The improvement in grinding efficiency in a conventional stirred mill, as the media became smaller, led to sand with a particle size of 0.5-0.9 mm being used as the media and the shaft speed increasing from 1,700 to 2,200 rpm. This highlighted the importance of stirrer design to ensure that there was a uniform energy spectrum throughout the mill and that very high-viscosity fluids could be agitated properly. Netzsch developed an agitator that consisted of eccentrically mounted hollow discs on a rotating shaft that were staggered to form a screw that moved the media against the product flow. A typical use of these mills is for paint manufacture. The grinding beads can be glass, aluminum or zirconium oxide, or steel with a diameter of 0.5 mm and greater. The particles are usually ground to less than 5-10 pm. This type of mill has been adapted to grind fine-grained mineral ores to less than 7 pm. Figure 8.8 shows the IsaMill, an example of this type of mill.
The IsaMill uses 1-8-mm (0.04-0.31-in.) grinding media, which can be the ore itself, slag, sand, or steel or ceramic beads, and MIM Holdings Ltd. now uses thirteen 1.1-MW mills at three mines to produce particles that are -7 to -12 pm. The company also uses autogenous mills, tumbling ball mills, and Tower mills at three mines and is in a position to assess the link between power intensity, media size, and product size (see Table 8.2). Approximate product size of autogenous and ball mills is -50 pm and of fine — grinding mills is -5 pm.