Bridging the gap between engineering and geology, the aim of minerals engineering is to provide a better recovery in terms of the exploitation of deposits. Mineralogists from Liege and Madrid have specialized in the quantitative analysis of digital microscopic images. The objective is to better understand the intergrowth textures of the different minerals in the same rock. These techniques make it possible not only to understand how to better liberate the minerals captured in the rock for a better recovery, but also to rethink the entire recycling process for used metals.
The journey from the mines to the copper pipe that connects the central heating circuit in our homes is a long and arduous one. This is because copper, like most metals, is not found conveniently separated from rock particles simply waiting to be harvested. On a scale of the order of tens of microns, the different minerals present in the same rock are interwoven. Between the extraction of the rock and the metallurgy phase, there is a rigorous and critical stage which involves mineral liberation and the separation of the different particles to be found in the same lump of rock.
Since 1986, Eric Pirard, Professor of Mineral Resources and Geo-imaging at the University of Liege has brought his precious scientific knowledge to bear on this phase of production for metals. Assisted by Laura Pérez-Barnuevo, a doctoral student at the Polytechnic University of Madrid, he has studied the liberation characteristics of chalcopyrite (CuFeS2, a mineral composed of copper, iron and sulphur) from a mineral deposit located in Zambia (1), with a view to proposing and marketing an automatic digital imaging technique. This mineral presents three advantages. It exists in great quantities on our planet (copper exists in many sulphide forms, and is interesting to extract and study). Chalcopyrite is yellow in sharp contrast with other minerals and is therefore easy to identify and observe. Finally, this sulphide presents complex intergrowth textures with the other minerals that make up the rock in which it is found. In other words, copper is difficult to extract but displays a large mineralogical and textural diversity. These characteristics have led the researcher to concentrate on this mineral in order to develop a quantitative tool that might eventually be more widely used.
Worse than a needle in a haystack
The theory of liberation involves the separation of the different molecules in a rock. It literally consists of separating the minerals from each other. Gold serves as a good example. “In the collective imagination we think of a prospector panning for gold fragments in a river, but that was an artisanal activity practiced during the gold rushes of the 19th century”, Eric Pirard reminds us. Most gold-extraction, as is the case with other minerals, takes place in large factories that process hundreds of thousands of tons of material. With a ratio of barely 5 grams of gold to every tonne of rock “you may just as well search for a needle in a haystack. It would be easier to find a needle in fact, as it does not stick to the hay. All you need is a magnet”, the researcher adds, with irony.
Evidently, a magnet cannot be used to extract gold from a tonne of rock. Firstly, the five grams of gold are not bunched together but are spread and trapped throughout the rock. And these particles of gold rarely exceed one-tenth of a millimetre.