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 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.
