Morphological analysis of aggregates during silica precipitation

International audienceSilica is an important chemical for industrial use. The control of its morphology can be performed by selecting the way of synthesis and the operating conditions. If silica is produced by precipitation from sodium silicate and sulfuric acid, the silica powder consists of several size scales: nanoparticles (2-20nm), small aggregates (100-300nm), large agglomerates (10-100µm). Phenomena are nucleation, growth of nanoparticles, formation of aggregates and gelation. This study is focused on the period before gelation. Silica is produced by mixing equimolar sodium silicate and sulfuric acid solutions in a 3L-vessel. The addition of reactants is monitored by controlling the pH and the temperature. In this study the pH and the temperature values are respectively set to 9 and 85°C. A turbidimetric sensor is used to measure the turbidity as a function of light wavelength and time. Sampling is performed during the precipitation process and analyzed by means of dynamic light scattering (DLS) and small angle X-Ray scattering (SAXS). The kinetics of the precipitation is studied by comparing the data coming from these various devices for different times: total mass of silica, mean scattering cross section versus wavelength, hydrodynamic radius distribution and scattered intensity versus scattering angle. The concomitant analysis of these data leads to any information about the morphology of the aggregates. A modelling of the precipitation is presented and compared with the experimental results