A treatment process for high-magnesium desulfurization wastewater resource utilization zero discharge

Abstract

The application provides a high-magnesium desulfurization wastewater resourceization zero-emission treatment system, which comprises a pretreatment softening area, a sludge-water separation area, a nanofiltration salt separation area, a reverse osmosis concentration area and an evaporation crystallization area; wherein the pretreatment softening area comprises an adjusting pool, a 1# reaction system and a 2# reaction system; the sludge-water separation area comprises a sludge storage pool and a plate-frame dewatering machine; the nanofiltration salt separation area comprises a sand filtration system, an ultrafiltration membrane system and a nanofiltration membrane system, the nanofiltration membrane system comprises a primary nanofiltration membrane unit, a secondary nanofiltration membrane unit, a nanofiltration concentrated water pool and a nanofiltration water production pool; the reverse osmosis concentration area comprises a reverse osmosis membrane system I and a reverse osmosis membrane system II; and the evaporation crystallization area comprises a 1# evaporation crystallization unit and a 2# evaporation crystallization unit. The treatment process of the application can realize zero emission of wastewater, reduce the dosing cost, avoid the treatment cost and secondary pollution to the environment caused by a large amount of sludge, and realize the resourceization of calcium carbonate, magnesium sulfate and sodium chloride crystalline salt.

Description

Technical Field

[0001] This invention relates to the field of desulfurization wastewater treatment, and is particularly applicable to the treatment process of zero-discharge resource utilization of high-magnesium desulfurization wastewater. Background Technology

[0002] Currently, limestone-gypsum wet flue gas desulfurization (FGD) technology is widely used in major coal-fired power plants in my country. According to incomplete statistics, this technology accounts for 93.9% of all desulfurization technologies. This process has advantages such as stable operation, low cost of limestone raw materials, and desulfurization efficiency exceeding 90%. However, the operation of this desulfurization system also generates desulfurization wastewater. The quality of the desulfurization wastewater varies depending on the composition of the limestone raw materials. Generally speaking, the wastewater is acidic (pH 4-6), which can corrode equipment and pipelines. It also has a complex composition, with high concentrations of suspended solids, high calcium and magnesium hardness, high sulfate ion content, high TDS, and small amounts of COD and heavy metals. When limestone raw materials contain a large amount of dolomite (CaMg(CO3)2), a large amount of magnesium will be introduced into the desulfurization water. During the process, it is difficult to form precipitates and crystals that can be removed by cyclone and dehydration, resulting in the desulfurization wastewater discharged by the desulfurization system containing a large amount of magnesium ions, with a magnesium ion concentration of 4000-20000 mg / L, which is a typical high-magnesium wastewater.

[0003] Most coal-fired power plants still rely on traditional three-compartment treatment processes for desulfurization wastewater. The treated wastewater is primarily utilized through methods such as wet ash mixing, hydraulic slag removal, dust suppression at the ash disposal site, or flushing of the coal conveying system. These measures fail to address issues such as equipment corrosion and pipe blockage, chloride ion accumulation, and the high price of selling fly ash externally, thus failing to fundamentally solve the problem. The "Guidelines for Feasible Technologies for Pollution Prevention and Control in Thermal Power Plants" also clearly states that achieving near-zero wastewater discharge from coal-fired power plants is key to achieving zero discharge of desulfurization wastewater. Therefore, developing efficient, economical, and environmentally friendly desulfurization wastewater treatment processes is imperative.

[0004] In zero-discharge processes for desulfurization wastewater, common process routes fall into three main categories: "pretreatment softening + NF desalination + membrane concentration + MVR," "pretreatment softening + membrane concentration + MVR," and "pretreatment softening + flue gas evaporation." The main purpose of pretreatment softening is to remove calcium and magnesium ions from the wastewater. The required chemical agents mainly include lime, sodium carbonate, sodium hydroxide, and sodium sulfate. Three common softening combination processes are: ① Lime-sodium carbonate combined softening method: softening the desulfurization wastewater by adding lime and sodium carbonate. This process is currently used in most zero-discharge desulfurization wastewater projects; ② Lime-sodium sulfate combined softening method: softening the desulfurization wastewater by adding lime in conjunction with a calcium sulfate crystallization process; ③ Dual-alkali softening method: softening by adding sodium hydroxide and sodium carbonate. All three softening methods result in a large amount of calcium and magnesium salt precipitates, leading to high sludge treatment and disposal costs, secondary pollution of the surrounding environment, and a waste of valuable calcium and magnesium ions. Summary of the Invention

[0005] To overcome the problems existing in the prior art, this invention provides a zero-discharge treatment process for high-magnesium desulfurization wastewater, which reduces the cost of chemical dosing while achieving zero wastewater discharge, avoids the treatment costs and secondary pollution to the environment caused by the generation of a large amount of sludge, realizes the resource utilization of calcium carbonate, magnesium sulfate and sodium chloride crystal salts, and effectively reduces the CO2 emissions in flue gas.

[0006] To achieve the objectives of this invention, the following technical solution is adopted:

[0007] The present invention provides a zero-discharge treatment process for high-magnesium desulfurization wastewater, comprising: a pretreatment softening zone, a sludge-water separation zone, a nanofiltration desalination zone, a reverse osmosis concentration zone, and an evaporation crystallization zone;

[0008] Pretreatment softening zone: used to soften and clarify the high-magnesium desulfurization wastewater; the pretreatment softening zone includes an equalization tank, a No. 1 reaction system and a No. 2 reaction system, the No. 2 reaction system includes a reaction tower, a sedimentation tank II and a desulfurization tower;

[0009] The No. 1 reaction system is used to soften the high-magnesium desulfurization wastewater flowing into the equalization tank to remove heavy metals and suspended solids. The effluent from the No. 1 reaction system enters the reaction tower for treatment. The reaction tower is equipped with a flue gas inlet for introducing flue gas into the wastewater in the reaction tower to make its pH value 8.3-8.6. The overflow of the effluent from the reaction tower enters the sedimentation tank II for sedimentation. The bottom of the reaction tower and the sedimentation tank II are connected to the desulfurization tower.

[0010] Sludge-water separation zone: used to store heavy metals and suspended solids precipitated in sedimentation tank I of the pretreatment softening zone;

[0011] Nanofiltration desalination zone: used to separate the permeate from the pretreatment softening zone to obtain permeate containing magnesium ions and permeate containing sodium ions; the nanofiltration desalination zone includes a sand filtration system, an ultrafiltration membrane system and a nanofiltration membrane system, the nanofiltration membrane system includes a primary nanofiltration membrane unit, a secondary nanofiltration membrane unit, a nanofiltration concentrate tank and a nanofiltration permeate tank;

[0012] The permeate from the pretreatment softening zone enters the sand filtration system for filtration. The effluent from the sand filtration system enters the ultrafiltration membrane system for treatment. The treated permeate then enters the nanofiltration membrane system, where it is processed by the first-stage nanofiltration membrane unit to obtain nanofiltration permeate and nanofiltration concentrate. The nanofiltration permeate enters the second-stage nanofiltration membrane unit, and the nanofiltration concentrate enters the nanofiltration concentrate tank. The permeate from the second-stage nanofiltration membrane unit enters the nanofiltration permeate tank, and the concentrate from the second-stage nanofiltration membrane unit is returned to the first-stage nanofiltration membrane unit.

[0013] Reverse osmosis concentration zone: used to concentrate the effluent from the nanofiltration permeate tank and the nanofiltration concentrate tank through reverse osmosis; the reverse osmosis concentration zone includes a reverse osmosis membrane system I mainly composed of sodium chloride solution and a reverse osmosis membrane system II mainly composed of magnesium sulfate solution; the reverse osmosis membrane system I includes a seawater desalination reverse osmosis membrane unit, a No. 1 high-pressure reverse osmosis membrane unit, a reverse osmosis permeate tank and a sodium chloride recovery tank, and the reverse osmosis membrane system II includes a No. 2 high-pressure reverse osmosis membrane unit and a magnesium sulfate recovery tank;

[0014] The effluent from the nanofiltration permeate tank enters the desalination reverse osmosis membrane unit to obtain reverse osmosis permeate and reverse osmosis concentrate. The reverse osmosis permeate enters the reverse osmosis permeate tank, and the reverse osmosis concentrate enters the No. 1 high-pressure reverse osmosis membrane unit for treatment. The resulting permeate is returned to the desalination reverse osmosis membrane unit, and the concentrate enters the sodium chloride recovery tank. The effluent from the nanofiltration concentrate tank enters the No. 2 high-pressure reverse osmosis membrane unit for treatment. The resulting permeate enters the reverse osmosis permeate tank, and the concentrate enters the magnesium sulfate recovery tank.

[0015] Evaporation crystallization zone: used to evaporate and crystallize the effluent from the magnesium sulfate recovery tank and the sodium chloride recovery tank to obtain magnesium sulfate crystals and sodium chloride crystals, respectively; the evaporation crystallization zone includes evaporation crystallization unit 1 and evaporation crystallization unit 2;

[0016] The effluent from the magnesium sulfate recovery tank is treated in the No. 1 evaporation and crystallization unit to obtain magnesium sulfate crystals, and the effluent from the sodium chloride recovery tank is treated in the No. 2 evaporation and crystallization unit to obtain magnesium sulfate crystals.

[0017] In a specific embodiment of the treatment process provided by the present invention, the No. 1 reaction system includes a neutralization grid, a coagulation grid, a flocculation grid, and a sedimentation tank I connected in sequence. In some specific embodiments, each grid is interconnected through a connecting hole, and the high magnesium desulfurization wastewater to be treated can flow through the above-mentioned grids in sequence.

[0018] The neutralization tank, coagulation tank, and flocculation tank are each equipped with a dosing device. In some specific embodiments, the dosing device I on the neutralization tank is used to add lime slurry solution and organic sulfur to the neutralization tank. Specifically, a lime slurry solution with a concentration of 5% to 10% can be added to adjust the pH value of the wastewater to 8 to 9. The organic sulfur agent is used to remove heavy metals from the wastewater. The wastewater with adjusted pH value enters the coagulation tank through the connecting hole. The dosing device II on the coagulation tank is used to add iron salt to the coagulation tank. The iron salt has a coagulation aid effect on heavy metal precipitates and suspended solids. The wastewater after coagulation in the coagulation tank enters the flocculation tank through the connecting hole. The dosing device III on the flocculation tank is used to add polyacrylamide to the flocculation tank.

[0019] In some specific implementations, the neutralization tank, coagulation tank, and flocculation tank are all equipped with stirring devices. The stirring action of each device allows the wastewater in each tank to fully react with the added reagents, and causes the suspended solids and heavy metals in the treated wastewater to flow into sedimentation tank I.

[0020] In some specific embodiments, the sludge-water separation zone includes a sludge storage tank and a plate and frame dewatering machine connected in sequence. The sludge storage tank is connected to the outlet of sedimentation tank I and is used to receive the sediment from sedimentation tank I. The plate and frame dewatering machine is used to perform pressure filtration and dewatering treatment on the sediment in the sludge storage tank. In a specific embodiment, sedimentation tank I is equipped with inclined tubes. The sludge at the bottom enters the sludge storage tank through a sludge discharge pump and is then pumped to the plate and frame dewatering machine for sludge-water separation. The sludge cake formed after separation is transported off-site for disposal. The effluent from sedimentation tank I is transported to the inlet of the reaction tower through a transfer pump.

[0021] In the treatment process provided by this invention, a water distribution device is installed at the top of the reaction tower to uniformly distribute the desulfurization wastewater within the tower; a blower is installed on the side of the reaction tower to blow flue gas into the tower through the flue outlet of the desulfurization tower. The flue gas typically contains approximately 15% to 20% CO2. The wastewater reacts with CO2 within the reaction tower to form calcium carbonate. An internal circulation pump is installed in the reaction tower to achieve internal circulation of the solution within the tower. When the slurry density within the reaction tower reaches 2.7 g / cm³... 3 At that time, the slurry discharge pump is started to transport the calcium carbonate slurry to the desulfurization tower.

[0022] The effluent from the reaction tower overflows into sedimentation tank II. After sedimentation through inclined tubes, the effluent is pumped into the nanofiltration desalination zone. In some specific embodiments, an external circulation device is installed between the reaction tower and sedimentation tank II to fully settle the treated water in the reaction tower and sedimentation tank II. Some of the bottom sludge in sedimentation tank II can be pumped back into the reaction tower through the external circulation pump.

[0023] The ultrafiltration membrane system of the present invention includes an ultrafiltration membrane unit and an ultrafiltration permeate tank; the effluent from the sand filtration system enters the ultrafiltration membrane unit for treatment, and the treated permeate enters the ultrafiltration permeate tank.

[0024] In some specific implementations, the sand filtration system uses quartz sand filtration.

[0025] In a specific embodiment of the process of the present invention, the No. 1 evaporation crystallization unit and the No. 2 evaporation crystallization unit adopt an MVR evaporation crystallizer and / or a multi-effect evaporation crystallizer.

[0026] The above technical solution achieves the following technical effects:

[0027] The treatment process provided by this invention involves adding CO2 from power plant flue gas to the pretreatment softening zone, which softens the calcium ions in the wastewater to form calcium carbonate crystals. Magnesium sulfate and sodium chloride are then separated in the nanofiltration desalination zone, resulting in magnesium sulfate and sodium chloride crystals with high recovery rates.

[0028] This invention significantly reduces the cost of chemical dosing by combining a complete set of treatment processes, avoids the increased treatment and disposal costs and secondary pollution to the environment caused by the generation of large amounts of sludge, realizes the resource utilization of calcium carbonate, magnesium sulfate and sodium chloride crystal salts, and effectively reduces CO2 emissions in flue gas. Attached Figure Description

[0029] Figure 1 A simplified flowchart of the processing system provided by this invention;

[0030] The components are as follows: 1. Equalization tank; 2. Neutralization tank; 3. Coagulation tank; 4. Flocculation tank; 5. Sedimentation tank I; 6. Reaction tower; 7. Sedimentation tank II; 8. Sludge storage tank; 9. Plate and frame dewatering machine; 10. Desulfurization tower; 11. Sand filtration system; 12. Ultrafiltration membrane unit; 13. Ultrafiltration permeate tank; 14. Primary nanofiltration membrane unit; 15. Secondary nanofiltration membrane unit; 16. Nanofiltration concentrate tank; 17. Nanofiltration permeate tank; 18. Seawater desalination reverse osmosis membrane unit; 19. High-pressure reverse osmosis membrane unit #1; 20. High-pressure reverse osmosis membrane unit #2; 21. Reverse osmosis permeate tank; 22. Magnesium sulfate recovery tank; 23. Sodium chloride recovery tank; 24. Evaporation crystallization unit #1; 25. Evaporation crystallization unit #2. Detailed Implementation

[0031] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0033] like Figure 1 As shown, the high-magnesium desulfurization wastewater resource utilization and zero-discharge treatment system provided by the present invention includes five treatment units according to the wastewater flow direction: pretreatment softening zone, mud-water separation zone, nanofiltration desalination zone, reverse osmosis concentration zone, and evaporation crystallization zone.

[0034] Pretreatment softening zone: includes equalization tank 1, reaction system 1# and reaction system 2#, wherein reaction system 2# includes reaction tower 6, sedimentation tank II 7 and desulfurization tower 10;

[0035] Among them, the No. 1 reaction system is used to soften the high magnesium desulfurization wastewater flowing into the equalization tank 1 to remove heavy metals and suspended solids; the effluent of the No. 1 reaction system enters the reaction tower 6 for treatment. The reaction tower 6 is connected to the flue outlet of the desulfurization tower 10 and is used to introduce flue gas into the wastewater of the reaction tower 6 to make the pH value of the wastewater 8.3 to 8.6. The effluent of the reaction tower 6 overflows into the sedimentation tank II 7 and settles in the sedimentation tank II 7. The bottom of the reaction tower 6 and the sedimentation tank II 7 are connected to the desulfurization tower 10.

[0036] Sludge-water separation zone: used to store heavy metals and suspended solids precipitated in sedimentation tank I5 of the pretreatment softening zone;

[0037] Nanofiltration desalination zone: includes sand filtration system 11, ultrafiltration membrane system and nanofiltration membrane system. The nanofiltration membrane system includes primary nanofiltration membrane unit 14, secondary nanofiltration membrane unit 15, nanofiltration concentrate tank 16, and nanofiltration permeate tank 17.

[0038] The permeate from the pretreatment softening zone enters the sand filtration system 11 for filtration. The effluent from the sand filtration system 11 enters the ultrafiltration membrane system for treatment. The ultrafiltration membrane system includes an ultrafiltration membrane unit 12 and an ultrafiltration permeate tank 13. The treated permeate enters the nanofiltration membrane system, and after being treated by the primary nanofiltration membrane unit 14, nanofiltration permeate and nanofiltration concentrate are obtained. The nanofiltration permeate enters the secondary nanofiltration membrane unit 15, and the nanofiltration concentrate enters the nanofiltration concentrate tank 16. The permeate from the secondary nanofiltration membrane unit 15 enters the nanofiltration permeate tank 17, and the concentrate from the secondary nanofiltration membrane unit 15 is returned to the primary nanofiltration membrane unit 14.

[0039] Reverse osmosis concentration zone: includes reverse osmosis membrane system I, which mainly uses sodium chloride solution, and reverse osmosis membrane system II, which mainly uses magnesium sulfate solution; reverse osmosis membrane system I includes seawater desalination reverse osmosis membrane unit 18, No. 1 high-pressure reverse osmosis membrane unit 19, reverse osmosis permeate tank 21, and sodium chloride recovery tank 23; reverse osmosis membrane system II includes No. 2 high-pressure reverse osmosis membrane unit 20 and magnesium sulfate recovery tank 22.

[0040] The effluent from nanofiltration permeate tank 17 enters desalination reverse osmosis membrane unit 18 to obtain reverse osmosis permeate and reverse osmosis concentrate. The reverse osmosis permeate enters the reverse osmosis permeate tank 21, and the reverse osmosis concentrate enters high-pressure reverse osmosis membrane unit 19 for treatment. The resulting permeate is returned to desalination reverse osmosis membrane unit 18, and the concentrate enters sodium chloride recovery tank 23. The effluent from nanofiltration concentrate tank 16 enters high-pressure reverse osmosis membrane unit 20 for treatment. The resulting permeate enters the reverse osmosis permeate tank 21, and the concentrate enters magnesium sulfate recovery tank 22.

[0041] Evaporation crystallization zone: includes evaporation crystallization unit 1# 24 and evaporation crystallization unit 25; wherein, the effluent from magnesium sulfate recovery tank 22 enters the evaporation crystallization unit 1# 24 for treatment to obtain magnesium sulfate crystals; the effluent from sodium chloride recovery tank 23 enters the evaporation crystallization unit 2# 25 for treatment to obtain magnesium sulfate crystals.

[0042] In the specific treatment process, high-magnesium desulfurization wastewater enters equalization tank 1. After the water quality is homogenized in equalization tank 1, it is pumped to reaction system #1. Reaction system #1 includes neutralization tank 2, coagulation tank 3, flocculation tank 4, and sedimentation tank I 5, which are connected in sequence through connecting holes. Neutralization tank 2, coagulation tank 3, and flocculation tank 4 are each equipped with a dosing device. After the high-magnesium desulfurization wastewater enters neutralization tank 2, dosing device I adds a lime slurry solution with a concentration of 5% to 10% to neutralize the wastewater and maintain the pH value of the wastewater at 8-9. Then, the wastewater enters coagulation tank 3 through connecting holes. Dosing device II adds iron salts (e.g., PFS) and organic sulfur (e.g., organic sulfur TMT-15) to coagulation tank 3 to precipitate heavy metals in the wastewater and to aid coagulation of suspended solids. Wastewater from coagulation tank 3 enters flocculation tank 4 through a connecting hole. The dosing device III adds polyacrylamide to flocculation tank 4. Wastewater from flocculation tank 4 enters sedimentation tank I5 through a connecting hole. Sedimentation tank I5 is equipped with inclined tubes. The sludge at the bottom of sedimentation tank I5 enters sludge storage tank 8 through a sludge discharge pump and is then pumped to plate and frame dewatering machine 9 for sludge-water separation. The resulting sludge cake is transported off-site for disposal. The effluent from sedimentation tank I5 is connected to the inlet of reaction tower 6.

[0043] In some specific implementations, the neutralization tank 2, the coagulation tank 3, and the flocculation tank 4 are all equipped with stirring devices. The stirring action of each device enables the wastewater in each tank to fully react with the added reagents, and causes the suspended solids and heavy metals in the treated wastewater to flow into the sedimentation tank I5.

[0044] After the wastewater enters reaction tower 6 of reaction system #2, a water distribution device is installed at the top of reaction tower 6 to evenly distribute the desulfurization wastewater into the tower. A blower is installed on the side of reaction tower 6 to blow flue gas (CO2 content approximately 15%–20%) discharged from the flue outlet of desulfurization tower 10 into reaction tower 6. The pH value of the solution is adjusted to 8.3–8.6 by adding sodium hydroxide solution into reaction tower 6. The wastewater reacts with CO2 in reaction tower 6 to form calcium carbonate. An internal circulation pump is installed in reaction tower 6 to achieve internal circulation of the solution within reaction tower 6. When the slurry density in reaction tower 6 reaches 2.7 g / cm³... 3 At that time, the slurry discharge pump is started to transport the calcium carbonate slurry to the desulfurization tower 10. The effluent from the reaction tower 6 overflows into the sedimentation tank II7. The effluent after sedimentation in the inclined tube of the sedimentation tank II7 is pumped into the nanofiltration desalination zone. The sludge at the bottom of the sedimentation tank II7 is sent to the desulfurization tower 10 by the transfer pump, and calcium carbonate crystals are obtained after post-treatment.

[0045] In some specific embodiments, an external circulation device is provided between the reaction tower 6 and the sedimentation tank II 7 to fully precipitate the treated water in the reaction tower 6 and the sedimentation tank II 7.

[0046] Within the nanofiltration desalination zone, wastewater flows sequentially through a sand filtration system 11, an ultrafiltration membrane system, and a nanofiltration membrane system. The ultrafiltration membrane system includes an ultrafiltration membrane unit 12 and an ultrafiltration permeate tank 13. The sand filtration system 11 uses quartz sand for filtration. The effluent from the sand filtration system 11 enters the ultrafiltration membrane unit 12 of the ultrafiltration membrane system, and then flows into the ultrafiltration permeate tank 13. The ultrafiltration permeate tank 13 is pumped by a booster pump (No. 1) into the primary nanofiltration membrane unit 14. The permeate from the primary nanofiltration membrane unit 14 enters the secondary nanofiltration unit. The concentrate from the primary nanofiltration membrane unit 14 enters the nanofiltration concentrate tank 16, used to store a solution primarily composed of magnesium sulfate. The permeate from the secondary nanofiltration unit enters the nanofiltration permeate tank 17, used to store a solution primarily composed of sodium chloride. The concentrate from the secondary nanofiltration unit is returned to the primary nanofiltration membrane unit 14. The effluent from the nanofiltration concentrate tank 16 and the nanofiltration permeate tank 17 then enter the reverse osmosis concentration zone.

[0047] Within the reverse osmosis concentration zone, the effluent from nanofiltration permeate tank 17 enters desalination reverse osmosis membrane unit 18 via booster pump #2. The concentrate from desalination reverse osmosis membrane unit 18 enters high-pressure reverse osmosis membrane unit #19. The permeate from desalination reverse osmosis membrane unit 18 enters reverse osmosis permeate tank 21. The permeate from high-pressure reverse osmosis unit #1 is returned to desalination reverse osmosis membrane unit 18. The concentrate from high-pressure reverse osmosis unit #1 enters sodium chloride recovery tank 23. The effluent from nanofiltration concentrate tank 16 enters high-salt reverse osmosis unit #2 via booster pump #3. The permeate from high-pressure reverse osmosis membrane unit #20 enters reverse osmosis permeate tank 21. The concentrate from high-pressure reverse osmosis membrane unit #20 enters magnesium sulfate recovery tank 22. The effluent from magnesium sulfate recovery tank 22 and sodium chloride recovery tank 23 enter the evaporation crystallization zone, respectively.

[0048] Within the evaporation crystallization zone, the effluent from the sodium chloride recovery tank 23 is pumped to evaporation crystallization unit 1 24, ultimately yielding sodium chloride crystals; the magnesium sulfate recovery tank 22 is pumped to evaporation crystallization unit 25, ultimately yielding magnesium sulfate crystals. In some specific embodiments, evaporation crystallization unit 1 24 and evaporation crystallization unit 25 employ MVR evaporation crystallizers and / or multi-effect evaporation crystallizers.

[0049] Example

[0050] Taking the high-magnesium desulfurization wastewater (calcium ion concentration of 854 mg / L, magnesium ion concentration of 13712 mg / L, and TDS concentration of 86374 mg / L) from a domestic power plant as an example, the treatment process of this invention is used to treat the desulfurization wastewater generated by the plant:

[0051] The wastewater is sequentially passed through the five treatment zones mentioned above: pretreatment softening zone, mud-water separation zone, nanofiltration desalination zone, reverse osmosis concentration zone, and evaporation crystallization zone.

[0052] In the pretreatment softening zone, a 5% lime slurry solution was added to the No. 1 reaction system to control the pH of the wastewater to 8.5. An organic sulfur agent was added at a dosage of 15 mg / L, along with iron salts and PAM agents. Finally, the heavy metals and suspended solids in the wastewater were effectively removed by inclined tube sedimentation.

[0053] Wastewater enters the reaction tower of reaction system #2. Flue gas from the desulfurization system is blown into the reaction tower, with a residence time of 1.5 hours. Power plant flue gas (CO2 concentration between 15% and 20%) is introduced into the reaction tower, and the flow rate is controlled by a gas flow meter to ensure the molar ratio of CO2 to calcium ions in the wastewater is 1.2. Sodium hydroxide is added to the reaction tower to adjust the pH of the wastewater to maintain at 8.3. The reaction proceeds when the slurry density in the reaction tower reaches 2.7 g / cm³. 3At that time, the slurry discharge pump is started to discharge the calcium carbonate slurry into the desulfurization tower. After the calcium carbonate slurry is sampled and dried, the purity of calcium carbonate is determined to be 95.2%. The effluent from the reaction tower enters the sedimentation tank II and enters the nanofiltration desalination zone through inclined tube sedimentation. The sludge at the bottom of the reaction tower and sedimentation tank II is transported to the desulfurization tower through the external discharge pump.

[0054] After entering the nanofiltration desalination zone, the wastewater sequentially passes through a sand filtration system, an ultrafiltration system, and a secondary nanofiltration system. The concentrate from the secondary nanofiltration membrane unit is returned to the primary nanofiltration membrane unit. The sodium chloride solution and magnesium sulfate solution obtained from the secondary nanofiltration system then enter the reverse osmosis concentration zone. The sodium chloride solution passes through a desalination membrane unit and a No. 1 high-pressure reverse osmosis membrane unit, and the concentrate enters the No. 1 evaporation and crystallization unit, ultimately yielding a high-purity sodium chloride crystal product (98% purity, meeting the Class II standard of the national standard for industrial salt (GBT5462-2015)). The magnesium sulfate solution passes through a No. 2 high-pressure reverse osmosis unit, and the concentrate enters the No. 2 evaporation and crystallization unit, ultimately yielding a high-purity magnesium sulfate crystal product (98.4% recovery rate, 99.2% purity).

[0055] Comparative Example 1

[0056] Using the same wastewater and equipment as in the above embodiments, and with other conditions unchanged, lime slurry solution was added to the No. 1 reaction system and the grid tank in the pretreatment softening zone to adjust the pH value to 10.5. The amount of sludge pressed out by the plate and frame dewatering machine in the final sludge-water separation zone increased significantly, and the magnesium sulfate crystals obtained by the No. 2 evaporation and crystallization unit decreased significantly (the recovery rate was only 47%, and the purity was 98.7%), while the sodium chloride crystals obtained by the No. 1 evaporation and crystallization unit had a purity of 97.8%.

[0057] Therefore, in this comparative example, when the pH of the neutralization tank reaches 10.5, magnesium ions in the wastewater precipitate as magnesium hydroxide. Simultaneously, increasing the amount of lime slurry solution added also leads to supersaturation of the calcium sulfate solution in the wastewater, resulting in precipitation. Under these operating conditions, not only is a large amount of sludge required for off-site disposal generated, but a relatively small amount of magnesium sulfate is also obtained.

[0058] Comparative Example 2

[0059] Using the same wastewater and equipment as in the above embodiments, and with other conditions unchanged, the pH value was controlled at 9.0 in the reaction tower of the No. 2 reaction system in the pretreatment softening zone. After drying, the final calcium carbonate slurry showed a calcium carbonate purity of only 62%, containing 24% magnesium carbonate. At the same time, the magnesium sulfate crystals obtained by the No. 2 evaporation crystallization unit were significantly reduced (recovery rate 86%, purity 99%), while the sodium chloride crystals obtained by the No. 1 evaporation crystallization unit had a purity of 98.2%.

[0060] Therefore, in this comparative example, after the pH was raised to 9.0, some magnesium ions in the wastewater precipitated as magnesium carbonate, resulting in the calcium carbonate slurry containing magnesium carbonate in addition to calcium carbonate. Under these operating conditions, it was impossible to obtain a high-purity calcium carbonate slurry, leading to a lower yield of magnesium sulfate product.

Claims

1. A zero-discharge treatment process for high-magnesium desulfurization wastewater, characterized in that, The treatment system includes: a pretreatment softening zone, a mud-water separation zone, a nanofiltration desalination zone, a reverse osmosis concentration zone, and an evaporation crystallization zone; Pretreatment softening zone: used to soften and clarify the high-magnesium desulfurization wastewater; the pretreatment softening zone includes an equalization tank, a No. 1 reaction system and a No. 2 reaction system, the No. 2 reaction system includes a reaction tower, a sedimentation tank II and a desulfurization tower; The No. 1 reaction system is used to soften the high-magnesium desulfurization wastewater flowing into the equalization tank to remove heavy metals and suspended solids. The effluent from the No. 1 reaction system enters the reaction tower for treatment. The reaction tower is connected to the flue outlet of the desulfurization tower and is used to introduce flue gas into the wastewater in the reaction tower to make its pH value 8.3~8.

6. The overflow of the effluent from the reaction tower enters the sedimentation tank II for sedimentation. The bottom of the reaction tower and the sedimentation tank II are connected to the desulfurization tower. Sludge-water separation zone: used to store heavy metals and suspended solids precipitated in sedimentation tank I of the pretreatment softening zone; Nanofiltration desalination zone: used to separate the permeate from the pretreatment softening zone to obtain permeate containing magnesium ions and permeate containing sodium ions; the nanofiltration desalination zone includes a sand filtration system, an ultrafiltration membrane system and a nanofiltration membrane system, the nanofiltration membrane system includes a primary nanofiltration membrane unit, a secondary nanofiltration membrane unit, a nanofiltration concentrate tank and a nanofiltration permeate tank; The permeate from the pretreatment softening zone enters the sand filtration system for filtration. The effluent from the sand filtration system enters the ultrafiltration membrane system for treatment. The treated permeate then enters the nanofiltration membrane system, where it is processed by the first-stage nanofiltration membrane unit to obtain nanofiltration permeate and nanofiltration concentrate. The nanofiltration permeate enters the second-stage nanofiltration membrane unit, and the nanofiltration concentrate enters the nanofiltration concentrate tank. The permeate from the second-stage nanofiltration membrane unit enters the nanofiltration permeate tank, and the concentrate from the second-stage nanofiltration membrane unit is returned to the first-stage nanofiltration membrane unit. Reverse osmosis concentration zone: used to concentrate the effluent from the nanofiltration permeate tank and the nanofiltration concentrate tank through reverse osmosis; the reverse osmosis concentration zone includes a reverse osmosis membrane system I mainly composed of sodium chloride solution and a reverse osmosis membrane system II mainly composed of magnesium sulfate solution; the reverse osmosis membrane system I includes a seawater desalination reverse osmosis membrane unit, a No. 1 high-pressure reverse osmosis membrane unit, a reverse osmosis permeate tank and a sodium chloride recovery tank, and the reverse osmosis membrane system II includes a No. 2 high-pressure reverse osmosis membrane unit and a magnesium sulfate recovery tank; The effluent from the nanofiltration permeate tank enters the desalination reverse osmosis membrane unit to obtain reverse osmosis permeate and reverse osmosis concentrate. The reverse osmosis permeate enters the reverse osmosis permeate tank, and the reverse osmosis concentrate enters the No. 1 high-pressure reverse osmosis membrane unit for treatment. The resulting permeate is returned to the desalination reverse osmosis membrane unit, and the concentrate enters the sodium chloride recovery tank. The effluent from the nanofiltration concentrate tank enters the No. 2 high-pressure reverse osmosis membrane unit for treatment. The resulting permeate enters the reverse osmosis permeate tank, and the concentrate enters the magnesium sulfate recovery tank. Evaporation crystallization zone: used to evaporate and crystallize the effluent from the magnesium sulfate recovery tank and the sodium chloride recovery tank to obtain magnesium sulfate crystals and sodium chloride crystals, respectively; the evaporation crystallization zone includes evaporation crystallization unit 1 and evaporation crystallization unit 2; The effluent from the magnesium sulfate recovery tank is treated in the No. 1 evaporation and crystallization unit to obtain magnesium sulfate crystals, and the effluent from the sodium chloride recovery tank is treated in the No. 2 evaporation and crystallization unit to obtain magnesium sulfate crystals. The No. 1 reaction system includes a neutralization tank, a coagulation tank, a flocculation tank, and a sedimentation tank I connected in sequence, and each of the neutralization tank, coagulation tank, and flocculation tank is equipped with a dosing device; the pH value of the wastewater in the neutralization tank is 8-9, and the dosing device I in the neutralization tank is used to add lime slurry aqueous solution and organic sulfur to the neutralization tank; The effluent from sedimentation tank I is connected to the inlet of the reaction tower.

2. The processing technology according to claim 1, characterized in that, The dosing device II installed in the coagulation tank is used to add iron salts into the coagulation tank; The dosing device III installed in the flocculation grid is used to add polyacrylamide into the flocculation grid.

3. The processing technology according to claim 2, characterized in that, The neutralization tank, coagulation tank, and flocculation tank are all equipped with stirring devices.

4. The processing technology according to claim 3, characterized in that, An external circulation device is provided between the reaction tower and the sedimentation tank II to fully precipitate the treated water in the reaction tower and the sedimentation tank II.

5. The processing technology according to claim 4, characterized in that, The top of the reaction tower is equipped with a water distribution device for distributing the desulfurization wastewater inside the reaction tower; the side of the reaction tower is equipped with a blower for blowing the flue gas from the flue outlet of the desulfurization tower into the reaction tower.

6. The processing method according to any one of claims 1 to 5, characterized in that, The sludge-water separation zone includes a sludge storage tank and a plate and frame dewatering machine connected in sequence. The sludge storage tank is connected to the outlet of the sedimentation tank I and is used to receive the sediment from the sedimentation tank I; the plate and frame dewatering machine is used to dewater the sediment in the sludge storage tank.

7. The processing technology according to claim 6, characterized in that, The ultrafiltration membrane system includes an ultrafiltration membrane unit and an ultrafiltration permeate tank; The effluent from the sand filtration system enters the ultrafiltration membrane unit for treatment, and the treated permeate enters the ultrafiltration permeate tank.

8. The processing technology according to claim 7, characterized in that, The sand filtration system uses quartz sand for filtration.

9. The processing method according to any one of claims 1 to 5, 7, and 8, characterized in that, The No. 1 evaporation crystallization unit and the No. 2 evaporation crystallization unit adopt MVR evaporation crystallizer and / or multi-effect evaporation crystallizer.

Images