Method for purifying a phosphate containing solution

Abstract

The present invention concerns a method for purifying a phosphate containing solution using at least a first ion-exchange separation system and a second ion-exchange separation system, each IEX system comprising at least two columns connected in series; each column of said first IEX system comprising at least one strong acid cation resin, and each column of said second IEX system comprising at least one chelating resin.

Description

[0001] “METHOD FOR PURIFYING A PHOSPHATE CONTAINING SOLUTION” FIELD OF THE INVENTION

[0002] The present invention relates to the technical field of phosphate purification, such as phosphoric acid purification, via ion-exchange.

[0003] STATE OF THE ART

[0004] Phosphates such as for example phosphoric acid and phosphate salts such as for example potassium phosphate (mono, di, tri), calcium phosphate (mono, di, tri), sodium phosphates, iron phosphates are used in many applications in the food and animal feed sector, agriculture and in the pharma industry. Phosphoric acid is also used in the production of semiconductors and integrated circuits. Depending on the final application, different levels of purity are required. However, phosphates are often produced from materials containing many impurities. These materials include for example phosphates rocks such as apatite, hydroxyapatite, chlorapatite or rocks containing low levels of P2O5, ashes from wastewater treatment sludges incineration, bone ashes, residual acids from metallurgical treatments, industry side streams such as washing solutions from the chemical, phosphate, pharmaceutical, food-processing industries.

[0005] The present invention is concerned with the removal of metal ions, in particular Na, Ca, Mg, Fe, Cd, Pb, and Al ions from phosphate containing solutions.

[0006] EP 0647205 A1 discloses a process for removing iron from phosphoric acid prepared from a wet process. The document further discloses that iron (III) is reduced to iron (II) which can then be removed by cationic ion exchange.

[0007] US 2015166343 A1 discloses processes and methods for the recovery or the removal of metals, including the so-called “Minor Elements” consisting of iron, aluminum and magnesium, or their respective oxides, from a wet-process phosphoric acid using a continuous ion exchange approach. The document further discloses the use of a strong cationic exchange resin or equivalent material or composition capable of binding the minor element (ME) components, wherein the strong cationic resin is in the H+form.

[0008] However, some of the processes of the prior art do not always enable an efficient removal of Na, Ca, Mg, Fe, Cd, Pb, and Al ions and an efficient regeneration of the resins. There is thus a need for an alternative or im proved versatile method for purifying a phosphate containing solutions.

[0009] SUMMARY OF THE INVENTION

[0010] The inventors have surprisingly found that the present invention can at least partially fulfill the above identified need.

[0011] The present invention is concerned with a method for purifying a phosphate containing solution [hereafter, solution P] using at least a first ion-exchange separation system [hereafter, first IEX system] and a second ion-exchange separation system [hereafter, second IEX system], each IEX system comprising at least two columns connected in series; each column of said first IEX system comprising at least one strong acid cation resin, and each column of said second IEX system comprising at least one chelating resin; wherein said method comprises a series of steps sequences which are carried out in each IEX system, each sequence comprising at least the steps of: a. feeding a solution P to at least one column; b. collecting the solution P from at least one column different to the column used in step (a); c. regenerating at least one column; and wherein in each sequence, the solution P is fed in step (a) in a different column than the column used in step (a) of the previous sequence; with the proviso that the solution P fed in said second IEX system is the solution P collected from said first IEX system or that the solution P fed in said first IEX system is the solution P collected from said second IEX system. In the context of the present invention, the term “comprising” should not be interpreted as excluding features or elements other than those explicitly mentioned. It should be construed as specifying the presence of the features or elements indicated, but does not exclude the presence or addition of one or more other features or elements. Thus, the scope of the expression "a method comprising steps A and B" should not be limited to methods consisting only of steps A and B. Similarly, a composition comprising components A and B should not be limited to compositions consisting only of components A and B. Accordingly, the terms "comprising" and "including" encompass the terms more restrictive “consisting essentially of’ and “consisting of”.

[0012] In the context of the present invention, if an element or component is said to be selected from a list of recited elements or components, it should be understood that the element or component can also be any one of the individual recited elements or components in said list, or can also be selected from a group consisting of any two or more of the explicitly listed elements or components.

[0013] Furthermore, the terms “first”, “second”, “third” and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein. Furthermore, the various embodiments, although referred to as “preferred” are to be construed as exemplary manners in which the invention may be implemented rather than as limiting the scope of the invention.

[0014] Unless stated otherwise, all the percentages and ppm concentrations are respectively percentages by weight (= wt.%) and ppm in weight (= ppm). The present invention is concerned with a method for purifying a phosphate containing solution [hereafter, solution P],

[0015] In the context of the present invention, the term “phosphate containing solution” is intended to denote a solution, particularly an aqueous solution, comprising at least one phosphate compound. In particular, the term “phosphate compound” can be understood as a compound comprising a phosphate anion. Thus, said phosphate containing solution may be for example a solution comprising phosphoric acid, a solution comprising phosphate salts, or a solution comprising phosphoric acid and phosphate salts. Preferably, said solution P is a phosphoric acid solution.

[0016] Preferably, said solution P comprises, based on the total weight of said solution P, at least 5 wt.%, preferably at least 10 wt.%, more preferably at least 20 wt.%, of phosphorus expressed as P2O5 equivalent.

[0017] Preferably, said solution P comprises, based on the total weight of said solution P, at most 40 wt.%, preferably at most 35 wt.%, more preferably at most 30 wt.% of phosphorus expressed as P2O5 equivalent.

[0018] Preferably, said solution P comprises, based on the total weight of said solution P, at least 5 wt.% and at most 40 wt.%, more preferably at least 10 wt.% and at most 35 wt.%, even more preferably at least 15 wt.% and at most 30 wt.%, even more preferably at least 20 wt.% and at most 30 wt.%, even more preferably 25 wt.% of phosphorus expressed as P2O5 equivalent.

[0019] Said solution P may be filtered before used in said step (a).

[0020] In the context of the present invention, phosphate weight percentages represent the total amount of phosphorus atoms expressed as wt.% of P2O5 equivalent, and may also be expressed in weight percentages of H3PO4 equivalents, denoted wt.% of H3PO4. The correspondence between the two concentration units is defined by the relation: 1 wt.% of P2O5 = 0.7245 wt.% of H3PO4.

[0021] In addition, said solution P can comprise based on the total weight of said solution P at least 100 ppm, preferably at least 1000 ppm, more preferably at least 5000 ppm of Ca ions. If desired, said solution P can comprise based on the total weight of said solution P at most 25000 ppm, preferably at most 16000 ppm, more preferably at most 10000 ppm more preferably at most 5000 ppm of Ca ions. Thus, if desired, said solution P can comprise based on the total weight of said solution P at least 100 ppm and at most 25000 ppm, preferably at least 1000 ppm and at most 10000 ppm, more preferably at least and at most 5000 ppm of Ca ions.

[0022] In addition, said solution P can comprise based on the total weight of said solution P at least 50 ppm, preferably at least 300 ppm, more preferably at least 1000 ppm of Mg ions. If desired, said solution P can comprise based on the total weight of said solution P at most 10000 ppm, preferably at most 3000 ppm, more preferably at most 1000 ppm of Mg ions. Thus, if desired, said solution P can comprise based on the total weight of said solution P at least 50 ppm and at most 10000 ppm, preferably at least 300 ppm and at most 3000 ppm, more preferably at least and at most 1000 ppm of Mg ions.

[0023] In addition, said solution P can comprise based on the total weight of said solution P at least 50 ppm, preferably at least 500 ppm, more preferably at least 5000 ppm of Fe ions. Said solution P can comprise based on the total weight of said solution P at most 15000 ppm, preferably at most 10000 ppm, more preferably at most 5000 ppm of Fe ions. Thus, if desired, said solution P can comprise based on the total weight of said solution P at least 50 ppm and at most 15000 ppm, preferably at least 500 ppm and at most 10000 ppm, more preferably at least and at most 5000 ppm of Fe ions. In addition, said solution P can comprise based on the total weight of said solution P at least 50 ppm, preferably at least 400 ppm, more preferably at least 2000 ppm of Al ions. If desired, said solution P can comprise based on the total weight of said solution P at most 10000 ppm, preferably at most 4000 ppm, more preferably at most 2000 ppm of Al ions. Thus, if desired, said solution P can comprise based on the total weight of said solution P at least 50 ppm and at most 10000 ppm, preferably at least 400 ppm and at most 4000 ppm, more preferably at least and at most 2000 ppm of Al ions.

[0024] In addition, said solution P can comprise based on the total weight of said solution P at least 10 ppm, more preferably at least 20 ppm, even more preferably at least 50 ppm, preferably at least 200 ppm, more preferably at least 350 ppm of Na ions. If desired, said solution P can comprise based on the total weight of said solution P at most 10000 ppm, preferably at most 3000 ppm, more preferably at most 1000 ppm of Na ions. Thus, if desired, said solution P can comprise based on the total weight of said solution P at least 10 ppm and at most 10000 ppm, preferably at least 20 ppm and at most 3000 ppm, more preferably at least 50 ppm and at most 1000 ppm, even more preferably at least 350 ppm and at most 1000 ppm of Na ions.

[0025] In addition, said solution P can comprise based on the total weight of said solution P at least 50 ppm, preferably at least 300 ppm, more preferably at least 1000 ppm of K ions. If desired, said solution P can comprise based on the total weight of said solution P at most 10000 ppm, preferably at most 3000 ppm, more preferably at most 2000 ppm of K ions. Thus, if desired, said solution P can comprise based on the total weight of said solution P at least 50 ppm and at most 10000 ppm, preferably at least 300 ppm and at most 3000 ppm, more preferably at least 300 ppm and at most 2000 ppm of K ions. In addition, said solution P can comprise based on the total weight of said solution P at most 100 ppm, preferably at most 10 ppm, more preferably at most 5 ppm of Cd ions.

[0026] In addition, said solution P can comprise based on the total weight of said solution P at most 100 ppm, preferably at most 20 ppm, more preferably at most 10 ppm of Pb ions.

[0027] In a preferred embodiment, said solution P is a phosphoric acid solution comprising, based on the total weight of said solution P:

[0028] • at least 10 wt.% and at most 35 wt.% of phosphoric acid expressed as P2O5 equivalent,

[0029] • at least 500 ppm and at most 10000 ppm of Ca ions,

[0030] • at least 100 ppm and at most 3000 ppm of Mg ions,

[0031] • at least 100 ppm and at most 10000 ppm of Fe ions,

[0032] • at least 100 ppm and at most 10000 ppm of Al ions,

[0033] • at least 20 ppm and at most 10000 ppm of Na ions, at least 300 ppm and at most 10000 ppm of K ions,

[0034] • at most 20 ppm of Pb ions,

[0035] • at most 10 ppm of Cd ions.

[0036] In a preferred embodiment, said solution P is a phosphoric acid solution comprising, based on the total weight of said solution P:

[0037] • at least 10 wt.% and at most 35 wt.% of phosphoric acid expressed as P2O5 equivalent,

[0038] • at least 1000 ppm and at most 10000 ppm of Ca ions,

[0039] • at least 300 ppm and at most 3000 ppm of Mg ions,

[0040] • at least 500 ppm and at most 10000 ppm of Fe ions,

[0041] • at least 400 ppm and at most 10000 ppm of Al ions,

[0042] • at least 350 ppm and at most 10000 ppm of Na ions, • at least 300 ppm and at most 10000 ppm of K ions,

[0043] • at most 10 ppm of Pb ions,

[0044] • at most 5 ppm of Cd ions.

[0045] According to the present invention, at least a first ion-exchange separation system [hereafter, first IEX system] and a second ion-exchange separation system [hereafter, second IEX system] are used for purifying said solution P, each IEX system comprising at least two columns connected in series, preferably at least three columns connected in series. Said method comprises a series of steps sequences which are carried out in each IEX system, each sequence comprising at least the steps of: a. feeding a solution P to at least one column; b. collecting the solution P from at least one column different to the column used in step (a), preferably from at least one column adjacent, more preferably adjacent and downstream, to the column used in step (a); c. regenerating at least one column.

[0046] The at least one column regenerated in step (c) can be the column used in step (a) of the same sequence or a previous sequence or the previous sequence.

[0047] In other words, in a given step sequence, the solution P passes through the column wherein it is fed in step (a) and which can be seen as a loading column. Then, in the same sequence, the solution P passes through the column wherefrom the solution P is collected in step (b), which can be seen as a polishing column that guaranties that a sufficient amount of metal ions was removed from said solution P.

[0048] In particular, in each IEX system, the columns can be connected to each other. For example, said at least two columns, preferably said at least three columns can be connected in series. Preferably, said at least two columns or at least three columns can be connected in series via fluidic connection means such as for examples without being limited to: conduits or ducts, like pipes, tubes, channels, valves, ports and combinations thereof. The fluidic connection means may also comprise other elements such as fittings like screws / nuts and ferrules, or liquid-tight sealings. The IEX systems (said first, second, third IEX systems) can be connected to each other, preferably via fluidic connection means such as defined above.

[0049] Thus, in each IEX system, said at least two or three columns can be fluidically connected. Preferably, in each IEX system, said at least two or three columns can be fluidically connected during at least step (a) of feeding a solution P to at least one column and step (b) of collecting the solution P. The IEX systems can be fluidically connected to each other. Within the context of the present invention, if two elements are fluidically connected, it does not imply that a fluid always passes from one element to the other. Moreover, if two elements are connected to each other or fluidically connected to each other, it does not imply that other elements such as vessels, tanks or reactors are not present between the two elements which are connected. Thus, the term “connected” encompasses respectively “directly connected”, “indirectly connected”. In each sequence of the method according to the present invention, a solution P is fed into a different column than the column used in step (a) of the previous sequence. In each sequence in an IEX system, the solution P is different (i.e., not the solution P collected in step (b) of the previous sequence).

[0050] For example, in one embodiment wherein each IEX system comprises at least two or only two columns, a first solution P is fed in step (a) of a first sequence in one column and then collected in step (b) from a second column of the same IEX system. Then, the column used in step (a) is regenerated in step (c). Then, in the following sequence, a different solution P (i.e., a second solution P, not the solution P collected in step (b) of the previous sequence) is fed in step (a) into the second column of the IEX system (the column wherefrom the first solution P was collected in step (b) of the previous sequence). The solution P is then collected in step (b) from the other column of the same IEX system. Then, the column used in step (a) is regenerated in step (c).

[0051] In another embodiment wherein each IEX system comprises three columns, a first solution P is fed in step (a) of a first sequence into one column. Then, in the following sequence, a different solution P (i.e., a second solution P) is fed in step (a) into a second column of the IEX system. The second column wherein the second solution P is fed is the column wherefrom the first solution P was collected in step (b) of the previous sequence. In the same second sequence, the second solution P is then collected from the third column of the IEX system. In each sequence, the column which is not used in step (a) or (b) of the second sequence (or the column which was used in step (a) of the previous sequence) is regenerated in a step (c).

[0052] Therefore, if each IEX system has at least three columns, 2 columns out of 3 are operating and one is regenerated.

[0053] Therefore, it is clear that if each IEX system has at least three columns, the method according to the present invention has the advantage of enabling at least partially at the same time the purification of the phosphoric acid (steps (a) and (b)) and the regeneration step (c). This provides a tremendous gain of time and efficiency.

[0054] Moreover, the solution P which is fed in said second IEX system may be preferentially the solution P that is collected from said first IEX system. Alternatively, the solution P which is fed in said first IEX system may be preferentially the solution P that is collected from said second IEX system. In other words, a same solution P passes through both the first and the second IEX systems according to the method of the present invention. The use of at least two IEX systems containing at least two columns enables to have a versatile and modular method which can be adapted in many different ways depending on different needs and situations. For example, the method according to the present invention may comprise at least one additional step. The at least one additional step may for example be a step of iron (III) reduction to iron (II), a filtration (micro- or nanofiltration) step, a rinsing step or any other suitable step. Depending on the circumstances, said additional step can be carried out after said solution P was passed through the first IEX system and before it was passed through the second IEX system. Alternatively, said additional step can be carried out before or after passing said solution P through both the first and second IEX systems or after each IEX system.

[0055] Preferably, between each step sequence, the columns of each IEX system can be rinsed with at least one aqueous solution. The aqueous solution used for rinsing can have a pH comprised between 1 .5 and 7.

[0056] Preferably, the process according to the present invention comprise an iron reduction step wherein a reducing agent selected from the group consisting of Fe(0), hydrazine, Zn(0), I2, quinone, hydroquinone, CO, and mixtures thereof are used.

[0057] The iron reduction step can be carried out by mixing said solution P with at least one reducing agent, preferably with at least one reducing agent selected from the group consisting of Fe(0), hydrazine, Zn(0), I2, quinone, hydroquinone, CO, and mixtures thereof are used.

[0058] Preferably, said step (c) comprises at least the steps of: c1 . feeding at least one regenerating solution to said column being regenerated; and c2. collecting said regenerating solution from the column being regenerated; with the proviso that the regenerating solution fed in said first IEX system is a regenerating solution collected from said second IEX system. In other words, the regenerating solution fed in said first IEX system is preferably a regenerating solution which was used to regenerate said second IEX system.

[0059] In other words, the regenerating solution is preferably first passed through the column being regenerated in the second IEX system and then through the column being regenerated in the first IEX system. It was surprisingly found by the inventors that it enables to efficiently regenerate columns in the first and second IEX system with the same regenerating solution, thus rendering the method of the present invention more economical and efficient.

[0060] Preferably, said regenerating solution is an acidic solution or an alkaline solution.

[0061] Preferably, said regenerating solution is an acidic solution comprising at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid and mixtures thereof; wherein said acidic solution preferably comprises at least 5 wt.%, more preferably at least 10 wt.%, even more preferably at least 15 wt.% and preferably at most 25 wt.%, more preferably at most 35 wt.% of said acid, based on the total weight of said acidic solution.

[0062] Said regenerating solution preferably comprises based on the total weight of said acidic solution, at least 5 wt.% and at most 35 wt.% of said acid when used in said second IEX system and preferably comprises based on the total weight of said acidic solution, at least 15 wt.% and at most 25 wt.% of said acid when used in said first IEX system.

[0063] Preferably, said regenerating solution has a pH of at most 2, preferably at most 1 , even more preferably at most 0.1 , preferably at normal temperature and pressure conditions with a suitable pH meter. Suitable pH meters include for example Metrohm pH meters.

[0064] According to the present invention, each column of said first IEX system comprises at least one strong acid cation resin, and each column of said second IEX system comprises at least one chelating resin.

[0065] Preferably, said at least one strong acid cation resin is made of a crosslinked polymer comprising sulfonic acid moieties; and wherein said polymer is selected from the group consisting of polystyrene, polyacrylate, and combinations thereof.

[0066] Preferably, said at least one chelating resin comprises at least one crosslinked polymer and phosphonic acid and sulfonic acid moieties; and wherein said polymer is selected from the group consisting of polystyrene, polyacrylate, and combinations thereof. Moreover, preferably said chelating resin does not comprise amine and / or amide moieties.

[0067] A third IEX system may be used. Thus, in a preferred embodiment, the invention concerns a method for purifying a solution P using at least a first IEX system and a second IEX system and a third IEX system, each IEX system comprising at least two columns, preferably at least three columns, connected in series. Preferably, each column of said first IEX system comprises at least one strong acid cation resin, and each column of said second IEX system preferably comprises at least one chelating resin and each column of said third IEX system preferably comprises at least one strong acid cation resin. Moreover, said method according to this embodiment comprises a series of step sequences which are carried out in each IEX system as described above.

[0068] The use of at least three IEX systems containing at least two columns enables to have a versatile and modular method which can be adapted in many different ways depending on different needs and situations. For example, the method according to the present invention may comprise at least one additional step. The at least one additional step may for example be a step of iron (III) reduction to iron (II), a filtration (micro- or nano- filtration) step or any other suitable step. Depending on the circumstances, said additional step can be carried out after said solution P was passed through the first IEX system and before it was passed through the second IEX system. Alternatively, said additional step can be carried out before or after passing said solution P through both the first, second and third IEX systems or before or after each IEX system.

[0069] Preferably, in the same embodiment, an iron (III) to iron (II) reduction step can be carried out, preferably after the solution P is collected from the first IEX system and preferably before it is fed in the second IEX system.

[0070] BRIEF DESCRIPTION OF THE FIGURES

[0071] For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings in which:

[0072] • Figure 1 schematically illustrates an embodiment of the method according to the present invention.

[0073] • Figures 2a to 2d schematically illustrate two consecutive step sequences in an IEX system having two columns according to an embodiment of the present invention. A first step sequence is illustrated by Figures 2a (steps (a) et (b)) and 2b (step (c)) and a second step sequence is illustrated by Figures 2c (steps (a) and (b)) and 2d (step (c)). Thus, the columns and sequences illustrated in figures 2a-2d can be present in each IEX system.

[0074] • Figures 3a to 3c schematically illustrate three consecutive step sequences in an IEX system having three columns according to an embodiment of the present invention. A first step sequence is illustrated by Figures 3a, a second step sequence is illustrated by Figure 3b and third step sequence is illustrated by Figure 3c. Thus, the columns and sequences illustrated in figures 3a-2c can be present in each IEX system.

[0075] • Figure 4 schematically illustrates a particular embodiment wherein an iron (III) reduction step is carried out.

[0076] DESCRIPTION OF THE FIGURES

[0077] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto. The described functions are not limited by the described structures. The drawings described are only schematic and are non-limiting.

[0078] In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Elements that are designated by the same number may be the same.

[0079] According to an embodiment of the method according to the present invention illustrated at Figure 1 , a phosphate containing solution 101 can be fed 104 into a first IEX system 102 which is connected in series to a second IEX system 103. The solution P is then collected from said first IEX system 102 and fed 105 into a second IEX system 103. The purified solution P 107 is then collected from said second IEX system 103.

[0080] The columns of the first IEX system 102 are filled with a strong acid cation resin and the columns of the second IEX system 103 are filled with a chelating resin (not shown).

[0081] In one embodiment illustrated in Figure 2a-2d, each IEX system comprises two columns 202 and 203 connected in series. In a first steps sequence of the same embodiment illustrated in Figures 2a and 2b, a solution P is fed 201 into a first column 202, it then passes through the column 202, it is fed 204 into column 203 and is finally collected 205 from the column 203. In other words, the column 203 can be adjacent to column 202. In that step sequence, the column 202 can be considered a loading column and the column 203 can be considered a polishing column.

[0082] Then, said column 202 can be rinsed with a water solution (not shown). Then, in Figure 2b, a regenerating solution, preferably an HCI solution, is preferably used to regenerate said column 202. The regenerating solution is preferably fed 206 into the first column 202 and collected 207 from said first column 202. After regeneration, said column

[0083] 202 can be rinsed with a water solution (not shown). The column 203 can be in stand-by (temporarily not in use) during the regeneration step.

[0084] In a second step sequence of the same embodiment illustrated in Figures 2c and 2d, a solution P is fed 20T into said second column 203 which is adjacent to said first column 202, it then goes through the column

[0085] 203 and is fed 204’ into column 202 and is then collected 205’ from the column 202. Then, said column 203 can be rinsed with a water solution (not shown). Then, a regenerating solution, preferably an HCI solution is preferably used to regenerate said column 203. The regenerating solution is fed 206’ into the column 203 and collected 207’ from said column 203. After regeneration, said column 203 can be rinsed with a water solution. The column 202 can be in stand-by during the regeneration step.

[0086] The step sequences illustrated in figures 2a-2b and 2c-2d can be repeated as many times as required, depending for example on the purification stage required for solution P to be purified.

[0087] The solutions P which are fed 201 , 20T in the columns 202 and 203 respectively of the first IEX system 102 in each step sequence, can be of identical nature, they can come from the same batch of solution P. The solutions P which are collected 205, 205’ from columns 203 and 202 respectively in each step sequences in the first IEX system 102 are the solutions P which are fed 201 , 20T into columns 202 or 203 respectively in the second IEX system 103 in each step sequence. Thus, the solutions P which are fed into the columns 202 and 203 of the second IEX system 103 were previously passed through the first IEX system 102. The solution P which are collected 205, 205’ from columns 202 and 203 respectively in the second IEX system 103 are purified solutions P containing reduced amounts of Na, Ca, Mg, Fe, Cd, Pb, and Al ions.

[0088] The regenerating solution 108 is preferably first used to regenerate the column 202 and / or 203 of the second IEX system 103 by feeding 206, 206’ the regenerating solution 108 in the column 202 and / or 203 of the second IEX system 103, during a regenerating step (c). The regenerating solution 108 which is collected from the column 202 and / or 203 of the second IEX system 103 is then preferably fed 206, 206’ into the column 202 and / or 203 of the first IEX system 102 during a regenerating step (c). The regenerating solution is then collected 207 and / or 207’ from the columns of the first IEX system 102. The obtained regenerating solution 112 is advantageously charged with ions which were retained by the columns 202, 203 of the first and second IEX systems 102 and 103. The use of the same regenerating solution for regenerating the columns of both IEX systems enables to lower the quantity of regenerating solution while at the same time guarantying an efficient regeneration of the columns.

[0089] In another embodiment illustrated in Figures 3a-3c, each IEX system comprises three columns 302, 303 and 308 connected in series. In a first step sequence illustrated in Figure 3a, a solution P is fed 301 into a first column 302, it then goes through the column 302, it is collected and fed 304 into column 303 and is collected 305 from the column 303. After or at least partially at the same time, a regenerating solution, preferably an HCI solution, is used to regenerate said column 308. The regenerating solution is fed 306 into the third column 308 and collected 307 from said first column 308. It is thus clear that the use of three columns in each IEX system advantageously enables to carry out the regenerating step (c) at least partially at the same time as steps (a) and (b).

[0090] In a second sequence illustrated in Figure 3b, a solution P is fed 30T into said second column 303 which is adjacent to said first column

[0091] 302, it then goes through the columns 303 and is fed 304’ into column 308 and is then collected 305’ from the column 308. After or at least partially at the same time, a regenerating solution, preferably an HCI solution is used to regenerate said column 302. The regenerating solution is fed 306’ into the first column 302 and collected 307’ from said first column 302.

[0092] In a third sequence illustrated in Figure 3c, a solution P is fed 301 ” into said second column 308 which is adjacent to said second column 303, it then goes through the columns 308 and is fed 304” into column 302 and is then collected 305” from the column 302. After or at least partially at the same time, a regenerating solution, preferably an HCI solution is used to regenerate said second column 303. The regenerating solution is fed 306” into the second column 303 and collected 307” from said second column

[0093] 303.

[0094] The solutions P which are fed 301 , 30T, 301 ” in the columns 302, 303 and 308 respectively of the first IEX system can be of identical nature, they can come from the same batch of solution P. The solutions P which are collected 305, 305’, 305” from columns 303, 308 and 302 respectively in each step sequences in the first IEX system 102 are fed 301 , 30T, 301 ” into columns 302 or 303 or 308 respectively in one sequence in the second IEX system. The solutions P which are collected 305, 305’, 305” from columns 303, 308 and 302 respectively in the second IEX system are purified solutions P containing reduced amounts of Na, Ca, Mg, Fe, Cd, Pb, and Al ions.

[0095] The regenerating solution 108 is first used to regenerate the column(s) 308 and / or 302 and / or 303 of the second IEX system 103 by feeding 306, 306’, 306” respectively the regenerating solution 108 in the column(s) 308 and / or 302 and / or 303 of the second IEX system 103, during a regenerating step (c). The regenerating solution which is collected from the column 308 and / or 302 and / or 303 of the second IEX system 103 is then fed 306, 306’, 306” into the column(s) 308 and / or 302 and / or 303 of the first IEX system 102 during a regenerating step (c). The regenerating solution 112 is then collected from the column(s) 308 and / or 302 and / or 303 of the first IEX system 102. The obtained regenerating solution 112 is charged with ions which were retained by the columns 302, 303 and 308 of the first and second IEX systems 102 and 103. The use of the same regenerating solution for regenerating the columns of both IEX systems enables to lower the quantity of regenerating solution while at the same time guarantying an efficient regeneration of the columns.

[0096] In another embodiment according to the present invention illustrated in Figure 4, the method is identical as the method illustrated on Figure 1 , with the exception that an iron (III) to iron (II) reduction step 113 is carried out on the solution P collected 105 from said first IEX system. The iron (III) to iron (II) reduction step is carried out by mixing said solution P with at least one reducing agent selected from the group consisting of Fe(0), hydrazine, Zn(0), I2, quinone, hydroquione, CO, and mixtures thereof are used. Preferably, the method comprises a third IEX system (not shown) which is connected in series with the first IEX system. This third IEX system may be identical to said first IEX system. In this case, the solutions P which are collected 305, 305’, 305” from columns 303, 308 and 302 respectively in the first IEX system 103 are fed 301 , 30T, 301 ” into columns 302 or 303 or 308 respectively in one sequence in the third IEX system. The solutions P which are collected 305, 305’, 305” from columns 303, 308 and 302 respectively in the third IEX system are purified solutions P containing reduced amounts of Na, Ca, Mg, Fe, Cd, Pb, and Al ions. The regenerating solution 108 is first used to regenerate the column(s) 308 and / or 302 and / or 303 of the second IEX system 103 by feeding 306, 306’, 306” respectively the regenerating solution 108 in the column(s) 308 and / or 302 and / or 303 of the second IEX system 103, during a regenerating step (c). The regenerating solution which is collected from the column 308 and / or 302 and / or 303 of the second IEX system 103 is then fed 306, 306’, 306” into the column(s) 308 and / or 302 and / or 303 of the first IEX system 102 and / or the third IEX system during a regenerating step (c).

[0097] Methods of measurements

[0098] Within the context of the present invention phosphorus content expressed as wt.% of P2O5 can be measured by any method known by the skilled person in the art. For example, it can be measured by titration preferably with a standardized NaOH solution. It can also be measured by spectrometry via reaction with a molybdovanadate reagent to form a yellow complex, the intensity of which is proportional to the concentration of reactive phosphorus and thus be measured by a spectrometer (for example a HACH DR3900 spectrometer).

[0099] Within the context of the present invention, wt.% of metallic ions such as Na, Ca, Mg, Fe, Cd, Pb, and Al ions can be measured by any method known by the skilled in the art. For examples, the wt.% of these ions can be determined via ICP-OES (Inductively Coupled Plasma Optical Emission spectroscopy).

[0100] Example 1

[0101] A solution P which characteristics are summarized in table 1 below, was purified according to a method similar to the method illustrated on Figure 4 wherein an iron (III) reduction step is carried out between the first and the second IEX system. Moreover, a third IEX system connected to said second IEX system and identical to said second IEX system was used downstream of the second IEX system as described above. Thus, after having passed through the first IEX and the second IEX, the solution P was passed through the third IEX system. Each IEX system had three columns and were operated as illustrated on figure 3 and as described above. All the columns of the first IEX system were filled with a strong acid cation resin made of crosslinked polystyrene having sulfonic acid moieties. All the columns of the second and the third IEX systems were filled with a chelating resin made of crosslinked polystyrene having phosphonic acid and sulfonic acid moieties.

[0102] Table 1 - Characteristics of the solution P to be purified

[0103] The characteristics of the purified solution P obtained at the end of the process are summarized in table 2 below.

[0104] Table 2 - Characteristics of the solution P purified by the process according to the example of the present invention.

Claims

CLAIMS1 . A method for purifying a phosphate containing solution [hereafter, solution P] (101 ) using at least a first ion-exchange separation system(102) [hereafter, first IEX system] and a second ion-exchange separation system (103) [hereafter, second IEX system], each IEX system (102, 103) comprising at least two columns (202, 203) connected in series; each column (202, 203) of said first IEX system comprising at least one strong acid cation resin, and each column of said second IEX system comprising at least one chelating resin; wherein said method comprises a series of step sequences which are carried out in each IEX system (102, 103), each sequence comprising at least the steps of: a. feeding (201 , 20T) a solution P (101 ) to at least one column (202, 203); b. collecting (205, 205’) the solution P from at least one column (202, 203) different from the column used in step (a); c. regenerating at least one column (202, 203); and wherein in each sequence, a solution P (101 ) is fed (201 , 20T) in a column (202, 203) different than the column used in step (a) of the previous sequence; with the proviso that the solution P (101 ) fed in said second IEX system(103) is the solution P collected from said first IEX system (102) or that the solution P (101 ) fed in said first IEX system (102) is the solution P collected from said second IEX system (103).

2. A method according to claim 1 , wherein said step (c) comprises at least the steps of: c1. feeding at least one regenerating solution to said column being regenerated; andc2. collecting said regenerating solution from said column being regenerated; with the proviso that the regenerating solution fed in said first IEX system is a regenerating solution collected from said second IEX system.

3. A method according to claim 1 or claim 2, wherein said at least one strong acid cation resin is made of a crosslinked polymer comprising sulfonic acid moieties; and wherein said polymer is selected from the group consisting of polystyrene, polyacrylate, and combinations thereof.

4. A method according to any one of the preceding claims, wherein said at least one chelating resin comprises at least one crosslinked polymer and phosphonic acid and sulfonic acid moieties; and wherein said polymer is selected from the group consisting of polystyrene, polyacrylate, and combinations thereof.

5. A method according to any one of claims 2 to 4, wherein said regenerating solution is an acidic solution or an alkaline solution; preferably, said regenerating solution is an acidic solution comprising at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid and mixtures thereof; wherein said acidic solution preferably comprises at least 5 wt.%, more preferably at least 10 wt.%, even more preferably at least 15 wt.% and preferably at most 25 wt.%, more preferably at most 35 wt.% of said acid, based on the total weight of said acidic solution.

6. A method according to claim 5, wherein said acidic solution has a pH of at most 2, preferably at most 1 , even more preferably at most 0.1 .

7. A method according to any one of the preceding claims, wherein each IEX system comprises at least three columns connected in series.

8. A method according to any one of the preceding claims, wherein said solution P comprises based on the total weight of said solution P, atleast 5 wt.%, preferably at least 10 wt.%, more preferably at least 20 wt.% and preferably at most 40 wt.%, preferably at most 25 wt.% of P2O5.

9. A method according to any one of the preceding claims, wherein said solution P further comprises at least 100 ppm, preferably at least 1000 ppm, more preferably at least 5000 ppm of Ca ions.

10. A method according to any one of the preceding claims, wherein said solution P further comprises, at least 50 ppm, preferably at least 300 ppm, more preferably at least 1000 ppm of Mg ions.

11. A method according to any one of the preceding claims, wherein said solution P further comprises, at least 50 ppm, preferably at least 500 ppm, more preferably at least 5000 ppm of Fe ions.

12. A method according to any one of the preceding claims, wherein said solution P further comprises, at least 50 ppm, preferably at least 400 ppm, more preferably at least 2000 ppm of Al ions.

13. A method according to any one of the preceding claims, wherein said solution P comprises, at least 50 ppm, preferably at least 300 ppm, more preferably at least 1000 ppm of K ions.

14. A method according to any one of the preceding claims, wherein said solution P comprises, at least 10 ppm, more preferably at least 20 ppm, even more preferably at least 50 ppm, preferably at least 200 ppm, more preferably at least 350 ppm of Na ions.

15. A method according to claim 11 , comprising a step of reducing at least partially said Fe (III) ions; said step of reducing at least partially said Fe(lll) ions being preferably carried out before feeding said solution P into said first I EX system and / or said second I EX system.

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