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Silicon has been applied in many fields, such as high-purity semiconductors, high-temperature resistant materials, optical fiber connection materials, organosilicon compounds, and alloys. In this work, silicon was extracted from the Bayan Obo tailing by high-temperature electrolysis. The reduction process of Si(IV) and the effect of CaF2 content on the reduction process were studied in the SiO2–CaO–CaF2 melt. The Si(IV) ions in the SiO2–CaO system with 10 wt pct CaF2 were reduced by a one-step four-electrons transfer process. In the SiO2–CaO melt with 15, 20, and 30 wt pct CaF2, the reduction process of Si(IV) was a two-step two-electrons transfer process: Si(IV) + 2e → Si(II) and Si(II) + 2e → Si(0). The reduction process of Si(IV) to Si(II) was reversible and was controlled by diffusion. The diffusion coefficient was 1.52 × 10−8 cm2 s−1. Si and WSi2 were prepared under a current density of − 0.55 A cm−2 at 1738 K. It was found that the WSi2 alloys were formed because of the alloying of Si(IV) and W. [ABSTRACT FROM AUTHOR]

Bayan Obo tailings are rich in rare earth elements (REEs), iron, and other catalytic active substances. In this study, mine tailings were calcined at different temperatures and tested for the catalytic combustion of low-concentration methane. Upon calcination at 600°C, high catalytic activity was revealed, with 50% CH4 conversion at 587°C (space velocity of 12,000 mL/g h). The physicochemical properties of catalysts were characterized using thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, hydrogen temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). Compared to the raw ore sample, the diffraction peak intensity of Fe2O3 increased post calcination, whereas that of CeCO3F decreased. A porous structure appeared after the catalyst was calcined at 600°C. Additionally, Fe, Ce, Ti, and other metal elements were more highly dispersed on the catalyst surface. H2-TPR results revealed a broadening of the reduction temperature range for the catalyst calcined at 600°C and an increase in the reduction peak. XPS analysis indicated the presence of Ce in the form of Ce3+ and Ce4+ oxidation states and the coexistence of Fe in the form of Fe2+ and Fe3+. Moreover, XPS revealed a higher surface Oads/Olatt ratio. This study provides evidence for the green reuse of Bayan Obo mine tailings in secondary resources.

The rare earth minerals in Bayan Obo are poor, fine, scattered and variegated, so it is very difficult to achieve efficient separation. Superconducting magnetic separation technology is an efficient separation technology for refractory minerals. In this paper, the magnetic field induced by a matrix in a superconducting magnetic field is studied based on a finite element method and a separate numerical calculation method, and a model for the trajectories of rare earth mineral particles in Bayan Obo is obtained. The matching relationship between the matrix parameters and the capture radius is obtained through model calculation, as is the optimal capture radius and capture efficiency. The results show that the magnetic field gradient and magnetic field force decrease with increasing distance from the surface of the matrix and that the critical distance of the magnetic field induced by the matrix is 1.5 times the diameter of the matrix. The particle capture radius and capture efficiency are inversely proportional to the matrix diameter. When the matrix diameter is 0.06 mm, the capture efficiency is higher than at other diameters. The theoretical capture efficiency of rare earth mineral particles of size 20–38 µm is approximately 98%, but for particles below 10 µm, the capture efficiency is only 30%. The research results provide a quantitative selective capture law for the matrix diameter and particle diameter, which provides theoretical guidance for production practice. [ABSTRACT FROM AUTHOR]

The evolution of mineral phase structure during the reduction and melting separation of an rare earth (RE)-rich iron mineral (RER-IM) is investigated. The results show the iron oxides are reduced to their metallic iron or FeO at 1373 K. When reduction time is 180 min, the reduction degree is 84%. Both bastnaesite (RE(CO3)F) and monazite (REPO4) are transformed into Ca2RE8(SiO4)6O2 during carbothermic reduction at 1373 K. The mineral with a reduction degree of 84% is melt-separated in a graphite crucible at 1773 K for 20 min, the resulting slag contains 20.64% RE2O3, with RE existing in the form of Ca2RE8(SiO4)6O2. Moreover, P from the reduction of Ca3(PO4)2 dissolves in iron with a content ranging from 1.2 to 2.21%. The type of RE phase that occurs in the slag is related to the distribution of P between slag and iron. A low P content in the slag facilitates the formation of Ca2RE8(SiO4)6O2, but a high content in the slag favours Ca3RE2[(Si, P)O4]3F. Thus, it is confirmed that the RE phase structure is controlled by the distribution of P between slag and iron. [ABSTRACT FROM AUTHOR]

The phase relations in the CaO-SiO2-Ce2O3 system under the reducing atmosphere at 1873 K were determined by the conventional equilibrium and quenching method combined with scanning electron microscopy-electron probe microanalysis (SEM-EPMA) and X-ray diffraction (XRD) measurements on the quenched samples. Based on these analyses, a large part of the isothermal phase diagram was constructed. Furthermore, the thermodynamic activities of Ce2O3 in the melts at 1873 K were measured by the chemical equilibrium method. Using the measured activity data, an empirical formula to estimate the activity coefficient of Ce2O3 was proposed based on the regular solution model. It was found that, for the melts with the same Ce2O3 contents, the thermodynamic activities of Ce2O3 increase gradually with the rise of basicity (the ratio of CaO to SiO2) of the melts. This implies that, from the thermodynamic point of view, the increase of basicity is favorable to the enrichment and precipitation of Ce-containing mineral phases. [ABSTRACT FROM AUTHOR]

Impurity removal and modification of rare earth concentrate powder were conducted by roasting weak acid-weak alkali leaching to obtain the active components of denitrification catalysts. NH3 selective catalytic reduction catalyst samples were prepared by mixing and kneading with pseudo-γ-Al2O3 boehmite as carrier. The results showed that the Ce7O12 content in the active component samples increased and dispersed more evenly. The grain size of the samples was refined, the specific surface area increased, and the active sites exposed more. Ce coexists in the form of Ce3+ and Ce4+. Fe coexists in the form of Fe3+ and Fe2+, but Fe3+ is abundant. Some Ce, La, Nd, Pr, Fe, Mn, and other components formed solid melts during preparation, which increased the synergistic catalytic effect. The denitrification efficiency of the catalyst sample was 92.8% under the conditions of reaction temperature 400 °C, NO content was 600 ppm, NH3/NO ratio was 1.5, and O2 concentration was 4%. [ABSTRACT FROM AUTHOR]

Spray cooling is a key technology in the continuous casting process and has a marked influence on the product quality. In order to obtain the heat transfer characteristics, which are closer to the actual continuous casting to serve the design, prediction and simulation, we created an experimental laboratory setup to investigate heat transfer characteristics of air mist spray cooling during the continuous casting secondary cooling process. A 200-mm thick sample of carbon steel was heated above 1000 °C, and then cooled in a water flux range of 0.84 to 3.0 L/(m2∙s). Determination of the boundary conditions involved experimental work comprising an evaluation of the thermal history and the heat flux and heat transfer coefficient (HTC) at the casting surface using inverse heat conduction numerical schemes. The results show that the heat fluxes were characterized via boiling curves that were functions of the slab surface temperatures. The heat flux was determined to be 2.9 × 105 W/m2 in the range of 1100 to 800 °C with a water flux of 2.1 L/(m2∙s). The critical heat flux increased with the increase of water flux. The HTC was close to a linear function of water flux. We also obtained the relation between the HTC and the water flux in the transition boiling region for surface temperatures of 850 to 950 °C. [ABSTRACT FROM AUTHOR]

Abstract Mg 80– x Ni 20 Y x (x = 0–7) alloys were successfully prepared by using the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy measurements. The effects of yttrium content on the microstructure and hydrogen storage properties of the as-cast alloys were investigated. The alloys are composed of a primary phase of Mg 2 Ni, lamella eutectic composites of Mg + Mg 2 Ni, and some amount of YNi 3. The YNi 3 has completely transformed into in situ formed nanoscale YH 2 and YH 3 , with the formation of Mg 2 NiH 4. Pulverization of the alloy particles was observed during the hydrogen absorption and desorption cycles. However, the phase composition remains unchanged even after 20 hydrogenation cycles. Yttrium addition significantly improved the hydrogen-absorption kinetic performance of the alloy. In addition, pressure-composition-temperature measurements indicated that the entropy and enthalpy changes for the formation and decomposition of MgH 2 and Mg 2 NiH 4 gradually decreased with the increase of yttrium content. Highlights • YNi 3 , Mg 2 Ni and lamella eutectic Mg + Mg 2 Ni phases were observed in the alloys. • YNi 3 fully decomposed, with the formation of nanometer YH 2 and Mg 2 NiH 4. • Increasing Y content significantly improves the hydrogen absorption kinetics. • The decomposition peak temperature was reduced from 586 K to 523 K. • Δ H and Δ S of the Mg/MgH 2 and Mg 2 Ni/Mg 2 NiH 4 transformations decreased. [ABSTRACT FROM AUTHOR]

• The M-doped CePO4 catalysts were prepared by the coprecipitation method. • The incorporation of M improved the NH 3 -SCR activity on the CePO 4. • The reaction mechanism followed the Eley-Rideal and Langmuir-Hinshelwood mechanism. A series of Metals doped M-CePO 4 (M =Zr, Co, Mo, Cu) catalysts were prepared via the coprecipitation method for selective catalytic reduction of NO with NH 3 (NH 3 -SCR). The results showed that the NO x conversion of M-CePO 4 were higher than that of CePO 4 , especially the Mo-CePO 4 catalysts exhibited the best low-temperature activity and N 2 selectivity, the Mo-CePO 4 catalysts can reach about 99% at 300 °C and excellent 70 h long stability with the coexistence of SO 2 and H 2 O. The effect of metal elements doping on physiochemical properties of catalysts were characterized by XRD, BET, Raman, TEM, XPS, H 2 -TPR and NH 3 -TPD. SEM and TEM analysis. The results showed that part of M ions doped into CePO4 lattice resulted in lattice shrinkage. More importantly, the MO x species could not only increased the relative content of Ce3+ and surface adsorption oxygen (O′) of M-CePO 4 catalysts, but also increased the specific surface area of the catalysts. It could provide more reaction sites for active gas to promote the activation of NH 3 and enhance oxidation–reduction performance. The in-situ DRIFTS results suggested that NH 3 were adsorbed on both Lewis and Bronsted acid sites which can easily react with NO x gas. The NO species were adsorbed on Mo-CePO 4 in the form of bidentate nitrate. The main reactants of Mo-CePO 4 catalysts were NH 3 linked to Lewis acid sites and bidentate nitrate species on the catalytic surface. The Eley–Rideal and Langmuir–Hinshelwood mechanisms coexisted in the SCR reaction route. According to the characterization results of the catalysts, the mechanism of Mo promoting the activity of NH 3 -SCR at low temperature was proposed (Fig. 13.). NH 3 binds to Lewis acid centers, and NH4+ adsorbs on Brønsted acid centers, monotone nitrates and bidentate nitrates. Studies had shown that NH 2 is an important medium in the reaction of NO (gas phase) to produce NH 2 NO and then decompose into N 2 and H 2 O. As shown in Fig. 12(a), the reaction of NH 3 -SCR on the catalyst follows the Eley-Rideal (E-R) mechanism. The NH 3 Lewis acid sites and NH4+ acid sites adsorbed on Brønsted react with NO (gas phase). As shown in Fig. 12(b), monodentate and bidentate nitrates reacted with adsorbed NH 3 species, following the Langmuir-Hinshelwood (L-H) mechanism [51]. The Mo6+/Mo5+ and Ce4+/Ce3+ redox cycles on the catalyst surface participate in the whole process of the catalytic reaction. At this stage, the electron transfer process promoted the conversion of oxygen vacancy and unsaturated chemical bond, thus enhancing the adsorption activation capacity of NO and NH 3 , promoting the decomposition of NO molecule, and improving the catalytic performance. The –NH 2 s species were intermediate adsorbed and reacted with the activated nitrate, eventually becoming N 2. In the process of NO reduction, NH 3 showed selectivity to the reaction intermediates, preferentially reacted with monotone nitrate and bidentate nitrate. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, glass-ceramics were prepared from stainless steel slag and iron tailing. Consequently, unstable heavy metals were converted to stable. With the adding of stainless steel slag, crystallization temperature and the mechanical properties of the resultant glass-ceramics increased first and then decreased, and it was found that secondary phase (Spinel) differed from major phase (Augite). The results of toxicities leaching test and Tessier five step sequential extraction showed that Cr content in the exchangeable state, carbonate bind state, ferromanganese oxide state and organically bind state of the basic glass were 0.04%, 0.08%, 7.82% and 1.13%, respectively. However, in the glass-ceramics, they exhibited the low content of 0, 0.04%, 2.02% and 0.40%. The residual content of Cr reached to 90.93% in the glass, whereas 97.55% in the glass-ceramics, being higher than that in the glass. The results indicated that Cr enters the silicate lattice, so as to realize the curing of chromium. • Preparation of glass-ceramics by iron tailings and stainless steel slag. • The residual state (stable state) of chromium in glass-ceramics was 98.9%. • TCLP results demonstrate the superiority of glass-ceramics in solidification Cr. • Glass-ceramics samples have excellent physical properties. [ABSTRACT FROM AUTHOR]

REEs are selectively separated from REPM wastes. The separated REEs are recycled in the form of rare-earth metals which can be directly applied in industry. Forming anode channels make the separation of REEs in the inside of REPM wastes feasible. • Rare-earth metal was directly recycled from REPM waste. • Forming anodic channels facilitated the oxidation of rare-earth elements inside the REPM waste. • The formation parameter and process of channels were proposed. Tens of thousands of tons of rare-earth permanent magnet (REPM) wastes are produced worldwide every year. The recovery of rare-earth elements from the wastes can effectively reduce the exploitation of rare-earth minerals. The recovery of neodymium and praseodymium from a REPM waste is investigated by electrolysis in molten LiF-CaF 2 where no anode gas releases, making the process environmentally attractive. Neodymium and praseodymium are selectively oxidized from the REPM waste and transformed into rare-earth ions. To facilitate the oxidation of rare-earth elements inside the REPM waste, we intended to make REPM electrode form channels by oxidation of Nd-Pr and Nd 2 Fe 14 B alloys. By analyzing the microstructure of REPM electrode after forming channels, it can be found that the channels provide continuous transport access of the electrolyte to the inside of the REPM electrode. The separation rates of Nd and Pr increase with increasing current, while the rate of oxidation reaction is proportional to current. In addition, the oxidation of Fe initially occurs at the inside of the REPM electrode where rare-earth elements have been first depleted. The separated rare-earth ions are directly prepared as rare-earth metals at the cathode by electrolysis, leaving the porous Fe 2 B alloy and metallic Fe. [ABSTRACT FROM AUTHOR]

We proposed a simple and sustainable approach to recover rare earths and iron from an ultrafine NdFeB waste. Rare earths were directly recovered by one-step precipitation in hydrofluoric acid and the dissolved iron was recovered in the form of nano-scale FeF 2 (s) and Fe(s) by electrodeposition. The hydrofluoric acid, which has removed iron by electrodeposition, could be still used for the leaching of the waste, making the hydrofluoric acid recyclable. • A simple and sustainable approach was proposed to recover rare earths and iron from an ultrafine NdFeB waste. • Rare earths were directly recovered by one-step precipitation. • The chemicals used in this approach were recyclable. In this work, a simple and sustainable approach is proposed to recover rare earths and iron from an ultrafine NdFeB waste. Rare earths are directly recovered by one-step precipitation in hydrofluoric acid and the dissolved iron by electrodeposition. The rate constant and activation energy of the reaction of rare earth hydroxide with hydrofluoric acid are investigated to understand the kinetics of iron leaching and rare earth precipitation. The rare earth precipitation is much faster than the iron leaching, enabling the rare earths separation from iron after a complete iron dissolution. The cathodic deposits of iron were characterized by X-ray photoelectron spectrometer (XPS) and scanning electron microscope (SEM). The nano-scale FeF 2 (s) and Fe(s) were deposited on Pt electrode in hydrofluoric acid solution with pH 2.3 and pH 2.89, respectively. The reduction of Fe(III) to Fe(0) was investigated by cyclic voltammetry, chronopotentiometry and open circuit chronopotentiometry. This reduction was a quasi-reversible process which was controlled by diffusion. The temperature dependence of the diffusion coefficient was established as lnD = −9.84 − 1805.43/ T and the activation energy value of the diffusion process was calculated to be −15.01 kJ mol−1. The relationship between the Gibbs energy of intermetallic Fe-Pt formation and the temperature was established to be Δ G = −487.53 + 0.936 T. The hydrofluoric acid after the electrodeposition could be recycled for the leaching of the ultrafine NdFeB waste again. [ABSTRACT FROM AUTHOR]

Acid leaching and alkali roasting were used to remove impurities such as Ca and Si in Baiyun Obo rare earth concentrate. The effects of acid-base treatment on the physical and chemical properties of the samples were analyzed by scanning electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller characterization, X-ray photoelectron spectroscopy, H 2 -temperature-programmed reduction, NH 3 -temperature-programmed desorption (TPD), and NO-TPD. Results showed that the content of Ce 7 O 12 in the rare earth concentrates increased and the dispersion was uniform. The grains became smaller, the specific surface area of rare earth concentrates increased, and the active sites were more exposed. Ce coexisted in the form of Ce 3+ and Ce 4+ , whereas Fe coexisted in the form of Fe 3+ and Fe 2+ . The content of Fe 3+ was increased. The acid-base-treated rare earth concentrates had a denitration efficiency of 87.4% at a reaction temperature of 400 °C., Competing Interests: The authors declare no conflict of interest.