A foam bath for ores - part 2
What happens at the surface?
With a device known as an inverse gas chromatograph, Martin Rudolph can determine the surface properties of solids. Photo: HZDR/ Detlev Müller Download
“We can only make improvements when we know what exactly occurs during flotation, especially on the molecular level,” Rudolph believes. “When particles dispersed in water attach themselves to gas bubbles, three phases come together at once: solid, liquid and gas. We are interested in the interactions at the interfaces where the different materials make contact with one another.” The Freiberg scientists focus on the particle surfaces where the collector and depressant reagents are applied as well as the hydrodynamic processes.
It is not only the surface properties of the minerals that interest the researchers but also how different reagents change the interaction forces in the flotation cell – the tank in which the processes take place. Given the increasing complexity of the ores with the most finely dispersed elements, this is a challenge. By using atomic force microscopy (AFM), the Freiberg group is able to investigate such complex mineral phases and characterize their wetting properties. AFM is an imaging method which makes it possible to visualize very small-scale topographies down to molecules. The scientists use it to measure the tiniest forces and thus to track down the interactions. This allows them to map the forces on the surface of fused raw materials and calculate the impact of reagents.
“With the measuring tool we have developed on the basis of atomic force microscopy we can quickly define the right chemical cocktail for floating a new raw material,” says Rudolph. “As this used to be a costly process in terms of both time and money, industry is very interested.” In the next five years, he and his team want to develop a prototype apparatus for screening flotation chemicals.
Flotation cell dynamics
The reagents are not the only things that influence the results of flotation, but also hydrodynamic processes. The flow conditions determine how often a particle collides with a bubble and whether the particle attaches itself to it. In investigating the complex hydrodynamics in the flotation cell, the HIF scientists work together closely with HZDR’s Institute of Fluid Dynamics and especially the Division of Transport Processes at Interfaces. The head, Kerstin Eckert, is a specialist in this field. “We look at the microprocesses and the inner dynamics of flotation through a very fine lens,” the physicist explains.
This research group focuses, above all, on fine grains of under 20 micrometers and particles that are too large to upwell in the froth. When it comes to ultra-fine particles, conventional froth flotation reaches its limits, which has meant that many valuable materials have been lost. The subject is also relevant for resource recovery from old waste dumps and recycling. Flotation could be a method for recovering and reusing the fine graphite and lithium containing particles in old batteries. Being able to float larger particles is worthwhile because coarser grains save energy on grinding.
The five-strong research group set up by Kerstin Eckert a year ago investigates the different effects at the interfaces in the slurry and froth phase. “This includes the electrical charges on bubbles and particles and the forces like surface tension and capillary actions,” the physicist explains. “We want to know how the dividing water film is broken when attachment takes place. This has to happen very quickly while the particle and the bubble are still close enough together.” To increase the chances of collision and attachment, the researchers are also studying measures to intensify the process, such as the application of ultrasound.
From an experimental point of view, the processes are difficult to follow because flotation literally takes place in the dark. All you can see is the input material and the final products. In order to understand what happens inside the black box, Eckert’s department conducts model experiments on sub-processes using new measuring methods. Neutrons, for example, are very good at shining some “light” on the opaque frothing phase and the particle movements that take place in it. Just as important is the development of computer models.
