At the heart of every successful open-pit mining venture is a simple economic problem: how to remove the most valuable ore possible without removing too much unwanted rock? The economic problem is embodied in the stripping ratio, which is the amount of rock removed for every tonne of ore removed. The key to designing the pit is to design the ultimate pit limits to maximize the amount of ore removed without allowing the cost of removing rock to outweigh the cost of removing the ore.
The key to it all is the breakeven stripping ratio. The breakeven stripping ratio is where the revenue generated by one unit of ore equals the cost of removing that ore and rock. When the stripping ratio is above this point, it means that money is being lost. The stripping ratio is like a guide in pit design; it ensures that for every tonne of ore removed, the cost of removing rock does not outweigh its economic value.
Optimum recovery is seldom achieved by hewing directly to the final pit plan. Rather, the mine is subdivided into pushbacks or phases. This allows the mine to pursue high-grade material first and reap this as a cash flow to support later pushbacks. Engineers then order these pushbacks to delay as much overburden removal as possible into later years, maximizing the project’s net present value while assuring that all ore is recovered at some point in the future.
Pit design for our current mines relies upon sophisticated algorithms, such as the Lerchs-Grossmann algorithm, to work through block models. These algorithms assess the value of each rock block, comparing costs to mine that rock to revenues generated by its contained economic materials. The result is a three-dimensional optimal pit shell, which outlines the final pit boundaries where total value minus total cost is maximized. This mathematical approach guarantees that no ore is unrecoverable and that no waste rock is unnecessarily mined.
Optimum pit design is the art of shaping the mine to chase the ore. It combines physics, economics, and geometry to keep the stripping ratio in sync with profitability rather than in opposition to it.
