Aluminum is produced from its primary bauxite ore through the Bayer process. Although Al is important nowadays in the development of humanity, its production leads to the generation of a huge amount of waste, called red mud. Globally, the estimation of the stock of red mud is about 4 billion tons, with about 10 million tons located in Turkey. The presence of rare-earth elements (REEs) in crucial materials such as red mud makes it a major source of these elements. A number of methods have been developed for treating red mud, which are employed globally to recover valuable products. The application of a suitable method for REE extraction from red mud is a way to overcome the supply risk, contributing to reducing the environmental issues linked to red mud pollution. The current review summarizes the research on red mud processing and examines the viability of recovering REEs from red mud sustainably, utilizing hydrometallurgy and biohydrometallurgy.

Diglycolamides have been proposed for partitioning of trivalent actinides from high-level liquid waste (HLLW). Third-phase formation is an undesirable event during the course of solvent extraction of trivalent actinides from HLLW into the solution of N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane (n-DD). Polar reagents such as tri-n-butyl phosphate (TBP), and N,N-dihexyloctanamide (DHOA) have been added to the TEHDGA phase, as phase modifiers in significant concentration, to overcome the third-phase formation. To understand the role of these phase modifiers in controlling the third-phase formation, the extraction behaviour of nitric acid and the trivalent representative metal ion Nd(III) was studied in a binary solution containing TEHDGA and phase modifier in n-dodecane. The organic phase obtained after extraction was subjected to dynamic light-scattering studies to examine the aggregation behaviour of the reverse micelles formed upon extraction and to unravel the unique role of TBP and DHOA in controlling the third-phase formation. The study revealed that the addition of these modifiers brought down the average size of aggregates and their distribution in organic phase below the limiting aggregate size for third-phase formation and increased the dispersion of aggregates in the n-dodecane phase. Among the two phase modifiers proposed for trivalent actinide separation from HLLW, TBP has been identified as a promising reagent for minimizing the third-phase formation.

Phase modifiers have been added to the solvent phase to prevent the undesirable third phase formation during the course of solvent extraction. Bis(2-ethylhexyl)phosphoric acid (HDEHP) has been explored as a phase modifier during the extraction of Nd(III) from nitric acid medium in a binary solution of tetra-bis(2-ethylhexyl)diglycolamide (TEHDGA) and HDEHP in n-dodecane (n-DD). To unravel the peculiar role of HDEHP in this binary solution, the reverse micellar aggregates formed in the organic phase upon nitric acid and Nd(III) extraction were probed by Dynamic Light Scattering (DLS) and Fourier Transform Infrared (FTIR) spectroscopy. The aggregate size and its distribution in the organic phase increased with an increase in the concentration of nitric acid and Nd(III) in the aqueous phase. The limiting aggregate size for the formation of a third phase in 0.2 M TEHDGA/n-DD was determined to be ∼30 nm in 1 M nitric acid. The effect of HDEHP concentration on the aggregate size was studied and the concentration of HDEHP needed for preventing third phase formation during the course of Nd(III) extraction from 4 M nitric acid medium was optimized. The results revealed that employment of about 0.25 M HDEHP in 0.2 M TEHDGA/n-DD not only prevented third phase formation during the extraction of trivalent metal ions from nuclear wastes but also facilitated the extraction of trivalents into the binary solution, due to synergism.