A method for treating a sulfur-containing spent caustic

Abstract

The application provides a treatment method of sulfur-containing waste lye, which comprises the following steps: carrying out oil removal treatment on the waste lye, and then carrying out first-stage wet oxidation treatment; controlling reaction conditions to make 10%-90% of S 2‑ in the waste lye be converted into S2O3 2‑ ; adding a co-oxidation promoter into effluent water to carry out catalytic wet oxidation treatment, so as to realize synchronous oxidation of organic matters and S 2‑ , and the co-oxidation promoter is at least one selected from peroxide, persulfate, percarbonate and peroxymonosulfate. The application adopts two-stage wet oxidation, and the temperature of each stage is relatively low. By controlling the conditions of the first-stage wet oxidation, 10%-90% of S 2‑ in the waste lye is converted into S2O3 2‑ , and by adding a co-oxidation promoter into the second-stage wet oxidation, S2O3 2‑ reacts to generate free radicals which can accelerate the wet oxidation, so as to accelerate the degradation of the organic matters, and the purpose of treating waste with waste is realized. The reaction conditions of the two-stage wet oxidation are relatively mild, the energy consumption is reduced, and the COD removal efficiency is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, and specifically relates to a method for treating sulfur-containing alkaline wastewater. Background Technology

[0002] Currently, alkaline washing is commonly used to remove acidic gases such as CO2 and H2S from pyrolysis gas. This process generates a large amount of waste alkaline solution. On one hand, this waste alkaline solution contains not only residual NaOH but also inorganic salts such as Na2S and Na2CO3 generated during the washing process. On the other hand, the condensation of heavy components in the pyrolysis gas and the polymerization of dienes and aldehydes during alkaline washing introduce a large amount of organic matter into the waste alkaline solution. This type of waste alkaline solution is characterized by large volume and high pollutant concentration; therefore, its treatment effectiveness directly impacts the pass rate of wastewater treatment in ethylene chemical plants.

[0003] The significant advantages of wet oxidation technology are its wide applicability, effective treatment of various high-concentration organic wastewaters, high treatment efficiency, good COD removal, fast oxidation rate, short reaction residence time, and minimal secondary pollution. It also utilizes the system's reaction heat, requiring less energy and allowing for the full recovery and reuse of materials and energy. Currently, sulfur-containing ethylene, propylene, and refinery waste alkaline solutions typically achieve good treatment results using wet oxidation technology.

[0004] Patent CN103771457A provides a method for treating waste alkaline solution in propylene production. This method involves controlling the wet oxidation temperature to 150-230℃, the reaction pressure to 1.5-4.0 MPa, and the time to 45-90 minutes. This method can efficiently remove sulfur ions from the waste alkaline solution while simultaneously recovering sodium sulfate and reusing the sodium hydroxide after treatment, achieving zero emissions in the propylene production process. Although primarily targeting the removal of divalent sulfur ions, the COD of the effluent remains high, resulting in poor purity of the recovered sodium sulfate and the effluent being unsuitable for direct biochemical treatment.

[0005] Patent CN106746100A provides a method for treating ethylene refining waste alkaline solution, which removes organic matter, namely divalent sulfur ions, at 240~320℃, 6.0~20MPa, and 40~90min. The process includes steps such as air flotation for oil removal, high-temperature wet oxidation treatment, sulfuric acid treatment, activated carbon adsorption, adjustment of alkali concentration, and evaporation concentration. Although this invention can efficiently remove COD and sulfides from ethylene refining waste alkaline solution and recover sodium sulfate, the wet oxidation treatment requires a temperature above 240℃ and a pressure above 6MPa, which results in high energy consumption and strict requirements on the materials of the equipment.

[0006] Although the above-mentioned existing technologies can completely remove divalent sulfur from waste alkaline solutions, the COD of the effluent is still high. Increasing the temperature would lead to high energy consumption and higher requirements for the materials of the equipment. Therefore, it is necessary to seek a low-energy and green waste alkaline solution treatment technology. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for treating sulfur-containing waste alkaline solution. Through two-stage wet oxidation, both at relatively low temperatures, organic matter and sulfides can be removed simultaneously. This method shows promising application prospects in desulfurization and deodorization of waste alkaline solution and in improving the biodegradability of effluent.

[0008] The technical objective of this invention is achieved through the following technical solution:

[0009] A method for treating sulfur-containing waste alkaline solution includes the following:

[0010] (1) Remove oil from the waste alkaline solution;

[0011] (2) The waste alkaline solution after oil removal is subjected to primary wet oxidation treatment, and the reaction conditions are controlled so that the sulfur content in the waste alkaline solution is 10%-90%. 2- Transformed into S2O3 2- ;

[0012] (3) Add a co-oxidation promoter to the effluent from (2) to carry out catalytic wet oxidation treatment, thereby achieving the reaction of organic matter and sulfur. 2- The co-oxidation promoter is selected from at least one of peroxide, persulfate (PDS), percarbonate and perhydrosulfate for synchronous oxidation.

[0013] Furthermore, the control of reaction conditions mentioned in (2) includes controlling the reaction temperature, specifically making the temperature of the first-stage wet oxidation reaction below 200°C, preferably 100-199°C, more preferably 120-180°C, and most preferably 150-180°C, such as 155°C, 160°C, 165°C, 170°C and 175°C.

[0014] Furthermore, the control of reaction conditions described in (2) also includes the control of reaction time. The reaction time for primary wet oxidation is 60-180 min, preferably 80-120 min.

[0015] Furthermore, the control of reaction conditions described in (2) also includes the control of reaction pressure. The reaction pressure of the first-stage wet oxidation is 1.2-3.5 MPa, preferably 1.4-2.9 MPa.

[0016] Furthermore, in (2), the reaction conditions are preferably controlled so that the waste alkaline solution contains 30%-90%, more preferably 60%-90% S. 2- Transformed into S2O32- S 2- Transformed into S2O3 2- The determination was performed by indirect iodometric titration.

[0017] Furthermore, the co-oxidation accelerator is selected from at least one of hydrogen peroxide, sodium persulfate, ammonium persulfate, potassium persulfate, sodium percarbonate, potassium percarbonate, ammonium percarbonate, sodium peroxymonosulfate, ammonium peroxymonosulfate, and potassium peroxymonosulfate. It should be understood by those skilled in the art that sodium peroxymonosulfate, ammonium peroxymonosulfate, and potassium peroxymonosulfate are often added in the form of a compound salt (PMS), which also includes the bisulfate and sulfate of each cation.

[0018] Furthermore, the co-oxidation accelerator is configured according to its relationship with S2O3. 2- The molar ratio is 0.5:1 to 5:1, preferably 1:1 to 3:1, and more preferably 1:1 to 1:1.

[0019] Furthermore, the reaction temperature of the catalytic wet oxidation described in (3) is 200-240℃, preferably 210-230℃, and most preferably 210-220℃; the pressure is 3.5-4.5MPa, preferably 4.0-4.3MPa; and the reaction time is 30-60min, preferably 40-50min.

[0020] Furthermore, the oil removal treatment described in (1) involves adding an oil removal flocculant to the waste alkaline solution and removing the light oil in the waste alkaline solution by air flotation, so that the light oil content in the effluent is not higher than 15 mg / L, preferably not higher than 10 mg / L.

[0021] Furthermore, the air flotation is selected from one or a combination of two of vortex air flotation and dissolved air flotation, and the air flotation residence time is 10-120 min.

[0022] Furthermore, the oil-removing flocculant is an inorganic-organic hybrid flocculant. The inorganic component includes one or more inorganic flocculants containing silicon, aluminum, iron, or magnesium, and the organic component is cationic polyacrylamide.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Traditional single-stage wet oxidation treatment of waste alkaline solution often employs medium-to-high temperature wet oxidation or extends the reaction time to ensure low COD in the effluent. However, even with these methods, the removal effect on some recalcitrant organic compounds is not significant, and the COD content in the effluent remains at several thousand mg / L, which is detrimental to subsequent salt recovery and zero wastewater discharge. This invention employs a two-stage wet oxidation process, with each stage operating at a lower temperature. By controlling the conditions of the first-stage wet oxidation, the COD content in the effluent is reduced to a lower level. 2-10-90% is converted to S2O3 2- Furthermore, by adding a co-oxidation promoter in the second stage of wet oxidation, it reacts with S2O3. 2- The reaction generates free radicals that can accelerate wet oxidation, thereby accelerating the degradation of organic matter and achieving the goal of treating waste with waste. The reaction conditions of the two-stage wet oxidation are relatively mild, reducing energy consumption and greatly improving COD removal efficiency.

[0025] (2) Compared with traditional homogeneous catalytic wet oxidation, the co-oxidation promoter introduced in this invention will not cause pollution to the end of water treatment.

[0026] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0027] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0028] Unless otherwise specified, the experimental methods used in the following examples are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following examples were purchased from conventional biochemical reagent stores.

[0029] In the following examples, the total COD is S 2- The chemical oxygen demand (COD) required for the complete oxidation of organic matter. The COD of the sample was measured using a Hach COD analyzer. S2O3 2- The content was determined by indirect iodometric titration.

[0030] In the following examples, the waste alkaline solution undergoes sequential treatment including oil removal, primary wet oxidation, acid neutralization, secondary homogeneous catalytic wet oxidation, alkali adjustment, and filtration. Specific operational procedures are detailed in the examples.

[0031] Example 1

[0032] Source and properties of the waste alkaline solution to be treated: Waste alkaline solution from an ethylene plant, total COD (including S) 2- The converted COD content is 24627 mg / L, S 2- The concentration of organic matter was 19332 mg / L, the COD was approximately 5295 mg / L, and the oil content was 138 mg / L. The treatment process is as follows:

[0033] (1) Air flotation for oil removal: An oil-removing flocculant consisting of polyaluminum chloride (inorganic component) and cationic polyacrylamide (organic component) was added to the waste alkaline solution, and light oil was removed from the waste alkaline solution by air flotation. The light oil content in the effluent was detected to be 6.8 mg / L.

[0034] (2) Primary wet oxidation: The effluent from step (1) was subjected to wet oxidation at a temperature of 160℃ and a pressure of 2.6 MPa for a reaction time of 90 min. The S2O3 in the effluent was measured. 2- The concentration was 24764.3 mg / L, approximately 73.2% of the S. 2- Transformed into S2O3 2- .

[0035] (3) Catalytic wet oxidation: 20% sodium persulfate co-oxidation promoter is added to the effluent from step (2), and the amount added is based on the ratio of persulfate ions to S2O3. 2- The components were added at a molar ratio of 1:1, and wet oxidation was carried out at a temperature of 220℃ and a pressure of 4.0 MPa for 50 min. The COD of the effluent was measured to be 3176 mg / L, and the S... 2- The concentration of S2O3 is 0.9 mg / L. 2- It is 0.

[0036] Example 2

[0037] Except for the co-oxidation accelerator in step (3) being changed to sodium percarbonate, the amount added is based on the ratio of percarbonate to S2O3. 2- The molar ratio was 1:1, and all other steps were the same. The COD of the effluent was measured to be 3826.8 mg / L, and the S... 2- The concentration is 1.5 mg / L, S2O3 2- It is 0.

[0038] Example 3

[0039] Except that the co-oxidation promoter in step (3) is changed to hydrogen peroxide, the amount added is based on the ratio of hydrogen peroxide to S2O3. 2- The molar ratio was 1:1, and the other steps were the same. The COD of the effluent was measured to be 3927.3 mg / L, and the S... 2- The concentration was 1.3 mg / L, and the concentration of S2O3 was 1.3 mg / L. 2- It is 0.

[0040] Example 4

[0041] Except for step (3), where the co-oxidation accelerator is changed to sodium persulfate and added at a molar ratio of persulfate to thiosulfate of 2:1, the other steps are the same. The COD of the effluent was measured to be 2936.5 mg / L, and the S... 2- The concentration of S2O3 is 0.9 mg / L. 2- It is 0.

[0042] Example 5

[0043] (1) Air flotation for oil removal: The waste alkaline solution was the same as in Example 1, but polyferric sulfate and an oil-removing flocculant with cationic polyacrylamide as the organic component were added to it, and light oil was removed from the waste alkaline solution by air flotation. The light oil content in the effluent was detected to be 7.2 mg / L.

[0044] (2) Primary wet oxidation: The effluent from step (1) was subjected to wet oxidation at a temperature of 140℃ and a pressure of 2.0 MPa for a reaction time of 120 min. The S2O3 content of the effluent was measured. 2- The concentration was 29568 mg / L, approximately 87.4% of the S. 2- Transformed into S2O3 2- .

[0045] (3) Catalytic wet oxidation: Add S2O3 to the effluent from step (2). 2- The molar ratio of potassium persulfate to persulfate was 1:1. Wet oxidation was carried out at 220℃ and 4.0 MPa for 50 min. The detected COD of the effluent was 2762.8 mg / L, and the S... 2- The concentration of S2O3 is 1.0 mg / L. 2- It is 0.

[0046] Example 6

[0047] (1) Air flotation for oil removal: The waste alkaline solution was the same as in Example 1, but polyaluminum chloride and an oil-removing flocculant with cationic polyacrylamide as the organic component were added to it, and light oil was removed from the waste alkaline solution by air flotation. The light oil content in the effluent was detected to be 7.2 mg / L.

[0048] (2) Primary wet oxidation: The effluent from step (1) was subjected to wet oxidation at a temperature of 160℃ and a pressure of 2.6 MPa for a reaction time of 90 min. The S2O3 content of the effluent was measured. 2- The concentration was 24764.3 mg / L, approximately 73.2% of the S. 2- Transformed into S2O3 2- .

[0049] (3) Catalytic wet oxidation: Add S2O3 to the effluent from step (2). 2- The molar ratio of potassium persulfate to persulfate was 1:1. Wet oxidation was carried out at 240℃ and 4.5 MPa for 30 minutes. The detected COD of the effluent was 1936.5 mg / L, and the S... 2- 0 mg / L, S2O3 2- It is 0.

[0050] Comparative Example 1

[0051] Except for the absence of a co-oxidation accelerator in step (3), the other operating procedures were the same as in Example 1. The COD of the effluent was measured to be 4782 mg / L, and the S... 2- The concentration was 1.7 mg / L, and the concentration of S2O3 was 1.7 mg / L. 2- The concentration is 0 mg / L.

[0052] This indicates that the high COD content of the effluent suggests that wet oxidation achieves complete oxidation of sulfur ions, but has poor degradation performance for organic matter.

[0053] Comparative Example 2

[0054] Except for changing the wet oxidation conditions in steps (2) and (3) to a temperature of 220°C and a pressure of 4.0 MPa, and not adding a co-oxidation accelerator in step (3), the other operating procedures were the same as in Example 1. The COD of the effluent was detected to be 3874 mg / L, and the S... 2- The concentration was 1.1 mg / L, and the concentration of S2O3 was 1.1 mg / L. 2- The concentration is 0 mg / L.

[0055] It can be seen that although the temperature of the first-stage wet oxidation is increased to the same level as that of the second-stage wet oxidation, the COD removal rate still cannot achieve the effect of adding a co-oxidant after low temperature in Example 1.

[0056] Comparative Example 3

[0057] Using the ethylene waste alkaline solution from Example 1 as the treatment target, a single-stage high-temperature wet oxidation treatment was employed at a temperature of 240°C, a pressure of 4.5 MPa, and a reaction time of 140 min. The COD in the water was detected to be 3056.6 mg / L, and the S... 2- 0 mg / L, S2O3 2- The concentration is 0 mg / L.

[0058] This shows that when the temperature rises to 240°C, a similar COD treatment effect can only be achieved under the same reaction time as in Example 1.

Claims

1. A method for treating sulfur-containing waste alkaline solution, characterized in that, Includes the following: (1) Remove oil from the waste alkaline solution; (2) The waste alkali solution after oil removal is subjected to primary wet oxidation treatment, reaction conditions are controlled to make 60%-90% of S in the waste alkali solution be converted into S2O3 2- 2- The primary wet oxidation reaction temperature is 120-180℃, the reaction time is 60-180min, and the reaction pressure is 1.2-3.5Mpa;​ (3) Add a co-oxidation accelerator to the effluent of (2), wherein the co-oxidation accelerator is reacted with S2O3. 2- The ingredients are added in a molar ratio of 1:1 to 2:1 for catalytic wet oxidation treatment. The reaction temperature of the catalytic wet oxidation is 200-240℃, the pressure is 3.5-4.5MPa, and the reaction time is 30-60min, to achieve the reaction of organic matter and sulfur. 2- The co-oxidation promoter is hydrogen persulfate, which is used for synchronous oxidation.

2. The processing method according to claim 1, characterized in that, The co-oxidation accelerator is selected from at least one of sodium peroxymonosulfate, ammonium peroxymonosulfate, and potassium peroxymonosulfate.

3. The processing method according to claim 1, characterized in that, The oil removal treatment described in (1) involves adding an oil removal flocculant to the waste alkaline solution and removing the light oil in the waste alkaline solution by air flotation, so that the light oil content in the effluent is not higher than 15 mg / L.