3.1.4. Effect of ore to acid ratio
Two sets of experiments were performed by varying
the ore to acid ratio from 100 g/L to 590 g/L. In the first
set, 18 M sulfuric acid at 190 °C was used for leaching
the ore with continuous stirring for 2 h. Under these
conditions, the dissolution of rare earths did not exceed
60% and it decreased at lower solids concentration. The
residue contained either anhydrite or a mixture of gypsum
and anhydrite. The second set of experiments used
9 M sulfuric acid at 100 °C, the leaching increased with
the solids concentration, Table 3.