Arsenic removal method of fluosilicic acid

Abstract

The invention discloses an arsenic removal method of fluosilicic acid. Comprising the following steps: (1) adding a coagulant into an arsenic-containing fluosilicic acid solution to carry out a pre-coagulation reaction to generate a precipitate; (2) adding an oxidizing agent into the system in the step (1) to react with silicon tetrafluoride gas, standing and aging, and filtering to remove precipitate; and (3) adding a water absorbent into the filtrate in the step (2), and filtering to obtain the arsenic-removed fluosilicic acid solution. According to the method, arsenic (III) in the fluosilicic acid solution can be oxidized into arsenic (V) by adding excessive coagulant and oxidant, the arsenic (V) is precipitated and separated in the form, and then the precipitated excessive coagulant and oxidant are filtered and removed by adding concentrated sulfuric acid and other water absorbents. The whole process is simple, additives can be removed, and the obtained fluosilicic acid solution can be applied to preparation of anhydrous hydrogen fluoride.

Description

Technical Field

[0001] This invention relates to a method for removing arsenic from fluorosilicic acid, specifically a method for removing arsenic from fluorosilicic acid used in the preparation of hydrogen fluoride, and belongs to the field of arsenic removal technology. Background Technology

[0002] The production of wet-process phosphoric acid and phosphate fertilizers emits a large amount of fluorine-containing gases, which are absorbed by water to produce fluorosilicic acid as a byproduct. Its comprehensive utilization is of great significance for environmental protection, economic benefits, and resource utilization.

[0003] Fluorosilicic acid, a byproduct of wet-process phosphoric acid and phosphate fertilizer production, is typically an aqueous solution with a concentration of 10-40 wt%. Furthermore, arsenic from phosphate rock also partially enters the fluorosilicic acid, resulting in an arsenic content of approximately 10-500 mg / L. Fluorosilicic acid is a fundamental raw material for the production of many fluorine-containing compounds, such as fluorosilicates, silicon tetrafluoride, ammonium fluoride, aluminum fluoride, and hydrogen fluoride. Hydrogen fluoride, in particular, is a widely used precursor in fluorochemicals. As a key chemical material in battery manufacturing, its purity and cleanliness significantly impact battery yield and reliability. The arsenic content of the fluorosilicic acid raw material often determines the feasibility and economic viability of hydrogen fluoride production.

[0004] Direct chemical precipitation is the preferred method for arsenic removal due to its simplicity and low investment. Patent CN101033067A discloses a method for removing arsenic from fluorosilicic acid. This method involves maintaining the temperature and stirring speed of the fluorosilicic acid solution while adding a sulfide arsenic removal agent to precipitate arsenic sulfide. The precipitate is then filtered to obtain a fluorosilicic acid solution with low arsenic content. This process is simple and reduces non-mechanical losses of fluorosilicic acid. However, the addition of sulfides often introduces new elements into the system, leading to undesirable impurities in subsequent hydrogen fluoride production.

[0005] Therefore, when removing arsenic from the fluorosilicic acid solution used to prepare hydrogen fluoride, it is necessary to ensure the overall effectiveness of arsenic removal while also considering the simplification of the arsenic removal process and avoiding the introduction of new impurities. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a method for removing arsenic from fluorosilicic acid. The method employs an oxidation-precipitation process to separate arsenic from the system. Both the oxidant and precipitant used can be separated during the anhydrous hydrogen fluoride preparation process, without introducing new impurities. The fluorosilicic acid obtained after arsenic removal can be directly used to prepare high-purity and clean hydrofluoric acid products, thereby improving the yield and reliability of batteries.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0008] This invention discloses a method for removing arsenic from fluorosilicic acid, comprising the following steps:

[0009] (1) Add a coagulant to the arsenic-containing fluorosilicic acid solution to carry out a pre-coagulation reaction and generate a precipitate;

[0010] (2) Add an oxidant to the system in step (1) and react it with silicon tetrafluoride gas, let it stand and age, and filter to remove the precipitate;

[0011] (3) Add a desiccant to the filtrate from step (2), filter, and obtain an arsenic-free fluorosilicic acid solution.

[0012] In step (1) of this invention, a coagulant is added to the arsenic-containing fluorosilicic acid solution. Arsenic (V) in the system will react with the coagulant to form a precipitate, such as ferric arsenate precipitate.

[0013] The arsenic-containing fluorosilicic acid solution typically originates from the recovery of fluorine-containing tail gas during the production process. For example, in the production of wet-process phosphoric acid and phosphate fertilizer, the emitted fluorine-containing gas is absorbed by water to obtain a by-product fluorosilicic acid solution. The by-product fluorosilicic acid solution obtained by existing known wet-process phosphoric acid and phosphate fertilizer production processes, or the arsenic-containing fluorosilicic acid solution obtained by any other method, are all applicable to the method of this invention. For example, one method uses phosphate rock and concentrated sulfuric acid as raw materials to prepare crude phosphoric acid by decomposing mineral powder with inorganic acid. The emitted fluorine-containing gas is then absorbed by washing with water to obtain a fluorosilicic acid solution, which contains fluorosilicic acid, a large amount of water, and the remainder consists of free hydrogen fluoride, sulfates, free silicon, iron, lead, and other trace impurities such as arsenic. The arsenic element specifically manifests as arsenic compounds such as arsenic acid and arsenous acid, including arsenic(V) and arsenic(III).

[0014] In one specific embodiment of the present invention, the arsenic-containing fluorosilicic acid solution in step (1) is an aqueous solution of fluorosilicic acid, wherein the concentration of fluorosilicic acid is 15-20 wt%, for example 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, etc.

[0015] The arsenic content in the fluorosilicic acid solution is 10-500 mg / L, for example, 10 mg / L, 50 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, 250 mg / L, 300 mg / L, 350 mg / L, 400 mg / L, 450 mg / L, 500 mg / L, etc.

[0016] In one specific embodiment of the present invention, the coagulant in step (1) is selected from one or more of the preferred ferric sulfate salts and ferrous sulfate salts, preferably one or more of ferrous sulfate, ferric sulfate, and polyferric sulfate, and more preferably ferric sulfate;

[0017] The amount of coagulant added is 0.1-0.8% of the mass of the arsenic-containing fluorosilicic acid solution, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc., preferably 0.3-0.5%.

[0018] In one specific embodiment of the present invention, the pre-coagulation reaction is carried out by stirring and dissolving the added coagulant, and then allowing it to stand for 10-30 minutes, such as 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, etc. The reaction temperature is not specifically required, such as room temperature.

[0019] In step (2) of this invention, an oxidant is added to the system in step (1), and silicon tetrafluoride gas is introduced at the same time. Silicon tetrafluoride reacts with water to generate silicon dioxide precipitate and releases heat. During this process, arsenic (III) in the system is oxidized to arsenic (V) by the oxidant, and further reacts with the coagulant added in step (1) to generate precipitate. After the reaction is completed, the system is allowed to stand for aging, and the precipitate is removed by filtration, including silicon dioxide precipitate and precipitate generated by reaction with coagulant.

[0020] In one specific embodiment of the present invention, the oxidant in step (2) is selected from one or more of persulfate and persulfate, preferably one or more of potassium persulfate and sodium persulfate, and more preferably sodium persulfate;

[0021] The amount of oxidant added is 0.1-1% of the mass of the arsenic-containing fluorosilicic acid solution in step (1), for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, etc., preferably 0.2-0.4%.

[0022] In one specific embodiment of the present invention, the amount of silicon tetrafluoride gas added in step (2) satisfies the reaction pressure (i.e., silicon tetrafluoride gas is introduced into the reactor and the system is pressurized to a preset reaction pressure), wherein the reaction pressure is 0.1-0.5 MPa, for example 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, etc. In specific implementation, the silicon tetrafluoride gas can be fed continuously or intermittently.

[0023] In one specific embodiment of the present invention, the reaction in step (2) has a reaction temperature range of 20-60℃, such as 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, etc., preferably 30-40℃; and a reaction time of 30-60min, such as 30min, 35min, 40min, 45min, 50min, 55min, 60min, etc.

[0024] In one specific embodiment of the present invention, the static aging in step (2) is carried out for 3-8 hours, such as 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, etc., preferably 5-7 hours; there is no specific requirement for the aging temperature, such as aging at room temperature.

[0025] The filtrate obtained by filtration in step (2) of the present invention has an arsenic content reduced to below 1 mg / L, preferably below 0.5 mg / L, which is up to 99.9% lower than the content before arsenic removal.

[0026] In step (3) of the present invention, a desiccant is added to the filtrate of step (2). As the concentration of the desiccant in the solution increases, the excess coagulant and oxidant in the solution gradually precipitate out. The fluorosilicic acid solution for preparing hydrogen fluoride can be obtained by filtration. At the same time, the precipitated coagulant and oxidant are recovered by filtration precipitation.

[0027] In one specific embodiment of the present invention, the absorbent in step (3) is selected from sulfuric acid absorbents. The sulfuric acid absorbent can be concentrated sulfuric acid, fuming sulfuric acid, etc., or it can be a substance that can indirectly generate concentrated sulfuric acid, such as sulfur trioxide. It is preferred to be one or more of sulfur trioxide, fuming sulfuric acid, and concentrated sulfuric acid, and more preferably concentrated sulfuric acid.

[0028] The amount of the absorbent added is 3-9 times the mass of water in the filtrate, for example, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, etc.

[0029] In one specific embodiment of the present invention, after adding the absorbent in step (3) and mixing, the mixture is allowed to stand and then filtered. The standing time is 3-8 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, etc. There are no special requirements for the temperature; room temperature is fine.

[0030] The arsenic-free fluorosilicic acid solution obtained in step (3) of this invention mainly contains fluorosilicic acid, water, and sulfuric acid; the remainder consists of trace amounts of sulfates, free silicon, iron, lead, and other impurities, without introducing any new impurities.

[0031] The fluorosilicic acid solution contains 4-15 wt% fluorosilicic acid and 70-90 wt% sulfuric acid, with the remainder being water and trace amounts of sulfates, free silicon, iron, lead, and other impurities. This fluorosilicic acid solution can be directly used to prepare hydrogen fluoride.

[0032] In existing technologies, it is known that hydrogen fluoride can be produced by reacting fluorosilicic acid with sulfuric acid. Specifically, one preparation process for hydrogen fluoride involves: concentrating and filtering a low-concentration fluorosilicic acid solution to obtain a higher-concentration fluorosilicic acid solution; this solution then enters a reactor where it undergoes a decomposition reaction with sulfuric acid to obtain SiF4 and a mixed acid (a mixture of hydrofluoric acid and concentrated sulfuric acid). The mixed acid leaving the reactor is then evaporated, condensed, and distilled to obtain anhydrous hydrofluoric acid and fluorinated sulfuric acid. The fluorosilicic acid solution system obtained by arsenic removal in this invention includes both fluorosilicic acid and sulfuric acid, and can be directly used as the reaction raw material for this process.

[0033] Compared with existing technologies, the advantages of this invention lie in the fact that it first removes some arsenic from the fluorosilicic acid solution by forming a precipitate with pentavalent arsenic using a coagulant. Then, it concentrates and removes water by reacting silicon tetrafluoride with water to generate fluorosilicic acid. Simultaneously, the addition of the oxidant is integrated with the fluorosilicic acid concentration process, utilizing the heat and mass transfer during concentration to fully carry out the oxidation reaction without adding extra process steps and without generating new impurities after the reaction. The added oxidant and precipitant precipitate out due to differences in solubility after the addition of the desiccant, eliminating the influence of unreacted oxidant and precipitant on impurity elements in the preparation of anhydrous hydrogen fluoride.

[0034] This invention has a good arsenic removal effect. The overall process is combined with the process for preparing hydrogen fluoride from fluorosilicic acid. The process is simple, does not introduce new impurities, and has little harm. Detailed Implementation

[0035] The following embodiments will further illustrate the process provided by the present invention, but the present invention is not limited to the listed embodiments, and should also include any other known modifications within the scope of the claims of the present invention.

[0036] Analysis method:

[0037] The concentration of fluorosilicic acid was determined according to the national standard HG / T 2832-2020.

[0038] The arsenic content was determined using the national standard HJ 694-2014.

[0039] Raw materials and reagents:

[0040] Arsenic-containing fluorosilicic acid solution, fluorosilicic acid recovered as a byproduct of wet phosphoric acid concentration process;

[0041] Silicon tetrafluoride, 999%, Apco (Shanghai) Gas Co., Ltd.;

[0042] Sodium persulfate, 99%, Innochem;

[0043] Potassium persulfate, 99%, Innochem;

[0044] Ferrous sulfate heptahydrate, 99%, innochem;

[0045] Ferric sulfate, 99.9%, innochem;

[0046] Polyferric sulfate, Fe content 21%, innochem;

[0047] Concentrated sulfuric acid, 96%, inochem;

[0048] Fuming sulfuric acid, 20-30% free SO3, inochem.

[0049] Example 1

[0050] Arsenic-containing fluorosilicic acid solution: fluorosilicic acid concentration 18.2wt%, As content 328mg / L, the remainder being water and trace amounts of sulfate, free silicon, iron, and lead impurities.

[0051] (1) Take 0.5L (1.17kg / L) of arsenic-containing fluorosilicic acid solution into the PFA reactor, add 0.585g of ferric sulfate for pre-coagulation reaction, stir to dissolve and let stand at room temperature for 20min.

[0052] (2) Add 5.85 g of sodium persulfate, start stirring, introduce silicon tetrafluoride gas to adjust the system pressure to 0.1 MPa, and control the temperature at 30-40℃ for 60 min. After the reaction is complete, let the solution stand at room temperature for 8 h, filter the precipitate, and test the arsenic content of the filtrate to be 0.6 mg / L.

[0053] (3) Add 96% concentrated sulfuric acid to the filtrate to absorb water. The amount of concentrated sulfuric acid added is 6 times the mass of water in the filtrate. After thorough mixing, let stand at room temperature for 5 hours. Filter to recover excess sodium persulfate and ferric sulfate precipitate, and at the same time obtain arsenic-free fluorosilicic acid solution, in which the fluorosilicic acid content is 4wt%; the sulfuric acid content is 82wt%, and the remainder is water and trace amounts of sulfate, free silicon, iron, and lead impurities.

[0054] Example 2

[0055] Arsenic-containing fluorosilicic acid solution: fluorosilicic acid concentration 19.6wt%, As content 143mg / L, the remainder being water and trace amounts of sulfate, free silicon, iron, and lead impurities.

[0056] (1) Take 0.5L (1.18kg / L) of arsenic-containing fluorosilicic acid solution into the PFA reactor, add 4.72g of ferric sulfate for pre-coagulation reaction, stir to dissolve and let stand at room temperature for 20min.

[0057] (2) Add 0.59 g of sodium persulfate, start stirring, introduce silicon tetrafluoride gas to adjust the system pressure to 0.5 MPa, and control the temperature at 30-40℃ for 30 min. After the reaction is complete, let the solution stand at room temperature for 3 h, filter the precipitate, and test the arsenic content of the filtrate to 0.2 mg / L.

[0058] (3) Add 23% free SO3 fuming sulfuric acid to the filtrate to absorb water. The amount of fuming sulfuric acid added is 3 times the mass of water in the filtrate. After thorough mixing, let stand at room temperature for 3 hours. Filter to recover excess sodium persulfate and ferric sulfate precipitate, and at the same time obtain arsenic-free fluorosilicic acid solution, in which the fluorosilicic acid content is 14wt%; the sulfuric acid content is 79wt%, and the remainder is water and trace amounts of sulfate, free silicon, iron, lead impurities.

[0059] Example 3

[0060] Arsenic-containing fluorosilicic acid solution: fluorosilicic acid concentration 19.6wt%, As content 143mg / L, the remainder being water and trace amounts of sulfate, free silicon, iron, and lead impurities.

[0061] (1) Take 0.5L (1.18kg / L) of arsenic-containing fluorosilicic acid solution into the PFA reactor, add 1.9g of ferric sulfate for pre-coagulation reaction, stir to dissolve and let stand at room temperature for 20min.

[0062] (2) Add 2.36 g of sodium persulfate, start stirring, introduce silicon tetrafluoride gas to adjust the system pressure to 0.3 MPa, and control the temperature at 20-45℃ for 60 min. After the reaction is complete, let the solution stand at room temperature for 3 h, filter the precipitate, and test the arsenic content of the filtrate to be 0.7 mg / L.

[0063] (3) Add 23% free SO3 fuming sulfuric acid to the filtrate to absorb water. The amount of fuming sulfuric acid added is 3 times the mass of water in the filtrate. After thorough mixing, let stand at room temperature for 8 hours. Filter to recover excess sodium persulfate and ferric sulfate precipitate, and at the same time obtain arsenic-free fluorosilicic acid solution, in which the fluorosilicic acid content is 15wt%; the sulfuric acid content is 78wt%, and the remainder is water and trace amounts of sulfate, free silicon, iron, lead impurities.

[0064] Example 4

[0065] Arsenic-containing fluorosilicic acid solution: fluorosilicic acid concentration 18.2wt%, As content 328mg / L, the remainder being water and trace amounts of sulfate, free silicon, iron, and lead impurities.

[0066] (1) Take 0.5L (1.17kg / L) of arsenic-containing fluorosilicic acid solution into the PFA reactor, add 1.755g of ferric sulfate for pre-coagulation reaction, stir to dissolve and let stand at room temperature for 20min.

[0067] (2) Add 1.17 g of sodium persulfate, start stirring, introduce silicon tetrafluoride gas to adjust the system pressure to 0.1 MPa, and control the temperature at 55-60℃ for 60 min. After the reaction is complete, let the solution stand at room temperature for 7 h, filter the precipitate, and test the arsenic content of the filtrate to be 0.3 mg / L.

[0068] (3) Add 96% concentrated sulfuric acid to the filtrate to absorb water. The amount of concentrated sulfuric acid added is 6 times the mass of water in the filtrate. After thorough mixing, let stand at room temperature for 4 hours. Filter to recover excess sodium persulfate and ferric sulfate precipitate, and at the same time obtain arsenic-free fluorosilicic acid solution, in which the fluorosilicic acid content is 5wt%; the sulfuric acid content is 81wt%, and the remainder is water and trace amounts of sulfate, free silicon, iron, lead impurities.

[0069] Comparative Example 1

[0070] The method is the same as in Example 1, except that: ferric sulfate is not added in step (1), and other operations and conditions remain unchanged. In step (2), the arsenic content of the filtrate is tested and found to be 302 mg / L.

[0071] Comparative Example 2

[0072] The method is the same as in Example 1, except that sodium persulfate is not added in step (2), and other operations and conditions remain unchanged. The arsenic content of the filtrate in step (2) is 317 mg / L.

[0073] Comparative Example 3

[0074] The method is the same as in Example 1, except that in step (1), ferric sulfate is replaced with an equal mass of ferric chloride, and other operations and conditions remain unchanged. In step (2), the arsenic content of the filtrate is tested and found to be 1.3 mg / L.

[0075] The fluorosilicic acid solution obtained in step (3) introduces a new impurity, chlorine.

Claims

1. A method for removing arsenic from fluorosilicic acid, characterized in that, Includes the following steps: (1) Add a coagulant to the arsenic-containing fluorosilicic acid solution to carry out a pre-coagulation reaction and generate a precipitate; (2) Add an oxidant to the system in step (1) and react it with silicon tetrafluoride gas, let it stand and age, and filter to remove the precipitate; (3) Add a desiccant to the filtrate from step (2), filter, and obtain an arsenic-free fluorosilicic acid solution.

2. The arsenic removal method according to claim 1, characterized in that, The arsenic-containing fluorosilicic acid solution in step (1) is an aqueous solution of fluorosilicic acid, wherein the concentration of fluorosilicic acid is 15-20 wt%. Preferably, the arsenic content in the fluorosilicic acid solution is 10-500 mg / L.

3. The arsenic removal method according to claim 1, characterized in that, The coagulant in step (1) is selected from one or more of ferric sulfate and ferrous sulfate, preferably one or more of ferrous sulfate, ferric sulfate, and polyferric sulfate, and more preferably ferric sulfate; Preferably, the amount of coagulant added is 0.1-0.8% of the mass of the arsenic-containing fluorosilicic acid solution, more preferably 0.3-0.5%.

4. The arsenic removal method according to claim 1, characterized in that, In step (1), the pre-coagulation reaction is carried out by stirring and dissolving the added coagulant and then allowing it to stand for 10-30 minutes.

5. The arsenic removal method according to claim 1, characterized in that, The oxidant in step (2) is selected from one or more of persulfate and persulfate, preferably one or more of potassium persulfate and sodium persulfate, and more preferably sodium persulfate; Preferably, the amount of oxidant added is 0.1-1% of the mass of the arsenic-containing fluorosilicic acid solution in step (1), more preferably 0.2-0.4%.

6. The arsenic removal method according to claim 1, characterized in that, The amount of silicon tetrafluoride gas added in step (2) meets the reaction pressure, which is 0.1-0.5 MPa.

7. The arsenic removal method according to claim 1, characterized in that, The reaction described in step (2) is carried out at a temperature of 20-60℃, preferably 30-40℃, for a reaction time of 30-60 min; and / or The aging process described in step (2) involves a settling time of 3-8 hours, preferably 5-7 hours.

8. The arsenic removal method according to claim 1, characterized in that, The absorbent in step (3) is selected from sulfuric acid absorbents, preferably one or more of sulfur trioxide, fuming sulfuric acid, and concentrated sulfuric acid, more preferably concentrated sulfuric acid; Preferably, the amount of the desiccant added is 3-9 times the mass of water in the filtrate.

9. The arsenic removal method according to claim 1, characterized in that, After adding the absorbent in step (3) and mixing, let it stand and then filter. The standing time is 3-8 hours.

10. The arsenic removal method according to claim 1, characterized in that, The application of the arsenic-removing fluorosilicic acid solution described in step (3) in the preparation of hydrogen fluoride.