From the previous results, it is clear that hot sulfuric
acid attacked the ore solubilising the rare earths and
leaving a residue consisting mainly of gypsum and/or
anhydrite. These residues are shown in Fig. 3 with dehydration
of gypsum to form anhydrite appearing to be a
dissolution precipitation process (Farrah et al., 2004).
The experiments show that the formation of the two
calcium sulfate compounds in the residue depends
mainly on the reaction temperature. Below 80 °C, the ore sample seems to be partly decomposed, since both
yttrium fluorite and calcium sulfate (especially anhydrite)
were detected by XRD analysis.