Molten Salt Electrolysis Production Of High Purity Silicon And Silicon Alloys

The FFC Cambridge Process is a method to direct electrochemical reduction of oxides which are used as cathode to prepare metals, nonmetals or alloys below the melting point of metal. The direct electrochemical reduction of metal oxides take place on cathode through gaining electron. The oxygen ions become gas on the anode. The FFC Cambridge Process has become the hot topic of the research in recent years because it has a lot of advantages. For example, the FFC Cambridge Process is not only process flow short, being simple to operate, low energy levels and green but also fit the direct reduction of high-melting point metals.In this paper, the FFC Cambridge Process was used to prepare Si, Ni3Si and NiSi2, in molten CaCl2. A graphite crucible was used as the anode and SiO2, SiO2-NiO were used as cathode respectively. Silicon nanowire were prepared using nanometer SiO2as cathode.Pre-electrolysis can remove part of impurities in calcium chloride molten. The B concentration in calcium chloride molten fell from7ppmw to4.1ppmw, P fell from10.1ppmw to4.8ppmw, Fe from105.1ppmw to40.1ppmw. The removal rate of B, P, Fe was41.43%,52%and61.85%respectively. It was studied that the removal rate of B, P, Fe was affected by voltage, temperature and time. The experimental results showed that the removal rate of B, P, Fe raised4.39%,6%,9.23%respectively when the voltage was increased from2.5V to3.1V. The removal rate of B, P, Fe raised5.71%,3%,2.86%respectively when the temperature was increased from850℃to900℃. Therefore, raising the pre-electrolysis voltage, temperature and increasing the time will enhance the removal rate of impurities. This paper determined the reasonable conditions of pre-electrolytic process ultimately.The behavior of impurities in direct electrochemical reduction process were studied by comparing theoretical concentration and actual concentration of B, P, Fe, Al, Ca in silicon. The concentration of B, P in attained silicon were4.8ppmw and3.5ppmw when high purity quartz were used as cathode. The concentration of B, P in attained silicon were3.1ppmw and2.2ppmw after pre-electrolyzing the CaCl2molten. The results showed that reducing impurity content in the electrolyte could increase silicon’s purity. The concentration of B, P, Fe, Al, Ca in silicon were0.3ppmw, 0.2ppmw,0.4ppmw,0.1ppmw, less than O.lpmw respectively after the attained silicon were acid leaching by the mixture of3%HNO3and3%HF. This silicon accorded with the concentration requirement of impurities in solar grade silicon.The Si-Ni alloy were prepared by FFC Cambridge Process. The electrolytic products was Ni3Si when the molar ratio of SiO2to NiO was1:3. The electrolytic products was NiSi2when the molar ratio of SiO2to NiO was2:1. The result indicated that the corresponding intermetallic compounds of silicon and nickel would be prepared by setting a molar ratio of SiO2to NiO. The electrochemical reduction process of SiO2-NiO was as follows by studying the process of electrochemical reduction on cathode:First of all, SiO2were reduced to silicon by electrolysis, and then NiO were reduced to nickel. Si and Ni diffused into each other body and the formation of intermediate products were followed by Ni3Si, Ni2Si, NiSi. The sequence of alloying was Ni3Siâ†'Ni2Siâ†'NiSiâ†'NiSi2.