Research On Preparation Of Upgraded Metallurgical Grade Silicon With Carbonized Rice Husk By Electrothermal Metallurgy Process

Oil, coal and other traditional energy sources in the world are gradually exhausted, solar energy has been rapidly developed and become one of the most important renewable energy because of its safty, wide distribution and cleaness. Silicon is the most important material in solar cells, and reducing the cost of silicon material becomes the key factor in the development of the PV industry. However, the main production technology of solar grade silicon, improved-Siemens method, is monopolized by United States, Japan, Germany ect. Solar grade silicon used in China mainly relies on imports, since the enterprises in China are fail in having the core technology of the improved-Siemens method. The metallurgy methods of preparing solar grade silicon, which have the characteristics of low energy consumption, low cost and environmental friendly, will have the important significance for China to possess our intellectual property rights and to develop China's PV industry.The upgraded metallurgical grade silicon(Si%>99.60%, Fe?0.2%, Al?0.1%, Ca? 0.01%, P and B much lower than 120ppmw and 20ppmw in the MG-Si) is intermediate product to the preparation of solar grade polysilicon. In this thesis, we investigated the process to produce upgraded metallurgical grade silicon using electrothermal metallurgy process, with CRH as a carbon reduction. This technology will lay a foundation for the preparation of solar grade polysilicon in next stage. Carbonized rice husk(CRH) is an excellent reduction in Si smelting becanus it has a high carbon content(>50%) and a contain anount of SiO2 about 25%). The main contents include:(1) Remove of impurities of CRH; (2) Preparation and of physical properties of pellets; (3) The thermodynamics analysis and verification in electrothermal metallurgy process of preparing silicon with CRH powder as carbon reduction; (4) Preparation of silicon using CRH as reducing agent; (5) Preparation of upgraded metallurgical grade silicon using mixture(CRH and petrol coke) as reducing agent; (6) The removal of phosphorus in metallurgical grade silicon (MG-Si) by Ar-H2O gas mixtures.The main conclusions are as follow:(1) The optimal conditions of acid leaching of the CRH were obtained as follows:the size of the CRH particles below 75?m; hydrochloric acid 5wt%, reaction time 3h, temperature 60?, the ratio of leaching liquid to solid 14:1 and stirring speed 60r/min. At this optimal conditions, up to 96.41% of metallic element in the CRH can be removed and 66.68% of non-metallic element can be removed. At the same time, the effect of ultrasonic field is investigated. As a result, the elimination of metallic and non-metallic impurities reached 99.07% and 71.77% respectively. Removal of Impurities in CRH at high temperature in vacuum was also studied. The optimal conditions to remove impurities in the CRH were obtained as follows:the size of CRH particle is less than 75?m; the temperature holding time is 120min, the temperature is 1100? and the pressure is 70kPa. At the optimal conditions, 91.85% phosphorus (P) in the CRH can be removed, and sulphur (S) removing ratio in the CRH is up to 88.96%. Removing phosphate in the CRH is possible in vacuum. Higher vacuum will greatly decrease, the reaction temperature.The temperature to remove phosphorus is 1100? under the pressure of 70 kPa.(2) For the oxidation reaction between CRH and air, the activation energy and pre-exponential factor is 78.89 kJ·mol-1 and 2083.03min-1, respectively. The calculation showed that the theoretical ratio were silica:CRH:agglomerant=100:85.14:0.56 for using the CRH as reducing agent; the optimal conditions of preparing pellets using the CRH as reducing agent are:the particle size 75?m, pressing pressure 15MPa, dry agglomerant content 3% and moisture content 7wt%; At the optimum conditions, the compression strength is 3.0 MPa and porosity is 38.6%. The theoretical ratio of silica:CRH:petrol coke:agglomerant is 100:54.61:22.34:0.53 when the mixture as reducing agent, the optimum conditions of preparing pellets using mixture as reducing agent are the particle size 75?m, pressing pressure 20MPa, dry agglomerant content 3%, moisture content 6wt%, at the optimum conditions, the compression strength is 5.8 MPa and porosity is 25.2%.(3) The possible reactions between C and SiO2 was analyzed, and the Gibbs free energy each reaction and the temperature was obtained.The lowest temperature of each reaction was determined which was validated by HSC software.In process of reduction of SiO2 with C, there are formation and decomposition of SiO and SiC.When the temperature is higher than 1900?, Si can be found in the product. The SiC, SiO2 and a little amount of SiO were also found in the product.(4) It has been shown that the technology of preparing silicon with CRH is feasible. The high quality silicon with 99.32wt% Si was obtained which reaches the state-level industrial silicon standard (second grade, Si%?99.30%). The mainly impurities, such as Fe, Al, Ca in the produced Si are inferior to the state-level industrial silicon standard (A grade). The P (26ppmw) and B (15ppmw) in the Si are much lower than P (120-200ppmw) and B (20-60ppmw) in the MG-Si.(5) In process of smelting Si in submerged arc furnace with a mixture of the CRH and petrol coke as carbon reduction, the suitable output vottage is 25-30V when the output current is stabilized at 500A. when percentage of additional carbon content is 30% of the theoretic amount, the yield of Si is 30.7% and SiC in the slag is relatively small. The Si in the product reaches 99.68wt%, which is higher than state-level industrial silicon standard (first grade, Si%?99.60%), the mainly impurities such as Fe, Al, Ca in the produced Si are inferior to the state-level industrial silicon standard (A grade), the P (24ppmw) and B (14ppmw) in the produced Si are much lower than P (120-200ppmw) and B (20-60ppmw) in the MG-Si. It has surpassed the standard of metallurgical grade silicon, that is, the high quality silicon (UMG-Si) is obtained.(6) When Ar-H2O gas mixtures was used to refine the MG-Si, the optimal refining conditions are nozzle type of holes at bottom and side, refining time of 3 hours, refining temperature of 1793K, refining gas temperature of 373K, refining gas flow rate of 2 L/min. At this optimal condition, the phosphorus content in the MG-Si is reduced from 94 ppmw to 11ppmw. The phosphorus content in MG-silicon is effectively reduced by gas blowing refining. The water vapor in the refining gas was introduced into the molten silicon reacts with silicon to produce silicon oxide and H2. Part of H2 is dissolved into the molten silicon. The [H] reacts with impurity [P] in the molten silicon to form PH3 gas which can be taken out from the molten silicon with the Ar bubble.In this thesis, the upgraded metallurgical grade silicon has been obtained by electrothermal metallurgy process using mixture of the CRH and petrol coke as reducing agent. This technology will lay a foundation for the preparation of solar grade polysilicon in next stage.