Abstract:

We showcase the environmental benefits of a software solution leading to overall steeper pitwalls without compromising mine safety combined with the introduction of a mining carbon tax. The novelty of the work is twofold: 1) the effect of a carbon tax proportional to the emission produced by mining activities is accounted for in the determination of pushbacks and Ultimate Pit Limit to maximise the NPV, i.e. the environmental costs are not applied a posteriori but included concurrently into the pit optimization; this allows running credible sensitivity analyses on the effect of different values of carbon tax on the amount of ore extracted and mine profitability 2) we use a software, OptimalSlope, to determine automatically optimal profiles for the mine pitwalls.

We provide a theoretical framework to calculate the CO2 emission for all relevant mining activities at the block model level, i.e. for each block of the block model. Then additional costs in the form of mining cost adjustment factor, rehabilitation costs and processing cost adjustment factors were added to capture the effect of carbon tax in performing mine strategic optimization via the Geovia Whittle optimizer.
The Overall Slope Angle (OSA) of pitwalls plays a crucial role in the financial return and carbon emissions of an open pit mine: the higher the OSA is the less the amount of waste rock and in turn emissions and mining costs. In current practice, pitwall profiles are designed to be planar in cross-section within each rock layer, i.e. the profile inclination across each layer tends to be constant. A new slope design software, OptimalSlope, was employed to automatically determine the geotechnically optimal profile shape for the pitwall of mine each sector. OptimalSlope seeks the solution of a mathematical optimisation problem where the overall steepness of the pitwall to be designed is maximised for an assigned stratigraphy, rock properties (unit weight and strength) and Factor of Safety (FoS). The geometry of the benches (bench height, face inclination and minimum width) is provided as input. The optimal profiles obtained by OptimalSlope are always steeper than their planar counterparts (i.e. the planar profile exhibiting the same Factor of Safety) up to 8 degrees depending on rock type and bench geometry.

The Marvin copper deposit, with the block model data taken from MineLib, was used as a case study. We performed several pit designs based on different values of carbon tax and adopting either traditional planar or non-linear optimal pitwalls. In determining the ultimate pit limit and pushbacks, we sought to maximize the NPV and achieve an annual production schedule as uniform as possible.

From our analysis, it emerges that the adoption of optimal profiles realizes gains up to 32% NPV and reductions up to 25.5 million kg CO_2,eq and 1.6 billion MJ, which correspond to the CO₂emissions from the consumption of 2.5 million gal of diesel. Moreover, the relationship between carbon tax value and NPV looked to be linear in both cases of traditional planar and optimal pitwall profiles.

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Micah Nehring