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 from 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 the wet method. The process can furthermore allow the selective and separate recovery of cadmium, copper and arsenic. TECHNOLOGICAL BACKGROUND

[0002] “Wet process” phosphoric acid is obtained industrially in the form of a solution by attacking 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 phosphoric acid solution produced 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 many processes for manufacturing food products and mineral fertilizers. A very high degree of purity must be strictly observed, especially with regard to certain elements such as cadmium, arsenic and other heavy metals. Thus, the impurities present in the phosphoric acid solution can be problematic depending on the destination of the phosphoric acid prepared by the wet process. It is therefore known to carry out a treatment of 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 around 8.6 g / cm3 that is relatively rare in the Earth's crust. The frequent oxidation state of this element in nature is the Cd2+ ion. Among the properties of cadmium, it can be noted a very rapid volatility, a 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 mainly found 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, in which 84% of the Cadmium produced is used directly in batteries. It can also be used as a corrosion protectant in the electroplating industry.

[0007] Various treatments are used to decadmiumate industrial phosphoric acid obtained by the wet process: 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. However, the resins used are not selective in acidic media, which limits their use in concentrated phosphoric acid media.

[0008] WO2020 / 247502 A1 describes a method for removing and recovering cadmium wet-process phosphoric acid using commercial resins (Purolite SSC60TM, Purolite C100TM and Dowex MSCTM) having a functional group of type -SO32, H+ attached to a macroporous polystyrene gel matrix. To maintain the efficiency and lifetime of the resin, a pretreatment (removal of suspended and organic matter) of the 29% P2O5 phosphoric acid is necessary to avoid clogging of the resins. It has also been shown that this type of resin makes it possible to obtain a superior quality acid, close to the desired one, i.e. an elimination close to 99% by weight of cadmium compared to the initial state using a treatment ratio BV (acid) / BV (resin): 4 / 1 (BV = bed volume). 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 by 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 A1 makes it possible to produce high purity cadmium (purity of 80% to 99.9%). However, no information has been given regarding the stability of the resins in phosphoric acid, the time and the elution volume.

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

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

[0013] After passing the phosphoric acid, the elution of the mixed resin was carried out by a simple passage of distilled water. The elution of the anionic resin was carried out by passing 10% NH3. - Precipitation:

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

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

[0016] Few treatments for decoppering industrial phosphoric acid obtained by wet process using resins are known.

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

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

[0019] In phosphoric acid, arsenic is found in a predominantly two-valent form: 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) in 54% concentrated phosphoric acid is done either by the addition of a sodium hydrogen solution (NaHS) or by direct injection of hydrogen sulfide (H2S). The precipitate is separated by decantation (Jorf Lasfar-OCP). To date, no studies have been carried out on the removal of arsenic from phosphoric acid by ion exchange resins.

[0021] Current techniques for removing heavy metals based on resin filtration, such as cationic exchange and anionic exchange resins, can be used as a treatment for decadmiation, decopperation and dearseniation of phosphoric acid but do not currently allow selectivity to be achieved between cadmium, copper and arsenic.

[0022] Thus, a need remains for the provision of a new process for the removal of heavy metals from phosphoric acid, typically obtained by the wet process, in particular the removal of cadmium and copper and optionally arsenic from phosphoric acid, typically obtained by the wet process, which can be implemented on an industrial scale. The proposed process will advantageously make it possible to produce a phosphoric acid compatible with the manufacture of phosphate fertilizers of the MAP (Monoammonium phosphate) and DAP (Diammonium phosphate) type. Advantageously, the proposed process will make it possible to recover the separately removed metals such as cadmium and copper to produce pure products usable for usual commercial applications. BRIEF DESCRIPTION OF THE INVENTION

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

[0024] (a) optionally pretreating the phosphoric acid solution to remove 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 carrying 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 by 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 acid 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 SSTC60 resin + MTS9140 resin (c) as a function of BV([H3PO4]).

[0031] [Fig.4]: method of concentrating cadmium and copper.

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

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

[0034] The method proposed by the inventors meets the needs expressed.

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

[0036] (a) optionally pre-treating the phosphoric acid solution to remove 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 carried out on an ion exchange resin carrying 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 carried out on a resin functionalized by thiourea groups.

[0039] In certain embodiments, the proposed method further allows for 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 op tionally elution and precipitation of arsenic.

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

[0042] If the objective is to eliminate cadmium and copper, the phosphoric acid solution is passed over only one of the two resins: the resin functionalized by 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 acid solution phosphoric acid is passed through the two resins: the ion exchange resin carrying sulfonic groups and then the resin functionalized by thiourea groups. By implementing step d), a recovery of these three elements is possible.

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

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

[0046] Pretreatment of the phosphoric acid solution (raw phosphoric acid solution) is carried out to remove organic matter and suspended matter in the phosphoric acid solution. In particular, organic matter and suspended matter can significantly hinder the fixation of metal ions when the phosphoric acid solution passes over the resins. This step makes it possible to improve the performance and lifetime 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 of the cadmium, copper and arsenic type, regardless of its origin. The phosphoric acid solution is typically a wet-process phosphoric acid solution, i.e. a phosphoric acid solution obtained by a process comprising attacking the natural phosphate (contained in a sedimentary phosphate rock) with 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 from 10 to 45% by weight of P2 O5, preferably from 25 to 30% by weight of P2O5, for example 29% by weight of P2O5.

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

[0050] Preferably, at the end of the 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 through an ion exchange resin carrying sulfonic groups (fSOy)

[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 carried out can be passed over a resin ion exchanger carrying sulfonic groups.

[0053] The ion exchange resin carrying 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 particles in spherical form typically have a diameter ranging from 300 to 1200 μm.

[0055] An ion exchange resin carrying sulfonic groups useful in the context of step (b) of the process of the present invention is the SSTC60 resin supplied by the company Purolite or a resin from the Lewatit® range from the company 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 efficiency of impurities such as cadmium and copper is maximum up to 3 BV. On the other hand, it gradually decreases from 4 BV.

[0057] “BV”, for “bed volume” in English, designates 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 (BV acid) typically circulates through the bed of resin carrying sulfonic groups (BV resin) for a period of time ranging from 12 min to 15 min or at an extraction rate ranging from 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 over 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 carried out 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 (elimination of copper, cadmium, nickel, calcium, magnesium, etc.).

[0061] Step (c): passage over a resin functionalized by 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 solution obtained after treatment on the ion exchange resin and possibly pretreatment when this step is carried out) is passed over a resin functionalized by thiourea groups.

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

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

[0065] The MTS9140 resin is stable at very acidic pH and has a lifespan of more than 10 years. This resin also has several other advantages: - it does not retain P2O5: retention is zero for treated acid BVs; - it allows the implementation of the cadmium elimination process and low temperature copper, i.e. in the range of 25 to 40°C; - it does not require any specific conditioning treatment before use; - it does not generate any discharge, except for 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 phosphoric acid bed (BV acid) typically circulates through the resin bed (BV resin) for a period of time ranging from 2 min to 5 min or at an extraction rate ranging from 30 BV to 12 BV (acid) / h (equivalent to 50 to 20 mL of acid / min), preferably at room temperature.

[0068] The passage over the resin functionalized by 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 be carried out at temperatures preferably ranging from 20°C to 60°C.

[0069] Embodiments without implementing 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 on a bed of resin functionalized by thiourea groups, such as MTS9140 resin, so as to fix the cadmium and the copper.

[0071] Each phosphoric acid bed (BV acid) typically circulates through the resin bed (BV resin) for a period of time ranging from 2 min to 5 min, preferably 5 min, or at an extraction rate ranging from 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 by thiourea groups then shows a selectivity only for copper and cadmium compared to other impurities contained in the phosphoric acid solution despite their high contents such as iron, aluminum, calcium, magnesium, etc. The selectivity of the resin towards copper and cadmium results in a high acid / resin volumetric treatment ratio compared to conventional resins. The amount of acid treated in relation to the volume of the resin is very high. The acid / resin volumetric treatment ratio is equal to 15BV acid / 1BV resin to achieve a maximum removal of 93% by weight of cadmium and 95% by weight of copper.

[0073] A solution of decadmium and copper-free phosphoric acid is collected.

[0074] Embodiments with implementation of step (b)

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

[0076] The conditions for implementing step b) are as described above. Thus, each bed of phosphoric acid (BV acid) typically circulates through the bed of ion exchange resin bearing sulfonic groups (BV resin) for a period ranging from 12 min to 15 min, preferably 15 min, or at an extraction rate ranging from 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 circulates through the bed of resin functionalized by thiourea groups (BV resin) for a period ranging from 2 min to 5 min, preferably 5 min, or at an extraction rate ranging from 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 desarcinized phosphoric acid is collected. Interestingly, the integration of the ion exchange resin upstream of the resin functionalized by 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 method further allows for 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 open the way to subsequent recovery of the elements. isolated.

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

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

[0083] Embodiments without implementing step (b)

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

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

[0086] Following cadmium elution, copper is eluted from the resin by washing the resin using a thiourea solution in a hydrochloric acid solution, typically a 1M concentration thiourea solution dissolved in a 2M hydrochloric acid solution.

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

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

[0089] [Fig.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 nanofiltrations in batch mode. The concentration of cadmium and copper in the respective eluates is carried out by a series of two nanofiltrations with two 'closed loop' filtration stages by an organic nanofiltration membrane. In a first stage, the eluate is filtered through a nanofiltration membrane (NF1). In the second stage, it is the permeate of the first stage, which is filtered through a nanofiltration membrane NF2. The nanofiltration membrane NF2 used in the second stage may be identical to that of the first stage or it may be a different membrane but with a similar chemical structure.

[0090] Among the suitable membranes, mention will be made, without limitation, of organic membranes made of polyethersulfone, polyamide / polysulfone or others, positively charged in an acid 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 nanofiltration steps 1 and 2 is provided in order 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 as to be recovered with a complementary fraction of the concentrate from the 2nd step. The concentrate from the 2nd step must have a cadmium or copper concentration close to or higher than that of the eluate before sending it upstream of step 1. The fraction recovered from the 1st and 2nd steps is the total fraction recovered and not the fraction from the 2nd step or the 1st step 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] Precipitation of cadmium and copper can be achieved by bubbling H2S gas (from a reaction between NaHS and concentrated hydrochloric acid). Sulfides are very reactive with cadmium and copper forming stable precipitates: pure CdS with an orange color and pure CuS with a black color. The precipitation kinetics observed are very rapid, the precipitation time generally does not exceed 5 min.

[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 adding 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) is reacted with the copper ions to form a black precipitate. This black precipitate is pure copper sulfide CuS.

[0097] In the case of the solution containing cadmium, the formation of cadmium sulfide CdS occurred.

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

[0099] (a) optionally pretreatment of the phosphoric acid solution to remove 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 carried out on a resin functionalized with thiourea groups;

[0101] (d) elution, concentration and precipitation of copper and cadmium, typically carried out as follows:

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

[0103] (d2) elution of copper by passing a thiourea solution into 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 (dl) and (d2), typically by nanofiltration;

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

[0106] [Fig.5] is illustrative of such a method. In the illustrated method, a solution crude phosphoric acid at 29% by weight is subjected to pretreatment using activated carbon (1). The solution obtained is passed over a chelating resin comprising a polymer support based on styrene divinylbenzene on which thiourea functional groups are adsorbed so as to fix the cadmium and the copper (MTS resin) (2) to give a solution of cadmium-free and copper-free 29% phosphoric acid.

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

[0108] An elution of the copper is then carried out 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 steps of elution, concentration and precipitation of cadmium and copper are identical to those described previously.

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

[0113] Arsenic concentration is achieved thermally or it can be directly collected by liquid / liquid extraction or precipitation.

[0114] The precipitation of arsenic can be carried out according to methods well known to those skilled in the art. In particular, the precipitation can be carried out by adding H2S or Na2S.

[0115] Thus, in certain embodiments, the method of the present invention makes it possible to remove cadmium, copper and arsenic in a phosphoric acid solution, typically obtained by wet method, and to selectively and separately recover cadmium, copper and arsenic. The method 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 carried out on an ion exchange resin carrying sulfonic groups;

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

[0119] (d) elution, concentration and precipitation of copper, cadmium and arsenic, ty stitching carried out as follows:

[0120] (dl') elution of arsenic by passing water, typically distilled, over the resin nationalized 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] [Fig. 6] is illustrative of such a process. In the illustrated process, a crude 29% by weight phosphoric acid solution is subjected to pretreatment. The obtained pretreated 29% by weight phosphoric acid solution is passed over an ion exchange resin carrying sulfonic groups (SST resin). The partially demetallized collected phosphoric acid solution is then passed over 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 for illustrative purposes, but should in no way be considered as limiting the present invention. EXAMPLES

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

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

[0129] Chemical analyses of phosphoric acid filtered through the 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 ACP crude ACP after resin passage MTS9140 BV-3 BV-6 BV-9 BV-12 BV-15 P2O5 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 Fe2O3 3100 3100 3100 3100 3100 3100 k2o 200 200 200 200 200 200 MgO 5100 5100 5100 5100 5100 5100 Na2O 700 700 700 700 700 700 CaO 400 400 400 400 500 400

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

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

[0134] [Fig. 1] shows that the retentions of Cd and Cu are significantly higher than those of other elements (e.g. Fe, Al, V, Mg, etc.) for acid BVs lower than 8. The MTS9140 resin retains only Cd and Cu at acid BVs higher 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 in Tables 2 and 3 respectively.

[0136] [Tables2] Elements Al Fe Cd Cu Mg Ni V As Content (ppm) 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 (ppm) 0 0 0 46 0 0 0 0

[0139] Table 3: Chemical analysis of the acid thiourea solution after the passage of the resin

[0140] The results of 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 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 MTS9140 resin

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

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

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

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

[0149] The results are presented in Table 6.

[0150] [Tableauxô] ACP Impurities Pretreated ACP after resin passage SSTC60 BV-1 BV-2 BV-3 P2OS % 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 A12O3 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 %P2OS 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 as well as non-metallic impurities obtained by SSTC60 resin alone (BV of 1 to 3)

[0152] The STTC60 resin does not provide sufficient performance to have a decadmium, decopper and dearcinized phosphoric acid that meets industrial requirements. The STTC60 resin partially retains Cd and Cu and 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 filtration cascade on an STTC60 resin then an MTS9140 resin.

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

[0156] [Tables?] Impurities ACP Pretreated ACP 29% P2OS Cascade SSTC60 (BV-3) + MTS9140 (BV-6) p2o5% 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 ACP 29 P2O5 obtained by the cascade of SSTC60 resin + MTS9140 resin.

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

[0159] (Step 2 MTS9140: (T=ambient, P=atmospheric, contact time = 1BV / 5min, 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] [Fig.3] shows that arsenic was strongly retained by a cascade SSTC60 Resin + MTS9140 Resin. The combination SSTC60 Resin + MTS9140 Resin achieved arsenic selectivity.

Claims

Claims

1. A method 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 pre-treatment of the phosphoric acid solution to remove organic matter and suspended matter; (b) optionally passing the phosphoric acid solution, optionally obtained at the end of step (a), over an ion exchange resin carrying sulfonic groups; (c) passing the phosphoric acid solution, optionally obtained at the end of step (a) and / or step (b), over a resin functionalized by thiourea groups.

2. The method of claim 1 wherein the phosphoric acid solution containing cadmium, copper, and arsenic is a wet-process phosphoric acid solution.

3. Method according to one of the preceding claims, further allowing selective and separate recovery of cadmium and copper and optionally arsenic and then comprising the following additional steps: (d) selective elution, concentration and precipitation of cadmium and copper, and optionally arsenic.

4. Method according to one of the preceding claims in which the volume of phosphoric acid solution passing through the resin functionalized by thiourea groups varies from 1 to 16 BV, preferably from 3 to 7 BV.

5. Method according to one of the preceding claims in which the phosphoric acid solution passes through the resin functionalized by thiourea groups at a flow rate ranging from 30 to 12 BV per hour.

6. A method according to any preceding claim for removing cadmium, copper and arsenic and comprising step (b) and step (c).

7. A method according to any preceding claim for removing cadmium, copper and arsenic and comprising steps (a), (b) and (c).

8. A method according to claim 6 or 7 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.

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

10. Method according to one of claims 6 to 9 in which the selective elution, concentration and precipitation of cadmium, copper and arsenic comprise the following steps: (dl') elution of arsenic by passing water, typically distilled, over the resin functionalized by 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.

11. Method according to one of claims 3 to 5 in which the selective elution, concentration and precipitation of cadmium and copper comprise the following steps: (dl) 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, at the end of step (dl); (d3) concentration of cadmium and copper respectively in the eluates obtained at the end of steps (dl) and (d2), typically by nanofiltration; (d4) precipitation of cadmium and copper.

Images