| Moist porous pellets, when dried under excessively severe conditions, disintegrate into onion-shell like fragments. Known as spalling, this phenomenon is important in iron-ore pelletizing, fluid bed roasting of pelletized feed, and in ceramic processing. Critical to spalling is the wet-core/dry-shell receding interface. In this seminar our recent work to quantitatively explain this phenomenon will be reviewed.;The cause of the characteristic failure is shown to be a combination of thermal stresses, set up in the outer shell and at the shell/core interface, and pore pressure stresses resulting from the build up of steam pressure at the wet/dry interface. Magnitudes of these stresses are calculated using concepts and equations from continuum mechanics, our measured Young's modulus data and model-predicted pressure and temperature gradients.;To calculate the latter a comprehensive mathematical model of the drying process has been developed. All of the following: boundary layer heat transfer, capillary flow of water within the pellet, heat conduction within the duplex matrix, and diffusion and Darcy flow of water vapour escaping through the dry shell are accounted for. Relevant model predictions are temperature gradients, pressure buildup, degree of water saturation of the pores, transition from surface to shrinking-core drying and the tensile stresses, both radical and circumferential. These will be confronted with practical experience or experimental results to explain the spalling phenomenon and to elucidate features of intense drying. |
