Tower / Stirred Grinding 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.

Advantages of stirred media mills are low capital, low operating costs, low noise levels, efficient energy usage, minimal overgrinding and small footprints. Low speed stirred mills normally have an effective size reduction to about 15 microns (80% passing size) and high speed mills with reduction to about 5 microns (80% passing size). Dry stirred mills or attritors also exist.

- Xstrata Technology, Metso Minerals, Nippon Eirich Co., Union Process Inc. images.