Abstract: The extraction equilibrium of Sc(Ⅱ), Y(Ⅲ), Ln(Ⅲ)(Ln=Ho, Er, Yb, Lu) and Fe(Ⅲ) from HCl solutions with H(DEHP) in n-octane has been investigated. The order of extractability of the above elements is found as follows:ScLu>Yb>Er>Y>Ho. Y lies between Ho and Er. It is possible to separate Se(Ⅱ) from Fe(Ⅲ) and Ln(Ⅲ), and heavy lanthanide(Ⅲ) each other by H(DEHP).By the methods of slope analysis, saturation capacity and IR, NMR·spectra, the composition of extracted complexes of Sc(Ⅲ) and Yb(Ⅲ) with H(DEHP) has been determined to be Sc(HL₂)₃ and YbL₃(HL₂). The extraction mechanism of rare earths(Ⅲ) has been obtained.The equilibrium constants and thermodynamic functions of extraction reactions have also been calculated.

Key words: diethylhexyl phosphinic acid, extraction mechanism, scandium(Ⅲ), yttrium(Ⅲ), lanthanide(Ⅲ), iron(Ⅲ)