Abstract: In a hydrocyclone, particularly for deinking of paper pulp, a movable generally cylindrical plug is utilized to define an annular space near the accepts withdrawal conduit to effect release of obstructions should the annular space become clogged. The plug is biased by a spring, and moved against the spring pressure to enlarge the annular space automatically in response to a build up of back pressure. The plug also may be moved manually from exteriorly of the hydrocyclone housing, or may be operated by fluidic elements on a periodic basis to clean the annular space whether or not there has been a build up of back pressure. The hydrocyclone preferably is a gas sparged hydrocyclone, and recycled paper--with print on it--having a consistency of at least about 2% is treated to effect deinking.