How to recycle used or discarded permanent magnets

Abstract

How to recycle used or discarded permanent magnets. This method of recycling used or discarded permanent magnets and scrap from the production of such magnets consists in demagnetizing said permanent magnets and, after reducing them to powder, completely oxidizing the latter, then subjecting the products resulting from this oxidation to dissolution and finally filtration. The oxidation is carried out in two successive substeps: A first partial oxidation substep at a temperature below 500°C. Then a second complete oxidation substep at a higher temperature, in particular between 600 and 800°C.

Description

[Technical Field]

[0001] The present invention is aimed at a new method intended to recover rare earth elements present in used or discarded permanent magnets, or also in scrap from the production of such permanent magnets. More specifically, the permanent magnets under consideration are of the neodymium-iron (Nd-Fe-B) or samarium-cobalt (Sm-Co) type.

[0002] The method of the present invention is part of an approach aimed at satisfying both economic and environmental requirements.

[0003] In the economic sphere, the recovery of rare earths, which can be present in such magnets at up to 35% by weight, is a key factor, exacerbated by the current geopolitical situation: in fact, the increasing global demand for rare earths, combined with the limited number of rare earth-producing countries, may create market procurement risks for these metals, making the recovery of rare earths more topical than ever.

[0004] In the environmental field, the currently known recovery methods are not satisfactory, at least on an industrial scale, due to the simultaneous production of solid waste or of liquid waste. [Background technology]

[0005] background Currently known methods for recycling permanent magnets include two main families: the reprocessing of magnets without melting them, by forming new magnets directly from the powders thus recycled, so-called short-loop recycling; and The so-called long loop method, which requires melting the magnets and then separating the different rare earths with the aim of producing pure oxides and thus supplying the magnet market.

[0006] The present invention belongs to this second family.

[0007] These methods include, in particular, pyrometallurgy and / or hydrometallurgy. Hydrometallurgy is based on liquid-liquid extraction techniques and allows rare earths to be recovered from an acidic aqueous phase, typically involving total dissolution of the magnet followed by iron removal (in the context of neodymium-iron-boron magnets) by increasing the pH. Such hydrometallurgical recycling processes are described, for example, in documents US 5,362,459 or WO 96 / 00698. In the latter, the magnet is oxidized by heat treatment, after which the oxidized powder is dissolved in hydrochloric acid. Iron-neodymium separation is carried out by oxalic acid precipitation of neodymium oxalate, which has a much lower solubility than iron oxalate.

[0008] In the hydrometallurgical processes carried out, an excessive consumption of acid and then base for the removal of iron by neutralization can frequently be observed.

[0009] Another type of hydrometallurgical process aims at oxidizing, in whole or in part, the alloys contained in permanent magnets with a view to selectively dissolving rare earths, in particular: Thermal oxidation (high-temperature magnet calcination) that allows obtaining a mixture of iron oxide and rare earths High-pressure hydrothermal magnet oxidation, which makes it possible to obtain rare earth hydroxides soluble in acid medium and magnetite, which can be easily separated by its magnetic properties. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 5,362,459 [Patent Document 2] International Publication No. 96 / 00698 Summary of the Invention [Problem to be solved by the invention]

[0011] Even if, on a laboratory scale, various hydrometallurgical techniques are offered that make it possible to carry out selective recycling of rare earths, real difficulties can be observed on an industrial scale.

[0012] These include in particular the fact that during oxidation temperatures above 600° C. may be reached, which may cause agglomeration and sintering of parts of the material, preventing the obtaining of a uniform oxide phase.

[0013] If the temperature during oxidation is higher than 600-900°C, the formation of mixed oxide TRFeO3 (TR = rare earth) can be observed. This oxide is more refractory than rare earth oxides and requires more severe attack conditions, resulting in a decrease in efficiency or selectivity during melting. Uncontrolled oxidation of magnet powders results in temperatures that can exceed 1000°C, thus favoring the formation of mixed oxides.

[0014] Furthermore, during hydrothermal oxidation, the oxidation kinetics mainly depend on the total gas pressure in the treatment chamber (typically an autoclave). Typically, to achieve kinetics that allow the process to be carried out on an industrial scale, pressures of the order of 40 to 200 bar must be applied, thereby complicating the actual recycling plant.

[0015] The real difficulty arises from the currently practiced methods due to the exothermic nature of the oxidation reaction, which is not controlled and which can create serious problems during industrial implementation. [Means for solving the problem]

[0016] The present invention provides a method for recycling used or discarded permanent magnets and further scrap from the production of such magnets, carrying out such hydrometallurgical processes on an industrial scale, and overcoming the drawbacks of currently known processes.

[0017] Discussion of the Invention According to the invention, this recycling method consists in demagnetizing said permanent magnets and, after reducing them to powder, oxidizing the latter in conditions that make it possible to improve the recovery efficiency of rare earths and their dissolution selectivity with respect to iron or cobalt.

[0018] According to the invention, the aforementioned powder is subjected to complete oxidation. At the end of this oxidation step, the product obtained is subjected to a dissolution step and finally to a filtration step.

[0019] According to the invention, the oxidation step is carried out in two successive steps: a first substep at a partial oxidation temperature below 500°C; Then a second sub-step of complete oxidation at a higher temperature, in particular between 600 and 800°C.

[0020] The implementation of these two oxidation substeps allows for much more efficient control of the temperature, avoiding problems of agglomeration and sintering and limiting the formation of refractory mixed oxides of those aforementioned types.

[0021] Typically, in the context of recycling neodymium iron boron permanent magnets, the product resulting from the first oxidation sub-step has a mostly amorphous structure and is in the zero oxidation state (typically, TRFe 14 It is particularly characterized by the absence of rare earth-containing phases (TRFeB and TRFe4B), the absence of refractory mixed oxides of the TRFeO3 type, and the presence of iron metallic or in the form of iron oxide, in particular magnetite Fe3O4.

[0022] As a result, the product resulting from the second oxidation substep has a mostly crystalline structure and is particularly characterized by the absence of metallic iron or rare earth elements, the presence of iron in the form of oxides, mainly hematite (Fe2O3), and a limited proportion (typically less than 10% by weight) of refractory mixed oxides of the TRFeO3 type.

[0023] According to an advantageous feature of the invention, between the oxidation and dissolution steps, a step of comminution of the product resulting from the oxidation takes place, the purpose of which is to release the rare earth oxides outside the iron oxide layer that forms on the outside of the grains, which in turn makes it possible to accelerate the kinetics of the subsequent dissolution step.

[0024] Typically, this grinding step results in obtaining particles of 100 micrometers or less.

[0025] It can further be envisaged that this grinding step is carried out in a humid atmosphere in order to reduce the phenomenon of overdispersion of the fine powder.

[0026] Advantageously, the first oxidation substep is carried out at a temperature between 150 and 500°C, preferably between 400 and 500°C.

[0027] In parallel, this first oxidation substep can be carried out in air provided to a temperature that is less than or equal to 400°C.

[0028] However, if this temperature is higher than 400°C, and therefore in the range of 400°C to 500°C, then this substep must occur: under a moist inert gas mixture (nitrogen N2 or argon Ar); or Either air, but with an oxygen content of not more than 15% by volume;

[0029] The gas may be moist, with a water content between 5 and 50% by volume. If it is chosen to operate in the absence of oxygen, the gas must necessarily be moist to obtain the desired oxidation.

[0030] Indeed, if this partial pressure is higher than 50% by volume, the first substep may result in too high a temperature and excessive recondensation at the cold spots of the circuit, whereas if the partial pressure is lower than 5% by volume, the reaction will not proceed to the next step efficiently.

[0031] The duration of this first substep ranges from 10 minutes to 10 hours, preferably from 30 minutes to 2 hours.

[0032] According to the invention, the second oxidation substep in air is carried out at a temperature between 600°C and 800°C, preferably between 650°C and 750°C, and for a period of 10 minutes to 10 hours, preferably between 30 minutes and 2 hours.

[0033] Ideally, the particle size distribution of the demagnetized powder undergoing these oxidation steps ranges from fragments on the order of 1 millimeter to particle sizes on the order of 4 micrometers. Preferably, this particle size is in the range of 100 to 500 micrometers. Indeed, if this particle size is too small, an excessive dispersion of fine particles can be observed, which may foul filters and other existing ducts or end up in the furnace where this operation takes place. However, if this particle size is too large, then the size of the coarse particles will prevent complete oxidation of the powder, thus altering the efficiency of the method of the present invention.

[0034] According to another characteristic of the invention, the dissolution step is carried out in an acid medium, the acid carried out being advantageously chosen from the group consisting of hydrochloric acid, nitric acid and sulfuric acid.

[0035] Advantageously, the temperature of this dissolution step is from room temperature to 90° C., preferably between 60° C. and 90° C. Typically, if the temperature is lower than 60° C., the kinetics of this dissolution step are altered. However, if the temperature is higher than 90° C., excessive evaporation of water and of the acid carried out can be observed, necessitating recondensation, which complicates the process.

[0036] To optimize the recycling method of the present invention, the amount of acid applied during this dissolution step plays an important role. Typically, the stoichiometry of the acid is in the range of 1 to 2, preferably 1 to 1.2, relative to that of the rare earth.

[0037] Indeed, if the concentration of the acid is substoichiometric with respect to the rare earths, there is a loss of efficiency in the recycling of the rare earths that is desired to be obtained.

[0038] However, if the acid concentration is superstoichiometric, on the one hand, the amount of acid consumed is effectively excessive, and on the other hand, more iron than desired is dissolved, which then exists in the form of Fe(II) or Fe(III) ions and must subsequently be removed by adjusting the redox potential and the pH, resulting in an overconsumption of base. Furthermore, filtration of the thus reprecipitated iron is relatively less efficient than filtration of undissolved ferric oxide.

[0039] The duration of this dissolution step ranges from 1 to 10 hours, advantageously from 2 to 6 hours.

[0040] According to the invention, the oxidation step is carried out in a furnace, e.g. static muffle furnaces incorporating a chamber heated by gas or by electrical resistance, in which the powder is loaded into a crucible; passage kilns, i.e. heated tunnels through which crucibles containing powder are progressively advanced; rotary furnaces, i.e. tubes heated by gas or by electrical resistance, in which the powder circulates due to the combined effect of the tilt applied to the tube and its rotation; Fluidized bed reactors, i.e. reactors containing powders into which hot gases are injected at a velocity such that said powders are in motion and have flow characteristics approaching those of a fluid.

[0041] The manner in which the invention can be put into practice and the resulting advantages will appear better from the following examples, given by way of non-limiting indication, in connection with the accompanying drawings, in which: [Brief explanation of the drawings]

[0042] [Figure 1] FIG. 1 is a simplified representation illustrating the different steps of the method of the present invention. [Figure 2] FIG. 2 is a simplified representation that more specifically depicts the two oxidation steps of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0043] Detailed Description of the Invention According to the method of the present invention, whose main steps are depicted diagrammatically in Figure 1, the product to be recycled typically consists of used or discarded permanent magnets. Alternatively, it may consist of scrap resulting from the operation of the production of such magnets, said scrap also containing rare earths.

[0044] These recycled products are first demagnetized, which is typically achieved by heating above the Curie temperature, typically 350° C. for Nd—Fe—B magnets.

[0045] They are then reduced to a powder, preferably by a crushing step (100) followed by a grinding step (101), ideally to typically reach particle sizes in the range of 100 to 500 micrometers.

[0046] The powder thus obtained is then subjected to a first sub-oxidation (102) which results in the partial oxidation of the rare earths present in said powder.

[0047] For this purpose, the powder, stored for example in a tank (1) (Figure 2), is fed by means of a worm screw (3) into a first furnace (2), typically a static muffle furnace, a passage kiln, a rotary kiln or a fluidized bed reactor.

[0048] In this first furnace, a gas is present, for example a mixture of nitrogen or argon, with a water content between 5% and 50% by volume.

[0049] The temperature in this first furnace is typically between 250 and 500° C., and the duration of treatment in this furnace is ideally between 30 minutes and 2 hours.

[0050] This moist gas can be replaced by air. In this configuration, the temperature required in the furnace is then a maximum of 400° C., and the treatment time is then ideally between 30 minutes and 2 hours.

[0051] Whatever gas is present in the furnace, its purpose is to oxidize the rare earths present in the powder.

[0052] Once this first oxidation substep (102) has been carried out, the powder will then undergo a second oxidation substep (103) with the aim of achieving complete oxidation of the rare earths contained in said powder.

[0053] For this purpose, the powder stored in a hopper (4) at the outlet of the furnace (2) is conveyed here again by means of a worm screw (5) to a buffer tank (8).

[0054] During this second oxidation substep, the powder stored in the buffer tank (8) is conveyed by means of a worm screw (9) to the furnace (6), in which it is then subjected to a new treatment, this time exclusively in air, at a temperature between 500 and 800°C, preferably between 600 and 800°C. This furnace (6) can be of the same nature as the furnace (2). The duration of this treatment is also between 30 minutes and 2 hours.

[0055] The objective of this second substep is to achieve complete oxidation of the elements present in the powder.

[0056] The latter is then stored in a tank (7).

[0057] It is then subjected to grinding (104), especially in wet conditions, to obtain a particle size of less than 100 micrometers.

[0058] The thus reduced powder is then subjected to a dissolution step (105).

[0059] This dissolution step is carried out in an acid medium, advantageously with nitric acid, at a temperature between 60° C. and 90° C. For this purpose, the powder is conveyed into a reactor equipped with an agitator.

[0060] The amount of acid applied is such as to provide a stoichiometry of between 1 and 1.2 with respect to the rare earths. In practice, due to the relative uncertainty regarding the actual rare earth content in the powder, dissolution is driven by varying the pH. For this purpose, the pH at equilibrium, i.e., several hours after the start of this step, is in the range of 0 to 4, preferably 0.5 to 2.

[0061] The liquid / solid ratio is in the range of 1 to 5, preferably 1.5 to 3.

[0062] The duration of residence in the dissolution reactor is between 1 and 10 hours, preferably between 2 and 6 hours.

[0063] It should be clear that the overall method can be applied in the same way whatever the rare earth composition of the magnet: the efficiency or yield is the same for all rare earths with the same behavior.

[0064] After dissolution, filtration (106) is then carried out, in particular using a press filter, in order to separate the rare earths in the form of nitrates from the oxides, preferably iron oxide, in the solution.

[0065] The following examples were carried out from NdFeB magnets with the following compositions:

[0066] [Table 1]

[0067] The rare earth yield is calculated as the combined mass yield of Nd, Pr, Dy, and Tb elements.

[0068] Example 1 NdFeB magnets are powder (d 100 <250 μm) and the powder is treated in a rotary furnace at 500 °C for 4 hours under a moist nitrogen flow (450 l / h with 4% H2O). At the end of the treatment the mass gain is 12%. By mass gain is meant the ratio (final mass - initial mass) / initial mass.

[0069] The powder thus partially oxidized is again treated in a rotary furnace under air flow at 700° C. for 2 hours, the cumulative mass gain at the end of this treatment being 30%.

[0070] The powder obtained is placed in suspension in water and nitric acid is added in an excess of 10 mol % with respect to the rare earths contained in the powder.

[0071] The dissolution occurs at 90°C and a liquid / solid ratio of 2.

[0072] After filtration and washing of the resulting cake, the efficiency of dissolution, i.e., the molar amount of rare earths relative to the molar amount in the starting powder, for each rare earth is 95%. The total concentration of rare earths is 132 g / L and the concentration of iron is 0.01 g / L, i.e., a mass ratio of TR / Fe in the solution of 13,200. The final pH is 0.5.

[0073] Example 2 NdFeB magnets are powder (d 100 <250 μm) and the powder is processed in a muffle furnace with the following temperature gradient: - In air, the temperature rises to 400°C within an hour; - Then heat in air at 400°C for 4 hours; - in air in the furnace (6), the temperature rises to 600°C within 1 hour; - Then heat at 600°C under air flow for 4 hours.

[0074] The powder obtained is coarsely ground to break down agglomerates. At the end of this process, the mass gain is 27%. The powder is placed in suspension in water and hydrochloric acid is added in an excess of 220 mol% with respect to the rare earths contained in the powder. The dissolution occurs at 90°C and with a liquid / solid ratio of 5. After filtration and washing of the solids, the efficiency of dissolution of each rare earth is 95%. The total rare earth concentration is 44 g / l and the iron concentration is 54 g / l, i.e., a TR / Fe ratio of 0.81. The final pH is 3.

[0075] Example 3 NdFeB magnets are powder (d 100 The powder is reduced in size to a diameter of <250 μm, and 15 kg of the powder is processed in a fluidized bed reactor. The powder is first fluidized with nitrogen flow and water is injected to reach a temperature of nearly 500°C.

[0076] The water supply is then turned off and the nitrogen flow is replaced by an air flow up to a temperature of 850° C. This is the second oxidation substep (103).

[0077] The powder obtained is coarsely ground to break down agglomerates. At the end of this process, the mass gain is 30%. The powder obtained is placed in suspension in water, to which nitric acid is added in a 20% excess relative to the rare earths contained in the powder. The dissolution takes place at 90°C and with a liquid / solid ratio of 2. After filtration and washing of the solids, the efficiency of dissolution of each rare earth is 93%. The total rare earth concentration is 140 g / l and the iron concentration is 19 g / l, i.e., a TR / Fe mass ratio of 7.4. Therefore, there is a slight decrease in yield, but above all a very strong decrease in selectivity.

[0078] It can thus be observed that by carrying out the process of the present invention, in particular the oxidation of the powder in the two given steps and within the temperature ranges indicated, a yield of 95% and excellent selectivity can be obtained.

[0079] Furthermore, when nitric acid is replaced by hydrochloric acid, high iron concentrations are maintained despite a final pH of 3.0. Nitric acid is thus preferred because this practice contributes to optimizing rare earth / iron selectivity (iron is stable in solution) even at relatively high pH.

[0080] As a result, when the temperature of the oxidation is too high (Example 3), there is a slight decrease in yield, but above all a loss of selectivity.

Claims

1. A method for recycling used or discarded permanent magnets and scrap from the production of such magnets, consisting in demagnetizing said permanent magnets and pulverizing them, and then completely oxidizing the latter, and then subjecting the products resulting from this oxidation to dissolution and finally to filtration, characterized in that said oxidation is carried out in two successive substeps: a first substep of partial oxidation (102) at a temperature below 500° C., Then a second substep of complete oxidation (103) at higher temperatures, in particular between 600 and 800°C.

2. 2. A method for recycling permanent magnets according to claim 1, characterized in that between the steps of oxidation (102, 103) and melting (105) a step (104) of comminution of the products resulting from said oxidation is carried out.

3. 3. The method for recycling permanent magnets according to claim 2, characterized in that the crushing step (104) results in obtaining particles having a particle size of 100 micrometers or less.

4. 4. The method for recycling permanent magnets according to claim 2, wherein the grinding step (104) is carried out in a humid atmosphere.

5. 5. A method for recycling permanent magnets according to any one of claims 1 to 4, characterized in that the first oxidation substep (102) is carried out at a temperature between 150°C and 500°C, preferably between 400°C and 500°C.

6. 6. A method for recycling permanent magnets according to any one of claims 1 to 5, characterized in that the first oxidation substep (102) is carried out in air at a temperature below 400°C.

7. The first oxidation substep (102) is carried out with a wet inert gas mixture (nitrogen N ２ 6. The method for recycling permanent magnets according to claim 1, wherein the method is carried out under atmospheric pressure (or argon Ar).

8. 6. A method for recycling permanent magnets according to any one of claims 1 to 5, characterized in that the first oxidation substep (102) is carried out in air, but with an oxygen content of not more than 15% by volume.

9. 9. A method for recycling permanent magnets according to any one of claims 6 to 8, characterized in that the gas is moist with a water content between 5 and 50% by volume.

10. 10. A method for recycling permanent magnets according to any one of claims 1 to 9, characterized in that the duration of the first oxidation substep (102) is in the range of from 10 minutes to 10 hours, preferably from 30 minutes to 2 hours.

11. 11. A method for recycling permanent magnets according to any one of claims 1 to 10, characterized in that the second oxidation substep (103) in air is carried out at a temperature between 500 and 800°C, preferably between 600 and 800°C, and for a duration of between 10 minutes and 10 hours, preferably between 30 minutes and 2 hours.

12. 12. A method for recycling permanent magnets according to any one of claims 1 to 11, characterized in that the demagnetized powder consists of fragments of the order of 1 millimeter up to particle sizes of the order of 4 micrometers, preferably in the range of 100 micrometers to 500 micrometers.

13. 13. A method for recycling permanent magnets according to any one of claims 1 to 12, characterized in that the dissolution step (105) is carried out in an acid medium, the acid used being advantageously chosen from the group consisting of hydrochloric acid, nitric acid and sulfuric acid.

14. 14. A method for recycling permanent magnets according to claim 13, characterized in that the dissolving step (105) is carried out at room temperature up to 90°C, preferably between 60°C and 90°C.

15. 15. A method for recycling permanent magnets according to any one of claims 13 and 14, characterized in that the stoichiometry of the acid carried out is in the range of from 1 to 2, preferably from 1 to 1.2, with respect to the rare earths contained in the magnet.

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