Production method for calcium phosphate

Abstract

To provide a production method for calcium phosphate which can produce calcium phosphate having a low aluminum content from sewage sludge incinerated ash containing phosphorus and aluminum.SOLUTION: The method for producing calcium phosphate from sewage sludge incinerated ash containing phosphorus and aluminum includes the steps (1) to (10) described herein, wherein the phosphorus content in the separated liquid 1 is 0.5 to 20 wt% in terms of P2O5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing calcium phosphate. [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] Patent Document 1 describes that the iron phosphate obtained by the phosphate recovery method of Patent Document 1 contains almost no aluminum component, but the obtained iron phosphate contains Al2O3 at a mass ratio of about 10% of phosphoric acid (PO5) (Table 3 of Patent Document 1), and the aluminum content is not sufficiently low.

[0006] If the aluminum content of phosphorus resources is high, it will have a negative impact on productivity at the plant when the phosphorus resources are processed into various raw materials. Examples of negative impacts include aluminum phosphate precipitating on the inner wall of the concentration tank during the process of heating and concentrating phosphoric acid solution, which reduces heat transfer efficiency, and a portion of phosphorus precipitating as aluminum phosphate during the phosphoric acid purification process, which reduces the extraction rate of purified phosphoric acid and other factors, resulting in a decrease in productivity.

[0007] The present inventors have focused on calcium phosphate, which is one of the phosphorus resources, and an object of the present invention is to provide a method for producing calcium phosphate with a low aluminum content, which can be produced from sewage sludge incineration ash containing phosphorus and aluminum. [Means for solving the problem]

[0008] 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 [5]. [1] A method for producing calcium phosphate from sewage sludge incineration ash containing phosphorus and aluminum, comprising the following steps (1) to (10): A method for producing calcium phosphate, wherein the phosphorus content in the following separated liquid 1 is 0.5 to 20 wt% in terms of P2O5. (1) A step of mixing the incineration ash with an acid solution to obtain a slurry 1 having a pH of less than 0.5 (2) A step of subjecting the slurry 1 to solid-liquid separation to obtain a separated liquid 1. (3) A step of adding an iron compound to the separated solution 1 to obtain an iron-added separated solution 1. (4) A step of adding an alkali metal hydroxide to the iron-added separated solution 1 to adjust the pH to a range of 0.5 to 2.0, thereby obtaining a pH-adjusted separated solution 1. (5) A step of heating and stirring the pH-adjusted separated liquid 1 at 50°C or higher to obtain a slurry 2. (6) A step of subjecting the slurry 2 to solid-liquid separation to obtain a cake 2 (7) A step of mixing the cake 2 with an alkali metal hydroxide solution to obtain a slurry 3 having a pH of 12 or more. (8) A step of subjecting the slurry 3 to solid-liquid separation to obtain a separated liquid 3 (9) A step of mixing the separated liquid 3 with calcium hydroxide to obtain a slurry 4 (10) A step of subjecting the slurry 4 to solid-liquid separation to obtain calcium phosphate. [2] The method for producing calcium phosphate according to [1], wherein in the step (4), the pH is adjusted to within the range of 0.8 to 1.5. [3] The method for producing calcium phosphate according to [1] or [2], wherein the amount of the iron compound is an amount such that the molar ratio of iron to phosphorus (Fe / P) in the iron-added separated liquid 1 is 1.1 to 1.5. [4] The method for producing calcium phosphate according to any one of [1] to [3], wherein the stirring time in the step (5) is 0.5 to 10 hours. [5] The method for producing calcium phosphate according to any one of [1] to [4], wherein the amount of calcium hydroxide is such that the molar ratio of calcium to phosphorus (Ca / P) in the slurry 4 is 1.5 to 2.5. [Effects of the Invention]

[0009] According to the present invention, calcium phosphate with a low aluminum content can be produced from sewage sludge incineration ash containing phosphorus and aluminum. Furthermore, according to the production method of the present invention, iron hydroxide can be easily separated into solid and liquid during the process, and calcium phosphate can be produced in a shorter time. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is an SEM image of the iron hydroxide cake obtained by filtering the iron hydroxide slurry in Example 1. 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] A method for producing calcium phosphate from sewage sludge incineration ash containing phosphorus and aluminum, comprising the following steps (1) to (10), wherein the phosphorus content in the separated liquid 1 described below is 0.5 to 20 wt% calculated as P2O5. This method is hereinafter also referred to as production method (X). (1) A step of mixing the incineration ash with an acid solution to obtain a slurry 1 having a pH of less than 0.5 (2) A step of subjecting the slurry 1 to solid-liquid separation to obtain a separated liquid 1. (3) A step of adding an iron compound to the separated solution 1 to obtain an iron-added separated solution 1. (4) A step of adding an alkali metal hydroxide to the iron-added separated solution 1 to adjust the pH to a range of 0.5 to 2.0, thereby obtaining a pH-adjusted separated solution 1. (5) A step of heating and stirring the pH-adjusted separated liquid 1 at 50°C or higher to obtain a slurry 2. (6) A step of subjecting the slurry 2 to solid-liquid separation to obtain a cake 2 (7) A step of mixing the cake 2 with an alkali metal hydroxide solution to obtain a slurry 3 having a pH of 12 or more. (8) A step of subjecting the slurry 3 to solid-liquid separation to obtain a separated liquid 3 (9) A step of mixing the separated liquid 3 with calcium hydroxide to obtain a slurry 4 (10) A step of subjecting the slurry 4 to solid-liquid separation to obtain calcium phosphate.

[0013] The production method (X) can be rephrased as a method for producing high-purity calcium phosphate, a method for purifying calcium phosphate, or a method for recovering calcium phosphate.

[0014] In steps (1) to (10), 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.

[0015] In steps (1) to (10), 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.

[0016] In steps (1) to (10), 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.

[0017] In steps (1) to (10), the pH is a value measured at 25 to 30° C. by the method described in the Examples below.

[0018] <Sewage sludge incineration ash> The sewage sludge incineration ash used in the present invention contains phosphorus and aluminum. 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 and aluminum. 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 mixing the incineration ash with an acid solution to obtain a slurry 1 having a pH of less than 0.5. 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 at a pH of less than 0.5, and are eluted from the incineration ash at a pH of less than 0.5 and dissolved in slurry 1. The pH of slurry 1 in step (1) is preferably less than 0.3, and more preferably less than 0.1.

[0021] SiO2, Fe2O3, CaSO4, etc. contained in the incineration ash have low solubility at a pH of less than 0.5 and become insoluble in Slurry 1.

[0022] The acid used in the acid solution is not particularly limited, and known acids such as sulfuric acid, nitric acid, and acetic acid can be used. One or more types of acids may be used. If the calcium phosphate obtained in step (10) contains a large amount of chlorine, it may corrode metal equipment used in producing other products using calcium phosphate as a raw material. Therefore, the acid solution is a chlorine-free acid solution, more preferably a sulfuric acid solution.

[0023] The concentration of the acid in the acid solution varies depending on the type of acid and the composition of the incineration ash, but is used at a concentration such that the pH of the slurry 1 obtained by mixing with the incineration ash is less than 0.5. When the acid solution is a sulfuric acid solution, the concentration of sulfuric acid is preferably 0.3 to 3 mol / L, more preferably 0.5 to 2 mol / L, and even more preferably 0.8 to 1.2 mol / L.

[0024] The ratio of the amount of incineration ash to the amount of acid solution to be mixed is not particularly limited, and the incineration ash and the acid solution may be mixed at any ratio. The incineration ash and the acid solution 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, phosphates tend to be efficiently eluted.

[0025] It is preferable to further stir and / or heat the slurry 1, as this will facilitate efficient elution of the phosphate. 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. When the stirring time is within the above range, the phosphate tends to be efficiently eluted. The heating temperature is preferably 50° C. to 100° C., more preferably 60° C. to 95° C., and even more preferably 70° C. to 90° C. When the heating temperature is within the above range, the phosphate tends to be efficiently eluted.

[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). SiO2, Fe2O3, CaSO4, etc. are separated from the solid (cake 1), so a liquid (separated liquid 1) is obtained that has low contents of these substances and a high content of phosphates.

[0027] The content of phosphorus in the separated liquid 1 is 0.5 to 20 wt % in terms of P2O5. The phosphorus content in separated liquid 1 is preferably 1 to 10 wt% in terms of P2O5, and more preferably 1 to 5 wt% in terms of P2O5. When the phosphorus content is 1 to 5 wt% in terms of P2O5, the reaction efficiency between phosphorus and the iron compound in step (3) tends to be improved, and the iron phosphate obtained in step (6) has excellent filterability, so that iron phosphate with fewer impurities can be obtained as cake 2.

[0028] The production method (X) 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 washed cake 1.

[0029] The production method (X) may include the following steps (2a) to (2c) between the step (2) and the step (3). (2a) adding a sulfuric acid solution to the separated liquid 1 to adjust the pH to 0.1 or less to obtain a sulfuric acid-added separated liquid 1; (2b) A step of mixing the sulfuric acid-added separated liquid 1 with new sewage sludge incineration ash containing phosphorus and aluminum to obtain a slurry 1-1. (2c) A step of subjecting the slurry 1-1 to solid-liquid separation to obtain a separated liquid 1-2

[0030] Steps (2a) to (2c) may be repeated two or more times using separation liquid 1-2 in step (2a) instead of separation liquid 1. By performing acid extraction of incineration ash multiple times, separation liquid 1-2 will have a higher phosphate content, and can therefore be preferably used in place of separation liquid 1 in the next step (3).

[0031] <Process (3)> Step (3) is a step of adding an iron compound to the separated liquid 1 to obtain an iron-added separated liquid 1. By adding iron compounds to the separated solution 1, even if the amount of iron compounds contained in the incineration ash is small, the phosphate ions (PO4 3- ) can all be converted into iron phosphate.

[0032] The iron compound is not limited as long as it is a known iron compound, but trivalent iron ions (Fe 3+ ) is a phosphate ion (PO4 3- ) to form iron phosphate (FePO4), the iron compound is preferably an iron compound containing trivalent iron. Examples of iron compounds containing trivalent iron include ferric sulfate (Fe2(SO4)3), iron hydroxide (Fe(OH)3 (amorphous)), and iron nitrate (Fe(NO3)3). If the calcium phosphate obtained in step (10) contains a large amount of chlorine, it may corrode metal equipment used in producing other products using calcium phosphate as a raw material. Therefore, the iron compound is preferably an iron compound that does not contain chlorine atoms, and more preferably ferric sulfate (Fe(SO)).

[0033] The iron compound may be cake 3 produced in step (8) described below.

[0034] The amount of iron compound to be added to separated liquid 1 is not limited, but is preferably an amount such that the molar ratio of iron to phosphorus (Fe / P) in iron-added separated liquid 1 is 1.1 to 1.5. This allows the phosphate ions in separated liquid 1 to be efficiently converted into iron phosphate, which facilitates precipitation of iron phosphate in step (5).

[0035] The production method (X) preferably includes a step of measuring the iron content and phosphorus content of the separated liquid 1 between the step (2) and the step (3). This step makes it possible to determine the amount of iron compound to be added so that the molar ratio of iron to phosphorus (Fe / P) in the iron-added separated liquid 1 becomes a specific value. The method for measuring the iron content and phosphorus content is not limited, and known methods can be used, such as colorimetry, atomic absorption spectrometry, and inductively coupled plasma atomic emission spectrometry.

[0036] The production method (X) preferably includes a step of adding washing liquid 1 to separation liquid 1 before step (3), which can increase the recovery rate of phosphate.

[0037] <Process (4)> Step (4) is a step of adding an alkali metal hydroxide to the iron-added separated liquid 1 to adjust the pH to within the range of 0.5 to 2.0, thereby obtaining a pH-adjusted separated liquid 1. Adjusting the pH to within the range of 0.5 to 2.0 reduces the solubility of iron phosphate contained in the iron-added separated liquid 1. In step (4), almost no iron phosphate precipitates, and adjusting the pH to within the range of 0.5 to 2.0 improves the filterability of the iron phosphate obtained in step (6), allowing the production of cake 2 with a higher iron phosphate purity.

[0038] In the step (4), the pH is adjusted to preferably 0.8 to 1.5, more preferably 1.0 to 1.5, and even more preferably 1.1 to 1.3. Adjusting the pH to within the above range reduces the solubility to a degree that does not immediately precipitate iron phosphate, making it easier for iron phosphate to precipitate in the step (5).

[0039] The alkali metal hydroxide is not limited and any known alkali metal hydroxide can be used. Examples of the alkali metal hydroxide include sodium hydroxide and potassium hydroxide, with sodium hydroxide being preferred. The alkali metal hydroxide may be added as a solid or as an aqueous solution obtained by dissolving it in water, but is preferably added as an aqueous solution. Because alkali metal hydroxides are strong bases, the amount of solution required for pH adjustment is smaller than when ammonia is used, and the phosphorus concentration in the reaction solution does not become excessively low. Also, unlike ammonia, alkali metal hydroxides are non-volatile, so their concentration in the solution does not decrease even when heated, resulting in little change in pH.

[0040] The amount of alkali metal hydroxide to be added varies depending on the type of alkali metal hydroxide and the pH of the iron-added separated liquid 1, but an amount that makes the pH of the pH-adjusted separated liquid 1 fall within the range of 0.5 to 2.0 is used.

[0041] <Process (5)> Step (5) is a step of heating the pH-adjusted separated liquid 1 to 50° C. or higher and stirring it to obtain a slurry 2. By heating and stirring the pH-adjusted separated liquid 1 at 50°C or higher, the solubility of iron phosphate is further reduced, causing it to precipitate and become a slurry.

[0042] Although iron phosphate is hardly precipitated in step (4), it precipitates in step (5) by heating and stirring at 50°C or higher. By performing step (5), the particle size of the iron phosphate contained in cake 2 obtained in step (6) becomes uniform and the particle size distribution becomes sharp. As a result, the iron hydroxide contained in cake 3 obtained in step (8) is approximately spherical and relatively dispersible. In addition, the filterability of cake 3 obtained in step (8) is improved, and the time required for solid-liquid separation in step (8) is shortened. Furthermore, the improved filterability increases the recovery efficiency of separated liquid 3, which ultimately increases the total phosphorus yield of calcium phosphate obtained in step (10).

[0043] The heating temperature in step (5) is not limited as long as it is 50°C or higher, but is preferably 50°C to 100°C, more preferably 60°C to 100°C, even more preferably 70°C to 100°C, and particularly preferably 80°C to 95°C. When the heating temperature is within the above range, iron phosphate is more likely to precipitate, and the particle size of the iron phosphate is more likely to become uniform.

[0044] The stirring time in step (5) is not limited, but is preferably 0.5 to 10 hours, more preferably 0.5 to 5 hours, and even more preferably 0.5 to 4 hours. When the stirring time is within the above range, iron phosphate is more likely to precipitate and the particle size of the iron phosphate is more likely to become uniform.

[0045] <Process (6)> Step (6) is a step of subjecting the slurry 2 to solid-liquid separation to obtain a cake 2. By solid-liquid separation, the slurry 2 is separated into a solid (cake 2) and a liquid (separated liquid 2). Heavy metals and the like derived from sewage sludge incineration ash are separated into the liquid (separated liquid 2), so that iron phosphate with a low content of heavy metals and the like can be obtained as a solid (cake 2).

[0046] The production method (X) preferably includes a step between the step (6) and the step (7) of washing the cake 2 with water to obtain the washed cake 2. This allows the heavy metals and the like contained in the cake 2 to be further reduced. The cleaning solution produced in this step is called cleaning solution 2. After washing, the cake 2 may be dried before being subjected to the next step.

[0047] The production method (X) preferably includes, between step (6) and step (7): (6a) adding an alkali metal hydroxide to the separated liquid 2 to readjust the pH to within a range of 0.5 to 2.0 to obtain a pH-adjusted separated liquid 2; (6b) heating and stirring the pH-adjusted separated liquid 2 at 50°C or higher to obtain a slurry 2-1; (6c) a step of subjecting the slurry 2-1 to solid-liquid separation to obtain a cake 2-1; and (6d) adding the cake 2-1 to the cake 2 Further includes:

[0048] The phosphate ions remaining in the separated liquid 2 are reprecipitated as iron phosphate to obtain a solid (cake 2-1), so cake 2 to which cake 2-1 has been added has a higher phosphorus yield while maintaining a low aluminum content.

[0049] It is preferable to repeat steps (6a) to (6d) two or more times, which further increases the phosphorus yield of cake 2 to which cake 2-1 has been added.

[0050] <Process (7)> Step (7) is a step of mixing the cake 2 with an alkali metal hydroxide solution to obtain a slurry 3 having a pH of 12 or higher. In step (7), the iron phosphate contained in cake 2 dissolves, and the resulting iron ions precipitate as iron hydroxide.

[0051] The pH in step (7) is preferably at least 13, more preferably at least 14. When the pH is within the above range, iron phosphate dissolves more easily, and iron hydroxide precipitates more easily.

[0052] The alkali metal hydroxide is not limited and any known alkali metal hydroxide can be used. Examples of the alkali metal hydroxide include sodium hydroxide and potassium hydroxide, with sodium hydroxide being preferred. Alkali metal hydroxides are strong bases and do not easily volatilize. Therefore, compared with using ammonia for pH adjustment, the amount required for pH adjustment is small, so the phosphorus concentration in the solution does not decrease, and the pH change due to heating is small. Furthermore, even in a saturated solution, ammonia has a pH of approximately 12 to 13. When mixed with cake 2 and neutralized with iron phosphate, it is difficult to raise the pH of slurry 3 to 12 or higher, making it unsuitable for use in step (7). On the other hand, a small amount of alkali metal hydroxide can raise the pH of slurry 3 to 12 or higher.

[0053] The amount of alkali metal hydroxide varies depending on the type of alkali metal hydroxide and the amount of cake 2, but an amount that makes the pH of the slurry 3 12 or higher is used.

[0054] The ratio of the amount of cake 2 to the alkali metal hydroxide solution to be mixed is not particularly limited, and the cake 2 and the alkali metal hydroxide solution may be mixed at any ratio. Cake 2 and the alkali metal hydroxide solution are preferably mixed at a solid-liquid ratio (L / S) of 13 to 27 L / kg, more preferably 15 to 25 L / kg, and even more preferably 17 to 23 L / kg. Mixing at a solid-liquid ratio within the above ranges increases the rate at which iron hydroxide precipitates in step (7), increases the rate at which calcium phosphate precipitates in step (9), and tends to increase the yield of calcium phosphate obtained in step (10).

[0055] It is preferable to further stir and / or heat the slurry 3, since this makes it easier for iron hydroxide to precipitate. The stirring time is preferably 1 minute to 5 hours, more preferably 10 minutes to 4 hours, and even more preferably 30 minutes to 2 hours. When the stirring time is within the above range, iron hydroxide is more likely to precipitate. The heating temperature is preferably 40° C. to 80° C., more preferably 50° C. to 70° C., and even more preferably 55° C. to 65° C. When the heating temperature is within the above range, iron hydroxide is more likely to precipitate, and the fluidity of the slurry 3 is increased, improving filterability, which also has the effect of improving the phosphorus yield.

[0056] <Process (8)> Step (8) is a step of subjecting the slurry 3 to solid-liquid separation to obtain a separated liquid 3. By solid-liquid separation, the slurry 3 is separated into a solid (cake 3) and a liquid (separated liquid 3). Since iron hydroxide is mainly separated from the solid (cake 3), a liquid (separated liquid 3) with a low iron content and a high phosphate ion content is obtained.

[0057] Cake 3 mainly contains iron hydroxide and can be used as the iron compound in step (3). At the same time, cake 3 also contains alkali metal hydroxide, which, combined with the iron hydroxide being basic, can also be used as the alkali metal hydroxide for pH adjustment in step (4). Furthermore, if the pH becomes high when cake 3 is used to adjust the pH in step (4), the pH can also be adjusted using an acid (such as sulfuric acid).

[0058] <Process (9)> Step (9) is a step of mixing the separated liquid 3 with calcium hydroxide to obtain a slurry 4. In step (9), calcium phosphate formed from phosphate ions contained in the separated liquid 3 and calcium ions of calcium hydroxide is precipitated.

[0059] Calcium hydroxide (Ca(OH)2) is also called slaked lime or lime. There are no restrictions on the purity or grade of calcium hydroxide, but calcium hydroxide with a small amount of impurities such as chlorine and aluminum is preferred. Calcium hydroxide may be mixed with the separated liquid 3 in any of the following forms: powder, aqueous solution (also called lime water), or suspension (also called milk of lime). However, it is preferably mixed in powder form. This makes it easier to obtain calcium phosphate with a low aluminum content and also increases the phosphorus yield. Furthermore, mixing in powder form tends to result in better filterability of the calcium phosphate obtained in step (10) than mixing in suspension form.

[0060] It is preferable to further stir the slurry 4, since this makes it easier for calcium phosphate to precipitate. The stirring time is preferably 1 to 10 hours, more preferably 1 to 7 hours, and even more preferably 4 to 6 hours. When the stirring time is within the above range, calcium phosphate is more likely to precipitate.

[0061] The amount of calcium hydroxide to be mixed with the separated liquid 3 is not limited, but is preferably an amount that results in a calcium to phosphorus molar ratio (Ca / P) in the slurry 4 of 1.5 to 2.5, more preferably an amount that results in a calcium to phosphorus molar ratio of 1.67 to 2.3, and even more preferably an amount that results in a calcium to phosphorus molar ratio of 1.8 to 2.2 in the above range. When the calcium to phosphorus molar ratio is within the above range, calcium phosphate with a low aluminum content is likely to be obtained, and the phosphorus yield is likely to be high.

[0062] The production method (X) preferably includes a step of measuring the phosphorus content of the separated liquid 3 between the step (8) and the step (9). This step allows the amount of calcium hydroxide to be determined so that the molar ratio of calcium to phosphorus (Ca / P) in the slurry 4 becomes a specific value. The method for measuring the phosphorus content is not limited, and known methods can be used, such as colorimetry, atomic absorption spectrometry, and inductively coupled plasma atomic emission spectrometry.

[0063] <Process (10)> Step (10) is a step of subjecting the slurry 4 to solid-liquid separation to obtain calcium phosphate. By the solid-liquid separation, the slurry 4 is separated into a solid (cake 4) and a liquid (separated liquid 4). Since sodium hydroxide and the like are separated into the liquid (separated liquid 4), high-purity calcium phosphate is obtained as a solid (cake 4).

[0064] The production method (X) preferably includes, after the step (10), a step of washing the cake 4 with water to obtain a washed cake 4. This allows the content of sodium hydroxide and the like in the cake 4 to be further reduced. The washing liquid produced in this step is called washing liquid 4. After washing, the cake 4 is preferably dried. [Example]

[0065] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0066] <Raw materials and reagents> Incineration ash: Sewage sludge incineration ash (gift from the Tokyo Metropolitan Government Bureau of Sewerage) was used, with the composition ratios shown in Table 1. The method for measuring the composition of incineration ashes 1 and 2 will be described later.

[0067] [Table 1]

[0068] Sulfuric acid: In Production Example 1 and Comparative Example 1, (1+1) sulfuric acid (65%) (manufactured by Kanto Chemical Co., Ltd.) was used, and in Production Examples 2 to 5, 98% sulfuric acid (manufactured by Taiki Pharmaceutical Co., Ltd.) was used.

[0069] Sodium hydroxide: 40% NaOH (special grade) (manufactured by Kanto Chemical Co., Ltd.) was used in Production Example 1 and Comparative Example 1, and 48% NaOH (first grade) (manufactured by Kanto Chemical Co., Ltd.) was used in Production Examples 2 to 5.

[0070] Iron sulfate (III): Fe2(SO4)3·n hydrate (Kanto Chemical Co., Ltd. Grade 1) was used.

[0071] <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 to obtain 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)

[0072] (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 beaker was rinsed with pure water and the sample solution was transferred to a 500 mL measuring flask. The solution was filtered using No. 6 quantitative filter paper, and at least 150 mL of filtrate was collected. The resulting filtrate was used as the test solution.

[0073] (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.

[0074] 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).

[0075] 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.

[0076] (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 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 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).

[0077] Using the weighed values ​​(D) and (E), SO3 (wt%) was calculated according to the following formula (II). SO3 (wt%) = (D - E) × 0.343 × 100 / Dry sample (g) ··· Formula (II)

[0078] (Analysis of SiO2 (atomic absorption spectrometry)) After precisely weighing 5.0 g of the dry sample into a nickel crucible, approximately 1 - 2 mL of pure water and 10 g of potassium hydroxide were added and mixed well. The crucible was heated to dissolve the potassium hydroxide and completely remove the moisture. After cooling, it was transferred to a 50 mL beaker, water was added to dissolve the melt. After adding 1 drop of phenolphthalein solution, (1 + 2) hydrochloric acid was added little by little to the beaker to neutralize the solution, and then (1 + 2) hydrochloric acid was added in excess. After cooling, it was diluted with water to obtain a SiO2 measurement solution. The SiO2 measurement solution was subjected to measurement using an atomic absorption spectrometer (ZA3300, manufactured by Hitachi, Ltd.). The measurement conditions are shown in Table 2 below.

[0079] SiO2 (wt%) was calculated from the following formula (III). SiO2 (wt%) = (Atomic absorption (Si) × 2.1393 × 100) / (Dry sample (g) × 1 / 500 × 1000000) ··· Formula (III)

[0080] (Analysis of CaO, Fe2O3, Al2O3, MgO, Na2O (atomic absorption spectrometry)) The test solution was appropriately diluted with pure water and subjected to measurement using an atomic absorption spectrometer (ZA3300, manufactured by Hitachi, Ltd.) to measure the concentrations of Ca, Fe, Al, Mg, and Na. The concentrations of Ca, Fe, Al, Mg, and Na were measured as the concentrations (wt%) of CaO, Fe2O3, Al2O3, MgO, and Na2O, respectively. The measurement conditions are shown in Table 2 below. Note that the concentration measurement of Na was carried out under the conditions of "Na" when the concentration was 0 - 1 ppm and "Na (fluorescence)" when the concentration was 1 - 30 ppm.

[0081]

Table 2

[0083] <Particle size measurement> The particle size of the sample was measured using a particle size analyzer (Microtrac, Leeds & Northrup, Microtrac 9320-X100). The 10%, 50%, and 90% diameters of the cumulative distribution were defined as D10, D50 (median diameter), and D90, respectively.

[0084] <Observation by Scanning Electron Microscope (SEM)> The samples were observed using a scanning electron microscope (JSM-6010LA, manufactured by JEOL Ltd.).

[0085] <Evaluation of filterability> The filterability during vacuum filtration was evaluated according to the following criteria. A: Good filterability (the filtration process takes less than 5 minutes) B: Poor filterability (the filtration process took more than 30 minutes).

[0086] [Experiment 1] Recovery of iron phosphate from incineration ash [Manufacturing Example 1] A sulfuric acid solution with a concentration of 1 mol / L was added to 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, and slurry 1 was obtained. The pH of slurry 1 was approximately 0 to 0.3. It was heated for 2 hours while maintaining the temperature at 80°C. Thereafter, the slurry 1 was suction filtered using filter paper (Advantec, quantitative filter paper No. 5C) to recover the separated liquid 1 (incinerator ash extract) and the filter cake 1 (incinerator ash extract residue). In addition, the filter cake 1 was filtered and washed with water to recover the washing liquid 1.

[0087] The concentrations of P2O5, Al2O3, and Fe2O3 were measured in the separated solution 1. Note that the separated solution 1 was pretreated as described above to prepare it as a test solution before the measurement. Next, the separated liquid 1 was placed in another reaction vessel 2, and the washing liquid 1 was further added thereto so that the phosphoric acid concentration was equivalent to 1.87 wt% as P2O5, and the mixture was stirred and mixed to obtain a mixed liquid. Iron (III) sulfate was added to the reaction vessel 2 so that the molar ratio of iron to phosphorus (Fe / P, mol / mol) in the mixed solution was 1.1, and the mixture was stirred until completely dissolved. While measuring the pH of the liquid in reaction tank 2 (iron-added separated liquid 1) with a pH meter, sodium hydroxide was added to adjust the pH to 1.2, and a pH-adjusted separated liquid 1 was obtained. The mixture was then heated at 80°C for 1 hour with stirring to obtain slurry 2. The obtained slurry 2 was filtered under reduced pressure, and a separated liquid 2 and a filter cake 2 were collected. The filter cake 2 was washed with water and dried to obtain an iron phosphate precipitate 1.

[0088] The filterability of Slurry 2 was evaluated when it was filtered under reduced pressure. The results are shown in Table 3. The recovered iron phosphate precipitate 1 was used as a sample and its composition was analyzed by the method described above. The results are shown in Table 3.

[0089] [Manufacturing Example 2] Iron phosphate precipitate 1 was recovered from the incineration ash in the same manner as in Production Example 1, except that incineration ash 1 in Production Example 1 was replaced with incineration ash 2 and the target phosphoric acid concentration of the mixed solution in Reaction Tank 2 was set to 1.5 wt%, and its composition was analyzed. The results are shown in Table 3.

[0090] [Manufacturing Example 3] Iron phosphate precipitate 1 was recovered from the incineration ash in the same manner as in Production Example 1, except that incineration ash 1 in Production Example 1 was replaced with incineration ash 2, the phosphoric acid concentration of the mixed solution in reaction tank 2 was set to 1.5 wt%, and the stirring conditions after the addition of the sodium hydroxide solution were set to 80°C for 2 hours. The results are shown in Table 3.

[0091] [Manufacturing Example 4] Iron phosphate precipitate 1 was recovered from the incineration ash in the same manner as in Production Example 1, except that incineration ash 1 in Production Example 1 was replaced with incineration ash 2, the phosphoric acid concentration of the mixed solution in reaction tank 2 was 1.5% by weight, and the stirring conditions after the addition of the sodium hydroxide solution were 95°C for 2 hours. The results are shown in Table 3. Furthermore, for Production Example 4, the particle size of iron phosphate precipitate 1 was also measured.

[0092] [Manufacturing Example 5] Iron phosphate precipitate 1 was recovered from the incineration ash in the same manner as in Production Example 1, except that incineration ash 1 in Production Example 1 was replaced with incineration ash 2, the phosphoric acid concentration of the mixed solution in Reaction Tank 2 was set to 1.54 wt%, and the stirring conditions after the addition of the sodium hydroxide solution were set to 95°C for 4 hours. The results are shown in Table 3.

[0093] [Comparative Manufacturing Example 1] Comparative iron phosphate precipitate 1 was recovered from the incineration ash in the same manner as in Production Example 1, except that the phosphoric acid concentration of the mixed solution in Reaction Tank 2 in Production Example 1 was adjusted to 1.6 wt%, the pH was adjusted to 2.5 by adding sodium hydroxide solution, and the stirring conditions after the addition of the sodium hydroxide solution were changed to 40°C for 1 hour. The composition was analyzed and the particle size was measured. The results are shown in Table 3.

[0094] [Table 3]

[0095] In Production Examples 1 to 5, iron phosphates with lower aluminum contents were obtained compared to Comparative Production Example 1 (Table 3). Furthermore, in Production Examples 1 to 5, the filterability of the slurry 2 was superior to that of Comparative Production Example 1. Furthermore, the iron phosphate precipitate 1 obtained in Production Example 4 had a smaller difference between D10 and D90, a more uniform particle size, and a sharper particle size distribution than that obtained in Comparative Production Example 1.

[0096] [Experiment 2] Reprecipitation of iron phosphate [Manufacturing Example 6] The separated liquid 2 obtained in Production Example 1 was stirred, and while measuring the pH, NaOH was added to adjust the pH to 1.2, yielding a pH-adjusted separated liquid 2-1. The mixture was then further heated to 80°C and stirred for 1 hour to yield a slurry 2-1. The slurry 2-1 was filtered under reduced pressure to recover a separated liquid 2-1 and a filter cake 2-1. The filter cake 2-1 was washed and dried to obtain an iron phosphate precipitate 2. The composition of the iron phosphate precipitate 2 was analyzed.

[0097] Separation liquid 2-1 was stirred, and while measuring the pH, NaOH was added to adjust the pH to 1.5, yielding pH-adjusted separation liquid 2-2. The mixture was then further heated to 80°C and stirred for 1 hour to yield slurry 2-2. Slurry 2-2 was filtered (vacuum filtration) to recover separation liquid 2-2 and filter cake 2-2. Filter cake 2-2 was washed and dried to yield iron phosphate precipitate 3. The composition of iron phosphate precipitate 3 was analyzed.

[0098] Iron phosphate precipitate 2, iron phosphate precipitate 3, and iron phosphate precipitate 1 obtained in Production Example 1 were combined into one sample (iron phosphate precipitate 4). The composition of iron phosphate precipitate 4 was calculated by summing the analytical values ​​of iron phosphate precipitate 2, iron phosphate precipitate 3, and iron phosphate precipitate 1 obtained in Production Example 1. The yields of phosphorus, iron, and aluminum contained in iron phosphate precipitate 4 were calculated by setting the phosphorus, iron, and aluminum contents of iron-added separated liquid 1 at 100%. The results are shown in Table 4.

[0099] [Comparative Manufacturing Example 2] Comparative iron phosphate precipitate 5 was produced in the same manner as in Comparative Production Example 1. As in Production Example 6, the yields of phosphorus, iron, and aluminum contained in Comparative iron phosphate precipitate 5 were calculated based on the phosphorus, iron, and aluminum contents of iron-added separated liquid 1 being 100%, and the results are shown in Table 4.

[0100] [Table 4]

[0101] After recovering the iron phosphate precipitate obtained in Production Example 1, an alkali metal hydroxide was added again to the separated liquid 2, which was then heated and stirred, and the iron phosphate precipitate was recovered again. By repeating this process, it was confirmed that iron phosphate could be obtained while maintaining a low aluminum content, and that the phosphorus yield was high (Table 4).

[0102] [Experiment 3] Production of artificial phosphate rock (calcium phosphate) [Example 1] <Iron hydroxide precipitation process> A 1 mol / L NaOH solution was introduced into reaction tank 3 and heated to 60°C. While stirring the heated NaOH solution, iron phosphate precipitate 4 obtained in Production Example 6 was added to the reaction tank at a solid-liquid ratio L / S = 20 (L / kg), causing iron hydroxide to precipitate and producing slurry 3 (iron hydroxide slurry). The pH of slurry 3 was approximately 13 to 14. The mixture was then stirred for 1 hour while maintaining the temperature at 60°C. The iron hydroxide slurry was filtered under reduced pressure to recover separated liquid 3 and iron hydroxide cake 3. Iron hydroxide cake 3 was also filtered and washed with water to recover washed liquid 3. The time required for the step of filtering the iron hydroxide slurry under reduced pressure was measured, and the filterability (filtration time) was evaluated. The results are shown in Table 5. The composition of the separated liquid 3 was also analyzed. Iron hydroxide cake 3 was observed under SEM. The SEM photograph is shown in Figure 1.

[0103] <Calcium phosphate precipitation process> The separated liquid 3 obtained in the above step was introduced into a reaction vessel 4, and while stirring, powdered slaked lime (Ca(OH)2) (Fujifilm Wako Pure Chemical Industries, special grade reagent, product code: 034-16297) was added so that the calcium to phosphorus molar ratio (Ca / P, mol / mol) was 2.0, precipitating calcium phosphate and obtaining a slurry 4 (calcium phosphate slurry). After stirring for 5 hours at room temperature, the calcium phosphate slurry was suction filtered to recover a calcium phosphate cake 4. The calcium phosphate cake 4 was washed with water and dried to obtain an artificial phosphate rock (calcium phosphate).

[0104] The composition of the artificial phosphate rock (calcium phosphate) was analyzed, and the results are shown in Table 5.

[0105] [Comparative Example 1] Instead of iron phosphate precipitate 4 obtained in Production Example 6, comparative iron phosphate precipitate 1 obtained in Comparative Production Example 1 was used, and the iron hydroxide precipitation step and calcium phosphate precipitation step were carried out in the same manner as in Example 1 to obtain artificial phosphate rock (calcium phosphate), and the composition was analyzed. The results are shown in Table 5. In addition, the time required for the step of filtering the iron hydroxide slurry under reduced pressure was measured in the same manner as in Example 1, and the filterability (filtration time) was evaluated, and the results are shown in Table 5.

[0106] [Table 5]

[0107] In Example 1, an artificial phosphate rock was obtained that had a very low content of aluminum as an impurity, but a high phosphorus content (Table 5). By the method of Example 1, calcium phosphate with a low aluminum content was obtained. Furthermore, the time required for the step of filtering the iron hydroxide slurry in Example 1 was significantly shorter than that in Comparative Example 1. The particles constituting the iron hydroxide cake obtained by filtering the iron hydroxide slurry in Example 1 were approximately spherical (Fig. 1). In addition, in the calcium phosphate precipitation step of Example 1, when comparing the case where slaked lime was added in powder form with the case where it was added in suspension form, it was confirmed that the case where it was added in powder form had better filterability.

Claims

1. A method for producing calcium phosphate from sewage sludge incineration ash containing phosphorus and aluminum, comprising the following steps (1) to (10): The phosphorus content in the separated liquid 1 is P 2 O 5 A method for producing calcium phosphate, which is calculated as 0.5 to 20 wt %. (1) A step of mixing the incineration ash with an acid solution to obtain a slurry 1 having a pH of less than 0.5 (2) A step of subjecting the slurry 1 to solid-liquid separation to obtain a separated liquid 1 (3) A step of adding an iron compound to the separated solution 1 to obtain an iron-added separated solution 1. (4) A step of adding an alkali metal hydroxide to the iron-added separated solution 1 to adjust the pH to a range of 0.5 to 2.0, thereby obtaining a pH-adjusted separated solution 1. (5) A step of heating and stirring the pH-adjusted separated liquid 1 at 50°C or higher to obtain a slurry 2. (6) A step of separating the slurry 2 into solid and liquid to obtain a cake 2 (7) A step of mixing the cake 2 with an alkali metal hydroxide solution to obtain a slurry 3 having a pH of 12 or more. (8) A step of subjecting the slurry 3 to solid-liquid separation to obtain a separated liquid 3 (9) A step of mixing the separated liquid 3 with calcium hydroxide to obtain a slurry 4. (10) A step of subjecting the slurry 4 to solid-liquid separation to obtain calcium phosphate

2. 2. The method for producing calcium phosphate according to claim 1, wherein in the step (4), the pH is adjusted to a range of 0.8 to 1.

5.

3. 3. The method for producing calcium phosphate according to claim 1, wherein the amount of the iron compound is such that the molar ratio of iron to phosphorus (Fe / P) in the iron-added separated liquid 1 is 1.1 to 1.

5.

4. The method for producing calcium phosphate according to claim 1 or 2, wherein the stirring time in the step (5) is 0.5 to 10 hours.

5. 3. The method for producing calcium phosphate according to claim 1, wherein the amount of calcium hydroxide is such that the molar ratio of calcium to phosphorus (Ca / P) in the slurry 4 is 1.5 to 2.5.

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