Gravity Concentration at Ashanti Goldfields BIOX Plant Obuasi Mine; Ghana, W. Africa

CONTENTS 1. Introduction - Knelson Concentrator - Overview - Obuasi STP-Biox Plant Flowsheet 2. Testwork - Results from lab scale testing 3. Construction and Installation - Materials of Construction 4. Circuit Metallurgy - Operating parameters of the circuit - Impact of the Knelson Concentrator 5. Summary

ASHANTI - OBUASI OPERATIONS PLANT CAPACITY(Mtpa) PROCESS Pompora Treatment Plant (PTP) 1.8 Float, Roast,CIL Sulphide Treatment Plant (STP) 2.9 Float, Biox, CIL Oxide Treament Plant (OTP) 2.9 CIL Tailings Treatment Plant (TTP) 1.9 CIP Heap Leach 1.1 Agglomerated Grind Flotation Flot. Tail Conc. Re-grind BIOX CCD CIL

On-site Lab Test Work Objective: To determine the presence of free gold in the STP Biox circuit. - free gold - easily visible 1. Three Streams Tested a) CCD Thickener #5 U/F b) Biox-CIL Tailings c) Final Tailings (from flotation) 2. Considerations - severe corrosion of the lab test machine - corrosive nature of the Biox process (pH 1.3 - 1.8) - fine free gold - visible under the microscope - flat gold flakes - easily visible - flat shape attributed to grinding

Lab Test Work Summary 1. Photomicrographs and visual observations indicated the presence of free gold in all of the streams tested. 2. Free gold passed through the CIL circuit without complete leaching. 3. Internal work by Ashanti indicated that free gold is liberated in the Biox Process. 4. +90% of the liberated gold particles are <20 micron. 5. Any installation of a Knelson Concentrator in the Biox/CCD circuit would have to address the corrosive nature of this slurry (pH 1.3 - 1.8).

Cyclone No. 5 Thickener Knelson Concentrator Thickener Feed Box Concentrate Hopper No. 6 Thickener Table CIL

Modifications to the Knelson Concentrator Materials of Construction Evaluated a) Hastelloy C-276 b) Incolloy 825 c) Stainless Steel 904L d) Stainless Steel 304L - Ashanti indicated that 304L had performed very well for other equpiment located in that part of the plant. - The other alloys would have at least doubled the cost of manufacture compared to the standard unit.

Materials of Construction General Assembly Stainless Steel (304L) - Tail. Launder Linatex Rubber Lined - Motor Cover - General Assembly - Upper & Lower Frame Assemblies - Conc. Launders Electroless Nickel Plated - Upper & Lower Bearing Exteriors - Drive Sheave/Bushing

Operating Parameters Knelson Concentrator Hourly Tonnage 33 - 40 tonnes Pulp Density of Knelson Feed 25 - 27% (w/w) Temperature of Knelson Feed 30oC Slurry pH 2 - 3 Size distribution of Knelson Feed 90% passing 45 microns Concentrating Cycle Time 3 hours/cycle Knelson Fluidization Flow (USgpm) 130 gpm

Process Improvements Plant Au Recovery Prior to KC Installation 87% Plant Au Recovery After KC Installation 90 - 92% Percentage of Total Gold Recovered by KC 4 - 5% CIL Tails Grade Prior to KC Installation 2 - 3 g-Au/t CIL Tails Grade After KC Installation 1 - 1.5 g-Au/t

Knelson Concentrator Installation at AGC Obausi Mine; Ghana, W. Africa

SUMMARY & CONCLUSIONS 1. Lab testing identified free gold present in the CCD circuit, CIL tails and final tails. 2. Free gold passed through the CIL circuit without complete leaching. 3. Decision to install a Knelson Concentrator in the CCD circuit after the Biox process. 4. The materials of construction were modified to 304L stainless steel to withstand the severely corrosive nature of the slurry. 5. Installation was extremely successful with significant improvents to gold recovery.