High-Frequency Electromagnetic Purification of Silicon
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lobal developments in the energy sector with a drive for the production of clean, reliable, and affordable energy have stirred a rapid growth in the production of photovoltaic (PV) cells.[1] This has led to growth in silicon-based PV cells which control over 90 pct of the PV market share[2] either as single-crystalline silicon, or multicrystalline silicon (MC-Si), or amorphous silicon.[3] MC-Si produced by directional solidification of solar grade silicon melt dominates the silicon PV market. During the directional solidification process, SiC and Si3N4 particles within the molten silicon are pushed by the solidification front to the top of the ingot which is the last to solidify. Therefore, the top of the ingot is cut generating what is known as top-cut solar silicon scrap. The Si3N4 particles are mainly from the coating material on the inside surface of the silica crucible and from the air absorption during the solidification process. The SiC particles mainly stem from the carbon reduction process during the primary production of silicon.[4] For sustainability, the silicon in the top-cut silicon scrap must be recovered. Therefore, methods for removing nonmetallic particles, such as SiC and Si3N4, from the silicon are important in order to recycle top-cut silicon scrap to LUCAS NANA WIREDU DAMOAH, formerly Ph.D. Student with the Department of Materials Science & Engineering, Missouri University of Science and Technology (Missouri S&T), 223 McNutt Hall, Rolla, MO 65409-0340, is now Lecturer with the Department of Materials Science and Engineering, University of Ghana, Legon, Accra, Ghana. LIFENG ZHANG, Dean, is with the School of Metallurgical and Ecological Engineering, University of Science & Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted June 30, 2015. Article published online September 21, 2015. 2514—VOLUME 46B, DECEMBER 2015
become a useful feedstock for silicon solar cells manufacturing. Methods such as sedimentation, filtration, fractional melting, plasma refining, and electromagnetic (EM) separation hold the potential to purify SoG-Si. Although the density of SiC and Si3N4 particles are higher than the silicon melt, the sedimentation process poses an industrial implementation challenge since the natural sedimentation requires long periods of time.[5,6] Fishman[5,6] reported that during the sedimentation process, natural convection generates a recirculating flow pattern transporting particles to the top and directional solidification pushes the particles into the solidifying liquid at the top. However, these complications might be reduced by performing the sedimentation process in a different furnace than directional solidification. Furthermore, it is also well known that the settling velocity of particles is enhanced in convective melts, except for excessive flow velocities that may result in back diffusion of already settled particles.[7] Any method that could enhance the particle sedimentation velocity will be advantageous. Filtration using ceramic foam filters t
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