Comminution is the process for the liberation and size reduction of ores prior to separationprocesses. Generally, in mineral processing, grinding is done using rod, ball, autogenous, or semiautogenous mills. The fully autogenous grinding (AG) is the most cost- and process-efficient grinding by benefiting from eliminating steel grinding media. It is also a superior choice for downstream processes such as flotation of some sulphide minerals that are sensitive to the reducing conditions caused by iron chipped away from the steel media.

One of the main factors affecting the suitability of an ore for autogenous grinding is the ore competency, i.e., provide enough critical stones for the grinding process. Several experimental test routines exist and are used for assessing the viability of the ore for autogenous grinding. However, very little attention is given to the generation of fatigue in large stones experiencing repeated shocks in the mill. To investigate this, large pebbles sampled from industrial autogenous grinding mills for hard and soft ores which were categorized based on the grinding energy.

From the pebbles, small drill core samples were prepared and went through a series of fatigue cycle tests. Both hard and soft ores showed similar average resistance to failure in compression tests, but the hard ore had a consistent resistance with lower variations. The cores were scanned before and in-between fatigue tests at the highest reachable resolution, 1.5 µm voxel size. The outcome showed that higher number of micro-cracks were visible in the soft ore compared to the hard ore. The frequency of micro-crack development in the soft ore may be the reason for its lower specific grinding energy compared to the hard ore.

It is obvious that for realistic conditions in an autogenous grinding mill, stones are pre-weakened by fatigue before they fracture. Therefore, shortcut methods focusing only on running tests on fresh and small samples may operate in unrealistic conditions by ignoring the fatigue phenomenon. This is even more important for ores that are in between the soft and hard ore boundary or on the borderline to be considered as a competent ore for autogenous milling.