Chapter 7: Grinding Mills

Introduction

Grinding is the last stage in the process of comminution; in this stage the particles are reduced in size by a combination of impact and abrasion, either dry or in suspension in water. It is performed in rotating cylindrical steel vessels which contain a charge of loose crushing bodies the grinding medium which is free to move inside the mill, thus comminuting the ore particles. According to the ways by which motion is imparted to the charge, grinding mills are generally classified into two types: tumbling mills and stirred mills. In tumbling mills the mill shell is rotated and motion is imparted to the charge via the mill shell. The grinding medium may be steel rods, balls, or rock itself. Tumbling mills are typically employed in the mineral industry for coarse-grinding processes, in which particles between 5 and 250 mm are reduced in size to between 40 and 300 m. In stirred mills the mill shell with either a horizontal or a vertical orientation is stationary and motion is imparted to the charge by the movement of an internal stirrer. Fine grinding media inside the mill are agitated or rotated by a stirrer, which typically comprises a central shaft to which are attached pins or discs of various designs. Stirred mills find application in fine (15 40 m) and ultra-fine (<15 m) grinding.

All ores have an economic optimum particle size (Chapter 1), which will depend on many factors, including the extent to which the values are...