Abstract
In this work, the cause of abnormal color in ammonium sulfate products formed by flue gas desulfurization is revealed by investigating the conversion relationship between different sulfur-containing ions and their behavior in a sulfuric acid medium. Both thiosulfate (S(2)O(3)(2-)) and sulfite (SO(3)(2-) ＆ HSO(3)(-)) impurities affect the quality of ammonium sulfate. The S(2)O(3)(2-) is the main reason for the yellowing of the product due to the formation of sulfur impurities in concentrated sulfuric acid. To address the yellowing of ammonium sulfate products, a unified technology (US/O(3)), using ozone (O(3)) and ultrasonic waves (US) simultaneously, is exploited to remove both thiosulfate and sulfite impurities from the mother liquor. The effect of different reaction parameters on the degree of removal of thiosulfate and sulfite is investigated. The synergistic effect of ultrasound and ozone on ion oxidation is further explored and demonstrated by the comparative experiments with O(3) and US/O(3). Under the optimized conditions, the thiosulfate and sulfite concentration in the solution is 2.07 and 5.93 g/L, respectively, and the degree of removal is 91.39 and 90.83%, respectively. The product obtained after evaporation and crystallization is pure white and meets the national standard requirements for ammonium sulfate products. Under the same conditions, the US/O(3) process has apparent advantages, such as saving reaction time compared with the O(3) process alone. Introducing an ultrasonically intensified field improves the generation of oxidation radicals ·OH, (1)O(2), and ·O(2)(-) in the solution. Furthermore, the effectiveness of different oxidation components in the decolorization process is studied by adding other radical shielding agents using the US/O(3) process supplemented with EPR analysis. The order of the different oxidation components is O(3)(86.04%) > (1)O(2)(6.53%) > •OH(4.45%) > •O(2)(-)(2.97%) for the oxidation of thiosulfate, and it is O(3)(86.28%) > •OH(7.49%) > (1)O(2)(4.99%) > •O(2)(-)(1.25%) for the oxidation of sulfite.