Eagle Gold Mine — 2023 Technical Report

The 2023 Technical Report describes the crushing, heap leach, and ADR processing facilities designed to treat approximately 11.5 Mt/y of ore at an average head grade of 0.65 g/t Au for 76% recovery, with supporting design criteria and heap leach infrastructure details.

Report context

The Eagle Gold Mine 2023 Technical Report documents process design criteria, flowsheet development basis, and operating parameters for the crushing, heap leach pad (HLP), and ADR plant facilities as of the 2023 reporting period. Design criteria were originally developed during the 2016 Feasibility Study and updated to show current operations. The report notes that the primary HLP began operating in 2019 and that the mine has completed its fourth winter of leaching operations.

Processing route

Crushing and ore handling

Run-of-mine ore is trucked from open pits and dumped into a primary feed hopper. A 375-kW gyratory crusher reduces material from a maximum feed size of 1,000 mm to a product P80 of approximately 115 mm. The primary crushing plant operates year-round at 29,500 t/d. During scheduled secondary and tertiary plant downtime, primary crusher product is conveyed to a stockpile; otherwise it feeds directly onto the secondary crushing feed conveyor. Stockpiled material is reclaimed by front-end loader at 470 t/h and combined to achieve a 39,200 t/d feed rate.

Secondary crushing begins with a vibrating double-deck screen. Oversize feeds a 932-kW secondary cone crusher; screen undersize is conveyed to the tertiary crushing feed conveyor. Tertiary crushing uses three vibrating double-deck screens and three 932-kW cone crushers in reverse closed circuit. Tertiary screen undersize has a target P80 of 12.5 mm, and is transferred by overland conveyors to the HLP for stacking via grasshoppers feeding a radial stacker.

A contractor mobile crushing spread, capable of 500 t/h, can operate when the fixed crushing circuit is down. Mobile crusher product is stockpiled, then reclaimed by loader onto a grasshopper feeding the overland conveyor at 1,000 t/h.

Lime is added to the stockpile feed conveyor by screw feeder from a 200-t lime silo at approximately 1 kg/t of ore for pH control.

Heap leach pad

The primary HLP is located approximately 1.2 km north of the Eagle pit in Ann Gulch, with a base elevation of 880 masl and a planned top elevation of approximately 1,225 masl. It is designed in phases: Phase 1 accommodates approximately 27.0 Mt, Phase 2 adds 32 Mt, and Phase 3 accommodates the remainder, for a total of 92 Mt. The secondary HLP, located approximately 3 km east of the Eagle pit near the Olive pit in the Bawn Boy catchment, will accommodate the remaining estimated 63.3 Mt of ore (designed capacity 111 Mt), extending from 1,300 masl to approximately 1,470 masl at full height in Phase 2.

The HLP liner system comprises a composite geomembrane over a geosynthetic clay liner (GCL), selected because suitable on-site soils were insufficient for a 300-mm-thick compacted low-permeability layer. The GCL provides equivalent secondary containment with hydraulic conductivity of 1×10⁻⁶ cm/sec or lower. Free-draining granular material placed above the liner contains collection pipes that convey PLS and storm infiltration while minimizing hydraulic head. Piezometers are installed at strategic locations to monitor hydraulic head.

The PLS sump area has a double-geomembrane liner installed over a GCL, with a leak detection and recovery system (LDRS) between the geomembranes. Event ponds are similarly lined and equipped with LDRS. Temporary runoff interceptor ditches or berms are constructed before each HLP phase and are sized for the 100-year, 24-hour event, armoured with riprap. Event ponds are sized to contain a Probable Maximum Flood (PMF) event plus 24 hours of heap draindown after the in-heap pond reaches maximum capacity. The primary HLP event ponds have a combined operational storage capacity of approximately 340,000 m³ including 1 m freeboard, or 300,000 m³ without freeboard.

The lower section of the HLP acts as an in-heap pond for primary storage of PLS. A minimum 0.6-m-thick drainage rock layer (all passing 38 mm) transmits PLS to the collection system. Collection piping is arranged in a "herringbone" pattern. Within the PLS sump, three submersible pumps with 112-kW motors operate with two spares available.

Winter operating provisions include an in-valley heap configuration, in-heap PLS pond, track dozer with ripper assembly for frozen areas, barren solution heating, in-heap temperature monitoring, buried drip emitter lines, heat-traced and insulated pipelines, and generators for backup power.

Solution management

Barren solution, consisting of a cyanide-caustic mixture, is pumped to the HLP at a nominal rate of 2,070 m³/h and applied via drip emitters buried at least 1 m deep, spaced approximately 1 m apart along each cell and slope. A diesel-fired boiler rated at 10 M Btu/h can heat barren solution during initial loading before the HLP mass achieves thermal balance; the report notes the boiler has seen little use to date.

Pregnant solution is collected in the in-heap sump and pumped to the ADR plant. The PLS pipeline runs on surface for approximately 400 m from the primary HLP and approximately 4 km from the secondary HLP. A leak detection system with moisture sensing cable ties into the plant distributed control system.

Solution temperatures during winter leaching have not fallen below 3°C and have typically ranged between 4°C and 8°C, trending toward a steady state of approximately 5.5°C as the heap has matured.

ADR plant

Pregnant solution from the HLP sump passes across a stationary trash screen before entering two trains of five cascading carbon-in-columns (CIC). Solution flows countercurrent to carbon movement from column 1 to column 5. Solution overflowing from the final column passes through a safety screen to recover any flushed carbon before discharging to the barren solution sump. Cyanide, caustic, antiscalant, and make-up water are added to the barren sump as needed. Approximately 8 t/d of loaded carbon (4 t per train) is pumped from the first columns to the acid wash and stripping circuits. Carbon is advanced up the train, with reactivated carbon added to the fifth column.

Loaded carbon is acid washed with dilute nitric acid to remove calcium, magnesium, sodium salts, silica, and fine iron particles, followed by dilute caustic neutralization. Carbon stripping uses the ZADRA process in a strip vessel holding approximately 8.0 t of carbon, with solution containing approximately 1.5% sodium hydroxide and 0.2% sodium cyanide at 140°C and 450 kPa. A diesel-fired boiler maintains solution temperature; pregnant solution is cooled below boiling point by a heat recovery heat exchanger that transfers heat to incoming cold barren solution. Stripped carbon is pumped to the carbon-regeneration circuit.

Carbon regeneration uses a vibrating dewatering screen to remove transfer water and fine carbon ahead of a 333 kg/h diesel-fired horizontal kiln treating 8.0 t/d at approximately 650°C. Kiln discharge is quenched and pumped back to the CIC circuit. Carbon losses from attrition are compensated by adding new carbon mixed with fresh water in the carbon attrition tank.

Gold recovery involves three electrowinning cells operating in parallel, where gold plates onto knitted-mesh steel wool cathodes. Loaded cathodes are power washed to recover gold-bearing sludge, which is filtered, dried, mixed with fluxes (borax, silica, nitre, soda ash), and smelted in an induction furnace to produce gold doré and slag. Doré bars are weighed, stored in a vault, and transported off-site for refining. Slag is processed to recover entrained gold prills and re-melted.

Sodium cyanide briquettes are delivered in 1-t super sacks, mixed with caustic and water, and added to the barren tank at 0.35 kg/t of ore. Cyanide is also used in the carbon strip circuit at 0.2% concentration. Caustic is supplied in 25-kg bags and used for pH adjustment and acid neutralization; a 1.5% caustic solution is used in the carbon strip circuit. Nitric acid and antiscalant are supplied in 208-L drums and 1-t totes, respectively.

The assay and metallurgical laboratory is equipped for sample preparation and analyses by AA, fire assay, and cyanide-soluble methods, supporting approximately 3,000 samples per month for exploration, mining, environmental, and process operations. Most environmental samples are sent off-site to an accredited laboratory.

Key reported parameters

Technical qualifications

The report states that flowsheet development, operating parameters, and design criteria were based on metallurgical testwork results presented in Section 13 of the report. Process design criteria were developed during the 2016 Feasibility Study and updated with modifications to match current operations, with the note that criteria will be continually updated throughout operations. The report notes the primary HLP began operating in 2019, and that the mine has completed four winter periods of leaching with no material impacts to date. The boiler for solution heating has seen limited use as solution temperatures have not necessitated it. The report indicates the secondary HLP will be constructed and begin operations during 2029.

Source: Eagle Gold Mine , 2023 Technical Report, Sections 17, 17.1, 17.2, 17.2.4, 17.2.4.5, 17.2.4.6, 17.2.6, 17.2.7, and 17.2.7.4.