Process for recovering iodine from a phosphoric acid solution

Abstract

The present invention relates to a process for recovering iodine from a phosphoric acid solution comprising the following steps: supplying a phosphoric acid solution containing dissolved iodine, the iodine concentration in the solution being at least 30 ppm; oxidizing the phosphoric acid solution so as to convert the dissolved iodine into volatile iodine; entraining the volatile iodine from the oxidized solution resulting from step b) by blowing air into an absorption column; in the absorption column, absorbing the volatile iodine into a solution comprising an absorbent; concentrating the solution from step d) by reverse osmosis; and precipitating and recovering iodine crystals or an iodine salt. Figure for the abstract: Fig. 1

Description

Title of the invention: Process for recovering iodine from a phosphoric acid solution. FIELD OF THE INVENTION

[0001] The present invention relates to a process for recovering iodine from a phosphoric acid solution, in which a step of concentrating a solution containing iodine from phosphoric acid is implemented. The process of the invention makes it possible to obtain iodine crystals or iodine salts. STATE OF THE ART

[0002] Iodine is useful in various industrial applications, particularly in medicine, but also in metallurgy, agriculture, printing, photography, etc. In particular, iodine is a micronutrient necessary for the biosynthesis of thyroid hormones, which are essential for the regulation of cellular metabolism, as well as for normal growth and mental development (Lyday, PA and Kaiho, T. (2015). Iodine and Iodine Compounds. In Ullmann's Encyclopedia of Industrial Chemistry, (Ed.).). Global demand for iodine is therefore increasing, prompting manufacturers to develop methods for extracting iodine from natural resources.

[0003] To date, there are three main natural resources from which iodine is extracted: brines (underground brines or those from the oil and gas industry), nitrate ores, and phosphate ores. Iodine is generally recovered via two processes: an air-blowing process or an ion-exchange process (using resins or activated carbon).

[0004] Currently, Chile and Japan are the two main producers of iodine, supplying 70% and 20% of world production respectively (Benali et al., 2023; Kim et al., 2018; Krukowski, 2014; Lauterbach, 2014), Lyday, PA and Kaiho, T. (2015). Lodine and lodine Compounds. In Ullmann's Encyclopedia of Industrial Chemistry, (Ed).).

[0005] In Chile, iodine is extracted and isolated in the form of diiodine from caliche ore (nitrate ore) by a process comprising leaching the ore (i.e. dissolving the soluble salts), then an oxidation and / or reduction reaction of iodine followed by various separation techniques (extraction in kerosene, air-blowing entrainment, flotation, etc.).

[0006] Application JPH02184504A proposes a process for extracting iodine from a natural iodine-rich brine (containing 300 ppm of iodine) derived from natural gas extracted in Tokyo. The process is based on air-blown entrainment technology, in which brine containing iodine (in the form of iodide ions) feeds a first tower where it is acidified by hydrochloric acid and then mixed with chlorine to oxidize the iodide into gaseous diiodine I2. The diiodine I2 thus obtained is entrained by air blowing, then it is sent to a second column where it is absorbed and reduced by a solution containing sulfur dioxide.

[0007] Recently, techniques for recovering iodine as a by-product of phosphoric acid production from phosphate ores have been developed. In China, almost all iodine is obtained as a by-product of processing seaweed and phosphate ores. The Wengfu mine in China has reported iodine mass concentrations of 0.010% in its phosphates, with an annual production of 100 tonnes of iodine obtained as a by-product of phosphoric acid production from these phosphate ores.

[0008] The extraction of iodine from phosphate ores is carried out using a so-called thermal process or a so-called wet process.

[0009] The thermal process is a method for extracting iodine from the gas produced by the calcination of phosphate ore. The iodine is recovered by absorption in a sodium solution. However, this type of process is not widely used on an industrial scale due to the small quantity of calcined phosphate in China, the high cost, and the low iodine yield.

[0010] The so-called wet extraction process is therefore more commonly used. Wengfu Group has developed a process for extracting iodine from 29% P2O5 phosphoric acid by oxidation followed by air blowing, absorption and reduction in a solution containing sulfur dioxide (CN101318625 and CN100345751).

[0011] Patent CN101323435B describes another process for extracting iodine from a solution of fluorosilicic acid H2SIF6, obtained from the production of phosphoric acid, comprising a step of oxidation of the iodide present to I2 using chlorine gas, followed by an air blowing step and then an absorption step of the iodine in an aqueous solution of sulfur dioxide. The iodine is then precipitated by the addition of hydrogen peroxide.

[0012] The air-blown iodine entrainment method is the most widely used method for iodine extraction, particularly when iodine is isolated as a by-product of phosphoric acid production. However, this method requires specialized equipment necessitating significant investment. Furthermore, existing methods use sulfur dioxide, which is a toxic gas.

[0013] A need therefore remains for the development of a reliable, less expensive process for the extraction and recovery of iodine from the production of phosphoric acid. dangerous, less expensive and easy to integrate into existing industrial facilities. Description of the invention

[0014] The invention relates to a process for recovering iodine from a phosphoric acid solution comprising the following steps: a. supply of a phosphoric acid solution comprising dissolved iodine, the concentration of dissolved iodine in the solution being at least 30 ppm, b. oxidation of the phosphoric acid solution so as to convert the dissolved iodine into volatile iodine, c. air entrainment of volatile iodine towards an absorption column from the oxidized solution resulting from step b), d. in the absorption column, absorption of volatile iodine in a solution comprising an absorbent, e. concentration of the solution from step d) by reverse osmosis, f. precipitation and recovery of diiodine crystals or an iodine salt.

[0015] Other aspects of the invention are as described below. DESCRIPTION OF THE FIGURES

[0016] [Fig.1] Fig.1 represents a diagram of the iodine recovery process according to the invention.

[0017] [Fig. .2] Fig. .2 represents a diagram of the iodine recovery process according to the invention when integrated into a wet phosphoric acid production system by acid attack of phosphate ore.

[0018] [Fig.3] [Fig.3] Figure 3 represents an X-ray diffractogram of diiodine crystals obtained according to example 5.

[0019] [Fig.4] Figure [Fig.4] represents an X-ray diffractogram of iodide crystals money obtained according to example 6. DETAILED DESCRIPTION OF THE INVENTION

[0020] The inventors have developed a process that meets the expressed needs.

[0021] The process of the invention allows the recovery of iodine in the form of crystals, the iodine being diiodine of formula I₂(s) or an iodine salt, depending on the conditions used. The iodine salt is notably chosen from the group consisting of silver iodide A₃(s), potassium iodide K₂(s), and sodium iodide N₂(s). The designation (s) indicates that the species is in solid form.

[0022] The process of the invention has the advantage of not using sulfur dioxide or any other toxic gas. Moreover, it requires less energy and water than existing processes and does not generate industrial effluents.

[0023] The process of the invention can advantageously be integrated into a phosphoric acid production system, in particular by wet process by acid attack of phosphate ore, for example already existing. Step a)

[0024] The phosphoric acid solution of step a) comprises at least 30 ppm of dissolved iodine, preferably the concentration of dissolved iodine is in a range of 30 ppm to 100 ppm.

[0025] The iodine dissolved in the phosphoric acid solution is notably in the form of iodide ions, of formula LU, can also be found in the form of polyiodide ions, such as the triiodide ion I3, or iodate ions IO3.

[0026] The phosphoric acid solution is in particular a phosphoric acid solution having a phosphorus pentoxide (P2O5) content ranging from 10% to 45%, in particular from 23% to 52%, by volume relative to the total volume of the solution. Preferably, it is a phosphoric acid solution with a P2O5 content of 29% v / v.

[0027] The phosphoric acid solution is notably obtained from a wet process for preparing phosphoric acid by acid attack of phosphate ore. The acid attack is carried out in particular using sulfuric acid.

[0028] Thus, the phosphoric acid solution used in step a) can be the solution obtained at the end of the wet process for preparing phosphoric acid, that is, after separation from phosphogypsum. Such a solution has, in particular, a concentration of 29% v / v in P2O5.

[0029] Step a) can be implemented in a reactor called a boiler as shown in [Fig.1].

[0030] Alternatively, when the process of the invention is directly integrated into the phosphoric acid production system, the phosphoric acid solution used in step a) can be the solution obtained by mixing sulfuric acid and phosphate ore, i.e., the slurry comprising phosphoric acid, dissolved iodine, and solid phosphogypsum. In such an embodiment, the acid attack on the phosphate ore typically takes place in a first phosphate attack reactor as shown in [Fig. 2]. This acid attack is highly exothermic, and the resulting slurry is advantageously transported to a second cooled reactor, referred to as a flash cooler in [Fig. 2], which maintains the temperature within the slurry at a value less than or equal to 80°C. Step b)

[0031] Step b) of oxidation converts the iodine dissolved in the phosphoric acid solution into volatile iodine. In other words, the iodide, polyiodide, and iodate ions initially dissolved in the phosphoric acid solution in step a) are oxidized to volatile diiodine with the formula I2(g). The (g) designation indicates that the species is in gaseous form. Therefore, the diiodine is in gaseous form.

[0032] Oxidation is carried out by adding an oxidant to the phosphoric acid solution obtained in step a). The oxidant is, for example, chosen from the group consisting of hydrogen peroxide, chlorine, and sodium nitrite. Preferably, it is hydrogen peroxide, in particular an aqueous solution of hydrogen peroxide at 30% by volume in water. The use of hydrogen peroxide avoids the introduction of unwanted elements (such as sodium) into the phosphoric acid solution. The amount of oxidant added depends on the concentration of iodine dissolved in the phosphoric acid solution. Typically, when the oxidant is hydrogen peroxide, it is added at a rate of 0.5 molar equivalents relative to the amount of dissolved iodine. The amount of dissolved iodine is determined, in particular, by spectrophotometric analysis.

[0033] Step b) can be carried out without heating the phosphoric acid solution. In some embodiments, the phosphoric acid solution can be heated to a temperature ranging from 40°C to 120°C, preferably from 60°C to 100°C, upstream of or simultaneously with the addition of the oxidant. When the solution is heated upstream, the temperature reached before the addition of the oxidant is maintained throughout the addition process.

[0034] Step b) can be carried out in the same reactor called a boiler from step a).

[0035] Alternatively, when the process of the invention is directly integrated into the phosphoric acid production system, step b) is typically carried out within the flash cooler as shown in [Fig. 2]. As indicated above, this cooler maintains the temperature within the slurry at or below 80°C, typically between 70°C and 80°C, so that it is not necessary to heat the solution before or during the addition of the oxidant. Step c)

[0036] In step c), an air-blowing entrainment allows the volatile iodine obtained in step b) to be transported from the reactor in which steps a) and b) were carried out to an absorption column, as shown for example in [Fig.1].

[0037] Preferably, the oxidized solution is also kept under agitation during air blowing. Preferably, the oxidized solution from step b) is at a temperature ranging from 40°C to 120°C, preferably from 60°C to 100°C, in particular from 70°C to 90°C, for example at 80°C. When the process of the invention is implemented independently of a phosphoric acid production system, a heating system can be used to reach this temperature. The heating can be carried out simultaneously with the air blowing. When the process of the invention is directly integrated into the phosphoric acid production system, as indicated above, there is then no need to heat the solution.

[0038] Temperature and agitation promote the extraction of volatile iodine from the oxidized solution and its trapping within the airflow. A temperature above 25°C increases the vapor pressure of iodine and promotes its volatilization. The vapor pressure of iodine is 0.0406 kPa at 25°C, 0.287 kPa at 50°C, and 3.570 kPa at 90°C.

[0039] Air blowing is implemented at an air flow rate ranging from 0.1 L / min to 5 L / min. The air flow used can be an air flow, a nitrogen flow, or a mixture of both.

[0040] In some embodiments, the air-blowing drive is carried out using a vacuum pump.

[0041] Steps b) and c) can be successive or simultaneous. When they are simultaneous, this means that the volatile iodine obtained by oxidation is transported rapidly and continuously to the absorption column.

[0042] When the process of the invention is directly integrated into the phosphoric acid production system, the slurry comprising iodine-free phosphoric acid and solid phosphogypsum is conveyed to a filter where the solid phosphogypsum is separated from the phosphoric acid solution, typically at 29% v / v P2O5. In such an embodiment, gases other than volatile iodine are likely to escape from the oxidized slurry, such as Cl2. The gas mixture is then conveyed to the absorption column. Step d)

[0043] During the absorption step d), the volatile iodine, previously transported to the absorption column during step c), is absorbed into a liquid present in the absorption column, also called the absorption solution.

[0044] The absorption solution is a reducing solution which will allow the volatile iodine I2 to be reduced into iodide ions dissolved in the absorption solution.

[0045] The absorption solution is a solution, typically an aqueous solution, comprising an absorbent. The absorbent is notably chosen from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium sulfite (Na2SO3), ascorbic acid, or a mixture thereof. The concentration of the absorbent in the absorption solution is typically in the range of 1% to 10%, preferably 1% to 5%, by volume relative to the total volume of the solution.

[0046] The temperature within the absorption column is in a range from 20°C to 40°C, preferably from 20°C to 30°C.

[0047] Typically, the incoming iodine-laden air flows counter-currently to the absorption solution in order to maximize contact between the air and the solution and thus promote the Reduction of volatile iodine to iodide ions. Typically, the conversion rate of volatile iodine to iodide is in the range of 90% to 99%, preferably 95% to 99%. The air exiting the column, completely or essentially free of volatile iodine, can advantageously be reused in step c). By "essentially free of" is meant that the air exiting the column contains 10% or less by volume of volatile iodine. When the process of the invention is directly integrated into the phosphoric acid production system, this absorption column is added to the existing system by connecting it directly to the flash cooler as shown in [Fig. 2]. In such an embodiment, and as previously stated, a gaseous mixture including volatile iodine can be conveyed by blowing air to the absorption column. Volatile iodine is then separated from the gaseous mixture by being the only species absorbed into the reducing solution.

[0048] From the iodide ion concentration of the solution obtained in step d), and knowing the initial iodide ion concentration in step a), it is possible to determine the iodine extraction yield Rm within the absorption solution. This yield is typically in the range of 90% to 99%, preferably from 95% to 99%.

[0049] Steps c) and d) can be repeated sequentially several times, for example up to 10 times, to optimize iodine extraction and its absorption into the absorption solution. In other words, the air exiting the absorption column, completely or essentially devoid of volatile iodine, can be reintroduced into the reactor of step c) (the boiler or flash cooler) and recharged with volatile iodine upon further contact with the oxidized solution. The recharged air will then be reintroduced into the absorption column to concentrate the iodine absorption solution, and so on. Step e)

[0050] The solution from step d) is conveyed to a storage tank, such as for example shown in Figures 1 and 2, in which step e) of concentration by reverse osmosis is carried out.

[0051] The solution is thus concentrated so as to obtain a concentration in iodine, i.e. iodide ions, in the solution typically greater than or equal to 2000 ppm.

[0052] A person skilled in the art will be able to select a suitable membrane for implementing the reverse osmosis step of the process of the invention. Preferably, the membrane used is a semi-permeable membrane. In particular, it is a thin-film composite semi-permeable membrane, preferably comprising an ultra-thin top layer of active filtration arranged on a support. The top layer, for example, has a thickness ranging from 50 to 200 nm. This The top layer is typically made of a cross-linked polymer, particularly a cross-linked polyamide. The substrate on which this top layer rests is preferably porous. It may, for example, be made of a polymer material. This is typically a material composed of porous polysulfone and polyester fibers, arranged in a grid pattern to ensure the membrane's mechanical stability.

[0053] Step e) can be carried out in several cycles, i.e. the solution passed through the membrane can be filtered again one or more times through said membrane until the desired iodine concentration is reached.

[0054] Using reverse osmosis to perform this concentration step minimizes the volume of solution to be treated in the subsequent precipitation step. Furthermore, reverse osmosis maximizes concentration and allows for the regeneration of iodine-free water. Step f)

[0055] In step f), the precipitation of diiodine crystals I2 (commonly referred to as iodine crystals) or of crystals of an iodine salt, such as Agi, Nal or Kl, is carried out. When an iodine salt is precipitated, it is preferably silver iodide.

[0056] Initially, regardless of the species to be precipitated, the solution from step e) is acidified by adding a strong acid until a pH ranging from 1 to 4 is obtained, preferably equal to 1. The strong acid is, for example, chosen from the group consisting of sulfuric acid and nitric acid. Preferably, it is sulfuric acid, for example at a concentration ranging from 1 / 100M to 5M.

[0057] The precipitation of iodine in the form of diiodine crystals (I₂) can then be carried out by adding an oxidant to the acidified solution. The oxidant is, for example, chosen from the group consisting of hydrogen peroxide, H₂O₂. This is in particular an aqueous solution of hydrogen peroxide at 30% by volume in water. The amount of oxidant added depends on the iodine concentration in the solution. Typically, when the oxidant is hydrogen peroxide, it is added at a rate of 0.5 molar equivalents relative to the amount of iodide ions, this amount being typically determined by spectrophotometric analysis. The conversion of iodide ions into iodine crystals (I₂) by oxidation with hydrogen peroxide is carried out according to the following equation (1):

[0058] H2O2+?.ï + 7.lf^I2 + 2H2O (1)

[0059] Alternatively, the precipitation of silver iodide crystals of formula Agi from the concentrated solution obtained in step e) is carried out by adding a silver salt, for example silver nitrate (AgNO3), to the acidified solution. In this embodiment, the strong acid added to acidify the solution obtained in step e) is typically nitric acid. The amount of silver salt, particularly silver nitrate, added depends on the iodine concentration in the solution. Typically, the salt, for example silver nitrate, is added at a rate of 1 molar equivalent relative to the amount of iodide ions. In the case of silver nitrate, it is added according to the following equation (2): [006°] AgNO + j ^AgI+ NO3 (2)

[0061] Other iodine salts can be precipitated, depending on the absorbent used in step d). For example, when the absorbent used is NaOH (sodium hydroxide), sodium iodide is formed in the absorption solution. Sodium iodide crystals Nal can thus be precipitated following step e), for example by simple evaporation. Similarly, if the absorbent used in step d) is KOH (potassium hydroxide), it will be possible to precipitate potassium iodide crystals Kl.

[0062] Thus, step f) may include the precipitation of either iodine crystals, i.e., diiodine crystals (I2), or crystals of an iodine salt, for example, silver iodide, sodium iodide, or potassium iodide. If an iodine salt is precipitated, it is preferably silver iodide.

[0063] A filtration step is then carried out to recover the iodine crystals or an iodine salt. The filtration can be carried out using any suitable conventional filtration method known to those skilled in the art.

[0064] The recovered iodine or iodine salt crystals typically have a purity greater than 95%, in particular equal to 99%. EXAMPLES

[0065] The following examples were carried out in an installation as illustrated in [Fig.1].

[0066] Example 1: Absorption of iodine in a sodium hydroxide (NaOH) solution with a concentration of 1 wt%

[0067] Iodine extraction was carried out in accordance with the invention by following these steps: a. supply of IL of phosphoric acid 25% v / v of P2O5 containing 30.15 mg / L of iodine; b. oxidation of iodide ions to volatile iodine by adding 0.4%(v / v) of an aqueous hydrogen peroxide solution at 30% v / v relative to the total volume of the solution; c. Heating the oxidized solution under stirring for 30 min at a temperature ranging from 80°C to 100°C to prevent the iodine from remaining in solution, trapping of iodine by a large volume of air or N2 circulating at a flow rate of 1 L.min and transport of volatile iodine to an absorption column d. absorption of iodine in a volume of 100 ml of a NaOH solution at 1% (m / m).

[0068] The absorption solution after absorption of iodine has a pale yellow color specific to the presence of iodine.

[0069] The results of chemical analyses by UV-Visible spectroscopy (SHIMADZU UV-1900i UV-VIS instrument) show that the solution from step d) contains iodide ions. Table 1 illustrates the extraction yield, denoted RM and expressed as a percentage, obtained during the extraction of Example 1, repeated 3 times.

[0070] Table 1: Iodine extraction yields in 1% NaOH. Experience QC in I (mg.Ll) Cf in I (mgU1) Rm(%) 1 30.15 0.55 98 2 30.15 0.65 97 3 30.15 0.45 98

[0071] Example 2: Absorption of iodine in a sodium sulfate (Na₂SO₃) solution ) of concentration 1% by weight

[0072] Iodine extraction was carried out in accordance with the invention by the following steps: a. supply of IL of phosphoric acid 25% v / v of P2O5 containing 30.15 mg / L of iodine; b. oxidation of iodide ions to volatile iodine by adding 0.4%(v / v) of an aqueous hydrogen peroxide solution at 30% v / v relative to the total volume of the solution; c. Heating the oxidized solution under stirring for 30 min at a temperature ranging from 80°C to 100°C to prevent the iodine from remaining in solution, trapping the iodine by a large volume of air or N2 circulating at a flow rate of 1 L.min 1 and transporting the volatile iodine to an absorption column d. absorption of iodine in a volume of 100 ml of a 1% (w / w) Na2SO3 solution.

[0073] The absorption solution after absorption of iodine has a pale yellow color specific to the presence of iodine.

[0074] The results of the chemical analyses by UV-Visible spectroscopy show that the solution from step d) contains iodide ions. Table 2 illustrates the yield extraction, noted RM and expressed as a %, obtained during the extraction of example 2, repeated 3 times.

[0075] Table 2: Iodine extraction yields in %Na2SO3. Experience Cq in I (mgl^) Cf in I (mgU1) Rm(%) 1 30.15 0.95 96.8 2 30.15 0.35 98.9 3 30.15 0.48 98.4

[0076] Example 3 (comparative): Absorption of iodine in water

[0077] In this example, the extraction of iodine is carried out according to the same steps a) to d) as those of examples 1 and 2 but water is used instead of the absorption solution in step d).

[0078] The solution obtained in step d) is not colored. The results of the chemical analysis by UV-Visible spectroscopy show that the solution does not contain iodide ions, which shows that iodine has no affinity for water and that its solubility in water is very low or even zero.

[0079] Example 4 (comparative): No precipitation of iodine-12 without a reverse osmosis step

[0080] In this example, an extraction process similar to that of Example 1 is implemented. The example is repeated on 10 batches of IL of phosphoric acid having an initial concentration of T ions of 31 mg.L' each, then the final absorption solutions are collected and combined.

[0081] A brown solution is obtained, typical of the presence of iodine in solution.

[0082] An aqueous solution of hydrogen peroxide is then added at a concentration of 0.04% equivalents to attempt to precipitate the iodine crystals I2. However, no precipitation is observed due to the iodine concentration being too low in the solution (290mg / L) (see table 3).

[0083] Table 3: Effect of iodine concentration on the efficiency of iodine precipitation. Experiment: Volume of ACP (L) CQenI (mg.L1) Mass of I in the trapping solution (mg) Precipitation I2 (g) Without reverse osmosis 10 31 290 0 (brown solution, no precipitation)

[0084] Example 5: Reverse osmosis step before the precipitation of iodine 12.

[0085] Iodine extraction was carried out in accordance with the invention by following these steps: a. supply of 10L of phosphoric acid 25% v / v of P2O5 containing 30.15 mg / L of iodine; b. oxidation of iodide ions to volatile iodine by adding 0.4% (v / v) of an aqueous hydrogen peroxide solution at 30% v / v relative to the total volume of the solution; c. Heating the oxidized solution under stirring for 30 min at a temperature ranging from 80°C to 100°C to prevent the iodine from remaining in solution, trapping the iodine by a large volume of air or N2 circulating at a flow rate of 1 L.min 1 and transporting the volatile iodine to an absorption column; d. absorption of iodine in a volume of 100 ml of a 1% (w / w) NaOH solution; e. concentration of the solution from step d) by reverse osmosis using the "Flat membrane, AK, PA-TFC, RO, 90mm 5 / PK" from Linkman group, f. acidification of the solution from step e) to a pH equal to 1 by adding IM sulfuric acid then adding hydrogen peroxide at a rate of 0.1% equivalent to precipitate the iodine crystals.

[0086] The iodine crystals obtained have a physical appearance identical to that of commercial iodine (same color, same size). Physical analyses by XRD (see [Fig. 3]) and chemical analyses by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy, see Table 5) show that the product obtained has a purity of at least 98%. The diffractometer used is the Bruker AXS D8 Advance XRD. The optical emission spectrometer used is the PerkinElmer Avio 500, which allows for the analysis of trace elements after digestion of the iodine obtained with HClO4. Titration with 0.1N sodium thiosulfate is used to calculate the mass of I2 crystals obtained. Table 4 illustrates the concentration of I- ions in the solution before and after the reverse osmosis step and the retention R (%).

[0087] Table 4: Effect of reverse osmosis on iodine precipitation. CfenI in the absorption solution (step d) (mg.L') Volume (L) CfenI in the concentrated solution (step e) (mg.L1) Retention (%) Precipitation of I2 With osmosis in vers e 290 1 493 70 Black crystals ( I2(s))

[0088] “The retention R corresponds to the capacity of the membrane to concentrate the iodide solution, R(%)=1-

[0089] (Cpermeate / cfeed) * 100)

[0090] Table 5: Chemical composition of the I2 crystals obtained by ICP-OES Chemical composition K (%m) P (%m) Cu (mg.k g') Mo (mg.k g') Pb (mg.k g') I2 by titration Early purity (ICP-OES) Iodine crystals 0.001 0.08 0.1344 0.06 0.262 98 99.9 Example 6: Precipitation of silver iodide Agi

[0091] In this example, the same steps a) to d) of Example 5 are implemented. In comparative test 1, step d) is directly followed by the next step f') and no reverse osmosis concentration step is performed:

[0092] f') acidification of the solution from step e) to a pH ranging from 1 to 6 by addition of nitric acid IM then addition of 2.3 mL of silver nitrate IM.

[0093] In test 2 according to the method of the invention, a reverse osmosis concentration step e) is carried out between step d) and step f'). Table 6 illustrates these two tests and the mass of silver iodide extracted in both cases.

[0094] The silver iodide crystals (a yellow precipitate) obtained have a physical appearance (color, size) identical to that of commercial silver iodide. Physical analyses by XRD show that it is indeed pure Agl (see [Fig. 4]).

[0095] Table 6: Effect of reverse osmosis on the precipitation of silver iodide. Volume of ACP (L) CO (mg / l) Mass of I in the absorption solution (mg) Mass of extracted Agi (g) Test 1 10 31 290 0.5 (precipitate already one) Test 2 10 31 493 0.8 (precipitate already one)

[0096] It is noted that the yield of extracted Agi is higher when a reverse osmosis step is carried out (test 2).

Claims

Demands

1. A process for recovering iodine from a phosphoric acid solution comprising the following steps: a. supplying a phosphoric acid solution comprising dissolved iodine, the concentration of dissolved iodine in the solution being at least 30 ppm, b. oxidizing the phosphoric acid solution so as to convert the dissolved iodine into volatile iodine, c. entraining by air blowing the volatile iodine from the oxidized solution resulting from step b) to an absorption column, d. in the absorption column, absorption of the volatile iodine into a solution comprising an absorbent, e. concentration of the solution from step d) by reverse osmosis, f. precipitation and recovery of diiodine crystals or of an iodine salt.

2. A process for recovering iodine according to claim 1, wherein the phosphoric acid solution supplied in step a) is a phosphoric acid solution of 29% v / v P2O5.

3. A method for recovering iodine according to claim 1 or 2, wherein the oxidation in step b) is carried out by adding hydrogen peroxide to the phosphoric acid solution from step a).

4. A process for recovering iodine according to any one of claims 1 to 3, wherein in step c), the oxidized solution is at a temperature ranging from 40°C to 120°C.

5. A method for recovering iodine according to any one of claims 1 to 4, wherein steps b) and c) are simultaneous.

6. A method for recovering iodine according to any one of claims 1 to 5, wherein the absorption solution comprises an absorbent selected from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium sulfite (Na2SO3), ascorbic acid, or a mixture thereof.

7. A method for recovering iodine according to any one of claims 1 to 6, wherein steps c) and d) are repeated serially several times.

8. A method for recovering iodine according to any one of claims 1 to 7, wherein the solution from step d) is concentrated in step e) so as to obtain an iodine concentration in the solution greater than or equal to 2000 ppm.

9. A process for recovering iodine according to any one of claims 1 to 8, wherein diiodine crystals are precipitated in step f) by the addition of an oxidant.

10. A process for recovering iodine according to any one of claims 1 to 8, wherein iodine salt crystals are precipitated during step f).

11. A process for recovering iodine according to claim 10, wherein the precipitation is carried out using silver nitrate and the iodine salt is silver iodide.

12. A process for recovering iodine according to any one of claims 1 to 11, wherein the concentrated solution from step e) is acidified to a pH of 1 to 4 before the precipitation of iodine crystals or an iodine salt.

13. A process for recovering iodine according to any one of claims 1 to 12, integrated into the phosphoric acid production chain, in particular by wet process by acid attack of phosphate ore.

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