Arsenic removal from lead-zinc smelter ash by NaOH-H2O2 leaching

Leaching kinetics of fluorine during the aluminum removal from spent Li-ion battery cathode materials

The high content of aluminum (Al) impurity in the recycled cathode powder seriously affects the extraction efficiency of Nickel, Cobalt, Manganese, and Lithium resources and the actual commercial value of recycled materials, so Al removal is crucially important to conform to the industrial standard of spent Li-ion battery cathode materials. In this work, we systematically investigated the leaching process and optimum conditions associated with Al removal from the cathode powder materials collected in a wet cathode-powder peeling and recycling production line of spent Li-ion batteries (LIBs). Moreover, we specifically studied the leaching of fluorine (F) synergistically happened along with the removal process of Al, which was not concerned about in other studies, but one of the key factors affecting pollution prevention in the recovery process. The mechanism of the whole process including the leaching of Al and F from the cathode powder was indicated by using NMR, FTIR, and XPS, and a defluoridation process was preliminarily investigated in this study. The leaching kinetics of Al could be successfully described by the shrinking core model, controlled by the diffusion process and the activation energy was 11.14 kJ/mol. While, the leaching of F was attributed to the dissolution of LiPF6 and decomposition of PVDF, and the kinetics associated was described by Avrami model. The interaction of Al and F is advantageous to realize the defluoridation to some degree. It is expected that our investigation will provide theoretical support for the large-scale recycling of spent LIBs.

A novel strategy for arsenic removal from acid wastewater via strong reduction processing

Due to the high-acidic arsenic-containing wastewater pollution greatly threatening human health and ecological safety, a simple and efficient method for reducing arsenic was proposed in this paper to solve this problem. By using potassium borohydride (KBH₄) as a reducing agent, the soluble arsenic was converted into the gaseous arsine (AsH₃) or solid arsenic (As⁰) to achieve the purpose of removing arsenic in wastewater. By exploring the reaction kinetics of the arsenic removal process, it was found that the fast reaction stage (0-2 min) conformed to pseudo-first-order kinetics. The removal rate of arsenic increased to over 73% in 0.5 min, and reaction equilibrium was reached after 30 min. Various influence factors including arsenic valence, aeration, addition method, concentrations of reducing agent, and hydrogen ion (H⁺) were investigated. The results showed that As(III) was easier to be removed by reduction than As(V), while adding KBH₄ in multiples and aeration were both favorable to the removal of arsenic. Increased concentration of KBH₄ also enhanced the removal of arsenic. Appropriate H⁺ concentration contributed to the arsenic removal, but excessive H⁺ concentration conversely has an inhibitory effect. The maximum removal rate of arsenic was 95.87%, with the maximum removal capacity of 45.50 mg/g. Based on the XRD and SEM-EDS analysis of residue, amorphous arsenic (As⁰) with a mass ratio of more than 94.52% was generated after the reduction of soluble arsenic. Our study demonstrated that the reaction mechanism of reductive degradation is soluble arsenic with hydrogen radicals (H•) to form arsenic (As⁰) and arsine (AsH₃) (in the molar ratio of 6:1). Although the generated solid arsenic (As⁰) is convenient for the soluble arsenic removal from wastewater, attention must be paid to the formation of AsH₃, and strategies for AsH₃ treatment should be considered.

Soil and plant contamination by potentially toxic and emerging elements and the associated human health risk in some Egyptian environments

The aim of this work was to assess the origins, mobility, bioavailability and potential health risks of V, Cr, Co, As, Se, Mo, Cd, Sn and Sb, which are not sufficiently studied in the terrestrial environment of Egypt. This has been carried out by employing a combination of chemical fractionation, plants uptake, mathematical modeling and risk assessment approaches on a wide range of soils and plants sampled from industrial, urban and agricultural locations across Egypt. The contents of As, Cd, Sn and Sb were elevated in the soils of some urban and industrial locations within Cairo, although their soil geo-accumulation (I_geo) indices remained ≤ 2, indicating only moderate contamination. Selenium showed moderate to heavy contamination levels (I_geo up to 4.7) in all sampling locations, and Sb was highly elevated (I_geo = 7.1; extreme contamination) in one industrial location. Therefore, Se was the most important contributor to the pollution load followed by Sb and Cd. Both principle component analysis (of total content) and geochemical fractionation (by sequential extraction) suggested that V, Cr and Co are mostly of geogenic origin, while Se and Sb contents appear to be highly influenced by anthropogenic inputs. The most mobile and bioavailable element was Cd with a large non-residual fraction in all soils (76% of total Cd). The bio-concentration factors of Cd in leafy and fruiting plants were 50 times larger than other elements (except Mo) indicating preferential systematic plant uptake of Cd. Risk assessment models showed an overall low noncarcinogenic and carcinogenic risks to the population of Egypt due to the studied elements with only a few anomalies.

A closed-circuit cycle process for recovery of carbon and valuable components from spent carbon cathode by hydrothermal acid-leaching method

Spent carbon cathode (SCC) as a hazardous solid waste produced in aluminum electrolysis industry, contains plenty valuable components but generate a seriously threat to the environment. This paper focus on a closed-circuit cycle process for direct treatment of SCC based on the hydrothermal acid-leaching method. Thermodynamic calculation, single factor experiment, orthogonal experiment and kinetic study are utilized to obtain the leaching properties of impurities, optimize the leaching conditions, study the influence of conditions on leaching, and capture the restriction factors of leaching. The results indicate that the carbon content of the treated SCC can reach 97.3% when the leaching condition attach the optimal (liquid-solid ratio of 25 mL/g, temperature of 413 K, time of 270 min and acid concentration of 4 mol/L), and liquid-solid ratio is regarded as the crucial factor influencing on that. In addition, the activation energy of impurities reaches 6.25 kJ/mol and the whole leaching process is controlled by the diffusion extent. Finally, the filtrate after the hydrothermal acid leaching is treated, and calcium fluoride, cryolite and sodium chloride are successfully separated. The proposed process eliminates the harm of SCC to the environment, and completes a closed-circuit cycle for the treatment of SCC and recovery of valuable components. It enriches the hydrometallurgical processes of SCC, and provides an attractive scheme for the treatment of SCC.

Co-treatment of copper smelting flue dust and arsenic sulfide residue by a pyrometallurgical approach for simultaneous removal and recovery of arsenic

As the typical hazardous arsenic pollutants, copper smelting flue dust (CSFD) and arsenic sulfide residue (ASR) are produced extensively during copper smelting process, which pose significant pressure on environmental protection and green development of the copper industry. This work proposed an economic, efficient, and applicable approach to treat waste with waste, in which the simultaneous removal and recovery of As from CSFD and ASR were realized by a roasting process, with adding sulfuric acid, at a relatively low temperature (300-350 ℃). The thermodynamic analysis and experiments confirmed that the main phases of As₂S₃ and S₀ in the ASR were used as a reductant for reducing As(Ⅴ) in the CSFD, and the introduction of sulfuric acid favorably enhanced the thermodynamic driving force and greatly lowered the reaction temperature. The results indicated that removal and behavior of As were highly dependent on the mass ratio of ASR to CSFD, roasting temperature, and H₂SO₄ dosage. By regulating the parameters, the species As₂S₃, As₂O₅, and arsenate were all converted to volatile As₂O₃, which could be captured and deposited in cold water. In the optimized co-treatment, a satisfied As removal efficiency of 96.12% was achieved, while getting the 97.03% pure As₂O₃.

Mechanochemical activation on selective leaching of arsenic from copper smelting flue dusts

A novel application, including mechanochemical pre-treating and alkali leaching, for arsenic selective leaching from copper smelter flue dusts (CSFUs) was developed to overcome the disadvantages of hydrometallurgical methods. Compared with raw CSFU powders, the mechanical-activated ones showed higher maximum arsenic leaching efficiency (increased by ~20%), and lower apparent activation energy (decreased by ~7 kJ·mol^-1). Furthermore, this novel process only consumed half of alkali and sulfides and needed one-third of the leaching time to compare with the ones used in the traditional alkali leaching process. The promoting effect of mechanical force on arsenic leaching firstly relied on the physical property changes of CSFU powders, including a decrease of particle sizes and an increase of the specific surface. Secondly, mechanochemical force converted As⁵⁺ species into reduced phases (e.g. As₂O₃, NaAsO₂), and thio-arsenates (e.g. AsO₂S₂^3-, AsO₃S^3-), which could spur its leaching due to their stronger mobilities in the alkali solution within sulfides. Finally, mechanochemical activation could be facilitated to separate discrete soluble arsenic species or incorporated ones from sulfate minerals in the CSFUs. This work may have important implications for the development of new eco-friendly technologies for purifying arsenic-bearing materials.

Remediation of Soil in a Deserted Arsenic Plant Site Using Synthesised MgAlFe-LDHs

Layered double hydroxides (LDHs) are promising soil contamination amendment agents for its efficient absorbing abilities. However, the application of LDHs in remediation of heavy metal contaminated soil are to be developed. In this study, we synthesized MgAlFe-LDHs by introducing Fe³⁺ into interlayer of the MgAl-LDHs using co-precipitation method. X-ray diffraction (XRD), Fourier transform infrared spectrometer (FT-IR) and scanning electron microscope (SEM) were employed to characterized the micro structure of MgAlFe-LDHs. And then pot incubation and pilot experiments were conducted to investigate the heavy metal removal efficiencies of MgAlFe-LDHs and its potential being applicated in As contaminated soil amendment from a deserted arsenic plant site. Incubation experiments showed that the MgAlFe-LDHs had a higher removal efficiency on arsenic contaminated soil compared to other agents. And the results of pilot experiments indicated that the MgAlFe-LDHs can immobilize up to 90% of the As in soil with 5% (w/w) addition. Based on the results above, MgAlFe-LDHs are promising materials amending the heavy metal contaminated soil with practical application value.

Efficient removal and recovery of arsenic from copper smelting flue dust by a roasting method: Process optimization, phase transformation and mechanism investigation

The efficient removal and recovery of arsenic from copper smelting flue dust have received widespread attention due to its extremely high toxicity and carcinogenicity. In this research, a roasting method used for treating the dust at a relatively low temperature (300-400 ℃), with adding sulfuric acid and bitumite, was proposed, in which the reduction of As(Ⅴ) and oxidation of arsenic sulfides were achieved simultaneously. It was proved by thermodynamic analysis and experiments that adding sulfuric acid was favorable for the removal of arsenic, through enhancing the thermodynamic driving force and promoting the transformation of arsenate and arsenic sulfides to As₂O₃. The phase transformation of arsenic was analyzed using XRD, SEM-EDS and XPS, which indicated that coal addition, roasting temperature and H₂SO₄ dosage play essential roles in arsenic removal. Based on the lab-scale experiments, the optimal conditions for arsenic removal were found to be at the roasting temperature of 300-400 °C, roasting time of 2-3 h, coal addition of 5% and H₂SO₄ dosage of 0.2-0.3 mL/g. Around 98% of arsenic was volatilized from the dust, while arsenic content in the residue was decreased to 0.57%. Eventually, arsenic was recovered as As₂O₃ with a high purity of 99.05%.

Alkali circulating leaching of arsenic from copper smelter dust based on arsenic-alkali efficient separation

Leaching arsenic from solid waste selectively and removing arsenic from alkaline leachate efficiently are two key points in alkali treatment of copper smelter dust, and the latter is challenging. In this study, composite salt precipitation of magnesium ammonium arsenate (NH₄MgAsO₄·6H₂O), similar to magnesium ammonium phosphate (NH₄MgPO₄·6H₂O), was proposed to solve the difficult problem of separation arsenic from alkali. Based on the thermodynamic analysis, the selective leaching of arsenic from copper smelting dust was carried out in the NaOH-Na₂S system. In the alkali leaching system, more than 80% arsenic can be leached out from the dust with the diffusion-controlled type in the Avrami model, while the leaching rates of valuable metals are less than 0.5%. For the strong alkaline leachate containing arsenic obtained by alkali leaching, the selective removal of arsenic was achieved by adding magnesium salt and ammonium salt. With the change of the amount of magnesium salt and ammonium salt, the sedimentation performance and composition of the arsenic slag changed accordingly. At the mole ratio of NH₄⁺: As = 8:1 and Mg²⁺: As = 1.5:1, 96.38% of arsenic was removed, and the content of arsenic in the arsenic slag composed of MgNH₄AsO₄·6H₂O reached 28.96%. On this basis, the circulating alkali leaching of copper smelter dust based on arsenic-alkali separation was successfully carried out. The whole scheme is not only economical and safe, but also achieves the reuse of wastewater without secondary pollution, which provides an alternative solution for the treatment of arsenic containing solid waste.

Selective Pre-leaching of Tellurium From Telluride-Type Gold Concentrate

With a telluride-type gold ore flotation concentrate as the research object, the Na₂S + NaOH collaborative leaching process was applied to selectively separate tellurium before the cyanide leaching of gold and silver. The effects of process parameters including the type of leaching agent, the amount of leaching agent, liquid-solid ratio, leaching temperature, and leaching time on the leaching rate of tellurium were investigated. The results showed that the tellurium leaching rate could reach 78.14% under the optimum conditions of −0.038 mm (95%) grinding fineness, 80 g/L Na₂S concentration, 30 g/L NaOH concentration, 4:1 liquid-solid ratio, 80°C leaching temperature and 3 h′s leaching time. The kinetic analysis showed that the leaching process of tellurium from telluride-type gold concentrate was a mixed type of chemical reaction control and diffusion control. The grain parameter in the leaching process was 0.26263 and the apparent activation energy E = 17.12 kJ/mol. Tellurium could be pre-leached from the telluride-type gold flotation concentrate through the Na₂S + NaOH alkaline leaching process to achieve the effective separation of tellurium from noble metals, which, when eliminating the adverse effects of telluride on the leaching of gold and silver, provides new ideas for the extraction of rare element tellurium.

Silicon recovery from diamond wire saw silicon powder waste with hydrochloric acid pretreatment: An investigation of Al dissolution behavior

Silicon recovery from diamond wire saw silicon powder (DWSSP) waste is of great significance for increasing production profits and alleviating hazardous effects on the ecological environment. The purity of recovered silicon powder is determined by the purification efficiency during acid leaching pretreatment. Because the metallic impurities present in DWSSP are mostly physically mixed rather than chemically bound, the reaction rate is very fast in the initial stage of acid leaching, whereas it is difficult to dissolve the retained impurities in the later stage with the depletion of metal fragments adhered on the surface of the silicon matrix. Many prior studies have failed to consider the retained metallic impurities that reside in the inner silicon particle surfaces. Therefore, this study investigates the dissolution behavior of retained impurities via the dissolution of Al in HCl solution as an example. Thermodynamic results indicate that the Al dissolution process is dominated by entropic changes (ΔS⁰), rather than enthalpic changes (ΔH⁰). Furthermore, the dissolution behavior of Al is in accordance with the diffusion-controlled step in the Avrami mode, and the kinetic parameters were found to be A=5.85×10⁷, E_a=49.27kJ·mol^-1, and m<1. The determined dissolution behavior provides a clear understanding of the removal of retained metallic impurities from DWSSP via an acid leaching pretreatment. This study provides enlightenment for the further purification of silicon recovered from DWSSP waste.

Experimental studies on removal of arsenites from industrial effluents using tridodecylamine supported liquid membrane

In this article, we report the efficient removal of arsenic in the form of arsenite (As(III)) from an aqueous solution which was applied on the industrial effluents. A flat-sheet polypropylene-supported liquid membrane (SLM) was clamped between the feed phase and strip phase in a liquid membrane permeator setup using tridodecylamine (TDDA) as carrier for extraction of As(III). We have optimized the reaction in changing different parameters such as metal, acid concentration in feed phase, strip phase concentration, and carrier concentration for the maximum As(III) transport through the SLM. The flux value, time of extraction, the stoichiometry of the complex formed, and membrane stability were also investigated. As a result of different experiments performed, we find the best conditions of 0.1 mol/L of TDDA, 1.0 mol/L of HCl in feed phase at optimum pH 1, and 1.0 mol/L of NaOH in stripping phase for the maximum As(III) removal. The optimized reaction was utilized on effluent collected from different industries. During repeated set of experiments on a single polypropylene membrane, it was found that it could withstand five consecutive experiments. Moreover, as high as 93% of extraction efficiency was achieved in 180 min.

Microwave-enhanced selective leaching of arsenic from copper smelting flue dusts

Alkaline sulphide leaching processes have been widely used for the arsenic selective removal from metallurgical wastes. However, high consumption of energy and chemicals as well as long extraction time have forced the industries to find more cost-effective and eco-friendly separation techniques. In this work, a feasible microwave-enhanced leaching process was developed for arsenic selective removal from copper smelter dusts, and arsenic leaching efficiency could reach ∼98 % after leaching for 10 min with 0.5 mol·L^-1 alkaline solution, in contrast to 86 % removal for the conventional electrical-heating leaching with even a more intensive alkali content (1.0 mol·L^-1) and a longer extract time (1.5 h). Furthermore, energy cost was cut down to one-tenth of the conventional leaching process. Two enhancing mechanisms were proposed: 1) rapid oxidation of As(III) of the dusts occurred under microwave irradiation reduced the energy potential for arsenic dissolving in alkali media; 2) cracks and fissures formed in the dust particles after microwave, coupled with the temporary and localized superheating in the bulk liquid, would dramatically enhance arsenic leaching kinetics and decrease the activation energy value from 42.88 to 35.81 kJ·mol^-1 (40-70 ℃). This work may have important implications to the development of new technologies to purify arsenic-bearing materials.

Arsenic removal from highly-acidic wastewater with high arsenic content by copper-chloride synergistic reduction

A synergistic combination of chloride and copper powder was proposed as a new method to reductively remove arsenic from highly-acidic wastewater with high arsenic content (HAWA). As(III) was reduced to As(0) by copper powder in the presence of chloride and were effectively removed from HAWA. The procedure to remove arsenic was optimized as follows: initial H⁺ concentration of 5 mol L^-1, Cu-to-As molar ratio of 8, Cl-to-As molar ratio of 10, a reaction temperature of 60 °C, copper powder particle size of 68-24 μm, and a stirring speed of 300 r min^-1. Under these optimal conditions, the removal rate of arsenic was close to 100%. Kinetics results suggested that the arsenic removal process was controlled by both diffusion and chemical reactions with an apparent activation energy of 29.78 kJ mol^-1. The XRD results showed that the removed arsenic in the residue existed primarily in the form of AsCu₃ alloy.

Mineralogy and Pretreatment of a Refractory Gold Deposit in Zambia

The technological mineralogy of a gold deposit located in North-Western province of Zambia was carried out by using X-ray fluorescence spectroscopy (XRF), X-ray diffraction spectroscopy (XRD), and scanning electron microscope (SEM). The results showed that gold was highly dispersed in gold-bearing minerals such as pyrite, arsenopyrite, and some gangues in the form of natural gold and electrum. The gold grade in the mineral was 15.96 g/t and the particle size distribution of gold was extremely uneven. Most of the gold particles were less than 10 μm and wrapped with gold-bearing minerals, making it difficult to achieve liberation during grinding. According to the characteristics of the refractory gold deposit, the gravity–flotation combined beneficiation process was used to recover the liberated coarse gold and the fine gold in the sulphides. The closed-circuit experiments obtained excellent indicators. The grade and recovery of gold in the gravity separation concentrates reached 91.24 g/t and 57.58%, respectively. The grade and recovery of gold in the flotation concentrates were 49.44 g/t and 33.36%, respectively. The total recovery of gold was 90.94%. The gravity–flotation combined beneficiation pretreatment process provided a feasible method for the refractory gold ore and ensured the effective recovery of gold.

Selective Separation of Arsenic from Lead Smelter Flue Dust by Alkaline Pressure Oxidative Leaching

This study investigated the feasibility of using an alkaline pressure oxidative leaching process to treat lead smelter flue dust containing extremely high levels of arsenic with the aim of achieving the selective separation of arsenic. The effects of different parameters including NaOH concentration, oxygen partial pressure, liquid-to-solid ratio, temperature, and time for the extraction of arsenic were investigated based on thermodynamic calculation. The results indicated that the leaching efficiency of arsenic reached 95.6% under the optimized leaching conditions: 80 g/L of NaOH concentration, 1.0 MPa of oxygen partial pressure, 8 mL/g of liquid-to-solid ratio, 120 °C of temperature, 2.0 h of time. Meanwhile, the leaching efficiencies of antimony, cadmium, indium and lead were less than 4.0%, basically achieving the selective separation of arsenic from lead smelter flue dust. More than 99.0% of arsenic was converted into calcium arsenate product and thus separated from the leach solution by a causticization process with CaO after other metal impurities were removed from the solution with the addition of Na2S. The optimized causticization conditions were established as: 4.0 of the mole ratio of calcium to arsenic, temperature of 80 °C, reaction time of 2.0 h. The resulting product of calcium arsenate may be used for producing metallic arsenic.