Effect of Si Content on the Morphology Evolution of the Si Primary Dendrites in Al-Si Alloy Solvent Refining Process

Abstract

Solvent refining with Al-Si alloy is a promising purification method for production of solar-grade silicon (SoG-Si) feedstock owing to the advantages of low production cost and high impurity removal efficiency. In this process, larger refined Si primary dendrites are easily collected after acid leaching, which is favorable to recovery, thereby to reduce the production cost. Hence, the growth behavior of the precipitated Si crystal must be investigated systematically. In present work, the morphology evolution of solidified Al-Si alloys with a wide range of Si content (30~70 wt.%) was analyzed. The typical plate-like Si primary dendrites grown following the twin plane re-entrance edge (TPRE) mechanism formed in all alloy compositions. As increasing the Si content from 30 wt.% to 50 wt.%, the Si primary dendrites underwent a coarsening process attributed to the preferred growth along <211> and <111> directions, leading to an increase in the experimental recovery rate. However, the preferred growth along <211> direction was inhibited when the Si content is higher than 55 wt.%. Moreover, the broken effect originating from grain collision and thermal stress on the Si primary dendrites was enhanced as further increasing the Si content, resulting in a decrease in the experimental recovery rate. Therefore, the optimum composition is determined as Al-50~55 wt.% Si for solvent refining solution, based on the cost reduction consideration.