Method for recovery of Nd.SUB.2.Fe.SUB.14.B grains from bulk sintered Nd—Fe—B magnets and/or magnet scraps by electrochemical etching

Abstract

The invention relates to a method for recovery of Nd.sub.2Fe.sub.14B grains from bulk sintered Nd—Fe—B magnets and/or magnet scraps. In this method the Nd—Fe—B magnets (1) and/or magnet scraps are anodically oxidized using a non-aqueous liquid electrolyte (5), said anodic oxidation releasing the Nd.sub.2Fe.sub.14B grains (6) in said Nd—Fe—B magnets (1) and/or magnet scraps. The released Nd.sub.2Fe.sub.14B grains (6) are collected during and/or after said anodic oxidation. The proposed method allows a more environmental friendly and cost-effective way for recycling EOL Nd—Fe—B magnets/Nd—Fe—B magnet scraps.

Claims

1. A method for recovery of Nd.sub.2Fe.sub.14B grains from bulk sintered Nd—Fe—B magnets and/or magnet scraps, in which method the Nd—Fe—B magnets and/or magnet scraps are anodically oxidized using a non-aqueous liquid electrolyte, said anodic oxidation releasing the Nd.sub.2Fe.sub.14B grains in said Nd—Fe—B magnets and/or magnet scraps, wherein the released Nd.sub.2Fe.sub.14B grains are collected magnetically during and/or after said anodic oxidation.

2. The method according to claim 1, characterized in that said anodic oxidation is performed in an electrochemical cell having an anode at least in part formed of said Nd—Fe—B magnets and/or magnet scraps.

3. The method according to claim 2, characterized in that a cathode of the electrochemical cell is formed of Cu.

4. The method according to claim 1, characterized in that said anodic oxidation is performed in a three-electrode electrochemical cell having an anode at least in part formed of said Nd—Fe—B magnets and/or magnet scraps, a cathode and a reference electrode.

5. The method according to claim 4, characterized in that the cathode is formed of Cu and the reference electrode is formed of a Pt material.

6. The method according to claim 1, characterized in that a non-aqueous solvent in which etched Nd-rich phases of the Nd—Fe—B magnets and/or magnet scraps dissolve is used as said liquid electrolyte.

7. The method according to claim 6, characterized in that the Nd-rich phases dissolved in the liquid electrolyte are also recovered by separating said Nd-rich phases from the electrolyte.

8. The method according to claim 1, characterized in that dimethylformamide is used as said liquid electrolyte.

9. The method according to claim 1, characterized in that an additive compound is added to said liquid electrolyte in order to enhance electrical conductivity of the electrolyte.

10. The method according to claim 9, characterized in that 0.05-0.3 mol L.sup.−1 FeCl.sub.2 is added as said additive compound.

11. The method according to claim 1, characterized in that the anodic oxidation is carried out at a temperature in a range between 0° C. and about 90° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The proposed method will be described in the following by way of example in connection with the accompanying figures showing:

(2) FIG. 1 a schematic illustration of an electrochemical cell for recovering Nd.sub.2Fe.sub.14B grains from Nd—Fe—B magnets according to the proposed method;

(3) FIG. 2 Backscattered electron images of (a) Bulk sintered Nd—Fe—B scrap before electrochemical etching, (b) Bulk sintered Nd—Fe—B scrap after electrochemical etching (15 min), (c) Collected magnetic powder after electrochemical etching (360 min) and (d) Collected Nd-rich phase by filtration after electrochemical etching (360 min) (Etching conditions: 2 mA.Math.cm.sup.−2, room temperature, no stirring);

(4) FIG. 3 XRD patterns of the collected magnetic particles in comparison with reflections characteristic of the Nd.sub.2Fe.sub.14B phase; and

(5) FIG. 4 a diagram showing the concentration of Nd.sup.3+ and Dy.sup.3+ in the electrolyte after electrochemical etching.

EXAMPLE FOR CARRYING OUT THE INVENTION

(6) In the following example of the proposed method Nd.sub.2Fe.sub.14B grains are recovered from an EOF Nd—Fe—B magnet using dimethylformamide as the liquid electrolyte.

(7) Regents

(8) Dimethylformamide (DMF, >99%) was purchased from Sigma-Aldrich, Germany. Prior to using, molecular sieves (4A, Sigma-Aldrich, China) which were dried under vacuum at 160° C. for more than 24 hours was added into DMF to remove the water. FeCl.sub.2.4H.sub.2O (>99.99%, Sigma-Aldrich) were dehydrated under vacuum at 140° C. for 24 hours. All dried chemicals were stored inside a closed bottle in an argon filled glove box with water and oxygen content below 1 ppm. The water concentration which was determined by Karl Fischer titration (C20S, Mettler-Toledo, Switzerland) in the electrolyte was less than 50 ppm. The sintered bulk Nd—Fe—B magnet waste (chemical composition: 66.34 wt. % Fe, 22.10 wt. % Nd, 5.78 wt. % Dy, 5.78 wt. % other elements) used in this example was supplied by Magneti Ljubljana d.d (Ljubljana, Slovenia). Prior to experiment, these bulk magnets were thermally demagnetized and mechanically polished to remove the coating.

(9) Instrumentation

(10) With reference to the schematic illustration in FIG. 1, an electrochemical cell usable in the proposed method compromises the Nd—Fe—B magnet scrap 1 (15 mm*30 mm*2 mm) the coating of which was removed prior processing as the anode, a metallic counter electrode 2 (10 mm*10 mm) as the cathode and e.g. a Pt wire 3 (0.5 mm diameter) as the reference electrode. For work in the laboratory, the electrodes were attached to the electrical conductors 4 for connection to electrical instrumentation power supply (not shown). The cathode as shown can be a substrate material such as copper in the form of foil or plate. The electrolyte 5 that surrounds the three electrodes must be a non-aqueous solvent. Organic solvents such as dimethylformamide (DMF), acetonitrile, ethanol et. al. and deep eutectic solvents (DESs) such as choline chloride-ethylene glycol are suitable for this purpose. The particles illustrated at 6 in FIG. 1 are Nd2Fe14B grains that fall out of the Nd—Fe—B magnet 1 forming the anode after the Nd-rich grain boundary phase is etched away electrochemically, collected by an external (commercial Nd—Fe—B) magnet 8. The non-magnetic particles 7 that fell out of the Nd—Fe—B magnet 1 forming the anode together with Nd2Fe14B grains 6 are Nd-based oxides.

EXAMPLE

(11) The proposed method is performed in this example using the three-electrode cell of FIG. 1 with 15 mL DMF containing 0.3 M FeCl.sub.2 at room temperature. A Nd—Fe—B magnet (15 mm*30 mm*2 mm) is served as the anode, a Cu foil (10 mm*10 mm) is used as the cathode and a Pt wire (0.5 mm diameter) is applied as the reference electrode. Electrochemical etching of the Nd—Fe—B magnet started by applying the current density of 2-48 mA.Math.cm.sup.−2 on the anode. The morphology of the anode was examined by SEM. In order to collect enough particles and investigate the etching efficiency of REEs, an electrochemical etching experiment with applied current density of 2 mA.Math.cm.sup.−2 for 6 hours was conducted. Since the magnetic particles formed after etching tend to be attracted by the anode, therefore, for magnetic particles collection, at every one hour interval the etched anode was manually put close to the external magnet to separate these particles from anode. The metallic Nd which was not completely etched in the grain boundaries dropped down to the bottom of the cell together with NdO.sub.x and was collected by filtration of the electrolyte after etching. After electrochemical etching, the collected particles are collected magnetically and washed 3 times using DMF. The cleaned particles are then put in a vacuum chamber to evacuate DMF overnight for further characterization. The REEs concentration in the solution was measured by ICP-MS.

(12) The initial sintered bulk Nd—Fe—B magnet (FIG. 2a (BEI SEM)) consists of Nd.sub.2Fe.sub.14B matrix phase (grey) with the grains size of ˜10 μm, Nd.sub.2Fe.sub.4B.sub.x phase found in triple pockets (light grey), together with Nd.sub.2O.sub.3 phase (white). On the grain boundaries the Nd-rich (Nd—Fe) grain boundary phase (bright phase) is present, however the analysis with the SEM is not reliable in our case. This bright intergranular phase is Nd-rich, the structure of which most probably consists of α-Nd (fcc) and a mixture of different Nd-based oxides (dhcp-Nd.sub.2O.sub.3, fcc-NdO, complex-Nd.sub.2O.sub.3 and h-Nd.sub.2O.sub.3) [29]. FIG. 2b (BEI SEM) shows a representative microstructure of the Nd—Fe—B magnet surface after electrochemical etching at 2 mA.Math.cm.sup.−2 for 15 min under room temperature. It can be observed that the metallic Nd-rich grain boundary (bright phase, FIG. 2b) was etched away, exposing the grains of Nd.sub.2Fe.sub.14B matrix phase and leaving behind the Nd-oxide phases (white phase on FIG. 2b) which are not prone to be electrochemically oxidised i.e. etched. Some vacancies observed on the surface of the magnet after etching indicate that some grains of the Nd.sub.2Fe.sub.14B matrix phase are detached from the magnet body. The etching front is pronounced on the Nd—Fe—B magnets, where the surface grains are more affected by etching (pores, holes) than the interior grains.

(13) After etching at 2 mA.Math.cm.sup.−2 for 360 min, powders were collected. The magnetic fraction of the etched powder that was collected and separated via external permanent magnet is shown in FIG. 2c (BEI SEM). It can be observed that this collected powder consists of the grey phase and white phase and the magnetic grains of the grey phase are not connected with each other. The EDS result (EDS: Energy Dispersive X-ray Spectrometry) shows that the composition of the grey phase (Nd.sub.2Fe.sub.14B matrix phase) and white phase (Nd-oxide, most probably Nd.sub.2O.sub.3) are similar to those in FIG. 2b. The measured grain size of the grey phase varies from 5 and 10 μm, which is consistent to the initial grain size of Nd.sub.2Fe.sub.14B matrix phase in the pristine as-sintered magnet. Some pores/holes observed in each grey phase are possibly due to the etching of Nd-rich phase inside the grains. Based on the obtained results, 67.2% of Nd—Fe—B magnet were recovered in the form of the Nd.sub.2Fe.sub.14B grains. FIG. 2d (BEI SEM) shows that the filtered particles are either the Nd-oxide phases (round particles) and Nd-based oxides and alloy phases (elongated ribbed particles), that resulted from the anodic etching of Nd-based alloy during electrolysis.

(14) An XRD pattern of the magnetic powders mainly shows reflections characteristic of Nd.sub.2Fe.sub.14B phase (Reference PDF: 04-005-2711) (FIG. 3) which indicates that the grey phase in both FIGS. 2b and c is Nd.sub.2Fe.sub.14B phase.

(15) The etched Nd-rich phase which was dissolved in DMF after filtration was measured by ICP-MS (Inductively Coupled Plasma—Mass Spectrometry) (see FIG. 4). The etching rate of Nd.sup.3+ and Dy.sup.3+ were calculated as 2.3668 and 0.685 mg L.sup.−1 min.sup.−1, respectively.

REFERENCES

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