METHOD FOR PRODUCING POLYPHOSPHORIC ACID AND DEVICE FOR SUCH A METHOD

Description

FIELD OF INVENTION

[0001] The present invention relates to a process and apparatus for producing polyphosphoric acid (PPA) from phosphoric acid solutions. The process of the present invention is particularly efficient and makes it possible, on the one hand, to produce PPA solutions at concentrations higher than those currently available on the market and, on the other hand, to reduce the energy consumption required to produce said polyphosphoric acid solutions at concentrations comparable to those currently available on the market. The present invention allows for better management of the combustion gases produced by the process before their release into the atmosphere. Finally, the present invention makes it possible to produce a polyphosphoric acid solution of particularly high concentration. TECHNOLOGICAL BACKGROUND

[0002] Polyphosphoric acid (PPA) is a viscous liquid that can be produced, notably from phosphoric acid. It has applications as a dehydrating agent in the fine chemicals and polymer industries, the pharmaceutical industry, the petrochemical industry, and the manufacture of various chemicals. 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 commonly referred to as tripolyphosphoric acid. For n > 3, it is simply called polyphosphoric acid, regardless of the value of n. When n = 1, it has the formula of orthophosphoric acid. PPA can be produced, in particular, by the dehydration and polycondensation of orthophosphoric acid, H₃PO₄, according to chemical equation (1).This yields an aqueous solution of polyphosphoric acid whose molecular weight distribution of species depends, among other things, on the polycondensation temperature (= Tpc).

[0003] Polyphosphoric acid most often occurs as linear chains. However, cyclic, metaphosphoric-type, or even branched forms can also exist. As illustrated in the Figure 1 The polycondensation temperature determines the equilibrium concentration of H3PO4 in the liquid state ( Figure 1(a) ) and the latter determines the weight distribution of molecular species, condensed or not, according to the different values ​​of n ( Figure 1(c)Thus, an aqueous solution of phosphoric acid with a concentration below approximately 61% will have a weight fraction consisting predominantly of H₃PO₄ molecules, that is, composed almost exclusively of H₃PO₄ molecules (or HO[P(OH)(O)O]ₙH with n = 1). As the concentration increases, the solution contains more and more polymerized molecules, the value of n increasing with the concentration of P₂O₅ equivalents as indicated on the Figure 1(c) .

[0004] The dehydration and polycondensation of phosphoric acid to polyphosphoric acid requires the evaporation of water molecules, which necessitates an energy input, generally in the form of heat. Patent EP2411325 B1 reviews several known processes for the production of polyphosphoric acid and describes a new 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. The process described in patent EP2411325 B1 comprises: a step of spraying a P0 feed solution from the contactor into a flame located in an upper part of a combustion chamber in order to form a polyphosphoric acid solution by polycondensation accompanied by the formation of hot combustion gases, a step of separating the polyphosphoric acid solution and the hot combustion gases, and optionally, a step of contacting the hot gases thus separated in a gas-acid contactor with the P0 feed solution from the contactor before the step of spraying the latter into the flame.

[0005] Several advantages result from the contacting step in the gas-acid contactor of the hot gases with the contactor's feed solution P0. First, the contactor's feed solution P0 is preheated and concentrated by water evaporation before the spraying step into the flame, which significantly reduces the energy requirements during polycondensation in the combustion chamber. Second, the combustion gases may contain droplets or vapors of phosphoric acid which, through exchange during contact with the contactor's feed solution P0, increase the concentration and thus form an enriched phosphoric acid solution before the spraying step. The term "enriched phosphoric acid solution" refers to a phosphoric acid solution containing between 5 and 80% P₂O₅.Finally, the contact stage contributes to reducing the temperature of the combustion gases before they are released.

[0006] The present invention constitutes an improvement on the process described in EP2411325 B1; it maintains the advantages obtained by that process while substantially increasing the yield and reducing energy consumption. Furthermore, the process of the present invention makes it possible to produce polyphosphoric acid solutions at concentrations and purities never before achieved, with concentrations far exceeding 86% P₂O₅. The present invention and its advantages are described in more detail in the following sections. SUMMARY OF THE INVENTION

[0007] 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 producing polyphosphoric acid P3 comprising the following steps: (a) introduce into a gas-acid contactor a supply stream F0 of a phosphoric acid feed solution P0 having a mass concentration, xp0, of between 0 and 70% P2O5, (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 stream F0 of the contactor and the recirculation stream F2 with the combustion gases G1 to form, on the one hand, an enriched phosphoric acid solution P1 having a mass concentration, xp1, which is greater than xp0 (xp1 > xp0) and, on the other hand, contacted combustion gases G3, (e) separate the contacted combustion gases G3 from the enriched phosphoric acid solution P1, and then vent the contacted combustion gases G3 from the contactor gas-acid contactor and remove the enriched phosphoric acid solution P1 from the gas-acid contactor.(f) to form from said enriched phosphoric acid solution P1, on the one hand, a recirculating stream F2 of recirculated enriched phosphoric acid solution P2 for introduction 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 for introduction into a combustion chamber, (g) to spray through a burning flame in the upper part of the combustion chamber a mixing stream Fm of a mixing solution Pm of phosphoric acid, having a mass concentration xpm greater than that of the contactor feed stream F0 formed by, on the one hand, the enriched phosphoric acid solution P1 and optionally, on the other hand, a direct feed stream Fd of a direct aqueous feed solution Pd of phosphoric acid at a mass concentration, xd, of at least 20%, preferably of at least 40%,to: evaporate water and thus concentrate the mixture solution Pm, polymerize the molecules of the phosphoric acid mixture solution Pm to form a polyphosphoric acid solution P3 and form combustion gases G1, (h) separate the polyphosphoric acid solution P3 from the combustion gases G1 and recover the polyphosphoric acid solution P3, and transfer the combustion gases G1 into the gas-acid contactor (1) as defined in step (c).

[0008] In a preferred embodiment of the invention, the contactor feed solution P0 comprises a concentration xp0 of at least 54%, preferably at least 58% or even at least 60% P2O5. The flow rate Q0 of the contactor 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 / (hMW), preferably between 500 and 2500 kg / (hMW).

[0009] The enriched phosphoric acid solution P1 is identical to the recirculated phosphoric acid solution P2 and comprises a concentration xp1 preferably between 5 and 80%, preferably at least 60%, even more preferably at least 62% or even at least 65% 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%.

[0010] The direct feed solution of phosphoric acid Pd may comprise a concentration xpd of 20 to 80%, preferably at least 60%, even more preferably at least 62% or even at least 65% P 2 O 5 . The flow rate Qd of the direct feed solution Pd into the combustion chamber expressed per unit of rated power [MW -1< ] of the combustion chamber is preferably between 0 and 1500 kg / (h MW), preferably between 400 and 1000 kg / (h MW).

[0011] In one embodiment of the invention, the feed solution P0 may comprise at least 40% P2O5, preferably at least 50% or at least 54%, more preferably at least 58% or even at least 60% P2O5.

[0012] In another variant of the invention, the feed solution P0 comprises less than 40% P2O5, preferably less than 30% or less than 20%, still more preferably less than 5% or even 0% P2O5 and in which a flow rate Qd of the direct feed solution Pd is preferably non-zero.

[0013] The mixing solution Pm may comprise a concentration xpm of 15 to 80%, preferably at least 65%, even more preferably at least 70%, or even at least 75% of P₂O₅. The flow rate, Qd, of the direct feed solution Pd is preferably non-zero. The flow rate Qm of the mixing solution Pm into the combustion chamber, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably between 600 and 3000 kg / (hMW), preferably between 900 and 2000 kg / (hMW).

[0014] The polyphosphoric acid solution P3 may comprise a concentration xp3 of at least 76% equivalent in P2O5 units, preferably greater than 80%, particularly preferably greater than 88%, or is preferably between 76 and 90%, again preferably between 86 and 88%. The flow rate Q3 of the polyphosphoric acid solution P3 in the combustion chamber, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably between 240 and 1500 kg / (hMW), preferably between 600 and 3000 kg / (hMW).

[0015] The supply flow F0 and recirculation flow F2 can be either, mixed before their introduction into the gas-acid contactor to form a flow of a mixture of the contactor's feed 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 contactor's feed solution P0 and the recirculated enriched phosphoric acid solution P2.

[0016] Similarly, the direct feed flow Fd and the spray solution flow Fp are either, mixed to form the mixture stream Fm before being sprayed into the flame in the combustion chamber, or sprayed separately into the combustion chamber to form the mixture stream Fm in the flame or just before reaching the flame.

[0017] The contact between the contactor 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), a 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%.

[0018] The present invention also relates to a device for producing polyphosphoric acid P3 according to a process as discussed above, comprising, (A) a combustion chamber having: an enriched phosphoric acid inlet into the combustion chamber allowing the introduction at a flow rate of an enriched phosphoric acid solution P1 in sprayed form into a combustion unit, a direct feed inlet into the combustion chamber or upstream of the enriched phosphoric acid inlet allowing the introduction of a direct feed solution Pd or a mixture of direct feed solutions Pd 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: ∘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 polyphosphoric acid outlet from the combustion chamber to recover a liquid phase, arranged downstream of the combustion unit which is itself arranged downstream of the enriched phosphoric acid inlet, a combustion gas exhaust outlet G1 from the flame (B), a gas-acid contactor having a contact supply inlet connected to a source of a contactor feed solution P0, allowing the introduction of a contact supply solution P0 at a contact supply 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 supply inlet, allowing the introduction of a recirculated enriched phosphoric acid solution P2 at a recirculation rate, the contact supply and / or recirculation inlets and the gas inlet being arranged to allow, on the one hand,∘a contact between the contact supply flow F0 and recirculation flow F2 to form a flow of a mixture of the contactor supply 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 enriched phosphoric acid outlets, (C) a combustion gas fluid connection linking one end coupled to the combustion gas outlet of the combustion chamber, to one end (6d) coupled to the combustion gas inlet in the gas-acid contactor, (D) a first spray fluid connection linking an upstream end 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 (2pu) in the combustion chamber, Characterized in that the device also includes (E) a recirculating fluid connection (3, 3r) connecting an upstream end coupled to a recirculated enriched phosphoric acid outlet (1pd) of the gas-acid contactor (1) or to a branch point (5) with the first fluid connection (3), to a downstream end (3r) coupled to the recirculation inlet (1 pru) of the gas-acid contactor (1) or to a contact supply connection (3a) supplying the gas-acid contactor with contactor 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 fluid connection (3p) 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 in the recirculating fluid connection (3r) at a value less than 50%, preferably less than 10%, preferably less than 5%, 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%. BRIEF DESCRIPTION OF THE FIGURES.

[0019] Various aspects of the present invention are illustrated in the following Figures. Figure 1 :graphically illustrates 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 molecular species 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

[0020] 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 fluid flow. 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 3 The 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 used alone, therefore defines a mass flow rate. Conventions and Definitions

[0021] 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.

[0022] It is understood in this text by the expressions "Phosphoric acid solution", an aqueous solution comprising HO[P(OH)(O)O]nH, with n ≥ 1; "Orthophosphoric acid", an aqueous solution comprising predominantly HO[P(OH)(O)O]nH, with n = 1, i.e. an aqueous solution of phosphoric acid containing less than 61% by weight of P2O5; "Polyphosphoric acid solution" (= PPA), an aqueous solution comprising predominantly HO[P(OH)(O)O]nH, with n > 1; i.e. an aqueous solution of phosphoric acid containing more than 76% by weight of P2O5; "Polycondensation of phosphoric or orthophosphoric acid", the polycondensation of the molecules considered as represented by equations (1) and / or (2) below. An aqueous solution containing phosphorus is a solution containing phosphorus dissolved 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 figure 1(c) These species can be present in the form of ions. Process - gas-acid contacts

[0023] The process of the present invention comprises introducing the following flows into a gas-acid contactor (1). A feed flow F0 of feed solution P0 from the contactor at a feed rate Q0. The feed solution P0 is an aqueous solution of phosphoric acid with a mass concentration xp0 between 0 and 70% P₂O₅. In one embodiment of the present invention, the solution P0 is water. In this case, another source of phosphoric acid containing P₂O₅, called the direct feed solution Pd, is required for the production of polyphosphoric acid P3; this production is discussed further in the section "Process - Polycondensation". In an alternative embodiment of the present invention, the solution P0 is a phosphoric acid solution and includes P₂O₅ (xp0>0). For example, the contact feed solution P0 may include a concentration xp0 of at least 40% P2O5, preferably at least 50%, or even at least 54% or at least 58%, again preferably at least 60% or even at least 65% P2O5.The feed solution P0 generally comprises less than 62% P2O5, but higher concentrations of the feed solution do not interfere with the process, especially in the absence of a direct feed solution source Pd of phosphoric acid (see the section "Process - Polycondensation"). At such concentrations, the feed solution P0 consists mainly of orthophosphoric acid H3PO4 in aqueous solution (cf. . Figure 1(c)), but may contain condensed molecules (n > 1). The flow rate Q0 of solution P0 in the contactor, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably between 100 and 3000 kg / (hMW), preferably between 500 and 2500 kg / (hMW). A recirculation flow rate F2 of recirculated enriched phosphoric acid solution P2 will be defined in detail later. The flow rate Q2 of recirculated enriched phosphoric acid solution P2 in the contactor, expressed per unit of rated power [MW⁻¹] of the combustion chamber, may be between 300 and 120,000 kg / (hMW), preferably between 600 and 100,000 kg / (hMW), preferably between 9,000 kg / (hMW) and 90,000 kg / (hMW).The recirculation stream F2 of recirculated enriched phosphoric acid solution P2 results from the contact of a mixture of the feed stream F0 and recirculation stream F2 of recirculated enriched phosphoric acid solution P2 from a previous cycle with a flue gas stream G1. A flue gas stream G1 is formed during the polycondensation of a mixture stream Fm of a phosphoric acid solution Pm with a mass concentration xpm higher than that of the contact feed stream F0 in a combustion chamber; both of these streams will be described and discussed in detail later. The flue gas stream G1 comprises various phosphorus-containing molecules, in the form of droplets or vapors, carried from the combustion chamber to the gas-acid contactor. For example, the flue gas stream G1 may contain between 0.5 and 40% P₂O₅.

[0024] 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.

[0025] In a preferred variant, illustrated in the Figures 2 & 4The supply flow F0 and recirculation flow F2 are mixed before entering 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. 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).

[0026] In an alternative version, illustrated in Figures 3 & 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 (1pu) for F0 and a separate recirculation flow inlet (1pru) for F2.

[0027] The contact between the feed stream F0 and the recirculation stream 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 streams flow 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 to refer to 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.

[0028] 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.

[0029] 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-containing molecules transported by the flue gases as droplets and / or vapor are carried away by the streams F0 and F2, allowing the formation of the enriched phosphoric acid solution P1, characterized by a P₂O₅ content greater than or equal to that of each of the streams F0 and F2.On the other hand, a heat exchange takes place between the hot combustion gases, at a temperature Tg1 of the order of 500 to 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).

[0030] The enriched phosphoric acid solution P1 and the contacted combustion gases G3, formed as a result of contacting the phosphoric acid feed and recirculation streams F0 & F2 with the combustion gases G1, are then separated by means of separation well known to those skilled in the art, such as a gravity or centrifugal separator, a coalescer, a demister, a mattress, baffles, etc. The contacted combustion gases G3 are then discharged from the gas-acid contactor (1) for further treatment at a temperature substantially lower than that of the contacted combustion gases G3 introduced into said gas-acid contactor.Since most of the phosphorus contained in the G1 flue gas stream is 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.

[0031] The contacted flue gases G3 can also be scrubbed after exiting the gas-acid contactor with an aqueous scrubbing solution to dissolve and remove undesirable compounds (e.g., fluorinated, chlorinated, or sulfur compounds, etc.) before being 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

[0032] The enriched phosphoric acid solution P1 may have a concentration xp1 of 5 to 80% P₂O₅, preferably at least 60%, even more preferably at least 62%, or even at least 65%. The concentration of the enriched phosphoric acid solution P1 naturally depends on the concentration of the feed solution P0 and the flue gas flow G1. As discussed below, the concentration of the spray solution Pp is generally higher than that of the feed solution P0.

[0033] The enriched phosphoric acid solution P1 is divided into two distinct streams before, during, or after its evacuation from the gas-acid contactor: 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).

[0034] 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 undergone no alteration between their formation in the gas-acid contactor and their division into two separate recirculation flows F2 and spray flows Fp. The temperatures, Tp and T2, of solutions Pp and P2 are also substantially identical to the temperature T1 of solution P1, which can range from 120 to 400°C. Under steady-state conditions, solutions Pp and P2 preferably contain a higher P2O5 equivalent value than the contactor's feed solution P0. This is due to two main reasons.

[0035] 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 500 to 600°C (cf. Figure 3 ), leads to the evaporation of some of the water contained in the aqueous phase of the phosphoric acid solutions P0 and P2, which de facto increases the concentration of P2O5.

[0036] Secondly, as we will discuss later, the combustion gases G1 formed during the polycondensation, among other things, of the spray stream Fp of spray solution Pp in the combustion chamber include P2O5 in the form of droplets or vapors. The combustion gases G1 may comprise between 0.5 and 40% P2O5, preferably between 1 and 30%, preferably between 2 and 25%, or between 5 and 20% (cf. Figure 3). During contact with the feed and recirculation streams of P0 solutions 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 phosphorus-containing molecules than before contact, generally less than 0.1% P2O5 (cf. Figure 3 ). By this transfer of molecules to the acid mixture, the concentration of P 2 O 5 in the latter increases.

[0037] 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.

[0038] In an alternative variant, the enriched phosphoric acid solution P1 exits the gas-acid contactor through a first fluidic connection (3) or (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 by any other means well known and used industrially to distribute a flow between 2 feeds (for example, T or Y pipes with sets of regulated or unregulated valves).

[0039] 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 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.

[0040] 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 (F1 being the sum of the spray flow Fp and the recirculation flow F2) 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 even more 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 enriched phosphoric acid solution Pp to be further enriched in P2O5.

[0041] 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%).

[0042] 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 90,000 kg / (hMW). Thus, the flow rate Qp of the spray solution Pp flowing to 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 - Flux Fp, Fd and Fm

[0043] A mixing flow Fm of a mixing solution Pm having a mass concentration xpm greater than that of the contact feed flow F0 is sprayed through a flame burning in the upper part of a combustion chamber (2) for: evaporate water and thus concentrate the mixture solution Pm, obtain by polymerization polyphosphoric acid P3 and form combustion gases G1.

[0044] The mixing flow Fm is formed at least by the spray flow Fp which is introduced into the combustion chamber (2). In a preferred embodiment, the mixing flow Fm is formed, on the one hand, by the spray flow Fp and, on the other hand, by a direct feed flow Fd of a direct feed solution Pd of phosphoric acid.

[0045] In the variant including a direct feed flow Fd, the direct feed solution of phosphoric acid Pd comprises a concentration xd of at least 20%, preferably at least 40%. Preferably, the concentration xd is from 20 to 80%, preferably at least 60%, even more preferably at least 62%, or even at least 65% P₂O₅. The flow rate Qd of the direct feed solution Pd in ​​the combustion chamber, expressed per unit of rated power [MW⁻¹] of the combustion chamber, is preferably between 0 and 1500 kg / (h MW), preferably between 100 and 1000 kg / (h MW), preferably between 400 and 700 kg / (h MW). A flow rate Qd = 0 kg / (h MW) corresponds, of course, to the variant of the present invention not including a direct feed flow Fd. The temperature of the direct feed solution Fd can be between 20 and 200°C, preferably between 40 and 150°C, and even more preferably between 50 and 100°C.Preheating the Fd solution is advantageous in terms of the combustion efficiency of the Fm mixture solution in the flame.

[0046] In a steady-state production environment, the ratio, Qp / Q0, between the spray flow rate Qp of the spray flow Fp and the feed flow rate Q0 of the supply flow 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 F1 flow exiting the gas-liquid contactor. The value of this ratio may decrease as the value of the direct feed flow rate, Qd, increases.

[0047] The ratio of flow rates Qd / (Qp + Qd) between the direct feed flow rate Qd and the sum of the spray flow rates Qp and direct feed flow rates Qd represents the fraction of direct feed solution flow rate Pd entering the combustion chamber. The value of this ratio depends on inter alia of the P2O5 content of the feed solution P0, which is determining for the P2O5 content of the spray solution Pp.

[0048] In cases where the feed solution P0 contains at least 40% P₂O₅, preferably at least 50% or at least 54%, and even more preferably at least 58% or at least 60% P₂O₅, the concentration of the spray solution is high and the flow rate ratio Qd / (Qp + Qd) can be low; that is, the direct feed solution input Pd can be low or even zero. For example, the ratio Qd / (Qp + Qd) can be between 0 and 50%, preferably between 5 and 30% or between 10 and 20%.

[0049] If the feed solution P0 contains less than 40% P₂O₅, preferably less than 30% or less than 20%, even more preferably less than 5%, or even 0% P₂O₅, the P₂O₅ concentration in the spray solution is low, or even zero. It may then be advantageous, or even necessary in the case of a 0% P₂O₅ (= water) content, to increase the mass concentration of the mixing solution Pm by introducing the feed solution directly into the flame at a flow rate Qd. The ratio Qd / (Qp + Qd) is then preferably between 40 and 99%, even more preferably between 50 and 90% or between 60 and 75%. It is preferable, if the contact feed solution P0 comprises a concentration xp0 between 0 and 49%, preferably between 1 and 45%, even more preferably at most 40%, or even at most 20% or even at most 10% P 2 O 5, that the direct feed rate Qd is then preferably non-zero.

[0050] For a flow rate Qd > 0, the direct feed stream Fd can be sprayed directly into the flame to form the mixing stream Fm either directly into or upstream of the flame. Alternatively, the direct feed stream can be pre-mixed with the spray stream Fp before introducing the mixture Fm of the two streams into the combustion chamber. In the first case, the combustion chamber would include a separate inlet for introducing the direct feed stream Fd. In the second case, the mixing of the spray stream Fp and the direct feed stream Fd would take place upstream of the enriched phosphoric acid inlet (2pu). The ratio of the direct feed stream Fd introduced into the combustion chamber to the feed stream F0 introduced into the contactor is as described above.

[0051] Regardless of the value of the ratio Qd / (Qp + Qd), the mixing solution Pm preferably comprises a concentration xpm of 20 to 80%, preferably at least 40%, preferably at least 65%, preferably at least 70%, or even at least 75% P₂O₅. The flow rate Qm of the solution Pm in the combustion chamber is the sum of the spray flow rates Qp and the direct feed flow rates Qd. Expressed per unit of rated power [MW⁻¹] of the combustion chamber, the mixing flow rate Qm is preferably between 600 and 3000 kg / (h MW), preferably between 900 and 2000 kg / (h MW). Process - Polycondensation

[0052] The combustion in the flame of the mixture solution Pm forms a polyphosphoric acid solution P3 by polycondensation of the phosphoric acid molecules contained in the solution Pm and forms combustion gases G1.

[0053] The temperature reached by the polyphosphoric acid P3 solution in the flame is an important parameter of the process since the concentration of P2O5 obtained in this solution depends on it, as shown in the graph of the Figure 1(a) It is also important to maintain the Pm mixture solution in contact with the flame and combustion gases for a sufficient residence time at the polycondensation temperature Tpc required to obtain the expected P2O5 concentration and ensure the transformation into polyphosphoric acid.

[0054] 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 with an excess of 1 to 5% by volume of air, and preferably between 2 and 3% by volume. 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 1000°C, preferably at least 1500°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 50-400°C and, on the other hand, because the polycondensation reaction and the evaporation of water molecules from the solution and formed during polycondensation are energy-intensive.

[0055] In the absence of a direct feed flow Fd (i.e., Qd = 0), only the spray solution Pp is sprayed into the flame. If the direct feed flow rate Qd is non-zero, the direct feed flow Fd and the spray solution flow Fp can be mixed to form the mixture flow Fm before being sprayed into the flame in the combustion chamber, as illustrated in the Figures 2 & 5Alternatively, the two streams 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 .

[0056] It is preferable that the mixture solution sprayed into the flame reaches 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 a high concentration of P2O5, on the order of 86% and above, equivalent in P2O5 units 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 apparatus. The mixture solution Pm, which comprises orthophosphoric acid molecules and polyphosphoric acid polymer chains (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 according to chemical equation (2) (with m ≥ 1 and r ≥ 1):

[0057] The polyphosphoric acid solution P3 thus formed is then separated from the combustion gases G1 formed during the polycondensation of phosphoric acid in a gas-liquid separator (9). The polyphosphoric acid solution P3 is recovered while the combustion gases G1 are transferred to the gas-acid contactor (1) to be contacted with the feed stream F0 and recirculation stream F2, as described above.

[0058] The F3 flux of the recovered polyphosphoric acid solution P3 can have a high temperature of the order of 350 to 700°C, preferably 400 to 650°C, depending on the polycondensation temperature Tpc used. It is preferable to cool the P3 solution in a heat exchanger (11) (cf. Figure 2) at a temperature of around 100 to 200°C, for example between 110 and 150°C, preferably between 120 and 130°C, which allows a greater choice of materials for the storage tank of the polyphosphoric acid thus formed and cooled, while maintaining the solution in a liquid state.

[0059] The polyphosphoric acid P3 thus formed and recovered has a higher concentration 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 of the polyphosphoric acid P3 solution is generally greater than 76%, preferably greater than 80%, particularly preferably greater than 86%, or at least 87% and even at least 88%. For example, it can be between 76% and 90%, preferably between 86% and 89% P₂O₅. P₂O₅ contents exceeding 86% in wet processes are achievable with the method of the present invention.

[0060] The flow rate Q3 of the polyphosphoric acid solution P3 in the combustion chamber is representative of the polyphosphoric acid production capacity P3. Expressed per unit of nominal power [MW-1<] of the combustion chamber, the flow rate Q3 is preferably between 240 and 1500 kg / (h MW), preferably between 500 and 1000 kg / (h MW).

[0061] The G1 combustion gases consist primarily of phosphorus-containing molecules, which can be present in quantities ranging from 0.5 to 40% by weight of P₂O₅, generally between 0.5 and 35% by weight of P₂O₅, preferably between 1 and 30%, preferably between 2 and 25%, or between 5 and 20% by weight of P₂O₅. The temperature Tg1 of the transferred G1 combustion gases is significantly lower than the temperature that 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, mainly to evaporate the water produced by the polycondensation reaction. The combustion gases enter the acid-gas contactor at a temperature Tg1 which is on the order of the polycondensation temperature Tpc, and is generally between 400 and 700°C, preferably between 500 and 600°C. Process - Recirculation loop and combustion gas

[0062] 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%).

[0063] 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.

[0064] The recirculation loop is the major distinguishing feature between the present invention and the method described in EP2411325 B1. 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 smaller 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 ideally between 1 and 7%. In a process according to EP2411325 B1 comprising contacting the combustion gases with the feed stream F0 (i.e., Qg1 > 0, Q0 > 0) but no recirculation stream F2 (i.e., Q2 = 0), the ratio (Qg1 / Q0) is considerably larger, with values ​​above 60%, generally above 100%, indicative of a combustion gas flow rate Qg1 greater than the supply flow rate Q0 of supply solution P0 of the contactor.

[0065] 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.

[0066] On the one hand, the transfer of phosphorus-containing molecules in droplets and / or vapors from the combustion gases G1 to the mixture stream of solutions P0 and P2 is significantly greater. The P₂O₅ concentration in the spray solution formed upon contact with the combustion gases is therefore higher than if the flow rate ratio Qg1 / (Q0+Q2) had been higher, as in EP2411325 B1. This improved gas / liquid contact allows for better recovery of phosphorus 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.

[0067] On the other hand, with such flow ratios, the temperature Tg3 of the combustion gases G3 after their contact with the fluxes F0 and F2 of phosphoric acid solutions 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

[0068] 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)

[0069] The combustion chamber (2) allows for the polycondensation of the mixing solution Pm by spraying it into the flame. The mixing solution Pm is formed from the spray solution Pp and, if necessary, mixed with the direct feed solution Pd to form a polyphosphoric acid solution P3. The walls of the combustion chamber must withstand the corrosive nature of the spray solutions Pp and direct feed Pd and the high temperatures within it; it is preferable that the walls be made of silicon carbide or amorphous carbon. Double walls can be used with an inert gas or the combustion gases circulating between the two walls, which can have advantages in terms of wall temperature and impermeability to (poly)phosphoric acid solutions.

[0070] 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 + Qd), 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 direct feed inlet(s) (2pdu) allowing the introduction of a direct feed solution Pd at a flow rate Qd separate from the spray inlet(s) (2pu) (cf. Figures 3 5). A supply of an inert gas, such as nitrogen, may be provided to optimize the spraying of the spray solution Pp and / or direct feed Pd and / or mixture Pm, which may have a significant viscosity at the inlet of the combustion chamber.

[0071] 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.

[0072] The combustion chamber (2) is equipped with a gas-liquid separator (9) to separate the resulting polyphosphoric acid solution P3 from the combustion gases G1. For example, the combustion gases can be separated from the polyphosphoric acid solution by increasing the transverse flow area, which reduces the flow velocity and therefore the kinetic energy of the gas and polyphosphoric acid flows. As the flows are downward, 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 polyphosphoric acid droplets P3 continue their downward flow under gravity.

[0073] The combustion chamber (2) has a polyphosphoric acid outlet (2pd) for recovering a liquid phase, located downstream of the combustion unit, which is itself located downstream of the enriched phosphoric acid inlet (2pu) and the direct feed inlet (2pdu). 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 polyphosphoric acid thus produced (not shown).Preferably, the device includes a heat exchanger (11) arranged between the polyphosphoric acid outlet (2pd) and the storage tank, in order to cool the polyphosphoric acid solution from a temperature of about 350 and 650°C to a temperature of about 100 to 150°C when it reaches the storage tank.

[0074] Finally, the combustion chamber (2) is equipped with a flue gas outlet G1 for the combustion gases produced by the flame. These flue gases have a temperature Tg1 on the order of the polycondensation temperature Tpc and do not require cooling before being introduced into the gas-acid contactor. Device - gas-acid contactor (2)

[0075] The gas-acid contactor (1) allows heating and increasing the concentration of P2O5 of the contactor feed solution introduced into the contactor, before its entry into the combustion chamber (2) in order to optimize the efficiency and energy consumption of the polycondensation reaction.

[0076] The gas-acid contactor (1) has a supply inlet (1pu) connected to a source of either the contactor's supply solution P0 or a mixture of the contactor's supply solution P0 and enriched phosphoric acid solution P2. As discussed above, the contactor's supply solution P0 comprises between 0 and 70% P₂O₅, at least 40%, preferably at least 50%, preferably at least 54%, even more preferably at least 58%, or even at least 60% P₂O₅. The supply inlet (1pu) must be configured to allow the introduction of the contactor's supply solution P0 at a supply rate Q₀ or the introduction of the mixture of the contactor's supply 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).

[0077] 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.

[0078] 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.

[0079] 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.

[0080] The gas inlet (1gu) is preferably arranged so that the combustion gases G1 (and designated G2 upon contact) flow co-currently with the feed flow F0 and recirculation flow F2 of phosphoric acid solution. However, it is possible to arrange the gas inlet so that the combustion gases flow counter-currently to the flows F0 and F2.

[0081] 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.

[0082] 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.

[0083] 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).

[0084] 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 for the contacted combustion gases G3 located downstream of the combustion gas outlet (1gd) of the gas-acid contactor, allowing the removal of any undesirable compounds such as, for example, fluorinated or sulfur compounds that the gases may contain before being released into the atmosphere.

[0085] 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 500 to 600°C.

[0086] The device is equipped with a fluidic spray connection (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 enriched phosphoric acid inlet (2pu) of the combustion chamber (2). Since the enriched phosphoric acid solution P1 has a higher temperature and P2O5 concentration than the contactor's feed solution P0, the polycondensation efficiency in the combustion chamber is improved.Since the transfer of phosphorus-containing molecules from the combustion gases G1 to the mixture of feed and recirculation solutions of phosphoric acid P0 and P2 is better than the transfer between the combustion gases G1 and the feed solution P0 of the contactor described in the device of EP2411325 B1, the efficiency obtained with the device and method of the present invention is superior to that obtained in EP2411325 B1.

[0087] This improved transfer of phosphoric acid molecules and the yield of the polycondensation reaction is made possible by the recirculation loop, which allows a portion of the phosphoric acid solution 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 an outlet of recirculated enriched phosphoric acid solution (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.

[0088] 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%.

[0089] 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.

[0090] 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.

[0091] 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,

[0092] 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),

[0093] In a third variant (not shown), the means for controlling 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 lifetime. Polyphosphoric acid solution P3

[0094] Spraying a Pm mixture solution obtained by the process of the present invention into a flame allows the production of polyphosphoric acid P3 solutions with characteristics unmatched to date. Thanks to the high P2O5 content of the Pm spray solution specific to the process of the present invention and its high temperature upon entering the combustion chamber, polyphosphoric acid solutions with very high P2O5 concentrations can be produced by wet mixing, the value of which depends on inter alia of the polycondensation temperature.

[0095] In the case where the polycondensation temperature, Tpc, of the spray solution Pp during the polycondensation reaction reaches at least 500°C, preferably at least 600°C or at least 650°C, a polyphosphoric acid solution P3, never before produced, is obtained in the form of a viscous liquid, comprising a P2O5 content of at least 86%, preferably at least 87%, even more preferably at least 88%, a very low organic compound content expressed as Total Organic Carbon (TOC) of less than or equal to 100 ppm, preferably less than or equal to 80 ppm, and a very low SiO2 content of less than or equal to 100 ppm, preferably less than or equal to 80 ppm.

[0096] Preferably, the polyphosphoric acid solution P3 of the present invention has an Fe content of less than or equal to 10 ppm, preferably less than or equal to 5 ppm, an Sb content of less than or equal to 10 ppm, preferably less than or equal to 5 ppm, and an SO4 content of less than or equal to 11 ppm, preferably less than or equal to 5 ppm. The contents of the various compounds above are expressed relative to the polyphosphoric acid solution. Example of a completed project

[0097] A pilot device according to the present invention has been constructed and tested. It comprises a gas-acid contactor (1) and a combustion chamber (2). The fluidic connections include a spray fluidic connection (3p) that is completely separate and independent of the recirculation connection (3r) as illustrated in the Figure 2 Each of the fluid connections for spraying (3p) and recirculation (3r) is equipped with a pump (4, 4r) to control and maintain a constant flow rate ratio Qp / (Qp+Q2) at various values ​​between 0.75 and 6.7%. The combustion unit is connected to a natural gas source and an air supply with controllable flow rates to regulate the flame temperature. The combustion chamber has a direct feed inlet (2mu) as illustrated in the Figure 4 .

[0098] Table 1 shows the concentration ranges for P₂O₅, flow rates, and temperatures of the different phosphoric acid flows P₀ to P₃ and combustion gases G₁, G₃ tested with the pilot device. The positions of said flows in a device according to the present invention are shown in Table 1. Figure 3 The polycondensation temperature Tpc was between 400 and 650°C. Table 1: Composition ranges, flow rates, and temperatures of the different flows with Qd = 0 Flow Fluid Concentration (% P2O5) Q (kg / h) T (°C) F0 P0 (liquid) > 40% P 2 O 5 Q0 = 105-445 T0 = 20 - 100 FP P1 (liquid) > 62% (> 70%) P 2 O 5 Qp = 150-400 T1 = 200 - 300 F2 P2 (liquid) > 62% (> 70%) P 2 O 5 Q2 = 1,000-20,000 T2 = 200 - 300 Fd Pd (liq) 0 Qd = 0 Td = NA gas G1 (gas) 0.5 - 10% P 2 O 5 Qg1 = 200-400 Tg1 = 500 - 600 F3 P3 (liquid) > 76% (> 86%) P 2 O 5 Q3 = 150- 250 T3 = 400 - 650 gas G3 (gas) < 1% P 2 O 5 Qg3 = 200-400 Tg3 = 50 - 100

[0099] Table 1 shows that with a feed solution P0 comprising 62% P2O5, and a polycondensation temperature Tpc = 630°C, a polyphosphoric acid solution P3 comprising 88% P2O5 was produced with the pilot device by applying a flow rate ratio, Qp / (Qp + Q2) = 1%.

[0100] 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 °C xpi %[P 2 0 5 ] Qi (kq / (h MW) min max min max min max F0 20 100 0% 70% 100 3000 F1 200 300 5% 80% 600 123000 F2 200 300 5% 80% 300 120000 FP 200 300 5% 80% 300 3000 Fd 50 200 20% 80% 0 1500 FM 20 300 20% 80% 600 3000 F3 350 700 76% 88% 240 1500 G1 500 600 0.5% 5 -- -- G3 100 250 0 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 1pru Recirculating enriched phosphoric acid inlet, P2, into the gas-acid contactor (optional). 2 Combustion chamber 2c Combustion unit 2pd Polyphosphoric acid, P3, outlet from the combustion chamber 2pdu Direct feed solution inlet, Pd, into the combustion chamber 2pu Inlet of spray acid solution, 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) or of the spray fluidic connection (3p) 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 Flow of polyphosphoric acid solution Fd Direct feed flow of direct feed solution Pd FP Spray stream of enriched phosphoric acid solution FM Mixing flux of mixing solution Fm (=Fd+ Fp) G1 Combustion gas G3 Contacted combustion gases P0 Contactor 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 Polyphosphoric acid Pd Direct feeding solution Pp Spray solution PM Mixture solution (= Pd + Pp) Q0 Supply flow rate of the P0 supply solution of the contactor Q1 Enriched phosphoric acid flow rate at the contactor outlet (= Q2 + Qp) Q2 Recirculation flow rate of the recirculated enriched phosphoric acid solution Q3 Polyphosphoric acid flow rate Qg1 Flow of combustion gases to the gas-acid contactor (1) Qg2 Combustion gas flow rate in the gas-acid contactor (1) When Direct feed solution flow rate Pd Qp Spraying rate of the spraying solution Pp Qm Flow rate of the mixing solution Pm Gg3 Flow rate of contacted combustion gases outside the gas-acid contactor (1) 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 Temperature of the polyphosphoric acid solution P3 Tg1 Combustion gas temperature G1 Tg3 Temperature of contacted combustion gases G3

Claims

1. Process for the production of polyphosphoric acid P3 comprising the following stages: (a) introducing, into a gas / acid contactor (1), a feed stream F0 of the contactor of a feed solution P0 of phosphoric acid which exhibits a concentration by weight, xp0, of between 0% and 70% P2OS, (b) introducing, into the gas / acid contactor (1), a recirculation stream F2 of recirculated enriched phosphoric acid solution P2, (c) introducing, into the gas / acid contactor (1), combustion gases G1, (d) bringing into contact the feed stream F0 of the contactor, the recirculation stream F2 and the combustion gases G1, in order to form, on the one hand, • an enriched phosphoric acid solution P1 comprising a concentration by weight, xp1, 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, then • discharging the contacted combustion gases G3 from the gas / acid contactor (1) and • taking the enriched phosphoric acid solution P1 out of 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, in order to introduce it into the gas / acid contactor (1) as defined in stage (b) and, on the other hand, • a spray stream Fp of the enriched phosphoric acid solution P1, in order to introduce it into a combustion chamber (2), (g) spraying, through a burning flame in the upper part of the combustion chamber (2), a mixing stream Fm of a mixing solution Pm of phosphoric acid, having a concentration by weight xpm which is greater than that of the feed stream F0 of the contactor formed by, on the one hand, • the enriched phosphoric acid solution P1 and optionally, on the other hand, • a direct feed stream Fd of a direct aqueous feed solution Pd of phosphoric acid at a concentration by weight, xd, of at least 20%, preferably of at least 40%, in order: • to evaporate water and thus to concentrate the mixing solution Pm, • to polymerize the molecules of the mixing solution Pm of phosphoric acid in order to form a polyphosphoric acid solution P3 and • to form combustion gases G1, (h) separating the polyphosphoric acid solution P3 from the combustion gases G1 and • recovering the polyphosphoric acid solution P3, and • transferring the combustion gases G1 into the gas / acid contactor (1) as defined in stage (c).

2. Process according to Claim 1, in which: • the feed solution P0 of the contactor comprises a concentration xp0 of 0% to 70%, preferably at least 54%, more preferably at least 58% or even at least 60% P2O5, and in which • a flow rate Q0 of the feed solution P0 of the contactor in the contactor, expressed per nominal power unit [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 phosphoric acid solution P2 and comprises a concentration xp1 of 5% to 80%, preferably at least 60%, more preferably at least 62% or even at least 65% P2O5, and in which • a total flow rate, Q1 = (Qp + Q2), of the solution P1 outside the contactor, expressed per nominal power unit [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), of the flow rate by weight Qp of the spray stream Fp to the total flow rate by weight (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 phosphoric acid direct feed solution Pd comprises a concentration xpd of 20% to 80%, preferably at least 60%, more preferably at least 62% or even at least 65% P2O5, and in which • a flow rate Qd of the direct feed solution Pd into the combustion chamber, expressed per nominal power unit [MW-1] of the combustion chamber, is preferably between 0 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: • the feed solution P0 comprises at least 40% P2O5, preferably at least 50% or at least 54%, more preferably at least 58% or even at least 60% P2O5.

6. Process according to any one of Claims 1 to 4, in which: • the feed solution P0 comprises less than 40% P2O5, preferably less than 30% or less than 20%, more preferably less than 5% or even 0% P2O5, and in which a flow rate Qd of the direct feed solution Pd is nonzero.

7. Process according to any one of the preceding claims, in which: • the mixing solution Pm comprises a concentration xpm of 15% to 80%, preferably at least 40%, preferably at least 65%, more preferably at least 70% or even at least 75% of P2O5, and in which a flow rate Qd of the direct feed solution Pd is preferably nonzero, • a flow rate Qm of the mixing solution Pm in the combustion chamber, expressed by nominal power unit [MW-1] of the combustion chamber, is preferably between 600 and 3000 kg / (h MW), preferably between 900 and 2000 kg / (h MW).

8. Process according to any one of the preceding claims, in which: • the polyphosphoric acid solution P3 comprises a concentration xp3 of at least 76% equivalent in P2O5 units, preferably greater than 80%, particularly preferably greater than 88%, or is preferably between 76% and 90%, more preferably between 86% and 88%, and in which • the flow rate Q3 of the polyphosphoric acid solution P3 in the combustion chamber, expressed per nominal power unit [MW-1] of the combustion chamber, is preferably between 240 and 1500 kg / (h MW), preferably between 600 and 3000 kg / (h MW).

9. 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, in order to form a stream of a mixture of the feed solution P0 of the contactor and of the recirculated enriched phosphoric acid solution P2, or • brought into contact after having been introduced separately into the gas / acid contactor, in order to form a stream of a mixture of the feed solution P0 of the contactor and of the recirculated enriched phosphoric acid solution P2.

10. Process according to any one of the preceding claims, in which the direct feed stream Fd and the spray solution stream Fp are either: • mixed in order to form the mixing stream Fm before being sprayed into the flame in the combustion chamber, or • sprayed separately into the combustion chamber in order to form the mixing stream Fm in the flame or immediately before reaching the flame.

11. Process according to any one of the preceding claims, in which contact between the feed stream F0 of the contactor and the recirculation stream F2 and the combustion gases G1 in stage (d) is carried out co-currentwise or countercurrentwise, preferably co-currentwise, by flowing from an upper part toward a lower part of the gas / acid contactor, and in which, during the contacting stage (d), the ratio (Qg1 / (Q0 + Q2)) between a flow rate by weight Qg1 of the combustion gas G1 introduced into the gas / acid contactor (1) and a total flow rate by weight (Q0 + Q2) of the contact feed stream F0 and of the recirculation feed 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%.

12. Device for the production of polyphosphoric acid P3 following a process according to any one of the preceding claims, comprising: (A) a combustion chamber (2) exhibiting: • an enriched phosphoric acid inlet (2pu) in the combustion chamber, making possible the introduction at a flow rate of an enriched phosphoric acid solution P1 in the sprayed form into a combustion unit (2c), • an inlet for direct feeding (2pdu) into the combustion chamber or upstream of the enriched phosphoric acid inlet (2pu), making possible the introduction of a direct feed solution Pd or of a mixture of direct feed solution Pd and of enriched phosphoric acid solution P1 in the 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 fuel, said combustion unit comprising: o a burner, o fluid connections between the burner and, on the one hand, a source of oxygen and, on the other hand, a source of fuel (10) making it possible to feed the flame, • an outlet for polyphosphoric acid (2pd) from the combustion chamber in order to recover a liquid phase, and arranged downstream of the combustion unit, which is itself arranged downstream of the enriched phosphoric acid inlet (2pu), • an outlet for a discharge of combustion gas G1 resulting from the flame, (B) a gas / acid contactor (1) exhibiting • a contact feed inlet (1pu) connected to a source of a feed solution P0 of the contactor, making possible the introduction at a contact feed flow rate Q0 of a feed solution P0 of the contactor, • an inlet for combustion gases (1gu) making possible the introduction, into the gas / acid contactor, of the combustion gases G1 at a flow rate Qg1, • a recirculation inlet (1pru) identical to or different from the contact feed inlet (1pu), making possible the introduction of a recirculated enriched phosphoric acid solution P2 at a recirculation flow rate Q2, • the contact feed inlet (1pu) and / or the recirculation inlet (1pru) and the gas inlet (1gu) being arranged in order to make possible, on the one hand, o contact between the contact feed stream F0 and the recirculation stream F2, in order to form a stream of a mixture of the feed solution P0 of the contactor and of the recirculated enriched phosphoric acid solution P2 and, on the other hand, o contact of the mixture thus formed with the combustion gases G1, • one or more outlets for enriched phosphoric acid (1pd), (C) a combustion gas fluid connection (6) linking one end (6u), coupled to the outlet for discharge of the combustion gases from the combustion chamber (2), to an end (6d) coupled to the inlet for combustion gases (1gu) into the gas / acid contactor (1), (D) a first spray fluid connection (3p) linking an upstream end (3u), coupled: • to the outlet for enriched phosphoric acid (1pd) of the gas / acid contactor (1) or • to a branch point (5) with a first fluid connection (3) which is coupled to the outlet for enriched phosphoric acid (1pd), to a downstream end (3d) coupled to the enriched phosphoric acid inlet (2pu) into the combustion chamber (2), characterized in that the device additionally comprises: (E) a recirculation fluid connection (3, 3r) linking an upstream end, coupled, • to an outlet for recirculated enriched phosphoric acid (1pd) of the gas / acid contactor (1) or • to a branch point (5) with the first fluid connection (3), to a downstream end (3r) coupled, • to the recirculation inlet (1pru) of the gas / acid contactor (1) or • to a contact feed connection (3a) feeding the gas / acid contactor with feed solution P0 for the contactor, and (F) means for controlling and maintaining a ratio, Qp / (Qp + Q2), between a spray flow rate by weight Qp, flowing in the spray fluid connection (3p), and a total flow rate by weight (Qp + Q2) defined as the sum of the spray flow rate by weight Qp and of a recirculation flow rate by weight Q2 flowing in the recirculation fluid connection (3r), at a value of less than 50%, preferably less than 10%, preferably less than 5%, more preferably of less than 2.5%, and in which the ratio Qp / (Qp + Q2) has a value of greater than 0.1%, preferably of greater than 0.5%.