This report describes the proposed processing facility for the Tuvatu gold project, based on a flotation and carbon-in-leach flowsheet designed for 219,000 tonnes per annum.
Report context
The Tuvatu Gold Project Preliminary Economic Assessment technical report, dated June 2015, was prepared for Lion One Metals Limited. Section 17 of the report describes the proposed recovery methods and processing facility design for the project, with a nominal capacity of 219,000 tonnes per annum at a design rate of 600 tonnes per day, based on 91% overall availability and a life-of-mine average feed grade of 11.3 g/t Au. The plant is designed to operate 365 days per year, 24 hours per day.
Processing route
Crushing circuit
The proposed flowsheet commences with a secondary crushing system designed for 1,000 tonnes per day, operating 24 hours per day at 75% overall system mechanical availability. The crushing system is sized at this capacity to allow for potential day-shift-only operation, over-crushing prior to relines, or future expansion. The report notes that the crushing index is based on a single test and feed size is assumed. The circuit was simulated using PlantDesigner and Bruno simulation software, but the report cautions that due to hardness and equipment specifications, a tertiary crushing circuit may be required in practice to achieve necessary size reduction.
Mineralized material from underground mining feeds a vibrating grizzly and primary jaw crusher system in open circuit, producing a product size (P80) of 105 mm, with a top size of 600 mm assumed from the underground mine. The secondary screen is an inclined, dual-deck vibrating screen with 50 mm and 15 mm openings on upper and lower panels respectively. Oversize from both screen decks reports to a secondary cone crusher in closed circuit with the secondary screen. The cone crusher has a maximum motor power of 250 kW and a closed side setting range from 10 to 25 mm depending on mantle profile. Undersized material is conveyed to a covered stockpile with 600 tonnes live capacity, supporting 24 hours of process plant operations when the crushing plant is not operating.
Grinding circuit
The grinding circuit consists of an open circuit primary ball mill and a secondary ball mill with gravity circuit in closed circuit with hydrocyclones. The circuit operates at 91% mechanical availability with annual throughput of 219,000 tonnes. Mill sizing was calculated based on the 75th percentile of Bond Work test results. The report recommends additional work in the next engineering phase to confirm hardness indices and mill sizing parameters.
Both ball mills are identical, with a diameter of 3.0 m and effective grinding length of 4.6 m, each equipped with a 522 kW motor. The estimated power draw range for both mills is 425 to 450 kW. The ball mill hydrocyclone cluster classifies feed slurry, with coarse underflow feeding the secondary ball mill or a gravity separation circuit. Cyclones are designed with 300% recirculating load. Cyclone overflow at 80% passing 75 µm flows by gravity to the flotation circuit.
A single gravity concentrator treats approximately 25% of the cyclone underflow. Testwork indicates approximately 38% of gold will be recovered to a gravity concentrate, which reports to a concentrating table for further upgrading and direct smelting to doré.
Flotation circuit
The flotation circuit was sized based on testwork performed by METCON and AMMTEC. A scale-up factor of 2.5 was applied to the flotation time used for cell sizing, within the common range of 2 to 3 for sulphide rougher flotation. The circuit comprises a conditioning tank with 12 minutes conditioning time and a single bank of six 10 m³ self-aspirating flotation cells arranged in series.
The material floats at natural pH of approximately 7.6. Potassium amyl xanthate (PAX) is added as collecting agent, and a frother is added as needed. Flotation concentrate overflows to a trash screen and then to the flotation concentrate thickener. Flotation tailings report to the flotation tailings thickener.
Regrinding and carbon-in-leach circuits
Float concentrate thickened underflow proceeds to the regrind circuit. Testwork indicates regrinding to 20 µm P80 or less is necessary to extract precious metals from the sulphide concentrate. The flowsheet proposes a stirred mill circuit for regrinding. The target regrind size is 80% passing 20 microns, though the report notes mineralogy indicates the regrind size may need to be finer and additional studies are required.
After regrinding, slurry reports to the float concentrate CIL circuit. Both the float concentrate CIL and float tails CIL circuits use six tanks in a single train, providing 24 hours of leaching. Slaked lime slurry maintains pH at 11.0 to prevent hydrogen cyanide gas evolution. Oxygen is sparged in the float concentrate CIL tanks at approximately 0.05 Nm³/h/m³, while process air is added to the float tails CIL tanks. Float concentrate CIL tailings report to the float tails CIL circuit for additional precious metal recovery.
Cyanide detoxification
Combined leached flotation tailings report to cyanide destruction using the SO₂/Air process with copper sulphate addition, targeting less than 1 ppm CN WAD in the discharge solution. Un-optimized detoxification testwork indicates the process can reduce residual cyanide to less than 2 ppm CN WAD. The report recommends a trade-off study for a cyanide recovery thickener prior to detoxification in the next phase.
Adsorption, desorption, and refining
Loaded carbon from the CIL circuits is pumped to the ADR plant. Carbon is acid washed with hydrochloric acid to remove calcium deposits, iron particles, silica, magnesium, and sodium salts. Elution uses a solution of approximately 1% sodium hydroxide and 0.1 to 0.2% sodium cyanide at 138 to 140 °C and 450 kPa, circulated at approximately 2.0 bed volumes per hour.
Pregnant solution passes through heat exchangers to the gold room for electrowinning, producing gold sludge. Sludge is filtered, dried, mixed with fluxes, and smelted in a diesel-powered direct fire furnace to produce gold doré. Stripped carbon is thermally regenerated in a diesel-fired horizontal kiln.
Key reported parameters
| Parameter | Units | Design Value | Basis |
|---|---|---|---|
| Plant nominal capacity | tpa | 219,000 | Design |
| Plant design rate | tpd | 600 | Design |
| Overall plant availability | % | 91 | Design |
| Life-of-mine average feed grade | g/t Au | 11.3 | Design |
| ROM moisture | % | 5.0 | Testwork |
| Bulk density | t/m³ | 1.6 | Testwork |
| Bond Ball Mill Work Index (BWi, 75th) | kWh/t | 18.6 | Testwork |
| Bond Rod Mill Work Index (RWi, 75th) | kWh/t | 19.8 | Testwork |
| Crushing Work Index (CWi) | kWh/t | 12.5 | Testwork |
| Crushing availability | % | 75 | Design |
| Crushing throughput | tpd | 1,000 | Design |
| Primary crusher product size (P80) | mm | 105 | Design |
| Secondary crusher product size (P80) | mm | 30 | Design |
| Primary grinding product size (P80) | µm | 1,000 | Design |
| Secondary grinding product size (P80) | µm | 75 | Design |
| Regrind product size (P80) | µm | 20 | Design |
| Gravity recovery to concentrate | % | 38 | Testwork |
| Flotation mass recovery | % of feed | 20.0 | Design |
| CIL retention time (both circuits) | h | 24 | Design |
| Carbon concentration in CIL | g/L | 20.0 | Design |
| Gold recovery to bullion (LOM) | % | 86.3 | Design estimate |
| Cyanide detoxification target CN WAD | ppm | 1 | Design |
Project website: https://liononemetals.com/
Technical qualifications
The report states that at the preliminary level of this study, testwork results, industry data, and assumptions have been used to make reasonable estimates for equipment sizing, which will be reviewed in the next engineering phase. Caution is advised regarding crushing plant sizing, as the crushing index is based on a single test and feed size is assumed; a tertiary crushing circuit may be required in practice. Mill sizing is based on the 75th percentile of Bond Work test results, and additional work is recommended to confirm hardness indices. Flotation testwork is preliminary, with a scale-up factor of 2.5 applied. Un-optimized detoxification testwork has been performed. The report recommends including metallurgical samples from the first three years of operation in future testwork programs to better define precious metal recoveries.
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Source: *Tuvatu Gold Project , Preliminary Economic Assessment*, National Instrument 43-101 Technical Report, Lion One Metals Limited, June 2015, Sections 17.0 to 17.10, 26.0 and 27.0.
