Separation of fibre pulp suspensions containing relatively heavy contaminants

A hydrocyclone unit for separating a fibre pulp suspension containing relatively heavy contaminants, comprising:

• a housing forming an elongate tapering separation chamber having a base end and an apex end,
• at least one suspension inlet member on the housing designed to feed the suspension to be separated tangentially into the separation chamber at the base end thereof, such that the incoming suspension forms a vortex, in which the heavy contaminants are pulled by centrifugal forces radially outwardly and the fibres are pushed by drag forces radially inwardly, whereby a central fraction of the suspension substantially containing fibres is created centrally in the vortex and a reject fraction containing heavy contaminants and some fibres is created radially outwardly in the separation chamber,
• one and only one reject fraction outlet for discharging the reject fraction from the separation chamber, the reject fraction outlet being axially situated at the apex end of the separation chamber,
• a central accept fraction outlet at the base end of the separation chamber for discharging the central fraction, and
• at least one fluid injection member for injecting a new fluid into the separation chamber,
• wherein the fluid injection member injects the new fluid tangentially into the separation chamber at a distance from the apex end of the separation chamber which is at least 40% of the length of the separation chamber, such that the injected new fluid increases the rotational speed of a portion of the vortex in the separation chamber to increase the separation efficiency with respect to fibres existing in said vortex portion.