Graphite exploration – not all graphite is created equal

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This article was written by Executive Consultant, Andrew Scogings.

Flake graphite projects are essentially no different from other mineral exploration projects, which require targets to be ranked before committing to costly drilling, assaying and metallurgical tests. Graphite exploration follows a similar path to other minerals, often from discovery of an outcrop or geophysical anomaly, followed up by methods such as field mapping, trenching, more detailed geophysics, drilling, analysis of the graphite content, mineralogical examination and metallurgical testing. Data generated in this way, if successful, could lead to the estimation of a Mineral Resource and Ore Reserve which underpin technical studies such as Scoping, Pre Feasibility or Feasibility Studies.

But the question to be asked right at the outset is: what constitutes the best graphite exploration target at a project?

Many junior graphite explorers over the past few years have espoused the notion that 'biggest is best'. So, should graphite exploration targets be ranked on possible tonnage and grade similar to the way other commodities such as metals often are?

My opinion is that, although resource tonnes and graphitic carbon content (grade) are important metrics in evaluating exploration / mining projects, the overall picture is more complex and there are other factors to be considered.

This is because natural graphite is an industrial mineral and is a lot more complex than it appears at face value - as there are a diverse (and sometimes bewildering) number of market-related specifications. At the bare minimum, flake size may be used to describe graphite products - see table below:

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Natural graphite is an example of a truly multi-functional industrial mineral. It has specific physical properties such as flexibility (and hence the ability to be rolled into tiny spheres for battery anodes), it’s soft and hence a lubricant, it conducts electricity, is stable at high temperatures and it can be expanded and made into foil for heat sinks in computers - this list is almost endless as are the specifications for individual markets.

The mention of the word 'markets' reminds me of that well-known saying by Peter Harben "Without a market, an industrial mineral deposit is merely a geological curiosity". Similarly, as noted by Border and Butt (2014) concerning the modifying factors for industrial minerals “Without a potential market, there can be no resource; without a good knowledge of the planned market (volume, price and competition), there is no reserve”.

This says a lot about ranking of graphite exploration targets, as the graphite will need to meet market specifications such as: moisture, chemical purity, particle size distribution, particle shape, mechanical strength, bulk density, thermal resistance, expandability and conductivity.

Therefore it is potentially misleading to simply rank graphite exploration targets based on tonnage and the contained mineral percentage for flake graphite projects. For example, a graphite explorer may hope to define a Mineral Resource of 100 million tonnes at 10% total graphitic carbon (TGC) at greater than 94% purity, but this conveys very little about the quality of product that may be possible to liberate. It's all well and good to have a huge tonnage of contained graphite (the example above has 10 million tonnes of contained graphite) but bear in mind that the global flake graphite market is under 1 million tonnes per year, hence the scale of production and expected market penetration should be realistic. In this regard, it's worth noting that many successful flake graphite mines currently produce around 7,000-20,000 tpa (e.g. Africa, Europe and China), though there are exceptions that turn out 50,000-70,000 tpa (e.g. China and Brazil) and a mine in Mozambique that is targeting ~300,000 tpa.

The explorer should therefore devote some time and money up front evaluating not only potential tonnage and grade, but also potential product quality before spending big dollars on drilling and further evaluation such as metallurgy / process tests.

Other considerations when evaluating graphite targets include the economics of mining and processing, as these are impacted by factors such as the size and geometry of the deposit, contained graphite and recoveries. The deposit geometry affects the strip ratio and, the higher the contained graphite, the less ore has to be mined and processed, as long as saleable product is delivered. Processing less ore results in lower production costs and less waste going to tailing dams, all of which can have a major impact on capital and operating expenditure. But the overriding factor is likely to be PRODUCT QUALITY as, without markets, the project is doomed to failure.

Finally, the explorer should consider logistics and accessibility to power, water, labour and transport routes as the cost of trucking and shipping often exceeds the value of industrial mineral products.

The TanzOz example

TanzOz is a private Australian company with a graphite exploration project in southern Tanzania. The tenement was initially surveyed regionally by means of VTEM (an airborne EM method known as Versatile Time Domain Electromagnetics) which highlighted numerous conductive targets that might be associated with graphitic rocks.

VTEM map of the TanzOz project. Red and yellow indicate the most conductive lithologies. Map grid 5km x 5km

But the question then was - how to rank the VTEM targets and where to spend the exploration dollars on drilling and subsequent metallurgical / process tests? TanzOz adopted the following sequence:

1) Dig trenches across the most conductive VTEM anomalies

2) Map the trenches, estimating visual graphite flake size and concentration and collect samples of graphitic rocks

3) Submit samples for chemical anlaysis of graphitic carbon and also have thin sections cut of selected samples for microscope examination

4) Fixed Loop Electromagnetic (FLEM) surveys were carried out over several targets, to determine dip and strike of conductive rocks. This assisted with predicting the expected depth of mineralisation when drilling.

Following this process, Targets CD and NP were selected for diamond core and reverse circulation drilling; these locations are about 3km apart and interestingly turned out to have quite different characteristics.

Cross section through Target NP, showing TGC assays and modelled FLEM plate (thin red line). Red bars 5-10% TGC; Orange bars 2-5% TGC

Example of a trench in the south western part of the project

Graphite gneiss sample collected from a trench, for petrographic examination

High-grade graphitic gneiss (~20% TGC) exposed in a shallow trench at Target CD

Although Target CD has much higher grade (graphite content) than Target NP, the liberated flake size distribution of CD isn't as favourable as at NP. In addition, the NP graphite has better expandability that CD graphite. Differences between these two deposits are illustrated below.

Bimodal flake graphite populations seen in thin section from Target CD. The larger flakes are in excess of 1 mm long, while the small flakes are about 0.2 mm in length.

Single graphite population seen in thin section from Target NP. The flakes are about 0.5 to 1 mm in length; however it's important to bear in mind that in situ flake size doesn't necessarily indicate the final liberated flake size distribution of a product.

The table above shows head grade (TGC) of metallurgical sample S1 (Target NP) and S2 (Target CD). The head grade of S2 is almost double that of S1. Differences in SiO2, Fe2O3, CaO and MgO between the two samples reflect the underlying mineralogy of the rocks. S1 is a quartzite that consists mainly of quartz and feldspar plus graphite, compared with S2 which is amphibolitic and consists predominantly of feldspar, hornblende and minor carbonates, plus graphite.

Thin section of hornblende plagioclase rock from Target CD

The tables above show flotation test results for samples from Targets CD and NP. Note the much higher mass retained on the 150 micron screen for sample S1 from the lower-grade Target NP

The next question to address was - how do these two targets compare when tested according to market performance specifications? For example, flake graphite can be evaluated for its ability to expand after chemical treatment for use in graphite foils and flame retardants products . TanzOz found that the +300 micron fraction of S1 expanded about 50% more than the +300 micron fraction of S2. This highlights that 'not all graphite is created equal' and just how important it is to test individual samples, not composites which would have masked this variability.

What can we conclude?

  • Graphite exploration targets should initially be ranked according to geophysical data, field mapping and trenching (indicative of graphite mineralisation extent), grade and perhaps some preliminary drilling (one hole per target?).
  • Ascertain in situ flake size, textural relationships between graphite and gangue minerals and overall host rock mineralogy early on in the project. Address questions such as - are there bimodal populations with some very small flakes that may be difficult to liberate? Is the graphite interleaved with other flaky minerals such as mica, or with clay minerals which could be difficult to physically separate?
  • Use a combination of thin sections and / or mineral liberation analysis and first pass metallurgical / process test work on outcrop / trench / drill samples to evaluate in situ and processed characteristics. This will provide data such as potential flake size and purity, which will impact on the likely 'basket price' for concentrate.
  •  Run some preliminary market-related tests to assess if the extracted graphite can actually find markets; e.g. bulk density, thermal stability, degree of crystallinity, or expandability.
  • Finally, select the best target/s and drill!

Bibliography

Border, S. and Butt, B.C., 2014. Mineral Resources and Ore Reserves of Industrial Minerals – Markets and Other Modifying Factors, in Mineral Resource and Ore Reserve Estimation – The AusIMM Guide to Good Practice, pp 467 – 472, Monograph 30 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Harben P.W. (1999). The industrial minerals handbook, 3rd edition. Industrial Minerals Information Ltd, London.

O’Driscoll, M. (2019). Industrial Minerals Basics Executive Primer. IMFORMED Rendezvous, Paris, 8-10 April 2019.

Scogings, A.J. (2015). Graphite exploration – the importance of planning. Industrial Minerals Magazine, December 2015, 42-46.

Scogings, A. J. (2015). Graphite – from discovery to resource. Heilongjiang under the microscope. Industrial Minerals Events, 5th graphite and graphene conference, 8-9 December 2015, London.

Scogings, A.J. (2016). Not all graphite is created equal. Australian Graphite Conference, Paydirt Media, 22 March 2016, Perth.

Scogings, A. J. (2017). What makes graphite projects tick? Industrial Minerals Events, 6th graphite and graphene conference, 16 March 2017, Berlin.

Scogings, A.J. (2017). Reporting graphite Exploration Results according to the JORC Code. Australian Graphite Conference, Paydirt Media, 27 April 2017, Perth.

Scogings, A.J. (2017). Graphite. SME Mining Engineering, July 2017, Vol. 69, 54-56. Society for Mining, Metallurgy & Exploration.

Scogings, A.J. (2018). Graphite. SME Mining Engineering, July 2018. Vol. 70, 54-57. Society for Mining, Metallurgy & Exploration.

Scogings, A.J. (2019). Graphite & Li Pegmatite Mineral Resources ‘looks can be deceiving!’ Battery Minerals Conference, Paydirt Media, 12 March 2019, Perth.

Scogings, A., and Chesters, J. (2014). Graphite: The six steps to striking success. Industrial Minerals Magazine, December 2014, 37-44.

Scogings, A., Chesters, J. and Shaw, W. (2015). Rank and file: Assessing graphite projects on credentials. Industrial Minerals Magazine, August 2015, 50-55.

Scogings, A.J. and Evans, D. (2019). Graphite. SME Mineral Processing & Extractive Metallurgy Handbook, 1735-1742. Eds. Dunne, R.C., Kawatra, S.K. & Young, C.A., Society for Mining, Metallurgy & Exploration. ISBN 978-0-87335-385-4.

Shaw, W.J. and Scogings, A.J (2017). The importance of geometallurgical test work for industrial minerals projects. Tenth International Mining Geology Conference 2017, Hobart, Tasmania, 20-22 September 2017.


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