PURPOSE: To condense Mg vapor contg. impurities in a molten Al-Mg alloy and separation recover the Mg from the impurities by contacting the vapor to the molten alloy.

CONSTITUTION: A molten Al-Mg alloy is held in furnace 4 heated with burner 5. The tips of rising reflux pipe 2a and falling reflux pipe 2b connected through reflux condenser 2 are dipped in the molten alloy. Condenser 2 is evacuated with an exhaust pipe to draw molten Al-Mg alloy 6 into pipes 2a, 2b and condenser 2. An inert gas such as Ar is introduced into the lower part of pipe 2a from pipe 3 to circulate alloy 6 through pipes 2a, 2b, condenser 2 and furnace 4. On the other hand, MgO-contg. raw material is reduced with Fe-Si, etc. in hermetically sealed reduction furnace 1, and Mg vapor contg. impurities such as raw material powder is condensed and absorbed in alloy 6 in condenser 2. Since the impurities rise to the surface of alloy 6 in furnace 4, they are separated from the alloy.