Solvent extraction of chromium (III) from spent tanning liquors with Aliquat 336

Studies are reported on the recovery of chromium(III) from industrial spent tanning liquors by liquid–liquid extraction with trioctyl methylammonium chloride (Aliquat 336), sulphate, and hydroxide. The optimal conditions of extraction of chromium(III) with Aliquat 336 from these liquors were selected taking account of their composition and using empirical dependencies established in previous studies on model solutions of chromium(III). Since the ionic strength of industrial spent chromium baths was very high, suitable dilution was needed to obtain efficient extraction of chromium(III). Effects of volume phase ratio, temperature as well as of anions of quaternary ammonium compounds on extraction of chromium(III) were examined. Irrespective of the composition of the tested liquors the yield of chromium(III) extraction under optimal conditions was equal to 98.6–99.9% after 15 min of phase contact and did not depend on volume ratio of the aqueous to the organic phases within the range from 1 to 5. Small increase in the temperature positively affects the extraction of Cr(III) and, therefore, compensates in part for the negative effect of ionic strength of the feed solution. Moreover, application of trioctyl methylammonium sulphate and/or hydroxide instead of Aliquat 336 allows partial reduction of the negative effect of chlorides on Cr(III) extraction. Chromium(III) is efficiently stripped from loaded organic phases with sulphuric acid. Resultant regenerated organic phases can be reused in succeeding extraction of chromium(III) with the yield ranging from 94.8% to 99.7% depending on composition of the initial feed solution.