S K Sahu

Recovery of rare earths from spent NdFeB magnets of wind turbine: Leaching and kinetic aspects

Increasing demands of rare earth (RE) metals for advanced technological applications coupled with the scarcity of primary resources have led to the development of processes to treat secondary resources like scraps or end of life products that are often rich in such metals. Spent NdFeB magnet may serve as a potential source of rare earths containing around ∼30% of neodymium and other rare earths. In the present investigation, a pyro-hydrometallurgical process has been developed to recover rare earth elements (Nd, Pr and Dy) from the spent wind turbine magnet. The spent magnet is demagnetized and roasted at 1123 K to convert rare earths and iron to their respective oxides. Roasting of the magnet not only provides selectivity, but enhances the leaching efficiency also. The leaching of the roasted sample with 0.5 M hydrochloric acid at 368 K, 100 g/L pulp density and 500 rpm for 300 min selectively recovers the rare earth elements almost quantitatively leaving iron oxide in the residue. Leaching of rare earth elements with hydrochloric acid follows the mixed controlled kinetic model with activation energy (Ea) of 30.1 kJ/mol in the temperature range 348-368 K. The leaching mechanism is further established by characterizing the leach residues obtained at different time intervals by scanning electron microscopy- energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Individual rare earth elements from the leach solution containing 16.8 g/L of Nd, 3.8 g/L Pr, 0.28 g/L of Dy and other minor impurity elements could be separated by solvent extraction. However, mixed rare earth oxide of 99% purity was produced by oxalate precipitation followed by roasting. The leach residue comprising of pure hematite has a potential to be used as pigment or can find other applications.

Arsenic Removal from Spent Liquor Generated during Processing of Vanadium Sludge

During production of ammonium meta-vanadate from vanadium sludge an arsenic containing (~2.2g/L) spent liquor is generated. Safe disposal of such liquor without arsenic contamination to the environment is essential. Therefore, various methods such as ion-exchange, adsorption, solvent extraction and precipitation were applied for recovery of arsenic from the spent liquor. Based on the Eh-pH diagram of As-H2O, the arsenic based species were delineated. For ion exchange Lewatit FO 36 and Amberlite IRA 400 Cl- resins were used to extract arsenic from the spent liquor. With Lewatit FO 36 a maximum of 20% arsenic was recovered at A/R ratio of 50. Amberlite IRA 400 Cl- on the other hand didnt extract arsenic at all. A two stage solvent extraction process using TBP as an extractant, with a recovery of 97% arsenic from the spent liquor was developed. From the loaded TBP almost complete stripping of arsenic was obtained with 0.1-0.5M NaOH solution. Precipitation of arsenic from the spent liquor as copper arsenate or iron arsenate was also examined. During the fixation of arsenic with iron, 70% arsenic was recovered in the precipitate. This process needs to be evaluated further with the aim of using the iron hydroxide containing arsenic for different applications.