Mixing in granular systems and applied coating technology

Abstract

The purpose of this thesis was to augment the research conducted by Dr. William Rowe, providing further understanding of the process parameters involved in bulk coatings utilizing sodium silicate slurries, and a basic understanding of the behavior of granular materials in rotary drum mixers. Rotary mixers are prime means for creating such coatings. Continuous mixers are preferred over batch models, in order to obtain high production volumes. The mixing time is difficult to predict in a continuous process, whereas in a batch system it is easily controlled. The particle flow behavior must be characterized, as the amount of time a given particle spends in the coating zone will affect its coating thickness and quality. Factors influencing residence time in the coating zone include granule particle size, amount of slurry added and its viscosity, and mixer flight arrangement. The effect of varying process conditions on residence time distribution will be examined, as well as coating transfer. The results of the experiments can be used to develop a more thorough understanding of granule behavior in continuous mixers, and will be of significance for agricultural seed coating, minerals processing, and pharmaceutical manufacture. X-ray Fluorescence Spectroscopy and copper tracer granules were used to study the residence time distribution of granules in a rotary drum mixer. The mean residence time is inversely related to granule particle size. Mean residence time (MRT) decreases with increasing particle size. Thus, the time spent in the coating zone will be greater for granules of smaller particle size. Separate tracer measurements were used to study the effect of slurry yield stress on granule flow behavior. Two regimes were found. The MRT increases with increasing viscosity, exhibits a maximum, and then begins to decrease. Visual analysis and copper tracer granules were used to study the effect of applied moisture on flow behavior. Increasing moisture had little effect until a threshold value, at which point, mean residence time increased significantly. The threshold is predicted to coincide with the emergence of capillary forces between particles. Inductively Coupled Plasma Atomic Emission Spectroscopy and copper tracer granules were used to examine the bulk flow behavior of granules in an industrial scale coating process. The bulk flow behavior was found to be much more plug-like than that of the laboratory mixer. The residence time distribution was narrower and the mean residence time was significantly shorter than that of the laboratory mixer. The coating process under investigation was found to have inadequate dispersion of the pigment particles, and to produce non-uniform coatings. An optical and a scanning electron microscope and an electrokinetic sonic amplitude measurement device were used to study to slurry formulation and coating quality. The ESA experiments demonstrated that the surfactants currently in use have little to no effect on the dispersion of titanium dioxide or kaolin clay. A study of the temperature distribution throughout the length of a laboratory mixer was conducted in order to test the accuracy of the heat and moisture model developed by Dr. Rowe. The predicted mixer temperature profile was found to not quite agree with that of the actual laboratory mixer. A coating transfer study was used to examine the transfer of coating slurry between granules throughout the length of the mixer. The results corroborate Dr. Rowe's conclusion that the majority of slurry coating occurs very soon after slurry application.