Synthesis of organic phosphinic acids and studies on the relationship between their structure and extraction–separation performance of heavy rare earths from HNO3 solutions

Efficient new extractant design relies on systematic structure–property studies. However, systematic structure–property research on extractants of organic phosphinic acids is far from sufficient. To make up for this deficiency, twelve organic phosphinic acid extractants (including nine dialkylphosphinic acids, two dicycloalkylphosphinic acids and one diarylphosphinic acid) were synthesized with free-radical-addition-reaction method and Grignard-reaction method. Reaction rules, advantages and disadvantages of these two methods were discussed respectively. The extraction ability and separation performance of the synthesized organic phosphinic acids for Tm, Yb and Lu from HNO3 solutions were evaluated and compared. For dialkylphosphinic acids the nearer the branched chain moved to P atom, the more reduction generated on the extraction ability. Dialkylphosphinic acids without branched chain in α-C or β-C (e.g. HYY-3, HYY-5, HYY-7 and HYY-9) exhibited strong extraction ability as P507. On the contrary, dialkylphosphinic acids with branched chain in α-C (e.g. HYY-4, HYY-8) and dicyclohexylphosphinic acids exhibited poor extraction ability as Cyanex 272. New compound Di-(2,3-dimethylbutyl)-phosphinic acid (HYY-2) exhibited good extraction ability and separation performance and its separation factors of Lu/Yb, Yb/Tm and Lu/Tm were 1.42, 3.10 and 4.40 respectively, apparently superior to Cyanex 272 (βLu/Yb = 1.31, βYb/Tm = 3.25, βLu/Tm = 4.26).