Continuous Optimisation Strategy for Grinding Circuits

Achieving the Maximum Performance from your Grinding Circuit & Liners

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The condition of the mill liner is an excellent indicator of chargebehaviour and can indicate specific operational problems.

Optimising the grinding circuit will save you money and will ensurethat your liners will perform as expected.

The grinding circuit needs a more modern optimisation approach. Acontinuous optimisation approach is essential for the moderngrinding plant.

The grinding circuit can not be optimised without considering themill liner quality and design.

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Grinding Circuit Performance is dependent on:

Throughput Required final product size Total power draw Equipment & product quality

LINERS, cyclones, screens

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The design flow sheet can be improved by changing:

Feed size or conditions

Operating variablesExamples: ball load, ball size, cyclone spigot

ProductsDifferent mill liners, cyclones, screens

Making calculated changes to operating variables is essential for optimum operation

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Common problems: Low throughput Incorrect final product size Specific operational problems (roping cyclone)

Product Problems:High wear on mill linersCyclone wearSpillage and vibration

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Cause of problems

Change in ore type or properties -harder or softer part of ore body

Change in crushing procedure, different F80

Change in equipment performance (Wear)

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This slow down in performance costs money

In most cases liner damage can be alleviated by adjusting the process

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60 mins

Traditionally, the changes made to improve circuit performance are based on only an hours snapshot

of the circuits performance

Is the snapshot survey still representative for year 10?

Plant Survey

Typical Optimisation Procedure

Data Analysis

Identify Variables/ Constraints

Fine-tune Variables to Achieve

Optimised State

Take samples of all streams over periods of one hour. Structural dimensions of equipment and operational parameters from the control room are collected.

Attempt to remove trouble spots by making adjustments and testing response.

Check the liner profile to check for clues about circuit performance.

Monitor the performance of equipment during the survey.

All samples must be sized and assays can be done to measure

content for valuables.

Plant Survey

Calibrate the collected data against mathematical equations that represent the behaviour of each equipment piece.

Data Analysis

The operational variables that can be easily changed must be identified. There is no use in suggesting unrealistic changes to circuit. Example: The cyclone spigot can be changed, but the only possible sizes are 60 mm, 70 mm and 80 mm.

The constraints are identified to ensure that the circuit is operating within its limitations. Example: Every cyclone cluster has a maximum pressure limit.

Identify Variables / Constraints

Fine-tune the available variables until the required conditions are satisfied. Making these changes will result in meeting the

required conditions.

Fine-tune Variables to Achieve

Optimised State

= USD 35,000 PER DAY NET GOLD INCREASE

Clients Problems:

Lower than expected throughput at 1700 tph vs Mineralitys estimated throughput of 1760 tph from initial assessment.

Final product was too coarse.

Harder ore expected in future; unsure how to operate with this ore.

After Optimisation

Throughput increased from 1700 tph to 1740 tph.

Required grind also achieved, with P80 = 106 m

Recommendations:

Increase SAG Ball load

Increase SAG Mill speed

Open pebble ports

Change Crusher Gap

Change Cyclone Spigot & Vortex Finder

Change water additions

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After making recommended changes the circuit may perform better temporarily.

However, the recommendations are only valid as long as the snapshot survey is representative and the inputs are unchanged.

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Unexpected Changes

The optimisation strategy may be invalid shortly after study completed. The optimisation report is rarely referenced in years to come.

The ore may become harder through the years which will drop throughput/damage liners

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60 mins

2 main problems with traditional approach to optimisation:

1. Operating Strategies, Choice of Variables based on 60 minute snapshot survey

2. Most optimisation studies solve problems for only a single scenario

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An ongoing optimisation advice strategy will:

Ensure operation of circuit close to maximum potential

Alleviate wear and problems with mill liners

Continuous Optimisation

Continuous Optimisation Strategy -Demonstration

Grinding circuit was built in 2002.Year 1-2: Good performance

Year 3: Throughput dropped 20 %(Snapshot survey and optimisationdiscussed)

The approach to solving the following scenarios will now be shownYear 4: Major drop in throughputYear 5: Severe cyclone ropingYear 6: Very soft feed material

1 2 3 4 5 - nFeed (tph) 153Primary Crusher Gap (inch) 6.5

Primary Mill 1 2 3 4 5 - nPower (kW) 2500Speed (rpm) 13Mill Water Addition (m3/h) 25

Secondary Mill 1 2 3 4 5 - nPower (kW) 2000Speed (rpm) 15Mill Water Addition (m3/h) 17

Cyclone Cluster 1 2 3 4 5 - nPressure (kPa) 93Number Operating 7Stream 7 Volumetric Flow (m3/h) 2150

1 2 3 4 5 - nStream 5 SAG Undersize P80 (um) 723Stream 8 Cyclone Underflow P80 (um) 213Stream 8 Cyclone Overflow P80 (um) 75

Continuous Inputs to Calibrate Model

Control Samples to Take to Calibrate Model

Calibration Models Continuously Refined

Primary Mill ScreenConst 1 Const 1Const 2 Const 2Const 3

Primary Mill Cyclone ClusterConst 1 Const 1Const 2 Const 2Const 3

Continuous Calibration

This unique approach allows for continuous upgrading of simulation base, instead of

using outdated, non-representative snapshot data

Enter VariableFeedOre Hardness (kWh/t) 14Ore Density 2.8

Primary MillBall Load (% Vol) 20Ball Size (mm) 150Speed (rpm) 15

ScreenOpenings (mm) 10

Secondary MillBall Load (% Vol) 30Ball Size (mm) 65Speed (rpm) 15

CyclonesNumber Operating 6Spigot Size (mm) 80Vortex Finder (m) 150

Effect of Variable ChangeThe following information about every stream

Dry Rate (tph)% SolidsSize Distribution

Primary MillPower (kW)Total Load (% Vol)

Secondary MillPower (kW)Recirculating Load (%)

CyclonesOperating Pressure (kPa)

Case Studies

Year 4: Major drop in throughputYear 5: Severe cyclone ropingYear 6: Very soft feed material

Continuous Optimisation Strategy

Day to day issues easily solved by referencing the Minerality optimisation

approach

PROCESS PRODUCTS

RIGHT PROCESS+

RIGHT PRODUCTS

What variables can you change to achieve maximum throughput?

Continuous optimisation strategy

Mill Liners Mill Lifters Discharge Grates Screens Column Flotation Conveyor Audits Wear and Spillage Control

Opt

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Minerality and Tega Industries provide a very unique consulting service.It is essential for us to provide the grinding circuit operator and plantmanager with:

Solution to achieve the best performance (throughput, final productsize, power draw) with high quality liners which are performing well.

Ability to continuously update the initial mathematical presentationof the grinding circuit.

High quality continuous strategy to solve challenges which may befaced long after our consulting job is finished.