The 2019 technical report describes the existing New Liberty processing facility, its historical operating performance from 2017 to early 2019, and the process design criteria and recommendations for treating underground and satellite ore sources.
Report context
This article is based on the PFS Mineral Resource and Mineral Reserve Update for the New Liberty Gold Mine, Liberia, with the processing methods chapter dated from the 2019 technical report (CSA Global Report Nº R148.2019). The report describes a processing facility already in operation, treating oxide resources from the New Liberty mine alongside fresh ore from satellite deposits. Planned development includes processing underground ore sources from the Larjor, Kinjor South/North, and Marvoe deposits, plus ore mined from the Ndablama deposit, at the existing plant.
Processing route
Existing plant design and commissioning
The original process plant was designed, built, and commissioned in July 2011 by DRA, an international engineering company, to treat 1.3 Mt/a of ore, corresponding to a mill feed of 165 t/hr. Metallurgical testwork results and industry norms were used to define the process design criteria for the New Liberty Plant.
Since start-up, a number of operational issues affected plant performance. In late 2016, following a change of project ownership, the operational issues were evaluated and measures identified to improve performance. Planned modifications to the plant, some already implemented and some planned for late 2019, were designed to increase throughput in stages up to 205 t/hr (1.6 Mt/a), while achieving the original design gold recovery of 90–95%.
Flowsheet description
The process flowsheet is an industry-standard arrangement comprising:
- Crushing
- Ore stockpiling
- Grinding and classification
- Gravity gold recovery
- Thickening
- Gold extraction by cyanidation in a CIL circuit
Gold is recovered from activated carbon by acid washing, elution, and electrowinning, followed by smelting to produce gold doré. The CIL tailings undergo cyanide detoxification followed by arsenic precipitation from tailings solution prior to disposal in the tailings dam.
Circuit modifications and grinding
In 2013, the circuit configuration was upgraded to two-stage crushing followed by two-stage milling. The primary ball mill consists of a 3.8 m by 4.9 m EGL ball mill, fitted with a 1.9 MW motor, operated in series with a VTM-500, fitted with a 0.2 MW motor. The design throughput remained the same; however, a finer grind size P80 of 75 µm to the CIL circuit was targeted to improve recoveries.
In 2018, a tertiary crusher was commissioned, which contributed to an increase in throughput rate compared with 2014 as a result of a finer F80 to the ball mill for an annualised production rate of 1.5 Mt/a.
The original plant design incorporates a comminution circuit with a ball mill and regrind Vertimill circuit to treat crushed ore at a design feed rate of 165 t/hr dry solids, producing a final product with a nominal P80 of 75 µm, for a rated throughput of 1.3 Mt/a. Design crushing and concentrator utilisation was 68.5% and 91.5%, respectively.
Requirements for target throughput
Based on the Bond Rod and Ball Mill Work indices reported in the testwork section, the existing grinding circuit has sufficient installed grinding capacity to process the underground ore sources from the New Liberty mine at an annual production rate of 1.4 Mt/a, targeting a grind size P80 of 75 µm. However, when processing the harder ore from Ndablama alone, the annual production rate is predicted to decrease to 1.2 Mt/a at a grind size P80 of 75 µm.
To achieve the study throughput rate of 1.4 Mt/a when treating the UG ore types, the operation will need to ensure:
- Continuous ore feed from the mine
- Three-stage crushing to achieve a final product size of 100% minus 12 mm
- Two-stage grinding: primary ball mill and secondary Vertimill
- Product size P80 of 75 µm reporting to the CIL circuit
- Overall plant availability maintained between 90% and 92%
For processing harder ore from Ndablama, it would be recommended to reduce the grind size to show the coarser liberation size for gold noted in this ore.
Operational performance history
Records of historical process plant performance at New Liberty are shown in Table 1.
Table 1: New Liberty historical plant performance (2017–2019)
| Month | Tonnes milled | Feed grade (g/t) | Recovery (%) | Plant utilisation (%) | Comment |
|---|---|---|---|---|---|
| Jan-17 | 99,280 | 1.62 | 85.0% | 93.0% | |
| Feb-17 | 81,436 | 2.05 | 90.0% | 89.0% | |
| Mar-17 | 99,597 | 2.24 | 90.8% | 95.0% | |
| Apr-17 | 86,636 | 1.82 | 90.0% | 84.0% | |
| May-17 | 93,236 | 1.93 | 84.0% | 87.0% | |
| Jun-17 | 114,664 | 2.19 | 89.0% | 97.0% | |
| Jul-17 | 85,686 | 2.62 | 90.0% | 69.0% | Low tonnes due to insufficient stock on ROM |
| Aug-17 | 99,367 | 2.67 | 91.0% | 80.0% | |
| Sep-17 | 68,998 | 2.43 | 92.2% | 57.0% | Low tonnes due to insufficient stock on ROM and plant breakdowns |
| Oct-17 | 78,449 | 3.48 | 91.0% | 62.0% | Low tonnes due to insufficient stock on ROM and pit flooding and plant breakdowns |
| Nov-17 | 111,406 | 2.88 | 89.0% | 94.0% | |
| Dec-17 | 124,612 | 2.74 | 89.4% | 98.0% | |
| Jan-18 | 117,194 | 3.33 | 87.7% | 93.6% | Low tonnes due to tertiary crusher not commissioned |
| Feb-18 | 101,923 | 2.78 | 87.4% | 100.0% | Low tonnes due to tertiary crusher not commissioned |
| Mar-18 | 124,472 | 2.62 | 86.0% | 91.4% | |
| Apr-18 | 113,469 | 2.73 | 87.1% | 79.8% | Low tonnes due to insufficient stock on ROM |
| May-18 | 132,911 | 2.98 | 87.7% | 90.2% | |
| Jun-18 | 105,771 | 2.68 | 87.5% | 81.5% | Low tonnes due to insufficient stock on ROM |
| Jul-18 | 101,513 | 2.83 | 90.5% | 80.9% | Low tonnes due to insufficient stock on ROM |
| Aug-18 | 120,323 | 2.67 | 89.9% | 83.3% | Low tonnes due to insufficient stock on ROM |
| Sep-18 | 131,807 | 2.94 | 88.4% | 91.1% | |
| Oct-18 | 124,986 | 2.54 | 89.9% | 94.1% | |
| Nov-18 | 124,488 | 2.23 | 88.4% | 95.7% | |
| Dec-18 | 112,339 | 2.55 | 88.5% | 80.2% | Low tonnes due to insufficient stock on ROM |
| Jan-19 | 122,600 | 2.38 | 87.5% | 79.5% | Low tonnes due to insufficient stock on ROM |
Project website: https://avesoro.com/Corporate/OurHistory
Project website: https://www.mining-technology.com/projects/new-liberty-gold-mine/
Project website: https://www.draglobal.com/projects/new-liberty-gold-mine/
Overall, the plant delivered an average throughput of 105,000 tonnes milled per month (1.3 Mt/a) with a maximum in the period of 132,911 tonnes milled per month (1.6 Mt/a). Low throughputs are by and large a consequence of limitations in mine production rate, in combination with low plant availability which was 85% over the period.
Predicted metallurgical recoveries
The metallurgical testwork recoveries obtained by gravity-leach tests were carried out on samples representing underground ore sources from New Liberty and fresh ore from Ndablama. The current process plant configuration includes all the unit processes to ensure target gold recovery is achieved, including gravity, intensive cyanidation leach, and conventional CIL/ADR circuits.
Testing showed that the New Liberty ore types have a significant gravity recoverable gold (GRG) component, and therefore it will be essential to maximise gravity recovery using the existing Falcon concentrator. It will also be important to ensure that the gold head grade to the CIL circuit does not spike in the event the Falcon concentrator is off-line for maintenance.
At an annual production rate of 1.4 Mt/a, the current CIL circuit has a calculated leach residence in the range of 8–10 hours. This is on the low side in the event the Falcon concentrator is offline.
Recommended optimisation areas
To ensure that the target gold recovery of 90% is achieved, the report recommends investigating:
- Installation of a second Falcon concentrator in the primary grinding circuit to maximise the GRG recovery
- Ensuring that the leach tanks are injected with pure dissolved oxygen to increase leach kinetics
- Maximising recovery of GRG in the gravity circuit to relieve residence times in the leach section
- Operating the Acacia leach reactor to increase kinetics in leaching the gravity concentrate
- Considering addition of oxygen to the CIL circuit to increase leach kinetics
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Original design feed rate | 165 t/hr | Design |
| Original design throughput | 1.3 Mt/a | Design |
| Current (2019) throughput rate | 205 t/hr (1.6 Mt/a) | Design target |
| Average historical throughput | 105,000 t/month (1.3 Mt/a) | Actual 2017–2019 |
| Maximum historical throughput | 132,911 t/month (1.6 Mt/a) | Actual May 2018 |
| Design gold recovery | 90–95% | Design |
| Target gold recovery | 90% | Recommended |
| Target grind size, P80 | 75 µm | Design |
| Historical plant availability | 85% | Actual 2017–2019 |
| Design crushing utilisation | 68.5% | Design |
| Design concentrator utilisation | 91.5% | Design |
| Predicted throughput (UG ore) | 1.4 Mt/a | Predicted |
| Predicted throughput (Ndablama ore) | 1.2 Mt/a | Predicted |
| CIL leach residence time | 8–10 hours | Calculated at 1.4 Mt/a |
| Primary ball mill | 3.8 m x 4.9 m EGL, 1.9 MW | Design |
| Regrind mill | VTM-500, 0.2 MW | Design |
| Crushing stages | Three-stage (2018 onwards) | Implemented |
Technical qualifications
The report notes several limitations and operational considerations:
- Operational issues since start-up have affected plant performance, with low throughputs attributed to limitations in mine production rate and plant availability of 85% over the period 2017–2019.
- Low tonnes in several months were directly attributed to insufficient stock on the ROM pad and plant breakdowns, with the tertiary crusher not commissioned in early 2018 also a contributing factor.
- When processing harder ore from Ndablama alone, the annual production rate is predicted to decrease to 1.2 Mt/a.
- The CIL circuit leach residence time of 8–10 hours at 1.4 Mt/a is considered on the low side if the Falcon concentrator is offline, creating risk of recovery loss.
- Testing indicated a significant gravity recoverable gold component in New Liberty ore types, making gravity recovery maximisation essential.
- Recommendations to achieve target recovery include a second Falcon concentrator and oxygen injection to the CIL circuit, both of which require further investigation.
Source: CSA Global Report Nº R148.2019, PFS Mineral Resource and Mineral Reserve Update for the New Liberty Gold Mine, Liberia, Section 17 (Recovery Methods).
