The Santa Fe Project will use a conventional heap leach and carbon adsorption process to recover gold and silver.
Report context
This report section, dated 2024, describes the recovery methods that will be used for the Santa Fe Project. The processing route is based on standard heap leaching technology with supporting testwork and process design criteria.
Processing route
Summary
Material for the Santa Fe Project will be processed using standard heap leaching methods. The general process flow consists of three-stage crushing of run-of-mine (ROM) material, conveying and stacking of crushed material on a heap leach pad, application of dilute sodium cyanide solution, collection of effluent solution, and recovery of gold and silver through carbon adsorption, desorption and reactivation (ADR), followed by electrowinning and refining to produce a doré product.
Crushing
Run-of-mine material will be trucked from open pits and dumped into a feed hopper equipped with a vibrating grizzly screen. Oversize material will report to a primary jaw crusher after passing under a rock breaker, while grizzly undersize will bypass the crusher. Both products will combine on a discharge conveyor feeding an outdoor coarse ore stockpile.
Material will be reclaimed from the stockpile through a subterranean tunnel, with a conveyor transporting it to an open-circuit secondary crushing and screening circuit. The oversize from an inclined screen will be fed to a standard cone crusher, while screen undersize will combine with secondary crusher product on a tertiary feed conveyor.
The tertiary crushing circuit consists of an inclined screen and cone crusher in closed circuit. Oversize material will be returned to the crusher, while undersize material meeting the final product specification of P80 12.7 mm will be deposited on the final product conveyor. A bin with two hours of surge capacity will store the crushed product, and dust collection and fogger systems will manage airborne fines.
Conveying and Stacking
Crushed material will be reclaimed from the bottom of the bin and conveyed by three overland conveyors toward the heap. Pebble lime will be dosed onto the material via a screw feeder from an adjacent silo for pH buffering during leaching. A series of ramp, grasshopper and index conveyors will deliver material to a radial stacker for retreat stacking in 10 m lifts. The area in front of stacked material will be ripped with a dozer to maintain heap permeability. Upon completion of a lift, conveyors will be repositioned to the top of the new lift and the surface will be cross-ripped prior to stacking.
Heap Leaching
Irrigation piping, consisting of HDPE pipe and drip tubing, will be placed on the stacked material and buried with a dozer. Solution will be applied at a rate of 10 L/h/m², percolating by gravity through the active lift and lower lifts. The total leach cycle of 70 days is the design basis, with a total nominal flow of 648 m³/h to the heap.
The heap leach pad will be constructed in two phases, with capacities of approximately 15 million tonnes each. It will consist of a compacted low-permeability soil layer, a geomembrane liner, and a free-draining crushed gravel drainage layer to limit liner head to 0.7 m. The pad will be built on a northward slope to reduce earthwork, with a maximum pad height of 50 m above the liner.
Solution Collection and Storage
The heap leach facility is designed to be zero-discharge. Pregnant and event solution ponds will contain process solutions and runoff from a 100-year, 24-hour storm event. The pregnant solution pond has a capacity of 18,000 m³ and the event solution pond 138,000 m³. Solution from submersible pumps in the pregnant pond will be directed to the ADR plant.
Carbon Adsorption
Gold and silver will be recovered using a train of five open-top up-flow carbon-in-column (CIC) tanks containing activated coconut-shell carbon. Pregnant solution will be pumped through the tanks, with carbon transferred upstream over several days. Loaded carbon from the first tank will be advanced to the desorption circuit at a gold loading of 2,700 g/t. Recoveries of gold and silver from solution are expected to be approximately 98% and 80% respectively. Solution overflowing the final tank will pass over a carbon safety screen to capture carbon, with fines collected for off-site processing. The barren solution will be returned to the heap, with antiscalant dosed upstream to prevent scale formation.
Carbon Desorption and Reactivation
Loaded carbon will be transferred to an acid wash vessel and treated with dilute hydrochloric acid to dissolve calcium carbonate scale. Steps include fresh water rinses, acid circulation with pH maintained below 2.0 for a minimum of one hour, spent acid transfer to waste, and a dilute caustic soda rinse. Carbon will then be transferred to the elution vessel.
Elution will use the Pressure Zadra process at approximately 135°C and 340 kPag with a small amount of cyanide and caustic soda in the eluant. The elution cycle is expected to last 18 hours, including transfer, elution and rinsing. Pregnant eluate will pass through heat exchangers to reduce temperature below 80°C prior to electrowinning. Barren eluant will be returned to a storage tank until residual gold concentration is below 10 ppm.
Eluted carbon will be transferred to the regeneration kiln feed hopper via a screw feeder, dewatered on a static screen, and thermally reactivated. Hot carbon will be quenched in water, screened to remove fines, and returned to the adsorption circuit. New carbon will be attrited in a carbon conditioning tank before being added to the carbon circuit.
Electrowinning and Refining
The eluate will pass through two electrolytic cells in parallel where gold and silver will deposit on steel wool cathodes. Barren eluant will be returned to the elution tank, with about one-third of the solution discarded after each strip and replaced with fresh caustic soda and sodium cyanide solution. Loaded cathodes will be harvested weekly by pressure washing, with the precipitate sludge transferred to a filter press. The filter cake will be dried and processed through a mercury retort to vaporize elemental mercury, which will be condensed and collected in flasks for off-site transport. The remaining precipitate will be smelted with fluxing agents to produce doré bars.
Emissions Control Systems
Emissions from the electrowinning cells, kiln and furnace will be treated to minimize release of pollutants, particularly mercury vapor. Electrowinning cell exhaust will be drawn through a double-deck bed of adsorbent carbon. Kiln exhaust will pass through a wet scrubber, heater and adsorbent carbon bed. Furnace exhaust will be captured by an overhead fume hood, pass through a particulate filter bag house, and through a carbon adsorption bed containing sulphur or iodine-impregnated carbon to remove mercury vapor before atmospheric venting.
Reagents
Several reagents will be stored and used on site. Pebble quicklime (CaO) will be delivered by bulk pneumatic trucks in 23-tonne loads and stored in a 150-tonne silo, added via screw conveyor for pH stabilization. Sodium cyanide will be delivered as 30% solution in 5.5-tonne loads and stored in a tank at the ADR plant, with additions for leaching and carbon desorption. Antiscalant will be delivered in 15 m³ tanker loads. Hydrochloric acid at 36% concentration and caustic soda as 50% sodium hydroxide solution will also be stored in the reagent area. Activated carbon in 0.5-tonne bags will be stored on site for makeup.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Processing rate | t/d | 12,500 | Design |
| Annual throughput | Mt/year | 4.563 | Design |
| Crushing availability | % | 75 | Design |
| Final crushed product size, P80 | mm | 12.7 | Design |
| Stacking availability | % | 85 | Design |
| LOM tonnage to heap leach | t | 27,731,098 | Design |
| LOM average gold grade | g/t | 0.63 | Design |
| LOM average silver grade | g/t | 3.26 | Design |
| LOM gold extraction by heap leaching | % | 60.1 | Design |
| LOM silver extraction by heap leaching | % | 24.6 | Design |
| Cyanide consumption | kg/t | 0.33 | Design |
| Lime consumption | kg/t | 3.37 | Design |
| Pregnant solution flow (nominal) | m³/h | 592 | Design |
| Carbon ADR processing capacity per batch | t | 3 | Design |
| Solution application rate | L/h/m² | 10 | Design |
| Leach cycle | d | 70 | Design |
| Pregnant solution pond capacity | m³ | 18,000 | Design |
| Event solution pond capacity | m³ | 138,000 | Design |
| Elution temperature | °C | 135 | Design |
| Elution pressure | kPag | 340 | Design |
| Gold recovery from solution | % | 98 | Expected |
| Silver recovery from solution | % | 80 | Expected |
| Gold loading on carbon | g/t | 2,700 | Expected |
| Elution duration | h | 18 | Expected |
| Elution barren solution residual gold | ppm | <10 | Expected |
| Gold recovered in year of stacking | % | 85 | Assumption |
| Gold recovered in subsequent year | % | 15 | Assumption |
| Silver recovered in year of stacking | % | 65 | Assumption |
| Silver recovered in subsequent year | % | 35 | Assumption |
| Barren eluant discard per strip cycle | % | ~33 | Design |
| Processing power (total attached) | MW | 4.706 | Design |
| Processing power (peak load) | MW | 3.542 | Design |
| Processing power (average load) | MW | 2.495 | Design |
Project website: https://www.nsenergybusiness.com/projects/santa-fe-project-us/
Technical qualifications
The heap leach pad design was based on the 2024 Preliminary Economic Assessment and is stated to meet or exceed North American standards. However, the report notes that actual standards for subsequent stages should be carefully considered and implemented to ensure environmental impacts are mitigated as required by prevailing laws, regulations and international standards. Heap leach recovery values and production schedules are based on assumptions outlined in Section 13. The production schedule assumes 85% of recoverable gold is recovered in the year of stacking with 15% in the following year, and for silver, 65% and 35% respectively, showing slower leaching kinetics for silver.
Source: Santa Fe Project 2024 Technical Report, Section 17.0 Recovery Methods and associated tables and figures.
