This article summarizes the conceptual processing route, design criteria, and personnel requirements for the Cangrejos Gold-Copper Project as presented in the June 1, 2022 NI 43-101 Technical Report.
Report context
The Cangrejos Gold-Copper Project, located in Ecuador, is the subject of a Preliminary Economic Assessment (PEA) dated effective June 1, 2022. The processing information in this report is conceptual in nature, based on metallurgical test data and trade-off studies that evaluated alternative recovery processes and comminution circuits.
Processing route
Comminution and grinding
The conceptual flowsheet begins with primary crushing, where run-of-mine material is dumped into a primary crusher sized for 80,000 tpd. The primary crusher and overland conveyor structures are designed for that throughput, though conveyor drives are sized for 40,000 tpd and designed for quick change-out to support expansion.
Material is conveyed to a coarse material stockpile, then withdrawn by apron feeders to a double-deck banana screen. Screen oversize reports to two secondary cone crushers operating in parallel, with crusher discharge returned to the screen. Screen undersize feeds a log washer to remove clay that could hamper subsequent operations.
Log washer oversize reports to a washing screen, with oversize fed to a single HPGR with 3,200 kW dual drives (6,400 kW total). HPGR discharge is returned to the washing screen in closed circuit. Screen undersize is split to two ball milling circuits, each in closed circuit with hydrocyclones. Cyclone overflow has a particle size distribution of 80% passing 90 µm.
Bulk flotation and regrind
Ball mill cyclone overflow feeds the bulk rougher flotation circuit, comprising two sets of tank cells operating in parallel with approximately 30 minutes retention time. Reagents include potassium amyl xanthate (PAX), Aero Promotor 208 (A208), MIBC, and Dowfroth 250 (DF250).
Bulk rougher concentrate is reground in a Vertimill to P80 35 µm, then cleaned through three stages of cleaner flotation plus cleaner-scavenger flotation. Third cleaner concentrate feeds the bulk concentrate thickener.
Copper-molybdenum separation
Bulk concentrate thickener underflow is pumped to an agitated conditioning tank where sodium hydrosulfide (NaHS) depresses copper minerals. The conditioned slurry feeds the molybdenum flotation circuit comprising rougher flotation, five stages of cleaner flotation, and cleaner scavenger flotation. Fuel oil and MIBC are used as molybdenum circuit reagents. Fifth cleaner concentrate is the final molybdenum concentrate.
Sand flotation and CIL
Bulk rougher flotation tailings are classified in sand flotation cyclones. Cyclone overflow reports to final tailings; underflow feeds sand flotation cells. Sand flotation tailings combine with cyclone overflow to report to final tailings.
Sand flotation concentrate combines with bulk cleaner-scavenger tailings, passes through a trash screen, and feeds the CIL feed thickener. Thickener underflow is pumped to a pH conditioning tank where lime adjusts pH to approximately 11.0. Slurry flows by gravity through five CIL tanks in series, designed for approximately 24 hours total retention. Sodium cyanide is added to CIL tank one; process air is injected into all tanks to provide oxygen for leaching.
Fresh and/or regenerated carbon is added to CIL tank five and advanced counter-current to slurry flow. Loaded carbon from tank one is screened and collected in a holding vessel.
Cyanide detoxification and carbon regeneration
CIL tailings are detoxified using the sulfur dioxide-air process. Sodium metabisulfite provides sulfur dioxide, air is supplied by blowers, copper sulfate catalyzes the reaction, and milk of lime controls pH between 8.0 and 9.0. The circuit includes two agitated tanks designed for approximately 2.5 hours residence time.
Carbon stripping uses a batch system with a pressure strip vessel designed for 2 tonnes of carbon per cycle. Sodium cyanide and caustic soda are added to prepare strip solution, with a water heater and heat exchanger increasing operating temperature and pressure. Barren and pregnant solution tanks are included.
Pregnant solution is pumped through three electrowinning cells where gold and silver plate onto cathodes. A hydrochloric acid wash removes inorganic carbon contaminants, and an electric rotary carbon regeneration kiln removes organic contaminants. Stripped and regenerated carbon discharges to a quench tank, then passes through a carbon sizing screen before returning to CIL tank five. A carbon attrition tank prepares new carbon.
Cathodes are power washed; electrowinning sludge is filtered, dried, mixed with fluxes, and smelted in a gas-fired furnace to produce gold-silver doré for off-site refining.
Tailings handling
Detoxified leach tailings, sand flotation cyclone overflow, and sand flotation tailings combine in the tailings box and flow to two parallel tailings thickeners. Underflows feed a filtration plant with nine pressure plate and frame filters (eight operating, one standby). Dewatered tailings are conveyed to a dry stack tailings facility via overland conveyor, placed using portable conveyors and a stacker.
The CIL circuit nominally processes 10% of plant feed: approximately 4,000 tpd during the first five years and 8,000 tpd after expansion.
Reagents and plant support
Reagents include PAX, MIBC, fuel oil, milk of lime, NaHS, antifoam, sodium metabisulfite, copper sulfate, flocculant, and sodium cyanide. Utilities include fresh water, process water, fire water, air blowers, air compressors, standby power generation, and gland water.
Plant support systems include analytical and metallurgical laboratories, truck scale, hoists, cranes, pumps, automated samplers and analyzers, and process control systems with PLCs and control rooms.
Process personnel
Process staffing is based on four crews, with two crews working 12-hour shifts while two are off. Personnel include process management, metallurgists, shift foremen, operators, and maintenance staff, with expatriate roles for defined durations to train local employees.
Key reported parameters
| Parameter | Units | Value | Basis |
|---|---|---|---|
| Plant throughput, Years 1–5 | tpd | 40,000 | Production schedule |
| Plant throughput, Years 1–5 | Mtpa | 14.6 | Production schedule |
| Plant throughput, Years 6–25 | tpd | 80,000 | Production schedule |
| Plant throughput, Years 6–25 | Mtpa | 29.2 | Production schedule |
| Plant availability, primary crushing | % | 75 | ONIX |
| Plant availability, other | % | 92 | ONIX |
| Head grade Au | g/t | 0.556 | Production schedule |
| Head grade Ag | g/t | 0.7 | Production schedule |
| Head grade Cu | % | 0.10 | Production schedule |
| Head grade Mo | ppm | 20 | Production schedule |
| Specific gravity | g/cc | 2.7 | Plenge |
| Bond Ball Mill Work Index | kWh/t | 17.1 | FLS |
| Bond Abrasion Index | g | 0.3511 | FLS |
| Primary grind size P80 | µm | 90 | Plenge |
| Concentrate regrind size P80 | µm | 35 | Plenge |
| Bulk rougher flotation time | minutes | 12 | Plenge data, factored |
| Tailings thickener sizing | m²/tpd | 0.125 | Pocock |
| CIL retention time | hours | 24 | Conceptual design |
Project website: https://www.wheatonpm.com/portfolio/development-projects/cangrejos/default.aspx
Technical qualifications
The processing information is based on a conceptual PEA-level flowsheet derived from metallurgical test data and trade-off studies. No pilot plant or full-scale operating data are presented for the Cangrejos process. Reported throughputs, head grades, grind sizes, flotation times, and equipment counts are design criteria, not confirmed operating performance. Labor tables show staffing used as a basis for the PEA, including expatriate positions of specified durations. All costs referenced are inclusive of process operating costs presented in Section 21 of the source report.
Source: Cangrejos Gold-Copper Project – Ecuador, NI 43-101 Technical Report, effective date June 1, 2022, Section 17 Recovery Methods.
