Experimental Investigation on Flowability and Compaction Behavior of Spray Granulated Submicron Alumina Granules

Aqueous slurry with various solid loadings (up to 40？wt%) of alumina powder (D50？=？300？nm) with suitable rheological properties were spray dried into granules. Solid loading and feed rate of the slurry are found to have a prominent effect on the shape and size distribution of granules. Powder flow measurements exhibited a cohesive index of 28.45 signifying an extremely cohesive flow due to high surface area and irregular morphology. Finer sizes though it offers high geometrical surface area it leads to more surface contacts and hence, high interparticle friction. Spherical morphology achieved through optimum spray drying parameters significantly reduced the cohesive index to 6.45 indicating free flow behavior. Compaction studies of the spray-dried granules and corresponding plot of relative density versus compaction pressure revealed an agglomerate strength of 500？MPa followed by a plateau-like behavior reaching a maximum in the relative density of 59%-60% of the theoretical values. 1. Introduction Flow properties of the powders are dictated by the collective forces acting on individual particle such as vander waals and electrostatic forces, surface tension, interlocking as well as friction. Compaction process of ceramic powders basically involves die filing and particle rearrangement under applied stress followed by brittle fracture and bonding between the particles [1–6]. Further, the removal of the applied pressure and ejection of the compact completes the compaction process. Flow variables such as energy required to ensure an acceptable flow, sensitivity to flow rate as consequence of the previous collective forces, and distribution of the particles under applied stress play a major role in determining the quality of the formed compact. Flow properties of the fluids and pastes are characterized in terms of their rheological properties but similar treatment cannot be applied to the powders. Several experimental techniques have been developed to determine the flow behaviors of the powders. The flow properties are generally characterized by physical measures such as angle of repose, flow and shear based methods and even correlated with tap density and green strength of the compact [7–9]. Flowability of powder is a complex parameter that cannot be described with a single number and there is no universal model in existence to predict powder flow behaviour in every situation. Particulate flow characteristics are complex and flow properties are a combined effect of physical and environmental variables. Hence, flowability is a combination of physical