Molecular thermodynamic model for solvent extraction of mineral acids by tri-n-butyl phosphate (TBP)

Removal or recovery of acids by solvent extraction is highly relevant for recycling, process control, and wastewater decontamination, especially in the hydrometallurgical industry. The construction and optimization of such processes would benefit from a molecular thermodynamic model that can predict liquid–liquid equilibria. Past attempts resulted in models that were not very predictive. Therefore, a new approach was followed and demonstrated for the extraction of the mineral acids (inorganic acids) HNO3, HCl, H2SO4, H3PO4, and H3AsO4 by tri–n–butyl phosphate (TBP). The semi-empirical OLI Mixed-Solvent Electrolyte (MSE) framework was used to construct the thermodynamic model for calculating the liquid–liquid equilibria. Contrary to previous attempts, this framework allows the description of both the aqueous and organic phases in one model, and it can accurately deal with the non-ideal behavior of concentrated electrolyte solutions. The best agreement between calculated and experimental distribution data was achieved by assuming that the extraction of mineral acids occurs via protonation of TBP and coextraction of the anions. At very high acid concentrations, also neutral acid molecules are transferred to the organic phase. The high accuracy of the thermodynamic model for all mineral acid systems considered in this study is an indication that this approach for modeling liquid–liquid equilibria is a universal one. Furthermore, the extraction of mineral acids can be predicted in mixed-acid systems and acid-salt systems that were not used to construct the model.