Mechanism and experimental study on the recovery of rare earth elements from neodymium iron boron waste by NaBF4 fluorination method

Abstract

Pyrometallurgical recovery of rare-earth elements from NdFeB wastes is affected by the quality of the raw materials, and mixed rare-earth products add little value. Therefore, this study investigates NaBF₄ fluoride roasting for the recovery of rare-earth elements and its underlying mechanisms. Reasonable roasting temperatures were determined based on thermodynamic calculations, and single-factor experiments were conducted. When roasted at 600 °C for 30 min with 65 % NaBF₄, the fluorination rate of rare-earth elements reached 95.83 %. The composition of the clinker mesophase after roasting under different conditions was analyzed. At high temperatures, oxygen heteroatoms entered the crystal lattice of the rare-earth fluorides, which were subsequently removed during acid leaching, thereby reducing the recovery rate. A three-factor, three-level Box–Behnken test was conducted using the response surface methodology to analyze the effects of various factors and their interactions on the fluorination rate. Thus, an experimental basis for the recovery of rare-earth elements from NdFeB by fluoridation roasting was established. The roasting temperature had the greatest effect on the fluorination rate, followed by the roasting time and amount of NaBF₄. The model predicted an optimal fluorination rate of 98.63 % when the material was roasted at 573 °C for 25 min with 59 % NaBF₄. The clinker was acid-leached under optimal conditions (2.5 h at 80 °C with 9 M HCl and a liquid–solid ratio of 4 mL/g), and the Purity of fluorinated rare earths reached 99.39%. These results provide theoretical and experimental support for the application of pyrometallurgy in the recovery of rare-earth elements from NdFeB wastes.