Effects of mill design and process parameters in milling dry extrudates

• The cone mill is more sensitive to changes in impeller speed than the hammer mill.
• The comil provides a narrower particle size distribution (large mean particle size).
• Both mills exhibit equal poly-dispersity (small mean particle size).
• Grater screens provide higher milling capacity than round holed screens.

An experimental study was performed to characterize two continuous mills for their ability to mill alumina–magnesia extrudates. The effect of mill parameters, namely, the screen aperture size, and impeller speed on the particle size distribution of the milled product was quantified for a conical screen mill and a hammer mill. In general, the conical screen mill was found to be more sensitive to changes in impeller speed compared to the hammer mill. The effect of impeller speed in case of the hammer mill was non-monotonic while the increasing speeds led to reduction in particle size in case of the cone mill, for the same screen aperture size. The effect of aperture screen size was observed to play a dominant role in dictating particle size distribution of the product material for both mills. In case of the cone mill, grated type screens exhibited higher milling capacity than round screens with equivalent apertures.Lastly, a study comparing the statistical particle size distribution parameters was performed for process design purposes. It was deduced that, if the desired particle size is greater, the comil provides a narrower particle size distributions than the hammer mill; whereas if the desired particle size is smaller, both mills exhibit similar poly-dispersity. The study provided insight into fundamental breakage mechanisms for both mill classes. Breakage in the hammer mill occurs primarily due to the impact of the hammers and large particles may often leak through the mill without sufficient breakage. Breakage in the comil is more gradual as the impeller sweeps a wide area generally ensuring sufficient breakage of particles before they exit the milling chamber.

Figure: Effect of impeller speed on change in particle size distribution parameters.Figure optionsDownload as PowerPoint slide