This article details the proposed processing route for the Kora Project, based on conventional crushing, grinding, gravity recovery, flash flotation, and conventional flotation to produce a gold-silver-copper concentrate and dorè bars.
Report context
The NI 43-101 Technical Report for the Kora Project describes a processing plant designed to treat underground ore from the Kora and Judd deposits at a proposed throughput of 1.2 million tonnes per annum. The process plant design and technology are described as well proven in the industry, making use of metallurgical unit processes already used in the existing Kainantu processing plant treating these ore types. Major unit operations and equipment are sized with a 20% design margin. The crushing plant utilization is designed for 68.5% (6,000 hours per year), while the grinding and flotation plant utilization is designed for 91.3% (8,000 hours per year), supported by crushed ore storage, standby equipment in critical areas, and back-up power from an on-site diesel power station.
Processing route
Crushing and Ore Storage
The proposed single stage open circuit crushing plant will prepare Run of Mine ore for feed to the grinding circuit. ROM ore is delivered into a 150 tonne capacity ROM bin equipped with a static grizzly screen with 600 mm by 600 mm aperture. Mining will be required to supply ore at a maximum nominal particle size of 600 mm. An apron feeder beneath the ROM bin feeds ore into a 160 kW, 810 mm by 1,370 mm primary jaw crusher which reduces ore from an F80 of 317 mm to a P80 of 90 mm, operating with a closed side setting of 100 mm.
Crushed ore discharges into a surge bin with a maximum live capacity of 75 tonnes, providing 30 minutes of mill feed at design throughput. A 1,800 tonne capacity emergency static stockpile, equivalent to approximately 12 hours of mill feed storage, is provided for use during crushing circuit downtime.
Grinding and Classification
The grinding circuit comprises a conventional two stage SAB milling circuit, consisting of a single grate discharge SAG mill and a single trunnion overflow ball mill. The SAG mill operates in open circuit, while the ball mill operates in closed circuit with hydro-cyclones. The SAG mill is 5.8 m diameter by 3.35 m long, equipped with a 1,850 kW variable speed drive capable of varying mill speed within 60% to 80% of critical speed. The ball mill is 4.2 m diameter by 6.85 m long, equipped with a 1,850 kW motor operating at a fixed speed of 75% critical speed.
The grinding circuit reduces primary crushed feed to a product size of 80% passing 106 µm for downstream flotation. The cyclone cluster consists of 8 by 400 mm diameter cyclones.
Flash Flotation
A Metso SK240 dual outlet flash flotation cell will treat a portion of the cyclone underflow stream to recover gold, silver and copper bearing minerals at relatively coarse particle size. The flash flotation cell is capable of treating up to 240 tonnes per hour of feed, equivalent to 40% to 50% of the recirculating load. The concentrate is pumped to the final concentrate thickener. The mid-point tailings offtake discharges low density fine slurry that gravitates to the mill discharge hopper. The bottom tailings discharge gravitates to the ball mill for further grinding.
Gravity Circuit
The gravity circuit consists of a horizontal vibrating gravity screen and a single SB1350 Falcon centrifugal concentrator treating a portion of the cyclone underflow stream. Feed passes over a 1.5 m wide by 3.6 m long horizontal vibrating screen with 2.5 mm by 18 mm apertures. Undersize reports to the centrifugal concentrator where precious metals amenable to gravity recovery are collected into a primary gravity concentrate. The gravity concentrate is discharged on a periodic regular basis.
Gold Recovery
Gravity concentrates are processed using wet shaking tables to produce a final concentrate for direct smelting to doré. The table separation circuits comprise a rougher shaking table (Holman 8000) and a cleaner table (Gemini 250). Final concentrates are smelted in a diesel fired furnace to produce doré.
Flotation Circuit
The flotation circuit produces a gold copper concentrate for downstream thickening and filtration. The circuit consists of a rougher conditioning tank, a bank of rougher/scavenger cells, cleaner conditioning tank, two stages of cleaning and one stage of cleaner scavenging.
The flotation circuit equipment includes:
- One 47 m³ rougher conditioning tank
- Six 40 m³ rougher/scavenger cells in series
- One 20 m³ cleaner conditioning tank
- Four 10 m³ cleaner cells in series
- Four 10 m³ cleaner scavenger cells in series
- Two 5 m³ recleaner cells in series
Laboratory conditioning and flotation times from test work formed the basis for equipment sizing with industry standard scale-up factors applied. The existing processing plant flotation retention times were also considered in the interpretation and process design.
Concentrate Thickening, Filtration and Handling
Concentrates from the flotation circuit and flash flotation circuit are combined and thickened by a 9 m diameter hi-rate concentrate thickener before reporting to a batch concentrate pressure filter containing 50 chambers with 1.2 m by 1.2 m plates, providing a total installed filter area of 115 m². Dewatered cake is stored in a storage shed before being loaded into 20 ft sea-containers for dispatch from the mine site.
Key reported parameters
| Parameter | Units | Value | Basis |
|---|---|---|---|
| Annual Plant Throughput | M tpa dry | 1.2 | Design |
| Ore SG | t/m³ | 2.78 | Design |
| Ore Moisture | % | 5 | Design |
| Gold Head Grade | g/t Au | 10 | Design |
| Copper Head Grade | % Cu | 1.0 | Design |
| Overall Gold Recovery (Gravity & Flotation) | % | 93.0 | Design |
| Gravity Gold Recovery | % | 15 | Design |
| Overall Copper Recovery | % | 95.0 | Design |
| Crushing Circuit Annual Operating Hours | hr/y | 6,000 | Design |
| Process Plant Annual Operating Hours | hr/y | 8,000 | Design |
| Crushing Plant Feed Rate | t/hr | 200 | Design |
| Process Plant Feed Rate | t/hr | 150 | Design |
| Crushing Work Index, CWi (85th Percentile) | kWhr/t | 16.4 | Design |
| Bond Ball Mill Work Index, BWi (85th Percentile) | kWhr/t | 16.8 | Design |
| SMC Test Parameter, A*b (15th Percentile) | 56.4 | Design | |
| Bond Abrasion Index, Ai (Average) | 0.129 | Design | |
| Grinding Circuit Product Size, P80 | µm | 106 | Design |
| Rougher/Scavenger Flotation Feed Density | % solids (w/w) | 30 | Design |
| Cleaner Flotation Feed Density | % solids (w/w) | 20 | Design |
| Rougher/Scavenger Laboratory Flotation Time | min | 10 | Testwork |
| Cleaner Laboratory Flotation Time | min | 4 | Testwork |
| Cleaner Scavenger Laboratory Flotation Time | min | 4 | Testwork |
| Scale-up Factor | 2.5 | Design | |
| Recleaner Laboratory Flotation Time | min | 4 | Testwork |
| Total Mass Pull to Concentrate | % | 5 | Design |
| Concentrate Thickener Settling Flux | t/m²/hr | 0.25 | Design |
| Concentrate Thickener Underflow Density | % solids (w/w) | 68 | Design |
| Concentrate Filter Utilization | % | 80 | Design |
| Concentrate Filter Discharge Cake Moisture | % | 10.5 | Design |
| Tailings Thickener Settling Flux | t/m²/hr | 0.7 | Design |
| Tailings Thickener Underflow Slurry Density | % solids (w/w) | 59 | Design |
Project website: https://koraproject.org/our-projects/kora-project/community-outreach-kora/
Technical qualifications
The report states that the process plant design and technology is well proven in the industry. The flowsheet makes use of metallurgical unit processes already used in the existing Kainantu processing plant treating these ore types. The report notes that the flotation residence time was based on aerated cell volume divided by average slurry flowrate, with effective scale-up ratio calculated based on flotation time divided by laboratory flotation time. Flotation times have effectively been scaled-up higher than the industry standard of two to three times the bench scale tests as a result of the 20% design margin being applied in the basis for cell sizing. The flotation reagent addition regime was selected based on test work, and the plant design will allow space to facilitate future installation of a xanthate reagent mixing, storage and distribution system should this be necessary.
*Source: NI 43-101 Technical Report , Kora Project, Section 17 Recovery Methods*
