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Ion flotation for the recovery of the metals

Is there a better way of starting your first blog post of the project you are working on than giving some general information about your research and the main technique you are going to use? Probably, but I will start with the latter.

My name is Demian Kalebić, writing from Leuven, Belgium, I am the ESR5 of the MSCA project SULTAN and the third Croatian writing a SULTAN blog post. As one of the few chemists among the ESRs of the project, my blog will not be based on mining, but more on the extraction of those valuable metals. Being a synthetic organic chemist, I am also not so experienced in that field, but up to this point of my PhD, I learned a lot about that absorbing topic. Especially interesting to me is the technique which I am investigating – ion flotation.  Even though the name is pretty straightforward and clearly states that it is one of the techniques that uses foam to extract the metal ions from solution, it doesn’t tell much about this rather captivating method which belongs to a whole group of flotation methods, some with more peculiar names such as gamma flotation and even piggyback flotation.

I will always remember my friend, a computer scientist, whose reaction when I first told him what the topic of my PhD was:

“Do you know what a metal is? You can’t possibly extract metals with foam. Do you even know what foam is?”.

Like most chemists I knew what foam was, I knew which conditions need to be met for the foam formation, I knew about the molecular structure of the surface-active compounds – surfactants, but with ion flotation I was not that well acquainted. How does it all happen, where does it happen, how does the process look like, there were many question marks floating around. I will try to answer all of them and hopefully provide more insight into this technique by the end of this blog post.

Figure 1. Different types of foam. Source: https://www.freeimages.com

So firstly, to avoid misconception, I must define some terms:

Foam – a gas trapped in a liquid or solid. Suds is the most well-known foam, but a sponge or a camping mat is also basically – foam.

Surfactant – short for surface-active agent, compounds that lower the surface tension of their solution. They contain hydrophilic (water-soluble) groups and a hydrophobic (water-insoluble) groups in their chemical structure. We use them in our everyday lives in detergents and shampoos. Not all of them are safe for the environment though – especially not sulfate ones, but they still can be found in many of our household items.

Metal ions – forms of metals most often occurring in aqueous solutions or salts as cations with different charges, depending on the specific metal.

Ion flotation can be performed by using a surfactant in combination with a different molecule – called a chelating agent, which can form a complex with the metal ion by chemically binding to it, so-called coordinating. The formed complex is then captured in the surfactant foam. That often brings in more parameters in the equation for an optimization of an ion flotation procedure. A convenient solution is to have the surfactant properties and the metal-binding properties in the same molecule. Such surfactants are called chelating surfactants and their design and synthesis is the topic of my research. We are also trying to investigate alternative sources for synthesis of these compounds such as natural products and widely abundant materials, preferably from waste streams. If a chelating surfactant is to be used in ion flotation, it should be negatively charged. That way, it can form neutral (uncharged) complexes by chemically binding to the metal ion in solution. That complex can then be floated out of the solution. Charged complexes can affect the efficiency of flotation as they are more water-soluble. As the formed complexes are very stable, it makes the method useful for recovery of metals (present in very small numbers) in very small concentration, such as the critical metals, rare-earths, noble and base metals or removal of toxic metals like cadmium (which Panagiotis, ESR10, will investigate).

In the extractive metallurgy, leaching is a process in which metals or ores are dissolved using various aqueous solutions (in hydrometallurgy) or using extractants and organic solvents (in solvometallurgy). Research on different leaching techniques such as microwave leaching, high-pressure leaching and bioleaching in the reprocessing of tailings will be done by Maja ESR7, Tamara ESR8 and Belsonia ESR9, respectively. Ion flotation is very useful for treating aqueous leachates from which the majority of the metals of importance has been removed yielding a dilute aqueous solution in which metals of great value still persist, and as water is the solvent often used for the leaching process that makes ideal conditions for ion flotation.

So far, ion flotation hasn’t been used successfully for the recovery of metals from leachates in the tailings reprocessing. On the other hand, a similar flotation technique – froth flotation is readily used in the recovery of sulfide ores, but its application in flotation of tailings is impeded because of the size of the tailing particles. Nevertheless, better experts on the topic of froth flotation definitely are Feliciana and Ana-Luiza, ESR4 and ESR6 respectively.

Figure 2. Laboratory setup of ion flotation. Special thanks to Panagiotis, ESR10 for the setup.

In practice, the process of ion flotation starts by mixing the chelating surfactant (or a surfactant and a ligand) and the metal-ion mixture followed by introduction of air or nitrogen gas from the bottom of the solution, which causes the formation of the foam. What happens then is very similar to the process that happens with the beer when you pour it in a glass when the air and carbon dioxide trapped inside the liquid rush towards the surface creating foam. That is not the same kind of foam that you get during ion flotation, but it is similar and serves a nice depiction of the flotation process. The ion-flotation foam, now bearing bound metal-ions in form of a complex, needs to be stable for a longer period of time than the one on top of beer so that it can be collected, otherwise our metal-extraction process would definitely not be so efficient. That can be achieved by design and synthesis of surfactants of different chemical structures to form better, more stable foam, especially after it forms a complex with the metal ions.

Not only can metal ions be extracted from their aqueous solution this way, but we can also selectively extract specific metal ions from metal-ion mixtures. In an ideal case, that can be done by simply changing the conditions of the solution such as making it more acidic or basic. This brings great economic and ecological importance to the design and synthesis of surfactants whose properties enable selective extraction of specific metal ions from their mixtures.

The following step in metal recovery is to dissolve the foam in an appropriate solvent, separate the metal ion from the chelating surfactant – a process called stripping, and then isolate the metal in its elementary, well known state by electrolysis. These metals are then ready to be used as fundamental components of many of our devices and facilities.

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