The force acting on the magnetic impurities in alternative magnetic field was analyzed

and the magnetic forces formed in alternative and steady magnetic field were compared. The results show that two kinds of magnetic forces can be united

and the moving rules of magnetism particles can be described with the same theory also. Both electromagnetic force and magnetic force can drive the paramagnetic iron-rich phase impurities in Al-Si alloy move. Electromagnetic separation experiment shows that the electromagnetic force is dominant in driving impurities particles to move to separator inner side with the magnetic induction intensity more than 145 mT while magnetic force is dominant in driving the impurities to move to the outer side of separator with the magnetic induction intensity less than 120 mT

which can guide the structure design of separator.