What is Purified Graphite?

In the field of materials science, graphite is widely used with its unique physical and chemical properties, from pencil leads to high-tech products. However, there are many impurities in natural graphite, which seriously affects the performance. And purified graphite has become the key, which can help it play a greater value, promote the development of technology.

Preparation method of purified graphite

Physical method

Flotation method:

Separation is achieved based on the difference in the physical properties of the surface of graphite and impurities. Especially the different wettability. The natural hydrophobicity of graphite is good, and the impurity minerals are hydrophilic. The graphite ore is ground and mixed with water to form pulp, and collector and foaming agent are added. The collector makes the graphite more hydrophobic and easier to adhere to bubbles. The foaming agent produces stable bubbles. Through the air to the pulp, the graphite floats up with the bubbles to form a foam layer. And the impurities remain at the bottom to achieve separation.

The process is simple, low cost, suitable for large-scale treatment of graphite ore. But it is difficult to remove fine-grained embedment impurities, the purification effect is limited. Generally can only increase the purity of graphite to 80%-90%.

High temperature sublimation method:

Use of graphite and impurities sublimation temperature is different purification. The melting point of graphite is as high as 3652℃, it is difficult to melt under atmospheric pressure. Under high temperature vacuum, the temperature is above 2700℃. Impurities such as silicon, aluminum and iron are preferentially sublimed, and graphite basically remains solid. Put the graphite into a special high temperature furnace. At 10^–3 -10^–5 Pa high vacuum temperature to 2800-3000 ℃, impurities sublimate after being extracted by vacuum pump. This can get more than 99.99% purity of graphite. However, the equipment is expensive, the energy consumption is large, and the production scale is limited.

Chemical method

Alkali-acid method:

Commonly using chemical purification method, reaction with alkali and acid to convert impurities into soluble substances to remove. First, mix graphite and sodium hydroxide in proportion, roast them at 500-700 ℃. And impurities such as silica and alumina react to produce sodium silicate and sodium metaaluminate. The roasted products were immersed in water and filtered to remove soluble salt. Then add hydrochloric acid to the filter residue, iron, calcium and other metal impurities to produce soluble chloride. After filtration, washing purified graphite. The process is mature, the equipment is simple, the purification effect is good, can increase the purity of graphite to 95%-99%. But it will produce a lot of waste water waste residue, polluting the environment.

Hydrofluoric acid method:

The use of hydrofluoric acid and impurities purification. Hydrofluoric acid reacts with impurities such as silicon dioxide to form volatile silicon tetrafluoride gas. The graphite is mixed with hydrofluoric acid in proportion, reacts at the appropriate temperature, filters and washes after the end to obtain high-purity graphite. This can increase the purity of graphite to more than 99%. However, hydrofluoric acid is highly corrosive and toxic, has high safety requirements for equipment and personnel. And it is difficult to treat fluorine-containing wastewater, and the environmental protection cost is high.

Purified graphite performance characterization

Purity test means

Chemical analysis:

Using instruments to quantitatively analyze graphite impurity elements to determine the purity. Like ICP-MS can simultaneously measure a variety of trace elements, ultra-high sensitivity, can detect very low content of impurities. AAS is mainly focused on metal impurities, by atomizing the sample and measuring the degree of light absorption. The impurity concentration is accurately calculated, and then the purity of graphite is obtained.

Ash determination method:

The graphite sample is burned at 950-1000 ℃ to constant weight, volatile organic matter, etc.. The remaining ash mass fraction can indirectly reflect the purity of graphite. The operation is simple, but only the total impurity content can be known. And the specific impurity types can not be defined.

Microstructure observation

Scanning electron microscope (SEM)

Used to observe the microscopic situation of graphite, which can clearly show the layered structure, crystal morphology and impurity distribution. At high resolution, you can even see microscopic features such as crystal defects. It helps us understand the properties of graphite and the effects of purification on its structure.

Transmission electron microscopy (TEM) :

Allows in-depth study of graphite microstructure, such as lattice structure. It penetrates thin samples and uses electron diffraction and imaging techniques to reveal atomic arrangements. Helping to study changes in graphite crystal structure during purification.

Performance Test

Conductivity test:

The conductivity of graphite itself is good, and it is better after purification. Four probe method is commonly used to measure electrical resistance and calculate electrical conductivity, which is the key to measure. In lithium-ion batteries, the graphite anode with high conductivity can improve the efficiency of battery charging and discharging.

Thermal stability test:

Thermal stability is critical for graphite in high temperature applications. The mass and heat changes were analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) respectively. To evaluate the performance stability of graphite at different temperatures.

Purified graphite applications

Lithium-ion batteries

In lithium-ion batteries, graphite is a common anode material. Purified graphite has higher theoretical specific capacity and better cycle stability, which can reduce the obstruction of impurities to lithium-ion activities. It can reduce internal resistance, improve battery energy density, charge and discharge efficiency and rate performance, extend cycle life. And it can meet the demand for high-performance batteries such as electric vehicles.

Semiconductor manufacturing

Semiconductor manufacturing requires very high material purity. You can use the purified graphite in production equipment graphite boats, fixtures and other components. Because of its very low impurity content, it can avoid polluting semiconductor devices. Its high purity and thermal stability can meet the high temperature, high precision process requirements. To ensure the high-quality production of semiconductor devices.

Nuclear industry

In the nuclear industry, graphite is used as a neutron moderator and reflective material. Purified graphite neutron deceleration performance is good, chemical stability is strong. It can effectively control the speed and distribution of neutrons, reduce the interference of impurities on nuclear reactions. It improves reactor safety and operation efficiency, and is of great significance in advanced nuclear reactors.

High-end lubricating materials

Graphite has good lubrication performance and is an important raw material for high-end lubricating materials. Lubrication materials made of purified graphite can maintain excellent lubrication performance under extreme conditions such as high temperature, high pressure and high vacuum. It can meet the demanding needs of equipment in the aerospace field, reduce component wear.

Conclusion

Purified graphite can effectively remove impurities and improve performance. And different purification methods have advantages and disadvantages, which should be comprehensively considered. It has great application value in many fields and promotes industrial development. In the future, more efficient, environmentally friendly and low-cost purification technologies will be developed and applied.