Method for removing heavy metals contained in phosphoric acid solutions

Abstract

The present invention relates to a method for removing cadmium and copper and optionally arsenic contained in phosphoric acid solutions typically obtained in wet processes. The process also enables selective and separate recovery of cadmium, copper and arsenic.

Description

Technical Field

[0001] This invention relates to a method for removing cadmium and copper, and optionally arsenic, from a phosphoric acid solution typically obtained in a wet process. The method also allows for the selective and separate recovery of cadmium, copper, and arsenic. Background Technology

[0002] Wet-process phosphoric acid is industrially obtained in solution form by decomposing natural phosphates using strong acids (such as hydrochloric acid, nitric acid, and / or sulfuric acid). Depending on the source of the natural phosphate, the resulting phosphoric acid solution contains varying concentrations of metallic and non-metallic impurities, particularly cadmium (Cd), copper (Cu), and arsenic (As).

[0003] In recent years, the requirements for phosphoric acid users have become increasingly stringent. Phosphoric acid is used in many food and inorganic fertilizer manufacturing processes. Extremely high purity must be maintained, especially for elements such as cadmium, arsenic, and certain other heavy metals. Therefore, depending on the end use of phosphoric acid prepared by wet processes, impurities present in the phosphoric acid solution can cause problems. Consequently, it is known to treat wet process phosphoric acid solutions 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. 3 Furthermore, it is relatively rare in the Earth's crust. The oxidation state of this element in nature is often as the ionic form Cd. 2+ Cadmium's properties include very rapid volatility and high reactivity with oxygen, sulfur dioxide, sulfur trioxide, and hydrochloric acid, thus forming compounds such as cadmium oxide (CdO), cadmium carbonate (CdCO3), cadmium sulfide (CdS), and cadmium chloride (CdCl2).

[0005] From a geological perspective, pure cadmium is a bluish-white metal, but it does not exist in nature in this form. It primarily occurs as an accompaniment to zinc in lead-zinc or lead-copper-zinc ores. Cadmium concentration is generally related to the zinc content in the ore.

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

[0007] Various treatments are used to remove cadmium from industrial phosphoric acid using wet processes: precipitation, crystallization, liquid-liquid extraction, membrane extraction, 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 pose risks associated with flammability and explosiveness. Several methods for treating phosphoric acid using resins have also been proposed. However, the resins used lack selectivity in acidic media, limiting their use in concentrated phosphoric acid media.

[0008] WO2020 / 247502 A1 describes a method for removing and recovering cadmium from phosphoric acid obtained by a wet process using commercial resins (Purolite SSC60TM, Purolite C100TM, and Dowex MSCTM), said resin having -SO3 groups attached to a macroporous polystyrene gel matrix. 2-- H + Type of functional groups. To maintain resin effectiveness and service life, pretreatment with 29% phosphoric acid P2O5 is required (to remove suspended and organic matter) to avoid clogging the resin. It has also been shown that using a treatment ratio of BV(acid) / BV(resin): 4 / 1 (BV = bed volume), this type of resin can achieve excellent quality acid (close to the desired quality, i.e., removal of approximately 99% by weight of cadmium compared to the initial state). The saturated resin is washed with softened water to remove traces of impregnated acid. Cadmium and residual impurities are eluted with a dilute sulfuric acid solution. WO2020 / 247502 A1 indicates that the sulfonic acid resin used is generally selective for divalent metals (e.g., cadmium and magnesium). WO2020 / 247502 A1 also indicates that there may be no affinity between polyvalent ions (e.g., Fe and Al) and the resin.

[0009] The method proposed in WO2020 / 247502 A1 can produce high-purity cadmium (80% to 99.9% purity). However, no information is given regarding the stability of the resin in phosphoric acid, elution time, and elution volume.

[0010] EP 0463674A2 and EP 0536466B1 disclose a method for removing cadmium from concentrated phosphoric acid (28% to 61% P2O5). This is achieved by adding halide ions X. - (I) - / Br - To form CdX + Ions are used in the upstream pretreatment steps of the resin. In EP0463674A2, the S861 resin used is supplied by Rohm and Haas. It is effective against CdX. + The ions exhibit very high affinity and are characterized by their stability in phosphoric acid media. The obtained pretreated acid solution is passed through a resin column comprising 250 mL of S861 resin at a flow rate of 1250 mL / min, i.e., a resin-acid treatment ratio varying from 25 BV to 35 BV depending on the volume of acid supplied. Cadmium is eluted by bubbling distilled water to obtain a solution containing cadmium at concentrations ranging from 0.55 g / L to 1.13 g / L.

[0011] EP 0536466B1 proposes the removal of Cd and bromine from concentrated phosphoric acid using ion exchange resins. For this purpose, two types of resins are used: the first is a cation exchange resin (Chelex 20), and the second is an anion exchange resin (Amberlite® IRA-93). The method consists of the following steps: - Passing crude acid through the resin: The process is carried out in a cascade of mixed resins (30% Chelex 20 + 70% IRA-93) and anion exchange resins. This method is able to remove 90% of the Cd content present in 42% concentrated phosphoric acid P2O5.

[0012] - Washing: After passing phosphoric acid, the mixed resin is eluted by simply passing distilled water. The anion exchange resin is eluted by passing 10% NH3.

[0013] - Sedimentation: Cadmium sulfide (CdS) is formed by precipitating cadmium through the addition of sodium sulfide (Na2S).

[0014] Copper is considered one of the earliest metals used by humankind. Copper is characterized by well-known properties such as excellent thermal and electrical conductivity, and high corrosion resistance. Due to its physicochemical properties, copper is widely used as a conductor and in copper-cellulose electrolytes for lithium-ion batteries and high-density batteries.

[0015] It is known that there are few processes for removing copper from industrial phosphoric acid obtained through a wet process involving resins.

[0016] US4452768 discloses a method for removing heavy metals, particularly Cu and Cd. A concentrated phosphoric acid solution (3% to 80% P2O5) is passed through a carbon column impregnated with a diorganodithiophosphate at a temperature between 10°C and 100°C. 98% of cadmium and copper are removed. Regeneration is then achieved by passing a concentrated hydrochloric acid solution through the column.

[0017] This method has several advantages, such as the purified phosphoric acid requiring no pretreatment and the use of minimal chemical reagents. In contrast, its removal kinetics are very slow, its selectivity between copper and cadmium is low, and it requires high concentrations of regenerant.

[0018] In phosphoric acid, two main valence states of arsenic were found: arsenite (III), which in the long term will be converted to arsenate (V).

[0019] Arsenic was removed from 54% concentrated phosphoric acid as arsenic sulfide (AsS) or arsenic pentasulfide (As2S5) by adding sodium hydrosulfide (NaHS) solution or by direct injection of hydrogen sulfide (H2S). The precipitate was separated by decantation (Jorf Lasfar-OCP). To date, there have been no studies on the removal of arsenic from phosphoric acid using ion exchange resins.

[0020] Current heavy metal removal technologies based on resin filtration (such as cation exchange resins and anion exchange resins) can be used to remove cadmium, copper, and arsenic from phosphoric acid, but they currently lack selectivity for cadmium, copper, and arsenic.

[0021] Therefore, there remains a need for a novel method that can be implemented on an industrial scale for removing heavy metals, particularly cadmium and copper, and optionally arsenic, from phosphoric acid (typically obtained via a wet process). The proposed method would advantageously enable the production of phosphoric acid compatible with the manufacture of phosphate fertilizers of the monoammonium phosphate (MAP) and diammonium phosphate (DAP) types. Advantageously, the proposed method would be able to recover the removed metals (e.g., cadmium and copper) separately, thereby producing a pure product suitable for conventional commercial applications. Summary of the Invention

[0022] This invention relates to a method for removing cadmium and copper, and optionally arsenic, from a phosphoric acid solution containing cadmium, copper, and arsenic impurities, the method comprising the following steps: (a) Optionally, the phosphoric acid solution is pretreated to remove organic matter and suspended solids; (b) Optionally, pass the phosphoric acid solution that can be obtained at the end of step (a) through an ion exchange resin with sulfonic acid groups; (c) Make the phosphoric acid solution that can be obtained at the end of step (a) and / or step (b) by using a resin functionalized with thiourea groups.

[0023] Other aspects of the invention are described below and in the claims. Attached Figure Description

[0024] Figure 1 The retention rate of MTS9140 resin for metallic and non-metallic impurities varies with acid BV ([H3PO4]).

[0025] Figure 2 Diffraction patterns of CdS (a) and CuS (b).

[0026] Figure 3 The retention rates of metallic and non-metallic impurities for MTS9140 (a), SSTC60 (b), and the cascade of SSTC60+MTS9140 (c) vary with BV ([H3PO4]).

[0027] Figure 4 : A method for concentrating cadmium and copper.

[0028] Figure 5 : Schematic diagram of a method for removing and recovering cadmium and copper contained in a phosphoric acid solution.

[0029] Figure 6 : Schematic diagram of a method for removing and recovering cadmium, copper and arsenic contained in a phosphoric acid solution. Detailed Implementation

[0030] The method proposed by the inventor satisfies the expressed need.

[0031] A method for removing cadmium and copper, and optionally arsenic, from a phosphoric acid solution containing cadmium, copper, and arsenic impurities includes the following steps: (a) Optionally, the phosphoric acid solution is pretreated to remove organic matter and suspended solids; (b) Optionally, pass the phosphoric acid solution obtained at the end of step (a) (when this step is performed) through a solution containing sulfonic acid groups (SO3). - ion exchange resins; (c) Make the phosphoric acid solution that can be obtained at the end of step (a) and / or step (b) (when the step is performed) by using a resin functionalized with a thiourea group.

[0032] In some embodiments, the proposed method is also capable of selectively and separately recovering cadmium and copper, and optionally recovering arsenic. Therefore, the method includes the following additional steps: (d) Selectively elute, concentrate and precipitate cadmium and copper, and optionally elute and precipitate arsenic.

[0033] Therefore, depending on whether step b) is performed, the method of the present invention may or may not remove arsenic.

[0034] If the objective is to remove cadmium and copper, the phosphoric acid solution is passed through only one of two resins: a resin functionalized with thiourea groups. Both elements can be recovered by implementing step d).

[0035] If the objective is to remove cadmium, copper, and arsenic, the phosphoric acid solution is passed through two resins: an ion exchange resin with sulfonic acid groups followed by a resin functionalized with thiourea groups. These three elements can be recovered by implementing step d).

[0036] The steps that have been implemented or can be implemented are detailed below.

[0037] Step (a): Pretreatment of phosphoric acid solution The phosphoric acid solution (crude phosphoric acid solution) is pretreated to remove organic matter and suspended matter. Specifically, when the phosphoric acid solution passes through the resin, the organic matter and suspended matter significantly hinder the binding of metal ions. This step improves the performance and lifespan of the resin used in the proposed method.

[0038] The method according to the invention can be used for any phosphoric acid solution containing impurities of the cadmium, copper, and arsenic types, regardless of its source. The phosphoric acid solution is typically obtained by a wet process, i.e., by a method involving the decomposition of natural phosphates (contained in sedimentary phosphate rocks) with strong acids (e.g., hydrochloric acid, nitric acid, and / or sulfuric acid). Most of the impurities present in the natural phosphate (phosphate rock) are found in the resulting phosphoric acid.

[0039] Phosphoric acid solutions typically contain 10% to 45% by weight of P2O5, preferably 25% to 30% by weight of P2O5, for example 29% by weight of P2O5.

[0040] Pretreatment can be precipitation, adsorption, coagulation / flocculation, filtration, and / or ultrafiltration. Activated carbon pretreatment is commonly used. Activated carbon treatment typically removes more than 98% by weight of organic matter and suspended matter from phosphoric acid solutions.

[0041] Preferably, at the end of the pretreatment, the phosphoric acid solution has a solid 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.

[0042] Step (b): via sulfonic acid groups (SO3) - ion exchange resin When this step is performed, the phosphoric acid solution (crude phosphoric acid solution) or the phosphoric acid solution obtained from step (a) (the solution obtained after pretreatment) can be passed through an ion exchange resin with sulfonic acid groups.

[0043] The exchange capacity of ion exchange resins with sulfonic acid groups is typically 3.8 equivalents / kg dry resin.

[0044] Ion exchange resins with sulfonic acid groups are typically in gel form. The diameter of the spherical particles typically ranges from 300 µm to 1200 µm.

[0045] In step (b) of the method of the present invention, the ion exchange resin with sulfonic acid groups that can be used is SSTC60 supplied by Purolite or the Lewatit® series of resins obtained from Lanxess.

[0046] The volume of phosphoric acid solution passing through an ion exchange resin with sulfonic acid groups typically ranges from 1 BV to 20 BV, preferably from 1 BV to 5 BV. Extraction efficiencies for impurities such as cadmium and copper are up to a maximum of 3 BV. In contrast, it gradually decreases from 4 BV.

[0047] “BV” or “bed volume” refers to the volume of the resin bed. Therefore, 1 BV of phosphoric acid solution corresponds to an equivalent volume of resin bed.

[0048] Each phosphate bed (acid BV) typically flows through a bed of resin with sulfonic acid groups (resin BV) for a time ranging from 12 to 15 minutes, or has an extraction flow rate ranging from 5 BV to 4 BV (acid) / hour (equivalent to 8.33 to 6.66 mL acid / min), preferably at room temperature.

[0049] The process involving ion exchange resins with sulfonic acid groups is typically carried out at room temperature (20°C to 25°C), but can also be performed at temperatures up to 60°C. Therefore, this step can be carried out at temperatures preferably in the range of 20°C to 60°C.

[0050] The phosphoric acid solution collected at the end of step (b) is partially demetallized (removing copper, cadmium, nickel, calcium, magnesium, etc.).

[0051] Step (c): Using a resin functionalized with thiourea groups The phosphoric acid solution (crude phosphoric acid solution) or the phosphoric acid solution obtained after step (a) (the solution obtained after pretreatment when this step is performed) or the phosphoric acid solution obtained after step (b) (the solution eluted during the passage of the solution obtained after treatment on the ion exchange resin and possible pretreatment when this step is performed) is obtained by using a resin functionalized with thiourea groups.

[0052] The exchange capacity of resins functionalized with thiourea groups is typically 1 liter per kilogram of dry resin. The diameter of the spherical particles typically ranges from 300 µm to 1200 µm.

[0053] Resins functionalized with thiourea groups may include a styrene-based divinylbenzene polymer support to which the thiourea functional groups are adsorbed. In step (c) of the method of the present invention, the thiourea-functionalized resins available are MTS9140 from Purolite or the Lewatit® series from Lanxess.

[0054] MTS9140 is stable at very acidic pH levels and has a service life of over 10 years. This resin also offers several other advantages: - It does not retain P2O5: the retention rate of BV in the treated acid is zero; - It allows for the implementation of cadmium and copper removal methods at low temperatures (i.e., in the range of 25°C to 40°C); - It requires no special adjustment or treatment before use; - It produces no emissions other than the equipment cleaning solution.

[0055] The volume of phosphoric acid solution using a resin functionalized with thiourea groups typically ranges from 1 BV to 16 BV, preferably from 3 BV to 7 BV.

[0056] Each phosphate bed (acid BV) typically flows through the resin bed (resin BV) for a time ranging from 2 to 5 minutes, or has an extraction rate ranging from 30 BV to 12 BV (acid) / hour (equivalent to 50 to 20 mL acid / min), preferably at room temperature.

[0057] The use of resins functionalized with thiourea groups is typically carried out at room temperature (20°C to 25°C), but can also be carried out at temperatures up to 60°C. Therefore, this step can be performed at temperatures preferably in the range of 20°C to 60°C.

[0058] Implementation plan that does not implement step (b) When the method of the present invention does not include step (b), the crude phosphoric acid solution or the pretreated phosphoric acid solution obtained at the end of step (a) is circulated in a bed of a thiourea group-functionalized resin (e.g., MTS9140) to fix cadmium and copper.

[0059] Each phosphate bed (acid BV) typically flows through the resin bed (resin BV) for a time ranging from 2 to 5 minutes (preferably 5 minutes), or has an extraction rate ranging from 30 BV to 12 BV, preferably 12 BV (acid) / hour (equivalent to 20 mL acid / min), preferably at room temperature. Cadmium and copper are immobilized via thiourea functional groups.

[0060] Compared to other impurities (such as iron, aluminum, calcium, magnesium, etc.) contained in the phosphoric acid solution, despite their higher concentrations, the resin functionalized with thiourea groups exhibits selectivity only for copper and cadmium. This selectivity for copper and cadmium results in a higher volumetric acid / resin treatment ratio compared to conventional resins. The amount of acid treated relative to the resin volume is very high. The volumetric acid / resin treatment ratio is equal to 15 BV acid / 1 BV resin, achieving a maximum removal of 93 wt% cadmium and 95 wt% copper.

[0061] Collect a phosphoric acid solution containing removed cadmium and copper.

[0062] Implementation plan for step (b) When the method of the present invention includes step (b), then the phosphoric acid solution (i.e., the partially demetallized phosphoric acid solution (removing copper, cadmium, nickel, calcium, magnesium, etc.) eluted during passage through an ion exchange resin with sulfonic acid groups is circulated in a bed of resin functionalized with thiourea groups (e.g., MTS9140) to fix arsenic.

[0063] The implementation conditions for step b) are as described above. Therefore, the time for each phosphate bed (acid BV) to flow through the bed of ion exchange resin with sulfonic acid groups (resin BV) typically ranges from 12 min to 15 min (preferably 15 min), or has an extraction rate ranging from 5 BV / h to 4 BV / h, preferably 4 BV (acid) / h (equivalent to 8.33 mL acid / min to 6.66 mL acid / min) and room temperature.

[0064] Each phosphate bed (acid BV) typically flows through a thiourea-functionalized resin bed (resin BV) for a time ranging from 2 to 5 minutes, preferably 5 minutes, or has an extraction rate ranging from 30 BV / h to 12 BV / h, preferably 12 BV (acid) / h (equivalent to 50 mL acid / min to 20 mL acid / min) and room temperature.

[0065] A phosphoric acid solution containing removed arsenic was collected. Interestingly, the integration of an ion exchange resin upstream of a thiourea-functionalized resin resulted in arsenic selectivity for the thiourea-functionalized resin.

[0066] At the end of steps (b) and (c), the phosphoric acid solution typically has a cadmium content of less than 2 ppm, a copper content of less than 2 ppm, and an arsenic content of less than 2 ppm.

[0067] As described above, in some embodiments, the proposed method is also capable of selectively and separately recovering cadmium and copper, and optionally recovering arsenic. It then includes additional steps of eluting, concentrating, and selectively precipitating cadmium and copper, and optionally eluting, concentrating, and selectively precipitating arsenic. These steps lay the foundation for subsequent recovery of the separated elements.

[0068] Step (d): Elution, concentration, and precipitation of cadmium, copper, and arsenic. Conventional methods are used to elute, concentrate, and precipitate cadmium, copper, and arsenic. For example, cadmium and copper in the collected eluents can be concentrated through one or more consecutive nanofiltration processes.

[0069] Implementation plan that does not implement step (b) When resins functionalized with thiourea groups are filled / saturated with cadmium and copper, the resin is washed with water (usually distilled water). This washing removes traces of phosphoric acid solution entrained in the resin and elutes the cadmium. The eluent containing cadmium is collected.

[0070] For alternative sites, acidic, alkaline, or neutral eluents can be used to elute cadmium.

[0071] At the end of cadmium elution, copper is eluted from the resin by washing it with a thiourea solution in hydrochloric acid (typically a 1 M concentration thiourea solution dissolved in 2 M hydrochloric acid).

[0072] This step allows for complete copper recovery and full resin regeneration. The eluent containing copper is collected.

[0073] The resulting eluates, containing cadmium and copper respectively, are then concentrated. Concentration can be achieved through nanofiltration.

[0074] Figure 4 An example method for concentrating cadmium and copper in their respective eluents is schematically illustrated. In the illustrated method, the cadmium and copper concentration steps are carried out via a cascade of two nanofiltrations in a batch mode. The cadmium and copper in their respective eluents are concentrated through a series of two nanofiltrations having two closed-loop filtration steps via organic nanofiltration membranes. In the first step, the eluent is filtered over a nanofiltration membrane (NF1). In the second step, the permeate obtained from the first step is filtered through a nanofiltration membrane NF2. The nanofiltration membrane NF2 used in the second step can be the same as the nanofiltration membrane used in the first step, or it can be a different membrane but with a similar chemical structure.

[0075] Suitable membranes include (but are not intended to be any limitation) organic membranes made of polyethersulfone, polyamide / polysulfone, etc. (which are positively charged in acidic media), especially membranes sold by companies Koch, Filmtec, Osmonics and Nadir under the trademarks MP, NF, Desal or PES respectively.

[0076] To regulate the cadmium or copper content, a recycling loop is provided for the concentrate (permeate) obtained from nanofiltration steps 1 and 2. In the first nanofiltration step, the concentration of cadmium or copper must be maintained higher than the eluent concentration to allow for recovery along with the supplementary fraction from the second step concentrate. Before sending the second step concentrate upstream of the first step, the cadmium or copper concentration of the second step concentrate must be close to or higher than the cadmium or copper concentration of the eluent. The fraction recovered from both steps is the total recovered fraction, not the fraction recovered from either step 2 or step 1 alone. Three-quarters of the eluent to be treated will be filtered through the membrane (permeate), and the remaining quarter will be directed to the CdS or CuS precipitation step.

[0077] After concentration, cadmium and copper precipitate.

[0078] Precipitation of cadmium and copper can be induced by blowing in H₂S gas (produced by the reaction between NaHS and concentrated hydrochloric acid). The sulfides are highly reactive with cadmium and copper, forming stable precipitates: pure orange CdS and pure black CuS. The observed precipitation kinetics are very rapid, and the precipitation time typically does not exceed 5 minutes.

[0079] Alternatively, CuS precipitate can be obtained by heat treatment (T>220°C) of the thiourea elution solution.

[0080] The precipitate is collected by filtration, decantation, or centrifugation. The solution obtained through separation can be regenerated by adding H2S.

[0081] In a copper-containing solution, H2S obtained through the temperature degradation (heat treatment) of thiourea reacts with copper ions to form a black precipitate. This black precipitate is pure copper sulfide, CuS.

[0082] In the presence of cadmium in the solution, cadmium sulfide (CdS) is formed.

[0083] Therefore, in some embodiments, the method of the present invention is capable of removing cadmium and copper from a phosphoric acid solution (typically obtained via a wet process) and selectively and separately recovering cadmium and copper. The method then includes the following steps: (a) Optionally, the phosphoric acid solution is pretreated to remove organic matter and suspended solids; (c) Make the phosphoric acid solution that can be obtained at the end of step (a) (when this step is performed) by using a resin functionalized with a thiourea group; (d) Elution, concentration, and precipitation of copper and cadmium are typically performed as follows: (d1) Cadmium is eluted by passing water (usually distilled water) through a functionalized resin; (d2) At the end of step (d1), copper is eluted by passing a thiourea solution in hydrochloric acid solution through a functionalized resin; (d3) Cadmium and copper in the eluents obtained in steps (d1) and (d2) are concentrated, typically by nanofiltration; (d4) Precipitation of cadmium and copper.

[0084] Figure 5 This method is illustrated. In the illustrated method, a 29% by weight crude phosphoric acid solution is pretreated with activated carbon (1). The resulting solution is then passed through a chelating resin comprising a styrene-divinylbenzene-based polymer carrier adsorbed with thiourea functional groups, thereby binding cadmium and copper (MTS resin) (2) to produce a 29% phosphoric acid solution with removed cadmium and copper.

[0085] Cadmium was eluted by passing distilled water through a resin (3). The eluted solution (water + cadmium) was then ultrafiltered (4).

[0086] Then, copper was eluted by passing an acidic thiourea solution through a resin (3'). The eluted solution (water + cadmium) was then ultrafiltered (4).

[0087] Finally, cadmium and copper are precipitated by blowing in H2S gas (5).

[0088] Implementation plan for step (b) The elution, concentration, and precipitation steps for cadmium and copper are the same as those described above.

[0089] When a resin functionalized with thiourea groups is saturated with arsenic, the resin is washed with water (usually distilled water) to elute the arsenic.

[0090] Arsenic can be thermally concentrated, or it can be collected directly through liquid / liquid extraction or precipitation.

[0091] Arsenic can be precipitated using methods known to those skilled in the art. In particular, precipitation can be achieved by adding H₂S or Na₂S.

[0092] Therefore, in some embodiments, the method of the present invention is capable of removing cadmium, copper, and arsenic from phosphoric acid solutions (typically obtained via wet processes) and selectively and separately recovering cadmium, copper, and arsenic. The method then includes the following steps: (a) Optionally, the phosphoric acid solution is pretreated to remove organic matter and suspended solids; (b) Pass the phosphoric acid solution that can be obtained at the end of step (a) (when step (a) is performed) through an ion exchange resin with sulfonic acid groups; (c) The phosphoric acid solution obtained at the end of step (b) is made by using a resin functionalized with thiourea groups; (d) Elution, concentration, and precipitation of copper, cadmium, and arsenic are typically performed as follows: (d1') Arsenic is eluted by using water (usually distilled water) with a resin functionalized with thiourea groups; (d2') Eluting copper and cadmium; (d3') Cadmium and copper in the eluent obtained in step (d2') are concentrated separately, usually by nanofiltration; (d4) Precipitation of cadmium, copper and arsenic.

[0093] Figure 6 This method is illustrated. In the illustrated method, a 29 wt% crude phosphoric acid solution is pretreated. The resulting pretreated 29 wt% phosphoric acid solution is passed through an ion exchange resin (SST) with sulfonic acid groups. Then, the collected partially demetallized phosphoric acid solution is passed through a thiourea-functionalized resin (MTS). Copper, cadmium, and arsenic are eluted, concentrated, and precipitated.

[0094] The phosphoric acid obtained by the method of the present invention conforms to European standards and can be used for a variety of purposes, such as the production of monoammonium phosphate (MAP) and diammonium phosphate (DAP).

[0095] The following embodiments are provided for illustrative purposes and should not be construed as limiting the invention in any way.

[0096] Example Example 1: Recycling of Cadmium and Copper: MTS9140 Only Recycle cadmium and copper according to the following steps: - Activated carbon pretreatment of phosphoric acid (PA); - Combine cadmium and copper into MTS9140 chelating resin; - Cadmium is recovered by elution through the introduction of softened water; - Copper is recovered by elution with a thiourea solution in hydrochloric acid; - Concentrate cadmium and copper in the eluent through nanofiltration; - Precipitate cadmium and copper.

[0097] Chemical analysis of phosphoric acid filtered through MTS9140 showed that the resin strongly retained Cd and Cu.

[0098] Table 1 presents the concentrations of metallic and non-metallic impurities obtained from phosphoric acid treated with MTS9140 alone.

[0099] [Table 1] Table 1: Concentrations of major and minor metallic impurities and non-metallic impurities (BV 1 to 15) obtained solely through MTS9140 (T = room temperature, P = atmospheric pressure, contact time = 1 BV / 5 min, acid percolation rate = 12 BV / h (20 mL / min)) Figure 1 The results show that when the acid BV is less than 8, the retention rates of Cd and Cu are significantly higher than those of other elements (e.g., Fe, Al, V, Mg, etc.). When the acid BV is greater than 12 BV up to 15 BV, MTS9140 still retains Cd and Cu. These results indicate that MTS9140 has a strong affinity for Cd and Cu.

[0100] The chemical analyses of the cadmium and copper eluent solutions are presented in Tables 2 and 3, respectively.

[0101] [Table 2] Table 2: Chemical analysis of deionized water after resin treatment [Table 3] Table 3: Chemical analysis of the acidic thiourea solution after resin treatment The results of chemical analysis using inductively coupled plasma (ICP) analysis showed that the final product was pure (purity >99%) (Tables 4 and 5).

[0102] [Table 4] Table 4: Chemical analysis of CdS based on MTS9140 [Table 5] Table 5: Chemical analysis of CuS based on MTS9140 The obtained product is in powder form. CdS powder is orange, and CuS powder is black.

[0103] Physical X-ray diffraction (XRD) results confirmed that the final product was pure. Figure 2 ).

[0104] Example 2: Recycling of Cadmium and Copper: STTC60 Only In this other embodiment, a cadmium and copper recovery method similar to that of Example 1 is used, but STTC60 resin is used.

[0105] The results are shown in Table 6.

[0106] [Table 6] Table 6: Concentrations of major and minor metallic impurities and non-metallic impurities obtained solely through SSTC60 (BV 1 to 3) STTC60 is insufficient to produce industrially required cadmium-, copper-, and arsenic-free phosphoric acid. STTC60 partially retains Cs and Cu, but not As.

[0107] Example 3: Arsenic selectivity: cascade of SSTC60 + MTS9140 In this embodiment, a method similar to that in Embodiment 1 is used, but with two filtering steps: cascading STTC60 followed by MTS9140.

[0108] Table 7 shows the concentrations of metallic and non-metallic impurities obtained from phosphoric acid obtained by removing metals and arsenic through an SSTC60+MTS9140 cascade.

[0109] [Table 7] Table 7: Concentrations of major and minor metal impurities and non-metallic impurities in 29% PA P2O5 obtained by SSTC60+MTS9140 cascade.

[0110] (Step 1 SSTC60: T = room temperature, P = atmospheric pressure, contact time / 15 min, acid BV is 1 to 3) (Step 2 MTS9140: T = room temperature, P = atmospheric pressure, contact time / 1 BV / 5 min, acid BV is 1 to 6) Chemical analysis of phosphoric acid after SSTC60 showed that the resin partially retained Cd and Cu, but not As.

[0111] Figure 3 This indicates that arsenic is strongly retained through the SSTC60+MTS9140 cascade. The combination of SSTC60+MTS9140 can achieve arsenic selectivity.

Claims

1. A method for removing cadmium and copper, and optionally removing arsenic, from a phosphoric acid solution containing cadmium, copper, and arsenic impurities, the method comprising the steps of: (a) Optionally, the phosphoric acid solution is pretreated to remove organic matter and suspended solids; (b) Optionally, pass the phosphoric acid solution that can be obtained at the end of step (a) through an ion exchange resin with sulfonic acid groups; (c) Make the phosphoric acid solution that can be obtained at the end of step (a) and / or step (b) by using a resin functionalized with thiourea groups.

2. The method according to claim 1, wherein, The phosphoric acid solution containing cadmium, copper, and arsenic is a phosphoric acid solution obtained through a wet process.

3. The method according to any one of the preceding claims, further comprising the ability to selectively and separately recover cadmium and copper and optionally recover arsenic, and further comprising the following additional steps: (d) Selectively elute, concentrate and precipitate cadmium and copper and optionally elute, concentrate and precipitate arsenic.

4. The method according to any one of the preceding claims, wherein, The volume of the phosphoric acid solution using a resin functionalized with thiourea groups varies from 1 BV to 16 BV, preferably from 3 BV to 7 BV.

5. The method according to any one of the preceding claims, wherein, Phosphoric acid solution is obtained by using a resin functionalized with thiourea groups at a flow rate of 30 BV / h to 12 BV / h.

6. The method according to any one of the preceding claims, the method being used to remove cadmium, copper and arsenic, and comprising steps (b) and (c).

7. The method according to any one of the preceding claims, the method being used to remove cadmium, copper and arsenic, and comprising steps (a), (b) and (c).

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

9. The method according to claim 6, 7 or 8, wherein, Phosphoric acid solution is passed through an ion exchange resin with sulfonic acid groups at a flow rate of 4 BV / h to 5 BV / h.

10. The method according to any one of claims 6 to 9, wherein, The selective elution, concentration, and precipitation of cadmium, copper, and arsenic involve the following steps: (d1') Arsenic is eluted by using water with a resin functionalized with thiourea groups, typically distilled water; (d2') Eluting copper and cadmium; (d3') Cadmium and copper in the eluent obtained in step (d2') are concentrated separately, usually by nanofiltration; (d4) Precipitation of cadmium, copper and arsenic.

11. The method according to any one of claims 3 to 5, wherein, The selective elution, concentration, and precipitation of cadmium and copper include the following steps: (d1) Cadmium is eluted by passing water through a functionalized resin, typically distilled water; (d2) At the end of step (d1), copper is eluted by passing a thiourea solution in hydrochloric acid solution through a functionalized resin; (d3) Cadmium and copper in the eluents obtained in steps (d1) and (d2) are concentrated, typically by nanofiltration; (d4) Cadmium and copper precipitate.

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