Long Term Planning for Open Pit Mines
Introduction
The profitable exploitation of a mineral deposit requires considerable evaluation and planning. First it must be determined what portion of the deposit is economical to mine (the mineable reserve) and by what mining method. Then considerable effort must be made in planning and scheduling the extraction of the mineable reserve in terms of mining sequence, mining rate, mine design, equipment requirements, etc. The objective of this effort is to determine the most profitable extraction plan and highest rate of return on the money invested
These tasks are performed under the realm of long term planning (LTP). They involve working with estimated rather than exact data, looking years instead of days into the future, and dealing with evolving economic criteria, e.g. future metal prices. The uncertain nature of mineral evaluation data requires that LTP tasks be done over and over again, evaluating sensitivities, redefining key assumptions, and incorporating new dat
Because of the repetitive nature of these tasks and the need to evaluate large mineral deposits with extraction lives of 50 years or more, long term planners have been using computer software since the 1960s to assist in their evaluations. Mintec, Inc. was one of the first mining software companies to offer LTP software and continues to do so to this day.
LTP Tasks
The evaluation of a mineral deposit typically follows two stages of study depending on data available and degree of accuracy required. Stage one involves scoping level studies of orebodies and includes the following tasks:
These tasks provide quick estimates of ore reserves, mine life, and mine profitability; and sensitivity of these estimates to metal prices, recoveries, costs, etc. These tasks can be done with minimal geologic data and simplified costs (scoping level studies) or with complex well-defined orebody models and detailed actual operating costs (continuing studies at existing mines). Mathematical optimization methods (Lerchs-Grossmann for pit limit optimization and Mixed Integer Linear Programming for scheduling optimization) are used during these tasks.
Depending on the results of stage one optimization studies, the mine planning effort may or may not move on to a more detailed design and scheduling stage. This second stage involves the following tasks:
These stage two tasks refine the first stage study results and provide the level of accuracy required by feasibility studies for new deposits and expansion studies at existing mines. These tasks are part of on-going evaluations at existing mines to incorporate new drilling data, evaluate changing economic conditions, updating past plans to account for annual mining progress, etc. These second stage tasks are more time-consuming than the first stage tasks because of their refined level of detail, especially in the mine design work. The results of the first stage optimization tasks provide important guidelines for the detailed work and identify the best scenarios upon which to perform the more time-consuming stage two tasks.