(270b) Responsive Discrete Element Model for Preparation of Advanced Materials by Mechanical Milling Using an Attritor Mill

Various research groups [1, 2, 3] have identified mechanical milling as a suitable technique to prepare advanced reactive materials. With growing demand for such mechanically alloyed and nanocomposite materials, there is need for predictive models enabling one to transfer synthesis from the laboratory to larger scale (and, possibly, operation principle) milling devices.  Due to lack of mechanistic models in literature, a trial and error approach for scale-up is the current methodology, which is both imprecise and inefficient. In order to overcome this inadequacy, a concept of milling dose was suggested to describe milling progress in different mills [4,5]. The milling dose D_m is defined as the rate of energy transfer from the milling tools to the powder, E_d, multiplied by the milling time, t, and divided by mass of the milled powder, m_p: D_m=(E_d.t)/m_p

 In our previous work, the concept of milling dose was considered for three milling devices: shaker, planetary and attritor mill. The shaker and planetary mills are used in laboratory scale preparations and attritor is more suited for commercial scale manufacture. In experiments, the same starting powders were used in each mill.  The milling time, t, for each mill was measured while the final product with the desired yield strength (serving as an indicator of material refinement) was prepared.  Theoretical models of the three mills were set up using discrete element method (DEM) [6] and the energy dissipation rate, E_d, for the milling dose was computed.  The milling dose values for the shaker and planetary mills were close to each other, as expected.  However, the computed energy dissipation rate for the attritor was too high, resulting in a higher than expected milling dose. 

 Detailed analysis of the milling tool interactions in the attritor mill showed that from time to time, balls are predicted to jam, resulting in very strong forces exerted onto the milling media from the impeller moving at a constant, pre-set speed.  In experiments, similar jamming events can also occur; however, they cause small changes in the impeller speed.  Thus, the variations in the force are attenuated.  Initially, to account for the computationally predicted events associated with unrealistically high forces, a screening scheme was proposed, in which any events involving such high forces were discarded [6].  The resulting, corrected milling dose for the attritor was much closer to that predicted for other milling devices. 

 The objective of this effort is to modify the DEM model for the attritor mill to imitate the response of impeller to the jamming events, i.e., to allow the instantaneous changes in its speed, depending on the packing of the milling balls. To develop a modified model, the impeller is described in the code as a complicated particle, or an element similar to milling balls.  It is rotated using several driving cylinders placed at its top, around its circumference.  The rotation rate is thus fixed for the driving cylinders, but not for the impeller.  When a jamming event occurs in the mill, impeller can slip relative to its driving cylinders, so that its rotation speed is reduced momentarily.  Once the ball packing is adjusted inside the mill, the impeller speed returns to its pre-set value. 

 In order to compare predicted and actual variations in the impeller rotation speed, the measurements are made using a controller equipped with data acquisition available for the attritor mill.  Details of model development and its experimental validation will be presented. The implications for the preparation of advanced reactive materials using mechanical milling will also be discussed. 

REFERENCES:

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