Seven preparation methods of expandable graphite

Expandable graphite is a kind of intercalation compound that uses physical or chemical methods to insert foreign materials with different properties into graphite sheets. It can be rapidly expanded at high temperatures to produce expanded graphite. As a new type of carbon functional material, it has many special properties and can be widely used in electrode materials, petrochemical industry, fire retardant, oil-absorbing materials, environmental protection, medical dressings, military smoke, and antistatic coatings. Therefore, the study of its preparation is particularly important.

Chemical oxidation

Chemical oxidation is a traditional method to prepare expandable graphite. In this method, natural flake graphite is evenly mixed with the appropriate amount of oxidant and intercalation agent, controlled by a certain temperature, continuously stirred, washed, filtered, and dried to obtain expandable graphite. Chemical oxidation has become a relatively mature method with the advantages of simple equipment, convenient operation, and low cost.

The chemical oxidation process includes oxidation and intercalation. The oxidation of graphite is the basic condition for the formation of expandable graphite because whether the intercalation reaction can proceed smoothly depends on the degree of an opening between graphite layers. However, natural graphite has excellent stability and resistance to acid and alkali at room temperature, so it does not react with acid and alkali. Therefore, the addition of an oxidant becomes a key component in chemical oxidation.

There are many kinds of oxidants. The commonly used oxidants are solid (such as potassium permanganate, potassium dichromate, chromium trioxide, potassium chlorate, etc.), or some liquid oxidants (such as hydrogen peroxide, nitric acid, etc.). In recent years, it is found that potassium permanganate is the main oxidant in the preparation of expandable graphite.

Under the action of oxidant, graphite is oxidized and the neutral network macromolecules of the graphite layer become planar macromolecules with positive charges. Due to the repulsive effect of the same-sex positive charges, the distance between graphite layers increases, which provides channels and spaces for intercalation agents to enter into graphite layers smoothly. In the preparation process of expandable graphite, the intercalation agent is mainly acid. In recent years, researchers have mainly used sulfuric acid, nitric acid, phosphoric acid, perchloric acid, mixed acid, and glacial acetic acid.

Electrochemical method

The electrochemical method is to use the water solution of the insert as the electrolyte under constant current, and the graphite and metal materials (stainless steel, platinum plate, lead plate, titanium plate, etc.) are used as the composite anode, and metal materials are inserted into the electrolyte as the cathode to form a closed circuit; Or, the graphite is suspended in the electrolyte, and the anode and cathode plates are inserted in the electrolyte at the same time. The anode oxidation is carried out by energizing the two electrodes. The surface of graphite is oxidized to carbon positive ions. Under the interaction of electrostatic gravity and concentration difference diffusion, acid ion or other polar interposing agent ions are embedded into graphite layer to form expandable graphite.

Compared with chemical oxidation, the whole process of electrochemical preparation of expandable graphite does not need to use an oxidant. The treatment capacity is large and the amount of corrosive substances is small. After the reaction, the electrolyte can be recycled, the amount of acid solution is reduced, the cost is saved, the environmental pollution is reduced, the damage to equipment is low, and the service life is prolonged. In recent years, the electrochemical method has become the first choice for the preparation of expandable graphite by many enterprises with many advantages.

Gas-phase diffusion method (double-chamber method)

The gas-phase diffusion method is to prepare expandable graphite by intercalation reaction between intercalation agent and graphite in gaseous form. In general, graphite and insert are placed at both ends of the heat-resistant glass reactor respectively, and then they are vacuum-sealed, so it is also called the double-chamber method. Halide eg and alkali metal eg are often synthesized by this method in the industry.

Advantages: the structure and order of the product can be controlled, and the reactants and products can be easily separated.

Disadvantages: the reaction device is relatively complex and difficult to operate, which limits the output. Moreover, the reaction needs to be carried out at a high temperature, which takes a long time. Moreover, the reaction conditions are very demanding, and the preparation environment must be a vacuum. Therefore, the production cost is relatively high, which is not suitable for large-scale production and application.

Mixed liquid phase method

The mixed liquid phase method is to prepare expandable graphite by directly mixing the insert material with graphite and heating reaction under the protection of flowing inert gas or sealing system. It is generally used to synthesize alkali graphite intercalation compounds (GICs).

Advantages: the reaction process is simple and the reaction speed is fast. The expandable graphite with a certain structure and composition can be achieved by changing the ratio of graphite raw materials and inserts, which is more suitable for mass production.

Disadvantages: the products formed are unstable, it is difficult to deal with the free intercalation materials attached to the surface of GICs, and it is difficult to ensure the consistency of graphite intercalation compounds in large-scale synthesis.

Melting method

The melting method is to prepare expandable graphite by mixing graphite with intercalation material and heating. It is based on the property that the eutectic of each component can reduce the melting point of the system (lower than the melting point of each component). It is a method to prepare ternary or multicomponent GICs by inserting two or more substances (which must be able to form a molten salt system) into the graphite layer at the same time. It is generally used to prepare metal chloride GICs.

Advantages: the synthetic product is stable, easy to wash, simple reaction device, low reaction temperature, short reaction time, suitable for large-scale production.

Disadvantages: it is difficult to control the order structure and composition of the product in the reaction process, and it is difficult to ensure the consistency of the order structure and composition of the product in large-scale synthesis.

Pressurization method

In the pressure method, the graphite matrix is mixed with alkaline earth metal and rare earth metal powder, and m-gics are formed by reaction under pressure.

Disadvantages: only when the vapor pressure of metal exceeds a certain threshold, the insertion reaction can be carried out; however, if the temperature is too high, it is easy for the metal and graphite to form carbides and react negatively, so the reaction temperature must be controlled within a certain range. The insertion reaction temperature of rare earth metal is very high, so pressure must be applied to reduce the reaction temperature. This method is suitable for the preparation of metal GICs with a low melting point, but the device is complex and the operation requirements are strict.

Explosion method

In the explosion method, pyrotechnic composition or mixture made of graphite and expansion agents such as KClO4, Mg (ClO4) 2 · nH2O, Zn (NO3) 2 · nH2O is usually used. When it is heated, graphite will oxidize and intercalate at the same time to form an intercalation compound, which will be expanded in an “explosive” way to obtain expanded graphite. When metal salts are used as expansion agents, the products are more complex, including not only expanded graphite but also metal oxides, which can modify the surface of expanded graphite; But when the expansion agent is HClO4, the product is relatively single, only expanded graphite. This method is easy to operate, short reaction time, and can be designed, but the purity of the product is low, and the risk of the experimental process is high, so this method is generally not used to prepare expandable graphite in practice.

In addition to the above-mentioned preparation methods, the synthesis methods of GICs include the catalyst method, photochemical method, etc. These synthesis methods have their advantages and disadvantages, and the synthesis objects are not the same, so they are rarely used. At present, the chemical oxidation method and electrochemical method are the most common preparation methods. The electrochemical method, in particular, has attracted more and more attention in recent years due to its advantages of less environmental pollution, low cost, large treatment capacity, and recyclability of acid solution.