Multistage process for removing sulfur dioxide from stack gases

In a process for removing sulfur dioxide from stack gases which includes the steps of contacting stack gases with an aqueous wash liquid comprising a solution of one or more alkali metal compounds thereby to form a spent absorbent liquid containing the bisulfite, sulfite and sulfate salts of said alkali metal and then subsequently recovering at least a portion of the alkali metal ion values by reacting said bisulfite, sulfite and sulfate salts in said spent absorbent liquid with one or more calcium compounds to precipitate calcium sulfite and calcium sulfate, the improvement comprising (1) carrying out the step of reacting said bisulfite, sulfite and sulfate salts with said calcium compounds in multiple stages, said spent absorbent liquid introduced into the first stage having a total oxidizable sulfur concentration of at least about 0.1 molar, a ratio of bisulfite to sulfite ions of at lest one and a sulfate ion concentration of between about 0.1 and 2.8 molar whereby the reaction conditions of said first of said multiple stages are maintained to effect the precipitation of a solids mixture of calcium sulfate and calcium sulfite in said first stage, said solids mixture having greatly enhanced settling and filtration characteristics, (2) limiting the mean residence time of said solids mixture in said first stage to not more than 30 minutes, and (3) transferring at least a portion of said solids mixture of calcium sulfate and calcium sulfite formed in said first stage to one or more succeeding stages to serve as seed crystals for the further precipitation of calcium salts.