The Dugbe Gold Project Feasibility Study describes a 5.0 Mtpa conventional milling and gravity gold recovery circuit with carbon-in-leach (CIL) recovery, designed for the Tuzon and Dugbe F open pit ores.
Report context
This processing overview is drawn from the Dugbe Gold Project NI 43-101 Technical Report, a Feasibility Study prepared for Pasofino Gold Ltd. The report describes the process plant design basis, flowsheet selection, and major equipment specifications for the proposed 5.0 Mtpa operation. The design was developed from testwork findings, desktop trade-off studies, and DRA design information.
Processing route
Crushing and stockpiling
The primary crushing circuit is designed around a single gyratory crusher with a dual-sided truck tip for CAT777 90t trucks. The selection followed a trade-off assessment comparing a single gyratory crusher against dual jaw crushers. The gyratory option received an overall ranking of 3.4 versus 3.3 for the jaw crusher option, with the gyratory selected for its ability to accommodate a wider ROM top size, reduced belt tear susceptibility, higher throughput capability, and more robust loading arrangement.
The crusher circuit is designed for 910 tph dry throughput with a target product size of P80 = 125mm. The design uses crusher work index values ranging from 16.6 to 22.4 kWh/t based on historical SMC crusher index testwork and drop weight test database correlations. Primary crusher discharge is conveyed to a 12,500t live capacity mill feed stockpile, providing approximately 20 hours of mill production capacity.
Milling and classification
The milling circuit is designed for 625 tph dry feed and incorporates a primary SAG mill in reverse open circuit, a secondary ball mill in reverse closed circuit with hydrocyclones, and a tertiary Vertimill® in reverse closed circuit with a dedicated hydrocyclone set.
Major comminution equipment includes:
- A gyratory crusher with 375 kW installed power
- A SAG mill (8.53m diameter × 5.79m EGL) with 8,000 kW installed power, grate discharge type
- An in-circuit pebble crusher (HP300 or equivalent) with 220 kW installed power
- A secondary ball mill (7.92m diameter × 12.19m EGL, overflow discharge) with 16,000 kW installed power
- A Tertiary Vertimill® VTM 3000 with 2,235 kW installed power
The SAG mill utilises load cell control and a variable speed drive. Pebble crusher product is recycled to the primary mill feed conveyor. SAG mill discharge reports to the secondary ball mill discharge sump via a trommel screen.
The milling circuit was sized using breakage functions derived from grindmill, Bond work index, and jar mill testwork from the 2021/2022 FS comminution programme, based on the 85th percentile ore hardness.
Gravity recovery
The gravity circuit treats tertiary cyclone underflow at 800 tph dry, corresponding to a modelled gravity recovery of approximately 18–20%. This recovery is noted as being at the lower end of the typical threshold for a full-scale gravity circuit, but is recommended for recovery of high specific gravity species such as maldonite.
The circuit comprises two Knelson 48-inch concentrators operated in parallel, with scalping screens ahead of each unit. Gravity concentrate is processed in a single intensive leach reactor (ILR) with approximately 14 hours residence time. The ILR operates with elevated cyanide levels, and pregnant solution is treated in a dedicated electrowinning cell in the Gold Room. ILR barren residue is returned to the tertiary mill discharge sump.
Pre-leach thickening and CIL
Tertiary cyclone overflow gravitates to a horizontal vibrating trash removal screen to remove coarse ore, wood, and organic material. Screen underflow reports to a pre-leach thickener, designed with a unit thickening rate of 0.50 t/h/m² to achieve a CIL feed solids concentration of 45 wt%.
The CIL circuit consists of eight tanks, each 3,250 m³, in series with a total nominal residence time of 24 hours. Each tank is fitted with an agitator, four air spargers for oxygen supply, interstage screens, bypass facilities, and a recessed impeller pump. A carbon concentration of 10 g/L is maintained, with loaded carbon harvested in 12 t batches. The design includes a spare interstage screen, screen cleaning facilities, spillage pumps, and a tower crane for maintenance.
The design feed rate to CIL is 625 tph dry at 45 wt% solids. Cyanide is dosed to the first tank with a TAC control system, with provision for addition to the second tank. Lime is added to the first three tanks for pH control.
Carbon elution and regeneration
Loaded carbon is processed in 12 t batches on a 12-hour cycle using the pressure split AARL elution procedure. The circuit is designed for two batches per day, processing approximately 18.5 t of carbon daily based on an expected average loading of 900 g/t Au. Eluted carbon is regenerated in a rotary kiln with a design capacity of 1,200 kg/hr over 18 hours/day. Regenerated carbon is quenched, screened, and returned to the CIL circuit.
Gold is recovered from eluate via atmospheric sludging electrowinning cells, with sludge smelted in an induction furnace to produce gold doré.
Tailings and water management
CIL tank discharge passes through a carbon safety screen before reporting to the first of two CCD thickeners. The CCD circuit is designed for a unit thickening rate of 0.50 t/h/m² to achieve final tailings at 40 wt% solids with average CN WAD below 50 ppm.
CCD overflow is collected as cyanide process water, with a bleed stream treated in a conventional SO₂/air cyanide destruction circuit. The design includes sodium metabisulphite (SMBS) dosing at 20% w/v, with a design solution throughput of 1,250 m³/hr and 60 minutes residence time. The circuit achieves a pH of 8.5 through controlled lime addition, with sparged air providing oxygen and soluble copper acting as catalyst. Nominal CCD solution treatment is 400–600 m³/hr.
Raw water is supplied from the Geebo River in year one, with TSF-treated water used from year two. A two-stage ultrafiltration/reverse osmosis plant is designed with phased capacity up to 3.7M m³/annum, with modules of 120 m³/hr each. The plant includes prefiltration for iron and manganese removal plus a second-stage gypsum reactor for sulphate removal.
Reagent facilities cover sodium cyanide, caustic soda, hydrated lime, SMBS, copper sulphate, hydrochloric acid, lead nitrate, flocculant, activated carbon, and grinding media.
Key reported parameters
| Parameter | Unit | Value |
|---|---|---|
| Process plant capacity | Mtpa dry | 5.0 |
| Crushing circuit run time | h/annum | 5,493 |
| Crushing plant throughput | t/h dry | 910 |
| Milling & CIL run time | h/annum | 7,996 |
| Milling & CIL feed rate | t/h dry | 625 |
| Average LOM head grade | g/t Au | 1.30 |
| Average LOM gold recovery | % | 83.0 |
| Maximum ROM feed size | mm | 1,000 |
| Primary SAG mill feed F80 | mm | 125 |
| CIL target feed size F80 | µm | 53 |
| CIL design residence time | h | 24 |
| CIL feed solids concentration | wt% | 45 |
| LOM average CIL cyanide consumption | kg/t | 0.70 |
| LOM average CIL lime consumption | kg/t | 1.00 |
| Elution circuit type | , | Split AARL |
| Elution capacity per batch | t | 12.0 |
| Maximum elution frequency | batches/day | 2.0 |
| Final tailings target CN WAD | ppm | <50 |
| Gravity concentration modelled recovery | % | 18–20 |
| Gravity circuit feed rate | t/h dry | 800 |
| CIL tank volume each | m³ | 3,250 |
| Number of CIL tanks | , | 8 |
| Carbon concentration in CIL | g/L | 10 |
| Loaded carbon loading | g/t Au | 900 |
| Carbon regeneration kiln capacity | kg/hr | 1,200 |
| Mill feed stockpile live capacity | t | 12,500 |
| CCD unit thickening rate | t/h/m² | 0.50 |
| CN destruction design flow | m³/hr | 1,250 |
| CN destruction residence time | min | 60 |
Project website: https://www.pasofinogold.com/dugbe-gold-project
Technical qualifications
The gravity-CIL flowsheet was selected over a gravity-flotation-CIL alternative based on a differential cost assessment. The gravity-CIL option was estimated to achieve 83% average LOM gold recovery versus 88% for the flotation option. The assessment showed the gravity-CIL flowsheet with USD 6.1/t lower operating cost and marginally higher return, although the gravity recovery percentage is acknowledged as at the lower end of typical justification thresholds.
The report notes the primary crushing trade-off produced similar rankings for both options, with the gyratory selected mainly for operational robustness despite higher capital cost.
Operational water supply from the Geebo River is planned for year one only, with TSF-treated water from year two. Discharge treatment capacity is designed for 0.7M to 3.7M m³/annum, with higher treatment rates required during upset conditions.
Source
Dugbe Gold Project NI 43-101 Technical Report, Feasibility Study, prepared for Pasofino Gold Ltd., Section 17 (Recovery Methods), Report Reference DRA-JLREBR4638-NI43-101-REP Rev 0.
