Removal of fluoride from waste acid using lanthanum chloride: Defluoridation behavior and reaction kinetics of recovery process

Waste acid would bring serious fluoride pollution. In this study, a fluoride removal method for waste acid using lanthanum chloride (LaCl3) was proposed. This process consists of three parts, removal of fluoride, collection of lanthanum, and transformation of lanthanum fluoride (LaF3). The E-pH diagram of the La-F-H2O system was constructed to learn about the composition variation of the system with pH. The removal efficiency of fluoride was 98.03%, and F− was reduced to 20 mg/L under the optimal conditions of the n(LaCl3):n(F−) of 1:1, the temperature of 80 °C, and the reaction time of 8 h. The residual lanthanum in the solution was collected as sodium lanthanum sulfate (NaLa(SO4)2) precipitate by sodium sulfate (Na2SO4), and the collection efficiency of lanthanum was 98.05%. The defluoridation product LaF3 was transformed to lanthanum hydroxide (La(OH)3) by sodium hydroxide (NaOH) solution, and the transformation efficiency was 93.44%. The adsorption phenomenon of La(OH)3 to F− was found in the transformation process, which is not conducive to the recovery of La. The kinetics of the transformation process of LaF3 to La(OH)3 shows that the rate-controlling step is the diffusion of the solid product layer, and the average apparent activation energy is 37.99 kJ/mol at 25–100 °C.