Abstract: In the extraction method for preparing KH₂PO₄, one of the key processes is the selective extraction of HCl over H₃PO₄. In our work, extraction kinetic studies have been carried out in a microfluidic device with a coaxial microchannel, using the extractant of 33.3% (by volume) trioctylamine (TOA) dissolved in n-octanol,with different aqueous phases: the HCl solution, the H₃PO₄ solution, and H₃PO₄ and KCl solutions of different concentrations. The changes of the extraction efficiency of HCl and H₃PO₄ and the selectivity for HCl along with the residence timewere investigated.Wefound that fast extraction kinetics could be realized in microfluidic devices, and that HCl could be extracted faster than H₃PO₄ due to smallermass transfer resistance and much stronger reaction between HCl and TOA. For the extraction of H₃PO₄ and KCl solutions, the selectivity for HCl first increased and then decreased when TOAwas in excess of H₃PO₄ in the initial feeds, and in contrast, always increasedwhen H₃PO₄was in excess of TOA in the initial feeds. The diverse changes of selectivity for HCl alongwith the residence timeindicate that a dynamic control of selectivity in microfluidic devices may be important and accessible for improving the KH₂PO₄ conversion efficiency in extraction method.

Key words: Kinetics, Selective extraction, Microchannel, TOA, HCl, H₃PO₄