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

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.

Separation and Recovery of Refined Si from Al–Si Melt by Modified Czochralski Method

Separation of refined silicon from Al–Si melt is still a puzzle for the solvent refining process, resulting in considerable waste of acid and silicon powder. A novel modified Czochralski method within the Al–Si alloy is proposed. After the modified Czochralski process, a large amount of refined Si particles was enriched around the seed crystalline Si and separated from the Al–Si melt. As for the Al–28%Si with the pulling rate of 0.001 mm/min, the recovery of refined Si in the pulled-up alloy (PUA) sample is 21.5%, an improvement of 22% compared with the theoretical value, which is much larger 1.99 times than that in the remained alloy (RA) sample. The content of impurities in the PUA is much less than that in the RA sample, which indicates that the modified Czochralski method is effective to improve the removal fraction of impurities. The apparent segregation coefficients of boron (B) and phosphorus (P) in the PUA and RA samples were evaluated. These results demonstrate that the modified Czochralski method for the alloy system is an effective way to enrich and separate refined silicon from the Al–Si melt, which provide a potential and clean production of solar grade silicon (SoG-Si) for the future industrial application.