3.1.1. Effect of sulfuric acid concentration.
Fig. 1 shows that 9 M sulfuric acid dissolved 96% of
the total rare earths when stirred for 2 h at 100 °C. Using
more concentrated acid (18 M) appears to cause
reprecipitation of the rare earths from the sulfate leach
liquor decreasing the leaching efficiency to 57%. For the
9Msulfuric acid, the insoluble residue was identified by
XRD as gypsum, while with 3 M a mixture of gypsum
and fluorite was in the residue.