Method for removing sulfur dioxide from exhaust gas

Claim 1

In a method for removing sulfur dioxide from an exhaust gas wherein the exhaust gas is contacted with an aqueous scrubbing solution containing an organic acid alkali salt in a contact zone to absorb the sulfur dioxide in the scrubbing solution in the form of alkali sulfite and alkali sulfate, wherein at least a portion of the solution is oxidized with oxygen or air to convert the alkali sulfite into alkali sulfate and, thereafter, calcium carbonate or calcium hydroxide is added to the resultant solution to convert the alkali sulfate to calcium sulfate, and the calcium sulfate is separated from the reaction solution and the resultant filtrate, along with any non-oxidized portion of said aqueous solution, is recycled for contact with said exhaust gas, the improvement comprising (a) using as the aqueous scrubbing solution feed to the contact zone, a solution containing at least 0.3 percent by weight of solid crystalline gypsum; (b) controlling the concentration of alkali sulfite in the aqueous solution exiting the contact zone at not more than 2 percent by weight; and (c) concentrating a part of the effluent from the oxidizing step to convert the chlorine ions contained therein into a solid salt and separating the formed salt.