Method and arrangement for nitrogen recovery

Abstract

A method of recovery of ammonium phosphate comprises providing a start material (100) comprising struvite into a first aqueous process liquid (101) comprising monoammonium phosphate. The first aqueous process liquid (101) is mixed. Thereby the start material (100) comprising struvite is converted into monoammonium phosphate and / or diammonium phosphate in solution and a residue (103) comprising newberyite as a precipitate. The residue (103) is separated from the first aqueous process liquid (101), thereby giving a second aqueous process liquid (104) comprising monoammonium phosphate and / or diammonium phosphate. At least a part of said second aqueous process liquid (104) or a part of an aqueous process liquid (107, 112) emanating therefrom is returned into the first aqueous process liquid. Phosphoric acid (110) is added. A method for extracting nitrogen from reject water, an arrangement (1) for recovery of monoammonium phosphate and a system for extracting nitrogen from reject water is also disclosed.

Description

[0001] METHOD AND ARRANGEMENT FOR NITROGEN RECOVERY

[0002] TECHNICAL FIELD

[0003] The present technology relates in general to recovery of valuable compounds from wastewater and in particular to recovery of ammonium phosphate from nitrogen-rich water.

[0004] BACKGROUND

[0005] Nitrogen is essential to life and is among the nutrients consumed in the largest quantities by all organisms. Ammonia (NH3) is synthesized at a massive scale by the fertilizer industry. Approximately 100 Mt of reactive nitrogen is synthesized annually worldwide using the Haber-Bosch process, which converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using a finely divided iron metal catalyst. The process requires high pressures and temperatures to drive the reaction forward, thus it is energy and resource demanding.

[0006] Humans and animals excrete a significant fraction of the nutrients contained in the food they ingest. Alongside other agricultural sources, these nutrients find their way back into the environment as municipal wastewater effluents, which contain significant amounts of phosphorus and nitrogen.

[0007] In typical environmental conditions the majority of nitrogen in wastewaters exists as dissolved ammonium ions. To achieve nitrogen discharge goals, wastewater treatment plants employ biological nitrogen removal processes, such as nitrification, which is commonly followed by denitrification to ultimately convert ammonia to atmospheric nitrogen. However, biological processes and other existing processes for nitrogen removal from effluents are costly and complex.

[0008] Phosphorus is also an important element essential to life. The release of phosphorous to surface waters, and its consequent contribution to eutrophication, has also led to increasing concerns about water quality. Policies have therefore been implemented throughout the world, to reduce the levels of phosphorus entering surface waters, by the implementation of technologies to remove phosphorus from domestic and industrial wastewater. In contrast to nitrogen, mineral phosphorus resources are considered limited and finite. In addition, most of the world 's phosphorus reserves are controlled by only few countries. Therefore, there is an increasing interest for recycling and beneficial re-use of the phosphorus present in wastes.

[0009] Struvite is a crystal, which is formed with equal molar concentrations of magnesium, ammonium and phosphate combined with six water molecules (MgNH4PO4-6H2O). The molecular weight of struvite is 245.43 g per mole, and it is sparingly soluble under neutral and alkaline conditions but readily soluble in acid. Struvite may be precipitated from wastewater if the magnesium to other nutrient ratio (N+P, or K+P) is sufficient and the pH is adjusted to neutral or alkaline levels. Precipitation of struvite from wastewater has been used for producing struvite as a fertilizer, although struvite is not an optimal fertilizer because it contains too much magnesium in relation to nitrogen, phosphorus or potassium. Moreover, other applications of struvite precipitation have so far mainly been focused on recovery of phosphorus as most wastewaters contain sufficient ammonium for phosphorus removal as struvite and the only addition required for struvite precipitation is typically a magnesium source. However, ammonium nitrogen is normally present in effluents at much higher concentrations compared to phosphorus. In order to remove nitrogen in form of struvite from wastewater, large amounts of external phosphorus and magnesium are needed. However, it is impractical to convert high quality sources of phosphorus to struvite just in order to recover ammonium-nitrogen. Such a process would also not be economically viable since the commercial value of struvite is low.

[0010] Nevertheless, struvite may be converted to newberyite (MgHPCU-SHaO) which precipitates and leaves ammonium ions in the solution. The current conversion process uses sulphate and consists of mixing a struvite slurry with sulfuric acid which has been disclosed in WO2020 / 242366. The acid dissolves the struvite and newberyite precipitates. The nitrogen and the sulfate from the acid form an ammonium sulfate (AMS) solution. The use of other mineral acids has also been disclosed in WO2020 / 256622.

[0011] The sulphate process generates a dilute solution that is evaporated for up- concentration and / or solid production. Ammonium sulfate may be recirculated in the process to increase the concentration of the ammonium sulfate going into the evaporator and thereby generating a more concentrated product and thereby reducing the amount of water having to be evaporated. At least one of the problems with this process is that the solubility of newberyite increases with increased ammonium sulfate concentration, therefore a further polishing step is required to remove magnesium and phosphorus.

[0012] The present inventors have also found that the ammonium sulfate concentration limits the conversion reaction. At an ammonium sulfate concentration above 30% w / w (2.7 M AMS or 5.2 M N), a poorer conversion has been observed as well as magnesium precipitating with sulphate and ammonium as a double salt, thereby the need for separation leads to increased losses of magnesium and phosphorus.

[0013] In order to avoid newberyite losses (Mg and P) to the product, a polishing step is required, wherein ammonia (or other base) is added to the AMS product solution. The dissolved newberyite then precipitates as struvite. The losses without the polishing step are too costly. This is true for a liquid product, but the present inventors have also experienced that for a solid product, where there is recirculation from the evaporator, the losses of Mg to the AMS product as a double salt, remains more expensive than the polishing step.

[0014] Thus, there is a need for an improved method and an arrangement that may enable improved recovery of valuable compounds, such as ammonium, from nitrogen-rich wastewaters.

[0015] SUMMARY

[0016] A general object for the present technology is to improve the recovery of valuable ammonium substances from nitrogen-rich waters.

[0017] The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims.

[0018] In general words, in a first aspect, a method of recovery of ammonium phosphate comprises providing a start material comprising struvite into a first aqueous process liquid comprising monoammonium phosphate. The first aqueous process liquid and the start material is mixed at a temperature of at least of 10°C. Thereby the start material comprising struvite is converted into monoammonium phosphate (MAP) and / or diammonium phosphate (DAP) in solution and a residue comprising newberyite as a precipitate. The residue comprising newberyite is separated from the first aqueous process liquid. Thereby a second aqueous process liquid is given, which comprises monoammonium phosphate and / or diammonium phosphate. At least a part of the second aqueous process liquid or a part of an aqueous process liquid emanating from the second aqueous process liquid is returned into the first aqueous process liquid. Phosphoric acid is also added during the process.

[0019] In a second aspect, a method for extracting nitrogen from reject water comprises exposing reject water comprising ammonium ions for substances comprising newberyite at neutral or basic pH, causing at least a part of the newberyite to form struvite. A material comprising struvite is separated from the exposed reject water. Ammonium phosphate is recovered by a method according to the first aspect. At least a part of the residue comprising newberyite is reused in a subsequent step of exposing reject water comprising ammonium ions for substances comprising newberyite.

[0020] In a third aspect, an arrangement for recovery of monoammonium phosphate comprises a mixing reactor, a first separator, a return arrangement, and an input for phosphoric acid. The mixing reactor has a first mixer input for a first aqueous process liquid comprising monoammonium phosphate, and a second mixer input for introducing a start material comprising struvite into the first aqueous process liquid. The mixing reactor has further a first mixing equipment configured for mixing said start material (100) and said first aqueous process liquid (101), thereby converting the start material comprising struvite into a mix of monoammonium phosphate and / or diammonium phosphate in solution and a residue comprising newberyite as a precipitate. The mixing reactor has also a mixer output for the mix. The first separator has an input for introducing the mix from the mixing reactor. The first separator is configured for separating the residue from the monoammonium phosphate and / or diammonium phosphate in solution. Thereby a second aqueous process liquid is given, which comprises monoammonium phosphate and / or diammonium phosphate. The first separator has a first separator output for the residue and a second separator output for the second aqueous process liquid. The return arrangement is configured for returning at least a part of the second aqueous process liquid or a part of an aqueous process liquid emanating from the second aqueous process liquid into the first aqueous process liquid.

[0021] In a fourth aspect, a system for extracting nitrogen from reject water comprises an exposing reactor and a separator. The exposing reactor has an input for reject water comprising ammonium ions, an input for substances comprising newberyite, and an output for a slurry comprising struvite. The exposing reactor is configured for exposing the reject water comprising ammonium ions for the substances comprising newberyite at neutral or basic pH, causing at least a part the newberyite to form the struvite. The separator is configured to separate a material comprising struvite from the slurry comprising struvite. The system for extracting nitrogen further comprises an arrangement for recovery of ammonium phosphate according to the third aspect. A reuse equipment is configured for reusing at least a part of the residue comprising newberyite in the exposing reactor.

[0022] One advantage with the proposed technology is that an efficient recovery of ammonium can be achieved without significant losses of magnesium. Other advantages will be appreciated when reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

[0024] FIG. 1 is a schematic drawing of an embodiment of an arrangement for recovery of ammonium phosphate.

[0025] FIG. 2 is a schematic drawing of an embodiment of a system for extracting nitrogen from reject water.

[0026] FIG. 3 is a schematic illustration of the transfer of ammonium ions in a struvite-phosphoric acid-ammonium phosphate chemical system.

[0027] FIG. 4 is a flow diagram illustrating steps of a first embodiment of a method of recovery of ammonium phosphate.

[0028] FIG. 5 is a flow diagram illustrating steps of an embodiment of a method for extracting nitrogen from reject water.

[0029] FIG. 6 is a flow diagram illustrating steps of a second embodiment of a method of recovery of ammonium phosphate.

[0030] FIG. 7 is a flow diagram illustrating steps of a third embodiment of a method of recovery of ammonium phosphate.

[0031] FIG. 8 is a flow diagram illustrating steps of a fourth embodiment of a method of recovery of ammonium phosphate.

[0032] FIG. 9 is a flow diagram illustrating steps of a fifth embodiment of a method of recovery of ammonium phosphate.

[0033] FIG. 10 is a part flow diagram illustrating steps of a sixth embodiment of a method of recovery of ammonium phosphate.

[0034] FIG. 11 is a part flow diagram illustrating steps of a seventh embodiment of a method of recovery of ammonium phosphate.

[0035] FIG. 12 is a schematic drawing of another embodiment of an arrangement for recovery of ammonium phosphate.

[0036] FIG. 13 is a schematic drawing of yet another embodiment of an arrangement for recovery of ammonium phosphate. FIG. 14 is a schematic drawing of yet another embodiment of an arrangement for recovery of ammonium phosphate.

[0037] FIG. 15 is a schematic drawing of yet another embodiment of an arrangement for recovery of ammonium phosphate.

[0038] FIG. 16 is a schematic drawing of yet another embodiment of an arrangement for recovery of ammonium phosphate.

[0039] FIG. 17 is a diagram illustrating the concentration over time of dissolved P, dissolved N and N / P solids during the conversion at high MAP / DAP ratio.

[0040] FIG. 18 is a diagram illustrating the solubility of N, P and Mg in MAP solution.

[0041] DETAILED DESCRIPTION

[0042] Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

[0043] As disclosed above, struvite may be converted to newberyite which precipitates and leaves ammonium ions in the solution. A common priorart conversion process uses sulphate and consists of mixing a struvite slurry with sulfuric acid (H2SO4). The acid dissolves the struvite and newberyite precipitates. The nitrogen and the sulfate from the acid form an ammonium sulfate (AMS) solution. The AMS concentration of the prior art conversion process limits the process in that, the process generates a dilute solution that is evaporated for up-concentration and / or solid production. AMS can be recirculated in the prior art process, herein called the AMS process, to increase the concentration of the AMS going into the evaporator and thereby generating a more concentrated product and reducing the amount of water to evaporate. However, the solubility of newberyite increases with increased AMS concentration, so a magnesium (Mg) and phosphorous (P) polishing step (post-struvite) is required in this process. As sulphate and ammonium precipitates as a double salt with magnesium, the separation of magnesium and phosphorous is leading to increased losses of Mg and P. Thus, in order to avoid newberyite losses (Mg and P) to the product of the AMS process, a polishing step is required, wherein ammonia (NH3) (or another base) has to be added to the AMS comprising solution. The dissolved newberyite then may be precipitated as struvite. The losses without this polishing step are too costly to be usable on an industrial scale process. This is true for a liquid product but calculations have also shown that for a solid product, wherein there is recirculation from an evaporator, the losses of Mg to the product (as a double salt) remains more expensive than the polishing step. Thus, there is a need for an improved method for recovery of ammonium from struvite.

[0044] In a series of initial experiments, it was found that by using phosphoric acid (H3PO4) instead of sulfuric acid, ammonium phosphate, typically monoammonium phosphate (MAP), may be generated instead of AMS. Such a process leads to that H3PO4 may be added directly to struvite to generate newberyite and a solution comprising MAP. Further, it was also found that a solution comprising MAP may moreover be mixed with struvite to generate newberyite and a solution comprising MAP / DAP.

[0045] In order to achieve an efficient and commercially attractive method for recovery of ammonium, preferably monoammonium phosphate or a mix of diammonium phosphate and monoammonium phosphate, from struvite and without significant losses of magnesium, two main operations have to be performed. Figure 1 shows an arrangement 1 for performing the general principle of a method of recovery of ammonium phosphate according to the present invention. In a first operation, a start material 100 comprising struvite is mixed with an aqueous process liquid 101 comprising at least monoammonium phosphate. This may be performed at an elevated temperature, but temperatures as low as 10°C has also been used. This should preferably be performed in an efficiently and environmental-friendly manner, giving a second aqueous process liquid 104, which comprises at least one of monoammonium phosphate and diammonium phosphate, and / or another aqueous process liquid 107 emanating from the second aqueous process liquid 104, and a residue 103; 103’ comprising at least newberyite as a precipitate. The precipitated residue 103; 103’ may in a second operation be easily separated from the second aqueous process liquid comprising at least one of monoammonium phosphate and diammonium phosphate in an easily processable form. Phosphoric acid 110 may further be added. Ammonium phosphate, i.e. monoammonium phosphate and / or diammonium phosphate, may be taken out of the process as precipitated ammonium phosphate 108 and or as a fluid product 108’ of ammonium phosphate 108. Further, at least a part of the remaining second aqueous process liquid 104, which comprises at least one of monoammonium phosphate and diammonium phosphate, and / or the other aqueous process liquid 107, is recirculated to the aqueous process liquid 101 in order to minimize losses of ammonium products.

[0046] This general principle of a method of recovery of ammonium phosphate can be used in any application where it is requested to decompose struvite for recovering ammonium therefrom.

[0047] A particularly advantageous application where these ideas are applicable is as a part for extracting nitrogen from reject water. Figure 2 illustrates schematically a system 9 for extracting nitrogen from reject water 99. An exposing reactor 2 has an input for reject water 99 comprising ammonium ions, an input for substances 103, 103’ comprising newberyite, and an output for a slurry comprising struvite. The exposing reactor 2 is configured for exposing the reject water 99 comprising ammonium ions for the substances 103, 103’ comprising newberyite in neutral or basic pH environment, causing at least a part the newberyite to form the struvite. A separator 3 is configured to separate a material 100 comprising struvite from the slurry comprising struvite. A struvite piping 5 connects the exposing reactor 2 and the separator 3 to an arrangement 1 for recovery of ammonium phosphate, according to Figure 1. A reuse equipment 4 is configured for reusing at least a part of a residue comprising newberyite in the exposing reactor as substances 103, 103’ comprising newberyite.

[0048] Figure 3 is a schematic illustration of the transfer of ammonium ions in a struvite-phosphoric acid-ammonium phosphate chemical system. In an acid pH liquid system of a first aqueous process liquid comprising ammonium phosphate, the ammonium ion in struvite is extracted, thereby forming newberyite. The ammonium ion is associated with the content of the liquid system. If phosphoric acid is present, the ammonium ion may form monoammonium phosphate. If monoammonium phosphate is present, the ammonium ion may form diammonium phosphate. Preferably the first aqueous process liquid comprises at least 10% monoammonium phosphate.

[0049] Experiments have been performed with conversion of struvite into newberyite in with different contents of the first aqueous process liquid and at different temperatures. The reactions were allowed to take place for 120 minutes. Some results are seen in Table 1.

[0050] Table 1. Conversion rates of Sturvite into newberyite. The N / P ratio indicates how much diammonium phosphate there is in the first aqueous process liquid after reaction, expressed as a ratio compared to monoammonium phosphate. The N / P ratio of pure monoammonium phosphate is 1.0, and the N / P ratio of pure diammonium phosphate is 2.0. The N / P ratio increases during the conversion and the figures in the table represent the final N / P ratio after 120 minutes. At temperatures of 60°C, a total conversion was achieved, even for a N / P ratio of 1.45 (not in the table). The pH of the process liquid increases with the N / P ratio, but even at as high pH levels as 5.7, a conversion was present. However, at N / P ratios of more than 1.5, no conversion was detected, which was considered to be due to a too high pH level. In a preferred embodiment, an amount of the start material in relation to a volume of the first aqueous process liquid is controlled to cause a process liquid after conversion, in the description further below denoted as a second aqueous process liquid, to comprise a mole ratio between ammonium and phosphate in solution in the range of 1 to 1.45.

[0051] The control of the N / P ratio can be based either on knowledge of the ingoing substances and experience from operation. The control of the N / P ratio may also be based on measurements, e.g. the pH of the liquids. For each temperature and concentration (or density), there is a direct relation between the N / P ratio and the pH. Control may also be based on conductivity measurements.

[0052] When considering the process temperatures, it seems to be unnecessary to heat the first aqueous process liquid to very high temperatures. In one embodiment, the step of mixing the first aqueous process liquid is therefore performed at a temperature below 80°C, preferably below 60°C.

[0053] In some applications, a full conversion is required. In such applications, a low final N / P ratio and / or a high temperature is preferred. However, in other applications, such as in e.g. for extracting nitrogen from reject water, a complete conversion is not absolutely necessary, since the struvite / newberyite is circulated between the reject water exposing reactor and the arrangement for recovery of ammonium phosphate. In such systems, other considerations, such as loss of magnesium from the system, may be more important. In one embodiment, an amount of the start material in relation to a volume of the first aqueous process liquid is controlled to cause the second aqueous process liquid to comprise a mole ratio between ammonium and phosphate in solution in the range of 1.2 to 1.4.

[0054] It is also seen from Table 1 that a conversion is possible to achieve also at low temperatures. For a final N / P ratio of 1.0, i.e. where phosphoric acid reacted with ammonium to form monoammonium phosphate, temperatures of 10°C were indeed enough to achieve a full conversion. Even for a final N / P ratio of 1.2 (not shown in the table), a temperature as low as 40°C could give a full conversion, thus also involving monoammonium phosphate reacting with ammonium to form diammonium phosphate.

[0055] Figure 4 illustrates a flow diagram of steps of a first embodiment of a method of recovery of ammonium phosphate. In step S10, a start material comprising struvite is provided into a first aqueous process liquid comprising monoammonium phosphate. The struvite in the present disclosure may have its origin from many different sources. Struvite precipitated from reject water is one typical source, but many other sources are possible as well. The start material may also comprise other substances, e.g. newberyite.

[0056] In step S30, the first aqueous process liquid, including the start material, is mixed at a temperature of at least 10°C. Thereby the start material comprising struvite is converted into monoammonium phosphate and / or diammonium phosphate in solution and a residue comprising newberyite as a precipitate. Conversion is thus possible also at very moderate temperatures, which reduces the requests for adding heating energy. Depending on the final composition of the process liquid and the process temperature, the residue may also comprise some struvite, as a result of an incomplete conversion.

[0057] In step S40, the residue comprising newberyite is separated from the first aqueous process liquid. Thereby a second aqueous process liquid is given, which comprises monoammonium phosphate and / or diammonium phosphate.

[0058] In step S21, phosphoric acid is added. This step S21 of adding phosphoric acid is in this embodiment performed into at least a part of the second aqueous process liquid. This gives a third aqueous process liquid comprising monoammonium phosphate. Preferably, the phosphoric acid is provided in an amount suitable for converting any diammonium phosphate in the second aqueous process liquid into monoammonium phosphate in the third aqueous process liquid.

[0059] In step S70, the third aqueous process liquid is split into two parts. In step S75, one of the parts of the third aqueous process liquid is exited as a product.

[0060] In step S50, a part of an aqueous process liquid emanating from the second aqueous process liquid is in this embodiment returned into the first aqueous process liquid. In this embodiment, the step of returning S50 comprises the step S51, in which a part of the third aqueous process liquid is returned into the first aqueous process liquid. This embodiment thus gives rise to a final product in the form of a liquid solution of monoammonium phosphate. Such monoammonium phosphate solutions are attractive e.g. for nutrition purposes.

[0061] The method of recovery of ammonium phosphate can be used as one step S3 in a larger process. Figure 5 is a flow diagram illustrating steps of an embodiment of a method for extracting nitrogen from reject water. This is in analogy with the system of Figure 2. In step SI , reject water comprising ammonium ions is exposed for substances comprising newberyite at neutral or basic pH. This causes at least a part the newberyite to form struvite. In step S2, a material comprising struvite is separated from the exposed reject water. In step S3, ammonium phosphate is recovered by a method according to any of the embodiments presented elsewhere in this disclosure. One non-limiting embodiment of this was described in Figure 4. In step S4, at least a part of the residue comprising newberyite, emanating from the ammonium phosphate recovering step, is reused in a subsequent step of exposing reject water comprising ammonium ions for substances comprising newberyite.

[0062] Figure 6 is a flow diagram illustrating steps of another embodiment of a method of recovery of ammonium phosphate. Steps S10, S30 and S40 are similar to the ones illustrated in Figure 4, and are therefore not described again.

[0063] In step S71, the second aqueous process liquid is split into two parts. One part of the second aqueous process liquid is in step S76 exited as a product. This embodiment thus gives rise to a final product in the form of a liquid solution of a mix of monoammonium phosphate and diammonium phosphate. Such monoammonium / diammonium phosphate solutions are attractive e.g. for nutrition purposes. In step S22, phosphoric acid is added into a remaining part of the second aqueous process liquid. Thereby a third aqueous process liquid comprising monoammonium phosphate is given. Preferably, the phosphoric acid is provided in an amount suitable for converting any diammonium phosphate in the second aqueous process liquid into monoammonium phosphate in the third aqueous process liquid. The step of returning S50 comprises the step S51, in which the third aqueous process liquid is returned.

[0064] This embodiment has the advantage of that the added amount of phosphoric acid is less than in the previous embodiment, thereby reducing the costs for the input chemicals. The exited product is a mix of monoammonium phosphate and diammonium phosphate, which is useful in many different applications.

[0065] In both embodiments of Figures 4 and 6, the exited product is a liquid product. This may be advantageous if the product is to be transported by pipes. This is thus indeed useful in many applications.

[0066] However, in other applications, solid products comprising ammonium phosphates are to prefer. Solid products are for instance less heavy compared to corresponding liquid products, which may save transportation costs. In order to achieve a solid product, water has to be removed. This removal of water can be performed on the exited product as such, or somewhere in the previous process.

[0067] Figure 7 illustrates a flow diagram of steps of another embodiment of a method of recoveiy of monoammonium phosphate. Steps S10, S30 and S40 are similar as in previous embodiments, and are not described again. In step S60, the separated second aqueous process liquid is concentrated. The second aqueous process liquid is in this embodiment a solution of diluted monoammonium phosphate and diammonium phosphate. The step of concentrating S60 is in this embodiment performed by evaporation S62 of water. Thereby a concentrated solution of monoammonium phosphate and diammonium phosphate can be formed.

[0068] In step S25, phosphoric acid is in this embodiment added to the concentrated solution of monoammonium phosphate and diammonium phosphate, causing precipitation of monoammonium phosphate from a fourth aqueous process liquid saturated by monoammonium phosphate. The third aqueous process liquid is thus an aqueous process liquid emanating from the second aqueous process liquid. The phosphoric acid added in step S25 is preferably added in a molar amount in the range of 1 : 1.2 to 1.2: 1, preferably 1 : 1, relative to a molar amount of struvite converted into newberyite in the first aqueous process liquid during the step of mixing S30. This ensures that the concentration levels of ammonium phosphates in different parts of the process are kept relatively constant during time.

[0069] In step S50, a part of an aqueous process liquid emanating from the second aqueous process liquid is in this embodiment after the step of separating the precipitated monoammonium phosphate returned into the first aqueous process liquid. In this embodiment, the step of returning S50 comprises the step S52, in which the fourth aqueous process liquid, comprising a saturated solution of monoammonium phosphate, is returned into the first aqueous process liquid.

[0070] In step S72, the precipitated monoammonium phosphate is separated from the fourth aqueous liquid. In step S77, solid monoammonium phosphate is exited as a product.

[0071] This embodiment thereby uses the limited solubility of monoammonium phosphate to produce the solid product that is exited. However, this embodiment has the disadvantage of using evaporation, which costs a lot of energy. Such a process needs, besides the evaporation equipment as such, some addition of energy, e.g. heat. Some of the heat energy may be reused, by condensing the evaporated water in a controlled manner and use the condensation heat from such a process for heating of a subsequent part of a second aqueous process liquid. Also, as will be discussed further below, by using the condensed water in the process, the disadvantages can be further reduced.

[0072] This embodiment has furthermore the additional advantage that the precipitation of monoammonium phosphate can be performed separately from the concentration process. This typically facilitates the necessary equipment.

[0073] Figure 8 illustrates a flow diagram of steps of another embodiment of the method of recovery of monoammonium phosphate. In this embodiment, the method of recovery of monoammonium phosphate comprises a step S20 of adding phosphoric acid already to the first aqueous process liquid. This ensures that the second aqueous process liquid comprises monoammonium phosphate. Further, in this embodiment, the method comprises the further step S60, wherein the separated second aqueous process liquid, which is a diluted monoammonium phosphate solution, is concentrated. In step S60, the concentration may be performed by evaporation S62 of water, thereby causing precipitation of monoammonium phosphate crystals, from a fourth aqueous process liquid saturated by monoammonium phosphate.

[0074] Figure 8 also illustrates that in this embodiment of the method of recoveiy of monoammonium phosphate the further step S72 is performed, wherein the precipitated monoammonium phosphate from the fourth aqueous process liquid is separated. Also, in a further step S77, the precipitated monoammonium phosphate is exited from the fourth aqueous process liquid as a solid product. Further, the method comprises the step S50, and the part step S52 as in the embodiment of Figure 7.

[0075] This embodiment has the advantage of giving a solid monoammonium phosphate product exiting the process. The adding of the phosphoric directly into the first aqueous process liquid gives an advantageous possibility to control the second aqueous process liquid to achieve a targeted composition.

[0076] The idea of providing phosphoric acid already into the first aqueous process liquid may also be used without concentration processes. Figure 9 illustrates a flow diagram of steps of another embodiment of the method of recovery of monoammonium phosphate. In this embodiment of the method, the steps S10, S20, S30 and S40 are basically the same as in the embodiment of Figure 8. However, instead of a further treatment of the second aqueous process liquid, the second aqueous process liquid can be used as it is for providing the return of MAP to the into the first aqueous process liquid as well as a final product.

[0077] To this end, after step S40 the embodiment comprises the step S71 in which the second aqueous process liquid is split into two parts. In step S50, which comprises the further step S53, a part of the second aqueous process liquid, comprising a solution of monoammonium phosphate, given by step S40, is returned into the first aqueous process liquid. A remaining part of the second aqueous process liquid is exited S76 as a product, comprising MAP in a liquid.

[0078] This embodiment has the advantage of having very few process steps. When turning the above presented ideas into a commercially interesting process, further considerations may be done. In a circular process system, the ideal situation is that all active substances used in the process are extractable and reused in subsequent processes. In practice there is, however, always different kinds of losses of chemical substances, even if the main process design is circular. In the present system, the loss of magnesium and phosphate from the second aqueous process liquid is one such consideration.

[0079] Ammonium phosphate may follow with the newberyite residue when the second aqueous process liquid is separated. This leads to that the amount of the intended final product is reduced. Furthermore, if this newberyite e.g. is to be used for extracting ammonium from reject water, this will call for additional makeup of an external magnesium source to bring back the ammonium phosphate as struvite again. It is therefore preferred if the residue comprising newberyite is washed before it leaves the process. The wash water will thus comprise at least some ammonium phosphate, which can be provided back into the system, e.g. by adding it to the first aqueous process liquid.

[0080] The main disadvantage of washing the newberyite residue is that water is introduced into the system. If e.g. the embodiments of Figures 4 and 6 are used, such addition of water will show up as a reduced concentration of the final liquid product. If e.g. the embodiments of Figures 7 and 8 are used, the evaporation has to take care of more water. Some compromises are therefore recommended.

[0081] Model calculations on a method according to Figure 4 has been performed. Without any washing, it was estimated that up to one third of all ammonium phosphate could follow with newberyite. The weight of the makeup with magnesium chloride in a reject water stage would then correspond to about 7% compared to the total weight of the final ammonium phosphate. A concentration of 38% monoammonium phosphate in the final product was estimated to be achievable.

[0082] If the residue of newberyite was washed with an amount of water having a weight corresponding to half the weight of the residue of newberyite, the estimated return of all ammonium phosphate was pushed down to 9% and the makeup of magnesium was reduced to less than 2%. However, the concentration was reduced to 27%. Furthermore, by doubling the amount of wash water reduced the estimated return of ammonium phosphate to 1% and reduced the makeup of magnesium was reduced to less than 0.5%. The final product concentration of monoammonium phosphate then became just below 20%. The degree of washing is therefore preferably decided on considering the requests for a final product concentration or the costs for additional evaporation.

[0083] In Figure 10, a part flow diagram illustrating steps of an embodiment of a method of recovery of ammonium phosphate. This part flow diagram can be combined with any of the previous embodiments. The residue comprising newberyite separated in step S40 is in step S80 washed, which gives washed newberyite and wash water. The wash water may be reused. In a further step S90, at least a part of the wash water from step S80 is returned to the first aqueous process liquid. Any ammonium phosphate and / or magnesium being captured by the wash water is thereby returned into the process again.

[0084] Figure 11 illustrates a part flow diagram of steps of an embodiment of the method of recovery of monoammonium phosphate. This embodiment provides further steps to the embodiments described in connection with Figures 7 and 8, here above. This embodiment comprises the further step S69, wherein the evaporated water in step S62 is condensed into condensed water. Further, the residue comprising newberyite separated in step S40 is washed S80 with the condensed water from step S70, which gives washed newberyite and wash water.

[0085] This reduces the need for fresh water to be entered into the system for the washing purposes. Also, if the condensed water comprises trace levels of other chemicals from the process, such a water may have to be cleaned before returning it to the nature. However, by using this water as wash water, such demands are omitted, since the trace levels of contamination are typically neglectable during a washing process.

[0086] Also here, the wash water may be reused. In the further step S90, at least a part of the wash water from step S80 is returned to the first aqueous process liquid. Thereby the need for purification of the wash water will disappear, since the contaminants will be provided back into the system.

[0087] The magnesium content in the circular process described above is ideally constant. However, if the exited final product is a liquid comprising ammonium phosphates, this liquid will always comprise also some minor amounts of magnesium. The magnesium loss thus has to be compensated for. In experiments, it was found that the solubility of magnesium also depends on the other dissolved substances in the liquid. In conversion liquids with different N / P ratios in the liquid between 1.0 and 1.4, the amount of dissolved magnesium was measured. The concentration of magnesium was almost constant over time, which indicates that an equilibrium was reached. For a N / P ratio in the liquid of 1.4, a magnesium level of 1.0- 1.2 g / L was measured. For a N / P ratio in the liquid of 1.0, a magnesium level of 2.2-2.3 g / L was measured. In other words, the losses of magnesium by the liquid final products were reduced by increasing the N / P ratio in the liquid during the conversion. In one embodiment, an amount of the start material in relation to a volume of the first aqueous process liquid is controlled to cause the second aqueous process liquid to comprise a mole ratio between ammonium and phosphate in solution in the range of 1.2 to 1.4.

[0088] Figure 12 illustrates schematically an arrangement 1 for recovery of ammonium phosphate. A mixing reactor 10 has a first mixer input 14 for a first aqueous process liquid 101 comprising monoammonium phosphate. The mixing reactor 10 further comprises a second mixer input 12 for introducing a start material 100, comprising struvite, into the first aqueous process liquid 101. The mixing reactor 10 even further comprises a first mixing equipment 18, also referred to as a reaction promoter equipment, is configured for mixing the input streams, i.e. mixing the start material 100 and the first aqueous process liquid 101. The mixing equipment 18 may also comprise a heater if a process temperature higher than the temperatures of the input streams is requested. The purpose of the mixing equipment 18 is for promoting the conversion of the start material 100 comprising struvite into a mix 102 of monoammonium phosphate and / or diammonium phosphate in solution and a residue 103 comprising newberyite as a precipitate. A mixer output 20 of the mixing reactor 10 is provided for exiting the mix 102 from the mixing reactor 10.

[0089] The arrangement 1 for recovery of ammonium phosphate comprises a first separator 40 having an input 42 for introducing the mix 102 from the mixing reactor 10. The mix is typically in the form of a slurry, and the transfer between the mixing reactor 10 and the first separator 40 may be assisted by different kinds of pumping equipment. The first separator 40 is configured for separating the residue 103 from the monoammonium phosphate and / or diammonium phosphate in solution. The separation gives a second aqueous process liquid 104 comprising monoammonium phosphate and / or diammonium phosphate. The first separator 40 has also a first separator output 44 for the residue 103, and a second separator output 46 for the second aqueous process liquid 104.

[0090] Any suitable equipment for solid liquid separation can be used such as centrifuge, vacuum belt filter, vertical press filter, horizontal press filter, drum filter, etc. A typical dry matter content for Newberyite after filtration and washing is generally > 90%. For easy pumping of the Newberyite after filtration and washing it can be mixed with water to form a slurry that can be pumped to the reactor exposing it to reject water to be treated.

[0091] The arrangement 1 for recovery of ammonium phosphate further comprises a neutralizer 50 connected by an input 54 to the second separator output 46 for the second aqueous process liquid 104. An input 52 for phosphoric acid 110 is connected to the neutralizer 50 for exposing a stream of the second aqueous process liquid 104 for phosphoric acid 110. This exposure gives rise to a third aqueous process liquid 112 comprising monoammonium phosphate. The diammonium phosphate in the second aqueous process liquid 104 thereby reacts and forms monoammonium phosphate.

[0092] The arrangement 1 for recovery of ammonium phosphate also comprises a return arrangement 90. The return arrangement 90 is configured for returning at least a part of the second aqueous process liquid 104 or a part of an aqueous process liquid emanating from the second aqueous process liquid 104 into the first aqueous process liquid 101. In the present embodiment, the return arrangement 90 is configured for returning a part of the third aqueous process liquid 112 into the first aqueous process liquid 101. The neutralizer 50 has furthermore an output 56 for a remaining part of the third aqueous process liquid 112 as a fluid product of monoammonium phosphate 108’.

[0093] In this embodiment, the arrangement 1 for recovery of ammonium phosphate further comprises a controller 95. The controller 95 is configured for controlling an amount of the start material 100 in relation to a volume of the first aqueous process liquid 101. The requested amount causes the second aqueous process liquid 104 to comprise a mole ratio between ammonium and phosphate in solution in the range of 1 to 1.45. Preferably, the mole ratio between ammonium and phosphate in solution is in the range of 1.2 to 1.4.

[0094] In the embodiment of Fig. 12, the arrangement 1 for recovery of ammonium phosphate further comprising a washing arrangement 30 for washing the residue 103 comprising newberyite by fresh water 113. The washing results in washed newberyite 103’ and wash water 105”. A wash water return arrangement 31 is configured for returning at least a part of the wash water 105” to the mixing reactor 10.

[0095] In figure 13, another embodiment of an arrangement 1 for recovery of ammonium phosphate is schematically illustrated. Most parts are similar to the embodiment of Figure 12 and are not described again. In this embodiment, the arrangement 1 for recovery of ammonium phosphate comprises an output 56 for exiting a part of the second aqueous process liquid 104 as a fluid product of a mix of monoammonium phosphate and diammonium phosphate 108”. The neutralizer 50 is connected to input a remaining part of the second aqueous process liquid 104 by the input 54. The input 52 for phosphoric acid 110 is connected to the neutralizer 50 for exposing a stream of the second aqueous process liquid 104 for phosphoric acid 110. This exposure gives a third aqueous process liquid 112 comprising monoammonium phosphate. The diammonium phosphate in the second aqueous process liquid 104 thereby reacts and forms monoammonium phosphate. The return arrangement 90 is configured for returning of the third aqueous process liquid 112 into the first aqueous process liquid 101.

[0096] In figure 14, another embodiment of the arrangement 1 for recovery of ammonium phosphate have many parts in common with the previously presented embodiments. These parts are not described again. This embodiment further comprises an evaporator 60 having an input 64 for the second aqueous process liquid 104. The evaporator 60 is configured for heating the second aqueous process liquid 104, thus causing evaporation of water 105 from the second aqueous process liquid 104. As a result of the evaporation of water, a concentrated solution 106 of monoammonium phosphate and diammonium phosphate is formed. The evaporator 60 has a first evaporator output 66 for the evaporated water 105 and a second evaporator output 68 for the concentrated solution 106 of monoammonium phosphate and diammonium phosphate.

[0097] Further, according to figure 14 the arrangement 1 for recovery of monoammonium phosphate in this embodiment further comprises a crystallizer 80. The crystallizer 80 has a first crystallizer input 82 for the concentrated solution 106 of monoammonium phosphate and diammonium phosphate and a second crystallizer input 84 for phosphoric acid 110. The crystallizer 80 is in this embodiment configured for mixing the concentrated solution 106 of monoammonium phosphate and diammonium phosphate with the phosphoric acid 110, causing precipitation of monoammonium phosphate 108 from a fourth aqueous process liquid 107 saturated by monoammonium phosphate. The crystallizer 80 has a first crystallizer output 86 for the precipitated monoammonium phosphate 108 and a second crystallizer output 89 for the fourth aqueous process liquid 107. The second crystallizer output 89 is in this embodiment connected to the return arrangement 90. Thus, the return arrangement 90 is in this embodiment configured for returning the concentrated solution 107 of monoammonium phosphate, being an aqueous process liquid emanating from the second aqueous process liquid 104, into the first aqueous process liquid 101.

[0098] In figure 15 another embodiment of the arrangement 1 for recovery of monoammonium phosphate is illustrated. Parts that are in common with other previous embodiments are not described again. In this embodiment the mixing reactor 10 has a third mixer input 17 for adding phosphoric acid 110 to the first aqueous process liquid 101. The third mixer input 17 for adding phosphoric acid 110 may optionally be combined with the first mixer input 1 .

[0099] Further, the embodiment of the arrangement 1 for recovery of monoammonium phosphate according to figure 7 comprises an evaporator 60 having an input 64 for the second aqueous process liquid 104. The evaporator 60 is in this embodiment configured for heating the second aqueous process liquid 104, thus causing evaporation of water 105 from the second aqueous process liquid 104. The evaporation of water causes precipitation of monoammonium phosphate crystals 108, from a fourth aqueous process liquid 107 saturated by monoammonium phosphate. The evaporator 60 has a second separator 88, separating the precipitated monoammonium phosphate crystals 108, from the fourth aqueous process liquid 107. The evaporator 60 of this embodiment has a first evaporator output 66 for the evaporated water 105, a second evaporator output 67 for the precipitated and separated monoammonium phosphate crystals 108 and a third evaporator output 69 for the remaining concentrated solution 107 of monoammonium phosphate. The third crystallizer output 69 may be connected to the return arrangement 90. Thus, the return arrangement 90 is in this embodiment configured for returning the concentrated solution 107 of monoammonium phosphate, being an aqueous process liquid emanating from the second aqueous process liquid 104, into the first aqueous process liquid 101.

[0100] In both embodiments as illustrated by figure 14 and figure 15, the arrangement 1 for recovery of monoammonium phosphate further comprises a condenser 70. The condenser 70 is connected to the first output 66 of the evaporator 60, and is configured for condensing the evaporated water 105 into condensed water 105’. According to both these embodiments, a washer arrangement 30 is connected to the first separator output 44 and an output from the condenser 70. Thus, the washer arrangement 30 is configured for washing the residue 103 with the condensed water 105’. The result of this washing is washed newberyite 103’, which can be exited for any external use, and wash water 105”. The washer arrangement 30 is in these embodiments also connected to the mixing reactor 10 for returning wash water 105” used for washing the residue 103 into the mixing reactor 10.

[0101] In the embodiment of the arrangement 1 for recovery of monoammonium phosphate of figure 16, the mixing reactor 10 has a third mixer input 17 for adding phosphoric acid 110 to the first aqueous process liquid 101. The third mixer input 17 for adding phosphoric acid 110 may optionally be combined with the first mixer input 14.

[0102] In this embodiment, it is further illustrated that the return arrangement 90 is configured for returning a part of the second aqueous process liquid 104, comprising a solution of monoammonium phosphate, into the first aqueous process liquid 101. The remaining part of the second aqueous process liquid is exited from the first separator 40 as a fluid product of monoammonium phosphate 108’. In a series of initial experiments, and as being demonstrated in the embodiments here above, it was found that that it is possible to convert struvite to newberyite by mixing a solution comprising MAP with struvite to achieve full conversion of the struvite into newberyite and a solution comprising MAP / DAP. With the term “MAP / DAP comprising solution” or “MAP / DAP solution” means a process liquid that comprises at least one of MAP and DAP, such as the second aqueous process liquid 104 of the inventive concept. Similarly, the term “MAP comprising solution” or “MAP solution” means a process liquid that comprises mainly MAP, such as the third aqueous process liquid 107 of the inventive concept. MAP as crystals may be precipitated from the process liquid below a certain process temperature. Hence, pure MAP product may be achieved in high yields with the present method. This is further confirmed by the set of experiments presented here below.

[0103] Experiment 1. MAP or MAP / DAP background during conversion

[0104] A MAP solution of around 45% wt / wt or 5 M (i.e. saturated solution at 60°C) was mixed with struvite. The concentrations as a function of time are shown in Figure 17. The concentration level of dissolved phosphate was almost unchanged, which indicates that the phosphate ions of the struvite did not go into solution. At the same time, the amount of solids comprising both N and P, e.g. struvite, rapidly became very small at the same time as the levels of dissolved N increased. This behavior is explained in that a full conversion of struvite into newberyite was achieved, which yielded a highly concentrated MAP / DAP comprising solution.

[0105] Furthermore, H3PO4 was added to struvite, which resulted in a full conversion of struvite to newberyite and yielded a MAP solution of around 30 % wt / wt or 2.3 M (i.e. a saturation limit at 40°C). These experiments show that a full conversion of struvite into newberyite can be obtained with very high MAP / DAP background. This behavior is different from the case where sulphuric acid is used for conversion of struvite. In the sulphuric acid case, the solubility of newberyite increased with increased acid concentration, prohibiting the use of highly concentrated solutions due to the loss of magnesium into the solution. By instead using MAP and / or DAP solutions or phosphoric acid, the concentrations could be very high without significantly influencing the solubility of newberyite. This makes it possible to achieve a very efficient conversion.

[0106] Experiment 2. Evaporation of water from MAP solution

[0107] In Figure 18, some results from experiments concerning the evaporation process are presented. A time diagram illustrates the evaporation process on the MAP solution with minor levels of dissolved magnesium. During the beginning of the evaporation, the concentrations of Mg as well as ammonium nitrogen and phosphorus in solution increases, which is consistent with the reduced amount of water. After some evaporation time, the concentration levels of ammonium nitrogen and phosphorus reaches a saturation limit and are no more increasing, which indicates that MAP is precipitating. However, in this evaporation range, the Mg concentration continues to increase, which proves that the magnesium stays in the solution.

[0108] Solid MAP crystals can thereby be separated from the saturated MAP solution, comprising some dissolved magnesium. The solid MAP crystals are almost completely free from magnesium. The saturated MAP solution can thereafter be returned into the process again, saving the magnesium within the process and providing MAP to the initial mixing part process. From a MAP / DAP solution, MAP solids would be precipitated by reactive crystallization. H3PO4 was added to the MAP / DAP solution to convert all DAP to MAP and due to the lower solubility of MAP in MAP / DAP than in MAP, the MAP crystals will precipitate. This would mean all Mg stays in the MAP solution after crystallization.

[0109] The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

CLAIMS1. A method of recovery of ammonium phosphate, characterized by comprising the steps of:- providing (S10) a start material (100) comprising struvite into a first aqueous process liquid (101) comprising monoammonium phosphate;- mixing (S30) said start material (100) and said first aqueous process liquid (101) at a temperature of at least of 10°C, thereby converting said start material (100) comprising struvite into at least one of monoammonium phosphate and diammonium phosphate in solution and a residue (103) comprising newberyite as a precipitate;- separating (S40) said residue (103) comprising newberyite from said first aqueous process liquid (101), thereby giving a second aqueous process liquid (104) comprising at least one of monoammonium phosphate and diammonium phosphate;- returning (S50) at least a part of said second aqueous process liquid (104) or a part of an aqueous process liquid emanating from said second aqueous process liquid (104) into said first aqueous process liquid (101); and- adding (S20; S21; S22; S23) phosphoric acid (110).

2. The method according to claim 1, characterized in that said first aqueous process liquid (101) comprises at least 10% monoammonium phosphate.

3. The method according to claim 1 or 2, characterized by the further step of controlling an amount of said start material in relation to a volume of said first aqueous process liquid (101) causing said second aqueous process liquid (104) to comprise a mole ratio between ammonium and phosphate in solution in the range of 1 to 1.45, preferably in the range of 1.2 to 1.4.

4. The method according to any of the claims 1 to 3, characterized by the further step of washing (S80) said residue (103) comprising newbeiyite, giving washed newberyite (103’) and wash water (105”).

5. The method according to claim 4, characterized by the further step of:- returning (S90) at least a part of said wash water (105”) to said first aqueous process liquid (101).

6. The method according to any of the claims 1 to 5, characterized in that said step of adding (S21) phosphoric acid (110) is performed into at least a part of said second aqueous process liquid (104), giving a third aqueous process liquid (112) comprising monoammonium phosphate, wherein said step of returning (S50) comprises returning (S51) of a part of said third aqueous process liquid (112), wherein a remaining part of said third aqueous process liquid (112) is exited (S75) as a product.

7. The method according to any of the claims 1 to 5, characterized in that a part of said second aqueous process liquid (104) is exited (S76) as a product, wherein said step of adding (S22) phosphoric acid (110) is performed into a remaining part of said second aqueous process liquid (104), giving a third aqueous process liquid (112) comprising monoammonium phosphate, wherein said step of returning (S50) comprises returning (S51) of said third aqueous process liquid (112).

8. The method according to claim 6 or 7, characterized in that said second aqueous process liquid (104) comprises diammonium phosphate, wherein said step of adding (S21; S22) phosphoric acid (110) causes conversion of diammonium phosphate to monoammonium phosphate.

9. The method according to any of the claims 1 to 5, characterized by the further step of:- concentrating (S60) said separated second aqueous process liquid (104), being a solution of diluted monoammonium phosphate and diammonium phosphate, by evaporating (S62) water (105), forming a concentrated solution (106) of monoammonium phosphate and diammonium phosphate; wherein said step of adding phosphoric acid (110) comprises adding (S25) phosphoric acid (110) to said concentrated solution (106) of monoammonium phosphate and diammonium phosphate, causing precipitation of monoammonium phosphate (108) from a fourth aqueous process liquid (107) saturated by monoammonium phosphate.

10. The method according to any of the claims 1 to 5, characterized in that said step of adding phosphoric acid comprises adding (S20) phosphoric acid (110) to said first aqueous process liquid (101), whereby said second aqueous process liquid (104) comprises monoammonium phosphate.

11. The method according to claim 10, characterized by the further step of:- concentrating (S60) said separated second aqueous process liquid (104), being a diluted monoammonium phosphate solution, by evaporating water (105), causing precipitation of monoammonium phosphate crystals (108), from a fourth aqueous process liquid (107) saturated by monoammonium phosphate.

12. The method according to claim 9 or 11 , characterized by the further step of:- separating (S72) said precipitated monoammounium phosphate (108) from said fourth aqueous process liquid (107) and exiting (S77) as a solid product;wherein said step of returning (S50) comprises returning, after said step of separating (S75) said precipitated monoammounium phosphate (108), said fourth aqueous process liquid (107), comprising a saturated solution of monoammonium phosphate, into said first aqueous process liquid (101).

13. The method according to any of the claims 9, 11 or 12, characterized by the further step of:- condensing (S70) said evaporated water (105) into condensed water (105’); wherein said step of washing (S80) said residue (103) comprising newberyite is performed with said condensed water (105’).

14. The method according to claim 10, characterized in that said step of returning (S50) comprises returning a part of said second aqueous process liquid (104), comprising a solution of monoammonium phosphate, into said first aqueous process liquid (101), wherein a remaining part of said second aqueous process liquid (104) is exited (S79) as a product.

15. The method according to any of the claims 1 to 14, characterized in that said added (S25) phosphoric acid is added in a molar amount in the range of 1: 1.2 to 1.2: 1, preferably 1: 1, relative to a molar amount of struvite converted into newberyite in said first aqueous process liquid (101) during said step of mixing (S30).

16. The method according to any of the claims 1 to 15, characterized by said step of mixing (S30) said first aqueous process liquid (101) is performed at a temperature below 80°C, preferably below 60°C.

17. A method for extracting nitrogen from reject water, characterized by comprising the steps of:- exposing reject water comprising ammonium ions for substances comprising newberyite at neutral or basic pH, causing at least a part said newberyite to form struvite;- separating a material comprising struvite from said exposed reject water;- recovering monoammonium phosphate by a method according to any of the claims 1 to 16; and- reusing at least a part of said residue (103) comprising newberyite in a subsequent step of exposing reject water comprising ammonium ions for substances comprising newberyite.

18. An arrangement (1) for recovery of ammonium phosphate, characterized by comprising:- a mixing reactor (10), having a first mixer input (14) for a first aqueous process liquid (101) comprising monoammonium phosphate; a second mixer input (12) for introducing a start material (100) comprising struvite into said first aqueous process liquid (101); a first mixing equipment (18) configured for mixing said start material (100) and said first aqueous process liquid (101), thereby converting said start material (100) comprising struvite into a mix (102) of at least one of monoammonium phosphate and diammonium phosphate in solution and a residue (103) comprising newberyite as a precipitate, and a mixer output (20) for said mix (102);- a first separator (40) having an input (42) for introducing said mix (102) from said mixing reactor (10), wherein said first separator (40) is configured for separating said residue (103) from said at least one of monoammonium phosphate and diammonium phosphate in solution, thereby giving a second aqueous process liquid (104) comprising at least one of monoammonium phosphate and diammonium phosphate,a first separator output (44) for said residue (103), a second separator output (46) for said second aqueous process liquid (104);- a return arrangement (90) configured for returning at least a part of said second aqueous process liquid (104) or a part of an aqueous process liquid emanating from said second aqueous process liquid (104) into said first aqueous process liquid (101); and- an input (14; 52; 84) for phosphoric acid (110).

19. The arrangement according to claim 18, characterized by further comprising a controller (95), configured for controlling an amount of said start material (100) in relation to a volume of said first aqueous process liquid (101) causing said second aqueous process liquid (104) to comprise a mole ratio between ammonium and phosphate in solution in the range of 1 to 1.45, preferably in the range of 1.2 to 1.4.

20. The arrangement according to claim 18 or 19, characterized by further comprising a washing arrangement for washing (S80) said residue(103) comprising newberyite, giving washed newberyite (103’) and wash water (105”).

21. The arrangement according to claim 20, characterized by a wash water return arrangement configured for returning at least a part of said wash water (105”) to said mixing reactor (10).

22. The arrangement according to any of the claims 18 to 21, characterized by further comprising a neutralizer (50) connected to said second separator output (46) for said second aqueous process liquid (104), wherein said input (52) for phosphoric acid (110) is connected to said neutralizer (50) for exposing a stream of said second aqueous process liquid(104) for phosphoric acid (110), giving a third aqueous process liquid (112)comprising monoammonium phosphate, wherein said return arrangement (90) is configured for returning a part of said third aqueous process liquid (112) into said first aqueous process liquid (101), wherein said neutralizer (50) has an output (56) for a remaining part of said third aqueous process liquid (112) as a fluid product of monoammonium phosphate (108’).

23. The arrangement according to any of the claims 18 to 21, characterized by further comprising an output (56) for exiting a part of said second aqueous process liquid (104) as a fluid product of a mix of monoammonium phosphate and diammonium phosphate (108”), and a neutralizer (50) connected to input a remaining part of said second aqueous process liquid (104), wherein said input (52) for phosphoric acid (110) is connected to said neutralizer (50) for exposing a stream of said second aqueous process liquid (104) for phosphoric acid (110), giving a third aqueous process liquid (112) comprising monoammonium phosphate, wherein said return arrangement (90) is configured for returning of said third aqueous process liquid (112) into said first aqueous process liquid (101).

24. The arrangement (1) according to any of the claims 18 to 21, characterized by further comprising:- an evaporator (60) having an input (64) for said second aqueous process liquid (104), wherein said evaporator (60) being configured for heating said second aqueous process liquid (104) causing evaporation of water (105) from said second aqueous process liquid (104), forming a concentrated solution (106) of monoammonium phosphate and diammonium phosphate, wherein said evaporator (60) has a first evaporator output (66) for said evaporated water (105) and a second evaporator output (68) for said concentrated solution (106) of monoammonium phosphate and diammonium phosphate; and- a crystallizer (80) having a first crystallizer input (82) for said concentrated solution (106) of monoammonium phosphate and diammonium phosphate and a second crystallizer input (84) for phosphoric acid (110), wherein said crystallizer (80) is configured for mixing said concentrated solution (106) of monoammonium phosphate and diammonium phosphate and said phosphoric acid (110), causing precipitation of monoammonium phosphate (108) from a fourth aqueous process liquid (107) saturated by monoammonium phosphate, wherein said crystallizer (80) has a first crystallizer output (86) for said precipitated monoammonium phosphate (108) and a second crystallizer output (89) for said fourth aqueous process liquid (107), wherein said second crystallizer output (89) is connected to said return arrangement (90).

25. The arrangement (1) according to any of the claims 18 to 21, characterized in that said mixing reactor (10) has a third mixer input (17) for adding phosphoric acid (110) to said first aqueous process liquid (101), wherein said third mixer input (17) for adding phosphoric acid (110) optionally is combined with said first mixer input (14).

26. The arrangement (1) according to claim 25, characterized by further comprising:- an evaporator (60) having an input (64) for said second aqueous process liquid (104), wherein said evaporator (60) being configured for heating said second aqueous process liquid (104) causing evaporation of water (105) from said second aqueous process liquid (104), causing precipitation of monoammonium phosphate crystals (108), from a fourth aqueous process liquid (107) saturated by monoammonium phosphate, wherein said evaporator (60) has a first evaporator output (66) for said evaporated water (105), a second evaporator output (67) for said precipitated monoammonium phosphate crystals (108) and a third evaporator output (69) for said fourth aqueous process liquid (107)saturated by monoammonium phosphate, wherein said third evaporator output (69) is connected to said return arrangement (90).

27. The arrangement (1) according to claim 24 or 26, characterized by further comprising:- a condenser (70), connected to said first output (66) of said evaporator (60), for condensing said evaporated water (105) into condensed water (105’); wherein said washer arrangement (30) is configured for washing said residue (103) with said condensed water (105’).

28. The arrangement (1) according to claim any of the claims 18 to 21, characterized in that said return arrangement (90) is configured for returning a part of the second aqueous process liquid (104), comprising a solution of monoammonium phosphate, into said first aqueous process liquid (101), and for exiting a remaining part of said second aqueous process liquid is exited as a fluid product of monoammonium phosphate (108’).

29. A system for extracting nitrogen from reject water, characterized by comprising:- an exposing reactor, having an input for reject water comprising ammonium ions, an input for substances comprising newberyite, and an output for a slurry comprising struvite, wherein said exposing reactor is configured for exposing said reject water comprising ammonium ions for said substances comprising newberyite at neutral or basic pH, causing at least a part said newberyite to form said struvite;- a separator configured to separate a material comprising struvite from said slurry comprising struvite;- an arrangement (1) for recovery of ammonium phosphate according to any of the claim 18 to 28; and- a reuse equipment configured for reusing at least a part of said residue (103) comprising newberyite in said exposing reactor.

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