Process for purifying waste water containing ammonium sulfate

A process for treating waste water containing ammonium sulfate to recover elemental sulfur, which comprises the steps of:

• a. reacting said waste water with a mixture of calcium oxide and calcium sulfide and with recycled ammonium sulfite from step (f), the molar ratio CaS/CaO being at least 0.5:1 and the amounts of the reactants being sufficient to form simultaneously insoluble calcium sulfate, insoluble calcium sulfite, and ammonium sulfide dissolved in the aqueous phase and then separating the resulting mixture of insoluble calcium sulfate and calcium sulfite from the aqueous phase,
• b. heating the mixture of calcium sulfite and calcium sulfate with carbon to the temperature of reduction of calcium sulfate to obtain a gas containing sulfur dioxide and a mixture of calcium oxide and calcium sulfide, and recycling said mixture to step (a),
• c. stripping the aqueous phase obtained in step (a) to separate therefrom a gas containing hydrogen sulfide and ammonia, and discharging resultant residual aqueous phase,
• d. reacting the gas separated in step (c) with sulfur dioxide, thereby producing sulfur and ammonia containing gas, and separating said sulfur,
• e. dissolving at least a fraction of the ammonia containing gas recovered from step (d) with water together with the gas containing sulfur dioxide recovered from step (b), thereby forming ammonium sulfite,
• f. recycling to step (a) at least a fraction of the ammonium sulfite recovered from step (e).