METHOD FOR THE PREPARATION OF MIXED RARE EARTH METAL CARBONATES FROM MONAZITE

Abstract

The present invention relates to a method for the preparation of mixed rare earth metal carbonates from monazite, this method comprising a first leaching with an alkaline hydroxide and a two-step second leaching in the presence of hydrochloric acid.

Description

1 / 57 “METHOD FOR THE PREPARATION OF MIXED RARE EARTH METAL CARBONATES FROM MONAZITE FIELD OF THE INVENTION

[0001] The present invention relates to a method for preparing mixed rare earth metal carbonates from monazite. FUNDAMENTALS

[0002] Rare earth metals are high-value-added components in a wide range of applications, such as catalysts, materials for optical applications, magnets, and batteries, among others. These elements are often critical materials in a wide range of technological and electronic devices used daily. Although these elements are relatively abundant on Earth, there are very few raw materials where rare earth elements are concentrated. These materials include, among others, monazite and bastnasite. Due to a growing demand for rare earth metals for such applications, efforts have been dedicated to developing processes for extracting rare earth metals from ores containing high concentrations of these metals, such as monazite.

[0003] US patent application 2,811,411 A1 discloses a method for processing monazite sand to produce trisodium phosphate by treating the monazite sand with sodium hydroxide, thereby producing a metal hydroxide cake and a trisodium phosphate solution which is subsequently isolated by crystallization. The application also briefly discloses a treatment of the resulting metal hydroxide cake by acid leaching with HCl or HNO3. Petition 870250081351, dated 10 / 09 / 2025, page 6 / 79 2 / 57 Furthermore, it is also disclosed that thorium and rare earth uranium can then be recovered from the solution by various methods, for example, by solvent extraction. The document, however, does not disclose details about such methods of rare earth metal extraction.

[0004] Patent application GB 674,400 discloses a multi-stage process for extracting rare earth metal carbonates from monazite, comprising the following stages:

[0005] 1) Preparation of monazite until it reaches a particle size of 200 mesh (i.e., 75 gm);

[0006] 2) Supply of an aqueous solution of NaOH with 55-70% by weight (preferably 65%);

[0007] 3) Gradually adding the NaOH solution to the monazite particles, so that the weight ratio of NaOH for monazite, be it 1, at a temperature of 140-145°C, for 2-5 h;

[0008] 4) Dilution of the paste resulting from the stage 4, at a temperature above 55 °C, until a NaOH concentration of 13% by weight (130 g per liter) is reached;

[0009] 5) Separation of precipitated metallic hydroxides by filtration or decantation;

[0010] 6) Crystallization of trisodium phosphate by cooling;

[0011] 7) Filtration of trisodium phosphate;

[0012] 8) Recovery of trisodium phosphate with 90% ore yield;

[0013] 9) Recovery and recirculation of NaOH having a concentration of approximately 13% for use in dilution step 4; Petition 870250081351, dated 10 / 09 / 2025, page 7 / 79 3 / 57

[0014] 10) Concentration of recirculated NaOH for use in stage 3;

[0015] 11) (optional): additional decantation of the metal hydroxides separated in stage 5 at a temperature above 25 °C;

[0016] 12) Filtration and washing of metal hydroxide cake;

[0017] 13) Addition of excess hydrochloric acid until all metallic hydroxides dissolve and precipitation of Th and U species by adding rare earth metal carbonates until pH 4 is reached. Th and U species begin to dissolve at pH values ​​below 3.2;

[0018] 14) Filtration of Th and U species to produce a liquid phase comprising rare earth metal chlorides;

[0019] 15) The aforementioned rare earth metal chlorides were recovered with more than 95% of the rare earth metals rendered soluble by the attack;

[0020] 16) Precipitation of mixed rare earth metal carbonate by treatment with sodium carbonate.

[0021] CK Gupta and N. Krishnamurthy (1992) Extractive metallurgy of rare earths, International Materials Reviews, 37:1, 197-248, discloses a similar multistage process for extracting rare earth metal carbonates from monazite. In particular, this process employs an acid leaching step using hydrochloric acid at a pH between 3.4 and 4. This pH is useful for selectively dissolving the rare earth chlorides while retaining the radioactive waste (Th and U) in solid form. This selective extraction is disclosed as Petition 870250081351, dated 10 / 09 / 2025, page 8 / 79 4 / 57 essential for producing mixed carbonates of non-radioactive rare earth metals.

[0022] Feng Xie et al. disclose in Minerals Engineering 2013, vol. 56, p. 10-28, a process for the extraction of mixed rare earth metal carbonates, comprising: 1) a basic leaching step of monazite particles, producing a liquid fraction A, comprising sodium triphosphate, and a solid fraction B, comprising rare earth metal hydroxide salts; 2) Separate solid B from liquid A; 3) an acid leaching step in hydrochloric acid of solid B, which produces a solid fraction C, comprising thorium and uranium hydroxide salts, and a liquid fraction D, comprising rare earth metal chloride salts; and 4) Separate solid C from liquid D.

[0023] Patent application CN 111 187 926 A discloses a process for the preparation of mixed rare earth chlorides from monazite, comprising: 1) an acid leaching step of monazite particles using hydrochloric acid with the aim of reducing the amount of calcium in the monazite particles and producing a solid residue that is isolated and then subjected to a basic leaching step of monazite particles, producing a liquid fraction A, comprising sodium triphosphate, and a solid fraction B, comprising rare earth metal hydroxide salts; 2) Separate solid B from liquid A; 3) an acid leaching step in acid Petition 870250081351, dated 10 / 09 / 2025, page 9 / 79 5 / 57 hydrochloric acid of solid B, which produces a solid fraction C, comprising thorium and uranium hydroxide salts, and a liquid fraction D, comprising rare earth metal chloride salts; and 4) Separate solid C from liquid D.

[0024] Mellodee et al. disclose in the Journal of Radioanalytical and Nuclear Chemistry 2014, vol. 303, no. 2, 1393-1398, a process for the extraction of mixed rare earth metal carbonates comprising: 1) a basic leaching step of monazite particles, producing a liquid fraction A, comprising sodium triphosphate, and a solid fraction B, comprising rare earth metal hydroxide salts; 2) Separate solid B from liquid A; 3) an acid leaching step in hydrochloric acid of solid B, which produces a solid fraction C, comprising thorium and uranium hydroxide salts, and a liquid fraction D, comprising rare earth metal chloride salts; 4) Separate solid C from liquid D; 5) remove radium and lead from D by (i) adding barium chloride and sodium sulfate to liquid D, thereby producing radium sulfate, and (ii) adding sodium sulfide to liquid D, thereby producing lead sulfide.

[0025] Patent application CN1045010C discloses a process for extracting rare earth metal compounds from a mixture of bastnasite and monazite using phosphonic acid as an extractant (P507).

[0026] Patent application CN111020242 discloses a process for the preparation of mixed metal chlorides of Petition 870250081351, dated 10 / 09 / 2025, page 10 / 79 6 / 57 rare earths from monazite particles, as well as the recovery of uranium and thorium compounds.

[0027] US patent application 2010 / 018347 discloses a chemical separation process for removing radium-228 from thorium-232-containing materials, such as monazite.

[0028] Several processes for extracting rare earth metals from monazite and other ores are disclosed in G. Balachandran: “Extraction of Rare Earths for Advanced Applications” 2014, Elsevier Science, pages 1291-1340.

[0029] Based on what is disclosed in the art, it is concluded that there is still a need to provide improved methods for extracting mixed rare earth metal carbonates from monazite. SUMMARY OF THE INVENTION

[0030] After exhaustive research, the inventors developed a process for the extraction of mixed rare earth metal carbonates from monazite, characterized by a high recovery yield of rare earth metals while avoiding Naturally Occurring Radioactive Materials (NORMs) in the products. The inventors found, in particular, that when the acid leaching step is carried out in two leaching stages, employing a first leaching stage at a pH between 1 and 2.5 and a second leaching stage at a pH between 3.2 and 4, the recovery of rare earth metals is improved. The specific acid leaching step of the invention advantageously allows for the unexpectedly effective removal of radioactive species of Th and U. Furthermore, the quantities of other metals, in addition to the rare earth metals, are reduced. Petition 870250081351, dated 10 / 09 / 2025, page 11 / 79 7 / 57 of rare earth elements, such as Fe, Al, and Pb, in the fraction comprising rare earth metal salts, are advantageously reduced, which further facilitates downstream operation and purification of rare earth metal carbonates.

[0031] In certain embodiments, the invention process presents additional advantages, as it allows, in certain embodiments, the efficient isolation of trisodium phosphate, which is a commercially valuable product, since said product is isolated by evaporation of a liquid fraction. Recovery of the vapor stream from said evaporation step allows the recirculation of water and energy at various points in the process where these are needed. Furthermore, the liquid fraction resulting from the precipitation of trisodium phosphate can be recycled in the process.

[0032] In certain embodiments, the invention process allows, in particular, the production of mixed rare earth metal carbonates that are substantially free of cerium species, since it allows the isolation of cerium (IV) hydroxide as a commercially valuable process product. Furthermore, the mixed rare earth metal carbonates produced are substantially free of radioactive components.

[0033] Thus, in a first aspect, the invention relates to a process for the preparation of mixed rare earth metal carbonate from monazite, said process comprising: (a) subjecting monazite particles to a leaching step with an aqueous solution of alkaline hydroxide, thus obtaining a mixture of: - a liquid fraction A comprising phosphate Petition 870250081351, dated 10 / 09 / 2025, page 12 / 79 8 / 57 trisodium and - a solid B comprising mixed hydroxide salts of the metals comprised in monazite; (b) separate solid B from solution A, preferably by filtration; (c) a leaching step of solid B comprising: (c-1) a first leaching step of solid B with an aqueous solution of hydrochloric acid, feeding said hydrochloric acid solution so that the resulting mixture has a pH between 1 and 2.5; (c-2) a second leaching stage after the first leaching stage (c-1), where the pH is adjusted to a value between 3.2 and 4 by the addition of an alkaline hydroxide; thus obtaining a mixture of: - a solid fraction C comprising thorium and / or uranium hydroxide salts, and - a liquid fraction D comprising rare earth metal chloride salts; and (d) separate the solid C from the liquid fraction D, preferably by filtration. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Fig. 1 shows a general scheme of the invention process. DETAILED DESCRIPTION

[0035] All terms as used in this application, unless otherwise indicated, shall be understood in their common meaning as known in the art. Petition 870250081351, dated 10 / 09 / 2025, page 13 / 79 9 / 57 Other more specific definitions for certain terms as used in this application are as set forth below and are intended to be applied uniformly throughout the descriptive report and claims, unless a definition expressly set forth otherwise provides a broader definition.

[0036] For the purposes of the invention, any particular ranges or values ​​provided include the lower and upper extreme points of the range. The ranges provided, such as temperatures, times, molar ratio, volumetric ratio and the like, should be considered approximate (i.e., with a margin of variation of 5% around the indicated point), unless specifically indicated.

[0037] In the context of the invention, the term “mixed rare earth metal carbonate” refers to a mixture of rare earth metal carbonate salts. The term “mixed rare earth metal carbonate” refers more particularly to a mixture of cerium Ce, lanthanum La, neodymium Nd, praseodymium Pr, europium Eu, gadolinium Gd, dysprosium Dy, yttrium Y, scandium Sc, samarium Sm, holmium Ho, erbium Er, thulium Tm, lutetium Lu, terbium Tb carbonate salts. It is preferred that the term "mixed rare earth metal carbonate" refers to a mixture of cerium carbonate salts Ce, lanthanum carbonates La, neodymium carbonates Nd, praseodymium carbonates Pr, europium carbonates Eu, gadolinium carbonates Gd, dysprosium carbonates Dy, yttrium carbonates Y, and samarium carbonates Sm. It is more preferred that the term "mixed rare earth metal carbonate" refers to a mixture of lanthanum carbonate salts La, neodymium carbonates Nd, praseodymium carbonates Pr, europium carbonates Eu, gadolinium carbonates Gd, dysprosium carbonates Dy, yttrium carbonates Y, and samarium carbonates Sm. Similarly, the term "mixed rare earth metal chloride" is Petition 870250081351, dated 10 / 09 / 2025, page 14 / 79 10 / 57 refers to a mixture of rare earth metal chloride salts.

[0038] In the context of the invention, the term monazite refers to a phosphate mineral containing rare earth elements and comprising rare earth metal phosphates, said rare earth metals being as listed above.

[0039] In the context of the invention, the term “diameter”, when referring to the diameter of a particle, refers to an average particle diameter, whereby 80% of the particle population has a diameter below a certain value. This term is also commonly referred to in the art as “D80”.

[0040] As mentioned above, a first aspect of the invention relates to a process for the preparation of mixed rare earth metal carbonate from monazite, said process comprising: (a) subjecting monazite particles to a leaching step with an aqueous solution of alkaline hydroxide, thus obtaining a mixture of: - a liquid fraction A comprising trisodium phosphate and - a solid B comprising mixed hydroxide salts of the metals comprised in monazite; (b) separate solid B from solution A, preferably by filtration; (c) a leaching step of solid B comprising: (c-1) a first leaching step of solid B with an aqueous solution of hydrochloric acid, Petition 870250081351, dated 10 / 09 / 2025, page 15 / 79 11 / 57 feeding the aforementioned aqueous hydrochloric acid solution so that the resulting mixture has a pH between 1 and 2.5; (c-2) a second leaching stage after the first leaching stage (c-1), where the pH is adjusted to a value between 3.2 and 4 by the addition of an alkaline hydroxide; thus obtaining a mixture of: - a solid fraction C comprising thorium and uranium hydroxide salts, and - a liquid fraction D comprising rare earth metal chloride salts; and (d) separate the solid C from the liquid fraction D, preferably by filtration.

[0041] Monazite comprises phosphate salts of rare earth elements. As defined above, the process of the invention comprises a first basic leaching step, followed by a second acidic leaching step. The objective of the first basic leaching step (a) is to break down the phosphate matrix comprising the rare earth metallic elements. This is achieved by treating the monazite ore in a concentrated alkaline medium so as to precipitate the hydroxide salts of the metals comprised in the monazite. The main chemical reactions that occur during this step are: CePO4 (s) + 3 NaOH (a) ^ Ce(OH)a (s) + NaaPO4(a) LaPO4 (s) + 3 NaOH (a) ^ La(OH)a (s) + NaaPO4 (a) NdPO4 (s) + 3 NaOH (a) ^ Nd(OH)a (s) + NaaPO4(a) PrPO4 (s) + 3 NaOH (a) ^ Pr(OH)3 (s) + Na3PO4(a) EuPO4 (s) + 3 NaOH (a) ^ Eu(OH)3 (s) + Na3PO4(a) Petition 870250081351, dated 10 / 09 / 2025, p. 16 / 79 12 / 57 GdPO4 (s) + 3 NaOH (a) ^ Gd(OH)a (s) + NaaPO4 (a) YPO4(s) + 3 NaOH(a) ^ Y(OH)3(s) + Na3PO4(a) SmPO4 (s) + 3 NaOH (a) ^ Sm(OH)3 (s) + Na3PO4 (a) Pb3(PO4)2 (s) + 6 NaOH (a) ^ 3 Pb(OH)2 (s) + 2 Na3PO4 (a) FeOOH (s) + 1 H2O (a) ^ Fe(OH)3 (a) S1O2 (s) + 2 NaOH (a)^ Na2SiO3 (s) + H2O (a)

[0042] In a preferred embodiment of the invention process, the monazite particles of step (a) have a particle diameter of less than 100 μm.

[0043] In a further preferred embodiment of the invention process, the alkaline hydroxide is selected from potassium hydroxide, sodium hydroxide and mixtures thereof; preferably, it is sodium hydroxide.

[0044] In a further preferred embodiment of the invention process, the alkaline hydroxide is sodium hydroxide and the weight ratio of sodium hydroxide to monazite is between 1.5 and 2.5. When the alkaline is other than sodium hydroxide, the skilled technician will adapt the weight ratio of the alkaline hydroxide accordingly.

[0045] In another preferred embodiment of the invention process, the aqueous alkaline hydroxide solution of step (a) comprises said alkaline hydroxide in an amount such that, when said alkaline hydroxide is sodium hydroxide, the aqueous sodium hydroxide solution will comprise sodium hydroxide in an amount of between 50 and 60% by weight; preferably, about 55% by weight.

[0046] In another preferred embodiment of the invention process, step (a) is carried out in such a way that the weight ratio of alkaline hydroxide to monazite is such that, when Petition 870250081351, dated 10 / 09 / 2025, p. 17 / 79 13 / 57 The aforementioned alkaline hydroxide is sodium hydroxide; the weight ratio of sodium hydroxide to monazite is between 3:2 and 3:1, preferably around 2:1. The technician will know how to adapt the weight ratio of alkaline hydroxide to monazite when alkaline hydroxides other than sodium hydroxide are used.

[0047] In another preferred embodiment of the invention process, step (a) is carried out at a temperature between 130 and 160°C, preferably 135 to 155°C, more preferably 135 to 145°C, even more preferably about 142°C.

[0048] In another preferred embodiment of the invention process, step (a) has a duration of at least 3 hours and up to 5 hours, preferably 4 hours.

[0049] In another preferred embodiment of the invention process, step (a) further comprises diluting the mixture obtained, at around 100 °C, with water, so as to achieve an alkaline hydroxide concentration such that, when said alkaline hydroxide is sodium hydroxide, the sodium hydroxide is at a concentration of about 25% by weight. Said dilution step is carried out after the basic leaching step.

[0050] In another preferred embodiment of the invention process, step (a) is carried out in such a way that: (i) the monazite particles from step (a) have a particle diameter of less than 100 μm; (ii) the alkaline hydroxide is sodium hydroxide and the aqueous alkaline hydroxide solution of step (a) comprises sodium hydroxide in an amount of between 50 and 60% by weight; preferably about 55% by weight; Petition 870250081351, dated 10 / 09 / 2025, page 18 / 79 14 / 57 (iii) the alkaline hydroxide is sodium hydroxide and the weight ratio of sodium hydroxide to monazite is about 2:1; (iv) step (a) is carried out at a temperature of about 130 to 150°C, preferably 142°C; (v) step (a) lasts at least 3 hours and up to 5 hours, preferably 4 hours; and (vi) step (a) also includes diluting the mixture obtained at around 100°C with water, so as to achieve a sodium hydroxide concentration of about 25% by weight.

[0051] As mentioned earlier, the basic leaching step is followed by an acidic leaching step, comprising steps (c) and (d), as defined previously. The objective of this step is to dissolve the metallic hydroxide salts comprised in the solid fraction B obtained in the first basic leaching step in a liquid phase, transforming them into their chloride salts. Careful choice of pH in this step allows for improved selectivity of extraction of certain metals relative to others.

[0052] The following chemical reactions occur, in particular, during this stage: Ce(OH)3 (s) + 3 HCl (a) ^ CeCl3 (a) + 3 H2O(a) La(OH)3 (s) + 3 HCl (a) ^ LaCl3 (a) + 3 H2O(a) Nd(OH)3 (s) + 3 HCl (a) ^ NdCl3 (a) + 3 H2O(a) Pr(OH)3 (s) + 3 HCl (a) ^ PrCl3 (a) + 3 H2O(a) Eu(OH)3 (s) + 3 HCl (a) ^ EuCl3 (a) + 3 H2O(a) Gd(OH)3 (s) + 3 HCl (a) ^ GdCl3 (a) + 3 H2O(a) Dy(OH)3 (s) + 3 HCl (a) ^ DyCl3 (a) + 3 H2O(a) Y(OH)3 (s) + 3 HCl (a) ^ YCl3 (a) + 3 H2O (a) Petition 870250081351, dated 10 / 09 / 2025, p. 19 / 79 15 / 57 Sm(OH)3 (s) + 3 HCl (a) ^ SmCla (a) + 3 H2O(a) Pb(OH)2 (s) + 2 HCl (a) ^ PbCl2 (a) + 2 H2O(a) Fe(OH)3 (s) + 3 HCl (a) ^ FeCl3 (a) + 3 H2O(a) Al2O3 (s) + 6 HCl (a) ^ 2 AlCl3 (a) + 3 H2O (a) NaAlO2 (s) + 4 HCl (a) ^ AlCls (a) + NaCl (a) + 2 H2O (a)

[0053] In another preferred embodiment of the invention process, step (c) is that in which step (c-1) and / or step (c-2) are carried out at a temperature between 80 and 95 °C, preferably at 90 °C, and / or for at least 70 minutes, preferably for 90 minutes.

[0054] In another preferred embodiment of the invention process, step (c) is that in which step (c-1) is carried out at a pH of at least 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 or 1.9 and not higher than 2.5.

[0055] In another preferred embodiment of the invention process, step (c) is that in which step (c-1) is carried out at a pH of at least 1 and less than 2.4, 2.3, 2.2 or 2.1.

[0056] In another preferred embodiment of the invention process, step (c) is that in which step (c-1) is carried out at a pH of about 2.

[0057] In another preferred embodiment of the invention process, step (c) is that in which the pH of the mixture resulting from step (c-2) is adjusted to a value between 3.2 and 4, preferably 3.6, by adding potassium hydroxide.

[0058] In another preferred embodiment of the invention process, step (c) is that in which step (c-2) is carried out at a pH of at least 3.3, 3.4 or 3.5 and not higher than 4. Petition 870250081351, dated 10 / 09 / 2025, p. 20 / 79 16 / 57

[0059] In another preferred embodiment of the invention process, step (c) is that in which the pH of the mixture resulting from step (c-2) is adjusted to a value of at least 3.2 and less than 4, 3.9, 3.8 or 3.7.

[0060] In another preferred embodiment of the invention process, step (c) is that in which step (c-2) is carried out at a pH of about 3.6.

[0061] In another preferred embodiment of the invention process, step (c) is that in which solid B is fed as a suspension, at a concentration of between 200 and 500 g per liter, preferably about 300 g per liter.

[0062] In another preferred embodiment of the invention process, step (c) is that in which: (i) step (c-1) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, more preferably for 90 minutes; (ii) step (c-1) is carried out at a pH of about 2; (iii) step (c-2) is carried out at a pH of about 3.6; (iv) step (c-2) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, preferably for 90 minutes; (v) the pH of the mixture from step (c-1) is adjusted to 3.6 by adding an alkaline hydroxide, such as potassium hydroxide; and (vi) solid B is fed as a suspension at a concentration of between 200 and 500 g per liter, preferably about 300 g per liter. Petition 870250081351, dated 10 / 09 / 2025, p. 21 / 79 17 / 57

[0063] Thus, in a more preferred embodiment of the invention process, step (a) is that in which: (i) the monazite particles in step (a) have a particle diameter smaller than 100 μm; (ii) the alkaline hydroxide is sodium hydroxide and the aqueous alkaline hydroxide solution of step (a) comprises sodium hydroxide in an amount of between 50 and 60% by weight; preferably about 55% by weight; (iii) the alkaline hydroxide is sodium hydroxide and the weight ratio of sodium hydroxide to monazite is about 2:1; (iv) step (a) is carried out at a temperature of about 130 to 150°C, preferably 142°C; (v) step (a) lasts at least 3 hours and up to 5 hours, preferably 4 hours; and (vi) step (a) further comprises diluting the mixture obtained at 100 °C with water, so as to achieve a sodium hydroxide concentration of about 25% by weight; and step (c) is that in which: (i) step (c-1) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, preferably for 90 minutes; (ii) step (c-1) is carried out at a pH of about 2; (iii) step (c-2) is carried out at a pH of about 3.6; (iv) step (c-2) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, preferably for 90 minutes; Petition 870250081351, dated 10 / 09 / 2025, p. 22 / 79 18 / 57 (v) the pH of the mixture from step (c-1) is adjusted to 3.6 by adding an alkaline hydroxide, such as potassium hydroxide; and (vi) solid B is fed as a suspension at a concentration of between 200 and 500 g per liter, preferably about 300 g per liter.

[0064] The process disclosed above advantageously allows for a reduction in the amount of non-rare earth metals, such as Th, U, Pb, Fe, and Al, extracted along with the rare earth metals in the liquid fraction D resulting from the acid leaching step. This allows for easier preparation of mixed rare earth metal carbonates with an improved purity profile, since the liquid fraction D, resulting from the acid leaching step, comprises a smaller amount of undesirable components, such as Th, U, Pb, Fe, and Al, than processes known in the art where acid leaching is carried out at a constant pH.

[0065] The invention process may include additional steps aimed at further reducing the amount of impurities in the fractions comprising rare earth metals and converting said fraction into mixed rare earth metal carbonates.

[0066] In particular, the invention process is intended to comprise the step of substantially removing all Ra(II) and / or Pb(II) species from the liquid fraction D, preferably by precipitation of RaSO4 and / or PbS. This step advantageously allows for the reduction of the radioactivity of the mixed rare earth metal fraction.

[0067] Thus, in a preferred embodiment, the Petition 870250081351, dated 10 / 09 / 2025, page 23 / 79 19 / 57 The invention process further comprises the following steps: (e) bring liquid fraction D into contact with a source of barium(II), a source of sulfate and a source of sulfide, so as to substantially precipitate all Ra(II) and Pb(II) species contained in said liquid fraction, in the form of RaSO4 and PbS; thus obtaining: - a solid fraction E comprising lead sulfide and barium and radium sulfate salts, and - a liquid fraction F comprising mixed salts of rare earth metal chlorides; (f) separate the liquid fraction F from the solid fraction E, preferably by filtration.

[0068] In the aforementioned preferred modality, step (e) preferably comprises the steps of: (e-1) precipitate radium sulfate salts by placing liquid fraction D in contact with a barium(II) source and a sulfate source; (e-2) precipitate lead sulfide by placing the product obtained in step (e-1) in contact with a source of sulfide(II).

[0069] In a preferred embodiment, in step (e1), the barium source is barium chloride. It is also preferred that the sulfate source be potassium sulfate.

[0070] In the aforementioned preferred embodiments, step (e-1) is the one in which the following chemical reaction occurs: K2SO4 (a) + BaCl2 (a) + RaCl2 (a)^ BaSO4 (s) + RaSO4 (s) + 4 KCl (a)

[0071] In a preferred embodiment, in step (e2), the sulfide (II) source is sodium sulfide. In said preferred embodiment, step (e-2) is that in which the following chemical reaction occurs: Petition 870250081351, dated 10 / 09 / 2025, p. 24 / 79 20 / 57 PbCl2 (a) + Na2S (a) ^ PbS <s>+ 2 NaCl (a)

[0072] In the aforementioned preferred embodiment, the aforementioned step (e-1) preferably satisfies one or more of, preferably all of, the following conditions: (i) the source of sulfate is potassium sulfate; (ii) the amount of sulfate source is such that the sulfate concentration in the middle of step (f) is between 5 and 15 g per liter, preferably 10 g per liter; (iii) the source of barium(II) is barium(II) chloride; (iv) the amount of barium(II) source is such that the molar ratio of Ba(II) to sulfate is between 1:5 and 1:15, preferably 1:10; (v) the temperature is between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C; and / or (vi) the reaction time is at least 45 minutes, preferably around 60 minutes;

[0073] In the aforementioned preferred embodiment, the aforementioned step (e-2) preferably satisfies one or more of, preferably all of, the following conditions: (i) the pH of the reaction medium in step (e-2) is 3.6 and is optionally adjusted by adding an alkaline hydroxide, such as potassium hydroxide; (ii) the source of sulfide(II) is sodium sulfide; (iii) the amount of sulfide(II) source is such that the molar ratio of lead to sulfide is approximately 1:1; and / or (iv) the temperature is between 50°C and 90°C; Petition 870250081351, dated 10 / 09 / 2025, p. 25 / 79 21 / 57 preferably between 60°C and 80°C; more preferably around 70°C.

[0074] The combination of steps (e) and (f) produces a liquid fraction F comprising rare earth metal chlorides that is substantially free of Ra(II) and Pb(II) species. These species typically involve radioactive isotopes of Ra(II) and Pb(II). Thus, the combination of steps (e) and (f) allows the production of a liquid fraction that is substantially free of radioactive isotopes of radium and lead, in any of their known cationic forms. In particular, the liquid fraction F advantageously meets the regulatory requirements of Spanish RD783 / 2001 and RD 1439 / 2010.

[0075] It is further intended that the invention process includes the step of substantially removing all Ce(III) cations in the liquid fraction comprising rare earth metal chloride salts, preferably by oxidative precipitation of cerium(IV) hydroxide. This step is advantageous because it allows isolating a valuable rare earth metal, namely cerium, from the rare earth metal mixture. As will be obvious to the expert, this step can be carried out on any liquid fraction comprising cerium(III) chloride obtained in the invention process. For example, it can be carried out on liquid fraction D, resulting from the acid leaching step, or on liquid fraction F, resulting from the removal of Ra(II) and Pb(II) species. However, it is preferred that this step be carried out on liquid fraction F.

[0076] Thus, in a preferred embodiment, the invention process further comprises the steps of: Petition 870250081351, dated 10 / 09 / 2025, page 26 / 79 22 / 57 (g) to substantially convert all Ce(III) cations from liquid fraction F into Ce(IV) hydroxide by subjecting said liquid fraction F to oxidative conditions; thus obtaining: - a solid fraction G consisting essentially of cerium(IV) hydroxide; and - a liquid fraction H comprising rare earth metal chloride salts; and (h) separate the liquid fraction H from the solid fraction G.

[0077] As mentioned above, the aforementioned step can be performed on the net fraction D resulting from step (d).

[0078] In the aforementioned preferred embodiment, step (g) preferably comprises contacting the liquid fraction F with a hypochlorite salt, such as sodium hypochlorite; more preferably, step (g) satisfies one or more of the following conditions: (i) the temperature is between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C; (ii) the pH of the reaction medium is between 3 and 4, preferably around 3.5; and / or (iii) sodium hypochlorite is present in an amount of at least 2 moles for each mole of cerium(III) chloride in the liquid fraction F, preferably 3 moles for each 2 moles of cerium(III) chloride in the liquid fraction F.

[0079] The invention process is suitable for the production of mixed rare earth metal carbonates. Petition 870250081351, dated 10 / 09 / 2025, page 27 / 79 23 / 57 Thus, the invention process is intended to comprise the step of converting a liquid fraction comprising rare earth metal chloride salts, such as the liquid fractions D, F and H of the process disclosed in this document, into solid rare earth metal mixed carbonates; preferably by precipitation of the rare earth metal carbonate salts.

[0080] The choice of the liquid fraction, chosen as feedstock for this step, depends on the purity profile of the target rare earth metal mixed carbonates. Therefore, it is preferable that the aforementioned precipitation step of the rare earth metal carbonate salts be carried out on the liquid fraction F or H, since these fractions are substantially free of radioactive species of Ra(II) and Pb(II). As a person skilled in the art will realize, the desirability of the presence of large amounts of cerium in the rare earth metal mixed carbonates will determine the choice of fraction F instead of fraction H as feedstock, and vice versa. However, it is preferable that this step be carried out on the liquid fraction H.

[0081] Thus, the invention process may also include the following steps: (i) placing the liquid fraction H in contact with an alkaline carbonate salt, such as sodium carbonate, so as to substantially precipitate all the rare earth metals comprised in said fraction, in the form of carbonate salts; thus obtaining - a solid fraction I consisting of mixed carbonate salts of rare earth metals, and Petition 870250081351, dated 10 / 09 / 2025, p. 28 / 79 24 / 57 - a liquid fraction J consisting of rare earth metal chloride salts and (j) separate the solid fraction I from the liquid fraction J; preferably by filtration.

[0082] The aforementioned process can be performed on either of the fractions D or F.

[0083] Step (i) of the invention process is preferably carried out at a temperature between 50°C and 100°C; preferably around 70°C.

[0084] Step (i) of the invention process is preferably carried out at a pH between 5.5 and 6.5, preferably around 6.

[0085] The alkaline carbonate salt, used in step (i), is preferably sodium carbonate. Said alkaline carbonate is preferably used as an aqueous solution, wherein the concentration of the alkaline carbonate is such that, when said alkaline carbonate is sodium carbonate, the aqueous solution will have an alkaline carbonate concentration of between 150 and 200 grams per liter. An expert in the field will know which concentration to use when employing carbonate salts other than sodium carbonate.

[0086] In the aforementioned preferred embodiment, step (i) preferably satisfies one or more of, preferably all of, the following conditions: (i) the temperature is 70°C; (ii) sodium carbonate is added as an aqueous solution having a concentration of approximately 180 grams per liter; and / or (ii) the amount of alkaline carbonate is such Petition 870250081351, dated 10 / 09 / 2025, p. 29 / 79 25 / 57 that the pH of the solution is 6.

[0087] Thus, in a more preferred embodiment of the invention process, step (a) is that in which: (i) the monazite particles in step (a) have a particle diameter smaller than 100 μm; (ii) the alkaline hydroxide is sodium hydroxide and the aqueous sodium hydroxide solution of step (a) comprises sodium hydroxide in an amount of between 50 and 60% by weight; preferably about 55% by weight; (iii) Alkaline hydroxide is sodium hydroxide and the weight ratio of sodium hydroxide to monazite is about 2:1; (iv) step (a) is carried out at a temperature of about 130 to 150°C, preferably 142°C; (v) step (a) lasts at least 3 hours and up to 5 hours, preferably 4 hours; and (vi) step (a) further comprises diluting the mixture obtained at 100°C with water, so as to achieve a sodium hydroxide concentration of about 25% by weight; Step (c) is the one in which: (i) step (c-1) is carried out at a temperature between 80 and 95 °C, preferably at 90 °C and / or for at least 70 minutes, preferably for 90 minutes; (ii) step (c-1) is carried out at a pH of about 2; (iii) step (c-2) is carried out at a pH of about 3.6; (iv) step (c-2) is carried out at a temperature between 80 and 95 °C, preferably at 90 °C and / or for at least 70 minutes, preferably for 90 minutes; Petition 870250081351, dated 10 / 09 / 2025, page 30 / 79 26 / 57 (v) the pH of the mixture from step (c-1) is adjusted to 3.6 by adding an alkaline hydroxide, such as potassium hydroxide; and (vi) solid B is fed as a suspension at a concentration of between 200 and 500 g per liter, preferably about 300 g per liter; The process also includes the following steps: (e-1) precipitate the radium sulfate salts by contacting the liquid fraction D with a barium(II) source and a sulfate source, wherein said step (e-1) satisfies the following conditions: (i) the source of sulfate is potassium sulfate; (ii) the amount of sulfate source is such that the sulfate concentration, in the middle of step (f), is between 5 and 15 g per liter, preferably 10 g per liter; (iii) the source of barium(II) is barium(II) chloride; (iv) the amount of barium(II) source is such that the molar ratio of Ba(II) to sulfate is between 1:5 and 1:15, preferably 1:10; the temperature is between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C; and (vi) the reaction time is at least 45 minutes, preferably around 60 minutes; (e-2) precipitate lead sulfide by placing the product obtained in step (e-1) in contact with a source of sulfide(II), wherein said step (e-2) satisfies the following conditions: Petition 870250081351, dated 10 / 09 / 2025, p. 31 / 79 27 / 57 (i) the pH of the reaction medium in step (e-2) is 3.6 and is optionally adjusted by adding an alkaline hydroxide, such as potassium hydroxide; (ii) the source of sulfide(II) is sodium sulfide; (iii) the amount of sulfide(II) source is such that the molar ratio of lead to sulfide is about 1:1; and (iv) the temperature is between 50 °C and 90 °C; preferably between 60 °C and 80 °C; more preferably about 70 °C; (f) separate the liquid fraction F from the solid fraction E by filtration; (g) to substantially convert all Ce(III) cations in the liquid fraction F into Ce(IV) hydroxide by subjecting said liquid fraction F to oxidative conditions; thus obtaining: - a solid fraction G consisting essentially of cerium(IV) hydroxide; and - a liquid fraction H comprising rare earth metal chloride salts; wherein said step (g) satisfies the following conditions: (i) the temperature is 70°C; (ii) the pH of the reaction medium is between 3 and 4, preferably about 3.5; and (iii) sodium hypochlorite is present in an amount of at least 2 moles for each mole of cerium(III) chloride in the liquid fraction F, preferably 3 moles for each 2 moles of cerium(III) chloride in the liquid fraction. Petition 870250081351, dated 10 / 09 / 2025, page 32 / 79 28 / 57 F; (h) separate the liquid fraction H from the solid fraction G; (i) placing the liquid fraction H in contact with an alkaline carbonate salt, such as sodium carbonate, so as to substantially precipitate all the rare earth metals comprised in said fraction, in the form of carbonate salts; thus obtaining - a solid fraction I consisting of mixed carbonate salts of rare earth metals, and - a liquid fraction J comprising rare earth metal chloride salts; wherein step (i) satisfies the following conditions: (i) the temperature is 70 °C; (ii) the alkaline carbonate is sodium carbonate and is added as an aqueous solution having a concentration of about 180 grams per liter; and / or (ii) the amount of alkaline carbonate is such that the pH of the solution is 6; and (j) separate the solid fraction I from the liquid fraction. J by filtration.

[0088] Basic leaching steps (a)-(b) produce a liquid fraction that is rich in trisodium phosphate, which is a valuable product. The invention process is further intended to include steps for recovering this compound.

[0089] Thus, in a preferred embodiment, the invention process further comprises the step of: (k) precipitate trisodium phosphate from liquid fraction A and Petition 870250081351, dated 10 / 09 / 2025, page 33 / 79 29 / 57 (l) isolate the said trisodium phosphate by solid-liquid separation, thereby obtaining the trisodium phosphate and a liquid fraction K; preferably by filtration.

[0090] Methods for precipitating trisodium phosphate are known in the art and include, for example, cooling the liquid fraction A. However, it is preferred that trisodium phosphate be precipitated by evaporation of water.

[0091] The aforementioned water evaporation step preferably satisfies one or more, preferably all of, the following conditions: (i) the water evaporation step comprises heating liquid fraction A to a temperature of 128 °C for 90 minutes, (ii) the evaporated water is further condensed and optionally recirculated in the dilution step of step (b), where step (b) comprises diluting the mixture at 100 °C with water to achieve an alkaline hydroxide concentration of about 25% by weight, (iii) the trisodium phosphate is precipitated at a temperature between 20 °C and 40 °C; preferably 30 °C, for a period between 60 and 120 minutes, preferably 90 minutes; and / or (iv) the trisodium phosphate is isolated by filtration.

[0092] It is further intended that the liquid fraction K, which is rich in sodium hydroxide, be recirculated in step (a) of the process of the invention. The liquid fraction K, however, also comprises silicate species that are quick to accumulate in the process during recirculation of the Petition 870250081351, dated 10 / 09 / 2025, page 34 / 79 30 / 57 of the aforementioned fraction, in step (a).

[0093] A preferred embodiment of the invention process therefore comprises the step of removing the silicate species from said liquid fraction K.

[0094] It is also preferred that the removal of silicate species be carried out by precipitation of calcium silicate, according to the following reaction: Na2SiO3 (a) + CaO (s) + H2O (a) ^ CaSiO3 (s) + 2 NaOH (a)

[0095] Thus, the invention process also includes the following steps: (m) bring the liquid fraction K into contact with calcium oxide in a quantity sufficient to substantially precipitate all silicate salts contained in said liquid fraction in the form of calcium silicate; thus obtaining: - a solid fraction L comprising calcium silicate, and - a liquid fraction M enriched in sodium hydroxide, and (n) isolate said liquid fraction M by solid-liquid separation.

[0096] The resulting liquid fraction M advantageously has a reduced amount of silicate species and is suitable for feeding into step (a) of the process as a source of sodium hydroxide, with a reduced risk of silicate species accumulation in the process.

[0097] The invention process can therefore comprise the step of feeding the liquid fraction M to step (a).

[0098] Thus, in a more preferred modality of Petition 870250081351, dated 10 / 09 / 2025, page 35 / 79 31 / 57 process of the invention, step (a) is that in which: (i) the monazite particles from step (a) have a particle diameter of less than 100 μm; (ii) the alkaline hydroxide is sodium hydroxide and the aqueous sodium hydroxide solution of step (a) comprises sodium hydroxide in an amount of between 50 and 60% by weight; preferably, about 55% by weight; (iii) the alkaline hydroxide is sodium hydroxide and the weight ratio of sodium hydroxide to monazite is about 2:1; (iv) step (a) is carried out at a temperature of about 130 to 150°C, preferably 142°C; (v) step (a) lasts at least 3 hours and up to 5 hours, preferably 4 hours; and (vi) step (a) further comprises diluting the mixture obtained at 100°C with water, so as to achieve a sodium hydroxide concentration of about 25% by weight; Step (c) is the one in which: (i) step (c-1) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, preferably for 90 minutes; (ii) step (c-1) is carried out at a pH of about 2; (iii) step (c-2) is carried out at a pH of about 3.6; (iv) step (c-2) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, preferably for 90 minutes; (v) the pH of the mixture from step (c-1) is adjusted to 3.6 by adding an alkaline hydroxide, such as Petition 870250081351, dated 10 / 09 / 2025, p. 36 / 79 32 / 57 potassium hydroxide; and (vi) solid B is fed as a suspension at a concentration of between 200 and 500 g per liter, preferably about 300 g per liter; and the process also includes the following steps: (e-1) precipitate the radium sulfate salts by contacting the liquid fraction D with a barium(II) source and a sulfate source, wherein said step (e-1) satisfies the following conditions: (i) the source of sulfate is potassium sulfate; (ii) the amount of sulfate source is such that the sulfate concentration in the middle of step (f) is between 5 and 15 g per liter, preferably 10 g per liter; (iii) the source of barium(II) is barium(II) chloride; (iv) the amount of barium(II) source is such that the molar ratio of Ba(II) to sulfate is between 1:5 and 1:15, preferably 1:10; the temperature is between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C; and (vi) the reaction time is at least 45 minutes, preferably around 60 minutes; (e-2) precipitate lead sulfide by placing the product obtained in step (e-1) in contact with a source of sulfide(II), wherein said step (e-2) satisfies the following conditions: (i) the pH of the reaction medium in step (e-2) is 3.6 and is optionally adjusted by the addition of a hydroxide Petition 870250081351, dated 10 / 09 / 2025, page 37 / 79 33 / 57 alkaline, such as potassium hydroxide; (ii) the sulfide(II) source is sodium sulfide, (iii) the amount of sulfide(II) source is such that the molar ratio of lead to sulfide is approximately 1:1; and (iv) the temperature is 70 °C; (f) separate the liquid fraction F from the solid fraction And by filtration; (g) convert substantially all cations Ce(III) of the liquid fraction F in Ce(IV) hydroxide, subjecting said liquid fraction F to oxidative conditions; thus obtaining: - a solid fraction G consisting essentially of cerium(IV) hydroxide; and - a liquid fraction H comprising rare earth metal chloride salts; wherein said step (g) satisfies the following conditions: (i) the temperature is 70°C; (ii) the pH of the reaction medium is between 3 and 4, preferably about 3.5; and (iii) sodium hypochlorite is present in an amount of at least 2 moles for each mole of cerium(III) chloride in the liquid fraction F, preferably 3 moles for each 2 moles of cerium(III) chloride in the liquid fraction F; (h) separate the liquid fraction H from the solid fraction G; (i) placing the liquid fraction H in contact with a Petition 870250081351, dated 10 / 09 / 2025, page 38 / 79 34 / 57 alkaline carbonate salt, such as sodium carbonate, in order to substantially precipitate all the rare earth metals included in the said fraction, in the form of carbonate salts; thus obtaining - a solid fraction I consisting of mixed carbonate salts of rare earth metals, and - a liquid fraction J comprising rare earth metal chloride salts; wherein step (i) satisfies the following conditions: (i) the temperature is 70°C; (ii) sodium carbonate is added as an aqueous solution having a concentration of about 180 grams per liter; and / or (ii) the amount of alkaline carbonate is such that the pH of the solution is 6; (j) separate the solid fraction I from the liquid fraction J by filtration; (k) precipitate trisodium phosphate from liquid fraction A by evaporation of water, wherein step (k) satisfies the following conditions: (i) the water evaporation step comprises heating liquid fraction A to a temperature of 128°C for 90 minutes, (ii) the evaporated water is further condensed and optionally recirculated in the dilution step of step (b), where step (b) comprises diluting the mixture at 100°C with water to achieve a sodium hydroxide concentration of about 25% by weight, (iii) trisodium phosphate is precipitated at a temperature between 20°C and 40°C; preferably 30°C, Petition 870250081351, dated 10 / 09 / 2025, page 39 / 79 35 / 57 during a period of between 60 and 120 minutes, preferably 90 minutes (l) isolate said trisodium phosphate by solid-liquid separation, thus obtaining the trisodium phosphate and a liquid fraction K by filtration; (m) bring the liquid fraction K into contact with calcium oxide in a quantity sufficient to substantially precipitate all silicate salts contained in said liquid fraction in the form of calcium silicate; thus obtaining: - a solid fraction L comprising calcium silicate, and - a liquid fraction M enriched in sodium hydroxide, (n) isolate said liquid fraction M by solid-liquid separation, and (o) optionally, feed the liquid fraction M to step (a).

[0099] As mentioned above, the invention process advantageously allows obtaining mixed rare earth metal carbonate with high extraction yields, while minimizing the amount of impurities such as Th, U, Pb, Fe and Al.

[00100] In a more preferred embodiment, the invention process satisfies one or more of the following conditions: (i) the process allows the recovery of at least 71% of the cerium present in monazite, in the form of cerium(IV) hydroxide; (ii) the process allows for the recovery of hair Petition 870250081351, dated 10 / 09 / 2025, page 40 / 79 36 / 57 minus 73% of the lanthanum present in monazite, in the form of mixed rare earth metal carbonate; (iii) the process allows the recovery of at least 59% of the neodymium present in monazite, in the form of mixed rare earth metal carbonate; (iv) the process allows the recovery of at least 59% of the praseodymium present in monazite, in the form of mixed rare earth metal carbonate; (v) the process allows the recovery of at least 44% of the europium present in monazite, in the form of mixed rare earth metal carbonate; (vi) the process allows the recovery of at least 50% of the gadolinium present in monazite, in the form of mixed rare earth metal carbonate; (vii) the process allows the recovery of at least 37% of the dysprosium present in monazite, in the form of mixed rare earth metal carbonate; (vii i) the process allows the recovery of at least 57% of the yttrium present in monazite, in the form of mixed rare earth metal carbonate; and / or (ix ) the process allows the recovery of at least 53% of the samarium present in monazite, in the form of mixed rare earth metal carbonate.

[00101] In a more preferred embodiment, the invention process satisfies one or more of the following conditions: (i) the process produces mixed rare earth metal carbonates where the Fe content is less than 0.01% by dry weight; (ii) the process produces mixed carbonates of Petition 870250081351, dated 10 / 09 / 2025, p. 41 / 79 37 / 57 rare earth metals where the Th content is less than 0.01% by dry weight; (iii) the process produces mixed rare earth metal carbonates in which the U content is less than 0.01% by dry weight; and / or (iv) the process produces mixed rare earth metal carbonates in which the Al content is less than 0.6% by dry weight.

[00102] Throughout the description and claims, the word "comprises" and variations thereof are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word "comprises" encompasses the instances of "consists of" and "consists essentially of". The objectives, advantages, and additional features of the invention will become apparent to those skilled in the art by examining the description or may be learned by practicing the invention. The following examples are provided by way of illustration and are not intended to be limiting of the present invention. EXAMPLES Raw materials used

[00103] The raw material used in the invention process is a mineral concentrate of monazite ore, obtained by concentrating the monazite fraction of an extracted mineral extract.

[00104] Different monazite concentrates with the following average composition, shown in Table 1, expressed as % by dry weight, were used in the steps of the process described below: Petition 870250081351, dated 10 / 09 / 2025, page 42 / 79 38 / 57 Average composition (% by weight) [PO4] 10-25 [Ce] 5-15 [La] 2-10 [Pr] 0.5-3 [Nd] 3-10 [Sm] 0.5-3 [Eu] 0.1-1 [Gd] 0.1-3 [Dy] 0.05-1 [Y] 0.1-1 [U] 0.005-0.02 [Th] 0.01-0.2 [Fe] 5-20 [Al] 1-10 [Si] 5-20 [Pb] 0.005-0.5 Table 1

[00105] The actual specific gravity and apparent specific gravity of the aforementioned raw materials were determined to be 3.26 g / cm3 and 1.07 g / cm3, respectively. The varying quantities of each component of monazite in the monazite sample are responsible for the variations in the compositions observed in the liquid and solid effluents obtained at each stage of the process detailed below.

[00106] In the following procedure, the composition of the liquid fractions for each of the process steps was determined as follows: - The NaOH content was determined by acid-base titration, following well-established procedures known to skilled technicians; - The phosphate, iron, aluminum, silicon, and lead content was determined by ICP-OES, following well-established procedures known to skilled technicians; Petition 870250081351, dated 10 / 09 / 2025, page 43 / 79 39 / 57 - The rare earth metal content was determined by ICP-MS, following well-established procedures known to skilled technicians.

[00107] In the following procedure, the composition of the solid fractions for each of the process steps was determined as follows: - Iron and lead content: a weighed sample of the solid was digested with aqua regia and the iron and lead content of the liquid phase was determined by ICP-OES, following well-established procedures known to skilled technicians; - Phosphate, aluminum, and silicon content: a weighed sample of the solid was digested with a sodium peroxide solution, and the phosphate content of the liquid phase was determined by ICP-OES, following well-established procedures known to skilled technicians; - Rare earth metal content: a weighed sample of the solid was digested with a sodium peroxide solution and the rare earth metal content was determined by ICP-MS, following well-established procedures known to skilled technicians.

[00108] ICP-OES measurements were performed using a Perkin Elmer Optima 8300 instrument. ICP-MS measurements were performed using a NexION 300X instrument. General procedure for the preparation of mixed rare earth metal carbonate from monazite

[00109] The process for extracting mixed rare earth metal carbonate from monazite of the invention is summarized in Figure 1. The process of the invention comprises Petition 870250081351, dated 10 / 09 / 2025, page 44 / 79 40 / 57 the process steps defined in the first aspect of the invention. Basic leaching step (step (a))

[00110] This step was performed by placing the monazite concentrate sample used, having the composition shown in Table 2a (540 g), in contact with a 55% by weight aqueous solution of sodium hydroxide (prepared from 1080 g of NaOH and 884 mL of H2O), in which the weight ratio of sodium hydroxide to monazite was 2:1, at a temperature of 142 °C, for a period of 4 hours. After this treatment, the mixture was diluted with water at 100 °C to achieve a sodium hydroxide concentration of 25% by weight. The precipitated metal hydroxide salts were separated by filtration (solid B1). The filtration operation also produces a liquid fraction A1. Composition of monazite (% by weight) [PO4] 11.81 [Ce] 7.52 [La] 3.52 [Pr] 1.07 [Nd] 4.27 [Sm] 0.62 [Eu] 0.11 [Gd] 0.36 [Dy] 0.07 [Y] 0.16 [U] <0.01 [Th] 0.07 [Fe] 12.38 Petition 870250081351, dated 10 / 09 / 2025, page 45 / 79 41 / 57 [Al] 4.95 [Si] 18.57 [Pb] 0.05 Table 2a

[00111] The experimental procedure was as follows:

[00112] Step (a): 884 grams of water were loaded into the reactor. 1080 grams of NaOH were slowly added to the reactor to obtain a [NaOH] concentration of 55% by weight. The solution was kept under vigorous stirring. When the NaOH solution reached 90°C, 540 grams (dry weight) of monazite concentrate were slowly added to the reactor. The reaction medium was gradually heated to 115-120-125-130-140°C. When the temperature reached 141-142°C, the reaction medium was maintained under these conditions for 4 hours. After 4 hours, the reaction medium was allowed to cool to 100°C, and then 1910 mL of water were added to the reactor. This diluted mixture was stirred for 1 hour.

[00113] Step (b): The reaction paste was filtered and the resulting cake was washed with 1 L of water. After filtration, two streams were obtained: -[i] liquid fraction A1 (Phosphate Liquor Solution), which consists of a mixture of the first filtrate and the washing liquor. -[ii] solid B1, comprising washed cake of rare earth metal hydroxide salts.

[00114] Table 2b shows the composition of the isolated fractions: Liquid fraction A1 (g / L) Solid B1 (% by weight) [PO4] 16.1 0.720 Petition 870250081351, dated 10 / 09 / 2025, page 46 / 79 42 / 57 [Ce] 0.002 9.000 [La] <0.001 3.360 [Pr] <0.001 1.150 [Nd] 0.001 4.390 [Sm] <0.001 0.843 [I] <0.001 0.116 [Gd] <0.001 0.365 [Dy] <0.001 0.071 [Y] 0.004 0.199 [U] 0.005 <0.010 [Th] <0.001 0.062 [Fe] 0.213 14.100 [Al] 0.282 5.690 [Si] 13.4 11.500 [Pb] 0.051 <0.100 [NaOH] 12.4% by weight - Table 2b

[00115] The yield (expressed as a percentage) of leaching of phosphate species in the liquid fraction, calculated as (i) the ratio of the weight amount of phosphate in the liquid fraction resulting from the leaching step to the weight amount of phosphate in the monazite concentrate, or as (ii) the ratio of the difference between the weight amount of phosphate in the monazite concentrate and the weight amount of phosphate in solid B1 to the weight amount of phosphate in the monazite concentrate, is 95%.In contrast, the leaching yield of rare earth metals in the liquid fraction, calculated as (i) the ratio of the weight quantity of rare earth metals in the liquid fraction, resulting from the leaching step, to the weight quantity of rare earth metals in the monazite concentrate, or as (ii) the ratio of the difference between the weight quantity of rare earth metals in the monazite concentrate and the weight quantity of rare earth metals in solid B1, to the weight quantity of rare earth metals in solid B1. Petition 870250081351, dated 10 / 09 / 2025, page 47 / 79 43 / 57 monazite concentrate, is insignificant. As the expert will realize, both approaches to yield determination provide identical results. That is, substantially all rare earth metal cations present in the monazite particles are contained in the solid fraction B1 after this step. Isolation of trisodium phosphate (Steps (k)-(l))

[00116] Step (k): The liquid fraction A1, resulting from step (a) defined above, was evaporated at 128 °C, during 1.5 h. The steam produced is compressed again and recycled to minimize steam consumption in this operation. The condensed steam can be returned as a supply for boiler replenishment and / or used as a hot water supply, which is used to wash solids in one or more of the caustic leaching, silicon removal, and cerium precipitation stages. In this way, the total water and energy consumption throughout the process is optimized. The resulting mixture was then cooled to 30 °C by means of a cooling coil to allow the precipitation of trisodium phosphate.

[00117] Step(l): The precipitated trisodium phosphate was isolated by filtration, thus producing a liquid fraction K1 and a solid fraction consisting essentially of trisodium phosphate.

[00118] Table 3 shows the composition of the isolated fractions: Net fraction K1 (g / L) Trisodium phosphate (% by weight) [PO4] 16.1 46.50 [Na] 0.002 32.60 [U] 0.005 <0.01 Petition 870250081351, dated 10 / 09 / 2025, page 48 / 79 44 / 57 [Th] <0.001 <0.001 [Fe] 0.213 - [Al] 0.282 0.095 [Si] 13.4 0.772 [Pb] 0.051 <0.100 [NaOH] 38.80% by weight - Table 3

[00119] The yield (expressed as a percentage) of trisodium phosphate isolation for this step is 79%. This yield is calculated as (i) the ratio of the weight amount of phosphate in the isolated trisodium phosphate to the weight amount of phosphate in liquid fraction A1, or as (ii) the ratio of the difference between the weight amount of phosphate in liquid fraction A1 and the weight amount of said phosphate in liquid fraction K1, to the weight amount of said phosphate in liquid fraction A1. As the expert will realize, both approaches to determining the yield provide identical results. Desilication stage (stages (m)-(n))

[00120] The liquid fraction K1, resulting from step (l) described above, was subjected to a desilication step by adding calcium oxide, in order to promote the precipitation of calcium silicate, according to the following reaction: Na2SiO3 (a) + CaO (s) + 1 H2O (a) ^ CaSiO3 (s) + 2 NaOH (s)

[00121] The experimental procedure was as follows:

[00122] Step (m): A total of 1460 mL of liquid fraction K1 was charged into a glass beaker. The temperature was raised to 120°C under vigorous stirring. A total of 77 grams (dry weight) of calcium oxide was Petition 870250081351, dated 10 / 09 / 2025, page 49 / 79 45 / 57 added to the reaction mixture and left for 1.5 hours.

[00123] Step (n): Filtration of the resulting paste produced two streams: a liquid fraction M1, which is enriched in NaOH, and a solid fraction L1, consisting of calcium silicate. This step advantageously allows the recirculation of said fraction M1 to step (a) of the process, which is rich in sodium hydroxide (45% by weight), without accumulation of silicate species in the process.

[00124] Table 4 shows the composition of the isolated fractions: Liquid fraction M1 (g / L) Solid fraction L1 (% by weight) [PO4] 0.778 3.61 [Na] -21.6 [U] 0.001 <0.01 [Th] <0.001 <0.01 [Fe] 0.316 0.718 [Al] 0.486 0.168 [Si] 4.11 11.70 [Pb] 0.106 <0.10 [NaOH] 42.90% - Table 4

[00125] The measured yield (expressed as a percentage) of silicate precipitation was 89%, calculated as (i) the ratio of the weight of silicate in the solid fraction L1, resulting from steps (m) and (n), to the weight of silicate in the liquid fraction K1, or as (ii) the ratio of the difference between the weight of silicate in the liquid fraction K1 and the weight of said silicate in the liquid fraction M1, resulting from steps (m) and (n), to the weight of said silicate in the liquid fraction K1. As the expert will realize, both approaches to determining the yield provide identical results. The liquid fraction M1 is Petition 870250081351, dated 10 / 09 / 2025, page 50 / 79 46 / 57 suitable for feeding in step (a). Acid leaching stage (stages (c)-(d))

[00126] In the following experiments, a solid B2 was used, obtainable from steps (a) and (b) performed as described above in a monazite sample having the composition described in Table 5, the said solid having the composition described in Table 5: Initial Monazite (% by weight) Solid B2 (% by weight) [PO4] 15.6 0.95 [Ce] 7.73 8.87 [La] 3.37 3.56 [Pr] 1.11 1.17 [Nd] nd 4.60 [Sm] 0.867 0.75 [Eu] 0.161 0.15 [Gd] 0.514 0.45 [Dy] 0.099 0.09 [Y] 0.214 0.18 [U] 0.01 0.01 [Th] 0.083 0.07 [Fe] 15.63 14.3 [Al] 3.85 5.38 [Si] 11.03 10.1 [Pb] 0.0083 0.05 Table 5

[00127] Step (c): Solid B2 obtained in step (a) was subjected to an acid leaching step, which occurred in two stages. Solid B2 (180 g) was suspended in water at a concentration of 300 g / L. The reaction paste was heated to 90°C with vigorous stirring. The pH of the resulting solution was adjusted to pH 2 or 3.6, as indicated in Table 6, with 37% (w / v) hydrochloric acid. The resulting mixture was left at 90°C for 90 minutes or 180 minutes, as indicated in Table 6. In entry 3 of Table 6, the pH of Petition 870250081351, dated 10 / 09 / 2025, page 51 / 79 The 47 / 57 solution was then adjusted from 2 to 3.6 by adding an aqueous solution of KOH (300 g / L), and the resulting mixture was further left at 90 °C for 90 minutes.

[00128] Step (d): The resulting suspension was then filtered, thus producing a solid residue C2, comprising Th and U, and a liquid fraction D2, comprising rare earth metal chloride salts.

[00129] Table 6 shows the leaching efficiency obtained for certain elements, for an acid leaching step of solid B2 carried out at different pH values ​​and for different periods. Leaching efficiencies are expressed as a percentage and calculated as (i) the ratio of the weight amount of an element in the liquid fraction resulting from step (c) to the weight amount of the same element in solid B2 obtained in step (a), or as (ii) the ratio of the difference between the weight amount of a rare earth metal in solid B2 and the weight amount of said rare earth metal in the solid fraction resulting from step (c), to the weight amount of the same element in solid B2. As will be apparent to the expert, both approaches to yield determination provide identical results. Entries 1 and 2 are provided as comparative examples, while entry 3 represents an example of a process according to the invention. Input 1 2 3 pH 3.6 2 2, then 3.6 Residence time (h) 3 3 1.5 at pH=2 + 1.5 at pH=3.6 Leaching Efficiency (%) Ce 79 99 84 La 86 99 93 Petition 870250081351, dated 10 / 09 / 2025, page 52 / 79 48 / 57 Pr 79 99 90 Nd 83 99 89 Sm 73 92 84 Eu 74 92 82 Gd 76 92 84 Dy 69 91 75 Y 72 90 80 U 30 45 29 Th 20 15 - Fe 7 5 0 Al 11 45 5 Si 4 10 5 Pb 32 83 52 Table 6

[00130] The results in Table 6 show that when the acid leaching step is carried out at pH 2 (entry 2), the leaching efficiency of rare earth metals will be very high, at the cost of co-extraction of other elements such as Fe, Al, Si, Pb, U, and Th in large quantities. The results in entry 3 show a balance between the high recovery yields of rare earths and the low solubilization of Al, Si, U, or Th in the liquid fraction, these elements being known to be detrimental to the downstream process. In particular, U and Th are of particular relevance as they are radioactive elements and therefore need to be treated separately. Residues containing Si make filtration operations more difficult, and the presence of Al is known to negatively affect downstream extraction processes.

[00131] Table 7 shows the composition of the isolated fractions resulting from step (d): Liquid fraction D2 (g / L) Solid fraction C2 (%) Ce 15.9 1.95 La 6.9 0.349 Petition 870250081351, dated 10 / 09 / 2025, page 53 / 79 49 / 57 Pr 2.34 0.164 Nd 8.45 0.676 Sm 1.9 0.196 Eu 0.316 0.038 Gd 0.968 0.105 Dy 0.158 0.032 Y 0.295 0.051 U <0.001 <0.01 Th <0.001 0.122 Fe <0.005 20.5 Al 0.034 8.58 Si 0.068 13.7 Pb 0.066 0.023 Table 7 Deactivation of radionuclides (steps (e)-(f))

[00132] A liquid fraction D3, obtained from the process comprising steps (a)-(d) described above and having the composition shown in Table 8, was used in the present steps (e)-(f). This composition also includes minimum amounts of the radioisotopes Ra and Pb (not shown in the composition tables published in this document). Liquid D3 (g / L) [Ce] 15.24 [La] 6.62 [Pr] 1.85 [Nd] 7.16 [Sm] 1.35 [Eu] 0.22 [Gd] 0.605 [Dy] 0.101 [Y] 0.239 [U] <0.001 [Th] <0.001 [Fe] 0.071 [Al] 0.199 [Si] 0.055 [Pb] 0.081 Table 8 Petition 870250081351, dated 10 / 09 / 2025, page 54 / 79 50 / 57

[00133] The liquid D resulting from step (d) comprises minute radioactive amounts of Ra and Pb, which must be removed to provide a mixed rare earth metal carbonate that is not radioactive. This is achieved by the consecutive selective precipitation of radium sulfate and lead sulfide.

[00134] Step (e) was performed in continuous mode, with a continuous inflow of liquid D3, under the following conditions and using the following reagents: - Liquid inlet flow rate D: 2 L / h Temperature: 70°C - Sulfate additive: K2SO4 from a stock solution at 90 g / L, to maintain a sulfate concentration in the medium of 10 g / L. Residence time: 30 minutes. - Barium additive: BaCl2, from a stock solution at 75 g / L, was added in an amount such that the molar ratio of barium to sulfate was 1:10 in the medium. Residence time: 30 minutes. - pH adjusting agent: KOH from a stock solution at 300 g / L, to adjust the pH of the medium to 3.6. Residence time: 30 minutes. - Sulfide additive: Na2S, from a stock solution at 1.2 g / L, was added in an amount such that the molar ratio of lead chloride to sodium sulfide was 1:1. Residence time: 60 minutes. - Total residence time: 150 minutes.

[00135] Step (f): The reaction paste was continuously filtered (step (g)) and two main streams were generated. Petition 870250081351, dated 10 / 09 / 2025, page 55 / 79 51 / 57 - a liquid fraction F3 comprising a rare earth chloride solution, substantially free of radioactive elements. - a solid fraction E3 comprising barium sulfate, radium sulfate and lead sulfide.

[00136] Table 9 shows the composition of fractions E3 and F3, as well as the yield (expressed as a %) of the co-extraction of rare earth metal salts in the solid fraction E3, with said yield (expressed as a percentage) calculated as (i) the ratio of the weight of a rare earth metal in fraction E3 to the weight of the same rare earth metal in liquid fraction D3, or as (ii) the ratio of the difference between the weight of a rare earth metal in liquid fraction D3 and the weight of said rare earth metal in liquid fraction F3 to the weight of said rare earth metal in liquid fraction D3. As the expert will notice, both approaches to determining the yield provide identical results: Liquid fraction F3 (g / L) Solid fraction E3 (%) Co-extraction yield in E3 (%) Ce 11.3 14.3 7 La 5 5.7 6 Pr 1.37 1.68 7 Nd 5.4 5.7 6 Sm 1.09 0.64 4 Eu 0.18 0.13 5 Gd 0.53 0.3 4 Dy 0.09 0.017 1 Y 0.22 0.01 0 U <0.001 0.01 - Th <0.001 0.01 - Petition 870250081351, dated 10 / 09 / 2025, pp. 56 / 79 52 / 57 Fe 0.019 0.51 - Al 0.152 0.34 - Si 0.042 0.35 - Pb 0.065 0.1 - Table 9

[00137] Radioisotope analysis of the liquid and solid fractions E3 and F3 reveals that the liquid fraction is substantially free of radioactive elements and meets the requirements of the applicable regulations relating to RD783 / 2001 and RD 1439 / 201 of Spain. Isolation of cerium hydroxide (steps (g)-(h))

[00138] A liquid fraction F4, obtained from a process comprising steps (a)-(f) described above and having the composition shown in Table 10, was used in the present steps (g)-(h): Liquid F4 (g / L) [Ce] 11.8 [La] 5.86 [Pr] 1.98 [Nd] 6.96 [Sm] 1.41 [Eu] 0.229 [Gd] 0.77 [Dy] 0.136 [Y] 0.243 [U] <0.001 [Th] <0.001 [Fe] 0.003 [Al] <0.005 [Si] 0.068 [Pb] 0.063 Table 10

[00139] The liquid fraction F4, resulting from step (f), was treated with sodium hypochlorite (stock solution at 76.6 g / L) and potassium hydroxide in order to promote the precipitation of cerium(IV) hydroxide according to the reaction Petition 870250081351, dated 10 / 09 / 2025, pp. 57 / 79 53 / 57 Chemistry: CeCl3 (a) + NaClO (a) + 6 KOH (a) + H2O (a) ^ 2 Ce(OH)4 (s) + 6 KCl (a)

[00140] NaClO was added in an amount such as to maintain 3 times the stoichiometric dose, according to the reaction described previously. The pH of the liquid fraction F4 was adjusted to 3.5 by the addition of potassium hydroxide (stock solution at 100 g / L), and additional KOH was added in an amount of 0.7 times the stoichiometric dose, according to the reaction described previously.

[00141] The experimental procedure was as follows:

[00142] Stage (g): 900 mL of liquid F4 were loaded into a 2 L glass reactor. The solution was stirred and heated to 70°C. The pH of the solution was adjusted to 3.5 with a small addition of KOH. - 108 mL of oxidizing reagent (NaClO solution) 76.6 g / L) were progressively added to the solution over the course of one hour. - 90 mL of precipitating agent (KOH solution) 100 g / L) were progressively added to the solution. - 90 mL of precipitating agent (KOH solution) 100 g / L) were progressively added to the solution.

[00143] Step (h): After 3 hours, the reaction paste was filtered and two main streams were generated: (i) a liquid fraction H4, comprising rare earth metal chloride salts, and (ii) a solid fraction G4, comprising cerium hydroxide. Petition 870250081351, dated 10 / 09 / 2025, pp. 58 / 79 54 / 57

[00144] Table 11 shows the composition of the isolated fractions resulting from step (h): Liquid fraction H4 (g / L) Solid fraction G4 (% by weight) [Ce] 0.001 63.4 [La] 4.61 0.33 [Pr] 1.48 0.57 [Nd] 5.37 2.10 [Sm] 0.995 0.7 [Eu] 0.169 0.133 [Gd] 0.552 0.340 [Dy] 0.102 0.132 [Y] 0.205 0.087 [U] <0.001 <0.010 [Th] <0.001 <0.010 [Fe] 0.002 - [Al] 0.098 <0.010 [Si] 0.049 0.569 [Pb] 0.032 0.12 Table L1

[00145] Table 12 shows the efficiency of this precipitation step, expressed as a percentage and calculated as (i) the ratio of the weight of a metal in the solid fraction G4 to the weight of said metal in the liquid fraction F4, or as (ii) the ratio of the difference between the weight of a metal in the liquid fraction F4 and the weight of said metal in the liquid fraction H4 to the weight of said metal in the liquid fraction F4. As the expert will notice, both approaches to determining the yield provide identical results. Petition 870250081351, dated 10 / 09 / 2025, pp. 59 / 79 55 / 57 Yield of metal precipitation: Ce (%) 99 La (%) 6 Pr (%) 10 Nd (%) 10 Sm (%) 15 Eu (%) 11 Gd (%) 14 Dy (%) 9 Y (%) 0 Table 12 Precipitation of mixed rare earth metal carbonate (steps (i)-(j))

[00146] A liquid fraction H5, obtained using the process comprising steps (a)-(h) described above and having the composition shown in Table 13, was used in the present example: Liquid H5 (g / L) [Ce] 0.01 [La] 5.4 [Pr] 1.43 [Nd] 5.73 [Sm] 0.96 [Eu] 0.16 [Gd] 0.59 [Dy] 0.09 [Y] 0.215 [U] <0.001 [Th] <0.001 [Fe] <0.002 [Al] 0.034 [Si] 0.023 [Pb] 0.050 Table 13

[00147] Stage (i):

[00148] The pH of the liquid fraction H5 was adjusted to Petition 870250081351, dated 10 / 09 / 2025, pp. 60 / 79 56 / 57 pH 6 by adding an aqueous solution of sodium carbonate at a concentration of 180 g / L, and the resulting mixture was left at 70°C for one hour. This step allowed the precipitation of mixed rare earth metal carbonate.

[00149] Step (j): The precipitated mixed rare earth metal carbonate was isolated from the mixture by filtration.

[00150] Table 14 shows the composition of the isolated solid and liquid fractions of rare earth metal carbonate resulting from step (j): Liquid fraction (g / L) Solid fraction (%) [Ce] <0.001 0.037 [La] 0.003 20.9 [Pr] <0.001 5.66 [Nd] 0.001 23.8 [Sm] <0.001 3.73 [Eu] <0.001 0.641 [Gd] <0.001 2.33 [Dy] <0.001 0.367 [Y] <0.001 0.982 [U] <0.001 <0.01 [Th] <0.001 <0.01 [Fe] 0.010 <0.010 [Al] <0.005 0.586 [Si] 0.006 0.419 [Pb] <0.050 0.149 Table 14

[00151] The yield (expressed as a percentage) of the precipitation step, calculated as (i) the ratio of the weight amount of a rare earth metal in the solid fraction resulting from step (j) to the weight amount of said rare earth element in the liquid fraction H5, or as (ii) the ratio of the difference between the amount in Petition 870250081351, dated 10 / 09 / 2025, pp. 61 / 79 57 / 57 weight of a rare earth metal, in the liquid fraction H5, and the weight amount of said rare earth metal, in the liquid fraction resulting from step (j), for the weight amount of said rare earth element, in the liquid fraction H5, was 99% for each of La, Pr, Nd, Sm, Eu, Gd, Dy and Y, while a yield of 88% for the carbonate precipitation step was observed for Ce. As will be perceived by those skilled in the art, both approaches to the determination of yield provide identical results. Petition 870250081351, dated 10 / 09 / 2025, pp. 62 / 79< / s>

Claims

1 / 10 CLAIMS 1. Process for the preparation of mixed rare earth metal carbonate from monazite, said process characterized in that it comprises: (a) subjecting monazite particles to a leaching step with an aqueous solution of alkaline hydroxide, thereby obtaining a mixture of: - a liquid fraction A comprising trisodium phosphate and - a solid B comprising mixed hydroxide salts of the metals comprised in monazite; (b) separating solid B from solution A, preferably by filtration; (c) a leaching step of solid B comprising: (c-1) a first leaching step of solid B with an aqueous solution of hydrochloric acid, feeding said aqueous solution of hydrochloric acid in such a way that the resulting mixture has a pH between 1 and 2.5; (c-2) a second leaching stage after the first leaching stage (c-1), where the pH is adjusted to a value between 3.2 and 4 by the addition of an alkaline hydroxide;thus obtaining a mixture of: - a solid fraction C comprising thorium and uranium hydroxide salts, and - a liquid fraction D comprising rare earth metal chloride salts; and (d) separating the solid C from the liquid fraction D, preferably by filtration. Petition 870250081351, dated 10 / 09 / 2025, page 63 / 79 2 / 10; 2. Process according to claim 1, characterized in that the monazite particles of step (a) have a particle diameter of less than 100 μm.

3. Process according to any one of claims 1 to 2, characterized in that the alkaline hydroxide of step (a) is sodium hydroxide and the aqueous sodium hydroxide solution of step (a) comprises sodium hydroxide in an amount of between 50 and 60% by weight; preferably about 55% by weight.

4. Process according to any one of claims 1 to 3, characterized in that the alkaline hydroxide of step (a) is sodium hydroxide and the weight ratio of sodium hydroxide to monazite is about 2:

1.

5. Process, according to any one of claims 1 to 4, characterized in that step (a) is carried out at a temperature of about 130 to 150°C, preferably 142°C.

6. Process, according to any one of claims 1 to 5, characterized in that step (a) has a duration of at least 3 hours and up to 5 hours, preferably 4 hours.

7. Process according to any one of claims 1 to 6, characterized in that step (a) further comprises diluting the mixture obtained at 100°C with water, so as to achieve a sodium hydroxide concentration of about 25% by weight.

8. Process, according to any one of claims 1 to 7, characterized in that step (c-1) is carried out at a temperature between 80 and 95°C, Petition 870250081351, dated 10 / 09 / 2025, page 64 / 79 3 / 10 preferably at 90°C, and / or for at least 70 minutes, preferably for 90 minutes.

9. Process, according to any one of claims 1 to 8, characterized in that step (c-1) is carried out at a pH of about 2.

10. Process, according to any one of claims 1 to 9, characterized in that step (c-2) is carried out at a pH of about 3.

6.

11. Process, according to any one of claims 1 to 10, characterized in that step (c-2) is carried out at a temperature between 80 and 95°C, preferably at 90°C and / or for at least 70 minutes, preferably for 90 minutes.

12. Process, according to any one of claims 1 to 11, characterized in that the pH of the mixture in step (c-1) is adjusted to 3.6 by the addition of an alkaline hydroxide, such as potassium hydroxide.

13. Process, according to any one of claims 1 to 12, characterized in that solid B is fed as a suspension at a concentration of between 200 and 500 g per liter, preferably about 300 g per liter.

14. Process, according to any one of claims 1 to 13, characterized in that it further comprises the steps of: (e) bringing the liquid fraction D into contact with a source of barium(II), a source of sulfate and a source of sulfide, so as to substantially precipitate all species of Ra(II) and Pb(II) comprised in said liquid fraction, in the form of RaSCg and PbS; thus obtaining: Petition 870250081351, dated 10 / 09 / 2025, page 65 / 79 4 / 10 - a solid fraction E comprising lead sulfide and barium and radium sulfate salts, and - a liquid fraction F comprising mixed rare earth metal chloride salts; (f) separating the liquid fraction F from the solid fraction E, preferably by filtration.

15. Process according to claim 14, characterized in that step (e) comprises the steps of: (e-1) precipitating radium sulfate salts by contacting the liquid fraction D with a source of barium(II) and a source of sulfate; (e-2) precipitating lead sulfide by contacting the product obtained in step (e-1) with a source of sulfide(II).

16. Process according to claim 15, characterized in that the sulfate source is potassium sulfate.

17. Process, according to any one of claims 15 to 16, characterized in that the amount of sulfate source is such that the sulfate concentration in the middle of step (f) is between 5 and 15 g per liter, preferably 10 g per liter.

18. Process according to any one of claims 15 to 17, characterized in that the source of barium(II) is barium(II) chloride.

19. Process, according to any one of claims 15 to 18, characterized in that the amount of barium(II) source is such that the molar ratio of Ba(II) to sulfate is between 1:5 and 1:15, preferably Petition 870250081351, dated 10 / 09 / 2025, page 66 / 79 5 / 10 of 1:

10.

20. Process, according to any one of claims 15 to 19, characterized in that step (e-1) is carried out at a temperature between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C.

21. Process, according to any one of claims 15 to 20, characterized in that the reaction time of step (e-1) is at least 45 minutes, preferably about 60 minutes.

22. Process, according to any one of claims 15 to 21, characterized in that the pH of the reaction medium of step (e-2) is 3.6 and is optionally adjusted by the addition of an alkaline hydroxide, such as potassium hydroxide.

23. Process according to any one of claims 15 to 22, characterized in that the source of sulfide(II) is sodium sulfide.

24. Process, according to any one of claims 15 to 23, characterized in that the amount of sulfide(II) source is such that the molar ratio of lead to sulfide is about 1:

1.

25. Process, according to any one of claims 15 to 24, characterized in that step (e-2) is carried out at a temperature between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C.

26. Process, according to any one of claims 14 to 25, characterized in that it further comprises the steps of: Petition 870250081351, dated 10 / 09 / 2025, page 67 / 79 6 / 10 (g) substantially converting all Ce(III) cations of the liquid fraction F into Ce(IV) hydroxide by subjecting said liquid fraction F to oxidative conditions; thereby obtaining: - a solid fraction G consisting essentially of cerium(IV) hydroxide; and - a liquid fraction H comprising rare earth metal chloride salts; and (h) separating the liquid fraction H from the solid fraction G.

27. Process according to claim 26, characterized in that step (g) comprises bringing the liquid fraction F into contact with sodium hypochlorite.

28. Process according to any one of claims 26 to 27, characterized in that step (g) is carried out at a temperature between 50°C and 90°C; preferably between 60°C and 80°C; more preferably around 70°C.

29. Process according to any one of claims 26 to 28, characterized in that the pH of the reaction medium of step (g) is between 3 and 4, preferably about 3.

5.

30. Process according to any one of claims 26 to 29, characterized in that step (g) comprises bringing the liquid fraction F into contact with sodium hypochlorite which is present in an amount of at least 2 moles for each mole of cerium(III) chloride in the liquid fraction F, preferably 3 moles for each 2 moles of cerium(III) chloride in the liquid fraction F.

31. Process, according to any of the claims 26 to 30 of Petition 870250081351, of 10 / 09 / 2025, pp. 68 / 79 7 / 10, characterized in that it further comprises the steps of: (i) bringing the liquid fraction H into contact with an alkaline carbonate salt so as to substantially precipitate all the rare earth metals comprised in said fraction, in the form of carbonate salts; thus obtaining - a solid fraction I consisting of mixed carbonate salts of rare earth metals, and a liquid fraction J comprising chloride salts of rare earth metals and (j) separating the solid fraction I from the liquid fraction J; preferably by filtration.

32. Process according to claim 31, characterized in that step (i) is carried out at a temperature between 50°C and 100°C; preferably around 70°C.

33. Process, according to any one of claims 31 to 32, characterized in that the alkaline carbonate salt is sodium carbonate and is preferably added as an aqueous solution having a concentration of between 150 and 200 grams per liter, preferably about 180 grams per liter.

34. Process, according to any one of claims 31 to 33, characterized in that the amount of alkaline carbonate salt is such that the pH of the solution in step (i) is 6.

35. Process, according to any one of claims 1 to 34, characterized in that it further comprises the step of: Petition 870250081351, dated 10 / 09 / 2025, page 69 / 79 8 / 10 (k) precipitating trisodium phosphate from liquid fraction A and (l) isolating said trisodium phosphate by solid-liquid separation, thereby obtaining trisodium phosphate and a liquid fraction K; preferably by filtration.

36. Process according to claim 35, characterized in that trisodium phosphate is precipitated by evaporation of water.

37. Process according to claim 36, characterized in that the water evaporation step comprises heating liquid fraction A to a temperature of 128 °C for 90 minutes.

38. Process, according to any one of claims 36 to 37, characterized in that the evaporated water is additionally condensed and optionally recirculated in the dilution step of step (b), where step (b) comprises diluting the mixture at 100°C with water, so as to achieve a sodium hydroxide concentration of about 25% by weight.

39. Process, according to any one of claims 35 to 38, characterized in that trisodium phosphate is precipitated at a temperature between 20°C and 40°C; preferably 30°C for a period between 60 and 120 minutes, preferably 90 minutes.

40. Process, according to any one of claims 35 to 39, characterized in that trisodium phosphate is isolated by filtration.

41. Process, according to any one of claims 35 to 40, characterized in that Petition 870250081351, dated 10 / 09 / 2025, pp. 70 / 79 9 / 10 further comprises the steps of: (m) bringing the liquid fraction K into contact with calcium oxide in a sufficient quantity to substantially precipitate all the silicate salts comprised in said liquid fraction, in the form of calcium silicate; thus obtaining: - a solid fraction L comprising calcium silicate, and - a liquid fraction M enriched in sodium hydroxide, and (n) isolating said liquid fraction M by solid-liquid separation.

42. Process according to claim 41, characterized in that it further comprises the step of feeding the liquid fraction M to step (a).

43. Process, according to any one of claims 1 to 42, characterized in that it allows the recovery of at least 71% of the cerium present in monazite, in the form of cerium(IV) hydroxide.

44. Process, according to any one of claims 1 to 43, characterized in that it allows the recovery of at least 73% of the lanthanum present in monazite, in the form of mixed rare earth metal carbonate.

45. A process, according to any one of claims 1 to 44, characterized in that it allows the recovery of at least 59% of the neodymium present in monazite, in the form of mixed rare earth metal carbonate.

46. ​​Process, according to any of the Petition 870250081351, dated 10 / 09 / 2025, pp. 71 / 79 10 / 10 claims 1 to 45, characterized by the fact that it allows the recovery of at least 59% of the praseodymium present in monazite, in the form of mixed rare earth metal carbonate.

47. Process, according to any one of claims 1 to 4, characterized in that it allows the recovery of at least 44% of the europium present in monazite, in the form of mixed rare earth metal carbonate.

48. A process, according to any one of claims 1 to 47, characterized in that it allows the recovery of at least 50% of the gadolinium present in monazite, in the form of a mixed rare earth metal carbonate.

49. A process, according to any one of claims 1 to 48, characterized in that it allows the recovery of at least 37% of the dysprosium present in monazite, in the form of mixed rare earth metal carbonate.

50. Process, according to any one of claims 1 to 49, characterized in that it allows the recovery of at least 57% of the yttrium present in monazite, in the form of mixed rare earth metal carbonate.

51. Process, according to any one of claims 1 to 50, characterized in that it allows the recovery of at least 53% of the samarium present in monazite, in the form of mixed rare earth metal carbonate. Petition 870250081351, dated 10 / 09 / 2025, pp. 72 / 79

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