Kinetic study of cobalt extraction from bis(2,2,4-trimethylpentyl) phosphinic acid.

Abstract

A modified Lewis cell was used to study the kinetics of the extraction of cobalt from sulfate solution into bis(2,4,4-trimethylpentyl) phosphinic acid (HDTMPP), commercially known as Cyanex 272. The pH was controlled with an auto-titration technique instead of using the traditional buffer solution which could affect or change the extraction mechanism. A "gravity leg" was used to control the position of the liquid-liquid interface level in the cell. A study of the extraction of cobalt into HDTMPP was carried out to determine the effect of the following variables on the extraction, namely: (i) aqueous phase cobalt concentration, (ii) aqueous phase ionic strength, (iii) interfacial area, (iv) extractant concentration, (v) modifier concentration in the organic phase, (vi) pH, (vii) hydrodynamic conditions in both phases and (viii) temperature. It was found that the mechanism of the extraction process was more accurately described using activities rather than concentrations. Both the aqueous phase cobalt activities and concentrations were calculated at high concentrations, up to 0.50 kmol m$\sp{-3}.$ The extraction resistance was found to arise from (1) the aqueous phase boundary layer, (2) the interface and (3) the organic phase boundary layer. These three resistances were observed to be independent of each other. The interfacial resistance was dependent on pH. The boundary layer resistances were dependent on the hydrodynamic conditions of the aqueous and the organic phases. The thickness of the aqueous phase boundary layer was estimated. The apparent activation energy of this extraction process was found to be 39.4 kJ mol$\sp{-1}.$ The extraction rate was found to be a function of the pH and depended on: the aqueous phase cobalt concentration, the extractant concentration and the hydrodynamic conditions in both the aqueous and the organic phases. A model describing the extraction process was found to be:$$\rm J={A\times a\sb{Co}\times a\sb{H\sb2A\sb2}\over3.69\times10\sp6Re\sb{aq}\sp{-1/2}+4.89 \times10\sp5Re\sb{org}\sp{-1/2}+1.52\times10\sp9a\sb{H}}.$$