Last week, we (re)introduced the "mine-to-mill" optimisation approach. In this first article we show how changing blasting fragmentation (also known as run-of-mine or ROM) can improve milling performance.
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In mining, the key objective of upstream activities is to remove in-situ rock volumes in sequence and to transfer them as efficiently as possible to different destinations based on their value (i.e. grade/metal content). The key objective of downstream activities is to extract value from the material provided by the upstream. However, the form in which fragmented ore arrives at the plant significantly influences how efficiently –– and profitably –– it is processed through multiple stages of size reduction, classification, and beneficiation until a saleable product is presented to market.
Mine-to-mill aims to improve overall value by establishing links between upstream and downstream activities, beginning with drill-and-blast practice, then transporting ore feed to the process plant. To accomplish this, mine-to-mill generally requires more energy-intensive blast designs (using higher powder factors) than the base case design to increase the amount of fines (below 10mm) in the muckpile. The modified particle size distribution (PSD) then allows for better performance at the coarse grinding stage (i.e. SAG mill) in the form of increased milling throughput.
This article explains how blasting fragmentation (also known as run-of-mine or ROM) affects milling performance.
Drill-and-Blast: First step in the comminution process
Rock fragmentation is considered one of the most important aspects of production blasting because it impacts both the costs of drilling and blasting and the efficiency of subsystems, such as loading, hauling, crushing and milling.
Rock fragmentation is influenced by parameters, which can be categorized as:
- controllable (e.g. drillhole geometry, sequence and explosive properties), and
- uncontrollable (rock mass properties).
The mismatch between blast design parameters and rock mass properties causes energy dissipation in rock blasting. When the explosive energy releases, apart from useful rock fragmentation and displacement, a considerable portion of that explosive energy is wasted in the form of undesirable side effects, such as ground vibration, noise, flyrock, over-breaks, ore loss, and dilution. However, a well-designed blast can result in “good” fragmentation and fewer side effects.
Typically, blasting is considered “good” when drill-and-blast costs are minimal and it fragments rock into particles that are fine enough and loose enough to be easily loaded and hauled away to the process plant. With the mine-to-mill approach, on the other hand, the degree of blast fragmentation is driven by feed-size requirements defined by the process plant. While producing the finer fragmentation commonly results in higher drill-and-blast costs, however, this increase can be justified in light of that fact that ore comminution is the most energy-intensive process in almost all mines. Therefore, a quality ore feed specifically tailored in size and metal content to meet milling requirements not only improves process performance in short-term, but also assists with significantly increased overall value over life-of-mine.
The crushing and grinding process is more or less efficient depending on the ROM size distribution, particularly for autogenous (AG) and semi-autogenous (SAG) milling, because a significant proportion of the grinding media (in AG mills, all of it) is comprised of ore feed. The more closely ore feed size distributions match downstream requirements, the more efficient, and therefore more profitable the processing will be.
How blasting impacts comminution
Crushing and grinding are comminution stages through which ROM size distribution is reduced to a certain size range within which most valuable minerals are liberated for later beneficiation stages, e.g. flotation. Since blasting is the first step in the process of breaking rock down to a specific size fraction, it has a significant effect on subsequent breakage processes by comminution machines.
Mine-to-mill involves increasing the amount of breakage achieved in blasting to decrease the amount of grinding effort required at the AG/SAG milling stage. This moves a notable proportion of size-reduction load back to the mine, where application of energy is more cost-efficient than at the mill. It is worth to be noted that, in general, AG mill performance is better with coarser feed because it requires large enough rock particles to act as grinding media and break smaller rocks.
At a constant ore feed hardness, any change in the feed size will affect the breakage characteristics of the SAG mill. That is, for a finer feed size, a decrease of the charge volume and therefore the power draw is expectable – either because relatively larger proportions of ore feed already meet (free grind material) or that in a shorter time will satisfy the size requirement for being discharged – which results in a higher throughput. In other words, it is the charge that changes owing to a different feed size.
However, it is important to remember that while conducting more intense blasting will generally increase the SAG mill throughput, it will also have an impact on the next stages, ball milling circuit performance and recovery, and this impact should be understood and accounted for.
Conclusion
Tailoring blast fragmentation by performing more intense blasts (~2-3 times) generates significant differences in the fines and intermediate size fractions relative to a standard blasting practice. This strategy often helps unlocking additional milling capacity, and ultimately helps better align production objectives along the entire value chain.
At the same time, any changes in feed size should be based on a proper understanding of the impact on the following stages and should consider equipment design and operational constraints. There are always risks to changing any practice, and with mine-to-mill in particular, there are risks to mitigate and competing priorities to balance.
In our next article, read 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.
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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