A cement mill is a horizontal cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis, imparting a tumbling and cascading action to the balls inside cement mill. Material fed through the cement mill is crushed by impact and ground by attrition between the balls inside cement mill. The grinding media inside cement mill are usually made of high-chromium steel. The smaller grades are occasionally cylindrical ("pebs") rather than spherical. There exists a speed of rotation (the "critical speed") at which the contents of cement mill would simply ride over the roof of the cement mill due to centrifugal action. The critical speed (rpm) is given by: nC = 42.29/√d, where d is the internal diameter in metres. Cement mill is normally operated at around 75% of critical speed, so a cement mill with diameter 5 metres will turn at around 14 rpm.
The cement mill is usually divided into at least two chambers,(Depends upon feed input size presently cement mill installed with Roller Press are mostly single chambered), allowing the use of different sizes of grinding media inside cement mill. Large balls are used at the inlet of cement mill, to crush clinker nodules (which can be over 25 mm in diameter). Ball diameter of cement mill here is in the range 60-80 mm. In a two-chamber cement mill, the media in the second chamber are typically in the range 15-40 mm, although media inside cement mill down to 5 mm are sometimes encountered. As a general rule, the size of media inside cement mill has to match the size of material being ground: large media can't produce the ultra-fine particles required in the finished cement, but small media can't break large clinker particles inside cement mill. Mills with as many as four chambers, allowing a tight segregation of media sizes, were once used, but this is now becoming rare. Alternatives to multi-chamber cement mill are:
pairs of cement mill, run in tandem, charged with different-sized media.
use of alternative technology (see Roll-presses below) to crush the clinker prior to fine-grinding in a cement mill.