Liquid extraction of metals from sulphate solutions by alkylphosphoric acids. II. Kinetics of the extraction of copper and cobalt with di‐(2‐ethyl hexyl) phosphoric acid

Kinetic studies of the transfer of copper and cobalt (separately) between aqueous solutions of their sulphates and organic phases consisting of solutions of di(2‐ethyl hexyl) phosphoric acid (EHPA) in kerosene were made by an ‘equilibrium extraction’ technique, in which radioactive isotopes of the metals were transferred between phases which were otherwise in equilibrium. The total resistance to mass transfer was found to be greater than that predicted from the mass‐transfer resistances of the aqueous and organic phases, the additional resistance to mass transfer being between 30 and 75% of the total resistance. When the concentration of free EHPA in the organic phase was greater than 0·01 g mole/l (which is approximately equivalent to a ratio of the metal concentration in the organic phase to the initial organic‐phase EHPA concentration of less than 0·25), expressions for these additional resistances were obtained from consideration of the kinetics of the extraction reaction, with a rate‐controlling step equivalent to the reaction of a metal ion with an EHPA dimer to form a metal‐EHPA compound and a hydrogen ion. At lower organic‐phase‐free EHPA concentrations, the additional resistances were lower and did not show any marked dependence on pH or reactant concentrations.