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 °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.