Process for purifying a sulfur dioxide containing gas, by washing with an ammonia aqueous solution, comprising the following steps:(a) contacting a sulfur dioxide containing gas with ammonia and/or ammonium sulfite in the presence of water, to form a solution containing ammonium bisulfite and/or sulfite;(b) reacting at least a portion of the solution obtained in step (a) with ammonium bisulfate, so as to decompose the ammonium bisulfite and sulfite and to obtain ammonium sulfate as an aqueous solution and gaseous sulfur dioxide;(c) recovering the sulfur dioxide;(d) heating the aqueous solution obtained in step (b) for vaporizing at least a portion of the water;(e) heating the product from step (d) to convert the ammonium sulfate to ammonium bisulfate and gaseous ammonia;(f) separating the gaseous ammonia obtained in step (e) and feeding it to step (a) either as such or as ammonium sulfite and,(g) feeding to step (b) the ammonium bisulfate obtained in step (e) either as such or after dissolution in water, wherein, in order to avoid an accumulation of ammonium sulfate and/or bisulfate, due the undesirable formation of ammonium sulfate, at least a portion of the ammonium sulfate and/or bisulfate from step (e) is heated with a molybdenum and/or tungsten compound for converting a portion of said ammonium sulfate and/or bisulfate to sulfur dioxide and ammonia, which are fed back to step (a).