Graphite

Graphite
Graphite
General
Category	Native mineral
Formula; (repeating unit)	Carbon, C
Crystal system	Hexagonal (6/m 2/m 2/m)
Identification
Color	Steel black, to gray
Crystal habit	Tabular, six-sided foliated masses, granular to compacted masses
Cleavage	Perfect in one direction
Fracture	Flaky, otherwise rough when not on cleavage
Mohs scale hardness	1 - 2
Luster	metallic, earthy
Streak	Black
Density	2.09–2.23 g/cm³
Refractive index	Opaque
Pleochroism	None
Solubility	Molten Ni

Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): "to draw/write", for its use in pencils) is one of the allotropes of carbon. Unlike diamond, graphite is an electrical conductor, and can be used, for instance, in the electrodes of an arc lamp. Graphite holds the distinction of being the most stable form of solid carbon ever discovered. It may be considered the highest grade of coal, just above anthracite and alternatively called meta-anthracite, although it is not normally used as fuel because it is hard to ignite.

There are three principal types of natural graphite, each occurring in different types of ore deposit: (1) Crystalline flake graphite (or flake graphite for short) occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken and when broken the edges can be irregular or angular; (2) Amorphous graphite occurs as fine particles and is the result of thermal metamorphism of coal, the last stage of coalification, and is sometimes called meta-anthracite. Very fine flake graphite is sometimes called amorphous in the trade; (3) Lump graphite (also called vein graphite) occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin.

The name "graphite fiber" is also sometimes used to refer to carbon fibre or carbon fibre reinforced plastic.

Occurrence

Minerals associated with graphite include quartz, calcite, micas, iron meteorites, and tourmalines. In 2005, China was the top producer of graphite with about 80% world share followed by India and Brazil.

Graphite has various other characteristics. Thin flakes are flexible but inelastic, the mineral can leave black marks on hands and paper, it conducts electricity, and displays superlubricity. Its best field indicators are softness, luster, density and streak.

According to the USGS, world production of natural graphite in 2005 was 1.05 million tonnes, of which the following major exporters produced: China produced 720,000 tonnes, Brazil 76,500 tonnes, Canada 30,000 tonnes, and Mexico (amorphous) 11,143 tonnes. In addition, there are two specialist producers: Sri Lanka produced 3,000 tonnes in 2005 of lump or vein graphite, and Madagascar produced 15,000 tonnes, a large portion of it "crucible grade" or very large flake flake graphite. Some other producers produce very small amounts of "crucible grade".

According to the USGS, U.S. (synthetic) graphite electrode production in 2005 was 146,000 tonnes valued at $391 million, and high-modulus graphite (carbon) fiber production in 2005 was 7,020 tonnes valued at $134 million.

graphite's unit cell
ball-and-stick model of a graphite layer
side view of layer stacking
plan view of layer stacking

Detailed properties

The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another.

Graphite can conduct electricity due to the vast electron delocalization within the carbon layers. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.

Graphite and graphite powder is valued in industrial applications for its self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are solely due to the loose interlamellar coupling between sheets in the structure. However, it has been shown that in a vacuum environment (such as in technologies for use in space), graphite is a very poor lubricant. This observation led to the discovery that the lubrication is due to the presence of fluids between the layers, such as air and water, which are naturally adsorbed from the environment. This molecular property is unlike other layered, dry lubricants such as molybdenum disulfide. Recent studies suggest that an effect called superlubricity can also account for graphite's lubricating properties. The use of graphite is limited by its tendency to facilitate pitting corrosion in some stainless steels, and to promote galvanic corrosion between dissimilar metals (due to its electrical conductivity). It is also corrosive to aluminium in presence of moisture. For this reason, the US Air Force banned its use as a lubricant in aluminium aircraft [1], and discouraged its use in aluminium-containing automatic weapons [2]. Even graphite pencil marks on aluminium parts may facilitate corrosion [3]. Another high-temperature lubricant, hexagonal boron nitride, has the same molecular structural as graphite. It is sometimes called white graphite, due to its similar properties.

When a large number of crystallographic defects binds these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic carbon. This material is useful for blood-contacting implants such as prosthetic heart valves. It is also highly diamagnetic, thus it will float in mid-air above a strong magnet.

Graphite forms intercalation compounds with some metals and small molecules. In these compounds, the host molecule or atom gets "sandwiched" between the graphite layers, resulting in compounds with variable stoichiometry. A prominent example of an intercalation compound is potassium graphite, denoted by the formula KC₈.

Natural and crystalline graphites are not often used in pure form as structural materials, due to their shear-planes, brittleness and inconsistent mechanical properties.

History

Some time before 1565 (some sources say as early as 1500), an enormous deposit of graphite was discovered at the site of Seathwaite Fell near Borrowdale, Cumbria, England, which the locals found very useful for marking sheep. This particular deposit of graphite was extremely pure and solid, and could easily be sawn into sticks. This remains the only deposit of graphite found in this solid form.^[1]

Uses

According to the USGS, U.S. consumption of natural graphite in 2004-05 averaged 43,800 tonnes in end uses such as refractories, steelmaking, expanded graphite, brake linings, and foundry facings-lubricants. GAN (Graphite Advocate News) import-export statistics for 2006 and 2007 indicate the consumption will continue at that level unless steelmaking carbon raiser takes a drastic drop.

Refractories: This end-use begins before 1900 with the graphite crucible used to hold molten metal; this is now a minor part of refractories. In the mid1980s, the carbon-magnesite brick became important, and a bit later the alumina-graphite shape. Currently the order of importance is alumina-graphite shapes, carbon-magnesite brick, monolithics (gunning and ramming mixes), and then crucibles. Crucibles began using very large flake graphite, and carbon-magnesite brick requiring not quite so large flake graphite; for these and others there is now much more flexibility in size of flake required, and amorphous graphite is no longer restricted to low-end refractories. Alumina-graphite shapes are used as continuous casting ware, such as nozzles and troughs, to convey the molten steel from ladle to mould, and carbon magnesite bricks line steel converters and electric arc furnaces to withstand extreme temperatures. High-purity monolithics are often used as a continuous furnace lining instead of the carbon-magnesite bricks. The U.S. and European refractories industry had a crisis in 2000-2003, with an indifferent market for steel and a declining refractory consumption per tonne of steel underlying firm buyouts and many plant closings. Many of the plant closings resulted from the RHI acquisition of Harbison-Walker Refractories; some plants had their equipment auctioned off. Since much of the lost capacity was for carbon-magnesite brick, graphite consumption within refractories area moved towards alumina-graphite shapes and monolithics, and away from the brick. The major source of carbon-magnesite brick is now imports from China. Almost all of the above refractories are used to make steel and account for 75% of refractory consumption; the rest is used by a variety of industries, such as cement. According to the USGS, 2005 U.S. natural graphite consumption in refractories was 11,800 tonnes.

Steelmaking: Natural graphite in this end use mostly goes into carbon raising in molten steel, although it can be used to lubricate the dies used to extrude hot steel. Supplying carbon raiser is very competitive, therefore subject to cut-throat pricing from alternatives such as synthetic graphite powder, petroleum coke, and other forms of carbon. A carbon raiser is added to increase the carbon content of the steel to the specified level. A GAN consumption estimate based on USGS U.S. graphite consumption statistics indicates that 10,500 tonnes was used in this end-use in 2005.

Expanded Graphite (including foil and packings): Expanded graphite is made by immersing natural flake graphite in a bath of chromic acid, then concentrated sulfuric acid, which forces the crystal lattice planes apart, thus expanding the graphite. The expanded graphite can be used to make graphite foil or used directly as "hot top" compound to insulate molten metal in a ladle or red-hot steel ingots and decrease heat loss, or as firestops fitted around a firedoor (During a fire, the graphite expands and chars to resist fire penetration and spread.), or to make high-performance gasket material for high-temperature use. After being made into graphite foil, the foil is machined and assembled into the bipolar plates in fuel cells. The foil is made into heat sinks for laptop computers which keeps them cool while saving weight, and is made into a foil laminate that can be used in valve packings or made into gaskets. Old-style packings are now a minor member of this grouping: fine flake graphite in oils or greases for uses requiring heat resistance. A GAN estimate of current U.S. natural graphite consumption in this end use is 8,000 tonnes.

Brake Linings: Natural amorphous and fine flake graphite are used in brake linings or brake shoes for heavier (nonautomotive) vehicles, and became important with the need to substitute for asbestos. This use has been important for quite some time, but nonasbestos organic (NAO) compositions are beginning to cost graphite market share. A brake-lining industry shake-out with some plant closings has not helped either, nor has an indifferent automotive market. According to the USGS, U.S. natural graphite consumption in brake linings was 6,510 tonnes in 2005.

Foundry facings and lubricants: A foundry facing or mold wash is a water-based paint of amorphous or fine flake graphite. Painting the inside of a mold with it and letting it dry leaves a fine graphite coat that will ease separation of the object cast after the hot metal has cooled. Graphite lubricants are specialty items for use at very high or very low temperatures, as a wire die extrusion lubricant, an antiseize agent, a gear lubricant for mining machinery, and to lubricate locks. Having low-grit graphite, or even better no-grit graphite (ultra high purity), is highly desirable. It can be used as a dry powder, in water or oil, or as colloidal graphite (a permanent suspension in a liquid). An estimate based on USGS graphite consumption statistics indicates that 2,200 tonnes was used in this end-use in 2005.

Natural graphite is the substance used as the marking material ("lead") in common pencils, in zinc-carbon batteries, in electric motor brushes, and in some other minor uses.

The major forms of synthetic graphite are as follows:

Synthetic graphite electrodes: These electrodes carry the electricity that heats electric arc furnaces, the vast majority steel furnaces. They are made from petroleum coke after it is mixed with petroleum pitch, extruded and shaped, then baked to sinter it, and then graphitized by heating it above the temperature that converts carbon to graphite. They can vary in size from 6 in. long to 6 ft. in diameter. The graphite electrode market is shrinking: plasma-arc furnaces (no electrodes) are often replacing electric arc furnaces, and the electric arc furnace itself is getting more efficient and making more steel per tonne of electrode. An estimate based on USGS data indicates that graphite electrode consumption was 197,000 tonnes in 2005.

Synthetic graphite powder and scrap: The powder is made by heating powdered petroleum coke above the temperature of graphitization, sometimes with minor modifications. The graphite scrap comes from pieces of unusable electrode material (in the manufacturing stage or after use) and lathe turnings, usually after crushing and sizing. Most synthetic graphite powder goes to carbon raising in steel (competing with natural graphite), with some used in batteries and brake linings. According to the USGS, U.S. synthetic graphite powder and scrap production was 95,000 tonnes in 2001 (latest data).

Graphite (carbon) fiber and carbon nanotubes are also used in carbon fiber reinforced plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC). Products made from carbon fiber graphite composites include fishing rods, golf clubs, and bicycle frames, and have been successfully employed in reinforced concrete. The mechanical properties of carbon fiber graphite-reinforced plastic composites and grey cast iron are strongly influenced by the role of graphite in these materials. In this context, the term "(100%) graphite" is often loosely used to refer to a pure mixture of carbon reinforcement and resin, while the term "composite" is used for composite materials with additional ingredients.

Synthetic graphite also finds use as a matrix and neutron moderator within nuclear reactors. Its low neutron cross section also recommends it for use in proposed fusion reactors. Care must be taken that reactor-grade graphite is free of neutron absorbing materials such as boron, widely used as the seed electrode in commercial graphite deposition systems-- this caused the failure of the Germans' World War II graphite-based nuclear reactors. Since they could not isolate the difficulty they were forced to use far more expensive heavy water moderators. Graphite used for nuclear reactors is often referred to as Nuclear Graphite.

Graphite has been used in at least three radar absorbent materials. It was mixed with rubber in Sumpf and Schornsteinfeger, which were used on U-boat snorkels to reduce their radar cross section. It was also used in tiles on early F-117 Nighthawks.

Graphite milling

One industrial form of processing the mineral graphite is through the milling process. In that process graphite is ground to a fine powder for use as a slurry in oil drilling; in zirconium silicate, sodium silicate and isopropyl alcohol coatings for foundry molds; and for calcined petroleum coke, which is used as a carbon raiser in the steel industry (Earth Metrics, 1989). Rough graphite is typically ground and packaged at a graphite mill; often the more complex formulations are also mixed and packaged at the mill facility. Environmental impacts from graphite mills consist of air pollution including fine particulate exposure of workers and also soil contamination from powder spillages leading to heavy metals contaminations of soil. Dust masks are normally worn by workers during the production process to avoid worker exposure to the fine airborne graphite and zircon silicate.

Graphite recycling

The most common way graphite is recycled occurs when synthetic graphite electrodes (or anodes or cathodes) are either manufactured and pieces are cut off or lathe turnings are discarded, or the electrode (or other) are used all the way down to the electrode holder. A new electrode replaces the old one , but a sizeable piece of the old electrode remains. This is crushed and sized, and the resulting graphite powder is mostly used to raise the carbon content of molten steel. Graphite-containing refractories are sometimes also recycled , but often not because of their graphite: the largest-volume items, such as carbon-magnesite bricks that contain only 15%-25% graphite, usually contain too little graphite. However, some recycled carbon-magnesite brick is used as the basis for furnace repair materials, and also crushed carbon-magnesite brick is used in slag conditioners. While crucibles have a high graphite content, the volume of crucibles used and then recycled is very small.

A high-quality flake graphite product that closely resembles natural flake graphite can be made from steelmaking kish. Kish is a large-volume near-molten waste skimmed from the molten iron feed to a basic oxygen furnace, and is a mix of graphite (precipitated out of the supersaturated iron), lime-rich slag, and some iron. The iron is recycled on site, so what is left is a mixture of graphite and slag. The best recovery process uses hydraulic classification (Which utilizes a flow of water to separate minerals by specific gravity: graphite is light and settles nearly last.) to get a 70% graphite rough concentrate. Leaching this concentrate with hydrochloric acid gives a 95% graphite product with a flake size ranging from 10 mesh down.

Media

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References

^ "Pencil". 2007-08-07. Retrieved 2007-08-07.

C.Michael Hogan, Marc Papineau et al., Phase I Environmental Site Assessment, Asbury Graphite Mill, 2426-2500 Kirkham Street, Oakland, California, Earth Metrics report 10292.001, December 18, 1989
Klein, Cornelis and Cornelius S. Hurlbut, Jr. (1985) Manual of Mineralogy: after Dana 20^th ed. ISBN 0-471-80580-7
Taylor, Harold A., "Graphite", Financial Times Executive Commodity Reports (London: Mining Journal Books ltd.) 2000 ISBN 1-84083-332-7
Taylor, Harold A., "Graphite", Industrial Minerals and Rocks, 7th ed. (Littleton, CO AIME-Society of Mining Engineers) 2005 ISBN 0-87335-233-5

External links

[1] "Pencil". 2007-08-07. Retrieved 2007-08-07.

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