| dc.description.abstract | The experimental methods were employed to investigate the granulation behavior of powder materials and binder liquid under the impeller and chopper mixing force. The main research topics include the granulation mechanisms, the velocity field of granules, the morphology of pellet, and the growth behaviors.
The agglomeration equipment— a high-shear mixer was designed and prepared at first. The raw materials included calcium carbonate powders with an average particle size of 32-75 μm and been operated on low impeller speed (300 rpm). Three major agglomeration mechanisms are discussed in this study: nucleation, consolidation and coalescence. The influence of binder viscosity and content are further presented.
The second research involved the effects of different initial volume fill ratios of the granulator on granular agglomeration. Calcium carbonate powders, with mean granule sizes of 75-150 μm were used as the raw material. Experiments of fill ratios are conducted for three conditions of parameters, (i) PEG 6000 and impeller speed at 500 rpm, (ii) PEG 6000 and impeller speed at 700 rpm, and (iii) PEG 4000 and impeller speed at 500 rpm. The range in granule size at the end of agglomeration increased as initial fill ratio increased. Low Vacuum Scanning Electron Microscope (LV-SEM) images of the surface structure of the granules are shown and the pellet shapes are also presented by the aspect ratio during the nucleation and final stages. On the other hand, the particle image velocimetry (PIV) experiments were also conducted by recording the moving granules during the mixing period with a high-speed Complementary Metal-Oxide-Semiconductor (CMOS) camera.
The third research was to investigate the effects of the different surface properties of powders on granular agglomeration. Three different powders, with mean granule sizes of 75-150 μm were used as the raw material: calcium carbonate, calcium sulfate, and sodium carbonate. The wetting properties of the raw materials were measured with a contact angle instrument. The results indicate that the speed at which the droplets sink into the powder bed and the contact angle of binder droplets on the powder surface play important roles in determining the progress of the agglomeration process. Several types of agglomeration were found: a slurry state, only nucleation, snowballing, and induction growth behavior. The heterogeneity and homogeneity of the dispersion brought about by the coalescence, layering and slow-wetting behaviors were analyzed. A heterogeneous dispersion leads to induction behavior and subsequent growth, but a homogeneous dispersion leads to little or no nucleation and non-enlargement of size.
| en_US |