Ranjit Kumar Biswas

The titled system has been investigated from the modelling point of view. The equilibration time for the system is 45 min, and 5% (v/v) heptane-1-ol in the organic phase is used as a de-emulsifier. The factors affecting the extent of extractions are [Ti(IV)], pH(eq), [Cyanex 301], [SO42-] and temperature (T). The selected levels are high (+) and low (-) for these factors in the present investigation. Model equation for the extraction of Ti(IV) by Cyanex 301 is determined from 25 full factorial design. However, the success of the factorial design depends on the linear relationship between yield and factor. Plots of log CD vs log [Ti(IV)], pH(eq), log [Cyanex 301] and log [SO42-] are curves. Logistic functions involving these factors are considered in designing. While [Ti(IV)] < 1.00 g/L and [HA](o) > 0.10 mol/L, considered logistic functions viz. -log(1+316.2 ([Ti(IV)], mol/L)), -log (10-pH + 229 ´10-2pH), log ([HA](o), mol/L), -log (1+0.79 ([SO42-], mol/L)) and absolute temperature are abbreviated as M, P, E, S and T, respectively. Model is log CD = 5.847 + 0.964 M + 0.909 P + 2 E + 0.995 S – (1437.5/T). The experimental model illustrates that there is no interaction effect between the factors under investigation.

The titled system has been investigated using a single drop technique and flux (F) method of data treatment. In calculating fluxes, the time for drop formation and coalescence (?t = 0.75 s) is considered. At lower concentration region of vanadium, V(V), the rate of V(V) transfer is directly proportional to its concentration. With the increase in [V(V)], slowly extractable V(V) species are formed gradually resulting in decrease in rate. Rate is directly proportional to the square root of [TOA]. It is reported that the most favorable extractable V(V) species exists in maximum amount at pH 2, and so, log F vs pH plots show a maximum at pH~2. The flux of V(V) transfer in most cases is found to be decreased with increasing [NO3-]. Reaction orders with respect to [V(V)], pH and [NO3-] have been attempted to be estimated, but in vein due to non-linearity of plots in most cases. Activation energies for the present system is ~ -12 kJ/mol. This is unexpected but explained. Due to irregular variation of the composition of extractable species with [V(V)], [H+]; as well as temperature; it is not possible to elucidate the value of rate constant (k) and mechanism in simple way.

The kinetics of Ti(IV) extraction by Cyanex 301 (HA) were investigated by measuring initial flux of Ti(IV) transfer (F, kmol/m²s), using a Lewis cell, operated at 3 Hz. The empirical flux equation at 298 K is found to be as follows: F (kmol/m²s) = 10^–4.288 [Ti(IV)] (1 + 447 [H⁺])^–1 [HA]_(o) (1 + 1.18[SO₄^2–])^–1. The activation energy, E_a, has been measured to be within 37–60 kJ/mol, depending on experimental parameters and temperature region. The ΔS^± value is always highly negative. Analysis of the flux equation has been done, given various parametric conditions, to elucidate the mechanism of extraction. The rate-determining chemical reaction step, in most parametric conditions, appears to be as follows: TiO²⁺ + A^– → TiOA⁺; and this step occurs via an S_N2 mechanism as suggested by high negative ΔS^± values. However, in certain cases, the extraction process appears as intermediate controlled as supported by E_a value of less than ~48 kJ/mol.

The rate of acidic sulfate-acetato solution stripping of Mn²⁺ from kerosene solution of Mn²⁺-Cyanex 272 complex (MnA₂) is investigated using the falling single drop technique. To study the kinetics of this process, the reaction orders and the value of backward extraction rate constant (k_b) have been determined to get the Mn²⁺-transfer flux equation in backward extraction as: F_b = 10^−4.88 [MnA₂]_(o) (1 + 0.002 [H⁺]⁻¹)⁻¹ (1 + 5.13 ). The energy of activation (E_a), entropy variation on activation (ΔS^‡), and the enthalpy variation on activation (ΔH^‡) have also been determined. It is noticed that the reaction orders with respect to [H⁺] and , and the values of E_a, ΔS^‡, and ΔH^‡ depend on the concentration regions of H⁺ and used in stripping. The analysis of flux equation at low-concentration region of H⁺ and points out that the dissociation of A⁻ from MnA⁺ is a rate-controlling chemical reaction step. On the other hand, at high-concentration regions of H⁺ and , the rate determining chemical reaction step is the replacement of A⁻ in MnA⁺ by . In other conditions, the process is either diffusion or intermediate control. High negative ΔS^‡ values indicate that the rate controlling chemical reaction steps occur via S_N2 mechanisms. Rate data have been compared with the equilibrium data for the Mn²⁺-Cyanex 272 extraction system.