Recovery of fluoride values from spent pot-lining: Precipitation of an aluminium hydroxyfluoride hydrate product

Treatment of spent pot-lining (SPL) from aluminium smelting cells by a two-stage leaching scheme comprising a water wash and an Al3+ leach and fluoride recovery as an aluminium hydroxyfluoride product has been studied for extraction of fluoride and then recovery as smelter grade AlF3. The NaF content of a −1.18 mm size fraction was removed by the water wash, while the more refractory Na3AlF6 and CaF2 were removed by treatment with 0.34 M Al3+ solution at 25 °C for 24 h, which yielded an overall fluoride extraction of 76–86 mol%. Mathematical modelling using experimental stability constant data was carried out to predict the effect of combining solutions and identify ways to manipulate the solution equilibria to maximise fluoride precipitation yields. The predictions were then tested experimentally. In the 4.5–5.5 pH range, selective precipitation of fluoride as an aluminium hydroxyfluoride hydrate product was achieved by neutralisation of the combined solutions with addition of 2 M NaOH solution. Higher pH values lead to the co-precipitation of hydrolysed sodium fluoroaluminates. Characterisation of precipitates using X-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectroscopy, thermogravimetric analysis, differential thermal analysis, aluminium and fluoride determination have pointed out both the AlF2(OH)·1.4H2O stoichiometry and possible thermal decomposition pathways to yield a dehydrated aluminium hydroxyfluoride product, AlF2(OH), that could be used for smelter grade AlF3 production. The kinetics of hydrolysis are such that nucleation dominates while particle growth is restricted. Techniques to allow slow hydrolysis are necessary to form smelter grade AlF3.