Calcium Sulfite Oxidation and Crystal Growth in the Process of Calcium Carbide Residue to Produce Gypsum

Calcium carbide residue mainly composed of Ca(OH)(2) can be used in wet flue gas desulfurization (FGD). Different from the traditional limestone-gypsum wet FGD, calcium carbide residue rapidly reacts with SO2 and generates numerous fine particulates. For the fine calcium sulfite particulates, the oxidation rate to CaSO4 is much lower than the sulfite formation rate, resulting in a low fraction of CaSO4 in the product and a difficult dewatering of the sludge. In order to improve the oxidation rate and the fraction of CaSO4 in the product, experiments of the calcium sulfite oxidation were conducted in a cylindrical flat-bottomed bottle placed in a water bath. The optimal oxidation reaction conditions are obtained: pH = 4.0, T = 60 degrees C. Four metal ions, Mn2+, Fe3+, Cu2+ and Co2+ are added in the slurry to improve the oxidation rate. The results show that only Mn2+ ion, or Mn2+ and Fe3+ ions together can obviously increase the oxidation rate. The products with Mn2+ and Fe3+ additives are observed as needle-like crystallites. The co-precipitation compound detected in the products indicates that the sulfite oxidation rate is relatively low for Fe3+ and Co2+ additives. Mn2+ additive is an excellent additive due to a high oxidation rate and a needle-like crystallite for the product, and can be recycled many times.