Wastewater treatment method

Abstract

This invention provides a wastewater treatment method, comprising mixing a sodium sulfate solution and a first crystal discharged from a crystallizer in a reactor, using the sodium sulfate solution to transform the first crystal into a second crystal, and washing water; wherein the first crystal contains anhydrous sodium sulfate and colored impurities, and the second crystal contains sodium sulfate decahydrate. Using the technical solution of this invention to treat coking wastewater or other recalcitrant high-salt wastewater can improve the purity, especially the whiteness, of sodium sulfate products at a lower disposal cost, enabling them to meet the industrial standards for anhydrous sodium sulfate.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a wastewater treatment method. Background Technology

[0002] In recent years, with the rapid development of my country's economy, the amount of wastewater generated by industries such as power, petrochemicals, and coal chemicals has been increasing both domestically and internationally. Zero wastewater discharge is the ultimate goal required by the industrial wastewater sector and a crucial link in achieving sustainable and stable development. Nanofiltration desalination technology is commonly used in the development of zero-discharge technologies. During nanofiltration desalination, sodium sulfate and organic matter are generally enriched on the concentrate side. Although advanced oxidation processes remove organic matter upstream, they cannot completely remove organic matter from coking wastewater and other recalcitrant organic wastewater. This results in a large amount of organic matter being enriched on the concentrate side during nanofiltration membrane concentration and reduction, which then affects the quality of sodium sulfate during evaporation and crystallization, hindering its resource utilization. Therefore, the proper treatment and utilization of sodium sulfate is a critical part of the zero-discharge wastewater process.

[0003] In addition, anhydrous sodium sulfate produced by thermal salt-nitrate co-production process, thermal nitrate removal process or the above-mentioned nanofiltration process usually contains organic matter and non-ferrous metal complexes, especially metal complexes of Fe ions, which causes the whiteness of the obtained anhydrous sodium sulfate to fail to meet the standards for use of industrial sodium sulfate. Summary of the Invention

[0004] To address the aforementioned technical problems in the prior art, the present invention provides a wastewater treatment method, comprising: mixing a sodium sulfate solution and a first crystal discharged from a crystallizer in a reactor; converting the first crystal with the sodium sulfate solution to obtain a second crystal and wash water; wherein the first crystal contains anhydrous sodium sulfate and colored impurities, and the second crystal contains sodium sulfate decahydrate.

[0005] Existing technologies typically use freezing processes to remove colored impurities such as organic matter and metal complexes from sodium sulfate solutions, but freezing processes are costly. This application achieves purification results comparable to freezing processes through crystal transformation, which can significantly improve economic efficiency for large-scale industrial production.

[0006] In the method of this invention, forming a second crystallizer whose main component is sodium sulfate decahydrate is essential. The process of forming sodium sulfate decahydrate removes colored impurities such as organic matter and non-ferrous metal complexes from anhydrous sodium sulfate obtained by thermal salt-nitrate co-production processes, thermal nitrate removal processes, or nanofiltration processes. These colored impurities typically cause the whiteness of the first crystallizer, measured according to GB / T5950-2008 "Methods for Measuring Whiteness of Building Materials and Non-metallic Mineral Products," to be below 80. Sodium sulfate decahydrate is a crystal form that does not easily carry impurities. During the process of forming sodium sulfate decahydrate through low-temperature crystal transformation, these colored impurities remain in the wash water, thereby achieving the goal of ensuring that the whiteness of the obtained sodium sulfate product meets the relevant usage standards.

[0007] Unless otherwise specified, the whiteness mentioned in this invention is measured in accordance with GB / T 5950-2008 "Methods for measuring the whiteness of building materials and non-metallic mineral products".

[0008] Preferably, the whiteness of the first crystal is 70 or higher; the temperature of the sodium sulfate solution is 0–25°C, preferably 5–25°C. The crystal transformation process of this invention is particularly suitable for first crystals with a whiteness of 70 or higher. When the whiteness of the first crystal is lower than 70, it is usually necessary to perform more than three crystal transformation processes to obtain a sodium sulfate product with a whiteness that meets the relevant usage standards. In this case, the existing freezing process is more economical.

[0009] The method of the present invention, when carried out under the aforementioned limited low-temperature conditions, yields a second crystal mainly containing sodium sulfate decahydrate. Using a sodium sulfate solution at temperatures above 25°C for crystal transformation results in colored impurities, affecting the purity of the second crystal. This typically requires more than three crystal transformation operations, offering no significant cost advantage compared to existing freezing processes, and may even prevent the formation of sodium sulfate decahydrate.

[0010] Preferably, the sodium sulfate solution is a saturated sodium sulfate solution; more preferably, the sodium sulfate solution and the first crystal are mixed at a mass ratio of 1:(0.5 to 2); in a preferred embodiment of the present invention, the sodium sulfate solution and the first crystal are typically mixed at a mass ratio of about 1:1.

[0011] In this invention, the sodium sulfate solution used for crystal transformation is a saturated sodium sulfate solution, and it is preferably mixed with the first crystal at a mass ratio of 1:(0.5-2) to convert the anhydrous sodium sulfate in the wastewater into sodium sulfate decahydrate, that is, to separate the colored metal complexes or organic matter in the wastewater from the sodium sulfate and form colorless sodium sulfate decahydrate crystals.

[0012] Preferably, the temperature of the first crystallizer discharged from the crystallizer is 50-60°C. The first crystallizer is used directly in subsequent processes after exiting the crystallizer.

[0013] Preferably, the above method further includes evaporating and crystallizing the second crystal and drying it to obtain a crystalline salt product.

[0014] The purpose of evaporation crystallization is to remove the water of crystallization, and the purpose of drying is to ensure that the obtained crystalline salt product meets the moisture content requirements for industrial anhydrous sodium sulfate as specified in GB / T6009-2014. Preferably, the moisture content of the crystalline salt product is below 1.5%, more preferably below 0.5%, and most preferably below 0.1%.

[0015] Preferably, the whiteness of the crystalline salt product at least meets the Class II index of industrial anhydrous sodium sulfate as specified in GB / T 6009-2014, and more preferably meets the Class I index.

[0016] Of course, if a single crystal transformation is insufficient to achieve the desired whiteness of the crystalline salt product, the crystal transformation can be performed multiple times until the whiteness of the crystalline salt product at least meets the Class II standard for industrial anhydrous sodium sulfate as specified in GB / T 6009-2014, preferably the Class I standard. Preferably, the multiple transformations are performed twice. When using the method of this invention to prepare sodium sulfate products via crystal transformation, the number of crystal transformation processes is typically limited to two.

[0017] Preferably, the wash water is returned to the crystallizer to produce the first crystal.

[0018] Performing more than three crystal transformations on wastewater generates excessive wash water, increasing the operating pressure of the wastewater treatment system and reducing the overall economic efficiency of the wastewater treatment process. Furthermore, in this invention, a saturated sodium sulfate solution must be used for crystal transformation; otherwise, the sodium sulfate concentration in the wash water will be too low, increasing the operating pressure of the crystallizer when it is returned to it.

[0019] Preferably, the above method further includes pretreatment of the wastewater, the pretreatment including nanofiltration of the wastewater to obtain nanofiltration concentrate and nanofiltration permeate containing sodium sulfate and organic impurities, and introducing the nanofiltration concentrate into the crystallizer to obtain a first crystal containing anhydrous sodium sulfate.

[0020] Preferably, the above method further includes treating the wastewater with a salt-nitrate co-production process to obtain products containing sodium sulfate and sodium chloride, respectively, wherein the product containing sodium sulfate is dissolved and then enters the crystallizer.

[0021] The method of this invention is suitable for treating coking wastewater and other recalcitrant high-salt wastewater to obtain sodium sulfate product. Using the technical solution of this invention, the whiteness of sodium sulfate product can be improved with lower treatment costs, enabling it to meet the Class II, preferably Class I, indicators of industrial anhydrous sodium sulfate as specified in GB / T 6009-2014. Attached Figure Description

[0022] Figure 1 A flowchart of the wastewater treatment method of the present invention is shown.

[0023] Figure label:

[0024] 1-Crystallizer; 2-Reactor; 3-Evaporation crystallization device; 4-Drying device. Detailed Implementation

[0025] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0026] It should be noted that in this specification, similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0027] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0028] In the description of this embodiment, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set up," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this embodiment based on the specific circumstances.

[0029] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0030] Example 1

[0031] According to one embodiment of the present invention, coking wastewater is subjected to advanced oxidation and nanofiltration to obtain nanofiltration concentrate and nanofiltration permeate containing sodium sulfate and organic impurities. The nanofiltration concentrate is introduced into crystallizer 1 and evaporated therein to obtain a first crystal. The first crystal is mainly composed of anhydrous sodium sulfate and has a distinct color. Its whiteness is measured to be 78 according to GB / T 5950-2008 "Methods for Measuring Whiteness of Building Materials and Non-metallic Mineral Products". The advanced oxidation and nanofiltration are carried out according to methods known in the prior art. The purpose of advanced oxidation is to initially remove organic matter from the wastewater, and the purpose of nanofiltration is to enrich sodium sulfate in the wastewater.

[0032] Approximately 20 kg of the first crystallized product is discharged from crystallizer 1 and fed into reactor 2. The temperature of the first crystallized product discharged from crystallizer 1 is approximately 50°C. Then, a saturated sodium sulfate solution at approximately 10°C is added to reactor 2, with a mass ratio of saturated sodium sulfate solution to the first crystallized product of 1:1. The mixture is stirred continuously for 30 minutes to ensure a complete crystallization reaction. After the crystallization reaction is complete, a second crystallized product and wash water are obtained. The whiteness of the second crystallized product is 84. The wash water is returned to crystallizer 1 for continued production of the first crystallized product.

[0033] Subsequently, the second crystallizer enters the evaporation crystallization device 3 to remove the water of crystallization from the second crystallizer, and then enters the drying device 4 for further dehydration and drying to obtain a crystalline salt product. The whiteness of the crystalline salt product is 89, and its comprehensive properties meet the Class I index of industrial anhydrous sodium sulfate specified in GB / T 6009-2014.

[0034] Example 2

[0035] According to another embodiment of the present invention, coking wastewater is subjected to salt-nitrate co-production treatment to obtain products mainly composed of anhydrous sodium sulfate and sodium chloride. The product containing sodium sulfate is dissolved and introduced into crystallizer 1, where it is evaporated to obtain a first crystal. The first crystal is mainly composed of anhydrous sodium sulfate, has low visual whiteness, and a distinct color. Its whiteness is measured to be 71 according to GB / T5950-2008 "Methods for Measurement of Whiteness of Building Materials and Non-metallic Mineral Products". The salt-nitrate co-production is carried out according to methods known in the prior art, utilizing the different solubility characteristics of sodium sulfate and sodium chloride in solution to separate sodium sulfate and sodium chloride in the wastewater.

[0036] Approximately 30 kg of the first crystallized material was discharged from crystallizer 1 and fed into reactor 2. The temperature of the first crystallized material discharged from crystallizer 1 was approximately 55°C. Then, a saturated sodium sulfate solution at 5°C was added to reactor 2, with a mass ratio of saturated sodium sulfate solution to the first crystallized material of 1:1.2. The mixture was stirred continuously for 40 minutes to ensure a complete crystallization reaction. After the crystallization reaction, a second crystallized material and wash water were obtained. The wash water was returned to crystallizer 1 for continued production of the first crystallized material. The whiteness of the second crystallized material was measured according to GB / T 5950-2008 "Methods for Measuring Whiteness of Building Materials and Non-metallic Mineral Products," and the whiteness was found to be 80. Subsequently, the second crystallized material was mixed with a saturated sodium sulfate solution at 5°C at a mass ratio of 1:1.2, and stirred in reactor 2 for 30 minutes to undergo a second crystallization reaction, obtaining a second crystallized material' and wash water. The whiteness of the second crystallized material was 87. The wash water was returned to crystallizer 1 for continued production of the first crystallized material.

[0037] Subsequently, the second crystallizer ' enters the evaporation crystallization device 3 to remove the water of crystallization from the second crystallizer ', and then enters the drying device 4 for further dehydration and drying to obtain a crystalline salt product. The whiteness of the crystalline salt product is 92, and its comprehensive properties meet the Class I index of industrial anhydrous sodium sulfate specified in GB / T 6009-2014.

[0038] The method of this invention treats coking wastewater or other recalcitrant high-salt wastewater, enabling the thorough purification and extraction of sodium sulfate from the wastewater through low-temperature solution crystallization. In particular, the crystallization process enhances the whiteness of the crystalline salt product, ensuring that the final anhydrous sodium sulfate product meets or exceeds the Class II standard for industrial anhydrous sodium sulfate as specified in GB / T 6009-2014. This method can be used as a thermal process for sodium sulfate in projects where the whiteness cannot meet requirements due to the influence of organic matter and other chromogenic groups. It achieves full utilization of resources and offers significant economic benefits.

[0039] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A wastewater treatment method, characterized in that, The method includes: mixing a saturated sodium sulfate solution and a first crystal discharged from a crystallizer in a reactor; converting the first crystal with the saturated sodium sulfate solution to obtain a second crystal; and washing water. The temperature of the saturated sodium sulfate solution is 0-10°C, and the temperature of the first crystal is 50-60°C. The saturated sodium sulfate solution and the first crystal are mixed at a mass ratio of 1:(0.5-2). The first crystal contains anhydrous sodium sulfate and colored impurities, and the second crystal contains sodium sulfate decahydrate. The colored impurities contain organic matter and non-ferrous metal complexes.

2. The method as described in claim 1, characterized in that, The whiteness of the first crystal is above 70; and / or the temperature of the saturated sodium sulfate solution is 5-10°C.

3. The method as described in claim 1 or 2, characterized in that, The method further includes evaporating and crystallizing the second crystal and drying it to obtain a crystalline salt product.

4. The method as described in claim 3, characterized in that, The whiteness of the crystalline salt product must at least meet the Class II index of industrial anhydrous sodium sulfate as specified in GB / T 6009-2014.

5. The method as described in claim 4, characterized in that, The transformation process is repeated multiple times until the whiteness of the crystalline salt product meets at least the Class II index of industrial anhydrous sodium sulfate specified in GB / T 6009-2014.

6. The method as described in claim 1 or 2, characterized in that, The wash water is returned to the crystallizer to produce the first crystal.

7. The method as described in claim 1 or 2, characterized in that, The method further includes pretreatment of the wastewater, the pretreatment comprising nanofiltration of the wastewater to obtain nanofiltration concentrate and nanofiltration permeate containing sodium sulfate and organic impurities, and introducing the nanofiltration concentrate into the crystallizer to obtain a first crystal containing anhydrous sodium sulfate.

8. The method as described in claim 1 or 2, characterized in that, The method further includes treating the wastewater with a salt-nitrate co-production process to obtain products containing sodium sulfate and sodium chloride, respectively. The product containing sodium sulfate is dissolved and then fed into the crystallizer.

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