Paramount Gold Nevada's September 2020 feasibility study for the Grassy Mountain gold project in Oregon replaces the 2018 PFS flowsheet with a lower-capital design treating 248,365 tons a year over a 7.8-year life.
The NI 43-101 Technical Report on Feasibility Study for the Grassy Mountain Project, Oregon, USA, was prepared by Ausenco, MDA, Golder and others for Paramount Gold Nevada Corp. with an effective date of September 15, 2020. The testwork showed the gold-silver mineralization is amenable to cyanide leaching. The plant is a 750 ton/d circuit of two-stage crushing, a ball mill, carbon-in-leach (CIL), elution and electrowinning, which the report describes as well-known, conventional unit operations. In the feasibility study, the 2018 PFS flowsheet was changed to a simpler, lower-capital alternative.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Report effective date | September 15, 2020 | Not applicable | Ausenco and others for Paramount Gold Nevada Corp. |
| Throughput | 750 tons/d (34 tons/h) | Not applicable | 248,365 tons/a; short tons |
| Mine life | 7.8 | years | Not applicable |
| LOM grade | 0.206 oz/ton Au, 0.293 oz/ton Ag | Not applicable | Design grade 0.266 oz/ton Au |
| Plant availability | 91.3 | % | 7,998 h/a; crushing 70% |
| Bond ball mill work index (75th percentile) | 26.9 | kWh/ton | Rod work index 22.3 kWh/ton |
| Primary crusher | Metso C80 jaw or equivalent | Not applicable | Secondary Metso HP200 cone |
| Ball mill | 12 x 16 ft, 1,341 hp installed | Not applicable | Up to 350% circulating load |
| Grind | P80 150 mesh (106 µm) | Not applicable | Not applicable |
| Leach-CIL | 1 pre-aeration + 2 leach + 7 CIL tanks | Not applicable | 42,250 gallons each |
| Reagents | 0.68 lb/ton NaCN, 2.1 lb/ton lime | Not applicable | Leach addition |
| Elution | Pressure Zadra, 2.2 ton batches | Not applicable | Mercury retort before smelting |
| Cyanide destruction | SO2/air | Not applicable | Ahead of the TSF |
Overview
The plant runs two shifts a day, 365 days a year, and makes doré bars. The gold room runs every week of the year, with two operating days and one smelting day a week. The 2020 flowsheet comprises two-stage crushing, grinding, a hybrid leach-CIL circuit with pre-aeration, mercury removal and cyanide destruction. Crushing and grinding were sized using Ausenco's Bruno and Ausgrind simulations.
Key Process Stages
Crushing. Ore from the underground mine is tipped into a ROM hopper, or onto an outdoor stockpile when the crushers are down. A vibrating pan feeder feeds the primary jaw crusher. A coarse ore screen sends oversize through a surge bin and vibrating feeder to a secondary cone crusher, whose product returns to the screen. Screen undersize goes by a weighed conveyor to the fine ore bin. A belt magnet and metal detector protect the equipment. The circuit averages 45 tons/h.
Grinding. A 12 ft by 16 ft single-pinion ball mill with a 1,341 hp fixed-speed motor runs at 72% solids in closed circuit with a cyclone cluster, at up to 350% circulating load. Cyclone overflow at about 45% solids and P80 150 mesh passes a trash screen to pre-aeration.
Pre-aeration and leaching. Testwork showed that pre-aeration with low-pressure air reduces cyanide consumption and improves recovery, and that the optimum leach time for gold is 24 hours. Lime (2.1 lb/ton) and cyanide (0.68 lb/ton) are added in the two leach tanks.
Hybrid leach-CIL. Seven CIL tanks follow the two leach tanks, all of the same size. The report explains that because gold leaches quickly, about 85% is expected to dissolve before adsorption. That gives higher carbon loadings in the first CIL tank, lower soluble losses and a smaller elution circuit. Carbon advances counter-current once a day. Tails pass a detox tank and a carbon safety screen.
Elution and refining. Loaded carbon is stripped in a pressure Zadra circuit in 2.2 ton batches, and the gold and silver are electrowon. The precipitate is dried in a mercury retort and smelted to doré. Mercury is shipped off site to a third party for storage. Carbon is reactivated in a kiln.
Cyanide destruction and tailings. CIL tails are treated by the SO2/air process, with sodium metabisulphite, hydrated lime and copper, before being pumped to the tailings storage facility.
Additional Interesting Data and Summary
The ore is hard, with 75th-percentile Bond indices of 22.3 kWh/ton (rod) and 26.9 kWh/ton (ball) and an average abrasion index of 0.641 g. Using identical tank sizes for leach and CIL simplifies access and reduces the spare parts needed.
In summary, the 2020 Grassy Mountain feasibility study is a small, simple gold plant. It treats hard, fast-leaching underground ore at 750 tons/d by crushing, single-stage ball milling to 106 µm and a hybrid leach-CIL, with mercury retorting and SO2/air cyanide destruction.
Key Processes
- Two-stage jaw and cone crushing
- Single-stage ball milling with cyclones to 106 µm
- Pre-aeration
- Hybrid leach-CIL (two leach and seven CIL tanks)
- Pressure Zadra elution and electrowinning
- Mercury retorting and smelting to doré
- Kiln carbon regeneration
- SO2/air cyanide destruction and tailings storage
Source: Grassy Mountain Project, Oregon, USA, NI 43-101 Technical Report on Feasibility Study, prepared for Paramount Gold Nevada Corp., effective date September 15, 2020.
Earlier report summaries
Earlier versions of this profile, based on older technical reports, are retained below for source traceability. Dates and figures reflect each report’s own effective date and are superseded by the current information above.
Grassy Mountain — 2018 Technical Report
This report presents the proposed mineral processing design for the Grassy Mountain gold-silver project, based on historical comminution data and 2017 laboratory testwork.
Report context
This technical report, dated July 9, 2018, was prepared as a Preliminary Feasibility and Technical Report for the Grassy Mountain Deposit in Oregon, on behalf of Paramount Gold Nevada Corp. The processing section describes a conventional carbon-in-leach plant designed for a nominal 750 short tons per day of underground ore feed.
Processing route
Crushing and Stockpile
The proposed three-stage crushing circuit will operate at a nominal 42 tons per hour using a single-toggle jaw crusher (35-inch × 26-inch) and cone crushers for secondary and tertiary reduction. The fine ore will report to a conical crushed-ore stockpile with 24 hours of live capacity.
Grinding and Classification
Primary grinding will use an 11-foot diameter × 17-foot EGL ball mill driven by a 1,072-hp single-pinion motor, in closed circuit with hydro-cyclones. The circuit is designed for a product size P80 of 100 mesh, with a circulation load of 350% of new feed and slurry density of 72% solids entering the mill.
Gravity Concentration and Intensive Leaching
A centrifugal gravity concentrator will recover gravity-recoverable gold from approximately 33% of the cyclone underflow, with a scalping screen removing particles larger than 10 mesh. Cycle time is 45 minutes, producing about 46 lb of concentrate per cycle. The concentrate will be processed in an intensive-leach reactor for approximately 20 hours per batch.
Leaching
A pre-aeration tank will be included ahead of the CIL circuit to passivate sulfides and reduce cyanide and lime consumption. The CIL circuit will consist of one pre-aeration tank and seven leach/adsorption tanks, with a total retention time of 24 hours. Laboratory testwork identified the need for all seven leach stages at a carbon concentration of 1.56 lb/ft³ and a carbon advance rate of 3.3 tons per day.
Carbon Management
Loaded carbon will be pumped to an acid-wash column using a 3% hydrochloric-acid solution, then eluted using a pressure Zadra circuit at 275°F. The elution column will contain 3.3 tons of carbon, with seven CIL strips per week. Carbon regeneration will occur in a kiln heated to 1,380°F, followed by a water quench tank and fines removal via a carbon-sizing screen. New carbon will be pre-attritted to break off sharp corners before addition to the leach circuit.
Cyanide Detoxification
Cyanide destruction will use an SO₂/air process with two hours residence time, targeting a WAD cyanide level below 0.1 ppm. Sodium metabisulphite will be delivered in 1-ton bulk bags, mixed to 20% solution strength, and dosed using positive-displacement metering pumps. Copper sulfate will serve as a catalyst, mixed to 15% solution strength in a combined mixing/storage tank.
Reagents
Estimated reagent consumptions per ton processed include: sodium cyanide at 0.93 lb, lime at 8.0 lb, and sodium metabisulphite at 3.92 lb. Sodium hydroxide will be delivered in 275-gallon totes at 50% solution strength and used for pH control and electrolyte. Cyanide will be stored as 1-ton bulk bags and mixed to a 28% solution strength.
Process Water Services
Raw water from borehole wells will be stored in a 200,000-gallon tank and supplied to the plant via duty/standby pumps. Potable water will be treated on-site, and gland water will be supplied from the raw-water tank. Process water will be stored separately and distributed by duty/standby pumps. Decant water from the TSF will be reclaimed and reused.
Gold Room
The gold room will house electrowinning cells, a smelting furnace, and associated support equipment within a secured access envelope. Sludge will be cleaned manually from cells weekly, dewatered in pressure filters, and dried overnight in the oven before smelting. No mercury retort was included as no mercury-bearing minerals were identified.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Nominal throughput | tons/day | 750 | Design |
| Annual throughput | tons/year | 273,750 | Design |
| Plant availability | % | 91.3 | Design |
| Crusher availability | % | 70 | Design |
| Hourly plant capacity | tons/hour | 31 | Design |
| Design grade – Au | oz/ton | 0.24 | Design |
| Design grade – Ag | oz/ton | 0.34 | Design |
| Ball mill work index | kWh/ton | 19.0 | Testwork |
| Ball mill power | hp | 1,072 | Design |
| Feed particle size (F80) | inch | 0.394 | Design |
| Product particle size (P80) | mesh | 100 | Design |
| Gravity gold recovery (overall) | % | 10 | Testwork (20% high-grade, ~10% LOM average) |
| Total leach time | hours | 24 | Testwork |
| Pre-aeration tanks | # | 1 | Design |
| Leach/adsorption tanks | # | 7 | Design |
| Cyanide addition | lb/ton | 0.82 | Design |
| Lime addition | lb/ton | 8.0 | Design |
| Carbon concentration | lb/ft³ | 1.56 | Testwork |
| Carbon loading (Au + Ag) | oz/ton | 175 | Design |
| Elution method | , | Pressure Zadra | Design |
| Carbon batch size | ton | 3.3 | Design |
| CIL strips per week | # | 7 | Design |
| Gravity leach electrowinning batches/week | # | 7 | Design |
| Detoxification method | , | SO₂/air | Testwork |
| Detoxification residence time | hours | 2 | Design |
| WAD cyanide target | ppm | <0.1 | Testwork |
| SMBS addition | lb/ton | 3.92 | Design |
| Process power consumption | kWh/ton | 53.3 | Design |
Project website: https://paramountnevada.com/Grassy-Mountain-Gold
Technical qualifications
A portion of the gravity-circuit recoveries relied on GRG testwork that exhibited issues, causing uncertainty and conservatism in the expected range; additional testwork may show the gravity circuit could be eliminated in favor of whole-ore leaching. Laboratory leach modeling indicated CIP was marginally better than CIL but required more than double the carbon elution and regeneration capacity; a trade-off study was recommended for the next engineering phase. No mercury-bearing minerals were identified, but no mercury retort was included in the design.
Source: Preliminary Feasibility and Technical Report, Grassy Mountain Deposit, Oregon, July 9, 2018, Sections 17.0, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6, and 17.7.
