Abstract: A process for producing an anhydrous alkali metal hydrosulfite comprises contacting hydrate crystals of an alkali metal hydrosulfite with an alkaline aqueous solution of a dehydrating agent having a pH of at least about 7. The slurry formed is maintained at or above the transition temperature to covert the hydrate crystals to anhydrous crystals of the alkali metal hydrosulfite. The process permits the conversion of hydrate crystals to produce anhydrous crystals of alkali metal hydrosulfite without substantial dissolving or decomposition of the alkali metal hydrosulfite.

Abstract: Disclosed is a process of producing anhydrous sodium dithionite by contacting sulfur dioxide-containing formamide and sodium amalgam, separating a sodium dithionite-containing formamide solution from the sodium amalgam and precipitating anhydrous sodium dithionite from the product formamide solution by mixing the sodium dithionite-containing formamide solution with isopropanol, n-propanol, gamma-butyrolactone, tetrahydrofuran or N-methyl-2-pyrrolidone. The sulfur dioxide may be contacted countercurrently with the sodium amalgam in a continuous process. A neutralizing agent can optionally be added to the sodium dithionite-containing formamide solution separated from the amalgam to reduce acidic decomposition of the sodium dithionite.

Abstract: A rotating disc reactor for production of sodium hydrosulfite comprising a reactor main housing containing a plurality of elongated reaction chambers with at least partially arcuate interior periphery. Each reaction compartment contains a top gaseous layer, an intermediate buffer solution layer, and a bottom sodium amalgam layer. Positioned in each compartment is a rotatable shaft with a plurality of rotatable discs mounted thereon for reacting and mixing the three layers of reactants. The reactor includes means for distributing amalgam and means for distributing recirculated buffer solution into each reaction compartment. At least one of the side walls of the housing is connected to an end box having inlet and outlet means for circulating the amalgam and the sodium hydrosulfite product. The reactor further includes a sulfur dioxide inlet manifold having an arcuate interior which is offset from the reactor housing.