Lemhi Gold Project: Two-Phase Whole Ore Leach Flowsheet Designed for Expansion

The Lemhi Gold Project is an advanced gold and silver development project in Idaho, USA, owned by Freeman Gold Corp. The project, which is the subject of a June 29, 2026 NI 43-101 Technical Report and Feasibility Study, is designed around a staged expansion strategy. Year one through four sees a single-stage crushing circuit, SAG mill, ball milling, and whole ore leach recovery at 6,849 tonnes per day. From year five onward, the plant adds tertiary milling and an extra leach tank, lifting throughput to 8,219 tonnes per day. The process plant operates on two twelve-hour shifts per day, 365 days per year.

Critical Data

Overview

The recovery methods at Lemhi follow a conventional gold processing route. Ore moves through crushing, grinding, cyanide leaching, carbon adsorption, desorption, and finally precious metal recovery to doré. The flowsheet design draws on previous test work programs performed on the deposit, Ausenco's database of reference projects, and in-house process modelling. Flowsheet optimization investigations supported the process design around the comminution circuit, equipment deferrals to the expansion phase, dewatering equipment, and safety considerations based on feasibility study test work.

The plant design intentionally defers certain equipment to the expansion phase. This staged approach lets the project match capacity to life of mine requirements. Beyond the tertiary mill and additional leach tank, all other process plant aspects remain the same between phases.

Key Process Stages

The crushing circuit is a one-stage process reducing mined ore from an F80 of 330 mm. Material hauled from the mine dumps into a coarse ore hopper with a static grizzly, feeding a vibrating grizzly feeder through an apron feeder. Oversized ore gets broken down by a rock breaker before being rehandled into the hopper. Vibrating grizzly oversize goes to a jaw crusher and blends with undersize on the discharge conveyor to a surge bin. A belt feeder withdraws material to feed the grinding circuit. Overflow from the bin goes to a crushed ore stockpile, and when the crushing circuit is not operating, a front-end loader reintroduces stockpiled material via the surge bin. The crushing plant is sized at the outset for Phase 2 throughput.

An overland conveyor delivers crushed material to the grinding circuit, which pairs a SAG mill with a ball mill in closed configuration with cyclones. The circuit is sized on a feed size of 55 mm and product size of 120 µm. SAG mill slurry discharges onto a rubber-lined trommel screen, with oversize returning to the mill for reprocessing. Trommel undersize combines with ball mill discharge in the cyclone pump box, where slurry dilutes with process water before pumping to the cyclone cluster. Cyclone overflow at 44% solids w/w reports to a trash screen then the leach circuit. Underflow returns to the ball mill. In the expansion phase, a tertiary mill with a secondary cyclone is added, with secondary cyclone underflow reporting to the tertiary mill to maintain the product size.

The leach and adsorption circuit consists of two leach tanks and six carbon-in-leach adsorption tanks, all mechanically agitated. Residence time is 24 hours at 44% w/w solids. Regenerated carbon returns to the last CIL tank and advances counter-currently using carbon transfer pumps. Carbon concentrations of 12 g/L are required in all CIL tanks. Loaded carbon from CIL tank 1 transfers via a recessed impeller pump to a loaded carbon recovery screen. Following elution, barren carbon is screened and pumped back to CIL tank 6. Tailings slurry from CIL 6 flows to an interstage thickener to recover gold bearing solution and reduce precious metals plant losses, with flocculant added to promote settling. Thickener underflow at approximately 60% solids goes to cyanide detoxification tanks.

The carbon acid wash step treats loaded carbon with weak hydrochloric acid at a 2 BV/h rinse rate to remove impurities and residual leaching reagents. The acid solution fills the column from the bottom up, soaks, then rinses to the final tailings pump box. The gold stripping circuit uses the Anglo-American Research Laboratory process. A cold cyanide wash step removes cyanide-soluble copper from the carbon, with carbon soaked in a weak NaOH and NaCN solution for 20 minutes. The elution sequence injects water mixed with cyanide and sodium hydroxide to achieve a 2.0% w/w NaOH and 4.0% w/w NaCN solution. During pre-heat and soak, the solution circulates through the column and heater until reaching 95°C. Additional heated water pumps through the column at 2.0 bed volumes per hour, with temperature increased to 120°C under pressure. Strip solution passes through a recovery heat exchanger to the pregnant eluate tank.

Stripped carbon dewaters over a screen feeding an electric rotary kiln via screw feeder. The kiln operates at 750°C in superheated steam to restore carbon activity. Quenched carbon pumps over a sizing screen to remove undersized fragments, with fresh carbon added as needed to compensate for attrition losses. Oversize reports to the last CIL tank while undersize goes to final tailings.

Gold recovery from pregnant solution uses four electrowinning cells with stainless steel mesh cathodes. Gold deposits on cathodes and barren solution returns to the leach circuit. Gold-rich sludge washes off cathodes with high-pressure spray water into a sludge hopper, then filters and dries. A mercury retort boils off mercury stripped from the elution circuit and plated on the cathodes. Gas from the retort and regeneration kiln treats in a gas scrubber. The material then mixes with fluxes and smelts in a diesel-fired furnace to produce gold doré within a secure, supervised gold room.

Tailings from the interstage thickener underflow go to two parallel cyanide detoxification tanks with 90 minutes total retention. The sulfur dioxide/air process reduces weak acid dissociable cyanide concentrations to a maximum of 5.0 mg/L. The reaction holds at pH 8-9 through lime addition, with sodium metabisulfite supplying sulfur dioxide and copper sulphate as catalyst. Each tank is mechanically agitated with oxygen sparging. Following detoxification, slurry pumps to the tailings thickener, and underflow at approximately 60% solids flows by gravity via overland pipeline to two agitated filter feed tanks, then to a filtration circuit for dewatering to 85% solids. Three tailings filter presses operate with two in service and one standby. Filter cake discharges onto dedicated conveyors, and filtrate pumps back to the tailings thickener.

Additional Interesting Data and Summary

The oxygen plant uses vacuum swing adsorption technology, producing 92% purity oxygen at 6 bar (g) for cyanide leaching and detoxification circuits. Raw water consumption is approximately 392,000 cubic metres per annum. Process water consists of tailings thickener overflow, with raw water for makeup. Fire water stores in a dedicated volume within the raw water tank. High-pressure air comes from compressors collected in a plant air receiver, with a dryer supplying instrument air. Reagent consumptions were estimated from test work results, with notable annual rates including sodium cyanide at 2,436 t/a in the initial phase and 1,380 t/a in expansion, SMBS at 5,338 t/a and 5,010 t/a respectively, and quicklime at 2,146 t/a and 1,935 t/a.

The plant design includes several safety and containment features. Reagent handling includes unloading and storage facilities, mixing and storage tanks, and feeding equipment for each required reagent. Compatible reagents sit in dedicated containment areas, with ventilation, fire protection, eyewash stations, and safety data sheet stations throughout. Sumps and sump pumps provide spillage control. The gold room has access control, intruder detection, and closed-circuit television.

Phase 2 additions include the tertiary mill with cyclone operating in open circuit, plus the additional leach tank to maintain 24 hours residence time at the expanded throughput. All other plant design remains as described for Phase 1. The staged expansion approach allows the plant to match capacity with mine production while deferring capital equipment to when it is needed.

Key Processes

• Single-stage jaw crushing from F80 330 mm to P80 55 mm
• SAG mill and ball mill in closed configuration with cyclones to P80 120 µm at 44% solids w/w
• Tertiary vertical tower mill in expansion phase with dedicated cyclone in open circuit
• Two leach tanks and six CIL adsorption tanks with 24 hours residence time and 12 g/L carbon concentration
• Interstage thickening to recover gold bearing solution before detoxification
• Carbon acid wash with hydrochloric acid at 2 BV/h
• AARL gold elution with cold cyanide wash for copper removal
• Electric rotary kiln carbon regeneration at 750°C in superheated steam
• Electrowinning with four cells and stainless steel mesh cathodes, followed by mercury retort and smelting
• Two-stage cyanide detoxification using SO₂/air process with 90 minutes retention
• Tailings thickening and filtration to 85% solids for dry stack disposal

Source: Lemhi Gold Project NI 43-101 Technical Report and Feasibility Study, Idaho, USA, June 29, 2026. Project website: Lemhi Gold Project