Gray, black, steel

It conducts electricity well, it’s relatively soft, it has a metallic shine, it’s heat stable, it's slippery to the touch and well-known for its flaky texture – all of that applies to graphite. We get acquainted with this mineral in childhood when we get hold of a simple pencil. But besides rods, graphite has many other useful applications. Read about the properties, characteristics, processing, and use of graphite in our material.

The story of the first pencil

Scientists cannot understand, when the ancient people began using graphite. The thing is, its coloring properties resemble other minerals, in particular molybdenite, so people could draw using different fossils. The first recorded use of graphite was the ancient utensils of the Boian culture, painted with graphite 4,000 BC. But unfortunately, the oldest pencils appeared not in the Neolithic era.

The widespread use of graphite in drawing began after an accident in England. In the middle of the 16th century, there was a violent storm in the county of Cumberland. Many old and heavy trees were knocked upside down. The shepherds found a solid colorant in the resulting pits that resembled coal. Local artisans assumed that it was a kind of tin and called the mineral “drawing tin.” It was only in 1789 that graphite acquired its present name, thanks to the German geologist Abraham Werner. It is based on the Ancient Greek word “γράφω” which means “I write.”

Properties and characteristics

Graphite has a number of useful properties that are actively used today in the industrial process. Without going into details, the basic characteristics are enough to get acquainted with graphite:

1. Conducts electric current
2. Possesses low hardness (1 on the Mohs scale)
3. Density: 2.08–2.23 g/cm³
4. Color: gray, black, steel / metallic shine
5. Stable when heated in the absence of air
6. Greasy, slippery to the touch (contains 10–12% of impurities of clays and iron oxides)
7. When rubbed, it exfoliates into separate flakes (for example, a pencil trace on paper consists of consecutively laid graphite flakes)

Origin and extraction

Natural graphite forms at high temperatures in volcanic and igneous rocks, namely pegmatites and skarns. It can be found in quartz veins with wolframite and other minerals. Graphite is common in crystalline schists, gneisses, and marbles. Large deposits are formed as a result of the thermal decomposition of hard coal into coal deposits. A well-known graphite deposit is the Tunguska basin. The rocks containing graphite often have accompanying materials: quartz, pyrite, garnets, spinel. In very small amounts, graphite is present in meteorites.

In addition to natural graphite, there’s an artificial one as well. The scientists learned to obtain it in different ways.

• Acheson graphite. Produced from a mixture of coke and pitch when heated to 2800 °C.
• Recrystallized graphite. The main method is the thermomechanical treatment of a mixture of coke, pitch, natural graphite, and carbide-forming elements.
• Pyrolytic graphite. Produced by pyrolysis (thermal decomposition) from gaseous hydrocarbons at a temperature of 1400–1500 °C in a vacuum, followed by an increase in temperature to 2500–3000 °C at a pressure of 50 MPa. The result is pyro- or electrographite.
• Blast furnace graphite. Released during slow cooling of large masses of cast iron.
• Carbide graphite. Formed by thermal decomposition of carbides.

Application and processing

Graphite has many applications. It is used in the production of heating elements, as it has high electrical conductivity and is resistant to almost any aggressive aqueous solution. Graphite is used in solid lubricants, in the production of current collectors, for thermal protection of the nose part of ballistic missile warheads and reusable spacecraft. We’d like to speak on some particular options for using graphite in practice.

Waste solidification

Scientists have learned not only to synthesize graphite but to process it into other substances with improved qualities. This is how thermally expanded graphite was obtained. The technology is as follows: the original crystalline graphite is oxidized with molecules and ions of sulfuric or nitric acid in the presence of an oxidizing agent. The oxidized graphite is washed and dried and then subjected to high-temperature treatment at high speed. Thanks to the speed of the process, the introduced sulfuric acid decomposes from the graphite crystal lattice, and further gaseous products are released. They press onto the intercrystalline space and wedge the graphite structure, expanding it. Then the material is rolled, reinforced (strength is increased with additional fixture), additives are added to improve the properties, and then pressed. Thermally expanded graphite is characterized by high specific strength and low bulk density. It looks like powder. It is used in the nuclear industry to solidify radioactive oil waste.

Neutron moderation

Graphite also plays an important role in the nuclear industry in neutron moderation. However, natural graphite is not suitable for this purpose, as it has too many impurities. Reactor graphite is obtained from a mixture of petroleum coke and coal tar. They’re pressed into blocks and thermally treated at high temperatures. Neutron moderators are extremely important in a nuclear reaction: without them, the reaction may not take place.

Calibration material

There is also pyrolytic graphite which is used as a calibration material in microscopic research. This type of graphite was created artificially by heating coke and pitch. It is most frequently used in scanning tunneling and atomic force microscopy.

Filling for plastics

Graphite-reinforced plastics are composite materials based on carbon-graphite fillers and polymer binders. The composition may include natural, crucible, or colloidal graphite. Graphite-reinforced plastic is used to produce sliding electrical contacts, friction units for agricultural machinery, pumps, dry compressors, separators for water emulsion mediums, chemically resistant equipment units, heat exchangers for aggressive mediums.

Gold and diamonds

Russian scientists studied graphite samples and found that the gold content reached 17.8 g/t. This is comparable to the level of gold mines and makes graphite ores a promising place for gold mining, since the deposits are most often located in areas with developed infrastructure, and not in places with harsh natural conditions. It is also possible to obtain synthetic diamonds from graphite.

The article is based on open sources.

Photo: mindatnh.org