Ultrafine particles, with particle size less than 1 µm, tend to be difficult for users to handle. Therefore, they are often assembled into larger granules for ease of handling and to allow controlled delivery in their desired functions. Engineers who manufacture granules want to be able to control and predict the structures formed and ultimately predict the functional performance of the granular products. The overall aim of this project is to engineer structure for TiO2 (titanium dioxide) type pigments using a controlled spray drying method. It is also important to gain a detailed understanding of the morphology control techniques and the associated properties of spray-dried granules.
The project first considers the colloidal characterisations of two distinctive TiO2 slurry systems in order to develop an overall understanding of the feed materials for the subsequent spray drying experiments. The morphology, size and surface of primary particles from two slurries system were examined using SEM, TEM and EDX analysis. The surface charge measurements were performed to identify the iso-electric point (IEP) at which system start to aggregate for two systems. The slurries were re-formulated with controlled colloidal stability by adjusting pH of system to IEP and to non-IEP conditions. The following particle size, particle settling, and rheology measurements confirmed the difference between slurry at IEP and non-IEP condition in terms of colloidal structure.
Initial drying work was performed using a pilot-plant scale ProCepT spray drying using an ultrasonic at 120ºC. It has been found the variations in colloidal structure of a droplet is responsible for the assemblies of primary particles during the drying process. For the spray drying of alumina-coated TiO2 system at all three concentration levels, spherical morphology was found from drying of slurries at IEP condition, whereas a buckled morphology was seen from drying of slurries at non-IEP condition. The Morphologi G3 image analysis indicated a size range of 10-100 µm for all spray-dried granules, regardless of their external morphology. For the spray drying of non-coated TiO2 system at all three concentration levels, constant spherical morphology was obtained from drying of slurries at both IEP and non-IEP conditions. In fact, a buckled morphology was expected from drying a “well-dispersed” system. The colloidal stability was not proved to affect the morphology in spray drying of the non-coated TiO2 system.
The subsequent drying work was conducted at 200ºC using the same ultrasonic nozzle to investigate the temperature effect and were conducted at 120ºC using a 0.6 mm bi-fluid nozzle to investigate the atomisation effect to the product. The revised drying process conditions were found to have minor effect on the spray-dried morphology for both two TiO2 systems. The obtained morphologies were same as the ones prepared from 120ºC drying using an ultrasonic nozzle.
The 20%wt slurries at both IEP and non-IEP conditions from two TiO2 particles systems were dried at 120ºC using a more controlled single droplet drying rig to verify the morphologies from spray drying experiment. For alumina-coated system, the morphologies obtained from single droplet drying were consistent from spray drying work. However, a buckled morphology was obtained from drying of non-coated system at non-IEP condition which was different from its spherical morphology obtained from spray drying study. The morphology forming mechanism was proposed based on the experimental results and calculated Péclet number, and the key driving force for structure buckling was identified to be capillary force. The droplet size effect on morphology was studied by drying the “well-dispersed” droplets with varied size from two TiO2 systems. Through the estimation of central void of spray-dried granule, the morphology was proved to be dependent on the hollowness of granule and particle-particle interaction in shell of droplet.
It was found the investigated spray drying process conditions cannot affect the porosity, internal structure, surface area, moisture adsorption and desorption, and the density of the spray-dried granule, whereas the alumina-coating layer on particle surface exhibited a significant effect on these properties. As the colloidal stability was proved to be important to morphology control, it allows a more research gaps for slurry formulation development.
