WO2018185163A1 - Method for separating lanthanides from a metal alloy - Google Patents

Abstract

The application concerns a method for separating lanthanide from a metal alloy comprising at least one transition metal and at least one lanthanide, said method comprising the steps of: a) providing a metal alloy comprising at least one transition metal and at least one lanthanide, b) bringing the metal alloy into contact with an aqueous solution of phosphoric acid, thus obtaining a medium comprising a liquid phase L1 and a solid phase S1 comprising the phosphates of at least one lanthanide, c) filtering the medium to separate the liquid phase L1 and the solid phase S1 comprising the phosphates of at least one lanthanide.

Description

 Process for separating lanthanides from a metal alloy

The present invention relates to a process for separating lanthanide from a metal alloy comprising at least one transition metal and at least one lanthanide, a typical case of a permanent magnet.

 Lanthanides are used in many applications, including those related to renewable energies. The processes for extracting these metals are very expensive. One of the ways to obtain them is to recycle them from the materials that contain them, for example metal alloys such as permanent magnets or nickel-metal hydride NiMH batteries. Recycling also has the advantage of avoiding the often harmful environmental impact of lanthanide mining. A recycling of permanent magnets pyrometallurgically at temperatures of 1000 ° C was described in 2015 by Fraunhofer. This recycling implements extreme temperature conditions and is therefore expensive. In addition, it does not separate lanthanides from other metals.

 The development of lanthanide separation method from metal alloy containing it is therefore required, ideally in milder conditions.

 Innocenzi et al. (Journal of Power Sources, 21 1, 2012, 184-191) report a process for separating lanthanides from nickel-metal hydride NiMH batteries comprising two successive stages of adding an aqueous solution of sulfuric acid to dissolve both lanthanides and transition metals, these two steps being separated by filtration. The lanthanide sulphates remain in the sulfuric acid solution. It is necessary to add a base to induce the selective precipitation of lanthanide sulfates and to be able to separate them from the nickel remaining in solution by filtration. 5 steps are therefore necessary to recover the lanthanides from the metal alloy.

 It would be advantageous to have a less complex method, including less steps.

 For this purpose, the invention provides a process for separating lanthanide from a metal alloy, said method comprising the steps of:

a) providing a metal alloy comprising at least one transition metal and at least one lanthanide,

b) contacting the metal alloy with an aqueous solution of phosphoric acid, whereby a medium comprising a liquid phase L1 and a solid phase S1 comprising the phosphates of at least one lanthanide, c) separating the liquid phase L1 and the solid phase S1 comprising the phosphates of at least one lanthanide.

 The process thus makes it possible to separate the lanthanide (s) from a metal alloy in only two steps. The lanthanide (s) is (are) recovered in the form of lanthanide phosphate and is (are) advantageously directly recoverable (s).

 The method comprises a step a) of providing a metal alloy comprising at least one transition metal and at least one lanthanide.

The metal alloy comprises at least one transition metal, sometimes at least two transition metals. Generally, the transition metal is selected from the elements of the 4 ^th transition period, preferably from cobalt (Co), iron (Fe), manganese (Mn) and nickel (Ni).

 The metal alloy may comprise one or more other metals, in particular chosen from poor metals, such as zinc (Zn) or aluminum (Al), and metalloids, for example boron (B).

 The metal alloy comprises at least one lanthanide, sometimes at least two lanthanides, for example two or three. The lanthanide is in particular chosen from lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium or lutetium.

 In one embodiment, the metal alloy comprises samarium and cobalt.

 In another embodiment, the metal alloy comprises neodymium, iron and boron.

 Generally, in the metal alloy, the cumulative proportion of elements other than a transition metal, a lanthanide or a metalloid is less than 5% by weight, especially less than 1% by weight, or even less than 0.1% by weight. weight. Preferably, the metal alloy consists of one or more transition metal (s), one or more lanthanide (s) and optionally one or more metalloids (s).

 The metal alloy may or may not be coated, for example a metal coating or a phosphate salt coating. The metal coating may comprise a metal of a nature identical to or different from that or those of the metal alloy.

In the metal alloy, the mass ratio of transition (es) metal to (x) lanthanide (s) can vary to a very large extent, and is generally from 0.1 / 100 to 100/0. , 1. Advantageously, the method can be implemented on a metal alloy comprising only traces of lanthanides. Generally the alloy metal comprises at least 0.01% by weight of lanthanide (s), in particular at least 0.1% by weight, preferably at least 1% by weight.

 In one embodiment, the metal alloy is a permanent magnet, typically of the samarium / cobalt (SmCo) or neodymium / iron / boron (NdFeB) type. These magnets may include one or more other lanthanide (s) and / or additional transition metal (s). Generally, SmCo magnets do not contain lanthanide other than samarium. On the other hand, magnets of NdFeB type may contain, in addition to neodymium, one or more other lanthanides, in particular dysprosium and / or praseodymium.

 The permanent magnet may or may not be coated, generally with a phosphate salt coating or a metal coating, for example a NiCuNi coating.

 In another embodiment, the metal alloy is a negative electrode from a NiMH nickel metal hydride battery.

 The metal alloy may be in its original form (in its mass form, with dimensions typically being of the order of a centimeter), in the form of pieces, grain or powder. The metal alloy can be milled or not. When the metal alloy is a permanent magnet, it is preferably unmilled since grinding of permanent magnets is generally difficult. The process is preferably free of grinding step.

The method comprises a step b) of bringing the metal alloy into contact with an aqueous solution of phosphoric acid, whereby a medium comprising a liquid phase L1 and a solid phase S1 comprising the phosphates of at least one lanthanide. Phosphoric acid is also called orthophosphoric acid (H ₃ PO ₄ ).

 During step b), in contact with the phosphoric acid solution, the transition metal (s) present in the metal alloy undergo (ssent) leaching and dissolve in solution, while lanthanide phosphates form and precipitate, typically as a white precipitate. The process according to the invention is based on the difference in solubility between lanthanides and transition metals in phosphoric acid medium. The process thus makes it possible to easily separate the metal and the lanthanide from the metal alloy in a single step, without any adjustment of the pH being necessary after the addition of the aqueous solution.

When the metal alloy comprises several lanthanides, the lanthanide phosphates precipitate, whatever their nature, and the solid phase S1 then comprises a mixture of lanthanide phosphates. Similarly, when the metal alloy comprises several transition metals, these dissolve in solution, whatever their nature, and the liquid phase L1 then comprises a mixture of transition metals. The process therefore advantageously makes it possible to separate the lanthanides from the transition metals, independently of the number of lanthanides or the number of transition metals initially present in the metal alloy.

 The contacting of the metal alloy with the aqueous solution of phosphoric acid can be carried out by spraying an aqueous solution of phosphoric acid on the metal alloy, or more generally by immersing the metal alloy in a solution aqueous phosphoric acid.

Step b) of the process can be carried out with an aqueous solution of phosphoric acid since the concentration of phosphoric acid is sufficient to attack the alloy under the effect of H ⁺ ions. The higher this concentration, the lower the proportion of lanthanide phosphate remaining solubilized in the liquid phase L1, and the lanthanide phosphates are in the solid phase S1. Preferably, the concentration of phosphoric acid in the aqueous solution is at least 0.1 mol / l, especially at least 1 mol / l, preferably at least 5 mol / l.

It is not necessary that the aqueous solution of phosphoric acid implemented in step b) be very acidic. Typically, its acidity is less than 1 mol / L of H ⁺ ions. Preferably, the aqueous phosphoric acid solution is free of sulfuric acid or sulfate ion and / or acid of which at least one of the pKa at 25 ° C is less than 1.0, such as acid. hydrochloric, hydrofluoric, sulfuric ...

 The mass ratio of the aqueous phosphoric acid solution to the metal alloy may vary to a large extent, it being understood that there should be at least as much phosphate in the aqueous solution as lanthanide and metal to form lanthanide phosphates and metal phosphates. The lower the mass ratio, the less expensive the process.

 Generally, step b) is carried out at a temperature of 5 ° C to 70 ° C, especially 15 to 40 ° C, preferably at room temperature (of the order of 25 ° C). The phosphates of lanthanides precipitate the more rapidly as the temperature is high.

Those skilled in the art are able to adapt the reaction time, in particular taking into account the precipitation time of lanthanide phosphates, which is related to the phosphoric acid concentration of the aqueous solution and the temperature during step b). Typically, the higher the phosphoric acid concentration of the aqueous solution and / or the higher the temperature, the shorter the duration of step b). The duration of step b) is generally from 1 to 48 hours, typically of the order of 24 hours. Step b) can be accompanied by a release of hydrogen from the attack of metals of the metal alloy by hydronium ions (H ⁺ ). This results in a dissolution of the metal in the aqueous phase in the form of an ion, and the reduction of H ⁺ ions which then form dihydrogen. In one embodiment of the process, the product hydrogen is recovered.

 The method comprises a step c) of separating the liquid phase L1 and the solid phase S1 comprising the phosphates of at least one lanthanide, generally by centrifugation or filtration, preferably by filtration. In the latter case, step c) consists in filtering the medium to separate the liquid phase L1 (filtrate) and the solid phase S1 (filtered) comprising the phosphates of at least one lanthanide. This step c) thus makes it possible to recover the phosphates of at least one lanthanide, generally in hydrated form.

 The process may comprise, after step c), a step c ') of drying the solid phase S1, for example at a temperature between 80 and 150 ° C.

 Advantageously, the solid phase S1 recovered at the end of step c) (or c ')) comprises a cumulative mass proportion of transition metal of less than 5%, especially less than 3%. Typically, the solid phase S1 recovered at the end of step c) (or c ')) comprises more than 90% by weight, in particular more than 95% by weight of lanthanide phosphate, or even more than 97% by weight of lanthanide phosphate. These proportions can easily be determined, for example by X-ray diffraction (XRD) or energy dispersive spectroscopy (EDX).

 The method may comprise a step d) of increasing the pH of the liquid phase L1 obtained in step c) to at least 4.0, whereby a second medium comprising a liquid phase L2 and a phase is obtained. solid S2 comprising at least one transition metal.

Such an increase in the pH of the liquid phase L1 leads to the precipitation of the transition metal (es) it contains, generally in the form of metal phosphates, to form the solid phase S2. When the liquid phase L1 comprises several transition metals (and therefore when the metal alloy used in the process comprises several transition metals), these precipitate during the pH increase beyond 4.0, whatever or their nature, and the solid phase S2 then comprises a mixture of transition metals, generally in the form of metal phosphates. The process therefore advantageously makes it possible to recover all the transition metals, independently of the number of transition metals initially present in the metal alloy. Preferably, the liquid phase pH L1 is increased to at least 4.5, even up to at least 5.0, preferably to a value of 5.5 to 7.0.

This increase in pH is generally carried out by adding a base, which may be of any kind, for example KOH, NaOH, K ₂ CO ₃ or Na ₂ CO ₃ . Generally, it is a solution of sodium hydroxide.

 Precipitation of transition metal (es) is generally very rapid, and the duration of step d) is typically of the order of a few minutes. Generally, step d) is carried out at a temperature of 5 ° C to 70 ° C, in particular 15 to 40 ° C, preferably at room temperature (of the order of 25 ° C). Preferably, all the steps of the process are carried out at ambient temperature and at atmospheric pressure.

 The method may comprise a step e) of separating the liquid phase L2 and the solid phase S2 comprising at least one transition metal, generally by centrifugation or filtration, preferably by filtration. In the latter case, step e) consists in filtering the second medium to separate the liquid phase L 2 (filtrate) and the solid phase S 2 (filtered) comprising at least one transition metal. This step advantageously makes it possible to recover the transition metals. Also, the process according to the invention makes it possible not only to recover the lanthanides, but also to recover the transition metals.

 The method may comprise, after step e), a step e ') of drying the solid phase S2, for example at a temperature between 80 and 1000 ° C. The transition metal phosphates obtained are generally multicrystalline. Drying by heating at temperatures up to 1000 ° C generally makes it possible to obtain them in a particular crystalline form and better recoverable for certain applications. For example, iron phosphate crystallizes after 850 ° C in hexagonal form and can thus be used in positive ion battery electrodes.

Advantageously, the solid phase S2 recovered at the end of step e) (or e ')) comprises a cumulative mass proportion of lanthanide of less than 5%, especially less than 3%. This proportion can easily be determined, for example by X-ray diffraction (XRD) or energy dispersive spectroscopy (EDS). The solid S 2 may comprise, in addition to metal phosphates, phosphates derived from the cation of the base used in step d). For example, when NaOH is used in step b), the S2 solid typically comprises sodium phosphate. This can not usually be removed by washing in aqueous solution solid S2. The inventors assume that sodium is present in the crystalline mesh of the solid S2.

The liquid phase L 2 obtained at the end of step e) is an aqueous solution generally comprising less than 1% by weight of metal and less than 1% by weight of lanthanide. It contains phosphoric acid (as base and / or acid, depending on its pH). For example, when sodium hydroxide is used as a base in stage d), the liquid phase L2 is an aqueous solution containing phosphoric acid (in the form of phosphoric acid, mono, bis and / or sodium triphosphate depending on the pH of the liquid phase L2) and Na ⁺ ions.

 This solution can advantageously be recycled and reused in the process by adding phosphoric acid. Thus, the method can include:

 a step f) of adding phosphoric acid to the liquid phase L 2 obtained in step e) to obtain an aqueous solution of phosphoric acid, and then

 the repetition of step a) and of step b), wherein the aqueous phosphoric acid solution used in step b) is the aqueous phosphoric acid solution obtained in step f). The process can thus be repeated without it being necessary to remove the aqueous solution.

 The process can be carried out continuously or batchwise.

 Advantageously, the method allows:

 to separate and recycle, from a metal alloy, the lanthanides in the form of lanthanide phosphates of a purity such that it is possible to recover them and / or to reuse them without prior purification,

 - Separate and recycle, from a metal alloy, the metals in the form of metal phosphates.

 The process is very simple in that it only implements aqueous solutions and does not require high temperatures and / or pressures. The aqueous phosphoric acid solution used in step b) is not a very acidic solution and its handling is dangerous neither for the operator nor for the environment. It allows the recycling of permanent magnets in much milder and less expensive conditions than those of existing processes.

The process can be implemented easily regardless of the amount of metal alloy to be treated, for example microgram to kilogram. The invention is illustrated in view of the figures and examples which follow.

Figure 1 illustrates the reaction scheme of the method in the embodiment where it comprises steps d), e) and f).

 FIG. 2 represents the mass proportion in an aqueous solution of

H ₃ PO4 at 0.1 mol / L (left part), at 1 mol / L (central part) or at 6 mol / L (right part), Sm (black striped histograms) and Co (white histograms) d a "model" permanent magnet SmCo leached as a function of time in hours, in an aqueous solution.

 FIG. 3 represents the mass proportion, in an aqueous solution of

H ₃ PO4 at 0.1 mol / L (left part), at 1 mol / L (central part) or at 6 mol / L (right part), Nd (black striped histograms) and Fe (white histograms) d a NdFeB "model" permanent magnet leached as a function of time in hours. In the following examples, 85% phosphoric acid was from Sigma

Aldrich, and 10% sodium hydroxide from Roth. The glassware was cleaned in H ₂ SO 4 / H ₂ O ₂ and then rinsed with ultrapure water (18.2 Ω and <3 ppb total organic carbon, Millipore Elix + Gradient) before use.

 An Varian 720-ES Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used to determine the chemical compositions of L1 liquid phase after leaching, L2 liquid phase and permanent magnets.

 The solid phases S1 and S2 were analyzed chemically and structurally by scanning microscopy (SEM) with SEM LEO S440 and SEM Ultra 55 devices equipped with an energy dispersive X-ray detector (EDAX Phoenix). Powder X-ray diffraction spectra (XRD) were made using an X'Pert Pro MPD (PANalytical) apparatus in a Bragg-Brentano geometry with the Ka wavelength of the copper (1. 5419 A).

Example 1: From permanent magnets "models"

• Step a): permanent magnets used

The ideal samarium / cobalt (SmCo) or neodymium / iron / boron (NdFeB) permanent magnets were from Sigma Aldrich and their chemical compositions were determined by ICP-OES (Table 1). They were in powder form and were used as is, without any modification. NdFeB SmCo

 N / A 33.39 Sm 33.10

Fe 65.18 Co 66.6

 B 1, 30 Ca 0.04

 Pr 0.08 C 0.01

 C 0.01 0.20

 If 0.02 Ni 0.02

 0 0.02 Fe 0.03

 Table 1: Composition of NdFeB and SmCo model permanent magnets in% by weight (only for concentrations greater than 0.01%) according to ICP-EOS analyzes.

 • Steps b) and c)

 The permanent magnets were dissolved in three different solutions of phosphoric acid at 0.1, 1 and 6 mol / L, adjusting the proportion of solution so that the lanthanide concentration was in each case 0.01 mol / L.

 For 6 hours, solution samples were taken and analyzed by

ICP-OES to determine dissolution kinetics. The results are shown in FIGS. 2 and 3. Regardless of the permanent magnet, the dissolution of the transition metal and that of the lanthanide are not concomitant. The concentration of lanthanide in solution decreases with time.

After 6 hours, the SmCo is leached in a solution of H ₃ PO ₄ at 1 mol / L.

Less than 20% of the Sm is in solution, while 100% of the Co is dissolved. The result is even more impressive in a solution of H ₃ PO ₄ at 6 mol / L, since only 6% of Sm is present in solution after 6 hours.

 Similar results are obtained for the permanent magnet NdFeB, but with different kinetics: 26% of Nd are still in solution after 6h in the most concentrated solution.

 For both permanent magnets, the decrease in lanthanide concentration is accompanied by the appearance of a white precipitate at the bottom of the beaker.

The experiments were reproduced, but this time by immersing the permanent magnets in an aqueous solution at 5 mol / l of H ₃ PO ₄ for 24 hours in order to promote the precipitation.

After 24h, the formed S1 precipitates were filtered and dried and analyzed by SEM-EDX. The precipitates showed a porous structure. The results are provided in Table 2. In both cases, electron-backscattering (EBS) analysis confirmed the homogeneity of the chemical composition. precipitate S1 obtained from the SmCo precipitate S1 obtained from the NdFeB

Sm 21 Nd 14

 P 19 P 19

 0 60 0 66

 Co 0 Fe 1

Table 2: Quantification in% by EDX of S1 precipitates obtained from a permanent magnet either SmCo or NdFeB, after they have been immersed in a 5 mol / L aqueous solution of H ₃ PQ ₄ .

 In the case of the S1 precipitate obtained from the SmCo, the chemical analyzes show that Sm, P and O are present, but the cobalt has not been identified, and this at all the analysis points tested.

 The results are similar in the case of the precipitate S2 obtained from NdFeB. The precipitate contained only 1 atomic% iron.

 • Steps d) and e)

 The pH of the filtrates L1 obtained following the filtration of the precipitates S1 was around 1 irrespective of the type of permanent magnet used initially. The liquid phase L1 obtained from the permanent magnet SmCo was pink, whereas that obtained from the permanent magnet NdFeB was colorless.

 The pH was adjusted to 6 by addition of an aqueous sodium hydroxide solution, which led to the formation of a second precipitate, of violet color for that resulting from SmCo, or yellow for that resulting from NdFeB. This was filtered and dried to obtain powder solids S2, which were analyzed in the same way as S1 solids.

 The results of the EDX analyzes are provided in Table 3.

 Precipitated S2 obtained from SmCo Precipitated S2 obtained from NdFeB

 Co 15 Fe 13

 P 15 P 15

 0 60 0 60

 Na 8 Na 1

 Sm 2 Nd 1

Table 3: Quantification in% by EDX of S2 precipitates obtained from a permanent magnet, either SmCo or NdFeB, after having been immersed in a 5 mol / L aqueous solution of H ₃ PO ₄ , filtered the solution, to have recovered the filtrate, to have adjusted the pH of the solution to 6 then to have filtered the precipitates S2.

These analyzes show that these S2 solids comprise the transition metals. They are probably in the form of metal phosphate, as the analyzes reveal the presence of P and O. A very small amount of lanthanide remains present. Na, from the sodium hydroxide solution used to increase the pH, is also present.

 The XRD analyzes did not identify the crystalline phases because the S2 solids are polycrystalline or amorphous.

The liquid L2 phases obtained following the filtration of the S2 precipitates were analyzed by ICP. These analyzes demonstrated the absence of metal and lanthanide in solution. These liquid phases can be removed, or reused in the process by adding H ₃ PO ₄ .

Example 2: From "industrial" permanent magnets

 • Step a): permanent magnets used

 The SmCo permanent magnets were either coated with a Ni / Cu / Ni metal alloy or uncoated, and the NdFeB permanent magnets were either Ni / Cu / Ni coated or phosphate coated.

In order to study their compositions, the permanent magnets were totally dissolved in a solution of HNO ₃ at 5 mol / L and the chemical composition determined by ICP is given in Table 4.

Table 4: Composition of the "industrial" permanent magnets NdFeB and SmCo in% by weight (only for concentrations greater than 0.01%) after leaching in HN0 ₃ mol L ^"1 according to the ICP-EOS analyzes.

Additional elements are present with respect to the permanent magnets "models" of Example 1: - The permanent magnet SmCo, coated or not, surprisingly comprises around 15% by weight of iron,

 - The permanent magnet SmCo, coated or not with NiCuNi, surprisingly comprises copper in the same proportions

 The permanent magnet NdFeB comprises three different lanthanides: neodymium, dysprosium and praseodymium. These last two are used to decrease the Curie temperature of permanent magnets.

 • Steps b) and c), then d) and e)

The experiments detailed in Example 1 were reproduced by immersing the permanent magnets in an aqueous solution at 5 mol / L H ₃ PO ₄ for 24 hours, then filtering the medium to obtain the solid phase S 1 and the liquid phase L 1 . The pH of the liquid phase L1 was then adjusted around 6, which led to the precipitation of the S2 solid, which was filtered and dried. The conditions and results of the EDX analyzes of solids S1 and S2 are given in Table 5.

Table 5: Experimental conditions and quantization in mass% by EDX of the precipitates S1 and D2 obtained from the permanent magnets of Table 4 after they have been immersed in a 5 mol / L aqueous solution of H ₃ PO ₄ , having filtered the solution, drying the solid S1, recovering the L1 filtrate, adjusting its pH to 6 and then filtering the precipitate and drying the solid S2.

 S1 solids from permanent magnets A and B (SmCo uncoated or coated with NiCuNi) are very similar, with 4% by weight of Sm, the rest of P and O. These solids are free of Co and Fe, which proves that these remained in solution L1. The scanning electron microscopy (SEM) analyzes show a granular structure.

 Solids S1 from permanent magnets C and D (NdFeB coated with NiCuNi or phosphate) also have similar granular structures according to SEM analyzes, but they have different chemical compositions. Pr and Nd are in fact present in greater proportions in the solid S2 derived from the permanent magnet D than the C. Iron is present in a very small amount in the solid S2 derived from the permanent magnet D. Despite this, the Steps b) and c) are also effective using industrial permanent magnets because the lanthanide separation takes place.

 Solids S2 from permanent magnets C and D (NdFeB coated with NiCuNi or phosphate) also have granular structures with some massive crystals. The compositions shown in Table 5 correspond to those of the granular structure. Only 1% by weight of Sm was found in these S1 solids, which shows that the majority of Sm had precipitated in steps b) and is present in the solid S1. The crystals observed are cobalt phosphate.

Claims

1. A process for separating lanthanide from a metal alloy, said method comprising the steps of: a) providing a metal alloy comprising at least one transition metal and at least one lanthanide, b) contacting the metal alloy with an aqueous solution of phosphoric acid, whereby a medium comprising a liquid phase L1 and a solid phase S1 comprising the phosphates of at least one lanthanide, c) separating the liquid phase L1 and the solid phase S1 comprising the phosphates of at least one lanthanide. 2. - The method of claim 1, wherein the metal alloy comprises at least two lanthanides and / or at least two transition metals. 3. - Method according to claim 1 or 2, wherein the metal alloy is a permanent magnet, preferably a permanent magnet samarium / cobalt or neodymium / iron / boron. 4. A process according to claim 3, wherein the permanent magnet is coated, in particular with a metal coating or a phosphate coating. 5. - Method according to claim 1 or 2, wherein the metal alloy is a negative electrode from a nickel-metal hydride battery. 6. - Process according to any one of claims 1 to 5, wherein the concentration of phosphoric acid in the aqueous solution is at least 0.1 mol / L, especially at least 1 mol / L, of preferably at least 5 mol / L. 7. A process according to any one of claims 1 to 6, wherein the separation of step c) is carried out by filtration or centrifugation. 8. A process according to any one of claims 1 to 7, comprising, after step c), the steps of: d) increasing the pH of the liquid phase L1 obtained in step c) to at least 4.0, whereby a second medium is obtained comprising a liquid phase L2 and a solid phase S2 comprising at least one metal of transition, e) separating the liquid phase L2 and the solid phase S2 comprising at least one transition metal, preferably by centrifugation or by filtration. 9. - The method of claim 8, wherein, in step d), the pH of the liquid phase L1 is increased to at least 4.5, or even up to at least 5.0, preferably at a value ranging from 5.5 to 7.0. 10. - Method according to claim 8 or 9, comprising:  a step f) of adding phosphoric acid to the liquid phase L 2 obtained in step e) to obtain an aqueous solution of phosphoric acid, and then  the repetition of step a) and of step b), wherein the aqueous phosphoric acid solution used in step b) is the aqueous phosphoric acid solution obtained in step f).