Studies of the reaction of crystalline calcium carbonate with aqueous solutions of NH 4F, KF and NaF

Abstract

The reaction between solid calcium carbonate and the aqueous fluorides NH 4F, KF, and NaF has been completely investigated. The reaction of CaCO 3 (solid) is completely independent of the dimensions of its polycrystalline particles and gives calcium fluoride. The calcium fluoride is formed in the same form and size as the grains of the original calcium carbonate. A course crystalline fluorite is formed at a satisfactory rate and with a sufficiently high mechanical strength to be of industrial interest. The course of the reaction appears to involve penetration of the fluoride solution into the body of a grain through voids which develop in the solid material owing to the formation of polycrystalline CaF 2 with a different molar volume as compared with CaCO 3. Data were obtained on the rate of formation and nature of the fluoride formed. The fluorite which is formed around the dissolving calcite was shown by X-ray diffraction and electron microscopy to have a polycrystalline aggregated structure and an estimate is made of crystallite size. The fluorite grains are pseudomorphs of the calcite crystals and there is crystallographic orientation of the product with respect to the parent phase.