METHOD FOR TREATING A PHOSPHORIC ACID SOLUTION CONTAINING HEAVY METALS BY IONIC FLOTATION

Abstract

The present invention relates to a method for treating a phosphoric acid solution comprising heavy metals by ionic flotation, in which the phosphoric acid solution is mixed with an ionizable surfactant and an adsorbent solid.

Description

1 / 21 “METHOD FOR TREATING A PHOSPHORIC ACID SOLUTION CONTAINING HEAVY METALS BY IONIC FLOTATION” Technical Field of the Invention

[001] The present invention relates to a method of treating a phosphoric acid solution comprising heavy metals by ion flotation, in which the phosphoric acid solution is mixed with an ionizable surfactant and an adsorbent solid with a particle size ranging from 5 to 160 μm. Background of the Invention

[002] Phosphoric acid (H3PO4) can be produced mainly by two methods: a wet process and a thermal process. The wet process is the most widely used. Phosphoric acid from the wet process is obtained by attacking natural phosphate with a strong acid, such as hydrochloric acid, nitric acid and / or sulfuric acid.

[003] Industrial phosphoric acid is especially a precursor in the manufacture of phosphate fertilizers. However, it contains elements that can pollute soils or groundwater, such as cadmium (Cd), copper (Cu), arsenic (As), zinc (Zn), lead (Pb), nickel (Ni), or chromium (Cr).

[004] Cadmium levels in fertilizers are particularly monitored by the European Parliament and other institutions that require cadmium limits in phosphate fertilizers. Consequently, there is interest in reducing the concentration of this element as much as possible, and also in reducing the levels of other heavy metals as much as possible.

[005] Several methods have been developed to remove heavy metals, such as precipitation, solvent extraction, adsorption, ion exchange, cocrystallization, membrane techniques and ion flotation.

[006] For example, patent applications EP 0 099 804 and EP 0 203 076 describe a method for treating phosphoric acid by ionic flotation. Petition 870250083062, dated 09 / 15 / 2025, page 41 / 68 2 / 21 This method consists of reducing the levels of Cd, As, and U in a phosphoric acid solution by adding a dithiophosphate. Although this method removes large quantities of heavy metals, it also presents significant losses of phosphoric acid, exceeding 30%, which does not allow for the industrial exploitation of the method at an acceptable cost.

[007] Documents FR 2 530 161 and EP 0 099 804 describe a method for treating an acidic medium, in particular a sulfuric acid solution by wet means, using the principle of ion flotation. The method comprises the following steps. There is no mention of the use of an adsorbent.

[008] US patent 4,844,873 describes a more efficient ion flotation method on an industrial scale than the methods described, for example, in patent FR 2,530,161. There is no mention of the use of an adsorbent.

[009] US documents 4,452,768, 4,479,924 and 4,713,229 describe methods for removing heavy metals from a sulfuric acid solution by a wet method. Crude phosphoric acid is contacted with an adsorbent and a diorganyldithiophosphoric acid ester, and the purified phosphoric acid is separated from the ester and the adsorbent. Separation is carried out by decantation, centrifugation, filtration or in a column. The adsorbents used may be activated carbon, carbon black, lignite, diatomaceous earth, silica gel, synthetic silicic acids, a porous resin, silicate or zeolites.

[0010] Ion exchange methods were also considered, in application WO 2004 / 083118 A1, for the removal of heavy metals. The recovery yields obtained are only approximately 50% for cadmium.

[0011] US patent application 450016 A describes a method for liquid-liquid extraction of heavy metals from phosphoric acid, being the Petition 870250083062, dated 09 / 15 / 2025, page 42 / 68 3 / 21 extraction performed using a dialkyldithiophosphoric acid ester solution. The disadvantage of this method lies in the need to perform a pretreatment step of the phosphoric acid solution. The problem with liquid-liquid extraction is the phase separation step, in which the emulsified or dissolved dialkyldithiophosphoric acid ester can be removed with the separated phosphoric acid, requiring an additional posttreatment step.

[0012] Thus, state-of-the-art methods have disadvantages in terms of removing heavy metals or other valuable elements from industrial phosphoric acid. In particular, they require difficult treatment conditions, sophisticated equipment, large quantities of reagents, incompatibility with all concentrations of phosphoric acid, or the need to perform pre- or post-treatment of the phosphoric acid.

[0013] There is therefore a need for new methods to facilitate the recovery of heavy metals from a phosphoric acid solution that are feasible on a large scale and at a moderate cost. Brief Description of the Invention

[0014] The present invention relates to a method for treating a phosphoric acid solution containing heavy metals by ionic flotation, the method comprising the following steps: (i) prepare a mixture comprising a phosphoric acid solution, an ionizable surfactant, and an adsorbent solid having a particle size greater than 90% by volume in the range of 5 to 160 μm, as determined by laser diffraction particle size analysis, (ii) inject gas upwards into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant, and the adsorbent solid, (iii) separate the phosphoric acid solution from the foams containing Petition 870250083062, dated 09 / 15 / 2025, page 43 / 68 4 / 21 heavy metals.

[0015] Other aspects of the invention are described below and in the claims. Brief Description of the Drawings

[0016] Figure 1: Example of a laboratory-scale flotation column.

[0017] Figure 2: Example of a pilot-scale flotation column. Detailed Description of the Invention

[0018] The inventors have developed a method that meets the expressed needs. The proposed method for treating a phosphoric acid solution, typically obtained by the wet process, for the removal of heavy metals by ion flotation does not present the disadvantages of the prior art. It involves the use of a small amount of solid adsorbent, resulting in greater efficiency in the recovery of phosphoric acid, while limiting the amounts of reagent used.

[0019] The various embodiments presented throughout the description can be used alone or in combination with each other, without limitation on the number of combinations.

[0020] Thus, the present invention relates to a method of treating a phosphoric acid solution containing heavy metals by ionic flotation, the method comprising the following steps: (i) prepare a mixture comprising a phosphoric acid solution, an ionizable surfactant, and an adsorbent solid having a particle size greater than 90% by volume in the range of 5 to 160 μm, as determined by laser diffraction particle size analysis, (ii) inject gas upwards into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant, and the solid Petition 870250083062, dated 09 / 15 / 2025, pp. 44 / 68 5 / 21 adsorbent, (iii) separate the phosphoric acid solution from the foams containing heavy metals.

[0021] In some embodiments, the present invention relates to a method of treating a phosphoric acid solution containing heavy metals by ionic flotation, the method comprising the following steps: (i') mix the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having a particle size greater than 90% by volume of 5 to 160 μm, (ii) inject gas upwards into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid, (iii) separate the phosphoric acid solution from the foams containing heavy metals.

[0022] Thus, in certain embodiments, the preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having a particle size of more than 90% by volume ranging from 5 to 160 μm (step (i)) is carried out by mixing the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having a particle size of more than 90% by volume ranging from 5 to 160 μm.

[0023] In other embodiments, the adsorbent is not added to the phosphoric acid solution, but is present in the phosphoric acid solution as a byproduct of phosphoric acid manufacturing. The adsorbent is phosphogypsum in these embodiments.

[0024] Advantageously, the treatment method according to the invention allows the removal of heavy metals present in a phosphoric acid solution by means of a reduced amount of reagents, in particular Petition 870250083062, dated 09 / 15 / 2025, pp. 45 / 68 6 / 21 a reduced amount of ionizable surfactant, typically an amount ranging from 0.5 to 2 g / kg of solution. The heavy metals removed from the phosphoric acid solution by the method according to the invention are advantageously cadmium, copper, arsenic, lead, nickel, chromium and zinc.

[0025] In particular, the method according to the invention allows for the removal of at least 90% by weight of the cadmium present in the phosphoric acid solution; advantageously, at least 99% by weight of the cadmium is removed.

[0026] Another advantage of the method according to the invention is to limit the losses of phosphoric acid.

[0027] Advantageously, the treatment method does not require any prior treatment or subsequent post-treatment of the phosphoric acid solution.

[0028] The advantages of the method according to the invention make it particularly applicable to the treatment of phosphoric acid solutions on an industrial scale.

[0029] The ion flotation treatment method can be implemented at a temperature ranging from 10 to 90 °C, typically from 15 to 90 °C or from 20 to 80 °C. The temperature is chosen to be lower than the degradation temperature of the reagents introduced in step (i), in particular, the temperature is lower than the degradation temperature of the ionizable surfactant. Generally, the temperature at which steps (i) and (ii) are implemented is identical. Stage (i)

[0030] The method of the present invention comprises the preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid with a particle size greater than 90% by volume ranging from 5 to 160 μm.

[0031] The adsorbent solid can be added to the acid solution. Petition 870250083062, dated 09 / 15 / 2025, pp. 46 / 68 7 / 21 phosphoric acid to be treated (see step i') or it may be present in the phosphoric acid solution as a byproduct of phosphoric acid production. In the latter case, it is phosphogypsum.

[0032] The use of phosphogypsum represents a significant advantage. When phosphogypsum is already present in the phosphoric acid solution as a byproduct, it eliminates the need for additional treatment of the phosphoric acid to add an adsorbent. The phosphoric acid solution used in the method is typically an unpurified solution to remove the phosphogypsum. This approach not only optimizes the use of available resources but also reduces the costs and environmental impact associated with adding an external adsorbent. Furthermore, the phosphogypsum present in the phosphoric acid as a byproduct contains organic materials that, from a technical point of view, increase the hydrophobicity of the medium. This property promotes the flotation of collector (surfactant)-metal ion complexes and contributes to the formation of stable foams without the need for the addition of other additives.Furthermore, the particle size of phosphogypsum (particle size ranging from 5 to 160 μm) is suitable to allow the particles to be transported by gas bubbles to the surface, facilitating their recovery in foams.

[0033] Although phosphogypsum may be present in the phosphoric acid solution to be treated as a byproduct, this does not exclude the possibility that in some embodiments it may be added to the phosphoric acid solution to be treated (see step i'). Mixing stage (i')

[0034] In some embodiments, the method according to the invention comprises a step (i') of mixing the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having a particle size of more than 90% by volume ranging from 5 to 160 μm.

[0035] Step (i') can be performed with agitation to promote Petition 870250083062, dated 09 / 15 / 2025, pp. 47 / 68 8 / 21 foam formation. Generally, the mixing stage lasts from 5 seconds to 15 minutes or from 1 to 5 minutes.

[0036] In some embodiments, the phosphoric acid solution is first mixed with the adsorbent solid and then mixed with the ionizable surfactant. The step of mixing the phosphoric acid solution with the adsorbent solid is hereinafter referred to as the “conditioning step”.

[0037] In some embodiments, the phosphoric acid solution is first mixed with the ionizable surfactant and then mixed with the adsorbent solid.

[0038] In some embodiments, the phosphoric acid solution is mixed simultaneously with the ionizable surfactant and the adsorbent solid.

[0039] Advantageously, reducing the contact time between the element to be floated (metal ions) and the collector (ionizable surfactant) before implementing the flotation step allows limiting, or even overcoming, liberation problems, which refer to the separation between metal ions in solution and the collector. Phosphoric acid solution

[0040] The method according to the invention can be implemented for any type of phosphoric acid solution, regardless of its origin. The phosphoric acid solution is commonly obtained through a wet production process. Generally, the phosphoric acid solution has a phosphoric acid concentration ranging from 5 to 65% by weight of P2O5, generally ranging from 10 to 60% by weight of P2O5, or else ranging from 25 to 55% by weight of P2O5, relative to the total weight of the phosphoric acid solution.

[0041] Generally, phosphoric acid solution contains from 0.1 to Petition 870250083062, dated 09 / 15 / 2025, pp. 48 / 68 9 / 21 6% by weight of impurities relative to the weight of P2O5.

[0042] The phosphoric acid solution normally has a cadmium content of less than or equal to 100 ppm, or less than or equal to 50 ppm, and normally greater than or equal to 1 ppm, for example, ranging from 1 to 100 ppm, or even ranging from 1 to 50 ppm or then ranging from 1 to 35 ppm.

[0043] Phosphoric acid solution typically has an arsenic content of less than or equal to 50 ppm, or less than or equal to 35 ppm, typically ranging from 1 to 30 ppm.

[0044] In some embodiments, the phosphoric acid solution is a 29% by weight solution of P2O5. Such solutions generally comprise 5 to 20 ppm of cadmium.

[0045] In some embodiments, the phosphoric acid solution is a 54% by weight solution of P2O5. Such solutions generally comprise 10 to 35 ppm of cadmium.

[0046] Phosphoric acid solution typically has a solids content of less than or equal to 3% by weight, typically ranging from 0.1 to 3% by weight or from 0.1 to 1% by weight of solids relative to the total weight of the solution. A low solids content in the phosphoric acid solution promotes foam formation with a reduced amount of ionizable surfactant. Commonly, a phosphoric acid solution with a solids content of less than or equal to 3% is obtained after a pretreatment step by precipitation, adsorption, coagulation / flocculation, filtration and / or ultrafiltration, typically filtration.

[0047] Alternatively, a phosphoric acid solution with a solids content greater than 3% by weight, typically between 3 and 5% by weight, can also be treated in the method according to the invention. Ionizable surfactant

[0048] In the method of the present invention, the phosphoric acid solution is mixed with an ionizable surfactant. Petition 870250083062, dated 09 / 15 / 2025, pp. 49 / 68 10 / 21

[0049] The term “ionizable surfactant” designates a surfactant that has at least one supporting hydrocarbon chain and a polar head, typically a negatively charged polar head.

[0050] The ionizable surfactant acts as a "scavenger," that is, it will form a complex with heavy metals. This complex is then adsorbed onto the adsorbent solid used in the method of the present invention.

[0051] The ionizable surfactant may be selected from the group consisting of dithiophosphate derivatives, dithiophosphinate derivatives, xanthate derivatives and mixtures thereof.

[0052] Typically, the ionizable surfactant is selected from the group consisting of compounds of formula (I), compounds of formula (II) and compounds of formula (III): R ---------------O s S R·!----OS Dithiophosphate (I) Dithiophosphinate (II) © s r3---o--^s Xanthate (III) wherein R1, R2 and R3 are independently linear or branched alkyl groups comprising from 1 to 30 carbon atoms or from 2 to 25 carbon atoms or from 4 to 20 carbon atoms or from 6 to 15 carbon atoms.

[0053] In some embodiments, the phosphoric acid solution is mixed with 0.1 to 5%, preferably 0.5 to 4%, or 1 to 3%, by weight of ionizable surfactant based on the total weight of P2O5 in the acid solution. Petition 870250083062, dated 09 / 15 / 2025, pages 50 / 68 11 / 21 phosphoric. SOLID ADSORBENT

[0054] In the method of the present invention, the phosphoric acid solution is mixed with an adsorbent solid having, in more than 90% by volume, a particle size ranging from 5 to 160 μm, preferably from 10 to 120 μm.

[0055] The term “adsorbent solid” designates a solid that adsorbs substances dissolved in phosphoric acid.

[0056] The adsorbent solid may be selected from the group consisting of phosphogypsum, activated carbon, zeolites and mixtures thereof. Preferably, the adsorbent solid is phosphogypsum.

[0057] The use of a solid adsorbent to perform ion flotation enables the formation of complexes with the ionizable surfactant and impurities in the form of foam, thus improving the buoyancy of the impurities in the presence of gas bubbles. The specific size of the adsorbent particles ensures flotation. It allows the particles to be transported by gas bubbles to the surface, facilitating their recovery in the foams.

[0058] The use of a solid adsorbent also makes it possible to limit the amount of ionizable surfactant needed to remove impurities, while at the same time limiting the losses of phosphoric acid that can be carried along with the impurities in the form of foam.

[0059] In some embodiments, the phosphoric acid solution is mixed with 0.1 to 5%, preferably 0.5 to 4%, or 1 to 3%, by weight of adsorbent solid based on the total weight of P2O5 in the phosphoric acid solution.

[0060] In some embodiments, the adsorbent solid has a particle size, greater than 90% by volume, ranging from 5 to 160 µm, typically ranging from 10 to 120 µm. The particle size is Petition 870250083062, dated 09 / 15 / 2025, pp. 51 / 68 12 / 21 is typically chosen to allow the solid to be transported by air bubbles. Particle size is commonly determined by laser diffraction particle size analysis, for example, using the Malvern Mastersizer 2000 (wet process).

[0061] Optionally, the phosphoric acid solution may be mixed with one or more conventional agents, such as a foaming agent, a reducing agent, and mixtures thereof. These agents aim to facilitate foam formation and enhance the treatment of heavy metals by ion flotation.

[0062] The use of a reducing agent allows limiting the amount of ionizable surfactant. The reducing agent is typically selected from the group consisting of iron powder, red phosphorus, iron(II) sulfate, iodine, hydrazine and mixtures thereof.

[0063] When the phosphoric acid solution is first mixed with the adsorbent solid and then with the ionizable surfactant, the reducing agent is normally added in the conditioning step.

[0064] The amount of reducing agent used is determined based on the ionic charge of the phosphoric acid solution, according to the general knowledge of the person skilled in the art. Ionic charge usually refers to the charge on ionic species, including metallic elements, to be removed from the phosphoric acid solution. In some embodiments, the phosphoric acid solution is mixed with 0.08 to 0.7%, preferably 0.1 to 0.5%, by weight, of reducing agent, based on the weight of P2O5 in the phosphoric acid solution.

[0065] Foaming agents facilitate the formation of foams containing the heavy metals to be removed. The foaming agent may be, for example, methyl isobutyl ketone, commonly abbreviated as MIBK, polypropylene glycol ether, polypropylene glycol alkyl ether, polyglycol alkyl ether, Petition 870250083062, dated 09 / 15 / 2025, pages 52 / 68 13 / 21 aliphatic alcohols, cyclic aliphatic alcohols (pine oils) or any other similar agent.

[0066] Foaming agents are normally added with or after the addition of the ionizable surfactant.

[0067] The amount of foaming agent used is determined based on the foaming character of the phosphoric acid solution, according to the general knowledge of the person skilled in the art. In some embodiments, the phosphoric acid solution is mixed with 0.02 to 2% by weight of foaming agent, based on the total weight of P2O5 in the phosphoric acid solution. Typically, the phosphoric acid solution is mixed with 0.06 to 1.5% by weight, or 0.1 to 1% by weight, or 0.3 to 0.5% by weight of foaming agent, based on the total weight of P2O5 in the phosphoric acid solution. Gas injection stage (ii)

[0068] The gas injection step (ii) allows the foams containing the heavy metals to be removed, and the ionizable surfactant and adsorbent to float towards the surface of the phosphoric acid solution. For this, the gas is injected in such a way as to form homogeneous gas bubbles which, after adsorption by the foams, will transport them by flotation to the surface of the activation solution.

[0069] The surface of the phosphoric acid solution is defined according to the direction of Earth's gravitational acceleration. It corresponds to the upper part, usually at the top, of the device in which the ion flotation treatment method is implemented.

[0070] Gas bubbles can be formed by any means known to a person skilled in the art, for example, by a porous bottom, sintered glass, or by one or more injector nozzles.

[0071] The gas injected in step (ii) may be air, nitrogen or any other gas that is inert with respect to the species present. Petition 870250083062, dated 09 / 15 / 2025, pp. 53 / 68 14 / 21

[0072] Gas injection in step (ii) can be carried out at a constant flow rate, typically between 200 and 400 ml / min, or between 300 and 600 ml / min, or between 250 and 800 ml / min. The flow rate is chosen based on the concentration of phosphoric acid in P2O5, the mass of phosphoric acid to be treated and the volume of the ion flotation column used.

[0073] Step (ii) is carried out with stirring to ensure a homogeneous distribution of gas bubbles in the phosphoric acid solution. Stirring may be carried out by any means known to those skilled in the art, such as mechanical stirring, like a rotor, or magnetic stirring. Stirring may be carried out, for example, at a speed of 100 to 120 rpm (revolutions per minute).

[0074] Generally, step (ii) can be performed for 5 seconds to 30 minutes, typically 5 seconds to 5 minutes. Separation stage (iii)

[0075] The treatment method comprises a step (iii) of separating foams containing heavy metals from the treated phosphoric acid solution.

[0076] The heavy metals removed by the method according to the invention are generally cadmium, copper, arsenic, lead, nickel, chromium and zinc.

[0077] The recovered foams also comprise the ionizable surfactant, the adsorbent and possibly other optional compounds that may be mixed in step (i). The recovered foams may optionally be treated again to separate and recover the extracted heavy metals.

[0078] Generally, the treated phosphoric acid solution obtained at the end of step (iii) is recovered after separating the foams.

[0079] Foams are commonly recovered in step (iii) in Petition 870250083062, dated 09 / 15 / 2025, pp. 54 / 68 15 / 21 top portion of the phosphoric acid solution treated by any means known to a person skilled in the art. For example, flotation foams may flow into a recovery tank.

[0080] Typically, the method according to the invention allows obtaining a high recovery rate. The recovery rate refers to the ratio between the mass of the treated phosphoric acid solution and the initial mass of the phosphoric acid solution. Thus, the recovery rate is greater than or equal to 90% by weight, typically ranging from 90 to 99% by weight of the initial mass of the phosphoric acid solution.

[0081] The mass ratio of recovered foams / treated phosphoric acid solution obtained at the end of step (iii) may be less than or equal to 3% by weight, typically less than or equal to 2% by weight, or less than or equal to 1% by weight, or less than or equal to 0.5% or greater than or equal to 0.1% by weight.

[0082] Typically, the phosphoric acid solution separated in step (iii) has a heavy metal content reduced by at least 90% or at least 99% by weight compared to the amount initially present in the phosphoric acid solution.

[0083] Thus, the phosphoric acid solution separated in step (iii) typically comprises less than 1.3 ppm of cadmium or 0.01 to 1 ppm of cadmium or 0.06 to 0.8 ppm of cadmium or 0.14 to 0.34 ppm of cadmium.

[0084] More particularly, the phosphoric acid solution separated in step (iii) has an arsenic content reduced by at least 93% or at least 99% by weight relative to the initial amount of arsenic in the phosphoric acid solution.

[0085] Thus, the phosphoric acid solution separated in step (iii) typically comprises less than 0.65 ppm of arsenic, or 0 to 0.44 ppm of arsenic, or 0.02 to 0.28 ppm of arsenic, or 0.07 to 0.16 ppm of arsenic.

[0086] Reducing the content of heavy metals, particularly Petition 870250083062, dated 09 / 15 / 2025, pages 55 / 68 16 / 21 cadmium and arsenic, can also be called heavy metal yield in foams. The values ​​defined above, therefore, apply to the definition of heavy metal yields in foams, in particular arsenic and cadmium yields.

[0087] Advantageously, the treatment method according to the invention does not comprise a subsequent post-treatment step of the phosphoric acid solution. In other words, the treatment method according to the invention typically comprises steps (i), (ii) and (iii), as described above.

[0088] The figures illustrate, in a non-limiting manner, devices that can implement the ion flotation treatment method according to the invention.

[0089] In some embodiments, the method according to the invention is carried out in a flotation device, such as a flotation column combined with a foam recovery tank at the top of the column, as shown schematically in Figure 1.

[0090] Part I, referred to as the treatment part, comprises the flotation column, which consists of a glass column (1) filled with phosphate ore pulp conditioned with the flotation collector according to the invention. Gas is introduced at the bottom of the column and gas bubbles are formed by the passage of the gas through the sintered glass (2). The gas is generated by a gas generator (3) and its flow rate is controlled by a flow rate meter (4). The pulp is agitated by a magnetic stir bar (5) with a magnetic stirrer (6), which allows for good distribution of the gas bubbles (7). Foams (8) form in contact with the gas bubbles. The foams are then carried to the top of the column to a foam evacuation zone (9) corresponding to Part II, referred to as the separation part. The foams (8) are then discharged into a foam recovery tank (10). Petition 870250083062, dated 09 / 15 / 2025, pages 56 / 68 17 / 21

[0091] In some embodiments, the treatment method according to the invention can be implemented in a device such as a flotation column shown schematically in Figure 2.

[0092] The ion flotation column in Figure 2 consists of a glass column (11) equipped with sampling and / or feed valves (12). Gas is introduced through a gas inlet (15), and gas bubbles (13) are formed by the passage of gas through the sintered glass (14). The gas is generated by a gas generator (17) and its flow rate is controlled by a flow rate meter (16). Foams (18 and 20) form in contact with the gas bubbles and are carried to the top of the column corresponding to the foam evacuation zone (19). The foams (18 and 20) then discharge into a foam recovery tank (21).

[0093] When the method of the present invention is carried out in a flotation column, as illustrated in Figure 1 or 2, the method typically comprises, in a first stage, a conditioning step (addition of an adsorbent solid to the phosphoric acid solution previously placed in the flotation column) and then a step of adding a surfactant to the mixture obtained at the end of the conditioning step. Subsequently, a gas is injected into the column to perform ion flotation.

[0094] The residence time in the flotation device is generally less than 30 minutes, preferably between 5 seconds and 5 minutes. Examples

[0095] The following examples, which are not exhaustive, illustrate examples of embodiments of the invention.

[0096] Adsorbent = phosphogypsum; it is a byproduct of the manufacture of phosphoric acid by a wet process.

[0097] Ionizable surfactant = sodium dithiophosphinate with formula Petition 870250083062, dated 09 / 15 / 2025, pages 57 / 68 18 / 21 molecular (C4Hg)2PS2Na, as per formula (II) described above.

[0098] The amounts of ionizable surfactant are expressed in g / kg of P2O5, which corresponds to the amount in grams of ionizable surfactant per kilogram of P2O5 in the phosphoric acid solution. Example 1a: phosphoric acid treatment with 29% P2O5

[0099] 184 g of wet phosphoric acid with 29% P2O5 are placed in an ion flotation column, as shown in Figure 1.

[00100] The adsorbent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution, followed by the ionizable surfactant (2, 4 and 6 g / kg of P2O5). The mixture is stirred at 120 rpm. After a few seconds under these conditions, air is introduced in the form of bubbles into the flotation column at a constant flow rate of 300 ml / min. Stirring is kept constant throughout all stages of the flotation method.

[00101] After a few seconds of air bubble injection, the flotation method is complete. The heavy metals are then recovered as foam in the foam recovery tank, and the phosphoric acid solution is treated in the ion flotation column.

[00102] Table 1 below summarizes the results of heavy metal separation based on the amount of ionizable surfactant used. Test Quantity of ionizable surfactant (g / kg) R%(Cd) R%(As) R%(Cu) 1 2 91.10 90.87 96.41 2 4 99.07 95.22 99.99 3 6 97.09 99.99 99.97 Table 1: Results of the analyses of the phosphoric acid treatment with 29% of P2O5

[00103] R% (Cs, As, Cu) designates the mass yield of cadmium, arsenic or copper recovery in the foams relative to the initial quantities of phosphoric acid solution to be treated.

[00104] The phosphoric acid solution is recovered with a Petition 870250083062, dated 09 / 15 / 2025, pp. 58 / 68 19 / 21 recovery rate of 99%. The P2O5 content is identical and is 29%. Example 1b: phosphoric acid treatment with 29% P2O5

[00105] The protocol of Example 1a is reproduced in a quantity of 11 kg of moist phosphoric acid with 29% P2O5.

[00106] 11 kg of wet phosphoric acid with 29% P2O5 are placed in an ion flotation column suitable for the amount of phosphoric acid to be treated, as shown in Figure 2.

[00107] The adsorbent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution, followed by the ionizable surfactant (4 g / kg of P2O5). The mixture is stirred at 120 rpm. After a few seconds under these conditions, air is introduced into the flotation column in the form of bubbles at a constant flow rate of 800 ml / min. Stirring is kept constant throughout all stages of the flotation method.

[00108] After a few seconds of air bubble injection, the flotation method is complete. The heavy metals are then recovered as foam in the foam recovery tank, and the phosphoric acid solution is treated in the flotation column.

[00109] Table 2 below summarizes the results of heavy metal separation based on the amount of ionizable surfactant used. Test Quantity of ionizable surfactant (g / kg) R%(Cd) R%(As) R%(Cu) R%(V) R%(Zn) R%(U) 4 4 99.67 95.82 97.56 57.42 41.28 54.25 Table 2: Results of the analyses of the phosphoric acid treatment with 29% of P2O5

[00110] R% (Cs, As, Cu) designates the mass yield of cadmium, arsenic, or copper recovery in the foams relative to the initial quantities of phosphoric acid solution to be treated.

[00111] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 29% P2O5. Petition 870250083062, dated 09 / 15 / 2025, pp. 59 / 68 20 / 21

[00112] This example shows that the treatment method according to the present invention makes it possible to treat large quantities of phosphoric acid solution while maintaining excellent recovery yields of heavy metals, in particular cadmium, arsenic and copper. Example 2: Treatment of phosphoric acid with 54% P2O5 on a scale LABORATORY

[00113] 243 g of wet phosphoric acid with 54% P2O5 are placed in an ion flotation column, as shown in Figure 1.

[00114] The adsorbent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution under stirring at 120 rpm, and then the ionizable surfactant (at 2 and 6 g / kg of P2O5) is added. The mixture is stirred at 120 rpm. After a few seconds under these conditions, air is introduced in the form of bubbles into the flotation column at a constant flow rate of 600 ml / min. Stirring is kept constant throughout all stages of the flotation method.

[00115] After a few seconds of air bubble injection, the flotation method is complete. The heavy metals are then recovered as foam in the foam recovery tank, and the phosphoric acid solution is treated in the ion flotation column.

[00116] Table 3 below summarizes the results of heavy metal separation based on the amount of ionizable surfactant used. Test Quantity of ionizable surfactant (g / kg) R%(Cd) R%(As) R%(Cr) R%(Zn) R%(Ni) R%(Cu) 4 2 97.02 95.93 52.4 50.09 55.4 99.99 5 6 99.01 98.99 53.62 54.64 59.24 99.99 Table 3: Results of the analyses of the phosphoric acid treatment with 54%

[00117] R% (Cd, As, Cu) designates the yield of Petition 870250083062, dated 09 / 15 / 2025, pp. 60 / 68 21 / 21 recovery of cadmium, arsenic or copper in foams.

[00118] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 54% P2O5.

[00119] This example illustrates that the treatment method according to the present invention makes it possible to treat phosphoric acid solutions with a high concentration of P2O5, while maintaining excellent recovery yields of heavy metals, in particular cadmium, arsenic and copper. Petition 870250083062, dated 09 / 15 / 2025, pp. 61 / 68

Claims

1 / 4 Claims 1. METHOD FOR TREATING A PHOSPHORIC ACID SOLUTION CONTAINING HEAVY METALS BY ION FLOTATION, characterized by the method comprising the following steps: (i) preparing a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having a particle size greater than 90% by volume in the range of 5 to 160 μm, as determined by laser diffraction particle size analysis, (ii) injecting gas upwards into the mixture of step (i) with agitation to form foams comprising the heavy metals, the surfactant and the adsorbent solid, (iii) separating the phosphoric acid solution from the foams containing heavy metals.

2. TREATMENT METHOD, according to claim 1, characterized in that step (i) is carried out by mixing a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having a particle size of more than 90% by volume ranging from 5 to 160 μm.

3. TREATMENT METHOD according to claim 2, characterized in that the phosphoric acid solution is first mixed with the adsorbent solid and then mixed with the ionizable surfactant.

4. TREATMENT METHOD, according to any one of claims 1 to 3, characterized by the phosphoric acid solution having one or more of the following characteristics: - a phosphoric acid concentration ranging from 5 to 65% by weight of P2O5, - a cadmium content ranging from 1 to 100 ppm, - an arsenic content less than or equal to 50 ppm, Petition 870250083062, dated 15 / 09 / 2025, p. 62 / 68 2 / 4 a solids content less than or equal to 3% by weight.

5. TREATMENT METHOD, according to any one of claims 2 to 4, characterized in that the phosphoric acid solution is mixed in step (i) with 0.1 to 5% by weight of adsorbent solid based on the total weight of P2O5 in the phosphoric acid solution.

6. TREATMENT METHOD, according to any one of claims 1 to 5, characterized in that the adsorbent solid has a particle size, of more than 90% by volume, in the range of 10 to 120 µm.

7. TREATMENT METHOD, according to any one of claims 1 to 6, characterized in that the adsorbent solid is selected from the group consisting of phosphogypsum, activated carbon, zeolites and mixtures thereof.

8. TREATMENT METHOD, according to any one of claims 1 to 7, characterized in that the ionizable surfactant is selected from the group consisting of dithiophosphate derivatives, dithiophosphinate derivatives, xanthate derivatives and mixtures thereof.

9. TREATMENT METHOD, according to claim 8, characterized by the ionizable surfactant being selected from the group consisting of compounds of formula (I), compounds of formula (II) and compounds of formula (III): Formula (I) R1---o 'S / ρ<θ ---os Dithiophosphate (I) Formula (II) Dithiophosphinate (II) Petition 870250083062, dated 09 / 15 / 2025, p. 63 / 68 3 / 4 Formula (III) Θ s r3—O— Xanthate (III) where R1, R2 and R3 are independently linear or branched alkyl groups comprising from 1 to 30 carbon atoms.

10. TREATMENT METHOD, according to any one of claims 1 to 9, characterized in that the phosphoric acid solution is mixed in step (i) with 0.1 to 5% by weight of ionizable surfactant based on the total weight of P2O5 in the phosphoric acid solution.

11. TREATMENT METHOD, according to any one of claims 1 to 10, characterized in that the phosphoric acid solution is further mixed with a foaming agent, preferably selected from the group consisting of methyl isobutyl ketone, a reducing agent and mixtures thereof.

12. TREATMENT METHOD, according to claim 11, characterized in that the phosphoric acid solution is mixed with a reducing agent, the reducing agent being selected from the group consisting of iron powder, red phosphorus, iron(II) sulfate, iodine, hydrazine and mixtures thereof.

13. TREATMENT METHOD, according to any one of claims 11 to 12, characterized in that the phosphoric acid solution is mixed with 0.08 to 0.7% by weight of ionizable surfactant based on the total weight of P2O5 in the phosphoric acid solution.

14. TREATMENT METHOD, according to any one of claims 1 to 13, characterized by being carried out at a temperature ranging from 20 to 80 °C. Petition 870250083062, dated 15 / 09 / 2025, p. 64 / 68 4 / 4 15. TREATMENT METHOD, according to any one of claims 1 to 14, characterized in that the gas injected in step (ii) is air, nitrogen or any other inert gas.

16. TREATMENT METHOD, according to any one of claims 1 to 15, characterized by gas injection in step (ii) being carried out at a constant rate. Petition 870250083062, dated 09 / 15 / 2025, pp. 65 / 68

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