Process for removing a coating comprising zinc applied to the surface of neodymium-iron-boron magnets

Abstract

The present invention relates to a process for removing a coating comprising zinc applied to the surface of Neodymium-Iron-Boron (NdFeB) magnets comprising the steps of: a) stirring the magnets in an aqueous solution of an organic acid so as to allow the organic acid solution to be spread evenly over the entire surface of the magnets, for a predetermined treatment time, wherein the ratio between the volume of the aqueous solution of organic acid and the mass of the magnets is comprised between 0.21 / Kg and 1001 / Kg; b) separating the magnets from the aqueous solution of organic acid. The process of the present invention has the advantage of completely removing the Zn coating applied to the surface of NdFeB magnets.

Description

[0001] Process for removing a coating comprising zinc applied to the surface of neodymium-iron- boron magnets

[0002] Field of the invention

[0003] The present invention relates to a process for removing a coating comprising zinc applied to the surface of Neodymium-Iron-Boron (NdFeB) magnets.

[0004] Prior art

[0005] The coating of a magnet surface with zinc (Zn), particularly the coating of the surface of Neodymium-Iron-Boron (NdFeB) magnets with Zn, is a widely used practice in various industrial fields. The Zn coating prevents the corrosion of the coated magnets.

[0006] However, when these coated magnets need to be disposed of, for example, by recycling through grinding, the presence of surface Zn poses a problem. If the Zn-coated magnets were recycled as is, without undergoing a coating removal treatment, they would be unsuitable as a secondary raw material for the production of new NdFeB magnets. A too-high Zn concentration would result in a significant reduction in the magnetic capability of the new magnet produced from the recycled Zn-coated magnet, rendering it either unusable or poorly performing. For this reason, it is essential to pretreat the Zn-coated NdFeB magnets to remove the Zn present on their surface before proceeding with the various phases of the recycling process.

[0007] Two techniques are known for removing or reducing the amount of Zn from the surface of NdFeB magnets: manual sandblasting and the HPMS process (hydrogen decrepitation of permanent magnet scraps) followed by a sieving step.

[0008] The sandblasting technique involves manually removing the Zn coating using a sandblaster, which removes the Zn coating through the abrasive action of sand impacting at high speed against the Zn coating. However, this technique requires a large amount of time, making it impractical on a large scale and inefficient from an economic standpoint. The HPMS process, followed by sieving, involves treating the Zn-coated magnets with hydrogen. The hydrogen reacts with both the Zn and the magnetic particles according to the hydrogen decrepitation reaction, which leads to the demagnetization and pulverization of the magnets. Subsequently, the powder thus obtained is ground using jet milling to further reduce the size of the magnet particles. The magnet, being inherently more brittle than Zn (which, being a metal, is more malleable and less brittle), forms particles of a generally smaller size than those of Zn during pulverization. Exploiting this physical characteristic, the subsequent sieving step allows the separation of magnet particles from Zn particles. However, this technique is not capable of reducing the Zn content to concentrations sufficient to produce a high-quality final product.

[0009] There is therefore a recognized need in the market for a process capable of completely removing the Zn coating applied to the surface of Neodymium-Iron-Boron (NdFeB) magnets.

[0010] Summary of the invention

[0011] The object of the present invention is therefore to provide a process capable of completely removing the Zn coating applied to the surface of Neodymium-Iron-Boron (NdFeB) magnets. This object is achieved through a process for removing a coating comprising zinc applied to the surface of NdFeB magnets as defined in the appended claims, the definitions of which form an integral part of the present description.

[0012] Brief description of the figures

[0013] The invention will be better understood from the following detailed description of its preferred embodiments, provided by way of example and not limitation, with reference to the appended figures, wherein:

[0014] -Figure 1 shows a non-limiting example of a reactor used in the process according to the present invention. In the appended figures, identical or similar elements are denoted by the same reference numerals.

[0015] Detailed description of the invention

[0016] A first object of the present invention is a process for removing a coating comprising zinc applied to the surface of Neodymium- Iron-Boron (NdFeB) magnets comprising the steps of: a) stirring the magnets in an aqueous solution of an organic acid so as to allow the organic acid solution to be spread evenly over the entire surface of the magnets, for a predetermined treatment time, wherein the ratio between the volume of the aqueous solution of organic acid and the mass of the magnets is comprised between 0.2 1 / Kg and 100 1 / Kg; b) separating the magnets from the aqueous solution of organic acid and optionally rinsing said magnets with washing water to remove any organic acid and / or zinc residues from the surface of the magnets.

[0017] In the context of the present description and the appended claims, the term “coating” refers to one (or more) layer(s) of material that fully or partially covers the surface of the magnet.

[0018] According to a preferred embodiment of the process of the present invention, the stirring action in step a) is preferably conducted manually by a human operator or mechanically with the aid of a mechanical agitator, which in turn is operated and controlled either by a human operator or by a computer program.

[0019] According to a preferred embodiment of the process of the present invention, the separating action in step b) is preferably conducted manually by a human operator or mechanically with the aid of a mechanical separator, which in turn is operated and controlled either by a human operator or by a computer program.

[0020] According to a preferred embodiment of the process of the present invention, the rinsing action in step c) is preferably conducted with running washing water or by cyclically immersing and removing the magnets from a washing water bath. According to a preferred embodiment of the process of the present invention, the organic acid is preferably a weak acid and more preferably selected from: citric acid, malic acid, maleic acid, tartaric acid, aconitic acid, acetic acid, or mixtures thereof. Even more preferably, the organic acid is citric acid. Preferably, the acids used in the process of the present invention are weak organic acids that are highly soluble or miscible in water.

[0021] With reference to Figure 1, according to a preferred embodiment of the process of the present invention, step a) is preferably conducted in a reactor 10 (Figure 1 represents a preferred, non-binding embodiment of the reactor 10) having an inner chamber 12 into which the aqueous solution of organic acid is previously poured, wherein the inner chamber 12 is at least partially occupied by a basket 14 permeable to the aqueous solution of organic acid, wherein the basket 14 is suitable for receiving said magnets, wherein the basket 14 is a rotating basket, and step a) is conducted by rotating said basket 14 to provoke collisions between the magnets and / or between the magnets and the walls of the basket 14, and to allow the organic acid solution to be spread evenly over the entire surface of the magnets.

[0022] According to a preferred embodiment of the process of the present invention, the inner chamber 12 preferably has a volume comprised between 100 dm3and 200 dm3, and more preferably has a volume of 145 dm3.

[0023] According to a preferred embodiment of the process of the present invention, the action of pouring the aqueous solution of organic acid into the inner chamber 12 is preferably conducted manually by a human operator or mechanically with the aid of a mechanical device, which in turn is operated and controlled either by a human operator or by a computer program.

[0024] In an alternative embodiment of the process of the present invention, the inner chamber 12 is preferably completely occupied by the basket 14, which is permeable to the aqueous solution of organic acid.

[0025] According to a preferred embodiment of the process of the present invention, the action of rotating said basket 14 is preferably conducted manually by a human operator or mechanically with the aid of a mechanical device (even more preferably by means of a driving shaft), which in turn is operated and controlled either by a human operator or by a computer program.

[0026] According to a preferred embodiment of the process of the present invention, the inner chamber 12 also preferably comprises at least one admission opening 16 at the basket 14; preferably, the reactor 10 also comprises a movable door 18 positioned at the admission opening 16. The door 18 is preferably attached to the reactor 10 via a hinge 20, allowing the door 18 to move between a closed configuration, in which the door 18 covers the admission opening 16 and isolates the inner chamber 12 from the external environment of the reactor 10, and an open configuration, in which the door 18 does not cover the admission opening 16, allowing the inner chamber 12 to communicate with the external environment of the reactor 10.

[0027] According to a preferred embodiment of the process of the present invention, the basket 14 preferably has a cylindrical shape and also comprises an admission opening aligned with the admission opening 16 of the inner chamber 12. Preferably, the basket 14 is made of a mesh, a grid, or a membrane being permeable to the aqueous solution of organic acid.

[0028] According to a preferred embodiment of the process of the present invention, the aqueous solution of organic acid fills half of the basket 14, and the basket 14 rotates partially immersed in the solution. The rotational movement is preferably transmitted to the basket 14 via a driving shaft that transfers the rotation of an electric motor. Preferably, three idler rollers hold the basket 14 in position inside the inner chamber 12 and facilitate its rotation (the three idler rollers are preferably fixed cylinders with free rotation).

[0029] According to a preferred embodiment of the process of the present invention, the reactor 10 preferably further comprises an agitator device positioned in the inner chamber and / or in the basket. Preferably, the agitator device increases the number of collisions occurring between the magnets and / or between the magnets and the walls of the basket 14, thereby promoting the diffusion of the organic acid solution over the entire surface of the magnets. Preferred, non-binding embodiments of the agitator device include a mechanical arm, a tilting or rotating platform, or a vibrating element.

[0030] According to a preferred embodiment of the process of the present invention, step a) preferably further comprises stirring in the presence of solid bodies, inert to the aqueous solution of organic acid, in free motion, so as to increase the number of collisions occurring between the magnets and improve the diffusion of the organic acid solution over the entire surface of the magnets. Preferred, non-binding embodiments of the solid bodies include spheres, balls, polygonal solids, and irregular polygonal solids. According to a preferred embodiment of the process of the present invention, the solid bodies are preferably inserted into the basket 14 and said bodies are free to move within the basket during its rotation, so as to increase the number of collisions occurring between the magnets and improve the diffusion of the organic acid solution over the entire surface of the magnets.

[0031] According to a preferred embodiment of the process of the present invention, the internal surface of the basket 14 preferably has asperities and / or the surface of the solid bodies preferably has asperities. Preferably, the asperities are protrusions having a pyramidal, domed, or trapezoidal shape.

[0032] According to a preferred embodiment of the process of the present invention, step a) is preferably conducted for a predetermined treatment time comprised between 5 minutes and 120 minutes, and more preferably between 15 minutes and 25 minutes. Step a) is preferably conducted at a rotational speed of the basket 14 comprised between 10 revolutions per minute and 40 revolutions per minute, and more preferably between 20 revolutions per minute and 30 revolutions per minute.

[0033] According to a preferred embodiment of the process of the present invention, the coating is a zinc coating.

[0034] According to a preferred embodiment of the process of the present invention, the aqueous solution of organic acid preferably has a concentration comprised between 2% wt / vol and 40% wt / vol, with the % being expressed with respect to the weight of the solute with respect to the volume of the aqueous solution of organic acid, and more preferably has a concentration of 10% wt / vol.

[0035] Below, a preferred embodiment of the process of the present invention is briefly described. Preferably, the process of the present invention comprises the steps of: inserting a quantity comprised between 5 Kg and 50 Kg of magnets coated with a coating comprising Zn into the basket 14 located inside the reactor 10. Pouring into the basket 14 a solution of citric acid having a concentration comprised between 5% wt / vol and 30% wt / vol. The basket 14 has a volume of 12 dm3, while the inner chamber 12 of the reactor 10 has a volume of 145 dm3. Rotating the basket 14 containing the magnets in the citric acid solution. During the process, the Zn coating reacts with the citric acid according to the reaction:

[0036] Zn(S) + 2AcOH(aq) —> Zn2+(aq) + 2AcO_(aq) + H2(g). The Zn dissolves into the solution, and the rotational motion of the basket 14 causes the magnets to slide against one another, creating a turbulent motion that facilitates the reaction by improving molecular diffusion on the surface of the magnets. Additionally, the motion allows the reaction to homogenize across the entire surface of the magnets. The reactor rotational speed is comprised between 10 revolutions per minute and 40 revolutions per minute. The process timing is comprised between 5 minutes and 120 minutes. The process of the present invention is based on three principles: chemical etching by the organic acid solution (preferably citric acid), rotation of the basket 14 creating strong turbulence that increases the reaction speed, the high selectivity of the organic acid (preferably citric acid) for Zn, which advantageously results in minimal etching of the magnet by the acid. Preferably, the organic acid solution (preferably citric acid) can be reused numerous times because the amount of Zn removed per cycle is small, being only a layer of about 20 micrometres on the surface of the magnet, and thus little organic acid (preferably citric acid) is consumed. After a certain number of cycles, the consumed organic acid can be replenished; if the organic acid is in solid form, it can be added without significantly increasing the solution volume since it is a solid highly soluble in water, if the organic acid is in liquid form, such as for example acetic acid, it can be added without the risk of generating a large amount of waste due to the increased solution volume, this because the amount of acid consumed per reaction cycle is very small, and acetic acid is advantageously a liquid composed of approximately 100% acetic acid. At the end of the process, the magnets are optionally rinsed with water to wash off the organic acid solution (preferably citric acid). Thus, the developed process is practically free of liquid waste, except for very small amounts. In the process of the present invention, the organic acid is preferably citric acid. Advantageously, citric acid is a weak organic acid, which means that even if added in large amounts to the solution, it does not produce a particularly acidic solution. This characteristic allows for large quantities of citric acid in the solution but with moderate reactivity, ensuring proper management of the process of the present invention, as a highly reactive aqueous organic acid solution would cause overly aggressive chemical reactions on the magnet. Despite magnets being inherently chemically reactive materials, citric acid has proven to be an excellent reagent because it removes the zinc coating without damaging the underlying magnet.

[0037] Advantageously, the process of the present invention is therefore capable of completely removing the Zn coating applied to the surface of Neodymium-Iron-Boron (NdFeB) magnets.

[0038] Examples

[0039] A preferred embodiment of the present invention is provided below by way of a non limiting example.

[0040] Example 1 : Process for removing a coating comprising Zn applied to the surface of Neodymium-Iron-Boron (NdFeB) magnets.

[0041] 30 Kg of magnets coated with a Zn coating were placed inside the basket 14 of the reactor 10. An aqueous solution of citric acid with a concentration of 10% wt / vol was introduced into the basket 14. The basket 14 has a volume of 12 dm3, while the reactor has a volume of 145 dm3. Subsequently, the basket 14 (containing the magnets and the aqueous solution of citric acid) was rotated to simulate a tumbling action, this step was conducted for 20 minutes. The rotational speed of the reactor was set to 25 revolutions per minute. The magnets were removed from the basket 14. The magnets were rinsed with water to wash off the aqueous solution of citric acid and the Zn.

Claims

CLAIMS1. Process for removing a coating comprising zinc applied to the surface of Neodymium- Iron-Boron (NdFeB) magnets comprising the steps of:(a) stirring the magnets in an aqueous solution of an organic acid so as to allow the organic acid solution to be spread evenly over the entire surface of the magnets, for a predetermined treatment time, wherein the ratio between the volume of the aqueous solution of organic acid and the mass of the magnets is comprised between 0.2 1 / Kg and 100 1 / Kg; b) separating the magnets from the aqueous solution of organic acid and optionally rinsing said magnets with washing water in order to remove any organic acid and / or zinc residue from the surface of the magnets.

2. Process according to claim 1, wherein the organic acid is a weak acid and is preferably selected from: citric acid, malic acid, maleic acid, tartaric acid, aconitic acid, acetic acid or mixtures thereof.

3. Process according to claim 1 or 2, wherein step a) is conducted in a reactor having an inner chamber into which the aqueous solution of organic acid is previously poured, wherein the inner chamber is at least partially occupied by a basket permeable to the aqueous solution of organic acid, wherein the basket is suitable for receiving said magnets, wherein the basket is a rotating basket and step a) is conducted by rotating said basket in order to provoke collisions between said magnets and / or collisions between the magnets and the walls of the basket and in order to allow the organic acid solution to be spread evenly over the entire surface of the magnets.

4. Process according to claim 3, wherein the reactor further comprises an agitator device arranged in the inner chamber and / or in the basket.

5. Process according to any one of claims 1 to 4, wherein step a) further comprises stirring in the presence of solid bodies, inert to the aqueous solution of organic acid, in free motion, so as to increase the number of collisions occurring between the magnets and to improve the spreading of the organic acid solution over the entire surface of the magnets.

6. Process according to claim 3, wherein an inner surface of the basket has asperities; or according to claim 5, wherein the surface of the solid bodies has asperities.

7. Process according to any one of claims 1 to 6, wherein step a) is conducted for a predetermined treatment time comprised between 5 minutes and 120 minutes, preferably comprised between 15 minutes and 25 minutes.

8. Process according to any one of claims 1 to 7, wherein the coating is a zinc coating.

9. Process according to any one of claims 1 to 8, wherein the aqueous solution of organic acid has a concentration comprised between 2% wt / vol and 40% wt / vol, the % being expressed with respect to the weight of the solute with respect to the volume of the aqueous solution of organic acid, and preferably has a concentration of 10% wt / vol.