Behaviour of the rare earths during goethite (α-FeOOH) precipitation from sulphate-based solutions

Goethite (alpha-FeOOH) precipitation is an effective means of removing iron in a filterable form from processing solutions, and the behaviour of the rare earths during goethite precipitation was investigated. Increasing concentrations of the light rare earth ions, to 10 g/L, yielded precipitates having nearly constant Fe contents. Both the SO4 and REE contents increased in a linear manner as the concentration of the light rare earth ions increased, and REE contents as high as 0.5% Gd were obtained. The products made from the more dilute light rare earth solutions consisted of dominant goethite (alpha-FeOOH) with trace-to-minor amounts of lepidocrocite (gamma-FeOOH), but the amount of lepidocrocite decreased significantly with increasing atomic number or with increasing concentration of a given rare earth ion. Increasing concentrations of Ce3+ gave similar results except that partial oxidation of Ce3+ resulted in minor CeO2 contents in some of the precipitates. Increasing concentrations, to 10 g/L, of the heavy rare earth ions yielded precipitates having essentially constant Fe contents, and goethite was the only product detected. In contrast, both the rare earth and SO4 contents of the precipitates increased in a linear manner as the concentrations of the heavy rare earth ions increased. The adsorption of sulphate ions on goethite seems to be responsible for the observed SO4 contents of the precipitates. The extent of rare earth incorporation in goethite increases as the atomic number of the rare earth increases, and a 1.4% Lu content was realized from a 5 g/L Lu3+ solution. However, it is not clear whether the rare earth ions are adsorbed on the goethite or are incorporated in the goethite structure, and further work is required to clarify the mode of rare earth incorporation.