A method for selectively leaching phosphorus from sludge incineration ash and recycling phosphorus resources

Abstract

The present application belongs to the technical field of solid waste resource utilization, and discloses a method for selectively leaching phosphorus from sludge incineration ash and recycling phosphorus resources. The sludge incineration ash is mixed with calcium oxide, and then is pressed into a shape under a pressure of 50-200 MPa, and is modified by heating to 800-1200 DEG C to obtain modified ash. The modified ash is added into a dilute acid solution for selective leaching, and the pH value of the solution is controlled to 1.0-4.0 during the leaching process. After leaching, the phosphorus-containing leaching solution is obtained by filtration separation. The pH value of the phosphorus-containing leaching solution is adjusted to 5.0-9.0 to form a precipitate, and high-quality phosphate is obtained. The method can leach and separate more than 95% of the phosphorus in the sludge incineration ash, and a calcium phosphate product with a P2O5 content of more than 37% is obtained. The present application can efficiently and at low cost separate and extract the phosphorus resources in the sludge incineration ash, realize high-value utilization of the sludge incineration ash, and bring significant environmental and economic benefits, and promote the establishment of a circular economy.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to a method for selectively leaching and recovering phosphorus resources from sludge incineration ash. Background Technology

[0002] Phosphorus is an essential nutrient element, necessary for the life activities of all organisms, and plays a crucial role in agricultural development. my country has a large population and a huge demand for food, requiring substantial phosphorus resources to ensure high-quality agricultural development. Currently, phosphorus mainly comes from phosphate rock, but its reserves are limited. In nature, phosphorus diffuses primarily from high-concentration phosphate rock to low-concentration natural environments, with almost no chemical cycle, resulting in a unidirectional and non-renewable flow within the biosphere. my country has relatively abundant phosphorus resources and is the world's largest producer of phosphate rock. To address future phosphorus resource shortages, it is urgent to find phosphorus-rich substances to replace phosphate rock. Sludge incineration ash is a solid waste generated from the treatment of domestic and industrial wastewater. Phosphorus in sludge incineration ash mainly exists in the form of iron phosphate and aluminum phosphate salts, with a phosphorus content as high as 11%–20%, comparable to low-grade phosphate rock, thus considered a potential phosphorus resource. Effective separation and recovery of phosphorus from sludge incineration ash can not only prevent environmental pollution but also alleviate my country's phosphorus resource shortage to some extent. However, sludge incineration ash also contains some metal elements, making phosphorus recovery difficult. Therefore, how to efficiently extract phosphorus from sludge incineration ash, while effectively separating it from other metal elements to recover high-purity phosphorus products, is an urgent problem to be solved.

[0003] Currently, wet processing is widely used for phosphorus recovery from sludge incineration ash. Its principle is to use acid to break the chemical bonds between metals and phosphates in the sludge incineration ash. By increasing the acid concentration, the dissolution of phosphorus compounds is promoted, dissolving the phosphorus in the sludge incineration ash into the leachate. The phosphorus product is then recovered from the solution through precipitation or crystallization. However, wet processing is usually carried out under strong acid conditions, which inevitably leads to the dissolution of other metal elements, resulting in a high content of impurities in the leachate, making it difficult to directly separate and extract high-purity phosphorus products. Typically, purification and impurity removal methods are needed to reduce the concentration of metal elements in the leachate.

[0004] For example, Chinese patent CN117208870A discloses a method for extracting hydroxyapatite from sludge incineration ash. This invention involves leaching the sludge incineration ash in a hydrochloric acid solution of a set concentration. During the acid leaching process, a large number of metal elements dissolve into the leachate, increasing the difficulty of recovering high-purity phosphorus products from the leachate. Therefore, pretreatment technology is needed to reduce the concentration of impurity ions in the leachate before extracting and recovering high-purity phosphorus products. CN117401657A discloses a method for recovering phosphorus resources from sludge incineration ash, which uses lanthanum-based nano-adsorbents to selectively adsorb phosphates in the leachate, achieving the separation of phosphates from heavy metals. CN114427033A discloses a method for separating heavy metals and recovering phosphorus and iron from sludge incineration ash. This method involves heavy metal precipitation, organic extraction, and back-extraction to recover iron ions from the leachate, obtaining ferric chloride flocculent products, and then adding a calcium source to the filtrate to obtain phosphorus products. In addition, commonly used purification and impurity removal methods include cation exchange resins and continuous extraction technology. Although the aforementioned patents and purification methods have recovered calcium phosphate products to some extent, they suffer from problems such as huge acid consumption, harsh reaction conditions, complex processes, and high processing costs, making them difficult to apply industrially. Summary of the Invention

[0005] To address the difficulties in separating and recovering phosphorus resources from sludge incineration ash and the high processing costs, the technical problem to be solved by this invention is: to selectively leach phosphorus elements from sludge incineration ash without purifying the leachate, and to directly recover high-quality phosphate products through chemical precipitation, thereby achieving efficient and low-cost recovery of phosphorus resources from sludge incineration ash and improving its utilization value.

[0006] To address the aforementioned technical problems, this invention provides a method for selectively leaching phosphorus from sludge incineration ash and recovering phosphorus resources: First, sludge incineration ash is thoroughly mixed with a certain amount of calcium oxide and pressed into shape. Then, it undergoes high-temperature heating for modification treatment to obtain modified ash. Next, the crushed and ground modified ash is added to a dilute acid solution for selective leaching. During the leaching process, the pH value of the solution is controlled to promote the leaching of phosphorus from the modified ash while preventing the leaching of other metal elements. The residue after leaching can be used as road construction material. Finally, an alkaline substance is added to raise the pH value of the phosphorus-containing leachate, causing phosphate ions to precipitate and yield high-quality phosphate. The principle of this invention is as follows:

[0007] (1) Different chemical agents are often used to remove phosphorus in the wastewater treatment process. These agents are mainly Al 3+ Fe 3+ Fe 2+Salt and lime react with phosphorus ions in wastewater to form insoluble phosphate precipitates. After incineration, the resulting sludge contains phosphorus primarily in the forms of Al(PO3)3, AlPO4, and FePO4. This invention addresses this by adding calcium oxide for high-temperature modification. The calcium oxide reacts with aluminum and iron phosphate salts (Al / Fe-P) in the incinerator ash, replacing the aluminum and iron ions to form calcium phosphate salts (Ca-P). The aluminum and iron ions are then distributed in other mineral phases, primarily in the forms of Al2O3 and Fe2O3. This high-temperature modification promotes the decomposition and transformation of insoluble phosphates, achieving the separation of phosphorus from metals such as iron and aluminum. When the modified ash is leached in a dilute acid solution, calcium phosphate salts dissolve more readily, allowing a large amount of phosphorus to enter the leachate. Meanwhile, mineral phases such as Al2O3 and Fe2O3 have low solubility and are difficult to dissolve in dilute acid solutions, thus preventing the leaching of aluminum and iron and achieving phosphorus separation.

[0008] (2) When the modified ash is leached in dilute acid solution, the phosphorus-containing mineral phase will undergo the following reaction:

[0009] Ca3(PO4)2+6H + =3Ca 2+ +2H3PO4

[0010] During the leaching process, H in the dilute acid solution + The phosphate ions are continuously consumed, causing the pH of the leachate to rise. If the pH of the leachate is too high, the dissolved phosphate ions will react with metal ions to form precipitates again. When the pH of the leachate is too low, in addition to the easily soluble calcium phosphate salt, other aluminum and iron-containing mineral phases in the modified ash will also dissolve, which is not conducive to selective leaching. Therefore, the method of this invention requires the continuous addition of dilute hydrochloric acid or dilute nitric acid solution during leaching to adjust the pH of the leachate, ensuring that the calcium phosphate salt is fully dissolved, while inhibiting the re-formation of phosphate ions into precipitates.

[0011] (3) The leachate contains a large amount of calcium and phosphate ions, and a small amount of silicate and aluminum ions, with almost no iron ions. Calcium phosphate has very low solubility in neutral or alkaline solutions, and phosphorus can be recovered from the leachate by forming calcium phosphate precipitate. This invention raises the pH value of the solution by adding alkaline substances such as calcium oxide, calcium hydroxide, or ammonia to the leachate, causing calcium ions and phosphate ions to form calcium phosphate precipitate. When the pH value is too low, aluminum ions will precipitate, resulting in an excessively high aluminum content in the separated phosphate product, making it unsuitable as a phosphate fertilizer; when the pH value is too high, both phosphate and silicate ions will precipitate, resulting in an excessively high silicon content and a low phosphorus content in the obtained phosphate product. Therefore, it is necessary to control the pH value during the precipitation process to precipitate most of the phosphate ions while inhibiting the precipitation of aluminum and silicate ions.

[0012] The specific technical solution of the present invention is as follows:

[0013] This invention provides a method for selectively leaching phosphorus from sludge incineration ash and recovering phosphorus resources, comprising the following steps:

[0014] (1) Mix sludge incineration ash with calcium oxide, press it into shape at 50-200 MPa, then heat it to 800-1200℃ for modification treatment, and obtain modified ash after cooling.

[0015] (2) After crushing the modified ash obtained in step (1), mix it with water and carry out selective leaching. During the leaching process, the pH value of the solution is adjusted to 1.0 to 4.0 to promote the leaching of phosphorus in the modified ash and avoid the leaching of other metal elements. After leaching, filter and separate to obtain phosphorus-containing leachate and residue. The residue can be used as road construction material.

[0016] (3) Adjust the pH of the phosphorus-containing leachate obtained in step (2) to 5.0-9.0 so that phosphate ions form a precipitate. After solid-liquid separation, phosphate is obtained and can be used as phosphate fertilizer.

[0017] In the above technical solution, further, the mass ratio of calcium oxide to sludge incineration ash in step (1) is 1:10 to 1:3.

[0018] In the above technical solution, the reaction time of the modification treatment in step (1) is 1 to 4 hours.

[0019] In the above technical solution, the modified ash described in step (2) is further crushed and ground to make its particle size less than or equal to 0.3 mm for selective leaching.

[0020] In the above technical solution, further, in step (2), a dilute acid solution is used to adjust the pH value; preferably, the dilute acid solution is one or both of dilute hydrochloric acid and dilute nitric acid solution.

[0021] In the above technical solution, further, in step (2), the mass ratio of modified ash to aqueous solution is 1:100 to 1:10; the selective leaching is carried out under normal temperature and pressure conditions, and the solution is mechanically stirred during leaching at a stirring rate of 50 to 300 r / min and a leaching time of 20 to 90 min.

[0022] In the above technical solution, the solution filtration separation in step (2) is further completed by vacuum filtration or squeeze filtration.

[0023] In the above technical solution, further, in step (3), an alkaline substance is added to the phosphorus-containing leachate to adjust the pH value. Preferably, the alkaline substance is calcium oxide, calcium hydroxide, or ammonia. During the pH adjustment process, the solution is stirred, and the stirring rate is controlled at 30-300 r / min.

[0024] In the above technical solution, the solid-liquid separation in step (3) is to first let the leachate stand to allow the solution and precipitate to separate into layers, then squeeze and filter or centrifuge the lower precipitate to extract the white precipitate, and then dry the precipitate to obtain phosphate, thereby achieving effective recovery of phosphorus resources.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) This invention proposes a new method for selectively separating phosphorus and recovering high-purity phosphate from sludge incineration ash. This method can efficiently and cost-effectively extract phosphorus resources from sludge incineration ash, realize the comprehensive utilization of all its components, solve the problem of difficult treatment of sludge incineration ash, develop a new phosphorus resource, and bring significant economic and environmental benefits, thus promoting the establishment of a circular economy.

[0027] (2) This invention uses high-temperature calcium oxide modification treatment on sludge incineration ash to promote the decomposition of insoluble phosphates into easily soluble Ca3(PO4)2, while aluminum and iron metals exist in the insoluble Al2O3 and Fe2O3 mineral phases. When the modified ash is leached in acid, Ca3(PO4)2 dissolves more easily, which can improve the dissolution rate of phosphorus and reduce acid consumption, thus lowering the leaching cost; while the insoluble Al2O3 and Fe2O3 are retained in the residue, achieving the separation of phosphorus from iron and aluminum metals, and achieving the purpose of selective leaching.

[0028] (3) The residue mainly contains valuable components such as SiO2, Al2O3, Fe2O3, and CaO, and the P2O5 content is low, indicating that the phosphorus element has been effectively separated. The remaining residue can be used as building material, realizing the comprehensive utilization of each component in the incineration ash.

[0029] (4) After selective leaching of the modified ash obtained by the present invention in acid solution, the concentration of metal elements such as aluminum and iron in the leachate is low. Therefore, there is no need to purify and remove impurities from the leachate. Calcium phosphate with a P2O5 content of more than 37% can be directly recovered through chemical precipitation and used as phosphate fertilizer.

[0030] (5) The raw materials used in this invention are all inexpensive acids and bases. The whole process is simple, low-cost, and highly efficient, with good market prospects and can be applied to industrial production on a large scale. Attached Figure Description

[0031] Figure 1 A process flow diagram for selectively leaching phosphorus from sludge incineration ash and recovering phosphorus resources;

[0032] Figure 2 The image shown is the XRD pattern of the modified incineration ash in the example.

[0033] Figure 3 The leaching rate (R) of the main elements before and after the modification of the incinerator ash in the examples is shown. M );

[0034] Figure 4 This is a photograph of calcium phosphate separated and extracted from the modified ash in the example;

[0035] Figure 5 The image shown is the XRD pattern of calcium phosphate separated and extracted from the modified ash in the example. Detailed Implementation

[0036] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0037] Example

[0038] A method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash involves first thoroughly mixing calcium oxide and sludge incineration ash at a mass ratio of 1:5, then pressing the mixture into a sample at 100 MPa using a press. The sample is then placed in a high-temperature electric furnace and heated to 1000℃ for 2 hours for modification treatment. After cooling, modified ash is obtained. The modified ash is then crushed and ground to a particle size of less than 0.15 mm. The chemical composition of the sludge incineration ash before and after modification was determined using X-ray fluorescence spectrometry (XRF), as shown in Table 1. The main components of sludge incineration ash are SiO2, P2O5, and Al2O3, with P2O5 content as high as 23.51%, making it an ideal resource for phosphorus recovery. The modified ash still contains a high content of P2O5, indicating high recovery value.

[0039] Table 1. Chemical composition of sludge incineration ash before and after modification (mass fraction / %)

[0040]

[0041] Mineral phase analysis of the modified ash was performed using X-ray diffraction (XRD), such as... Figure 2 As shown, the phosphorus-containing mineral phase in the modified ash is mainly Ca3(PO4)2, while aluminum and iron elements mainly exist in the form of Al2O3 and Fe2O3. When the modified ash is leached in dilute acid solution, Ca3(PO4)2 dissolves more readily, while Al2O3 and Fe2O3 mineral phases have lower solubility and are difficult to dissolve in dilute acid solution. This avoids the leaching of aluminum and iron metal elements, creating favorable conditions for subsequent phosphorus separation and recovery.

[0042] The sludge incineration ash before and after modification was mixed with aqueous solution at a mass ratio of 1:100 and selectively leached at 298K. During leaching, the solution was mechanically stirred at a rate of 200 r / min, and dilute hydrochloric acid solution was continuously added to control the pH of the leachate to 2.0. After 60 min of reaction, solid-liquid separation was performed by vacuum filtration to obtain a phosphorus-containing leachate and residue. The main chemical components of the residue were measured after drying, as shown in Table 2. The residue modified according to this invention contains a large amount of valuable components such as SiO2, Al2O3, Fe2O3, and CaO, while the P2O5 content is low, indicating that phosphorus was effectively separated and the residue can be used as road construction material. In contrast, the unmodified sludge incineration ash residue has a high P2O5 content, indicating that most of the phosphorus in the incineration ash did not dissolve during the leaching process, and effective phosphorus recovery cannot be achieved.

[0043] Table 2. Main chemical components of the residue (mass fraction / %)

[0044]

[0045] The mass concentration of each element in the leachate was determined by inductively coupled plasma atomic emission spectrometry (ICP-OES), and the dissolution rate of each element in the acid solution before and after the incinerator ash was calculated using the following formula.

[0046]

[0047] In equation (1), R M C represents the dissolution rate of element M in the leachate; M V represents the mass concentration of element M in the leachate, in mg / L; V represents the final volume of the solution, in L; m M Let M be the mass of element M in the ash from sludge incineration, in grams.

[0048] Calculate the leaching rate (R) of the main elements in the incinerator ash before and after the modification. M ),like Figure 3 As shown, the leaching rate of phosphorus in unmodified sludge incineration ash is only 48%, while the leaching rate of magnesium reaches 80%, aluminum exceeds 10%, and iron exceeds 3%. Under the same conditions, most of the phosphorus and calcium elements in the modified ash are dissolved and separated, with the phosphorus leaching rate exceeding 95%, the magnesium leaching rate below 43%, and the iron and aluminum leaching rates below 3%. This indicates that after modification by the method of this invention, most of the metal elements in the modified ash are retained in the residue, achieving ideal selective leaching.

[0049] After selective leaching, the phosphorus-containing leachate mainly contains calcium and phosphate ions. Saturated lime water is added to the prepared phosphorus-containing leachate to raise the pH to 7.5, at which point a large amount of white precipitate is formed. The turbid leachate is allowed to stand for a period of time to allow the solution and precipitate to separate into layers. Finally, the lower precipitate is recovered by centrifugation.

[0050] Table 3 shows the concentrations of various elements in the leachate from the modified ash and the supernatant after separation and precipitation. Calculations indicate that 99.92% of the phosphate ions in the leachate precipitate, while the precipitation rates of aluminum and silicate ions are relatively low, at 25.62% and 16.27%, respectively, achieving effective phosphorus recovery.

[0051] Table 3. Concentrations (mg / L) of various elements in the leachate of the modified ash and the supernatant after separation and precipitation.

[0052]

[0053] After drying the white precipitate, the following was obtained: Figure 4 The phosphate product shown is analyzed using XRF, and its chemical composition is shown in Table 4. The main components of this phosphate are CaO and P2O5, with P2O5 content of 37.51%, while the contents of Al2O3, SiO2, and Fe2O3 are low. Therefore, it can be used as a slow-release phosphate fertilizer.

[0054] Table 4. Chemical composition of phosphates recovered from the leachate (mass fraction / %)

[0055]

[0056] The XRD results of the recovered phosphate products are as follows: Figure 5 As shown, its main mineral phase is hydroxyapatite, which can be used as a high-efficiency, environmentally friendly slow-release phosphate fertilizer, and also as a chemical raw material.

[0057] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method for selectively leaching phosphorus from sludge incineration ash and recovering phosphorus resources, characterized in that: Includes the following steps: (1) Mix sludge incineration ash with calcium oxide, press it into shape at 50~200MPa, then heat it to 800~1200℃ for modification treatment, and obtain modified ash after cooling. (2) After crushing the modified ash obtained in step (1), mix it with water and carry out selective leaching. During the leaching process, the pH value of the solution is adjusted to 1.0~4.

0. After leaching, filter and separate to obtain phosphorus-containing leachate. The mass ratio of calcium oxide to sludge incineration ash in step (1) is 1:10 to 1:

3. The modified ash described in step (2) is crushed to a particle size of less than or equal to 0.3 mm for selective leaching.

2. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 1, characterized in that: The reaction time for step (1) of the modification treatment is 1 to 4 hours.

3. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 1, characterized in that: In step (2), a dilute acid solution is used to adjust the pH value.

4. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 3, characterized in that: The dilute acid solution is one or both of dilute hydrochloric acid and dilute nitric acid solutions.

5. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 1, characterized in that: In step (2), the mass ratio of modified ash to aqueous solution is 1:100 to 1:10; the selective leaching is carried out under normal temperature and pressure conditions, and the solution is mechanically stirred during leaching at a stirring rate of 50 to 300 r / min and a leaching time of 20 to 90 min.

6. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 1, characterized in that: The filtration separation in step (2) is vacuum filtration or squeeze filtration.

7. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 1, characterized in that, The phosphorus-containing leachate obtained in step (2) is adjusted to pH 5.0-9.0 to form a precipitate, and phosphate is obtained after solid-liquid separation.

8. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 7, characterized in that: An alkaline substance is added to the phosphorus-containing leachate to adjust the pH value.

9. The method for selectively leaching phosphorus and recovering phosphorus resources from sludge incineration ash according to claim 8, characterized in that: The alkaline substance is calcium oxide, calcium hydroxide, or ammonia; the solution is stirred during pH adjustment, and the stirring rate is controlled at 30~300 r / min.

10. The method for selectively leaching phosphorus from sludge incineration ash and recovering phosphorus resources according to claim 7, characterized in that: The solid-liquid separation process involves first allowing the leachate to stand, causing the solution and precipitate to separate into layers. Then, the lower precipitate is extracted by squeezing, filtering, or centrifuging. After drying, the precipitate is obtained as phosphate.

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