SILICON PRODUCTION, REFINING & RECYCLING FOR PHOTOVOLTAIC CELLS

Refining of metallurgical grade silicon is one of the most challenging scientific topics for metallurgists and material scientists. Slag refining is one of the promising refining methods which could remove impurities like phosphorus and boron from the silicon matrix. Published data are available for high purity silicon only whereas slag refining of metallurgical grade silicon is feebly studied. In the present work metallurgical grade silicon has been treated with the slag of Na2O-CaO-SiO2 system. After cooling down the microstructure of silicon has been studied in comparison with the untreated silicon. The composition of the slag has been varied with concentrations of Na2O from 10 to 30 wt% at a fixed ratio CaO/SiO2=1.21 to ensure a high basicity of the slag.Metallurgical grade silicon contains about 2 wt% of impurities, with major impurity elements Fe, Al, Ti and Ca. They form many small inclusions, which are randomly distributed throughout the matrix. B and P are dissolved in the silicon matrix, which is the main difficulty for their refining. The main impurity phase in inclusions is FeSi2. After slag treatment no more small inclusions have been observed and impurities have been found enriched in clear grain boundaries, whereas along with FeSi2 a large amount of CaSi2 has been detected exceeding its initial content. Thermodynamic analysis of possible reactions shows the probability for CaSi2 formation only at high temperatures in an argon atmosphere from CaO and silicon. This conclusion should be taken into account for study of the slag refining process of metallurgical grade silicon. Additionally, in the authors’ previous work on alloying, the impurity phase FeSi2Ti has been found capable to absorb up to 0.4 wt% of phosphorus, and this observation has been confirmed in slag treatment experiments. This fact indicates a concurrence between possible processes: Oxidation of phosphorus by a slag and dissolution of phosphorus in the Fe-Si-Ti intermetallic phases. It could point to a different mechanism of phosphorus refining in comparison to boron refining.