The write stuff

Materials World magazine
,
1 Feb 2015

In the first half of a special focus on graphite, Guy Richards charts its uses throughout history and examines some trends in supply and demand. 

To a layperson, graphite is simply the active ingredient in a pencil. But in industry its wide range of uses – as a refractory material, in electric motors, as a lubricant and in nuclear reactors, for example – make it a key commodity, and thanks to the advent of new technologies and emerging markets, this common mineral’s importance is set to grow even more.

Its use extends back through the millennia. Current knowledge dates the earliest application of what we now call graphite to at least 6,000 years ago, by Neolithic cultures in south-eastern Europe as a ceramic paint for decorating pottery, but it does not appear to have come into more widespread use, this time for drawing, until the Middle Ages.

As Daniel Thompson, in his book The Materials and Techniques of Medieval Painting (Allen and Unwin, 1956), says, ‘There are occasional references in 14th and 15th Century texts to the use of a soft black stone. This stone was presumably graphite… The evidence suggests that is was used in the Middle Ages… chiefly for drawing’.
Medieval artists had found that graphite was not only better than lead styli (as used by scribes on wax tablets since Roman times) and charcoal for drawing, but it was easier to erase and could be drawn over with ink. Sticks of graphite snap easily though, so at first they were wrapped in string then later encased in wood, heralding the form of the modern pencil.

In the mid-1500s – historical sources aren’t precise here – a huge deposit of very pure, solid graphite was uncovered in Barrowdale, in the north-west of England, after a storm toppled a tree growing above it. It remains the largest of its kind ever found, and for a time it gave England a monopoly on the world’s pencil supplies, as it was only this deposit that could be cut into high-quality pencil form without further processing.

However, this monopoly disappeared during the Napoleonic Wars, at the end of the 18th Century. France could no longer import pencils from England, but it was discovered (by two men, independently and a few years apart) that poorer-grade graphite could be used instead, by crushing it to remove its impurities, mixing it with clay, then forming it into sticks and firing in a kiln. They both also found that varying the clay-to-graphite ratio produced sticks of different levels of blackness and hardness, which is essentially how pencil grades of HB, 2H and others are still made.

Back in Elizabethan Barrowdale, farmers had been quick to realise that the graphite could be used to mark their sheep, but it also began to take on a military significance, thanks to its refractory properties.

Graphite’s value as a refractory material had already been known for some time. As an example, in 2007, a team from University College London published research into the manufacture of crucibles in continental Europe from post-Medieval times. In their paper, they say, ‘This study has demonstrated the existence of a large-scale international trade of crucibles since the Renaissance… the dark, smooth, usually graphitic crucibles from Bavaria and the surrounding region [were] used in a variety of pyrotechnological contexts, including copper metallurgy, ore assaying and coin minting.’ By contrast, the Barrowdale deposit was being used to line moulds for cannonballs, as it allowed for smoother and rounder balls that could be fired further. This gave it strategic importance, so it was taken over by Elizabeth I’s Government and locals were barred from it – such was its value, the mined graphite was transported to London under guard.

Graphite was usually called black lead or plumbago (from the Latin plumbium for lead, because of its appearance) until 1789, when German mineralogist AG Werner coined its modern name, from the Greek for ‘writing stone’. As the Industrial Revolution gathered pace, the use of natural graphite continued to grow, still largely as a refractory material and as a result of the mechanisation of pencil production by companies such as AW Faber, the forebear of the modern Faber-Castell group.

In the mid-1890s, a US chemist named EG Acheson discovered a way to make synthetic graphite. He found that if silicon carbide (SiC) – also called carborundum, which Acheson is also credited with discovering – is heated to about 4,150°C then the silicon component vapourises, leaving behind graphitic carbon. He was awarded a patent for the process in 1896 and started commercial production the following year.
Synthetic graphite is purer than the natural form, and in recent years demand for it has been driven by its use as the negative electrode in lithium-ion batteries for laptops, smartphones and electric cars, with natural graphite accounting for only about 5% of this demand. But synthetic graphite is expensive to produce – up to five times that of the best natural graphite – so battery manufacturers are looking to increase their use of natural

Meeting this fresh demand is feasible in principle, as natural graphite occurs in reasonable abundance in many parts of the world, but doing so hinges on its quality. Graphite comes in three forms – amorphous, flake and vein/lump. The amorphous form (70–75% carbon) is the most common and is used in pencils, dry lubrication and refractory applications. Flake graphite (85–90% carbon) has extremely low electrical resistance, so this is the form required for applications such as batteries and carbon electrodes, while the vein/lump form (90–96% carbon) is suitable for many of the same applications as flake but is regarded as the most valuable, as it requires the least primary processing.

Note that no mention is made here of its use to make graphene. That’s because, although it is widely touted as the material of the moment, the market for graphene is still in its infancy, and questions remain among some industry observers as to its relevance to the natural graphite mining industry.

In terms of overall world production and demand, according to statistics portal Statista, in 2013 the largest producers – China, India, Brazil, North Korea and Canada – put about 1.13Mt of graphite onto the market. 

China has dominated world production, with about 70% of the share, but it has been closing its graphite mines in response to environmental health concerns and its stated intention to keep more of its graphite for domestic use. This will of course severely curtail global supplies over the next few years, yet at the same time it is estimated that by 2020, world graphite consumption will be 1.9Mt and demand for flake graphite for lithium-ion batteries alone will outstrip total current production levels.

To put this in perspective, the US Geological Survey estimates the graphite market to be 10 times the size of that for rare earth elements and about the same size as that for nickel, but the pressures on supply and demand will take time to ease as new mines come onstream. As a result, the USA – which produces no natural graphite of its own – and the EU states have included it on their lists of critical materials.
All of this makes graphite a mineral to watch. In the next issue, we’ll look in more detail at how the market for it over the next few years is likely to play out.


Production and processing

Graphite occurs in metamorphic and igneous rocks, and is mined around the world using open-pit and underground methods. While flake and amorphous graphites are mined in both ways, lump (vein) graphite is only mined underground in Sri Lanka, a minor producer, whose 2013 output was 5,000t, according to statistics portal Statista.

As outlined in the main article, flake graphite is likely to be in particular demand in the next few years, and most of the world’s production is processed by crushing, grinding and then flotation, which typically involves using a series of flotation cells to purify the concentrate. This can sometimes be difficult, as graphite can coat particles of the gangue, which float off with the graphite and therefore lower the concentrate’s purity. In these circumstances the gangue is acid-leached – hydrofluoric acid for a silicate gangue, hydrochloric acid for a carbonate.
The graphite can then be screened according to different flake sizes, to give either standard blends or custom ones for particular customers. Rough graphite is usually processed at a graphite mill, while purer or more complex formulations are treated, mixed and packaged at an upgrade facility.


For more on graphite, read Post-Medieval Crucible Production And Distribution: A Study Of Materials And Materialities, Martinón-Torres M and Rehren YH, Archaeometry 51, pp49–74