Treatment of sodium dithionite reaction mixture

A method is taught for increasing the yield of anhydrous sodium dithionite by adding an organic compound that is thiosulfate reactive to a batch reactor containing a puddle solution of methanol and fed with formic acid or an alkali formate as a first feed, an aqueous alkali compound as a second feed, an aqueous alkali formate solution as a third feed, and a methanolic SO₂ solution as a fourth feed. This organic compound may be added prior to, combined with, or concurrently with one of the four feeds to the reactor. Preferably, it is added concurrently with the third feed and throughout the entire course of the reaction, ending with the beginning of the cooling period. A suitable addition rate is 0.4-0.6 wt.%/minute, preferably 0.5 wt.%/minute. All of the organic compound is consumed, and at least a portion of the thiosulfate ion is destroyed. The organic compound is selected from the group consisting of epoxy compounds having the formula or halogenated hydrocarbons having the general formula R₂X or XR₂X, R₁ being hydrogen, an alkyl group containing from 1 to 8 carbon atoms, a halogenated alkyl group containing from 1 to 2 carbon atoms, a phenyl group, or a substituted phenyl group. The compound represented by this formula includes ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, and styrene oxide. R₂ is a primary or secondary alkyl group containing from 1 to 8 carbon atoms, an allyl group or a 2-methylallyl or 2-ethylallyl group, and X is a halogen atom. Suitable compounds include methyl iodide and allyl chloride.