Characteristics of rare-earth elements extraction from tholeiitic basalt by hydrochloric acid nitric acid mixture - Polymorphic effect of sesquioxides of lanthanides

The interesting features that rare-earth elements (REE) exhibit in HCl-HNO3 acid extraction from powdered tholeiitic basalt are reported here. Cerium which can be oxidized to tetravalent shows the highest extraction percentage. The lower extractability of Eu compared with Sm and Gd is attributed to the high chemical resistance of feldspar in which divalent Eu is preferentially incorporated Based on the extractability, the stable trivalent rare-earth elements, La, Nd, Sm, Gd, Dy, Er, Yb and Lu are divided into three subgroups; {La, Nd}, {Sm}, and {Gd, Dy, Er, Yb, Lu}. This differentiation of the stable trivalent REE is inferred to be related to the polymorphic crystal structures of A-, B- and C-type sesquioxides of REE. Barium shows an extractability identical with that of La-Nd subgroup.