COMMINUTION & CRUSHING
an enormous number of top-sized balls tumbling at any given time, Cornish says.
Larger diameter( and therefore heavier) balls will result in greater kinetic energy per impact, meaning that they can break larger and harder rocks more effectively. In theory, larger balls create a finer particle size distribution( PSD) per impact than smaller balls on the same size rock, perhaps leading to accelerated fine particle production.“ While it is true that there are fewer balls per unit volume and thus less frequent impacting events than with smaller diameter balls, it is also true that there are still an enormous number of balls tumbling within the mill,” Cornish said.
In SAG mills, there is evidence that rock breakage rates can be increased balls, thereby leading to lower media consumption.
At a higher level, larger balls make the SAG mill easier to operate and maintain the ball load, boosting the fresh feed rate( assuming no other plant constraints) and offering the ability to decrease the mill speed and / or ball load, and decreased pebble production enables optimisation of the pebble crushing circuit. The improved mill stability enables further optimisation of rock load control leading to a more uniform PSD delivery to the ball mill circuit, Cornish says.
Leveraging any of these opportunities effectively leads to increased metal production at a lower overall operating expense, according to Cornish.
At a minimum, larger grinding balls of premium quality will result in lower media consumption in SAG mill applications with all else held constant, Brian Cornish says
by using larger grinding media – hence the title of Cornish’ s presentation – and the improvement can be measured as a combination of throughput rate, specific energy consumption and pebble production rate. The physical benefits of using larger balls are summarised as follows:
• The increased kinetic energy per impact provides an increased rock breakage rate that expands operational flexibility to accommodate varying SAG mill feed size and ore competencies;
• The larger( and heavier) balls are more likely to be released earlier from the lifter resulting in an equal or lower trajectory from shoulder to toe;
• Larger balls provide a more stable grinding environment, with a decrease in transient and potentially dangerous ball trajectories that may result in direct ball-on-liner impact events; and
• The exposed surface area of a ball charge with larger diameter balls is less than that of a charge with smaller diameter
Yet, any plant management team considering using larger SAG balls must be sure to adhere to the industry standard change management protocols and be aware of the mill internal dynamics and impact on liner design.
Ball drop test work cited by Cornish demonstrated that a larger ball will create increased rock breakage( a decreased P80 size) with any given drop height within the mill and this can be translated directly to actual SAG mill operation. Given this fact, the concurrent finer grinding within the rotating kidney should be more efficient.
Simulations indicate that, as the ball top size increases, there is increasing diversity within the charge that ultimately enables the mill to develop a wider operating range and greater stability, Cornish says. This diversity results in healthy increases in the desirable abrasion and impact breakage and a decrease in the undesirable collisions that can cause liner wear and breakage if sustained over extended periods.
Industrial case studies reveal that SAG milling operations have benefited from using a larger top size ball. Such benefits include an increase in feed rate or a decrease in pebble production, but there will always be an increased range of operation and improved accommodation of variable ore conditions.“ At a minimum, larger balls of premium quality will result in lower media consumption with all else held constant,” Cornish says.
Geared for secondary grinding
As the world’ s only horizontal fine grinding mill, the IsaMill™ has been an important part of mineral processing circuits since its introduction in the late 1990s.
Achieving ultrafine grind sizes below 10 microns, it has always been billed as unlocking previously untreatable orebodies by owner Glencore Technology.
One of the recent applications the IsaMill is being lined up for is secondary grinding with a test at Hudbay’ s Stall Mill site in Manitoba – involving 8-mm and 10-mm ceramic media – demonstrating over 50 % reductions in specific energy consumption compared with traditional tumbling mills.
Kai Johnston, Glencore Technology Metallurgist, said since this trial the company has been further evaluating concept level testing at coarser feed sizes of + 1 mm using the coarser grinding medias available on the market.
“ Our second demonstration trial identified disc designs for improved coarse milling performance, media retainment mechanisms and optimum operating conditions for further energy efficiency,” he told IM.“ The current focus, as a result of this latest trial, has centred around evaluating / developing materials handling solutions for treating even coarser feeds, but we are always on the hunt for more testing opportunities in coarser applications.”
While most of the baseline data on the IsaMill compares its use to traditional SABC circuits, Johnston was able to also point to pilot studies that pitted it against some of the other grinding pairings featured in newer flowsheets.
He explained:“ There has been lots of chatter among the industry around potential dry primary grinding options like the HPGR to not only reduce circuit water consumption but also provide energy reductions. Some older published literature from piloting shows that a HPGR / ball milling circuit demonstrated a 17 % reduction in energy compared to a cone crusher / ball mill circuit. HPGR / IsaMill testing then concluded a further 9 % reduction in energy requirements when compared to the HPGR / ball milling circuit, which further shows that this concept is technically feasible.”
He added:“ We expect coarser grind targets for a CPF design, and some newer
36 International Mining | MARCH 2026