Patent ID: 7968076

Claim:
A process for preparing sodium dithionite, comprising the steps of (a) preparing a sodium dithionite mother liquor from a synthesis batch comprising sodium formate, sulfur dioxide, and an alkaline sodium compound in aqueous methanolic solution, wherein said sodium dithionite mother liquor comprises sodium dithionite; (b) isolating sodium dithionite from the sodium dithionite mother liquor of step (a) to form a residual mother liquor; (c) separating methanol from the residual mother liquor of step (b); (d) lowering the pH of the residual mother liquor of step (c) to a value in the range of from 4 to 6; (e) separating the residual mother liquor of step (d) into a thiosulfate-rich, first part stream and a thiosulfate-poor, sodium formate-containing, second part stream by means of nanofiltration; (f) concentrating the sodium formate in the second part-stream of step (e) to form a concentrated second part-stream; and (g) recycling the concentrated second part-stream of step (f) back to step (a).