Abstract: Mechanism extraction of Sc(Ⅲ)from hydrochloric acid solution by di(1-methylheptyl)methyl phosphonate(abbreviated as P350)in hoptane has been studied. The composition ofthe complex was determined to be ScCl-3.3P350 by the metbod of slope. The extraction rea-ction can be expressed as:Sc³⁺+3Cl^- + 3P350_(o)⇔ScCl₃.3P35_(o)The HCl and H₂O were extracted as 3HCl.2H₂O.3P350 likely through the reaction: 3H⁺3Cl^-2H₂O+3P350_(o)⇔3HCl.2H-2O.3P350_(o) The effects of concentration of HCl, extractant and temperatures on the exraction equi-librium were discussed in detail. The equilibrium constant(K_ex)and the thermodynamicfunctions(△H^o, △S^o, and △G^o)of the extraction reaction were calculated.

Key words: di(methylheptyl)methyl phosphonate, extraction, scandium, mechanism