Polycrystalline silicon wastewater zero discharge treatment system and treatment method

Abstract

The invention belongs to the technical field of wastewater treatment, and particularly relates to a zero-discharge treatment system and method for polycrystalline silicon wastewater. Comprising a pretreatment system, a membrane treatment system and an evaporative crystallization system which are sequentially connected along a water flow direction, the pretreatment system comprises a sodium sulfate softening and precipitating unit, a dual-alkali softening and precipitating unit, an enhanced hardness and silicon removal unit, a ceramic membrane filtering unit, a decarburization unit and an ion exchange unit; the membrane treatment system comprises a COD (Chemical Oxygen Demand) purification membrane unit and a two-stage nanofiltration unit; the evaporative crystallization system comprises an evaporation unit, a circulating evaporative crystallization unit, a separation unit and a mother liquor drying unit. According to the method, high-value industrial byproduct gypsum is generated through sodium sulfate softening precipitation in the pretreatment stage, meanwhile, the cost of a softening agent is reduced, sodium sulfate generated by a membrane treatment system is recycled, the investment cost of an evaporative crystallization system is reduced, and resource utilization of wastewater treatment byproducts and sustainable development of the environment are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of wastewater treatment, and particularly relates to a zero-discharge treatment system and method for polysilicon wastewater. Background Art

[0002] Saline wastewater generally refers to wastewater containing organic matter and at least 3.5% total dissolved solids. In the production wastewater of industries such as mining, coal chemical industry, and polysilicon, relatively high hardness is usually accompanied. Direct discharge or reuse of such high-salt and high-hardness wastewater without treatment may lead to a series of problems such as soil salinization, groundwater mineralization, equipment scaling, and pipeline blockage. With the increasingly prominent contradiction between water resource supply and demand and the increasingly strict national environmental protection policies, the discharge standards for industrial wastewater are gradually increasing, and zero-discharge technology has become the mainstream trend in the development of industrial water treatment.

[0003] At present, zero discharge of industrial wastewater is widely applied in wastewater treatment such as coal chemical industry, chlor-alkali industry, and power plants, while zero discharge of wastewater is mostly not achieved in the polysilicon industry. The current mainstream processes for polysilicon production include the improved Siemens process, the silane process, etc. After the tail gas in the production process is absorbed and neutralized by lime water + alkali solution, the effluent contains a large amount of calcium salts and sodium salts, which belongs to typical high-salt and high-hardness wastewater. In addition, the wastewater also contains pollutants such as silicon, suspended solids, and COD. Based on the above characteristics of polysilicon wastewater, in order to achieve zero discharge of wastewater, a targeted process route needs to be given. The conventional industrial wastewater zero-discharge process route is pretreatment + membrane concentration + evaporation crystallization. Among them, there are generally problems of large dosage of medicaments and high operation cost in the hardening removal stage of pretreatment. In addition, in order to completely remove sulfates in the current traditional high-salt wastewater zero-discharge process, usually two sets of evaporation crystallization systems for sodium chloride and sodium sulfate are set, which brings high construction costs. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a zero-discharge treatment system and method for polysilicon wastewater. While solving the problem of high medicament cost in the traditional zero-discharge process, the process route is optimized, and the requirements for wastewater reuse and up-to-standard discharge in polysilicon production are met to achieve zero discharge of wastewater; not only high-value industrial by-product gypsum is generated through sodium sulfate softening precipitation in the pretreatment stage, while reducing the softening medicament cost, but also the sodium sulfate generated by the membrane treatment system is recycled, reducing the investment cost of the evaporation crystallization system, and realizing the resource utilization of wastewater treatment by-products and the sustainable development of the environment.

[0005] The present invention solves the above technical problems through the following technical solutions.

[0006] One object of the present invention is to provide a zero-discharge treatment system for polysilicon wastewater, including a pretreatment system, a membrane treatment system, and an evaporation crystallization system connected in sequence along the water flow direction.

[0007] The pretreatment system includes a sodium sulfate softening and precipitation unit, a dual-alkali method softening and precipitation unit, a strengthening hard and silicon removal unit, a ceramic membrane filtration unit, a decarbonation unit, and an ion exchange unit, which are connected in sequence along the water flow direction.

[0008] The membrane treatment system includes a COD purification membrane unit, a first nanofiltration unit, and a second nanofiltration unit, which are connected in sequence along the water flow direction; the ion exchange unit is connected to the COD purification membrane unit, and the first nanofiltration unit is connected to the sodium sulfate softening and precipitation unit.

[0009] The evaporation and crystallization system includes an MVR falling film evaporation unit, a circulating evaporation and crystallization unit, a separation unit, and a mother liquor drying unit, which are connected in sequence along the water flow direction; the MVR falling film evaporation unit is connected to the second nanofiltration unit, and the circulating evaporation and crystallization unit is connected to the COD purification membrane unit.

[0010] Furthermore, the sodium sulfate softening and precipitation unit includes a sodium sulfate softening unit, a sludge storage unit, a filtration unit, and a filtrate collection unit, which are connected in sequence along the water flow direction; the first nanofiltration unit is connected to the sodium sulfate softening unit, the sludge storage unit is connected to the dual-alkali method softening and precipitation unit, the strengthening hard and silicon removal unit, and the ceramic membrane filtration unit, and the filtrate collection unit is connected to the dual-alkali method softening and precipitation unit.

[0011] Furthermore, the pretreatment system further includes an adjustment and homogenization unit, which is connected to the sodium sulfate softening and precipitation unit; the ion exchange unit is connected to a regeneration liquid collection unit, and the regeneration liquid collection unit is connected to the adjustment and homogenization unit.

[0012] Furthermore, the second nanofiltration unit is connected in a reflux manner to the first nanofiltration unit.

[0013] Furthermore, the circulating evaporation and crystallization unit includes a first MVR forced circulation evaporation and crystallization unit and a second MVR forced circulation evaporation and crystallization unit; the separation unit includes a first solid-liquid separation unit and a second solid-liquid separation unit; the second solid-liquid separation unit is connected to the mother liquor drying unit; along the water flow direction, the first MVR forced circulation evaporation and crystallization unit is connected to the first solid-liquid separation unit, the first solid-liquid separation unit is connected to the second MVR forced circulation evaporation and crystallization unit, the second MVR forced circulation evaporation and crystallization unit is connected to the second solid-liquid separation unit, the first solid-liquid separation unit is connected in a reflux manner to the first MVR forced circulation evaporation and crystallization unit, the second solid-liquid separation unit is connected in a reflux manner to the second MVR forced circulation evaporation and crystallization unit, and the COD purification membrane unit is connected to the second MVR forced circulation evaporation and crystallization unit.

[0014] The second object of the present invention is to provide a method for zero-emission treatment of polysilicon wastewater, which is carried out by using the above polysilicon wastewater zero-emission treatment system and includes the following steps: S1. The wastewater from polysilicon production first enters the conditioning and equalization unit to equalize the quality and quantity of the wastewater, and then enters the sodium sulfate softening unit to remove Ca 2+ in the wastewater. Meanwhile, seeds are added to the sodium sulfate softening unit to induce the crystallization and nucleation of calcium sulfate dihydrate, forming pre-hardness-removed wastewater.

[0015] S2. The primary wastewater is successively precipitated in the sludge storage unit and separated in the filtration unit to obtain filtered wastewater. The filtered wastewater is discharged to the double-alkali softening and precipitation unit through the filtrate collection unit to remove Ca 2+ and Mg 2+ in the filtered wastewater. Then, it is discharged to the enhanced hardness and silicon removal unit through the double-alkali softening and precipitation unit to remove the total silicon in the hardness-removed wastewater and stabilize the indexes of the hardness-removed wastewater, forming hardness and silicon removal wastewater. Meanwhile, the sludge storage unit serves as a sludge treatment system to receive the sludge discharged from the double-alkali softening and precipitation unit, the enhanced hardness and silicon removal unit, and the ceramic membrane filtration unit.

[0016] S3. The hardness and silicon removal wastewater is discharged to the ceramic membrane filtration unit through the enhanced hardness and silicon removal unit to remove the SS and colloid in the hardness and silicon removal wastewater. Then, it is discharged to the decarbonation unit through the ceramic membrane filtration unit to remove carbonates and bicarbonates, forming decarbonated wastewater. The decarbonated wastewater is discharged to the ion exchange unit to remove the remaining Ca 2+ and Mg 2+ to form ion exchange product water and generate regeneration liquid. After being collected by the regeneration liquid collection unit, the regeneration liquid is discharged to the homogenization unit.

[0017] S4. The ion exchange product water is discharged to the COD purification membrane unit through the ion exchange unit to remove the COD in the ion exchange product water, forming COD purification membrane concentrated water and COD purification product water. The COD purification membrane concentrated water enters the second MVR forced circulation evaporation and crystallization unit. The COD purification product water successively enters the first nanofiltration unit and the second nanofiltration unit to separate Cl - and SO4 2- and organic substances. Among them, the first nanofiltration unit filters to form first nanofiltration product water and first nanofiltration concentrated water. The first nanofiltration product water enters the second nanofiltration unit, and the first nanofiltration concentrated water is discharged to the sodium sulfate softening unit to participate in the removal of Ca 2+ in the wastewater from polysilicon production as a reactant. The second nanofiltration unit filters to produce second nanofiltration product water and second nanofiltration concentrated water, and the second nanofiltration concentrated water flows back into the first nanofiltration unit.

[0018] S5. The secondary reverse osmosis permeate enters the MVR falling film evaporation unit to evaporate the reverse osmosis permeate, forming a concentrated mother liquor. After the concentrated mother liquor is crystallized by the first MVR forced circulation evaporation crystallization unit and separated by the first solid-liquid separation unit, industrial salt and the first enriched mother liquor are formed. After the first enriched mother liquor and the COD purification membrane concentrate enter the second MVR forced circulation evaporation crystallization unit for secondary crystallization and the second solid-liquid separation unit for secondary separation, the first miscellaneous salt and the second enriched mother liquor are formed. The second enriched mother liquor is dried by the mother liquor drying unit to form organic matter and the second miscellaneous salt, achieving the reduction treatment and zero discharge of organic pollutants in the wastewater from polysilicon production.

[0019] Further, the concentration of Ca in the wastewater from polysilicon production 2+ is ≥ 3000 mg / L.

[0020] Further, the concentration of Ca in the filtered wastewater 2+ is 1000 mg / L - 2000 mg / L, the crystal seed is calcium sulfate dihydrate, and the dosage is 5 g / L - 20 g / L.

[0021] Further, the concentration of Ca in the secondary wastewater for hardness and silicon removal 2+ is ≤ 40 mg / L, and the total silicon concentration is ≤ 10 mg / L.

[0022] Further, the sludge density index of the effluent after the ceramic membrane filtration unit is ≤ 5, and the bicarbonate concentration of the effluent is ≤ 50 mg / L. Calculated by the dosage of CaCO3, the total hardness of the decarbonated wastewater is ≤ 5 mg / L.

[0023] The present invention has the following beneficial effects compared with the prior art: (1) The zero-discharge treatment system for crystalline silicon wastewater provided by the present invention includes a pretreatment system, a membrane treatment system, and an evaporation crystallization system connected in sequence along the water flow direction. The purpose of the pretreatment system is to remove total hardness, total silicon, suspended solids, and decarbonation. The membrane treatment system uses a COD purification membrane and a two-stage reverse osmosis process to remove some organic pollutants in the wastewater and separate monovalent and divalent ions. The purpose of the evaporation crystallization system is to produce sodium chloride product salt while separating organic pollutants and miscellaneous salts, optimizing the process route while solving the problem of high reagent cost in traditional zero-discharge processes, and meeting the requirements of wastewater reuse and up-to-standard discharge in polysilicon production, achieving zero discharge of wastewater. Not only high-value industrial by-product gypsum is produced through sodium sulfate softening precipitation in the pretreatment stage, while reducing the softening reagent cost, but also the sodium sulfate generated by the membrane treatment system is recycled, reducing the investment cost of the evaporation crystallization system, and realizing the resource utilization of wastewater treatment by-products and the sustainable development of the environment.

[0024] (2) The pretreatment system adopted in the present invention uses a combination of sodium sulfate softening and precipitation, double-alkali method softening and precipitation, enhanced hardness and silicon removal, and ion exchange for hardness removal. It not only strictly controls the Ca 2+ to be stabilized at a very low level to prevent the impact on the process equipment of the membrane treatment system and the evaporation crystallization system, but also when treating high-salt and high-hardness wastewater in industries such as polysilicon, since most of the Ca 2+ is removed by sodium sulfate precipitation, compared with the conventional double-alkali method softening process, the chemical agent cost can be significantly reduced. At the same time, the sodium sulfate softening and precipitation can produce high-value industrial by-product gypsum, which has more application fields and greater resource utilization value compared with the calcium carbonate product of the double-alkali method softening.

[0025] (3) In view of the characteristics that the wastewater generated from polysilicon contains organic substances, the present invention sets a COD purification membrane unit in the membrane treatment system to ensure the stable removal of COD in the wastewater and prevent the impact on the operation of each unit in the nanofiltration and evaporation crystallization systems. At the same time, after the COD in the concentrated water of the nanofiltration unit circulates back into the main process, through the COD purification membrane unit, the organic pollutants in the wastewater are effectively removed, so as to maintain the COD in the influent of the nanofiltration unit at an appropriate level and ensure the stability of the whole process. In addition, the concentrated water of the COD purification membrane unit goes to the evaporation crystallization system, and after being jointly treated by processes such as evaporation, crystallization, solid-liquid separation, and mother liquor drying, finally forms a solid product to be discharged from the system, realizing the reduction treatment and zero discharge of organic pollutants in the wastewater.

[0026] (4) The evaporation crystallization system of the present invention not only realizes the crystallization and recovery of salts such as sodium chloride, but also realizes the drying and recovery of organic substances through the MVR evaporation crystallization unit and the separation unit, thus realizing the zero discharge of polysilicon wastewater and the recovery of waste resources, with significant ecological and environmental benefits. Since most of the sulfates are recycled in the first-stage nanofiltration unit, a small amount of sodium sulfate generated by evaporation crystallization is discharged from the system as miscellaneous salt. Therefore, compared with the traditional evaporation crystallization system, the sodium sulfate evaporation crystallization unit is reduced, and the engineering construction cost is lowered.

[0027] (5) On the basis of selecting a lower-cost sodium sulfate chemical agent for hardness removal in the pretreatment system, the present invention makes the sulfates in the first-stage nanofiltration concentrated water generated by the first-stage nanofiltration unit in the membrane treatment system flow back to the sodium sulfate softening unit, so that the by-products in the membrane treatment are recycled, further reducing the amount of externally added chemical agents, saving the chemical agent cost, and improving the resource recycling utilization rate. And since most of the sulfates are recycled in the first-stage nanofiltration unit, a small amount of sodium sulfate generated by evaporation crystallization is discharged from the system as miscellaneous salt. Therefore, compared with the traditional evaporation crystallization system, the sodium sulfate evaporation crystallization unit is reduced, and the engineering construction cost is lowered.

[0028] (6) The sodium sulfate softening and precipitation system in the pretreatment system of the present invention consists of a sodium sulfate softening unit, a sludge storage unit, a filtration unit, and a filtrate collection unit. Among them, the sludge storage unit and the filtration unit not only participate in the solid-liquid separation of calcium sulfate precipitation as the main process of softening to ensure the normal operation of subsequent treatment units, but also serve as a sludge treatment system to receive the sludge discharged from the double-alkali method softening and precipitation unit, the enhanced hardness and silicon removal unit, and the ceramic membrane filtration unit, achieving multiple functions in one, which is an optimization attempt for the existing zero-discharge process. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the overall process flow of the zero-discharge treatment system for polysilicon wastewater of the present invention.

[0030] Figure 2 It is a schematic diagram of the process flow of the pretreatment system of the present invention.

[0031] Figure 3 It is a schematic diagram of the process flow of the membrane treatment system of the present invention.

[0032] Figure 4 It is a schematic diagram of the process flow of the evaporation and crystallization system of the present invention.

[0033] Illustration: 1. Pretreatment system, 1-1. Sodium sulfate softening and precipitation unit, 1-11. Sodium sulfate softening unit, 1-12. Sludge storage unit, 1-13. Filtration unit, 1-14. Filtrate collection unit, 1-2. Double-alkali method softening and precipitation unit, 1-3. Enhanced hardness and silicon removal unit, 1-4. Ceramic membrane filtration unit, 1-5. Decarbonation unit, 1-6. Ion exchange unit, 1-7. Regenerated liquid collection unit, 2. Membrane treatment system, 2-1. COD purification membrane unit, 2-2. First-stage nanofiltration unit, 2-3. Second-stage nanofiltration unit, 3. Evaporation and crystallization system, 3-1. MVR falling-film evaporation unit, 3-2. Circulating evaporation and crystallization unit, 3-21. First MVR forced-circulation evaporation and crystallization unit, 3-22. Second MVR forced-circulation evaporation and crystallization unit, 3-3. Separation unit, 3-31. First solid-liquid separation unit, 3-32. Second solid-liquid separation unit, 3-4. Mother liquor drying unit, 4. Adjusting and homogenizing unit. DETAILED DESCRIPTION OF THE INVENTION

[0034] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0035] It should be noted that the professional terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention. Unless otherwise specifically stated, various raw materials, reagents, and equipment used in the following embodiments of the present invention can be obtained through market purchase or prepared by existing methods.

[0036] It should be noted that the professional terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention. In the present invention, certain terms are used to refer to specific components. Those skilled in the art should understand that technicians may use different nouns to refer to the same component.

[0037] The following is a further description through specific embodiments.

[0038] Example 1 The present invention provides a zero-discharge treatment system for polysilicon wastewater, as Figure 1 shown, including: A pretreatment system 1, a membrane treatment system 2, and an evaporation and crystallization system 3 connected in sequence along the water flow direction; the pretreatment system 1 includes a sodium sulfate softening and precipitation unit 1-1, a double-alkali method softening and precipitation unit 1-2, a strengthening hard and silicon removal unit 1-3, a ceramic membrane filtration unit 1-4, a decarbonation unit 1-5, and an ion exchange unit 1-6 connected in sequence along the water flow direction; the membrane treatment system 2 includes a COD purification membrane unit 2-1, a first-stage nanofiltration unit 2-2, and a second-stage nanofiltration unit 2-3 connected in sequence along the water flow direction; the ion exchange unit 1-6 is connected to the COD purification membrane unit 2-1, and the first-stage nanofiltration unit 2-2 is connected to the sodium sulfate softening and precipitation unit 1-1; the evaporation and crystallization system 3 includes an MVR falling-film evaporation unit 3-1, a circulating evaporation and crystallization unit 3-2, a separation unit 3-3, and a mother liquor drying unit 3-4 connected in sequence along the water flow direction; the MVR falling-film evaporation unit 3-1 is connected to the second-stage nanofiltration unit 2-3, and the circulating evaporation and crystallization unit 3-2 is connected to the COD purification membrane unit 2-1.

[0039] In the present invention, the purpose of the pretreatment system 1 is to achieve the removal of total hardness, total silicon, suspended solids, and decarbonation. Among them, the dosing agents of the sodium sulfate softening and precipitation unit 1-1 are sodium sulfate and sulfates generated by the first-stage nanofiltration unit 2-2, so that sodium sulfate and Ca in the wastewater 2+ are mixed and reacted, and most of the Ca in the wastewater is removed by sodium sulfate precipitation 2+ , and at the same time, calcium sulfate dihydrate seeds are added during the reaction, and the function is to induce the crystallization nucleation of calcium sulfate dihydrate, thereby shortening the reaction time; the dosing agents of the double-alkali method softening and precipitation unit 1-2 can be NaOH, Na2CO3, PFS, or PAM, further removing Ca in the wastewater 2+ and Mg 2+; The optional chemicals added to the enhanced hardness and silica removal unit 1-3 are NaOH, Na2CO3, PFS, PAM or NaAlO2, which can ensure the stability of the effluent index of the previous hardness removal step and remove the total silica in the wastewater; the ceramic membrane filtration unit 1-4 mainly functions to filter SS and colloids in the wastewater; the chemical added to the decarbonation unit 1-5 is mainly hydrochloric acid, and its main function is to remove the residual carbonates and bicarbonates in the wastewater to prevent them from affecting the operation of the membrane treatment system 2 unit; the main function of the ion exchange unit 1-6 is to deeply remove the residual Ca 2+ , Mg 2+ ; The pretreatment system 1 of the present invention adopts a hardness removal combination method of sodium sulfate softening precipitation + double-alkali method softening precipitation + enhanced hardness and silica removal + ion exchange, which can not only strictly control the Ca 2+ at a very low level to prevent it from affecting the process equipment of the membrane treatment system 2 and the evaporation crystallization system 3, but also when treating high-salt and high-hard wastewater in industries such as polysilicon, since most of the Ca 2+ is removed by sodium sulfate precipitation, compared with the conventional double-alkali method softening process, the chemical cost can be greatly reduced. At the same time, the sodium sulfate softening precipitation can produce high-value industrial by-product gypsum, which has more application fields and greater resource utilization value compared with the calcium carbonate produced by the double-alkali method softening product.

[0040] The membrane treatment system 2 adopts a COD purification membrane and a two-stage nanofiltration process to achieve the purpose of removing some organic pollutants in the wastewater and separating monovalent and divalent ions. Among them, the main functions of the COD purification membrane unit 2-1 are: on the one hand, to remove COD in the wastewater to prevent organic matter from fouling the nanofiltration unit and affecting normal operation; on the other hand, the first-stage nanofiltration concentrate produced by the first-stage nanofiltration unit 2-2 is recycled to the sodium sulfate softening precipitation unit 1-1, and the residual COD in the process of the first-stage nanofiltration concentrate circulation is treated by the COD purification membrane unit 2-1; the main functions of the first-stage nanofiltration unit 2-2 and the second-stage nanofiltration unit 2-3 are to separate monovalent ions mainly composed of Cl - and divalent ions mainly composed of SO4 2- as well as organic matter, etc., to provide conditions for the subsequent evaporation crystallization system 3 to produce sodium chloride industrial salt. In view of the characteristics of the wastewater containing organic matter generated by polysilicon, the COD purification membrane unit 2-1 is set in the membrane treatment system 2 of the present invention to ensure the stable removal of COD in the wastewater and prevent it from affecting the operation of each unit in the nanofiltration and evaporation crystallization systems 3. At the same time, after the COD in the nanofiltration unit concentrate is recycled into the sodium sulfate softening precipitation unit 1-1, the organic pollutants in the wastewater are effectively removed through the COD purification membrane unit 2-1, so as to maintain the COD in the nanofiltration unit influent at an appropriate level and ensure the stability of the entire process.

[0041] The purpose of the evaporation crystallization system 3 is to produce sodium chloride product salt while separating organic pollutants and miscellaneous salts. Among them, the main function of the MVR falling film evaporation unit 3-1 is to realize the evaporation of the solution; the main function of the circulating evaporation crystallization unit 3-2 is to crystallize and precipitate NaCl, miscellaneous salts, etc.; the separation unit 3-3 realizes the solid-liquid separation of crystals and supernatant; the mother liquor drying unit 3-4 is mainly used for the drying and recovery of organic substances. The evaporation crystallization system 3 of the present invention not only realizes the crystallization and recovery of salts such as sodium chloride, but also realizes the drying and recovery of organic substances through the circulating evaporation crystallization unit 3-2 and the separation unit 3-3, thereby realizing the zero discharge of polysilicon wastewater and the recovery of waste resources, and having significant ecological and environmental benefits.

[0042] In a specific embodiment, as Figure 2 shown, the sodium sulfate softening and precipitation unit 1-1 includes a sodium sulfate softening unit 1-11, a sludge storage unit 1-12, a filtration unit 1-13, and a filtrate collection unit 1-14 connected in sequence along the water flow direction. The first-stage nanofiltration unit 2-2 is connected to the sodium sulfate softening unit 1-11. The sludge storage unit 1-12 is connected to the double-alkali method softening and precipitation unit 1-2, the enhanced hardness and silicon removal unit 1-3, and the ceramic membrane filtration unit 1-4. The filtrate collection unit 1-14 is connected to the double-alkali method softening and precipitation unit 1-2. In the present invention, the sodium sulfate softening unit 1-11 causes Ca 2+ in the wastewater to precipitate by adding chemicals, stores the precipitate generated in the previous step through the sludge storage unit 1-12, then performs solid-liquid separation through the filtration unit 1-13, and the filtrate is collected and stored by the filtrate collection unit 1-14; the sodium sulfate softening and precipitation unit 1-1 is composed of a sodium sulfate softening unit 1-11, a sludge storage unit 1-12, a filtration unit 1-13, and a filtrate collection unit 1-14. Among them, the sludge storage unit 1-12 and the filtration unit 1-13 not only participate in the solid-liquid separation of calcium sulfate precipitation as the main process of softening to ensure the normal operation of the subsequent treatment unit, but also serve as a sludge treatment system to receive the sludge from the double-alkali method softening and precipitation unit 1-2, the enhanced hardness and silicon removal unit 1-3, and the ceramic membrane filtration unit 1-4, realizing multiple functions in one, which is an optimization attempt for the existing zero-discharge process.

[0043] In the present invention, through the connection between the first-stage nanofiltration unit 2-2 and the sodium sulfate softening unit 1-11, the first-stage nanofiltration concentrate generated by the filtration of the first-stage nanofiltration unit 2-2 completely returns to the sodium sulfate softening unit 1-11 in the pretreatment system 1, where sulfate participates as a reactant in the removal of Ca 2+ in the raw water. The externally added sodium sulfate dosage of the sodium sulfate softening unit 1-11 is composed of the original sulfate amount, the recycled sulfate amount, and the Ca 2+The requirements are jointly determined. Furthermore, on the basis that the pretreatment system 1 selects the lower-cost sodium sulfate reagent for hardness removal, the sulfate in the first-stage nanofiltration concentrate generated by the first-stage nanofiltration unit 2-2 in the membrane treatment system 2 is refluxed to the sodium sulfate softening unit 1-11, so that the by-products in the membrane treatment are recycled. The amount of externally added sodium sulfate reagent in the sodium sulfate softening unit 1-11 is determined jointly by the original amount of sulfate, the refluxed amount of sulfate, and the Ca in the effluent of this unit 2+ The requirements are jointly determined, thereby further reducing the amount of externally added reagent, saving the reagent cost, and improving the resource recycling rate. And since most of the sulfate is recycled in the first-stage nanofiltration unit 2-2, a small amount of sodium sulfate generated by evaporation and crystallization is discharged from the system as miscellaneous salt. Therefore, compared with the traditional evaporation and crystallization system 3, the sodium sulfate evaporation and crystallization unit is reduced, and the engineering construction cost is lowered.

[0044] In summary, this process system not only produces high-value industrial by-product gypsum through sodium sulfate softening and precipitation in the pretreatment stage, while reducing the softening reagent cost, but also recycles the sodium sulfate generated by the membrane treatment system 2, reduces the investment cost of the evaporation and crystallization system 3, and realizes the resource utilization of wastewater treatment by-products and the sustainable development of the environment.

[0045] In a specific embodiment, as Figure 2 shown, the pretreatment system 1 further includes an adjustment and homogenization unit 4. The adjustment and homogenization unit 4 is connected to the sodium sulfate softening and precipitation unit 1-1. The ion exchange unit 1-6 is connected to a regeneration liquid collection unit 1-7, and the regeneration liquid collection unit 1-7 is connected to the adjustment and homogenization unit 4. In the present invention, the adjustment and homogenization unit 4 is mainly used to homogenize and equalize the wastewater. After the wastewater is homogenized and equalized by the adjustment and homogenization unit 4, it is discharged to the sodium sulfate softening and precipitation unit 1-1. The ion exchange unit 1-6 is connected to a regeneration liquid collection unit 1-7. The regeneration liquid generated by the ion exchange of the ion exchange resin in the ion exchange unit 1-6 is collected by the regeneration liquid collection unit 1-7 and refluxed to the adjustment and homogenization unit 4 to achieve the resource recycling rate.

[0046] In a specific embodiment, as Figure 3 shown, the second-stage nanofiltration unit 2-3 is reflux-connected to the first-stage nanofiltration unit 2-2.

[0047] In a specific embodiment, as Figure 4As shown, the cyclic evaporation crystallization unit 3-2 includes a first MVR forced circulation evaporation crystallization unit 3-21 and a second MVR forced circulation evaporation crystallization unit 3-22. The separation unit 3-3 includes a first solid-liquid separation unit 3-31 and a second solid-liquid separation unit 3-32. The second solid-liquid separation unit 3-32 is connected to the mother liquor drying unit 3-4. Along the water flow direction, the first MVR forced circulation evaporation crystallization unit 3-21 is connected to the first solid-liquid separation unit 3-31. The first solid-liquid separation unit 3-31 is connected to the second MVR forced circulation evaporation crystallization unit 3-22. The second MVR forced circulation evaporation crystallization unit 3-22 is connected to the second solid-liquid separation unit 3-32. The first solid-liquid separation unit 3-31 is reflux-connected to the first MVR forced circulation evaporation crystallization unit 3-21. The second solid-liquid separation unit 3-32 is reflux-connected to the second MVR forced circulation evaporation crystallization unit 3-22. The COD purification membrane unit 2-1 is connected to the second MVR forced circulation evaporation crystallization unit 3-22.

[0048] In the present invention, through a two-stage nanofiltration process, the purpose of removing some organic pollutants in the wastewater and separating monovalent and divalent ions is achieved. And the secondary nanofiltration concentrated water generated by the secondary nanofiltration unit 2-3 is reflux-connected to the primary nanofiltration unit 2-2 to further improve the removal efficiency. The COD purification membrane unit 2-1 is connected to the second MVR forced circulation evaporation crystallization unit 3-22. The COD purification membrane concentrated water generated by the COD purification membrane unit 2-1 is discharged to the second MVR forced circulation evaporation crystallization unit 3-22. After being jointly treated by processes such as evaporation, crystallization, solid-liquid separation, and mother liquor drying, a solid product is finally formed and discharged from the system, realizing the reduction treatment and zero discharge of organic pollutants in the wastewater. The mother liquor generated by the first solid-liquid separation unit 3-31 and the second solid-liquid separation unit 3-32 are respectively refluxed to the first MVR forced circulation evaporation crystallization unit 3-21 and the second MVR forced circulation evaporation crystallization unit 3-22 for re-crystallization. The mother liquor generated by the separation unit 3-3 is dried into salt by the mother liquor drying unit 3-4 to prevent the high-concentration mother liquor from refluxing to the system and causing water quality deterioration.

[0049] The zero-discharge treatment method using the above polysilicon wastewater zero-discharge treatment system includes the following steps: S1. The wastewater from polysilicon production first enters the conditioning and homogenization unit 4 to homogenize and equalize the wastewater, and then enters the sodium sulfate softening unit 1-11 to remove most of the Ca 2+ in the wastewater. At the same time, seeds are added to the sodium sulfate softening unit 1-11 to induce the crystallization nucleation of calcium sulfate dihydrate, forming pre-softened wastewater.

[0050] S2. The pre-hardness removal wastewater is successively precipitated by the sludge storage unit 1-12 and separated by the filtration unit 1-13 to obtain filtered wastewater. The filtered wastewater is discharged to the double-alkali method softening and precipitation unit 1-2 through the filtrate collection unit 1-14 to further remove Ca 2+ and Mg 2+ in the filtered wastewater. Then, it is discharged to the enhanced hardening and silicon removal unit 1-3 through the double-alkali method softening and precipitation unit 1-2 to remove the total silicon in the hardening removal wastewater and stabilize the indexes of the hardening removal wastewater, forming hardening and silicon removal wastewater. At the same time, the sludge storage unit 1-12 serves as a sludge treatment system, receiving the sludge from the double-alkali method softening and precipitation unit 1-2, the enhanced hardening and silicon removal unit 1-3, and the ceramic membrane filtration unit 1-4, realizing multiple functions in one.

[0051] S3. The hardening and silicon removal wastewater is discharged to the ceramic membrane filtration unit 1-4 through the hardening and silicon removal wastewater discharge to remove SS and colloid in the hardening and silicon removal wastewater. Then, it is discharged to the decarbonation unit 1-5 through the ceramic membrane filtration unit 1-4 to remove carbonates and bicarbonates, forming decarbonated wastewater. The decarbonated wastewater is discharged to the ion exchange unit 1-6 to further remove the residual Ca 2+ and Mg 2+ , forming ion exchange product water and generating regeneration liquid. After the regeneration liquid is collected by the regeneration liquid collection unit 1-7, it is discharged to the homogenization unit.

[0052] S4. The ion exchange product water is discharged to the COD purification membrane unit 2-1 through the ion exchange unit 1-6 to remove the COD in the ion exchange product water, forming COD purification membrane concentrated water and COD purification product water. The COD purification membrane concentrated water enters the second MVR forced circulation evaporation and crystallization unit 3-22, and the COD purification product water enters the first nanofiltration unit 2-2 to filter and form first nanofiltration product water and first nanofiltration concentrated water. The first nanofiltration product water enters the second nanofiltration unit 2-3 to filter and form second nanofiltration product water and second nanofiltration concentrated water. The first nanofiltration unit 2-2 and the second nanofiltration unit 2-3 mainly separate Cl - and SO4 2- and organic substances. The second nanofiltration concentrated water is refluxed into the first nanofiltration unit 2-2. The first nanofiltration concentrated water is completely refluxed and discharged, and after being mixed with the wastewater discharged from the regulation and homogenization unit 4, it enters the sodium sulfate softening unit 1-11. The sulfate in the first nanofiltration concentrated water participates as a reactant in the removal of Ca 2+ in the raw water.

[0053] S5. The secondary reverse osmosis permeate enters the MVR falling film evaporation unit 3-1 for evaporating the reverse osmosis permeate to form a concentrated mother liquor. After crystallization in the first MVR forced circulation evaporation crystallization unit 3-21 and separation in the first solid-liquid separation unit 3-31, industrial salt and a first enriched mother liquor are formed. The first enriched mother liquor and the COD purification membrane concentrate enter the second MVR forced circulation evaporation crystallization unit 3-22 for secondary crystallization and the second solid-liquid separation unit 3-32 for secondary separation, forming a first miscellaneous salt and a second enriched mother liquor. The second enriched mother liquor is dried in the mother liquor drying unit 3-4 to form organic matter and a second miscellaneous salt, achieving the reduction treatment and zero discharge of organic pollutants in the wastewater from polysilicon production.

[0054] In a specific embodiment, the concentration of Ca in the wastewater from polysilicon production 2+ ≥ 3000 mg / L. It should be noted that the wastewater from polysilicon production targeted by the present invention is high-salt and high-hardness wastewater. For wastewater with lower hardness, i.e., the concentration of Ca 2+ <3000 mg / L, although the cost of sodium sulfate is lower than that of sodium carbonate / sodium hydroxide agents, there are limitations such as low Ca 2+ removal efficiency and slow reaction rate in the softening and precipitation of sodium sulfate. Therefore, the concentration of Ca 2+ in the high-salt and high-hardness wastewater is limited in this patent.

[0055] In a specific embodiment, the concentration of Ca in the pre-hardness-removed wastewater 2+ is 1000 mg / L - 2000 mg / L, the seed crystal is calcium sulfate dihydrate, and the dosage is 5 g / L - 20 g / L. It should be noted that after treatment in the sodium sulfate softening unit 1-11, the concentration of Ca 2+ in the effluent is controlled at 1000 mg / L - 2000 mg / L. The purpose of limiting the concentration of Ca 2+ in the effluent of this unit is to achieve the best technical and economic effect of the hardness removal process combination with the subsequent double-alkali method softening and precipitation unit 1-2. The type of seed crystal added in the sodium sulfate softening unit 1-11 is solid calcium sulfate dihydrate. The addition of the seed crystal can promote the nucleation process of calcium sulfate crystals, greatly shorten the reaction time, and thus save the floor area of the structure. However, with the excessive dosage, this promoting effect gradually becomes less obvious, and at the same time, the agent cost will increase.

[0056] In a specific embodiment, the concentration of Ca in the hardness- and silicon-removed wastewater 2+ ≤ 40 mg / L, and the total silicon concentration ≤ 10 mg / L.

[0057] In a specific embodiment, the sludge density index of the effluent after treatment in the ceramic membrane filtration unit 1-4 ≤ 5, the concentration of bicarbonate in the effluent ≤ 50 mg / L; calculated by the dosage of CaCO3, the total hardness of the decarbonated wastewater ≤ 5 mg / L.

[0058] Application Example The polysilicon tail gas washing wastewater was treated with the polysilicon wastewater zero - discharge treatment system and treatment method provided in Example 1. Among them, the flow rate of the wastewater was 30 m 3 / h, and the Ca 2+ in the wastewater transported to the conditioning and homogenization unit 4 was 6753 mg / L, SO4 2- was 1521 mg / L, Si was 65 mg / L, TDS was 38187 mg / L, and COD was 305 mg / L.

[0059] In the first - stage nanofiltration concentrate produced by the first - stage nanofiltration unit 2 - 2, Ca 2+ was 22 mg / L, SO4 2- was 45818 mg / L, Si was 20 mg / L, COD was 338 mg / L, and the flow rate of the first - stage nanofiltration concentrate was 30 m 3 / h.

[0060] The wastewater in the conditioning and homogenization unit 4 and the first - stage nanofiltration concentrate were mixed and then entered the sodium sulfate softening unit 1 - 11. After mixing and entering the sodium sulfate softening unit 1 - 11, Ca 2+ was 5100 mg / L, SO4 2- was 10639 mg / L, and Si was 53 mg / L.

[0061] The chemicals added in the sodium sulfate softening unit 1 - 11 were a 10% (mass concentration) Na2SO3 solution and calcium sulfate dihydrate seeds. Among them, the flow rate of the Na2SO3 solution was 1.7 m 3 / h, the dosage of calcium sulfate dihydrate seeds was 10 g / L, and the designed effluent Ca 2+ in the sodium sulfate softening unit 1 - 11 was reduced to 1000 mg / L. After actual reaction, the Ca 2+ in the supernatant was 865 mg / L, SO4 2- was 11000 mg / L.

[0062] The effluent of the sodium sulfate softening unit 1 - 11 entered the sludge storage unit 1 - 12, the filtration unit 1 - 13, and the filtrate collection unit 1 - 14 in sequence. The effluent of the filtrate collection unit 1 - 14 entered the double - alkali method softening and precipitation unit 1 - 2. The chemicals added in the double - alkali method softening and precipitation unit 1 - 2 were a 40% (mass concentration) NaOH solution, a 10% (mass concentration) Na2CO3 solution, PFS, and PAM. The flow rate of the NaOH solution was 1 m 3 / h, the dosage of the Na2CO3 solution was 6 L / h, the dosage of PFS was 50 L / h, and the dosage of PAM was 200 L / h. The Ca 2+ in the effluent of the double - alkali method softening and precipitation unit 1 - 2 was 150 mg / L.

[0063] The effluent from the dual-alkali softening and precipitation unit 1-2 enters the enhanced hardness and silica removal unit 1-3. The chemicals added in the enhanced hardness and silica removal unit 1-3 are sodium aluminate solution with a mass concentration of 10%, PFS, and PAM. The dosing amount of the sodium aluminate solution is 10 L / h, the dosing amount of PFS is 5 L / h, and the dosing amount of PAM is 110 L / h. The Ca in the effluent of the enhanced hardness and silica removal unit 1-3 2+ : 36 mg / L, SO4 2- : 20 mg / L, total alkalinity: 150 mg / L.

[0064] The effluent from the enhanced hardness and silica removal unit 1-3 enters the ceramic membrane filtration unit 1-4; the water production flux of the ceramic membrane filtration unit 1-4 is 242 LMH.

[0065] The effluent from the ceramic membrane filtration unit 1-4 enters the decarbonation unit 1-5. The chemical added in the decarbonation unit 1-5 is hydrochloric acid with a mass concentration of 31%. The dosing amount of hydrochloric acid is 10 L / h. The Ca in the effluent of the decarbonation unit 1-5 2+ : 40 mg / L, total alkalinity: 40 mg / L.

[0066] The effluent from the decarbonation unit 1-5 enters the ion exchange unit 1-6. The Ca in the effluent of the ion exchange unit 1-6 2+ : 5 mg / L.

[0067] The effluent from the ion exchange unit 1-6 enters the COD purification membrane unit 2-1. The COD of the effluent of the COD purification membrane unit 2-1 is 152 mg / L, and the COD of the concentrated water of the COD purification membrane is 3711 mg / L. The effluent from the COD purification membrane unit 2-1 enters the first-stage nanofiltration unit 2-2, and the concentrated water of the COD purification membrane enters the second MVR forced circulation evaporation and crystallization unit 3-22.

[0068] The recovery rate of the first-stage nanofiltration unit 2-2 is 80%. The SO4 in the effluent of the first-stage nanofiltration unit 2-2 2- : 477 mg / L. The effluent from the first-stage nanofiltration unit 2-2 enters the second-stage nanofiltration unit 2-3. The first-stage nanofiltration unit 2-2 produces first-stage nanofiltration concentrated water, and the SO4 in the first-stage nanofiltration concentrated water 2- : 45818 mg / L. The first-stage nanofiltration concentrated water is refluxed and discharged and mixed with the wastewater discharged from the regulation and homogenization unit 4 and then enters the sodium sulfate softening unit 1-11.

[0069] The recovery rate of the second-stage nanofiltration unit 2-3 is 85%. The SO4 in the effluent of the second-stage nanofiltration unit 2-3 2- : 48 mg / L. The effluent from the second-stage nanofiltration unit 2-3 enters the MVR falling film evaporation unit 3-1 and the circulation evaporation and crystallization unit 3-2 in sequence. The second-stage nanofiltration unit 2-3 produces second-stage nanofiltration concentrated water, and the SO4 in the second-stage nanofiltration concentrated water 2-: 2911 mg / L. The secondary nanofiltration concentrated water is recycled to the primary nanofiltration unit 2-2.

[0070] The evaporation crystallization system 3 processes water at a rate of 31 t / h, with a total evaporation rate of 29 t / h, a salt output of 1 t / h, and the moisture content of the salt being 3%; the discharged concentrated mother liquor volume is 0.2 t / h to 0.5 t / h; it enters the separation unit 3-3, and the separation unit 3-3 separates solid and liquid to obtain sodium chloride salt. The sodium chloride product meets the superior requirements of refined industrial salt in GB / T 5462 "Industrial Salt". The concentrated mother liquor generated by the solid-liquid separation in the separation unit 3-3 enters the mother liquor drying unit 3-4, and the moisture content of the miscellaneous salt after the drying treatment in the mother liquor drying unit 3-4 is ≤ 10%.

[0071] To demonstrate the technical and economic advantages of sodium sulfate softening + double-alkali method softening in the pretreatment system of the present invention compared with the traditional double-alkali method softening process, a comparative analysis of the chemical dosage and chemical cost was carried out through experiments, as shown in Table 1 below. The reference market prices of the chemicals sodium sulfate, sodium carbonate, caustic soda, and concentrated hydrochloric acid used for chemical addition are 400 yuan / t, 2000 yuan / t, 880 yuan / t, and 400 yuan / t respectively.

[0072] As can be seen from Table 1, the cost of the traditional double-alkali method softening chemicals and the comprehensive chemicals of sulfate softening + double-alkali method softening is the sum of the costs of each chemical dosage. Among them, the cost of the traditional double-alkali method softening chemicals is 33.4 yuan / t, and the cost of the comprehensive chemicals of sulfate softening + double-alkali method softening is 20.8 yuan / t, which is 37.7% lower than that of the double-alkali method. Therefore, sulfate softening + double-alkali method softening has significant economic benefits in terms of chemical cost.

[0073] Table 1 Chemical dosage and chemical cost of sulfate softening + double-alkali method softening and double-alkali method softening

[0074] It should be noted that when the present invention involves numerical ranges, it should be understood that any value between the two endpoints of each numerical range and the two endpoints can be selected. Since the adopted step methods are the same as those in the embodiments, in order to prevent repetition, the present invention describes the preferred embodiments. Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0075] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A zero-emission treatment system for polysilicon wastewater, characterized in that, It includes a pretreatment system (1), a membrane treatment system (2) and an evaporation crystallization system (3) connected in sequence along the water flow direction; The pretreatment system (1) includes a sodium sulfate softening and precipitation unit (1-1), a double-alkali method softening and precipitation unit (1-2), a strengthening hard and silicon removal unit (1-3), a ceramic membrane filtration unit (1-4), a decarbonation unit (1-5) and an ion exchange unit (1-6) connected in sequence along the water flow direction; The membrane treatment system (2) includes a COD purification membrane unit (2-1), a first-stage nanofiltration unit (2-2) and a second-stage nanofiltration unit (2-3) connected in sequence along the water flow direction; The ion exchange unit (1-6) is connected to the COD purification membrane unit (2-1), and the first-stage nanofiltration unit (2-2) is connected to the sodium sulfate softening and precipitation unit (1-1); The evaporation crystallization system (3) includes an MVR falling film evaporation unit (3-1), a circulating evaporation crystallization unit (3-2), a separation unit (3-3) and a mother liquor drying unit (3-4) connected in sequence along the water flow direction; The MVR falling film evaporation unit (3-1) is connected to the second-stage nanofiltration unit (2-3), and the circulating evaporation crystallization unit (3-2) is connected to the COD purification membrane unit (2-1).

2. The zero-discharge treatment system for polysilicon wastewater according to claim 1, wherein The sodium sulfate softening and precipitation unit (1-1) includes a sodium sulfate softening unit (1-11), a sludge storage unit (1-12), a filtration unit (1-13) and a filtrate collection unit (1-14) connected in sequence along the water flow direction. The first-stage nanofiltration unit (2-2) is connected to the sodium sulfate softening unit (1-11). The sludge storage unit (1-12) is connected to the double-alkali method softening and precipitation unit (1-2), the strengthening hard and silicon removal unit (1-3) and the ceramic membrane filtration unit (1-4). The filtrate collection unit (1-14) is connected to the double-alkali method softening and precipitation unit (1-2).

3. The polysilicon wastewater zero-discharge treatment system according to claim 2, wherein The pretreatment system (1) further includes a conditioning and homogenizing unit (4). The conditioning and homogenizing unit (4) is connected to the sodium sulfate softening unit (1-11). The ion exchange unit (1-6) is connected to a regeneration liquid collection unit (1-7), and the regeneration liquid collection unit (1-7) is connected to the conditioning and homogenizing unit (4).

4. The zero-discharge treatment system for polysilicon wastewater according to claim 3, wherein, The second-stage nanofiltration unit (2-3) is reflux-connected to the first-stage nanofiltration unit (2-2).

5. The polysilicon wastewater zero-discharge treatment system according to claim 4, characterized in that, The cyclic evaporation crystallization unit (3-2) includes a first MVR forced circulation evaporation crystallization unit (3-21) and a second MVR forced circulation evaporation crystallization unit (3-22). The separation unit (3-3) includes a first solid-liquid separation unit (3-31) and a second solid-liquid separation unit (3-32). The second solid-liquid separation unit (3-32) is connected to the mother liquor drying unit (3-4). Along the water flow direction, the first MVR forced circulation evaporation crystallization unit (3-21) is connected to the first solid-liquid separation unit (3-31), the first solid-liquid separation unit (3-31) is connected to the second MVR forced circulation evaporation crystallization unit (3-22), the second MVR forced circulation evaporation crystallization unit (3-22) is connected to the second solid-liquid separation unit (3-32), the first solid-liquid separation unit (3-31) is reflux-connected to the first MVR forced circulation evaporation crystallization unit (3-21), the second solid-liquid separation unit (3-32) is reflux-connected to the second MVR forced circulation evaporation crystallization unit (3-22), and the COD purification membrane unit (2-1) is connected to the second MVR forced circulation evaporation crystallization unit (3-22).

6. A zero-emission treatment method for polysilicon wastewater, characterized in that, It is carried out by using the polysilicon wastewater zero-discharge treatment system described in claim 5, and includes the following steps: The wastewater from polysilicon production first enters the conditioning and homogenization unit (4) to homogenize and equalize the wastewater, and then enters the sodium sulfate softening unit (1-11) to remove Ca 2+ in the wastewater. Meanwhile, seeds are added to the sodium sulfate softening unit (1-11) to induce the crystallization and nucleation of calcium sulfate dihydrate, forming pre-hardness-removed wastewater; The pre-hardness removal wastewater is sequentially precipitated by the sludge storage unit (1-12) and separated by the filtration unit (1-13) to obtain filtered wastewater. The filtered wastewater is discharged to the double-alkali method softening and precipitation unit (1-2) through the filtrate collection unit (1-14) to remove Ca 2+ and Mg 2+ in the filtered wastewater. Then, it is discharged to the enhanced hardness and silicon removal unit (1-3) through the double-alkali method softening and precipitation unit (1-2) to remove the total silicon in the hardness removal wastewater and stabilize the indexes of the hardness removal wastewater, forming hardness and silicon removal wastewater. At the same time, the sludge storage unit (1-12) serves as a sludge treatment system and receives the sludge discharged from the double-alkali method softening and precipitation unit (1-2), the enhanced hardness and silicon removal unit (1-3), and the ceramic membrane filtration unit (1-4); The hard-removing and silicon-removing wastewater is discharged to the ceramic membrane filtration unit (1-4) through the enhanced hard-removing and silicon-removing unit (1-3) to remove SS and colloids, and then discharged to the decarbonization unit (1-5) through the ceramic membrane filtration unit (1-4) to remove carbonates and bicarbonates, forming decarbonized wastewater. The decarbonized wastewater is discharged to the ion exchange unit (1-6) to remove residual Ca 2+ and Mg 2+ , forming ion exchange product water and generating a regeneration liquid. After being collected by the regeneration liquid collection unit (1-7), the regeneration liquid is discharged to the homogenization unit; The ion-exchanged product water is discharged to the COD purification membrane unit (2-1) through the ion exchange unit (1-6) to remove the COD in the ion-exchanged product water, forming COD purification membrane concentrated water and COD purification product water. The COD purification membrane concentrated water enters the second MVR forced circulation evaporation crystallization unit (3-22). The COD purification product water successively enters the first-stage nanofiltration unit (2-2) and the second-stage nanofiltration unit (2-3) to separate Cl - and SO4 2- and organic matters. Among them, the first-stage nanofiltration unit (2-2) filters to form first-stage nanofiltration product water and first-stage nanofiltration concentrated water. The first-stage nanofiltration product water enters the second-stage nanofiltration unit (2-3), and the first-stage nanofiltration concentrated water is discharged to the sodium sulfate softening unit (1-11) to participate as a reactant in the removal of Ca 2+ in the wastewater for polysilicon production. The second-stage nanofiltration unit (2-3) filters to produce second-stage nanofiltration product water and second-stage nanofiltration concentrated water, and the second-stage nanofiltration concentrated water is refluxed into the first-stage nanofiltration unit (2-2); The secondary nanofiltration produced water enters the MVR falling film evaporation unit (3-1) for evaporating the nanofiltration produced water to form a concentrated mother liquor. After crystallization by the first MVR forced circulation evaporation crystallization unit (3-21) and separation by the first solid-liquid separation unit (3-31), industrial salt and a first enriched mother liquor are formed. After secondary crystallization by the second MVR forced circulation evaporation crystallization unit (3-22) and secondary separation by the second solid-liquid separation unit (3-32), a first miscellaneous salt and a second enriched mother liquor are formed. The second enriched mother liquor is dried by the mother liquor drying unit (3-4) to form organic matter and a second miscellaneous salt; realizing the reduction treatment and zero discharge of organic pollutants in the wastewater from polysilicon production.

7. The zero-emission treatment method for polysilicon wastewater according to claim 6, wherein, Ca in the wastewater from polysilicon production 2+ Concentration ≥ 3000 mg / L.

8. The zero-emission treatment method for polysilicon wastewater according to claim 6, wherein Filter Ca in wastewater 2+ The concentration is 1000 mg / L to 2000 mg / L, the seed crystal is calcium sulfate dihydrate, and the dosage is 5 g / L to 20 g / L.

9. The zero-discharge treatment method for polysilicon wastewater according to claim 6, characterized in that, Ca in the wastewater from hard silicon removal 2+ Concentration ≤ 40 mg / L, total silicon concentration ≤ 10 mg / L.

10. The zero-emission treatment method for polysilicon wastewater according to claim 6, wherein The sludge density index of the effluent after treatment by the ceramic membrane filtration unit (1-4) is ≤5, and the bicarbonate concentration of the effluent is ≤50 mg / L; calculated by the dosage of CaCO3, the total hardness of the decarbonated wastewater is ≤5 mg / L.

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