Authors: Pearce, S; Warner, J; Sinclair, E; Pearce, J; Olds, W; Weber, P
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Acid and metalliferous drainage (AMD) management plans are generally developed as part of a site’s closure plan to inhibit or mitigate the generation and release of AMD for sites with problematic materials. They are typically constructed around a body of knowledge involving multiple geoscience and environmental disciplines. However, despite the volume and degree of scientific investigations completed, if the waste rock classification system and therefore AMD management plan developed is not practical and does not take into consideration other site drivers such as production, its successful implementation and adoption is unlikely. A common weakness of AMD plans developed based on industry best practice is that they often fail the practicality test, as the characterisation process produces ambiguous outcomes such as the classification of material as uncertain with respect to acid generating potential. As part of optimising the characterisation process, a site specific waste rock classification process flow diagram approach is discussed herein. The development and use of a process flow diagram to optimise the testing regime opposed to a traditional matrix-style system can reduce the number of tests required, the cost of testing, and the time required to make informed classification decisions. However, to be confident in the use of a process flow method for waste rock classification requires detailed knowledge of site geology and geochemistry; and the completion of a suitable sampling program, incorporating acid base accounting (ABA) before the development of a process flow method. A flow process often only requires basic parameters for the classification of a block of waste, arriving at the parameter boundary values that incorporate the results of several detailed second and third geochemical testing phases. The use of acid buffering characteristic curves, kinetic net acid generation tests and large scale site column tests allow the consideration of kinetic factors for a risk-based operational waste rock classification that differentiates between different degrees of potentially acid forming (PAF) materials. This differentiation allows more control over AMD management and subsequently reduces closure risk. At the Martabe Gold Mine, Sumatra, following a detailed ABA classification program, a process flow methodology and specialised rapid field testing program for geochemical classification of waste rock was developed as a tool for the operational management of overburden. This is used as a quality control or verification phase for confirmation of the geochemical waste rock block model, and ensures that waste rock is correctly identified in the field and handled as per the management plan. Application of this approach is discussed in this paper. At the Escarpment Coal Mine, West Coast, New Zealand, a new process flow method for geochemical classification is being trialled. Results indicate that classification by a process flow method results in far fewer samples being classified as uncertain compared to the current resource consent matrix-style classification. Results presented in this paper indicate that ABA data and field column leach trials validate this approach.

Keywords: AMD management, waste rock classification, block model validation

Pearce, S, Warner, J, Sinclair, E, Pearce, J, Olds, W & Weber, P 2016, 'A risk-based approach using process flow diagrams for operational waste rock classification — case studies', in AB Fourie & M Tibbett (eds), Proceedings of the 11th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 649-660.

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