The precipitation and solubility of aluminium hydroxyfluoride hydrate between 30 and 70 °C

• Precipitation of AlF2OH at 25 °C is favoured with a solution F:Al of 1.6 and a pH of 4–5.
• Higher F:Al (> 2) and pH (> 5) cause co-precipitation of sodium fluoroaluminates.
• The pKsp for AlF2OH at 25 °C was found to be 24.52 with a ΔHR of − 23.48 kJ/mol.
• The high ΔHR indicates that temperature is effective for inducing precipitation.

This paper considers the recovery of fluoride from spent pot-lining (SPL) in the form of aluminium hydroxyfluoride (AHF), AlFx(OH)3 − x, with the intent of recycling the fluoride back into Al smelting pots. This study uses ion speciation equilibria calculations combined with dissolution experiments to examine the solubility (and thus potential yield of recovery) for the crystallization of AHF from synthetic solutions. Results suggest that when sodium hydroxide is used for pH adjustment, a high fluoride to aluminium (F:Al) ratio can be problematic because of the competitive co-precipitation of sodium fluoroaluminates. Further, for an F:Al ratio of 1.6 and pH of 4.9, the desired high purity AHF product can be produced. Results from solubility experiments strongly suggest that the AHF has a molecular formula of AlF2OH·H2O and yields a pKsp at 25 °C of 24.52 ± 0.05 and a heat of reaction (ΔHR) of − 23.48 ± 2.9 kJ/mol for the following reaction:AlF2OHs⇆Al3+aq+2F−aq+OH−aq.The primary means for inducing precipitation of AHF is pH adjustment/control, with temperature being a secondary means. The precipitation of AHF is beset by the competitive co-precipitation of other minerals, therefore restricting the operating conditions of the precipitation process for high purity AHF to a low pH (4–5). High temperatures (50–70 °C) are expected to be advantageous for precipitation owing to the solubility behaviour observed.