Crystallization of hydrosulfite

Claim 1

IN PROCESS OF CRYSTALLIZING ALKALI METAL DITHIONITE DIHYDRATE FROM AN AQUEOUS SOLUTION OBTAINED IN PREPARING ALKALI METAL DITHIONITE AND CONTAINING AN ALCOHOL, THE STEPS OF (A) SEEDING SAID SOLUTION BY ADDING PART OF A THICKENED DIHYDRATE CRYSTAL SLURRY, OBTAINED SUBSEQUENTLY IN THE PROCESS, TO SAID SOLUTION IN A VOLUME LESS THAN THAT OF SAID SOLUTION, (B) EXPOSING THE MIXTURE OF SAID SOLUTION AND SAID SLURRY TO A REDUCED PRESSURE SO AS TO EFFECT COOLING THEREOF BY EVAPORATION OF LIQUID FROM SAID MIXTURE WHILE MAINTAINING THE AMOUNT OF ALCOHOL SUBSTANTIALLY CONSTANT DURING VACUUM COOLING BY CONDENSING THE ALCOHOL AT THE TOP OF THE VACUUM COOLING REGION AS THE ALCOHOL EVAPORATES, AND RETURNING THE ALCOHOL TO THE MIXTURE AND SUSPENSION IN THE VACUUM COOLING REGION, (C) PASSING THE THUS COOLED MIXTURE THROUGH A CRYSTALLIZATION ZONE SO AS TO EFFECT CRYSTALLIZATION OF ALKALI METAL DITHIONITE DIHYDRATE THEREFROM, (D) SEPARATING SAID MIXTURE IN AS THICKENING ZONE INTO A THICKENED CRYSTAL SLURRY AND A SUBSTANTIALLY CLEAR SUPERNATANT SOLUTION, AND (E) RETURNING PART OF THE SLURRY TO THE NEW SOLUTION FOR THE INTIAL SEEDING STEP.