This report presents the proposed processing methods for the Granite Creek Mine Project, including heap leach and carbon-in-leach circuits with associated crushing, agglomeration, adsorption-desorption-recovery, and solution management systems.
Report context
The Granite Creek Mine Project, as described in this 2021 technical report, would employ open pit mining with two alternative processing routes: a conventional heap leach system and a carbon-in-leach (CIL) system, both operating on a 365-day-per-year, 24-hour-per-day basis. The report details the proposed design parameters for each processing route, including crushing circuits, agglomeration (for heap leach), leaching, carbon adsorption, gold recovery, and solution management infrastructure. All information presented shows proposed design, with no historical operating data or testwork results included in the processing sections of this report.
Processing route
Heap Leach Process Description
The heap leach system would use crushed run-of-mine (ROM) material at a nominal size of 2 inches (51 mm). The crushed material would be agglomerated with lime and cement, as necessary, and transported to the heap leach pad via conveyor belt.
The heap leach would consist of a lined area using a linear low-density polyethylene (LLDPE) liner with an over liner of sized material to facilitate drainage and protect the liner during initial stacking. Drainage pipes would be placed within this over liner to conduct leach solution to centralized solution collection ponds. Crushed material would be stacked in lifts on the lined pad by a radial stacker fed by a series of jump or grasshopper conveyors from the agglomeration circuit. Lifts are targeted at 20 feet (6 meters) in height, with a total heap height of 200 feet (60 meters).
Once a suitable area has been stacked (cell), the cell would be irrigated with dilute cyanide solution. Stacking would continue to advance, and each area would be irrigated for a set period (primary leach cycle). The solution leaches gold and silver from the heap materials and is transported to the recovery circuit as pregnant leach solution (PLS).
PLS would be collected in a dedicated pond and either recirculated or processed in the Adsorption-Desorption-Recovery (ADR) plant. Gold in solution would be collected on activated carbon in a series of five carbon-in-column (CIC) vessels. The depleted barren solution would report back to the heap leach barren pond for reagent adjustment prior to recirculation.
Once gold loading on carbon reaches a specific setpoint (e.g., 3,000 g/t in the lead column), carbon is advanced and a set amount removed for gold recovery. Gold recovery would take place through stripping the activated carbon using a specifically designed process (ZADRA or Anglo American Research Laboratory are typical). Gold would be stripped from the carbon into an enriched solution that reports to an electrowinning circuit where gold is recovered as a sludge that is ultimately smelted into high purity gold bars.
The heap leach is typically designed to have multiple lifts installed, with each new lift placed on top of the last until the heap reaches its ultimate height. The configuration is heavily dependent on material permeability characteristics, terrain available, and geotechnical aspects of the site.
Crushing Circuit (Heap Leach)
The crusher has been designed to process approximately 9,370 short tons per day (tpd) (8,500 metric tonnes per day) on a 24-hour basis. The design crushing rate would be 389 short tons per hour (354 metric tonnes per hour).
ROM feed would pass over a vibrating grizzly with a 6-inch (150-mm) opening. Undersize would report directly to the jaw crusher discharge conveyor while oversize would feed the jaw crusher. The jaw crusher would crush to a nominal 6-inch (150-mm), with crushed product reporting to a vibrating screen for scalping undersize prior to secondary crushing. The double-deck screen would be equipped with a top deck with 4-inch (100-mm) openings and a lower deck with 2-inch (51-mm) openings. Screen undersize would report to the final product conveyor, and screen oversize would be fed to an HP 500 crusher, or equivalent, with a closed side setting of 1 inch (25 mm). Discharge from the crushers would fall onto the final product conveyor. The secondary crushing circuit would be operated in open circuit. This crushing circuit would be capable of achieving a product of 100% passing two inches or a P80 of approximately 1½ inches (37.5 mm).
Agglomeration (Heap Leach)
Final crushed product would be conveyed to a rotary drum agglomerator. Barren cyanide solution and cement/lime would be added to the material prior to mixing. The target would be to deliver approximately 50% of the total cyanide demand to the material while not exceeding 8 to 10% moisture by weight. Agglomerated material would be conveyed via a combination of overland and grasshopper conveyors to a prepared permanent leach pad.
Material would be stacked using a slewing radial stacker to lift heights of 20 feet (6 meters). Stacking would be conducted in retreat mode during creation of each leach cell. The agglomerated mixture would be allowed to cure for several days prior to solution application.
Heap Leach Circuit
Material would be stacked for sufficient period to allow enough surface area for irrigation. Irrigation would be provided by an emitter-type system designed to deliver 0.005 gallons per minute per square foot (gpm/ft²) (12 liters per hour per square meter [lph/m²]). The heap would be placed under primary irrigation for a period of approximately 60 days. After primary leach, irrigation would be discontinued and advanced to the next cell. No rinse phase is included because of the multiple lift system employed. Subsequent lifts would be placed on top of the previous lift up to a total of 10 lifts. Rinsing will be conducted as part of final closure.
PLS would flow from the heap leach pad to the PLS sump by gravity. Solution would be pumped from the sump to the recovery circuit, with excess solution diverted to the PLS pond. Solution would be collected from each heap cell by a series of drainpipes under the heap that transport solution to perimeter piping. Solution can be directed to either the PLS or Event Pond piping. Storm water collected during heavy precipitation events can be diverted to an event pond and used as fresh make up water to the circuit.
Adsorption, Desorption, Recovery (ADR) – Heap Leach
During normal operations, PLS solution would be pumped to the CIC tanks. The CIC circuit would consist of one train of five CIC vessels, each containing four tons of carbon. Carbon is advanced counter-current to the PLS flow as the first tank in the series reaches its loading limit. The target carbon loading would be 3,000 g/t of gold. Carbon would be advanced by recessed impeller pumps.
Loaded carbon from the first tank would be pumped across a loaded carbon screen to the acid wash column. Screen underflow would be returned to the PLS flow. Fine carbon from the screen underflow would be stockpiled and sent for separate off-site recovery.
Barren solution exiting the last CIC vessel would be returned to the heap leach barren solution pond after passing through a carbon safety screen. Loaded carbon would be acid washed with dilute nitric acid to remove calcium and adsorbed metals. Spent acid would be neutralized and disposed. After acid washing, carbon would be passed to an elution column. Elution would be conducted by the modified ZADRA system. A solution of caustic and cyanide would be passed through the elution column to remove adsorbed gold. The rich electrolyte would be pumped to electrowinning cells, where gold and silver are recovered on the cathodes. Cathodes would be washed, and recovered sludge would be refined in a conventional induction furnace after drying. The circuit is designed to conduct two strip cycles per day. Doré produced would be assayed and stored in a vault before being shipped off-site for payment.
Barren carbon from the elution column would be returned to the CIC circuit after passing across a carbon sizing screen. Fine carbon from screen underflow would be stockpiled and sent for separate off-site recovery. Approximately 50% of barren carbon would report to an indirect fired kiln for thermal regeneration. Regenerated carbon would report to a quench tank before being pumped to the carbon sizing screen. Fresh makeup carbon would first be sent to an attrition tank for fines removal before being pumped to the carbon sizing screen. Fine carbon from screen underflow would be captured in a plate and frame filter.
Conceptual Heap Leach Pad and Pond Design
The heap leach facility (HLF) would consist of the following components: heap leach pad, liner system, leachate (solution) collection system, storm pond, stormwater management system, and freshwater supply.
To minimize capital expenditure, the heap leach pad has been designed in phases, with each phase requiring advanced expansion of the engineered pad. The HLF would be constructed in three phases, with pad foundation preparation, liner installation, and collection piping advanced as the leach pad expands. The capacity of each stacking stage includes an initial three-year period and two additional two-year periods.
The initial HLF development (Phase 1) would also include full development of the solution handling system, storm pond, and perimeter diversion ditches prior to commencing material stacking and leaching.
The heap leach pad would consist of a perimeter berm, pad liner system, and leachate collection system to collect and convey leachate solution to the ADR plant, which should be located adjacent to the HLF. The leach pad would have an approximate final footprint area of 5,651,052 square feet (525,000 square meters). The heap leach pad is designed to be operated as a fully drained system with no leachate storage within the HLF. Prior to the start of each development stage, the pad foundation must be prepared, involving stripping topsoil and vegetation and removal of any rocks. A minimum pad grade of 1% to 2% would be required.
A double liner system would be employed with two layers of synthetic material. The double liner system would consist of: a 1.6-foot-thick (0.5-meter-thick) over liner (1.5-inch [38-mm] minus with less than 10% fines content) using mineralized rock; an 80-mil (2-mm) LLDPE geomembrane; a 1-foot-thick (0.3-meter-thick) compacted low permeability soil liner; a Leak Detection and Recovery System (LDRS); and a60-mil (1.5-mm) LLDPE geomembrane.
Development of the heap leach liner would be constructed in three phases, with pad expansions proposed after three years of initial production to meet material stacking requirements.
A protective layer of approximately 1.5 feet (½ meter) of coarse crushed mineralized material/waste would be placed over the entire liner system footprint to protect the liner integrity from damage during material placement.
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 (five meters) apart under the entire pad footprint.
The LDRS is designed to capture and convey any solution that may leak through the overlying primary geomembrane layer. The LDRS consists of a 1-foot-thick (0.3-meter-thick) sand layer embedded with 4-inch (100-mm) diameter perforated CPT collection pipes.
The Event Pond is designed to provide storage for excess leachate and runoff generated during rainfall events, with storage capacity to contain the excess HLF 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 pond storage capacity: 1,136,461 cubic feet (32,181 cubic meters).
The PLS pond and Barren tank are designed to provide storage for leachate and ADR plant return solutions. The PLS Pond is designed with a capacity of approximately 636,970 cubic feet (18,037 cubic meters). The Barren tank is designed to hold 5 minutes of solution at a capacity of 1,624 cubic feet (46 cubic meters).
The engineered double liner system designed for the ponds uses: a60-mil (1.5 mm) HDPE geomembrane; a1-foot-thick (0.3-meter-thick) low permeability soil liner; a geosynthetic “geonet” drainage layer; and a60-mil HDPE geomembrane. It is recommended that HDPE geomembrane be used for the pond liner system rather than LLDPE.
The surface water management system consists of a series of ditches constructed around the perimeter of the HLF to intercept overland surface runoff around the HLF pad and to convey surface water away from the active site. Ditches are designed to convey the1 in 100-year 24-hour duration storm event with minimum freeboard of 1-foot (0.3 meters).
CIL Process Description
The CIL system would use crushed ROM material at a nominal size of approximately 1/2 inch (17 mm). Crushed material would advance to a ball mill where lime would be added to control pH levels in the CIL circuit.
A pre-leach thickener would then be used to thicken the grinded material, and flocculant would be added to improve settling rates. The thickener underflow would then be pumped to a series of CIL tanks in which the slurry flows sequentially. As gold is extracted via cyanidation, it is quickly adsorbed onto carbon. Carbon would flow counter-current to the slurry and would be recovered in the first tank. The loaded carbon would then be treated with an acid wash to remove any deposits and would be pumped to an elution column. A stripping solution strips gold off the loaded carbon. The pregnant solution would then be pumped to the electrowinning process and the precious metal cathodes are smelted into Dore bars.
A regeneration kiln is used for the reactivation of barren carbon from the elution column. This reactivated carbon is pumped back into the last CIL tank. Fresh carbon may
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Recovery route | Crushing, grinding, pre-leach thickening, CIL, elution and electrowinning | Proposed design |
| Gold adsorption | Counter-current carbon adsorption in CIL tanks | Proposed design |
Project website: https://www.i80gold.com/
Technical qualifications
The report presents a proposed recovery flowsheet. Values not stated in the source are not inferred.
