ROLE OF THE EXTRACTION EQUILIBRIUM-CONSTANT IN THE COUNTERCURRENT MULTISTAGE SOLVENT EXTRACTION-STRIPPING PROCESS FOR METAL-IONS

Computer simulation of a steady-state countercurrent multistage metal solvent extraction-stripping process (ESP) using cation-exchange reage nts shows that there is a value of extraction equilibrium constant, K, yielding the maximum metal recovery when the other operational parame ters are constant. Qualitatively, this is because the larger K makes s tripping more difficult. Steady-state local linearization reveals the symbolic relation behind the numerical results. The coefficient of d(l n K) in the differential equilibrium relation extended to the counterc urrent multistage process is the weighted average of the partial deriv ative, in each stage, of the equilibrium organic-metal molarity with r espect to In K under constant equilibrium aqueous-metal molarity, wher e the weight is related to the partial derivative of the equilibrium o rganic-metal molarity with respect to the equilibrium aqueous-metal mo larity under constant K. The balance of these coefficient values for t he extraction and stripping sections of ESP determines the trend of th e recovery with varying K: at the maximum recovery, these values are e qual to each other. The stagewise plot of the partial derivative of th e equilibrium organic-metal molarity with respect to In K under consta nt equilibrium aqueous-metal molarity versus organic-metal loading rat io clarifies the relation between the trend of the recovery with varyi ng K and the organic-metal loading.