Ball mill efficiency
Milling operations are known to be notoriously energy inefficient. Ball mill grinding is based on a work index value developed to directly reflect industrial milling practices. Mill selection is based on this value and related empirical equations. Improved grinding efficiency requires making a change from an energy per ton basis to an efficient particle breakage basis.
Recently, investigators concluded that grinding efficiency is limited to less than 20%, and grinding efficiency can be increased beyond this limit by making three changes in operating practice as follows:
- Use correct make-up media size.
- Operate the mill in a cataracting mode.
- Control milling circuit in a modified mode.
Ball mill Operating Mode:
In an operating ball mill, there are five distinct modes of media action each having a direct effect on particle breakage.
- First, in a slowly rotating mill when the angle of repose is exceeded the media load slides, tumbles and cascades down to the toe.
- Second, as speed increases, some of the media is held against the shell until gravitational force takes over then this media follows a separate cataracting path to an impact zone. This is a mixed cascade, cataract grinding action.
- Third, as speed increases, essentially all of the media is cataracting.
- Fourth, at high speeds the outer layers of media approach centrifugation.
- Overflow mills have a fifth mode and that is a turbulent toe zone in combination with the previous four.
Grinding action changes with each of these variations. Abrasion grinding predominates at cascading speeds with impact grinding at cataracting speeds. That is, at a specified constant kW, changing mill geometry and operating parameters will change the effective quantity and quality of impacting media per unit of energy.
Until a specific media relationship per ton of ore and not kWh per ton is understood as the dominating role in grinding and this understanding faithfully reflected in mill design and operation, energy efficiencies will remain low. A cataracting charge is necessary for efficient impact breakage in ball mill grinding.
Ball Mill Speed
Paths of outer media at cataracting speed indicate direct impact on the mill shell. At 70% critical speed, impacts are downward shearing on an upward moving charge, at 81.354% critical impacts are more direct on a side ward moving charge, and at 90% critical impacts are downward shearing on a downward moving charge. Particle breakage is a function of the type (quality) of impact.
Impact density, the number of media impacts per second per unit area, can be calculated from the media size distribution for a makeup media size. Using a makeup media size of 2 in. and disregarding the minus half inch fraction, impact densities (impacts per second per square inch) for the impact zone of the three speeds are shown for each mill in the following drawing. The impact zone area changes with speed and is smallest for the slowest speed.
Tons of impacting media per square foot of impact area per minute are 87.6, 43.9 and 37.3 for the three different speeds. The lowest speed has the smallest impact area and highest impact density. In this torrential smothering down-pour there is no opportunity for wildly bouncing media. In overflow ball mills, the concept of media falling through a bath of slurry and robbing impact energy is dubious.
However, there are some random media paths above the mass. These are attributed to individual balls momentarily held in place by lifters, by adhesion, and by wedging against liner joints and lifters. Since both media and shell linings are coated with slurry, all impacts will break or abrade ore particles. Each impacting mode may be favored for specific grinding objectives.
For example, a regrind mill may favor all cascade or mixed cascade/cataract mode. A problem of chip buildup may be solved by changing the impact zone (changing mill speed). By controlling media load and mill speed the impacting media can be focused into a narrow or broad zone with a narrow or broad spread of impact energies.