Hemlo Mine — 2025 Technical Report

This report describes the conventional crushing, grinding, gravity concentration, cyanide leaching, carbon-in-pulp, elution, electrowinning, refining, cyanide detoxification, and flotation processing route at the Williams Mill.

Report context

This technical report on the Hemlo Mine, Ontario, Canada, is dated October 27, 2025 (SLR Project No.: ADV-TO-00122). The report presents the processing flowsheet for the Williams Mill, which has been in operation since 1985. The report includes descriptions of the existing plant configuration, historical operating data from 1985 through 2024, and proposed modifications for a potential restart of open pit operations.

Processing route

Crushing

Ore and waste rock from the underground mine are crushed by jaw crushers located underground and transported separately to a 300 t waste rock bin and a 300 t ore bin. Ore and waste are reclaimed from their respective bins by feeders and conveyed to a transfer tower where they are hoisted separately to the surface. The ore is then transported to two coarse ore surge bins near the mill, each with a live capacity of 4,000 t and a nominal capacity of 6,000 t. Waste rock is delivered to a load-out bin for trucking to waste rock storage.

When the open pit was operating, run-of-mine ore from the pit was trucked to the dump pocket of a 42 ft x 65 ft gyratory crusher where it was crushed to approximately 80% passing 90 mm. The crushed ore was conveyed to a stockpile with a nominal capacity of 40,000 t. The stockpiled ore was reclaimed by a front-end loader into a loading hopper and conveyed onto the coarse ore surge bin feed conveyor and delivered to the two coarse ore surge bins. After the closure of the open pit mine, the gyratory crusher, crushed ore transfer conveyors, and stockpiling conveyors were decommissioned but left in place. Restarting the open pit operation will require refurbishment of the crusher and conveyors.

Grinding

The grinding area comprises two parallel lines, each consisting of a semi-autogenous grinding (SAG) mill, a ball mill, and associated classification cyclone cluster, with a capacity of approximately 225 tph. The crushed ore surge bins are each dedicated to feeding one grinding line. The SAG mills are 6.71 m in diameter and 3.66 m long (22 ft diameter x 12 ft long), each with a pebble screen. The ball mills are 4.88 m in diameter x 6.40 m long (16 ft diameter x 21 ft long). All of the mills have 2,600 kW motors. The SAG mill pebble screen oversize is recycled back to the feed of each respective SAG mill. The pebble screen undersize and ball mill discharge are combined in each line and then classified by cyclones. The target particle size of the cyclone overflow is approximately 80% passing 75 µm. The overflow from the two cyclone clusters is combined and flows by gravity to a linear trash screen prior to being pumped to a 65 m conventional pre-leach thickener.

The two lines operate independently, and the closure of the open pit has allowed the mill to be operated using only a single grinding line since 2021. Both lines are fully operation-ready, and each line has been used alternately over the last five years.

The grinding circuit can be operated in either a split mode or a blend mode. In the split milling mode, ore can be preferentially directed to one or the other grinding line. Historically, underground ore was significantly higher in gold grades than open pit ore. The purpose of split milling was to allow for a finer grind on the more-grind-sensitive underground ore, while maximizing throughput of open pit ore through the other grinding line. The blend mode involves directing a mix of ores to the two coarse ore bins, allowing the bin levels to be maintained at 60%.

Gravity Concentration

In 2021, a gravity circuit was installed on grinding line 1 for gravity gold recovery. The circuit includes a Knelson concentrator and an intensive leach reactor (ILR). Subsequently, piping was installed to allow the circuit to be fed from grinding line 2 when that line is in use. Currently, the gravity circuit can only be fed from one grinding line at a time. The Knelson concentrate is processed in the ILR, producing a high-grade gold solution that is transferred to the pregnant solution tank at the gold room, where the gold is recovered by electrowinning.

Consideration is being given to the installation of a second Knelson concentrator, which would allow both grinding lines to feed the gravity circuit simultaneously. The concentrate from the second Knelson concentrator would be treated in the existing ILR, which has sufficient capacity.

Leaching and Carbon Adsorption

The pre-leach thickener underflow, at a solids content between approximately 50% and 54% solids, is pumped to the cyanide leach circuit, which consists of nine 12.2 m diameter x 13.0 m high tanks (normally eight in operation and one on standby). At a mill throughput of 10,000 tpd, the tanks allow for 24 hours of leach retention time. At current throughputs, retention time would be from 48 hours to more than 60 hours. Each leach tank is equipped with a dual-propeller agitator. Sodium cyanide solution is added to the leach circuit, and hydrated lime slurry is added to maintain the slurry pH at 10.5 or higher. The first two leach tanks are aerated with oxygen gas, generated from liquid oxygen.

The discharge slurry from the final leach tank flows by gravity to three trash screens with apertures of 28 mesh (595 µm). The screen undersize is pumped to the CIP circuit. The CIP slurry solid density is approximately 50% to 54% solids. The six, 8.0 m diameter x 9.2 m high CIP tanks are each equipped with a dual-propeller agitator, an in-tank carbon transfer pump, and an inter-stage screen to retain the carbon within each tank. The carbon is pumped counter-current to the slurry flow. Loaded carbon leaves the CIP circuit from the first CIP tank, and stripped, regenerated carbon is added into the last CIP tank. The carbon concentration in the CIP tanks is approximately 33 g/L to 35 g/L of slurry.

Carbon Elution and Regeneration

The loaded carbon leaving the first CIP tank is transferred to the carbon elution circuit, which has a capacity of 7 t per carbon batch. The leach residue flows by gravity to three vibrating carbon safety screens to capture any fine carbon that escapes the CIP circuit. Loaded carbon from the CIP circuit is washed with a diluted acid solution and stripped in a high temperature and pressure elution process. The stripped carbon is regenerated in a rotary kiln. The barren solution from the elution circuit is circulated back to the leach and elution circuits.

Refining

The gold in the pregnant solution from carbon elution and from the ILR is recovered by electrowinning. The gold sludge produced from the electrowinning circuit is dried, treated in a retort to remove mercury, and then smelted in an electric induction furnace to produce doré bars.

Tailings Detoxification, Flotation, and Disposal

A portion of the mill tailings is sent to the Williams paste plant to produce backfill for the underground mine. The portion required for backfill ranges from 30% to 50% of the total tonnage produced from the underground feed only.

Cyanide destruction and sulphide flotation circuits were added and commissioned in 2020. The CIP tailings stream is pumped to the cyanide destruction circuit, which reduces weak acid dissociable (WAD) cyanide to below 1 ppm, and then to flotation (excluding the tailings required for paste backfill). In flotation, a potassium amyl xanthate (PAX), a polyglycol frother, and air are added to remove sulphide minerals, thereby producing a non-acid generating (NAG) tailings stream and a sulphide concentrate. The sulphide concentrate is deposited in a paddock within the tailings management facility (TMF), while the flotation tailings are deposited in the Williams Basin within the TMF. Both streams are pumped, unthickened, approximately 4 km to 6 km to the TMF. Dilution water is added to the flotation tailings at the mill to maintain velocity and prevent solids from settling within the pipe to the TMF. The residual cyanide in the tailings degrades seasonally by natural attenuation. The water in the TMF is recycled back to the mill as process make-up water, while the excess supernatant is treated during the summer in an effluent treatment plant adjacent to the TMF prior to being discharged into the environment.

Reagents and Consumables

Key consumables and reagents include grinding media, hydrated lime, sodium cyanide, cyanide destruction chemicals (sodium metabisulphite and copper sulphate), and flotation reagents. The Hemlo ore is described as a low reagent-consuming ore with requirements from 2022 to 2024 of approximately 0.21 kg/t sodium cyanide, and 0.5 kg/t hydrated lime. Oxygen consumption has decreased from 0.47 kg/t in 2022 to 0.23 kg/t in 2024. Grinding media consumption averaged 1.09 kg/t from 2022 to 2024.

Water Requirements

A recent focus in the mill operations has been maximizing the water recycle from the TMF to the plant to minimize additional water being introduced into the circuit as water volumes within the TMF became challenging to cope with. Consequently, the use of raw water has been significantly reduced and was negligible from August 2024 through to April 2025 (the latest information provided).

Expansion (Proposed)

The potential for restarting open pit operations to supplement the underground ore supply has been assessed. Processing throughput could return to 10,000 tpd or more. This would require:

• Refurbishment of the primary gyratory crusher, crushed ore conveyor and stockpile conveyors
• Replacement of the stockpile reclaim conveyor and reclaim hopper (conveying crushed ore to the coarse ore bins)
• Installation of a second Knelson concentrator in the grinding area
• Installation of an additional cyanide destrution tank
• Installation of additional tailings flotation cells.

Additionally, to assist with the water balance in the TMF, as well as the collection and pumping of reclaim water, a tailings thickener may be installed near the TMF.

All other equipment is expected to be adequate to achieve the proposed throughputs.

Key reported parameters

Technical qualifications

The report notes that a simplified process flowsheet for the Williams Mill is presented in Figure 17-1, which is not reproduced in this text. A flowsheet showing the mill expansion requirements is presented in Figure 17-2. The gyratory crusher and associated conveyors for the open pit operation were decommissioned but left in place, and restarting the open pit operation will require refurbishment. The expansion requirements for a return to 10,000 tpd processing are identified as proposed modifications. The report does not provide actual operating performance data such as recovery rates, availability, or operating costs beyond the consumable rates reported for 2022 to 2024. The water recycle information is noted as the latest information provided through April 2025.

Source: Hemlo Mine , 2025 Technical Report, Carcetti Capital Corp., Section 17.0 Recovery Methods, dated October 27, 2025.