Process for salt extraction from hydrogen-sulphide scrubber solution

Abstract

This invention relates a method for the selective removal of alkali metal salts of sulphate (SO.sub.4.sup.2-), and thiosulphate (S.sub.2 O.sub.3.sup.2-) from hydrogen sulphide (H.sub.2 S) scrubber solutions of the liquid redox type using an electrodialysis system. In the process of this invention the H.sub.2 S scrubber solution is directed to the diluting compartments within an electrodialysis stack, while a collecting solution with a minimal initial salt content is directed to the concentrating compartments. With the application of a direct current a portion of the alkali metal salts of sulphate and thiosulphate present in the scrubber solution are transported through ion selective membranes into the collecting solution. Essentially all of the metal organic chelate reagents and carbonate ions initially present in the scrubber solution are retained in the desalted solution and can be recycled to the H.sub.2 S scrubbing process. Organic fouling of the anionic membranes is avoided by maintaining the organic weight fraction of the total dissolved solids in the diluting solution less than 0.15 and preferably less than 0.05. High current efficiency is obtained at salt concentrations above 5 normal in the diluting solution by maintaining the salt concentration of the collecting solution below that in the diluting solution.