graphite

Graphite

General
CategoryNative mineral
Chemical formulaCarbon, C
Identification
ColorSteel black, to gray
Crystal habitTabular, six-sided foliated masses, granular to compacted masses
Crystal systemHexagonal (6/m 2/m 2/m)
CleavagePerfect in one direction
FractureFlaky, otherwise rough when not on cleavage
Mohs Scale hardness1 - 2
Lustermetallic, earthy
Refractive indexOpaque
PleochroismNone
StreakBlack
Density2.09–2.23 g/cm³
SolubilityMolten 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

Enlarge picture
Graphite ore
Enlarge picture
Graphite output in 2005
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


Structure

Each carbon atom is covalently bonded to three other surrounding carbon atoms. The flat sheets of carbon atoms are bonded into hexagonal structures. These exist in layers, which are not covalently connected to the surrounding layers. Instead, different layers are connected together by weak forces called van der Waals forces much like those of mica.

The unit cell dimensions are a = b = 2.456 ångströms = 245.6 picometers, c = 6.694 Å = 669.4 pm. The carbon-carbon bond length in the bulk form is 1.418 Å (141.8 pm), and the interlayer spacing is c/2 = 3.347 Å (334.7 pm).

Enlarge picture
Crystal structure of graphite
Each carbon atom possesses an sp² orbital hybridisation. The pi orbital electrons delocalized across the hexagonal atomic sheets of carbon contribute to graphite's conductivity. In an oriented piece of graphite, conductivity parallel to these sheets is greater than that perpendicular to these sheets.

The bond between the atoms within a layer is stronger than the bond of diamond, but the force between two layers of graphite is weak. Therefore, layers of it can slip over each other making it soft.

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 prior to 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. The locals found that it was very useful for marking sheep. This particular deposit of graphite was extremely pure and solid, and it could easily be sawn into sticks. This was and remains the only deposit of graphite ever 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

See also

References

1. ^ (2007-08-07). Retrieved on 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 20th 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

A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties.
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A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. A chemical formula is also a short way of showing how a chemical reaction occurs.
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4, 2
(mildly acidic oxide)
Electronegativity 2.55 (Pauling scale)
Ionization energies
(more) 1st: 1086.5 kJmol−1
2nd: 2352.6 kJmol−1
3rd: 4620.5 kJmol−1

Atomic radius 70 pm
Atomic radius (calc.
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habit of crystals.

The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals.
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Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the typical metamorphic rock sequence of slate, phyllite, schist and gneiss.
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A crystal system is a category of space groups, which characterize symmetry of structures in three dimensions with translational symmetry in three directions, having a discrete class of point groups.
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hexagonal crystal system is one of the 7 lattice point groups (see Hexagonal_lattice). It has the same symmetry as a right prism with a hexagonal base. There is only one hexagonal Bravais lattice, which has six atoms per unit cell.
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Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types:
  • Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes.

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fracture is the (local) separation of a body into two, or more, pieces under the action of stress.

The word fracture is often applied to bones of living creatures, or to crystals or crystalline materials, such as gemstones or metal.
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The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in
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Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock, or mineral.
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The streak (also called powder color) of a mineral is the color of the powder produced when it is dragged across an unweathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a
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In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
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Solubility is a physical property referring to the ability for a given substance, the solute, to dissolve in a solvent.[1] It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution.
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Abraham Gottlob Werner (September 25, 1749 – June 30, 1817), was a German geologist who set out a controversial theory about the stratification of the Earth's crust and coined the now obsolete word Neptunism.
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Greek}}} 
Writing system: Greek alphabet 
Official status
Official language of:  Greece
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recognised as minority language in parts of:
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pencil is a writing instrument or drawing instrument consisting of a thin stick of pigment (usually graphite, but can also be coloured pigment or charcoal) and clay, usually encased in a thin wood cylinder though paper and plastic sheaths are also used.
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Diamond is one of the best known allotropes of carbon, whose hardness and high dispersion of light make it useful for industrial applications and jewelry. Diamond is the hardest known natural mineral, which makes it an excellent abrasive and makes it hold polish and lustre
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Diamond is an allotrope of carbon. It is the hardest known natural material and the third-hardest known material after aggregated diamond nanorods and ultrahard fullerite. Its hardness and high dispersion of light make it useful for industrial applications and jewelry.
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In science and engineering, conductors, such as copper or aluminum, are materials with atoms have loosely held valence electrons. See electrical conduction.

Conductors in context


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An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by the scientist Michael Faraday from the Greek words elektron
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arc lamp is the general term for a class of lamps that produce light by an electric arc (or voltaic arc). The lamp consists of two electrodes typically made of tungsten which are separated by a gas.
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Coal (IPA: /ˈkəʊl/) is a fossil fuel formed in swamp ecosystems where plant remains were saved by water and mud from oxidization and biodegradation.
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Anthracite (Greek Ανθρακίτης, literally "a form of coal", from Anthrax [Άνθραξ], coal) is a hard, compact variety of mineral coal that has a high luster.
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Carbon fibre or carbon fiber can refer to carbon filament thread, or to felt or woven cloth made from those carbon filaments. By extension, the term is also used informally to mean any composite material made with carbon filament, such as carbon fiber reinforced plastic.
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Carbon fiber reinforced plastic (CFRP or CRP), is a very strong, light and expensive composite material or fiber reinforced plastic. Similar to glass-reinforced plastic, which is sometimes simply called fiberglass, the composite material is commonly referred to by
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Quartz (from German Quarz  [1]) is the second most common mineral in the Earth's continental crust, feldspar being the first.
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calcite]] The carbonate mineral, calcite, is a chemical or biochemical calcium carbonate corresponding to the formula CaCO3 and is one of the most widely distributed minerals on the Earth's surface.
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