Hydrocyclone separation of hydrogen decrepitated NdFeB

Abstract

Hydrogen decrepitation (HD) is an effective and environmentally friendly technique for recycling of neodymium-iron-boron (NdFeB) magnets. During the HD process, the NdFeB breaks down into a matrix phase (Nd2 Fe14 BHx) and RE-rich grain boundary phase. The grain boundary phase in the HD powder is <2 µm in size. Recycled NdFeB material has a higher oxygen content compared to the primary source material. This additional oxygen mainly occurs at the Rare Earth (RE) rich grain boundary phase (GBP), because rare earth elements oxidise rapidly when exposed to air. This higher oxygen level in the material results in a drop in density, coercivity, and remanence of sintered NdFeB magnets. The particle size of the GBP is too small to separate by sieving or conventional screening technology. In this work, an attempt has been made to separate the GBP from the matrix phase using a hydrocyclone, and to optimise the separation process. HD powder, obtained from hard disk drive (HDD) scrap NdFeB sintered magnets, was used as a starting material and passed through a hydrocyclone a total number of six times. The X-ray fluorescence (XRF) analysis and sieve analysis of overflows showed the matrix phase had been directed to the underflow while the GBP was directed to the overflow. The optimum separation was achieved with three passes. Underflow and overflow samples were further analysed using an optical microscope and MagScan and matrix phase particles were found to be magnetic.