How the Circulation Process Works
The circulation grinding mill is a combination of a grinding mill and a large holding tank which is generally about ten times the size of the mill. The mill is filled with media and contains grids which restrain the media while the slurry is allowed to pass through.
The unique principle of the circulation mill is the high circulating (pumping) rate. The entire contents of the holding tank are passed through the mill at least once every 7-1/2 minutes, or about 8 times per hour.
This high pumping rate results in a faster grind and a narrower particle size distribution. This phenomenon is explained by the principle of preferential grinding (See Figure 1). The fast pumping stream through the agitated media bed makes the circulation mill grinding chamber act as a dynamic sieve or filter, allowing the fine particles to pass and move quickly through, while the coarser particles follow a more tortuous path through the media bed.
With the circulation process, unlike a continuous machine with the slurry making a single pass, the material makes many passes through the grinding chamber until the desired particle size is obtained.
Grinding media used in circulation mills range in size from 3/32" to 1/4" in diameter. Chrome, steel, steatite, and zirconium oxide are commonly used media.