Abstract: The invention relates to a continuously working crystallizer in the shape of an oblong vessel with at its one end a supply and at its other end a discharge for the solution, with a rotatable driven horizontal shaft extending in lengthwise direction of the vessel and provided with discs extending in transverse direction, said discs being provided with a passage opening for the solution, said discs dividing the vessel in a number of sections succeeding in lengthwise direction to each other, each section having one or more cooled wall surfaces and conduits being provided for supplying and discharging a cooling fluid which is flowing along these cooled wall surfaces.