A method for recovering phosphorus from sludge incineration ash

Abstract

The application discloses a method for recovering phosphorus elements in sludge incineration ash, comprising: pre-treatment, preliminary acid leaching impurity removal treatment, intensified acid leaching phosphorus extraction treatment, phosphorus precipitation recovery and mother liquor recycling. The application removes impurities and selectively extracts phosphorus elements by a two-stage acid leaching process, cooperates phosphoric acid leaching and complexing promoters to improve the phosphorus recovery rate, and the application realizes the recycling of mother liquor and the synergistic recovery of calcium and magnesium resources, reduces reagent consumption and environmental pollution, and combines microwave and ultrasonic intensification technology to significantly shorten the reaction time, improve the leaching efficiency and product purity, and the obtained high-purity struvite can be used as high-quality slow-release fertilizer. The overall process realizes the efficient, clean and high-value recovery of phosphorus resources in sludge ash.

Description

Technical Field

[0001] This invention relates to the field of sludge treatment technology, specifically to a method for recovering phosphorus from ash in sludge incineration. Background Technology

[0002] With the accelerating pace of urbanization, the amount of sludge produced by wastewater treatment plants is increasing daily. Sludge incineration, as an effective means of reducing volume and rendering the sludge harmless, has been widely applied in the field of sludge disposal. However, the ash from incinerated sludge is rich in phosphorus, with its phosphorus content accounting for more than 80% of the total phosphorus in the original sludge. This makes the ash from incinerated sludge a highly promising "urban mine" resource.

[0003] Phosphorus is a non-renewable strategic resource, playing an irreplaceable role in key sectors of the national economy such as fertilizers, food, and pharmaceuticals. Traditionally, phosphorus resources rely primarily on phosphate mining, but this process not only consumes limited natural resources but also causes severe damage to the ecological environment. Therefore, recovering phosphorus from secondary resources such as sludge ash from incineration, and achieving resource recycling, is of great economic and environmental significance for ensuring resource security and promoting sustainable development.

[0004] Currently, the main technologies for recovering phosphorus from incinerated sludge ash include acid leaching, alkaline leaching, and thermal treatment. However, these existing technologies generally have many drawbacks:

[0005] Acid leaching methods, such as using sulfuric acid or hydrochloric acid, although the leaching efficiency of phosphorus is relatively high, have the disadvantages of high acid consumption and easy introduction of a large number of impurity ions such as aluminum, iron, calcium and magnesium, which may lead to severe equipment corrosion and complicated subsequent treatment of the leachate.

[0006] Alkali leaching: has poor selectivity for phosphorus and usually requires high energy consumption.

[0007] Heat treatment: This method generally requires high temperatures, resulting in huge energy consumption and high operating costs.

[0008] Furthermore, phosphorus in the ash of incinerated sludge mainly exists in the form of insoluble calcium phosphate compounds. The ash also contains a large amount of interfering elements such as aluminum, iron, and silicon, posing significant technical challenges to the efficient and selective extraction of phosphorus. In summary, existing technologies generally suffer from low phosphorus recovery rates, high process costs, and a tendency to cause secondary pollution, making it difficult to meet the demands for efficient, green, and economical resource recovery. Therefore, we propose a method for recovering phosphorus from sludge incineration ash. Summary of the Invention

[0009] The purpose of this invention is to provide a method for recovering phosphorus from ash in sludge incineration, in order to solve the problems that need to be addressed in the background art.

[0010] To achieve the above objectives, the present invention provides the following technical solution: a method for recovering phosphorus from sludge incineration ash, the method comprising the following steps:

[0011] Step 1: Pre-treatment: Dry and grind the ash from sludge incineration. After drying and grinding, sieve to obtain uniformly dried sludge incineration ash powder.

[0012] Step 2: Preliminary acid leaching for impurity removal: Pour the ash powder from the sludge incineration into a mixer, then pour in a dilute inorganic acid solution, and stir and react at a temperature of 25℃-60℃ for 0.5-2 hours. After the reaction is completed, perform solid-liquid separation, discard the filtrate, and retain the filter residue.

[0013] Step 3: Enhanced acid leaching for phosphorus extraction: Mix the filter residue obtained in Step 2 with phosphoric acid solution, stir and react at 60℃-80℃ for 1-3 hours, and after the reaction is completed, perform solid-liquid separation and collect filtrate A;

[0014] Step 4: Phosphorus precipitation recovery: Add a mixture of magnesium salt and ammonium salt to filtrate A from Step 3, adjust the pH of filtrate A to 8.5-10.0, and then stir the reaction at 25℃-40℃ for 0.5-1.5h to generate struvite precipitate. After the reaction is completed, perform solid-liquid separation, collect the precipitate, and then wash and dry the precipitate to obtain high-purity struvite product and mother liquor residue.

[0015] Step 5: Mother liquor recycling: Add phosphate, magnesium salt and ammonium salt to the mother liquor residue obtained after solid-liquid separation in step 4 and then recycle it to step 3.

[0016] Phosphorus in the ash of incinerated sludge mainly exists in the form of insoluble calcium phosphate compounds. At the same time, the ash also contains a large number of interfering elements such as aluminum, iron, and silicon, which poses a huge technical challenge to the efficient and selective extraction of phosphorus. In summary, existing technologies generally suffer from drawbacks such as low phosphorus recovery rates, high process costs, and a tendency to cause secondary pollution, making it difficult to meet the demands for efficient, green, and economical resource recovery. This invention effectively solves the selective problem of phosphorus recovery from sludge incineration ash through a two-stage leaching process. The initial acid leaching step preferentially removes a large amount of interfering impurities such as calcium and magnesium, creating favorable conditions for subsequent phosphorus extraction. The enhanced acid leaching phosphorus extraction step utilizes phosphoric acid as a leaching agent, which effectively dissolves insoluble calcium phosphate while its phosphate ions inhibit calcium dissolution, thus significantly improving the selectivity and efficiency of phosphorus leaching. Secondly, the mother liquor generated after phosphorus precipitation is used to replenish the consumed reagents and reused in the enhanced acid leaching step. This not only significantly reduces the consumption cost of chemical reagents such as phosphate, magnesium salts, and ammonium salts but also significantly reduces the generation and discharge of process wastewater, reducing the risk of secondary environmental pollution from the source and conforming to the concept of green and sustainable development. Finally, the process flow is clear and logically rigorous, decomposing the complex sludge incineration ash resource recovery process into controllable unit operations, making the entire recovery process easy to scale up industrially and achieve continuous production. It not only successfully transforms waste sludge incineration ash into high-purity struvite, a high-value phosphate fertilizer product, but also provides a practical and feasible technical path to solve the problem of phosphorus resource shortage, with significant economic and environmental benefits.

[0017] As a further description of the above technical solution:

[0018] In step two, the dilute inorganic acid solution is selected from sulfuric acid or hydrochloric acid with a concentration of 0.1-1.0 mol / L, and the mass ratio of the sludge incineration ash powder to the dilute inorganic acid solution is 1:5-15.

[0019] As a further description of the above technical solution:

[0020] In step three, the concentration of the phosphoric acid solution is 1.5-3.0 mol / L, and the mass ratio of the filter residue to the phosphoric acid solution is 1:5-10.

[0021] As a further description of the above technical solution:

[0022] In step five, the mother liquor is treated with activated carbon or ion exchange resin before circulation, and the concentration of phosphate ions is detected. Based on the detected concentration of phosphate ions, appropriate mass fractions of phosphoric acid, magnesium sulfate, and ammonium chloride are added to the initial concentration before returning to step three for use.

[0023] As a further description of the above technical solution:

[0024] It also includes a calcium resource regeneration step, in which the calcium and magnesium-containing acid leaching filtrate produced in step two is evaporated and concentrated, cooled and crystallized to precipitate gypsum, and the filtrate A obtained in step three and the mother liquor residue after solid-liquid separation in step four are evaporated and concentrated under reduced pressure, then cooled to room temperature, and magnesium sulfate heptahydrate crystals are precipitated and filtered to obtain magnesium sulfate crystals, which are dried to obtain magnesium sulfate. After removing phosphoric acid from the mother liquor residue, it is returned to step three for recycling. The mother liquor residue in step four is treated by adsorbing magnesium ions through a chelating ion exchange resin, and then eluted with 0.5-2.0 mol / L hydrochloric acid to obtain a magnesium chloride solution.

[0025] As a further description of the above technical solution:

[0026] In step three, a complexing accelerator with a concentration of 0.01-0.1 mol / L is added to the phosphoric acid solution. The complexing accelerator is selected from citric acid or tartaric acid.

[0027] As a further description of the above technical solution:

[0028] The complexation accelerator is added to the phosphoric acid solution in a pulsed, segmented manner. During the enhanced acid leaching and phosphorus extraction process, the complexation accelerator is added every 20-30 minutes. The total amount of the complexation accelerator remains unchanged and is added in 2-3 separate additions.

[0029] As a further description of the above technical solution:

[0030] In step three, the enhanced acid leaching and phosphorus extraction treatment employs microwave-assisted heating. The microwave-assisted heating has a power of 300-800W and a frequency of 2.45 GHz, achieving rapid and uniform heating at 60℃-80℃ and shortening the reaction time to 0.5-1.5h. In step four, the struvite precipitation is assisted by ultrasonic precipitation. The ultrasonic frequency is 20-40kHz, and the ultrasonic power density is 0.3-0.8W / mL.

[0031] As a further description of the above technical solution:

[0032] Before the preliminary acid leaching and impurity removal treatment in step two, the sludge incineration ash is subjected to low-temperature thermochemical treatment. The sludge incineration ash is hydrothermally reacted with a phosphoric acid solution with a concentration of 0.5-1.0 mol / L in an environment of 60℃-100℃ for 1-2 hours. After cooling, the solid and liquid are separated, and the obtained filter residue is further subjected to the preliminary acid leaching and impurity removal step in step two.

[0033] As a further description of the above technical solution:

[0034] In step four, phosphorus precipitation recovery, the pH of filtrate A is adjusted using a two-stage pH adjustment method:

[0035] First stage: Adjust the pH of filtrate A to 7.5–8.0, and slowly add 50% magnesium salt and ammonium salt to generate seed crystals;

[0036] Second stage: Add the remaining magnesium salt and ammonium salt to adjust the pH value to 9.0-9.5 to promote crystal growth.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. This invention effectively solves the core problem of poor selectivity and low efficiency in phosphorus recovery from sludge incineration ash by employing a two-stage leaching process combining "preliminary acid leaching for impurity removal" and "enhanced acid leaching for phosphorus extraction." The first step uses a low-concentration dilute inorganic acid for pretreatment, preferentially dissolving the large amounts of alkaline earth metals such as calcium and magnesium, as well as some heavy metal impurities present in the ash, significantly reducing the concentration of interfering ions in the subsequent phosphorus extraction process. The second step uses phosphoric acid as the main leaching agent, which not only effectively dissolves insoluble phosphorus compounds, primarily hydroxyapatite, but also, due to its own phosphate ions, inhibits further calcium dissolution. This significantly improved the selectivity of phosphorus leaching. Furthermore, by introducing a complexation promoter and employing a pulsed, segmented addition method, the decomposition rate of the organic complexing agent under high-temperature, acidic conditions was effectively slowed, allowing it to function more persistently. This further enhanced the selective dissolution of the target phosphorus minerals and significantly inhibited the co-dissolution of impurities such as iron and aluminum. Finally, through two-stage pH adjustment and ultrasonic-assisted precipitation technology, the generated struvite precipitate exhibited uniform crystal nuclei and good growth, resulting in a product with high crystallinity, narrow particle size distribution, and high purity, making it suitable for recycling and use as a high-quality slow-release fertilizer.

[0039] 2. Secondly, the mother liquor generated after phosphorus precipitation in step four of this invention, after purification with activated carbon or ion exchange resin, is replenished with consumed phosphate, magnesium salts, and ammonium salts to the initial concentration, and can then be reused in the enhanced acid leaching process of step three. This not only significantly reduces the consumption of fresh chemical reagents and the discharge of process wastewater, but also effectively reduces the overall operating cost. In addition, the calcium- and magnesium-containing acid leaching filtrate generated in step two can be used to recover gypsum through evaporation, concentration, and cooling crystallization. The filtrate A from step three and the mother liquor residue from step four can be concentrated by vacuum evaporation before recycling to precipitate magnesium sulfate heptahydrate crystals, thus recovering magnesium resources. For magnesium ions, which are more difficult to treat, chelating ion exchange resin is used for adsorption and hydrochloric acid elution to obtain magnesium chloride solution, which is then used to prepare the precipitation step. This series of designs achieves the full recovery of multiple valuable elements such as phosphorus, calcium, and magnesium, transforming waste into multiple high-value-added products, resulting in significant economic and environmental benefits.

[0040] 3. Finally, before the leaching step, a low-temperature thermochemical treatment step is set up to pretreat the incinerated sludge ash. This process does not completely dissolve the sludge ash, but rather uses a chemical reaction to convert the originally insoluble hydroxyapatite (HA) in the incinerated sludge ash into intermediate products such as octacalcium phosphate (OCP), which are more soluble. This achieves selective activation of the phosphate minerals. The material that has undergone this activation treatment has a faster phosphorus release rate and higher leaching efficiency in the subsequent main leaching stage. More importantly, since the phosphate minerals have been pretreated into intermediate products such as octacalcium phosphate (OCP), which are more soluble, the acid concentration and amount required for the main leaching process can be reduced accordingly, thereby effectively reducing the expensive phosphoric acid consumption and the risk of equipment corrosion. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the system flow of the present invention. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Example 1:

[0044] Please see Figure 1 This invention provides a technical solution: a method for recovering phosphorus from sludge incineration ash, the method comprising the following steps:

[0045] Step 1: Pre-treatment: Dry and grind the ash from sludge incineration. After drying and grinding, sieve to obtain uniformly dried sludge incineration ash powder.

[0046] Step 2: Preliminary acid leaching for impurity removal: Pour the sludge incineration ash powder into a mixer, then add a dilute inorganic acid solution. Stir and react at 25℃-60℃ for 0.5-2 hours. After the reaction, perform solid-liquid separation, discard the filtrate, and retain the filter residue. This step aims to selectively dissolve easily soluble alkaline earth metals (such as calcium and magnesium) and some heavy metals in the ash, reducing their content and creating conditions for subsequent efficient phosphate leaching. The dilute inorganic acid solution is selected from sulfuric acid or hydrochloric acid with a concentration of 0.1-1.0 mol / L, and the mass ratio of the sludge incineration ash powder to the dilute inorganic acid solution is 1:5-15.

[0047] Step 3: Enhanced acid leaching for phosphorus extraction: Mix the filter residue obtained in Step 2 with a phosphoric acid solution and stir at 60℃-80℃ for 1-3 hours. After the reaction, perform solid-liquid separation and collect filtrate A. Phosphoric acid is used as a leaching agent, which can effectively dissolve insoluble phosphorus compounds such as hydroxyapatite in the ash. At the same time, phosphate ions can inhibit further dissolution of calcium, thereby improving the selectivity of phosphorus leaching. The concentration of the phosphoric acid solution is 1.5-3.0 mol / L, and the mass ratio of the filter residue to the phosphoric acid solution is 1:5-10.

[0048] Step 4: Phosphorus precipitation recovery: Add a mixture of magnesium salt and ammonium salt to filtrate A from Step 3, adjust the pH of filtrate A to 8.5-10.0, and then stir the reaction at 25℃-40℃ for 0.5-1.5h to generate struvite precipitate. After the reaction is completed, perform solid-liquid separation, collect the precipitate, and then wash and dry the precipitate to obtain high-purity struvite product and mother liquor residue. The amount of magnesium salt and ammonium salt added is controlled according to the molar ratio of n(Mg²⁺):n(NH₄⁺):n(PO₄³⁻) = 1.0-1.2:1.0-1.2:1.0.

[0049] Step 5: Mother liquor recycling: Phosphate, magnesium salts, and ammonium salts are added to the mother liquor residue obtained after solid-liquid separation in Step 4, and then recycled to Step 3 for enhanced acid leaching phosphorus extraction. This achieves reagent recycling, reduces operating costs, and minimizes wastewater discharge. The separated mother liquor mainly contains phosphate, magnesium salts, and ammonium salts.

[0050] The two-stage leaching process effectively solves the selective problem of phosphorus recovery from sludge incineration ash. The initial acid leaching step removes a large amount of interfering impurities such as calcium and magnesium, creating favorable conditions for subsequent phosphorus extraction. The enhanced acid leaching step uses phosphoric acid as a leaching agent, which effectively dissolves insoluble calcium phosphate while its phosphate ions inhibit calcium dissolution, thus significantly improving the selectivity and efficiency of phosphorus leaching. Secondly, the mother liquor generated after phosphorus precipitation is used to replenish the consumed reagents and then reused in the enhanced acid leaching step. This not only significantly reduces the consumption cost of chemical reagents such as phosphate, magnesium salts, and ammonium salts, but also significantly reduces the generation and discharge of process wastewater, reducing the risk of secondary environmental pollution from the source, which is in line with the concept of green and sustainable development. Finally, the process flow is clear and logically rigorous, decomposing the complex sludge incineration ash resource recovery process into controllable unit operations, making the entire recovery process easy to scale up industrially and achieve continuous production. It not only successfully transforms waste sludge incineration ash into high-purity struvite, a high-value phosphate fertilizer product, but also provides a practical and feasible technical path to solve the problem of phosphorus resource shortage, with significant economic and environmental benefits.

[0051] Example 2:

[0052] Please see Figure 1In step five, the mother liquor is treated with activated carbon or ion exchange resin before circulation to remove heavy metal ions, and the concentration of phosphate ions is detected. Based on the detected concentration of phosphate ions, appropriate mass fractions of phosphoric acid, magnesium sulfate and ammonium chloride are added to the initial concentration before returning to step three for use.

[0053] By using activated carbon adsorption or ion exchange resin treatment before circulation, these harmful heavy metals and organic pollutants can be efficiently removed, ensuring the purity of the circulating mother liquor. This fundamentally prevents the unlimited accumulation of impurities in the closed-loop system and ensures the long-term, stable, and reliable operation of the entire recovery process.

[0054] Please see Figure 1 The process also includes a calcium resource regeneration step, in which the calcium and magnesium-containing acid leaching filtrate produced in step two is evaporated and concentrated, cooled and crystallized to precipitate gypsum. The filtrate A obtained in step three and the mother liquor residue after solid-liquid separation in step four are evaporated and concentrated under reduced pressure, then cooled to room temperature, and magnesium sulfate heptahydrate crystals are precipitated and filtered to obtain magnesium sulfate crystals. After drying, magnesium sulfate is obtained. Then, the mother liquor residue is returned to step three for recycling after phosphoric acid removal. The mother liquor residue in step four is treated by adsorbing magnesium ions through a chelating ion exchange resin, and then eluted with 0.5-2.0 mol / L hydrochloric acid to obtain magnesium chloride solution, which is used to prepare magnesium salts for addition in step four.

[0055] The filtrate is rich in calcium ions. After evaporation, concentration and cooling crystallization, gypsum can be obtained as a by-product. The filtrate A from step three and the mother liquor residue from step four are combined and concentrated by vacuum evaporation and cooling to precipitate magnesium sulfate heptahydrate crystals. After drying, industrial or agricultural grade magnesium sulfate products are obtained. Magnesium can be further adsorbed and recovered from the residual mother liquor after the above treatment by utilizing the high selectivity of chelating ion exchange resin for magnesium ions. The magnesium is then eluted with hydrochloric acid to obtain a magnesium chloride solution. This solution can be directly used as the magnesium source for the struvite precipitation in step four, forming an internal small cycle that greatly reduces the demand for external magnesium salts. This series of operations transforms the acid leaching filtrate and mother liquor, which might otherwise be disposed of as hazardous waste, into a variety of high-value-added commodities such as gypsum, magnesium sulfate, and magnesium chloride solution. This upgrades the single phosphorus recovery process into a comprehensive resource recovery plant, significantly improving the overall profitability and economic feasibility of the project. Moreover, this setup removes a large amount of calcium and magnesium from the system in the form of products in advance, effectively preventing their accumulation in subsequent cycles and mitigating the negative impact on the phosphorus leaching and precipitation processes. This helps maintain the high efficiency and stability of the main reaction. The magnesium chloride solution prepared by recycling is reused in the precipitation step, forming an "internal cycle" of magnesium, which significantly reduces the cost of using fresh magnesium salts. Through evaporation, concentration, and crystallization, soluble salts are converted into solid products, greatly reducing the volume of waste liquid and total dissolved solids content that need to be disposed of in the final stage, thereby reducing the difficulty of wastewater treatment and environmental pressure.

[0056] Please see Figure 1 In step three, a complexing accelerator with a concentration of 0.01-0.1 mol / L is added to the phosphoric acid solution. The complexing accelerator is selected from citric acid or tartaric acid. The addition of the complexing accelerator enhances the selective dissolution of hydroxyapatite and inhibits the co-dissolution of iron and aluminum. The complexing accelerator is added to the phosphoric acid solution in a pulsed, segmented manner. During the enhanced acid leaching and phosphorus extraction process, the complexing accelerator is added every 20-30 minutes. The total amount of complexing accelerator added remains unchanged and is added in 2-3 times. By controlling the addition rhythm, the degradation rate is slowed down, and the effective concentration maintenance time in the reaction system is increased, thereby enhancing the selective dissolution efficiency of hydroxyapatite and reducing the co-dissolution rate of impurity ions such as iron and aluminum. This breaks through the traditional mode of adding the accelerator in one go and proposes a "pulse-type addition" strategy, which solves the problem of easy decomposition and inactivation of organic complexing agents under high temperature and acidic conditions, and significantly improves its utilization rate and selective leaching effect.

[0057] Please see Figure 1 In step three, the enhanced acid leaching phosphorus extraction treatment employs microwave-assisted heating with a microwave power of 300-800W and a frequency of 2.45 GHz. This achieves rapid and uniform heating at 60℃-80℃, shortening the reaction time to 0.5-1.5h and simultaneously increasing the phosphorus leaching rate by 5-15%. In step four, the struvite precipitation is assisted by ultrasonic precipitation with a frequency of 20-40kHz and a power density of 0.3-0.8W / mL. This promotes uniform crystal nucleus formation and growth, resulting in struvite particles with a narrower particle size distribution and increased crystallinity, facilitating subsequent screening and fertilizer application.

[0058] Microwave-assisted heating is employed, where microwaves directly penetrate the material, causing internal friction and heat generation within polar molecules. This achieves heating from the inside out, rapidly and uniformly heating the reaction system to the target temperature within a short time. Due to its high efficiency and uniformity, the chemical reaction kinetics are accelerated, significantly shortening the reaction time to 0.5-1.5 hours, a reduction of 30%-50% compared to traditional methods. This directly improves the processing capacity of the unit equipment and increases the overall production efficiency of the process. Microwave heating not only increases the reaction rate but also effectively disrupts the crystal structure of insoluble phosphorus minerals such as hydroxyapatite, promoting their dissolution, thanks to its uniform and rapid heating characteristics. Simultaneously, ultrasonic assistance is added during struvite precipitation. The cavitation effect of ultrasound in the liquid generates localized high temperatures, high pressures, and strong shock waves, instantly breaking the supersaturated state of the solution and inducing the formation of numerous uniform crystal nuclei. Moreover, the stirring effect of ultrasound is far stronger than mechanical stirring, effectively preventing crystal agglomeration and allowing crystals to grow in a more uniform environment. The final result is struvite particles with a narrower particle size distribution and higher crystallinity.

[0059] Before the preliminary acid leaching and impurity removal treatment in step two, the sludge incineration ash is subjected to low-temperature thermochemical treatment. The sludge incineration ash is hydrothermally reacted with a phosphoric acid solution with a concentration of 0.5-1.0 mol / L at an environment of 60℃-100℃ for 1-2 hours. This step results in incomplete dissolution of the sludge incineration ash, only changing the crystal structure. After cooling, solid and liquid separation occurs, and the obtained filter residue continues to undergo the preliminary acid leaching and impurity removal step in step two to improve phosphorus leaching efficiency and significantly increase the subsequent leaching rate. OCP is more soluble in acid than HA, and phosphorus is released faster. Because the pretreatment has "activated" the phosphorus minerals, the amount of acid used in the main leaching stage is reduced, and selective transformation is achieved. Impurity minerals such as Fe and Al are not easily transformed in a similar way, thus achieving selective activation.

[0060] Please see Figure 1 In step four, phosphorus precipitation recovery, the pH of filtrate A is adjusted using a two-stage pH adjustment method:

[0061] First stage: Adjust the pH of filtrate A to 7.5–8.0, and slowly add 50% magnesium salt and ammonium salt to generate seed crystals;

[0062] Second stage: Add the remaining magnesium salt and ammonium salt to adjust the pH value to 9.0-9.5 to promote crystal growth.

[0063] The two-stage pH adjustment avoids the large amount of amorphous precipitates or co-precipitates of impurities such as ferric hydroxide and magnesium ammonium phosphate caused by raising the pH at one time. The resulting struvite crystals are complete, with uniform particle size, purity ≥92%, and impurity metal content reduced by more than 40%.

[0064] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for recovering phosphorus from ash in sludge incineration, characterized in that: The method for recovering phosphorus from sludge incineration ash includes the following steps: Step 1: Pre-treatment: Dry and grind the ash from sludge incineration. After drying and grinding, sieve to obtain uniformly dried sludge incineration ash powder. Step 2: Preliminary acid leaching for impurity removal: Pour the ash powder from the sludge incineration into a mixer, then pour in a dilute inorganic acid solution, and stir and react at a temperature of 25℃-60℃ for 0.5-2 hours. After the reaction is completed, perform solid-liquid separation, discard the filtrate, and retain the filter residue. Step 3: Enhanced acid leaching for phosphorus extraction: Mix the filter residue obtained in Step 2 with the phosphoric acid solution and stir at 60℃-80℃ for 1-3 hours. After the reaction, perform solid-liquid separation and collect filtrate A. Add a complexing accelerator with a concentration of 0.01-0.1 mol / L to the phosphoric acid solution. The complexing accelerator is selected from citric acid or tartaric acid. The complexing accelerator is added to the phosphoric acid solution in a pulsed, segmented manner. During the enhanced acid leaching for phosphorus extraction, the complexing accelerator is added every 20-30 minutes. The total amount of complexing accelerator added remains unchanged and is added in 2-3 times. Step 4: Phosphorus precipitation recovery: Add a mixture of magnesium salt and ammonium salt to filtrate A from Step 3, adjust the pH of filtrate A to 8.5-10.0, and then stir the reaction at 25℃-40℃ for 0.5-1.5h to generate struvite precipitate. After the reaction is completed, perform solid-liquid separation, collect the precipitate, and then wash and dry the precipitate to obtain high-purity struvite product and mother liquor residue. Step 5: Mother liquor recycling: Add phosphate, magnesium salt and ammonium salt to the mother liquor residue obtained after solid-liquid separation in step 4 and then recycle it to step 3; In step two, the dilute inorganic acid solution used is hydrochloric acid with a concentration of 0.1-1.0 mol / L, and the mass ratio of the sludge incineration ash powder to the dilute inorganic acid solution is 1:5-15.

2. The method for recovering phosphorus from ash in sludge incineration according to claim 1, characterized in that: In step three, the concentration of the phosphoric acid solution is 1.5-3.0 mol / L, and the mass ratio of the filter residue to the phosphoric acid solution is 1:5-10.

3. The method for recovering phosphorus from ash in sludge incineration according to claim 2, characterized in that: In step five, the mother liquor is treated with activated carbon or ion exchange resin before circulation, and the concentration of phosphate ions is detected. Based on the detected concentration of phosphate ions, the corresponding mass fractions of phosphoric acid, magnesium sulfate, and ammonium chloride are added to the initial concentration before returning to step three for use.

4. The method for recovering phosphorus from ash in sludge incineration according to claim 3, characterized in that: It also includes a calcium resource regeneration step, in which the calcium and magnesium-containing acid leaching filtrate produced in step two is evaporated and concentrated, cooled and crystallized to precipitate gypsum, and the filtrate A obtained in step three and the mother liquor residue after solid-liquid separation in step four are evaporated and concentrated under reduced pressure, then cooled to room temperature, and magnesium sulfate heptahydrate crystals are precipitated and filtered to obtain magnesium sulfate crystals, which are dried to obtain magnesium sulfate. After removing phosphoric acid from the mother liquor residue, it is returned to step three for recycling. The mother liquor residue in step four is treated by adsorbing magnesium ions through a chelating ion exchange resin, and then eluted with 0.5-2.0 mol / L hydrochloric acid to obtain a magnesium chloride solution.

5. A method for recovering phosphorus from ash in sludge incineration according to claim 4, characterized in that: In step three, the enhanced acid leaching and phosphorus extraction treatment employs microwave-assisted heating. The microwave-assisted heating has a power of 300-800W and a frequency of 2.45 GHz, achieving rapid and uniform heating at 60℃-80℃ and shortening the reaction time to 0.5-1.5h. In step four, the struvite precipitation is assisted by ultrasonic precipitation. The ultrasonic frequency is 20-40kHz, and the ultrasonic power density is 0.3-0.8W / mL.

6. A method for recovering phosphorus from ash in sludge incineration according to claim 5, characterized in that: Before the preliminary acid leaching and impurity removal treatment in step two, the sludge incineration ash is subjected to low-temperature thermochemical treatment. The sludge incineration ash is hydrothermally reacted with a phosphoric acid solution with a concentration of 0.5-1.0 mol / L in an environment of 60℃-100℃ for 1-2 hours. After cooling, the solid and liquid are separated, and the obtained filter residue is further subjected to the preliminary acid leaching and impurity removal step in step two.

7. A method for recovering phosphorus from ash in sludge incineration according to claim 6, characterized in that: In step four, phosphorus precipitation recovery, a two-stage pH adjustment method is used to adjust the pH of filtrate A: First stage: Adjust the pH of filtrate A to 7.5–8.0, and slowly add 50% magnesium salt and ammonium salt to generate seed crystals; Second stage: Add the remaining magnesium salt and ammonium salt to adjust the pH value to 9.0-9.5 to promote crystal growth.

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