Fundamental Study On Preparation Of Metallic Silicon By Silicon Cutting

In recent years, the exploration and use of energy is growing continuously with the rapid development of human society. Solar photovoltaic is considered by many scientists as a sunrise industry in the21st century for no pollution, rich resource reserves, security, universality, etc. At present, solar-grade polysilicon is produced mainly by modified Siemens process. Because the improved Siemens process and other chemical production processes hold some shortcomings such as high-cost, high energy-consumption, technical monopoly and so on, it is extremely necessary to develop a new process with low-energy, low-cost and low pollution for solar-grade polysilicon production. However, recently the process of producing metallurgical silicon gets attention from more and more research institutes and enterprises concerned for the advantages like low-cost, universality and low energy-consumption.However, in view of the great variance in the available separation properties of these impurities in condensed matter, such as segregation coefficient, oxidation-reduction and saturation vapor pressure, metallurgical silicon in metallurgy purification process cannot complete the separation of all the impurities and silicon with only one unit operation. In this paper, based on the source of impurities in metallurgical silicon, a process for the production of high-purity silicon with low B/P is proposed with high-purity quartz sand and silicon cutting powder as raw materials. After being purified, the purity of silica can reach more than99.99%. The silica and the recycled waste generated during wafer cutting are mixed according to a certain proportion, put into high-frequency induction furnace designed by ourselves, and then silicon with low B/P is produced through carbon thermal reduction reaction. The polysilicon produced can meet the requirement of solar-grade polysilicon just after a simple directional solidification. In this paper, two conclusions about two aspects are drawn from theoretical and experimental research.First one is about the purification of silica as raw material, as follows:(1) The distribution and physical characteristics of the metal impurities in silica are studied. The metal impurities Fe and Ti exist in silica mainly in form of oxide, while metal impurities Al, K, Na and Ca exist in silica tetrahedra in the form of solid solution, forming aluminosilicates. By XRD analysis, the aluminosilicates existed in silica mainly include: feldspar, illite and kaolinite. (2) Through the study of structure evolution of aluminosilicates under acid condition and corrosion-resistance degree against different acids, it is known that feldspathic aluminosilicates turn into illites or kaolinites under acid condition. The stability of anorthite is worst under acid condition, while the stability of the K-feldspar is best. The corrosion resistance of aluminosilicate against HF is worst, but when reacting with Ca, the new precipitate CaF2will be generated. So HF+HC1mixed acid system as a purification method is established with HF as major component.(3) The aluminosilicate in silica is of diversity, leading to that high-temperature activation cannot modify all aluminosilicates in silica under a certain temperature. For this reason, firstly pickling is proposed for the structure evolution of the silica impurities in the experiment, and then high-purity activation is applied as purification method. Compared with the process that high-temperature activation first and then pickling method, Al and K contents in silica are reduced by76.6%and66.5%respectively by this method. The purity of SiO2in silica reaches99.995%.Second one is about the reaction of silicon cutting powder and high-purity quartz, as follows:(1) Through thermodynamic data calculation and the calculation by HSC thermodynamic software, the thermodynamic feasibility of producing silicon metal with low B/P by silicon cutting powder and high-purity quartz is studied. By thermodynamic calculation, it can be seen that the theoretical temperature Si generation by the reaction of SiC and SiO2is between1900℃and2000℃. And during the whole process, the reaction of SiO gas and SiC requires the highest temperature.(2) By thermogravimetric analysis, the chemical reaction in initial stage of the reaction of SiC and SiO2belongs to restriction aspects. As the reaction proceeds and the temperature increases, the diffusion of SiO to SiC surface through product layer Si becomes restriction aspect.(3) The efficiencies of restriction aspects in reaction are improved by the research of different reacting temperatures and the application of powder forming, and meanwhile SiO loss is reduced.