CIM Bulletin, Vol. 97, No. 1082, 2004
S. Fuentes S. and J. Cáceres S.
During the past fifteen years, block caving mines have experienced huge changes in design, planning, and administration. These changes have allowed operators to drastically reduce costs, improve equipment productivity, and improve overall business performance. Because of this, the block/panel caving mining system is challenging actual open pit operations to maximize the Net Present Value (NPV) of the business. The main improvements are mine design, mine planning, and mine management.
These improvements have encouraged the re-evaluation of underground operations at on-going and closed open pits, and challenge surface mining applications as the only low-cost mining method.
Recently, many analyses of large open pit operations have shown the economic advantage of changing the mining method instead of continuing to operate as an open pit mine. Marginal improvement of the business’ value can usually be verified during the last operational years of an open pit, making the mining method change a huge opportunity for the business and block caving the best available choice under certain technical constraints.
However, as usual, the main issues are: the risks involved in this decision; how to change or to adapt the infrastructure; skills and knowledge; and the organization’s ability to accept change. The authors could not yet find an algorithm to simultaneously manage and generate the proper mining schedule for both methods (underground and open pit), without considering the economic evaluation.
Other conditions to consider are that these kinds of analyses are carried out when there is not enough time to completely plan an underground project. As a result, decisions are made according to whether they are politically rather than technically correct. There are only a few examples of projects that started as open pits that knew in advance when the mining method needed to be changed.
The analysis proposed to find the open pit — underground limit was based on marginal push back compared to a new underground mine plan for each configuration. The net present value of the global scenario, as a decision parameter, gives the main guideline for the proper border limit.
To do this marginal comparison, it is necessary to have a base underground scenario, setting up the main parameters for these operations, such as start-up time, maximum production rate, mining costs profile, main investment through time, profitability, etc.
Basically, the economic value of the open pit plan, including each new expansion, must be higher than the economic reduction in the new underground mining project. The exercise has to be repeated enough times to identify the economic optimum as shown the figure.
The most common results are that the open pit’s economic limit is much closer in time than the analysis developed as a stand-alone open pit.
This paper describes a strategy to estimate the optimum open pit to underground transition time during the life of a mine.