A semi-autogenous grinding (SAG) mill is the largest piece of capital equipment and generally the focal point of a mining operation. This great big mill stands at the beginning of the comminution circuit, in which ore goes through various grinders, crushers, etc., to break the rock down into smaller, more useable pieces. With large motors rated to tens of thousands of horsepower, large slowly rotating bearings and variable-speed drives, it’s the largest energy consumer in the mine and also tends to be the production facility’s bottleneck.
It’s also the focal point for a solid case made for implementing advanced process control (APC) in mining—specifically the ore comminution process.
With recent technical advancements, APC is no longer the technical hurdle it once was in the mining industry even five to 10 years ago. There was still a lack of confidence then in what APC could do for mining, according to Robert Zwick, president of Automan Controls. “The new players were in oil and gas, and then it migrated more to chemical and pulp and paper,” he said. “Now we’re seeing it as really something in the mining industry too.”
APC strategies simultaneously target the top four major mineral concerns: energy efficiency, recovery and grades or extraction ratio, throughput rates, and operational efficiency and process stability. And Automan Controls—which delivers operations technology projects with a focus on heavy industry—has seen the benefits that APC can bring to optimize the performance of plants and their processing equipment.
With mine project costs growing out of control, APC enables mines to move operating points closer to their limits, which means getting more out of the capital expenditure, Zwick explained to attendees of Schneider Electric’s recent Foxboro User Group conference in San Antonio, Texas.
New mines are getting larger and larger, Zwick noted, with a typical mining project costing $5 billion to $6 billion. Mines are getting increasingly remote as well, and infrastructure costs are pushing up the capital equipment expenditure. “Once a capital project is finished, management starts looking at opportunities to improve production and availability of the plant,” Zwick said.
APC uses models to reduce the variability of a process. You’re then able to change set points to a targeted constraint. “You’ve got the ability to find interactions between loops as well as the ability to control multiple set points at a time even if the process has large lag times or dead times,” Zwick explained. “It’s a very powerful method.”
APC enables you to operate closer to whatever your constraints are—whether they’re related to engineering, physical limits, processes, environment or other factors. “You’re able to run closer to a variety of constraints to a wide variety of factors,” he said. “Once you stabilize your process, you’re able to change it so you’re running closer to the limits.”
Automan Controls uses Aveva’s SimSci APC, which is model-based predictive control software that Zwick said is “quite easy to use.” He also referenced its large installed base in heavy industry, including oil and gas, pulp and paper, and mining.
In a comminution circuit, which continues on from the SAG mill through various other grinders and crushers, there are a number of KPIs to measure performance: grinding rate, specific energy consumption, recycle rate, particle specifications, engineering design constraints, Pareto constraints, and overall equipment effectiveness (OEE). “OEE numbers are surprisingly low,” Zwick commented, noting that best in class is 80-85 percent, with 70 percent being more typical. “There are all sorts of software tools that are being made available. But at the end of the day, they’re all being used to bump up that OEE.”
An important way to measure the benefits of APC is through the use of a net present value (NPV) analysis, which more accurately shows the payback from an improved mean time between repair (MTBR) or mean time between failures (MTBF). It can also be used to capture investment opportunities related to OEE. “ROI is a common number to justify projects, but it doesn’t capture everything,” Zwick said. “NPV is used commonly in investment strategies on Wall Street. It steadily discounts future cash flow as the time horizon goes out in the future.” This is done to adjust for the risk of an investment opportunity and to account for the time value of money, he added.
NPV, which is determined by calculating the costs and benefits for each period of an investment, presents a more accurate picture for assets with a finite life. This fits well with mines, Zwick said, which typically have a finite life of 25-30 years.
One example Zwick showed of APC’s ability to extend the lifetime of a plant without a major shutdown focused on the liners that have to be replaced from time to time in the SAG mills. Not only does the liner cost about $5 million itself, but any time it has to be replaced means taking the equipment offline as well. The ultimate goal, of course, is to maximize the fresh feed throughput.
There is a whole slew of variables that can affect that throughput as well as the wear and tear on the liner. Operators will typically adjust a variety of manipulated variables to achieve their goals—increasing or decreasing the feed rate, changing water addition or adjusting mill feed density, or changing mill speed. They will not be able to achieve the goals as well as APC models can, however, which can take into account several significant process control constraints.
Zwick laid out his proof of APC’s impact in detail. But a key benefit was an MTBR increase of 25 percent. Through his NPV model analysis, he could show a significant cash inflow at about year five with the improvement in MTBR. A simple payback or ROI model doesn’t fully capture the economic benefits of technologies that improve MTBR, he noted. “NPV is a way to communicate some of these benefits that normally aren’t looked at.”
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