The behaviour of scandium, yttrium and uranium during jarosite precipitation

The behaviour of scandium, yttrium and uranium during the precipitation of potassium jarosite and sodium jarosite was investigated in laboratory experiments. The end-member scandium, yttrium and uranium analogues of potassium jarosite could not be synthesized from iron-free solutions under the acidic conditions required for jarosite formation. However, scandium contents as high as 2.70 wt.% Sc were detected in potassium jarosite precipitates, and supporting mineralogical studies indicate that Sc(III) substitutes for Fe(III) in the jarosite structure. The extent of Sc incorporation in potassium jarosite increases slightly as the pH of the solution increases, and this behaviour is consistent with the enhanced hydrolysis of Sc(III) ions at the higher pH values. Nevertheless, the scandium molar partitioning coefficients are only about 0.2, indicating that Fe is preferentially precipitated relative to Sc in acid media. Increasing concentrations of yttrium have little effect on the composition of either potassium jarosite or sodium jarosite synthesized at 98 °C from solutions at pH 1.6. Although the yttrium contents increased systematically as the yttrium concentration increased to 8 g/L, the yttrium contents of the potassium jarosite precipitates were < 0.082 wt.% Y, Similar results were obtained for potassium jarosite made using a slow addition method or precipitated from predominantly chloride media. The yttrium content of the potassium jarosite precipitates increased linearly from 0.023 to 0.111 wt.% Y as the solution pH increased from 0.8 to 2.2. In contrast, the yttrium content of the precipitates is nearly independent of the K2SO4 concentration. Uranium is not significantly incorporated in jarosite-type compounds; for example, a sodium jarosite precipitate made from a solution containing 25 g/L UO2SO4 contained < 0.0032 wt.% U.