This study investigates the removal of aluminum and iron from rare earth element (REE) containing solutions by solvent extraction with saponified naphthenic acid and by hydrolysis-precipitation. The results emphasize both, the preferential application as well as limitations of every method. We find that emulsification occurring during the solvent extraction of aluminum is caused by its slow extraction rate in comparison to the neutralization reaction and by the proximity of the pH value required for aluminum extraction and the pH value at which hydrolysis of aluminum occurs. However, by choosing a long shaking time of at least 4 h, the emulsion recedes. The formation of emulsion can be avoided by strict control of pH value during the extraction. Moreover, the loading capacity of the organic phase with aluminum is limited due to the strong increase in viscosity of the organic phase with increasing aluminum concentration and due to the gel formation. Regarding the extraction of iron, the amount of extracted ions is limited due to the overlap of the pH range required for the extraction with pH range in which sparingly soluble iron oxides/hydroxides are formed. In summary, aluminum and iron can be simultaneously removed from REE-sulfate solution by solvent extraction with saponified naphthenic acid in one extraction stage only from diluted solutions. However, in comparison to the hydrolysis-precipitation method, a higher purity of the solution is achieved. A complete removal of aluminum and iron from concentrated solutions can be achieved in two stages. First, the content of aluminum and iron should be reduced by hydrolysis-precipitation. After that, a high-purity solution can be obtained by subsequent solvent extraction by saponified naphthenic acid.

Eudialyte, a sodium rich zirconosilicate, is one of the promising sources for REEs (rare earth elements), particularly for HREEs + Y (heavy rare earth elements and yttrium). The key challenge in hydrometallurgical processing is the prevention of silica gel formation and REE separation from resulting multi-element leach solutions. This study deals with the selective extraction of REE from eudialyte concentrate by selective roasting. In this method, metal ions are converted into sulfates, followed by the decomposition of non-REE sulfates in a roasting step and the water leaching of the calcine. The effect of acid addition, roasting temperature, roasting time, pulp density and leaching time is studied. For sufficient conversion of REEs into sulfates, sulfuric acid is added in excess. At a roasting temperature of ≥750 °C sulfates of zirconium, hafnium, niobium, aluminum and iron decompose into sparingly soluble compounds, while REE and manganese sulfates remain stable up to a roasting time of 120 min. The silica present in the calcine is found to be metastable even after roasting. The amount of leached Si⁴⁺ is dependent predominantly on the pH value of the leaching medium. Applying the method, REEs can be efficiently separated from zirconium, hafnium, niobium, aluminum and iron. However, only diluted solutions can be produced. Water leaching of calcine at high solid/liquid ratios causes REE losses resulting from formation of double sulfates and gypsum. The acid excess removed from the reaction mixture in the roasting stage can be simply recovered by treatment of the gas phase.