Liquid separation methods

Abstract

A liquid separation method (10) for separating mono ammonium phosphate ("MAP") (20) in aqueous solution, includes: • a receiving step in which a process liquid which comprises water and MAP is received; • a cooling step in which a controlled cooling process is applied to the process liquid to induce the crystallization (14) of MAP; • a separating step (16) in which the crystallised MAP is separated from the remaining process liquid.

Description

[0001] Liquid Separation Methods

[0002] Technical Field

[0003] The present invention relates to liquid separation methods, particularly but not exclusively mono ammonium phosphate solution separation methods..

[0004] Background

[0005] Mono ammonium phosphate (“MAP”) solutions often comprise part of liquid waste streams.

[0006] In one example, municipal or industrial waste water comprises struvite, a hydrate of MAP, which conventionally is removed by precipitation by dosing with magnesium salts.

[0007] In another example, agro-industrial and fertiliser production processes result in waste streams comprising mixtures of MAP with other salts such as diammonium phosphate (“DAP”), ammonium sulphate (“AS”) and sodium sulphate. Conventionally, the MAP is separated by evaporation crystallisation, which is energy intensive.

[0008] In another example, fire extinguisher powder (hereinafter referred to as “FEP”) is used in certain types of fire extinguishers. FEP comprises two main components: mono ammonium phosphate (“MAP”) and ammonium sulphate (“AS”).

[0009] To ensure that the FEP remains free flowing and available for use after long periods of non-use, FEP particles are coated with silicone oil. The silicone oil is hydrophobic and prevents moisture uptake by the FEP which could lead to agglomeration of the FEP and cause operational failure. The FEP may also comprise small amounts of insoluble ingredients such as Fuller’s Earth, mica, calcium carbonate and amorphous silica. After expiry, the FEP requires disposal. MAP and AS can be utilised as fertiliser but the presence of silicone oil prevents this, since it affects the handling and solubility of the MAP and AS particles. Removing the silicone oil to a sufficient extent at an economic cost has proved problematic. Hence most FEP has traditionally been sent to landfill, which is increasingly expensive and environmentally unfriendly.

[0010] Some methods have been proposed for processing FEP for re-use, but these have utilised volatile organic solvents, such as acetone, as solvents for the silicone oil. Such solvents pose health, safety and disposal risks.

[0011] Recently, new methods have been developed in which the FEP is processed by contact with water, then filtered to remove the water insoluble components, which include the silicone oil. The filtrate liquid comprises the water-soluble components, including the MAP and AS.

[0012] Conventionally, the filtrate liquid can be used as a base for liquid fertiliser or can be concentrated by heating and then cooled to cause crystallisation of the soluble salts, which are filtered and dried. The resultant dried material is a mixture of MAP and AS. The processes of heating, cooling, filtering and drying are energy intensive and hence relatively expensive.

[0013] In this specification, the word “substantially” is used to include the meaning of “exactly or for practical purposes, as will be understood by a skilled person in the technical field”. This includes some variation from “exactly” because of practical considerations. Statements of Invention

[0014] According to a first aspect of the present invention, there is provided a liquid separation method for separating mono ammonium phosphate (“MAP”) in aqueous solution, the method including :

[0015] • a receiving step in which a process liquid which comprises water and MAP is received;

[0016] • a cooling step in which a controlled cooling process is applied to the process liquid to induce the crystallization of MAP;

[0017] • a separating step in which the crystallised MAP is separated from the remaining process liquid.

[0018] Possibly, in the cooling step, the cooling process cools the process liquid to an MAP crystallisation temperature.

[0019] Possibly, at the MAP crystallisation temperature, the process liquid is at an MAP eutectic temperature and the MAP is at an MAP eutectic concentration. Possibly, the MAP eutectic concentration is in the range 17-19% w / w.

[0020] Possibly, the cooling process includes flash cooling to induce rapid crystallization of MAP.

[0021] Possibly, the concentration of MAP in the initial process liquid is no more than 10% w / w and the MAP crystallisation temperature is in the range from -6°C to 0°C.

[0022] Possibly, the concentration of MAP in the process liquid is at least 25% w / w and the MAP crystallisation temperature is in the range from 0°C to 9°C.

[0023] Possibly, the method includes a concentration step before the cooling step, which concentrates the MAP to greater than 10% w / w and more preferably at least 25%. Possibly at the MAP eutectic temperature, both crystalline MAP and ice form.

[0024] Possibly, in the separating step, the crystallised MAP and ice is separated from the remaining process liquid by a separation process, which may comprise skimming, filtration, gravity separation and / or centrifuging.

[0025] Possibly, the process liquid includes a plurality of salts and may comprise a primary salt and a secondary salt. Possibly, MAP comprises the primary salt.

[0026] Possibly, the secondary salt crystallises at a secondary crystallisation temperature, which may be lower than the MAP crystallisation temperature.

[0027] Possibly, the method includes the step of cooling the process liquid to the secondary crystallisation temperature.

[0028] Possibly, at the secondary crystallisation temperature, the process liquid is at a secondary salt eutectic temperature and the secondary salt is at a eutectic concentration.

[0029] Possibly, the step of cooling the process liquid to the secondary crystallisation temperature occurs after the step of cooling the process liquid to the MAP crystallisation temperature, and after the step of separating the crystallised MAP and ice from the process liquid.

[0030] Possibly, the method includes a secondary separating step, after the step of cooling the process liquid to the secondary crystallisation temperature, to separate the secondary salt and ice from the process liquid.

[0031] Possibly, the secondary salt comprises one of potassium salts, sodium salts, calcium salts, magnesium salts, ammonium salts and / or rare-earth phosphates. Possibly, the process liquid is a waste stream, possibly a waste stream from municipal waste, mining, mineral, metallurgical, battery, agro-industrial or chemical processing, refining and / or manufacturing.

[0032] Possibly, the process liquid comprises other salts which may have different crystallisation temperatures. Possibly, the method includes the steps of sequentially cooling the process liquid to each of the different crystallisation temperatures. Possibly, at each crystallisation temperature, the respective salt is separated from the process liquid.

[0033] Possibly, the method includes the step or steps of holding the process liquid at the or each respective crystallisation temperature for a respective holding time, to permit crystallisation to occur. Possibly, the holding time is dependent on the salt.

[0034] Possibly, the secondary salt comprises ammonium sulphate (“AS”), which may have a secondary crystallisation temperature of no higher than -15 °C, possibly no higher than 18°C and may be no lower than -25 °C, possibly no lower than -20°C and is desirably 19°C.

[0035] Possibly, the process liquid is a waste stream comprising fertiliser residues, agro-industrial brines and chemical effluents.

[0036] Possibly, the process liquid is derived from processing fire extinguisher powder (“FEP”), and possibly the FEP comprises expired FEP.

[0037] Possibly, in deriving the process liquid, the FEP has been processed to remove silicone oil by a process which does not use alcohol.

[0038] Possibly, the process liquid comprises water, MAP and AS. Possibly, the process liquid is substantially free of silicone oil, which may comprise less than 1 % w / w, desirably less than 0.5% w / w and optimally less than 0.1 % w / w.

[0039] Possibly, the process liquid is substantially free of alcohol, which may comprise less than 5% w / w, desirably less than 2% w / w and optimally less than 1 % w / w.

[0040] Possibly, the crystals forming at the MAP crystallisation temperature comprise MAP, and may comprise a greater proportion of MAP than AS.

[0041] Possibly, the MAP crystallisation temperature is a eutectic temperature at which both MAP crystals and ice are formed. Possibly, the MAP eutectic temperature varies depending on the initial concentration of MAP in the process liquid.

[0042] Possibly, at initial lower concentrations of no more than 10% w / w, the MAP eutectic temperature is in the range of -3°C to -6°C.

[0043] Possibly, if the initial concentration of MAP in the process liquid is at least 25% w / w, the eutectic temperature can increase to 9° C.

[0044] Possibly, the eutectic concentration of the MAP is in the range of 16% to 20% w / w of the process liquid.

[0045] Possibly, after cooling to the MAP crystallisation temperature, the crystals formed at the MAP crystallisation temperature are removed, possibly, by a separation process, which may comprise skimming, filtration, gravity separation and / or centrifuging.

[0046] Possibly, the remaining process liquid is further cooled in a second crystalliser.

[0047] Possibly, in the second crystallisation step, the crystals forming at the second crystallisation temperature comprise AS, and may comprise a greater proportion of AS than MAP. Possibly, the secondary crystallisation temperature is a eutectic temperature in the range of -18°C to -20°C. The eutectic concentration of the AS is in the range of 37% to 41 % w / w of the remaining process liquid.

[0048] Possibly, after cooling to the second crystallisation temperature, the crystals formed at the second crystallisation temperature are removed, possibly, by a separator in a separation process, which may comprise skimming, filtration, gravity separation and / or centrifuging.

[0049] Possibly, the method further comprises drying and purifying the separated MAP crystals to achieve high purity.

[0050] Possibly, the method is for recovering MAP derived from expired FEP.

[0051] Possibly, the method includes mixing the FEP with water, possibly to form a soluble component, which may comprise an aqueous solution of MAP and AS and an insoluble component.

[0052] Possibly, the method includes separating the soluble component from the insoluble component, possibly by filtration.

[0053] Possibly, the soluble component comprises the process liquid.

[0054] Possibly, in the separating step, the crystallized MAP is separated from the remaining solution using filtration or centrifugation.

[0055] Possibly, no heat is applied to the process liquid to effect the separation.

[0056] According to a second aspect of the present invention, there is provided a mono ammonium sulphate solution separation method for separating mono ammonium phosphate (“MAP”) in aqueous solution, the method including : • a receiving step in which a process liquid which comprises water and MAP is received;

[0057] • a cooling step in which a controlled cooling process is applied to the process liquid to induce the crystallization of MAP;

[0058] • a separating step in which the crystallised MAP is separated from the remaining process liquid.

[0059] According to a third aspect of the present invention, there is provided a liquid processing apparatus for carrying out the method described above, the apparatus including: a cooling system capable of implementing various cooling techniques; a crystallization unit for MAP; a separation unit for recovering the MAP crystals.

[0060] The apparatus may be arranged to provide continuous crystallisation of the MAP.

[0061] Possibly, the apparatus includes a mixing tank for dissolving the fire extinguisher powder.

[0062] Possibly, the apparatus includes any of the features described in any of the preceding statements or following description. Possibly, the method includes any of the steps described in any of the preceding statements or following description.

[0063] Figures

[0064] Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:-

[0065] Fig. 1 is a block diagram showing steps in a liquid separation method;

[0066] Fig. 2 is a block diagram of liquid separation apparatus. Description

[0067] Referring to the drawings, a liquid separation method 10 for separating mono ammonium phosphate (“MAP”) 20 in aqueous solution, includes:

[0068] • a receiving step in which a process liquid which comprises water and MAP is received;

[0069] • a cooling step in which a controlled cooling process is applied to the process liquid to induce the crystallization 14 of MAP;

[0070] • a separating step 16 in which the crystallised MAP is separated from the remaining process liquid.

[0071] In the cooling step, the cooling process cools the process liquid to an MAP crystallisation temperature.

[0072] At the MAP crystallisation temperature, the process liquid is at an MAP eutectic temperature and the MAP is at an MAP eutectic concentration. The MAP eutectic concentration is in the range 17-19% w / w.

[0073] In one example, the cooling process could include flash cooling to induce rapid crystallization of MAP.

[0074] In one example, the concentration of MAP in the process liquid is no more than 10% w / w and the MAP crystallisation temperature is in the range from -6°C to 0°C.

[0075] In another example, the concentration of MAP in the process liquid is at least 25% w / w and the MAP crystallisation temperature is in the range from 0°C to 9°C.

[0076] The method could include a concentration step before the cooling step, which concentrates the MAP to greater than 10% w / w and more preferably at least 25%. At the MAP eutectic temperature, both crystalline MAP and ice form.

[0077] In the separating step, the crystallised MAP and ice are separated from the remaining process liquid by a separation process, which could comprise skimming, filtration, gravity separation and / or centrifuging.

[0078] The process liquid includes a plurality of salts and may comprise a primary salt and a secondary salt. MAP comprises the primary salt.

[0079] The secondary salt crystallises at a secondary crystallisation temperature, which is lower than the MAP crystallisation temperature.

[0080] The method includes the step of cooling the process liquid to the secondary crystallisation temperature.

[0081] At the secondary crystallisation temperature, the process liquid is at a secondary salt eutectic temperature and the secondary salt is at a eutectic concentration.

[0082] The step of cooling the process liquid to the secondary crystallisation temperature occurs after the step of cooling the process liquid to the MAP crystallisation temperature, and after the step of separating the crystallised MAP and ice from the process liquid.

[0083] The method includes a secondary separating step, after the step of cooling the process liquid to the secondary crystallisation temperature, to separate the secondary salt and ice from the process liquid.

[0084] The remaining process liquid could be processed further or recycled to the process liquid feed.

[0085] The secondary salt could comprise one of potassium salts, sodium salts, calcium salts, magnesium salts, ammonium salts and / or rare-earth phosphates. The process liquid could be a waste stream, possibly a waste stream from municipal waste, mining, mineral, metallurgical, battery, agro-industrial or chemical processing, refining and / or manufacturing.

[0086] The method includes the step or steps of holding the process liquid at the or each respective crystallisation temperature for a respective holding time, to permit crystallisation to occur. The holding time is dependent on the salt.

[0087] In one example, the secondary salt comprises ammonium sulphate (“AS”), which may has a secondary crystallisation temperature of no higher than -15°C, possibly no higher than 18°C and may be no lower than -25 °C, possibly no lower than -20°C and is desirably 19°C.

[0088] The process liquid could be a waste stream comprising fertiliser residues, agroindustrial brines and chemical effluents.

[0089] Process Liquid derived from FEP

[0090] In a further example, the process liquid is derived from processing fire extinguisher powder (“FEP”), and could comprise expired FEP.

[0091] Desirably, in deriving the process liquid, the FEP has been processed to remove silicone oil by a process which does not use alcohol.

[0092] In this example, the process liquid comprises water, MAP and AS.

[0093] The process liquid is substantially free of silicone oil, which comprises less than 1 % w / w, desirably less than 0.5% w / w and optimally less than 0.1 % w / w.

[0094] The process liquid is substantially free of alcohol, which comprises less than 5% w / w, desirably less than 2% w / w and optimally less than 1 % w / w. The crystals forming at the MAP crystallisation temperature comprise MAP, and comprise a greater proportion of MAP than AS.

[0095] The MAP crystallisation temperature is a eutectic temperature at which both MAP crystals 20 and ice 18 are formed. This eutectic temperature can vary depending on the initial concentration of MAP in the process liquid.. At lower concentrations of no more than 10% w / w, the MAP eutectic temperature is in the range of -3°C to -6°C.

[0096] If the initial concentration of MAP in the process liquid is at least 25% w / w, the eutectic temperature can increase to 9° C.

[0097] The eutectic concentration of the MAP is in the range of 16% to 20% w / w of the process liquid.

[0098] After cooling to the MAP crystallisation temperature, the crystals formed at the MAP crystallisation temperature are removed, by a separator 116 in a separation process 16, which may comprise skimming, filtration, gravity separation and / or centrifuging.

[0099] The remaining process liquid is further cooled in a second crystalliser 122.

[0100] In the second crystallisation step 22, the crystals forming at the second crystallisation temperature comprise AS, and may comprise a greater proportion of AS than MAP.

[0101] In this example, the secondary crystallisation temperature is a eutectic temperature in the range of -18°C to -20°C. The eutectic concentration of the AS is in the range of 37% to 41 % w / w of the remaining process liquid.

[0102] After cooling to the second crystallisation temperature, the crystals formed at the second crystallisation temperature are removed, by a separator 124 in a separation step 24, which may comprise skimming, filtration, gravity separation and / or centrifuging.

[0103] The method further comprises drying and purifying the separated MAP crystals to achieve high purity.

[0104] Advantageously, no heat is applied to the process liquid to effect the separation and the energy costs in comparison with evaporative separation are significantly lower.

[0105] Further discussion of FEP derived methods

[0106] 1 ) The FEP is processed to remove silicone oil.

[0107] The solid feed is typically ABC40 FEP: ~30-45% w / w MAP (target ~40%), balance largely AS, plus minor insolubles (silicone-treated fillers).

[0108] The clarified process liquor after dissolution / filtration is generally ~50-90% more exact ~65-90% water more exact: 75-90% (TDS ~10-25% w / w), depending on batch and any pre-concentration. For EFC we may preconcentrate to bring MAP into its crystallisation window (so MAP can nucleate near 0 °C or even above 0 °C).

[0109] Typical out going (non-concentrated) ranges seen in our analyses of our current liquid product:

[0110] • MAP (as NH4H2PO41 reported as P2O5): ~4-10% w / w equivalent

[0111] • AS (reported as S042-1 SO3): ~6-11 % w / w equivalent

[0112] — Balance water + traces — water ~80-90% w / w

[0113] For MAP-first EFC, we target the MAP eutectic domain (~18% w / w MAP); with ~2.5x pre-concentration, MAP has crystallised for us at ~+9 °C (details below).

[0114] 2) Alcohols (methanol / ethanol) in the process feed No added alcoholsA / OCs in our process. We run a water-only dissolution / clarification pathway specifically to avoid solvent hazards and freezing-point depression.

[0115] 3) Silicone oil content

[0116] FEP contains silicone oils / additives (to keep it free-flowing). After dissolution & filtration, silicone partitions mainly to the insoluble fraction and the clarified filtrate which comprises the process liquid is <1 % w / w silicone oil, typically much lower, so substantially free of silicone oil (<1 % w / w, desirably <0.5%, optimally <0.1 %).

[0117] 4) The Applicant surprisingly realised that the eutectic behaviour of solutions could be used as an alternative to evaporative separation. The invention lies in the recognition and exploitation of specific eutectic behaviours of phosphate systems, more specifically MAP systems, more specifically in the MAP-AS- H2O system, applied to FEP-derived liquors:

[0118] In single salt systems (salt-water) eutectics MAP-H2O (NH4H2PO4-water): eutectic at « 18 w / w MAP with ice at « -6 °C.

[0119] AS-H2O ((NH4)2SO4-water): eutectic at « 39 wt% AS with ice at « -19 °C.

[0120] These are the “theoretical” binary eutectic points commonly used as anchors for design. When both MAP and AS are in the same liquor (ternary system) You no longer have a single “point” per salt; you have phase fields and isotherms across composition-temperature space. The order of crystallisation still holds: at compositions relevant to FEP derived liquors, MAP crystallises first at warmer temps, and AS requires deeper sub-zero.

[0121] MAP crystallises first at relatively warm sub-zero (« -6 °C at ~18% w / w MAP); with pre-concentration, MAP crystallised for us at ~+9 °C.

[0122] AS crystallises later at deeper sub-zero (« -19 °C at ~39% w / w AS). This sequential crystallisation delivers two separate high-purity products (MAP then AS) directly from FEP liquors, avoiding mixed cakes typical of evaporative crystallisers.

[0123] From a phase-diagram standpoint: both binary boundaries (MAP / ice and AS / ice) and the ternary isotherms govern the windows. Our contribution is defining the practical temperature-composition domains and process steps (including pre-concentration and washing protocols) that make MAP-first separation work on EFEP liquors (which carry silicone traces and other impurities) — something not taught / obvious in prior EFC literature focused on other salts.

[0124] 5) Key experimental findings:

[0125] Batch / static trials on FEP-derived liquors confirm:

[0126] • MAP appears first; AS only at lower temp (some AS observed by -16 °C).

[0127] • ~2.5x pre-concentration enabled MAP crystallisation at ~+9 °C (no ice burden), basically up to this point we can take water out (evap)

[0128] • Washing raised MAP purity; K shows anomalous retention (potential cocrystallisation with ammonium dihydrogen phosphate).

[0129] • Recommendation: continuous pilot for operability / selectivity validation.

[0130] 6) Synergy between Phos Cycle process and EFC

[0131] The PhosCycle process uniquely produces the process liquid which is clean, being free from alcohol and silicone oil which reduces fouling and improves MAP-first selectivity.

[0132] 7) Washability / impurity protocols tailored to MAP from FEP

[0133] Ca / Mg / Na wash out readily while K shows anomalous retention (cocrystallisation risk). The washing / recrystallisation steps achieve technical- grade MAP (which is at 95%) even though we have also obtained 99% purity and clean AS. 8) Other applications

[0134] The same principle can be used to separate:

[0135] • MAP from other salts;

[0136] • MAP from AS, Na2SO4, K2SO4, DAP, and struvite-forming systems

[0137] • Production of technical-grade salts (MAP, AS, SOP, Na2SO4, struvite / MAP 6H2O) from multi-salt waste liquors by EFC, with >95% purity.

[0138] • phosphate / sulfate-rich brines, including fertiliser mother liquors, food / pharma phosphates, and battery-grade phosphoric acid processes.

[0139] • phosphate and sulfate salts suitable for fertiliser and technical / industrial applications (detergents, flame retardants, food-grade phosphates).

[0140] • multi-salt brines with eutectics >10 °C apart, enabling selective crystallisation of at least one phosphate salt.

[0141] • rare-earth- and transition-metal-containing brines (including WPA pond waters, red mud leachates, magnet recycling liquors), selectively crystallising bulk salts while concentrating REE fractions.

[0142] Advantageously, the use of EFC (eutectic freeze crystallisation) reduces energy intensity of phosphate recovery compared to evaporation.

[0143] Other Modifications

[0144] Various other modifications could be made without departing from the scope of the invention.

[0145] Any of the features or steps of any of the embodiments shown or described could be combined in any suitable way, within the scope of the overall disclosure of this document.

[0146] Final Remarks

[0147] There is thus provided a liquid processing method with a number of advantages over conventional methods. In particular, the Applicant has realised that the difference in eutectic points for AS and MAP solutions can be exploited to separate AS from MAP and provide AS and MAP in crystalline form of relatively high purity. The process described avoids the heating and cooling of conventional processes and therefore is energy efficient and cheaper.

Claims

1. ClaimsA liquid separation method for separating mono ammonium phosphate (“MAP”) in aqueous solution, the method including :• a receiving step in which a process liquid which comprises water and MAP is received;• a cooling step in which a controlled cooling process is applied to the process liquid to induce the crystallization of MAP;• a separating step in which the crystallised MAP is separated from the remaining process liquid.

2. A method according to any one of the preceding claims, in which in the cooling step, the cooling process cools the process liquid to an MAP crystallisation temperature.

3. A method according to any one of the preceding claims, in which, at the MAP crystallisation temperature, the process liquid is at an MAP eutectic temperature and the MAP is at an MAP eutectic concentration.

4. A method according to any one of the preceding claims, in which the MAP eutectic concentration is in the range 17-19% w / w.

5. A method according to any one of the preceding claims, in which the cooling process includes flash cooling to induce rapid crystallization of MAP.

6. A method according to any one of the preceding claims, in which the concentration of MAP in the process liquid is no more than 10% w / w and the MAP crystallisation temperature is in the range from -6°C to 0°C.

7. A method according to any one of the preceding claims, in which the concentration of MAP in the initial process liquid is at least 25% and the MAP crystallisation temperature is in the range from 0°C to 9°C.

8. A method according to any one of the preceding claims, in which the method includes a concentration step before the cooling step, which concentrates the MAP to greater than 10% w / w and more preferably at least 25%.

9. A method according to any one of the preceding claims, in which at the MAP eutectic temperature, both crystalline MAP and ice form.

10. A method according to any one of the preceding claims, in which in the separating step, the crystallised MAP and ice is separated from the remaining process liquid by a separation process, which may comprise skimming, filtration, gravity separation and / or centrifuging.

11. A method according to any one of the preceding claims, in which the process liquid includes a plurality of salts and may comprise a primary salt and a secondary salt.

12. A method according to any one of the preceding claims, in which MAP comprises the primary salt.

13. A method according to any one of the preceding claims, in which the secondary salt crystallises at a secondary crystallisation temperature, which may be lower than the MAP crystallisation temperature.

14. A method according to any one of the preceding claims, in which the method includes the step of cooling the process liquid to the secondary crystallisation temperature.

15. A method according to any one of the preceding claims, in which at the secondary crystallisation temperature, the process liquid is at a secondary salt eutectic temperature and the secondary salt is at a eutectic concentration.

16. A method according to any one of the preceding claims, in which the step of cooling the process liquid to the secondary crystallisation temperature occurs after the step of cooling the process liquid to the MAP crystallisation temperature, and after the step of separating the crystallised MAP and ice from the process liquid.

17. A method according to any one of the preceding claims, in which the method includes a secondary separating step, after the step of cooling the process liquid to the secondary crystallisation temperature, to separate the secondary salt and ice from the process liquid.

18. A method according to any one of the preceding claims, in which the secondary salt comprises one of potassium salts, sodium salts, calcium salts, magnesium salts, ammonium salts and / or rare-earth phosphates.

19. A method according to any one of the preceding claims, in which the process liquid is a waste stream, possibly a waste stream from municipal waste, mining, mineral, metallurgical, battery, agro-industrial or chemical processing, refining and / or manufacturing.

20. A method according to any one of the preceding claims, in which the process liquid comprises other salts which may have different crystallisation temperatures.

21. A method according to any one of the preceding claims, in which the method includes the steps of sequentially cooling the process liquid to each of the different crystallisation temperatures.

22. A method according to any one of the preceding claims, in which at each crystallisation temperature, the respective salt is separated from the process liquid.2123. A method according to any one of the preceding claims, in which the method includes the step or steps of holding the process liquid at the or each respective crystallisation temperature for a respective holding time, to permit crystallisation to occur.

24. A method according to any one of the preceding claims, in which the holding time is dependent on the salt.

25. A method according to any one of the preceding claims, in which the secondary salt comprises ammonium sulphate (“AS”), which may have a secondary crystallisation temperature of no higher than -15 °C, possibly no higher than 18°C and may be no lower than -25 °C, possibly no lower than -20°C and is desirably 19°C.

26. A method according to any one of the preceding claims, in which the process liquid is a waste stream comprising fertiliser residues, agroindustrial brines and chemical effluents.

27. A method according to any one of the preceding claims, in which the process liquid is derived from processing fire extinguisher powder (“FEP”), and possibly the FEP comprises expired FEP.

28. A method according to any one of the preceding claims, in which the process liquid comprises water, MAP and AS.

29. A method according to any one of the preceding claims, in which the process liquid is substantially free of silicone oil, which may comprise less than 1 % w / w, desirably less than 0.5% w / w and optimally less than 0.1 % w / w.

30. A method according to any one of the preceding claims, in which the process liquid is substantially free of alcohol, which may comprise less than 5% w / w, desirably less than 2% w / w and optimally less than 1 % w / w.2231. A method according to any one of the preceding claims, in which the crystals forming at the MAP crystallisation temperature comprise MAP, and may comprise a greater proportion of MAP than AS.

32. A method according to claim 1 , in which the MAP crystallisation temperature is a eutectic temperature at which both MAP crystals and ice are formed.

33. A method according to claim 1 , in which the MAP eutectic temperature varies depending on the initial concentration of MAP in the process liquid.

34. A method according to claim 1 , in which at lower initial concentrations of no more than 10% w / w, the MAP eutectic temperature is in the range of -3°C to -6°C.

35. A method according to claim 1 , in which if the initial concentration of MAP in the process liquid is at least 25% w / w, the eutectic temperature can increase to 9° C.

36. A method according to claim 1 , in which the eutectic concentration of the MAP is in the range of 16% to 20% w / w of the process liquid.

37. A method according to any one of the preceding claims, in which after cooling to the MAP crystallisation temperature, the crystals formed at the MAP crystallisation temperature are removed by a separation process, which may comprise skimming, filtration, gravity separation and / or centrifuging.

38. A method according to any one of the preceding claims, in which the crystals forming at the second crystallisation temperature comprise AS, and may comprise a greater proportion of AS than MAP.2339. A method according to any one of the preceding claims, in which the secondary crystallisation temperature is a eutectic temperature in the range of -18°C to -20°C and the eutectic concentration of the AS is in the range of 37% to 41 % w / w of the remaining process liquid.

40. A method according to any one of the preceding claims, in which after cooling to the second crystallisation temperature, the crystals formed at the second crystallisation temperature are removed, A method according to any one of the preceding claims, in which by a separation process, which may comprise skimming, filtration, gravity separation and / or centrifuging.41 . A method according to any one of the preceding claims, in which the method further comprises drying and purifying the separated MAP crystals to achieve high purity.

42. A method according to any one of the preceding claims, in which the method is for recovering MAP from FEP.

43. A method according to any one of the preceding claims, in which the method includes mixing the FEP with water, possibly to form a soluble component, which may comprise an aqueous solution of MAP and AS and an insoluble component.

44. A method according to any one of the preceding claims, in which the method includes separating the soluble component from the insoluble component, possibly by filtration.

45. A method according to any one of the preceding claims, in which the soluble component comprises the process liquid.

46. A method according to any one of the preceding claims, in which in the separating step, the crystallized MAP is separated from the remaining solution using filtration or centrifugation.2447. A method according to any one of the preceding claims, in which no heat is applied to the process liquid to effect the separation.

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