COMMINUTION & CRUSHING
ore application, a SAG / AG mill paired with a Vertimill ® in a gold processing plant, an HRC HPGR and Vertimill combination in iron ore, as well as a Vertimill into a HIGmill™ for copper concentrators.
“ We are starting to see this thinking across the board,” he explained.“ Even in projects that have historically been super sensitive to upfront cost, the thinking has shifted towards energy efficiency, lower operating costs and sustainability.”
The sales of stirred mills, such as the Vertimill, HIGmill, Stirred Media Detritor mill and Vertical Power Mill, have increased in line with this. Metso reported earlier this year that it had sold over 20 Vertimill grinding mills with a combined installed power of 67 MW in 2025.
With these stirred mills delivering up to 35 % energy savings compared with conventional ball mills, the total power saved amounts to approximately 36,550 kW, the OEM claims, equivalent to avoiding 135,000 t / y of CO 2 emissions, based on 8,500 operating hours per year. To further support the assessment of flowsheet alternatives, Metso recently launched what it has termed the Sustainable Flowsheet Explorer which allows customers to explore various pairings.
Against this backdrop, Hoetzel is excited about where the holistic flowsheet thinking goes next.
“ Once you consider the opportunity to grind to a coarser fraction, we then need to think about the interim step before flotation,” he said.“ How do we match the comminution equipment and the separation and sizing equipment to make this process more efficient?”
He went some way to answering this:“ Taking this holistic view and the aim to find the best solution for the task at hand, we can explore whether it is a single-stage segment in closed circuit with a hydrocyclone, or whether it is an HPGR with a screen or an air classifier, or maybe something even more interesting.
“ What if you could combine comminution and classification in a single step for further simplification? Could that be something to include in a flowsheet of the future?”
Embracing the variability
At the 58th Annual Canadian Mineral Processors Operators Conference, in January, Maarten van de Vijfeijken, Product Marketing Manager Grinding at ABB Process Industries, and Mark Sherman, Mining & Metals Technical Director for Fluor Canada, took to the stage to discuss,‘ Variable speed on ball mills: More than just a nice-to-have feature?’
The underlying report of this presentation went some way to discussing if variable speed ball mills can offer any additional practical advantages besides their inherent mechanical and electrical soft-start properties, talking up the potential to reduce liner and grinding media wear, increase throughput capacity, improve grinding duty and downstream recovery, enhance grinding efficiency and reduce the carbon footprint. The discussion aligns well with this article. Traditionally, ball mills were equipped with fixed speed motors because the higher upfront cost of variable speed motors could not be justified, and changes in ball mill energy demands were managed via changes to the recirculating load and mill ball charge volumes.
“ Whilst this operating philosophy was justified several decades ago, it may be surprising today given that ball mills are often the highest energy and steel media consumers in a concentrator,” van de Vijfeijken and Sherman wrote.“ To those interested in energy and operational efficiencies, the authors propose that the ball mills present themselves as an obvious starting point for optimisation measures. Therefore, the question is: does variable speed for a ball mill offer any practical or economic advantages for the modern-day concentrator operator?”
The answer to this question comes down to the expenditure and productivity‘ buckets’ mine sites and engineering firms are focused on. A capital-only focus could lead to fixed speed motors being preferred exclusively should the cost dynamic established decades ago continue; variable speed motors should be in the running for those more inclined to look at operating costs and total cost of ownership metrics.
The authors said:“ The omnipresent challenge for circuit designers and owners’ teams is finding the balance between capital efficiency and operational flexibility. Design conventions, some of which were developed during times when capital was expensive and power and steel media costs were comparatively cheap, are now anachronistic, and potentially detrimental to the design of a contemporary grinding circuit.”
When selecting the motor power, it is also important the design team accounts for any additional losses incurred by, if applicable, mechanical drivetrain components installed between the motor and the mill shell, such as the gearbox, pinion and ring gear, they say, recommending selection and installation from the outset of the power that the selected mill size can draw and probably will draw for the majority of its operating life, rather than installing a motor that is 10-15 % greater than the mill’ s expected power draw according to the‘ design point’.
“ Care must be taken when specifying the design point for calculating ball mill power demand, especially if the proposed ball mill circuit will be treating a wide range of feed rates, feed sizes and feed hardnesses,” they added.
Changes to the cost of variable speed drive technology, coupled with increasing gear drive capabilities, provide more design options for the owners’ teams, they argue. Much more powerful gearless drive systems providing superior drive efficiency and mill availability are also on offer, with the first 24 MW gearless driven ball mill expected to be commissioned this year.“ Gearless driven ball mills, up to 34 MW, are being offered by the major mill and drive suppliers,” the authors added.
Practical benefits of variable speed driven ball mills cited in the paper included:
• Providing more consistent power draws over the life of the shell liner;
• The ability to compensate for errors in the modelling of the shell liner’ s geometry effect on charge motion;
• Providing lower grinding power and reduced grinding media wear during commissioning and ramping up;
• Providing proper protection against a‘ frozen charge’, as well as a frozen charge remover function; and
• The inherent variability of orebodies creates different and opposing energy demands for the SAG and ball mill, which can be met efficiently and conveniently by variable speed control of both the SAG and ball mills.
The authors added:“ Most importantly, the variable speed drive provides superior flexibility in power draw delivery when operating over a wide range of grinding power demands, which has a direct impact on power and media consumption rates, liner wear rates, recovery in the downstream process and minimising the detrimental effects of over-grinding the target minerals.”
Bigger ball media
Another CMP Operators Conference paper that caught IM’ s eye came from Brian Cornish, Director of Application Engineering – Grinding Media Division at ME Elecmetal.
Cornish’ s grinding media insights came to the front in a presentation titled:‘ SAG media: Bigger is Better’, in which he went into granular detail on the evolution of ball size and ball loads over time.
“ Ball size and ball loads have generally increased in recent years; in past decades, a 100 mm ball size was desirable with an 8 % steel load,” Cornish said.“ Today, 140 mm and even 160 mm balls are common in 40-ft( 12.2-m) diameter SAG mills with 28 MW motors at steel loads of 22 %.”
In such a case, a ball load of 800 t is maintained by daily addition of 27 t of 140 mm balls each weighing 11.2 kg( 2,500 balls / day). Despite the large size, this is
34 International Mining | MARCH 2026