Mechanochemical Heat Generation Balance for a Vibratory Mill

With increasing emphasis on sustainable and efficient reaction technology, mechanochemistry has posed itself as an emerging field due to its operation at nominally ambient conditions and without the need for solvents. Reactions are driven by the mechanical force of impacts, from collisions between milling spheres, between milling spheres and the wall, and onto media trapped between the milling spheres and walls. Currently, no operational and material dependent framework exists to quantify heat generation in bench scale vibratory mills as a function of milling parameters, and as such, we seek to establish parameters for understanding heat generation. Thermal behavior was quantified using a high-speed infrared thermographic camera. The impacts of milling balls were tracked by acoustic analysis paired with image recognition of trajectories of balls in a transparent vessel. Three distinct heating regimes were identified, corresponding to frequencies between 0-12 Hz, 12-25 Hz, and above 25 Hz, each with distinct ball dynamics and heating mechanisms. Low frequencies result in minimal heating from a rolling ball motion, intermediate frequencies result in a mixture of ball-wall collisions and rolling friction driving greater heat generation, and at high frequencies, rapid temperature increases result from the onset of head-on impacts. Each regime also saw different steady state temperatures. For controlled mill operation, understanding the impact behavior is vital to ensure the operating conditions meet the designed conditions. Analysis of the impact of grinding media material properties on heat generation was also performed. Experiments varying material properties reveal a strong correlation to elastic modulus, with a higher elastic modulus leading to higher heat generation. Additional trials with a varying number of balls but a constant mass showed that fewer collisions with higher energy cause greater heat generation than a greater number of low-energy collisions. These results establish infrared thermography coupled with acoustic monitoring as an effective approach to quantify heat generation in bench scale vibratory mills. Establishing this framework supports the development of cohesive kinetic models for mechanochemistry.