Process for purifying impure phosphate-containing acidic solutions and apparatus for applying same - Patents.com

Abstract

The present invention relates to a process and apparatus for purifying an impure phosphate-containing acidic solution through a nanofiltration station comprising several nanofiltration membrane units, each comprising a retentate side and a permeate side separated by a membrane, the process comprising feeding the phosphate-containing acidic solution through an entry line to a first membrane unit of n 2:1 membrane units arranged in series, the nth permeate exiting the nth membrane unit forming a nanofiltered phosphate solution. The gist of the invention is the provision of at least one permeate recirculation loop branching off from the retentate side of the first membrane unit and closing the loop at the entry line to combine at least one of the three permeates with the phosphate-containing acidic solution.

Description

[Technical Field]

[0001] The present invention relates to a process for purifying impurity-containing phosphate-containing acidic solutions, such as phosphoric acid solutions, and an apparatus for carrying out said process. The process and apparatus include a novel nanofiltration station designed to enhance both the P2O5 yield of the phosphate-containing acidic solution and the rate of impurity removal therefrom. [Background technology]

[0002] Phosphate-containing acidic solutions, including but not limited to phosphoric acid solutions, are commercially available as so-called "merchant grade," "technical grade," or "feed grade" solutions, or can be produced from secondary phosphate sources, such as fly ash product attacks. However, these grades contain significant amounts of impurities in ionic form that must be removed for many higher-performance applications, such as in the food, pharmaceutical, chemical, and steel industries. Impurities typically include cations such as Al, Ca, Cu, Cr, Fe, K, Mg, Mn, Mo, Na, Ni, Sr, Ti, Cd, As, V, and Zn. For example, Al, Ca, Cr, Fe, and Mg are fairly common impurities that are generally present in higher amounts. However, the suitability of each impurity depends on the end use, which determines the required removal rate for each impurity type. Nanofiltration has been successfully used in several processes to purify phosphate-containing acidic solutions, such as phosphoric acid solutions.

[0003] Nanofiltration is a membrane filtration-based process that uses nanometer-sized pores that pass through the membrane. As illustrated in Figure 10(a), a nanofiltration membrane unit (M1) for nanofiltration includes a retentate side and a permeate side separated by a membrane. Nanofiltration membranes typically have pore sizes of 1 to 10 nanometers, smaller than those used in microfiltration and ultrafiltration but slightly larger than those used in reverse osmosis. The nanofiltration membrane unit is fed with an impurity-containing solution on its retentate side. The impurity-lean permeate passes through the membrane from the retentate side of the nanofiltration membrane unit to the permeate side and exits the nanofiltration membrane unit. The impurity-rich retentate is retained on the retentate side of the nanofiltration membrane unit and exits the retentate side of the nanofiltration membrane unit. In the context of the present invention, the terms "lean and rich in impurities" refer to the impurity content of the solution fed to the nanofiltration membrane unit and are completely independent of the impurity content of any other solution in the process. For example, if a process includes two or more nanofiltration membrane units, depending on their respective locations, the impurity-lean permeate of a first nanofiltration membrane unit may have a higher impurity content than the impurity-rich retentate of a second nanofiltration membrane unit.

[0004] Nanofiltration has been used in many industrial applications. For example, Australian Patent Application Publication No. 2008202302 describes a reverse osmosis and nanofiltration process for purifying water. US Patent Application Publication No. 20120238777 describes a nanofiltration process for recovering sugars at the end of refining, allowing for substantial energy savings by reducing the amount of water evaporated. US Patent No. 6,083,670 describes a nanofiltration process or rejuvenation treatment of photoresist developer wastewater containing primarily photoresist and tetraalkylammonium (TAA) ions.

[0005] Nanofiltration for the purification of phosphate-containing acidic solutions is described in WO2013133684, where two nanofiltration membrane units are arranged in series as illustrated in Figure 11(c) (reference numbers in parentheses correspond to the reference numbers used in WO2013133684). The membrane is an organic nanofiltration membrane having adsorbed thereon at least one water-soluble polymer containing at least one amine functional group, one aromatic amine functional group, one acid functional group, and / or one alcohol functional group. By arranging two nanofiltration membrane units in series, impurity removal is enhanced but the yield of P2O5 is reduced.

[0006] U.S. Patent No. 5,945,000 describes a nanofiltration process for purifying a phosphoric acid solution using two (or even three) nanofiltration membrane units arranged in series, as described in WO 2013133684 discussed above, in which the retentate of both nanofiltration membrane units is recycled in a recirculation loop with the feed solution of either the same nanofiltration membrane unit or the first nanofiltration membrane unit, as illustrated in Figures 11(a) and 11(b) (the reference numbers in parentheses correspond to the reference numbers used in U.S. Patent No. 5,945,000). In this way, the yield of P2O5 is enhanced compared to the process of WO 2013133684. However, the recirculation loop formed between the retentate side of the first and second nanofiltration membrane units and the feed solution of the first and, optionally, second nanofiltration membrane units has the disadvantage of increasing the impurity content in the feed solution. Therefore, as stated in U.S. Patent No. 5,945,000, retentate solutions must be bled at regular intervals before entering the corresponding nanofiltration membrane units to reduce the amount of impurities entering the nanofiltration membrane units, which can quickly rise to unacceptable levels when "the contaminant level in the [retentate] solution [...] becomes sufficiently high, based on the amount of phosphate, that it is no longer economical to recycle the [retentate] solution." U.S. Patent No. 5,945,000 also teaches reducing the temperature of the phosphate-containing acidic solution to a temperature between 1 and 32°C (30°F and 90°F) to increase membrane life. Summary of the Invention

[0007] Given the current low market price for phosphate, there is a need for a purification process that enhances both the yield of P2O5 and the removal of impurities. The purification process is preferably fully continuous without the need for bleeding at regular time intervals.

[0008] The present invention proposes a fully continuous process for purifying phosphate-containing acidic solutions with increased P2O5 yield and impurity removal. These and other advantages of the present invention are described in more detail in the following sections.

[0009] The object of the present invention has been achieved by a process for purifying an impure phosphate-containing acidic solution (P1), which comprises the following steps: In the step of supplying the phosphate-containing acidic solution (P1) to a nanofiltration station via an entry line to produce a nanofiltered phosphate solution (P2), the nanofiltration station n membrane units (M1 to Mn) (where n ≥ 1) arranged in series; o a first recovery membrane unit (Mr1) and optionally a second recovery membrane unit (Mr2) arranged in series with the first recovery membrane unit (Mr1); o optionally a first exit membrane unit (Me1); Including, each of the membrane units (M1-Mn, Mr1, Mr2, Me1) includes a retentate side and a permeate side separated by a membrane; forming an input phosphate solution (Pf) by combining the phosphate-containing acidic solution (P1) with one or more other flows; Input phosphate solution (Pf) in two streams: o A first permeate (Pp1) lean to impurities and o Impurity-enriched first retentate (Pr1) feeding an input phosphate solution (Pf) to a first membrane unit (M1) for separation into if n>1, feeding at least a portion of the first permeate (Pp1) to a second membrane unit (M2), and so on, until at least a portion of the (n-1)th permeate has been fed to an nth membrane unit (Mn); First retentate (Pr1) into two streams: o The first harvest permeate (Ppr1) which is lean to impurities and o Impurity-rich first recovered retentate (Prr1) feeding at least a portion of the first retentate (Pr1) to a first recovery membrane unit (Mr1) for separation into at least a portion of the first collection permeate (Ppr1) o an entry line for combining with the phosphate-containing acidic solution (P1) to contribute to the formation of an input phosphate solution (Pf), and / or o The first recovered permeate (Pr1) is split into two streams: The first exit permeate (Ppe1) is lean and The first exit retentate (Pre1), which is rich in impurities First exit membrane (Me1) for separation into and Optionally split the first recovery retentate (Prr1) into two streams: A second harvested permeate (Ppr2) leaner in impurities and Impurity-rich second recovery retentate (Prr2) feeding at least a portion of the first recovery retentate (Prrl) to a second recovery membrane (Mr2) for separation into - sending the nanofiltered phosphate solution (P2) from the nanofiltration station through the permeate side of the nth membrane unit (Mn); Includes:

[0010] The gist of the present invention is providing one or more permeate recirculation loops in fluid communication with the entry line (1e) and the permeate side of one or more of the first or second recovery membrane units (Mr1, Mr2) or the first exit membrane unit (Me1); feeding at least a portion of one or more of the first or second recovery permeates (Ppr1, Ppr2) or the first exit permeate (Ppe1) to an entry line and combining at least a portion with a phosphate-containing acidic solution (P1) to form an input phosphate solution (Pf); Includes:

[0011] In one embodiment, n>1, and each of the n membrane units (M1 to Mn) sequentially separates the first to (n-1)th permeates (Pp1 to Pp(n-1)) into two streams: The second to nth permeates (Pp2 to Ppn) are impurity-poor and Impurity-rich retentates 2 to n (Pr2 to Prn) In this case, At least a portion of each of the first to (n-1)th permeates (Pp1 to Pp(n-1)) is fed to the retentate side of the next second to nth membrane units (M2 to Mn) located downstream of the series of n membrane units (M1 to Mn).

[0012] At least a portion of each of the second to nth retentates (Pr2 to Prn) is fed to the retentate side of the first recovery membrane unit (Mr1) and / or to the retentate side of the previous first to (n-1)th membrane units (M1 to M(n-1)) located upstream of the series of n membrane units (M1 to Mn).

[0013] According to one embodiment of the present invention, at least a portion of the first recovered retentate (Prr1) discharged from the first recovery membrane unit (Mr1) can be fed to the retentate side of the second recovery membrane unit (Mr2), in which case at least a portion of the second recovered permeate (Ppr2) can be fed to the retentate side of the second recovery membrane unit (Mr2). be fed to an entry line to form one of one or more permeate recirculation loops and to be combined with a phosphate-containing acid solution (P1) to contribute to the formation of an input phosphate solution (Pf); and / or It leaves the nanofiltration unit and can be recovered as nanofiltered phosphate recovery solution (P2r).

[0014] At least a portion of the second recovered permeate (Ppr2) can be fed to the entry line as a component of the input phosphate solution (Pf) to form one of one or more permeate recirculation loops. For example, it is possible to have no permeate recirculation loop in fluid communication between the permeate side of the first recovered membrane unit (Mr1) and the entry line because there is already a second permeate recirculation loop formed between the retentate side of the second recovered membrane unit (Mr2) and the entry line.

[0015] In a preferred embodiment, at least a portion, preferably 10 to 100 wt.%, of the first recovered permeate (Ppr1) is fed to an exit membrane unit (Me1), and at least a portion, preferably 10 to 100 wt.%, of the first exit permeate (Ppe1) is fed to a membrane separator (Me1) for separating the first exit permeate (Ppe1) into two streams: A second, impurity-lean exit permeate (Ppe2) and The second exit retentate (Pre2), which is rich in impurities The retentate is fed to the retentate side of the second exit membrane unit (Me2) for separation into

[0016] At least a portion of the second exit permeate (Ppe2) can be discharged from the nanofiltration station to form a nanofiltration phosphate exit solution (P2e). At least a portion of the second exit retentate (Pre2) is fed to an entry line to form a retentate recycle loop and is combined with the phosphate-containing acid solution (P1) to contribute to the formation of the input phosphate solution (Pf). The retentate recycle loop should not be confused with the permeate recycle loop, the latter circulating solution typically (but not necessarily) having a lower impurity concentration than the retentate.

[0017] In some embodiments, First harvest permeate (Ppr1) A second harvested permeate (Ppr2), or First exit permeant (Ppe1) Preferably, 100 wt.% of one or more of is. It is fed to the entry line to form one or more of the permeate recirculation loops and is combined with the phosphate-containing acidic solution (P1) to contribute to the formation of the input phosphate solution (Pf). In a preferred embodiment, the nanofiltration station includes a single permeate recirculation loop formed between the first recovered permeate (Ppr1) and the entry line.

[0018] The nanofiltered phosphate solution (P2, P2e, P2r) can be further processed. For example, the nanofiltered phosphate solution (P2) discharged from the nanofiltration unit can be fed to an ion exchange station containing an ion exchange resin configured to remove residual cations, preferably mono- and divalent cations, remaining in the nanofiltered phosphate solution (P2) and to form a purified phosphate solution (P3) lean to impurities.

[0019] The phosphate-containing acidic solution (P1) preferably comprises 2 to 25% P2O5, preferably 15 to 21% P2O5, particles greater than 1 μm and less than 100 ppm, preferably particles greater than 1 μm and less than 50 ppm, more preferably less than 10 ppm, and most preferably less than 1 ppm; less than 3 wt.% total organic carbon (TOC), preferably 1 wt.% TOC or less; preferably not more than 4 wt% SO4, preferably not more than 1000 ppm SO4; Includes:

[0020] The impurities contained in the phosphate-containing acidic solution (P1) may include Al, Ca, Cr, Fe, Mg, and in this case, P2 has a removal rate of these impurities of at least 90 wt.%, preferably at least 95 wt.%, more preferably at least 98 wt.%, or even at least 99 wt.%, based on P1.

[0021] The present invention also relates to an apparatus for purifying an impure phosphate-containing acidic solution (P1) in the process discussed above, the apparatus comprising a nanofiltration station in fluid communication with an entry line (1e) in fluid communication with a source of phosphate-containing acidic solution (P1) for supplying the phosphate-containing acidic solution (P1) to the nanofiltration station (2), and an exit line (2e) for delivering the nanofiltered phosphate solution (P2) from the nanofiltration station. n membrane units (M1 to Mn) (where n ≥ 1) arranged in series; a first recovery membrane unit (Mr1) and optionally a second recovery membrane unit (Mr2) arranged in series with the first recovery membrane; optionally a first exit membrane unit (Me1); Includes:

[0022] Each of the above-mentioned membrane units (M1-Mn, Mr1, Mr2, Me1) includes a retentate side and a permeate side separated by a membrane, where: The retentate side of the first membrane unit (M1) an input line in fluid communication with the entry line for supplying an input phosphate solution (Pf) to the first membrane unit; and an outlet line in fluid communication with the retentate side of the first recovery membrane unit (Mr1) for supplying the first retentate (Pr1) thereto; in fluid communication with The permeate side of the first membrane recovery unit (Mr1) is an entry line or chamber for combining at least a portion, preferably 10-100 wt.%, of the first harvest permeate (Ppr1) with a phosphate-containing acidic solution (P1) to contribute to the formation of an input phosphate solution (Pf); and / or o the retentate side of the first exit membrane unit (Me1) for feeding at least a portion of the first collected permeate (Ppr1) to the first exit membrane unit (Me1). in fluid communication with The retentate side of the first recovery membrane unit (Mr1) is optionally in fluid communication with the retentate side of the second recovery membrane unit (Mr2).

[0023] The apparatus of the present invention differs from prior art apparatus in that at least one permeate recirculation loop is formed by including fluid communication between an entry line or chamber and the permeate side of one or more of the first recovery membrane unit (Mr1), the second recovery membrane unit (Mr2), or the first exit membrane unit (Me1).

[0024] In a preferred embodiment, the permeate side of the first membrane unit (M1) is in fluid communication with the retentate side of the second membrane unit (M2), whose permeate side is in fluid communication with the third membrane unit (M3), and so on up to the nth membrane unit (Mn), whose permeate side is connected to the exit line. o the first to (n-1)th membrane units preceding a given membrane unit, and / or o First collection membrane unit (Mr1) The retentate side of the [Brief explanation of the drawings]

[0025] [Figure 1]FIG. 1 depicts schematic diagrams of process and apparatus embodiments according to the present invention, where (a) is a general embodiment including a single membrane unit, and (b)-(d) are preferred embodiments of the general embodiment of FIG. 1(a). [Figure 2] FIG. 2 shows schematic diagrams of process and apparatus embodiments according to the present invention, where (a), (b), (d), (e), and (g) include a single permeate recycle loop, and (c), (f), and (h) include two permeate recycle loops. [Figure 3] FIG. 3 shows schematic diagrams of process and apparatus embodiments according to the present invention, where (a) includes one or two permeate recirculation loops, and (b) includes one to three permeate recirculation loops with two or more membrane units. [Figure 4] FIG. 4(a) shows a nanofiltration station with an ion exchange column downstream of the nanofiltration station, and (b) shows a pre-purification station upstream of the nanofiltration station. [Figure 5] FIG. 5 shows the P2O5 yield and impurity removal for a process according to the present invention including a single permeate recycle loop from the first recovery membrane unit, where in (a) 100 wt. % of the first recovery permeate is recycled, and in (b) 10 wt. % of the first recovery permeate is recycled and 90 wt. % exits the nanofiltration station as the nanofiltration phosphate recovery solution. [Figure 6] FIG. 6 shows the P2O5 yield and impurity rejection for a process according to the present invention including a single permeate recycle loop from the first exit membrane unit, where in (a) 100 wt.% of the first exit permeate is recirculated and in (b) 10 wt.% of the first exit permeate is recirculated and 90 wt.% exits the nanofiltration station as the nanofiltered phosphate exit solution. [Figure 7]FIG. 7 shows the P2O5 yield and impurity rejection for processes according to the present invention, where (a) includes a single permeate recycle loop from the first recovery membrane unit, where 5 wt. % of the first recovery permeate is recycled, 65 wt. % exits the nanofiltration station as nanofiltration recovery solution, and 30 wt. % is fed to the first exit membrane unit for recovery of the first exit permeate as nanofiltration phosphate exit solution, and (b) is identical to (a), except that the first exit permeate is recycled in the second permeate loop instead of exiting the nanofiltration station. [Figure 8] FIG. 8 shows the impurity removal rate of the process according to US Pat. No. 5,945,000 illustrated in FIG. 11(b) with some assumptions made to produce results comparable to those of FIG. [Figure 9] FIG. 9 shows the impurity rejection of a process according to the present invention based on the prior art apparatus of U.S. Pat. No. 5,945,000 illustrated in FIG. 8, with the addition of a first recovery membrane unit and a first recovery permeate recycle loop initiated therefrom. [Figure 10] FIG. 10 shows a nanofiltration membrane unit in (a) a schematic diagram, (b) an arrangement of several membranes arranged in series and in parallel to form a single nanofiltration membrane unit (inspired by FIG. 4 of U.S. Pat. No. 5,945,000), and (c) an arrangement of several membranes arranged in series to form part of the nanofiltration membrane unit of FIG. 10(b) (inspired by FIG. 5 of U.S. Pat. No. 5,945,000). [Figure 11] FIG. 11 shows (a) and (b) two embodiments of the process and apparatus according to U.S. Pat. No. 5,945,000, and (c) an embodiment according to WO2013133684. DETAILED DESCRIPTION OF THE INVENTION

[0026] The process and apparatus of the present invention are for purifying an impure phosphate-containing acidic solution (P1), such as, but not limited to, a phosphoric acid solution, and the process comprises feeding the phosphate-containing acidic solution (P1) via an entry line (1e) to a nanofiltration station (2) to produce a nanofiltered phosphate solution (P2) which exits the nanofiltration station (2) via an exit line (2e). For example, as shown in Figure 3(b), the nanofiltration station (2) n membrane units (M1 to Mn) (where n ≥ 1) arranged in series; a first recovery membrane unit (Mr1) and optionally a second recovery membrane unit (Mr2) arranged in series with the first recovery membrane unit (Mr1); optionally a first exit membrane unit (Me1) and optionally a second exit membrane unit (Me2); Includes:

[0027] Each of the nanofiltration membrane units (M1-Mn, Mr1, Mr2, Mel) described above includes a retentate side and a permeate side separated by a membrane. A solution containing impurities is fed to the retentate side of the nanofiltration membrane unit. The impurity-lean permeate passes through the membrane into the permeate side of the nanofiltration membrane unit, where it exits the nanofiltration membrane unit. The impurity-rich retentate is retained by the membrane and exits the retentate side of the nanofiltration membrane unit. As explained above, "lean" and "rich" in impurities refer to a given nanofiltration membrane unit where the permeate is leaner in impurities than the solution fed to the nanofiltration membrane unit and is itself leaner in impurities than the retentate. Or, conversely, the retentate is richer in impurities than the solution fed to the nanofiltration membrane unit and is itself richer in impurities than the permeate.

[0028] According to the present invention, the input phosphate solution (Pf) is formed by combining the phosphate-containing acidic solution (P1) with one or more other flows. The input phosphate solution (Pf) is formed by combining the input phosphate solution (Pf) with two streams: The first permeate (Pp1) is lean and Impurity-rich first retentate (Pr1) The resulting mixture is fed to a first membrane unit (M1) for separation into

[0029] When at least two membrane units are arranged in series (n>1), at least a portion of the first permeate (Pp1) is fed to the second membrane unit (M2), and so on, until at least a portion of the (n-1)th permeate is fed to the nth membrane unit (Mn). The permeate (Ppn) of the nth membrane unit (Mn) forms the nanofiltered phosphate solution (P2) and exits the nanofiltration station (2). When the apparatus includes a single membrane unit (M1), for example, as illustrated in FIG. 1(a), the nanofiltered phosphate solution (P2) exiting the nanofiltration station is formed by the first permeate (Pp1).

[0030] At least a portion of the first retentate (Pr1) is separated into two streams: The first harvested permeate (Ppr1) is lean and Impurity-rich first recovery retentate (Prr1) The retentate side of the first membrane unit (M1) is fed to the retentate side of the first recovery membrane unit (Mr1) for separation into

[0031] In this document, the expression "at least a portion" of a flow should be interpreted as a non-zero portion of the flow, preferably a portion comprising 10-100 wt.% of the flow, more preferably 20-95 wt.% of the flow, more preferably 35-85 wt.% of the flow, more preferably 40-75 wt.%, and more preferably 50-65 wt.%. Similarly, the expression "only a portion" of a flow should be interpreted as a portion >0 and <100 wt.% of the flow, preferably a portion comprising 10-95 wt.% of the flow, more preferably 35-85 wt.% of the flow, more preferably 40-75 wt.% of the flow, and more preferably 50-65 wt.% of the flow.

[0032] All or part of the first harvest permeate (Ppr1) can be fed to different locations in the nanofiltration station (2). In particular, the first harvest permeate (Ppr1) is into the entry line (1e), either directly or via a chamber (2c), to combine with the phosphate-containing acidic solution (P1) so as to contribute to the formation of the input phosphate solution (Pf), on the one hand to form one of the one or more other flows to be combined with the phosphate-containing acidic solution (P1), and on the other hand to form a first permeate recirculation loop; and / or The first collected permeate (Ppr1) is split into two streams: o The first exit permeate (Ppe1) which is lean to impurities and o The first exit retentate (Pre1) enriched in impurities on the retentate side of the first exit membrane (Me1) to separate Supplied.

[0033] At least a portion of the first recovery retentate (Prrl) is optionally separated into two streams: A second harvested permeate (Ppr2) leaner in impurities and Impurity-rich second recovery retentate (Prr2) The resulting mixture can be fed to a second collection membrane (Mr2) for separation into

[0034] The present invention is characterized by forming one or more permeate recirculation loops fluidly connecting the permeate side of one or more of the first or second recovery membrane units (Mr1, Mr2) or the first exit membrane unit (Me1) with an entry line (1e). One or more of the first or second recovery permeates (Ppr1, Ppr2) or at least a portion of the first exit permeate (Ppe1) is fed to the entry line (1e) and combined with a phosphate-containing acidic solution (P1) to form an input phosphate solution (Pf).

[0035] In other words, one or more permeate recirculation loops are formed when permeate flows from the permeate side of a given nanofiltration membrane unit and loops back to the entry line (1e) or to the chamber (2c) attached to the entry line (1e).

[0036] In the figures, the following criteria apply: each arrow represents a fluid communication between two components of the device, e.g., a pipe, a tube, etc. Although not necessarily specified, each fluid communication represented by an arrow may include any one or more of: a pump to increase pressure; a dilution source to control the viscosity of the solution, e.g., an aqueous solution, e.g., water or acid water or dilute phosphoric acid; a heat exchanger to control the temperature of the solution; a buffer to compensate for flow rate differences between two sequential nanofiltration membrane units of the device;

[0037] A nanofiltration membrane unit generally includes a single inlet and two outlets, including a retentate outlet and a permeate outlet. To avoid excessive line crossings and to clarify the diagram, it is noted that the two arrows can lead to or leave the retentate or permeate sides of the nanofiltration membrane unit. This does not mean that a nanofiltration membrane unit has more than one inlet or more than one retentate or permeate outlet, but simply that the two flows merge upstream before entering the nanofiltration membrane unit or split downstream after exiting it. "Upstream" and "downstream" are defined herein with reference to the direction of solution flow during the purification process.

[0038] Valves are illustrated (opposite triangles) when the same solution can flow through two different fluid communication lines. The portion of the solution flowing in each of the two lines is controllable. That is, each valve can be closed, partially open, or fully open. In specific examples, for example, in FIG. 7(a), the wt.% portion flowing in each direction is indicated (e.g., "Ppr1-65" corresponds to 65 wt.% of Ppr1 flowing into the entry line (1e)). When a valve cannot be closed, a flow rate of "Q>0" is indicated. For example, in FIG. 1(a), the dotted circles identify three possible permeate recirculation loops, at least one of which must be at least partially open (i.e., Q>0) to ensure that at least one permeate recirculation loop is formed in the process.

[0039] The values ​​of impurity amounts and P2O5 yield shown in Figures 5 to 9 are calculated based on the measured performance of each individual nanofiltration membrane unit (M1 to Mn, Mr1, Mr2, Me1, Me2).

[0040] Phosphate-containing acidic solution (P1) Phosphate solution, as used herein, refers to HO[P(OH)(O)O] nH, where n≧1. The phosphate-containing acidic solution (P1) and all phosphate-containing solutions in and downstream of the nanofiltration station (2) refer to solutions containing phosphorus dissolved as orthophosphates and / or polyphosphates, the respective contents of which depend on the P2O5 content of the solution.

[0041] Unless otherwise indicated, all % and ppm concentrations are expressed as weight % (= wt.%) and weight ppm (= ppm). Because phosphate can exist in various forms in solution, the phosphate content of a solution is expressed as % equivalent P2O5, denoted by %P2O5, as is well known and used in the art. The phosphate concentration of a solution is sometimes expressed in the art by % equivalent H3PO4. For information, 1% P2O5 corresponds to 1.38% H3PO4.

[0042] If the raw phosphate solution (P0) does not contain elements harmful to the service life of the nanofiltration membrane, it can be fed directly to the nanofiltration station (2) as a phosphate-containing acidic solution (P1). Just as any solution flowing through the apparatus of the present invention can be diluted between any two adjacent units or stations, particularly to control its viscosity, the raw phosphate solution (P0) can be diluted with a solution such as water or a dilute phosphoric acid solution. Alternatively, the raw phosphate solution (P0) can be pretreated to remove suspended solid particles larger than 10 μm, preferably larger than 5 μm, more preferably larger than 1 μm, more preferably larger than 0.5 μm, more preferably larger than 0.22 μm, more preferably larger than 0.05 μm, organic matter and oily residues, as well as elements that pose a risk of precipitation, such as Ca or Ba, which may precipitate as sulfate salts. The raw phosphate solution (P0) can be pretreated to eliminate as many particles as possible, for example to produce a phosphate-containing acidic solution (P1) containing at least 1 μm particles of 100 ppm or less, preferably less than 50 ppm, more preferably less than 10 ppm, and most preferably less than 1 ppm.

[0043] Particles with a diameter greater than 1 μm. The phosphate-containing acidic solution (P1) preferably contains 4 wt% or less SO4, preferably 2.5 wt% or less, preferably 1 wt% or less, preferably 0.5 wt% or less, preferably 1000 ppm or less SO4. P0 can be pretreated to produce a phosphate-containing acidic solution (P1) containing 3 wt% or less TOC (=total organic carbon), 1 wt% or less TOC, preferably 500 ppm or less TOC, preferably 200 ppm or less TOC, preferably 100 ppm or less TOC. Depending on the type of nanofiltration membrane used, arsenic and sulfate may not be efficiently separated by the nanofiltration membrane and are preferably removed either before or after, preferably before, the nanofiltration station (2). The pretreated solution thus forms a phosphate-containing acidic solution (P1) that can be fed to the nanofiltration station (2).

[0044] The origin of the raw phosphate solution (P0) and the nature of its pretreatment to form the phosphate-containing acidic solution (P1) determine the concentration of impurities present in P1. The end use of the purified phosphate solution determines the impurity removal rate achieved for each impurity type. As discussed below, the present invention proposes a nanofiltration station (2) that enhances both the P2O5 yield and the impurity removal rate to produce nanofiltered phosphate solutions (P2, P2r, P2e) that are ready for use in a specific application or can be fed to a subsequent purification station, such as an ion exchange column (3). The nanofiltered phosphate solution (P2) obtainable by the nanofiltration station (2) of the present invention can have at least 90 wt.%, preferably at least 95 wt.%, more preferably at least 98 wt.%, or even at least 99 wt.% of the impurities consisting of Al, Ca, Cr, Fe, and Mg removed from the phosphate-containing acidic solution (P1).

[0045] The starting phosphate solution (P0) may have 5-85 wt.% P2O5, preferably 10-75 wt.% P2O5, preferably 15-62 wt.% P2O5, preferably 17-54 wt.% P2O5, preferably 37-52 wt.% P2O5, preferably 25-30 wt.% P2O5. The phosphate-containing acidic solution (P1) is preferably diluted to produce a P2O5 content comprised between 2 and 25%, preferably between 5 and 23%, preferably between 10 and 22%, preferably between 15 and 21%, preferably between 17 and 18%. The phosphate-containing acidic solution (P1) preferably has a pH of 2 or less, preferably 1 or less, preferably 0.5 or less.

[0046] Nanofiltration membrane and nanofiltration membrane unit Nanofiltration membranes are capable of separating specific ions with a low molecular weight cutoff of about 150-200 Da for uncharged particles. Nanofiltration membranes are operated at high pressures of about 1-6 MPa, preferably about 3-5 MPa. Each membrane is usually in the form of a rolled film forming a tube. Other membrane geometries are available, and the present invention is not limited to any particular geometry. As shown in Figures 10(b) and 10(c) (inspired by Figures 4 and 5 of U.S. Pat. No. 5,945,000), a single nanofiltration membrane unit may contain several membranes arranged in series (see (m11-m1k) in Figures 10(b) and 10(c) herein) or even in parallel (see (m11-m21) in Figure 10(b) herein). The use of each nanofiltration membrane unit is a feed solution containing impurities and fed to the retentate side of the nanofiltration membrane unit; a lean permeate solution (i.e., containing fewer impurities than the feed solution) exiting the permeate side of the nanofiltration membrane unit, and An impurity-rich retentate solution (i.e., containing more impurities than the feed solution) exiting the retentate side of the nanofiltration membrane unit. It is characterized by:

[0047] Suspended solid materials should be avoided in the solution fed to the nanofiltration membrane, as solid particles can inhibit the membrane by loss of permeability and can degrade it by abrasion. Similarly, the risk of precipitation increases with increasing concentration of specific impurities. For example, Ca or Ba can precipitate as sulfate salts. The impurity concentration of the solution can be reduced by diluting the solution with the addition of water and / or phosphoric acid solution. The addition of water and / or phosphoric acid solution can also be useful to control the viscosity of the solution fed to the membrane. According to the present invention, water and / or phosphoric acid solution can be added to the solution at any stage between two nanofiltration membrane units, if necessary.

[0048] In the present invention, the nanofiltration membrane is preferably selected for the removal of metals, such as Al, Ca, Cr, Fe, Mg, Sr, V, and particularly divalent and trivalent cations, under acidic conditions. Because the solutions flowing through the various nanofiltration membrane units (M1-Mn, Mr1, Mr2, Mel, and Me2) are acidic, the nanofiltration membrane must be resistant to acidic pH. The nanofiltration membrane used in the present invention is preferably a composite membrane comprising a porous support membrane supporting a polymer film. The polymer film can be selected from the group consisting of polyolefins, polysulfones, polyethers, polysulfonamides, polyamines, polysulfides, and melamine polymers. Polysulfonamides are preferred. The polymer film preferably has a thickness of 2 μm or less, preferably 1 μm or less.

[0049] The porous support membrane can be chosen from polyamide, polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinyl chloride, ceramic, or porous glass. Polysulfone is preferred for the porous support membrane. The porous support membrane can have a thickness comprised between 1 and 250 μm, preferably between 50 and 100 μm.

[0050] 1st to nth membrane units (M1 to Mn) The process and apparatus of the present invention may include only the first membrane unit (M1), for example, as illustrated in Figure 1(a). Alternatively, as illustrated in Figures 2(b) and 3(b), they may sequentially separate each of the first through (n-1) permeates (Pp1 through Pp(n-1)) into two streams, respectively: The second to nth permeates (Pp2 to Ppn) are impurity-poor and Impurity-rich retentates 2 to n (Pr2 to Prn) The nanofiltration station may include more than one (n>1) membrane unit arranged in series to separate the first through (n-1) permeates (Pp1 through Pp(n-1)) in sequence, where at least a portion of each of the first through (n-1) permeates (Pp1 through Pp(n-1)) is fed to the retentate side of the next second through nth membrane units (M2 through Mn) downstream of the series of n membrane units (M1 through Mn). At least a portion of the nth permeate (Ppn) exits the nanofiltration station to form the nanofiltered phosphate solution (P2).

[0051] Two or more membrane units (M1 to Mn, n>1) are arranged in series where at least a portion of a first permeate (Pp1) is fed to the retentate side of a second membrane unit (M2), and so on, until at least a portion of the (n-1)th permeate is fed to the nth membrane unit (Mn).

[0052] In a preferred embodiment, n = 2 or 3 membrane units (M1, M2, M3). Arranging several membrane units (M1-Mn) in series results in higher impurity removal rates than can be achieved with a single first membrane unit (M1), but the P2O5 yield is substantially reduced.

[0053] As proposed in U.S. Patent No. 5,945,000, the first retainer (Pr1) is returned to the retainer side (M1) of the same first membrane unit, and each of the second to nth retainers (Pr2 to Prn) can be supplied to the previous retainer side by a series of first to (n - 1)th membrane units (M1 to M(n - 1)) (see Figs. 2(b), 3(b), and 11(b)), or can be returned to the retainer side of the same membrane unit (M2 to Mn) (see Fig. 11(a)). However, as discussed in the background of the invention, such a retainer recirculation loop results in the accumulation of impurities in the solution supplied to the previous or the same membrane unit. The process may thus require sequential bleeding of the recirculated solution at regular intervals to reduce the amount of impurities present in the solution before supply to the nanofiltration membrane unit. This is necessary to ensure the quality of the final product. This is clarified by comparing the amount of impurities present in P1 and Pf in the process according to U.S. Patent No. 5,945,000 illustrated in Fig. 8, where, after 5 loops, the input phosphate solution (Pf) has a higher impurity concentration (c(Pf)) than P1 (c(P1) = 657 ppm < c(Pf) = 1707 ppm, resulting in a relative increase in impurities Δcr = (c(Pf) - c(P1)) / c(P1) = 160% between P1 and Pf).

[0054] According to the invention, firstly, there is at least one permeate recirculation loop branching off from the retentate side of the first membrane unit (M1) and returning via one or more additional nanofiltration membrane units (Mr1, Mr2, Me1) to a return point located in the entry line (1e), making it possible to reduce the amount of impurities present in the input phosphate solution (Pf) fed to the first membrane unit (M1) compared to the amount present in the phosphate-containing acidic solution (P1). In contrast to the 160% increase in impurity concentration between P1 and Pf observed in the apparatus according to U.S. Pat. No. 5,945,000 discussed above with reference to FIG. 8, according to the present invention illustrated in FIG. 9, the input phosphate solution (Pf) has a lower impurity content than P1 (c(P1)=657 ppm>c(Pf)=554 ppm, resulting in a relative reduction in impurities between P1 and Pf, Δcr=−16%), thanks to the first recovered permeate recycle loop (1e → Pf → M1 → Mr1 → 1e) defined above and described in detail below, where the relative variation Δcr=(c(P1)−c(Pf) / c(P1)×100%, and c(P1) and c(Pf) are the impurity concentrations in solutions P1 and Pf.

[0055] Additionally, in the present invention, as with the first retentate (Pr1), it is possible to feed at least a portion of each of the second through nth retentates (Pr2-Prn) to the retentate side of the first recovery membrane unit (Mr1) (see Figures 2(b) and 3(b)). This has the effect of reducing impurities upstream of any of the second through nth membrane units (M2-Mn), while also significantly increasing the yield of P2O5, since any P2O5 remaining in the successive retentates (Pr2-Prn) is recycled through the nanofiltration membrane unit. The gist of the present invention is the presence of at least one permeate recirculation loop branching off from the retentate side of the first (and optionally each of the second to nth) membrane units (M1 to Mn), passing through the first recovery membrane unit (Mr1), and then flowing from the permeate side of one or more of the first or second recovery membrane units (Mr1, Mr2) or the first exit membrane unit (Me1) back to the entry line (1e).

[0056] Recirculation Loop As discussed above with reference to the first to nth membrane units (M1 to Mn), currently the first to nth nanofiltration membrane units are arranged in series where the permeate (Pp1) of the first nanofiltration membrane unit (M1) is fed to the retentate side of the second nanofiltration membrane unit, and so on up to the nth nanofiltration membrane unit.

[0057] n>1 nanofiltration membrane units are arranged in parallel if the same feed solution is fed to the retentate sides of n nanofiltration membrane units. In contrast, the fluid communication between the retentate side of a first nanofiltration membrane unit and the retentate side of a second nanofiltration membrane unit forms a bifurcation. The nanofiltration unit that begins the bifurcation to form a recirculation loop is called a bifurcated nanofiltration membrane unit.

[0058] A recirculation loop is formed when one branch returns from the branch nanofiltration membrane unit to a return point located in the entry line (1e) or is in direct fluid communication with the entry line (1e), such as through a chamber (2c). One or more nanofiltration units can be interposed between the branch nanofiltration unit and the return point.

[0059] A permeate recirculation loop is formed when a line is connected to the permeate side of a nanofiltration unit located immediately upstream (according to the solution flow) of the return point of the entry line (1e). Similarly, a retentate recirculation loop is formed when a line is connected to the retentate side of the nanofiltration membrane unit. A particular recirculation loop is referred to by the name of the nanofiltration membrane unit located immediately upstream of the return point. For example, the first and second recovered permeate recirculation loops defined above include fluid communication between the permeate sides of the first and second recovered membrane units (Mr1, Mr2), respectively, and the entry line (1e) or chamber (2c) (see Figures 2(a)-2(c) and 2(f)-2(h)). Similarly, the first exit permeate recirculation loop includes fluid communication between the permeate side of the first exit membrane unit (Me1) and the entry line (1e) (see Figures 2(d)-2(f) and 2(h)).

[0060] The process and apparatus of the present invention requires at least one of three permeate recycle loops defined as follows: A first recovered permeate recirculation loop, as illustrated in FIG. 2(a), is formed as follows: a phosphate-containing acidic solution (P1) is mixed with one or more other flows to form an input phosphate solution (Pf) fed to a first membrane unit (M1). At least a portion of the first retentate (Pr1) is fed from the retentate side of the first membrane unit (M1) to the retentate side of the first recovery membrane unit (Mr1), forming a branch toward the first recovery membrane unit (Mr1). At least a portion of the first recovered permeate (Ppr1) is fed from the permeate side of the first recovery membrane unit (Mr1) to an entry line (1e) or to a chamber (2c), forming one of one or more other flows. The first recovered recirculation loop follows the following flow path: 1e → M1 → Mr1 → 1e. A first exit permeate recirculation loop, as illustrated in Figure 2(e), is formed as follows: the phosphate-containing acidic solution (P1) is mixed with one or more other flows to form an input phosphate solution (Pf) that is fed to the first membrane unit (M1). At least a portion of the first retentate (Pr1) is fed from the retentate side of the first membrane unit (M1) to the retentate side of the first recovery membrane unit (Mr1). At least a portion of the first recovery permeate (Ppr1) is fed from the permeate side of the first recovery membrane unit (Mr1) to the retentate side of the first exit membrane unit (Me1). At least a portion of the first exit permeate (Ppe1) is fed from the permeate side of the first exit membrane unit (Me1) to the entry line (1e) or to chamber (2c) to form one of the one or more other flows. The first exit recirculation loop follows the following flow path: 1e → M1 → Mr1 → Me1 → 1e. A second recovered permeate recirculation loop, as illustrated in FIG. 2(g), is formed as follows: the phosphate-containing acidic solution (P1) is mixed with one or more other flows to form an input phosphate solution (Pf) fed to the first membrane unit (M1). At least a portion of the first retentate (Pr1) is fed from the retentate side of the first membrane unit (M1) to the retentate side of the first recovery membrane unit (Mr1). At least a portion of the first recovered retentate (Pr1) is fed to the retentate side of the second recovery membrane unit (Mr2). At least a portion of the second recovered permeate (Ppr2) is fed from the permeate side of the second recovery membrane unit (Mr2) to the entry line (1e) or to chamber (2c) to form one of the one or more other flows. The second recovered recirculation loop follows the following flow path: 1e → M1 → Mr1 → Mr2 → 1e.

[0061] Any two or three of the above-mentioned permeate recycle circuits can be formed simultaneously according to the present invention depending on the yield and impurity removal target set required by the end use of the purified phosphate acid solution.

[0062] All of the above-mentioned permeate recirculation loops branch off from the retentate side of the first membrane unit (M1) and feed the first retentate (Pr1) to the retentate side of the first recovery membrane unit (Mr1), and the three permeate recirculation loops defined above follow different flow paths to the same return point of the entry line (1e) or to the chamber (2c) provided in the entry line (1e).

[0063] The process of the present invention comprises a step of forming an input phosphate solution (Pf) by combining a phosphate-containing acidic solution (P1) with one or more other flows. The one or more other flows reach an entry line (1e) that forms a return point for one or more corresponding recirculation loops. According to the present invention, at least one of the one or more flows must reach a return point from at least one of the first or second recovered permeate recirculation loop or the first exit permeate recirculation loop. As shown in Figures 1(a) and 3(b), in addition to the permeate recirculation loop described above, a flow can also reach a return point from the retentate side of the second exit membrane unit (Me2) via a second retentate exit recirculation loop.

[0064] The return point can be formed by direct connection of the last line to the entry line (1e), which forms fluid communication with the nanofiltration membrane unit located immediately upstream of the entry line (1e). Alternatively, the last line can be coupled to a chamber (2c) provided in the entry line to form the return point, as illustrated, for example, in Figures 1(a) and 3(b). Chamber (2c) can include one or more functions, including mixing with a static or dynamic mixer, forming a buffer, cooling the solution, etc.

[0065] The permeate recycle loop described above is · The impurity concentration (c(Pf)) of the input phosphate solution (Pf) that is lower than c(P1) (i.e., c(Pf) < c(P1)) is supplied to the first membrane unit (M1) to reduce the impurity concentration (c(P1)) of the phosphate-containing acidic solution (P1) so as to enhance the efficacy of the nanofiltration membrane. · Increase the P2O5 yield by recycling P2O5 retained in the first retentate (Pr1) of the first membrane unit (M1) that may be lost and optionally those in the subsequent second to nth membrane units (M2 - Mn) through the first recovery membrane (Mr1). · Manage the viscosity of the input phosphate solution (Pf). Combine a plurality of advantages, including the above.

[0066] The first recovery permeate recycle loop The first recovery permeate recycle loop is the shortest among the three permeate recycle loops of the present invention. In all three as permeate recycle loops, the first recovery permeate recycle loop branches from the retentate side of the first membrane unit (M1) and supplies the first retentate (Pr1) to the retentate side of the first recovery membrane unit (Mr1). The first recovery permeate (Ppr1) that has permeated through the nanofiltration membrane of the first recovery membrane unit (Mr1) flows out from the permeate side of the first recovery membrane unit (Mr1) and reaches the return point of the entry line (1e) (or the chamber (2c) provided in the entry line (1e)).

[0067] Figure 2(a) illustrates an embodiment including a first recycled permeate recirculation loop that includes a single first membrane unit (M1) and a single recovery membrane unit (Mr1) and forms the only permeate recirculation loop of the apparatus. The entire first recycled permeate (Ppr1) is recycled to the return point of the entry line (1e). Figure 2(b) shows a similar apparatus including n=3 membrane units (M1-M3). As indicated by the valves, at least a portion of each of the second (=(n-1)) and third (=n) retentates (Pr2, Pr3) is branched from the retentate side of the second and third membrane units (M2, M3) and fed to the retentate side of the first recycled membrane unit (Mr1), where it contributes to the supply of the first recycled permeate recirculation loop together with the first retentate (Pr1).

[0068] As discussed above with reference to FIG. 8 (prior art = U.S. Pat. No. 5,945,000) and FIG. 9 (present invention), providing the prior art apparatus of FIG. 8 with the first recovered permeate recycle loop illustrated in FIG. 9 reduces the impurity selectivity from 1707 ppm to 554 ppm (i.e., =(c(Pf FIG.8 )-c(Pf FIG.9 )) / c(Pf FIG.8 ) = (1707 - 554) / 1707 = 68% reduction). Rather than increasing the impurity concentration in the input phosphate solution (Pf) by Δcr = (c(Pf) - c(P1)) / c(P1) = 160% relative to P1, this has the unexpected effect of making the impurity concentration in the input phosphate solution (Pf) Δcr = -16% lower than in P1, which of course has a beneficial effect on the impurity removal rate of the process.

[0069] It is not necessary to recycle all (=100%) of the first harvest permeate (Ppr1) directly to the return point of the entry line (1e); instead, it is possible to recycle only a portion of it. For example, the complementary portion that is not directly recycled to the entry point can be leaving nanofiltration station (2) as a nanofiltration recovery solution (P2r) that can be used for lower performance applications, where the 90% portion of the first recovery permeate (Ppr1-90=P2r) leaving nanofiltration station (2) has an impurity rejection of about 86 wt.% compared to 94 wt.% rejection with nanofiltration phosphate solution (P2), as shown in Figure 5(b); or into the retentate side of the first exit membrane unit (Me1) to form a first exit permeate recirculation loop and / or to produce a high quality nanofiltration exit solution (P2e) formed by the first or second exit permeates (Ppe1, Ppe2) that can be used for high performance applications, or Either one is possible.

[0070] The first recovered retentate (Prrl) can be delivered from the nanofiltration station (2) or alternatively fed to a second recovered membrane unit (Mr2), in which case a second recovered permeate recirculation loop can be formed.

[0071] Second recovered permeate recycle loop At high impurity concentrations reported in Figures 5-7 and 9, six to almost eight times the phosphate-containing acidic solution (P1), the first recovery retentate (Prrr1) can be delivered from the nanofiltration station (2). However, in preferred embodiments, at least a portion of the first recovery retentate (Prrr1) is not delivered from the nanofiltration station (2) but instead branches off from the first recovery membrane unit and is sent to a second recovery membrane unit (Mr2). Note that a branch is formed when fluid communication is established between the retentate sides of two adjacent nanofiltration membrane units. Figures 1(a)-1(d), 2(c), 2(g), 2(h), and 3(b) illustrate various embodiments including a second recovery membrane unit (Mr2) branching off from the first recovery membrane unit (Mr1).

[0072] At least a portion (or all) of the second recovered permeate (Ppr2) is fed back to the entry line (1e) (or chamber (2c)) to form a second recovered permeate recirculation loop. A complementary portion can be output from the nanofiltration station (2) as a moderately purified nanofiltration recovery solution (P2r). Providing a second recovered permeate recirculation loop allows for increased P2O5 yield by recycling and treating the P2O5 that may be output from the nanofiltration station (2) in the first recovered retentate (Prr1). The second recovered retentate (Prr2) can be output from the nanofiltration station (2). Although possible, including a third recovery membrane unit branched off from the second recovery membrane unit (Mr2) and fed with the second recovered retentate (Prr2) is not considered economically attractive. It is preferable to find an alternative use for the second recovered retentate (Prr2) output from the nanofiltration station (2).

[0073] Figure 2(g) shows There is no permeate recirculation loop in fluid communication between the permeate side of the first recovery membrane unit (Mr1) and the entry line (1e), and At least a portion of the second recovered permeate (Ppr2) is fed to the entry line (1e) as a component of the input phosphate solution (Pf) to form a second recovered permeate recirculation loop; Illustrate an embodiment.

[0074] In other words, the second recycled permeate loop is the only permeate recycled loop in Figure 2(g) that returns to entry line (1e). Because the impurity concentration in the second recycled permeate (Ppr2) is higher than that of the first recycled permeate (Ppr1) with the same reported P2O5 concentration, which is itself higher than that of the first exit permeate (Ppe1), the beneficial reduction of impurities in the input phosphate solution (Pf) relative to the phosphate-containing acidic solution (P1) is reduced if the system includes only a second recycled permeate recycled loop.

[0075] When using a second recovered permeate recycle loop, it is preferable to simultaneously use a first recovered permeate recycle loop, as illustrated in Figure 2(c), which allows for further reduction in the impurity concentration of the input phosphate solution (Pf) relative to the phosphate-containing acidic solution (P1).

[0076] First Exit Permeate Recycle Loop For example, as illustrated in Figures 2(d)-2(f) and 2(h), a portion or all of the first harvest permeate (Ppr1) can flow into the retentate side of the first exit membrane unit (Me1). Figures 2(d), 2(e), and 2(h) show embodiments of the apparatus in which 100% of the first harvest permeate (Ppr1) is fed to the permeate side of the first exit membrane unit (Me1), and in Figure 2(f) only a portion of Ppr1 is fed to the first exit membrane unit (Me1), with the complementary portion being recycled to entry line (1e) via the first harvest permeate recycle loop discussed above.

[0077] At least a portion of the first exit permeate (Ppe1) that permeates the nanofiltration membrane to the permeate side of the first exit membrane unit (Me1) can be recycled to the entry line (1e) via fluid communication between the first exit membrane unit (Me1) and the entry line (1e) or chamber (2c), thus defining a first exit recirculation loop. In the embodiment illustrated in Figures 2(e), 2(f), and 2(h), 100% of the first exit permeate (Ppe1) is recycled to the entry point of the entry line (1e) or chamber (2c) via the first exit permeate recirculation loop. In Figure 2(d), only a portion of Ppe1 is recycled. The complementary portion can be delivered from the nanofiltration unit as nanofiltration phosphate exit solution (P2e). As can be seen from Figure 6(b), the nanofiltered phosphate exit solution with 99 wt.% impurity rejection (P2e) is of higher purity than the nanofiltered phosphate solution with excellent but lower impurity rejection of 94 wt.% (P2).

[0078] Because the first exit permeate (Ppel) has a high purity level, recycling at least a portion of it to the entry line (Ie) advantageously reduces the impurity concentration in the input phosphate solution (Pf) relative to the phosphate-containing acidic solution (P1). This is illustrated by comparing the impurity concentrations in the input phosphate solution (Pf) in Figure 6(a), where 100% of Ppel is recycled to the entry line, and Figure 6(b), where only 10% of Ppel is recycled. It can be seen that when 100% of Ppel is recycled, the impurity content in the input phosphate solution (Pf) in Figure 6(a) was 907 ppm, and when the portion of Ppel being recycled was reduced to 10%, as illustrated in Figure 6(b), it rose to 991 ppm. Note that in both cases, Pf has a lower impurity concentration than the phosphate-containing acidic solution (P1). To be clear, it was standardized to 1000 ppm.

[0079] The same conclusion is reached by comparing the impurity concentrations of the input phosphate solution (Pf) in Figure 7(a), where 100% of Ppel is output from nanofiltration station (2) as nanofiltered phosphate exit solution (P2e), and Figure 7(b), where 100% of Ppel is recycled to entry line (1e). It can be seen that the impurity content in the input phosphate solution (Pf) in Figure 7(a) is 940 ppm when 100% of Ppel is output from nanofiltration station (2) as P2e, and it decreases to 914 ppm when 100% of Ppel is recycled to entry line (1e) as illustrated in Figure 7(b).

[0080] At least a portion of the first exit retentate (Pre1) can be recycled back to the retentate side of the first recovery membrane unit (Mr1), as illustrated in Figures 2(d)-2(h), 3(a), 3(b), 4(a), 4(b), 6(a), 6(b), 7(a), and 7(b). This first exit retentate recycle loop reduces the impurity concentration entering the retentate side of the first recovery membrane unit (Mr1). For example, Figure 6(b) shows that the first retentate (Pr1) has an impurity concentration of 5591 ppm, which, in the absence of the first exit retentate recycle loop, would be fed directly to the retentate side of the first recovery membrane unit (Mr1). The first exit retentate (Pre1) has an impurity concentration of 718 ppm, which, when combined with the first retentate (Pr1), contributes to a reduction in the impurity concentration of the solution Pre1+Pre1 fed to the first membrane recovery unit (Mr1) to 4296 ppm. This is important because it enhances the efficiency of all permeate recycle loops and the quality of the nanofiltered phosphate solution exiting nanofiltration station (2). (Note that the impurity concentrations in Figures 5-9 are not absolute but are based on P1 normalized to 1000 ppm.) The first exit retentate recycle loop also advantageously increases the P2O5 yield by recycling P2O5 that would otherwise be discharged from nanofiltration station (2).

[0081] Second Exit Membrane Unit (Me2) In one embodiment of the present invention, at least a portion, preferably 10 to 100 wt. %, of the first exit permeate (Ppel) is separated into two streams: A second, impurity-lean exit permeate (Ppe2) and The second exit retentate (Pre2), which is rich in impurities The retentate is fed to the retentate side of a second exit membrane unit (Me2) arranged in series with the first exit membrane unit (Me1) for separation into

[0082] Figures 1(a)-1(d) and 3(b) illustrate various embodiments of an apparatus including first and second exit membrane units (Me1, Me2) arranged in series. If 100% of the first exit permeate (Ppe1) is fed to the second exit membrane unit (Me2), the first exit permeate recirculation loop does not exist, and according to the present invention, the permeate side of at least one of the first or second recovery membrane units (Mr1, Mr2) must form a first or second recovery permeate recirculation loop together with the entry line (1e). Such an embodiment is illustrated in Figure 1(b), which includes a first recovery permeate recirculation loop, and in Figure 1(d), which includes a second recovery permeate recirculation loop.

[0083] Alternatively, only a portion of the first exit permeate is fed to the second exit membrane unit (Me2), and the complementary portion is recycled to the entry line (1e) via the first exit permeate recycle loop. Such embodiments are illustrated in Figures 1(a), 1(c), and 3(b). In these embodiments, a first exit permeate recycle loop is formed, and the presence of the first and / or second recovered permeate recycle loops is not required.

[0084] The second exit permeate (Ppe2) has a significantly lower yield but a much higher level of purification with a higher impurity removal rate than that of the already very high first exit permeate (Ppe1). Therefore, the second exit permeate (Ppe2) can advantageously be delivered from the nanofiltration station (2) as nanofiltered phosphate exit solution (P2e) and recovered for high performance applications.

[0085] The second exit retentate (Pre2) can advantageously be recycled to the entry line (1e) forming a second exit retentate recirculation loop formed between the retentate side of the second exit membrane unit (Me2) and the entry line (1e), as illustrated in Figures 1(a) to 1(d) and 3(b).

[0086] Nanofiltration station (2) and ion exchange station (3) In a preferred embodiment illustrated in Figures 4(a) and 4(b), the nanofiltered phosphate solution (P2) discharged from the nanofiltration unit (2) can be fed to an ion exchange station (3) containing an ion exchange resin (3x) configured to remove residual cations, preferably monovalent and divalent cations, remaining in the nanofiltered phosphate solution (P2) and to form a purified phosphoric acid solution (P3) lean to impurities. For example, the ion exchange station (3) can be configured to remove Na + , K. + , Mg +2 , Ca +2 Cations can be removed, including one or more of:

[0087] As shown in Figure 4(a), both the nanofiltered phosphate solutions, including the nanofiltered recovered and exit phosphate solutions (P2r, P2e), can be fed to ion exchange stations (3r, 3e) containing ion exchange resins (3x) for further processing of the nanofiltered phosphate solutions (P2r, P2e) described above.

[0088] The ion exchange resin (3x) is preferably a strong acid cation exchange resin. For example, the strong acid cation exchange resin may comprise polystyrene beads cross-linked with divinylbenzene and sulfonic acid functional groups. Ion exchange resins (3x) suitable for the present invention may be available from Dow, Lanxess, Mitsubishi, etc.

[0089] After treatment in the ion exchange station (3), the nanofiltered phosphate solution (P2, P2r, P2r) is preferably concentrated to a content of at least 40%, preferably at least 50%, more preferably at least 60%, or at least 62%, or even at least 76% P2O5, based on the total weight of the nanofiltered phosphate solution. Concentration of the solution can be achieved by any means known in the art for evaporating water.

[0090] Device The present application also relates to an apparatus for purifying an impure phosphate-containing acidic solution (P1) in the process discussed above. The apparatus comprises a nanofiltration station (2) in fluid communication with an entry line (1e) for feeding the phosphate-containing acidic solution (P1) to the nanofiltration station (2) and an exit line (2e) for delivering the nanofiltered phosphate solution (P2) from the nanofiltration station. The nanofiltration station (2) comprises: n membrane units (M1 to Mn) (where n ≥ 1) arranged in series; a first recovery membrane unit (Mr1) and optionally a second recovery membrane unit (Mr2) arranged in series with the first recovery membrane; optionally a first exit membrane unit (Me1); Each of the above-mentioned membrane units (M1 to Mn, Mr1, Mr2, Me1) includes a retentate side and a permeate side separated by a membrane.

[0091] The retentate side of the first membrane unit (M1) an input line (1e) in fluid communication with the entry line for supplying an input phosphate solution (Pf) to the first membrane unit; and an outlet line in fluid communication with the retentate side of the first recovery membrane unit (Mr1) for supplying the first retentate (Pr1) thereto; and an outlet line in fluid communication with the retentate side of the first recovery membrane unit (Mr1) for supplying the first retentate (Pr1) thereto; In fluid communication with the

[0092] The permeate side of the first collection membrane unit (Mr1) is an entry line (1e) or chamber (2c) for combining at least a portion, preferably 10 to 100 wt.%, of the first recovered permeate (Ppr1) with a phosphate-containing acidic solution (P1) so as to contribute to the formation of an input phosphate solution (Pf); and / or a retentate side of the first exit membrane unit (Me1) for feeding at least a portion of the first collected permeate (Ppr1) to the first exit membrane unit (Me1). In fluid communication with the

[0093] The retentate side of the first recovery membrane unit (Mr1) is optionally in fluid communication with the retentate side of the second recovery membrane unit (Mr2).

[0094] The gist of the device of the present invention is one or more permeate sides of the first membrane recovery unit (Mr1) or the second membrane recovery unit (Mr2) or the first exit membrane unit (Me1); an entry line (1e) or chamber (2c); forming at least one permeate recirculation loop by including fluid communication between the

[0095] Note that the entry line (1e) carrying the phosphate-containing acidic solution (P1) terminates at a return point where the permeate recirculation loop is joined to the entry line (1e) to form the input line, and the input phosphate solution is fed to the retentate side of the first membrane unit (M1). If the entry line (1e) comprises a chamber (2c), the entry line is located upstream of the chamber (2c), and the input line is located downstream of the chamber (2c).

[0096] Figures 1-7 and 9 illustrate various embodiments of the device according to the invention discussed above. A selection of preferred devices is reviewed below. Figure 1(a) with a single membrane unit (M1) and Figure 3(b) with n=3 membrane units (M1-M3) provide a schematic summary of most embodiments of the present invention, showing that valves can be opened, closed, or partially opened to stop flow or, alternatively, allow at least a portion of the solution, i.e., a non-zero portion of the flow, preferably 10-100 wt.% of the flow, more preferably 20-95 wt.% of the flow, more preferably 35-85 wt.%, more preferably 40-75 wt.%, and more preferably 50-65 wt.% of the flow, to flow therethrough. The remaining Figures 1-7 and 9 show specific variations of the devices of Figures 1(a) and 3(b). As explained above, the figures do not show any pumps or other accessories. It is clear that the present invention is not limited to the absence or presence of any pumps, water or other solvent inlets, buffers, heat exchangers, mixers (static or dynamic), valves, etc., located either inside or outside the nanofiltration station 2 depending on the requirements. It is within the ability of a person skilled in the art to determine whether any one of the above mentioned components is required, where to place it, and determine its dimensions.

[0097] Single permeate recirculation loop The simplest form of the device according to the present invention comprises a single permeate recirculation loop in which 100% of the permeate flows from the permeate side of the last nanofiltration membrane unit upstream of the entry line (1e) and returns to the entry line (1e). Figures 2(a) and 5(a) illustrate a first embodiment having only a first recovered permeate recirculation loop with fluid communication between the permeate side of the first recovery membrane unit (Mr1) and the entry line (1e). Figures 2(e) and 6(a) illustrate a second embodiment having only a first exit permeate recirculation loop with fluid communication between the permeate side of the first exit membrane unit (Me1) and the entry line (1e). Finally, Figure 2(g) illustrates a third embodiment having only a second recovered permeate recirculation loop with fluid communication between the permeate side of the second recovery membrane unit (Mr2) and the entry line (1e).

[0098] While 100% of the permeate is recycled in the corresponding permeate recycle loop in each of the first through third embodiments described above, only a portion of the permeate can be recycled, with a complementary portion of the permeate being delivered from nanofiltration station (2) as the nanofiltration phosphate recovery or exit solution (P2r, P2e). Corresponding embodiments having a split of the permeate between the corresponding permeate recycle loop and the exit line from nanofiltration station (2) are illustrated in Figure 5(b) for the first embodiment having a first recovery permeate recycle loop and Figures 2(d) and 6(b) for the second embodiment having a first exit permeate recycle loop.

[0099] Figures 5(a) and 5(b) and Figures 6(a) and 6(b) list the P2O5 yields and impurity rejections obtained for an embodiment in which 100% of the permeate is recycled and corresponding embodiments in which only a portion of the permeate is recycled. Comparing Figures 5(a) and 5(b) reveals that in a single first recycled permeate recycle loop, the P2O5 yield at the same impurity rejection of 94% is enhanced by recycling 100% of the first recycled permeate (Ppr1) to entry line (1e), with less concern about recycling only a portion of the first recycled permeate (Ppr1) to entry line (1e), since the P2O5 yields for full and partial recycling of the first recycled permeate (Ppr1) are 88% vs. 80%, respectively.

[0100] 6(a) and 6(b), it can be seen that in a single first exit permeate recycle loop, partial or complete recycle of the first exit permeate (Ppe1) does not significantly change the nanofiltration performance, including the P2O5 yield and impurity rejection. However, because the first exit permeate has a very high impurity rejection of 99%, a portion of it can be sent from the nanofiltration station (2) and used in high-profile applications requiring high purity levels.

[0101] As shown in Figures 1(b)-1(d), the apparatus of the present invention can include a single permeate recycle loop, as well as a second exit retentate recycle loop with fluid communication established between the retentate side of the second exit membrane unit (Me2) and the entry line (1e), which provides the advantage of producing a highly purified nanofiltered phosphate exit solution (P2e) while simultaneously increasing the total P2O5 yield.

[0102] 2 or 3 permeate recirculation loops Two or more permeate recycle loops can be provided to recycle the corresponding permeate back to the entry line (1e). Adding a permeate recycle loop increases the P2O5 yield and also enhances impurity removal, as long as the additional loop reduces the impurity concentration in the entry line (1e). A first example includes first and second recycled permeate recycle loops, as illustrated in Figure 2(c). In Figure 2(c), depending on the valve settings, only a portion of the second recycled permeate (Ppr2) is recycled to the entry line (1e), and a complementary portion is delivered from the nanofiltration station (2) as nanofiltration recycled phosphate solution (P2r). However, this solution (P2r) itself has limited P2O5 content and impurity removal, and is of limited interest. However, recycling to the second recycled recycle loop is of great interest, as it enhances the P2O5 yield and reduces the impurity concentration in Pf relative to P1.

[0103] A second example involving two recycle loops is illustrated in Figure 2(f), with a first recovery recycle loop and a first exit recycle loop. This embodiment and similar ones are also depicted in Figures 7(a) and 7(b), along with corresponding values ​​of P2O5 and impurity content. It can be seen that both P2O5 yield and impurity removal are very good, especially for the embodiment of Figure 7(b).

[0104] A third example involving two recycle loops is illustrated in Figure 2(h) with a second recovery recycle loop and a first exit recycle loop. This embodiment involves the combination of a second recovery permeate (Ppr2) and a first exit permeate (Ppel), which have significantly different concentrations of both P2O5 and impurities. As such, this embodiment is limited to certain end uses.

[0105] Of course, the apparatus according to the present invention may simultaneously include all three permeate recirculation loops by at least partially opening appropriately set valves to have three recirculation fluid connections connecting the permeate side of each of the first and second recovery membrane units (Mr1, Mr2) and the first exit membrane unit (Me1) to the entry line (1e), as illustrated in Figures 1(a) and 3(b). [Example]

[0106] Figures 5-7 and 9 show examples of various embodiments of the present invention, along with the corresponding values ​​of PO and impurity content at various stages of the nanofiltration process. Figure 8 shows the corresponding results for a comparative apparatus described in U.S. Pat. No. 5,945,000. The example apparatus of Figure 9 according to the present invention is a modification of the prior art apparatus of Figure 8, with the addition of a first recovery membrane unit (Mr1) and a first recovery recycling loop to obtain an apparatus according to the present invention. The mass recovery of each nanofiltration membrane unit in Figures 8 and 9 is shown in the box representing it (e.g., "83%" shown for the first membrane unit (M1) in both Figures 8 and 9 indicates that 83 wt.% of the input phosphate solution (Pf) passed through the nanofiltration membrane of the first membrane unit (M1)). The nanofiltration units contained TFC-type (=thin film composite) nanofiltration membranes and were tested in a pilot plant using feed solutions with various PO and impurity concentrations. The values ​​of P2O5 and impurity content at each stage of a specific device were calculated from empirical results measured on individual or specific configurations of nanofiltration membrane units. These calculations suggest that the viscosity of the feed solutions to each nanofiltration unit must be optimized before entering their respective retentate sides. In practice, this can be done by adding water to the feed solutions or by controlling their temperature.

[0107] Examples are discussed above to demonstrate that the apparatus and process architecture of the present invention can be modified to achieve predefined purification goals corresponding to specific end uses.

[0108] Comparing the example of FIG. 9 with the comparative prior art example illustrated in FIG. 8, it can be seen that higher impurity removal rates and higher P2O5 yields are achieved using the apparatus and process of the present invention. [Explanation of symbols]

[0109] 1 Pre-refining station 1e Entry Line 2 Nanofiltration Station 2e Exit Line 2c Chamber 3. Ion exchange station supplied with nanofiltration solution P2 3e Ion exchange station supplied with nanofiltration exit solution P2e 3r Ion exchange station supplied with nanofiltration recovery solution P2r 3x Ion Exchange Resin M1-Mn 1st to nth membrane units Me1, Me2 First and second exit membrane units Mr1, Mr2 First and second recovery membrane units P1 Acidic solution containing phosphate P2 Nanofiltered Phosphate Solution P2r Nanofiltration Phosphate Recovery Solution P2e Nanofiltration Phosphate Exit Solution P3 Purified Phosphate Solution Pf: Input phosphate solution to the first membrane Pp1-n Permeates from membrane stations 1 to n Pr1-n: 1st to nth retentates of 1st to nth membrane stations Ppe1, Ppe2: First and second exit permeants Pre1, Pre2 First and second exit holdings Ppr1, Ppr2 First and second collected permeates Prr1, Prr2 First and second recovery retentates

Claims

1. A process for purifying an impure phosphate-containing acidic solution (P1) comprising the steps of: supplying said phosphate-containing acidic solution (P1) to a nanofiltration station (2) via an entry line (1e) to produce a nanofiltered phosphate solution (P2), said phosphate-containing acidic solution (P1) having a pH of less than or equal to 2, said nanofiltration station (2) n membrane units (M1 to Mn) (where n≧1) arranged in series; o a first collection membrane unit (Mr1); and Including, providing said membrane units (M1-Mn, Mr1), each comprising a retentate side and a permeate side separated by a membrane; - forming an input phosphate solution (Pf) by combining said phosphate-containing acidic solution (P1) with one or more other flows; The input phosphate solution (Pf) is divided into two streams: o a first permeate (Pp1) lean to impurities, and o Impurity-enriched first retentate (Pr1) feeding said input phosphate solution (Pf) to a first membrane unit (M1) for separation into if n>1, feeding at least a portion of said first permeate (Pp1) to a second membrane unit (M2), and so on, until at least a portion of the (n-1)th permeate has been fed to an nth membrane unit (Mn); The first retentate (Pr1) is separated into two streams: o a first harvested permeate (Ppr1) lean in impurities; and o Impurity-enriched first recovered retentate (Prr1) feeding at least a portion of said first retentate (Pr1) to said first recovery membrane unit (Mr1) for separation into - sending the nanofiltered phosphate solution (P2) from the nanofiltration station (2) through the permeate side of the nth membrane unit (Mn); including: providing a permeate recirculation loop in fluid communication between the permeate side of said first recovery membrane unit (Mr1) and said entry line (1e); feeding at least a portion of said first recovered permeate (Ppr1) to said entry line (1e) and combining with said phosphate-containing acidic solution (P1) to form said input phosphate solution (Pf); A process characterized by

2. 2. The process according to claim 1, wherein n>1 and each of the n membrane units (M1 to Mn) sequentially separates each of the first to (n-1)th permeates (Pp1 to Pp(n-1)) into two streams: impurity-depleted permeates 2 to n (Pp2 to Ppn); and Impurity-rich retentates 2 to n (Pr2 to Prn) are arranged in series to separate at least a portion of each of the successive first through (n-1)th permeates (Pp1 through Pp(n-1)) is fed to the retentate side of the next second through nth membrane units (M2 through Mn) downstream of the series of n membrane units (M1 through Mn); at least a portion of each of the second to nth retentates (Pr2 to Prn) is fed to the retentate side of the first recovery membrane unit (Mr1) and / or to the retentate side of the first to (n-1)th membrane units (M1 to M(n-1)) upstream of the series of n membrane units (M1 to Mn); A process characterized by:

3. 3. The process according to claim 1 or 2, a second recovery membrane unit (Mr2) arranged in series with the first recovery membrane unit (Mr1); The first recovered retentate (Prr1) is separated into two streams: a second recovered permeate (Ppr2) lean in impurities, and An impurity-enriched second recovered retentate (Prr2) at least a portion of the first recovery retentate (Prr1) discharged from the first recovery membrane unit (Mr1) is fed to the retentate side of the second recovery membrane unit (Mr2) to separate At least a portion of the second recovered permeate (Ppr2) is fed into said entry line (1e) to form a permeate recycle loop and to combine with said phosphate-containing acidic solution (P1) to contribute to the formation of said input phosphate solution (Pf); and / or - discharged from said nanofiltration station (2) and recovered as nanofiltered phosphate recovery solution (P2r); A process characterized by:

4. 4. The process of claim 3, no permeate recirculation loop is provided in fluid communication between the permeate side of the first recovery membrane unit (Mr1) and the entry line (1e); and at least a portion of said second recovered permeate (Ppr2) is fed to said entry line (1e) as a component of said input phosphate solution (Pf) to form one of one or more permeate recirculation loops; A process characterized by:

5. In the process according to any one of claims 1 to 4, a first exit membrane unit (Me1) disposed in series with the first recovery membrane unit (Mr1), the first exit membrane unit (Me1) including a retentate side and a permeate side separated by a membrane; at least a portion of the first recovered permeate (Ppr1), The first recovered permeate (Pr1) is separated into two streams: a first exit permeate (Ppe1) lean in impurities, and an impurity-rich first exit retentate (Pre1); and feeding the first exit membrane (Me1) for separation into providing a permeate recirculation loop in fluid communication between the permeate side of the first exit membrane unit (Me1) and the entry line (1e).

6. 6. The process according to claim 5, wherein at least a portion of the first recovered permeate (Ppr1) is fed to the first exit membrane unit (Me1), and at least a portion of the first exit permeate (Ppe1) is fed to the first exit membrane unit (Me1), which separates the first exit permeate (Ppe1) into two streams: a second exit permeate (Ppe2) lean in impurities, and An impurity-enriched second exit retentate (Pre2) and the second exit permeate (Ppe2) is fed to the retentate side of a second exit membrane unit (Me2) for separation into a nanofiltration phosphate exit solution (P2e), at least a portion of the second exit permeate (Ppe2) is discharged from the nanofiltration station (2) to form a nanofiltration phosphate exit solution (P2e), and at least a portion of the second exit retentate (Pre2) is fed to the entry line (1e) to form a retentate recirculation loop and to be combined with the phosphate-containing acidic solution (P1) to contribute to the formation of the input phosphate solution (Pf).

7. 6. The process according to any one of claims 1 to 5, the first recovered permeate (Ppr1) the second recovered permeate (Ppr2) according to claim 3 or 4, or The first exit permeate (Ppe1) according to claim 5 or 6. is fed to said entry line (1e) to form one or more permeate recycle loops and to combine with said phosphate-containing acidic solution (P1) to contribute to the formation of said input phosphate solution (Pf).

8. 7. The process according to any one of claims 1 to 6, characterized in that it comprises a single permeate recirculation loop formed between the first recovered permeate (Ppr1) and the entry line (1e).

9. 8. The process according to any one of claims 1 to 7, characterized in that the nanofiltered phosphate solution (P2) discharged from the nanofiltration station (2) is fed to an ion exchange station (3) containing ion exchange resins (3x) configured to remove residual cations remaining in the nanofiltered phosphate solution (P2) and to form a purified phosphoric acid solution (P3) lean to impurities.

10. 9. The process according to any one of claims 1 to 8, wherein the phosphate-containing acidic solution (P1) is ・ 2 to 25% P ２ O5 and particles greater than 1 μm and less than 100 ppm; - less than 3 wt.% total organic carbon (TOC); - 4 wt% or less SO4; A process comprising:

11. 10. The process of any one of claims 1 to 9, wherein the impurities include Al, Ca, Cr, Fe, Mg, and P2 has a removal rate of these impurities of at least 90 wt. % based on P1.

12. 11. An apparatus for purifying an impure phosphate-containing acidic solution (P1) in a process according to any one of claims 1 to 10, said apparatus comprising a nanofiltration station (2) in fluid communication with an entry line (1e) for feeding said phosphate-containing acidic solution (P1) to said nanofiltration station (2) and with an exit line (2e) for delivering a nanofiltered phosphate solution (P2) from said nanofiltration station, said nanofiltration station (2) comprising: a source of said phosphate-containing acidic solution (P1) having a pH of 2 or less in fluid communication with said entry line (1e); n membrane units (M1 to Mn) (where n≧1) arranged in series; First recovery membrane unit (Mr1) Including, Each of the aforementioned membrane units (M1-Mn, Mr1, Me1) comprises a retentate side and a permeate side separated by a membrane; the retentate side of the first membrane unit (M1) o an input line in fluid communication with said entry line (1e) for supplying an input phosphate solution (Pf) to said first membrane unit; and an outlet line in fluid communication with the retentate side of the first recovery membrane unit (Mr1) for supplying the first retentate (Pr1) thereto; in fluid communication with the permeate side of the first recovery membrane unit (Mr1) o in fluid communication with said entry line (1e) or chamber (2c) for combining at least a portion of the first recovered permeate (Ppr1) with said phosphate-containing acidic solution (P1) to contribute to the formation of said input phosphate solution (Pf); The apparatus is characterized in that a permeate recirculation loop is formed by including fluid communication between said entry line (1e) or chamber (2c) and the permeate side of said first recovery membrane unit (Mr1).

13. The device according to claim 12, a second recovery membrane unit (Mr2) comprising a retentate side and a permeate side separated by a membrane, in fluid communication with the retentate side of the second recovery membrane unit (Mr2); a second recovery membrane unit (Mr2) disposed in series on the retentate side of the first recovery membrane unit (Mr1); and / or a first exit membrane unit (Me1) comprising a retentate side and a permeate side separated by a membrane, a first exit membrane unit (Me1) connected to the retentate side for supplying at least a portion of the first recovered permeate (Ppr1) to the first exit membrane unit (Me1); a first exit membrane unit (Me1) disposed in series on the permeate side of the first recovery membrane unit (Mr1); Including, and including fluid communication between said entry line (1e) or chamber (2c) and the permeate side of said second recovery membrane unit (Mr2) or said first exit membrane unit (Me1), thereby forming at least one permeate recirculation loop.

14. 14. The device according to claim 12 or 13, the permeate side of the first membrane unit (M1) is in fluid communication with the retentate side of a second membrane unit (M2), the permeate side of which is in fluid communication with a third membrane unit (M3), and so on up to the nth membrane unit (Mn), the permeate side of which is coupled to the exit line (2e); The retentate side of the second to nth membrane units (M2 to Mn) o the first to (n-1)th membrane units preceding a given membrane unit, and / or o First collection membrane unit (Mr1) in fluid communication with the retentate side of the

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