a hydrocyclone comprising a vertical - axis separating chamber having an upper cylindrical portion and a lower , coaxial conically - tapering portion has : a tangential inlet at its upper end for a suspension to be classified ; an upper , axial outlet for the overflow containing finer particles separated in the hydrocyclone in use ; a lower axial outlet for the underflow containing coarser particles ; and a hollow spigot surrounding the upper outlet and projecting into the separating chamber . the separation of coarse particles from the overflow is improved by the provision of an extension tube extending coaxially from the spigot into the separating chamber .