An apparatus for tribocharging particles of electrically insulating materials has a rotatable drum (1), made of a metallic material, for containing the particles. The drum has an inlet (11), for charging material, and an outlet (13), for discharging material. The drum further has means (7) for rotating the drum about an axis of rotation, the orientation of the axis of rotation being such that, in operation of the apparatus, the particles tumble against each other and against the walls of the vessel, thereby becoming tribocharged. A fines removal mesh screening means (10) is attached to an outer surface of the drum, allowing fines to escape from the drum. An electrostatic separation tower (20), having tapering walls and at least one pair of electrodes (24) arranged on opposite walls of the tower, is aligned to receive discharged material from the drum. The tower separates the particles, falling from the drum into the tower (20), in an electric field generated between the electrodes (24).