2021 Vol. 28, No. 9
As the world’s second largest economy experiencing rapid economic growth, China has a huge demand for metals and energy. In recent years, China ranks first, among all the countries in the world, in the production and consumption of several metals such as copper, gold, and rare earth elements. Bioleaching, which is an approach for mining low grade and refractory ores, has been applied in industrial production, and bioleaching has made great contributions to the development of the Chinese mining industry. The exploration and application of bioleaching in China are reviewed in this study. Production and consumption trends of several metals in China over the past decade are reviewed. Technological processes at key bioleaching operations in China, such as at the Zijinshan Copper Mine and Mianhuakeng Uranium Mine, are presented. Also, the current challenges faced by bioleaching operations in China are introduced. Moreover, prospects such as efficiency improvement and environmental protection are proposed based on the current situation in the Chinese bioleaching industry.
Phenolic compounds are widely present in domestic and industrial sewage and have serious environmental hazards. Electrochemical oxidation (EO) is one of the most promising methods for sewage degradation because of its high efficiency, environmental compatibility, and safety. In this work, we present an in-depth overview of the mechanism and factors affecting the degradation of phenolic compounds by EO. In particular, the effects of treatment of phenolic compounds with different anode materials are discussed in detail. The non-active anode shows higher degradation efficiency, less intermediate accumulation, and lower energy consumption than the active anode. EO combined with other treatment methods (biological, photo, and Fenton) presents advantages, such as low energy consumption and high degradation rate. Meanwhile, the remaining drawbacks of the EO process in the phenolic compound treatment system have been discussed. Furthermore, future research directions are put forward to improve the feasibility of the practical application of EO technology.
A series of laboratory investigations are conducted to analyze the effect of flocculant type on the spatial morphology and microstructural characteristics of flocs during the flocculation and settling of tailings. Four flocculant types (i.e., ZYZ, JYC-2, ZYD, and JYC-1) are considered in this study. The fractal characteristics and internal structures of tailings flocs with different flocculant types and settlement heights are analyzed by conducting scanning electron microscopy and X-ray micro-computed tomography scanning experiments based on the fractal theory. Results show that unclassified tailings flocs are irregular clusters with fractal characteristics, and the flocculation effect of the four flocculant types has the following trend: ZYZ > JYC-2 > ZYD > JYC-1. The size and average grayscale value of tailings flocs decrease with the increase in settlement height. The average grayscale values at the top and bottom are 144 and 103, respectively. The settlement height remarkably affects the pore distribution pattern, as reflected in the constructed three-dimensional pore model of tailings flocs. The top part of flocs has relatively good penetration, whereas the bottom part of flocs has mostly dispersed pores. The number of pores increases exponentially with the increase in settlement height. By contrast, the size of pores initially increases and subsequently decreases with the increase in settlement height.
Phosphogypsum (PG) is a typical by-product of phosphoric acid and phosphate fertilizers during acid digestion. The application of cemented paste backfill (CPB) has been feasibly investigated for the remediation of PG. The present study evaluated fluorine immobilization mechanisms and attempted to construct a related thermodynamic and geochemical modeling to describe the related stabilization performance. Physico-chemical and mineralogical analyses were performed on PG and hardened PG-based CPB (PCPB). The correlated macro- and microstructural properties were obtained from the analysis of the combination of unconfined compressive strength and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy imaging. Acid/base-dependent leaching tests were performed to ascertain fluoride leachability. In addition, Gibbs Energy Minimization Software and PHREEQC were applied as tools to characterize the PCPB hydration and deduce its geochemical characteristics. The results proved that multiple factors are involved in fluorine stabilization, among which the calcium silicate hydrate gel was found to be associated with retention. Although the quantitative comparison with the experimental data shows that further modification should be introduced into the simulation before being used as a predictive implement to determine PG management options, the importance of acid/base concentration in regulating the leaching behavior was confirmed. Moreover, the modeling enabled the identification of the impurity phases controlling the stability and leachability.
Reducing NOx emission of iron ore sintering process in a cost effective manner is a challenge for the iron and steel industry at present. Effects of the proportion of mill scale and coke breeze on the NOx emission, strength of sinter, and sinter indexes were studied by combustion and sinter pot tests. Results showed that the peak value of NO concentration, total of NO emission, and fuel-N conversion rate gradually decreased as the proportions of the mill scale increased because NO was reduced to N2 by Fe3O4, FeO, and Fe in the mill scale. The strength of sinter reached the highest value at 8.0wt% mill scale due to the formation of minerals with low melting point. The fuel-N conversion rate slightly fluctuated and total NOx emission significantly decreased with the decreased proportions of coke breeze because CO formation and content of N element in the sintered mixture decreased. However, the sinter strength also decreased due to the decrease in the amount of the melting minerals. Furthermore, results of the sinter pot tests indicated that NOx emission decreased. The sinter indexes performed well when the proportions of mill scale and coke breeze were 8.0wt% and 3.70wt% respectively in the sintered mixture.
The aim of this study was to investigate the phase transformation and kinetics of the solid-state reaction of CaO−V2O5, which is the predominant binary mixture involved in the vanadium recovery process. Thermal analysis, X-ray diffraction spectroscopy, scanning electron microscopy, and energy dispersive spectrometry were used to characterize the solid-state reaction of the samples. The extent of the solid reaction was derived using the preliminary quantitative phase analysis of the X-ray patterns. The results indicate that the solid reaction of the CaO−V2O5 mixture is strongly influenced by the reaction temperature and CaO/V2O5 mole ratio. The transformation of calcium vanadate involves a step-by-step reaction of CaO−V2O5, CaO−CaV2O6, and CaO−Ca2V2O7 depending on the CaO/V2O5 mole ratio. The kinetic data of the solid reaction of the CaO−V2O5 (1:1) mixture followed a second-order reaction model. The activation energy (Ea) and preexponential factor (A) were determined to be 145.38 kJ/mol, and 3.67 × 108 min−1, respectively.
X-ray powder diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetry, differential scanning calorimetry, and mass spectrometry have been used to study the products of nickel-containing pyrrhotite tailings oxidation by oxygen in the air. The kinetic triplets of oxidation, namely, activation energy (Ea), pre-exponential factor (A), and reaction model (f(α)) being a function of the conversion degree (α), were adjusted by regression analysis. In case of a two-stage process representation, the first step proceeds under autocatalysis control and ends at α = 0.42. The kinetic triplet in the first step is Ea = 262.2 kJ/mol, lg A = 14.53 s−1, and f(α) = (1 – α)4.11(1 + 1.51 × 10–4α). For the second step, the process is controlled by the two-dimensional diffusion of the reactants in the layer of oxidation products. The kinetic triplet in the second step is Еa = 215.0 kJ/mol, lg A = 10.28 s−1, and f(α) = (–ln(1 – α))–1. The obtained empirical formulae for the rate of pyrrhotite tailings oxidation reliably describe the macro-mechanism of the process and can be used to design automatization systems for roasting these materials.
The recycling of spent LiFePO4 batteries has received extensive attention due to its environmental impact and economic benefit. In the pretreatment process of spent LiFePO4 batteries, the separation of active materials and current collectors determines the difficulty of the recovery process and product quality. In this work, a facile and efficient pretreatment process is first proposed. After only freezing the electrode pieces and immersing them in boiling water, LiFePO4 materials were peeled from the Al foil. Then, after roasting under an inert atmosphere and sieving, all the cathode and anode active materials were easily and efficiently separated from the Al and Cu foils. The active materials were subjected to acid leaching, and the leaching solution was further used to prepare FePO4 and Li2CO3. Finally, the battery-grade FePO4 and Li2CO3 were used to re-synthesize LiFePO4/C via the carbon thermal reduction method. The discharge capacities of re-synthesized LiFePO4/C cathode were 144.2, 139.0, 133.2, 125.5, and 110.5 mA·h·g−1 at rates of 0.1, 0.5, 1, 2, and 5 C, which satisfies the requirement for middle-end LiFePO4 batteries. The whole process is environmental and has great potential for industrial-scale recycling of spent lithium-ion batteries.
Surface deterioration occurs more easily in nickel-rich cathode materials with the increase of nickel content. To simultaneously prevent deterioration of active cathode materials and improve the electrochemical performance of the nickel-rich cathode material, the surface of nickel-rich LiNi0.6Co0.2Mn0.2O2 cathode material is decorated with the stable structure and conductive Li3PO4 by a facile method. The LiNi0.6Co0.2Mn0.2O2–1wt%, 2wt%, 3wt%Li3PO4 samples deliver a high-capacity retention of more than 85% after 100 cycles at 1 C under a high voltage of 4.5 V. The effect of different coating amounts (0–5wt%) for the LiNi0.6Co0.2Mn0.2O2 cathode is analyzed in detail. Results show that 2wt% coating of Li3PO4 gives better performance compared to other coating concentrations. Detailed analysis of the structure of the samples during the charge−discharge process is performed by in-situ X-ray diffraction. It is indicated that the modification for LiNi0.6Co0.2Mn0.2O2 cathode could protect the well-layered structure under high voltages. In consequence, the electrochemical performance of modified samples is greatly improved.
The interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints with a nickel-based filler metal during aging was studied in detail to elucidate the mechanism of premature failures of this kind of joints. The results showed that not only a band of granular Cr23C6 carbides were formed along the fusion boundary in the ferritic steel during aging, but also a large number of granular or plate-like Cr23C6 carbides, which have a cube–cube orientation relationship with the matrix, were also precipitated on the weld metal side of the fusion boundary, making this zone be etched more easily than the other zone and become a dark etched band. Stacking faults were found in some Cr23C6 carbides. In the as-welded state, deformation twins were observed in the weld metal with a fully austenitic structure. The peak microhardness was shifted from the ferritic steel side to the weld metal side of the fusion boundary after aging and the peak value increased significantly. Based on the experimental results, a mechanism of premature failures of the joints was proposed.
The present study focuses on interface microstructure and joint formation. AA6061 aluminum alloy (Al) and commercial pure titanium (Ti) joints were welded by ultrasonic spot welding (USW). The welding energy was 1100–3200 J. The Al–Ti joint appearance and interface microstructure were observed mainly via optical microscopy and field emission scanning electron microscopy. Results indicated that a good joint can be achieved only with proper welding energy of 2150 J. No significant intermetallic compound (IMC) was found under all conditions. The high energy barriers of Al–Ti and difficulties in diffusion were the main reasons for the absence of IMC according to kinetic analysis. The heat input is crucial for the material plastic flow and bonding area, which plays an important role in the joint formation.
The formation mechanism of the bonding between compound cast Al/Ti bimetal during a heat treatment regime was investigated. Commercially pure Al was cast and melt on a Ti bar in a steel tube, followed by heat treatment on the compound cast Ti/Al bimetal for different periods of time once the Al melt was solidified. No bonding was observed between the two metals after the initial casting, which can be attributed to the presence of oxide films on the liquid Al and solid Ti alloys and the trapped atmosphere between them. The effect of these layers in preventing the formation of bonding was eliminated after heat treating the cast part at ~973 K (~700°C) for at least 15 min, and the metals started to bond with each other. A detailed description of this bonding mechanism is presented in this paper.
Cu–graphene (Gr) composite thin films were prepared by electrodeposition route using in-house synthesized Gr sheets. The Gr sheets were synthesized by the electrochemical exfoliation route using 1 M HClO4 acid as electrolyte. The Gr sheets were confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The (002) plane of Gr sheets was observed at 2θ of 25.66°. The (002) plane confirmed the crystal structure of carbon peaks. The stretching vibration of C=C bond at a wavelength of 1577 cm−1 and other functional groups of carboxyl and epoxide groups were observed from FTIR. TEM confirmed the transparent structure of Gr sheets. The prepared Gr sheets were used as reinforcement at concentrations of 0.1 and 0.3 g/L with a copper matrix to synthesize the Cu–Gr composite. The prepared composite thin films were characterized by XRD, SEM, and energy-dispersion spectrometry (EDS) for morphological and analytical studies. The presence of Gr sheets in Cu–Gr composite was confirmed by EDS analysis. The prepared Cu–Gr nanocomposite thin film showed higher corrosion resistance compared with pure copper thin films in 3.5wt% NaCl, as confirmed by Tafel plots. Electrochemical impedance spectroscopy complimented the above results and showed that 0.3 g/L composite film achieved the highest film resistance.
Al2O3 nanoparticles and MCrAlY/nano-Al2O3 nanocomposite powder (M = Ni, Co, or NiCo) were produced using high-energy ball milling. The MCrAlY/nano-Al2O3 coating was deposited by selecting an optimum nanocomposite powder as feedstock for high-velocity oxygen fuel thermal spraying. The morphological and microstructural examinations of the Al2O3 nanoparticles and the commercial MCrAlY and MCrAlY/nano-Al2O3 nanocomposite powders were investigated using X-ray diffraction analysis, field-emission scanning electron microscopy coupled with electron dispersed spectroscopy, and transmission electron microscopy. The structural investigations and Williamson–Hall results demonstrated that the ball-milled Al2O3 powder after 48 h has the smallest crystallite size and the highest amount of lattice strain among the as-received and ball-milled Al2O3 owing to its optimal nanocrystalline structure. In the case of developing MCrAlY/nano-Al2O3 nanocomposite powder, the particle size of the nanocomposite powders decreased with increasing mechanical-milling duration of the powder mixture.