From a block of stone to rock flour 

In order to separate gold from other minerals it is necessary to break up the rock. It is then ground into a fine flour which requires the use of heavy duty equipments. First, the blocks of stone are broken up in an industrial crusher that looks like a gigantic nut crusher. The pieces of rock which are a few centimetres in size are then placed in a ball mill (a large rotating drum filled with steel balls which are a speciality of Magotteaux, a Belgian company based in Vaux-sous-Chèvremont). A fine flour, the grains of which are only a few tenths of a millimetre thick, is then produced by the ball mill. 

The challenge facing the production process is to produce flour that is sufficiently fine so that only particles of gold and rock are left. “All mineralogists want to reach a liberation level of 100% but that would be the ideal level. This is the aim of the research we are conducting. We want to be able to maximize this liberation which, at the present time, is not always efficient”.

The extent of this liberation is crucial because, at this stage of production, the flour is still only a mishmash of the different minerals which made up the crushed rock. “Before the metallurgy phase, the minerals that will be used during the other stages still need to be separated”.

There are several separation techniques. For gold, differences in density are used. A 1 mm3  particle of gold has a mass of 19 milligrams and that of the rock is 2.5 milligrams. For other minerals containing sulphide such as chalcopyrite, the technique used is flotation. “We use the hydrophobic property of most sulphide minerals whereas the others are hydrophilic. We add a foaming agent to a vat containing a cubic meter of water and the foaming agent rises to the surface creating air bubbles. We then add the mineral particles from the crushing process,” explains the researcher. The hydrophobic particles tend to adhere to the air bubbles. In this way they are separated from the rock particles and form a froth that has a high concentration of chalcopyrite. “Two concentrates are formed when the rock flour is added. One is rich in sulphide and the other is not. It is mainly at this stage of the production that we intervene. Our study consists of observing what goes in and what comes out, and verifying if the separation has been sufficient or not which is of vital importance because, on an industrial scale, the crushing phase of the rock is the most taxing in terms of metal production. It is a very costly and energy-consuming phase involving thousands of tonnes of rock”

By repeating the process, this froth becomes more and more concentrated until a high percentage of chalcopyrite is reached. It can then be sent to metallurgy factories where the copper will be extracted and refined. Given that one tonne of rock only results in around twenty kilograms of copper, any tool or method of expertise that makes it possible to avoid wasting any of these very valuable particles will be welcome. 

The separation techniques for minerals are more efficient when the liberation phase functions well also and this is the stumbling block for engineers today.