Method for separating and purifying heavy rare earth elements by means of liquid / liquid extraction

Abstract

The invention relates to a method for producing a mixture of terbium-dysprosium or dysprosium from a mixture of rare earth elements in a solution, said method comprising at least two consecutive steps of separation by liquid / liquid extraction, carried out using two different solvents, S1 and S2: - the first separation step is carried out using the solvent S1, which comprises at least one anionic extraction agent, and - the second separation step is carried out using the solvent S2, which comprises at least one cationic extraction agent.

Description

[0001] PROCESS FOR SEPARATION AND PURIFICATION OF RARE EARTHS

[0002] HEAVY BY LIQUID / LIQUID EXTRACTION

[0003] FIELD OF THE INVENTION

[0004] The invention falls within the field of separation of rare earths, and in particular heavy rare earths (HREs).

[0005] Rare earths are a group of metals with similar chemical properties, including the fourteen stable elements of the lanthanide family: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), as well as scandium (Sc) and yttrium (Y), which are transition metals.

[0006] In particular, rare earths with the highest atomic numbers are referred to as "heavy rare earths." Associated with these are yttrium, which, although having a lower atomic number, has chemical properties similar to erbium.

[0007] Their specific physical properties, particularly their magnetic properties, have led to their use in many industries, particularly in renewable energies, such as wind turbines and electric vehicles.

[0008] The term "rare earths" as used herein describes heavy rare earths.

[0009] Rare earths are generally extracted from ores such as monazite, xenotime where they are present in the form of phosphates, bastnaesite where they are present in the form of fluorocarbonates or in certain clays where they are present in ionic form.

[0010] Their extraction traditionally begins with the processing of ores, which after several stages of hydrometallurgical operations, leads to aqueous solutions of nitrates or chlorides of rare earth mixtures. These elements must then be isolated so that they can be further recovered. To this end, the invention relates to a process for separating rare earths contained in an acid solution comprising the rare earth elements Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y.

[0011] PRIOR STATE OF THE ART

[0012] Separating rare earth elements from each other is complex because these elements have similar chemical properties, making them difficult to isolate.

[0013] The most widely used processes involve at least one liquid-liquid extraction using at least one solvent. To date, the most effective methods rely on the selectivity of this extraction solvent, which is organic in nature, with respect to rare earths, the choice of which depends on the rare earth that one wishes to isolate. Selectivity means the capacity to separate two elements

[0014] In the specific case of heavy rare earths, the type of solvent used depends on the acid used when dissolving the rare earth concentrate obtained after processing the ores. Thus, in the case of hydrochloric solutions, only organophosphorus acid molecules have sufficient selectivity to separate the heavy rare earths from each other. For nitric solutions, the molecules used are organophosphorus acids as well as amines, and in particular quaternary ammonium salts.

[0015] Organophosphorus acid molecules have the following formulas: phosphoric acid ester phosphonic acid ester phosphinic acid where R1 and R2 are independently selected from the group comprising linear or branched alkyl or aryl, aromatic radicals.

[0016] In particular, document EP 0156735 describes a process for separating lanthanides and yttrium by liquid-liquid extraction, using an extraction solvent comprising a phosphinic acid, such as di-n-octylphosphinic acid or bis(2-ethylhexyl)phosphinic acid.

[0017] Organophosphorus acids, although highly selective, cause the consumption of significant amounts of acid and base in the separation process, due to the driving force of liquid-liquid equilibrium, which is related to the energy required to achieve liquid-liquid equilibrium, and which depends on the acidity of the aqueous medium, according to the following reaction: where TR represents a rare earth element, HL represents organophosphorus acid and L the base associated with it.

[0018] Extraction of the rare earth element requires shifting the equilibrium to the right by neutralizing the acidity of the aqueous medium by adding base. The aqueous phase is then acidified by adding acid, the equilibrium is shifted to the left, and the rare earth element is then in aqueous solution in the form of a trivalent ion. The solvent is thus regenerated. However, any addition of acid (HCl or HNO3) or base (NaOH or NH4OH) generates saline effluents (NaCl OR NH4NO3), generally released into the environment.

[0019] The amount of base and acid required increases with the atomic number of the rare earth element. It becomes particularly important for the heavier elements (Ho, Er, Tm, Yb, and Lu) and for yttrium. In particular, mixtures from ionic clay and xenotime ores are rich in heavy rare earths, and their separation therefore requires a high amount of acid and base.

[0020] For example, document FR 2460275 describes a process for separating rare earths using an extraction solvent comprising a phosphonic acid, the mono-2-ethylhexyl ester of 2-ethylhexylphosphonic acid. However, this process is expensive in terms of acid.

[0021] Another category of extraction solvents is known for its selectivity towards heavy rare earths in nitrate medium, in particular the heavier elements (Ho, Er, Tm, Yb and Lu) and yttrium. These are quaternary ammonium extraction solvents, which correspond to the following formula: where R1, R2, R3 and R4 are independently selected from the group consisting of alkyls of more than 4 carbon atoms.

[0022] Document US 3,640,678 describes in particular a process for purifying yttrium from a solution of heavy rare earths, using quaternary ammonium nitrate marketed under the name Aliquat™ 336. These extractants Aliquat 336 or Adogen 464 HF are quaternary ammoniums of formula tri-C8-C10 alkylmethyl ammonium marketed in the form of chlorides. They are transformed into nitrate to be advantageously used in the separation of rare earths.

[0023] These molecules do not require additional consumption of acid and base during the implementation of the process because the driving force of the liquid-liquid equilibrium is not linked to the acidity of the aqueous phase, but only to the nitrate concentration, according to the following reaction: where TR represents a rare earth element, R CH N represents a quaternary ammonium.

[0024] Extraction of the rare earth element recovered in the aqueous phase requires shifting the equilibrium to the right by increasing the concentration of nitrate in the aqueous phase, by evaporation of the aqueous solution, then shifting the equilibrium to the left, by adding water, in order to recycle the solvent.

[0025] However, when the mixture of heavy rare earths to be treated contains a significant proportion of the lightest elements (Sm, Eu, Gd), or even small quantities of "light" rare earths (La, Ce, Pr, Nd), the organic phase flow rate required for their extraction becomes very high, which requires large mixer-settlers and generates excessive investment costs. Indeed, the organic phase flow rate is linked to the quantity of TR to be extracted (that which passes into the solvent). However, with the implementation of Aliquat, the selectivity is in the order Sm>Eu>Gd...>Yb>Lu. In concentrates containing mainly Sm Eu Gd, it is necessary to extract the majority TR, which leads to a high solvent flow rate.

[0026] Current processes for separating mixtures rich in heavy rare earths require significant amounts of acid and base for their operation, which generally results in significant costs related to the price of reagents, and generates a significant volume of saline effluent, which must be recycled or discharged into the environment. In fact, current separation processes each have their limitations:

[0027] ■ those using organophosphorus acid: high consumption of acid and base, particularly for the extraction of the heaviest rare earths;

[0028] ■ those which use a quaternary ammonium: a high solvent flow rate, in particular for mixtures containing mainly Sm, Eu, Gd.

[0029] STATEMENT OF THE INVENTION

[0030] The Applicant found that by combining an extraction solvent comprising a quaternary ammonium compound and an extraction solvent comprising an organophosphorus acid compound in the rare earth separation process, it was possible to retain the advantages associated with each of these molecules while greatly limiting the associated disadvantages.

[0031] Thus, the present invention relates to a process for producing a mixture of terbium-dysprosium or dysprosium from a mixture of rare earths in solution comprising at least two consecutive stages of separation by liquid-liquid extraction, carried out using two different solvents, making it possible to limit the consumption of base and acid linked to the operation of the process and to generate less saline effluent.

[0032] The invention relates to a process for producing a terbium-dysprosium or dysprosium mixture from a mixture of rare earths in solution, said process comprising at least two consecutive steps of separation by liquid-liquid extraction, carried out using two different solvents, SI and S2:

[0033] ■ the first separation step is carried out using the SI solvent which includes at least one anionic extraction agent,

[0034] ■ the second separation step is carried out using solvent S2 which includes at least one cationic extraction agent.

[0035] Preferably, the mixture of rare earths in solution comprises a first group of rare earths TRLM comprising the elements Sm, Eu, Gd, Tb and Dy, and a second group of rare earths TRLL comprising the elements Ho, Er, Tm, Yb, Lu and Y.

[0036] The first separation step preferentially separates the elements included in the TRLL group from the elements included in the TRLM group.

[0037] The second separation step preferentially separates the elements included in the TRLM group to isolate the terbium-dysprosium mixture or dysprosium.

[0038] The rare earth mixture in solution can be a solution from ores rich in heavy rare earths, preferably xenotime or ionic clays.

[0039] According to one embodiment, the mixture of rare earths in solution is a mixture of rare earth nitrates in solution.

[0040] The rare earth mixture in solution may comprise at least 50 mol% TRLL, preferably at least 70 mol% TRLL.

[0041] According to one embodiment, the mixture of rare earths in solution is subjected to a first separation step using the solvent S 1 , generating a fraction comprising the elements included in the TRLM group, said fraction subsequently being subjected to a second separation step using the solvent S2, one of the products of said second separation being a mixture of terbium-dysprosium or dysprosium. According to another embodiment, the mixture of rare earths in solution is subjected to a first separation step using the solvent S 1 , generating a fraction comprising the elements included in the TRLM group, said fraction subsequently being mixed with a solution pre-enriched in heavy rare earths from ores rich in light rare earths, preferably monazite or bastnaesite, said fraction being subjected to a second separation step using the solvent S2.

[0042] Ores, such as monazite or bastnaesite, mainly comprise so-called "light" rare earths. A liquid-liquid extraction step can first be carried out to separate the "light" rare earths from the heavy rare earths. The mixture of heavy rare earths thus obtained can then be subjected to the process which is the subject of the present invention.

[0043] Ores comprising mainly heavy rare earths, such as xenotime or ionic clays, can be subjected to an initial separation between "light" and heavy rare earths.

[0044] The heavy rare earth pre-enriched solution from light rare earth rich ores preferably comprises at least 80 mol% of heavy rare earths, preferably at least 95 mol% of heavy rare earths, said heavy rare earths comprising at least 60 mol% of elements included in the TRLM group, preferably at least 80 mol% of elements included in the TRLM group.

[0045] In this embodiment, the product of the second separation step containing terbium and dysprosium is subjected to a third separation step using solvent S2 generating a mixture of terbium-dysprosium or dysprosium.

[0046] Alternatively, the separation steps are carried out using mixer-decanter batteries or liquid-liquid extraction columns.

[0047] Advantageously, the anionic extracting agent is methyl trialkyl ammonium nitrate. For example, the anionic extracting agent may be methyl trioctyl ammonium nitrate.

[0048] The cationic extractant is preferably selected from the group consisting of alkyl-phosphonic derivatives, alkyl-phosphinic derivatives and mixtures thereof, for example 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH(EH)P), trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (known under the trade name CYANEX®272), a mixture of phosphonic acid and phosphinic acid (known under the trade name CYANEX®572). The extraction agents have a concentration in the solvents which makes it possible to maintain a viscosity of less than 20 cP, preferably a concentration of between 0.1 and 2 mol / L, preferably between 0.5 and 1.5 mol / L for organophosphorus acids, and between 0.1 and 0.8 mol / L, and preferably between 0.3 and 0.6 mol / L for quaternary ammoniums.

[0049] Solvents SI and S2 may further comprise aliphatic or aromatic hydrocarbons of 5 to 16 carbon atoms, and optionally at least one modifier chosen from fatty alcohols.

[0050] Aliphatic or aromatic hydrocarbons can be diluents that solubilize the extraction agents, and do not modify their chemical properties.

[0051] Examples of aliphatic or aromatic hydrocarbons include hexane, heptane, kerosene, toluene, benzene, Solvesso™ 150 (aromatic hydrocarbon), ketones and alcohols.

[0052] The invention can be implemented in a battery of multi-stage liquid-liquid mixer-settlers or in a multi-stage liquid-liquid extraction column, preferably operating countercurrently.

[0053] The process of the invention is preferably carried out at a temperature between 20°C and 90°C. This temperature is preferably lower than the flash point of the mixture.

[0054] The process according to the invention makes it possible to reduce the quantities of base and acid used in the process.

[0055] BRIEF DESCRIPTION OF THE FIGURES

[0056] The manner in which the invention can be implemented and the advantages which result therefrom will become more apparent from the following examples of implementation, given for informational and non-limiting purposes, in support of the appended figures.

[0057] Figure 1 is a schematic representation of the method according to the invention corresponding to Example 1, in two liquid-liquid extraction batteries.

[0058] Figure 2 is a schematic representation of the process corresponding to Comparative Example 1, in two liquid-liquid extraction batteries.

[0059] Figure 3 is a schematic representation of the method according to the invention corresponding to example 2, in two liquid-liquid extraction batteries.

[0060] Figure 4 is a schematic representation of the process corresponding to Comparative Example 2, in two liquid-liquid extraction batteries. Figure 5 is a schematic representation of the process according to the invention corresponding to Example 3, in three liquid-liquid extraction batteries.

[0061] Figure 6 is a schematic representation of the process corresponding to Comparative Example 3, in two liquid-liquid extraction batteries.

[0062] EXAMPLES OF CARRYING OUT THE INVENTION

[0063] The examples below were simulated using software, in this case called “PAREX+”, developed by the Applicant, the basic data necessary for the calculations of which were previously obtained by laboratory tests according to the different chemical systems presented.

[0064] Liquid-liquid extraction batteries were powered by two types of solutions:

[0065] ■ a solution of heavy rare earths derived from a xenotime-type raw material;

[0066] ■ a first solution of heavy rare earths from a bastnaesite type ore and a second solution of heavy rare earths from an ionic clay type ore.

[0067] Example 1: Process for producing Dy from a solution of a mixture of heavy rare earth nitrates, derived from a xenotime-type ore according to the invention

[0068] The method of Example 1, implementing the invention, is carried out using the following device:

[0069] ■ a first liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 48-stage extraction section (32), a 33-stage washing section (33) and a 6-stage extraction section (34), these three sections being connected to each other,

[0070] ■ a second liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 13-stage extraction section (35), a 14-stage washing section (36) and a 6-stage extraction section (37), these three sections being connected to each other.

[0071] The first extraction section is fed with an aqueous solution consisting of a mixture of heavy rare earth nitrates (Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y), with a flow rate of 240 L / h (1), the composition of which is detailed in tables 1 and 2.

[0072] The solution is extracted in the extraction section (2). The SI solvent, composed of 30% Aliquat™ 336 in nitrate form and 70% Solvesso™ 150 by volume, is injected at a flow rate of 6500 L / h (3).=Here, Aliquat™ 336 is the anionic extraction agent. This product is marketed in the chloride form. In the context of the invention, it is pretreated in order to transform it into the nitrate form.

[0073] An aqueous raffinate comprising mainly the rare earths Ho, Er, Tm, Yb, Lu and Y is obtained (4).

[0074] Part of the raffinate is reinjected into the extraction section at a flow rate of 385 L / h (5), after concentration in order to load the solvent.

[0075] About 576 L / h of water is evaporated (30).

[0076] An aqueous extract comprising mainly the rare earths Sm, Eu, Gd, Tb and Dy is obtained and fed into the second extraction battery with a flow rate of 54 L / h (6).

[0077] The solvent undergoes washing before being introduced for re-extraction by injecting into the washing section either part of the extract or water with a flow rate of 608 L / h (7).

[0078] Approximately 317 L / h of water from the extract is evaporated (31).

[0079] The extraction section of the second battery is supplied with an aqueous ammonia solution with a HO' concentration of 4 mol / L with a flow rate of 120 L / h (8).

[0080] An aqueous nitric acid solution with a concentration of H +of 3.2 mol / L is fed into the re-extraction section with a flow rate of 198 L / h (8).

[0081] Solvent S2 is fed into the second battery at a flow rate of 1200 L / h (10). Solvent S2 is composed of 67% kerosene and 33% HEH(EH)P, by volume. Here, HEH(EH)P is the cationic extraction agent.

[0082] An aqueous raffinate composed mainly of Sm, Eu, Gd and Tb is obtained (11).

[0083] An extract comprising 90.83 g / L of Dy is recovered (12). The extract then undergoes another wash by being reinjected into the washing section with a flow rate of 114 L / h (13), this wash being carried out either by a part of the extract or by an acid solution.

[0084] Aqueous effluents are recovered at the outlet of the second battery section (14). They mainly contain ammonium nitrate, and can be used, for example, as fertilizer. The process according to the invention uses two solvents SI and S2, consecutively.

[0085] The summary diagram of the process of example 1 according to the invention is shown in figure 1. The quantities of rare earths and the flow rates are shown in tables 1 and 2.

[0086] Table 1 Table 2 Comparative example 1: Process for producing Dy from a solution of a mixture of heavy rare earth chlorides from a xenotime-type ore

[0087] This comparative example results from the industrial processes known to date, which operate using the chloride route.

[0088] The process is carried out using the following device:

[0089] ■ a first liquid-liquid extraction battery of the mixer-decanter type operating in countercurrent, consisting of a 15-stage extraction section (38), a 17-stage washing section (39) and a 6-stage re-extraction section (40), these three sections being connected to each other, ■ a second liquid-liquid extraction battery of the mixer-decanter type operating in countercurrent, consisting of a 12-stage extraction section (41), a 12-stage washing section (42) and a 6-stage re-extraction section (43), these three sections being connected to each other.

[0090] The first extraction section is fed with an aqueous solution consisting of a mixture of heavy rare earth chlorides, with a flow rate of 427 L / h (1), the composition of which is detailed in tables 3 and 4.

[0091] An aqueous sodium hydroxide solution with a HO' concentration of 8 mol / L is fed into the first battery with a flow rate of 284 L / h (2).

[0092] An aqueous solution of hydrochloric acid with a concentration of H + of 5.5 mol / L is fed into the re-extraction section with a flow rate of 482 L / h (3).

[0093] Solvent S2 is fed into the first battery at a flow rate of 5680 L / h (4). Solvent S2 is composed of 67% kerosene and 33% HEH(EH)P, by volume. Here, HEH(EH)P is the cationic extraction agent.

[0094] An aqueous raffinate comprising mainly the rare earths Sm, Eu, Gd, Tb and Dy is obtained (5).

[0095] Aqueous effluents (approximately 719 L / h) are recovered at the outlet of the first battery (8) and are released into the environment.

[0096] An aqueous extract comprising mainly the rare earths Ho, Er, Tm, Yb, Lu and Y is obtained and fed into the second extraction battery (6).

[0097] The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 136 L / h (7).

[0098] The extraction section of the second battery is supplied with an aqueous sodium hydroxide solution with a HO' concentration of 8 mol / L with a flow rate of 50 L / h (9).

[0099] An aqueous solution of hydrochloric acid with a concentration of H + of 5 mol / L is fed into the re-extraction section with a flow rate of 92 L / h (10).

[0100] Solvent S2 is fed into the second battery at a flow rate of 1000 L / h (11).

[0101] An aqueous raffinate comprising mainly the elements Sm, Eu, Gd and Tb is obtained. An extract composed of 158.99 g / L of Dy is recovered (13). The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 57 L / h (14). Aqueous effluents containing only NaCl are recovered at the outlet of the second battery section (15) and are released into the environment.

[0102] This process uses only one solvent, solvent S2. The summary diagram of the process of comparative example 1 is shown in Figure 2. The quantities of rare earths and the flow rates are shown in Tables 3 and 4.

[0103] Table 3 Table 4 Comparison of the processes of Example 1 and Comparative Example 1 with respect to the consumption of reagents and the production of saline effluents

[0104] The process according to the invention makes it possible to divide the molar consumption of acid by 5 (10.0 moles of acid per kg of heavy rare earths compared to 50.0 moles for the process which is not according to the invention) and to divide the molar consumption of base by 6.9 (7.6 moles per kg of heavy rare earths compared to 52.5 moles for the process which is not according to the invention). Furthermore, the process according to the invention generates ammonium nitrate in a quantity very much lower than the sodium chloride generated by the process which is not according to the invention (0.61 kg per kg of heavy rare earths compared to 3.07 kg for the process which is not according to the invention). Ammonium nitrate can easily be recovered in the fertilizer industry. NaCl is released into the environment.

[0105] The values ​​are reported in Table 5.

[0106] Table 5 Example 2: Process for producing a mixture of Tb and Dy from a solution of a mixture of heavy rare earth nitrates from a xenotime-type ore according to the invention

[0107] The method according to the invention is carried out using the following device:

[0108] ■ a first liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 48-stage extraction section (44), a 33-stage washing section (45) and a 6-stage re-extraction section (46), these three sections being connected to each other,

[0109] ■ a second liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 12-stage extraction section (47), a 5-stage washing section (48) and a 6-stage re-extraction section (49), these three sections being connected to each other.

[0110] The first extraction section is fed with an aqueous solution consisting of a mixture of heavy rare earth nitrates, with a flow rate of 240 L / h (1), the composition of which is detailed in tables 6 and 7.

[0111] An acidic aqueous solution with a concentration of H + of 0.1 mol / L is fed into the re-extraction section with a flow rate of 867 L / h (3).

[0112] The SI solvent, composed of 30% Aliquat™ 336 and 70% Solvesso™ 150 by volume, is injected at a flow rate of 6500 L / h (3). Here, Aliquat™ 336 is the anionic extraction agent. It is previously transformed into nitrate.

[0113] An aqueous raffinate comprising mainly the rare earths Ho, Er, Tm, Yb, Lu and Y is obtained (4).

[0114] A portion of the raffinate containing the unextracted rare earths is reinjected into the extraction section at a flow rate of 385 L / h (5).

[0115] About 576 L / h of water is evaporated (30).

[0116] An aqueous extract comprising mainly the rare earths Sm, Eu, Gd, Tb and Dy is obtained and fed into the second extraction battery with a flow rate of 54 L / h (6).

[0117] The solvent is washed before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 608 L / h (7). Approximately 317 L / h of water from the extract is evaporated (31).

[0118] The extraction section of the second battery is supplied with an aqueous ammonia solution with a HO' concentration of 4 mol / L with a flow rate of 125 L / h (8).

[0119] An aqueous solution of nitric acid with a concentration of H + of 3.2 mol / L is fed into the re-extraction section of the second battery with a flow rate of 208 L / h (9).

[0120] Solvent S2 is fed into the second battery at a flow rate of 1250 L / h (10). Solvent S2 is composed of 67% kerosene and 33% HEH(EH)P, by volume. Here, HEH(EH)P is the cationic extraction agent.

[0121] An aqueous raffinate composed mainly of Sm, Eu and Gd is obtained (11).

[0122] An extract comprising 11.43 g / L of Tb and 78.16 g / L of Dy is recovered (12). The solvent is washed before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 111 L / h (13).

[0123] Aqueous effluents are collected at the outlet of the second battery section (14). They contain only ammonium nitrate and can be used as fertilizer.

[0124] The summary diagram of the process of Example 2 is shown in Figure 3. The quantities of rare earths and the flow rates are shown in Tables 6 and 7.

[0125] Table 6

[0126] Table 7 Comparative example 2: Process for producing a mixture of Tb and Dy from a solution of a mixture of heavy rare earth chlorides from a xenotime-type ore

[0127] The method is carried out using the following device: ■ a first liquid-liquid extraction battery of the mixer-settler type operating in countercurrent, consisting of a 15-stage extraction section (50), a 17-stage washing section (51) and a 6-stage re-extraction section (52), these three sections being connected to each other, ■ a second liquid-liquid extraction battery of the mixer-settler type operating in countercurrent, consisting of a 10-stage extraction section (53), a 4-stage washing section (54) and a 6-stage re-extraction section (55), these three sections being connected to each other.

[0128] The first extraction section is fed with a mixture of two aqueous solutions including a mixture of heavy rare earth chlorides, with a flow rate of 427 L / h (1), the composition of which is detailed in tables 8 and 9.

[0129] An aqueous sodium hydroxide solution with a HO concentration of 8 mol / L is fed into the first battery (2).

[0130] An aqueous solution of hydrochloric acid at a concentration of H + of 5.5 mol / L is fed into the re-extraction section with a flow rate of 482 L / h (3).

[0131] Solvent S2 is fed into the first battery with a flow rate of 5680 L / h (4). Solvent S2 is composed of 67% kerosene and 33% HEH(EH)P, by volume. Here, HEH(EH)P is the cationic extraction agent.

[0132] An aqueous raffinate comprising mainly the rare earths Sm, Eu, Gd, Tb and Dy is obtained and fed into the second battery (5).

[0133] Aqueous effluents (approximately 719 L / h) are recovered at the outlet of the first battery (8). They consist of NaCl and are released into the environment.

[0134] An aqueous extract comprising mainly the rare earths Ho, Er, Tm, Yb, Lu and Y is obtained (6).

[0135] The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 136 L / h (7).

[0136] The extraction section of the second battery is supplied with an aqueous sodium hydroxide solution with a HO' concentration of 8 mol / L with a flow rate of 63 L / h (9).

[0137] A concentrated aqueous solution of hydrochloric acid in H + at 5 mol / L is fed into the re-extraction section with a flow rate of 117 L / h (10).

[0138] The solvent S2 is fed into the second battery with a flow rate of 1250 L / h (11).

[0139] An aqueous raffinate composed mainly of Sm, Eu and Gd is obtained (12). An extract composed of 21.869 g / L of Tb and 149.58 g / L of Dy is recovered (13). The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 73 L / h (14). Aqueous effluents (approximately 169 L / h) are recovered at the outlet of the second battery section (15). They contain only NaCl and are discharged into the environment.

[0140] This process uses only one solvent, solvent S2.

[0141] The summary diagram of the process of comparative example 2 is shown in Figure 4.

[0142] The quantities of rare earths and the flow rates are reported in Tables 8 and 9.

[0143] Table 8 Table 9

[0144] Comparison of the processes of Example 2 and Comparative Example 2 with respect to the consumption of reagents and the production of saline effluents

[0145] The process of the invention makes it possible to divide the molar consumption of acid by 4.8 (10.6 moles of acid per kg of TRL compared to 51.0 moles for the process which is not according to the invention) and to divide the molar consumption of base by 5.5 (8.0 moles per kg of TRL compared to 44.1 moles for the process which is not according to the invention). Furthermore, the process of the invention generates ammonium nitrate in a quantity which is very much lower than the sodium chloride generated by the process which is not according to the invention (0.64 kg per kg of TRL compared to 2.58 kg for the process which is not according to the invention). This ammonium nitrate can be used as a fertilizer, even though sodium chloride is released into the environment.

[0146] The values ​​are reported in Table 10.

[0147] Table 10 Example 3: Process for producing Dy from a first solution of heavy rare earth nitrates from a bastnaesite type ore and a second solution of heavy rare earth nitrates from an ionic clay type ore, according to the invention

[0148] The process is carried out using the following device:

[0149] ■ a first liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 25-stage extraction section (56), a 28-stage washing section (57) and a 6-stage re-extraction section (58), these three sections being connected to each other,

[0150] ■ a second liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 19-stage extraction section (59), a 14-stage washing section (60) and a 6-stage re-extraction section (61), these three sections being connected to each other,

[0151] ■ a third liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of an 18-stage extraction section (62), a 16-stage washing section (63) and a 6-stage re-extraction section (64), these three sections being connected to each other.

[0152] The first extraction section is fed with an aqueous solution consisting of a mixture of heavy rare earth nitrates from an ionic clay type ore, with a flow rate of 240 L / h (1), the composition of which is detailed in tables 11 and 12.

[0153] The solution is extracted in the re-extraction section (2).

[0154] The SI solvent, composed of 30% Aliquat™ 336 and 70% Solvesso™ 150 by volume, is injected at a flow rate of 6100 L / h (3). Aliquat™ 336 is the anionic extraction agent. It undergoes treatment for transformation into nitrate form.

[0155] An aqueous raffinate comprising mainly the rare earths Ho, Er, Tm, Yb, Lu and Y is obtained (4).

[0156] An aqueous extract comprising mainly the rare earths Sm, Eu, Gd, Tb, Dy, Ho, Er and Y, is obtained and fed into the second extraction battery with a flow rate of 368 L / h (5).

[0157] The solvent is washed before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 330 L / h (6). Approximately 220 L / h of water are evaporated (30).

[0158] The second battery is supplied with a solution comprising a mixture of heavy rare earths from a bastnasite-type ore (7), the composition of which is reported in tables 11 and 12.

[0159] The extraction section of the second battery is supplied with an aqueous ammonia solution, with a HO' concentration of 4 mol / L, with a flow rate of 1227 L / h (8).

[0160] An aqueous solution of nitric acid with a concentration of H + of 3.5 mol / L is fed into the re-extraction section with a flow rate of 5171 L / h (9).

[0161] Solvent S2 is fed into the second battery at a flow rate of 12270 L / h (10). Solvent S2 is composed of 67% kerosene and 33% HEH(EH)P, by volume. Here, HEH(EH)P is the cationic extractant.

[0162] An aqueous raffinate composed mainly of Sm, Eu, Gd, Tb and Dy is obtained (11).

[0163] An extract composed mainly of Ho, Er, Tm, Yb, Lu and Y is recovered (12). The solvent is washed before being introduced for re-extraction by injecting part of the extract into the washing section with a flow rate of 716 L / h (13).

[0164] Aqueous effluents are recovered at the outlet of the second battery section (14). They consist of ammonium nitrate and can be used as fertilizer.

[0165] The aqueous raffinate composed mainly of Sm, Eu, Gd, Tb and Dy is fed into the third battery.

[0166] An aqueous ammonia solution with a HO' concentration of 4 mol / L is fed into the extraction section of the third battery with a flow rate of 800 L / h (15).

[0167] An aqueous solution of nitric acid with a concentration of H + of 3.2 mol / L is fed into the re-extraction section with a flow rate of 1268 L / h (16).

[0168] Solvent S2 is fed into the third battery with a flow rate of 8000 L / h (17).

[0169] An aqueous raffinate composed mainly of Sm, Eu, Gd and Tb is obtained (18). An extract comprising 89.40 g / L of Dy is recovered (19). The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 710 L / h (20). Aqueous effluents (2639 L / h) are recovered at the outlet of the third battery (21). They consist of ammonium nitrate, and can be used as fertilizer.

[0170] The summary diagram of the process of example 3 according to the invention is shown in figure

[0171] 5. The quantities of rare earths and the flow rates are reported in Tables 11 and 12.

[0172] Table 11 Table 12 Comparative example 3: Process for producing Dy from a mixed solution composed of a mixture of heavy rare earth chlorides from a bastnaesite type ore and a mixture of heavy rare earth chlorides from an ionic clay type ore. The process is carried out using the following device:

[0173] ■ a first liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 14-stage extraction section (65), a 13-stage washing section (66) and a 6-stage re-extraction section (67), these three sections being connected to each other, ■ a second liquid-liquid extraction battery of the mixer-decanter type operating in counter-current, consisting of a 12-stage extraction section (68), an 8-stage washing section (69) and a 6-stage re-extraction section (70), these three sections being connected to each other. The first extraction section is fed with a mixture of two aqueous solutions consisting of heavy rare earth chlorides, one from a bastnaesite type ore and the other from an ionic clay type ore, with a flow rate of 2480 L / h (1), the composition of which is detailed in tables 13 and 14.

[0174] An aqueous sodium hydroxide solution with a HO' concentration of 8 mol / L is fed into the first battery with a flow rate of 1825 L / h (2).

[0175] An aqueous solution of hydrochloric acid with a concentration of H + of 5.5 mol / L is fed into the re-extraction section with a flow rate of 5188 L / h (3).

[0176] Solvent S2 is fed into the first battery with a flow rate of 36500 L / h (4). Solvent S2 is composed of 67% kerosene and 33% HEH(EH)P, by volume. Here, HEH(EH)P is the cationic extraction agent.

[0177] An aqueous raffinate comprising mainly the rare earths Sm, Eu, Gd, Tb and Dy is obtained and fed into the second battery (5).

[0178] Aqueous effluents (5468 L / h) are recovered at the outlet of the first battery (8). These effluents are composed of sodium chloride and are released into the environment.

[0179] An aqueous extract comprising mainly the rare earths Ho, Er, Tm, Yb, Lu and Y is obtained (6).

[0180] The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section with a flow rate of 1165 L / h (7).

[0181] The extraction section of the second battery is supplied with a sodium hydroxide solution with a HO' concentration of 8 mol / L with a flow rate of 400 L / h (9).

[0182] An aqueous solution of hydrochloric acid with a concentration of H + of 5 mol / L is fed into the re-extraction section with a flow rate of 713 L / h (10).

[0183] The solvent S2 is fed into the second battery with a flow rate of 8000 L / h (11).

[0184] An aqueous raffinate composed mainly of Sm, Eu, Gd and Tb is obtained (12). An extract comprising 237.35 g / L of Dy is recovered (13). The solvent undergoes washing before being introduced for re-extraction by injecting part of the extract into the washing section, with a flow rate of 404 L / h (14). Aqueous effluents (approximately 1172 L / h) are recovered at the outlet of the second battery section (15). They consist of sodium chloride and are discharged into the environment.

[0185] This process uses only one solvent, solvent S2.

[0186] The summary diagram of the process of comparative example 3 is shown in Figure 6.

[0187] The quantities of rare earths and the flow rates are reported in Tables 13 and 14.

[0188] Table 13 Table 14 Comparison of the processes of Example 3 and Comparative Example 3 with respect to the consumption of reagents and the production of saline effluents

[0189] The process according to the invention makes it possible to reduce the molar consumption of acid by 30% (43.0 moles of acid per kg of heavy rare earths compared to 62.0 moles for the process which is not according to the invention) and the molar consumption of base by 54% (15.8 moles per kg of heavy rare earths compared to 34.6 moles for the process which is not according to the invention). Furthermore, the process of the invention generates ammonium nitrate in a quantity substantially lower than the sodium chloride generated by the process which is not according to the invention (1.26 kg per kg of heavy rare earths compared to 2.02 kg for the process which is not according to the invention). In addition, the ammonium nitrate can be used as a fertilizer, while the sodium chloride is released into the environment.

[0190] The values ​​are reported in Table 15.

[0191] Table 15

[0192] Table 16 summarizes the different examples detailed above. Table 16

Claims

CLAIMS 1. Process for producing a mixture of terbium-dysprosium or dysprosium from a mixture of rare earths in solution, said process comprising at least two consecutive steps of separation by liquid-liquid extraction, carried out using two different solvents, SI and S2: ■ the first separation step is carried out using the solvent SI which includes at least one anionic extraction agent, ■ the second separation step is carried out using solvent S2 which includes at least one cationic extraction agent.

2. A method of producing a terbium-dysprosium or dysprosium mixture according to claim 1 wherein the mixture of rare earths in solution comprises a first group of rare earths TRLM comprising the elements Sm, Eu, Gd, Tb and Dy, and a second group of rare earths TRLL comprising the elements Ho, Er, Tm, Yb, Lu and Y.

3. A method for producing a terbium-dysprosium or dysprosium mixture according to claim 2, wherein the first separation step separates the elements included in the TRLL group from the elements included in the TRLM group.

4. A method for producing a terbium-dysprosium or dysprosium mixture according to one of claims 2 and 3, wherein the second separation step separates the elements included in the TRLM group from each other to isolate the terbium-dysprosium or dysprosium mixture.

5. A method for producing a terbium-dysprosium or dysprosium mixture according to one of claims 1 to 4, wherein the mixture of rare earths in solution is a solution derived from ores rich in heavy rare earths, preferably xenotime or ionic clays.

6. A method for producing a terbium-dysprosium or dysprosium mixture according to one of claims 2 to 5, wherein the mixture of rare earths in solution comprises at least 50 mol% of TRLL, preferably at least 70 mol% of TRLL.

7. A method for producing a terbium-dysprosium or dysprosium mixture according to one of claims 2 to 6, wherein the mixture of rare earths in solution is subjected to a first separation step using the solvent SI, generating a fraction comprising the elements included in the TRLM group, said fraction subsequently being subjected to a second separation step using the solvent S2, the product of said second separation being a terbium-dysprosium or dysprosium mixture.

8. A method for producing a terbium-dysprosium or dysprosium mixture according to one of claims 2 to 5, wherein the mixture of rare earths in solution is subjected to a first separation step using the solvent SI, generating a fraction comprising the elements included in the TRLM group, said fraction being subsequently mixed with a solution pre-enriched in heavy rare earths from ores rich in light rare earths, preferably monazite or bastnaesite, said fraction being subjected to a second separation step using the solvent S2.

9. A method for producing a terbium-dysprosium or dysprosium mixture according to claim 8, wherein the product of the second separation step containing terbium and dysprosium is subjected to a third separation step using solvent S2 generating a terbium-dysprosium or dysprosium mixture.

10. Process for producing a mixture of terbium-dysprosium or dysprosium according to one of claims 1 to 9, in which said separation steps are carried out by means of batteries of mixer-decanters or liquid-liquid extraction columns.

11. A process for producing a terbium-dysprosium or dysprosium mixture according to one of claims 1 to 10, wherein the anionic extracting agent is methyl trialkyl ammonium nitrate.

12. A method for producing a terbium-dysprosium or dysprosium mixture according to one of claims 1 to 11, wherein the cationic extracting agent is selected from the group consisting of alkyl-phosphonic derivatives, alkyl-phosphinic derivatives and mixtures thereof.

13. A method for producing a mixture of terbium-dysprosium or dysprosium according to one of claims 1 to 12, wherein the mixture of rare earths in solution is a mixture of rare earth nitrates in solution.

14. A method for producing a mixture of terbium-dysprosium or dysprosium according to one of claims 1 to 13, wherein the solvents S1 and S2 further comprise aliphatic or aromatic hydrocarbons of 5 to 16 carbon atoms, and optionally at least one modifier chosen from fatty alcohols.

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