Don't give up yet. There is a way to optimise your waste dumping schedule...

The recent Covid-19 pandemic has really forced mines all over the world to start thinking more about different ways to cut down costs. If you ask me, there is no better place to start than cutting down costs associated with hauling waste material (material of no economic value) in open pit mining environments. In my experience here at Datamine Africa, material handling in open pit mines can easily account for up to 50% of the mine’s operating costs. A large portion of the material handling costs is for hauling waste material to the waste dumps.

With that in mind, it is quite clear that any effective strategy to reduce the cost of waste hauling and dumping could translate into a big cost saving. The two common strategies for reducing the waste hauling costs include minimising the haulage distances or improving the productivity (and efficiencies) to reduce the overall unit cost of handling waste material. Some of the available options to improve productivity and efficiencies include, but are not limited to, upsizing your equipment, improving equipment availabilities and utilisations, providing better operating conditions, and employing a dispatch system. The question is: What can open pit mines do to effectively optimise the waste dumping schedules through reduction of the haulage costs?

It is safe to say that mine planning has been and still is primarily focussed on ore production more than anything. The reason is simply because the main objective of most mines is to maximise the Net Present Value of the mine through constant production of quality ore. It is more of a normal behaviour for waste material mining, hauling, and dumping to receive little attention when scheduling. More often than not, the problem of waste dumping is resolved by treating the waste dump as a single dumping point with a known volumetric capacity. This approach involves matching the known volumetric capacity of the waste dump (volumetrics established through CAD dump designing) with the waste volume to be mined out of the pit. This penny plain approach significantly improves the solution time. However, the outcome of this approach often contains little to no information about the actual spatial dumping locations. The lack of detailed information regarding spatial waste material dumping could potentially result in misalignment between the dump design and modelling as well as what happens on the ground. This could ultimately result in the operation failing to achieve the long-term objectives such as waste dump height, capacity, footprint, and slope angles.

In most cases, waste rock spatial dumping is a problem that is often left to the short-term planning engineers to address. The short-term planning engineers try to solve the waste rock spatial dumping challenge using the principle of availability (dumping space availability) and looking for the shortest haul to minimise haulage costs in the short-term. This approach tries to answer the “where should we dump next to minimise costs?” question in order to reduce hauling costs in the short term. Due to the absence of long-term guidance, this approach could be problematic and could also result in extra rehandling costs in the near future. With time, the impact on the long-term design objectives and NPV can be devastating.

There are three common ways in which mines typically build and progress their dumps over time to try and deal with haulage distances.

Short-to-long progression approach

This approach hauls waste material for short distances during the earlier years of the Life of Mine. The haulage distances increase as mining goes deeper and dumping moves further away as well. In the illustration below you can see that waste material in pushback 1 (in blue) is hauled the shortest distance to dumping location 1 (in blue as well). Waste material in pushback 4, which is the deepest, is hauled the furthest to dump location 4. The advantage of this approach is that the hauling costs may be significantly lower in the earlier years of the operation. Since the Net Present Value is a function of time, this could have a notable, positive impact on the NPV. However, later on the Life of Mine you might struggle with the hauling costs and this could potentially harm your operation. Think of what would happen if the price of diesel significantly increases later in the Life of Mine, you may end up in a tight, unpleasant corner. Then, you may survive by creating temporary dumps everywhere to reduce hauling. This might kill you even more!

Figure 1: Short-to-long dump progression.

Long-to-short progression approach

With this strategy, you start dumping far from the pit in the earlier years of the Life of Mine so that as the pit gets deeper, you will now be dumping closer to the pit for a more consistent haulage over the years. In the illustration below you can see that waste material in pushback 1 (in blue) is hauled the longest distance to dumping location 1 (in blue as well). Waste material in pushback 4 which is the deepest is hauled the shortest to dump location 4. As I said, the advantage of this approach is that you could achieve a more consistence haulage distance profile over the Life of Mine which could be healthier for your trucks. However, the disadvantage is that you may need a big capital injection in the earlier years of the Life of Mine in order to handle the stripping as well as the excessive travelling distances over the earlier years. 

Figure 2: Long-to-short dump progression.

Centred progression approach

This approach tries to find the balance between the two other approaches discussed already. The dump progresses in both directions – a portion progressing closer to the pit and another portion progression away from the pit. This approach may provide better haulage distances and have a better impact on the NPV over time. However, this approach is deemed not provide too much flexibility for waste dumping and may introduce numerous operational challenges. The lack of flexibility may affect productivities and efficiency and consequently increase the unit cost of handling waste material (even though hauling costs are managed well).

Figure 3: centred dump progression.

I am sure you are now asking yourself what is the optimal dump progression strategy?

Analysing the overall haulage costs under these 3 dump progressions sequence discussed above is a very time-consuming exercise as you can imagine. We should also keep in mind is that there could be other dump progression strategies that are not listed above which could be more economical and friendly for your operation. For one to reach an optimal waste dumping solution, one must determine the haulage distances and costs from each waste block in the pit to all possible dumping destinations assigned to that waste block. The more waste blocks and possible destinations you have, the more complex (and manually out of your hands) the problem becomes. One must also take into consideration the timing costs for hauling waste material i.e. the cost of hauling to one dumping location in different time periods due to the time value of money.

Figure 4: Minemax Tempo showing the cycle times for one block to all possible dumping locations on the waste dump.

The way we plan to mine waste (addressed by the pit production schedule) and the subsequent hauling and dumping of the waste materials are really not discreet activities. Therefore, the waste dump schedule (volumetrics and spatial representations) must be fully integrated with an optimised pit schedule. Awesome optimisation tools such as Minemax Tempo and Datamine’s AutoScheduler (for tactical planning) as well as Minemax Scheduler and Datamine's Studio NPVS (for strategic planning) are designed to optimise both your pit and waste dumping schedules to satisfy the different constraints and objectives on your mine such as maximising ore production, minimising haulage costs, minimising the opportunity costs or a combination of different constraints and objectives.

These tools are designed to rapidly determine the haulage distances (and consequently costs) from each waste block in the pit to all the possible dumping destinations assigned to the waste block. The tools also use a global lookahead scheduling approach to solve your schedule. In simple terms, this means that they look ahead to the end of the scheduling horizon (instead of scheduling period by period). So, the tools have the capability to consider the timing cost of hauling waste material – basically answering the question “is it cheaper for me to dump at this specific location today or 2 years down the line”? The answer to this question talks to the time value of money.

In addition to establishing an optimal spatial dumping sequence, these tools can also help you answer the question of how far and high you should dump. As material accumulates on the waste dump, the dumps normally expand horizontally and vertically (more lifts) as well. This results in increasing hauling distances and costs as well. So, when trying to optimise waste dumping by minimising hauling costs, it is equally important to figure out how far (horizontally) should you expand the dump and how high (number of lifts) should you build the dump as well. When you consider your operation’s haulage costs and efficiencies, you will discover that there is definitely a sweet spot between dumping far and high. You can try and find the spot through manual means or you can fire the optimisation tools and have coffee while the magic happens. If you go the optimisation route, there are two benefits here: you can define dump progressions in such a way that minimises the actual costs as well as time costs and you would have had a cup of coffee (and maybe some sleep).

The next important topic that is spoken less of is whether mines should do dump spatial and plan compliance or not? Well, I have said a lot here. Tune in…