Particle de-agglomeration with an in-line rotor-stator mixer at different solids loadings and viscosities

In-line rotor-stators are used in a range of energy intensive processes but there is relatively little published work with these devices on which to base process design. This study was performed to investigate the performance of an in-line rotor-stator for the de-agglomeration of nanoparticle clusters in a liquid with the objective of determining the effects of solids loading (up to 15%wt) and continuous phase viscosity (up to 100 mPa·s) on the mechanisms and kinetics of breakup and dispersion fineness. A Silverson 150/250MS rotor-stator equipped with the EMSC (Emulsor) screen was used in the recirculation loop of a stirred tank charged with 100 litres of pre-dispersion. It was shown that the power number values previously obtained at Reynolds numbers greater than 200,000 are constant at Reynolds numbers as low as 2,400. It was found that the breakup kinetics were not significantly affected by the solids loading, within the range covered in this study. Whilst 10 and 15%wt. pre-dispersions in water were non-Newtonian, during the course of deagglomeration, the dispersion rheology changes resulting in a Newtonian final dispersion of a low viscosity- only slightly higher than that of water. On the other hand, when the viscosity of the continuous phase was increased, the de-agglomeration became slower even though the solids concentration was low (1%wt.) and the flow through the rotor-stator was still turbulent. This indicates that it is the flow conditions around the particle and not the bulk rheology of the dispersion that determines the kinetics of the de-agglomeration process. Breakup mechanism was found to be erosion and the dispersion fineness was determined by the size of aggregates.