Colloidal silica (CS) is a promising raw material for refractory castable ceramics. It consists of stable suspensions of synthetic amorphous silica nanoparticles that behave simultaneously as liquid medium and binder for ceramic particles and as a porogenic agent and highly reactive source of silica to promote in-situ reactions. The setting mechanism of CS balances two opposite effects. Adding more CS to a suspension increases the bonding potential for gelling reactions and strengthening; on the other hand, it also introduces more water into the system, enhancing pore content. Such effects can be advantageously employed in the preparation of porous structures from aqueous suspensions and applied as high-temperature thermal insulators. The present study addresses the production of porous structures of in-situ mullite attained from aqueous suspensions of highly porous transition alumina particles bonded with colloidal silica. Different grades of CS and transition aluminas were combined to present suitable workability (flowability and gelling time) and to generate stoichiometric mullite or mullite-alumina porous structures after sintering.
The purpose of this study was to investigate the porosity behavior of dust cakes formed by phosphatic concentrate along filtration time. Filtration trials were performed for four different pressure drops. The fabric filter used as filter medium was a polyester felt. The dust cakes obtained were submitted to a hardening process and their images were acquired with the use of a SEM. The images generated by the SEM were analyzed by using an image analyzing program that supplied the cake porosity values. Porosity values were also estimated using Ergun’s modified equation. Results show that porosity presents higher values when calculated using the image analyzing program, but has the same behavior as the results found with Ergun’s equation. It was also observed that porosity is not constant during filtration; therefore it can be concluded that the dust cake is compressible.
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