2024 Technical Report — Lamaque Complex (Sigma Mill)

This report describes the processing methods and equipment at the Sigma mill, which has a demonstrated annual throughput capacity above 900 ktpa.

Report context

This technical report for the Lamaque Complex in Québec, Canada, is dated 2024. The Sigma mill, situated at the East entrance of the city of Val-d’Or, started operation in 1937 and underwent a complete rebuild before operations restarted in March 2019. The current configuration consists of two-stage crushing, primary and secondary grinding, gravity concentration, cyanide leaching, carbon-in-pulp recovery, and gold refining. Minor debottlenecking investments are planned to ensure processing capacity keeps pace with increased mine production.

Processing route

Crushing Circuit

Ore from the mine is trucked to the Sigma mill site 24 hours per day. The crushing circuit operates during a 12-hour day shift and comprises a grizzly screen, a rock breaker, a 110 kW crusher with an opening of 1,120 mm × 870 mm, and a 294 kW secondary cone crusher in closed circuit with a triple deck screen. The final screened product is conveyed to a covered stockpile. In winter, calcium chloride is added to crushed material on the conveyor belt to prevent ore freezing.

Ore Storage

The mill is fed from a covered stockpile above a reclaim tunnel equipped with three apron feeders. A 100-tonne quicklime silo located near the ore storage silo feeds solid quicklime via a screw feeder directly onto the ore silo discharge conveyor.

Grinding Circuit

The grinding circuit includes a 2.74 m × 3.65 m, 300 kW rod mill and a 3.51 m × 4.27 m, 750 kW ball mill. The rod mill operates in open circuit, and the primary ball mill is closed with cyclones. A portion of the cyclone underflow is sent to the gravity circuit, with the remainder returned to the primary ball mill. The cyclone overflow, with a grind size P80 of 75 µm to 100 µm, proceeds to the secondary ball mill pumpbox. The secondary ball mill is 3.66 m × 4.27 m, 930 kW and operates in closed circuit with a second set of cyclones. The targeted grind size from the secondary ball mill is a P80 of 40 µm. A lead nitrate system is installed in the grinding circuit but is currently not in use.

Gravity Circuit

The gravity circuit incorporates a static screen and two gravity concentrators, each capable of 80 tonnes/hr solids. The gravity concentrate is treated on shaker tables, with the table concentrate further processed in the refinery.

Thickening

The trash screen underflow is pumped to the pre-leach thickener feed box. Two 9.14 m diameter high-rate thickeners are installed, supplied by a flocculant preparation system. Thickener overflow gravity feeds into the grinding water tank, and underflow is pumped to the first leach tank.

Leach Circuit

After thickening to approximately 50% solids, underflow slurry is pumped to the leach circuit, which currently consists of seven tanks with a total of 10,475 m³ active leach volume, translating to a leaching residence time of over 70 hours at current processing rates. Slurry flows by gravity between tanks, and each tank can be bypassed for maintenance. A second 40 tonne quicklime silo was recently installed near the main plant building to feed a lime slaker and milk of lime storage tank for pH control in the leach and cyanide destruction circuits.

Cyanidation

Sodium cyanide is fed to the leach circuit and the elution circuit. The sodium cyanide tank is located in an annex of the mill building with its own containment area and truck delivery pad, which is also used for sodium hydroxide and lime deliveries.

Gold Extraction Circuit

Leach circuit discharge flows through a sampler before feeding the carbon-in-pulp circuit, composed of one larger 280 m³ tank and six smaller 170 m³ tanks. Slurry flows by gravity between tanks, and new inter-stage screens prevent carbon transfer with slurry. Carbon is pumped counter-current to slurry flow using vertical pumps. Fresh pre-attritioned carbon is fed to the last tank via the regenerated carbon vibrating screen. After the carbon-in-pulp circuit, slurry proceeds to two parallel vibrating safety screens to recover smaller carbon particles.

Elution and Carbon Regeneration

Gold loaded carbon is pumped periodically from the first carbon-in-pulp tank onto a screen, with oversize flowing into a bin followed by the acid wash column, which soaks carbon with hydrochloric acid for about 2 hours. Carbon is then transferred to a 3-tonne capacity pressure elution column using the ZADRA process, which uses a high temperature, high pressure sodium cyanide and caustic solution to elute gold. The enriched solution is cooled and transferred to electrowinning cells. Carbon from the elution column is transferred to a dewatering screen, with oversize feeding the regeneration kiln, which is equipped with a 240-kW electric heater and can regenerate a maximum of 160 kg/h carbon. Bagged carbon fines are sent to a third-party smelter.

Refining

Two electrowinning cells operate in parallel with stainless steel cathodes. Gold is removed from cathodes by pressure washing, and the resulting gold sludge is pumped to a filter press. The filter cake is dried, and mercury is removed in a mercury retort unit before mixing with flux and melting in an induction furnace. Doré ingots are stored in a vault. The retort unit and furnace also treat shaking table concentrate.

Cyanide Destruction and Related Reagents

Carbon-in-pulp tailings are pumped to a cyanide destruction tank equipped with an agitator mechanism and compressed air line. Sodium metabisulphite is used as the SO₂ source, and copper sulfate is added as needed to catalyze the reaction. Milk of lime is used for pH control.

Tailings and Water Supply

Two tailings pump boxes, each connected to two tailings pumps in series with a third spare set, send plant tailings to the tailings pond. The plant is equipped with a grinding water tank and a recirculated water tank. Fresh water averaging about 65 m³/h is required for reagent mixing, gland water, gravity concentrators, carbon screens, and elution and carbon regeneration. A planned conversion of the spare pre-leach thickener to a tailings thickener in 2025 and addition of a paste backfill plant in 2026 will alter the water balance.

Air Supply Services

A third low pressure air compressor will be installed to meet expanded leach, carbon-in-pulp, and cyanide destruction compressed air requirements, with two compressors in operation and one stand-by unit. Plant compressed air is supplied by two high pressure air compressors with one operating and one on stand-by.

Key reported parameters

Technical qualifications

The report notes that the lead nitrate system in the grinding circuit is currently not in use. The Stockwork zone is not considered in the current analysis for lower Triangle zone recoveries. The planned conversion of the spare pre-leach thickener to a tailings thickener in 2025 and addition of a paste backfill plant in 2026 will alter the water balance. Future power requirements for the paste backfill plant will be analyzed based on siting, sizing, and other design parameters and may require additional power infrastructure. The water balance will change with these planned additions.

*Source: 2024 Technical Report , Lamaque Complex, Québec, Canada, Section 17 Recovery Methods.*