Roles of Extractant Concentration and Flow Rate of Organic Phase in Countercurrent Multistage Metal Solvent Extraction-Stripping Process for Metal Ions.

Abstract

The effects of the extractant concentration, CHAo, and the flow rate of the organic phase, S, on the metal recovery in a steady-state countercurrent multi-stage metal solvent extraction-stripping process (ESP) using cation-exchange reagents have been assessed by computer simulation. The results show that, with increasing CHAo or S, (i) the recovery fraction monotonically increases when the number of stages in the extraction or stripping section (N or N′, respectively) is unity and (ii) the recovery fraction first increases, then reaches a maximum, and thereafter very slowly decreases when N and N′ are larger than unity. The optimum combination of CHAo and S will be determined by using the equi-recovery-fraction and equi-operating-cost curves. In order to obtain in-depth understanding of the simulation results, steady-state local linearization (SLL) analysis which theoretically considers the infinitesimal variations in the metal concentrations in each stage caused by the infinitesimal variation in the operational parameters has been done. As a result, it is proved that the balance between the quantities ψ (CHAo) of the extraction and stripping sections determines the trend of the recovery fraction with CHAo, where ψ (CHAo) is the partial derivative of the metal molarity in the organic phase at the outlet of the extraction or stripping section with respect to CHAo: At the maximum recovery fraction, these values are equal to each other. Similar results are obtained also in the effect of S. Furthermore, it is proved that, with increasing CHAo or S, (i) the maximum of the recovery fraction never appears when N or N′ is unity and (ii) the decrease in the recovery fraction after reaching the maximum is much slower than the increase before reaching the maximum when N and N′ are larger than unity.