(70z) Change in the properties of beds granulated in disk granulators

During tumbling of a powder bed wetted with liquid, material particles are agglomerated. Both, formation of granules and an increase of the bed moisture content cause changes in bulk density and inner friction coefficients as well as the coefficients of material friction against the granulator surface. They in turn, determine energy transfer to the granulated material. In the study, a change of the above mentioned parameters during batch granulation was investigated by comparing results obtained with a change in granulation degree and torque on the granulator shaft. Experiments were made in three disk granulators of diameters 0.5, 0.7 and 1 m, using in all cases rotational speeds equal to 0.2 of critical velocity. Variable parameters were the degree of disk filling and the angle of inclination of the disk axis to the level. The experimental material was bentonite with particles of diameters ranging from 0 to 0.16 mm (dav = 0.056 mm), specific density 2420 kg/m3 and bulk density 790 kg/m3. The material was wetted drop-wise with water during the granulation at constant liquid flow rate Q = 0.7 ml/s. Analysis of results allowed us to formulate a preliminary conclusion that the angle of disk axis inclination in the applied range of its changes had no effect on the process and layer properties. In all cases a linear decrease of bulk density of the bed during granulation was reported. When observing change of the coefficient of friction between the bed and disk surface during granulation, three ranges of change in this parameter were found. At the beginning, there was an increase of the coefficient, in the second, short period, the coefficient decreased slightly only to increase again in the third period. A change in torque on the granulator shaft was also analysed. It was found, that like in the case of friction coefficient, three ranges could be distinguished here as well. In the first, initial range, a distinct increase of unit moment reaching 50% is observed, in the second period there is a drop, and in the third period the coefficient is constant.