Method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treatment process

Abstract

A method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treatment process is disclosed. The method may comprise leaching the iron phosphate-based material with an acid; precipitating at least a part of the iron leached from the iron phosphate-based material as ferrous oxalate, thereby forming a precipitated ferrous oxalate and a first liquid phase comprising at least a part of the phosphate contained in the iron phosphate-based material; separating the precipitated ferrous oxalate from the first liquid phase, thereby obtaining the precipitated ferrous oxalate and the first liquid phase, and collecting the first liquid phase; optionally adding an acid to the precipitated ferrous oxalate, thereby dissolving the ferrous oxalate; and optionally recovering oxalic acid from the dissolved ferrous oxalate at least partially.

Description

[0001] METHOD FOR SEPARATING IRON AND PHOSPHORUS FROM AN IRON PHOSPHATE-BASED MATERIAL OBTAINABLE FROM A WASTE WATER TREATMENT PROCESS TECHNICAL FIELD The present disclosure relates to a method for separatingiron and phosphorus from an iron phosphate-based material obtain- able from a waste water treatment process. BACKGROUND Waste water is typically treated e.g. in municipal and other waste water treatment processes to reduce the amount of components which may be harmful to the environment, such as phos- phorus. As environmental regulations restricting phosphorus emis- sions tend to become stricter, there is a need to reduce the amount of phosphorus in waste water conveyed to a waste water treatmentand ultimately to the environment. There may also be a desire torecover and utilize the phosphorus. Waste water treatment processes may reduce the amount ofphosphorus by dosing and / or controlling an iron coagulant to awaste flow containing phosphate. Such waste water treatment pro-cesses may produce sludges containing iron phosphate-based mate- rials, such as iron phosphate e.g. in the form of vivianite-like structures, FePO4-like structures, and / or Fe-OH-PO4-like struc- tures. From an economic and environmental perspective, it may be desirable to be able to recover the iron and / or the phosphate from such materials at least partially. SUMMARY This Summary is provided to introduce a selection of con- cepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. A method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treatmentprocess is disclosed. The method may compriseleaching the iron phosphate-based material with an acid; precipitating at least a part of the iron leached from the iron phosphate-based material as ferrous oxalate, thereby forming a precipitated ferrous oxalate and a first liquid phase comprising at least a part of the phosphate contained in the iron phosphate-based material; separating the precipitated ferrous oxalate from the first liquid phase, thereby obtaining the precipitated ferrous oxalate and the first liquid phase, and collecting the first liquid phase; optionally adding an acid to the precipitated ferrous oxalate, thereby dissolving the ferrous oxalate; and optionally recovering oxalic acid from the dissolved fer- rous oxalate at least partially. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the described embodiments and consti- tute a part of this specification, illustrate various advantageous features and examples of their combinations. In the drawings: Figure 1 shows a flow chart representation of an embodi-ment of a method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treatment process; Figure 2A shows a flow chart representation of anotherembodiment of a method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treat- ment process; Figure 2B shows a flow chart representation of anotherembodiment of a method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treat- ment process; Figure 3 illustrates schematically an embodiment of anexemplary waste water treatment process;Figure 4 shows an X-ray diffraction graph (XRD) showingthat the precipitate is ferrous oxalate and calcium oxalate; Figure 5A illustrates mass concentrations of iron andphosphorus in the collected leachate solution, liquid solution, non-dissolved residue, and ferrous oxalate; Figure 5B shows iron and phosphorus mass balance at eachstep; Figure 6A illustrates mass concentrations of iron andphosphorus in the collected leachate solution, liquid solution, non-dissolved residue, and ferrous oxalate; Figure 6B shows iron and phosphorus mass balance at eachstep; Figures 7A and 7B show the grey green precipitate obtainedin an example of the method; Figure 8 illustrates the EDS elemental distribution ofthe grey green precipitate, indicating that it is ferrous sulphate; Figures 9A and 9B show the appearance of ferrous oxalateprecipitate samples A and B, respectively; Figure 10 shows ferric reduction in sulfuric acid andphosphoric acid leachates; Figure 11 shows ferric reduction in oxalic acid leachate;and Figure 12 illustrates the formation of a small amount ofstrengite precipitate. DETAILED DESCRIPTION A method for separating iron and phosphorus from an iron phosphate-based material obtainable from a waste water treatmentprocess is disclosed.The method may compriseleaching the iron phosphate-based material with an acid; precipitating at least a part of the iron leached from the iron phosphate-based material as ferrous oxalate, therebyforming a precipitated ferrous oxalate and a first liquid phasecomprising at least a part of the phosphate contained in the iron phosphate-based material; separating the precipitated ferrous oxalate from thefirst liquid phase, thereby obtaining the precipitated ferrousoxalate and the first liquid phase, and collecting the first liquidphase; optionally adding an acid to the precipitated ferrous oxalate, thereby dissolving the ferrous oxalate; and optionally recovering oxalic acid from the dissolved fer-rous oxalate at least partially.With the method, it may be possible to separating ironand phosphorus from the iron phosphate-based material efficientlyand also to recover at least a part of the iron and phosphorus contained therein. The oxalate and / or oxalic acid used in the method may also be recovered and possibly also reused. The method may thus be a method for separating and re-covering iron and phosphorus from an iron phosphate-based material obtainable from a waste water treatment process. When the oxalic acid is recovered from the dissolved fer-rous oxalate, solid ferrous sulphate and / or a second liquid phasecontaining the iron ions in the dissolved ferrous oxalate (or atleast a part of them) may be thereby obtained. The iron ions inthe dissolved ferrous oxalate may further be collected and / or re-covered e.g. as the second liquid phase remaining from recoveringthe oxalic acid, the second liquid phase containing the dissolvediron ions (or at least a part of the dissolved iron ions). Thesecond liquid phase containing the dissolved iron ions and / or thesolid ferrous sulphate may be processed further, if desired, and / orreused in dissolving more ferrous oxalate. Reusing the second liq-uid phase containing the dissolved iron ions may increase the ironcontent in the second liquid phase as well as increase its valueas a recycled water treatment chemical or as a raw material for water treatment chemicals. The second liquid phase thus collected and / or recovered may be obtained in a relatively concentrated form, and it may be further used as or processed into a product, e.g. a product suit-able for use as an iron coagulant and / or for use in binding H2S ina biogas reactor. The second liquid phase and / or the product maybe used in a waste water treatment process to remove phosphatefrom waste flows containing phosphate. Additionally or alterna-tively, it / they may be used for binding H2S in a biogas reactor. The phosphate thus collected in and optionally recovered in or from the first liquid phase, in the form of dissolved phos- phate and / or phosphoric acid, may be further used as such as a leaching acid in the present method or in any other process, orprocessed into e.g. a fertilizer product, or it may be purified toobtain pure phosphoric acid. Such pure phosphoric acid may be usede.g. as a leaching acid in the method, as such, or for producinga fertilizer product.The precipitated ferrous oxalate thus obtained may, at least in some embodiments, be collected and optionally used fur- ther. It is not always necessary to add an acid to the precipitated ferrous oxalate, thereby dissolving the ferrous oxalate, and / or torecover oxalic acid from the dissolved ferrous oxalate at leastpartially. The recovery rates achieved may be relatively high.Further, reagents such as acids used in the method may be reused and recycled at least partially. Energy consumption mayremain reasonable or relatively low. The use of virgin resourcesmay also be reduced. The method may thus be relatively sustainable. The iron phosphate-based materials do not sinter or ag- glomerate to a significant extent, and they are readily leached in the method according to one or more embodiments described in this specification. Acid consumption may also remain reasonable or relatively low, for example compared to various other raw materials. In iron phosphate-based materials, the concentration of phosphorus may bein the range of about 5 - 16.6 % (w / w), preferably about 5 – 12.3% (w / w), more preferably about 5 – 12 % (w / w), more preferablyabout 5.6 - 11.7 % (w / w) based on dry matter of the iron phosphate-based material. The concentration of iron may be in the range ofabout 19 – 40 % (w / w), preferably about 19.5 – 36.5 % (w / w) basedon dry matter of the iron phosphate-based material. The remaindermay be mainly organic matter. Less acid is required to leach such materials, as compared e.g. to materials containing a large pro- portion of metal oxides. In some iron phosphate-based materials such as vivianite and / or vivianite-like structures, the concentration of phosphorusmay be in the range of about 8 – 12.3 % (w / w), preferably about 8– 12 % (w / w), more preferably about 8 – 11.7 % (w / w) based on drymatter of the iron phosphate-based material. The concentration ofiron in such iron phosphate-based materials may be in the range ofabout 19 – 33.4 % (w / w), preferably about 19 – 27 % (w / w), morepreferably about 19.5 – 27 % (w / w) based on dry matter of the ironphosphate-based material. In some iron phosphate-based materials, such as FePO4-like structures and / or Fe-OH-PO4 like structures separated from atertiary treatment stage, i.e. post-precipitation, by centrifuga-tion, filtration (e.g. disc filter), flotation, settling, magneticseparation, gravimetric separation, hydrocyclone, or any combina-tion thereof, or any other suitable method for solid-liquid sepa-ration, in particular using centrifugation, filtration (e.g. disc filter), flotation, settling, or any other suitable method for solid-liquid separation, the concentration of phosphorus may be inthe range of 5 – 8 %, preferably 5.6 – 7.9 %, (w / w) based on drymatter of the iron phosphate-based material; the concentration ofiron may be in the range of 30 – 37 %, preferably 30.4 – 36.5 %,(w / w) based on dry matter of the iron phosphate-based material.Such an iron phosphate-based material may be further purified bymagnetic separation, gravimetric separation, hydrocyclone, flota-tion, or any combination thereof. Such purification may increasethe proportion of iron and / or phosphorus in the iron phosphate- based material. The acid used to leach the iron phosphate-based material may comprise or be oxalic acid, HCl, H2SO4, citric acid, H3PO4, and / or any mixture or combination thereof. The acid used to leach the iron phosphate-based materialmay be an acid solution, i.e. a solution of the acid and / or solidacid (e.g. oxalic acid). In other words, the iron phosphate-basedmaterial may be leached with a solution of the acid and / or with asolid acid. The solution of the acid and / or solid acid may be mixedwith the iron phosphate-based material, and the mixture may bee.g. agitated. The acid solution may be an aqueous solution of theacid. When the iron phosphate-based material is leached withthe acid, the iron phosphate-based material may at least partiallydissolve, thus producing a leached solution containing the dissolved iron and phosphate. This reaction may be exemplifiede.g. for vivianite by the following equation:Fe3(PO4)2∙8H2O + 6H+→ 3Fe2++ 2H3PO4+ 8H2O The precipitation of the at least the part of the ironleached from the iron phosphate-based material as ferrous oxalate may be exemplified by the following equation: 2Fe2++ H2C2O4→ Fe2C2O4∙2H2O + 2H+The acid used to leach the iron phosphate-based material may be the same as or different from the acid optionally added tothe precipitated ferrous oxalate. Said acids may be selected in-dependently from each other. In other words, the iron phosphate-based material may be leached with a first acid. A second acid maybe added to the precipitated ferrous oxalate, thereby dissolvingthe ferrous oxalate. The first and second acid may thus be the same or different acid. The (second) acid added to the precipi- tated ferrous oxalate may be an acid other than oxalic acid. The pH of the leached solution may be e.g. about 4 orlower, or preferably about 2 or lower, or in the range of about 0– 4, preferably in the range of about 0 – 2. The phosphate dissolvedin the leached solution may be partially in the form of phosphoricacid and partially in the form of dihydrogen phosphate and / orhydrogen phosphate ion(s). At pH 4 and lower, the phosphate may bemainly in the form of dihydrogen phosphate and / or phosphoric acid;very small amounts of the phosphate may be in the form of hydrogen phosphate. The (second) acid that is optionally added to the pre- cipitated ferrous oxalate, thereby dissolving the ferrous oxalate, may comprise or be e.g. HCl, H2SO4, and / or any mixture or combina- tion thereof. The (first) acid used to leach the iron phosphate-based material may comprise or be oxalic acid. In such an embodiment,the oxalate from the oxalic acid may precipitate at least a partof the iron leached from the iron phosphate-based material asferrous oxalate, thereby forming the precipitated ferrous oxalateand the first liquid phase comprising at least a part of thephosphate contained in the iron phosphate-based material. In other words, the method may comprise leaching the iron phosphate-based material with oxalicacid (e.g. a solution of oxalic acid and / or solid oxalic acid) andprecipitating at least a part of the iron contained in the iron phosphate-based material as ferrous oxalate, thereby forming theprecipitated ferrous oxalate and the first liquid phase.In other words, the method for separating iron and phos-phorus from the iron phosphate-based material obtainable from awaste water treatment process may compriseleaching the iron phosphate-based material with oxalicacid (e.g. a solution of oxalic acid and / or solid oxalic acid);precipitating at least a part of the iron contained in the iron phosphate-based material as ferrous oxalate, therebyforming a precipitated ferrous oxalate and the first liquid phase;separating the precipitated ferrous oxalate from the first liquid phase, thereby obtaining the precipitated ferrous oxalate and the first liquid phase, and collecting the first liquid phase; optionally adding an acid to the precipitated ferrous oxalate, thereby dissolving the ferrous oxalate; and optionally recovering oxalic acid from the dissolved fer-rous oxalate at least partially.The oxalic acid may be in the form of an aqueous oxalicacid solution and / or in solid form, when used in the leaching.In such an embodiment, the leaching and the precipitation may be performed simultaneously; thus acid consumption may be re-duced. However, if the iron phosphate-based material contains sol-ids that are not soluble in the oxalic acid leaching, such as acidinsoluble matter (for example, quartz), the remaining insolublesolids may also end up in the precipitate containing the precipi- tated ferrous oxalate. The (first) acid used to leach the iron phosphate-based material may be an acid other than oxalic acid. The method may thus comprise leaching the iron phosphate-based material with the acid (i.e. a first acid) into a leached solution; optionally separating any remaining solid material sus-pended in the leached solution from the leached solution; adding oxalic acid and / or an oxalate into the leachedsolution, thereby precipitating at least a part of the iron con-tained in the iron phosphate-based material as ferrous oxalate, thereby forming the precipitated ferrous oxalate and the firstliquid phase comprising at least a part of the phosphate containedin the iron phosphate-based material; separating the precipitated ferrous oxalate from the first liquid phase, thereby obtaining the precipitated ferrous oxalate and the first liquid phase, and collecting the first liquid phase; optionally adding an acid (i.e. a second acid) to theprecipitated ferrous oxalate, thereby dissolving the ferrous oxa-late; andoptionally recovering oxalic acid from the dissolved fer-rous oxalate at least partially.In such an embodiment, the (first) acid used to leach the iron phosphate-based material may be other than oxalic acid. The (first) acid used to leach the iron phosphate-based material maycomprise or be e.g. HCl, H2SO4, citric acid, H3PO4, and / or anymixture or combination thereof. H2SO4, i.e. sulphuric acid, is typically cost-effective. However, sulphate ions remaining in the leached solution as impu- rities may require removal or purification in subsequent steps, which may increase costs. H3PO4may be a good choice for the leaching acid in many situations, as it does not add impurities to the leached solution and may assist in achieving a high concentration of phosphoricacid in the first liquid phase. However, increasing concentrationsof phosphate may reduce the recovery efficiency of iron and phos- phorus, while chloride and sulphate ions do not have a marked effect on recovery efficiency. Citric acid may be well suited as the leaching acid as it may enhance the leaching of iron and phosphorus. If the iron phos- phate-based material contains sulphides, citric acid or the com- bination or mixture of citric acid and HCl may be used as the leaching acid. Due to the buffering capacity of citric acid, itcan leach phosphorus and iron at higher pH (e.g. pH of about 3)without dissolving significant amounts of sulphides. The pH of the leached solution may be e.g. about 4 orlower, or preferably about 2 or lower, or in the range of about 0– 4, preferably in the range of about 0 – 2.The oxalic acid and / or an oxalate may be e.g. in the formof an aqueous solution and / or in solid form when added into theleached solution. The precipitated ferrous oxalate (and optionally any re- maining insoluble solids) may be separated from the first liquid phase e.g. by filtering, centrifuging, settling, flotation, using a hydrocyclone, or by any other suitable method for separating a solid phase from a liquid phase. The (second) acid that is optionally added to the pre-cipitated ferrous oxalate, thereby dissolving the ferrous oxalate, may be an acid other than oxalic acid. The (second) acid that is added to the precipitated ferrous oxalate, thereby dissolving theferrous oxalate, may comprise or be e.g. HCl, H2SO4, and / or anymixture or combination thereof. The (second) acid and the precipitated ferrous oxalate may be heated. The heating may improve the solubility of ferrous oxalate. As the solubility is increased, more ferrous oxalate isdissolved, and subsequently the recovery of the oxalic acid may beimproved. The acid and the precipitated ferrous oxalate may beheated e.g. to a temperature of about 60 °C or higher, for exampleto a temperature in the range of about 60 – 80 °C, or preferablyin the range of about 65 – 80 °C, or more preferably to a temper-ature of about 70 °C, to dissolve the precipitated ferrous oxalate. The (second) acid that is optionally added to the pre- cipitated ferrous oxalate, thereby dissolving the ferrous oxalate, may comprise or be HCl. In such an embodiment, the dissolved fer- rous ions in the solution may be enriched to a certain concentra-tion. The dissolved ferrous chloride may be concentrated, for ex-ample by evaporation.The (second) acid that is optionally added to the pre- cipitated ferrous oxalate, thereby dissolving the ferrous oxalate,may comprise or be H2SO4. In such embodiments, the iron may pre-cipitate at least partially as solid ferrous sulphate from thesolution containing the dissolved oxalate (and optionally a part of the dissolved iron ions). The precipitated solid ferroussulphate may be separated from the solution comprising at least apart of the dissolved oxalate. The precipitated solid ferrous sul-phate may be separated after heating the (second) acid (i.e. theH2SO4) and the precipitated ferrous oxalate. The ferrous sulphatereadily precipitates e.g. in high concentrated sulphuric acid(e.g. 9 mol / L), so precipitating the ferrous sulphate may notrequire cooling or e.g. crystallizing. When the (second) acid andthe precipitated ferrous oxalate have been heated to dissolve theferrous oxalate, cooling may to some extent assist in the pre-cipitation of the ferrous sulphate. Upon cooling, the oxalic acidmay subsequently precipitate (at least partially). The solid fer-rous sulphate may be filtered or otherwise separated from thesolution, in which the dissolved oxalate remains at least par-tially. The solid ferrous sulphate may be reused in water treat-ment, as a raw material for an iron coagulant, or as a precursorfor a battery material or battery production. The heating of the(second) acid (i.e. the H2SO4) and the precipitated ferrous oxalatemay improve the solubility of the precipitated ferrous oxalate. Asthe solubility is increased, more ferrous oxalate is dissolved,and subsequently the recovery of ferrous sulphate may be increased.The solution, in which the dissolved oxalate remains at leastpartially, may be reused to dissolve ferrous oxalate. Subse-quently, the oxalic acid may be recovered at least partially from the solution, in which the dissolved oxalate remains at least partially, for example by crystallizing. The molar ratio of H+ions to phosphorus upon leaching the iron phosphate-based material with the acid may be e.g. atleast 0.5, or preferably at least 1, or in the range of 0.5 – 7.5,or preferably in the range of 1 – 3.About 50% phosphorus leaching efficiency can be achievedusing a molar ratio of about 1. Phosphoric acid as the leachingacid tends to be less effective. For HCl and H2SO4, a molar ratioof H+ ions to phosphorus in the range of 1 to 3 may lead to approx.50 - 90% phosphorus leaching efficiency. For H3PO4, a molar ratioof H+ ions to phosphorus of approx. 3 may lead to at least 50 %phosphorus leaching efficiency. Any remaining insoluble solids may optionally be sepa- rated from the dissolved ferrous oxalate e.g. by filtering,centrifuging, settling, flotation, using a hydrocyclone, or by anyother suitable method for separating a solid phase from a liquid phase. The dissolved oxalate may be recovered e.g. as oxalic acid. The dissolved oxalate may be recovered e.g. by crystallizing the dissolved oxalate as oxalic acid. The crystallizing may be done by cooling crystallization, evaporation, vacuum cooling,salting, using seeds or by any other suitable method. The dissolvedoxalate may be crystallized e.g. by cooling. The dissolved oxalatemay be cooled, for example, to a temperature of about 5 °C orlower. At such a temperature, the yield may be increased. The recovered dissolved oxalate and / or the crystallizedoxalic acid may be reused e.g. in the leaching the iron phosphate-based material with oxalic acid and / or added to the leached solu- tion, thereby precipitating ferrous oxalate. When the dissolved oxalate has been recovered as oxalicacid, any remaining liquid phase, i.e. the second liquid phasecontaining dissolved iron ions from the dissolved ferrous oxalate, may be collected. The second liquid phase, i.e. the residual acidic liquid,from which the oxalic acid has been crystallized or otherwiserecovered, may be reused as the (second) acid or in addition tothe (second) acid that is added to the precipitated ferrous oxa-late, thereby dissolving the ferrous oxalate, used as an ironcoagulant in waste water treatment process, as a raw material forproducing an iron coagulant in a waste water treatment process,for binding H2S in a biogas reactor, and / or in any other process.Waste water flows incoming into the waste water treatment process may contain phosphorus, for example in the form of phos- phate. To remove phosphate in the waste water treatment process, an iron coagulant may be controlled and / or dosed to a waste flow containing phosphate and thereby precipitating at least a part of the phosphate. The waste flow may be a waste flow incoming to and / or produced in the waste water treatment process. The waste water treatment process may comprise a primary treatment stage, a secondary treatment stage, and optionally a tertiary treatment stage. The waste water treatment process may further comprise athickener and / or a digester. The iron coagulant may be dosed to any suitable dosing point in the waste water treatment process,such as prior to or to the primary treatment stage, to primaryeffluent or to secondary treatment stage (biological treatmentstage), to the secondary effluent, to the tertiary treatment stageto precipitate phosphorus, to any separated sludge such as primary,secondary or tertiary sludge or any mixture or combination ofthese, prior to or into the thickener, after the thickener, and / orprior to or into the digester. Iron phosphates are formed, andthey can be FePO4-like structures, Fe-OH-POH4-like structures, and / or vivianite-like structures. For example, the waste water treatment process may con- tain activated sludge, which may be contacted with the waste water flow incoming to the waste water treatment process, for example after a primary treatment, in which solid material may be removedin a primary clarifier. The activated sludge mainly comprises mi-crobial biomass. The iron coagulant may be controlled and / or dosedinto the activated sludge, such that the precipitate thereby formedmay form a part of the sludge. A part of the sludge may be removedand anaerobically digested. More vivianite-like structures may beformed during the anaerobic digestion.The chemistry of iron phosphate-based materials obtaina-ble from waste water treatment processes may be complex. Variousforms of iron phosphate-based materials may be formed in waste water treatment process, they may vary, and their structures maybe challenging to characterise.The iron phosphate-based material may comprise or be inthe form of Fex(OH)y(PO4)z-like structures. In such structures, x,y and z may vary depending on the crystal lattice of the ironphosphate-based material. Such structures may be formed in the waste water treatment process. The iron phosphate-based material may comprise or be inthe form of vivianite-like structures, of FePO4-like structures, of Fe-OH-PO4-like structures, and / or any mixture or combination thereof. At least 30 %, or at least 50 % (w / w) of the iron phos-phate-based material based on the dry weight of the iron-phosphatematerial may be in the form of vivianite-like structures, of FePO4- like structures, of Fe-OH-PO4-like structures, or any mixture or combination thereof. The percentages may be calculated on the basis of thetotal dry weight of the iron phosphate-based material when theiron and phosphorus contents of the iron phosphate-based materialare known. Such structures may be obtainable by dosing and / or con-trolling an iron coagulant to a waste flow containing phosphateand thereby precipitating at least a part of the phosphate. For example, they may be obtainable by post-precipitation of phospho-rus in the waste water treatment process in the tertiary treatmentstage. The iron coagulant may be dosed and / or controlled to theprimary treatment, to primary effluent or to secondary treatment (biological treatment), to the secondary effluent, and / or to thetertiary treatment. Such structures may be obtainable e.g. by theprocess for recovering phosphates from wastewater described in WO 2017 / 108930. The iron coagulant may comprise e.g. iron chloride, ironsulphate, iron chlorosulphate, iron hydroxide or any mixture orcombination thereof. The iron phosphate-based material may, at least in someembodiments, comprise or be in the form of vivianite-like struc-tures. The vivianite-like structures may be understood as refer- ring to vivianite, as well as structures similar to vivianite, such as baricite and / or metavivianite. Vivianite is a hydratediron phosphate mineral of the general structural formula(Fe2+Fe2+2(PO4)2·8H2O or Fe2+3(PO4)2·8H2O). Vivianite, or the vivi-anite-like structures, may contain an amount of manganese (Mn2+),magnesium (Mg2+), and / or calcium (Ca2+) ions substituting the fer- rous iron (Fe2+) in the structure. The iron phosphate-based material may comprise metal ions (other than iron) as impurities, such as manganese (Mn2+), magne-sium (Mg2+), and / or calcium (Ca2+) ions. These metal ions may alsoprecipitate in the presence of oxalic acid and / or an oxalate asmetal oxalates. This may be exemplified e.g. by the followingequations: 2Mg2++ H2C2O4→ Mg2C2O4∙2H2O + 2H+2Ca2+ + H C O → Ca C O +2 2 4 2 2 4∙XH2O + 2HIn some iron phosphate-based materials, at least 70 %, orat least 80 % (w / w) of the iron phosphate-based material may be inthe form of vivianite-like structures and / or vivianite. The per-centage may be calculated on the basis of the total weight ofsolids in the iron phosphate-based material when iron and phos-phorus contents of the iron phosphate-based material are known. The iron phosphate-based material may, at least in someembodiments, comprise or be vivianite.The iron phosphate-based material may, at least in someembodiments, comprise or be in the form FePO4-like structures. TheFePO4 in such FePO4-like structures is likely to contain crystalwater (probably two crystal waters). Therefore they may be con-sidered to be FePO4∙2H2O-like structures. The exact composition ofsuch FePO4-like structures may however be very challenging to char-acterise. These iron phosphate-based material comprising FePO4-like structures may further comprise ferric hydroxides (FeOH com-pounds, for example Fe2+or Fe3+hydroxides). In embodiments in which the iron phosphate-based materialcomprises or is in the form of FePO4-like structures, the ironphosphate-based material may be leached with oxalic acid. This isbecause ferric oxalate is relatively soluble in water; remaining insoluble solids may then be separated from the leached solution e.g. by filtration or any other suitable method for separating a solid phase from a liquid phase. The iron phosphate-based material may comprise or be in the form of Fe-OH-PO4-like structures. The vivianite-like structures, FePO4-like structures, Fe- OH-PO4-like structures, or any mixture or combination thereof maybe obtainable from sludge obtainable by dosing and / or controllingan iron coagulant to a waste flow containing phosphate and thereby precipitating at least a part of the phosphate as the vivianite-like structures, FePO4-like structures, Fe-OH-PO4-like structuresand / or any mixture or combination thereof. The vivianite-like structures, FePO4-like structures, Fe- OH-PO4-like structures, and / or any mixture or combination thereofmay be obtainable by separation of the vivianite-like structures,FePO4-like structures, Fe-OH-PO4-like structures and / or any mix- ture or combination thereof from sludge obtainable by dosing and / orcontrolling an iron coagulant to a waste flow containing phosphateand thereby precipitating at least a part of the phosphate as thevivianite-like structures, FePO4-like structures, Fe-OH-PO4-likestructures and / or any mixture or combination thereof. The vivi-anite-like structures, FePO4-like structures, Fe-OH-PO4-like structures or any mixture or combination thereof may be separated from the sludge e.g. by magnetic separation, gravity separation,flotation, a hydrocyclone, or any combination thereof. Such sepa-ration methods may allow for selectively separating the vivianite- like structures, FePO4-like structures, Fe-OH-PO4-like structures or any mixture or combination thereof from other solid componentsor impurities of the sludge. The separated material may be furtherpurified by treating it further with magnetic separation, gravity separation, flotation, hydrocyclone or any combination thereof. Some iron phosphate-based materials, such as vivianite- like structures, FePO4-like structures, Fe-OH-PO4-like structuresand / or any mixture or combination thereof obtainable by separationfrom a tertiary treatment stage, i.e. post-precipitation, may beseparated by solid-liquid separation by e.g. settling, flotation,centrifugation, filtration (e.g. using a disc filter), or by mag-netic separation, gravimetric separation, hydrocyclone, flotation,or any combination thereof. Such materials may contain a relativelylow proportion of other materials or impurities. However, if de- sired, they may be further purified e.g. by magnetic separation,gravity separation, flotation, by a hydrocyclone, or any combina-tion thereof. The iron phosphate-based material may be obtainable fromdigested sludge, in particular anaerobically digested sludge fromthe waste water treatment process.At least vivianite and vivianite-like structures, andother FePO4-like structures and Fe-OH-PO4-like structures, mayhave paramagnetic properties. They may thus be magnetically sepa-rated from digested sludge, for example using the ViviMag® sepa-ration technology. In other words, the iron phosphate-based mate- rial may be magnetically separated, for example using the ViviMag® separation technology. A method for such magnetic separation isdescribed e.g. in WO 2018 / 169395 (for example, page 5, line 13 –page 6, line 3; page 8, line 1 – page 13, line 33), which is hereinincorporated in its entirety. The iron phosphate-based material may be obtainable e.g. by the process for recovering phosphates from wastewater describedin WO 2017 / 108930 (for example, page 3, line 31 – page 10, line15), which is herein incorporated in its entirety.The iron phosphate-based material may be provided in dry form. Drying the iron phosphate-based material, such as vivi- anite-like structures, may lead to changes in the material, forexample oxidation of the iron therein, or from materials such asvivianite-like structures, FePO4-like structures, and Fe-OH-PO4-like structures, losing crystal waters of the iron phosphate-basedmaterial therein. The iron phosphate-based material may therefore be dried such that the iron contained therein is not oxidized or crystal water not lost to a significant extent. The iron phosphate-based material may be provided in theform of a slurry. Such a slurry may have a water content of at least about65 % (w / w), or a water content in the range of about 65 – 90 %(w / w), preferably in the range of about 67.8 – 86 % (w / w), basedon the total weight of the slurry. Depending on the source of theiron phosphate-based material, a typical water content for theslurry may be e.g. in the range of about 80.3 – 86 % (w / w), or inthe range of about 67.8 – 76 % (w / w), based on the total weight ofthe slurry. However, the water content is not particularly limited and may be decreased further by any solid-liquid separation method, for example centrifugation or filtration. The water content of the slurry may depend e.g. on how the iron phosphate-based material has been obtained from the waste water treatment process. A high water content may cause issues in the leaching, for example when phosphoric acid is used for the leaching. The iron phosphate-based material may be leached with the acid by adding the acid into the slurry. The (first) acid used to leach the iron phosphate-based material may be added as an acid solution, i.e. a solution of theacid. The (first) acid solution thus contains a liquid having avolume. The iron phosphate-based material may be leached with the(first) acid by adding the acid solution into the slurry such thatthe ratio of the volume of the liquid of the acid solution to thedry weight of the solid material of the slurry is in the range ofabout 2 to 180 mL / g. The iron phosphate-based material may beleached with the acid by adding the acid solution to the iron phosphate-based material such that the ratio of the volume of the liquid in the acid solution to the dry weight of the solid materialin the iron phosphate-based material is in the range of 5 to 50mL / g, when dried iron phosphate-based material is leached (i.e.when the iron phosphate-based material is provided in dry form).The ratio of the volume of the liquid of the acid solution to thedry weight of the solid material of the slurry or the iron phos-phate-based material may be expressed e.g. as mL / g.The ratio of the volume of the liquid of the acid solution to the dry weight of the solid material of the slurry may have an effect on leaching efficiency and / or on iron and phosphorus re- covery efficiency. A smaller ratio may, in general, reduce the iron and phosphorus recovery efficiency. When the acid is phos- phoric acid, increasing the ratio may increase phosphorus recovery efficiency. The pH of the slurry after adding the acid into the slurrymay be e.g. about 4 or lower, or preferably about 2 or lower, orin the range of about 0 – 4, or preferably in the range of about0 – 2.Iron in the iron phosphate-based material or in variousstages of the method may be in the form of ferrous iron (Fe2+, i.e.Fe(II)), ferric iron (Fe3+, i.e. Fe(III)), or any mixture of com-bination thereof. The iron may be present at least partially asferrous iron. However, at least in some embodiments, a proportion of the iron in the iron phosphate-based material may be in the form of ferric iron. Ferric ions do not readily precipitate as oxalates, because ferric oxalate is more water soluble than ferrous oxalate. Therefore there may be a need to convert ferric iron present in the iron phosphate-based material at least partially into ferrous iron prior to or when precipitating at least a partof the iron leached from the iron phosphate-based material as theferrous oxalate. The method may comprise reducing ferric iron present in the iron phosphate-based material at least partially into ferrous iron prior to or when precipitating at least a part of the ironleached from the iron phosphate-based material as the ferrous ox-alate. The reducing of the ferric iron may be done when theferric iron is dissolved in the leached solution. H2SO4 may be awell-suited acid for the leaching prior to the reduction. Alter-natively or additionally, the reducing of the ferric iron may be done in the presence of oxalate ions. The ferric iron may be reduced e.g. by adding metallic iron (Fe0) such that it may react with ferric iron to thereby reduce the ferric iron into ferrous iron at least partially. Additionally or alternatively, ferric oxalate may be at least partially photolytically decomposed into ferrous oxalate. Iron(III) oxalate, Fe3+(C2O4)33–, is a photoactive metal organiccomplex which may undergo photolysis following photoexcitation.The reaction upon the photoexcitation may be expressed as follows: 2Fe3+(C2O4)33-→ 2Fe2+(C2O4)22-+ 2CO2+ C2O42-The iron ions from the dissolved ferrous oxalate may becollected and / or recovered e.g. by collecting and / or recoveringthe second liquid phase from the dissolved ferrous oxalate fromwhich the oxalate has been separated e.g. by crystallizing.The collected or recovered iron ions in the second liquidphase may be e.g. in the form of an iron salt, such as iron chloride and / or sulphate and / or chlorosulphate (depending e.g. on the acidadded to the precipitated ferrous oxalate). The iron salt is in adissolved form in the second liquid phase. The method may thus further comprise collecting the sec-ond liquid phase remaining from the recovering of the oxalic acidfrom the dissolved ferrous oxalate at least partially, the secondliquid phase containing dissolved iron ions from the dissolved ferrous oxalate. The method may further comprise processing the collectedsecond liquid phase containing the dissolved iron ions from the dissolved ferrous oxalate into a product suitable for use as aniron coagulant. The method may further comprise using the secondliquid phase and / or the product as an iron coagulant in a wastewater treatment process. The waste water treatment process in whichthe recovered and processed iron ions are used as an iron coagulantmay be the same or different from the waste water treatment process from which the iron phosphate-based material is obtainable or ob- tained. The method may further comprise processing the collectedsecond liquid phase containing the dissolved iron ions from thedissolved ferrous oxalate into a product suitable for use forbinding H2S in a biogas reactor. The method may further compriseusing the product for binding H2S in a biogas reactor.The separated first liquid phase may comprise at least a part of the phosphate contained in the iron phosphate-based mate- rial. The method may further comprise purifying the first liq-uid phase, thereby obtaining a purified liquid phase comprising at least a part of the phosphate contained in the iron phosphate- based material. The method may further comprise purifying the separatedfirst liquid phase by ion exchange, thereby obtaining a purified liquid phase comprising at least a part of the phosphate containedin the iron phosphate-based material. The ion exchange may removecations from the separated first liquid phase. The method may further comprise processing the collectedfirst liquid phase into a form suitable for use as a fertilizer product, or into a fertilizer product. For example, ammonia solution may be added to the col-lected first liquid phase so as to thereby produce ammonium phos- phate. The ammonium phosphate may be in a form suitable for use as a fertilizer product, or it may be further processed into a formsuitable for use as a fertilizer product. For example, the ammoniumphosphate may be crystallized and optionally granulated. The method may further comprise processing the collectedfirst liquid phase into an acid form suitable for use in leaching the iron phosphate-based material into a leached solution in the method according to one or more embodiments described in thisspecification. For example, the recovered first liquid phase maybe processed into phosphoric acid. The method may further compriseusing the processed first liquid phase in the acid form in leaching the iron phosphate-based material into a leached solution in the method according to one or more embodiments described in this specification. EXAMPLES Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawings. The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification. Figure 1 shows a flow chart representation of anembodiment of a method for separating iron and phosphorus from an iron phosphate-based material 101 obtainable from a waste watertreatment process (WWTP) 102. The details of the waste watertreatment process 102 are not illustrated in this Fig. for clarity. The iron phosphate-based material 101 may be provided e.g. in the form of a slurry, or as dried. At 103, the iron phosphate-based material 101 is leached with an acid 104, i.e. a first acid. In this exemplary embodiment,the acid 104 is oxalic acid, which may be added in solid formand / or e.g. as an aqueous solution. The acid 104 thus dissolvesthe iron phosphate-based material 101 at least partially. If at least a part of the iron leached from the iron phosphate-based material 101 is in the form of ferrous iron, i.e. ferrous iron ions (Fe2+), it will readily precipitate as ferrous oxalate. Theresulting leachate 105 is a mixture which may contain theprecipitated ferrous oxalate; optionally any other remaining solidmaterial, for example solid impurities originally present in theiron phosphate-based material 101; and a first liquid phase 108comprising at least a part of the phosphate contained in the ironphosphate-based material in the form of dihydrogen phosphates,hydrogen phosphates and / or phosphoric acid.At 106, the precipitated ferrous oxalate 107 is separatedfrom the first liquid phase 108, thereby obtaining the precipitatedferrous oxalate 107 and the first liquid phase 108. Any remainingsolid material in the leachate 105 may be separated together with the precipitated ferrous oxalate 107. The separation at 106 may be done e.g. by filtering, centrifuging, settling, flotation, usinga hydrocyclone, any combination thereof, or by any other suitablemethod for separating a solid phase from a liquid phase. The first liquid phase 108 thus separated at 106 may becollected. It may then be utilized further or e.g. disposed. ThepH of the leachate 105 may be e.g. about 4 or lower, or preferablyabout 2 or lower, or in the range of about 0 – 4, or preferably inthe range of about 0 – 2. The pH of the first liquid phase 108 maythus be the same or similar. Phosphate present in the first liquid phase 108 may thus be at least partially in the form of phosphoric acid, and partially in the form of dihydrogen phosphate ([H2PO4]−), i.e. in the form of dihydrogen phosphate salts. For example, about 50 % (w / w) or more of the phosphate in the first liquid phase 108 may be in the form of phosphoric acid, when the pH is about 2 orlower. At a pH of about 4 or lower, a very small proportion of thephosphate may be in the form of hydrogen phosphate ([HPO4]2−), i.e. in the form of hydrogen phosphate salts. The leachate 105 mayadditionally comprise organic phosphate, which may not bedissolved and may therefore not end up in the first liquid phase108. The organic phosphate, and other possible or optionalinsoluble impurities, may thus be separated together with theprecipitated ferrous oxalate 107. The first liquid phase 108 collected may be further purified at 109. For example, it may be purified at 109 by ionexchange. The resulting purified liquid phase 110 may then becollected. However, the purification at 109 is optional and maydepend e.g. on what is intended with the first liquid phase 108after it has been collected. The phosphorus is thus separated and recovered from the iron phosphate-based material 101 in the form of the first liquid phase 108 or the purified liquid phase 110. The first liquid phase 108 may be reused to leach theiron phosphate-based material at 103 in addition to the oxalicacid 104 (illustrated with a dotted arrow). The phosphorusconcentration of the first liquid phase 108 may be thereforeincreased, and the first liquid phase 108 may be richer inphosphorus. However, the collected first liquid phase 108 or thepurified liquid phase 110 may, if desired, be further processed at 111, for example into a form 112 suitable for use as a fertilizerproduct, or e.g. into a fertilizer product or e.g. moreconcentrated phosphoric acid. By way of example, the pH of thecollected first liquid phase 108 or the purified liquid phase 110may be adjusted, and / or the phosphate present in the collected first liquid phase 108 or the purified liquid phase 110 may beprocessed into a dry form 112 at 111. Or, for example, ammoniasolution may be added at 111 to the first liquid phase 108 or thepurified liquid phase 110, so as to thereby produce ammoniumphosphate. The ammonium phosphate may be in a form suitable foruse as a fertilizer product and may be used as such, or it may befurther processed at 111 into a form suitable for use as a fertilizer product. For example, the ammonium phosphate may becrystallized and optionally granulated. The collected first liquidphase 108 and / or the purified liquid phase 110 may contain phosphoric acid. Additionally or alternatively, the collected first liquid phase 108 and / or the purified liquid phase 110,e.g.after concentrating of the phosphoric acid in 111, may be usedas phosphoric acid in another process, for example as a raw material in the production of a fertilizer or of a battery material. At 113, an acid 114, i.e. a second acid, is added to theprecipitated ferrous oxalate 107. The acid 114 is other than oxalicacid. For example, the acid may comprise or be HCl, H2SO4, and / orany mixture or combination thereof. The ferrous oxalate 107 maythen dissolve. The resulting dissolved ferrous oxalate 115, i.e. a solution in which the ferrous oxalate 107 is dissolved in theform of ferrous ions and oxalate ions, may be separated from anyremaining solid material at 116, if desired. The separated dis-solved ferrous oxalate 117 may then be processed further at 118.The dissolved iron ions 119, from the dissolved ferrous oxalate(i.e. the dissolved ferrous ions) may be collected or recovered at118. The dissolved oxalate from ferrous oxalate 117 may be crys-tallized at 118 as oxalic acid 120 and recovered at leastpartially. The crystallizing may be done by cooling crystalliza-tion, evaporation, vacuum , salting, or by any other suitable method. The dissolved oxalate may be crystallized e.g. by coolingthe dissolved oxalate, for example to a temperature of 5 °C orlower. At such a temperature, the yield may be increased.However, in some embodiments, the precipitated ferrousoxalate 107 may be collected and optionally processed further asdesired. In such embodiments, the precipitated ferrous oxalate 107 does not need to proceed to 113 and further as illustrated in this Fig. The method may differ, when the (second) acid 114 com- prises or is H2SO4. When the (second) acid 114 and the precipitatedferrous oxalate 107 have been mixed and heated, and the dissolvedferrous oxalate resulting from the heating is cooled at 118, fer- rous sulphate may precipitate (at least partially). When the cool- ing is continued, the oxalic acid may precipitate (at least par-tially). Thus, at 118, the precipitated ferrous sulphate may firstbe separated, and subsequently the oxalic acid 120 may be crys- tallized. These phases are not illustrated in the figure for sim-plicity. The ferrous sulphate may be collected at 118 and option-ally processed further, if desired, and / or used in the waste water treatment process 102 or another waste water treatment process. The solid ferrous sulphate may be reused e.g. as a precursor for a battery material or battery production. The crystallized oxalic acid 120 may be reused at 103 forleaching as the (first) acid 104 or in addition to the (first)acid 104, as illustrated by the dotted arrow. When the oxalic acid 120 has been recovered, any remainingliquid phase, i.e. the second liquid phase 119 may also becollected. It may contain e.g. at least a part of the acid 114 added to the precipitated ferrous oxalate 107 at 113, and at least a part of the dissolved iron ions from the dissolved iron oxalate.Thus the second liquid phase 119 separated from the oxalic acid at118 may e.g. be used further at 113 as the acid 114 or in additionto the acid 114 added to the precipitated ferrous oxalate 107 at113, as illustrated with a dotted arrow. In particular, if the acid 114 comprises or is HCl, the second liquid phase 119 may bereused at 113 in addition to the HCl as the acid 114. Before that it may be processed further such that it is suitable for use asthe acid 114 or as a part of the acid 114, but such processing maynot always be necessary. When the second liquid phase 119 is recycled and reused at 113, the Fe concentration of the secondliquid phase 119 may be increased, and the second liquid phase 119may be richer in Fe. The iron is thus separated from the iron phosphate-basedmaterial 101 in the form of the second liquid phase 119 containingthe dissolved iron ions from the dissolved ferrous oxalate, or, insome embodiments, as solid ferrous sulphate. Optionally, the collected second liquid phase 119 may befurther recovered and / or processed at 121 into a product 122. Forexample, the collected second liquid phase 119 may be furtherprocessed at 121 into a product 122 suitable for use as an ironcoagulant. The product 122 may be further used e.g. as an ironcoagulant in the same waste water treatment process 102, asillustrated by the dotted arrow, or in a different waste waterprocess. The second liquid phase 119 as such may be further usede.g. as an iron coagulant in the same waste water treatment process102 (as illustrated by the dotted arrow) or in a different wastewater process. Figure 2A shows a flow chart representation of anotherembodiment of a method for separating iron and phosphorus from an iron phosphate-based material 201 obtainable from a waste watertreatment process (WWTP) 202. This embodiment of the method issimilar to the one shown in Fig. 1, except that oxalic acid is notused as the acid 204 in the leaching stage 203. Instead, the(first) acid 204 used to leach the iron phosphate-based material at 203 may comprise or be e.g. HCl, H2SO4, citric acid, H3PO4,and / or any mixture or combination thereof. In some embodiments, amixture or combination of oxalic acid, and at least one of HCl,H2SO4, citric acid, H3PO4may be used, for example when the iron is mainly ferric iron and would not precipitate readily at this stage.At 203, the iron phosphate-based material 201 is leached into aleached solution 205. The leached solution 205 may then proceed e.g. directly to 226. There may be solid material 223 suspended in the leachedsolution 205. For example, the solid material 223 may contain asludge residue. Optionally, at 224, any remaining solid material223 suspended in the leached solution 205 may be separated fromthe leached solution 205, for example by filtering, centrifuging,settling, flotation, using a hydrocyclone, or by any other suitable method. The liquid phase thus obtained, i.e. the purified leachedsolution 242, may contain at least a part of the of the ironcontained in the iron phosphate-based material 201 as dissolved iron. Optionally, at 225, any ferric iron present in the puri-fied leached solution 242 may be reduced at least partially intoferrous iron. This may be done e.g. by adding metallic iron to the leached solution. A similar reduction reaction of ferric iron couldalso be included e.g. in the method as shown in Fig. 1. A reducedleached solution 243 may thus be obtained. At 226, oxalic acid and / or an oxalate 227 is added intothe leached solution 205, to the purified leached solution 242, or to the reduced leached solution 243, depending on whether theoptional stages 224 and / or 225 are present in the method. At leasta part of the iron contained in the iron phosphate-based material201, which is dissolved in the leached solution 205, in the puri-fied leached solution 242, or in the reduced leached solution 243,will readily precipitate as ferrous oxalate. Thus a precipitatedferrous oxalate 207 and a first liquid phase 208 comprising atleast a part of the phosphate contained in the iron phosphate-based material are formed at 226.At 206, the precipitated ferrous oxalate 207 is separated from the first liquid phase 208, thereby obtaining the precipitatedferrous oxalate 207 and the first liquid phase 208. The separationmay be done e.g. by filtering, centrifuging, settling, flotation, using a hydrocyclone, or by any other suitable method for separating a solid phase from a liquid phase. The first liquid phase 208 may be collected at 206 and / orprocessed e.g. as described in the context of Fig. 1. The firstliquid phase 208 may be reused to leach the iron phosphate-basedmaterial at 203 as the acid 204 or in addition to the acid 204(illustrated with a dotted arrow). The phosphorus concentration ofthe first liquid phase 208 may be therefore increased, and thefirst liquid phase 208 may be richer in phosphorus. The firstliquid phase 208 collected may be further purified at 209. Forexample, it may be purified at 209 by ion exchange. The resulting purified liquid phase 210 may then be collected. However, thepurification at 209 is optional and may depend e.g. on what isintended with the first liquid phase 208 after it has been collected. The phosphorus is thus separated and recovered from the iron phosphate-based material 201 in the form of the first liquid phase 208 or the purified liquid phase 210. However, the collected first liquid phase 208 or the purified liquid phase 210 may, if desired, be further processed at211, for example into a form 212 suitable for use as a fertilizerproduct or as a battery material, or e.g. into a fertilizer productor into battery material. By way of example, the pH of the collected first liquid phase 208 or the purified liquid phase 210 may be adjusted, and / or the phosphate present in the collected first liquid phase 208 or the purified liquid phase 210 may be processed into a dry form at 211. Or, for example, ammonia solution may be added at 211 to the first liquid phase 208 or the purifiedliquid phase 210, so as to thereby produce ammonium phosphate. Theammonium phosphate may be in a form suitable for use as a fertilizer product, or it may be further processed at 211 into a form 212 suitable for use as a fertilizer product. For example, the ammonium phosphate may be crystallized and optionallygranulated. The collected first liquid phase 208 or the purifiedliquid phase 210 may, if desired, also be further processed at 211, for example into a more concentrated phosphorus containing liquid 212, for example by evaporation of water to a moreconcentrated phosphoric acid. The formed phosphoric acid may berecycled and reused in leaching more iron phosphate-based material in 203 and / or for the production of a battery material and / or any other process utilizing phosphoric acid. At 213, an acid 214, i.e. a second acid, is added to the precipitated ferrous oxalate 207. The acid 214 is other than oxalic acid. The acid 214 may be the same or different as the acid 204 used in the leaching at 203. For example, the acid 214 may comprise or be HCl. The ferrous oxalate 207 may then dissolve. The dissolvedferrous oxalate 215 may then be processed further at 218. Thedissolved oxalate 215 may be crystallized as oxalic acid 220 andrecovered. However, in some embodiments, the precipitated ferrousoxalate 207 may be collected and optionally processed further asdesired. In such embodiments, the precipitated ferrous oxalate 207 does not need to proceed to 213 and further as illustrated in this Fig. The crystallized oxalic acid 220 may be reused at 226 forprecipitating ferrous oxalate as the oxalic acid 227 or in additionto the oxalic acid 227, as illustrated by the dotted arrow. When the oxalic acid 220 has been recovered, any remainingliquid phase, i.e. the second liquid phase 219, may also becollected at 218. It may contain e.g. at least a part of the acid214 added to the precipitated ferrous oxalate 207 at 213, and at least a part of the dissolved iron ions from the dissolved ironoxalate. Thus the second liquid phase 219 separated from the oxalicacid at 218 may e.g. be used further at 213 as the acid 214 or inaddition to the acid 214 added to the precipitated ferrous oxalate 207 at 213, as illustrated with a dotted arrow. In particular, if the acid 214 comprises or is HCl, the second liquid phase 219 maybe reused at 213 in addition to the HCl as the acid 214. Beforethat it may be processed further such that it is suitable for useas the acid 214 or as a part of the acid 214, but such processingmay not always be necessary. When the second liquid phase 219 isrecycled and reused at 213, the Fe concentration of the secondliquid phase 219 may be increased, and the second liquid phase 219may be richer in Fe. The iron is thus separated from the iron phosphate-based material 201 in the form of the second liquid phase 219 containingthe dissolved iron ions from the dissolved ferrous oxalate 215.The second liquid phase 219 as such may be further usede.g. as an iron coagulant in the same waste water treatment process202 (as illustrated with a dotted arrow) or in a different wastewater process, and / or it may be used to control H2S formation inbiogas production, and / or it may be used as a raw material forbattery material production. Optionally, the collected second liquid phase 219 may befurther recovered and / or processed at 221 into a product 222. Forexample, the collected second liquid phase 219 may be furtherprocessed at 221 into a product 222 suitable for use as an ironcoagulant and / or to control H2S formation in biogas productionand / or as a raw material for battery material production. Theproduct 222 may be further used e.g. as an iron coagulant in thesame waste water treatment process 202, as illustrated by a dottedarrow, or in a different waste water process.Figure 2B shows a flow chart representation of anotherembodiment of a method for separating iron and phosphorus from an iron phosphate-based material 201 obtainable from a waste water treatment process (WWTP) 202. This embodiment of the method isotherwise similar to the one shown in Fig. 2A, except that the(second) acid 214 comprises or is H2SO4. The (second) acid, i.e.the H2SO4, and the ferrous oxalate 207 may be heated at 213 toimprove the dissolution of the ferrous oxalate 207. In suchembodiments, the ferrous ions dissolved may precipitate at 213 at least partially as solid ferrous sulphate 246 from the solution 215 containing at least a part of the dissolved ferrous oxalate.The ferrous sulphate readily precipitates e.g. in highconcentrated sulphuric acid (e.g. 9 mol / L), so precipitating theferrous sulphate may not require cooling or e.g. crystallizing.When the (second) acid 214 and the precipitated ferrous oxalate207 have been heated to dissolve the ferrous oxalate at 213,cooling may to some extent assist in the precipitation of the ferrous sulphate. The solid ferrous sulphate 246 precipitated at 213 may be separated at 216 from the solution 215 comprising at least a part of the dissolved oxalate, for example using any suitable solid- liquid separation method, such as centrifugation, filtration, flotation, settling, or any combination thereof. The precipitated solid ferrous sulphate 246 may be separated after the heating ofthe acid 214 and the precipitated ferrous oxalate 207 at 213. Asolution 217 comprising at least a part of the dissolved oxalate is also obtained at 216. Upon cooling, the oxalic acid may subsequently precipitate (at least partially), or the oxalic acid may becrystallized at 218 from the solution 217 and used further as described e.g. in the context of Fig. 2A. The solid ferroussulphate 246 separated at 216 may be e.g. processed further at 221and reused in water treatment or as a raw material for an ironcoagulant or as a precursor for a battery material or batteryproduction as described e.g. in the context of Fig. 2A.A similar stage as 216 for separating solid ferrous sulphate 246 could also be included in the method as described in Fig. 1, when the (second) acid 114 comprises or is H2SO4, to obtain solid ferrous sulphate. Figure 3 illustrates schematically an embodiment of anexemplary waste water treatment process (WWTP) 302. Awaste flow, such as a waste water stream 327, for ex-ample municipal waste water stream, may be conducted to the wastewater treatment process 302. The waste water treatment process 302may comprise a primary treatment stage 328 and a secondary treat- ment stage 330. The first stage of the waste water treatment process 301may be the primary treatment stage 328. The primary treatment stage328 may comprise e.g. a primary sedimentation tank or basin 329,to which the waste water stream 327 is conducted.From the primary treatment stage 328, the waste waterstream 327 may be conducted to the secondary treatment stage 330.The secondary treatment stage 330 may comprise a biological treat-ment stage 331 (including e.g. an aeration tank) and a secondaryclarification (sedimentation) stage 332. The waste water treatment process 302 may further com-prise a tertiary treatment stage 333. However, many waste water treatment processes do not comprise a tertiary treatment stage. The tertiary treatment stage 333 may comprise e.g. a flocculation stage and / or a precipitation stage 334. The waste water stream maybe emitted as emitted waste water 336 after the tertiary treatmentstage 333, or in embodiments in which there is no tertiary treat- ment stage, after the secondarytreatment stage 330. An iron flocculant 335 may be dosed e.g. in or prior tothe primary treatment stage 328, such as in the waste water stream 327 incoming to the primary sedimentation tank or basin 329 in order to flocculate / precipitate at least a part of phosphorus pre-sent in the waste water stream 327. A sludge 338 containing precipitated iron phosphate-based material may be obtained from the primary treatment stage 328, in particular from the primary sedimentation tank or basin 329. The iron flocculant 335 may, additionally or alterna- tively, be dosed e.g. in the secondary treatment stage 330, for example to the biological treatment stage 331 and / or to the sec-ondary clarification stage 332, or to the waste water stream 344incoming to the secondary treatment stage 330. In some embodiments, at least a part of sludge from the secondary clarification stage332 may be returned to the biological treatment stage 331 (e.g. inthe aeration tank) in the secondary treatment, to the primary treatment stage 328, and / or to the waste water stream 327 incomingto the primary treatment stage 328 as return sludge 337. The ironcoagulant 335 may then be dosed in the return sludge 337. A sludge338’ containing precipitated iron phosphate-based material may be obtained from the secondary treatment stage 330. The iron coagulant 335 may, additionally or alterna-tively, be dosed e.g. in the tertiary treatment stage 333, and / orto the waste water stream 345 incoming to the tertiary treatmentstage 333, in order to flocculate / precipitate at least a part ofphosphorus present in the waste water stream 327. This may beconsidered as post-precipitation of phosphorus. The iron coagulant 335 may flocculate and / or precipitate at least a part of the phosphorus present in the waste water stream 327 as an iron phosphate-based material, for example in the form of vivianite-like structures, of FePO4-like structures, of Fe-OH- PO4-like structures, and / or any mixture or combination thereof. Such iron phosphate-based materials 301 may be separated from the sludge 338 from the primary treatment stage 328 and / or from the sludge 338’ from the secondary treatment stage 330, e.g.from the secondary clarification stage 332 at a separation stage341. Prior to the separation at the separation stage 341, thesludge 338, 338’ from the primary treatment stage 328 and / or sec- ondary clarification stage 332 may be thickened in a thickener 339. Prior to the separation at the separation stage 341, thesludge 338, 338’ from the primary treatment stage 328 and / or secondary clarification stage 332, which may optionally have beenthickened in the thickener 339, may be anaerobically digested ina digester 340. In anaerobic digestion, ferric iron present in thesludge may be reduced at least partially into ferrous iron, which results in the formation of vivianite and vivianite-like struc- tures. The anaerobic digestion may involve various other reactions as well. For example, phosphates may be released, and they mayreact with iron present and may be precipitated as iron phosphate-based materials, such as vivianite and vivianite-like structures. In the separation stage 341, the iron phosphate-basedmaterial 301 may be separated from the sludge(s) 338, 338’ e.g. by magnetic separation, gravity separation, flotation, by a hydrocy- clone, or any combination thereof. Alternatively or additionally, iron phosphate-based ma-terials may be separated as tertiary sludge 338” and / or as theiron phosphate-based material 301’ from the tertiary treatmentstage 333, e.g. from the flocculation stage and / or precipitationstage 334. The tertiary sludge 338” may be separated as part ofa tertiary treatment stage 334 or after the tertiary treatmentstage 334 by settling, flotation, centrifugation, filtering (e.g.using a disc filter), magnetic separation, gravity separation, hydrocyclone, and / or any combination thereof, or by any othersuitable method. The tertiary treatment stage 334 may thus includea separation unit. The tertiary sludge 338” may be in the form ofan iron phosphate-based material, such as vivianite-like struc-tures, FePO4-like structures, and / or Fe-OH-PO4-like structures, and treated according to the method according to one or more em- bodiments described in this specification. The iron phosphate- based material 301’ may be separated from tertiary sludge 338”. Ifdesired, a separation stage 341’ may separate more liquid from theiron phosphate-based material that is the tertiary sludge 338” andmay purify the tertiary sludge 338”. The separation stage 341’ mayutilize e.g. settling, centrifugation, filtering (e.g. using adisc filter), flotation, magnetic separation, hydrocyclone, grav-ity separation, or any combination thereof, or any other suitableseparation method. If desired, the iron phosphate-based materialthat is the tertiary sludge 338” thus separated may be furtherpurified at the separation stage 341’ e.g. by flotation, magnetic separation, gravity separation, by a hydrocyclone, or any combi- nation thereof. The sludge 338” from the tertiary treatment stage 333 canalso be at least partially recycled back to primary treatment stage328, to incoming wastewater 327, or to primary effluent, to thesecondary treatment stage 330, and / or to the biological treatmentstage 331 (not shown for clarity).The sludge 338” from the tertiary treatment stage 333 canbe at least partially directed to the optional thickener 339 and / or to the optional digester 340 and / or to the separation stage 341. Thus all the sludges 338, 338’ and 338’’ may be directed to the same separation stage 341 (optionally via the thickener 339 and / or the digester 340). In other embodiments, the sludge 338’’ from the tertiary treatment stage 333 may be at least partially directed to its own separation stage 341’. The composition of the iron phosphate-based materials 301, 301’ obtainable from the waste water treatment process 302may be similar or different. Their compositions may depend e.g. onthe stages, the chemistry of the stages of the waste water treat-ment process 302, and / or the iron coagulant(s) dosed. At leastvivianite and vivianite-like structures, as well as other FePO4-like structures and / or Fe-OH-PO4 like structures, may have para-magnetic properties. They may thus be magnetically separated fromthe sludge(s), for example using the ViviMag® separation technol-ogy. The separation stage 341 may thus magnetically separate theiron phosphate-based material 301 from the sludge(s). EXAMPLE 1 1 g of recovered vivianite slurry (total solids 28 %, measured at 40°C in the oven) was added into a 50 ml tube that was used as a reaction container, an acid was added into the tube and the reaction mixture was shaken and let to dissolve for 2 hours. The H+ / P molar ratio was about 3 (Table 1) and 1,5 (Table 2). Oxalic acid, hydrochloric acid, sulphuric acid and citric acid were used. The used acid volumes and concentrations as well as the observed iron and phosphorus leaching efficiences from the recov- ered vivianite are presented in Tables 1-2. Table 1. Acid leaching results for iron and phosphorus from recovered vivianite slurry when using different acids at different concentrations. H+ / P molar ratio about 3. Concentra-Fe leaching % P leaching % pH at the endTest Volume of tion ofmean stdev.Pnumber Acidacid, ml acid, Mmean stdev.P mean stdev.P1 Oxalic acid 5 0.300 0.77 0.00 90.94 0.78 1.63 0.042 Oxalic acid 10 0.150 1.48 0.09 94.61 0.01 1.685 0.0253 Oxalic acid 20 0.075 3.61 0.38 98.37 1.30 1.845 0.0054 Oxalic acid 50 0.030 6.06 0.40 93.25 0.61 2 05 HCl 0.6 5.000 88.70 0.33 89.02 0.346 HCl 1 3.000 89.37 0.38 89.63 0.717 HCl 3 1.000 90.90 2.68 93.36 1.668 HCl 5 0.600 87.32 2.44 89.35 0.63 1.055 0.0059 HCl 10 0.300 93.14 1.17 94.13 0.39 1.445 0.01510 HCl 20 0.150 91.76 2.25 93.91 2.26 1.67 0.0111 HCl 50 0.060 91.53 1.41 93.21 2.25 1.85 0.0312 H2SO4 5 0.300 84.05 2.94 89.77 0.25 1.595 0.00513 H2SO4 10 0.150 87.14 1.65 88.86 0.52 1.725 0.05514 H2SO4 20 0.075 87.57 1.01 90.82 0.05 1.87 015 H2SO4 50 0.030 96.02 0.05 97.11 0.04 2.085 0.01516 H3PO4 5 0.428 67.68 1.27 58.83 0.48 2.5 017 H3PO4 10 0.214 71.53 0.04 55.75 7.71 2.615 0.01518 H3PO4 20 0.107 78.54 1.14 57.38 0.17 2.795 0.02519 H3PO4 50 0.043 86.54 1.14 86.59 6.06 3.025 0.025

[0002] 20 Citric acid 5 0.212 77.64 4.86 83.95 0.20 3.05 0.0321 Citric acid 10 0.106 90.34 0.22 91.05 0.84 2.98 0.0522 Citric acid 20 0.053 91.72 0.77 92.78 1.35 3.055 0.02523 Citric acid 50 0.021 90.48 2.29 94.84 0.14 3.07 0.02Table 2. Acid leaching results for iron and phosphorus from recovered vivianite slurry when using different acids at different concentrations. H+ / P molar ratio about 1.5. Fe leaching % pH at the end Test num- Volume of Concentration of P leaching % ber Acidacid, ml acid, Mmean stdev.P mean stdev.P mean stdev.P24 Oxalic acid 5 0.150 7.75 0.31 58.02 1.79 3.01 0.0325 Oxalic acid 10 0.075 8.80 0.30 62.03 2.32 3.33 0.0526 Oxalic acid 20 0.038 13.22 0.65 59.70 0.79 3.02 0.3727 Oxalic acid 50 0.015 46.49 0.77 52.15 1.77 2.835 0.14528 HCl 5 0.300 55.09 2.11 52.67 2.64 2.86 0.0329 HCl 10 0.150 56.86 0.67 55.35 0.50 3.035 0.01530 HCl 20 0.075 64.26 0.07 62.72 0.09 3.325 0.02531 HCl 50 0.030 68.81 0.56 67.40 0.40 3.465 0.00532 H2SO4 5 0.150 59.63 1.37 58.32 1.16 3.155 0.02533 H2SO4 10 0.075 65.35 0.27 62.50 0.04 3.26 0.0234 H2SO4 20 0.038 68.44 0.96 68.14 0.25 3.42 0.0135 H2SO4 50 0.015 71.34 0.21 70.53 0.17 3.68 0.0136 H3PO4 5 0.214 33.65 0.21 29.94 0.45 3.18 0.01

[0003] H3PO4 10 0.107 35.19 0.62 37.86 0.91 3.25 0.04H3PO4 20 0.054 42.76 0.77 45.43 0.87 3.31 0.02H3PO4 50 0.021 46.67 1.04 59.41 0.29 3.58 0.01Citric acid 5 0.106 59.49 0.32 61.79 1.69 3.37 0.09Citric acid 10 0.053 71.05 1.24 74.41 1.49 3.555 0.025Citric acid 20 0.027 38.25 0.85 68.35 0.54 3.51 0.01Citric acid 50 0,011 50,71 2,25 69,84 2,44 3.595 0.025

[0004] EXAMPLE 2 1 g of dry recovered vivianite was added into a 50 ml tube that was used as a reaction container, an acid was added into the tube and the reaction mixture was shaken and let to dissolve for 2 hours. The H+ / P molar ratio was about 3. Phosphoric acid were used. The liquid to solid ratio (acid to vivianite) (L / S), acid concentrations as well as the observed iron and phosphorus leaching efficiencies from the recovered vivianite are presented in Table 3. Table 3. Acid leaching results for iron and phosphorus from dry recovered vivianite when using phosphoric acid at dif- ferent liquid to solid ratios to vivianite and acid concentrations. H+ / P molar ratio about 1.5 Liq- Fe leaching ef- P leaching ef- uid / ficiency % ficiency % Test solid Concent- num- ratio ration of Stdev.P Stdev.P ber Acid(L / S) acid, M Mean(n=2) Mean(n=2) 44 H3PO4 5 1.538 68.53 0.23 60.79 1.2945 H3PO4 10 0.769 58.30 2.27 57.73 1.2846 H3PO4 20 0.385 68.59 0.26 75.29 7.2347 H3PO4 50 0.154 69.81 0.84 64.69 3.48EXAMPLE 3 40 g of the same recovered vivianite slurry as in Example 1 was taken and leached with oxalic acid. The same liquid / slurry ratio as in Test 1, that is 5ml acid / 1 g slurry, was used and also the acid concentration was the same 0.300 M as in Test 1. The reaction mixture was mixed in a measuring flask for 2 hours. The mixture was then filtrated by 0.45 µm PES filter membrane to col-lect leachate. After centrifugation, the filter cake was washedtwice with deionized water and then freeze-dried to obtain 10,3 gof precipitate. The precipitate contained 23.5 wt% iron, 1.75wt%calcium, and 1.5% magnesium. It is estimated to contain 90wt% metal oxalate compounds (ferrous oxalate dihydrate, calcium oxalate, andmagnesium oxalate dihydrate) The x-ray diffraction analysis showthe precipitate is ferrous oxalate and calcium oxalate (Figure 4). This precipitate was further leached in the second acid, that was 6 M hydrochloric acid, with a liquid to solid ratio of 10 ml / g, at70°C. 5g of precipitate was added to 50 ml of 6M hydrochloric acidsolution. In the acid leaching process, the application of strongacid and heat led to the partial dissolution of organic matter, itwas observed that the solution acquired a brown coloration. Usinga Buchner funnel, the hot mixture from the second acid leaching was filtered to remove undissolved substances. Subsequently, the solution was cooled to 4°C and maintained for 12 hours resultingin the formation of needle-like transparent crystals, which can beclearly observed. 1,34 g of oxalic acid dihydrate crystal and aniron-rich liquid was obtained.EXAMPLE 4 In this example, 800 ml of 0.06 M hydrochloric acid wasused to leach 40 g of recovered vivianite slurry as in Example 1.The reaction mixture was mixed in a plastic bottle for 2 hours. Post-leaching, the 0.45 µm PES membrane filter was employed to remove insolubles, and 7.56 g of oxalic acid dihydrate was added to the liquid phase. The oxalate precipitation reaction, conducted at 200 rpm on a shaker for 2 hours, resulted in 6.4 g of precipi- tate. Analysis indicated that 60% of the oxalic acid reacted with iron and calcium ions during precipitation, resulting in approxi- mately 10% calcium oxalate hydrate and 90% iron oxalate dihydrate in the precipitate. Subsequently, 30 ml of 6M hydrochloric acid at 70°C dissolved 3 g of the collected precipitate, which completely dissolved. Therefore no filtration was required. The solution was then cooled to 4°C and maintained for 12 hours, leading to the crystallization of oxalic acid dihydrate. 1.21 g of crystallized oxalic acid dihydrate was recovered (60% recovery efficiency of oxalate in the solid). Using the residual solution from this crys- tallization, a further 3 g of precipitate was dissolved at 70°C, and after repeating the cooling and crystallization steps, 2.07 g of oxalic acid dihydrate was collected in the second step (98% recovery efficiency of oxalate in the solid). The iron content in the remaining liquid was >5wt%. EXAMPLE 5 The experimental procedure is as follows: Initially, a solution of pure phosphoric acid was employed to dissolve the solid vivianite recovered, maintaining a liquid-to-solid ratio (L / S) of20ml / g and a phosphoric acid to dry recovered vivianite of7.69mmol / g. The mixture was placed on a shaker at 200 rpm for twohours. Subsequently, the mixed solution was centrifuged at 8000rpm, and the supernatant was filtrated to separate the leachatefrom the solid residues. The collected leachate was labeled asPA1. Notably, "First PA1", and "Second PA1" were markedrespectively to distinguish between duplicates.In the next step, 4mM of oxalic acid per 20ml of leachateis introduced into the PA1 solution, during which a noticeable transition in solution color to golden yellow occurs, suggestingof the formation of ferrous oxalate. Following a one-hour shakingperiod at 200 rpm, the mixture was settled down for 8 hours. Theresulting mixture was then subjected to centrifugation to collectthe supernatant, labeled the supernatant as OA1, while theprecipitated solid, ferrous oxalate, underwent washing withdeionized water before collection, thereby marking the conclusion of the first cycle. For the subsequent cycles, the supernatant acquired from the preceding cycle serves as the leaching agent, with all other parameters held constant except for the phosphoric acid to dry recovered vivianite ratio. The mixture was placed on a shaker at 200 rpm for two hours. Subsequently, the mixed solution was centrifuged at 8000 rpm, and the supernatant was filtrated to separate the leachate from the solid residues. The collected leachate was labeled as PA2. 4mM of oxalic acid per 20ml of leachate is introduced into the PA2 solution. Following a one-hour shaking period at 200 rpm, the mixture was settled down for 8 hours. The resulting mixture was then subjected to centrifugation to collect the supernatant, labeled the supernatant as OA2, while the precipitated solid, ferrous oxalate, underwent washing with deionized water before collection, thereby marking the conclusionof the second cycle. The cycles in this experiment repeated for atotal of five cycles, with each subsequent cycle replicating thesteps of the second cycle. The amount of 4 mM oxalic acid per 20mlleachate was not optimized for maximum ferrous oxalate FeOAformation in OA1 – OA5, and therefore from the first cycle to thefifth cycle, the Fe concentration and amount entering the liquidoutput from OA1 to OA5 accumulated. In the process of theinvention, the oxalic acid dosing amount to OA1 – OA5 can beincreased to minimize the iron concentration and / or amount in theliquid output OA1 – OA5 and to maximize the amount of ferrousoxalate FeOA. Example 5.11. First cycle^ A solution of 300 ml of 11.9g / L P phosphoric acid wasemployed to leach 15 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^288ml leachate solution was collected. And 4.77g vacuumfreeze-dried non-dissolved residue was obtained. ^7.056g oxalic acid dihydrate was added to 280ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 272ml liquid solution wascollected. And 7.75g vacuum freeze-dried FeOA was obtained. 2. Second cycle^ 270ml liquid solution obtained from the first cycle wasemployed to leach 13.5 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter.^ 263ml leachate solution was collected. And 2.15g vacuumfreeze-dried non-dissolved residue was obtained.^ 6.3g oxalic acid dihydrate was added to 250ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid.^ After centrifugation, 242ml liquid solution wascollected. And 7.55g vacuum freeze-dried FeOA was obtained. rd cycle^ 230ml liquid solution obtained from the second cycle wasemployed to leach 11.5 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter.^ 221ml leachate solution was collected. And 1.81g vacuumfreeze-dried non-dissolved residue was obtained.^ 5.292g oxalic acid dihydrate was added to 210ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid.^ After centrifugation, 206ml liquid solution wascollected. And 6.17g vacuum freeze-dried FeOA was obtained. rth cycle^ 190ml liquid solution obtained from the second cycle wasemployed to leach 9.5 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter.^ 182ml leachate solution was collected. And 1.44g vacuumfreeze-dried non-dissolved residue was obtained.^ 4.284g oxalic acid dihydrate was added to 170ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 166ml liquid solution wascollected. And 4.49g vacuum freeze-dried FeOA was obtained. 5. Fifth cycle^ 160ml liquid solution obtained from the second cycle wasemployed to leach 8 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^153ml leachate solution was collected. And 1.39g vacuumfreeze-dried non-dissolved residue was obtained. ^3.276g oxalic acid dihydrate was added to 130ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 126ml liquid solution wascollected. And 2.83g vacuum freeze-dried FeOA was obtained. Figures 5A and 5B show the liquid volume and vacuumfreeze-dried solid mass obtained at each step of the first 5-cycle process, the mass concentrations of iron and phosphorus in the collected leachate solution, liquid solution, non-dissolved residue, and ferrous oxalate, as well as the iron and phosphorus mass balance at each step. Example 5.2 1. First cycle^ A solution of 300 ml of 11.9g / L P phosphoric acid wasemployed to leach 15 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^288ml leachate solution was collected. And 5.71g vacuumfreeze-dried non-dissolved residue was obtained.^ 7.056g oxalic acid dihydrate was added to 280ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 272ml liquid solution wascollected. And 7.12g vacuum freeze-dried FeOA was obtained.2. Second cycle^ 270ml liquid solution obtained from the first cycle wasemployed to leach 13.5 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^262ml leachate solution was collected. And 2.25g vacuumfreeze-dried non-dissolved residue was obtained. ^6.3g oxalic acid dihydrate was added to 250ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 244ml liquid solution wascollected. And 7.43g vacuum freeze-dried FeOA was obtained.3. Third cycle^ 230ml liquid solution obtained from the second cycle wasemployed to leach 11.5 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^220ml leachate solution was collected. And 1.66g vacuumfreeze-dried non-dissolved residue was obtained. ^5.292g oxalic acid dihydrate was added to 210ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid.^ After centrifugation, 206ml liquid solution wascollected. And 5.66g vacuum freeze-dried FeOA was obtained. 4. Fourth cycle^ 200ml liquid solution obtained from the second cycle wasemployed to leach 10 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^193ml leachate solution was collected. And 1.71g vacuumfreeze-dried non-dissolved residue was obtained. ^4.536g oxalic acid dihydrate was added to 180ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 176ml liquid solution wascollected. And 4.52g vacuum freeze-dried FeOA was obtained. 5. Fifth cycle^ 160ml liquid solution obtained from the second cycle wasemployed to leach 8 g of dry recovered vivianite for 2h. The mixture was subjected to centrifugation and filtration by 0.45 µm PES filter membrane to remove non- dissolved matter. ^153ml leachate solution was collected. And 1.35g vacuumfreeze-dried non-dissolved residue was obtained. ^3.276g oxalic acid dihydrate was added to 130ml collectedleachate solution from the acid leaching step. The mixed solution was shaken for one hour and sat for 8 hours. The mixture was subjected to centrifugation to separate ferrous oxalate (FeOA) solid from liquid. ^After centrifugation, 127ml liquid solution wascollected. And 2.82g vacuum freeze-dried FeOA was obtained. Figures 6A and 6B show the liquid volume and vacuumfreeze-dried solid mass obtained at each step of the first 5-cycle process, the mass concentrations of iron and phosphorus in the collected leachate solution, liquid solution, non-dissolved residue, and ferrous oxalate, as well as the iron and phosphorus mass balance at each step. EXAMPLE 6 5g of oxalate precipitate from Example 5 was added to 50ml of 9M sulfuric acid at 70 degrees, using a magnetic stirrer to mix During the stirring process, the color of mixture quickly changed from golden yellow to grey green, which may indicate the dissolution of ferrous oxalate and the precipitation of ferroussulfate (Figures 7A and 7B). A small portion of the precipitatewas dissolved in 1M HCl and potassium thiocyanate was added into the dissolved precipitate to identify with a colour reaction if ferric iron was present. Pink or blood red coloration indicates if ferric iron is present. No pink or blood red coloration was ob-served, which indicates that the concentration of ferric iron inthe precipitate is very low or there is no ferric iron. Scanning electron microscope energy dispersive spectroscopy (SEM-EDS) givenelemental distribution (Figure 8) shows iron and sulphur molarratio in the grey green precipitate is close to 1, indicating theprecipitate is ferrous sulphate. A Buchner funnel was used for vacuum filtration while the solution was hot, resulting in about 6.8 g of dry solid and fil- trated oxalate solution. The filtrate solution was then cooled to 4°C and maintained for 12 hours to obtain about 2g oxalic acid dihydrate. EXAMPLE 7 1g of ferric phosphate-based material from tertiarytreatment was added into a 50 ml tube that was used as a reaction container, hydrochloric acid, sulfuric acid, oxalic acid, orphosphoric acid was added into the tube and the reaction mixturewas shaken and let to dissolve for 2 hours. The used acid volumes and concentrations as well as the observed iron and phosphorusleaching efficiencies from the ferric phosphate-based material arepresented in Tables 4 and 5. The elemental composition of ferric phosphate-based material from tertiary treatment was 28±1 wt-% Fe, 5.9±0.1 wt-% P, 2.6±0.1 wt-% Ca and 0.2±0.1 wt-% Mg.

[0005] Table 4. Acid leaching results for iron and phosphorus when using different acids atdifferent concentrations and different liquid-to-solid ratios of ferric phosphate-based material.Liquid / Fe leaching efficiency % P leaching efficiency %The Test solid Acid concentration number ratioMean Stdev.P (n=2) Mean Stdev.P (n=2)of acid, M (mL / g) 48 HCl 50 0.04 1.13 0.29 0.00 0.0049 HCl 50 0.08 11.81 1.65 0.98 0.0550 HCl 50 0.12 19.70 0.70 6.50 0.0451 HCl 50 0.16 34.20 2.64 20.45 1.5852 HCl 50 0.2 46.90 2.14 37.15 4.2053 HCl 50 0.24 64.21 7.44 62.09 0.4454 HCl 50 0.28 71.52 4.21 78.34 0.0355 HCl 50 0.32 92.21 7.99 96.11 0.5756 HCl 5 1 26.14 1.15 17.56 0.8057 HCl 10 0.5 22.13 0.29 12.90 0.1158 HCl 20 0.25 18.51 0.62 8.35 0.1159 HCl 50 0.1 14.12 0.06 3.20 0.0060 HCl 5 2.4 96.5 1.9 100.5 1.261 HCl 10 1.2 101.8 0.4 103.7 0.362 HCl 20 0.6 104.4 1.9 106.4 2.363 HCl 50 0.24 100.7 0.1 103.1 0.364 H2SO4 50 0.02 0.45 0.04 0.00 0.0065 H2SO4 50 0.04 4.84 0.06 1.11 0.01

[0006] H2SO4 50 0.06 15.06 0.04 8.12 0.15H2SO4 50 0.08 30.31 0.02 22.35 0.19H2SO4 50 0.1 47.32 1.64 42.38 0.35H2SO4 50 0.12 67.47 0.18 64.76 0.17H2SO4 50 0.14 67.23 8.90 86.79 0.13H2SO4 50 0.16 85.65 6.85 97.08 0.87H2SO4 5 0.5 17.25 1.80 10.76 1.17H2SO4 10 0.25 14.16 0.10 7.33 0.70H2SO4 20 0.125 11.49 0.22 5.95 0.28H2SO4 50 0.05 8.83 0.13 3.18 0.07H2SO4 5 1 78.23 1.23 75.28 7.43H2SO4 10 0.5 68.13 2.12 70.03 1.70H2SO4 20 0.25 59.19 1.62 59.90 1.17H2SO4 50 0.125 46.22 1.97 40.56 0.26Oxalic acid 50 0.02 2.69 0.01 0.00 0.00Oxalic acid 50 0.04 9.79 0.00 1.58 0.02Oxalic acid 50 0.06 22.14 0.69 7.77 0.50Oxalic acid 50 0.08 34.02 0.02 22.68 0.26Oxalic acid 50 0.1 49.08 0.14 40.82 0.31Oxalic acid 50 0.12 64.23 0.44 61.35 0.29Oxalic acid 50 0.14 70.39 6.06 78.82 0.16Oxalic acid 50 0.16 80.69 5.52 91.33 0.47Oxalic acid 5 0.5 10.04 0.10 2.99 0.04Oxalic acid 10 0.25 11.55 0.06 2.97 0.08

[0007] Oxalic acid 20 0.125 13.11 0.03 3.24 0.01Oxalic acid 50 0.05 14.34 0.03 3.61 0.24Oxalic acid 5 1 57.58 0.38 55.08 0.65Oxalic acid 10 0.5 55.19 0.76 48.79 0.03Oxalic acid 20 0.25 53.79 0.67 46.53 0.39Oxalic acid 50 0.125 49.83 0.04 40.76 0.06

[0008] Table 5. Phosphoric acid leaching: This ferric phosphate material had an adsorption capacityfor phosphorus, so the leaching efficiency of phosphorus is negative. Liquid / Fe leaching efficiency % P leaching efficiency %solid The Test ratio concentration Stdev.P Stdev.P number Acid(mL / g) of acid, M Mean(n=2) Mean(n=2) 96 H3PO4 50 0.04 0.0 0.0 -119.7 2.397 H3PO4 50 0.08 12.5 0.1 -152.7 0.198 H3PO4 50 0.12 33.5 3.2 -159.0 6.799 H3PO4 50 0.16 52.1 15.2 -27.0 4.9100 H3PO4 50 0.2 66.4 15.4 -15.4 1.4101 H3PO4 50 0.24 70.7 0.7 -36.8 3.8

[0009] EXAMPLE 8 Ferrous oxalate precipitate samples from Example 5 were collected after (A) only oxalic acid leaching of vivianite slurry (B) using phosphoric acid to dissolve the vivianite slurry and removing the insoluble matter, then oxalic acid was added to pre-cipitate ferrous oxalate. Compositions of the samples are shown inTable 6. The appearance of the samples is shown in Figs. 9A (sample A) and 9B (sample B). Table 6. Ferrous oxalate precipitates Content Unit Sample A Sample BFe wt% 21±1 31±1Ca wt% 1.45±0.05 0.48±0.1Mg wt% 0.79±0.01 0.28±0.01P wt% 0.14±0.02 0.15±0.04Calcula- ted fer- rouswt% 67.5±3.2 99.6±3.2oxalate dihydrate EXAMPLE 9 To prepare the solution for ferric reduction experiments,20 g of ferric phosphate-based material from tertiary treatmentwas added into a 500 ml container, 400 mL of 0.375M sulfuric acidor oxalic acid was added into the container and the reactionmixture was shaken and let to dissolve for 2 hours. In addition,10 g of ferric phosphate-based material from tertiary treatmentwas added into a 500 ml container, 500 mL of 1M phosphoric acidwas added into the container and the reaction mixture was shakenand let to dissolve for 2 hours. After filtering the leachate to remove insoluble matter (Initial iron and phosphorus concentrations are shown in Table 7“Reduction time 0 min”), 100 mL of the filtered leachate was addedto a beaker, and then the leachate was heated to 50 degrees Celsius.Zerovalent iron was added after 0 minutes according to the ratio of zerovalent iron to ferric iron in the leachate shown in theTable 7. During the reduction reaction, heating was continued tomaintain the temperature of the leachate, and the solution wasstirred at 300 rpm. The reaction followed the equation:2^^^^ + ^^(^) → 3^^^^ Table 7. Zero- Zero valent valent iron iron to P powder ferric Ferric Ferrous concent added, leaching molar Reduction concentra concentra ration, g Acid ratio time, min tion, g / L tion, g / L g / L H3PO4 0.5 0 0.13 6.01 32.610.5 10 8.71 0.69 33.470.3 0.5 20 8.70 0.69 33.860.5 30 8.98 0.66 34.070.5 0 0.18 10.26 2.120.5 10 11.97 1.39 1.670.53 0.5 20 14.40 0.18 1.84H2SO4 0.5 30 16.06 -0.03 2.110.5 0 1.74 7.65 2.030.5 10 2.51 6.79 2.070.47 Oxalic0.5 20 2.50 6.89 2.09acid0.5 30 2.54 6.97 2.131.5 0 1.74 7.65 2.031.5 10 2.02 4.90 2.101.41 Oxalic1.5 20 1.63 4.98 2.12acid1.5 30 1.49 4.98 2.15Oxalic3 0 1.74 7.65 2.032.82 acid3 10 0.04 0.05 2.153 20 0.04 0.04 2.173 30 0.04 0.05 2.20Figure 10 shows ferric reduction in sulfuric acid andphosphoric acid leachates. Figure 11 shows ferric reduction in oxalic acid leachate.Excluding the leachate of sulfuric acid, the phosphorusconcentration in the other leachates exhibited a slight increaseduring the reduction reaction progresses. This increase is at- tributable to the evaporation of a minor quantity of water, which occurs as a result of maintaining the reaction temperature. The phosphorus concentration in the sulfuric acid leachate first de- creased and then increased. This was likely due to the formation of a small amount of strengite precipitate, shown in Figure 12. As the reaction time continued, the precipitate dissolved. In the oxalic acid leachate, it was necessary to use threetimes the amount of zero-valent iron to fully reduce the ferriciron. This requirement is likely to arise because, in the presenceof oxalic acid, the ferrous iron generated during the reduction forms a ferrous oxalate precipitate. This precipitate envelops the iron powder, thereby inhibiting the continuation of the reduction reaction. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects / embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure. It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implementedin various ways. The embodiments are thus not limited to theexamples described above; instead they may vary within the scope of the claims. The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method, a product, an arrangement, or a use, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term “comprising” or “including” is used in this specification to mean including the feature(s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.

Claims

CLAIMS 1. A method for separating iron and phosphorus from aniron phosphate-based material obtainable from a waste water treat-ment process, wherein the method comprisesleaching the iron phosphate-based material with an acid; precipitating at least a part of the iron leached fromthe iron phosphate-based material as ferrous oxalate, therebyforming a precipitated ferrous oxalate and a first liquid phasecomprising at least a part of the phosphate contained in the iron phosphate-based material; separating the precipitated ferrous oxalate from thefirst liquid phase, thereby obtaining the precipitated ferrousoxalate and the first liquid phase, and collecting the first liquidphase; optionally adding an acid to the precipitated ferrousoxalate, thereby dissolving the ferrous oxalate; andoptionally recovering oxalic acid from the dissolved fer-rous oxalate at least partially.

2. The method according to claim 1, wherein the methodcomprises leaching the iron phosphate-based material with oxalic acid; precipitating at least a part of the iron contained in the iron phosphate-based material as ferrous oxalate, therebyforming the precipitated ferrous oxalate and the first liquidphase; separating the precipitated ferrous oxalate from thefirst liquid phase, thereby obtaining the precipitated ferrousoxalate and the first liquid phase, and collecting the first liquidphase; optionally adding the acid to the precipitated ferrous oxalate, thereby dissolving the ferrous oxalate; and optionally recovering oxalic acid from the dissolved fer-rous oxalate at least partially.

3. The method according to claim 1, wherein the method comprises leaching the iron phosphate-based material with the acid into a leached solution;optionally separating any remaining solid material sus-pended in the leached solution from the leached solution;adding oxalic acid and / or an oxalate into the leachedsolution, thereby precipitating at least a part of the iron con-tained in the iron phosphate-based material as ferrous oxalate,thereby forming the precipitated ferrous oxalate and the firstliquid phase comprising at least a part of the phosphate containedin the iron phosphate-based material; separating the precipitated ferrous oxalate from thefirst liquid phase, thereby obtaining the precipitated ferrousoxalate and the first liquid phase, and collecting the first liquidphase; optionally adding an acid to the precipitated ferrousoxalate, thereby dissolving the ferrous oxalate; andoptionally recovering oxalic acid from the dissolved fer-rous oxalate at least partially.

4. The method according to claim 3, wherein the acid usedto leach the iron phosphate-based material comprises or is HCl,H2SO4, citric acid, H3PO4, and / or any mixture or combinationthereof.

5. The method according to any one of claims 1 - 4,wherein the oxalic acid is recovered from the dissolved ferrousoxalate at least partially by crystallizing the oxalic acid fromthe dissolved oxalate.

6. The method according to any one of claims 1 – 5,wherein the method further comprises purifying the separated firstliquid phase by ion exchange, thereby obtaining a purified liquidphase comprising at least a part of the phosphate contained in theiron phosphate-based material.

7. The method according to any one of claims 1 - 6,wherein the iron phosphate-based material comprises or is in the form of vivianite-like structures, of FePO4-like structures, of Fe-OH-PO4-like structures, and / or of any mixture or combination thereof.

8. The method according to claim 7, wherein the vivi- anite-like structures, FePO4-like structures, Fe-OH-PO4-likestructures, and / or any mixture or combination thereof are obtain-able by separation of the vivianite-like structures, FePO4-likestructures, Fe-OH-PO4-like structures and / or any mixture or com-bination thereof from sludge obtainable by dosing and / or control-ling an iron coagulant to a waste flow containing phosphate andthereby precipitating at least a part of the phosphate as thevivianite-like structures, FePO4-like structures, Fe-OH-PO4-likestructures and / or any mixture or combination thereof.

9. The method according to any one of claims 1 – 8,wherein the acid that is added to the precipitated ferrous oxalate,thereby dissolving the ferrous oxalate, comprises or is HCl, H2SO4,H3PO4, or any mixture or combination thereof.

10. The method according to any one of claims 1 – 9,wherein the iron phosphate-based material is provided in the form of a slurry, and the iron phosphate-based material is optionally leached with the acid by adding the acid as an acid solution into the slurry such that the ratio of the volume of the liquid of the acid solution to the dry weight of the solid material of the slurryis in the range of about 2 to 180 mL / g.

11. The method according to any one of claims 1 – 10,wherein the iron phosphate-based material is provided in dry form.

12. The method according to any one of claims 1 – 11,wherein the method comprises reducing ferric iron present in theiron phosphate-based material at least partially into ferrous ironprior to or when precipitating at least a part of the iron leachedfrom the iron phosphate-based material as the ferrous oxalate.

13. The method according to any one of claims 1 – 12,wherein the molar ratio of H+ ions to phosphorus upon leaching theiron phosphate-based material with the acid is at least 0.5, or atleast 1, or in the range of 0.5 – 7.5, or in the range of 1 – 3.

14. The method according to any one of claims 1 - 13,wherein the method further comprises collecting a second liquidphase remaining from the recovering of the oxalic acid from thedissolved ferrous oxalate at least partially, the second liquidphase containing dissolved iron ions from the dissolved ferrousoxalate.

15. The method according to claim 14, wherein the methodfurther comprises processing the collected second liquid phase containing the dissolved iron ions from the dissolved ferrous ox- alateinto a product suitable for use as an iron coagulant andoptionally using the processed product as an iron coagulant in a waste water treatment process.

16. The method according to any one of claims 1 – 15,wherein the method further comprises processing the collected first liquid phase into a form suitable for use as a fertilizer product.

Images