RECYCLING PROCESS FOR USED OR WASTE PERMANENT MAGNETS

Abstract

This recycling process for used or discarded permanent magnets and production scraps of such magnets consists, after demagnetizing the permanent magnets and reducing them to powder, of completely oxidizing this powder, then subjecting the resulting product to dissolution and finally filtration. The oxidation is carried out in two successive sub-steps: – a first sub-step of partial oxidation at a temperature not exceeding 500 °C; – then a second sub-step of total oxidation at a higher temperature, specifically between 600 and 800 °C.

Description

Title of the invention: METHOD FOR RECYCLING USED OR WASTE PERMANENT MAGNETS Field of invention

[0001] The invention relates to a new process for recovering rare earth elements present in used or discarded permanent magnets, or in production scraps of such permanent magnets. More specifically, the permanent magnets in question are of the neodymium-iron (Nd-Fe-B) or samarium-cobalt (Sm-Co) type.

[0002] The process of the invention is part of an approach aimed at satisfying both economic and ecological conditions.

[0003] From an economic standpoint, the recovery of rare earth elements, which can be present in such magnets at up to 35% by weight, is proving to be a determining factor, further exacerbated by the current geopolitical context. Indeed, combined with a growth in global demand for rare earth elements, as well as a limited number of rare earth producing countries, which could lead to supply risks for the markets of these metals, the recovery of rare earth elements is more relevant than ever.

[0004] From an ecological standpoint, the recovery processes known to date are not satisfactory, at least on an industrial scale, due to the simultaneous production of solid waste or liquid effluents. Prior art

[0005] Among the various processes for recycling permanent magnets known to date, two main families stand out, respectively: - so-called short-loop recycling, which reprocesses magnets without dissolving them and by reforming magnets directly from the recycled powder, and - the so-called long loop processes, which require the dissolution of magnets, then the separation of the different rare earths in order to produce pure oxides and thus supply the magnet market.

[0006] The invention falls into this second family.

[0007] These processes include, in particular, pyrometallurgy and / or hydrometallurgy. The hydrometallurgical process is based on the liquid-liquid extraction technique, which allows the recovery of rare earth elements from an acidic aqueous phase and typically involves the complete dissolution of the magnet followed by the removal of iron (in the case of neodymium-iron-boron magnets) by raising the pH. Such hydrometallurgical recycling processes have, for example, been described in the document US 5,362,459 or WO 96 / 00698. In the latter, the magnet is oxidized by heat treatment, followed by dissolution of the oxidized powder with hydrochloric acid. The iron-neodymium separation is achieved by oxalic precipitation of neodymium oxalate, which has a much lower solubility than iron oxalate.

[0008] Among the hydrometallurgical processes implemented, excessive consumption of acid is frequently observed, followed by consumption of base to remove iron by neutralization.

[0009] Other types of hydrometallurgical processes aim to oxidize, in whole or in part, the alloys contained in permanent magnets, in order to achieve the selective dissolution of rare earths, and in particular: - thermal oxidation (calcination of magnets at high temperature) allowing to obtain a mixture of iron oxide and rare earths; - the hydrothermal oxidation of high-pressure magnets, which allows the obtaining of rare earth hydroxide, soluble in acidic medium, and magnetite, easily separable due to its magnetic character.

[0010] While, at the laboratory scale, the various hydrometallurgical techniques proposed have made it possible to carry out the selective recycling of rare earths, real difficulties are observed at the industrial scale.

[0011] Among these, we can notably mention that during oxidation, temperatures above 600°C are likely to be reached, which can lead to the agglomeration and sintering of part of the material, preventing the obtaining of a homogeneous oxidized phase.

[0012] When oxidation temperatures exceed 600-900°C, a mixed oxide, TRFeO3 (TR = rare earth elements), is formed. This oxide is more refractory than rare earth oxides and requires more severe etching conditions, resulting in a loss of yield or a loss of selectivity during dissolution. Uncontrolled oxidation of the magnet powder leads to temperatures exceeding 1000°C, thus promoting the formation of mixed oxides.

[0013] Furthermore, during hydrothermal oxidation, the oxidation kinetics depend primarily on the total gas pressure within the treatment vessel (typically an autoclave). Typically, in order to achieve acceptable kinetics for implementing a process on an industrial scale, pressures on the order of 40 to 200 bar must be applied, consequently complicating the recycling installation itself.

[0014] The processes implemented to date present a real difficulty arising from the Texothermicity of the oxidation reaction, which is not controlled, and which is sustained likely to cause serious problems during industrial implementation.

[0015] The invention proposes a process for recycling used or discarded permanent magnets, and in addition production scraps of such magnets, implementing such a hydrometallurgical process on an industrial scale and overcoming the disadvantages of processes known to date. Description of the invention

[0016] According to the invention, this recycling process consists, after demagnetizing said permanent magnets and, after reducing them to powder, in oxidizing the latter under conditions allowing the improvement of the recovery yield of rare earths and the selectivity of dissolution with respect to iron or cobalt.

[0017] According to the invention, said powder is subjected to total oxidation. Following this oxidation step, the product obtained is subjected to a dissolution step, and finally to a filtration step.

[0018] According to the invention, the oxidation step is carried out in two successive sub-steps: - a first partial oxidation sub-step at a temperature not exceeding 500°C, - then a second sub-step of total oxidation at a higher temperature, and in particular at a temperature between 600 and 800 °C.

[0019] The implementation of these two oxidation substeps makes it possible to control the temperature much more effectively, avoiding agglomeration and sintering problems and limiting the formation of refractory mixed oxide of the type mentioned previously.

[0020] Typically, in the context of recycling neodymium-iron-boron type permanent magnets, the product from the first oxidation substep has a predominantly amorphous structure, and is characterized in particular by the absence of phase containing rare earths in the zero oxidation state (typically TR2Fei4B and TRFe4B), the absence of refractory mixed oxide of the type TRFeO3, the presence of iron in the form of metallic iron or iron oxide in particular magnetite Fe3O4.

[0021] Consequently, the product from the second oxidation substep has a predominantly crystalline structure, and is characterized in particular by the absence of iron or rare earths in the metallic state, the presence of iron in the form of oxide, mainly hematite (Fe2O3) as well as a limited proportion (typically less than 10% by mass) of refractory mixed oxide of the type TRFeO3.

[0022] According to an advantageous feature of the invention, a grinding step for the products resulting from oxidation occurs between the oxidation and dissolution steps. This grinding step is intended to release the earth oxides. rare outside the layer of iron oxides that forms on the outside of the grains. It also helps to accelerate the kinetics of the subsequent dissolution stage.

[0023] Typically, this grinding step results in the production of particles less than or equal to 100 micrometers.

[0024] It may also be envisaged to carry out this grinding step under a humid atmosphere, in order to reduce the phenomena of excessive flight of fines.

[0025] Advantageously, the first oxidation substep is carried out at a temperature between 150 and 500°C, preferably between 400 and 500°C.

[0026] In parallel, this first oxidation substep can be carried out in air, provided that the temperature is less than or equal to 400 °C.

[0027] On the other hand, if this temperature is greater than 400 °C, and therefore between 400 and 500 °C, this substep must occur: - either under a mixture of inert gas (nitrogen N2 or argon Ar) that is humid; - either in air but with an oxygen content not exceeding 15% by volume.

[0028] This gas may be humid 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 humid to obtain the desired oxidation.

[0029] Indeed, if this partial pressure is greater than 50% by volume, the first substep can lead to an excessively high temperature and excessive recondensation at the cold spots of the circuit. If the partial pressure is less than 5% by volume, the reaction progresses insufficiently to proceed to the next step efficiently.

[0030] The duration of this first sub-step is between 10 minutes and 10 hours, preferably between 30 minutes and 2 hours.

[0031] According to the invention, the second sub-step of oxidation in air is carried out at a temperature between 600 and 800 °C, preferably between 650 and 750 °C, for a period of 10 minutes to 10 hours, preferably between 30 minutes and two hours.

[0032] Ideally, the particle size of the demagnetized powder subjected to these oxidation steps consists of fragments on the order of millimeters, down to a particle size of approximately 4 micrometers. Preferably, this particle size is between 100 and 500 micrometers. Indeed, if this particle size is too small, there is an excessive release of fine particles, which can clog the filters and other pipes present in or leading to the furnace in which this operation takes place. If, on the other hand, this particle size is too large, the size of the coarse particles prevents complete oxidation of the powder and consequently reduces the efficiency of the process of the invention.

[0033] According to another feature of the invention, the dissolution step is carried out in an acidic medium, the acid used being advantageously chosen from the group including hydrochloric acid, nitric acid and sulfuric acid.

[0034] Advantageously, the temperature of this dissolution step is carried out at ambient temperature up to 90°C, and preferably between 60 and 90°C. Typically, if this temperature is below 60°C, the kinetics of this dissolution step are altered. If, on the other hand, this temperature is above 90°C, excessive evaporation of water and the acid used occurs, requiring recondensation, and thus complicating the process.

[0035] In order to optimize the recycling process of the invention, the dosage of the acid during this dissolution step plays an important role. Typically, the stoichiometry of the acid is 1 to 2 with respect to the rare earths, preferably 1 to 1.2.

[0036] Indeed, if the acid concentration is substoichiometric with respect to rare earths, we lose the rare earth recycling yield that we wish to obtain.

[0037] Conversely, if the acid concentration is over-stoichiometric, on the one hand, the amount of acid consumed is de facto excessive, and on the other hand, more iron is dissolved than desired, the iron then being present in the form of Fe(II) or Fe(III) ions, which must then be eliminated by adjusting the redox potential and pH, consequently leading to overconsumption of base. Furthermore, the filtration of the iron thus reprecipitated is relatively less efficient compared to the filtration of untreated ferric oxide.

[0038] The duration of this dissolution step is between 1 and 10 hours, advantageously between 2 and 6 hours.

[0039] According to the invention, the oxidation step is carried out within a furnace, and for example: - a static muffle furnace incorporating a chamber heated by gas or by electrical resistances in which the powder is loaded into crucibles, - a through furnace, that is to say a heated tunnel in which a crucible containing the powder advances progressively, - a rotary kiln, that is to say a tube heated by gas or by electrical resistances and in which the powder circulates by the combined effect of the slope imposed on the tube and its rotation, - of a fluidized bed furnace, that is to say a reactor containing the powder into which hot gas is injected at speeds such that said powder is set in motion and has flow characteristics close to those of a fluid. Brief description of the figures

[0040] The manner in which the invention can be implemented and the resulting advantages will be more apparent from the following example of implementation, given by way of illustration and not limitation, in support of the attached figures.

[0041] Fig. 1 is a schematic representation illustrating the different stages of the process of the invention.

[0042] Figure 2 is a schematic representation illustrating more specifically the two oxidation steps of the process of the invention. Detailed description of the invention

[0043] According to the process of the invention, the main steps of which are schematically illustrated in [Fig. 1], the recycled product typically consists of used or discarded permanent magnets. Alternatively, it may consist of scrap resulting from the production of such magnets, said scrap also containing rare earth elements.

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

[0045] Then they are reduced to powder in order to typically achieve a particle size ideally between 100 and 500 micrometers, in this case by a crushing step (100), followed by a grinding step (101).

[0046] The powder thus obtained then undergoes a first sub-oxidation (102), in order to achieve a partial oxidation of the rare earths present in said powder.

[0047] For this purpose, the powder, for example stored in a reservoir (1) ([Fig.2]), feeds a first furnace (2), typically a static muffle furnace, a pass-through furnace, a rotary furnace or a fluidized bed furnace, by means of a screw (3).

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

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

[0050] This inert, water-laden 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 ideally between 30 minutes and two hours.

[0051] Regardless of the gas present in the furnace, the objective sought is the oxidation of 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), the objective of which is to achieve the total 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 oven (2) is conveyed, again by means of a screw conveyor (5) to the level of a buffer tank (8).

[0054] During this second oxidation substep, the powder, stored in the buffer tank (8), is conveyed by means of a screw conveyor (9) to a furnace (6), where it undergoes further treatment, this time limited to air, at a temperature between 600 and 800 °C. This furnace (6) may be of the same type as the furnace (2). The duration of this treatment is also between thirty minutes and two hours.

[0055] The objective of this second sub-step is to achieve the total oxidation of the elements present in the powder.

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

[0057] It then undergoes grinding (104), in particular in a wet process, so as to result in a particle size less than or equal to 100 micrometers.

[0058] Then, the reduced powder then undergoes a dissolution step (105).

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

[0060] The acid dosage is such that a stoichiometry of between 1 and 1.2 is obtained with respect to the rare earths. In practice, due to the relative uncertainty regarding the actual rare earth content of the powder, the dissolution is controlled by adjusting the pH. To this end, the equilibrium pH, that is, a few hours after the start of this step, is between 0 and 4, preferably between 0.5 and 2.

[0061] The liquid / solid ratio is between 1 and 5, preferably between 1.5 and 3.

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

[0063] It should be noted that the entire process can be applied in the same way regardless of the rare earth composition of the magnets. The yield is identical for all rare earths that have the same behavior.

[0064] After dissolution, filtration (106) is carried out, in particular using a filter press, so as to separate the rare earths, then in the form of nitrates in solution and the oxide(s), in this case iron.

[0065] The following examples were made from NdFeB magnets having the following composition:

[0066] [Tab 1] Nd Pr Dy Tb Fe B Al Co Cu Zn 24% 2.7% 2.3% 0.4% 61% 0.9% 0.4% 1.3% 0.2% 0.3%

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

[0068] Example 1

[0069] NdFeB magnets are ground into powder (du < 250 µm) and the powder is treated in a rotary kiln for 4 hours at 500 °C under a flow of wet nitrogen (450 L / h at 4% H2O). The mass gain after this treatment is 12%. Mass gain is defined as the ratio (final mass - initial mass) / initial mass.

[0070] The partially oxidized powder is then reprocessed in a rotary kiln for 2 hours at 700 °C under an air flow. The cumulative mass increase after this treatment is 30%.

[0071] The resulting powder is suspended in water, and nitric acid is added with an excess of 10% by moles relative to the rare earths contained in the powder.

[0072] This dissolution takes place at 90 °C with a liquid / solid ratio of 2.

[0073] After filtration and washing of the resulting cake, the dissolution yield, i.e., the amount in moles of rare earths divided by the amount in moles of rare earths in the initial powder for each rare earth, is 95%. The total concentration of rare earths is 132 g / L and the concentration of Fe is 0.01 g / L, resulting in a TR / Fe mass ratio in solution of 13200. The final pH is 0.5.

[0074] Example 2

[0075] NdFeB magnets are reduced to powder (du₀ < 250 µm) and the powder is processed in a muffle furnace with the following temperature ramp: - temperature rise to 400 °C in 1 hour under air; - then treatment for 4 hours at 400 °C under air; - temperature rise to 600 °C in 1 hour under air in the oven (6); - then treatment for 4 hours at 600 °C under airflow.

[0076] The resulting powder is coarsely ground to break up any agglomerates. The mass gain after this treatment is 27%. The powder is suspended in water, and hydrochloric acid is added at an excess of 220% by mole relative to the rare earths contained in the powder. This dissolution takes place at 90 °C with a liquid / solid ratio of 5. After filtration and washing of the cake, the dissolution yield of each rare earth is 95%. The total concentration of rare earths is 44 g / L and the concentration of Fe is 54 g / L, resulting in a TR / Fe ratio of 0.81. The final pH is 3.0.

[0077] Example 3

[0078] NdFeB magnets are ground into powder (diO₂ < 250 µm) and 15 kg of powder are processed in a fluidized bed reactor. The powder is first fluidized by a flow of nitrogen and water is injected until a temperature close to 500 °C is reached.

[0079] The water is then shut off and the nitrogen flow is replaced by an air flow until a temperature of 850 °C is reached. This is the second oxidation substep (103).

[0080] The resulting powder is coarsely ground to break up the agglomerates. The mass gain after this treatment is 30%. The resulting powder is then placed in A suspension in water and nitric acid are added at a concentration 20% above the concentration of rare earth elements in the powder. This dissolution takes place at 90 °C with a liquid / solid ratio of 2. After filtration and washing of the cake, the dissolution yield of each rare earth element is 93%. The total concentration of rare earth elements is 140 g / L and the Fe concentration is 19 g / L, resulting in a rare earth / Fe mass ratio of 7.4. A slight loss of yield is therefore observed, but more importantly, a very significant decrease in selectivity.

[0081] It is thus observed that, by implementing the process of the invention, and in particular an oxidation of the powders in two controlled stages, and within the indicated temperature ranges, yields of 95% and excellent selectivity are obtained.

[0082] Moreover, when nitric acid is replaced by hydrochloric acid, a high iron concentration is maintained despite a final pH of 3.0. Nitric acid is therefore preferred since its implementation helps to optimize rare earth / iron selectivity, iron being stable in solution, even at relatively high pH.

[0083] As a corollary, when the oxidation temperature is too high (example 3), a slight loss of yield is observed, but above all, a loss of selectivity.

Claims

Claims

1. A method for recycling used or discarded permanent magnets and production scraps of such magnets, consisting, after having demagnetized said permanent magnets and having reduced them to powder, in completely oxidizing the latter, then in subjecting the product resulting from this oxidation to dissolution and finally to filtration, characterized in that the oxidation is carried out in two successive sub-steps: - a first sub-step of partial oxidation (102) at a temperature not exceeding 500 °C; - then a second sub-step of total oxidation (103) at a higher temperature, and in particular at a temperature between 600 and 800 °C

2. V-. Method for recycling permanent magnets according to claim 1, characterized in that between the oxidation (102, 103) and dissolution (105) steps there is a step of grinding (104) the products resulting from the oxidation.

3. Method for recycling permanent magnets according to claim 2, characterized in that the grinding step (104) results in obtaining particles with a particle size less than or equal to 100 micrometers.

4. Method for recycling permanent magnets according to one of claims 2 and 3, characterized in that the grinding step (104) is carried out in a humid atmosphere.

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

6. Method for recycling permanent magnets according to one of claims 1 to 5, characterized in that the first oxidation sub-step (102) is carried out in air with a temperature at most equal to 400°C.

7. Method for recycling permanent magnets according to one of claims 1 to 5, characterized in that the first oxidation sub-step (102) is carried out under a mixture of humid inert gas (nitrogen N2 or argon Ar).

8. Method for recycling permanent magnets according to one of claims 1 to 5, characterized in that the first oxidation sub-step (102) is carried out in air but with an oxygen content not exceeding 15% by volume.

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

10. Method for recycling permanent magnets according to one of claims 1 to 9, characterized in that the duration of the first oxidation sub-step (102) is between 10 minutes and 10 hours, preferably between 30 minutes and 2 hours.

11. Method for recycling permanent magnets according to one of claims 1 to 10, characterized in that the second sub-step of oxidation in air (103) is carried out at a temperature between 600 and 800°C, for a duration of 10 minutes to 10 hours, preferably between 30 minutes and two hours.

12. Method for recycling permanent magnets according to one of claims 1 to 11, characterized in that the demagnetized powder consists of fragments of the order of a millimeter, up to a particle size of the order of 4 micrometers, and preferably between 100 and 500 micrometers.

13. Method for recycling permanent magnets according to 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 comprising hydrochloric acid, nitric acid and sulfuric acid.

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

15. Method for recycling permanent magnets according to one of claims 13 and 14, characterized in that the stoichiometry of the acid used is 1 to 2 relative to the rare earths contained in the magnets, and preferably between 1 and 1.2.

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