Selective removal of zinc from basic oxygen furnace sludges

The present research investigated the removal of zinc from BOF (basic oxygen furnace) sludges in an attempt to reuse the iron content of the sludge in the production of sinter. The study consisted of two main steps: (i) the characterization of fine and coarse fractions of BOF sludges and (ii) the hybrid treatment of sludge to selectively remove zinc from iron. The characterization study showed that the Fe/Zn ratio is 132.6 in the coarse fraction and 11.6 in the fine fraction; the amount of zinc proved to be 8.5 times higher in the later fraction, while the total content of iron ranged between 50 and 60% in both fractions. Zinc could also be identified in the sludge as zincite (ZnO) and franklinite (ZnO.Fe2O3), whose last phase is quite stable and insoluble in NaOH solutions. Other characteristics, including the granulometry, morphology, and toxicity of both fractions of the sludge, were also determined. To improve the zinc removal efficiency, franklinite must be decomposed by a thermal treatment. For this reason, a hybrid route was investigated using the fine fraction of the sludge. The effect of various materials and reagents was evaluated concerning the decomposition of franklinite, in which NaOH was found to be the most effective. Operating variables, such as temperature, time, and NaOH/sludge ratio were studied. The treatment was found to be quite efficient in the selective removal of zinc from the sludge (zinc removal higher than 90% was obtained with practically no iron extraction), thus improving the Fe/Zn ratio in the fine fraction to approximately 200.