Process for purifying a sulfur dioxide containing gas

Claim 1

A process for efficiently purifying a sulfur dioxide containing gas, which comprises the steps of 1. contacting said sulfur dioxide containing gas with an aqueous ammonia absorption solution in a first absorption zone to absorb the major part of said sulfur dioxide, thereby obtaining an ammonium sulfite solution (A) and an effluent gas containing residual sulfur dioxide and ammonia, and separating said solution (A) from said effluent gas, 2. contacting said separated effluent gas with a dilute aqueous ammonium sulfite solution (B) in a second absorption zone to absorb residual sulfur dioxide and ammonia thereby producing an ammonium sulfite solution (C) and a gas practically free of sulfur dioxide and ammonia, and 3. electrodialyzing the solution (C) in at least one multi-stage electrodialysis cell comprising a plurality of alternatively cation-permselective and anion-permselective membranes, thereby producing a relatively concentrated ammonium sulfite solution (D) and a dilute ammonium sulfite solution (E), separately recovering each of the solutions (D) and (E), and recycling solution (E) to step (2) to constitute at least a portion of the absorption solution (B).