(70u) The effect of wetting conditions on wet drum granulation kinetics

Granulation of fine-grained materials is a complex processing operation that depends on many parameters referring to media properties, applied equipment and process parameters. To produce granulated material, it is required in most cases to add to powder a certain amount of binding liquid. The type of liquid, its quantity, and also the method of supply determine intraparticle bonds, which in turn have a deciding effect on the nucleation and growth of granules, and in this way on granulation kinetics. The effect of wetting conditions (the amount of supplied binding liquid and degree of liquid jet break-up) on the kinetics of fine-grained bed granulation in a horizontal rotary drum is estimated in the study. Granulation was a batch process carried out in a granulator 0.5 m in diameter, at constant rotational speed of 20 rpm and constant filling of the drum volume equal to 10%. A model experimental material was dolomite flour of grains < 350 mm, and the binding liquid was distilled water. The process of granulation consisted of two stages: wetting and granulation. At the stage of wetting, a bed tumbling in the drum was sprayed with different amounts of binding liquid (from the determined range) of specified droplet size, at the constant volumetric flow rate Qw= 0.012 m3/h. The droplet size was changed using different rates of air flow through pneumatic spray nozzles Qa= 1.0 to 3.0 m3/h, and applying a sprayer that would supply the liquid in the form of droplets of regular size equal to ca. 3.5 mm, uniformly along the drum. The droplet size distribution in the stream supplied by spraying nozzles, at a specified break-up degree q=Qw/Qa, was measured by a DANTEC laser analyser. When the wetting stage ended, the proper granulation started which lasted until the moment when water pressed onto the granule surface caused intensive sticking of the product to drum walls, thus disturbing the process. At this stage, samples were taken at specified time intervals. On this basis, instantaneous properties of the granulated feed were determined (particle size distribution, moisture content, particle porosity, liquid saturation degree in the granules). A significant effect of the quantity and size of droplets of the added binding liquid on the rate of changes that occurred in the granulated bed was found. Equations of agglomerate growth kinetics for the tested stages of drum granulation were derived, taking into account variable conditions of binding liquid supply.