In the framework of the European Training Network for Zero-Waste Valorisation of Bauxite Residue (REDMUD), an integrated process was developed by KU Leuven and RWTH Aachen University to selectively recover metallic iron and rare earths elements from slags produced during smelting of bauxite residue. The work is published in Hydrometallurgy (PTJ/RMR, Leuven 28/01/2019).
Towards an efficient recovery of rare earth elements from bauxite residue (red mud)
Bauxite residue is a by-product that originates during the Bayer process of alumina production from bauxite mineral ores. Every year, approximately 2 tonnes of bauxite residue is generated for every tonne of alumina extracted from the Bayer process. It is estimated that about 160 million tonnes of bauxite residue are produced every year, while approximately 4 billion tonnes have already been stockpiled. Disposal and long-term storage of such waste volumes occupy a lot of land. In turn, this results in major costs and liabilities for alumina producers.
Some treatments already exist to further utilize this bauxite residue, e.g., bioremediation and utilisation in building materials. However, bauxite residue also contains valuable metals such as rare-earth elements (REEs), in minor but non-negligible concentration. In the recovery of these valuable metals from bauxite residue, direct acid leaching is a common applied method. However, the co-dissolution of silicon and iron represents a serious drawback in the downstream processing (e.g., solvent extraction or ion exchange).
A high concentration of silicon in the leach solution can lead to silica gel formation, which significantly affects the leaching efficiency as the gel solution can no longer be filtered. Meanwhile, the co-dissolution of iron is detrimental as it is difficult to separate it from the rare-earths, particularly from scandium, requiring a large quantity of reagents during the downstream processing. Therefore, iron must be removed in advance in order to improve the extraction and selectivity of REEs.
Integrated process to recover rare earths and metallic iron
In this work, bauxite residue was mixed with different proportions of lignite coke, silica and lime, and smelted in an electric arc furnace for iron separation, so that different slags with low iron content but rich in rare-earths were generated. The slags were cooled down at two different cooling rates (see Fig. 1). Subsequently, the leaching behaviour of selected rare earth elements (Sc, Y, La, Nd) by high-pressure acid leaching of the slag was analysed. Firstly, the REEs recovery by acid leaching of a slag with the highest content of aluminium, silicon and iron was studied. The acid concentration of two different mineral acids (HCl and H2SO4) and the leaching temperature were investigated. Secondly, the optimal conditions for REEs recovery from bauxite residue slags were established in terms of maximizing the extraction yield of REEs with the lowest dissolution of major elements (aluminium, iron, silicon, titanium). Thirdly, the untreated bauxite residue and the remaining slags (also generated by reductive smelting) were studied under optimal conditions to evaluate the effect of different chemical composition, mineralogy and morphology on the recovery of REEs. The treatment of bauxite residue and bauxite residue slags by HPAL was compared in terms of selected REEs, iron, aluminium and titanium concentration
Fig. 1: Flow sheet for reductive smelting of bauxite residue, followed by high-pressure acid leaching of the slag arising during the smelting process
Full reference paper
Rodolfo Marin Rivera, Buhle Xakalashe, Ghania Ounoughene, Koen Binnemans, Bernd Friedrich, Tom Van Gerven, Selective rare earth element extraction using high-pressure acid leaching of slags arising from the smelting of bauxite residue, Hydrometallurgy, 184 (2019) 162–174 – https://doi.org/10.1016/j.hydromet.2019.01.005
The research has received funding from the European Community’s Horizon 2020 Programme (H2020/2014–2019) under Grant Agreement No. 636876 (MSCA- ETN REDMUD)
Bio Rodolfo Marin Rivera
On January 23rd, 2018, Rodolfo Marin Rivera obtained his PhD degree in Chemical Engineering. He successfully defended his PhD thesis entitled ‘Innovative technologies for rare earth element recovery from bauxite residue‘. The research was supervised by Prof. Tom Van Gerven (promoter), Prof. Koen Binnemans (co-promotor) and Dr. Ghania Ounoughene (co-promotor). Dr. Marin Rivera obtained his degree in the framework of the European Training Network for Zero-Waste Valorisation of Bauxite Residue (REDMUD).