Process for reducing molten ammonium sulfates containing metallic impurities to ammonia and sulfur dioxide

In a process for reducing ammonium sulfate containing at least iron sulfate as an impurity, wherein said impure ammonium sulfate is progressively introduced into a reaction zone at 150.degree.-450.degree. C, is reacted in the molten state with molecular sulfur, hydrogen sulfide and/or ammonium thiosulfate to form a sulfur-dioxide containing gas, and the so-formed gas is discharged, the improvement comprising maintaining the iron sulfate content of the impure ammonium sulfate in the reaction zone in the range of 1 to 15% by weight by (a) withdrawing a fraction of said impure ammonium sulfate from said bath at a sufficient rate to maintain the 1-15% by weight concentration of iron sulfate in the reaction zone, (b) treating the withdrawn fraction with water and ammonia so as to precipitate iron compounds, (c) separating the iron compound precipitate from the remaining ammonium sulfate fraction, of low iron content, and (d) feeding back the ammonium sulfate fraction obtained from step (c) to the reaction zone.