The second important characteristic is that the section does not take account the texture of the intergrowths in the third dimension of the particle which could be very different. “This is a basic principle of stereology. If we take as an example a cake with layers of vanilla and chocolate. When the cake is cut transversely and I observe a layer of vanilla, I cannot make the deduction that it is made up of 100% vanilla. One section does not tell us anything. We would need several cross-sections of the same particle, but this is impossible. Today, as we study several particles we only make random cuts”

The creation of several indices has made it possible to better classify the particles according to the two problems mentioned above, and to better identify the nature of a deposit to enable more economical exploration. “The first index that emerged from the calculation by means of pixels made it possible to take account of the volume proportion, and therefore to establish the percentage of chalcopyrite in the particle. The second index is that of surface exposition. When the separation technique used is froth flotation, for example, the hydrophobic minerals must be at the surface of the particles so that the latter adhere to the bubbles and rise to the surface”. For example, a rim-type particle could contain 80% chalcopyrite and not float to the surface. 

These two first indices already make it possible to take account of the way the particles could react to crushing and the separation phase. But this is not yet sufficient to classify them into families. For example, the stockwork and emulsion particles will have indices that are similar, that is to say, the same percentage of chalcopyrite, and will be poorly represented on the surface. “To obtain an even more precise vision, we draw random lines. Then we measure the length covered and we look at the distances covered by the yellow phases, therefore chalcopyrite, and at what frequency”

 In the case where the yellow phases are short and numerous, we are dealing with an emulsion-type particle, in the case where the yellow phases are long and not very numerous, we are dealing with stockwork particles, etc. “All these indices make it possible to calculate and identify particle textures in different ways. This procedure may seem off-putting, but the addition of these results gives a more precise statistical analysis of the comparison and classification of these particles into identifiable families. This allows for a better exploitation of the raw material.”

Beyond the texture of rocks

The article is part of much wider research. Eric Pirard aims to develop an automatic optical microscope dedicated to the needs of the mineral industry and to perfect a marketable quantification technique for minerals. 

The researcher also mentions his knowledge with regard to improving recycling conditions for metals. “The crushing techniques that work for rocks also work for GSMs or for solar panels, for example”, the scientist explains. The constraints and problems encountered are the same. A way has to be found to liberate the different metals at the lowest possible cost in terms of economics and energy. Also, with regard to the exponential increase in the world population and the tools it uses, particularly in developing countries, it is vitally important that the researcher devotes his time to these procedures involving engineering, geology, recycling and exploitation of rock. “There has been a lot of talk about closing mines in Europe”, here the researcher takes a jump forward in time, “I predict that in the future mines will be soon be reopened”.