Grinding Technology Extends Horizons
With miners worldwide facing lower-grade feeds, fine-tuning energy-hungry grinding circuits is becoming increasingly important
By Simon Walker, European Editor
As Allan Boylston, director for process engineering development and sales at Metso, explained to E&MJ, “One main challenge we see for our mining clients in today’s markets is how to balance the increased production needed to meet a rising demand for metals, with the fact that ore grades around the world continue to fall. What this means is that even to maintain current levels of production, mines need to process more and more raw ore, which requires additional energy as they ramp up production.
“It also translates in many cases to higher investment in equipment and higher operating costs,” he went on. “In order to be profitable, mines have to really zero in on ways to improve operational efficiency. The big trends to address this are related to looking at the many options that can help to lower energy costs as production demands increase. It becomes important to look at improvement opportunities across the entire process to find ways to drive down costs.”
Putting this into the perspective of grinding technology, Boylston noted that there is now more openness to looking at new applications for older technology. “For example, high-pressure grinding rolls (HPGRs) have been around a long time in limestone, cement and kimberlite operations, but applications involving hard ores are relatively new,” he said. “With their higher energy efficiency, HPGRs are getting a second look as an alternative to more traditional grinding in certain applications.”
Meeting Technical Challenges
Boylston went on to list some other strategies that mines are employing to improve operational efficiency and making better use of energy — to drive down costs. These include improved blasting practices to reduce the feed size to the primary crushers, optimizing mill liners for longer service life and energy savings, and the increased use of advanced process controls to make operations more efficient and to reduce energy consumption.
His colleague, Håkan Ståhlbröst, Metso’s global application support manager for mill lining, added that there is a trend to replace small- and medium-sized mills by fewer, but significantly larger mills to handle the high capacities required. “Large mills require fast and safe maintenance, something we will continue to improve,” he said. “However, large mills also require large amounts of energy to operate, so I believe that the focus on efficiency will increase.
“Traditional horizontal mills are generally not particularly energy-efficient,” Ståhlbröst explained. “Only a very small part of the energy is transformed into efficient grinding, with the remaining energy channeled into liner and media wear, heat and noise. In terms of technology, there is not much that can be done with the mill itself except for the motor and drive, but it is often possible to control the process better.
Metso’s global consulting and labs manager, Suzy Lynch-Watson, highlighted mill liners as being one area where the company has been focusing technological development, citing the example of its Megaliner, made from rubber and steel. “The liners can use less material than previously since we use a ‘skip row’ design, where the liners now have only twothirds of the number of lifters as before,” she said. “Using this design, we can still achieve the same lift and charge trajectory but with less liner weight compared to steel lining. The lower weight can make it possible to load more in the mill, and together with an optimized design, you can produce more tons, and in some cases, increase efficiency.”
Looking beyond products, Lynch-Watson explained that Metso has also made technical improvements by combining its services to find solutions that help clients achieve lower energy and higher production levels. “Of course, the grinding mill itself needs to be assessed, but the process steps before and after the mill all play a part in driving the overall performance of the circuit,” she pointed out.
Lynch-Watson added that plant operators are looking at different options in terms of equipment selection to keep energy costs in check as production demands continue to increase. “For example, we are seeing interest in Metso’s Vertimill stirred milling technology, particularly in cases where a plant needs extra milling capacity or wants to achieve a finer grind,” she said.
“In the past, plants would invest in a second ball mill to run in parallel with their current one to add the additional capacity, but this is not always an energy-efficient way of tackling the problem. Recent research by Malcolm Powell and Sam Palandiandy at the JKMRC showed that using a tertiary Vertimill to increase circuit capacity can be 25% more energy-efficient compared with adding extra ball milling capacity in a secondary grinding service.”
Broadening the Perspective
To find out how experts at FLSmidth view grinding technology developments and trends, E&MJ spoke to David Rahal, product manager for fine grinding; Jack Meegan, global product line manager for liners and wear parts; and application engineer Garret Barthold, who looked at the issues from the field perspective.
In response to a question on technical improvements that the company has made in recent years to improve the efficiency of its grinding mills, and to make them more energy-efficient, Barthold noted that FLSmidth recognizes that tumbling mills — such as ball and SAG mills — are high-energy systems and that their efficiency can indeed deteriorate over time. “We keep monitoring mills in the field in order to be able to see when the energy efficiency begins to drop off,” he said. “That way, we can provide operators with guidance to help them maintain their mills so as to be as efficient as possible.
“For example, when mill liners wear beyond a certain point, online monitoring lets us see when the efficiency goes down,” he said. “That’s not usually a sudden drop, unless there is an actual failure in the liner material. It’s more a question of countering wear with appropriate preventive maintenance.”
Meegan expanded on this: “We are always looking at liner profiles in order to maximize the mill’s efficiency for particle breakage,” he said. “It’s a case of always trying to stay ahead of the curve, and we already offer the technologies — such as acoustic monitoring and expert control systems — that can help operators understand better what’s actually happening inside the mill.
“Our acoustic monitoring technology is becoming used much more widely, especially in SAG mills, and we are looking at using existing sensor technologies more effectively as well as developing new sensors — taking a more holistic approach,” he added.
Turning to future trends in the way grinding equipment is designed and used, especially as throughput tonnages continue to increase, Rahal pointed out that existing technologies are increasingly being used in a wider range of applications. “Take vertical roller mills, for example,” he said. “These were developed for the cement industry but there is now strong interest in transferring the concept to mineral processing as well.
“Feeding the output from HPGRs into vertical roller mills is one possibility,” he explained. “Stirred mills used to be used for regrind operations; now operators are looking at running SAG mills together with stirred mills, which can be much more efficient than tumbling mills in the right conditions.
“Wear on vertical mill rollers is obviously an issue, but that is something that can be addressed by using materials that are appropriate to the abrasiveness of the ore being ground,” he agreed.
Vertical Roller Mills
Indeed, as Loesche — one of the world’s leading suppliers of vertical roller mills (VRMs) — explained to E&MJ, transferring VRM technology from the cement industry to mineral processing began quite some time ago. In a paper presented at Comminution 2016, organized by MEI Conferences and held in Cape Town in April 2016, Pieter Jacobs and others described 16 years of operating a Loesche VRM at the Foskor phosphate mine at Phalaborwa in South Africa.
The 5-m-diameter, four-roller VRM operates at a capacity of 580 metric tons per hour (mt/h). In 2010, Foskor installed a tertiary crusher followed by a conventional wet rod- and ball-milling circuit in parallel to the dry VRM system, allowing the company to make direct comparisons between the two in terms of wear, process flexibility and downstream recovery.
In summary, the authors reported that in trials, the VRM’s operating cost of ZAR21.90/mt handled was marginally lower than the conventional milling circuit at ZAR22.61/mt, although the VRM was not being used at full capacity. Its media and wear costs were markedly lower, at ZAR1.88/mt against ZAR2.86/mt, not including the replacement of the mill liners.
And apatite is not the only mineral on which Loesche has carried out VRM test work. In a presentation last November, the company’s Dr. Carsten Gerold noted that to date, 114 trials have been undertaken, on iron, copper, gold, phosphate, platinum and zinc ores, among a number of others. This work has taken place both at Loesche’s test center in Germany, and also using the company’s containerized mobile ore-grinding plant (the OGPmobile).
The first Loesche VRM to be used for grinding copper matte went into service with Kennecott Utah Copper in 1996, Gerold pointed out, with the company having supplied its VRM technology to every Outotec copper flash smelting and flash converting installation since then. A further five mills have been supplied to copper smelters in China over the past eight years. In another application, a VRM is used to grind tin-smelting slag for use in cemented underground backfill, while later this year, a 250-mt/h unit will be commissioned in Turkey to grind sulphide ore at a copper-gold mine there.
Benefits from using the VRM in this application, producing feed for flotation, include higher flotation kinetics, with non-oxidized fresh mineral surfaces that maintain the natural hydrophoby of the sulphides. The edgy particle shape produced in the mill is also beneficial, as is the steep particle-size distribution with less ultra-fines, all of which helps to improve the flotation performance with a greatly reduced reagent dosage, he said.
Gerold also cited work done in the early 2000s by Loesche and Anglo American to develop a dry, energy-efficient grinding circuit. Using zinc ore from Anglo’s Gamsberg project in South Africa (since sold to Vedanta), the research looked at various crushing and grinding combinations in terms of their energy requirement and effect on flotation performance.
Data generated from the testwork showed that using an AG/SAG/ball mill route to grind to a P80 of 65 µm used 21.26 kWh/mt of energy, while the combination of crusher, HPGR and ball mill was cheaper at 15.55 kWh/mt. For comparison, using a cone crusher before a VRM in airflow mode used 13.65 kWh/ mt, the cheapest option of all, with an HPGR/VRM circuit having an energy demand of 14.22 kWh/mt.
Putting Bad Practice Right
E&MJ asked what the panels typically see as being bad grinding practice when they visit mine sites, and what advice they give to help operators correct this. Rob Edwards, senior sales development manager, solutions function at Metso, noted that one key thing to remember is that even with the most modern and up-to-date equipment, performance in the field is still dependent on how the equipment is operated. “Most of the process bottlenecks we run into are not caused by the machines themselves but rather have resulted from the way the comminution equipment has been configured or is operated,” he said. “The four most common issues encountered revolve around the grinding media used in the mill, how the mill speed and liner angles are paired, relying too heavily on rules of thumb related to production, and finally a lack of communication between the mine and the plant.”
All three experts at FLSmidth were in agreement that the most common problem encountered is where the grinding media are allowed to impact directly on the mill liners, either because of an insufficient charge in the mill or through the rotational speed being too high. “This can lead to premature damage to the liners, and is a poor use of power,” Meegan stated. “Power, media and liners are the highcost items in mill operation, and by allowing direct impacts an operator is going to use more media and reduce the liner life.”
Rahal took a wider view. “It’s often the case that ancillary equipment is not being operated correctly,” he said. “It’s important to look at the whole comminution island, not just the grinding mills in isolation. “As an example, a recent plant optimization program allowed the operator to increase throughput from 1,400 to 1,700 tons per hour by adjusting the ball mill cyclone configuration,” he added. “Another possibility would be to introduce a pebble crusher into the circuit as a means of making the whole system more efficient.”
Metso’s Edwards looked more closely at grinding media. “We run into situations where the media is too large for the expected grind. The result ends up being high wear rates to both the liners and the media and a very inefficient use of energy. “You can also run into poor-quality media that splits or else wears in a way that results in disc-shaped media that does not roll as it should within the mill. This media consumes mill volume and power but the grinding is very inefficient due to the motion of the charge. All of these factors add up to inefficient use of energy. The solution to this would be to conduct ongoing calculations to ensure the optimal size of grinding media is used, and to keep a close eye on media quality,” he suggested.
Taking a Holistic Approach
As a summary, Rahal from FLSmidth commented that today, since many operators do not have funding available for new milling equipment, they are eager to get more out of what they already have. “Customers are asking us for support in making their existing equipment more efficient,” he explained, “and a lot of development work is now being focused on just that.
“There is a growing trend for operators asking for plant audits,” he went on. “They want to be able to use existing equipment better, even if that involves flowsheet changes. There’s also strong interest in digital data, the Internet of Things, and customers expect better data collection and analysis to provide them with a better understanding of how their equipment is performing and how to improve it further.”
Lynch-Watson (Metso): “Looking at the performance of a single mill or piece of equipment is not always the right approach. You really need to look at how all the different pieces of equipment in the circuit work together to find the most energy-efficient combination. It comes down to finding the right approach to the right application.”
Edwards: “A lack of communication between the mine and the plant can be classified as a poor grinding practice. A plant needs to have good visibility on incoming feed materials, in particular the ratio between coarse and fine run of mine ore, which can vary significantly. Without good visibility, the plant may not be able to react quickly enough to adjust its operating parameters and therefore lose efficiency and overall performance.”
Ståhlbröst: “Many companies are just trying to accomplish one thing at a time, lowering the price or increasing equipment longevity instead of considering how to improve the efficiency of the entire grinding process and reduce the cost per processed ton. In some cases, investment may be required, or making changes in the process or equipment to solve problems or achieve set objectives. “There really is no one-size-fits-all solution,” he explained.
SAG Mill Optimization
While trial-and-error was often the rule of thumb for mill optimization in the past, sophisticated modelling techniques can now provide operators with much clearer guidance. A paper presented at the SAG 2015 conference in Vancouver, organized by the SAG Conference Foundation, described how modelling had been used at First Quantum Minerals’ Kansanshi copper-gold mine in Zambia.
Following modeling of the circuits using JKSimMet software, the work involved running separate studies on how varying operating conditions affected the performance of the SAG mills. Ore characterization tests showed the sulphide ore to be the most competent while oxide ore was found to be the least competent. The authors noted that: “at the operational level, the average specific energy consumption associated with each circuit reflects the characteristic strength hierarchy observed amongst the ore types, while at the planning level the differences in installed power between the circuits shows the impact of the reserve variability on process design decisions. From the analysis of a series of surveys conducted on the three circuits, this study has shown that in addition to influencing equipment selection and design, operating strategies are profoundly influenced by the breakage characteristics of the ore processed.”
The authors concluded that: “for soft ores there is advantage in operating with high volumetric filling degrees in the SAG mill. Besides both being characterized as soft with respect to abrasion resistance, the mixed and oxide ores responded similarly to changes in SAG mill filling during operation. The mixed ore circuit experienced a 14% increase in throughput when the SAG volumetric filling degree changed from 21.9% to 37.2%, with little change in grind. Similarly, a 2% increase in volumetric filling for the oxide SAG mill, though small in comparison, led to a noticeable increase in throughput accompanied by a 10% increase in the sub-75µm fraction in the circuit product.
“However, for the medium to hard ore it appears there is advantage in operating the SAG mill with low volumetric filling degrees,” they said. “When the volumetric filling of the sulphide SAG mill was reduced from 33.7% to 26.6% the circuit throughput increased by 20%, though the final product grind coarsened as shown by a 16% decrease in sub-75µm in the circuit product. “Thus, the filling degrees at which the relatively softer ores perform well were around 32%-37% while the harder ore performed well around 26% volumetric filling degree in the SAG mill.”
In another paper presented at the same conference, Paul Staples from Ausenco took a critical look at why SAG mills are not always as successful in operation as the comminution circuit designers had expected. “There are several well-established methodologies for designing and predicting energy requirements of AG/SAG-based circuits,” Staples noted. “The recent problems originate either from unrepresentative sampling, poor testing protocols, inappropriate interpretation of data and definition of design criteria, wrong mill sizing methodologies, poor project management decisions or a combination of these.”
Staples went on to discuss three projects where SAG mill issues required subsequent rectification: Cadia in Australia, Sossego in Brazil and Phoenix in the USA. “Cadia’s issues were associated with scale up of pilot plant data,” he noted, “Sossego’s were similar but were compounded by liner and grate design issues, while issues at Phoenix were only overcome with the addition of secondary crushing prior to SAG milling.”
In addition, Staples identified five recent major projects that reportedly did not meet design throughput on start-up: Copper Mountain, Andacollo, Mount Milligan, Esperanza and Malartic. All of the projects mentioned, he went on, represent a minority of those developed using SAG mills in comminution circuits, while all of them treat ore of above-average competency, are generally of modest head grade, and are hence very sensitive to project capital and operating costs.
“The current skepticism within financial circles of the ability of consulting and engineering firms to predict SAG mill throughput should not be considered as a lack of sophistication of the available testing methods or the maturity of SAG milling technology,” Staples concluded. “Rather it reflects issues in properly applying the available techniques for prudent SAG mill design when processing competent ores, including benchmarking of performance.”