Removal of Impurities from Metallurgical Grade Silicon by Liquation Refining Method

Abstract

To develop a new method of refining silicon for use in solar cell materials, liquation refining with Al-Si alloying was proposed for purification of metallurgical grade silicon (MG-Si). The morphologies of impurities, especially for phosphorus and iron, before and after purification were analyzed. By alloying MG-Si with Al, phosphorus in MG-Si formed Al-Si-Ca-P impurity phase and iron formed skeletal-shaped α-Al8SiFe2 phase into the Al-Si eutectic phase from MG-Si with the presence of Al. It can be concluded that removal mechanism of phosphorus and iron was not only a segregating process, but also a recombining process within certain elements to form specified impurity phases. By centrifugal separation, average separation ratio was 75.73% and average recovery ratio was 82.52% for Al-45%Si system, which is better than traditional separation methods and avoids considerable loss of Al and acid. After refining for four times, mass fractions of phosphorus and boron were reduced to 0.46 × 10−6 and 0.21 × 10−6 respectively. For transition metal impurities, Fe, Ti and Mn, removal fractions of those all were more than 95%.

Keywords

• metallurgical grade silicon
• liquation refining
• al-si alloy
• phosphorus
• iron