Stabilising the SAG at St IvesControlLogix joins forces with advanced process control algorithm, ‘Manta Cube’, toachieve operational stability on a closed-loop single-stage SAG mill at Western Australia’sGold Fields St Ives gold mine.

SolutionsThe new control solution comprised:

• 7 Allen-Bradley ControlLogix multi-disciplinarycontrollers

• 7 RSView SE HMIs

• ControlNet and EtherNet/IP communications

• ‘Structured Text’ code development andFunction Block programming

• Independent globalised tagging

To provide ongoing technical support, the solutionalso comprised:

• TechConnect support program

ResultsThe Rockwell Automation control solution inconjunction with Manta Cube control algorithmenabled:

• Management of control systems using familiarstandard control methods

• The SAG mill to exceed its design throughput of550 dry tonnes per hour

• A 6.1 per cent increase in treated orethroughput with less labour

• Hydrocyclone pump failure to be reduced to 25 per cent

• Stable and sustainable SAG operation

‘Expert control’ is an area of process control often shrouded in mystique.Typically used to control complex multi-variable process applications,these so-called ‘knowledge-based’ control systems are founded on a widerange of predictive, adaptive and learning algorithms.

Yavuz Atasoy, Metallurgical Superintendent at the Gold Fields St Ives minein Western Australia, holds strong beliefs about the use of any controlsystems – expert or otherwise. “We hear many stories about sitesimplementing ‘trial’ expert control systems that ultimately spend much oftheir time switched off by the operators,” he says. “We believe that anexpert system utilisation should be 100 per cent as a minimum!Acceptance by the operations team is the key issue here. Our view is that ifthe controller is really doing its job, it should never be turned off.”

Atasoy, who heads up the mine’s Lefroy gold processing mill, is wellqualified to make this comment. He and his team have recently deployedan advanced multi-variable control system to control the Lefroy mill’scentrepiece – its 11m-diameter semi-autogenous grinding (SAG) mill. Akey indicator of the success of this deployment is that the new controlsystem runs continuously. The mill operators demand and depend on it.

Developed on the Rockwell Automation® ControlLogix® plant automationplatform, the SAG mill’s new control system uses a unique process control

Gold Fields St Ives SAG mill main feed conveyor with classification tower and hydrocyclone classifiers on top, a key element ofthe comminution circuit.

algorithm called the ‘Manta Cube’.Developed by South Australianprocess control engineering group,Manta Controls, the Manta Cube is aninnovative alternative to thetraditional expert system. At theLefroy mill, the Manta Cubeaccurately monitors and responds tothe dynamic behaviour and multi-variable nature of the SAG mill and itsassociated processing circuit. Tomake this happen, Manta Controlshas taken advantage of the uniqueadvanced process controlfunctionality of the mill’s existingControlLogix platform, coupled withthe open nature of the ControlLogixdevelopment environment.

Closed-loop SAG Located at Kambalda, 80km south ofKalgoorlie, the Lefroy mill comprisesfour main process circuits:comminution, gravity separation,leach/adsorption, and carbonhandling/electrowinning. Thecomminution circuit largelycomprises primary crushing, coarse-ore storage and feeding, a bank of tenhydrocyclone classifiers, and the SAGmill – an enormous tumbling millmounted on load-cells and poweredby a 13MW wrap-around drive.

Control-system wise, the entireLefroy mill is handled by a RockwellAutomation Integrated Architecturesolution, comprised of sevenseparate ControlLogix systemscoupled with RSView® SupervisoryEdition™ (SE) PC-based human-machine interface (HMI). End-to-endplant wide connectivity is achievedthrough the NetLinx open networkcommunications architecture –primarily ControlNet™ for controller-to-I/O rack connectivity, andEtherNet/IP for peer-to-peer andcontroller-to-HMI connectivity.

The prime objective on any SAG milloperation is to adjust the variousprocess parameters (mill in-feed, SAGmill speed, cyclone operations andpressure, and so on) to ensure that theSAG mill weight remains as close aspossible to the target weight.Maintaining the mill weight to set

point as close as possible ensuresprecision control of the key parametersthat impact on mill throughput:grind/particle size, product density andtonnage throughput. In short, itimpacts directly on the productionbottom line.

Post commissioning, Atasoy notedproblems with the SAG mill’soperations – both in the way it wasoperated, and the throughput. “Withan open-circuit SAG, operation isrelatively straightforward,” Atasoy

says. “The challenge here at Lefroy isthat the SAG mill is in closed loopwith the cyclone underflows and thepebble crusher. We also don’t have aball mill, so the SAG mill does all thegrinding. As a result, it is a verychallenging process control situation.”

Atasoy noted a lack of stability in theoperation of the mill, and a criticalneed to micro-manage the milloperations. “We don’t blend the oreto make the feed uniform at Lefroy,”he says. “We have the ore feedcoming from different sources, whichmeans they have different hardness,mineralogical compositions andgrades. This coupled with theunpredictable nature of the SAG millsclosed-loop feed, made it verydifficult to control the SAG weight ona sustainable basis.”

The inherent instability impactedseverely on the operations procedureand mill throughput. “The milloperators had to keep a constant eyeon the grinding circuit to ensurecontinuity of the operation withoutcreating spillages or stoppages. Theyneeded to be there all the time,playing with the operationalparameters to achieve target KPIs,such as grind size, tonnes per hourand density” says Atasoy. A furthergoal was to achieve SAG mill designthroughput – 550 dry tonnes per

hour – in a sustainable and stablemanner. “We were occasionallyachieving 550, but it wasn’t stable.We really needed to see stability,even with the disturbances that werecoming into the mill,” he says.

Open platform;collaborativedevelopmentAtasoy approached managingdirector of Manta Controls, JohnKarageorgos, to discuss potentialsolutions for the mill. High on his listwas a system that would not onlyachieve the SAG mill stability goals,but would also win wide acceptancewith the operations andmaintenance crew.

Yavuz Atasoy, metallurgical superintendent at the Gold Fields St Ives mine (standing), and Craig Waywood, programmer/commstech: impressed that the Manta Cube algorithm could reside on the existing ControlLogix platform.

The Manta Cube impressed Atasoyand Lefroy mill programmer/commstech, Craig Waywood, first andforemost because it could reside onthe existing ControlLogix controllerplatform. “We were really attracted bythe fact that we could apply MantaCube step-by-step and customise itto our conditions,” Waywood says. “Bybeing embedded in the openplatform ControlLogix controller, itallowed our people to work closelywith Manta Controls and be involvedin the Cube’s development.”

The powerful Manta Cube is made upof four distinct parts: ‘the Cube’, whichdetermines the SAG mill operatingmode; the Cube ‘expert decisionmatrix’, which determines what isrequired to return the SAG mill to therequired operating band; the Cube‘engine’, which handles the coremulti-variable control and decouplingtechniques; and the Cube ‘optimiser’,which optimises the key controlobjectives, such as SAG mill output.

Karageorgos explains thatapplication of the Cube algorithm

The Lefroy mill’s centrepiece is the 11m-diameter SAG mill, which utilises anadvanced multi-variable control systemfounded on ControlLogix.

demands specific high-end processcontrol functionality. “We need twodevelopment tools to make the Cubehappen: a high-level text-based codedevelopment environment, plus arich library of pre-configured processcontrol function blocks. We cannot

do without either of these,” he says.The need for a text-based codedevelopment environment is met bythe ControlLogix platform’sStructured Text offering. This high-level language allowed MantaControls to tailor the Manta Cube’s‘optimiser’ routines.

The need for process control functionblocks was easily met by

ControlLogix’s process FunctionBlock library – a broad library of over50 pre-configured process controlinstructions, such as the PIDenhanced (PIDE) controller, lead-lag,logical instructions, alarms,select/limit instructions, scaling,

multipliers and so on. These blockswere essential to allow MantaControls to build the Cube’s ‘engine’.

In all, Manta Controls spent aroundone and a half years at the site, firstinstalling the Cube, then ‘retuning’the mill controls. “We wanted it thatway,” says Atasoy, “We wanted ourpeople to follow it all, step-by-step.”Manta Controls installed the first

“I was aware that ControlLogix could handle processcontrol, but unfortunately, these functions weren’tused in the mill at initial commissioning... I’ve beenpleasantly surprised by ControlLogix functionality,most particularly the functionality of the PIDE block.”

Cube on the SAG mill in mid-2004,then, when things started to stabilise,began work on the remainder of themill loops. “It’s like tuning a piano,”Karageorgos says. “You have to besystematic.”

New age process controlSurprisingly, neither Karageorgos norLefroy’s Waywood had workedwith the ControlLogix platformprior to the Lefroy SAG millupgrade. Both were wellexperienced in process control,and both were long-term usersof distributed control systems(DCS). “I was aware thatControlLogix could handleprocess control, butunfortunately, these functionsweren’t used in the mill atinitial commissioning,” saysWaywood. “I’ve beenpleasantly surprised byControlLogix functionality,most particularly thefunctionality of the PIDE block.”

Both Waywood and Karageorgospoint to the ease with whichControlLogix met accepted processcontrol operational protocols: it has aglobalised tag system that isindependent of controller and I/Ohardware, advanced process controlfunctionality such as PIDE control,and seamless connectivity with theRSView SE HMI platform.

Karageorgos found the transitionfrom DCS to ControlLogix a verypositive move. “ControlLogixprovided the required flexibilityduring Cube development,” he says.“By contrast, the legacy DCSsdemand a certain structure –deviation from the structure issometimes not possible.”

Publication PROCES-AP021A-EN-P – December 2008 Copyright ©2008 Rockwell Automation, Inc. All Rights Reserved. Printed in Australia.

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A case-in-point he cites is achievingbumpless transfer duringcascade/auto switching on a cascadeloop configuration involving acalculation between the inner loopand outer loop. “You can’t do this onsome legacy DCSs, but you can withthe ControlLogix PIDE function block.You simply tell the block what valueto track,” he says.

He also points to the ease ofconfiguration using the RSLogix™5000 configuration software. “It’s neatand easy to use. It actually lays out theprogram so that you can easily seewhat is going on in terms of fault-finding. It’s nice to work onControlLogix—using the ControlLogixStructured Text is a lot quicker thanusing the equivalent coding systemsof legacy DCS,” he says.

Positive move, positivemoodThe success of the Lefroy SAG millManta Cube is immediately obvious.The stability realised in throughput,

density and product grind size – evenunder significant productiondisturbances – has impacted rightthrough to the Lefroy leach circuit.The SAG mill now exceeds its 550 drytonnes per hour design throughput ina stable and sustainable manner, andits weight histograms indicate a post-commissioning narrowing of the millweight operating band of around 40

per cent. The overall processstability extends well beyondcomminution to the entire mill,resulting in a measurable 6.1per cent increase in treated orethroughput. All this can bedirectly attributable to theinnovative Manta Cube controlalgorithm.

While these metrics areimportant, the acid test for anycontrol system upgrade is,according to Atasoy, how wellaccepted it is in the controlroom. “The mood in thecontrol room is greatly

improved,” he says. “Theoperators don’t have to sit and playwith the set points all the time. It’sbasically a hands-free operation now,and operators are free to focus onother parts of the circuit – not justthe SAG mill. We are producing moretreated ore with less labour.”

“ControlLogix gives us everything weneed,” says Karageorgos. “It’s a vendorsupported product backed by acompany with global reach – this isan essential for us. The Manta Cube isnow seen as a real alternative toconventional expert control, and weare currently fielding queries fromaround the world. It’s reassuring toknow that wherever Manta Controlsis in the world, we can rely onRockwell Automation.”

RSView Supervisory Edition (SE) software provides the user interface.