Removal of impurities from metallurgical grade silicon by addition of ZnO to calcium silicate slag

• A dual agitation to melt is provided by both zinc vapor and electromagnetic force.
• Both ZnO and SiO2 in slag can react with boron in silicon.
• Boron was reduced from 12.94 ppmw to 1.52 ppmw after refining.
• Removal efficiency of Al and Fe reaches to 99.76% and 97.47%.

Slag refining is one of the most effective methods to remove impurities from metallurgical grade silicon. In this paper, ZnO was added to a binary 50% CaO-50% SiO2 slag system with a mass ratio of 2–30% to study the impurities removal from metallurgical grade silicon. The results showed that boron was reduced from 12.94 ppmw to 2.18 ppmw by a 46% CaO-46% SiO2-8% ZnO slag system. Compared to the 50% CaO-50% SiO2 slag system, slag refining with ZnO addition showed a 20% increase in removal efficiency and a 40% increase in the distribution coefficient. After acid leaching, boron was further reduced to 1.52 ppmw with a removal efficiency of 88.25%. By a combined process of slag refining and acid leaching, impurities Al and Fe were reduced from 817 ppmw and 3800 ppmw to 1.2 ppmw and 96 ppmw with removal efficiencies of 99.8% and 97.5%, respectively. The evaporation loss of the generated metallic zinc during slag refining was greater than 99.7%. After acid leaching, the minimum zinc content in silicon was only 4 ppmw.

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