Experimental study of aluminum–fluoride complexation in near-neutral and alkaline solutions to 300 °C

Abstract

Solubility of boehmite (from 90 to 300 °C) and gibbsite (at 44.5 °C) was measured as a function of NaF concentration in alkaline NH 4OH-NH 4Cl-(NaCl) solutions at saturated vapor pressure. The marked increase of boehmite and gibbsite solubility in the presence of fluorine is explained by the formation of Al(OH) 2F 2 − according to Al(OH) 4 −+2F −=Al(OH) 2F 2 −+2OH −. Log K values regressed from these data at 44.5, 90, 150, 200, and 300 °C are −6.51±0.15, −5.06±0.10, −3.98±0.10, −3.49±0.15, and −2.86±0.20, respectively. The dependence of log K on temperature is precisely described (within ±0.07 log unit) by the function p K=−log K=7297.15/ T( K)−81.53+11.29·ln T( K) whose differentiation with respect to T yields the following thermodynamic properties for this reaction: log K 298.15=−7.26, Δ r H° 298.15=75.28 kJ/mol, and Δ r S° 298.15=113.4 J/mol K.