Vapour Phase Extraction: A Re-Evaluation

Abstract

Research undertaken at the University of the Witwatersrand has pursued the recovery of valuable metals using vapour-phase extraction. VPE is a process in which an organic ligand reacts with an element of interest; the product, an organo-metallic complex, reports to the vapour/gas phase and so is separated from the bulk material. Contacting with the solid particulate material occurs in a fluidized bed. The stable organo-metallic complexes, being removed, are condensed and recovered. VPE seems a promising technology and so the use of acetylacetone (acac) to extract several metals from various sources was investigated at Wits. The extractions of (1) iron from iron ore fines and synthetic sources and (2) chromium and tantalum from synthetic samples were investigated; so, too, was (3) aluminium from fly ash and synthetic sources; (4) vanadium from depleted catalysts; and (5) gold from tailings. Gold and tantalum cannot be extracted by acac at all, yet it was thought that they could be. Ligands may also not react with—or react in the same way with—different structural states of the same compound; this was not considered in previous studies, which only mentioned that higher extractions were obtained from synthetic samples. The underlying causes were not adequately considered. Essential properties—such as phase and composition—were not identified, much less characterized. Other limitations include fractionation and fluidization. If the grade of the material is very low, there may not be sufficient metal recovered to isolate it by means of fractionation. This would require high selectivity, which precludes the use of acac. A lack of fluidization prevented reagents from reaching the target species, starving the reactions. Thus, the conditions needed to undertake kinetic studies were not met, invalidating previous tests and their findings. Previous work which reported success has been shown to be unreliable or misguided by experimental anomalies. However, the possibility of using such a process cannot be ruled out altogether. That is to say, alternative ligands can be considered, but only if they can be shown to react with a target species in the phase in which it occurs. The mechanisms involved in the process need to be properly understood and employed before a process can be conceived and developed.