NO881237L - PROCEDURE FOR EXTRACTION OF RARE EARTH ELEMENTS - Google Patents

Abstract

Rare-earth elements present in aqueous solutions are separated using a solvent by a process which consists in performing a liquid- liquid extraction between an aqueous phase containing the rare-earth nitrates and an organic phase, characterised in that a mixture of organophosphorus compounds is used as extraction agent. <IMAGE>

Description

The present invention relates to a method for the separation, by means of a solvent, of rare earth elements contained in aqueous solutions.

In particular, the present invention relates to a method for separation by means of liquid-liquid extraction of the rare earth elements in aqueous nitrate solutions of these elements.

The term "rare earth elements" used in accordance with the present invention includes the rare earth elements called lanthanides and having atomic numbers ranging from 57 to 71 and yttrium with atomic number equal to 39-

According to the present invention, by the "cerium-earth elements" is meant the lighter elements of the rare-earth elements beginning with lanthanum and equal to neodymium corresponding to the atomic number and by the "yttrium-earth elements" is meant the heavier elements of the rare-earth elements, being one begins with samarium and ends with lutetium and extensive yttrium.

It is well known that the rare earth elements are difficult to separate, which has to do with the fact that the differences in terms of properties of the rare earth elements in relation to the adjacent elements are very small.

Methods have thus been developed for the separation of elements that are very close, which have been of particular importance in the separation of rare earth elements and are known as liquid-liquid extraction. The method is based on the selective extraction of one of the rare earth elements by starting from the solution in which the elements are contained and extraction with a solvent that is not miscible with the solution.

As examples of solvents for extraction, certain organophosphorus compounds are already used in solution in an organic diluent to carry out the separation of rare earth elements. It is particularly known to use the tri-n-octylphosphine oxide called TOPO. Nevertheless, TOPO is in the form of a solid substance at room temperature and has a reduced solubility in common diluents of the kerosene type, and its use is discouraged in certain areas of application, particularly in the separation of rare earth elements in a nitrate solution and in concentrated solution.

Furthermore, certain separations, e.g. it is difficult to separate yttrium from other yttrium-earth elements due to their great similarity in terms of properties and likewise this has to do with, in industrial conditions that utilize rare earth ores, that yttrium is in the presence of relatively large amounts of yttrium-earth elements, amounts that are clearly consistent with the composition of the rare earth ores .

It has been found that the extraction agents defined in the following avoid the above-mentioned disadvantages and have a very good selectivity towards all the rare earth elements.

The present invention relates to a method for the separation of rare earth elements that are contained in an aqueous solution comprising at least two of the rare earth elements, by means of liquid-liquid extraction between an aqueous phase containing the nitrates of the rare earth elements to be separated and an organic phase containing an extraction agent consisting of an organophosphorus compound, characterized in that the above extraction agent consists of at least two organophosphorus compounds with the formula (I) and (I<*>), respectively:

in which:

- in formula (I):

represent R^, R^, R^» which are the same or different and optionally substituted, a hydrogen atom or a straight-chain or branched hydrocarbon radical, which is saturated or unsaturated or a cyclic hydrocarbon radical, which is saturated or unsaturated;

the sum of the number of carbon atoms for the radicals R^, Rp,

R^ is at least equal to 12 carbon atoms;

in formula (I'):

R^, R^, Rg, which are the same or different and optionally substituted, represent a hydrogen atom or a straight-chain or branched hydrocarbon radical, which is saturated or unsaturated or a cyclic hydrocarbon radical, which is saturated or unsaturated;

the sum of the number of carbon atoms for the radicals R^, R,-,

Rg is at least equal to 12 carbon atoms.

It is specified that the radicals R^, R^, R^> R^, R^,

Rg can carry substituent groups such as e.g. hydroxy or nitro groups; the lower alkoxy radicals with a small number of carbon atoms, e.g. 1 to 4 atoms; the cyano groups, etc.

The radicals R^, R2, R^>R^»R^»Rg are a number

carbon atoms which generally vary between 1 and 18 carbon atoms and preferably between 2 and 8 carbon atoms.

They are chosen in such a way that the number of carbon atoms in the radicals R.j, R.sub.2 and R.sub.2, or R.sub.1, R.sub.- and R.sub.g is at least equal to 12 carbon atoms and preferably more than 18.

As examples of the radicals R^, R^, R^>R^iR5Rg

mention may be made of the radicals methyl, ethyl, n-propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 1-methyl-butyl, isopentyl, tert-pentyl, neo-pentyl; following common alkyl radicals: n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexdecyl, n- heptadecyl, n-octadecyl; as well as the corresponding branched alkyl radicals and cycloalkyl radicals, especially the cyclohexyl radical.

Particularly suitable for use in the present invention is a mixture of two extractants, in which one of the extractants has formula (I) in which R 1 , R 2 , R 2 are identical straight chain alkyl radicals, i.e. the radicals R 1 , R 2 , R 2 are straight chain alkyl radicals with the same

number of carbon atoms, and the second of the agents has formula (I') in which R^, R^, Rg are identical straight-chain alkyl radical-

is, i.e. that the radicals R^, R^, Rg are straight-chain alkyl radicals with the same number of carbon atoms, but these have a different number of carbon atoms in relation to the radicals R i , R2, R^-

The extractant of formula (I) in which R^, R2, R^ are identical straight-chain alkyl radicals can be together with an extractant of formula (I') in which:

R^, R^, Rg are identical branched alkyl radicals, i.e.

that the radicals R^, R^, Rg are branched alkyl radicals with the same number of carbon atoms,

R4>R5>Rg are different branched alkyl radicals,

i.e. that at least one of the radicals is different from the other two,

Rjj, Rj-, Rg are straight chain branched alkyl radicals,

i.e. that at least one straight-chain alkyl radical and at least one branched alkyl radical are simultaneously present.

The ratio between the two organophosphorus compounds of formula (I) and (I<1>) is determined so that one has a mixture of phosphine oxides which are liquid at room temperature. The resulting mixture is miscible with commonly used diluents in all proportions.

As examples of preferred mixtures of phosphine oxides according to the present invention, mention may be made of a mixture of tri-(n-hexylphosphine)oxide and tri(n-octylphosphine)oxide, and in particular Cyanex 923 from Societe American Cyanamid.

'in

Another popular extraction agent is Cyanex 925 which is a mixture consisting of 85% bis-2,4,4-trimethylpentyl-n-octyl-phosphine oxide and 15% tri-2,4,4-trimethylpentyl-phosphine oxide.

It has been found that the above-mentioned extractants show very good selectivity towards all the rare earth elements and especially yttrium and the yttrium-earth elements especially gadolinium to lutetium, in contrast to the classic neutral organophosphorus extractants such as e.g. tri-butyl phosphate.

A particularly adapted application of the method according to the invention is a method for separating yttrium

from other yttrium-earth elements: yttrium can be represented by from 5 to 80% by weight of the total yttrium-earth elements: the % part is expressed in parts by weight of yttrium oxide in relation to the parts by weight of the oxides of the yttrium rare-earth elements, inclusive yttrium.

Another thing of interest for the above-mentioned extractants is that they allow one to operate in the presence of highly concentrated aqueous phases of nitrates of rare earth elements (more than 300 g/l) and one obtains highly concentrated organic phases, from more than 50 g to 100 g/l and more of rare earth elements expressed as oxides.

According to the method of the present invention, the aqueous phase which is brought into contact with the extractant may optionally consist of an aqueous solution which has been prepared, by re-dissolving with the aid of nitric acid, the hydroxides obtained after digestion of the ore containing the rare earth elements with sodium hydroxide, such as monazite, bastnazite and xenotime. You can also use all other salt solutions of rare earth elements after the anion present has been replaced with a nitrate anion.

The method according to the invention can be used with the solutions as they are or rather after they have been subjected to a prior concentration.

In general, liquid-liquid extraction is applied to the aqueous nitrate solutions of rare earth elements with a concentration, expressed as oxides of the rare earth elements, between 20 g/l and 500 g/l: the given range is not of a critical nature. In a preferred embodiment, the solution has a concentration between 100 g/l and 500 g/l.

The solution has an acid content that generally varies between • 0.01 N and 3.0 N.

The organic phase according to the method of the present invention optionally contains, without the extractant, an organic diluent. As sensitive diluents used, one can use those which are usually used to carry out liquid-liquid extractions. Among these, mention can be made of the aliphatic or cycloaliphatic hydrocarbons such as e.g. hexane, heptane, dodecane, cyclohexane, isoparaffin and petroleum fractions of the cerosene or isoparaffin type: the aromatic hydrocarbons, such as e.g. benzene, toluene, ethylbenzene, cylene, the petroleum fractions consisting of a mixture of alkylbenzenes, especially the Solvesso type mixtures (from la Société Exxon) and the halogenated hydrocarbons such as chloroform, carbon tetrachloride, 1,2-dichloroethylene, monochlorobenzene.

A mixture of these diluents can also be used.

In a preferred embodiment, an aliphatic hydrocarbon is selected.

The extraction ability of the organic solution against the rare earth elements increases when the concentration of the extract ion agent in the organic phase increases; however, the separation factors of the elements among themselves do not change appreciably with the concentration of the extractant. Thus, the concentration of the extractant in the organic phase is not a critical factor according to the invention and can vary within wide limits. It can vary from 5% by volume in the organic phase when the extractant is in solution in a diluent to approximately 100% when the extractant is used in pure form.

Preferably, the concentration is chosen between 50 and 100% by volume of the organic phase.

The organic phase in accordance with the invention can also contain various modifiers, one of the most important purposes of which is to improve the hydrodynamic properties of the system without changing the extractive properties of the organophosphorus compounds. Among the compounds that fit well, one can mention the compounds with an alcohol function and particularly heavy alcohols whose number of carbon atoms is between 4 and 15 and the heavy phenols, as well as various other compounds such as certain phosphorous esters such as tributyl phosphate. A ratio comprising between 3 and 20% by volume in the organic phase is generally preferred. In the choice of extraction conditions, the concentration of nitrate ions is important as it facilitates the extraction of the rare earth element(s) to be extracted in the organic phase. The concentration of nitrate ions can vary between 1 and 10 mol/litre and is preferably chosen between 2 and 9 mol/litre.

If necessary, the concentration of nitrate ions can be increased by adding those used, e.g. by means of an aqueous solution of nitric acid, an aqueous solution of a salt in the form of nitrate, especially ammonium nitrate. During the extraction phase, the organic phase is brought into contact with the aqueous phase at a temperature that is not of a critical nature, it can generally be chosen between 10 and 80°C and is most often between 30 and 60°C.

The ratio between supply quantity of organic phase and aqueous phase is in accordance with the concentration of the extraction agent and it is chosen in accordance with the concentration of the rare earth elements to be extracted in the aqueous phase and in accordance with commonly used apparatus in liquid-liquid extraction.

When certain rare earth elements have been extracted into the organic phase in small quantities, but they should have remained in the aqueous phase, it is sometimes necessary to carry out a washing step after this extraction.

In the washing step, the organic phase is washed with water, preferably deionized water or by means of an aqueous solution containing nitrate ions such as a dilute aqueous nitric acid solution or an aqueous solution of nitrates of rare earth elements of the same type as the one to be extracted and which can be contained in the part of the aqueous phase that is obtained in the subsequent recovery procedure of the extraction solvent.

When the washing is carried out using a nitric acid solution, the concentration is chosen to be less than 1N and, preferably, between 10"^ and 10~^N. When using an aqueous solution of nitrates of rare earth elements, the concentration is expressed as oxides of the earth elements between 5 and 500 g/l, preferably between 100 and 500 g/l.

After extraction and washing followed by a separation of the aqueous phase and the organic phase, a recovery step of the extraction solvent is carried out.

The rare earth element(s) extracted over the organic phase is separated by bringing the latter into contact with water, preferably deionised water or with an acidic aqueous solution which, e.g. an aqueous solution of nitric acid, sulfuric acid, hydrochloric acid or perchloric acid. Nitric acid is preferably used.

The concentration of the acidic solution is chosen below 1N and preferably between 10~<3> and 10"<1>N.

The extracted rare earth element(s) is collected in the aqueous phase while the extraction solvent is recycled to the extraction step. Recycling is not necessary according to the present invention, but it is desirable for economic reasons.

In the following, a formula is defined which allows separation in accordance with the method of the present invention and which will be explained, at least two rare earth elements into N rare earth elements.

In the case of separation of two rare earth elements symbolized by TR^ and TR2, the separation factor between these rare earth elements is defined as a ratio of the partition coefficients of TR^ and TR2

The distribution coefficient is equal to the ratio between the concentration of TR.J (or TR^) in the organic phase and the concentration of TR.J (or TR2) in the aqueous phase.

For the separation between TR^ and TR2 to be possible, must

F be different from 1.

If TR^ is the rare earth element that has a distribution coefficient that is elevated, F is in this case greater than 1.

According to the present invention, the separation of at least two rare earth elements TR^ and TR2 is carried out

by liquid-liquid extraction between an aqueous phase containing at least the nitrates of these rare earth elements and an organic phase containing the above-mentioned extractant, by proceeding as follows:

- in a first step, the repair is carried out between TR^

and TR2 extraction of TR^ into the organic phase;

TR2 essentially remains in the aqueous phase

- in a second step, the selective washing of the organic phase containing TR^ and a small amount of TR2 is carried out

by means of an aqueous solution containing nitrate ions which allow the removal of TR21 from the organic phase by transfer to the aqueous phase - the organic phase and the aqueous phase are finally separated - in a third step, the recovery of the extraction solvent is carried out by the organic phase is brought into contact with an acidic aqueous solution.

The conditions relating to the completion of each stage are given above.

According to the present invention, a mixture of n rare earth elements can be separated in the form of two subgroups by comparing each subgroup with TR^ and TR2.

If you have a mixture of n rare earth elements to be separated individually, the order of the three steps defined above is repeated (n - 1) times to separate all the rare earth elements from each other.

The practical execution of the invention can be carried out according to known counter-flow techniques as explained above, but also with the help of co-flow techniques and mixed flow techniques which are well known to those skilled in the field.

The various steps in the contact treatment can especially be carried out in classic devices for liquid-liquid extraction that work in countercurrent. Such devices generally comprise several system steps with mixing/settling devices or packed and/or stirred columns, means to repeat the extraction operations, selective washing and recovery of rare earth elements in the aqueous phase and recovery of the extraction solvent. The following examples illustrate the invention. In the examples, the given % is expressed as weight %, unless otherwise given.

EXAMPLE 1

Separation factor of yttrium towards a lanthanide in a nitrate environment using a mixture of phosphino sliders, liquid, in an aromatic hydrocarbon (Solvesso 150).

The mixture of rare earth elements is a solution of nitrates of the earth elements with a concentration expressed as oxides of the earth elements of 500 g/l distributed as follows:

As extraction, a mixture of trialkylphosphine oxides consisting of 60% tri(n-hexyl)phosphine oxide and 40% tri-(n-octyl)phosphine oxide, marketed by Société American Cyanamid under the trade name ti Cyanex 923.<i>(

The above extraction agent is used in solution in an aromatic hydrocarbon, petroleum fraction Solvesso 150, consisting of a mixture of alkylbenzene and especially dimethylethylbenzene and tetramethylbenzene in a ratio of 1 mol/liter and the obtained tetramethylbenzene in a ratio of 1 mol/liter and the obtained mixture is extraction- the solvent.

The aqueous phase consisting of the solution of nitrates of the soil elements is brought into contact with the organic phase consisting of the extraction solvent: the volume ratio between the phases is equal to 1:1.

The extraction is carried out at room temperature.

This allows determination of the separation factors Fj^/y for the lanthanide (Ln)/yttrium pairs given in the following table:

The values for the separation factors given above allow calculation of the conditions that can be used to separate yttrium from other rare earth elements according to classical liquid-liquid extraction.

It is noted that the mixture of phosphine oxides given above shows, compared to yttrium, a very good selectivity for the heavier yttrium earth elements.

EXAMPLE 2

Separation factor of yttrium over a lanthanide in a nitrate environment using a mixture of phosphine oxides, liquid, in an aliphatic hydrocarbon (kerosene).

The same extraction agent as mentioned in ex. 1 is used in solution with kerosene, in a ratio of 75% by volume and the mixture obtained is the extraction solvent.

The aqueous phase consisting of the nitrate solution of the soil elements defined in ex. 1 is brought into contact at 50°C with the organic phase consisting of the extraction solvent: the volume ratio between the phases is equal to 1:1.

The following table gives the separation factors Fj^/y ^orParene lanthanide (Ln)/yttrium, as well as the amount of extraction solvent, in equilibrium, expressed as g oxide of earth elements per litres.

It is noted that the mixture of phosphine oxides defined according to the invention is well adapted to a separation of yttrium/heavier yttrium earth elements.

EXAMPLE 3

Process for the separation of yttrium from a mixture containing nitrates of thulium, ytterbium and lutetium

This example is carried out by the following method: illustrated in fig. 1.

The apparatus used for the separation of the above-mentioned rare earth elements comprises: a first battery for liquid-liquid extraction with several stage of the mixing device-deposition device type, which operates in countercurrent and consists of an extraction part (a) comprising 36 theoretical stages and a washing part (a') comprising 14 theoretical stages,

a section for recovery-recovery of the extraction-solvent (b) comprising 10 theoretical steps.

The initial mixture of rare earth elements to be separated is a solution of nitrates of the earth elements with an acidity of 0.1N and a concentration, expressed as oxides of the earth elements, of 270 g/l, distributed as follows:

The extractant used is a mixture of tri-alkyl-U (i

phosphine oxides (Cyanex 923) as in example 1.

The above extracting agent is in solution with the kerosene, in a ratio of 75% by volume and the resulting mixture is called extraction solvent.

Before the detailed description of the various operations, it is specified that one chooses, as inlet and outlet for the units extraction-washing and recovery-extraction, the direction of circulation for the organic phase.

The sequence of steps is carried out as follows:

one introduces, at the outlet of the extraction unit (a) at (1), the solution of the nitrates of the soil elements to be separated at a supply quantity of 161 l/h; ..

one introduces, at the inlet to the extraction unit (a)

at (2), the extraction solvent at a feed rate of 2135 l/h;

deionized water is introduced at (5), at the outlet of the section for counter-extraction <b) and in countercurrent with the organic phase, at a supply quantity of 1060 l/h;

an aqueous solution of nitrates of rare earth elements is collected, at the inlet to the counter-extraction section (b) at (6), which is concentrated by evaporation to a concentration, expressed as oxides of the earth elements, of 450 g/l with the following distribution: 8.0% Y2O^,

15.0% Tm 2 O 3 , 70% Yb 2 and 7.0% Lu 2 . One takes out 5 l/h which makes up the production and the rest 150 l/h, supplied at (3) of the liquid unit (a') to carry out return flow;

one collection at the inlet to the washing unit (a) at (4),

in a quantity of 711 l/h, an aqueous solution of yttrium nitrate which is very pure and with a concentration expressed as Y2C>2 of 59 g/l and which contains less than 10% of

the other rare earth elements expressed as oxides;

one recovers, at the end of the recovery-recovery section (b) at (7), purified extraction solvent which can be recycled at (2) in the extraction unit (a) in the same amount: nevertheless, recycling is unnecessary in the present invention, but the desirable for financial reasons.

The process described allows obtaining yttrium starting from a mixture of rare earth elements, with an excellent extraction yield as it is 99.8% and with a very high degree of purity as it is 99.9999%.

Claims (23)

1. Process for the separation of rare earth elements contained in an aqueous solution comprising mist two of these rare earth elements, by liquid-liquid extraction between an aqueous phase containing the nitrates of the rare earth elements to be separated and an organic phase containing an extractant consisting of an organophos -for connectIse, characterized in that the above-mentioned extractant consists of at least two organophosphorus compounds of formula (I) and (I'), respectively: in which : - in formula (I): R , R 2 , R 3 , which are the same or different and optionally substituted, represent a hydrogen atom or a straight-chain or branched hydrocarbon radical, which is saturated or unsaturated or a cyclic hydrocarbon radical, which is saturated or unsaturated; the sum of the number of carbon atoms for the radicals R^ , R,,, R^ is at least equal to 12 carbon atoms; in formula (I' ): represents R 1 , R 1 -, R 8 , which are the same or different and optionally substituted, a hydrogen atom or a straight-chain or branched, saturated or unsaturated, hydrocarbon radical or a cyclic hydrocarbon radical, which is saturated or unsaturated; •,- the sum of these number of carbon atoms for the radicals R^ , R^ , Rg are at least equal to 12 carbon atoms. 2. Process as stated in claim 1, characterized in that the above-mentioned organophosphorus compounds have the formulas (I) and (I') in which R^ , R2 , R^ > Rjj, R^ , Rg have from 1 and 18 carbon atoms and preferably from 2 and 8 carbon atoms. 3. Method as stated in claims 1 and 2, characterized in that the above-mentioned organophosphorus compound has the formula (I) in which the sum of the number of carbon atoms for the radicals R^ , R2 , and R^ is at least equal to 18 carbon atoms. 4. Process as stated in claims 1 to 3, characterized in that the above-mentioned organophosphorus compound has the formula (I') in which the sum of the number of carbon atoms for the radicals R^ , R< - and Rg is at least equal to 18 carbon atoms. 5. Method as stated in claims 1 to 4, characterized in that the extractant consists of an organophosphorus compound of formula (I) in which R2 , R^' are straight-chain identical alkyl radicals and an organophosphorus compound of formula (I') in which R^ , R^ Rg are straight-chain identical alkyl radicals with a number of carbon atoms different from the radicals R ^1, E^ , R^ . 6. Method as stated in claims 1 to 4, characterized in that the extractant consists of an organophosphorus compound of formula (I) in which R^ , R2 , R^ are straight-chain identical alkyl radicals and an organophosphorus compound of formula (I') in which R^ , R ^ , Rg are branched identical alkyl radicals or f-or-branched different alkyl radicals or straight-chain or branched alkyl radicals or straight-chain or branched alkyl radicals. 7. Method as stated in claims 1 to 6, characterized in that the extractant is a mixture of tri(n-hexylphosphine) oxide and tri(n-octylphosphine) oxide or a mixture of bis(-2,4,4-trimethylpentyl) n-octyl-phosphine and tri(2,4,4-tri-methylpentyl)phosphine oxide. 8. Method as stated in claims 1 to 7, characterized in that the aqueous solution of nitrates of rare earth elements has a concentration, expressed as oxides of rare earth elements, between 20 g/l and 500 g/l, preferably between 100 g/l and 500 g/l. 9. Method as stated in claim 8, characterized in that the aqueous solution of nitrates of rare earth elements has an acidity between 0.01 N and 3.0 N. 10. Method as stated in claims 1 to 9, characterized in that the organic phase also contains at least one organic diluent selected from the group consisting of aliphatic or cycloaliphatic hydrocarbons, petroleum fractions of the cerosene or isoparaffin type, aromatic hydrocarbons, petroleum fractions of the Solvesso type , halogenated hydrocarbons. 11. Method as stated in claim 1, characterized in that the concentration of the above-mentioned extractant in the organic phase is between 5 and 100% by volume of the organic phase, preferably between 50 and 100%. 12. Method as stated in claims 1 to 11, characterized in that the organic phase also contains at least one modifying agent selected from the group consisting of compounds with an alcohol function, the phosphoester. 13. Method as stated in claim 12, characterized in that the concentration of the modifying agent in the organic phase is between 3 and 20% by volume of the organic phase. 14. Method as stated in claims 1 to 13, characterized in that the concentration of the nitrates in the water phase during the extraction can vary between 1 and 10 mol/litre, preferably between 2 and 9 mol/litre 15. Method as stated in claims 1 to 14, characterized in that the extraction temperature is between 10 and 80°C 16. Method as stated in claims 1 to 14, characterized in that after the extraction step, one proceeds with a washing operation of the organic phase with water or with the help of an aqueous solution containing the nitrates. 17- Method as stated in claim 16, characterized in that the washing solution is an aqueous solution of nitric acid with a concentration less than 1N, preferably between 10"^ and 10 <1> N and/or an aqueous solution of nitrates of rare earth elements if concentration expressed as oxides of the rare earth elements is between 5 and 500 g/l, preferably between 100 and 500 g/l. 18. Method as stated in claims 1 - 17, characterized in that, after the extraction step and the subsequent washing which is followed by a separation of the aqueous phase and the organic phase, a recovery step of the extraction solvent is carried out by bringing the organic phase in contact with water or with an acidic aqueous solution. 19. Method as stated in claim 18, characterized in that the acidic aqueous solution is an aqueous solution of nitric acid, sulfuric acid, hydrochloric acid, perchloric acid; preferably nitric acid. 20. Method as stated in claim 19, characterized in that the concentration of the acidic vanidge solution is less than 1N, preferably between 10 <3> and 10 <-1> N. 21. Method as stated in claims 1 to 20, characterized in that, in a first step, in order to separate at least two rare earth elements or two subgroups of rare earth elements, the aqueous solution of nitrates of rare earth elements is brought into contact with an organic phase consisting of a diluent and the above-mentioned extractant to extract the rare earth element or the subgroups of the rare earth elements into the organic phase with a partition coefficient that is increased; wherein the second rare earth element or the second subgroup of rare earth elements essentially remains in the aqueous phase; in a second step, the selective washing of the organic phase is carried out by means of an aqueous solution containing nitrate ions to eliminate the small amount of the rare earth element or the subgroup of the rare earth element which has been extracted to a moderate extent into the organic phase by basing to said phase from the aqueous phase; the organic phase is then separated from the aqueous phase; in a third step, recovery of the extraction solvent is carried out by bringing the organic phase into contact with water or with an acidic aqueous solution. 22. Method as stated in claim 21, characterized in that yttrium is separated from the yttrium earth elements. 23. Method as stated in claim 22, characterized in that yttrium, expressed as oxide, represents from 5 to 80% of the total weight of the yttrium rare earth elements expressed as oxides.