Electrochemical treatment of spent NdFeB magnet in organic acid for recovery of rare earths and other metal values

Abstract

The spent Neodymium-Iron–Boron (NdFeB) magnet is potential secondary resource of rare earth elements. Conventional hydrometallurgical processing of spent NdFeB magnet requires energy intensive steps viz. crushing, grinding, roasting etc. and is associated with the discharge of acidic effluents in the environment. The chemical dissolution of waste magnet using biodegradable organic acids is also associated with the major drawback of poor leaching efficiency. The present study is focused on the electrochemical dissolution of spent NdFeB magnet in organic acid for the recovery of rare earths and other metal values. The electrochemical dissolution studies were carried out using citric acid as an electrolyte. The dissolution of spent NdFeB magnet in citric acid was studied with and without electrochemical effect. It was found that the dissolution of NdFeB magnet in citric acid enhanced significantly under electrochemical effect as compared to chemical dissolution. The reaction mechanism of electrochemical dissolution was also determined. The effect of various parameters such as citric acid concentration, current density, stirring speed, bath temperature etc. were studied for electrochemical dissolution of spent NdFeB magnet. The anodic dissolution efficiency and energy consumption were also evaluated. A solvent extraction method using 1M di-(2-ethylhexyl) phosphoric acid (D2EHPA) as an extractant was developed to extract rare earths selectively and quantitatively from the electrolytic liquor. From the loaded organic, mixed oxalates of rare earths were recovered quantitatively by precipitation stripping using oxalic acid solution as stripping agent. Mixed oxides of neodymium, praseodymium and dysprosium of purity 99.9% were obtained by calcination at 1073 K. Iron oxide (98.6% pure) was also produced as by-product of the process. The solid products were characterized by chemical analysis, XRD, SEM and TG-DTA. Enhanced dissolution of metals in citric acid under electrochemical effect is the main advantage of this electrochemical process. The developed process is clean and avoids energy intensive steps viz. crushing, grinding and roasting.