System and method for synergistically preparing potassium struvite from biomass ash and sludge ash

Abstract

The invention discloses a system and method for preparing potassium struvite through cooperation of biomass ash and sludge ash, the system comprises an acid leaching module, a heavy metal separation module, a crystallization module and a drying granulation module, the acid leaching module is used for leaching and separating phosphorus and potassium elements in the biomass ash and the sludge ash; an inlet of the acid leaching module is communicated with biomass ash and sludge ash, and a supernatant outlet of the acid leaching module is communicated with an inlet of the heavy metal separation module; the heavy metal separation module is used for deeply removing heavy metal elements in the acid leaching supernate; a purified liquid outlet of the heavy metal separation module is communicated with an inlet of the crystallization module; the crystallization module is used for crystallizing the purified liquid to obtain potassium struvite crystals; a crystal wet material outlet of the crystallization module is communicated with an inlet of the drying granulation module; and the drying and granulating module is used for forming the crystal wet material to obtain the potassium struvite. The biomass ash and the sludge ash can be retreated, and the potassium struvite can be efficiently prepared.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to a system and method for the synergistic preparation of potassium struvite from biomass ash and sludge ash. Background Technology

[0002] With the rapid advancement of urbanization and the continuous increase in sewage treatment volume, sludge production has shown a dramatic upward trend. After sludge incineration, phosphorus remains in the ash, which achieves a certain degree of initial phosphorus enrichment. However, sludge ash often contains pollutants such as heavy metals. If directly applied to land, it will cause serious pollution to soil, water bodies, and other environmental elements, posing a significant environmental risk. Meanwhile, biomass utilization produces biomass ash, which is rich in potassium but relatively low in phosphorus. Currently, the main method for disposing of these two types of ash is landfill. This crude and simplistic approach not only results in a huge waste of valuable resources such as phosphorus and potassium but also further increases the environmental burden, hindering sustainable development.

[0003] Traditional phosphate fertilizer production heavily relies on phosphate rock; however, global phosphate rock reserves are limited, while potash fertilizer mainly comes from potash mines. With the continuous development of agriculture, the demand for potash fertilizer continues to grow. Against this backdrop, developing alternative potassium struvite sources is particularly urgent and important. Existing research has attempted to recover phosphorus from sludge ash and potassium from biomass ash, but utilizing these two types of ash alone has revealed numerous problems. For example, the nutrients are limited, failing to meet the comprehensive nutrient requirements of crops; heavy metal migration is a significant issue, making it difficult to effectively control the flow of heavy metals during recycling, potentially posing a threat to the environment and the quality of agricultural products. However, co-processing sludge ash and biomass ash can achieve complementary phosphorus and potassium nutrients, fully leveraging the advantages of both types of ash, while also reducing equipment investment costs, opening up new avenues for the resource utilization of waste and fertilizer production.

[0004] Despite the numerous advantages and potential of co-processing sludge ash and biomass ash, a series of technical challenges remain in practical operation. A key technology to overcome is how to efficiently extract phosphorus from sludge ash and potassium from biomass ash, and effectively combine the two to produce high-quality potassium struvite. Simultaneously, how to effectively separate heavy metal ions using appropriate methods, such as resin adsorption, during the co-processing process to ensure that the prepared potassium struvite meets environmental and quality standards and avoids adverse effects of heavy metals on the environment and crops is also an important issue that urgently needs to be addressed. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention proposes a method for the synergistic preparation of potassium struvite based on biomass ash and sludge ash. This method can effectively utilize the potassium resources in biomass ash and the phosphorus resources in sludge ash to finally prepare potassium struvite, greatly reducing the preparation cost of potassium struvite and realizing the resource utilization of multiple solid wastes. This solves the current problem of synergistic treatment and utilization of multi-source organic solid waste.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a system for the co-preparation of potassium struvite from biomass ash and sludge ash, the system comprising an acid leaching module, a heavy metal separation module, a crystallization module, and a drying and granulation module, wherein: The acid leaching module is used to leach and separate phosphorus and potassium elements from biomass ash and sludge ash; the inlet of the acid leaching module is connected to the biomass ash and sludge ash, and the outlet of the supernatant of the acid leaching module is connected to the inlet of the heavy metal separation module. The heavy metal separation module is used for the deep removal of heavy metal elements from the acid leaching supernatant; the purified liquid outlet of the heavy metal separation module is connected to the inlet of the crystallization module. The crystallization module is used to crystallize the purified liquid to obtain potassium struvite crystals; the wet crystal outlet of the crystallization module is connected to the inlet of the drying and granulation module; The drying and granulation module is used for the molding process of wet crystal materials to obtain potassium struvite.

[0007] Furthermore, the acid leaching module includes a stirring device, a sedimentation tank, and a filter. The stirring device is installed in the sedimentation tank, which is used to hold the acid leaching solution. The outlet of the sedimentation tank is connected to the inlet of the filter. The supernatant is obtained by filtration through the filter, and the outlet of the supernatant is connected to the inlet of the heavy metal separation module.

[0008] Furthermore, the acid leaching solution is a sulfuric acid solution with a pH value of 1.5~2.2; biomass ash and sludge ash are added to the sedimentation tank at a potassium-to-phosphorus ratio of 1.05:1, the total amount of biomass ash and sludge ash in the sedimentation tank is less than or equal to the mass ratio of sulfuric acid solution to 1:8, the stirring temperature is 20℃~60℃, and the mixture is stirred and dissolved before settling.

[0009] Furthermore, the heavy metal separation module includes a heavy metal chelating resin column and a precision filter device. The inlet of the heavy metal chelating resin column is connected to the supernatant outlet of the acid leaching module, the outlet of the heavy metal chelating resin column is connected to the inlet of the precision filter device, and the purified liquid outlet of the precision filter device is connected to the inlet of the crystallization module.

[0010] Furthermore, the heavy metal chelating resin column uses sulfur-based chelating resin, which dynamically adsorbs under pH conditions of 3.0~4.5, with the flow rate controlled at 2BV / h~3BV / h and the resin bed height-to-diameter ratio ≥5:1; The precision filtration device uses a ceramic membrane with a pore size of 0.45μm. After the sulfur-based chelating resin is adsorbed, the solution is filtered through the ceramic membrane to obtain a purified solution with a heavy metal content of <1mg / L.

[0011] Furthermore, the crystallization module includes a reactor and a crystallizer. The first inlet of the reactor is connected to the purified liquid outlet of the heavy metal separation module, the second inlet of the reactor is connected to the magnesium salt source, the outlet of the reactor is connected to the inlet of the crystallizer, and the outlet of the crystallizer is connected to the inlet of the drying and granulation module.

[0012] Furthermore, the reactor is used for the initial crystallization of potassium struvite. KOH is used in the reactor to adjust the pH of the purification solution to 9.5~10.5. The purification solution and magnesium salt are fed in a molar ratio of K:Mg:P=1.05:1:1. The reactor is stirred and reacted at 50±2℃ to initially crystallize potassium struvite crystals with a particle size distribution of 1μm~100μm. In the crystallizer, potassium struvite crystals are cooled to 25°C at a gradient of 0.5°C / min, aged, and then centrifuged to obtain uniform wet crystals with a particle size of 50μm~200μm.

[0013] Furthermore, the drying and granulation module includes a belt filter press, a dryer, a mixer, and a granulator. The wet crystal outlet of the crystallization module is connected to the inlet of the belt filter press, the outlet of the belt filter press is connected to the inlet of the dryer, the outlet of the dryer is connected to the first inlet of the mixer, the second inlet of the mixer is connected to the binder source, and the outlet of the mixer is connected to the inlet of the granulator.

[0014] Furthermore, a humic acid adhesive is used as the binder.

[0015] The present invention also provides a method for preparing potassium struvite by co-processing biomass ash and sludge ash, using the above-mentioned system for preparing potassium struvite by co-processing biomass ash and sludge ash.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: This invention provides a system for the co-processing of biomass ash and sludge ash to prepare potassium struvite. Through the combined processing of an acid leaching module, a heavy metal separation module, a crystallization module, and a drying and granulation module, the system achieves the co-processing of biomass ash and sludge ash, realizes the efficient extraction of phosphorus and potassium elements and the simultaneous passivation of heavy metals, and avoids the problems of high energy consumption and long processing time of traditional landfill and composting treatment methods by disposing of biomass ash and sludge ash on a large scale, thereby improving the processing efficiency. Furthermore, this invention employs a sulfuric acid solution with a pH of 1.5–2.2 for acid leaching, efficiently separating phosphorus and potassium elements and improving recycling efficiency. Simultaneously, a sulfur-based chelating resin column is used for deep removal of heavy metals, and the acid leaching solution, after resin adsorption, is directly used for potassium struvite synthesis. The mother liquor recycling rate exceeds 95%, avoiding the high energy consumption problem of multiple concentrations in traditional processes. This achieves resource utilization of various solid wastes and significantly reduces the preparation cost of potassium struvite.

[0017] Furthermore, the system of this invention employs a 0.45μm ceramic membrane for precision filtration, obtaining a purified liquid with a heavy metal content of <1 mg / L, effectively solving the heavy metal pollution problem in existing technologies. In the reaction vessel, the pH value of the purified liquid is adjusted by KOH, which then acts synergistically with magnesium salts to achieve the directional conversion of phosphorus and potassium, generating high-quality potassium struvite crystals. Gradient cooling crystallization and centrifugal separation processes are used to obtain uniformly sized wet crystals, ensuring product quality stability and consistency, and providing a guarantee for the production of high-quality potassium struvite.

[0018] Furthermore, the system of this invention utilizes a combination of equipment such as a belt filter press, dryer, mixer, and granulator to achieve rapid dehydration, uniform mixing, and molding of wet materials, thereby improving production efficiency. Moreover, the prepared potassium struvite can be used to produce potassium-phosphate compound fertilizer, which exhibits a phosphorus release rate of <20% in the soil within 24 hours, far lower than traditional phosphate fertilizers (>80%), meeting the GB / T 23348-2009 slow-release fertilizer standard. Simultaneously, the large-scale disposal of biomass ash and sludge ash avoids the high energy consumption and long processing times of traditional landfill and composting methods, truly achieving the goal of "turning waste into treasure," and demonstrating significant environmental and economic benefits.

[0019] This invention proposes an innovative method for the co-preparation of potassium struvite based on biomass ash and sludge ash. This method fully utilizes the potassium resources in biomass ash and the phosphorus resources in sludge ash, effectively solving the problem of co-processing and utilizing multi-source organic solid waste through a specific process. First, the biomass ash and sludge ash are acid-leached with a sulfuric acid solution at a specific pH value to achieve efficient leaching and separation of phosphorus and potassium elements. Then, heavy metals are deeply removed using a sulfur-based chelating resin column, and the purified liquid is obtained through precision filtration. Next, the pH value is adjusted in a reaction vessel, and the mixture synergistically reacts with magnesium salts to directionally convert phosphorus and potassium into potassium struvite crystals. Finally, through gradient cooling crystallization, centrifugal separation, dehydration, mixing, and granulation, high-quality potassium struvite is prepared, significantly reducing preparation costs and realizing the resource utilization of solid waste. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a system for the synergistic preparation of potassium struvite from biomass ash and sludge ash according to the present invention; In the attached diagram: 1. Biomass ash; 2. Sludge ash; 3. Magnesium salt; 4. Humic acid binder; 5. Stirring device; 6. Sedimentation tank; 7. Filter; 8. Heavy metal chelating resin column; 9. Precision filtration device; 10. Reactor; 11. Crystallizer; 12. Belt filter press; 13. Dryer; 14. Mixing machine; 15. Granulator; 16. Final product. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0022] Example 1 Biomass ash is rich in potassium, and sludge ash is rich in phosphorus; both have extremely high resource utilization value. However, current disposal methods for these two types of ash mainly involve landfilling and stockpiling, resulting in significant resource waste. Furthermore, this extensive treatment method also poses a risk of heavy metal leaching pollution. This invention proposes a system for the co-production of potassium struvite from biomass ash and sludge ash, which can be used for large-scale disposal of both. Figure 1 As shown, the system consists of an acid leaching module, a heavy metal separation module, a crystallization module, and a drying and granulation module, wherein: The acid leaching module is used for the leaching and separation of phosphorus and potassium elements in ash. It includes a stirring device 5, a sedimentation tank 6, and a filter 7. The stirring device 5 is set in the sedimentation tank 6, and a sulfuric acid solution with a pH of 1.5~2.2 is added to the sedimentation tank 6. Biomass ash and sludge ash are added to the sedimentation tank 6 through the inlet. The mass ratio of ash to sulfuric acid solution is ≤1:8. The stirring temperature is 20~60℃, and the mixture is stirred for 2 hours at a stirring speed of 200 rpm. Phosphorus and potassium dissolve in the acid leaching solution, and then the mixture is allowed to stand in the sedimentation tank 6. The outlet of the sedimentation tank 6 is connected to the inlet of the filter 7. The supernatant is obtained by filtration through the filter 7. At this time, the supernatant still contains a large amount of heavy metal elements, which need to be further separated and purified. The outlet of the filter 7 is connected to the inlet of the heavy metal separation module.

[0023] The heavy metal separation module is used for the deep removal of heavy metal elements in the acid leaching supernatant. It includes a heavy metal chelating resin column 8 and a precision filter device 9. The inlet of the heavy metal chelating resin column 8 is connected to the outlet of the filter 7, the outlet of the heavy metal chelating resin column 8 is connected to the inlet of the precision filter device 9, and the outlet of the precision filter device 9 is connected to the inlet of the crystallization module to introduce the purified liquid into the crystallization module. Preferably, the heavy metal chelating resin column 8 uses a sulfur-based chelating resin, which dynamically adsorbs heavy metals under pH conditions of 3.0–4.5, with a flow rate controlled at 2–3 BV / h and a resin bed height-to-diameter ratio ≥ 5:1. The resin loaded with heavy metals is regenerated using a 5% (w / w) HNO3 solution, with a regeneration efficiency ≥ 95%. The sulfur-based chelating resin can efficiently remove heavy metal ions, especially exhibiting a strong affinity for mercury and lead, even in environments containing large amounts of Na+. + Ca 2+ Mg 2+ It can still efficiently capture trace heavy metals in wastewater; heavy metals can be eluted with 1 M HCl dilute acid or Na2S2O3 solution, and the resin regeneration rate is high; moreover, sulfur-based resin will not introduce excessive sulfur. 2- To avoid H2S poisoning.

[0024] Preferably, the precision filtration device 9 employs a ceramic membrane with a pore size of 0.45 μm. After the sulfur-based chelating resin is adsorbed, the solution is filtered through the ceramic membrane to obtain a purified solution with a heavy metal content of <1 mg / L (compliant with GB / T 23349-2020). The ceramic membrane enables fine filtration of the solution, facilitating the subsequent acquisition of a high-purity phosphorus and potassium solution. The ceramic membrane is resistant to strong acids and alkalis, has high mechanical strength, can withstand frequent backwashing or high-pressure operation, has low operating costs, and is easy to clean and regenerate.

[0025] The crystallization module is used for the directional conversion of phosphorus and potassium nutrients, including a reactor 10 and a crystallizer 11. The first inlet of the reactor 10 is connected to the outlet of the precision filter 9, the second inlet of the reactor 10 is connected to the magnesium salt source, the outlet of the reactor 10 is connected to the inlet of the crystallizer 11, and the outlet of the crystallizer 11 is connected to the drying and granulation module. Preferably, the pH of the purification solution is adjusted to 9.5~10.5 using KOH in the reactor 10; wherein the purification solution and magnesium salt (MgCl2·6H2O) are fed in a molar ratio of K:Mg:P=1.05:1:1, and the reaction is carried out in the reactor 10 at 50±2℃ for 2 h (200 rpm), and potassium struvite (KMgPO4·6H2O) crystals are initially formed with a particle size distribution of 1μm~100 μm.

[0026] Preferably, potassium struvite (KMgPO4·6H2O) is transferred to crystallizer 11 and cooled to 25°C at a gradient of 0.5°C / min. After aging for 12 h, uniform wet crystals with a particle size of 50μm~200μm are obtained after centrifugation. The mother liquor outlet of crystallizer 11 is connected to the inlet of sedimentation tank 6 to return the mother liquor to sedimentation tank 6 for recycling in the acid leaching process.

[0027] The drying and granulation module is used for the molding process of wet crystal material to obtain potassium struvite. It includes a belt filter press 12, a dryer 13, a mixer 14, and a granulator 15. The wet material outlet of the crystallizer 11 is connected to the inlet of the belt filter press 12. The outlet of the belt filter press 12 is connected to the inlet of the dryer 13. The outlet of the dryer 13 is connected to the first inlet of the mixer 14. The second inlet of the mixer 14 is connected to the binder source. The outlet of the mixer 14 is connected to the inlet of the granulator 15. The outlet of the granulator 15 yields the final product 16, in which the K2O content is 25-35% and the P2O5 content is 35-50%.

[0028] Preferably, the adhesive is a humic acid adhesive.

[0029] The main operating flow of the system of this invention is as follows: First, the raw materials are proportioned, with biomass ash 1 and sludge ash 2 entering the acid leaching module at a potassium-to-phosphorus ratio of 1.05:1. The acid leaching module is located at the beginning of the system, and its main components are a stirring device 5 and a sedimentation tank 6, used to dissolve phosphorus in the biomass ash and potassium in the sludge ash. The supernatant from the acid leaching then passes through a filter 7 and enters the heavy metal separation module. The main components of this module are a heavy metal chelating resin column 8 and a precision filter 9. The heavy metal chelating resin column 8 is used for preliminary purification, and the precision filter 9 performs secondary purification to ensure the safety of the final product. The crystallization module is located after the heavy metal separation module, and its main components are a reactor 10 and a crystallizer 11. First, a certain amount of magnesium salt 3 is added to the reactor 10 at a magnesium-to-phosphorus ratio of 1. Under controlled reaction conditions, potassium struvite crystals are initially synthesized with a particle size distribution of 1–100 μm. The crystallizer 11, through graded discharge and precise, slow cooling, prepares uniform crystals of 50–200 μm, obtaining high-quality potassium struvite. The drying and granulation module is located after the crystallization module. The main body of the equipment consists of a belt filter press 12, a dryer 13, a mixer 14, and a granulator 15. First, the filter press 11 is used to reduce the moisture content of the material to obtain a filter cake with low moisture content. Then, the material is dehydrated by the dryer 12. Next, the material enters the mixer 14, and humic acid binder 4 is added to the mixer at the same time. The powder and humic acid binder are mixed and spherical particles are obtained in the granulator 15, which is the final product 16.

[0030] Example 2 This application also provides a method for the co-production of potassium struvite from biomass ash and sludge ash, comprising: mixing biomass ash and sludge ash to achieve nutrient complementarity; using acid leaching to optimize the dissolution efficiency of phosphorus and potassium by controlling the pH value; dynamic adsorption control to remove heavy metal pollutants; adjusting the pH value and adding magnesium salts to form potassium struvite crystals; crystallization grading and aging treatment to improve the quality of the crystals; dehydration, drying and granulation to complete the molding process of the wet crystal material to obtain potassium struvite. This embodiment effectively promotes the dissolution of phosphorus and potassium by precisely controlling the pH value of the sulfuric acid solution, while inhibiting the activity of heavy metals, achieving effective resource separation. This embodiment utilizes the differences in the solubility characteristics of different elements at specific pH values, as well as the specific binding of heavy metals with sulfur-based chelating resins, achieving the dual goals of nutrient extraction and heavy metal passivation. The technology in this embodiment significantly improves the recovery rate of phosphorus and potassium, while effectively reducing the leaching toxicity of heavy metals, ensuring the safety and quality of potassium struvite.

[0031] Furthermore, during acid leaching, the pH value of the sulfuric acid solution is controlled between 1.5 and 2.2 to ensure maximum solubility of phosphorus and potassium while minimizing the dissolution of heavy metals, thereby improving the efficiency and safety of subsequent treatments.

[0032] Furthermore, the mass ratio of ash to sulfuric acid solution during the mixing process is ≤1:8 to ensure sufficient contact and dissolution efficiency of the raw materials, while avoiding resource waste and environmental pollution caused by excessive acid solution.

[0033] Furthermore, the stirring temperature is controlled between 20 and 60°C. Appropriate temperatures can accelerate the dissolution process and improve the dissolution efficiency of the elements. This embodiment ensures efficient dissolution of phosphorus and potassium while avoiding increased energy consumption and equipment wear caused by high temperatures, achieving resource conservation and economic benefits.

[0034] Furthermore, the dynamic adsorption flow rate was controlled at 2 BV / h to 3 BV / h. This flow rate setting aims to balance adsorption efficiency and treatment time, ensuring sufficient removal of heavy metals without prolonging the treatment cycle. In this embodiment, heavy metal ions were effectively removed, improving the quality of the purified solution and providing high-quality raw materials for the subsequent synthesis of potassium struvite.

[0035] Furthermore, the aspect ratio of the resin bed used in the dynamic adsorption control is ≥5:1. In this embodiment, the larger the aspect ratio of the resin bed, the longer the solution passes through the resin bed, and the better the adsorption effect.

[0036] Furthermore, resin regeneration is achieved using a 5% (w / w) HNO3 solution. In this embodiment, the nitric acid solution breaks down the metal complexes on the resin, releasing the adsorbed heavy metals and thus regenerating the resin. This ensures the long-term stability of the resin, reduces the frequency of resin replacement, and achieves both economic and environmental benefits.

[0037] Furthermore, the purified solution is filtered using a ceramic membrane with a pore size of 0.45 μm. In this embodiment, the ceramic membrane filtration effectively traps suspended particles and incompletely removed heavy metals through pore size sieving, ensuring the purity of the purified solution.

[0038] Furthermore, the pH of the purification solution was adjusted to 9.5-10.5 using KOH. In this embodiment, the pH value affects the charge state of ions in the solution and the equilibrium of the precipitation reaction; a suitable alkaline environment is conducive to the formation of potassium struvite. The technique in this embodiment significantly improves the yield and quality of potassium struvite.

[0039] Furthermore, the reaction temperature during the potassium struvite crystallization process was controlled at 50±2℃, and the stirring speed at 200 rpm. This choice of reaction conditions balances crystallization efficiency and crystal quality, and is beneficial for forming uniform and stable potassium struvite crystals. Temperature and stirring speed affect the nucleation and growth process of crystals; appropriate conditions can promote uniform crystal distribution and good morphology.

[0040] Furthermore, the cooling rate in the crystallizer was controlled at 0.5℃ / min, and the aging time was 12 h. This setting of cooling rate and aging time ensured the maturity and stability of the crystals, which is beneficial for obtaining potassium struvite with uniform particle size. In this embodiment, the cooling rate and aging time affected the crystal growth and aggregation process; appropriate conditions can promote uniform growth and close arrangement of crystals. The technology in this embodiment significantly improved the particle size uniformity and stability of potassium struvite crystals, providing a guarantee for the subsequent physical properties of potassium struvite.

[0041] Furthermore, the mother liquor recycling rate is >95%. In this embodiment, mother liquor recycling improves the overall element recovery rate and reduces waste emissions by refluxing unreacted solution.

[0042] Furthermore, the potassium struvite prepared in this embodiment has a slow-release effect, which can control the release rate of phosphorus, avoid rapid phosphorus loss, and improve the plant's absorption and utilization rate of phosphorus. The technology in this embodiment significantly improves the slow-release performance of potassium struvite, reduces the number of fertilizations, and achieves efficient resource utilization and environmental friendliness.

[0043] This invention discloses a system and method for the co-processing of biomass ash and sludge ash to prepare potassium struvite. The system includes: an acid leaching module for soaking the mixed ash to obtain a leaching solution containing phosphorus and potassium elements; a heavy metal separation module for deep removal of heavy metals from the acid leaching solution to obtain a phosphorus and potassium solution that meets safety production standards; a crystallization module for the directional conversion of phosphorus and potassium components, improving the efficiency of slow-release phosphorus and potassium fertilizer preparation; and a drying and granulation module for dehydrating, drying, and granulating the potassium struvite precipitate to obtain a shaped product. This invention enables the reprocessing of biomass ash and sludge ash, as well as the high-efficiency preparation of potassium struvite.

Claims

1. A system for the co-production of potassium struvite from biomass ash and sludge ash, characterized in that, The system includes an acid leaching module, a heavy metal separation module, a crystallization module, and a drying and granulation module, wherein: The acid leaching module is used to leach and separate phosphorus and potassium elements from biomass ash and sludge ash; the inlet of the acid leaching module is connected to the biomass ash and sludge ash, and the outlet of the supernatant of the acid leaching module is connected to the inlet of the heavy metal separation module. The heavy metal separation module is used for the deep removal of heavy metal elements from the acid leaching supernatant; the purified liquid outlet of the heavy metal separation module is connected to the inlet of the crystallization module. The crystallization module is used to crystallize the purified liquid to obtain potassium struvite crystals; the wet crystal outlet of the crystallization module is connected to the inlet of the drying and granulation module; The drying and granulation module is used for the molding process of wet crystal materials to obtain potassium struvite.

2. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 1, characterized in that, The acid leaching module includes a stirring device, a sedimentation tank, and a filter. The stirring device is installed in the sedimentation tank, which is used to hold the acid leaching solution. The outlet of the sedimentation tank is connected to the inlet of the filter. The supernatant is obtained by filtration through the filter, and the outlet of the supernatant is connected to the inlet of the heavy metal separation module.

3. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 2, characterized in that, The acid leaching solution is a sulfuric acid solution with a pH value of 1.5 to 2.2; Biomass ash and sludge ash are added to the sedimentation tank at a potassium-to-phosphorus ratio of 1.05:

1. The total amount of biomass ash and sludge ash in the sedimentation tank is less than or equal to the mass ratio of sulfuric acid solution to 1:

8. The stirring temperature is 20℃~60℃. After stirring and dissolving, the mixture settles.

4. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 1, characterized in that, The heavy metal separation module includes a heavy metal chelating resin column and a precision filter. The inlet of the heavy metal chelating resin column is connected to the supernatant outlet of the acid leaching module, the outlet of the heavy metal chelating resin column is connected to the inlet of the precision filter, and the purified liquid outlet of the precision filter is connected to the inlet of the crystallization module.

5. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 4, characterized in that, The heavy metal chelating resin column uses sulfur-based chelating resin, which dynamically adsorbs under pH conditions of 3.0~4.5, with the flow rate controlled at 2BV / h~3BV / h and the resin bed height-to-diameter ratio ≥5:

1. The precision filtration device uses a ceramic membrane with a pore size of 0.45μm. After the sulfur-based chelating resin is adsorbed, the solution is filtered through the ceramic membrane to obtain a purified solution with a heavy metal content of <1mg / L.

6. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 1, characterized in that, The crystallization module includes a reactor and a crystallizer. The first inlet of the reactor is connected to the purified liquid outlet of the heavy metal separation module, the second inlet of the reactor is connected to the magnesium salt source, the outlet of the reactor is connected to the inlet of the crystallizer, and the outlet of the crystallizer is connected to the inlet of the drying and granulation module.

7. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 6, characterized in that, The reactor is used for the initial crystallization of potassium struvite. KOH is used in the reactor to adjust the pH of the purification solution to 9.5~10.

5. The purification solution and magnesium salt are fed in a molar ratio of K:Mg:P=1.05:1:

1. The reactor is stirred and reacted at 50±2℃ to initially crystallize potassium struvite crystals with a particle size distribution of 1μm~100μm. In the crystallizer, potassium struvite crystals are cooled to 25°C at a gradient of 0.5°C / min, aged, and then centrifuged to obtain uniform wet crystals with a particle size of 50μm~200μm.

8. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 1, characterized in that, The drying and granulation module includes a belt filter press, a dryer, a mixer, and a granulator. The wet crystal outlet of the crystallization module is connected to the inlet of the belt filter press, the outlet of the belt filter press is connected to the inlet of the dryer, the outlet of the dryer is connected to the first inlet of the mixer, the second inlet of the mixer is connected to the binder source, and the outlet of the mixer is connected to the inlet of the granulator.

9. The system for co-preparing potassium struvite from biomass ash and sludge ash according to claim 8, characterized in that, The adhesive used is a humic acid adhesive.

10. A method for co-preparing potassium struvite from biomass ash and sludge ash, characterized in that, The method described in any one of claims 1 to 9 is used to prepare potassium struvite by co-processing biomass ash and sludge ash.

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