Seawater desalination sludge heat pump drying-mineralization recycling method

Abstract

The application discloses a seawater desalination sludge heat pump drying-mineralization recycling method, which comprises the following steps: homogenizing, screening and removing impurities and mechanically dewatering seawater desalination sludge to obtain dewatered sludge; grading countercurrent desalination of the dewatered sludge through a first washing section and a second washing section to obtain salt-regulated sludge; mixing and forming the salt-regulated sludge and back-mixed formed material to perform closed cycle heat pump drying, and dividing dried condensate water into high-salt condensate water and low-salt condensate water according to a preset conductivity threshold; crushing, grinding and grading the dried sludge, and then performing acidification and mineral release and buffer stable mineralization of the low-salt condensate water, seawater desalination produced water and carbon dioxide gas to obtain mineralized produced water and mineralized residual material, so that the seawater desalination sludge is dried, desalted and mineralized and recycled.

Description

Technical Field

[0001] This invention relates to the field of seawater desalination by-product sludge treatment and resource recycling technology, and in particular to a heat pump drying-mineralization recycling method for seawater desalination sludge. Background Technology

[0002] Seawater desalination projects typically generate saline sludge during operation. This type of sludge mainly originates from brine softening and sedimentation, membrane cleaning and neutralization sedimentation, and scale and flocculated sediments from the cleaning discharge of low-temperature multi-effect seawater desalination units. Because seawater desalination sludge is usually characterized by high water content, high chloride content, and the presence of calcium and magnesium mineral components, simply dewatering and transporting it or landfilling it as ordinary sludge would not only increase the subsequent disposal load but also make it difficult to effectively utilize the calcium and magnesium components that can be used for mineralization of desalination permeate.

[0003] Existing technologies already include online removal solutions for sludge from seawater desalination cleaning. For example, patent announcement number CN115159602B discloses a low-temperature multi-effect seawater desalination cleaning sludge online removal system. This system includes cleaning pipelines, an acid washing tank, a sludge dewatering device, multiple heat exchanger assemblies, and connecting pipelines. It can remove insoluble scale, slime, and flocculent matter from the cleaning solution and achieve solid-liquid separation through the sludge dewatering device, providing conditions for subsequent treatment. The focus of this existing technology is the online removal of sludge and impurities during the seawater desalination cleaning process, in order to reduce the adverse effects of impurities in the cleaning solution on the operation of seawater desalination equipment.

[0004] However, the aforementioned existing technologies mainly address the online removal of sludge, scale, and flocculent matter from seawater desalination cleaning solutions. Their treatment of the removed sludge primarily focuses on dewatering and subsequent disposal, without establishing a continuous treatment process specifically addressing the high salt content of seawater desalination sludge and the need for reuse of its calcium and magnesium components. Specifically, after dewatering, this type of sludge may still contain chlorides and other soluble salts. Direct drying or reuse can easily lead to these soluble salts entering the drying condensate, low-salt recycled water, or mineralized permeate, limiting the reuse of the drying condensate and causing fluctuations in the quality of subsequent mineralized permeate. Furthermore, without proper desalination, drying, and mineralization contact treatment, the calcium and magnesium components are difficult to stably participate in the mineralization treatment of seawater desalination permeate.

[0005] Therefore, the main technical problem that the existing technology still needs to solve is: how to reduce the impact of soluble salts in seawater desalination sludge on the quality of condensate reuse and mineralized water by graded desalination, heat pump drying and condensate diversion and reuse, and mineralization contact treatment, and to enable the calcium and magnesium components in seawater desalination sludge to participate in the mineralization treatment of seawater desalination water. Summary of the Invention

[0006] To overcome the aforementioned technical deficiencies, the present invention aims to provide a method for the heat pump drying-mineralization and reuse of seawater desalination sludge. This invention solves the technical problem of soluble salts in seawater desalination sludge affecting condensate reuse and the quality of mineralized permeable water, and enables the calcium and magnesium components to participate in the mineralization treatment of seawater desalination permeable water, through a continuous closed-loop treatment scheme comprising "staged countercurrent desalination, closed-loop heat pump drying condensate water diversion and reuse according to conductivity, acidification and mineral release of dried sludge powder and buffer stabilization mineralization, and bidirectional recirculation of mineralized residue."

[0007] This invention discloses a method for heat pump drying-mineralization and reuse of seawater desalination sludge, comprising the following steps: S1. Receive seawater desalination sludge, homogenize, screen to remove impurities and mechanically dewater the seawater desalination sludge to obtain dewatered sludge; S2. The dewatered sludge is fed into a graded countercurrent desalination process, which includes a first washing section and a second washing section. In the second washing section, low-salt recycled water is used to wash the dewatered sludge. In the first washing section, the second washing filtrate generated from the second washing section is used to wash the dewatered sludge. The first washing filtrate generated from the first washing section is then sent to a high-salt wastewater treatment process to obtain salt-controlled sludge. S3. Mix the salt-regulating sludge with the remixed molding material to form shaped sludge. Then, send the shaped sludge into a closed-loop heat pump drying process. The closed-loop heat pump drying process divides the dried condensate into high-salt condensate and low-salt condensate according to a preset conductivity threshold. The dried condensate with a conductivity greater than the preset conductivity threshold is used as high-salt condensate, and the dried condensate with a conductivity not greater than the preset conductivity threshold is used as low-salt condensate. The high-salt condensate is returned to the first washing section, and the low-salt condensate is returned to the second washing section or sent to the mineralization contact process as low-salt reclaimed water. S4. The dried sludge obtained from the closed-loop heat pump drying process is crushed, ground and classified to obtain dried sludge powder. S5. The dried sludge powder, low-salt condensate, seawater desalination product water and carbon dioxide gas are fed into the mineralization contact process. The mineralization contact process includes an acidification and release section and a buffer stabilization section. In the acidification and release section, carbon dioxide gas is introduced to allow the calcium and magnesium components in the dried sludge powder to enter the liquid phase. In the buffer stabilization section, the remaining mineralized material as seed crystals is added and the amount of carbon dioxide gas introduced is adjusted to obtain a mineralized mixture. S6. Perform solid-liquid separation on the mineralized mixture to obtain mineralized water and mineralized residue. Return a portion of the mineralized residue to step S3 as remixing molding material, and return the other portion of the mineralized residue to step S5 as seed crystal.

[0008] Preferably, in step S1, the seawater desalination sludge includes brine softening and settling sludge and membrane cleaning and neutralizing settling sludge. The brine softening and settling sludge and the membrane cleaning and neutralizing settling sludge are mixed according to the total calcium and magnesium content test results and then enter mechanical dewatering.

[0009] Preferably, the total calcium and magnesium content test results are obtained through acid leaching test solution. When the total mass concentration of calcium and magnesium ions in the acid leaching test solution is lower than the preset mineralization feed limit, the corresponding batch of seawater desalination sludge is sent to the non-mineralized sludge treatment process.

[0010] Preferably, in step S2, an intermediate pressure filtration step is set between the first washing section and the second washing section. The sludge treated by the first washing section is subjected to the intermediate pressure filtration step, the intermediate filtrate obtained by the intermediate pressure filtration step is returned to the first washing section, and the intermediate filter cake obtained by the intermediate pressure filtration step enters the second washing section.

[0011] Preferably, in step S2, a terminal pressure filtration step is set after the second washing section. The terminal filtrate obtained from the terminal pressure filtration step is subjected to chloride ion detection. When the chloride ion mass concentration in the terminal filtrate is higher than the preset desalination limit, the terminal filter cake obtained from the terminal pressure filtration step is returned to the second washing section for washing again.

[0012] Preferably, in step S3, the remixed molding material includes the mineralized residue obtained in step S6 and the dried sludge powder obtained in step S4. The remixed molding material is mixed with the salt-controlled sludge to form molded sludge with continuous pores.

[0013] Preferably, in step S3, the closed-loop heat pump drying process includes a front-end low-temperature drying section, a middle-end constant-temperature dehydration section, and a terminal low-humidity drying section. The dried condensate generated in the front-end low-temperature drying section with a conductivity greater than a preset conductivity threshold is used as high-salt condensate, and the dried condensate generated in the middle-end constant-temperature dehydration section and the terminal low-humidity drying section with a conductivity not greater than a preset conductivity threshold is used as low-salt condensate.

[0014] Preferably, the inlet air temperature of the front low-temperature drying section is 35°C to 50°C, the inlet air temperature of the middle constant-temperature dehydration section is 50°C to 70°C, and the inlet air temperature of the terminal low-humidity drying section is 40°C to 60°C, and the relative humidity of the circulating air in the terminal low-humidity drying section is lower than that of the circulating air in the middle constant-temperature dehydration section.

[0015] Preferably, in step S3, the closed-loop heat pump drying process includes a heat pump evaporation dehumidification zone. Before entering the heat pump evaporation dehumidification zone, the circulating air in the closed-loop heat pump drying process passes through a dust interception zone and a salt spray collection zone in sequence. The dust intercepted in the dust interception zone is returned to step S4, and the liquid intercepted in the salt spray collection zone is returned to the first washing section.

[0016] Preferably, in step S4, the dried sludge is classified into mineralized powder and skeleton powder. The mineralized powder enters step S5, and the skeleton powder is returned to step S3 as part of the remixed molding material.

[0017] Preferably, the mineralized powder is powder that passes through a 120-mesh sieve, and the skeleton powder is powder that does not pass through a 120-mesh sieve but passes through a 40-mesh sieve. Powder that does not pass through a 40-mesh sieve is returned to step S4 for further grinding.

[0018] Preferably, in step S5, the pH value of the acidification and ore release section is controlled to be 5.6 to 6.6, the pH value of the buffer stabilization section is controlled to be 7.0 to 8.4, and a solid retention zone is set between the acidification and ore release section and the buffer stabilization section. Unreacted dried sludge powder in the solid retention zone enters the buffer stabilization section with the liquid flow.

[0019] Preferably, in the buffer stabilization section, the remaining mineralized material returned as seed crystals is added first, and then the amount of carbon dioxide gas introduced is adjusted so that the suspended solids in the mineralized mixture form solid particles containing calcium and magnesium carbonates before solid-liquid separation.

[0020] Preferably, in step S6, the mineralized water is divided into qualified mineralized water and mineralized water to be reprocessed after online detection. The qualified mineralized water enters the seawater desalination water post-treatment pipeline, and the mineralized water to be reprocessed is returned to the acidification and mineral release section.

[0021] Preferably, the online detection includes pH value detection, conductivity detection, total hardness detection, turbidity detection, and chloride ion detection. Based on the results of the online detection, the amount of low-salt recycled water added in the second washing section, the ratio of drying condensate in the closed-loop heat pump drying process, the amount of dried sludge powder added, the amount of carbon dioxide gas introduced, the amount of mineralized residue returned to step S3, and the amount of mineralized residue returned to step S5 are adjusted.

[0022] Compared with existing technologies, the above technical solution has the following advantages: 1. This invention employs a staged countercurrent desalination process, where the second washing filtrate from the second washing stage is returned to the first washing stage for reuse, while the first washing filtrate from the first washing stage is sent to a high-salinity wastewater treatment process. Simultaneously, chloride ion detection in the terminal filter cake is used to determine whether it needs to be returned to the second washing stage for further washing, thus achieving staged reduction of soluble salts in seawater desalination sludge before drying. According to data from the embodiments, the chloride ion mass fraction of the sludge before drying or subsequent treatment decreased from 3.6% in the comparative example to 0.82%, indicating that this staged countercurrent desalination process can effectively reduce the soluble salt content in the sludge.

[0023] 2. In the closed-loop heat pump drying process, this invention separates the dried condensate into high-salt and low-salt condensate according to a preset conductivity threshold. The high-salt condensate is returned to the first washing section, while the low-salt condensate is returned to the second washing section or sent to the mineralization contact process as low-salt recycled water. This avoids mixing and reusing dried condensate with different salinity levels. According to the data from the embodiment, low-salt condensate accounts for 68.5% of all dried condensate, the average conductivity of high-salt condensate is 7120 μS / cm, and the average conductivity of low-salt condensate is 1460 μS / cm, indicating that the dried condensate can form a relatively clear recycling path after conductivity-based diversion.

[0024] 3. This invention involves crushing, grinding, and classifying dried sludge to obtain dried sludge powder. This powder is then contacted with low-salt condensate, desalination permeate, and carbon dioxide gas in an acidification and mineral release section. This allows the calcium and magnesium components in the dried sludge powder, existing as carbonates, hydroxides, or mixed precipitates, to enter the liquid phase. A buffer stabilization section then forms a mineralized mixture. According to the example data, the total hardness of the mineralized permeate reaches 82 mg / L (calculated as calcium carbonate), indicating that the calcium and magnesium components in the desalination sludge can serve as a mineralization source for the mineralization treatment of desalination permeate.

[0025] 4. This invention incorporates an acidification and release stage and a buffer stabilization stage in the mineralization contact process. The acidification and release stage promotes the release of calcium and magnesium components from the dried sludge powder. The buffer stabilization stage, by adding mineralization residue as seed crystals and adjusting the carbon dioxide gas flow rate, causes suspended solids in the mineralization mixture to form calcium and magnesium carbonate-containing solid particles before solid-liquid separation, thereby reducing water quality fluctuations caused by single acidification treatment. Based on the results of a 72-hour continuous operation test, the pH value of the mineralized permeate remained between 7.35 and 7.92, and the total hardness remained between 75 mg / L and 90 mg / L, indicating that this two-stage mineralization treatment method is beneficial for maintaining stable mineralized permeate water quality.

[0026] 5. This invention divides the mineralization residue obtained from solid-liquid separation into two parts. One part is returned as remixing material to the step of mixing and molding the salt-regulating sludge with the remixing material. The other part is returned as seed crystals to the mineralization contact process. The mineralization residue returned to the molding step can improve the pore structure of the molded sludge as a solid skeleton, and the mineralization residue returned to the mineralization contact process can participate in the formation of solid particles in the buffer stabilization section as seed crystals. According to the data from the embodiments, the recycling rate of mineralization residue reaches 76%, and the relative amount of discharged solids is reduced to 24% based on the comparative example of 100%, indicating that this bidirectional recirculation method can reduce the amount of residual solids discharged.

[0027] 6. This invention classifies dried sludge into mineralized powder and skeletonized powder. The mineralized powder enters the mineralization contact process, while the skeletonized powder is returned to the molding process as part of the remixing molding material. The smaller particle size of the mineralized powder provides a larger contact area, which is beneficial for the release of calcium and magnesium components. The relatively larger particle size of the skeletonized powder allows it to mix with the salt-controlled sludge to form molded sludge with continuous pores. This facilitates the passage of circulating air through the molded sludge in the closed-loop heat pump drying process, thereby reducing the risk of local agglomeration and surface crusting.

[0028] 7. This invention ensures that the circulating air in the closed-loop heat pump drying process passes sequentially through a dust interception zone and a salt spray collection zone before entering the heat pump evaporation and dehumidification zone. Dust trapped in the dust interception zone is returned to the drying sludge crushing, grinding, and grading steps, while liquid trapped in the salt spray collection zone is returned to the first washing section. This arrangement reduces the entry of salt-containing dust and salt spray droplets into the heat pump evaporation and dehumidification zone, thereby helping to reduce fluctuations in the quality of the drying condensate and minimizing salt interference in subsequent mineralization contact processes.

[0029] 8. This invention performs pH, conductivity, total hardness, turbidity, and chloride ion detection on the mineralized permeate. Based on the online detection results, it adjusts the dosage of low-salt recycled water in the second washing stage, the drying condensate flow ratio in the closed-loop heat pump drying process, the dosage of dried sludge powder, the amount of carbon dioxide gas introduced, the amount of mineralized residue returned to the molding step, and the amount of mineralized residue returned to the mineralization contact process. According to the test results after 72 hours of continuous operation, the conductivity of the mineralized permeate remained between 410 μS / cm and 620 μS / cm, the turbidity remained between 0.35 NTU and 0.58 NTU, and the chloride ion concentration remained between 105 mg / L and 132 mg / L. This indicates that the online detection and feedback adjustment method can reduce the impact of changes in the composition of different batches of seawater desalination sludge on the quality of the mineralized permeate. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the process for the seawater desalination sludge heat pump drying-mineralization and reuse method provided in an embodiment of the present invention. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. The following embodiments are used to illustrate the implementation of the present invention. Equivalent substitutions or conventional adjustments made by those skilled in the art based on the disclosure in this specification can all be considered as other implementations of the present invention.

[0032] like Figure 1As shown in the figure, this embodiment provides a method for heat pump drying-mineralization and reuse of seawater desalination sludge. This method takes seawater desalination sludge generated during the operation of seawater desalination projects as the treatment object. Through homogenization, screening and impurity removal, mechanical dewatering, graded countercurrent desalination, closed-loop heat pump drying, drying sludge crushing, grinding and grading, mineralization contact, solid-liquid separation and online detection and adjustment, the soluble salt content in the seawater desalination sludge is controlled in stages, the drying condensate can be diverted and reused according to the difference in conductivity, and the calcium and magnesium components in the seawater desalination sludge participate in the mineralization treatment of seawater desalination product water.

[0033] In this embodiment, the seawater desalination sludge includes brine softening and sedimentation sludge and membrane cleaning neutralization and sedimentation sludge. The brine softening and sedimentation sludge mainly originates from calcium- and magnesium-containing precipitates formed during the brine softening process of seawater desalination. These precipitates constitute the main source of calcium and magnesium components that can be released in the subsequent mineralization contact process. The membrane cleaning neutralization and sedimentation sludge mainly originates from precipitates formed after neutralization of the cleaning solution from reverse osmosis membranes, nanofiltration membranes, or pretreatment membrane modules. To ensure a relatively stable source of calcium and magnesium for the materials entering the subsequent mineralization contact process, representative samples of both the brine softening and sedimentation sludge and the membrane cleaning neutralization and sedimentation sludge are collected after receiving the seawater desalination sludge. The total mass concentration of calcium and magnesium ions is then detected using an acid leaching test solution. Specifically, the collected sludge samples are dried and then leached with hydrochloric acid solution. After filtration, an acid leaching test solution is obtained, and the total mass concentration of calcium and magnesium ions in the acid leaching test solution is then detected. The preset mineralization feed limit can be determined based on the total hardness target of the mineralized water, for example, set to 300 mg / L to 1000 mg / L. When the total mass concentration of calcium and magnesium ions in the acid leaching test solution corresponding to a batch of seawater desalination sludge is lower than the preset mineralization feed limit, the batch of seawater desalination sludge is sent to the non-mineralized sludge treatment process; when the total mass concentration of calcium and magnesium ions in the acid leaching test solution corresponding to the batch of seawater desalination sludge reaches the preset mineralization feed limit, the batch of seawater desalination sludge is used as the subsequent treatment target for homogenization, screening and impurity removal, and mechanical dewatering. The non-mineralized sludge treatment process can be a general sludge dewatering and off-site transportation process, a solidification process, or other sludge disposal processes that do not enter the mineralization contact process. In this way, sludge with excessively low calcium and magnesium content can be avoided from entering the heat pump drying and mineralization contact processes, thus stabilizing the composition of mineralized raw materials from the source.

[0034] In this embodiment, the brine softening and settling sludge and the membrane cleaning neutralization and settling sludge are blended according to the total calcium and magnesium content test results to ensure that the blended seawater desalination sludge has a suitable calcium and magnesium content. The blended seawater desalination sludge is first homogenized by low-speed stirring to ensure uniform mixing of sludge from different sources. It is then screened to remove shell fragments, sand, plastic fragments, and other impurities unsuitable for subsequent drying and mineralization processes. The screened seawater desalination sludge undergoes mechanical dewatering, which can be achieved using plate and frame filter press, belt filter press, or centrifugal dewatering. In this embodiment, plate and frame filter press is used for mechanical dewatering, with the filter pressure controlled at 0.5 MPa to 0.8 MPa, resulting in dewatered sludge. This dewatered sludge still contains a certain amount of soluble chlorides and other salts, therefore it is not suitable for direct drying or mineralization.

[0035] To reduce the impact of soluble salts in seawater desalination sludge on subsequent drying condensate and mineralization permeate, this embodiment sends the dewatered sludge into a staged countercurrent desalination process. The staged countercurrent desalination process includes a first washing section and a second washing section. The second washing section uses low-salt recycled water to wash the dewatered sludge, while the first washing section uses the second washing filtrate produced in the second washing section to wash the dewatered sludge. That is, the high-cleanliness, low-salt recycled water is preferentially used in the second washing section to further desalinate the sludge that has already undergone the previous washing stage; the second washing filtrate produced in the second washing section still has a certain washing capacity and is returned to the first washing section for pre-washing the initially high-salt dewatered sludge. The first washing filtrate produced in the first washing section has a high salt content and is sent to a high-salt wastewater treatment process. The high-salt wastewater treatment process can be a concentrated brine treatment process within the plant, an evaporation and crystallization process, or other centralized high-salt wastewater treatment processes.

[0036] An intermediate filtration step is set between the first and second washing sections. After being treated in the first washing section, the dewatered sludge enters the intermediate filtration step, which yields intermediate filtrate and intermediate filter cake. The intermediate filtrate is returned to the first washing section, and the intermediate filter cake enters the second washing section. By setting an intermediate filtration step, the entrainment of high-salt liquid from the first washing section into the second washing section can be reduced, thereby reducing the consumption of low-salt recycled water in the second washing section. After the second washing section, a terminal filtration step is set, yielding terminal filtrate and terminal filter cake. Chloride ion concentration in the terminal filtrate is detected. If the chloride ion concentration in the terminal filtrate is higher than a preset desalination limit, the terminal filter cake is returned to the second washing section for further washing; if the chloride ion concentration in the terminal filtrate is not higher than the preset desalination limit, the terminal filter cake is used as salt-regulating sludge and enters the subsequent step of mixing and molding the salt-regulating sludge with the recycled molding material. In this embodiment, the preset desalination limit can be determined according to the water quality requirements of the mineralized product water, for example, set to 800 mg / L to 1500 mg / L.

[0037] After obtaining the salt-conditioning sludge, it is mixed with a remixing molding material to form molded sludge. The remixing molding material includes the mineralization residue obtained from subsequent solid-liquid separation and dried sludge powder obtained after crushing, grinding, and grading the dried sludge. The dried sludge powder used as the remixing molding material is preferably skeletal powder or dried sludge powder that has not entered the mineralization contact process. After mixing the remixing molding material with the salt-conditioning sludge, molded sludge with continuous pores can be formed. These continuous pores allow circulating air in the closed-loop heat pump drying process to pass more evenly through the interior of the molded sludge, reducing the probability of local agglomeration and surface crusting. In this embodiment, the molded sludge can be strip-shaped, sheet-shaped, or granular, preferably strip-shaped. No additional organic binder is added during the molding process to reduce the risk of introducing organic matter into the subsequent mineralization permeate.

[0038] The formed sludge enters a closed-loop heat pump drying process. This process includes a pre-drying low-temperature drying section, a mid-drying constant-temperature dehydration section, and a final low-humidity drying section. The inlet air temperature of the pre-drying low-temperature drying section is controlled at 35°C to 50°C to allow surface water and some entrained salt spray to migrate out of the formed sludge more gently. The inlet air temperature of the mid-drying constant-temperature dehydration section is controlled at 50°C to 70°C to complete the main dehydration process of the formed sludge. The inlet air temperature of the final low-humidity drying section is controlled at 40°C to 60°C, and the relative humidity of the circulating air in the final low-humidity drying section is lower than that in the mid-drying constant-temperature dehydration section. This reduces residual moisture in the dried sludge and improves the stability of subsequent crushing and grinding.

[0039] In the closed-loop heat pump drying process, the circulating air passes through a dust interception zone and a salt spray collection zone before entering the heat pump evaporation and dehumidification zone. The dust interception zone traps sludge particles carried by the circulating air, and the trapped dust is returned to the steps of sludge crushing, grinding, and grading in the drying process. The salt spray collection zone traps salt spray droplets carried by the circulating air, and the trapped liquid is returned to the first washing section. After the circulating air enters the heat pump evaporation and dehumidification zone, the moisture in it condenses to form drying condensate. In the closed-loop heat pump drying process, the difference in conductivity in the drying condensate can mainly come from salt spray droplets entrained on the surface of the formed sludge, salt-containing micro-droplets carried by the circulating air, hygroscopic entrainment of salt-containing dust, and the difference in the content of entrainment in the airflow at different drying stages. Therefore, this embodiment uses the dust interception zone, the salt spray collection zone, and a preset conductivity threshold to divert the drying condensate formed at different stages for graded reuse.

[0040] The closed-loop heat pump drying process divides the dried condensate into high-salt and low-salt condensate according to a preset conductivity threshold. Dried condensate with a conductivity greater than the preset threshold is designated as high-salt condensate, while condensate with a conductivity not greater than the preset threshold is designated as low-salt condensate. High-salt condensate is returned to the first washing section, while low-salt condensate is returned to the second washing section or sent to the mineralization contact process as low-salt recycled water. In this embodiment, the preset conductivity threshold can be set to 2500 μS / cm to 5000 μS / cm. This preset conductivity threshold can be determined based on the water quality requirements of the low-salt recycled water in the second washing section and the conductivity control requirements of the mineralization product water, ensuring that the low-salt condensate meets the water requirements of either the second washing section or the mineralization contact process. This diversion method prevents all dried condensate from indiscriminately entering the second washing section or the mineralization contact process, thereby reducing fluctuations in the conductivity of the mineralization product water.

[0041] After obtaining dried sludge through a closed-loop heat pump drying process, the dried sludge is crushed, ground, and classified to obtain dried sludge powder. Further, the dried sludge is classified into mineralized powder and skeleton powder. The mineralized powder enters the mineralization contact process, while the skeleton powder is returned as part of the back-mixing molding material to the step of mixing and molding the salt-controlled sludge with the back-mixing molding material. In this embodiment, the mineralized powder is powder that passes through a 120-mesh sieve, and the skeleton powder is powder that does not pass through the 120-mesh sieve but passes through a 40-mesh sieve. The powder that does not pass through the 40-mesh sieve is returned to the steps of crushing, grinding, and classifying the dried sludge for further grinding. Through the functional classification of mineralized powder and skeleton powder, powders with smaller particle sizes and larger specific surface areas can enter the mineralization contact process, promoting the release of calcium and magnesium components; simultaneously, powders with a certain particle skeleton function are returned to the step of mixing and molding the salt-controlled sludge with the back-mixing molding material, improving the pore structure of the molded sludge.

[0042] The mineralization contact process includes an acidification and release section and a buffer stabilization section. Dried sludge powder, low-salt condensate, desalination permeate, and carbon dioxide gas are fed into the mineralization contact process. The carbon dioxide gas can be externally supplied or be carbon dioxide-containing tail gas from the plant area after dust removal, cooling, and purification. The low-salt condensate is used to supplement the water volume in the mineralization contact process and reduce the consumption of fresh desalination permeate for pulping. Carbon dioxide gas is introduced into the acidification and release section to create a weakly acidic environment in the liquid phase, allowing the calcium and magnesium components in the dried sludge powder to enter the liquid phase. In the acidification and release section, the carbon dioxide gas dissolves into the liquid phase, creating a weakly acidic environment that causes the calcium and magnesium components in the dried sludge powder, existing in the form of carbonates, hydroxides, or mixed precipitates, to dissolve, forming a liquid phase system containing calcium and magnesium ions. In this embodiment, the pH value of the acidification and release section is controlled at 5.6 to 6.6, the stirring speed is controlled at 80 r / min to 250 r / min, and the residence time is controlled at 15 min to 60 min. A solid retention zone is set up between the acidification and ore release section and the buffer stabilization section. Unreacted dried sludge powder in the solid retention zone enters the buffer stabilization section with the liquid flow, so that the solids that have not yet fully released calcium and magnesium components can continue to participate in subsequent reactions.

[0043] The pH value of the buffer stabilization section is controlled between 7.0 and 8.4. In the buffer stabilization section, the mineralized residue returned as seed crystals is added first, and then the carbon dioxide gas flow rate is adjusted to allow the suspended solids in the mineralized mixture to form calcium-magnesium carbonate-containing solid particles before solid-liquid separation. The mineralized residue, acting as seed crystals in the buffer stabilization section, provides an attachment nucleus for the formation of calcium-magnesium carbonate solid particles and facilitates subsequent solid-liquid separation. In continuous operation, the mineralized residue returned as seed crystals is the same as that obtained from the previous cycle's solid-liquid separation; for initial startup, the mineralized residue obtained from previous trial runs or the same batch of mineralized mixture obtained after solid-liquid separation can be used as the starting seed crystals. The buffer stabilization section yields a mineralized mixture. Solid-liquid separation is then performed on the mineralized mixture to obtain mineralized permeate and mineralized residue. Solid-liquid separation can be achieved through sedimentation, filtration, ceramic membrane filtration, or centrifugation. In this embodiment, a combination of sedimentation and filtration is used. First, large suspended particles in the mineralized mixture are reduced by sedimentation, and then mineralized water is obtained by filtration.

[0044] The mineralization residue is recycled in two parts. One part is returned as remixing material to the step of mixing and molding the salt-regulating sludge with the remixing material to form molded sludge; the other part is returned as seed crystals to the mineralization contact process. When the mineralization residue is returned to the step of mixing and molding the salt-regulating sludge with the remixing material, it can improve the pore structure of the molded sludge as a solid skeleton; when it is returned to the mineralization contact process, it can participate in the formation of solid particles in the buffer stabilization section as seed crystals. Through the bidirectional recirculation of the mineralization residue, the amount of residual solids discharged can be reduced, and the material coupling relationship between the drying and mineralization stages can be enhanced. In this embodiment, the mineralization residue returned to the step of mixing and molding the salt-regulating sludge with the remixing material accounts for 45% of the total mineralization residue, the mineralization residue returned to the mineralization contact process accounts for 31% of the total mineralization residue, and the remaining mineralization residue is discharged as solids for subsequent treatment. In other embodiments, the steps of returning the mineralized residue to the salt-regulating sludge and mixing it with the remixed molding material, and the proportion of returning it to the mineralization contact process, can be adjusted according to the pore state of the molded sludge, the turbidity of the mineralized mixture, and the total hardness of the mineralized permeate.

[0045] After online testing, the mineralized wastewater is divided into qualified mineralized wastewater and wastewater awaiting reprocessing. Online testing includes pH, conductivity, total hardness, turbidity, and chloride ion detection. When the pH, conductivity, total hardness, turbidity, and chloride ion content of the mineralized wastewater all meet the preset effluent conditions, it is sent to the seawater desalination wastewater post-treatment pipeline as qualified mineralized wastewater. When any of the tested items does not meet the preset effluent conditions, it is returned to the acidification and mineral release section as wastewater awaiting reprocessing. For example, the preset effluent conditions may include: pH 7.0 to 8.5, conductivity not exceeding 800 μS / cm, total hardness 60 mg / L to 120 mg / L, turbidity not exceeding 1.0 NTU (calculated as calcium carbonate), and chloride ion concentration not exceeding 150 mg / L. The online detection results are also used to adjust the amount of low-salt recycled water added in the second washing section, the ratio of drying condensate flow in the closed-loop heat pump drying process, the amount of dried sludge powder added, the amount of carbon dioxide gas introduced, the amount of mineralized residue returned to the step of mixing and molding sludge with recycled molding material, and the amount of mineralized residue returned to the mineralization contact process. Specifically, when the chloride ion detection result is high, the amount of low-salt recycled water added in the second washing section is increased, and the proportion of high-salt condensate returned to the first washing section is increased; when the conductivity detection result is high, the proportion of low-salt condensate entering the mineralization contact process is reduced; when the total hardness detection result is low, the amount of dried sludge powder added is increased or the residence time in the acidification and mineral release section is extended; when the pH value is low, the amount of carbon dioxide gas introduced is reduced and the amount of mineralized residue returned as seed crystals is increased; when the turbidity detection result is high, the amount of mineralized residue used as seed crystals in the buffer stabilization section is increased and the solid-liquid separation time is extended.

[0046] To verify the effectiveness of this embodiment, a comparative experiment was conducted using brine softening and sedimentation sludge and membrane washing neutralization and sedimentation sludge from the same seawater desalination plant. The comparative model was set up according to the prior art described in the background section, which involves online removal and mechanical dewatering of seawater desalination sludge before subsequent treatment. No staged countercurrent desalination process, no closed-loop heat pump drying condensate diversion and reuse process based on conductivity, and no process for using dried sludge powder in the mineralization treatment of seawater desalination permeate. This embodiment employs the seawater desalination sludge heat pump drying-mineralization reuse method described in this invention. Under the experimental conditions, the preset conductivity threshold for this embodiment was 3500 μS / cm, the pH value of the acidification and mineralization release section was controlled at 6.2, the pH value of the buffer stabilization section was controlled at 7.8, and the low-salt reclaimed water in the second washing section mainly used low-salt condensate. Before the mineralized permeate entered the post-treatment pipeline, pH, conductivity, total hardness, turbidity, and chloride ion levels were measured. The above experimental data represents a set of test results under continuous operating conditions. It is possible that other numerical results may be obtained under conditions of different sources of seawater desalination sludge, different desalination rates, and different mineralization retention times. Comparison results are shown in Table 1.

[0047] Table 1 Comparison of Detection Items between Comparative Examples and Embodiments

[0048] The above experimental results show that, after adopting the method of this embodiment, the chloride ion mass fraction of the sludge before drying or subsequent treatment decreased from 3.6% to 0.82%, indicating that the staged countercurrent desalination process can reduce the soluble salt content in the sludge. After the drying condensate is diverted through a preset conductivity threshold, it can be distinguished into high-salt condensate and low-salt condensate. Among them, the low-salt condensate with a conductivity not greater than the preset conductivity threshold accounts for 68.5% of all drying condensate. The high-salt condensate is returned to the first washing section, and the low-salt condensate is returned to the second washing section or sent to the mineralization contact process, avoiding the direct entry of high-salt condensate into the mineralization contact process. After the dried sludge powder is treated by the acidification and release mineralization section and the buffer stabilization section, it can make the sludge more suitable for marine environments. The calcium and magnesium components in the desalination sludge participate in the mineralization treatment of seawater desalination permeate, resulting in a total hardness of 82 mg / L (calculated as calcium carbonate). The chloride ion concentration and turbidity of the mineralized permeate remain at low levels, indicating that a stable coordination can be formed between the staged desalination, condensate diversion, and mineralization contact processes. The recycling rate of mineralization residues reaches 76%, with the steps of returning salt-regulating sludge to mix with remixed molding material and the two parts of mineralization residues returned to the mineralization contact process constituting a two-way reflux. The relative amount of discharged solids is reduced to 24% based on a 100% comparative ratio, indicating that the two-way reflux of mineralization residues can reduce the amount of residual solids discharged and provide recyclable solid materials for the molding and mineralization processes. Based on the results of the above comparative experiments, it can be seen that under the same sludge source conditions, this embodiment can reduce the chloride ion mass fraction of sludge before drying or subsequent treatment from 3.6% to 0.82%, and make the proportion of low-salt condensate water to all drying condensate water reach 68.5%, while making the total hardness of mineralized water reach 82 mg / L, calculated as calcium carbonate. This proves that there is a continuous coordination relationship between the staged countercurrent desalination, the diversion and reuse of drying condensate water, and the mineralization contact process.

[0049] Furthermore, during a 72-hour continuous operation test, the pH of the mineralized permeate in the embodiment remained between 7.35 and 7.92, the conductivity between 410 μS / cm and 620 μS / cm, the total hardness between 75 mg / L and 90 mg / L, the turbidity between 0.35 NTU and 0.58 NTU, and the chloride ion concentration between 105 mg / L and 132 mg / L. These results indicate that online detection and feedback control can be used to adjust the dosage of low-salt recycled water in the second washing stage, the ratio of drying condensate flow, the dosage of dried sludge powder, the carbon dioxide gas injection rate, and the return rate of residual mineralized material, thereby reducing the impact of variations in the composition of different batches of seawater desalination sludge on the quality of the mineralized permeate.

[0050] Therefore, this embodiment reduces the soluble salt content in seawater desalination sludge through a graded countercurrent desalination process, dries the sludge through a closed-loop heat pump drying process and reuses the drying condensate by diverting it according to conductivity, allows calcium and magnesium components to participate in the mineralization treatment of seawater desalination product through acidification and mineral release and buffer stabilization mineralization of dried sludge powder, and forms a solid material cycle through the steps of returning the mineralized residue to salt control sludge and mixing it with the remixing molding material and the mineralization contact process. Thus, it provides a continuous treatment method for the drying, salt control and mineralization reuse of seawater desalination sludge.

[0051] It should be noted that the embodiments of the present invention have better implementability and are not intended to limit the present invention in any way. Any person skilled in the art may use the above-disclosed technical content to change or modify it into equivalent effective embodiments. However, any modifications or equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for heat pump drying-mineralization and reuse of seawater desalination sludge, characterized in that, Includes the following steps: S1. Receive seawater desalination sludge, homogenize, screen to remove impurities and mechanically dewater the seawater desalination sludge to obtain dewatered sludge; S2. The dewatered sludge is fed into a graded countercurrent desalination process, which includes a first washing section and a second washing section. In the second washing section, the dewatered sludge is washed with low-salt recycled water. In the first washing section, the dewatered sludge is washed with the second washing filtrate generated in the second washing section. The first washing filtrate generated in the first washing section is then fed into a high-salt wastewater treatment process to obtain salt-controlled sludge. S3. The salt-regulating sludge is mixed with the remixing molding material to form shaped sludge, and the shaped sludge is sent to a closed-loop heat pump drying process. The closed-loop heat pump drying process divides the drying condensate into high-salt condensate and low-salt condensate according to a preset conductivity threshold. The drying condensate with a conductivity greater than the preset conductivity threshold is the high-salt condensate, and the drying condensate with a conductivity not greater than the preset conductivity threshold is the low-salt condensate. The high-salt condensate is returned to the first washing section, and the low-salt condensate is returned to the second washing section or sent to the mineralization contact process as low-salt recycled water. S4. The dried sludge obtained from the closed-loop heat pump drying process is crushed, ground and classified to obtain dried sludge powder. S5. The dried sludge powder, the low-salt condensate, the seawater desalination product water, and the carbon dioxide gas are fed into the mineralization contact process. The mineralization contact process includes an acidification and release section and a buffer stabilization section. In the acidification and release section, the carbon dioxide gas is introduced to allow the calcium and magnesium components in the dried sludge powder to enter the liquid phase. In the buffer stabilization section, the remaining mineralization material as seed crystals is added and the amount of carbon dioxide gas introduced is adjusted to obtain a mineralization mixture. S6. Perform solid-liquid separation on the mineralized mixture to obtain mineralized water and the remaining mineralized material. Return a portion of the remaining mineralized material to step S3 as the remixing molding material, and return the other portion of the remaining mineralized material to step S5 as seed crystals.

2. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 1, characterized in that, In step S1, the seawater desalination sludge includes brine softening and sedimentation sludge and membrane cleaning and neutralization sedimentation sludge. The brine softening and sedimentation sludge and the membrane cleaning and neutralization sedimentation sludge are mixed according to the total calcium and magnesium content test results and then enter the mechanical dewatering process.

3. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 2, characterized in that, The total calcium and magnesium content detection results are obtained through acid leaching test solution. When the total mass concentration of calcium and magnesium ions in the acid leaching test solution is lower than the preset mineralization feed limit, the corresponding batch of seawater desalination sludge is sent to the non-mineralized sludge treatment process.

4. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 1, characterized in that, In step S2, an intermediate pressure filtration step is set between the first washing section and the second washing section. The intermediate pressure filtration step is performed on the sludge treated by the first washing section. The intermediate filtrate obtained by the intermediate pressure filtration step is returned to the first washing section, and the intermediate filter cake obtained by the intermediate pressure filtration step enters the second washing section.

5. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 4, characterized in that, In step S2, a terminal pressure filtration step is set after the second washing section. The terminal filtrate obtained by the terminal pressure filtration step is subjected to chloride ion detection. When the chloride ion mass concentration in the terminal filtrate is higher than the preset desalination limit, the terminal filter cake obtained by the terminal pressure filtration step is returned to the second washing section for washing again.

6. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 1, characterized in that, In step S3, the remixed molding material includes the mineralized residue obtained in step S6 and the dried sludge powder obtained in step S4. The remixed molding material is mixed with the salt-controlled sludge to form the molded sludge containing continuous pores.

7. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 1, characterized in that, In step S3, the closed-loop heat pump drying process includes a front-end low-temperature drying section, a middle-end constant-temperature dehydration section, and a terminal low-humidity drying section. The dried condensate generated in the front-end low-temperature drying section with a conductivity greater than the preset conductivity threshold is used as the high-salt condensate. The dried condensate generated in the middle-end constant-temperature dehydration section and the terminal low-humidity drying section with a conductivity not greater than the preset conductivity threshold is used as the low-salt condensate.

8. The seawater desalination sludge heat pump drying-mineralization and reuse method according to claim 7, characterized in that, The inlet air temperature of the front low-temperature drying section is 35°C to 50°C, the inlet air temperature of the middle constant-temperature dehydration section is 50°C to 70°C, and the inlet air temperature of the terminal low-humidity drying section is 40°C to 60°C. The relative humidity of the circulating air in the terminal low-humidity drying section is lower than that of the circulating air in the middle constant-temperature dehydration section.

9. The method for heat pump drying-mineralization and reuse of seawater desalination sludge according to claim 1, characterized in that, In step S3, the closed-loop heat pump drying process includes a heat pump evaporation dehumidification zone. Before entering the heat pump evaporation dehumidification zone, the circulating air in the closed-loop heat pump drying process passes through a dust interception zone and a salt spray collection zone in sequence. The dust intercepted by the dust interception zone is returned to step S4, and the liquid intercepted by the salt spray collection zone is returned to the first washing section.

10. The method for heat pump drying-mineralization and reuse of seawater desalination sludge according to claim 1, characterized in that, In step S4, the dried sludge is classified into mineralized powder and skeleton powder. The mineralized powder enters step S5, and the skeleton powder is returned to step S3 as part of the remixed molding material.

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