In our last post, 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.
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.
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