2.3 Multi-stage countercurrent extraction procedures

The three-stage countercurrent extraction as shown in Figure 1 was performed using a separatory funnels at a laboratory scale[26]. 0.4 mol/L TMPD dissolved in CCl4 and the SL brine without acidification (pH=4.60) was used as organic phase and feed brine, respectively. Firstly, the prepared fresh organic (20 mL) and feed brine (20 mL) were added to the second funnel. After the mixture reached a balance, the phase separation was set. Then, the aqueous phase (A2) was poured into the first funnel and the organic phase (O2) into the third funnel. Equal volumes of the fresh organic and brine were then added to the first and the third stage for the mixture and separation, respectively. Then, the organic phase (O1) and aqueous phase (A3) were simultaneously poured into the second funnel, and both the raffinate and the loaded organic samples were obtained. In this way, the whole countercurrent extraction process could be recycled by introducing the feed brine and organic intermittently. After a series of extraction and product removal cycles, the system stabilized. The whole five-stage extraction and stripping process experiment (section 3.10) was carried out in the same way.