Remember the old arcade games from the 80s? No fancy hyper-realistic graphics, no third dimension. Nonetheless, in just a few pixels, you could live the most amazing adventures: Rescue abducted princesses, save Earth from alien invasions, flying spaceships through asteroids and… eat a lot of fruit while staying away from indigestible ghosts.
Yes, that was Pacman’s thing. The yellow pal was on a mission to seek and collect all the pieces of fruit that were disseminated within a labyrinth, avoiding contact with the ghosts.
What does that have to do with solvent extraction? Regrettably, there are no arcades at CHROMIC facilities, yet Pacman’s lifestyle comes in handy to describe the object of this Getting to Know, as we’ll see in a moment.
After the “tea preparing” stage (i.e. leaching stage) of the metal recovery process, we were left with our valuable metals (Cr, Mo, V, Nb) dissolved in a complex solution containing solvents and some impurities: The final step of the recovery process is then that of separating the metals from the solution, and possibly enrich them. That can be achieved very efficiently by cunningly combining different chemical techniques, one of which is solvent extraction.
CHROMIC Podcast – Episode 6 is about solvent extraction. Click here to listen to the podcast
This method is used to separate compounds based on their solubility in two different immiscible liquids, which in most cases are an aqueous solution containing the target element(s) and an organic compound. To do that, the two liquids (or “phases”) are mixed, so that the solutes can distribute between them until equilibrium is established, and the two liquids are separated again. The transfer of species from one phase to the other is driven by the chemical potential, which by the end of the reaction brings the whole system to a more stable energetic configuration. The liquid containing the desired solute is called the “extract” and what is left behind in the other solution is called the “raffinate”.
Going back to the analogy we started from, we can conveniently link the elements at play with the characters and dynamics of the beloved arcade game. The organic compound is represented by Pacman itself, the fruits are the valuable metals, and the ghosts are the impurities which Pacman does not want to get into contact. The solutions are immiscible, but they can be mixed through stirring, so Pacman comes into contact with the fruits, transferring them into the organic phase. When Pacman has explored the whole labyrinth, the mixing stops and the two solutions separate, leaving the ghosts and impurities in the aqueous leachate, whereas Pacman and the fruits are located in the organic phase. In a second step another solution can be added, which encourages Pacman to release the fruits into the aqueous phase, so to finally get a pure solution of our valuable metals.
Aside from this simplified image, the process is indeed in principle pretty straightforward, but in practice very challenging. First off, in case of the reactive extraction of metal ions selecting the right extractants, modifiers and solventsfor the task is far from being trivial. A number of features have to be taken into account: The ability of the extractant to bind to the target metal to a much larger extent than the rest of the components in the mixture; the irreversibility of the reaction must be guaranteed, in order for the dissolved components not to go back to their previous form; the compound formed after the reaction has taken place must be easily recoverable. Other factors that affect selection for the composition of the organic phase are its solubility in the aqueous phase and its long term stability; for industrial applications other factors like low toxicity are important, too. . Furthermore, the conditions under which the extractive reaction takes place greatly impact on the final result, and need to be fine-tuned. For instance, it is very important to maintain a stablepH and a constant temperature of the compound during the extraction process, as well as to find the right residence time (the time in which the two solutions are in contact) and the suitable phase ratio so that the reaction is optimized.
Solvent extraction is widely used both on small – chemical laboratories – and industrial scale, due to its cost-effectiveness and capacity of separating the required components without altering their properties. It is for instance applied in the production of fine organic compounds, the production of vegetable oils and biodiesel, the processing of perfumes. It is also employed in the petrochemical refining industries, where extraction allowsthe procurement of pure petroleum from the impurity-filled crude oil. From a hydrometallurgical standpoint, the ability to selectively separate out even very similar metals makes solvent extraction the way to go for separation and purification of elements like uranium and plutonium, cobalt and nickel, as well as rare earth elements.
Depending on the application, different devices and apparatus can be used to perform solvent extraction. Those commonly include so called separatory funnels (at lab scale), and machines that bring the two liquids into contact with each other, like extraction columns and mixer-settlers.
In CHROMIC the research on solvent extraction to recover Cr, V, Mo and Nb is brought forward by HZDR, while other methods which are to be combined with solvent extraction, like selective precipitation and sorbent materials are investigated respectively by BFI and FehS, and VITO. Lastly, TUK focuses on the final processing.
No arcade game characters are being harmed during the process.