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An alternating-current (ac) powered high-gradient magnetic separator (HGMS) was compared with a conventional (direct-current powered) HGMS. Over all tested conditions with magnetite/ silica mixtures, the ac HGMS gave significantly higher grades of magnetite in the magnetic product. The probable reason is that the alternating field causes vibration of the bed of particles held on the matrix which dislodges entrapped nonmagnetic particles. This was supported by testing the HGMS with mechanical vibration of the matrix which gave even higher grades than the ac HGMS. A model of grade, based on the probability of void filling, was developed which fitted the grade versus the feed composition data.