Abstract:Aiming at the separation problem of iron and chromium in sulfuric acid leaching solution of electroplating sludge, this paper presented a method of “preferential complexing-hydrolysis precipitation” to separate ferro-chrome and reduce the loss of chromium while removing iron. The complexing mechanism of sodium formate with ferric sulfate, chromium sulfate, mixed solution of ferric sulfate and chromium sulfate was studied comprehensively by UV spectrophotometry and mass spectrometry. The results show that both chromium and ferric ions can form complexes with sodium formate. But under the same conditions, the complexing of Fe³⁺ and sodium formate in aqueous solution is prior to that of Cr³⁺. The calculation of reaction heat effect shows that these reactions are endothermic and the high temperature is favorable for these reactions especially ferric than chromium. The effects of amount of sodium formate, reaction temperature and holding time on the separation efficiency between iron and chromium were studied. The results show that when the molar ratio of HCOO^-/Cr³⁺ is 5.5, reaction temperature is 90 ℃, holding time is 6 h, the pH value of the solution is adjusted to 2.5 with addition of alkali and the reaction time is 30 min, the iron precipitation rate is 93.65%, the chromium precipitation rate is only 13.37%. The separation effect of Cr³⁺ and Fe³⁺ is good.

Key words: separation of iron and chromium; electroplating sludge; complexing; hydrolysis; precipitation