More than two decades ago, the mining industry started exploring the positive effect of blast-induced fragment conditioning on downstream processes. The phenomenon, colloquially called microfracturing, has been linked to increased comminution throughput, reduced energy consumption and enhanced ore recovery performance in controlled environments.
However, microfracturing remains a somewhat elusive concept with limited application in mine operations, and companies rarely deem the phenomenon a viable option to improve mining performance. Nowadays, microfracturing is perceived as a highly complex phenomenon to dive into in search of value.
In retrospect, this grim scenario seems to have stemmed from the inability to properly apply the microfracturing concept with improvement purposes rather than the lack of technical merit of the phenomenon itself. In addition, widely-spread optimisation approaches such as mine to mill and
mine to leach have almost exclusively relied on the fragment size distribution and displacement to improve mining performance, shifting the attention of mining companies away from microfracturing.
Evidence produced over the years suggests that any attempt to successfully introduce blast-induced fragment conditioning into the value chain of mine operations should consider at least the following five aspects.
1. Concept and purpose
Inducing fractures inside the fragments resulting from blasting represents a means to an end. As such, an increase in the degree of fracturing is not a direct descriptor of the ability of the phenomenon to positively impact subsequent processes. It is crucial to keep in mind that the ultimate goal must be to alter the physical properties of the rock up to achieving either a measurable reduction in fragment strength, an improvement in fragment permeability or an increase in ore exposure – or ideally achieving all at the same time.
2. Controlling factors
From a pure drill and blast perspective, stress intensity and loading rate produced by the detonation of explosives are the main factors controlling microfracturing. In this sense, borehole diameter, burden and spacing, type and density of explosive, charging configuration, and sequencing and timing can all promote the creation of fractures inside fragments when purposely designed.
“Geometric properties of a blast” - Image from: O-Pitblast RedPocket
Intact rock's elastic and mechanical properties also contribute to microfractures creation up to a certain extent. However, rock matrix properties (e.g. grain shape, size and strength, grains physical arrangement and pores occurrence) are key to unlocking insight into rock susceptibility to blast-induced fragment conditioning. This piece of information will be crucial to scaling the phenomenon up for full-scale operational applications.
In opposition, macro-scale factors relevant for other blasting results, such as detonation gas flow and rock mass properties, have a negligible impact – if any - on microfracturing.
3. Phenomenon Scale
Microfracturing takes place at a scale of millimetres or centimetres, which differs highly from the scale at which the rest of the adjacent operations occur. From a practical standpoint, there is a mismatch between the scale of microfracturing and the degree of precision and selectivity that any other mining process can achieve (e.g. geology mapping , drilling, and explosive charging. This condition makes the phenomenon hard to measure, control and integrate through the standard approaches, methodologies and technologies used in the past.
Emergent technologies may help provide both the required accuracy needed to capture the phenomenon on a daily basis and the tools to successfully link the phenomenon with the rest of the operations that happen at a larger scale. Some examples of these technologies are automatic explosive manufacturing units, real-time data acquisition systems, advanced simulation software, and AI-driven data analytics.
4. Microfracture Types
Typically, the microfracturing concept refers to the occurrence of fractures in generic terms, using the number and extent of fractures per area as the primary descriptors. However, microfracturing involves the creation of three types of fractures that are produced under different circumstances.
(1) Intergranular fractures: formed along grain boundaries;
(2) Transgranular fractures: created across multiple mineral grains, and
(3) Intragranular fractures: generated inside grains.
“Representation of the 3 types of fractures” - Image from: Research Gate
Experimental work has shown that stress intensity directly affects the type of fractures created inside the fragments. In relative terms, low stress can only produce intergranular fractures, whereas high stress can produce all three types of fractures.
This differentiation is crucial for improving leaching performance because certain types of fractures can be linked to a higher degree of ore exposure, depending on the particular characteristics of the mineral of interest.
5. Sampling and testing
Unlike the case of fragmentation and displacement, sampling with microfracturing purposes highly disrupts the mining operation as it requires collecting physical fragments from the muckpile. In addition, the limited number of rock fragments that it is possible to collect from each muckpile represents a challenge in terms of significance and representativeness. Therefore, microfracturing-focused studies must target well-defined sectors within each blast where conditions of interest are present (e.g. specific ore grades and rock strength.
Regarding testing, microscopy (https://www.earth.ox.ac.uk/~oesis/micro/) and X-ray tomography are the two techniques to measure microfracturing directly, both of which are time-consuming and expensive. However, some indirect methods can make the measuring process more cost-efficient. These techniques are based on the variation of physical and mechanical properties of unblasted and blasted rock (e.g. apparent porosity test and point load test.
“High Pressure Porosity System” – Image from: Grace Instrument
“Point Load Tester” – Image from: Grance Instrument
Note that when applying mechanical testing, it is vital to separate the fragment samples according to different size fractions to minimise the influence of this parameter on the results.
Despite the evident challenges, microfracturing can be a valuable option to unlock additional value, especially in operations when other most common sources of improvement have been exhausted. However, this value will only be realised if companies rethink their approach to blasting optimisation and adopt a more expansive improvement methodology.