The Long Valley project would use a conventional crushed-run-of-mine heap leach system with Merrill-Crowe precious metal recovery, as described in the June 7, 2021 preliminary economic assessment technical report.
Report context
This technical report dated June 7, 2021, describes a preliminary economic assessment (PEA) for the Long Valley project. The project would employ open pit mining with a conventional heap leach system operating 365 days per year, 24 hours per day. The report presents proposed design parameters, including crushing, agglomeration, heap leaching, and precious metal recovery using the Merrill-Crowe process.
Processing route
Crushing circuit
The proposed crushing circuit is designed to process approximately 26,400 short tons per day (23,900 metric tonnes per day) on a 24-hour basis with 80% availability, equating to a design crushing rate of 1,100 short tons per hour (997 metric tonnes per hour).
Run-of-mine feed would pass over a vibrating grizzly with 3-inch (75-mm) openings. Undersize material would report directly to the jaw crusher discharge conveyor, while oversize material would feed a jaw crusher. The jaw crusher would reduce material to a nominal 7 inches (175 mm), with crushed product conveyed to a screen feed bin. A vibrating feeder beneath the bin would feed a double deck screen with a 5-inch (125-mm) top deck and a 1.5-inch (37.5-mm) lower deck. Screen undersize would report to the final product conveyor; screen oversize would be split into two streams feeding two standard 4-foot cone crushers with a closed side setting of 1.2 inches (30 mm). The secondary crushing circuit would operate in open circuit and is designed to achieve a P80 of 1½ inches (37.5 mm).
Agglomeration
Final crushed product would be conveyed to a rotary drum agglomerator. Barren cyanide solution and cement/lime would be added prior to mixing. The target is to deliver approximately 50% of total cyanide demand to the economic mineralized material while not exceeding 8 to 10% moisture by weight. Agglomerated material would be conveyed via overland and grasshopper conveyors to the permanent leach pad. The agglomerated mixture would be allowed to cure for several days prior to solution application.
Heap leach pad
The heap leach pad would have an approximate final footprint of 6,781,263 square feet (630,000 square meters). Mineralized material would be stacked in lifts targeted at 32 feet (10 meters) in height using a radial stacker, with stacking conducted in retreat mode during creation of each leach cell. Total heap height is designed for 328 feet (100 meters), with up to 10 lifts. The heap would be placed under primary irrigation for approximately 90 days per cell. No rinse phase is included because of the multiple lift system; rinsing would be conducted as part of final closure.
The heap leach pad would operate as a fully drained system with no leachate storage within the heap leach facility. Irrigation would be provided by an emitter-type system designed to deliver 0.005 gpm/ft² (12 lph/m²). The heap leach facility would be constructed in three phases, with each phase requiring advanced expansion. The initial phase includes full development of the solution handling system, storm pond, and perimeter diversion ditches prior to mineralized material stacking.
Liner system
The pad would employ an engineered composite double liner system installed under the entirety of the heap leach facility. The double liner system consists of:
- 1.6-foot-thick (0.5-meter-thick) over liner using 1.5-inch (38-mm) minus economic mineralized material with less than 10% fines
- 80-mil (2-mm) LLDPE geomembrane
- 1-foot-thick (0.3-meter-thick) compacted low permeability soil liner
- Leak Detection and Recovery System (LDRS)
- 60-mil (1.5-mm) LLDPE geomembrane
LLDPE was proposed for the geomembrane liner systems due to higher interface friction values, ease of installation in cold climates, good performance under high confining stresses, and higher allowable strain where moderate settlement may occur.
The liner system would be constructed in three phases, with pad expansions proposed after three years of initial production. A 1-foot-thick (0.3-meter-thick) bedding sand layer would be placed on the face of the confining embankment directly underneath the second (bottom) geomembrane liner. A protective layer of approximately 1.5 feet (0.5 meter) of coarse crushed mineralized material/waste would be placed over the entire liner system footprint.
Solution collection system
The solution collection system consists of lateral collection pipes, collection header pipes, main header collection pipes, and leachate collection sumps. Lateral collection pipes would be spaced approximately 16 feet (5 meters) apart under the entire pad footprint. The main header pipes would be positioned along the centerline of each heap leach pad cell and terminate at the upstream toe of the perimeter berm at the leachate collection ditch. All piping would be constructed from perforated corrugated plastic tubing embedded within the over liner layer.
The Leak Detection and Recovery System would consist of a 1-foot-thick (0.3-meter-thick) sand layer embedded with 4-inch (100-mm) diameter perforated CPT collection pipes. Leakage would be conveyed to an LDRS sump at the downstream toe of the heap leach facility, with a level-switch controlled submersible sump pump transferring recovered solution via a pipe installed within the LDRS sand layer. Monitoring would be undertaken by recording pump operating hours.
Ponds and solution storage
The PLS pond would be situated immediately down gradient of the heap leach facility. The PLS pond is designed to contain up to 24 hours of solution assuming a maximum irrigation rate of 0.005 gpm/ft² (12 lph/m²), with a capacity of approximately 1,108,633 cubic feet (31,393 cubic meters). The Barren tank is designed to hold 5 minutes of solution at a capacity of 3,885 cubic feet (110 cubic meters).
The Event Pond is designed to provide storage for excess leachate and runoff from rainfall events, with storage capacity to contain the excess heap leach facility leachate and surface runoff from the 1 in 100-year 24-hour storm event without discharge, and overflow designed to discharge the 1 in 200-year 24-hour storm event. Total runoff estimates for the 1 in 100-year 24-hour storm event are 2,375,971 cubic feet (67,280 cubic meters), with a 10% additional factor of safety giving a total pond storage capacity of 2,635,569 cubic feet (74,631 cubic meters). The Event Pond pump station is designed to drain the storm volume over approximately ten days.
Pond liner systems would use HDPE geomembrane rather than LLDPE due to higher ultraviolet resistance for exposed pond surfaces. The pond liner system consists of:
- 60-mil (1.5 mm) HDPE geomembrane
- 1-foot-thick (0.3-meter-thick) low permeability soil liner
- Geosynthetic "geonet" drainage layer
- 60-mil HDPE geomembrane
Recovery plant – Merrill-Crowe
A Merrill-Crowe plant is included for gold and silver recovery due to potentially high silver solution grades. An Adsorption-Desorption Recovery plant was noted as preferred but further test work is required to validate solution tenors; capital costs were reported as nearly identical for the two options. The anticipated silver to gold concentration ratio is 3.9 to 1.
PLS solution would report to a series of pressure leaf clarifiers to remove suspended solids, with clarification filters coated with diatomaceous earth as required. Suspended solids concentration after clarification would typically be less than 5 ppm. After clarification, solution would feed to a de-aeration tower (Crowe tower) where a vacuum pump removes dissolved oxygen, with dissolved oxygen concentration targeted for less than 1 ppm.
Zinc powder would be added to precipitate gold and silver, with lead nitrate added at approximately 10% of the zinc rate. Zinc would be added in excess of the stoichiometric quantity (4x), with lower grade solutions requiring a higher proportion of zinc addition. Additional cyanide would be added to ensure precipitation reactions.
Gold and silver precipitates would be collected within recessed plate and frame precipitate filter presses (2x), with precipitate filters emptied on a weekly basis. Before opening, filters would be purged with low pressure compressed air. Precipitate would be dried, retorted to remove mercury, mixed with fluxes, and smelted in an electric melting furnace to produce doré for shipment to a third-party refinery.
Storm water management
The surface water management system would consist of perimeter ditches around the heap leach facility to intercept overland surface runoff. Ditch design criteria include: conveyance of the 1 in 100-year 24-hour duration storm event, minimum freeboard of 1 foot (0.3 meters), minimum ditch grade of 0.01 foot/foot, side slopes of 2H:1V, and trapezoidal channel shape. Storm water collected during heavy precipitation events can be diverted to an event pond and used as fresh make-up water to the circuit.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Crushing rate | 26,400 tpd (23,900 metric tpd) | Design |
| Crushing rate | 1,100 short tons/hour (997 metric tonnes/hour) | Design |
| Crusher availability | 80% | Design |
| Crushed product size | P80 1½ inches (37.5 mm) | Design |
| Heap lift height | 32 feet (10 meters) | Design |
| Total heap height | 328 feet (100 meters) | Design |
| Maximum lifts | 10 | Design |
| Primary leach cycle | Approximately 90 days | Design |
| Irrigation rate | 0.005 gpm/ft² (12 lph/m²) | Design |
| Heap leach pad footprint | 6,781,263 ft² (630,000 m²) | Design |
| PLS pond capacity | 1,108,633 ft³ (31,393 m³) | Design |
| PLS pond storage time | 24 hours at max irrigation | Design |
| Barren tank capacity | 3,885 ft³ (110 m³) | Design |
| Barren tank retention | 5 minutes | Design |
| Event pond capacity | 2,635,569 ft³ (74,631 m³) | Design |
| Event pond storm event | 1 in 100-year 24-hour | Design |
| Event pond overflow event | 1 in 200-year 24-hour | Design |
| Event pond drain time | Approximately 10 days | Design |
| Anticipated silver:gold ratio | 3.9:1 | Predicted |
| Suspended solids after clarification | Less than 5 ppm | Design |
| De-aeration dissolved oxygen target | Less than 1 ppm | Design |
| Lead nitrate addition rate | Approximately 10% of zinc rate | Design |
| Zinc addition | 4x stoichiometric excess | Design |
| LDRS sand layer | 1 foot (0.3 meters) thick | Design |
| LDRS collection pipe | 4-inch (100-mm) perforated CPT | Design |
| Lateral collection pipe spacing | Approximately 16 feet (5 meters) | Design |
| Over liner | 1.6-foot (0.5-meter) 1.5-inch minus | Design |
| Over liner fines content | Less than 10% | Design |
| Geomembrane (pad) | 80-mil (2-mm) LLDPE | Design |
| Geomembrane (pond) | 60-mil (1.5-mm) HDPE | Design |
| Low permeability soil liner | 1-foot (0.3-meter) thick | Design |
Project website: https://koremining.com/projects/long-valley/overview/
Technical qualifications
The report is a preliminary economic assessment and states that the recovery plant would utilize the Merrill-Crowe system rather than an Adsorption-Desorption Recovery plant because further test work is required to validate solution tenors. The heap leach pad design states that the liner system is designed assuming fully saturated solution storage conditions, and foundation preparation must be completed prior to the start of each development stage. The report recommends HDPE geomembrane for pond liner systems rather than LLDPE due to higher ultraviolet resistance for exposed surfaces. No rinse phase is included in heap operations because of the multiple lift system employed; rinsing would be conducted as part of final closure. The event pond storage capacity includes a 10% additional factor of safety over the estimated 1 in 100-year 24-hour storm runoff volume.
Source: Long Valley Project , 2021 Technical Report, June 7, 2021, Sections 17.14, Recovery Methods, Process Description, and related subsections.
