Mineralogical Transformations in Altasteel Electric Arc Furnace Dust Roasted with Na₂CO₃ and Secondary Ferrite-Forming Additives

Abstract

The effect of secondary additives, such as CaCO 3 and MnCO 3 , on the mineralogical transformations in Altasteel EAF dust (9.4% Zn, 31.9% Fe) during roasting with Na 2 CO 3 , and metal extractions during hot water and H 2 SO 4 leaching of the roasted residues, was studied using a combination of techniques, including Design of Experiments testing, x-ray diffraction, scanning electron microscope/energy dispersive xray analysis and chemical analysis. The research objective was to promote the formation of acid insoluble metal ferrites during roasting to try to lower iron extractions during acid leaching. Neither additive was effective in reducing iron extractions due to the unexpected mineralogical changes caused during roasting by the addition of these secondary additives to this poorly crystalline EAF dust. Low CaCO 3 or MnCO 3 additions promoted the formation of manganese rich iron oxides (e.g., Mn 2 FeO 4 ), instead of MnFe 2 O 4 , which caused more iron to be available to react with Na 2 CO 3 during roasting to form acid soluble NaFeO 2 . Increased CaCO 3 additions further increased iron extractions, by further increasing the formation of Mn 2 FeO 4 ferrites, but increased MnCO 3 additions at roasting temperatures above 950°C led to the formation of a ZnFe 2 O 4 -MnFe 2 O 4 solid solution. While this resulted in lower iron extractions, lower overall zinc recoveries during acid leaching were also observed.