Phoenix Gold Mill Review Draws on 2015 and 2018 Operating Campaigns at the 1,250–2,500 t/d SAG-Ball, Gravity and CIL Plant

Rubicon Minerals' 2020 technical report for the Phoenix gold project at Cochenour, Ontario, describes the mill as built and the lessons from its two short runs. The 2018 bulk sample campaign recovered 92.9% to 96.7% of the gold.

The National Instrument 43-101 Technical Report for the Phoenix Gold Project Mineral Resource Update, Cochenour, Ontario, was prepared for Rubicon Minerals Corporation by T. Maunula & Associates Consulting and others, with an effective date of December 6, 2019 and a report date of January 3, 2020. Rubicon built the mill between 2013 and 2015, with tailings management and a paste backfill plant. The mill operated from May to November 2015 at an average head grade of 3.02 g/t Au and produced 5,610 oz. It then went into care and maintenance. In 2018 it was recommissioned to batch-process material from three stopes in the F2 Gold Zone and 7,620 t of low-grade material: 43,250 t in total, producing 5,669 oz Au. The mill then returned to care and maintenance.

Critical Data

Overview

Ore hoisted from underground is held in a 277 t raw ore bin beside the headframe and conveyed to an ore storage bin. Two apron feeders reclaim it to a two-stage grinding circuit: an open-circuit SAG mill and a ball mill closed with hydrocyclones. Part of the cyclone underflow feeds a gravity circuit, and the gravity concentrate is upgraded on a table and smelted. Cyclone overflow is trash screened, thickened and leached in a pre-aeration tank and six CIL tanks. Carbon is acid washed and stripped, and gold is electrowon and smelted to doré in an induction furnace. Tailings pass SO2/O2 cyanide destruction and go to the tailings management facility (TMF). The paste backfill and tailings filtration plants are installed but not commissioned. An Actiflo metals treatment system was commissioned in mid-2018 for effluent.

Key Process Stages

Ore handling. An underground grizzly with 23 cm openings and a rock breaker on the 305 m Level size the ore before hoisting. A 48-inch underground jaw crusher was bought but never installed. For future operation, a modular semi-mobile surface crusher beside the headframe will replace it. In 2015 the mill was fed uncrushed ore through the reclaim system, and the feed rate could not be held steady.

Grinding. Process water is added at the SAG feed chute. All SAG discharge passes a primary sampler, which in both campaigns under-reported head grade because gold accumulated in the ball mill. About 75% of the cyclone underflow returns to the ball mill and the rest goes to gravity.

Gravity. Cyclone underflow is screened and fed to the Knelson concentrators. The concentrate goes through a magnet and a gravity table, then is calcined and smelted. The circuit ran only 42% of the time in 2015 and recovered 14% of the gold. In 2018, wash cycles were cut from 60 to 30 minutes, doubling mass pull to 50 kg/h per unit. Gravity recovery rose to 49.4% with one cyclone and up to 57.3% with two, and cyanide consumption fell by 16% against design. Screening the table feed at 841 µm raised the table concentrate from 48% to over 65% gold.

Thickening and CIL. A 14 m thickener raises the slurry to 50% solids, and lime neutralizes the acidic peat-bog water. The pre-aeration tank gives four hours to restore dissolved oxygen to 8 mg/L and break down flocculant. Six CIL tanks of about six hours each hold 10.2 t of carbon per tank at 20 g/L. Kemix basket screens were replaced with Derrick conversion kits. Carbon loading peaked at 1,218 g/t in 2018, against 1,600 g/t in 2015.

Elution and refinery. Loaded carbon is washed and acid washed in 3 to 5% hydrochloric acid. It is stripped through heat-recovery exchangers and a heater, and the pregnant solution is electrowon in two parallel cells with stainless steel cathodes. Sludge is filter pressed, calcined, fluxed and smelted. Stripped carbon is reactivated in a kiln and returned to the final CIL tank.

Cyanide destruction and tailings. A safety screen recovers fine carbon. Oxygen produced on site and liquid SO2 (or SMBS), copper sulphate and lime destroy the cyanide before the tailings go to the TMF or the paste plant buffer tank. When completed, the paste plant will filter tailings on two disc filters and mix the cake with fly ash and Portland cement.

Additional Interesting Data and Summary

The 2015 start-up had grate failures, trash screen overflows and coarse rock in the CIL. About 200 t of gravel had to be vacuumed out of CIL Tank B. Fine solids building up in the process water tank damaged unlined pumps, valves and piping. In 2018, cyanide was added in stages inside the mill, which left high residual free cyanide in the last CIL tanks. The PEA includes capital for leach tanks outside the mill building. Rubicon is also considering expanding the gravity circuit to over 3,000 kg/d of mass pull before restarting.

In summary, the 2020 Phoenix report covers an existing single-line SAG-ball mill with gravity and CIL. Its two short campaigns exposed commissioning and design problems, many of them fixed in 2018, when gold recovery reached 92.9 to 96.7%.

Key Processes

• Underground grizzly sizing and hoisting (surface crusher planned)
• Open-circuit SAG and closed-circuit ball milling
• Knelson gravity concentration and tabling
• Pre-leach thickening and pre-aeration
• Carbon-in-leach
• Acid wash, elution, electrowinning and induction smelting to doré
• SO2/O2 cyanide destruction
• Paste backfill (installed, not commissioned)

Source: National Instrument 43-101 Technical Report for the Phoenix Gold Project Mineral Resource Update, Cochenour, Ontario, prepared for Rubicon Minerals Corporation by T. Maunula & Associates Consulting Inc. and others, effective date December 6, 2019.