Method for producing phosphorus compound solution

Abstract

To provide a method for producing a phosphorus compound solution from sewage sludge incineration ash containing phosphorus, using a reduced amount of sulfuric acid.SOLUTION: A method for producing a phosphorus compound solution from sewage sludge incineration ash containing phosphorus comprises steps (1) to (5) described in the specification.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a phosphorus compound solution. [Background technology]

[0002] Phosphorus resources, such as phosphate rock, are useful raw materials used in a variety of fields, as they can be used for industrial purposes or as fertilizer raw materials. However, in recent years, changes in the global situation have led to problems such as rising prices for phosphate rock, and new means of stably obtaining high-purity phosphorus resources other than phosphate rock are needed. In addition, in recent years, attention has been focused on the Sustainable Development Goals (SDGs), raising questions about the state of society. Among the new economic systems aimed at achieving the SDGs, the circular economy is attracting attention from the perspective of environmental and economic goals.

[0003] It is believed that phosphorus resources can be recycled and recovered from sewage sludge incineration ash, steelmaking slag, industrial wastewater, etc., and various studies are being conducted. For example, Patent Document 1 describes a phosphate recovery method comprising: a dissolution step of adding an acid-containing solution to incineration ash and / or incineration fly ash containing phosphate radicals to adjust the pH to 1.6 or less, thereby dissolving the phosphate radicals in the ash in the acid-containing solution; a residue separation step of separating insoluble residue from the acid-containing solution; and an alkaline component addition step of adding an alkaline component to the solution from which the insoluble residue has been separated to adjust the pH to a range of 1.8 to 2.2; the iron compound addition step of adding an iron compound is carried out at any step before the alkaline component addition step; and a recovery step of recovering iron phosphate after the alkaline component addition step, and it describes that phosphorus can be recovered and reused at low cost. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-130903 Summary of the Invention [Problem to be solved by the invention]

[0005] When hydrochloric acid is used to extract phosphoric acid from incineration ash, chlorine remains in the final phosphorus compound obtained. If there is a large amount of residual chlorine, it may corrode the metal equipment used to manufacture other products using the phosphorus compound as a raw material, so phosphorus compounds with low chlorine residues are required. In order to obtain phosphorus compounds with low chlorine residues from incineration ash, sulfuric acid must be used to extract the phosphoric acid.

[0006] In the method described in Patent Document 1, the acid added to the incineration ash is not used in the subsequent steps and is discarded, which is undesirable from an environmental and economic standpoint.

[0007] Therefore, the present inventors decided to develop a technology that can reduce the amount of sulfuric acid used and efficiently extract phosphorus from incineration ash.

[0008] An object of the present invention is to provide a method for producing a phosphorus compound solution from phosphorus-containing sewage sludge incineration ash, which method uses a small amount of sulfuric acid. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems, and as a result have found that the above problems can be solved by providing the following configuration, which has led to the completion of the present invention. The present invention relates to, for example, the following [1] to [3]. [1] A method for producing a phosphorus compound solution from phosphorus-containing sewage sludge incineration ash, comprising the following steps (1) to (5): (1) A step of mixing the incineration ash with a 0.25 to 2.5 mol / L sulfuric acid solution to obtain a slurry 1. (2) A step of subjecting the slurry 1 to solid-liquid separation to obtain a separated liquid 1. (3) adding a sulfuric acid solution to the separated liquid 1 to adjust the pH to 0.1 or less to obtain a pH-adjusted separated liquid 1; (4) A step of mixing the pH-adjusted separated liquid 1 with phosphorus-containing sewage sludge incineration ash to obtain a slurry 2. (5) A step of subjecting the slurry 2 to solid-liquid separation to obtain a phosphorus compound solution as a separated liquid 2. [2] The method for producing a phosphorus compound solution according to [1], wherein in the step (1), the solid-liquid ratio (L / S) between the incineration ash and the sulfuric acid solution is 5 to 20 L / kg, and in the step (4), the solid-liquid ratio (L / S) between the incineration ash and the pH-adjusted separated liquid 1 is 5 to 20 L / kg. [3] The method for producing a phosphorus compound solution according to [1] or [2], wherein the steps (3) to (5) are repeated at least once using the separation liquid 2 instead of the separation liquid 1. [Effects of the Invention]

[0010] According to the present invention, a phosphorus compound solution can be efficiently produced from phosphorus-containing sewage sludge incineration ash, and the amount of sulfuric acid used can be reduced. DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, the present invention will be described in detail. Unless otherwise specified, the expression "A to B" regarding a numerical range means that it is equal to or greater than A and equal to or less than B. Furthermore, % means % by mass.

[0012] The method for producing a phosphorus compound solution of the present invention is a method for producing a phosphorus compound solution from phosphorus-containing sewage sludge incineration ash, and includes the following steps (1) to (5). This method is hereinafter also referred to as production method (Y). (1) A step of mixing the incineration ash with a 0.25 to 2.5 mol / L sulfuric acid solution to obtain a slurry 1 (2) A step of subjecting the slurry 1 to solid-liquid separation to obtain a separated liquid 1. (3) adding a sulfuric acid solution to the separated liquid 1 to adjust the pH to 0.1 or less to obtain a pH-adjusted separated liquid 1; (4) A step of mixing the pH-adjusted separated liquid 1 with phosphorus-containing sewage sludge incineration ash to obtain a slurry 2. (5) A step of subjecting the slurry 2 to solid-liquid separation to obtain a phosphorus compound solution as a separated liquid 2.

[0013] The production method (Y) can be rephrased as a method for producing a high-purity phosphorus compound solution, a method for purifying a phosphorus compound, or a method for recovering a phosphorus compound.

[0014] In the production method (Y), the phosphorus compound is a general term for compounds containing phosphorus element, and although there are no limitations on the chemical structure thereof, it is mainly a phosphate compound.

[0015] In steps (1) to (5), the mixing method is not limited, and known methods such as stirring mixing, pump circulation mixing, and pipeline mixing can be used, but stirring mixing is preferred.

[0016] In steps (1) to (5), the equipment used for stirring is not limited and known equipment can be used, for example, a stirrer, a propeller, a turbine, or a paddle.

[0017] In steps (1) to (5), the method of solid-liquid separation is not limited, and known methods such as filtration using a filter medium, decantation, and centrifugation can be used. When solid-liquid separation is carried out by filtration, known filter media such as filter paper, membrane filters, and fiber filters can be used, and filter paper is preferably used. The filtration may be performed by natural filtration, vacuum filtration, pressure filtration, or centrifugal filtration, but vacuum filtration is preferred. The temperature at which solid-liquid separation is carried out is not limited, but room temperature (for example, 10 to 30° C.) is preferred.

[0018] <Sewage sludge incineration ash> The sewage sludge incineration ash used in the present invention contains phosphorus. Sewage sludge incineration ash is a residue obtained by incinerating activated sludge generated in sewage treatment, and there are no particular restrictions on its form or composition as long as it contains phosphorus. As the sewage sludge incineration ash, for example, sewage sludge incineration ash, and incineration ash of sludge from human waste treatment facilities and various wastewater treatment devices can be used.

[0019] In sewage treatment, flocculants such as polyferric sulfate and aluminum sulfate, and flocculation, precipitation, and dewatering aids such as hydrated lime (Ca(OH)2) are added to sewage for the purposes of flocculation and sedimentation of activated sludge containing phosphorus, and dewatering of the activated sludge. Therefore, in sewage sludge incineration ash, phosphorus exists mainly as phosphates, represented by iron phosphate (FePO4, Fe3(PO4)2, etc.), aluminum phosphate (AlPO4, etc.), and calcium phosphate (Ca3(PO4)2, Ca5(PO4)3OH, etc.).

[0020] <Process (1)> Step (1) is a step of obtaining a slurry 1 by mixing the incineration ash with a 0.25 to 2.5 mol / L sulfuric acid solution. In step (1), phosphates contained in the incineration ash, mainly iron phosphates (FePO4, Fe3(PO4)2, etc.), aluminum phosphates (AlPO4, etc.), and calcium phosphates (Ca3(PO4)2, Ca5(PO4)3OH, etc.) are eluted with acid. The solubility of phosphates varies greatly depending on the pH, but iron phosphate, aluminum phosphate, and calcium phosphate have sufficiently high solubility in a 0.25 to 2.5 mol / L sulfuric acid solution that they are eluted from the incineration ash and dissolved in slurry 1.

[0021] SiO2, Fe2O3, CaSO4, etc. contained in the incineration ash have low solubility in a 0.25 to 2.5 mol / L sulfuric acid solution and become insoluble in the slurry 1.

[0022] The 0.25 to 2.5 mol / L sulfuric acid solution (hereinafter also referred to as sulfuric acid solution A) may contain sulfuric acid at this concentration, and may also contain known acids such as nitric acid and acetic acid. The concentration of the sulfuric acid solution A is preferably 0.5 to 2 mol / L, more preferably 0.8 to 1.5 mol / L.

[0023] If the concentration of sulfuric acid solution A is higher than the upper limit, the calcium ions extracted from the incineration ash react with sulfate ions from the sulfuric acid to form calcium sulfate, and the rate at which calcium sulfate precipitates becomes excessively fast. As a result, the phosphorus compounds extracted from the incineration ash are easily taken up by calcium sulfate, becoming insoluble in slurry 1, and the final yield of phosphorus compounds is likely to decrease. If the concentration of the sulfuric acid solution A is lower than the above lower limit, the phosphorus compounds contained in the incineration ash cannot be sufficiently extracted, and the final yield of phosphorus compounds tends to decrease.

[0024] The ratio of the amounts of the incineration ash and the sulfuric acid solution A to be mixed is not particularly limited, and the incineration ash and the sulfuric acid solution A may be mixed at any ratio. The incineration ash and sulfuric acid solution A are mixed preferably at a solid-liquid ratio (L / S, Liquid / Solid) of 5 to 20 L / kg, more preferably 7 to 15 L / kg, and even more preferably 8 to 12 L / kg. When mixed at a solid-liquid ratio within the above range, phosphorus compounds are likely to be efficiently eluted.

[0025] The slurry 1 is preferably further stirred and / or heated. The stirring time is preferably 1 minute to 10 hours, more preferably 10 minutes to 5 hours, even more preferably 30 minutes to 4 hours, and particularly preferably 1 hour to 3 hours. The heating temperature is preferably 50 to 100°C, more preferably 60 to 95°C, and even more preferably 70 to 90°C. Stirring and / or heating facilitates efficient elution of the phosphorus compound.

[0026] <Process (2)> Step (2) is a step of obtaining separated liquid 1 by solid-liquid separation of slurry 1. By solid-liquid separation, the slurry 1 is separated into a solid (cake 1) and a liquid (separated liquid 1). Since SiO2, Fe2O3, CaSO4, etc. are separated from the solid (cake 1), a liquid (separated liquid 1) is obtained that has low contents of these compounds and a high content of phosphorus compounds.

[0027] The content of phosphorus in the separated liquid 1 is preferably 0.5 to 20 wt % in terms of P2O5, more preferably 1 to 10 wt %, and even more preferably 1 to 5 wt % in terms of P2O5.

[0028] The production method (Y) may include a step of washing the cake 1 with water to obtain a washing liquid 1. The cake produced in this step is referred to as a washed cake 1.

[0029] <Process (3)> Step (3) is a step of adding a sulfuric acid solution to the separated liquid 1 to adjust the pH to 0.1 or less, thereby obtaining a pH-adjusted separated liquid 1. In step (3), the pH of the separated liquid 1 is adjusted to elute phosphorus compounds contained in the incineration ash to be mixed in the next step (4).

[0030] The concentration of the sulfuric acid solution in step (3) (hereinafter also referred to as sulfuric acid solution B) is not limited, but is preferably 0.4 to 18.3 mol / L, more preferably 0.5 to 18.3 mol / L, even more preferably 2.0 to 18.3 mol / L, and particularly preferably 9.0 to 18.3 mol / L. The sulfuric acid solution B may contain, in addition to sulfuric acid, a known acid such as nitric acid or acetic acid. The amount of sulfuric acid solution B to be added to separation liquid 1 is not limited, and it is sufficient to use an amount that can adjust the pH of separation liquid 1 to 0.1 or less.

[0031] The pH is a value measured at 25 to 30° C. by the method described in the Examples below.

[0032] <Process (4)> Step (4) is a step of mixing the pH-adjusted separated liquid 1 with phosphorus-containing sewage sludge incineration ash to obtain a slurry 2. In step (4), new phosphorus-containing sewage sludge incineration ash is used, and the phosphates contained therein, mainly FePO4, AlPO4, and Ca3(PO4)2, are eluted using a pH-adjusted separation solution 1. SiO2, Fe2O3, CaSO4, etc. contained in the incineration ash have low solubility in the pH-adjusted separation liquid 1 and become insoluble in the slurry 2.

[0033] The concentration of sulfuric acid contained in the pH-adjusted separated liquid 1 is such that the rate at which calcium sulfate is generated and precipitated as a result of the reaction between calcium ions extracted from the incineration ash and sulfate ions from the sulfuric acid is not excessively high, which makes it easy to prevent phosphorus compounds extracted from the incineration ash from being taken up by calcium sulfate and becoming insoluble in the slurry 2. This tends to increase the final yield of phosphorus compounds.

[0034] The mixing ratio of the incineration ash and the pH-adjusted separated liquid 1 is not particularly limited, and fresh incineration ash and the pH-adjusted separated liquid 1 may be mixed in any desired ratio. The incineration ash and the pH-adjusted separated liquid 1 are mixed preferably at a solid-liquid ratio (L / S) of 5 to 20 L / kg, more preferably 7 to 15 L / kg, and even more preferably 8 to 12 L / kg. When mixed at a solid-liquid ratio within the above range, phosphorus compounds tend to be efficiently eluted.

[0035] The slurry 2 is preferably further stirred and / or heated. The stirring time is preferably 1 minute to 5 hours, more preferably 30 minutes to 4 hours, and even more preferably 1 hour to 3 hours. The heating temperature is preferably 50 to 100°C, more preferably 60 to 95°C, and even more preferably 70 to 90°C. Stirring and / or heating facilitates efficient elution of the phosphorus compound.

[0036] <Process (5)> Step (5) is a step of subjecting the slurry 2 to solid-liquid separation to obtain a phosphorus compound solution as a separated liquid 2. By solid-liquid separation, the slurry 2 is separated into a solid (cake 2) and a liquid (separated liquid 2). Since SiO2, Fe2O3, CaSO4, etc. are separated from the solid (cake 2), the content of these compounds is low, and a phosphorus compound solution with a high content of phosphorus compounds is obtained as separated liquid 2. The phosphorus compounds contained in separated liquid 2 are mainly phosphate ions (PO4 3- )

[0037] The production method (Y) may include a step of washing the cake 2 with water to obtain a washing liquid 2. The cake produced in this step is referred to as a washed cake 2.

[0038] In the production method (Y), steps (3) to (5) are preferably repeated at least once, more preferably twice or more, using separation liquid 2 instead of separation liquid 1. This allows a phosphorus compound solution with a high phosphorus compound content to be obtained, and the amount of sulfuric acid used to be further reduced.

[0039] The separated liquid 2 obtained in step (5) can be used in a method for producing iron phosphate by adding an alkaline solution to adjust the pH and precipitate iron phosphate, or a method for producing calcium phosphate by adding an alkaline solution to iron phosphate to adjust the pH and dissolve the iron phosphate to obtain an alkaline solution containing phosphoric acid, and a method for producing calcium phosphate by adding a calcium salt to the alkaline solution containing phosphoric acid. Compared to a single sulfuric acid extraction, the separated liquid 2 has a higher concentration of phosphorus compounds, allowing for efficient production of iron phosphate and calcium phosphate. Furthermore, the amount of sulfuric acid relative to phosphoric acid is reduced, resulting in a reduction in the amount of alkaline solution used for pH adjustment. [Example]

[0040] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0041] <Raw materials and reagents> Incineration ash: Sewage sludge incineration ash (gift from the Tokyo Metropolitan Government Bureau of Sewerage) was used, having the composition ratio shown in Table 1. The method for measuring the composition of incineration ash 1 will be described later.

[0042] [Table 1]

[0043] Sulfuric acid: (1+1) sulfuric acid (65%) (Kanto Chemical Co., Ltd.)

[0044] <Measurement of sample composition ratio> (moisture) After accurately weighing a 50 mm diameter flat weighing bottle to obtain the accurate weighing value (A), 5.0 g of sample was quickly placed in the 50 mm diameter flat weighing bottle, the lid was replaced, and the sample was accurately weighed (B), obtaining the accurate weighing value (B). The lid was removed, and the sample was dried at 105°C (in a fan dryer) for 5 hours. The lid was replaced, the sample was allowed to cool in a silica gel desiccator for 30 minutes, and the sample was accurately weighed to obtain the accurate weighing value (C). This sample will be referred to as the dried sample below. The moisture content (wt %) was calculated using the precise weighing values ​​(A) to (C) according to the following formula (I). Moisture (weight %)=(BC)×100 / (BA)...Formula (I)

[0045] (Pretreatment) Approximately 2.3 g of the dried sample was accurately weighed into a 200 mL PYREX® conical beaker and slightly moistened with water. After adding 10 mL of nitric acid and 30 mL of hydrochloric acid, the conical beaker was placed on the heating element of a preheated electric heater and gently boiled for at least 30 minutes. After allowing to cool, the sample solution was rinsed with pure water and transferred to a 500 mL measuring flask. The solution was filtered using No. 6 quantitative filter paper, and at least 150 mL of the separated solution (filtrate) was collected. The resulting separated solution was used as the test solution.

[0046] (P2O5 analysis (colorimetric method)) 1. Preparation of color developing solution Dissolve 1.12 g of ammonium metavanadate (NH4VO3) in 200-300 mL of pure water and add 250 mL of nitric acid. While stirring the solution, add ammonium molybdate ((NH4)6Mo7O 24 27 g of 4H2O was added, and then purified water was added to make 1 L. The mixture was stored in a colored bottle. Before use, it was filtered.

[0047] 2. Preparation of P2O5 Standard Solution 19.17 g of KH2PO4 was dissolved in pure water, and 10 mL of nitric acid was added to make exactly 1 L to obtain a P2O5 standard stock solution (10 mg / mL). This was then diluted to prepare a P2O5 standard solution (0.2 mg / mL) and a P2O5 standard solution (0.3 mg / mL).

[0048] 3 Quantitative P2O5 standard sample solutions of different concentrations were prepared using the P2O5 standard solution. The test solutions were appropriately diluted with pure water. The color-developing solution was added to the P2O5 standard sample solution and the test solution, and then mixed. The solution was left at room temperature for 30 minutes, and the absorbance at 420 nm was measured. The P2O5 concentration (wt%) of the test solution was calculated from the obtained absorbance using a colorimetric method.

[0049] (SO3 analysis (BaSO4 gravimetric method)) 5.0 g of the dried sample was weighed into a weighing bottle and transferred to a 500 mL PYLEX beaker using water. 30 mL of hydrochloric acid and 10 mL of perchloric acid were added. After cooling, the mixture was diluted to 100 mL with pure water and 10 mL of hydrochloric acid was added. The mixture was heated to dissolve and filtered using No. 6 quantitative filter paper. 20 mL of hydroxylamine hydrochloride solution was added to the separated liquid (filtrate) and heated for 5 minutes. 10 mL of 10% barium chloride solution was added, diluted to 300 mL with pure water, boiled, and aged for at least 3 hours. The mixture was then filtered using No. 6 quantitative filter paper and washed with hot water. The resulting precipitate and the filter paper used were dried in an electric furnace and a silica gel desiccator, then weighed to obtain the weight (D). The precipitate was removed from the filter paper with a brush and weighed to obtain the weight (E).

[0050] Using the weighed values ​​(D) and (E), SO3 (wt%) was calculated according to the following formula (II). SO3 (wt%) = (DE) × 0.343 × 100 / g of dry sample... Equation (II)

[0051] (SiO2 analysis (atomic absorption spectrometry)) 5.0 g of dried sample was precisely weighed into a nickel crucible, and then approximately 1-2 mL of pure water and 10 g of potassium hydroxide were added and mixed. The crucible was heated to dissolve the potassium hydroxide and completely remove the moisture. After cooling, the sample was transferred to a 50 mL beaker, and water was added to dissolve the molten material. After adding one drop of phenolphthalein solution, (1 + 2) hydrochloric acid was added little by little to the beaker to neutralize the solution, and then an excess of (1 + 2) hydrochloric acid was added. After cooling, the sample was diluted with water to obtain an SiO2 measurement solution. The SiO2 measurement solution was subjected to measurement using an atomic absorption spectrophotometer (ZA3300, Hitachi, Ltd.). The measurement conditions are shown in Table 2 below.

[0052] SiO2 (wt%) was calculated from the following formula (III). SiO2 (wt%) = (atomic absorption (Si) × 2.1393 × 100) / (dry sample (g) × 1 / 500 × 1,000,000) Equation (III)

[0053] (Analysis of CaO, Fe2O3, Al2O3, MgO, Na2O (atomic absorption spectrometry)) The test solution was diluted appropriately with pure water and subjected to measurement using an atomic absorption spectrophotometer (ZA3300, Hitachi, Ltd.) to measure the concentrations of Ca, Fe, Al, Mg, and Na. The concentrations (wt%) of Ca, Fe, Al, Mg, and Na were all measured as CaO, Fe2O3, Al2O3, MgO, and Na2O, respectively. The measurement conditions are shown in Table 2 below. The Na concentration was measured under the conditions of "Na" when the concentration was 0 to 1 ppm, and "Na (flame)" when the concentration was 1 to 30 ppm.

[0054] [Table 2]

[0055] [Example 1] (First acid extraction) A sulfuric acid solution with a concentration of 1 mol / L was placed in reaction tank 1 and heated to 80°C. While stirring the sulfuric acid solution at 80°C, incineration ash 1 listed in Table 1 was added to reaction tank 1 so that the solid-liquid ratio L / S was 10 L / kg, to obtain slurry 1. The mixture was heated for 2 hours while maintaining the temperature at 80°C. Slurry 1 was then subjected to solid-liquid separation (suction filtration) using filter paper (Advantec, quantitative filter paper No. 5C type) to recover separated liquid 1 (incinerator ash extract liquid 1) and filter cake 1 (incinerator ash extract residue 2). Filter cake 1 was also filtered and washed with water to recover washing liquid 1. The P2O5 content in filter cake 1 after washing was measured.

[0056] (Second acid extraction) The separated liquid 1 was placed in another reaction vessel 2, and a sulfuric acid solution with a concentration of 10.3 mol / L (65 wt%) was added to adjust the pH to 0.09, thereby obtaining a pH-adjusted separated liquid 1. Thereafter, fresh incineration ash 1 was added to the reaction tank 2 so that the solid-liquid ratio L / S was 10 L / kg, and a slurry 2 was obtained. After heating and stirring for 2 hours while maintaining the temperature at 80°C, Slurry 2 was suction filtered to recover Separation Liquid 2 (Incineration Ash Extract Liquid 2) and Filter Cake 2 (Incineration Ash Extract Residue 2). Filter Cake 2 was also filtered and washed with water to recover Washing Liquid 2. The P2O5 content in Filter Cake 2 after washing was measured.

[0057] (Third acid extraction) The separated liquid 2 was placed in another reaction vessel 3, and a sulfuric acid solution with a concentration of 10.3 mol / L (65 wt%) was added to adjust the pH to 0.08, thereby obtaining a pH-adjusted separated liquid 2. Thereafter, fresh incineration ash 1 was added to the reaction tank 3 so that the solid-liquid ratio L / S was 10 L / kg, and a slurry 3 was obtained. After heating and stirring for 2 hours while maintaining the temperature at 80°C, Slurry 3 was suction filtered to recover Separation Liquid 3 (Incineration Ash Extract Liquid 3) and Filter Cake 3 (Incineration Ash Extract Residue 3). In addition, Filter Cake 3 was filtered and washed with water to recover Washing Liquid 3. The P2O5 content in Filter Cake 3 after washing was measured.

[0058] (4th acid extraction) The separated liquid 3 was placed in another reaction vessel 4, and a sulfuric acid solution with a concentration of 10.3 mol / L (65 wt%) was added to adjust the pH to 0.08, thereby obtaining a pH-adjusted separated liquid 3. Thereafter, fresh incineration ash 1 was added to the reaction tank 4 so that the solid-liquid ratio L / S was 10 L / kg, and a slurry 4 was obtained. After heating and stirring for 2 hours while maintaining the temperature at 80°C, the slurry 4 was suction filtered to recover a separated liquid 4 (incinerator ash extract liquid 4) and a filter cake 4 (incinerator ash extract residue 4). The filter cake 4 was also filtered and washed with water to recover a washing liquid 4. The P2O5 content in the filter cake 4 after washing was measured.

[0059] (5th acid extraction) The separated liquid 4 was placed in another reaction vessel 5, and a sulfuric acid solution with a concentration of 10.3 mol / L (65 wt%) was added to adjust the pH to 0.08, thereby obtaining a pH-adjusted separated liquid 4. Thereafter, fresh incineration ash 1 was added to the reaction tank 5 so that the solid-liquid ratio L / S became 10 L / kg, and a slurry 5 was obtained. After heating and stirring for 2 hours while maintaining the temperature at 80°C, the slurry 5 was subjected to suction filtration to recover a separated liquid 5 (incinerator ash extract liquid 5) and a filter cake 5 (incinerator ash extract residue 5). The filter cake 5 was also filtered and washed with water to recover a washing liquid 5. The PO content in the filter cake 5 after washing was measured.

[0060] The phosphorus yield (residual rate) of filter cakes 1 to 5 was calculated from the following formula IV. The mass of the incineration ash before extraction is S0, its phosphorus concentration (P2O5 equivalent) is s0 (wt%), the mass of filter cake 1 is S1, and its phosphorus concentration (P2O5 equivalent) is s1 (wt%). Phosphorus yield (residual rate) of filter cake 1 = (S1 × s1) / (S0 × s0) × 100 ...(Formula IV) In the second and subsequent (nth) extraction operations, the phosphorus yield of filter cake n was calculated using the following formula (V). Phosphorus yield (residual rate) of filter cake n = (Sn × sn) / (S0 × s0) × 100 ...(Formula V)

[0061] Furthermore, the yield of phosphorus in the separated liquid 1 (incinerated ash extract) was calculated from the following formula VI. The mass of the incineration ash before extraction is S0, and its phosphorus concentration (equivalent to P2O5) is s0 (wt%). Phosphorus yield (extraction rate) of separated liquid 1 = 100 - phosphorus yield (residual rate) of filter cake 1 (Formula VI)

[0062] In addition, in the second and subsequent (nth) extraction operations, the phosphorus yield in the used separated liquids 2 to 5(n) was calculated from the phosphorus yield (residual rate) of the filter cakes 2 to 5(n) using the following formula VII. Phosphorus yield (extraction rate) of separated liquid n = 100 - phosphorus yield of filter cake n ...(Formula VII)

[0063] The pH measurements were carried out at 25 to 30°C using a HORIBA LAQUA benchtop pH meter F-72S and a composite electrode: a HORIBA LAQUA Long ToupH electrode (9680S-10D).

[0064] [Comparative Example 1] After separation liquid 1 was placed in reaction tank 2, the same procedures as in Example 1 were carried out up to the second acid extraction, except that sulfuric acid was not added to adjust the pH. Filter cake 2 obtained in the second acid extraction was washed. The contents of P2O5 in separation liquid 2 and in filter cake 2 after washing were measured. The phosphorus yield in separation liquid 2 was calculated in the same manner as in the example.

[0065] Comparative Example 2 The same procedures as in Example 1 were carried out up to the third acid extraction, except that after the separated liquid 1 was placed in the reaction tank 2, sulfuric acid was not added to adjust the pH, and the temperature during stirring after mixing with new incineration ash 1 was changed to 90°C. The filter cake 3 obtained in the third acid extraction was washed. The P2O5 contents in the separated liquid 3 and the filter cake 3 after washing were measured. The phosphorus yield in the separated liquid 3 was calculated in the same manner as in the examples.

[0066] Table 3 shows the phosphorus yields (%) in separated solutions 1 to 5 of Example 1 (first to fifth acid extractions), separated solutions 1 to 2 of Comparative Example 1, and separated solutions 1 to 3 of Comparative Example 2.

[0067] [Table 3]

[0068] From Table 3, it was confirmed that the phosphorus yield did not decrease significantly even when acid extraction was repeated, and phosphorus could be extracted efficiently in Example 1. On the other hand, in Comparative Examples 1 and 2, since the pH was not adjusted, it was confirmed that when acid extraction was repeated, the phosphorus yield decreased significantly from the second extraction onwards.

[0069] Table 4 shows the amount (g) of sulfuric acid solution with a concentration of 10.3 mol / L (65 wt%) used when the first to fifth sulfuric acid extractions were performed in Example 1. For comparison, the amount of sulfuric acid used when sulfuric acid extraction was performed in a single extraction is also shown. It was confirmed that the method of Example 1 reduced the amount of sulfuric acid used to about half that when sulfuric acid extraction was performed in a single extraction.

[0070] [Table 4]

Claims

1. A method for producing a phosphorus compound solution from phosphorus-containing sewage sludge incineration ash, comprising the following steps (1) to (5): (1) A step of mixing the incineration ash with a 0.25 to 2.5 mol / L sulfuric acid solution to obtain a slurry 1 (2) A step of subjecting the slurry 1 to solid-liquid separation to obtain a separated liquid 1 (3) A step of adding a sulfuric acid solution to the separated liquid 1 to adjust the pH to 0.1 or less, thereby obtaining a pH-adjusted separated liquid 1. (4) A step of mixing the pH-adjusted separated liquid 1 with phosphorus-containing sewage sludge incineration ash to obtain a slurry 2. (5) A step of subjecting the slurry 2 to solid-liquid separation to obtain a phosphorus compound solution as a separated liquid 2.

2. In the step (1), the solid-liquid ratio (L / S) of the incineration ash and the sulfuric acid solution is 5 to 20 L / kg, and in the step (4), the solid-liquid ratio (L / S) of the incineration ash and the pH-adjusted separation liquid 1 is 5 to 20 L / kg. The method for producing a phosphorus compound solution according to claim 1.

3. 3. The method for producing a phosphorus compound solution according to claim 1, wherein the steps (3) to (5) are repeated at least once using the separation liquid 2 instead of the separation liquid 1.

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