Editorial for Special Issue “Comminution and Comminution Circuits Optimisation: 3rd Edition”

Comminution is crucial to mining and mineral processing operations, as valuable minerals cannot be extracted if they are not liberated. This process spans from blasting to mineral processing, where the run-of-mine material is crushed and ground prior to separation. As articulated in this Special Issue, the efficiency of the separation stage, which determines the degree of mineral recovery, depends not only on the efficiency of the comminution stages but also on the understanding of the ore characteristics. However, achieving efficiency in comminution, which is widely known for a lion’s share of the world’s energy, has long been a mammoth task, given the non-revealing nature of comminution equipment, often referred to as “black boxes”. The interactions among the operational parameters of the mills or crushers further complicate their understanding, making the use of techniques such as the discrete element method (DEM) helpful for optimising machines for size reduction. Considerable progress has been made in this area; nevertheless, the work is still far from exhaustive, especially considering global challenges such as environmental and energy crises, high-grade ore depletion, and unsustainable production. Thus, the research published in this Special Issue aims to address some of these challenges, providing important insights into blasting, ore pre-treatment and characterisation, optimal crushing and grinding, and the numerical assessment of comminution circuits.

In this vein, Contribution 1 used the Kuz–Ram model and Bond equation to assess the impact of surface blast design parameters—such as burden, spacing, stemming, and powder factor—on the fragmentation size and overall performance of a comminution circuit processing sedimentary copper-bearing ore, in terms of energy and throughput. The study resulted in a 20% increase in comminution circuit throughput, a 29% decrease in specific energy consumption, and a 12% total operating cost reduction, all of which contributed to improved overall fragmentation. Although higher powder factors were found to enhance processing, fragmentation that is too fine may lead to increased equipment wear and maintenance costs if an adequate balance is not maintained.

Focusing on the crushing stage in the comminution chain—in particular, the jaw crusher—Contribution 2 investigated the influence of mineralogical and physical properties on ore breakage behaviour. The authors explored the potential for the selective liberation of valuable minerals and determined that selective liberation was possible at relatively coarser particle sizes, which in turn improved ore beneficiation with lower energy consumption. They found that chalcopyrite and pentlandite exhibited better liberation at particle sizes below 400 μm, achieving over 50% liberation in the finest fraction, whilst magnetite was more enriched in particles smaller than 125 μm. The study emphasised the importance of considering mineralogical parameters when designing energy-efficient and selective comminution strategies.

A semi-autogenous (SAG) mill was assessed by Contribution 3, who aimed to characterise the stress and impact loading experienced by the grinding media in these mills to help design stronger grinding media capable of withstanding stress and damage risk. The authors employed the DEM to evaluate the intensity of collisions using impact energy spectra (IES). The IES showed that higher ball loads resulted in more severe ball-on-ball impacts, increasing the risk of ball fracture. This study also showed that, while a higher mill speed is necessary to generate greater collision energy for ore breakage, it may also promote ball fracture. In SAG mills, bigger balls are preferred due to their high energy, which efficiently breaks particles; however, they are more vulnerable to fracture. Thus, the efficient operation of a SAG mill requires an optimum ball size to reduce ball damage, an optimum mill speed to balance ore breakage and media wear, and an optimum ball-to-ore ratio to balance ore cushioning and grinding efficiency.

Expanding on the milling stage of the mineral beneficiation chain, an integrated approach—combining a computer and the DEM coupled with the smoothed particle hydrodynamics (DEM-SPH)—was employed by Contribution 4 to calibrate the digital twin of a laboratory ball mill. They examined various milling scenarios, which included balls only, balls with ore, and balls with slurry, and simulated mill load behaviour. Their proposed method enabled the development of a numerical testing regime that can predict the grinding behaviour of the mill under different operational conditions without causing disruptive and costly changes in milling operations. The study combined experimental and simulation approaches, assessing the mill’s behaviour under both dry and wet milling conditions, supported by image processing and the DualSPHysics framework. This research resulted in the creation of a calibrated digital twin, facilitating more efficient processing and energy utilisation during milling. Although the study was conducted using copper ore, the approach can be extended to other ores and milling environments. Designing optimised and energy-efficient milling operations is a critical step toward sustainable industrial practices, and this work provides a means to achieve that without the need for extensive physical testing.

Using vibrating disc mills, Contribution 5 investigated how the tap density of graphite is affected by grinding time. The study found that smaller particle sizes had higher tap densities than larger ones, while particles with a higher degree of sphericity resulted in lower tap densities. The authors attributed this behaviour to the impurities in the graphite, which affect packing and particle alignment during tapping. Consequently, graphite grinding using the vibratory disc mill can be optimised using exponential models, leading to energy and time savings.

Contribution 6 focused on optimising the rotational speed of the agitator in a vertical stirred mill, with the ultimate goal of enhancing the grinding efficiency. It is well established that increasing agitator speed enhances grinding efficiency, albeit with increased energy consumption. In this work, the authors successfully identified what they termed as the critical speed selection point, which balances grinding efficiency and energy consumption. This parameter is particularly significant, as even small efficiency gains can substantially reduce energy costs on an industrial scale. Notably, the evaluation index proposed in this study uses larger values to indicate greater grinding efficiency. From this perspective, an inverse proportional relationship was observed between rotational speed and grinding efficiency for a fixed milling time.

Several researchers have contributed to the optimisation of comminution circuits from the perspective of ore characteristics rather than comminution machinery. Contribution 7 established that the Bond Work Index can be applied to non-standard feed particle size distributions, with the help of a correction factor. This factor adjusts the work index to align with the standard feed conditions. For non-standard feed particles, the model was recommended for P100 = 75 μm tests.

Another method for measuring rock breakage energy and force is the Geopyörä Breakage Test, which uses two counter-rotating wheels to nip and crush rock samples between rollers with a tightly controlled gap. Contribution 8 employed this method and determined that it requires much smaller sample sizes and can accommodate broader, more detailed orebody characterisation.

An effective method for improving ore grindability is through pretreatment, as demonstrated by Contribution 9 who applied a hybrid of thermal and mechanical pretreatments before grinding. The Bond Work index tests revealed that microwave pretreatment alone significantly enhanced grindability compared to other individual treatment methods; however, prolonged microwave treatment time can cause iron ore particles to fuse, negatively affecting grindability. Mechanical pretreatment alone enhanced grindability by generating cracks, while furnace treatment was found to be the least effective. The combination of microwave and mechanical treatments yielded the best results in terms of grindability and energy savings. Nevertheless, a comprehensive energy assessment is necessary to account for the energy consumed during heating and compression.

In conclusion, the papers published in this Special Issue address interesting topics from various stages of comminution across the mineral beneficiation chain, ranging from blasting and crushing to milling. Ore pretreatment methods were also analysed, along with the grinding tests necessary for the optimisation of comminution circuits, providing valuable insights. The research presented in this Special Issue will be beneficial to both the mining and mineral processing industry and the academic community.