The Mechanism of P Removal by Solvent Refining in Al-Si-P System
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THE exploitation of green and renewable energy is more and more valued due to the exhaustion of traditional energy resources and the increasing requirement of environment protection.[1,2] As a result, solar cell production (especially poly-Si solar cell) increased rapidly in the past decade, and so does the immense demand for solar-grade Si (SoG-Si).[3] Currently, the production process of SoG-Si, via the traditional Siemens process[4] or its modified alternatives, is fairly energy intensive and environment-unfriendly.[5] Aiming at reducing energy consumption and increasing productivity, new green metallurgical refining processes of SoG-Si using metallurgical grade Si (MG-Si) as a starting material have been developed. These include directional solidification,[6] plasma treatment,[7] vaporization refining,[8] slag treatment,[9] and solvent refining.[10] As is known to all, P is difficult to remove from Si and should be reduced to less than 1 ppmw in SoG-Si. Because the segregation coefficient of P in Si, 0.35,[11] is considerably larger than those of most metallic impurities in Si, the ordinary directional solidification refining is not effective for the removal of P. On the other BOYUAN BAN, XIAOLONG BAI, and YANLEI LI, Graduate Students, JINGWEI LI, Engineer, and JIAN CHEN, Professor, are with the Key Laboratory of Novel Thin Film Solar Cells, Institute of Applied Technology, Chinese Academy of Sciences, Hefei 230088, China Contact e-mail: [email protected] SONGYUAN DAI, Professor, is with the Key Laboratory of Novel Thin Film Solar Cells, Institute of Applied Technology, Chinese Academy of Sciences, and also with State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China. Manuscript submitted May 4, 2015. Article published online September 8, 2015. 2430—VOLUME 46B, DECEMBER 2015
hand, P has high vapor pressure in molten Si[8] and can be removed from the molten Si by using vaporization refining. However, the vaporization refining process still requires a high temperature and long operation time. The high cost of the equipment and large energy consumption limit this technique from industrial application. Solvent refining with Al-Si alloy is a promising process to produce SoG-Si at large scale with low cost.[10,12–14] The purification can be achieved by redistributing the impurities in the solid/liquid interface. Compared with the directional solidification of Si [1700 K to 1800 K (1427 °C to 1527 °C)], solvent refining [1000 K to 1500 K (727 °C to 1227 °C)] is carried out at much lower temperature, and the removal of P is more efficient. This process is conducted by (1) alloying MG-Si with Al to obtain hyper-eutectic Al-Si melt, (2) cooling and solidifying the Al-Si melt to grow primary Si phases, and obtaining solidified ingots, and (3) acid leaching the solidified ingots to collect the primary Si phases. Our recent research proved that kinetics and interaction with metal elements are two important factors controlling the P removal efficienc
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