Migration and distributions of zinc, lead and arsenic within sinter bed during updraft pre-reductive sintering of iron-bearing wastes

Abstract

An updraft pre-reductive sintering process was proposed to convert the iron and steelmaking wastes with high Zn and Pb content into a pre-reduced sinter with low Zn and Pb suitable for blast furnace smelting process. In this work, the migration and distribution characteristics of Zn, Pb and As along the vertical direction of sinter bed were particularly studied by analyzing the compositions of quenched samples obtained from different bed depth. The microstructure of the quenched samples and the micro-area composition analysis were performed by using scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). The result showed that the sintering bed can be roughly divided into five different zones along the bed, i.e., sinter product, combustion and pre-reducing, drying and preheating, over wet and original material zones from the bottom to the top. The content of Zn, Pb and As varies with different zones. Generally, the contents of above-mentioned elements reach their minimums in the product sinter zone and while that reach the maximums in the drying-preheating zone. In contrast, the combustion zone contains intermediate contents of Zn, Pb and As. The migration of Zn, Pb and As in the sinter bed occurs according to the following rule: The ZnO, PbO and As2O5 were first reduced and volatilized in the combustion and pre-reducing zone, and then re-oxidized and condensed on the surface of the dried and preheated balls in the drying and preheating zone. As the combustion zone moves upward, the ZnO, PbO and As2O5 that condensed on surface of dried and preheated balls would be re-reduced, re-volatilized, re-oxidized and re-condensed at upper zones until the sintering process reaches the burnt-through point when the Zn, Pb and As volatiles enter into the flue gas.