Method for recycling used or waste permanent magnets

Abstract

The invention relates to a method for recycling used or waste permanent magnets and scrap from the production of such magnets, the method consisting, after demagnetising and reducing the permanent magnets to a powder, in completely oxidising this powder, then in dissolving the product resulting from this oxidation and finally in filtering this product. The oxidation is carried out in two consecutive sub-steps:  a first sub-step of partial oxidation at a temperature of no more than 500°C;  and then a second sub-step of complete oxidation at a higher temperature, and in particular at a temperature of between 600 and 800°C.

Description

[0001] RECYCLING PROCESS FOR USED OR SCRAP PERMANENT MAGNETS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a new process for recovering rare earths present in used or discarded permanent magnets, or present in the production waste of such permanent magnets. More specifically, the permanent magnets in question are of the neodymium and iron (Nd-Fe-B) type or samarium and cobalt (Sm-Co) type.

[0004] The method of the invention is part of an approach aimed at satisfying both economic and ecological conditions.

[0005] On the economic front, the recovery of rare earths, which can be included in such magnets 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 earths, as well as a limited number of rare earth producing countries, likely to generate risks of supplying markets with these metals, the recovery of rare earths is more relevant than ever.

[0006] In ecological terms, 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.

[0007] PRIOR STATE OF THE ART

[0008] Among the various permanent magnet recycling processes known to date, there are two essential families, respectively:

[0009] ■ so-called short-loop recycling, which reprocesses the magnets without dissolving them and by reforming magnets directly from the powder thus recycled, and

[0010] ■ so-called long-loop processes, which require dissolving the magnets, then separating the different rare earths with the aim of producing pure oxides and thus supplying the magnet market. The invention falls into this second family.

[0011] These processes include pyrometallurgy and / or hydrometallurgy. The hydrometallurgical process is based on the liquid-liquid extraction technique for recovering rare earths from an acidic aqueous phase, and typically involves total dissolution of the magnet followed by elimination of the iron (in the case of neodymium-iron-boron magnets) by raising the pH. Such hydrometallurgical recycling processes have, for example, been described in document US 5,362,459 or in document WO 96 / 00698. In the latter, the magnet is oxidized by heat treatment, followed by dissolution of the oxidized powder by hydrochloric acid. The iron-neodymium separation is carried out by oxalic precipitation of neodymium oxalate, which has a much lower solubility than iron oxalate.

[0012] Among the hydrometallurgical processes implemented, excessive acid consumption is frequently observed, followed by base consumption to eliminate iron by neutralization.

[0013] Other types of hydrometallurgical processes aim to oxidize, in whole or in part, the alloys contained in permanent magnets, with a view to achieving the selective dissolution of rare earths, and in particular:

[0014] ■ thermal oxidation (calcination of magnets at high temperature) to obtain a mixture of iron oxide and rare earths;

[0015] ■ hydrothermal oxidation of magnets at high pressure, which allows the production of rare earth hydroxide, soluble in an acid medium, and magnetite, easily separable due to its magnetic nature.

[0016] While, on a laboratory scale, the various hydrometallurgical techniques proposed have made it possible to carry out the selective recycling of rare earths, real difficulties are observed on an industrial scale.

[0017] Among these, we can notably mention that during oxidation, temperatures above 600°C are likely to be reached, which can lead to agglomeration and sintering of part of the material, preventing the production of a homogeneous oxidized phase. When the temperatures during oxidation are above 600-900 °C, the formation of a mixed oxide TRFeOa (TR = rare earths) is observed. This oxide is more refractory than rare earth oxides and requires more severe attack conditions, causing a loss of efficiency or a loss of selectivity during dissolution. Uncontrolled oxidation of magnet powder results in temperatures that can exceed 1000 °C, thus promoting the formation of mixed oxides.

[0018] Furthermore, during hydrothermal oxidation, the oxidation kinetics mainly depends on the total pressure of the gases within the treatment enclosure (typically an autoclave). Typically, in order to achieve acceptable kinetics for the implementation of a process on an industrial scale, pressures of the order of 40 to 200 bars must be applied, consequently complicating the recycling installation itself.

[0019] The processes implemented to date result in a real difficulty arising from the exothermicity of the oxidation reaction, which is not controlled, and which is likely to cause serious problems during industrial implementation.

[0020] The invention proposes a method for recycling used or discarded permanent magnets, and also production waste from such magnets, implementing such a hydrometallurgical process on an industrial scale and overcoming the drawbacks of the processes known to date.

[0021] STATEMENT OF THE INVENTION

[0022] According to the invention, this recycling process consists, after having demagnetized said permanent magnets and, after having reduced 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.

[0023] 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. According to the invention, the oxidation step is carried out in two successive sub-steps:

[0024] ■ a first sub-step of partial oxidation at a temperature not exceeding 500°C,

[0025] ■ then a second sub-step of total oxidation at a higher temperature, and in particular at a temperature between 600 and 800°C.

[0026] The implementation of these two oxidation sub-steps allows the temperature to be controlled much more efficiently, avoiding problems of agglomeration and sintering and limiting the formation of refractory mixed oxide of the type mentioned above.

[0027] Typically, in the recycling of permanent magnets of the neodymium-iron-boron type, the product resulting from the first oxidation sub-step has a mainly amorphous structure, and is notably characterized by the absence of a phase containing rare earths in the zero oxidation state (typically TFLFe B and TRFe4B), the absence of refractory mixed oxide of the TRFeOa type, the presence of iron in the form of metallic iron or iron oxide, in particular magnetite FeiCU

[0028] Correspondingly, the product from the second oxidation sub-stage has a mainly crystalline structure, and is notably characterized by the absence of iron or rare earths in the metallic state, the presence of iron in the form of oxide, mainly hematite (FejOs) as well as a limited proportion (typically less than 10% by mass) of refractory mixed oxide of the TRFeOa type.

[0029] According to an advantageous characteristic of the invention, a step of grinding the products resulting from the oxidation takes place between the oxidation and dissolution stages. This grinding step is intended to release the rare earth oxides from the layer of iron oxides which forms outside the grains. It also makes it possible to accelerate the kinetics of the subsequent dissolution stage.

[0030] Typically, this grinding step results in particles smaller than or equal to 100 micrometers. It may also be considered to carry out this grinding step in a humid atmosphere, in order to reduce excessive fly-away of fines.

[0031] Advantageously, the first oxidation sub-step is carried out at a temperature between 150 and 500°C, preferably between 400 and 500°C.

[0032] At the same time, this first oxidation sub-step can be carried out in air, provided that the temperature is less than or equal to 400°C.

[0033] On the other hand, if this temperature is higher than 400°C, and therefore between 400 and 500°C, this sub-step must take place:

[0034] ■ either under a mixture of humid inert gas (nitrogen N2 or argon Ar);

[0035] ■ either in air but with an oxygen content not exceeding 15% by volume.

[0036] This gas can be humid with a water content of 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.

[0037] Indeed, if this partial pressure is greater than 50% by volume, the said first sub-step can lead to an excessively high temperature and excessive recondensation at the cold points of the circuit. If the said partial pressure is less than 5% by volume, the progress of the reaction is insufficient to move on to the next step efficiently.

[0038] The duration of this first sub-step is between 10 minutes and 10 hours, preferably between 30 minutes and 2 hours.

[0039] 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. Ideally, the particle size of the demagnetized powder subjected to these oxidation steps consists of fragments of the order of a millimeter, up to a particle size of the order of 4 micrometers. Preferably, this particle size is between 100 and 500 micrometers. Indeed, if this particle size is too small, an excessive flight of fine particles is observed, likely to clog the filters and other pipes present or leading into the furnace in which this operation takes place. If, on the other hand, this particle size is too large, the size of the particles, which are then coarse, prevents total oxidation of the powder and consequently impairs the effectiveness of the process of the invention.

[0040] According to another characteristic of the invention, the dissolution step is carried out in an acid medium, the acid used being advantageously chosen from the group comprising hydrochloric acid, nitric acid and sulfuric acid.

[0041] Advantageously, the temperature of this dissolution step is carried out at room temperature up to 90°C, and preferably between 60 and 90°C. Typically, if this temperature is lower than 60°C, the kinetics of this dissolution step are altered. If, on the other hand, this temperature is higher than 90°C, excessive evaporation of water and the acid used is observed, requiring recondensation, and therefore complicating the process.

[0042] 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 relative to the rare earths, preferably 1 to 1.2.

[0043] Indeed, if the acid concentration is sub-stoichiometric compared to the rare earths, we lose the recycling efficiency of the rare earths that we want to obtain.

[0044] On the other hand, if the acid concentration is over-stoichiometric, on the one hand, the quantity of acid consumed is de facto excessive, and on the other hand, more iron is dissolved than desired, iron then present in the form of Fe(II) or Fe(III) ions, which should then be eliminated by adjusting the redox potential and the pH, and consequently causing an overconsumption of base. In addition, the filtration of the iron thus re-precipitated is relatively less effective compared to the filtration of the unattacked ferric oxide. The duration of this dissolution step is between 1 and 10 hours, advantageously between 2 and 6 hours.

[0045] According to the invention, the oxidation step is carried out within a furnace, and for example:

[0046] ■ a static muffle furnace incorporating a chamber heated by gas or electrical resistors in which the powder is loaded into crucibles,

[0047] ■ a passage furnace, that is to say a heated tunnel in which a crucible containing the powder advances progressively,

[0048] ■ 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,

[0049] ■ a fluidized bed furnace, that is to say a reactor containing the powder into which the hot gas is injected at speeds such that said powder is set in motion and has flow characteristics close to those of a fluid.

[0050] BRIEF DESCRIPTION OF THE FIGURES

[0051] The manner in which the invention can be implemented and the advantages which result therefrom will emerge more clearly from the following example of implementation, given for informational and non-limiting purposes, with the support of the appended figures.

[0052] Figure 1 is a schematic representation illustrating the different steps of the method of the invention.

[0053] Figure 2 is a schematic representation illustrating more specifically the two oxidation stages of the process of the invention.

[0054] DETAILED DESCRIPTION OF THE INVENTION

[0055] According to the method of the invention, the main steps of which are schematically illustrated in Figure 1, the recycling product typically consists of used or discarded permanent magnets. Alternatively, it may consist of scrap resulting from the production operations of such magnets, said scrap also containing rare earths. These recycling products are first demagnetized. This demagnetization is typically obtained by heating above the Curie temperature, and typically 350°C for Nd-Fe-B magnets.

[0056] Then they are reduced to powder in order to typically reach a particle size ideally between 100 and 500 micrometers, in this case by a crushing step (100), followed by a grinding step (101).

[0057] The powder thus obtained then undergoes a first under-oxidation (102), in order to achieve a partial oxidation of the rare earths present in said powder.

[0058] For this purpose, the powder, for example stored in a tank (1) (figure 2), feeds a first furnace (2), typically a static muffle furnace, a passage furnace, a rotary furnace or a fluidized bed furnace, by means of a worm screw (3).

[0059] 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.

[0060] The temperature in this first oven is typically between 250 and 500°C, and the treatment time in this oven is ideally between 30 minutes and two hours.

[0061] This inert gas containing water can be replaced by air. In this configuration, the temperature in the oven should be a maximum of 400°C, and the treatment time should ideally be between 30 minutes and two hours.

[0062] Whatever the gas present in the furnace, the objective sought is the oxidation of the rare earths present in the powder.

[0063] Once this first oxidation sub-step (102) has been carried out, the powder will then undergo a second oxidation sub-step (103), the objective of which is to achieve the total oxidation of the rare earths contained in said powder. For this purpose, the powder stored in a hopper (4) at the outlet of the furnace (2), is conveyed, again by means of an endless screw (5) to a buffer tank (8).

[0064] During this second oxidation sub-step, the powder, stored in the buffer tank (8), is conveyed by means of an endless screw (9) to a furnace (6), in which it undergoes a new treatment, this time limited to air, at a temperature between 500 and 800°C, preferably between 600 and 800°C. This furnace (6) may be of the same nature as the furnace (2). The duration of this treatment is also between thirty minutes and two hours.

[0065] The objective of this second sub-step is to achieve total oxidation of the elements present in the powder.

[0066] The latter is then stored in a tank (7).

[0067] It then undergoes grinding (104), particularly in a wet process, so as to achieve a particle size less than or equal to 100 micrometers.

[0068] Then, the powder thus reduced undergoes a dissolution step (105).

[0069] 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 a stirrer.

[0070] The acid dosage is such that a stoichiometry of between 1 and 1.2 is obtained relative to the rare earths. In practice, due to the relative uncertainty surrounding the actual rare earth content of the powder, the dissolution is controlled by adjusting the pH. For this purpose, the equilibrium pH, i.e. a few hours after the start of this step, is between 0 and 4, preferably between 0.5 and 2.

[0071] The liquid / solid ratio is between 1 and 5, preferably between 1.5 and 3.

[0072] The residence time in the dissolution reactor is between 1 and 10 hours, preferably between 2 and 6 hours. 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.

[0073] After dissolution, filtration is carried out (106), in particular using a filter press, in order to separate the rare earths, then in the form of nitrates in solution, and the oxide(s), in the form of iron.

[0074] The following examples were made from NdFeB magnets having the following composition:

[0075] The rare earth yield is calculated as a mass yield on the sum of the elements Nd, Pr, Dy and Tb.

[0076] Example 1

[0077] NdFeB magnets are ground into powder (dioo < 250 pm) and the powder is treated in a rotary kiln for 4 hours at 500 °C under a flow of wet nitrogen (450 1 / h at 4% H2O). The mass gain after this treatment is 12%. By mass gain, we mean the ratio (final mass - initial mass) / initial mass.

[0078] The partially oxidized powder is then treated again in a rotary kiln for 2 hours at 700°C under air flow. The cumulative mass gain at the end of this treatment is 30%.

[0079] The resulting powder is suspended in water, and nitric acid is added with an excess of 10% in moles relative to the rare earths contained in the powder.

[0080] This dissolution takes place at 90°C with a liquid / solid ratio of 2. After filtration and washing of the cake resulting from this filtration, the dissolution yield, i.e. the quantity in moles of rare earths over the quantity in moles of rare earths in the initial powder of each rare earth is 95%. The total concentration of rare earths is 132 g / 1 and the concentration of Fe is 0.01 g / 1, i.e. a mass ratio TR / Fe in solution of 13200. The final pH is 0.5.

[0081] NdFeB magnets are reduced to powder (dioo < 250 pm) and the powder is treated 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 in air; temperature rise to 600 °C in 1 hour under air in furnace (6); then treatment for 4 hours at 600 °C under air flow.

[0082] The powder obtained is coarsely ground to break up the agglomerates. The mass gain at the end of this treatment is 27%. The powder is suspended in water and hydrochloric acid is added with an excess of 220% in moles compared 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 efficiency 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, i.e. a TR / Fe ratio of 0.81. The final pH is 3.0.

[0083] NdFeB magnets are ground into powder (dioo < 250 pm) and 15 kg of powder are processed in a fluidized bed reactor. The powder is first fluidized by a nitrogen flow and water is injected until a temperature of around 500 °C is reached.

[0084] The water is then cut off and the nitrogen flow replaced by an air flow up to a temperature of 850 °C. This is the second oxidation sub-step (103). The powder obtained is coarsely ground to break up the agglomerates. The mass gain at the end of this treatment is 30%. The powder obtained is suspended in water and nitric acid is added with a 20% excess compared to the rare earths contained 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 is 93%. The total concentration of rare earths is 140 g / 1 and the concentration of Fe is 19 g / 1, i.e. a TR / Fe mass ratio of 7.4. We therefore observe a slight loss of yield, but above all a very clear reduction in selectivity.

[0085] It is thus noted that by implementing the process of the invention, and in particular an oxidation of the powders in two controlled stages, and in the temperature ranges indicated, yields of 95% and excellent selectivity are obtained.

[0086] Furthermore, 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 use helps to optimize rare earth / iron selectivity, iron being stable in solution, even at relatively high pH.

[0087] Correspondingly, when the oxidation temperature is too high (example 3), we observe a slight loss of yield, but above all, a loss of selectivity.

Claims

CLAIMS Process 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-stages: ■ a first partial oxidation sub-step (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. 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) of the products resulting from the oxidation. Method for recycling permanent magnets according to claim 2, characterized in that the grinding step (104) results in particles with a particle size less than or equal to 100 micrometers. 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. 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.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 500 and 800°C, preferably 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. Method for 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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