Method for desulfurizing exhaust gas by alkali sulphite-gypsum process

Claim 1

In a process for desulfurizing exhaust gas containing oxides of sulfur, including sulfur dioxide, and oxygen, which comprises (a) contact-reacting said exhaust gas with a solution containing an alkali metal sulfite to absorb a major portion of the sulfur dioxide and thus form an absorption liquid containing the alkali metal sulfite, the corresponding alkali metal bisulfite and the corresponding alkali metal sulfate, (b) adding lime or lime stone to the absorption liquid to cause a double decomposition reaction between the lime or lime stone and the alkali metal bisulfite contained in the absorption liquid and thus produce the corresponding alkali metal sulfite and calcium sulfite, (c) separating the calcium sulfite from the resultant mixture and recycling the filtrate obtained therefrom to step (a), and (d) converting the separated calcium sulfite to produce gypsum, the improvement which comprises suspending a portion of the separated calcium sulfite in a portion of the absorption liquid formed in step (a), while adding sulfuric acid to the resultant suspension to lower the pH value thereof to from 2 to 4 to (1) convert the alkali metal sulfate contained in said portion of the absorption liquid to the corresponding alkali metal bisulfite and (2) convert the suspended calcium sulfite to gypsum, separating the gypsum from the resultant mixture and recycling the filtrate obtained therefrom to step (b), the remaining portion of the separated calcium sulfite being subjected to oxidation.