Presentation to The Mill Optimisation Summit 2011 Presented by: Paul Wilson Technology Manager...
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Transcript of Presentation to The Mill Optimisation Summit 2011 Presented by: Paul Wilson Technology Manager...
Presentation to The Mill Optimisation Summit 2011
Presented by:Paul WilsonTechnology Manager Calibre Automation, Communications & Technology Group
Optimising SAG mill throughput:A case study in tuning
Two SAG mills
1
• Porgera Mine, PNG
• 4.5 Megawatt, variable speed drive
• About 500 tonnes per hour per mill
• Highly variable lithology with grinding factors from 6 to 18 kilowatt hours per tonne
The mills
2
The mills
3
The mills were often unstable, seen as oscillations in the feedrate trend graphs
Unstable behaviour
4
Loss of production
7
As much as 15% on bad days
Up to 380 ounces of gold per day on bad days
At $425 US per ounce = $160,000 per day
You could hire a very good plant operator for that kind of money
Natural instability
8
Poor tuning causes natural instability
Operator-caused instability
8
Poor operator skills also forces instability
The control system
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Highest levelMinnovex expert system control
Optional top level controlConstraint control
Mid-levelClosed loop control
Bottom levelDelta-V distributed control system
The Minnovex expert system
9
A fuzzy rules based artificial intelligence system
Running on a G2 expert system shell
Running on a Windows NT PC platform
Takes data from, & feeds setpoints to, the loop controllers on the Delta-V
Performance comparison
9
Expert system control is far better than poor operator control
Why mills go unstable (#1)
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Mills stall (bog or centrifuge). The behaviour at maximum throughput is highly non-linear
Why mills go unstable (#2)
9
Dynamic behaviour of a mill is type 1
Control engineers recognise that type 1 systems are more likely to be unstable than type 0 systems
Caused by the inherent integration in the mill transfer function
Mill load (level) is the integral of the nett feedrate
level = ∫ (Qin – Qout).dt
This induces a -90o phase shift in the transfer function which leads to reduced stability
Why mills go unstable (#2)
10
Why mills go unstable (#2)
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The integration causes a phase shift
Simulated integral response
11
1. When a mill stalls it stops working. 2. The mill fills with unground material.3. It takes time to grind out the rock and get the
outflow going again
So: the control system / plant operator must be:
PATIENT
Why mills go unstable (#3)
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The result of operator impatience
13
Mill under tuned expert system control
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Section A to B is the maximum speed of recovery to prevent stalling the mill again
Expert system recovers from a motor overheat event
13
The result
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• In 2004 / 2005 mill production rose from850,000 ounces to 1,000,000 ounces
• At $425 US per ounce that was $63.75 million US increase per year
Not possible without increasing SAG mill throughput
Extra energy used
7
The difference in energy usage between the unstable zone and the stable zone is:
the unstable zone averages
25% more motor energy per tonne of product
than the stable zone and produces
15% less product
Which adds 10% to the energy costs for the remainder of the processing plant
How was it done?
30
• Develop a mathematical model• Use trend plots and tests to characterise
the mill (find the characteristics of mill behaviour)
• Estimate fastest possible recovery times on the worst-case ores
• Retune the expert system rules for robust, always-stable behaviour
How was it done?
30
Block diagram model of mill behaviour
How was it done?
30
Transfer function of mill load to ore feedrate
Additional development
30
The decision to secondary crush the harder ores.
A secondary crusher was installed. With a bit of clever mathematics we were able to estimate SAG mill grinding factor at the primary crusher. We used this to feed some of the hard ore (GF > 10 kWhr per tonne) through the secondary crusher thus increasing SAG mill throughput on the harder ore.
Questions
31
Your questions are welcome
Plant characterisation & transfer function development is a complex process. I am happy to discuss some of the methods afterwards with anyone interested.