An apparatus and a process for crystallizing substances from solutions or dispersions containing these substances, in a crystallizer containing a classifying zone ( 3 ), comprise a) an inner and an outer circulation system ( 1; 2 ), the inner circulation system ( 1 ) being present in the crystallizer, the inlet of the outer circulation system ( 2 ) being connected to the inner circulation system ( 1 ) via the classifying zone ( 3 ), the outer circulation system, being present outside the crystallizer, the outlet of the outer circulation system ( 2 ) being connected to the inner circulation system ( 1 ) of the crystallizer, and a means for dissolving crystals being arranged in the outer circulation system ( 2 ), before its outlet, b) an inflow ( 4 ) for solution and/or dispersion, which inflow is present on the crystallizer or on the outer circulation system, and c) an outflow ( 5 ) for dispersions, which outflow is arranged on the crystallizer or on the outer circulation system. The novel apparatus has the special feature that a line ( 8 ) connecting the outer and inner circulation systems ( 1; 2 ) to one another and intended for transporting (recycling) dispersion and/or a line ( 8 ) for transporting (recycling) dispersion are additionally present, in which both its entrance and its exit are connected to the inner circulation system ( 1 ).