Efficient two-step separation of vanadium, scandium, and heavy rare-earth elements from gallium and germanium in high-value fly ash: Coupling solid-phase and solvent extraction

Geopolitical factors have significantly disrupted global supply chains for critical metals such as gallium, germanium, and rare-earth elements (REEs). This study examined 18 unconventional feedstock samples—including fly ash, raw coal, plasma slag, and gypsum fertilizer—to determine critical material content, estimate metal oxide equivalent values (MOEVs) with current market prices, and develop a system for extraction and separation. A recovery framework was developed to prioritize critical metal extraction with a particular focus on fly ash resulting from fuel blends containing 10–15 % petroleum coke (petcoke). Petcoke fly ash contains as high as 1.5 g of vanadium per kilogram of ash and has an estimated MOEV of ∼$80 per ton when only considering vanadium, gallium, germanium, and scandium. The recovery process involves three stages: (i) leaching metals using 6.0 M nitric acid, (ii) separating heavy rare-earth elements with a TODGA-organosilica packed-bed column, (iii) and extracting vanadium with D2EHPA solvent extraction. This method achieved > 30 % vanadium recovery using a 1:1 organic-to-aqueous ratio, 15 min reaction time, and a solvent mixture of 10 % D2EHPA in kerosene at pH 1.5 or higher. The TODGA-organosilica packed bed column strip solution contained > 75 % pure rare-earth elements (based on multivalent cation mass in solution), with more than 95 % of V, Ga, and Ge remaining in solution. Vanadium recovery via solvent extraction with D2EHPA started with an original leach solution of < 0.2 % vanadium, while the strip solution contained a concentrated vanadium and iron matrix (>34 % vanadium and 56 % iron). The findings demonstrate the potential of extracting vanadium, gallium, germanium, scandium and other critical metals from coal fly ash and industrial waste, offering a promising opportunity for resource recovery.