Recovery of Rare Earths from Bauxite Residue (Red Mud)

Promovendus/a: Chenna Rao Borra

Promotoren: Prof. dr. ir. Thomas Van Gerven, Prof. dr. Koen Binnemans, Prof. dr. Yiannis Pontikes

Long term storage of bauxite residue is harmful to the environment due to the residual sodium hydroxide content. Some of the bauxite residues contain valuable minor metals like rare-earth elements (REEs). Therefore, recovery of REEs with or without other metals from bauxite residue, and utilization of the left-over residue in other applications like building materials can solve the handling and storage problem of bauxite residue.

First, selective recovery of REEs over major elements such as iron was studied by direct acid leaching. During leaching, either the recovery of REEs was low or the dissolution of iron was high. The high amount of iron going into solution during leaching poses problems during the extraction of REEs from leach solution. Therefore, iron was removed from bauxite residue in the form of pig iron prior to leaching by high temperature smelting process in presence of carbon and flux. The slag generated during smelting was leached with acids. The selectivity of REEs over iron was clearly improved. However, smelting of bauxite residue requires high amount of energy due to the presence of high amount of alumina in bauxite residue, which require high amount of flux. Therefore, the removal (and recovery) of alumina from bauxite residue by sodium carbonate roasting prior to smelting was studied. The sample after alumina removal was smelted without any added flux and it was possible to obtain a clear slag-metal separation. REEs were successfully recovered from slag by acid leaching from alumina-poor slags.

An alternative process called sulfation-roasting-leaching, was also developed to selectively leach the REEs. In this process bauxite residue was mixed with water and concentrated H2SO4 followed by drying, roasting and finally leaching of the roasted product in water. REEs were selectively leached by this process however, the scandium recovery is low. The residue generated in this process, could be used in cementitious binders.

Preliminary energy and economic analyses show that alkali roasting-­smelting-­leaching and sulfation-­roasting-­leaching are promising processes for treatment of bauxite residue.