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)

Abstract

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.