The article discusses how the efficiency of the ball mill grinding circuit has been greatly enhanced with the advent of the O-Sepa cage-type separator. Unique design features allow the O-Sepa to provide exceptional performance by delivering high separator efficiency and broad system flexibility. Because the air and material forces are balanced throughout the classification zone, precise separation takes place which results in average capacity increases of 30% and reductions of 20 to 30% in specific power consumption when producing ordinary cement. Also discussed are conventional classifiers, high efficiency classifiers, separator design, and other aspects of the subject.