Method for regenerating iron-EDTA solutions used for scrubbing plug gases

Claim 1

In the known method which comprises contacting industrial exhaust gases containing nitrogen oxides and sulfur dioxide in an absorption zone with a circulating flow of absorbing solution containing an iron EDTA complex salt and ammonium sulfite, said absorption zone being maintained under conditions whereby said absorbing solution will absorb nitrogen oxides and sulfur dioxide from said exhaust gases, the improvement in the method of treating said absorbing solution which comprises: (a) withdrawing absorbing solution from said absorption zone and supplying make-up absorbing solution to said absorption zone to take the place of the removed absorbing solution, (b) contacting at least a portion of said withdrawn absorbing solution with an oxygen-containing gas in an oxidation zone to thereby generate a SO 2 -containing gas, (c) contacting the absorbing solution removed from said oxidation zone with sulfuric acid to thereby acidify the solution and generate a SO 2 -containing gas, cooling the thus acid-treated absorbing solution to crystallize EDTA, and then separating the crystallized EDTA from the cooled absorbent solution, (d) heating the absorbent solution resulting from step (c) to thereby decompose the dithionate, imidodisulfate, and amidosulfate contained therein and to generate a SO 2 -containing gas, (e) contacting the solution obtained from step (d) with ammonia to thereby precipitate an iron compound, and then separating said iron compound from the treated solution, (f) concentrating the solution resulting from step (e), crystallizing ammonium sulfate from the concentrated solution, and separating the crystallized ammonium sulfate from its mother liquor, (g) dispersing and dissolving in water both the crystallized EDTA separated in step (c) and the iron compound separated from step (e) and incorporating said water solution and dispersion into the make-up absorbing solution set forth in step (a), (h) conducting the SO 2 -containing gas resulting from steps (b), (c) and (d) to a zone for the preparation of said make-up solution, (i) introducing an aqueous ammonia solution into the zone for producing said make-up absorbent solution so as to maintain the pH of the absorbing solution between 5.5 and 7, and (j) adding the mother liquor from step (f) to the solution obtained after the oxidation in accordance with step (b) and introducing the mixture into step (c).