Method for using phosphate sludge for the production of rare earths, calcium carbonate and silicates

Abstract

The present invention relates to a method for extracting rare earths (REs) from phosphate sludge. In addition to rare earths, the method makes it possible to recover and use silica and calcium carbonate contained in phosphate sludge.

Description

[0001] Process for the valorization of phosphate sludge for the production of rare earths, calcium carbonate and silicates

[0002] FIELD OF INVENTION

[0003] The present invention relates to a process for extracting rare earth elements (REEs) from phosphate sludge. In addition to rare earth elements, the process allows for the recovery and valorization of silica and calcium carbonate contained in the phosphate sludge.

[0004] TECHNOLOGICAL BACKGROUND

[0005] Phosphate beneficiation by washing and / or flotation, to concentrate apatite, generates co-products known as phosphate sludge (PS).

[0006] These phosphate rocks are essentially a phosphate waste rock, poor in fluorapatite and rich in calcite, dolomite, and quartz. This co-product also contains a wide variety of valuable elements, namely rare earth elements, iron, magnesium, etc. Table 1 shows the mass fractions of elements in the Moroccan phosphate rocks analyzed at the UM6P-Jorf Lasfar laboratory, Morocco.

[0007] [Table 1]

[0008] Element P2O5CaO C2SiO 2 T F Fe2Ô3MgÔ ÂÎ2Ô3k2Ô Na2Ô SÔ3C.Ôrg TRs

[0009] 16.25 35.20 13 20.18 2.12 0.94 1.91 3.51 1.18 1.05 0.74 0.18 0.0418 in %

[0010] Table 1: Chemical composition of Moroccan BP

[0011] Over time, several methods have been developed to valorize phosphate rock waste (PRW). For example, a research team developed a thermal transformation process aimed at using PRW as a lightweight granular material. However, these methods have mainly focused on exploiting the carbonates present in PRW [1]. It has also been proposed to valorize phosphate waste, but this time before beneficiation, by directly treating the low-grade phosphorite. Despite using a combination of flotation and acid leaching, a final rare earth oxide (REO) concentration of only 1.59% was obtained, with a recovery rate of 72.69% [2].

[0012] A need remains for a process to recover valuable elements from bioplastics, particularly a process for extracting trace elements (TIs), that is both efficient and easy to implement. Advantageously, this process will allow for the isolation and recovery of other bioplastic components, such as silica, fluoroapatite, and calcium carbonate.

[0013] BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a process for extracting rare earth elements from phosphate sludge comprising the following steps:

[0014] (1) treatment of phosphate sludge by an acidic saline solution leading to the obtaining of a solid-liquid mixture M1;

[0015] (2) separation of mixture M1 and collection of a solid P1 and a solution L1;

[0016] (3) flotation of solid P1 leading to the formation of a floated solid-liquid mixture M2 and a solid-liquid mixture M3;

[0017] (4) separation of mixture M2 and collection of a solid P2 and a solution L2;

[0018] (5) separation of mixture M3 and collection of a solid P3 and a solution L2;

[0019] (6) acid leaching of solid P3 leading to the obtaining of a solid-liquid mixture M4;

[0020] (7) separation of mixture M4 and collection of a solid P4 and a solution S1; and

[0021] (8) retrieval of TRs from solution S1.

[0022] Other aspects of the invention are as described below and in the claims.

[0023] FIGURES

[0024] [Fig.1]: XRD analysis of solids PO and P1

[0025] [Fig.2]: XRD analysis of solids P1, P2 and P3

[0026] [Fig.3]: XRD analysis of precipitates P3 and P4

[0027] [Fig. 4]: XRD analysis of precipitate P9

[0028] DEFINITIONS

[0029] The term "rare earths" refers to the rare earth elements called lanthanides with atomic numbers from 57 to 71 inclusive, yttrium with atomic number 39 and scandium with atomic number 21.

[0030] The expression "ambient temperature" means, for the purposes of the present invention, a temperature within a range of 20°C to 25°C.

[0031] The term "leaching" in the context of the present invention means the extraction of a soluble compound from a solid residue by means of washing operations. When the washing is carried out using a basic solution, it is referred to as "basic leaching," while it is referred to as "acid leaching" when the washing is carried out using an acidic solution.

[0032] The term "phosphate sludge" refers to all ores resulting from phosphate beneficiation processes. This includes, but is not limited to, ores obtained during crushing, washing, flotation, desliming, or any other process aimed at benefiting phosphate ore.

[0033] DETAILED DESCRIPTION OF THE INVENTION

[0034] The inventors have developed a process for extracting rare earth elements (REEs) from phosphate sludge. In addition to rare earth elements, the process according to the invention makes it possible to recover and valorize the silica (SiO2) and calcium carbonate contained in the phosphate sludge.

[0035] The proposed process for extracting rare earth elements from phosphate sludge includes the following steps:

[0036] (1) treatment of phosphate sludge by an acidic saline solution leading to the obtaining of a solid-liquid mixture M1;

[0037] (2) separation of mixture M1 and collection of a solid P1 and a solution L1;

[0038] (3) flotation of solid P1 leading to the formation of a floated solid-liquid mixture M2 and a solid-liquid mixture M3;

[0039] (4) separation of mixture M2 and collection of a solid P2 and a solution L2;

[0040] (5) separation of mixture M3 and collection of a solid P3 and a solution L2;

[0041] (6) acid leaching of solid P3 leading to the obtaining of a solid-liquid mixture M4;

[0042] (7) separation of mixture M4 and collection of a solid P4 and a solution S1; and

[0043] (8) retrieval of TRs from solution S1.

[0044] The steps of the process of the invention can be as described in detail below.

[0045] The phosphate sludge used in the process of the present invention may be obtained from the washing, flotation or any other stage of the phosphate beneficiation process.

[0046] Steps (1) and (2) (“Phase A”)

[0047] Steps (1) and (2) allow the TRs to be concentrated.

[0048] In the first stage, the phosphate sludge (PO) is treated with an acidic saline solution (LO). Typically, the phosphate sludge is agitated in the acidic saline solution. The agitation speed generally varies from 100 to 1000 revolutions per minute, preferably from 500 to 800 revolutions per minute. The treatment is carried out for a duration generally ranging from 20 minutes to 2 hours, preferably from 1 to 2 hours.

[0049] Acidic saline solution is typically an acidic aqueous saline solution.

[0050] The acidic saline solution comprises a salt fraction consisting of one or more salts, typically one or more chloride salts, including but not limited to NaCl, KCl, MgCh, and CaCh. The acidic saline solution generally comprises at least 1 g / L of salts, preferably 5 to 40 g / L of salts, and even more preferably 20 to 30 g / L of salts.

[0051] The acidic saline solution comprises an acid fraction consisting of one or more acids, typically selected from mineral acids, such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, or mixtures thereof, or organic acids such as acetic acid, citric acid, fatty acids comprising 4 to 20 carbon atoms, such as butyric acid (C4), lauric acid (C12), or mixtures of mineral and organic acids. The acidic saline solution generally comprises an organic acid mass concentration of 5 to 20%, preferably 8 to 15%. The acidic saline solution generally comprises a mineral acid mass concentration of 0.5 to 2%, preferably 0.5 to 1%.

[0052] The treatment is typically carried out at a temperature ranging from 10 to 40°C, preferably from 20 to 30°C, or even at room temperature.

[0053] At the end of the treatment, a solid-liquid mixture M1 is obtained. The solid:liquid ratio (Kg / L) preferably varies from 1:10 to 1:200, ideally from 1:100 to 1:150.

[0054] In a second step, the solid-liquid mixture M1 is separated. The separation is typically carried out by vacuum filtration. Alternatively, it can be carried out by centrifugation or decantation, for example for a period of 1 to 3 hours, preferably 1 to 2 hours.

[0055] The separation allows the collection of a solid P1 and a solution L1.

[0056] The P1 solid contains TRs. This solid can be described as TR-enriched, meaning its TR concentration is higher than that of phosphate sludge (approximately 420 ppm). Generally, the P1 solid has a TR content exceeding 850 ppm.

[0057] Solution L1 contains calcium carbonate. Solution L1 is a solution rich in calcium carbonate (CaCO3).

[0058] Optionally, the P1 solid can then be crushed. Crushing increases the contact surface area for the flotation operation.

[0059] Stages (3), (4) and (5) (“Phase B”)

[0060] Steps (3) to (5) allow us to obtain a new solid richer in TRs than that obtained at the end of step (2) and high purity silica.

[0061] Thus, in a third step, the flotation of solid P1 is carried out. The flotation is performed conventionally. Typically, solid P1 is introduced into a flotation system, such as a flotation tube (Hallimond tube) or a flotation cell, containing distilled water. The solid-to-liquid ratio (kg / L) generally ranges from 1:10 to 1:2, preferably from 1:6 to 1:4. The mixture is stirred until a homogeneous suspension is formed, which generally takes from 1 to 10 minutes, typically 5 minutes. The stirring speed generally ranges from 100 to 500 rpm, preferably from 200 to 400 rpm. An amino acid solution (collector) with a concentration ranging from 1 M to 10 M, preferably from 4 to 7 M, is then added to the suspension. The ratio (Kg / Kg) between the amine and the solid generally varies from 1:5 to 4:5, preferably from 2:5 to 3:5.The amine is preferably chosen from primary, secondary, tertiary or quaternary amines, having carbon chains comprising 4 to 20 linear or branched and / or functionalized carbon atoms, such as for example dodecylamine, hexadecylamine and octadecylamine.

[0062] The mixture is then conditioned for 1 to 10 minutes, typically 5 minutes, with agitation at 100 to 500 revolutions per minute, preferably 200 to 400 revolutions per minute. After conditioning, air bubbles are introduced into the mixture to cause the particles (silica) to float to the surface of the liquid. These bubbles are generated by an airflow typically ranging from 3 to 12 liters per minute, preferably 5 to 8 liters per minute. Flotation is carried out for 5 to 15 minutes, preferably 8 to 10 minutes.

[0063] The floating particles are retained on the surface of the flotation tube in the scum. After the flotation time, the scum, also referred to as the floated solid-liquid mixture M2, and the pulp containing unfloated residues, also referred to as the solid-liquid mixture M3, are collected separately.

[0064] The M2 mixture is separated. Separation is typically carried out by vacuum filtration. Alternatively, it can be carried out by centrifugation or by decantation, for example by decantation for a period of 1 to 3 hours, preferably 1 to 2 hours.

[0065] It allows the collection of a solid P2 and a solution L2.

[0066] The P2 solid consists mainly of silica. It has a mass purity ranging from 90 to 99% silica.

[0067] Solution L2 includes the amine that did not react during flotation.

[0068] The M3 mixture is separated. Separation is typically carried out by vacuum filtration. Alternatively, it can be carried out by centrifugation or decantation, for example for a period of 10 to 20 minutes.

[0069] It allows the collection of a solid P3 and a solution L2.

[0070] Solid P3 contains TRs. This solid can be described as TR-enriched, meaning its TR concentration is higher than that of solid P1 and phosphate sludge. The SiC concentration of this solid is lower than that of solid P1. The TR concentration of solid P3 is generally greater than or equal to 1200 ppm.

[0071] Solution L2 includes the amine that did not react during flotation. Steps (6) and (7) ("Phase C")

[0072] Steps (6) and (7) yield an acidic solution rich in TRs and fluoroapatite. Acid leaching of solid P3 is then carried out. The leaching is performed conventionally. Typically, solid P3 is stirred in an acidic solution. The acids used are typically mineral acids (e.g., sulfuric, nitric, hydrochloric acid, etc., or mixtures thereof), organic acids (e.g., acetic acid, citric acid, fatty acids containing 4 to 20 carbon atoms, or mixtures thereof), and mineral / organic acid mixtures. The mass concentration of the various acidic solutions is generally 4 to 20%, preferably 8 to 15%. The stirring speed typically ranges from 100 to 700 rpm, preferably 300 to 500 rpm. The solid:liquid ratio (Kg / L) generally varies from 1:2 to 1:15, preferably from 1:8 to 1:12.

[0073] Leaching is typically carried out at a temperature ranging from 20 to 40 °C, preferably from 25 to 30 °C.

[0074] The leaching time is generally 1 to 4 hours, preferably 1 to 2 hours.

[0075] At the end of the leaching process, a solid-liquid mixture M4 is obtained. The leaching efficiency is generally from 50 to 99%.

[0076] The solid-liquid mixture M4 is separated. Separation is typically carried out by vacuum filtration. Alternatively, it can be carried out by centrifugation or decantation, for example by decantation for a period of 5 to 30 minutes, preferably 10 to 20 minutes.

[0077] The separation allows the collection of a solid P4 and a solution S1.

[0078] Solid P4 contains fluoroapatite. The solid can be described as a fluoroapatite-rich solid (i.e., it consists mainly of fluoroapatite, typically about 65% by mass) and TR-depleted, i.e., its TR concentration is lower than that of solid P3.

[0079] Solution S1 contains TRs. The solution can be described as a TR-rich solution. It typically has a TR content greater than or equal to 120 ppm.

[0080] Step 8 (“Phase D”)

[0081] Step 8 allows you to retrieve the TRs.

[0082] TRs can be recovered by solid / liquid extraction, liquid / liquid extraction or by direct precipitation or with ligands (extraction agents, complexing agents).

[0083] In particular, TRs can be precipitated as hydroxide.

[0084] Precipitation is typically achieved by adding a basic solution to solution S1. The basic solution typically has a concentration ranging from 1 to 12 M, preferably from 5 to 10 M. The base can be sodium hydroxide, potassium hydroxide, magnesium hydroxide, tetramethylammonium hydroxide, or any other hydroxide. The basic solution is preferably added dropwise while maintaining a stirring speed of 100 to 400 rpm, preferably 100 to 200 rpm. The addition under stirring is continued until the pH of the solution reaches a value of 5 to 7, ideally 6 to 7. At the end of this step, a solid-liquid mixture M5 is obtained.

[0085] The resulting solid-liquid mixture M5 is separated. This separation is typically carried out by centrifugation followed by filtration. Alternatively, it can be performed under vacuum. The separation yields a solid P5 and a solution S2.

[0086] Solid P5 comprises TR hydroxides. The solid can be described as a TR hydroxide-rich solid.

[0087] Solution S2 can be reused to precipitate TRs as hydroxide or to recover other valuable elements present within it. Partial dissolution of solid P5 can then be carried out. The TR oxides are dissolved at this stage. Partial dissolution of solid P5 is achieved using a dilute acid solution. The acid is typically a mineral acid or a mixture of mineral acids. The concentration of this acid solution is generally 1 to 5 M, preferably 1 to 3 M. The dilute acid solution is typically added dropwise while maintaining an agitation level of 100 to 400 rpm, preferably 100 to 200 rpm. The addition under stirring is continued until the pH of the solution reaches a value from 1 to 4, ideally from 1 to 2. At the end of this step, a solid-liquid mixture M6 is obtained.

[0088] The resulting liquid-solid mixture M6 can then be separated. Separation is typically carried out by centrifugation followed by filtration. Alternatively, it can be performed under vacuum.

[0089] The separation allows the collection of a solid P6 and a solution S3.

[0090] Solution S3 comprises the TR oxides. It can be described as a TR oxide-rich solution. The TR oxides are dissolved at this stage.

[0091] Solid P6 is depleted in TRs hydroxide, meaning its TRs concentration is lower than that of solid P5. Solid P6 can be subjected to further partial dissolution as previously described (in conjunction with solid P5) or used to recover other valuable elements (Ca, Mg, Cd, etc.) present within it. The TRs contained in solution S3 can then be precipitated as TRs oxalates. An oxalic acid solution with a concentration of 1 to 5 M, preferably 1 to 3 M, can thus be added to solution S3. The ratio between solution S3 and oxalic acid (V / V) is typically 1:5. The addition is generally carried out under stirring, at speeds ranging from 100 to 400 rpm, preferably 100 to 200 rpm. The precipitation pH of TR oxalates is typically between 2 and 4, preferably between 2 and 3.Preferably, drops of sodium hydroxide are added at this stage to adjust the pH of the mixture to approximately 2.5. The addition of sodium hydroxide promotes the precipitation of oxalates from TRs.

[0092] Precipitation is preferably carried out at a temperature ranging from 10 to 40°C, preferably from 20 to 30°C, or even at ambient temperature.

[0093] At the end of this step, a solid-liquid mixture M7 is obtained.

[0094] The resulting liquid-solid mixture M7 can then be separated. Separation is typically carried out by centrifugation followed by filtration. Alternatively, it can be performed under vacuum.

[0095] The separation allows the collection of a solid P7 and a solution S4.

[0096] Solid P7 comprises TR oxalates. It can be described as a TR oxalate-rich solid.

[0097] The S4 solution can be subjected to the precipitation step described above (precipitation in the form of TR oxalates) or used to valorize the other valuable elements (Al, Fe, ...) that are present in it.

[0098] Finally, solid P7 can be subjected to a calcination step. Calcination is generally carried out at a temperature ranging from 300 to 1200 °C, preferably from 800 to 1000 °C, for a duration of 1 to 5 hours, preferably 2 to 3 hours. Calcination removes residual water and organic matter from solid P7 and converts TRs oxalates into TRs oxide. A solid P8 very rich in TRs oxide is thus obtained. Its mass concentration of TRs oxide is greater than or equal to 21%.

[0099] After recovery, the TRs can be separated in an appropriate manner, for example by magnetic separation and / or gravimetry and / or ion chromatography.

[0100] TRs can also be separated by liquid / liquid and / or liquid / solid extraction.

[0101] Step 9 (“Phase E”)

[0102] A regeneration step for the calcium carbonate contained in solution L1 can also be implemented. First, solution L1 can be evaporated by fractional evaporation (slow or accelerated), for example, using a multi-stage evaporation system or any other equivalent system, to obtain a distillate and a solid-liquid mixture M8. The system temperature typically ranges from 15 to 200°C, preferably from 40 to 100°C. The evaporation rate of the initial volume of solution L1 is 10 to 95%, preferably 70 to 95%, to ensure selective precipitation while achieving an optimal calcium carbonate recovery rate.

[0103] The resulting liquid-solid mixture M8 can then be separated. Separation is typically carried out by vacuum filtration, preferably with sintered glass of 4 µm porosity.

[0104] The separation allows the collection of a solid P9 and a solution L01.

[0105] Solid P9 comprises calcium carbonate. It exhibits a high purity of calcium carbonate: typically 90 to 99% by mass. The calcium carbonate recovery yield is generally greater than 95% by mass.

[0106] Solution S5 contains salts. It can be described as an acidic solution rich in salts (NaCl and KCl).

[0107] Solid P9 can be washed with distilled water to remove traces of salts and organic acid, thus improving product purity. The washing solution can be combined with solution S5.

[0108] Solution S5 can be combined with distilled water to give an acidic saline solution (L01) similar to that used in step (1) of the process (L0) and thus be used in this step (1) of the process. The solution resulting from this combination with distilled water (L01) has a salinity of 5 to 40 g / l, preferably 20 to 30 g / l, a density of 1.015 to 1.040, preferably 1.025 to 1.030, a pH of 1 to 4, preferably 2 to 3, and a conductivity of 30 to 40 mS / cm, preferably 32 to 35 mS / cm.

[0109] The following examples are given for illustrative purposes only, but should in no way be considered as limiting the present invention.

[0110] EXAMPLES

[0111] Analysis methods

[0112] The following analytical methods are used:

[0113] 1. X-ray diffraction (XRD): identification of phases in solids; 2. Energy-dispersive spectroscopy (EDS): elemental chemical analysis of solids

[0114] 3. Inductively coupled plasma mass spectrometry (ICP-MS): elemental analysis of TRs in solids and liquids;

[0115] 4. Atomic absorption spectrometry (AAS): elemental analysis of other elements in solids and liquids.

[0116] Example 1: Salt-acid concentration of TRs, flotation of SiC>2, acid leaching, fluorapatite recovery, TR precipitation and CaCCh regeneration

[0117] Phase A:

[0118] In a 5 L reactor, 200 g of phosphate sludge (PO) is combined with 25 L of a saline / acid solution LO. This solution consists of 25 g / L of salts (1:1 NaCl:KCl) and a 10% mass concentration of acetic acid, with a conductivity of 56 mS, a density of 1.030 g / mL, and a pH of 4.5. The resulting suspension is stirred using a mechanical stirrer operating at 600 rpm for one hour at room temperature. The suspension is then filtered under vacuum, yielding a CaCO₃-rich solution L1 and a TR-rich precipitate P1.

[0119] The masses and chemical analysis data are compiled in Table 2. Analyses and characterizations were then carried out on the PO and P1 solids to assess the modifications. The results of the XRD analyses are presented in Figure 1.

[0120] Table 2 shows that the solubility of PO in solution L0 is 4.25 g / l and the concentration of TRs has been multiplied by a factor of more than 2, thus indicating a concentration yield of TRs of 94.13%.

[0121] [Table 2]

[0122] Solid Mass (g) TRs (ppm)

[0123] PO 200 418

[0124] P1 93.7 840

[0125] Table 2: TRs content in PO and P1 solids

[0126] XRD analyses performed on solids PO and P1 (Figure 2) reveal that PO consists of four major phases: fluoroapatite, dolomite, calcite, and quartz. After the salt-acid concentration process, the calcite phase is eliminated; this change is apparent in the spectrum of precipitate P1. This evolution is attributable to the dissolution of calcium carbonates during the concentration of TRs.

[0127] Phase B:

[0128] In a 180 mL Hallimond tube apparatus, 25 g of precipitate P1 is introduced along with 150 mL of distilled water. This suspension is then stirred using a magnetic stirrer at 200 rpm for 5 minutes at room temperature. Subsequently, 10 mL of a 5 M industrial amine is added to the pulp, and this mixture is stirred again at 200 rpm for another 5 minutes. After this step, a flow rate of 5 L / min is maintained for 10 minutes to induce air bubble formation. The floating particles are collected, filtered, washed with 10 mL of distilled water, and dried for one hour in an oven at 60 °C to yield solid P2. As for the pulp remaining in the tube, it is filtered under vacuum and the resulting P3 precipitate is washed with 10 ml of distilled water, then dried for one hour at 60 °C.The resulting solution is kept for later reuse in phase B of the process.

[0129] The masses and chemical analysis data are compiled in Table 3. Analyses and characterizations were then carried out on precipitates P1, P2, and P3 to assess the modifications. The results of the XRD analyses are presented in Figure 2.

[0130] Table 3 indicates that the concentration of TRs in residue P3 increased by a factor of 1.5 (equivalent to a total concentration factor of 2.5). Concurrently, a negligible amount was transferred to solid P2 during the flotation process, confirming a TR concentration yield of approximately 94.34%.

[0131] [Table 3]

[0132] Solid Mass (g) TRs (ppm)

[0133] P1 25 840

[0134] P2 7.61 130

[0135] P3 16.51 1200

[0136] Table 3: TRs content in solids P1, P2 and P3

[0137] XRD analyses performed on solids P1, P2, and P3 (see Figure 2) indicate that solid P2 is composed exclusively of quartz, which is as expected. Meanwhile, crystal analysis of precipitate P3 shows that the intensity of the characteristic quartz peaks has decreased, and that the residue is now mainly composed of fluorapatite and dolomite.

[0138] Phase C:

[0139] In a 200 mL beaker, 5 g of precipitate P3 is mixed with 50 mL of hydrochloric acid adjusted to a mass concentration of 10%. This is carried out under magnetic stirring at a speed of 500 rpm for 1 hour at room temperature. After this time, the mixture is subjected to vacuum filtration, yielding a solution S1 enriched in TRs and a precipitate P4 depleted in TRs and rich in fluorapatite.

[0140] The chemical analysis data for solution S1 are detailed in Table 4. The results of the XRD analyses carried out on precipitate P4 are shown in Figure 3.

[0141] Table 4 indicates that the leaching efficiency is 85%. The P4 leach residue has a mass of 2.5 and a TRs content of 350 ppm.

[0142] [Table 4]

[0143] Mass / Volume Matrix TRs (ppm)

[0144] P3 5 g 1200

[0145] P4 2.5 350

[0146] S1 50 ml 102 (1024 taking into account the dilution)

[0147] Table 4: TR contents in matrices P3, P4 and S1

[0148] XRD analysis of precipitate P4 (Figure 3) reveals that this precipitate is now mainly composed of fluorapatite, while the proportion of dolomite has decreased, since the solubility of dolomite in dilute acids is much greater than that of fluorapatite.

[0149] Phase D:

[0150] A 20 mL volume of solution S1 is introduced into a beaker equipped with a pH meter. Stirring is then initiated using a magnetic stirrer, and a 5 M sodium hydroxide solution is added dropwise while maintaining a stirring speed of 300 rpm. This process continues until the pH of the solution reaches 6.4. At this pH, a precipitate forms. The resulting mixture is then centrifuged to recover precipitate P5, which is rich in TRs, while solution S2, depleted in TRs, is retained for later use.

[0151] The recovered precipitate P5 is placed in a 30 mL beaker equipped with a pH meter. Five mL of distilled water are added, and the mixture is stirred at 300 rpm. Then, 2 M nitric acid is added dropwise until the pH reaches 1.5. Following this, the mixture is centrifuged to separate solution S3, in which all the TRs are dissolved, while precipitate P6, depleted in TRs, is retained for later use.

[0152] The resulting solution S3 is placed in a 30 mL beaker also equipped with a pH meter. Then, 5 mL of 2 M oxalic acid are added to the mixture, which is stirred at 200 rpm. The initial pH of the solution is 1.2. Drops of sodium hydroxide are then added until the pH reaches 2.5, causing the formation of a precipitate. The mixture is finally centrifuged to collect precipitate P7, which is calcined in a furnace at 900 °C for 2 hours to obtain precipitate P8.

[0153] The chemical analysis data for solution S1 and precipitate P8 are detailed in Table 5.

[0154] Table 5 indicates that the final precipitation yield is 82.35%. And the final TR concentration is 21,000 ppm, or 2.85% OTR.

[0155] [Table 5]

[0156] Mass / Volume Matrix TRs (ppm)

[0157] S1 20 ml 1200

[0158] P8 80 mg 21,000

[0159] Table 5: TR contents in matrices S1 and residual and S1

[0160] Phase E:

[0161] One liter of solution L1 is introduced into a distillation system. The distillation process is maintained at 100°C until 85% of the initial volume of solution L1 has evaporated. Following this evaporation, two components are obtained: distillate and a solid-liquid mixture. This mixture is then subjected to vacuum separation using a sintered glass device. This allows for the recovery of, on the one hand, a precipitate P9, which is washed with 5 mL of distilled water to remove traces of NaCl, KCl, and acetic acid, resulting in a high-purity precipitate P9 containing CaCO3. On the other hand, a filtrate concentrated in salts and acetic acid is collected. This filtrate is then mixed with the distillate and the wash water, forming a new solution LOE with properties similar to those of solution LO.

[0162] Table 6 presents a comparison between the initial LO solution and the LOE solution obtained after the recycling process. The chemical properties of the LO and LOE solutions are summarized in Table 6. The XRD analysis of precipitate P9 is shown in Figure 4.

[0163] [Table 6]

[0164] Salinity, Conductivity

[0165] Solution, ,1pH Density,

[0166] (g / l) (mS / cm)

[0167] LO 26 2.08 1.026 33.5

[0168] LO after 27 97 2 0.05 1.027 33.2 recycling

[0169] Table 6: Properties of LO solutions before and after recycling

[0170] The LOE solution obtained through the recycling process was reused for salt-acid concentration, according to the process of the invention. Experimental tests demonstrated that this solution provides a PO solubility of 4.25 g / L and an enrichment factor of 2, results similar to those obtained for the initial LO solution. This observation confirms the effectiveness of the recycling system, showing that the recycled solution retains its properties and its ability to promote the solubility and enrichment of phosphate sludge.

[0171] XRD analysis performed on precipitate P9 (figure 4) reveals that this precipitate has a purity of 99% CaCCh.

[0172] Example 2 (comparative example): Saline concentration alone of TRs in BP

[0173] In this example, a process for concentrating TRs in BP(PO), similar to that described in Example 1, is implemented. However, this time, in phase A, the process takes place in a saline medium without the addition of acid. No modifications have been made to the other phases. Phase A:

[0174] Table 7 shows that the solubility of the PO solid in the saline medium alone is 1.9 g / l, which shows a decrease in solubility of 2.2 times; this decrease also had an impact on the enrichment factor, which does not exceed 1.3.

[0175] [Table 7]

[0176] _ , Mass TRs

[0177] So hde

[0178] (g) (ppm)

[0179] PO 200 418

[0180] P1 151 543

[0181] Table 7: TRs content in PO and P1 precipitates

[0182] Phase B:

[0183] According to Table 8, precipitate P3 has a TR enrichment factor of 1.5, similar to Example 1. The TR yield reaches 95.30%.

[0184] [Table 8]

[0185] " , . , x Mass TRs

[0186] Rushed...

[0187] (g) (ppm)

[0188] P1 25 543

[0189] P2 7, 31 92.5

[0190] P3 16.05 793

[0191] Table 8: TRs content in solids P1, P2 and P3

[0192] Phase C:

[0193] Table 9 shows that the leaching efficiency of TRs is 80%.

[0194] [Table 9]

[0195] Mass / Volume Matrix TR (ppm)

[0196] P3 5 g 793

[0197] P4 2.7 g 293

[0198] 63

[0199] S1 50 ml (630 taking into account the dilution) Table 9: TRs content in matrices P3, P4 and S1

[0200] Phase D:

[0201] The precipitation results were similar to the results obtained in phase D of example 1.

[0202] Phase E:

[0203] Table 10 presents a comparison between the initial LO solution and that obtained after the recycling process.

[0204] [Table 10]

[0205] Conductivity

[0206] Solution Salinity (g / l) pH Density . _ . .

[0207] (mS / cm)

[0208] L0 25.12 6.231 1.015 41.8

[0209] LOE 25.08 5.612 1.015 41.2

[0210] Table 10: Properties of LO solutions before and after recycling

[0211] Experimental tests revealed that the LO solution obtained after recycling exhibits a PO solid solubility of 4.31. This value is slightly higher than that obtained with the initial LO solution. This slight increase can potentially be attributed to the reduction in pH in the solution after recycling. Furthermore, the enrichment factor also decreased from 1.3 to 1.4.

[0212] Example 3 (comparative example): Saline concentration of TRs alone using KCl instead of NaCl

[0213] In this example, a process for concentrating TRs in BP(PO), similar to that described in Example 2, is implemented. However, this time, in phase A, the process takes place in a saline medium using 25 g / L KCl. No modifications were made to the other phases.

[0214] Phase A:

[0215] Table 11 shows that the solubility of the PO solid in this saline medium is 1.34 g / l, which marks a decrease in solubility of 1.26 times; this decrease also had an impact on the enrichment factor, which does not exceed 1.1. [Table 11]

[0216] " , . , x . Mass TRs

[0217] Rushed...

[0218] (g) (ppm)

[0219] P0 200 418

[0220] P1 162 508

[0221] Table 11: TRs content in solids PO and P1 The results for Phases B, C, D and E are similar to the results obtained for example 2.

[0222] References:

[0223] [1] M. Loutou, M. Hajjaji, M. Mansori, C. Favotto, and R. Hakkou, “Phosphate sludge: Thermal transformation and use as lightweight aggregate material,” J Environ Manage, vol.

[0224] 130, pp. 354-360, Nov. 2013, doi: 10.1016 / j.jenvman.2013.09.004.

[0225] [2] “CN114703385A : Process method for extracting phosphorus and rare earth from rare earth-containing low-grade phosphate ore,” 2022

Claims

DEMANDS 1. Process for extracting rare earth elements from phosphate sludge comprising the following steps: (1) treatment of phosphate sludge by an acidic saline solution leading to the obtaining of a solid-liquid mixture M1; (2) separation of mixture M1 and collection of a solid P1 and a solution L1; (3) flotation of solid P1 leading to the formation of a floated solid-liquid mixture M2 and a solid-liquid mixture M3; (4) separation of mixture M2 and collection of a solid P2 and a solution L2; (5) separation of mixture M3 and collection of a solid P3 and a solution L2; (6) acid leaching of solid P3 leading to the obtaining of a solid-liquid mixture M4; (7) separation of mixture M4 and collection of a solid P4 and a solution S1; and (8) retrieval of TRs from solution S1.

2. A process according to claim 1, wherein the acidic saline solution comprises one or more salts selected from chloride salts, preferably NaCl, KCl, MgCh and CaCh, and one or more acids selected from mineral acids, preferably selected from sulfuric acid, nitric acid, hydrochloric acid and phosphoric acid, organic acids, preferably acetic acid, citric acid and fatty acids comprising from 4 to 20 carbon atoms or mixtures of mineral and organic acids.

3. A method according to claim 1 or 2, wherein the separation of steps (2), (4), (5) and (7) is carried out by vacuum filtration, centrifugation or decantation.

4. A method according to any one of claims 1 to 3, wherein the recovery of TRs from solution S1 according to step (8) is carried out by precipitation as hydroxide.

5. A process according to claim 4, wherein the precipitation of TRs in the form of oxide is carried out by adding a basic solution to solution S1 leading to the formation of a solid-liquid mixture M5.

6. A method according to claim 5, wherein the basic solution is a solution of sodium hydroxide, potassium hydroxide, magnesium hydroxide, or tetramethylammonium hydroxide.

7. A method according to claim 5 or 6, further comprising separation of the mixture M5 and collection of a solid P5 and a solution S2.

8. A process according to claim 7, further comprising a partial redissolution of the solid P5 by the addition of a dilute acidic solution leading to the formation of a solid-liquid mixture M6.

9. A method according to claim 8, further comprising separation of the mixture M6 and collection of a solid P6 and a solution S3.

10. A process according to claim 9, further comprising the precipitation of TRs as oxalate from solution S3.

11. A process according to claim 10, further comprising calcining TRs in the form of oxalate to give TRs oxides.

12. A process according to any one of the preceding claims, further comprising evaporation of solution L1 by fractional evaporation leading to a solid comprising calcium carbonate.

13. A method according to any one of the preceding claims, wherein the solid P2 comprises from 90 to 99% by weight of silica.

14. A method according to any one of the preceding claims further comprising a step of separating the TRs recovered in step 8.

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