You may have heard about flotation, so many times in the mining world. But what is that actually?
Get educated about this processing technique by reading our recent article!
After leaving you hanging for a while, I am now ready to talk about froth flotation as minerals process technology. As I have already revealed in my first blog “Finnish wood”, the idea of the project is the development of new cellulose-based chemicals, having selective affinity towards selected minerals surfaces, thus behaving as efficient collectors. There is an urgent need to replace petroleum-based chemicals. Indeed, bio-renewable resources are appealing candidates.
What does froth flotation mean?
Froth is synonymous to indicate foam. Foam can be liquid or solid; also, Demian (ESR5) explained what a foam is in his first blog. We are dealing with a liquid foam, a colloidal dispersion in which a gas is dispersed in a continuous liquid phase. Foams can be simply generated by shaking, agitation, rotation, and mixing a surfactant solution so that air bubbles become entrapped.
Examples of liquid foam are present in our daily life (Figure 1); imagine the simple gesture of washing our hands or when in the early morning, we make a cappuccino to start our day. Surfactants or proteins contained respectively in the soap or milk contribute to the foam formation.
Figure 1. Daily life liquid foams.
Then, flotation is the action of floating in a liquid or a gas. Froth flotation represents the most versatile separation method in mineral processing. It takes advantage of the different surface minerals properties to selectively separate more hydrophobic minerals from less ones towards interactions with air bubbles.
Flotation has been initially developed to treat the sulfide minerals of copper, lead, and zinc. Nowadays, it is largely used for non sulfide minerals, i.e. oxides, carbonate, and silicate minerals, also for non-metallic minerals such as fluorite, talc, and phosphates.
This process is a physicochemical process that involves three phases: solids, water, and air. The ore, natural sediment also containing valuable minerals, is dispersed in water, while air is injected under stirring (Figure 2).
Figure 2. Froth flotation cell.
The air bubbles introduced in the agitator collide with minerals particles, which start to slide around it. While sliding towards the bottom of the bubbles, the hydrophobic particles will become attached, and the hydrophilic ones will fall away; finally, the bubbles carry the attached particles to the froth, which is then collected to the top of the cell.
Chemical and physical variables play an important role in a successful flotation, which is a trade-off between recovery and grade. The chemical variables including collectors, regulators and frothers, regulate the transition between the hydrophilic and hydrophobic state. Collectors adsorb on mineral surfaces, increasing their hydrophobicity and favouring bubble attachment. Regulators either activate or depress mineral attachment to air bubbles. Moreover, they control particle dispersion and the pH of the system. Frothers help produce the fine bubbles necessary to increase collision rates and maintain a reasonably stable froth. Physical variables refer to the ore properties, such as particle size and composition, and the machine-derived factors such as air rate and bubble size.
Two types of flotation can occur; when the valuable mineral is transferred to the froth and the gangue or tailings are left in the pulp, we refer to direct flotation; reverse flotation happens when the gangue is separated into the float fraction. Chemico-physical properties of the minerals play a key role for selecting the collectors and setting the flotation parameters. All minerals are classified into nonpolar or polar types according to their surface characteristics. Relatively weak molecular bonds characterize the surfaces of nonpolar minerals. The nonpolar surfaces do not readily attach to the water dipoles, in consequence, they are naturally hydrophobic. Minerals with strong covalent or ionic surface bonding, on the other hand, are known as polar types and react strongly with water molecules; these minerals are naturally hydrophilic.
Regarding my current research, after functionalizing the cellulose nanoparticles towards environmentally friendly reactions, the focus is investigating the interactions and the efficiency of these new bio-based chemicals with the selected minerals.
Which kind of minerals am I working with?
Pyrite, which is a naturally hydrophobic sulfide mineral, and quartz which is a naturally hydrophilic silicate mineral. The main efficient collectors for pyrite and quartz are classified as anionic and cationic. More in-depth sulfhydryl and thiol chemicals, which contain -SH group have been largely studied for sulfide minerals recovery. Meanwhile, amines, especially dodecylamine, are excellent collectors for quartz.
Two general adsorption mechanisms can occur between the collectors and the minerals. In the flotation of sulfide minerals the adsorption, known as chemisorption, involves chemical bond formation. The electron donor sulfur atom, contained in the sulfhydryl collector, forms a chemical bond with the metal cation. Physical adsorption, so electrostatic interactions, concerns the adsorption of quartz on the dodecylamine surface. Mineral particles in water carry a surface charge, which electrostatically interacts with the ionic group of the collectors.
Despite their high efficiency, both sulfhydryl and amine chemicals are not environmentally friendly. This urges for greener alternatives such as cellulose nanoparticles, which represent promising candidates in froth flotation for replacing conventional collectors.
Did I catch a bit of your attention? Do you want to know how these newly synthesized cellulose-based chemicals will perform as collectors in froth flotation? Well, stay tuned with our website and research 😊
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- Hartmann M., Sirviö J. A., Sliz R., Laitinen O., Liimatainen H., Ämmälä A., Fabritius T., Illikainen M. (2016), Interactions between aminated cellulose nanocrystals and quartz: Adsorption and wettability studies, https://doi.org/10.1016/j.colsurfa.2015.10.022
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