A crystallizer for crystallizing solute contained in a solution has first (2) and second (1) communicating chambers, for the solution, and a heat exchanger (20) for keeping a state of supersaturation therein. A first circuit extracts from a calm zone (15) of the latter chamber a first stream of liquid containing small crystals and recycles this stream into the first chamber. A second circuit extracts from the second chamber a second stream of liquid containing coarse crystals and brings this second stream into the first chamber. The downstream end (26) of the first circuit runs into the second circuit so that the first and second liquid streams are made to flow in the same direction into said second circuit.