Open-pit optimization aims to extract the maximum economic value from the deposit by determining the most efficient configuration of mining operations. One of the key concepts in this regard is the stripping ratio, which represents the volume of waste material per ton of ore extracted. To determine the economic borders of the mine, engineers use the Break-Even Stripping Ratio (BESR) or the maximum amount of waste material that can be economically extracted to expose ore. Essentially, it is the point where waste stripping costs become equal to the net profits from the extracted ore.
First of all, calculation of the break-even strip ratio implies assessment of the basic economic parameters of the mine. The first parameter to calculate is the value of the recoverable ore, which depends on the price of commodities, ore grade, and recovery percentage. At the same time, the cost of all ore production should be assessed, including drilling, blasting, loading and milling, but excluding waste stripping costs. Finally, the last parameter to identify is the stripping cost, which refers to the unit cost of excavation and transportation of waste material to disposal site.
The calculation of this limit uses a straightforward mathematical equation. BESR equals the difference between ore revenue and the cost of ore production, divided by the waste stripping cost. Mathematically speaking, BESR = (Revenue − Ore Production Cost) / Waste Stripping Cost. In case the ratio calculated for some block of ore exceeds the physical stripping ratio, extraction of that ore becomes economically viable. Otherwise, if the physical ratio is higher, mining such a block leads to losses.
Modern open-pit optimization uses spatial algorithms such as Lerchs-G Grossmann approach to perform this calculation. Such algorithms assess the 3-D block model and apply the concept of BESR to determine the Ultimate Pit Limit (UPL), which means the geometric limits of the pit. Using the break-even threshold, the software automatically deletes from consideration any blocks for which the stripping costs are higher than the value of ore and, thus, prevents the operation at a loss when expanding the pit.
At that, the break-even strip ratio is not a static parameter, but a dynamic one that responds to changes in commodity prices and ore grade. Operational efficiency greatly influences this calculation because improvements in the transportation system of the mine allow reducing the unit stripping costs, thus raising the BESR and digging deeper pits (Samimi Namin et al., 2023). As a result, engineers perform sensitivity analyses to adapt pit designs in response to changes in variables.
In conclusion, the break-even strip ratio plays a crucial role in open-pit optimization as an economic criterion. Comparing the mineral value to ore production and stripping costs, the BESR allows defining the optimal pit size and depth. Accurate calculation of this limit provides the basis for the economically viable design of mining operation.
References
Samimi Namin, F., Ghasemzadeh, H., & Aghajari, A. M. (2023). A comprehensive approach to selecting mine transportation system using AHP and FUZZY-TOPSIS. Decision Making and Analysis, 23–39. https://doi.org/10.55976/dma.12023117323-39
