| Experiments were conducted to measure the dissolution and crystallization rates of calcium sulfite hemihydrate in aqueous solutions typical of flue gas desulfurization processes. A stagnant mass transfer model integrated with solution equilibrium was developed to predict CaSO(,3)(.) 1/2H(,2)O dissolution rates as a function of pH, temperature, solution composition, and particle size. The crystal growth rate was found to be a strong function of relative supersaturation and strongly inhibited by dissolved sulfate. The growth rate per unit BET surface area, R' (mole/cm('2)-min), is given by: 9.7 x 10('-4) exp (-10250/RT) x (RS(,CaSO(,3)) - 1)('2) x RS(,CaSO(,4))('-1), where RS(,CaSO(,3)) and RS(,CaSO(,4)) are the relative saturations with respect to calcium sulfite (CaSO(,3)(.) 1/2H(,2)O) and gypsum (CaSO(,4)(.)2H(,2)O) respective. Scanning electron microscopy and infrared spectroscopy demonstrated that solids generated in the presence of dissolved sulfate contained solid solution sulfate and crystallized as agglomerates of very thin platelets. In the absence of solid or dissolved sulfate, the solids were agglomerates of well-formed columnar, hexagonal crystals. |
