Efficient recovery of fluorine from wet-process phosphoric acid using silicon powder as a new and eco-friendly reagent

The recovery and utilization of fluorine resources from wet-process phosphoric acid (WPA) is of great importance in protecting environment and providing cheap raw materials for the fluorine chemical industry. Evaporation concentration method is widely used due to its simplicity and low operating expenses. However, the lack of effective reagents limits the further development. In this study, silicon powder (SP) was firstly used to enhance the fluorine recovery during the concentration process of WPA. The gasification performance and behavior in reaction system (SP dosage, stirring speed, and concentration temperature) and absorptive system (initial mass of absorptive water) were investigated in detail. The results showed that SP had an excellent performance for fluorine recovery, which was superior to traditional reagents like diatomite, white carbon black (WCB) and zeolite powder (ZP). Under the optimal conditions, the fluorine recovery could reach an impressively high percentage of 81.13%, exhibiting a significant increase of 31.08% compared with blank concentration. Furthermore, SP could selectively and efficiently recover fluorine without affecting the quality of WPA. Spillover kinetics suggested that the rate of reaction between SP and fluorine was slow, with an apparent reaction order of 0.56. During the concentration process of WPA, SP showed a better tendency to react with H2SiF6 and HF. The addition of SP could increase the production of SiF4 gas and reduce the formation of fluorine precipitates. These two pathways are responsible for the improvement of fluorine recovery. Overall, this research provides a valuable reference for the efficient recovery of fluorine from WPA.