Process for removing arsenic, cadmium and copper from phosphoric acid

Abstract

The present invention relates to a method for purifying a phosphoric acid solution comprising a step of removing arsenic from a phosphoric acid solution using an extraction agent of formula (I): (I) The removal step can be carried out upstream or downstream of a conventional step for purifying a phosphoric acid solution by liquid-liquid extraction or by nanofiltration on a membrane. Figure for the abstract: Fig. 1

Description

Title of the invention: Process for removing arsenic, cadmium and copper from phosphoric acid. FIELD OF THE INVENTION

[0001] The present invention relates to a process for the dearsening of phosphoric acid implemented during a phosphoric acid purification process. The dearsening process of the invention may further include the simultaneous removal of copper and cadmium from the phosphoric acid to be purified. STATE OF THE ART

[0002] The composition and purity of phosphoric acid depend on the origin of the natural mineral phosphate from which the phosphoric acid is derived, and especially on its manufacturing process. There are two main methods for manufacturing phosphoric acid from natural mineral phosphate: the thermal process and the wet process.

[0003] In a thermal process, phosphoric acid is obtained by oxidizing phosphorus by calcination to obtain phosphoric anhydride P2O5, which is then hydrated to phosphoric acid. Phosphoric acid obtained thermally has excellent purity. However, it is very expensive due to the high costs associated with the energy-intensive and highly polluting thermal reduction process. Consequently, it is generally reserved for use in a few high-tech industries, such as pharmaceuticals, detergent manufacturing, and food production.

[0004] In a wet process, phosphoric acid is obtained by direct attack of mineral phosphate with a strong acid (sulfuric, hydrochloric, or nitric). The phosphoric acid produced by this method contains impurities, such as fluorine, organic matter, metallic cations (cadmium, copper, etc.), and arsenic, which are more or less problematic depending on the intended use.

[0005] Several methods exist for the purification of phosphoric acid from the wet process, the best known of which involve solvent extraction or membrane filtration.

[0006] Solvent extraction is one of the most widely used techniques for the purification of phosphoric acid obtained by wet extraction. It was primarily studied by Almela et al. (Fluid Ph. Equilibria. 1998, 745(2), 301-310). This study described the removal of cadmium from phosphoric acid at various concentrations using a commercial complexing agent called Cyanex-302.

[0007] Another study conducted by Touati et al. (Hydrometallurgy, 2009, 95, 135-140) explored the possibility of removing cadmium from phosphoric acid using di(2-ethylhexyl) dithiophosphoric acid (D2EHDTPA) as an extraction agent, through a liquid-liquid extraction process.

[0008] Berkalou et al. (Int. J. Adv. Eng. Technol, 2020, 77(6), 28-35) synthesized an extraction agent of the formula CnHi8N2O to extract cadmium from phosphoric acid by liquid-liquid extraction. However, in this study, toxic benzene was used as the solvent. Furthermore, this method is not effective at removing cadmium present at high concentrations in phosphoric acid.

[0009] US patent 3,970,741 describes a method for purifying phosphoric acid by extraction using a mixed solvent mainly composed of an ether containing 5 to 7 carbon atoms and an ester resulting from the reaction of phosphoric acid with alcohols containing 3 to 5 carbon atoms. This method relies on the extraction of P2O5 and does not remove metallic impurities, thus requiring an additional purification step.

[0010] EP patent 0 030 487 describes the continuous extraction of a phosphoric acid solution using an organic solvent immiscible with water, such as alkyl phosphates or C4-C9 aliphatic alcohols. The charged organic phase is then re-extracted with water to obtain an aqueous phase of phosphoric acid.

[0011] US patent 3,375,068 describes a liquid-liquid extraction method involving the use of an organic amide, specifically A / A-dimethylcaprylamide-capramide (DCCA), dissolved in an inert solvent, for example, isoamyl alcohol. This method is proposed primarily for the extraction of trivalent iron from phosphoric acid solutions. Furthermore, by adjusting the concentration of the amide, it is possible to extract phosphoric acid itself. It is important to note that this method requires the use of substantial concentrations of the organic ligand, even for the extraction of trace amounts of metallic impurities.

[0012] Nanofiltration techniques for phosphoric acid using membranes in aqueous media are also described. However, few exist for concentrated media due to the limited number of membranes capable of withstanding the entire pH range and high temperature.

[0013] Application WO 01 / 89654 describes nanofiltration membranes that remain stable in acidic media. These membranes are made of a specific polymer matrix based on polysulfonamides. However, they are not suitable for industrial use in the phosphoric acid preparation process.

[0014] Application WO2013 / 133684 describes a process for purifying a phosphoric acid solution by nanofiltration, involving the use of an organic nanofiltration membrane onto which is adsorbed at least one water-soluble polymer containing functional groups such as an amine, an aromatic amine, an acid, and / or an alcohol. The application shows that the developed membrane has a Lifespan of more than 6 months but does not remove non-metallic impurities such as fluorine, sulfates and arsenic.

[0015] Existing membrane techniques and liquid-liquid extraction lead to high value added products but do not allow the simultaneous removal of arsenic, thus requiring the addition of an additional dearsening step upstream of these techniques.

[0016] Several methods have been described for dearsening phosphoric acid, including ion exchange, precipitation and adsorption.

[0017] The removal of arsenic by the addition of sodium sulfide (NaHS) is a method commonly used in the phosphate industry. However, the use of NaHS has major drawbacks due to the release of hydrogen sulfide (H₂S), a toxic and foul-smelling gas. H₂S is not only dangerous if inhaled in large quantities, but it can also cause corrosion of metal equipment and infrastructure. Furthermore, its unpleasant odor makes working in the surrounding area difficult.

[0018] Another significant drawback of using sodium sulfide (NaHS) for the dearsening of phosphoric acid is the increased residual sodium content in the final product. Neither liquid-liquid extraction nor nanofiltration can remove the sodium, necessitating further treatment to eliminate it and achieve the required quality.

[0019] A need therefore remains for a phosphoric acid purification process including a disarsening step that does not generate toxic and undesirable by-products, this step being able to be implemented without the need to change the installation. Summary of the invention

[0020] The invention relates to a process for purifying a phosphoric acid solution comprising a step of dearsening a phosphoric acid solution using an extraction agent of formula (I): s (I) RioPOR 2 ï -*■ sz

[0021] in which

[0022] Ri and R2 are independently of each other a linear or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 12 carbon atoms, or a aryl group, with hydrocarbon and aryl groups optionally substituted, and

[0023] Z+ is a counter ion, in particular chosen from Na+, K+ and NH4+.

[0024] The invention also relates to the use of an extraction agent of formula (I) as defined above for the disarmenization, decopper removal and / or decadmization of a phosphoric acid solution.

[0025] The process of the invention can also be defined according to the following optional characteristics, taken alone or in combination where technically possible:

[0026] Other aspects of the invention are as described below. DESCRIPTION OF THE FIGURES

[0027] [Fig.1] represents a schematic illustration of the process of the invention according to the second variant with the disarsening step b') downstream of the step c') of purification of phosphoric acid by liquid-liquid extraction or by nanofiltration by membrane.

[0028] [Fig. 2] represents a schematic illustration of the process of the invention according to the first variant with the disarsening step b) upstream of the step c) of purification of phosphoric acid by liquid-liquid extraction or by nanofiltration by membrane. DETAILED DESCRIPTION OF THE INVENTION

[0029] The proposed process meets the stated requirements. Advantageously, it allows the simultaneous removal of other impurities, in particular metallic cations such as copper and cadmium.

[0030] The process of the invention includes a step of dearsening phosphoric acid using an extraction agent of formula (I): | œ R^OP-OR? i ... 4. ' s Z

[0031] in which

[0032] Ri and R2 are independently of each other a linear or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 12 carbon atoms, or an aryl group, the hydrocarbon and aryl groups being optionally substituted, and

[0033] Z+ is a counter ion, in particular chosen from Na+, K+ and NH4+.

[0034] The extraction agent of formula (I), of the dialkyl dithiophosphate type, is capable of selectively binding arsenic, as well as copper and cadmium ions present in Phosphoric acid. In phosphoric acid solution, arsenic is found primarily in the form of arsenite ions (AsO33) or arsenate ions (AsO43). This arsenic removal step can be carried out upstream or downstream of a conventional phosphoric acid purification step by liquid-liquid extraction or membrane nanofiltration, which removes any impurities other than arsenic. When carried out upstream of a conventional purification step, the extraction agent of formula (I) allows the extraction of any copper and cadmium ions that may be present from the phosphoric acid solution, in addition to arsenic.When carried out downstream of a conventional purification step, the copper and cadmium ions have generally been removed, in part or in full, by the liquid-liquid extraction or membrane nanofiltration purification step, so that the agent of formula (I) mainly extracts the arsenic present in the phosphoric acid solution.

[0035] The process of the invention is advantageously implemented in an existing acid purification plant, avoiding the need to construct a new, separate treatment plant. Indeed, the arsenic removal step can be added directly to a phosphoric acid purification process, either upstream or downstream of conventional liquid-liquid extraction or nanofiltration purification steps, without requiring a change of industrial plant.

[0036] Advantageously, prior to the disarsening step or the so-called classic purification step by liquid-liquid extraction or nanofiltration, depending on the order in which these two steps are implemented, the process of the invention includes a pretreatment step of phosphoric acid.

[0037] In a first embodiment of the invention, the process of the invention comprises the following successive steps:

[0038] a) pretreatment of a phosphoric acid solution,

[0039] b) dearsening of the phosphoric acid solution from step a) using a extraction agent of formula (I) as described above, then

[0040] c) purification of the phosphoric acid solution from step b) by liquid-liquid extraction or by membrane nanofiltration.

[0041] In a second embodiment of the invention, the process of the invention comprises the following successive steps:

[0042] a) pretreatment of a phosphoric acid solution,

[0043] c') purification of the phosphoric acid solution from step a) by extraction liquid-liquid or by nanofiltration using a membrane, then

[0044] b') dearsening of the phosphoric acid solution from step c') using of an extraction agent of formula (I) as described above.

[0045] In this description, the terms "phosphoric acid" and "phosphoric acid solution" refer to an aqueous solution of phosphoric acid having a phosphorus pentoxide (P2O5) content ranging from 10% to 60%, in particular from 23% to 55%, by volume relative to the total volume of the solution. Preferably, it is a phosphoric acid solution with a concentration of 29% v / v or 54% v / v P2O5. The phosphoric acid solution used in the purification process of the invention is prepared from mineral phosphate by a wet process, that is, by reacting mineral phosphate with a strong acid. Step a): Pretreatment

[0046] This pretreatment step makes it possible in particular to remove solid impurities, at least some of the fluorine and at least some of the sulfates initially present in a phosphoric acid solution.

[0047] At the end of this pretreatment step, the phosphoric acid solution advantageously possesses: - a solids content (SC) of less than 1%, preferably less than 0.1%, and in particular less than 0.07%, by weight relative to the total weight of the solution, - a sulfate content of less than 0.5%, in particular less than 0.39%, by weight relative to the total weight of the solution, - an organic matter content of less than 1000 ppm, in particular less than 300 ppm, and / or - a fluoride content of less than 0.2%, in particular less than 0.1%, by weight relative to the total weight of the solution.

[0048] A person skilled in the art knows the pretreatment techniques to be implemented to achieve the aforementioned rates and contents.

[0049] Typically, pretreatment step a) includes a desulfation step of the phosphoric acid solution by adding phosphate rock and / or a defluorination step by adding calcium carbonate to the phosphoric acid solution. These desulfation and fluoridation steps can be carried out using conventional methods known to those skilled in the art. For example, the sulfate content in phosphoric acid can be reduced / removed by using phosphates, and the fluorine content can be reduced / removed by using sodium carbonate.

[0050] Filtration is then typically carried out in step a) to filter out the impurities to be removed and recover a phosphoric acid solution meeting the aforementioned conditions. Filtration can be performed using any suitable conventional filtration method known to those skilled in the art. Step b) or b'): Arsenic removal

[0051] In this step b) or b'), the extraction agent of formula (I) is added to the phosphoric acid solution from step a) or c').

[0052] The formula extraction agent (I) has the following formula: s ® R-jO-P-ORa s"z +

[0053] in which

[0054] Ri and R2 are independently of each other a linear or branched hydrocarbon group, saturated or unsaturated, comprising from 1 to 12 carbon atoms, or an aryl group, the hydrocarbon and aryl groups being optionally substituted, and

[0055] Z+ is a counter ion, in particular chosen from Na+, K+, and NH4+.

[0056] The term "optionally substituted" means that the group in question is optionally substituted by one or more, preferably 1 to 4, and in particular 1 or 2, substituents chosen from the group consisting of a linear or branched, unsaturated or saturated Ci-C6 hydrocarbon group, an OH group, an aryl group, NRaRb, CORC, CO2Rd, and ORe, in which Ra to Re represents, independently of each other, H or Ci-C6 alkyl.

[0057] The term "hydrocarbon group" refers to a group comprising carbon and hydrogen atoms, monovalent, saturated or unsaturated, linear or branched. It notably covers alkyl groups.

[0058] The term "alkyl group in Ci-Ci2" refers to a monovalent saturated hydrocarbon chain, linear or branched, comprising 1 to 12, preferably 1 to 6, carbon atoms. Examples include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, and hexyl groups.

[0059] The term “aryl” designates an aromatic hydrocarbon group, preferably comprising 6 to 10 carbon atoms, and comprising one or more fused rings, such as for example a phenyl or naphthyl group.

[0060] In some embodiments, Ri and R2 are independently of each other a non-substituted Ci-Ci2 hydrocarbon group, preferably Ci-C6, linear or branched, saturated or unsaturated, or an aryl group, the aryl group being preferably a phenyl, optionally substituted by a linear or branched Ci-Ci0 hydrocarbon group, saturated or unsaturated.

[0061] Preferably, Ri and R2 are independently of each other an alkyl in CrC 12, in particular in Ci-C6, preferably chosen from a methyl, an ethyl and a propyl.

[0062] In a preferred embodiment, the extraction agent is as follows: S EtO-P-OEt i _ + S Na

[0063] In some embodiments, the extraction agent is added to the phosphoric acid solution in an amount ranging from 0.05% to 2%, in particular from 0.1% to 1.5% by weight, preferably from 0.1% to 0.5% by weight, relative to the total weight of the solution. Preferably, the extraction agent is added at a rate of 0.1% by weight relative to the total weight of the solution.

[0064] The extraction agent of formula (I) is specific to the following species to be extracted: arsenic, copper, and cadmium. The extraction agent thus allows the extraction of arsenic ions, as well as copper and cadmium ions when present (in the case where step b) is carried out upstream of step c) according to the first embodiment of the invention), via the formation of a complex between the extraction agent of formula (I) and the species to be extracted. At a concentration of the extraction agent of 0.05% to 2% by weight, relative to the total weight of the solution, the latter is selective for arsenic, copper, and cadmium. In other words, at this concentration, the extraction agent does not allow the extraction of any species other than arsenic, copper, and cadmium.

[0065] In particular, an ion exchange is carried out in which the counter ion Z+ initially present in the extraction agent is exchanged with an ionic species to be extracted, namely Cu2+, Cd2+. The extraction agent is further capable of forming a stable complex with arsenic (in the form of arsenite or arsenate), for example thanks to sulfur atoms acting as bond donors.

[0066] The complex formed between the agent of formula (I) and the species to be extracted is then removed. Step b) or b') of disarsening therefore involves the formation of a complex between the agent of formula (I) and the species to be extracted and its removal from the phosphoric acid solution, the species to be extracted being arsenic and, if present, cadmium and copper ions.

[0067] Typically, step b) in the first variant of the invention therefore comprises the following successive substeps:

[0068] bl) adding the extraction agent of formula (I) to the phosphoric acid solution from step a) and forming a complex between the extraction agent of formula (I) and arsenic, copper and / or cadmium;

[0069] b2) elimination of the complex formed in step b 1) of the phosphoric acid solution.

[0070] Typically, step b') in the second variant of the invention therefore comprises the following successive substeps:

[0071] bl') addition of the extraction agent of formula (I) to the phosphoric acid solution from step c') and formation of a complex between the extraction agent of formula (I) and arsenic;

[0072] b2') elimination of the complex formed in step bl') from the acid solution phosphoric.

[0073] Step bl) or bl') is in particular carried out at a temperature ranging from 20°C to 80°C, preferably at room temperature, i.e. ranging from 20°C to 40°C.

[0074] The elimination step b2) or b2') can in particular be carried out either by liquid-liquid extraction by adding an organic solvent that is not miscible with the phosphoric acid solution, or by precipitation and then filtration.

[0075] In the case of liquid-liquid extraction, an organic solvent immiscible with the phosphoric acid solution and in which the complex formed between the extractant of formula (I) and the species to be extracted is soluble, is added to the phosphoric acid solution following the addition of the extraction agent of formula (I). The organic solvent is, in particular, a nonpolar and aprotic solvent, for example, chosen from hydrocarbons, ethers, and mixtures thereof, such as kerosene, diisopropyl ether, and mixtures thereof. The organic solvent is typically added in a 1:1 volume ratio with the phosphoric acid solution. Decantation and separation steps of the organic and aqueous phases then allow the recovery of a phosphoric acid solution free of the complex formed and thus of the targeted impurities. The decantation-separation cycle can be repeated as many times as necessary to remove all of the organic phase.

[0076] The liquid-liquid extraction step can be repeated several times, typically 2 or 3 times, if necessary to remove all of the complex formed.

[0077] In the case of extraction by precipitation and filtration, the complex formed between the agent of formula (I) and the species to be extracted precipitates in the phosphoric acid solution, in the form of a solid precipitate which is removed by filtration.

[0078] Step b) or b') may optionally be repeated several times, particularly step b) in the first embodiment of the invention. The extraction agent of formula (I) has a greater affinity for copper and cadmium ions than for arsenic. Therefore, the arsenic will complex with the extraction agent once the copper and then cadmium ions have been removed. Step b) is thus advantageously repeated two or three times, in particular three times.

[0079] For example, step b) may include the following successive steps:

[0080] bl.a) adding the extraction agent of formula (I) to the phosphoric acid solution from step a) and forming a complex between the extraction agent of formula (I) and the copper ions;

[0081] b2.a) elimination of the complex formed in step bl.a) from the acid solution phosphoric;

[0082] bl.b) adding the extraction agent of formula (I) to the phosphoric acid solution from step b2.a) and forming a complex between the extraction agent of formula (I) and the cadmium ions;

[0083] b2.b) elimination of the complex formed in step bl.b) from the acid solution phosphoric;

[0084] bl.c) adding the extraction agent of formula (I) to the phosphoric acid solution from step b2.b) and forming a complex between the extraction agent of formula (I) and arsenic;

[0085] b2.c) removal of the complex formed in step bl.c) from the acid solution phosphoric.

[0086] Following step b) or b'), the extraction agent of formula (I) can be regenerated either by evaporation of the organic solvent, or by extraction using hot water, followed by neutralization using an acidic solution. Step c) or c'): Purification

[0087] In this step c) or c'), the phosphoric acid solution from step b) or a) respectively is implemented by liquid-liquid extraction or by membrane nanofiltration according to methods known to those skilled in the art.

[0088] In the first variant of the invention, step c) makes it possible to eliminate all remaining impurities in the phosphoric acid solution from step b), already free of arsenic, copper ions and cadmium ions.

[0089] In the second variant of the invention, step c') makes it possible to eliminate all impurities present in the phosphoric acid solution from step a), including copper ions and cadmium ions, with the exception of arsenic.

[0090] The impurities removed during step c) or c') are impurities typically present in phosphoric acid solutions obtained by wet process from phosphate ores and which are not removed during pretreatment step a). These are, in particular, metallic cations.

[0091] In some embodiments, the liquid-liquid extraction in step c) or c') is as described in Radhika, S. et al., Separation and purification technology, 2010, 75(3), 295-302.

[0092] In certain embodiments, the membrane nanofiltration in step c) or c') is as described in WO2013133684A1. The membrane used is in particular chosen from an organic nanofiltration membrane onto which is adsorbed at least one water-soluble polymer comprising at least one amino function, one aromatic amino function, one acid function and / or one alcohol function.

[0093] An example of the process of the invention according to the second embodiment is shown in [Fig. 1]. The illustrated process comprises, successively after the introduction of a volume V0 of a crude phosphoric acid solution: a defluorination step 1, a desulfation step 2 (steps 1 and 2 constituting the pretreatment of a phosphoric acid solution), a purification step 3 of the phosphoric acid solution by nanofiltration or liquid-liquid extraction, a dearsening step 4, followed by clarification (solid-liquid separation) and concentration steps 5 and 6, subsequent to the process of the invention. The dearsening step 4 comprises the successive substeps A, B, and C: A - liquid-liquid extraction, B - decantation, and C - clarification. V1 corresponds to the volume of the pretreated solution.

[0094] An example of the process of the invention according to the first embodiment is shown in [Fig. 2]. The illustrated process comprises, successively after the introduction of a volume V0 of a crude phosphoric acid solution: a defluorination step 1, a desulfation step 2 (steps 1 and 2 constituting the pretreatment of a phosphoric acid solution), a step 3 for the removal of copper, cadmium, and arsenic, a step 4 for the purification of the phosphoric acid solution by nanofiltration or liquid-liquid extraction, followed by clarification and concentration steps 5 and 6, subsequent to the process of the invention. Step 3 comprises the successive substeps A to G: A - liquid-liquid extraction, B - decantation, C - second liquid-liquid extraction, D - decantation, E - third liquid-liquid extraction, F - decantation, G - clarification. V1 corresponds to the volume of the pretreated solution. EXAMPLES

[0095] In the following examples, the formula extraction agent (I) used is the following agent 1: S EtO-P-OEt S Na 1

[0096] Example 1 (first variant of the invention): removal of Cu2+, Cd2+ and As, using 1% of the extraction agent I in kerosene or diisopropyl ether (DiPE)

[0097] In this example, following a pretreatment (step a)), the dearsening (step b) of a 29% and 54% P2O5 phosphoric acid solution was carried out according to the first embodiment of the invention by adding 1% (10 g) of the extraction agent I relative to the total weight of the phosphoric acid solution (1 kg). The resulting solution undergoes Next, a liquid-liquid extraction is performed by adding 1 kg of organic solvent, either kerosene or diisopropyl ether (DiPE), and stirring the mixture for 20 min. The mixture is then decanted, and the organic and aqueous phases are separated.

[0098] Table 1 shows the concentrations of metallic and non-metallic impurities in the phosphoric acid solution before and after implementation of step b). The results show that during step b) using the extraction agent of formula (I), all of the arsenic, Cu2+ and Cd2+ ions are removed, regardless of the solvent used.

[0099] On the other hand, the extraction agent 1 weakly removes other cationic impurities present such as chromium, vanadium ions etc., demonstrating the specificity of this agent with respect to arsenic, copper and cadmium.

[0100] Table 1 Concentrations of metallic and non-metallic impurities in a 29% or 54% P2O5 phosphoric acid solution before and after the addition of agent 1 and liquid-liquid extraction with kerosene or DiPE.

[0101] [Tables 1] Elements Industrial acid 29% Acid 29% treated Industrial acid 54% Acid 54% treated Content after step a) Content after step eb); Solvent = Kerosene Content after step eb); Solvent = DiPE Content after step a) Content after step eb); Solvent = Kerosene Content after step b); Solvent = DiPE P2O5(%) 25.99 25.04 25.21 50.45 49.87 48.35 Cu (ppm) 30.8 «0 «0 51.9 0.9 0 Cd (ppm) 17.8 «0 «0 18.3 0 0 As (ppm) 16.0 «0 «0 19.2 5.92 2.3 Fe (%) 0.21 0.21 0.20 0.31 0.31 0.31 Al (%) 0.11 0.24 0.23 0.15 0.15 0.15 Cr (ppm) 120.0 102.0 97.29 217.4 205.8 213.3 Ni (ppm) 18.9 12.5 7.4 50.7 39.3 32.7 V (ppm) 98.2 86.3 82.9 190.5 186.6 186.4 Zn (ppm) 234.6 168.5 156.6 336.0 328.4 330.0

[0102] Example 2 (first variant of the invention): Removal of Cu2+, Cd2+ and As in cascade in three extraction steps using 0.1% of the extraction agent 1 in kerosene or diisopropyl ether (DiPE) at each step.

[0103] In this example, following a pretreatment (step a)), the dearsening (step b)) of a 29% or 54% P2O5 phosphoric acid solution was carried out according to the first embodiment of the invention by adding 0.1% (1 g) by weight of the extraction agent 1 relative to the total weight of the phosphoric acid solution (1 kg). The resulting solution then undergoes liquid-liquid extraction by adding 1 kg of an organic solvent, either kerosene or diisopropyl ether (DiPE), and stirring the mixture for 20 min. The mixture is then decanted, and the organic and aqueous phases are separated. Step b) is repeated three times in succession. In other words, after separation of the aqueous and organic phases, the aqueous phase is again treated by adding 0.1% of the extraction agent 1 and then extracted with the solvent. This operation is then repeated a third time.

[0104] Tables 2 and 3 show the concentrations of metallic and non-metallic impurities in the phosphoric acid solution before the first repetition of step b) and after the third repetition of step b). The results demonstrate that by adding extraction agent 1 successively in the three steps, copper is removed in the first repetition (RI), followed by cadmium in the second repetition (R2), and then arsenic in the third repetition (R3). This highlights the selectivity of the agent in the following order: Cu, Cd, then As. These results apply to a 29% P2O5 phosphoric acid solution.

[0105] For a 54% P2O5 phosphoric acid solution, it is observed that the affinity of the extraction agent 1 for each of the species to be extracted can be modified. Thus, arsenic and copper are both removed after the repeated RI, and cadmium is removed subsequently. Indeed, at high concentrations, phosphoric acid can alter chemical equilibria and cause different elimination patterns.

[0106] Table 2 Concentrations of metallic and non-metallic impurities before and after each repetition of step b) in a 29% p2O5 phosphoric acid solution

[0107] [Tables2] Elements Contents in acid after step a) Contents in treated acid Solvent used: kerosene Contents in treated acid Solvent used: DiPE after RI after R2 after R3 after R1 after R2 after R3 P2O5(%) 25.99 25.82 25.43 25.37 25.32 24.93 24.83 Cu (ppm) 30.8 «0 «0 «0 «0 «0 «0 Cd (ppm) 17.8 15.7 1.84 0 4.5 0.8 0.5 As (ppm) 16.0 14.4 6.7 1.4 8.3 4.2 0.8 Fe (%) 0.21 0.23 0.24 0.23 0.21 0.20 0.21 Al (%) 0.11 0.19 0.20 0.19 0.19 0.19 0.18 Cr (ppm) 120.0 111.8 113.4 110.3 109.8 110.3 112.3 Ni (ppm) 18.9 17.7 17.3 15.2 18.3 18.7 16.8 V (ppm) 98.2 94.4 95.5 92.7 95.4 94.8 94.3 Zn (ppm) 234.6 191.01 206.1 187.5 220.4 214.5 218.4

[0108] Table 3 Concentrations of metallic and non-metallic impurities before and after each repetition of step b) in a 54% p2O5 phosphoric acid solution

[0109] [Tables3] Elements Contents in acid after step a) Contents in treated acid Solvent used: kerosene Contents in treated acid Solvent used: DiPE After R1 After R2 After R3 After RI After R2 After R3 P2O5(%) 50.69 50.33 49.04 48.88 47.46 44.35 42.37 Cu (ppm) 64.3 2.4 0 0 0 0 0 Cd (ppm) 19.6 8.9 0 0 4.3 0.04 0 As (ppm) 17.0 2.8 2.6 1.9 0.2 0 0 Fe (%) 0.46 0.46 0.46 0.45 0.49 0.5 0.49 Al (%) 0.38 0.38 0.38 0.37 0.41 0.42 0.42 Cr (ppm) 274.6 255.4 257.6 249.1 270.7 286.9 289.7 Ni (ppm) 51.2 47.4 47.3 44.1 51.0 55.4 53.4 V (ppm) 214.3 200.3 200.9 193.9 215.03 235.7 229.5 Zn (ppm) 391.7 365.0 365.3 337.5 384.2 416.1 406.4

[0110] Example 3 (first variant of the invention): removal of Cu2+, Cd2+ and As, using 0.1% of the extraction agent I in a mixture of tributyl phosphate (TBP) and diisopropyl ether (DiPE).

[0111] In this example, following a pretreatment (step a)), the disarsening (step b)) a 29% phosphoric acid solution (P2O5) was prepared according to the first A variant of the invention uses 0.1% (1 g) by weight of the extraction agent I relative to the total weight of the phosphoric acid solution (1 kg). The resulting solution then undergoes liquid-liquid extraction by adding 1 kg of a mixture of organic solvents, consisting of a combination of 10% by weight of (TBP) and 90% by volume of Diisopropyl Ether (DiPE). The mixture is stirred for 20 min, then decanted, and the organic and aqueous phases are separated.

[0112] In parallel, a 29% P2O5 (1kg) phosphoric acid solution underwent liquid-liquid extraction under the same conditions but without the addition of extraction agent 1, in order to evaluate the impact of this agent on the removal of targeted impurities (Cu, Cd, and As).

[0113] Table 4 shows the concentrations of metallic and non-metallic impurities in the phosphoric acid solution before and after liquid-liquid extraction in the presence or absence of extraction agent 1 in a mixture of 10% TBP and 90% DiPE. Under these conditions, the results confirm that the extraction agent according to the invention strongly promotes the removal of copper, cadmium, and arsenic.

[0114] Table 4: Concentrations of metallic and non-metallic impurities before and after treatment in the presence (=according to step b) or in the absence of agent 1 at 0.1% by weight in a 29% P2O5 phosphoric acid solution

[0115] [Tables4] Elements Contents in acid after step a) Contents in acid treated according to step b) Contents in acid extracted without addition of agent 1 P2O5 (%) 25.99 24.55 24.14 Cu (ppm) 30.8 27.0 Cd (ppm) 17.8 15.3 As (ppm) 16.0 11.3 Fe (%) 0.21 0.22 0.20 Al (%) 0.23 0.25 0.22 Cr (ppm) 120.0 112.1 96.6 Ni (ppm) 18.9 18.6 19.1 V (ppm) 98.2 95.1 82.0 Zn (ppm) 234.6 173.5 169.6

[0116] Example 4 (first variant of the invention): removal of Cu2+, Cd2+ and As using 0.1% of an extraction agent 1 and by precipitation, without the use of organic solvents

[0117] In this example, following a pretreatment (step a)), the dearsening (step b)) of a 29% or 54% P2O5 phosphoric acid solution was carried out according to the first embodiment of the invention using 0.1% (1 g) by weight of the extraction agent 1 relative to the total weight of the phosphoric acid solution (1 kg). The complex formed between the agent 1 and the species to be extracted is precipitated and filtered through calcium sulfate or activated carbon. Step b) is repeated three times. In other words, after filtration and recovery of the aqueous phase, it is treated again by adding 0.1% of the extraction agent 1, then precipitated and filtered. This operation is repeated a third time.

[0118] Table 5 shows the concentrations of metallic and non-metallic impurities in the 29% or 54% P2O5 phosphoric acid solution before and after this three-step treatment, without the use of organic solvents.

[0119] The results demonstrate the ability of extraction agent 1 to remove As by precipitation, followed by simple filtration, without requiring the use of an organic solvent. As removal is favored after the removal of Cu and Cd. Under these conditions, As removal from the 54% P2O5 phosphoric acid solution was carried out in replicate 2 (R2), while in the case of the 29% P2O5 phosphoric acid solution, it was carried out in replicate 3 (R3).

[0120] Table 5: Concentrations of metallic and non-metallic impurities obtained in phosphoric acid after treatment in the presence of extraction agent 1 without solvent

[0121] [Tables5] Elements Contents in acid at 29% after step ea) Contents in treated acid Contents in acid at 54% after step ea) Contents in treated acid After R1 After R2 After R3 After RI After R2 P2O5(%) 25.99 23.94 23.65 22.30 52.69 53.06 52.21 Cu (ppm) 30.8 0 0 0 64.3 0 0 Cd (ppm) 17.8 0 0 0 19.6 2.1 1.5 As (ppm) 16.0 8.9 0 0 17.0 0.7 0.2 Fe (%) 0.21 0.23 0.22 0.22 0.46 0.44 0.43 Al (%) 0.11 0.18 0.18 0.18 0.38 0.28 0.28 Cr (ppm) 120.0 108.6 104.4 104.3 274.6 273.6 274.9 Ni (ppm) 18.9 18.2 17.4 15.2 51.2 44.5 39.37 V (ppm) 98.2 91.9 88.4 88.3 214.3 230.27 227.21 Zn (ppm) 234.6 198.4 184.0 185.9 391.7 354.39 295.02

[0122] Example 5 (second variant of the invention): elimination of arsenic during step b')

[0123] In this example, following a pretreatment (step a)) and a membrane nanofiltration purification step (step c')), the dearsenation (step b')) of a 29% P2O5 phosphoric acid solution was carried out according to the second embodiment of the invention using 0.1% (1 g) by weight of the extraction agent 1 relative to the total weight of the phosphoric acid solution (1 kg). The resulting solution then undergoes liquid-liquid extraction by adding 1 kg of organic solvent, either kerosene or diisopropyl ether (DiPE), and stirring the mixture for 20 min. The mixture is then decanted, and the organic and aqueous phases are separated.

[0124] Table 6 shows the concentrations of metallic and non-metallic impurities in the phosphoric acid solution before and after step b'). The results indicate that the extraction agent 1 completely removes arsenic from the DIPE solvent and almost completely from the kerosene. For both solvents, the quality of the phosphoric acid solution meets the requirement of European standards, which is less than 3 ppm arsenic.

[0125] Table 6: Concentrations of metallic and non-metallic impurities in 29% P2O5 phosphoric acid before and after step b').

[0126] [Tableauxô] Elements Content in acid after step c') Content in acid after step b') Solvent = kerosene Content in acid after step b') Solvent = DiPE P2O5 (%) 25.96 25.67 26.03 Cu (ppm) 3.05 0 0 Cd (ppm) 1.72 0 0 As (ppm) 15.26 1.4 0 Fe (%) 0.01 0.01 0.01 Al (%) 0.01 0.01 0.01 Cr (ppm) 0.4 0.4 0.4 Ni (ppm) 0.7 0.2 0.4 v (ppm) 3,4 3,4 3,2 Zn (ppm) 16,8 16,8 15,8

Claims

Demands

1. A process for purifying a phosphoric acid solution comprising a step of dearsening a phosphoric acid solution using an extraction agent of formula (I): S RtO—P—OR2 J, SZ in which Ri and R2 are independently of each other a linear or branched hydrocarbon group, saturated or unsaturated, comprising from 1 to 12 carbon atoms, or an aryl group, the hydrocarbon and aryl groups being optionally substituted, and Z+ is a counter ion, in particular selected from Na+, K+ and NH4+.

2. A method according to claim 1, characterized in that in formula (I), Ri and R2 are independently of each other an alkyl in CrCi2, in particular in Ci-C6, preferably chosen from a methyl, an ethyl and a propyl.

3. A process according to claim 1 or 2, comprising the following successive steps: a) pretreatment of a phosphoric acid solution, b) dearsening of the phosphoric acid solution from step a) using an extraction agent of formula (I), then c) purification of the phosphoric acid solution from step b) by liquid-liquid extraction or by membrane nanofiltration.

4. A method according to claim 3, wherein copper ions and cadmium ions are removed from the phosphoric acid solution using the extraction agent of formula (I) in step b).

5. A process according to claim 1 or 2, comprising the following successive steps: a) pretreatment of a phosphoric acid solution, c') purification of the phosphoric acid solution from step a) by liquid-liquid extraction or by membrane nanofiltration, then b') dearsening of the phosphoric acid solution from step c') using an extraction agent of formula (I).

6. A process according to any one of claims 1 to 5, wherein the phosphoric acid solution from step a) has the following contents: - a solids content (SC) of less than 1%, preferably less than 0.1%, in particular less than 0.07%, by weight relative to the total weight of the solution, - a sulfate content of less than 0.5%, in particular less than 0.39%, by weight relative to the total weight of the solution, - an organic carbon content of less than 1000 ppm, in particular less than 300 ppm, and / or - a fluorine content of less than 0.2%, in particular less than 0.1%, by weight relative to the total weight of the solution.

7. A method according to any one of claims 1 to 6, wherein step b) or b') includes the formation of a complex between the agent of formula (I) and the species to be extracted and the removal of this complex from the phosphoric acid solution, the species to be extracted being arsenic and, if present, cadmium and copper ions.

8. A method according to claim 7, wherein the removal of the complex is carried out by liquid-liquid extraction using an organic solvent immiscible with the phosphoric acid solution and in which the complex is soluble, or by precipitation and then filtration of the complex.

9. A method according to any one of claims 1 to 8, wherein step b) or b') is repeated several times, in particular 3 times.

Images