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Author | Ding Chaohui
Datang Environmental Industry Group Co., Ltd
Abstract: This article introduces the zero discharge process of crystal salt purification. As an efficient wastewater treatment and reuse technology, the zero discharge process of crystal salt purification has a salt removal rate of up to 92%. The solid salt separated from crystallization is utilized as a resource to produce solid salt products that can meet industrial standards, improve economic benefits, and avoid environmental pollution and water resource recovery from high salinity wastewater.
Keywords: desulfurization wastewater; Crystalline salt purification; Wastewater treatment; Solid salt; Zero emissions
1. Overview of wastewater from a certain power plant. After investigating the 2 # 300MW coal-fired unit of a power plant, the preliminary analysis of wastewater from the power plant can be divided into two categories: ordinary wastewater, which has undergone mature treatment processes such as flocculation, sedimentation, neutralization, and pressure filtration to achieve comprehensive utilization of power plant wastewater; High salinity wastewater, including some acid alkali regeneration water, circulating efflux wastewater (with a salt content of over 5000mg/L), and desulfurization wastewater (around 20000mg/L), has a high salt content. According to the latest national environmental protection requirements, it must meet the zero discharge requirements for desalination.
2 Zero Emission Treatment Technology 2.1 Scheme Introduction and Comparison 2.1.1 Salt Mixing Process
The main process route is pretreatment+reduction+evaporation crystallization. The main advantages of this process are relatively simple system, fewer processing steps, easy operation control, average equipment investment, and average operating costs; The disadvantage is that a large amount of solid salt waste is generated, and the disposal cost is high. This plan is mainly used for early zero emission projects.
2.1.2 Flue spray process
The main process route is pretreatment+reduction+flue spray drying. The main advantage of this process is that the investment cost and operating cost are relatively low, making it easy to control; The disadvantage is that direct spraying of high concentration mixed salt concentrate into the flue will have adverse effects such as scaling, fouling, and corrosion. The evaluation of various impacts after long-term operation still needs to be investigated.
2.1.3 Crystalline Salt Purification Zero Emission Process
At present, the mainstream process of desulfurization wastewater in China is salt separation purification process, which uses nanofiltration membrane for salt separation, reverse osmosis membrane for reduction and water Recycling, and finally evaporative crystallization. This solution can achieve true zero emissions, with no other waste discharged from the system except for dry sludge; The crystallized sodium chloride can be sold as a product, greatly reducing waste disposal costs while also compensating for a portion of operating costs. Based on the situation of this project, we recommend choosing the zero discharge process of crystal salt purification to achieve the goal of resource utilization and reduction treatment of desulfurization wastewater.
2.2 Explanation of zero discharge process flow for crystal salt purification 2.2.1 Pretreatment unit
2.2.1.1 Reaction sedimentation tank
The main purpose of the reaction sedimentation tank is to reduce the concentration of Ca2+, Mg2+, SiO2, suspended solids, etc., reduce their impact on the evaporation unit, and remove some SO42- ions to ensure stable inlet water quality of the nanofiltration unit. In this project, lime milk, sodium carbonate, PAC, PAM and other chemicals are added to the reaction sedimentation tank to ensure the removal effect. The large amount of CaCO3, Mg (OH) 2, CaSO4 and other precipitates generated in the reaction are separated by PAC and PAM in the sedimentation zone, forming a sludge discharge system. The sludge is discharged into the sludge collection tank, filtered and transported outward, and the supernatant enters the tubular microfiltration membrane. Design a reaction sedimentation tank, with a designed flow rate of 35m3/h in the reaction zone and 130m3/h in the sedimentation zone.
2.2.1.2 Tubular microfiltration membrane
The structure of a tubular membrane is that the membrane is cast inside a porous material tube. After the water flow containing the filtered substance (solid) passes through the membrane, it then passes through the porous support material and enters the water production side (where the water is purified). The solid particles intercepted by the membrane do not stay on the surface of the membrane under the push of water flow, but rather play a certain scouring role on the membrane surface to prevent pollutants from staying on the membrane surface. The supernatant in the reaction sedimentation tank is pumped into the tubular microfiltration system. Driven by the pressure and speed, the waste water is filtered through the tubular microfiltration membrane in a large flow cross flow way to separate Suspended solids from the liquid. In each membrane column, the flow rate of wastewater pumped through the membrane tube is very high, forming parallel turbulence on the membrane surface, generating a shear effect that plays a role in cleaning the membrane.
The tubular microfiltration unit mainly consists of a circulating pump, tubular microfiltration membrane and membrane frame, cleaning device, relevant control valves, and matching pipelines. Design 2 sets of tubular microfiltration membranes, each with a processing capacity of 18m3/h. The effluent from the tubular microfiltration membrane enters the ultrafiltration tank and the pH value is adjusted by adding acid.
2.2.2 Nanofiltration unit
Due to the close content of sodium chloride and sodium sulfate in wastewater, extracting a single salt alone can cause significant losses and increase the amount of miscellaneous salt treatment. Therefore, nanofiltration separation is considered to separate sodium chloride and sodium sulfate, achieving the goal of salt separation.
Nanofiltration membrane is essentially a loose reverse osmosis membrane, which, due to its special membrane material, has high permeability to monovalent ions (with minimal loss of NaCl after penetrating the membrane), and high interception ability to divalent ions (almost all sulfate ions are intercepted). After nanofiltration, the salt content on the fresh water side is mainly sodium chloride (with a content of over 97%), with only a very small amount of sodium sulfate. The concentrated water side is mainly sodium sulfate, containing some sodium chloride.
There are two sets of nanofiltration membranes, each with a processing capacity of 18m3/h, of which 13m3/h is produced on the fresh water side and enters the sodium chloride water tank; The concentrated water on the concentrated water side is 5m3/h, which flows back to the raw water inlet of the reaction sedimentation tank.
2.2.3 Seawater reverse osmosis unit
There are two key conditions for water production of reverse osmosis facilities: one is a selective membrane, which we call Semipermeable membrane; The second is a certain amount of pressure.
Among various impurities in water, soluble salts are the most difficult to remove. Therefore, the water purification effect of reverse osmosis is often determined based on the level of desalination rate. The desalination rate of reverse osmosis is mainly determined by the selectivity of reverse osmosis Semipermeable membrane.
The main water produced on the NF fresh water side of this project is sodium chloride, with a TDS of around 10000mg/L. The seawater reverse osmosis membrane is used for further concentration and reduction. This project selects high-quality anti pollution seawater reverse osmosis membranes based on the characteristics of water quality, which have many advantages such as smooth membrane surface and strong anti pollution ability, and can ensure that the produced water quality meets the established standards for discharge and long-term stable operation.
Design one set of sodium chloride SWRO device, each with a processing capacity of 25m3/h, a production capacity of 17.5m3/h, and a concentrated water capacity of 7.5m3/h, arranged in a two-stage manner. The produced water enters the reverse osmosis water tank.
2.2.4 DTRO unit
DT membrane technology, also known as disc tube membrane technology, is a patented membrane separation equipment. The disc tube membrane module has a patented flow channel design form, which adopts an open flow channel. The material enters the pressure vessel through the inlet and flows from the channel between the guide plate and the shell to the other end of the module. At the other end of the flange, the material liquid enters the guide plate through 8 channels. The processed liquid quickly flows through the filter membrane at the shortest distance, then reverses 180 degrees to another membrane surface, and flows from the slot in the center of the guide disc to the next guide disc, forming a double "S" shaped route on the membrane surface from the circumference of the guide disc to the center of the circle, then to the circumference, and finally to the center of the circle. The concentrated liquid finally flows out from the flange at the feeding end. The distance between the two guide plates of the DT component is 4mm, and there are convex points arranged in a certain way on the surface of the guide plate. This special Hydraulics design allows the treatment fluid to flow through the membrane surface under pressure, forming turbulence when encountering bump collision, increasing the transmission rate and self-cleaning function, thus effectively avoiding membrane blockage and concentration polarization, and successfully extending the service life of the membrane. When cleaning, it is also easy to clean the scale on the membrane, ensuring that the disc tube membrane group is suitable for treating wastewater with high turbidity and high sand content coefficient, and adapting to even worse inlet conditions.
2.2.5 STRO unit
STRO is a reliable and permeable separation method. Permeation describes the mutual transfer of low concentration solution and high concentration solution on both sides of Semipermeable membrane. The membrane allows water molecules to pass through and intercepts salt and other components of water. The Semipermeable membrane allows water molecules to diffuse from the low concentration side to the high concentration side until both sides reach the osmotic equilibrium of the same salt concentration. During reverse osmosis, pressure acts on the concentrated water side to promote water molecules to pass through the Semipermeable membrane. Most dissolved salts, organic matter, bacteria, and solid suspensions cannot be discharged through the membrane along with the system concentrate. The produced water can be used without further treatment.
2.2.6 Sludge treatment unit
The sludge discharged from the reaction sedimentation tank is transported to the sludge collection tank through a sludge pump. After being conditioned and concentrated in the sludge collection tank, it is sent to the filter press for filtration and external transportation.
2.2.7 Auxiliary dosing unit
The auxiliary dosing unit mainly includes the following devices: reverse osmosis cleaning device, DTRO/STRO cleaning device, lime milk dosing device, sodium carbonate dosing device, hydrochloric acid dosing device, sodium hydroxide dosing device, coagulant aid dosing device, coagulant dosing device, Sodium hypochlorite dosing device, scale inhibitor dosing device, reducing agent dosing device, etc.
2.2.8 Evaporative crystallization unit MVC+MVR (mechanical steam recompression)
This project uses a set of "MVC+MVR" evaporation crystallization units to treat the concentrated brine generated by DTRO, with a processing capacity of 3m3/h (NaC). After high magnification concentration, it is subjected to salt crystallization, and high-quality condensate is recovered while obtaining NaCl that meets industrial salt standards.
After preheating, the concentrated brine enters the evaporative crystallization system and exchanges heat in the heater to reach the designed flash temperature of the system. The circulating liquid enters the crystallizer and completes flash evaporation at the liquid level. The secondary steam is heated and boosted by the compressor as heating steam and enters the shell side of the heat exchanger. The condensate is collected through the condensate tank and enters the preheating system to recover heat.
Conclusion 3: Through the introduction of the zero discharge process for crystal salt purification, the following conclusion can be drawn: as an efficient wastewater treatment and reuse and zero discharge technology scheme, the salt removal rate can reach 92%. The resource utilization of solid salts separated from crystallization can produce solid salt products that meet industrial standards, improving economic benefits while avoiding environmental pollution and water resource recovery from high salinity wastewater
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