(216c) Scale-up Studies of the Spiral Air Jet Mill

The focus of the present work is the analysis and optimization of an air jet mill, which is a versatile equipment used in many industries for fine-grinding powders to sizes less than around 10 microns. The efficiency of the grinding process in an air jet mill typically varies from 2% to 4%, depending on the design and operating parameters so even small efficiency improvements can result in significant energy savings [1]. Amongst many of the available machines, spiral air jet mills are attracting attention because of their advantages like absence of the moving parts and low-temperature rise during grinding, resulting in better temperature control with minimal maintenance. Spiral air jet mills boast an exciting ability: not only can they grind feed particles, but they can also classify the resulting material as a product. This distinctive feature truly sets these machines apart.

From the impetus of the literature, as a first step, we study the gas flow in the mill using Computational Fluid Dynamics (CFD) with OpenFOAM and actual machine behavior through experiments. The spin number is the ratio of tangential velocity to the radial velocity of the flow. A particle inside the jet mill experiences the drag toward the outlet in the radially inwards direction and centrifugal force in the radially outward direction. A balance of the two forces defines a cut size of the particle at which both the forces are equal and opposite in direction. As per the resultant equation, the cut size is inversely proportional to the spin number, assuming other variables to be constant. Thus, as the system is dilute of particles, getting the flow behaviour of a single phase helps to predict the cut size variation.

The scaleup analysis of the spiral air jet mill involves an experimental study using various sizes of cylindrical mill chambers operated in a semi-batch mode. Initially, a fixed base case is established, where the diameter of the chamber is varied while maintaining a constant height. Subsequently, the height of the chamber is varied while keeping the diameter constant, similar to the base case scenario. During the experimental runs, the mill is fed in batches while the outlet is kept open to allow for continuous collection of the product over a specified duration. Each case is conducted under identical conditions to ensure consistency of the results. Following the experimental runs, both the mill material and the corresponding product undergo analysis. The size distribution of the particles is recorded, providing us with valuable statistical data for further analysis and interpretation.

The unimodal feed in the spiral air jet mill leads to a trimodal product, indicating the presence of multiple parallel grinding mechanisms within the mill. According to M.M. Dhakate et al. [2], the major contributors (by weight) to this trimodal distribution are erosion and chipping, followed by initial breakage and impact breakage. Enhancing the mill chamber in the radial direction has been shown to yield a finer mean product, with a similar trend observed in the leftover mill material. Conversely, varying the height of the mill chamber resulted in a finer product, but no clear trend was observed for the leftover mill material. It appears that the effect of radial variation dominates over the height variation of the mill chamber.