Kinetics of backward extraction of Mn(II) from Mn-D2EHP complex in kerosene to hydrochloric acid medium using single drop technique

• Published in: Hydrometallurgy 1997-10, Vol.46 (3), p.349-362
• Main Authors: Biswas, R.K., Habib, M.A., Bari, M.F.
• Format: Article
• Language: English
• ISSN: 0304-386X; 1879-1158
• DOI: 10.1016/S0304-386X(97)00030-3

The extraction equilibrium constant, K ex for the extraction of Mn(II) from hydrochloric acid medium by di-2-ethylhexylphosphoric acid (D2EHPA or H 2A 2) in kerosene (mostly aliphatic) has been determined to be 10 −2.16. The rate of backward extraction of Mn(II) from Mn-D2EHP complex dissolved in kerosene by hydrochloric acid has been measured by the single rising drop technique and it is found that the flux equation at 30 ± 1°C is: J (kmol/m 2s) = 10 −4.08[Mn-D2EHP] (o)(1 + 0.011[H +] −1) −1(1 + [H 2A 2] (o) 0.5) −1. Analysis of this rate equation suggests that the process is almost chemically controlled at lower aqueous acidity and higher free D2EHPA concentration regions; whereas it is almost diffusion controlled at higher aqueous acidity and lower free D2EHPA concentration regions. In the investigated D2EHPA concentration (0.04-0.50 kmol/m 3) and aqueous acidity (pH = 1.0–3.0) regions, the process is intermediate controlled. The activation energy depends on the back-extraction parameters and is of the order of ∼ 20–∼ 40 kJ/mol. This low value of activation energy supports the conclusion that the process is intermediate controlled. Temperature dependence data give ΔH ± and ΔS ± values of 20–40 kJ/mol and (−180)–(−250) J/K mol, respectively. The high negative value of ΔS ± suggests that the chemical controlling step occurs via S N2 mechanism.