Reaction mechanism of fluoride conversion into BF_4~- during sulphuric acid leaching of roasted bastnaesite

The bastnaesite used in hydrometallurgy usually contains 7%-11% fluorine,and the conversion of fluorine into high-value products is the key to achieving green production of rare earths and improving the comprehensive utilization of bastnaesite.In order to recover fluorine from bastnaesite in the form of KBF4,the mechanism of F~-conversion to BF_4~-in sulphuric acid leaching solution of roasted bastnaesite was studied by using E_h-pH diagram and simulation experiment.It shows that the formation of BF_4~-is affected by pH in the absence of rare earths.BF_4~-is hydrolyzed to BF_3 OH~-and F~-when the pH is greater than 3.9,and part of F~-exists as HF_2~-when pH is lower than 2.In the presence of La~(3+),the formation of BF_4~-is mainly affected by LaF3 when pH is greater than 0,and in the case that the pH is lower than 2,it is mainly affected by HF_2~-.When Ce4+is present in solution,CeF_2~(2+) can exist stably in sulphuric acid solution.Bringing down the pH can reduce the stability of CeF_2~(2+) and increase the BF_4~-conversion rate.Based on these results,KBF_4 was prepared in the alkaline solution of bastnaesite,and the conversion of BF_4~-is 84.31%.This provides a theoretical basis for the recovery of fluorine from bastnaesite in the form of KBF_4.