Low Leaching Rate Research Articles

Suppression mechanism contributing to the low leaching rate of cesium from incineration bottom ash

After the Fukushima Daiichi nuclear accident, municipal solid waste (MSW) contaminated with radiocesium was generated. In Japan, approximately 80% of MSW by weight has been incinerated. As consequence, radiocesium was retained in incineration residue after the accident. Among the incineration residues, fly ash with high radioactivity was transported to an interim storage facility and special treatment has been carried out. While, bottom ash with radioactivity levels of <8000 Bq kg−1 wet has been directly deposited in conventional MSW landfills. The radiocesium leaching from bottom ash is low, but the reason for this low leaching remains unclear. In this study, leaching tests and microscopic observations of bottom ash containing stable Cs were conducted to investigate the mechanism making Cs leaching low. It is noted that the ash used was simulated ash created from combusting refuse derived fuel (RDF) to which stable Cs was added, not real radioactive ash. Based on pH dependence testing, the Cs leaching increased as the pH decreased. The amount of Cs leached in the neutral range was 2–4% mass of the total content. Electron probe microanalysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy confirmed that certain particles in bottom ash contain concentrated Cs. These particles were found to comprise aluminum, silicon, potassium, and oxygen at their core, surrounded by concentrated Cs. Raman microscopy suggested that these particles are microcline. Co-heating of microcline with Cs carbonate led to the condensation of Cs in a manner similar to particles observed in bottom ash, and Cs was captured in a glassy substance formed on the microcline surface, which reduced the leaching of Cs.

The effect of biopolymer stabilisation on biostimulated or bioaugmented mine residue for potential technosol production

Mine waste can be transformed into technosol as an ecological strategy. Despite its importance to soil functions, biological activity is often overlooked. Biopolymers can serve as innovative tools for bioremediation, facilitating chemical reactions and creating networks to encapsulate contaminants. This work aims to assess the use of bioleached and stabilised residues from a tungsten mine for technosol production. The first objective was to evaluate mine tailings for their bioleaching potential by biostimulation or bioaugmentation with strain Diaphorobacter polyhydroxybutyrativorans B2A2W2. The second was to evaluate the effect of Portland cement or biopolymers such as Carboxymethyl Cellulose (CMC) or Xanthan Gum (XG) on the stabilisation of bioleached residues. The impact of biopolymers on residues’ characteristics, such as metal leaching, number of cultivable microorganisms, compression strength and ecotoxicity was evaluated using flow systems. Over time, bioleached metallic elements decreased, except for iron (Fe). Biostimulated and stabilised residues exhibited similar trends; both CMC and cement showed low leaching rates and viable microorganisms in the same order (106 CFU × ml−1). However, bioaugmented residue stabilised with XG showed 106 CFU × ml−1 viable microorganisms and increased 2.2-fold Fe leaching than BA_Control. CMC addition to bioaugmented residue reduced 5.9-fold Fe leaching and increased 100-fold viable microorganisms. By utilising both biological and engineering approaches to characterise the technosol, this study contributes to advancing knowledge of technosol production. The residues biostimulated and stabilised with CMC produced a material useful for bio-applications, with low toxicity and metal leaching, useful for bio-applications. XG was the best stabiliser for geotechnical engineering applications, with improved compression strength. In conclusion, the study demonstrates the usefulness of biopolymer treatment for residues and emphasises the importance of selecting the appropriate biopolymer for the intended function of technosols.

Nitric acid dissolution of germanium overlay in hard zinc slag to enhance germanium leaching and optimization of response surface methodology

With the rapid development of global digital economy and aerospace, the gap between the supply and demand of germanium is expanding, and the establishment of a new process for the deep leaching of germanium from hard zinc slag is imminent. In this paper, on the basis of analyzing the reasons for the low germanium leaching rate from hard zinc slag, a new process of germanium leaching enhanced by HNO3 dissolution of externally wrapped ZnFe2O4 is established, and response surface optimization is carried out. The PbggGe4O7 phase in hard zinc slag is externally wrapped with ZnFe2O4 phase, and the non-reaction between ZnFe2O4 and hydrochloric acid is the main reason for the low leaching rate of germanium from hard zinc slag, so it is necessary to add HNO3 to dissolve the external ZnFe2O4 in leaching, and then use hydrochloric acid to leach the PbGe4O7 containing germanium in the interior. The potential pH diagram of the Pb-Ge-Cl-H2O system was also plotted, indicating that increasing the concentration of chloride ions during hydrochloric acid leaching contributes to the generation of GeCl4 at low acidity. When the hydrochloric acid concentration was 134.65 g/L, the liquid–solid ratio was 6, the theoretical dosage of HNO3 was 0.3, and the leaching time was 236 min, the optimum leaching rate of germanium was 93.72%, which was 23.72% higher than that of germanium leaching from hard zinc slag.

A low-temperature molten salt synthesis strategy of Eu2Ti2O7 ceramic enabling ultra-durable glass-ceramic waste forms

The conventional solid-phase sintering method for the preparation of Ln2Ti2O7 ceramics using oxides as reactants generally requires higher temperature, larger pressure, and greater energy consumption. In this paper, we synthesize a Eu2Ti2O7 ceramic powder in relatively low-temperature molten salt (KCl–NaCl) using Eu2O3 powder and TiO2 powder as reactants at 1000 °C. The results of XRD, SEM, and element mapping indicate that the product have high phase purity, good crystal morphology and uniform chemical composition. Moreover, Eu2Ti2O7-based glass-ceramics are obtained by sintering with different contents of borosilicate glass. The SEM images of glass-ceramic show that almost no obvious defects or voids are found on the surface of the samples. The values of water absorption for all glass-ceramic waste forms are less than 0.5 %. With the increase of ceramic embedding rate, the hardness of the glass-ceramics first increases and then decreases, while the density gradually increases. When the embedding rate increased to 30 wt%, the hardness of glass-ceramic reaches a maximum of 19.53 GPa. The glass-ceramic waste form embedded with 30 wt% Eu2Ti2O7 exhibits very stable chemical durability with a low leaching rate of 1.42 × 10−8 g m−2 d−1 at 90 °C even after 28 days.

Enhanced Fenton-like catalysis via interfacial regulation of g-C3N4 for efficient aromatic organic pollutant degradation

For the efficient degradation of organic pollutants with the goal of reducing the water environment pollution, we employed an alkaline hydrothermal treatment on primeval g-C3N4 to synthesize a hydroxyl-grafted g-C3N4 (CN-0.5) material, from which we engineered a novel Fenton-like catalyst, known as Cu–CN-0.5. The introduction of numerous hydroxyl functional groups allowed the CN-0.5 substrate to stably fix active copper oxide particles through surface complexation, resulting in a low Cu leaching rate during a Cu–CN-0.5 Fenton-like process. A sequence of characterization techniques and theoretical calculations uncovered that interfacial complexation induced charge redistribution on the Cu–CN-0.5 surface. Specifically, some of the π electrons in the tris-s-triazine units were transferred to the copper oxide particles along the newly formed chemical bonds (C(π)-O-Cu), forming a π-deficient area on the tris-s-triazine plane near the complexation site. In a typical Cu–CN-0.5 Fenton-like process, a stable π-π interaction was established due to the favorable positive-negative match of electrostatic potential between the aromatic pollutants and π-deficient areas, leading to a significant improvement in Cu–CN-0.5's adsorption capacity for aromatic pollutants. Furthermore, pollutants also delivered electrons to the Cu–CN-0.5 Fenton-like system via a “through-space” approach, which suppressed the futile oxidation of H2O2 in reducing the high-valent Cu2+ and significantly improved the generation efficiency of •OH with high oxidative capacity. As expected, Cu–CN-0.5 not only exhibited an efficient Fenton degradation for several typical aromatic organic pollutants, but also demonstrated both a low metal leaching rate (0.12 mg/L) and a H2O2 utilization rate exceeding 80%. The distinctive Fenton degradation mechanism substantiated the potential of the as-prepared material for effective wastewater treatment applications.

A hydrometallurgical process flowsheet for recovering MoO3 from Molybdenite

In this study, a comprehensive hydrometallurgical processing of molybdenite (MoS2) concentrate was investigated. This investigation involved a novel approach combining mechanical activation, leaching, and polyelectrolyte extraction methods. The integrated method effectively addressed the challenge of low leaching rate of molybdenite, resulting in the successful production of high-purity molybdenum trioxide. Milled molybdenite samples were analyzed by different methods of X-ray diffraction, atomic force and electron microscopy, and BET. The increasing trend of the specific surface area during milling was determined by a model fitting which was useful for optimization of milling time. Several leaching reagents were studied to achieve high molybdenum dissolution. The most promising results were achieved through a two-hour process, yielding an impressive leaching efficiency of 80% and a resulting Mo concentration of 6700 mg/L. Molybdenum recovery was efficiently carried out through polyelectrolyte extraction, as confirmed by ICP and CHNS analyses, demonstrating selective precipitation of molybdenum from the solution. The subsequent calcination of the precipitated molybdenum(VI) compound resulted in the production of high-purity molybdenum trioxide. Furthermore, a conceptual hydrometallurgical treatment process for molybdenite concentrate was proposed, aiming to recover molybdenum, sulfuric acid, and copper. This proposed process presents a promising avenue for further exploration in pilot plant studies within the molybdenum industry.

Enhanced chalcopyrite leaching by mechanical activation: New insights from microstructure

Bioleaching is an effective technique for treating low-grade copper sulfide ore, but low leaching rate limit its industrial application of chalcopyrite bioleaching. Many passivation film compositions have been hypothesized to explain the difficulty of chalcopyrite leaching, but the essential reason remain unclear. Mechanical activation can not only reduce the solid particle size, but also result in lattice defects and induce redox reactions, and even affect the electronic structure of the solid. Therefore, the mechanical activation enhanced chalcopyrite leaching and its influence mechanism was investigated. The main conclusions are as follows: mechanical activation can significantly reduce chalcopyrite particle size, simultaneously, microscopic strain and cell volume increased; the mechanical activation changed the energy band structure of chalcopyrite—the positions of the conduction band and the valence band were positively shifted, implying that Fe3+ could more easily capture electrons from the chalcopyrite surface to be reduced, thus the leaching rate of chalcopyrite increased significantly in in Fe3+ solution. The results show that the passivation film may affect the leaching of chalcopyrite, but the structural characteristics of chalcopyrite is the essential reason for the difficulty of chalcopyrite leaching.

Electro-controlled membrane coupling heterogeneous electro-Fenton (EM-HEF) in treating acetaminophen: performance, mechanism, DFT calculation, and application

The utilization of hybrid electro-Fenton (EF) processes has gained considerable attention, which can combine the advantages of two or more treatment methods resulting in efficient removal of refractory pollutants from water. Here, an electro-controlled membrane coupling heterogeneous EF (EM-HEF) system was constructed for the degradation of acetaminophen (APAP) in water. In the EM-HEF system, a bifunctional ACF/CC-FeOCl-Cu cathode material was synthesized to enhance the HEF reaction, while a tubular Ti microfiltration membrane was used as both cathode and membrane. Degradation experiments indicated the EM-HEF system exhibited remarkable removal efficiency (97.0 %) towards APAP within 30 min under the conditions of flow rate at 60 mL min−1, initial pH at 7.1, and current density at 3 mA cm−2. Notably, the cathode materials showed low metal leaching rate and high APAP removal rate in the 10 cyclic experiments. The common inorganic anions (Cl−, HCO3−, HPO42−, and NO3−) did not inhibit the removal of APAP significantly, whereas humic acid exhibited a more pronounced inhibitory effect only at concentrations exceeding 10 mg L−1. Quenching experiments and electron spin resonance (ESR) tests confirmed the pivotal role of hydroxyl radicals in the decomposition of APAP. Combining density functional theory (DFT) and LC-MS results, three pathways were proposed. The overall toxicity of intermediates was relieved during EM-HEF degradation of APAP. Furthermore, it also achieved good performance in the treatment of landfill leachate, with increasing BOD5/COD ratio from 0.11 to 0.34. This work provides an efficient strategy for the removal of refractory pollutants in water.

Mechanism Analysis and Experimental Research on Leaching Zn from Zinc Oxide Dust with an Ultrasound-Enhanced NH3-NH4Cl-H2O System

Zinc oxide dust (ZOD) is an industrial solid waste produced in the production process of wet smelting Zn, with large output and great pollution to the environment. The recycling of metallurgical solid waste such as zinc oxide dust is very important to achieve the sustainable development of the circular economy. An experimental study of zinc (Zn) leaching from zinc oxide dust using an ultrasound-enhanced ammonia–ammonium chloride system was performed. The effects of ultrasonic power, leaching time, total ammonia concentration, and other factors on the leaching rate of zinc from zinc oxide dust were investigated. The results revealed that the leaching rate of Zn reached up to 80.70% under the condition of ultrasound power of 1000 W, reaction time of 15 min, total ammonia concentration of 6 mol/L, [NH3]:[NH4+] of 1:1, L/S of 5:1, temperature of 45 °C, and stirring speed of 100 r/min. The conventional leaching was conducted under similar conditions, except that the time was controlled to 40 min and the zinc leaching rate was 71.15%. The leaching rate of Zn in the ultrasound condition was improved by 9.55% compared with that in the conventional leaching process. XRD, laser particle size, and SEM-EDS analyses were conducted to study the leaching residues of ZOD. The analysis results showed that in the ultrasound condition, the largest leaching rate of soluble ZnO phases was achieved after 15 min of leaching. Under the ammoniacal system, it was difficult to leach ZnFe2O4, Zn2SiO4, and ZnS phases, which partly accounted for the low zinc leaching rate. Additionally, through ultrasound-enhanced treatment, the ZnO particles encapsulated in ZOD particles were broken into smaller sizes and exposed to the leaching solution. Thus, the leaching rate of Zn was improved. The experimental results show that ultrasound can tremendously improve the effect of Zn extraction from ZOD, shorten reaction time, and help reduce energy consumption and environmental pollution, making it a promising application in the treatment of secondary Zn resources.

Superior performance of oxygen vacancy-enriched Cu-Co3O4/urushiol-rGO/peroxymonosulfate for hypophosphite and phosphite removal by enhancing singlet oxygen

The treatment of wastewater containing hypophosphite [P(I)] and phosphite [P(III)] is challenged by limitations of traditional Fenton oxidation such as low efficiency, secondary pollution and high costs. This study introduced a facile solvent-thermal method to synthesize Cu-Co3O4 nanoparticles uniformly loaded on graphene (Cu-Co3O4/U-rGO) through the reduction and coordination effects of urushiol (U). As prepared Cu-Co3O4/U-rGO exhibited excellent activity in activating peroxymonosulfate (PMS) for the oxidation of P(I)/P(III) to phosphate [P(V)] (0.229 min−1), along with high stability and reusability (91.5 % after 6 cycles), low metal leaching rate (Co: 0.2 mg/L, Cu: 0.05 mg/L), insensitivity to common anions in water and a wide pH range (3–11). The activation mechanism involved the synergistic effects from both urushiol and graphene, which promoted redox of Cu+/Cu2+ and Co2+/Co3+ and induced abundant oxygen vacancies for PMS activation to produce singlet oxygen. Furthermore, the Cu-Co3O4/U-rGO/PMS was also excellent in the oxidative removal of organic phosphorus. This study is expected to advance strategies for the treatment of P(I)/P(III)-rich wastewater and provide new insights for the development of low-cost, highly efficient heterogeneous catalysts with abundant oxygen vacancies.

Insight on the degradation of P-chlorophenol based on the Co-g-C3N4/diatomite composite photo-Fenton process

The non-metallic photocatalyst g-C3N4 is characterized by non-toxicity and straightforward preparation. Still, the drawbacks of low specific surface area and fast carrier complexation rate limit its practical application in photocatalysis. In this work, Co-doped g-C3N4 was prepared on the surface of diatomite (DE) through a simple one-step calcination method and used to degrade 4-chlorophenol (4-CP). Photocatalytic experiments showed that Co-g-C3N4/DE removed up to 98.7% of 4-CP. The doping of Co could capture the photogenerated electrons, thereby reducing the recombination of photocarriers and accelerating the oxidation–reduction reaction of Co3+/Co2+ and H2O2, promoting the generation of OH. Introducing DE improved the aggregation of g-C3N4 and expanded its surface area. Moreover, the Co-g-C3N4/DE composite material exhibited a low metal leaching rate and good reusability throughout the experiments. In addition, based on DFT calculations, the ROS attack mechanism of 4-CP degradation was speculated and calculated. The final product of 4-CP was found to pose a more minor environmental threat, according to the T.E.S.T. In short, the prepared Co-g-C3N4/DE composite realized the capture of photogenerated electrons and the acceleration of photocatalytic reactions, which are expected to solve the challenges in wastewater treatment.

Waste rice noodle-based CQDs/ZnO composite nanorod array on steel wire mesh: Preparation and photocatalytic capability

In order to develop cost-effective photocatalytic devices, a nanorod array-structured carbon quantum dot/zinc oxide (CQDs/ZnO) composite was synthesized on a steel wire mesh substrate using waste rice noodle (WRN) as the raw material. The incorporation of CQDs derived from WRN played a crucial role in controlling the morphology of the ZnO-based array. By optimizing the loading of CQDs, the CQDs/ZnO composite achieved a regular hexagonal prism nanorod array distribution and exhibited highly efficient photocatalytic degradation of various organic pollutants. For instance, in the case of methylene blue, the CQDs/ZnO composite demonstrated a remarkable degradation rate of 99.4% within 90 min, with a high degradation rate constant of 0.033 min−1. Moreover, the composite material could be recycled and reused for five photocatalytic cycles without a significant decrease in its degradation performance, surpassing the performance of the pure ZnO array. Furthermore, the resulting CQDs/ZnO composite array exhibited effective photocatalytic degradation for other organic dyes such as malachite green, methyl violet, basic fuchsin, and rhodamine B. Additionally, this composite material was successfully applied to real water purification, achieving a high mineralization efficiency of 48% and a low leaching rate of Zn of 1.2% in a river water sample after a 90-minute photocatalytic process. The introduction of CQDs derived from WRN into the ZnO array led to efficient electron-hole pair separation, enabling more photogenerated electrons to reduce O2 and more photogenerated holes to oxidize H2O. This resulted in enhanced radical generation and improved photocatalytic degradation of organic pollutants, showcasing the superior performance of the CQDs/ZnO composite array.

Study on Electrochemical Behavior of Oxidized Pyrite in Alkaline Electrolyte

Nowadays, refractory gold ore production around the world accounts for about 30% of total gold production, and the low leaching rate of such gold concentrate seriously limits the efficient utilization of gold resources. The alkaline preoxidation process can improve the leaching rate of this kind of gold deposit and has good development and application prospects. Therefore, it is of great significance to study the oxidation and dissolution behavior of pyrite in an alkaline environment. In this paper, the oxidation process of gold-bearing pyrite in an alkaline electrolyte was studied using electrochemical techniques, and the oxidation products of a pyrite electrode in an alkaline solution were characterized using XPS, SEM, and other analytical methods. The results show that the optimum pH for pyrite electrochemical oxidative dissolution is about 12, and the oxidation potential of pyrite should be above 0.8 V. In the process of alkaline oxidative dissolution of pyrite, part of the S element enters the electrolyte in the forms of Sx2−, S2O32−, SO32−, and SO42−, and a small amount of the S element is adsorbed on the surface of the electrode in the form of S0 and becomes a part of the passive layer. The Fe element is adsorbed on the surface of the electrode in the forms of Fe(OH)2, Fe2O3, and Fe2(SO4)3, which become the main components of the passivation layer. This study provides a theoretical basis and reference data for the chemical preoxidation treatment of gold-bearing sulfide ores.

Mineralogical Characteristics and Refractory Properties of Arsenic-bearing Gold Concentrate

The arsenic-bearing gold ore in Anhui, China is an extremely refractory gold ore, and the cyanide leaching rate of gold concentrate is only 30%. The mineralogical factors of the low leaching rate were discussed by process mineralogical analysis. The results demonstrate that the gold concentrate contains 20.30 g/t gold, 3.39% arsenic, 29.80% sulfur, and 4.10% iron. Gold exists in the occurrence states of metallic mineral-wrapped gold, gangue mineral-wrapped gold, and fissure gold, which account for 64.90%, 8.31%, and 26.79% respectively. Pyrite, arsenopyrite, realgar, sphalerite, and pyrrhotite are the main metallic minerals in the gold concentrate. Among them, pyrite accounts for 51.20%, and arsenopyrite accounts for 24.30%. Gangue minerals include feldspar, sericite, quartz, calcite, and chlorite. Among them, feldspar, sericite, and quartz account for 30.30%, 22.70%, and 22.10% respectively. In addition, the grain size distribution and distribution characteristics of various minerals are obtained, and the texture and structure of the gold ore are analyzed. Pyrite and arsenic phase are the main factors causing the low gold leaching rate. The presence of carbonate minerals and clay minerals also has an effect on the rate of gold leaching. The oxidation pretreatment can increase the gold leaching rate to about 94%. The findings provide a theoretical foundation for the selection of manufacturing techniques in businesses and play an important guiding role in the development of similar ores.

Study on the extraction of poor scandium from low-grade rare earth ores

ABSTRACT Based on the analysis of chemical composition and the phase of scandium-containing clay type rare earth ore in a place in China, it is found that the content of scandium in the ore is 18.55 g/t. Due to the low leaching rate of direct acid leaching, the effect of the roasting process on the leaching rate of scandium from the raw ore was investigated. By comparing the operation of the two leaching processes, oxidised roasting-hydrochloric acid leaching and alkali roasting-hydrochloric acid leaching, we obtain the optimal technical conditions for both leaching processes. At the same time, the mechanism of alkali roasting was investigated and the conclusions obtained are of some reference for the extraction of scandium from low-grade rare earth ores.

Study on Mechanism of Oxygen Oxidation Leaching with Low Acid for High Acid Consumption Sandstone Uranium Deposit

In view of the problems of high acid consumption, the blockage of ore-bearing beds caused by iron ion precipitation and a low leaching rate in the conventional acid leaching of a sandstone-type uranium deposit, the author put forward “low-acid and oxygen leaching technology” research. In order to further clarify the mechanism of leaching sandstone-type uranium ore with low acid and oxygen, the oxidation mechanism of ferrous ion under acidic conditions, the influencing factors of ferrous ion oxidation process, the kinetic simulation of oxygen oxidizing uranium minerals, and the interaction of uranium and iron precipitation in acidic solution were analyzed and studied, and the mechanism of oxygen oxidizing uranium and uranium leaching under low acid conditions was explored. The results show that under the condition of low acidity, the kinetics that ferrous oxidized by oxygen is between the first-order and second-order reaction, which can reduce the iron ion precipitation and then reduce the influence of iron ion precipitation on uranium leaching, so as to improve the uranium leaching rate. This study confirmed the feasibility of the oxygen oxidation of uranium and uranium leaching under low acidity conditions, which has a good guiding significance for the effective leaching of uranium deposits with a high acid consumption.

Galvanic Biomining: A Low-Carbon Hydrometallurgical Process for Efficient Resource Recovery and Power Generation

Biomining/bioleaching has been widely considered as an alternative to traditional technologies in processing ores and solid wastes mainly due to its environmental and economic advantages, but the low leaching rate and adverse microorganism–environment interactions cause the severe restriction of industrial application. Here, we first report galvanic biomining that achieves both efficient resource recovery and power generation based on the Fe2+/Fe3+ redox couple of indirect/noncontact bioleaching. Ore oxidation, oxidant regeneration, and power generation are achieved in the oxidation leaching section, the bio-oxidation section, and the galvanic biomining reactor, respectively. At various temperatures and in the presence of toxic ions, leaching rates are approximately 10 times higher than those of conventional bioleaching, which represents a breakthrough of the above bottleneck. The power generation, bacterial carbon sequestration, and total carbon sequestration are 23.5 kW h, 34.4, and 2510 kg, respectively, per ton of copper produced. The galvanic biomining effectively regenerates oxidants at room temperature and significantly reduces power consumption and the carbon footprint.

Ultrasonic enhanced hydrazine sulfate acid leaching of low-grade germanium dust

By studying the principle of ultrasonic-enhanced leaching and the mechanism of reductive acid leaching of valuable metals, a new method of ultrasonic-enhanced hydrazine sulfate leaching of Zn and Ge in zinc oxide dust in the sulfuric acid system has been proved to be very effective. The phase of Ge was determined by the segmented dissolution method, and it was found that insoluble Ge mainly existed in the form of GeS2 and tetragonal phase germanium dioxide. Comparing the solubility of GeS2 and GeS in a hot high-acid system, it was found that GeS was more easily dissolved. And Ge4+ is more prone to hydrolysis at high acid concentration, resulting in germanium precipitation again. In addition, the presence of more Fe3+ in the solution will increase the potential of the system and inhibit the leaching of zinc ferrate. Therefore, the reduction method is used to reduce Ge4+ and Fe3+ to Ge2+ and Fe2+, which can accelerate the dissolution of zinc ferrate and effectively avoid the low leaching rate of zinc and germanium caused by the hydrolytic precipitation of Fe3+ and Ge4+. The valence states of Fe and Ge in the leachate were analyzed by ICP and XPS, and it was found that they mainly existed in the leachate in the divalent form, which indicated that Fe3+ and Ge4+ had been successfully reduced to Fe2+ and Ge2+. Some black substances were formed in the leaching process. EPMA analysis showed that the material was the oxide of silicon and aluminum, and the number of black substances increased gradually with the passage of leaching time, which led to the decreased metal leaching rate. The leaching kinetics is used to study the mechanism of the leaching process. By comparing the fitting degree of multiple models, the final mixed controlled model is determined to control the whole leaching reaction. Therefore, it is finally determined that when the initial acidity of 140 g/L, the liquid–solid ratio of 7 mL/g, the dosage of the reducing agent is 1/3 of the molar mass of Fe in zinc oxide dust, and ultrasonic intensity of 300 W is used to leach for 60 min at 333 K, Zn and Ge have the best leaching efficiency and economic benefits, and the leaching rates are 96.56 % and 94.62 %, respectively. Compared with conventional leaching, the leaching rate of Zn is increased by 6 %, and that of Ge is increased by 11 %.

Catalytic oxidation of methylene blue by using Ni-Fe bimetallic catalyst/NaClO system: Performance, kinetics, mechanism, and DFT calculations

Nickel-iron bimetallic catalysts (Ni-Fe/Al2O3) with activated alumina as the carrier were prepared by impregnation pyrolysis, and the structure and morphology of the catalysts were analyzed by characterization via X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, programmed temperature rise reduction, and X-ray photoelectron spectroscopy. The prepared Ni-Fe/Al2O3 was used to catalyze sodium hypochlorite (NaClO) for oxidative degradation of methylene blue (MB). The effects of the dosage of iron nitrate in the preparation process and the catalyst dosage, effective chlorine concentration, pH, temperature, initial MB concentration, and coexisting ions on the MB removal effect in the oxidative degradation experiment were investigated. Then, the oxidative degradation mechanism and practical application prospects of the system were examined. Results showed that the Ni and Fe nitrate mass ratio of 5:1 was favorable for the dispersion of Ni elements in the catalyst preparation process. When the temperature was 45 °C, the initial concentration of MB was 50 mg/L, the dosage of Ni-Fe/Al2O3 was 20 g/L, the initial effective chlorine concentration was 1.76%, and the initial pH was 4.0. The removal rate of MB could reach 98.98% within 40 min, and the degradation process was in accordance with quasi primary kinetics, with reaction rate constant of 0.0873 min−1. The experimental result of free radical quenching and electron paramagnetic resonance indicated that the addition of Ni-Fe/Al2O3 induced the decomposition of NaClO to produce a large amount of singlet oxygen (1O2), 1O2 and HOCl worked together to oxidatively degrade MB. Ni3+/Ni2+ redox cycles play an important role in activating HOCl to produce active species. The common inorganic anions (Cl−, NO3−, and SO42−) did not inhibit the removal of MB significantly, whereas humic acid showed a more significant inhibitory effect only at concentrations greater than 20 mg/L. In the five recycling experiments, the catalysts showed low metal leaching rate and high MB removal rate. Ni-Fe/Al2O3 was reusable, and it showed excellent application potential.

Numerical modeling of gas-solid two-phase flow in a plasma melting furnace

Due to the low leaching rate of heavy metals and high energy utilization, the plasma melting furnace is widely used in the harmless treatment of waste incineration. In this paper, the gas-solid two-phase flow in the plasma melting furnace was numerically modelled by means of Euler-Lagrange approach. Conclusions can be drawn that the raw fly ash should be pelletized in order to successfully participate in the melting reaction and effectively avoid the collisional erosion with the graphite cathode. The velocity and temperature fields in the furnace showed asymmetry due to the interaction between the flue gas and slag particles. The heat-up degree of fly ash was much greater than that of SiO 2 particle while the speed discrepancy was small. The average temperature of slag particles in the effective melting region was relatively higher, which is more beneficial to the melting reaction. In addition, three evaluation indicators were established to analyze the distribution of slag particles on the bath surface. The mass proportions of different slag particles in the effective melting region were >40%, the maximum value of their radial distribution can be obtained in the radius range of 0.9–1.0 m, and the mass ratios of fly ash and SiO 2 particles in the same radius domain were overall closer to that of entering the furnace. Finally, the collisional erosion effect of slag particles on the graphite cathode was discussed in detail. • Particle flow in the plasma melting furnace is studied by Euler-Lagrange approach. • Fly ash pelletizing facilitates melting reaction and avoids collisional erosion. • Distribution of particles on bath surface can be evaluated by three key indicators. • High portion of particles in the effective melting region benefits melting reaction.