Abstract
Rare earths, e.g. neodymium (Nd), praseodymium (Pr) and dysprosium (Dy), are abundant in the rare earth sintered magnet scrap (Nd-Fe-B scrap), but their recycling is tedious and costly due to the high content of impurity Fe. Herein, a novel approach was developed to effectively recycle rare earths from the scrap via an integrated acid dissolution and hematite precipitation method. The scrap contained 63.4% Fe, 21.6% Nd, 8.1% Pr and 3.9% Dy. It was dissolved in nitric, hydrochloric and sulfuric acids, separately. Nearly all impurity Fe in the scrap was converted to Fe3+ in nitric acid but was converted to Fe2+ in hydrochloric and sulfuric acids. After hydrothermal treatment, the rare earths in the three acids were almost unchanged. From nitric acid, 77.6% of total Fe was removed, but total Fe was not from the hydrochloric and sulfuric acids. By adding glucose, the removal of total Fe was further increased to 99.7% in nitric acid, and 97% of rare earths remained. The major mechanism underlying total Fe removal in nitric acid was the hydrolysis of Fe3+ into hematite, which was promoted by the consumption of nitrate during glucose oxidation. This method effectively recycled rare metals from the waste Nd-Fe-B scrap and showed great potential for industrial application.
Highlights
Many approaches have been developed for rare earth recovery, which was generally initiated by dissolving the scrap in acids, such as sulfuric, hydrochloric and nitric acids
Nitric acid was optimal for dissolving the scrap, and most the impurity Fe in the scrap was converted into Fe3+
In the following hydrothermal treatment, 77.6% of the total Fe in nitric acid was removed as hematite particles
Summary
Many approaches have been developed for rare earth recovery, which was generally initiated by dissolving the scrap in acids, such as sulfuric, hydrochloric and nitric acids. When Fe3+ oxyhydroxide was converted to the well-crystallised Fe oxides, the two adjacent Fe-OH bonds on Fe3+ oxyhydroxide were dehydrated to form the Fe-O-Fe bond[18,19], and the average number of coordination sites on Fe3+ oxyhydroxide decreased[20,21], thereby subsequently reducing the precipitation of rare earths. He et al reported that 90.7% of Fe3+ was eliminated as hematite when the Fe3+/Zn2+-bearing sulfuric acid solution was hydrothermally treated at 210 °C for 2 h with the addition of H2O222. This is the first report on the effective removal of impurity Fe from a rare earth-bearing solution with high rare earth retention
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