This feasibility study update describes a copper concentrator flowsheet designed for a nominal throughput of 300 mtph, incorporating grinding, flotation, regrinding, and concentrate dewatering.
Report context
The Copperwood Project feasibility study update is dated April 2023. The processing section of this report is based on the previous technical report prepared by Lycopodium and G Mining in June 2018. Comprehensive testwork programs have been completed on samples from mineralized zones, including repeatability of the results, providing a good understanding of the flotation circuit and associated process plant design. The current process plant design is described as lean and fit to purpose. No technical modifications were made during this feasibility update; therefore, this section has largely been reproduced from the previous technical report.
Processing route
Crushed Ore Reclaim
Crushed ore from the underground mine will be conveyed to a crushed ore transfer conveyor equipped with a weight scale. This conveyor will discharge onto a bidirectional or reversible conveyor that feeds the crushed ore bins. The two crushed ore bins will be equipped with two pan feeders each to reclaim material onto the SAG mill feed conveyor, which will also be equipped with a weight scale for measuring and controlling the SAG mill feed rate. A surplus ore feeding system, comprising a mobile hopper or feeder and conveyor, will allow ore material to be fed to the crushed ore bin via a front-end loader from the ore stockpile when required.
Grinding and Classification
The grinding circuit will receive ore at a nominal top size of 203 mm with an 80% passing size of 150 mm. The circuit will consist of a SAG mill operating in closed circuit with a screen and a ball mill operating in closed circuit with a cyclone cluster.
The SAG mill will be a 7.92 m diameter by 4.21 m EGL mill with a 5,500-kW motor, operating with a 12% to 15% ball charge. Ore will be fed to the SAG mill at a controlled rate, nominally 300 dry mtph, and water will be added to the feed chute to achieve the desired milling feed density. Flotation reagents, including sodium hydrosulphide (NaSH), alkylaryl dithiophosphate (A-249) and sodium isobutyl Xanthate (SIBX), will also be added to the mill feed. Product from the SAG mill will discharge over a grate, with oversize reporting to the scats bunker where it will be periodically removed. Grate undersize will be pumped to the SAG mill discharge screen, a single-deck inclined screen with a width of 2.4 m and length of 3.7 m, having a screen deck aperture of 2.0 mm. Screen oversize will be recycled back to the SAG mill and the undersize will gravitate to the cyclone feed pump box where it will be further diluted to achieve the required cyclone feed density.
The cyclone feed pumps will deliver slurry to the cyclone cluster. Cyclone underflow will gravitate to the ball mill, while cyclone overflow will gravitate to the trash screen. The ball mill will be a 5.80 m diameter by 9.86 m EGL overflow mill, with a 5,500-kW fixed speed motor, operating with between 30% to 35% ball charge. Product from the ball mill will discharge over a trommel, with oversize reporting to the rejects bin. Trommel undersize will gravitate back to the cyclone feed hopper to be classified again.
A separate layout model was developed to accommodate flash flotation. Space has been identified in the building for future installation of a flash flotation circuit if required.
Rougher Flotation
Cyclone overflow will gravitate to the trash screen, which will be a linear screen designed to remove foreign material prior to flotation. Screen undersize will gravitate to the rougher conditioner tank. SIBX, A-249, frother, and a sodium silicate-carboxymethyl cellulose sodium mixture (SS/CMC) will be added into the rougher conditioner tank. Process water can be added if required to dilute the feed to the appropriate slurry density.
The rougher flotation cells will consist of eight 130 m³ forced air tank cells in series. Rougher concentrate will gravitate into the regrind cyclone feed hopper. The first rougher flotation cell is installed such that the concentrate from the first tank can be directed to the second cleaner flotation circuit, bypassing the regrind circuit during operations if required. The rougher tailings will gravitate to the flotation tails pump box. The facility to dose SIBX, frother, A-249 and n-Dodecyl Mercaptan (NDM) along the rougher flotation cells train will be provided so that stage collector and frother additions can be used if required.
Regrind
Rougher concentrate and second cleaner tailings will report to the regrind cyclone feed pump box. The slurry will be pumped to the regrind cyclone cluster by the regrind cyclone feed pumps. The cyclone underflow will gravitate to the regrind mill where water and pH modifier (if required) will be added to achieve the desired milling density and desired operating pH respectively. The regrind mill will be a vertical mill and grinding will be achieved via attrition and abrasion of the particles in contact with steel media. Mill discharge will gravitate back to the regrind cyclone feed hopper for classification in the regrind cyclones. Regrind cyclone overflow will gravitate to the cleaner conditioner tank.
Cleaner Flotation
Cleaner flotation will consist of three stages of closed-circuit cleaning. The final arrangement includes recirculation of the first cleaner scavenger concentrate and tailings to the regrinding or first cleaner circuit and rougher last cells (scavenger) respectively.
Regrind cyclone overflow will gravitate to the cleaner conditioner tank. NaSH, pH modifier and SS/CMC will be added to this tank. The first cleaner flotation cells will consist of six 18 m³ trough cells in series. First cleaner concentrate will gravitate to the first cleaner concentrate, while the first cleaner tailings will gravitate to the first cleaner scavenger flotation cells. The first cleaner concentrate will be pumped to the second cleaner flotation cells.
The first cleaner scavenger flotation cells will consist of seven 18 m³ trough cells in series. A pH modifier, an A-249 and an SIBX will be added to the first cleaner scavenger flotation feed box. First cleaner scavenger concentrate will be collected in a pump box and will be pumped back to the rougher flotation circuit. First cleaner scavenger tailings will gravitate to a pump box from where the material is pumped to the flotation tailings pump box.
The second cleaner flotation cells will consist of six 8 m³ trough cells in series. A pH modifier and an SIBX will be added to the second cleaner flotation feed box. Second cleaner concentrate will be collected in a pump box and will be pumped to the third cleaner flotation circuit. Second cleaner tailings will be collected in a pump box and will be pumped to the regrind cyclone feed pump box.
The third cleaner flotation cells will consist of six 2 m³ trough cells in series. Third cleaner concentrate will be collected in a pump box and will be pumped to the concentrate thickener. Third cleaner tailings will gravitate to the first cleaner concentrate pump box.
Concentrate Thickening and Filtration
Final concentrate at 15.1 mtph solid will be pumped to a 16 m diameter high-rate concentrate thickener, along with filtrate return from the filtration area. Flocculant stock solution will be further diluted to 0.25% w/w with process water in an in-line mixer prior to addition to the concentrate thickener. Thickener overflow at a flow rate of 41.6 m³/h will gravitate to the process water tank for re-use.
Concentrate thickener underflow, at approximately 60% solids w/w, will be pumped to the agitated concentrate filter feed tank by one operating, with one standby, 3 x 2 concentrate thickener underflow pump. This tank will provide 12 hours of surge capacity between the thickener and filter. Thickened concentrate will be pumped batch wise to the concentrate filter press by filter feed pumps (1 operating, 1 standby). The filter for 35 mtph (235 m² area) will remove water from the concentrate to meet the target moisture of approximately 9% w/w using a series of pressing and air blowing steps. After the desired filtration time of approximately 12 minutes, the filter press will open and discharge concentrate directly to the floor of the concentrate shed.
A front-end loader will be used to remove the concentrate from beneath the filter press and transfer it to the adjacent 542 t concentrate storage areas. Concentrate will be loaded into the loadout hopper by the front-end loader when required, then transferred to concentrate trucks via a 900 mm wide concentrate feeder and 750 mm wide truck loading conveyor equipped with a weight scale.
Tailings Handling
Rougher and first cleaner scavenger tailings will be combined in a mixing box from where a final flotation sampler will take a sample to the on-stream analyzer for metallurgical and process control purposes. The mixing box discharge will combine with a number of intermittent reagent sump pump streams in the flotation tails pump box. Flotation tailings will be pumped to the tailings disposal facility.
Key reported parameters
| Parameter | Units | Value | Source |
|---|---|---|---|
| Plant Throughput | mtph | 300 | Highland |
| Head Grade – LoM | % Cu | 1.35 | Highland |
| Head Grade – Design | % Cu | 2.2 | Highland |
| Head Grade – Design | g/t Ag | 3.41 | Highland |
| Bond Crusher Work Index (CWi) | kWh/t | 20.3 | Consultant |
| Bond Ball Mill Work Index (BWi) | kWh/t | 16.2 | Testwork |
| SMC Axb | 34.5 | Consultant | |
| Bond Abrasion Index (Ai) | g | 0.014 | Testwork |
| Concentrator Feed Size (F80) | mm | 150 | Testwork |
| Grind Size (P80) | µm | 45 | Testwork |
| Rougher Residence Time – Laboratory | min | 50 | Testwork |
| Cleaner 1 Residence Time – Laboratory | min | 6 | Testwork |
| Cleaner 1 Scavenger Residence Time – Laboratory | min | 10 | Testwork |
| Cleaner 2 Residence Time – Laboratory | min | 5 | Testwork |
| Cleaner 3 Residence Time – Laboratory | min | 3 | Testwork |
| Regrind Mill Product Size (P80) | µm | 20 | Testwork |
| Concentrate Production Rate | t/h | 15.1 | Calculated |
| Target Concentrate Grade | % Cu | 24.7 | Highland |
| Target Overall Recovery | % | 86 | Highland |
| Concentrate Thickener Solids Loading | t/m² h | 0.20 | Lycopodium |
| Filter Solids Loading | kg/m² h | 160 | Lycopodium |
Project website: https://www.highlandcopper.com/projects/copperwood-project/
*Note: Design Axb value derived from the 85th percentile ranking of specific energies determined for each individual ore type.*
Technical qualifications
The process plant availability of 91.3% is planned for the first two years and 95% in the years after. This availability will be achieved during the third year of operation through good asset management, use of standby equipment in critical areas, and reliable grid power supply.
The comminution testwork has focused on the Copperwood Main Zone, which represents 75% of the mineral resources. The remaining 25% of resources lies to the east of the Main Zone, comprising the Bridge zone, Section 6, and Section 5. The comminution testwork for these zones has been analyzed separately, but the grinding characteristics have revealed small differences from the Main Zone parameters. The design of the circuit and equipment sizing will be based primarily on the Main Zone due to the small difference in grinding characteristics and the percentage of these zones in the orebody.
The DFS comminution design criteria will be based on the 85th percentile values of 13.9 kWh/t for the BWi and 34.5 Axb for the impact breakage SMC test. The selected 85th percentile values indicate that Main Zone ore has a high resistance to grinding both in terms of impact and abrasion energy requirement.
The flotation circuit configuration, residence times, reagent addition rates, and concentrate mass recoveries have been selected based on the metallurgical testwork conducted at SGS in 2018 and earlier work conducted at Metcon in 2011, with consideration of the design head grades. Ongoing optimization metallurgical testwork confirmed process selection and number of flotation stages; however, flotation residence time, flowsheet configuration, and reagents may need adjustments according to the final results.
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*Source: Copperwood Project Feasibility Study Update, April 2023. Sections 1.16, 17.1-17.13.*
