A foam bath for ores
This article was published in the HZDR magazine "Discovered" 2/2017.
In nature, raw materials do seldom occur in a pure form. The valuable particles in the ores first have to be painstakingly separated from other materials and enriched. The leading method for doing this is flotation. HZDR scientists examine its underlying mechanisms and microprocesses with the aim of optimizing industrial processes and improving the extraction efficiency in raw materials processing.
Anything lighter than water floats towards the surface. This is the basic principle underlying material separation by froth flotation. Without it, it would be almost impossible to utilize many raw materials. “Unlike other methods, flotation is highly suited to separating fine particles,” says Martin Rudolph, who heads the Division of Processing at the Helmholtz Institute Freiberg for Resource Technology (HIF). “Although ever tinier fractions of the valuable minerals are being found in the deposits.”
His ten-strong research group at HIF and his HZDR-colleagues from the Institute of Fluid Dynamics are thus searching for new ways of separating even the tiniest particles and improving the resource and energy efficiency of flotation processes in industry. As around the world several billion tons are floated to extract raw materials every year, there is enormous potential for optimization. Froth flotation is not only used in processing ores but, increasingly, in material recycling.
The method utilizes the differing surface properties of mineral particles which determine to what extent gas bubbles can accumulate there. If you add gas bubbles to a fluid containing finely ground particles, the bubbles attach themselves to particles with a hydrophobic, that is, water-repellent surface. This causes upwelling and they rise to the top while the unwanted hydrophilic grains remain in the fluid phase, the slurry. On the surface of the fluid the “good” particles with bubbles attached form a layer of froth which is continually skimmed off. The rest of the particles, the so-called gangue, are pumped off at the end of the process.
How to stay dry in water
But how do you make the gas bubbles in the flotation cell collect the right particles and carry them to the surface? It is only to surfaces that are hydrophobic and therefore poorly wetted by water that the gas has a chance of attaching itself. Typically, however, mineral grains in water are well wetted. To ensure that, in the end, the good ones end up in the pot – or, in this case, the froth – the valuable substances’ wettability is chemically modified. “Reagents, so-called collectors, that are tailored to the respective valuable material make their surface hydrophobic while the other components remain hydrophilic,” Martin Rudolph explains. “This means froth flotation can be customized to separate a wide range of minerals.”
In addition to collectors, other activators like so-called depressants are used which improve the wettability of the unwanted grains, causing them to remain in the flotation cell. Chemicals called frothers help to stabilize the froth with the valuable particles so that the grains do not sink down into the fluid again. In order to float well, the grains need to be between 20 and 250 micrometers in size. The ore is ground as finely as necessary and selectively hydrophobized with collectors. It then enters the flotation cell as a watery slurry where a mixer generates and finely disperses air. The objective is to achieve as high an extraction rate as possible and this depends on various factors: the density of the slurry, the amount of gas added, the size of the particles and bubbles, the reagents used, and fluid dynamics – all of these need optimizing.