A crystallizer comprises a crystallizer body 1 having a generally cylindrical upper part and a lower part conjoined with the other part and of a generally conical shape, apex lowermost. A mixing zone 4 is provided within the upper part of the body 1, to which zone are supplied an aqueous separating solution and an organic solvent containing extracted ions of a metal. The tank defining the mixing zone 4 has a pipe 6 descending therefrom, down to the lower part of the body 1 and beneath the pipe 6 is provided a fluidizing liquid inlet 8. Separating solution may be removed from the body through a ring pipe 17 and discharge pipe 16 by means of a pump 9, at least a portion of the removed solution being recirculated through the fluidizing inlet 8. The temperature of such recirculated solution may be adjusted as necessary. Crystals are discharged through pipe 12, whereas organic solvent may be removed from settling zone 5, through pipe 11. In operation, the extracted ions in the solvent are transferred to the separating solution to form a metal salt there, which salt is barely soluble in the solution and consequently crystal nuclei start to form in the descending pipe 6, which crystals are developed in zone 7 of the body 1, for subsequent removal through the pipe 12.