A comprehensive treatment method for the process tail gas of lithium hexafluorophosphate

Abstract

The present invention relates to a comprehensive treatment method for the process tail gas of lithium hexafluorophosphate, comprising the following steps: 1) absorbing the tail gas containing phosphorus fluoride and phosphorus chloride with water to obtain a mixed acid containing hydrogen chloride, hydrogen fluoride and phosphoric acid; 2) electrolyzing the mixed acid obtained in step 1) until the hydrogen chloride in the mixed acid is completely electrolyzed to obtain a solution; 3) heating the solution obtained in step 2) to cause the water and hydrofluoric acid to escape by heat, and then obtaining hydrofluoric acid after cooling. Through electrolysis and phosphorus removal treatment, fluorine, chlorine and phosphorus in the process tail gas of lithium hexafluorophosphate are fully separated, and hydrofluoric acid and chlorine are respectively prepared. The chlorine can be recycled for the preparation of phosphorus pentachloride in the production of lithium hexafluorophosphate, realizing the recycling of chlorine. At the same time, industrial-grade hydrofluoric acid can be obtained, and the added value of the product is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of tail gas treatment, and specifically relates to a comprehensive treatment method for the tail gas of the lithium hexafluorophosphate process. Background Art

[0002] Currently, the most widely used production process of lithium hexafluorophosphate in industry is the hydrogen fluoride solvent method. In this process, phosphorus pentachloride and lithium fluoride are introduced into an anhydrous hydrogen fluoride solution for reaction to obtain an anhydrous hydrogen fluoride solution of lithium hexafluorophosphate, which is then crystallized and dried to prepare lithium hexafluorophosphate. A large amount of waste gas is generated during the reaction and drying stages. The main components of the waste gas are hydrogen chloride, and there are also some phosphorus pentafluoride, nitrogen, phosphorus pentachloride, hydrogen fluoride, etc. Industrially, the waste gas is generally pressurized and condensed to separate hydrogen fluoride, and then the remaining waste gas is absorbed by water to obtain fluorine-containing hydrochloric acid for metal pickling. This tail gas treatment method has problems such as incomplete separation of fluorine and chlorine in the tail gas and low added value of the product.

[0003] Patent CN201610000694.8 discloses a method for treating the tail gas of lithium hexafluorophosphate. In this process, the tail gas is pressurized and condensed to obtain anhydrous hydrogen fluoride; the remaining tail gas is reacted with lithium chloride to further remove hydrogen fluoride; after the remaining tail gas is absorbed by water, hydrochloric acid with a low fluorine content is prepared, and the remaining gas is discharged after alkali washing. However, the reaction rate between lithium chloride and hydrogen fluoride is slow and it is a reversible reaction, still unable to completely separate hydrogen fluoride and hydrogen chloride, and the obtained hydrochloric acid product still contains hydrogen fluoride, with low product purity and low added value.

[0004] In patent CN202110829838.1, a method for separating and recovering hydrofluoric acid from a fluorine-containing mixed acid is disclosed. By utilizing the difference in redox strength between different ions, the mixed acid containing hydrofluoric acid, hydrochloric acid, and nitric acid is electrolyzed to separate Cl - 、NO3 - into gaseous chlorine and nitrogen dioxide, thereby obtaining high-quality hydrofluoric acid with fewer impurities.

[0005] However, in the present application, the tail gas to be treated also contains phosphorus, and the above method cannot complete the separation of fluorine and phosphorus. Summary of the Invention

[0006] In view of the above-mentioned defects existing in the prior art, the present invention provides a comprehensive treatment method for the tail gas of the lithium hexafluorophosphate process. The specific scheme is as follows:

[0007] A comprehensive treatment method for the tail gas of the lithium hexafluorophosphate process includes the following steps:

[0008] 1) Absorb the tail gas containing phosphorus fluoride and phosphorus chloride with water to obtain a mixed acid containing hydrogen chloride, hydrogen fluoride, and phosphoric acid;

[0009] 2) Electrolyze the mixed acid obtained in step 1) until the hydrogen chloride in the mixed acid is completely electrolyzed to obtain a solution.

[0010] 3) Heat up the solution obtained in step 2), control the temperature at 50 - 80 °C and the pressure at -0.08 MPa to -0.09 MPa, so that water and hydrofluoric acid escape by heating, and then obtain hydrofluoric acid after cooling.

[0011] Preferably, the tail gas in step 1) is generated in the lithium hexafluorophosphate production process.

[0012] In the process of preparing lithium hexafluorophosphate by the hydrogen fluoride solvent method, the main components of the tail gas are hydrogen chloride, phosphorus pentafluoride, nitrogen, hydrogen fluoride, and phosphorus pentachloride. After being absorbed by water, the following reactions occur:

[0013] PF5 + 4H2O → H3PO4 + 5HF

[0014] PCl5 + 4H2O → H3PO4 + 5HCl

[0015] A mixed acid of phosphoric acid, hydrogen fluoride, and hydrogen chloride is obtained.

[0016] During the electrolysis process, the electrolysis order of the mixed acid in the anode chamber is as follows:

[0017] Cl - >OH - >F - 、PO4 3- (No electrolysis occurs)

[0018] Electrolysis will continuously consume the hydrogen chloride in the mixed acid and leave the electrolysis chamber in the form of chlorine and hydrogen.

[0019] Monitor the gas components in the anode chamber during electrolysis. When oxygen is generated in the anode chamber, the anode starts to electrolyze OH - , generating oxygen, and thus it can be judged that the hydrogen chloride in the mixed acid has been consumed. At this time, only hydrogen fluoride and phosphoric acid remain in the mixed acid. Then, the mixed acid can obtain industrial-grade hydrogen fluoride after removing phosphorus.

[0020] Preferably, in step 2), the chlorine generated by electrolysis is used to prepare phosphorus pentachloride. Specifically, chlorine can react with phosphorus trichloride to prepare phosphorus pentachloride, and the phosphorus pentachloride is recycled for the preparation of lithium hexafluorophosphate.

[0021] Preferably, the hydrofluoric acid obtained in step 3) is used for the tail gas absorption in step 1) until high-concentration hydrofluoric acid is obtained in step 3). After three electrolysis treatments, 40% grade I hydrofluoric acid can be obtained.

[0022] Preferably, in step 1), the electrolysis of the mixed acid is carried out in the anode chamber of the electrolytic cell. The electrodes in the anode chamber are made of materials resistant to hydrofluoric acid corrosion, such as graphite electrodes and platinum electrodes.

[0023] Preferably, a diaphragm is provided between the cathode chamber and the anode chamber of the electrolytic cell. The diaphragm can separate the anode and cathode chambers, prevent ion migration, and facilitate the separation of electrolysis products.

[0024] Preferably, the diaphragm is a perfluorosulfonic acid type proton exchange membrane. This type of diaphragm is resistant to mixed acid corrosion.

[0025] Preferably, the current density during electrolysis is 100 - 500 A / m 2 . When it is lower than this range, the electrolysis speed is too slow and the efficiency is too low; when it is higher than this range, the electrolysis speed is too fast and the control difficulty is high.

[0026] Preferably, the temperature during electrolysis is 10 - 15 °C. Temperature affects the electrolysis efficiency. When it is lower than this range, the electrolysis efficiency decreases; when it is higher than this range, the improvement of electrolysis efficiency is not obvious.

[0027] Through electrolysis and dephosphorization treatment, this process fully separates fluorine, chlorine, and phosphorus in the tail gas of the lithium hexafluorophosphate process, and prepares hydrofluoric acid and chlorine respectively. Chlorine can be recycled for the preparation of phosphorus pentachloride in the production of lithium hexafluorophosphate, realizing the recycling of chlorine. At the same time, industrial - grade hydrofluoric acid can be obtained, and the product has high added value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described below with reference to the drawings and embodiments.

[0029] Figure 1 It is a schematic flow chart of the method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present invention will be clearly described below with reference to the drawings in the embodiments of the present invention and specific embodiments. The description here is only used to explain the present invention, but not to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present invention.

[0031] Embodiment

[0032] As Figure 1 shown, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention.

[0033] The technical solutions in the present invention will be clearly and completely described below in combination with specific embodiments.

[0034] Embodiment 1

[0035] This embodiment provides a comprehensive treatment method for the tail gas of the lithium hexafluorophosphate process:

[0036] (1) Absorb the tail gas of the lithium hexafluorophosphate process with water to obtain a mixed acid containing 35% hydrogen chloride, 7% hydrogen fluoride, and 4% phosphoric acid;

[0037] (2) Feed the mixed acid obtained in step (1) into the anode chamber of the electrolytic cell. Use a perfluorosulfonic acid type proton exchange membrane to separate the anode and cathode chambers. Add water to the cathode chamber. Use carbon as the electrolytic electrodes at both the anode and cathode. Carry out electrolysis at a temperature of 10 °C and a current density of 100 A / m 2 . Chlorine is generated at the anode and hydrogen is generated at the cathode;

[0038] (3) When oxygen is detected in the anode chamber in step (2), stop the electrolysis. Heat up the solution in the anode chamber, control the temperature at 50 - 80 °C and the pressure at -0.08 MPa to -0.09 MPa, so that water and hydrofluoric acid escape by heating, and then obtain 11.5% hydrofluoric acid after cooling.

[0039] (4) Use the 11.5% hydrofluoric acid obtained in step (3) to continue absorbing the tail gas of lithium hexafluorophosphate to obtain a mixed acid containing 33.5% hydrogen chloride, 19.6% hydrogen fluoride, and 3.7% phosphoric acid.

[0040] (5) Repeat steps (2) and (3) for the mixed acid obtained in step (4) to obtain hydrofluoric acid with a concentration of 31.2%.

[0041] (6) The chlorine obtained in steps (2) and (5) can be used for the production of phosphorus pentachloride and recycled to the lithium hexafluorophosphate production process.

[0042] Example 2

[0043] This embodiment provides a comprehensive treatment method for the tail gas of the lithium hexafluorophosphate process:

[0044] (1) Absorb the tail gas of the lithium hexafluorophosphate process with water to obtain a mixed acid containing 30% hydrogen chloride, 15% hydrogen fluoride, and 8% phosphoric acid;

[0045] (2) Feed the mixed acid obtained in step (1) into the anode chamber of the electrolytic cell. Use a perfluorosulfonic acid type proton exchange membrane to separate the anode and cathode chambers. Add the mixed acid in step (1) to the cathode chamber. Use carbon as the electrolytic electrodes at both the anode and cathode. Carry out electrolysis at a temperature of 13 °C and a current density of 300 A / m 2 . Chlorine is generated at the anode and hydrogen is generated at the cathode;

[0046] (3) When oxygen is detected in the anode chamber in step (2), stop the electrolysis, heat up the solution in the anode chamber, control the temperature at 50 - 80 °C and the pressure at -0.08 MPa to -0.09 MPa, so that water and hydrofluoric acid escape due to heat, and then obtain hydrofluoric acid with a concentration of 24.0% after cooling;

[0047] (4) The chlorine gas obtained in step (2) can be used for the production of phosphorus pentachloride and recycled to the production process of lithium hexafluorophosphate.

[0048] Example 3

[0049] This example provides a comprehensive treatment method for the tail gas of the lithium hexafluorophosphate process:

[0050] (1) Absorb the tail gas of the lithium hexafluorophosphate process with water to obtain a mixed acid containing 32% hydrogen chloride, 15% hydrogen fluoride, and 3% phosphoric acid;

[0051] (2) Feed the mixed acid obtained in step (1) into the anode chamber of the electrolytic cell, separate the anode and cathode chambers with a perfluorosulfonic acid type proton exchange membrane, add water to the cathode chamber, use carbon as the electrolysis electrodes at both the anode and cathode, and perform electrolysis at a temperature of 15 °C and a current density of 500 A / m 2 under the condition, chlorine gas is generated at the anode and hydrogen gas is generated at the cathode;

[0052] (3) When oxygen is detected in the anode chamber in step (2), stop the electrolysis, heat up the solution in the anode chamber, control the temperature at 50 - 80 °C and the pressure at -0.08 MPa to -0.09 MPa, so that water and hydrofluoric acid escape due to heat, and then obtain hydrofluoric acid with a concentration of 23.1% after cooling;

[0053] (4) Use the 23.1% hydrofluoric acid obtained in step (3) to continue absorbing the tail gas of lithium hexafluorophosphate to obtain a mixed acid containing 32.4% hydrogen chloride, 26.8% hydrogen fluoride, and 2.6% phosphoric acid.

[0054] (5) Repeat step (2) and step (3) for the mixed acid obtained in step (4) to obtain hydrofluoric acid with a concentration of 41.2%, and dilute it with water to obtain 40% industrial grade hydrofluoric acid.

[0055] (6) The chlorine gas obtained in steps (2) and (5) can be used for the production of phosphorus pentachloride and recycled to the production process of lithium hexafluorophosphate.

[0056] Conduct component analysis on the hydrofluoric acid obtained in Examples 1 - 3, and the results are as follows in the table:

[0057]

[0058] It can be seen that high-purity industrial hydrofluoric acid can be recovered by this method, which can generate relatively high economic benefits.

Claims

1. A comprehensive treatment method for the process tail gas of lithium hexafluorophosphate, characterized in that, It includes the following steps: 1) Absorb the tail gas containing phosphorus fluoride and phosphorus chloride with water to obtain a mixed acid containing hydrogen chloride, hydrogen fluoride and phosphoric acid; 2) Electrolyze the mixed acid obtained in step 1) until the hydrogen chloride in the mixed acid is completely electrolyzed to obtain a solution; 3) Heat up the solution obtained in step 2), control the temperature at 50-80 °C and the pressure at -0.08 MPa to -0.09 MPa, so that water and hydrofluoric acid escape by heating, and then obtain hydrofluoric acid after cooling; The tail gas in step 1) is generated in the production process of lithium hexafluorophosphate; The hydrofluoric acid obtained in step 3) is used for the tail gas absorption in step 1) until high-concentration hydrofluoric acid is obtained in step 3).

2. The comprehensive treatment method of the process tail gas of lithium hexafluorophosphate according to claim 1, wherein: In step 2), the chlorine gas generated by electrolysis is used to prepare phosphorus pentachloride.

3. The comprehensive treatment method for the process tail gas of lithium hexafluorophosphate according to claim 1, characterized in that: In step 2), the electrolysis of the mixed acid is carried out in the anode chamber of the electrolytic cell.

4. The comprehensive treatment method for the process tail gas of lithium hexafluorophosphate according to claim 3, characterized in that: A diaphragm is provided between the cathode chamber and the anode chamber of the electrolytic cell.

5. The comprehensive treatment method for the process tail gas of lithium hexafluorophosphate according to claim 4, characterized in that: The diaphragm is a perfluorosulfonic acid type proton exchange membrane.

6. The comprehensive treatment method for the process tail gas of lithium hexafluorophosphate according to claim 3, characterized in that: The current density during electrolysis is 100-500 A / m2.

7. The comprehensive treatment method of the process tail gas of lithium hexafluorophosphate according to claim 3, characterized in that: The temperature during electrolysis is 10-15 °C.

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