Complex Formation and Metal Extraction

Trialkyl and triaryl phosphates, in common with their phosphonate, phosphinate and phosphine oxide analogues, will complex heavy metals in solution and some of these complexes can be isolated as crystalline solids. Complexing affinity increases with increasing basicity of the donor phosphoryl compound (R being constant)  

Esters of pyrophosphoric acid (Table 5.27) are also efficient complexing agents as well as dial-kylamino derivatives such as HMPA and OMPA (Chapter 7) and diphosphonates (Chapter 6.18). Typical complexes formed by tributyl phosphate (TBP) are 

High metal coordination numbers are often found in complexes of this type (12.71). 

(BuO)3PO, and related esters such as dibutyl phosphate, (BuO)2P(0)OH (DBP), and bis-2-ethylhexyl phosphate (HDEP) find important uses in the extraction of rare earth, actinide and other heavy metals from mineral sources, and their recovery from waste products of the atomic energy industry. A solution of TBP in kerosine 

Differences in the extraction coefficients with different cations may be utilised in the separation of uranium, transition metals and rare earths. This is a well-established method for the extraction of U in the processing of nnclear fuels. Hafnium ean also be separated from zirconium by this technique, using TBP. 

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