Leaching kinetics of neodymium in sulfuric acid from E-scrap of NdFeB permanent magnet

The leaching kinetics of neodymium in NdFeB permanent magnet powder was analyzed for the purpose of recovery of neodymium in sulfuric acid ($H_2SO_4$) from E-scrap (electric scrap) of NdFeB permanent magnet powder treated by oxidation roasting to form a reactant. The reaction was conducted with $H_2SO_4$ concentrations ranging from 2.5 to 3.5M, a pulp density of 110.8 g/L, an agitation speed of 750 rpm, and a temperature range of 30 to $70^{\circ}C$. After 4 h of leaching, the neodymium content in the E-scrap powders was completely converted into a neodymium sulfate ($Nd_2(SO_4)_3$) solution phase in $H_2SO_4$ in the condition of $70^{\circ}C$ and 3.0M $H_2SO_4$. Based on a shrinking core model with sphere shape, the leaching mechanism of neodymium was determined by the rate-determining step of the ash layer diffusion. Generally, the solubility of pure rare earth elements in $H_2SO_4$ is decreased with an increase in leaching temperatures. However, the leaching rate of the neodymium in E-scrap powders increased with the leaching temperatures in this study because the ash layer included in the E-scrap powder provided resistance against the leaching. Using the Arrhenius expression, the apparent activation energy values were determined to be $2.26kJmol^{-1}$ in 2.5M $H_2SO_4$ and $2.77kJmol^{-1}$ in 3.0 M $H_2SO_4$.