Scheduling Methods

• Scheduling Methods
• • Manual Scheduling
  • Automated Scheduling
  • • Heuristic Scheduling
    • Schedule Optimization

There are a number of different methods and associated tools for mine scheduling. Here we will consider manual scheduling and various types of automated scheduling.

Manual Scheduling

Although automated scheduling tools are now widely available, the majority of mining operations still use spread sheets to support a manual scheduling process. Typically, block model information including geometry, tonnage and grade information is available on one sheet. On another sheet is a representation of the schedule where blocks are manually chosen to be scheduled in certain time periods. When a block is scheduled, summary information such as ore, waste movement and grades are updated. To use these spread sheets, the mining engineer usually has detailed knowledge of the pits being scheduled such that practical schedules are generated. However, if there are constraints on the schedule such as those that can be associated with achieving grade targets, the scheduling process can be meticulously arduous. Even if there are not strict grade constraints, it is often next to impossible for an engineer to know if there is significant room for improving the value of a given schedule.

Automated Scheduling

Automated scheduling is a means for automatically computing a schedule through computer software. While automated scheduling relieves the mining engineer from the tedium of generating and evaluating schedules, it is important to know how the scheduling is taking place. This is important because some methods of automated scheduling are not able to satisfy scheduling constraints due to the way the automated scheduling occurs.

Heuristic Scheduling

Heuristic scheduling is a form of automated scheduling where the scheduling algorithm is based upon rules for selecting blocks. Rules are usually related to the production constraints and possibly to some measure of a block’s value. Typically a schedule is constructed by iteratively selecting blocks, one block at a time. The advantage of this approach is its speed in constructing a schedule. One of the disadvantages is that it is not guaranteed to produce truly optimal schedules. Additionally, if satisfying grade constraints are important, heuristic algorithms cannot guarantee a schedule that satisfies the constraints, even if a schedule that satisfies grade constraints is known to exist.

Schedule Optimization

Schedule optimization often uses a mathematical model to represent the mine and its production constraints. Optimization algorithms which operate on this model (simplex, branch and bound, dynamic programming and others) are used to automatically compute a schedule that not only satisfies the production constraints, but also optimizes the schedule. Normally it is net present value (NPV) that is optimized, although other parameters can be optimized as well.

Minemax Scheduler is an optimizing scheduling tool that uses a mixed integer linear programming (MILP) model of the constraints, financials and production targets. It uses a branch and cut algorithm to optimize this MILP model.