(245g) Aggregates Size/Stracture and Rheological Properties of Nano-Sized Goethite Aqueous Suspension

Properties of aqueous goethite suspensions and the kinetics of de-aggregation of dry goethite aggregates strongly depend on hydrodynamic conditions and chemical compositions of suspension. The current understanding of the behaviour of the suspensions of nano-particles and the kinetic of de-aggregation is rather limited. In this paper the effect of power input, pH and concentrations of goethite powder on aggregate size, kinetics of de-aggregation and rheological properties of suspensions has been investigated. The experiments were carried in an ultrasonic device of maximum frequency of 24 kHz. Particle size distributions, rheological properties of suspension as well as morphology of aggregates and suspension at different energy density were measured using Malvern 2000, controlled stress rheometer (AR 1000, TA Instruments, UK) and optical microscopy, Scanning Electron Microscopy and Environmental Scanning Electron Microscope. At pH=3 dry powder aggregates (up to 300 ?Ým) were broken into approx 100nm nano-particles by erosion and rupture at energy density of approximately 3.0 MJ/kg. The kinetic of breakage was dependent of concentration of nano-particles with faster breakage at lower concentrations and during breakage secondary aggregation was not observed. The aggregate size distribution was bimodal with the first mode of 100 nm and second mode ranging from 1 to 30 ?Ým. Large aggregate size can be related to energy dispassion rate by a power law model with an exponent of 0.55 which is consistent with literature data. Up to 20% mass fraction of goethite, suspension of nano-particles was Newtonian with viscosity increasing slightly with particles concentration from 1.2 to 3.7 mPas. At pH >6 simultaneous breakage of large aggregates and aggregation of small aggregates was observed after certain processing time. At this pH even at energy density of 4.0 MJ/kg it was impossible to break primary aggregates (approx 300-400 nm) into single nano-particles. The suspensions of those aggregate was strongly shear thinning with the yield stress of between 2.5 and 6 Pa. The highest yield stress was observed at pH=9 corresponding to iso-electric point. Analysis of the images of aggregates at different processing time indicate that the dry powder aggregates were broken initially broken (as at pH =3) but subsequently nano-particles aggregated into 2-20?Ým aggregates. The results indicating that particle size, morphology and rheology of goethite aqueous suspensions are controlled by complex particle-particle and particle-solution interactions are discussed in terms of presence of active surface groups as well as van der Waals, electrostatic and hydration forces. It appears that the magnitude of each of those forces strongly depend on pH.

Acknowledgment: This work is a part of PROFORM ("Transforming Nano-particles into Sustainable Consumer Products Through Advanced Product and Process Formulation" EC Reference NMP4-CT-2004-505645) project which is partially funded by the 6th Framework Programme of EC.