3.5 Multi-stage countercurrent process

As shown in Fig. 14, a five-stage extraction and stripping process experiment was performed in separating funnels. 0.4 mol/L TMPD dissolved in CCl4 was used as organic phase, and filtered brine (presented in Table 1) as feed solution. The phase ratios (O/A) of the extraction and stripping sections were 1/1 and 2/1, respectively. The organic phase is recycled throughout the extraction process. The whole extraction process was performed 25 cycles until the extraction equilibrium became stable. Table 6 showed the main components of the stripping solution samples which were obtained after the system was stabilized. It can be seen that the main impurity ion in the stripping liquor is Mg2+, and the concentration is less than 20 mg/L, while the concentration of other impurity ions is very low. The boron-rich solution obtained can be used for the production of high-purity borax or boric acid products. The results indicated that TMPD is a highly selective extractant for boron extraction. The results of extraction and stripping efficiency are shown in Fig. 15. The results showed that the overall extraction efficiency of boric acid was 97.12%, and the stripping efficiency was 88.98%. It can be seen that the stripping efficiency is not ideal, because a slight emulsification occasionally occurs in the stripping section during the experiment. Although the phase separation is not affected, it has a negative effect on mass transfer, which makes the loaded boron organic phase cannot be stripped completely, resulting in a decreasing trend of extraction efficiency. The emulsification phenomenon did not occur in single-stage experiments, but appeared in whole extraction process experiments, which also illustrates the importance of multi-stage countercurrent experiments to verify the stability and feasibility of the process.