AuNax(N(CN)2)x+2]− (x = 1, 2, 3, or 4).

Biomass Waste Valorization by Acidic and Basic Leaching Process for Thermochemical Applications

Inorganic minerals present in biomass feedstock can cause various technical problems during thermochemical conversions such as fouling, ash slagging and agglomeration, alkali deposition, and surface corrosion. To counter such problems, it is recommended to demineralize the biomass by treating it with a suitable extractive reagent. In this study, cotton stalk (CS) and coconut husk (CH) has been treated with HCl, HNO3, and NaOH reagents using different concentration levels. The effect of treatment on thermal, physical, and chemical properties of the biomass has been examined using ash analysis, ultimate analysis, scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), calorimetry, and thermogravimetric analysis (TGA). From this study, HCl appeared to be the most efficient leaching reagent as it removed the maximum 96.5% and 91.2% ash forming components from CS and CH samples, respectively and increased the heating value by 6.03% and 7.19%, respectively. On the contrary, NaOH treatment has devalued the biomass due to a decrease in the heating value by 10.42% and 11.87% for CS and CH samples, respectively despite the ash reduction by 51.4% and 59.1%. The adverse effects of NaOH treatment have been attributed to the delignification of the biomass samples.

LiMn2O4 prepared from waste lithium ion batteries through sol-gel process

Abstract The core issues that restrict the recycling of waste lithium-ion batteries (LIBs) are the difficulty of separation and the secondary pollution that occurs during the recycling process. In this paper, new LiMn2O4 (LMO) was prepared directly from the leaching solution of waste LIBs, which avoided the complex separation process. Citric acid was used as the leaching reagent, and glucose was used as the reducing agent. The structural properties of the resynthesized LMOs calcined at different temperatures were studied by X-ray diffraction (XRD) and further refined by the Rietveld method. XRD and Rietveld refinement confirmed that all the samples had cubic spinel structures, and the lattice parameters increased with increasing temperature. The surface morphology of the final products was characterized by SEM, TEM and XPS to determine the crystal structure, morphology and surface chemical constitution. The electrochemical properties were also tested. The results showed that the new LiMn2O4 prepared from waste LIBs possessed good electrochemical properties, and this method for recycling waste lithium-ion batteries is feasible.