The production process of precipitated silica used as reinforcing fillersincludes a filtration step carried out to collect silica agglomerates in the synthesized silica dispersion and then to eliminate salts and impurities beforedrying. The purpose of this study is to correlate the multilevel structure ofsilica agglomerates to the mechanical and hydrodynamic properties of filter cakes with or without a washing step. For that, we have chosen different reinforcing silicas which differ from the way the synthesis is conducted and performed the filtration at several operating pressures. The structural, mechanical and hydrodynamic properties of filter cakes were investigated by small angle scattering, osmotic compression and filtration and washing experiments. It appears that for each system two characteristic quantities : the maximum diameter of aggregates and the percolation threshold corresponding to the formation of a network that begins to resist to compression, are correlated to the hydraulic permeability of filter cakes. Moreover, we show that the calculated permeability using a Brinkman model is in quite reasonable agreement with experimental data. Finally, we demonstrate that the elimination of salts and impurities in filter cakes during the washing step affects neither the local organization of filter cakes nor the permeability.
