SAG milling is arguably the most commonly used method of comminution in mineral processing. So why are so many SAG mills operating sub-optimally and why are they considered to be so difficult to control?

Operators often consider the basic controls provided during commissioning are as “good as it gets.” Those that do seek improved performance often reach for a packaged, third-party solution. Few consider the capabilities that they have already paid for—in their PCS.

The third-party package is an expensive and unnecessary capital expenditure. It adds yet another computer system to the control ecosystem and requires integration, maintenance and expert proprietary knowledge.

Optimize underperforming assets and remove the need for capital expenditure: Optimal control of the SAG mill can be achieved by using existing DCS or PLC.

It is commonly assumed SAG mill control can only be successfully implemented using “black-box” solutions; this is just not true. Black-box solutions are referring to advanced control algorithms or solutions which are contained in a separate computer, interfaced to the PCS. Another common name is “off-theshelf” solutions, which is a misnomer since these systems require a considerable amount of setup and configuration.

SAG mill control fundamentally means managing the load in the mill by adjusting the feed rate of ore to the mill and/or manipulating the rotational speed of the mill. The best way to measure load is to mount the mill on load cells. The load cells are calibrated to measure the total weight of water, grinding media (balls) and ore in the mill, but not the weight of the mill itself. While many mills today are mounted on load cells, others employ bearing oil pressure as a proxy measurement of mill load.

Schematic illustration of a control strategy recently implemented on a large variable-speed SAG mill at MMG’s
Century Zinc operation in Australia.

In contrast to an expert or black box system, the control loop approach was built using standard blocks, connected as shown. This control strategy has been in operation since January 2007 and controls the mill with minimal operator involvement over 95% of the time. (For more information on this project, please contact the author.)

So, in summary: Why use the existing Plant Control System to improve SAG mill control? Consider these points:
• No capital expenditure.
• One system controls all of the plant (no split responsibilities).
• No additional learning curve for operations and maintenance personnel. Control strategies implemented in the PCS can be developed and maintained by existing plant personnel.
• No proprietary software or system requiring specialist support.
• Control loop structures degrade gracefully— this is a natural consequence of the loop “topology.”
• Many modern DCS systems are equipped with a third or fourth generation computer language that can be used to implement complex logic structures which human operators perform.
• There is no need to deal with interfaces to separate computers. Complexity is reduced while robustness is increased.

Andrew Thornton (andrew.thornton@ mipac.com.au) is a mineral processing and advanced process control expert and MIPAC’s principal process control engineer.

The Twiflex braking system shown here can bring this large mill to a full stop in less than two seconds. Mill Drives and Brakes Muscle Up Recent years have seen a trend toward fewer comminution machines per process line, particularly as grinding mills have increased considerably in size. Today, autogenous grinding (AG) mills as large as 40-ft in diameter with motor power in excess of 20 MW are being ordered and installed as producers look for higher production and economy of scale in their process plants. Semiautogenous grinding (SAG) mills up to 44-ft diameter with 35-MW gearless mill drives (GMDs) are available. In January, for example, a 1,210-ton AG mill—considered to be the largest ever built—was shipped from the CITIC Heavy Machinery plant in China to the Sino Iron project in Australia. The mill, the first of five to be installed at Sino, has a diameter of 12.2 m (40 ft), is 15 m (49.2 ft) long, and will be powered by a 28-MW Siemens GMD. Siemens also supplied the gearless drives for a 36-ft SAG mill with a rated power of 15 MW and a 26-ft ball mill with 15.5 MW of rated power recently installed at the Minera Los Pelambres copper operation in Chile. These large, powerful gearless drive mills require improved control systems, ranging from the sophisticated controllers that monitor and protect the mill’s ringmotor and frequencyconversion electronics from electrical damage, to the braking systems that provide safe, controlled stops. And, as these larger units replace multiple smaller mills and are often installed at sites in remote locations, reliability and operational stability become increasingly critical factors in achieving production targets. In order to improve mill performance ABB, in 2009, replaced its PSR GMD controller with the AC 800PEC version, which it has used since 2001 in converters and drives for power transmission, excitation systems for motors and generators, and wind power applications. This upgrade, according to the company, provides a number of improvements in reliability, service availability and hardware simplification, as well as faster testing and commissioning procedures. One of the principal improvements offered by the new controller is “power ride-through.” If the main power supply voltage drops below 85% of normal, for example, or if a phase angle deviation is detected, the controller can activate the ridethrough function, which immediately ramps down the stator current to zero, followed by thyristor pulse blocking. The rotating mass of the mill load and the rotor inertia keep the mill turning during the ride-through period, which is adjustable from 0 to 1 second, with 200 ms the default. As long as mill speed remains above a specified level during the ride-through, the system will release the thyristor pulses and will power the mill slowly up to the previous speed when supply voltage returns. If the voltage doesn’t recover within the specified interval or if mill speed drops too much, a fast shutdown is performed and the system is disconnected from the electrical network. Stopping large mills—either as part of a planned event or in an emergency—also requires upgraded equipment. U.K.-based Twiflex Ltd., which specializes in providing advanced braking technology for industrial applications, recently delivered their largest grinding mill braking system to Metso Minerals Industries, Inc. for installation on gearless grinding mills installed at Boliden Mineral’s Aitik mine in northern Sweden, the largest open-pit copper mine in Europe. An expansion program (Aitik 36) will raise production capacity from 18 million mt/y to 36 million mt/y when completed this year. The project includes a wide range of processing equipment at the new Aitik concentrator, ranging from the large grinding mills to 52 Outotec TankCell flotation machines, three pressure filters and molybdenum and gold recovery circuits. At the Aitik concentrator, two 22.5-MW GMDs supplied by ABB power 38-ft (11.6-m) diameter x 45-ft (13.7-m) long AG grinding mills. Twiflex said its braking system is designed specifically for grinding mill installations and provides both static and dynamic braking functions. In static operation, the braking system holds the mill motionless during liner replacement and general mill maintenance. For dynamic operation, the system can operate in two modes, stopping the mill from full speed in an emergency or providing inching/creeping capability in the event of bearing lubrication problems or power failures. These modes, respectively, require steady, controlled application of the brakes as well as quick activation to provide the accurate stoppage required by the operator. For the Aitik project, Twiflex supplied eight VMS-DP brake calipers with pedestals plus a hydraulic power pack for each mill. The VMS-DP caliper has an adjustable braking force from 590 to 737 kN and is a floating-spring applied, hydraulically retracted brake suitable for disc/flange thicknesses from 117 mm to 130 mm. The brake can be used on installations with a braking path of at least 7.6-m outside diameter or larger, and 7-m inside diameter. The hydraulic power pack offers versatile brake control as it allows both local and remote operation for inching and creeping duties through a control panel. According to Twiflex, the braking systems installed at Aitik can generate up to 38 MNm braking torque for each mill, acting on a 12.97-m flange diameter. The VMS-DP calipers weigh 1.85 mt each and are able to deliver 940 kN clamping force. The system is able to stop a mill in less than 2 seconds.