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

Solubility of boehmite (from 90 to 300 °C) and gibbsite (at 44.5 °C) was measured as a function of NaF concentration in alkaline NH4OH-NH4Cl-(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)2F2− according to Al(OH)4−+2F−=Al(OH)2F2−+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 pK=−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 K298.15=−7.26, ΔrH°298.15=75.28 kJ/mol, and ΔrS°298.15=113.4 J/mol K.