Red mud (bauxite-residue) is leached with a caustic (acid) solution. Scandium dissolves best near a moderate temperature — too cool and the reaction stalls, too hot and iron/aluminium co-dissolve. Higher causticity pulls more scandium into solution up to a point, then starts dragging along impurities. Fe/Si impurity load directly eats into both recovery and selectivity. Agitation speeds up how fast the batch approaches its equilibrium values; it doesn't change the equilibrium itself.
- Recovery peaks near ≈90 °C and drops off as causticity or impurity load rise.
- Selectivity favours a mid-range causticity (~62%) and falls as impurity load climbs.
- Loss to residue grows with excess heat and impurity load.