Papermaking wastewater zero discharge treatment system and method using an alkali recovery causticizing section

Abstract

The application belongs to the technical field of industrial wastewater treatment and resource utilization, and discloses a papermaking wastewater zero discharge treatment system and method using an alkali recovery causticizing section. After hardness and organic matter in the wastewater are removed by a deep treatment unit, the papermaking wastewater enters a membrane method salt separation unit. After total silicon and hardness in the wastewater are removed by a concentrated water silicon and hardness removal unit, concentrated water enters a concentrated water concentration unit. After the concentrated liquid is mixed with pulp black liquor, the concentrated liquid enters a black liquor thermal concentration unit of the alkali recovery section. After water produced by the membrane method salt separation unit is concentrated by a water concentration unit, the water enters a water silicon and hardness removal unit to further remove hardness and total silicon in the wastewater, and then enters a secondary concentration unit. Low-concentration sludge produced by the deep treatment unit, the concentrated water silicon and hardness removal unit and the water silicon and hardness removal unit is dewatered by a sludge dewatering unit. The application fully utilizes the alkali recovery section, realizes cyclic utilization of sulfate and calcium salt, and realizes sludge reduction and resource utilization while ensuring wastewater recycling.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment and resource utilization technology. Specifically, it relates to a zero-discharge treatment system and method for papermaking wastewater using an alkali recovery causticizing stage. Background Technology

[0002] Improving resource and energy utilization efficiency is an inherent requirement for the green development of the paper industry. The reduction, reuse, and recycling of raw materials have become a continuous focus for paper companies. The alkali recovery section, while processing pulping black liquor, recovers most of the chemicals required in the papermaking process (such as sodium hydroxide and sodium sulfide), and has become a standard feature of large-scale integrated pulp and paper enterprises in recent years. The alkali recovery section mainly includes a black liquor thermal concentration unit, an alkali recovery furnace (equipped with dust removal devices, flue gas denitrification devices, and alkali ash dechlorination and potassium removal devices), a causticizing unit (equipped with green liquor dissolution and filtration devices, lime digestion devices, and causticizers), and a white mud lime kiln calcination unit (white mud filtration devices, white mud concentration and dewatering devices, lime kiln, dust removal devices, and denitrification devices).

[0003] Most existing zero-discharge wastewater systems include multiple membrane concentration units, concentrate treatment units, and evaporation and crystallization units. The evaporation and crystallization units are similar in function to the black liquor thermal concentration and potassium chloride removal devices in the alkali recovery section of paper mills. The chemical sludge generated by the zero-discharge system, after dewatering, has a composition similar to the white mud from the alkali recovery unit. Utilizing some process units in the alkali recovery section to achieve zero discharge of paper mill wastewater will significantly reduce project investment and operating costs, resulting in significant economic and environmental benefits. Summary of the Invention

[0004] This invention focuses on solving the technical problems related to zero discharge and resource utilization of papermaking wastewater. It provides a treatment system and method for zero discharge of papermaking wastewater by utilizing the alkali recovery and causticizing process. By making full use of the alkali recovery process, it achieves the recycling of sulfate and calcium salts, as well as the reduction and resource utilization of sludge, while ensuring the recycling of wastewater. It can be widely applied to wastewater treatment scenarios in papermaking enterprises in water-scarce and water-sensitive areas.

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

[0006] According to one aspect of the present invention, a zero-discharge treatment system for papermaking wastewater using an alkali recovery causticizing section is provided, comprising a deep treatment unit, a membrane desalination unit, a concentrate desiliconization and hardening unit, a concentrate concentration unit, a product water concentration unit, a product water hardening and desiliconization unit, a secondary concentration unit, and a sludge dewatering unit.

[0007] After pretreatment and anaerobic-aerobic biological treatment, the papermaking wastewater is further treated by the advanced treatment unit to remove hardness and organic matter before entering the membrane desalination unit.

[0008] The concentrated water from the membrane desalination unit is then processed by the concentrated water desiliconization and hardening unit to remove total silicon and hardness from the wastewater before entering the concentrated water concentration unit for further concentration. The concentrated liquid is mixed with pulping black liquor and then enters the black liquor thermal concentration unit of the alkali recovery section. After concentration, it enters the alkali recovery and causticizing unit. The sodium sulfate in the wastewater eventually forms white liquor (mainly composed of NaOH and Na2S) which is reused in the cooking section.

[0009] The permeate from the membrane separation unit is concentrated in the permeate concentration unit and then enters the permeate hardness and total silicon removal unit to further remove hardness and total silicon in the wastewater before entering the secondary concentration unit. The secondary concentrate from the secondary concentration unit is used for ash filtration and washing. The filtrate enters the potassium chloride removal unit in the alkali recovery section, where chlorides in the water are finally extracted and purified in the form of sodium chloride and potassium chloride.

[0010] The water produced by the concentrated water unit, the product water unit, and the secondary concentration unit is mixed and used as production water for papermaking enterprises. The low-concentration sludge produced by the deep treatment unit, the concentrated water desiliconization and hardening unit, and the product water desiliconization and hardening unit is dewatered by the sludge dewatering unit and then mixed with white mud before entering the lime kiln calcination unit. The calcium carbonate in the sludge is ultimately recycled as calcium oxide in the causticizing unit.

[0011] Furthermore, the advanced treatment unit includes a hardening sedimentation tank, a pre-oxidation tank, and an aerated biological filter; the hardening sedimentation tank is supplemented with calcium hydroxide, sodium hydroxide, sodium carbonate, anionic polyacrylamide, and sulfuric acid, and the pre-oxidation tank is supplemented with hydrogen peroxide and ozone.

[0012] Furthermore, the membrane-based salt separation unit includes an ultrafiltration device and a nanofiltration device, wherein the ultrafiltration device uses an immersed PVDF hollow fiber membrane, and the nanofiltration device adopts a single-stage three-section configuration.

[0013] Furthermore, the concentrated water desiliconization and hardening unit includes a desiliconization sedimentation tank and a hardening sedimentation tank; sodium aluminate, sulfuric acid, ferric sulfate and anionic polyacrylamide are added to the desiliconization sedimentation tank; sodium hydroxide, sodium carbonate, ferric sulfate and anionic polyacrylamide are added to the hardening sedimentation tank.

[0014] Furthermore, the concentrated wastewater unit includes an ultrafiltration unit, a neutralization and decarbonization unit, a primary concentration unit, and a secondary concentration unit; wherein, the ultrafiltration unit uses an immersed PTFE hollow fiber membrane; the neutralization and decarbonization unit adds sulfuric acid to adjust the pH to weakly acidic, and removes alkalinity and carbon dioxide from the wastewater by stripping; the primary concentration unit uses an antifouling spiral wound reverse osmosis membrane; and the secondary concentration unit uses a butterfly reverse osmosis membrane.

[0015] Furthermore, the product water concentration unit includes two concentration devices; wherein, the first concentration device adopts an industrial-grade brackish water reverse osmosis membrane, and the second concentration device adopts an industrial-grade anti-fouling reverse osmosis membrane.

[0016] Furthermore, the water hardening and desiliconization unit includes a hardening sedimentation tank and a silicon removal sedimentation tank; the hardening sedimentation tank is supplemented with calcium hydroxide, sodium hydroxide, sodium carbonate, ferric sulfate, and anionic polyacrylamide; the silicon removal sedimentation tank is supplemented with sodium aluminate, sulfuric acid, ferric sulfate, and anionic polyacrylamide.

[0017] Furthermore, the secondary concentration unit includes an ultrafiltration device, a first-stage concentration device, and a second-stage concentration device; wherein, the ultrafiltration device adopts an immersed PTFE hollow fiber membrane, the first-stage concentration device adopts an anti-fouling spiral wound reverse osmosis membrane, and the second-stage concentration device adopts a butterfly reverse osmosis membrane.

[0018] Furthermore, the sludge dewatering unit includes a sludge thickening tank and a sludge dewatering device.

[0019] According to another aspect of the present invention, a method for zero-discharge treatment of papermaking wastewater using an alkali recovery causticizing section is provided, based on the above-described zero-discharge treatment system for papermaking wastewater using an alkali recovery causticizing section, comprising:

[0020] After pretreatment and anaerobic-aerobic biological treatment, the wastewater discharged from the paper mill enters the advanced treatment unit for further removal of hardness and organic matter. The effluent from the advanced treatment unit meets the following requirements: COD ≤ 100 mg / L, Ca... 2+ ≤50mg / L, SS≤30mg / L;

[0021] The effluent from the advanced treatment unit passes through the membrane desalination unit, where residual COD and sulfate in the wastewater are concentrated and separated into concentrated water, while chlorides in the wastewater are retained in the product water. The desalination effect meets the following requirements: product water rate ≥ 90%, sulfate retention rate ≥ 90%.

[0022] The concentrate from the membrane desalination unit is then treated by the concentrate desiliconization and hardness removal unit to remove total silicon and hardness from the wastewater; the effluent quality meets the following requirements: total silicon ≤ 20 mg / L, Ca 2+≤50mg / L, Mg 2+ ≤10mg / L, SS≤20mg / L;

[0023] Wastewater from the concentrated water desiliconization and hardening unit is further concentrated in the concentrated water thickening unit and then mixed with low-concentration black liquor discharged from the pulping section. It then enters the black liquor thermal thickening unit of the alkali recovery section. After thermal thickening, it enters the alkali recovery and causticizing unit. The sodium sulfate in the wastewater ultimately forms white liquor (mainly composed of NaOH and Na2S) which is recycled to the cooking section. The treatment effect of the concentrated water thickening unit is as follows: TDS ≥ 4.5% after thickening, thickening ratio ≥ 6, desalination rate ≥ 97%.

[0024] The permeate from the membrane desalination unit enters the permeate concentration unit for multi-stage concentration, with a concentration ratio ≥20 and a desalination rate ≥95%.

[0025] The concentrated water from the product water concentration unit is then processed by the product water hardening and silica removal unit to remove hardness and total silica from the wastewater, and the effluent quality meets the following requirements: Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, total silicon ≤20mg / L;

[0026] Wastewater from the hardening and desiliconization unit is further concentrated to a salt content ≥10% in the secondary concentration unit and then used to wash away the large amount of soda ash generated in the alkali recovery section. The filtrate enters the potassium chloride removal unit to extract sodium chloride and potassium chloride; the desalination rate of the secondary concentration unit is ≥97%.

[0027] The permeate from the concentrate unit, the product water concentration unit, and the secondary concentration unit is mixed and then used for production water in paper mills.

[0028] The low-concentration sludge from the deep treatment unit, the concentrated water desiliconization and hardening unit, and the permeate desiliconization and hardening unit is concentrated and dewatered to a moisture content of ≤60% by the sludge dewatering unit. After being mixed with the white sludge produced by the causticizing unit, it is then fed into a lime kiln for calcination. The calcium carbonate in the sludge eventually forms calcium oxide, which is then recycled to the causticizing unit.

[0029] The beneficial effects of this invention are:

[0030] (i) The treatment system and method of the present invention realize the recycling of wastewater, sulfate and calcium salt, and simultaneously reduce the amount of raw materials such as sodium sulfate and limestone consumed in the alkali recovery section. The sludge generated in the water treatment process is reduced and recycled, which fully implements the green development concept of "reduction, reuse and recycling" of water resources and raw materials, and significantly reduces the impact of papermaking enterprises on regional water resources and water environment.

[0031] (2) The treatment system and method of the present invention make full use of the alkali recovery section supporting the pulp and paper enterprises, realizing the co - construction and sharing of the water treatment facilities and the alkali recovery section, avoiding the duplicate construction of the evaporation concentration and crystallization systems, and reducing the overall construction and operation costs. The treatment system and method couple the membrane - based salt separation unit, solving the corrosion problem of the chloride ions to the alkali recovery section.

[0032] (3) The treatment system and method of the present invention make full use of the tolerance and interception capabilities of nanofiltration membranes, wide - channel and strongly anti - fouling spiral - wound reverse osmosis, plate - and - frame reverse osmosis, etc. for organic matters in wastewater, avoiding the use of processes such as Fenton oxidation, adsorption, electrolysis, etc. to remove organic matters, and minimizing the increase in the salt content and sludge production in the wastewater caused by the water treatment process, thereby reducing the comprehensive operation cost.

[0033] (4) The treatment system and treatment method of the present invention fully consider the water quality characteristics of papermaking wastewater with high alkalinity and high hardness. In each stage of hardness removal, quicklime produced by the lime kiln in the alkali recovery section is preferably used instead of sodium hydroxide to remove hardness by using the alkalinity in the wastewater, greatly reducing the chemical agent cost of the hardness removal process.

[0034] (5) The treatment system and method of the present invention fully consider the collaborative treatment capabilities of each process unit, the water quality characteristics and scaling tendencies at each stage, and optimize the selection of scale inhibitors and membrane modules to maximize the concentration ratio of each concentration unit as much as possible. For example, the concentration ratio of the membrane - based salt separation unit reaches more than 10, and the concentration ratio of the product water concentration unit reaches more than 20. By increasing the concentration ratio, the process flow is shortened as much as possible, thereby reducing the investment and operation costs and improving the stability of the system simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 FIG. is a schematic structural diagram of a zero - discharge treatment system for papermaking wastewater using the alkali recovery causticizing section provided by the present invention.

[0036] In the above figure: 01, advanced treatment unit; 02, membrane - based salt separation unit; 03, concentrated water silicon and hardness removal unit; 04, concentrated water concentration unit; 05, product water concentration unit; 06, product water hardness and silicon removal unit; 07, secondary concentration unit; 08, sludge dewatering unit. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] In order to further understand the content, features and effects of the present invention, the following embodiments are cited and described in detail in conjunction with the drawings as follows:

[0038] As Figure 1As shown in the figure, this embodiment provides a zero-discharge treatment system for papermaking wastewater using an alkali recovery causticizing section, including a deep treatment unit 01, a membrane desalination unit 02, a concentrated water desiliconization and hardening unit 03, a concentrated water concentration unit 04, a product water concentration unit 05, a product water dehardening and desiliconization unit 06, a secondary concentration unit 07, and a sludge dewatering unit 08.

[0039] The influent to the advanced treatment unit 01 comes from the effluent of the secondary sedimentation tank of the paper mill's wastewater treatment system. After pretreatment, anaerobic and aerobic biological treatment, and sludge-water separation in the secondary sedimentation tank, the supernatant passes sequentially through three devices included in the advanced treatment unit 01: a hardening sedimentation tank, an ozone-hydrogen peroxide combined oxidation system, and an aerated biological filter. This process aims to minimize calcium ions and organic matter in the wastewater, creating conditions for subsequent treatment systems. The advanced treatment unit 01 includes a hardening sedimentation tank, a pre-oxidation tank, and an aerated biological filter. The hardening sedimentation tank is treated with calcium hydroxide, sodium hydroxide, sodium carbonate, anionic polyacrylamide, and sulfuric acid. It is equipped with two reaction zones, a flocculation zone, an inclined plate sedimentation zone, and a final neutralization zone. The residence time in both reaction zones is ≥10 min, the residence time in the flocculation zone is ≥20 min, and the surface loading of the sedimentation zone is ≤8 m³. 3 / m 2 • h, the residence time in the terminal neutralization zone is ≥5 min; the residence time in the pre-oxidation tank is 20-40 min, the ozone dosage is 30-50 mg / L, the hydrogen peroxide dosage is 50-100 mg / L, and the pH is controlled at 8-9 during operation; the residence time in the aerated biological filter is 2-3 h, the ceramsite depth is 3.5 m, and the air-to-water ratio is 2.5-3.5.

[0040] The influent to the membrane-based salinization unit 02 comes from the effluent of the aerated biological filter in the advanced treatment unit 01. After passing through the ultrafiltration and nanofiltration devices included in the membrane-based salinization unit 02, organic matter, calcium and magnesium ions, sulfate ions, etc., in the wastewater are separated and concentrated into concentrated water, while sodium, potassium, chloride ions, etc., remain in the product water. The ultrafiltration device uses a submerged PVDF hollow fiber ultrafiltration membrane with an operating flux ≥30L / m³. 2 • h, water production rate ≥90%, effluent SDI ≤3; the nanofiltration unit adopts a single-stage three-stage configuration, with a water production rate ≥90%, sulfate rejection rate ≥90%, and operating flux of 17~20 L / m 2 ·h.

[0041] The influent to the concentrated water desiliconization and hardness removal unit 03 comes from the concentrated water produced by the membrane desalination unit 02. It sequentially passes through the desiliconization and hardness removal sedimentation tanks included in the concentrated water desiliconization and hardness removal unit 03 to remove total silicon and hardness, while simultaneously removing some organic matter. Sodium aluminate, sulfuric acid, ferric sulfate, and anionic polyacrylamide are added to the desiliconization sedimentation tank. The operating pH is controlled at 8–9, the residence time in the reaction tank is ≥10 min, the residence time in the adsorption zone is ≥20 min, the residence time in the flocculation zone is ≥20 min, and the surface loading rate in the sedimentation zone is ≤8 m³.3 / m 2 • h; In addition to adding sodium hydroxide, sodium carbonate, ferric sulfate, and anionic polyacrylamide to the hard sedimentation tank, the operating pH is controlled between 10.8 and 11.2, with residence time in both reaction zones ≥ 10 min, residence time in the flocculation zone ≥ 20 min, and surface loading in the sedimentation zone ≤ 8 m². 3 / m 2 ·h.

[0042] The feed water to the concentrate concentration unit 04 comes from the supernatant of the concentrate desiliconization and hardening unit 03. It is further concentrated by passing through an ultrafiltration unit, a neutralization and decarbonization unit, a primary concentration unit, and a secondary concentration unit within the concentrate concentration unit 04. The concentrated liquid is mixed with low-concentration black liquor and then enters the black liquor thermal concentration unit in the alkali recovery causticizing section. The permeate is mixed with permeate from other concentration units and reused in the enterprise's production. The concentrate concentration unit sequentially includes an ultrafiltration unit, a neutralization and decarbonization unit, a primary concentration unit, and a secondary concentration unit. The ultrafiltration unit uses an immersed PTFE hollow fiber membrane, with an operating pH controlled between 10.8 and 11.2 and an operating flux ≥ 50 L / m³. 2 • h; Sulfuric acid is added to the decarbonization unit, and the pH is controlled at 6.5-7.0 after neutralization. Alkalinity and carbon dioxide in the wastewater are removed by aeration and stripping; The first-stage thickening unit uses a fouling-resistant spiral wound reverse osmosis membrane with an operating flux of 10-15 L / m 2 •h, equipped with a concentrate recirculation system; preferably, the anti-fouling spiral wound reverse osmosis membrane adopts a spiral wound reverse osmosis membrane with wide flow channels and strong anti-fouling performance, more preferably, an anti-fouling spiral wound reverse osmosis membrane with a flow channel width of 34mil and a hydrophilic coating on the surface; the two-stage concentration unit adopts a disc reverse osmosis system with an operating flux of 10-15L / m 2 •h, with a matching reflux system.

[0043] The feed water to the permeate water concentration unit 05 comes from the permeate water of the membrane desalination unit 02. It undergoes concentration treatment through two concentration units within the unit. The permeate water is then mixed with permeate water from other concentration units and used for production in the paper mill. The permeate water concentration unit 05 includes two concentration units. The first unit uses an industrial-grade brackish water reverse osmosis membrane, configured in a two-stage, single-stage configuration, with an operating flux of 18–20 L / m³. 2 The second concentration unit uses industrial-grade anti-fouling reverse osmosis membranes, with a two-stage configuration and an operating flux of 18–20 L / m³. 2 ·h.

[0044] The influent to the permeate water hardening and desiliconization unit 06 comes from the concentrate of the permeate water concentration unit 05. It sequentially passes through the hardening and desiliconization settling tank and the desiliconization settling tank within the permeate water hardening and desiliconization unit 06 to remove hardness and total silica from the concentrate, while simultaneously removing some organic matter. The hardening settling tank is supplemented with calcium hydroxide, sodium hydroxide, sodium carbonate, ferric sulfate, and anionic polyacrylamide. The operating pH is controlled between 10.8 and 11.2. The residence time in both reaction zones is ≥10 min, the residence time in the flocculation zone is ≥20 min, and the surface loading of the settling zone is ≤8 m³. 3 / m 2 • h; Sodium aluminate, sulfuric acid, ferric sulfate, and anionic polyacrylamide are added to the silica removal sedimentation tank. The operating pH is controlled at 8–9. The residence time in the reaction tank is ≥10 min, the residence time in the adsorption zone is ≥20 min, the residence time in the flocculation zone is ≥20 min, and the surface loading of the sedimentation zone is ≤8 m². 3 / m 2 ·h.

[0045] The influent to the secondary concentration unit 07 comes from the supernatant of the permeate water descaling and desiliconization unit 06. It undergoes concentration treatment sequentially through an ultrafiltration unit, a primary concentration unit, and a secondary concentration unit within the secondary concentration unit 07. The concentrated solution is used for ash washing in the alkali recovery causticizing section and finally enters the potassium chloride removal unit. The permeate water from the secondary concentration unit is mixed with permeate water from other concentration units and then used for production water in the paper mill. The secondary concentration unit 07 includes an ultrafiltration unit, a primary concentration unit, and a secondary concentration unit. The ultrafiltration unit uses an submerged PTFE hollow fiber membrane with an operating flux ≥ 50 L / m³. 2 •h; The first-stage concentration unit uses anti-fouling spiral wound reverse osmosis membranes, with a two-stage configuration and an operating flux of 10–15 L / m. 2 •h; Preferably, the anti-fouling spiral-wound reverse osmosis membrane is a spiral-wound reverse osmosis membrane with wide flow channels and strong anti-fouling performance; more preferably, it is an anti-fouling spiral-wound reverse osmosis membrane with a flow channel width of 34mil and a hydrophilic coating on the surface; the two-stage concentration unit adopts a disc-type reverse osmosis with an operating flux of 10-15L / m 2 •h, equipped with a concentrate reflux device.

[0046] The feed for sludge dewatering unit 08 comes from low-concentration sludge discharged from the advanced treatment unit 01, the concentrated water desiliconization and hardening unit 03, and the permeate water desiliconization and hardening unit 06. After being processed by the sludge thickening device and the sludge dewatering device, the sludge cake is mixed with the white mud produced in the alkali recovery causticizing section and then enters the lime kiln calcination unit. The dewatering liquid and the concentrated supernatant are returned to the advanced treatment unit 01. Sludge dewatering unit 08 includes a sludge thickening device and a sludge dewatering device. The sludge thickening device uses a gravity thickening tank, and the solids load is controlled at 40-80 kgDS / m³. 2 The dewatering unit uses a high-pressure diaphragm plate and frame dewatering device with a pressing capacity of 2-4 kgDS / m³. 2The operating cycle is 1.5 to 4 hours, and the pressing pressure is 2.0 to 2.5 MPa.

[0047] Based on the above system, the present invention also provides a method for zero-discharge treatment of papermaking wastewater using an alkali recovery causticizing stage, comprising the following processes:

[0048] The supernatant from the secondary sedimentation tank of the wastewater treatment system in the paper mill undergoes advanced treatment unit 01 to remove hardness and organic matter, achieving the following effluent quality requirements: COD ≤ 100 mg / L, Ca... 2+ ≤50mg / L, SS≤30mg / L; In this invention, SS represents suspended solids;

[0049] Wastewater from the advanced treatment unit 01 undergoes salt separation and concentration treatment via the membrane salt separation unit 02. This separates and concentrates divalent ions such as calcium, magnesium, and sulfate, along with organic matter, into concentrated water, while monovalent ions such as sodium, potassium, and chloride remain in the product water. The salt separation effect meets the following requirements: product water rate ≥ 90%, sulfate rejection rate ≥ 90%.

[0050] The concentrate from membrane desalination unit 02 is then treated by concentrate desiliconization and hardness removal unit 03 to remove total silicon and hardness from the wastewater; the effluent quality meets the following requirements: total silicon ≤ 20 mg / L, Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, SS≤20mg / L;

[0051] Wastewater from the concentrate desiliconization and hardening unit 03 is further concentrated in the concentrate thickening unit 04 and then mixed with low-concentration black liquor discharged from the pulping section before entering the black liquor thermal thickening unit in the alkali recovery section. After thermal thickening, it enters the alkali recovery and causticizing unit, where the sodium sulfate in the wastewater ultimately forms white liquor (mainly composed of NaOH and Na2S) which is recycled to the cooking section. The treatment effect of the concentrate thickening unit is as follows: TDS of the concentrated wastewater ≥ 4.5%, concentration ratio ≥ 6, and desalination rate ≥ 97%.

[0052] The permeate from membrane desalination unit 02 is concentrated in permeate concentration unit 05, with a concentration ratio ≥20 and a desalination rate ≥95%.

[0053] The concentrated water from the product water concentration unit 05 enters the product water hardness and total silicon removal unit 06 to remove hardness and total silicon from the wastewater, and the effluent quality meets the following requirements: Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, total silicon ≤20mg / L;

[0054] Wastewater from the hardening and silicon removal unit 06 is further concentrated to TDS ≥ 10% in the secondary concentration unit 07 and then used to recover a large amount of alkali ash generated in the filtration and washing alkali recovery section. The filtrate enters the potassium chloride removal unit to extract sodium chloride and potassium chloride; the desalination rate of the secondary concentration unit is ≥ 97%.

[0055] The permeate from the concentrate unit 04, the product water concentration unit 05, and the secondary concentration unit 07 is mixed and then used for production water in papermaking enterprises.

[0056] Low-concentration sludge from the deep treatment unit 01, the concentrated water desiliconization and hardening unit 03, and the permeate water desiliconization and hardening unit 06 is concentrated and dewatered to a moisture content of ≤60% by the sludge dewatering unit. After being mixed with white sludge produced by the causticizing unit, it is fed into a lime kiln for calcination. The calcium carbonate in the sludge eventually forms calcium oxide and is recycled to the causticizing unit.

[0057] The above-mentioned treatment is continuous and can realize the recycling of water resources, sulfates, and calcium salts, as well as the reduction and resource utilization of sludge through the alkali recovery caustic process. At the same time, it reduces the amount of industrial raw materials such as sodium sulfate and limestone consumed by the alkali recovery process, thereby significantly reducing the overall operating cost.

[0058] The following experimental examples further illustrate the content and effects of the treatment system and method for achieving zero discharge of papermaking wastewater using an alkali recovery causticizing section provided by this invention:

[0059] Experimental Example 1:

[0060] A large paper manufacturing enterprise in North China has five pulping and paper production lines, with a capacity of approximately 900,000 tons / year for high-grade kraft paper, 700,000 tons / year for high-grade corrugated paper, and 500,000 tons / year for high-grade coated white board paper. It is equipped with a sulfate-based delignification continuous pulping process with a processing capacity of 1500 BDt / d, including an alkali recovery system and a circulating cooling water system. The black liquor evaporation and concentration unit has a processing capacity of 800 t / h, the alkali recovery furnace has a processing capacity of 3500 t / d, and the lime kiln has a processing capacity of 600 t / d (calculated as CaO). Currently, the plant's wastewater treatment facilities include a screen and enhanced primary sedimentation, cooling and water quality conditioning, an IC anaerobic reactor, an aerobic reaction tank, a secondary sedimentation tank, shallow air flotation, Fenton oxidation, and a tertiary sedimentation tank. The effluent is discharged downstream to the industrial park's wastewater treatment plant for further treatment, with a total discharge volume of approximately 40,000 tons / day. To completely alleviate the pressure of the paper manufacturing enterprise on local water resources and the water environment, a zero-discharge wastewater project is being implemented, using the effluent from the secondary sedimentation tank as the water source. The water quality of the effluent from the secondary sedimentation tank is shown in the table below:

[0061]

[0062] The above water quality indicators show that the effluent from the secondary sedimentation tank has high hardness, alkalinity, and COD concentration, and also contains a small amount of potassium ions. The molar ratio of calcium ions to bicarbonate ions is approximately 8.7:14, and the mass ratio of sodium to potassium is approximately 5:1. Its zero-discharge system, using the treatment system and method provided by this invention, can achieve a reclaimed water production capacity of 39,000 m³. 3 The per-day water production rate can reach over 98%, with TDS levels controlled below 200 mg / L and chloride ion levels controlled below 100 mg / L. Approximately 640 m³ of concentrated sulfate solution is produced daily. 3 (Production volume approximately 27m) 3 The concentration of sodium chloride (approximately 4.8% salt content) is equivalent to 3.3% of the black liquor evaporation and concentration unit's processing capacity, simultaneously reducing sodium sulfate consumption by approximately 22 t / d (about 70% of the sodium sulfate demand); and producing approximately 252 m³ of concentrated sodium chloride solution daily. 3 (Production volume approximately 10.5m) 3 / h, with a salt content of about 10.7%, which is equivalent to about 60% of the washing water volume of alkali lime. The potassium chloride production of the potassium chloride removal unit increases by 2.2 tons per day; about 78.2t / d of sludge is produced daily, of which about 87% is calcium carbonate, which is equivalent to 6.4% of the lime kiln's processing capacity, and the amount of limestone used can be reduced by about 68t per day.

[0063] Experimental Example 2:

[0064] A large paper mill in East China has a total annual production capacity of 1.5 million tons for its chemical pulping production line, 500,000 tons for its functional cardboard production line, 450,000 tons for its kraft paper production line, 100,000 tons for its food packaging paper production line, and 100,000 tons for its medical supplies release paper production line. It is equipped with an alkali recovery system and corresponding wastewater treatment systems. The black liquor evaporation and concentration unit has a capacity of 1500 t / h, the alkali recovery furnace has a capacity of 6400 t / d, and the lime kiln has a capacity of 1200 t / d (calculated as CaO). Currently, the plant's wastewater treatment facilities include a screen and enhanced primary sedimentation, cooling and water quality conditioning, anaerobic reactors, aerobic reactors, and a secondary sedimentation tank, with a total discharge of approximately 85,000 tons / day. To completely alleviate the pressure of the paper mill on local water resources and the water environment, a zero-discharge wastewater project is implemented, using the effluent from the secondary sedimentation tank as the water source. The water quality of the secondary sedimentation tank effluent is shown in the table below.

[0065]

[0066] The above water quality indicators show that the effluent from the secondary sedimentation tank has high hardness, alkalinity, and COD concentration, and also contains a small amount of potassium ions. The molar ratio of calcium ions to bicarbonate ions is approximately 5:8, and the mass ratio of sodium to potassium is approximately 6:1. Its zero-discharge system uses the treatment system and method provided by this invention, with a total reclaimed water production of approximately 82,800 tons / day. TDS can be controlled below 200 mg / L, chloride ions below 100 mg / L, and the water production rate reaches over 98%. Approximately 1700 m³ of concentrated sulfate solution is produced daily. 3 (Production volume approximately 71m) 3 The concentration of sodium chloride (approximately 4.6% salt content) is equivalent to 4.7% of the black liquor evaporation and concentration unit's processing capacity, simultaneously reducing sodium sulfate consumption by approximately 62 t / d (about 72% of the total sodium sulfate demand); and producing approximately 482 m³ of concentrated sodium chloride solution per day. 3 (Production volume approximately 20m) 3 The potassium chloride production of the dechlorination unit increases by 4.4 tons per day; it produces about 187 tons of dried sludge per day, of which about 88% is calcium carbonate, equivalent to 7.7% of the lime kiln's processing capacity, and can reduce limestone consumption by about 166 tons per day.

[0067] It is evident that, for the application scenario of zero discharge of papermaking wastewater, the treatment system and method provided by this invention can realize the recycling of wastewater, sulfate, and calcium salts, simultaneously achieving sludge reduction and resource recovery, reducing the demand for sodium sulfate and limestone in the alkali recovery causticizing section, fully utilizing the process units of the alkali recovery causticizing section, and significantly reducing the construction and operation costs of the wastewater zero discharge system; compared with similar processes, the process flow is shorter and the engineering cost is lower; therefore, this invention has broad application prospects in the field of zero discharge and resource recovery of papermaking wastewater.

[0068] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many specific modifications under the guidance of the present invention without departing from the spirit and scope of the claims, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section, characterized in that, It includes a deep treatment unit, a membrane desalination unit, a concentrated water desiliconization and hardening unit, a concentrated water concentration unit, a permeate concentration unit, a permeate hardening and desiliconization unit, a secondary concentration unit, and a sludge dewatering unit. After pretreatment and anaerobic-aerobic biological treatment, the papermaking wastewater is further treated by the advanced treatment unit to remove hardness and organic matter before entering the membrane desalination unit. The concentrated water from the membrane desalination unit is then processed by the concentrated water desiliconization and hardening unit to remove total silicon and hardness from the wastewater before entering the concentrated water concentration unit for further concentration. The concentrated liquid is mixed with pulping black liquor and then enters the black liquor thermal concentration unit of the alkali recovery section. After concentration, it enters the alkali recovery and causticizing unit. The sodium sulfate in the wastewater is finally used to form white liquor for reuse in the cooking section. The permeate from the membrane separation unit is concentrated in the permeate concentration unit and then enters the permeate hardness and total silicon removal unit to further remove hardness and total silicon in the wastewater before entering the secondary concentration unit. The secondary concentrate from the secondary concentration unit is used for ash filtration and washing. The filtrate enters the potassium chloride removal unit in the alkali recovery section, where chlorides in the water are finally extracted and purified in the form of sodium chloride and potassium chloride. The water produced by the concentrated water unit, the product water unit, and the secondary concentration unit is mixed and used as production water for papermaking enterprises. The low-concentration sludge produced by the deep treatment unit, the concentrated water desiliconization and hardening unit, and the product water desiliconization and hardening unit is dewatered by the sludge dewatering unit and then mixed with white mud before entering the lime kiln calcination unit. The calcium carbonate in the sludge is ultimately recycled as calcium oxide in the causticizing unit.

2. The zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The advanced treatment unit includes a hardening sedimentation tank, a pre-oxidation tank, and an aerated biological filter. The hardening sedimentation tank is treated with calcium hydroxide, sodium hydroxide, sodium carbonate, anionic polyacrylamide, and sulfuric acid, while the pre-oxidation tank is treated with hydrogen peroxide and ozone.

3. The zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The membrane-based salt separation unit includes an ultrafiltration device and a nanofiltration device, wherein the ultrafiltration device uses an immersed PVDF hollow fiber membrane, and the nanofiltration device adopts a single-stage three-section configuration.

4. The zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The concentrated water desiliconization and hardening unit includes a desiliconization sedimentation tank and a hardening sedimentation tank; sodium aluminate, sulfuric acid, ferric sulfate and anionic polyacrylamide are added to the desiliconization sedimentation tank; sodium hydroxide, sodium carbonate, ferric sulfate and anionic polyacrylamide are added to the hardening sedimentation tank.

5. A zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The concentrated wastewater unit includes an ultrafiltration unit, a neutralization and decarbonization unit, a primary concentration unit, and a secondary concentration unit; wherein, the ultrafiltration unit uses an immersed PTFE hollow fiber membrane; the neutralization and decarbonization unit adds sulfuric acid to adjust the pH to weakly acidic, and removes alkalinity and carbon dioxide from the wastewater by stripping; the primary concentration unit uses an antifouling spiral wound reverse osmosis membrane; and the secondary concentration unit uses a butterfly reverse osmosis membrane.

6. The zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The water concentration unit includes two concentration devices; the first concentration device uses an industrial-grade brackish water reverse osmosis membrane, and the second concentration device uses an industrial-grade anti-fouling reverse osmosis membrane.

7. A zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The water hardening and desiliconization unit includes a hardening sedimentation tank and a silicon removal sedimentation tank; the hardening sedimentation tank is supplemented with calcium hydroxide, sodium hydroxide, sodium carbonate, ferric sulfate, and anionic polyacrylamide; the silicon removal sedimentation tank is supplemented with sodium aluminate, sulfuric acid, ferric sulfate, and anionic polyacrylamide.

8. A zero-discharge treatment system for papermaking wastewater utilizing an alkali recovery causticizing section according to claim 1, characterized in that, The secondary concentration unit includes an ultrafiltration device, a primary concentration device, and a secondary concentration device; wherein, the ultrafiltration device adopts an immersed PTFE hollow fiber membrane, the primary concentration device adopts an anti-fouling spiral wound reverse osmosis membrane, and the secondary concentration device adopts a butterfly reverse osmosis membrane.

9. A method for zero-discharge treatment of papermaking wastewater using an alkali recovery causticizing stage, characterized in that, Based on the zero-discharge treatment system for papermaking wastewater utilizing the alkali recovery causticizing section as described in any one of claims 1 to 8, including: After pretreatment and anaerobic-aerobic biological treatment, the wastewater discharged from the paper mill enters the advanced treatment unit for further removal of hardness and organic matter. The effluent from the advanced treatment unit meets the following requirements: COD ≤ 100 mg / L, Ca... 2+ ≤50mg / L, SS≤30mg / L; The effluent from the advanced treatment unit passes through the membrane desalination unit, where residual COD and sulfate in the wastewater are concentrated and separated into concentrated water, while chlorides in the wastewater are retained in the product water. The desalination effect meets the following requirements: product water rate ≥ 90%, sulfate retention rate ≥ 90%. The concentrate from the membrane desalination unit is then treated by the concentrate desiliconization and hardness removal unit to remove total silicon and hardness from the wastewater; the effluent quality meets the following requirements: total silicon ≤ 20 mg / L, Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, SS≤20mg / L; Wastewater from the concentrated water desiliconization and hardening unit is further concentrated in the concentrated water thickening unit and then mixed with low-concentration black liquor discharged from the pulping section. It then enters the black liquor thermal thickening unit of the alkali recovery section. After thermal thickening, it enters the alkali recovery and causticizing unit. The sodium sulfate in the wastewater ultimately forms white liquor which is reused in the cooking section. The treatment effect of the concentrated water thickening unit is as follows: TDS ≥ 4.5% after thickening, thickening ratio ≥ 6, desalination rate ≥ 97%. The permeate from the membrane separation unit enters the permeate concentration unit for multi-stage concentration, with a concentration ratio ≥20 and a desalination rate ≥95%. The concentrated water from the product water concentration unit is then processed by the product water hardening and silica removal unit to remove hardness and total silica from the wastewater, and the effluent quality meets the following requirements: Ca 2+ ≤50mg / L, Mg 2+ ≤10mg / L, total silicon ≤20mg / L; Wastewater from the hardening and desiliconization unit is further concentrated to a salt content ≥10% in the secondary concentration unit and then used to wash away the large amount of soda ash generated in the alkali recovery section. The filtrate enters the potassium chloride removal unit to extract sodium chloride and potassium chloride; the desalination rate of the secondary concentration unit is ≥97%. The permeate from the concentrate unit, the product water concentration unit, and the secondary concentration unit is mixed and then used for production water in paper mills. The low-concentration sludge from the deep treatment unit, the concentrated water desiliconization and hardening unit, and the permeate desiliconization and hardening unit is concentrated and dewatered to a moisture content of ≤60% by the sludge dewatering unit. After being mixed with the white sludge produced by the causticizing unit, it is then fed into a lime kiln for calcination. The calcium carbonate in the sludge eventually forms calcium oxide, which is then recycled to the causticizing unit.

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