A zero-discharge treatment system and method for polysilicon wastewater

Abstract

The present invention belongs to the technical field of wastewater treatment, and specifically relates to a zero-discharge treatment system and method for polysilicon wastewater. It comprises a pretreatment system, a membrane treatment system, and an evaporation crystallization system, which are sequentially connected along the direction of water flow; the pretreatment system comprises a sodium sulfate softening and precipitation unit, a double-alkali softening and precipitation unit, an enhanced hardness and silicon removal unit, a ceramic membrane filtration unit, a decarbonization unit, and an ion exchange unit; the membrane treatment system comprises a COD purification membrane unit and a two-stage nanofiltration unit; and the evaporation crystallization system comprises an evaporation unit, a circulating evaporation crystallization unit, a separation unit, and a mother liquor drying unit. The present invention not only produces high-value industrial by-product gypsum through sodium sulfate softening and precipitation in the pretreatment stage, while reducing the cost of softening agents, but also recycles the sodium sulfate produced by the membrane treatment system, reducing the investment cost of the evaporation crystallization system, and achieving resource utilization of wastewater treatment by-products and sustainable environmental development.

Description

Technical Field

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

[0002] Salt wastewater typically refers to wastewater containing organic matter and at least 3.5% total dissolved solids. In industries such as mining, coal chemical industry, and polysilicon, production wastewater is often accompanied by high hardness. Direct discharge or reuse of this high-salt, high-hardness wastewater without treatment can lead to a range of problems, including soil salinization, groundwater mineralization, equipment scaling, and pipe blockage. With the growing imbalance between water supply and demand and increasingly stringent national environmental protection policies, industrial wastewater discharge standards are gradually being tightened, and zero-emission technology has become a mainstream trend in industrial water treatment.

[0003] At present, zero discharge of industrial wastewater is widely used in wastewater treatment in coal chemical industry, chlor-alkali, power plants, etc., but in most polysilicon industries, zero discharge of wastewater has not yet been achieved. The current mainstream processes for polysilicon production include the modified Siemens method and the silane method. The tail gas in the production process is absorbed and neutralized by lime water + alkali solution, and the effluent contains a large amount of calcium salts and sodium salts, which is a 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, a targeted process route needs to be given to achieve zero discharge of wastewater. The conventional zero-discharge process route for industrial wastewater is pretreatment + membrane concentration + evaporation crystallization. Among them, the pretreatment and hardness removal stage generally have the problems of large dosage of reagents and high operating costs. In addition, in order to completely remove sulfate, the current traditional high-salt wastewater zero-discharge process usually has two evaporation crystallization systems for sodium chloride and sodium sulfate, which brings high construction costs. Summary of the Invention

[0004] To address the above-mentioned issues, the present invention provides a zero-discharge polysilicon wastewater treatment system and method. This system addresses the high reagent costs associated with traditional zero-discharge processes while optimizing the process route. Furthermore, it meets the requirements for polysilicon production wastewater reuse and standard discharge, achieving zero wastewater discharge. Not only does it produce high-value industrial byproduct gypsum through sodium sulfate softening and precipitation during the pretreatment stage, reducing softening reagent costs, but it also recycles the sodium sulfate produced by the membrane treatment system, reducing the investment cost of the evaporation and crystallization system. This system achieves resource utilization of wastewater treatment byproducts and sustainable environmental development.

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

[0006] One of the purposes of the present invention is to provide a zero-discharge treatment system for polysilicon wastewater, comprising 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 double alkali softening and precipitation unit, an enhanced hardness and silicon removal unit, a ceramic membrane filtration unit, a decarbonization unit and an ion exchange unit which are sequentially connected along the water flow direction.

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

[0009] The evaporation crystallization system includes an MVR falling film evaporation unit, a circulating evaporation crystallization unit, a separation unit and a mother liquor drying unit which are sequentially connected along the water flow direction; the MVR falling film evaporation unit is connected to the secondary nanofiltration unit, and the circulating evaporation 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 connected in sequence along the water flow direction, the first-level nanofiltration unit is connected to the sodium sulfate softening unit, the sludge storage units are connected to the dual-alkali softening and precipitation unit, the enhanced hardness and silicon removal unit and the ceramic membrane filtration unit, and the filtrate collection unit is connected to the dual-alkali softening and precipitation unit.

[0011] Furthermore, the pretreatment system also includes a regulating and homogenizing unit, which is connected to the sodium sulfate softening and precipitation unit; the ion exchange unit is connected to a regeneration liquid collecting unit, which is connected to the regulating and homogenizing unit.

[0012] Furthermore, the secondary nanofiltration unit is reflux-connected to the primary nanofiltration unit.

[0013] Furthermore, the circulating evaporation crystallization unit includes a first MVR forced circulation evaporation crystallization unit and a second MVR forced circulation evaporation 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 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 crystallization unit, the second MVR forced circulation evaporation crystallization unit is connected to the second solid-liquid separation unit, the first solid-liquid separation unit reflux is connected to the first MVR forced circulation evaporation crystallization unit, the second solid-liquid separation unit reflux is connected to the second MVR forced circulation evaporation crystallization unit, and the COD purification membrane unit is connected to the second MVR forced circulation evaporation crystallization unit.

[0014] A second object of the present invention is to provide a method for treating polysilicon wastewater with zero discharge, which is carried out using the above-mentioned polysilicon wastewater zero discharge treatment system and includes the following steps:

[0015] S1. The wastewater from polysilicon production first enters the homogenization unit to homogenize and equalize the wastewater, and then enters the sodium sulfate softening unit to remove Ca in the wastewater. 2+ At the same time, seed crystals are added to the sodium sulfate softening unit to induce the nucleation of calcium sulfate dihydrate crystals to form pre-hard wastewater.

[0016] S2. The primary wastewater is sequentially 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 precipitation unit through the filtrate collection unit to remove Ca in the filtered wastewater. 2+ and Mg 2+ , and then discharged to the enhanced hardness and silicon removal unit through the double alkali softening and sedimentation unit to remove the total silicon in the hardness removal wastewater and stabilize the indicators of the hardness removal wastewater to form hardness and silicon removal wastewater. At the same time, the sludge storage unit serves as a sludge treatment system, receiving the sludge discharged from the double alkali softening and sedimentation unit, the enhanced hardness and silicon removal unit and the ceramic membrane filtration unit.

[0017] 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 SS and colloids in the hardness and silicon removal wastewater, and then discharged to the decarbonization unit through the ceramic membrane filtration unit to remove carbonate and bicarbonate to form decarbonized wastewater. The decarbonized wastewater is discharged to the ion exchange unit to remove residual Ca 2+ and Mg 2+ , forming ion exchange water and generating regeneration liquid, the regeneration liquid is collected by the regeneration liquid collection unit and discharged to the homogenization unit.

[0018] S4, ion exchange water is discharged to the COD purification membrane unit through the ion exchange unit to remove COD in the ion exchange water, forming COD purification membrane concentrated water and COD purified water. The COD purification membrane concentrated water enters the second MVR forced circulation evaporation crystallization unit, and the COD purified water enters the first nanofiltration unit and the second nanofiltration unit in turn to separate Cl - and SO4 2- The first-level nanofiltration unit filters to form first-level nanofiltration water and first-level nanofiltration concentrated water. The first-level nanofiltration water enters the second-level nanofiltration unit, and the first-level nanofiltration concentrated water is discharged to the sodium sulfate softening unit to participate in the Ca in the wastewater of polysilicon production as a reactant. 2+ The secondary nanofiltration unit filters to produce secondary nanofiltration product water and secondary nanofiltration concentrated water, and the secondary nanofiltration concentrated water flows back to the primary nanofiltration unit.

[0019] S5. The secondary nanofiltration water product enters the MVR falling film evaporation unit, which is used to evaporate the nanofiltration water product to form a concentrated mother liquor. The concentrated mother liquor is crystallized in the first MVR forced circulation evaporation crystallization unit and separated in the first solid-liquid separation unit to form industrial salt and a first enriched mother liquor. The first enriched mother liquor and COD purification membrane concentrated water enter the second MVR forced circulation evaporation crystallization unit for secondary crystallization and the second solid-liquid separation unit for secondary separation to form a first miscellaneous salt and a second enriched mother liquor. The second enriched mother liquor is dried in the mother liquor drying unit to form organic matter and a second miscellaneous salt; thus, the reduction treatment and zero emission of organic pollutants in the wastewater of polysilicon production are achieved.

[0020] Furthermore, Ca in wastewater from polysilicon production 2+ Concentration ≥3000mg / L.

[0021] Furthermore, the Ca in the filtered wastewater 2+ The concentration is 1000mg / L~2000mg / L, the seed crystal is calcium sulfate dihydrate, and the dosage is 5g / L~20g / L.

[0022] Furthermore, the secondary wastewater is used to remove the Ca in the hard and silicon wastewater. 2+ Concentration ≤40mg / L, total silicon concentration ≤10mg / L.

[0023] Furthermore, after treatment by the ceramic membrane filtration unit, the effluent sludge density index is ≤5, and the effluent bicarbonate concentration is ≤50mg / L; based on the amount of CaCO3 used, the total hardness of the decarbonization wastewater is ≤5mg / L.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The zero-discharge treatment system for crystalline silicon wastewater provided by the present invention comprises a pretreatment system, a membrane treatment system and an evaporation crystallization system connected in sequence along the direction of water flow, wherein the purpose of the pretreatment system is to remove total hardness, total silicon, suspended solids and decarbonization, the membrane treatment system adopts a COD purification membrane and a two-stage nanofiltration process to remove some organic pollutants in the wastewater and separate monovalent and divalent ions, and the purpose of the evaporation crystallization system is to separate organic pollutants and miscellaneous salts while producing sodium chloride product salt, so as to solve the problem of high reagent cost in the traditional zero-discharge process and optimize the process route, and meet the requirements for reuse and standard discharge of polysilicon production wastewater, thereby achieving zero wastewater discharge; not only does high-value industrial by-product gypsum be produced by sodium sulfate softening precipitation in the pretreatment stage, while reducing the cost of softening reagents, but the sodium sulfate produced by the membrane treatment system is also recycled, reducing the investment cost of the evaporation crystallization system, and achieving resource utilization of wastewater treatment by-products and sustainable development of the environment.

[0026] (2) The pretreatment system adopted by the present invention adopts a combination of sodium sulfate softening precipitation + double alkali softening precipitation + enhanced hardness and silicon removal + ion exchange to strictly control Ca 2+ It is stable at a very low level to prevent the impact on the membrane treatment system and evaporation crystallization system process equipment. In addition, when treating high-salt and high-hardness wastewater in industries such as polysilicon, most of the Ca 2+ Removal through sodium sulfate precipitation significantly reduces reagent costs compared to conventional dual-alkali softening processes. Furthermore, sodium sulfate softening precipitation produces a high-value industrial byproduct, gypsum, which has more applications and greater resource utilization value than the dual-alkali softening product, calcium carbonate.

[0027] (3) In view of the fact that wastewater generated by polysilicon contains organic matter, 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 it from affecting 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 is circulated into the main process, it passes through the COD purification membrane unit to effectively remove the organic pollutants in the wastewater, thereby maintaining the COD in the influent of the nanofiltration unit at an appropriate level and ensuring the stability of the entire process. In addition, the concentrated water of the COD purification membrane unit is sent to the evaporation crystallization system, and is treated by evaporation, crystallization, solid-liquid separation and mother liquor drying, and finally forms a solid product discharge system, thereby achieving the reduction of organic pollutants in the wastewater and zero discharge.

[0028] (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 matter through the MVR evaporation crystallization unit and separation unit, thereby achieving zero discharge of polysilicon wastewater and waste resource recovery, with significant ecological and environmental benefits. Since most of the sulfate is recycled in the primary nanofiltration unit, the small amount of sodium sulfate produced by evaporation crystallization is discharged from the system as miscellaneous salts. Therefore, compared with traditional evaporation crystallization systems, the number of sodium sulfate evaporation crystallization units is reduced, thereby reducing engineering construction costs.

[0029] (5) In the present invention, based on the selection of a lower-cost sodium sulfate agent for hardness removal in the pretreatment system, the sulfate in the primary nanofiltration concentrated water produced by the primary nanofiltration unit in the membrane treatment system is refluxed to the sodium sulfate softening unit, so that the by-products of the membrane treatment are recycled, thereby further reducing the amount of added chemicals, saving chemical costs, and improving the resource recycling rate. Moreover, since most of the sulfate is refluxed and recycled in the primary nanofiltration unit, the small amount of sodium sulfate produced by evaporation and crystallization is discharged from the system as miscellaneous salts. Therefore, compared with the traditional evaporation and crystallization system, the sodium sulfate evaporation and crystallization unit is reduced, thereby reducing the engineering construction cost.

[0030] (6) The sodium sulfate softening and precipitation system in the pretreatment system of the present invention is composed of a sodium sulfate softening unit, a sludge storage unit, a filtration unit and a filtrate collection unit. The sludge storage unit and the filtration unit are the main softening processes that participate in the solid-liquid separation of calcium sulfate precipitation to ensure the normal operation of the subsequent treatment units. At the same time, they also serve as a sludge treatment system to receive sludge discharged from the dual-alkali softening and precipitation unit, the enhanced hardness and silicon removal unit and the ceramic membrane filtration unit, thus achieving multi-purpose integration and being an attempt to optimize the existing zero-emission process. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the overall process of the zero-discharge treatment system for polysilicon wastewater of the present invention.

[0032] Figure 2 This is a schematic diagram of the process flow of the pretreatment system of the present invention.

[0033] Figure 3 It is a schematic diagram of the process flow of the membrane treatment system of the present invention.

[0034] Figure 4 It is a schematic diagram of the process flow of the evaporation crystallization system of the present invention.

[0035] Illustration:

[0036] 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 softening and precipitation unit, 1-3. Enhanced hardness and silicon removal unit, 1-4. Ceramic membrane filtration unit, 1-5. Decarbonization unit, 1-6. Ion exchange unit, 1-7. Regeneration liquid collection unit, 2. Membrane treatment system, 2-1. COD purification membrane unit Element, 2-2, first-stage nanofiltration unit, 2-3, second-stage nanofiltration unit, 3, evaporation crystallization system, 3-1, MVR falling film evaporation unit, 3-2, circulation evaporation crystallization unit, 3-21, first MVR forced circulation evaporation crystallization unit, 3-22, second MVR forced circulation evaporation 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, homogenization adjustment unit. DETAILED DESCRIPTION

[0037] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0038] 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 scope of protection of the present invention. Unless otherwise specified, the various raw materials, reagents and equipment used in the following embodiments of the present invention can be purchased from the market or prepared by existing methods.

[0039] 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 scope of protection of the present invention. Certain terms are used in the present invention to refer to specific components. Those skilled in the art will understand that technicians may use different terms to refer to the same component.

[0040] The following is further described through specific examples.

[0041] Example 1

[0042] The present invention provides a zero-discharge treatment system for polysilicon wastewater, such as Figure 1 Shown, including:

[0043] A pretreatment system 1, a membrane treatment system 2 and an evaporation crystallization system 3 are 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 softening and precipitation unit 1-2, an enhanced hardness and silicon removal unit 1-3, a ceramic membrane filtration unit 1-4, a decarbonization 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 primary nanofiltration unit 2-2 and a secondary 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 primary 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 secondary nanofiltration unit 2-3, and the circulating evaporation crystallization unit 3-2 is connected to the COD purification membrane unit 2-1.

[0044] In the present invention, the purpose of the pretreatment system 1 is to remove total hardness, total silicon, suspended solids and decarbonization, wherein the sodium sulfate softening precipitation unit 1-1 is fed with sodium sulfate and the first-level nanofiltration unit 2-2 produces sulfate, so that sodium sulfate and Ca in wastewater are 2+ Mixed reaction, most of the Ca in the wastewater is removed by sodium sulfate precipitation 2+ At the same time, calcium sulfate dihydrate seeds are added in the reaction to induce calcium sulfate dihydrate crystal nucleation, thereby shortening the reaction time; the dosing agent of the double alkali softening precipitation unit 1-2 can be NaOH, Na2CO3, PFS or PAM to further remove Ca 2+ and Mg2+ ; The optional dosing agents for the enhanced hardness and silicon removal units 1-3 include NaOH, Na2CO3, PFS, PAM or NaAlO2 to ensure the stability of the effluent indicators of the previous hardness removal step and remove the total silicon in the wastewater; the ceramic membrane filtration units 1-4 are mainly used to filter SS and colloids in the wastewater; the dosing agent for the decarbonization units 1-5 is mainly hydrochloric acid, which is mainly used to remove residual carbonates and bicarbonates in the wastewater to prevent the operation of the membrane treatment system 2 unit from being affected; the ion exchange units 1-6 are mainly used to deeply remove residual Ca in the wastewater 2+ Mg 2+ The pretreatment system 1 of the present invention adopts a combination of sodium sulfate softening precipitation + double alkali softening precipitation + enhanced hardness and silicon removal + ion exchange to remove hardness, which not only strictly controls Ca 2+ It is stable at a very low level to prevent the impact on the membrane treatment system 2 and the evaporation crystallization system 3 process equipment. In addition, when treating high-salt and high-hardness wastewater in industries such as polysilicon, most of the Ca 2+ Removal through sodium sulfate precipitation significantly reduces reagent costs compared to conventional dual-alkali softening processes. Furthermore, sodium sulfate softening precipitation produces a high-value industrial byproduct, gypsum, which has more applications and greater resource utilization value than the dual-alkali softening product, calcium carbonate.

[0045] The membrane treatment system 2 adopts COD purification membrane and 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, removing COD in the wastewater to prevent organic matter from blocking the nanofiltration unit and affecting normal operation; on the other hand, the primary nanofiltration concentrated water produced by the primary nanofiltration unit 2-2 is returned to the sodium sulfate softening precipitation unit 1-1 and treated by the COD purification membrane unit 2-1. The primary nanofiltration unit 2-2 and the secondary nanofiltration unit 2-3 are mainly used to separate the COD in the form of Cl - The main monovalent ions and SO4 2- The main divalent ions and organic matter, etc., provide conditions for the subsequent evaporation and crystallization system 3 to produce sodium chloride industrial salt. In view of the fact that the wastewater generated by polysilicon contains organic matter, the present invention sets a COD purification membrane unit 2-1 in the membrane treatment system 2 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 and crystallization system 3. At the same time, after the COD in the concentrated water of the nanofiltration unit is circulated into the sodium sulfate softening and precipitation unit 1-1, it passes through the COD purification membrane unit 2-1 to effectively remove organic pollutants in the wastewater, thereby maintaining the COD in the influent of the nanofiltration unit at an appropriate level and ensuring the stability of the entire process.

[0046] The purpose of evaporation and crystallization system 3 is to simultaneously produce sodium chloride product salt and separate organic contaminants and miscellaneous salts. MVR falling-film evaporation unit 3-1 primarily evaporates the solution; circulating evaporation and crystallization unit 3-2 primarily crystallizes NaCl and miscellaneous salts; separation unit 3-3 separates the crystals from the supernatant; and mother liquor drying unit 3-4 is primarily used for drying and recovering organic matter. The evaporation and crystallization system 3 of the present invention not only crystallizes and recovers salts such as sodium chloride, but also, through circulating evaporation and crystallization unit 3-2 and separation unit 3-3, recovers organic matter through drying. This achieves zero discharge of polysilicon wastewater and waste resource recovery, resulting in significant ecological and environmental benefits.

[0047] In a specific embodiment, Figure 2 As 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, a primary 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 softening and precipitation unit 1-2, the enhanced hardness and silicon removal unit 1-3 and the ceramic membrane filtration unit 1-4, and the filtrate collection unit 1-14 is connected to the double alkali softening and precipitation unit 1-2. In the present invention, the sodium sulfate softening unit 1-11 makes the Ca in the wastewater 2+ Sedimentation: The precipitate produced by the previous reaction is stored in the sludge storage unit 1-12, and then solid-liquid separation is carried out through the filtration unit 1-13, and the filtrate is collected and stored through the filtrate collection unit 1-14; the sodium sulfate softening 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, wherein 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 softening process to ensure the normal operation of the subsequent treatment units, but also serve as a sludge treatment system to accept sludge from the dual-alkali softening precipitation unit 1-2, the enhanced hardness and silicon removal unit 1-3 and the ceramic membrane filtration unit 1-4, realizing multi-purpose integration and an attempt to optimize the existing zero-emission process.

[0048] In the present invention, the primary nanofiltration unit 2-2 is connected to the sodium sulfate softening unit 1-11, and the primary nanofiltration concentrated water generated by the primary nanofiltration unit 2-2 is completely refluxed to the sodium sulfate softening unit 1-11 in the pretreatment system 1, wherein sulfate is used as a reactant to participate in the sodium sulfate softening in the raw water. 2+ The amount of sodium sulfate added to the sodium sulfate softening unit 1-11 is determined by the original sulfate amount, the reflux sulfate amount and the Ca 2+It is required to jointly determine that, based on the selection of lower-cost sodium sulfate reagent for hardness removal in the pretreatment system 1, the sulfate in the primary nanofiltration concentrated water produced by the primary 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 sodium sulfate added to the sodium sulfate softening unit 1-11 is calculated from the original sulfate amount, the reflux sulfate amount and the Ca content of the effluent of this unit. 2+ The requirements are jointly determined, thereby further reducing the dosage of added chemicals, saving chemical costs, and improving resource recycling. In addition, since the vast majority of sulfate is recycled in the primary nanofiltration unit 2-2, the small amount of sodium sulfate produced by evaporation and crystallization is discharged from the system as miscellaneous salts. Therefore, compared with the traditional evaporation and crystallization system 3, the sodium sulfate evaporation and crystallization unit is reduced, reducing engineering construction costs.

[0049] In summary, this process system not only produces high-value industrial by-product gypsum through sodium sulfate softening precipitation in the pretreatment stage, while reducing the cost of softening agents, but also recycles the sodium sulfate produced by the membrane treatment system 2, reducing the investment cost of the evaporation crystallization system 3, and realizing the resource utilization of wastewater treatment by-products and sustainable development of the environment.

[0050] In a specific embodiment, Figure 2 As shown, the pretreatment system 1 also includes a regulating and homogenizing unit 4, which is connected to the sodium sulfate softening and precipitation unit 1-1, and the ion exchange unit 1-6 is connected to the regeneration liquid collecting unit 1-7, and the regeneration liquid collecting unit 1-7 is connected to the regulating and homogenizing unit 4. In the present invention, the regulating and homogenizing unit 4 is mainly used to homogenize and equalize the wastewater. After the wastewater is homogenized and equalized by the regulating and homogenizing unit 4, it is discharged to the sodium sulfate softening and precipitation unit 1-1. The ion exchange unit 1-6 is connected to the regeneration liquid collecting 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 collecting unit 1-7 and refluxed to the regulating and homogenizing unit 4 to achieve resource recycling efficiency.

[0051] In a specific embodiment, Figure 3 As shown, the secondary nanofiltration unit 2-3 is reflux-connected to the primary nanofiltration unit 2-2.

[0052] In a specific embodiment, Figure 4As shown, the circulating 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 refluxed and connected to the first MVR forced circulation evaporation crystallization unit 3-21, the second solid-liquid separation unit 3-32 is refluxed and 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.

[0053] In the present invention, a two-stage nanofiltration process is used to achieve the purpose of removing some organic pollutants in wastewater and separating monovalent and divalent ions, and the secondary nanofiltration concentrated water produced by the secondary nanofiltration unit 2-3 is refluxed to connect the primary nanofiltration unit 2-2, further improving the removal efficiency. The COD purification membrane unit 2-1 is connected to the second MVR forced circulation evaporation crystallization unit 3-22, and the COD purification membrane concentrated water produced by the COD purification membrane unit 2-1 is discharged to the second MVR forced circulation evaporation crystallization unit 3-22. After evaporation, crystallization, solid-liquid separation and mother liquor drying, a solid product discharge system is finally formed to achieve the reduction of organic pollutants in wastewater and zero emission. The mother liquor produced 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 produced 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 flowing back into the system and causing water quality deterioration.

[0054] The zero-discharge treatment method using the above-mentioned polysilicon wastewater zero-discharge treatment system includes the following steps:

[0055] S1. The wastewater from polysilicon production first enters the conditioning 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 in the wastewater. 2+ At the same time, seed crystals are added to the sodium sulfate softening unit 1-11 to induce the nucleation of calcium sulfate dihydrate crystals to form pre-hard wastewater.

[0056] S2, pre-hard wastewater is sequentially precipitated in sludge storage unit 1-12 and separated in filtration unit 1-13 to obtain filtered wastewater, which is then discharged to double alkali softening precipitation unit 1-2 through filtrate collection unit 1-14 to further remove Ca in the filtered wastewater. 2+ and Mg 2+ , and then discharged to the enhanced hardness and silicon removal unit 1-3 through the double alkali softening and sedimentation unit 1-2 to remove the total silicon in the hardness removal wastewater and stabilize the indicators of the hardness removal wastewater to form hardness and silicon removal wastewater; at the same time, the sludge storage unit 1-12 serves as a sludge treatment system, receiving sludge from the double alkali softening and sedimentation unit 1-2, the enhanced hardness and silicon removal unit 1-3 and the ceramic membrane filtration unit 1-4, realizing multi-purpose integration.

[0057] S3, the hardness and silicon removal wastewater is discharged to the ceramic membrane filtration unit 1-4 to remove SS and colloids in the hardness and silicon removal wastewater, and then discharged to the decarbonization unit 1-5 through the ceramic membrane filtration unit 1-4 to remove carbonate and bicarbonate to form decarbonized wastewater, and the decarbonized wastewater is discharged to the ion exchange unit 1-6 to further remove residual Ca 2+ and Mg 2+ , forming ion exchange water and generating regeneration liquid, the regeneration liquid is collected by the regeneration liquid collecting units 1-7 and discharged to the homogenizing unit.

[0058] S4, ion exchange product water is discharged to COD purification membrane unit 2-1 through ion exchange unit 1-6 to remove COD in ion exchange product water, forming COD purification membrane concentrated water and COD purified product water. COD purification membrane concentrated water enters the second MVR forced circulation evaporation crystallization unit 3-22, COD purified product water enters the first-level nanofiltration unit 2-2, filtered to form first-level nanofiltration product water and first-level nanofiltration concentrated water, first-level nanofiltration product water enters the second-level nanofiltration unit 2-3, filtered to form second-level nanofiltration product water and second-level nanofiltration concentrated water, first-level nanofiltration unit 2-2 and second-level nanofiltration unit 2-3 mainly separate Cl - and SO4 2- The secondary nanofiltration concentrated water is returned to the primary nanofiltration unit 2-2, and the primary nanofiltration concentrated water is completely returned and discharged and mixed with the wastewater discharged from the regulating homogenization unit 4 and then enters the sodium sulfate softening unit 1-11, wherein the sulfate in the primary nanofiltration concentrated water acts as a reactant to participate in the Ca 2+ Remove.

[0059] S5. The secondary nanofiltration product water enters the MVR falling film evaporation unit 3-1, which is used to evaporate the nanofiltration product water to form a concentrated mother liquor. The concentrated mother liquor is crystallized in the first MVR forced circulation evaporation crystallization unit 3-21 and separated in the first solid-liquid separation unit 3-31 to form industrial salt and a first enriched mother liquor. The first enriched mother liquor and the COD purification membrane concentrated water 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 to form 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. This realizes the reduction treatment and zero emission of organic pollutants in the wastewater of polysilicon production.

[0060] In a specific embodiment, Ca in wastewater from polysilicon production 2+ Concentration ≥ 3000mg / 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. Ca 2+ Concentration is less than 3000mg / L. Although sodium sulfate is cheaper than sodium carbonate / sodium hydroxide, sodium sulfate softening precipitation has Ca 2+ Due to the limitations of low removal efficiency and slow reaction rate, this patent is suitable for high-salt and high-hardness wastewater Ca 2+ The concentration is limited.

[0061] In a specific embodiment, the Ca in hard wastewater is pre-removed 2+ The concentration is 1000mg / L~2000mg / L, the seed crystal is calcium sulfate dihydrate, and the dosage is 5g / L~20g / L. It should be noted that the wastewater is treated by sodium sulfate softening unit 1-11 and the effluent Ca 2+ The concentration is controlled at 1000mg / L~2000mg / L. Limit the effluent Ca in this unit 2+ The concentration is designed to achieve optimal technical and economic results in combination with the subsequent dual-alkali softening and precipitation unit 1-2 for hardness removal. The seed crystals added to the sodium sulfate softening unit 1-11 are solid calcium sulfate dihydrate. This seeding promotes calcium sulfate crystal nucleation, significantly shortening reaction time and thus saving floor space. However, with excessive addition, this promoting effect gradually diminishes, and reagent costs also increase.

[0062] In a specific embodiment, the Ca in the hardness and silicon removal wastewater is removed. 2+ Concentration ≤40mg / L, total silicon concentration ≤10mg / L.

[0063] In a specific embodiment, after treatment by the ceramic membrane filtration units 1-4, the effluent sludge density index is ≤5, and the effluent bicarbonate concentration is ≤50 mg / L; based on the amount of CaCO3 used, the total hardness of the decarbonization wastewater is ≤5 mg / L.

[0064] Application Examples

[0065] The polysilicon wastewater zero discharge treatment system and treatment method provided in Example 1 are used to treat the polysilicon tail gas washing wastewater, wherein the wastewater flow rate is 30m 3 / h, transported to the wastewater Ca in the regulating homogenization unit 4 2+ :6753mg / L, SO4 2- : 1521mg / L, Si: 65mg / L, TDS: 38187mg / L, COD: 305mg / L.

[0066] Ca in the primary nanofiltration concentrate produced by the primary nanofiltration unit 2-2 2+ :22mg / L、SO4 2- : 45818mg / L, Si: 20mg / L, COD: 338mg / L, the flow rate of the primary nanofiltration concentrated water is 30m 3 / h.

[0067] The wastewater in the homogenization unit 4 is mixed with the concentrated water of the first-level nanofiltration and then enters the sodium sulfate softening unit 1-11. 2+ :5100mg / L, SO4 2- :10639mg / L, Si:53mg / L.

[0068] The reagents added to the sodium sulfate softening unit 1-11 are 10% mass concentration of Na2SO3 solution and dihydrate calcium sulfate seed crystals, wherein the flow rate of Na2SO3 solution is 1.7m 3 / h, the dosage of calcium sulfate dihydrate seed crystal is 10g / L, and the sodium sulfate softening unit 1-11 is designed to produce water Ca 2+ After the reaction, the Ca content in the supernatant was reduced to 1000 mg / L. 2+ :865mg / L、SO4 2- :11000mg / L.

[0069] The effluent from the sodium sulfate softening unit 1-11 enters the sludge storage unit 1-12, the filtration unit 1-13 and the filtrate collection unit 1-14 in sequence. The effluent from the filtrate collection unit 1-14 enters the double alkali softening and precipitation unit 1-2. The dosing reagents for the double alkali softening and precipitation unit 1-2 are 40% mass concentration NaOH solution, 10% mass concentration Na2CO3 solution, PFS and PAM. The flow rate of the NaOH solution is 1m 3 / h, the dosage of Na2CO3 solution is 6L / h, the dosage of PFS is 50L / h, the dosage of PAM is 200L / h, and the Ca content in the effluent of the double alkali softening and precipitation unit 1-2 is 2+ :150mg / L.

[0070] The effluent from the double alkali softening and sedimentation unit 1-2 enters the enhanced hardness and silicon removal unit 1-3. The dosing reagents in the enhanced hardness and silicon removal unit 1-3 are sodium aluminate solution, PFS and PAM with a mass concentration of 10%. The dosing rate of sodium aluminate solution is 10L / h, the dosing rate of PFS is 5L / h, and the dosing rate of PAM is 110L / h. The Ca content in the effluent from the enhanced hardness and silicon removal unit 1-3 is 2.3%. 2+ :36mg / L、SO4 2- : 20mg / L, total alkalinity: 150mg / L.

[0071] The effluent from the enhanced hardness and silicon 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 242LMH.

[0072] The effluent from the ceramic membrane filtration unit 1-4 enters the decarbonization unit 1-5. The dosing agent for the decarbonization unit 1-5 is hydrochloric acid with a mass concentration of 31%. The dosing rate of hydrochloric acid is 10L / h. The Ca 2+ : 40mg / L, total alkalinity: 40mg / L.

[0073] The effluent from the decarbonization unit 1-5 enters the ion exchange unit 1-6, and the effluent from the ion exchange unit 1-6 is Ca 2+ :5mg / L.

[0074] The effluent from the ion exchange unit 1-6 enters the COD purification membrane unit 2-1. The COD of the effluent from the COD purification membrane unit 2-1 is 152 mg / L. The COD of the concentrated water from the COD purification membrane is 3711 mg / L. The effluent from the COD purification membrane unit 2-1 enters the primary nanofiltration unit 2-2. The concentrated water from the COD purification membrane enters the second MVR forced circulation evaporation crystallization unit 3-22.

[0075] The recovery rate of the first-stage nanofiltration unit 2-2 is 80%, and the SO4 in the effluent of the first-stage nanofiltration unit 2-2 is 2- : 477mg / L, the effluent from the primary nanofiltration unit 2-2 enters the secondary nanofiltration unit 2-3, the primary nanofiltration unit 2-2 produces primary nanofiltration concentrated water, SO4 2- : 45818 mg / L, the primary nanofiltration concentrated water reflux discharge is mixed with the wastewater discharged from the regulating homogenization unit 4 and then enters the sodium sulfate softening unit 1-11.

[0076] The recovery rate of the secondary nanofiltration unit 2-3 is 85%, and the SO4 in the effluent of the secondary nanofiltration unit 2-3 is 2- : 48mg / L, the effluent from the secondary nanofiltration unit 2-3 enters the MVR falling film evaporation unit 3-1 and the circulating evaporation crystallization unit 3-2 in sequence, and the secondary nanofiltration unit 2-3 produces secondary nanofiltration concentrated water, and SO4 2-: 2911 mg / L, the secondary nanofiltration concentrated water is returned to the primary nanofiltration unit 2-2.

[0077] The evaporation crystallization system 3 processes 31 t / h of water, has a total evaporation capacity of 29 t / h, a salt output of 1 t / h, and a salt moisture content of 3%; the amount of enriched mother liquor discharged is 0.2 t / h to 0.5 t / h; it enters the separation unit 3-3, where solid-liquid separation is performed to obtain sodium chloride salt. The sodium chloride product meets the requirements of GB / T5462 "Industrial Salt" for the superior grade of refined industrial salt. The enriched mother liquor produced by the solid-liquid separation in the separation unit 3-3 enters the mother liquor drying unit 3-4. After drying treatment in the mother liquor drying unit 3-4, the moisture content of the impurity salt is ≦10%.

[0078] To demonstrate the technical and economic advantages of the sodium sulfate softening + double-alkali softening process in the pretreatment system of the present invention compared with the traditional double-alkali softening process, a comparative analysis of the dosage and reagent costs was conducted through experiments, as shown in Table 1 below. The reference market prices of the sodium sulfate, sodium carbonate, caustic soda, and concentrated hydrochloric acid reagents used for dosing are 400 yuan / t, 2000 yuan / t, 880 yuan / t, and 400 yuan / t, respectively.

[0079] As shown in Table 1, the cost of the traditional dual-alkali softening agent and the combined sulfate softening and dual-alkali softening agent is the sum of the costs of each agent dosage. Among them, the cost of the traditional dual-alkali softening agent is 33.4 yuan / t, and the combined sulfate softening and dual-alkali softening agent cost is 20.8 yuan / t, which is 37.7% lower than the dual-alkali softening agent. Therefore, the sulfate softening and dual-alkali softening method has significant economic benefits in terms of agent cost.

[0080] Table 1 Dosage and cost of sulfate softening + double alkali softening and double alkali softening

[0081]

[0082] It should be noted that when numerical ranges are mentioned in the present invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints may be selected. Since the steps and methods used are the same as those in the embodiments, in order to avoid redundancy, the present invention describes preferred embodiments. Although preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they understand the basic inventive concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0083] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A polysilicon wastewater zero discharge treatment system, characterized in that: It includes a pretreatment system (1), a membrane treatment system (2) and an evaporation crystallization system (3) which are sequentially connected along the water flow direction; The pretreatment system (1) includes a sodium sulfate softening and precipitation unit (1-1), a double alkali softening and precipitation unit (1-2), an enhanced hardness and silicon removal unit (1-3), a ceramic membrane filtration unit (1-4), a decarbonization 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 primary nanofiltration unit (2-2), and a secondary 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 primary nanofiltration unit (2-2) is connected to the sodium sulfate softening 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) which are sequentially connected along the water flow direction; the MVR falling film evaporation unit (3-1) is connected to the secondary nanofiltration unit (2-3), and the circulating evaporation crystallization unit (3-2) is connected to the COD purification membrane unit (2-1); 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) which are sequentially connected 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 dual-alkali softening and precipitation unit (1-2), the enhanced hardness and silicon removal unit (1-3) and the ceramic membrane filtration unit (1-4); and the filtrate collection unit (1-14) is connected to the dual-alkali softening and precipitation unit (1-2).

2. The polysilicon wastewater zero discharge treatment system according to claim 1, characterized in that: The pretreatment system (1) further includes a regulating homogenizing unit (4), the regulating homogenizing unit (4) is connected to the sodium sulfate softening unit (1-11), the ion exchange unit (1-6) is connected to the regeneration liquid collecting unit (1-7), and the regeneration liquid collecting unit (1-7) is connected to the regulating homogenizing unit (4).

3. The polysilicon wastewater zero discharge treatment system according to claim 2, characterized in that: The secondary nanofiltration unit (2-3) is reflux-connected to the primary nanofiltration unit (2-2).

4. The polysilicon wastewater zero discharge treatment system according to claim 3, characterized in that: The circulating 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 refluxed and connected to the first MVR forced circulation evaporation crystallization unit (3-21), the second solid-liquid separation unit (3-32) is refluxed and 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).

5. A zero-discharge treatment method for polysilicon wastewater, characterized in that: The method of using the polysilicon wastewater zero discharge treatment system according to claim 4 comprises the following steps: The wastewater from polysilicon production first enters the homogenization unit (4) to homogenize and equalize the wastewater, and then enters the sodium sulfate softening unit (1-11) to remove Ca 2+ , while adding seed crystals to the sodium sulfate softening unit (1-11) to induce calcium sulfate dihydrate crystal nucleation to form pre-hard wastewater; The pre-hardening wastewater is sequentially precipitated in the sludge storage unit (1-12) and separated in the filtration unit (1-13) to obtain filtered wastewater, which is then discharged to the double alkali softening precipitation unit (1-2) via the filtrate collection unit (1-14) to remove Ca in the filtered wastewater. 2+ and Mg 2+ , and then discharged to the enhanced hardness and silicon removal unit (1-3) through the dual alkali softening and sedimentation unit (1-2), to remove the total silicon in the hardness removal wastewater and stabilize the indicators of the hardness removal wastewater to form hardness and silicon removal wastewater. At the same time, the sludge storage unit (1-12) serves as a sludge treatment system, receiving the sludge discharged from the dual alkali softening and sedimentation unit (1-2), the enhanced hardness and silicon removal unit (1-3) and the ceramic membrane filtration unit (1-4); The hardness and silicon removal wastewater is discharged to the ceramic membrane filtration unit (1-4) through the enhanced hardness and silicon removal 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 carbonate and bicarbonate to form 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 water and generating regeneration liquid, the regeneration liquid is collected by the regeneration liquid collection unit (1-7) and discharged to the homogenization unit; The ion exchange product water is discharged to the COD purification membrane unit (2-1) through the ion exchange unit (1-6), and the COD in the ion exchange product water is removed to form COD purification membrane concentrated water and COD purified product water. The COD purification membrane concentrated water enters the second MVR forced circulation evaporation crystallization unit (3-22), and the COD purified product water enters the first nanofiltration unit (2-2) and the second nanofiltration unit (2-3) in turn to separate Cl - and SO4 2- and organic matter, wherein the first-level nanofiltration unit (2-2) filters to form first-level nanofiltration product water and first-level nanofiltration concentrated water, the first-level nanofiltration product water enters the second-level nanofiltration unit (2-3), and the first-level nanofiltration concentrated water is discharged to the sodium sulfate softening unit (1-11), and participates in the Ca in the wastewater of polysilicon production as a reactant 2+ After removal, the secondary nanofiltration unit (2-3) filters to produce secondary nanofiltration product water and secondary nanofiltration concentrated water, and the secondary nanofiltration concentrated water flows back into the primary nanofiltration unit (2-2); The secondary nanofiltration product water enters the MVR falling film evaporation unit (3-1) for evaporating the nanofiltration product water to form a concentrated mother liquor. The concentrated mother liquor is crystallized in the first MVR forced circulation evaporation crystallization unit (3-21) and separated in the first solid-liquid separation unit (3-31) to form industrial salt and a first enriched mother liquor. The first enriched mother liquor and the COD purification membrane concentrated water 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 to form a first impurity 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 impurity salt. This realizes the reduction treatment and zero emission of organic pollutants in the wastewater of polysilicon production.

6. The zero-discharge treatment method for polysilicon wastewater according to claim 5, characterized in that: Ca in wastewater from polysilicon production 2+ Concentration ≥3000mg / L.

7. The zero-discharge treatment method for polysilicon wastewater according to claim 5, characterized in that: Filtering Ca in wastewater 2+ The concentration is 1000mg / L~2000mg / L, the seed crystal is calcium sulfate dihydrate, and the dosage is 5g / L~20g / L.

8. The zero-discharge treatment method for polysilicon wastewater according to claim 5, characterized in that: Removal of Ca from hard and silicon wastewater 2+ Concentration ≤40mg / L, total silicon concentration ≤10mg / L.

9. The zero-discharge treatment method for polysilicon wastewater according to claim 5, characterized in that: After treatment by the ceramic membrane filtration unit (1-4), the effluent sludge density index is ≤5, and the effluent bicarbonate concentration is ≤50mg / L; based on the amount of CaCO3 used, the total hardness of the decarbonization wastewater is ≤5mg / L.

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