| Developing a low-cost solar grade (SoG) silicon feedstock is both requirement of continued rapid expansion of the photovoltaic (PV) market and a very important role in national energy security strategies. This thesis work has been funded by Specialized Research Fund for the Doctoral Program of Higher Education(Grant No.200805331120), Hunan Provincial Innovation Foundation For Postgraduate (CX2009B036) and Innovation Foundation of Central South University(2010bsxt02). After systematic analyzing of the advantages and disadvantages of "solidification refining with Al-Si melts" method, this paper focuses on the study of electrochemical preparation of high pure Si and Si alloys. The reduction potentials of elements in fluoride molten salt were calculated based on Gibbs free energy, and the electrodeposition behavior of Si on solid and liquid electrodes were studied. Furthermore, electrowinning of Si in Na3AlF6-SiO2molten salt was studied systematically considering the electrolysis parameters, electrolysis cell and composition of electrodes. Meanwhile, three-liquid-layer electrorefining and similar horizontal electrorefining of Si were proposed and carried out. The main results of the thesis can be summarized as follows:(1) Reduction potential of each element is calculated based on the Gibbs free energy of fluoride, the conditions of co-deposition of impurities and Si in complex fluoride molten salt are also discussed. Reduction potentials of impurities such as Sr, Mg and Ca are more negative than Si, which means that they can’t co-deposit with Si. When the concentration of impurities such as Fe, Cu and P are greater than the calculated minimal concentration in complex fluoride molten salt, they would be reduced prior to Si and bring bad influence on the purity of deposited Si.(2) Si electrodeposition behavior in complex fluoride molten salt is revealed by electrochemical measurements. Silicon diffusion into liquid cathode is modelled and evolution of diffusion coefficient and Si activity with concentration is deduced by a mathematic approach based on the electrochemical results. Cyclic voltammograms (CVs) showed that the reduction order of ions in Na3AlF6-LiF melts are Si, Al and Na. Maintaining the concentration and adjusting the reduce potetial of Si ions in eletrolyte, Si can be successfully depostied without codeposition of Al and other impurities. Si diffusion coefficient in Al-Si alloy is much less than that in Cu-Si alloy. When the Si concentration in Al-Si alloy are3.230×10-5mol/cm3and6.457×10-5mol/cm3, Si activity coefficiency are0.387and0.435respectively.(3) This study demonstrates the feasibility of continous Si alloy and very pure Si production directly from SiO2by electrowinning using liquid electrodes. The influence law of electrolysis parameters, electrolysis cell and electrodes materials on current efficiency of electrowinning and purity of deposits is revealed. Solid deposited Si is more pure than99.9%, the dominant impurities are Al, Fe and Ti. The experimental works with200A scale indicate that the average Si content in Al-Si alloy after electrolysis is found above18%, and the highest Si content in top area of solidified Al-Si alloy is near40%. Compared with the MG-Si, Si crystals in Al-Si alloy prepared from200A and20h electrolysis are much more pure, impurities such as B and P are decreased from5.1and21.4to3.3and10.2ppmw respectively. Moreover, concentration of impurities B, Ti, Ni and Cu are all less than10ppmw.(4) Taking advantage of the knowledge of Al electrorefining, the influence law of electrolysis parameters, electrolysis cell and electrodes materials on current efficiency of electrorefining and purity of deposits is revealed. Deposited silicon particles are found embedded in electrolyte. Furthermore, with increasing operation time and current density, re-combination of silicon particles is revealed and yield a larger size Al-Si alloy ball of2cm. Compared with the smelting Al-Si alloy, electrorefined Al-Si alloy shows a remarkable reduction of B and P concentration, decreasing from32.7and32.5to4.8and12.4ppmw respectively. B and P concentration in Si crystals separated from electrorefined Al-Si alloy are as lower as2.2and6.5ppmw respectively. Al-Si alloy obtained from electrochemical methods are more appropriate as raw materials for the alloy solidification refining method.(5) A modified horizontal electrorefining cell was developed and the uneven current density distribution does not bring bad influences on the electrochemical process. Aluminum electrorefining could run steady under current density as high as800mA/cm2and current efficiency can reach95%. Under experimental scale, Si electrorefining using horizontal cell indicates that Cu-8wt%Si alloy is more appropriate than A1acting as cathode and collecting the deposited Si.(6) An innovative route for production of SoG-Si is raised with the combination of molten salt electrochemical method and alloy solidification physical method. Although this process isn’t optimized yet, electrochemical method is believed that can satisfy the demand of high pure raw materials and recycle the high pure Al during the physical production lines. Compared with the Siemens process, this route is simple and saves a great deal of energy, only consuming less than58.4kWh per kg... |
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