The dust collected in bag filters at the end of the EAF fume extraction system is the final product of a series of phenomena, such as the emission of particles from the steel bath, the transport of these particles by the gas flow in the fume extraction system, the in-flight physico-chemical transformations they undergo, etc. The results of the morphological analysis of the EAF dust show that the dust formation process takes place in two steps: first, the emission of dust “precursors”, i.e. vapors, metal droplets, and solid particles, inside the furnace; second, the conversion of those precursors into dust by agglomeration and physico-chemical transformations.
From the different types of particles displayed previously, five emission mechanisms of dust precursors have been identified (see figure 11):
volatilization, especially localized at the hot spots in the arc zone (1) and the oxygen jet zone (1’), but taking place as well in the CO bubbles;
projection of droplets at the impact points of the arc (2) and of the oxygen jet (2’) on the steel bath;
projection of fine droplets by bursting of CO bubbles (3) coming from the decarburization of the steel bath;
direct fly-off of solid particles (5) during the introduction of powder materials into the EAF (scrap, coal for slag foaming, additions, recycled dust, etc.).
Figure 11. Schematic representation of the mechanisms of dust emission in EAF According to the preceding analysis and experimental quantifications of each mechanism made by Birat et al. , the prevailing mechanisms of dust precursor emission appear to be the volatilization (27 % of the dust) and the bursting of CO bubbles (60 % of the dust). The direct fly-off of solid particles remains very limited if sufficient operating cautions are taken. As for the projections at the impact points of the arc or of the oxygen jet, most of them fail to be carried up by the fume extraction system, due to their size, and fall back into the liquid bath.
The precursors are further transformed during their transport within the furnace and then in the fume extraction system. They can undergo physical transformations: condensation of the vapors, rapid solidification of the fine projections in contact with a colder atmosphere, in-flight agglomeration and coalescence of dust particles. The precursors can also be modified by chemical reactions (e.g. oxidation) with the carrier gas, whose temperature and composition vary, and, they can possibly react with other precursor particles. For a reaction between condensed phases (liquid or solid) to occur, particles must first be brought into contact. Therefore, there is a strong link between the mechanisms of agglomeration and the chemical evolution .