The determination of the kinetics of ice-brine interfaces from data on temperature-gradient zone migration

Abstract

An analysis is made of existing experimental data for the temperature-gradient migration of droplets, containing either NaCl or KCl solution, through ice. In contrast with earlier treatments the present analysis employs properly calculated values for the diffusivities of NaCl and KCl in saturated aqueous solution. In the present study droplet motion is shown to be controlled partly by the reactions at the ice-solution interfaces. Since the data are consistent with the operation of interfacial mechanisms either involving ‘normal growth', or requiring the presence of screw dislocations or ‘two-dimensional’ nuclei, the mechanism of the interfacial reaction cannot be decided. However, the choice of the ‘normal growth’ or dislocation mechanisms requires a dependence of the appropriate kinetic coefficients on salt concentration, whereas no such dependence is required if a nucleation mechanism is assumed. It is not possible to determine the kinetic coefficients for the nucleation mechanisms; but values are obtained for the coefficients of the ‘normal growth’ and screw-dislocation mechanisms, for both the NaCl and KCl systems, for liquidus (droplet) temperatures between 260 and 266°K(K). The dependence of the latter coefficients on concentration was much more pronounced in the NaCl system than in the KCl system. For the case of ‘normal growth', for example, the kinetic coefficient was about 3 × 10⁻³ mm s⁻¹ °K⁻¹ (3 μm s⁻¹ K⁻¹) for a concentration of NaCl equivalent to a liquidus temperature of 263°K(K); for a similar concentration of KCl, the coefficient was about 30 × 10⁻³ mm s⁻¹ °K⁻¹ (30 μm s⁻¹ K⁻¹).