Utilization of methyltrichlorosilane as a novel and efficient reagent to enhance fluorine recovery from wet-process phosphoric acid

Abstract

Recovering fluorine from wet-process phosphoric acid (WPA) by the evaporation concentration method has been widely used due to its simplicity and cheapness. However, the low fluorine recovery reduces the WPA quality, which in turn has detrimental effects on fertilizer production and leads to a waste of valuable fluorine resources. In this work, a cost-effective organic liquid, methyltrichlorosilane (MTS), was first employed as the gasification reagent in the WPA concentration process to promote fluorine recovery. The effects of several important parameters including MTS dosage, initial mass of absorptive water, addition stages, stirring speed, and concentration temperature, were investigated. The first three factors emerged as pivotal determinants of the fluorine recovery and the results revealed a remarkable 87.94% recovery of fluorine from WPA, signifying a substantial enhancement compared to the recovery achieved in the absence of MTS, which amounted to 40.36%. The defluorination kinetics data was well-fitted with a second-order model, and showed an activation energy of 23.77 kJ/mol, highlighting the strong interaction between MTS and fluorine in WPA. In addition, liquid MTS could recover fluorine selectively and efficiently without compromising the quality of WPA and exhibited superior fluorine recovery when compared to traditional solid mineral reagents such as diatomite and white carbon black. Further analysis revealed the governing mechanism for fluorine recovery that the addition of MTS prevented the conversion of liquid HF and H2SiF6 to solid Na2SiF6 and K2SiF6, favoring SiF4 gas production. Overall, this research provides valuable insights for enhancing fluorine recovery during the WPA concentration process and developing environmentally friendly reagents.