Spheronisation of a basket screen-extruded paste using screens of different hole diameters

Abstract

In order to gain a better understanding of wiping mode screen extrusion-spheronisation process, a 45wt% MCC/water paste was extruded using a basket screen extruder and the extrudate size and shape evolution during spheronisation tests were investigated in this study. Following the regular formulation tests, 30g of extrudates with random lengths were employed as a starting charge for each spheronisation test. Two screens with 1 and 2mm diameter holes were used to generate extrudates of the two diameters.It was found that the mathematical relationship between time to complete spheronisation (tend) and spheroniser plate rotational speeds (ω) varied with extrudate diameters. For the 1mm diameter extrudates, tend scaled with ω−3.8, which was close to that predicted by a simple collision model, whereas the product of tend and ω was relatively constant for the 2mm extrudates. Projected images and SEM images of the dry pellets showed that the shape of 1mm diameter material during spheronisation went through a dumb-bell stage, and the dumb-bells folded over together to form pellets. In comparison, there were few dumb-bells observed during spheronisation of 2mm extrudates, and agglomeration was the dominant scenario by which fines attached to big segments to form ellipses and to be rounded into spheres. In the case of the 2mm extrudates, the aspect ratio was changed mainly with the total number of rotations, and the trend showed little variation between different spheronisation speeds. Based on the observation, it was able to propose a phenomenological model to illustrate pellet shape evolution as well as a semi-empirical mathematical relationship to demonstrate the correlation between processing parameters at an optimal condition for each case. The former emphasised the differences in the shape change route of extrudate with different diameters. The later provided a quantitative guide for optimisation of a spheronisation process. The results suggested that the strain experienced by the paste during extrusion has played an important role in extrudate breakage at the early stage of spheronisation. The size of extrudate segments, after breakage, then determined their spheronisation behaviour during following processing.