Modelling of Macraes POX Circuit May 2006

Acknowledgements OceanaGold GRD Minproc Brent Hill Tony Frater David King Quenton Johnston Nevin Scagliotta Adrian Marin

Presentation Outline Background Macraes POX circuit Integration of Reefton concentrates Modelling Metsim model calibration Model prediction of increased throughput Conclusion/Recommendation

Johannesburg Office Belo Horizonte Office

Macraes Processing Background Historical Processing Small scale operation from 1862 until oz gold and 100 t scheelite recovered Modern Processing (Since 1990) Crush / Grind / Flotation / CIL Crush / Grind / Flotation / Fine grind / CIL Crush / Grind / Flotation / Fine grind / POX / CIL

Modern Project History Major Projects 1.5 Mt/a sulphide treatment plant – Mt/a expansion – 1994 MREP 4.5 Mt/a Increase for sulphide and oxide capacity Newmont POX technology 170 t/d BOC cryogenic oxygen plant Smaller Projects Unit cell installation Reclaim circuit 0.5 Mt/a oxide mill Autoclave optimisation Current capacity approximately 6 Mt/a

Macraes Processing Issues 1 Massive sulphide orebody hosting FeS2 / FeAsS Muscovite / quartz/ chlorite / siderite in gangue Presence of organic carbon, double refractory Variability. Low and high preg-robbing ore types 50% to 80% CIL recovery without POX Poor recovery with “conventional” POX

Macraes Processing Issues 2 Newmont technology required for “controlled” POX Limestone for free acid control Washing for chlorides Scale formation in autoclave

Macraes POX Circuit Design Concentrate grade % S 3.5 m dia. x 12.6 m 2:1 semi-elliptical ends 4 agitator, 3 compartment vessel 225°C and 3,140 kPag Koch Pyroflex membrane and AP302

Autoclave Scaled Agitator

Reefton Processing Orebody Native gold with minor sulphides in quartz veins Gold in FeS2, FeAsS, Sb2S3 Processing Crush / Grind / Flotation / Filtration / Transport Concentrate at 17.1 % S No organic carbon Highly refractory, complete oxidation required

Reefton / Macraes Integration Additional S oxidation requirement Oxygen plant constraint Autoclave retention time constraint Differing POX conditions Requirement for modelling to optimise capacity

History of Macraes POX Modelling Spreadsheet POX model developed and verified Single-compartment Metsim model developed Three-compartment Metsim model developed POX chemistry modified based on XRD results Thermodynamic data sources consolidated

Plant Trials and Model Calibration Plant trial in March 04 generated 23 data sets Solids and solution assays recorded Operating conditions recorded: Autoclave Pressure Temperatures in C1, C2 and C3 Cooling water to C1,C2 and C3 Oxygen flow rate and purity Overall oxidation from feed and discharge assays Compartment oxidation inferred from heat balance

Sulphur Analysis Discrepancy Trial data:for 98% oxidation, 20 t/h CW added Model results:for 98% oxidation, 16 t/h CW added Site assay 10% of the total S (TS) is sulphate S No TS reported for the trial data No free acid in discharge reported Can not do overall S balance calculation

MLA Mineralogy Investigation MLA used for quantitative mineralogy investigation MLA results 2% of TS is sulfate S Site assay 15% sulfate S for the same sample Revised S and gangue mineralogy according to MLA

Plant Trials in 10/04 and 01/05 Updated trial data collection template Additional data for heat/mass balance Updated mineralogy data used Good correlation between models and assays No heat adjustment factor required

Plant Trials in 10/04 and 01/05

Plant High Throughput Trials in 07/05 In July 2005 eight plant trials run Four data sets from scaled autoclave and Four sets from “clean” autoclave Scaled agitators show poorer oxygen dispersion Scaled sets average oxygen utilisation is 79% “Clean” sets average oxygen utilisation is 85%

Plant Trials in 07/05

Model to Predict Various Scenarios Plot leach kinetics for all plant trials Use average kinetic curve for further modelling Scenarios modelled: Grade: 10%, 12% and 14% total S Throughput: 2.7, 2.8, 2.9, 3.0, 3.1 and 3.3 t/h TS Constant oxygen partial pressure Oxygen: 7 t/h

The Final Kinetic Curve Used for Scenario Modelling

Scenario Modelling Results For 10% S and 12% S -C1 temp drops with higher throughput For 14% S -C1 maintains 225°C for all scenarios modelled

Scenario Modelling Results

Above 2.7 t/h TS, oxygen constrained Increasing throughput, decreases RT for ≤ 12% S Increasing throughput, increases RT for ≥ 14% S For 14% S the RT is over 50 mins The autoclave is not constrained by RT at 14%

Conclusions Metsim a useful framework for plant optimisation/design Careful selection of chemistry and thermodynamic data Plant trial data for model calibration Modelling can assist in plant optimisation and future design