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.