Extraction of Lithium Using TBP and the Noncoordinating Cation Exchanger Tetraphenylborate: Principles of Selectivity from Sodium and Higher-Valent Cations

Extraction of lithium, calcium, and some other metal cations using tetraphenylborate/tributyl phosphate mixtures has been studied. The possibility of a qualitative change in the cation exchange extraction series towards the preferential recovery of lowly-charged (singly-charged) metal cations was shown for these systems. A change in the separation factors for singly- over multiply-charged cations in this system, compared to the known commercially available extractants such as alkylphosphoric, carboxylic, and other organic acids, is several orders of magnitude and increases when passing from doubly- to triply-charged metal cations, reaching, for example, values of 10(3) and more for the lithium/yttrium couple. It was assumed that the change in the extraction series was caused by the instability of the salts of the multiply-charged metal cations compared to that for the singly-charged cations due to the ionic character of the bond with the extracted metals and structural problems caused by the large volume of the tetraphenylborate anion. The investigated system can serve as a model for the development of a new class of extractants having a higher selectivity for singly-charged metal cations, in particular, for lithium.