In this article, you will learn about the expected risks of adopting mine-to-mill optimization strategies and the offsets available to balance installed power between the SAG and ball milling units to take advantage of potential mine-to-mill gains.
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As we discussed in our last post, blast design can be leveraged to provide the process plant with feed size distributions (PSD) that significantly improve process performance. In addition, a finer fragmentation generated by performing relatively high-energy blasts, also makes it easier to excavate and transport material within the mine. Therefore, changing PSD in the upstream can determine the economics of many mining projects as it impacts the value chain.
At the same time, however, it can be difficult to control high-energy blasting outcomes, which increases the risk of side effects, such as backbreak, ground vibration, air-blast, and flyrock. The approach turns blast movement control into a challenge, which can also result in ore loss and dilution, which can negatively impact process recovery and hence lead to a significant loss in the overall value over time.
In this article, in addition to a review of expected risks and offsets, we specifically discuss how ore loss, ore dilution and also loss of recovery might cause diminishing long-term profit, and nullify effectiveness of optimisation strategies adopted in the upstream.
Importance of power balance between SAG and ball milling stages
Before implementing the mine-to-mill approach, operations must understand the impact of applied changes on the process and how it may influence interaction between different units of process, including crushing, coarse grinding, fine grinding, and flotation. It is important to note that removing a bottleneck without respecting the next stage operational constraints may result in building a new bottleneck, which would effectively nullify upstream optimization practices. In this article, we consider a SABC circuit (SAG-Ball Mill-Secondary Crusher) to briefly explain how a mismatch between different units may become problematic.
At the SAG milling stage, one of the main operational challenges imposed by feed size and competence variability is the challenge of maintaining a steady load due to changes in SAG mill comminution characteristics and discharge rate. In SABC circuits (depending on operational constraints of a process plant), changes associated with feed size and its properties may induce imbalance of installed power between the SAG and ball milling units. The imbalance can result in the circuit shifting from SAG to ball mill limitation – increasing the risk of underutilization of available power and hence throughput limit.
Coarser and harder particles tend to accumulate and dominate the mill content, limiting the throughput, while softer and smaller particles (smaller than the SAG screen size) empty the mill quickly, increasing risk of liner damage by the steel media.
For a competent ore domain, intense blasting can help generate a finer feed, offsetting difficulty in grinding a harder ore type at the SAG milling stage – resulting in a slightly a coarser ‘transfer size’ (or coarser SAG mill product) being transferred to the ball milling stage at an increased rate – which can limit capacity at the ball milling stage. For a soft ore type, a feed with a high proportion of soft particles in SABC grinding circuits can also limit throughput at the ball milling stage. Because in both cases, an increased discharge rate at the SAG milling stage (often followed by a relatively coarser product) may overload ball mills/ hydrocylone circuit, resulting in increased pressure on recirculating load (~>300%), which often restricts the capacity or “hydrocylones overflow rate”. Another barely discussed scenario might be when crushed SAG pebbles (which are generally comprised of harder material) are diverted to the ball milling stage with the aim of improving SAG mill throughput. As a result, ball mills will process material which may be significantly harder than the “initial” circuit feed. In all these scenarios, the ball milling stage may become "the next" operational bottleneck within the value chain. Therefore, not being able to harmonize the interaction between SAG and ball milling units after applying changes in the feed size or in response to change of ore properties, can result in not taking advantage of finer fragmentation from optimized blasting practices.
There are various operational strategies operations may adopt to mitigate the risk of shifting bottleneck from SAG mill to ball mills. One such lever is to increase hydrocyclone cut-size (overflow P80) to reduce circulating load – however, this must be coupled with a review of possible negative impact on flotation as the coarser P80 might have a negative impact on recovery. It is worth noting that in many cases, it is required to apply several strategies in addition to enlarging P80, which might include changing the pebble port size, SAG discharge screen aperture (transfer size), etc.
To maintain long-term benefits, domain-based blasting strategies should be established for tailoring fragmentation in favor of downstream feed requirements. This would help optimize the process plant performance through balancing SAG and ball mills priorities and support increased SAG throughput.
Ore loss, dilution and process recovery
High-energy blasting requires good rock mass characterization for grade boundaries and advanced understanding of blast movement or it can result in remarkable financial losses in form of ore losses, dilution and poor recovery at the flotation stage.
Ore dilution occurs when waste/ lower-value material is sent to the process plant – diluting ROM head grade leading to both decreased metal yield (recovery) and increased cost of grinding. Ore loss occurs when valuable mineral is sent to the waste dump, decreasing ore-reserve utilization and causing significant loss of value over life-of-mine. The graph below illustrates how ore dilution and ore loss could potentially diminish or improve NPV of a copper mine.
As it is shown in the graph, the metal yield and recovery has a significant influence on NPV. The results suggest the importance of establishing grade control strategies at upstream and downstream stages and quantifying value at specific time periods to evaluate their effectiveness. A sustainable grade control strategy requires well-developed pre-blast and post-blast strategies, including measurement, modelling, optimization, and value quantification.
Conclusion
Adopting a mine-to-mill optimization approach can improve the productivity of the entire value chain, if associated risks and offsets are reviewed and understood. It is necessary to establish built-in strategies at the blast design stage and through the process plant and between its units to control change of PSD side effects and effectively manage ore variability to ensure optimal performance (throughput and recovery).
In our next article, we discuss how novel techniques and new technologies can further improve mines upstream and downstream activities, and help taking the conventional mine-to-mill approach to the next level.
About the expert
Farhad FARAMARZI is a Senior Mining Industry Consultant at
GEOVIA Dassault Systemes with over 10 year experience in Research, Consulting and Industry. Farhad holds BEng, MEng in Mining, and is specialised in Drill & Blast optimisation. He has worked in Drill & Blast specialist and superintendent positions - designed, led and surveyed over 100 full-scale production blasts at some large iron and copper open-pit mines. Farhad’s main area of expertise was built during his PhD in the Mineral Processing field at the Julius Kruttschnitt Mineral Research Centre (JKMRC) where he broadened his skillset and specialised in ore breakage characterisation, performance improvement, value-chain optimisation, modelling and simulation with several accomplished projects for Anglo American & BHP in this space.
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