The extraction equilibrium of aluminum from hydrochloric, nitric and sulfuric acid solutions was studied at 298K by using 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester in DOSB diluent. The ionic strength of all the aqueous solutions was fixed to 3 so that the activity coefficient of species in aqueous phase might be constant during extraction. The extraction was highly dependent on pH and was expected to be an ion-exchange reaction. Little difference was observed in the extraction

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... ria between hydrochloric and nitric acid solutions. In sulfuric acid solution, the distribution ratio of aluminum between the two phases was about one-two hundredth of those obtained in the other acid solutions. The formation mechanism of aluminum complex ion in the aqueous solution was discussed in detail, and it was concluded that the librium constants were 10-1.16, 10-1.32 and 10-1.16 for hydrochloric, nitric and sulfuric acid solutions, respectively.