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.