Selection criteria of diluents of tri-n-butyl phosphate for recovering neodymium(III) from nitrate solutions

Diluent plays an important role in the solvent extraction of metals. The selection of a proper diluent is important since it affects the economics of the process. The effect of different diluents (aliphatic, mixed aliphatic-aromatic and aromatic) on the solvent extraction of Nd(III) by the neutral extractant tri-n-butylphosphate (TBP) from nitrate aqueous feed solutions was studied with variation of the following process parameters: extraction kinetics, phase disengagement time, TBP concentration, nitrate concentration, loading capacity of TBP and aqueous-to-organic phase volume ratio. The present study shows that the nature of the diluent has no effect on the extraction kinetics of Nd(III) by TBP. Phase disengagement times were relatively faster for aromatic diluents compared to aliphatic diluents. Conversely, extraction efficiencies were the highest for aliphatic diluents, slightly lower for mixed aliphatic-aromatic diluents and much lower for aromatic diluents. The poorer extraction efficiencies of aromatic diluents may be due to the lower concentration of free extractant as a result of the stronger interactions of the diluent with water and/or of the diluent with the extractant. The differences in extraction performance between aliphatic and aromatic diluents decrease with increasing nitrate concentration in the aqueous feed solution. Thus, the negative effect on the extraction of the aromatics in the diluent can be compensated by the positive effect of a higher concentration of salting-out nitrate ions in the feed. The present results reveal that the selection of the diluent can be preferably based on its cost, safety and biodegradability rather than on its physico-chemical properties, since the physico-chemical properties have a limited influence on the extraction of Nd(III) by TBP at highly concentrated nitrate solutions.