Fox Complex Expansion — 2022 Technical Report

The report presents a modified processing flowsheet for the Fox Mill at the Stock mine site, supported by testwork and past operational performance.

Report context

The NI 43-101 technical report, dated January 2022 (Project No. 246279), describes proposed modifications to the Fox Mill to increase overall tonnage throughput and process harder materials from multiple deposits including Froome, Grey Fox, Stock West, and Fuller. The report is prepared for McEwen Mining Inc. as a preliminary economic assessment (PEA).

Processing route

Existing and Proposed Flowsheet

The overall flow of the existing plant is maintained with modifications to increase tonnage throughput and process harder materials at higher rates. The simplified process flowsheet aligns with recommendations from this PEA.

Mineralized material delivered to the Fox Mill is stockpiled and bucket-blended into a portable crusher. The material continues through a three-stage crusher and is fed to a primary ball mill operating in closed-circuit. Sized material is transferred to regrind with two ball mills operating in parallel in closed-circuit. Final sized product is leached with cyanide and recovered to activated carbon. Gold is stripped from the carbon and recovered with electrowinning cells and poured into doré bars. Stripped carbon is acid washed, regenerated, and returned to the leach circuit.

Historical Operating Data

When the PEA study work started, plant operations processed material on an on/off schedule based on mining rates and typically ran at 1,600 t/d with a target grind of 75 μm. The mill now operates continuously with a 24/7 schedule.

Proposed Design Changes

Operational changes required within the plant include increasing the target production size to 130 μm and reducing the closed side setting on the secondary crusher to 12 mm. The larger product size requires less grinding and the tighter CSS reduces the workload on the tertiary crusher, which is the current plant bottleneck. Since the site visit, the primary crusher has been rebuilt, improving availability.

Recommendations for operational improvements that are not imperative to production but will improve operation and reduce risk of not meeting throughput rates include moving the portable crusher to discharge to the apron feeder and increasing the width of the conveyors around the secondary and tertiary crushers feeding the sizing screen.

Crushing Circuit

Plant feed involves a pre-crush step with a Cedarapids portable crusher. Wood recommends relocating the portable crusher such that the product discharges to the apron feeder. Feed to the crushing circuit first passes over a fines screen to remove undersize material, though the installed screen does not currently operate properly and all feed is transferred to the Kemco C-95N jaw crusher. This screen should be replaced with screens designed to manage wet, sticky material. McEwen is currently replacing the existing screen with a Weir scalping screen.

Material less than 11 mm reports to a bypass conveyor and goes directly to the fine ore bin. Material larger than 11 mm and less than 50 mm bypasses the jaw crusher. Oversize material is crushed in the jaw crusher to a transfer size P100 of 133 mm.

Jaw crusher product and intermediate size material from the fines screen are fed to the Sandvik secondary cone crusher operating in open circuit. Wood recommends the CSS be reduced to 12 mm to reduce the P80 to 15.6 mm. Product from the secondary and tertiary cone crushers combine on conveyor #2 that feeds conveyor #3, which feeds the sizing screen. Wood recommends increasing the width of conveyor #2 and #3 to 762 mm. The sizing screen is a Metso banana screen with undersize as final product to the fine ore bin and oversize fed to the tertiary crusher. The tertiary crusher is a Metso cone crusher and is often overloaded but with crusher circuit modifications should operate at 60 to 70% of installed power.

Grinding Circuit

Crushed material is fed to the primary ball mill via conveyor from the fine ore bin. Wood recommends increasing the diameter of the primary ball mill to 4.3 m to utilize installed power. An engineering study should be completed to confirm that a larger shell will fit without major modifications and that items such as bearings can withstand the additional weight and increased volume of total charge.

The primary ball mill is operated in closed circuit with three hydrocyclones and one stand-by. Underflow is returned to the ball mill and overflow is sent to the secondary cyclone feed pumpbox. Underflow of the secondary cyclone cluster, with five cyclones operating and one stand-by, is split to two regrind mills operating in parallel. Wood recommends increasing the target grind size to P80 130 μm to meet tonnage production targets. With the larger primary ball mill and coarser transfer and product size, Wood recommends using larger size grinding media: 89 mm grinding balls for the primary ball mill and 38 mm grinding balls for the regrind mills.

Leach Circuit

The back end of the process plant is adequate to process the increased production levels. Wood does not recommend any changes to the leach circuit.

Ground slurry is thickened and pumped to the first of two leach tanks. Cyanide is added to the first tank, leach tank #3. Lime is currently added to the primary mill cyclone pumpbox for pH control. There are two concrete pedestals available if additional leach volume is required in the future.

Leached slurry overflows to the carbon-in-leach circuit with six tanks. The first two tanks do not contain carbon and loaded carbon is removed from CIL tank #3. If additional carbon and residence time is required in the future, interstage screens and carbon transfer pumps could be installed to the first two tanks to convert them to CIL. Tailings pass over a safety screen before being pumped to the tailings dam.

Carbon Stripping and Electrowinning

Carbon from the loaded carbon screen is transferred to the loaded carbon tank in three tonne batches to the carbon strip vessel. There is a parallel system to strip and acid wash a one tonne batch that is not in use. Gold is removed from the carbon using a Zadra process with a concentrated solution of caustic and cyanide at 142 °C. Hot pregnant solution is cooled through a heat exchanger before going into an electrowinning cell where gold plates onto stainless steel cathodes.

Stripped carbon is not currently being acid washed. Wood recommends reinstating this practice. The circuit has the capability to acid wash carbon in three tonne batches with 3% nitric acid. Acid washed carbon is split to two carbon regeneration kilns that can each process 1.5 t/d. Regenerated carbon is cooled in a quench tank, passed over a carbon sizing screen into a conditioning tank before being pumped back to the CIL circuit.

Tailings and Water Management

A new 16 m tailings thickener is presented as an addition to the current flowsheet to send thickened tails to the tailings facility and return overflow water back to the mill solution tank. Underflow will target 60 to 65% solids to the tailings facility. Additional reclaimed water is recovered from tailings via a barge-mounted pump. Water can also be pumped to water treatment tanks for cyanide destruction using air, sodium metabisulphite, and copper sulphate. Ferric sulphate can be added to precipitate arsenic when required. Treated water flows to a polishing pond before discharging to the environment.

Key reported parameters

Technical qualifications

The report states that testwork and past operational performance supports the flowsheet as appropriate to process mineralized material from all the deposits. The design criteria were used to determine recommendations for this PEA. Wood recommends that an engineering study be completed to confirm that a larger primary ball mill shell will fit without major modifications and that bearings can withstand the additional weight and increased volume of total charge.

Source: Fox Complex Expansion, NI 43-101 Technical Report, McEwen Mining Inc., Project No. 246279, January 2022, Section 17.0 Recovery Methods.