THERMAL-DECOMPOSITION OF SOLID WEGSCHEIDERITE, NA2CO3.3NAHCO3

The thermal decomposition of solid wegscheiderite, Na2CO3.3NaHCO3, has been studied over the temperature range 350-465 K in atmospheres of pure nitrogen and pure carbon dioxide. At high temperatures, in both gases, the decomposition is controlled by a phase-boundary process having an activation energy of ca. 30 kJ mol-1. The frequency factor for this process is very low in both gases. At low temperatures, nucleation-and-growth processes control the decomposition; in nitrogen the reaction follows Avrami-Erofeyev kinetics with n = 2, with an activation energy of 64 kJ mol-1, while in carbon dioxide a first-order mechanism is rate-controlling, with an activation energy of 94 kJ mol-1. Electron micrographs are presented which generally support the suggested mechanisms. Comparisons are made with the decomposition of sodium hydrogencarbonate.