In mining, the strength of rock is required by engineers for a range of applications ranging from designing underground tunnels to selecting appropriate comminution devices. Measuring the strength of rocks can be achieved by a number of commercially available tests which are based on rock mechanics and mineral processing characterisation procedures.
In rock mechanics characterisation, a wide range of controlled tests are used to quantify the strength of rocks; each test has predefined protocols, procedures and sampling preparations aimed to minimise variability of strength due to the effect of rock shapes. However, in mineral processing characterisations, no strict protocols and procedures are followed to determine the strength of rock. Strength is determined empirically from the degree of breakage when a controlled energy is applied to crushed rock samples, such as when using either the JKMRC Drop Weight or the JKMRC Rotary Breakage testers.
It has been reported in the literature of both rock mechanics and mineral processing characterisation that varying rock shapes respond differently with varying measured strengths. To test the robustness and the extent of variability of the two characterisation methods, an initial hypothesis was tested:
• “The strengths of various rock shapes for the same rock type should rank the same in various test methods.” -
This is because the strength is predetermined by the rock sample shape and not by the type of the test device used.
Therefore, the measured response in various test devices is expected to provide a consistent strength ranking of the
different shapes of rock used.
Initial results showed that the strength ranking of rock shapes in various tests using rock mechanics characterisation methods were consistent. However, tests conducted using mineral processing characterisation showed inconsistencies in the strength ranking of similar rock shapes; therefore contradicting the hypothesis. The competency of rock is an important measurement to select the correct comminution device in circuit design. Incorrect selection of comminution devices based on incorrect rock strength measurements would have consequences on the overall circuit performance.
Controlled tests were conducted to determine the reasons for the differences between the two characterisation methods. It was shown that the empirical methods used for mineral processing characterisation do not take into account the actual particle dimension but instead use the passing screen size. This sizing protocol biases the strength measurement of flaky or flat shapes, making them appear less competent. Using mineral processing characterisation methods and after correcting for actual particle dimensions, flaky shapes are shown to be more competent than non-flaky shapes, corresponding with the strength ranking as observed in rock mechanics characterisation.
With the extensive insights gained into rock shape strengths from both rock mechanics and mineral processing characterisation, a fundamental breakage model was developed that can predict the strength of crushed rock for any shapes in drop weight test conditions. The model development was based on the following hypothesis:
• “If a unified relationship between rock shapes and strength response is known, then it should be possible to predict
the strengths of varying rock shapes based on one strength parameter.” - If strength was predetermined by the rock
sample shape and not by the type of the test device used, then it could be assumed that the dependence of shape and
strength are inter-related and the prediction of failure strength of any rock shapes could be determined by one shape
The mechanistic breakage model uses rock mechanics principles to predict the strength of crushed rock shapes tested in mineral processing characterisations. The model is novel in that it links both fields of sciences; rock mechanics and mineral processing characterisation. The model has wide application for determining the distribution of the strength of different rock shapes fed into various comminution devices. To determine the influence of rock shapes in AG / SAG milling, a third hypothesis was explored:
• “Because of strength variation of rock shapes, various proportions of rock shapes can be used to improve the
grinding efficiency in AG / SAG milling.” – The estimations of energy utilisation and throughput in tumbling mills are
determined from the standard mineral processing characterisation test work. If the mineral processing characterisation
shows clear differences of strengths of various shapes, then it can be assumed that the throughputs in tumbling mills
can be controlled by the type of rock shape used.
Controlled experiments on blends of rock shape showed that mill throughput increased and the grind became coarser with the addition of weaker rock shapes. The insights gained showed that various rock shapes behaved differently for various breakage mechanisms within the AG / SAG breakage environment. The work presents an opportunity to incorporate rock shapes as an optimisation strategy to increase performance of comminution circuits.