The present invention comprises a device that uses controlled magnetic fields to screen, concentrate and/or separate ferromagnetic or magnetic particles from other particles with which they are mixed, where the separation can be carried out as a result of a difference in ferromagnetic grade, or depending on variations in the properties of the controlled field. Ferromagnetic particles with different masses and sizes can be separated by controlling the intensity and frequency of the applied field. The device is characterised by a combination of frequencies and intensities of rotary magnetic fields generated by a triphase power supply system by means of windings that are independent for each electric phase and are mounted in a core (1) formed by suitably laminated and cut ferromagnetic plates. The core (1) comprises 12 poles and a chamfered pivot (2), and operates according to the triphase magnetic field principle, with phases applied in a differentiated manner by a different winding (4, 5 and 6) for each phase. The core is mounted inside a tube (11) made of a non-magnetic material (12) with a semi-cylindrical cover (12), the cores are arranged side by side (14) in holders, and the device comprises a feed slope (15), besides supports (16) for fitting the hexagonal pivots of the cores (17).