The Challenge

Acid rock drainage (ARD) testing as practised by the mining industry is in need for reinvention. With the global financial liability associated with ARD estimated at US$100 billion (Tremblay and Hogan, 2001), now is the time to reduce costs, increase knowledge, prevent environmental impacts and reshape environmental ARD testing.  How can Best Practice ARD sampling as recommended by the Australian Government (Price, 2009) be achieved and ARD accurately predicted when using such costly tests and outdated protocols? The scale of the problem increases further when considering that 20-25 Gt of waste rock is produced globally by the mining industry (Lottermoser, 2010). Lower grade deposits are being mined (Mudd, 2007), and whilst much research is conducted as to how to process and extract the value, how should the additional waste rock be most appropriately managed?

ARD at Haulage Creek, Mt Lyell Mine, Tasmania

The Current Approach

Current approaches either directly utilise, or are modifications of, the wheel approach (Morin and Hutt, 1997). This recommends the use of several categories of tests (e.g., acid-base-accounting, whole-rock geochemistry, field and kinetic tests). Limitations of these tests are often discussed in the scientific literature, with the pertinent question being:  how relevant is it for the mining industry to use costly geochemical tests developed in the 1970s? The wheel approach and its derivatives provide little information on how best to use the tests concurrently, and whilst mineralogy is identified as a test category, few guidelines are given as to what type of mineralogical work is required. Furthermore, texture, a factor known to significantly impact on ARD formation, is not even considered in its own right.

The Solution

The answer is two-fold.  First, a logical and integrated approach must be adopted. Second, accurate small-scale, low-cost tests must be utilised, which when appropriately used, allow for Best Practice sampling, and the geological variability of the future mine waste to be examined. Based on this requirement, the geochemistry-mineralogy-texture or ‘GMT’ approach was developed (Parbhakar-Fox et al., 2011).  This approach comprises of three stages following an initial grouping of samples based on lithological ARD characteristics.

1) Stage-one is a mandatory pre-screening stage, which focusses on utilising small-scale, inexpensive geochemical and mineralogical tests as well as textural evaluations. Data from these are concurrently used for waste classification, and allows for samples identified as potentially acid forming, or as have an acid neutralising capacity to progress forward. Non-acid forming samples are not further tested.

2) Stage-two is the quantification of the acid forming or neutralising potential, using geochemical tests.

3) Stage-three is the realisation of the controls on sulphide oxidation, using advanced analytical techniques including laser-ablation ICPMS and MLA-SEM available at the University of Tasmania.

 The integrated approach: GMT + Geometallurgy

This approach has been tested at several mine sites in Australia, where improved waste characterisation was demonstrated. Whilst this approach is a significant improvement to the wheel, Parbhakar-Fox et al. (2013) identified further opportunities to improve ARD prediction and waste characterisation by integrating geometallurgical datasets, thus proposing an environmental geometallurgy approach. This should be adopted where geometallurgical data has been collected, thus adding-value to existing datasets and allows for conservative ARD domaining. The output data facilitates decision making regarding which samples require GMT testing, a decision which cannot be made based on sulphur values alone (as is often attempted). The mining industry ultimately desires simple and accurate field ARD tests with waste classification results rapidly determined in the core-shed or rock-face. Whilst tools and tests exist which can assist (e.g., handheld-XRF, pH tests), their application has to be optimised and proven before they can be routinely implemented.

Establishing the use of new field-based ARD tests is the way forward.

‘You can’t reinvent the wheel’ – I disagree; we can and must reinvent the wheel by using the integrated GMT approach, geometallurgy and field tests in order to bring ARD prediction into the 21^st century.  

References

Lottermoser, B.G., 2010. Mine Wastes: Characterization, Treatment and Environmental Impacts, 3rd edition, Springer-Verlag, Berlin Heidelberg, pp. 400.

Morin, K.A., Hutt, N.M., 1997, Environmental geochemistry of minesite drainage: Practical theory and case studies, MDAG Publishing, Vancouver, British Colombia, 1997, ISBN 0-9682039-0-6. Mudd, G.M., 2007. An assessment of the sustainability of the mining industry in Australia, Australian Journal of Multi-Disciplinary Engineering, v.5, p.1-12.

Parbhakar-Fox, A.K, Edraki, M, Walters, S, and Bradshaw, D, 2011. Development of a textural index for the prediction of acid rock drainage, Minerals Engineering, 24: 1277-1287.

Parbhakar-Fox, A.K., Lottermoser, B.G., Bradshaw, D. 2013. Cost-Effective Means for Identifying ARD Risks – Integration of the Geochemistry-Mineralogy-Texture (GMT) Approach and Geometallurgical Techniques.  The 2^nd AUSIMM International Geometallurgy Conference, Brisbane, Queensland, p.143-154.

Price, W.A., 2009. Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials, CANMET Mining and Mineral Sciences Laboratories, 579p.

Tremblay, G.A., Hogan, C.M., 2001. Mine Environment Neutral Drainage (MEND) Manual 5.4.2d: Prevention and Control, Canada Centre for Mineral and Energy Technology, Natural Resources Canada, Ottawa, 352 pp.