Hydrogen Decrepitation of NdFeB End-of-Life Magnets with the Preliminary Three-Step Surface Cleaning

Abstract

The hydrogen decrepitation of end-of-life NdFeB magnetic alloys was studied. End-of-life magnets extracted from PC hard drives were taken as a material for the experiments. The choice of these items is governed by their low cost, easy removal from PC, and small size. The magnets were demagnetized at 623 K in a medium vacuum for 4 h, which was followed by three-step cleaning of their surface to remove the electroplated coating, oxidized surface layer, and adsorbed impurities and moisture. The first step of the cleaning was sandblasting with slag shot fed at a pressure of 300 kPa. The second step was chemical etching with solutions of dilute acids (1–3% HCl, HNO₃, or H₂SO₄ in distilled water), followed by washing in acetone. The third step was heat treatment in a vacuum, consisting in rapid heating in a shaft furnace of magnet samples placed after etching in an autoclave, with several exposures in the temperature interval 373–573 K in a medium vacuum. After these operations, the samples were quenched by placing the hot autoclave into a vessel with ice-cold water, preceded by filling the autoclave with argon. Then, alloys of the NdFeB system without removing from the autoclave were subjected to hydrogen decrepitation performed in the temperature interval from 298 to 473 K at an excess hydrogen pressure of 30 to 210 kPa to determine how these parameters influence the properties of the hydride powders obtained and the amount of hydrogen taken up. The hydrogen for the decrepitation was generated by direct desorption from the heated hydride of the LaNi₅ alloy. In the pressure range 30–70 kPa and room temperature, alloys of the NdFeB system transform into hydrides. The chemical reaction with hydrogen starts instantaneously without a period of primary hydrogen adsorption. The hydride powders obtained, containing no less than 0.459 wt % hydrogen, are very brittle and can be finely milled (100-g portion) in a planetary ball mill within 6 min to obtain a powder with the particle size smaller than 20 μm. The analysis of the surface of the finely milled hydride powder particles revealed no oxygen (≤1 wt %).