optimisation of a sag mill through trajectory and power ... · pdf file-mill filling -ball...

OPTIMISATION OF A SAG MILL THROUGH TRAJECTORY AND

POWER MODELLING

Paul Toor

© Metso © Metso

- Problem Statement

- Case Study

- Trajectory Modelling

- Power Modelling

- Identifying Optimum Region of Operation

- Results & Benefits

- Summary & Conclusion

September 2015 Metplant Optimisation of a SAG Mill Through Trajectory and Power Modelling 2

Presentation Summary

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Mill performance is very sensitive to total filling.

September 2015 Metplant Optimisation of a SAG Mill Through Trajectory and Power Modelling

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Powell, M.S., van der Westhuizen, A.P. & Mainza, A.N. (2009). Applying grindcurves to mill operation and optimisation. Minerals. Engineering, 625–632.

Problem Statement

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September 2015 Metplant Optimisation of a SAG Mill Through Trajectory and Power Modelling

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However no mill filling

percentage is reported to the

control room, instead a

calculated mill load is

provided.

Problem Statement

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The reported mill load is a arbitrary measure and is comprised of liner weight, the charge weight plus an offset. The inclusion of liner weight is particularly problematic as it is dynamic.


Load Reading= Charge mass + Liner mass + Offset

Problem Statement

© Metso © Metso September 2015 Metplant Optimisation of a SAG Mill Through Trajectory and Power Modelling 6

Toor, P., Franke, J., Powell, M.S., Perkins, T., (2011), Quantifying the Influence of Liner Shape on Mill Performance. Proceedings International autogenous and semiautogenous grinding technology 2011, Sep. 25-28, Ed. Flintoff et al, Published CIM. .

Problem Statement



Problem Statement

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Ball trajectory is another critical parameter. •Overthrow causes liner damage and reduced efficiency.

•Underthrow causes damping of grinding action also leading to reduced efficiency.


Problem Statement

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The assessment of ball trajectory cannot be done in isolation as the toe of the charge needs to be determined to make any meaningful assessment


Problem Statement

Presenter

Presentation Notes

Thus both total percent mill filling and mill trajectory are critical parameters; however neither is reported or available to metallurgy or processing teams in real time or a regular basis.


Proposed Methodology

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•SABC

•Fixed Speed

•F80 50-100 mm (Dependant on ore source).

•Ball Top Size 125 mm

•Very little autogenous grinding. (BAG Mill)


Background Case Study

Presenter

Presentation Notes

Fine Feed, very little autogenous grinding occurring. BAG mill.


Trajectory Modelling


• Trajectories calculated at two week intervals.

• Outermost trajectory when liners new.

• Innermost trajectory when liners fully worn.



• 25 % Mill Filling required when liners are new

• Decrease in trajectory constant over time.

• Virtually impossible to overthrow media after 4 weeks of liner wear.

• Significant depletion in trajectory. Liner redesign recommend.


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A model which relates the two most critical parameters in SAG milling

(Ball charge and Total Charge (%)) to a readily available output, Power

(kW).


JKMRC Power Model


Power Curves

Presenter

Presentation Notes

For a given ball filling and total filling there is a unique power solution.


Power Curves


Ball Charge Is Reasonably Estimated from Inspections

and Mass Balancing

Power Curves

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Example: Power = 1400 kW Ball Charge = 15% Total Charge=21.5%


Power Curves


Power Curves: Determining Optimum Regions


Target Power <97% Or 1450 kW



Target Power >1300 kW=86.67%



Target Mill Filling < 25%



Target Mill Filling > 18%



Target Ball Filling < 17.5%



Target Ball Filling > 12.5%



Case 1 Tph:~220 Power:~ 1300 kW Ball Charge:-13.3% Mill Load:~ 60t Mill Filling=21% Rock Filling=7.7% Rock to Ball Ratio~1:2

Case 2 Tph:- 205 Power:- 1410 kW Ball Charge:- 11 % Mill Load:~65 t Mill Filling=28 % Rock Filling=17% Rock to Ball Ratio~3:2

Real World Example

Presenter

Presentation Notes

From this comparison it is easy to see how trouble shooting becomes much easier utilising Mill Filling knowledge as opposed to load data.

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Real World Example

© Metso © Metso September 2015 Metplant Optimisation of a SAG Mill Through Trajectory and Power Modelling

41

Real World Example


Case 2 Tph:- 210 Power:- 1410 Ball Charge:- 11 % Mill Load:~65 t Mill Filling=28 % Rock Filling=17% Rock to Ball Ratio~3:2

Actions Required

• Reduction in mill load set point 15t

• Addition of 2 tonnes of media

Real World Example

Presenter

Presentation Notes

A reduction in 15 tonne is significant in this case. With site typically decreasing the load setpoint 2-5 tonnes. An example how the decrease in liner weight can mask the change in mill filling.

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•Allows the calculation and tracking of critical parameters - Mill Filling - Ball Filling - Rock Filling

•Data can be coupled with Trajectory models for improved insight

•Not affected by liner mass

•Use of actual mill parameters rather than inference from mill load and or power.


Power Curves: Major Benefits

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•Power curves/modelling utilised on a daily basis on site to determine ball charging regime and load set point.

•Improved mill performance

- Mill operating more consistently due to tight regulation of rock and ball filling. - Faster ramp ups post relines

•Methodology can be utilised for both fix and variable speed mills.


Power Curves: Major Benefits

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• Load cells are adequate in informing the DCS, metallurgist and operators the rate of change of mill filling. However they do not provide information on the total mill filling or the composition of rock and ball.

•Mill filling is a critical parameter, with its optimisation the mill may be optimised.

•Power Modelling allows for an accurate method to determine mill filling in real time with no or few mill stoppages.


Conclusion

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•Having developed power models it is reasonably straight forward to determine optimum operating regions.

•Furthermore ensuring the mill remains in the optimum region becomes significantly easier with knowledge of Total, Ball and Rock Filling.

•The end result being increased mill performance and improved liner management.


Conclusion

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