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Graphite

GRAPHITE

 

 

GRAPHITE is a form of elemental carbon. It is one of the three allotropic forms which the element carbon exists in nature, the other two being coal and diamond. It crystallises in hexagonal system in platy form but it is rare that perfect crystals of graphite have been found. Graphite is of metamorphic origin and usually found as veins, lenses, pockets and as thin laminae disseminated in the gneisses, schists and phyllites. Depending upon the mode of occurrence and origin, it is graded into three forms:

  • Flake - found in metamorphosed rocks as vein deposits.

  • Crystalline (lumpy) - found as fissure filled veins.

  • Cryptocrystalline (amorphous) - form in metamorphosed coal beds.

It has black to steel grey colour, usually leaves a black streak on the hand when touched because of its extreme softness and greasiness. It is opaque even in the finest particles. Graphite is a good conductor of heat and electricity. It has high refractoriness. It can stand a temperature upto 3000ºC in an inert atmosphere though in the presence of oxygen it burns between 620ºC to 720º C. It is unaffected by most of the acids and reagents but yields graphitic acid on treatment with a mixture of potassium nitrate and nitric acid.

In nature, graphite is found usually in mechanical association with felspar, mica, quartz, pyroxene, rutile, pyrites, and apatite. These impurities are associated with vein graphite. The impurities with amorphous graphite are shale, slate, sandstone, quartz and limestone.

Graphite is found in almost every country. Plentiful reserves, however, are possessed the following countries:

  • Ceylon

  • Malagasy

  • Mexico

  • West Germany

  • North & South Korea

Hardness

Associated Minerals

Streak

Colour

characteristics

Luster

Field Indicators

1 - 2

quartz
calcite
micas
iron
meteorites
tourmalines

black gray to brownish gray

black, silver

thin flakes are flexible but inelastic, mineral can leave black marks on hands and paper, weakly conducts electricity

metallic to dull

softness, luster, density and streak.

 

Graphite Mining
Graphite is mined by both open pit and underground methods. Graphite usually needs beneficiation. This may be carried out by hand-picking the pieces of gangue (rock) and hand-screening the product or by crushing the rock and floating out the graphite. Beneficiation by flotation encounters the difficulty that graphite is very soft and "marks" (coats) the particles of gangue. This makes the "marked" gangue particles float off with the graphite, yielding impure concentrate. There are two ways of obtaining a commercial concentrate or product: repeated regrinding and floating (up to seven times) to purify the concentrate, or by acid leaching (dissolving) the gangue with (for a silicate gangue) or (for a carbonate gangue).
In milling, the incoming graphite products and concentrates can be ground before being classified (sized or screened), with the coarser flake size fractions (below 8 mesh, 8–20 mesh, 20–50 mesh) carefully preserved, and then the carbon contents are determined. Some standard blends can be prepared from the different fractions, each with a certain flake size distribution and carbon content. Custom blends can also be made for individual customers who want a certain flake size distribution and carbon content. If flake size is unimportant, the concentrate can be ground more freely. Typical end products include a fine powder for use as a slurry in oil drilling and coatings for foundry molds, carbon raiser in the steel industry (Synthetic graphite powder and powdered petroleum coke can also be used as carbon raiser). 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.