It seems like everyone is over-tasked and short on time these days, so we have to be as efficient as possible when we work. We don’t want to keep re-inventing the wheel – why waste time figuring out a solution when someone else has already done so? It doesn’t make sense to start a task from the beginning when the groundwork has already been laid for you.
This is especially important in setting up a strategic Life-of-Mine (LOM) schedule from scratch. Your model will be used over an extended period of time, and many major decisions will be based on it, so you want to make sure you have covered all the bases in your initial setup.
To help you to think through the different aspects of this task, I have put together a list of questions for mining engineers to consider when creating a new strategic LOM schedule. Each question is followed by a short explanation, and there is a handy checklist for you to download for future use.
The questions are grouped into five sections:
- Material in the Pit
- Material Destinations
- Moving Material to Destinations
- Processing of Material at Destinations
Material in the Pit
1. What’s your reserve data?
Strategic schedules are spatially based on the block level, where a block model defines material in the pit. While block model specifics may vary, at a minimum these models should include the following attributes:
- The pit and/or phase in which the block is to be mined
- Geographic location (X,Y,Z)
- Spatial dimensions (x-width, y-width, z-height)
- Mass (tonnage or density)
- Grades (metal, contaminants, etc.)
2. Are ore and waste pre-determined, or will the model determine your cut-off grade?
When you predetermine ore and waste blocks for processing and waste dumping, then the model is a fixed cut-off grade model and the cut-off grade is an input. When the model chooses which blocks to process and waste, then the model is a variable cut-off grade model, and the cut-off grade is an output. Variable cut-off grade models are usually able to produce schedules of higher value than those produced by fixed cut-off grade models. For more information on this, read the article: How Much Value is Your Cut-Off Grade Devouring?
3. What are your processing options for ore material?
Mined ore is often processed at various destinations, e.g., mill and leach pad. While these multiple processing options can complicate finding an optimal schedule, they can also be a source of great value, leading to a greater profit.
4. Are the processing options predetermined, or will the optimization determine them?
When you pre-determine each block’s processing destination, the optimization is unable to capitalize on opportunities created through the combination of varied processing recoveries, costs, capacities, and available ore.
If you allow the optimization to determine each block’s destination while solving the model, those trade-offs are evaluated and exploited to find a schedule of greater value.
5. Are intermediate stockpiles available?
If intermediate stockpiles are available, material can be set aside so as to later achieve specific blends for processing or sale.
You can also use stockpiles to set aside lower grade ore, exposing the deeper, higher-grade ore for earlier processing, so as to generate a higher discounted profit.
6. Will you consider capital expenditure to expand capacity?
You may have the option to expand infrastructure capacity over the life of mine.
If you consider future expansions of processing capacity (e.g., increase mill capacity) or mining capacity (e.g., purchase additional trucks), you may be able to add extra value to the schedule.
The key to maximizing schedule value lies in providing the optimization model with all feasible options, so that it can search through those options to find the most valuable use of your resources.
Moving Material to Destinations
7. Will tonnage or trucking constrain your material movement?
You can constrain your mining capacity by tonnes or by trucking, and attribute costs accordingly.
If you constrain mining capacity by tonnage, then you will require an approximation approach to varying costs (e.g., incremental bench mining cost).
If you constrain mining capacity by trucking, then you can calculate haulage costs directly. For background on this, read the article: Why Accurate Trucking In A Mine Plan Is Essential
8. What are your truck cycle times?
Implementing truck-based constraints and costing requires accurate cycle times from each block to each of that block’s possible destinations. If those possible destinations include a waste dump, then you will require cycle times to either an average location on each lift, or to the centroid of each dump block. Make sure that your truck cycle times take appropriate account of congestion effects that may be occurring in your mine’s haul road network, particularly at key haul road intersections and around your loading and dumping points.
9. Will any additional trucks have varying capacity based on truck life?
Your truck fleet can be considered fixed, or you can model a fleet capacity expansion through capital expenditure. As you consider purchasing additional trucks, you can also account for decreasing annual capacity as they age and require more maintenance. For more information read the article: Truck Purchase or Rebuild: An Optimal Approach
10. What are the requirements for your waste dump(s)?
If it is sufficient to model waste dumps at an aggregated level, then for haulage you may assume dumping at some average location(s) in the dump.
If you want the schedule to account for accurate waste haulage requirements, then you will want to consider detailed placement at dump blocks on the lifts.
If you have different types of waste (potentially acid-forming, soil, or hard rock waste), then you will need to model particular placements for them on your waste dump(s).
Processing of Material at Destinations
11. What are your processing requirements and capacities?
Processing destinations can have minimum requirements and/or maximum processing capacities that constrain the schedule. Along with mining precedences, processing requirements and capacities limit those schedules that are operationally acceptable. These might include minimum grade, blending limits, a maximum number of mill hours, or total tonnage per period.
12. Do your processing plants have any blending requirements?
If there are blending constraints on any of your processing destinations, they can be grade-based or rock-type based. If stockpiles are in place, you can use them to achieve these blends.
13. Is your processing throughput constant for all ore types?
The throughput can be constant for all ore blocks, or can vary by ore type or other attributes.
14. What are the recoveries and processing costs at processing destinations?
Do these recoveries and costs vary by rock type, grade, or something else? The profitability of processing ore at different destinations is a function of recoveries and costs, in conjunction with capacities and ore availability. Accurately modelling the costs and recoveries is a key to distinguishing between the values of different schedules.
15. What do you need to report on, and how will you present the report details?
Finally, you need to consider how to break down your reports, e.g., by pit, phase, or rock type?
Beyond key performance indicators (KPIs), you may need to consider reporting on outputs such as optimal mill capacity, optimal leach pad location and a number of required waste dumps or fleet purchase decisions.
For your convenience, I have created a quick checklist including these 15 questions. You can download this resource Checklist of 15 Must-ask questions for your strategic mine plan and always have it handy for your next strategic planning task.
Are there any important questions you would like to see added to this list? I would like to hear from you on social media, or you can send your comments directly to me using the contact form.