Castle Mountain Project — 2021 Feasibility Study

The Feasibility Study proposes a Phase 2 expansion at the existing Castle Mountain heap leach operation, adding a 50,000 ton/d run-of-mine heap leach and a 3,500 ton/d crushing, milling and Carbon-in-Leach plant.

Report context

The Castle Mountain Project Feasibility Study, dated 17 March 2021 and identified as M3-PN190397 Revision 0, describes the proposed Phase 2 expansion of a current operation that processes 14,000 short ton of ore per day via run-of-mine heap leaching. The study covers processing facilities design, supported by metallurgical testwork and analysis from Section 13 of the report and, where appropriate, past production results.

Processing route

Heap leaching and Carbon-in-Column circuit

The expanded heap leach facility will process 50,000 ton/d (45,400 t/d) of run-of-mine ore at an average life-of-mine grade of 0.012 oz/ton (0.41 g/t). The existing heap leach pad will be expanded using a double-liner system consisting of prepared subgrade, a vadose zone monitoring system, a secondary 80 mil LLDPE geomembrane liner, a two-foot gravel leak detection layer, a primary 80 mil LLDPE geomembrane liner, and a two-foot gravel and perforated HDPE piping network. The Phase 2 leach pad will encompass approximately 480 acres within the project's permitted 3,910-acre Mine Property boundary, with ultimate stack height of 300 ft above adjacent native grades.

Ore will be stacked in 25 ft lifts. Pebble quicklime will be added for pH control at an estimated consumption of 2.35 lb/ton based on metallurgical testwork. Drip emitters will apply dilute cyanide solution at 0.004 gpm/ft². The primary leach cycle is 80 days, selected based on testwork. Barren solution will be pumped at 12,800 gpm via four vertical turbine pumps; pregnant solution flow is 12,000 gpm.

A Carbon-in-Column (CIC) circuit comprising two trains of five cascading carbon columns will recover gold and silver from leach solution. Each train has five 18-ft diameter columns. Barren solution from the final column discharges via a safety screen. Loaded carbon from the CIC circuit (12 tons daily) will be pumped to acid wash and stripping.

The Phase 2 heap leach pad will be developed as a single pad abutted to the Phase 1 heap in four campaigns (Cells 2A, 2B, 2C, 2D) over the mine life. Cell 2A will accommodate 78 Mton of ore; Cell 2B, 29 Mton; Cell 2C, 74 Mton; Cell 2D, 50 Mton. Two event ponds,the existing Phase 1 pond (23.5 M gal) and a new Phase 2 pond (100 M gal),will manage storm solution, with the Phase 2 pond designed for a 100-year, 24-hour precipitation event.

Crushing and crushed ore storage

Ore from the open pit will feed a two-stage crushing plant processing 3,500 ton/d. The crushing circuit operates at 75% availability (194 ton/h nominal throughput) to allow for catch-up. Run-of-mine ore passes a static grizzly with 24-inch openings, then a vibrating grizzly feeder. Oversize feeds a 49-in × 37-in jaw crusher (175 hp) with a closed side setting of 4 in. Undersize combines with crusher discharge on the coarse ore transfer conveyor.

An 8-ft × 20-ft double deck inclined screen with 1-in top deck and ½-in bottom deck apertures classifies the material. Oversize from both decks reports to a surge bin feeding a 60-in standard cone crusher (500 hp) in closed circuit with the screen. Undersize (minus ½ inch) is the final product, conveyed to a fine ore bin. The fine ore bin is steel construction, fully enclosed, with 3,500 tons live storage (approximately 24 hours capacity). Two reclaim belt feeders with variable frequency drives control feed to the ball mill.

A two-stage crushing system was selected after a trade-off study that investigated 2-3 stages of crushing as well as a SAG mill and pebble handling circuit; based on preliminary pricing, two stages offered optimized economics.

Grinding and gravity concentration

The grinding circuit comprises a single 16.5-ft-diameter × 21-ft-long overflow ball mill (3,300 hp) with rubber liners, a trommel screen, and a retractable feed spout. The mill operates in closed circuit with a cluster of four 12-inch cyclones (2 operating, 2 standby) at 300% circulating load. Grinding targets a final product P80 of 100 mesh (150 μm) at a nominal fresh feed rate of 162 ton/h (147 t/h). The mill can operate at 55%-70% solids pulp density.

A portion of cyclone underflow feeds a semi-continuous batch gravity concentrator (48-in bowl, 60 hp) via a 10-mesh scalping screen. Gravity concentrate is collected in a storage hopper and transferred daily to an intensive leach reactor (ILR) for processing with concentrated cyanide, caustic solution, and LeachAid. Pregnant solution from the ILR reports to a dedicated electrowinning cell.

Leach/CIL circuit

Cyclone overflow passes through a trash screen to a 68-ft-diameter high-rate pre-leach thickener. Thickened slurry at 45%-50% solids is pumped to the Leach/CIL circuit. The hybrid circuit comprises seven agitated tanks, with the first one or two tanks operating without carbon to improve leaching efficiency. Total retention time is approximately 30 hours at 45% solids. Slurry pH at the circuit is expected at 10.5; milk of lime may be added for pH adjustment. Sodium cyanide may be added to any tank.

Slurry advances through the tanks using submerged vertical carbon pumper interstage screens. Tanks contain 10-15 g/L of 6×12 mesh granular activated carbon. Loaded carbon (3-ton batches daily) is sent to desorption. Slurry from the last tank passes through a 28-mesh safety screen before reporting to the cyanide recovery thickener.

Tallings management

The cyanide recovery thickener (68-ft diameter, high-rate) thickens CIL discharge to approximately 60% solids. Overflow solution containing cyanide is recycled to the barren solution tank. Underflow is pumped to cyanide detoxification, where the SO₂/oxygen process using sodium metabisulfite and oxygen (with copper sulfate as catalyst as needed) oxidizes cyanide to cyanate. Detoxification uses a single agitated tank with approximately 120 minutes residence time based on testwork.

Detoxified slurry is filtered by three pressure filters (8.2 ft × 8.2 ft, 64 chambers, mixed pack polypropylene plates), with 2 operating and 1 standby. Each full cycle is estimated at 16 minutes. Filter cake at approximately 18% moisture (wet weight basis) is stockpiled for haulage to the filtered tailings facility. Filtrate returns to the grinding circuit.

Carbon handling and refining

A common carbon handling facility processes carbon from both the CIC and CIL circuits. Acid wash uses two 6-ton capacity fiber-reinforced plastic vessels with circulating 3% hydrochloric acid. Stripping uses a pressure Zadra circuit in two stainless steel strip vessels (each 6-ton capacity), circulating 290°F caustic cyanide solution. A propane-fired thermal fluid heating system supplies heat.

Carbon regeneration uses a horizontal propane-fired indirect rotary kiln (5 ft diameter × 50 ft long) treating 18 tons of carbon per day at up to 1,500°F. Kiln off-gas passes through a scrubber with sulfur-impregnated carbon for mercury vapor removal.

Pregnant strip solution is processed through four electrowinning cells (0-2000 amps, 0-9 volts each) in parallel. Sludge is filtered, dried in a mercury retort furnace (1,200°F), and refined in an electric induction furnace (450 kW) to produce gold and silver doré bars weighing approximately 35-40 pounds.

Key reported parameters

Technical qualifications

• The design parameters are supported by past production results where appropriate.
• Ore grades and relative amounts of mill ore and heap leach ore vary throughout mine life.
• Plant design includes flexibility for treatment of all ore types as per testwork at design throughput.
• The leach pad lining system is designed in accordance with California Code of Regulations Title 27 requirements and International Cyanide Code recommendations.
• Reagent consumption rates are based on metallurgical testwork.
• Liners for crushers and grinding mill have been estimated based on ore hardness and experience at similar operations.
• Grinding media consumption estimated on a lb/ton basis, mill power draw and abrasion index.

Source: Technical Report on the Castle Mountain Project Feasibility Study, 17 March 2021, Section 17 Recovery Methods.