A device for hydraulically classifying solid fragments includes two side-by-side liquid-holding compartments between which liquid is continuously circulated; the liquid flows through a flow tube connecting the lower region of the first compartment with the second compartment, then flows upwardly through a throat zone in the first compartment and then over a weir back to the second compartment. The flow tube has a convergent mouth in the second compartment and a divergent outlet in the first compartment. A pump is arranged to withdraw a fraction of the circulating liquid from the second compartment and to inject that liquid into an eductor arranged at the mouth of the flow tube in the second compartment. Solids classification is accomplished at the throat zone: heavier solids sink through the throat zone, and lighter solids are carried upward in the rising current over the weir and onto a drainage screen.