EP0541158A1 - Process for extracting cerium from an aqueous solution containing a mixture of rare earth elements - Google Patents

Abstract

Process for the selective extraction of cerium from an aqueous solution of rare-earth elements, according to which alkali metal hypochlorite (15) is added to the aqueous solution (13) while the pH is maintained at a value above 4.5, to precipitate the cerium in the form of ceric hydroxide and from the resulting slurry there are separated (19, 25, 28, 47), on the one hand, a precipitate of ceric hydroxide (48) and, on the other hand, a mother liquor (32) containing the other elements in the dissolved state. <IMAGE>

Description

The invention relates to a method for the selective extraction of cerium from an aqueous solution of salts of rare earth elements, comprising a dissolved cerium salt.

The industrial extraction of rare earth elements from their ore (for example monazite or bastnasite) involves obtaining an aqueous solution of salts of said elements, from which these elements are selectively collected by an operation of fractionation, which may include treatment on a cation exchanger. When the aqueous solution contains a high content of trivalent cerium salt, it is desirable to extract the latter from the solution before subjecting the latter to the fractionation operation. To this end, it has been proposed to oxidize the trivalent cerium to precipitate ceric hydroxide which is separated by filtration. Various oxidants have been suggested to oxidize trivalent cerium, in particular hydrogen peroxide and sodium hypochlorite (Kiyoe Umeda "Separação des Elementos das Terras Raras Individuais, por associação das Técnicas de Precipitação Homogênea e de Troca Iônica" Dissertação de mestrado, Escola Politécnica da Universidade de Sao Paulo, 1973; Pavanin, LA et al., Publ ACIESP n ° 36, 1982, pages 225-244, Chemical Abstracts n ° 98-208981; Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Vol. 5, 1979, pages 318-319).

In processes using sodium hypochlorite, this reagent is usually used in the form of an aqueous solution at a pH between 3 and 4.5. Although these known hypochlorite processes have given good results on a laboratory scale, they have proved impractical on an industrial scale, because of the very low filtration speed, due to the gelatinous nature of the reaction medium. subject to filtration. These known methods also require a significant amount of sodium hypochlorite, significantly greater than the stoichiometric amount.

The invention remedies these drawbacks by providing a means which makes it possible to facilitate and accelerate the separation of the precipitate of ceric hydroxide in the case of the hypochlorite process and thus to make this process operational on an industrial scale.

Consequently, the invention relates to a process for the selective extraction of cerium from an aqueous solution of a mixture of rare earth elements comprising a trivalent cerium salt, according to which alkali metal hypochlorite is added to the aqueous solution for precipitating the cerium in the state of ceric hydroxide and separating from the resulting broth, on the one hand a precipitate of ceric hydroxide and, on the other hand, mother liquor containing the other elements in the dissolved state ; according to the invention, the pH is maintained at a value greater than 4.5 during the addition of the alkali metal hypochlorite to the aqueous solution.

In the process according to the invention, the alkali metal hypochlorite must be used in an amount sufficient to convert all of the trivalent cerium salt into ceric hydroxide (tetravalent cerium hydroxide). The alkali metal hypochlorite is generally used in the form of an aqueous solution. This is advantageously an aqueous solution of sodium hypochlorite.

According to the invention, the pH of the reaction medium is maintained above 4.5 during the addition of the alkali metal hypochlorite to the aqueous solution. Maintaining the pH above 4.5 can be easily achieved by regulating the flow rate of the alkali metal hypochlorite solution.

The working temperature must be lower than the boiling temperature of the reaction mixture. It can advantageously be room temperature.

In the process according to the invention, maintaining the pH above 4.5 during the addition of the alkali metal hypochlorite has a double advantage. On the one hand, it reduces the consumption of alkali metal hypochlorite; on the other hand, it gives the slurry a remarkable fluidity, allowing easy and rapid separation of the precipitate of ceric hydroxide. The process according to the invention lends itself well to easy, rapid and economical separation of the ceric hydroxide by a decantation or filtration operation.

It is preferred, according to a particular embodiment of the process, to select a pH value of between 6 and 7, preferably between 6 and 6.5. These pH values optimize the subsequent separation of the ceric hydroxide and the mother water by a decantation and / or filtration operation.

After the addition of the alkali metal hypochlorite, the pH of the reaction medium tends to decrease in order to stabilize at a value substantially between 3.5 and 4.5 during the precipitation of the ceric hydroxide. After the ceric hydroxide has precipitated, it may prove advantageous, in accordance with the invention, to lower the pH further, for example between 2.5 and 3.5, so as to redissolve the hydroxides of the other rare earth elements which coprecipitated with ceric hydroxide. The pH can be lowered by adding hydrochloric acid to the broth.

The cerium content in mother water is practically negligible. The other rare earth elements are present in the form of dissolved salts. To collect them selectively, the mother liquor is subjected to a fractionation treatment. Solution fractionation treatments are well known in the technique of extracting rare earth elements and include, for example, treatment on a cation exchanger.

In the process according to the invention, the broth generally contains active chlorine which it is desirable to remove, for example before treating the mother water on a cation exchanger. To this end, in a particular embodiment of the process according to the invention, the broth is subjected to a dechlorination treatment by physical entrainment in an inert gas, and then a sufficient quantity of hyposulfite is added thereto. of alkali metal (eg sodium) to remove the active chlorine it contains. In this variant of the invention, the gas must be chosen from those which are inert with respect to ceric hydroxide and the constituents of mother water and which are insoluble in it. It can for example be air, argon or nitrogen. Air is preferred.

In the process according to the invention, the origin of the aqueous solution of salts of rare earth elements is not critical.

The invention finds, for example, a particularly advantageous application for the treatment of aqueous solutions of chlorides of rare earth elements, obtained by means of the known method comprising the treatment of a precipitate of hydroxides of said elements with hydrochloric acid, this precipitate being produced by the reaction of a monazite ore (phosphate ore of rare earth elements) with an aqueous solution of alkali metal hydroxide, for example sodium. This known process is notably described in the document Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Vol. 5, 1979, pages 318-319 and in the document Mineral chemistry treaty, Mineral technology, third part, K. Winnacker and L. Küchler, translated by A. Zundel, Ed. Eyrolles, Paris, 1965, pages 434-436) .

The method according to the invention allows efficient extraction of cerium, with a high yield, generally between 97 and 99% by weight, the cerium obtained having a degree of purity of 95 to 99% by weight. Compared to the known process in which the treatment with the alkali metal hypochlorite solution is carried out at a pH below 4.5, the process according to the invention saves on the consumption of alkali metal hypochlorite. It also facilitates the separation of the ceric hydroxide by filtration.

Special features and details of the invention will emerge from the following description of the two appended figures, which show the diagrams of two particular embodiments of the method according to the invention. In these figures, the same reference notations designate identical elements.

In FIG. 1, the reference notation 1 designates a phosphate ore of rare earth elements. It may be a monazite ore whose deposits exist in particular in the United States, India and Brazil and which contains, in addition to cerium, for example yttrium and elements belonging to the lanthanide group (in particular lanthanum, praseodymium, neodymium, samarium, gadolinium, ytterbium and dysprosium).

The ore 1, ground, is treated in a reaction chamber 2 with a sufficient quantity of an aqueous solution of sodium hydroxide 3 to convert the phosphates into hydroxides. Is withdrawn from the reaction chamber 2, a suspension 4 of hydroxides of rare earth elements in an aqueous solution of sodium phosphate. The suspension 4 is treated on a filter 5 where the aqueous sodium phosphate solution 6 and a precipitate 7 are separated, containing the rare earth elements in the form of hydroxides. The precipitate 7 is sent to a reaction chamber 8, where it is treated with an aqueous solution of hydrochloric acid 9 in an amount sufficient to dissolve the rare earth elements in the form of chlorides. The liquid 10 collected from the chamber 8 is treated on a filter 11 to remove the insoluble matter 12 (in particular thorium and silica from the gangue of the ore). An aqueous solution 13 of chlorides of the rare earth elements of the ore is collected from the filter 11.

The solution 13 is introduced into a reaction chamber 14 and an aqueous solution 15 of sodium hypochlorite is added to it in an amount sufficient to oxidize the trivalent cerium to tetravalent cerium. In accordance with the invention, the introduction of the aqueous sodium hypochlorite solution 15 into the chamber 14 is regulated, so as to maintain the pH of the reaction medium permanently between 6 and 6.5. During the introduction of the sodium hypochlorite solution into chamber 14, the trivalent cerium is gradually oxidized to tetravalent cerium. Immediately after the end of the introduction of the sodium hypochlorite solution 15 into the chamber 14, one sends the reaction mixture 16 to a maturation chamber 17 from which a broth 19 of ceric hydroxide is collected. The broth is sent to a chamber 20, where its pH is lowered to a value equal to or less than 4 by the addition of an aqueous solution of hydrochloric acid 21. The broth 25 collected from the chamber 20 contains almost all of the cerium from ore 1 in the form of insoluble ceric hydroxide and the other rare earth elements from ore 1 in the form of dissolved chlorides. It is introduced at the head of a column 26 supplied, at the base, by a stream of air 27. In the column 26, the slurry 25 circulates in the opposite direction to the ascending stream of air which leaves the column at 18. The air stream 27 has the function of extracting the chlorine contained in the slurry 25. The air stream 18 leaving the column 26 is therefore charged with chlorine. It is treated in an absorption column 22 with a sufficient quantity of an aqueous solution of sodium hydroxide 23 to form an aqueous solution of sodium hypochlorite 24 which is sent to the reaction chamber 14. The dechlorinated air leaves column 22 at 49.

The broth 28 which is collected at the bottom of the column 26 still contains a little active chlorine originating from the treatment with sodium hypochlorite. To remove this active chlorine, the slurry 28 is introduced into a reaction chamber 29 where an aqueous solution 30 of sodium hyposulfite is added to it. The dechlorinated broth 47 collected from the chamber 29 is sent to a filter 31 where a declorated precipitate of ceric hydroxide 48 and a dechlorinated aqueous solution 32 are separated. The aqueous solution 32 collected from the filter 31 is treated in a manner known per se cation exchanger 33 to selectively extract the rare earth elements therein. Information on the cation exchanger 33 and on the process for extracting the rare earth elements from solution 32 can be found in patents US-A-2 798 789 and US-A-3 228 750. It is desirable to submit the precipitate of ceric hydroxide 48 in a wash with acidulated water, to remove the metallic elements which could contaminate it. Washing with water acidulated can be operated on the filter 31. The acidulated water collected after washing can be recycled in the reaction chamber 8, with the hydrochloric acid solution 9.

The following examples serve to illustrate the invention.

Example 1 (according to the invention)

This example is described with reference to Figure 2.

A monazite ore was treated as described in the description of FIG. 1 and an aqueous solution 13 of chlorides of rare earth elements was collected, containing 55.0 g of said elements (expressed in the form of oxides) by liter. In addition, an aqueous solution of sodium hypochlorite 15 was used, having a weight content of active chlorine of 13%. The two solutions were introduced into the reaction chamber 14, in a continuous process and at the respective flow rates of 12.0 l / h for the chloride solution 13 and 8.0 l / h for the hypochlorite solution 15. The steps of the process shown diagrammatically in FIG. 1 were then followed. At the end of the process, a precipitate 48 rich in cerium and a mother liquor 32 containing the other rare earth elements of the ore were collected.

Precipitate 48 was subjected to two successive washes with water. For this purpose, it was introduced into a mixing chamber 36 with acidulated water 35 [water acidulated with hydrochloric acid (pH = 3)] and the resulting dispersion 37 was sent to a filter 38. A second precipitate 39 and a mother liquor 40 were collected. The precipitate 39 was treated with water 41 in a second mixing chamber 42. The resulting dispersion 43 was filtered through a filter 44, from which a precipitate 45 and mother liquor 46.

The concentration and composition of the starting solution 13, the concentration and the composition of the mother liquors 32, 40 and 46 and the composition of the precipitates 48, 39 and 45 are recorded in Table 1. The table provides the weight contents (in %) of the elements expressed in the form of oxides. With regard to precipitates 39 and 48, the balance at 100% of the precipitate consists mainly of sodium chloride.

Examples 2, 3 and 4 serve to show the progress made by the invention. Example 2 is in accordance with the invention, while Examples 3 and 4 are reference examples, not in accordance with the invention.

In these examples, the operation was carried out as described above with reference to FIG. 1 and an aqueous solution 13 of chlorides of rare earth elements was obtained, containing 46.3 g of said elements (expressed in the form of oxides) per liter. The composition of solution 13 is mentioned in Table 2. In this table, the weight of the elements (considered as oxides) is expressed in% of the total weight of the elements in the dry state (and considered as state of oxides). Table 2 Composition (% by weight) Concentration (g / l) La₂O₃ CeO₂ About Nd₂O₃ Sm₂O₃ 22.4 47.5 5.1 17.7 2.8 46.3

In addition, an aqueous solution of sodium hypochlorite 15 was used, having a weight content of active chlorine of 13%.

The two solutions 13 and 15 were introduced into the reaction chamber 14, at the respective flow rates of 12.0 l / h for solution 13 and 8.0 l / h for solution 15.

After passing through the maturation chamber 17, the reaction mixture 19 was collected, filtered and the filtrate was analyzed.

Example 2 (according to the invention)

In this example, the pH of the reaction medium in chamber 14 was kept at the constant value 6.8.

The reaction mixture 19 was filtered, the filtrate was collected and analyzed. Table 3 provides the composition of the filtrate.

In this table, the concentration of the solution (g / l) and its weight composition (%) have the same definition as in Table 2. Table 3 Composition (% by weight) Concentration (g / l) La₂O₃ CeO₂ About Nd₂O₃ Sm₂O₃ 45 0.33 9.61 32.5 4.68 15.5

A comparison of Tables 2 and 3 shows that a large fraction of the cerium has passed through the filter cake.

Example 3 (reference example)

The procedure was as in Example 2, with the only difference that the pH of the reaction medium in chamber 14 was fixed at 3, by introduction of hydrochloric acid in solution 13.

The results of the test (composition of the filtrate obtained after filtration of the mixture 19) are reproduced in Table 4. Table 4 Composition (% by weight) Concentration (g / l) La₂O₃ CeO₂ About Nd₂O₃ Sm₂O₃ 23.1 46.0 5.3 18.3 2.8 26.9

Example 4 (reference example)

The test of Example 3 was repeated, setting the pH value in reaction chamber 14 to 4.5, by introduction of hydrochloric acid into solution 13.

The results of the test are shown in Table 5. Table 5 Composition (% by weight) Concentration (g / l) La₂O₃ CeO₂ About Nd₂O₃ Sm₂O₃ 23.5 45.0 5.36 18.6 2.9 25.6

A comparison of the results of Tables 4 and 5 with Table 2 shows that in Examples 3 and 4, almost all of the cerium remained dissolved.

The results of Examples 2, 3 and 4 are collated in Table 6. Table 6 Composition (% by weight) Concentration (g / l) La₂O₃ CeO₂ About Nd₂O₃ Sm₂O₃ Solution 13 22.4 47.5 5.1 17.7 2.8 46.3 Example 2 45.0 0.33 9.61 32.5 4.68 15.5 Example 3 23.1 46.0 5.3 18.3 2.8 26.9 Example 4 23.5 45.0 5.36 18.6 2.9 25.6

It emerges from Examples 2, 3 and 4 that, all other things being equal, the pH value in the reaction chamber 14 a has an important action on the precipitation of the ceric hydroxide. At a pH above 4.5 (Example 2), a very large fraction of ceric hydroxide precipitated; on the other hand, at a pH equal to or less than 4.5 (Examples 3 and 4), only a tiny fraction of ceric hydroxide precipitated. It is deduced therefrom that at a pH equal to or less than 4.5 (Examples 3 and 4), the precipitation of ceric hydroxide would have required a greater quantity of sodium hypochlorite.

Claims (7)

Process for the selective extraction of cerium from an aqueous solution of rare earth elements containing a trivalent cerium salt, according to which alkali metal hypochlorite (15) is added to the aqueous solution (13) to precipitate the cerium in the form of ceric hydroxide, and the resulting broth is separated (19, 25, 28, 47), on the one hand, a precipitate of ceric hydroxide (48) and, on the other hand, a mother liquor ( 32) containing the other elements in the dissolved state, characterized in that the pH is maintained at a value greater than 4.5 during the addition of the alkali metal hypochlorite to the aqueous solution. Method according to claim 1, characterized in that the pH is maintained at a value between 6 and 7. Method according to claim 1 or 2, characterized in that after precipitating the ceric hydroxide, the pH of the broth (19) is adjusted to a value between 2.5 and 3.5. Process according to any one of Claims 1 to 3, characterized in that the broth (25) is subjected to dechlorination by entrainment using an inert gas (27) and a sufficient quantity of hyposulfite is then added thereto of alkali metal (30) to remove the active chlorine it contains. Process according to any one of Claims 1 to 4, characterized in that the cerium salt of the aqueous solution (13) is cerium chloride. Process according to any one of Claims 1 to 5, characterized in that the alkali metal is sodium. Process according to any one of Claims 1 to 6, characterized in that the aqueous solution (13) is an aqueous solution of chlorides of rare earth elements, obtained by treating with hydrochloric acid (9) a precipitate of hydroxides of said elements (7) resulting from the reaction of a phosphate ore of rare earth elements (1) with an aqueous solution of alkali metal hydroxide (3 ).

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