Kinetics of Forward Extraction of V(IV) in V(IV)-SO2- 4(H+, NA+)-Cyanex 302-Kerosene System Using Lewis Cell Technique

Abstract

Kinetics of V(IV) extraction from acidic sulfate medium by Cyanex 302 (2,4,4-trimethylpentylmonothio phosphinic acid, [H2A2](o)) dissolved in kerosene were investigated using a Lewis cell operated at 3 Hz and flux (F) method of data treatment. The F (kmol/m2s) is inversely proportional to (1 + 0.01 [V(IV)]−1), (1 + 2000 [H+]), and (1 + 0.089 ) and directly proportional to (1 + 0.2 []). The activation energy (E a ) depends on the temperature region and is a function of reactants (f(R)). When log f(R) <−1.0, E a > 48 kJ/mol and when log f(R)>−0.3, E a < 20 kJ/mol. Within log f(R) values of −1.0 to −0.3, E a varies within 20–50 kJ/mol. The rate constant (k) at 293 K is 10−7.335 kmol/m2 s. The entropy changes in activation (ΔS ‡) were measured as negative value always. The complicated empirical rate equation was analyzed to provide extraction mechanisms in different parametric conditions, being supported by E a values. In most cases, the extraction process is either diffusion controlled or mixed controlled; a mixed-controlled process can be converted to a diffusion-controlled process at a lower temperature region and to a chemical-controlled process at a higher temperature region. For systems with log f(R) <−1.0, the process is completely chemical controlled. The rate-determining chemical reaction steps in order are: → ; and → at lower and higher concentration regions of . The negative ΔS ‡ values suggest that the rate-determining chemical reaction steps occur by SN2 mechanisms.