Dehydration of ammonium magnesium chloride hexahydrate (ammonium carnallite)

Abstract

The dehydration of ammonium carnallite (NH₄MgCl₃·6H₂O) has been studied in the temperature range 373–453 K under partial pressures of water vapour between 1 × 10^–3 N m^–2 and 2.7 kN m^–2. At lower temperatures a dihydrate is formed by a phase-boundary controlled process, having an activation energy varying with water-vapour pressure between 65 and 125 kJ mol^–1. At higher temperatures and low water-vapour pressures the anhydrous chloride is formed in a single stage following first-order kinetics with an activation energy close to 40 kJ mol^–1. Intermediate temperatures and vapour pressures cause the reaction to proceed in two stages, both of which are controlled by phase-boundary processes; Arrhenius parameters have been derived for these also. Enthalpy data are given for the reactions as well as provisional X-ray diffraction data for the products.