This technical report describes the proposed processing facilities for gold, silver, and copper recovery from the Cerro del Gallo deposit using heap leaching and SART technology, supported by testwork results.
Report context
This technical report for the Cerro del Gallo Heap Leach Project, dated January 31, 2020, presents engineering and design criteria developed for a proposed processing plant incorporating crushing, heap leaching, SART, carbon adsorption-desorption-recovery (ADR), and gold refining. The processing route is based on testwork that indicated the ore is amenable to heap leaching for recovery of gold, silver, and copper.
Processing route
Crushing and agglomeration
The proposed design comprises a three-stage crushing system with an open primary circuit and closed secondary and tertiary circuits, operating seven days per week, 24 hours per day. Run-of-mine (ROM) material is delivered by haul truck into a 200-tonne ROM feed bin. A permanent rock breaker handles oversized material. ROM material is fed by a vibrating grizzly feeder; oversize is crushed by a primary jaw crusher operated in open circuit to produce 80% passing 145 mm. The primary product is stockpiled by a coarse ore stockpile feed stacker conveyor.
Material reclaimed from the primary crushed stockpile is conveyed to the secondary crushing circuit, which includes two double deck vibrating screens and two cone crushers operated in closed circuit with a product size of 80% passing 36 mm. Secondary screen undersize is conveyed to a secondary product storage bin, then transferred to the tertiary crushing circuit, which consists of an HPGR crusher operated in closed circuit with a fine screening plant. The final crushed product is designed for 80% passing 4 to 6 mm.
Tertiary screen undersize is stockpiled by a fine ore stacking conveyor. Material from the fine ore stockpile is conveyed to a splitting chute feeding two parallel agglomeration drums. Cement is added from two 150-tonne silos, with the addition rate controlled by weightometers. Cement consumption is estimated in the range of 84 to 334 tonnes per day depending on material type, with a life-of-mine average of 166 tonnes per day. Barren process solution is added in the agglomeration drums, and agglomerated material is conveyed to the stacking system.
Heap conveying and stacking
The heap leach is designed to be constructed in eight-meter lifts using a mobile conveyor stacking system. The Phase 1 system consists of an overland conveyor, 23 mobile grasshopper conveyors, an index feed conveyor, a horizontal index conveyor, and a radial stacker. Phases 2 through 4 will increase the leach area without additional equipment. After a lift has finished leaching and is sufficiently drained, a new lift is stacked over the old lift. The old lift is cross-ripped with a dozer prior to stacking to break up compacted ore. Stacked lifts progress in a stair-step manner.
Heap leaching
The proposed design specifies a single-stage leach system with no intermediate solution use. Irrigation uses a drip-tube system. Antiscale agent is added to the suction of barren and pregnant solution pumps.
The total leach cycle is designed for 120 days, based on metallurgical testwork. The first stage of 40 days applies solution at a nominal rate of 12 L/h/m², and the second stage of 80 days applies solution at 6 L/h/m². Leach solutions have an approximate cyanide concentration of 1500 ppm when applied to the heap. Two horizontal centrifugal pumps operating in parallel at the barren tank provide barren solution application. High-strength sodium cyanide solution and antiscale agent are added to the suction side of barren leach pumps. The combined nominal flow to the heap is 1,280 m³/h.
Heap leach facility design
The heap leach facility (HLF) was designed by Golder for an ore capacity of approximately 92 million tonnes using a dry ore density of 1.6 t/m³. The 139.8-hectare HLF has a maximum heap height of 80 meters. Ore is designed to be stacked at 16,667 tpd. The leach pad is divided into four construction phases providing approximately 1.2 million square meters of lined surface.
The foundation includes an underdrain system within natural drainages. Fill material for perimeter roads, process ponds, and leach pad foundations is intended to be sourced from waste rock mined from the pit.
The composite liner system consists of, from top to bottom: a 700-mm thick drainage layer overliner with solution collection pipes; a 1.5-mm thick, single-sided textured HDPE geomembrane; a 300-mm thick compacted soil liner bedding with permeability no greater than 1 x 10⁻⁵ cm/sec or a geosynthetic clay liner (GCL) with permeability no greater than 5 x 10⁻⁹ cm/sec; and prepared subgrade.
The Pregnant and Event Ponds use a composite liner with an additional secondary 1.5 mm HDPE geomembrane and geonet layers for dual-containment and leak detection. The Satellite Pond, not expected to receive fluids during average climate conditions, is designed with a single geomembrane liner system.
Solution collection cells use geomembrane-lined cell separation berms. Perforated collection pipes of 100 to 450 mm diameter N-12 corrugated polyethylene (CPE) are within the drainage layer. Cell outlet pipes are 600 mm diameter HDPE solid wall. The collection system is sized to maintain a maximum 700 mm hydraulic head on the liner system.
Storm water diversion channels are sized for the 1 in 100-year, 24-hour storm event. A Gypsum Pond, with double liner system, is designed to contain 2 years of gypsum produced from SART operations.
SART plant
Pregnant solution is treated in a SART plant for removal of copper and silver prior to the ADR plant for gold recovery.
Copper precipitation: Concentrated sulfuric acid (98 wt%) is diluted to 30 wt% and combined with pregnant solution in an in-line mixer to acidify to pH 4.0-4.5. The current design is for 1,400 m³/hr of pregnant solution (nominal 1,280 m³/hr). Thickener underflow recycle slurry at 15-43 m³/hr and 10-25 wt% solids is combined with fresh sodium hydrosulfide solution (25 wt%) in the Copper Recycle Mix Tank. Acidified pregnant solution, combined with recycled copper precipitate, passes through three agitated precipitation tanks with a total residence time of 15 minutes. Copper sulfide thickener overflow gravity flows to the Neutralization Reactor; underflow is recycled or advanced to filtration.
Thickener underflow at 2.0-5.6 m³/hr and 10-25 wt% solids is neutralized with caustic solution (20 wt%) to pH 7-8 in the Filter Feed Tank. Two filter presses operate with batch cycles every 4-8 hours. Filter cake is produced at 0.728 tph as 40 wt% cake (0.296 tph dry cake) based on 90% copper and 95% silver precipitation. Cake is conveyed to drying pads, sized to 100% minus 5 mm, and loaded into 20-ton containers or 1-tonne bulk bags.
Caustic scrubber systems: Tanks and sumps in the copper area are ventilated to a caustic scrubbing system with two packed towers. The normal operating case treats 6,800 Nm³/hr with 100 ppmv HCN and H₂S, discharging at 0.2 ppmv. An emergency scrubber treats a burst of 17,100 Nm³/hr at 76,800 ppmv H₂S and 61,300 ppmv HCN for 5 minutes.
Solution neutralization: Acidified thickener overflow is neutralized with slaked lime and recycled gypsum thickener underflow using a high-density sludge process. Neutralization occurs in a tank of approximately 5 minutes residence time. Gypsum thickener overflow gravity flows to the ADR plant. Underflow is recycled or advanced to storage.
Metal recovery (ADR)
The proposed ADR plant uses a single train of five cascade-type, open-top up-flow carbon adsorption columns, each with capacity for 10 tonnes of activated carbon. Loading is continuous; approximately 5 tonnes of carbon per day are expected to be treated, with stripping occurring about 7 times per week, each lasting 12 to 16 hours.
Carbon acid wash involves circulating dilute acid solution at pH 1.0-2.0 for 4 to 6 hours per 10-tonne batch.
Desorption uses a Zadra pressure elution circuit with hot caustic solution at 135°C and approximately 340 kPa (50 psig). Each cycle processes a 10-tonne batch in approximately 18 hours.
The electrowinning circuit operates in series with elution using stainless steel cathodes at a current density of approximately 50 amperes per square meter of anode surface. Loaded cathodes are removed about once or twice per week. Sludge is filtered, dried, and smelted in an electric furnace with fluxes to produce doré bullion.
Thermal carbon regeneration is performed after every elution cycle at approximately 750°C for ten minutes.
Key reported parameters
| Parameter | Design value | Basis |
|---|---|---|
| Nominal crushing capacity | 16,667 t/d (696 t/h) | Design |
| Design crushing capacity | 22,200 t/d (925 t/h) | Design |
| Design conveying capacity | 26,700 t/d (1,110 t/h) | Design |
| Crushing product size | 5 mm (80% passing) | Design |
| Operating days per year | 360 | Design |
| Operating time (crushing, conveying, agglomeration, stacking) | 75% | Design |
| Ore copper grade (average) | 0.09% | Mine plan estimate |
| Leach cycle (total) | 120 days | Design based on testwork |
| Phase 1 leach duration | 40 days | Design |
| Phase 1 application rate | 12 L/h/m² | Design |
| Phase 2 leach duration | 80 days | Design |
| Phase 2 application rate | 6 L/h/m² | Design |
| Barren solution flow (average) | 1,280 m³/hr | Design |
| Barren solution flow (design) | 1,400 m³/hr | Design |
| Cyanide concentration in leach solution | ~1500 ppm | Design |
| SART & ADR feed flow (design) | 1,400 m³/hr | Design |
| HLF ore capacity | ~92 Mt | Golder design |
| HLF lined area | ~1.2 million m² | Golder design |
| HLF maximum heap height | 80 m | Golder design |
| Heap lift height | 8 m | Design |
| Copper precipitation efficiency | 90% | Testwork basis |
| Silver precipitation efficiency | 95% | Testwork basis |
| Copper filter cake production | 0.728 tph (wet), 0.296 tph (dry) | Design calculation |
| Filter cake copper grade (maximum) | 61.6 wt% | Design assay estimate |
| Filter cake silver grade (maximum) | 1.174% | Design assay estimate |
| Cement consumption (LOM average) | 166 t/d | Design estimate |
| Cement consumption range | 84–334 t/d | Design estimate |
| Make-up water (average climate, Phase 1) | 84.9 m³/hr average | Water balance |
| Make-up water (average climate, Phase 4) | 53.7 m³/hr average | Water balance |
| Make-up water (1 in 100 dry year, all phases) | 127.3 m³/hr average | Water balance |
| Rinsing water treatment capacity (average year) | 127,000 m³/year | Design for reclamation |
| Rinsing water treatment capacity (1 in 100 wet year) | 620,000 m³/year | Design for reclamation |
Technical qualifications
The report specifies this is a pre-feasibility level evaluation. Several criteria are identified as assumptions rather than measured data. The water balance includes assumed values for as-mined moisture content and specific moisture retention of ore and waste rock. The report notes that material suitable for use as soil liner bedding appears to be sparse within the limits and immediate proximity of the HLF and WRD, and may need to be imported from local off-site sources. The process pond storage criteria include a 0.6 meter freeboard requirement across all ponds. The emergency scrubber gas burst calculations are noted as being based on review of plant operating data from a metal sulfide leach process with concentrated sulfuric acid.
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Source: Cerro del Gallo Heap Leach Project, NI 43-101 Technical Report, January 31, 2020, Sections 17 Recovery Methods.
