| OCR Text |
Show temperature exceeds the melting point of pyrrhotite (1356 K) as ~ell as the eutectic temperature of the Fe-S-O system (1213 K), due to the exothermic nature of the oxidation step. As discussed before, if particle impaction occurs during this stage, it ~ill adhere and contribute to deposit buildup. As the oxygen content of the melt increases, and crosses beyond the eutectic composition into the two phase region, a driving force for crystallization of magnetite is established. For example, at 1500 K, as the sulfur mass fraction ( Sulfur/(Sulfur+lron) ) in the melt goes belo~ 0.12, equilibrium calculations predict that magnetite should crystallize out of the melt; at 1400 K it can occur at a sulfur mass fraction of 0.21. Figure 5d demonstrates the initiation of crystal formation at 17 weight percent sulfur, as evidenced by the triangular-shaped structures at certain locations on the particle. As oxidation proceeds, continued crystallization results in the formation of a honeycomb-like structure on the spherical (or cenospheric) particle (Figure Se). Coalescence of individual crystallites results in an assembly of hollow inverted pyramid-like substructures on the particle surface (Figures 5f and Sg). This step is completed in conjunction ~ith the complete loss of sulfur from the particles. Although these samples have not been analyzed for the oxide phase, previov~ Hossbauer analysis7 of particles obtained under similar conditions indicates they are predominantly magnetite. The next two micrographs (Figures 5h and Si) sho~ particles ~hich have experienced longer residence times in the hot zone of the furnace, possibly representing hematite formed from the oxidation of magnetite. Stickiness Behavior Capture efficiencies of partially oxidized pyrite particles were determined in the manner described in the experimental section. The capture efficiency is the fraction of particles that both impact and stick ~ith impaction efficiency determined from Stokes number considerations. A higher value of the Stokes number ~ill result in higher impaction efficiencies. 12 Figure 6 sho~s data for capture efficiencies for values of the Stokes parameter 9 |
