Timmins West Mine — 2021 Technical Report

The Timmins West Mine delivers gold ore by highway truck to the Bell Creek Mill, a facility configured with crushing, single-stage grinding with gravity recovery, and a carbon-in-leach and carbon-in-pulp circuit.

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Timmins West Mine ore is transported via surface highway trucks to the Bell Creek Mill for processing. The Bell Creek Mill site is positioned roughly 6.7 kilometres north of Highway 101 in South Porcupine, Ontario. This designed 5,468 tonne per day facility combines a one-stage crushing circuit, an ore storage dome, a single-stage grinding circuit with gravity recovery, and a subsequent pre-oxidation and cyanidation stage treating slurry in carbon-in-leach and carbon-in-pulp circuits. Ore from LSG's Bell Creek Mine is also processed through the same mill.

The Bell Creek mill was originally established as a conventional gold processing plant that used cyanidation with gravity and carbon-in-pulp recovery. Between 1987 and 1994, the operation processed 576,017 short tonnes of Bell Creek ore at a grade of 0.196 ounce per short ton, recovering roughly 112,739 ounces of gold, with historical recovery near 93 percent. In the period before 2002, ore from the Marlhill Mine, Owl Creek open pit, and Hoyle Pond Mine was also treated, and the facility's capacity grew from its original 350 tonnes per day to 1,500 tonnes per day. LSG acquired the mill in 2008 and restarted it in 2009 at a rate of 1,000 tonnes per day. Capacity rose to 2,000 tonnes per day in the fourth quarter of 2010 and then to 2,500 tonnes per day in 2011. The Phase 2 expansion, which brought design capacity to 3,300 tonnes per day, was completed in the third quarter of 2013. The plant has exceeded this design capacity since that time. Further debottlenecking in 2020 allowed a maximum one-day tonnage of 6,000 tonnes per day, and combined with a 91.14 percent run time and 2 percent moisture, the facility achieved an average throughput of 5,369 tonnes per day.

Processing results for Timmins West Mine material in 2020 showed high performance, with 997,918 tonnes processed at an average gold grade of 2.59 grams per tonne and a recovery of 97.30 percent. Gravity recovery averaged 32 percent during this operational period. Gold recovery from all Timmins West Mine material has met expectations set by pre-startup test work, and the ore has produced a consistent high recovery and consistent grade. The average grind size required to achieve these recoveries is a P80 of 75 microns. All reagent consumptions remained at expected levels for the different ore types processed.

Critical Data

Overview

The Bell Creek Mill handles ore from both the Timmins West Mine and LSG's Bell Creek Mine, with Timmins West ore arriving by truck. The plant is designed around a conventional gold recovery flowsheet that has been expanded multiple times since its original construction in 1987. The current configuration achieves throughputs above its design capacity, and 2020 processing demonstrated strong recoveries of Timmins West material. The circuit combines gravity concentration with carbon-in-leach and carbon-in-pulp recovery, with a target grind size of 80 percent passing 75 microns.

Key Process Stages

Ore from the Timmins West Mine and Bell Creek Mine is delivered to the mill by surface highway trucks. At the truck dump, material is dumped directly onto a 16-inch by 16-inch grizzly, where a remote-controlled rockbreaker breaks up oversized material. An apron feeder feeds the ore to a series of conveyors that report to a scalping grizzly feeder in the crushing building. The grizzly feeder has openings of 3.5 inches, and the oversize reports to a C110HD Metso jaw crusher set to a 4-inch closed side setting. The crusher discharge combines with the material passing the 3.5-inch grizzly, and the combined stream reports to the ore storage dome. This dome has a 24,000-metric-tonne storage capacity, of which 6,000 tonnes is live. Three apron feeders draw ore from the dome, conveying it to the SAG mill building.

The grinding circuit uses a single 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 was originally a ball mill, converted by fitting 0.5-inch grates and a trommel with 0.5-inch openings. Oversize from the trommel is collected in a bin and fed back into the SAG mill feed chute. Undersize from the trommel enters a pumpbox that feeds a cyclopac with six outlets. Five of these outlets are equipped with 20-inch Krebs gMAX cyclones, with the sixth capped and available for future expansion. The SAG cyclone overflow reports to the thickener feed box, while underflow returns to the SAG mill. A portion of the cyclone underflow feeds a 30-inch Knelson concentrator for gravity recovery. The Knelson concentrate is collected in a hopper and pumped daily to the refinery, while the tails flow by gravity back to the SAG mill. The target grind is 80 percent passing 200 mesh.

Flocculent is added to the cyclone overflow, which is pumped to a 20-metre diameter thickener. The slurry from the cyclones contains 25 to 35 percent solids by weight, and the thickener underflow is 55 to 60 percent solids by weight. Thickener overflow water is pumped to the process water tank and reused in the grinding process. The thickener underflow slurry feeds the leach circuit, which 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 pyrrhotite in the ore and to maintain a target dissolved oxygen level needed for efficient gold dissolution in cyanide. Cyanide is added to leach tank number four or five. Three CIL tanks equipped with Kemix screens follow, with a total volume of 7,500 cubic metres, holding 8 to 10 grams of carbon per litre of slurry. The circuit reaches equilibrium for carbon loading with loaded carbon grades in the range of 2,500 to 5,500 grams per tonne. Loaded carbon is pumped from CIL tank number five, screened, washed, and transferred to a loaded carbon tank. Carbon in the CIP and CIL tanks advances counter-current to slurry flow. Slurry from CIL tank number one reports to the CIP circuit, split into two parallel trains of three CIP tanks with roughly 45 grams of carbon per litre of slurry.

Gold recovery from carbon is a batch process. Carbon is stripped at a rate of 3.5 tonnes per batch, with a 24-hour turnaround time between batches. Carbon is cleaned with acid, reactivated using a kiln, and reused in the circuit. The loaded solution from the strip circuit flows through two electro-winning cells in the refinery, where gold collects on the cathodes as sludge. Cells are washed weekly, and the sludge is collected in filter bags and dried. The dried sludge is mixed with reagents and melted in an induction furnace, with gold bullion bars poured when the melt is complete. Gravity concentrate from the Knelson concentrator is transferred to the refinery, where a gravity table increases the gold content. The concentrate is dried, mixed with reagents, and melted in the induction furnace. The gravity concentrate and the carbon-in-pulp gold sludge are melted separately because they contain different amounts of reagents, a practice that allows more accurate determination of recoveries for each circuit.

The daily metallurgical balance uses tonnage from the four-roller belt weightometer on the SAG mill feed conveyor, with total tonnage corrected for moisture combined with daily sample assays. Samples are assayed to typical standards under a QA/QC program that ensures integrity of the assay laboratory processes. Main components contributing to the daily balance include thickener underflow solids and solution, the weight of gravity gold collected, estimated grade and moisture of that gravity gold, and tailings sample solids and solution. This daily balance is a best estimate of production and is reconciled monthly with circuit inventory and bullion poured. All circuit areas are sampled for tank level, percent solids, solids grade, solution grade, carbon concentration, and carbon grade. Since the carbon contains most of the gold in inventory, sampling is performed with strict care.

Additional Interesting Data and Summary

The Bell Creek Mill's processing performance with Timmins West ore has met pre-startup expectations, delivering high and consistent recovery with consistent grade. The plant achieved an average throughput of 5,369 tonnes per day in 2020 with a 91.14 percent run time while processing Timmins West material at 997,918 tonnes for the year. The metallurgical balance is conducted daily as a best estimate, with monthly reconciliation against circuit inventory and bullion production. Final cleanout of the electro-winning cell is done under security control, with all sludge collected and dried. Washed cathodes from the cells are weighed, and the weights recorded to identify any plating buildup. Dried cell sludge and gravity concentrate from the same period are smelted together, with bullion bars poured, stamped, and weighed. Bullion samples are assayed at the Bell Creek Lab, and results are used in the metallurgical balance.

Key Processes

• Crushing: single-stage jaw crusher, 4-inch closed side setting
• Grinding: single SAG mill, 22-foot diameter by 36.5-foot long, twin 6,250 hp motors
• Gravity recovery: 30-inch Knelson concentrator
• Leaching: five agitated tanks, pure oxygen sparging, total volume 1,940 cubic metres
• Carbon-in-leach: three tanks with Kemix screens, total volume 7,500 cubic metres
• Carbon-in-pulp: two parallel trains of three tanks, approximately 45 g/L carbon
• Elution: batch stripping at 3.5 tonnes per batch, 24-hour turnaround
• Refining: electro-winning, sludge collection, induction furnace melting, bullion pouring
• Metallurgical balance: daily, reconciled monthly
• Target grind: 80 percent passing 200 mesh (75 microns P80)

Source: Timmins West Mine , 2021 Technical Report, 2021-06-30. Project website: Timmins West Mine

Project website: Timmins West Mine, 2021 Technical Report