Process for freezing crystallization treatment of desulfurization wastewater by using flue gas waste heat

Abstract

The invention discloses a process for freezing crystallization treatment of desulfurization wastewater by using flue gas waste heat. Comprising a flue gas heat exchanger (1), an ammonia-water absorption heat pump (3), an electric heat pump (15), a first-stage pre-cooling unit (9), a second-stage pre-cooling unit (11), a third-stage pre-cooling unit (13), an ice maker (5), an ice-water separator (7) and an evaporator (10). The flue gas heat exchanger (1) is respectively connected with the ammonia-water absorption heat pump (3) and the electric heat pump (15); the ammonia-water absorption heat pump (3) is respectively connected with the ice maker (5) and the three-stage pre-cooling unit (13); the electric heating pump (15) is connected with the evaporator (10); the first-stage pre-cooling unit (9) is connected with the second-stage pre-cooling unit (11); the second-stage pre-cooling unit (11) is connected with the third-stage pre-cooling unit (13); the third-stage pre-cooling unit (13) is connected with the ice maker (5); the ice maker (5) is connected with the ice-water separator (7); the ice-water separator (7) is respectively connected with the secondary precooling unit (11) and the primary precooling unit (9); and the secondary precooling unit (11) is connected with the evaporator (10). Aiming at waste heat utilization of industrial flue gas, low-temperature waste heat at an inlet of a sintering flue gas chimney is utilized, waste heat flue gas is used as a driving heat source through a heat pump, refrigeration and heat supply are used for a'freezing crystallization and evaporative crystallization 'process of desulfurization waste water, and efficient utilization of industrial low-temperature flue gas waste heat is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of utilization of flue gas waste heat and treatment of industrial high-salt wastewater in the steel industry, and relates to a zero-emission treatment process for desulfurization wastewater, and specifically to a process for using low-temperature flue gas waste heat for freezing and crystallization treatment of desulfurization wastewater. Background Art

[0002] There are many sources of waste heat in the flue gas of steel plants, and the emission volume is large. Direct emission causes a lot of energy loss. For example, after sintering flue gas dust removal, desulfurization and denitrification, the flue gas volume at the chimney inlet is large, the temperature is about 95°C, and the humidity is about 15%. The flue gas contains a lot of heat energy, and direct emission causes a lot of heat energy loss.

[0003] At the same time, the "limestone-gypsum" wet flue gas desulfurization produces a large amount of wastewater, and the wastewater contains a large amount of salts. These salts are mainly chloride ions, with a chloride ion content of about 2% (wt%). High concentrations of chloride ions can cause corrosion to equipment and pipelines, and lead to wastewater discharge that does not meet standards. In the future, "zero discharge" of desulfurization wastewater will become a hot spot for industrial wastewater management.

[0004] At present, the zero-emission process of desulfurization wastewater is mainly achieved by the "pretreatment + membrane method + thermal evaporation method". The pretreatment mainly removes some impurities by adding chemical reagents for precipitation, and concentrates the wastewater through a reverse osmosis membrane. The concentrated wastewater is evaporated to obtain solid salt. For the field of high-salt wastewater, the traditional process has a limited life of the membrane, and the concentration multiple will be restricted by the composition and concentration of the wastewater. A large amount of wastewater requires a lot of heat energy through the back-end thermal evaporation process. The high-cost and high-energy consumption process limits the promotion and application of this technology.

[0005] Freeze crystallization has the advantages of high wastewater concentration ratio and low energy consumption.

[0006] The purpose of freezing crystallization in the desulfurization wastewater desalination process is to concentrate the wastewater, replacing the membrane concentration in the traditional process. Compared with the membrane concentration process, the main advantages of freezing crystallization of high-salt wastewater are: first, the concentration multiple of freezing crystallization is higher, which is more than twice the membrane concentration multiple; second, freezing crystallization replaces membrane concentration for wastewater concentration, which solves the problem of high membrane loss and high operating costs caused by high-concentration wastewater.

[0007] Freezing crystallization consumes less energy than evaporation crystallization.

[0008] Ice is a single mineral rock, so water will automatically remove impurities during the crystallization process. The latent heat of water is 334.7 kJ / kg, which is only 1 / 7 of its latent heat of vaporization. The energy consumption of freezing crystallization desalination is lower than that of evaporation crystallization desalination. The corrosive components in the wastewater at low temperatures have lower requirements for equipment materials, and freezing crystallization can remove inorganic salts, microorganisms, organic matter, etc. at the same time.

[0009] Based on the market demand for industrial waste heat utilization and zero discharge of desulfurization wastewater, the present invention application considers adopting the process of "freezing crystallization + evaporation crystallization" to achieve the effect of zero discharge of desulfurization wastewater.

[0010] The prior art and existing defects related to the present invention are analyzed as follows.

[0011] Patent document CN 114656086 A discloses a method and system for fully utilizing desulfurization wastewater based on waste heat method and freezing method for salt separation. The document mainly realizes salt separation based on the difference in solubility of sodium sulfate and sodium chloride with temperature. The desulfurization wastewater is cooled from 50°C to 10°C, and the solubility of sodium sulfate decreases significantly with decreasing temperature, while the solubility of sodium chloride does not change much with decreasing temperature. The sodium sulfate decahydrate crystals are first filtered and separated, and then the sodium chloride is separated by evaporation and crystallization. The disadvantage of this process is that it is highly selective for the type of salt, and the method cannot be generally applied to all salts. Only salts whose solubility decreases significantly with decreasing temperature can be separated by lowering the temperature and crystallizing the salt. However, many salts whose solubility does not change significantly with decreasing temperature cannot be separated by cooling crystallization. For example, sodium chloride can only be separated by evaporation and crystallization.

[0012] Patent document CN 216445186 U discloses an integrated system of heat pump heating and wastewater evaporation and crystallization driven by flue gas waste heat. The document uses the waste heat of the flue gas at the boiler outlet as the driving heat source, and evaporates and crystallizes high-salt wastewater such as desulfurization wastewater through a waste heat heat pump to desalinate. The disadvantage of this process is that the energy consumption required for direct evaporation and crystallization of desulfurization wastewater is high. Summary of the invention

[0013] In view of the deficiencies in the prior art, the present invention provides a process for using flue gas waste heat for freezing and crystallization treatment of desulfurization wastewater.

[0014] To achieve the purpose of the present invention, one aspect of the present invention provides a system for treating desulfurization wastewater by freezing and crystallizing flue gas waste heat, comprising a flue gas heat exchanger, an ammonia-water absorption heat pump, an electric heat pump, a primary precooling unit, a secondary precooling unit, a tertiary precooling unit, an ice maker, an ice-water separator, and an evaporator; the flue gas heat exchanger is respectively connected to the ammonia-water absorption heat pump and the electric heat pump; the ammonia-water absorption heat pump is respectively connected to the ice maker and the tertiary precooling unit; the electric heat pump is connected to the evaporator; the primary precooling unit is connected to the secondary precooling unit; the secondary precooling unit is connected to the tertiary precooling unit; the tertiary precooling unit is connected to the ice maker; the ice maker is connected to the ice-water separator; the ice-water separator is respectively connected to the secondary precooling unit and the primary precooling unit; the secondary precooling unit is connected to the evaporator.

[0015] According to an embodiment of the present invention, the flue gas heat exchanger is connected to an ammonia-water absorption heat pump and an electric heat pump through a circulating water pipe. The ammonia-water absorption heat pump is connected to the circulating water pipe on the left side and to the refrigerant ethylene glycol aqueous solution pipe on the right side; the upper side is connected to the heat dissipation medium air or cooling water.

[0016] According to an embodiment of the present invention, the electric heat pump is connected to a circulating water pipe on the left side and to a saturated steam pipe on the right side.

[0017] According to an embodiment of the present invention, the right side of the primary precooling unit is connected to the desulfurization wastewater inlet, the upper side is connected to the ice crystal tube inlet, and the lower side is the ice melt water outlet after heat exchange; the upper side of the ice crystal tube is connected to the ice-water separator.

[0018] According to an embodiment of the present invention, the secondary precooling unit is connected to the primary precooling unit on the right side, is connected to the ice-water separator through a concentrated brine pipe on the upper side, and is connected to the evaporator on the lower side.

[0019] According to an embodiment of the present invention, the right side of the three-stage pre-cooling unit is connected to the secondary pre-cooling unit, the left side is the inlet and outlet of the refrigerant ethylene glycol aqueous solution pipe, and the upper side is the inlet for waste water to enter the ice maker; the lower part of the ice maker is connected to the three-stage pre-cooling unit, the left side is the inlet and outlet of the refrigerant ethylene glycol aqueous solution pipe, and the upper side is the outlet of the ice crystal / concentrated brine pipe; the left side of the ice-water separator is the ice crystal / concentrated brine pipe, the right side is separated into an ice crystal tube, and the lower side is separated into a concentrated brine tube; the left side of the evaporator is connected to a saturated steam pipe, the upper side is connected to the concentrated brine after heat exchange, the lower side is the solid salt outlet, and the right side is the water vapor generated by evaporation and crystallization condensed into condensed water.

[0020] Another aspect of the present invention provides a method for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization, the steps of which include: (1) After dust removal, desulfurization and denitrification, the flue gas at the chimney inlet is heat exchanged by the flue gas heat exchanger, and the temperature of the circulating water in the circulating water pipe increases. The circulating water after heat exchange is used in two parts. One part of the circulating water is the driving heat source of the ammonia-water absorption heat pump, and the other part of the circulating water is the driving heat source of the electric heat pump; (2) The cold energy generated by the ammonia-water absorption heat pump is transmitted through the refrigerant ethylene glycol aqueous solution pipe. The output temperature of the refrigerant ethylene glycol aqueous solution is lower than -5°C, providing a cold source for the three-stage precooling unit of the desulfurization wastewater and the freezing crystallization of the ice maker; the heat energy generated by the electric heat pump is saturated steam, which provides a heat source for the evaporation crystallization of the evaporator; (3) After the refrigerant ethylene glycol aqueous solution pipe outputs cold energy, it circulates in the system, providing a cold source to the ice maker and the three-stage pre-cooling unit through the ethylene glycol aqueous solution refrigerant; the return state of the heat source saturated steam pipe is condensed water; (4) The temperature of the desulfurization wastewater is reduced after passing through the primary precooling unit. The cold source of the primary precooling unit is the ice crystals separated by the ice-water separator; (5) The wastewater after the primary precooling unit enters the secondary precooling unit. The cold source is the concentrated brine separated by the ice-water separator. The temperature of the wastewater after the secondary precooling unit continues to decrease. (6) The desulfurization wastewater enters the tertiary precooling unit. The cold source of the tertiary precooling unit is ethylene glycol aqueous solution. The temperature of the wastewater after precooling is lower than -2°C; (7) The desulfurized wastewater precooled by the three-stage precooling unit enters the ice-making machine for freezing and crystallization. The inlet temperature of the wastewater at the ice-making machine is -2~-3°C, and the outlet of the freezing and crystallization is a mixture of ice crystals and concentrated brine, with a temperature below -4°C; (8) The mixture of ice crystals and concentrated brine is separated into ice crystals and concentrated brine by an ice-water separator; (9) After the concentrated brine passes through the secondary precooling unit and exchanges heat with the desulfurization wastewater, its temperature rises and it evaporates and crystallizes through the evaporator. The heat source of the evaporator is saturated steam, and the evaporation product of the concentrated brine is solid mixed salt. The water vapor generated during the evaporation process is condensed into condensed water; (10) After the desulfurization wastewater is precooled in the primary precooling unit, the ice crystals melt into water, and the desalination rate of the water is higher than 70%. It is used as industrial circulating water, and the flow ratio of ice melt water to concentrated brine is higher than 2:1.

[0021] According to an embodiment of the present invention, in step (8), the temperature of the ice crystals is 0°C, and the temperature of the concentrated brine is -4°C.

[0022] According to an embodiment of the present invention, the solid mixed salt in step (9) includes sodium chloride, calcium chloride, magnesium chloride and calcium sulfate.

[0023] The present invention makes full use of the waste heat of sintering flue gas, adopts heat pump equipment for cooling and heating, and provides an energy source for freezing crystallization + thermal evaporation crystallization to treat desulfurization wastewater. The process fully considers the utilization of industrial low-temperature waste heat, energy classification utilization, and low-energy consumption process, and plays a role in desalination of desulfurization wastewater and coordinated treatment of multiple pollutants such as suspended solids and organic matter, thereby achieving the goal of low energy consumption and zero emission in desulfurization wastewater treatment.

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

[0025] (1) The present invention aims at utilizing the waste heat of industrial flue gas. It utilizes the low-temperature waste heat at the inlet of the sintering flue gas chimney, uses the waste heat flue gas as the driving heat source through a heat pump, and uses the "freezing crystallization + evaporation crystallization" process for cooling and heating desulfurization wastewater, thereby realizing the efficient utilization of the waste heat of low-temperature industrial flue gas.

[0026] (2) The present invention makes efficient use of system energy. The freezing crystallization process and thermal evaporation process adopted are fully and efficiently distributed for energy. The low-temperature ice crystals and low-temperature concentrated brine after freezing crystallization are used to pre-cool the desulfurization wastewater, which greatly reduces the external energy consumption of the system.

[0027] (3) The present invention reduces the operating energy consumption under the premise of achieving the goal of zero discharge of desulfurized wastewater. The combined process of "freezing crystallization + thermal evaporation crystallization" of desulfurized wastewater is adopted. The freezing crystallization controls the temperature below 0 degrees. The water crystallizes while the salt does not crystallize, and the brine is separated. The final product is industrial circulating water, condensed water and solid salt. The freezing crystallization desalination process adopted has a desalination rate of 70% for the ice melt water obtained. The freezing crystallization can simultaneously remove pollutants such as suspended matter, organic matter, and microorganisms in the wastewater, and the water can be directly used for industrial circulating water; the concentrated brine separated by freezing crystallization is 1 / 3 of the original wastewater volume, which is higher than the traditional membrane osmosis concentration multiple. The reduced amount of concentrated brine reduces the energy consumption of subsequent thermal evaporation. At the same time, the concentrated brine is evaporated and crystallized with the heat source provided by the heat pump. Compared with the traditional "reverse osmosis membrane + thermal evaporation" "zero discharge" process, the operating cost of this process is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a process flow chart of flue gas waste heat utilization technology for freezing crystallization treatment of desulfurization wastewater.

[0029] In the figure: 1- flue gas heat exchanger; 2- circulating water pipe; 3- ammonia-water absorption heat pump; 4- ethylene glycol aqueous solution pipe; 5- ice maker; 6- ice crystal / concentrated brine pipe; 7- ice-water separator; 8- ice crystal pipe; 9- primary precooling unit; 10- evaporator; 11- secondary precooling unit; 12-concentrated brine pipe; 13- tertiary precooling unit; 14- saturated steam pipe; 15- electric heat pump. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to the accompanying drawings.

[0031] The present invention is a technical route developed with the low-temperature waste heat utilization of flue gas in the steel industry, graded utilization of energy, and zero discharge of desulfurization wastewater as the innovative points.

[0032] A system for treating desulfurization wastewater by freezing and crystallizing flue gas waste heat, comprising a flue gas heat exchanger 1, an ammonia-water absorption heat pump 3, an electric heat pump 15, a primary precooling unit 9, a secondary precooling unit 11, a tertiary precooling unit 13, an ice maker 5, an ice-water separator 7, and an evaporator 10; the flue gas heat exchanger 1 is connected to the ammonia-water absorption heat pump 3 and the electric heat pump 15 respectively; the ammonia-water absorption heat pump 3 is connected to the ice maker 5 and the tertiary precooling unit 13 respectively; the electric heat pump 15 is connected to the evaporator 10; the primary precooling unit 9 is connected to the secondary precooling unit 11; the secondary precooling unit 11 is connected to the tertiary precooling unit 13; the tertiary precooling unit 13 is connected to the ice maker 5; the ice maker 5 is connected to the ice-water separator 7; the ice-water separator 7 is connected to the secondary precooling unit 11 and the primary precooling unit 9 respectively; the secondary precooling unit 11 is connected to the evaporator 10.

[0033] The flue gas heat exchanger 1 is connected to the ammonia-water absorption heat pump 3 and the electric heat pump 15 through the circulating water pipe 2. The ammonia-water absorption heat pump 3 is connected to the circulating water pipe 2 on the left side and to the refrigerant ethylene glycol aqueous solution pipe 4 on the right side; the upper side is connected to the heat dissipation medium air or cooling water.

[0034] The electric heat pump 15 is connected to the circulating water pipe 2 on the left side and to the saturated steam pipe 14 on the right side.

[0035] The first-level precooling unit 9 is connected to the desulfurization wastewater inlet on the right side, connected to the ice crystal tube 8 inlet on the upper side, and the ice melt water outlet after heat exchange on the lower side; the upper side of the ice crystal tube 8 is connected to the ice-water separator 7.

[0036] The secondary precooling unit 11 is connected to the primary precooling unit 9 on the right side, connected to the ice-water separator 7 through the concentrated brine pipe 12 on the upper side, and connected to the evaporator 10 on the lower side.

[0037] The right side of the three-stage precooling unit 13 is connected to the two-stage precooling unit 11, the left side is the inlet and outlet of the refrigerant ethylene glycol aqueous solution pipe 4, and the upper side is the inlet for waste water to enter the ice maker 5; the lower part of the ice maker 5 is connected to the three-stage precooling unit 13, the left side is the inlet and outlet of the refrigerant ethylene glycol aqueous solution pipe 4, and the upper side is the outlet of the ice crystal / concentrated brine pipe 6; the left side of the ice-water separator 7 is the ice crystal / concentrated brine pipe 6, the right side is separated into the ice crystal pipe 8, and the lower side is separated into the concentrated brine pipe 12; the left side of the evaporator 10 is connected to the saturated steam pipe 14, the upper side is connected to the concentrated brine after heat exchange, the lower side is the solid salt outlet, and the right side is the water vapor generated by evaporation and crystallization condensed into condensed water.

[0038] The process can take the flue gas temperature at the chimney inlet of the sintering machine after dust removal, desulfurization and denitrification is about 95℃, and the humidity is about 15%. After heat exchange in the flue gas heat exchanger 1, the flue gas temperature returning to the chimney is about 85℃. The medium of the flue gas heat exchanger 1 is circulating water, and the temperature of the circulating water increases.

[0039] Furthermore, the circulating water after heat exchange is used in two parts, one part of the circulating water is the driving heat source of the ammonia-water absorption heat pump 3 , and the other part of the circulating water is the driving heat source of the electric heat pump 15 .

[0040] Furthermore, the cold energy prepared by the ammonia-water absorption heat pump 3 can be transmitted through the refrigerant ethylene glycol aqueous solution pipe 4. The output temperature of the refrigerant ethylene glycol aqueous solution is lower than -5°C, providing a cold source for the desulfurization wastewater tertiary precooling unit 13 and the ice maker 5 for freezing and crystallization; the heat energy prepared by the electric heat pump 15 is saturated steam, providing a heat source for the evaporation and crystallization of the evaporator 10.

[0041] Furthermore, the refrigerant ethylene glycol aqueous solution pipe 4 outputs cold energy and circulates in the system; the return state of the heat source saturated steam pipe 14 is condensed water.

[0042] The temperature of the desulfurization wastewater produced by the wet desulfurization of the sintering machine is about 40°C and the salinity is about 3%.

[0043] The freezing point of saline wastewater decreases with the increase of salt concentration. In order to avoid salt crystallization and control the formation of ice crystals that are easy to separate, the freezing point of desulfurization wastewater with a salinity of 3% is controlled at -2°C~-4°C, that is, the temperature of the wastewater entering the ice maker 5 is controlled to be lower than -2°C, and the temperature of the ice crystals / concentrated brine 6 leaving the ice maker is controlled to be lower than -4°C.

[0044] Furthermore, the temperature of the desulfurized wastewater is reduced after passing through the primary precooling unit 9 , and the cold source of the primary precooling unit 9 is the ice crystals separated by the ice-water separator 7 .

[0045] Furthermore, the wastewater after the primary precooling unit enters the secondary precooling unit 11, the cold source is the concentrated brine separated by the ice-water separator 7, and the temperature of the wastewater after the secondary precooling unit 11 continues to decrease.

[0046] Furthermore, the desulfurized wastewater enters the tertiary precooling unit 13, the cold source of which is ethylene glycol aqueous solution, and the temperature of the wastewater after precooling is about -2°C.

[0047] Furthermore, the desulfurized wastewater after the tertiary precooling unit 13 enters the ice maker 5 for freezing and crystallization. The inlet temperature of the wastewater in the ice maker 5 is -2 to -3°C, and the freezing and crystallization outlet is a mixture of ice crystals and concentrated brine, and the temperature is -4°C at this time.

[0048] Further, the mixture of ice crystals at -4°C and concentrated brine is separated into ice crystals and concentrated brine 1 by an ice-water separator 7, wherein the temperature of the ice crystals is 0°C and the temperature of the concentrated brine is -4°C.

[0049] Furthermore, the concentrated brine is heated after heat exchange with the desulfurization wastewater in the secondary precooling unit 11, and is evaporated and crystallized through the evaporator 10. The heat source of the evaporator 10 is saturated steam, and the evaporation product of the concentrated brine is solid mixed salt; the solid salt includes sodium chloride, calcium chloride, magnesium chloride, calcium sulfate, etc. The water vapor generated in the evaporation process is condensed into condensed water.

[0050] Furthermore, after the desulfurization wastewater is pre-cooled by ice crystals, the ice crystals melt into water, and the desalination rate of the water reaches 70%, which can be used as industrial circulating water. The flow ratio of ice melt water to concentrated brine is higher than 2:1.

[0051] The present invention treats desulfurization wastewater at 40°C and 3% salinity. The temperature of the low-temperature waste heat flue gas provided is 95°C and the humidity is 15%. The final products are industrial circulating water, solid mixed salt, and condensed water. The process is innovative in terms of flue gas waste heat utilization, energy graded utilization, freezing crystallization, and zero discharge of high-salt wastewater. A set of process methods for treating high-salt wastewater with waste heat to achieve zero discharge has been developed. Compared with the traditional "reverse osmosis membrane + thermal evaporation" "zero discharge" process, the operating cost is greatly reduced.

[0052] The present invention is not limited to the specific details of the above solutions, and any modifications and adjustments made within the ideas and principles of the present invention are within the protection scope of the present invention.

Claims

1. A flue gas waste heat is used for freezing and crystallization to treat desulfurization wastewater system, characterized in that The invention comprises a flue gas heat exchanger (1), an ammonia-water absorption heat pump (3), an electric heat pump (15), a primary precooling unit (9), a secondary precooling unit (11), a tertiary precooling unit (13), an ice maker (5), an ice-water separator (7), and an evaporator (10); the flue gas heat exchanger (1) is connected to the ammonia-water absorption heat pump (3) and the electric heat pump (15) respectively; the ammonia-water absorption heat pump (3) is connected to the ice maker (5) and the tertiary precooling unit (13) respectively; the electric heat pump (1 5) is connected to the evaporator (10); the primary precooling unit (9) is connected to the secondary precooling unit (11); the secondary precooling unit (11) is connected to the tertiary precooling unit (13); the tertiary precooling unit (13) is connected to the ice maker (5); the ice maker (5) is connected to the ice-water separator (7); the ice-water separator (7) is respectively connected to the secondary precooling unit (11) and the primary precooling unit (9); the secondary precooling unit (11) is connected to the evaporator (10).

2. The system for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 1 is characterized in that The flue gas heat exchanger (1) is connected to an ammonia-water absorption heat pump (3) and an electric heat pump (15) via a circulating water pipe (2); the ammonia-water absorption heat pump (3) is connected to the circulating water pipe (2) on the left side and to a refrigerant ethylene glycol aqueous solution pipe (4) on the right side; and is connected to a heat dissipation medium, air or cooling water, on the upper side.

3. The system for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 2 is characterized in that The electric heat pump (15) is connected to the circulating water pipe (2) on the left side and to the saturated steam pipe (14) on the right side.

4. The system for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 1 is characterized in that The first-stage precooling unit (9) is connected to the desulfurization wastewater inlet on the right side, connected to the ice crystal tube (8) inlet on the upper side, and is an ice melt water outlet after heat exchange on the lower side; the upper side of the ice crystal tube (8) is connected to the ice-water separator (7).

5. The system for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 1 is characterized in that The secondary precooling unit (11) is connected to the primary precooling unit (9) on the right side, connected to the ice-water separator (7) via a concentrated brine pipe (12) on the upper side, and connected to the evaporator (10) on the lower side.

6. The system for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 3 is characterized in that The right side of the three-stage precooling unit (13) is connected to the two-stage precooling unit (11), the left side is the inlet and outlet of the refrigerant ethylene glycol aqueous solution pipe (4), and the upper side is the inlet of waste water entering the ice making machine (5); the lower part of the ice making machine (5) is connected to the three-stage precooling unit (13), the left side is the inlet and outlet of the refrigerant ethylene glycol aqueous solution pipe (4), and the upper side is the outlet of the ice crystal / concentrated brine pipe (6); the left side of the ice-water separator (7) is the ice crystal / concentrated brine pipe (6), the right side is separated into an ice crystal pipe (8), and the lower side is separated into a concentrated brine pipe (12); the left side of the evaporator (10) is connected to the saturated steam pipe (14), the upper side is connected to the concentrated brine after heat exchange, the lower side is the solid salt outlet, and the right side is the water vapor generated by evaporation and crystallization condensed into condensed water.

7. A method for treating desulfurization wastewater by using flue gas waste heat from the system according to any one of claims 1 to 6 for freezing and crystallization, wherein the characteristic steps are include: (1) After dust removal, desulfurization and denitrification, the flue gas at the chimney inlet is heat exchanged in the flue gas heat exchanger (1), and the temperature of the circulating water in the circulating water pipe (2) increases. The circulating water after heat exchange is used in two parts, one part of the circulating water is the driving heat source of the ammonia-water absorption heat pump (3), and the other part of the circulating water is the driving heat source of the electric heat pump (15); (2) The cold energy generated by the ammonia-water absorption heat pump (3) is transmitted through the refrigerant ethylene glycol aqueous solution pipe (4). The output temperature of the refrigerant ethylene glycol aqueous solution is lower than -5°C, which provides a cold source for the desulfurization wastewater tertiary precooling unit (13) and the ice maker (5) for freezing and crystallization. The heat energy generated by the electric heat pump (15) is saturated steam, which provides a heat source for the evaporation and crystallization of the evaporator (10). (3) The refrigerant ethylene glycol aqueous solution pipe (4) outputs cold energy and circulates in the system, providing a cold source to the ice maker (5) and the three-stage pre-cooling unit (13) through the ethylene glycol aqueous solution refrigerant; The return state of the heat source saturated steam pipe (14) is condensed water; (4) The temperature of the desulfurized wastewater is reduced after passing through the primary precooling unit (9), and the cold source of the primary precooling unit (9) is the ice crystals separated by the ice-water separator (7); (5) The wastewater after the primary precooling unit (9) enters the secondary precooling unit (11), and the cold source is the concentrated brine separated by the ice-water separator (7). The temperature of the wastewater after the secondary precooling unit (11) continues to decrease; (6) The desulfurization wastewater enters the tertiary precooling unit (13). The cold source of the tertiary precooling unit (13) is ethylene glycol aqueous solution. The temperature of the wastewater after precooling is lower than -2°C. (7) The desulfurized wastewater precooled by the tertiary precooling unit (13) enters the ice maker (5) for freezing and crystallization. The inlet temperature of the wastewater at the ice maker (5) is -2 to -3°C, and the outlet of the freezing and crystallization is a mixture of ice crystals and concentrated brine, with a temperature lower than -4°C; (8) The mixture of ice crystals and concentrated brine is separated into ice crystals and concentrated brine by an ice-water separator (7); (9) The concentrated brine is heated by the secondary precooling unit (11) and the desulfurization wastewater, and then evaporates and crystallizes through the evaporator (10). The heat source of the evaporator (10) is saturated steam, and the evaporation product of the concentrated brine is solid mixed salt. The water vapor generated during the evaporation process is condensed into condensed water; (10) After the desulfurization wastewater is precooled in the primary precooling unit (9), the ice crystals melt into water, and the desalination rate of the water is higher than 70%. It is used as industrial circulating water, and the flow ratio of ice melt water to concentrated brine is higher than 2:

1.

8. The method for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 7 is characterized in that In step (8), the temperature of the ice crystals is 0°C, and the temperature of the concentrated brine is -4°C.

9. The method for treating desulfurization wastewater by using flue gas waste heat for freezing and crystallization according to claim 7, characterized in that The solid mixed salt in step (9) includes sodium chloride, calcium chloride, magnesium chloride and calcium sulfate.

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