The 2021 technical report describes the proposed processing route for the Metates Sulphide Heap Leach Project Phase 1, including crushing, oxidation on an on-off pad, cyanide heap leaching, and gold and silver recovery by Merrill-Crowe.
Report context
The Metates Sulphide Heap Leach Project Phase 1 technical report, dated 30 August 2021 (Revision 1), documents the planned processing facilities for a nominal 15,000 tpd heap leach operation at 90% overall availability. The report was prepared as a Form 43-101F1 Technical Report and presents the design basis for the recovery methods, associated infrastructure, reagent consumption, water use, and power demand.
Processing route
Crushing and stacking
The proposed crushing plant comprises three stages: a jaw crusher for primary crushing, a standard cone crusher for secondary crushing, and a short-head cone crusher for tertiary crushing. Run-of-mine material is delivered by mine trucks to a dump pocket with three-truckload capacity (273 tonnes), then fed by a variable speed apron feeder to a vibrating grizzly set at 130 mm opening. Grizzly undersize bypasses the primary crusher, blending with primary crusher product to give an estimated nominal P80 of 128 mm.
The combined material is conveyed to a double-deck secondary screen (63 mm and 20 mm openings) that removes -20 mm material to the final crushed product. Secondary crusher oversize, crushed at a close side setting of 30 mm, is conveyed to a double-deck tertiary screen (30 mm and 20 mm openings) in closed circuit with a short-head tertiary crusher set at 20 mm CSS. Final crushed product is targeted to a P80 of 13 mm.
Oxidation pad
The oxidation pad is designed as an on-off pad, 438 m wide by 961 m long including access corridors, with a useable stacking area of 360 m by 871 m (31.4 hectares). The pad is lined with HDPE overlaid with a 1-m layer of permeable aggregate, with aeration lines embedded in the liner cover above the phreatic level. The pad is divided into 13 cells separated by curbs built into the HDPE liner, with one cell always empty as a buffer between stacking and emptying operations. Material in each cell is planned for an oxidation cycle of 90 to 180 days. Aeration is supplied by two blowers, one operating and one standby, each delivering 64,200 Nm³/h at 52 kPa(g).
Crushed material is mixed in a rotating drum with alkaline solution before being stacked to a planned lift height of 20 feet (6 m), with the stacker capable of reaching 25 feet (7.5 m) to allow for slump. Stacking uses a system of overland conveyors, a tripper conveyor, 19 ramp-portable conveyors, 12 portable conveyors, a radial stacker, and support conveyors. During oxidation, aeration is continuously injected at the heap base while alkaline solution is irrigated from the top; the pH of solutions draining from each cell is monitored, with solution diverted to regeneration once pH falls below a threshold of about pH 9.5.
After the oxidation cycle, material is rinsed with raw water to remove acid and sulfate, allowed to drain, then reclaimed by front end loaders to a mobile hopper and portable conveyors feeding a reclaim overland conveyor. Oxidized material is transferred to the dedicated leach pad by overland conveyors, with an overland tripper conveyor and radial stacker used for heap building.
Cyanide heap leaching
Oxidized material is stacked on a dedicated valley-fill leach pad starting at the lowest point, closest to Rio San Juan de Camarones. Header pipes and drip emitters deliver barren cyanide solution at a planned irrigation rate of 10 L/h/m² for up to 60 days of primary leach, with cyanide concentrations of 1 to 1.5 kg per tonne of solution. Pregnant solution is collected at the pad base and flows by gravity to a pregnant solution pond before being pumped to the Merrill-Crowe plant.
Gold and silver recovery
The Merrill-Crowe plant has a design capacity of 680 m³/h (3,000 gpm) of pregnant solution. Pregnant solution is clarified, deaerated, then contacted with zinc dust in a mixing cone. Precipitated gold and silver are collected in filter presses, dried in a retort to remove moisture and mercury, fluxed, and smelted in a natural gas-fired furnace to produce doré bars for shipment.
Reagents
Key reagents and projected dosages are as follows:
| Reagent | Area or Point of Addition | Dosage | Unit |
|---|---|---|---|
| Sodium cyanide | Leach, Barren Solution Pond & MC | 0.5 | kg/t |
| Lime | Oxidation | 1.22 | kg/kg S to be oxidized |
| Lime | Leach, Overland Conveyor to HLP | 0.5 | kg/tonne |
| Na2CO3 | Oxidation | 0.6 | kg/tonne solution |
| Zn dust | Merrill-Crowe | 25.8 | kg/kOz Au |
| Zn dust | Merrill-Crowe | 47.1 | kg/kOz Ag |
| Lead Nitrate | Merrill-Crowe | 15 | ppm in PLS |
| Diatomaceous Earth (DE) | Merrill-Crowe | 45.4 | kg/filter batch |
| Melting Flux | Refining | 5.5 | g/oz of metal |
| Flocculant | Oxidation Solution Regeneration | 20 | g/tonne precipitate |
| Antiscalant | Barren Solution, Pregnant Solution | 6 | kg/tonne of solution |
Project website: https://chesapeakegold.com/metates/
Solution regeneration
The oxidation solution regeneration process is described as currently proprietary. It neutralizes acid produced during oxidation, restoring solution pH to at least 10.5 and at most 11. Two agitated regeneration tanks are planned, with precipitates thickened in a high-rate thickener; 85 to 90% of the thickener underflow is recycled to the regeneration tanks as seed for crystallization. The remaining underflow disposal method is to be determined once sufficient sample is available, using USEPA synthetic precipitation leaching procedure testing, with options including co-mingling with mine waste or separate lined storage.
Control systems
A crusher control room serves the primary crushing area. A central control room located within the Merrill-Crowe plant building will monitor and control crushing, screening, material handling, reagents, pumping, aeration, and utility systems via a distributed control system. A computer room adjacent to the central control room will host engineering workstations, supervisory computers, historical trending, management information systems server, programming terminal, network and communications equipment, and printers. Local video display terminals are selectively provided on the plant floor for occasional local monitoring and control. Vendor-supplied local panels will be interfaced with the DCS.
Water and power consumption
Total projected raw water consumption is 95 m³/h, comprising 74 m³/h for the process plant, 20 m³/h for mine dust control, and 1.25 m³/h for potable use. Connected power load is 11,691 kW, with 7,742 kW drawn in a typical year, equivalent to about 11 kWh per tonne of material processed and $1.11 per tonne in power cost.
Key reported parameters
| Parameter | Reported Value | Basis |
|---|---|---|
| Nominal plant capacity | 15,000 tpd | Design |
| Overall availability | 90% | Design |
| Final crushed product size | P80 13 mm | Design target |
| Oxidation cycle time | 90–180 days | Estimated, from testwork to be confirmed |
| Oxidation pad useable area | 31.4 ha (360 m x 871 m) | Design |
| Number of oxidation cells | 13 cells, 1 always empty | Design |
| Aeration blower capacity | 64,200 Nm³/h each at 52 kPa(g) | Design, 1 operating + 1 standby |
| Irrigation rate | 10 L/h/m² | Design |
| Primary leach duration | Up to 60 days | Design |
| Merrill-Crowe capacity | 680 m³/h (3,000 gpm) | Design |
| Raw water makeup | 95 m³/h total | Projection |
| Connected power load | 11,691 kW | Design |
| Typical annual power draw | 7,742 kW | Projection |
| Specific power consumption | ~11 kWh/tonne | Projection |
| Solution pH regeneration target | 10.5–11 | Design |
| THP regeneration underflow recycle | 85–90% | Design |
Technical qualifications
The report notes that the solution regeneration process is currently proprietary. Oxidation time projections are to be derived from testing of composites and variability samples, and will be further refined from measurements during operations. Disposal of regeneration precipitates will be determined when enough sample is produced during testing using the USEPA synthetic precipitation leaching procedure. The oxidation process is stated to occur rapidly at the beginning of the oxidation cycle and taper off towards the end, with solution regeneration expected to be needed more for cells in early stages of oxidation.
Source: Metates Sulphide Heap Leach Project , Phase 1, Form 43-101F1 Technical Report, 30 August 2021, Revision 1, M3-PN210035, Sections 17 and associated tables and figures.
