Boron removal from silicon by slag refining using Na2O-SiO2 in industrial applications

Boron (B) removal by slag refining using Na₂O-SiO₂ was investigated in industrial applications. The experimental results showed that the reasonable ratio range of slag to silicon is about 0.7–0.8; the suitable holding time is about 30 min; the concentration of B is reduced from 1.90 ppmw to 0.17 ppmw by three times slag re?ning; and the removal efficiency of B reaches 91.1%. Moreover, it is discussed that B in silicon is more inclined to be oxidized by Na₂O than SiO₂ according to thermodynamic analysis and then volatilized to the atmosphere in the form of Na₂B₂O₄ according to kinetic analysis.     

Removal of Boron from Molten Silicon using Na2O-CaO-SiO2 Slags

The global shortage of solar grade silicon for production of solar cells has motivated a lot of research on the refining of metallurgical grade silicon. However, approaches to upgrade metallurgical grade silicon have been mainly handicapped by difficulties in reducing boron and phosphorus levels. In the present study, the possibility of refining metallurgical grade silicon to remove impurity boron using Na₂O-CaO-SiO₂ slags was investigated. Before slag melting, a process of mixing pulverized slags and silicon under an action of mechanical force was used to provide a higher probability of contact and reaction of slags and silicon. The melting time was reduced with an increase in contact area A _S , resulting in improved efficiency of boron removal. The parameters, including the slag basicity, and the weight ratio of silicon to slag were discussed. The process of slag treatment was performed twice and directional solidification was conducted to promote the separation of slags from silicon. A maximum value of L _B (5.81) was obtained when the basicity was 1.21, and the weight ratio of silicon to slag was 5.     

Effects of Slag Refining on Boron Removal from Metallurgical-Grade Silicon Using Recycled Slag with Active Component

A new slag refining method to remove boron from metallurgical silicon (MG-Si) by mixed reused slag and active component was proposed. The silicon was refined with CaCl₂-CaO-SiO₂ slag system in this work. The boron removal efficiency under different temperature and holding time was investigated, particularly for the recycle of the used slag mixed with different amount of CaCl₂ in the 2^nd and the 3^rd run. The multiple slag refining was adopted to test the effect of reused slag; the boron removal ratio was 96% and 94% at 1923 K and 1873 K, respectively. These methods could significantly reduce the slag amount in refining. The total transfer coefficient of boron was also derived by the boron content. The overall boron removal mechanism and effect of slag recycle on boron removal was also discussed.     

Impurities Removal From Metallurgical Grade Silicon Using Gas Blowing Refining Techniques

The removal of impurities from metallurgical grade silicon using the O₂ and H₂O-O₂ gas blowing techniques was firstly studied by thermodynamics. The relationships between the boron content in refined silicon and the equilibrium partial pressures of gaseous boride species were established, which shows a theoretical limitation for boron removal from metallurgical grade silicon using the H₂O-O₂ gas blowing technique. The data also showed that the impurity boron in silicon was mainly volatilized in the form of B₃H₃O₆, BHO₂ and BO and the volatilization of boric hydrate species was much more than that of the oxide species. The impurities removal from metallurgical grade silicon including Fe, Al, Ca, Ti, B, P and C was studied using an O₂ gas blowing in a ladle and in succession a mixed Ar-H₂O-O₂ gas blowing was operated in a DC arc furnace for boron removal. It showed a removal efficiency higher than 90 % for Al, Ca and 50 % for B using the O₂ gas blowing technique in the ladle. Impurity boron was reduced from 35 ppmw to 18 ppmw in the ladle and it was once again reduced to 0.6 ppmw using an Ar-H₂O-O₂ gas blowing technique in the DC arc furnace for a systematic pressure of 5 Pa when the ratio of H₂O to O₂ and the refining times are 2:1 and 12 min, respectively.     

Kinetics of Boron Removal from Metallurgical Grade Silicon using a Slag Refining Technique Based on CaO-SiO2 Binary System

The removal of non-metallic impurity boron in metallurgical grade silicon (MG-Si) can be carried out by using a slag refining technique based on CaO-SiO₂ system. However, the boron removal in depth in the slag refining process is limited by the kinetic conditions. The refining agents based on the binary and ternary slag systems CaO-SiO₂, CaO-SiO₂-LiF, CaO-SiO₂-Li₂O, and CaO-SiO₂-K₂CO₃ were used to remove boron in this paper. The corresponding kinetic equations of boron removal using these slags were established by fitting the relationship between refining time and boron concentration in the refined silicon. It was determined that the rate coefficients of boron removal (k_B) are 0.25, 0.24, 0.26, and 0.57, respectively, using CaO-SiO₂, CaO-SiO₂-LiF, CaO-SiO₂-Li₂O, and CaO-SiO₂-K₂CO₃ slag refining. It was found that the mass transfer of boron in silicon is the rate controlling step for boron removal using 40%CaO-40%SiO₂-20%LiF, 40%CaO-40%SiO₂-20%Li₂O, and 40%CaO-40%SiO₂-20%K₂CO₃ slag refining. It is opposite to 50%CaO-50%SiO₂ slag refining that the mass transfer of boron in slag becomes the controlling step for boron removal.     

Removal of Boron from Molten Si and Si-Cu Using Ammonia Containing Gas

All the reported thermodynamics analysis implied that ammonia is a promising reagent for removing boron from silicon, but efforts to employ it in silicon refining have failed in practice. As such, there are few reports detailing this process. In this study, this process was analyzed experimentally. Various concentrations of ammonia were introduced to remove boron from silicon in the temperature range of 1200–1550 ^°C with a total pressure of 1 atm. Boron containing nitrides precipitates were detected in the furnace tube. The effect of ammonia content in the feeding gas was explored. It implied that higher ammonia partial pressures promote the boron removal. The experimental results have suggested that ammonia could remove boron from silicon in the form of volatiles, such as BH_x (x = 1, 2, 3) and B₃ H ₆ N ₃, in practice. The reaction-rate constant was limited to 10^?6– 10^?7 m/s in pure ammonia at 1450 ^°C. Moreover, a higher ammonia flow rate resulted in lower boron removal ratio. It was indicated that the rate determining steps of boron removal and silicon loss in this process were the chemical reaction at the surface of the melt and the transport of ammonia from gas phase to the surface, respectively. The relationship of the boron-removal rate with temperature followed a “V”-shaped curve, which implied the limit of thermodynamic factors at high temperature and the limit of kinetic factors at temperatures lower than 1300 ^°C. Based on the analysis results, temperature-programed reaction was designed to promote the boron-removal efficiency doubled. Cu was used to decrease the liquidus temperature of Si based alloy in the process. As a result, more than 80% boron in Si-Cu alloy could be removed.     

复合熔析精炼去除工业硅中的非金属杂质硼

在1550℃下将CaO-SiO2-Al2O3三元渣与Sn-Si合金混合精炼,降温熔析去除工业硅中的杂质B,考察了渣系光学碱度及合金组成对B去除的影响及作用机理. 结果表明,渣系光学碱度增大,BO1.5的活度系数降低明显,增加了B的分配系数,即增强了精炼效果. Sn-Si合金中Sn比例从0增至70%时,B在渣相与合金相间的分配系数从3.16提高至13.8,硅中B含量最低为0.89′10-6,最高除硼率为93.3%;当Sn比例大于30%时,合金粘度大幅降低,B的分配系数提高;当Sn比例为70%、合金粘度为0.61 mPa×s时,渣系CaO-SiO2-20%Al2O3与CaO-SiO2-40%Al2O3精炼工业硅所得最大B分配系数分别为10.1和12.3.     

The effect of the addition of alumina on the viscosity,surface tension,and foaming efficiency of 2.5(CaO/SiO2)-xAl2O3-yFeO-MgO melts

Viscosity and surface tension strongly influence the efficiency of slag foam in metallurgical processes. An excellent foaming slag preserves heat and lowers the cost of smelting in an electric furnace. In this study, we investigated the viscosity, surface tension, and foaming efficiency of a 2.5CaO/SiO₂-xAl₂O₃-yFeO-MgO slag. We also investigated the different valence oxygen ions by X-ray photoelectron spectroscopy (XPS). The results showed that with a gradual increase in the content of Al₂O₃, the viscosity initially increased and then decreased, and the changes in surface tension followed a similar pattern. The change in viscosity was caused by the increase in the degree of polymerization of the slag, which was determined by the competitive relationship between polymerization and the reduction in the stability of the overall network structure. Adding a small amount of Al₂O₃ to the slag slightly increased the number of Al–O–Al structures, whereas adding a large amount of the Al₂O₃ led to the formation of low-strength Al–O–Si structures, which reduced the stability of the network structure, thus reducing the viscosity. Because the surface tension is related to the concentration of non-bridging oxygens (NBOs), when the NBO content increased, the instability of the surface structure caused an increase in energy, thus increasing the surface tension. In addition, the CaO–SiO₂–5MgO-xAl₂O₃-yFeO five-element oxide in this study had the lowest surface tension at the same NBO concentration, which positively contributed to slag foaming. Finally, When the Al₂O₃ content in the system increased from 5.1 to 15.7 wt%, the foaming efficiency increased from 24.2 to 69.2 (minute‧centimeters), an increase of 286%.     

A bimetallic molybdenum (VI) and stannum (IV) catalyst for the transesterification of dimethyl oxalate with phenol

The transesterification of dimethyl oxalate (DMO) with phenol to methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) was carried out in liquid phase under facile catalytic conditions. A series of bimetallic, MoO₃–SnO₂/SiO₂, catalysts with various Mo and Sn content were prepared by sequential impregnation of cationic Mo species and cationic Sn complexes using impregnation method. The effects of mass ratio of Mo:Sn, amount of Sn additive, and Mo(Sn) laoding amount on activities of transesterification of dimethyl oxalate with phenol were investigated. The evaluation results showed that MoO₃–SnO₂/SiO₂ catalyst with 14 wt% Mo(Sn) content performed best, giving 74.6% DMO conversion and 99.5% selectivity to MPO and DPO. This new heterogeneous catalyst provided not only an excellent selectivity (99% to MPO and DPO) and high yield of MPO and DPO but also a simple and practical protocol for DPC synthesis.     

Role of Oxygen Potential and Oxygen Ions on Phosphorus Removal from Silicon via Addition of FeO into Slag

In present work, the role of the oxygen potential (P_O2) and oxygen ion (O²⁻) concentration for removing phosphorus (P) during CaO–SiO₂–Al₂O₃–Fe_xO slag refining was studied by on-line measurement of oxygen activity in molten silicon (Si), FactSage calculation, Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The results show that the addition of FeO from 0 to 9.25 wt% in slag can increase the activity of dissolved oxygen (a_[O]) in Si and the mole fraction of O²⁻ in slag. Moreover, the increase of O²⁻ concentration leads to the increase of non-bridge oxygen (NBO). The value of L_P (the partition ratio of phosphorous between slag and Si shows a first increase and then decrease trend and reaches a maximum value of 1.95 at 5 ± 0.1 wt% FeO. It is believed that the increase of a_[O] and NBO can promote the removal of P as FeO content is less than 5 ± 0.1 wt%. the chain structure unit (Q²) of silicate network as the main intermediate structure to capture PO₄³⁻ from the charge compensation of P₂O₅ by O²⁻ to form the sheet structure unit Q³(Si and P). When FeO content is increased to more than 5 ± 0.1 wt%, L_P value gradually decreases although the values of NBO and a_[O] are increasing. NBO plays a leading role in this process, it can be speculated that more NBO can depolymerize the Q³ (Si and P) to destroy the stability of P in silicate network. As a result, a mount of PO₄³⁻ is present at the interface to prevent the oxidation of phosphorous, which leads to the decrease of L_P value.

     

The role of MgSiN2 during the sintering process of silicon nitride ceramic

The reaction process between MgSiN₂ SiO₂ and Si₃N₄ was investigated by analyzing the composition change of the powder mixture of 61 wt% MgSiN₂, 34 wt% SiO₂ and 5 wt% α-Si₃N₄ after heat treatment at different temperatures. The phase and chemical compositions of the grain boundary phase in the silicon nitride ceramic was analyzed by x-ray diffraction, transmission electron microscope, and energy-dispersive x-ray spectroscopy. The results demonstrated that MgSiN₂ reacted with the surface silica and Si₃N₄ to form Mg–Si–O–N liquid phase, which promoted the consolidation densification of silicon nitride powders through liquid-phase sintering mechanism. The amount of Mg–Si–O–N glass boundary phase using MgSiN₂ as additives is much less than that using the same amount of MgO additive, owing to the lower oxygen concentration and higher nitrogen content.     

Synthesis of SiO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript> gels in water free conditions

A series of SiO₂–SnO₂ samples with various Sn/Si molar ratios (0.05–1.0) have been synthesized by the sol–gel technique from (Tetraethylorthosilicate) TEOS and Sn(CH₃COO)₄ precursors in water free conditions. The synthesis applied allowed obtaining the final product in monolithic (nonfractured upon drying) form with no use of drying control chemical additives. All samples are characterized by thermal analysis, XRD, and FTIR. The low temperature nitrogen adsorption measurements indicate the presence of both micro and mesopores. The samples containing less than 20 wt% of SnO₂ show much higher surface area than SiO₂ gel. The appearance of new bands at 1,048 and 882 cm⁻¹ in the FTIR spectra could be related to stretching vibrations of the three dimensional Si–O–Sn network, which suggests that tin component has replaced silicon atoms in Si–O–Si structure.     

Calculation and Characterization of Silicon-Boron Phases in Metallurgical Grade Silicon

Introduction

It is a challenge that boron is removed in upgrading metallurgical grade silicon (MG-Si) to solar grade silicon (SoG-Si) with a metallurgical process. In this research, the Si-B binary system is thermodynamically assessed and characterized by using the CompuTherm Pandat software, which is helpful to the boron removal in refining MG-Si to SoG-Si.

Methods

The Si-B binary system was thermodynamically assessed and characterized by using the CompuTherm Pandat software. The SEM-EDS and XRD technologies were used.

Results

The solution phases, including Liquid, diamond-Si and ??-B were treated as substitutional solution phases, of which the Gibbs energies were expressed with Redlich?CKister polynomial functions. Meanwhile, the compounds, SiB₃, SiB₆, SiB_n, were modeled as stoichiometric compounds. The thermodynamic parameters formulating the Gibbs energies of various phases were obtained and the equilibria and transition of the phases were discussed. According to XRD technology, the intermediate phase SiB₄ was obtained and observed. It was tested that the phase SiB₄ existed objectively although it is not found in the assessed Si-B binary phase diagram.

Discussion

The existent forms for Si-B phases in the MG-Si melt were forecast and tested to be SiB₆ and Liquid.     

Removal of Impurities from Metallurgical Grade Silicon During Ga-Si Solvent Refining

Purification of metallurgical grade silicon (MG-Si), using gallium as the impurity getter has been investigated. The technique involves growing Si dendrites from an alloy of MG-Si with Ga, followed by their separation by acid leaching. The morphologies of impurity phases in the MG-Si and the Ga-Si alloy were investigated during the solvent refining process. Effective segregation ratios of B and P in the Ga-Si system were calculated. Most metallic impurities formed silicides, such as Si-Fe-Ga-Mn or Si-Fe-Ga impurity phases, which segregated to the grain boundaries of Si or into the Ga phase during the Ga-Si solvent refining process. After purification, the refined Si is plate-like with < 111 > crystallographic orientation, and the removal fraction of B and P was 83.28 % and 14.84 % respectively when the Si proportion was 25 % in the Ga-Si alloy. The segregation ratios of B and P were determined to be 0.15 and 0.83 when the solid fraction of Si was 0.25. The effective removal of B and P by a solidification refining process with a Ga-Si melt is clarified.     

Cold Simulation of momentum transfer of a metal droplet falling through slag systems possessing net-work structured fluids

Most of the high temperature smelting/refining processes involve falling of metal droplets through the slag systems. The slag systems comprising oxides viz., Al₂O₃, SiO₂, CaO, FeO, MgO, MnO etc., are essentially produced from the removal of ash and gangue from coke and/or ore during the metal extraction. At high temperatures these oxides form a network structured fluids. Momentum transfer phenomena of a falling sphere through these slag systems significantly influence the kinetics of the refining reactions occurring in the high temperature metallurgical reactors. It governs the metal quality including their productivity. The paper discusses cold simulation of the momentum transfer of a metal droplet falling through the slag systems possessing network structure.     

One‐step synthesis of ordered Sn‐substituted SBA‐16 mesoporous materials using prepared silica source of rice husk and their selectively catalytic activity

Highly ordered SBA‐16 silica mesoporous materials were synthesised hydro‐solvothermally under the acidic medium using SiO₂/F127/BuOH/HCl/H₂O gel. Pure SiO₂ powders were prepared from inexpensive and environmentally friendly silica source of rice husk. The pore size of the materials could be optimised by using a blend of P123 and F127 templates. Sn‐substituted SBA‐16 mesoporous materials were yielded via the direct injection of stannic chloride into the fixed gel in acidic medium. X‐ray diffraction, N₂ adsorption, scanning electron microscope/transmission electron microscope results suggest that tin ions were incorporated into the Si‐SBA‐16 framework by isomorphous substitution between Sn and Si ions. Elemental analysis indicates that tin can be substituted in the range of Si/Sn = 21.4–10.5. UV–vis, XPS, TPR‐H₂, TPD‐NH₃ results reveal that tin atoms are highly dispersed in 4+ oxidation state and mostly occupy in the silica framework. The degree of tin incorporation into silica framework can easily be controlled by a simply adjustment of the H₂O and HCl molar ratios. The mesoporous Sn‐SBA‐16 materials were an active benzylation catalyst with almost 100% selectivity to monoalkylated product in alkylation of aromatics with benzyl chloride. © 2011 Canadian Society for Chemical Engineering     

TPD study for direct synthesis of methylchlorosilanes

Temperature programmed desorption (TPD) was performed to obtain information about the role of silicon, catalyst (copper), and promoters (zinc and tin), and to characterize the active sites for the formation of silanes. Use of infrared spectroscopy allowed rapid analysis of the gas-phase product composition. During TPD where methyl chloride was used as an adsorbate, methyl chloride (MeCl), hydrogen chloride (HC1), methane (CH₄) and silanes were produced from contact masses. Although dimethyldichlorosilane (DMDC), methyltrichlorosilane (MTCS), methyltrichlorosilane (MTCS), trimethylchlorosilane (TMCS) and dimethylchlorosilane (DMCS) were produced during the direct reaction, tetrachlorosilane (QCS), trichlorosilane (TCS), methyltrichlorosilane (MTCS), and methyldichlorosilane (MDCS) were the major silanes observed during the TPD. Zinc promotion to silicon-copper contact mass (CuSi mass) increased the production of TCS, while tin promotion decreased the production of silanes having H atom, and increased the production of MTCS. Copromotion of 0.5 wt% zinc and 0.2 wt% tin increased the MTCS production further; however, the copromotion of zinc (0.5 wt%) and tin containing a small amount of tin (0.01 wt%) produced QCS as a major silane product. The silicon sites having two or three surface species such as CH₃, Cl and H were proposed as the active sites responsible for the formation of silanes, and the silicon sites of = SiCl₂ and =Si(CH₃)Cl were the most abundant under the steady state condition.     

Fabrication of uniform SnO2–SiO2–Pt composite nanofibres via co-electrospinning

We successfully fabricated uniform SnO₂–SiO₂–Pt composite nanofibres (NFs) by using a co-electrospinning technique, in which we set up two coaxial capillaries. Morphology control of NFs was investigated, along with their structural properties and chemical compositions. Furthermore, to systematically investigate the morphological changes in SnO₂–SiO₂–Pt composite NFs, the relative weight ratios of the Sn precursor to the Si precursor including the 4 wt% Pt precursor were controlled at 3:1, 1:1, and 1:3. To demonstrate the formation mechanism of the composite NFs, the precursor positions of the shell section and the core section in co-electrospinning were reversed. The resultant composite NFs were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). These results showed that in the case of the optimum weight ratio (1:1) of the Sn precursor in the shell section to the Si precursor including the 4 wt% Pt precursor in the core section, SnO₂ and Pt nanoparticles were uniformly grown on SiO₂ NFs, implying the successful formation of uniform SnO₂–SiO₂–Pt composite NFs.     

Supported RuB amorphous alloy mediated 1 atm hydrogenation of carbonyl compounds under ambient temperature

Tin promoted RuB amorphous alloy supported on SiO₂ was prepared by a novel reductant impregnation method for the hydrogenation of carbonyl compounds to the corresponding alcohol under atmospheric H₂ and ambient temperature. The as-prepared Ru–Sn–B/SiO₂ catalyst presented high activity for hydrogenation of carbonyl compounds even under atmospheric H₂. The TEM images showed that addition of tin improved the dispersion of RuB particles, and then significantly increased the conversion of carbonyl compounds. Based on the XPS spectra and catalytic performance, tin in Ru–Sn–B/SiO₂ catalyst was supposed to play a role of adsorbing and activating the CO bond of carbonyl group during the hydrogenation.     

Effect of boron on the stability of Ni catalysts during steam methane reforming

Ni catalysts promoted with 0.5 and 1.0 wt% boron were synthesized, characterized and tested during steam methane reforming, to evaluate the effect of boron on the deactivation behavior. Boron adsorbs on the γ-Al₂O₃ support and on the Ni particles and 1.0 wt% boron is found to enhance the stability without compromising the activity. Catalytic studies at 800 °C, 1 atm, a stoichiometric methane to steam ratio, and space velocities of 330,000 cm³/(h g_cat) show that promotion with 1.0 wt% boron reduces the rate of deactivation by a factor of 3 and increases the initial methane conversion from 56% for the unpromoted catalyst to 61%. Temperature-programmed oxidation (TPO) and scanning electron microscopy (SEM) studies confirm the formation of carbonaceous deposits and illustrate that 1.0 wt% boron reduces the amount of deposited carbon by 80%.