3.2. Rare earth precipitation
The dissolved rare earths should be removed from the
leach liquor before they become incorporated again in
gypsum (Koopman and Witkamp, 2000). In a typical
experiment, 50 g of ground ore sample was digested
using 250 mL of 9 Msulfuric acid at 100 °C for 2 h. The
slurry was filtered and the gypsum residue thoroughly
washed with distilled water. The filtrate and washes
were combined and made up to 1 L giving a solution
with the composition in Table 4.