Numerical investigation on phase change cooling and crystallization of a molten blast furnace slag droplet

To realize the dual goals of high efficient heat recovery and high value-added utilization of blast furnace (BF) slag by the dry heat recovery technology, an enthalpy-based model is established to analyze the phase change cooling and crystallization of a molten BF slag droplet. In the present model, besides the consideration in both the variable physical properties of BF slag and the phase change temperature range of crystal and glassy phase, the effect of crystal phase content on the enthalpy-temperature curve is firstly taken into account due to the coupling relationships between phase change heat transfer and crystallization inside the BF slag droplet. As the results, the evolutions of temperature and crystal phase content in the BF slag droplet are obtained for an air cooling process. The effects of cooling air velocity and temperature as well as droplet diameter and initial temperature are discussed on the phase change heat transfer and crystallization process. The numerical results indicate that a slow cooling rate leads to the precipitation of crystal phase and thus more latent heat release, which gives rise to an obvious decrease in the cooling rate. Moreover, to achieve the dual goals for the droplet with a diameter of 5 mm, 773-973 K is an appropriate temperature of cooling air, correspondingly, 0.87-1.96 m·s−1 is the optimal air velocity. The optimal air velocity reduces about 72% when the droplet diameter decreases to 4 mm.