This article was written by Executive Consultant, Andrew Scogings.
Graphite is an allotrope of carbon and is grey to black, opaque, very soft, has a low density and a metallic lustre. It is flexible and exhibits both non-metallic and metallic properties, making it suitable for diverse industrial applications. Physical properties include specific gravity of 2.2, and Mohs hardness of 1-2. Flake graphite is produced and sold according to specifications, the most basic of which is size faction for which there commonly accepted market descriptions such as Fine, Small, Medium, Large and Extra Large (see table below).
Global natural graphite production is approximately 1 to 1.2 million tonnes (Mt) per annum, of which flake graphite is estimated to be about 600-800 kt per year, amorphous graphite about 300 kt and vein graphite around 4 kt.
The leading world producers of flake graphite in 2018 were China and Brazil. Mozambique is ramping up production significantly, having produced around 0.1 Mt in 2018 – it is anticipated to overtake Brazil and become the second largest global producer in 2019. China is also seeing increased flake graphite production, mainly from Heilongjiang Province, to supply the battery anode market.
Amorphous graphite is produced mainly in China and Mexico. Sri Lanka is the sole producer of vein graphite.
The major usages of graphite are in refractories, batteries, expandable graphite, plus brake linings, lubricants and steelmaking / foundry operations.
Batteries are the fastest growing market for flake graphite. Chinese producers use small / fine flake (China Specification LG -194; 80% < 100 mesh, Carbon >94%, moisture <0.5%) to make spherical graphite for battery anode applications. The conversion ratio for flake to spherical graphite is estimated to be around 30-40%, hence the production of spherical graphite is expected to generate significant tonnages of fine (‘amorphous’) graphite by-product and drive prices down at that end of the market. Exports of spherical graphite from China increased by around 40% in 2018 compared with 2017 (Benchmark Mineral Intelligence, 12 December 2018).
Expandable graphite is another market expected to grow, for applications such as fire retardation, insulation and heat transmission applications. As a rule of thumb, the highest expansion rates are achieved by large and extra large flake products generally >80 mesh (>180 micron).
Natural graphite deposits occur in three main geological settings: flake graphite disseminated in metamorphosed sedimentary rocks such as gneiss or schist, amorphous graphite formed by metamorphism of coal or carbon-rich sediments, and vein or lump graphite filling fractures in granitic country rock.
The term ‘amorphous graphite’ is a commercial term and is misleading, as all graphite has a crystalline structure though the degree of crystallinity may vary. Fine crystalline flake is often described in the trade as ‘amorphous’, leading to confusion when analysing graphite production and markets.
Graphite may be manufactured synthetically from carbon-bearing raw materials such as petroleum coke. Synthetic graphite is far higher-priced than natural graphite.
Natural graphite products typically contain mineral impurities, which may include silicate and sulphide minerals such as quartz, mica or pyrite in the case of flake graphite. Amorphous graphite derived from coal may contain sedimentary rock impurities such as shale, sandstone, quartzite or limestone.
Graphite is explored for by methods such as field mapping, outcrop sampling, trenching, geophysics, drilling, and analysis of the graphite content followed by mineralogical and metallurgical testing.
Given that graphite conducts electricity, various electromagnetic (EM) methods can be highly effective exploration tools for graphite mineralisation.
There are two main methods of drilling for natural graphite: reverse circulation percussion (RC) and diamond core drilling (DD). Auger drilling may occasionally be used to explore highly-weathered clayey mineralisation. RC is a useful way of infill drilling to demonstrate geological and grade continuity, as it is less costly than DD; however RC drill chips and powder are not suitable for metallurgical / process tests. DD is the preferred method of exploration drilling for graphite, as the graphite and host rock are relatively undisturbed when retrieved as core and hence appropriate for metallurgical testwork.
Carbon may be present in rocks in various forms including organic carbon, carbonates or graphitic carbon. Carbon in rocks may be reported as total carbon (organic carbon + carbon in carbonate minerals + carbon as graphite) or as total graphitic carbon (total carbon – (organic + carbonate carbon).
Therefore, when total graphitic carbon (TGC) is to be reported, organic carbon and carbon in carbonate minerals such as calcite should be removed before analysing TGC.
Exploration samples can be examined in thin section under a petrographic microscope, which is a relatively affordable and quick way of estimating the in situ graphite flake size distribution, gangue mineralogy and likely liberation characteristics. Optical microscopy may be complemented by methods such as XRD, QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) and MLA (Mineral Liberation Analyser, or automated SEM).
Assaying for graphitic carbon quantifies the amount of graphite contained within a deposit, but does not indicate the amount, size distribution or purity of graphite that may be recoverable.
Therefore, samples should be tested at a metallurgical laboratory, which would typically run comminution and flotation tests to produce graphite concentrates (products) for further product performance tests and evaluation by potential customers.
Performance and characterisation tests include bulk density, crystallinity, Loss on Ignition (LOI), chemical purity, expandability and spheronisation amongst others.
Global flake graphite exploration
There has been intensive exploration for graphite by publicly-listed companies since about 2012, with most activities aimed at flake graphite deposits in Australia, Canada, Mozambique and Tanzania. The biggest Mineral Resource tonnages were discovered in northern Mozambique and southern / central Tanzania. Other target countries include Brazil, Finland, Madagascar, Malawi, Sweden and the USA.
Publicly reported exploration has added approximately 5.3 billion tonnes to the global flake graphite Mineral Resource base, at an average grade of just under 9% graphite. The reported deposits range in size from about 2 million tonnes (Mt) to over 1,000 Mt, while grades in individual deposits range from approximately 2% to as high as around 25% graphite (TGC).
The size distribution and purity of liberated flakes is important to project economics, as this affects product marketability and sales price. As a general rule of thumb, larger flakes attract a higher price than small flakes, e.g. extra large flakes >300 micron may attract a price several times that of small flakes <150 micron.
Mining and production
Flake graphite deposits occur as lens-like to tabular bodies and are generally mined opencast, though some high-grade flake graphite deposits are mined underground in Germany and Norway and until recently, in Zimbabwe. Most flake graphite deposits mined opencast contain between about 5% and 15% graphite, whereas underground mines have grades of around 20-30% graphite.
Some flake graphite deposits are deeply weathered and described as ‘free-dig’ given that explosives are not needed to extract the ore – examples include producing mines in Brazil and Madagascar and some exploration projects in Malawi and Australia.
Vein deposits have complex geometry, are narrow and are selectively mined underground in Sri Lanka. Amorphous graphite is mined underground, mainly in China, using methods similar to coal mining.
The most commonly used beneficiation method for flake graphite is flotation, while acids or alkalis may be used to leach out impurities from concentrates. Graphite ore is crushed and ground in rod mills, before flotation, followed by several regrind and cleaner flotation stages and final drying, screening and packaging. Vein graphite is often purified by hand-sorting.
Most spherical graphite for lithium-ion battery anodes is manufactured in China and then exported to Japan or Korea for final coating. The process involves micronising and spheronising the graphite flakes followed by purification (commonly acid, alkaline or heat treatment) before being coated and made into anodes. The conversion ratio for flake to spherical graphite is estimated to be around 30-40%.
Graphite products are typically specified, at a minimum, by particle size distribution and carbon content (purity) as shown in the table below for some refractory products. There are no set industry specifications, although in countries such as China the government has established national standards based mainly on carbon content and size category.
Chinese flake graphite standards are based on four categories LC, LG, LZ and LD according to carbon content. These four categories are subdivided into 212 grades according to retained percentage by mesh size, fixed carbon, volatile content and moisture.
Other specifications agreed to by the producer and customer may be moisture, volatile content, bulk density, crystallinity, specific surface area, impurities, peak oxidation temperature and expansion volume for example.
Reporting graphite Mineral Resources and Ore Reserves
Clause 49 of the JORC Code 2012 requires that: “For minerals that are defined by a specification, the Mineral Resource or Ore Reserve estimation must be reported in terms of the mineral or minerals on which the project is to be based and must include the specification of those minerals.”
Natural graphite is an industrial mineral produced to customer specifications and therefore ASX and NZX listed companies are required to report Mineral Resources, Ore Reserves (and Exploration Results) in accordance with Clause 49 of the JORC Code.
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