Kinetics of sodium fluosilicate precipitation

The formation of Na₂SiF₆ by discontinuous precipitation of dilute H₂SiF₆ with a 40% excess of an aqueous solution of NaCl under various conditions was studied. The values of induction time, number of crystals formed, their final size and their habit were determined during precipitation from solutions, whose initial supersaturation was 2 < S < 11. At S < 7–8 the crystals were formed by heterogeneous nucleation, whereas at S ≳ 7–8 homogeneous nucleation mechanism began to prevail. Once formed, the Na₂SiF₆ crystals were growing according to the screw-dislocation mechanism till they reached visible size; the corresponding values of kinetic order of nucleation and of the growth rate constant were g = 1.35 and k_g = 4.32 × 10⁻⁸ cm^2.05 sec⁻¹ g^−0.35, resp. The value of interfacial tension on the phase boundary Na₂SiF₆ crystal — saturated solution was determined (σ ∼ 52 erg/cm²). The resulting Na₂SiF₆ crystals conformed to log-normal distribution irrespective of conditions of precipitation. The dependence of the final size of crystals on supersaturation exhibited a maximum at S ∼ 6. Crystals of Na₂SiF₆ had a hexagonal habit, which was near to a spherical form at lower supersaturations, while dendritic crystals were formed at higher supersaturations.