Critical review of the thermodynamics of the Na-Al-O system

The free energies of formation of various phases in the Na-Al-O systems are critically reviewed for the composition range between pure alumina and sodium-aluminate. The results clearly indicate that in the temperature range between 600 and 1100 K neither-alumina nor'-alumina (Na₂O.5Al₂O₃) are stable with respect to sodium; the most stable phase in that range being sodium aluminate. Least square fitting and statistical analysis of all the available data on the free energy of formation of-alumina deemed accurate yields G _form ⁰ (from oxides) = –8772+1.53T (J/mole of O) (600 T 1200K) If the composition of-alumina is assumed to be Na₂0.11Al₂O₃ then G _form ⁰ (Na_1/17Al_11/17O, from elements) = –565100+109.6T (J mole of O) (600 T 1200 K) If the composition is assumed to be 19 then: G _form ⁰ (Na_1/14Al_9/14O, from elements) = –564000+109.6T (J/mole of O) (600 T 1200K) Based on the only study to date, the best estimate for the free energy of formation of-alumina is G _form ⁰ (Na_1/8Al_5/8O, from the elements) = –567860+114T (J/mole of O) (600 T 1200 K)The uncertainty (95% confidence band) is estimated to be about ±200 J/mole of O. Based on these results-alumina should be thermodynamically stable at lower temperatures, but become unstable at temperatures >900 K and should, kinetics permitting, dissociate into-alumina and sodium aluminate. Experimental evidence, however exists that conclusively shows that-alumina is thermodynamically stable at much higher temperatures than 900 K which leads to the conclusion that G °_form for-alumina reported to date is too high and should be more negative.