Abstract

Comminution of rocks to extract target minerals is the most energy-intensive step in the mining process. Small improvements in the efficiency of comminution have the potential for substantial energy savings and economic gains. In-situ mining rocks mechanical characterisation can lead to a better control of comminution processes with energy reduction. Laser-based ultrasonic and photoacoustic techniques are well adapted to online monitoring when parts are moving and provide a rich, broadband signature that may be correlated to important elastic and non-elastic properties of rocks that have impact on comminution. The resulting measurements could then be used for ore sorting, maximizing the efficiency of the comminution operation. Here we present a series of measurements performed on rock samples, highlighting the range of information delivered by these techniques.

First, we perform a series of contact and laser-based ultrasonic measurements of the elastic velocities in a “pulse-echo” configuration where elastic waves gets reflected at the back of the sample, while being generated and detected on the front side of the sample. We show that among a sample selection of 24 different minerals, the measured shear wave velocity strongly correlates with the hardness of the material.

Next, we investigate photo-acoustic measurements on rocks. By focusing a pulsed laser into a small spot on a rock sample, we cause a small damage at the surface that in turn generates an acoustic (sound) wave that radiates away from the damage location. By placing a microphone close to the sample, we are able to record the photoacoustic signature of the material. We repeat this process hundreds of times by using a laser with a high repetition rate, in order to observe the evolution of the photoacoustic signature as the damage increases. We show that for each mineral sample, the amplitude of the photoacoustic signal trends towards a lower limit that we interpret to be linked to the damage threshold of the material.

Finally, we used laser-based generation and detection of ultrasonic waves to analyse the microstructural noise in a group of rock samples. In this configuration, elastic waves are generated by a pulsed laser focused into a small spot on the sample surface. By quickly heating the sample, it causes a small ablation at the surface, which leads to a compressional wave propagating inside the material from the ablation point. Depending on the internal structure of the rock (e.g. grains, microfractures), some of the energy is diffracted back towards the surface, causing a displacement that is then measured with a laser-based interferometer focused at the generation location. We show that for frequencies on the order of a few MHz, the recorded signal is much stronger in coarse-grained samples (diorite) than in fine samples (andesite), allowing us to categorize rock types.

In conclusion, we show that laser-based ultrasonic and photoacoustic techniques provide a wealth of information that can be combined with additional measurements in order to optimize comminution or to perform ore sorting in a mining environment.