Industrial residues high in Fe, Zn and Pb? Non-aqueous solvent extraction is a no brainer !

Fe, Zn and Pb are found in many industrial residues and their effective separation from a deep eutectic solvent was made possible in this work using commercial extractants. The process was validated using mixer-settlers, bringing it one step closer to real life realization.

Non – aqueous solvent extraction of Fe, Zn and Pb

Fe, Zn and Pb are present in metallurgical waste at such amounts that their recovery can lead to viable business (Figure 1). Waste from the zinc production in the form of jarosite contain approximately 40% Fe, 9% Zn and 8% Pb while a typical fayalite slag from primary copper production usually contains 40% Fe, 0.25% Pb and 3% Zn. Besides, Pb being a toxic element makes the valorisation of those materials a necessity prior to their disposal. Aiming for a holistic exploitation route, the leaching, separation and recovery of those metals was studied in the framework of the SOCRATES project (https://etn-socrates.eu/).

Being relatively new in the metal processing field, deep eutectic solvents (DESs) are on a mission to make a disruption and lead in new metal exploitation routes. Carried out jointly by the University of Leicester and KU Leuven, this work utilised DESs as alternatives to the aqueous phase in the solvent extraction of Fe, Zn and Pb. The researchers investigated the selective extraction of Fe over Zn and Pb from a feed of Ethaline (1 : 2 molar ratio of choline chloride : ethylene glycol) containing the three metals, by a commercial mixture of trialkylphosphine oxides (Cyanex 923) diluted in an aliphatic diluent. 95% of Fe was extracted from the Ethaline feed with minor co-extraction of Zn(II), while Pb(II) was not extracted. The subsequent separation of Zn from Pb was carried out using the basic extractant Aliquat 336 at an 80% efficiency. For all the experiments, the equilibration time and the extractant concentration were optimized to highlight for maximum efficiency. After the extractions, Fe and Zn were stripped back to aqueous phases using oxalic acid and ammonia respectively, closing the loop in the successful selective extraction of the metals. A process flowsheet was then created to showcase the possibilities for holistic recovery of the three metals using both solvent extraction and electrochemistry (Figure 1).

Figure 2 Proposed flowsheet for the separation and recovery of Fe, Zn and Pb.

Upon successful separation in batch, the process was also upscaled in a countercurrent mixer-settler set-up (Figure 2). The concept was also successfully validated at the continuous flow mode, resulting in the high separation and purification of the metal phases, each containing one metal.

The mixer-settler set-up demonstration highlighted that non-aqueous solvent extraction has potential for large scale utilisation. The process as designed within the specific framework has the advantage that can be fitted in large metal processing flowsheets to facilitate the holistic metal extraction of the specific metals but also to be adjusted for the extraction of other metals. For precious metals in particular that have high market prices, non-aqueous solvent extraction can have high potential for high-return business.

Stylianos Spathariotis, Nand Peeters, 03.09.2020

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