Four-step eco-friendly energy efficient recycling of contaminated Nd2Fe14B sludge and coercivity enhancement by reducing oxygen content

Syed Kamran Haider,Young Soo Kang,Dongsoo Kim

doi:10.1038/s41598-021-01382-4

Abstract

Complete recycling of Nd2Fe14B sludge by chemical methods has gained significance in recent years, however, it is not easy to recycle highly contaminant sludge and obtain product with good magnetic properties. Herein we report a simple four-step process to recycle the Nd2Fe14B sludge containing ~ 10% of contaminants. Sludge was leached in H2SO4 and selectively co-precipitated in two steps. In the first co-precipitation, Al3+ and Cu2+ were removed at pH 6. Thereafter, in the second co-precipitation Fe2+ and RE3+ sulfates were converted to the Fe and RE hydroxides. By annealing at 800 °C RE and Fe hydroxides precipitates were converted to the oxides and residual carbon was oxidized to CO2. After the addition of boric acid, Fe and RE oxides were reduced and diffused to the (Nd-RE)2Fe14B by calciothermic reduction diffusion. Removal of CaO by washing with D.I. water in glove box reduced the oxygen content (~ 0.7%), improved crystallinity and enhanced the magnetic properties significantly. Coercivity increased more than three times (from 242.71 to 800.55 kA/m) and Mr value was also enhanced up to more than 20% (from 0.481 to 0.605 T). In this green process Na2SO4 and Ca(OH)2 were produced as by-product those are non-hazardous and were removed conveniently.

Highlights

Among permanent magnets, Nd2Fe14B type hard magnets exhibit the highest recorded BHmax[1,2,3,4,5]
It was concluded that oxygen content reduces the crystallinity Nd2Fe14B because no oxide is detected in the X-ray diffraction (XRD) patterns
Contaminated Nd2Fe14B sludge was recycled by four-step chemical process

Summary

Introduction

Among permanent magnets, Nd2Fe14B type hard magnets exhibit the highest recorded BHmax[1,2,3,4,5]. Magnetic properties are further enhanced by the removal of contamination and reduction of oxygen content. The precipitates were washed thrice with de-ionized water to remove the N a2SO4 and NaOH and annealed at 800 °C to convert all Fe and RE hydroxides to oxides. Oxides produced from both the selective and regular co-precipitation were mixed with the boric acid and C aH2 in two separate experiments.

Results

Conclusion