Sigma Mill

In 2014, Integra acquired the Sigma Mill and Mine Complex, a fully permitted 2,400 tonne per day mill and tailings facility, for $7 million in cash and stock. The mill last operated in 2012 and has an estimated replacement value of $100 million. Located at the east entrance of the city of Val-d’Or, directly across the street from the Lamaque South property, the mill can process 2,400 tonnes per day. Integra’s aggressive growth strategy is designed to harness the potential expansion of the mill to its previous 5,000 tonnes per day capacity. Integra’s main deposits and high quality growth targets are located less than 5 kilometers from the Sigma Mill.

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Mill Circuit Flow

Crushing Circuit

The plant feed is first dumped on a heavy duty grizzly. An impact hammer breaks the oversize rock directly on it.

The crushing circuit is composed of a vibrating grizzly feeder followed by a jaw crusher. A cone crusher is used to obtain the final crushing size, and this is installed in closed circuit with triple deck screen. The final screen product is transported by conveyor belt to the fine ore bin for storage.

The bin is equipped with an apron feeder. A second apron feeder could be used to feed the grinding circuit from an outdoor stockpile. The fine ore bin has an approximate live capacity of 2400 tonnes.

All the equipment within the crushing circuit is required for an operation at 450,000 tpa. The crushing capacity is such that all crushing work required to sustain the planned production can be done in a single 12 hour shift per day. The maximum capacity of the crushing circuit is around 210-230 tph.

Grinding and Gravity Circuits

The grinding circuit is composed of a primary rod mill and two ball mills. However, for the time being, a 75 µm grind size is being considered, and this would only require one of the two ball mills to be refurbished at the planned 450,000 tpa production capacity.

The rod mill runs in open circuit. The ball mill is operated in closed circuit with cyclones. The cyclone cluster is composed of fourteen cyclones, four units of which would be in operation and ten in standby. The underflow product from three cyclones feeds the ball mill, one cyclone underflow product is sent to the gravity recovery circuit, whereas all the overflows are sent to the thickener.

Lime, the pH controller, is added to the mill feed conveyor, and sodium cyanide can be added to the milling circuit if desired.

The gravity circuit is composed of two Knelson concentrators operating as parallel units to recover free gold. Initially only one of the units would be put in service. The Knelson concentrate is processed on a shaking table. The gold concentrate is then further treated in the refinery.

Thickener, Leach and Carbon-in-Pulp Circuits

The cyclone overflow is sent to a trash screen. The underflow goes to the leach circuit after it has been thickened by a high-rate thickener. The plant is equipped with two thickeners, but for a production rate of 450,000 tpa, only one would be rehabilitated.

In the leach circuit, cyanide is used to dissolve the gold. The circuit is composed of five tanks for a total of approximately 5100 m³ of active leach volume. Each tank is equipped with an agitator mechanism and compressed air lines.

The discharge of the leach circuit flows to the CIP circuit, composed of one larger tank and six smaller tanks. However for operation at 450,000 tpa, only the first large tank and four of the smaller tanks would be put in service. The slurry goes from one tank to the other by gravity. Inter-stage screens prevent carbon from being carried away with the slurry. Carbon is pumped counter-current to the slurry.

The combined volume of the leach and CIP circuits provides a residence time of approximately 72 hours.

Elution, Refinery and Carbon Regeneration

Loaded carbon is pumped from the first CIP tank onto a screen, which returns the underflow slurry to the tank. Carbon is then sent to an acid wash column to eliminate carbonates that make elution more difficult. From there, the carbon goes into a 3-tonne capacity elution column.

The elution solution is pre-heated via heat exchange with the hot pregnant solution and then further heated using an electric heater. The cooled pregnant solution is sent to the electrolysis cells located in the refinery. There are two electrolysis cells running in parallel. Gold forms on “stainless steel wool” cathodes. Cathodes are pressure washed and the recovered sludge is dry-out before melted with flux in an induction furnace. The drying oven and furnace are also used to treat the shaking table concentrate. The doré ingots are stored in a safety vault.

Carbon from elution is regenerated in a rotary furnace, cooled, screened and returned to the last CIP tank. Fresh carbon is added as needed via the cooling circuit.

Tailings and Cyanide Destruction

After going through the CIP circuit, the slurry proceeds onto a safety screen to recover any coarse carbon particles that may have passed through the inter-stage screen. The plant is equipped with two screens, but initially only one will be put into service. All wash water from carbon screening is stored in a tank, whereas the overflow, which contains fine loaded carbon, is filled into bags.

In the cyanide destruction tank, reagents and air are used to reduce cyanide concentrations to environmentally acceptable levels. For the purpose of this PEA, the use of sodium metabisulphite has been assumed. New tanks and pumps are required for this reagent. The plant is already equipped with copper sulphate tanks. Once through cyanide destruction, the slurry is pumped to the tailings pond.