A method separating magnesium and boron from ascharite by vacuum thermal reduction using calcium carbide as redutant was studied experimentally. The reason that led to low reduction ratio of MgO in the vacuum thermal reduction process was researched by phase analysis of calcined ascharite and the influencing factors were analyzed. The results show that magnesium in the calcined ascharite is mainly involved in magnesium borate and magnesium silicate

reduction of which is difficult and only small amount is free state of MgO

which is the main cause of low reduction ratio. The addition of CaO during calcination of ascharite can promote formation of more free state MgO and reduction of more MgO. As long as enough CaO is added

magnesium and boron can be effectively separated and reduction rate can be over 85%. The main phases in reduction slag are CaO and 3CaO·B2O3. The reduction slag can be used as raw material for production of alkali free glass fiber. So the comprehensive utilization of ascharite ore can be realized.