1 into an end 2 of which is fed a stream of particulate material in suspension in a gaseous fluid the duct is divided into two equal legs so that two streams move past a solenoid magnet 3 disposed centrally within the duct passing its vertically disposed faces 4 and 5 respectively alternatively as shown in fig4 the magnet may be disposed between two ducts so as to act on materials fed through both ducts the magnetic material being deflected axially and radially inwardly and passing into at least one collector channel through openings in the walls of the ducts the magnet is enclosed in a smoothly contoured fairing 13 to reduce turbulence the shape of the two legs of the duct at the sides of the fairing being such as to direct the flows towards receiver ducts 6 and 7 respectively the magnetic forces will act across the flows towards the faces 4 and 5 and also towards the central axis of the solenoid magnet hence the relatively more magnetic material in the stream will be deflected inwards and travel towards the openings 8 and 9 respectively leading to an outlet duct 10 it is a particular characteristic of this invention that use is made of the combined directional effects of the external stray magentic field of a solenoid magnet the circular solenoid is designed to generate field gradients and hence directional magnetic forces which increase axially towards the faces of the solenoid as well as radially towards its axis in consequence magnetic particles approaching the solenoid from 2.