Very often in SAG circuits, the ball-mill circuit is neglected. In many operations where economics dictate that throughput is worth more economically than the required sacrifice in grind, the focus shifts to throughput so much that the available ball mill power is not used to the fullest extent. Even in those operations where a firm grind target is not adhered to and attainable throughput governs the balance of the circuit, it is foolish not to take full advantage of the installed grinding power. Ores for which recovery is grind insensitive in the range of the typical operation are unusual.
Given that mills are charged to the target ball charge with reasonably sized media, and the feed to the ball mill circuit is not so coarse as to cause constant scatting, the key to efficient ball-mill circuit operation is efficient classifier operation. The standard classifier for ball mill circuits is the hydrocyclone. Ensuring that the finest and most efficient cyclone cut involves selecting the appropriate cyclone configuration for the ranges of grinds that will be encountered. The apex (spigot) size can be manipulated to deliver the maximum underflow density at the target operating conditions, with the vortex finder tailored for the desired product size. With a given configuration, adding the maximum amount of water (subject to cyclone feed-pump limitations, the minimum overflow density, and cyclone pressure) will generally result in attaining the finest possible grind. Employing a control system to maximize water addition to the cyclone feed pump (subject to pump capacities and downstream densities constraints) is often employed successfully to maximize ball mill circuit grind.
There is strong evidence supporting the concept of using a mixed-size make-up ball to attain incremental grinding efficiencies in ball mills. There is little reason to believe that the steady-state media size distribution resulting from the wear rate of the make-up ball size corresponds to the optimum ball size based on the mills’ feed and target grind. In general, a mixed make-up ball-charging regime improves grinding efficiency, with greatest benefits seen for single-stage milling applications with large size reductions.
Nonetheless, most operations tend to use a single-sized make-up ball for reasons of convenience. There is less conclusive evidence for the removal of fine steel from ball charges. While some operations claim an anecdotal benefit from removal of fine steel, unpublished studies by the author indicate a substantial benefit from the use of fine media (less than 12mm) as a supplement to a conventionally sized make-up ball when grinding a gold ore to an 80% size of 75 mm. It is possible that removal of ball chips, which may tend to float due to a shape factor and likely contribute very little to grinding, could result in an improvement in grinding efficiency.
