Kuixian Wei

Boron removal in purifying metallurgical grade silicon by CaO−SiO2 slag refining

Abstract Boron removal from metallurgical grade silicon (MG-Si) using a calcium silicate slag was studied. The results show that it is impossible basically to remove boron using a pure SiO 2 refining. The oxidizing ability of CaO–SiO 2 slag for boron removal was characterized by establishing the thermodynamic relationship between the distribution coefficient of boron ( L B ) and the activities of SiO 2 and CaO. The experimental results show that the distribution coefficient and the removal efficiency of boron are greatly improved with the increase of CaO proportion in the slag. The maximal value of L B reaches 1.57 with a slag composition of 60%CaO-40%SiO 2 (mass fraction). The boron content in the refined silicon is reduced from 18×10 −6 to 1.8×10 −6 using slag refining at 1600 °C for 3 h with a CaO–SiO 2 /MG-Si ratio of 2.5, and the removal efficiency of boron reaches 90%.

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.

Study on Al Removal from MG-Si by Vacuum Refining

Aluminum is one of the main impurity elements in metallurgical-grade silicon (MG-Si). The methods of aluminum removal from MG-Si are slag refining, directional solidification, acid leaching and vacuum evaporation refining, respectively. Based on the theoretical calculation, the vacuum evaporation refining experiments for aluminum removal were carried out in this paper. The effects of refining time and temperature on the removal efficiency of aluminum were investigated. The results show that the vacuum evaporation is an efficient refining technique for removing impurity aluminum from MG-Si. The content of aluminum in MG-Si can be reduced from 1120 × 10−6 to 427 × 10−6, and its removal efficiency is 61.9 %. Meanwhile, the removal efficiency of aluminum is improved with the increasing refining time and temperature during vacuum evaporation.

Cr(VI) Removal from Aqueous by Adsorption on Amine-Functionalized Mesoporous Silica Prepared from Silica Fume

Amino-functionalized mesoporous silica MCM-41 materials have been prepared to develop efficient adsorbents of Cr(VI) in wastewater, using silica fume as silica source. Functionalization with amino groups has been carried out by using grafting method. The materials have been characterized by means of X-ray diffraction (XRD), nitrogen (N2) adsorption-desorption, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Adsorption potential of the material for Cr(VI) removal from aqueous solution was investigated by varying experimental conditions such as pH, initial metal concentration, and contact time. The equilibrium data were analyzed using the Langmuir and Freundlich isotherm by linear regression analysis, and the results show that the adsorption equilibrium data obeyed the Langmuir model. In addition, the kinetics analysis revealed that the overall adsorption process was successfully fitted with the pseudo-second-order kinetic model.

Study on Volatilization Rate of Silicon Melt at Vacuum

Abstract In this paper, the theoretical volatilization rate of silicon was analyzed at different temperatures in order to study the temperature influence on the volatilization rate of silicon under vacuum conditions. The experimental volatilization rates of silicon were obtained by experimental measurements.

Predicting the Electricity Consumption and the Exergetic Efficiency of a Submerged Arc Furnace with Raw Materials using an Artificial Neural Network

The problem of higher electricity consumption and lower exergy efficiency in the submerged arc furnace process of the silicon industry needs to be urgently solved. However, various raw materials play important roles in the electricity consumption and exergy efficiency of a submerged arc furnace during silicon production. An artificial neural network (ANN) method was used to model the final strain of the electricity consumption and exergy efficiency with varying silica, coke, coal and electrode. The measured strain versus predicted strain by the model was compared using the R2 coefficient. The results showed that the exergy efficiency and the electricity consumption values of the testing data are R2= 0.9918 and R2 = 0.9896, respectively, in a very short time with low error levels. They clearly indicate the adequacy of the model proposed for prediction of the exergy efficiency and the electricity consumption with different raw materials in the mixture of carbonaceous raw materials in the furnace. Additionally, there is good agreement between the actual and predicted values. Therefore, this developed ANN model is useful to guide the decision about the use of raw materials in silicon production under the condition of lower electricity consumption and higher exergy efficiency.

Application of a Waste Carbon Material as the Carbonaceous Reductant During Silicon Production

It is important to study the application of alternative carbon reductants for industrial silicon smelting to reduce consumption of carbonaceous reducing agents, electricity, and CO2 emissions during silicon production. In this study, an industrial experiment was carried out in an 8 MVA submerged arc furnace using waste carbon material in place of approximately 20% partial reducing agents. The system was analyzed for silicon yield, power consumption, overall energy efficiency, CO2 emissions, and the utilization rate of carbonaceous materials. The system improved the efficiency of carbonaceous materials and decreased power consumption using alternative carbon reductants. The results have showed that use of waste carbon materials reduced carbon emissions per ton silicon by more than 19.14% and specific CO2 emissions decreased to 0.865 t.

Effect of Rapid Heat Treatment on the Crystal Defect Evolution and Electrical Properties of Highly Efficient Polycrystalline Silicon

The existence of a large number of crystal defects in polycrystalline Si(poly-Si) has a significant impact on its electrical property. In order to solve this problem, this study adopts the industrially produced native efficient poly-Si wafers, with 120 s rapid heat treatment experimental conditions under different temperatures. The evolution of the poly-Si crystal defects such as the grain boundary and dislocation as well as the changes in the electrical properties of the samples before and after annealing have been analyzed.. The results show that the annealing process causes a significant reduction in the defects in the samples significantly and improvement in the electrical performance. After annealing at 1200 ∘C, the dislocation density of ploy-Si decreases to 710 cm−2 from 1120 cm−2, revealing a drop of 36.61%. Furthermore, a 1.62% reduction in the high Σ value (Σ27) grain boundary and 3.19% increase in the Σ3 grain boundaries. After the heat treatment, the minority carrier lifetime of the sample increases by up to 0.6 μs. In addition, the size of the grains increases and the dislocation density reduces while the grain orientation is not changed during the heat treatment. The results show that the performance of poly-Si does not linearly improve with temperature, but is related to the crystal structure of Si.

A Study of the Performance of Submerged Arc Furnace Smelting of Industrial Silicon

The consumption of energy and carbonaceous reductants is one of the more important factors that significantly influence the cost of industrial silicon production. The effect of raw materials on the silicon yield and the energy consumption in the process of producing silicon was investigated in this study using data collected from various submerged arc furnaces. Following analysis of a large quantity of industrial data, the results showed that a 12.5 MVA submerged arc furnace was the most efficient in utilizing the consumption of coal. In addition, the 8 MVA furnaces consumed the petroleum coke (petcoke) more efficiently in a silicon production process. To decrease the raw materials consumption and the energy loss in an industrial silicon production process, the petcoke/coal ratios for the 8 MVA and 12.5 MVA furnaces should be 1.25∼1.4 and 0.4∼0.6, respectively, for the production of one ton of silicon.

Boron Removal from Silicon Using Secondary Refining Techniques by Metallurgical Method

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.