| Among renewable energy, solar power has many advantages, such as safe, reliable, clean, widely distributed, no mechanical moving parts and so on, it has been attracted the world's attention. At the same time, photovoltaic power generation is the more mature technology in renewable energy, Many countries in the world, especially the United States, Japan, Germany and other developed countries have launched a large scale photovoltaic development plan to stimulate the rapid development of photovoltaic industry. The rapid growth of photovoltaic industry gave birth to a large demand for silicon. Currently, the major faced obstacles are the high cost and the shortage of silicon in the photovoltaic industry application. Reduce the cost of silicon is the key to reduce the cost of silicon solar cells without affecting the efficiency of solar cells.Currently, raw materials used for the production of SoG-Si are mainly from the scraps of microelectronics industry. Taking into account the size, purity and the cost in production process of silicon used in microelectronics industry and photovoltaic field are different, meantime, in order to meet the urgent demand of PV industry for silicon and get rid of the dependence of SoG-Si on electronic grade silicon (EG-Si). Therefore, many countries begin to carry out the low cost production technology research for solar grade silicon (SoG-Si).A new metallurgical purification route is proposed and the specific purification process route is as follows:①Acid pretreatment of metallurgical grade silicon (MG-Si),②Electromagnetic induction slag refining,③Electron beam refining and④Directional solidification based on the analysis for the presence forms and distribution of different impurities in MG-Si. The results show that the above four refining process have better removal effect for different types of impurities in silicon. SoG-Si ingot with the final purity of 5N-6N has been prepared successfully. The results of each refining process are as follows:Hydrochloric acid and hydrofluoric acid were used in this paper. Contrast test results showed that acid effect under hydrochloric acid was better than that of hydrofluoric acid. Not only pickling process would produce insoluble material under hydrofluoric acid but also hydrofluoric acid was easy to volatile and exploded. So hydrochloric acid was selected as the final pickling solution. Optimize process parameters under hydrochloric acid were as follows: solid-liquid ratio 1:20,15wt%,80,10h, 100um.Under above conditions, the main impurities removal efficiency of aluminum, iron and calcium in silicon reached 75.6wt%,79.3wt% and 61.7wt%, respectively. The purity of silicon increased from 99% to 99.9% after acid pickling. Ultrasound vibration was used during pickling process and acoustic streaming and acoustic cavitation could remove impurities which were not fully exposed at the slit of grain boundary, so it could increase the effect of acid pickling. But there was no removal effect for B, P and other non-metallic impurities in MG-Si with pickling process and pickling could not remove all metal impurities in silicon because pickling limit. Although pickling only just as a pretreatment process, it was a very important and indispensable link.The effect of different refining temperature, time and slag agent components on the removal rules of boron was mainly studied under SiO2-CaO-Na2O and SiO2-CaO-Al2O3 systems. Meantime, the thermodynamics and kinetics of the removal rules of boron was also discussed. The results showed that impurities removal effect under SiO2-CaO-Al2O3 slag system was better than that of SiO2-CaO-Na2O slag system. The content of boron elements in silicon was successfully reduced from the original 15 ppmw to 2 ppmw under SiO2-CaO-Al2O3 slag system. Refining process parameters were as follows:slag dosage was 10wt%, temperature was 1823K and time was 2h. At the same time, metal impurities Al, Ca and Mg were also well removed and the removal efficiency of them reached 85.0%,50.2%, and 66.7%, respectively. Electromagnetic induction slag refining could improve and accelerate the removal process of boron because the mass transfer coefficient of impurity elements in the boundary layer was strengthened by electromagnetic stirring.In this paper, MG-Si was refined by electron beam melting furnace which designed by us and the thermodynamics and kinetics of impurities removal were also discussed. The results showed that the volatile reactions of metal impurities aluminum and calcium and nonmetallic impurities phosphorus were all first order reaction (that is a single atom of volatile reaction). Therefore, the free volatile process of impurity elements spread from the gas boundary layer to gas phase was the control link of whole volatilization process. The content of main metallic impurities besides iron, aluminum and titanium in silicon all reduced to below 0.1 ppmw, especially the content of phosphorus reduced to 0.16 ppmw when the electron beam power was 20kW and refining time was 30 minutes. The expression of impurity content in silicon as electron beam power and refinement time was derived as follows: The calculated value was consistent with the experimental value. Impurity content in silicon was inversely proportional to the feed speed because the relationship between impurity content and refining time was a negative exponential. Therefore, in order to increase productivity and reduced costs, electron beam power used in this paper was 30kW for continuous refined. The required speed of the feed silicon was about 0.1kg/min when the phosphorus content in export molten silicon was lppmw.Silicon directional solidification process was studied in detail in this paper via utilizing an electron beam directional solidification furnace which designed by ourselves. The results showed that a silicon ingot with very smooth surface quality and no defect was obtained by the use of quartz ceramic crucible coated with silicon nitride coating when the superheat of silicon melt was 130K and the directional solidification rate was 3.0×10-5m/s. Ingot with the diameter of 70 mm and the length of 210mm and the grain size of columnar crystal at the central of silicon ingot with the width of 3～4mm and the length of 10～30mm was obtained. Critical purification height of silicon ingot was about 140mm which was approximately 66.7% of the total ingot height. The contents of aluminum, iron and titanium and other metal impurities in ingot were controlled below 0.1ppmw, the content of boron and phosphorus was below 0.6ppmw and 0.1ppmw, the content of carbon, oxygen and nitrogen were basic controlled below 10ppmw. The purity of silicon ingot had reached more than 5N and various impurities content in silicon ingot had reached the required impurity range of solar grade silicon. |
PDF Full Text Request