METHOD FOR RECYCLING RESIDUAL SOLUTIONS COMPRISING PHOSPHORUS AND DEVICE FOR SUCH A METHOD

Description

FIELD OF INVENTION

[0001] The present invention relates to a method and device for purifying residual phosphoric acid solutions, such as those from industrial or agro-industrial processes, containing phosphorus in the form of orthophosphate and / or polyphosphate species, as well as undesirable volatile matter that makes their subsequent use difficult or even impossible. The method of the present invention is particularly effective and allows for the removal of undesirable volatile matter from residual phosphoric acid solutions and, where applicable, the production of phosphoric acid solutions at high concentrations while minimizing the energy consumption required to produce purified phosphoric or polyphosphoric acid solutions. The present invention also allows for improved management of the combustion gases produced by the process before their release into the atmosphere. TECHNOLOGICAL BACKGROUND

[0002] Many industrial and agro-industrial processes generate aqueous waste solutions containing phosphorus. These solutions are classified as waste because they cannot be used without prior treatment in these same industries. The phosphorus in these solutions may be present as orthophosphate or polyphosphate species, depending on the P₂O₅ content. Due to the presence of undesirable dissolved volatiles such as fluorine, sulfur, carbon, etc., these waste solutions generally cannot be recycled in processes for producing higher value-added materials, such as purified phosphoric acid or polyphosphoric acid (PPA) solutions, nor in the production of phosphate salts. For this reason, these volatiles are referred to hereafter as " undesirable volatile matterTo date, these high-potential residual solutions are treated as simple waste with very low value and high polluting power.

[0003] Polyphosphoric acid (PPA) is a viscous liquid that can be produced, notably from phosphoric acid. PPA has the general formula HO[P(OH)(O)O]nH, with n > 1. When n = 2, PPA is commonly called pyrophosphoric acid; when n = 3, it is called tripolyphosphoric acid. For n > 3, it is simply called polyphosphoric acid, regardless of the value of n. PPA can be produced by the dehydration and polycondensation of orthophosphoric acid, H3PO4, according to equation (1). This yields an aqueous solution of polyphosphoric acid whose molecular species distribution depends, among other things, on the polycondensation temperature, Tpc.

[0004] Polyphosphoric acid most often occurs as linear chains. However, cyclic, metaphosphoric, or branched forms can also exist. As illustrated in the Figure 1 , the polycondensation temperature determines the equivalent phosphoric acid concentration in P 2 O 5 units at equilibrium in the liquid state ( Figure 1(a) ) and the latter determines the distribution of molecular species (i.e., the distribution of phosphoric acid in different forms with different n values ​​( Figure 1(c) Thus, an aqueous solution of phosphoric acid with an equivalent concentration of less than approximately 61% in P₂O₅ units will consist predominantly of H₃PO₄ molecules. As the equivalent concentration of P₂O₅ units increases, this indicates that the solution contains more and more polymerized molecules, with the value of n increasing with the concentration of equivalent P₂O₅ units as indicated on the graph. Figure 1(c) .

[0005] The dehydration and polycondensation of a phosphoric acid solution into polyphosphoric acid requires the evaporation of water molecules, which necessitates the input of heat energy. Patent EP2411325 B1 reviews several known processes for the production of polyphosphoric acid and describes a novel wet process, compared to the reviewed processes, that offers high energy efficiency and drastically reduces environmental impact. This patent describes a device resistant to the very harsh operating conditions of polyphosphoric acid production, thus limiting maintenance costs, ensuring equipment durability, and guaranteeing the production of high-quality polyphosphoric acid without contamination during the manufacturing process.

[0006] It would be advantageous to produce a high-value material such as purified phosphoric acid or polyphosphoric acid from low-value waste solutions. However, the use of such waste solutions in a process for producing purified phosphoric acid or PPA as described in EP2411325 B1 is not possible due to the presence of undesirable volatile compounds in these solutions.

[0007] JP2000178014 describes a process for recovering phosphoric acid from recovery solutions, in which a recovery solution containing phosphorus molecules is incinerated at a temperature of 900 to 1000°C. The combustion gases containing phosphorus molecules are cooled in a cooler and the phosphorus molecules are recovered as phosphoric acid.

[0008] The present invention proposes a more efficient process than that described in JP2000178014 for the production of purified phosphoric acid or PPA from residual solutions containing phosphorus from generally industrial processes, which, until now, have simply been treated as waste. The present invention and its advantages are described in more detail in the following sections. SUMMARY OF THE INVENTION

[0009] The present invention is described in the attached independent claims. Preferred variants are defined in the dependent claims. In particular, the present invention relates to a process for purifying an aqueous waste solution containing phosphorus molecules and undesirable volatile matter, comprising the following steps: (a) introduce into a gas-acid contactor a supply stream F0 of a feed solution P0 which is aqueous and comprises phosphorus molecules preferably in the form of orthophosphate-type species at a mass concentration, xp0, of between 0 and 54% equivalent in P2O5 units, (b) introduce into the gas-acid contactor a recirculation stream F2 of recirculated enriched phosphoric acid solution P2, (c) introduce into the gas-acid contactor combustion gases G1, (d) contact the supply streams F0 and F2 and the combustion gases G1 to form in the gas-acid contactor, on the one hand, an enriched phosphoric acid solution P1 comprising a mass concentration, xp1, of P2O5 content which is greater than xp0 (xp1 > xp0) and, on the other hand, contacted combustion gases G3, (e) separate the contacted combustion gases G3 of the enriched phosphoric acid solution P1,then vent the contacted combustion gases G3 from the gas-acid contactor, and remove the enriched phosphoric acid solution P1 from the gas-acid contactor (1), (f) form from said enriched phosphoric acid solution P1, on the one hand, a recirculating stream F2 of recirculated enriched phosphoric acid solution P2 to introduce it into the gas-acid contactor (1) as defined in step (b) and, on the other hand, a spray stream Fp of the enriched phosphoric acid solution P1 to introduce it into a combustion chamber, (g) spray through a burning flame in the upper part of the combustion chamber a mixing stream Fm of a mixing solution Pm comprising phosphorus at a mass concentration, xpm, and undesirable volatilizable materials, the mixing stream being formed by, on the one hand, the enriched phosphoric acid solution P1 and optionally, on the other hand,a residual stream Fr of an aqueous residual solution Pr comprising a mass concentration, xpr, of at least 1% P2O5, to: evaporate water and thus concentrate the mixture solution Pm, possibly oxidize and in all cases evaporate undesirable volatilizable impurities, form combustion gases G1, and form a combustion solution P3 having a mass concentration, xp3, of P2O5 greater than the concentration of the mixture solution Pm, and a volatilizable impurity content lower than that of the mixture solution Pm, (h) separate the combustion solution P3 from the combustion gases G1 and recover the combustion solution P3, and transfer the combustion gases G1 into the gas-acid contactor (1) as defined in step (c), in which undesirable volatilisable materials are introduced into the residual stream Fr of the aqueous residual solution Pr and / or into the feed stream F0 of the feed solution P0.

[0010] The feed solution P0 may have a concentration xp0 of between 0.1 and 50%, preferably between 1 and 35%, and preferably between 5 and 20% P₂O₅. In some cases, the feed solution may also include undesirable volatile matter, but this is not essential, particularly when a residual solution Pr containing undesirable volatile matter is added at the combustion chamber, as explained below. The flow rate, Q0, of the feed solution P0 in the contactor, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably between 100 and 3000 kg / (h MW), and preferably between 500 and 2500 kg / (h MW).

[0011] The enriched phosphoric acid solution P1 is identical to the recirculated enriched phosphoric acid solution P2 and comprises a phosphorus concentration xp1 preferably greater than or equal to 1%, preferably less than 60%, more preferably between 5 and 50%, preferably between 10 and 40% P2O5. The total flow rate, Q1 = (Qp + Q2), of the solution P1 out of the contactor expressed per unit of rated power [MW-1<] of the combustion chamber is preferably between 600 and 123000 kg / (h MW), preferably between 1000 and 50000 kg / (h MW). The ratio, Qp / (Qp + Q2), between the mass flow rate Qp of the spray flow Fp and the total mass flow rate (Qp + Q2) is preferably less than 50%, preferably less than 10%, preferably less than 5%, even more preferably less than 2.5% and in which the ratio Qp / (Qp + Q2) is greater than 0.1%, preferably greater than 0.5%.

[0012] The residual solution Pr may include a phosphorus concentration xpr greater than or equal to 2%, preferably at least 5%, more preferably at least 10%, and more preferably at least 20% P₂O₅. The residual solution Pr includes an undesirable volatile matter concentration xpv of at least 5 ppm, preferably at least 10 ppm, more preferably at least 100 ppm, more preferably at least 1%, more preferably at least 5%, more preferably at least 10%, and more preferably at least 25% by weight relative to the total weight of the solution. The flow rate Qr of the residual solution Pr in the combustion chamber, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably non-zero and preferably between 5 and 1500 kg / (hMW), and more preferably between 400 and 1000 kg / (hMW).If the flow rate Qr of the residual solution Pr in the combustion chamber is zero, then the feed solution P0 must include a non-zero concentration xpv of undesirable volatilizable matter, for example of at least 5 ppm, preferably at least 10 ppm, preferably at least 100 ppm, preferably at least 1%, preferably at least 5%, preferably at least 10%, again preferably at least 25% by weight relative to the total weight of the solution.

[0013] The ratio Qr / (Qr + Q0) can be between 0 and 99%, preferably between 5 and 90%, more preferably between 10 and 80%, or between 15 and 45%. The mixing flow Fm can include a phosphorus concentration xpm, preferably greater than 1% P₂O₅ (xpm > 1% P₂O₅), and includes undesirable volatilizable matter from the residual solution Pr and / or the feed solution P0. In this document, the flow rates Q0, Qp, and Qr are mass flow rates of the feed solution P0, enriched phosphoric acid P1, and residual solution Pr, respectively.

[0014] The Pm mixture solution may include a concentration xpm greater than 2%, preferably greater than 5%, more preferably greater than 20%, more preferably greater than 30%, preferably greater than 40%, and more preferably between 45 and 60% P2O5. The Pm mixture solution may include a concentration xpv of undesirable volatile matter preferably of at least 5 ppm, preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, preferably of at least 5%, preferably of at least 10%, more preferably of at least 25% by weight relative to the total weight of the solution. The flow rate Qm of the mixing solution Pm in the combustion chamber expressed per unit of nominal power [MW -1< ] of the combustion chamber is preferably between 305 and 3000 kg / (h MW), preferably between 200 and 2000 kg / (h MW).

[0015] The combustion solution P3 may comprise a phosphorus concentration xp3 greater than 1% equivalent in P2O5 units, preferably greater than 10%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30 and 76%. The flow rate Q3 of the combustion solution P3 out of the combustion chamber expressed per unit of rated power [MW-1] of the combustion chamber is preferably between 240 and 1500 kg / (h MW), preferably between 600 and 3000 kg / (h MW).

[0016] The supply flow F0 and recirculation flow F2 can either be mixed before being introduced into the gas-acid contactor to form a flow of a mixture of the supply solution P0 and the recirculated enriched phosphoric acid solution P2, or contacted after being introduced separately into the gas-acid contactor to form a flow of a mixture of the supply solution F0 and the recirculated enriched phosphoric acid solution P2.

[0017] It is preferable for the residual flow rate Qr of the residual solution Pr to be non-zero. The residual flow rates Fr and Fp can then be either mixed to form the mixture flow rate Fm before being sprayed into the flame in the combustion chamber, or sprayed separately into the combustion chamber to form the mixture flow rate Fm in the flame or just before reaching the flame. The residual solution Pr, and optionally the feed solution P0 and therefore the spray solution Pp, contain undesirable volatile matter. It is also possible that only the spray solution Pp contains undesirable volatile matter, for example, in the case of a flow rate Qr = 0. However, it is preferable for the residual flow rate Qr to be non-zero.

[0018] The contact between the supply flow F0 and recirculation flow F2 and the combustion gases G1 in step (d) can be co-current or counter-current, preferably co-current, flowing from an upper to a lower part of the gas-acid contactor. During the contact step (d), the ratio (Qg1 / (Q0+Q2)) between a mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor and a total mass flow rate (Q0 + Q2) of the contact supply flow F0 and recirculation flow F2 introduced into the gas-acid contactor is preferably between 0.1 and 50%, preferably between 0.5 and 10%, and even more preferably between 1 and 7%.

[0019] The present invention also relates to a device for producing purified phosphoric acid (P3) according to a process according to any one of the preceding claims, comprising: (A) a combustion chamber having: a spray inlet into the combustion chamber allowing the introduction at a flow rate of a solution of enriched phosphoric acid P1 in sprayed form into a combustion unit, a residue inlet into the combustion chamber or upstream of the spray inlet allowing the introduction of a residual solution Pr or a mixture of residual solutions Pr and enriched phosphoric acid P1 in sprayed form into a combustion unit, the combustion unit being arranged in the upper part of the combustion chamber, and being capable of forming a flame having a temperature of at least 1500°C by combustion of a fuel, said combustion unit comprising: o a burner, fluidic connections between the burner and, on the one hand, an oxygen source and, on the other hand, a fuel source for supplying the flame,a combustion outlet from the combustion chamber to recover a combustion solution P3 in liquid phase, arranged downstream of the combustion unit which is itself arranged downstream of the spray and residue inlet, a combustion gas exhaust outlet G1 from the flame (B), a gas-acid contactor having a supply inlet connected to a source of a feed solution P0, allowing the introduction of a feed solution P0 at a contact feed rate Q0, a combustion gas inlet allowing the introduction of combustion gases G1 into the gas-acid contactor at a flow rate Qg1, a recirculation inlet identical or different from the contact feed inlet, allowing the introduction of a recirculated enriched phosphoric acid solution P2 at a recirculation flow rate Q2, the supply and / or recirculation inlets and the gas inlet being arranged to allow, on the one hand,∘ a contact between the feed flow F0 and the recirculation flow F2 to form a flow of a mixture of the feed solution P0 and the recirculated enriched phosphoric acid solution P2 and, on the other hand ∘ a contact of the mixture thus formed with the combustion gases G1, one or more outlets of enriched phosphoric acid, (C) a flue gas fluid connection linking one end coupled to the flue gas outlet of the combustion chamber, to one end coupled to the flue gas inlet in the gas-acid contactor, (D) a first spray fluid connection linking an upstream end (3u) coupled to the enriched phosphoric acid outlet of the gas-acid contactor or to a branch point with a first fluid connection which is coupled to the enriched phosphoric acid outlet, to a downstream end coupled to the enriched phosphoric acid inlet in the combustion chamber, , Characterized in This thatthe device further comprises (E) a recirculating fluidic connection linking an upstream end coupled to a recirculated enriched phosphoric acid outlet (1pd) of the gas-acid contactor or to a branch point with the first fluidic connection, to a downstream end coupled to the recirculation inlet of the gas-acid contactor or to a supply connection supplying the gas-acid contactor with feed solution P0, and (F) means for controlling and maintaining a ratio, Qp / (Qp + Q2), between a spray mass flow rate Qp flowing in the spray fluidic connection (3p) and a total mass flow rate (Qp + Q2) defined as the sum of the spray mass flow rate Qp and a recirculating mass flow rate Q2 flowing in the recirculating fluidic connection to a value less than 50%, preferably less than 10%, preferably less than 5%, and even more preferably less than 2%.5% and in which the ratio Qp / (Qp + Q2) has a value greater than 0.1%, preferably greater than 0.5%.

[0020] The residue inlet is preferably in fluidic communication with a source of residual solution Pr which is aqueous and includes phosphorus molecules in orthophosphate and / or polyphosphate form and undesirable volatilizable matter. BRIEF DESCRIPTION OF THE FIGURES.

[0021] Various aspects of the present invention are illustrated in the following Figures. Figure 1 :illustrates graphically the relationship between boiling point and P2O5 concentration at equilibrium of the liquid phase (part (a)) and vapor phase (part (b)), as well as the relationship between P2O5 concentration and the weight distribution of phosphoric acid molecules according to different values ​​of n (part (c)). Figure 2: illustrates a variant of the device according to the present invention. Figure 3:reports values ​​of a selection of parameters illustrative of the process according to the present invention. Figure 4: illustrates a variant of the device according to the present invention. Figure 5: illustrates a variant of the device according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] THE Figures 2 to 5 illustrate the process and non-exhaustive variations of devices for implementing said process. In the following, the term "flow," represented by the letter "F," is used in its commonly accepted interpretation of simply the flow of a fluid. Only the Figure 3 indicates the flows with the letter "F". The other figures illustrating devices indicate flow rates "Q" corresponding to the flows "F" of the Figure 3The term "flow rate," represented by the letter "Q," characterizes the mass of the flow per unit of time and is expressed in [kg / s] or [kg / h]. The term "flow rate," even when used alone, therefore defines a mass flow rate. In order to express the flow rates as a function of the combustion chamber's nominal power, the mass flow rates of the different flows will subsequently be reported in units of [kg / (h MW)], which represents a flow rate per unit of nominal power [MW⁻¹] of the combustion chamber.

[0023] Since the combustion solution P3 resulting from the combustion of the mixture solution Pm obtained at the end of the process of the present invention can comprise purified phosphoric acid alone or in mixture with polymerized molecules whose respective concentrations vary according to the temperature and the concentration in P2O5 of the mixture solution Pm contacting the flame of the combustion chamber (cf. Figure 1 ), which itself depends inter aliaRegarding the concentration in the residual solution Pr used, the following will refer to solution P3, obtained by the expression "purified phosphoric acid solution," even though it is clear that this solution may also contain polymerized molecules and therefore PPA, and may also contain impurities. It is also evident that the purity level of solution P3 will depend on the applications for which this purified acid is intended, and that the presence of certain ions will not always be contraindicated in some applications. Conventions and Definitions

[0024] Unless otherwise stated in this patent, the term concentration is used to express mass concentrations (weight percentages, w / o). To define the content of the species that primarily concerns us, when referring to the concentration of phosphoric acid solutions or others, we must understand the content by weight expressed in units of P₂O₅ equivalents, which we will write as "% eq. P₂O₅" or simply "% P₂O₅". With regard to gaseous streams, such as combustion gases, where several species of interest may coexist depending on the operating conditions, in gaseous or liquid form (for example, by droplet entrainment), or even possibly as a solid (fumes), the concentration in these streams is also expressed in units of P₂O₅ equivalents (by weight, w / o). The ionic dissociation of the species of interest is not considered in this text.For your information, the concentration of a phosphoric acid solution can also sometimes be expressed in units of H3PO4 equivalents. The correspondence between the two units of concentration is defined by the relation: 1 eq. P2O5 = 0.7245 eq. H3PO4.

[0025] It is understood in this text by the expressions "Phosphoric acid solution", an aqueous solution comprising HO[P(OH)(O)O] n H, with n ≥1; "Orthophosphoric acid", an aqueous solution comprising predominantly HO[P(OH)(O)O] n H, with n = 1, i.e. an aqueous solution of phosphoric acid containing less than 61% by weight of P 2 O 5; "Polyphosphoric acid solution" (= PPA), an aqueous solution comprising predominantly HO[P(OH)(O)O] n H, with n > 1; i.e. an aqueous solution of phosphoric acid containing more than 76% by weight of P 2 O 5; "Polycondensation of phosphoric or orthophosphoric acid", the polycondensation of the molecules considered as represented by equations (1) and / or (2) below. "Aqueous solution containing phosphorus" means a solution containing dissolved phosphorus in the form of orthophosphate or polyphosphate species. Depending on the P₂O₅ content of these solutions, the orthophosphate or polyphosphate species may be present as shown in [reference]. figure 1(c) These species can be present in the form of ions. Process - gas-acid contacts

[0026] The process of the present invention comprises introducing the following flows into a gas-acid contactor (1). A supply flow F0 of feed solution P0 from the contactor at a supply rate, Q0. The feed solution P0 is an aqueous solution preferably containing phosphorus, preferably in the form of orthophosphate-type species (cf. Figure 1) at a mass concentration xp0 of between 0 and 54%, preferably from 0.1 to 50%, more preferably from 1 to 35%, preferably from 5 to 20% P₂O₅. The feed solution F0 allows the return to the combustion chamber of phosphorus-containing species that would have left it with the combustion gases G1. In a variant of the present invention, the solution P0 does not contain phosphorus (xp0 = 0) and may be water. The solution P0 may contain compounds that can react with undesirable volatile substances to destroy them, for example, hydrogen peroxide, chlorate or nitrate ions. These compounds may, for example, react with dissolved carbonaceous materials to form carbon dioxide (CO₂), which is readily volatilized.

[0027] In an alternative embodiment of the present invention, the solution P0 is a source of phosphorus and comprises P₂O₅. For example, the contact feed solution P0 may comprise a phosphorus concentration xp0 of 5 to 54%, preferably less than 45%, and even more preferably between 10 and 35% P₂O₅. Higher concentrations of the feed solution do not interfere with the process and allow for an increase in the phosphorus concentration xpm of the mixing solution, Pm, reaching the flame in the combustion chamber. The P₂O₅ content in the purified phosphoric acid solution, P3, at the end of the process can thus be varied.

[0028] The flow rate Q0 of the solution P0 in the contactor expressed per unit of nominal power [MW -1< ] of the combustion chamber is preferably between 100 and 3000 kg / (h MW), preferably between 500 and 2500 kg / (h MW), or between 1000 and 2000 kg / (h MW).

[0029] In one embodiment of the present invention, the solution P0 comprises undesirable volatile materials such as typically carbon, fluorine, chlorine, sulfur, and nitrogen in soluble form (ionic or insoluble). For example, these may be present in the solution P0 in concentrations xpv of undesirable volatile materials of at least 5 ppm (parts per million), or at least 10 ppm, preferably at least 100 ppm. Preferably, the concentration xpv of undesirable volatile materials is less than 5%, preferably less than 2% by weight of total organic carbon relative to the total weight of the solution. For example, the solution P0 may comprise at least 10 ppm of fluorine, or at least 100 ppm of fluorine, or at least 1% fluorine. Depending on the application, such solutions are unusable as is.

[0030] If the P0 solution does not include undesirable volatile materials, then a recirculating solution Pr containing such undesirable volatile materials is added at the combustion chamber level to produce a mixture solution including undesirable volatile materials in the concentrations indicated above.

[0031] To avoid excessive fouling in the gas-acid contactor, it is preferable that most or all of the undesirable volatile materials be introduced directly into the combustion chamber, contained in the residual solution Pr. A recirculation flow F2 of recirculated enriched phosphoric acid solution P2 will be defined in detail later. The flow rate of the recirculated enriched phosphoric acid solution P2 in the contactor, expressed per unit of rated power [MW⁻¹] of the combustion chamber, can be between 300 and 120,000 kg / (hMW), preferably between 600 and 100,000 kg / (hMW), preferably greater than 9,000 kg / (hMW), and preferably between 1,500 and 80,000 kg / (hMW). The recirculation flow F2 of recirculated enriched phosphoric acid solution P2 results from the contacting of a mixture of the feed flow F0 and the recirculation flow F2 of recirculated enriched phosphoric acid solution P2 from a previous cycle with a flue gas flow G1.A combustion gas stream G1 is formed during the combustion of a mixture stream Fm of a mixing solution Pm containing phosphorus at a mass concentration xpm higher than that of the contact feed stream F0 in a combustion chamber. This stream will be described and discussed in detail later. The combustion gas stream G1 includes phosphorus molecules in the form of droplets or vapors, carried from the combustion chamber to the gas-acid contactor. For example, the combustion gas stream G1 may contain between 0.1 and 15% P₂O₅, e.g., between 0.5 and 13%, or between 1 and 10% P₂O₅, preferably between 2 and 5% P₂O₅. In addition, the combustion gases include undesirable volatilized matter resulting from the combustion of the mixing solution Pm.

[0032] The supply flow F0 and recirculation flow F2 and the combustion gases G1 are therefore brought into contact with each other in the gas-acid contactor (we are therefore talking about a direct gas-acid contactor) to form, on the one hand, an enriched phosphoric acid solution P1 and, on the other hand, contacted combustion gases G3.

[0033] In a preferred variant, illustrated in the Figures 2In step 4, the supply flow F0 and recirculation flow F2 are mixed before being introduced into the gas-acid contactor to form a flow of a mixture of the supply solution P0, the contactor, and the recirculated enriched phosphoric acid solution P2. This is achieved by connecting a recirculation line (3r) carrying the flow F2 and a supply line (3a) carrying the flow F0 upstream of an inlet (1pu) of the gas-acid contactor (1). The pressures in the recirculation (3r) and supply (3a) lines must be controlled to prevent liquid backflow into either of the two connected lines. Figure 2 shows a supply line (3a) connected to the recirculation line (3r), while the Figure 4illustrates a recirculation line (3r) connected to a supply line (3a). In both configurations, the combustion gases G1 are then brought into contact with the solution flow mixture (F0+F2) thus formed, after the introduction of the latter into the gas-acid contactor (1).

[0034] In an alternative version, illustrated in Figures 3 In step 5, the supply flow F0 and recirculation flow F2 are contacted after being introduced separately into the gas-acid contactor to form a flow of a mixture of the contactor's supply solution P0 and the recirculated enriched phosphoric acid solution P2. The phosphoric acid flows F0 and F2 and the combustion gas G1 are thus all brought into contact within the gas-acid contactor. It is sufficient to provide in the gas-acid contactor a separate supply flow inlet (1 pu) for F0 and a separate recirculation flow inlet (1 pru) for F2.

[0035] The contact between the feed flow F0 and recirculation flow F2, or their mixture (F0+F2), and the combustion gases G1 in the gas-acid contactor can be achieved by co-current or counter-current flow contact. In particular, the liquid phases flow downwards in the direction of gravity, and the gaseous phase flows upwards. In a preferred embodiment, the two or three flows co-currently from an upper to a lower part of the gas-acid contactor. In the context of the present invention, the terms "upper" and "lower" are understood in the direction of the Earth's gravitational forces extending towards the Earth's center of gravity. Thus, in the absence of pressure gradients, a liquid naturally flows from the upper part of a reactor to its lower part, which is located downstream of the upper part, in the direction of Earth's gravity.

[0036] It is possible to contact the G1 combustion gases with the phosphoric acid supply and recirculation flows F0&F2 or their mixture (F0+F2) by guiding the combustion gases in a flow transverse to those of the phosphoric acids. However, contact via co-current flows is preferred.

[0037] The contact between the feed stream F0 and the recirculation stream F2, or their mixture (F0+F2), and the flue gases G1 forms a solution of enriched phosphoric acid P1 and contacted flue gases G3. This contact can be achieved by percolating the streams through a packing material that withstands the operating conditions. During the contact between the flue gases G1 and the feed streams F0 and F2, exchanges occur. On the one hand, phosphorus molecules transported by the flue gases as droplets and vapor are carried away by the streams F0 and F2, allowing the formation of the enriched phosphoric acid solution P1, which has a P₂O₅ content higher than that of either stream F0 or F2.On the other hand, a heat exchange takes place between the hot combustion gases, at a temperature Tg1 between 200 and 600°C, and the aqueous solutions of the fluxes F0 and F2 which are at lower temperatures, as indicated in the . Figure 3 The contacted gases G3 are therefore at a temperature Tg3 < Tg1, facilitating their subsequent treatment for release into the atmosphere. At the same time, the enriched phosphoric acid solution P1 is thus at a temperature T1 higher than that of the mixture of solutions P0 and P2; T1 is higher than the temperature T0 of the feed solution P0 (which can be on the order of 20 to 200°C) and is approximately equal to the temperature T2 of the recirculated enriched phosphoric acid solution P2 since it is the same solution at both ends of the recirculation loop (3r).

[0038] The enriched phosphoric acid solution P1 and the contacted combustion gases G3 formed following contact between the phosphoric acid supply and recirculation streams F0 and F2 and the combustion gases G1 are then separated by means of separation well known to those skilled in the art, such as a centrifugal or gravity separator, a coalescer, a demister, a mattress, baffles, etc. The contacted combustion gases G3 are then discharged from the gas-acid contactor (1) at a temperature substantially lower than that of the contacted combustion gases G3 introduced into said gas-acid contactor for further treatment.Since most of the phosphorus molecules contained in the G1 flue gas stream are transferred into the F1 stream of enriched phosphoric acid solution P1 upon contact of the G1 stream with the F0 and F2 streams, the contacted gas stream G3 is much poorer in P2O5 than the G1 flue gas stream with contents that may be less than 1% P2O5.

[0039] The contacted flue gases (G3) containing undesirable volatilized matter can also be scrubbed after exiting the gas-acid contactor with an aqueous scrubbing solution to dissolve and remove undesirable compounds such as fluorinated or chlorinated compounds, SOs, etc., before the gases are released into the atmosphere. Other treatments of the contacted flue gases (G3) are possible, including, for example, condensation of the gases in an indirect condenser. The enriched phosphoric acid solution (P1) is also exited from the gas-acid contactor, separately from the contacted flue gases (G3). Process - flow F1 and division into flows Fp and F2

[0040] The enriched phosphoric acid solution P1 may have a P₂O₅ concentration xp1 of 1% or greater, preferably less than 60%, and more preferably between 5% and 50%, preferably between 10% and 40% P₂O₅. The P₂O₅ concentration of the enriched phosphoric acid solution P1 depends, of course, on the P₂O₅ concentration of the feed solution, P₀, and the flue gas stream, G₁. As discussed below, the phosphorus concentration of the enriched phosphoric acid solution P1 is generally higher than that of the feed solution, P₀.

[0041] Before, during or after its evacuation from the gas-acid contactor, the enriched phosphoric acid solution P1 is divided into two distinct streams: a recirculation flow F2 of recirculated enriched phosphoric acid solution P2 to introduce it into the gas-acid contactor (1) through a recirculation loop (3r) to bring it into contact with the supply flow F0 and the combustion gases G1 as described above, and a spray flow Fp of a spray solution Pp to introduce it into a combustion chamber (2).

[0042] The spray solution Pp and the recirculated enriched phosphoric acid solution P2 are identical in composition to each other and to the enriched phosphoric acid solution P1 (P1 = Pp = P2) since they have not undergone any alteration between their formation in the gas-acid contactor and their division into two separate recirculation streams F2 and spray streams Fp. The temperatures Tp and T2 of solutions Pp and P2 are also substantially identical to the temperature T1 of solution P1, which can be in the range of 100 to 300°C. Under steady-state conditions, solutions Pp and P2 preferably have a higher P₂O₅ concentration than the contactor feed solution P0. For example, solutions Pp and P2 may have phosphorus concentrations ranging from 1 to 60%, preferably from 5 to 50% P₂O₅, and preferably from 10 to 40% P₂O₅. This can be explained by two main reasons.

[0043] Firstly, the contacting of the phosphoric acid supply and recirculation flows with the combustion gases G1 which are at a higher temperature Tg1 of the order of 300 to 600°C (cf. Figure 3 ), causes the evaporation of some of the water contained in the aqueous phase of solutions P0 and P2, which in fact increases the concentration of P2O5 in solutions P1, Pp and P2.

[0044] Secondly, as we will discuss later, the combustion gases G1 formed during the combustion of the mixture stream Fm of the mixture solution Pm in the combustion chamber include droplets or vapors of phosphoric acid. The combustion gases G1 may comprise between 0.1 and 15% P2O5, preferably between 0.5 and 13%, preferably between 1 and 10%, preferably between 2 and 5%, (cf. Figure 3). Upon contact with the feed streams of solution P0 and recirculated enriched phosphoric acid P2, most of these molecules are transferred from the flue gases to the acid solution mixture (P0+P2). After contact, the contacted flue gases G3 contain much fewer P2O5 molecules than the gas G1 before contact, generally less than 1%; preferably less than 0.5%, advantageously less than 0.1% by weight (cf. Figure 3 ). By this transfer of P2O5 molecules to the acid mixture, the concentration of P2O5 in the latter increases.

[0045] In one embodiment of the invention, the enriched phosphoric acid solution P1 is divided into two streams: a spray stream Fp and a recirculation stream F2 at the outlet of the gas-acid contactor, in a spray fluidic connection (3p) and a recirculation fluidic connection (3r), respectively, as illustrated in the Figure 2. Each of the fluidic connections (3p) and (3r) is equipped with a pumping system (4, 4r) to ensure the flow rates and a spray flow Fp at a spray flow rate Qp towards a combustion chamber (2) and to drive the recirculation flow F2 at a flow rate Q2 towards the gas-acid contactor thus forming a recirculation loop.

[0046] In an alternative variant, the enriched phosphoric acid solution P1 exits the gas-acid contactor through a first fluidic connection (3, 3u) which is common and splits into two at a branch point (5) in a "T" or "Y" configuration. One branch is the spray fluidic connection (3p), which carries the spray flow Fp at a flow rate Qp to a combustion chamber (2), and the other is a recirculation fluidic connection (3r), which carries the recirculation flow F2 at a flow rate Q2 back to the gas-acid contactor, thus forming a recirculation loop. Various variants of this configuration, including a branch point (5), are illustrated in the following: Figures 3 to 5 . The spray flow rates Qp and recirculation flow rates Q2 can be ensured by one or more valves (cf. Figure 3 And 4 ), by pumps (4, 4r) on each of the branches of the branching point (5) (cf. Figure 5) and / or by spray (3p) and recirculation (3r) duct sections sized to obtain the desired flow rates or other means well known and used industrially to distribute a flow between 2 feeds (for example, T-pipes, Y-pipes with sets of regulated valves).

[0047] The enriched phosphoric acid solution P1 exits the contactor at a total flow rate, Q1 = (Qp + Q2). The total flow rate Q1, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably between 600 and 123,000 kg / (hMW) or between 1,000 and 120,000 kg / (hMW), preferably between 12,000 and 100,000 kg / (hMW). As discussed above, the enriched phosphoric acid (P1) flow F1 is divided into two flows Fp and F2, each with a spray flow rate Qp and a recirculation flow rate Q2. The division into two flows can occur before, at, or after the gas-acid contactor outlet. The spray flow rates Qp and recirculation flow rates Q2 must be determined according to inter alia of the capacity of the combustion chamber and the gas-acid contactor, of the temperature of the combustion gases G1 and their P2O5 content.

[0048] In a steady-state production environment, the ratio, Qp / (Qp + Q2), between the mass flow rate Qp of the spray flow Fp and the total mass flow rate (Qp + Q2) of the enriched phosphoric acid flow F1 (which is actually the sum of the spray flow rates Qp and recirculation flow rates Q2) is preferably less than 50%, preferably less than 20%, and even more preferably less than 10%. In a preferred embodiment of the invention, the ratio Qp / (Qp + Q2) is less than 5%, preferably less than 4%, even more preferably less than 2.5%, and even less than 2%. The ratio Qp / (Qp + Q2) is preferably greater than 0.1%, or even greater than 0.2%, and preferably greater than 0.5%. Increasing the flow rate Q2 relative to the flow rate Qp allows, on the one hand, for the combustion gases G1 to be cooled to a lower temperature, which is necessary before their evacuation and, on the other hand, for the P2O5 content of the enriched phosphoric acid solution Pp to be further enriched.

[0049] The ratio, Q2 / (Qp + Q2), between the mass flow rate Q2 of the recirculation flow F2 and the total mass flow rate (Qp + Q2), is of course the complement of the ratio Qp / (Qp + Q2), the sum of which equals 100%. The recirculation flow rate Q2 is therefore preferably greater than or equal to the spray flow rate Qp and, in some preferred variants, is considerably greater than Qp, with a flow rate ratio, Qp / Q2, ranging from 0.1 / 99.9 to 49 / 51 (= 0.1 to 96%). Preferably, the flow rate ratio Qp / Q2 is between 1 / 99 and 5 / 95 (= 1 to 5.3%). Preferably, the flow rate ratio Qp / Q2 is between 1 / 99 and 4 / 95 (= 1 to 4.2%).

[0050] As discussed above, the flow rate Q2 of the recirculated enriched phosphoric acid solution P2 in the contactor, expressed per unit of rated power [MW⁻¹] of the combustion chamber, can be between 300 and 120,000 kg / (hMW), preferably between 600 and 110,000 kg / (hMW), and preferably between 9,000 and 100,000 kg / (hMW). Thus, the flow rate Qp of the spray solution Pp flowing into the combustion chamber, expressed per unit of rated power [MW⁻¹] of the combustion chamber, can be between 300 and 3,000 kg / (hMW), preferably between 600 and 2,000 kg / (hMW), and preferably between 1,000 and 1,500 kg / (hMW). Process - FP, FR and FM streams

[0051] A mixing stream Fm of a mixing solution Pm comprising undesirable volatile matter at a non-zero mass concentration xpv and phosphorus at a mass concentration xpm preferably higher than that of the contact feed stream F0 is formed by the spray stream Fp, optionally mixed with a residual stream Fr of an aqueous residual solution Pr from the residues of a previous industrial process. If the feed solution F0 does not contain undesirable volatile matter, then mixing the feed stream P0 with a residual stream Fr is mandatory. Otherwise, it is optional, but preferred. The mixing stream Fm is sprayed through a flame burning in the upper part of a combustion chamber (2) to: oxidize the undesirable volatile substances and volatilize them, forming combustion gases G1 carrying away the undesirable volatile substances thus volatilized (= undesirable volatile substances) and thus purifying the residual solution Pr, evaporating water and thus concentrating the mixture solution Pm, forming a purified phosphoric acid solution P3. Depending on the initial P2O5 content, polymerize purified phosphoric acid molecules into polyphosphoric acid.

[0052] The residual flow Fr includes: a mass concentration, xpr, of at least 1% P 2 O 5 , a mass concentration, xer, of water, and undesirable volatilizable matter.

[0053] The residual stream Fr comprises a phosphorus concentration xpr of at least 1% or at least 5%, preferably at least 10%, more preferably at least 15%, and more preferably at least 20% P₂O₅. The flow rate Qr of the residual solution Pr in the combustion chamber, expressed per unit of the combustion chamber's rated power [MW⁻¹], is preferably non-zero and preferably between 5 and 1500 kg / (hMW), and more preferably between 400 and 1000 kg / (hMW). The temperature of the residual solution Fr can be between 20 and 200°C, preferably between 40 and 150°C, and more preferably between 50 and 100°C. Preheating the solution Fr is advantageous in terms of the combustion efficiency of the mixture solution Fm in the flame.

[0054] The residual solution Pr, and optionally the feed solution P0, is or preferably contains a solution originating from industry. This solution can be generated by plant cleaning or during routine production or maintenance operations in industries such as metallurgy, food processing, pharmaceuticals, and chemicals, particularly during the production of phosphate salts or fertilizers. These solutions are generally difficult to recycle in their current state due to their levels of various pollutants, notably soluble organic matter residues, and their low phosphorus concentrations. They must therefore be treated before being concentrated. The residual solution can also originate from phosphorus recovery processes using so-called "secondary" raw materials, which are solid compounds containing phosphorus other than phosphate ore.Examples include bone meal or bone ash, sewage sludge or sludge ash, pig and chicken manure or manure ash, etc. These residual solutions, also called acid solutions or waste solutions, contain P₂O₅ but often also undesirable volatile substances such as carbon, fluorine, chlorine, sulfur, and nitrogen in soluble (ionic or non-ionic) form. The concentrations of these undesirable volatile substances naturally depend on the origin of the residual solution.For example, they may be present in the residual solution Fr in concentrations xpv of undesirable volatile matter of at least 5 ppm (parts per million), preferably at least 10 ppm, preferably at least 100 ppm, preferably at least 1%, preferably at least 5% by weight of total organic carbon relative to the total weight of the solution, or at least 10 ppm of fluorine, at least 100 ppm of fluorine, or at least 1% of fluorine. Depending on the application, these solutions are unusable as is.

[0055] In a steady-state production environment, the ratio, Qp / Q0, between the spray flow rate Qp Fp and the feed flow rate Q0 F0 is preferably between 100 and 250%, preferably between 101 and 140%, and preferably between 110 and 115%. This ratio may be higher than 100% because the contact of the Q0 and Q2 flows with the combustion gases in the gas-liquid contactor increases the mass of the flow, F1, exiting the gas-liquid contactor. The value of this ratio may decrease as the residual flow rate, Qr, increases.

[0056] The ratio of flow rates Qr / (Qp + Qr) between the residual flow rate Qr and the sum of the spray flow rates Qp and residual flow rates Qr represents the fraction of the residual solution Pr flow rate entering the combustion chamber. The value of this ratio depends on inter aliathe P2O5 and undesirable volatile matter content of the residual solution and / or the feed solution, which are determining for the P2O5 and undesirable volatile matter content of the mixing solution Pm. For example, the ratio Qr / (Qp + Qr) may be between 0 and 94%, preferably between 5 and 90%, even more preferably between 10 and 80%, or between 15 and 45%,

[0057] Regardless of the value of the ratio Qr / (Qp + Qr), the mixing solution Pm preferably comprises a phosphorus concentration xpm greater than 1%, preferably greater than 2% or 5%, more preferably greater than 20%, more preferably greater than 30%, more preferably greater than 40%, and more preferably between 45 and 60% P₂O₅. The flow rate Qm of the solution Pm in the combustion chamber is the sum of the spray flow rates Qp and the residual solution flow rates Qr. Expressed per unit of rated power [MW⁻¹] of the combustion chamber, the mixing flow rate Qm is preferably between 305 and 3000 kg / (h MW), more preferably between 900 and 2000 kg / (h MW). Process - Evaporation of Undesirable Volatile Substances and Concentration in P2O5

[0058] A primary function of the combustion chamber is to degrade, if necessary by oxidation, and then vaporize any undesirable volatile materials present in the residual solution. A second function of the combustion chamber is to evaporate water present in the solutions to concentrate the residual and feed solutions. A third (optional) function is the polycondensation of phosphate molecules into polyphosphoric acid (PPA). The distribution of species present in the resulting solution depends on the P₂O₅ concentration of the mixture solution Fm reaching the combustion flame, as well as the polycondensation temperature. As can be seen in the Figure 1(c)PPA only forms if the P2O5 content is sufficiently high, approximately 60% P2O5, which is higher than the phosphorus content typically found in residual solutions Fr. If PPA is desired, it is then necessary to increase the phosphorus content of the mixing solution Fm by feeding the combustion chamber with a spray solution Fp having a higher phosphorus concentration or with a residual solution Fr having a higher phosphorus concentration.

[0059] The combustion in the flame of the mixture solution Pm therefore forms, on the one hand, combustion gases G1 formed by the evaporation of water and, in particular, undesirable volatilizable materials and, on the other hand, a combustion solution P3 which is in the liquid state and comprising phosphorus and, if the concentration in P 2 O 5 and the polycondensation temperature Tpc are sufficient, species polymerized by polycondensation of the phosphoric acid contained in the solution Pm.

[0060] The temperature reached by the mixture solution Pm in the flame is an important parameter of the process since it allows the volatilization of undesirable volatile compounds present in the residual solution and therefore in the mixture solution. The concentration of P₂O₅ obtained in the combustion solution P₃ is also dependent on it, as shown in the graph of the Figure 1(a)It is also important to keep the mixture solution containing phosphoric acid in contact with the flame and combustion gases for a sufficient time to allow the water to evaporate and for polycondensation to eventually occur.

[0061] The flame is fueled by a combustible material and an oxygen source, typically air or, for higher temperatures, oxygen. The flame is preferably a slightly oxidizing flame, preferably containing between 1 and 5% excess air. The fuel is preferably natural gas, butane, propane, or any other fuel, whether gaseous or liquid. In the absence of atomization of the Pm mixture solution, the flame preferably reaches a theoretical temperature of at least 750°C, preferably at least 1000°C, and even more preferably at least 1700°C, for example, 1800°C ± 50°C. In the process of the present invention, the temperature increase is instantaneously limited because, on the one hand, the mixing solution Pm is supplied at a lower temperature Tm, on the order of 20-300°C and, on the other hand, because the evaporation of water molecules from the solution is energy-intensive.

[0062] The residual stream Fr and the spray stream Fp can be mixed to form the mixed stream Fm before being sprayed into the flame in the combustion chamber, as illustrated in the Figures 2 5. Alternatively, the two streams Fr and Fp can be sprayed separately into the combustion chamber to form the mixture stream Fm in the flame or just before reaching the flame, as illustrated in the Figures 3 4.

[0063] The P3 combustion solution, which consists of a purified phosphoric acid solution, is therefore an aqueous solution of phosphoric acid which may contain polymerized species depending on the P2O5 content present in the solution (cf. Figure 1 ).

[0064] If the production of PPA is desired, it is preferable that the mixture solution sprayed into the flame reach a polycondensation temperature Tpc of at least 400°C, preferably at least 500°C and even above 550°C, or even on the order of 650°C or 700°C, for a predetermined polycondensation time. A high polycondensation temperature Tpc makes it possible to obtain polyphosphoric acid solutions with high P2O5 concentrations, on the order of 86% and above, with longer chain lengths n (e.g., n ≥ 5 to 12) (cf. Figure 1(c)The temperatures required for the polycondensation of phosphoric acid necessitate chemically and thermally resistant materials for the various components of the reaction device. The mixture solution Pm, which comprises orthophosphoric acid molecules and optionally polyphosphoric acid oligomers (of m+1 condensed units), undergoes a polycondensation reaction under the influence of temperature to release water and form longer polymer chains, according to equation (1) described above and equation (2) (with m ≥ 1 and r ≥ 1):

[0065] The combustion solution P3 thus formed is then separated from the combustion gases G1 formed during the evaporation of water and undesirable volatile matter and possibly from the polycondensation of polyphosphoric acid in a gas-liquid separator (9). The combustion solution P3, containing phosphoric acid and optionally polyphosphoric acid and practically free of undesirable volatile matter, is recovered while the combustion gases G1 are transferred to the gas-acid contactor (1) to be brought into contact with the supply stream F0 and recirculation stream F2, as described above.

[0066] The recovered combustion solution P3 flux F3 can have a high temperature on the order of 150 to 700°C, preferably 200 to 650°C, preferably 300 to 500°C, depending on the vaporization temperature of the undesirable volatiles contained in the mixture solution, Pm. Indeed, the temperature required for the volatilization of the volatilizable substances present in the mixture solution Pm varies according to the nature of the substances present in the mixture solution Pm. It is preferable to cool the P3 solution in a heat exchanger (11) (cf. Figure 2 ) at a temperature below T3 which allows a greater choice of materials for the storage tank of the phosphoric acid (and possibly purified polyphosphoric) thus formed and cooled, while maintaining the solution in a liquid state.

[0067] The combustion solution P3, comprising purified phosphoric (and possibly polyphosphoric) acid, thus formed and recovered, has a lower concentration of undesirable volatile matter than the mixture solution Pm. For example, the combustion solution P3 contains less than 50% of the undesirable volatile matter contained in the residual solution Pr, preferably less than 70%, even more preferably less than 80% or less than 90%, and ideally less than 95% or 99%. The combustion solution P3 has a higher concentration of P₂O₅ than the mixture solution Pm. This is due to the evaporation of a large portion of the water in the solution as it passes through the flame.The concentration xp3 of P2O5 in the combustion solution P3 is normally greater than 10% P2O5, preferably greater than 15%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30 and 76%.

[0068] The flow rate Q3 of the combustion solution P3 out of the combustion chamber is representative of the phosphoric acid purification capacity. Expressed per unit of rated power [MW⁻¹] of the combustion chamber, the flow rate Q3 is preferably between 240 and 1500 kg / (hMW), preferably between 500 and 1000 kg / (hMW).

[0069] The G1 combustion gases consist mainly of CO2, O2, and H2O, and also of undesirable volatile matter such as nitrogen oxides (NOx), sulfur oxides, fluorinated and chlorinated compounds, organic matter, and phosphorus-containing molecules. The latter may be present in quantities ranging from 0.1 to 15% by weight of P2O5, depending on the concentration xpm of the mixing solution Pm. Generally, the P2O5 content in the G1 combustion gases varies between 0.5 and 13% by weight, preferably between 1 and 10%, and preferably between 2 and 5% P2O5.The temperature Tg1 of the transferred combustion gases G1 is significantly lower than the temperature the flame can reach because, as discussed above, the temperature in the combustion unit drops during the polycondensation reaction, which requires a great deal of energy, primarily to evaporate the water produced by the polycondensation reaction. The combustion gases enter the acid-gas contactor at a temperature Tg1 that is on the order of the polycondensation temperature Tpc, and is generally between 200 and 600°C, preferably between 400 and 500°C. Process - Recirculation loop and combustion gases

[0070] As discussed above, a recirculating fraction of the enriched phosphoric acid solution P1 exiting the gas-acid contactor (1) is reintroduced into the gas-acid contactor, thus forming a recirculation loop, while a spray fraction Pp is routed to the combustion chamber (2). The recirculating fraction is preferably greater than or equal to the spray fraction and ideally considerably greater than the spray fraction, with ratios Qp / Q2 of the spray flow rate Qp to the recirculation flow rate Q2 ranging from 0.1 / 99.9 to 49 / 51 (0.1 to 96%). Preferably, the ratio of flow rates Qp / Q2 is between 1 / 99 and 5 / 95 (1 to 5.3%).

[0071] Upon introduction into the gas-acid contactor, the supply stream F0 and recirculation stream F2 can be mixed before entering the gas-acid contactor to form a stream of a mixture of the contact feed solution P0 and the recirculated enriched phosphoric acid solution P2, as illustrated in the Figures 2 4. Alternatively, the F0 and F2 streams can be contacted after being introduced separately into the gas-acid contactor to form a stream of a mixture of the contact feed solution P0 and the recirculated enriched phosphoric acid solution P2, as illustrated in the Figures 3 5.

[0072] The recirculation loop is an important element of the present invention. The main consequence of introducing such a recirculation loop is that the ratio (Qg1 / (Q0+Q2)) between the mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor (1) and the total mass flow rate (Q0 + Q2) of the supply flow F0 and recirculation flow F2 introduced into the gas-acid contactor (1) is much lower than in the absence of such a recirculation loop. The ratio (Qg1 / (Q0+Q2)) according to the present invention is preferably between 0.1 and 50%, more preferably between 0.5 and 20% or less than 10%, and is ideally between 1 and 7%. In the absence of such a recirculation loop (i.e., Qg1 > 0, Q0 > 0 and Q2 = 0), the ratio (Qg1 / Q0) is considerably larger, with values ​​greater than 60%, generally greater than 100%, indicative of a combustion gas flow rate Qg1 greater than the supply flow rate Q0 of supply solution P0 of the contactor.

[0073] The recirculation loop thus allows control of the ratio between the combustion gas flow rate G1 and the total flow rate (Q0+Q2) of phosphoric acid feed solution P0 and recirculated enriched phosphoric acid solution P2. In particular, it allows a considerable increase in the mass of phosphoric acid solution in contact with the combustion gas. This has several advantages.

[0074] On the one hand, the transfer of P₂O₅ droplets and vapors from the combustion gases G1 to the mixture stream of solutions P0 and P2 is significantly greater. Consequently, the P₂O₅ concentration in the spray solution formed upon contact with the combustion gases is higher than if the flow rate ratio Qg1 / (Q0+Q2) had been higher. This improved gas / liquid contact allows for better recovery of the P₂O₅ from the combustion gases G1 by the enriched phosphoric acid solution P1. Furthermore, the combustion gases G3, after contact with streams F0 and F2, are thus cleaned of their P₂O₅ content, reducing the need for treatment before release into the atmosphere.

[0075] On the other hand, with such flow ratios, the temperature Tg3 of the combustion gases G3 after their contact with the flows F0 and F2 is reduced much more efficiently than in the process described in EP2411325 B1, thus not requiring any other heat exchanger (or at least one of lesser capacity), which is essential in the process of EP2411325 B1 to lower the temperature of the combustion gases to a value acceptable for their release into the atmosphere. Device

[0076] The method of the present invention can be implemented in a device comprising a combustion chamber (2), a gas-acid contactor (1), and various fluid connections between the combustion chamber and the gas-acid contactor. It is clear that the device may comprise several combustion chambers and / or several gas-acid contactors positioned in parallel or in series. Device - combustion chamber (2)

[0077] The combustion chamber (2) allows the combustion of the mixture solution Pm by spraying it into the flame. The mixture solution Pm is formed from the spray solution Pp mixed with the residual solution Pr to form a combustion solution P3 comprising phosphoric (and possibly polyphosphoric) acid purified of undesirable volatile matter. The walls of the combustion chamber must withstand the corrosive nature of the spray solutions Pp and the residual Pr, as well as the high temperatures within it. Preferably, the walls should be made of silicon carbide or amorphous carbon. Double walls can be used, with an inert gas or combustion gases circulating between the two walls. This can offer advantages in terms of wall temperature and impermeability to (poly)phosphoric acid solutions.

[0078] The combustion chamber (2) has one or more spray inlet(s) (2pu) into the combustion chamber allowing the introduction of a spray solution Pp at a flow rate Qp, or a mixture solution Pm at a flow rate (Qp + Qr), in sprayed form into a combustion unit located in an upper part of the combustion chamber (cf. Figures 2 &4). In a variant of the invention, the combustion chamber may include one or more residue inlet(s) (2pdu) allowing the introduction of a residual solution Pr at a flow rate Qr, separate from the spray inlet(s) (2pu) (cf. Figures 3µ). A supply of an inert gas, such as nitrogen, can be provided to optimize the atomization of the spray solution Pp and residues Pr and / or mixture Pm, which may have significant viscosity at the combustion chamber inlet. The residue inlet (2pdu) is in fluidic communication with a source of residual solution (Pr) that is aqueous and contains phosphorus and undesirable volatilizable matter.

[0079] The combustion chamber (2) comprises a combustion unit (2c) arranged in the upper part of the combustion chamber, capable of producing a flame with a temperature of at least 1000°C, preferably at least 1500°C, and even at least 1700°C, preferably 1800°C ± 50°C, by combustion of a fuel in the presence of oxygen. The flame temperature can be controlled by varying the oxygen flow rate supplied to the flame. The combustion unit comprises: A burner, fluid connections between the burner and, on the one hand, an oxygen source and, on the other hand, a fuel source (10) to feed the flame. Controlling the ratio between the fuel and oxygen supplies to the burner allows control of the flame temperature. Preferably, the fuel used is chosen from natural gas, methane, butane, or propane. The oxygen source is generally air or oxygen.

[0080] The combustion chamber (2) is equipped with a gas-liquid separator (9) to separate the resulting combustion solution P3 from the combustion gases G1. For example, the combustion gases can be separated from the combustion solution by increasing the transverse flow area, which reduces the flow velocity and therefore the kinetic energy of the G1 and combustion gas flows F3. As the flows are from top to bottom, the decrease in their kinetic energy causes the gases to slow down and can be deflected towards a baffle that guides them to the combustion gas outlet. Due to their higher density, the purified phosphoric and possibly polyphosphoric acid droplets from the combustion solution P3 continue their downward flow by gravity.

[0081] The combustion chamber (2) has a combustion outlet (2pd) for recovering a liquid phase of purified (poly)phosphoric acid. This outlet is located downstream of the combustion unit, which is itself located downstream of the mixing or spray and residue inlet. The term "downstream" refers to the flow direction of the spray solutions Pp and polyphosphoric acid P3 within the combustion chamber. As explained above, the flow direction is preferably from top to bottom, following the direction of gravity. The device can therefore be equipped with a storage tank for the phosphoric acid thus produced (not shown).Preferably, the device includes a heat exchanger (11) arranged between the combustion outlet (2pd) and the storage tank, in order to cool the combustion solution P3 from a temperature of about 200 and 650°C to a temperature of about 100 to 150°C when it reaches the storage tank.

[0082] Finally, the combustion chamber (2) is equipped with a flue gas outlet G1 for the combustion gases emanating from the flame. These flue gases are laden with droplets and vapors of P2O5 and undesirable volatilized matter. They have a temperature Tg1 and do not require cooling before being introduced into the gas-acid contactor. Device - gas-acid contactor (2)

[0083] The gas-acid contactor (1) allows heating and increasing the equivalent concentration in P2O5 units of the feed solution introduced into the contactor, before its entry into the combustion chamber (2) in order to optimize the purification yield of phosphoric acid and the energy consumption of the polycondensation reaction.

[0084] The gas-acid contactor (1) has a supply inlet (1pu) connected to a source of either a contactor feed solution P0 or a mixture of the contactor feed solution P0 and enriched phosphoric acid solution P2. As discussed above, the contactor feed solution P0 comprises between 0 and 54% P₂O₅, preferably from 0.1 to 50%, preferably from 1 to 35%, and even more preferably between 15 and 20% P₂O₅. The supply inlet (1pu) must be sized to allow the introduction of the contactor feed solution P0 at a feed rate Q₀ or the introduction of the mixture of the contactor feed solution P0 and recirculated enriched phosphoric acid solution P2 at a flow rate (Q₀ + Q₂). The recirculated enriched phosphoric acid solution, P2, can also be introduced into a recirculated enriched phosphoric acid inlet (1pru), P2, separate from the feed inlet (1pu).

[0085] The gas-acid contactor (1) is preferably a direct contactor. It includes a flue gas inlet (1gu) allowing the introduction of flue gases G1 from the flue gas outlet G1 into the gas-acid contactor. The supply inlet (1gu) must be sized to allow the introduction of the flue gases G1 at a flow rate Qg1. As discussed above, the flue gases G1, when brought into contact with the contactor's feed solution P0, (a) increase the temperature of the contactor's feed solution P0, (b) evaporate some of the water from the contactor's feed solution P0, and (c) exchange the droplets and vapors of P2O5 contained in the flue gas G1 with the solution P0.

[0086] The gas-acid contactor (1) is equipped with a recirculation inlet (1pru), allowing the introduction of a recirculated enriched phosphoric acid solution P2. In one embodiment of the invention, the flows F0 and F2 are mixed before being introduced into the gas-acid contactor, and the recirculation inlet is then the same as the feed inlet (1pu). In an alternative embodiment, the feed (1pu) and recirculation (1pru) inlets are separate. The recirculation inlet must be sized to allow the introduction of the recirculated enriched phosphoric acid solution P2 at a feed rate Q2.

[0087] The gas inlet (1gu), the supply inlet (1pu) and, if separate from the latter, the recirculation inlet (1pru) are arranged to allow, on the one hand, a contact between the supply flow F0 and recirculation flow F2 to form a flow (F0 + F2) of a mixture (P0+P2) of the supply solution P0 of the contactor and the recirculated enriched phosphoric acid solution P2 and, on the other hand, a contact of the flow of the mixture thus formed with the combustion gas flow G1.

[0088] The gas inlet (1gu) is preferably arranged so that the combustion gases G1 (and designated G2 upon contact) flow co-currently with the phosphoric acid supply flow F0 and recirculation flow F2. However, it is possible to arrange the gas inlet so that the combustion gases flow counter-currently to the flows F0 and F2.

[0089] The gas-acid contactor preferably includes a filling material through which the supply flow F0 and recirculation flow F2 of phosphoric acid solutions percolate. The filling material is preferably arranged on a perforated support, for example a support grid.

[0090] The gas-acid contactor (1) comprises one or more enriched phosphoric acid outlets (1pd, 1prd). The enriched phosphoric acid outlet(s) (1pd, 1prd) are positioned downstream of the gas inlet (1gu), which is itself positioned downstream of the feed inlet (1pu) and, if separate from the feed inlet, the recirculation inlet (1pru). The term "downstream" refers to the direction of flow of the feed and recirculation streams of the phosphoric acid feed solution and the recirculated enriched phosphoric acid P2 in the gas-acid contactor. The enriched phosphoric acid outlet(s) (1pd, 1prd) allow the enriched phosphoric acid solution P1, formed in the gas-acid contactor by the contact between the streams F0 and F2 and the combustion gases G1, to exit the gas-acid contactor.

[0091] The gas-acid contactor (1) includes a gas-liquid separator for separating liquids from gases after contact between the combustion gases G1 and the solutions P0 and P1. For example, the gas-acid contactor may include a defoamer that recovers any liquid droplets present in the contacted combustion gas G3 before it exits via the gas outlet (1gd).

[0092] The gas-acid contactor (1) also includes a combustion gas outlet (1gd), allowing the contacted combustion gases G3 to be evacuated from the gas-acid contactor after their contact with the mixture of solutions P0 and P2. The device can be followed by a combustion gas scrubbing tower G3 located downstream of the combustion gas outlet (1gd) of the gas-acid contactor, allowing the removal of any fluorinated and sulfur compounds that the gases contain before they are released into the atmosphere.

[0093] The device is equipped with a flue gas fluid connection (6) linking one end (6u) coupled to the flue gas outlet of the combustion chamber (2), to one end (6d) coupled to the flue gas inlet (1gu) in the gas-acid contactor (1). The temperature in this fluid connection (6) should preferably be kept as high as possible so that at the inlet (1gu) in the gas-acid contactor, the flue gases G1 have a temperature as close as possible to the temperature Tg1 they have at the outlet of the combustion chamber, i.e., approximately 200 to 600°C.

[0094] The device is equipped with a fluidic connection (3, 3p) linking an upstream end (3u) coupled to the enriched phosphoric acid outlet (1pd) of the acid gas contactor (1), to a downstream end (3d) coupled to the spray inlet (2pu) of the combustion chamber (2). Since the enriched phosphoric acid solution P1 has recovered most of the phosphorus molecules carried away by the combustion gases G1, the fluidic connection (3, 3p) allows these molecules to be reinjected into the combustion chamber to obtain a combustion solution P3 that is as rich as possible in P₂O₅. The enriched phosphoric acid solution P1 has a higher temperature than the contactor's feed solution P0, which allows for better management of the process's heat energy by injecting a solution already at a relatively high temperature into the combustion chamber.In cases where PPA production is desired, the highest concentration of P2O5 and the highest temperature of the enriched phosphoric acid solution P1 allow for increased concentration yield in the combustion chamber.

[0095] This improved transfer of phosphoric acid molecules and concentration yield is made possible by the recirculation loop, which allows a portion of the phosphoric acid flow P1 exiting the gas-acid contactor to be reintroduced into the same contactor. The device also includes a fluidic recirculation connection (3r) linking an upstream end coupled either to a recirculated enriched phosphoric acid outlet (1prd) of the gas-acid contactor (1), either at a branch point (5v) with the first fluidic connection (3), or at a branch point (4r) with the first fluidic connection (3u), at a downstream end (3r) coupled to the recirculation inlet (1pru) or (1pu) of the gas-acid contactor.

[0096] The device is provided with means for controlling and maintaining a ratio, Qp / (Qp + Q2), between a spray mass flow rate Qp flowing into the first fluidic connection (3) and a total mass flow rate (Qp + Q2) defined as the sum of the spray mass flow rate Qp and a recirculation mass flow rate Q2 flowing into the recirculation fluidic connection (3r) at a value less than 50%, preferably less than 10%, preferably less than 5%, even more preferably less than 2.5% and in which the ratio Qp / (Qp + Q2) has a value greater than 0.1%, preferably greater than 0.5%.

[0097] As illustrated in the Figure 2The fluid connections (3p) and (3r) can be separated along their entire length between the gas-acid contactor and the combustion chamber. On the one hand, the spray fluid connection (3p) links a first enriched phosphoric acid outlet (1pd) to the enriched phosphoric acid inlet (2pu) in the combustion chamber. On the other hand, the recirculation fluid connection (3r) links a second enriched phosphoric acid outlet (1prd) to the recirculated enriched phosphoric acid inlet (1pu) of the gas-acid contactor or to the supply connection (3a) that provides the gas-acid contactor with the contactor's feed solution P0. Each of the spray (3p) and recirculation (3r) fluid connections is equipped with a pump (4, 4r) sized to maintain the ratio Qp / (Qp + Q2) at a desired value, or with a fluid transfer system.

[0098] In an alternative variant illustrated to Figures 3 to 5The gas-acid contactor is equipped with a single outlet (1pd) of enriched phosphoric acid P1, which is coupled to a first fluidic connection (3). The upstream portions of the spray (3p) and recirculation (3r) fluidic connections are coupled to a branch point (5), thus forming a T or Y branch with the first fluidic connection (3). In this variant, various means can be used to control and maintain the ratio, Qp / (Qp + Q2), at the desired value.

[0099] In a first variant illustrated at the Figure 5The means for ensuring a ratio, Qp / (Qp + Q2) at the desired value include a pump (4) arranged on the spray fluid connection (3p) and having a pumping capacity of a liquid at a spray flow rate Qp and a recirculation pump (4r) arranged on the recirculation fluid connection (3r) and having a pumping capacity of a liquid at a recirculation flow rate Q2,

[0100] In a second variant illustrated with Figures 3 And 4, the means for ensuring a ratio, Qp / (Qp + Q2) include a pump (4) arranged on the first fluidic connection (3) upstream of the branch point (5) and having a pumping capacity of a liquid at a main flow rate (Qp+ Q2) and one or more valves (5v) (e.g., a three-way valve) arranged at the branch point (5) and allowing the main flow rate to be divided into a spray flow rate Qp to the spray fluidic connection (3p) and a recirculation flow rate Q2 to the recirculation fluidic connection (3r),

[0101] In a third variant (not shown), the means for ensuring the ratio, Qp / (Qp + Q2), include a pump (4) arranged on the first fluidic connection (3) upstream of the branch point (5) and having a pumping capacity of a liquid at a main flow rate (Qp + Q2), and piping forming the spray (3p) and recirculation (3r) fluidic connections, sized to obtain the desired ratio Qp / (Qp + Q2). This solution is less flexible than the first two in that, once the piping is sized, the ratio Qp / (Qp + Q2) cannot be easily varied, which is not necessarily a problem if the ratio is not to vary during the device's service life.

[0102] In a fourth variant (not illustrated), the means for ensuring the ratio, Qp / (Qp + Q2) include a pump (4) arranged on the first fluidic connection (3) upstream of the branch point (5) and having a capacity to pump a liquid at a main flow rate (Qp+ Q2) and forming the spray (3p) and recirculation (3r) fluidic connections and valves set to obtain the desired ratio Qp / (Qp + Q2).

[0103] Table 2 lists a series of value ranges for the different parameters suitable for implementing the process of the present invention. Table 2: Examples of parameter values ​​adapted to the process of the present invention Ti xpi Qi °C %[P 2 O 5 ] [kg / (h MW)] min max Min max Min max F0 20 200 ≥0% 54% 100 3000 F1 100 300 > F0 60% 600 123000 F2 100 300 > F0, = F1 60% 300 120000 FP 100 300 > F0, = F1 60% 300 3000 Fr 20 200 ≥ 1% 50% 5 1500 FM 20 300 ≥ 1% 60% 305 3000 F3 150 700 > FM, 76% 240 1500 G1 200 600 0.1% 15% -- -- G3 100 250 < G1 1% -- -- # Characteristic 1 Gas-acid contactor 1gd Gas-acid contactor combustion gas outlet 1gu Combustion gas inlet to the gas-acid contactor 1pd Outlet of enriched phosphoric acid solution, P1, from the gas-acid contactor 1pu Inlet for the contact solution, P0, or mixture (P0+P2) in the gas-acid contactor 1 pr Recirculating enriched phosphoric acid inlet, P2, into the gas-acid contactor (optional). 2 Combustion chamber 2c Combustion unit 2pd Combustion outlet, P3, of the combustion chamber 2pdu Residual inlet, Pr, into the combustion chamber 2pu Spray solution inlet, Pp, into the combustion chamber or combined inlet of direct feed streams and spray streams 3 First fluidic connection 3a Fluidic power supply connection 3d Downstream end of the first fluidic connection (3) 3p Fluidic connection for spraying 3r Fluid recirculation connection to the gas-acid contactor (1) 3rd Downstream end of the fluidic recirculation connection (3r) 3u Upstream end of the spray fluid connection (3p) or of the first fluid connection (3) 4 Pump 4r Recirculation pump 5v Valve or set of valves (e.g., three-way valve) 6 Fluid connection of combustion gases 6d Flue gas connection outlet 6u Combustion gas connection inlet 10 Fuel source for the combustion unit (10) 11 Heat exchanger FP Spray stream of enriched phosphoric acid solution F2 Recirculation flow of recirculated enriched phosphoric acid solution F3 Combustion solution flow Fr Residual flow of residual solution Pr FP Spray stream of enriched phosphoric acid solution FM Mixing flux of mixing solution Fm (Fr + Fp) G1 Combustion gas G3 Contacted combustion gases P0 Power supply solution P0+P2 Mixture of the contactor feed solution P0 and the recirculated enriched phosphoric acid solution P2 P1 enriched phosphoric acid solution P2 Recirculating enriched phosphoric acid solution P3 Combustion solution Pr residual solution Pp Spray solution PM Mixture solution (= Pr + Pp) Q0 Supply flow rate of the P0 supply solution of the contactor Q1 Enriched phosphoric acid debid at the contactor outlet (= Q2 + Qp) Q2 Recirculation flow rate of the recirculated enriched phosphoric acid solution Q3 Combustion solution flow rate Qg1 Flow of combustion gases to the gas-acid contactor (1) Qg2 Combustion gas flow rate in the gas-acid contactor (1) Gg3 Flow rate of contacted combustion gases outside the gas-acid contactor (1) Qm Flow rate of the mixing solution Pm Qp Spraying rate of the spraying solution Pp Qr Flow rate of the residual solution Pr T0 Temperature of the contactor's supply solution P0 T1 Temperature of the enriched phosphoric acid solution P1 T2 Temperature of the recirculated enriched phosphoric acid solution P2 T3 Combustion solution temperature P3 Tg1 Combustion gas temperature G1 Tg3 Temperature of contacted combustion gases G3

Claims

1. Process for purifying an aqueous residual solution comprising phosphorus molecules and undesirable volatilizable materials, comprising the following steps: (a) introducing into a gas-acid contactor (1) a feed stream F0 of a feed solution P0 which is aqueous and comprising phosphorus molecules preferably in the form of species of orthophosphate type at a mass concentration, xp0, of between 0 and 54% equivalent of P2O5 units, (b) introducing into the gas-acid contactor (1) a recirculation stream F2 of recirculated enriched phosphoric acid P2, (c) introducing into the gas-acid contactor (1) combustion gases G1, (d) contacting the feed stream F0 and the recirculation stream F2 and the combustion gases G1 to form in the gas-acid contactor, on the one hand, • an enriched phosphoric acid solution P1 comprising a mass concentration, xp1, with a P2O5 content which is greater than xp0 (xp1 > xp0) and, on the other hand, • contacted combustion gases G3, (e) separating the contacted combustion gases G3 from the enriched phosphoric acid solution P1, and then • evacuating the contacted combustion gases G3 from the gas-acid contactor (1), and • removing the enriched phosphoric acid solution P1 from the gas-acid contactor (1), (f) forming from said enriched phosphoric acid solution P1, on the one hand, • a recirculation stream F2 of recirculated enriched phosphoric acid solution P2 to introduce it into the gas-acid contactor (1) as defined in step (b) and, on the other hand, • a spraying stream Fp of the enriched phosphoric acid solution P1 to introduce it into a combustion chamber (2), (g) spraying through a flame burning in the upper part of the combustion chamber (2) a mixing stream Fm of a mixing solution (Pm) comprising phosphorus at a mass concentration, xpm, and undesirable volatilizable materials, the mixing stream being formed by, on the one hand, • the enriched phosphoric acid solution P1 and optionally, on the other hand, • a residual steam Fr of an aqueous residual solution Pr comprising a mass concentration, xpr, of at least 1% P2O5, to: • evaporate the water and thus concentrate the mixing solution Pm, • optionally oxidize and in any case evaporate the undesirable volatilizable impurities, • form combustion gases G1, and • form a combustion solution P3 having - a mass concentration, xp3, of P2O5 which is higher than the concentration of the mixing solution Pm, and - a content of volatilizable impurities which is lower than that of the mixing solution Pm, (h) separating the combustion solution P3 from the combustion gases G1 and • recovering the combustion solution P3, and • transferring the combustion gases G1 into the gas-acid contactor (1) as defined in step (c).

2. Process according to Claim 1, in which • the feed solution P0 comprises a concentration xp0 of between 0.1% and 50%, preferably from 1% to 35%, preferably from 5% to 20% P2O5 and in which • a flow rate Q0 of the feed solution P0 into the contactor expressed in terms of per nominal power units [MW-1] of the combustion chamber is preferably between 100 and 3000 kg / (h MW), preferably between 500 and 2500 kg / (h MW).

3. Process according to Claim 1 or 2, in which • the enriched phosphoric acid solution P1 is identical to the recirculated enriched phosphoric acid solution P2 and comprises a phosphorus concentration xp1 of greater than or equal to 1%, preferably less than 60%, more preferably between 5% and 50%, preferably between 10% and 40% P2O5 and in which • a total flow rate, Q1 = (Qp + Q2), of the solution P1 out of the contactor expressed in terms of per nominal power units [MW-1] of the combustion chamber is preferably between 600 and 123 000 kg / (h MW) , preferably between 1000 and 50 000 kg / (h MW) and • a ratio, Qp / (Qp + Q2), between the mass flow rate Qp of the spraying stream Fp and the total mass flow rate (Qp + Q2) is preferably less than 50%, preferably less than 10%, preferably less than 5%, more preferably less than 2.5% and in which the ratio Qp / (Qp + Q2) is greater than 0.1%, preferably greater than 0.5%.

4. Process according to any one of the preceding claims, in which • the residual solution Pr comprises a phosphorus concentration xpr of greater than or equal to 2%, preferably at least 5%, more preferably at least 10%, preferably at least 20% P2O5. • the residual solution Pr comprises a concentration xpv of undesirable volatilizable materials of at least 5 ppm, preferably at least 10 ppm, preferably at least 100 ppm, preferably at least 1%, preferably at least 5%, preferably at least 10%, more preferably at least 25% by weight relative to the total weight of the solution, and in which • a flow rate Qr of the residual solution Pr in the combustion chamber expressed in nominal power units [MW-1] of the combustion chamber is non-zero and preferably between 5 and 1500 kg / (h MW), preferably between 400 and 1000 kg / (h MW).

5. Process according to any one of the preceding claims, in which • a ratio Qr / (Qr + Q0) is between 0 and 99%, preferably between 5% and 90%, more preferably between 10% and 80%, or between 15% and 45%, and in which • the mixing stream (Fm) comprises a phosphorus concentration (xpm) of greater than 1% P2O5 (xpm > 1% P2O5) and in which Q0, Qp and Qr are mass flow rates of the feed solution (P0), of the enriched phosphoric acid solution P1 and of the residual solution Pr, respectively.

6. Process according to any one of the preceding claims, in which • the mixing solution Pm comprises a concentration xpm of greater than 2%, preferably greater than 5%, more preferably greater than 20%, more preferably greater than 30%, preferably greater than 40%, and more preferably between 45% and 60% P2O5, • the mixing solution Pm comprises a concentration xpv of undesirable volatilizable materials of at least 5 ppm, preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, preferably of at least 5%, preferably of at least 10%, more preferably of at least 25% by weight relative to the total weight of the solution, and in which • a flowrate Qm of the mixing solution Pm in the combustion chamber expressed in terms of per nominal power units [MW-1] of the combustion chamber is preferably between 305 and 3000 kg / (h MW), preferably between 200 and 2000 kg / (h MW).

7. Process according to any one of the preceding claims, in which • the combustion solution P3 comprises a phosphorus concentration xp3 of greater than 1% equivalent of P2O5 units, preferably greater than 10%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30% and 76%, and in which • the flow rate Q3 of the combustion solution P3 outside the combustion chamber expressed in nominal power units [MW-1] of the combustion chamber is preferably between 240 and 1500 kg / (h MW), preferably between 600 and 3000 kg / (h MW).

8. Process according to any one of the preceding claims, in which the feed stream F0 and the recirculation stream F2 are either • mixed before they are introduced into the gas-acid contactor to form a stream of a mixture of the feed solution P0 and of the recirculated enriched phosphoric acid solution P2, or • contacted after having been introduced separately into the gas-acid contactor to form a stream of a mixture of the feed solution P0 and of the recirculated enriched phosphoric acid solution P2.

9. Process according to any one of the preceding claims, in which • a residual flow rate Qr of the residual solution Pr is non-zero, • the residual solution Pr and, preferably, the spraying solution Pp comprise undesirable volatilizable materials, and in which the residual stream Fr and the spraying stream Fp are either • mixed to form the mixing stream Fm before being sprayed into the flame in the combustion chamber, or • sprayed separately into the combustion chamber to form the mixing stream Fm in the flame or just before reaching the flame.

10. Process according to any one of the preceding claims, in which the contact between the feed stream F0 and the recirculation stream F2 and the combustion gases G1 in step (d) takes place co-currently or countercurrently, preferably co-currently by flowing from an upper part to a lower part of the gas-acid contactor and in which, during the contact step (d), a ratio (Qg1 / (Q0 + Q2)) between a mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor (1) and a total mass flow rate (Q0 + Q2) of the contact feed stream F0 and the recirculation stream F2 introduced into the gas-acid contactor (1), is between 0.1% and 50%, preferably between 0.5% and 10%, more preferably between 1% and 7%.

11. Device for producing purified phosphoric acid P3 according to a process according to any one of the preceding claims, comprising: (A) a combustion chamber (2) having: • a spraying inlet (2pu) in the combustion chamber for introducing at a flow rate an enriched phosphoric acid solution P1 in sprayed form into a combustion unit (2c), • a residue inlet (2pdu) in the combustion chamber or upstream of the spraying inlet (2pu) for introducing a residual solution Pr or a mixture of residual solution Pr and of enriched phosphoric acid solution (P1) in sprayed form into a combustion unit (2c), • the combustion unit (2c) being arranged in the upper part of the combustion chamber, and being capable of forming a flame having a temperature of at least 1500°C by combustion of a combustible, said combustion unit comprising: o a burner, ∘ fluid connections between the burner and, on the one hand, an oxygen source and, on the other hand, a combustible source (10) for feeding the flame, • a combustion outlet (2pd) of the combustion chamber for recovering a combustion solution P3 in liquid phase, and arranged downstream of the combustion unit, which is itself arranged downstream of the spraying (2pu) and residue (2pdu) inlet, • an evacuation outlet for combustion gases G1 obtained from the flame, (B) a gas-acid contactor (1) having • a feed inlet (1pu) connected to a source of a feed solution P0, for introducing at a contact feed flow rate Q0 a feed solution P0, • a combustion gas inlet (1gu) for introducing into the gas-acid contactor combustion gases G1 at a flow rate Qg1, • a recirculation inlet (1pru) which is identical to or different from the contact feed inlet (1pu), for introducing a recirculated enriched phosphoric acid solution P2 at a recirculation flow rate Q2, • the feed (1pu) and / or recirculation (1pru) inlets and the gas inlet (1gu) being arranged to allow, on the one hand, ∘ contact between the feed stream F0 and the recirculation stream F2 to form a stream of a mixture of the feed solution P0 and of the recirculated enriched phosphoric acid solution P2 and, on the other hand ∘ contact of the mixture thus formed with the combustion gases G1, • one or more enriched phosphoric acid outlets (1pd), (C) a combustion gas fluidic connection (6) connecting an end (6u) coupled to the combustion gas evacuation outlet of the combustion chamber (2), to an end (6d) coupled to the combustion gas inlet (1gu) in the gas-acid contactor (1), (D) a first spraying fluidic connection (3p) connecting an upstream end (3u) coupled • to the enriched phosphoric acid outlet (1pd) of the gas-acid contactor (1) or • to a branching point (5) with a first fluidic connection (3) which is coupled to the enriched phosphoric acid outlet (1pd), to a downstream end (3d) coupled to the enriched phosphoric acid inlet (2pu) in the combustion chamber (2), characterized in that the device also comprises (E) a recirculation fluidic connection (3r) connecting an upstream end coupled • to a recirculated enriched phosphoric acid outlet (1pd) of the gas-acid contactor (1) or • to a branching point (5) with the first fluidic connection (3), to a downstream end coupled • to the recirculation inlet (1pru) of the gas-acid contactor (1) or • to a feed connection (3a) feeding the gas-acid contactor with feed solution P0, and (F) means for controlling and maintaining a ratio, Qp / (Qp + Q2), between a spraying mass flow rate Qp flowing in the spraying fluidic connection (3p) and a total mass flow rate (Qp + Q2) defined as the sum of the spraying mass flow rate Qp and of a recirculation mass flow rate Q2 flowing in the recirculation fluidic connection (3r) at a value of less than 50%, preferably less than 10%, preferably less than 5%, more preferably less than 2.5% and in which the ratio Qp / (Qp + Q2) has a value of greater than 0.1%, preferably greater than 0.5%.

12. Device according to Claim 11, in which the residue inlet (2pdu) is in fluidic communication with a source of a residual solution Pr which is aqueous and comprises phosphorus molecules in orthophosphate and / or polyphosphate form and undesirable volatilizable materials.