The Eskay Creek Project is designed as a two-phase gold and silver flotation concentrator, with initial throughput of 3.0 Mt/a expanding to 3.7 Mt/a.
Report context
This technical report presents the recovery methods for the Eskay Creek Project as described in the NI 43-101 Technical Report and Feasibility Study dated September 2022. The report documents the proposed process plant design, including comminution, flotation, concentrate dewatering, and tailings disposal. The project is based on a feasibility-level study, with the processing route selected from testwork analysis and subsequent economic assessment. The process design is presented in two phases: initial operation from years 1 to 5 and expansion from year 6 onward.
Processing route
Overall process design
The project will be constructed in two distinct phases. Initial operation of 3.0 Mt/a for years 1 to 5 includes a single-stage jaw crushing circuit fed from the open pit mine, a coarse ore stockpile with reclaim system, and primary grinding consisting of a semi-autogenous grinding (SAG) mill, pebble crusher (installed for year 4 operations), and ball mill in closed circuit with hydrocyclones. The initial phase also includes rougher flotation with concentrate regrind and two stages of cleaning, rougher tails slimes classification via two stages of hydrocyclones, secondary grinding with ball mill and IsaMill plus scavenger flotation, fines flotation with two stages of cleaning, concentrate thickening, filtration, drying and storage, and concentrate load-out by front-end loader. Final tailings will be pumped to the TMSF.
Expansion to 3.7 Mt/a for the remaining mine life includes the initial equipment with the addition of a secondary cone crushing circuit, a second ball mill and extra cyclones in the primary grinding circuit, and an additional IsaMill in the secondary grinding circuit, all installed for year 6 operation.
Crushing and stockpile
The crushing facility initially consists of a single-stage crushing circuit processing run-of-mine ore at a nominal rate of 489 t/h at 70% availability for years 1 to 5. Following installation of the secondary crusher, nominal processing rate will be 603 t/h at 70% availability for year 6 onward. Major equipment includes a stationary ROM bin grizzly, ROM surge bin, primary crusher apron feeder, vibrating grizzly, primary jaw crusher, and secondary screen and cone crusher installed for the expansion. The coarse ore stockpile is uncovered with reclaim via apron feeders. The overland conveyor from the primary crusher to the coarse ore stockpile has a route length of 725 m with a 150 m lift and is fitted with a weightometer. Live stockpile capacity is approximately 8 hours initially and approximately 6.5 hours after expansion.
Grinding and classification
The primary grinding circuit for years 1 to 3 consists of a SAG mill and ball mill in closed circuit with classifying cyclones. A pebble crusher begins operating at the start of year 4. A second ball mill is installed in a new building starting at the beginning of year 6. The proposed ball mill circulating load is a nominal 250% of new feed. Primary grinding is designed for a product size P80 of 100 µm. The SAG mill is driven by a single wound rotor drive motor with liquid resistance starter and slip energy recovery unit for variable speed operation. Process water is added to the SAG mill to achieve a slurry density of approximately 70% solids by weight. Cyclone overflow reports to a trash screen, with undersize flowing by gravity to rougher flotation.
Flotation and regrinding
The flotation area comprises a primary rougher and cleaner circuit, rougher scavenger circuit, and fines rougher and cleaner circuit. Regrinding of rougher concentrate occurs prior to cleaning. Rougher tailings are classified for slimes separation prior to secondary grinding and scavenger flotation. Conventional tank cells are used in all flotation circuits. Flotation reagents include potassium amyl xanthate (PAX; collector), copper sulphate (promoter), and methyl isobutyl carbinol (MIBC; frother). The regrind mill targets a discharge size P80 of 15 µm. Slimes classification overflow targets P80 of 20 µm, and the secondary regrinding circuit targets a final discharge P80 of 30 µm. Rougher concentrate is pumped through the regrind mill cyclone cluster, with cyclone underflow ground in the regrind mill. Cleaner 2 concentrate gravitates to the concentrate thickener. Rougher flotation tailings are pumped to the slimes classification circuit. Underflow from both classification stages feeds the secondary grinding circuit. For the expansion, extra cyclones are added to the secondary grinding cyclopaks, and a portion of dewatering cyclone underflow is pumped to the new IsaMill in the expansion building.
Concentrate dewatering
Cleaner 2 concentrate combines with fines cleaner 2 concentrate and is pumped to a static trash screen, then to a feed de-aeration tank and high-rate thickener. Flocculant is added to enhance settling. Thickener underflow solids density ranges from 40% to 55% w/w depending on ore feed. The filter feed tank provides 12 hours of surge capacity. Concentrate is filtered in one of two vertical plate pressure filters to produce filter cake at approximately 15% w/w moisture. A Holo-Flite dryer reduces moisture to below 13.5% w/w for transport. Dried concentrate is stored in a normally empty storage area with nominal capacity of 4 days or 2,407 t.
Tailings disposal
Scavenger tailings, cleaner 1 tailings, fines rougher tailings, and fines cleaner 1 tailings combined with excess contact water are pumped to the TMSF. In-line flocculant is added to enhance settling and minimize turbidity.
Reagents and consumables
PAX is supplied in 1,000 kg bulk bags as a dry reagent and mixed to a 15% w/w solution, with consumption of approximately 2,100 t/y. Copper sulphate pentahydrate is supplied in 1,200 kg bulk bags and mixed to 15% w/w, with consumption of approximately 1,800 t/y. MIBC is supplied in 810 kg IBC totes and dosed directly, with consumption of approximately 450 t/y. Flocculant MF336 (or similar) is supplied in 25 kg bags and mixed to 0.25% w/v, with consumption of approximately 10.100 t/y split evenly between concentrate and tailings thickening.
Services
Fresh water is sourced from borehole wells, with consumption of approximately 2,330 m³/d for years 1-5 and 2,820 m³/d for years 6+. The fresh water tank provides 10 hours of capacity. Potable water is treated on site with 44 hours of storage tank live capacity. Process water consists predominantly of mine dewatering, contact water, concentrate thickener overflow, and TMSF reclaim water as required, with 1.2 h of live storage capacity.
Process control strategy
Field instruments provide inputs to programmable logic controllers (PLCs), with each process area controlled by a single PLC. PLCs collect drive and instrument status, provide drive control and process interlocking, and perform PID control for process control loops. Control is performed in two centralized control rooms. HMI operator stations provide graphical representation of process equipment and perform interlocks, level control, alarms, trends, and report generation.
Key reported parameters
| Parameter | Units | Year 1–5 | Year 6+ |
|---|---|---|---|
| Ore throughput (base case) | Mt/y | 3.0 | 3.7 |
| Daily throughput, average | kt/d | 8.22 | 10.14 |
| Process plant capacity, nominal @ 92% availability | t/h | 372 | 459 |
| Process plant availability | % | 92 | 92 |
| Crushing plant availability | % | 70 | 70 |
| Filter plant availability | % | 85 | 85 |
| Average head grade, Au | g/t | 2.99 (life of mine) | , |
| Average head grade, Ag | g/t | 78.55 (life of mine) | , |
| Recovery to concentrate, mass | % plant feed | 5–10 | 5–10 |
| Concentrate grade, Au | g/t | 25–50 | 25–50 |
| Primary grinding product size P80 | µm | 100 | 100 |
| Regrind mill discharge P80 | µm | 15 | 15 |
| Slimes classification overflow P80 | µm | 20 | 20 |
| Secondary grinding discharge P80 | µm | 30 | 30 |
| Thickener underflow density | % w/w | 55 | 55 |
| Filter cake moisture, nominal | % w/w | 15 | 15 |
| Dryer cake moisture | % w/w | <13.5 | <13.5 |
| Ball mill circulating load | % new feed | 250 | 250 |
Project website: https://skeenagoldsilver.com/eskay-creek/
Project website: https://skeenaeskaycreek.com/
Technical qualifications
The report states that testwork was thoroughly analysed with several process route options considered in the initial feasibility study stages. The selected process route was chosen as best suited to the testwork results and subsequent economic analysis. Recoveries and process parameters are presented as design criteria and annual production estimates; they do not represent actual operating data. Annual gold recoveries range from 76% to 87% and silver recoveries from 82% to 90% across the mine life. Comminution parameters, including Bond work indices and A×b values, vary by period, showing changing ore characteristics. Grinding media consumption figures are estimated from power input and industry-standard calculation methods based on expected mill operating conditions and abrasion indices from testwork. The report notes that the process plant design incorporates flexibility for treatment of all ore types. Equipment specifications in the major equipment list include "or equivalent" designations for several items, indicating that final equipment selection may vary.
Source: Eskay Creek Project, NI 43-101 Technical Report and Feasibility Study, September 2022, Section 17 Recovery Methods.
