USE OF SYNERGISTIC MIXTURE OF EXTRACTANTS FOR EXTRACTING RARE EARTH ELEMENTS FROM AN AQUEOUS MEDIUM COMPRISING PHOSPHORIC ACID

Abstract

The use of a synergistic mixture of extractants for extracting at least one rare earth element from an aqueous medium comprising phosphoric acid. The mixture comprises: —a first extractant of formula (I):

##STR00001##

wherein R.sub.1 and R.sub.2, which are identical or different, represent a linear or branched, saturated or unsaturated hydrocarbon group, comprising from 6 to 12 carbon atoms, or a phenyl group optionally substituted by a linear or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 10 carbon atoms; and —a second extractant of formula (II):

##STR00002##

in which R.sub.3 represents a linear or branched alkyl group, comprising from 6 to 12 carbon atoms.

Use of the synergistic mixture in the treatment of phosphate minerals with a view to recovering the rare earth elements contained in the minerals.

Claims

1. A method for extracting at least one rare earth from an aqueous medium comprising phosphoric acid, said method comprising contacting the aqueous medium with a mixture of extractants comprising: a first extractant of formula (I): ##STR00005## wherein R.sup.1 and R.sup.2, the same or different, represent a saturated or unsaturated, linear or branched hydrocarbon group, comprising from 6 to 12 carbon atoms, or a phenyl group, optionally substituted by a saturated or unsaturated, linear or branched hydrocarbon group, comprising from 1 to 10 carbon atoms; and a second extractant or formula (II): ##STR00006## wherein R.sup.3 represents a linear or branched alkyl group, comprising from 6 to 12 carbon atoms; and then separating the aqueous medium from the mixture of extractants.

2. The method of claim 1, wherein R.sup.1 and R.sup.2 represent a linear or branched alkyl group, comprising from 6 to 12 carbon atoms, or a phenyl group substituted by a linear or branched alkyl group comprising from 1 to 10 carbon atoms.

3. The method of claim 2, wherein R.sup.1 and R.sup.2 represent a linear or branched alkyl group comprising from 8 to 10 carbon atoms, or a phenyl group substituted by a linear or branched alkyl group, comprising from 6 to 10 carbon atoms.

4. The method of claim 1, wherein R.sup.1 and R.sup.2 are identical to each other.

5. The method of claim 1, wherein the first extractant is di(2-ethylhexyl)phosphoric acid.

6. The method of claim 1, wherein R.sup.3 represents a linear or branched alkyl group comprising from 8 to 10 carbon atoms.

7. The method of claim 6, wherein the second extractant is N,N,N′,N′-tetraoctyl-diglycolamide.

8. The method of claim 1, wherein the mixture of extractants comprises di(2-ethylhexyl)phosphoric acid and N,N,N′,N′-tetraoctyl-diglycol amide.

9. The method of claim 1, wherein the mixture of extractants is used in solution in an organic diluent.

10. The method of claim 1, wherein contacting the aqueous medium with the mixture of extractants comprises contacting the aqueous medium with an organic solution that is not miscible with water, comprising the mixture of extractants in an organic diluent, and separating the aqueous medium from the mixture of extractants comprises separating the aqueous medium from the organic solution, whereby an organic solution comprising at least the rare earth is obtained.

11. The method of claim 10, wherein the organic solution comprises from 0.2 mol/L to 2 mol/L of the first extractant and from 0.05 mol/L to 2 mol/L of the second extractant.

12. The method of claim 10, further comprising contacting the organic solution comprising at least the rare earth with an acid or basic aqueous solution, and then separating the organic solution from the aqueous solution, whereby an aqueous solution comprising at least the rare earth is obtained.

13. The method of claim 1, wherein the aqueous medium comprises from 0.5 mol/L to 10 mol/L of phosphoric acid.

14. The method of claim 1, wherein the aqueous medium is an aqueous solution of phosphoric acid resulting from the lixiviation of a phosphate ore by sulfuric acid.

15. The method of claim 1, wherein the rare earth is yttrium, lanthanum, neodymium, dysprosium, or ytterbium.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0067] FIGS. 1A and 1B illustrate the results of extraction tests that were performed on aqueous phases of phosphoric acid comprising five REs—namely yttrium, lanthanum, neodymium, dysprosium and ytterbium—as well as iron, using organic phases comprising 0.5 mol/L of D2EHPA and a variable molar concentration of TODGA; FIG. 1A shows the change in the distribution coefficients of the REs and of iron, denoted D.sub.M and set out on a logarithmic scale, as a function of the molar concentration of TODGA, denoted [TODGA], in the organic phases, while FIG. 1B shows the change in the separation factors between the REs and the iron, denoted FS.sub.TR/Fe and also set out on a logarithmic scale, as a function of the concentration of TODGA; by way of comparison, the D.sub.M and FS.sub.TR/Fe values obtained under the same operating conditions with an organic phase comprising only D2EHPA as extractant are also indicated in these figures.

[0068] FIG. 2 illustrates the results of extraction tests that were performed on aqueous phases of phosphoric acid comprising the aforementioned five REs and iron, using organic phases comprising a variable molar concentration of D2EHPA and 0.5 mol/L of TODGA; more specifically, this figure shows the change in the distribution coefficients of the REs and of iron, denoted D.sub.M and set out on an arithmetic scale, as a function of the molar concentration of D2EHPA, denoted [D2EHPA], in the organic phases.

[0069] FIG. 3 illustrates the stripping yields, denoted R.sub.M and expressed as %, as obtained during stripping tests that were performed on an organic phase comprising the aforementioned five REs, iron, 0.5 mol/L of D2EHPA and 0.25 mol/L of TODGA, using various acidic aqueous phases.

[0070] FIGS. 4A and 4B illustrate, by way of comparison, the results of extraction tests that were performed on aqueous phases of phosphoric acid comprising the aforementioned five REs and iron, using organic phases comprising only D2EHPA as extractant, at variable molar concentrations; FIG. 4A shows the change in the distribution coefficients of the REs and of iron, denoted D.sub.M and set out on a logarithmic scale, as a function of the molar concentration of D2EHPA, denoted [D2EHPA], in the organic phases, while FIG. 4B shows the separation factors between the RE and the iron, denoted FS.sub.TR/Fe, obtained with an organic phase comprising 1 mol/L of D2EHPA.

[0071] FIG. 5 illustrates, by way of comparison, the results of extraction tests that were performed on aqueous phases of phosphoric acid comprising the aforementioned five REs and iron, using organic phases comprising 0.5 mol/L of D2EHPA and a variable molar concentration of TBP; more specifically, this figure shows the change in the distribution coefficients of the REs and of iron, denoted D.sub.M and set out on an arithmetic scale as a function of the molar concentration of TBP, denoted [TBP], in the organic phases; the D.sub.M values obtained under the same operating conditions with an organic phase comprising only D2EHPA as extractant are also indicated in this figure.

[0072] FIG. 6 illustrates, by way of comparison, the results of extraction tests that were performed on aqueous phases of phosphoric acid comprising the aforementioned five REs and iron, using organic phases comprising 0.5 mol/L of D2EHPA and a variable molar concentration of TOPO; more specifically, this figure shows the change in the distribution coefficients of the REs and of iron, denoted D.sub.M and set out on an arithmetic scale as a function of the molar concentration of TOPO, denoted [TOPO], in the organic phases; the D.sub.Ms obtained under the same operating conditions with an organic phase comprising only D2EHPA as extractant are also indicated in this figure.

[0073] In FIGS. 3 and 4B, the error bars correspond to a relative uncertainty of 10% that encompasses the various analytical and experimental uncertainties.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0074] The extraction tests that are reported in the following examples were all performed using, as aqueous phases, aliquots of a synthetic aqueous solution, representative of aqueous solutions of phosphoric acid actually obtained during the production of H.sub.3PO.sub.4 by lixiviation of natural phosphates with H.sub.2SO.sub.4.

[0075] This synthetic aqueous solution comprises, apart from phosphoric acid, three light REs, namely yttrium, lanthanum and neodymium, two heavy REs, namely dysprosium and ytterbium, and a major and detrimental impurity, namely iron. It was prepared by dissolving oxides of the corresponding metals at the +3 oxidation state in a solution of concentrated H.sub.3PO.sub.4 and then adjusting the concentration of H.sub.3PO.sub.4 of this solution to 4.6 mol/L.

[0076] Its composition by mass of metallic elements is presented in table I below.

TABLE-US-00001 TABLE I Elements Y La Nd Dy Yb Fe [C] in mg/L 216 195 173 204 267 1525

[0077] The organic phases were prepared using Isane™ IP185 as organic diluent and prebalanced by contact with an aqueous solution comprising 4.6 mol/L of H.sub.3PO.sub.4.

[0078] Moreover, the extraction and stripping tests that are reported in the following examples were all performed in microtubes with volumes of less than 1.5 mL, at a temperature of 45° C., using a ratio by volume between the organic phases and the aqueous phases (O/A) equal to 1 and subjecting these phases to a single contact of 20 minutes, under stirring by means of a Vibrax™ stirrer. After centrifugation, the organic and aqueous phases were separated by settling.

[0079] The distribution coefficients, the separation factors and the stripping yields were determined in accordance with the conventions of the field of liquid-liquid extractions, namely that: [0080] the distribution coefficient of a metallic element M, denoted D.sub.M, between two phases, respectively organic and aqueous, is equal to:

[00001]$D_M=\frac{{\left[M\right]}_{org,f}}{{\left[M\right]}_{aq,f}}=\frac{{\left[M\right]}_{aq,i}-{\left[M\right]}_{aq,f}}{{\left[M\right]}_{aq,f}}$

with:

[0081] [M].sub.org,f=concentration of M in the organic phase after extraction (or stripping),

[0082] [M].sub.aq,f=concentration of M in the aqueous phase after extraction (or stripping), and

[0083] [M].sub.aq,i=concentration of M in the aqueous phase before extraction (or stripping); [0084] the separation factor between two metallic elements M1 and M2, denoted FS.sub.M1/M2, is equal to:

[00002]$F{S}_{M1/M2}=\frac{D_{M1}}{D_{M2}}$

with:

[0085] D.sub.M1=distribution coefficient of the metallic element M1, and

[0086] D.sub.M2=distribution coefficient of the metallic element M2; [0087] the stripping yield of a metallic element M, denoted R.sub.M, of an organic phase is equal to:

[00003]$R_M=\frac{{\left[M\right]}_{aq,f}}{{\left[M\right]}_{org,i}}\times 100$

with:

[0088] [M].sub.aq,f=concentration of M in the aqueous phase after stripping, and

[0089] [M].sub.org,i=concentration of M in the organic phase before stripping.

[0090] The multi-element analyses of the aqueous or organic phases comprising the REs (initial synthetic solution, aqueous and organic phases after extraction, aqueous phases after stripping, etc.) were performed by atomic emission spectrometry the source of which is an argon plasma generated by inductive coupling, after dilution to bring the metallic elements to measurable concentrations.

Example 1: Extraction of the REs by Mixtures of Extractants D2EHPA/TODGA in Accordance with the Invention

1.1—Extraction Tests

[0091] *First Series of Tests:

[0092] Extraction tests were performed using, as organic phases, solutions comprising 0.5 mol/L of D2EHPA and of TODGA at a concentration of 0.1 mol/L, 0.25 mol/L, 0.5 mol/L or 1 mol/L and, by way of comparison, a solution comprising 0.5 mol/L of D2EHPA but free from TODGA.

[0093] FIG. 1A illustrates the change in the distribution coefficients of the REs and of iron, D.sub.M, obtained at the end of these tests as a function of the concentration of TODGA in the organic phases.

[0094] As shown in this figure, adding TODGA to D2EHPA results in a high increase in the distribution coefficients of the REs and therefore in their extraction of an aqueous phase of phosphoric acid.

[0095] The D2EHPA/TODGA mixtures have a particularly high affinity for dysprosium, ytterbium and yttrium.

[0096] The affinity thereof is substantially lower for lanthanum and neodymium but remains nevertheless high (D.sub.La.sup.max=2.3) if it is compared with that which D2EHPA alone has (D.sub.La<0.01).

[0097] Moreover, adding TODGA to D2EHPA results in a drop in the distribution coefficient of iron by a factor of 3 compared with that obtained with D2EHPA alone. This distribution coefficient is very small (D.sub.Fe=0.005) for concentrations of TODGA ranging up to 0.25 mol/L, which clearly shows the excellent selectivity for REs towards iron that the D2EHPA/TODGA mixtures have.

[0098] The change in the separation factors between the REs and iron, FS.sub.TR/Fe, as a function of the concentration of TODGA in the organic phases is illustrated in FIG. 1B.

[0099] This figure shows that the maximum values of FS.sub.TR/Fe are very significant for heavy REs such as ytterbium (FS.sub.Yb/Fe>12,500) and remain very satisfactory for light REs such as lanthanum (FS.sub.La/Fe=460).

[0100] By way of comparison, the best separation factor between RE and iron that is obtained for D2EHPA alone, at a concentration of 0.5 mol/L in organic phase, is the separation factor between ytterbium and iron, which is at a minimum 20 times less (FS.sub.Yb/Fe≈600) than that obtained with D2EHPA/TODGA mixtures.

[0101] *Second Series of Tests:

[0102] Tests were performed using, as organic phases, solutions comprising 0.5 mol/L of TODGA and D2EHPA at a concentration of 0.1 mol/L, 0.5 mol/L, 1 mol/L or 2 mol/L.

[0103] FIG. 2 illustrates the change in the distribution coefficients of the REs and iron, D.sub.M, obtained at the end of these tests as a function of the concentration of D2EHPA in the organic phases.

[0104] As shown in this figure, adding D2EHPA to TODGA results in a high increase in the distribution coefficients of the REs and therefore in the extraction thereof in an aqueous phase of phosphoric acid.

[0105] A concentration of D2EHPA greater than 0.1 mol/L in the mixture is necessary for obtaining good extraction performance for all the REs. By way of example, the distribution coefficient of lanthanum is very small when the concentration of D2EHPA in the mixture is 0.1 mol/L (D.sub.La=0.06) but increases very significantly when the concentration of D2EHPA in the mixture is 0.5 mol/L (D.sub.La=2.2). By way of comparison, the affinity for lanthanum of D2EHPA alone at a concentration of 0.5 mol/L is very small (D.sub.La=0.01).

[0106] These results corroborate those of reference [5], namely that TODGA alone does not make it possible to extract REs from an aqueous phase of phosphoric acid.

[0107] On the other hand, they show that using a concentration of D2EHPA at least equal to that of TODGA and preferably twice as great as that of TODGA makes it possible to obtain a good extraction of all the REs.

1.2—Stripping Tests

[0108] Stripping tests were performed using: [0109] as organic phases: aliquots of the organic phase resulting from the extraction test that was performed at point 1.1 above with a mixture comprising 0.5 mol/L of D2EHPA and 0.25 mol/L of TODGA; and as aqueous phases: aqueous solutions comprising: [0110] either 0.5 mol/L, 1 mol/L or 6 mol/L of H.sub.2SO.sub.4, [0111] or 1 mol/L of H.sub.2SO.sub.4 and 0.125 mol/L of Na.sub.2SO.sub.4, [0112] or 5 mol/L or 10 mol/L of H.sub.3PO.sub.4.

[0113] FIG. 3 illustrates the stripping yields, R.sub.M, obtained at the end of these tests.

[0114] As shown by this figure, aqueous solutions of highly concentrated H.sub.3PO.sub.4 (5 mol/L or 10 mol/L) make it possible also to strip lanthanum and neodymium as well as iron but do not make it possible to strip yttrium, dysprosium and ytterbium, which remain in organic phase.

[0115] Aqueous solutions of dilute H.sub.2SO.sub.4 (0.5 mol/L and 1 mol/L) make it possible to strip almost quantitatively lanthanum and neodymium, partially yttrium, dysprosium and ytterbium. A 1 mol/L solution of H.sub.2SO.sub.4 allows better stripping selectivity of the REs towards iron than a 0.5 mol/L solution of H.sub.2SO.sub.4. Adding Na.sub.2SO.sub.4 to the extent of 0.125 mol/L further improves this selectivity but to the detriment of a drop in the stripping yields of yttrium, neodymium and dysprosium.

[0116] FIG. 3 also shows that iron is partially but selectively stripped by an aqueous solution comprising 6 mol/L of H2504.

[0117] It is therefore possible to envisage the implementation of a scheme in which the organic phase resulting from the extraction of REs would be subjected to a step of washing by an aqueous solution comprising for example 6 mol/L of H.sub.2SO.sub.4 in order to selectively eliminate the iron present in this organic phase, before being subjected to a step of stripping of the REs, for example by means of a solution of dilute H.sub.2SO.sub.4, optionally with Na.sub.2SO.sub.4 added.

[0118] The results obtained with the aqueous solution comprising both sulfuric acid and sodium sulfate would also make it possible to envisage a scheme wherein the REs present in aqueous solution after stripping would be recovered by precipitation, for example in the form of double sulfates of RE and sodium, carbonates, oxalates, etc., this type of precipitation being known in the literature.

Example 2: Extraction of REs by D2EHPA Alone (Comparative Example)

[0119] By way of comparison, extraction tests were performed using, as organic phases, solutions comprising 0.1 mol/L, 0.5 mol/L, 1 mol/L, 1.5 mol/L or 2 mol/L of D2EHPA.

[0120] FIG. 4A illustrates the change in the distribution coefficients of the REs and of iron, D.sub.M, obtained at the end of these tests as a function of the concentration of D2EHPA in the organic phases.

[0121] This figure shows clearly that the extraction of the REs by D2EHPA decreases with the increase in the ionic radius of the REs. Thus D2EHPA has good affinity for the REs with a small ionic radius such as yttrium, dysprosium and ytterbium but makes it impossible or almost impossible to extract REs with a higher ionic radius such as lanthanum and neodymium (D.sub.M<0.1 whatever the concentration of D2EHPA in organic phase).

[0122] Moreover, the separation factors between the REs and iron, FS.sub.TR/Fe, obtained with a concentration of D2EHPA of 1 mol/L in organic phase are set out in FIG. 4B. These separation factors are satisfactory for ytterbium (FS.sub.Yb/Fe=500). On the other hand, they are not at all satisfactory for the other REs.

[0123] It should be noted that no significant variation in these separation factors is observed as a function of the concentration of D2EHPA in organic phase.

Example 3: Extraction of the REs by D2EHPA/TBP and D2EHPA/TOPO Mixtures

Comparative Example

[0124] Since TODGA is a solvating extractant, extraction tests were performed in order to check whether mixtures comprising D2EHPA and a solvating extractant other than TODGA would be liable to have the same synergic effect as that observed when D2EHPA is used in a mixture with TODGA.

[0125] These extraction tests were performed using, as organic phases, solutions comprising 0.5 mol/L of D2EHPA and: [0126] either tri-n-butyl phosphate (or TBP) at a concentration of 0.1 mol/L, 0.25 mol/L or 0.5 mol/L; [0127] or trioctylphosphine oxide (or TOPO) at a concentration of 0.1 mol/L, 0.25 mol/L or 0.5 mol/L.

[0128] The results obtained at the end of these tests are illustrated in terms of distribution coefficients, D.sub.M, in FIG. 5 for the D2EHPA/TBP mixtures and in FIG. 6 for the D2EHPA/TOPO mixtures.

[0129] As shown in FIG. 5, adding TBP to D2EHPA results in an appreciable reduction in the distribution coefficients of the REs and therefore in their extraction of an aqueous phase of phosphoric acid, except in the case of lanthanum and neodymium since these are not already extracted by D2EHPA alone.

[0130] This reduction in D.sub.M, which is all the greater as the concentration of TBP in organic phase increases, thus shows the existence of an antagonistic effect of D2EHPA/TBP mixtures on the extraction of REs from an aqueous solution of phosphoric acid.

[0131] In a similar manner, FIG. 6 shows a constant reduction in the distribution coefficients of the REs as a function of the concentration of TOPO in organic phase, there also showing the existence of an antagonistic effect of D2EHPA/TOPO mixtures on the extraction of REs from an aqueous solution of phosphoric acid.

[0132] These results corroborate those given in the aforementioned references [2] and [3] for D2EHPA/TBP and D2EHPA/TOPO mixtures, and confirm that the use of a mixture comprising an organophosphoric acid such as D2EHPA and a solvating extractant in principle has no interest if it is wished to extract REs from an aqueous solution comprising phosphoric acid.

REFERENCES CITED

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