The Imbo Gold Project has been designed with a 5 Mt/a processing plant based on gravity and carbon-in-leach technologies, incorporating SAB and SABC comminution circuits depending on ore type.
Report context
This technical report, dated 2023, describes the proposed processing facilities for the Imbo Gold Project (Adumbi deposit). The process plant design was developed from interpretation of preliminary economic assessment (PEA) test work, with comminution circuit simulations conducted by Orway Mineral Consultants (OMC). The report presents design criteria for oxide, transition, and sulphide (fresh) ore types.
Processing route
Comminution
The proposed comminution circuit consists of single-stage primary crushing using a jaw crusher (C160 or equivalent, 250 kW installed power) to produce a crushed product size (P80) of 180 mm. Crushed ore is directed to a surge bin, with provision for diversion to an emergency stockpile from which ore can be reclaimed by front-end loader when the crushing circuit is offline.
For oxide ores, the circuit operates as a SAG mill and ball mill combination (SAB circuit). For transition and sulphide ores, a pebble crusher (HP200 cone crusher or equivalent, 132 kW installed power) is added to treat SAG mill pebbles, forming an SABC circuit.
The SAG mill has a diameter of 9.15 m with effective grinding length of 4.35 m and 7,400 kW installed power, and grate discharge arrangement. The ball mill has a diameter of 6.70 m with effective grinding length of 11.00 m and 9,000 kW installed power, with overflow discharge arrangement. The milling circuit operates in closed circuit with a cyclone cluster comprising 18 cyclones (400 CVD10 model, 400 mm diameter). The target grind size (P80) is 75 µm.
Gravity concentration
A portion of cyclone underflow is bled to a gravity concentrator to recover coarse free gold. The centrifugal gravity concentrator operates in batch mode. Gravity concentrate is treated by intensive cyanidation in a leach reactor, followed by electrowinning to recover gold to bullion. Design gravity gold recovery ranges from 26.6% for sulphide to 36.8% for oxide ore.
Carbon-in-leach (CIL) circuit
The CIL circuit comprises one pre-leach tank and 11 CIL tanks in series, providing 24 hours residence time. Leach slurry density is 40% w/w for oxide and transition ores and 50% w/w for sulphide ore. Oxygen is sparged into the pre-leach tank and CIL tanks. Cyanide and lime are dosed through ring mains. Carbon is moved countercurrent to slurry flow using interstage carbon transfer pumps.
Tailings detoxification
CIL tails are processed through a two-stage INCO air/SO₂ cyanide detoxification system using sodium metabisulphite and copper sulphate solution, followed by arsenic precipitation with ferrous sulphate. Detoxified tailings are then pumped to the tailings storage facility.
Elution and gold recovery
Loaded carbon is acid washed (12 t batch) using dilute hydrochloric acid, then stripped using a pressure Zadra elution system operating at 125 °C under 300–350 kPa pressure. Electrowinning recovers gold from the pregnant eluate. Design elution frequency is seven strips per week. Stripped carbon is regenerated in a kiln before being returned to the CIL circuit. Gold sludge is calcined at approximately 800 °C and smelted at 1,200–1,400 °C to produce bullion bars.
Reagents and services
Consumables include sodium cyanide (bulk bags), hydrated lime (1 t bulk bags), caustic soda, SMBS, copper sulphate, ferrous sulphate, sulphuric acid, grinding media (125 mm and 50 mm steel balls), and activated carbon. Oxygen is supplied by a dedicated oxygen plant. Process water is sourced from raw water abstraction and return water from the tailings facility.
Key reported parameters
| Item | Unit | Oxide | Transition | Sulphide |
|---|---|---|---|---|
| Plant Throughput | Mt/a | 5.0 | 5.0 | 5.0 |
| Gold Head Grade | g/t Au | 2.25 | 3.2 | 4.00 |
| Design Gold Recovery | % | 91.82 | 90.38 | 80.1 to 89.83 |
| Crushing Plant Utilisation | % | 65.0 | 65.0 | 65.0 |
| Plant Availability | % | 91.32 | 91.32 | 91.32 |
| Comminution Circuit | – | Primary Crushing and SAB | Primary Crushing and SABC | Primary Crushing and SABC |
| Crush Size, P80 | mm | 180 | 180 | 180 |
| Grind Size, P80 | µm | 75 | 75 | 75 |
| Mill Pinion Power | kW | 5,112 | 4,075 | 3,670 |
| Grinding Media Consumption | kg/t | 0.780 | 0.547 | 0.503 |
| Gravity Gold Recovery | % | 36.8 | 31.6 | 26.6 |
| Leach/CIL Residence Time | h | 24 | 24 | 24 |
| Leach Slurry Density | % w/w | 40 | 40 | 50 |
| Number of Pre-leach Tanks | – | 1 | 1 | 1 |
| Number of CIL Tanks/Stages | – | 11 | 11 | 11 |
| Cyanide Consumption | kg/t | 0.99 | 1.32 | 1.31 |
| Lime Consumption | kg/t | 3.64 | 5.40 | 3.61 |
| Elution Circuit Type | – | Pressure Zadra | Pressure Zadra | Pressure Zadra |
| Elution Circuit Size | t | 12 | 12 | 12 |
| Frequency of Elution | strips/week | 7 | 7 | 7 |
Project website: https://www.loncor.com/projects/mineral-resources
Comminution test work data:
| Parameter | Unit | Oxide | Transition | Sulphide |
|---|---|---|---|---|
| Ai | g | 0.1878 | 0.2519 | 0.3409 |
| CWi | kWh/t | – | – | 15 |
| BRWi | kWh/t | 12.7 | 12.7 | 14.6 |
| BBWi | kWh/t | 11.8 | 13.7 | 14.2 |
| SG | – | 2.85 | 3.07 | 3.12 |
Technical qualifications
The process plant design presented in this report is based on interpretation of PEA test work results, not on definitive feasibility study or operational data. The report states that the flowsheet was developed from interpretation of PEA test work and simulations conducted by Orway Mineral Consultants (OMC). Proposed infrastructure includes hydroelectric power, tailings storage facility, and accommodation camps, with further studies required to optimise the power generation concept. The report notes that Loncor has engaged with local contractors who have provided budget tenders for plant buildings and accommodation camps, requiring further refinement of quotes.
The process design criteria are explicitly labelled as design values, distinguishing them from historical operating data or confirmed performance. The report acknowledges that further studies are required for certain infrastructure elements, and no operating data from an existing plant at this project is presented.
Source: Imbo Gold Project , 2023 Technical Report, Sections 17.1–17.5, Section 17, and Section 1.15
