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

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 M_r 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.