The complex of (TOAH)2Fe(OH)(SO4)2 reacts with sulfuric acid
with stoichiometric ratio of 1:1.5 according to the reaction displayed
in Fig. 9. Fe(II) needs to be stripped completely and the
hydrolysis of Fe(III) should be prevented at an appropriate ratio.
Excessive amount of sulfuric acid is needed in the stripping liquor.
Therefore, the phase ratio of volumes of organic and aqueous solutions
is a key influence factor that needs to be evaluated firstly.
The stripping experiments were performed with a variation of
ratios at a temperature of 25 ◦C and the mixtures of the two phases
were agitated for 20 min. To strip Fe(III) completely and simultaneously
avoid too much sulfuric acid leaving in the strip liquor,
a comparison of stripping yield was made between the stripping
with a counter current of two stages and a single stage. The effect
of phase ratio for the counter current stripping on the pH of the
stripping liquor is plotted in Fig. 10. It indicates that the pH of the
stripping liquor with the run number ofthe stripping can be divided
into two groups. One group has a pH platform for ratios from 5:1 to
6:1 after the second run of the stripping, and the pH of the platform
increases with the increase of the ratio. The other group does not
have a pH platform for ratios from 6.5:1 to 7:1, and the pH increases
continually with the run number. When pH is greater than 1.4, the
hydrolysis of Fe(III) in the WSA occurs and the extraction of the
hydrolysis species takes place as Eq. (14). Apparently, the favorable
ratio is 6:1 since there is enough sulfuric acid to depress the
hydrolysis of Fe(III) and not too much amount of sulfuric acid to be