Liquid – liquid equilibria of quaternary system phosphoric acid/sulfuric acid/water/tri-n-butyl phosphate and the formation of extraction complex at 303.2 K

• Obtain the liquid – liquid equilibrium data of quaternary system H3PO4–H2SO4–H2O-TBP.
• Hand, Othmer – Tobias and Bachman equations are used to test reliability of data.
• The extraction complex of phosphoric acid and TBP molecules is analyzed.

Nowadays, with the increasing scarcity of phosphate ore, many researchers are focused on the purification of wet – process phosphoric acid, which is leaching with strong acids. It produces mixtures of phosphoric acid and sulfuric acid (or nitric acid, or hydrochloric acid), which requires a separation process of mixed acids. This paper conducts an experiment about liquid – liquid equilibrium data of quaternary system H3PO4 – H2SO4 – H2O – TBP (tri – n – butyl phosphate). It determines mass percents of phosphoric acid in the two equilibria phases, and analyzes density, viscosity of two equilibrium phases. Equilibria data obtained are correlated by the Hand equation, Othmer – Tobias equation and Bachman equation, which all show good linear fitting. In addition, the formations of extraction complexes are analyzed with Pitzer's model and Sergievskii-Dannus's model, which calculates the thermodynamic equilibrium constant of extraction process.