System for improving the whiteness of wastewater by-product salt

Abstract

The utility model provides a system for improving the whiteness of wastewater byproduct salts, the system includes a mother liquor acquisition module, a mother liquor pretreatment module and a subcritical oxidation treatment module which are arranged in sequence, the mother liquor acquisition module is connected to the subcritical oxidation treatment module; the mother liquor acquisition module performs membrane concentration and salt separation of the wastewater and evaporation and crystallization to obtain evaporation and crystallization mother liquor; the mother liquor pretreatment module performs mother liquor pretreatment on the evaporation and crystallization mother liquor to obtain a first product water; the subcritical oxidation treatment module performs subcritical oxidation treatment on the first product water and obtains a product, and sends the product to the mother liquor acquisition module. The system for improving the whiteness of wastewater byproduct salts provided by the utility model can effectively remove organic matter and chromaticity in the mother liquor by using the subcritical oxidation treatment module, with low investment cost and high treatment efficiency. Moreover, the product obtained after being treated by the subcritical oxidation treatment module can be returned to the mother liquor acquisition module to form a circulation treatment system for the evaporation and crystallization mother liquor, thereby improving the whiteness of wastewater byproduct salts.

Description

Technical Field

[0001] The utility model relates to the technical field of wastewater treatment, in particular to a system for improving the whiteness of wastewater by-product salt. Background Art

[0002] In order to achieve wastewater reuse and reduce water waste, and also because of the increasing environmental protection requirements, more and more companies are carrying out zero wastewater discharge treatment. At present, the coal chemical industry is the most implemented in China's zero wastewater discharge, but the steel industry and mining industry are also gradually implementing it.

[0003] The wastewater zero discharge process mainly consists of pretreatment, membrane concentration and salt separation, and evaporation and crystallization. This wastewater zero discharge process often produces by-product sodium chloride and by-product sodium sulfate products. In order to realize the resource utilization of by-products, by-products must meet the corresponding standards before they can be accepted by users. At present, the coal chemical industry, steel industry, and coking industry all have group standards for by-product industrial salt, and the coal chemical industry has a group standard for by-product sodium sulfate. At the same time, industrial anhydrous sodium sulfate products also have national standards GB / T 6009-2014 "Industrial Anhydrous Sodium Sulfate". The whiteness (R457) of Class I and Class II products in the industrial anhydrous sodium sulfate standard is ≥82%. The industrial dry salt and industrial wet salt in the coking wastewater by-product industrial salt and coal chemical by-product industrial salt standards have whiteness (R457) indicators. The industrial dry salt in T / CISA 227-2022 "Coking Wastewater By-product Industrial Salt" requires whiteness ≥55%. It can be seen that both industrial salt and anhydrous sodium sulfate have whiteness requirements. Moreover, downstream customers have high requirements for product whiteness. If the product is yellow, it is generally difficult to sell.

[0004] In the wastewater from the coal chemical industry and the coking industry, due to the high concentration of organic matter and chromaticity, the whiteness of the anhydrous sodium sulfate product produced after zero-discharge treatment of the wastewater is between 40% and 70%, with large fluctuations, making it difficult to stably meet the standard requirements, resulting in difficulties in product sales and low sales prices.

[0005] In addition, in the wastewater zero-discharge treatment process, the existing processes for improving salt whiteness are mainly activated carbon adsorption and ozone oxidation processes. These two processes mainly treat the organic matter and color of the evaporation crystallization process feed. The higher the salinity of the wastewater, the lower the treatment efficiency. The conventional organic matter treatment efficiency is within 50% (TDS within 100,000 mg / L), and although the color can be treated to a certain extent, due to the characteristics of continuous concentration and enrichment inside the evaporation crystallization, the color of the evaporation crystallization mother liquor will still increase, resulting in the salt whiteness failing to meet the standard.

[0006] In addition, whiteness is often directly related to the concentration of organic matter in the wastewater. A high concentration of organic matter in the wastewater in the zero-emission system not only affects whiteness, but also affects the operation of the evaporation crystallization system due to the increase in boiling point. In order to maintain the stable operation of the evaporation crystallization system and ensure the purity of the product, the mother liquor needs to be discharged continuously, and the mother liquor will produce impurities after drying. Utility Model Content

[0007] The utility model aims to provide a system for improving the whiteness of by-product salt of wastewater, so as to solve one or more problems existing in the prior art, such as the whiteness of anhydrous sodium sulfate product produced after zero-discharge treatment of wastewater with high organic matter concentration and chroma is difficult to stably meet the standard requirements, the treatment efficiency of activated carbon adsorption and ozone oxidation processes for wastewater with high organic matter concentration and chroma is low and the whiteness of by-product salt cannot meet the standard, and the high organic matter concentration in wastewater easily affects the operation of evaporation crystallization system and increases the output of impurities salt.

[0008] To achieve the above-mentioned purpose, the utility model is implemented through the following technical solutions: a system for improving the whiteness of wastewater by-product salt, the system comprising a mother liquor acquisition module, a mother liquor pretreatment module and a subcritical oxidation treatment module which are arranged in sequence, and the mother liquor acquisition module is connected to the subcritical oxidation treatment module; the mother liquor acquisition module is configured to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater to obtain evaporation and crystallization mother liquor; the mother liquor pretreatment module is configured to perform mother liquor pretreatment on the evaporation and crystallization mother liquor to obtain a first product water; the subcritical oxidation treatment module is configured to: perform subcritical oxidation treatment on the first product water to obtain a product, and transport the product to the mother liquor acquisition module.

[0009] Optionally, the mother liquor acquisition module includes a wastewater pretreatment unit, a membrane concentration and salt separation unit and an evaporation and crystallization unit arranged in sequence; the wastewater pretreatment unit includes a coagulation and sedimentation device and a resin softening device arranged in sequence; the membrane concentration and salt separation unit includes a membrane concentration device and a nanofiltration salt separation device arranged in sequence; the evaporation and crystallization unit includes an evaporation and crystallization raw material tank, an evaporation and crystallization device and an evaporation and crystallization mother liquor tank arranged in sequence, and an evaporation and crystallization condensation water tank and a tail gas treatment device connected to the evaporation and crystallization device; the evaporation and crystallization mother liquor tank and the evaporation and crystallization condensation water tank are both connected to the mother liquor pretreatment module.

[0010] Optionally, the product includes a liquid product and a gas product, the evaporation crystallization raw material tank and the tail gas treatment device are both connected to the subcritical oxidation treatment module, and the subcritical oxidation treatment module is further configured to: transport the liquid product to the evaporation crystallization raw material tank, and transport the gas product to the tail gas treatment device.

[0011] Optionally, the mother liquor pretreatment module includes a mother liquor dilution unit, a filtration unit and a produced water storage unit which are arranged in sequence; the mother liquor dilution unit is configured to dilute the evaporation and crystallization mother liquor according to a preset dilution ratio to obtain a diluted liquid; the filtration unit is configured to filter the diluted liquid to obtain the first produced water; the produced water storage unit is used to store the first produced water.

[0012] Optionally, the preset dilution ratio is the volume ratio of the evaporation crystallization mother liquor to the diluent, and the preset dilution ratio is not less than 0.5 and not more than 2.

[0013] Optionally, a stirring device is provided in the mother liquor dilution unit.

[0014] Optionally, the subcritical oxidation treatment module includes a pressurizing unit, a heating unit and an oxidation unit; the pressurizing unit is configured to pressurize the first produced water to a preset subcritical pressure to obtain a second produced water; the heating unit is configured to heat the second produced water to a preset subcritical temperature to obtain a third produced water; the oxidation unit is configured to: perform subcritical oxidation treatment on the third produced water to obtain an oxidized liquid; and perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module.

[0015] Optionally, the preset subcritical pressure is not less than 60 bar and not more than 100 bar; the preset subcritical temperature is not less than 260°C and not more than 300°C.

[0016] Optionally, the pressurizing unit includes a booster pump, a filtering device and a high-pressure pump which are arranged in sequence, and the oxidation unit includes an oxidation reactor, an air compressor and a gas-liquid separator; the booster pump is connected to the mother liquor pretreatment module, the high-pressure pump is connected to the input end of the heating unit through an output pipe, and the output end of the air compressor is connected to the output pipe; the output end of the heating unit is connected to the input end of the oxidation reactor; the oxidation reactor is configured to perform subcritical oxidation treatment on the third produced water to obtain the oxidized liquid, and transport the oxidized liquid to the gas-liquid separator; the output end of the gas-liquid separator is connected to the mother liquor acquisition module, and the gas-liquid separator is configured to perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module.

[0017] Optionally, the heating unit includes a heat exchanger and a heater, the output pipe is connected to the liquid inlet of the heat exchanger, the liquid outlet of the heat exchanger is connected to the input end of the heater, and the output end of the heater is connected to the input end of the oxidation reactor; the output end of the oxidation reactor is connected to the heat source inlet of the heat exchanger, and the heat source outlet of the heat exchanger is connected to the input end of the gas-liquid separator.

[0018] Compared with the prior art, the system for improving the whiteness of wastewater by-product salt provided by the utility model has the following advantages:

[0019] Beneficial effects:

[0020] The utility model provides a system for improving the whiteness of wastewater by-product salts, the system includes a mother liquor acquisition module, a mother liquor pretreatment module and a subcritical oxidation treatment module arranged in sequence, and the mother liquor acquisition module is connected to the subcritical oxidation treatment module; the mother liquor acquisition module is configured to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater to obtain the evaporation and crystallization mother liquor; the mother liquor pretreatment module is configured to perform mother liquor pretreatment on the evaporation and crystallization mother liquor to obtain the first produced water; the subcritical oxidation treatment module is configured to: perform subcritical oxidation treatment on the first produced water and obtain a product, and transport the product to the mother liquor acquisition module. Thus, the utility model provides a system for improving the whiteness of wastewater by-product salts, by setting a mother liquor acquisition module (including but not limited to a membrane concentration and salt separation unit and an evaporation and crystallization unit) to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater, and can obtain evaporation and crystallization mother liquor. By setting a mother liquor pretreatment module (including but not limited to a mother liquor dilution unit and a filtration unit), the evaporation and crystallization mother liquor can be diluted and impurity removal treatment can be performed, thereby removing impurities in the evaporation and crystallization mother liquor, laying a foundation for obtaining by-product salts with higher whiteness. By setting a subcritical oxidation treatment module behind the mother liquor pretreatment module to efficiently oxidize the first produced water obtained by the mother liquor pretreatment module, the organic matter and chromaticity in the first produced water can be effectively removed, and compared with the traditional oxidation treatment equipment, the subcritical oxidation treatment module has a lower investment cost and a higher treatment efficiency; at the same time, since the temperature of the evaporated crystallization mother liquor is relatively high, when it is transported from the mother liquor pretreatment module to the subcritical oxidation treatment module, the energy consumption of the subcritical oxidation treatment module can be reduced, thereby reducing the overall cost. By connecting the mother liquor acquisition module with the subcritical oxidation treatment module, the product obtained after treatment by the subcritical oxidation treatment module can be returned to the mother liquor acquisition module, forming a circulating treatment system for the evaporated crystallization mother liquor, thereby improving the whiteness of the wastewater byproduct salt; and the temperature of the product obtained after treatment by the subcritical oxidation treatment module is also relatively high, and after returning to the mother liquor acquisition module, the energy consumption of the mother liquor acquisition module can be reduced, thereby further saving costs.

[0021] Furthermore, the subcritical oxidation treatment module includes a pressurizing unit, a heating unit and an oxidation unit; the pressurizing unit is configured to pressurize the first produced water to a preset subcritical pressure to obtain a second produced water; the heating unit is configured to heat the second produced water to a preset subcritical temperature to obtain a third produced water; the oxidation unit is configured to: perform subcritical oxidation treatment on the third produced water to obtain an oxidized liquid; and perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module. Therefore, the system for improving the whiteness of wastewater by-product salt provided by the utility model pressurizes the first produced water obtained after pre-treatment of the mother liquor to a preset subcritical pressure to obtain the second produced water through the pressurizing unit, and then heats the second produced water to a preset subcritical temperature through the heating unit to obtain the third produced water, which lays a foundation for the subsequent oxidation unit to perform subcritical oxidation treatment on the third produced water. Subcritical oxidation treatment of the third produced water by the oxidation unit can effectively remove organic matter and color in the evaporation and crystallization mother liquor; gas-liquid separation treatment of the oxidized liquid obtained after subcritical oxidation treatment and conveying the obtained product to the mother liquor acquisition module lays the foundation for the recycling treatment of the evaporation and crystallization mother liquor.

[0022] Furthermore, the pressurizing unit includes a booster pump, a filtering device and a high-pressure pump which are arranged in sequence, and the oxidation unit includes an oxidation reactor, an air compressor and a gas-liquid separator; the booster pump is connected to the mother liquor pretreatment module, the high-pressure pump is connected to the input end of the heating unit through an output pipe, and the output end of the air compressor is connected to the output pipe; the output end of the heating unit is connected to the input end of the oxidation reactor; the oxidation reactor is configured to perform subcritical oxidation treatment on the third produced water to obtain the oxidized liquid, and transport the oxidized liquid to the gas-liquid separator; the output end of the gas-liquid separator is connected to the mother liquor acquisition module, and the gas-liquid separator is configured to perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module. Therefore, the system for improving the whiteness of wastewater byproduct salt provided by the utility model can provide a stable feed pressure for the high-pressure pump by initially pressurizing the first produced water through a booster pump; by setting a filtering device, impurities in the first produced water can be further removed, thereby improving the working stability of the subsequent high-pressure pump and oxidation reactor; by setting a high-pressure pump, the first produced water can be pressurized to a preset subcritical pressure. By connecting the output pipe of the high-pressure pump to the input end of the heating unit, and the output end of the air compressor to the output pipe, the second produced water obtained after being treated by the high-pressure pump can be mixed with the high-pressure air supplied by the air compressor and transported to the heating unit for heating. By connecting the output end of the heating unit to the input end of the oxidation reactor, the third produced water obtained after being heated by the heating unit can be transported to the oxidation reactor for subcritical oxidation treatment to obtain an oxidized liquid. By setting a gas-liquid separator, the oxidized liquid can be subjected to gas-liquid separation treatment to obtain a product, and the output end of the gas-liquid separator is connected to the mother liquor acquisition module, so that the product can be transported to the mother liquor acquisition module.

[0023] Furthermore, the heating unit includes a heat exchanger and a heater, the output pipe is connected to the liquid inlet of the heat exchanger, the liquid outlet of the heat exchanger is connected to the input end of the heater, and the output end of the heater is connected to the input end of the oxidation reactor; the output end of the oxidation reactor is connected to the heat source inlet of the heat exchanger, and the heat source outlet of the heat exchanger is connected to the input end of the gas-liquid separator. Therefore, the system for improving the whiteness of wastewater by-product salt provided by the utility model can improve energy utilization by setting a heat exchanger so that the second produced water can exchange heat with the oxidizing liquid. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A structural block diagram of a system for improving the whiteness of wastewater by-product salt provided in Example 1 of the utility model;

[0025] Figure 2A process flow chart of a system for improving the whiteness of wastewater by-product salt provided in Example 1 of the utility model;

[0026] Figure 3 A schematic flow chart of a method for improving the whiteness of wastewater by-product salt provided in Example 2 of the utility model;

[0027] The following are the descriptions of the reference numerals:

[0028] 1-mother liquor acquisition module, 11-wastewater pretreatment unit, 12-membrane concentration and salt separation unit, 13-evaporation and crystallization unit, 131-evaporation and crystallization raw material tank, 132-evaporation and crystallization device, 133-evaporation and crystallization mother liquor tank, 134-evaporation and crystallization condensation water tank, 135-tail gas treatment device, 2-mother liquor pretreatment module, 21-mother liquor dilution unit, 211-mother liquor dilution tank, 22-filtration unit, 221-mother liquor filter, 23-product water storage Unit, 231-filtration water production tank, 3-subcritical oxidation treatment module, 31-pressurization unit, 311-boosting pump, 312-filtration device, 313-high-pressure pump, 32-heating unit, 321-heat exchanger, 3211-liquid inlet, 3212-liquid outlet, 3213-heat source inlet, 3214-heat source outlet, 322-heater, 33-oxidation unit, 331-oxidation reactor, 332-air compressor, 333-gas-liquid separator. DETAILED DESCRIPTION

[0029] The following is a further detailed description of the system for improving the whiteness of wastewater by-product salt proposed by the utility model in combination with the accompanying drawings and specific embodiments. According to the following description, the advantages and features of the utility model will be clearer. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the embodiment of the utility model. In order to make the purpose, features and advantages of the utility model more obvious and easy to understand, please refer to the accompanying drawings. It should be noted that the structure, proportion, size, etc. illustrated by the accompanying drawings of this specification are only used to match the content disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the utility model. Any modification of the structure, change of the proportional relationship or adjustment of the size, in the case of the same or similar effect and purpose that can be achieved by the utility model, should still fall within the scope of the technical content disclosed by the utility model. The specific design features of the utility model disclosed herein include, for example, specific size, direction, position and shape, which will be determined in part by the specific environment to be applied and used. Furthermore, in the embodiments described below, the same reference numerals may be used in common among different drawings to indicate the same parts or parts having the same functions, and their repeated descriptions may be omitted.

[0030] Embodiment 1

[0031] This embodiment provides a system for improving the whiteness of wastewater by-product salt. Figure 1 and Figure 2 , Figure 1 A structural block diagram of a system for improving the whiteness of wastewater by-product salt provided in this embodiment; Figure 2 The process flow chart of the system for improving the whiteness of wastewater by-product salt provided in this embodiment. Figure 1 and Figure 2 It can be seen that the system includes a mother liquor acquisition module 1, a mother liquor pretreatment module 2 and a subcritical oxidation treatment module 3 which are arranged in sequence, and the mother liquor acquisition module 1 is connected to the subcritical oxidation treatment module 3; the mother liquor acquisition module 1 is configured to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater to obtain the evaporation and crystallization mother liquor; the mother liquor pretreatment module 2 is configured to perform mother liquor pretreatment on the evaporation and crystallization mother liquor to obtain the first product water; the subcritical oxidation treatment module 3 is configured to: perform subcritical oxidation treatment on the first product water and obtain a product, and transport the product to the mother liquor acquisition module 1. Therefore, the system for improving the whiteness of wastewater by-product salt provided in this embodiment can obtain evaporation and crystallization mother liquor by setting the mother liquor acquisition module 1 (including but not limited to the membrane concentration and salt separation unit 12 and the evaporation and crystallization unit 13) to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater. By setting up a mother liquor pretreatment module 2 (including but not limited to a mother liquor dilution unit 21 and a filtering unit 22), the evaporation crystallization mother liquor can be diluted and impurity removed, thereby removing impurities in the evaporation crystallization mother liquor, laying the foundation for obtaining a by-product salt with higher whiteness. By setting up a subcritical oxidation treatment module 3 behind the mother liquor pretreatment module 2 to efficiently oxidize the first produced water obtained by the mother liquor pretreatment module 2, the organic matter and chromaticity in the first produced water can be effectively removed, and compared with traditional oxidation treatment equipment, the subcritical oxidation treatment module 3 has a lower investment cost and a higher treatment efficiency; at the same time, due to the high temperature of the evaporation crystallization mother liquor, when it is transported from the mother liquor pretreatment module 2 to the subcritical oxidation treatment module 3, the energy consumption of the subcritical oxidation treatment module 3 can be reduced, thereby reducing the overall cost. By connecting the mother liquor acquisition module 1 with the subcritical oxidation treatment module 3, the product obtained after being treated by the subcritical oxidation treatment module 3 can be returned to the mother liquor acquisition module 1, forming a circulating treatment system for evaporation and crystallization of the mother liquor, thereby improving the whiteness of the wastewater by-product salt; moreover, the temperature of the product obtained after being treated by the subcritical oxidation treatment module 3 is also relatively high. After returning to the mother liquor acquisition module 1, the energy consumption of the mother liquor acquisition module 1 can be reduced, thereby further saving costs.

[0032] It should be noted that the utility model does not impose too many restrictions on the wastewater. For example, the wastewater can be, but is not limited to, coking wastewater. For example, the evaporation crystallization mother liquor obtained can include sodium sulfate evaporation crystallization mother liquor and sodium chloride evaporation crystallization mother liquor. The sodium sulfate evaporation crystallization mother liquor has a TDS concentration of 200,000 mg / L-500,000 mg / L, a COD concentration of 3,000 mg / L-50,000 mg / L, a chloride ion concentration of 30,000 mg / L-100,000 mg / L, and a chromaticity of 4,000-10,000; the sodium chloride evaporation crystallization mother liquor has a TDS concentration of 200,000 mg / L-500,000 mg / L, a COD concentration of 3,000 mg / L-50,000 mg / L, a chloride ion concentration of 100,000 mg / L-200,000 mg / L, and a chromaticity of 500-1,500.

[0033] Preferably, the mother liquor acquisition module 1 includes a wastewater pretreatment unit 11, a membrane concentration and salt separation unit 12 and an evaporation and crystallization unit 13 which are arranged in sequence; the wastewater pretreatment unit 11 includes a coagulation and sedimentation device (not shown in the figure) and a resin softening device (not shown in the figure) which are arranged in sequence; the membrane concentration and salt separation unit 12 includes a membrane concentration device (not shown in the figure) and a nanofiltration and salt separation device (not shown in the figure) which are arranged in sequence; the evaporation and crystallization unit 13 includes an evaporation and crystallization raw material tank 131, an evaporation and crystallization device 132 and an evaporation and crystallization mother liquor tank 133 which are arranged in sequence, and an evaporation and crystallization condensation water tank 134 and a tail gas treatment device 135 which are connected to the evaporation and crystallization device 132; the evaporation and crystallization mother liquor tank 133 and the evaporation and crystallization condensation water tank 134 are both connected to the mother liquor pretreatment module 2. Thus, by setting up the wastewater pretreatment unit 11, suspended matter and particulate matter in the wastewater can be removed; by setting up the membrane concentration and salt separation unit 12, the preliminary separation of different types of salts can be achieved, which can provide purer raw materials for subsequent evaporation and crystallization; by setting up the evaporation and crystallization unit 13, the effective separation of salt and water can be achieved, and the evaporation and crystallization mother liquor can be obtained. By connecting the evaporation and crystallization condensation water tank 134 set in the evaporation and crystallization unit 13 with the mother liquor pretreatment module 2, the condensed water in the evaporation and crystallization condensation water tank 134 can be used in the mother liquor pretreatment module 2, so that the rational use of resources can be achieved.

[0034] For example, in some exemplary embodiments, in the evaporation crystallization unit 13, the discharge amount of the evaporation crystallization mother liquor is preferably 5%-10% of the total evaporated water amount. Thus, the operation efficiency and crystallization product quality of the evaporation crystallization unit 13 can be improved and the cost of subsequent processing can be reduced.

[0035] Further, the product includes a liquid product and a gas product, and the evaporation crystallization raw material tank 131 and the tail gas treatment device 135 are both connected to the subcritical oxidation treatment module 3, and the subcritical oxidation treatment module 3 is also configured to: transport the liquid product to the evaporation crystallization raw material tank 131, and transport the gas product to the tail gas treatment device 135. Thus, by connecting the evaporation crystallization raw material tank 131 with the subcritical oxidation treatment module 3, the liquid product obtained after being treated by the subcritical oxidation treatment module 3 can be returned to the evaporation crystallization raw material tank 131 for further evaporation crystallization treatment, forming a circulating treatment system for the evaporation crystallization mother liquor, thereby improving the whiteness of the wastewater by-product salt, and further improving the stability of the operation of the evaporation crystallization unit 13, and reducing the amount of impurity salt generated. By connecting the tail gas treatment device 135 in the evaporation crystallization unit 13 with the subcritical oxidation treatment module 3, the gas product obtained after being treated by the subcritical oxidation treatment module 3 can be transported to the tail gas treatment device 135 for treatment and then discharged, which can effectively reduce environmental pollution and energy consumption.

[0036] Preferably, the mother liquor pretreatment module 2 includes a mother liquor dilution unit 21, a filtration unit 22 and a produced water storage unit 23 which are arranged in sequence; the mother liquor dilution unit 21 is configured to dilute the evaporation crystallization mother liquor according to a preset dilution ratio to obtain a diluted liquid; the filtration unit 22 is configured to filter the diluted liquid to obtain the first produced water; the produced water storage unit 23 is used to store the first produced water. Therefore, by setting the mother liquor dilution unit 21 to dilute the evaporation crystallization mother liquor, it is helpful to leave some soluble impurities in the evaporation crystallization mother liquor, and when the evaporation crystallization treatment is performed later, a higher purity product can be obtained. By setting the filtration unit 22 to remove impurities in the diluted liquid, the working efficiency of the subsequent subcritical oxidation treatment module 3 can be improved.

[0037] Preferably, the mother liquor dilution unit 21 includes a mother liquor dilution tank 211, and the evaporation crystallization mother liquor tank 133 and the evaporation crystallization condensation water tank 134 in the evaporation crystallization unit 13 are both connected to the mother liquor dilution tank 211. As a preferred embodiment, a stirring device can be provided in the mother liquor dilution tank 211. Specifically, the general process of diluting the evaporation crystallization mother liquor is as follows: the condensed water in the evaporation crystallization condensation water tank 134 is transported to the mother liquor dilution tank 211 by a pump, and the condensed water is used as a diluent to dilute the evaporation crystallization mother liquor transported to the mother liquor dilution tank 211 through the evaporation crystallization mother liquor tank 133 according to a preset dilution ratio to obtain a dilution liquid. Further, the filtering unit 22 includes a mother liquor filter 221, and the output end of the mother liquor dilution tank 211 is connected to the mother liquor filter 221. The dilution liquid is filtered by the mother liquor filter 221 to remove impurities in the dilution liquid and obtain the first produced water. The produced water storage unit 23 includes a filtered water production tank 231 . The output end of the mother liquid filter 221 is connected to the filtered water production tank 231 , and the first produced water is stored in the filtered water production tank 231 .

[0038] For example, in some exemplary embodiments, the preset dilution ratio may be a volume ratio of the evaporation crystallization mother liquor to the diluent, and the preset dilution ratio is not less than 0.5 and not more than 2.

[0039] Further, the subcritical oxidation treatment module 3 includes a pressurizing unit 31, a heating unit 32 and an oxidation unit 33; the pressurizing unit 31 is configured to pressurize the first produced water to a preset subcritical pressure to obtain a second produced water; the heating unit 32 is configured to heat the second produced water to a preset subcritical temperature to obtain a third produced water; the oxidation unit 33 is configured to: perform subcritical oxidation treatment on the third produced water to obtain an oxidized liquid; and perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module 1. Thus, the first produced water obtained after the mother liquor pretreatment is pressurized to a preset subcritical pressure by the pressurizing unit 31 to obtain the second produced water, and then the second produced water is heated to a preset subcritical temperature by the heating unit 32 to obtain the third produced water, which lays a foundation for the subsequent oxidation unit 33 to perform subcritical oxidation treatment on the third produced water. Subcritical oxidation treatment of the third produced water by the oxidation unit 33 can effectively remove organic matter and chromaticity in the evaporation crystallization mother liquor; gas-liquid separation treatment of the oxidized liquid obtained after the subcritical oxidation treatment and the obtained product are transported to the mother liquor acquisition module 1, which lays the foundation for the recycling treatment of the evaporation crystallization mother liquor.

[0040] For example, in some exemplary embodiments, the preset subcritical pressure is not less than 60 bar and not more than 100 bar; the preset subcritical temperature is not less than 260° C. and not more than 300° C. Thus, the treatment effect of the subcritical oxidation treatment module 3 can be enhanced.

[0041] Preferably, the pressurizing unit 31 includes a booster pump 311, a filtering device 312 and a high-pressure pump 313 which are arranged in sequence, and the oxidation unit 33 includes an oxidation reactor 331, an air compressor 332 and a gas-liquid separator 333; the booster pump 311 is connected to the mother liquor pretreatment module 2, the high-pressure pump 313 is connected to the input end of the heating unit 32 through an output pipe (not shown in the figure), and the output end of the air compressor 332 is connected to the output pipe; the output end of the heating unit 32 is connected to the input end of the oxidation reactor 331; the oxidation reactor 331 is configured to perform subcritical oxidation treatment on the third produced water to obtain the oxidized liquid, and transport the oxidized liquid to the gas-liquid separator 333; the output end of the gas-liquid separator 333 is connected to the mother liquor acquisition module 1, and the gas-liquid separator 333 is configured to perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module 1. Thus, the first produced water is initially pressurized by the booster pump 311, which can provide a stable feed pressure for the high-pressure pump 313; by setting the filter device 312, impurities in the first produced water can be further removed, so that the working stability of the subsequent high-pressure pump 313 and the oxidation reactor 331 can be improved; by setting the high-pressure pump 313, the first produced water can be pressurized to a preset subcritical pressure. By connecting the output pipe of the high-pressure pump 313 to the input end of the heating unit 32, and the output end of the air compressor 332 to the output pipe, the second produced water obtained after being processed by the high-pressure pump 313 can be mixed with the high-pressure air supplied by the air compressor 332 and transported to the heating unit 32 for heating. By connecting the output end of the heating unit 32 to the input end of the oxidation reactor 331, the third produced water obtained after being heated by the heating unit 32 can be transported to the oxidation reactor 331 for subcritical oxidation treatment to obtain an oxidized liquid. By setting up a gas-liquid separator 333 , the oxidizing liquid can be subjected to gas-liquid separation to obtain a product, and the output end of the gas-liquid separator 333 is connected to the mother liquid acquisition module 1 , so that the product can be transported to the mother liquid acquisition module 1 .

[0042] Specifically, the booster pump 311 is connected to the filtered water production tank 231 in the mother liquor pretreatment module 2, and the first water produced in the filtered water production tank 231 is transported to the booster pump 311 for preliminary pressurization. In addition, the gas-liquid separator 333 has a liquid inlet (not shown in the figure), a liquid outlet (not shown in the figure) and an exhaust port (not shown in the figure). The liquid inlet is used to receive the oxidized liquid obtained after being treated by the oxidation reactor 331, and the gas-liquid separator 333 performs gas-liquid separation treatment on the oxidized liquid to obtain a liquid phase product and a gas phase product. The liquid outlet is connected to the evaporation crystallization raw material tank 131 in the mother liquor acquisition module 1, and the liquid phase product is transported from the liquid outlet to the evaporation crystallization raw material tank 131; the exhaust port is connected to the tail gas treatment device 135 in the mother liquor acquisition module 1, and the gas phase product is transported from the exhaust port to the tail gas treatment device 135.

[0043] It should be noted that the present invention does not impose too many restrictions on the structure of the heating unit 32. For example, in one exemplary embodiment, the heating unit 32 may include a heat exchanger 321 and a heater 322, the output pipe is connected to the liquid inlet 3211 of the heat exchanger 321, the liquid outlet 3212 of the heat exchanger 321 is connected to the input end of the heater 322, and the output end of the heater 322 is connected to the input end of the oxidation reactor 331; the output end of the oxidation reactor 331 is connected to the heat source inlet 3213 of the heat exchanger 321, and the heat source outlet 3214 of the heat exchanger 321 is connected to the input end of the gas-liquid separator 333. Therefore, by setting the heat exchanger 321, the second produced water can exchange heat with the oxidizing liquid, thereby improving the energy utilization rate. Specifically, the first produced water is pressurized to 60 bar-100 bar by the booster pump 311 and the high-pressure pump 313 to obtain the second produced water; the second produced water is mixed with the high-pressure air supplied by the air compressor 332 in the output pipeline and then enters from the liquid inlet 3211 of the heat exchanger 321, and then is transported to the heater 322 from the liquid outlet 3212 of the heat exchanger 321, and the temperature of the second produced water is increased to 260°C-300°C by steam or heat transfer oil to obtain the third produced water; then, the third produced water is discharged from the outlet 3212 of the heater 322. The outlet is transported to the oxidation reactor 331, which is filled with a catalyst. The third produced water is subjected to subcritical oxidation treatment under a pressure condition of 60 bar-100 bar and a temperature condition of 260°C-300°C to obtain an oxidized liquid. Then, the oxidized liquid enters from the heat source inlet 3213 of the heat exchanger 321, exchanges heat with the second produced water in the heat exchanger 321, and is then transported from the heat source outlet 3214 of the heat exchanger 321 to the liquid inlet of the gas-liquid separator 333 for gas-liquid separation treatment.

[0044] In some other embodiments, the heating unit 32 may also only include a heater 322, the output pipe is connected to the input end of the heater 322, the output end of the heater 322 is connected to the input end of the oxidation reactor 331; the output end of the oxidation reactor 331 is connected to the liquid inlet of the gas-liquid separator 333.

[0045] Embodiment 2

[0046] This embodiment provides a method for improving the whiteness of wastewater by-product salt, which is used in the system for improving the whiteness of wastewater by-product salt described in any of the above embodiments. Figure 3 , Figure 3 The schematic diagram of the process for improving the whiteness of wastewater by-product salt provided in this embodiment. Figure 3 It can be seen that the method comprises:

[0047] S100: transporting the wastewater to the mother liquor acquisition module 1 for membrane concentration, salt separation and evaporation and crystallization to obtain evaporation and crystallization mother liquor;

[0048] S200: transporting the evaporation and crystallization mother liquor to the mother liquor pretreatment module 2 for mother liquor pretreatment to obtain first product water;

[0049] S300: transporting the first produced water to the subcritical oxidation treatment module 3 for subcritical oxidation treatment to obtain a product;

[0050] S400: transporting the product to the mother liquor acquisition module 1.

[0051] Therefore, the method for improving the whiteness of wastewater by-product salt provided in this embodiment is to transport the wastewater to the mother liquor acquisition module 1 for membrane concentration and salt separation and evaporation and crystallization to obtain evaporation and crystallization mother liquor, and then transport the evaporation and crystallization mother liquor to the mother liquor pretreatment module 2 for mother liquor pretreatment to obtain the first product water, and then transport the first product water to the subcritical oxidation treatment module 3 for subcritical oxidation treatment to obtain the product, and finally transport the product back to the mother liquor acquisition module 1, so as to form a circulation treatment system of the evaporation and crystallization mother liquor, thereby improving the whiteness of the wastewater by-product salt.

[0052] In order to make the present invention more easily understood, the process of treating coking wastewater by using the method for improving the whiteness of wastewater by-product salt provided in this embodiment is described by way of example.

[0053] First, the coking wastewater is transported to the mother liquor acquisition module 1 for wastewater treatment; the mother liquor acquisition module 1 includes a wastewater pretreatment unit 11, a membrane concentration and salt separation unit 12 and an evaporation and crystallization unit 13 which are arranged in sequence. The coking wastewater is first subjected to coagulation and precipitation and resin softening treatment by the wastewater pretreatment unit 11, and then subjected to membrane concentration treatment and nanofiltration and salt separation treatment by the membrane concentration and salt separation unit 12, and then subjected to evaporation and crystallization treatment by the evaporation and crystallization unit 13 to obtain sodium sulfate evaporation and crystallization mother liquor and sodium chloride evaporation and crystallization mother liquor. Among them, the COD concentration of the sodium sulfate evaporation and crystallization mother liquor is 31862 mg / L, the conductivity is 452000 μs / cm, the pH value is 9.96, and the chromaticity is 10000; the COD concentration of the sodium chloride evaporation and crystallization mother liquor is 47060 mg / L, the conductivity is 686800 μs / cm, the pH value is 10.65, and the chromaticity is 1000.

[0054] Then, the sodium sulfate evaporation crystallization mother liquor and the sodium chloride evaporation crystallization mother liquor are respectively transported to the mother liquor pretreatment module 2 for mother liquor pretreatment; the mother liquor pretreatment module 2 includes a mother liquor dilution unit 21, a filtration unit 22 and a produced water storage unit 23 which are arranged in sequence. The sodium sulfate evaporation crystallization mother liquor and the sodium chloride evaporation crystallization mother liquor are diluted by the mother liquor dilution unit 21, wherein the sodium sulfate evaporation crystallization mother liquor is diluted according to the volume ratio of the sodium sulfate evaporation crystallization mother liquor to the diluent of 1:1 to obtain a dilution of the sodium sulfate evaporation crystallization mother liquor, the COD concentration of the dilution is 14700 mg / L, the conductivity is 143100 μs / cm, the pH value is 9.82, and the chromaticity is 8000; the sodium chloride evaporation crystallization mother liquor is diluted according to the volume ratio of the sodium chloride evaporation crystallization mother liquor to the diluent of 1:2 to obtain a dilution of the sodium chloride evaporation crystallization mother liquor, the COD concentration of the dilution is 13030 mg / L, the conductivity is 148300 μs / cm, the pH value is 10.09, and the chromaticity is 500. Then, the dilution of the sodium sulfate evaporation crystallization mother liquor and the dilution of the sodium chloride evaporation crystallization mother liquor are filtered by the filtering unit 22 respectively, and stored in the produced water storage unit 23.

[0055] Next, the diluted liquid of the sodium sulfate evaporation crystallization mother liquor and the diluted liquid of the sodium chloride evaporation crystallization mother liquor in the water production storage unit 23 are respectively transported to the subcritical oxidation treatment module 3, and subcritical oxidation treatment is carried out at a temperature of 270°C to obtain the oxidized liquid of the sodium sulfate evaporation crystallization mother liquor and the oxidized liquid of the sodium chloride evaporation crystallization mother liquor, respectively, wherein the COD concentration of the oxidized liquid of the sodium sulfate evaporation crystallization mother liquor is 1667 mg / L, the electrical conductivity is 180400 μs / cm, the pH value is 7.91, and the chromaticity is 100; the COD concentration of the oxidized liquid of the sodium chloride evaporation crystallization mother liquor is 8485 mg / L, the electrical conductivity is 168400 μs / cm, the pH value is 7.72, and the chromaticity is 100.

[0056] It can be seen that the COD removal rates of the sodium sulfate evaporation crystallization mother liquor and the sodium chloride evaporation crystallization mother liquor after treatment by the subcritical oxidation treatment module 3 are 88.66% and 34.88% respectively, the chroma can be within 100 times, the chroma removal rate can reach up to 98%, and the salt whiteness of the treated sodium sulfate evaporation crystallization mother liquor and the sodium chloride evaporation crystallization mother liquor can reach 82%.

[0057] In summary, the system for improving the whiteness of by-product salts in wastewater provided by the utility model has the following advantages: the system for improving the whiteness of by-product salts in wastewater provided by the utility model comprises a mother liquor acquisition module, a mother liquor pretreatment module and a subcritical oxidation treatment module which are arranged in sequence, and the mother liquor acquisition module is connected to the subcritical oxidation treatment module; the mother liquor acquisition module is configured to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater to obtain the evaporation and crystallization mother liquor; the mother liquor pretreatment module is configured to perform mother liquor pretreatment on the evaporation and crystallization mother liquor to obtain the first produced water; the subcritical oxidation treatment module is configured to perform subcritical oxidation treatment on the first produced water and obtain a product, and to transport the product to the mother liquor acquisition module. Therefore, the system for improving the whiteness of by-product salts in wastewater provided by the utility model can obtain evaporation and crystallization mother liquor by setting a mother liquor acquisition module (including but not limited to a membrane concentration and salt separation unit and an evaporation and crystallization unit) to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater. By setting up a mother liquor pretreatment module (including but not limited to a mother liquor dilution unit and a filtration unit), the evaporation crystallization mother liquor can be diluted and impurity removed, thereby removing impurities in the evaporation crystallization mother liquor, laying the foundation for obtaining by-product salts with higher whiteness. By setting up a subcritical oxidation treatment module behind the mother liquor pretreatment module to efficiently oxidize the first produced water obtained by the mother liquor pretreatment module, the organic matter and chromaticity in the first produced water can be effectively removed, and compared with traditional oxidation treatment equipment, the subcritical oxidation treatment module has a lower investment cost and higher treatment efficiency; at the same time, due to the high temperature of the evaporation crystallization mother liquor, when it is transported from the mother liquor pretreatment module to the subcritical oxidation treatment module, the energy consumption of the subcritical oxidation treatment module can be reduced, thereby reducing the overall cost. By connecting the mother liquor acquisition module with the subcritical oxidation treatment module, the product obtained after treatment in the subcritical oxidation treatment module can be returned to the mother liquor acquisition module, forming a circulating treatment system for evaporation and crystallization of the mother liquor, thereby improving the whiteness of the wastewater by-product salt; moreover, the temperature of the product obtained after treatment in the subcritical oxidation treatment module is also relatively high. After returning to the mother liquor acquisition module, the energy consumption of the mother liquor acquisition module can be reduced, thereby further saving costs.

[0058] Furthermore, the subcritical oxidation treatment module includes a pressurizing unit, a heating unit and an oxidation unit; the pressurizing unit is configured to pressurize the first produced water to a preset subcritical pressure to obtain a second produced water; the heating unit is configured to heat the second produced water to a preset subcritical temperature to obtain a third produced water; the oxidation unit is configured to: perform subcritical oxidation treatment on the third produced water to obtain an oxidized liquid; and perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module. Therefore, the system for improving the whiteness of wastewater by-product salt provided by the utility model pressurizes the first produced water obtained after pre-treatment of the mother liquor to a preset subcritical pressure to obtain the second produced water through the pressurizing unit, and then heats the second produced water to a preset subcritical temperature through the heating unit to obtain the third produced water, which lays a foundation for the subsequent oxidation unit to perform subcritical oxidation treatment on the third produced water. Subcritical oxidation treatment of the third produced water by the oxidation unit can effectively remove organic matter and color in the evaporation and crystallization mother liquor; gas-liquid separation treatment of the oxidized liquid obtained after subcritical oxidation treatment and conveying the obtained product to the mother liquor acquisition module lays the foundation for the recycling treatment of the evaporation and crystallization mother liquor.

[0059] Furthermore, the pressurizing unit includes a booster pump, a filtering device and a high-pressure pump which are arranged in sequence, and the oxidation unit includes an oxidation reactor, an air compressor and a gas-liquid separator; the booster pump is connected to the mother liquor pretreatment module, the high-pressure pump is connected to the input end of the heating unit through an output pipe, and the output end of the air compressor is connected to the output pipe; the output end of the heating unit is connected to the input end of the oxidation reactor; the oxidation reactor is configured to perform subcritical oxidation treatment on the third produced water to obtain the oxidized liquid, and transport the oxidized liquid to the gas-liquid separator; the output end of the gas-liquid separator is connected to the mother liquor acquisition module, and the gas-liquid separator is configured to perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module. Therefore, the system for improving the whiteness of wastewater byproduct salt provided by the utility model can provide a stable feed pressure for the high-pressure pump by initially pressurizing the first produced water through a booster pump; by setting a filtering device, impurities in the first produced water can be further removed, thereby improving the working stability of the subsequent high-pressure pump and oxidation reactor; by setting a high-pressure pump, the first produced water can be pressurized to a preset subcritical pressure. By connecting the output pipe of the high-pressure pump to the input end of the heating unit, and the output end of the air compressor to the output pipe, the second produced water obtained after being treated by the high-pressure pump can be mixed with the high-pressure air supplied by the air compressor and transported to the heating unit for heating. By connecting the output end of the heating unit to the input end of the oxidation reactor, the third produced water obtained after being heated by the heating unit can be transported to the oxidation reactor for subcritical oxidation treatment to obtain an oxidized liquid. By setting a gas-liquid separator, the oxidized liquid can be subjected to gas-liquid separation treatment to obtain a product, and the output end of the gas-liquid separator is connected to the mother liquor acquisition module, so that the product can be transported to the mother liquor acquisition module.

[0060] Furthermore, the heating unit includes a heat exchanger and a heater, the output pipe is connected to the liquid inlet of the heat exchanger, the liquid outlet of the heat exchanger is connected to the input end of the heater, and the output end of the heater is connected to the input end of the oxidation reactor; the output end of the oxidation reactor is connected to the heat source inlet of the heat exchanger, and the heat source outlet of the heat exchanger is connected to the input end of the gas-liquid separator. Therefore, the system for improving the whiteness of wastewater by-product salt provided by the utility model can improve energy utilization by setting a heat exchanger so that the second produced water can exchange heat with the oxidizing liquid.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the utility model rather than to limit it. Although the utility model has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the utility model can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the utility model.

Claims

1. A system for improving the whiteness of wastewater by-product salt, characterized in that: The system comprises a mother liquor acquisition module, a mother liquor pretreatment module and a subcritical oxidation treatment module which are arranged in sequence, and the mother liquor acquisition module is connected to the subcritical oxidation treatment module; The mother liquor acquisition module is configured to perform membrane concentration and salt separation and evaporation and crystallization treatment on the wastewater to obtain evaporation and crystallization mother liquor; The mother liquor pretreatment module is configured to perform mother liquor pretreatment on the evaporation and crystallization mother liquor to obtain first product water; The subcritical oxidation treatment module is configured to: perform subcritical oxidation treatment on the first produced water to obtain a product, and transport the product to the mother liquor acquisition module.

2. The system for improving the whiteness of wastewater by-product salt according to claim 1, characterized in that: The mother liquor acquisition module includes a wastewater pretreatment unit, a membrane concentration and salt separation unit and an evaporation and crystallization unit which are arranged in sequence; the wastewater pretreatment unit includes a coagulation and sedimentation device and a resin softening device which are arranged in sequence; the membrane concentration and salt separation unit includes a membrane concentration device and a nanofiltration and salt separation device which are arranged in sequence; the evaporation and crystallization unit includes an evaporation and crystallization raw material tank, an evaporation and crystallization device and an evaporation and crystallization mother liquor tank which are arranged in sequence, and an evaporation and crystallization condensation water tank and a tail gas treatment device which are connected to the evaporation and crystallization device; the evaporation and crystallization mother liquor tank and the evaporation and crystallization condensation water tank are both connected to the mother liquor pretreatment module.

3. The system for improving the whiteness of wastewater by-product salt according to claim 2, characterized in that: The products include liquid products and gas products. The evaporation and crystallization raw material tank and the tail gas treatment device are both connected to the subcritical oxidation treatment module. The subcritical oxidation treatment module is also configured to: transport the liquid products to the evaporation and crystallization raw material tank, and transport the gas products to the tail gas treatment device.

4. The system for improving the whiteness of wastewater by-product salt according to claim 1, characterized in that: The mother liquor pretreatment module comprises a mother liquor dilution unit, a filtering unit and a produced water storage unit which are arranged in sequence; the mother liquor dilution unit is configured to dilute the evaporation crystallization mother liquor according to a preset dilution ratio to obtain a diluted liquid; the filtering unit is configured to filter the diluted liquid to obtain the first produced water; The produced water storage unit is used to store the first produced water.

5. The system for improving the whiteness of wastewater by-product salt according to claim 4, characterized in that: The preset dilution ratio is the volume ratio of the evaporation crystallization mother liquor to the diluent, and the preset dilution ratio is not less than 0.5 and not more than 2.

6. The system for improving the whiteness of wastewater by-product salt according to claim 4, characterized in that: The mother liquor dilution unit is provided with a stirring device.

7. The system for improving the whiteness of wastewater by-product salt according to claim 1, characterized in that: The subcritical oxidation treatment module includes a pressurizing unit, a heating unit and an oxidation unit; The pressurizing unit is configured to pressurize the first produced water to a preset subcritical pressure to obtain second produced water; the heating unit is configured to heat the second produced water to a preset subcritical temperature to obtain third produced water; The oxidation unit is configured to: perform subcritical oxidation treatment on the third produced water to obtain an oxidized liquid; and perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module.

8. The system for improving the whiteness of wastewater by-product salt according to claim 7, characterized in that: The preset subcritical pressure is not less than 60 bar and not more than 100 bar; the preset subcritical temperature is not less than 260°C and not more than 300°C.

9. The system for improving the whiteness of wastewater by-product salt according to claim 7, characterized in that: The pressurizing unit includes a booster pump, a filtering device and a high-pressure pump arranged in sequence, and the oxidation unit includes an oxidation reactor, an air compressor and a gas-liquid separator; The boost pump is connected to the mother liquor pretreatment module, the high-pressure pump is connected to the input end of the heating unit through an output pipe, and the output end of the air compressor is connected to the output pipe; the output end of the heating unit is connected to the input end of the oxidation reactor; the oxidation reactor is configured to perform subcritical oxidation treatment on the third produced water to obtain the oxidized liquid, and transport the oxidized liquid to the gas-liquid separator; the output end of the gas-liquid separator is connected to the mother liquor acquisition module, and the gas-liquid separator is configured to perform gas-liquid separation treatment on the oxidized liquid to obtain the product, and transport the product to the mother liquor acquisition module.

10. The system for improving the whiteness of wastewater by-product salt according to claim 9, characterized in that: The heating unit includes a heat exchanger and a heater, the output pipe is connected to the liquid inlet of the heat exchanger, the liquid outlet of the heat exchanger is connected to the input end of the heater, and the output end of the heater is connected to the input end of the oxidation reactor; the output end of the oxidation reactor is connected to the heat source inlet of the heat exchanger, and the heat source outlet of the heat exchanger is connected to the input end of the gas-liquid separator.

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