Phosphate adsorbent and method for producing the same

Abstract

To provide a phosphate adsorbent exhibiting enhanced phosphate adsorption performance.SOLUTION: A phosphate adsorbent comprising CaO MgO 2SiO2 and at least one selected from the group consisting of MgO, CaO MgO SiO2, 2CaO MgO 2SiO2, 2MgO SiO2, and 2CaO SiO2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a phosphate adsorbent and a method for producing the same. [Background technology]

[0002] Phosphorus (P) is used in a variety of applications, such as agricultural fertilizers, surfactants, and animal feed additives, and is an element that constitutes substances essential to daily life. In particular, P is a component of phosphoric acid, one of the three elements (nitrogen, phosphate, and potassium) used in fertilizers. Phosphorus production is also increasing due to factors such as increased food demand due to factors such as a growing world population and improved living standards in developing countries. Phosphorus is primarily extracted as phosphate rock. Due to this increased demand for phosphorus, phosphorus resources are decreasing, raising concerns about resource depletion.

[0003] From the viewpoint of reusing phosphorus resources, phosphorus components are currently recovered at sewage treatment plants and urine treatment plants. For example, Patent Document 1 discloses a system and method for recovering phosphorus components from urine wastewater using a purification device equipped with a column packed with a phosphorus adsorbent. Furthermore, Patent Document 2 discloses a wastewater treatment method for recovering phosphorus components by thermal hydrolysis and drying of sewage sludge.

[0004] Meanwhile, Japan produces approximately 7.5 million tons of rice annually, of which approximately 1.5 million tons, or one-fifth, is rice husks. Of these, 65% are reused, while the remaining 35% is disposed of as waste. Methods for reuse include compost and bedding, but most of the husks are incinerated. In the past, open burning was used for incineration, but this is now restricted due to the toxic gases it produces, which cause environmental pollution. Therefore, there is a need to explore effective ways to utilize the husks and find appropriate disposal methods.

[0005] From the viewpoint of recovering phosphorus components and effectively utilizing rice husks, Patent Document 3 discloses a phosphorus recovery material made of a charcoal made of rice husks carrying calcium, and a method for producing the same. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-054592 [Patent Document 2] Japanese Patent Application Publication No. 2020-157299 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-075706 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the inventors have conducted research and found that there is room for further improvement of the phosphorus recovery material and its manufacturing method described in Patent Document 3. The present disclosure has been made in light of this problem. That is, a primary object of the present disclosure is to provide a phosphate adsorbent that can be manufactured at lower cost and has superior phosphorus component recovery ability (phosphate adsorption ability). This will enable reduction in the amount of rice husk emissions and conservation of phosphorus resources. Another object of the present disclosure is to provide a method for manufacturing such a phosphate adsorbent.

[0008] That is, the phosphate adsorbent according to one embodiment of the present disclosure has MgO·CaO·2SiO2 and At least one selected from the group consisting of MgO, CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2 and 2CaO·SiO2 The present invention comprises: Further, a method for producing a phosphate adsorbent according to an embodiment of the present disclosure includes the steps of: A method for producing the phosphate adsorbent, comprising the steps of: Soaking rice husks in a mixed aqueous solution containing a water-soluble calcium compound and a magnesium salt; Burning the soaked rice husks. The present invention comprises: [Effects of the Invention]

[0009] The present disclosure can provide a phosphate adsorbent that can be produced at lower cost and has superior phosphate adsorption capacity, as well as a method for producing such a phosphate adsorbent. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows the XRD spectra of the phosphate adsorbents of Examples 1 to 4. [Figure 2] FIG. 2 shows the XRD spectra of the phosphate adsorbents of Examples 5 to 8. [Figure 3] FIG. 3 shows the XRD spectra of the phosphate adsorbents of Examples 9 to 12. [Figure 4] FIG. 4 shows the XRD spectra of the phosphate adsorbents of Examples 13 to 16. [Figure 5] FIG. 5 shows the relationship between the specific surface area and the firing temperature of the phosphate adsorbents of Examples 1 to 4. [Figure 6] FIG. 6 shows the relationship between the specific surface area and the firing temperature of the phosphate adsorbents of Examples 5 to 8. [Figure 7] FIG. 7 shows the relationship between the specific surface area and the firing temperature of the phosphate adsorbents of Examples 9 to 12. [Figure 8] FIG. 8 shows the relationship between the specific surface area and the firing temperature of the phosphate adsorbents of Examples 13 to 16. [Figure 9] FIG. 9 shows the phosphate removal rates of the phosphate adsorbents of Examples 1 to 4 prepared at a firing temperature of 600°C. [Figure 10] FIG. 10 shows the phosphate removal rates of the phosphate adsorbents of Examples 5 to 8 prepared at a firing temperature of 800°C. [Figure 11] FIG. 11 shows the phosphate removal rates of the phosphate adsorbents of Examples 9 to 12 prepared at a firing temperature of 1000°C. [Figure 12] FIG. 12 shows the phosphate removal rates of the phosphate adsorbents of Examples 13 to 16 prepared at a firing temperature of 1300°C. [Figure 13] FIG. 13 shows the phosphate removal rates of the phosphate adsorbents of Examples 1 to 4 prepared at an aqueous solution concentration of 0.25 mol / L. [Figure 14] FIG. 14 shows the phosphate removal rates of the phosphate adsorbents of Examples 5 to 8 prepared at an aqueous solution concentration of 0.50 mol / L. [Figure 15] FIG. 15 shows the phosphate removal rates of the phosphate adsorbents of Examples 9 to 12 prepared in aqueous solutions with a concentration of 1.0 mol / L. [Figure 16] FIG. 16 shows the phosphate removal rates of the phosphate adsorbents of Examples 13 to 16 prepared in aqueous solutions with a concentration of 2.0 mol / L. [Figure 17] FIG. 17 shows the specific surface areas of the phosphate adsorbents of Examples 9 to 12, Reference Example 1, and Comparative Examples 1 and 2. [Figure 18] FIG. 18 shows the specific surface areas of the phosphate adsorbents of Examples 9 to 12, Reference Example 1, and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] The "phosphate adsorbent" and "method for producing a phosphate adsorbent (method for producing a phosphate adsorbent)" of the present disclosure will be described in detail below. The description will be made with reference to the drawings as necessary.

[0012] Numerical ranges mentioned in this specification are intended to include the lower and upper limit values ​​themselves, unless otherwise specified, such as "less than," "smaller," or "greater than." That is, for example, a numerical range such as 1 to 10 is interpreted as including the lower limit of 1 and the upper limit of 10.

[0013] In this specification, the term "aqueous solution to be removed" refers to an aqueous solution that contains dissolved phosphorus and is a target for removing at least a portion of the dissolved phosphorus. Here, "dissolved phosphorus" in this specification refers to all phosphorus-containing compounds dissolved in water (more specifically, the aqueous solution to be removed), such as phosphoric acid and its salts (phosphates).

[0014] In this specification, the term "phosphate adsorption capacity (phosphate removal capacity)" refers to the ability to adsorb (remove) dissolved phosphorus in an aqueous solution that is the target for removal.

[0015] <First embodiment: Phosphate adsorbent> The first embodiment relates to a phosphate adsorbent. The phosphate adsorbent according to the first embodiment is CaO·MgO·2SiO2 and At least one selected from the group consisting of MgO, CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2 and 2CaO·SiO2 The present invention comprises:

[0016] (Method for identifying components in phosphorus adsorbents) The method for identifying the components in the phosphate adsorbent is as follows. Using a powder X-ray diffractometer (Rigaku "MiniFlex"), measure the X-ray diffraction (XRD) spectrum under the following conditions: scan speed 3.0° / min, sampling width 0.10, diffraction angle (diffraction angle) 3°-90°, number of accumulations 3, and X-ray source CuKα. Identify the sample from the shape (XRD pattern) in the measured XRD spectrum.

[0017] [Mechanism of action] The phosphate adsorbent according to the first embodiment has excellent phosphate adsorption capacity, and the reason for this is presumed to be as follows, without being bound by any particular theory. The phosphate adsorbent according to the first embodiment comprises CaO·MgO·2SiO2 and at least one selected from the group consisting of MgO, CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2, and 2CaO·SiO2. Therefore, when the phosphate adsorbent is added to an aqueous solution containing dissolved phosphorus, silicic acid and calcium ions are believed to be eluted from the surface of at least the CaO·MgO·2SiO2 into the aqueous solution. The eluted silicic acid and calcium ions are believed to react with dissolved phosphorus in the aqueous solution to produce octalcium phosphate ((Ca8H2(PO4)6·5H2O)). In this way, dissolved phosphorus in the aqueous solution can be recovered. Therefore, the phosphate adsorbent according to this embodiment is believed to have excellent phosphate adsorption ability. However, this embodiment is not limited to this mechanism of action.

[0018] (Composition of phosphate adsorbent) The phosphate adsorbent comprises CaO·MgO·2SiO2 (diopside) and at least one selected from the group consisting of MgO (periclase), CaO·MgO·SiO2 (monticellite), 2CaO·MgO·2SiO2 (akermanite), 2MgO·SiO2 (forstonite), and 2CaO·SiO2 (larnite).

[0019] In one embodiment, the phosphate adsorbent comprises CaO·MgO·2SiO2, MgO, and at least one selected from the group consisting of CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2, and 2CaO·SiO2Si. In one embodiment, the phosphate adsorbent comprises CaO·MgO·2SiO2, MgO, CaO·MgO·SiO2 and / or 2CaO·MgO·2SiO2.

[0020] Specifically, the phosphate adsorbent may comprise CaO·MgO·2SiO2, MgO, and 2CaO·MgO·2SiO2. The phosphate adsorbent may comprise CaO·MgO·2SiO2, MgO, and 2MgO·SiO2. The phosphate adsorbent may comprise CaO·MgO·2SiO2, MgO, 2CaO·MgO·2SiO2, and 2MgO·SiO2. The phosphate adsorbent may comprise CaO·MgO·2SiO2, MgO, 2CaO·SiO2, and CaO·MgO·SiO2. The phosphate adsorbent may comprise CaO·MgO·2SiO2, MgO, 2MgO·SiO2, 2CaO·SiO2, and CaO·MgO·SiO2. The composition of the phosphate adsorbent can be confirmed by the peaks in the XRD spectrum of the phosphate adsorbent.

[0021] The phosphate adsorbent may further comprise a carbonized component and / or SiO2 (cristobalite). Specifically, the phosphate adsorbent may comprise MgO·CaO·2SiO2, MgO, 2CaO·MgO·2SiO2, 2MgO·SiO2, and SiO2. The carbonized component may be derived, for example, from incomplete burning of rice husks. In this case, the carbonized component is, for example, charcoal. The carbonized component may be porous. A porous carbonized component may have a relatively large specific surface area.

[0022] (Phosphate adsorbent components) -MgO·CaO·2SiO2 (diopside)- MgO·CaO·2SiO2 (diopside) can be dissolved in the target aqueous solution. This allows MgO·CaO·2SiO2 to dissolve into the target aqueous solution as its individual components (MgO, CaO, SiO2). In this case, it can react with water molecules in the target aqueous solution to become acidic, creating an acidic atmosphere on the surface of the phosphate adsorbent. Furthermore, CaO dissolved from MgO·CaO·2SiO2 can react with dissolved phosphorus to produce octacalcium phosphate. Furthermore, MgO·CaO·2SiO2 can form nuclei for octacalcium phosphate. This is due to epitaxial growth of octacalcium phosphate on the surface of MgO·CaO·2SiO2.

[0023] The phosphate adsorbent may comprise a crystalline phase (diopside crystalline phase) of MgO·CaO·2SiO2, and preferably comprises the diopside crystalline phase on the surface of the phosphate adsorbent. That is, in a preferred embodiment, the crystalline phase of MgO·CaO·2SiO2 constitutes the surface of the phosphate adsorbent. The diopside crystalline phase forms crystalline nuclei of octacalcium phosphate on its surface, allowing the octacalcium phosphate to grow (epitaxially). Therefore, when the crystalline phase of MgO·CaO·2SiO2 constitutes the surface of the phosphate adsorbent, dissolved phosphorus in the aqueous solution to be removed is easily captured as octacalcium phosphate. That is, in such a case, the phosphate adsorption capacity of the phosphate adsorbent is improved.

[0024] It can be confirmed from the XRD spectrum that the phosphate adsorbent contains a crystalline phase of MgO·CaO·2SiO2 on its surface.

[0025] In a more preferred embodiment, MgO·CaO·2SiO2 comprises a diopside crystalline phase, and the crystallinity of the diopside crystalline phase is low, and more preferably, the crystallinity of the MgO·CaO·2SiO2 crystalline phase (diopside crystalline phase) on the surface of the phosphate adsorbent is low. The low crystallinity of the diopside crystalline phase can be determined by the intensity ratio of peaks assigned to the diopside crystalline phase in an XRD spectrum. The determination method will be described in detail in the Examples.

[0026] When MgO·CaO·2SiO2 has a diopside crystalline phase on the surface of a phosphate adsorbent, it can form octocalcium phosphate crystal nuclei in the target aqueous solution, as described above. On the other hand, if the diopside crystalline phase on the surface of MgO·CaO·2SiO2 is low, MgO·CaO·2SiO2 is more likely to dissolve into the target aqueous solution. As a result, the concentration of the raw material for octocalcium phosphate (CaO) increases around the phosphate adsorbent, and the area around the phosphate adsorbent becomes an acidic atmosphere, promoting the production of octocalcium phosphate and making it easier to capture dissolved phosphorus.

[0027] Preferably, in an X-ray diffraction spectrum measured with a powder X-ray diffractometer under conditions of a sampling width of 0.10, a diffraction angle of 3° to 90°, and an X-ray source of CuKα, the ratio I / I of the peak intensity I to the reference peak intensity I at a diffraction angle 2θ of around 29.9° is 2% or less. Here, a diffraction angle 2θ of approximately 29.9° refers to a diffraction angle 2θ of 29.9° or its vicinity, for example, 29.9±1.5°, 29.9±1.4°, 29.9±1.3°, 29.9±1.2°, 29.9±1.1°, 29.9±1.0°, 29.9±0.9°, 29.9±0.8°, 29.9±0.7°, 29.9±0.6°, 29.9±0.5°, 29.9±0.4°, 29.9±0.3°, 29.9±0.2°, or 29.9±0.1°.

[0028] The phosphate adsorbent (reference sample) assigned to the reference peak intensity I0 was prepared by a solid-state reaction method, in which a mixture of CaCO3, MgO, and SiO2 in a 1:1:1 molar ratio was pre-calcined at 1100°C for 2 hours, and then calcined at 1300°C for 2 hours. In other words, the reference sample phosphate adsorbent herein refers to a phosphate adsorbent in which crystallization has progressed completely and in which the peak intensity of the XRD spectrum does not substantially increase even after further calcination. Here, "substantially no increase in peak intensity of the XRD spectrum" means that the increase in peak intensity is within the range of the measurement error (measurement error, instrument resolution, etc.) and no significant difference is observed.

[0029] When the X-ray diffraction peak intensity ratio I / I0 is below 2%, the crystallinity of the diopside crystal phase contained in MgO·CaO·2SiO2 is low. Low crystallinity facilitates elution of silicic acid and calcium ions from MgO·CaO·2SiO2 into the target aqueous solution. When silicic acid elutes into the target aqueous solution, the pH near the surface of the MgO·CaO·2SiO2 drops, creating an acidic environment. In this case, calcium ions react with dissolved phosphorus to form octacalcium phosphate ((Ca8H2(PO4)6·5H2O)). Furthermore, when calcium ions elute into the target aqueous solution, the reaction rate increases, and calcium ions react with phosphate derivatives to form octacalcium phosphate ((Ca8H2(PO4)6·5H2O)). This further improves phosphate adsorption capacity.

[0030] The content of MgO·CaO·2SiO2 is, for example, 8.8 to 60.7 mol % relative to the substance amount (mol) of the phosphate adsorbent. The method for determining the content will be described in detail in the Examples.

[0031] MgO·CaO·2SiO2 is composed of MgO, CaO, and SiO2. In MgO·CaO·2SiO2, these components (MgO, CaO, and SiO2) exist in a ratio of MgO:CaO:SiO2 = 1:1:2. However, these components may have a slightly different composition ratio from the above. If MgO·CaO·2SiO2 has a slightly different composition ratio from the above, the crystallinity of MgO·CaO·2SiO2 decreases. From the perspective of improving the phosphate adsorption capacity of the phosphate adsorbent, a low crystallinity is preferable. If the crystallinity of MgO·CaO·2SiO2 is low, silicic acid and calcium ions from the phosphate adsorbent are more likely to dissolve from the phosphate adsorbent into the target aqueous solution (containing dissolved phosphorus and from which at least a portion of the dissolved phosphorus is to be removed), which results in a chemical reaction with dissolved phosphorus (e.g., phosphoric acid) and facilitates the capture of dissolved phosphorus. The crystallinity of MgO·CaO·2SiO2 can be determined based on the XRD spectrum. Details will be described in the Examples.

[0032] -MgO(pecurose)- When the phosphate adsorbent contains MgO, the content of MgO is, for example, 10.7 to 59.6 mol % relative to the substance amount of the phosphate adsorbent. The method for determining the content will be described in detail in the Examples.

[0033] As described above, the content of MgO in the phosphate adsorbent is a relative physical property value based on the amount of substance (moles) of the phosphate adsorbent.

[0034] MgO may be present on the surface of the phosphate adsorbent. If MgO is present on the surface of the phosphate adsorbent, the MgO is likely to dissolve into the aqueous solution to be removed, and as a result, the vicinity of the surface of the phosphate adsorbent is likely to become an acidic atmosphere in the aqueous solution to be removed.

[0035] -CaO·MgO·SiO2 (Monticellite)- When the phosphate adsorbent contains CaO·MgO·SiO2, the content of CaO·MgO·SiO2 is, for example, 5.6 to 26.5 mol % relative to the amount of substance of the phosphate adsorbent. The method for determining the content will be described in detail in the Examples.

[0036] CaO·MgO·SiO2 is composed of CaO, MgO, and SiO2. In CaO·MgO·SiO2, these components (CaO, MgO, and SiO2) exist in a ratio of CaO:MgO:SiO2 = 1:1:1. However, these components may have a slightly different composition ratio in CaO·MgO·SiO2. If the composition ratio of CaO·MgO·SiO2 differs slightly from the above, the crystallinity of the CaO·MgO·SiO2 decreases. From the perspective of improving the phosphate adsorption capacity of the phosphate adsorbent, a low crystallinity is preferable. If the crystallinity of CaO·MgO·SiO2 is low, silicic acid and calcium ions are more likely to be eluted from the phosphate adsorbent, which then chemically reacts with dissolved phosphorus (e.g., phosphoric acid) and makes it easier to capture dissolved phosphorus.

[0037] -2CaO·MgO·2SiO2 (akermanite)- When the phosphate adsorbent contains 2CaO·MgO·2SiO2, the content of 2CaO·MgO·2SiO2 is, for example, 5.3 to 23.1 mol % relative to the amount of substance of the phosphate adsorbent. The method for determining the content will be described in detail in the Examples.

[0038] 2CaO·MgO·2SiO2 is composed of CaO, MgO, and SiO2. In 2CaO·MgO·2SiO2, these components (CaO, MgO, and SiO2) exist in a ratio of CaO:MgO:SiO2 = 2:1:2. However, these components may have a slightly different composition ratio in 2CaO·MgO·2SiO2. If the composition ratio of 2CaO·MgO·2SiO2 differs slightly from the above, the crystallinity of 2CaO·MgO·2SiO2 decreases. A low crystallinity is preferable from the perspective of improving the phosphate adsorption capacity of phosphate adsorbents. A low crystallinity of 2CaO·MgO·2SiO2 makes it easier for silicic acid and calcium ions to dissolve from the phosphate adsorbent, which then chemically reacts with dissolved phosphorus (phosphate) and makes it easier to capture dissolved phosphorus.

[0039] -2MgO·SiO2 (forstonite)- When the phosphate adsorbent contains 2MgO·SiO2, the 2MgO·SiO2 dissolves from the phosphate adsorbent in the aqueous solution to be removed, creating an acidic atmosphere near the surface of the phosphate remover. This facilitates the production of octacalcium phosphate, improving the phosphate adsorption capacity of the phosphate adsorbent. When the phosphate adsorbent contains 2MgO·SiO2, the content of 2MgO·SiO2 is, for example, 2.0 to 44.4 mol% relative to the amount of substance of the phosphate adsorbent. The method for determining the content is described in detail in the Examples.

[0040] -2CaO·SiO2(larnite)- When a phosphate adsorbent contains 2CaO·SiO2, the 2CaO·SiO2 dissolves from the phosphate adsorbent in the aqueous solution to be removed, creating an acidic atmosphere near the surface of the phosphate remover. This facilitates the production of octacalcium phosphate, improving the phosphate adsorption capacity of the phosphate adsorbent. When the phosphate adsorbent contains 2CaO·SiO2, the content of 2CaO·SiO2 is, for example, 5.6 to 14.7 mol % relative to the substance amount of the phosphate adsorbent. The method for determining the content will be described in detail in the Examples.

[0041] -SiO2 (cristobalite)- The content of SiO2 is, for example, 0 to 10.4 mol % relative to the substance amount of the phosphate adsorbent. The method for determining the content will be described in detail in the Examples.

[0042] <Second embodiment: Method for producing phosphate adsorbent> The second embodiment relates to a method for producing a phosphate adsorbent. The method for producing a phosphate adsorbent according to the second embodiment includes the steps of: A method for producing a phosphate adsorbent according to the first embodiment, comprising: Soaking rice husks in an aqueous solution containing a water-soluble calcium compound and a water-soluble magnesium compound (hereinafter also referred to as the "soaking step"); Burning the soaked rice husks (hereinafter also referred to as the "burning process"); The present invention comprises:

[0043] The method for producing a phosphate adsorbent according to the second embodiment can produce the phosphate adsorbent at lower cost. The method for producing a phosphate adsorbent according to the second embodiment includes a soaking step and a calcining step, in which the soaking step involves soaking rice husks in an aqueous solution of a water-soluble calcium compound and a water-soluble magnesium compound, and the calcining step involves calcining the soaked rice husks. Therefore, the phosphate adsorbent can be produced relatively easily without requiring complicated equipment.

[0044] Furthermore, the method for producing a phosphate adsorbent according to the second embodiment uses rice husks, which are normally treated as waste, to re-recover and reuse phosphorus components, eliminating the need to extract phosphorus components from phosphate rock, and contributing to the realization of a sustainable society. Furthermore, geopolitical risks arising from political, economic and conflict situations can be avoided, and a stable supply of phosphorus components can be ensured domestically.

[0045] [Motivation behind the invention of this disclosure: Manufacturing method] The present inventors have focused on providing a phosphate adsorbent that effectively utilizes silicon dioxide derived from rice husks and has phosphate adsorption capacity equivalent to that of phosphate adsorbents prepared by conventional sol-gel methods. When using the conventional sol-gel method, it was necessary to extract and purify silicon dioxide from rice husks by calcining them. Furthermore, it was necessary to produce and purify diopside from the silicon dioxide obtained. This method proved to be too costly and involved too many steps in the production of phosphate adsorbents. Based on the above technical knowledge, the present inventors came up with the idea of ​​producing a phosphate adsorbent by adding component elements other than silicon dioxide that make up the phosphate adsorbent to rice husks and then firing the husks. Furthermore, it was found that the phosphate adsorbent prepared by this method does not require purification because not only diopside but also other components (at least one selected from the group consisting of MgO, CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2 and 2CaO·SiO2) have phosphate adsorption ability. In this way, a simpler method for directly producing a phosphate adsorbent has been devised.

[0046] An example of a method for producing a phosphate adsorbent according to the second embodiment will be described below. The method for producing a phosphate adsorbent includes, for example, a soaking step, a drying step, a firing step, and a pulverization step.

[0047] (Immersion process: Immersion in a mixed aqueous solution of a water-soluble calcium compound and a water-soluble magnesium compound) In the soaking step, the rice husks are soaked in a mixed aqueous solution containing a water-soluble calcium compound and a water-soluble magnesium compound.

[0048] The soaking step may involve placing the rice husks in the mixed aqueous solution and subjecting them to a shaking treatment. The temperature for the shaking treatment is, for example, room temperature (25°C) to 40°C. The shaking treatment time is, for example, 1 to 10 hours. The shaking treatment may be performed by periodically vibrating the sealed container or by rotating it with a vibrator or magnetic stirrer.

[0049] The concentrations of the water-soluble calcium compound and the water-soluble magnesium compound contained in the mixed aqueous solution are each independently 0.25 mol / L to 2.0 mol / L. When the concentrations of the water-soluble calcium compound and the water-soluble magnesium compound are each independently 0.25 to 2.0 mol / L, an appropriate amount of water-soluble magnesium compound and water-soluble calcium compound is present on the surface of the phosphate adsorbent. Therefore, when the phosphate adsorbent is present in the aqueous solution to be removed, the water-soluble magnesium compound and the water-soluble calcium compound are eluted from the surface of the phosphate adsorbent, easily forming an atmosphere with a relatively high concentration of water-soluble magnesium compound and water-soluble calcium compound around (near) the surface of the phosphate adsorbent. This makes it easy for a chemical reaction with dissolved phosphorus to occur around the surface of the phosphate adsorbent, resulting in the formation of octacalcium phosphate on the surface of the phosphate adsorbent.

[0050] As used herein, the term "rice husk" refers to the relatively hard outermost shell of grains (e.g., rice, barley, wheat, millet, and barnyard millet). The rice husk may be derived from a plant of the Poaceae family. In other words, in this case, the rice husk is rice husk.

[0051] Examples of water-soluble calcium compounds include calcium hydroxide (Ca(OH)2) and its salts (calcium salts). Examples of calcium salts include inorganic and organic acid salts of calcium. Examples of inorganic acid salts of calcium include calcium halides (more specifically, calcium bromide, calcium chloride, and calcium iodide) and calcium nitrate. Examples of organic acid salts of calcium include calcium acetate and calcium citrate.

[0052] Examples of water-soluble magnesium compounds include magnesium hydroxide (Mg(OH)2) and its salts (magnesium salts). Examples of magnesium salts include inorganic and organic acid salts of magnesium. Examples of inorganic acid salts of magnesium include magnesium halides (more specifically, magnesium bromide, magnesium chloride, and magnesium iodide) and magnesium nitrate. Examples of organic acid salts of magnesium include magnesium acetate and magnesium citrate.

[0053] Among these, the water-soluble calcium compound is preferably a calcium salt, more preferably an inorganic acid salt of calcium, and even more preferably calcium chloride. The water-soluble magnesium compound is preferably a magnesium salt, more preferably an inorganic acid salt of magnesium, and even more preferably magnesium chloride.

[0054] (Drying process: Drying of soaked rice husks) After shaking, remove the rice husks from the mixed aqueous solution, filter them, and thoroughly dry them.

[0055] (Burning process: Burning of soaked rice husks) In the calcination step, the dried rice husks are placed in a calciner and calcined. The calcination temperature is, for example, 600°C to 1300°C. From the viewpoint of increasing the content of MgO·CaO·2SiO2 in the phosphate adsorbent, the calcination temperature is preferably 800°C to 1300°C. From the viewpoint of decreasing the crystallinity of MgO·CaO·2SiO2 in the phosphate adsorbent, the calcination temperature is preferably 600°C to 1000°C, more preferably 600°C to 800°C. From the viewpoint of improving the phosphate adsorption capacity of the phosphate adsorbent, the calcination temperature is preferably 600°C to 1000°C, more preferably 600°C to 800°C. When the calcination temperature is 600°C to 800°C, the crystallinity of the inorganic compound in the phosphate adsorbent produced by calcination is not too high. Therefore, silicic acid and calcium ions are easily eluted from the phosphate adsorbent, undergo a chemical reaction with phosphoric acid, and are easily captured on the surface of the phosphate adsorbent. The baking temperature is, for example, 2 to 3 hours. After baking, the mixture may be left to return to room temperature (25°C).

[0056] (Crushing process: Crushing burnt rice husks) In the pulverization step, the burned rice husks are pulverized. For example, the pulverization step may be carried out in an agate mortar until the husks pass through a sieve (mesh opening: 250 μm).

[0057] Aspects of the phosphate adsorbent and the method for producing the phosphate adsorbent according to the present disclosure are as follows. <1> CaO·MgO·2SiO2 and At least one selected from the group consisting of MgO, CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2 and 2CaO·SiO2 A phosphate adsorbent comprising: <2> CaO·MgO·2SiO2 and MgO and At least one selected from the group consisting of CaO·MgO·SiO2, 2CaO·MgO·2SiO2, 2MgO·SiO2 and 2CaO·SiO2 comprising <1> The phosphate remover according to claim 1. <3> CaO·MgO·2SiO2 and MgO and CaO·MgO·SiO2 and / or 2CaO·MgO·2SiO2 comprising <1> or <2> The phosphate remover according to claim 1. <4> Further comprising a carbonized component and / or SiO2; <1> ~ <3> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <5> The MgO·CaO·2SiO2 comprises a diopside crystalline phase, <1> ~ <4> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <6> The diopside crystalline phase has low crystallinity, <5> The phosphate adsorbent according to claim 1. <7> In the X-ray diffraction spectrum measured by a powder X-ray diffractometer under the conditions of a sampling width of 0.10, a diffraction angle of 3° to 90°, and an X-ray source of CuKα, The ratio I / I0 of the peak intensity I to the reference peak intensity I0 at a diffraction angle 2θ of approximately 29.9° is 2% or less, The phosphate adsorbent assigned to the reference peak intensity I0 is prepared by a solid-state reaction method in which a mixture of CaCO3, MgO, and SiO2 in a molar ratio of 1:1:1 is pre-baked at a baking temperature of 1100°C for 2 hours, and then baked at a baking temperature of 1300°C for 2 hours. <1> ~ <6> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <8> The content of the MgO·CaO·2SiO2 is 8.8 to 60.7 mol% based on the amount of the substance of the phosphate adsorbent. <1> ~ <7> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <9> The MgO is included, The content of the MgO is 10.7 to 59.6 mol% relative to the amount of the phosphate adsorbent. <1> ~ <8> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <10> The CaO·MgO·SiO2 is included, 3. The phosphate adsorbent according to claim 1, wherein the content of said CaO·MgO·SiO2 is 5.6 to 26.5 mol % based on the amount of substance of the phosphate adsorbent. <11> The 2CaO·MgO·2SiO2 is included, The content of the 2CaO·MgO·2SiO2 is 5.3 to 23.1 mol% relative to the amount of the phosphate adsorbent. <1> ~ <10> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <12> The 2MgO SiO2 is included, The content of the 2MgO·SiO2 is 2.0 to 44.4 mol % relative to the amount of the substance of the phosphate adsorbent. <1> ~ <11> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <13> The 2CaO SiO2 is included, The content of the 2CaO·SiO2 is 5.6 to 14.7 mol% relative to the substance amount of the phosphate adsorbent. <1> ~ <12> 10. The phosphate adsorbent according to claim 9, wherein the phosphate adsorbent is a phosphate binder. <14> <1> ~ <13> A method for producing the phosphate adsorbent according to any one of the above, Immersing rice husks in a mixed aqueous solution containing a water-soluble calcium compound and a water-soluble magnesium compound; Burning the soaked rice husks. A method for producing a phosphate adsorbent, comprising: <15> The rice husk is derived from a plant of the family Poaceae. <14> A method for producing the phosphate adsorbent described in <16> The water-soluble calcium compound is calcium chloride, and the water-soluble magnesium compound is magnesium chloride. <14> or <15> A method for producing the phosphate adsorbent described in <17> The firing is carried out at a temperature of 600°C to 1300°C. <14> ~ <16> 10. A method for producing the phosphate adsorbent according to any one of the above. <18> the concentrations of the water-soluble calcium compound and the water-soluble magnesium compound in the mixed aqueous solution are each independently 0.25 mol / L to 2.0 mol / L; <14> ~ <17> 10. A method for producing the phosphate adsorbent according to any one of the above.

[0058] Although the embodiments have been described above, it will be understood that various changes in form and details may be made without departing from the spirit and scope of the claims. [Example]

[0059] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. The preparation, evaluation, and measurement of the phosphate adsorbent were carried out at room temperature and atmospheric pressure (23°C and atmospheric pressure: 1 atm) in an open system unless otherwise specified.

[0060] <1. Preparation of phosphate adsorbent> [Example 1] (Soaking process: soaking rice husks in an aqueous solution of a water-soluble calcium compound and a water-soluble magnesium compound) A mixture was obtained by mixing calcium chloride dihydrate (Fujifilm Wako Pure Chemical Corporation, special reagent grade, CaCl2·2H2O) and magnesium chloride hexahydrate (Fujifilm Wako Pure Chemical Corporation, special reagent grade, MgCl2·6H2O). The resulting mixture was then poured into 60 mL of purified water and stirred. The calcium chloride and magnesium chloride concentrations were 0.25 mol / L, respectively. This resulted in the preparation of a mixed aqueous solution of calcium chloride and magnesium chloride. 30 g of rice husks (brand "Aichi no Kaori", manufactured in 2018) were added to 60 mL of the resulting mixed aqueous solution (mixed aqueous solution of calcium chloride and magnesium chloride). The mixture was shaken at 25°C for 6 hours using a small shaking high-temperature oven ("PIC-1015" manufactured by AS ONE Corporation).

[0061] (Drying process: Drying of soaked rice husks) After the shaking treatment, the rice husks were filtered out of the mixed aqueous solution and then thoroughly dried.

[0062] (Burning process: Burning of soaked rice husks) The dried rice husks were placed in a high-temperature heating electric furnace ("NL-2025D" manufactured by Motoyama Corporation). The temperature inside the furnace was raised to 600°C at a heating rate of 18°C / min, and the temperature inside the furnace (firing temperature) was maintained at 600°C for 2 hours to burn the dried rice husks. After burning, the burned material was left in the furnace (natural cooling) until the temperature inside the furnace returned to room temperature (25°C).

[0063] (Crushing process: Crushing burnt rice husks) The burned rice husks were pulverized in an agate mortar until they completely passed through a sieve (mesh opening: 250 μm). The obtained powder was used as the phosphate adsorbent of Example 1. Table 1 summarizes the production method and production conditions for the phosphate adsorbent.

[0064] [Examples 2 to 16] Phosphate adsorbents were prepared in the same manner as in Example 1, except that the concentration of the metal aqueous solution (concentration of calcium chloride and magnesium chloride) was changed from 0.25 mol / L to 0.50, 1.0, or 2.0 mol / L, and / or the firing temperature was changed from 600°C to 800, 1000, or 1300°C. [Table 1]

[0065] [Reference example 1] (Diopside synthesized by the sol-gel method) 29.52 g (0.125 mol) of calcium nitrate tetrahydrate (Fujifilm Wako Pure Chemical Industries, Ltd., special reagent grade, Ca(NO3)2·4H2O), 25.41 g (0.125 mol) of magnesium chloride hexahydrate (Fujifilm Wako Pure Chemical Industries, Ltd., special reagent grade, MgCl2·6H2O), and 55.4 mL (0.250 mol) of tetraethoxysilane (Fujifilm Wako Pure Chemical Industries, Ltd., special reagent grade, Si(OC2H5)4) were weighed out to a molar ratio (Ca:Mg:Si) of 1:1:2.

[0066] The calcium nitrate tetrahydrate and magnesium chloride hexahydrate were dissolved in 150 mL of ethanol. The tetraethoxysilane was then added to obtain a mixed solution. The mixed solution was stirred for 1 hour and allowed to stand at 80°C for 24 hours to obtain a colorless, transparent gel.

[0067] This gel was left to stand at 80°C for 2 to 3 days to be thoroughly dried. After drying, it was pulverized to obtain a gel powder. The gel powder was sintered at 650°C for 2 hours using a tabletop electric furnace ("AMF-20" manufactured by Asahi Rika Seisakusho Co., Ltd.). It was then pulverized in a mortar. The obtained powder was used as the phosphate adsorbent of Reference Example 1.

[0068] [Comparative Example 1] (Diopside derived from natural minerals) Diopside ore (a natural mineral from Shandong Province, China (where the ore is mined), manufactured in 2017) was pulverized until it passed through a sieve (mesh opening: 53 μm). The obtained powder was used as the phosphate adsorbent of Comparative Example 1.

[0069] Comparative Example 2 (Diopside synthesized by solid-state reaction method) Calcium carbonate (Fujifilm Wako Pure Chemical Industries, Ltd., special reagent grade, CaCO3), magnesium oxide (Fujifilm Wako Pure Chemical Industries, Ltd., special reagent grade, MgO), and silicon dioxide (Fujifilm Wako Pure Chemical Industries, Ltd., special reagent grade, SiO2) were weighed out to a molar ratio of 1:1:1 (i.e., a weight ratio (CaCO3:MgO:SiO2) of 38.4:15.5:46.1).

[0070] The weighed calcium carbonate, magnesium oxide, and silicon dioxide were mixed while adding pure water appropriately to obtain a mixture.

[0071] The mixture was placed in a high-temperature heating electric furnace ("NL-2025D" manufactured by Motoyama Corporation). Pre-firing was performed at a furnace temperature of 1100°C for 2 hours. Thereafter, main firing was performed at a furnace temperature of 1300°C for 2 hours to obtain a fired product. The fired product was pulverized until it completely passed through a sieve (mesh opening: 53 μm). The obtained powder was used as the phosphate adsorbent of Comparative Example 2.

[0072] [Comparative Examples 3 to 4] A phosphate adsorbent of Comparative Example 3 was prepared in the same manner as in Example 1, except that the rice husks were not immersed in the metal aqueous solution. A phosphate adsorbent of Comparative Example 4 was prepared in the same manner as in Example 1, except that the rice husks were not immersed in the metal aqueous solution and the calcination temperature was changed.

[0073] <2.Measurement method> [2-1. Measurement of X-ray diffraction spectrum: Identification analysis (determination of the composition of phosphate remover)] (Determining the components contained in phosphate removers) The X-ray diffraction spectrum of the phosphate adsorbent was measured using a powder X-ray diffractometer (MiniFlax, manufactured by Rigaku Holdings Corporation) under the following conditions: sampling width 0.10°, diffraction angle 3°-90°, accumulation 3 times, and X-ray source CuKα. The signal intensity (peak intensity) of the measured XRD spectrum was used to determine the presence of specific components assigned to specific signals based on the following criteria. Based on these results, the composition of the phosphate remover was determined. The results are shown in Tables 2 and 3. (standard) The component is contained in the phosphate remover: signal intensity is 5 cps or more The component is not included in the phosphate remover: signal intensity is less than 5 cps

[0074] When there are multiple peaks attributable to a specific component in the XRD spectrum, the peak with the greatest intensity was used for the determination. Here, the peak intensity in this specification refers to the maximum value of the peak intensity in the XRD spectrum, not the integrated value.

[0075] Although the components of the phosphate remover listed in Tables 2 to 3 may not be shown in the XRD spectra of Figures 1 to 3, this does not mean that the components were not identified in Figures 1 to 3.

[0076] (Determination of the content of components contained in phosphate remover) -Diopside content- The sum of the peak intensities of the components (components contained in the phosphate remover) identified in the above (Determination of the components contained in the phosphate remover) I total In this case, when there are multiple peaks attributed to a specific component, the peak with the greatest peak intensity among them is used. The peak intensity I of a specific component (e.g., diopside) in the obtained XRD spectrum is calculated. 特定の成分 , and the sum of the peak intensities I total From formula (1): C 特定の成分 (mol%)=(I 特定の成分 / I total )×100···(1) Using this, the content of a specific component C 特定の成分 was calculated.

[0077] The calculated content C of the components in the phosphate adsorbent is summarized in Tables 2 and 3.

[0078] [2-2. Evaluation of crystallinity of phosphate adsorbents: Crystallinity of diopside] In the XRD spectra of the phosphate adsorbents of Examples 1 to 16 measured by the method described in 2-1, the peak intensity at a diffraction angle 2θ of approximately 29.9° (hereinafter also referred to as the "strongest peak intensity") attributed to diopside was the strongest among the multiple peaks attributed to diopside in the XRD spectrum.

[0079] The maximum peak intensity (or diffraction intensity; unit: cps) of the XRD spectrum of Examples 1 to 16 was measured. In calculating the ratio I / I0, a reference peak intensity I0 was determined as a comparison target. The reference peak intensity I0 was the intensity of the strongest peak (strongest reference peak intensity) among multiple peaks in the XRD spectrum of the phosphate adsorbent prepared by the solid-state reaction method (Comparative Example 2).

[0080] The ratio I / I0 (no unit) of the peak intensity I at a diffraction angle 2θ of about 29.9° in the XRD spectra of Examples 1 to 16 to the strongest reference peak intensity I0 of Comparative Example 2 was calculated. The degree of crystallinity of diopside was determined from the calculated peak intensity ratio I / I0 based on the following criteria. (Judgment criteria) Diopside has a high degree of crystallinity: the calculated peak intensity ratio I / I0 is less than 2% Diopside has low crystallinity: the calculated peak intensity ratio I / I0 is greater than 2%

[0081] [2-3. Surface area measurement] Using a specific surface area measuring device (Monosorb MS-21 manufactured by Yuasa Ionics Co., Ltd.), the surface area of ​​the phosphate adsorbent was measured under carrier gas conditions (a mixture of nitrogen gas and helium gas) using the Brunauer-Emmett-Teller (BET) single-point method.

[0082] [2-4. Phosphate adsorption measurement using the molybdenum blue method] (Preparation of measurement samples) The phosphate adsorption capacity of the phosphate adsorbent was evaluated using the molybdenum blue method. Potassium dihydrogen phosphate (KH2PO4, special grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in purified water to prepare a potassium dihydrogen phosphate aqueous solution (100 μmol / L). Hereinafter, this potassium dihydrogen phosphate concentration will also be referred to as the "initial concentration." 0.1 g of phosphate adsorbent was added to 100 mL of the prepared potassium dihydrogen phosphate aqueous solution and stirred using a stir bar and a magnetic stirrer. Samples were taken 30 and 60 minutes later. Stirring was initiated immediately after adding the phosphate adsorbent to the potassium dihydrogen phosphate aqueous solution. 30 minutes after the start of stirring, 0.5 mL of acidic ascorbic acid solution was added to the sample, which was then inverted and stirred with the lid on. Two minutes after the addition of the acidic ascorbic acid solution, an additional 0.5 mL of acidic molybdenum solution was added, and the solution was inverted and stirred with the lid on. This liquid sample was stirred for an additional 20 minutes to prepare the measurement sample.

[0083] For the sample taken 60 minutes after the start of stirring, a measurement sample was prepared in the same manner as for the sample taken 30 minutes after the start of stirring.

[0084] (measurement) The measurement sample was placed in a spectrophotometer (Hitachi, Ltd., "U-5100") and the absorbance at 883 nm was measured. The obtained absorbance was compared with the known molar extinction coefficient (ε) at 883 nm. 883 ) and the optical path length, the potassium dihydrogen phosphate concentration in the measurement sample (hereinafter also referred to as "potassium dihydrogen phosphate concentration after 30 minutes of stirring") was calculated using the Beer-Lambert equation. The absorbance at 883 nm of the sample 60 minutes after the start of stirring was measured in the same manner as for the sample 30 minutes after the start of stirring, and the potassium dihydrogen phosphate concentration in the measured sample (hereinafter also referred to as "potassium dihydrogen phosphate concentration after 60 minutes of stirring") was calculated.

[0085] (Calculation of phosphate removal rate) The initial concentration of potassium dihydrogen phosphate [H2KPO4] (unit: mol / L) and the resulting concentration of potassium dihydrogen phosphate [H2KPO4] after T minutes of stirring T (unit: mol / L) (T = 30 minutes or 60 minutes) from the following formula (3):

number

[0086] [2-5. Surface observation of phosphate adsorbent: Scanning electron microscope image] The measurement sample was prepared by adding 0.1 g of phosphate adsorbent to 100 mL of potassium dihydrogen phosphate aqueous solution and stirring the mixture (2-4). SEM images of the phosphate adsorbent surface were obtained using a scanning electron microscope (Hitachi, Ltd., "S-2600N").

[0087] <3. Conclusion and Discussion> [3-1. XRD Spectrum] (Identification (composition) of phosphate adsorbent) Figure 1 shows the XRD spectra of the phosphate adsorbents of Examples 1 to 4. Figure 2 shows the XRD spectra of the phosphate adsorbents of Examples 5 to 8. Figure 3 shows the XRD spectra of the phosphate adsorbents of Examples 9 to 12. Figure 4 shows the XRD spectra of the phosphate adsorbents of Examples 13 to 16. In Figures 1 to 4, the horizontal axis represents the diffraction angle (unit: °), and the vertical axis represents the signal intensity (or diffraction intensity; unit: arbitrary unit). In Figures 1 to 4, the symbol ▲ represents a peak attributed to diopside (CaO·MgO·2SiO2). The symbol ● represents a peak attributed to perecryse (MgO). The symbol + represents a peak attributed to cristobalite (SiO2). The symbol ○ represents a peak attributed to akermanite (2CaO·MgO·2SiO2). The symbol ■ represents a peak attributed to forstonite (2MgO·SiO2). The symbol × indicates a peak attributed to larnite (2CaO·SiO2), and the symbol ◆ indicates monticellite (CaO·MgO·SiO2).

[0088] As shown in Figure 1, the XRD spectrum of Example 1 had multiple peaks. These peaks were assigned to diopside (CaO-MgO-2SiO2) (represented by ▲ in Figure 1), perecryse (MgO) (represented by ● in Figure 1), and forsnite (2MgO SiO2) (represented by ■ in Figure 1), respectively.

[0089] Based on the above assignments, it was determined that the phosphate adsorbent of Example 1 contained at least diopside (CaO-MgO-2SiO2), perecryse (MgO), and forstnite (2MgO SiO2). The results are shown in Table 2.

[0090] Furthermore, the content of each component in the phosphate adsorbent was calculated from the intensity of the XRD spectrum. The results are shown in Tables 2 and 3. The numerical values ​​(unit: mol%) in Tables 2 and 3 represent the content of the component. The content of each component represents the content based on the total amount of substance (total number of moles) of all components contained in the phosphate remover.

[0091] The XRD spectra of Examples 2 to 16 were assigned from the XRD spectrum in the same manner as Example 1. The results are shown in Tables 2 and 3.

[0092] [Table 2]

[0093] [Table 3]

[0094] (Dependence of peak intensity on calcination temperature in XRD spectra) As shown in Figures 1 to 4, the shapes of the XRD spectra of all the phosphate adsorbents were almost the same. The peak intensities (signal intensities) are summarized in Table 4. In this specification, the signal intensity refers to the peak intensity (maximum signal intensity) rather than the peak integral value.

[0095] [Table 4]

[0096] As shown in Figure 1 and Table 4, when the concentration of the metal aqueous solution is fixed and the calcination temperature is increased, the peak intensity increases. This indicates that the crystallinity of the inorganic compounds that make up the phosphate adsorbent increases with increasing calcination temperature.

[0097] (Dependence of peak intensity in XRD spectrum on concentration of metallic sulphur solution) 2 and 3, it can be seen that the peak intensity increases when the concentration of the metal aqueous solution is increased while the calcination temperature is fixed. This indicates that the crystallinity of the inorganic compound constituting the phosphate adsorbent increases with the increase in the concentration of the metal aqueous solution.

[0098] [3-2. Specific surface area of ​​phosphate adsorbent] (Overall trend of specific surface area) 5 to 8 show the relationship between the specific surface area and the firing temperature of the phosphate adsorbents of Examples 1 to 16. Table 5 shows the specific surface area of ​​the phosphate remover. In Table 5, "-" in the "Ratio I / I0" column indicates that no measurement was performed.

[0099] [Table 5]

[0100] As shown in Figures 5 to 8 and Table 5 (note that the scale of the vertical axis in Figures 7 and 8 is smaller than that in Figures 5 and 6), 2 The phosphate adsorbents prepared in Examples 1-2 and 5-6, which had a calcination temperature of 600°C or 800°C and an aqueous metal solution concentration of 0.25 mol / L or 0.50 mol / L, had a specific surface area of ​​1 / g or more. As a general trend, it was found that the phosphate removers prepared at relatively low aqueous solution concentrations and relatively low calcination temperatures had relatively large specific surface areas.

[0101] The tendency for the specific surface area to increase when the firing temperature is relatively low suggests that when the firing temperature is relatively high, the charcoal from the rice husks disappears completely, and the pores on the SiO2 surface become blocked by CaO and MgO and disappear. In addition, the tendency for the specific surface area to increase when the metal aqueous solution concentration is relatively low suggests that when the metal aqueous solution concentration is relatively high, the pores on the SiO2 surface are blocked by MgO and CaO.

[0102] (Relationship with metal aqueous solution concentration) For example, in Examples 1, 5, 9 and 13, as shown in Table 5, it was observed that when the concentration of the aqueous metal solution was increased at the same firing temperature of 600° C., the specific surface area tended to decrease.

[0103] (Relationship with firing temperature) For example, in Examples 1 to 4, as shown in Table 5 and FIG. 5, it was observed that when the firing temperature was increased at the same metal aqueous solution concentration of 0.25 mol / L, the specific surface area tended to decrease.

[0104] (Relationship between immersion in metal aqueous solution and specific surface area) As shown in Table 5, comparisons at the same calcination temperature (comparison between Comparative Example 3 and Example 1, and comparison between Comparative Example 4 and Example 2) revealed that the phosphate adsorbents of Examples 1 and 2, which were immersed in an aqueous metal solution, had a larger specific surface area than the phosphate adsorbents of Comparative Examples 3 and 4, which were not immersed in an aqueous metal solution. This suggests that immersion of the rice husks in an aqueous metal solution coated the surface of SiO2 with MgO and CaO, suppressing the disappearance of pores during the manufacturing process of the phosphate adsorbent, resulting in an increase in the specific surface area.

[0105] (carbide) When the phosphate adsorbents of Examples 1 to 16 were observed (visual appearance observation), black charcoal was observed in the phosphate adsorbents fired at 600°C. On the other hand, no black charcoal was observed in the phosphate adsorbents fired at 800°C, 100°C, or 1300°C. From these observation results, it is considered that when the phosphate adsorbent was fired at 600°C, complete incineration was not achieved (i.e., not all organic matter was converted to carbon dioxide), and charcoal remained. Therefore, it is considered that the specific surface area of ​​the phosphate adsorbents fired at 600°C (Examples 1, 5, 9, and 13) includes the specific surface area of ​​the charcoal.

[0106] (Relationship between specific surface area of ​​phosphate adsorbent and phosphate removal rate) As shown in Table 5, when the specific surface area and phosphate removal rate of Examples 1, 5, 9, and 13 are compared, a slight increase in the phosphate removal rate was observed from Examples 1 to 13, despite the large increase in specific surface area. This suggests that the specific surface area of ​​the phosphate adsorbent makes a small contribution to the phosphate removal rate of the phosphate adsorbent in the systems of Examples 1, 5, 9, and 13. Because the specific surface area is a physical property that reflects physical adsorption, it is thought that the contribution of physical adsorption is small in the systems of Examples 1, 5, 9, and 13.

[0107] [3-3. Phosphate removal rate] (Relationship with firing temperature) 9 to 12 show the phosphoric acid removal rates for Examples 1, 5, 9, and 13 (calcination temperature 600°C), Examples 2, 6, 10, and 14 (calcination temperature 800°C), Examples 3, 7, 11, and 15 (calcination temperature 1000°C), and Examples 4, 8, 12, and 16 (calcination temperature 1300°C). In the examples using calcination temperatures of 600°C, 800°C, and 1000°C, the phosphoric acid removal rates were approximately the same (80%). In Examples 4, 8, 12, and 16, where the firing temperature was 1300°C, the phosphoric acid removal rates of Examples 4 and 8 were smaller than those of Examples 12 and 16.

[0108] The average value of the phosphate removal rate was calculated for each example in which the firing temperature was the same, and is summarized in Table 6. As shown in Table 6, it was found that the phosphate removal rate (phosphate adsorption rate) decreased as the firing temperature increased.

[0109] [Table 6]

[0110] These results showed that, as a general trend, phosphate adsorbents prepared at relatively low firing temperatures (i.e., phosphate adsorbents with a lower degree of crystallinity) had higher phosphate adsorption capacity. This is thought to be because phosphate adsorbents with a lower degree of crystallinity are more likely to be eluted as silicate and calcium ions from the phosphate removal agent components (i.e., the amount eluted is greater), and are more likely to react with phosphate derivatives to produce octacalcium phosphate ((Ca8H2(PO4)6·5H2O)). Therefore, phosphate adsorbents prepared at relatively low firing temperatures are thought to have excellent phosphate adsorption capacity.

[0111] (Relationship with metal aqueous solution concentration) 13 to 16 show the phosphoric acid removal rates of Examples 1 to 4, Examples 5 to 8, Examples 9 to 12, and Examples 13 to 16. In Examples 13 to 16 with an aqueous solution concentration of 2.0 mol / L and Examples 9 to 12 with an aqueous solution concentration of 1.0 mol / L, the phosphoric acid removal rates were almost the same (80%). In Examples 5 to 8, where the aqueous solution concentration was 0.50 mol / L, the phosphoric acid removal rate in Example 8 was lower than in Examples 5 to 7. In Examples 1 to 4, where the aqueous solution concentration was 0.25 mol / L, the phosphoric acid removal rate in Example 4 was lower than in Examples 1 to 3.

[0112] The average value of the phosphate removal rate was calculated for each example in which the firing temperature was the same, and the results are summarized in Table 7. As shown in Table 7, it was found that, as the concentration of the metal aqueous solution increased, the phosphate removal rate (phosphate adsorption rate) increased overall.

[0113] [Table 7]

[0114] These results suggest that phosphate adsorbents prepared with a relatively high concentration of metal solution are more likely to dissolve as silicate and calcium ions from the components of the adsorbent (i.e., the amount of dissolution is large), and are more likely to react with phosphoric acid derivatives to produce octacalcium phosphate ((Ca8H2(PO4)6·5H2O)). Therefore, phosphate adsorbents prepared at a relatively low firing temperature are thought to have excellent phosphate adsorption capacity.

[0115] (Relationship with specific surface area: Table 7) Regarding the relationship between the phosphate removal rate and the metal aqueous solution concentration, as mentioned using Table 7, it was observed that when the calcination temperature was fixed and the aqueous solution concentration was increased, the phosphate removal rate tended to increase. On the other hand, regarding the relationship between the specific surface area and the metal aqueous solution concentration, as mentioned using Table 5, it was observed that when the calcination temperature was fixed and the aqueous solution concentration was increased, the specific surface area tended to decrease. In other words, it was found that with the phosphate remover of the example, when the metal aqueous solution concentration was increased, the phosphate removal rate increased despite the decrease in specific surface area.

[0116] The following two modes are thought to be the modes in which dissolved phosphorus in the aqueous solution to be removed is captured by the phosphate removing agent. In one mode of capture, the dissolved phosphorus in the aqueous solution to be removed forms octacalcium phosphate with calcium ions and silicic acid derived from the phosphate removing agent, and crystals grow on the surface of the phosphate removing agent, thereby being removed from the aqueous solution to be removed. On the other hand, in another mode of capture, the dissolved phosphorus in the aqueous solution to be removed is thought to be bound to the surface of the phosphate removing agent by a relatively small force such as an intermolecular force (van der Waals force), and captured.

[0117] The tendency seen in Tables 5 and 7 (in the phosphate removers of the Examples, the phosphate removal rate tends to increase when the metal aqueous solution concentration increases, despite a decrease in specific surface area) suggests that in the phosphate removers of the Examples, the latter capture mode resulting from the specific surface area contributes relatively little to the phosphate removal ability, while the former capture mode contributes relatively much to the phosphate removal ability. In other words, the phosphate removers of the Examples are thought to remove dissolved phosphorus from the target aqueous solution mainly in the former mode.

[0118] (Relationship with specific surface area: Comparison with comparative examples and reference examples) Fig. 17 shows the phosphate removal rates of the phosphate adsorbents of Examples 9 to 12, Reference Example 1, and Comparative Examples 1 and 2. As shown in Fig. 17, the phosphate removal rates of the phosphate adsorbents of Examples 9 to 12 were very high, ranging from 79 to 91.1%, and were almost equivalent to the phosphate removal rate of Reference Example 1. In contrast, the phosphate removal rates of the phosphate adsorbents of Comparative Examples 1 and 2 were much lower than those of Examples 9 to 12.

[0119] Fig. 18 shows the specific surface areas of the phosphate adsorbents of Examples 9 to 12, Reference Example 1, and Comparative Examples 1 and 2. As shown in Fig. 18, the specific surface areas of the phosphate adsorbents of Examples 9 to 12 were much smaller than the specific surface area of ​​the phosphate adsorbent of Reference Example 1 (phosphate adsorbent derived from the sol-gel method) and slightly larger than the specific surface area of ​​the phosphate adsorbent of Comparative Example 12.

[0120] As shown in Figure 18, the phosphate adsorbents of Examples 9 to 12 exhibited phosphate removal rates (phosphate removal capacity) almost equivalent to that of the phosphate adsorbent of Reference Example 1 (a benchmark prior art phosphate removal agent), despite having a much smaller specific surface area than the phosphate adsorbent of Reference Example 1. Thus, no strong correlation was found between the phosphate removal rate (phosphate removal capacity) and the specific surface area of ​​the phosphate removers of the Examples of the present disclosure. As mentioned in the discussion using Tables 5 and 7, the phosphate removers of the Examples removed dissolved phosphorus from the target aqueous solution mainly in the former manner, suggesting that the mechanism of phosphate removal differs from that of the conventional phosphate remover (Reference Example 1). [Industrial Applicability]

[0121] The phosphate removing agent according to the present disclosure can remove dissolved phosphorus from an aqueous solution. For example, the phosphate removing agent according to the present disclosure can be added to an aqueous solution containing phosphorus, such as sewage in a sewage treatment facility or manure in a manure treatment facility, to remove dissolved phosphorus from the aqueous solution and effectively recover the phosphorus component.

Claims

1. CaO・MgO・2SiO ２ and, MgO, CaO・MgO・SiO ２ , 2CaO・MgO・2SiO ２ , 2MgO.SiO ２ and 2CaO.SiO ２ and at least one selected from the group consisting of A phosphate adsorbent comprising:

2. CaO・MgO・2SiO ２ and, MgO, CaO・MgO・SiO ２ , 2CaO・MgO・2SiO ２ , 2MgO.SiO ２ and 2CaO.SiO ２ and at least one selected from the group consisting of The phosphate remover of claim 1, comprising:

3. CaO・MgO・2SiO ２ and, MgO, CaO・MgO・SiO ２ and / or 2CaO.MgO.2SiO ２ and The phosphate remover according to claim 1 or 2, comprising:

4. Furthermore, carbonized components and / or SiO ２ The phosphate adsorbent according to claim 1 or 2, comprising:

5. The CaO.MgO.2SiO ２ The phosphate adsorbent according to claim 1 or 2, comprising a diopside crystalline phase.

6. In the X-ray diffraction spectrum measured by a powder X-ray diffractometer under the conditions of a sampling width of 0.10, a diffraction angle of 3° to 90°, and an X-ray source of CuKα, Reference peak intensity I at a diffraction angle 2θ of approximately 29.9° ０ Ratio of peak intensity I to I / I ０ is 2% or less, The reference peak intensity I ０ The phosphate adsorbent is CaCO ３ , MgO and SiO ２ 3. The phosphate adsorbent according to claim 1 or 2, which is prepared by a solid-state reaction method in which a mixture of the above components in a molar ratio of 1:1:1 is pre-baked at a baking temperature of 1100°C for 2 hours, and then baked at a baking temperature of 1300°C for 2 hours.

7. The CaO.MgO.2SiO ２ The phosphate adsorbent according to claim 1 or 2, wherein the content of is 8.8 to 60.7 mol % based on the amount of substance of the phosphate adsorbent.

8. The MgO is included, 3. The phosphate adsorbent according to claim 1, wherein the content of said MgO is 10.7 to 59.6 mol % based on the amount of substance of the phosphate adsorbent.

9. The CaO.MgO.SiO ２ Contains, The CaO.MgO.SiO ２ The phosphate adsorbent according to claim 1 or 2, wherein the content of is 5.6 to 26.5 mol % based on the amount of substance of the phosphate adsorbent.

10. The 2CaO.MgO.2SiO ２ Contains, The 2CaO.MgO.2SiO ２ The phosphate adsorbent according to claim 1 or 2, wherein the content of is 5.3 to 23.1 mol % based on the amount of substance of the phosphate adsorbent.

11. The 2MgO.SiO ２ Contains, The 2MgO.SiO ２ The phosphate adsorbent according to claim 1 or 2, wherein the content of is 2.0 to 44.4 mol % relative to the amount of substance of the phosphate adsorbent.

12. The 2CaO.SiO ２ Contains, The 2CaO.SiO ２ The phosphate adsorbent according to claim 1 or 2, wherein the content of is 5.6 to 14.7 mol % based on the amount of substance of the phosphate adsorbent.

13. A method for producing the phosphate adsorbent according to claim 1 or 2, comprising the steps of: Immersing rice husks in a mixed aqueous solution containing a water-soluble calcium compound and a water-soluble magnesium compound; Burning the soaked rice husks; A method for producing a phosphate adsorbent, comprising:

14. The method for producing a phosphate adsorbent according to claim 13, wherein the rice husk is derived from a plant of the Poaceae family.

15. 14. The method for producing a phosphate adsorbent according to claim 13, wherein the water-soluble calcium compound is calcium chloride and the water-soluble magnesium compound is magnesium chloride.

16. The method for producing a phosphate adsorbent according to claim 13, wherein the calcination is carried out at a temperature of 600°C to 1300°C.

17. The method for producing a phosphate adsorbent according to claim 13, wherein the concentrations of the water-soluble calcium compound and the water-soluble magnesium compound contained in the mixed aqueous solution are each independently 0.25 mol / L to 2.0 mol / L.

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