The System Sodium Fluoride—Alumina Investigated by Quenching Methods

Alumina was found to react with sodium fluoride on fusion to produce sodium aluminate and cryolite according to the reaction 6NaF + 2Al₂2c₃= 3NaA10₂+ Na₃A1F₆. An insoluble sodium aluminate phase was observed under the polarizing microscope in samples quenched from as high as 1400°C. The equilibrium crystallization temperature of sodium fluoride in the presence of solid sodium aluminate was found to be slightly depressed with added alumina. A maximum lowering of 6°C was found for a starting alumina content of 5.4%. Further alumina additions resulted in the secondary precipitation of β-Al₂O₃. The shallow depression of the sodium fluoride crystallization temperature and the observed limited alumina solubility are attributed to the formation of cryolite. The composition of the liquid in equilibrium with sodium aluminate and sodium fluoride or sodium aluminate and β-alumina is represented in terms of the pseudo-ternary system NaF-Al₂O₃-Na₃A1F₆.