This report details the proposed addition of a 5 Mtpa flotation plant at the Esaase mining site with overland conveying to the existing Obotan processing site, alongside expansion of the Phase 1 whole ore leach circuit to 3.8 Mtpa.
Report context
The Pre-Feasibility Study for the Asanko Gold Mine Phase 2 (AGM Phase 2) was prepared as a technical report under the project reference Rev 2, with report identifier TGHDP0220. The study covers the expansion of the existing Asanko Gold Mine located in Ghana, which commenced Phase 1 operations with a 3 Mtpa carbon-in-leach (CIL) plant treating material from the Obotan pits. Phase 2 proposes the addition of a 5 Mtpa flotation plant with ore feed from the Esaase mining site.
Processing route
Phase 1 existing design and Phase 2 expansion basis
The Phase 1 processing plant design is based on a single stage crushing, semi-autogenous grinding (SAG) and SAG/ball mill closed circuit (SABC) followed by a CIL plant treating 3 million tonnes per annum from the Obotan pits. The flow sheet includes a single stage jaw crusher feeding either onto a live stockpile or directly into an open circuit SAG mill with pebble crusher, and a ball milling unit in closed circuit with classification cyclones. A gravity recovery circuit treats a portion of the cyclone underflow stream to recover coarse free gold from the recirculating load. This process flow sheet is noted in the report as having been historically proven during Resolute Mining Ltd operations from 1998 to 2002 for Obotan region ores.
Under Phase 2, the Phase 1 circuit will be operated at a maximum of 3.8 Mtpa by processing additional oxide material. The whole ore leach circuit design remains as described with additional leaching capacity.
Phase 2 flotation plant design
Phase 2 includes the addition of a 5 Mtpa flotation plant consisting of a run-of-mine (ROM) handling and two-stage crushing circuit located at the Esaase mining site, followed by an overland conveying circuit to transport crushed material to the Obotan processing site. The gravity recovery, milling, flotation, concentrate regrind and CIL circuits will be located at the Obotan site.
Provision is made for intermediate stockpiling and interlinking conveyors between the Phase 1 whole ore leach circuit and the Phase 2 flotation circuit expansion to allow for optimisation of different ore types to either processing facility. The design makes provision for feeding oxide material from the Esaase pits to the CIL circuit and fresh material from the Obotan pits to the flotation circuit.
Esaase crushing and conveying
ROM ore will be treated in an open circuit primary crushing circuit with two tips, each with a dedicated ROM bin and crusher. One hybrid crusher and one jaw crusher will be installed initially, allowing oxide material to be crushed with the hybrid crusher while more competent material is crushed by the jaw crusher. Primary crusher product at P80 of 175 mm will be fed to a secondary cone crusher for further size reduction to P80 of 75 mm. A bypass will allow wet oxide material to bypass the secondary crusher stage. Secondary crusher product will be conveyed onto a single overland conveyor to the Phase 1 processing site.
Intermediate stockpiling and Obotan fresh material crushing
The overland conveyor transfers material to a transfer point where oxide or transitional material reports to an intermediate oxide stockpile feed conveyor, while fresh material reports to the mill stockpile feed conveyor. Crushed oxide material can be fed either to the CIL plant milling circuit or the flotation plant milling circuit. Crushed fresh material feeds directly to a live stockpile for the flotation plant ball milling circuit.
A secondary crushing stage is provided for Obotan fresh material from the CIL plant live stockpile. A single jaw crusher will crush Obotan fresh material from F100 250 mm to P100 90 mm before transfer to the flotation plant fresh ore stockpile.
Ball milling circuit
A ball milling circuit will treat material at a design feed rate of 635 t/h dry solids, operating in closed circuit with a hydrocyclone cluster. Mill density will be controlled at approximately 70% solids by mass. Cyclone overflow at 35% to 38% solids by mass will be gravity fed to the flotation circuit.
Primary gravity concentration
The primary gravity concentrator circuit will include two independent Knelson QS-48 concentrators with capacity to each treat 400 t/h. Feed material will be pre-screened on vibrating screens. Concentrators are expected to produce approximately 3 t/d of high grade concentrate, which will report to a batch intensive dissolution reactor.
Flotation circuit
Cyclone overflow from the ball milling circuit will gravitate to a flotation circuit comprising a single bank of seven 130 m³ forced air rougher flotation cells in series. PAX collector, dithiophosphate co-collector (Aero 9967), frother and copper sulphate will be added at staged addition points. Flotation concentrate will be collected in an agitated concentrate sump before being pumped to the pre-leach thickener. Flotation tailings will be pumped to a final tailings disposal tank.
Pre-leach thickening and regrind
A high rate thickener will increase flotation concentrate density to 40% to 45% solids by mass. Thickener underflow will be pumped to the regrind mill and secondary gravity concentration circuit. The secondary gravity concentrator is expected to produce approximately 1 t/d of high grade concentrate. Tailings from the gravity concentrator will be treated in a stirred media mill, with regrind mill product pumped to the CIL circuit.
Concentrate CIL circuit
The CIL circuit will consist of seven 390 m³ tanks in series with counter-current carbon flow. The circuit will be operated to achieve a design carbon loading of approximately 2500 g/t Au, with four inter-tank carbon transfers per day translating to a loaded carbon batch size of 5 t/d.
Cyanide destruction
CIL tailings will be pumped to a detox circuit with two 150 m³ tanks in series, each agitated and sparged with air. Detoxification will use the SO₂/Air process with copper sulphate added to catalyse the reaction, operated to achieve cyanide concentration of below 25 ppm CN WAD in the combined final tailings stream. Detoxified concentrate tailings will be pumped together with flotation circuit tailings to a common 8 Mtpa tailings storage facility shared with the Phase 1 circuit.
Elution and gold recovery
Loaded carbon will be batch treated in a split Anglo American Research Laboratories (AARL) elution circuit consisting of an acid wash vessel and 5 t elution column with heater facility. The CIL carbon will be treated in 5 t batches once every 24 hours. Elution uses a heated solution of sodium cyanide and caustic soda. Pregnant solution will be routed to electrowinning. The electrowinning and gold room will be shared with the Phase 1 facilities.
Historical operating basis
The report notes that the Phase 1 process flow sheet was historically proven a successful processing route for Obotan region ores during Resolute Mining Ltd operations from 1998 to 2002.
Key reported parameters
| Parameter | Units | Phase 1 Gravity-CIL Circuit (Design) | Phase 2 Gravity-Flotation-Conc CIL Circuit (Design) |
|---|---|---|---|
| Plant capacity | t/a | 3,800,000 | 5,000,000 |
| LOM average head grade | Au g/t | 1.76 | 1.66 |
| Fresh sulphides grade | Au g/t | 2.116 | 1.63 |
| Transitional grade | Au g/t | 1.35 | 1.99 |
| Oxides grade | Au g/t | 1.36 | 1.64 |
| LOM average overall recovery | % | 91.11 | 90.65 |
| Gravity recovery (LOM average) | % | 39.9 | Not stated separately |
| Crushing plant feed rate | tph | 741 | 819 |
| Milling plant feed rate | tph | 475 | 633 |
| ROM feed size (F100) | mm | 800 | 800 |
| Flotation feed size (F80) | µm | n/a | 75 |
| Flotation residence time | min | n/a | 45 |
| Flotation slurry feed density | % w/w | n/a | 38.8 |
| Flotation mass pull to concentrate | % | n/a | 9 – 12 |
| Leach feed size (F80) | µm | 106 | 15–20 |
| Leach residence time | h | 24.0 | 24.0 |
| Leach slurry feed density | % w/w | 45.0 | 37.6 |
| Number of CIL tanks | # | 7 | 7 |
| LOM average CIL cyanide consumption | kg/t | 0.91 | 6.48 |
| LOM average lime consumption | kg/t | 0.92 | 4.0 |
| Elution circuit type | , | Split AARL | Split AARL |
| Elution circuit size | t | 5.0 | 5.0 |
| Frequency of elution | batches/day | 2 | 1 |
| Cyanide destruction process | , | SO₂/Air Process | SO₂/Air Process |
Project website: https://www.asanko.com/
Notes on recovery values:
- Phase 1 recovery (91.11%) is based on test work and includes calculated discount factors for CIL solution gold losses (0.01 g/l Au in solution at 42% solids), fine carbon gold losses (40 g carbon per ton milled at 50 g/t Au), and a scale-up/ramp-up discount of 0.3% over life of mine.
- Phase 2 recovery (90.65%) is based on test work and includes calculated discount factors for concentrate CIL solution gold losses (0.04 g/l Au in solution at 35% solids), fine carbon gold losses (10 g carbon per ton milled at 50 g/t Au), and a scale-up/ramp-up discount of 0.54% over life of mine.
Technical qualifications
The Phase 2 capital cost estimate was developed to a nominal accuracy of -/+15%, inclusive of allowances for contingencies and estimating inaccuracies of 8.5% in aggregate (amounting to US$27 million). The Phase 1 capital cost estimate was developed to a nominal accuracy of -/+10%, with 8.3% aggregate contingency allowances (US$22.75 million). The engineering has been developed to support these capital cost estimate accuracies.
The process description is based on a pre-feasibility study level of engineering and testwork. Performance parameters are design criteria derived from testwork and include calculated discount factors for losses and scale-up effects. The report states that economics are preliminary in nature and there is no certainty that financial outcomes as presented will be realised.
Source: Asanko Gold Inc, Asanko Gold Mine Phase 2 Pre-Feasibility Study, Reference: Rev 2, Our Ref: TGHDP0220. Relevant sections: 1.14 Recovery Methods, 1.15 Operating Costs, 1.16 Capital Costs, 1.17 Economic Analysis, 17 Recovery Methods, 17.1 AGM Phase 1, 17.2 AGM Phase 2.
