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INCREASING THROUGHPUT THROUGH AG2 AT

GUELB MOGHREIN MINE

In Autogenous Grinding (AG), rock-on-rock and rock on liner impacts are themain causes of breakage. With this in mind, attention must be paid to thefollowing:

1. FEED SIZE: The reason for this is obvious. To recap, large, competentrocks are essential in mill feed to act as the means of converting electrical

energy to mechanical energy for comminution of the remainder of thecharge. Some mines have gone to the extent of changing fragmentation inthe pit to ensure this is achieved. The results shown below are from amine the author was involved with some years back, this increase inthroughput was as a result of better fragmentation in the pit.

Increased AG mill throughput attributed to better ore fragmentation

2. MILL DENSITY: When converting from SAG to AG it requires a paradigmshift in operating practice regarding mill density. The two major reasonsfor this are: 1) The material needs to be evacuated from the AG mill much

faster in order to prevent a buildup in mill load, and

2) Less cushioning is required. I.e. there are no balls to becoated, and the effect of cushioning is not desired as westrive for rock-on-rock impact.

WITH THE TWO AFOREMENTIONED FACTS IN MIND, A MILL DENSITY INTHE RANGE 68-72% SOLIDS MIGHT BE DESIRABLE. This is not fixed andvaries from location to location based on ore mineralogy and its effect onslurry rheology, trial and error will ascertain this. The results below show

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the effect of lowering mill density on power draw.

Effect of lowering mill density onAG mills power draw (kW)

3. LINER TYPE: There is no recorded evidence in base metals processing ofrubber liners being used in AG mills! The reason being that rubber linersabsorb the impact of the rocks and help to dissipate the mechanicalenergy, thus leading to inefficient breakage. The underlying reason beingenergy loss due to the plasticity of the rubber. This is explained by thecoefficient of restitution. Steel will have coefficient of restitution rangingfrom 0.35 to 0.60 and this figure increases with time as work hardening

takes place. On the other hand, rubber will have coefficient of restitutionin the range of 0.55 to 0.85 and this figure decreases with time. Mostreputable foundries are capable of measuring up rubber liners anddesigning steel liners as a replacement. Eclipse foundry of South Africasent two members of staff to Frontier a few years back and theymeasured up existing liners and designed new ones.

4. LINER DESIGN, LIFTER HEIGHT and LEADING FACE ANGLE: A reputablefoundry will have milling software capable of simulating the trajectory ofthe charge with steel liners. This will often be different from the existingrubber liners. Overthrow, or under throw, will lead to inefficient grinding.Typical simulation results from such software package are shown below.

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Milling software used in steel liner design

Milling software used in steel liner design

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5. OPERATING CONDITIONS: The following matrix can be used as a guide tomaximize throughput, this is subject to change as more operatingdata/experience is gathered.

Table showing operating guidelines for AG2

No MILLLOAD

POWERDRAW

HPGRLOAD

ACTION

1 DOWN DOWN UP INCREASE MILL DENSITY

2 DOWN DOWN DOWN INCREASE MILL FEED

3 UP UP UP DECREASE MILL FEED

4 UP UP DOWN DECREASE MILL DENSITY

5 UP DOWN DOWN DECREASE MILL DENSITY(OVERLOADING/BOGGING IMMINENT)

6 DOWN UP UP DECREASE MILL DENSITY(FINES ARE BEING FLUSHED OUT)

Typical milling circuit

Ramoutar SeecharranPlant Superintendent23rd August 2010