In 2010, a 40-foot, 22-megawatt semi-autogenous grinding (SAG) mill set a

world record for grinding power when the Esperanza copper mine put the

device, manufactured by FLSmidth, into operation. With low-grade reserves

and a high daily throughput of around 90,000 tonnes, three-year-old Esperanza

is impressive, but may not the best model for future mines around the world.

By Eavan Moore

BREAKING DOWN

The 40-foot, 22-megawatt SAG mill atAntofagasta's Esperanza copper mine and begs thequestion how much larger can these mills grow?

COMMINUTION

“If you take a simplistic view of demand increase and decli-ning head grades over the next 20 years, you might have to beprocessing four times the tonnage that we currently process tomake the amount of copper that the world needs,” says JoePease, CEO of Xstrata Technology. He notes that energy pricesare rising, energy security is falling, and available deposits arebecoming more remote and more challenging to extract, furtherincreasing the energy needed for each of those tonnes.

If the only problem were accommodating higher tonnages,more grinding power in a standard SAG mill-ball mill proces-sing circuit would suffice. And that is part of the answer. HarriLehto, technology manager, grinding processes at Outotec, saysgiant tumbling mills are “more or less a must” for any equip-ment manufacturer wishing to be taken seriously. Incrementalincreases in mill size have brought the maximum rating up to22 megawatts, while 42-foot, 28-megawatt SAG mills have beendesigned (though not installed).

But can they go bigger? “We have been stuck at 40 feet forover 10 years,” says Steve Morrell, a long-time comminutionresearcher and current managing director of SMC Testing PtyLtd. The bigger SAG mills use gearless drives, and a number ofrecent drive failures have led to a general reluctance to push theenvelope beyond the 28-megawatt maximum.

Energy efficiencyThe looming problem is not tonnage in itself but in the cost

of tonnage, exacerbated by tumbling mills’ inherent inefficiency.Rotating containers tossing rock and steel spend most of thepower they consume on generating heat. Ball mills use as littleas one per cent of their energy draw to break rock.

The industry is discussing – and in some cases adopting –more complex but potentially more effective and energy-efficientmilling circuits that do not rely exclusively on scaling up tum-bling mills. In terms of energy use, it can be more cost-effectiveto prepare ore for the ball mill by sending it through multiplecrushing stages rather than recirculating it through a SAG mill.The same staged approach to grinding divides the circuit intocoarser- and finer-grinding equipment to more efficiently recoverthe materials available at different sizes.

The last two decades of mill development have producedmore equipment suited for size-specific crushing and grin-ding. High-pressure grinding rolls (HPGR), typically used inthe final stage of crushing, compress the coarse ore betweentwo cylinders. They may use 10 to 20 per cent less power thanSAG mills.

On the other end of the size spectrum, stirred mills grindfine fractions using a rotating shaft within a stationary shell.Stirred mills cut energy use because only the shaft and discsneed to be rotated, and because the mills can use small, high-velocity grinding media with a much greater surface area forgrinding. They are typically used to regrind ore once it has pas-sed through a ball mill.

“These technologies have been discussed for many years,”says Walter Valery, global senior vice-president at Metso ProcessTechnology and Innovation. “For example, we proposed a cir-cuit flowsheet with HPGR followed by stirred mills in conjunc-tion with high-intensity blasting as an energy-efficientalternative over 10 years ago. However, only recently are weseeing HPGR technology considered in most prefeasibility andfeasibility studies.”

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Costs and benefitsBoth HPGR and stirred mills have made inroads. One of the

two most widespread suppliers of stirred mills, MetsoMiningand Construction Technology, has a total of 39 installations inCanada. Paul Cousin, vice-president of metallurgy at AgnicoEagle Mines, says his company is considering using stirred milltechnology to regrind the ore from its LaRonde mine in Que-bec. What he has heard suggests that the mills yield a betterend-product, adding not just energy efficiency but overall costpayback.

Nonetheless, the inherent conserva-tism of the industry slows adoption ofproven technologies, says JonathanAllen, product manager for stirred millsat Metso. “Everybody in mineral proces-sing knows ball mills,” he says. “Sowhen a new supplier brings in a newtechnology, no matter what it is, if it’sunique to one supplier, you’ve got eve-rybody else out there saying it’s a badidea. The market penetration just takesa bit of time in our industry.”

Capacity is another concern, saysAllen. Stirred mills were first used forregrinding so they have a small through-put relative to ball mills, but as their capacity has increased withthe move to whole ore grinding, so has interest. In 2008, repla-cing one 12-megawatt ball mill would have required a series ofanywhere from two to eight smaller stirred mills. Five yearslater, Metso is working on a 4.5-megawatt version of its Verti-mill, or vertical stirred mill, and Xstrata Technology offers aneight-megawatt horizontal IsaMill. Stirred mill installations havemultiplied in the last few years. This includes large-scale adop-tion by Anglo Platinum, which extensively uses horizontal stir-red mills for tertiary grinding as well as for regrinding.

While HPGR technology at first promises large increases inenergy efficiency, more holistic evaluations of HPGR erode thecost savings that “energy efficiency” measures imply. They may

have lower operating costs and offer certain savings – for exam-ple, no need for steel grinding media – but HPGR also requireadditional auxiliary equipment, precrushing, extra screens,conveyors, storage and dust extraction. Brian Putland, presidentof Toronto-based Orway Mineral Consultants, explains thatthose extra items can add up to a high-capital investment com-pared to SAG installations. Capital costs reflect energy costs aswell, argues Putland: A full evaluation of the energy used would

include the manufacturing and shippingof equipment and wear items.

Alan Muir, vice-president of metal-lurgy at AngloGold Ashanti, says thecompany considers HPGR at any newproject but has only installed one suchcircuit, at its new Tropicana mine in Aus-tralia. “That decision was really driven bythe very hard nature of the ore and thecost of on-site power generation, whichis extremely high,” he says. Tropicanaruns up power costs of 27 to 30 US centsper kilowatt-hour.

“If we take South America, wherethere’s a lot of hydropower production,there the cost is typically in the region of

9 to 12 US cents per kilowatt-hour,” adds Muir. “So there, itdoesn’t really stack up. You would spend more money on capi-tal equipment and not have the savings on power that you needto offset that.”

In Canada, cheap hydropower puts energy costs lower onthe priority list. About 90 percent of Putland’s clients go witha conventional SAG and ball mill set-up after considering alter-native methods. Even Far North projects, where power isexpensive, find that the costs of covering, heating, operating,or maintaining additional equipment can outweigh the energybenefits.

Cousin says HPGR were among the options considered butdiscarded at Agnico Eagle’s evaluation-stage Meliadine gold

“The standard should be that the plant iscustom-designed

for exactly what the ore body needs.”Joe Pease, Xstrata Technology

The last five years haveseen a quick increase in

the power rating ofstirred mills.

Glencore’s McArthur River zinc mine inAustralia uses stirred mills for bothSAG mill discharge and ultrafineregrinding.

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project in Nunavut. “It was our belief that HPGR could be well-suited on an energy basis, because of the high cost of producingpower out there,” he explains. “But to our surprise, the designof the overall comminution circuit including an HPGR portionproved to be not as efficient, in terms of overall economics, asa more conventional approach of SAG-ball milling.”

A better circuitSteve Walters, research director of

the industry-funded project Coopera-tive Research Centre for OptimisingResource Extraction (CRC ORE),stresses that focusing on the energy effi-ciency of equipment misses the point.“It’s like rating a washing machine andnot checking that it cleans the clothes,”he says. “We can argue about the effi-ciencies of HPGR comminution pro-cesses versus a SAG. That’s not the realquestion here. The performance metricshouldn’t be the efficiency of the acti-vity; it should be the useful output.”

By “output,” Walters does not meanthroughput: he means metal, the ulti-mate unit of success. At the University ofQueensland in Australia, CRC ORE andthe Julius Kruttschnitt Mineral ResearchCentre (JKMRC), researchers are working on approaches thatprocess less rock and recover more metal: in other words, howto do as little work as possible on as little ore as possible.

Malcolm Powell, chair of the Anglo-American Centre forSustainable Comminution at JKMRC, suggests that mines stopblending different ore grades and start separating differentgrades into different streams, to be processed in different cir-cuits. The tools to do this already exist; bulk grade detectorscan already sort out ore, and some large operations already havemultiple ore streams, simply because they have too much

production volume to process in one mill. That presents anopportunity to apply what Powell calls “flexible circuits” thatrespond to a specific ore, via conventional or novel millingmethods as appropriate.

AngloGold Ashanti is following this approach at its projectsin development. “Just blending everything together through a

single circuit is probably not optimal,”agrees Muir. “The trick will be to deve-lop circuits which have the flexibility toadapt to different ore types and main-tain optimal processing even when theore changes.”

Ore body knowledge is a critical partof this approach, and that is one reasonthat it has not been more readily takenup, explains Pease. He estimates thatknowing what can be done with the oreis about 80 per cent of the work needed.With that in place, each circuit ought torequire less equipment because it hasbeen designed with clinical precision.But the individual nature of the testwork and resulting flow sheet can beoff-putting.

“In theory, it’s there,” says Pease. “Wehave the drill core. We have quantitativemineralogy. We have diagnostic crushing

and comminution and laboratory tests. We can really map the orebody and map the metallurgical response and custom-design aflow sheet for it. I’m not sure we always make use of that as muchas we can, because it seems expensive and time-consuming andtakes a fair bit of expertise. It doesn’t suit fast-track engineeringand the flow sheet that comes out of that is individual. Perhapsfrom the position of the board of directors, they’re saying ‘Uniquedesign sounds like risky.’ You need to be able to explain to themthat it means custom-designed to be lowest cost for this ore, withsavings far outweighing the investment in time and design.”

“There’s a big issue here as to whether they

will retreat to what they think is safe, or

whether they’ll evolveinto something which is

going to be moreefficient, but also more

profitable.” Steve Walters, CRC ORE

Paul Cousin, vice-president of metallurgy at Agnico Eagle Mines, says his company is considering

using stirred mill technology to regrind the ore fromits LaRonde mine (pictured here) in Quebec.

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High-pressure grinding rolls rely on compression rather thanimpact forces to break rock, and can provide energy savings.

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Integrating the mine siteWhat any mine can and should do, many agree, is to

improve cross-silo communication. That is critical to Pease andWalters’ suggested strategies, which focus on doing work asearly as possible: blasting selectively to produce a better feed,using pre-concentration and crushing to reduce the work doneby more energy-intensive grinding equipment, and producingthe highest grade possible in concentrate, since smelting usesfar more energy to remove the same impurities. At Agnico Eagle,talk within the operations team now includes discussion of howdrilling patterns could increase costs on the mining end butreduce the cost of operating comminution equipment.

Powell says the potential benefits of adopting more complexcontrols are apparent and well-supported on a simulation level;the challenge is to demonstrate and quantify those benefits inpractice. Given the risk-averse climate, only a few mines havevolunteered to try out JKMRC initiatives. But Powell believesthat the industry as a whole is moving toward a more integrated,mine-to-smelter perspective. “It’s not obvious in the way busi-nesses are run yet,” he says, “but the way we’re talking to indus-try and the way we think about the problem now is much morea systems approach.”

The upside to the downturnIn the midst of the last mining boom, when skills were scarce

and services expensive, redesigning comminution circuits tooka backseat to getting projects up and running. Mining compa-nies’ current financial challenges have put a new premium onoperating what they have with greater efficiency.

Grant Ballantyne, a research fellow at JKMRC, has seen atti-tudes change within 18 months. During a workshop held bythe Coalition for Eco-Efficient Comminution in 2012, whenmost metal prices were rising or had hit a plateau, attendeesemphasized throughput as the financial driver of their commi-nution choices. At another CEEC workshop in July 2013, all

16 attendees – from vendors, operations and enginee-ring companies – came seeking efficiency and pro-ductivity increases. “The push for operationalefficiency seems to be increasing,” he says.

But Walters is not positive that the downturn willinspire innovation. “The mining industry is right onthe cusp of change,” he says. “Some mines have star-ted to take a bit more of a flexible approach to howthey deal with their ores, but there are very few ofthem. So given the change in the industry dynamics,there’s a big issue here as to whether they will retreatto what they think is safe, or whether they’ll evolveinto something which is going to be more efficient,but also more profitable.”

While major innovations are often left to majorcompanies that can afford to open themselves up tosuch risk, Valery’s experience suggests that solutionscould come out of smaller mines as well. “Smallercompanies often have more difficulties raising capitalthan larger companies and therefore need to lowercapital expenditures and operating expenditures toget their projects off the ground,” he explains. “In

order to do this, they are more receptive to innovative or alter-native solutions, which are riskier than the conventional ones.”

The future of comminutionWhat will a standard circuit look like in 10 years’ time?

“The standard will be that there is no standard,” Pease ans-wers. “I think that’s sort of the problem at the moment, is thatfor lack of resources, we design a standard plant. The standardshould be that the plant is custom-designed for exactly whatthis ore body needs. And so the standard will consider first ofall how much fragmentation is done in the mine; it will consi-der what can be done with pre-concentration to remove reallycoarse gangue early on; it will then consider a stage grindingand flotation flow sheet to minimize grinding energy. It’ll usethe least amount of grinding energy on the lowest possibletonnage. And that standard approach will build, I believe,smaller, more efficient plants.”

Can comminution be done away with altogether? Muirthinks that is a question worth posing, and he plans to addressit in a keynote lecture next year. “It’s a little bit provocative tosay, however, we are being pushed into the corner by risingcosts, dropping feed grades and higher throughput rates, andall we’re trying to do is tweak the existing technologies that wehave, where we should be really spending more money onexploring new technology based on different science,” he com-ments. HPGR use compression force to break down particlesrather than the typical impact forces employed in tumblingmills. “If compression is more efficient than impact (at finersizes) we need to explore what is more efficient than compres-sion. We have also started to look at whether we can do in situleaching, which would eliminate the need for mining and sur-face operations, grinding and all that kind of stuff. It’s not a tech-nology I can switch on tomorrow.” But, he says, “I think it’s timewe started looking at that as an opportunity worthy of seriousconsideration.” CIM

A number of manufacturers, including Xstrata Technology with its Isamill (pictured here), offerstirred mills.

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