Process for removing heavy metals contained in phosphoric acid solutions

Abstract

This disclosure relates to a process for removing cadmium and copper, and optionally arsenic, from phosphoric acid solutions typically obtained by wet methods. The process can also allow for the selective and separate recovery of cadmium, copper, and arsenic.

Description

Title of the invention: Process for removing heavy metals contained in phosphoric acid solutions. FIELD OF THE INVENTION

[0001] The present invention relates to a process for removing cadmium and copper, and optionally arsenic, from phosphoric acid solutions typically obtained by wet methods. The process can further enable the selective and separate recovery of cadmium, copper, and arsenic. TECHNOLOGICAL BACKGROUND

[0002] Wet process phosphoric acid is industrially obtained as a solution by reacting natural phosphate with a strong acid such as hydrochloric acid, nitric acid, and / or sulfuric acid. Depending on the origin of the natural phosphate, the resulting phosphoric acid solution contains metallic and non-metallic impurities, including cadmium (Cd), copper (Cu), and arsenic (As), at varying concentrations.

[0003] In recent years, the requirements of phosphoric acid users have become increasingly stringent. Phosphoric acid is used in numerous manufacturing processes for food products and mineral fertilizers. A very high degree of purity must be rigorously maintained, especially with regard to certain elements such as cadmium, arsenic, and other heavy metals. Thus, impurities present in the phosphoric acid solution can be problematic depending on the intended use of the wet-process phosphoric acid. It is therefore known to treat the wet-process phosphoric acid solution to remove heavy metals (e.g., precipitation, crystallization, liquid-liquid extraction, and ion exchange).

[0004] Cadmium is a heavy metal with a density of approximately 8.6 g / cm³ and is relatively rare in the Earth's crust. The common oxidation state of this element in nature is the Cd²⁺ ion. Among the properties of cadmium, it is worth noting its very rapid volatility and high reactivity with oxygen, sulfur dioxide and trioxide, and hydrochloric acid to form compounds such as cadmium oxide (CdO), carbonates (CdCO₃), sulfides (CdS), and chlorides (CdCl₂).

[0005] Geologically, pure cadmium is a blue-white metal, but it does not exist as such in nature. It is found mainly associated with zinc, in the form of lead-zinc or lead-copper-zinc ores. The cadmium concentration is generally related to the zinc content of the ores.

[0006] Cadmium is a metal widely used in industry, with 84% of the cadmium produced being used directly in batteries. It can also be used as a corrosion inhibitor in the electroplating industry.

[0007] Various treatments are used to decadmize industrial phosphoric acid obtained by wet extraction: precipitation, crystallization, liquid-liquid extraction, membranes, and ion exchange. Liquid-liquid extraction is the most widely used on an industrial scale. However, this technique requires the use of organic solvents, which present risks related to their flammability and explosiveness. Several processes for treating phosphoric acid with resins have also been proposed. Nevertheless, the resins used are not selective in acidic media, which limits their use in concentrated phosphoric acid environments.

[0008] WO2020 / 247502 A1 describes a process for the removal and recovery of cadmium Phosphoric acid obtained via the wet process using commercial resins (Purolite SSC60TM, Purolite C100TM, and Dowex MSCTM) with a -SO32, H+ functional group attached to a macroporous polystyrene gel matrix. To maintain the resin's effectiveness and lifespan, pretreatment (removal of suspended and organic matter) of the 29% P2O5 phosphoric acid is necessary to prevent resin clogging. It has also been shown that this type of resin yields a superior quality acid, close to the desired level, i.e., a removal of approximately 99% by weight of cadmium compared to the initial state, using a treatment ratio (BV = bed volume) of 4:1. The saturated resin is washed with demineralized water to remove traces of the acid impregnated in the resin.The elution of cadmium and retained impurities is carried out with a dilute sulfuric acid solution. WO2020 / 247502 Al shows that the sulfonic resins used are generally selective for divalent metals such as cadmium and magnesium. WO2020 / 247502 Al also shows that no affinity can exist between multivalent ions (e.g., Fe and Al) and the resin.

[0009] The process proposed in WO2020 / 247502 Al makes it possible to produce high-purity cadmium (purity of 80% to 99.9%). However, no information has been given concerning the stability of the resins in phosphoric acid, the elution time and volume.

[0010] EP0463674A2 and EP 0536466B1 propose a process for the removal of cadmium contained in concentrated phosphoric acid (28-61% P2O5). A pretreatment step upstream of the resin is carried out by adding halide ions X (L / Br) to form CdX+ ions. In EP0463674A2, the S861 resin used is supplied by Rhom and Haas. It has a very high affinity for CdX+ ions and is characterized by its stability in phosphoric acid. The pretreated acid solution obtained was passed through the resin column containing 250 ml of S861 resin at a flow rate of 1250 ml / min, resulting in a resin-to-acid treatment ratio varying from 25 to 35 BV depending on the volume of acid fed. Cadmium was eluted by passing distilled water through the column, yielding a solution containing cadmium at a concentration of 0.55 g / L to 1.13 g / L.

[0011] EP 0536466B1 proposes the removal of Cd and bromine from concentrated phosphoric acid by the use of ion-exchange resins. Two types of resins are used for this purpose: the first is a cationic resin (Chelex20) and the second is an anionic resin (AMBERLITE® IRA-93). This process consists of the following steps: - Passage of the crude acid through the resin:

[0012] The process was carried out in a cascade of mixed resin (30% Chelex 20 + 70% IRA-93) and anionic resin. This process allows for the removal of 90% of the Cd content present in the concentrated phosphoric acid 42% P2O5 - Elution:

[0013] After treatment with phosphoric acid, the mixed resin was eluted by simple treatment with distilled water. The anionic resin was eluted by treatment with 10% NH3. - Precipitation:

[0014] Cadmium precipitation was achieved by adding sodium sulfide (Na2S) to form cadmium sulfide (CdS).

[0015] Copper is considered one of the first metals used by humankind. Copper is characterized by well-known properties such as exceptional thermal and electrical conductivity, high corrosion resistance, etc. Due to its physicochemical properties, copper is widely used as a conductor, as a copper-cellulose electrolyte for lithium batteries, and in high-density batteries.

[0016] Few treatments for removing copper from industrial phosphoric acid obtained by wet methods using resin-based processes are known.

[0017] US4452768 proposes a process for the removal of heavy metals in particular the Cu and Cd. A concentrated phosphoric acid solution (3 to 80% P2O5) is passed through a carbon column impregnated with diorganyldithiophosphoric acid at a temperature between 10 and 100°C. Cadmium and copper are removed at a rate of 98%. Regeneration is then carried out by passing a concentrated hydrochloric acid solution through the column.

[0018] This process has several advantages, such as the fact that the purified phosphoric acid does not undergo preliminary treatment and it uses few chemical reagents. On the other hand, its removal kinetics are very slow, its selectivity is low between copper and cadmium, and it requires a high concentration of regenerating agent.

[0019] In phosphoric acid, arsenic is found in a predominant form with two valences: arsenite (III) which transforms into arsenate (V) in the long term.

[0020] The removal of arsenic in the form of arsenic sulfide (AsS) or arsenic pentasulfide (As₂S₅) from 54% concentrated phosphoric acid is carried out either by the addition of a sodium hydrogen oxide (NaHS) solution or by direct injection of hydrogen sulfide (H₂S). The precipitate is separated by decantation (Jorf Lasfar-OCP). To date, no studies have been conducted on the removal of arsenic from phosphoric acid using ion-exchange resins.

[0021] Current heavy metal removal techniques based on resin filtration, such as cation exchange and anion exchange resins, can be used as a decadmium, copper and arsenic removal treatment for phosphoric acid but do not currently allow selectivity between cadmium, copper and arsenic.

[0022] Thus, a need remains for a new process for removing heavy metals from phosphoric acid, typically obtained by wet means, in particular for removing cadmium and copper, and optionally arsenic, from phosphoric acid, typically obtained by wet means, that can be implemented on an industrial scale. The proposed process will advantageously produce phosphoric acid compatible with the manufacture of MAP (monoammonium phosphate) and DAP (diammonium phosphate) type phosphate fertilizers. Advantageously, the proposed process will allow the separately removed metals, such as cadmium and copper, to be recovered to produce pure products usable for common commercial applications. BRIEF DESCRIPTION OF THE INVENTION

[0023] The present invention relates to a process for removing cadmium and copper, and optionally arsenic, contained in a phosphoric acid solution containing cadmium, copper and arsenic impurities, comprising the following steps:

[0024] (a) optionally pretreatment of the phosphoric acid solution to eliminate organic matter and suspended matter;

[0025] (b) optionally passing the phosphoric acid solution, possibly obtained at the end of step (a), on an ion exchange resin bearing sulfonic groups;

[0026] (c) passage of the phosphoric acid solution, possibly obtained at the end of step (a) and / or step (b), on a resin functionalized with thiourea groups.

[0027] Other aspects of the invention are as described below and in the claims. FIGURES

[0028] [Fig.l]: Evolution of the retention of metallic and non-metallic impurities by the MTS9140 resin as a function of acidic BV([H3PO4]).

[0029] [Fig.2]: CdS (a) and CuS (b) difractogram.

[0030] [Fig.3]: Evolution of the retention of metallic and non-metallic impurities by MTS9140 Resin alone (a), SSTC60 Resin (b) and a cascade of SSTC60 Resin + MTS9140 Resin (c) depending on BV([H3PO4]).

[0031] [Fig.4]: Method for concentrating cadmium and copper.

[0032] [Fig. 5]: Diagram of a process for the removal and recovery of cadmium and copper contained in a phosphoric acid solution.

[0033] [Fig. 6]: Diagram of a process for the removal and recovery of cadmium, of copper and arsenic contained in a phosphoric acid solution. DETAILED DESCRIPTION OF THE INVENTION

[0034] The process proposed by the inventors meets the stated needs.

[0035] The process for removing cadmium and copper, and optionally arsenic, contained in a phosphoric acid solution containing cadmium, copper, and arsenic impurities, comprises the following steps:

[0036] (a) optionally pretreatment of the phosphoric acid solution to eliminate organic matter and suspended matter;

[0037] (b) optionally passing the phosphoric acid solution, possibly obtained at the end of step (a) when this step is implemented, on an ion exchange resin bearing sulfonic groups (SO3);

[0038] (c) passage of the phosphoric acid solution, possibly obtained at the end of step (a) and / or step (b) when this step is implemented, on a resin functionalized with thiourea groups.

[0039] In certain embodiments, the proposed process also allows for the selective and separate recovery of cadmium and copper, and optionally arsenic. It then comprises the following additional steps:

[0040] (d) selective elution, concentration and precipitation of cadmium and copper, and optionally elution and precipitation of arsenic.

[0041] Thus, depending on whether or not step b is implemented, the process of the present invention makes it possible to eliminate arsenic or not.

[0042] If the objective is to remove cadmium and copper, the phosphoric acid solution is passed over only one of the two resins: the resin functionalized with thiourea groups. By implementing step d), recovery of these two elements is possible.

[0043] If the objective is to remove cadmium, copper, and arsenic, the phosphoric acid solution is passed over two resins: the ion-exchange resin bearing sulfonic groups and then the resin functionalized with thiourea groups. By implementing step d), recovery of these three elements is possible.

[0044] The steps implemented, or that can be implemented, are as described below in detail.

[0045] Step (a): Pretreatment of the phosphoric acid solution

[0046] The pretreatment of the phosphoric acid solution (crude phosphoric acid solution) is carried out to remove organic matter and suspended solids from the phosphoric acid solution. In particular, organic matter and suspended solids can significantly hinder the binding of metal ions when the phosphoric acid solution passes over the resins. This step improves the performance and lifespan of the resins used in the proposed process.

[0047] The process according to the invention can be used for any phosphoric acid solution containing impurities such as cadmium, copper, and arsenic, regardless of its origin. The phosphoric acid solution is typically a wet-process phosphoric acid solution, that is, a phosphoric acid solution obtained by a process comprising the attack of natural phosphate (contained in sedimentary phosphate rock) by a strong acid such as hydrochloric acid, nitric acid, and / or sulfuric acid. The majority of the impurities contained in the natural phosphate (the phosphate rock) are found in the phosphoric acid formed.

[0048] The phosphoric acid solution typically comprises 10 to 45% by weight of P2O5, preferably 25 to 30% by weight of P2O5, for example 29% by weight of P2O5.

[0049] The pretreatment may be by precipitation, adsorption, coagulation / flocculation, filtration and / or ultrafiltration. It is generally carried out using activated carbon. Activated carbon treatment typically removes more than 98% by weight of the organic matter and suspended solids contained in the phosphoric acid solution.

[0050] Preferably, after pretreatment, the phosphoric acid solution has a solids content of less than 1% by weight, preferably less than 0.7% by weight, and an organic carbon content of less than 1000 ppm, preferably less than or equal to 100 ppm.

[0051] Step (b): passage over an ion exchange resin bearing sulfonic groups (SO 3 - )

[0052] The phosphoric acid solution (crude phosphoric acid solution) or the phosphoric acid solution obtained at the end of step (a) (solution obtained after pretreatment) when this step is implemented can be passed over an ion exchange resin bearing sulfonic groups.

[0053] The ion exchange resin bearing sulfonic groups typically has a mass exchange capacity of 3.8 eq / Kg of dry resin.

[0054] The ion exchange resin bearing sulfonic groups is typically in the form of a gel. The spherical particles typically have a diameter ranging from 300 to 1200 pm.

[0055] An ion exchange resin bearing sulfonic groups useful in the context of step (b) of the process of the present invention is the SSTC60 resin supplied by Purolite or a resin from the Lewatit® range of Lanxess.

[0056] The volume of phosphoric acid solution passing through the ion-exchange resin bearing sulfonic groups typically varies from 1 to 20 BV, preferably from 1 to 5 BV. The extraction yield of impurities such as cadmium and copper is highest up to 3 BV. However, it gradually decreases from 4 BV onwards.

[0057] “BV”, for “bed volume”, refers to the volume of the resin bed. Thus, 1BV of a phosphoric acid solution corresponds to a volume equivalent to the resin bed.

[0058] Each bed of phosphoric acid (acid BV) typically flows through the bed of resin bearing sulfonic groups (resin BV) for a duration of 12 min to 15 min or at an extraction rate of 5 BV to 4 BV (acid) / h (equivalent to 8.33 to 6.66 mL of acid / min), preferably at room temperature.

[0059] The passage through the ion exchange resin bearing sulfonic groups is typically carried out at room temperature (20-25°C) but can be carried out at temperatures up to 60°C. Thus, this step can be implemented at temperatures preferably ranging from 20°C to 60°C.

[0060] The phosphoric acid solution collected at the end of step (b) is partially demetallized (removal of copper, cadmium, nickel, calcium, magnesium, ...).

[0061] Step (c): Passing over a resin functionalized with thiourea groups

[0062] The phosphoric acid solution (crude phosphoric acid solution) or the phosphoric acid solution obtained at the end of step (a) (solution obtained after pretreatment when this step is carried out) or the phosphoric acid solution obtained at the end of step (b) (solution eluted during the passage of the the solution obtained after treatment on the ion exchange resin and possibly pre-treatment when this step is implemented) is passed over a resin functionalized with thiourea groups.

[0063] The resin functionalized with thiourea groups typically exhibits a mass exchange capacity of IL / Kg of dry resin. The spherical particles typically have a diameter ranging from 300 to 1200 pm.

[0064] The resin functionalized with thiourea groups may comprise a styrene divinylbenzene-based polymer support onto which thiourea functional groups are adsorbed. A resin functionalized with thiourea groups useful in step (c) of the process of the present invention is the MTS9140 resin supplied by Purolite or a resin from the Lewatit® range by Lanxess.

[0065] The MTS9140 resin is stable at very acidic pH and has a lifespan of over 10 years. This resin also offers several other advantages: - it does not retain P2O5: retention is zero for treated acidic BVs; - it enables the implementation of the cadmium removal process and copper at low temperature, that is to say in a range of 25 to 40°C; - it does not require any specific conditioning treatment before use; - it does not generate any waste, with the exception of equipment cleaning solutions.

[0066] The volume of phosphoric acid solution passing through the resin functionalized by thiourea groups typically varies from 1 to 16 BV, preferably from 3 to 7 BV.

[0067] Each bed of phosphoric acid (BV acid) typically circulates through the bed of resin (BV resin) for a duration of 2 min to 5 min or at an extraction rate of 30 BV to 12 BV (acid) / h (equivalent to 50 to 20 mL of acid / min), preferably at room temperature.

[0068] The passage through the resin functionalized with thiourea groups is typically carried out at room temperature (20-25°C) but can be carried out at temperatures up to 60°C. Thus, this step can preferably be carried out at temperatures ranging from 20°C to 60°C.

[0069] Embodiments without implementation of step (b)

[0070] When the process of the present invention does not include step (b), the crude or pretreated phosphoric acid solution obtained at the end of step (a) is circulated over a bed of resin functionalized with thiourea groups, such as MTS9140 resin, so as to fix cadmium and copper.

[0071] Each bed of phosphoric acid (acid BV) typically circulates through the resin bed (resin BV) for a duration of 2 to 5 minutes, preferably 5 minutes, or at an extraction rate of 30 to 12 BV per hour, preferably 12 BV (acid) / h (equivalent to 20 mL of acid / min), preferably at room temperature. The fixation of cadmium and copper is ensured by the thiourea functional groups.

[0072] The resin functionalized with thiourea groups then exhibits selectivity only for copper and cadmium compared to other impurities present in the phosphoric acid solution, such as iron, aluminum, calcium, magnesium, etc., despite their high concentrations. The resin's selectivity for copper and cadmium is reflected in a high acid / resin volumetric treatment ratio compared to conventional resins. The amount of acid treated relative to the resin volume is very high. The acid / resin volumetric treatment ratio is 15 BV acid / 1 BV resin to achieve a maximum removal of 93% by weight of cadmium and 95% by weight of copper.

[0073] A solution of decadmized and decopper-free phosphoric acid is collected.

[0074] Embodiments with implementation of step (b)

[0075] When the process of the present invention includes step (b), the phosphoric acid solution eluted during the passage over the ion exchange resin bearing sulfonic groups, i.e. a partially demetallized phosphoric acid solution (removal of copper, cadmium, nickel, calcium, magnesium, ...), is then circulated over a bed of resin functionalized by thiourea groups, such as the MTS9140 resin, so as to fix the arsenic.

[0076] The implementation conditions of step b) are as described above. Thus, each bed of phosphoric acid (acid BV) typically circulates through the bed of ion exchange resin bearing sulfonic groups (resin BV) for a duration of 12 min to 15 min, preferably 15 min, or at an extraction rate of 5 to 4 BV per hour, preferably 4 BV (acid) / h (equivalent to 8.33 to 6.66 mL of acid / min) and at room temperature.

[0077] Each bed of phosphoric acid (BV acid) typically flows through the bed of resin functionalized with thiourea groups (BV resin) for a duration of 2 min to 5 min, preferably 5 min, or at an extraction rate of 30 to 12 BV per hour, preferably 12 BV (acid) / h (equivalent to 50 to 20 mL of acid / min) and at room temperature.

[0078] A solution of de-arcinated phosphoric acid is collected. Interestingly, the integration of the ion-exchange resin upstream of the resin functionalized with thiourea groups makes the latter selective for arsenic.

[0079] At the end of steps (b) and (c), the phosphoric acid solution typically has a cadmium level of less than 2 ppm, a copper level of less than 2 ppm and an arsenic level of less than 2 ppm.

[0080] As indicated above, in certain embodiments, the proposed process also allows for the selective and separate recovery of cadmium and copper, and optionally arsenic. It then comprises additional steps of selective elution, concentration, and precipitation of cadmium and copper, and optionally arsenic. These steps pave the way for the subsequent valorization of the isolated elements.

[0081] Step (d): elution, concentration and precipitation of cadmium, copper and arsenic

[0082] The elution, concentration and precipitation of cadmium, copper and arsenic are carried out using conventional methods. The concentration of cadmium and copper in the different collected eluates can, for example, be achieved by one or more successive nanofiltrations.

[0083] Embodiments without implementation of step (b)

[0084] When the resin functionalized with thiourea groups is loaded / saturated with cadmium and copper, the resin is washed with water, typically distilled water. This washing removes traces of the phosphoric acid solution carried into the resin and elutes the cadmium. The eluate containing the cadmium is collected.

[0085] Alternatively, cadmium can be eluted using an acidic, basic or neutral eluent.

[0086] After the elution of cadmium, copper is eluted from the resin by washing the resin using a thiourea solution in a hydrochloric acid solution, typically a thiourea solution of concentration IM dissolved in a 2M hydrochloric acid solution.

[0087] This step allows for total copper recovery and complete resin regeneration. The eluate containing the copper is collected.

[0088] The eluates obtained, containing cadmium and copper respectively, are then concentrated. The concentration can be achieved by nanofiltration.

[0089] Figure 4 schematically illustrates an example of a method for concentrating cadmium and copper in their respective eluates. In the illustrated method, the cadmium and copper concentration step is carried out by a cascade of two batch nanofiltrations. The concentration of cadmium and copper in the respective eluates is achieved by a series of two two-stage 'closed-loop' nanofiltrations using an organic nanofiltration membrane. In the first stage, the eluate is filtered through a nanofiltration membrane (NF1). In the The second step involves filtering the permeate from the first step through an NF2 nanofiltration membrane. The NF2 nanofiltration membrane used in the second step can be identical to that of the first step or a different membrane with a similar chemical structure.

[0090] Among the suitable membranes, mention will be made, by way of non-limiting, of organic membranes in polyethersulfone, polyamide / polysulfone or others, positively charged in acidic medium, in particular marketed under the brands MP, NF, Desal or PES respectively by the companies Koch, Filmtec, Osmonics and Nadir.

[0091] A recirculation loop for the concentrates (retentates) from the nanofiltration steps Steps 1 and 2 are designed to adjust the cadmium or copper content. In the first nanofiltration step, the cadmium or copper concentration must remain higher than that of the eluate so that it can be recovered along with a complementary fraction of the concentrate from the second step. The second-step concentrate must have a cadmium or copper concentration close to or higher than that of the eluate before being sent upstream of step 1. The fraction recovered from steps 1 and 2 is the total fraction recovered, not the fraction from step 2 or step 1 alone. Three-quarters of the eluate flow to be treated will be filtered through the membranes (permeate), and the remaining quarter will be sent to the CdS or CuS precipitation step.

[0092] After concentration, cadmium and copper are precipitated.

[0093] Cadmium and copper can be precipitated by bubbling H₂S gas (produced by a reaction between NaHS and concentrated hydrochloric acid). The sulfides are highly reactive with cadmium and copper, forming stable precipitates: pure CdS, which is orange, and pure CuS, which is black. The observed precipitation kinetics are very rapid; the precipitation time generally does not exceed 5 minutes.

[0094] The CuS precipitate could alternatively be obtained by heat treatment of the thiourea elution solution (T>220°C).

[0095] The precipitates are collected by filtration, decantation or centrifugation. The solutions obtained by separation can be regenerated by the addition of H2S.

[0096] In the case of the copper-containing solution, the H2S obtained by the degradation of thiourea under the effect of temperature (heat treatment) reacts with copper ions to form a black precipitate. This black precipitate is pure copper sulfide CuS.

[0097] In the case of the solution containing cadmium, cadmium sulfide CdS was formed.

[0098] Thus, in certain embodiments, the process of the present invention makes it possible to eliminate cadmium and copper in a phosphoric acid solution, typically obtained by wet process, and to selectively and separately recover cadmium and copper. The process then comprises the following steps:

[0099] (a) optionally pretreatment of the phosphoric acid solution to eliminate organic matter and suspended matter;

[0100] (c) passage of the phosphoric acid solution, possibly obtained at the end of step (a) when this step is implemented, on a resin functionalized with thiourea groups;

[0101] (d) elution, concentration and precipitation of copper and cadmium, typically carried out in the following manner:

[0102] (dl) elution of cadmium by passing water, typically distilled, over the functionalized resin;

[0103] (d2) elution of copper by passing a thiourea solution through a solution hydrochloric acid on the functionalized resin, at the end of step (dl);

[0104] (d3) concentration of cadmium and copper respectively in the eluates obtained at the outcome of steps (d1) and (d2), typically by nanofiltration;

[0105] (d4) precipitation of cadmium and copper.

[0106] Figure 5 illustrates such a process. In the illustrated process, a crude 29% by weight phosphoric acid solution is pretreated with activated carbon (1). The resulting solution is passed over a chelating resin comprising a styrene divinylbenzene-based polymer support onto which thiourea functional groups are adsorbed to bind cadmium and copper (MTS resin) (2) to give a cadmium- and copper-free 29% phosphoric acid solution.

[0107] Cadmium is eluted by passing distilled water over the resin (3). The eluted solution (water + cadmium) is subjected to ultrafiltration (4).

[0108] Copper is then eluted by passing an acidic thiourea solution over the resin (3')- The eluted solution (water + Cadmium) is subjected to ultrafiltration (4).

[0109] Finally, the precipitation of cadmium and copper is carried out by bubbling H2S gas (5).

[0110] Embodiments with implementation of step (b)

[0111] The elution, concentration and precipitation steps of cadmium and copper are identical to those described previously.

[0112] When the resin functionalized with thiourea groups is saturated with arsenic, the resin is washed with water, typically distilled water, allowing the arsenic to be eluted.

[0113] The concentration of arsenic is carried out by thermal means or it can be directly collected by liquid / liquid extraction or precipitation.

[0114] Arsenic precipitation can be carried out using methods well known to those skilled in the art. In particular, precipitation can be carried out by the addition of H2S or Na2S.

[0115] Thus, in certain embodiments, the process of the present invention makes it possible to remove cadmium, copper, and arsenic from a phosphoric acid solution, typically obtained by wet means, and to selectively and separately recover the cadmium, copper, and arsenic. The process then comprises the following steps:

[0116] (a) optionally pretreatment of the phosphoric acid solution to remove organic matter and suspended matter;

[0117] (b) passage of the phosphoric acid solution, possibly obtained at the end of step (a) when step (a) is implemented, on an ion exchange resin bearing sulfonic groups;

[0118] (c) passing the phosphoric acid solution obtained at the end of step (b) through a resin functionalized by thiourea groups;

[0119] (d) elution, concentration and precipitation of copper, cadmium and arsenic, typically carried out in the following way:

[0120] (dl') elution of arsenic by passing water, typically distilled, over the resin functionalized by thiourea groups;

[0121] (d2') elution of copper and cadmium;

[0122] (d3') concentration of cadmium and copper respectively in the eluates obtained at the outcome of step (d2'), typically by nanofiltration;

[0123] (d4) precipitation of cadmium, copper and arsenic.

[0124] Figure 6 illustrates such a process. In the illustrated process, a crude 29 wt% phosphoric acid solution is pretreated. The resulting pretreated 29 wt% phosphoric acid solution is passed through an ion-exchange resin bearing sulfonic groups (SST resin). The partially demetallized collected phosphoric acid solution is then passed through a resin functionalized with thiourea groups (MTS resin). Elution, concentration, and precipitation of copper, cadmium, and arsenic are then carried out.

[0125] The phosphoric acid obtained by the process of the present invention meets European standards and can be used for various purposes such as the production of MAP (Mono Ammonium Phosphoric) and DAP (Dia Ammonium Phosphoric).

[0126] The following examples are given by way of illustration, but shall in no way be considered as limiting the present invention. EXAMPLES

[0127] Example 1: Valorization of cadmium and copper: MTS9140 resin alone

[0128] The recovery of cadmium and copper was carried out in several stages as follows: - Pretreatment of phosphoric acid (ACP) with activated carbon; - Fixation of cadmium and copper onto the MTS9140 chelating resin; - Recovery of cadmium by elution through the passage of demineralized water; - Copper recovery by elution through a thiourea solution in hydrochloric acid; - Concentration of cadmium and copper in the eluates by nanofiltration; - Precipitation of cadmium and copper.

[0129] Chemical analyses of phosphoric acid filtered through MTS9140 resin show that this resin strongly retains Cd and Cu.

[0130] Table 1 shows the concentrations of metallic and non-metallic impurities obtained in phosphoric acid treated with MTS9140 resin alone.

[0131] [Tables 1 Impurities Raw ACP ACP after resin treatment MTS9140 BV-3 BV-6 BV-9 BV-12 BV-15 P 2 05 26.68 26.68 26.68 26.68 26.68 26.68 As 12.92 12.34 12.92 12.92 12.92 12.92 Cd 22.94 0 0.46 0.76 1.12 1.52 Cu 32 0.40 0.55 0.74 1.16 1.55 Ni 37.94 37.11 37.94 37.94 37.94 37.94 V 137.04 128.29 131.60 136.59 137.04 137.04 A12O3 1800 1800 1800 1800 1800 1800 Fe 2 O 3 3100 3100 3100 3100 3100 3100 k2o 200 200 200 200 200 200 MgO 5100 5100 5100 5100 5100 5100 Na 2 O 700 700 700 700 700 700 CaO 400 400 400 400 500 400

[0132] Table 1: Concentration of major and minor metallic impurities and non-metallic impurities obtained with MTS9140 resin alone (BV from 1 to 15)

[0133] (T=ambient, P=atmospheric, contact time = 1 BV / 5 min, Percolation rate) acid = 12 BV / h (20 ml / min)

[0134] Figure 1 shows that the retention of Cd and Cu is significantly greater than that of other elements (e.g., Fe, Al, V, Mg, ...) for acidic BVs below 8. The MTS9140 resin retains Cd and Cu with acidic BVs greater than 12, up to 15 BV. This result shows that the MTS9140 resin has a strong affinity for Cd and Cu.

[0135] The chemical analyses of the cadmium and copper elution solution are presented respectively in Tables 2 and 3.

[0136] [Tables2] Elements Al Fe Cd Cu Mg Ni V As Content (p pm) 0 0 135 6 0 0 0 0

[0137] Table 2: Chemical analysis of deionized water after passing through the resin

[0138] [Tables3] Elements Al Fe Cd Cu Mg Ni V As Content (p pm) 0 0 0 46 0 0 0 0

[0139] Table 3: Chemical analysis of the acidic thiourea solution after passing the resin

[0140] The results of the chemical analysis by inductively coupled plasma (ICP) analytical technique show that the final products are pure (purity > 99%) (Tables 4 and 5).

[0141] [Tables4] Characteristic Value Test Method Cd Content (%) 77.71 Elemental Analysis after Drying S Content (%) 22.01 Elemental Analysis after Drying

[0142] Table 4: Chemical analysis of CdS according to the MTS9140 resin

[0143] [Tables5] Characteristic Value Test Method Cu Content (%) 45 Elemental Analysis after Drying S Content (%) 33 Elemental Analysis after Drying

[0144] Table 5: Chemical analysis of CuS according to the MTS9140 resin

[0145] The products obtained are in the form of powders. The CdS powder is orange and the CuS powder is black.

[0146] The results of the physical analysis by X-ray diffraction (XRD) confirm that the final products are pure ([Fig.2]).

[0147] Example 2: Valorization of cadmium and copper: STTC60 resin alone

[0148] In this other example, a cadmium and copper recovery process similar to that of example 1 is implemented, but using STTC60 resin.

[0149] The results are presented in Table 6.

[0150] [Tableauxô] Impurities ACP Pretreated ACP after resin passage SSTC60 BV-1 BV-2 BV-3 P2O2 s% 24-27 24.66 25.67 25.16 As ppm 11 9.5 10.04 10.62 Cd ppm 15.16 0 0.61 4.17 Cu ppm 28.43 0 0.46 2.77 Ni 26.33 0.8 0.96 3.10 V 112 92 93 99 Fe2O3 1459.50 1459.10 1500.54 1544.54 Al2O3 1095.89 950 953.98 1012.65 K2O 300 50 75 183 MgO 4485 1415 1817.5 2225 %TS 0.29 0.19 0.25 0.3 %P 2 OS 24-27 24.66 25.67 25.16 As 11 9.5 10.04 10.62

[0151] Table 6: Concentration of major and minor metallic impurities and non-metallic impurities obtained with SSTC60 resin alone (BV from 1 to 3)

[0152] The STTC60 resin does not provide sufficient performance to obtain decadmized, decopper-free, and dearcinized phosphoric acid that meets industrial requirements. The STTC60 resin partially retains Cd and Cu but does not retain As.

[0153] Example 3: Arsenic selectivity: cascade of SSTC60 Resin + MTS9140 Resin

[0154] In this example, a process similar to that of Example 1 is implemented, but with a two-stage cascade of filtration on an STTC60 resin and then an MTS9140 resin.

[0155] Table 7 shows the concentrations of metallic and non-metallic impurities obtained in the demetallized and de-arcinized phosphoric acid from the SSTC60 Resin + MTS9140 Resin cascade.

[0156] [Tables?] ACP Impurities Pretreated ACP 29% P2O5 Cascade SSTC60 (BV-3) + MTS9140 (BV- 6) p205% 26.68 26.68 As ppm 18 1.68 Cd 18 1.56 Cu 35 1 Fe2O3 1493 1645 A12O3 1500 1600 Na2O 325 287 k2o 310 228 MgO 4875 3371 CaO 451 262 Ni 30 10 V 131 143 %TS 0.1 - %F 0.84 -

[0157] Table 7: Concentration of major and minor metallic impurities as well as non-metallic impurities of the ACP 29 P2O5 obtained by cascade the SSTC60 resin + the MTS9140 resin.

[0158] (Step 1 SSTC60: T=ambient, P=atmospheric, contact time / 15min, acid BV of 1 to 3)

[0159] (Step 2 MTS9140: (T=ambient, P=atmospheric, contact time = 1 BV / 5 min, acid BV from 1 to 6)

[0160] Chemical analyses of phosphoric acid after SSTC60 resin show that this resin partially retains Cd and Cu and does not retain As.

[0161] Figure 3 shows that arsenic was strongly retained by a cascade of SSTC60 resin + MTS9140 resin. The combination of SSTC60 resin + MTS9140 resin provided selectivity for arsenic.

Claims

Demands

1. A process for removing cadmium and copper, and optionally arsenic, from a phosphoric acid solution containing cadmium, copper and arsenic impurities, comprising the following steps: (a) optionally pretreatment of the phosphoric acid solution to remove organic matter and suspended matter; (b) optionally passing the phosphoric acid solution, possibly obtained at the end of step (a), through an ion-exchange resin bearing sulfonic groups; (c) passing the phosphoric acid solution, possibly obtained at the end of step (a) and / or step (b), through a resin functionalized with thiourea groups; (d) selective elution, concentration and precipitation of cadmium and copper, and optionally arsenic.

2. A process according to claim 1 wherein the phosphoric acid solution containing cadmium, copper, and arsenic is a phosphoric acid solution obtained by a wet process.

3. A process according to any one of the preceding claims wherein the volume of phosphoric acid solution passing through the resin functionalized with thiourea groups varies from 1 to 16 BV, preferably from 3 to 7 BV.

4. A method according to any one of the preceding claims wherein the phosphoric acid solution passes through the resin functionalized with thiourea groups at a flow rate of 30 to 12 BV per hour.

5. A method according to any one of the preceding claims for removing cadmium, copper and arsenic and comprising step (b), step (c) and step (d).

6. A process according to any one of the preceding claims for removing cadmium, copper and arsenic and comprising steps (a), (b), (c) and (d).

7. A method according to claim 5 or 6 wherein the volume of phosphoric acid solution passing through the ion exchange resin varies from 1 to 20 BV, preferably from 1 to 5 BV.

8. A method according to any one of claims 5, 6 or 7 wherein the phosphoric acid solution passes through the ion exchange resin bearing sulfonic groups at a flow rate of 4 to 5 BV per hour.

9. A process according to any one of claims 5 to 8 wherein the selective elution, concentration and precipitation of cadmium, copper and arsenic comprise the following steps: (d1') elution of arsenic by passing water, typically distilled, over the resin functionalized with thiourea groups; (d2') elution of copper and cadmium; (d3') concentration of cadmium and copper respectively in the eluates obtained at the end of step (d2'), typically by nanofiltration; (d4) precipitation of cadmium, copper and arsenic.

10. A process according to any one of claims 1 to 4 wherein the selective elution, concentration and precipitation of cadmium and copper comprise the following steps: (d1) elution of cadmium by passing water, typically distilled, over the functionalized resin; (d2) elution of copper by passing a thiourea solution in a hydrochloric acid solution over the functionalized resin, after step (d1); (d3) concentration of cadmium and copper respectively in the eluates obtained after steps (d1) and (d2), typically by nanofiltration; (d4) precipitation of cadmium and copper.

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