Method for treating phosphoric acid by ionic flotation

Abstract

The present invention relates to a method for treating a phosphoric acid solution comprising heavy metals by ionic flotation, in which the phosphoric acid solution is mixed with an ionizable surfactant and an adsorbent solid.

Description

[0001] Process for treating phosphoric acid by ionic flotation

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a method for treating a phosphoric acid solution comprising heavy metals by ionic flotation in which the phosphoric acid solution is mixed with an ionizable surfactant and an adsorbent solid having a particle size ranging from 5 to 160 μm.

[0004] STATE OF THE ART

[0005] Phosphoric acid (H3PO4) can be produced mainly by two processes: a wet process and a thermal process. The wet process is the most widely used. Wet-process phosphoric acid is obtained by attacking natural phosphate with a strong acid such as hydrochloric acid, nitric acid, and / or sulfuric acid.

[0006] Industrial phosphoric acid is a precursor to the manufacture of phosphate fertilizers. However, it contains elements that can pollute soil or groundwater, such as cadmium (Cd), copper (Cu), arsenic (As), zinc (Zn), lead (Pb), nickel (Ni) and chromium (Cr).

[0007] Cadmium levels in fertilizers are under particular scrutiny by the European Parliament and other institutions, which require limits on cadmium in phosphate fertilizers. Hence the importance of reducing the concentration of this element as low as possible and also reducing the levels of other heavy metals as much as possible.

[0008] Various processes have been developed to remove heavy metals, such as precipitation, solvent extraction, adsorption, ion exchange, co-crystallization, membrane techniques and ion flotation.

[0009] For example, patent applications EP0099804 and EP0203076 describe a process for treating phosphoric acid by ionic flotation. This process involves reducing the Cd, As, and U contents in a phosphoric acid solution by adding a dithiophosphate. Although this process has high removal rates for heavy metals, it also has significant losses of phosphoric acid, greater than 30%, which do not allow industrial exploitation of the process at an acceptable cost. FR2530161 and EP0099804 describe a process for treating an acidic medium, in particular a wet sulfuric acid solution, using the principle of ionic flotation. The process comprises the following steps. No mention is made of the use of an adsorbent agent.

[0010] US4844873 describes an ion flotation process that is more efficient on an industrial scale than the processes described, for example, in FR2530161. There is no mention of the use of an adsorbent agent.

[0011] US4452768, US4479924 and US4713229 describe processes for removing heavy metals from a wet sulfuric acid solution. The crude phosphoric acid is contacted with an adsorbent and a diorganyldithiophosphoric acid ester, and then the purified phosphoric acid is separated from the ester and the adsorbent. The separation is carried out by decantation, centrifugation, filtration or on a column. The adsorbents used may be activated carbon, carbon black, lignite, diatomaceous earth, silica gel, synthetic silicic acids, poreus resin, silicate or zeolites.

[0012] Ion exchange processes have also been considered, in application WO2004 / 083118 A1, for the removal of heavy metals. The recovery efficiencies achieved are only about 50% of cadmium.

[0013] Patent application US4503016A describes a process for the liquid-liquid extraction of heavy metals from phosphoric acid, the extraction is carried out by a solution of dialkyl dithiophosphoric acid ester. The disadvantage of this process lies in the need to carry out a pretreatment step of the phosphoric acid solution. The problem with liquid-liquid extraction is the phase separation step where the emulsified or dissolved dialkyl dithiophosphoric acid ester is likely to be removed with the separated phosphoric acid, so it is necessary to have an additional post-treatment step.

[0014] Thus, the prior art processes have disadvantages in terms of removing heavy metals or other valuable elements from industrial phosphoric acid. They require, in particular, difficult treatment conditions, sophisticated equipment, high quantities of reagents, incompatibility with all concentrations of phosphoric acid, or the need to carry out pre- or post-treatment of the phosphoric acid.

[0015] There is, therefore, a need for new methods to facilitate the recovery of heavy metals from a phosphoric acid solution that are feasible on a large scale at moderate cost. SUMMARY OF THE INVENTION

[0016] The present invention relates to a method for treating a phosphoric acid solution containing heavy metals by ionic flotation, the method comprising the following steps:

[0017] (i) preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 pm as determined by laser diffraction particle size analysis,

[0018] (ii) injecting gas upwards into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid,

[0019] (iii) separation of the phosphoric acid solution and the foams containing heavy metals.

[0020] Other aspects of the invention are as described below and in the claims.

[0021] FIGURES

[0022] Figure 1: Example of a laboratory-scale flotation column

[0023] Figure 2: Example of a pilot-scale flotation column

[0024] DETAILED DESCRIPTION OF THE INVENTION

[0025] The inventors have developed a process that meets the expressed needs. The proposed process for treating a phosphoric acid solution, typically derived from the wet process, for the removal of heavy metals by ionic flotation does not have the drawbacks of the prior art. It includes the use of a small amount of adsorbent solid resulting in an improvement in the recovery yield of phosphoric acid while limiting the quantities of reagent used.

[0026] The various embodiments presented throughout the description may be used alone or in combination with each other, without limitation of combination.

[0027] Thus, the present invention relates to a method for treating a phosphoric acid solution containing heavy metals by ionic flotation, the method comprising the following steps:

[0028] (i) preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 pm as determined by laser diffraction particle size analysis, (ii) injection of gas upwardly into the mixture resulting from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid,

[0029] (iii) separation of the phosphoric acid solution and the foams containing heavy metals.

[0030] In some embodiments, the present invention relates to a method of treating a phosphoric acid solution containing heavy metals by ion flotation, the method comprising the following steps:

[0031] (i') mixing the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 pm,

[0032] (ii) injecting gas upwards into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid,

[0033] (iii) separation of the phosphoric acid solution and the foams containing heavy metals.

[0034] Thus, in certain embodiments, the preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 μm (step (i)) is carried out by mixing the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 μm.

[0035] In other embodiments, the adsorbent agent is not added to the phosphoric acid solution but is present in that solution as a by-product of phosphoric acid manufacture. The adsorbent agent is phosphogypsum in these embodiments.

[0036] Advantageously, the treatment method according to the invention makes it possible to eliminate the heavy metals present in a phosphoric acid solution by means of a reduced quantity of reagents, in particular a reduced quantity of ionizable surfactant, typically an amount ranging from 0.5 to 2 g / kg of solution. The heavy metals eliminated from the phosphoric acid solution by the method according to the invention are advantageously cadmium, copper, arsenic, lead, nickel, chromium and zinc. In particular, the method according to the invention makes it possible to eliminate at least 90% by weight of the cadmium present in the phosphoric acid solution, advantageously at least 99% by weight of the cadmium is eliminated.

[0037] Another advantage of the process according to the invention is to limit losses of phosphoric acid.

[0038] Advantageously, the treatment process does not require any prior treatment or subsequent post-treatment of the phosphoric acid solution.

[0039] The advantages of the process according to the invention make it particularly applicable to the treatment of phosphoric acid solutions on an industrial scale.

[0040] The ion flotation treatment process can be carried out at a temperature ranging from 10 to 90°C, typically from 15 to 90°C or from 20 to 80°C. The temperature is chosen to be lower than the degradation temperature of the reagents introduced in step (i), in particular the temperature is lower than the degradation temperature of the ionizable surfactant. Generally, the temperature for carrying out steps (i) and (ii) is identical.

[0041] The process of the present invention comprises the preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 μm.

[0042] The adsorbent solid can be added to the phosphoric acid solution to be treated (see step i') or be present in the phosphoric acid solution as a by-product of phosphoric acid production. In the latter case, it is phosphogypsum.

[0043] The use of phosphogypsum represents a significant advantage. Phosphogypsum, when already present in the phosphoric acid solution as a by-product, eliminates the need to further process the phosphoric acid and then add an adsorbent. The phosphoric acid solution used in the process is then typically an unpurified solution to remove the phosphogypsum. This approach not only optimizes the use of available resources but also reduces the costs and environmental impact associated with the addition of an external adsorbent. Furthermore, the phosphogypsum present in the phosphoric acid as a by-product contains organic matter that, from a technical point of view, increases the hydrophobicity of the medium. This property promotes the flotation of collector (surfactant)-metal ion complexes and contributes to the formation of stable foams, without the need for other additives.Furthermore, the size of the phosphogypsum particles (particle size ranging from 5 to 160 pm) is adapted to allow the particles to be transported by gas bubbles to the surface, facilitating their recovery in the foams.

[0044] While phosphogypsum may be present in the phosphoric acid solution to be treated as a by-product, this does not preclude, in embodiments, it being added to the phosphoric acid solution to be treated (see step i')).

[0045] In certain embodiments, the method according to the invention comprises a step (i') of mixing the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 μm.

[0046] Step (i') can be carried out with stirring to promote foam formation. Generally, the mixing step lasts from 5 seconds to 15 minutes or from 1 to 5 minutes.

[0047] In some embodiments, the phosphoric acid solution is mixed firstly with the adsorbent solid and then secondly with the ionizable surfactant. The step of mixing the phosphoric acid solution with the adsorbent solid is referred to hereinafter as the “conditioning step”.

[0048] In some embodiments, the phosphoric acid solution is mixed first with the ionizable surfactant and then with the adsorbent solid.

[0049] In some embodiments, the phosphoric acid solution is mixed simultaneously with the ionizable surfactant and the adsorbent solid.

[0050] Advantageously, reducing the contact time between the element to be floated (metal ions) and the collector (ionizable surfactant) before implementing the flotation step makes it possible to limit, or even overcome, the problems of release which designates the separation between the metal ions in solution and the collector. Phosphoric acid solution

[0051] The method according to the invention can be implemented for any type of phosphoric acid solution, regardless of its origin. The phosphoric acid solution commonly comes from a wet production process. Generally, the phosphoric acid solution has a phosphoric acid concentration ranging from 5 to 65% by weight of P2O5, generally ranging from 10 to 60% by weight of P2O5, or even ranging from 25 to 55% by weight of P2O5, relative to the total weight of the phosphoric acid solution.

[0052] Typically, phosphoric acid solution contains 0.1 to 6% by weight of impurities relative to the weight of P2O5.

[0053] The phosphoric acid solution typically has a cadmium content of less than or equal to 100 ppm, or less than or equal to 50 ppm, and typically greater than or equal to 1 ppm, for example ranging from 1 to 100 ppm, or even ranging from 1 to 50 ppm or even ranging from 1 to 35 ppm.

[0054] The phosphoric acid solution typically has an arsenic content of less than or equal to 50 ppm, or less than or equal to 35 ppm, typically ranging from 1 to 30 ppm.

[0055] In some embodiments, the phosphoric acid solution is a 29% by weight solution of P2O5. Such solutions generally comprise from 5 to 20 ppm of cadmium.

[0056] In some embodiments, the phosphoric acid solution is a 54% by weight solution of P2O5. Such solutions generally comprise from 10 to 35 ppm of cadmium.

[0057] The phosphoric acid solution typically has a solids content of less than or equal to 3% by weight, typically ranging from 0.1 to 3% by weight or from 0.1 to 1% by weight of solids relative to the total weight of the solution. A low solids content in the phosphoric acid solution promotes the formation of foams with a reduced amount of ionizable surfactant. Commonly, a phosphoric acid solution with a solids content of less than or equal to 3% is obtained after a pretreatment step by precipitation, adsorption, coagulation / flocculation, filtration and / or ultrafiltration, typically filtration.

[0058] Alternatively, a phosphoric acid solution having a solids content greater than 3% by weight, typically between 3 and 5% by weight, may also be treated in the process according to the invention. Ionizable surfactant

[0059] In the method of the present invention, the phosphoric acid solution is mixed with an ionizable surfactant.

[0060] The term "ionizable surfactant" means a surfactant having at least one apolar hydrocarbon chain and a polar head, typically a negatively charged polar head.

[0061] The ionizable surfactant acts as a "collector", meaning that it forms a complex with the heavy metals. This complex is then adsorbed onto the adsorbent solid used in the process of the present invention.

[0062] The ionizable surfactant may be selected from the group consisting of dithiophosphate derivatives, dithiophosphinate derivatives, xanthate derivatives and mixtures thereof.

[0063] Typically, the ionizable surfactant is selected from the group consisting of compounds of formula (I), compounds of formula (II) and compounds of formula (III):

[0064] Dithiophosphinate (II)

[0065] Xanthate (III) in which R1, R2 and R3 are, independently, linear or branched alkyl groups comprising from 1 to 30 carbon atoms or from 2 to 25 carbon atoms or from 4 to 20 carbon atoms or from 6 to 15 carbon atoms. In certain embodiments, the phosphoric acid solution is mixed with 0.1 to 5%, preferably 0.5 to 4%, or from 1 to 3%, by weight of ionizable surfactant relative to the total weight of P2O5 in the phosphoric acid solution.

[0066] Adsorbent solid

[0067] In the process of the present invention, the phosphoric acid solution is mixed with an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 pm, preferably from 10 to 120 pm.

[0068] The term "adsorbent solid" refers to a solid that adsorbs substances dissolved in phosphoric acid.

[0069] The adsorbent solid may be selected from the group consisting of phosphogypsum, activated carbon, zeolites and mixtures thereof. Preferably, the adsorbent solid is phosphogypsum.

[0070] The use of an adsorbent solid to carry out ionic flotation allows the formation of complexes with the ionizable surfactant and the impurities in the form of foam and thus promotes the buoyancy of the impurities in the presence of gas bubbles. The particular particle size of the adsorbent agent ensures flotation. It allows the particles to be transported by the gas bubbles to the surface, facilitating their recovery in the foams.

[0071] The use of an adsorbent solid also makes it possible to limit the quantity of ionizable surfactant required to remove impurities while limiting losses of phosphoric acid which can be carried along with the impurities in the form of foam.

[0072] In some embodiments, the phosphoric acid solution is mixed with 0.1 to 5%, preferably 0.5 to 4%, or 1 to 3%, by weight of adsorbent solid relative to the total weight of P2O5 in the phosphoric acid solution.

[0073] In some embodiments, the adsorbent solid has a particle size, at more than 90% by volume, ranging from 5 to 160 μm, typically ranging from 10 to 120 μm. The particle size is typically chosen so as to allow the solid to be transported by air bubbles. The particle size is commonly determined by laser diffraction particle size analysis, for example using the Malvern Mastersizer 2000 particle size analyzer (wet method). Optionally, the phosphoric acid solution may be mixed with one or more other conventional agents, such as a foaming agent, a reducing agent and mixtures thereof. These agents are intended to facilitate the formation of foams and to improve the treatment of heavy metals by ion flotation.

[0074] The use of a reducing agent limits the amount of ionizable surfactant. The reducing agent is typically selected from the group consisting of iron powder, red phosphorus, iron(II) sulfate, iodine, hydrazine, and mixtures thereof.

[0075] When the phosphoric acid solution is mixed first with the adsorbent solid and then with the ionizable surfactant, the reducing agent is typically added at the conditioning stage.

[0076] The amount of reducing agent used is determined based on the ionic charge of the phosphoric acid solution according to the general knowledge of a person skilled in the art. Ionic charge typically refers to the charge of ionic species including the metallic elements to be removed from the phosphoric acid solution. In some embodiments, the phosphoric acid solution is mixed with 0.08 to 0.7%, preferably 0.1 to 0.5%, by weight of reducing agent relative to the weight of P2O5 in the phosphoric acid solution.

[0077] Foaming agents facilitate the formation of foams containing heavy metals to be removed. The foaming agent may be, for example, methyl isobutyl ketone commonly abbreviated as MIBC, polypropylene glycol ether, alkyl polypropylene glycol ether, alkyl polyglycol ether, aliphatic alcohols, cyclic aliphatic alcohols (pine oils) or any other similar agent.

[0078] Foaming agents are typically added with or after the ionizable surfactant.

[0079] The amount of foaming agent used is determined based on the foaming character of the phosphoric acid solution according to the general knowledge of a person skilled in the art. In some embodiments, the phosphoric acid solution is mixed with 0.02 to 2% by weight of foaming agent relative to the total weight of P2O5 in the phosphoric acid solution. Typically, the phosphoric acid solution is mixed with 0.06 to 1.5% by weight or 0.1 to 1% by weight or 0.3 to 0.5% by weight of foaming agent relative to the total weight of P2O5 in the phosphoric acid solution.

[0080] Step (ii) of gas injection allows the foams comprising the heavy metals to be removed, the ionizable surfactant and the adsorbent agent to float to the surface of the phosphoric acid solution. To do this, the gas is injected so as to form homogeneous gas bubbles which, after adsorption with the foams, will transport the foams by flotation to the surface of the activation solution.

[0081] The surface of the phosphoric acid solution is defined according to the direction of acceleration of Earth's weightlessness. It corresponds to the upper part, typically the top, of the device in which the ion flotation treatment process is carried out.

[0082] The gas bubbles can be formed by any means known to those skilled in the art, for example by a porous base, sintered glass or by one or more injection nozzles.

[0083] The gas injected in step (ii) may be air, nitrogen, or any other gas inert to the species present.

[0084] The gas injection in step (ii) can be carried out at a constant flow rate, typically at a flow rate ranging from 200 to 400 ml / min or from 300 to 600 ml / min or from 250 to 800 ml / min. The flow rate is chosen according to the concentration of phosphoric acid in P2O5, the mass of phosphoric acid to be treated and the volume of the ion flotation column used.

[0085] Step (ii) is carried out with stirring in order to have a homogeneous distribution of the gas bubbles in the phosphoric acid solution. Stirring can be ensured by any means known to those skilled in the art, such as for example mechanical stirring such as a rotor or magnetic stirring. Stirring can for example be carried out at a speed ranging from 100 to 120 rpm (rotations per minute).

[0086] Generally step (ii) can be performed for 5 seconds to 30 minutes, typically 5 seconds to 5 minutes.

[0087] The treatment process comprises a step (iii) of separating the foams containing heavy metals from the treated phosphoric acid solution. The heavy metals removed by the process according to the invention are generally cadmium, copper, arsenic, lead, nickel, chromium and zinc.

[0088] The recovered foams further comprise the ionizable surfactant, the adsorbent agent and possibly the other optional compounds that can be mixed in step (i). The recovered foams can optionally be reprocessed in order to separate and recover the extracted heavy metals.

[0089] Generally, the treated phosphoric acid solution obtained at the end of step (iii) is recovered once the foams have been separated.

[0090] Typically, the foams are recovered in step (iii) from the upper part of the treated phosphoric acid solution by any means known to those skilled in the art. For example, the flotation foams may be discharged into a recovery tank.

[0091] Typically, the method according to the invention makes it possible to obtain a high recovery rate. The recovery rate designates the ratio between the mass of the treated phosphoric acid solution and the initial mass of the phosphoric acid solution. Thus, the recovery rate is greater than or equal to 90% by weight, typically ranging from 90 to 99% by weight of the initial mass of the phosphoric acid solution.

[0092] The mass ratio of recovered foams / treated phosphoric acid solution obtained at the end of step (iii) may be less than or equal to 3% by weight, typically less than or equal to 2% by weight, or less than or equal to 1% by weight, or less than or equal to 0.5% or even greater than or equal to 0.1% by weight.

[0093] Typically, the phosphoric acid solution separated in step (iii) has a heavy metal content reduced by at least 90% or at least 99% by weight relative to the amount initially present in the phosphoric acid solution.

[0094] Thus, the phosphoric acid solution separated in step (iii) typically comprises less than 1.3 ppm cadmium or 0.01 to 1 ppm cadmium or 0.06 to 0.8 ppm cadmium or 0.14 to 0.34 ppm cadmium.

[0095] More particularly, the phosphoric acid solution separated in step (iii) has an arsenic content reduced by at least 93% or at least 99% by weight relative to the initial amount of arsenic in the phosphoric acid solution. Thus, the phosphoric acid solution separated in step (iii) typically comprises less than 0.65 ppm of arsenic, or from 0 to 0.44 ppm of arsenic or from 0.02 to 0.28 ppm of arsenic or from 0.07 to 0.16 ppm of arsenic.

[0096] The reduction of heavy metal content, especially cadmium and arsenic, can also be referred to as heavy metal yield in foams. The values ​​defined above therefore apply to define heavy metal yields in foams and in particular arsenic and cadmium yields.

[0097] Advantageously, the treatment method according to the invention does not comprise a subsequent step of post-treatment of the phosphoric acid solution. In other words, the treatment method according to the invention comprises, and typically consists of, steps (i), (ii) and (iii) as described above.

[0098] The figures illustrate in a non-limiting manner devices capable of implementing the ion flotation treatment method according to the invention.

[0099] In certain embodiments, the method according to the invention is implemented in a flotation device, such as a flotation column combined with a froth recovery tank in the upper part of the column, as shown schematically in Figure 1.

[0100] Part I, called the treatment part, comprises the flotation column which consists of a glass column 1 filled with the phosphate ore pulp conditioned with the flotation collector according to the invention. The gas is introduced into the bottom of the column, the gas bubbles are formed by the passage of the gas through the sintered glass 2. The gas is generated by a gas generator 3 and its flow rate is controlled by a flow meter 4. The pulp is stirred by a magnetic bar 5 with a magnetic stirrer 6 which makes it possible to obtain a good distribution of the gas bubbles 7. The foams 8 are formed on contact with the gas bubbles. The foams are then entrained at the top of the column in a foam discharge zone 9 corresponding to part II, called the separation part. The foams 8 then flow into a foam recovery tank 10.

[0101] In certain embodiments, the treatment method according to the invention can be implemented in a device such as a flotation column shown schematically in Figure 2.

[0102] The ion flotation column of Figure 2 consists of a glass column 11 equipped with sampling and / or feed valves 12. The gas is introduced via a gas inlet 15, the gas bubbles 13 are formed by the passage of the gas through the sintered glass 14. The gas is generated by a gas generator 17 and its flow rate is controlled by a flow meter 16. The foams 18 and 20 are formed on contact with the gas bubbles and are entrained in the upper part of the column corresponding to the foam discharge zone 19. The foams 18 and 20 then flow into a foam recovery tank 21.

[0103] When the method of the present invention is implemented in a flotation column as illustrated in Figure 1 or 2, the method typically comprises firstly a conditioning step (addition of an adsorbent solid to the phosphoric acid solution previously placed in the flotation column) then a step of adding a surfactant to the mixture obtained at the end of the conditioning step. Then, a gas is injected into the column so as to carry out the ionic flotation.

[0104] The residence time in the flotation device is generally less than 30 minutes, preferably between 5 seconds and 5 minutes.

[0105] EXAMPLES

[0106] The following non-restrictive examples illustrate exemplary embodiments of the invention.

[0107] Adsorbent agent = Phosphogypsum; it is a by-product of wet phosphoric acid manufacturing.

[0108] Ionizable surfactant = sodium dithiophosphinate having the chemical formula (C4Hg)2PS2Na, according to formula (II) described above.

[0109] The quantities of ionizable surfactant are expressed in g / kg of P2O5, which corresponds to the quantity in grams of ionizable surfactant per kilogram of P2O5 of the phosphoric acid solution.

[0110] Example 1: Treatment of phosphoric acid 29% P2O5

[0111] 184g of wet process phosphoric acid at 29% P2O5, are placed in an ionic flotation column as shown in Figure 1. The adsorbent agent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution and then the ionizable surfactant (2.4 and 6 g / kg of P2O5) is added. The mixture is stirred at 120rpm. After a few seconds under these conditions, air is introduced in the form of bubbles into the flotation column at a constant flow rate of 300ml / min. Stirring is maintained constant during all stages of the flotation process.

[0112] After a few seconds of injecting air bubbles, the flotation process is complete. The heavy metals are then recovered as foams in the foam recovery tank, and the phosphoric acid solution is treated in the ion flotation column.

[0113] Table 1 below summarizes the heavy metal separation results based on the amount of ionizable surfactant used.

[0114] R% (Cd, As, Cu) denotes the mass recovery yield of cadmium, arsenic or copper in the foams relative to the initial quantities of the phosphoric acid solution to be treated.

[0115] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 29% P2O5.

[0116] Example lb: Treatment of 29% phosphoric acid

[0117] The protocol of example 1a is reproduced on a quantity of 11 kg of wet process phosphoric acid at 29% P2O5. 11 kg of wet process phosphoric acid at 29% P2O5 are placed in an ionic flotation column adapted to the quantity of phosphoric acid to be treated as shown in figure 2.

[0118] The adsorbent agent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution and then the ionizable surfactant (4g / kg of P2O5) is added. The mixture is stirred at 120rpm. After a few seconds under these conditions, air is introduced in the form of bubbles into the flotation column at a constant flow rate of 800ml / min. Agitation is maintained constant during all stages of the flotation process.

[0119] After a few seconds of injecting air bubbles, the flotation process is complete. The heavy metals are then recovered as froth in the froth collection tank, and the phosphoric acid solution is treated in the flotation column.

[0120] Table 2 below summarizes the heavy metal separation results based on the amount of ionizable surfactant used.

[0121] Table 2: Results of phosphoric acid treatment analyses 29% P2O5

[0122] R% (Cd, As, Cu) denotes the mass recovery yield of cadmium, arsenic or copper in the foams relative to the initial quantities of the phosphoric acid solution to be treated.

[0123] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 29% P2O5.

[0124] This example shows that the treatment method according to the present invention makes it possible to treat large quantities of phosphoric acid solution while maintaining excellent recovery yields of heavy metals, in particular cadmium, arsenic and copper.

[0125] Example 2: Treatment of 54% phosphoric acid on a laboratory scale

[0126] 243g of wet process phosphoric acid at 54% P2O5, are placed in an ionic flotation column as shown in Figure 1.

[0127] The adsorbent agent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution while stirring at 120 rpm, then the ionizable surfactant (at 2 and 6 g / kg of P2O5) is added. The mixture is stirred at 120 rpm. After a few seconds under these conditions, air is introduced in the form of bubbles into the flotation column at a constant flow rate of 600 ml / min. Agitation is maintained constant during all stages of the flotation process. After a few seconds of air bubble injection, the flotation process is completed. The heavy metals are then recovered in the form of foams in the foam recovery tank, and the phosphoric acid solution is treated in the ionic flotation column.

[0128] Table 3 below summarizes the heavy metal separation results based on the amount of ionizable surfactant used.

[0129] Table 3: Results of phosphoric acid treatment analyses 54%

[0130] R% (Cd, As, Cu) denotes the recovery efficiency of cadmium, arsenic or copper in foams.

[0131] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 54% P2O5. This example illustrates that the treatment method according to the present invention makes it possible to treat phosphoric acid solutions having a high P2O5 concentration while maintaining excellent recovery yields of heavy metals, in particular cadmium, arsenic and copper.

Claims

CLAIMS 1. A method of treating a phosphoric acid solution containing heavy metals by ionic flotation, the method comprising the following steps: (i) preparation of a mixture comprising a phosphoric acid solution, an ionizable surfactant and an adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 pm as determined by laser diffraction particle size analysis, (ii) injecting gas upwards into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid, (iii) separation of the phosphoric acid solution and the foams containing heavy metals.

2. Treatment method according to claim 1 in which step (i) is carried out by mixing the phosphoric acid solution, the ionizable surfactant and the adsorbent solid having at more than 90% by volume a particle size ranging from 5 to 160 μm.

3. Treatment method according to claim 2, in which the phosphoric acid solution is mixed firstly with the adsorbent solid and then secondly with the ionizable surfactant.

4. Treatment method according to any one of claims 1 to 3, in which the phosphoric acid solution has one or more of the following characteristics: - a phosphoric acid concentration ranging from 5 to 65% by weight of P2O5, - a cadmium content ranging from 1 to 100 ppm, - an arsenic content of less than or equal to 50 ppm, - a solids content less than or equal to 3% by weight.

5. Treatment method according to any one of claims 2 to 4, wherein the phosphoric acid solution is mixed in step (i) with 0.1 to 5% by weight of adsorbent solid relative to the total weight of P2O5 in the phosphoric acid solution.

6. Treatment method according to any one of claims 1 to 5, in which the adsorbent solid has a particle size, at more than 90% by volume, ranging from 10 to 120 μm.

7. Treatment method according to any one of claims 1 to 6, in which the adsorbent solid is selected from the group consisting of phosphogypsum, activated carbon, zeolites and mixtures thereof.

8. A treatment method according to any one of claims 1 to 7, wherein the ionizable surfactant is selected from the group consisting of dithiophosphate derivatives, dithiophosphinate derivatives, xanthate derivatives and mixtures thereof.

9. A treatment method according to claim 8, wherein the ionizable surfactant is selected from the group consisting of compounds of formula (I), compounds of formula (II) and compounds of formula (III): Formula (I) Dithiophosphinate (II) Formula (III) Xanthate (III) in which R1, R2 and R3 are, independently, linear or branched alkyl groups comprising from 1 to 30 carbon atoms.

10. A treatment method according to any one of claims 1 to 9, wherein the phosphoric acid solution is mixed in step (i) with 0.1 to 5% by weight of ionizable surfactant relative to the total weight of P2O5 in the phosphoric acid solution.

11. A treatment method according to any one of claims 1 to 10, wherein the phosphoric acid solution is further mixed with a foaming agent, preferably selected from the group consisting of methyl isobutyl ketone, a reducing agent and mixtures thereof.

12. A treatment method according to claim 11, wherein the phosphoric acid solution is mixed with a reducing agent, the reducing agent being selected from the group consisting of iron powder, red phosphorus, iron (II) sulfate, iodine, hydrazine, and mixtures thereof.

13. A treatment method according to any one of claims 11 or 12, wherein the phosphoric acid solution is mixed with 0.08 to 0.7% by weight of reducing agent relative to the weight of P2O5 in the phosphoric acid solution.

14. Treatment method according to any one of claims 1 to 13, carried out at a temperature ranging from 20 to 80°C.

15. Treatment method according to any one of claims 1 to 14, in which the gas injected in step (ii) is air, nitrogen, or any other inert gas.

16. Treatment method according to any one of claims 1 to 15, in which the injection of gas in step (ii) is carried out at a constant flow rate.

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