Reflected light microscopy, electron microscopy, electron spectroscopy, and x-ray diffraction mineralogical characterization of electric arc furnace (EAF) dusts

The secondary steel industry in the United States generates 600,000 tons of electric arc furnace (EAF) dust each year. Over 5,000,000 tons of EAF dust are generated annually world wide. The dust, which is removed from off-gasses from the high temperature furnaces, is considered by the Environmental Protection Agency to be hazardous waste due to its leachable lead, cadmium, and chromium. In addition to hazardous components, EAF dust contains valuable zinc, averaging 19%. This study was initiated to determine the mineralogy and characterization of the dust to aid in the processing of the dust for metals recovery and rendering the remaining material non-hazardous. Reflected light microscopy, scanning electron microscopy-energy dispersive spectroscopy, electron probe microanalysis, transmission electron microscopy-energy dispersive spectroscopy, X-ray diffraction, Mössbauer spectroscopy, auger electron spectroscopy, electron spectroscopy for chemical analysis, and cathodoluminescence microscopy were applied to the study of EAF carbon steel (CSD) and stainless steel (SSD) dusts to determine the mineralogy and character of the dusts and some processed dusts. EAF dust is composed primarily of spinel solid solution crystals ((Zn,Fe,Mn,Mg,Ca)(Fe,Mn,AI,Cr)2O4) and zincite (ZnO). Most of the spinel crystals are crystallized within a Ca-Fe-Si glass matrix. Some of the \u3c 1 µm spinels are euhedral crystals. Zincite is predominantly in the fines. Large zincite forms rounded aggregates and \u3c1 µm zincite forms elongated crystals. The abundance of zincite increases with increasing zinc content. Coke, sylvite, hematite, metallic iron, and periclase are minor phases in all of the CSD studied. Halite, fluorite, and wustite are minor phases in most of the CSD. Calcite is present in some of the dusts. SSD varied from the CSD in a greater abundance of sylvite and the presence of halite and 304 stainless steel as minor phases in all the SSD. Lead is present in clusters of 200 – 500 Å spheres in association with phosphorus. Lead is present on the surface of particles to a few atomic levels deep in association with oxygen, fluorine, sodium, zinc, and/or chlorine in the CSD. Lead is not present on the surface of SSD, which have sylvite and halite surface coatings. Chromium is present as Cr3+ in solid solution in the spinel crystals. Leachable hexavalent chromium was not identified and is suspected to be present in very small quantities, with most of the chromium (0.23% total chromium) as trivalent chromium in the spinel crystals. Cadmium averages 250 ppm in the CSD studied and its mineralogical form was not identified. Chlorine is present as sylvite, halite, hydrophilite, Cl-bearing lime, and Cl-bearing coke. Fluorine is present as fluorite. Chlorine and fluorine are deleterious constituents to the processing of the dust. Furnace processing recovers zinc through volatization. Residues from an experimental flash furnace were studied to determine the mineralogical form of the unrecovered zinc. One to two percent zinc was retained in the slags in solid solution with wustite. Pyrohydrolysis is another experimental technique being tested for EAF dust treatment. Residues from pyrohydrolysis tests show the silica and tungstite additives reacted with the dust forming willemite and scheelite. This process eliminated zincite, sylvite, halite, and fluorite. Some of the scheelite formed rims surrounding the spinel crystals, which would deter tungsten recovery and reusability. Reflected light microscopy, in conjunction with electron probe microanalysis and X-ray diffraction, have been the primary research techniques employed in this comprehensive study of EAF dusts. The mineralogy and character of EAF dusts are more fully identified and understood by the research presented in this dissertation. This is vital information for future recycling of EAF dust for metals recovery and processing to render the dust non-hazardous --Abstract, pages iii-v