Solvent Extraction of Mn(II), Ni(II) and Zn(II) by Cyanex 272: Equilibrium and Kinetic Studies

Abstract

The solvent extraction systems: Mn(II)-SO, -Ac Cyanex 272-kerosene, Ni(II)- SO4-Ac Cyanex 272-kerosene and Zn(II)-SO4 Cyanex 272-kerosene have been thoroughly investigated from equilibrium and kinetic point of views. From the dependence of extraction ratio (at constant equilibrium values of other parameters) on various parametric concentrations, the equations for extraction ratios have been derived at 303 K as: log Dmn=-6.17+2 pHm) + log [H2Azam)-log (1 + 1.9 [SO"]) log Di-11.16+2 pH)+ log [H2A2)-log (1+ 6.92 [SO."])-log [Ac"]; at [H2A2]) <0.05 kmol/m log DN-11.56+2 pH)+3 log [H2A2km)-log (1+6.92 [SO, D)-log [Ac]; at [H2A2 >0.10 kmol/m log Dzn --3.11+2 pH)+ log [H2A2])-log (1+2 [SO,"]); at [H2A2] (en) <0.05 kmol/m3 log "Dzn -2.08 +2 pH() +2 log [H2A2](o)-log (1+2 [SO"]); at [H2A2]()>0.10 kmol/m where, the first terms on the right hand sides represent the logarithmic values of extraction equilibrium constants (Kex). The equilibrium extraction reactions in low concentration regions of extractant are suggested as M(II) + H2A2(0) [MA2](0) + 2 H; but disolvated and monosolvated species are extracted in the cases of Ni(II) and Zn(II), respectively, at high concentration regions of Cyanex 272 (H2A2). The extraction processes are found to be endothermic. The maximum loading capacities of the extractant are found to be 9.52 g Mn(II), 21.28 g Ni(II) and 11.5 g Zn(II) per 100 g extractant. The extracted species are strippable by dilute H2SO4, HNO3 and HCl solutions. In kinetics of forward extractions of the selected divalent metal ions by Cyanex 272, the rates have been measured the single falling drop technique at various extraction parametric concentrations in order to determine the orders of reaction with respect to various concentration terms and also to evaluate the log kr values. Rates have measured at various temperatures to determine E., AH* and AS* values at various parametric conditions for all systems under consideration. The rates of forward extractions of Mn(II), Ni(II) and Zn(II) at 303 K can be expressed respectively as: log F-3.6+ log [Mn(II)] +0.5 log [H2A2]()-log (1+ 105 [H])-log (1+1.58[SO"]) log Fr=-3.7+ log [Ni(11)] +0.5 log [H2A2](o)-log (1+ 10635 [H])-log (1+6.3[SO, ])-log (1 +0.55 [Ac"] log Fr=-8.4 + log [Zn(II)-log [H])+0.5 log [H2A2])+ log (1+ 1.07 [H2A]()) where, Fr represents flux of metal transfer from one phase to another and defined as rate per unit interfacial area. From the rate equations, the mechanisms of extractions are given. Invariably in all cases under investigation, the attachment of the first monomeric anion of the extractant (A') to the metal ion is the rate controlling (M2++ A slow [MA]'); which has been supported by high activation energy (>48 kJ/mol). However, in certain parametric conditions diffusions rather the chemical reaction stated become rate controlling, which is supported by low activation energy. In case of Ni(II) extraction, at high concentration region of extractant, the reaction: Ni2++ HA2 (int) → [NIHA2] becomes rate controlling. The highly negative AS* values in all cases suggest the chemical rate determining step occurs via SNo2 mechanism. The kinetics of stripping of metal ions from highly metal ion loaded organic phases by sulphuric acid solutions have been investigated by the single falling drop technique to derive respective stripping rate equations at 303 K. The stripping rate equations derived for stripping of Mn(II), Ni(II) and Zn(II), respectively, are: log F1 = -4.88 + log [MnA2])- 0.5 log (1+0.002 [H]') + log (1 +5.129 [SO,")) log F-4.35+ log [Ni-H2A2 complex])-log (1 + 1042 [H])-log ([H2A2])+2.5 [H2A2])+ log (1+6 [SO, ])+ log (1 + 3.2 [Ac']) log F-5.24+ log [Zn-HA, complex]) + log [H]-0.5 log [H2A2]) + log (1 +1.5 log [SO."]) The rate equations have been analyzed to give stripping mechanisms. It is found that the dissociation of second anion ligand from [MA2](o) is rate determining which occurs in the bulk aqueous phase (MA slow M2++ A), which is, supported by high Eq. values. This mechanism is valid for Mn(II) and Zn(II) covering all concentration region of free extractant in the organic phase; and also for Ni(II) in low concentration region of extractant, but at its high concentration region, the dissociation of dimeric anion (HA2) from [Ni(HA2)2. H2A2] (0) appears as rate determining. However, low E, value suggests this step occurs via SN2 mechanism. The extraction equilibrium constants (Ke) for Mn(II) and Zn(II) derived from equilibrium studies are matchable to those from respective kinetic studies (Kex = kr/k). But in the case of Ni(II), a deviation by a factor of 10 is obtained, which may be attributed to the loss buffer action resulting the change of interfacial pH. The possibilities of separations of the metal ions under consideration from their mixtures by Cyanex 272 have been theoretically evaluated and it has been shown that the mutual almost complete separation of this metal ion by single or by at least two stage extractions by Cyanex 272 solution in kerosene is possible.