PROCESSES FOR SELECTIVE RECOVERY OF RARE EARTH METALS PRESENT IN ACIDIC AQUEOUS PHASES RESULTING FROM THE TREATMENT OF SPENT OR SCRAPPED PERMANENT MAGNETS

Abstract

The invention relates to a hydrometallurgical process which makes it possible to selectively recover at least one “heavy” rare earth metal, i.e. a rare earth metal with an atomic number at least equal to 62, that is in an acidic aqueous phase resulting from the treatment of spent or scrapped permanent magnets. It also relates to a hydrometallurgical process which makes it possible to selectively recover, on the one hand, at least one heavy rare earth metal present in an acidic aqueous phase resulting from the treatment of spent or scrapped permanent magnets and, on the other hand, at least one “light” rare earth metal, i.e. a rare earth metal with an atomic number at most equal to 61, that is also in this acidic aqueous phase. The invention has in particular an application in the recycling of rare earth metals present in spent or scrapped permanent magnets of the type Neodymium-Iron-Boron (or NdFeB) and, in particular, dysprosium, praseodymium and neodymium, and also in the recycling of samarium present in spent or scrapped permanent magnets of the type samarium-cobalt (or SmCo).

Claims

1-29. (canceled)

30: A method for selective recovery of at least one rare earth metal TR1 having an atomic number at least equal to 62 from an acid aqueous phase A1, the aqueous phase A1 stemming from a processing of spent or scrapped permanent magnets and comprising one or more rare earth metals TR1, transition metals and a strong acid having a concentration from 0.2 mol/L to 6 mol/L, which method comprises: a) extracting the rare earth metal TR1 from the aqueous phase A1, the extraction comprising putting the aqueous phase A1 in contact with an organic phase not miscible with water, which comprises a diglycolamide having a total number of carbon atoms at least equal to 24 as an extractant, in an organic diluent, and then separating the aqueous and organic phases; b) washing the organic phase stemming from a), the washing comprising putting the organic phase in contact with an acid aqueous phase A2 which comprises a strong acid identical with the strong acid of the aqueous phase A1, at a concentration at most equal to the strong acid concentration of the aqueous phase A1, and then separating the aqueous and organic phases; and c) stripping the rare earth metal TR1 from the organic phase stemming from b), the stripping comprising putting the organic phase in contact with an acid aqueous phase A3 which has a pH at least equal to 3, and then separating the aqueous and organic phases.

31. The method of claim 30, wherein the diglycolamide has the formula: R.sup.1(R.sup.2)N—C(O)—CH.sub.2—O—CH.sub.2—C(O)—N(R.sup.3)R.sup.4 wherein each of R.sup.1 to R.sup.4 represents a linear or branched alkyl group comprising at least 5 carbon atoms.

32. The method of claim 30, wherein the diglycolamide is N,N,N′,N′-tetraoctyl-3-oxapentanediamide, N,N,N′N′-tetra(2-ethylhexyl)-3-oxapentanediamide, N,N,N′N′-tetradecyl-3-oxapentanediamide or N,N,N′,N′-tetradodecyl-3-oxapentanediamide.

33. The method of claim 1, wherein the organic phase comprises from 0.05 mol/L to 1 mol/L of the diglycolamide.

34. The method of claim 30, wherein the aqueous phase A2 comprises from 0.01 mol/L to 0.5 mol/L of the strong acid.

35. The method of claim 30, wherein the aqueous phase A3 comprises from 0.0001 mol/L to 0.001 mol/L of a strong acid.

36. The method of claim 30, wherein the strong acid of the aqueous phases A1 and A2 is nitric acid.

37. The method of claim 30, which comprises a cycle, the cycle comprising a), b), c) and a purification of the organic phase stemming from c).

38. The method of claim 30, wherein the aqueous phase A1 stems from the processing of neodymium-iron-boron permanent magnets in a strong acid and the rare earth metal TR1 is dysprosium.

39. A method for selective recovery of at least one rare earth metal TR1 having an atomic number at least equal to 62 and of at least one rare earth metal TR2 having an atomic number at most equal to 61 from an acid aqueous phase A1, the aqueous phase A1 stemming from a processing of spent or scrapped permanent magnets and comprising one or more rare earth metals TR1, one or more rare earth metals TR2, transition metals and a strong acid having a concentration from 0.2 mol/L to 6 mol/L, which method comprises: recovery of the rare earth metal TR1 from the aqueous phase A1, which recovery comprises: a) extracting the rare earth metal TR1 from the aqueous phase A1, the extraction comprising putting the aqueous phase A1 in contact with an organic phase not miscible with water, which comprises a diglycolamide having a total number of carbon atoms at least equal to 24 as an extractant, in an organic diluent, and then separating the aqueous and organic phases; b) washing the organic phase stemming from a), the washing comprising putting the organic phase in contact with an acid aqueous phase A2, which comprises a strong acid identical with the strong acid of the aqueous phase A1, at a concentration at most equal to the strong acid concentration of the aqueous phase A1, and then separating the aqueous and organic phases; and c) stripping the rare earth metal TR1 from the organic phase stemming from b), the stripping comprising putting the organic phase in contact with an acid aqueous phase A3 which has a pH at least equal to 3, and then separating the aqueous and organic phases; and recovery of the rare earth metal TR2 from the aqueous phase at stemming from a), which recovery comprises: d) extracting the rare earth metal TR2 from the aqueous phase stemming from a), the extraction comprising putting the aqueous phase in contact with a second organic phase not miscible with water, which comprises an extractant identical with the extractant of the first organic phase, in an organic diluent, and then separating the aqueous and organic phases; e) washing the organic phase stemming from d), the washing comprising putting the organic phase in contact with an acid aqueous phase A4, which comprises a strong acid identical with the strong acid of the aqueous phase A1, at a concentration at most equal to the strong acid concentration of the aqueous phase stemming from a), and then separating the aqueous and organic phases; and f) stripping the rare earth metal TR2 from the organic phase stemming from e), the stripping comprising putting the organic phase in contact with an acid aqueous phase A5 which has a pH at least equal to 3, and then separating the aqueous and organic phases.

40. The method of claim 39, wherein the diglycolamide has the formula: R.sup.1(R.sup.2)N—C(O)—CH.sub.2—O—CH.sub.2—C(O)—N(R.sup.3)R.sup.4 wherein each of R.sup.1 to R.sup.4 represents a linear or branched alkyl group comprising at least 5 carbon atoms.

41. The method of claim 39, wherein the diglycolamide is N,N,N′N′-tetraoctyl-3-oxapentanediamide, N,N,N′,N′-tetra(2-ethylhexyl)-3-oxapentanediamide, N,N,N′N′-tetradecyl-3-oxapentanediamide or N,N,N′N′-tetradodecyl-3-oxapentanediamide.

42. The method of claim 39, wherein the organic phase comprises from 0.05 mol/L to 1 mol/L of the diglycolamide.

43. The method of claim 39, wherein the aqueous phase A2 comprises from 0.01 to 0.5 mol/L of the strong acid.

44. The method of claim 39, wherein the aqueous phase A4 comprises from 0.2 mol/L to 4 mol/L of the strong acid.

45. The method of claim 39, wherein each of the aqueous phases A3 and A5 comprises from 0.0001 mol/L to 0.001 mol/L of a strong acid.

46. The method of claim 39, wherein the strong acid of the aqueous phases A1, A2 and A4 is nitric acid.

47. The method of claim 39, which comprises a first and a second cycle, the first cycle comprising a), b) and c), the second cycle comprising d), e) and f), the first and second cycles further comprising a purification of an organic phase formed by grouping the organic phases stemming from c) and f) and dividing the thereby purified organic phase into the first and second organic phases.

48. The method of claim 39, wherein the aqueous phase A1 stems from the processing of neodymium-iron-boron permanent magnets in a strong acid and comprises dysprosium as the rare earth metal TR1 and praseodymium and neodymium as the rare earth metals TR2.

49. A method for selective recovery of at least one rare earth metal TR1 having an atomic number at least equal to 62 and of at least one rare earth metal TR2 having an atomic number at most equal to 61 from an acid aqueous phase A1, the aqueous phase A1 stemming from a processing of spent or scrapped permanent magnets and comprising one or more rare earth metals TR1, one or more rare earth metals TR2, transition metals and a strong acid having a concentration from 0.2 mol/L to 6 mol/L, which method comprises: a) extracting the rare earth metal TR1 and the rare earth metal TR2 from the aqueous phase A1, the extraction comprising putting the aqueous phase A1 in contact with a first organic phase not miscible with water, which comprises a diglycolamide having a total number of carbon atoms at least equal to 24 as an extractant, in an organic diluent, and then separating the aqueous and organic phases, b) washing the organic phase stemming from a), the washing comprising putting the organic phase in contact with an acid aqueous phase A2, which comprises a strong acid identical with the strong acid of the aqueous phase A1, at a concentration at most equal to the strong acid concentration of the aqueous phase A1, and then separating the aqueous and organic phases; b′) stripping the rare earth metal TR2 from the organic phase stemming from b), the stripping comprising putting the organic phase in contact with an acid aqueous phase A5, which has a pH from 1 to 2.3, and then separating the aqueous and organic phases; b″) washing the aqueous phase stemming from b′), the washing comprising putting the aqueous phase in contact with a second organic phase not miscible with water, which comprises an extractant identical with the extractant of the first organic phase, in an organic diluent, and then separating the aqueous and organic phases; c) stripping the rare earth metal TR1 from the organic phase stemming from b′), the stripping comprising putting the organic phase in contact with an acid aqueous phase A3 which has a pH at least equal to 3, and then separating the aqueous and organic phases.

50. The method of claim 49, wherein the diglycolamide has the formula: R.sup.1(R.sup.2)N—C(O)—CH.sub.2—O—CH.sub.2—C(O)—N(R.sup.3)R.sup.4 wherein each of R.sup.1 to R.sup.4 represents a linear or branched alkyl group comprising at least 5 carbon atoms.

51. The method of claim 49, wherein the diglycolamide is N,N,N′,N′-tetraoctyl-3-oxapentanediamide, N,N,N′,N′-tetra(2-ethylhexyl)-3-oxapentanediamide, N,N,N′,N′-tetradecyl-3-oxapentanediamide or N,N,N′,N′-tetradodecyl-3-oxapentanediamide.

52. The method of claim 49, wherein the organic phase comprises from 0.05 mol/L to 1 mol/L of the diglycolamide.

53. The method of claim 49, wherein the aqueous phase A2 comprises from 0.2 mol/L to 4 mol/L of the strong acid.

54. The method of claim 49, wherein the aqueous phase A5 comprises from 0.005 mol/L to 0.1 mol/L of a strong acid.

55. The method of claim 49, wherein the aqueous phase A3 comprises from 0.0001 mol/L to 0.001 mol/L of a strong acid.

56. The method of claim 49, wherein the strong acid of the aqueous phases A1 and A2 is nitric acid.

57. The method of claim 49, which comprises a cycle, the cycle comprising a), b), b′), b″), c), a purification of the organic phase stemming from c) and a division of the thereby purified organic phase into the first and second organic phases.

58. The method of claim 49, wherein the aqueous phase A1 stems from the processing of neodymium-iron-boron permanent magnets in a strong acid and comprises dysprosium as the rare earth metal TR1 and praseodymium and neodymium as the rare earth metals TR2.

Description

SHORT DESCRIPTION OF THE FIGURES

[0085] FIG. 1 illustrates the principle of a preferred embodiment of the first method of the invention, applied to the processing, at an industrial scale, of a nitric aqueous phase stemming from the processing of spent or scrapped permanent magnets NdFeB, with view to selectively recovering dysprosium present in this aqueous phase.

[0086] FIG. 2 illustrates the principle of a first preferred embodiment of the second method of the invention, applied to the processing, at an industrial scale of a nitric aqueous phase stemming from the processing of spent or scrapped permanent magnets NdFeB, with view to selectively recovering dysprosium, praseodymium and neodymium present in this aqueous phase.

[0087] FIG. 3 illustrates the principle of a second preferred embodiment of the second method of the invention, applied to the processing, at an industrial scale, of a nitric aqueous phase stemming from the processing of spent or scrapped permanent magnets NdFeB, with view to selectively recovering dysprosium, praseodymium and neodymium present in this aqueous phase.

[0088] FIGS. 4A and 4B illustrate results of extraction tests carried out on synthetic nitric aqueous phases, comprising boron, iron, dysprosium, praseodymium and neodymium, and by using organic phases comprising 0.2 mol/L of TODGA, 5% (v/v) of n-octanol in n-dodecane; FIG. 4A shows the distribution coefficients, noted as D.sub.M, of the different metal elements obtained according to the nitric acid concentration, expressed in mol/L, of the tested aqueous phases while FIG. 4B shows the separation factors, respectively noted as FS.sub.Dy/Nd, FS.sub.Dy/Pr, FS.sub.Dy/B and FS.sub.Dy/Fe, between dysprosium and the other metal elements as well as the separation factor noted as FS.sub.Nd/Pr, between neodymium and praseodymium, obtained according to the nitric acid concentration, expressed in mol/L, of the tested aqueous phases.

[0089] FIG. 5 illustrates the scheme which was used for testing, in mixers-decanters, the preferred embodiment of the first method of the invention illustrated in FIG. 1.

[0090] FIG. 6 illustrates the scheme which was used for testing, in mixers-decanters, the second preferred embodiment of the second method of the invention illustrated in FIG. 3.

[0091] FIGS. 7A and 7B illustrate a scheme which may be used for the first preferred embodiment of the second method of the invention which is illustrated in FIG. 2; FIG. 7A corresponds to the steps of the first cycle of this embodiment except for the step of the purification of the organic phase while FIG. 7B corresponds to the steps of the second cycle of said embodiment except, there also, the step for purifying the organic phase.

[0092] In FIGS. 1 to 3, the rectangles referenced as 1 to 7 represent multistaged extractors such as those which are conventionally used for achieving liquid-liquid extractions at an industrial scale like for example, extractors consisting of batteries of mixers-decanters.

[0093] Moreover, in FIGS. 1 to 3 and 5 to 7B, the organic phase flows entering and leaving these extractors are symbolized with a solid line while the aqueous phase flows entering and leaving said extractors are symbolized by dotted lines.

DETAILED DISCUSSION OF THE INVENTION

Example 1: Principle Scheme of a Preferred Embodiment of the First Method of the Invention

[0094] Reference is made to FIG. 1 which illustrates the principle of a preferred embodiment of the first method of the invention, designed for processing, at an industrial scale, a nitric aqueous phase stemming from the processing of spent or scrapped permanent magnets NdFeB, with view to selectively recovering dysprosium which is the most interesting economically one of the heavy rare earth metals present in this aqueous phase.

[0095] This aqueous phase is for example an aqueous phase stemming from the dissolution in a nitric medium, supplemented with an oxidizer, of a powder of permanent magnets NdFeB as obtained in the aforementioned reference [2].

[0096] Such an aqueous phase, which is designated hereafter and in FIG. 1 as “Aqueous phase A1”, may comprise, according to the type of magnets from which it was obtained and from conditions under which the powder of magnets was dissolved in a nitric medium: from 0.2 mol/L to 6 mol/L of nitric acid, dysprosium, praseodymium, neodymium, iron and a certain number of metal impurities such as boron, nickel, copper, cobalt, chromium, aluminium, zirconium etc. This iron and these impurities will be grouped, in the following, under the term of “undesirable elements”.

[0097] In the present embodiment, the method comprises a single cycle which aims at selectively recovering the dysprosium present in the aqueous phase A1.

[0098] This cycle comprises: [0099] a first step, designated as “Dy extraction” in FIG. 1, which aims at extracting dysprosium from the aqueous phase A1 by means of an organic phase not miscible with water; [0100] a second step, designated as “Pr+Nd+undesirable elements washing” in FIG. 1, which aims at washing the organic phase stemming from the “Dy extraction” by means of an acid aqueous phase A2 for removing from this organic phase the metal elements other than dysprosium which may have been partly extracted during the “Dy extraction”; [0101] a third step, designated as “Dy stripping” in FIG. 1, which aims at stripping dysprosium from the organic phase stemming from the “Pr+Nd+undesirable elements washing” by means of an acid aqueous phase A3; and [0102] a fourth step, designated as “Organic phase purification” in FIG. 1, which aims at subjecting the organic phase from the “Dy stripping” to a series of processing operations allowing purifying this phase for its reuse in a subsequent cycle.

[0103] Practically, the “Dy extraction” is achieved in the extractor 1 by putting the aqueous phase A1 entering this extractor several times in contact, as a counter-current, with the organic phase which comprises a diglycolamide (noted as DGA in FIG. 1) in solution in an organic diluent.

[0104] As indicated earlier, the diglycolamide is selected from among lipophilic diglycolamides, i.e. for which the total number of carbon atoms is at least equal to 24 and more particularly, from among diglycolamides which fit the formula: R(R.sup.2)N—C(O)—CH.sub.2—O—CH.sub.2—C(O)—N(R.sup.3)R.sup.4 wherein R.sup.1 to R.sup.4 represent branched or linear alkyl groups, each comprising at least 5 carbon atoms and even better, at least 8 carbon atoms, preference being given to the diglycolamides in which R.sup.1 to R.sup.4 represent identical alkyl groups with each other, comprising from 8 to 12 carbon atoms.

[0105] This diglycolamide is for example TODGA, TEHDGA or TdDDGA which are used at a concentration typically ranging from 0.05 mol/L to 1 mol/L, preferably from 0.05 mol/L to 0.4 mol/L, this concentration being 0.2 mol/L for example.

[0106] The organic phase may further comprise, notably when the alkyl groups R.sup.1 to R.sup.4 of the diglycolamide comprise less than 12 carbon atoms, a phase modifier able to avoid the formation of a third phase, for example n-octanol (CH.sub.3(CH.sub.2).sub.6CH.sub.2OH), in which case the latter does not represent preferably more than 10% by volume of the volume of the organic phase.

[0107] As for the organic diluent, this is for example an aliphatic diluent such as n-dodecane, TPH or a kerosene such as Isane IP-185.

[0108] The organic phase leaving the extractor 1, which is loaded with dysprosium, is directed towards the extractor 2 dedicated to the “Pr+Nd+undesirable elements washing” while the aqueous phase leaving the extractor 1 (designated as “raffinate” in FIG. 1) is directed towards a unit for processing aqueous effluents of the method.

[0109] The “Pr+Nd+undesirable elements washing” is achieved in the extractor 2 by putting the organic phase entering this extractor several times into contact, as a counter-current, with the aqueous phase A2 which comprises nitric acid at a concentration which is at most equal to the nitric acid concentration of the aqueous phase A1 but which is preferably less than this concentration, each putting into contact being followed by a separation of the aqueous and organic phases. Typically, the nitric acid concentration of the aqueous phase A2 ranges from 0.01 mol/L to 0.5 mol/L depending on the nitric acid concentration of the aqueous phase A1 and preferably is equal to 0.01 mol/L.

[0110] The organic phase leaving the extractor 2 is directed towards the extractor 3 dedicated to the “Dy stripping” while the aqueous phase leaving the extractor 2 is sent back towards the extractor 1 where it joins up with the aqueous phase A1 and is added thereto.

[0111] The “Dy stripping” is achieved in the extractor 3 by putting the organic phase entering this extractor several times into contact, as a counter-current, with the aqueous phase A3 which comprises nitric acid as an acid and for which the nitric acid concentration is at most equal to 0.001 mol/L, typically comprised between 0.0001 mol/L and 0.001 mol/L and, preferably, equal to 0.001 mol/L, each putting into contact being followed by a separation of the aqueous and organic phases, and by preferably heating the extractor 3, typically to a temperature ranging from 40° C. to 55° C.

[0112] In order to facilitate the stripping of the dysprosium, the aqueous phase A3 may comprise, in addition to nitric acid, one or more compounds complexing the rare earth metals in an aqueous medium such as a hydrophilic diglycolamide, i.e. for which the total number of carbon atoms does not exceed 20, like TMDGA, TEDGA or TPDGA, a polyaminocarboxylic acid like HEDTA, NTA or DTPA, or a mono-, di- or tri-carboxylic acid like glycolic acid, malonic acid or mesoxalic acid.

[0113] At the end of the “Dy stripping”, an aqueous phase is obtained which only contains dysprosium as a metal element, and an organic phase which is directed towards the extractor 4 dedicated to the “Organic phase purification” in order to be subject to a series of processing operations (acid washings, alkaline washings, complexing washings, etc.) able to get rid of the possible degradation products, notably from hydrolysis, and of residual metal elements which it contains.

Example 2: Principle Scheme of a First Preferred Embodiment of the Second Method of the Invention

[0114] Now, reference is made to FIG. 2 which illustrates the principle of a first preferred embodiment of the second method of the invention, in which this method is designed for processing at an industrial scale, an acid aqueous phase A1 similar to the one treated in the Example 1 hereinbefore but with view to selectively recovering, not only the dysprosium but also the praseodymium and neodymium, and this, in two cycles, i.e.: a first cycle which aims at selectively recovering dysprosium present in the aqueous phase A1 and a second cycle which itself aims at selectively recovering praseodymium and neodymium present in the raffinate of the first cycle.

[0115] The first cycle comprises: [0116] a first step, designated as “Dy extraction” in FIG. 2, which aims at extracting dysprosium from the aqueous phase A1 by means of a first organic phase not miscible with water; [0117] a second step, designated as “Pr+Nd+undesirable elements washing” in FIG. 2, which aims at washing the organic phase stemming from the “Dy extraction” by means of an acid aqueous phase A2 for removing from this organic phase the metal elements other than dysprosium which may have been partly extracted during the “Dy extraction”; [0118] a third step, designated as “Dy stripping” in FIG. 2, which aims at stripping the dysprosium from the organic phase stemming from the “Pr+Nd+undesirable elements washing” by means of an acid aqueous phase A3;
while the second cycle comprises: [0119] a first step, designated as “Pr+Nd co-extraction” in FIG. 2, which aims at extracting praseodymium and neodymium from the aqueous phase stemming from the “Dy extraction” by means of a second organic phase having the same qualitative and quantitative composition as the first organic phase; [0120] a second step, designated as “Undesirable elements washing” in FIG. 2, which aims at washing the organic phase stemming from the “Pr+Nd co-extraction” by means of an acid aqueous phase A4 for removing from this organic phase the metal elements other than praseodymium and neodymium which may have been partly extracted during the “Pr+Nd co-extraction”; and [0121] a third step, designated as “Pr+Nd co-stripping” in FIG. 2, which aims at stripping praseodymium and neodymium from the organic phase stemming from the “Undesirable elements washing” by means of an acid aqueous phase A5.

[0122] The first and second cycles further comprise, a common step, designated as “Organic phase purification” in FIG. 2, which binds them with each other and which aims at subjecting the organic phase formed by the gathering of the organic phases respectively stemming from the “Dy stripping” and from the “Pr+Nd co-stripping” to a series of processing operations allowing purifying this phase for its reuse, after dividing it into two, in the subsequent first and second cycles.

[0123] The steps “Dy extraction”, “Pr+Nd+undesirable elements washing”, “Dy stripping” and “organic phase purification” are carried out, respectively in the extractors 1, 2, 3 and 7, in the same way as in Example 1.

[0124] On the other hand, unlike Example 1, the aqueous phase leaving the extractor 1 is directed towards the extractor 4 dedicated to the “Pr+Nd co-extraction” instead of being directed towards a unit for processing aqueous effluents of the method.

[0125] The “Pr+Nd co-extraction” is achieved by putting the aqueous phase entering the extractor 4 several times into contact, as a counter-current, with the second organic phase, each putting into contact being followed by a separation of the aqueous and organic phases. An increase in the concentration of nitric acid of the aqueous phase stemming from the “Dy extraction” may be achieved, before entry or during entry of this aqueous phase into the extractor 4, in order to promote the extraction of praseodymium and of neodymium by the diglycolamide.

[0126] The organic phase leaving the extractor 4, which is loaded with praseodymium and neodymium, is directed towards the extractor 5 dedicated to the “Undesirable elements washing” while the aqueous phase leaving the extractor 4 (designated as “raffinate” in FIG. 2) is directed towards a unit for processing aqueous effluents of the method.

[0127] The “Undesirable elements washing” is achieved in the extractor 5 by putting the organic phase entering this extractor several times into contact, as a counter-current, with the aqueous phase A4 which comprises nitric acid at a concentration which is at most equal to la nitric acid concentration of the aqueous phase stemming from the “Dy extraction” but which is preferably less than this concentration, each putting into contact being followed by a separation of the aqueous and organic phases. Typically, the nitric acid concentration of the aqueous phase A4 ranges from 0.2 mol/L to 4 mol/L depending on the nitric acid concentration of the aqueous phase stemming from the “Dy extraction” and preferably is equal to 1 mol/L.

[0128] The organic phase leaving the extractor 5 is directed towards the extractor 6 dedicated to the “Pr+Nd co-stripping” while the aqueous phase leaving the extractor 5 is sent back towards the extractor 4 where it joins up with the aqueous phase stemming from the “Dy extraction” and is added thereto.

[0129] The “Pr+Nd co-stripping” is achieved in the extractor 6, at room temperature or under hot conditions (45°-50° C.), by putting the organic phase entering this extractor several times in contact, as a counter-current, with the aqueous phase A5 which comprises nitric acid as an acid and for which the nitric acid concentration is at most equal to 0.001 mol/L, typically comprised between 0.0001 mol/L and 0.001 mol/L and is preferably equal to 0.001 mol/L, each putting into contact being followed by a separation of the aqueous and organic phases.

[0130] In order to promote the stripping of praseodymium and neodymium, the aqueous phase A5 may comprise, in addition to nitric acid, one or more agents complexing the rare earth metals of the same type as those which may be used for the “Dy stripping”.

[0131] At the end of the “Pr+Nd co-stripping”, an aqueous phase is obtained which does not contain any longer praseodymium and neodymium as metal elements, and an organic phase which joins up with the organic phase stemming from the “Dy stripping” and is directed, together with the latter, towards the extractor 7 dedicated to the “Organic phase purification”.

Example 3: Principle Scheme of a Second Preferred Embodiment of the Second Method of the Invention

[0132] Reference is made now to FIG. 3 which illustrates the principle of a second preferred embodiment of the second method of the invention, wherein this method is also designed for selectively recovering and at an industrial scale dysprosium, praseodymium and neodymium from an acid aqueous phase A1 similar to the one processed in Example 1 hereinbefore but in a single cycle.

[0133] This cycle comprises: [0134] a first step, designated as “Dy+Pr+Nd co-extraction” in FIG. 3, which aims at co-extracting dysprosium, praseodymium and neodymium from the aqueous phase A1 by means of a first organic phase not miscible with water; [0135] a second step, designated as “Undesirable elements washing” in FIG. 3, which aims at washing the organic phase stemming from the “Dy+Pr+Nd co-extraction” by means of an acid aqueous phase A2 for removing from this organic phase the metal elements other than dysprosium, praseodymium and neodymium which may have been partly extracted during the “Dy+Pr+Nd co-extraction”; [0136] a third step, designated as “Pr+Nd co-stripping” in FIG. 3, which aims at selectively stripping praseodymium and neodymium from the organic phase stemming from the “Undesirable elements washing” by means of an acid aqueous phase A5; [0137] a fourth step, designated as “Dy washing” in FIG. 3, which aims at removing from the aqueous phase stemming from the “Pr+Nd co-stripping” the dysprosium fraction that has been stripped during this co-stripping, by means of a second organic phase having the same qualitative et quantitative composition as the first organic phase; [0138] a fifth step, designated as “Dy stripping” in FIG. 3, which aims at selectively stripping dysprosium from the organic phase stemming from the “Pr+Nd co-stripping” by means of an acid aqueous phase A3; and [0139] a sixth step, designated as “Organic phase purification” in FIG. 3, which aims at subjecting the organic phase stemming from the “Dy stripping” to a series of processing operations allowing purifying this phase for its reuse, after having been divided into two, in a subsequent cycle.

[0140] The “Dy+Pr+Nd co-extraction” is achieved in the extractor 1 in the same way as the Dy extraction of Examples 1 and 2.

[0141] The organic phase leaving the extractor 1, which is loaded with dysprosium, praseodymium and neodymium, is directed towards the extractor 2 dedicated to the “Undesirable elements washing” while the aqueous phase leaving the extractor 1 (designated as “raffinate” in FIG. 3) is directed towards a unit for processing aqueous effluents of the method.

[0142] The “Undesirable elements washing” is achieved in the extractor 2 by putting the organic phase entering this extractor several times into contact, as a counter-current, with the aqueous phase A2 which, in this case, typically comprises a nitric acid concentration which is comprised between 0.2 mol/L and 4 mol/L depending on the nitric acid concentration of the aqueous phase A1 and preferably is equal to 1 mol/L, each putting into contact being followed by a separation of the aqueous and organic phases.

[0143] The organic phase leaving the extractor 2 is directed towards the extractor 4 dedicated to the “Pr+Nd co-stripping” while the aqueous phase leaving the extractor 2 is sent back towards the extractor 1 where it joins up with the aqueous phase A1 and is added thereto.

[0144] The “Pr+Nd co-stripping” is achieved in the extractor 4 in the same way as the “Pr+Nd co-stripping” of Example 2, except, on the one hand, the aqueous phase A5 has a nitric acid concentration ranging from 0.005 mol/L to 0.1 mol/L, for example 0.01 mol/L, and, on the other hand, it is achieved at room temperature, and this, so as to limit the amount of dysprosium which may be stripped together with praseodymium and neodymium.

[0145] The aqueous phase leaving the extractor 4 is sent back towards the extractor 3 dedicated to the “Dy washing” while the organic phase leaving the extractor 4 is directed towards the extractor 5 dedicated to the “Dy stripping”.

[0146] The “Dy washing” is achieved in the extractor 3 by putting the aqueous phase stemming from the extractor 4 several times in contact, as a counter-current, with the second organic phase, each putting into contact being followed by a separation of the aqueous and organic phases. At the end of this washing, an aqueous phase is obtained which does not contain any more praseodymium and neodymium as metal elements, and an organic phase which is directed towards the extractor 4 where it joins up with the organic phase stemming from the extractor 2 and is added thereto.

[0147] The “Dy stripping” is achieved in the extractor 5 in the same way as in Examples 1 and 2.

[0148] At the end of the “Dy stripping”, an aqueous phase is obtained which only contains dysprosium as a metal element, and an organic phase which is directed towards the extractor 6 dedicated to the “Purification of the organic phase”.

Example 4: Experimental Validation of the Invention

[0149] 4.1—Tests in Test Tubes:

[0150] In the tests which follow, the concentrations of the different metal elements in the aqueous solutions or phases, were all measured by atomic emission spectrometry with a plasma torch, further known under the acronym ICP-AES.

[0151] The concentrations of the metal elements in the organic phases were estimated after having stripped these elements in an aqueous phase which is strongly complexing (oxalic acid=0.5 mol/L; TEDGA=0.2 mol/L; HNO.sub.3=1 mol/L; volume ratio O/A=1/5; duration of the stirring=10 minutes; temperature=25° C.) and after having measured the concentrations of said elements in the aqueous phase obtained at the end of this stripping.

[0152] Moreover, the distribution coefficients and the separation factors were determined according to the conventions in the field of liquid-liquid extractions, i.e.: [0153] the distribution coefficient of a metal element M, noted as D.sub.M, between two phases, respectively an organic and aqueous phase, is equal to:

[00001]$D_M=\frac{{\left[M\right]}_{\mathrm{org}.}}{{\left[M\right]}_{\mathrm{aq}.}}$

with:

[0154] [M].sub.org.=concentration of the metal element in the organic phase at extraction equilibrium (in g/L); and

[M].sub.aq.=concentration of the metal element in the aqueous phase at extraction equilibrium (in g/L); [0155] the separation factor between two metal elements M1 and M2, noted as FS.sub.M1/M2, is equal to:

[00002]${\mathrm{FS}}_{M.Math..Math.1/M.Math..Math.2}=\frac{D_{M.Math..Math.1}}{D_{M.Math..Math.2}}$

with:

[0156] D.sub.M1=distribution coefficient of the metal element M1; and

[0157] D.sub.M2=distribution coefficient of the metal element M2.

[0158] 4.1.1—Extraction Tests Carried Out on Synthetic Nitric Aqueous Phases Comprising Boron, Iron, Praseodymium, Neodymium and Dysprosium:

[0159] Extraction tests are carried out in tubes, by using: [0160] as organic phases: phases comprising either 0.2 mol/L of TODGA in n-dodecane or 0.2 mol/L of TODGA and 5% (v/v) of n-octanol (as a phase modifier) in n-dodecane; and [0161] as aqueous phases: phases obtained by dissolution of boric acid and of hydrated iron nitrates, of praseodymium, of neodymium and dysprosium in an aqueous solution of nitric acid; all these phases have the boron, iron, praseodymium, neodymium and dysprosium concentrations which are indicated in the table I hereafter but their nitric acid concentration varies from 0.1 mol/L to 2.6 mol/L.

TABLE-US-00001 TABLE I Elements Concentrations (g/L) B 1.3 Fe 43.7 Pr 1.1 Nd 10.3 Dy 1.0

[0162] Each organic phase is put into contact, with stirring, with one of the aqueous phases, volume by volume, for 30 minutes at 25° C., and then these phases are separated from each other after centrifugation.

[0163] The results of these tests show that there is no formation of a third phase when the organic phase used for the extraction comprises n-octanol and this, regardless of the acidity of the tested aqueous phase. On the other hand, a third phase forms in the absence of n-octanol in the organic phase.

[0164] As visible in FIG. 4A, which illustrates the distribution coefficients of the different metal elements according to the nitric acid concentration of the tested aqueous phases, a quantitative extraction of dysprosium (D.sub.Dy>100) is obtained for the whole of these phases.

[0165] The iron, which is the most abundant element in the aqueous phases is very little extracted (D.sub.Fe<0.01).

[0166] Moreover, as shown in FIG. 4B, which illustrates the separation factors between the dysprosium and the other metal elements depending on the nitric acid concentration of the tested aqueous phases, the separation of dysprosium relatively to iron and boron is excellent since the separation factors FS.sub.Dy/Fe and FS.sub.Dy/B are respectively greater than 10,000 and 750. Further, separation factors FS.sub.Dy/Nd and FS.sub.Dy/Pr of greater than 10 are obtained regardless of the acidity of the tested aqueous phases.

[0167] These results show that it is therefore possible to easily recover dysprosium from an acid aqueous phase comprising high concentrations of iron, boron, neodymium and praseodymium by using a diglycolamide as an extractant.

[0168] 4.1.2—Recovery of Dysprosium, Neodymium and Praseodymium from an Aqueous Phase Stemming from the Dissolution of a Powder of Permanent Magnets NdFeB in a 5.15 M Nitric Medium:

[0169] An aqueous phase is prepared by dissolving a powder of scrapped permanent magnets NdFeB in a nitric medium 5.15 M supplemented with H.sub.2O.sub.2 (1% by volume) as described in the aforementioned reference [2], the powder of permanent magnets having itself been obtained by demagnetization of these magnets by means of a heat treatment in an oven (200° C.-5 hours), milling and then treatment by hydridation-dehydridation.

[0170] The concentrations of metal elements of the aqueous phase thereby obtained are indicated in the table II hereafter.

TABLE-US-00002 TABLE II Elements Concentrations (g/L) B 0.4 Fe 36 Co 0.2 Ni 0.8 Cu 0.5 Pr 2.9 Nd 11 Dy 0.2

Extraction Test:

[0171] The aqueous phase obtained earlier is subject to an extraction test which is conducted by using as an organic phase, a phase comprising 0.2 mol/L of TODGA and 5% (v/v) of n-octanol in n-dodecane. To do this, the aqueous phase is put with stirring in contact with this organic phase in a volume ratio O/A of 1, for 30 minutes at 25° C., and then these phases are separated from each other by decantation.

[0172] Table III hereafter shows the distribution coefficients of the different metal elements, the separation factors FS.sub.Dy/M and FS.sub.Pr/M as well as the concentration of the aqueous phase of nitric acid at equilibrium obtained for this extraction test.

TABLE-US-00003 TABLE III Elements D.sub.M FS.sub.Dy/M FS.sub.Pr/M [HNO.sub.3].sub.eq. B 0.06 383 8.3 4.76M Fe <0.004 >5,750 >125 Co <0.02 >1,150 >25 Ni <0.005 >4,600 >100 Cu <0.04 >575 >12.5 Pr 0.5 46 1.0 Nd 1.3 18 0.4 Dy 23 1.0 0.02

[0173] This table confirms the quantitative extraction of dysprosium (D.sub.Dy=23) obtained earlier during the extraction tests conducted at point 4.1.1 and before on synthetic nitric aqueous phases.

[0174] The strong selectivity which TODGA exhibits for dysprosium as compared with the other metal elements, including the light rare earth metals, is also confirmed since the separation factors FS.sub.Dy/Pr and FS.sub.Dy/Nd are respectively 46 and 18.

[0175] Finally, it shows that it is possible to efficiently separate, after having extracted dysprosium from the aqueous phase, didymium Pr+Nd of the other metal elements still present in this aqueous phase by using the same organic phase as the one used for extracting dysprosium.

[0176] Stripping Tests:

[0177] The organic phase obtained at the end of the extraction test hereinbefore is subject to a series of stripping tests which are carried out by using as an aqueous phase, an aqueous solution of nitric acid at 0.001 mol/L (pH 3).

[0178] To do this, aliquots of the aqueous phase are put into contact a first time (hereafter “contact 1”), with stirring, with aliquots of the organic phase in a volume ratio O/A of 1/5, for 30 minutes at 25° C., 40° C. or 55° C., and then these aliquots are separated from each other by decantation.

[0179] The concentrations of metal elements as well as the pH of the aliquots of aqueous phase thereby separated are measured, after which these aliquots are again put into contact (hereafter “contact 2”) with aliquots of the organic phase under the same conditions as earlier.

[0180] Table IV hereafter shows the distribution coefficients of dysprosium, neodymium and praseodymium obtained for these stripping tests. The pH exhibited by the aliquots of aqueous phase after each of the contacts 1 and 2 are also indicated in this table.

TABLE-US-00004 TABLE IV D.sub.M Stripping at 25° C. Stripping at 40° C. Stripping at 55° C. Contact 1 Contact 2 Contact 1 Contact 2 Contact 1 Contact 2 Elements Dy 20.2 0.9 3.8 0.65 0.73 0.86 Nd 0.60 0.15 0.13 <0.1 0.05 <0.05 Pr 0.6 <0.3 0.10 <0.1 0.05 <0.05 pH.sub.after contact 1.2 3.0 1.2 3.0 1.2 3.0

[0181] This table shows that at 25° C., stripping of dysprosium is only efficient at the second contact (D.sub.Dy<1) when the pH is greater than 1 (pH 3). It also shows that it is possible to improve the stripping of dysprosium at the first contact by carrying out this stripping at a higher temperature. Thus at 55° C., a distribution coefficient of less than 1 (D.sub.Dy=0.7) is obtained for dysprosium as soon as the first contact at a pH of 1.16 at equilibrium.

[0182] On the other hand, good stripping of the didymium Pr+Nd is observed as soon as the first contact and this, regardless of the temperature, this stripping being however improved by an increase in the temperature.

[0183] 4.1.3—Recovery of Dysprosium from an Aqueous Phase Stemming from the Dissolution of a Powder of Permanent Magnets NdFeB in a 0.4 M Nitric Medium:

[0184] An aqueous phase is prepared by dissolving a powder of scrapped permanent magnets NdFeB in a 0.4 M nitric medium in the same way as in point 4.1.2 hereinbefore, except that the aqueous solution used for dissolving the resulting powder from the hydridation-dehydridation process is an aqueous solution of nitric acid at 0.4M.

[0185] The concentrations of metal elements of the thereby obtained aqueous phase are indicated in table V hereafter.

TABLE-US-00005 TABLE V Elements Concentrations (g/L) B 0.4 Fe 32 Co 0.2 Ni 0.08 Cu 0.1 Pr 2.7 Nd 11 Dy 0.2

[0186] Then, from this aqueous phase, an extraction test is carried out followed by a series of stripping tests under the same conditions as those described under point 4.1.2 hereinbefore.

[0187] Table VI hereafter shows the distribution coefficients of the different metal elements, the separation factors FS.sub.Dy/M and FS.sub.Pr/M as well as the concentration of the aqueous phase of nitric acid at equilibrium obtained for the extraction test while table VII hereafter shows the distribution coefficients of dysprosium, neodymium and praseodymium obtained for the stripping tests. The pH exhibited by the aliquots of aqueous solution after contact 1 are also indicated in this table.

TABLE-US-00006 TABLE VI Elements D.sub.M FS.sub.Dy/M FS.sub.Pr/M [HNO.sub.3].sub.eq. B 0.12 225 14 0.34M Fe <0.002 >3,500 >857 Co <0.02 >1,350 >85 Ni <0.05 >540 >34 Cu <0.17 158 10 Pr 1.7 15.9 1.0 Nd 2.1 12.8 0.8 Dy 27 1.0 0.06

TABLE-US-00007 TABLE VII D.sub.M Stripping Stripping at 25° C. at 40° C. Stripping at 55° C. Contact 1 Contact 1 Contact 1 Elements Dy 5.4 0.76 0.23 Nd 0.17 0.05 0.02 Pr 0.11 0.04 0.02 pH.sub.after contact 2.0 2.0 2.0

[0188] Table VI shows that it is also possible to extract and efficiently separate dysprosium on the one hand, and the didymium Pr+Nd on the other hand, from the other metal elements (by considering the separation factors FS.sub.Pr/element as dimensioning for the separation) from an aqueous phase with a concentration much less than that of the aqueous phase used under point 4.1.2 hereinbefore (0.4 M versus 5.15 M). However, the selectivity of TODGA for dysprosium as compared with praseodymium and neodymium appears to be less than the one observed for the aqueous phase used under point 4.1.2 hereinbefore, so that it seems preferable that the solution from the dissolution of a powder of permanent magnets NdFeB from which dysprosium is extracted has a nitric acid concentration significantly greater than 0.4 M.

[0189] Table VII itself confirms that it is possible to strip the rare earth metals comprising TODGA as an extractant by means of a very diluted nitric acid aqueous phase (pH 3) and that this stripping is improved by increasing the temperature.

[0190] 4.1.4—Influence of the Nature of the Alkyl Chains of the Diglycolamide:

[0191] Extraction Tests are Carried Out in Tubes, by Using: [0192] as organic phases: phases comprising 0.2 mol/L of one of the following diglycolamides: TODGA, TEHDGA or TdDDGA, with or without 5% (v/v) of n-octanol (as a phase modifier), in n-dodecane; and [0193] as aqueous phases: the aqueous phases stemming from the dissolution of a powder of permanent magnets NdFeB in a 5.15 M and 0.4 M nitric medium, respectively prepared under points 4.1.2 and 4.1.3 hereinbefore.

[0194] These extraction tests are carried out under the same conditions as the ones described under point 4.1.2 hereinbefore.

[0195] Table VIII hereafter shows the distribution coefficients of dysprosium, of neodymium, of praseodymium, of iron and of boron as well as the pH exhibited by the aqueous phases at equilibrium.

TABLE-US-00008 TABLE VIII Organic phases TODGA TEHDGA TdDDGA 5% 0% 5% 0% 5% 0% octanol octanol octanol octanol octanol octanol Aqueous D.sub.Dy 23 3.sup.rd phase 3.sup.rd phase 3.sup.rd phase 24.9 25.3 phase D.sub.Nd 1.4 3.sup.rd phase 3.sup.rd phase 3.sup.rd phase 1.65 1.65 5.15M D.sub.Pr 0.5 3.sup.rd phase 3.sup.rd phase 3.sup.rd phase 1.61 1.62 D.sub.Fe <0.004 3.sup.rd phase 3.sup.rd phase 3.sup.rd phase 0.002 0.002 D.sub.B 0.06 3.sup.rd phase 3.sup.rd phase 3.sup.rd phase 0.09 0.03 [HNO.sub.3].sub.eq 4.76 3.sup.rd phase 3.sup.rd phase 3.sup.rd phase 4.71 4.88 (M) Aqueous D.sub.Dy 27 3.sup.rd phase 3.08 3.sup.rd phase 29.8 30.1 phase D.sub.Nd 2.4 3.sup.rd phase 3.90 3.sup.rd phase 2.42 2.55 0.4M D.sub.Pr 1.7 3.sup.rd phase 3.54 3.sup.rd phase 1.67 2.02 D.sub.Fe <0.002 3.sup.rd phase 0.001 3.sup.rd phase 0.001 <0.001 D.sub.B 0.08 3.sup.rd phase 0.13 3.sup.rd phase 0.17 0.04 [HNO.sub.3].sub.eq 0.34 3.sup.rd phase 0.33 3.sup.rd phase 0.31 0.33 (M)

[0196] This table first of all shows that a third phase does not form when TdDDGA is used as an extractant, even in the absence of the phase modifier in the organic phase. On the other hand, the use of TODGA or TEHDGA leads, in the absence of a phase modifier, to the formation of a third phase. A 3.sup.rd phase is also observed with TEHDGA with 5% of n-octanol in the case of the aqueous phase with higher acidity (HNO.sub.3 5.15 M).

[0197] As already illustrated in the literature, the loading capacity of TdDDGA is greater than those of the extractants TODGA and TEHDGA. The use of TdDDGA would therefore have the advantage of avoiding the use of a phase modifier in the working of the method of the invention.

[0198] This table also shows that in the case of TdDDGA, the distribution coefficients of dysprosium, neodymium, praseodymium, iron and boron are comparable between the tests carried out with and without n-octanol in the organic phases, which shows the absence of any impact of this phase modifier on the distribution coefficients of the metal elements.

[0199] The obtained distribution coefficients, for a same metal element, with TODGA and TdDDGA are very close to each other and this, regardless of the acidity of the aqueous phase. The rare earth metals are quantitatively extracted with TdDDGA with excellent selectivity towards the other metal elements. TdDDGA has a slightly greater extracting power than TODGA (notably in the case of praseodymium) while that, in the case of the aqueous phase with lower acidity (HNO.sub.3 0.4 M), TEHDGA exhibits a much lower extracting power than the two other diglycolamides and leads to lower selectivities among rare earth metals (Dy/Nd notably).

[0200] These results confirm the excellent capability which TODGA, TEHDGA and TdDDGA have for separating rare earth metals from the other metal elements and notably from iron. TdDDGA appears to be particularly well indicated for recovering dysprosium, and then praseodymium and neodymium, from an acid aqueous solution from the dissolution of a powder of permanent magnets because of its strong extracting power but also from the fact that it gives the possibility of avoiding the use of a phase modifier.

[0201] 4.2—Tests in Mixers-Decanters:

[0202] 4.2.1—Recovery of Dysprosium from an Aqueous Phase from the Dissolution of a Powder of Permanent Magnets NdFeB in a 1 M Nitric Medium:

[0203] A test is conducted in mixers-decanters for verifying the possibility of quantitatively recovering dysprosium and this, selectively towards the other metal elements, from an aqueous phase A1 stemming from the dissolution of a powder of permanent magnets NdFeB in a 1 M nitric medium, by applying the preferred embodiment of the first method of the invention which is illustrated in FIG. 1.

[0204] The installation used for carrying out this test is represented in FIG. 5.

[0205] As shown by this figure, the installation comprises three batteries of mixers-decanters: [0206] a first battery with 3 stages of mixers-decanters dedicated to the “Dy extraction”; [0207] a second battery with 5 stages of mixers-decanters dedicated to the “Pr+Nd+undesirable elements washing”; and [0208] a third battery with 8 stages of mixers-decanters dedicated to the “Dy stripping”.

[0209] The aqueous phase A1, which was prepared as described under points 4.1.2 and 4.1.3 hereinbefore, comprises 0.69 g/L of dysprosium, 0.020 g/L of praseodymium, 6.62 g/L of neodymium, 1.9 g/L of iron and 1 mol/L of nitric acid, while the organic phase used for the “Dy extraction” comprises 0.2 mol/L of TODGA and 5% (v/v) of n-octanol in TPH.

[0210] The steps “Dy extraction” and “Pr+Nd+undesirable elements washing” are carried out at room temperature while the “Dy stripping” is carried out at a temperature from 45° C. to 50° C.

[0211] After the “Dy stripping”, the organic phase is recycled at the head of the method after controlling its concentration in TODGA and a possible adjustment of this concentration.

[0212] The test is carried out continuously for 20 hours.

[0213] Its proper functioning is followed by analyzing regular samplings in diverse points of the installation.

[0214] At the end of the test, the different aqueous and organic phases are collected and analyzed with the purpose of evaluating the performances of the method.

[0215] The results of these analyses are reported in FIG. 5. They show that, under the conditions of the test, dysprosium is quantitatively recovered (about 99.7%) with an excellent degree of purity (>99.99%) towards the other metal elements present in the magnets (Nd, Pr, Fe, Co, B, Ni, Al, etc.). Dysprosium is moreover concentrated by a factor 6 with the method since its concentration, initially of 0.69 g/L in the aqueous phase A1, is 4.05 g/L in the aqueous phase stemming from the “Dy stripping”.

[0216] Further it should be noted that all the metal elements have a concentration of less than 0.001 g/L in the recycled organic phase.

[0217] 4.2.2—Recovery of Dysprosium, Neodymium and Praseodymium from an Aqueous Phase Stemming from the Dissolution of a Powder of Permanent Magnets NdFeB in a 1 M Nitric Medium:

[0218] A test is also carried out in mixers-decanters for verifying the possibility of quantitatively recovering dysprosium, praseodymium and neodymium, and this, selectively with regard to the other metal elements, from an aqueous phase A1 stemming from the dissolution of a powder of permanent magnets NdFeB in a 1 M nitric medium, by applying the second preferred embodiment of the second method of the invention which is illustrated in FIG. 3.

[0219] The installation used for carrying out this test is illustrated in FIG. 6.

[0220] As shown by this figure, the installation comprises five batteries of mixers-decanters: [0221] a first battery with 4 stages of mixers-decanters dedicated to the “Dy+Pr+Nd co-extraction”; [0222] a second battery with 4 stages of mixers-decanters dedicated to the “Undesirable elements washing”; [0223] a third battery with 5 stages of mixers-decanters dedicated to the “Pr+Nd co-stripping”; [0224] a fourth battery with 3 stages of mixers-decanters dedicated to the “Dy washing”; and [0225] a fifth battery with 8 stages dedicated to the “Dy stripping”.

[0226] The aqueous phase A1, which was prepared as described under points 4.1.2 and 4.1.3 hereinbefore, comprises 1.06 g/L of dysprosium, 6.7 g/L of praseodymium, 20.74 g/L of neodymium, 60.25 g/L of iron and 1 mol/L of nitric acid, while the organic phase used for the “Dy+Pr+Nd co-extraction” comprises 0.2 mol/L of TODGA and 5% (v/v) of n-octanol in TPH.

[0227] All the steps are carried out at room temperature except for the “Dy stripping” which is carried out at a temperature from 45° C. to 50° C.

[0228] After the “Dy stripping”, the organic phase is recycled at the head of the method after a control of its concentration of TODGA and an optional adjustment of this concentration.

[0229] The test is carried out continuously for 20 hours.

[0230] Its proper functioning is followed by analyzing regular samplings in diverse points of the installation.

[0231] At the end of the test, the different aqueous and organic phases are collected and analyzed with the purpose of evaluating the performances of the method.

[0232] The results of these analyses are reported in FIG. 6. They show that, under the conditions of the test, praseodymium and neodymium are recovered at more than 99.9% with an excellent degree of purity (>99.9%) with regard to the other metal elements present in the magnets (Fe, Co, B, etc.). Dysprosium is then recovered to about 99.5% with also an excellent degree of purity (>99.9%) relatively to said other metal elements.

Example 5: Detailed Scheme of an Example of the First Preferred Embodiment of the Second Method of the Invention

[0233] A mathematical model of extraction of rare earth metals and of iron from an aqueous solution stemming from the dissolution of a powder of permanent magnets NdFeB in a nitric medium by the first preferred embodiment of the second method of the invention illustrated in FIG. 2, by using an organic phase comprising TODGA as an extractant, and n-octanol as a phase modifier, was developed.

[0234] The model takes into account the distribution of the species of interest, here the rare earth metals, between an aqueous phase and an organic phase. In order to optimize the reliability of the model, the latter is based on a chemical description of the phenomena. On the one hand, the activity coefficients in the aqueous phase are taken into account so that the range of validity of the model is as wide as possible in terms of concentration of nitric acid in particular, and, on the other hand, the constants of the model are determined by numerical optimization of the experimental data.

[0235] In the present case, the model was developed from experimental data obtained under points 4.1.1, 4.1.2 and 4.1.3 of Example 4 hereinbefore with an organic phase comprising 0.2 mol/L of TODGA and 5% (v/v) of n-octanol in n-dodecane.

[0236] It was necessary to model the behavior of nitric acid since the extraction of this acid compete with that of the rare earth metals, and then to propose, from the optimization of the experimental data, complexes of rare earth metals. The complexes of the {TR(NO.sub.3).sub.3, TODGA.sub.n} type, wherein TR designates Dy, Pr or Nd and n varies from 1 to 3, were retained.

[0237] With each transferred species between an aqueous phase and an organic phase, a standard mathematical chemical equation was associated.

[0238] For nitric acid and the rare earth metals (TR), the mathematical equations are the following:

[00003]$K_n=\frac{\stackrel{\_}{\left[\left\{{\mathrm{HNO}}_3,{\mathrm{TODGA}}_n\right\}\right]}}{{\left({\gamma }_{{\mathrm{HNO}}_3}.Math.\left[{\mathrm{HNO}}_3\right]\right)}^m.Math.\stackrel{\_}{\left[\mathrm{TODGA}\right]}}$

where n varies from 1 to 3;

[00004]$K_n^{\prime }=\frac{\stackrel{\_}{\left[\left\{{\mathrm{TR}\left({\mathrm{NO}}_3\right)}_3,{\mathrm{TODGA}}_n\right\}\right]}}{{\gamma }_{{\mathrm{TR}\left({\mathrm{NO}}_3\right)}_3}.Math.\left[{\mathrm{TR}\left({\mathrm{NO}}_3\right)}_3\right].Math.{\stackrel{\_}{\left[\mathrm{TODGA}\right]}}^n}$

where TR=Dy, Pr or Nd and m varies from 1 to 3.

[0239] In these equations, γ.sub.TR(NO.sub.3).sub.3 and γ.sub.HNO.sub.3 designate the activity coefficients of the rare earth metal TR and of nitric acid respectively and the parameters K.sub.n and K′.sub.n are constants.

[0240] These constants were optimized in order to reproduce at best the experimental distribution coefficients. The comparison of the experimental data and of the data calculated with the model is shown in table IX hereafter.

TABLE-US-00009 TABLE IX [H.sup.+].sub.eq. D.sub.Fe D.sub.Fe D.sub.Dy D.sub.Dy D.sub.Nd D.sub.Nd D.sub.Pr D.sub.Pr (M) (exp.) (calc.) (exp.) (calc.) (exp.) (calc.) (exp.) (calc.) Synthetic nitric 0.109 0.0041 0.0038 119 104 7.6 4.5 3.4 4.1 aqueous phases 0.173 0.0032 0.0037 121 92 4.9 3.5 3.1 3.1 0.418 0.0033 0.0033 115 102 4.6 3.3 2.6 2.8 0.761 0.0031 0.0030 114 116 4.1 3.2 2.1 2.5 1.49 0.0024 0.0027 105 132 3.6 3.2 1.4 2.0 2.23 0.0019 0.0029 104 108 3.4 3.3 1.2 1.7 Aqueous phases 0.34 0.0014 0.0014 37 43 2.3 1.8 1.9 1.6 stemming from 2.57 0.0018 0.0030 197 139 13.4 3.3 6.8 2.1 the dissolution 4.76 0.0036 0.0037 34 32 1.5 1.2 0.6 0.5 of a powder of 5.04 0.0037 0.0034 32 32 2.4 1.2 0.9 0.5 magnets in nitric medium

[0241] The thereby developed mathematical model gave the possibility of resulting in the detailed scheme which is illustrated in FIGS. 7A and 7B, in which FIG. 7A corresponds to the first cycle dedicated to the selective recovery of dysprosium while FIG. 7B corresponds to the second cycle dedicated to the selective recovery of praseodymium and neodymium. In these figures, the step common to both purification cycles of the organic phase is however not shown.

[0242] According to this scheme, an extractor with 7 stages would be necessary, in the first cycle, for extracting more than 99.99% of dysprosium from the aqueous phase A1 (“Dy extraction”) while an extractor also with 7 stages would allow separating dysprosium from the other rare earth metals and from the other metal elements (“Pr+Nd+undesirable elements washing”) and attaining a purity of dysprosium of the order of 99.99%. An extractor with 5 stages would be necessary for quantitatively stripping dysprosium from the organic phase stemming from the “Pr+Nd+undesirable elements washing” (“Dy stripping”) but, considering the results shown in tables IV and VII hereinbefore, this number of stages may be reduced if the extractor is heated to a temperature ranging from 40 to 55° C.

[0243] In the second cycle, an extractor with 7 stages would be necessary for co-extracting more than 99.99% of praseodymium and of neodymium present in the aqueous phase stemming from the “Dy extraction” (“Pr+Nd co-extraction”) while an extractor with 3 stages would be sufficient for separating the Nd+Pr mixture from the other metal elements (“Undesirable elements washing”) and for attaining a purity of the didymium Pr+Nd greater than 99.99%. The stripping of neodymium and of praseodymium from the organic phase having been found to be easier than that of dysprosium, an extractor with 3 stages should be sufficient for quantitatively recovering the didymium Pr+Nd purified in an aqueous phase (“Pr+Nd co-stripping”).

[0244] The invention is not limited to the embodiments described in the examples hereinbefore. In particular, it is quite possible to adapt the scheme shown in FIGS. 7A and 7B, which was defined for treating an aqueous phase A1 comprising 5 mol/L of nitric acid, to the treatment of aqueous phases comprising another strong acid and/or having quite another acidity and notably, a much weaker acidity such as for example 0.4 mol/L of nitric acid.

CITED REFERENCES

[0245] [1] H. Narita and M. Tanaka, Solvent Extraction Research and Development, Japan, 2013, 20, 115-121 [0246] [2] International application WO 2014/064597