Bell Creek Mill — 2021 Technical Report

This report describes the design, history, process description, and 2021 operating results for the Bell Creek Mill, a gold processing facility using crushing, grinding with gravity recovery, and carbon-based cyanidation methods.

Report context

This technical report summary is dated June 30, 2021, and details the Bell Creek Mill processing plant, including its historic operations, current process flowsheet, and metallurgical results for material processed in 2020 and early 2021. The report provides an update to the project description and processing methods for the Bell Creek Mill, which also processes ore from the Timmins West Mine.

Processing route

Plant overview

The Bell Creek Mill is described as a designed 5,359 tonne per day processing plant. Ore from Bell Creek is hauled from the surface stockpile to the mill's crusher, located on site. The processing route consists of a one-stage crushing circuit, an ore storage dome, a one-stage grinding circuit with gravity recovery, followed by pre-oxidation and cyanidation of the slurry with carbon-in-leach (CIL) and carbon-in-pulp (CIP) recovery. Ore from the company's Timmins West Mine is also processed through the Bell Creek Mill.

Crushing and ore storage

Ore feed is dumped directly onto a 16-inch by 16-inch grizzly at the truck dump. A remote-controlled rockbreaker is used to break up oversized material. The ore is fed with an apron feeder to a series of conveyors that report to a scalping grizzly feeder in the crushing building. The openings between the fingers on the grizzly feeder are 3.5 inches, with the oversize reporting to a 44-inch by 34-inch C110HD Metso jaw crusher. The jaw crusher is set to a closed side setting of 4 inches. The discharge from the crusher is combined with the undersize material from the grizzly feeder and conveyed to the ore storage dome. The dome has a 24,000-metric-tonne storage capacity, of which 6,000 metric tonnes is live. Three apron feeders pull ore from the dome and convey it to the SAG mill building.

Grinding and gravity recovery

The grinding circuit consists of one 22-foot diameter by 36.5-foot long low aspect ratio Metso SAG mill, powered by twin 6,250 horsepower (4,600 kW) motors. This SAG mill is a repurposed ball mill that was converted to a SAG mill by installing half-inch grates and a trommel with half-inch openings. Oversize from the trommel reports to a collection bin which is fed back into the SAG mill feed chute. Undersize from the trommel reports to a pumpbox which feeds a cyclopac equipped with 6 outlets. Five of the outlets are fitted with 20-inch Krebs gMAX cyclones, and the other outlet is capped and available for possible future expansion. The SAG cyclone overflow reports to the thickener feed box and the underflow reports back to the SAG mill. A portion of the cyclone underflow is fed to a 30-inch Knelson concentrator. Knelson concentrate is collected in a hopper and is pumped daily to the refinery for further treatment, while the Knelson tails flow by gravity back to the SAG mill. Target grind is 80 percent passing 200 mesh.

Thickening and leaching

Flocculant is added to the cyclone overflow, which is pumped to a 20-meter diameter thickener. The slurry from the cyclones is 25 to 35 percent solids by weight, with the thickener underflow at 55 to 60 percent solids by weight. The thickener overflow water is pumped to the process water tank and reused in the grinding process. The thickener underflow slurry is pumped to the leach circuit. The leach circuit consists of five agitated tanks in series with a total volume of 1,940 cubic metres. Pure oxygen is sparged into the first three leach tanks to passivate the contained pyrrhotite in the ore, as well as to maintain a target dissolved oxygen level, which is required for efficient gold dissolution in cyanide. Cyanide is then added to leach tank 4 or 5.

Carbon-in-leach and carbon-in-pulp circuits

There are three CIL tanks equipped with Kemix screens, having a total volume of 7,500 cubic metres. The CIL tanks contain 8 to 10 grams of carbon per liter of slurry. The circuit is described to reach equilibrium for loading of the carbon, with the grade of the loaded carbon in the range of 2,500 to 5,500 grams per tonne. Loaded carbon is pumped from CIL tank 5, screened, washed, and then transferred to the loaded carbon tank. Carbon in the CIP and CIL tanks is advanced counter-current to the flow of slurry in the circuits.

The slurry from CIL tank 1 reports to the CIP circuit and is split into two trains of three CIP tanks in parallel, with approximately 45 grams of carbon per liter of slurry. Recovery of the gold from the carbon is a batch process, with carbon being stripped at a rate of 3.5 tonnes per batch. The turnaround time between batches is 24 hours. Carbon can be cleaned with acid, reactivated with the kiln, and reused in the circuit.

Refining and bullion production

The loaded solution from the strip circuit is passed through two electrowinning cells in the refinery. The gold collects on the cathodes in a sludge form. The cells are washed weekly, and the sludge is collected in filter bags and dried. The dried sludge is then mixed with reagents and melted in an induction furnace. Gold bullion bars are poured when the melt is completed.

The gravity gold material collected from the Knelson concentrator is transferred to the refinery, where a gravity table is used to increase the gold content. The concentrate is then dried, reagents are added, and the material is melted in the induction furnace. The gravity concentrate and the CIP gold sludge are melted separately due to the differing amounts of reagents used in each, and to more accurately determine recoveries in each circuit.

Metallurgical balance

A metallurgical balance is conducted daily based on the tonnage from the 4-roller belt weightometer located on the feed conveyor to the SAG mill. The total tonnage, corrected for moisture, and assays from the daily sample campaign are used to produce the balance. All samples are assayed in accordance with typical assay standards, and a QA/QC program is in place to ensure the integrity of the assay lab processes. The main components used to calculate the daily balance are the thickener underflow solids and solution, the weight of gravity gold collected, the estimated grade and moisture content of the gravity gold collected, and the tailings sample solids and solution. The daily metallurgical balance is a best estimate of daily production, which must then be reconciled with the circuit inventory and bullion poured; this reconciliation is performed on a monthly basis.

All areas of the circuit are sampled for tank level, percent solids, solids grade, solutions grade, carbon concentrations, and grade where applicable. As the carbon contains the majority of the gold in inventory, strict care is taken to ensure sampling is performed correctly. The final cleanout of the electrowinning cell is completed by the refiner or his designate, under security control. All sludge is collected and dried. The washed cathodes from the cells are weighed, and the weights are recorded to determine whether any plating buildup is occurring. The dried cell sludge and the gravity concentrate collected over the same period are smelted, and bullion bars are poured. The bars are stamped, and their weights are recorded and verified. Bullion samples are taken and are assayed at the Bell Creek Lab. These sample results are used in the metallurgical balance.

Key reported parameters

The following table summarizes the key reported parameters from the technical report, noting the basis for each value as design, actual, or historical operating data.

Technical qualifications

Specific limitations from the report include:

• The report notes that the plant demonstrated after the Phase 2 expansion that it could achieve higher throughput than the design capacity of 3,300 tpd. The 2020 debottlenecking resulted in a maximum one-day tonnage of 6,000 tpd, which, combined with 91.14 percent run time and 2 percent moisture, yields an average throughput of 5,359 tpd. This value is a calculated average based on operational performance, not a confirmed sustained design basis.
• The historical metallurgical data for 1987 to 1994 is presented for context, covering material from Bell Creek ore and additional tonnage from the Marlhill Mine, Owl Creek open pit, and Hoyle Pond Mine. These historical figures are provided as background and are not indicative of current or future performance.
• The report states that gold recovery from all Bell Creek material met expectations established by test work completed prior to plant start-up, and that all material yields consistent high recovery and consistent grade. The average grind size to achieve these recoveries is reported as a P80 of 75 micron for 2020. This is presented as actual operational data.
• The metallurgical balance is described as a best estimate of daily production, which must be reconciled with the circuit inventory and bullion poured on a monthly basis, indicating that daily figures are subject to adjustment.

Source: Bell Creek Mill , 2021 Technical Report, Recovery Methods section, dated June 30, 2021, pages 123-126.