South African Journal of Chemical Engineering (Jul 2020)
An attainable region perspective on the role of residence time distribution in ball milling
Abstract
The effect of material flow through a ball mill on the efficient production of particles was explored using the attainable region framework. The target efficiency was the production of floatable particles of size between 300 μm and 75 μm upon expending the lowest ball milling energy.To this end, a 3.2 m x 3.6 m ball mill in open circuit was assumed to process a feed of size less than 9.5 mm. A model of the milling circuit was then built for simulation under typical operating conditions. Feed flow-rates between 10 and 200 tph solids were simulated for several residence time distributions. First, the milling volume was set as a perfectly mixed reactor; then, gradually segmented into an increasing number of perfectly mixed reactors in series. The mass fraction of floatable particles in the product was monitored for the various combinations of operating conditions. The energy required to produce less than 300 μm particles was also estimated. Last, the collected simulation data was analysed using the attainable region and size-specific energy concepts.From the simulation results, it was found that the ball mill would produce -300 + 75 µm particles efficiently from a feed of size less than 9.5 mm at a rate of 75 tph. In terms of residence time distribution, milling performance hit a plateau once the number of perfectly mixed reactors in series reached 9 segments. Finally, it was shown that the throughput of the plug-flow milling reactor could be increased by 10% from the baseline of the perfectly-mixed mill. There may therefore be a motivation for the development of plug-flow-like milling technology.
