The method involves removing actinide present in rare earth solutions using phosphorus-containing organic acid solvents, and the rare earth preparation obtained by this method.

Abstract

The invention relates to a method for removing actinium from an aqueous solution containing rare earth sulfates, which includes the following steps: - contacting and mixing the aqueous solution with an organic solvent containing at least one cationic extractant; - separating the rare earth elements and actinium by liquid-liquid extraction; - contacting the organic solvent containing the rare earth elements, except for a portion of lanthanum, with an acidic solution, to produce a rare earth solution of which the actinium content is lower than the actinium content in the aqueous solution; this rare earth solution forms a rare earth preparation containing the rare earth elements Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, and the La content in this rare earth preparation is lower than the La content in the aqueous solution.

Description

[0001] METHOD FOR REMOVING ACTINIUM CONTAINED IN A RARE EARTH SOLUTION BY AN ORGANOPHOSPHORUS SOLVENT AND RARE EARTH COMPOSITIONS OBTAINED BY THIS METHOD FIELD OF THE INVENTION The invention falls within the general field of the treatment of aqueous solutions of rare earths. More specifically, it relates to a method for removing a radioactive compound from an aqueous solution, resulting from the treatment of rare earth ores, by liquid-liquid extraction. The invention also relates to a rare earth composition resulting from the liquid / liquid extraction of an initial aqueous solution of rare earth sulfates. PRIOR ART The ores from which rare earths are extracted may contain radioactive compounds, which must be separated from the rare earths. For example, monazite comprises all the radioactive isotopes of the thorium 232 families ( 232 Th), uranium 235 ( 235 U) and uranium 238 (238 U). Ores generally first undergo a first processing stage which consists of physical concentration (magnetic separation, flotation, etc.). However, during this stage, the radioactive isotopes present in the ore are not separated from the rare earths. During a second stage, chemical treatments are carried out on the concentrate obtained previously. During these chemical treatments, the radioactive equilibrium of the three radioactive families mentioned above is broken and each radioelement will evolve according to its own chemical properties, independently of the other elements in the radioactive family. Some isotopes decay quickly, while others, which have a radioactive half-life greater than a few days, must be eliminated to avoid contamination of the rare earths. For example, the following isotopes have a relatively long radioactive half-life: • Family of 232 Th: 232 Th, 228 Th, 228Ra, • Family of 238 U: 238 U, 234 U, 234 Th, 230 Th, 226 Ra, 210 Pb, 210 Po, • Family of 235 U: 235 U, 231 Pa, 227 Ac, 223 Ra. Most of these elements have chemical properties sufficiently different from those of the rare earths that their removal can be carried out using conventional hydrometallurgical techniques, such as precipitation and filtration. This is the case, for example, with the isotopes of thorium, uranium, radium (Ra), lead (Pb), polonium (Po) and protactinium (Pa). On the other hand, the case of the isotope 227Ac, which has a radioactive half-life of 22 years, is unique because its chemical properties are similar to those of rare earths, particularly lanthanum. When separating rare earths, actinium will follow the lanthanum and may accumulate in compounds containing lanthanum. It will then have to be disposed of in the radioactive waste stream, which requires additional costs. It is therefore necessary to implement processes for separating rare earths and actinium, allowing the isotope to be efficiently removed. 227Ac. Such methods for removing actinium are described, for example, in WO 2019 / 000014 where actinium is precipitated from solutions of rare earth chlorides or nitrates, which come from the processing of monazite or xenotime ores, by adding sulfate salts. The disadvantage of this method is that it requires the addition of large quantities of salt (in the order of hundreds of g / L) to cause precipitation. In addition, this method is relatively time-consuming. Furthermore, it does not allow sufficient removal of actinium. Other methods describe the separation by liquid-liquid extraction of rare earths and actinium. This extraction is carried out using organic solvents comprising molecules selective for rare earths, which are extracted into the organic phase while the actinium remains in the aqueous phase.Indeed, it is known to use as extracting agents, cationic agents that have a strong affinity with rare earths, such as organophosphorus acid derivatives. For example, document CN 85100148 describes a process for producing lanthanum oxides, by separating lanthanum and actinium present in a solution of rare earth nitrates. The liquid-liquid separation is carried out using an organic solvent comprising mono-2-ethylhexyl ester of 2-ethylhexylphosphonic acid (HEHEHP) or di(2-ethylhexyl)phosphoric acid ester (HDEHP or D2EHPA). Similarly, document KR 100351554 details a method for liquid-liquid separation of rare earths and actinides contained in solutions of rare earth nitrates, originating from radioactive liquid waste. The process involves the use of tributyl phosphate and phosphoric acid esters in the form of metal salts.Another problem related to the presence of actinium in rare earth solutions concerns transport. Indeed, an ore with a uranium content of 5000 ppm compared to the sum of the rare earths contained results in a content of. 227Ac which generates an activity equivalent to approximately 2.4 Bq / g of rare earths. However, beyond 1 Bq / g, the transport of such a mixture is subject to strict regulations concerning the transport of radioactive materials, known as "class 7". This requires the use of specialized carriers, which constitutes an additional constraint and cost. It is therefore necessary to limit the stages of transport of such products, in particular by directly treating the solutions from the ores, without resorting to intermediate stages. In addition, during rare earth production operations, intermediate manufacturing concentrates are subject to strict radiation protection regulations. In particular, concentrates whose activity linked to the presence of 227 Ac exceeds 0.1 Bq / g requires special monitoring of workers. E XPOSÉ DE L ' INVENTIONOne of the aims of the invention is to eliminate actinium from the production of a rare earth solution resulting from ore processing, to avoid the costly production, transport and subsequent processing of radioactive intermediate rare earth concentrates. This elimination of actinium is accompanied by the elimination of a portion of the lanthanum. Since this rare earth has little commercial value, this makes it possible to considerably reduce the total mass of the intermediate rare earth concentrate to be transported and very significantly reduces the cost of the separation processes implemented by users of said concentrate. Thus, the invention relates to a method for eliminating actinium, as well as a portion of the lanthanum, contained in an initial aqueous solution of rare earth sulfates, said aqueous solution being derived from ore processing.The method comprises the following steps: • contacting and mixing the initial aqueous solution with a water-immiscible organic solvent, said organic solvent comprising at least one cationic extraction agent, • separating by liquid-liquid extraction the rare earths and actinium, the rare earths, with the exception of a portion of the lanthanum and optionally cerium, being extracted in the organic solvent, • contacting the organic solvent comprising the rare earths, with the exception of a portion of the lanthanum, with an acid solution, to produce a rare earth solution having an actinium concentration lower than the actinium concentration of the initial aqueous solution of rare earth sulfates, said rare earth solution constituting a rare earth composition comprising the rare earths Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, and having a La concentration lower than that of the initial aqueous solution.The invention also relates to a rare earth composition resulting from the process according to the invention. The rare earth composition has a content of. 227 Ac less than 0.10 Bq / g of rare earths, preferably less than 0.01 Bq / g of rare earths, the alkaline earth and lead content of said rare earth composition being less than 1000 ppm relative to the amount of rare earths, preferably less than 100 ppm relative to the amount of rare earths, and said rare earth composition being in the form of a solid or in the form of an aqueous solution. ORGANIC SOLVENT According to the invention, the organic solvent comprises at least one cationic extraction agent which is an organophosphorus acid. Preferably, the cationic extraction agent is an organophosphorus acid chosen from the group comprising: - monoacid diesters of phosphoric acid, of formula I: - monoacid esters of phosphonic acids, of formula II: - formula III: and mixtures thereof, where R1 and R2 are the same or different, independently selected from the group consisting of linear or branched, aliphatic or aromatic alkyl or aryl radicals, of 1 to 20 carbon atoms, preferably of 4 to 20 carbon atoms. These compounds are particularly effective for complexing trivalent cations, such as rare earth cations. According to the invention, the rare earths are selected from the group comprising: scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and mixtures thereof. Advantageously, the cationic extraction agent may be chosen from the group consisting of di(2-ethylhexyl)phosphoric acid ester, mono-2-ethylhexyl ester of 2-ethylhexylphosphonic acid, bis(2,4,4-trimethylpentyl)phosphinic acid (known under the trade name CYANEX ® 272), the CYANEX® 572 (mixture of phosphonic acid and phosphinic acid), and mixtures thereof. Additionally, the organic solvent may comprise a diluent compound, selected from the group consisting of hydrocarbons of 5 to 16 carbon atoms, linear or branched, aliphatic or aromatic. For example, the diluent compound is selected from the group comprising: decane and its isomers, dodecane and its isomers, kerosene, toluene, aliphatic hydrocarbons having between 10 and 14 carbon atoms, hydrocarbons of 10 carbon atoms comprising at least one aromatic function, and mixtures thereof. As a diluent compound, mention may be made, for example, of Shellsol ® D70 (aliphatic hydrocarbon), Solvesso ®150 (aromatic hydrocarbon). The diluting compound dissolves the cationic extractant, and optionally the modifying compound. It is different from the cationic extractant. In addition to the cationic extraction agent and the diluent compound, the solvent may comprise at least one modifying compound, selected from the group consisting of neutral phosphoric acid esters, neutral phosphonic acid esters, linear or branched fatty alcohols, sulfoxides, carboxylic acids with linear or branched alkyl chains, the total number of carbon atoms of which is between 6 and 16. Preferably, the modifying compound is selected from the group comprising: tributyl phosphate, decan-1-ol, octan-1-ol, isodecan-1-ol, hexan-1-ol, dodecan-1-ol, 2-ethylhexanol, dibutyl sulfoxide, dihexyl sulfoxide, mixtures of sulfoxides derived from petroleum, 2-ethylhexanoic acid, neodecanoic acid, and mixtures thereof.For the purposes of the invention, the term "modifying compound" denotes a chemical species other than the cationic extraction agent and the diluent compound, capable of modifying certain properties of the organic solvent. For example, the modifying compound can delay the appearance of a third phase, assist in decantation or modify the extraction efficiencies. According to a particular embodiment, the organic solvent comprises at least one cationic extraction agent and at least one diluent compound. Alternatively, the organic solvent may comprise at least one cationic extraction agent, at least one diluent compound and at least one modifying compound. Preferably, the organic solvent has a concentration of cationic extraction agent of between 0.1 and 2 mol / L, preferably between 0.3 and 1.5 mol / L. Such a concentration allows the solvent to maintain a viscosity of less than 20 cP at room temperature, i.e. between 20°C and 27°C, preferably at 25°C.In a particular embodiment of the invention, the organic solvent comprises: - from 15% to 100% of at least one cationic extraction agent, - from 0% to 85% of at least one diluent compound, by weight, relative to the total weight of the organic solvent, the total being equal to 100%. In another embodiment of the invention, the organic solvent comprises: - from 15% to 70% of at least one cationic extraction agent, - from 0% to 85% of at least one diluent compound, - from 0% to 50% of at least one modifying compound, by weight, relative to the total weight of the organic solvent, the total being equal to 100%. INITIAL AQUEOUS SOLUTION OF RARE EARTH SULFATES The concentration of rare earths in the initial aqueous solution of earth sulfates can be between 100 mg / L and 40 g / L, or between 0.001 and 0.3 mol / L.Preferably, the initial aqueous solution of rare earth sulfates is obtained from the chemical treatment of a xenotime and / or monazite type ore concentrate, in particular with sulfuric acid. This concentrate can be obtained by physical concentration of the ores. Examples of physical concentration methods include magnetic separation and flotation, but the present invention is not limited to these methods. Alternatively, the initial aqueous solution of rare earth sulfates can be obtained from the treatment of ionic clays with ammonium sulfate, alkali sulfates or magnesium sulfate. Depending on the nature of the ore from which the solution is obtained, the activity of the initial aqueous solution is linked to the content of. 227Ac, is between 0.1 and 25 Bq / g of rare earths. LIQUID-LIQUID EXTRACTION In practice, the contacting and mixing of the initial aqueous solution of rare earth sulfates with the organic solvent can be carried out in a device comprising several stages, for example a battery of mixer-settlers or an extraction column. In this device, the initial aqueous solution and the organic solvent advantageously circulate countercurrently. Advantageously, the process is carried out at a temperature between 20°C and 60°C, preferably between 30°C and 50°C. Preferably, the device comprises an extraction section and a re-extraction section. It may also comprise a washing section. Each section may consist of 1 to 10 stages. According to one embodiment, the solvent may be saponified by a basic solution (ammonia, magnesia, etc.) with an OH- ion concentration of between 0.5 and 10 mol / L.Once the rare earths have been extracted into the organic solvent, the latter is brought into contact with an acid solution which is chosen from the group consisting of nitric acid, sulfuric acid or hydrochloric acid solutions. This contacting or washing of the organic solvent can be carried out in the washing section. In practice, a rare earth composition is obtained at the end of the liquid-liquid extraction comprising all or part of the rare earths initially present in the initial aqueous solution but whose La content is reduced. This composition, which is a solution of nitrates, sulfates or chlorides of rare earths, advantageously has an activity, linked to the actinium content, of less than 0.1 Bq / g of rare earths, preferably less than 0.01 Bq / g of rare earths.The method according to the invention therefore has the advantage of reducing the relative content of La compared to the other rare earths contained in the rare earth composition, compared to the initial aqueous solution of rare earth sulfates. Preferably, the mass percentage of lanthanum compared to the total mass percentage of rare earths is lower than that of the initial aqueous solution of rare earth sulfates. Preferably, the rare earth composition does not include lanthanum. The reduction in the La content is important from an economic point of view because, on the one hand, it makes it possible to reduce transport costs, if applicable, and on the other hand, it reduces the cost of separations carried out downstream, leading to the separated rare earths, in particular the production costs of pure praseodymium, neodymium, terbium and dysprosium.In general, an aqueous raffinate is also obtained whose lanthanum content relative to other rare earths is greater than 90%. According to one embodiment, the rare earth composition is precipitated using a precipitating agent selected from the group comprising: sodium carbonate, ammonium bicarbonate, sodium hydroxide, ammonia and oxalic acid or its alkali salts. The precipitation of the composition results in the formation of rare earth carbonates, oxycarbonates, hydroxycarbonates, hydroxides, hydroxynitrates or oxalates. Advantageously, the precipitates are calcined to obtain oxides. According to another embodiment, the rare earth composition is crystallized or cast at high temperature, preferably between 110°C and 150°C, preferably at atmospheric pressure.For example, crystallization of rare earth nitrate or chloride solutions generates crystallized nitrates or crystallized chlorides, while high-temperature casting of these solutions and subsequent cooling results in the formation of anhydrous nitrates or anhydrous chlorides. RARE EARTH COMPOSITION In practice, the rare earth composition has a total activity of less than 0.10 Bq / g of rare earths, preferably less than 0.01 Bq / g of rare earths. The total activity is related to the radioelement content, e.g. 226 Ra, 228 Ra, 232 Th, 238 U or 231 Pa, and their descendants of the composition. This composition advantageously has a content of 227Ac less than 0.10 Bq / g of rare earths. This rare earth composition may contain all or part of the rare earths and advantageously has a relative La content compared to the other rare earths reduced compared to the content in the initial aqueous solution. The rare earth composition is in the form of a solid or in the form of an aqueous solution. When the composition is in the form of a solid, it is in the form of a solid chosen from the group consisting of: carbonates, hydroxycarbonates, oxycarbonates, hydroxides, oxides, oxalates, hydroxynitrates, crystallized nitrates, anhydrous nitrates, crystallized chlorides and anhydrous chlorides of rare earths. When the composition is in the form of an aqueous solution, the rare earth composition is an aqueous solution chosen from the group consisting of aqueous solutions of nitrates, sulfates or chlorides of rare earths.Advantageously, the concentration of rare earths in the aqueous solution is between 1 g / L and 500 g / L. Preferably, the concentration of rare earths is between 50 and 450 g / L in a nitrate solution, between 5 and 30 g / L in an aqueous sulfate solution and between 50 and 200 g / L in an aqueous chloride solution. DETAILED DESCRIPTION OF THE FIGURES Figure 1 is a schematic representation of the process according to Example 1 of the invention, in a liquid-liquid extraction battery consisting of three sections. Figure 2 is a schematic representation of the process according to Example 2 of the invention, in a liquid-liquid extraction battery consisting of two sections. Figure 3 is a schematic representation of the process according to Example 3 of the invention, in a liquid-liquid extraction battery consisting of three sections.EXAMPLES OF CARRYING OUT THE INVENTION The examples below were simulated using software called “PAREX+”, implemented by the Applicant, the basic data of which necessary for the calculations were previously obtained by laboratory tests according to the different chemical systems presented. Two types of aqueous solutions resulting from the chemical treatment of rare earth ores are considered: - an aqueous solution of rare earth sulfates resulting from the treatment of a raw material of the ionic clay type; - an aqueous solution of rare earth sulfates resulting from the treatment of a raw material of the xenotime type.The examples below have been calculated for a battery-type installation of mixer-decanters operating continuously in counter-current and presenting the following performances: - the extraction efficiency of all rare earths with atomic number higher than cerium (Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) as well as that of yttrium is higher than 99%, - the purification rate of. 227 Ac relative to rare earths, which is equal to the ratio between the content of 2 27 Ac per gram of rare earths in the aqueous feed and the content of 227 Ac per gram of rare earths in the extracted aqueous solution is greater than 100. Example 1: Removal of 227Ac of a solution of rare earth sulfates obtained by chemical treatment of ionic clays with a solution of (NH4)2SO4The method according to the invention is carried out using the following device: a liquid-liquid extraction battery of the mixer-settler type operating in countercurrent, consisting of a one-stage loading section (34), a two-stage extraction section (35), a two-stage washing section (36) and a seven-stage re-extraction section (37), these four sections being interconnected. The extraction section (35) is fed with an aqueous solution (8) comprising a mixture of rare earth sulfates (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) and 227Ac, with a flow rate of 120800 L / h, the composition of which is detailed in Table 1. Solvent S1 (11), composed of 67% by volume of aliphatic kerosene and 33% by volume of HEHEHP (where HEHEHP is the cationic extraction agent), is injected at a flow rate of 10000 L / h into the loading section (34). Solvent S1 is saponified by an ammonia solution (9) with an OH- concentration of 4 mol / L, at a flow rate of 1000 L / h. An aqueous raffinate (12) comprising mainly La as well as almost all of the 227Ac is obtained with a flow rate of 126323 L / h. The measured activity is 27 Bq / g of rare earths. The solvent S1 is washed with a sulfuric acid solution (13) at a concentration of 0.7 mol / L, with a flow rate of 4248 L / h in the washing section (36). An aqueous nitric acid solution (10) with a concentration of 3.5 mol / L is fed into the re-extraction section (37) with a flow rate of 2523 L / h, thus allowing the regeneration of the solvent. An aqueous extract (14) of rare earths, whose La content has been significantly reduced compared to that of the feed solution, is obtained with a flow rate of 2636 L / h. This extract has a rare earth concentration of 31 g / L and an activity, related to the content of 227 Ac, from 1.3.10 -3 Bq / g of rare earths. The purification rate of 227 Ac relative to rare earths, which is equal to the ratio between the content of 227 Ac per gram of rare earths in the aqueous feed (8) and the content of 227Ac per gram of rare earths in the extracted aqueous solution (14) is under these conditions close to 5000. 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 table 1.

[0002] Table 1 Step Feed. Load. Reex. S1 Ref. Wash. Ext. Flow 8 9 10 11 12 13 14 Nature Aqueous Aqueous Aqueous Organic Aqueous Aqueous Aqueous Flow (L / h) 120800 1000 2523 10000 126323 4248 2636 TR Flow (kg TR / h) 106 25 81 La (g / L) 0.32 0.20 5.2 Ce (g / L) 0.04 0.0002 1.9 Pr (g / L) 0.05 <0.0001 2.4 Nd (g / L) 0.16 <0.0001 7.2 Sm (g / L) 0.02 <0.0001 1.1 Eu (g / L) <0.10 <0.0001 0.08 Gd (g / L) 0.02 <0.0001 1.1 Tb (g / L) <0.01 <0.0001 0.21 Dy (g / L) 0.02 <0.0001 1.1 Ho (g / L) 0.01 <0.0001 0.25 Er (g / L) 0.01 <0.0001 0.68 Tm (g / L) <0.001 <0.0001 0.08 Yb (g / L) 0.01 <0.0001 0.55 Lu (g / L) <0.001 <0.0001 0.08 Y (g / L) 0.19 <0.0001 8.8 2 27 Ac (Bq / g TR ) 6.35 27 1.2910 -3OH- (mol / L) 4 H2SO4 (mol / L) 0.001 0.00598 0.66HNO3 (mol / L) 3.5 2.32 HEHEHP (%vol) 33Kerosene (%vol) 67 The abbreviations correspond to the following steps: Feed: feeding of the aqueous rare earth solution (8) into the extraction section (35) Charge: loading of the ammonia solution (9) into the loading section (34) Reex: re-extraction of the rare earths by adding an aqueous nitric acid solution (10) into the re-extraction section (37) Ref: aqueous raffinate (12) from the loading section (34) Wash: washing, regeneration of the solvent using a sulfuric acid solution (13) in the washing section (36) Ext. : aqueous extract (14) from the re-extraction section (37) TR: rare earths Nature: nature of the flow gTR: gram of rare earths kgTR: kilogram of rare earths The extract solution (14) has an activity in 227 Ac equal to 1.3x10 -3 Bq / g of rare earths, or 4x10 -2Bq / liter of solution. This solution is precipitated in the form of carbonate by adding ammonium bicarbonate. After filtration, a wet rare earth carbonate is obtained containing 30% moisture and 40.4% rare earths. This solid has an activity in 227 Ac equal to 1.3x10 -3 Bq / g of rare earths, or 5.2x10 -4 Bq / g of solid. This solid is then calcined at 900°C. This produces an oxide containing 83.2% rare earths. This solid has an activity in 227 Ac equal to 1.3x10 -3 Bq / g of rare earths, or 1.1x10 -3 Bq / g of solid. Example 2: Elimination of 227Ac of a solution of rare earth sulfates obtained by treatment at 500°C of a xenotime concentrate with a sulfuric acid solution. The method according to the invention is carried out using the following device: a liquid-liquid extraction battery of the mixer-decanter type operating countercurrently, continuously, consisting of a loading section (38) of one stage, a two-stage extraction section (39), a washing section of one stage (40) and a re-extraction section of seven stages (41), these four sections being interconnected. The extraction section (39) is fed with an aqueous solution (15) comprising a mixture of rare earth sulfates (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) and 227Ac, with a flow rate of 7927 L / h, the composition of which is detailed in Table 2. The solvent S1 (18), composed of 67% by volume of aliphatic kerosene and 33% by volume of HEHEHP (cationic extraction agent) is injected at a flow rate of 6650 L / h into the loading section (38). The solvent S1 is saponified by an ammonia solution (16) with an OH- concentration of 4 mol / L at a flow rate of 665 L / h. An aqueous raffinate (19) comprising a very large majority of La as well as almost all of the 227Ac is obtained with a flow rate of 8966 L / h. The measured activity is 167 Bq / g of rare earths. The solvent S1 is washed with a sulfuric acid solution (20) at a concentration of 0.4 mol / L with a flow rate of 84 L / h in the washing section (40). An aqueous nitric acid solution (17) with a concentration of 3.5 mol / L is fed into the re-extraction section (41) with a flow rate of 2671 L / h. An aqueous extract (21) of rare earths, whose La content has been significantly reduced compared to that of the feed solution, is obtained with a flow rate of 2798 L / h. It has a rare earth concentration of 37 g / L and an activity, linked to the content of 227 Ac, from 2.27.10 -4 Bq / g of rare earths. The purification rate of 227 Ac, which is equal to the ratio between the content of 227 Ac per gram of rare earths in the aqueous feed (15) and the content of 227Ac per gram of rare earths in the extracted aqueous solution (21) is close to 5200. The summary diagram of the process of example 2 according to the invention is shown in figure 2. The quantities of rare earths and the flow rates are shown in table 2. Table 2 Feed. Charge. Reex. Solvent S1 Refine. Wash Ext. Flow 15 16 17 18 19 20 21 Nature Aqueous Aqueous Aqueous Organic Aqueous Aqueous Aqueous Flow (L / h) 7 927 665 2671 6650 8966 84 2798 TR Flow (kgTR / h) 104 1 103 La (g / L) 0.29 0.080 0.57Ce (g / L) 0.69 0.003 1.95 Pr (g / L) 0.10 <0.0001 0.28Nd (g / L) 0.46 <0.0001 1.31Sm (g / L) 0.30 <0.0001 0.85Eu (g / L) 0.07 <0.0001 0.18 Gd (g / L) 0.82 <0.0001 2.31Tb (g / L) 0.18 <0.0001 0.50Dy (g / L) 1.20 <0.0001 3.41 Ho (g / L) 0.26 <0.0001 0.73Er (g / L) 0.75 <0.0001 2.11Tm (g / L) 0.10 <0.0001 0.29 Yb (g / L) 0.61 <0.0001 1.73Lu (g / L) 0.08 <0.0001 0.24 Y (g / L) 7.20 <0.0001 20.39 2 27 Ac (Bq / gTR) 1.18 167 2.27.10 -4 O H - (mol / L) 4H2SO4(mol / L) 0.001 0.02 0.4HNO3 (mol / L) 3.5 2.34HEHEHP (%vol) 33Kerosene (%vol) 67 The abbreviations correspond to the following steps: Feed: feeding of the aqueous rare earth solution (15) into the extraction section (39) Charge: loading of the ammonia solution (16) into the loading section (38) Reex: re-extraction of the rare earths by adding an aqueous nitric acid solution (16) into the re-extraction section (41) Raff: aqueous raffinate (19) from the loading section (38) Ext. : aqueous extract (21) from the re-extraction section (41) TR: rare earths Nature: nature of the flow gTR: gram of rare earths kgTR: kilogram of rare earths The extract solution (21) has an activity in 227 Ac equal to 2.3x10 -4 Bq / g of rare earths, or 8.4x10 -3Bq / liter of solution. This solution is precipitated in the form of oxalate by adding oxalic acid. After filtration, a wet rare earth oxalate containing 30% moisture and 31.8% rare earths is obtained. This solid has an activity in 227 Ac equal to 2.3x10 -4 Bq / g of rare earths, or 7.2x10 -5 Bq / g of solid. This solid is then calcined at 900°C. This produces an oxide containing 83.2% rare earths. This solid has an activity in 227 Ac equal to 2.3x10-4 Bq / g rare earths, or 1.8x10 -4 Bq / g of solid. Example 3: Elimination of 227Ac of a solution of rare earth sulfates obtained by treatment at 500°C of a xenotime concentrate with a sulfuric acid solution. The method according to the invention is carried out using the following device: a liquid-liquid extraction battery of the mixer-decanter type operating countercurrently, continuously, consisting of a four-stage extraction section (42), a four-stage washing section (43) and a seven-stage re-extraction section (44), these three sections being interconnected. The extraction section (42) is fed with an aqueous solution (22) comprising a mixture of rare earth sulfates (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) and 227Ac, with a flow rate of 7927 L / h, the composition of which is detailed in Table 3. A solvent S2 (24), composed of 70% by volume of aliphatic kerosene and 30% by volume of HDEHP, (where HDEHP is the cationic extraction agent), is injected at a flow rate of 56000 L / h. An aqueous raffinate (25) comprising mainly La as well as almost all of the 227 Ac, is obtained with a flow rate of 8298 L / h. The measured activity is 105 Bq / g of rare earths. The solvent S2 is washed with a sulfuric acid solution (26) at a concentration +of 3.1 mol / L with a flow rate of 162 L / h in the washing section (43). An aqueous nitric acid solution (23) with a concentration of 5 mol / L is fed into the re-extraction section (44) with a flow rate of 36000 L / h. An aqueous extract (27) of rare earths, the La content of which has been significantly reduced compared to that of the feed solution, is obtained with a flow rate of 38775 L / h. This extract has a rare earth concentration of 2.7 g / L and an activity, linked to the content of 227 Ac, from 7,8.10 -3 Bq / g of rare earths. The purification rate in 227 Ac, which is equal to the ratio between the content of 227 Ac per gram of rare earths in the aqueous feed (22) and the content of 227 Ac per gram of rare earths in the extracted aqueous solution (27) is close to 150. The summary diagram of the process of example 3 according to the invention is shown in figure 3. The quantities of rare earths and the flow rates are shown in table 3.

[0003] Table 3 Step Power Supply Reex. S2 Refrig. Wash. Ext. Flow 22 23 24 25 26 27 Nature Aqueous Aqueous Organic Aqueous Aqueous Aqueous Flow (L / h) 7927 36000 56000 8298 162 38775 TR Flow (kgTR / h) 104 1 103 La (g / L) 0.29 0.130 0.03 Ce (g / L) 0.69 0.009 0.14 Pr (g / L) 0.10 <0.0001 0.02 Nd (g / L) 0.46 <0.0001 0.09 Sm (g / L) 0.30 <0.0001 0.06 Eu (g / L) 0.07 <0.0001 0.01 Gd (g / L) 0.82 <0.0001 0.17 Tb (g / L) 0.18 <0.0001 0.04 Dy (g / L) 1.20 <0.0001 0.25 Ho (g / L) 0.26 <0.0001 0.05 Er (g / L) 0.75 <0.0001 0.15 Tm (g / L) 0.10 <0.0001 0.02 Yb (g / L) 0.61 <0.0001 0.12 Lu (g / L) 0.08 <0.0001 0.02 Y (g / L) 7.20 <0.0001 1.47 2 27 Ac (Bq / gTR) 1.18 105 7.83.10 -3 H2SO4(mol / L) 0.001 0.40 3.1 HNO3 (N) 5 4.6 H DEHP (%vol) 30Kerosene (%vol) 70 The abbreviations correspond to the following steps: Feed: feeding of the aqueous solution of rare earths (22) into the extraction section (42) Reex: re-extraction of the rare earths by adding an aqueous solution of nitric acid (23) into the re-extraction section (44) Raff: aqueous raffinate (25) from the extraction section (42) Wash: washing, regeneration of the solvent (24) using a sulfuric acid solution (26) in the washing section (43) Ext: aqueous extract (27) from the re-extraction section (44) TR: rare earths Nature: nature of the flow g TR : gram of rare earths The extract solution (27) has an activity in 227 Ac equal to 7.8x10 -3 Bq / g of rare earths, or 2.1x10 -2Bq / liter of solution. This solution is precipitated in the form of hydroxynitrate by adding ammonia. After filtration, a wet rare earth hydroxynitrate containing 40% moisture and 36% rare earths is obtained. This solid has an activity in 227 Ac equal to 7.8x10 -3 Bq / g of rare earths, or 2.8 x 10 -3 Bq / g of solid. This solid is then calcined at 900°C. This produces an oxide containing 81.8% rare earths. This solid has an activity in 227 Ac equal to 7.8x10 -3 Bq / g rare earths, or 6.4x10 -3Bq / g of solid. In addition to the disadvantages of the state of the art listed above, it appears that certain prior art processes, such as that described in the publication “Cleaning rare earth elements from actinium”, Journal of Alloys and Compounds, vol. 225 (1995), pages 320-323, can only be implemented if the initial aqueous solution of rare earths is a solution of rare earth nitrates. This is also the case for the process described in document CN 85100148, which is carried out to purify solutions of lanthanum nitrates only. Furthermore, example 3 of document WO 2019 / 000014, which describes a process for purifying actinium by precipitation, applicable to solutions of rare earth sulfates, has a precipitation rate of 227 Ac for contents in 227 High Ac (70 Bq / L) of approximately 47%, for a rare earth precipitation rate of 2%. The purification rate in 227Ac is therefore: (1 − 2%)⁄ (1 − 47%) = 1.85 Which is clearly insufficient to make the solution compliant with international standards governing the transport of non-radioactive products.

Claims

CLAIMS 1. A method for removing actinium from an initial aqueous solution of rare earth sulfates, said aqueous solution being derived from ore processing, said method comprising the following steps: ^ contacting and mixing the initial aqueous solution with a water-immiscible organic solvent, said organic solvent comprising at least one cationic extraction agent, ^ separating the rare earths and actinium by liquid-liquid extraction, the rare earths except for a portion of the lanthanum being extracted into the organic solvent, ^ contacting the organic solvent comprising the rare earths, except for a portion of the lanthanum, with an acid solution, to produce a rare earth solution having an actinium concentration lower than the actinium concentration of the initial aqueous solution of rare earth sulfates,said rare earth solution constituting a rare earth composition comprising the rare earths Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, and having a La concentration lower than that of the initial aqueous solution.

2. A process for removing actinium according to claim 1, wherein the cationic extraction agent is an organophosphorus acid selected from the group comprising: ^ monoacid diesters of phosphoric acid, of formula I:, ^ monoacid esters of phosphonic acid, of formula II: ^ phosphinic acids, of formula III: and their where R1 and R2 are identical or different, independently selected from the group consisting of alkyl or aryl radicals, linear or branched, aliphatic or aromatic, of 1 to 20 carbon atoms, preferably of 4 to 20 carbon atoms.

3. Method for removing actinium according to claim 2, in which the cationic extraction agent is selected from the group consisting of di(2-ethylhexyl)phosphoric acid ester, mono-2-ethylhexyl ester of 2-ethylhexylphosphonic acid, bis(2,4,4-trimethylpentyl)phosphinic acid, and mixtures thereof.

4. Method for removing actinium according to one of claims 1 to 3, in which the organic solvent further comprises a diluent compound, chosen from the group consisting of hydrocarbons with 5 to 16 carbon atoms, linear or branched, aliphatic or aromatic. 5.Method for removing actinium according to one of claims 1 to 4, wherein the organic solvent has a concentration of cationic extraction agent of between 0.1 and 2 mol / L.

6. Method for removing actinium according to one of claims 1 to 5, wherein the initial aqueous solution of rare earth sulfates is obtained from chemical treatments of a concentrate of xenotime and / or monazite, in particular with sulfuric acid.

7. Method for removing actinium according to one of claims 1 to 5, wherein the initial aqueous solution of rare earth sulfates is obtained from the treatment of ionic clays with ammonium sulfate, alkali sulfates or magnesium sulfate.

8. Method for removing actinium according to one of claims 1 to 7, in which the concentration of rare earths in the initial aqueous solution of rare earth sulfates is between 0.001 and 0.30 mol / L.

9. Method for removing actinium according to one of claims 1 to 8, wherein the contacting of the initial aqueous solution of rare earth sulfates with the organic solvent is carried out in a device comprising several stages, where said aqueous solution and the organic solvent circulate in countercurrent.

10. Method for removing actinium according to one of claims 1 to 9, wherein the acid solution is chosen from the group comprising nitric acid, sulfuric acid or hydrochloric acid solutions. 11.

12. A method for removing actinium according to one of claims 1 to 10, wherein the rare earth composition is a solution of rare earth nitrates, sulfates or chlorides having an activity, related to the actinium content, of less than 0.10 Bq / g of rare earths, preferably less than 0.01 Bq / g of rare earths, and the mass percentage of lanthanum relative to the total mass percentage of rare earths is lower than that of the initial aqueous solution of rare earth sulfates. 13.A method of removing actinium according to one of claims 1 to 11, wherein the rare earth composition is crystallized or cast at high temperature, preferably between 110°C and 150°C.

14. Rare earth composition resulting from the method according to one of claims 1 to 13, ^ said rare earth composition having a content of. 227 Ac less than 0.10 Bq / g of rare earths, preferably less than 0.01 Bq / g of rare earths, ^ the alkaline earth and lead content of said rare earth composition being less than 1000 ppm relative to the amount of rare earths, preferably less than 100 ppm relative to the amount of rare earths, and ^ said rare earth composition being in the form of a solid or in the form of an aqueous solution.

15. Rare earth composition according to claim 14, wherein the total activity is less than 0.10 Bq / g of rare earths, preferably less than 0.01 Bq / g of rare earths.

16. Rare earth composition according to claim 14 or 15, characterized in that the rare earths belong to the group comprising La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y.

17. Rare earth composition according to claim 14, 15 or 16, wherein the rare earth composition is an aqueous solution selected from the group comprising aqueous solutions of nitrates, sulfates, or chlorides of rare earths.

18. Rare earth composition according to claim 17, wherein the concentration of rare earths in the aqueous solution is between 1 g / L and 500 g / L. 19.Rare earth composition according to one of claims 14 to 16, wherein the rare earth composition is in the form of a solid selected from the group comprising carbonates, hydroxycarbonates, oxycarbonates, hydroxides, hydroxynitrates, oxides, oxalates, crystallized nitrates, anhydrous nitrates, crystallized chlorides and anhydrous chlorides of rare earths.

20. Rare earth composition according to one of claims 14 to 19, wherein the mass percentage of lanthanum relative to the total mass percentage of rare earths contained in the composition is lower than that of the initial aqueous solution of rare earth sulfates.