Process 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. Abstract figure: Figure 2

Description

Title of the invention: Process for treating phosphoric acid by ionic flotation TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a process 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 qm. STATE OF THE ART

[0002] 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. Phosphoric acid from the wet process is obtained by attacking natural phosphate with a strong acid such as hydrochloric acid, nitric acid and / or sulfuric acid.

[0003] Industrial phosphoric acid is notably a precursor in the manufacture of phosphate fertilizers. However, it contains elements that can pollute soils or groundwater, such as cadmium (Cd), copper (Cu), arsenic (As), zinc (Zn), lead (Pb), nickel (Ni) or chromium (Cr).

[0004] Cadmium levels in fertilizers are closely monitored 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.

[0005] Different processes have been developed to remove heavy metals, such as precipitation, solvent extraction, adsorption, ion exchange, cocrystallization, membrane techniques or ion flotation.

[0006] For example, patent applications EP0099804 and EP0203076 describe a process for treating phosphoric acid by ionic flotation. This process consists of reducing the Cd, As and U contents contained in a phosphoric acid solution by adding a dithiophosphate. Although this process has significant removal rates of 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.

[0007] 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.

[0008] 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 that it is necessary to have in addition a post-treatment step.

[0009] Thus, the methods of the prior art 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.

[0010] There is, therefore, a need for new methods to facilitate the recovery of heavy metals from a phosphoric acid solution that can be carried out on a large scale at moderate cost. Summary of the invention

[0011] 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:

[0012] (i) mixing the phosphoric acid solution, an ionizable surfactant and an adsorbent solid having a particle size ranging from 5 to 160 qm at more than 90% by volume,

[0013] (ii) injecting gas upwardly into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid,

[0014] (iii) separation of the foams containing the heavy metals and the phosphoric acid solution.

[0015] Other aspects of the invention are as described below and in the claims. FIGURES

[0016] [Fig.l]: Example of a laboratory-scale flotation column

[0017] [Fig.2]: Example of a flotation column on pilot scale DETAILED DESCRIPTION OF THE INVENTION

[0018] The inventors have developed a process meeting the expressed needs. The process for treating a phosphoric acid solution, typically derived from the wet process, for the elimination of heavy metals by ionic flotation proposed does not have the disadvantages of the prior art. It includes a step of mixing a small quantity of adsorbent solid leading to the improvement of the recovery yield of phosphoric acid while limiting the quantities of reagent used.

[0019] The various embodiments presented throughout the description may be used alone or in combination with each other, without limitation of combination.

[0020] 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:

[0021] (i) mixing the phosphoric acid solution with an ionizable surfactant and an adsorbent solid having a particle size ranging from 5 to 160 qm at more than 90% by volume,

[0022] (ii) injecting gas upwardly into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid,

[0023] (iii) separation of the phosphoric acid solution and the foams containing the heavy metals.

[0024] 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. 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.

[0025] 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.

[0026] Another advantage of the process according to the invention is to limit losses of phosphoric acid.

[0027] Advantageously, the treatment method does not require any prior treatment or subsequent post-treatment.

[0028] The advantages of the process according to the invention make it particularly applicable to the treatment of phosphoric acid solutions on an industrial scale.

[0029] 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 implementation of steps (i) and (ii) is identical. Step (i) of mixing

[0030] The process according to the invention therefore 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 qm.

[0031] Step (i) may be carried out with stirring so as to promote the formation of foams. Generally, the mixing step lasts from 5 seconds to 15 minutes or from 1 to 5 minutes.

[0032] In certain 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 hereinafter referred to as the “conditioning step”.

[0033] In certain embodiments, the phosphoric acid solution is mixed firstly with the ionizable surfactant and then secondly with the adsorbent solid.

[0034] In some embodiments, the phosphoric acid solution is mixed simultaneously with the ionizable surfactant and the adsorbent solid.

[0035] Advantageously, the reduction of the contact time between the element to be floated (metal ions) and the collector (ionizable surfactant) before the implementation of the flotation step makes it possible to limit, or even to overcome, the problems of release which designates the separation between the metal ions in solution and the collector. Phosphoric acid solution

[0036] 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.

[0037] Generally, the phosphoric acid solution has from 0.1 to 6% by weight of impurities relative to the weight of P2O5.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] Alternatively, a phosphoric acid solution having a solids content greater than 3% by weight, typically between 3 and 5% by weight, can also be treated in the process according to the invention. Ionizable surfactant

[0044] In the process of the present invention, the phosphoric acid solution is mixed with an ionizable surfactant.

[0045] The term “ionizable surfactant” agent designates a surfactant having at least one apolar hydrocarbon chain and a polar head, typically a negatively charged polar head.

[0046] The ionizable surfactant acts as a “collector”, that is to say that it will form a complex with the heavy metals. This complex is then adsorbed onto the adsorbent solid used in the process of the present invention.

[0047] The ionizable surfactant may be selected from the group consisting of dithiophosphate derivatives, dithiophosphinate derivatives, xanthate derivatives and mixtures thereof.

[0048] Typically, the ionizable surfactant is selected from the group consisting of compounds of formula (I), compounds of formula (II) and compounds of formula (III):

[0049] [Chem.l]

[0050] Dithiophosphate (I)

[0051] [Chem.2]

[0052] Dithiophosphinate (II)

[0053] [Chem.3] S

[0054] Xanthate (III)

[0055] in which Rb 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.

[0056] 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 ionizable surfactant relative to the mass quantity of P2O5 in the phosphoric acid solution. Adsorbent solid

[0057] 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 qm, preferably from 10 to 120 qm.

[0058] The term "adsorbent solid" designates a solid which adsorbs substances dissolved in phosphoric acid.

[0059] The adsorbent solid may be selected from the group consisting of phosphogypsum, activated carbon, zeolites and mixtures thereof. Preferably, the adsorbent solid is phosphogypsum.

[0060] The use of an adsorbent solid to carry out ionic flotation makes it possible to form complexes with the ionizable surfactant and the impurities in the form of foam and thus promote the buoyancy of the impurities in the presence of gas bubbles.

[0061] The use of an adsorbent solid also makes it possible to limit the quantity of ionizable surfactant necessary to eliminate the impurities while limiting the losses of phosphoric acid which can be carried along with the impurities in the form of foam.

[0062] In certain 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 mass quantity of P2O5 in the phosphoric acid solution.

[0063] In some embodiments, the adsorbent solid has a particle size of more than 90% by volume, ranging from 5 to 160 pm, typically ranging from 10 to 120 pm. The particle size is typically chosen to allow the solid to be transported by air bubbles. The particle size is commonly determined by laser diffraction particle size analysis.

[0064] 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.

[0065] The use of a reducing agent makes it possible to limit 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.

[0066] When the phosphoric acid solution is mixed firstly with the adsorbent solid and then secondly with the ionizable surfactant, the reducing agent is typically added at the conditioning stage.

[0067] 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. The ionic charge typically refers to the charge of ionic species including the metallic elements to be removed from the phosphoric acid solution. In certain 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.

[0068] Foaming agents facilitate the formation of foams comprising the heavy metals to be removed. The foaming agent may be, for example, methyl isobutyl ketone commonly abbreviated MIBC, polypropylene glycol ether, alkyl polypropylene glycol ether, alkyl polyglycol ether, aliphatic alcohols, cyclic aliphatic alcohols (pine oils) or any other similar agent.

[0069] Foaming agents are typically added with or after the ionizable surfactant is added.

[0070] 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 certain embodiments, the phosphoric acid solution is mixed with 0.02 to 2% by weight of foaming agent relative to the mass quantity 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 relative to the mass quantity of P2O5 in the phosphoric acid solution. Step (ii) gas injection

[0071] The gas injection step (ü) makes it possible to float towards the surface of the phosphoric acid solution, the foams comprising the heavy metals to be eliminated, the ionizable surfactant and the adsorbent agent. 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.

[0072] 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 implemented.

[0073] The gas bubbles can be formed by any means known to those skilled in the art, for example by a porous bottom, sintered glass or even by one or more injection nozzles.

[0074] The gas injected in step (ii) may be air, nitrogen, or any other gas inert with respect to the species present.

[0075] The injection of gas 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 depending on the concentration of phosphoric acid in P2O5, the mass of phosphoric acid to be treated and the volume of the ion flotation column used.

[0076] 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).

[0077] Generally step (ii) can be carried out for from 5 seconds to 30 minutes, typically from 5 seconds to 5 minutes. Step (iii) of separation

[0078] The treatment method comprises a step (iii) of separating the foams containing the heavy metals from the treated phosphoric acid solution.

[0079] The heavy metals removed by the process according to the invention are generally cadmium, copper, arsenic, lead, nickel, chromium and zinc.

[0080] The recovered foams further comprise the ionizable surfactant, the adsorbent agent and optionally 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.

[0081] Generally, the treated phosphoric acid solution obtained at the end of step (iii) is recovered once the foams have been separated.

[0082] Commonly, the foams are recovered in step (iii) in the upper part of the treated phosphoric acid solution by any means known to those skilled in the art. For example, flotation froths can be discharged into a recovery tank.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] Thus, the phosphoric acid solution separated in step (iii) typically comprises less than 1.3 ppm of cadmium or from 0.01 to 1 ppm of cadmium or from 0.06 to 0.8 ppm of cadmium or from 0.14 to 0.34 ppm of cadmium.

[0087] 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.

[0088] 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.

[0089] The reduction of heavy metal content, in particular 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.

[0090] 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, typically consists of, steps (i), (ii) and (iii) as described above.

[0091] The figures illustrate in a non-limiting manner devices capable of implementing the ion flotation treatment method according to the invention.

[0092] 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 diagrammatically in [Fig.l].

[0093] Part I, called the treatment part, includes the flotation column which is composed 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.

[0094] In certain embodiments, the treatment method according to the invention can be implemented in a device such as a flotation column shown diagrammatically in [Fig.2],

[0095] The ion flotation column of [Fig.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.

[0096] When the process of the present invention is implemented in a flotation column as illustrated in [Fig.l] or 2, the process typically comprises, firstly, a conditioning step (addition of an adsorbent solid to the phosphoric acid solution previously placed in the flotation column) and 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.

[0097] The residence time in the flotation device is generally less than 30 minutes, preferably between 5 seconds and 5 minutes. EXAMPLES

[0098] The following non-restrictive examples illustrate examples of embodiments of the invention.

[0099] Adsorbent agent = Phosphogypsum; it is a by-product of wet phosphoric acid manufacturing.

[0100] Ionizable surfactant = sodium dithiophosphinate having the crude formula (C4H9)2PS2Na, according to the formula (II) described above.

[0101] 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 P2 O5 of the phosphoric acid solution.

[0102] Example 1a: Treatment of phosphoric acid 29% P2Os

[0103] 184g of wet process phosphoric acid at 29% P2O5, are placed in a ion flotation column as shown in [Fig.l].

[0104] 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 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 300 ml / min. Stirring is kept constant during all stages of the flotation process.

[0105] After a few seconds of injecting air bubbles, the flotation process is complete. 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 ion flotation column.

[0106] Table 1 below summarizes the heavy metal separation results as a function of the amount of ionizable surfactant used.

[0107] [Tableauxl] Test Quantity of ionizable surfactant (g / kg) R%(Cd) R%(As) R%(Cu) 1 2 91.10 90.87 96.41 2 4 99.07 95.22 99.99 3 6 97.09 99.99 99.97

[0108] Table 1: Results of phosphoric acid treatment analyses 29% P2O5

[0109] 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.

[0110] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 29% P2O5 [YES] Example 1b: Treatment of phosphoric acid 29% P2Os

[0112] The protocol of example 1a is reproduced on a quantity of 11 kg of wet process phosphoric acid at 29% P2O5.

[0113] 1 IKg of wet process phosphoric acid at 29% P2O5, are placed in a ion flotation column adapted to the quantity of phosphoric acid to be treated as shown in [Fig.2].

[0114] 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. Stirring is kept constant during all stages of the flotation process.

[0115] After a few seconds of injecting air bubbles, the flotation process is complete. 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 flotation column.

[0116] Table 2, below, summarizes the results of separation of heavy metals as a function of the quantity of ionizable surfactant used.

[0117] [Tables2] Test Quantity of ionizable surfactant (g / kg) R%(C d) R%(As) R%(Cu) R%(V) R%(Zn) R%(U) 4 4 99.67 95.82 97.56 57.42 41.28 54.25

[0118] Table 2: Results of phosphoric acid treatment analyses 29% P2O 5

[0119] 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.

[0120] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 29% P2O5

[0121] 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.

[0122] Example 2: Treatment of phosphoric acid 54% P2O^ on a laboratory scale

[0123] 243g of wet process phosphoric acid at 54% P2O5, are placed in a ion flotation column as shown in [Fig.l].

[0124] The adsorbent agent (1% by weight relative to the amount of P2O5 in the phosphoric acid solution) is added to the phosphoric acid solution with stirring at 120 rpm and 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. Stirring is kept constant during all stages of the flotation process.

[0125] After a few seconds of injecting air bubbles, the flotation process is complete. 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 ion flotation column.

[0126] Table 3, below, summarizes the results of separation of heavy metals as a function of the quantity of ionizable surfactant used.

[0127] [Tables3] Test Quantity of ionizable surfactant (g / kg) R%(Cd ) R%(As ) R%(Cr) R%(Zn) R%(Ni) R%(Cu ) 4 2 97.02 95.93 52.4 50.09 55.4 99.99 5 6 99.01 98.99 53.62 54.64 59.24 99.99

[0128] Table 3: Results of phosphoric acid treatment analyses 54%

[0129] R% (Cd, As, Cu) denotes the recovery yield of cadmium, arsenic or copper in the foams.

[0130] The phosphoric acid solution is recovered with a recovery rate of 99%. The P2O5 content is identical and is 54% P2O5

[0131] 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) mixing the 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 qm as determined by laser diffraction, (ii) injecting gas upwardly into the mixture from step (i) with stirring to form foams comprising the heavy metals, the surfactant and the adsorbent solid, (iii) separating the foams containing the heavy metals and the phosphoric acid solution.

2. Treatment method according to claim 1, in which the phosphoric acid solution is mixed firstly with the adsorbent solid and then secondly with the ionizable surfactant.

3. Treatment method according to any one of claims 1 or 2, 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 less than or equal to 50 ppm, - a solid content less than or equal to 3% by weight.

4. A treatment method according to any one of claims 1 to 3, wherein the phosphoric acid solution is mixed in step (i) with 0.1 to 5% by weight of adsorbent solid relative to the mass quantity of P2O5 in the phosphoric acid solution.

5. Treatment method according to any one of claims 1 to 4, in which the adsorbent solid has a particle size, at more than 90% by volume, ranging from 10 to 120 qm.

6. A treatment method according to any one of claims 1 to 5, wherein the adsorbent solid is selected from the group consisting of phosphogypsum, activated carbon, zeolites and mixtures thereof.

7. A treatment method according to any one of claims 1 to 6, wherein the ionizable surfactant is selected from the group consisting of dithiophosphate derivatives, dithiophos- derivatives phinates, xanthate derivatives and their mixtures.

8. A treatment method according to claim 7, 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) R'— P \ e Dithiophosphate (I) Formula (II) e r2-----s Dithiophosphinate (II) Formula (III) © S Et „ --------- - 3 -¼. w Xanthate (III) in which Rh R2 and R3 are, independently, linear or branched alkyl groups comprising from 1 to 30 carbon atoms.

9. A treatment method according to any one of claims 1 to 8, wherein the phosphoric acid solution is mixed in step (i) with 0.1 to 5% by weight of ionizable surfactant relative to the mass quantity of P2O5 in the phosphoric acid solution.

10. A treatment method according to any one of claims 1 to 9, wherein the phosphoric acid solution is further mixed with a foaming agent such as methyl isobutyl ketone, a reducing agent and mixtures thereof.

11. The treatment method of claim 10, wherein the reducing agent is selected from the group consisting of iron powder, red phosphorus, iron (II) sulfate, iodine, hydrazine, and mixtures thereof.

12. A treatment method according to any one of claims 10 or 11, 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

13.

14.

15. phosphoric acid solution. Treatment method according to any one of claims 1 to 12, carried out at a temperature ranging from 20 to 80°C. Treatment method according to any one of claims 1 to 13, in which the gas injected in step (ii) is air, nitrogen, or any other gas inert with respect to the species present. Treatment method according to any one of claims 1 to 14, wherein the injection of gas in step (ii) is carried out at a constant flow rate.

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