TREATMENT OF WASTEWATER CONTAINING PHOSPHATE AND FINES CONTROL PROCEDURES

Abstract

Methods and apparatus are provided for precipitating dissolved materials from an aqueous solution. In one embodiment, the method comprises introducing the aqueous solution into a reactor and introducing a magnesium (Mg) source into the reactor in a sufficient quantity to cause the dissolved materials to precipitate into crystals. The Mg source is introduced into the reactor in the form of particles of a Mg-containing material. The Mg source has a solubility in the aqueous solution of less than approximately 1 g / L. Alternatively, the concentration of Mg in the reactor is less than approximately 0.03 mol / L. In one embodiment, the apparatus comprises a reaction tank having an inlet and an outlet, and a hydration tank associated with the reaction tank and configured to hydrate a Mg source in an aqueous solvent, and introducing the Mg source as a hydrated suspension into the reaction tank.

Description

TREATMENT OF WASTEWATER CONTAINING PHOSPHATE AND FINES CONTROL PROCEDURES FIELD OF INVENTION The invention relates to apparatus and methods for precipitating dissolved materials. Some embodiments provide apparatus and methods for crystallizing materials such as struvite from aqueous solutions such as wastewater or water from industrial processes. For example, some embodiments relate to apparatus and methods for precipitating dissolved materials to form crystals while controlling for fines. BACKGROUND OF THE INVENTION Reactors in general, and fluidized bed reactors in particular, have been used to remove and recover phosphorus from solutions such as wastewater and industrial process water. Aqueous solutions from some sources contain significant concentrations of phosphorus, often in the form of phosphate. Such aqueous solutions can originate from a wide variety of sources. These include sources such as landfill leachate, agricultural runoff, industrial process effluents, industrial process water, municipal wastewater, animal waste, phosphogypsum pond water, and the like. If released into the environment without treatment, such aqueous solutions can result in excessive levels of phosphorus in receiving waters. Several phosphorus removal and recovery technologies exist. Some of these technologies utilize fluidized bed reactors to remove phosphorus from aqueous solutions by producing struvite (MgNH4PO4·6H2O) or a struvite analogue or phosphate compound in granular form. Magnesium can be added to the reactor to form struvite. Struvite can be formed by the following reaction: πλ 171 η / ηζηζ / Ε / γίΛΐ Mg2++ NH4++ PO43' + 6H2O ~ MgNH4PO4»6H2O Koch et al., Fluidized Bed Wastewater Treatment, US Patent No. e7622047, describes examples of reactors and procedures that can be applied to remove and recover phosphorus from aqueous solutions. One difficulty that sometimes arises in crystallization reactions is that the particle sizes produced by the reaction may not be as desired. For example, under certain operating conditions, a reactor may produce a large quantity of very small crystals (fines) instead of the larger crystals that are desired. Excessive fines production can result in low phosphate capture, as the fines can be carried away with the effluent from a crystallization reactor. Crystal sizes are affected by a wide variety of factors, including flow conditions, chemical composition, temperature, and so on. For example, if the loading rate (i.e., the mass of phosphorus (PPO4) added to a reactor (or portion thereof) per unit time) is too high, then undesirable fines will form.Load limitations affect the volume of aqueous solution that can be treated per unit of time, thereby affecting struvite production over time. References describing various crystallization procedures include: US8245625; US7942939; WO2006082341; US6946572; US6364914; WO9837938; US4666527; US3419899; US2209019; US4159194; US4263010; US5124265; US6660049; US5663456; AU2004320909; WO2012022099; WO2012134255. There remains a need for efficient reactors and processes for removing and recovering dissolved materials from solutions. There is a particular need for effective reactors and processes suitable for producing large particles of slightly soluble substances such as struvite, struvite analogues, and calcium phosphate. The preceding examples of the related technique and its associated limitations are intended to be illustrative and are not exhaustive. Other limitations of the related technique will become apparent to those skilled in the field after reviewing the descriptive report and examining the drawings. ηκ 171 η / η7Π7 / Ε / γΐΛΐ BRIEF DESCRIPTION OF THE INVENTION The following embodiments and aspects thereof are described and illustrated, along with apparatus, systems, tools, and procedures intended as examples and illustrations, without limiting their scope. In various embodiments, one or more of the problems described above have been reduced or eliminated, while other embodiments are aimed at other improvements. The aspects of the present invention relate to apparatus and methods for precipitating dissolved materials. The apparatus and methods have application, for example, in the growth of crystals of materials such as struvite, struvite analogues, and calcium phosphate. The apparatus and procedures, according to some embodiments, work to grow particles while avoiding or minimizing the production of fines by injecting an aqueous suspension of a low-solubility magnesium (Mg) source. By maintaining a low concentration of fines, the growth rates of larger particles can be improved, and phosphorus loss through fines can be minimized or eliminated. Some aspects of the invention provide methods for operating a reactor under high-growth conditions. For example, high-growth conditions may correspond to the loading conditions of a substance being produced. In some embodiments, the loading is above a threshold to achieve a high crystal growth rate. For example, in some embodiments, the loading may be 5 g PO4-P / min / m3 or 50 g PO4-P / min / m3 or 100 g PO4-P / min / m3 or 250 g PCU-P / min / m3 in a reactor (or in a portion of the reactor). In some embodiments, a supersaturation ratio (the ratio of the product of the concentrations of the substance's constituents to the product of their equilibrium concentrations) is above a threshold to achieve a high crystal growth rate. For example, in some embodiments, the supersaturation ratio for struvite or another crystallizing material may be 2 or more, 3 or more, or 5 or more in the reactor. In some embodiments, the substance is sparingly soluble in aqueous solution. The fines concentration can be kept below a threshold, thereby maintaining a growth rate of larger particles, by injecting an aqueous suspension of a low-solubility magnesium (Mg) source into the reactor (or a portion thereof) as described herein. In some embodiments, the procedure for precipitating dissolved materials from an aqueous solution involves introducing the aqueous solution containing the dissolved materials into a reactor and introducing a magnesium (Mg) source into the reactor. The Mg source is introduced in a sufficient quantity to cause the materials dissolved in the aqueous solution to precipitate as crystals. The Mg source may, for example, be in the form of particles of a Mg-containing material. The Mg source may, for example, have a solubility in the aqueous solution of less than approximately 1 g / L, or the concentration of Mg in the reactor that is available for the reaction to produce struvite (including the solids in the particles of the Mg-containing material) may be less than approximately 0.03 mol / L. In some embodiments, the Mg source is introduced into the reactor as a suspension. The suspension can be prepared by adding water to the Mg source and allowing the Mg source to soak for a hydration time before introducing the hydrated suspension into the reactor. The Mg source may, for example, have an aqueous solvent solubility of approximately 5 mg / L, approximately 500 mg / L, or approximately 150 mg / L. The concentration of Mg in the reactor may be approximately 0.1 mmol / L or approximately 0.03 mol / L after the introduction of the suspension. The Mg source may, for example, have a particle size of less than 50 µm, or from approximately 10 µm to approximately 30 µm, or from approximately 100 mesh to approximately 400 mesh, or from approximately 17 SGN to 100 SGN, for example. In some embodiments, the procedure further includes maintaining the aqueous solution at a pH above approximately 7. In some embodiments, the pH of the aqueous solution is maintained by adding an acid or a base to the hydrated suspension. In some embodiments, the pH of the aqueous solution is maintained by controlling the amount of the Mg source present in the aqueous solution. The amount of the Mg source present in the aqueous solution can be controlled by the following steps: measuring the pH of the aqueous solution in real time, comparing the measured pH with a target pH (or reference pH value), and adjusting the pH by introducing the Mg source into the aqueous solution so that the pH of the aqueous solution is controlled to be equal to or close to the target pH. In some embodiments, the Mg source is introduced into the reactor at a predetermined time interval, such as intervals of approximately 30 seconds to approximately 5 minutes. In some embodiments, the Mg source is a source of low solubility. The source of low solubility Mg may, for example, be MgO, Mg(OH)2, or a magnesium carbonate. Examples of magnesium carbonate may include one or more anhydrous salts of magnesite (MgCO3), barringtonite (MgCO3*2H2O), nesquehonite (MgCO3*3H2O), lansfordite (MgCO3*5H2O), artinite (MgCO3*Mg(OH)2·3H2O), hydromagnesite (4 MgCO3Mg(OH)2«4H2O), dipingite (4 MgCO3Mg(OH)2«5H2O), dolomitic lime and limestone. In some embodiments, the source of low-solubility Mg is MgO. The MgO can be prepared in any suitable manner. In some embodiments, the MgO is prepared at a calcination temperature for a period of time sufficient to produce MgO particles. The calcination temperature can, for example, be in the range of approximately 600 °C to approximately 1200 °C. The time used to produce MgO particles can, for example, be in the range of approximately 1 to 3 hours. The hydration time can, for example, be from approximately 15 minutes to approximately 36 hours. In some embodiments, an acid is added to the hydrated suspension before introducing the suspension into the reactor. The acid can be added, for example, at approximately 0.4:1 molar equivalents. The acid can be a strong acid such as hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). In some embodiments, the acid is a weak acid such as acetic acid, citric acid, or oxalic acid. In some embodiments, the Mg source is reacted with the aqueous solution for a reaction time of approximately 30 minutes to approximately 60 minutes. In some embodiments, the aqueous solution has a phosphorus concentration in the range of approximately 40 mg / l to approximately 10,000 mg / l. In some embodiments, a highly soluble Mg source such as MgCb or MgSO4 is introduced into the reactor in addition to the low-solubility Mg source. The highly soluble and low-solubility Mg sources can be introduced into the reactor simultaneously. Some aspects of the invention provide a fluidized bed reactor for precipitating dissolved materials from an aqueous solution. The fluidized bed reactor may comprise a reaction tank and a hydration tank. The reaction tank may include an inlet and an outlet. The hydration tank may be associated with the reaction tank and configured to hydrate a magnesium (Mg) source in an aqueous solvent and introduce the Mg source as a hydrated suspension into the reaction tank. In some embodiments, the reactor further includes a control valve. The control valve may be configured to regulate the flow of the hydrated suspension from the hydration tank to the reaction tank. In some embodiments, the reactor further includes a recycle path.The recycling route can be configured to take a solution from one part of the reaction tank and return at least a portion of the extracted solution to another part of the reaction tank. The recycling route may optionally include a fines treatment tank. In some embodiments, a solids separation device is located upstream of the fines treatment tank. The solids separation device can be configured to separate solids from the liquid in the recycling route. In some embodiments, the reactor also includes a pH probe. The pH probe can be configured to measure the pH of the aqueous solvent in the reaction tank. In some embodiments, a controller can be configured to receive an input from the pH probe. The controller can be configured to control the opening and closing of the control valve in response to a deviation of the pH of the aqueous solvent in the reaction tank from a desired pH reference value. In some embodiments, the hydration tank includes a first acid injector. The first acid injector can be configured to controllably dose an acid into the slurry in the hydration tank. In some embodiments, the reactor includes a second acid injector. The second acid injector can be configured to controllably dose an acid into the solution stream in the recycle path. The reactor may also include a base injector. The base injector can be configured to controllably dose a base into the solution stream in the reaction tank. In some embodiments, the base injector is located downstream of the acid injector in the recycle path. Another aspect provides a method for producing struvite or a struvite analogue. The method comprises providing a reactor vessel in which, in at least a portion of the reactor vessel, the cross-sectional area of ​​the reactor vessel increases with elevation, and a fluidized bed of size-separated granules is maintained in the reactor vessel portion by flowing a phosphate solution upward through the reactor portion. The upward flow velocity of the flowing solution decreases with elevation in the reactor portion. The method introduces fine particles of a low-solubility magnesium source into the reactor and allows the fine particles to disperse in the fluidized bed. Some of the granules are removed from the fluidized bed. The fine particles may comprise, for example, magnesium oxide.Fine particles can have, for example, sizes of SGN 100 or less. Fine particles can have, for example, diameters of 0.1 mm or less. In this procedure, the pH may be maintained at a reference pH value equal to or greater than 7.5 in at least part of the reactor vessel. The reference pH value may be at least pH 8 in some cases. The procedure may involve recycling the solution in the reactor vessel through a recycling route extending from a rise in the reactor vessel above the fluidized bed to a rise in the reactor vessel below the fluidized bed. Optionally, the procedure includes capturing or redissolving struvite particles in the recycling route. Fine particles of a low-solubility magnesium source can be introduced into the reactor vessel as a suspension. The suspension can be hydrated before introduction into the reactor vessel. Hydration can be achieved by mixing the fine particles with an aqueous solvent (e.g., water) and allowing the particles to remain in contact with the water for at least a few minutes before introducing the suspension into the reactor vessel. The suspension can be injected into the reactor vessel at a location below the fluidized bed. In some cases, an acid is mixed with the suspension before introducing it into the reactor vessel. The pH at a specific location within the reactor vessel can be controlled by the controlled addition of the suspension. In some cases, the suspension is buffered to an alkaline pH (e.g., at least pH 7.5 or at least approximately pH 8). In addition to the examples of aspects and realizations described above, other aspects and realizations will become evident with reference to the drawings and through the study of the following detailed descriptions. BRIEF DESCRIPTION OF THE FIGURES Examples of implementation are illustrated in the reference figures of the drawings. The implementations and figures disclosed in this document are intended to be considered illustrative rather than restrictive. FIG. 1A is a schematic diagram of an example reactor apparatus according to one example embodiment. FIG. 1B is a schematic diagram of an example reactor apparatus according to another example embodiment. FIG. 2 is a flowchart of a procedure according to an example implementation. FIG. 3 is a graph showing the relationship of phosphorus (P) removal (%) as a function of reaction time for an aqueous solution treated with ηβ I 7 I η / η7P7 / E / YILI sources of Mg. FIG. 4 is a graph showing the effect of temperature on the hydration of an aqueous suspension of MgO. FIG. 5 is a graph showing the effect of aqueous suspensions of Mg sources on P removal (%). FIG. 6 is a graph showing the effect of aqueous suspensions of Mg sources on Mg concentration. FIG. 7 is a graph showing the relationship of P removal (%) as a function of hydration time for aqueous solutions of Mg sources. FIG. 8 is a graph showing the effect of hydration temperature on the removal of P (%) by aqueous suspensions of a light burnt magnesium oxide product derived from natural magnesite ore. FIG. 9 is a graph showing the effect of hydration temperature on P removal (%) by aqueous suspensions of a synthetically derived light burnt magnesium oxide product. FIG. 10 is a graph showing the relationship of P removal (%) as a function of reaction time for aqueous suspensions of Mg sources. FIG. 11 is a graph showing the relationship of P removal (%) as a function of pH for aqueous suspensions of Mg sources. FIG. 12 is a graph showing the relationship of P removal (%) as a function of reaction time for aqueous suspensions of Mg sources. DETAILED DESCRIPTION OF THE INVENTION The following description provides specific details to offer a more complete understanding to experts in the field. However, well-known elements may not be shown or described in detail to avoid unnecessarily obscuring the information. Therefore, the description and drawings should be considered illustrative rather than restrictive. The present invention relates to the crystallization of materials from solution. Embodiments provide crystallization reactors and processes, as well as apparatus and processes for reducing the concentration of fines in crystallization reactors. Other embodiments provide crystallization reactors and processes for growing crystals of materials with low solubility (examples of which are struvite, struvite analogues, and calcium phosphate). The invention can be applied to control fines in the crystallization of any of a wide variety of chemicals from solution. The precipitation of struvite from aqueous solutions is used in this disclosure as a non-limiting example of an application of the invention. Some embodiments of the invention described below relate to crystallization reactors and processes in which magnesium is added to precipitate phosphorus from wastewater in the form of struvite or struvite analogues or a phosphate compound. For example, some embodiments provide crystallization reactors and processes in which magnesium is added to precipitate phosphorus from wastewater in the form of struvite (MgNH4PO4*6H2O) according to the following reaction: Mg2++ NH4+ + PO43' + 6H2O θ MgNH4PO4*6H2O Struvite can be in the form of granules. The granules can range in size from approximately 0.1 mm to 3 mm or larger. These examples correspond to embodiments that have significant commercial utility. The scope of the invention, however, is not limited to these examples. The expression "aqueous solution" or the term "solution" is used in the following description and claims to include aqueous solutions such as industrial and municipal wastewater, industrial process water, leachate, runoff, animal waste, effluent, phosphogypsum pond water, or the like. Some embodiments provide methods for treating municipal wastewater and / or animal waste. Some embodiments provide methods for treating other types of wastewater. Some embodiments provide methods for crystallizing materials using raw materials other than wastewater. πλ 171 η / ηζηζ / Ε / γίΛΐ The term charge is used in the following description and claims to denote the mass of phosphorus (PO4-P) added to a reactor (or portion of the reactor) per unit of time. The term saturation is used in the following description and claims to indicate a solution at its equilibrium point of the solution that cannot dissolve more than one solute under the current conditions. The term supersaturation is used in the following description and claims to indicate a solution containing more of a dissolved solute than could otherwise be dissolved by the solvent under the present conditions. One aspect of the invention relates to apparatus and methods for precipitating dissolved materials, such as struvite, struvite analogues, calcium, and phosphate. Other aspects of the invention relate to apparatus and methods for operating a reactor under high-growth conditions. In some embodiments, the reactor comprises a fluidized bed reactor. An example of a fluidized bed reactor 12 is shown in FIG. 1. The reactor 12 comprises an inlet 14, an outlet 16, and a reaction tank 18. An aqueous solution is introduced into the reaction tank 18 through the inlet 14. The inlet 14 is located below the outlet 16. The tank 18 is constructed such that the flow of aqueous solution in the reactor 12 is generally upward. Any crystals that may form in the reactor 12 will be pushed upward against the force of gravity by the flow of fluid in the reactor 12. The crystals may grow by sticking together and / or by further crystallization.When the flow rate decreases with elevation in tank 18, the particles composed of crystallization product will tend to sort by size, with the larger particles tending to settle in the lower parts of tank 18 and the smaller particles tending to settle higher up in tank 18. The aqueous solution flows into reaction tank 18 through inlet 14 and exits reaction tank 18 through outlet 16. In some embodiments, reactor 12 comprises a plurality of inlets and / or outlets. Inlet 14 may be located, for example, in or near the lower portion of the reaction tank 18. Inlet 16 may be located, for example, in or near the upper portion of the reaction tank 18. In some embodiments, inlet 14 is oriented upwards and a solution flow introduced from inlet 14 to the reaction tank 18 is directed upwards. In some embodiments, a process for preparing a crystalline product includes providing at least one ionic species that reacts to form the crystalline product from a material that produces the ionic species when dissolved and has low solubility. The material may be provided in the form of fine particles. For example, when the product is struvite, the ionic species may be magnesium ions (Mg2+), and the material may be magnesium oxide (MgO), which has low solubility in water. The MgO may be provided as a suspension of fine particles. Reactor 12 comprises a hydration tank 40. A source of low-solubility Mg is introduced into the hydration tank 40 through inlet 42. An aqueous solvent (e.g., water) is introduced into the hydration tank 40 through inlet 44. The low-solubility Mg source is hydrated with the aqueous solvent for a period of time sufficient to hydrate the Mg source to provide a hydrated suspension comprising free Mg2+ ions available in solution for the production of struvite in reaction tank 18. The hydrated suspension leaving the hydration tank 40 is supplied to reaction tank 18 through inlet 46. Inlet 46 may be located, for example, in or near the lower portion of reaction tank 18. In some embodiments, inlet 46 is directed upwards, and the flow of a hydrated suspension introduced from inlet 46 to reaction tank 18 is directed upwards.Under suitable reaction conditions, crystals (e.g., struvite crystals or other phosphorus-containing compounds in some embodiments) form in reaction tank 18 through the precipitation of materials dissolved in the solution (e.g., wastewater solution in some embodiments). The crystals may grow larger over time and can be sorted by size based on differences in fluid velocities in different regions within the reaction tank. For example, in some embodiments, the fluid flows upward in the reaction tank with a velocity that increases with depth (decreases with elevation).This can be achieved, for example, by providing a reaction tank that has a cross-sectional area that increases with elevation above the inlet and / or by providing recycling routes (described elsewhere in this document) that have outlets at different depths in the reaction tank 18. In such embodiments, the crystal-formed particles can move downwards as they increase in size (e.g., by accretion and / or aggregation with other crystals). The particles can eventually enter a collection zone in the reaction tank 18, from which they can be extracted for use as fertilizer or other applications. In some embodiments, the reaction tank 18 comprises a substantially vertically oriented conduit having a collection section and two or more sequential vertical sections above the collection section. The cross-sectional area of ​​the conduit may increase with increasing elevation from the bottom of the reaction tank 18 to the top. For example, the cross-sectional area may increase between adjacent sections. The number of conduit sections may vary. In some embodiments, the cross-sectional area increases smoothly. In some embodiments, the cross-sectional area increases gradually. In some embodiments, the reaction tank 18 is cone-shaped or horn-shaped, or otherwise configured to have a cross-sectional area that increases gradually with elevation above the bottom of the reaction tank 18.Tank 18 may have a round cross-section, but this is not necessarily the case. Inlet 14 may be located, for example, in or below the collection section. Some embodiments of the present invention may comprise a fluidized bed reactor of the type described in Britton, publication WO N.Q2012 / 119260, entitled Reactor for Precipitating Solutes from Wastewater and Associated Methods and / or of the type described in Britton, et al., publication WO N.s2015 / 003265, entitled Reactor Apparatus and Methods for Fines Control and / or of the type described in Koch and et al., United States patent N.s7622047, entitled Fluidized Bed Wastewater Treatment, which are incorporated herein by reference in their entirety for all purposes. The inventors have determined that, in some applications, it is desirable to be able to control the size of the product crystals (e.g., struvite crystals, struvite analogues, or other phosphorus-containing compounds) that form in the reactor by precipitation from dissolved materials. For example, it may be desirable to selectively precipitate and collect relatively large product crystals and / or granules (e.g., crystals and / or granules with a diameter >1 mm) when the product crystals aggregate to form product granules. One aspect of the invention relates to apparatus and procedures that provide control of fines (fines being very small crystals, for example, crystals with a diameter <100 pm can be described as fines) in a reactor. Many fines in a reactor can have sizes in the range of approximately 1 pm to approximately 10 pm. Fines can have an extremely large surface area to mass ratio compared to larger crystals. When a large number of fines are present in a crystallization reactor, a high proportion of crystal growth can occur on the surface of the fines, thereby reducing the growth rates of larger crystals. The inventors have realized that the production of fines in a reactor can be due to zones of high supersaturation (exceeding metastable limits and resulting in primary or secondary nucleation) or to the accumulation of previously formed crystals within the reactor. The production of fines could be reduced by operating a reactor with very low supersaturation. However, crystals may grow slowly under such conditions.It is desirable to be able to control the accumulation of fines within the reactor, especially when large product crystal sizes are desired (e.g., >1 mm). The inventors have realized that a reactor can operate under increasing load conditions by increasing the concentrations of one or more of the constituents of the struvite product, a struvite analogue, or another phosphorus-containing compound. For example, increasing the concentration of free Mg2+ ions can increase crystal growth. During the operation of the struvite crystallizer, and especially when operating with feedstock solutions with high phosphate concentrations (>100 mg / L of feedstocks of PO4-P and particularly >2,000 mg / L of PO4-P), and / or prolonged hydraulic retention times (>1 hour and particularly >12 hours) in a fluidized bed reactor (such as reactor 12), fines (crystals with a diameter <100 µm) have been found to accumulate in the reactor. In some cases, within a 6- to 12-hour operating period, almost all crystal formation / growth can occur as fines, either by primary / secondary nucleation or due to growth occurring mainly on the surface of existing fines retained in the reactor.This phenomenon is believed to occur as a result of a combination of increased secondary nucleation in the presence of high levels of fines combined with the overwhelming majority of the crystal surface area in the reactor being on the surface of the fines (which have much higher surface area to volume ratios than the larger, more desirable crystals with diameters of, for example, 1-5 mm). The rate at which fines accumulate can be controlled to some extent (and the period before uncontrolled fines production begins could be extended) by reducing the crystallization reaction rate and / or the supersaturation ratio and / or the loading and / or concentration of magnesium added to the reactor. However, if significant amounts of fines are present in the reactor (e.g., >5 mg / L of settled fines measured in the reactor's recycle stream, or turbidity >500 NTU), further increase in crystal size distribution or the growth of large crystals could be significantly affected by the presence of a large amount of fines. Typically, struvite crystal recovery procedures use soluble sources of Mg, such as magnesium chloride (MgCk) and / or magnesium sulfate (MgSO4), as a source of free Mg2+ for struvite production. nR 171 η / η7Π7 / E / γΐΛΐ Due to the high solubility of these Mg sources (MgCl₂ has a water solubility of approximately 54 g / L), the introduction of MgCb and / or MgS₄ leads almost immediately to a higher concentration of Mg²⁺ ions, increasing the supersaturation ratio in the localized area where the Mg source was introduced. Consequently, areas of high supersaturation can occur in the reactor (or portions of the reactor), thus producing fines, as described elsewhere in this document. For example, when the Mg source is MgCb, a portion (e.g., approximately 10–50%) of the struvite crystals produced may be present as fine particles (i.e., <20 µm in diameter). Such fines are too small to settle and be effectively retained in the reactor and therefore escape from the reactor and are lost with the treated effluent (i.e., fines loss). The inventors have empirically determined that when a Mg source with lower solubility than conventional soluble magnesium sources (e.g., MgCk and / or MgSO4) is used, the production of fines is eliminated or reduced, even with increased loading. Accordingly, one aspect of the invention provides methods comprising injecting a low-solubility Mg source into the reactor for the recovery of struvite crystals. In some embodiments, the processes include a hydration step of a low-solubility Mg source before injecting the resulting hydrated suspension into the reactor (or a portion of the reactor). The free Mg²⁺ ions are then available to react and form struvite in the reactor. For example, the processes may operate by forming an aqueous suspension of a source that releases Mg²⁺ ions upon dissolution. The Mg source can be mixed with an aqueous solvent for a period of time (i.e., the hydration time). In some embodiments, the hydrated suspension is circulated from a suspension tank around a circuit and back to the suspension tank. The fluid velocity in the circuit can be maintained high enough to prevent sedimentation of suspension particles within the circuit. The circuit can be arranged to pass close to the reactor. Injection of the hydrated suspension into the reactor can involve diverting a portion of the hydrated suspension flowing in the circuit into the reactor, for example, by means of a valve. FIG. 2 is a flow diagram of a process 100 according to an exemplary embodiment of the invention. Process 100 comprises step 110 of hydrating a low-solubility Mg source in an aqueous solvent and step 120 of injecting the resulting hydrated suspension into a reactor (or a portion of the reactor) to precipitate the dissolved materials (including phosphorus (PO4-P)) from an aqueous solution to form crystals. In some embodiments, the source of Mg is magnesium oxide (MgO). As shown in the reaction below, upon hydration, MgO forms a MgOH+ complex on the surface of the MgO particle, ultimately leading to the formation of 2 mol of OH(aq): MgO(surface) + H+(aq) —> MgOH+(surface) + OH (aq) —>· MgOH+»OH' (surface) —> Mg2+(aq) + 2OH'(aq) θ Mg(OH)2(s) The formation of 2OH(aC) increases and / or maintains the pH of the solution in the reactor at a level suitable for crystal formation and growth and, in some embodiments, reduces and / or eliminates the total consumption of the caustic substance for crystal precipitation in the reactor. In some embodiments, the use of a low-solubility Mg source, such as MgO or Mg(OH)2, can replace the need for a caustic substance (e.g., NaOH), thereby reducing or eliminating pH / supersaturation peaks at a caustic substance injection site in the reactor (or a portion of the reactor). This can reduce overall capital and / or operating costs by eliminating or reducing the need for a separate pH control reagent and the associated storage and dosing systems. Furthermore, by eliminating or reducing pH / supersaturation peaks, the production of fines can be reduced or eliminated.In some embodiments, the hydrated suspension is buffered to an alkaline pH. For example, the inventors have empirically determined that a hydrated suspension formed by hydrating MgO in an aqueous solvent is buffered to a pH of approximately 8.1. Suitable sources of low-solubility Mg may include: MgO, Mg(OH)2, magnesium carbonates (such as anhydrous magnesite salt (MgCOs), barringtonite di-, tri-, and pentahydrate (MgCO3*2H2O), nesquehonite (MgCOs*3H2O), and lansfordite (MgCO3*5H2O), respectively, artinite (MgCO3*Mg(OH)2*3H2O), hydromagnesite (4 MgCO3Mg(OH)2*4H2O), dipingite (4 MgCO3Mg(OH)2«5H2O), dolomitic lime, or limestone with a suitable Mg content). Such sources are typically less expensive than conventionally used soluble Mg sources (e.g., MgCk and MgSO4). In some embodiments, the low-solubility Mg source has a solubility in water (at a temperature of approximately 25 °C to approximately 30 °C) of less than approximately 1 g / L. In some embodiments, the low-solubility Mg source has a solubility in water (at a temperature of approximately 25 °C to approximately 30 °C) in the range of: approximately 5 mg / l to approximately 1 g / l, approximately 5 mg / l to approximately 500 mg / l, approximately 5 mg / l to approximately 150 mg / l, approximately 5 mg / l to approximately 100 mg / l, approximately 5 mg / l to approximately 90 mg / l, approximately 5 mg / l to approximately 80 mg / l, approximately 5 mg / l to approximately 70 mg / l, approximately 5 mg / l to approximately 60 mg / l, approximately 5 mg / l to approximately 50 mg / l, approximately 5 mg / l to approximately 40 mg / l, approximately 5 mg / l to approximately 30 mg / l, approximately 5 mg / l to approximately 20 mg / l, approximately 5 mg / l to approximately 10 mg / l. For example, MgO has a water solubility of approximately 86 mg / L (at 30 °C). Mg(OH)₂ has a water solubility of approximately 6.4 mg / L (at 25 °C).MgCOs has a water solubility of approximately 139 mg / l (at 25 °C). Those skilled in the field will recognize that some variability in solubility is common and that the solubility of a Mg source may differ depending on the supplier of that Mg source. For example, the solubility of a Mg source may differ depending on the calcination temperature and / or the calcination time used to prepare the Mg source, as described elsewhere in this document. In some embodiments, the low-solubility Mg source has a PLI 7 I η / η7P7 / E / YILI concentration in the reactor of less than approximately 0.03 mol / L. In some embodiments, the low-solubility Mg source has a concentration in the reactor in the range of: approximately 0.1 mmol / L to approximately 0.03 mol / L, approximately 0.1 mmol / L to approximately 0.02 mol / L, approximately 0.1 mmol / L to approximately 4 mmol / L, approximately 0.1 mmol / L to approximately 2.5 mmol / L, approximately 0.1 mmol / L to approximately 2.2 mmol / L, approximately 0.1 mmol / L to approximately 2.0 mmol / L, approximately 0.1 mmol / L to approximately 1.7 mmol / L, approximately 0.1 mmol / L to approximately 1.5 mmol / L, approximately 0.1 mmol / L to approximately 1.2 mmol / l, approximately 0.1 mmol / l, approximately 1.0 mmol / l, approximately 0.1 mmol / l, approximately 0.7 mmol / l, approximately 0.1 mmol / l, approximately 0.5 mmol / l, or approximately 0.1 mmol / l, approximately 0.25 mmol / l.For example, when the source of low solubility Mg is MgO, the concentration of Mg in the reactor can be approximately 2.1 mmol / min or less. When the source of low-solubility Mg is Mg(OH)₂, the Mg concentration in the reactor can be approximately 0.1 mmol / min or less. When the source of low-solubility Mg is MgCO₃, the Mg concentration in the reactor can be approximately 1.6 mmol / min or less. In some embodiments, the degree of hydration of the low-solubility Mg source is correlated with phosphorus removal (e.g., struvite crystal production). The degree of hydration can be controlled by one or more of the following: the duration of the hydration time, the heating of the Mg source and the aqueous solvent mixture, and the concentration of the hydrated slurry. For example, when the Mg source is MgO, the hydration time can be controlled to prevent or minimize the formation of solid Mg(OH)₂. Mg(OH)₂(s) can accumulate on the surface of the MgO particles, thereby decreasing the release rate of free Mg²⁺ and reducing crystal formation in the reactor. Since Mg(OH)2(s) is less soluble than MgO(s), the time period to redissolve Mg(OH)2(s) in an aqueous solvent to provide Mg2+ The free / dissolved amount of Mg for crystal formation is higher. In some embodiments, the hydration time for a low-solubility Mg source is controlled to improve crystal production. For example, the inventors have empirically demonstrated that hydrating MgO in an aqueous solvent for approximately 20 hours resulted in a struvite yield approximately 4–8% better than a hydration time of approximately 15 minutes or approximately 36 hours. In some embodiments, the desired hydration time for MgO is between approximately 15 minutes and approximately 36 hours. In some embodiments, the hydration time for MgO is between approximately 15 minutes and 1 hour. In some embodiments, the hydration time for MgO is between approximately 15 minutes and several hours (e.g., 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, or 20 hours). Those knowledgeable in the field will understand that the hydration time can be influenced by the reaction conditions and / or the source of Mg. For example, temperature, pressure, and / or the presence of other solutes in an aqueous solvent can affect the release and / or formation of free Mg²⁺ and / or Mg(OH)₂(s) when the source of low-solubility Mg is MgO. For instance, an aqueous suspension of MgO heats up as hydration progresses, especially when acid is added to the suspension. This heating can affect the hydration time. Consequently, the hydration time of MgO can be greater than 36 hours or less than 15 minutes. In some embodiments, when the source of low-solubility Mg is a substance other than MgO, the desired hydration time is greater than 15 minutes. In other embodiments, the hydration time ranges from approximately 15 minutes to several hours (e.g., from 15 minutes to 20 hours). Some examples of non-limiting hydration times are 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, 27 hours, 28 hours, 29 hours, 30 hours, 31 hours, 32 hours, 33 hours, 34 hours, 35 hours, 36 hours. To prepare the hydrated suspension, an aqueous solvent (e.g., water) is added to the source of low-solubility Mg. In some embodiments, the hydrated suspension contains approximately 5% to approximately 30% Mg by weight. In other embodiments, the hydrated suspension contains less than approximately 10% by weight. Since MgO absorbs water upon hydration, the hydrated suspension may become lumpy if too little aqueous solvent is used. Consequently, a hydrated MgO suspension containing approximately 25% to approximately 30% Mg by weight may be too dry, viscous, and / or thick to be easily mixed and pumped into the reactor (or a portion of the reactor). In some embodiments, the amount of soluble Mg in the hydrated suspension is approximately 5 mg / L. In other embodiments, the amount of soluble Mg in the hydrated suspension is approximately 0.5 mg / L. If the weight percentage of Mg in the hydrated suspension is too low, this can reduce crystal production and / or increase production times and / or lead to excessive water consumption, thereby increasing production costs. In some embodiments, the hydrated suspension is more effective when it is relatively dilute (e.g., containing less than approximately 10% Mg by weight). For example, crystal production can be increased when the Mg(OH)₂ hydrated suspension is dilute (e.g., containing less than approximately 5% Mg by weight). In some embodiments, Mg(OH)₂ is used as the low-solubility Mg source and is in the form of a suspension. Since the Mg(OH)₂ form is already hydrated, no further hydration time is required. Because Mg(OH)₂ is less soluble than MgO, using Mg(OH)₂ as a magnesium source may require more dilute suspensions to achieve a yield equivalent to that of MgO suspensions. Consequently, Mg(OH)₂ may be a less efficient Mg source than MgO in terms of struvite recovery and / or loading. Magnesium carbonates can be used as a source of Mg. However, the Hydration of magnesium carbonates can result in the production of carbon dioxide (CO2) gas, which could cause foaming problems and / or affect the pH of the suspension. CO2 gas is acidic, and therefore a caustic agent may be necessary to neutralize excess acid. Dolomitic lime or limestone can be used as a source of Mg. However, these sources of Mg contain calcium (Ca), which can interfere with struvite production and / or contaminate the resulting product with Ca. These products can precipitate more readily from wastewater, but may be less desirable as fertilizers than struvite. However, since MgO, Mg(OH)₂, magnesium carbonates, lime, and limestone are relatively inexpensive, they can be an attractive source of low-solubility Mg for struvite production. In some embodiments, the particle size of the low-solubility Mg source is controlled to prevent or minimize the hydrated suspension from clogging reactor orifices or pipes, such as collector orifices or similar. If the particle size exceeds the desired size, the hydrated suspension can clog the pipes to the point of impeding flow and / or causing damage to the pumping equipment.In some embodiments, the particle size of the low-solubility Mg source is in the range of: approximately 100 to approximately 400 mesh, approximately 100 to approximately 350 mesh, approximately 100 to approximately 300 mesh, approximately 100 to approximately 250 mesh, approximately 100 to approximately 200 mesh, approximately 150 to approximately 200 mesh, approximately 200 to approximately 400 mesh, approximately 250 to approximately 400 mesh, approximately 300 to approximately 400 mesh, or approximately 350 to approximately 400 mesh. In some embodiments, the MgO has a particle size of approximately 200 mesh. In some embodiments, the MgO has a particle size of approximately 400 mesh.In some embodiments, the source of low-solubility Mg is a fine powder having a particle size in the range of approximately 10 to approximately 50 pm, approximately 10 to approximately 40 pm, or approximately 12 to approximately 30 pm. In some embodiments, the MgO has a particle size in the range of approximately 17 SGN to approximately 100 SGN (size guide number). In some embodiments, the MgO has a particle size in the range of approximately 17 SGN to approximately 30 SGN. Those skilled in the art will recognize that the mesh size of a low-solubility Mg source can be affected by the calcination temperature and / or calcination time used to prepare the Mg source. For example, MgO can be prepared by calcining magnesium carbonate. In some embodiments, the calcination temperature is less than approximately 1200 °C. In some embodiments, the calcination temperature is in the range of approximately 600 °C to approximately 1200 °C. In some embodiments, the calcination time is less than approximately 3 hours. In some embodiments, the calcination time is in the range of approximately 1 to 3 hours. In some embodiments, the calcination time is approximately 2 hours. By optimizing the conditions for preparing the low-solubility Mg source, particle size, specific surface area, and reactivity can be controlled. For example, in some embodiments, a particle size of approximately 200 mesh is desirable. Such particles can be optimally retained by the reactor without clogging the system used to supply the low-solubility Mg source to the reactor and / or without the need for high-flow pumps. Without adhering to any particular theory, the inventors believe that using a low-solubility Mg source can prevent or reduce fines production due to the longer retention time of slow-dissolving, low-solubility Mg sources compared to conventional soluble Mg sources (e.g., MgCb and / or MgSO4). Low-solubility Mg sources slowly release free Mg2+ ions into solution, resulting in a substantially uniform distribution of Mg2+ ions throughout the reactor (or a portion thereof). This promotes crystallization throughout the reactor (or a portion thereof) and minimizes or prevents localized areas where the Mg concentration is significantly elevated.Therefore, the slow dissolution rate of low-solubility Mg sources can prevent or minimize areas where the supersaturation ratio is high enough to cause the production of a large number of fines. Another theory is that the presence of a low-solubility Mg source helps the fines produced in the reactor (or a portion of the reactor) to stick together or to other particles present in solution. The inventors analyzed the treated effluent by adding a low-solubility Mg source to an aqueous solution in a fluidized bed reactor at loading rates between 5 and 250 g P / min / m³ and observed no fine particles or MgO particles in the effluent or the recycle fluid. If the reduction or elimination of fines were solely due to a longer retention time, then the presence of the low-solubility Mg source would be expected in the treated effluent and / or the recycle fluid.Since the inventors empirically observed that the treated effluent and the recycled fluid did not contain a source of low-solubility Mg, it is possible that the source of low-solubility Mg is attached to the struvite crystals present in the reactor. Therefore, in some embodiments, the low-solubility Mg source (e.g., MgO) is introduced into a fluidized bed. The particles in the fluidized bed may comprise struvite, a struvite analogue, or monoammonium phosphate, for example. The low-solubility Mg source may be provided in the form of particles that would be removed from the reactor for given fluid flow rates. Interaction among the small MgO particles (or another low-solubility source such as an ionic species) can retain the small particles in the fluidized bed so that the particles supply Mg²⁺ (or another ionic species) throughout all or most of the fluidized bed. The distributed small particles can replenish the available Mg²⁺ (or other ionic species) as the available Mg²⁺ is consumed in the production of struvite or another product. The inventors have empirically determined that the resulting struvite crystals can be relatively pure, with little or no excess of nR I 7 I η / η7P7 / E / YILI observed. Mg in the product samples. Therefore, the inventors believe that the source of low-solubility Mg can essentially dissolve completely and be used to form struvite crystals. Further tests were conducted in which the reactor was fed with tap water essentially free of phosphate and ammonia and a hydrated MgO suspension, resulting in the injection and dilution of the MgO suspension in the water, but without a struvite formation reaction. In this case, a substantial portion of the injected MgO suspension was observed in the reactor recycle and effluent with the same results. The experiment was repeated with and without a fluidized bed of struvite particles present in the reactor, with the same results. This indicates that the struvite formation reaction is required for the hydrated MgO suspension particles to adhere to the fluidized bed. Without adhering to any particular theory, the inventors believe that forming a suspension by hydrating a low-solubility Mg source before adding the source to the reactor (or a portion of the reactor) can result in a uniform distribution of free Mg2+ ions throughout the reactor volume (or a portion of the reactor volume), thereby improving loading constraints and / or increasing struvite production and / or reducing fines production. For example, the limited solubility of the low-solubility Mg source may allow the localized supersaturation ratio around the Mg2+ injection point to remain relatively low compared to cases where highly soluble conventional Mg sources (e.g., MgCk and MgSO4) are injected into a similar reactor. In some embodiments, process 100 comprises an optional step 130 of adding an acid to the hydrated slurry. The acid can be added to the tank in which the slurry of low-solubility Mg source is hydrated or stored. The addition of acid can increase the struvite yield in the treatment of high-concentration wastewater (e.g., wastewater containing high amounts of fats, oils, and greases (FOG) or other organic components). The addition of approximately a molar ratio of 0.4:1 mol IT:Mg has been found to be effective in increasing the struvite yield. When the low-solubility Mg source is MgO, the acid provides H+(aq) to drive the formation of the MgOH+ complex in the first step of the hydration mechanism described elsewhere herein. The inventors have empirically determined that adding an appropriate amount of acid to the hydrated suspension can increase the production rate of struvite crystals and / or the overall struvite conversion efficiency, resulting in a higher yield of struvite crystals and / or allowing a higher reactor loading. For example, the addition of an acid in a 0.2:1 molar equivalent H+:Mg ratio increased the struvite crystal yield by approximately 1020%. In some embodiments, an amount of acid is added to the hydrated suspension to neutralize approximately 20% of any hydroxide formed upon hydration of the low-solubility Mg source. Suitable acids include strong acids, such as hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), etc., and weak acids, such as acetic acid, citric acid, oxalic acid, etc. In some embodiments, acids that interfere with struvite production (e.g., phosphoric acid (H3PO4)) and / or acids that are too weak to break down the MgO particles (or other low-solubility Mg source particles) are avoided. In one embodiment, the acid comprises sulfuric acid. For example, the acid may be 93%–98% sulfuric acid. Sulfuric acid is readily available at wet-process phosphoric acid plants and is therefore convenient to use when operating this process at or near a wet-process phosphoric acid plant. Sulfuric acid is generally one of the most cost-effective acids available on the market. In another embodiment, the acid comprises acetic acid (or another volatile fatty acid or mixture of volatile fatty acids), which is useful as a source of volatile fatty acids required for phosphate absorption from wastewater in treatment plants using enhanced biological phosphorus removal. Therefore, applying acetic acid or volatile fatty acids to wastewater in the recycling route creates an additional level of synergy. ηκ 171 η / η7Π7 / Ε / γΐΛΐ In some embodiments, in step 120, the acid is dosed to the hydrated suspension. For example, the acid can be added in liquid form to a hydration tank for the suspension. Weak acids, such as citric acid, which are available in dry form, can be added as a powder to the hydration tank or premixed with MgO before hydration. In some embodiments, in step 110 and / or step 120, the pH of the hydrated suspension is controlled by adding an appropriate amount of acid. The reactor design and / or reaction conditions can affect fines and / or struvite production and / or operating economics when the Mg source is of low solubility. For example, in some embodiments, the hydrated slurry is injected near the bottom of the reactor (or a portion of the reactor), as described elsewhere in this document. Since hydrated suspensions, such as hydrated MgO suspensions, can be prone to scaling and clogging, a recycle route can be used to adjust fluid flow rates within the hydrated suspension mixing and pumping system. The relatively high fluid velocity in the recycle route can prevent or minimize scaling and clogging from the low-solubility Mg source. In the embodiment illustrated in FIG. 1A, the reactor 12 comprises a recycle route 30. Not all fluidized bed reactors have a recycle route. However, a recycle route 30 can be advantageous as it provides a way to adjust the fluid flow rates within the reactor 12 without changing the rate at which the aqueous solution is introduced into the reaction tank 18 at inlet 14. The recycle route 30 is connected to receive or withdraw the solution from the reaction tank 18 and to return the solution to the reaction tank 18. Reactors according to some embodiments can provide a plurality of recycle routes 30. The illustrated recycling path 30 has an input end 30A and an output end 30B. In the embodiment of FIG. 1, the output 16 is connected to the input end 30A of the recycling path 30, although in other embodiments, the input end 30A of the recycling path 30 may be separate from the output nR I 7 I η / η7P7 / E / YILI 16. Outlet 16 can be connected to an effluent piping system 20. Inlet 14, outlet 16, recycle route 30, and effluent piping system 20 each comprise one or more valves that allow them to be turned on or off. Recycling route 30 is connected to remove solution from reaction tank 18 and return solution to reaction tank 18. In some embodiments, recycling route 30 returns solution to reaction tank 18 below a location where solution is received from reaction tank 18. In some embodiments, recycling route 30 shares inlet 14 and / or outlet 16 (e.g., inlet end 30A of recycling route 30 is in direct fluid communication with outlet 16, and / or outlet end 30B of recycling route 30 is in direct fluid communication with inlet 14). In other embodiments, recycling route 30 has one or more inlet ends separate from outlet 16 and / or one or more outlet ends in reaction tank 18 separate from inlet 14. When the solubility of a precipitating substance depends on pH, reactor 12 may include an acid injector 32 to apply acid to the solution stream. The acid can be injected, for example, into the recycle route 30. Acid injection lowers the pH of the solution in the recycle route 30 or in reactor 12 as a whole. A recycle route 30 with an acid injector 32 can act as a fines destruction circuit, selectively dissolving fines that are small enough to remain in suspension in the reactor's recycle route stream. Most of the larger crystals remain in the reaction tank 18 (for example, in the reaction tank's fluidized bed). In some embodiments, the acid applied in recycling route 30 can reduce the pH not only in recycling route 30 but also in reaction tank 18. The local pH reduction in recycling route 30 may be greater than the pH reduction in reaction tank 18, which is farther from the acid injector 32. Because the fines have a high surface area to volume ratio relative to the bulkier crystals, the fines tend to dissolve more rapidly by mass percent than the larger crystals. In some embodiments, the pH is increased in the recycling route (e.g., by neutralizing the acid with a base) before the recycling route rejoins tank 18. In some embodiments, the reaction tank 18 may comprise a base injector that injects a base (e.g., a substance that increases the concentration of OH ions in the solution and / or increases the pH of the solution) into the solution in the reaction tank 18 to raise or maintain the pH of the solution. An example base injector 40A is illustrated schematically in FIG. 1A. The base injector 40A may be connected to a controller and a measuring device (e.g., a pH probe). In some embodiments, the pH of the solution in reaction tank 18 is maintained at a relatively high pH (compared to some prior struvite precipitation procedures, for example, a pH > 7 or > 7.5). In some cases, a pH of 8 or 9 or higher is maintained in all or part of the reaction tank. The inventors have determined that maintaining certain struvite formation reactions at a higher pH can help drive the equilibrium reaction to completion. This, in turn, improves the phosphate removal efficiency. Specifically, the inventors have found that by increasing the reaction pH using MgO as the Mg source in the struvite formation reaction, an almost complete (98%) phosphate removal efficiency can be achieved from a given wastewater sample. The typical phosphate removal efficiency using conventional struvite formation reactions is significantly lower. The rate of addition of the hydrated suspension into reaction tank 18 can affect the efficiency of phosphate removal. The inventors have discovered that the pH of the solution in reaction tank 18 can be used as an indicator of the amount of hydrated suspension (or specifically the concentration of the low-solubility Mg source) present in the solution in the reactor at any given time. In some embodiments, the pH is maintained by adjusting the flow rate of the hydrated suspension into reaction tank 18 (or conversely, the injection of the hydrated suspension into the reaction tank is controlled based on the pH in reaction tank 18). It is also possible to control the addition of the hydrated suspension to maintain a desired Mg:P molar ratio in the reactor. In some embodiments, the pH of the solution in reaction tank 18 is controlled by the addition of the hydrated suspension. In some embodiments, reactor 12 includes a control valve 31 between reactor tank 18 and hydration tank 40. Control valve 31 controls the flow of the hydrated suspension out of hydration tank 40 and into reaction tank 18. In some embodiments, control valve 31 is connected to a controller 35. Controller 35 can receive input from a pH probe 33. pH probe 33 measures the pH of the solution in reaction tank 18. Controller 35 can control the opening and closing of control valve 31 in response to the pH of the solution in reaction tank 18. In some embodiments, pH probe 33 transmits real-time pH measurements of the solution in reaction tank 18 to controller 35.Control valve 31 can be opened to allow the flow of the hydrated suspension into reaction tank 18 in response to a deviation in the solution pH from the target pH. In some embodiments, the target pH is higher than that conventionally maintained in struvite precipitation procedures. In such embodiments, control valve 31 can be opened to allow the flow of the hydrated suspension into reaction tank 18 in response to a decrease in the solution pH from the target pH. The increased flow of the hydrated suspension through the fluidized bed of reaction tank 18 results in an increase in the solution pH. In some embodiments, control valve 31 can be configured to open at intervals of approximately 30 seconds to approximately 5 minutes. In an example embodiment illustrated in FIG. 1B, the reactor 12 includes a suspension circuit 48. The suspension circuit 48 circulates the hydrated suspension into and out of the hydration tank 40. The hydration tank 40 may comprise a mixer 41 for continuously mixing the hydrated suspension. The suspension circuit 48 may be in fluid connection with the inlet 46. This allows the hydrated suspension to flow out of the suspension circuit 48 and into the reaction tank 18. In some embodiments, the hydrated suspension circulates continuously out of the hydration tank 40 and into the reaction tank 18 through the suspension circuit 48. In some embodiments, the flow of the hydrated suspension from the suspension circuit 48 to the reaction tank 18 is controlled by valve 31.The inventors have determined that the circulation of the hydrated suspension within a suspension circuit maintains the flow of the hydrated suspension within the reactor at a desired rate that prevents the sedimentation of the Mg source particles. In some embodiments, a soluble Mg source (e.g., MgCb) can be injected into reaction tank 18 in addition to introducing the hydrated suspension containing a low-solubility Mg source. The soluble Mg source can optionally be injected into reaction tank 18 at the same time as the hydrated suspension. The soluble Mg source can be injected into reaction tank 18 through the same inlet or a different inlet as the low-solubility Mg source. Reaction time can affect struvite and / or fines production and / or operating economics when the Mg source is of low solubility. Due at least in part to the limited solubility of a low-solubility Mg source, reaction times to achieve the desired phosphorus removal are generally longer than those observed for conventional soluble Mg sources (e.g., MgCb and MgSO4). The low solubility of the low-solubility Mg source allows it to slowly release free Mg2+ ions over time. For example, reaction times of approximately 6 minutes are typical for soluble Mg sources. For low-solubility Mg sources, reaction times of approximately 30 to 60 minutes may be required to achieve the desired phosphorus removal. The reaction time can be controlled by decreasing the flow rate of the aqueous solution in the reactor. By increasing the reaction time, the low-solubility Mg source can be distributed uniformly throughout the reactor volume (or a portion thereof). This allows the localized supersaturation ratio around the injection point of the hydrated slurry to remain relatively low compared to conventional soluble Mg sources (e.g., MgCk and MgSO4), while maintaining a relatively high supersaturation in a substantial portion of the reactor (or a portion thereof) as the low-solubility Mg source particles continue to dissolve, thereby replenishing the free Mg2+ concentration throughout the fluidized bed. The loading rate can affect struvite production and / or fines formation and / or operating economics when the Mg source is of low solubility. For example, a fluidized bed reactor can operate under high growth conditions (i.e., high loading conditions) when a low-solubility Mg source is added to an aqueous solution in the reactor (or a portion of the reactor). In some embodiments, the loading rate is above a threshold to achieve a high crystal growth rate. For example, the inventors have empirically observed that the loading rate can be 5 g of PO4-P / min / m3 or higher, or 50 g of PO4-P / min / m3 or higher, or 100 g of PO4-P / min / m3 or higher, or 250 g of PO4-P / min / m3 or higher in a reactor (or a portion of the reactor).In some cases the PO4-P loading can be increased by a multiple of 2, 5, 7, 10, 17, 20, 25 or more while producing commercially acceptable struvite particles by changing from injection of a high solubility Mg source to injection of a lower solubility Mg source as described herein (e.g., when using a 10 wt% aqueous suspension of MgO), compared to a similar case where conventional soluble Mg sources (i.e., MgCla or MgSO4) are used (typically as 1% to 32% solutions of MgCb or MgSO4). Therefore, the load can be increased very significantly, for example, by a multiple of 5 or more or 10 or more or 25 times by employing a low solubility Mg source as described herein compared to a case in which a highly soluble Mg source is used. ΠΛI 7 I η / η7Π7 / Ε / ΥΙΛΙ In some embodiments, an aqueous solution having a relatively high phosphorus concentration (i.e., from approximately 2,000 to approximately 10,000 mg / L PO4-P) can be fed to the reactor at a relatively low flow rate to maintain a reactor loading. In some embodiments, an aqueous solution having a lower phosphorus concentration (i.e., from approximately 40 to approximately 600 mg / L PO4-P) can be fed to the reactor. In some embodiments, the reactor (or a portion of the reactor) has a hydraulic limitation (i.e., a maximum upward flow velocity and / or a minimum hydraulic retention time [HRT]) that can limit the maximum loading. For example, in some embodiments, the reaction tank 18 of reactor 12 comprises a substantially vertically oriented conduit having a collection section and two or more vertically sequential sections above the collection section.In such embodiments, the reactor can operate at an upward flow rate of approximately 250 cm / min in a collection section of the reaction tank and 60 cm / min in one or more of the vertically sequential sections above the collection section. In some embodiments, the fines can be separated from the solution in a recycle route and concentrated by sedimentation, filtration, centrifugation, or other solids separation techniques. The concentrated fines solids are then sent to a fines treatment tank where they are dissolved in a solution of reduced pH (e.g., an acidic solution) before being returned to the reaction tank and / or the recycle route. The fines treatment tank can be operated at an upward flow rate of approximately 7 cm / min.The fines treatment tank can be used to capture fines and / or particles from the low-solubility Mg source and return the low-solubility Mg source to the reaction tank and / or the recycling route for dissolution in crystal production. When the low-solubility Mg source settles quickly enough to be retained in the reactor, or otherwise agglomerates into growing struvite particles, then the low-solubility Mg source can be retained for a period longer than the HRT of the aqueous solution flowing through the reactor, potentially eliminating or reducing the HRT as the limiting factor for reactor loading and leaving the upward flow rate as the limiting factor. Example 1 Phosphorus removal with various Mg sources Samples of aqueous solution (dehydration liquors from anaerobic digesters of municipal wastewater treatment plants) were reacted with various sources of low-solubility Mg to determine the impact of the type of low-solubility Mg source on the removal (%) of phosphorus (P) as a function of reaction time (minutes). The test results are shown in FIG. 3. The samples were prepared as described in Table 1. In each case, a 10% MgO suspension was prepared. If acid was used, 93% sulfuric acid was added to the MgO suspension at a 0.4:1 mol H₂ per mol Mg ratio. The MgO suspension was then allowed to hydrate for 15 minutes before being added at a 1:1 Mg:P ratio to a flask of dehydrating liquor. The flask was allowed to react for 60 minutes, and the concentrations of solubles in the flask were then recorded to determine the extractions. After 20 hours of hydration, another flask of dehydrating liquor containing the MgO suspension at a 1:1 Mg:P ratio was added, allowed to react for 60 minutes, and the concentrations of solubles in the flask were then recorded. (IR I 7 I η / η7Π7 / Ε / ΥΙΛΙ Table 1 - Phosphorus removal with various Mg sources Mg Source Hydration Time Mg:P (mol:mol) Acid Base pH SITE 1 10% Aqueous Mg(OH)2 Suspension 1:1 SITE 1 0.57% Aqueous Mg(OH)2 Suspension 1.5:1 Mg Source Hydration Time Mg:P (mol:mol) Acid Base pH SITE 2 10% Mg(OH)2 aqueous suspension 0.25 h 1:1 SITE 2 10% Mg(OH)2 aqueous suspension 0.25 h 1.25:1 SITE 2 10% Mg(OH)2 aqueous suspension 1:1 SITE 2 32 wt% MgCl2 1:1 SITE 1 10% Mg(OH)2 aqueous suspension 1.25:1 SITE 1 0.23% Mg(OH)2 aqueous suspension 1:1 SITE 2 10% MgO aqueous suspension 0.25 h 1:1 Yes 0.4:1 mol H+:mol Mg SITE 2 10% MgO aqueous suspension % 0.25 h 1:1 Yes 0.4:1 mol of H+:mol Mg NaOH added >8 ΠΛI 7 I η / η7Π7 / Ε / ΥΙΛΙ Example 2: Effect of acid and hydration time on struvite recovery The following hydrated suspensions in water were prepared: (i) 10 wt% MgO, hydration time = 20 hours; (ii) 10 wt% MgO, hydration time = 15 minutes; (iii) 10 wt% MgO with 0.2:1 mol H2SO4:MgO, hydration time = 19 hours; and (iv) 10 wt% MgO with 0.2:1 mol H2SO4:MgO, retention time = 15 minutes. Each hydrated suspension was added to a sample of an aqueous solution (dehydration liquors from 10 anaerobic digesters of municipal wastewater treatment plants) and the solutions were mixed for 60 minutes. The solutions were analyzed to determine their P, N, and Mg content. At least 80% of the phosphorus (P) content of each aqueous solution was removed by the hydrated suspensions as shown in Table 3. The P removal (%) was greater when the hydrated suspension contained 0.2:1 mol H₂SO₄:MgO (i.e., suspensions (iii) and (iv)) and when the suspension had a longer hydration time (i.e., suspensions (i) and (iii)). Analysis of the Mg content indicated that less Mg was available for P removal in suspensions (i) and (iv). ΠΛI 7 I η / η7Π7 / Ε / ΥΙΛΙ Table 3: Effect of acid and hydration time on struvite recovery Suspension Hydration time (hours) HLSO^MgO (mol:mol) Mg:P (mol:mol) P removal (%) 19 0 1:1 90 i 0.25 0 1:1 84 iii 20 0.2:1 1:1 91 iv 0.25 0.2:1 1:1 87 Example 3 Effect of acid on the concentration of free Mg2+ ion Acid was added to the MgO hydrated suspensions prepared in Example 4. The final ratios were as follows: (i) 0.49:1 mol H₂SO₄:MgO; (ii) 0:1 mol H₂SO₄:MgO; (iii) 0.20:1 mol H₂SO₄:MgO; and (iv) 0.94:1 mol H₂SO₄:MgO. Each hydrated suspension was added to a sample of an aqueous solution (dehydration liquors from anaerobic digesters of municipal wastewater treatment plants). Additional acid was added to the samples with hydrated suspensions (i) and (iv) to reduce the pH below 9. Without being tied to a particular theory, the inventors believe that additional acid was needed to lower the pH because the mechanism for producing Mg(OH)2 from MgO includes the intermediate production of Mg2+(aq) and OH'(aq). The Mg2+(aq) ions are available for crystal production and the OH'(aq) ions remain free in solution available to form MgOH+»OH'(surface) and promote the production of additional Mg2+(aq) and OH'(aq), thus causing the pH to continue rising until Mg(OH)2(s) is formed. Without adhering to any particular theory, the inventors believe that the sample containing the hydrated suspension (iii) did not require additional acid because the MgO had likely been completely converted to Mg(OH)2, and consequently, the pH of the solution was controlled by the dissolution of Mg(OH)2 and the precipitation of struvite. The sample containing the hydrated suspension (v) required additional acid because the hydrated suspension had likely formed an initial layer of Mg(OH)2, and additional Mg2+(aq) and OH'(aq) ions had been released into the solution as MgOH+*OH'(Surface). The sample containing the hydrated suspension (i), which is believed to have been less hydrated than the sample containing the hydrated suspension (iii), required additional acid. It is likely that some unreacted MgO was hydrating during the production of struvite.Since the hydrated suspension (i) did not contain acid, was hydrated for a shorter period (i.e., 15 minutes), and the temperature was maintained at room temperature, it is believed that the water did not have enough time to diffuse into the MgO particles. Consequently, the initial surface layer of Mg(OH)₂ could still have formed and would have been available to react with the struvite. Once the struvite reaction was complete, several unreacted MgO sites would have remained (which would explain the low final Mg concentration observed). Example 4 Elimination of P (MqCk) A 32 wt% aqueous solution of MgCb was prepared and added to an aqueous solution (dehydration liquors from anaerobic digesters of municipal wastewater treatment plants) so that the Mg:P (mol / L) ratio was 1:1. The pH of the resulting solution was increased to 7.9 by adding 0.5 mL of NaOH(aq) and the solution was mixed for 60 minutes. The pH increased to 8.24 during the 60-minute mixing time. The removal of P was 92% and the solution contained 32 mg / l of Mg, 10.8 mg / l of P, 621 mg / l of NH3-N. Example 7 Alternative sources of MgO The reactivity differences between different MgO sources were compared. Figure 5 shows the % removal of P and Figure 6 shows the final concentration of Mg for several samples prepared using MgO from a first supplier (Supplier 1) and MgO from a second supplier (Supplier 2). Hydrated suspensions of MgO from the two sources were prepared as follows: ΠΛI 7 I η / η7Π7 / Ε / ΥΙΛΙ MgO Source Acid Hydration Time Supplier 2 Yes 0.2 mol of H2SO4: mg ~20 hours Supplier 1 Yes 0.2 mol of H2SO4: mg ~20 hours Supplier 2 Yes 0.2 mol of H2SO4: mg <15 minutes Supplier 1 Yes 0.2 mol of H2SO4: mg <15 minutes Supplier 2 0 ~20 hours Supplier 1 0 ~20 hours Supplier 2 0 <15 minutes Supplier 1 0 <15 minutes Each hydrated suspension was added to a sample of an aqueous solution (dehydration liquors from anaerobic digesters of municipal wastewater treatment plants) and the removal of P (%) was determined. The removal of P (%) was higher for the MgO from Supplier 2 than for the MgO from Supplier 1. This indicates that the suspensions of hydrated MgO Supplier 2 probably had limited Mg and that the source of MgOdel Supplier 1 is probably less reactive than the MgO source Supplier 2. However, the MgO source from Supplier 1 was more sensitive to hydration time and added acid. The % removal of P was higher when the hydration time was longer (i.e., ~20 hours vs. <15 minutes). The % removal of P was also higher when acid was added to the MgO suspension. It should be noted that the MgO suspensions from Supplier 1 were all hydrated at a lower temperature than the MgO suspensions from Supplier 2, since the heat of reaction of the MgO from Supplier 2 and water was higher than that of the MgO suspensions from Supplier 1. Furthermore, the final pH of the mixture of the Mg suspension and aqueous solution from Supplier 1 was approximately 0.1 lower than the pH of the mixture of the suspension and aqueous solution from Supplier 2. Consequently, no secondary acid addition was required for the mixtures containing an MgO suspension from Supplier 1.This may imply the presence of fewer MgOH+ sites for reaction and / or that the MgO suspension particles from Supplier 1 have a smaller surface area than the MgO suspension particles from Supplier 2. Since excessive pH appears to be less of a concern when using an MgO suspension from Supplier 1, MgO from Supplier 1 may be preferable for use in struvite recovery. Example 5 Effect of hydration time Aqueous suspensions of MgO from Supplier 2 and MgO from Supplier 1 were prepared and subjected to varying hydration times. An aqueous suspension of MgO hydrated for 48 hours appeared dry, indicating that the MgO had likely been converted to Mg(OH)₂(s). Aqueous suspensions of MgO from Supplier 2 and MgO from Supplier 1 were prepared with the following hydration times: 15 minutes, approximately 20 hours, and 36 hours. Each suspension was added to a sample of an aqueous solution containing P (dehydration liquors from anaerobic digesters of municipal wastewater treatment plants), and the P removal (%) was compared. The results are shown in Figure 7. The MgO suspensions hydrated for 36 hours were less effective at removing P than the MgO suspensions hydrated for 20 hours.MgO suspensions that were hydrated for 15 minutes were less effective for P removal. MgO suspensions prepared with acid effected greater P removal than MgO suspensions prepared without acid. Example 6 Effect of temperature The effect of temperature on hydration was tested by heating aqueous suspensions of MgO prepared from MgO from Supplier 2 and PLI 7 I η / η7P7 / E / YILI of MgO from Supplier 1 in the presence and absence of acid. The hydration time was 15 minutes. As shown in FIGS. 11 and 12, increasing the temperature from room temperature to over 55 °C had little effect on the removal of P (%) (i.e., Flasks 4, 13, 9, and 15). The addition of acid improved the removal of P (%) at room temperature (e.g., Flasks 2 and 7). Example 7 Effect of retention time The effect of reaction time on P removal (%) was tested for aqueous suspensions of MgO from Supplier 2 and MgO from Supplier 1 prepared with and without acid (10% MgO suspensions with and without a 0.2:1 molar dose of H₂SO₄:MgO). A fluidized bed reactor of the type described in Britton, WO N.s2012 / 119260, entitled "Reactor for Precipitating Solutes from Wastewater and Associated Methods," was used. The reactor had a volume of approximately 168 m³ to the bottom of the recycle collector. As shown in Figure 10, P removal (%) was determined after reaction times of 6.5 minutes (the reactor's HRT), 15 minutes, and 60 minutes. Figure 11 shows the pH of each aqueous solution. The removal of P (%) was greater for the 15-minute sample than for the 6.5-minute sample, which may be related to the observed pH increase of 0.2 after the longer reaction time. NaOH was added to raise the pH of each mixture above 8, and the percentage of phosphorus (P) removed was determined after reaction times of 6.5 minutes, 15 minutes, and 60 minutes. The Mg:P ratio (mol / mol) was 1.25:1 for each sample. The percentage of P removed was also determined for a sample of MgCb with added NaOH to raise the pH above 8. The results are shown in Figure 12. MgO suspensions showed an increase in P removal performance with longer reaction time and with the addition of acid; approaching the performance of soluble Mg (MgCL) and caustic substance sources at 60 minutes of reaction time. ΠΛI 7 I η / η7Π7 / Ε / ΥΙΛΙ Example 8 Large-scale demonstration Large-scale commercial fluidized bed reactors were used to test the use of a low-solubility Mg source, specifically MgO, in wastewater precipitation. Two large-scale demonstration experiments were conducted. Different MgO sources were used in the two experiments. In the first experiment, the MgO source was a fine powder of calcined magnesite with a particle size distribution in which approximately 90% of the particles were smaller than 75 µm. A target MgO concentration of approximately 12 wt% was maintained in the reaction tank. The feed rate was approximately 65 kg of PO₄-P per day. The pH of the solution in the reaction tank ranged from approximately 7.5 to approximately 8.1. The flow rate of the hydrated MgO slurry to the reaction tank was controlled by maintaining a specific Mg:P molar ratio in the reaction tank.Samples were collected and analyzed from the reaction tank to determine the amount of solids produced from the reaction and the amount of dissolved Mg remaining in the sample supernatant. The percentage of total solids and the amount of dissolved Mg detected in the collected samples were 18.75% and 0.566 mg / L, respectively. In the second experiment, the MgO source was a fine powder of calcined magnesite with a particle size of approximately 200 mesh (passing approximately 96%). A target MgO concentration of approximately 20 wt% was maintained in the reaction tank. The loading rate was approximately 195 kg of P-PO4 per day. The flow rate of the hydrated MgO slurry to the reaction tank was controlled by maintaining the pH between approximately 7.9 and 8. Samples were collected and analyzed from the reaction tank to determine the amount of solids produced from the reaction and the amount of dissolved Mg remaining in the sample supernatant. The percentage of total solids and the amount of dissolved Mg detected in the collected samples were 30.21% and 1.36 mg / L, respectively. The results of these experiments illustrate that when the reaction is complete, low amounts of free Mg are present. This suggests that most of the Mg added to the reaction tank was used to form struvite. Although several examples of aspects and embodiments have been discussed above, those skilled in the art will recognize certain modifications, permutations, additions, and subcombinations thereof. Therefore, the following appended claims and the claims presented below are intended to be interpreted to include all modifications, permutations, additions, and subcombinations that are consistent with the broader interpretation of the specification as a whole. Interpretation of the terms Unless the context clearly requires otherwise, throughout this descriptive report and the claims: • approximately a quoted value means a range of values ​​that is within plus or minus 10% of the quoted value (e.g., approximately 10 means in the range of 9 to 11); • includes, that includes, and similar terms should be considered in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is, in the sense of includes, but not in a limiting way; • in this document, previously, subsequently, and words of similar implied meaning, when used to describe this descriptive report, shall refer to this descriptive report as a whole, and not to particular portions of this descriptive report; • or, with reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list; • The singular forms uno, una and el, la also include the meaning of any appropriate plural form. The specific examples of systems, procedures, and devices described herein are for illustrative purposes only. These are merely examples. The technology provided herein may be applied to systems PLI 7 I η / η7P7 / E / YILI other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of the present invention. The present invention includes variations of the described embodiments that would be obvious to a person skilled in the art, including variations obtained by: substituting features, elements, and / or actions with equivalent features, elements, and / or actions; mixing and comparing features, elements, and / or actions from different embodiments; combining features, elements, and / or actions of the embodiments as described herein with features, elements, and / or actions from other technology; and / or omitting the combination of features, elements, and / or actions of the described embodiments. Therefore, it is intended that the following appended claims and the claims presented below be interpreted to include all modifications, permutations, additions, omissions, and subcombinations that may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

1. A process for precipitating dissolved materials from an aqueous solution, characterized in that it comprises the steps of: introducing the aqueous solution containing the dissolved materials into a reactor; and introducing a magnesium (Mg) source into the reactor in a sufficient amount to cause the materials dissolved in the aqueous solution to precipitate into crystals, wherein the Mg source is introduced into the reactor in the form of particles of a Mg-containing material, and wherein the Mg source has a solubility in the aqueous solution of less than approximately 1 g / L or the concentration of Mg available in the reactor is less than approximately 0.03 mol / L 2. The process according to claim 1, characterized in that the Mg source is introduced as a hydrated suspension.

3. The procedure according to claim 2, characterized in that it further comprises preparing the hydrated solution by adding water to the Mg source and soaking the Mg source for a hydration time before introducing the hydrated suspension into the reactor.

4. The process according to any of claims 1 to 3, characterized in that the Mg source has an aqueous solvent solubility of approximately 5 mg / L to approximately 500 mg / L 5. The process according to any of claims 1 to 3, characterized in that the Mg source has an aqueous solvent solubility of approximately 5 mg / l to approximately 150 mg / l.

6. The process according to any of claims 1 to 5, characterized in that it comprises maintaining the concentration of Mg in the reactor in the range of approximately 0.1 mmol / L to approximately 0.03 mol / L 7. The process according to any of claims 1 to 6, characterized in that the Mg source has a particle size of less than approximately 50 pm.

8. The process according to any of claims 1 to 6, characterized in that the Mg source has a particle size in the range of approximately 10 pm to approximately 30 pm.

9. The process according to any of claims 1 to 6, characterized in that the Mg source has a particle size in the range of approximately 100 mesh to approximately 400 mesh.

10. The process according to any of claims 1 to 9, characterized in that it further comprises maintaining the pH of the aqueous solution at a pH above approximately 7.

11. The process according to claim 10, characterized in that the pH of the aqueous solution is maintained by adding an acid or a base to the hydrated suspension.

12. The procedure according to claim 10, characterized in that the pH of the aqueous solution is maintained by controlling the amount of the Mg source present in the aqueous solution.

13. The method according to claim 12, characterized in that the amount of the Mg source present in the aqueous solution is controlled by: measuring the pH of the aqueous solution in real time; comparing the measured pH with a target pH; adjusting the pH by introducing the Mg source into the aqueous solution so that the pH of the aqueous solution is altered to the target pH.

14. The procedure according to claim 13, characterized in that the target pH is greater than 7.

15. The procedure according to any of claims 1 to 14, characterized in that it further comprises introducing the Mg source into the reactor within a predetermined time interval.

16. The procedure according to claim 15, characterized in that the preset time interval is from approximately 30 seconds to approximately 5 minutes.

17. The process according to any of claims 1 to 16, characterized in that the Mg source comprises a low solubility Mg source, the low solubility Mg source comprising one or more of: MgO, Mg(OH)2, and a magnesium carbonate.

18. The process according to claim 17, characterized in that the magnesium carbonate comprises one or more anhydrous salts of magnesite (MgCO3), barringtonite (MgCO3*2H2O), nesquehonite (MgCO3*3H2O), lansfordite (MgCO3*5H2O), artinite (MgCO3*Mg(OH)2·3H2O), hydromagnesite (4 MgCO3Mg(OH)2*4H2O), dipingite (4 MgCO3Mg(OH)2*5H2O), dolomitic lime, and limestone.

19. The process according to claim 17, characterized in that the low-solubility Mg source comprises MgO. nR 171 η / η7Π7 / E / γΐΛΐ 20. The process according to claim 19, characterized in that the MgO is prepared at a calcination temperature for a period of time sufficient to produce MgO particles and the calcination temperature is in the range of approximately 600 °C to approximately 1,200 °C.

21. The procedure according to claim 19 or 20, characterized in that the time period is in the range of approximately 1 to 3 hours.

22. The procedure according to claim 3, characterized in that the hydration time is between approximately 15 minutes and approximately 36 hours.

23. The process according to any of claims 1 to 22, characterized in that it further comprises adding an acid to the hydrated suspension before introducing the hydrated suspension into the reactor.

24. The procedure according to claim 24, characterized in that approximately 0.4:1 molar equivalents of the acid are added to the hydrated suspension.

25. The process according to claim 11 or 23, characterized in that the acid is a strong acid.

26. The process according to claim 25, characterized in that the acid is one or more of: hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid.

27. The process according to claim 11 or 23, characterized in that the acid is a weak acid. 171 η / η7Π7 / E / γΐΛΐ 28. The process according to claim 27, characterized in that the acid is acetic acid, citric acid, or oxalic acid.

29. The process according to any of claims 1 to 28, characterized in that the Mg source is reacted with the aqueous solution for a reaction time of less than approximately 60 minutes.

30. The process according to any of claims 1 to 28, characterized in that the Mg source is reacted with the aqueous solution for a reaction time of approximately 30 minutes to approximately 60 minutes.

31. The process according to any of claims 1 to 30, characterized in that the reactor tank is operated under high crystal growth conditions.

32. The process according to claim 31, characterized in that the reactor tank load is above a threshold to achieve a high crystal growth rate.

33. The process according to claim 31 or 32, characterized in that the loading is approximately 5 g of PO4-P / min / m3 or more in the reactor tank.

34. The process according to claim 31 or 32, characterized in that the loading is 50 g of PO4-P / min / m3 or more in the reactor tank.

35. The process according to claim 31 or 32, characterized in that the loading rate is 250 g of PO4-P / min / m3 or more in the reactor tank. nR I 7 I η / η7P7 / E / YILI 36. The process according to any of claims 33 to 35, characterized in that the aqueous solution has a phosphorus concentration in the range of approximately 40 mg / l to approximately 10,000 mg / l.

37. The process according to claim 1, characterized in that the Mg source comprises a high solubility Mg source and a low solubility Mg source, the high solubility Mg source comprising MgCb or MgSO4 and the low solubility Mg source comprising one or more of: MgO, Mg(OH)2, and a magnesium carbonate.

38. The process according to claim 37, characterized in that the high solubility Mg source and the low solubility Mg source are introduced into the reactor at the same time.

39. A fluidized bed type reactor for precipitating dissolved materials from an aqueous solution, characterized in that it comprises: a reaction tank having an inlet and an outlet; and a hydration tank associated with the reaction tank and configured to hydrate a magnesium (Mg) source in an aqueous solvent and introduce the Mg source as a hydrated suspension into the reaction tank.

40. The reactor according to claim 39, characterized in that it further comprises a control valve configured to control a flow of the hydrated suspension from the hydration tank to the reaction tank.

41. The reactor according to claim 39 or 40, characterized in that it further comprises a recycling route associated with the reaction tank and configured to take the solution from one part of the reaction tank and return at least a portion of the extracted solution to another part of the reaction tank. ηβ I 7 I η / η7P7 / E / YILI 42. The reactor according to any of claims 39 to 41, characterized in that it further comprises a pH probe for measuring the pH of the aqueous solvent in the reaction tank.

43. The reactor according to claim 42, characterized in that it further comprises a controller connected to the pH probe and the control valve, the controller controlling the opening and closing of the control valve in response to a change in pH of the aqueous solvent in the reaction tank.

44. The reactor according to any of claims 39 to 43, characterized in that the hydration tank comprises a first acid injector configured to controllably dose an acid into the hydrated suspension in the hydration tank.

45. The reactor according to claim 44, characterized in that the reactor comprises a second acid injector that is configured to controllably dose an acid into the solution flow in the recycling route.

46. ​​The reactor according to claim 45, characterized in that the reactor comprises a base injector that is configured to controllably dose a base into the solution flow in the reaction tank.

47. The reactor according to claim 46, characterized in that the base injector is located downstream of the acid injector.

48. The reactor according to any of claims 41 to 47, characterized in that the recycling route comprises a fines treatment tank.

49. The reactor according to claim 48, characterized in that it further comprises an upstream solids separation device nR I 7 I η / η7P7 / E / YILI of the fines treatment tank.

50. The reactor according to claim 49, characterized in that the recycling route comprises a solids separation device upstream of the fines treatment tank, and the solids separation device is configured to separate solids from the liquid in the recycling route.

51. The reactor according to claim 39 or 40, characterized in that it further comprises a suspension circuit configured to circulate the hydrated suspension into and out of the hydration tank.

52. The reactor according to claim 51, characterized in that it comprises an inlet connecting the suspension circuit to the reaction tank.

53. A process for producing struvite or a struvite analogue, characterized in that it comprises the steps of: providing a reactor vessel wherein, in at least a portion of the reactor vessel, the cross-sectional area of ​​the reactor vessel increases with elevation; maintaining a fluidized bed of size-separated granules in the portion of the reactor vessel by flowing a solution comprising phosphate upward through the portion of the reactor in which the upward velocity of the flowing solution decreases with elevation in the reactor portion; introducing fine particles of a low-solubility magnesium source into the reactor and allowing the fine particles to disperse in the fluidized bed; and removing some of the granules from the fluidized bed.

54. The process according to claim 53, characterized in that the fine particles comprise magnesium oxide. nR I 7 I η / η7Π7 / E / YILI 55. The process according to claim 53 or 54, characterized in that the fine particles have sizes of SGN 100 or less.

56. The process according to claim 53 or 54, characterized in that the fine particles have diameters of 0.1 mm or less.

57. The method according to any of claims 53 to 56, characterized in that it comprises maintaining a pH at a reference pH value that is equal to or greater than a pH of 7.5 in at least a portion of the reactor vessel.

58. The procedure according to claim 57, characterized in that the reference pH value is at least pH 8.

59. The process according to any of claims 53 to 58, characterized in that it comprises recycling the solution in the reactor vessel through a recycling route extending from a rise in the reactor vessel above the fluidized bed to a rise in the reactor vessel below the fluidized bed.

60. The process according to claim 59, characterized in that it comprises capturing or redissolving struvite particles in the recycling route.

61. The process according to any of claims 53 to 60, characterized in that the introduction of fine particles from a low-solubility magnesium source into the reactor comprises introducing a suspension of the fine particles into the reactor vessel.

62. The process according to claim 61, characterized in that it comprises preparing the suspension by mixing the fine particles with water and allowing the particles to remain in contact with the water for a period of at least a few minutes before introducing the suspension into the reactor vessel.

63. The method according to claim 61 or 62, characterized in that it comprises injecting the suspension into the reactor vessel at a location below the fluidized bed.

64. The process according to any of claims 61 to 63, characterized in that it comprises mixing an acid with the suspension before introducing the suspension into the reactor vessel.

65. The method according to any of claims 61 to 64, characterized in that it comprises controlling a pH at a location in the reactor vessel by adding the suspension.

66. The procedure according to any of claims 61 to 65, characterized in that the solution is buffered to an alkaline pH.

67. The process according to claim 66, characterized in that the solution is buffered to a pH of at least 7.

5.

68. The process according to claim 67, characterized in that the solution is buffered to a pH of approximately 8.

69. The process in accordance with any of claims 53 to 68, characterized in that the solution comprises municipal or agricultural wastewater.

70. An apparatus, characterized in that it comprises any novel inventive feature, combination of features, or subcombination of features disclosed herein.

71. A procedure, characterized in that it comprises any novel inventive step, action, combination of steps and / or actions or subcombination of steps and / or actions described herein.