NI 43-101 Technical Report and Prefeasibility Study for Marban Engineering

This prefeasibility study describes a proposed conventional leach and carbon-in-pulp processing plant designed for a nominal throughput of 16,438 t/d at the Marban Engineering Project.

Report context

The technical report, dated October 2022, presents the results of a prefeasibility study for the Marban Engineering Project. The study evaluated metallurgical testwork data and selected a processing route based on standard gold recovery technologies.

Processing route

Overall process design

The study analysed testwork results and reviewed several process route options during the initial study stages. Based on this analysis, a conventional leach and carbon-in-pulp process route was chosen. The unit operations selected are all typical for gold recovery and the proposed flowsheet uses standard processes and technologies.

Key operating criteria for the process plant include a nominal throughput of 16,438 t/d or 6.0 Mt/a, crushing plant availability of 70%, and plant availability of 92% for grinding, leach plant, and gold recovery operations.

Crushing and stockpiling

The crushing circuit is designed for an annual operating time of 6,132 hours at 70% availability at a capacity of 16,438 t/d. Ore is hauled from the mine and tipped into the run-of-mine bin, which has a static grizzly screen to remove oversize. A fixed rock breaker handles oversize rocks at the ROM bin feed. Ore is withdrawn by an apron feeder and passed through a vibrating grizzly feeder at 978 t/h to feed the jaw crusher. Primary crusher discharge is combined with vibrating grizzly undersize and conveyed to a vibrating double-deck secondary screen. Secondary screen oversize from both decks feeds the secondary cone crusher. Cone crusher discharge is combined with secondary screen undersize and conveyed to a covered stockpile providing approximately 9,000 t of live storage.

Mill feed is regulated at 744 t/h into the SAG mill via two belt feeders. Pebbles from the SAG mill are recirculated by a pebble conveyor back to the SAG mill feed conveyor.

Grinding

The grinding circuit consists of a SAG mill followed by a ball mill in closed circuit with hydrocyclones. The circuit is sized based on a SAG mill feed size of 80% passing 46 mm and a ball mill product of 80% passing 85 µm. SAG mill slurry discharges through a trommel screen; oversize pebbles are recycled to the SAG mill via conveyors. Trommel screen undersize discharges into the cyclone feed pumpbox. The SAG mill is powered by a variable speed drive.

The ball mill is fed by cyclone underflow. Ball mill discharge passes through a trommel; oversize is discharged to a scats bunker, while undersize goes to the cyclone feed pumpbox. No pebble crusher is included as no significant pebble generation is expected.

Cyclone overflow at 43.0% w/w solids is sent to a trash screen ahead of the leach circuit.

Gravity recovery circuit

The gravity circuit comprises one scalping screen and two centrifugal batch concentrators in parallel. Feed is directed from the cyclone feed pumpbox to the scalping screen. Gravity scalping screen oversize at +2 mm reports back to the ball mill.

Scalping screen undersize feeds the centrifugal concentrator. Gravity concentrate is collected in the concentrate storage cone and subsequently leached by the intensive cyanidation reactor circuit. Tailings from the gravity concentrator also report to the ball mill.

Intensive leaching reactor

Concentrate from the gravity circuit reports to the intensive leaching reactor to extract contained gold. Concentrate is directed to the ILR gravity concentrate storage cone and de-slimed before transfer to the ILR.

ILR leach solution (mixture of NaCN, NaOH, and an oxidant) is made up within the heated ILR reactor vessel feed tank. Leach solution is circulated through the reaction vessel and drained back into the feed tank. Leached residue is washed, and solid gravity leach tailings are pumped to the cyclone feed pumpbox.

ILR pregnant leach solution is pumped to the ILR pregnant solution tank in the gold room. ILR pregnant solution is treated in the gold room using a dedicated electrowinning cell; the sludge is combined with sludge from the carbon elution electrowinning cells and smelted.

Leach and adsorption circuit

The leach-adsorption circuit consists of four leach tanks and six carbon-in-pulp tanks. The circuit is fed by trash screen undersize. Barren solution from electrowinning cells is periodically transferred to the leach circuit. Total circuit residence time is 24 hours at 42.5% w/w solids.

Air is sparged into alternating tanks to maintain dissolved oxygen levels at 8 mg/L. Hydrated lime adjusts pH to 10.5. Cyanide solution is added to the first leach tank. Fresh or regenerated carbon is returned to the last CIP tank and advanced counter-currently. Slurry from the last CIP tank flows to cyanide detoxification.

Cyanide destruction

CIP tailings are discharged to the cyanide detoxification tank at 42.5% w/w solids. Cyanide destruction uses the SO₂/air method with reagents of air, lime, copper sulphate, and SO₂. The tank has a total residence time of 60 minutes to reduce weak acid dissociable cyanide concentration from 145 mg/L to less than 8 mg/L.

Detoxified tailings pass through a carbon safety screen; recovered carbon is collected for potential return to the CIP circuit, and screen undersize is pumped to the tailings thickener.

Tailings thickening

Detoxified tailings are thickened in a 38 m diameter high-rate thickener before discharge to the Tailings Storage Facility. Thickener overflow is reused as process water. Flocculant is added to improve settling rate.

Carbon acid wash, elution, and regeneration

Loaded carbon is treated with weak hydrochloric acid solution in an 11-tonne capacity acid wash column to remove deposits. Acid-washed carbon is transferred to the elution column.

The gold stripping circuit uses the Pressure Zadra process. A high cyanide, caustic solution is recirculated through a pressure elution column at 140°C. Precious metal-rich solution flows through electrowinning cells to deposit gold and silver on cathodes before recycling to the elution column.

Gold sludge is recovered from electrowinning cells, washed, filtered, dried, mixed with fluxes, and smelted in an electric induction furnace to produce gold doré.

Barren carbon is regenerated in an electric rotary kiln at 650° to 750°C in an atmosphere of superheated steam. Regenerated carbon is quenched, screened, and returned to the CIP circuit.

Key reported parameters

Technical qualifications

The technical report presents a prefeasibility study design based on testwork results and standard industry practices. The process flow diagram is cited as Figure 17-1 and the process plant layout as Figure 17-2, both sourced from Ausenco, 2022. Reagent consumptions are based on testwork results and standard industry practices. The study did not identify any significant technical limitations within the report sections reviewed.

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Source: NI 43-101 Technical Report and Prefeasibility Study for Marban Engineering, October 2022, Sections 17 Recovery Methods.