The Effects of Surfactants on the Nucleation and Growth of Calcium Sulfate Dihydrate in Phosphoric Acid Solution
Calcium sulfate dihydrate (gypsum) was crystallized from a reaction of calcium phosphate and sulfuric acid in phosphoric acid solution, at several different solution supersaturation values and in the absence and presence of several surfactants (Crysmod and Hiflo-S5). The nucleation and growth of the precipitate was examined through turbidity measurements, particle size distribution, and microscopy. Plots of (-1/Turbidity) vs. time revealed taht the crystallization reaction was approximately second order. Statistical analysis indicated that the presence of surfactants did not significantly effect either the induction period or the growth rate of the crystals. Particle size distribution was examined for one supersaturation valve studied, with and without each surfactant. The analysis suggested that both surfactants caused a shift in the size distribution for crystals formed without surfactant toward larger crystals. Microscopy indicated the production of larger, rhombic shaped crystals in the presence of either surfactant. In the absence of surfactant, the crystals were needle-like in shape. If used in the phosphoric acid industry, these surfactants could potentially improve phosphoric acid filtration rates because the larger, rhombic crystals would form a more porous gypsum filter cake, This study did not resolve any possible effects the surfactants might have on the rate of formation of the filter cake.