Ammonia → Ammonium Sulfite

In contrast, ammonia-based scrubbing processes have been used to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention and incorporated herein by reference. In these processes (also known as ammonium sulfate flue gas desulfurization, or AS FGD), the scrubbing solution is accumulated in a tank where the absorbed sulfur dioxide reacts with dissolved ammonia (NH3) to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction.