Effect of Cooling Rate on Phosphorus Removal During Al-Si Solvent Refining
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y market has grown rapidly in recent years. Along with that comes the strong demand for solar grade silicon and in turn economically efficient processes for silicon purification. To purify silicon by a metallurgical method, phosphorus removal is difficult due to its rather large segregation coefficient (valued at 0.35[1]) during silicon solidification, so regular directional solidification is not effective in its removal. Nowadays, electron beam treatment of silicon melt[2–4] is found to be useful for the removal of phosphorus. However, that is not an ideal process because of its high energy consumption. On the other hand, solvent refining with Al-Si alloy is found to be a promising process. It consists of three steps, first, alloying silicon with aluminum to form a hyper-eutectic Al-Si alloy, then, solidifying the alloy to grow primary silicon phases, which are considered as purified silicon, and finally, separating and collecting these primary silicon phases. Many works[5–11] have been conducted based on this method, both in experiments or theories. Yoshikawa et al.[5] studied the thermodynamics of solidification of the Al-Si alloys and discovered that the
YANLEI LI and BOYUAN BAN, Graduate Students, JINGWEI LI and TAOTAO ZHANG, Engineers, XIAOLONG BAI, Graduate Student, and JIAN CHEN, Professor, are with the Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China. Contact e-mail: [email protected] SONGYUAN DAI, Professor, formerly with the Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, is now with the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, P.R. China. Manuscript submitted June 16, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
segregation coefficient of phosphorus decreases with decrease in solidification temperature. Yet the kinetics of phosphorus removal during solidification of the Al-Si alloys has never been studied. That is the subject of our work to be presented here. Phosphorus removal in the Al-Si alloys with a series of cooling rates is investigated. A concept of apparent segregation coefficient of phosphorus in the Al-Si alloys solidification is then proposed and calculated. The raw materials used in this work are metallurgical grade silicon and industrial grade aluminum. Their impurities contents determined by inductively coupled plasma optical emission spectrometry (ICP-OES) are shown in Table I. About 250 g Al-Si alloy (20 pct wt Si-Al alloy, 30 pct wt Si-Al alloy, and 40 pct wt Si-Al alloy respectively) was put in an alumina crucible, which has a height of 100 mm and inner diameter of 55 mm, and the crucible was put in a resistance heating furnace. The furnace temperature was first raised to and kept at 1323 K (1050 °C) for 3 hours during which the melt was stirred with a quartz rod to mix it completely. Then the temperature of the furnace was lowered to 10 K (10 °C) above the liquidus te
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