| Currently, fossil fuels in the world are gradually exhausted, solar energy has been rapidly developed and become one of the most important renewable energy because of its safety, wide distribution and cleanness. As a foundation material for solar cells, silicon is necessary. However, the traditional production technology of solar grade silicon, improved-Siemens method, is high cost and monopolized by foreign country. The metallurgy methods of preparing solar grade silicon have characteristics of low energy consumption, low cost and environmental friendly. The research of preparation of solar grade silicon with metallurgy method will have the important significance for China breaking the international monopoly, possessing our intellectual property rights and developing China’s PV industry.In this thesis, we investigated the technology to produce high quality silicon using electrothermal metallurgy process, with purified raw materials. This technology will lay a foundation for the preparation of solar grade poly-silicon in next stage. As the impurities in the raw petroleum coke and silica can be brought into the product during the smelting process, the experiment was carried out firstly to purify this two kinds of raw materials separately for controlling the content of metal, P and B. The results are listed as follows.(1) The remove of impurity of petrol coke powder with ultrasonic acid leaching. The optimized conditions is:the particle size of petroleum coke was under 75 μm, the leaching time was 0.5 h, the liquid-solid ratio was 10:1, concentration of hydrochloric acid was 12 wt%, the leaching temperature was 50 ℃, the ultrasonic power was 270 W and the ultrasonic frequency was 45 kHz, under which condition the leach ratio of Fe was up to 99.15% and the leach ratio of P was up to 94.42%. The effect of removing impurities in the process of ultrasonic acid leaching was obvious.(2) The remove of impurity of silica with acid leaching. The optimized conditions of ultrasonic acid leaching and conventional acid leaching in stirring were obtained through the single factor experiments. The result showed that:ultrasonic acid leaching has an obvious advantage in both purification effect and purification rate. Meanwhile, The optimum conditions of ultrasonic acid leaching are:the particle of silica size was under 75 μm, the concentration of hydrochloric acid was 12 wt%, reaction temperature was 40 ℃, ultrasonic power was 300 W and ultrasonic frequency was 45 kHz. The best effect can be achieved after 20 min of leaching. With this optimum conditions, up to 99.15% Fe in silica can be removed.(3) The exploration of preparing high quality using purified raw materials. In submerged arc furnace, the melting experiments of two times were conducted. In the first experiment, the raw materials were silica and petroleum coke without purification treatment. In the second experiment, the raw materials that purified through acid leaching were selected. The results show that the content of product (Si) was up to 99.74%, which is far higher than the purity of the products with 98.02% for the first time. This result exceeds the standard of national class A industrial silicon. The content of metal impurities about Fe, Al, Ca are all lower than the standard of current national class A industrial silicon.The content of P and B is 10 ppm and 6 ppm respectively, both of which are under the content of P and B in the current industrial silicon. This research shows that the process of preparation of high quality silicon through purifying raw materials is entirely feasible. |
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