Metastable zone width and nucleation threshold of aluminium hydroxyfluoride hydrate

This paper considers the metastable zone width (MSZW) of aluminium hydroxyfluoride hydrate (AHF, AlF2OH.H2O) crystallizing solution, with a view to identifying suitable conditions for crystal growth. Speciation calculations combined with experimental validations have been used to estimate the degree of saturation at varying pHs leading to the identification of the metastable zone width (MSZW). From the results obtained, the solutions are saturated at pH 3.0 +/- 0.3 and remain metastable until the nucleation threshold is reached at pH 5.0 +/- 0.3. A narrowing of the MSZW at elevated temperatures has been observed and is attributed to the decrease in AHF solubility with increasing temperatures. This phenomenon was applied to obtain a tripling of the crystal particles in 30 minutes. The pH profile of the crystallizing solution is similar to that of acid/base titration curves and is found to be a quick and accurate method of estimating the MSZW. Using similar terminologies, the equivalent point corresponds to solution saturation while the midpoint corresponds to the nucleation threshold. These results are applicable in the recovery of fluoride values of spent pot lining.