COMMINUTION & CRUSHING
Sustainable size reduction
The old mineral processing paradigms are ripe for change , with a series of stakeholder demands leading mining companies to venture beyond tradition , Dan Gleeson reports
The comminution sector has had to evolve over the last few decades as grades have dropped and processing requirements have increased in terms of scale and complexity .
On top of this , higher capital costs to build concentrators has led companies to more readily look at ‘ sweating ’ existing assets to boost production or unlock new processing capacity , as opposed to construct new expensive facilities .
Fluor is cognisant of this trend , with its engineering teams well experienced in delivering large-scale gold and copper projects that address these wider industry challenges .
The engineering firm notes a significant change in the capacities of concentrators over the last few decades .
“ Whereas 10-15 years ago , the typical ‘ new ’ copper concentrator aimed for a capacity ranging between 100,000 t / d and 125,000 t / d , current designs are considering throughput rates of more than 150,000 t / d ,” Mark Sherman , Global Comminution SME ( Subject Matter Expert ), Fluor , told IM . “ Increased processing rates , combined with the desire to minimise capital requirements , lead to increased equipment sizes .”
Equipment design has had to keep up with these needs .
The fundamental design of the primary gyratory crusher hasn ’ t changed in decades , yet significant changes to various components of the drive system and the introduction of advanced process control ( APC ) have allowed crusher mantles to operate at higher speeds , Sherman said . This has seen the capacity of newgeneration machines rise by approximately 30 % across the sector .
“ When combined with the reduced planned maintenance requirements for the new generation of primary gyratories , primary crusher availability assumptions have increased from the typical 70 % utilisation rates used 20 years ago to the now more common 75-78 % utilisation rates ,” Sherman said .
Cone crusher technology capacity has also improved along similar lines , ie via increased crusher speed and APC .
When it comes to grinding , there has been a rise in the number of milling circuits incorporating high pressure grinding rolls ( HPGRs ), with this equipment often positioned in the flowsheet as not only the final stage of size reduction prior to milling in the classical ‘ single stage ball milling circuit ’, but also as pebble crushers for AG-SAG milling circuits .
“ While the often-predicted replacement of AG- SAG mills has yet to occur , the industry ’ s understanding of the attributes of each technology continues to grow and it is on this basis that Fluor proposes that the full benefits of the two technologies are yet to be realised by the industry , especially for high tonnage processing plants ,” Sherman said . “ Fluor has constructed a broad range of both AG-SAG- and HPGR-based milling circuits and is ideally placed to discuss with an operator how each of the grinding technologies impacts upon their processing needs over the life of mine .”
The dominance of the 40-ft ( 12.2 m ) diameter AG-SAG mills will , Fluor says , soon be usurped by the installation of the first of the 42-ft diameter grinding mills , with an order for another 42 ft SAG mill expected soon .
However , unlike the new generation of primary gyratory crushers , the assumed design utilisation rates for the large diameter SAG mills have reduced over the last couple of decades . Around
An FLS Tower Mill ( FTM ), a vertical steel media tower mill that can be used in a variety of mineral processing flowsheets
30 years ago , a SAG mill was assumed to operate at utilisation rates of plus-94 %, versus the current design assumption of 92 % annual run time .
“ This is a result of the challenges associated with the protective wear liners installed inside these mills , both in the numbers of liners required to line the mill and their service life ,” Sherman said .
Larger diameter SAG mills generate larger impact forces that reduce the life of wear liners . When combined with the higher number of liners that need to be replaced , the result is an increase in time required for maintenance .
“ We therefore have to account for increased liner handler capacity and changes to the liner ’ s materials of construction ,” Sherman said . “ Hybrid liners , such as liners constructed from steel alloy housed in a rubber matrix , have produced good results in plant trials in terms of service life and resistance to impact damage .”
When it comes to ball mills , the availability of 24 MW mills , which offer an approximate 10 % increase in power rating over the largest currently operating ball mills ( 22 MW ), have recently been announced .
The increase in ball mill power ratings might be attractive from a milling capacity point of view , but both the circuit designers and potential owners of the larger ball mills need to be wary of the total volume of slurry being processed by these machines , Sherman said , explaining that the discharge trunnion of the mill acts as an orifice , with a limit to how much slurry it can discharge . Exceeding this limit will detrimentally
34 International Mining | SEPTEMBER 2023