Challenges and Opportunitiesin Controlling Large Capacity

CircuitsDr. André Vien, P. Eng., Ph.D.Process Engineering Manager

Metso Operations and Controls Technology

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Outline

• Introduction• Primary Crushing• Feeders• Conveyors• Secondary Crushing• Screening• HPGR (HRC)• Grinding• Flotation• Summary

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Sec Scrn BinSec Crush Bin

Sec Scrn (2)

Sec Crusher (2)

Dry Bin

HRC Surge Bin

HRC Hopper/HRC

Wet Scrn Bin

Wet Scrn (2)

To Cyclone Feed Tank

Wet Bin

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Introduction

• Declining grades has led to the development of large capacity circuits totake advantage of economies of scale.

• Large equipment vendors have taken this opportunity to improve theirofferings.

- New crushers have more power and the reduction ratio is higher for the samefootprint. Some of the comminution is shifted back to crushers which are more energyefficient than tumbling mills.

- HPGRs are now more common (“new” type of crushing device that is more energyefficient).

- Large flotation cells have lower power density (kW/m3) due to better mixingmechanisms.

• Auxiliary loads, which were often ignored, are now included in studies.- 30 MW or more in conveying and screening.- 1 MW or more for cyclone feed pumps.

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Primary CrushingChallenges

• Crusher instant capacity approaching 9000 t/h (at 250 mm OSS).- This means 1 x 300 t truck every 2 min.- Or 1 x 500 t truck every 3 1/3 min.- Idle time consumes power that is not used (dissipated as heat).

• Example- Probability of arrival > 2 min is 0.43 (> 3.33 min is 0.24).- Probability of a dump > 2 min is 0.52 (> 3.33 min is 0.23).

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Primary CrushingChallenges

• Vault- Typically 1.5 to 2 trucks – 500 to 1000 t bin!

• Semi-mobile- Crusher itself 530 t.

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Primary CrusherOpportunities

• Blasting Practice- Improve fragmentation (smaller top size, more fines).- Decrease in dig time, truck loading time.- Increase in truck payload, loading performance.- Decrease in total load and haulage cost.- Increase mill throughput.

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Cost ($/t) Energy(kWh/t)Drill and Blasting 0.15 0.2Load and Haul 0.75 0.3Crushing 0.75 2.0Grinding 3.75 20.0

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Primary CrusherOpportunities

• Shovel bucket vision system- Gradation at shovel

• Can redirect oversize to manual processing to break large rocks.• Better crusher utilization with no oversize.• Information for blast optimization.

- Missing tooth detection• Can redirect load to manual sorting.• Prevent crusher jam.

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FeedersChallenges

• Capacity- High bed depth leads to large sloughing.- A 1% change in speed is a large tonnage change.- Using multiple feeders require a bin or equivalent to split – segregation.- Width limited to 3.3 m or by the downstream conveyor.

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Hopper Width 2.2Height of layer (m)

Feeder Speed (m/min) 0.25 0.5 0.75 1.5 23 160 321 481 962 12836 321 642 962 1925 25669 481 962 1443 2887 3849

12 642 1283 1925 3849 513215 802 1604 2406 4811 641518 962 1925 2887 5774 7698

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FeedersOpportunity

• Primary Crusher Discharge- Immediate take-away – minimal vault.- Possible with high bed depth to absorb the shock of falling rocks.- Need an intermediate stockpile to buffer feed rate to downstream unit operations.

• Segregation- Blending of coarse and fines with multiple feeders (within limits).

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ConveyorsChallenges

• Bed Thickness- Surcharge angle material dependent.

• Conveyor Width- Up to 2.2 m.

• Trough Angle- Up to 30 deg. (higher possible but depends on belt flexibility).

• Incline- Reduced capacity with angle of incline – longer conveyor and more spacing to achieve

required height.

• Lift Height- Larger bins means higher lifts and more power consumed.

• Belt speed- Limited by material (dust, abrasion, impact for large lumps).

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ConveyorsOpportunities

• Still has higher capacity compared to single comminution machine butcollecting more than one discharge can reach limit. This leads topotential for modularization.

• Downslope conveyors can generate power rather than brakes generatingheat.

• Can collect signals e.g. using Dupeline bus to reduce wiring.• Load sharing for long conveyors using several motors at different

locations reduces belt stress.

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Secondary CrushingChallenges

• Need 2 MP 2500 crushers to match primary crushing capacity- Splitter or surge bin.- Segregation.- Balancing load.

• Large top size- Large crusher feed opening.- Large reduction ratio (5-6).

• Need large feed bin to allow choke feeding- Frequent start/stop if capacity exceeds circuit feed introduces mechanical stress.

• Feed distribution is a challenge as due to horizontal speed of feeder- Segregation.- Uneven distribution.

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Secondary CrushingOpportunities

• Variable Speed- Match capacity with upstream and downstream capacity (limited range).- Reduce size of surge bin/stockpile.- Finer product and utilizing installed power when there is excess capacity.

• Choke feeding- More throughput.- Finer product.- More consistent use of power.- Less mechanical stress.

• Closed Circuiting- Top size control.- Crushing more energy efficient (not overcrushing) but auxiliary loads introduced.- Flexibility in balancing secondary and downstream operations.

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Tertiary CrushingChallenges

• Closed Circuit- Circulating loads not that large in % but large in t/h.- If wet screening, moisture can be an issue.- Segregation of recycle material vs fresh feed.- Matching crusher and screen capacity.

• Choke feeding- Large surge bins.

• Multiple units- Segregation.- Balancing load.

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Tertiary CrushingOpportunities

• Use an HPGR (HRC)- HRC more efficient than cone crusher (or standard HPGR).- Higher capacity (HRC 3000 can handle 5400 t/h – 3600 t/h fresh feed at 50% CL).- Higher availability (97% compared to 85%).- In closed circuit can produce feed to ball mill (5 to 8 mm).- Variable speed allows to adjust capacity with upstream/downstream operations.- Specific force has limited impact but could potentially be used to control circulating

load.

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ScreeningChallenges

• Multiple Units- Large (long) bin for feeding.- Segregation.- Blinding.

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ScreeningOpportunities

• Variable Motion (Ellipti-Flo)- Variable stroke angle.- Variable speed.- Allows for self-clearing (to a point).- Allows for changing material movement (retaining or forwarding).

• Balancing- Adjust feed rate for screening capacity.

• ScreenWatch- Monitoring screen mechanical state online.

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Ball MillingChallenges

• Multiple Units- Segregation.- Balancing load.

• Large Flows- Pumping limitations – parallel lines.- Gravity flow limitations.

• Large Dimensions- Larger elevation change required for cyclones.

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Ball MillingOpportunities

• Variable speed mill to adjust CL.• Larger cyclone cluster due to cyclone capacity limit allows for finer

cyclone feed pressure control (e.g. 1 out of 20 cyclones vs 1 out of 5).• Single cyclone feed pump (no standby) offers economy of piping and

valves. This simplifies control (no switching logic).• Instrumentation is more costly due to larger pipe diameter, however,

there are fewer instruments (fewer parallel lines) so overall incentive toinstrument the circuit is greater (lower installation and maintenancecosts).

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FlotationChallenges

• Multiple Lines- Proper distribution.

• Large Dimensions- Long stems for dart valves.- Longer distance from grating to froth (longer stems for level measurement)- Aspect ratio of a large mechanical cell is between old mechanical and columns. This

means higher froth depth but lower carrying capacity and lower lip length to carry froth.- Internal launders disrupt froth flow pattern and can lead to localize boiling (high local

Jg).

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FlotationOpportunities

• RTD is not perfect mixing so performance of a single cell is bettercompared to smaller flotation cells.

• Variable speed impeller (or variable speed pump for microcell) allows toadjust performance based on need (bubble size vs particle size).

• Fewer addition points (compared to many smaller cells) means moreopportunity to automate reagent addition with feedback.

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Summary

• There are challenges related to large equipment at every step in theplant.

• Limitations have shifted from the main equipment to auxiliary equipmentin some cases (e.g. feeders, conveyors, pumps).

• As you move downstream there is a need to use more parallel lines tohandle the load.

- Segregation and load balancing between parallel units are the main issues.

• Larger equipment has given vendors the opportunity to upgrade orprovide new offerings not only for higher capacity but higher efficiency aswell.

• Large equipment need variable speed to start. This can be takenadvantage of for balancing upstream/downstream circuits.

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