Method for catalyzing high-efficiency hydrolysis of hexafluorophosphate radicals in wastewater by high-valence metal cations

Abstract

The invention discloses a method for catalyzing high-efficiency hydrolysis of hexafluorophosphate radicals in wastewater by high-valence metal cations, which comprises the following steps of: enabling PF6 <-> to form a Lewis acid-base pair intermediate which is easily attacked by nucleophilic substances (water) by utilizing the characteristic that the high-valence metal cations in water have strong Lewis acidity and can be combined with fluorine atoms on the PF6 <->; the high-temperature hydrolysis reaction energy barrier of PF6 <-> is greatly reduced, preliminary pyrolysis of PF6 <-> is realized, a large amount of H < + > is generated, then, the H < + > generated by hydrolysis interferes with the electronic structure of PF6 <-> due to the strong polarization effect of the H < + >, the thermal stability of PF6 <-> can be further reduced, and high-valence metal cations and H < + > are synergistically catalyzed, so that efficient pyrolysis of hexafluorophosphate radicals is realized. The method provided by the invention has the advantages of high hydrolysis efficiency, low operation cost, no need of extreme acid-base environment, low equipment corrosion, easy realization of engineering application and the like.

Description

Technical Field

[0001] The invention relates to a method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations, and belongs to the technical field of sewage treatment. Background Art

[0002] Lithium hexafluorophosphate (LiPF 6 ) are widely used in new energy related industries involving lithium batteries due to their excellent electrochemical stability. With the vigorous development of the new energy industry, the environmental problems associated with its upstream and downstream industrial chains are becoming increasingly prominent. The German Helmholtz Center for Environmental Research has detected PF 6 - , combined with the PF confirmed by relevant toxicological studies 6 - It has extremely high biological toxicity and environmental persistence, and was identified as a new type of persistent and mobile chemical pollutant in 2021.

[0003] Due to PF 6 - The extremely high chemical stability makes traditional oxidation methods difficult to work, and its high biological toxicity also limits the application of traditional biochemical methods. 6 - The treatment process of hydrolysis is mostly acid pyrolysis, and its core principle is PF 6 - Due to its inherent property of reduced stability under high temperature and high pressure conditions, PF 6 - Hydrolyze into phosphate and fluoride ions. However, this type of process has high requirements on the corrosion resistance of the disposal equipment, and also requires additional treatment of highly toxic HF vapor, which leads to high operation and maintenance costs and harsh process operating conditions.

[0004] For this type of fluorinated inorganic anions with extremely high chemical stability (such as: PF 6 - , BF 4 - , AsF 6 - ), some studies have shown that the stability of the intermediate can be effectively reduced by constructing Lewis acid-base pairs. Theoretically, with the help of the strong Lewis acidity of high-valent metal cations, the PF 6 - One or more fluorine atoms of the PF bond to form Lewis acid-base intermediates, thereby reducing its thermal stability and achieving PF 6 - Initial pyrolysis produces a large amount of H + , then, the H +Due to its strong polarization effect, it interferes with PF 6 - The electronic structure can further reduce PF 6 - Thermal stability to achieve PF 6 - Efficient hydrolysis.

[0005] Based on this, the present invention develops a method for catalyzing PF in wastewater with high-valent metal cations. 6 - Highly efficient hydrolysis process, without additional pH adjustment, using the strong Lewis acidity of high-valent metal cations and H + Polarization effect, effectively reducing PF 6 - The pyrolysis energy barrier is reduced to achieve Lewis efficient pyrolysis. Compared with the traditional pyrolysis process, this method can effectively solve the PF 6 - It overcomes key challenges such as difficulty in hydrolysis and high requirements for equipment corrosion resistance, while also having the advantages of being environmentally friendly, economical and efficient, easy to operate, and easy to implement in engineering applications. Summary of the invention

[0006] For the existing wastewater hexafluorophosphate (PF 6 - ) In order to solve the problems of severe equipment corrosion and high treatment cost in the pyrolysis treatment process, the present invention provides a method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations, which has the advantages of being environmentally friendly, economical and efficient, simple to operate, and easy to implement in engineering applications.

[0007] The technical solution provided by the present invention to solve the above problems is: a method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations, the hydrolysis method comprising the following steps:

[0008] Step (1): Add PF 6 - The wastewater and high-valent metal cation solution are added into the magnetic stirring reactor, and the magnetic stirring reactor is placed in the heating jacket and the magnetic stirring is turned on to carry out high-temperature pyrolysis reaction. 6 - Form a Lewis acid-base metastable intermediate, reducing PF 6 - The thermal stability of 6 - The pyrolysis energy barrier is 6 - Initial pyrolysis;

[0009] Step (2): During the high-temperature pyrolysis process in the magnetic stirring reactor, water acts as a nucleophilic substance and reacts with the Lewis acid-base metastable intermediate formed in step (1) to promote the PF 6 - hydrolysis, while generating a large amount of H + ;

[0010] Step (3): H generated in step (2) + The pH value of the reaction environment in the magnetic stirring reactor is regulated. + The strong polarization effect interferes with PF 6 - electronic structure, further weakening the PF 6 - thermal stability, thereby reacting with H through high-valent metal Lewis acid-base intermediates + Strong polarization effect on PF 6 - Synergistic effect of efficient hydrolysis, ultimately achieving PF 6 -Hydrolysis.

[0011] Preferably, in step (2), the reaction equation of water and Lewis acid-base on the metastable intermediate is as shown in equation (1):

[0012]

[0013] Preferably, in step (3), H + Interference PF 6 The equations for the electronic structure of - are shown in equations (2), (3), and (4):

[0014]

[0015] HF-PF 5 →PF 5 +HF (3)

[0016]

[0017] Preferably, the magnetic stirring reactor has + There is no need to adjust the pH value of the water environment in the magnetic stirring reactor.

[0018] Preferably, in step (1), the amount of metal cations added to the high-valent metal cation solution is 50 to 100 mM, and the high-valent metal cations preferably include Al 3+ 、Zr 4+ Sn 4+ , Fe 3+ 、Ti 4+ At least one of .

[0019] Preferably, in step (1), PF 6- The concentration is 10~12mM.

[0020] Preferably, in step (1), the temperature range of high-temperature pyrolysis in the magnetic stirring reactor is 60 to 100° C., and the time of high-temperature pyrolysis is 3 to 6 hours.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1. The present invention uses high-valent metal cations and H + Synergistic catalysis, thus achieving PF 6 - Highly efficient pyrolysis without the need to add highly concentrated inorganic acid greatly reduces the corrosiveness to processing equipment.

[0023] 2. The metal salt used in the present invention is cheap and readily available, economical and efficient, and suitable for the requirements of large-scale production. In addition, the present invention has the advantages of simple process, simple operation, stable operation, and easy engineering application. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The fluoride ion and phosphate concentration diagrams of different reaction solutions before and after pyrolysis of the original solution of the present invention;

[0025] Figure 2 The low-valent metal cation catalyzed PF 6 - Pyrolysis performance comparison chart;

[0026] Figure 3 The high-valent metal cation catalyzes PF 6 - Pyrolysis performance comparison chart;

[0027] Figure 4 The high-valent metal cation and hexafluorophosphate (PF 6 - ) Schematic diagram of the formation of a Lewis acid-base pair metastable intermediate. DETAILED DESCRIPTION

[0028] This section will describe in detail the specific embodiments of the present invention. The experimental methods in the following embodiments are conventional methods unless otherwise specified. The purpose of the drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but they should not be understood as limiting the scope of protection of the present invention.

[0029] A preferred embodiment of the present invention is a method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations, the hydrolysis method comprising the following steps:

[0030] Step (1): Add PF 6 -wastewater and high-valent metal cation solution are added into a magnetic stirring reactor, and the magnetic stirring reactor is placed in a heating jacket and magnetic stirring is turned on for high-temperature pyrolysis. 6 -Forming Lewis acid-base metastable intermediates, reducing PF 6 - Thermal stability, i.e. reducing PF 6 - pyrolysis energy barrier, achieving PF 6 - Preliminary pyrolysis, e.g. Figure 4 As shown;

[0031] Step (2): During the high-temperature pyrolysis process in the magnetic stirring reactor, water acts as a nucleophilic substance and reacts with the Lewis acid-base metastable intermediate formed in step (1) to promote the PF 6 - hydrolysis, while generating a large amount of H + ;

[0032] Step (3): H generated in step (2) + The pH value of the reaction environment in the magnetic stirring reactor is regulated. + The strong polarization effect interferes with PF 6 - electronic structure, further weakening the PF 6 - thermal stability, thereby reacting with H through high-valent metal Lewis acid-base intermediates + Strong polarization effect on PF 6 - Synergistic effect of efficient hydrolysis, ultimately achieving PF 6 -Hydrolysis.

[0033] Furthermore, in step (2), the reaction equation of water and Lewis acid-base on the metastable intermediate is shown in equation (1):

[0034]

[0035] Furthermore, in step (3), H + Interference PF 6 The equations for the electronic structure of - are shown in equations (2), (3), and (4):

[0036]

[0037] HF-PF 5 →PF 5 +HF (3)

[0038]

[0039] Furthermore, due to the H +There is no need to adjust the pH value of the water environment in the magnetic stirring reactor.

[0040] Furthermore, in step (1), the amount of metal cation added to the high-valent metal cation solution is 50 to 100 mM, and the high-valent metal cation is preferably Al 3+ 、Zr 4+ Sn 4+ , Fe 3+ 、Ti 4+ At least one of .

[0041] Furthermore, in step (1), PF in the wastewater 6 - The concentration is 10~12mM.

[0042] Furthermore, in step (1), the temperature range of high-temperature pyrolysis in the magnetic stirring reactor is 60 to 100° C., and the time of high-temperature pyrolysis is 3 to 6 hours.

[0043] The hydrolysis principle of the present invention is to utilize the strong Lewis acidity of high-valent metal cations in water to react with PF 6 - The fluorine atoms on the PF4 6 - The Lewis acid-base intermediate is formed, which is susceptible to attack by nucleophilic substances (water), greatly reducing the PF 6 - High temperature hydrolysis reaction energy barrier and realization of PF 6 - Initial pyrolysis produces a large amount of H + , then, the H + Due to its strong polarization effect, it interferes with PF 6 - The electronic structure can further reduce PF 6 - Thermal stability of high-valent metal cations and H + Synergistic catalysis, thus achieving efficient thermal decomposition of hexafluorophosphate.

[0044] Example 1

[0045] Formulated PF 6 - The initial concentration is 10mM simulated reaction solution. After preparation, the fluoride ion and phosphate concentrations in the initial reaction solution are tested. Then, 50mL of 10mM hexafluorophosphate solution is transferred into the magnetic stirring reactor with a measuring cylinder. The magnetic stirring reactor is placed in a heating jacket, magnetic stirring is turned on, and the temperature is increased. After heating to 100 degrees Celsius, the reaction is carried out for 6 hours. After the reaction is completed, the reaction is cooled to room temperature, and the fluoride ion and phosphate concentrations in the mother liquor of the reaction solution after pyrolysis are tested.

[0046] like Figure 1 As shown, the concentrations of fluoride ions and phosphates in the reaction liquid mother liquor do not change much before and after the pyrolysis, which means that only about 10% of the hexafluorophosphate is completely hydrolyzed after the pyrolysis is completed. This shows that the hexafluorophosphate is very stable and is difficult to be attacked by water and pyrolyzed without the addition of high-valent metal cations to form Lewis acid-base metastable intermediates.

[0047] Example 2

[0048] Use a graduated cylinder to measure five 50 mL sets of 10 mM PF 6 - The solution was added into three 100 mL beakers, and different low-valent metal ion salts (Sn 2+ , Ca 2+ Mg 2+ ), so that the concentration of low-valent metal cations in the reaction solution is 50mM. After the low-valent metal ion salt in the beaker is completely dissolved, it is poured into three magnetic stirring reactors respectively, and the magnetic stirring reactor is placed in a heating jacket to turn on the magnetic stirring and heat up. After heating to 100 degrees Celsius, the constant temperature reaction is carried out for 6 hours. After the reaction is completed, it is cooled to room temperature, and the concentrations of fluoride ions and phosphates in the reaction solution with low-valent metal cations added after pyrolysis are tested.

[0049] like Figure 2 As shown, Ca 2+ Mg 2+ Effect of two low-valent metal cations on PF 6 - The pyrolysis of PF has no catalytic effect, which may be due to the inability of low-valent metal cations to react with PF 6 - Formation of metastable intermediates, however, Sn 2+ Because it is easily oxidized to Sn by fluorine in acidic solution 4+ , so it can play the same role as directly adding Sn 4+ Similar catalytic effects, this experiment proves the catalytic effect of high-valent metal cations on the high-temperature thermal decomposition of hexafluorophosphate.

[0050] Example 3

[0051] Use a graduated cylinder to measure five 50 mL sets of 10 mM PF 6 - The solution was added into five 100 mL beakers, and five common high-valent metal ion salts (Al 3+ 、Zr 4+ Sn 4+ , Fe 3+ 、Ti 4+), so that the concentration of high-valent metal cations in the reaction solution is 50mM. After the high-valent metal ion salt in the beaker is completely dissolved, it is poured into five magnetic stirring reactors respectively, and the magnetic stirring reactors are placed in a heating jacket to turn on magnetic stirring and heat up. After heating to 100 degrees Celsius, the reaction is carried out for 6 hours. After the reaction is completed, it is cooled to room temperature, and the concentrations of fluoride ions and phosphates in the reaction solution with high-valent metal cations added after pyrolysis are tested.

[0052] like Figure 3 As shown in Figure 2, after adding high-valent metal cations and completing the pyrolysis reaction, a large amount of hydrolysis of hexafluorophosphate in the reaction solution occurs. Among the five selected high-valent metal cations, the catalytic performance order is: Al 3+ >Fe 3+ >Sn 4+ >Ti 4+ >Zr 4 + , Al 3+ The catalytic effect of PF 6 - The hydrolysis amount of Zr can reach 94%. 4+ The catalytic effect of PF was the weakest, only 56%. 6 - The hydrolysis catalytic performance of Fe may be related to the content of metal cations in the reaction medium. 3+ Sn 4+ 、Ti 4+ 、Zr 4+ In the fourth group, there was obvious precipitation in the solution after the reaction was completed, which was presumed to be hydroxide precipitation. These precipitations were the main reason for the poor hydrolysis performance. Under the high-temperature hydrothermal environment, the formation of insoluble hydroxide precipitation led to a reduction in the free high-valent metal cations, thereby causing the catalytic performance to decline. The experimental results show that high-valent metal cations can effectively catalyze the efficient thermal decomposition of hexafluorophosphate in water. Combined with the results obtained in Example 1, it can be further explained that in the presence of high-valent metal cations and H + In the process of synergistic catalytic thermal decomposition of hexafluorophosphate, high-valent metal cations play a major catalytic role.

[0053] The above is only an explanation of the best embodiment of the present invention, which should not be construed as a limitation on the claims. The present invention is not limited to the above embodiments, and all changes made within the protection scope of the independent claims of the present invention are within the protection scope of the present invention.

Claims

1. A method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations, characterized in that: The hydrolysis method comprises the following steps: Step (1): Add PF6 - The wastewater and high-valent metal cation solution were added into the magnetic stirring reactor, and the magnetic stirring reactor was placed in the heating jacket and the magnetic stirring was turned on for high-temperature pyrolysis reaction. - Forming Lewis acid-base metastable intermediates, reducing PF6 - The thermal stability of PF6 - The pyrolysis energy barrier of PF6 - Initial pyrolysis; Step (2): During the high-temperature pyrolysis process in the magnetic stirring reactor, water acts as a nucleophilic substance and reacts with the Lewis acid-base metastable intermediate formed in step (1) to promote the PF6 - Hydrolysis, while generating a large amount of H + ; Step (3): H generated in step (2) + The pH value of the reaction environment in the magnetic stirring reactor is regulated. + The strong polarization effect interferes with PF6 - The electronic structure of PF6 - The thermal stability of the Lewis acid-base intermediates is + Strong polarization effect on PF6 - The synergistic effect of efficient hydrolysis ultimately achieves PF6 - of hydrolysis.

2. The method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations according to claim 1, characterized in that: In step (2), the reaction equation of water and Lewis acid-base on the metastable intermediate is shown in equation (1):

3. The method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations according to claim 1, characterized in that: In step (3), H + Interference PF6 - The equations for the electronic structure of are shown in equations (2), (3), and (4): HF-PF5→PF5+HF (3) 4. The method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations according to claim 1, characterized in that: The magnetic stirring reactor is + There is no need to adjust the pH value of the water environment in the magnetic stirring reactor.

5. The method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations according to claim 1, characterized in that: In step (1), the dosage of the metal cation in the high-valent metal cation solution is 50-100 mM.

6. The method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations according to claim 1, characterized in that: In step (1), PF6 - The concentration is 10~12mM.

7. The method for efficiently hydrolyzing hexafluorophosphate in wastewater catalyzed by high-valent metal cations according to claim 1, characterized in that: In step (1), the temperature range of high-temperature pyrolysis in the magnetic stirring reactor is 60 to 100° C., and the time of high-temperature pyrolysis is 3 to 6 hours.

Images