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Carbon is present in two natural crystalline allotropic forms—Graphite and diamond—each with its own separate crystal structure and properties.
The term graphite is derived from the Greek word “graphein,” which means to write. The material is typically grayish-black in color, opaque, and has a radiant black sheen. Graphite is a distinct material as it displays the properties of both a metal and a non-metal.
Although graphite is flexible, it is not elastic and has high electrical and thermal conductivity. It is also chemically inert and highly refractory. Since graphite displays low adsorption of X-rays and neutrons, it is very valuable in nuclear applications.
This uncommon combination of properties is due to graphite’s crystalline structure. The carbon atoms are set hexagonally in a planar condensed ring system. The layers are stacked parallel to each other. The atoms within the rings are bonded covalently, while the layers are loosely linked together by van der Waals forces. Graphite has a high degree of anisotropy, which is caused by two types of bonding acting in different crystallographic directions.
For example, graphite’s ability to develop a solid film lubricant is the outcome of these two contrasting chemical bonds. As weak Van der Waals forces control the bonding between each layer, they can slide against one another, making graphite an ideal lubricant. In 2000, worldwide graphite production was estimated to be about 602,000 tons, with China as the largest producer followed by India, Mexico, Brazil, and the Czech Republic.
Graphite can be divided into two main types—natural and synthetic.
Natural graphite is a mineral composed of graphitic carbon. It varies considerably in crystallinity. Most of the commercial (natural) graphites are mined, and typically contain other minerals. After graphite is mined, it usually requires a considerable amount of mineral processing like froth flotation to concentrate the graphite.
Natural graphite is an excellent conductor of heat and electricity, stable over a broad range of temperatures, and a highly refractory material with a high melting point of 3650 °C.
There are three types of natural graphite:
It is said that crystalline vein graphite came from crude oil deposits that have transformed into graphite through time, temperature, and pressure. Vein graphite fissures typically measure between 1 cm and 1 m in thickness and usually have a purity of more than 90%.
Although this type of graphite can be found globally, only Sri Lanka commercially mines it, using conventional shaft or surface mining techniques.
Amorphous graphite is the least graphitic among the natural graphites. However, the term “amorphous” is incorrect as the material is still crystalline. Amorphous graphite can be found as minute particles in beds of mesomorphic rocks such as coal, slate, or shale deposits. The graphite content varies from 25% to 85% according to the geological environment.
Conventional mining techniques are used to extract amorphous graphite, which occurs mainly in Mexico, North Korea, South Korea, and Austria.
Flake graphite can be found in metamorphic rocks evenly spread through the body of the ore or in concentrated lens-shaped pockets. The range of carbon concentrations varies from 5% to 40%. Graphite flake can be found as a lamella or scaly form in specific metamorphic rocks such as limestone, gneisses, and schists.
Froth flotation is used to extract flake graphite. “Floated” graphite has 80%–90% graphite content. Over 98% of flake graphite is made using chemical beneficiation processes. Flake graphite can be found in numerous places worldwide.
Synthetic graphite can be produced from coke and pitch. Although this graphite is not as crystalline as natural graphite, it is likely to have higher purity. There are basically two types of synthetic graphite. One is electrographite, pure carbon produced from coal tar pitch and calcined petroleum coke in an electric furnace. The second is synthetic graphite, produced by heating calcined petroleum pitch to 2800 °C.
Essentially, synthetic graphite has higher electrical resistance and porosity, and lower density. Its enhanced porosity makes it unsuitable for refractory applications.
Synthetic graphite contains mainly graphitic carbon that has been attained by graphitization, heat treatment of non-graphitic carbon, or chemical vapor deposition from hydrocarbons at temperatures over 2100 K.
On account of its high-temperature stability and chemical inertness, graphite is the perfect candidate for refractory material. It is used in the production of refractory bricks as well as “Mag-carbon” refractory bricks (Mg-C). In addition, graphite is used to produce crucibles, ladles, and molds for holding molten metals. Moreover, graphite is one of the most basic materials used in the production of functional refractories for the continuous casting of steel.
Here, graphite flake is mixed with alumina and zirconia, and then isostatically pressed to create components like stopper rods, subentry nozzles, and ladle shrouds used for regulating the flow of molten steel and also for safeguarding against oxidation. This type of material could also be used as protection for pyrometers.
In the production of iron, graphite blocks are used to produce a portion of the lining of the blast furnace. Their structural strength at high temperature, thermal shock resistance, high thermal conductivity, low thermal expansion, and good chemical resistance are highly important in this application.
The electrodes used in many electrical metallurgical furnaces are mass-produced from graphite, for instance, the electric arc furnaces used for processing steel.
In the chemical sector, graphite is employed in many high-temperature applications, like in the production of phosphorus and calcium carbide in arc furnaces. Graphite is used as an anode in specific aqueous electrolytic processes such as the production of halogens (chlorine and fluorine).
Large amounts of high-purity electrographite are used for producing moderator rods and reflector components in nuclear reactors. The suitability of electrographite comes from its low absorption of neutrons, high thermal conductivity, and high strength at higher temperatures.
Graphite is mainly used as an electrical material in the manufacture of carbon brushes in electric motors. Here, the component’s service life and performance largely depend on grade and structure.
Graphite is widely used as an engineering material across a variety of applications such as piston rings, thrust bearings, journal bearings, and vanes. Carbon-based seals are used in the fuel pumps and shafts of several aircraft jet engines.
Amorphous graphite is used in:
Areas where crystalline graphite can be used include:
Flake graphite is used in refractory applications, typically in secondary steelmaking. Apart from this, it may also be used in pencils, powder metallurgy, coatings, and lubricants.
Many of the sources of natural graphite are also used in the development of graphite foil.
Areas where synthetic graphites are used include:
The higher level of porosity of synthetic graphite makes it unsuitable in refractory applications.
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