LAUSR

Abstract The kinetics of Ti(IV)-extraction by Cyanex 302 (H 2 A 2 ) were investigated by measuring initial Ti(IV)-transfer flux, using a constant interfacial area stirring cell, operated at 3 Hz. The empirical flux equation, at 293 K, is: F = 10 −4.40 [Ti(IV)] (1 + 233[H + ]) −1 [H 2 A 2 ] (o) 0.5 (1 + 3.20[SO 4 2− ]) −1 (F in kmol/m 2  s and [ ] terms are in kmol/m 3 ). The activation energy, E a is measured to be (36–58) × 10 3  kJ/kmol depending on experimental conditions. The enthalpy change on activation, ΔS ± , is always highly negative. Analysis of the flux equation has been done, at various concentration regions of H + and SO 4 2− , to elucidate the mechanism of extraction. The rate-determining chemical reaction step, irrespective of extraction condition, appears as: TiO 2+  + A −  → TiOA + . This step occurs in the bulk aqueous phase via an S N 2 mechanism. The pseudo first order rate constant for the system is 7.5–55 times smaller than the maximum mass transfer coefficient attainable in a Lewis cell. This indicates that the process is chemically controlled or at least mixed controlled.