Rare Earth Element Phases in Bauxite Residue

Abstract: The purpose of present work was to provide mineralogical insight into the rare earth
element (REE) phases in bauxite residue to improve REE recovering technologies. Experimental
work was performed by electron probe microanalysis with energy dispersive as well as wavelength
dispersive spectroscopy and transmission electron microscopy. REEs are found as discrete mineral
particles in bauxite residue. Their sizes range from <1 µm to about 40 µm. In bauxite residue, the most
abundant REE bearing phases are light REE (LREE) ferrotitanates that form a solid solution between
the phases with major compositions (REE,Ca,Na)(Ti,Fe)O₃ and (Ca,Na)(Ti,Fe)O₃. These are secondary
phases formed during the Bayer process by an in-situ transformation of the precursor bauxite LREE
phases. Compared to natural systems, the indicated solid solution resembles loparite-perovskite series.
LREE particles often have a calcium ferrotitanate shell surrounding them that probably hinders their
solubility. Minor amount of LREE carbonate and phosphate minerals as well as manganese-associated
LREE phases are also present in bauxite residue. Heavy REEs occur in the same form as in bauxites,
namely as yttrium phosphates. These results show that the Bayer process has an impact on the initial
REE mineralogy contained in bauxite. Bauxite residue as well as selected bauxites are potentially
good sources of REEs.