Abstract: A reverse hydrocyclone cleaner for removing light contaminants from pulp slurry has a housing defining a hydrocyclone separating chamber. Pulp slurry is fed into the separating chamber adjacent an upper end thereof to form an outer helically and downwardly moving slurry stream relatively free from light contaminants, an inner pulp stream containing a substantial amount of the light contaminants and an air core within the inner stream. An overflow orifice is located adjacent an upper end of the separating chamber, and an underflow orifice is located adjacent a lower end of the separating chamber to remove the outer pulp stream relatively free from light contaminants. A centrally located blocking finger is located in the underflow orifice.