page_title: IsaMill
section_title: IsaMill Operating Principles
hierarchical_section_title: IsaMill / IsaMill Operating Principles
caption_attribution_description: English: This drawing provides a schematic view of the inside of an IsaMill, including showing the flow patterns of the grinding medium and the slurry particles that are being ground.
context_page_description: The IsaMill is an energy-efficient mineral industry grinding mill that was jointly developed in the 1990s by Mount Isa Mines Limited and Netzsch Feinmahltechnik, a German manufacturer of bead mills. The IsaMill is primarily known for its ultrafine grinding applications in the mining industry, but is also being used as a more efficient means of coarse grinding. By the end of 2008, over 70% of the IsaMill’s installed capacity was for conventional regrinding or mainstream grinding applications, with target product sizes ranging from 25 to 60 µm.
context_section_description: The IsaMill is a stirred-medium grinding mill, in which the grinding medium and the ore being ground are stirred rather than being subjected to the tumbling action of older high-throughput mills (such as ball mills and rod mills). Stirred mills often consist of stirrers mounted on a rotating shaft located along the central axis of the mill. The mixing chamber is filled with the grinding medium (normally sand, smelter slag, or ceramic or steel beads) and a suspension of water and ore particles, referred to in the minerals industry as a slurry. In contrast, ball mills, rod mills and other tumbling mills are only partially filled by the grinding medium and the ore.
  In stirred-medium mills, the stirrers set the contents of the mixing chamber in motion, causing intensive collisions between the grinding medium and the ore particles and between the ore particles themselves. The grinding action is by attrition and abrasion, in which very fine particles are chipped from the surfaces of larger particles, rather than impact breakage. This results in the generation of fine particles at greater energy efficiency than tumbling mills. For example, grinding a pyrite concentrate so that 80% of the particles are less than 12 µm (0.012 mm) consumes over 120 kilowatt-hours per tonne (kWh/t) of ore in a ball mill using 9 mm balls, but only 40 kWh/t in an IsaMill using a 2 mm grinding medium.
  The IsaMill usually consists of a series of eight disks mounted on a rotating shaft inside a cylindrical shell (see Figure 2). The mill is 70–80% filled with the grinding medium, and is operated under a pressure of 100 to 200 kilopascals. The disks contain slots to allow the ore slurry to pass from the feed end to the discharge end (see Figure 3). The area between each disk is effectively an individual grinding chamber, and the grinding medium is set in motion by the rotation of the disks, which accelerate the medium toward the shell. This action is most pronounced close to the disks. The medium flows back toward the shaft in the zone near the midpoint between the disks, creating a circulation of the grinding medium between each pair of disks, as shown in Figure 4. 
  The average residence time of the ore in the mill is 30–60 seconds. There is negligible short-circuiting of the grinding zone by the feed, as a result of having multiple grinding chambers in series.
  The ground product is separated from the grinding medium at the discharge end of the mill. This is achieved without using screens by using a patented product separator that consists of a rotor and a displacement body (see Figure 2 and Figure 4). The relatively short distance between the last disk results in a centrifugal action that forces coarse particles towards the mill shell, from where they flow back towards the feed end. This action retains the grinding medium within the mill.
  The product separator is a very important part of the IsaMill design. It avoids the need to use screens to separate the grinding medium from the ground particles. Using screens would make the mills high-maintenance, as they would be prone to blocking, necessitating frequent stoppages for cleaning.
  Fine particles are not as susceptible to the centrifugal forces and stay closer to the center of the mill, where they are discharged through the displacement body at a rate equal to the mill’s feed rate.
  The design of the IsaMill results in a sharp product size distribution, meaning that the IsaMill can operate in open circuit (i.e. without the need for an external separation of the discharged particles in screens or hydrocyclones to allow coarse over-size product to be returned to the mill for a second pass). It also means that there is less overgrinding at the finer end of the size distribution, such as occurs during the operation of tower mills.
caption: Figure 2. Schematic view of an IsaMill showing the principles of its operation.
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