A method for purifying carbon nanotubes by acid washing

Abstract

The application provides an acid pickling purification method of carbon nanotubes, and belongs to the technical field of carbon nanotube purification. The acid-resistant water-soluble dispersant is mixed with the carbon nanotubes to be purified and water to obtain modified carbon nanotube liquid; the modified carbon nanotube liquid is mixed with acid liquid to obtain purified carbon nanotube liquid; and the purified carbon nanotube liquid is subjected to solid-liquid separation, and the obtained solid material is washed to neutral and dried to obtain purified carbon nanotubes. In the acid pickling purification process, the acid-resistant water-soluble dispersant is introduced to improve the infiltration effect of the acid liquid on the carbon nanotubes, to make the acid liquid fully contact with the impurity particles, to complex transition metal impurities in the carbon nanotubes, to increase the solubility of transition metal ions in the acid liquid, to be beneficial to reducing the acid pickling frequency, to shorten the treatment period, and to be beneficial to reducing the acid liquid usage and the water usage, so that the production cost can be significantly reduced.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube purification technology, and more particularly to an acid washing purification method for carbon nanotubes. Background Technology

[0002] Carbon nanotubes are tubular one-dimensional nanomaterials, typically obtained by catalytic cracking and deposition of carbon-containing compounds, especially hydrocarbons, using transition metal catalysts. During this process, transition metal catalysts can remain in the coarse carbon nanotube powder. In applications requiring high purity, such as power batteries, a low content of transition metal impurities in the carbon nanotubes is necessary; therefore, further purification of the coarse carbon nanotube powder is required.

[0003] Currently, the mainstream purification methods fall into two main categories: chemical purification and physical purification. Chemical purification methods, such as acid washing, have unique advantages in removing transition metal impurities. For example, the reaction process is simple, generally limited to changes at the surface and interface, and less prone to forming overall structural defects. It can control the residual transition metal impurities in coarse carbon nanotube powder to within 500 ppm. However, traditional acid washing for carbon nanotubes typically requires a large amount of acid solution, a long processing time (over 15 hours), and heating (80–90°C) to achieve good results, resulting in high production costs. Furthermore, the large amount of acid used in the initial treatment requires a large amount of deionized water to wash away impurity ions and neutralize the carbon nanotube powder, generating a large amount of acidic washing solution, further increasing production and environmental costs significantly. Summary of the Invention

[0004] The purpose of this invention is to provide an acid washing and purification method for carbon nanotubes. The method provided by this invention has good acid washing and purification effect and low cost.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides an acid washing and purification method for carbon nanotubes, comprising the following steps:

[0007] (1) The carbon nanotubes to be purified, the acid-resistant water-soluble dispersant and water are mixed and modified to obtain the modified carbon nanotube solution.

[0008] (2) The modified carbon nanotube solution is mixed with acid solution and purified to obtain purified carbon nanotube solution.

[0009] (3) The purified carbon nanotube material is subjected to solid-liquid separation, and the resulting solid material is washed with water until neutral and then dried to obtain purified carbon nanotubes.

[0010] Preferably, in step (1), the length of the carbon nanotube to be purified is 10-200 μm and the outer diameter is 1-100 nm; the aggregate size of the carbon nanotube to be purified is 1-2000 μm.

[0011] Preferably, the acid-resistant water-soluble dispersant in step (1) includes one or more of polyvinylpyrrolidone, polyvinyl alcohol, hydroxyethylidene diphosphonic acid, cellulose compounds, and compounds containing polyoxyethylene ether functional groups.

[0012] Preferably, the cellulose compound in step (1) includes hydroxyethyl cellulose and / or hydroxypropyl methyl cellulose.

[0013] Preferably, the compound containing polyoxyethylene ether functional groups includes C 8-10 One or more of the following: glyceryl ester polyoxyethylene ether, fatty alcohol polyoxyethylene ether, dodecylphenol polyoxyethylene ether, and polyoxypropylene polyoxyethylene propylene glycol ether.

[0014] Preferably, in step (1), the mass ratio of the acid-resistant water-soluble dispersant to water is (5-35):100.

[0015] Preferably, in step (1), the mass ratio of the carbon nanotubes to be purified to the total mass of the acid-resistant water-soluble dispersant and water is (10-50):100.

[0016] Preferably, the temperature of the modification treatment in step (1) is 50-60°C and the time is 30-90 min; the modification treatment is carried out under stirring conditions.

[0017] Preferably, the acid solution in step (2) is hydrochloric acid, and the concentration of the hydrochloric acid is 36-38 wt%; the mass ratio of the hydrochloric acid to the modified carbon nanotube solution is 1:(4-30).

[0018] Preferably, the purification process in step (2) is carried out at a temperature of 50-60°C for 2-4 hours; the purification process is carried out under stirring conditions.

[0019] This invention provides an acid washing and purification method for carbon nanotubes, comprising the following steps: mixing the carbon nanotubes to be purified, an acid-resistant water-soluble dispersant, and water for modification treatment to obtain a modified carbon nanotube solution; mixing the modified carbon nanotube solution with an acid solution for purification treatment to obtain a purified carbon nanotube solution; performing solid-liquid separation on the purified carbon nanotube solution, washing the resulting solid material with water until neutral, and then drying to obtain purified carbon nanotubes. This invention introduces an acid-resistant water-soluble dispersant during the acid washing and purification process, which improves the wetting effect of the acid solution on the carbon nanotubes, allowing the acid solution to fully contact the impurity particles. Simultaneously, it complexes transition metal impurities in the carbon nanotubes, increasing the solubility of transition metal ions in the acid solution. This helps reduce the number of acid washing cycles, shortens the processing cycle, and also reduces the amount of acid and water used, significantly lowering production costs. The results of the examples show that, using the method provided by the present invention, the acid washing and purification time of carbon nanotubes to be purified can be shortened to less than 8 hours. The main iron impurities can be reduced from 500 ppm to less than 210 ppm after one acid washing and purification (i.e., one modification treatment and one purification treatment), and the cobalt impurities can be reduced from 105 ppm to less than 41 ppm after one acid washing and purification. The water consumption is also reduced as a result. Attached Figure Description

[0020] Figure 1 A flowchart of the acid washing and purification process for carbon nanotubes provided by this invention. Detailed Implementation

[0021] This invention provides an acid washing and purification method for carbon nanotubes, comprising the following steps:

[0022] (1) The carbon nanotubes to be purified, the acid-resistant water-soluble dispersant and water are mixed and modified to obtain the modified carbon nanotube solution.

[0023] (2) The modified carbon nanotube solution is mixed with acid solution and purified to obtain purified carbon nanotube solution.

[0024] (3) The purified carbon nanotube material is subjected to solid-liquid separation, and the resulting solid material is washed with water until neutral and then dried to obtain purified carbon nanotubes.

[0025] Figure 1 The following is a flowchart of the acid washing and purification process for carbon nanotubes provided by this invention, in conjunction with... Figure 1 The acid washing and purification method for carbon nanotubes in this invention is described in detail.

[0026] This invention involves mixing carbon nanotubes to be purified, an acid-resistant water-soluble dispersant, and water, followed by modification treatment to obtain a modified carbon nanotube slurry. In this invention, the carbon nanotubes to be purified are preferably coarse carbon nanotube powder (i.e., unpurified coarse carbon nanotube powder) prepared by catalytic cracking and deposition of carbon-containing compounds using a transition metal catalyst, and the carbon nanotubes to be purified contain residual transition metal catalyst. In this invention, the main impurities in the carbon nanotubes to be purified include iron and cobalt, with the iron content preferably being 3000–3500 ppm and the cobalt content preferably being 700–850 ppm. In this invention, the length of the carbon nanotubes to be purified is preferably 10–200 μm, more preferably 10–50 μm, and even more preferably 10–30 μm; the outer diameter is preferably 1–100 nm, more preferably 5–40 nm, and even more preferably 5–30 nm; the cluster size of the carbon nanotubes to be purified is preferably 1–2000 μm, more preferably 1–1000 μm, even more preferably 1–100 μm, and even more preferably 10–100 μm. In this invention, the coarse carbon nanotube powder prepared by catalytic cracking and deposition of carbon-containing compounds using a transition metal catalyst usually exists in the form of clusters. The cluster size of the carbon nanotubes to be purified in this invention specifically refers to the size of the clusters formed by the coarse carbon nanotube powder. In this invention, the water is preferably deionized water. In this invention, the acid-resistant water-soluble dispersant preferably includes one or more of polyvinylpyrrolidone, polyvinyl alcohol, hydroxyethylidene diphosphonic acid (HEDP), cellulose compounds, and compounds containing polyoxyethylene ether functional groups; the cellulose compounds preferably include hydroxyethyl cellulose (HEC) and / or hydroxypropyl methylcellulose (HPMC), and the compounds containing polyoxyethylene ether functional groups preferably include C 8-10 The present invention utilizes one or more of the following: glyceryl ester polyoxyethylene ether, fatty alcohol polyoxyethylene ether, dodecylphenol polyoxyethylene ether, and polyoxypropylene polyoxyethylene propylene glycol ether. Preferably, the present invention employs an acid-resistant water-soluble dispersant of the above types, which can increase the wettability of the acid solution on the carbon nanotubes, while simultaneously complexing transition metal impurities in the carbon nanotubes, increasing the solubility of transition metal ions in the acid solution, thereby improving the acid washing and purification effect; furthermore, the above-mentioned acid-resistant water-soluble dispersant can be removed by subsequent water washing without introducing new impurities.

[0027] In this invention, the preferred method for mixing the carbon nanotubes to be purified, the acid-resistant water-soluble dispersant, and water includes: dissolving the acid-resistant water-soluble dispersant in water to obtain an aqueous solution of the acid-resistant water-soluble dispersant; and mixing the aqueous solution of the acid-resistant water-soluble dispersant with the carbon nanotubes to be purified. In this invention, the preferred mass ratio of the acid-resistant water-soluble dispersant to water is (5-35):100, more preferably (10-30):100, and even more preferably (20-30):100; the preferred mass ratio of the carbon nanotubes to be purified to the total mass ratio of the aqueous solution of the acid-resistant water-soluble dispersant is (10-50):100, more preferably (15-40):100. The present invention preferably involves mixing an acid-resistant water-soluble dispersant with water, and achieving complete dissolution of the acid-resistant water-soluble dispersant in water under heating and stirring conditions; the stirring speed is preferably 100-120 r / min, more preferably 110-120 r / min; the heating temperature is preferably 50-60℃, more preferably 55-60℃; and the stirring and heating time is based on the complete dissolution of the acid-resistant water-soluble dispersant in water.

[0028] In this invention, the temperature of the modification treatment is preferably 50–60°C, more preferably 55–60°C; the time is preferably 30–90 min, more preferably 30–60 min; the modification treatment is preferably carried out under stirring conditions, and the stirring speed is preferably 60–120 r / min, more preferably 80–120 r / min. In this invention, during the modification treatment, the surface of the carbon nanotubes to be purified is fully wetted by an aqueous solution of an acid-resistant water-soluble dispersant.

[0029] In this invention, after the modification treatment, no post-treatment is required. The obtained modified carbon nanotube solution is directly mixed with an acid solution for purification to obtain a purified carbon nanotube solution. In this invention, the acid solution is preferably hydrochloric acid, the concentration of which is preferably 36-38 wt%, and the mass ratio of hydrochloric acid to the modified carbon nanotube solution is preferably 1:(4-30), more preferably 1:(4.5-20), and even more preferably 1:(5-10). In this invention, the purification treatment temperature is preferably 50-60℃, more preferably 55-60℃; the time is preferably 2-4 hours, more preferably 2-3 hours; the purification treatment is preferably carried out under stirring conditions, and the stirring speed is preferably 60-120 r / min, more preferably 80-120 r / min.

[0030] In this invention, the modification and purification processes of the carbon nanotubes to be purified are preferably carried out in a Teflon reactor equipped with a heating water jacket and a stirring device. The stirring device includes a stirring paddle and a stirring shaft, both of which are coated with Teflon.

[0031] After obtaining the purified carbon nanotube solution, this invention performs solid-liquid separation on the purified carbon nanotube solution, washes the resulting solid material with water until neutral, and then dries it to obtain the purified carbon nanotubes. This invention does not have a particular limitation on the method of solid-liquid separation; any method well-known to those skilled in the art can be used, such as filtration. In this invention, the washing liquid used for washing is preferably deionized water; the drying is preferably carried out in a continuous mesh belt furnace with a graphite liner.

[0032] The method provided by this invention purifies carbon nanotubes by acid washing. The main iron impurities can be reduced from 500 ppm to less than 210 ppm after a single acid washing purification process (i.e., one modification treatment and one purification treatment). Cobalt impurities can be reduced from 105 ppm to less than 41 ppm after a single acid washing purification process, demonstrating good purification results. To further improve the acid washing purification effect and make it suitable for applications requiring high purity, this invention preferably repeats the modification and purification treatment 2 to 6 times. Specifically, the solid material obtained after each solid-liquid separation is used as raw material for further modification and purification treatment. After the final solid-liquid separation, the obtained solid material is washed with water until neutral and then dried to obtain purified carbon nanotubes.

[0033] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0034] Example 1

[0035] In this embodiment, the carbon nanotubes to be purified are unpurified coarse carbon nanotube powders. The main impurities in the carbon nanotubes to be purified are iron content of 3000 ppm and cobalt content of 708 ppm. The length of the carbon nanotubes to be purified is 10-30 μm, the outer diameter is 5-30 nm, and the agglomeration size is 1-100 μm.

[0036] Polyvinylpyrrolidone and deionized water were placed in a Teflon reactor equipped with a heating water jacket and a stirring device (the stirring device includes a stirring paddle and a stirring shaft, the surfaces of which are coated with Teflon) at a mass ratio of 5:100. The mixture was stirred at 55°C and 120 r / min to completely dissolve the polyvinylpyrrolidone and obtain an aqueous solution of polyvinylpyrrolidone.

[0037] The carbon nanotubes to be purified were added to the reactor at a mass ratio of 100:15 (polyvinylpyrrolidone aqueous solution to carbon nanotubes to be purified), and the modification was carried out at 55℃ and 120r / min for 60min to obtain the modified carbon nanotube solution.

[0038] Hydrochloric acid with a concentration of 38wt% was added to the reactor at a mass ratio of 4:1 to modified carbon nanotube solution, and the mixture was purified for 3 hours at 60℃ and 120r / min to obtain purified carbon nanotube solution.

[0039] The purified carbon nanotube liquid was filtered, and the resulting solid material was washed with water until neutral. It was then dried in a continuous mesh belt furnace with a graphite liner to obtain purified carbon nanotubes with an iron impurity content of 210 ppm and a cobalt content of 41 ppm.

[0040] Example 2

[0041] In this embodiment, the carbon nanotubes to be purified are unpurified coarse carbon nanotube powders. The main impurities in the carbon nanotubes to be purified are iron with a content of 3500 ppm and cobalt with a content of 821 ppm. The length of the carbon nanotubes to be purified is 10-30 μm, the outer diameter is 5-30 nm, and the agglomeration size is 1-100 μm.

[0042] Hydroxyethyl cellulose (HEC) and deionized water were placed in a Teflon reactor equipped with a heating water jacket and a stirring device (the stirring device included a stirring paddle and a stirring shaft, both of which were coated with Teflon) at a mass ratio of 10:100. The mixture was stirred at 60°C and 120 r / min to completely dissolve the hydroxyethyl cellulose and obtain an aqueous solution of hydroxyethyl cellulose.

[0043] The carbon nanotubes to be purified were added to the reactor at a mass ratio of 100:10 to hydroxyethyl cellulose aqueous solution. The mixture was then subjected to a modification treatment at 60℃ and 120r / min for 60min to obtain the modified carbon nanotube solution.

[0044] Hydrochloric acid with a concentration of 38wt% was added to the reactor at a mass ratio of 5:1 to modified carbon nanotube solution, and the mixture was purified at 60℃ and 120r / min for 4 hours to obtain purified carbon nanotube solution.

[0045] The purified carbon nanotube liquid was filtered, and the resulting solid material was washed with water until neutral. It was then dried in a continuous mesh belt furnace with a graphite liner to obtain purified carbon nanotubes with an iron impurity content of 105 ppm and a cobalt content of 18 ppm.

[0046] Example 3

[0047] In this embodiment, the carbon nanotubes to be purified are unpurified coarse carbon nanotube powders. The main impurities in the carbon nanotubes to be purified are iron content of 3300 ppm and cobalt content of 800 ppm. The length of the carbon nanotubes to be purified is 10-60 μm, the outer diameter is 5-30 nm, and the agglomeration size is 10-1000 μm.

[0048] Hydroxyethylidene diphosphonic acid (HEDP) and deionized water were placed in a Teflon reactor equipped with a heating water jacket and a stirring device (the stirring device includes a stirring paddle and a stirring shaft, both of which are coated with Teflon) at a mass ratio of 30:100. The mixture was stirred at 55°C and 120 r / min to completely dissolve the hydroxyethylidene diphosphonic acid, thus obtaining an aqueous solution of hydroxyethylidene diphosphonic acid.

[0049] The carbon nanotubes to be purified were added to the reactor at a mass ratio of 100:10 to hydroxyethylidene diphosphonic acid aqueous solution. The modification treatment was carried out at 55℃ and 120r / min for 60min to obtain modified carbon nanotube solution.

[0050] Hydrochloric acid with a concentration of 38wt% was added to the reactor at a mass ratio of 5:1 to modified carbon nanotube solution, and the mixture was purified at 60℃ and 120r / min for 4 hours to obtain purified carbon nanotube solution.

[0051] The purified carbon nanotube liquid was filtered, and the resulting solid material was washed with water until neutral. It was then dried in a continuous mesh belt furnace with a graphite liner to obtain purified carbon nanotubes with an iron impurity content of 50 ppm and a cobalt content of 4.8 ppm.

[0052] Comparative Example 1

[0053] In this comparative example, the carbon nanotubes to be purified are unpurified coarse carbon nanotube powders. The main impurities in the carbon nanotubes to be purified are iron with a content of 3000 ppm and cobalt with a content of 720 ppm. The length of the carbon nanotubes to be purified is 10–60 μm, the outer diameter is 5–30 nm, and the aggregate size is 10–1000 μm.

[0054] Deionized water and carbon nanotubes to be purified were placed in a Teflon reactor equipped with a heating water jacket and a stirring device (the stirring device includes a stirring paddle and a stirring shaft, the surfaces of which are coated with Teflon) at a mass ratio of 100:10 to obtain an aqueous dispersion of carbon nanotubes.

[0055] Hydrochloric acid with a concentration of 38wt% was added to the reaction vessel at a mass ratio of 5:1 to carbon nanotube aqueous dispersion. The mixture was purified at 80℃ and 120r / min for 8 hours to obtain purified carbon nanotube solution.

[0056] The purified carbon nanotube liquid was filtered, and the resulting solid material was washed with water until neutral. It was then dried in a continuous mesh belt furnace with a graphite liner to obtain purified carbon nanotubes with an iron impurity content of 500 ppm and a cobalt impurity content of 105 ppm.

[0057] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for purifying carbon nanotubes by acid washing, comprising the following steps: The carbon nanotubes to be purified are unpurified coarse carbon nanotube powders. The main impurities in the carbon nanotubes to be purified are iron with a content of 3300 ppm and cobalt with a content of 800 ppm. The length of the carbon nanotubes to be purified is 10~60 μm, the outer diameter is 5~30 nm, and the aggregate size is 10~1000 μm. Hydroxyethylidene diphosphonic acid and deionized water were placed in a Teflon reactor equipped with a heating water jacket and a stirring device, with the stirring device including a stirring paddle and a stirring shaft. The surfaces of the stirring paddle and the stirring shaft were both coated with Teflon. The mixture was stirred at 55°C and 120 r / min to completely dissolve the hydroxyethylidene diphosphonic acid, thus obtaining an aqueous solution of hydroxyethylidene diphosphonic acid. The carbon nanotubes to be purified were added to the reactor at a mass ratio of 100:10 to hydroxyethylidene diphosphonic acid aqueous solution. The modification treatment was carried out at 55℃ and 120r / min for 60min to obtain modified carbon nanotube solution. Hydrochloric acid with a concentration of 38wt% was added to the reaction vessel at a mass ratio of 5:1 to modified carbon nanotube solution, and purified for 4 hours at 60℃ and 120r / min to obtain purified carbon nanotube solution. The purified carbon nanotube liquid was filtered, and the resulting solid material was washed with water until neutral. It was then dried in a continuous mesh belt furnace with a graphite liner to obtain purified carbon nanotubes with an iron impurity content of 50 ppm and a cobalt content of 4.8 ppm.

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