Process and apparatus to purify waste industrial gases

Abstract

Industrial waste gases containing sulfur dioxide, hydrogen fluoride, hydrochloric acid and sulfur trioxide are treated within a reaction tank in a first stage with a solution containing ammonium ions to increase the dew point of the gases. The gases are then cooled below the elevated dew point in a second stage below which a deflector system directs the gases along the wall of the reaction tank into a third stage where the gases are cooled by a spray mist of an ammonia mixture to form ammonium salts. The gases are then treated in a fourth stage at the bottom of the reaction tank with the solution containing ammonium ions to precipitate ammonium salts into a reservoir below the reaction tank. Purified waste gases are discharged into a cooler where a stripper removes entrained droplets of liquid before discharge into the atmosphere. The droplet size of the solution sprayed into the second stage is larger than the droplet size of the solution sprayed into the first stage. The solution sprayed into the reaction tank as well as the cooler has a concentration below its saturation limit and the gases are cooled to a temperature which is 0.1.degree. C. to 3.degree. C. below the dew point temperature. The dew point of the cooled gases in the second stage is below the temperature of the gases and sensible heat withdrawn from the gases at this stage is recovered. The precipitated ammonium salt solution in the reservoir contains ammonium ions and a partial flow of this solution is treated according to different embodiments of the processes.