Boron separation from metallurgical grade silicon (MG-Si) is still a puzzle for metallurgical route. A purification method to MG-Si using a synthetic CaO-CaCl2 slag (no SiO2 addition) treatment combined with an Ar-O-2-H2O gas blowing technique was studied. The mechanism of boron separation from silicon using the synthetic CaO-CaCl2 slag treatment and the Ar-O-2-H2O gas blowing was analyzed and elaborated. The results showed that the intermediate product SiO2, generated by the O-2 gas blowing, was combined into silicate with CaO slag reagent. Boron in silicon can be separated by both gaseous borides HBO, BOCl and borate CaO center dot B2O3. It was found that the boron separation can be further improved by separating soluble boron in the refined silicon using the acid leaching treatment. The total separation efficiency of boron in MG-Si reached 94.5% using the combined slag treatment and gas blowing refining technique followed an acid leaching treatment to the refined silicon. (C) 2017 Elsevier B.V. All rights reserved.