Purification Of Metallurgical Grade Silicon Research Articles

Purification of Metallurgical-grade Silicon by Sn-Si Solvent Refining with Different Tin Content

Purification of Metallurgical-grade Silicon by Sn-Si Solvent Refining with Different Tin Content

Features of Impurity Segregation and Microstructure of Si Ingots obtained by Electron-Beam Purification of Metallurgical Grade Silicon

Features of Impurity Segregation and Microstructure of Si Ingots obtained by Electron-Beam Purification of Metallurgical Grade Silicon

Hydrogen generation during the purification of metallurgical-grade silicon

Environmental and resource implications of metallurgical-grade silicon (MG-Si) slag with more than 15% of silicon become increasingly serious. The recycle or reuse of silicon slag is in urgent need. Recent reports increase about hydrogen production using wastes from silicon production. This paper primarily proposes hydrogen generation by reaction of silicon slag with water via wet grinding. Meanwhile using hydrogen generated realized the purification of MG-Si by flotation. During the process, Si is liberated from slag by wet grinding, with the slurry pH of 8.68. Hydrogen can continuously formed for more than 6h at 298 K. With silicon-water reaction going on, hydrogen content gradually increased to the highest value 73.77%, and then decreased. Hydrogen generated can be collected and used in metallurgical reduction and combustion fuels fields. It is significant to do further study on hydrogen generation via wet grinding, which may provide a new idea for hydrogen production and application.

Purification of metallurgical grade silicon by gas blowing and slag refining method: progress of researches

Purification of metallurgical grade silicon by gas blowing and slag refining method: progress of researches

Purification of Metallurgical-Grade Silicon by Acid Leaching

Purification of metallurgical-grade silicon (MG-Si), used in many applications such as the medical industry, polymer industry and the solar cells manufacturing. Removal of impurities from MG-Si is critical in these applications. The MG-Si studied in the present research contained various iron, calcium, and aluminum impurities. The hydrometallurgical purification method with different types of acids as solvents was chosen to refine MG-Si. Effects of hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid in combination with each other as a solvent for purification of MG-Si were investigated. Additionally, the effects of temperature and particle size of MG-Si were evaluated. In this study, in comparison with previous studies, samples with a larger particle size (in the range of 53 to 300 microns), less refining time (1 to 6 h) and relatively higher temperatures (30 to 70 ∘C) were studied. The results indicated that the highest purity of MG-Si was achieved by HCl (25%) + HF (5%) and followed by HCl (25%) + H2SO4 (30%), HNO3 (25%) + HCl (25%), HNO3 (5%) and H2SO4 (25%), respectively. It was also found that with particle size 53μm, applying a temperature of 50 ∘C for 6 h resulted in high efficiency and purity (99.96%) of MG-Si.

Purification of metallurgical-grade silicon using Si–Sn alloy in presence of Hf, Zr, or Ti

Hf, Zr, or Ti was employed as additive to enhance removal of the main impurities from metallurgical-grade Si (MG–Si) during the Si–Sn alloy refining process. The microstructure of Si-Sn alloy without/with Zr, Hf, or Ti was observed and analyzed by electron probe microanalysis (EPMA) to investigate the distribution of impurities. Compositions of intermetallic phases precipitated in Sn or among the grain boundaries of Si were quantitatively analyzed using EPMA. The mechanisms of their formation and the removal of impurities are discussed. The results of Si refining showed that the addition of Hf or Zr slightly enhanced the removal of Al. The high refining temperature in the Si-Sn alloy refining process was responsible for the low extraction of B. Most of Zr, Hf, or Ti was removed simultaneously with other impurities during the Si–Sn alloy refining process.

Assessment and Study on the Impact on Environment by Multi-crystalline Silicon Preparation by Metallurgical Route

The process of the production of multi-crystalline silicon is also that of incessant purification of metallurgical grade silicon, during which high energy consumption and environmental pollutants are inevitable. The paper, which is based on life cycle assessment (LCA), presents calculation and analysis on resource input, energy consumption, emissions into and comprehensive impact on environment generated from the whole process in which 1 kg of solar grade multi-crystalline silicon (SoG-Si) is produced with metallurgical route. The program is based on its research on four representative processes in metallurgical route, namely slag refining, wet purification, directional solidification and electron beam melting. Key factors determining environmental load are sought out through contribution analysis and improvement evaluation. Measures and suggestions on how to improve are proposed. For the first time, the LCA model of comparison of metallurgical route and modified Siemens process to SoG-Si environmental impact is constructed. The environmental impacts of SoG-Si production via metallurgical route is compared with purification via modified Siemens process. Results show that the comprehensive environmental impact of metallurgical route SoG-Si is only 49.57% of modified Siemens process SoG-Si. The metallurgical route enjoys obvious environmental advantage. The improvement should be focused on the reduction of electric power consumption in production processes.

Hydrometallurgical Purification of Metallurgical Grade Silicon with Hydrogen Peroxide in Hydrofluoric Acid

The effect of adding hydrogen peroxide (H2O2) as an oxidizing agent on purifying metallurgical grade silicon (MG-Si) by leaching with hydrofluoric acid (HF) was studied as a function of leaching temperature, particle size, leaching duration and concentration of leaching agents. It was found that the extraction capacity for metallic impurities could be significantly enhanced with introducing H2O2 into HF lixiviant showing litter dependence on HF concentration. By leaching with 1 mol· L-1 HF plus 2 mol· L-1 H2O2 for only 0.25 h at 55 ℃, the MG-Si purity could be upgraded from 99.74% to 99.96%, to 99.99% with further prolonging leaching duration. The sensitivity sequences of precipitates to each etchant were obtained through revealing the micro-structural evolution of MG-Si before and after etching. With the help of Raman spectrometry, it was reasonable that etching silicon starts with first oxidizing reaction by H2O2 and then dissolved by HF. Like Si-Si bonds, some Si-F and Si-O bonds could also be found on silicon surface after acid etching with HF plus H2O2 etchant. After acid leaching, the zigzag silicon surface had been detected by using transmission electron microscopy analysis and the formation mechanism was further discussed. Figure 1

Hydrometallurgical Purification of Metallurgical Grade Silicon with Hydrogen Peroxide in Hydrofluoric Acid

The effect of adding hydrogen peroxide (H2O2) as an oxidizing agent on purifying metallurgical grade silicon (MG-Si) by leaching with hydrofluoric acid (HF) was studied as a function of leaching temperature, particle size, leaching duration, and concentration of leaching agents. It was found that the extraction capacity for metallic impurities could be significantly enhanced with introduction of H2O2 into HF lixiviant with little dependence on HF concentration. Upon leaching with 1 mol·L–1 HF and 2 mol·L–1 H2O2 for only 0.25 h at 55 °C, the MG-Si purity could be upgraded from 99.74% to 99.96%, to 99.99% with further prolonging of leaching duration. The sensitivity sequences of precipitates to each etchant were obtained through revealing the microstructural evolution of MG-Si before and after etching. With the help of Raman spectrometry and transmission electron microscopy, the chemical etching mechanism is discussed.

Mechanism of boron removal from Si–Al melt by Ar–H2 gas mixtures

A new method about purification of metallurgical grade silicon (MG-Si) by a combination of Si–Al solvent refining and gas blowing treatment was proposed. The morphologies and transformation of impurity phases, especially for boron and iron in Si–Al melt were investigated during Ar–H2 gas blowing treatment. The mechanism of boron removal was discussed. The results indicate that gas blowing can refine grain size and increase nucleation of the primary Si. Boron can be effectively removed from MG-Si using the Ar–H2 gas blowing technique during the Si–Al solvent refining. Compared with the sample without gas blowing, the removal efficiency of boron increases from 45.83% to 74.73% after 2.5 h gas blowing. The main impurity phases containing boron are in the form of TiB2, AlB2 and VB compounds and iron-containing one is in the form of β-Al5FeSi intermetallic compound. Part of boron combines [H] to transform into gas BxHy (BH, BH2) and diffuses towards the surface of the melt and is volatilized by Ar–H2 gas blowing treatment under electromagnetic stirring.

Purification of metallurgical-grade silicon using zirconium as an impurity getter

Abstract 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.

Separation of boron and phosphorus from Cu-alloyed metallurgical grade silicon by CaO–SiO2–CaCl2 slag treatment

Abstract A purification method using copper (Cu) alloying and CaO–SiO 2 –CaCl 2 slag treatment had been studied in connection with the purification of metallurgical grade silicon (MG-Si). The principle of this method was that low-purity MG-Si alloyed with Cu to form Si–Cu alloy phase, and then the Si–Cu alloy was treated using CaO–SiO 2 –CaCl 2 slag to remove impurities. Finally, selective acid leaching was applied to remove the Si–Cu alloy phase, whereupon high-purity silicon could be recovered from the Si–Cu alloy. Effects of slag composition and Cu content on the removal of boron (B) and phosphorus (P) were also investigated. When the Si–50 wt% Cu alloy treated with 45 wt% CaO–45 wt% SiO 2 –10 wt% CaCl 2 slag for 30 min at 1550 °C, the final concentrations of B and P in the purified Si decreased from 3.12 to 0.35 ppmw and 17.14 to 7.27 ppmw, respectively. Furthermore, the corresponding removal mechanism was investigated. It was found that the reaction kinetics followed a first-order rate law with respect to B and P, and were controlled by mass transfer with mass transfer coefficients of 6.25 × 10 −4 cm s −1 and 2.55 × 10 −4 cm s −1 , respectively.

A Modified Oxidative Refinement Process for Removing Boron from Molten Silicon Under Enhanced Electromagnetic Force.

The removal of boron is one of the main challenges in the purification of metallurgical grade silicon destined for low-cost photovoltaic applications. However, boron is very difficult to remove in its elemental form due to its large segregation coefficient in silicon and its low vapor pressure. The removal of boron by slag treatment is today regarded as a highly promising method, but its refining efficiency is relatively low. Also, the reduction of boron by plasma treatment exhibits a high refining efficiency, but the processing cost is high due to the large amount of electricity consumed by the process. In this regard, the use of an oxidizing reactive gas in the refinement process offers some advantages both in terms of low energy consumption and promising refinement rates. Boron can be extracted in various gaseous forms as B(x)O(y) and/or B(x)H(z)O(y) phases, but the vapor pressure of B(x)H(z)O(y) is much greater than that of the other specie at a temperature of 1700 K. The present study reports a modified oxidative refining method designed to enhance the vaporization of boron as B(x)H(z)O(y) by blowing gaseous water onto the silicon melt in a segmented crucible to enhance the electromagnetic force, whereby the processing cost can be dramatically reduced due to the use of a reusable quartz crucible in a graphite crucible. An initial boron content of 13 ppm in the metallurgical grade silicon was significantly decreased to 0.3 ppm by the employment of 1.7SLM Ar + 100 ml/h H2O. Also, a mechanism capable of reducing boron based on thermodynamic considerations is proposed.

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 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.

Si Purity Control and Separation from Solidification of Al–30Si Melt under Pulse Electromagnetic Field

Purification of metallurgical-grade silicon (MG–Si) by a combination of Al-Si solidification refining and electromagnetic oscillating separation and acid leaching collection has been studied. The primary Si crystals and Al-Si alloy in hypereutectic Al-30%Si melt were separated during solidification under the pulse electromagnetic field (PEF). The results show that the Si content in Si-rich layer increases with increasing discharging frequency. The typical metallic impurities (Fe, Ti, and Ca) have removal fraction higher than 99.5%. The removal fractions of the impurities B and P which are more difficult to remove are over 90% and 85%, respectively.

Silicon purification in vacuum induction furnace

In this paper the removal of impurities from metallurgical grade silicon by melting in a vacuum induction furnace was studied, in order to produce solar grade silicon, which is the raw material of the photovoltaic cells to convert solar energy into electricity. The work was focused on the removal of phosphorus, which is one of the most difficult impurities to remove from silicon. Refining was obtained 119 ppm to 8 ppm of phosphorus in 3 hours at 1650°C under pressure of 0.2 Pa, obtained by a set of mechanical, “roots” type, and diffusing pumps. It was concluded that this method is technically feasible for purification of metallurgical grade silicon. The rates constants were obtained for phosphorus (kP=5•10-6 m/s), aluminum (kAl=2•10-6 m/s) and calcium (kCa=1•10-5 m/s). Silicon losses by vaporization during the test averaged around 13%.

Super Gravity Separation of Purified Si from Solvent Refining with the Al-Si Alloy System for Solar Grade Silicon

The purification of metallurgical grade silicon (MG-Si) by using a combination of solvent refining and super gravity separation was studied. MG-Si was first alloyed with aluminum and then solidified at different cooling rates in order to precipitate dendritic primary Si from the Al-Si alloy. Primary Si dendrites were separated under super gravity, resulting in the ejection of Al-Si eutectics from the solid Si-eutectics melt mixture. The effect of the gravity coefficient on the separation efficiency and then influence of the cooling rate and proportion of Si in the Al-Si alloy on the removal efficiency of Al entrainment were investigated. The results demonstrate that super gravity is an effective tool for the separation of primary Si dendrites from Al-Si alloy, with an optimum gravity coefficient of 280. When the proportion of Si was held constant, varying the cooling rate had almost no effect on the entrainment of Al in the separated Si. However, increasing the proportion of Si in the Al-Si alloy had the effect of reducing Al entrainment.

Control of silicon solidification and the impurities from an Al–Si melt

The investigation on purification of metallurgical grade silicon by solidification of hypereutectic Al–Si melt under the temperature gradient as an intensified separation way was carried out. Based on the available thermodynamic parameters and experimental data, the thermodynamic behavior and chemical composition of metallic impurities was studied in the solidification process. The principle for the silicon growth in the Al–Si melts was investigated. The results indicated that the refined silicon grains were successfully enriched at the top of the Al–Si alloy. Then the top part refined silicon was collected by aqua regia leaching. Electrorefining of the bottom part (Al–22%Si) was investigated effectively in view of recovering pure Si and Al. Additionally, according to previous investigation, the optimized technical process for SOG-Si production was proposed.

Effect of Na2O-SiO2 Slag Treatment on Hydrometallurgical Purification of Metallurgical Grade Silicon

The effects of Na2O-SiO2slag treatment on purification of metallurgical grade silicon by leaching with hydrogen fluoride have been investigated. A comparative analysis of microstructure evolution was carried out to examine the leaching behavior of impurities from metallurgical grade silicon. It was found that the distribution of metal impurities Al, Ca, Ti and Na, which co-deposited with Si and formed different intermetallic phases at grain boundaries, had manifest distinction between precipitated phase and silicon. Moreover, acid corrosion experiment results revealed that slag treatment improved the dissolution rate of metal impurities from metallurgical grade silicon as contrasted to that without slag treatment.

Purification of metallurgical-grade silicon by Sn–Si refining system with calcium addition

Purification of metallurgical-grade silicon (MG–Si) by a combination of solvent refining and super gravity separation and acid leaching has been studied. MG–Si was alloyed with tin, and based on this system, the removal of main impurities in MG–Si by solvent refining was investigated. Furthermore, phosphorus removal by calcium addition in molten Si and Sn–Si melt was also studied. Inductively Coupled Plasma (ICP) chemical analysis revealed main impurities including B and P could be efficiently removed by the Sn–Si process and acid leaching. The content of P further reduced when Ca was added to the Sn–Si refining system. Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM–EDS) analysis showed that the formation of compounds between P and several elements in the grain boundaries during the solvent refining process was an important routine of P removal. The maximum weight percent of P in P-containing impurity phases reached to 17.8% in the refined Si after the Sn–Si refining process with Ca addition.