The Eva Copper Project's Little Eva processing plant is designed to treat 31,200 t/d of blended sulphide and native copper ore through a three-stage crushing circuit with HPGR, ball milling, jigging, flotation, and regrind to produce a 28% copper concentrate.
Report context
This report presents the Feasibility Study Update for the Eva Copper Project, dated May 7, 2020, covering the processing facilities at the Little Eva plant in North West Queensland, Australia. The study describes a nominal ore throughput of 31,200 t/d (11.4 Mt/a) treating a feed blend of 75% sulphide ores and 25% native copper ores. The design basis incorporates metallurgical testwork completed between 1996 and 2016, with additional testwork conducted between 2018 and 2019. The processing plant is designed for a 15-year project life.
Processing route
Comminution circuit
The comminution flowsheet represents a key update from the prior feasibility study, changing from a SAG mill and pebble crushing circuit to a secondary crusher and HPGR design. The circuit consists of three crushing stages followed by ball milling.
Run-of-mine ore is delivered by 141-tonne haul trucks to a dump pocket with a live capacity of 440 tonnes. A gyratory crusher (Metso Mk III 42/65) performs primary crushing at a nominal rate of 1,733 t/h (1,925 t/h design) at 75% availability, with a 450-kW installed motor and a closed side setting of 130 mm. Primary crusher product is designed at P80 of 137 mm. Front-end loaders will re-handle ROM ore on the ROM pad as required to achieve the crushing circuit availability.
Secondary crushing uses a cone crusher fitted with a 933-kW motor operating in reverse closed circuit with a double-deck banana screen (4.2 m by 8.5 m). The screen is fitted with a 75 mm top deck and a 50 mm bottom deck. Screen undersize at P80 of 35 mm reports to the fine ore stockpile, which has a live capacity of 30,086 tonnes (approximately 21 hours). Two variable-speed apron feeders, each designed to provide 100% of the design rate, reclaim ore from the stockpile.
The HPGR circuit uses a 2.4 m by 1.65 m HPGR with two 5.4 MW drives, operating in closed circuit with two 4.2 m by 8.5 m double-deck wet screens with 10 mm top and 6 mm bottom deck apertures. The target transfer size to the ball mill circuit is P80 of 4 mm. Nominal and design circulating loads are set at 85% and 130% for conveyors respectively. Metal detectors on the HPGR feeder and screen oversize conveyor activate diverter gates to tramp metal bunkers.
The ball mill is 7.3 m diameter by 12.2 m effective grinding length, fitted with a 14.0 MW dual pinion variable frequency drive. Grinding circuit throughput is 1,413 t/h nominal and 1,700 t/h design at 92% availability. The ball mill operates in closed circuit with a 12-port cyclone cluster consisting of eight 800 mm diameter operating cyclones, two standby units, and two blanks dedicated to feed the rougher jigs. The cyclones are designed to operate with a feed density of 52% solids at 60 kPa and a circulating load of 300%. Ball mill grind product size is P80 of 165 µm.
Gravity concentration
A three-stage jigging circuit is incorporated into the ball milling circuit to recover coarse native copper. A portion of the cyclone cluster feed (typically 15%) gravitates to two IPJ3500 rougher jigs, limited to 500 t/h due to maximum rougher jig capacity. Rougher jig concentrate reports to a single IPJ2400 cleaner jig, and cleaner concentrate gravitates to a single IPJ1000 recleaner jig. Rougher and cleaner jig tailings return to the cyclone feed pump box. Recleaner jig tailings are recirculated to the cleaner jig. Recleaner jig concentrate reports to the gravity concentrate dewatering cone.
Flotation and regrind
The rougher flotation circuit consists of six 300 m³ conventional forced-air tank cells with an overall nominal residence time of 35 minutes. Provision is designed for sulphidization in the final two rougher flotation stages using sodium hydrosulphide. Rougher tailings are discharged to the 50 m diameter tailings thickener.
Rougher concentrate is directed to regrind in a Vertimill VTM1500 tower mill with 1,119 kW installed power, operating at a circulating load of 150% in closed circuit with a cyclone cluster. The regrind cyclone cluster has six ports with four 400 mm diameter operating cyclones and two in standby, operating at 80 kPa and producing a cyclone overflow at P80 of 48 µm.
A continuous gravity concentrator treating 25 t/h of solids processes approximately 16% of the regrind cyclone underflow stream to recover fine native copper, with concentrate reporting directly to the gravity concentrate dewatering cone.
The cleaner circuit consists of two 18 m³ DFR cells producing a high-grade concentrate to final concentrate. Six 18 m³ cleaner-scavenger DFRs send concentrate to three 6 m³ recleaner DFRs. Recleaner concentrate joins cleaner concentrate as final flotation concentrate. Recleaner tailings are recycled to the regrind cyclone feed pump box, and cleaner-scavenger tailings report to the tailings thickener.
Concentrate handling
Flotation concentrate is thickened in a 16 m diameter high-rate thickener with a Frothbuster system, specified at 65% solids underflow density. Thickener underflow is pumped to an agitated filter feed tank with 24 hours retention, then filtered in an automatic horizontal pressure filter with 140 m² filtration area to achieve 9% moisture. Filtered concentrate is stored in a covered building with capacity for 2,400 tonnes (three to four days of production) and loaded into trucks for transport.
Gravity concentrate is dewatered in a 1.8 m diameter dewatering cone at 70% solids, then transferred to a two-compartment drying paddock with total capacity of 200 tonnes for evaporation before truck loading.
Tailings and water systems
Rougher and cleaner-scavenger tailings are combined and thickened in a 50 m diameter tailings thickener designed for 63% solids underflow density. Thickener underflow is pumped by duty and standby pumps to the tailings storage facility, which uses subaerial deposition with decant solution recovery and under-drainage leak detection.
Process water is recycled from the tailings thickener overflow and supplemented with water recovered from the TSF. Concentrate thickener overflow solution is returned directly to the cyclone feed pump box to utilize residual flotation reagents. Fresh water is used for gland service, reagent preparation, filter press cloth wash, and process water make-up.
Reagents
The recommended reagent scheme includes potassium amyl xanthate (PAX) collector, alkyl dithiophosphate promoter Aero A3477, frother MIBC or Polyfroth H27, and sodium hydrosulphide sulphidizer. No pH modifier is required as the natural slurry pH of approximately 7.7 is suitable for copper flotation. PAX is prepared as a 20% strength solution, frother is added undiluted from 20-tonne tanker loads, and NaHS is supplied at 70% strength. A test reagent circuit is included for evaluating additional reagents.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Nominal throughput | t/d | 31,200 | Design |
| Annual throughput | Mt/a | 11.4 | Design |
| Feed blend | % sulphide / % native copper | 75 / 25 | Design |
| Feed grade – copper | % | 0.56 nominal; 0.46 average | Design |
| Feed grade – gold | g/t | 0.077 nominal; 0.047 average | Design |
| Primary crusher product P80 | mm | 137 | Design |
| Secondary crusher product P80 | mm | 35 | Design |
| HPGR product P80 | mm | 4 | Design |
| Ball mill product P80 | µm | 165 | Design |
| Regrind product P80 | µm | 48 | Design (regrind cyclone overflow) |
| Final concentrate grade – copper | % | 28 | Design |
| Final concentrate grade – gold | g/t | 3 | Design |
| Copper recovery to concentrate | % | 87.0 | Design (combined gravity and flotation) |
| Gold recovery to concentrate | % | 78.0 | Design (combined gravity and flotation) |
| Final concentrate moisture | % w/w | 9 | Design |
| Concentrate production – gravity | dmt/a | 1,484 | Design |
| Concentrate production – flotation | dmt/a | 192,129 | Design |
| Ball mill power | MW | 14.0 | Design |
| Tailings thickener diameter | m | 50 | Design |
| Bond ball mill work index | kWh/t | 16.5 | Testwork |
| Bond rod mill work index | kWh/t | 18.7 | Testwork |
| Bond crushing work index | kWh/t | 14.0 | Testwork |
| HPGR specific throughput (M-dot) | ts/m³h | 291 | Testwork |
Project website: https://www.mining.com/harmony-bets-big-on-copper-with-1-6b-eva-build/
Technical qualifications
The report notes that process design criteria were determined by Hatch in the 2018 Feasibility Study, then reviewed and revised by Ausenco considering new testwork results and the feed blend comprising sulphide and native copper ores. Throughput modelling used Ausenco's in-house comminution program Ausgrind, based on breakage testwork from 44 sample sets distributed across the pit. The design is specific to the blended ore and is based on testwork results from sample material originating from the various deposits. The report identifies that the proportion of copper production as gravity concentrate is anticipated to vary significantly over the life of mine. The nominated three to four days of flotation concentrate storage may not be sufficient during wet season road closures, with additional capacity potentially realized through shipping containers. Reagent storage capacity should be reviewed in the next phase to avoid potential production interruptions from wet season road closures.
Source: Eva Copper Project, NI 43-101 Technical Report, Feasibility Study Update, North West Queensland, Australia, May 7, 2020, Sections 1.17 and 17.
