Refining of Silicon by Solidification of Al-Si Melt

Abstract

Primary silicon crystals grown from an Al-Si melt has been investigated by solidifying directionally and under electromagnetic field. The goal of this thesis was to increase the size of the primary Si crystals and to agglomerate the crystals to one part of the melt. If achieved, this could simplify the following acid leaching process that is necessary to collect the crystals from the melt. Seven experiments were conducted in a resistance furnace with directional solidification to investigate the agglomeration and size of the Si crystals. A mono-crystalline Si seed was added to the Al-Si melt in three of these experiments as an attempt to increase the crystal growth. The impact of stirring in the melt before solidification was investigated. Al-Si melt was directionally solidified without the aid of seed in three experiments. The silicon content in the alloy and pulling rate during solidification was varied in these experiments to find the appropriate Si crystal growth conditions. The growth of silicon crystals from a mono-crystalline Si seed without aluminium was performed to investigate the impact aluminium had on the seeded growth. One experiment was conducted in an induction furnace to investigate the influence electromagnetic force has on the agglomeration of primary Si crystals. Agglomeration of Si primary crystals was found not to be successful for either directional solidification or electromagnetic force method, as the crystals were not gathered to one part of the melt. The size of the primary Si crystals was not as large as expected and addition of mono-crystalline Si seed did not improve the crystal size. A single Si crystal was successfully grown from a mono-crystalline Si seed when there was no aluminium in the melt.