Process for the removal of cerium from an aqueous solution containing rare earth elements

Abstract

A process for the extraction of cerium from an aqueous solution comprising REE, the process comprising cycles of adjusting the pH of the aqueous solution to 5-5.5 by the addition of NaOH, and then adding H2O2 until a pH of 3-5.5 in order to precipitate Ce(OH)4; wherein the amount of H2O2 in the second and subsequent cycles allows the pH to be maintained in the range of 3-5.5; wherein the cycles are repeated until the total amount of NaOH added to the aqueous solution reaches from 90% to 115% of the stoichiometric amount to convert all the rare earths into their hydroxides; separating by filtration the precipitate comprising Ce(OH)4 to obtain an aqueous solution mainly comprising non-Ce REE; and recovering the non-Ce REE from the aqueous solution. And use of the non-Ce rare earth elements recovered by the process above for the preparation of an article of manufacture.

Description

[0001] Process for the removal of cerium from an aqueous solution containing rare earth elements

[0002] This application claims the benefit of European Patent Application EP24383191.4 filed on October 30th, 2024.

[0003] Technical Field

[0004] The present invention relates to the field of extraction of metals from a substrate. Particularly, it relates to the removal of cerium from a solution comprising rare earth elements, more particularly, from a leachate in order to recover the targeted non-Ce rare earth elements from wastes containing them.

[0005] Background Art

[0006] Cerium is forming part of the rare earth elements (REE), which includes the 15 members of the lanthanides group (lanthanum, praseodymium, cerium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium) and yttrium and scandium. These elements play a crucial role in the high-tech industry, having diverse applications in electrical and electronic components, lasers, glass, magnetic materials, and industrial processes.

[0007] REE have very similar physical and chemical properties and therefore obtaining individual REE in industrial scale from their mineral sources or waste materials containing them is very complex.

[0008] Cerium is among the least valuable REE and in can be desirable to remove it from other REE.

[0009] Cerium is the only element in the REE series that can be stable in oxidized form. The difference in solubility between hydroxides of Ce(IV) and the other trivalent REE is sufficiently high to obtain a good separation of the first as a hydroxide. Generally, the separation of cerium is accomplished by the oxidation of Ce(lll) to Ce(IV) following by the selective dissolution of the trivalent REE. Ce4+precipitation in basic media is a process widely used. In nitric acid media cerium the oxidation of cerium is due to the high oxidising power of nitric acid. In a hydrochloric media the formation of Ce complexes takes place preventing the complete precipitation of C4+compounds. However, although the literature shows studies with numerous oxidising agents, including H2O2, the results are not satisfactory since complete precipitation of Ce is not achieved. As a consequence, by the processes known in the prior art, the other REE are usually contaminated with cerium or cerium is contaminated with other REE, depending on the desired purity of the product. CN115232965A discloses a method for removing cerium from high-cerium praseodymium neodymium rare earth feed liquid.

[0010] Nechaev, A. V. et al. (2016). "A rational approach to processing cerium-containing raw materials". Theoretical Foundations of Chemical Engineering, 50(5), 863-866 discloses a method of extracting cerium from rare-earth raw materials based on the oxidation of its trivalent ions with hydrogen peroxide, followed by the precipitation of hydroxide and additional acid treatment to remove trivalent impurities.

[0011] CN118813991A discloses an online cerium removal method for treating high-cerium praseodymium neodymium feed liquid.

[0012] Thus there is an interest in providing improved methods to extract cerium(IV) selectively from solutions comprising other REE with higher yield and / or purity.

[0013] Summary of Invention

[0014] The inventors have developed a process that allows removing cerium from an aqueous solution comprising cerium and other REE, the process yielding a solution of the other REE essentially free of cerium (i.e. , with amounts of cerium of up to below 0.1 wt.% of the cerium contained in the initial solution).

[0015] For instance, by obtaining a leachate from a substrate comprising cerium and other REE, cerium can be selectively removed and thus, it is possible to obtain a mixture of more valuable REE. This other REE are useful in some cutting-edge technologies such as for the manufacture of high-performance magnets.

[0016] Therefore, an aspect of the present invention relates to a process for the extraction of cerium from an aqueous solution comprising rare earth elements, the process comprising the steps of: a) providing an aqueous solution comprising cerium and other rare earth elements; b) adjusting the pH of the aqueous solution to a value from 5 to 5.5, particularly, from 5.2 to 5.5, by the addition of NaOH; c) adding to the aqueous solution an amount of H2O2 as oxidizing agent until obtaining a pH from 3 to 5.5 in order to precipitate cerium in the form of Ce(OH)4; d) repeating steps b) and c) until the total amount of NaOH added to the aqueous solution reaches from 90% to 115%, particularly 100%, of the stoichiometric amount to convert all the rare earths into their hydroxides; e) separating by filtration the precipitate comprising Ce(OH)4 to obtain an aqueous solution mainly comprising non-Ce rare earth elements; f) recovering the non-Ce rare earth elements from the aqueous solution.

[0017] Advantageously, the process of the invention allows recovering non-Ce REE present in a substrate (such as in magnets) with very high yield and high purity, that is, with a very low content of cerium. After cerium removal, the other REE can be individually recovered by processes well known in the art such as solvent extraction processes.

[0018] By carrying out the process of the present disclosure, the displacement of the equilibrium towards Ce4+is forced in each cycle of addition of NaOH and H2O2, what allows gradually extracting cerium (which precipitates in the form of Ce(OH)4) from the solution until achieving a extraction essentially complete. This occurs when the stoichiometric amount of NaOH to convert all rare earths to hydroxides has been reached. The stoichiometric amount of the pH adjusting agent is calculated on the basis of the number of moles required to convert all REE into hydroxides. For the purpose of calculating the NaOH stoichiometry, all REE are considered to have a valence of 3+ except Ce which is considered to have a valence of 4+. This amount can be known by prior analysis of the solution containing the REE and confirmed during precipitation. When this consumption of NaOH is reached, the addition of the oxidizing agent (H2O2) has practically no effect, so that the pH reduction is very small.

[0019] It has been shown that the working methodology and the control of the consumption of the pH adjusting agent are key to the complete removal of Ce.

[0020] Besides, by working at the mentioned conditions, no special filtration system is needed, as a laboratory vacuum filtration system provides very good filtration at room temperature.

[0021] A second aspect of the present invention relates to the use of the non-Ce REE recovered by the process defined herein for the preparation of an article of manufacture such as a high-performance magnet.

[0022] Brief Description of Drawings

[0023] Fig. 1 shows a scheme of the route for obtaining Ce-free high purity rare earth oxides comprising the separation of Ce from a leachate (particularly, of a leachate of a magnet, after removing iron from the leachate), the recovery of non-Ce rare earth as oxalates by precipitation; and the calcination of the non-Ce rare earth oxalates to obtain non-Ce rare earth oxides. It is also shown the possibility of recovering the REE coprecipitated with Ce in the Ce removal step.

[0024] Detailed description of the invention

[0025] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly through-out the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0026] As used herein, the term “leachate” refers to an aqueous solution comprising multiple metals therein, in particular, multiples REE including cerium. The metals may be dissolved or otherwise dispersed in the aqueous solution. A “leachate” may otherwise be referred to herein as a “leachate solution”.

[0027] As used herein, the term “leaching” of a substrate such as a magnet, is to be understood as a dissolving process by which the REE contained in the substrate are extracted into a liquid, in this case the aqueous leaching solvent system used for the extraction. The leaching process can be described as the process by which constituents of solid material are released into a contacting solvent under a set of chemical phenomena, which may include mineral dissolution, desorption, and complexation, and mass transport processes. In turn, these phenomena are affected by certain factors that can alter the rate or extent of leaching, such as, for example, the solvent used or the operating conditions.

[0028] As used herein, the term “recovery efficiency” of a component refers to the amount, by weight, of an element that is recovered relative to its content in the initial source (liquid solution or substrate), expressed as a percentage by weight.

[0029] As used herein, the term “removal efficiency” of a component refers to the amount, by weight, of an element that is removed relative to its content in the initial source (liquid solution or substrate), expressed as a percentage by weight.

[0030] As used herein, the concept “removal efficiency vs NaOH stoichiometry” means the weight of a component that is extracted or removed from the liquid solution with respect to its initial content expressed as a percentage versus the number of moles of NaOH used to convert the REE present in the liquid solution to the corresponding hydroxides with respect to the total number of moles required to convert all the REE present in the liquid solution to hydroxides expressed as a percentage.

[0031] The term "room temperature" refers to a temperature from 15 °C to 25 °C.

[0032] As used herein, the indefinite articles “a” and “an” are synonymous with “at least one” or “one or more.” Unless indicated otherwise, definite articles used herein, such as “the” also include the plural of the noun.

[0033] As mentioned above, an aspect of the invention relates to a process for the extraction of cerium from an aqueous solution comprising REE, the process comprising cycles of adjusting the pH of the aqueous solution to 5-5.5 by the addition of NaOH, and then adding H2O2 until a pH of 3 to 5.5, particularly until a pH of about 4, in order to precipitate Ce(OH)4; wherein the amount of H2O2 in the second and subsequent cycles is the same as in the first cycle; wherein the cycles are repeated until the total amount of NaOH added to the aqueous solution reaches from 90% to 115%, particularly from 90% to 110%, more particularly 100%, of the stoichiometric amount to convert all the rare earths into their hydroxides; separating by filtration the precipitate comprising Ce(OH)4 to obtain an aqueous solution mainly comprising non-Ce REE; and recovering the non-Ce REE from the aqueous solution.

[0034] In an embodiment of the process of the present disclosure, in step d), steps b) and c) are repeated until the total amount of NaOH added to the aqueous solution reaches from 90% to 110% (such as of 95%, 100%, 105%) of the stoichiometric amount to convert all the rare earths into their hydroxides.

[0035] In another embodiment, the process is carried out at a temperature from room temperature to 120 °C °C, or from 18 °C to 80 °C, or from 18 °C to 60 °C, or from 18 °C to 50 °C, or from 18 °C to 40 °C, particularly, from 18 °C to 30 °C.

[0036] Advantageously, the process of the present disclosure can be carried out at relatively low temperature such as from room temperature to 30 °C, i.e. , it is not necessary to heat the solution such as up to a temperature from 75-80 °C and / or to maintain stirring for long periods of time.

[0037] In another embodiment, optionally in combination with one or more features of the various embodiments described above, the pH in step b) is from 5.2 to 5.5.

[0038] The inventors have found that by carrying out the process of the present disclosure, by precipitating Ce(OH)4 under the conditions mentioned above, a leachate essentially free of Ce (that is with an amount of Ce lower than 1 wt.%, or lower than 0.5 wt.%, or lower than 0.2 wt.% with respect to its content in the initial leachate) can be obtained.

[0039] Hence, by precipitating the REE remaining in the solution, recovery efficiency of the non- Ce REE exceeding 99 wt.% of the non-Ce REE present in the initial source can be achieved.

[0040] Non-Ce REE can be recovered from the aqueous solution by precipitating their oxalates, which can then be calcined to obtain the corresponding oxides.

[0041] Therefore, in another embodiment, optionally in combination with one or more features of the various embodiments described above, step f) of recovering the non-Ce REE is carried out by precipitation of the REE in the form of the corresponding rare earth element oxalates, particularly, by the addition of oxalic acid or, alternatively, an oxalate salt such as sodium oxalate at a pH higher than 0.5 and up to 14 such as at pH 1.

[0042] Non-Ce rare earth oxalates can then be calcined to obtain the corresponding rare earth oxides, which can be used as starting materials for the manufacture of high-tech products.

[0043] Therefore, in another embodiment, optionally in combination with one or more features of the various embodiments described above, the rare earth oxalates are calcined, for instance, at a temperature equal or higher than 700 °C, particularly from 800 °C to 900 °C °C, to obtain the corresponding rare earth oxides.

[0044] In another embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the precipitate of cerium in the form of Ce(OH)4 of step c) further comprises coprecipitated non-Ce REE, and the process further comprises recovering the non-Ce REE by:

[0045] - calcining the precipitate comprising Ce(OH)4 and non-Ce REE to obtain a calcined product;

[0046] - dissolving the calcined product in HCI to obtain an acidic aqueous solution; and

[0047] - performing step f) as defined above and, optionally, also step g) as defined above, on the acidic aqueous solution; or, alternatively,

[0048] - dissolving the precipitate comprising Ce(OH)4 and non-Ce REE in HCI to obtain an acidic aqueous solution; and

[0049] - performing steps b) to f) as defined above and, optionally, also step g) as defined above, on the acidic aqueous solution.

[0050] As mentioned above, the process of the present disclosure can be useful for the recovery of non-Ce REE from substrates comprising cerium and other REE. It can also be useful for the separation of cerium from a mixture containing other REE and the subsequent recovery of the rest of REE.

[0051] Thus, in another embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the aqueous solution is a leachate of a substrate comprising cerium together with other additional REE and, optionally, other elements such as B, Mn, Co, Cu, Zn and Ni as impurities. Particularly, the additional REE are selected from the group consisting of Nd, Pr, Dy, Gd, Tb, Ho and Er.

[0052] In an example, the process is useful for the recovery of REE from magnets comprising cerium and other REE, being cerium a pollutant when it comes to high performance magnets.

[0053] In another embodiment of the process of the present disclosure, the substrate is a magnet such as a permanent magnet. Hence, in an embodiment of the process of the present disclosure, the aqueous solution comprising REE is a leachate obtained by the leaching of a magnet, particularly, of a permanent magnet. More particularly, the leachate is obtained by leaching of a magnet with hydrochloric acid to obtain a first liquor and, subsequently, removing iron from the first liquor, for instance, by precipitation (e.g., by a goethite precipitation process) to obtain the leachate.

[0054] In Fig. 1 a route comprising 6 steps has been depicted wherein, firstly, the separation of the most disturbing elements present in the magnets (Fe and Ce) from the main stream is carried out; then, high purity non-Ce rare earth oxalates are recovered by precipitation; and, finally, the rare earth oxalates are calcined to produce high purity rare earth oxides, free of Ce, that can be used, for instance, in the production of high performance magnets.

[0055] In a first step (leaching of the magnet) a magnet leachate is obtained by almost full dissolution of the REE elements of the magnet in HCI. This can be achieved by using various HCI concentrations and for all the magnet types studied if enough acidity is maintained in the leaching medium.

[0056] The REE magnets are essentially composed by an alloy of iron, rare earths (usually Nd) and boron in a particular atomic ratio (REE2Fei4B). Due to the reduction potential of the metals present in the REE magnets, those elements would lead to spontaneous oxidation and, thus, leaching reactions in a proton containing medium (acidic), leading to the formation of hydrogen gas molecules.

[0057] The magnets are leached by placing them in a reactor with a certain volume of water, where concentrated HCI is dosed controlling the working pH below 1 until full dissolution.

[0058] The solution is filtered to remove the undissolved possible residues and is transferred to the second step.

[0059] In a second step, the removal of the iron of the magnet leachate can be carried out, for instance, by goethite precipitation using an oxidizing agent (air and H2O2) at pH 3 and 80 °C, which allows the removal of virtually all the iron present in the magnet leachates (the concentration of Fe in the leachate after using this process can reach trace levels down to 15 mg / kg).

[0060] The removal of Ce from the leachate, was carried out in a third step by carrying the process for the extraction of Ce as disclosed herein above, that is, by oxidizing Ce(lll) to Ce(IV) and precipitating cerium in the form of Ce(OH)4 at a pH from 5 to 5.5 in order to obtain a leachate essentially free of Ce.

[0061] Although after the third step the cerium removal efficiency could be higher than 99 wt.%, it can result in the coprecipitation of some other REE. Thus, optionally, an additional fourth step can be carried out to recover the REE coprecipitated with Ce, for instance, by calcining the precipitated Ce(OH)4 and, subsequently, dissolving the obtained product in HCI. By applying this strategy, 95 wt.% of the non-Ce REE trapped in the Ce precipitate from the third step could be recovered and, optionally, returned to the main process stream, although still certain amount of Ce (about 6 wt.% of the Ce contained in the leachate from the second step) will remain.

[0062] The additional REE recovered in the fourth step will contain a higher amount of Ce than the one of main process stream, i.e. , the stream obtained after removal of Ce from the leachate after performing the third step. Therefore, by applying to the stream with the REE recovered in the fourth step (resulting from the third step after cerium precipitation), the fifth, and sixth and steps below, the rare earth oxides obtained may contain up to 6 wt.% of Ce.

[0063] After precipitation of the Ce(OH)4, the REE remaining in the leachate can be recovered by forming their oxalates, which will then precipitate. Hence, in a fifth step, precipitation of the REE in the form of oxalates can be performed by the addition of oxalic acid or sodium oxalate and adjustment of the solution to pH 1. In such a way, a recovery efficiency of REE exceeding 99 wt.% of the REE contained in the initial solution in the form of oxalates can be achieved.

[0064] Finally, a sixth step can be carried out wherein rare earth oxalates are calcined in order to form the corresponding rare earth oxides. As mentioned above, rare earth oxides could be obtained from oxalates by calcining the latter at 900 °C.

[0065] By carrying out the process comprising the first, second, third, fifth and sixth steps disclosed above, the obtained rare earth oxides will contain most impurities at trace level and a Ce content below 0.1 wt.% of Ce.

[0066] As a whole, the process comprising from the first to the sixth steps disclosed above allows recovering up to 98% of the rare earths present in magnets. Particularly, it is possible to recover:

[0067] 1) a first product in the form of rare earth oxides with very high purity (less than 0.1 wt.% of Ce and other impurities at trace level) and containing about 80 wt.% of the non-Ce REE present in the initial leachate; and

[0068] 2) a second product comprising about 20 wt.% of the non-Ce REE present in the initial leachate with a Ce content of up to 6% wt.% of Ce as the only significant impurity.

[0069] The leachate can be obtained from a substrate containing REE, particularly, such as a mineral source or a waste material. Some examples of REE-containing substrates include minerals containing rare-earth elements, coal fly ash, magnets, and waste materials.

[0070] Rare earth oxides are usually used as starting materials for the preparation of commercial products of considerable variety and technical importance, for instance, by co-reduction or reduction diffusion.

[0071] Accordingly, in another embodiment of the process of the present disclosure, optionally in combination with one or more features of the various embodiments described above, the process further comprises the manufacture of an article of manufacture such as a high performance magnet with the recovered non-Ce REE.

[0072] Thus, also forms part of the invention an article of manufacture, in particular, a high performance magnet, made of the non-Ce REE recovered by the process disclosed herein above.

[0073] Examples

[0074] Example 1

[0075] A permanent magnet (the native sample) was leached by adding concentrated hydrochloric acid (35% w / w HCI) as described above. The obtained solution was treated to remove Fe by goethite precipitation process, i.e., by the simultaneous addition of a NaOH solution and air / H2O2. During the goethite precipitation process the pH is kept between 2-3 and the temperature between 70-80 °C. The leachate solution obtained had a pH of 3.0-3.3 at 20 °C and contained REE such as Ce, Nd, Pr, Dy, and Gd, as well as impurities of B, Mn, Co, Cu, Zn and Ni.

[0076] The leachate solution was first adjusted to the working pH and then H2O2 and the pH adjusting agent (NaOH in the form of an aqueous solution), were added successively to maintain the pH at the desired value. The addition of H2O2 is not the main factor to obtain a complete precipitation of Ce from the rare earth leachate, but it is necessary to force the equilibrium between Ce3+and Ce4+that is stablished in hydrochloric medium at a specific acidic pH. The main parameter for the complete precipitation of Ce is the consumption of the pH adjusting agent, NaOH, combined with the final pH of the solution, which must be set between 5-5.5.

[0077] Particularly, the pH solution was first adjusted to 5.2-5.5 with the addition of an aqueous solution of NaOH at room temperature and then, 30% H2O2 was added until a pH from 3 to 5.5 was reached. This cycle pH adjustment to 5.2-5.5 with the addition of a NaOH solution and, subsequently, to pH 3-5 with the addition of 30% H2O2 was repeated as many times as needed using the same amount of H2O2 in each addition as that added in the first cycle. The decrease in pH in each cycle, i.e., after each addition of H2O2, was smaller than in the previous one. In the last cycles the decrease in pH was very low, around 0.2 pH units, so it seemed that the reaction was finished. Nevertheless, H2O2 was added until the amount of NaOH added reached the stoichiometric amount to convert all the REE present in the solution into the corresponding hydroxides (Ce as Ce(OH)4, which precipitates, and other REE as RE(OH)a, which remain in solution).

[0078] The Ce-free solution obtained contained approximately 80 wt.% of the non-Ce REE contained in the initial leachate solution and <0.5 wt.% of the Ce contained in the initial leachate solution. This solution was treated with oxalic acid or sodium oxalate and adjusted at pH 1 in order to precipitate the non-Ce REE in the form of their oxalate salt. Subsequent calcination at a temperature above 700 °C produced a mixture of rare earth oxides with a content of Ce lower than 0.1 wt.%.

[0079] The non-Ce REE co-precipitated with Ce(OH)4 were recovered by dissolving the cerium residue in HCI and treating it again by the same method and under the same conditions as described above, i.e., by i) carrying out successive cycles of NaOH and H2O2 additions in the conditions disclosed above in order to precipitate Ce(OH)4; ii) forming the oxalates of the REE remaining in solution; and iii) calcining the RE oxalates to form the corresponding oxides. It is verified that in this secondary non-Ce REE recovery the same distribution as in the mainstream was maintained for all the REE (about 80 wt.% of the non-Ce REE and <0.5 wt.% of Ce contained in the cerium precipitate were dissolved to the new leachate, and about 20 wt.% of the non-Ce REE were co-precipitate with Ce again).

[0080] Magnets extracted from hoverboards and containing Ce were leached into a 6M HCI solution with successive additions of 35% HCI to keep the pH below 0. The solution was then filtered, heated to 80 °C and adjusted to pH 3 with NaOH. Air and H2O2were bubbled to form goethite while NaOH was added to maintain the pH within the precipitation range of this compound. After Fe precipitation was completed, the solution was filtered and analysed for REE content, including Ce.

[0081] 50 grams of the solution containing Ce were taken at a temperature of 20 °C and treated by the proposed process. For this purpose, the solution was adjusted to pH 5.5 with NaOH solution, then 10 additions of H2O2 30% and NaOH 0.5M were made intermittently, adjusting the pH between 5.2 and 5.5 with NaOH after each addition of H2O2. After the last addition of H2O2 the pH was adjusted to 5.5 with NaOH in both tests. In total, 10 times the stoichiometric amount of H2O2 calculated for the oxidation of Ce3+to C4+was consumed. The NaOH consumption in each test was 103% and 104%, respectively, of the stoichiometric value needed to convert all REE to their corresponding hydroxides. The precipitation process took 1 to 1.5 hours to complete. The solution was then filtered in a laboratory vacuum filtration equipment.

[0082] Table 1 shows the results of the Ce removal by carrying out the process at room temperature (18 °C) and final pH of 5.5. The table shows the initial concentration of the leachate entering the Ce removal step (native sample) for the different elements analysed as well as the values obtained for tests 1 and 2. It can be seen that the Ce concentration decreases from 6.3 g / kg in the native solution to 37.4 and 6.4 mg / kg respectively in the final solutions of tests 1 and 2.

[0083] Table 1

[0084] Table 2 shows the results of the REE removal efficiency vs. NaOH consumption, expressing NaOH consumption as a percentage of the stoichiometric amount required to convert all REE to hydroxides. An increase in Ce removal efficiency is observed as the stoichiometry used increases, reaching up to 99.8% removal efficiency when the value of the stoichiometry reaches 104%. Compared to the content in the native sample, the Ce- free solution obtained contained 78-82 wt.% of total non-Ce REE and 0.2-0.9 wt.% Ce when the stoichiometric NaOH consumption was 103 and 104% of the stoichiometric value, respectively. For NaOH consumptions equivalent to 90% and 96% of the stoichiometric value the Ce removal efficiency from the aqueous solution was 92 wt.% and 97 wt.%, respectively.

[0085] Table 2

[0086] Example 2

[0087] From the solution used to carry out tests 1 and 2, 500 grams of the solution containing Ce were taken and heated to a temperature of 80 °C. The solution was adjusted to pH 5.5 with NaOH solution, then 4 additions of 30% H2O2 and 0.5M NaOH were added intermittently, adjusting the pH between 5.2 and 5.5 with NaOH after each addition of H2O2. After the last addition of H2O2 the pH was adjusted to 5.2. In total, 3 times the stoichiometric amount of H2O2 calculated for the oxidation of Ce3+to Ce4+was consumed. The final NaOH consumption was 111% of its stoichiometric value. A sample corresponding to 105% of its stoichiometric value was also taken during the process. The precipitation process took 2 hours to complete. The solution was then filtered in a laboratory vacuum filtration unit.

[0088] Table 3 shows the results of the Ce removal by carrying out the process at 80 °C temperature and a final pH of 5.2. In the table the initial concentration of the leachate before Ce removal and the values obtained in Test 3 are shown, for the different elements analysed, when the NaOH consumption reached 111% of the calculated stoichiometric value. It can be seen that the Ce concentration decreased from 6.3 g / kg in the native solution to 15 mg / kg in the Ce-free solution.

[0089] Table 3

[0090] Table 4 shows the results of the REE removal efficiency vs NaOH consumption expressed as a percentage of the stoichiometric amount required to convert all REE into hydroxides. An increase in Ce removal efficiency is observed with increasing NaOH consumption, reaching values above 99 wt.% in Ce removal efficiency at a slight excess of NaOH consumption.

[0091] Table 4

[0092] 99.1 wt.% of cerium was removed from the aqueous solution containing the REE when 105% of the stoichiometric amount of NaOH was consumed. Compared to the content in the native sample, the Ce-free solution obtained contained 75 wt.% of the total non-Ce REE and 0.9 wt.% Ce.

[0093] 99.6 wt.% of cerium was removed from the aqueous solution containing the REE when 111% of the stoichiometric amount of NaOH was consumed. Compared to the content in the native sample, the Ce-free solution obtained contained 74 wt.% of total non-Ce REE and 0.4 wt.% Ce.

[0094] Example 3

[0095] For the test 4, 56 grams of the above solution were taken. The solution was heated to a temperature of 80 °C and adjusted to pH 5.5 with NaOH solution, then 18 additions of 30% H2O2 and 0.5M NaOH were added intermittently, adjusting the pH between 5.2 and 5.5 with NaOH after each addition of H2O2. After the last addition of H2O2 the pH was adjusted to 5.5. In total, 12 times the stoichiometric amount of H2O2 calculated for the oxidation of Ce3+to C4+was consumed. The final NaOH consumption was 113% of its stoichiometric value. Two samples corresponding to 66% and 86% of the NaOH stoichiometric value were also taken during the process. The precipitation process took 2 hours to complete. The solution was then filtered in a laboratory vacuum filtration unit. Table 5 shows the results of the Ce removal by carrying out the process at 80 °C temperature and a final pH 5.5. In the table the initial concentration of the leachate before Ce removal and the values obtained in Test 4 are shown, for the different elements analysed, when the NaOH consumption reached 111% of the calculated stoichiometric value. It can be seen that the Ce concentration decreased from 6.3 g / kg in the native solution to 0.1 mg / kg in the Ce-free solution.

[0096] Table 5

[0097] Table 6 shows the results of the REE removal efficiency vs NaOH consumption expressed as a percentage of the stoichiometric amount required to convert all REE into hydroxides.

[0098] A high Ce removal efficiency (>99.9 wt.%) is observed with NaOH consumption is slightly higher than 100% of the calculated stoichiometric value. On the contrary, Ce removal efficiency is drastically reduced when this value is below 90 wt.% (a Ce removal efficiency of 84.7 wt % for a stoichiometry of 86% and a removal efficiency of 77 wt.% Ce for a stoichiometry of 66%).

[0099] Table 6

[0100] 99.9 wt.% of cerium was removed from the initial aqueous solution containing the REE when 113% of the stoichiometric amount of NaOH was consumed. Compared to the content in the native sample, the Ce-free solution obtained contained 74 wt.% of the total non-Ce REE and 0.1 wt.% of the Ce contained in the initial solution.

Claims

Claims1 . A process for the extraction of cerium from an aqueous solution comprising rare earth elements, the process comprising the steps of: a) providing an aqueous solution comprising cerium and other rare earth elements; b) adjusting the pH of the aqueous solution to a value from 5 to 5.5 by the addition of NaOH; c) adding to the aqueous solution an amount of H2O2 as oxidizing agent until obtaining a pH from 3 to 5.5 in order to precipitate cerium in the form of Ce(OH)4; d) repeating steps b) and c) until the total amount of NaOH added to the aqueous solution reaches from 90% to 115% of the stoichiometric amount to convert all the rare earths into their hydroxides; e) separating by filtration the precipitate comprising Ce(OH)4 to obtain an aqueous solution mainly comprising non-Ce rare earth elements; f) recovering the non-Ce rare earth elements from the aqueous solution.

2. The process of claim 1 , wherein in step d) the total amount of NaOH added to the aqueous solution reaches from 90% to 110% of the stoichiometric amount to convert all the rare earths into their hydroxides.

3. The process of claims 1 or 2, wherein the process is carried out at a temperature from room temperature to 120 °C.

4. The process of claim 3, wherein the temperature is from 18 °C to 30 °C.

5. The process of claims 1 to 4, wherein in step b) the pH is from 5.2 to 5.5.

6. The process of any one of claims 1 to 5, wherein step f) of recovering the non-Ce rare earth elements is carried out by precipitation of the rare earth elements oxalates.

7. The process of claim 6, wherein precipitation is carried out by the addition of oxalic acid, or alternatively, an oxalate salt at a pH higher than 0.5 and up to 14.

8. The process of claims 6 or 7, further comprising a step g) of calcining the rare earth element oxalates to obtain the corresponding rare earth element oxides.

9. The process of claim 8, wherein calcining is carried out at a temperature higher than 700 °C.

10. The process of any one of claims 1 to 9, wherein the precipitate of cerium in the form of Ce(OH)4 of step c) further comprises coprecipitated non-Ce rare earth elements, and the process further comprises recovering the non-Ce rare earth elements by:- calcining the precipitate comprising Ce(OH)4 and non-Ce rare earth elements to obtain a calcined product;- dissolving the calcined product in HCI to obtain an acidic aqueous solution; and- performing step f) as defined in claims 6 or 7 and, optionally, also step g) as defined in claims 8 or 9, on the acidic aqueous solution; or, alternatively,- dissolving the precipitate comprising Ce(OH)4 and non-Ce rare earth elements in HCI to obtain an acidic aqueous solution; and- performing steps b) to f) as defined in any one of claims 1 to 7 and, optionally, also step g) as defined in claims 8 or 9, on the acidic aqueous solution.

11. The process of any one of claims 1 to 10, wherein the aqueous solution is a leachate of a substrate comprising cerium together with other additional rare earth elements.

12. The process of any one of claims 1 to 11, wherein the additional rare earth elements are selected from the group consisting of Nd, Pr, Dy, Gd, Tb, Ho and Er.

13. The process of any one of claims 10 to 12, wherein the substrate is a magnet such as a permanent magnet.

14. The process of any one of claims 10 to 12, wherein the leachate is obtained by leaching a magnet with hydrochloric acid to obtain a first liquor and then removing iron from the first liquor.

15. The process of any one of claims 1 to 14, further comprising the manufacture of an article of manufacture such as a high performance magnet with the recovered non-Ce rare earth elements.

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