Moss Lake Deposit — 43-101 Technical Report

This report presents the proposed process plant design for the Moss Lake gold deposit based on conventional gravity and carbon-in-leach technology.

Report context

This technical report and mineral resource estimate for the Moss Lake Deposit was prepared in support of a 43-101 disclosure. The processing chapter (Section 17 – Recovery Methods) describes a proposed plant design intended to treat 14.0 million tonnes per annum of mineralized material using established Gravity/CIL technology.

Processing route

Crushing and stockpiling

The primary crushing plant is designed to treat 2,131 tonnes per hour for 18 hours per day. The run-of-mine (ROM) bin has a capacity of 200 m³ and is equipped with a 700 x 700 mm static grizzly. Oversize material will be broken by a mechanical rock breaker. An apron feeder withdraws material at a controlled rate onto a vibrating grizzly feeder; undersize at -150 mm joins the crusher discharge conveyor. The single-stage gyratory cone crusher operates at a closed side setting of 150 mm to produce 80% passing 150 mm. Crushed product is conveyed to a 15,000-tonne live conical stockpile. Dust suppression uses conveyor skirting and atomized water sprays.

Milling and classification

Reclaim from the stockpile is via long variable speed pan feeders, each sized for 550 tph. The milling circuit comprises a grate discharge SAG mill and an overflow discharge ball mill, capable of treating 1,700 tph of hard rock. The SAG mill operates at a ball load of 8% by volume up to a maximum 15% and has a variable speed drive. A pebble crusher with a closed side setting of 12 mm reduces pebbles from the SAG mill trommel (12 mm slotted apertures) before recirculation. An over-band electro-magnet and metal detector protect the pebble crusher.

The SAG mill discharge at P80 of 750 μm and 65% solids by weight passes over the trommel screen. Trommel undersize combines with ball mill discharge in a cyclone feed sump, diluted to 58-60% solids, and pumped to a hydrocyclone cluster. Cyclone overflow at P80 of 106 μm at approximately 40% solids by weight gravitates to a linear screen for trash removal. Underflow returns to the ball mill. A facility to bleed part of the cyclone underflow to the SAG mill is provided. A bypass conveyor allows material to feed directly to the ball mill during SAG mill maintenance.

Lime is dosed into the SAG mill with an option to bypass to the ball mill and additional lime added to the pre-leach tank as required.

Gravity recovery and intensive cyanidation

A portion of the cyclone underflow is bled to a scalping screen to remove +2 mm material. Screen undersize at approximately 47% solids feeds a centrifugal concentrator for free gold recovery. Concentrates are discharged into a Gekko ILR feed cone and leached batch-wise for 14-16 hours using an In-line Leach Reactor. Pregnant solution from the gravity circuit is pumped to a dedicated electrowinning cell with stainless steel wire mesh cathodes. The gravity tailings gravitate to the cyclone feed sump.

Pre-leach and carbon-in-leach (CIL)

Cyclone overflow after trash removal gravitates to a pre-leach/CIL circuit comprising ten tanks: two pre-leach tanks and six CIL tanks. Total effective residence time is 48 hours. Slurry flows through interconnecting launders. Each CIL tank is equipped with an inter-stage wedge wire cylindrical pumping screen to prevent carbon migration. Cyanide is dosed into the second pre-leach tank via a ring main; an automatic cyanide analyzer controls addition. Loaded carbon from CIL Tank 1 (or Tank 2 if Tank 1 is on bypass) is pumped to a loaded carbon screen; underflow returns to the CIL tank. Recessed impeller vertical spindle pumps transfer carbon upstream at intermittent intervals between CIL Tanks 2 to 6. Eluted or regenerated carbon is added to the last CIL tank.

Detoxification

CIL tailings slurry passes through a carbon safety screen to recover escaped carbon. Screen underflow gravitates to the detoxification circuit of three tanks. Free cyanide and weakly bound metal cyanide complexes are oxidized by addition of sulphur dioxide (supplied as sodium metabisulphite) and oxygen. A copper catalyst at approximately 50 mg/l is added as copper sulphate solution. Lime slurry neutralizes acid formed during the reaction.

Tailings disposal and water return

Slurry from the third detoxification tank gravitates to a final tails sump and is pumped to the tailings disposal and storage facilities. Process water from the tailings dam returns to a return water pond and is pumped via pontoon pumps to a return water tank, then to the plant process water tank.

Carbon regeneration and gold recovery

Loaded carbon is acid washed in a 2-tonne batch cone using dilute hydrochloric acid (~3% HCl) at 2 bed volumes per hour for 1 hour, then rinsed. Elution uses the pressurized Zadra method with cyanide-caustic solution at 2 bed volumes per hour, heated to 120°C via primary and secondary heat exchangers using thermic oil. The column operates at 300 to 350 kPa. The elution cycle runs approximately 14 hours. Pregnant eluate is directed to double sludging electrowinning cells. Spent electrolyte returns to the eluant tank in closed circuit.

Carbon is regenerated in an electric fired inclined kiln at 600 kg/hr, quenched, and screened before returning to the CIL circuit. A virgin carbon conditioning facility removes fines.

Gold smelting

Gold sludge from electrowinning is dried in calcining ovens at 800°C. Dried sludge is mixed with fluxes and smelted in an induction furnace at 1,200 to 1,400°C. Molten gold is poured into cascading bullion moulds; slag collects in a separate crucible. Gold bars are cleaned, stamped, and stored prior to dispatch.

Consumables and services

Grinding media: 100 mm balls for SAG mill and 65 mm balls for ball mill. Sodium cyanide delivered in 1-tonne bulk bags, dissolved batch-wise to 25% by weight. Caustic soda delivered in 25 kg bags, dissolved to 20% by weight. Quicklime delivered in 1-tonne bags, slaked in a detention lime slaker to produce lime slurry. Sodium metabisulphite delivered in 1-tonne bulk bags, dissolved to 25% by weight. Copper sulphate delivered in 25 kg bags on pallets, dissolved to 15% by weight. Carbon delivered in 500 kg bulk bags. Hydrochloric acid delivered in 290 kg (210 litre) drums at 33% concentration.

Compressed air: two compressors (one duty, one standby) provide 2,000 Nm³/hr at 750 kPa for process uses plus a dedicated instrument air compressor.

Water services: a pontoon with centrifugal pumps draws raw water from a storage facility. Return water from tailings and raw water top-up constitute process water. Dedicated pumps supply mill cooling, gravity concentrator fluidising water, dust suppression, reagent make-up, potable water, gland water, and fire water systems. Fire water includes electric and diesel driven pumps plus a jockey pump.

Key reported parameters

Technical qualifications

This report presents a proposed process plant design only. No historical operating data from a Moss Lake plant is presented. The report notes that laboratory tests on some mineralized material samples displayed high cyanide consumptions. No testwork results, metallurgical recoveries, or throughput performance data from pilot-scale or demonstration-scale operations are provided. The design basis and parameters described are those selected for the proposed plant and do not represent actual operating data.

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Source: 43-101 Technical Report and Mineral Resource Estimate – Moss Lake Deposit, Section 17 – Recovery Methods, document dated as referenced in the original report.