Phosphate-containing solid

Abstract

To provide a phosphate-containing solid and a method for producing the same, with the use of sewage sludge incineration ash as a raw material, which has a high proportion of citrate-soluble phosphoric acid relative to a total phosphoric acid, contains a low amount of free phosphate, and can be effectively utilized as a phosphate fertilizer.SOLUTION: A phosphate-containing solid is produced by reacting sewage sludge incineration ash and sulfuric acid under non-aqueous conditions to generate water-soluble or citrate-soluble phosphates,in which the quantities of the aforementioned sewage sludge incineration ash and sulfuric acid are controlled during this generation process to achieve a citrate-soluble phosphate ratio relative to a total phosphoric acid of the phosphate to be produced of 0.7 mass ratio or higher, and a free phosphate content of 8 mass% or less, and a manufacturing method thereof is provided.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a phosphate-containing solid material that contains phosphate, which is a reaction product of sewage sludge incineration ash and sulfuric acid, has a high ratio of citrate-soluble phosphoric acid to total phosphoric acid, and can be effectively used as a phosphate fertilizer, and a method for producing the same. [Background technology]

[0002] Phosphate rock, the raw material for phosphate fertilizer, is produced in limited areas, and its price fluctuates dramatically due to factors such as global population growth and geopolitical issues. Meanwhile, the phosphorus concentration in sludge has been gradually increasing as sewage treatment becomes more advanced, and it is said that the phosphorus concentration in incineration ash in particular has increased to a level comparable to that of phosphate rock. Furthermore, as the amount of heavy metals in sewage has been decreasing recently, the heavy metal content in sewage sludge incineration ash is now often sufficient to meet the standard for "microbial phosphate fertilizer." For this reason, attempts are being made to use sewage sludge incineration ash as a raw material for phosphate fertilizer.

[0003] The following techniques have been known as techniques for recovering phosphorus from waste generated during sewage treatment or for producing phosphate fertilizer. (i) A method in which a phosphorus adsorbent is added to the dehydrated filtrate of sewage sludge to recover phosphorus components, and the recovered phosphorus is used as phosphate fertilizer or a fertilizer raw material (Patent Publication No. 2019-155353). (b) A technology in which sulfuric acid is added to sewage sludge to dissolve phosphorus and aluminum, and the resulting solution is filtered to recover and reuse the phosphoric acid solution (Patent Publication No. 2015-199614). (c) A method in which sulfuric acid or hydrochloric acid is added to sewage sludge incineration ash to elute phosphorus, and the eluate is separated to recover and reuse the phosphorus (Japanese Patent Laid-Open Nos. 7-2511417 and 11-278814, etc.).

[0004] The conventional technology is a solution treatment method in which a large amount of sulfuric acid or the like is added to sewage sludge incineration ash or the like to elute phosphorus components, and the resulting phosphate solution is separated, recovered, and utilized. This conventional solution treatment requires a large amount of solution, and the amount of free phosphate is also large, so if the solution is dried as is, particles tend to stick together. Furthermore, in the conventional solution treatment, after the elution step using sulfuric acid or the like, the eluted phosphate solution is recovered by solid-liquid separation, and lime or the like is added to this to produce phosphate fertilizer. This makes fertilization time-consuming. Furthermore, there are problems with the burden on filtration equipment and the like, which increases running costs. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-155353 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-199614 [Patent Document 3] Japanese Patent Application Publication No. 7-251141 [Patent Document 4] Japanese Patent Application Publication No. 11-278814 Summary of the Invention [Problem to be solved by the invention]

[0006] To achieve sufficient effectiveness as a phosphate fertilizer, a sufficiently high proportion of citrate-soluble phosphate is required. However, since citrate-soluble phosphate generally accounts for approximately 40-50% of the total phosphate in sewage sludge, the fertilizer efficiency is low. It is difficult to directly use sewage sludge incineration ash as phosphate fertilizer, creating challenges in circulating phosphorus. Furthermore, conventional solution treatment methods for eluting phosphorus from sewage sludge, etc., require the separation of the solution containing the phosphorus, which increases the burden of fertilization.

[0007] The present invention overcomes the above-mentioned problems of the prior art and provides a phosphate-containing solid that is made from sewage sludge incineration ash as a raw material, has a high ratio of citrate-soluble phosphoric acid to total phosphoric acid, and has a small amount of free phosphoric acid, so that particles do not stick together and fertilization is easy.Furthermore, in its production, sewage sludge incineration ash is reacted with sulfuric acid without forming a solution, so that solid-liquid separation is not required, and a method for producing the same is provided. [Means for solving the problem]

[0008] The present invention relates to a phosphate-containing solid material having the following constitution and a method for producing the same. [1] A phosphate-containing solid containing water-soluble or citrate-soluble phosphates and reaction residues, which are reaction products of sewage sludge incineration ash and sulfuric acid, characterized in that the ratio of citrate-soluble phosphates to total phosphates is 0.7 mass ratio or more, and the content of free phosphates is 8 mass% or less. [2] The phosphate-containing solid material described in [1] above, which is used as a phosphate fertilizer. [3] A method for producing a phosphate-containing solid, comprising reacting sewage sludge incineration ash with sulfuric acid in a non-solvent state to produce water-soluble or citrate-soluble phosphates, controlling the amount of sulfuric acid relative to the sewage sludge incineration ash in this production process so that the ratio of citrate-soluble phosphoric acid to total phosphoric acid in the produced phosphates is 0.7 mass ratio or more, and the content of free phosphoric acid is 8 mass% or less, and recovering the produced phosphates and residue. [4] A method for producing a phosphoric acid-containing solid material according to claim 3, comprising adding 25 to 100 parts by mass of sulfuric acid with a concentration of 98% or more to 100 parts by mass of sewage sludge incineration ash having a phosphoric acid content of 20 to 40% by mass, thereby producing a phosphate salt in which the ratio of citrate-soluble phosphoric acid to the total phosphoric acid is 0.7 mass ratio or more and free phosphoric acid is 8 mass% or less, and the produced phosphate salt and residue are recovered. [5] A method for producing a phosphate-containing solid material according to [3] or [4] above, in which powdered sewage sludge incineration ash is granulated by rolling while being sprayed with a sulfuric acid solution.

[0009] [Phosphate-containing solids] The phosphate-containing solid of the present invention contains water-soluble or citrate-soluble phosphate, which is a reaction product of sewage sludge incineration ash and sulfuric acid, and reaction residue, and has a ratio of citrate-soluble phosphoric acid to total phosphoric acid of 0.7 mass ratio or more and a free phosphoric acid content of 8 mass% or less, and can be effectively used as a phosphate fertilizer.

[0010] The phosphate-containing solid of the present invention is made from sewage sludge incineration ash. This sewage sludge incineration ash is ash obtained by dehydrating and incinerating sludge such as sewage or wastewater, and general sewage sludge incineration ash can be used. The sewage sludge incineration ash, which is the raw material for the phosphate-containing solid of the present invention, preferably has a heavy metal content below the standard values ​​for bacterial phosphate fertilizer (Cd: 0.0005%, Pb: 0.01%, Ni: 0.03%, Hg: 0.0002%, As: 0.005%, Cr: 0.05%). Since many recent sewage sludge incineration ashes have heavy metal contents below the standard values, general sewage sludge incineration ash can generally be used.

[0011] The phosphate-containing solid of the present invention is a phosphate-containing solid containing a large amount of water-soluble or citric acid-soluble phosphate, which is a reaction product of sewage sludge incineration ash and sulfuric acid. Typical sewage sludge incineration ash contains, for example, approximately 25 to 40 mass% total phosphoric acid, but most of these phosphorus components are bound to metal components and immobilized, and exist in the form of, for example, iron phosphate or aluminum phosphate. Iron phosphate and aluminum phosphate are almost insoluble in water and in 2% citric acid, so they cannot be used as fertilizer components as they are.

[0012] The phosphate-containing solid material of the present invention contains a large amount of water-soluble or citric acid-soluble phosphates, which are reaction products of sulfuric acid reacting with poorly soluble iron phosphate, aluminum phosphate, etc. contained in the incineration ash of sewage sludge. For example, it is thought that iron phosphate, aluminum phosphate, etc. react with sulfuric acid to decompose, and the phosphate ions combine with alkali metals such as calcium, sodium, and potassium contained in the incineration ash to form water-soluble alkali phosphates.

[0013] In the phosphate-containing solid of the present invention, the ratio of citrate-soluble phosphoric acid (C-P2O5) to total phosphoric acid (T-P2O5) is 0.7 mass ratio or more (70 mass ratio or more), preferably 0.8 mass ratio or more (80 mass ratio or more). Hereinafter, the ratio of citrate-soluble phosphoric acid to total phosphoric acid (C / T-P2O5) is referred to as the citrate-soluble phosphoric acid ratio. The ratio of citrate-soluble phosphoric acid to the total phosphoric acid in general sewage sludge incineration ash before sulfuric acid reaction is approximately 35 to 45 mass %. Therefore, the ratio of citrate-soluble phosphoric acid contained in the phosphate-containing solid of the present invention is significantly higher, and when used as a phosphate fertilizer, excellent fertilizer effects can be obtained.

[0014] In the phosphate-containing solid of the present invention, the water-soluble or citric acid-soluble phosphate, which is the reaction product of sewage sludge incineration ash and sulfuric acid, is produced by a non-solution-based reaction, and therefore the content of free phosphoric acid is significantly low, 8% by mass or less. Specifically, the phosphate-containing solid of the present invention is a phosphate produced by reacting sewage sludge incineration ash with a sufficiently small amount of sulfuric acid so as not to produce free phosphoric acid during the production process. For example, it is a phosphate produced by a non-solution-based reaction in which sulfuric acid is sprayed onto sewage sludge incineration ash and reacted.

[0015] On the other hand, conventional phosphorus recovery methods involve a solution-based reaction using a large amount of sulfuric acid, which dissolves the phosphorus components and separates and recovers the eluate. This conventional solution-based treatment produces a large amount of free phosphorus, which, when dried as is, tends to cause particles to stick together, creating problems in storage and fertilization. Furthermore, since free phosphorus is washed away in a short period of time, a high content of free phosphorus makes it difficult to enhance the long-term fertilizer effect.

[0016] In conventional solution treatments in which sewage sludge incineration ash is treated with a large amount of sulfuric acid, the resulting phosphoric acid solution is recovered by solid-liquid separation and lime, etc. is added to produce phosphate fertilizer. On the other hand, the phosphate-containing solid material of the present invention is a solid material containing phosphate and reaction residues produced by reacting sewage sludge incineration ash with sulfuric acid without forming a solution, and since no lime, etc. is added, it contains almost no lime content other than that derived from sewage sludge incineration ash.

[0017] The phosphate-containing solid of the present invention may contain lime other than that derived from sewage sludge incineration ash in order to adjust the composition. Conventional phosphate fertilizers produced by adding lime or the like contain approximately 15 to 30 mass% of lime in terms of Ca, but when the phosphate-containing solid of the present invention contains lime other than that derived from sewage sludge incineration ash, the content does not need to be the same as the amount of lime in the conventional phosphate fertilizers, and may be less than 15 mass% in terms of Ca.

[0018] [Manufacturing method] The phosphate-containing solid of the present invention is prepared by reacting sewage sludge incineration ash with sulfuric acid in a non-solvent state to produce water-soluble or citrate-soluble phosphates, and in this production process, the amounts of the sewage sludge incineration ash and sulfuric acid are controlled so that the ratio of citrate-soluble phosphate (C-P2O5) to total phosphate (T-P2O5) in the reaction product phosphate (citric acid soluble phosphate ratio: C / T-P2O5) is 0.7 mass ratio (70 mass%) or more, preferably 0.8 mass ratio (80 mass%) or more, and the content of free phosphate is 8 mass% or less. The phosphate-containing solid of the present invention can be obtained by recovering the produced phosphates and residue.

[0019] In the production method of the present invention, sewage sludge incineration ash and sulfuric acid are reacted in a non-solution state. Specifically, a sufficiently small amount of sulfuric acid is used relative to the amount of sewage sludge incineration ash so as not to produce a solution. For example, 25 to 100 parts by mass of sulfuric acid with a concentration of 98% or higher is added to 100 parts by mass of sewage sludge incineration ash (phosphate content 20 to 40% by mass, preferably 25 to 35% by mass) and thoroughly stirred, or the above amount of sulfuric acid is sprayed on the sewage sludge incineration ash and reacted in this state, thereby increasing the ratio of citrate-soluble phosphoric acid to total phosphoric acid (citrate-soluble phosphoric acid ratio: C / T-P2O5) to 0.7 mass ratio or more, preferably 0.8 mass ratio or more, and producing a water-soluble or citrate-soluble phosphate with a free phosphoric acid content limited to 8 mass% or less.

[0020] If the amount of sulfuric acid added to the sewage sludge incineration ash is too small, water-soluble or citrate-soluble phosphate is not produced sufficiently, and the ratio of citrate-soluble phosphate to total phosphate (citrate-soluble phosphate ratio) cannot be increased to 0.7 mass ratio or more. On the other hand, if the amount of sulfuric acid added to the sewage sludge incineration ash is too large, the amount of free phosphate produced increases, and it is not possible to limit the amount of free phosphate to 8 mass% or less.

[0021] The amount of sulfuric acid to be added to the sewage sludge incineration ash is adjusted depending on the content of phosphoric acid contained in the incineration ash and the concentration of sulfuric acid. In the production method of the present invention, when a sufficiently small amount of sulfuric acid is sprayed onto the sewage sludge incineration ash to cause a reaction, a plate-type granulator or the like is used to roll the product at the same time as the spraying to produce a granular phosphate-containing solid. [Effects of the Invention]

[0022] The phosphate-containing solid of the present invention has a sufficiently high citrate-soluble phosphate ratio of 0.7 mass ratio or more, preferably 0.8 mass ratio or more, so that when used as a phosphate fertilizer, it can provide excellent fertilizer effects over a long period of time. Furthermore, since the amount of free phosphate in the phosphate-containing solid of the present invention is limited to 8 mass% or less, particles do not adhere to each other, maintaining good properties even during long-term storage and facilitating fertilization. Furthermore, the amount of phosphate washed away within a short period of time after application is significantly reduced, making it economical.

[0023] The production method of the present invention produces a phosphoric acid-containing material in a non-solution state, does not require solid-liquid separation, and is therefore easy to implement, with little burden on equipment. On the other hand, conventional methods in which sewage sludge incineration ash is treated with a large amount of sulfuric acid require many production steps, such as a step of eluting the phosphorus component, a step of solid-liquid separation after elution, and a step of making a fertilizer by adding lime or the like after solid-liquid separation, which places a heavy burden on equipment and work processes. [Brief explanation of the drawings]

[0024] [Figure 1] Graph showing test results of Example 1 [Example]

[0025] Examples of the present invention are presented below. The scope of the present invention is not limited to the following examples. For each sample, total phosphate, citrate-soluble phosphate, and water-soluble phosphate were measured using the ammonium vanadomolybdate spectrophotometric method in the Fertilizer Testing Methods (2023). Free phosphate was extracted using the method described in Section 5.30.1, Diethyl Ether Method, of the Fertilizer Analysis Methods (1992 edition), and the resulting solution was measured using the ammonium vanadomolybdate spectrophotometric method. Extraction was performed by shaking 2 g of sample in 100 mL of a 1:1 diethyl ether-acetone mixture for 30 minutes, filtering, and heating the filtrate to 80°C to dryness, followed by dissolving in water.

[0026] Example 1 Sewage sludge incineration ash A containing the components shown in Table 1 was used as the raw material. To 100 g of this sewage sludge incineration ash, 98% sulfuric acid was added in incremental amounts from 11.1 g to 100 g, and 200 g of the sulfuric acid was added, and the resulting phosphate was recovered. Furthermore, the amounts of total phosphate, citrate-soluble phosphate, water-soluble phosphate, and free phosphate were measured. The results are shown in Table 2 and Figure 1.

[0027] As shown in Table 2 and Figure 1, among samples Nos. 1 to 7, in which 11.1 to 100.0 g of sulfuric acid was added to 100 g of sewage sludge incineration ash, the citrate-soluble phosphate content of samples Nos. 3 to 8 was in the 18% to 25% range, and the citrate-soluble phosphate ratio (C / T-P2O5) was 0.8 to 1.0. Therefore, if 35.0 g of sulfuric acid is added to 100 g of sewage sludge incineration ash, the citrate-soluble phosphate ratio (C / T-P2O5) is generally 0.7 or higher. Furthermore, the free phosphate content of samples Nos. 2 to 7 was 8 mass% or less, indicating a low amount of free phosphate. On the other hand, in sample No. 8, in which 200 g of sulfuric acid was added to 100 g of sewage sludge incineration ash A, the amount of soluble phosphate was high, but the amount of free phosphate increased rapidly, and the product was a slurry, with no solid matter being obtained.

[0028] [Table 1]

[0029] [Table 2]

[0030] Example 2 Sewage sludge incineration ash B containing the components shown in Table 1 was used as the raw material. To 100 g of this sewage sludge incineration ash, 98% sulfuric acid was added in incremental amounts from 11.1 g to 100 g, and 200 g of the sulfuric acid was added, and the resulting phosphate was recovered. Furthermore, the amounts of total phosphate, citrate-soluble phosphate, water-soluble phosphate, and free phosphate were measured. The results are shown in Table 3.

[0031] As shown in Table 3, the citrate-soluble phosphate content of samples Nos. 11 to 15 was in the 24% to 34% range, and the citrate-soluble phosphate ratio (C / T-P2O5) was in the 0.7 to 0.97 range. The free phosphate content of samples Nos. 11 to 13 was 8% by mass or less, which is low. On the other hand, in samples No. 14 to 16, in which 81.8 to 200 g of sulfuric acid was added to 100 g of sewage sludge incineration ash B, the amount of soluble phosphate was high, but the amount of free phosphate increased rapidly, and the product of sample No. 16 was a slurry, and no solid matter was obtained.

[0032] [Table 3]

Claims

1. A phosphate-containing solid comprising water-soluble or citrate-soluble phosphates and reaction residues which are reaction products of sewage sludge incineration ash and sulfuric acid, wherein the ratio of citrate-soluble phosphate to total phosphate is 0.7 mass ratio or more, and the content of free phosphate is 8 mass% or less.

2. 2. The phosphate-containing solid material according to claim 1, which is used as a phosphate fertilizer.

3. A method for producing a phosphate-containing solid, comprising reacting sewage sludge incineration ash with sulfuric acid in a non-solvent state to produce a water-soluble or citrate-soluble phosphate, controlling the amount of sulfuric acid relative to the sewage sludge incineration ash in this production process so that the ratio of citrate-soluble phosphoric acid to total phosphoric acid in the produced phosphate is 0.7 mass ratio or more and the content of free phosphoric acid is 8 mass% or less, and recovering the produced phosphate and residue.

4. 4. The method for producing a phosphoric acid-containing solid according to claim 3, wherein 25 to 100 parts by mass of sulfuric acid with a concentration of 98% or more is added to 100 parts by weight of sewage sludge incineration ash having a phosphoric acid content of 20 to 40% by mass, thereby producing a phosphate salt in which the ratio of citrate-soluble phosphoric acid to the total phosphoric acid is 0.7 mass ratio or more and free phosphoric acid is 8 mass% or less, and the produced phosphate salt and residue are recovered.

5. 5. The method for producing a phosphate-containing solid material according to claim 3, wherein the powdered sewage sludge incineration ash is granulated by rolling while being sprayed with a sulfuric acid solution.

Images