The overflow and underflow effluents of a conventional hydrocyclone are discharged into enclosed vessels, so that the operating pressure may be manipulated for each effluent independently of other operating parameters. Each of the effluent vessels is equipped with a source of pressurized gas to form a gas blanket over the effluent at a controllable pressure. Such variable pressures in each vessel, in combination with the pressure of the incoming feed stream, provide the necessary parameters to implement pressure-differential-ratio control, thereby improving the efficiency of separation.