This technical report presents the processing route design for the Okanjande graphite project, based on historical operational experience and metallurgical test work.
Report context
The Preliminary Economic Assessment Study Update Report for the Okanjande Graphite Project, published 31 July 2023, describes a proposed relocation and redesign of the graphite processing plant. The existing plant at Okorusu, which processed Okanjande material during 2017-2018, will be moved to the Okanjande Mine site.
Processing route
Previous Imerys-Gecko Process (2017-2018)
The existing graphite processing plant at Okorusu was based on the Imerys Lac de Iles graphite operation in Quebec, Canada, and relied heavily on gravity spirals early in the process. According to the report, while spirals can separate gangue from graphite, it is paramount that the graphite is well liberated to produce an acceptable concentrate. This was not the case for the Okanjande graphite, and the plant struggled operationally to achieve nameplate production rates and concentrate grades.
Identified design and operational issues included the rod mill having a high reduction ratio target of 22:1 based on a crush size of P80 = 10mm and mill discharge size of P80 = 425µm. The rougher circuit was fed from the grinding circuit at over 46% solids, which reportedly created high viscosities and gangue entrainment issues, potentially triggering the use of the dispersant Betamin. Dowfroth 200, a strong poly glycol ether frother, produced a strong and persistent froth that caused entrainment issues. Collector and frother were only added to the 2nd cleaner stage, leading to elevated graphite losses.
Current Northern Graphite Process Design
The proposed design relocates the process plant to the Okanjande Mine site, achieving four main benefits: reduction of operating costs through eliminating approximately 90km of mineralized material hauling; elimination of space constraints for future expansions; sufficient real estate for tailings storage to accommodate life of mine operations; and reduction in carbon footprint through removal of road haulage and implementation of solar power generation.
The design incorporates findings from both early and recent test work programs and is intended to improve flake preservation and purity of graphite flake concentrate through the following changes: a split milling circuit to prevent excessive recirculation and reduce potential for flake degradation; provision for potentially implementing a flash flotation stage after the Autogenous Mill (AG mill); generation of flotation feed stream at optimal solids content of 30%; a Flotation Feed Buffer Tank to reduce fluctuations; removal of gravity spirals early in the process flow; removal of excessive recirculating streams; reduced flotation solids content to remove need for dispersants; addition of MIBC as a more suitable frother; allowance for reagent addition at all flotation banks; and reduction of raw water consumption through dry tailings disposal.
Front-End Comminution
The comminution circuit consists of a single crushing stage followed by an open circuit SAG mill and closed circuit Rod Mill with hydro-cyclone classification. Primary crushing will be fulfilled by contractor crushing using a modular jaw crusher with hopper feeder and stockpile feed conveyor. Crushed mineralized material will be conveyed to a 2400 tonne capacity stockpile. The SAG mill will be of grate discharge type with a trommel screen. Hydro-cyclone underflow feeds directly into the Rod Mill, while overflow feeds a Low Intensity Magnetic Drum Separator (LIMS). Magnetics from LIMS will be removed for disposal; non-magnetics feed the Flotation Feed buffer tank.
Flotation
The flotation circuit consists of existing flotation banks transferred to the Okanjande site. Ground ROM mineralized material at 30% solids feeds conditioning tanks where Diesel (collector) and MIBC (frothing agent) are dosed. Conditioning is followed by a Rougher stage, with rougher tails pumped to a rougher scavenger stage. Tails from all downstream cleaner stages are treated through cleaner scavenger cells.
Rougher concentrate is polished through a newly installed 2.6×7.5m wet scrubber with smooth rubber liners and ceramic grinding media, followed by a single cleaning stage. Polished concentrate passes through a repurposed 6.5×9.0m ball mill with smooth rubber liners and ceramic grinding media, then through two more cleaner stages with internal recycle. Cleaned concentrate is classified into coarse (>180µm) and fine (<180µm) fractions using SWECO screens. Both fractions are polished through vertical stirred media mills followed by two and three further cleaning stages respectively.
Concentrate Dewatering, Classification and Bagging
The existing Okorusu concentrate dewatering, classification and bagging section was constructed by Imerys-Gecko and is described as fit-for-purpose. Combined concentrate from flotation enters the concentrate thickener, then is pumped to a vacuum belt filter. Filter cake is discharged into a diesel-fired rotary drum dryer with dust extraction. Dried concentrate is pneumatically transported to storage bins and classified into four fractions: +300µm (ultra-coarse), -300 to +180µm (coarse), -180 to +106µm (middlings), and -106µm (fines). Fractions are stored in product bins with dedicated bagging stations for 1 tonne bulk bags.
Integrated Waste Dump (IWD)
Rougher and Cleaner Scavenger tails are pumped to the tailings dewatering section where they are filtered through plate and frame filter presses. Filtered tailings are discharged on a dry-tailings stockpile, then loaded, hauled and co-deposited with waste rock on the IWD facility. The IWD will be located on the north-western edge of the mining license above the band of the Swakop marble group. The IWD will be lined with a double HDPE liner with leak detection. Run-off water will be collected in a lined pond for neutralization and recovery to the process.
Production Plan
Production will ramp up rapidly as a brown fields operation with all initial overburden stripping completed during the 2017-2018 period. ROM head grade increases with depth, with average grade exceeding 5% from year 7. A flat concentrate graphite grade was assigned to the production plan, though the report notes that increased grade in the last 3 years could lead to improved concentrate grade or overall recovery.
Key reported parameters
| Parameter | Units | Value | Basis |
|---|---|---|---|
| Mean annual throughput | t/a | 631,450 | Design |
| Design throughput (nominal) | t/h | 87.1 | Design |
| Design throughput (peak) | t/h | 100 | Design |
| ROM head grade (average) | % TGC | 5.3 | Design assumption |
| Mean combined recovery | % | 92 | Design target |
| Mean combined concentrate grade | % C | 95-96 | Design target (different values in two mass balances) |
| Combined concentrate production | t/h | 4.0-4.4 | Design (variation between report sections) |
| Concentrate +300µm fraction | % m/m | 9 | Design flake distribution |
| Concentrate +180µm fraction | % m/m | 40 | Design flake distribution |
| Concentrate +106µm fraction | % m/m | 30 | Design flake distribution |
| Concentrate -106µm fraction | % m/m | 21 | Design flake distribution |
| Historic ROM head grade (processed) | % | 5.6 (2024) to 5.9 (2034) | Production plan projection |
| Total concentrate produced (LOM) | t | 311,031 | Production plan projection |
| Total graphite processed (LOM) | t | 321,174 | Production plan projection |
| Annual water consumption | m³/a | 105,390 | Design water balance |
| Diesel (flotation reagent) | kg/a | 57,462 | Estimated from historical and testwork |
| MIBC | kg/a | 31,573 | Estimated from historical and testwork |
| Primary crusher P80 nominal | mm | 120 | Design |
| SAG mill product P80 (estimate) | µm | 3000 | Design estimate |
| Rod mill product P80 (estimate) | µm | 325 | Design estimate |
| Flotation feed solids | % m/m | 30 | Design optimal |
Project website: https://www.northerngraphite.com/okanjande/
Technical qualifications
The report clearly distinguishes between design targets, historical operating data, and test work results. The design parameters for comminution include estimates for mill product sizes and mill discharge solids concentration. Reagent consumption rates are based on values obtained from historical graphite processing at the Okorusu plant and the recent SGS test work program. The mass balance is provided at two different head grades (5% and 5.3% TGC) with different concentrate grade targets (95% and 95.1% respectively). The report states that a flat concentrate graphite grade was assigned to the production plan, and the increased grade in later years could likely lead to improved achieved concentrate grade or overall graphite recovery.
_Source: Okanjande Graphite Project: Preliminary Economic Assessment Study Update Report, 31 July 2023, Sections 1.16, 1.17, 17, 17.1, 17.2, 17.3, 17.4, 17.5_
