RU2419586C1 - Method of producing graphite-based thermally expanding compound - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to production of graphite-based laminar compounds and may be used for production of carbon sorbents. Graphite weight 2 is placed into reactor 1. Rector 1, vessels 4 and 6 are evacuated by vacuum booster pump 9 to residual pressure of 13.3 Pa. Vessel 4 is cooled down by filling Dewar vacuum flask 7 with crushed ice. Iodine heptafluoride that passes in solid-phase state at 0°C is fed from initial vessel 3 into vessel 4. Weight 2 is treated by iodine heptafluoride gas phase at 78÷240 kPa and 18÷40°C. JF₇ surpluses are pumped by booster pump 9 from reactor 1 via cryogenic trap 6 to residual pressure not exceeding 13.3. Pa. Gasses not entrapped by trap 6 are neutralised at chemical absorber in column 10. iodine heptafluoride frozen in trap 6 is fed into cooled vessel 4 for reuse. Weight 2 of synthesised intercalated fluorinated graphite compound is withdrawn from reactor 1.

EFFECT: invention allows simplifying processing hardware and increasing efficiency.

2 cl, 3 ex, 4 dwg

Description

The invention relates to methods for producing layered graphite-based compounds capable of initiated thermal expansion, and can be used to prepare carbon adsorbents.

Thermally expanded graphite compounds (TEG) are finely dispersed carbon materials formed during the rapid thermal decomposition of graphite intercalates with volatile substances.

Known methods for producing thermally expanded compounds of graphite, which consist in the preliminary preparation of layered intercalated compounds of oxidized graphite by processing powder of natural flake graphite with concentrated nitric and glacial acetic acids [A.S. SU No. 1614350 A1, IPC 6 C01B 31/04. Priority from 05/19/89. Publ. 02/20/95] (analogue), concentrated nitric acid, acetic acid and organic compounds [A.S. SU No. 1476785 A1, IPC 6 C01B 31/04. Priority from 07/02/86. Publ. 02/20/95.] (Analog).

The disadvantage of analogues is that the thermal expansion of intercalated compounds of oxidized graphite requires high-speed heating (thermal shock) to a temperature of 950 ÷ 1250 ° C. Only in this case, pairs of acids create pressure in the graphite matrix, causing the separation of carbon layers (graphenes), which is visually recorded as an increase in the size of the graphite flakes. The coefficient of expansion (increase in volume) of graphite powder does not exceed 100 ÷ 150.

Known methods for producing an expanded form of graphite, differing from previous analogues in that intercalated compounds of fluorinated graphite (ISFG) are used as starting material [V. Makotchenko. et al. New forms of expanded graphite with increased sorption capacity / 2nd International Conference “Carbon: Fundamental Problems of Science, Materials Science, Technology”. Collection of abstracts. Moscow, 2003, p.141]. ISFG is obtained by low-temperature (up to 100 ° C) gas-phase fluorination of natural flake graphite with strong fluorooxidants - fluorohalogens: ClF ₃ , ClF ₅ , BrF ₃ , BrF ₅ .

In terms of parameters such as volume increase and surface area, expanded graphites from ISFG significantly exceed the known forms of TWG from intercalated compounds of oxidized graphite. Thus, the expansion coefficient of ISFG during shock thermal heating, preferably up to 600 ÷ 700 ° C, reaches 300 ÷ 500.

A disadvantage of the known methods for producing ISFG is that the strong fluoroxidants ClF ₃ and BrF ₃ are extremely hazardous chemicals. The process of gas-phase fluorination of graphite lasts dozens of hours, since the fluorination reaction products in the form of chlorofluoride (ClF) and bromine (BrF) monofluorides prevent the access of fresh portions of fluoroxidant to the graphite layer. For this reason, the known methods for producing ISPH remained at the laboratory level.

There are also known methods for producing ISFG through contact of graphite powder with the liquid phase of fluorohalides.

In the method of producing ISFG [US Patent No. 3962133, IPC B01J 27/12. Publ. 06/08/76] (analog) by treating graphite powder with the liquid phase of a solution of chlorine trifluoride in anhydrous hydrogen fluoride for 24-30 hours at a temperature of -78 ° C to + 22 ° C, there is no significant acceleration of the process of graphite fluorination and intercalation. The reaction products are thermally stable up to 590 ° C. The analogue method is absolutely not acceptable for industrial practice due to the tendency of liquid ClF ₃ to ignite and explode. In addition, additional preparation of the reaction oxidizing medium is required.

By the combination of the main features, the closest technical solution to the proposed one is the method of producing a thermally expanding compound based on graphite [Application RU No. 2007129419. IPC С01В 31/04. Priority of July 31, 2007. Publ. 02/10/2009. Bull. No. 4] (prototype), which includes the treatment of graphite-containing powder material with the liquid phase of iodine heptafluoride.

Of the known halide fluoroxides, JF _{7 is the} least dangerous. Graphite fluorination reactions proceed with the formation of iodine pentafluoride (JF ₅ ), the equilibrium vapor pressure of which over the liquid phase is approximately 20 times less than the equilibrium pressure JF ₇ . As a result, the total pressure in the reaction volume decreases. Iodine pentafluoride is fixed in the crystal lattice of graphite and does not prevent the access of JF ₇ to particles of graphite-containing powder material.

Hereinafter, the term "graphite-containing powder material" refers to the powder (grains) of natural or artificial graphite in a mixture with mineral impurities. The proportion of impurities in some commercial varieties of natural fine crystalline graphites can reach 25 wt.%.

The disadvantages of the prototype method include the cooling and thawing of closed process volumes containing graphite-containing powder material with liquid nitrogen, which lengthens the process cycle time and, as a result, leads to low productivity of plants for the production of intercalated fluorinated graphite compounds based on this method.

The present invention is aimed at simplifying the technical design and increasing the productivity of the method for producing thermally expandable compounds based on graphite.

The above objectives are achieved by a technical solution, the essence of which is that in a method for producing a thermally expanding compound based on graphite, which includes treating a graphite-containing powder material with iodine heptafluoride, a graphite-containing powder material is treated with the gas phase of iodine heptafluoride in a closed volume at a pressure of 78 ÷ 240 kPa.

In addition, the above tasks are achieved using an additional technical solution, consisting in the fact that the processing is carried out at a temperature of 18 ÷ 45 ° C.

The main distinguishing feature of the proposed method is the treatment of graphite-containing powder material with the gaseous phase of heptafluoride in a closed volume of iodine at an absolute pressure of 78 ÷ 240 kPa. This feature is new and significant, as it eliminates the inherent prototype disadvantages. Firstly, the consumption of iodine heptafluoride is significantly reduced, since it is spent only on the synthesis of ISPH. Secondly, the operations of freezing and thawing with liquid nitrogen of a process volume with graphite-containing powder material are excluded. Thirdly, the constancy of the pressure of the gas phase of iodine heptafluoride over graphite-containing powder material makes it possible to obtain ISPH powders with a stable chemical composition and the same expansion coefficient.

As shown by special studies of the authors, at an absolute pressure of more than 78 kPa, iodine heptafluoride is intensively introduced into the interlayer space of graphite to obtain an intercalated compound in the form of a fluoroxidized graphite matrix containing iodine fluoride heteroatoms. The heating of the ISFG powder to a temperature of 125 ° C and above, including without thermal shock, is accompanied by the formation of thermally expanded graphite in the form of a powdery material.

A pressure of JF ₇ below 78 kPa does not lead to the formation of inclusion compounds capable of intense thermal expansion. Beyond the lower limit of the claimed range, it is difficult to maintain a stable pressure of the gas phase of iodine heptafluoride in the industrial implementation of the method.

Processing of graphite-containing powder material with a gas phase JF ₇ at a pressure of more than 240 kPa can be performed only under conditions of additional heating of the vapor source (capacity) of iodine heptafluoride to a temperature of 40 ° C or higher, which technically immediately greatly complicates the implementation of the claimed method and makes it unsafe.

The claimed temperature range for the treatment of graphite-containing powder material with a gas phase JF ₇ provides comfortable (lower limit) and safe (upper limit) conditions for the implementation of the method.

Examples of the method are illustrated in graphic materials.

Figure 1 shows a diagram of a plant for the synthesis of intercalated compounds of fluorinated graphite, capable of thermal expansion, through the gas phase JF ₇ . Here: 1 - technological volume (reactor); 2 - a sample of graphite-containing powder material; 3 - initial capacity with JF ₇ ; 4 - capacity with liquid (solid) phase 5 JF ₇ ; 6 - cryogenic trap with a solid phase 5 JF ₇ for trapping excess iodine heptafluoride; 7, 8 - Dewar vessels; 9 - fore-vacuum pump; 10 - column with a chemical absorber; 11 - pressure gauge; 12 - micromanometer.

Figure 2, 3 and 4 are, respectively, micrographs of natural flake graphite powder, ISFG powder and particles of thermally expanded graphite.

The following are examples of the method.

Example 1. A portion of 2 flake graphite brand GT-1 (crucible graphite, ash content of 5-7 wt.%) Having an average particle size of 200 ÷ 300 μm and a bulk density of 0.45 g / cm ³ (see figure 2), weighing 1.0 grams are placed in a Nickel reactor 1 with a volume of 0.15 dm ³ (see figure 1). The reactor 1, tanks 4 and 6 with a volume of 0.27 dm ^{3 are} pumped out by a foreline pump 9 to a residual pressure of 13.3 Pa. Pressure monitoring in the system is carried out on a micromanometer 12. Stand for checking the vacuum density. Capacity 4 is cooled by pouring crushed ice into the Dewar 7. From the original tank 3, the tank 5 is filled with iodine heptafluoride, which at a temperature of 0 ° C becomes solid state. In this case, the vapor pressure JF ₇ above the surface of the solid phase 6 in a closed volume is set to ~ 78 kPa (the pressure in the system is monitored by a vacuum gauge 11). After opening the valve in the reactor 1, the same pressure of iodine heptafluoride is set in the technological volume, and it is kept constant during the entire processing time of the graphite-containing powder material. A portion of graphite is kept under pressure of the gas phase of iodine heptafluoride at room temperature (at least 18 ° C) for 12 ÷ 20 hours.

The capacity 4 is cut off from the reactor 1 and technological routes. The cryogenic trap 6 is frozen with liquid nitrogen poured into the Dewar vessel 8. The excess JF ₇ from the reactor 1 and the process lines is pumped by the foreline pump 9 through the cryogenic trap 6 to a residual pressure of not more than 13.3 Pa. The pressure in the system is controlled by a micromanometer 12. Gases not trapped in trap 6 are neutralized on a chemical absorber in column 10. Subsequently, iodine heptafluoride, frozen out in trap 6, is refluxed into a cooled container 5 for reuse.

Reactor 1 with a weighed portion of 2 synthesized ISFG is kept under dynamic vacuum for the purpose of squeezing, then it is filled with dried nitrogen and disconnected from the installation.

With this method of synthesis of ISFG, a portion of 2 graphite is processed under conditions of constant pressure of the gas phase of iodine heptafluoride, despite the absorption of fluorine oxidizer by a graphite matrix. The stabilization of the pressure of the gas phase occurs due to the constant sublimation of JF ₇ in the tank 4 and its entry into the process volume 1.

After unloading the sample 2, a light brown ISFG powder is obtained, weighing 3.4 g with a bulk density of 0.40 g / cm ³ (see FIG. 3). The powder is thermally stable to a temperature of 150 ° C.

ISFG powder is poured into a thin-walled nickel crucible, closed with a metal mesh and placed in an oven, previously heated to a temperature of 150 ° C. After heating the crucible, destructive decomposition of ISFG occurs with the formation of a downy material from expanded graphite particles having a length of up to 5–12 mm (see Fig. 4).

The expansion coefficient of ISFG powder is ~ 370, since the expansion of the graphite matrix occurs not only due to an increase in the vapor pressure of iodine fluorides, but also the formation of additional carbon compounds with fluorine. The density by volume of expanded graphite is approximately 1.2 dm ³ / g. The output of graphite in an expanded form is 0.84.

Example 2. A sample of 2 graphite grade TG-1 weighing 10.0 g is placed in reactor 1. The unit is prepared for operation according to example 1. The cooled tank 4 is filled with iodine heptafluoride from the original tank 3. The tank 4 is thawed, removing Dewar 7. At the same time, JF ₇ at room temperature (at least 18 ° C) it passes into a liquid with a vapor pressure above the surface of liquid phase 5 in a closed volume of ~ 140 kPa (20 ° C). The pressure in the system is controlled by a manovacuum meter 11. The valve is opened on the process volume 1 and the sample of graphite is kept under vapor pressure of iodine heptafluoride at room temperature for 6-8 hours. After unloading from the reactor 1, the weights of 2 receive 45.4 g of ISPH powder. The powder is thermally stable to a temperature of 130 ° C.

The coefficient of expansion of the powder ISFG is ~ 450. The density by volume of expanded graphite corresponds to approximately 1.0 dm ³ / g. The output of graphite in an expanded form is 0.82.

Processing graphite portions with a gas phase JF ₇ at a pressure equal to the equilibrium vapor pressure over the liquid phase of iodine heptafluoride allows one to obtain the same type of high-mass ISPH samples.

Example 3. A portion of 2 graphite grade TG-1 weighing 30.0 g is treated with the gas phase of iodine heptafluoride in the installation of figure 1 according to the conditions of example 2. The tank 4 with liquid iodine heptafluoride is heated to a temperature of 40 ° C. The vapor pressure JF ₇ above the liquid phase in a closed volume is set to ~ 240 kPa. The temperature of reactor 1 with a weighed graphite is maintained at about 45 ° C to prevent JF ₇ condensation, which is about 5 ° C higher than the temperature of vessel 4. After unloading of reactor 1, 158.2 g of ISPH powder is obtained, having a color from white to light brown . ISFG powder is thermally stable up to a temperature of 125 ° C.

The coefficient of expansion of the powder ISFG is ~ 500. The density by volume of expanded graphite corresponds to about 0.9 dm ³ / g. The output of graphite in an expanded form is 0.80.

It is understood that the invention is not limited to the examples given. Other examples of the implementation of the method within the scope of the proposed claims are possible.

Thermally expanded graphite was used as an adsorbent in wound dressings and allowed long-term (up to several days) and active evacuation of wound discharge. The mass of thermally expanded graphite in dressings due to the binding of wound exudate increased by 50 times, which exceeded the same indicator for all known brands of carbon adsorbents.

The proposed method for the synthesis of a thermally expanding compound based on graphite with the introduction of iodine heptafluoride into a graphite matrix is simple and safe from a technological point of view, does not require complex equipment, the synthesis is carried out in a relatively short time (6-8 hours versus 24 ÷ 30 hours in known methods) at a temperature of 18 ÷ 45 ° С. Excess reagents are reused. The synthesis of iodine heptafluoride is a mastered process that is not particularly difficult. The resulting compounds are resistant to prolonged exposure to water and moist air. Since the need for thermally expanding compounds based on graphite is great in various fields of science and technology, the great practical importance of the proposed method is very obvious.

Claims (2)

1. A method of producing a thermally expandable compound based on graphite, comprising treating a graphite-containing powder material in a closed volume with iodine heptafluoride, characterized in that the graphite-containing powder material is treated with a gas phase of iodine heptafluoride in a closed volume at a pressure of 78-240 kPa. 2. The method according to claim 1, characterized in that the treatment is carried out at a temperature of 18-40 ° C.

Images