Foam Separation of Cyanide Complexed by Iron

An experimental investigation is presented of the foam separation of cyanide, complexed with ferrous iron, using a cationic surfactant, ethylhexa-decyldimethylammonium bromide (EHDA-Br). The effects of pH over the range 5.8–11.4, of iron concentration, and of surfactant and cyanide concentrations on the separation of noncomplexed and complexed cyanide are established. Prior to surfactant addition and foaming, variation of the iron-to-cyanide ratio in the initial solutions produced different species of complexed cyanide, ranging from [Fe(CN)₆]^4-to polynucleated [FeFe(CN)₆]^2-, which, in turn, brought about markedly different solution characteristics. Initial solutions containing 0.206 and 0.351 moles of iron per mole of cyanide exhibited contrasting foam separation behavior, at pH 7 and below, but not at pH 8 and above. The fractional residuals of complexed cyanide were approximate linear functions of the ratios of surfactant to complexed cyanide in the initial (feed) solutions, enabling the prediction of required surfactant dosages and establishing the approximate formulas EHDA_3.9Fe(CN)₆ and EHDA_2.1 FeFe(CN)₆ at the two ratios. Operation at pH 7 or less and an iron-to-cyanide ratio of about 0.35 are recommended due to surfactant saving and reduction in noncomplexed cyanide.