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

摘要

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 SO_2 and generates numerous fine particulates. For the fine calcium sulfite particulates, the oxidation rate to CaSO_4 is much lower than the sulfite formation rate, resulting in a low fraction of CaSO_4 in the product and a difficult dewatering of the sludge. In order to improve the oxidation rate and the fraction of CaSO_4 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 ℃. Four metal ions, Mn~(2+), Fe~(3+), Cu~(2+) and Co~(2+) are added in the slurry to improve the oxidation rate. The results show that only Mn~(2+) ion, or Mn~(2+) and Fe~(3+) ions together can obviously increase the oxidation rate. The products with Mn~(2+) and Fe~(3+) 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 Fe~(3+) and Co~(2+) additives. Mn~(2+) 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.