Removal of contaminants and recovery of by-products from hot waste gas stream

A process for the removal of sulfur dioxide and other contaminants and the recovery of ammonium sulfate as a by-product from a hot waste gas stream comprising

• cooling said waste gas stream to a temperature below about 120.degree. F.,
• passing said waste gas stream in sequence through at least two separate absorption zones utilizing as an absorbent a recirculated aqueous salt solution of ammonium sulfite-bisulfite, the absorbent in the first absorption zone having an initial pH of from 7 to 9.0 and a salt concentration of 30 to 50%, the absorbent in the second absorption zone having an initial pH of from 5.2 to 7.0 and a salt concentration of 0.1 to 5%,
• withdrawing a portion of said aqueous ammonium sulfite-bisulfite salt solution from said first absorption zone,
• oxidizing said withdrawn ammonium sulfite-bisulfite salt solution in the presence of air at a temperature below about 200.degree. F. and a pressure between about 50 and 200 psig for a time sufficient to substantially convert said salts to sulfate salts and
• recycling the remaining ammonia and sulfur dioxide vapors to the first absorption zone.