The behaviour of the rare earth elements during gypsum (CaSO4·2H2O) precipitation

The behaviour of the rare earth elements at concentrations likely to be encountered during gypsum precipitation in a commercial process was investigated. The syntheses were done by slowly mixing pH-adjusted Li2SO4 and CaCl2 solutions such that significant variations in pH did not occur. Increasing concentrations of the light or heavy rare earth ions had no significant effect on the calcium (~ 24% Ca) or sulphate (~ 55% SO4) contents of the precipitates. However, the REE contents increased as the concentration of the rare earth ions increased from 0 to 8 g/L, although most of the increase occurred in the 0.2 to 2 g/L range. The maximum extent of REE incorporation occurred at praseodymium-neodymium and decreased for lighter or heavier rare earth elements. The maximum contents of the individual REEs were < 1.5%. The gypsum precipitates made from mixtures of the rare earth ions had reduced contents of the heavy rare earths relative to the light rare earths. This could be advantageous as the generally more valuable heavy REEs would tend to remain dissolved for subsequent recovery. The rare earth ions are structurally incorporated in gypsum where three Ca2 + ions are replaced by two trivalent rare earth ions and a vacant Ca2 + lattice site. Although the trivalent light rare earth ions have radii similar to that of divalent calcium, the charge differences limit the extent of solid state substitution, even where the ionic radii are very similar to that of Ca2 +.