Stirred Media Mills /Tower Mills
“Tower mills, low speed vertical stirred mills, and high speed stirred mills are used for fine and ultra fine grinding. In contrast to tumbling mills with outer rotating shells, tower and stirred mills impart motion to the charge by the movement of an internal stirrer. Tower and low speed vertical stirred mills have a double start helical rotating screw inside a stationary vertical grinding chamber filled with balls or pebbles. The feed (and mill water) enter the top of the mill and is reduced by attrition and abrasion as it falls down the mill and encounters the grinding media being agitated and lifted by the screw. The finely ground particles move to the top of the mill (water surface) to a settling classifier. Oversize particles are returned to the bottom of the tower for regrinding. Other low speed stirred media mills use a similar principle but use a vertical pin agitator and the media is typically high grade alluvial silica sand or small ceramic spheres. For ultra fine grinding the high speed stirred mill is used. Using a high speed rotating disc or pin stirrer and fine grinding media (sand, ceramics, etc) these mills can produce very fine product with high energy efficiencies.” (Mining Online Expo, Stirred Media Mills /Tower Mills, 6/145/2011)
Recent Journal Articles
Influence of process parameters on breakage kinetics and grinding limit at the nanoscale
(1751–1758)AIChE Journal 57 #7 (2011)
Knieke, Romeis and Peukert of the Friedrich-Alexander-University Erlangen-Nuremberg, Germany, identified a grinding limit where no further particle breakage occurs in long-term milling experiments such as a stirred media mill. During mechanical stressing of the particles, defects are generated in the crystalline lattice, which allows real fracture of nanoparticles. Below a critical size, defects cannot be stored or generated in the crystallites and the overall limit of grinding is reached. This limit is strongly influenced by material properties and hardly affected by most of the process conditions. However, the breakage kinetics strongly depend on the process parameters and suspension conditions as long as the grinding limit is not reached. Based on these findings, two mechanisms of nanoparticle breakage are proposed. Proper choice of process parameters saves not only up to 90% of the energy input to reach the grinding limit but also leads to a higher product quality in terms of crystallinity and less milling bead wear. (RDC 6/13/2011)
Review Articles
Comparison of the micromechanical aggregate properties of nanostructured aggregates with the stress conditions during stirred media milling
(4943-4952) Chemical Engineering Science 66 #21 (2011)
Schilde, Beinert and Kwade of the Institute for Particle Technology, Germany, reviews the results of nanoindenter comparing the measured micromechanical properties with dispersion results in a stirred media mill. The strength of the aggregates can be changed using different primary particle sizes. Generally, the maximum achievable product fineness and the efficiency of the dispersion process increases with decrease in aggregate strength and, thus, increasing primary particle size. With the help of the calculated stress energy distribution in the stirred media mill using the discrete element method and the measured fracture distribution of the aggregates measured via nanoindentation an effective dispersion fraction can be calculated. (RDC 8/29/2011)