Kinetic study on selective extraction of HCl and H₃PO₄ in a microfluidic device

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 time were investigated. We found that fast extraction kinetics could be realized in microfluidic devices, and that HCl could be extracted faster than H₃PO₄ due to smaller mass 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 TOA was in excess of H₃PO₄ in the initial feeds, and in contrast, always increased when H₃PO₄ was in excess of TOA in the initial feeds. The diverse changes of selectivity for HCl along with the residence time indicate that a dynamic control of selectivity in microfluidic devices may be important and accessible for improving the KH₂PO₄ conversion efficiency in extraction method.