Abstract: A hydrocyclone (10) is disclosed in which the inlet section (14) of the chamber (13) has a curved inner side wall surface (29) which is generally in the shape of a volute (28), for directing material received in use from the feed inlet port (17) in a rotational motion. In the embodiment shown, the volute (28) is ramped axially downward within the inlet section (14), in a direction towards the conical separating section (15), and turns through an angle of more than 270 angle degrees. The conical section has a central axis X-X, and comprises two segments 32, 34 each being of a frustoconical shape, and joined together end to end to form a generally conical separating chamber (15). An internal angle A located between an inner wall surface (50) of the so-formed conical separating chamber (15) and a line parallel to the central axis X-X is ideally less than (8) angle degrees, to provide a hydrocyclone design with beneficial operating parameters.