Treatment system and method for zero discharge of photovoltaic wastewater containing potassium coupled with extraction of potassium glauber's salt

Abstract

The present application belongs to the technical field of industrial wastewater treatment and resource utilization, and discloses a treatment system and method for zero discharge of photovoltaic wastewater containing potassium coupled with extraction of potassium mirabilite. After being pretreated by a wastewater treatment facility, the photovoltaic wastewater is treated by a first hard removal unit, a biological treatment unit, a first silicon and fluorine removal unit, and then enters a first concentration unit. The first concentrated water is treated by a second fluorine removal unit, a second silicon and hard removal unit, a deep softening and decarburization unit, and then enters a second concentration unit. The second concentrated water enters an evaporation crystallization unit. The water produced by the first and second concentration units and the condensed water produced by the evaporation crystallization unit are mixed and used for enterprise production. The chemical sludge and biological sludge produced by each treatment unit are dewatered by a sludge dewatering unit and then transported for disposal. The by-products of the evaporation crystallization unit are potassium mirabilite, sodium chloride and miscellaneous salt. The present application couples the extraction and purification of potassium mirabilite, and simultaneously realizes the recycling of wastewater, the high-value conversion of by-product salt and the reduction of solid waste.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment and resource utilization technology. Specifically, it relates to a treatment system and method for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater. Background Technology

[0002] Wastewater from the photovoltaic industry primarily originates from the cell manufacturing process and is characterized by large discharge volumes, concentrated flow, high salt content, high fluoride content, high total silicon content, and high hardness. Some photovoltaic cell manufacturers use large amounts of potassium hydroxide during silicon wafer cleaning, resulting in high concentrations of potassium ions in their wastewater. Potash fertilizer is one of the three essential fertilizers for agricultural production. Extracting potassium resources from mixed salts through technological innovation, and achieving the cascaded utilization and recycling of potassium, is an important way to address resource constraints and promote the green development of the potash fertilizer industry.

[0003] Most existing zero-discharge and resource recovery projects for potassium-containing wastewater recover potassium resources in the form of potassium chloride, often employing a combination of membrane and thermal separation processes. These processes are lengthy and energy-intensive. Sulfate ions in the wastewater are mostly discharged as calcium sulfate or mixed salts, resulting in high solid waste production and low economic value of byproducts. For zero-discharge applications of potassium-containing photovoltaic wastewater, there is an urgent need for short-process treatment systems and methods that can reduce solid waste production and increase the economic value of byproducts, aiming to lower overall operating costs. Summary of the Invention

[0004] This invention focuses on solving the technical problems related to zero discharge and resource utilization of potassium-containing photovoltaic wastewater, and provides a treatment system and method for zero discharge of potassium-containing photovoltaic wastewater coupled with the extraction of potassium sulfate. This invention achieves zero discharge and recycling of wastewater while recovering potassium and sulfate ions from the water, forming potassium sulfate with high economic value, which can be used as a raw material for the production of compound fertilizers or potassium sulfate. Simultaneously, most of the calcium ions in the wastewater are solidified into calcium carbonate particles for power plant desulfurization, realizing the reduction and resource utilization of solid waste.

[0005] To solve the above-mentioned technical problems, the present invention is implemented through the following technical solution:

[0006] According to one aspect of the present invention, a treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater is provided, comprising a primary hardening removal unit, a biological treatment unit, a primary silicon and fluoride removal unit, a primary concentration unit, a secondary fluoride removal unit, a secondary silicon and hardening removal unit, a deep softening and decarbonization unit, a secondary concentration unit, an evaporation and crystallization unit, and a sludge dewatering unit.

[0007] The production wastewater discharged from the photovoltaic enterprise's wastewater discharge outlet is treated by the primary hardening removal unit, the biological treatment unit, and the primary silicon and fluoride removal unit before entering the primary concentration unit. The primary concentrate from the primary concentration unit is treated by the secondary fluoride removal unit, the secondary silicon and hardening removal unit, and the deep softening and decarbonization unit before entering the secondary concentration unit. The secondary concentrate from the secondary concentration unit enters the evaporation and crystallization unit. The permeate from the primary and secondary concentration units and the condensate from the evaporation and crystallization unit are mixed and used as production water for the photovoltaic enterprise. The byproducts of the evaporation and crystallization unit are potassium sulfate, sodium chloride, and miscellaneous salts. The calcium carbonate granules, a byproduct of the primary hardening removal unit, are used for power plant desulfurization. The chemical sludge from the primary hardening removal unit, the primary silicon and fluoride removal unit, the secondary fluoride removal unit, and the secondary silicon and hardening removal unit, as well as the residual sludge from the biological treatment unit, are dewatered by the sludge dewatering unit and then transported for disposal.

[0008] Furthermore, the primary hardening unit includes a homogenization conditioning tank, a granulation fluidized bed, and a turbidity removal sedimentation tank; sodium hydroxide, sodium carbonate, and crystal nuclei are added to the granulation fluidized bed to form calcium carbonate particles; ferric chloride or ferric sulfate, anionic polyacrylamide, hydrochloric acid, or sulfuric acid are added to the turbidity removal sedimentation tank.

[0009] Furthermore, the biological treatment unit includes a denitrifying biofilter and a nitrifying biofilter; wherein sodium acetate or methanol is added to the denitrifying biofilter as a supplementary carbon source.

[0010] Furthermore, the primary silicon and fluoride removal unit includes a silicon removal precipitation tank and a fluoride removal precipitation tank; sodium aluminate, hydrochloric acid or sulfuric acid, ferric chloride or ferric sulfate, and anionic polyacrylamide are added to the reaction zone of the silicon removal precipitation tank; aluminum sulfate or aluminum chloride, sodium hydroxide, and anionic polyacrylamide are added to the reaction zone of the fluoride removal precipitation tank.

[0011] Furthermore, the primary concentration unit includes an ultrafiltration device and a reverse osmosis device.

[0012] Furthermore, the secondary defluorination unit employs a defluorination sedimentation tank, and calcium chloride or calcium carbonate, sodium dihydrogen phosphate or potassium dihydrogen phosphate, ferric sulfate or ferric chloride, sodium hydroxide, and anionic polyacrylamide are added.

[0013] Furthermore, the secondary silicon and hardening removal unit includes a silicon and hardening removal sedimentation tank and an ultrafiltration device; magnesium chloride or magnesium sulfate, sodium hydroxide, sodium carbonate, ferric sulfate or ferric chloride, and anionic polyacrylamide are added to the reaction zone of the silicon and hardening removal sedimentation tank; the ultrafiltration device uses an immersed PTFE hollow fiber membrane.

[0014] Furthermore, the deep softening and decarbonization unit includes an ion exchange device and a decarbonization device; the ion exchange device uses chelating resin to achieve deep softening, and sodium hydroxide and hydrochloric acid are used as regeneration agents; the decarbonization device adds hydrochloric acid or sulfuric acid to remove alkalinity from the wastewater by stripping.

[0015] Furthermore, the secondary concentration unit includes two reverse osmosis devices, one of which is a fouling-resistant high-pressure spiral reverse osmosis device, and the other is a butterfly reverse osmosis device.

[0016] Furthermore, the evaporation crystallization unit includes a feeding and mixing device, an MVR evaporation and concentration device, a potassium sulfate extraction device, a sodium chloride extraction device, and a miscellaneous salt drying device; the feeding and mixing device mainly includes a homogenizing tank and a sodium sulfate or potassium chloride mixing and dosing device; the potassium sulfate extraction device includes a cooling crystallization device and a dehydration and drying device; the sodium chloride extraction device includes an evaporation crystallization device and a dehydration and drying device; and the miscellaneous salt drying device includes an evaporation device and a drying device.

[0017] Furthermore, the sludge dewatering unit includes a sludge thickening device and a sludge dewatering device.

[0018] According to another aspect of the present invention, a method for treating potassium-containing photovoltaic wastewater with zero discharge coupled extraction of potassium mirabilite is provided, based on the above-mentioned treatment system for potassium-containing photovoltaic wastewater with zero discharge coupled extraction of potassium mirabilite, comprising the following steps:

[0019] Step 1: Wastewater from the photovoltaic enterprise's main wastewater discharge outlet undergoes hardening and turbidity removal treatment in the primary hardening unit, achieving the following effluent quality requirements: Ca 2+ ≤50mg / L, SS≤20mg / L; the particle size of the produced calcium carbonate particles is controlled within 1~3mm, and the purity is ≥90%;

[0020] Step 2: The wastewater from the primary hardening unit is treated by the biological treatment unit to remove organic matter, ammonia nitrogen and nitrate nitrogen. The effluent quality meets the following requirements: COD ≤30mg / L, ammonia nitrogen ≤3mg / L, total nitrogen ≤10mg / L.

[0021] Step 3: The wastewater from the biological treatment unit is passed through the primary silicon and fluoride removal unit to remove total silicon and fluoride, and simultaneously remove some organic matter. The effluent quality meets the following requirements: total silicon ≤ 20 mg / L, fluoride ≤ 2 mg / L, COD ≤ 20 mg / L.

[0022] Step 4: The wastewater from the primary silicon and fluoride removal unit is passed through the primary concentration unit to concentrate the salt and impurities to primary concentrate. The primary concentrate is used for production water in photovoltaic enterprises, with a concentration ratio ≥10 and a desalination rate ≥95%.

[0023] Step 5: The primary concentrate from the primary concentration unit is passed through the secondary defluorination unit to remove fluoride and suspended solids from the wastewater. The effluent quality meets the following requirements: fluoride ≤ 5 mg / L, SS ≤ 20 mg / L.

[0024] Step 6: The wastewater from the secondary defluorination unit is passed through the secondary silicon and hardness removal unit to remove total silicon, hardness, and turbidity, and the effluent quality meets the following requirements: Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, total silicon ≤20mg / L, turbidity ≤1NTU, SDI ≤5;

[0025] Step 7: The wastewater from the secondary silicon and hardness removal unit is further treated by the deep softening and decarbonization unit to remove hardness and alkalinity, and the effluent quality meets the following requirements: Ca 2+ ≤2mg / L, Mg 2+ ≤2mg / L, alkalinity (calculated as calcium carbonate) ≤50mg / L; pH controlled between 6.5 and 7.0;

[0026] Step 8: Wastewater from the deep softening and decarbonization unit is passed through the secondary concentration unit to concentrate salts and impurities into secondary concentrate. The secondary concentrate is used for production water in photovoltaic enterprises. The salt content of the secondary concentrate reaches 10-12%, and the desalination rate is ≥97%.

[0027] Step 9: The secondary concentrate from the secondary concentration unit enters the evaporation and crystallization unit, where the salts are finally formed into potassium sulfate, sodium chloride, and miscellaneous salts. The condensate is used for production water in photovoltaic enterprises; the potassium ion capture rate is ≥90%, the purity of the by-product potassium sulfate is ≥39% (calculated as K2O), and the purity of the by-product sodium chloride is ≥97.5%.

[0028] Step 10: The sludge from the primary hardening unit, biological treatment unit, primary silicon and fluoride removal unit, secondary fluoride removal unit, and secondary silicon and hardening removal unit is dewatered to a moisture content of ≤60% by the sludge dewatering unit and then transported off-site for disposal.

[0029] The beneficial effects of this invention are:

[0030] (i) The treatment system and method of the present invention, while realizing the recycling of industrial wastewater, recover potassium and sulfate ions in the wastewater to form potassium mirabilite with high economic value. This product can be used as an industrial raw material in the potassium sulfate and compound fertilizer industries, partially offsetting the treatment costs.

[0031] (ii) The treatment system and method of the present invention extract most of the calcium ions in the form of calcium carbonate particles for power plant desulfurization, thereby reducing the scale of general solid waste treatment and disposal, and realizing the resource utilization and recycling of calcium salts.

[0032] (III) The treatment system and method of the present invention, by adjusting the type and amount of reagents in each treatment unit, achieves targeted regulation of the concentration ratio of potassium ions and sulfate ions in wastewater, and can effectively control the purity and production capacity of by-product potassium sulfate and by-product sodium chloride.

[0033] (iv) The processing system and method of the present invention repeatedly use submerged ultrafiltration instead of sand filtration and pressure ultrafiltration; the reverse osmosis device of the first-stage concentration unit adopts a three-stage arrangement, and the reverse osmosis device of the second-stage concentration unit adopts a two-stage arrangement, which simplifies the two sets of pretreatment devices and shortens the overall process route to the greatest extent.

[0034] (v) The processing system and method of the present invention uses an immersed PTFE ultrafiltration membrane to remove suspended solids in the secondary silicon and hardening unit, which can withstand long-term high pH operation, reduces the excess coefficient of hardening agent in the secondary silicon and hardening unit, and thus reduces agent consumption.

[0035] (vi) The treatment system and method of the present invention, taking into account the low magnesium content of photovoltaic wastewater, adopts sodium aluminate desiliconization process in the first-stage silicon and fluoride removal unit, which minimizes the salt increase caused by the silicon removal process and reduces the overall treatment cost.

[0036] (vii) The treatment system and method of the present invention fully consider the synergistic removal function of each treatment unit for different pollutants, shorten the overall process flow, and avoid repeated addition of reagents. For example, the primary silicon and fluoride removal unit considers the use of aluminum salts to remove total silicon and fluorides while simultaneously removing some organic matter; the secondary silicon and fluoride removal unit utilizes the synergistic removal function of magnesium in the primary concentrate for total silicon, etc. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the structure of the potassium-containing photovoltaic wastewater zero-discharge coupled extraction potassium mirabilite treatment system provided by the present invention.

[0038] In the above diagram: 01, primary hardening unit; 02, biological treatment unit; 03, primary silicon and fluoride removal unit; 04, primary concentration unit; 05, secondary fluoride removal unit; 06, secondary silicon and hardening removal unit; 07, deep softening and decarbonization unit; 08, secondary concentration unit; 09, evaporation and crystallization unit; 10, sludge dewatering unit. Detailed Implementation

[0039] To further understand the invention's content, features, and effects, the following embodiments are provided, along with detailed descriptions in conjunction with the accompanying drawings:

[0040] like Figure 1As shown, this embodiment provides a treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater, including a primary hardening unit 01, a biological treatment unit 02, a primary silicon and fluoride removal unit 03, a primary concentration unit 04, a secondary fluoride removal unit 05, a secondary silicon and hardening removal unit 06, a deep softening and decarbonization unit 07, a secondary concentration unit 08, an evaporation and crystallization unit 09, and a sludge dewatering unit 10.

[0041] The treatment system provided by this invention can achieve targeted regulation of the ratio of potassium ions and sulfate ions in wastewater by rationally selecting the types of reagents in each treatment unit according to the ionic components of the wastewater to be treated. This allows for control of the yield and purity of by-product salts potassium sulfate and sodium chloride, reducing the yield of miscellaneous salts and saving operating costs.

[0042] The influent to the primary hardening unit 01 comes from the photovoltaic enterprise's main wastewater discharge outlet. After internal treatment by the photovoltaic enterprise, the wastewater, meeting industry emission standards, sequentially passes through three sets of devices included in the primary hardening unit 01: a homogenization and equalization tank, a granulation fluidized bed, and a turbidity removal sedimentation tank. This process minimizes the calcium ion and suspended solids content of the wastewater, creating conditions for subsequent biological treatment and concentration. Simultaneously, most of the calcium ions in the wastewater are extracted in the form of calcium carbonate granules. These granules are used for desulfurization in power plants, realizing the resource utilization and recycling of calcium salts, while significantly reducing sludge production.

[0043] The primary hardness removal unit 01 consists of a homogenization equalization tank, a granulation fluidized bed, and a turbidity removal sedimentation tank. The homogenization equalization tank has a residence time controlled at 4–8 hours and is equipped with water homogenization equipment to reduce the impact of water quality and quantity fluctuations on subsequent treatment systems. The granulation fluidized bed operates with a pH controlled at 10.3–10.5 and an upflow velocity controlled at 60–100 m / h. Sodium hydroxide, sodium carbonate, and crystal nuclei are added to form calcium carbonate particles with a particle size controlled at 1–3 mm and a purity ≥90%. The turbidity removal sedimentation tank includes four parts: a mixing zone, a flocculation zone, an inclined plate sedimentation zone, and a product water neutralization zone. The residence time in the mixing zone is ≥10 min, the residence time in the flocculation zone is ≥20 min, and the surface loading of the sedimentation zone is ≤8 m². 3 / m 2 • h, residence time in the neutralization zone of the product water ≥ 5 min, add ferric chloride or ferric sulfate, anionic polyacrylamide, hydrochloric acid or sulfuric acid.

[0044] The influent to biological treatment unit 02 comes from the effluent of the sedimentation tank in the primary hardness removal unit 01. After biological treatment, organic matter, ammonia nitrogen, and nitrate nitrogen are removed from the wastewater. Biological treatment unit 02 includes two sets of devices: a denitrifying biological filter and a nitrifying biological filter, equipped with a permeate recirculation system. The denitrification load is controlled at 0.5–1.0 kg NO3-N / m³. 3 • nitrification load is controlled at 0.3–0.5 kg NH3-N / m³. 3•d, organic matter load is controlled at 0.8–1.5 kg BOD5 / m³ 3 • The permeate recirculation ratio is controlled at 100–200%, and the filter media depth is controlled at 3.5–4 meters. Sodium acetate or methanol is added to the influent of the denitrification biological filter as a supplementary carbon source.

[0045] The influent to the primary silicon and fluoride removal unit 03 comes from the permeate from the biological treatment unit 02. The wastewater sequentially passes through a silicon removal sedimentation tank and a fluoride removal sedimentation tank to remove total silicon and fluoride, while simultaneously removing some organic matter. The primary silicon and fluoride removal unit 03 consists of a silicon removal sedimentation tank and a fluoride removal sedimentation tank. The residence time in the silicon removal sedimentation tank reaction tank is ≥10 min, the residence time in the adsorption zone is ≥20 min, the residence time in the flocculation zone is ≥20 min, and the surface loading of the sedimentation zone is ≤8 m³. 3 / m 2 • h; During operation, the pH should be controlled at 8-9, and sodium aluminate, hydrochloric acid or sulfuric acid, ferric chloride or ferric sulfate, and anionic polyacrylamide should be added. The weight ratio of sodium aluminate to total silica in the influent should be controlled at 1.8-2.2. The residence time in the defluorination sedimentation tank reaction tank should be ≥10 min, the residence time in the adsorption zone ≥20 min, the residence time in the flocculation zone ≥20 min, and the surface loading rate in the sedimentation zone ≤8 m². 3 / m 2 • h; During operation, the pH should be controlled at 6-7, and aluminum sulfate or aluminum chloride, sodium hydroxide, and anionic polyacrylamide should be added. The weight ratio of aluminum to fluorine should be controlled at 8-10.

[0046] The influent to the primary concentration unit 04 comes from the permeate from the primary silicon and fluoride removal unit 03. The wastewater is then concentrated via an ultrafiltration unit and a reverse osmosis unit. The primary permeate is used for production in the photovoltaic company, while the primary concentrate is discharged downstream for further treatment. The ultrafiltration unit uses a submerged PVDF hollow fiber ultrafiltration membrane with an operating flux ≥30 L / m³. 2 • h, permeate rate ≥ 90%, effluent SDI ≤ 3; reverse osmosis unit adopts a single-stage three-section layout, concentration ratio ≥ 10, operating flux 17~20 L / m 2 ·h.

[0047] The influent to the secondary defluoridation unit 05 comes from the primary concentrate discharged from the primary concentration unit 04, and is used to remove fluoride from the primary concentrate. The secondary defluoridation unit 05 employs a defluoridation sedimentation tank, with residence times in both mixing zones ≥10 min, the reaction zone ≥120 min, the flocculation zone ≥20 min, and the sedimentation zone surface loading ≤8 m³ / s. 3 / m 2 • During operation, the pH should be controlled at 6.5–8, and calcium chloride or calcium carbonate, sodium dihydrogen phosphate or potassium dihydrogen phosphate, ferric sulfate or ferric chloride, sodium hydroxide, and anionic polyacrylamide should be added. The phosphorus-fluorine molar ratio should be controlled at 5–6, and the calcium-fluorine molar ratio should be controlled at 14–16.

[0048] The influent to the secondary silica and hardness removal unit 06 comes from the effluent of the secondary fluoride removal unit 05, and is used to remove total silica, hardness, and suspended solids from the primary concentrate. The secondary silica and hardness removal unit 06 sequentially includes a silica and hardness removal sedimentation tank and an ultrafiltration device. The residence time in both mixing zones of the silica and hardness removal sedimentation tank is ≥10 min, the residence time in both reaction zones is ≥20 min, the residence time in the flocculation zone is ≥20 min, and the surface loading of the sedimentation zone is ≤8 m³. 3 / m 2 During operation, the pH is maintained between 10.8 and 11.2. Magnesium chloride or magnesium sulfate, sodium hydroxide, sodium carbonate, ferric sulfate or ferric chloride, and anionic polyacrylamide are added, with the magnesium-to-silicon weight ratio controlled between 3 and 5. The ultrafiltration unit uses a submerged PTFE hollow fiber membrane. During operation, the pH is maintained between 10.8 and 11.2, and the operating flux is ≥50 L / m³. 2 ·h.

[0049] The influent to the deep softening and decarbonization unit 07 comes from the effluent of the secondary silica and hardness removal unit 06. In the deep softening and decarbonization unit 07, the water undergoes sequential ion exchange and decarbonization treatment to further remove hardness and alkalinity. The regenerated wastewater from the deep softening process is returned to the secondary silica and hardness removal unit 06. The deep softening and decarbonization unit 07 consists of an ion exchange device and a decarbonization treatment device. The ion exchange device uses chelating resin to achieve deep softening, with the operating pH controlled between 10.8 and 11.2. The regeneration agents used are sodium hydroxide and hydrochloric acid. The decarbonization device removes alkalinity from the wastewater by adding hydrochloric acid or sulfuric acid followed by stripping, with the effluent pH controlled between 6.5 and 7.0.

[0050] The influent to the secondary concentration unit 08 comes from the effluent of the deep softening and decarbonization unit 07. The wastewater passes through two reverse osmosis concentration stages. The product water is used for photovoltaic production, while the concentrated water enters the evaporation and crystallization unit 09. The secondary concentration unit 08 comprises two reverse osmosis concentration stages, one of which uses a fouling-resistant high-pressure spiral wound reverse osmosis membrane, with an operating flux controlled at 10–15 L / m³. 2 •h, concentration ratio 1.5–2.0; preferably, the anti-fouling high-pressure spiral wound reverse osmosis membrane is a high-pressure spiral wound reverse osmosis membrane with wide flow channels and strong anti-fouling performance, more preferably, it is an anti-fouling high-pressure spiral wound reverse osmosis membrane with a flow channel width of 34mil and a hydrophilic coating on the surface, with an allowable operating pressure of 2.5–6.0 MPa; the two-stage reverse osmosis unit adopts a butterfly reverse osmosis, and the operating flux is controlled at 10–15 L / m 2 •h, concentration ratio 1.5 to 2.0.

[0051] The feed for evaporation crystallization unit 09 comes from the secondary concentrate produced by the secondary concentration unit. Salts in the wastewater are discharged from the system in the form of potassium sulfate, sodium chloride, and other salts. The condensate is mixed with the permeate from the primary and secondary concentration units and then used for production water in the photovoltaic enterprise. The evaporation crystallization unit achieves a potassium ion capture rate of ≥90%, with potassium sulfate purity ≥39% (based on K2O) and sodium chloride purity ≥97.5%. Evaporation crystallization unit 09 includes a feed mixing device, an MVR evaporation concentration device, a potassium sulfate extraction device, a sodium chloride extraction device, and a other salt drying device. The feeding and mixing unit mainly includes a homogenizing tank and equipment for mixing and adding sodium sulfate or potassium chloride. The amount of sodium sulfate or potassium chloride to be added is determined based on the potassium ion and sulfate ion content in the feed, aiming to ensure that the molar ratio of potassium ions to sulfate ions in the material entering the MVR evaporation and concentration unit is controlled at 1.5 to 1.6. The potassium sulfate preparation unit includes three sets of equipment: cooling crystallization, concentration and dehydration, and drying. The controlled temperature for cooling crystallization is 25 to 30°C. The sodium chloride extraction unit includes three sets of equipment: evaporation crystallization, concentration and dehydration, and drying. Most of the crystallization mother liquor is returned to the feeding and mixing unit to improve the potassium recovery rate, and a small amount enters the miscellaneous salt treatment unit. The miscellaneous salt drying unit includes two sets of equipment: evaporation and drying.

[0052] The feed for sludge dewatering unit 10 comes from the chemical sludge discharged from the primary hardening removal unit 01, the primary silicon and fluoride removal unit 03, the secondary fluoride removal unit 05, and the secondary silicon and hardening removal unit 06, as well as the excess sludge discharged from the biological treatment unit 02. The treated sludge cake is transported off-site for disposal, while the dewatering liquid and concentrated supernatant are returned to the primary hardening removal unit 01. Sludge dewatering unit 10 sequentially includes a sludge thickening device and a sludge dewatering device. The sludge thickening device uses a gravity thickening tank, with the solids load controlled at 30–60 kgDS / m³. 2 •d; The dewatering device adopts a high-pressure diaphragm plate and frame dewatering equipment with a pressing capacity of 2-4 kgDS / m³. 2 The operating cycle is 1.5 to 4 hours, and the pressing pressure is 2.0 to 2.5 MPa.

[0053] Based on the above-mentioned treatment system, the present invention also provides a method for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater, comprising the following steps:

[0054] Step 1: Wastewater from the photovoltaic enterprise's main wastewater discharge outlet undergoes primary hardening and turbidity removal treatment in the first-stage hardening unit 01, achieving the following effluent quality requirements: Ca 2+ ≤50mg / L, SS≤20mg / L; the particle size of the produced calcium carbonate particles is controlled between 1 and 3mm, and the purity is ≥90%. In this invention, SS represents suspended solids.

[0055] Step 2: Wastewater from the primary hardening unit 01 is treated by biological treatment unit 02 to remove organic matter, ammonia nitrogen and nitrate nitrogen. The effluent quality meets the following requirements: COD≤30mg / L, ammonia nitrogen≤3mg / L, total nitrogen≤10mg / L.

[0056] Step 3: Wastewater from biological treatment unit 02 passes through primary silicon and fluoride removal unit 03 to remove total silicon and fluoride, while simultaneously removing some organic matter. The effluent quality meets the following requirements: total silicon ≤ 20 mg / L, fluoride ≤ 2 mg / L, COD ≤ 20 mg / L.

[0057] Step 4: Wastewater from the primary silicon and fluoride removal unit 03 is concentrated in the primary concentration unit 04 to concentrate salts and impurities to primary concentrate. The primary concentrate is used for photovoltaic production water, with a concentration ratio ≥10 and a desalination rate ≥95%.

[0058] Step 5: The primary concentrate from the primary concentration unit 04 passes through the secondary defluorination unit 05 to remove fluoride and suspended solids from the wastewater. The effluent quality meets the following requirements: fluoride ≤ 5 mg / L, SS ≤ 20 mg / L.

[0059] Step 6: Wastewater from the secondary defluorination unit 05 passes through the secondary silicon and hardness removal unit 06 to remove total silicon, hardness, and turbidity. The effluent quality meets the following requirements: Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, total silicon ≤20mg / L, turbidity ≤1NTU, SDI ≤5.

[0060] Step 7: Wastewater from the secondary silicon and hardness removal unit 06 is further treated by the deep softening and decarbonization unit 07 to remove hardness and alkalinity, achieving the following effluent quality requirements: Ca 2+ ≤2mg / L, Mg 2+ ≤2mg / L, alkalinity (calculated as calcium carbonate) ≤50mg / L, pH controlled at 6.5~7.0;

[0061] Step 8: Wastewater from the deep softening and decarbonization unit 07 is further concentrated in the secondary concentration unit 08 to concentrate salts and impurities to secondary concentrate. The secondary concentrate is used for production water in photovoltaic enterprises. The salt content of the secondary concentrate reaches 10-12%, and the desalination rate is ≥97%.

[0062] Step 9: The secondary concentrate from the secondary concentration unit 08 enters the evaporation and crystallization unit 09, where the salts are finally formed into potassium sulfate, sodium chloride, and other salts. The condensate is used for photovoltaic production. The potassium ion capture rate is ≥90%, the purity of the by-product potassium sulfate is ≥39% (based on K2O), and the purity of the by-product sodium chloride is ≥97.5%.

[0063] Step 10: The sludge from the primary hardening unit 01, biological treatment unit 02, primary silicon and fluoride removal unit 03, secondary fluoride removal unit 05, and secondary silicon and hardening removal unit 06 is dewatered to a moisture content of ≤60% by the sludge dewatering unit 10 and then transported off-site for disposal.

[0064] The above steps, when run continuously, can achieve the recycling and zero discharge of photovoltaic wastewater. At the same time, potassium and sulfate ions in the wastewater are recovered in the form of potassium sulfate, and most of the calcium ions in the wastewater are recovered in the form of calcium carbonate particles. This achieves high-value conversion of by-product salts and reduction and partial resource utilization of general solid waste, thereby significantly reducing the overall operating cost.

[0065] The following experimental examples further illustrate the content and effects of the potassium-containing photovoltaic wastewater zero-discharge coupled extraction potassium mirabilite treatment system and method provided by this invention:

[0066] Experimental Example 1:

[0067] A photovoltaic cell manufacturer discharges 30,000 tons of wastewater per day. The wastewater quality is shown in the table below:

[0068]

[0069] The company uses a large amount of KOH in its battery cell processing, resulting in a high potassium ion content in its wastewater, with a potassium to sulfate molar ratio of approximately 3.7. Its zero-emission system adopts the treatment system and treatment method provided by this invention. The types of reagents used in each treatment unit are as follows: Sodium hydroxide, sodium carbonate, and crystal nuclei are added to the granulation fluidized bed in the primary hardening unit 01, and ferric sulfate, anionic polyacrylamide, and sulfuric acid are added to the turbidity removal sedimentation tank; methanol is added to the biological treatment unit 02; sodium aluminate, ferric sulfate, sulfuric acid, and anionic polyacrylamide are added to the silicon removal sedimentation tank in the primary silicon and fluoride removal unit 03, and aluminum sulfate, ferric sulfate, sodium hydroxide, and anionic polyacrylamide are added to the fluoride removal sedimentation tank; calcium chloride, sodium dihydrogen phosphate, sodium hydroxide, ferric sulfate, and anionic polyacrylamide are added to the secondary fluoride removal unit 05; magnesium chloride, sodium hydroxide, sodium carbonate, ferric sulfate, and anionic polyacrylamide are added to the secondary silicon and hardening unit 06; hydrochloric acid and sodium hydroxide are used to regenerate the ion exchange device in the deep softening and decarbonization unit 07, and sulfuric acid is added to the decarbonization device for neutralization and decarbonization; sodium sulfate is added to the evaporation crystallization unit 09 to replenish sulfate ions.

[0070] Using the treatment system and method provided by this invention, and adding various reagents according to the above-mentioned regulations to achieve the required treatment effect in each treatment unit, the final daily production is 65.2 tons of potassium sulfate (K2O≥39%), 49.5 tons of sodium chloride (purity≥97.5%), 14.5 tons of miscellaneous salts, and 17.5 tons of calcium carbonate granules; approximately 16.2 tons of sodium sulfate are added daily; and 74 tons of sludge cake (moisture content≤60%) are generated daily for off-site disposal. If the selling price of potassium sulfate is 2500 yuan / ton and the purchase price of sodium sulfate is 300 yuan / ton, after deducting the purchase cost of sodium sulfate, the daily revenue is 158,000 yuan, which can offset the zero-emission system operating cost of approximately 5.27 yuan / ton.

[0071] Experimental Example 2:

[0072] A photovoltaic cell manufacturer discharges 20,000 tons of wastewater per day. The wastewater quality is shown in the table below:

[0073]

[0074] The company used NaOH instead of KOH in some steps of the battery cell processing, but this was not a complete replacement. The potassium ion content in the wastewater remained high, with a potassium to sulfate molar ratio of approximately 1.97. Its zero-emission system uses the treatment system and method provided by this invention. The main difference between the reagents used in each treatment unit and in Example 1 is in the primary silicon and fluoride removal unit 03; the reagents used in the other treatment units are the same as in Example 1. The silicon removal sedimentation tank in the primary silicon and fluoride removal unit 03 uses sodium aluminate, ferric chloride, hydrochloric acid, and anionic polyacrylamide, while the fluoride removal sedimentation tank uses aluminum chloride, sodium hydroxide, ferric chloride, and anionic polyacrylamide.

[0075] Using the processing system and method provided by this invention, and adding various reagents according to the above-mentioned regulations to achieve the required treatment effect in each processing unit, the final daily production is 19 tons of potassium sulfate (K2O≥39%), 53.5 tons of sodium chloride (purity≥97.5%), 9.2 tons of miscellaneous salts, and 16.1 tons of calcium carbonate granules; approximately 2.5 tons of sodium sulfate are added daily; and 48.5 tons of sludge cake (moisture content≤60%) are generated daily for off-site disposal. If the selling price of potassium sulfate is 2500 yuan / ton and the purchase price of sodium sulfate is 300 yuan / ton, after deducting the purchase cost of sodium sulfate, the daily revenue is 46,700 yuan, which can offset the zero-emission system operating cost of approximately 2.33 yuan / ton.

[0076] It is evident that, for the application scenario of zero discharge of potassium-containing photovoltaic wastewater, the treatment system and method provided by this invention, while ensuring zero discharge and recycling of wastewater, also achieve high-value conversion of by-product salts, reduction of general solid waste, and effective recovery of potassium, calcium, and sulfur resources, significantly reducing the overall operating cost of the zero-discharge system. Compared with similar processes, the process flow is shorter and the engineering cost is lower. Therefore, this invention has broad application prospects in the field of zero discharge and resource utilization of potassium-containing wastewater.

[0077] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many specific modifications under the guidance of the present invention without departing from the spirit and scope of the claims, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater, characterized in that, It includes a primary hardening removal unit, a biological treatment unit, a primary silicon and fluoride removal unit, a primary concentration unit, a secondary fluoride removal unit, a secondary silicon and hardening removal unit, a deep softening and decarbonization unit, a secondary concentration unit, an evaporation and crystallization unit, and a sludge dewatering unit; The production wastewater discharged from the photovoltaic enterprise's wastewater discharge outlet is treated by the primary hardening removal unit, the biological treatment unit, and the primary silicon and fluoride removal unit before entering the primary concentration unit. The primary concentrate from the primary concentration unit is treated by the secondary fluoride removal unit, the secondary silicon and hardening removal unit, and the deep softening and decarbonization unit before entering the secondary concentration unit. The secondary concentrate from the secondary concentration unit enters the evaporation and crystallization unit. The permeate from the primary and secondary concentration units and the condensate from the evaporation and crystallization unit are mixed and used as production water for the photovoltaic enterprise. The byproducts of the evaporation and crystallization unit are potassium sulfate, sodium chloride, and miscellaneous salts. The calcium carbonate granules, a byproduct of the primary hardening removal unit, are used for power plant desulfurization. The chemical sludge from the primary hardening removal unit, the primary silicon and fluoride removal unit, the secondary fluoride removal unit, and the secondary silicon and hardening removal unit, as well as the residual sludge from the biological treatment unit, are dewatered by the sludge dewatering unit and then transported for disposal. The primary hardening unit includes a homogenization conditioning tank, a granulation fluidized bed, and a turbidity removal sedimentation tank; sodium hydroxide, sodium carbonate, and crystal nuclei are added to the granulation fluidized bed to form calcium carbonate particles; the turbidity removal sedimentation tank is added with either ferric chloride or ferric sulfate, either hydrochloric acid or sulfuric acid, and anionic polyacrylamide. The secondary defluorination unit uses a defluorination sedimentation tank, and adds one of calcium chloride or calcium carbonate, one of sodium dihydrogen phosphate or potassium dihydrogen phosphate, one of ferric sulfate or ferric chloride, sodium hydroxide and anionic polyacrylamide. The evaporation and crystallization unit includes a feeding and mixing device, an MVR evaporation and concentration device, a potassium sulfate extraction device, a sodium chloride extraction device, and a miscellaneous salt drying device. The feeding and mixing device mainly includes a homogenizing tank and a sodium sulfate or potassium chloride mixing and dosing device. The potassium sulfate extraction device includes a cooling crystallization device, a concentration, dehydration, and drying device. The sodium chloride extraction device includes an evaporation and crystallization device, a concentration, dehydration, and drying device. The miscellaneous salt drying device includes an evaporation device and a drying device.

2. The treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater according to claim 1, characterized in that, The biological treatment unit includes a denitrifying biological filter and a nitrifying biological filter; wherein sodium acetate or methanol is added to the denitrifying biological filter as a supplementary carbon source.

3. The treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater according to claim 1, characterized in that, The primary silicon and fluoride removal unit includes a silicon removal precipitation tank and a fluoride removal precipitation tank; the reaction zone of the silicon removal precipitation tank is added with either hydrochloric acid or sulfuric acid, either ferric chloride or ferric sulfate, sodium aluminate and anionic polyacrylamide; the reaction zone of the fluoride removal precipitation tank is added with either aluminum sulfate or aluminum chloride, sodium hydroxide and anionic polyacrylamide.

4. The treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater according to claim 1, characterized in that, The secondary silicon and hardening removal unit includes a silicon and hardening removal sedimentation tank and an ultrafiltration device; the reaction zone of the silicon and hardening removal sedimentation tank is added with one of magnesium chloride or magnesium sulfate, one of ferric sulfate or ferric chloride, sodium hydroxide, sodium carbonate and anionic polyacrylamide; the ultrafiltration device uses an immersed PTFE hollow fiber membrane.

5. The treatment system for zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater according to claim 1, characterized in that, The deep softening and decarbonization unit includes an ion exchange device and a decarbonization device; the ion exchange device uses chelating resin to achieve deep softening, and sodium hydroxide and hydrochloric acid are used as regeneration agents; the decarbonization device adds hydrochloric acid or sulfuric acid to remove alkalinity and carbon dioxide from the wastewater by stripping.

6. A method for treating potassium-containing photovoltaic wastewater with zero discharge coupled extraction of potassium mirabilite, characterized in that, The treatment system based on the zero-discharge coupled extraction of potassium mirabilite from potassium-containing photovoltaic wastewater as described in any one of claims 1 to 5 includes the following steps: Step 1: Wastewater from the photovoltaic enterprise's main wastewater discharge outlet undergoes hardening and turbidity removal treatment in the primary hardening unit, achieving the following effluent quality requirements: Ca 2+ ≤50mg / L, SS≤20mg / L; the particle size of the produced calcium carbonate particles is controlled within 1~3mm, and the purity is ≥90%; Step 2: The wastewater from the primary hardening unit is treated by the biological treatment unit to remove organic matter, ammonia nitrogen and nitrate nitrogen. The effluent quality meets the following requirements: COD ≤30mg / L, ammonia nitrogen ≤3mg / L, total nitrogen ≤10mg / L. Step 3: The wastewater from the biological treatment unit is passed through the primary silicon and fluoride removal unit to remove total silicon and fluoride, and simultaneously remove some organic matter. The effluent quality meets the following requirements: total silicon ≤ 20 mg / L, fluoride ≤ 2 mg / L, COD ≤ 20 mg / L. Step 4: The wastewater from the primary silicon and fluoride removal unit is passed through the primary concentration unit to concentrate the salt and impurities to primary concentrate. The primary concentrate is used for production water in photovoltaic enterprises, with a concentration ratio ≥10 and a desalination rate ≥95%. Step 5: The primary concentrate from the primary concentration unit is passed through the secondary defluorination unit to remove fluoride and suspended solids from the wastewater. The effluent quality meets the following requirements: fluoride ≤ 5 mg / L, SS ≤ 20 mg / L. Step 6: The wastewater from the secondary defluorination unit is passed through the secondary silicon and hardness removal unit to remove total silicon, hardness, and turbidity, and the effluent quality meets the following requirements: Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, total silicon ≤20mg / L, turbidity ≤1NTU, SDI ≤5; Step 7: The wastewater from the secondary silicon and hardness removal unit is further treated by the deep softening and decarbonization unit to remove hardness and alkalinity, and the effluent quality meets the following requirements: Ca 2+ ≤2mg / L, Mg 2+ ≤2mg / L, alkalinity ≤50mg / L; pH controlled between 6.5 and 7.0; Step 8: Wastewater from the deep softening and decarbonization unit is passed through the secondary concentration unit to concentrate salts and impurities into secondary concentrate. The secondary concentrate is used for production water in photovoltaic enterprises. The salt content of the secondary concentrate reaches 10-12%, and the desalination rate is ≥97%. Step 9: The secondary concentrate from the secondary concentration unit enters the evaporation and crystallization unit, where the salts are finally formed into potassium sulfate, sodium chloride, and miscellaneous salts. The condensate is used for production water in photovoltaic enterprises; the potassium ion capture rate is ≥90%, the purity of the by-product potassium sulfate is ≥39%, and the purity of the by-product sodium chloride is ≥97.5%. Step 10: The sludge from the primary hardening unit, biological treatment unit, primary silicon and fluoride removal unit, secondary fluoride removal unit, and secondary silicon and hardening removal unit is dewatered to a moisture content of ≤60% by the sludge dewatering unit and then transported off-site for disposal.

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