Precipitation and dissolution of KCaCl3 impurity in NaCl crystals

Impurity precipitation in NaCl crystals containing CaCl₂ and KCl in various proportions is studied by means of flotation density measurements. The paper is concerned with the determination and dissolution, the kinetics and activation energy, and the mechanism of nucleation. Two temperature regions for solid solution decomposition were revealed. The characteristic stages in density variation upon isochronal annealing are shown to be correlated with radical changes in the X-ray diffraction patterns. Evidence is given that the nucleation is heterogeneous for the stable phase and homogeneous for the metastable one. – The activation energy in the low-temperature region has been found to be 0.8 ± 0.1 eV, which suggests precipitation to be diffusion-limited. From the kinetics studies it follows that the composition of the metastable phase corresponds to KCaCl₃ and the rate-controlling stage in stoichiometric crystals is a K⁺ diffusion.