Digital imaging forbetter performance 

After crushing the rock, it will never be possible to have particles containing 100% chalcopyrite or 100% gangue. Most of the particles are mixed. In order to better understand and therefore react to the level of liberation, mineralogists observe samples of this rock flour with an optical microscope using reflected light. “The particles are placed on a resin and then sectioned before being polished. What interests us is the internal structure of the particles and not their surface. Tomography or 3D analysis could be used but the material is too compact and dense”. Traditionally, these observations are made with the naked eye and, from a qualitative point of view, make it possible to obtain more or less precise information on the degree of liberation of minerals.

Since the middle of the 1980s, Eric Pirard has aimed to go further and develop a quantitative approach. Therefore he no longer uses his eyes but rather a camera fitted to the microscope which digitally transcribes the observed image. “Today, digital imaging is a widely-used technology, even by private individuals. Twenty-five years ago we were almost pioneers, the researcher says with some pride”.

The images which are then sent to the computer are made up of millions of pixels which assume different colours according to the type of material they represent. These pixels make possible a series of very precise calculations. For example, chalcopyrite as we have said is a yellow mineral. In an image of a cross-section of a particle, all the yellow pixels represent chalcopyrite. It is possible to calculate to the nearest pixel, the percentage of chalcopyrite still present in each particle and to establish a liberation curve. If the particle contains 100%, 90% or 80% this means that the liberation of minerals was a success during the crushing phase. Yet, very often, there is an important percentage of other minerals present in these particles. The thorny question is to know whether it would useful to recrush tens of tons of flour in the crusher in an attempt to have a greater liberation or whether this would be a waste of time, money and energy.

Better classification of the texture of particles

Precisely establishing the percentage of different minerals in a particle was certainly very useful but did not take into account two important characteristics. Firstly, the intergrowths can be very different and more or less complex in nature. Professor Pirard’s team identified four main families of particles: The simple particles (a), the stockwork particles (b), the coated particles (c), and the emulsions (d). It should be noted that in nature, there is an intermediate series of textures that make it difficult to achieve an exhaustive and efficient classification.



Assuming there are four groups of particles, each one belonging to one of these families and each having a 50% proportion of chalcopyrite, then in the case of group C, if the flour is re-exposed to a longer time in the crusher, there is a good chance that the particles will be ground and broken up during the separation of the minerals. There is a greater possibility that the chalcopyrite will be better liberated. Another spell in the crusher could therefore be useful. If the particles now present a type-A texture, we can imagine a better liberation but this already appears to be more complicated. With regard to the B-texture intergrowth, if these particles are ground even more, they will certainly be smaller but will still present the same texture. The chalcopyrite will not have been liberated. Sending particles like this to be recrushed would quite simply be wasteful.