Process of treating exhaust gas containing nitrogen oxides and sulfur dioxide

Claim 1

In a process for treating an exhaust gas containing nitrogen oxides and sulfur dioxide, wherein said exhaust gas is brought into contact with an aqueous absorbing solution containing at least an iron chelate salt and an alkali sulfite for the production of absorption products, said absorbing solution containing said absorption products is cooled to crystallize and separate said absorption products by filtration, the filtrate is recycled to the above-mentioned step of contact between said exhaust gas and said absorbing solution, the thus obtained absorption products are reduced into ammonia or nitrogen and elementary sulfur, the improvement comprising the steps of: (a) heating the separated absorption products of potassium imidodisulfonate, potassium dithionate and potassium sulfate to a temperature of 250° to 400° C. to convert potassium dithionate into potassium sulfate and sulfur dioxide; (b) further heating a residual potassium imidodisulfonate to a temperature of 800° to 1000° C. in the presence of a carbonaceous substance selected from the group consisting of coal, coke, petroleum pitch and petroleum coke thereby decomposing said residual potassium imidodisulfonate into gaseous ammonia and at the same time reducing potassium sulfate into potassium polysulfide; and (c) heating potassium polysulfide in the presence of carbon dioxide and steam to a temperature of 350° to 500° C., preferably 380° to 450° C. thereby converting said potassium polysulfide into hydrogen sulfide, gaseous sulfur and potassium carbonate.