On Precipitation of Sparingly Soluble Fluoride Salts

The classical approaches for nucleation rate and diffusion-limited crystal growth are checked for their validity for sparingly soluble salts, i.e., calcium fluoride and strontium fluoride. Particle size distributions are calculated by population balance modeling as a function of the initial supersaturation and free lattice ion ratio. Theoretical results are compared with experimentally determined particle sizes which are measured by dynamic light scattering technique. Before measurement, suspensions were stabilized by dilution with deionized water or surfactant solution. The dilution of suspensions was done after solid liquid equilibrium had been reached. Consequently, particle formation mechanisms, such as nucleation and crystal growth which depend on supersaturation, are assumed to be unaffected. If supersaturation is calculated correctly by considering activities and ion complex formation, we found that experimental results are predictable by modeling with classical approaches. Hereby, interfacial energy is the only fitting parameter. Furthermore, the diffusion-limited crystal growth rate decreases with deviation from stoichiometric precipitation condition due to the lower transport flux of the shortfall ion.