Dissolution and recrystallization of alum to separate cesium, rubidium and potassium using their different precipitation kinetics to reach higher purity than solubility equilibria based purity of MAl(SO4)2 (M = Cs, Rb, K)

Alum is an important intermediate for the extraction and purification of Cs or Rb chemicals, due to its excellent separation ability for Cs, Rb and K. Although the solubility sequence K-alum > Rb-alum > Cs-alum has been well known, the separation factors among Cs, Rb and K during the formation of alum have not been determined. The formation of solid solution of alum controls the Cs or Rb content and Cs/Rb/K ratio in the sequentially precipitated Cs, Rb and K alums. In this study, the thermodynamic equilibrium separation factors were determined from the solid solution-aqueous solution equilibrium data of the two mixed alum systems: (i) RbAl(SO₄)₂, CsAl(SO₄)₂, and H₂O, and (ii) KAl(SO₄)₂, RbAl(SO₄)₂ and H₂O. The separation factor β_Cs/Rb varied from 48 to 2 with the increase of the Rb/Cs ratio in system (i), and β_Rb/K varied from 22 to 0.2 with the increase of the K/Rb ratio in system (ii). This suggests that the purification of Cs or Rb using alums is more efficient from raw materials containing high Cs/Rb or Rb/K ratio than that from low Cs/Rb or Rb/K ratio. In kinetic studies, the apparent separation factors β_Cs/Rb and β_Rb/K were highly affected by the recrystallization process of cooling or non-cooling and the initial composition of raw alum, showing asymmetric kinetic intensification. The separation factors β_Cs/Rb or β_Rb/K obtained in the “dissolution by heating and crystallization by cooling (DHCC)” process for the raw alum with high initial Cs/Rb or Rb/K molar ratio is several times larger than the equilibrium value expected from thermodynamics. Finally, CsAl(SO₄)₂ and RbAl(SO₄)₂ with a metal-based purity of 99.97 % and 99.94 %, respectively, were obtained by using the DHCC process 3–4 times.