Recycling methods for rare earth sintered magnets

Abstract

To provide a recycling method of a rare earth magnet, which enables recycling of a rare earth magnet by peeling a nickel plating provided on a surface of the rare earth magnet without degrading characteristics of the rare earth magnet, and thereby, can improve a product yield of the rare earth magnet.SOLUTION: Provided is a recycling method of a rare earth magnet, comprising immersing a rare earth magnet, which has on its surface a coating film containing nickel, in a peeling liquid comprising a derivative of nitrobenzene, ethylene diamine, and ammonia.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for recycling rare earth magnets, particularly a method for recycling rare earth magnets that removes a plating film formed on the surface of a neodymium magnet and re-uses the neodymium magnet.

Background Art

[0002] Rare earth magnets are essential functional materials for energy conservation and high functionality, and their application range extends to a wide range of fields from general household appliances such as air conditioners to automotive applications such as HEVs and EVs. With the global movement to promote EVs and the increasing demand for ultra-large capacity HDDs for data centers due to the spread of cloud services and video distribution, the production volume is expected to increase further in the future.

[0003] A general rare earth magnet is manufactured by pulverizing a raw material alloy adjusted to a predetermined composition in an inert gas atmosphere, powder compacting it to a certain size while applying a magnetic field, and sintering it in a vacuum or an inert gas atmosphere. The manufactured rare earth magnet is processed into a product shape by machining or grinding, and further surface-treated such as plating or painting to become a product. Although losses such as defective products and sludge occur in each process, recycling to reduce these losses is positioned as an important process from the viewpoints of effective utilization of rare resources, reduction of waste generation, and further reduction of the price of rare earth magnets. [[ID=十七]]

[0004] Among the above processes, in the surface treatment process, the surface of the rare earth magnet is treated to impart corrosion resistance to the rare earth magnet. Generally, Ni plating is often applied because it has good corrosion resistance and is easy to mass-produce. However, in this surface treatment process of applying Ni plating, there is a limit to improving the yield, and it is inevitable that a certain amount of plating defective products will occur. As described above, since rare earth magnets contain rare earth elements, which are precious resources, it has been considered to reuse the plating defective products.

[0005] For example, Patent Document 1 describes a technique for removing the Ni coating from rare earth alloys by electrolytic oxidation as a safe, easy, and inexpensive method for reusing waste materials with Ni coatings generated in the manufacturing process of Ni-coated rare earth alloys. It reports that electrolytic oxidation is the reverse reaction of electroplating, is easy to control, and can be carried out safely and inexpensively.

[0006] Furthermore, Patent Document 2 states that, since neodymium magnets are expensive, the plating failures that occur during the plating process are intended to be stripped and re-plated, and therefore the plating film is intended to be removed, which corrodes the base material of the object being treated, such as neodymium magnets. It has been reported that a non-cyanide immersion stripping agent for copper and Ni can be provided that can strip copper and Ni metals or alloys without causing damage. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 5-33074 [Patent Document 2] Japanese Patent Application Publication No. 7-138772 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the technology described in Patent Document 1 aims to remove the Ni plating film as a preliminary step before extracting rare earth elements from rare earth alloys, and does not mention the magnetic properties after Ni plating removal, which are important when recycling rare earth magnets.

[0009] Furthermore, the technology described in Patent Document 2 has room for improvement in terms of the peeling temperature (room temperature to 90°C, especially 70°C to 80°C). Although rare earth magnets do not corrode or dissolve when exposed to high temperatures in a basic solution, their properties may deteriorate when heated during the assembly of rare earth magnets.

[0010] Furthermore, the technology described in Patent Document 2 reports a plating peeling rate of typically 2 to 10 μm / hr at a processing temperature of 70 to 80°C, but this peeling rate is not sufficient from the standpoint of improving the efficiency of the recycling process.

[0011] This invention has been made in view of the above circumstances, and aims to provide a method for recycling rare earth magnets that allows for the recovery of the rare earth magnets as they are and subsequent replating, by selectively dissolving only the Ni plating on the magnet surface without causing any damage to the rare earth magnet substrate, which has low corrosion resistance. [Means for solving the problem]

[0012] As a result of diligent research to solve the above problems, the inventors of the present invention discovered that magnets can be recycled without degrading their magnetic properties by treating them with a liquid containing a nitrobenzene derivative and ethylenediamine, leading to the present invention.

[0013] In other words, the present invention provides the following method for recycling rare earth magnets. 1. A method for recycling rare earth magnets, characterized by immersing a rare earth magnet having a Ni-containing coating on its surface in a solution containing a nitrobenzene derivative, ethylenediamine, and ammonia. 2. The method for recycling rare earth magnets according to claim 1, characterized in that the derivative of nitrobenzene is at least one sodium nitrobenzenesulfonate selected from the group consisting of sodium o-nitrobenzenesulfonate, sodium m-nitrobenzenesulfonate, and sodium p-nitrobenzenesulfonate. 3. The method for recycling rare earth magnets according to claim 1 or 2, characterized in that the rare earth magnet having the Ni-containing coating on its surface is a defective product generated in the manufacturing process of rare earth magnets. 4. A method for recycling rare earth magnets according to any one of 1 to 3, characterized in that ultrasonic waves with a frequency of 20 to 100 kHz are applied to the stripping solution while the rare earth magnet having the Ni-containing coating on its surface is immersed in the stripping solution. 5. A method for recycling rare earth magnets according to any one of 1 to 4, characterized in that the immersion temperature of the rare earth magnet having the Ni-containing coating on its surface in the stripping solution is 20 to 70°C. 6. A method for recycling rare earth magnets according to any one of claims 1 to 5, characterized in that the pH of the stripping solution is 10.0 to 13.5. 7. A method for recycling rare earth magnets according to any one of 1 to 6, characterized in that the change in magnetic flux density and coercivity of the rare earth magnet before and after immersion in the stripping solution is 1.0% or less. [Effects of the Invention]

[0014] According to the present invention, the Ni plating applied to the surface of rare earth magnets can be removed without degrading the properties of the rare earth magnets, thereby enabling the recycling of the rare earth magnets and improving the product yield of the rare earth magnets. [Modes for carrying out the invention]

[0015] The rare earth magnets to which the recycling method for rare earth magnets of the present invention can be applied are rare earth magnets having a Ni-containing coating on their surface, and the method is particularly suitable for Nd sintered magnets that have been Ni-plated (especially electrolytic Ni-plated). Among rare earth magnets, defective products generated during the manufacturing process of rare earth magnets, particularly defective products having defects in the Ni coating, can be suitable for recycling. Defective products here refer mainly to those that have defects in the Ni plating film (such as blistering, scratches, unplated areas, or stains), but the magnet base itself does not have any problems in practical use. Such defective products can be used as good products by peeling off the coating and then re-treating the surface.

[0016] Furthermore, in addition to the method of re-surface treatment after stripping the coating as described above to reuse the magnets as good quality products, there is also a method of dissolving each element contained in the rare earth magnet in acid and then returning it to its raw material state by methods such as solvent extraction. In such methods, if Ni remains in the solution from which the elements contained in the rare earth magnet have been dissolved, there is a problem that it reduces the extraction efficiency of other elements that are more expensive than Ni. The Ni-containing coating stripping solution used in the recycling method of the present invention can effectively separate the Ni-containing coating from the rare earth magnet, and therefore can be applied not only to Ni-plated products that have large chips or cracks in the magnet base material that hinder practical use, but also as a pre-treatment for recycling to recover expensive elements from rare earth magnets contained in discarded HDDs and other items collected from the market.

[0017] The stripping solution used in the recycling method for rare earth magnets of the present invention for removing a Ni-containing coating from a rare earth magnet comprises an oxidizing agent that dissolves the Ni coating and the dissolved Ni 2+ The solution contains a chelating agent to stabilize the ions. As the oxidizing agent, a derivative of nitrobenzene is used, preferably at least one sodium nitrobenzenesulfonate selected from the group consisting of sodium o-nitrobenzenesulfonate, sodium m-nitrobenzenesulfonate, and sodium p-nitrobenzenesulfonate. As the chelating agent, ethylenediamine is used from the viewpoint of complex stability. In addition, ammonia water is added to easily achieve the appropriate pH for the peeling reaction for pH adjustment. Therefore, the peeling solution used in the rare earth magnet recycling method of the present invention to peel off the Ni-containing coating from the rare earth magnet can be said to be a solution containing a nitrobenzene derivative and ethylenediamine, with the pH adjusted with ammonia. Thus, in the rare earth magnet recycling method of the present invention, cyanide compounds such as sodium cyanide are not used as complexing agents, so there is no risk of pollution caused by cyanide wastewater.

[0018] In the method for recycling rare earth magnets of the present invention, for example, it can be carried out by immersing a rare earth magnet having a film containing Ni to be treated on its surface in the above-described stripping liquid contained in a stripping tank. The method of immersion is not particularly limited, and the amount of the stripping liquid and the amount of the rare earth magnet to be treated may be appropriately adjusted so that the rare earth magnet to be treated is sufficiently in contact with the stripping liquid. It can be applied either as a batch process or as a continuous process.

[0019] The amount of the nitrobenzene derivative contained in the above stripping liquid is preferably 20 to 50 g / L, more preferably 30 to 40 g / L. The saturated dissolution amount of the nitrobenzene derivative, particularly nitrobenzenesulfonate, in water is around 200 g / L. As long as the amount used is within the above range and the solvent evaporation amount is realistic (allowed up to 75 vol%), precipitation can be prevented.

[0020] Also, the amount of ethylenediamine contained in the above stripping liquid is preferably 50 to 150 g / L, more preferably 80 to 120 g / L. After dissolution, Ni exists in the form of 2+ [Ni(en)3]. Therefore, stoichiometrically, a three-fold amount of substance of the nitrobenzene derivative is required. Since the amount used is around ten-fold when within the above range, the complex can exist very stably. In addition, in order to prevent the concentration from decreasing due to volatilization, it is preferable to provide a lid on the stripping tank.

[0021] The pH of the above-mentioned stripping solution is preferably 10.0 to 13.5, more preferably 10.5 to 12.0. 25% aqueous ammonia can be applied for pH adjustment, and the usage amount thereof is preferably 10 to 80 g / L. Here, the pH immediately after adjusting the stripping solution is around 11.0, and the pH increases as the dissolution of Ni proceeds. When the pH exceeds 12.0, the stripping rate begins to gradually become dull, and the pH when finally losing the stripping ability is around 13.5. The stripping solution is 1 L and can dissolve 10 to 20 g of Ni. It is economical to use up the once-adjusted stripping solution until it loses its ability and then update it. Also, when the pH during adjustment is less than 10.0, the stripping rate significantly decreases. The pH value mentioned here is the measured value at 15 to 25 °C, and a commercially available measuring device can be used for measurement. The concentration of ammonia in the aqueous ammonia used for pH adjustment is not limited to 25% by mass and can be appropriately changed. When the concentration of ammonia in the aqueous ammonia used for pH adjustment is other than 25% by mass, the usage amount of the aqueous ammonia is, for example, the usage amount of ammonia equivalent to the case of using 25% aqueous ammonia with the above-mentioned usage amount. That is, when the concentration of ammonia in the aqueous ammonia used for pH adjustment is other than 25% by mass, the usage amount of the aqueous ammonia is preferably 10 to 80 g / L in terms of 25% aqueous ammonia.

[0022] Furthermore, the immersion temperature of the above-mentioned rare earth magnet in the above-mentioned stripping solution is preferably 20 to 70 °C, more preferably 45 to 55 °C. Within such a range, it is possible to suppress the decrease in the stripping rate of the film due to the low liquid temperature of the stripping solution and maintain good magnetic properties after assembling the rare earth magnet after stripping. Also, the pH of the stripping solution may exceed 13 at most, and the higher the liquid temperature, the greater the risk of thermal demagnetization of the Nd sintered magnet. Therefore, it is desirable that the stripping tank containing the stripping solution has a temperature control mechanism for preventing overheating in a heating facility such as a heater.

[0023] The stripping tank containing the stripping solution preferably has a barrel rotation or oscillating mechanism to prevent residual stripping. Such a mechanism allows for efficient removal of the coating. Although not particularly limited, rare earth magnets are prone to chipping after the coating is removed, so the rotation speed, especially when introducing a barrel rotation mechanism, is preferably 10 rpm or less. Adding chemical-resistant spheres (e.g., Al2O3 or Zr2O3) in an amount about twice the volume of the rare earth magnet to be stripped is highly effective in preventing chipping and cracking.

[0024] During the peeling process, from the viewpoint of improving the peeling speed, it is preferable to apply ultrasonic waves with a frequency of 20 to 100 kHz to the peeling solution while the rare earth magnets are immersed in the peeling solution. This improves the peeling speed of the coating and significantly reduces residual peeling caused by adhesion between the rare earth magnets and contact with the peeling basket. In this case, since it becomes difficult to control the liquid temperature due to the application of ultrasonic waves, it is preferable that the capacity of the peeling tank containing the peeling solution be 50 L or more. When the coating is peeled under these conditions, it is possible to peel off the Ni-containing coating at a rate of 5 to 20 μm / hr with the peeling solution immediately after preparation, making it possible to peel very efficiently compared to conventional techniques.

[0025] In the rare earth magnet recycling method of the present invention, the solution containing a nitrobenzene derivative, ethylenediamine, and ammonia removes the Ni-containing coating but hardly erodes the substrate of the rare earth magnet. As a result, the magnetic properties of the rare earth magnet are hardly changed before and after the removal of the Ni-containing coating. Consequently, it is possible to perform Ni plating again without any further treatment after the coating is removed, resulting in extremely high recycling efficiency. The change in residual magnetic flux density and coercivity before and after the coating removal is preferably 1.0% or less, more preferably 0.5%, taking into account variations between individual magnets and between lots. [Examples]

[0026] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples.

[0027] [Example 1] A stripping solution with the composition of 40 g / L of sodium m-nitrobenzenesulfonate, 100 g / L of ethylenediamine, and 40 g / L of 25 wt% aqueous ammonia was prepared and placed in a stripping tank. Nd sintered magnets measuring 7 mm in length, 7 mm in width, and 1.5 mm in thickness were immersed in the stripping solution for 96 hours under the conditions shown in Table 1, and the presence or absence of weight loss before and after immersion was confirmed. After that, each Nd sintered magnet was magnetized and heat-treated in a 120°C oven for 1 hour, then magnetized again, and its magnetic properties were measured using a pulse tracer (manufactured by Toei Kogyo Co., Ltd.). The results are shown in Table 1.

[0028] [Example 2] The test was carried out in the same manner as in Example 1, except that 15 g / L of Ni was dissolved and the pH became 13.

[0029] [Example 3] Seventy liters each of stripping solutions with the following compositions were prepared: sodium m-nitrobenzenesulfonate: 20 g / L, ethylenediamine: 100 g / L, and 25 wt% aqueous ammonia: 40 g / L, and placed in a stripping tank. The test was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0030] [Comparative Example 1] The test was carried out in the same manner as in Example 1, except that ethylenediamine was used at a concentration of 80 g / L and ethylenediaminetetraacetic acid was used as a chelating agent at a concentration of 20 g / L. The results are shown in Table 1.

[0031] [Comparative Example 2] The test was carried out in the same manner as in Example 3, except that the pH of the stripping solution was adjusted to 11 using ammonium chloride and oxalic acid instead of 25 wt% aqueous ammonia. The results are shown in Table 1.

[0032] [Table 1] *1 The solution used was one that was on the verge of losing its peeling ability. All other values ​​are from immediately after preparation. *2 The demagnetization curve was measured and compared using the value at the intersection with the line where Pc (permeance) = 0. *3 The demagnetization curve was measured, and the coercivity value at a magnetic flux density of 90% was used for comparison.

[0033] As shown in Table 1, when the stripping solutions of Examples 1 to 3 were used, the substrate of the Nd sintered magnet was not damaged at all before and after immersion, and the magnetic properties did not deteriorate at all. In contrast, when the stripping solutions of Comparative Examples 1 and 2 were used, the substrate of the Nd sintered magnet was damaged simply by immersion, and the magnetic properties deteriorated significantly. This is thought to be because components such as oxalate ions and chloride ions used to adjust the pH of the stripping solution damaged the substrate of the Nd sintered magnet.

[0034] As described above, 3.0 mm thick Nd sintered magnets for HDDs, which had a minimum thickness of 10 μm of electrolytic Ni plating, were immersed in the stripping solutions of Examples 1 and 3, which do not damage the substrate of the Nd sintered magnets. The process was carried out under conditions of a stripping solution temperature of 50°C and ultrasonic application at 26 kHz until the plating film was completely removed, and the weight loss was recorded. After confirmation of removal, a new Nd sintered magnet of the same shape was placed in the immersion solution, and this operation was repeated until some of the plating film remained, and the cumulative weight loss was recorded as the Ni stripping ability. The stripped magnets were then re-plated, and reliability tests (corrosion resistance, adhesion) were performed. The results are shown in Table 2.

[0035] [Table 2] *1 No rust was observed during a pressure cooker test (exposure to saturated steam at 120°C and 2 atm for 48 hours). *2 Tensile test result: 100 kg / cm 2 That's all.

[0036] The composition of the stripping solution and Table 2 suggest that the stripping ability depends on the amount of oxidizing agent (nitrobenzene derivative) contained in the stripping solution. Furthermore, Nd sintered magnets that had their plating film stripped using the stripping solutions of Examples 1 and 3 and then re-plated showed good corrosion resistance and adhesion.

[0037] As described above, the stripping solutions of Examples 1 to 3 did not damage the substrate of the Nd sintered magnets, thus not degrading their magnetic properties. Furthermore, no problems were caused with the plating properties during the re-plating process after stripping the plating film. Thus, it has been shown that the stripping solution of the present invention can strip the Ni plating applied to the surface of Nd sintered magnets without degrading the magnetic properties of the magnets, and the magnets can be recycled by re-plating, thereby improving product yield.

Claims

1. A rare earth magnet having a Ni-containing coating on its surface is immersed in a stripping solution containing a nitrobenzene derivative, ethylenediamine, and ammonia. The stripping tank containing the stripping liquid has a barrel rotation mechanism or a rocking mechanism. A method for recycling rare earth magnets, characterized in that the barrel rotation mechanism or oscillating mechanism includes the rare earth magnet and chemical-resistant spheres in an amount twice the volume of the rare earth magnet.

2. The method for recycling rare earth magnets according to claim 1, characterized in that the nitrobenzene derivative is at least one sodium nitrobenzenesulfonate selected from the group consisting of sodium o-nitrobenzenesulfonate, sodium m-nitrobenzenesulfonate, and sodium p-nitrobenzenesulfonate.

3. The method for recycling rare earth magnets according to claim 1 or 2, characterized in that the rare earth magnet having the Ni-containing coating on its surface is a defective product generated in the manufacturing process of rare earth magnets.

4. A method for recycling a rare earth magnet according to any one of claims 1 to 3, characterized in that ultrasonic waves with a frequency of 20 to 100 kHz are applied to the stripping solution while the rare earth magnet having the Ni-containing coating on its surface is immersed in the stripping solution.

5. A method for recycling rare earth magnets according to any one of claims 1 to 4, characterized in that the immersion temperature of the rare earth magnet having the Ni-containing coating on its surface in the stripping solution is 20 to 70°C.

6. The method for recycling rare earth magnets according to any one of claims 1 to 5, characterized in that the pH of the stripping solution is 10.0 to 13.

5.

7. A method for recycling rare earth magnets according to any one of claims 1 to 6, characterized in that the change in magnetic flux density and coercivity of the rare earth magnet before and after immersion in the stripping solution is 1.0% or less.

Images