A desulfurization wastewater heat method evaporation crystallization treatment system and method

Abstract

The application discloses a desulfurization wastewater heat evaporation crystallization treatment system and method, which comprises a pretreatment unit, a multi-effect flash evaporation concentration unit for producing desulfurization wastewater into concentrated liquid, a mechanical steam compression crystallization unit for solidifying the concentrated liquid into crystalline salt, and a mechanical steam compression crystallization unit composed of an evaporator, a steam compressor, a crystallizer and a crystal slurry treatment module, wherein the steam compressor is used for recycling the secondary steam after pressurization and temperature rise to the evaporator; wherein the final steam of the negative pressure flash module is input into the mechanical steam compression crystallization unit as initial steam, and the condensed water of the negative pressure flash module is input into the mechanical steam compression crystallization unit as production water. In use, the steam condensed water and the secondary steam of the multi-effect flash evaporation concentration unit are recycled, the two-step treatment processes are connected, the steam and water consumption is reduced, the heat consumption is lowered, and the system is simple.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of desulfurization wastewater treatment, and particularly relates to a desulfurization wastewater heat evaporation crystallization treatment system and method. BACKGROUND

[0002] In the wet desulfurization process of a coal-fired power plant, desulfurization wastewater contains high-concentration suspended solids, heavy metal ions and supersaturated sulfates (such as CaSO4 and CaCO3), and thus becomes one of the most difficult end wastewater to be treated in the power plant. In recent years, with the tightening of environmental protection policies, how to achieve the "zero discharge" of desulfurization wastewater has become a key direction of the industry's technical research.

[0003] However, in the prior art, the treatment of desulfurization wastewater is usually divided into concentration and reduction treatment and drying and solidification treatment, but the two treatment processes are relatively independent, so that different steam and water need to be used; and the desulfurization wastewater is prone to cause fouling on the surface of the heat exchange tube, which on the one hand reduces the flash evaporation efficiency, and on the other hand requires frequent maintenance and descaling, thereby reducing the production efficiency; and after flash evaporation, the concentrated liquid also needs to be treated by solidification, and in the MVR process solidification treatment, the problem of fouling on the heat exchange tube also exists. SUMMARY

[0004] In view of the problems of independent treatment process and easy fouling in the prior art desulfurization wastewater heat evaporation crystallization treatment, the present application is proposed.

[0005] Therefore, the present application aims to provide a desulfurization wastewater heat evaporation crystallization treatment system, which aims to: connect the treatment processes and reduce the amount of steam and water used.

[0006] To solve the above technical problems, the present application provides the following technical scheme: a desulfurization wastewater heat evaporation crystallization treatment system, comprising a pretreatment unit, which carries out hardness removal treatment on the desulfurization wastewater through a sodium carbonate-sodium hydroxide double-alkali softening device; a multi-effect flash evaporation concentration unit, which is used for producing concentrated liquid from the desulfurization wastewater, and comprises multiple negative pressure flash modules connected in series, the inside of each negative pressure flash module is driven by a forced circulation pump to circulate and heat the wastewater, the secondary steam generated by the negative pressure flash module at the front is used as the heat source of the negative pressure flash module at the back; a mechanical steam compression crystallization unit, which is used for solidifying the concentrated liquid into crystalline salt, and comprises an evaporator, a steam compressor, a crystallizer and a crystal slurry treatment module, the steam compressor is used for pressurizing and heating the secondary steam and then feeding it back to the evaporator for secondary use; wherein, the final steam of the negative pressure flash module is input into the mechanical steam compression crystallization unit as initial steam, and the crystal slurry treatment module comprises a thickener, a centrifugal machine and a mother liquor tank connected in sequence, the thickener is used for separating the crystal grains from the supernatant in the crystal slurry, the centrifugal machine is used for dehydrating the crystal grains, and the mother liquor tank is used for collecting the supernatant and the water separated by the centrifugal machine.

[0007] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, the mechanical steam compression crystallization unit further comprises a preheater arranged in front of the evaporator and used for preheating the concentrated liquid, and a gas-liquid separator arranged between the crystallizer and the steam compressor and used for separating the steam and the crystal slurry, the concentrated liquid and the saturated liquid are boiled and evaporated in the crystallizer, the secondary steam generated by the evaporation is input into the steam compressor, the saturated liquid generated in the crystallizer is mixed with the new concentrated liquid by a circulating pump and then re-input into the evaporator for heating, and the crystal slurry is transported to the thickener through the bottom discharge port.

[0008] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, the negative pressure flash modules in the multi-effect flash evaporation concentration unit are greater than or equal to two, each negative pressure flash module comprises a separator and a heater, and a forced circulation pump is arranged between the separator and the heater and used for circulating the flash desulfurization wastewater.

[0009] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, the negative pressure flash module at the front end is a one-effect flash module, the negative pressure flash module at the tail end is a last-effect flash module, the input end of the one-effect flash module is communicated with the pretreatment unit, and the output end of the last-effect flash module is communicated with the mechanical steam compression crystallization unit.

[0010] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, a density meter and a discharge pump are arranged at the discharge port of each separator, the density meter and the discharge pump are electrically connected, and the discharge pump is used for automatically outputting the concentrated wastewater meeting the density requirement.

[0011] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, wherein: each of the separator feed port is provided with a feed pump, the feed pump is provided with a feed electric valve, which is used to supplement the liquid level in each of the separators, the separator is provided with a liquid level meter, and the feed electric valve is electrically connected with the liquid level meter.

[0012] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, wherein: a control unit is further provided, the concentration of the concentrated liquid is monitored in real time through the density meter arranged at the discharge port of each of the separators, and the electric valve is connected in linkage to realize automatic adjustment of the liquid level of the evaporator.

[0013] As a preferred scheme of the desulfurization wastewater heat evaporation crystallization treatment system, wherein: the multi-effect flash evaporation concentration unit is further provided with a demister communicated with the negative pressure flash module, a condenser communicated with the demister, and a water collecting pool communicated with the condenser through a heat exchanger, and the water collecting pool is provided with a softening treatment device for reducing the hardness of the condensed water.

[0014] The application further provides a desulfurization wastewater treatment method for the desulfurization wastewater heat evaporation crystallization treatment system, which comprises the following steps:

[0015] S1: performing two-stage chemical softening treatment on the desulfurization wastewater to control the hardness of the desulfurization wastewater;

[0016] S2: performing multi-effect flash evaporation concentration on the pretreated desulfurization wastewater to generate concentrated liquid;

[0017] S3: performing gradient crystallization on the concentrated liquid to generate crystalline salt;

[0018] S4: performing resource utilization on the separated mixed salt after stabilization treatment.

[0019] The application has the advantages that: in use, the steam condensate water and the secondary steam of the multi-effect flash evaporation concentration unit are recycled, the secondary steam can be sent back to the low-pressure auxiliary steam for mixing and used as the flash evaporation heat source again, or can be sent into the mechanical steam compression crystallization unit for continuous use as the preheating heat source and the initial steam. The condensate water in the flash evaporation process is collected through the demister and finally condensed into the water collecting pool for collection, and can be optionally cooled by heat exchange with cooling water according to needs, and can be input into the mechanical steam compression crystallization unit as production water after softening treatment in the water collecting pool, so that the two-step treatment processes are connected, the steam and water consumption is reduced, the heat consumption is reduced, and the system is more simple.

[0020] The application also simultaneously carries out softening pretreatment on the desulfurization wastewater and the condensate water, so that the probability of the fouling of the heat exchange pipe can be effectively reduced, the heat exchange efficiency can be improved, the maintenance frequency can be reduced, and the production efficiency can be indirectly improved. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0022] Figure 1 It is a schematic diagram of the desulfurization wastewater heat evaporation crystallization treatment system of the present application.

[0023] Figure 2 It is a schematic diagram of the mechanical vapor compression crystallization unit of the desulfurization wastewater heat evaporation crystallization treatment system of the present application.

[0024] Figure 3 It is a schematic diagram of the pretreatment unit and the multi-effect flash evaporation concentration unit of the desulfurization wastewater heat evaporation crystallization treatment system of the present application.

[0025] Figure 4 It is a schematic diagram of the desulfurization wastewater heat evaporation crystallization treatment system of the present application when using three-effect flash evaporation concentration. DETAILED DESCRIPTION

[0026] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the drawings of the specification.

[0027] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of the present application, therefore the present application is not limited to the specific embodiments disclosed below.

[0028] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the present application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an independent or alternative embodiment that excludes other embodiments.

[0029] Thirdly, the present application is described in detail in combination with the schematic diagram, in the detailed description of the embodiments of the present application, for the convenience of description, the cross-sectional view of the device structure will be partially enlarged without the general proportion, and the schematic diagram is only an example, which should not limit the scope of protection of the present application herein. In addition, the three-dimensional spatial dimensions of length, width and depth should be included in actual production.

[0030] Embodiment 1

[0031] Reference Figures 1-4 For the first embodiment of the present application, a desulfurization wastewater heat evaporation crystallization treatment system is provided, which comprises a pretreatment unit 100, and the desulfurization wastewater is treated by hardness removal through a sodium carbonate-sodium hydroxide double-alkali softening device.

[0032] The pretreatment unit 100 comprises a desulfurization wastewater input 101 and a wastewater buffer pool 102, for example, the hardness of the desulfurization wastewater can be reduced to ≤50 mg / L through the sodium carbonate-sodium hydroxide double-alkali softening device, at the same time, the wastewater buffer pool 102 also plays a role in collecting and buffering the wastewater, avoiding the wastewater directly rushing into the multi-effect flash evaporation concentration unit 200, and the wastewater buffer pool 102 can also be more convenient to adjust the water quantity transported into the multi-effect flash evaporation concentration unit 200 by setting a wastewater delivery pump.

[0033] The multi-effect flash evaporation concentration unit 200 is used for producing concentrated liquid from the desulfurization wastewater, and the multi-effect flash evaporation concentration unit 200 comprises a plurality of negative pressure flash evaporation modules 201 connected in series, the inside of each negative pressure flash evaporation module 201 is driven by a forced circulation pump to circulate and heat the wastewater, the secondary steam generated by the negative pressure flash evaporation module 201 of the front effect is used as the heat source of the negative pressure flash evaporation module 201 of the rear effect, the steam input 103 of the low-pressure auxiliary steam of the unit is used as the heat source, the desulfurization wastewater is concentrated through the plurality of negative pressure flash evaporation modules 201 connected in series, in the concentration process, the steam is continuously transported to the next effect as the heat source of the next effect, wherein the negative pressure of each negative pressure flash evaporation module 201 is derived from the tail-end vacuum pump arranged at the last effect, the tail-end vacuum pump is communicated with each heater 201b, for discharging non-condensable gas and maintaining micro-negative pressure, at the same time, the condensed water generated by the multi-effect flash evaporation concentration unit 200 is finally collected in the water collecting pool 205, the condensed water after softening treatment can be used as cooling water or preheating water and input into the mechanical steam compression crystallization unit 300, according to the demand, the heat exchanger 206 can be used for heat exchange with the external cooling water, heat exchange with the concentrated liquid, preheating or cooling the steam, and the actual demand can be flexibly adjusted.

[0034] The mechanical vapor compression crystallization unit 300 is used to solidify the concentrated liquid into crystalline salt, and the mechanical vapor compression crystallization unit 300 is composed of an evaporator 302, a vapor compressor 303, a crystallizer 304 and a crystal slurry treatment module 305. The vapor compressor 303 pressurizes and heats the secondary steam and then returns it to the evaporator 302 for secondary use.

[0035] The final steam of the negative pressure flash evaporation module 201 is input into the mechanical vapor compression crystallization unit 300 as initial steam. The crystal slurry treatment module 305 includes a thickener 305a, a centrifuge 305b and a mother liquor tank 305c connected in sequence. The thickener 305a is used to separate the crystal grains in the crystal slurry from the supernatant. The centrifuge 305b is used to dehydrate the crystal grains. The mother liquor tank 305c collects the supernatant and the water removed by the centrifuge 305b.

[0036] The mechanical vapor compression crystallization unit 300 further includes a preheater 301 arranged in front of the evaporator 302, which is used to preheat the concentrated liquid. A gas-liquid separator is arranged between the crystallizer 304 and the vapor compressor 303, which is used to separate the steam and the crystal slurry. The concentrated liquid and the saturated liquid are boiled in the crystallizer 304 to generate secondary steam, which is input into the vapor compressor 303. The saturated liquid generated in the crystallizer 304 is mixed with the new concentrated liquid by a circulating pump 306 and then reenters the evaporator 302 for heating. The crystal slurry is transported to the thickener 305a through the bottom discharge port.

[0037] During use, the untreated desulfurization wastewater is injected into the wastewater buffer tank 102 through the desulfurization wastewater input 101. The wastewater buffer tank 102 is used to avoid the desulfurization wastewater being directly sent into the multi-effect flash evaporation concentration unit 200. A stirring device can be arranged in the wastewater buffer tank 102 to pretreat the desulfurization wastewater in different ways to prevent scaling or blockage in the pipeline. The desulfurization wastewater is transferred in the wastewater buffer tank 102 and treated to remove hardness. Then, the desulfurization wastewater is pumped into the multi-effect flash evaporation concentration unit 200 by a wastewater delivery pump. In the multi-effect flash evaporation concentration unit 200, the desulfurization wastewater is continuously flashed. Part of the water in the desulfurization wastewater is evaporated into steam. The final steam is output from the last negative pressure flash evaporation module 201 to the mechanical vapor compression crystallization unit 300 as initial steam. The remaining water and impurities become concentrated liquid, which is discharged from the concentrated liquid output 202 of the last negative pressure flash evaporation module 201 and then enters the mechanical vapor compression crystallization unit 300 for further treatment. The concentrated liquid first enters the concentrated liquid tank 301a and then is input into the preheater 301 through the feed pump 301b for preheating. The preheater 301 can collect the steam finally discharged from the multi-effect flash evaporation concentration unit 200 as long as it meets the use conditions of the preheater 301.

[0038] The preheater 301 preheats the concentrated solution to 60-80°C, and then the heated concentrated solution is input into the evaporator 302, where the concentrated solution is heated again by using hot steam, and the heated concentrated solution is sent into the crystallizer 304, where it is boiled and evaporated. The secondary steam generated in the evaporation process is transported to the steam compressor 303 after being separated by a gas-liquid separator. The outlet of the crystallizer 304 is connected to the gas-liquid separator, and the separated secondary steam is pressurized by the steam compressor 303 and reused.

[0039] The pressure and temperature in the steam compressor 303 are increased to become saturated steam, which is returned to the evaporator 302 as a heat source. The crystals precipitated after evaporation settle at the bottom of the crystallizer 304 to become crystal slurry, and the remaining saturated liquid enters the circulating pipe from the middle of the crystallizer 304 to continue to participate in evaporation. The crystal slurry enters the thickener 305a to thicken and precipitate, the supernatant in the thickener 305a overflows into the mother liquor tank 305c and is returned to the system to continue to participate in circulation, and the crystal grains at the bottom of the thickener 305a are discharged into the centrifuge 305b, which is dewatered to obtain crystalline salt.

[0040] Example 2

[0041] Reference Figures 1-2 For the second embodiment of the present application, the difference from the first embodiment is that the crystallizer 304 is used to process the concentrated solution into saturated liquid and crystal slurry, and a circulating pump 306 is provided between the crystallizer 304 and the evaporator 302 to transport the saturated liquid and the concentrated solution back to the evaporator 302 for heating and circulation. The saturated liquid enters the circulating pipe from the middle of the crystallizer 304, the input end of the circulating pump 306 is communicated with the saturated liquid outlet of the crystallizer 304 through the circulating pipe, the output end of the circulating pump 306 is communicated with the liquid inlet of the evaporator 302 through the circulating pipe, and the circulating pump 306 provides the circulating power.

[0042] A gas-liquid separator is provided between the crystallizer 304 and the steam compressor 303 to separate the secondary steam and the crystal slurry, and the secondary steam is transported back to the steam compressor 303 for secondary heating and then sent into the evaporator 302 to heat the concentrated solution and the saturated liquid. The concentrated solution and the saturated liquid are boiled and evaporated in the crystallizer 304, the saturated liquid generated in the crystallizer 304 is mixed with the new concentrated solution by the circulating pump 306 and then re-enters the evaporator 302 for heating, and the crystal slurry is transported to the thickener 305a through the bottom discharge outlet.

[0043] The supernatant generated in the process of the thickener 305a processing the crystal slurry is transported to the mother liquor tank 305c, and the separated water generated in the process of the centrifuge 305b dewatering the crystal grains is also transported to the mother liquor tank 305c, and the mother liquor tank 305c sends the collected liquid back to the crystallizer 304 for recrystallization. The mother liquor tank 305c collects the moisture in the thickener 305a and the centrifuge 305b, and after collection, the liquid is sent back to the crystallizer 304 for evaporation by the mother liquor pump 305d.

[0044] The evaporator 302 is divided into a tube pass and a shell pass, the concentrated liquid enters the tube pass, and steam enters the shell pass. The evaporator 302 is provided with a condensed water discharge port at the bottom of the shell pass, for discharging the condensed water in the evaporator 302 outward, and the steam releases latent heat to condense into water and enter the condensed liquid tank 302a. The condensed liquid tank 302a sends the high-temperature condensed liquid back to the preheater 301 through the condensed liquid pump 302b, and the condensed liquid is used as a heat source to preheat the feed in the preheater 301 to meet the discharge standard or is used for production processes.

[0045] The remaining structure is the same as that of example 1.

[0046] Example 3

[0047] With reference to Figures 3-4 For the third embodiment of the present application, the difference between this embodiment and the second embodiment is that the multi-effect flash concentration unit 200 is provided with two or more negative pressure flash modules 201, each of which includes a separator 201a and a heater 201b, and a forced circulation pump is arranged between the separator 201a and the heater 201b, for circulating the flash desulfurization wastewater.

[0048] The negative pressure flash module 201 at the front end is a one-effect flash module, and the negative pressure flash module 201 at the tail end is a last-effect flash module. The input end of the one-effect flash module is communicated with the pretreatment unit 100, and the output end of the last-effect flash module is communicated with the mechanical vapor compression crystallization unit 300. The pretreatment unit 100 is also provided with a low-pressure auxiliary steam input 103, which can input plant auxiliary steam at 250 DEG C.

[0049] A density meter and a discharge pump are arranged at the discharge port of each separator 201a, and the density meter and the discharge pump are electrically connected, for automatically discharging concentrated wastewater meeting the density requirement.

[0050] A feed pump is arranged at the feed port of each separator 201a, and a feed electric valve is arranged on the feed pump, for supplementing the liquid level in each separator 201a. A liquid level meter is arranged in the separator 201a, and the feed electric valve and the liquid level meter are electrically connected.

[0051] In the embodiment, three negative pressure flash modules 201 are provided as an example. During use, the desulfurization wastewater enters the first separator 201a through the wastewater delivery pump. The desulfurization wastewater in the first separator 201a is pumped to the first heater 201b tube side by the forced circulation pump, and exchanges heat with the low-pressure auxiliary steam from the unit. The heated desulfurization wastewater returns to the first separator 201a. The heated desulfurization wastewater boils in the vacuum environment of the first separator 201a, and part of the wastewater becomes steam and enters the second heater 201b shell side as the heat source of the second heater 201b. The remaining wastewater continues to circulate into the first heater 201b and exchanges heat with the steam. When the desulfurization wastewater is evaporated in the first separator 201a for multiple times to meet the standard requirements, the concentrated liquid generated by the first negative pressure flash module 201 enters the second negative pressure flash module 201, and is driven by the forced circulation pump of the second negative pressure flash module 201 to circulate and evaporate in the second separator 201a. The principles of the second, third, and last negative pressure flash modules 201 are the same.

[0052] The evaporation temperature of the first effect is about 76℃, the evaporation temperature of the second effect is about 66℃, and the evaporation temperature of the third effect is about 50℃. Through the three-effect setting, the processing water amount of the multi-effect flash concentration unit 200 is about 22m3 / h, the fresh water recovery amount is about 14m3 / h, and the concentration ratio reaches 2.75 times.

[0053] Further, the first heater 201b can also be provided with a condensate pipe to discharge the condensate condensed in the first heater 201b. After being discharged, the condensate can exchange heat with the high-temperature flue gas of the unit to generate steam again, which is mixed with the low-pressure auxiliary steam to serve as the heat source of the first heater 201b again and return to the cycle.

[0054] The multi-effect flash concentration unit 200 is also provided with a demister 203 communicating with the negative pressure flash module 201, a condenser 204 communicating with the demister 203, and a water collecting tank 205 communicating with the condenser 204 through a heat exchanger 206. The water collecting tank 205 is provided with a softening treatment device for reducing the hardness of the condensate.

[0055] The steam discharged from the first-effect separator 201a can also be provided with a demister 203 before entering the second-effect heater 201b, which also functions to collect water in the steam, and similarly, the steam discharged from the second-effect separator 201a can also be provided with a demister 203 before entering the third-effect heater 201b, and so on. Each demister 203 collects water in the steam and collects the water in the condenser 204, which is condensed into high-temperature condensed water. The high-temperature condensed water is sent to the heat exchanger 206 to exchange heat with the cooling water 207, and then sent to the water collecting tank 205, where the condensed water is softened. The treated condensed water can be used as heat exchange water for the multi-effect flash evaporation concentration unit 200 and the mechanical vapor compression crystallization unit 300, and is not prone to fouling. The total amount of cooling water 207 is about 440 m³ / h, and the temperature rise of the cooling water 207 is about 10°C (feed water temperature 32°C, return water temperature 42°C).

[0056] Further, the calcium sulfate in the desulfurization wastewater is used as a crystal seed, and the concentration is greater than 0.5%. When the desulfurization wastewater starts to evaporate, the calcium sulfate precipitated from the desulfurization wastewater is preferentially attached to the suspended crystal seed by using the characteristic that substances tend to be adsorbed on the surface of similar substances when they are crystallized, thereby avoiding deposition on the inner wall of the heating pipe and achieving the purpose of preventing fouling.

[0057] The remaining structure is the same as that of Example 2.

[0058] Example 4

[0059] Referring to Figures 3-4 As the fourth embodiment of the present application, the difference between this embodiment and the third embodiment is that a control unit is further provided. The concentration of the concentrated liquid is monitored in real time by the densitometer arranged at the discharge port of each separator 201a, and the electric valve is linked to realize automatic adjustment of the liquid level of the evaporator 302.

[0060] The intelligent control unit includes a PLC controller, which is used to dynamically adjust the speed of the compressor to match the change of the steam load. For example, the PLC controller takes the density as the first control parameter, and preferentially adjusts the flow of the circulating pump when the density is not up to standard. When the steam load fluctuates by more than ±5%, the compressor speed is started. A multi-point temperature sensor is arranged in the crystallizer 304 to monitor the temperature in the crystallizer 304. The valve opening degree sensor and the densitometer arranged at the crystal slurry discharge port of the crystallizer 304 are used to control the discharge of the crystal slurry in real time.

[0061] The opening and closing of each pump and valve and the flow rate are controlled in real time by the PLC controller, so as to realize automatic control of the system. For example, when the concentration of the concentrated liquid in the first separator 201a does not meet the standard, the PLC controller closes the valve at the connection between the first separator 201a and the second separator 201a, controls the forced circulation pump to circulate and flash evaporate the concentrated liquid in the first negative pressure flash evaporation module 201, until the concentration of the concentrated liquid meets the standard. At this time, the PLC controller receives the data of the density meter, confirms that the standard is met, and automatically opens the valve at the connection between the first separator 201a and the second separator 201a, so as to transport the concentrated liquid to the second separator 201a. Similarly, by using various sensors such as density meters and temperature sensors, automatic control of various valves, pumps and other equipment can be realized.

[0062] The rest of the structure is the same as that of Example 3.

[0063] Example 5

[0064] The embodiment also provides a desulfurization wastewater treatment method, which is used for the above-mentioned desulfurization wastewater heat evaporation crystallization treatment system and includes the following steps:

[0065] S1: performing two-stage chemical softening treatment on the desulfurization wastewater, and controlling the hardness of the desulfurization wastewater;

[0066] S2: performing multi-effect flash evaporation concentration on the pretreated desulfurization wastewater, to generate concentrated liquid;

[0067] S3: performing gradient crystallization on the concentrated liquid, to generate crystallized salt;

[0068] S4: performing resource utilization on the separated mixed salt after stabilization treatment.

[0069] Specifically, taking three-effect flash evaporation as an example.

[0070] S1.1: first-stage sodium carbonate softening, sodium carbonate solution is added to the desulfurization wastewater, the pH value is controlled to be 9.8-10.2, the pH value is the best pH range of calcium carbonate precipitation (pH < 9.8, incomplete precipitation, pH > 10.2, magnesium precipitation may be induced), the reaction time is ≥ 30 min, the dosage is 1.1 times of the molar concentration of Ca²⁺, the calcium hardness is reduced to ≤ 150 mg / L, calcium ions in the desulfurization wastewater are converted into calcium carbonate precipitate by adding sodium carbonate, the solubility product of calcium carbonate is much lower than that of calcium sulfate, calcium sulfate which is easy to form scale in the wastewater can be eliminated, hard scale in the subsequent evaporator can be prevented, calcium hardness can be efficiently removed, and thus the desulfurization wastewater is preliminarily softened.

[0071] S1.2: Secondary sodium hydroxide softening, continue to add sodium hydroxide solution until pH = 11.0-11.5, which is the optimal pH for magnesium hydroxide precipitation (pH < 11.0 has low precipitation rate, pH > 11.5 may be redissolved), reaction time ≥ 20 min, dosage is 1.1-1.3 times the molar concentration of Mg²⁺, so that the total hardness is ≤ 50 mg / L, adding sodium hydroxide to precipitate magnesium hydroxide and remove residual calcium hardness (using bicarbonate in wastewater to further precipitate residual calcium ions), magnesium hydroxide is easy to form sticky silicon-magnesium scale (combined with SiO2) in the evaporator, covering the magnesium ion concentration fluctuation + neutralizing sodium hydroxide in wastewater.

[0072] S1.3: After softening, the wastewater enters the buffer tank, the residence time is controlled for 1-2 h, and the stirring speed is 30-50 rpm.

[0073] Among them, calcium ions precipitate in the form of ceCaCO3 at pH = 9.5-10.5, while magnesium ions need pH > 10.5 to form ceMg(OH)2, if adjusted to pH = 11.0 in one step, ceCa(OH)2 will be generated (solubility 0.173 g / 100 g water), which has fine particles and is difficult to settle, resulting in high residual hardness, sodium carbonate is lower than sodium hydroxide in monovalence, and stepwise addition can reduce reagent cost, and residence and stirring in the buffer tank can make the precipitation complete, preventing unreacted reagents from entering the evaporator to cause scaling.

[0074] S2.1: One-effect flash evaporation, maintain operating pressure -0.06~-0.08 MPa, temperature 76±2℃, forced circulation flow rate ≥ 2.8 m / s, when the concentration liquid density reaches ≥ 1.076 g / cm³, transfer to two-effect.

[0075] S2.2: Two-effect flash evaporation, temperature 66±2℃, automatically supplement 10%-15% of the fresh steam of the secondary steam of the previous effect, when the concentration liquid density reaches ≥ 1.114 g / cm³, transfer to three-effect.

[0076] S2.3: Three-effect flash evaporation, temperature 50±1℃, when the concentration liquid density reaches ≥ 1.21 g / cm³, discharge the concentrated liquid, and the final concentration rate can reach 90%.

[0077] S3.1: Preheat control, preheat the concentrated liquid to 60-80℃ and then output.

[0078] S3.2: Secondary heating, heat the preheated concentrated liquid, use 125-135℃ steam in the evaporator 302 to exchange heat with the concentrated liquid, and the pipe flow rate is ≥ 2.0 m / s.

[0079] S3.3: The concentrated liquid after heating is fed into the crystallizer 304 to be boiled and evaporated at 80-85℃, the pressure is raised to 0.15-0.3 MPa, and the solid content of the crystal slurry after evaporation is 20-30wt%.

[0080] S3.4: The crystal slurry is fed into the thickener 305a to be thickened and precipitated, the residence time is 1.5-2h, and the solid content of the underflow is ≥45wt%.

[0081] S3.5: The crystal grains meeting the requirements are fed into the centrifuge 305b to be spun dry, the centrifuge 305b rotates at 1800-2200rpm, and the water content of the crystalline salt product after spinning is ≤8%.

[0082] In S3, the secondary steam in the crystallizer 304 can be returned to the steam compressor 303 to be raised in pressure and temperature, and then fed into the evaporator 302 again.

[0083] It is important to note that the construction and arrangement of the application shown in the various exemplary embodiments is illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described in the application. For example, elements shown as integrally formed can be constructed of multiple parts or elements, the position of elements can be reversed or otherwise varied, and the nature or number of elements or positions can be altered or varied. Accordingly, all such variations are intended to be included within the scope of the present application. The order or sequence of any process or method steps can be changed unless otherwise specified or clearly implied by the context. Thus, the present application is not limited to the specific embodiments described in this disclosure but is extended to encompass all such modifications and alterations.

[0084] In addition, for the purpose of providing a concise description of exemplary embodiments, all features will not necessarily be described (that is, in connection with the preferred embodiment, those features not considered by the inventor to be as important in practical implementation as the features being described) or can not be present in the disclosed embodiment. The disclosure as a whole is intended to be representative of the disclosure.

[0085] It should be noted that the above examples are only used to illustrate the technical solutions of the present application but not limit the present application, and although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the spirit and scope of the present application, and all should be included in the scope of the claims of the present application.

Claims

1. A desulfurization wastewater thermal evaporation crystallization treatment system characterized by: The application relates to a desulfurization wastewater treatment system, which comprises the following units: a pretreatment unit (100) which comprises a desulfurization wastewater input (101) and a wastewater buffer tank (102), and which is used for carrying out hardness removal treatment on the desulfurization wastewater through a sodium carbonate-sodium hydroxide double-alkali softening device; a multi-effect flash evaporation concentration unit (200) which is used for producing concentrated liquid from the desulfurization wastewater, and which comprises multiple-stage series connection negative pressure flash evaporation modules (201), the inside of each effect negative pressure flash evaporation module (201) is driven by a forced circulation pump to drive wastewater circulation heat exchange, secondary steam generated by the negative pressure flash evaporation module (201) of a previous effect is used as a heat source of the negative pressure flash evaporation module (201) of a later effect; a mechanical steam compression crystallization unit (300) which is used for solidifying the concentrated liquid into crystalline salt, and which comprises an evaporator (302), a steam compressor (303), a crystallizer (304) and a crystal slurry treatment module (305), the steam compressor (303) is used for pressurizing and heating the secondary steam and then feeding the secondary steam back to the evaporator (302) for secondary utilization; wherein the final steam of the negative pressure flash evaporation module (201) is input into the mechanical steam compression crystallization unit (300) as initial steam, the condensate water of the negative pressure flash evaporation module (201) is input into the mechanical steam compression crystallization unit (300) as production water, the crystal slurry treatment module (305) comprises a thickener (305a), a centrifugal machine (305b) and a mother liquor tank (305c) which are sequentially connected, the thickener (305a) is used for separating crystal grains from supernatant in the crystal slurry, the centrifugal machine (305b) is used for dehydrating the crystal grains, and the mother liquor tank (305c) is used for collecting the supernatant and water separated out from the centrifugal machine (305b); the multi-effect flash evaporation concentration unit (200) is further provided with a demister (203) which is in communication with the negative pressure flash evaporation module (201), a condenser (204) which is in communication with the demister (203), the condenser (204) is in communication with a water collecting tank (205) through a heat exchanger (206), and the water collecting tank (205) is provided with a softening treatment device which is used for reducing the hardness of the condensate water.

2. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 1, characterized in that: The crystallizer (304) is used for treating the concentrated liquid into saturated liquid and crystal slurry, a circulating pump (306) is arranged between the crystallizer (304) and the evaporator (302), and is used for feeding the saturated liquid and the concentrated liquid back to the evaporator (302) for heating circulation.

3. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 2, characterized in that: The mechanical steam compression crystallization unit (300) further comprises a preheater (301) which is arranged in front of the evaporator (302) and is used for preheating the concentrated liquid. A gas-liquid separator is arranged between the crystallizer (304) and the vapor compressor (303) to separate the vapor and the crystal slurry. The concentrated liquid and the saturated liquid are boiled and evaporated in the crystallizer (304), the secondary vapor generated by the evaporation is input into the vapor compressor (303), the saturated liquid generated in the crystallizer (304) is mixed with the new concentrated liquid by the circulating pump (306) and then re-enters the evaporator (302) for heating, and the crystal slurry is transported to the thickener (305a) through the bottom discharge port.

4. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 1 or 3, characterized in that: The multi-effect flash concentration unit (200) is provided with two or more negative pressure flash modules (201), each of which comprises a separator (201a) and a heater (201b), and a forced circulation pump is arranged between the separator (201a) and the heater (201b) to circulate the flash desulfurization wastewater.

5. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 4, characterized in that: The negative pressure flash module (201) at the front end is a one-effect flash module, and the negative pressure flash module (201) at the tail end is a last-effect flash module. The input end of the one-effect flash module is connected with the pretreatment unit (100), and the output end of the last-effect flash module is connected with the mechanical vapor compression crystallization unit (300).

6. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 5, characterized in that: A density meter and a discharge pump are arranged at the discharge port of each separator (201a), and the density meter and the discharge pump are electrically connected to automatically output the concentrated wastewater meeting the density requirement.

7. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 6, characterized in that: A feed pump is arranged at the feed port of each separator (201a), and a feed electric valve is arranged on the feed pump to supplement the liquid level in each separator (201a). A liquid level meter is arranged in each separator (201a), and the feed electric valve and the liquid level meter are electrically connected.

8. The desulfurization wastewater thermal evaporation crystallization treatment system according to claim 7, characterized in that: A control unit is further arranged to monitor the concentration of the concentrated liquid in real time through the density meter arranged at the discharge port of each separator (201a) and to automatically adjust the liquid level of the evaporator (302) through the linkage of the electric valve.

9. A desulfurization wastewater treatment method based on the desulfurization wastewater thermal evaporation crystallization treatment system according to any one of claims 1 to 8, characterized by: The method comprises the following steps: S1: two-stage chemical softening treatment is performed on the desulfurization wastewater to control the hardness of the desulfurization wastewater; S2: the pretreated desulfurization wastewater is subjected to multi-effect flash concentration to generate concentrated liquid; S3: gradient crystallization is performed on the concentrated liquid to generate crystalline salt; S4: the obtained mixed salt is subjected to stabilization treatment and then is used as a resource.

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