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.