TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The ammonium sulfate crystallization step comprises: passing the gas into the ammonium sulfate crystallization part through the gas inlet, where ammonium bisulfite in the slurry is converted into ammonium sulfite by an ammonia water feeding device, ammonium sulfite is rapidly oxidized to ammonium sulfate by an oxidation feeding device, spraying the slurry on the gas by a spray device through a circulating pump so that the gas is cooled and wetted, and condensing the ammonium sulfate in the slurry, where ammonium sulfate cannot absorb sulfur dioxide. The sulfur dioxide absorption step comprises: passing the gas into the sulfur dioxide absorption part through a steam raising cap, spraying to remove the sulfur dioxide after cooling and wetting. The demisting and ammonia removing step comprises: passing the gas into the demisting and ammonia removing part through the sulfur dioxide absorption part, where the demisting and ammonia removing part is provided with processed water for spraying and washing, controlling the escape of aerosol and ammonia of purified gas, and discharging the clean gas into a chimney from a gas outlet. The liquid-air ratio of circulating spray in the ammonium sulfate crystallization step is 8-20 L/m3, the gas is cooled to lower than 85 degrees C, the quality concentration of ammonium sulfate in the slurry is 8-30%, and pH is 6.0-7.5. The liquid-air ratio of circulating spray in the sulfur dioxide absorption step is 5-15 L/m3 and the pH of the slurry is 5.0-6.2 to maintain an appropriate proportion of ammonium bisulfite and ammonium sulfite and to ensure a good absorption effect of sulfur dioxide. The circulating slurry in the ammonium sulfate crystallization step is introduced from a circulating slot of the sulfur dioxide absorption part and is stopped by a valve. The demisting and ammonia removing part is provided with a demister, which has two layers and is used for spraying and washing with processed water.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The outlet of the circulating pump is provided with an ammonium sulfate discharge valve. The sulfur dioxide absorption part is provided with a steam raising cap, a second circulating spray device and a circulating slot, where the circulating slot is provided with a circulating liquid inlet and an ammonia water feeding port. The circulating slot is connected with the second circulating spray device by a circulating slot pump, and the outlet of the circulating slot pump is provided with a control valve for supplying slurry to the ammonium sulfate crystallization part. The demisting and ammonia removing part is provided with a demister and a washing device for the demister, where the upper side of the demister is provided with a gas outlet, and the demister is a double-layer demister.