Two stage continuous process and apparatus for crystallization

In an apparatus for producing crystals from a stream of solution comprising a heat exchange vessel for producing crystal nuclei whereby said solution is caused to become supersaturated with respect to said crystallizable component and to form said nuclei;

• a crystallizer vessel for recrystallizing whereby crystal growth is promoted and a separator vessel wherein a first conduit means connects said crystallizer vessel to said heat exchange vessel and a second conduit means connects said crystallizer vesel to said separator vessel so that said solution passes continuously from said heat exchange vessel to said crystallizer vessel wherein small crystals are formed in said heat exchange vessel and larger crystals are formed in said stream of solution in said crystallizer vessel and wherein crystals are separated from said solution in said separator vessel, the improvement comprising wherein the crystallizer is equipped with a means for rapidly mixing the stream of solution to produce a substantially homogenuous mixture therein and promote growth of said larger crystals whereby substantially all of said small crystal nuclei are dissolved thereby promoting growth of said larger crystals, and the apparatus includes a means for removing crystal-free liquor from said crystallizer vessel and means to recirculate crystal-free solution from said crystallizer vessel to said heat exchange vessel while retaining the crystals dispersed in said crystallizer vessel and a third conduit connected between said heat exchange vessel and said crystallizer vessel for carrying the crystal-free solution.