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.
