Equilibrium of the Extraction of V(IV) in the V(IV)-SO₄^2-(H⁺, Na⁺)—Cyanex 302-Kerosene System

Abstract

The title system has been investigated from the equilibrium point of view. Significant extraction occurs above pH 2. Equilibration time is 20 min. The extraction ratio (D) remains constant with increasing [V(IV)] of at least 0.50 g/L. It is inversely proportional to [H + ] 2 , [H + ] and [H + ] 0.3 in the lower pH (<2.25), medium pH (~2.90) and higher pH (~4.0) regions, respectively. Moreover, it is proportional to [Cyanex 302] 2 ; and [SO 4 2- ] 0 and [SO 4 2- ] -1 in the lower [SO 4 2- ] (<0.05 mol/L) and higher [SO 4 2- ] (>1 mol/L) regions, respectively. The apparent extraction equilibrium constant (K ex ) in 0.02 mol/L SO 4 2- medium and at 303 K is found to vary from 10 -3.447 to 10 1.508 with increasing equilibrium pH from 2.25 to 4.00. Various sulphated, hydrolyzed, hydrated and mixed sulphated hydrolyzed species of V(IV) have been considered at different extraction conditions to propose the extraction equilibrium reactions to form always [VO(HA 2 ) 2 ] as the extractable species. The system is highly temperature dependent with ?H value of ~90 kJ/mol and ~25 kJ/mol in lower and higher temperature regions, respectively. The calculated loading capacity is low (4.05 g V(IV)/100 g Cyanex 302). Kerosene is a better diluent over CHCl 3 , Cyclo-C 6 H 12 and CCl 4 ; but much better solvents are C 6 H 6 , C 6 H 5 CH 3 , n-C 7 H 16 ,C 6 H 4 (CH 3 ) 2 , petroleum benzin, 1,2-C 2 H 4 Cl 2 , C 6 H 5 Cl. Mineral acids (1 mol/L) are able to strip off V(IV) from the organic phase in a single-stage. Using Cyanex 302, almost complete separations of V(IV) from Cu(II) at pH 1.0 and from Ni(II) at pH (eq) 4.5 are possible in a single-stage of extraction; whereas, its separation from Zn(II) at pH (eq) 2.5, Co(II) at pH (eq) 3.5, Fe(III) at pH (eq) 2.0 and Ti(IV) at pH (eq) 2.5 will require counter-current multi-stage extractions.