Selenium removal agent water, preparation method of selenium removal agent and selenium removal agent

Abstract

The invention discloses selenium removal agent water, a preparation method of the selenium removal agent and the selenium removal agent, and belongs to the technical field of wastewater treatment.The selenium removal agent water is prepared through the following steps that a saturated sodium thiosulfate solution is prepared, dilute sulphuric acid is added, and a reaction mixing system A in a closed container is formed; mixing calcium oxide and lime nitrogen, adding the mixture into the obtained reaction mixed system A, stirring, introducing nitrogen for protection, reacting and filtering to obtain filtrate, namely selenium removal agent water, freezing and crystallizing the selenium removal agent water, and centrifugally drying to obtain a white or faint yellow crystalline solid selenium removal agent. The prepared selenium removal agent water and the selenium removal agent are developed, selenium in the solution can be precipitated in the form of a complex, the method has the advantages of being small in residue amount and easy to operate, and the purpose of deep selenium removal is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of wastewater treatment, and particularly relates to a selenium-removing reagent solution, a preparation method of the selenium-removing reagent, and the selenium-removing reagent. Background Art

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

[0003] At present, the commonly used deep treatment process for selenium-containing wastewater in industry is the ferric salt flocculation method. By using the flocculation of ferric salts and calcium salts, heavy metals, fluoride ions and other impurities in the solution are adsorbed to form neutralization slag for removal. For copper smelting industrial wastewater, an oxidation process needs to be added to oxidize selenite in the solution into selenate, otherwise the selenium removal effect is not good and the standard-reaching treatment cannot be achieved. At the same time, the presence of sulfate ions in copper smelting industrial wastewater will also affect the selenium removal effect. Therefore, barium salts need to be added first to remove sulfate ions to ensure the selenium removal effect. Aiming at the problems of the iron salt flocculation method for removing selenium from copper smelting selenium-containing wastewater, such as the co-precipitation neutralization slag needs to be treated twice, and the cost increases sharply due to the additional addition of hydrogen peroxide and barium salts, a selenium-removing reagent solution, a preparation method of the selenium-removing reagent, and the selenium-removing reagent are developed for the selenium removal of copper smelting industrial wastewater. The selenium in the solution is precipitated in the form of a complex, which has the advantages of less slag volume and simple operation, and achieves the purpose of deep selenium removal. Summary of the Invention

[0004] Aiming at the problems and deficiencies existing in the above-mentioned prior art, the present invention aims to provide a selenium-removing reagent solution, a preparation method of the selenium-removing reagent, and the selenium-removing reagent to solve the problems of complex operation, large slag volume and high alkali consumption of the existing iron salt flocculation selenium removal method.

[0005] A preparation method of a selenium-removing reagent solution provided by the present invention includes the following steps: Step A: Prepare a saturated sodium thiosulfate solution with pure water in a closed container, heat it to 50-60°C, and add dilute sulfuric acid. The concentration of the dilute sulfuric acid is 40-60%, and the molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1. Stir and react for 4-8 hours. The addition process of the sulfuric acid and the stirring reaction are both carried out under closed conditions. Observe the pressure inside the closed container. When the pressure inside the closed container reaches 0.4-0.6 Mpa and no longer changes, stop the stirring reaction to obtain the reaction mixture system A inside the closed container; Step B: Mix calcium oxide and calcium cyanamide, crush and sieve them, add the reaction mixture A obtained in Step A into the closed container, and adjust the pH of the reaction mixture A to 8 - 12; Step C: Continue to heat up to 70 - 90 °C, stir and charge nitrogen for protection, react for 100 - 130 h. The addition process of the above calcium oxide and calcium cyanamide and the stirring reaction are all carried out under closed conditions. After cooling to below 30 °C, filter. The obtained filtrate is the selenium-removing agent water.

[0006] Preferably, in Step A, the purity of sodium thiosulfate is not less than 98%, and the mass fraction of sodium thiosulfate in the saturated sodium thiosulfate aqueous solution is 30 - 35%.

[0007] 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.

[0008] Preferably, in Step B, after mixing calcium oxide and calcium cyanamide, the particle size of the mixed powder is 40 - 400 mesh.

[0009] Preferably, in Step B, the weight ratio of the mixture of calcium oxide and calcium cyanamide powder to the weight of the reaction mixture A is 1:5 - 10.

[0010] Preferably, in Step C, the purity of the charged nitrogen should not be less than 99%, and the nitrogen protection pressure is 0.4 - 0.6 Mpa.

[0011] A preparation method of a selenium-removing agent further includes the following steps: Step D: Freeze-crystallize the prepared selenium-removing agent at a temperature of -10 - 20 °C. After freeze-crystallizing for 4 - 8 h, centrifuge and dry to make the selenium-removing agent. The remaining filtrate can be returned to Step C for reaction together.

[0012] A selenium-removing agent, by weight, comprises 60 - 80% calcium thiocyanate, 15 - 30% calcium sulfate, 3 - 8% calcium sulfate and the balance of impurities. The preparation steps of the selenium-removing agent are as follows: Step A: Prepare a saturated sodium thiosulfate solution with pure water in a closed container, heat up to 50 - 60 °C, and add dilute sulfuric acid. The concentration of the dilute sulfuric acid is 40 - 60%, and the molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5 - 1. Stir and react for 4 - 8 h. The addition process of the above sulfuric acid and the stirring reaction are all carried out under closed conditions. Observe the internal pressure of the closed container. When the internal pressure of the closed container reaches 0.4 - 0.6 Mpa and no longer changes, stop the stirring reaction to obtain the reaction mixture A in the closed container; Step B: Mix calcium oxide and calcium cyanamide, crush and sieve them, add the reaction mixture A obtained in Step A into the closed container, and adjust the pH of the reaction mixture A to 8 - 12; Step C: Continue to heat up to 70 - 90 °C, stir and charge nitrogen for protection, react for 100 - 130 h. The addition process of the above calcium oxide and calcium cyanamide and the stirring reaction are all carried out under closed conditions. After cooling to below 30 °C, filter, and the obtained filtrate is the selenium-removing agent water; Step D: Freeze-crystallize the prepared selenium-removing agent water at a temperature of -10 - -20 °C. After freeze-crystallizing for 4 - 8 h, centrifuge and dry to make the selenium-removing agent. The remaining filtrate can be returned to Step C for reaction together.

[0013] The selenium-removing agent is a white or light yellow crystalline solid, which is easily soluble in water at room temperature and forms a transparent or slightly turbid solution.

[0014] In the present invention, by saturating a sodium thiosulfate solution in a closed container and reacting it with sulfuric acid, the reaction of sodium thiosulfate (Na2S2O3) and 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 release rate of SO2 can be precisely controlled to avoid pressure out of control caused by excessive gas. Since it is in a closed container, part of the SO2 dissolves in the water in the reaction system and forms a metastable acidic system of SO2 - Na2SO4 - Na2S2O3 - S, that is, the reaction mixture A.

[0015] In the present invention, the key technical point is that when adding the mixed powder of calcium oxide and calcium cyanamide to the closed container, it is necessary to ensure that the addition is carried out under a closed state in the closed container to prevent the overflow of SO2 gas in the closed container and ensure that the SO2 gas component in the closed container will not be lost due to this. In this case, then pressurize by introducing nitrogen, and the pressure of nitrogen pressurization is 0.4 - 0.6 Mpa. In the closed container, due to the metastable acidic system of SO2 - Na2SO4 - Na2S2O3 - S, there is an equilibrium between the liquid phase and the gas phase of SO2 in the closed container, and the internal pressure reaches 0.4 - 0.6 Mpa. After adding the mixed powder of calcium oxide and calcium cyanamide, the added calcium oxide (CaO) neutralizes the excessive sulfuric acid in the reaction mixture A, making the reaction mixture A alkaline, and the pH of the reaction mixture A is 8 - 12. In this case, the SO2 in the reaction mixture A and the discrete SO2 in the closed container dissolve to form SO3 under alkaline conditions 2- , SO2 makes it in the form of SO3 2-It remains in the solution in the form of [description]. The consumption of discrete SO2 gas in the sealed container reduces the pressure in the sealed container. Therefore, by maintaining the pressure with nitrogen at 0.4 - 0.6 Mpa, it can ensure that the SO2 gas in the sealed container can better enter the solution. At the same time, during the stirring reaction process, nitrogen protection is used to maintain an anaerobic reduction environment, inhibit oxidation side reactions, and ensure the stability of the system.

[0016] Under alkaline conditions in the reaction mixture system A, calcium cyanamide (CaCN2) hydrolyzes. Under the combined action of CaCN2 and SO2 - Na2SO4 - Na2S2O3 - S in the reaction mixture system A, calcium thiocyanate Ca(SCN)2, cyanamide (H2NCN), or aminocyanide (NH2CN) is formed. At the same time, part of the SO3 2- and Ca in the solution 2+ react to form CaSO3 precipitate. The CaSO3 precipitate can be filtered and then acid is added to form SO2 which can be recycled. These nitrogen - containing compounds such as calcium thiocyanate Ca(SCN)2, cyanamide (H2NCN), or aminocyanide (NH2CN) can be used as complexing agents, which can form stable selenium - nitrogen complexes (such as selenocyanates) with selenium and precipitate, thus achieving the removal of selenium from copper smelting industrial wastewater. It has the advantages of less slag volume and simple operation, achieving the purpose of deep selenium removal.

[0017] Since liquid SO2 needs to be stored in a low - temperature and high - pressure container, the equipment cost is high. The present invention uses the reaction system of sodium thiosulfate (solid) and sulfuric acid (liquid) to provide a SO2 gas source, making better use of the advantages that sodium thiosulfate (solid) and sulfuric acid (liquid) are easy to store and transport at room temperature, replacing liquid SO2, and thus achieving the purpose of reducing costs and improving safe preparation.

[0018] The main innovative points and beneficial effects of the present invention are as follows: (1) The selenium - removing agent aqueous solution or selenium - removing agent prepared by the present invention can be directly added to copper smelting industrial wastewater in a ratio of 1:100 - 500, and the reaction is carried out at 80 °C for 3 h. The operation is simple and convenient.

[0019] (2) The selenium - removing process subverts the traditional iron salt neutralization and flocculation selenium - removing scheme, with less slag produced, which can be directly returned to the smelting furnace for treatment, effectively solving the problem of large slag volume in the iron salt neutralization precipitation method and realizing clean production. Brief Description of the Drawings

[0020] Figure 1 It is the preparation flow chart of the selenium - removing agent water and selenium - removing agent of the present invention. Detailed Embodiments

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, the following further describes the present invention in detail with reference to the drawings and embodiments.

[0022] In the embodiments of the present invention, the pure water is distilled water.

[0023] Refer to Figure 1 , a method for preparing a water for removing selenium agent, comprising the following steps: Step A: Prepare a saturated sodium thiosulfate solution with pure water in a closed container, heat it up to 50-60°C, and add dilute sulfuric acid. The concentration of the dilute sulfuric acid is 40-60%, and the molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1. Stir and react for 4-8 hours. The addition process of the sulfuric acid and the stirring reaction are both carried out under closed conditions. Observe the pressure inside the closed container. When the pressure inside the closed container reaches 0.4-0.6 Mpa and no longer changes, stop the stirring reaction to obtain the reaction mixture system A inside the closed container; Step B: Mix and crush calcium oxide and calcium cyanamide and pass them through a sieve. Add the reaction mixture system A obtained in Step A into the closed container, and make the pH of the reaction mixture system A 8-12; Step C: Continue to heat up to 70-90°C, stir and fill with nitrogen for protection, and react for 100-130 hours. The addition process of the calcium oxide and calcium cyanamide and the stirring reaction are both carried out under closed conditions. After cooling to below 30°C, filter, and the obtained filtrate is the water for removing selenium agent.

[0024] The preparation of the water for removing selenium agent 1 is as follows: Take 500 g of pure water at room temperature (20°C), add 220 g of sodium thiosulfate to the pure water according to the saturated solubility of sodium thiosulfate, stir and mix to dissolve to obtain 720 g of a saturated sodium thiosulfate solution. The mass fraction of sodium thiosulfate in the obtained saturated sodium thiosulfate aqueous solution is 30.5%, and it is placed in a closed container and heated to 60°C. According to the molar ratio of sodium thiosulfate to sulfuric acid of 1:0.8, add 218.2 g of dilute sulfuric acid with a concentration of 50%, stir and react for 8 hours. The addition process of the sulfuric acid and the stirring reaction are both carried out under closed conditions. Observe the pressure inside the closed container. When the pressure inside the closed container reaches 0.4-0.6 Mpa and no longer changes, stop the stirring reaction to obtain the reaction mixture system A inside the closed container, denoted as the reaction mixture system A1. The theoretical total amount of the reaction mixture system A1 is 938.2 g.

[0025] Mix calcium oxide with a purity of not less than 98% and calcium cyanamide with a purity of not less than 98% in a weight ratio of 1:20, crush and sieve them. The particle size of the mixed powder is 200 mesh. According to the ratio that the weight of the mixed powder of calcium oxide and calcium cyanamide and the weight of reaction mixture system A1 is 1:5, take 187.6 g of the above mixed powder to make the pH of reaction mixture system A1 be 8 - 12. Then continue to heat up to 90 °C, stir and fill with nitrogen for protection, and react for 120 h. The addition process of the above calcium oxide and calcium cyanamide and the stirring reaction are all carried out under closed conditions. After cooling to below 30 °C, filter, and the obtained filtrate is the selenium-removing agent water 1.

[0026] The preparation method of selenium-removing agent water 2 is the same as above, and the specific preparation is shown in Table 1.

[0027] Table 1

[0028] Example 1: Preheat the copper smelting industrial wastewater to 80 °C, add the above selenium-removing agent water 1. After the copper smelting industrial wastewater and the selenium-removing agent water 1 are fully stirred and mixed according to a ratio of 1:100, keep the solution temperature at 80 °C and react for 3 h. The selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in the selenium-removing agent water 1 complex to form a precipitate, and then filter and separate. Before the reaction, the selenium content in the copper smelting industrial wastewater was measured to be 109.65 mg / L, and the selenium concentration in the filtrate after the reaction was 0.045 mg / L, meeting the requirement of the maximum allowable concentration of total selenium in the specified first-class discharge standard being 0.1 mg / L, achieving up-to-standard treatment.

[0029] Example 2: Preheat the copper smelting industrial wastewater to 80 °C, add the above selenium-removing agent water 2. After the copper smelting industrial wastewater and the selenium-removing agent water 1 are fully stirred and mixed according to a ratio of 1:100, keep the solution temperature at 80 °C and react for 3 h. The selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in the selenium-removing agent water 2 complex to form a precipitate, and then filter and separate. Before the reaction, the selenium content in the copper smelting industrial wastewater was measured to be 89.65 mg / L, and the selenium concentration in the filtrate after the reaction was 0.030 mg / L, meeting the requirement of the maximum allowable concentration of total selenium in the specified first-class discharge standard being 0.1 mg / L, achieving up-to-standard treatment.

[0030] Example 3: Sodium selenite and sodium selenate are used to prepare an aqueous solution with a selenium content of 100 mg / L, wherein the molar ratio of sodium selenite to sodium selenate is 2:1. The sodium selenite and sodium selenate aqueous solution is preheated to 80°C, and the selenium removal agent water 1 is added, and the addition ratio of the selenium removal agent water 1 to the sodium selenite and sodium selenate aqueous solution is 1:100. After sufficient stirring and mixing, the solution temperature is maintained at 80°C for 3 hours. The selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in the selenium removal agent water 2 are complexed to form a precipitate, which is filtered and separated. The selenium concentration in the filtrate after the reaction is 0.015 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, and achieves standard treatment.

[0031] Embodiment 4: The same as in Example 3, the added selenium removal agent water is selenium removal agent water 2, the addition ratio of selenium removal agent water 2 to sodium selenite and sodium selenate aqueous solution is 1:100, after sufficient stirring and mixing, the solution temperature is maintained at 80°C for 3 hours, and the selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in the selenium removal agent water 2 are complexed to form a precipitate, which is filtered and separated. The selenium concentration in the filtrate after the reaction is 0.018 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, and achieves standard treatment.

[0032] Reference Figure 1 The preparation method of the selenium removal agent is implemented in the same steps as the preparation steps of the selenium removal agent water. After the selenium removal agent water is obtained, it is further processed, including the following steps: Step D: freeze-crystallize the prepared selenium removal agent water at a freezing crystallization temperature of -10 to -20°C. After freezing crystallization for 4 to 8 hours, centrifuge and dry to obtain the selenium removal agent. The remaining filtrate can be returned to step C for reaction.

[0033] The preparation of selenium removal agent 1 is as follows: take the prepared selenium removal agent water 1, freeze and crystallize the selenium removal agent water 1 at -10~-20℃, freeze and crystallize for 4~8 hours, and then centrifuge and dry to prepare selenium removal agent 1. Selenium removal agent 1 is a white with light yellow crystalline solid, which is easily soluble in water at room temperature and forms a translucent, slightly turbid solution.

[0034] 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℃, freeze and crystallize for 4~8 hours, and then centrifuge and dry to prepare selenium removal agent 2. Selenium removal agent 2 is a white with light yellow crystalline solid, which is easily soluble in water at room temperature and forms a translucent, slightly turbid solution.

[0035] After testing, the specific components of selenium removal agent 1 and selenium removal agent 2, measured by weight, are shown in Table 2.

[0036] Table 2

[0037] Example 5: Same as Example 1, preheat the copper smelting industrial wastewater to 80 °C, add the above selenium-removing agent 1. After the copper smelting industrial wastewater and the selenium-removing agent water 1 are fully stirred and mixed according to a ratio of 1:300, keep the solution temperature at 80 °C and react for 3 h. The selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in the selenium-removing agent 1 complex to form a precipitate, and then filter and separate. Before the reaction, the measured selenium content in the copper smelting industrial wastewater is 109.65 mg / L. After the reaction, the selenium concentration in the filtrate is 0.082 mg / L. Compared with the selenium-removing agent water 1, the selenium-removing effect of the selenium-removing agent 1 is relatively weakened, but it meets the requirement of the maximum allowable concentration of total selenium in the specified first-class discharge standard, which is 0.1 mg / L, achieving up-to-standard treatment. Moreover, compared with the selenium-removing agent water 1, the selenium-removing agent 1 is convenient for long-term storage.

[0038] Example 6: Same as Example 2, preheat the copper smelting industrial wastewater to 80 °C, add the above selenium-removing agent 2. After the copper smelting industrial wastewater and the selenium-removing agent water 1 are fully stirred and mixed according to a ratio of 1:300, keep the solution temperature at 80 °C and react for 3 h. The selenium ions in the copper smelting industrial wastewater and the nitrogen-containing compounds in the selenium-removing agent 2 complex to form a precipitate, and then filter and separate. Before the reaction, the measured selenium content in the copper smelting industrial wastewater is 89.65 mg / L. After the reaction, the selenium concentration in the filtrate is 0.071 mg / L. Compared with the selenium-removing agent water 2, the selenium-removing effect of the selenium-removing agent 2 is relatively weakened, but it meets the requirement of the maximum allowable concentration of total selenium in the specified first-class discharge standard, which is 0.1 mg / L, achieving up-to-standard treatment. Moreover, compared with the selenium-removing agent water 2, the selenium-removing agent 2 is convenient for long-term storage.

[0039] Example 7: Same as Example 3, replace the selenium-removing agent water 1 with the selenium-removing agent 1, and the addition ratio of the selenium-removing agent 1 to the sodium selenite and sodium selenate aqueous solution is 1:400. After the reaction, the selenium concentration in the filtrate is 0.042 mg / L, meeting the requirement of the maximum allowable concentration of total selenium in the specified first-class discharge standard, which is 0.1 mg / L, achieving up-to-standard treatment.

[0040] Example 8: Same as Example 4, replace the selenium-removing agent water 2 with the selenium-removing agent 2, and the addition ratio of the selenium-removing agent 2 to the sodium selenite and sodium selenate aqueous solution is 1:400. After the reaction, the selenium concentration in the filtrate is 0.054 mg / L, meeting the requirement of the maximum allowable concentration of total selenium in the specified first-class discharge standard, which is 0.1 mg / L, achieving up-to-standard treatment.

[0041] Comparative Example 1: Using the sodium selenite and sodium selenate agents prepared in Example 2, an aqueous solution with a selenium content of 100 mg / L was prepared, and iron salt neutralization was used for selenium removal. The process of iron salt neutralization for selenium removal is as follows: Take 500 mL of selenious acid solution and place it in a beaker. Heat it to 80 °C, add FeCl3 at a concentration of 1.0 g / L, stir for 0.5 h, then adjust the pH to 7 with lime milk solution, stir at room temperature for 2 h, add 1‰ flocculant solution, let it stand and filter. The selenium concentration in the filtrate is 10.24 mg / L.

[0042] In Examples 1 to 8, compared with Comparative Example 1, the selenium removal effect of the selenium removal agent water or the selenium removal agent prepared by the present invention on copper smelting industrial wastewater is better than that of selenium removal by the iron salt neutralization method under the same conditions.

[0043] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing selenium-removing medicinal water, characterized in that: The steps include: Step A: prepare a saturated sodium thiosulfate solution with pure water in a closed container, heat it to 50-60° C., and add dilute sulfuric acid, wherein the concentration of the dilute sulfuric acid is 40-60%, and the molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1, and stir to react for 4-8 hours. The sulfuric acid addition process and stirring reaction are both carried out under closed conditions, and the pressure inside the closed container is observed. When the pressure inside the closed container reaches 0.4-0.6Mpa and no longer changes, stop stirring the reaction to obtain a reaction mixture system A in the closed container; Step B: Mix calcium oxide and lime nitrogen, grind and sieve, 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 to heat to 70-90°C, stir and fill with nitrogen for protection, react for 100-130h, the above calcium oxide and lime nitrogen addition process and stirring reaction are all carried out under closed conditions, after cooling to below 30°C, filter, and the filtrate obtained is selenium removal agent water.

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

4. The method for preparing selenium-removing medicinal water according to claim 1, characterized in that: In step B, after calcium oxide and lime nitrogen are mixed, the particle size of the mixed powder is 40-400 meshes.

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

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

7. A method for preparing a selenium removal agent, characterized in that: The method for preparing the selenium removal agent water according to claim 1 to claim 6 further comprises the following steps: Step D: freeze-crystallize the prepared selenium removal agent water at a freezing crystallization temperature of -10 to -20°C. After freezing crystallization for 4 to 8 hours, centrifuge and dry to obtain the selenium removal agent. The remaining filtrate can be returned to step C for reaction.

8. A selenium removal agent, characterized in that: The selenium removal agent comprises, by weight, 60-80% calcium thiocyanate, 15-30% calcium sulfate, 3-8% calcium sulfate and the remainder of impurities. The preparation steps of the selenium removal agent are as follows: Step A: Prepare a saturated sodium thiosulfate solution with pure water in a sealed container, heat it to 50-60°C, and add dilute sulfur. Acid, the concentration of dilute sulfuric acid is 40-60%, the molar ratio of sodium thiosulfate to sulfuric acid is 1:0.5-1, and the reaction is stirred for 4-8 hours. The sulfuric acid addition process and the stirring reaction are all carried out under closed conditions. The pressure inside the closed container is observed. When the pressure inside the closed container reaches 0.4-0.6Mpa and no longer changes, the stirring reaction is stopped to obtain a reaction mixture system A in the closed container; Step B: Mix calcium oxide and lime nitrogen, grind and sieve, 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 to heat to 70-90°C, stir and fill with nitrogen for protection, react for 100-130h, the above calcium oxide and lime nitrogen addition process and stirring reaction are all carried out under closed conditions, after cooling to below 30°C, filter, and the obtained filtrate is selenium removal agent water; Step D: freeze-crystallize the prepared selenium removal agent water at a freezing crystallization temperature of -10 to -20°C. After freezing crystallization for 4 to 8 hours, centrifuge and dry to obtain the selenium removal agent. The remaining filtrate can be returned to step C for reaction.

9. A selenium removal agent according to claim 8, characterized in that: The selenium removal agent is a white or light yellow crystalline solid, which is easily soluble in water at room temperature and forms a transparent or slightly turbid solution.

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