Leading mill supplier FLSmidth said the lead times for new equipment are getting longer.

Capacity and availability are key to success for grinding systems, while power and water savings are adding to the appeal of HPGRs

By Steve Fiscor, Editor-in-Chief

Miners rely on semi-autogenous grinding (SAG) mills and ball mills to liberate metals from ore. They are familiar pieces of machinery that are seen continuously turning at processing plants. Availability with this machinery is key. When these mills go down, the mine loses money every minute the machine remains off-line and the repair costs can be enormous. Initial engineering and design are important, but it doesn’t stop with installation. Some of the more successful mining operations today are using new tools to monitor performance, which can greatly extend their useful lives.

While horizontal tumbling mills have considerable market share in the grinding space, high pressure grinding rolls (HPGR) have earned their stripes in certain applications and they offer miners a cost savings opportunity in the form of reduced energy. These machines employ considerable force in a confined area to achieve substantial size reduction with dry, hard materials. Working with dry material could lend itself to other opportunities in the form of ore sorting and air classification.

Similar to the entrance of HPGRs 15 years ago, dry vertical roller milling (VRM) application could disrupt this space again. Another crossover form of dry comminution from the cement industry could offer similar savings, but it has not been widely accepted in mining yet.

Developments With Horizontal Mills

SAG mills grind ore by lifting it and a small amount of grinding media and dropping it inside a horizontal mill. Ball mills are smaller and use a higher concentration of iron balls to pummel the ore. The mills can operate in an open grinding circuit, feeding secondary grinding systems, or in a closed circuit generating a fine product. Single-stage SAG mills can be very beneficial in reducing capital costs for plants with a staged throughput approach, according to FLSmidth, which markets the Fuller-Traylor line of SAG mills that range from 14- to 40-feet (ft) in diameter. When future tonnage needs to be increased, the SAG Mill can feed downstream ball mills to boost capacity.

When it comes to the biggest of the big, FLSmidth has experienced a great deal of success with SAG and ball mills. “We just sold our 27th 40-ft-dia. SAG mill,” said Anthony Filidore, head of GPLM-Mining Comminution for FLSmidth. “Of the 43 40-ft SAG mills sold worldwide, 27 are FLSmidth machines. We have a 63% market share in this space.” The other suppliers are Metso:Outotec and CITIC.

When asked about plans for pursuing a larger machine, Filidore said most customers seem to be content with 40-ft-dia. SAG mills. “Although we have designs for 42-ft-dia. SAG mills ready to go and discussions with miners continue, most of the interest is in the 40-ft-dia. SAG mills,” Filidore said.

Designing them and building them are one thing, but getting them working to their full potential is another. A 42-ft SAG mill was designed and built for Minas Congas mine in Peru, but never installed. “The rise from 34- to 40-ft-dia. SAG mills was quick, but then stalled,” Filidore explained. “Today, we’re seeing a lot of 28-ft ball mills. Last month, we sold six 28-ft ball mills and six 40-ft SAG mills. We currently have 42 mills in manufacturing, which is the most activity since the 2006-2008 boom period.”

Like almost everything else, lead times for this equipment is growing. “If you want a 40-ft-dia SAG mill supplied by a Tier 1 manufacturer, such as Siempelkamp in Germany or Ferry Capitain in France, you’re looking at late 2023 for delivery,” Filidore said. “A year ago, we were 50 to 60 weeks out on delivery and now we’re pushing almost two years. Lead times during the last boom period grew to 140 weeks.”

Another difference between today’s situation and the last boom period is the escalation in costs for raw materials, power and transportation. Siempelkamp has made 56% of the 40-ft SAG Mills. Ferry Capitain has made 23% of the 40-ft SAG mills. CITIC-Censa in Spain makes many of the 40-ft shells. While resources, such as pig iron, nickel and chromium are still available, albeit at a higher price than six months ago, power supplies and costs in Europe have become a concern.

“Unlike the last boom, we didn’t have that cost escalation fear that castings would cost 20% more when we actually start to build the mill a year from now,” Filidore said. “Recently, we’ve seen the raw material prices decrease, but our sub-suppliers prices are absolutely not coming down. Some people will notice metal prices are down 3% in the last couple of months, but sub-suppliers have increased prices 10%, and a lot of that is fuel, electricity, etc. One supplier recently added a carbon energy tax. More sub-suppliers are also requiring down payments.” They are using  the down payments to buy materials and hedge against escalating costs.

Fastener Technology  Extends Mill Life

Valley Forge has multiple patented items that improve mill performance, including mill flange bolts, bolts that hold mill shells together and sealing bolts that hold mill liners in place. “We get involved with the mill build and we remain engaged offering bolt monitoring services and other concepts that extend the lives of SAG and ball mills,” Valley Forge & Bolt Mfg. Co. COO Bret Halley said.

The company’s SPC4 Load Indicating System and Max Load Indicating Fasteners hold the mill shell together guaranteeing clamp load for the joint.

A large SAG mill may have 150 bolts holding the flange together. Using Valley Forge’s SPC4, the engineers know the mill shell has been bolted together to the designed clamp load. “Torque measures the twisting force that is supposed to develop tension for the clamp load in the bolted joint,” Halley said. “We take that out of the equation and give the amount of stretch in each bolt at a percentage of that bolt’s capability. If an engineer or a mill OEM installs these bolts, they could check the clamp load at any time and take the guess work out of how well it was installed.”

A monitoring device clips on to the end of the bolt and reads the load in it. ““We can place a Wi-Fi unit on the SPC4s to ensure the mill is operating securely for the first three months or six years,” Halley said. Valley Forge recently completed a 19-year read on some of its first SPC4s at a mine in Chile. 

Torque is simply the way a twisting moment load is applied to a bolt to get it to stretch, explained James Brooks, director-new business developent for Valley Forge. “The whole idea of bolted joints and clamp load is to get that bolt to stretch inside its elastic zone, so the bolt is always trying to pull itself back together. Until we came on the scene, torque was the only way of measuring the stretch or tension in the bolt and that method is indirect and inaccurate. Now we have a super accurate method for measuring the tension in that bolt.”

Valley Forge has also applied this approach to flange bolts. “Most problematic flanges were likely installed with torque and, as soon as they get a departure at the flange, slurry escapes and they can never correct it until they take the entire mill apart and re-machine it,” Halley said.

Many of the bolts on the mills are T joints, where the bolt is fastened by a threaded hole on the mill shell. “Sometimes the clamp load is lowered in those regions to prevent damage to the mill shell,” Halley said. “Problems at these spots start occurring with five bolts leaking, and then it grows to 10 bolts, then 20, and it just keeps getting larger. Instead of replacing these bolts once every 30 or 40 years, the mine is replacing them annually, then every six months, and the downtime starts to add up.”

Valley Forge’s Flange Manager offers a way to tighten multiple bolts at the same time. “If a mine has a mill with 20 problematic bolts, they will often try to torque some of the center ones, only to find the outer ones have loosened,” Halley said. “Now they are chasing the load because there’s a gap in the mill shell. By pulling multiple bolts up at the same time, the Flange Manager eliminates torque chasing from bolt to bolt in an area that is basically spongy.”

The use of Maxbolt Load Indicating Fasteners and SPC4s for load verification has really improved mill installs. “We all know how things go. The torque wrench isn’t working so somebody fudges the numbers,” Halley said. “An inspector can very easily check the load on bolts with one of our meters. We had a mine where 19 bolts were twisted off in the holes and the contractor just pushed them back into the holes. The site inspector clipped our meter on to them and noticed they were all reading zero.”

Valley Forge has also developed remote sensing systems using the wireless meters that attach to the end of an SPC4. “We are seeing a trend toward remote monitoring and we have a new generation of wireless meter, which enables the user to change the parameters of the meter remotely using a web-based user interface,” Brooks said. “Instead of physically going to the mill, removing the device and reprogramming it, the user can change parameters remotely. They can also set alerts for low tension levels. When a bolt reaches 80% of design tension, this meter will send an alert saying that it has lost tension. The preventative maintenance benefits are enormous. You’re able to catch a bolted joint or a series of bolted joints as they start to lose tension, and you can do something about it before it becomes catastrophic.”

Valley Forge’s SPC4 Load Indicating System and Max Load Indicating Fasteners guarantee clamp load. Inset: A cut-away shows the bolt sealing the inside of the mill liner pocket.

Another popular Valley Forge product is the sealing mill-liner bolts. They are designed with seals under the bolt heads. “Leakers are big issues for the mines,” Halley said. “When slurry leaks from the mill through a loose bolt, it gets everything dirty. There are so many things that could go wrong in that situation. These bolts now seal the inside of the mill liner pocket, so that slurry can’t enter the bolt hole and get behind the liners.”

Valley Forge holds the patent for sealing the bolt from inside the mill. “Everybody used to seal it from the outside,” Halley said. “The slurry would leak into the mill shell and develop ‘racing.’ “Any time you can reduce racing or mill leaks by sealing it and keeping the slurry where it’s supposed to be, on top of the liners, the better the mill will perform in the long run,” Halley said.

HPGRs: The Sustainable Alternative

Any discussion about sustainable mining usually involves subjects such as energy efficiency grinding, lower emissions, lowering water usage, and HPGRs check all those boxes. “It’s a dry process and it is one of the most efficient comminution tools,” said Joe Dziedzina, product line manager for HPGRs and vertical roller mills for FLSmidth. “A lot of random action takes place inside tumbling mills and they waste a lot of energy. They are effective, but not really efficient. They do what they are intended to do, but they require more power, and with harder, more competent orebodies, power consumption jumps exponentially.”

If a SAG mill can grind it, then an HPGR will also work in that application. “Hard ores that are really abrasive and competent might give SAG mills trouble to the point where you might end up with crushers on both ends of the SAG mills,” Dziedzina said. “That’s where the HPGR really shines.”

The one thing that customers are starting to look at with HPGRs as well as with vertical roller mills, which is another product that FLSmidth is migrating from cement to mining, is that neither one of those tools use grinding media, Dziedzina explained. “Manufacturing 1 ton of grinding balls generates about 1.9 tons of CO2. The consumption of grinding media at some large concentrators could generate 100,000 tons of CO2 per year,” he said. Beyond the environmental aspect, a considerable amount of capital is invested in storing and handling grinding media.

Dziedzina said he noticed more interest in vertical roller mills at the 2019 SAG conference. “Up to that point, we were talking with customers about them, but they didn’t seem interested,” Dziedzina said. “There was a lot of buzz about vertical roller mills with some of the heavy hitters from that conference, and ever since then there has been increased interest in it.” Dziedzina thinks interest will only build as mining companies look for more sustainable ways to process materials.

HPGRs were a surprising change after 70 years of traditional milling and maybe vertical roller mills will be that next step, he said. “It’s something that’s near and dear to our hearts, of course, because FLSmidth has a long history with that product,” Dziedzina said. “On the cement side of its business, it’s the king of comminution. As with the HPGRs, it’s a very efficient grinding tool. It has some real advantages for consolidating the plant footprint, but there’s concerns over the wear surfaces, some of the exact same concerns miners had with HPGRs in the late 1990s, which is why it took 10 years to break through into mining.” FLSmidth supports three models of vertical roller mills, but the OK mill is the one they evaluated as the best candidate for mining.

As far as HPGRs, any significant improvements will probably be evolutionary. “HPGRs have a bit of reputation for creating a coarse product compared to grinding mills,” Dziedzina said. “In cement, however, it’s used to grind extremely fine, down to 40 microns. In a dry grinding process, you could use the HPGR in a closed circuit with air classifiers to replace the SAG mill and the ball mill in a flowsheet.”

It’s widely accepted that by replacing the SAG mill with an HPGR, the energy savings would be in the range of 17% to 20%, Dziedzina explained. “If an air classifier could be used to eliminate the ball mill, the efficiency would increase quite a bit, but high tonnage air classifiers haven’t really been proven yet,” Dziedzina said. “It’s close. There is a plant in Australia that will be using HPGRs in a closed circuit with air classifiers, and that will have a real impact on the industry.” That Australian installation is not using FLSmidth HPGRs.

For the HGPR application in a closed circuit with an air classifier, the ore would have to be dried and the plant would have to generate heat to dry it, and the efficiency number begins to drop pretty quick. With cement applications, where this is proven, they always have a hot air source from the kiln. “You might be able to do the same thing with a roaster at a gold mine,” Filidore said. “I can’t believe a gold miner would ever want to buy another ball mill for a roaster. In the future, they would buy a vertical roller mill because they already have the hot air source.”

“With energy savings, however, many of these studies fail to look at the full plant connectivity,” Dziedzina said. “They don’t normally include the conveyors or heating the plant. They usually do not consider all the extra material handling. It’s easy to compare the SAG mill to the HPGR and the extra cone crushers, and forget all the material handling and the extra people and the larger footprint. Energy savings is probably 30% at the best and probably closer to 15%, which is still a lot.”

Process-wise, Dziedzina sees rotating side plates as potential area for improvement. Referring to the situation as the The Game of Flanges, he likens it to a great race to see who can get rotating side plates or flange rolls to work effectively. “Everyone has tested them and everyone has tried their version of flange rolls, both OEMs and roll suppliers, and everyone knows that there’s a process advantage to not using static cheek plates, but there’s challenges in getting them to last,” Dziedzina said.

A huge amount of pressure builds at high press forces and pushes the flanges off. The pressure between the rolls is high, but it’s actually the lateral forces that push material toward the edges. That material starts to push on the flange, bending it, then a gap forms and material begins to pack into it with a jacking, wedging effect. It just keeps going until the flange fails or the fastener fails.

“People use scrapers to clean the corners, which is a great idea, but then the scraper wears,” Filidore said. “In three months, you have to change the scraper and it becomes another wear item. When you have an HPGR roll that lasts 10,000 hours, you want the flanges and cheek plates to last that long too, but they don’t. You either get a build-up in the corner and the thing just pops off or you can get skewing.”

Acknowledging they have their deficiencies, Dziedzina said cheek plates are super easy to change. “A pair of cheek plates can be changed in a couple of hours,” Dziedzina said. “If a flange breaks, then you’re talking some serious downtime taking the machine apart to access the rolls.” The solution, Dziedzina explained, is a physics problem that amounts to finding a way to relieve that stress or divert it somewhere else. 

Grinding as a Service

Weir Minerals has a large installed base of its Enduron HPGRs. “We are commissioning several installations and we anticipate to have more than 100 units in operation within the next 16 to 18 months,” said Bjorn Dierx, global product manager, Enduron HPGR, Weir Minerals. The company’s HPGR portfolio ranges from pilot machines to large format production machines, capable of processing more than 118,000 tons of ore per day.

Weir gained much of HPGR experience from iron ore grinding applications. More recently, other miners have expressed significant interest in these systems, especially large copper and gold operations where the orebodies are more difficult to process. “Of the last five major projects, which we have been fortunate to get the awards, three of them were copper,” Dierx said.

In the last three years or so, the mining industry has been looking at doing things a little bit differently, as they have realized the importance of becoming more energy efficient and consuming less water, Dierx explained. “ESG strategies are top of mind these days, with a good example being Weir Minerals’ HPGR and air classification partnership with Fortescue in Australia,” Dierx said. “With declining ore grades, there has been a natural push toward new approaches to meet these goals and maintain project economics.”

Dierx also admits that commissioning these new HPGR projects has been a challenge lately. Supply chain disruptions and travel restrictions have made it difficult to get all of the resources on-site. “Despite these disruptive hurdles, we have found new ways to keep people safe and support our customers during project execution,” Dierx said. “We have been able to comply with and set industry standards for COVID-19 protocols. Assembly teams in some cases are isolated from office teams, however, digital factory acceptance tests allowed the customer a “live experience” while being remote and special attention was required to mobilize the service  engineers to site safely. This relates not only to HPGR installations, but also the overall project commissioning. In some cases, mines have been forced to push commissioning back.”

With low energy consumption, low maintenance requirements and high availability, HPGRs have proven themselves in grinding circuits. “Existing operations are looking to become more effective, not only in the reduction of their all-in sustaining costs, which are rising due to the global supply chain disruptions, but also how they can actually get more tons through their existing processing plants,” Dierx said.

Comparing the energy savings between traditional milling and HPGRs, which is known for the fines generation, we often see a 25% direct savings, Dierx said. “Looking at the entire picture, especially in terms of reducing recirculating loads downstream, the savings could be as much as 40%,” Dierx said.

A Weir Minerals Enduron HPGR installation at Eldorado Gold’s Kisladag mine in Turkey.

The Enduron HPGR has been designed to maximize grinding pressure effectiveness and therefore size reduction at any feeding condition through the use of dynamic skewing. The company said its unique bearing design makes this possible. It also prevents bearing failure and enables the rollers to be quickly exchanged within a 24-hour shutdown.

“The fundamentals of our HPGR system are based around a very sustainable bearing arrangements that consists of multiple roller bearings with a separate thrust bearing as well,” Dierx said. “Since, we are using higher press forces, our system requires bearing arrangements to be more rigid. Without a rigid bearing arrangement, you would have very poor availability.”

Some HPGRs use large lateral walls, or cheek plates, to reduce the materials exiting the high pressure area horizontally without being properly crushed. Cheek plates, when self-adjusting, also accommodate effective roller sealing during skewing. The Enduron HPGR uses a spring-loaded lateral wall that skews alongside the rollers, maintaining a distance of 1 mm to prevent particles falling out the sides unreduced.

An HPGR has to be designed to withstand high pressures. “We are processing more and more material through larger machines,” Dierx said. “Initially we used a 2.2-m roll diameter and then 2.4-m roll diameters, and now we’re going 2.6- and 3-m diameter rolls, while staying true to our length-to-diameter ratio to minimize edge effect. It’s all about getting the right design and control philosophy to achieve the highest level of availability and maximum size reduction with the least amount of energy.”

Dierx does not recommend using non-OEM rolls. “The materials we select for our rolls, which includes the tungsten carbide pins that are distributed across the roll, have been specifically cured for the application,” Dierx said. “These rolls are designed to withstand high forces and provide consistent, even grinding profiles. Uneven wear across the processing surface will create fluctuations and severe vibrations in the hydraulic system. An uneven wear profile also leads to poor size reduction in that area and recycling product increases operating costs.”

If miners are looking for more predictability, especially with their balance sheets, they should use OEM parts and ask about service agreements, Dierx said. Weir Minerals, as an example, offers a monthly service fee with a quality and tonnage guarantee, essentially derisking the entire operation. Think of it as grinding as a service.