Fine particles, whose sizes are below 20 μm, account for the major part of EAF dust. A small proportion of those particles corresponds to monocristals of zinc oxide (figure 7). They are easily identifiable thanks to their facetted aspect. Their size rarely exceeds a few hundred nanometres.
Figure 7. Zincite monocristals
The other particles are spherical. Their size varies from 0.2 to 20 μm. We found three types of spheres which differ from each other because of their mineralogy:
homogeneous spheres whose composition corresponds either to the slag or to the steel bath with an enrichment in zinc; they are often hollow when they are larger than 2 or 3 μm (figure 8);
heterogeneous spheres made up of a slag phase and a steel phase enriched in zinc (figure 9); some of them display an iron-rich dendritic structure buried inside a vitreous phase;
The zincite spheres, like the monocristals, form during the condensation of the vapors of zinc contained in the EAF fumes [1, 2]. The other spheres represent the major part of the dust observed. They come from the projection of liquid droplets. On account of their sizes, they are thought to be emitted by the burst of CO-bubbles coming from the decarburization of the steel bath [1, 3, 4, 5].
The finest particles, whose sizes are lower than 2 or 3 μm, are frequently agglomerated to each other or around a bigger particle (figure 10). The sizes of these agglomerates vary from 5 to 20 μm; a few of them reach 50 μm and some show signs of partial sintering also noted by Cruells et al. . In this case, the agglomeration took place inside the furnace or the fume extraction ducts at high temperature.