A selenium removal agent water, a preparation method of a selenium removal agent and a selenium removal agent

Abstract

The application discloses a kind of selenium-removing agent water, preparation method of selenium-removing agent and selenium-removing agent, belong to wastewater treatment technical field, and the preparation steps of selenium-removing agent water are as follows: saturated sodium thiosulfate solution is configured, dilute sulfuric acid is added, and reaction mixture system A in a closed container is formed;Calcium oxide and lime nitrogen are mixed and added to the obtained reaction mixture system A, stirred and filled with nitrogen protection, filtered, and the obtained filtrate is selenium-removing agent water, after selenium-removing agent water is frozen crystallization, centrifugal drying is carried out, and selenium-removing agent in white or light yellow crystalline solid is prepared, the application is aimed at the selenium removal of copper smelting industrial wastewater, and the prepared selenium-removing agent water and selenium-removing agent can precipitate selenium in solution in the form of complex, with the advantages of small residue amount and simple operation, to achieve the purpose of deep selenium removal.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a selenium removal agent water, a method for preparing the selenium removal agent, and the selenium removal agent itself. Background Technology

[0002] Selenium (Se) is a metalloid chemical element. Selenium and its compounds are classified as Group 3 carcinogens. If the concentration in the environment is too high, it can easily have adverse effects on human health. With increasingly stringent environmental protection standards, the control requirements for selenium concentration in water are also becoming more stringent. The "Integrated Wastewater Discharge Standard GB 8978-1996" stipulates that the maximum allowable concentration of total selenium in the first-level discharge standard is 0.1 mg / L.

[0003] Currently, the commonly used advanced treatment process for selenium-containing wastewater in industry is the ferric salt flocculation method. This method utilizes the flocculation of ferric and calcium salts to adsorb heavy metals, fluoride ions, and other impurities in the solution, forming a neutralized slag for removal. For copper smelting wastewater, an oxidation process is required to oxidize selenite ions in the solution to selenate ions; otherwise, the selenium removal effect is poor and treatment standards cannot be met. Simultaneously, the presence of sulfate ions in copper smelting wastewater also affects the selenium removal effect; therefore, barium salts need to be added first to remove sulfate ions to ensure effective selenium removal. The co-precipitated neutralized slag from the ferric salt flocculation method for removing selenium from copper smelting wastewater requires secondary treatment, and the additional addition of hydrogen peroxide and barium salts leads to a significant increase in cost. To address the issue of selenium removal from copper smelting wastewater, a selenium removal agent solution, its preparation method, and the agent itself have been developed. This method precipitates selenium in the solution as a complex, offering advantages such as low slag volume and simple operation, achieving the goal of deep selenium removal. Summary of the Invention

[0004] In view of the problems and deficiencies in the existing technology, the present invention aims to provide a selenium removal agent water, a method for preparing the selenium removal agent, and the selenium removal agent, so as to solve the problems of complex operation, large slag volume, and high alkali consumption in the existing iron salt flocculation selenium removal method.

[0005] The present invention provides a method for preparing a selenium-removing agent water, comprising the following steps:

[0006] Step A: Prepare a saturated sodium thiosulfate solution in pure water in a sealed container, heat it to 50-60°C, and add dilute sulfuric acid with a concentration of 40-60%. The molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1. Stir the reaction for 4-8 hours. The addition of sulfuric acid and the stirring reaction are carried out under sealed conditions. Observe the pressure inside the sealed container. When the pressure inside the sealed container reaches 0.4-0.6 MPa and no longer changes, stop stirring the reaction to obtain the reaction mixture A in the sealed container.

[0007] Step B: Mix calcium oxide and calcium cyanamide, crush and sieve them, add the reaction mixture A obtained in step A into the sealed container, and make the pH of the reaction mixture A = 8~12;

[0008] Step C: Continue heating to 70~90℃, stir and purge with nitrogen for protection, react for 100~130h. The above process of adding calcium oxide and calcium cyanamide and stirring reaction are all carried out under closed conditions. After cooling to below 30℃, filter, and the obtained filtrate is the selenium removal agent water.

[0009] Preferably, in step A, the purity of the sodium thiosulfate is not less than 98%, and the mass fraction of sodium thiosulfate in the saturated sodium thiosulfate aqueous solution is 30-35%.

[0010] Preferably, in step B, the purity of calcium oxide or calcium cyanamide is not less than 98%, and the weight ratio of calcium oxide to calcium cyanamide is 1:10~20.

[0011] Preferably, in step B, after calcium oxide and calcium cyanamide are mixed, the particle size of the mixed powder is 40-400 mesh.

[0012] Preferably, in step B, the weight ratio of the powder after mixing calcium oxide and calcium cyanamide to the weight ratio of the reaction mixture A is 1:5~10.

[0013] Preferably, in step C, the purity of the nitrogen gas being introduced should be no less than 99%, and the nitrogen protection pressure should be 0.4~0.6 MPa.

[0014] A method for preparing a selenium removal agent further includes the following steps:

[0015] Step D: The prepared selenium removal agent is subjected to freeze crystallization at a temperature of -10 to -20°C for 4 to 8 hours. After freezing and crystallization, it is centrifuged and dried to obtain the selenium removal agent. The remaining filtrate can be returned to step C for further reaction.

[0016] A selenium removal agent, by weight, comprises 60-80% calcium thiocyanate, 15-30% calcium sulfate, 3-8% calcium sulfate, and the balance being impurities. The preparation steps of the selenium removal agent are as follows:

[0017] Step A: Prepare a saturated sodium thiosulfate solution in pure water in a sealed container, heat it to 50-60°C, and add dilute sulfuric acid with a concentration of 40-60%. The molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1. Stir the reaction for 4-8 hours. The addition of sulfuric acid and the stirring reaction are carried out under sealed conditions. Observe the pressure inside the sealed container. When the pressure inside the sealed container reaches 0.4-0.6 MPa and no longer changes, stop stirring the reaction to obtain the reaction mixture A in the sealed container.

[0018] Step B: Mix calcium oxide and calcium cyanamide, crush and sieve them, add the reaction mixture A obtained in step A into the sealed container, and make the pH of the reaction mixture A = 8~12;

[0019] Step C: Continue heating to 70~90℃, stir and purge with nitrogen for protection, react for 100~130h. The above process of adding calcium oxide and calcium cyanamide and stirring reaction are all carried out under closed conditions. After cooling to below 30℃, filter, and the obtained filtrate is the selenium removal agent water.

[0020] Step D: The prepared selenium removal agent water is subjected to freeze crystallization at a temperature of -10 to -20°C for 4 to 8 hours. After freezing and crystallization, it is centrifuged and dried to prepare the selenium removal agent. The remaining filtrate can be returned to step C for reaction.

[0021] The selenium removal agent is a white or pale yellow crystalline solid that is easily soluble in water at room temperature, forming a transparent or slightly turbid solution.

[0022] This invention involves saturating a sodium thiosulfate solution in a sealed container and reacting it with sulfuric acid. The reaction of sodium thiosulfate (Na2S2O3) with sulfuric acid is: Na2S2O3 + H2SO4 → Na2SO4 + S↓ + SO2↑ + H2O. By adjusting the sulfuric acid concentration (40~60%) and the molar ratio (Na2S2O3:H2SO4 = 1:0.5~1), the SO2 release rate can be precisely controlled, avoiding excessive gas and pressure runaway. Since SO2 is partially dissolved in the water in the reaction system within the sealed container, a metastable acidic system of SO2-Na2SO4-Na2S2O3-S is formed, i.e., reaction mixture A.

[0023] The key technical point of this invention lies in ensuring the sealed state of the container when adding the mixed powder of calcium oxide and calcium cyanamide to prevent the leakage of SO2 gas and ensure that the SO2 gas components in the sealed container are not lost. Under these conditions, nitrogen gas is introduced to maintain pressure (0.4-0.6 MPa). In the metastable acidic system of SO2-Na2SO4-Na2S2O3-S, SO2 exists in a liquid-gas phase equilibrium within the sealed container, reaching an internal pressure of 0.4-0.6 MPa. After adding the mixed powder of calcium oxide and calcium cyanamide, the added calcium oxide (CaO) neutralizes the excess sulfuric acid in reaction mixture A, making reaction mixture A alkaline (pH 8-12). Under these conditions, the SO2 in reaction mixture A, as well as the SO2 dispersed within the sealed container, dissolves to form SO3. 2- SO2 makes SO3 2-The SO2 gas is retained in the solution in its discrete form. The consumption of SO2 gas in the closed container reduces the pressure inside the container. Therefore, using nitrogen to maintain the pressure at 0.4~0.6 MPa can ensure that SO2 gas in the closed container can enter the solution better. At the same time, during the stirring reaction, nitrogen protection is used to maintain an anaerobic reducing environment, suppress oxidation side reactions, and ensure the stability of the system.

[0024] Under alkaline conditions, calcium cyanamide (CaCN2) in reaction mixture A undergoes hydrolysis. Combined with the action of SO2-Na2SO4-Na2S2O3-S in reaction mixture A, it produces calcium thiocyanate (Ca(SCN)2), cyanamide (H2NCN), or aminocyanide (NH2CN), while some SO3 is also produced. 2- and Ca in the solution 2+ The reaction forms CaSO3 precipitate. This CaSO3 precipitate can be filtered and then recycled by adding acid to form SO2. These nitrogen-containing compounds, such as calcium thiocyanate (Ca(SCN)2), cyanamide (H2NCN), or aminocyanide (NH2CN), can act as complexing agents, forming stable selenium-nitrogen complexes (such as selenocyanate) precipitates with selenium. This achieves selenium removal from copper smelting wastewater, offering advantages such as low slag volume and simple operation, thus achieving deep selenium removal.

[0025] Since liquid SO2 requires storage in low-temperature, high-pressure containers, the equipment cost is high. This invention utilizes the reaction system of sodium thiosulfate (solid) and sulfuric acid (liquid) to provide the gas source for SO2, making better use of the advantages of sodium thiosulfate (solid) and sulfuric acid (liquid) being easy to store and transport at room temperature, thereby replacing liquid SO2, and thus achieving the purpose of reducing costs and improving safe preparation.

[0026] The main innovations and beneficial effects of this invention are as follows:

[0027] (1) The selenium removal agent aqueous solution or selenium removal agent prepared by the present invention can be directly added to copper smelting industrial wastewater at a ratio of 1:100~500 and reacted at 80℃ for 3 hours. The operation is simple and convenient.

[0028] (2) The selenium removal process overturns the traditional iron salt neutralization flocculation selenium removal scheme, produces less slag, and can be directly returned to the smelting furnace for processing, effectively solving the problem of large slag volume in the iron salt neutralization precipitation method and realizing clean production. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating the preparation process of the selenium-removing agent water and the selenium-removing agent of the present invention. Detailed Implementation

[0030] To make the technical problems, technical solutions and beneficial effects of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] In this embodiment of the invention, the pure water is distilled water.

[0032] Reference Figure 1 The preparation method of selenium-removing agent water includes the following steps:

[0033] Step A: Prepare a saturated sodium thiosulfate solution in pure water in a sealed container, heat it to 50-60°C, and add dilute sulfuric acid with a concentration of 40-60%. The molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1. Stir the reaction for 4-8 hours. The addition of sulfuric acid and the stirring reaction are carried out under sealed conditions. Observe the pressure inside the sealed container. When the pressure inside the sealed container reaches 0.4-0.6 MPa and no longer changes, stop stirring the reaction to obtain the reaction mixture A in the sealed container.

[0034] Step B: Mix calcium oxide and calcium cyanamide, crush and sieve them, add the reaction mixture A obtained in step A into the sealed container, and make the pH of the reaction mixture A = 8~12;

[0035] Step C: Continue heating to 70~90℃, stir and purge with nitrogen for protection, react for 100~130h. The above process of adding calcium oxide and calcium cyanamide and stirring reaction are all carried out under closed conditions. After cooling to below 30℃, filter, and the obtained filtrate is the selenium removal agent water.

[0036] The preparation of selenium removal agent water 1 is as follows: Take 500g of pure water at room temperature (20℃), add 220g of sodium thiosulfate to the pure water according to the saturated solubility of sodium thiosulfate, stir and mix to dissolve, and obtain 720g of saturated sodium thiosulfate solution. The mass fraction of sodium thiosulfate in the saturated sodium thiosulfate aqueous solution is 30.5%. Place it in a sealed container, heat it to 60℃, and add 218.2g of 50% dilute sulfuric acid according to the molar ratio of sodium thiosulfate to sulfuric acid of 1:0.8. Stir and react for 8h. The above sulfuric acid addition process and stirring reaction are all carried out under sealed conditions. Observe the pressure inside the sealed container. When the pressure inside the sealed container reaches 0.4~0.6Mpa and no longer changes, stop stirring and react to obtain reaction mixture A in the sealed container, which is denoted as reaction mixture A1. The theoretical total amount of reaction mixture A1 is 938.2g.

[0037] Calcium oxide with a purity of not less than 98% and calcium cyanamide with a purity of not less than 98% were mixed, pulverized, and sieved at a weight ratio of 1:20. The particle size of the mixed powder was 200 mesh. 187.6 g of the mixed powder was taken according to the weight ratio of the powder after mixing calcium oxide and calcium cyanamide to the weight of the reaction mixture A1 of 1:5. The pH of the reaction mixture A1 was adjusted to 8-12. The temperature was raised to 90°C, stirred, and nitrogen gas was introduced for protection. The reaction was carried out for 120 h. The addition of calcium oxide and calcium cyanamide and the stirring reaction were all carried out under closed conditions. After cooling to below 30°C, the mixture was filtered. The obtained filtrate is the selenium removal agent water 1.

[0038] The preparation method of selenium removal agent water 2 is the same as above, and the specific preparation is shown in Table 1.

[0039] Table 1

[0040]

[0041] Example 1:

[0042] The copper smelting industrial wastewater was preheated to 80℃, and the aforementioned selenium removal agent (water 1) was added. The wastewater and agent were thoroughly mixed at a 1:100 ratio, and the solution temperature was maintained at 80℃ for 3 hours. Selenium ions in the wastewater and nitrogen-containing compounds in the agent formed a precipitate, which was then separated by filtration. Before the reaction, the selenium content in the wastewater was measured to be 109.65 mg / L. After the reaction, the selenium concentration in the filtrate was 0.045 mg / L, meeting the requirement of the highest permissible total selenium concentration of 0.1 mg / L in the Class I discharge standard, thus achieving compliant treatment.

[0043] Example 2:

[0044] The copper smelting industrial wastewater was preheated to 80℃, and the aforementioned selenium removal agent 2 was added. The wastewater and agent 1 were mixed thoroughly at a ratio of 1:100. The mixture was then kept at 80℃ for 3 hours. Selenium ions in the wastewater and nitrogen-containing compounds in agent 2 complexed to form a precipitate, which was then separated by filtration. Before the reaction, the selenium content in the wastewater was measured to be 89.65 mg / L. After the reaction, the selenium concentration in the filtrate was 0.030 mg / L, meeting the requirement of the Class I discharge standard of a maximum allowable total selenium concentration of 0.1 mg / L, thus achieving compliant treatment.

[0045] Example 3:

[0046] An aqueous solution with a selenium content of 100 mg / L was prepared using sodium selenite and sodium selenate reagents, with a molar ratio of sodium selenite to sodium selenate of 2:1. The sodium selenite and sodium selenate aqueous solution was preheated to 80°C. Selenium removal reagent 1 (water 1) was added at a ratio of 1:100 to the sodium selenite and sodium selenate aqueous solution. After thorough mixing, the solution temperature was maintained at 80°C for 3 hours. Selenium ions in the copper smelting industrial wastewater complexed with nitrogen-containing compounds in selenium removal reagent 2 to form a precipitate, which was then separated by filtration. The selenium concentration in the filtrate after the reaction was 0.015 mg / L, meeting the requirement of the maximum allowable concentration of total selenium of 0.1 mg / L in the Class I emission standard, thus achieving compliant treatment.

[0047] Example 4:

[0048] Similar to Example 3, the added selenium removal agent water was selenium removal agent water 2. The addition ratio of selenium removal agent water 2 to sodium selenite and sodium selenate aqueous solution was 1:100. After thorough stirring and mixing, the solution temperature was maintained at 80℃ for 3 hours. Selenium ions in the copper smelting industrial wastewater complexed with nitrogen-containing compounds in selenium removal agent water 2 to form a precipitate, which was then separated by filtration. The selenium concentration in the filtrate after the reaction was 0.018 mg / L, meeting the requirement of the maximum allowable concentration of total selenium of 0.1 mg / L in the specified Class I discharge standard, thus achieving compliant treatment.

[0049] Reference Figure 1 The preparation method of the selenium removal agent is the same as that of the selenium removal agent water preparation method. After obtaining the selenium removal agent water, further processing is carried out, including the following steps:

[0050] Step D: The prepared selenium removal agent water is subjected to freeze crystallization at a temperature of -10 to -20°C for 4 to 8 hours. After freezing and crystallization, it is centrifuged and dried to prepare the selenium removal agent. The remaining filtrate can be returned to step C for reaction.

[0051] The preparation of selenium removal agent 1 is as follows: the prepared selenium removal agent water 1 is taken and frozen at -10~-20℃ for 4~8h. After freezing and crystallizing, it is centrifuged and dried to prepare selenium removal agent 1. Selenium removal agent 1 is a white crystalline solid with a light yellow tinge. It is easily soluble in water at room temperature and forms a semi-transparent, slightly turbid solution.

[0052] The preparation of selenium removal agent 2 is as follows: take the prepared selenium removal agent water 2, freeze and crystallize the selenium removal agent water 2 at -10~-20℃, after freezing and crystallizing for 4~8h, centrifuge and dry to prepare selenium removal agent 2. Selenium removal agent 2 is a white crystalline solid with a light yellow tinge. It is easily soluble in water at room temperature and forms a semi-transparent, slightly turbid solution.

[0053] According to the test results, the specific components of selenium removal agent 1 and selenium removal agent 2 by weight are shown in Table 2.

[0054] Table 2

[0055]

[0056] Example 5:

[0057] Similar to Example 1, the copper smelting industrial wastewater was preheated to 80°C, and the above-mentioned selenium removal agent 1 was added. The copper smelting industrial wastewater and selenium removal agent water 1 were thoroughly mixed at a ratio of 1:300. After thorough mixing, the solution temperature was maintained at 80°C for 3 hours. The selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in selenium removal agent 1 complexed to form a precipitate, which was then separated by filtration. Before the reaction, the selenium content in the copper smelting industrial wastewater was measured to be 109.65 mg / L. After the reaction, the selenium concentration in the filtrate was 0.082 mg / L. Compared with selenium removal agent water 1, the selenium removal effect of selenium removal agent 1 was relatively weaker, but it still met the requirement of the maximum allowable concentration of total selenium of 0.1 mg / L in the first-level discharge standard, achieving compliance treatment. Moreover, compared with selenium removal agent water 1, selenium removal agent 1 is easier to store for a long time.

[0058] Example 6:

[0059] Similar to Example 2, the copper smelting industrial wastewater was preheated to 80°C, and the above-mentioned selenium removal agent 2 was added. The copper smelting industrial wastewater and selenium removal agent water 1 were thoroughly mixed at a ratio of 1:300. After thorough mixing, the solution temperature was maintained at 80°C for 3 hours. The selenium ions in the copper smelting industrial wastewater complexed with the nitrogen-containing compounds in selenium removal agent 2 to form a precipitate, which was then separated by filtration. Before the reaction, the selenium content in the copper smelting industrial wastewater was measured to be 89.65 mg / L. After the reaction, the selenium concentration in the filtrate was 0.071 mg / L. Compared with selenium removal agent water 2, the selenium removal effect of selenium removal agent 2 was relatively weaker, but it still met the requirement of the maximum allowable concentration of total selenium of 0.1 mg / L in the first-level discharge standard, achieving compliance treatment. Moreover, compared with selenium removal agent water 2, selenium removal agent 2 is easier to store for a long time.

[0060] Example 7:

[0061] Similar to Example 3, the selenium removal agent 1 was replaced with selenium removal agent 1, and the addition ratio of selenium removal agent 1 to sodium selenite and sodium selenate aqueous solution was 1:400. After the reaction, the selenium concentration in the filtrate was 0.042 mg / L, which meets the requirement of the maximum allowable concentration of total selenium of 0.1 mg / L in the first-level emission standard, thus achieving the standard treatment.

[0062] Example 8:

[0063] Similar to Example 4, the selenium removal agent 2 was replaced with selenium removal agent 2, and the addition ratio of selenium removal agent 2 to sodium selenite and sodium selenate aqueous solution was 1:400. After the reaction, the selenium concentration in the filtrate was 0.054 mg / L, which meets the requirement of the maximum allowable concentration of total selenium of 0.1 mg / L in the first-level emission standard, thus achieving the standard treatment.

[0064] Comparative Example 1:

[0065] Using the sodium selenite and sodium selenate reagents prepared in Example 2, an aqueous solution with a selenium content of 100 mg / L was prepared. Selenium removal was carried out by iron salt neutralization. The iron salt neutralization process for selenium removal was as follows: 500 mL of selenite solution was placed in a beaker, heated to 80 °C, 1.0 g / L of FeCl3 was added, and after stirring for 0.5 h, the pH was adjusted to 7 with lime milk solution. After stirring at room temperature for 2 h, 1‰ flocculant solution was added, and the solution was allowed to stand and filter. The selenium concentration in the filtrate was 10.24 mg / L.

[0066] Examples 1 to 8, compared with Comparative Example 1, show that the selenium removal agent water or selenium removal agent prepared by the present invention, under the same conditions, has a better effect on removing selenium from copper smelting industrial wastewater than the effect of selenium removal by iron salt neutralization.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a selenium-removing agent water, characterized in that, Includes the following steps: Step A: Prepare a saturated sodium thiosulfate solution in pure water in a sealed container, heat it to 50-60°C, and add dilute sulfuric acid with a concentration of 40-60%. The molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1. Stir the reaction for 4-8 hours. The addition of sulfuric acid and the stirring reaction are carried out under sealed conditions. Observe the pressure inside the sealed container. When the pressure inside the sealed container reaches 0.4-0.6 MPa and no longer changes, stop stirring the reaction to obtain the reaction mixture A in the sealed container. Step B: Mix calcium oxide and calcium cyanamide, crush and sieve them, add the reaction mixture A obtained in step A into the sealed container, and make the pH of the reaction mixture A = 8~12; Step C: Continue heating to 70~90℃, stir and purge with nitrogen for protection, react for 100~130h. The above process of adding calcium oxide and calcium cyanamide and stirring reaction are all carried out under closed conditions. After cooling to below 30℃, filter, and the obtained filtrate is the selenium removal agent water. Under alkaline conditions, calcium cyanamide hydrolyzes in reaction mixture A, and together with SO2-Na2SO4-Na2S2O3-S in reaction mixture A, calcium thiocyanate Ca(SCN)2, cyanamide H2NCN, or aminocyanide NH2CN are generated.

2. The method for preparing a selenium-removing agent water according to claim 1, characterized in that, In step A, the purity of the sodium thiosulfate is not less than 98%, and the mass fraction of sodium thiosulfate in the saturated sodium thiosulfate aqueous solution is 30-35%.

3. The method for preparing a selenium-removing agent water according to claim 1, characterized in that, In step B, the purity of calcium oxide or calcium cyanamide is not less than 98%, and the weight ratio of calcium oxide to calcium cyanamide is 1:10~20.

4. The method for preparing a selenium-removing agent water according to claim 1, characterized in that, In step B, after calcium oxide and calcium cyanamide are mixed, the particle size of the mixed powder is 40-400 mesh.

5. The method for preparing a selenium-removing agent water according to claim 1, characterized in that, In step B, the weight ratio of the powder after mixing calcium oxide and calcium cyanamide to the weight ratio of the reaction mixture A is 1:5~10.

6. The method for preparing a selenium-removing agent water according to claim 1, characterized in that, In step C, the purity of the nitrogen gas should be no less than 99%, and the nitrogen protection pressure should be 0.4~0.6 MPa.

7. A method for preparing a selenium removal agent, characterized in that, The method for preparing a selenium-removing agent water according to any one of claims 1 to 6 further includes the following steps: Step D: The prepared selenium removal agent water is subjected to freeze crystallization at a temperature of -10~-20℃ for 4~8 hours. After freezing and crystallization, it is centrifuged and dried to prepare the selenium removal agent. The centrifugation waste liquid is returned to step C for reaction together. The selenium removal agent, by weight, comprises 60-80% calcium thiocyanate, 15-30% sodium sulfate, 3-8% calcium sulfate, and the balance being impurities.

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