Method for removing graphite metal-based impurities by using low-concentration hydrochloric acid in cooperation with surface activity and pressure oxygen method

Abstract

The invention relates to a method for removing graphite metal-based impurities through surface activity and pressure oxygen method combined low-concentration hydrochloric acid, and aims to solve the problem that the use amount of hydrochloric acid and nitric acid is large in a process for removing natural flake graphite metal-based impurities by mixing high-concentration hydrochloric acid and nitric acid. The purification method comprises the following steps: 1, mixing natural spherical graphite, sodium hydroxide and industrial pure water, and carrying out heat preservation treatment at the temperature of 380-440 DEG C; 2, uniformly stirring the graphite subjected to alkali melting treatment and hydrochloric acid; 3, adding absolute ethyl alcohol and hydrogen peroxide into the graphite-hydrochloric acid material to obtain a mixed material; 4, introducing oxygen into the reaction kettle, and raising the temperature to 75-85 DEG C; 5, carrying out suction filtration, solid-liquid separation, washing and drying on the acid-leached material; and 6, carrying out demagnetization treatment. According to the method, hydrogen peroxide and high-pressure oxygen are added to replace low-concentration nitric acid to serve as oxidizing agents, a small amount of absolute ethyl alcohol and high pressure of 10 atmospheric pressure oxygen are added synchronously, and the content of purified spherical graphite iron-based impurities can be controlled within 0.002%.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical purification of natural graphite, and particularly relates to a process for removing metal-based impurities from natural spherical graphite for lithium-ion batteries. Background Art

[0002] Among the associated impurities of natural flake graphite, there are often metal-based impurities that are difficult to corrode and dissolve, such as iron trioxide, copper sulfide, titanium dioxide, chromium sesquioxide, etc. When using spherical graphite prepared from natural flake graphite as the negative electrode material for lithium-ion batteries, the content of some metal elements is usually required to be below 0.002%, such as iron, chromium, nickel, copper, etc. At present, most enterprises carry out step-by-step purification of the spherical graphite prepared from this natural flake graphite. First, hydrofluoric acid or molten sodium hydroxide is used to remove silicon impurities, and then a mixture of hydrochloric acid and nitric acid with a higher concentration is used to remove metal-based impurities. The two acids are usually mixed in the ratio of "aqua regia", that is, the molar ratio of concentrated hydrochloric acid to concentrated nitric acid is 3:1, and the acid leaching is carried out according to the mass ratio of the mixed acid to graphite of 1.5:1. Usually, after acid leaching at 80 °C for 8 h, washing is carried out. Although the advantage of the above process is that the metal element content is relatively easy to control within 0.002%, the disadvantages are that the consumption of hydrochloric acid and nitric acid is large, the cost is high, and the total amount of ammonia nitrogen in the wastewater is not easy to control. Therefore, the large-scale production of high-purity spherical graphite prepared from natural flake graphite for lithium-ion batteries is restricted. Therefore, developing a method for effectively removing metal-based impurities from graphite by the synergistic effect of surface activity and lower-concentration hydrochloric acid is one of the technologies urgently needed in the industry. Summary of the Invention

[0003] The purpose of the present invention is to solve the problems of large consumption of hydrochloric acid and nitric acid and high cost in the process of using a mixture of high-concentration hydrochloric acid and nitric acid to remove metal-based impurities from natural flake graphite, and to provide a method for removing metal-based impurities from graphite by the synergistic effect of surface activity and pressure oxygen method with lower-concentration hydrochloric acid.

[0004] The method for removing metal-based impurities from graphite by the synergistic effect of surface activity and pressure oxygen method with lower-concentration hydrochloric acid of the present invention is realized according to the following steps:

[0005] 1. Mix natural spherical graphite, flaky sodium hydroxide and industrial pure water according to a mass ratio of 1:(0.15 - 0.25):0.4, and carry out heat preservation treatment at a temperature of 380 - 440 °C under nitrogen protection. After washing and drying, the graphite after alkali fusion treatment is obtained;

[0006] 2. Put the graphite after alkali fusion treatment and 10% hydrochloric acid with a mass concentration of 1:1.4 - 1.6 into an (anti-corrosion type) closed reaction kettle, and stir evenly to obtain a graphite-hydrochloric acid material;

[0007] III. Add absolute ethanol and hydrogen peroxide with a mass concentration of 7.5% to the graphite-hydrochloric acid material according to a mass ratio of 10% hydrochloric acid, absolute ethanol, and hydrogen peroxide with a mass concentration of 7.5% being 10:(0.4 - 0.6):(0.08 - 0.14) to obtain a mixed material;

[0008] IV. Place the mixed material in a closed reaction kettle, introduce oxygen into the closed reaction kettle to make the pressure of the closed reaction kettle reach 8 - 12 atm, heat the closed reaction kettle to 75 - 85 °C, cool down and relieve pressure after heat preservation treatment, and collect the material after acid leaching;

[0009] V. Filter the acid-leached material to separate the solid and liquid. Wash the obtained filter cake with industrial pure water multiple times until the pH of the washing liquid of the final filter cake is 5 - 6, and obtain the washed graphite after drying;

[0010] VI. Perform demagnetization treatment on the washed graphite to obtain purified graphite.

[0011] In the method for removing graphite metal-based impurities by the surface activity synergistic pressure oxygen method with a lower concentration of hydrochloric acid in the present invention, a single lower concentration of about 10% hydrochloric acid is used as the acid leaching solution. By adding hydrogen peroxide and high-pressure oxygen to replace low-concentration nitric acid as the oxidant, peroxyhydrochloric acid with stronger corrosiveness is generated. Synchronously, a small amount of absolute ethanol is added as a surfactant and the high-pressure corrosion effect of 10 atmospheric pressures of oxygen. After stirring at 80 °C for 8 h and then washing, the content of iron-based impurities in the purified spherical graphite can be controlled within 0.002%. Compared with the acid leaching solution prepared from existing concentrated hydrochloric acid and concentrated nitric acid, the hydrochloric acid consumption is lower, so it is easier to realize large-scale, green, and low-cost production of qualified natural spherical graphite anode materials for lithium ions. Description of the Drawings

[0012] Figure 1 It is an elemental surface scanning picture of the purified spherical graphite in the example. Detailed Embodiments

[0013] Detailed Embodiment 1: The method for removing graphite metal-based impurities by the surface activity synergistic pressure oxygen method with a lower concentration of hydrochloric acid in this embodiment is implemented according to the following steps:

[0014] I. Mix natural spherical graphite, flaky sodium hydroxide, and industrial pure water according to a mass ratio of 1:(0.15 - 0.25):0.4, and perform heat preservation treatment at a temperature of 380 - 440 °C under nitrogen protection. After washing and drying, obtain the graphite after alkali fusion treatment;

[0015] II. Put the graphite after alkali fusion treatment and 10% hydrochloric acid according to a mass ratio of 1:1.4 - 1.6 into an (anti-corrosion type) closed reaction kettle, and stir evenly to obtain a graphite-hydrochloric acid material;

[0016] III. Add absolute ethanol and 7.5% hydrogen peroxide solution by mass ratio of 10% hydrochloric acid, absolute ethanol and 7.5% hydrogen peroxide solution being 10:(0.4 - 0.6):(0.08 - 0.14) to the graphite-hydrochloric acid material to obtain a mixed material;

[0017] IV. Place the mixed material in a closed reaction kettle, introduce oxygen into the closed reaction kettle to make the pressure of the closed reaction kettle reach 8 - 12 atm, heat the closed reaction kettle to 75 - 85 °C, cool down and relieve pressure after heat preservation treatment, and collect the material after acid leaching;

[0018] V. Filter the acid-leached material to separate solid and liquid, wash the obtained filter cake with industrial pure water for multiple times until the pH of the washing liquid of the final filter cake is 5 - 6, and obtain the washed graphite after drying;

[0019] VI. Perform demagnetization treatment on the washed graphite to obtain purified graphite.

[0020] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that the fixed carbon content of the natural spherical graphite in Step I is 93% - 95%.

[0021] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 or 2 is that heat preservation treatment is carried out at a temperature of 400 °C for 1 - 1.5 h in Step I.

[0022] Specific Embodiment 4: The difference between this embodiment and any one of Specific Embodiments 1 to 3 is that the graphite after alkali fusion treatment and 10% hydrochloric acid are put into an (anti-corrosion type) closed reaction kettle by a mass ratio of 1:1.5 in Step II.

[0023] Specific Embodiment 5: The difference between this embodiment and Specific Embodiment 4 is that absolute ethanol and 7.5% hydrogen peroxide solution are added to the graphite-hydrochloric acid material by a mass ratio of 10% hydrochloric acid, absolute ethanol and 7.5% hydrogen peroxide solution being 10:0.5:0.1 in Step III.

[0024] Specific Embodiment 6: The difference between this embodiment and any one of Specific Embodiments 1 to 5 is that oxygen is introduced into the closed reaction kettle to make the pressure of the closed reaction kettle reach 10 atm in Step IV.

[0025] Specific Embodiment 7: The difference between this embodiment and any one of Specific Embodiments 1 to 6 is that the closed reaction kettle is heated to 75 - 85 °C and heat preservation treatment is carried out for 8 - 12 h in Step IV.

[0026] Specific Embodiment 8: The difference between this embodiment and any one of Specific Embodiments 1 to 7 is that the drying temperature in Step V is 400 - 450 °C.

[0027] Embodiment Nine: The difference between this embodiment and any one of Embodiments One to Eight is that in Step Six, the washed graphite is subjected to dry demagnetization treatment.

[0028] Embodiment Ten: The difference between this embodiment and any one of Embodiments One to Nine is that during the demagnetization treatment in Step Six, the medium induction magnetic field is controlled to be 12000 - 15000 GS.

[0029] Example: The method for removing metal - based impurities from graphite by the synergistic effect of surfactant and pressurized oxygen with low - concentration hydrochloric acid is implemented according to the following steps:

[0030] I. Mix natural spherical graphite, flaky sodium hydroxide, and industrial pure water according to a mass ratio of 1:0.2:0.4, carry out heat preservation treatment at a temperature of 400 °C for 1 h under nitrogen protection, and obtain the graphitized product after alkali fusion treatment through cleaning and drying;

[0031] This step uses molten sodium hydroxide to remove silicon - based impurities, so that metal - based impurities are fully exposed, facilitating the next - step large - area contact and reaction between the acid leaching solution and the metal - element impurities contained in the spherical graphite.

[0032] II. Put the graphitized product after alkali fusion treatment and 10% hydrochloric acid by mass ratio of 1:1.5 into an anti - corrosion sealed reaction kettle, and mechanically stir evenly at a speed of 200 r / min to obtain a graphite - hydrochloric acid material. This step minimizes the concentration and dosage of hydrochloric acid.

[0033] III. Add absolute ethanol and 7.5% hydrogen peroxide by mass ratio of 10% hydrochloric acid, absolute ethanol, and 7.5% hydrogen peroxide of 10:0.5:0.1 to the graphite - hydrochloric acid material to obtain a mixed material.

[0034] IV. Introduce oxygen into the sealed reaction kettle to make the pressure of the sealed reaction kettle reach 10 atm, heat the sealed reaction kettle to 80 °C, keep it warm for 8 h, then cool down and release the pressure, and collect the material after acid leaching.

[0035] V. Filter the acid - leached material to separate the solid and liquid. The obtained filter cake is washed with industrial pure water multiple times until the pH of the washing liquid of the final filter cake is 6, and then dried to obtain the washed graphite; during the drying process, the material is placed in a crucible made of non - metal materials such as alumina and quartz to avoid corrosion of the metal crucible and secondary introduction of metal - based impurities. This step ensures that the chlorides generated from the residual metal - based impurities in the graphite are minimized or re - introduction of impurities is avoided.

[0036] VI. Carry out dry demagnetization treatment on the washed graphite, and control the medium induction magnetic field to reach above 12000 GS during the demagnetization process to obtain purified graphite.

[0037] Table 1 below lists the contents of impurity elements on graphite after purification of the acid leaching solution in this embodiment. Table 2 is a table showing the influence of impurity element contents after purification under different process conditions.

[0038] Table 1 Contents of impurity elements after purification of the acid leaching solution in this embodiment (μg / g)

[0039]

[0040] Table 2 Table showing the influence of each factor in this embodiment on the content of impurity elements after purification (μg / g)

[0041]

[0042] Natural flake graphite undergoes a process of multiple grinding and selection. Most impurities adhere to the graphite surface in the form of fine particle flakes. Crystalline graphite itself is relatively soft and is prone to contacting the surface of impurities during abrasion and collision, thus coating the impurities. This coating film-like multi-layer graphene forms a protection for the impurities, has strong hydrophobicity and prevents the leaching solution from contacting the impurities. During the preparation of spherical graphite, some impurity particles will be further encapsulated. In the acid leaching process of the present invention, a surfactant, hydrogen peroxide, and a closed pressurization process are added. Among them, the role of the surfactant is to promote the hydrophilicity of the graphite film on the surface of the impurities, and to push the dilute hydrochloric acid to slowly penetrate along the cracks between the graphite impurities and the graphite film coating, thereby improving the leaching of ferromagnetic impurity minerals. The role of adding hydrogen peroxide is as follows: First, the addition of H 2 O 2 significantly increases the self-corrosion potential and current density, the corrosion tendency is aggravated, and the metal will not be passivated; second, the hydrogen evolution potential shifts negatively and the limiting diffusion current increases. The addition of H 2 O 2 increases the effective oxygen concentration and promotes the cathodic reduction rate, thereby accelerating the corrosion of ferromagnetic impurities. The role of closed pressurization is as follows: First, the high-pressure state delays the decomposition of hydrogen peroxide; second, it further promotes hydrogen peroxide to effectively convert the sulfur in metal sulfide minerals into sulfur dioxide, and under a certain pressure, it becomes sulfurous acid or salt; third, oxygen is pressurized at 10 atmospheres during the acid leaching process, which not only strengthens the oxidation effect, but also promotes the leaching solution to further penetrate along the corrosion cracks. This plays a dual role with the addition of the surfactant and promotes the leaching of metals and oxides. The elemental surface scanning pictures of the purified spherical graphite are as Figure 1 shown. The addition of a surfactant, hydrogen peroxide, and a closed pressurization process during the acid leaching process can replace the acid leaching solution of spherical graphite prepared from existing concentrated hydrochloric acid and concentrated nitric acid. In particular, the contents of ferromagnetic impurities such as chromium and nickel can be controlled to less than 0.002%.

Claims

1. A method for removing graphite metal-based impurities by using a surface active agent in combination with a pressurized oxygen method and a relatively low concentration of hydrochloric acid, characterized in that The method for removing graphite metal-based impurities by using low-concentration hydrochloric acid with the surface active agent in cooperation with the pressure oxygen method is implemented according to the following steps:

1. Mix natural spherical graphite, flake sodium hydroxide and industrial pure water in a mass ratio of 1: (0.15-0.25): 0.4, perform heat preservation treatment at a temperature of 380-440°C under nitrogen protection, and obtain alkali-melted graphite after washing and drying; 2. Put the alkali-melted graphite and 10% hydrochloric acid in a closed reactor at a mass ratio of 1:1.4-1.6, and stir evenly to obtain a graphite-hydrochloric acid material; 3. Adding anhydrous ethanol and 7.5% hydrogen peroxide to the graphite-hydrochloric acid material in a mass ratio of 10% hydrochloric acid, anhydrous ethanol and 7.5% hydrogen peroxide to obtain a mixed material; 4. The mixed material is placed in a closed reactor, oxygen is introduced into the closed reactor to make the pressure of the closed reactor reach 8-12 atm, the closed reactor is heated to 75-85°C, the temperature is lowered and the pressure is released after heat preservation treatment, and the material after acid leaching is collected; 5. The acid-leached material is filtered to separate the solid and liquid, and the obtained filter cake is washed with industrial pure water for multiple times until the pH of the washing liquid of the filter cake is 5-6, and the washed graphite is obtained after drying; 6. Demagnetize the washed graphite to obtain purified graphite.

2. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressure oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that The fixed carbon content of the natural spherical graphite in step 1 is 93% to 95%.

3. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressure oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that In step 1, the temperature is kept at 400° C. for 1 to 1.5 hours.

4. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressurized oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that In step 2, the graphite after alkali melting treatment and hydrochloric acid with a mass concentration of 10% are placed in a closed reactor at a mass ratio of 1:1.

5.

5. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressure oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that In step 3, anhydrous ethanol and 7.5% hydrogen peroxide are added to the graphite-hydrochloric acid material in a mass ratio of 10:0.5:0.1 among 10% hydrochloric acid, anhydrous ethanol and 7.5% hydrogen peroxide.

6. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressurized oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that In step 4, oxygen is introduced into the closed reactor to make the pressure of the closed reactor reach 10 atm.

7. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressurized oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that In step 4, the temperature of the sealed reactor is raised to 75-85° C. and kept warm for 8-12 hours.

8. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressure oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that The drying temperature in step 5 is 400-450°C.

9. The method for removing graphite metal-based impurities by using a surface active agent in cooperation with a pressurized oxygen method and a relatively low concentration of hydrochloric acid according to claim 1, characterized in that In step six, the washed graphite is subjected to dry demagnetization treatment.

10. The method for removing graphite metal-based impurities by using surface active agent in cooperation with pressure oxygen method and low concentration hydrochloric acid according to claim 9, characterized in that In step 6, during the demagnetization process, the medium induced magnetic field is controlled to be 12000-15000GS.

Images