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Abstract Ti-bearing blast-furnace slag is a metallurgical waste but a promising Ti resource. In this study, Ti and Al–Si alloys can be prepared directly by separating a Ti–Si–Al alloy that was prepared by reducing Ti and Si from Ti-bearing blast-furnace slag with Al. The technology of directional solidification with electromagnetic force was employed to carry out the separation of the Ti–Si–Al alloy. When additions of CaO and Al were 24 wt% and 40 wt%, respectively, the composition of the obtained Ti–Si–Al alloy was 31.8 wt% Ti, 22.7 wt% Si, 28.5 wt% Al, and 10.2 wt% Fe. Ti 5 Si 4 , TiSi, Al–Si–Ti (τ 2), Al–Si–Fe (τ 4), and Al–Si alloys were prepared in sequence after separating the Ti–Si–Al alloy using directional solidification with electromagnetic force. However, when the composition of the Ti–Si–Al alloy was changed to 21.2 wt% Ti, 48.5 wt% Si, 19 wt% Al, and 6.8 wt% Fe, the TiSi 2 , Si, Al–Si–Fe and Al–Si alloys were prepared in sequence. This study will propose an approach to employ the metallurgical waste, titanium-bearing blast-furnace slag, to prepare Ti and Al–Si alloys. Highlights • An approach to employ Ti-bearing blast-furnace slag to prepare Ti and Al–Si alloys. • Ti and Al–Si alloys were prepared directly by separating a Ti–Si–Al alloy. • Electromagnetic directional solidification was employed to separate Ti–Si–Al alloy. [ABSTRACT FROM AUTHOR]

In order to discuss the effects of transition metals such as Zr, Hf, and Ti on the removal of impurities (B, P, Fe, Al, Ca, Ti, V, Cu, Ni, Mn, Zr, and Hf) in the purification of metallurgical-grade Si (MG-Si) with Al–Si alloy, the impurity phases formed after solidification were investigated by electron probe microanalysis (EPMA) and their removal by acid leaching was performed. EPMA investigation showed that β-Al 5 FeSi was the main impurity phase aside from Al in the sample without additives. However, after adding Zr, Hf, or Ti, the ZrSi 2 , HfSi 2 , or TiSi 2 compound dissolved in small amounts of Al was the most observed phase besides β-Al 5 FeSi and Al. Impurities such as V, Ni, and Mn were more likely to coexist with β-Al 5 FeSi when Hf or Zr was employed as the impurity getter. However, V preferentially coexisted with TiSi 2 when Ti was added. The leaching results showed that the addition of Zr, Hf, or Ti significantly enhanced the extraction of B and slightly enhanced that of other impurities. The added Zr, Hf, and Ti were eliminated efficiently with the other impurities, reaching extraction ratios of 99.998%, 99.9994%, and 99.997%, respectively. [ABSTRACT FROM AUTHOR]

Hf was employed as an impurity getter to enhance the removal of impurities from metallurgical-grade Si (MG-Si) via the solidification of Si or a Si-33 wt% Al solvent. The leaching behaviors of the impurities (B, Fe, Al, Ca, P, Zr, Ti, V, Mn, Hf, and Ni) within MG-Si, in the presence of 5 wt% Hf, were investigated using various leaching approaches. Compared with aqua regia and HF, HCl + HF was determined to be the optimal lixiviant for the elimination of impurities from Hf-containing MG-Si. The use of a combination of HCl + HF and aqua regia reduced the quantity of impurities from 6126 ppmw to 94 ppmw. Eh-pH diagrams were calculated to discuss the leaching of HfSi 2 in aqua regia and HF solutions. The presence of Hf in the MG-Si enhanced the removal of impurities, especially P, which cannot be efficiently removed via solidification refining and hydrometallurgical treatments. Hf-containing Si-Al solvent refining is considered the most efficient approach for the elimination of impurities (except Al). The removal fractions of B and P were 94.2% and 86.2%, respectively, achieved via the solidification of the Si-33 wt% Al solvent. Moreover, 99.94% and 99.9996% of the Hf, used as an impurity getter, could be eliminated through the solidification of the Si and Si-33wt% Al solvent, respectively, decreasing from 50,000 ppmw, to 28 ppmw and 0.2 ppmw, respectively. [ABSTRACT FROM AUTHOR]

Zr was employed as an impurity getter to enhance removal of impurities from MG-Si because of its strong affinity for a wide range of impurities. Different lixiviants, HCl + HF, HF and aqua regia, were employed to investigate the leaching behavior of MG-Si at 348 K for 2.5 h with and without Zr addition. Typical precipitates at Si grain boundaries before and after acid leaching were observed and analyzed via scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The leaching results show that the order of efficiency of the lixiviants, from highest to lowest, is HCl + HF > HF > aqua regia. HCl + HF is the optimal lixiviant for purification of MG-Si with and without Zr addition. The presence of Zr in MG-Si can significantly enhance the efficient extraction of impurities, especially P and Al, which cannot be removed efficiently using conventional hydrometallurgical technologies. The total fraction of impurities removed (Fe, Al, Ca, B, P, Zr, Ti, V, Mn, Cu and Ni) increased from 94.2 to 97.5% after HCl + HF leaching. In addition, 99.9% of the impurity getter Zr was removed, with its concentration decreasing from 50,000 ppmw to 49 ppmw after HCl + HF leaching. The residual amount of Zr in the refined Si can be further removed by directional-solidification purification because of its extremely small segregation coefficient. [ABSTRACT FROM AUTHOR]

A small amount of Hf was employed as a new additive to improve B removal in the electromagnetic solidification refinement of Si with an Al–Si melt, because Hf has a very strong affinity for B. The segregation ratio of Hf between the solid Si and Al–Si melt was estimated to range from 4.9 × 10−6to 8.8 × 10−7for Al concentrations of 0 to 64 at.%, respectively. The activity coefficient of Hf in solid Si at its infinite dilution was also estimated. A small addition of Hf (<1025 parts per million atoms, ppma) significantly improved the B removal. It was confirmed that the use of an increased Hf addition, slower cooling rate, and Al-rich Al–Si melt as the refining solvent removed B more efficiently. B in Si could be removed as much as 98.2% with 410 ppma Hf addition when the liquidus temperature of the Al–Si melt was 1173 K and the cooling rate was 4.5–7.6 K min–1. The B content in Si could be controlled from 153 ppma to 2.7 ppma, which meets the acceptable level for solar-grade Si. [ABSTRACT FROM PUBLISHER]

Impurity removal, the purification process from metallurgical grade silicon (MG-Si) required to obtain solar grade silicon (SoG-Si), is crucial to the preparation of silicon-based solar cells. Some processes for boron removal by metallurgical method were reviewed. Secondary refining techniques, including gas blowing, slag treatment, plasma refining, solvent refining and other refining silicon techniques were summarized. The effects of gas species and slag systems on boron removal efficiency were emphatically discussed. Experimental and theoretical investigations show that a united technique of combining water vapor and oxygen gases blowing with slag treatment containing chloride or fluoride has achieved an amazing improvement for boron removal from molten silicon. Plasma refining and solvent refining also display high efficiency but acid leaching treatments, vacuum volatilization, electron beam and directional solidifications are hardly effective to boron removal. As for the potential industrial application of this united technique, the authors propose that some experimental and theoretical studies in dynamics should be further explored. [ABSTRACT FROM AUTHOR]

• A novel purification technique of ultrasonic-Cu catalyzed chemical leaching (U-CuCCL) was proposed. • The rapid thermal processing is beneficial for the internal impurities diffuse to the porous silicon surface. • The impurities removal mechanism during U-CuCCL was investigated. • The purity of silicon powders could be improved from 99.32% to 99.995%. The present study proposed a novel and efficient ultrasonic-Cu catalyzed chemical leaching (U-CuCCL) method to purify large-sized industrial silicon powders. Different from the traditional ultrasonic-HF (U-HF) leaching method, U-CuCCL and U-CuCCL combined rapid thermal processing (U-CuCCL + RTP) were performed to investigate the efficiency of removing the main impurities Fe, Al, Ca, Ti, Ni, V, Cu, and Mn. The evolution of typical precipitates phases on the surface of silicon before and after leaching were observed and analyzed by electron probe micro analyzer. The results show that the impurities removal can be significantly improved under the ultrasonically strengthen process, especially the U-CuCCL process shows a high-efficient impurities removal efficiency. After the U-CuCCL, the etched silicon powders accompanied with numerous porous structure are obtained which is beneficial for the removal of impurities. Notably, it was found that the rapid thermal processing is beneficial for the residual impurities diffuse to the porous layer surface and the purity of silicon powder can be significantly increased from 99.3% to 99.995%. [ABSTRACT FROM AUTHOR]

The preparation of solar-grade silicon (99.9999 wt%) with low-cost technology is facing challenges. In the present study, a novel metal-catalyzed chemical corrosion (MCCC) method on purifying industrial silicon powder, consisting of HF concentration, corroding temperature, H 2 O 2 concentration and corroding duration were studied. The evolution of typical precipitates phases on the surface of silicon to each etchant were investigate before and after etching with/without MCCC via EPMA (electron probe micro analyzer) equipped with EDS (energy dispersive X-ray spectroscopy). The results showed that numerous microporous structures applied on the surface of silicon powder by MCCC are beneficial for the removal of impurities. After MCCC, the purity of silicon powder can be improved from 99.48 to 99.99 wt%, which is higher than 99.93 wt% obtained without MCCC. It was found that the removal reaction of boron and phosphorus by MCCC from industrial silicon powder in accordance well with the cracking shrinking kinetic model, which was confirmed to be controlled by both of interfacial transfer and diffusion through the product layer process. Furthermore, the chemical reaction mechanism of MCCC was revealed. The impurities deep removal under leaching step would greatly relieve successive impurities removal stress, even shorten the traditional metallurgical routes, which is of great significance to high-effective and clean produce solar grade silicon for industrial use. Unlabelled Image • A novel metal-catalyzed chemical corrosion(MCCC) purification technique was proposed. • Numerous micro-scale "channels" intensify contact between impurities and etchants. • The chemical reaction mechanism and kinetics analysis of MCCC were firstly studied. • The purity of silicon powders could be improved from 99.48 to 99.99%. [ABSTRACT FROM AUTHOR]

Abstract A multi-field coupling model of heat-flow-thermal stress was established to investigate the influence of Marangoni effect on the growth quality of multi-crystalline silicon (mc-Si) during the vacuum directional solidification (VDS) process. The simulation results showed that the Marangoni effect has a significant effect on the distribution of temperature and thermal stress as well during the VDS process due to the increased velocity of melts. The enhanced flow incurred more homogeneous temperature distribution of silicon melts and reduced radial temperature gradient, which then leads to the flatter solid/liquid (s/l) interface. However, as temperature gradient of the crystal increased, thermal stress of the silicon ingot was strengthened as a result. The growth quality of crystal was more desirable when adopting a pulling-down rate of 10 µm/s, in which case the (111) surface was advantageous throughout the entire crystal growth process by XRD detection. Finally, the reliability of the numerical simulation result was verified by the experiment. [ABSTRACT FROM AUTHOR]

In the present work, a continuous growth model of primary silicon from Al-Si melts was established based on the experimental and numerical data to analyze the separation mechanism of primary silicon from Al-Si alloy with high silicon content during electromagnetic directional solidification process. Results show that the separation of primary Si from Al-Si melts depends on the growth rate of primary Si phase and the solidification rate of the melts. Moreover, the separation and enrichment effects of primary Si during the electromagnetic directional solidification process could be improved by enhancing the bottom cooling conditions of crucible and the stirring strength of Al-Si alloy melts. Finally, the distribution and composition of phases and microstructures of the Al-Si alloy after the electromagnetic directional solidification technique were also analyzed and discussed. [ABSTRACT FROM AUTHOR]

The depressing texturation and stubborn directional saw marks on diamond wire sawing (DWS) polysilicon wafers has resulted in severe limitations for the mass application of DWS technology in PV market. Herein, a simple and low-cost copper-assisted chemical etching method was presented to controllably nano-texturing DWS polysilicon wafers. The work firstly reveals the adjustment of anion species in copper salts can readily control the nanostructure during the copper-catalyzed etching process. After etching, the obtained inverted pyramid structure can effectively reduce the reflectivity of the silicon wafer surface, the reflectance can be as low as 5.8% in the wavelength range of 300–1100 nm. Moreover, the surface saw marks are removed after etching, and the novel texture method guarantee the low recombination rates. The effective texturing method shows a promising potential application in the photovoltaic field. [ABSTRACT FROM AUTHOR]

In this study, a novel metal assisted chemical leaching (MACL) was proposed to remove impurities from metallurgical grade silicon (MG-Si). HF acid leaching, one step MACL (1-MACL, HF + AgNO 3 ) and two step MACL (2-MACL, HF + AgNO 3  + H 2 O 2 ) were employed to purify MG-Si. The typical precipitates at Si grain boundaries before and after leaching were observed and analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The leaching results show that the impurities removal efficiency is 2-MACL > 1-MACL > HF acid leaching. After 2-MACL, the impurities (Fe, Al, Ca, Ti, Mn, Ni, Cu, V, Cr, Ag, B and P) concentration can be reduced from 4550.40 ppmw to 106.09 ppmw and the purity of MG-Si increased from 99.55 to 99.99%. It is notable that the introduction of micro-scale “channels” can facilitate the impurities removal when compared to solely HF leaching, especially for some non-dissolving metallic impurities like copper, calcium and aluminum. It should be noted that 2-MACL has obvious effect on the removal of non-metallic impurities B and P. [ABSTRACT FROM AUTHOR]

In electromagnetic directional solidification, the silicon phase cannot always be completely separated, resulting in considerable waste of power and silicon. This study investigated the electromagnetic separation of silicon by using electromagnetic induction-heated directional solidification furnaces at varying frequencies. Two frequencies were applied to separate silicon from aluminum–silicon melts. Numerical simulation results indicated that a low frequency (3 kHz) could substantially enhance the separation of silicon from aluminum–silicon melts under an alternating electromagnetic field, which could increase the speed of the melts to 0.92 cm/s. Experimental results showed that separation efficiency could exceed 85% at a pulling rate of 10 μm/s when a furnace at a frequency of 3 kHz was used. This method can potentially meet the requirements of manufacturing low-cost solar cells for industrial use. [ABSTRACT FROM AUTHOR]

Mass transfer of boron within silicon and slag is a critical component in boron removal from metallurgical grade silicon by slag refining. To clarify the mechanism of B mass transfer in slag, the present work reports an experimental kinetic study of B removal. The diffusion coefficient of boron oxide in a binary 0.37CaO-0.63SiO 2 (mass fraction) slag was determined by a capillary-bath diffusion device. The mass transfer coefficient of B 2 O 3 in the binary slag is calculated according to the experimental observations during slag refining, and the thickness of boundary layer between reaction interface and slag is obtained. The results show that the diffusion coefficient of B 2 O 3 in 0.37CaO-0.63SiO 2 slag is 5.24 × 10 −9  m 2 ·s −1 at 1723 K; the mass transfer coefficient is 6.2 × 10 −6  m·s −1 at the same condition. The thickness of the effective boundary layer between silicon-slag reaction interface and slag is 0.85 mm. It is confirmed that the mass transfer of B 2 O 3 in binary 0.37CaO-0.63SiO 2 slag is the rate-controlling step for B removal using calcium silicate slag refining. [ABSTRACT FROM AUTHOR]

To explore the potential of acid leaching in purification metallurgical grade silicon (MG-Si) for solar cells, the effect of acetic acid on the leaching behavior was investigated in the present work by focusing on the behavior of impurities affected by the addition of acetic acid to a conventional acid mixture composed of hydrochloric acid and hydrofluoric acid. The etching results reveal that the HCl–HF–CH3COOH mixture is a better lixiviant for dissolving impurity inclusions in MG-Si. The extraction yield of impurities in HCl–HF–CH3COOH leaching was found to increase by 7% compared to conventional HCl–HF leaching. [ABSTRACT FROM PUBLISHER]

Slag refining is one of the most effective methods to remove impurities from metallurgical grade silicon. In this paper, ZnO was added to a binary 50% CaO-50% SiO 2 slag system with a mass ratio of 2–30% to study the impurities removal from metallurgical grade silicon. The results showed that boron was reduced from 12.94 ppmw to 2.18 ppmw by a 46% CaO-46% SiO 2 -8% ZnO slag system. Compared to the 50% CaO-50% SiO 2 slag system, slag refining with ZnO addition showed a 20% increase in removal efficiency and a 40% increase in the distribution coefficient. After acid leaching, boron was further reduced to 1.52 ppmw with a removal efficiency of 88.25%. By a combined process of slag refining and acid leaching, impurities Al and Fe were reduced from 817 ppmw and 3800 ppmw to 1.2 ppmw and 96 ppmw with removal efficiencies of 99.8% and 97.5%, respectively. The evaporation loss of the generated metallic zinc during slag refining was greater than 99.7%. After acid leaching, the minimum zinc content in silicon was only 4 ppmw. [ABSTRACT FROM AUTHOR]

The installed capacity of solar photovoltaic power generation has grown rapidly in the last decades. With the rapid development of the photovoltaic industry, the demand for Si wafers, which are integral to solar cells, has grown dramatically. In the manufacture of Si wafers, the traditional loose abrasive sawing method (LAS) has gradually been replaced by the diamond-wire sawing method (DWS). However, during the diamond-wire wafer sawing process, approximately 35%−40% of the crystalline Si becomes diamond-wire sawing silicon waste (DSSW). Therefore, DSSW represents a resource worth recycling due to its low levels of impurities and high silicon content. Furthermore, recycling prevents DSSW from becoming environmental pollution and eliminates disposal costs. This review provides an overview of the recycling and reutilization of DSSW based on an extensive literature survey. In view of the rapid increase in DSSW production and current purification bottleneck of < 5 N, in-situ utilizations may be more feasible, such as the preparation of silicon containing alloys and functional ceramic materials, which not only frees from the complex purification process, but has a huge demand. Finally, based on the review, future prospects are proposed, aiming to identify research directions with significant potential in the resource utilization of DSSW and other silicon wastes. [Display omitted] ● DSSW with high silicon content represents a resource worth recovery and recycling. ● The proper purification process is a trade-off between cost and purity. ● In-situ utilizations of DSSW may be more feasible strategies. ● Modification facilitates real application of DSSW in lithium-ion batteries. ● DSSW has scalable applications in energy conversion and environmental catalysis. [ABSTRACT FROM AUTHOR]

The installed capacity of solar photovoltaic power generation has grown rapidly in the last decades. With the rapid development of the photovoltaic industry, the demand for Si wafers, which are integral to solar cells, has grown dramatically. In the manufacture of Si wafers, the traditional loose abrasive sawing method (LAS) has gradually been replaced by the diamond-wire sawing method (DWS). However, during the diamond-wire wafer sawing process, approximately 35%−40% of the crystalline Si becomes diamond-wire sawing silicon waste (DSSW). Therefore, DSSW represents a resource worth recycling due to its low levels of impurities and high silicon content. Furthermore, recycling prevents DSSW from becoming environmental pollution and eliminates disposal costs. This review provides an overview of the recycling and reutilization of DSSW based on an extensive literature survey. In view of the rapid increase in DSSW production and current purification bottleneck of < 5 N, in-situ utilizations may be more feasible, such as the preparation of silicon containing alloys and functional ceramic materials, which not only frees from the complex purification process, but has a huge demand. Finally, based on the review, future prospects are proposed, aiming to identify research directions with significant potential in the resource utilization of DSSW and other silicon wastes. [Display omitted] ● DSSW with high silicon content represents a resource worth recovery and recycling. ● The proper purification process is a trade-off between cost and purity. ● In-situ utilizations of DSSW may be more feasible strategies. ● Modification facilitates real application of DSSW in lithium-ion batteries. ● DSSW has scalable applications in energy conversion and environmental catalysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • Refractory silicides and eutectic Si/MSi 2 (M: Nb, Ta, Mo, or W) alloys were prepared by separation Si–M solvent. • High-purity silicon can be obtained by acid leaching of eutectic Si/MSi 2 alloy. • A new approach for simultaneous preparation of high-purity refractory silicides, eutectic Si/MSi 2 alloys, and Si. • Mechanism separation and impurity removal of Si–M alloy was discussed. Refractory silicides, eutectic Si/MSi 2 (M: Nb, Ta, Mo, or W) alloys, and high-purity silicon are materials that have been used in many high-tech industries. Today, they are prepared using different technologies, i.e., each of them is prepared using an independent technology. This study proposes a new method to simultaneously prepare three high-purity materials (refractory silicides, eutectic Si/MSi 2 alloys, and silicon) using electromagnetic directional crystallization. The Si–M (Si–Nb, Si–Ta, Si–Mo, or Si–W) solvents are separated using electromagnetic directional crystallization, and then the MSi 2 (NbSi 2 , TaSi 2 , MoSi 2 , or WSi 2) and eutectic Si/MSi 2 (Si/NbSi 2 , Si/TaSi 2 , Si/MoSi 2 , or Si/WSi 2) alloys are precipitated in sequence because the crystallization temperature of MSi 2 is higher than that of the eutectic Si/MSi 2 alloys. Additionally, owing to the segregation behavior of impurities between the solid and liquid phases, impurities in the Si–M solvents can be eliminated efficiently to prepare high-purity MSi 2 and eutectic Si/MSi 2 alloys. In addition, as the eutectic Si/MSi 2 alloys consist of Si and MSi 2 phases, high-purity Si powders (>99.98%) can be obtained by separating the Si and MSi 2 phases using acid leaching. The results indicate the feasibility of the new approach in preparing refractory silicides, eutectic Si/MSi 2 alloys, and high-purity Si. [ABSTRACT FROM AUTHOR]