The role of Al13-polymers in the recovery of rare earth elements from acid mine drainage through pH neutralization

Rare Earth Elements (REE) are crucial to the development of new technologies, given their strategic importance, a wide range of actions has been implemented to ensure sustainable and affordable supplies. Acid Mine Drainage (AMD) besides being one of the major environmental issues faced by the mining industry is a naturally occurring process involving a considerable amount of REE. The recovery of REE from AMD, which is considered a secondary source, has been investigated worldwide over the past few years aiming at sustainability. The current work seeks to provide an understanding of the geochemistry of the REE in an AMD system and also of its respective treatment through pH neutralization, as a contribution to the development of novel REE recovery technologies. The approach proposed herein employed acid mine water and sediment sampling, ICP-OES, XRD, SEM-EDS, TEM, 27Al MAS-NMR and XPS analysis, BCR sequential extraction and geochemical modelling by PHREEQC. The results showed that the REE content in the AMD investigated is very high being Ln3+, LnSO4+ and LnF2+ the dominant species in the impoundments at pH ± 3.5. The neutralization of the AMD with an alkali agent promoted a high REE recovery and produced an amorphous precipitate containing 14% of REE oxides. This precipitate contains Al13-polymers and accounts for the REE removal when the pH is raised to 8, thus promoting adsorption and entrapment simultaneously. Sequential extraction revealed that the REE could be efficiently leached with the use of acetic acid, thus supporting novel and promising recovery technologies through co-precipitation processes.