The Pulacayo Phase I feasibility study defines a 1,000 t/d polymetallic concentrator designed to produce separate lead and zinc concentrates with silver reporting predominantly to the lead concentrate.
Report context
The Pulacayo 1 000 t/d Phase I Feasibility Study – NI 43-101 Technical Report presents the process plant design for the Pulacayo project. The report, referenced as 090644-3-0000-20-IFI-100, documents a concentrator designed to treat 360,000 t/y of polymetallic ore over a 12.5 year mine life. The design basis was developed from metallurgical testwork performed by various laboratories on behalf of Apogee Silver Ltd., with assumed data from similar projects used where testwork results were unavailable.
Processing route
Process Design Basis
The Pulacayo plant is designed to process 1,000 t/d of ore through conventional crushing, screening, clays washing and screening, grinding, lead flotation, zinc flotation, concentrate filtration and storage, reagents storage and handling, tailings storage and disposal, backfill, and auxiliary services. The design includes carbon columns to remove copper ions in solution and organic flotation reagents from recirculated process water.
The process design basis specifies a design silver grade of 240 g/t, a design lead grade of 1.1% and a design zinc grade of 1.92%. The plant is designed with an overall availability of 86.2%, providing 7,446 operating hours per year across 360 operating days. Crushing operates on two 12-hour shifts, while the concentrator operates on three 8-hour shifts.
Ore from the underground mine is delivered to the crushing area by Gramby self-dumping cars through a railway system, with the crushing area located on surface 1.43 km from the adit.
Crushing and Screening
Run of mine ore is dumped onto a fixed grizzly with 400 mm bar spacings into a coarse ore bin. A 500 mm by 750 mm single toggle jaw crusher, installed with a 75 kW de-rated motor and set at a 50 mm close side setting, provides primary crushing. A self-cleaning moving belt magnet, fixed magnet and metal detector protect the secondary cone crusher from tramp metal.
The jaw crusher discharge feeds a 4,800 mm by 1,800 mm double deck primary vibrating screen with 25 mm upper and 12 mm lower decks. Both decks are equipped with polyurethane screen panels and polyurethane weir bars spaced at 900 mm intervals. Four bar pressure spray nozzles wash clays off rock particles. Oversize fractions report to a 1,300 mm diameter cone crusher with a 12 mm close side setting and 200 kW de-rated motor, operating in closed circuit with the screen.
Screen undersize is directed to a dewatering screen with an upper deck of 6.5 mm and lower deck of 0.4 mm. The crushed product has a nominal top size of 12 mm, with 85% passing 6.5 mm. The undersize fraction is directed through an in-line strainer to retain clayish material and recover water for high pressure spray and hosing. An alternative configuration using a scrubber with trommel in place of the primary screen is under consideration for detail engineering.
Grinding and Classification
Each of the two parallel ball mill circuits receives ore from its fine ore bin via an electromagnetic vibrating feeder onto the mill feed conveyor belt. Each 2,700 mm by 3,600 mm ball mill with rubber liners is closed-circuited with a cluster cyclone. De-rated power is 560 kW per mill.
Milled ore slurry at 70.9% solids by weight discharges from each ball mill through a trommel screen into a pump box, is diluted with process water and pumped to a D-6 cyclone cluster (4 operating, 2 stand-by) with 150 mm diameter cyclones. Cyclone underflow returns to the ball mill feed. Cyclone overflow at 80% passing 74 microns reports to a rotospiral trash screen (2 mm) for fibre removal before flotation.
Lead Flotation Circuit
Milled slurry is directed to high intensity conditioning tanks in the lead flotation circuit where iron pyrite depressor (NaCN), Sodium Ethyl Xanthate (SEX) collector, Sasfroth 200 and pH modifier (Ca(OH)₂) are added. After conditioning, slurry is pumped to the lead rougher flotation cell (Imhoflot G cell 1.4). Tailings report to a lead rougher/scavenger cell then a lead scavenger cell, both Imhoflot G cell 1.4 units. Lead rougher, rougher/scavenger and scavenger concentrates are directed to the lead cleaner circuit. SEX collector, KU5 (CMC) and ZnSO₄.7H₂O:NaCN complex are added to the cleaner feed pump boxes.
The lead cleaner circuit has two flotation cells in series. Final lead concentrate flows to a surge tank and is pumped to a lead recessed plate pressure filter for dewatering to below 10% moisture. Filtered solution is pumped to the carbon columns to adsorb dissolved metal ions.
Zinc Flotation and Paste Tailings
Lead circuit tailing is directed to a high intensity conditioning tank in the zinc flotation circuit where pH modifier (Ca(OH)₂), CuSO₄.5H₂O sphalerite activator, Aero 3418A collector and Sasfroth 200 are added. The slurry is pumped to the zinc rougher cell (Imhoflot G cell 1.4), with tailings directed to the zinc rougher/scavenger cell and then the zinc scavenger cell. Zinc rougher, rougher/scavenger and scavenger concentrates are directed to the zinc cleaner circuit, which has two Imhoflot G cell 0.8 flotation cells in series.
Final zinc concentrate flows to a surge tank and is pumped to a zinc recessed plate pressure filter for dewatering to below 10% moisture. The filter size is 1.4 m by 8.58 m (W x L). Filtered solution reports to the carbon columns.
Final zinc flotation tailings are pumped to an 8.4 m diameter paste thickener. Thickener underflow is either pumped to the tailings storage facility at 70% solids or to the backfill plant, where paste is prepared by mixing thickened tailings at 65% solids with cement at 6% w/w. The paste plant is designed to treat 100% of tailings.
Carbon Columns and Water Systems
The lead concentrate filter filtrate, zinc concentrate filter filtrate, paste thickener overflow and run-off water report to three carbon in column (CIC) tanks in series where copper ions and organic flotation reagents are adsorbed onto 6 x 12 mesh granular coconut carbon. The train consists of three 2 m diameter by 1 m high columns. Treated water is pumped to the process water tank.
Fresh water tanks receive water from the Yana Pollera reservoir for reagent mixing, general services, potable water treatment, camp and firewater supply. Plant spillage is contained in a lined dam and pumped to the mill discharge sump. Emergency power is provided by a 75 kW diesel generator serving the paste thickener rake, underflow pump and general lighting.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Plant throughput | t/d | 1,000 | Design |
| Annual throughput | t/y | 360,000 | Design |
| Life of mine | y | 12.5 | Design |
| Operating days | d/y | 360 | Design |
| Milling availability | % | 86.2 | Design |
| Crushing availability | % | 86.2 | Design |
| Design silver grade | g/t | 240 | Design |
| Silver grade range | g/t | 135.6 – 487.3 | Testwork composites |
| Silver grade mean | g/t | 302.3 | Testwork composites |
| Design lead grade | % | 1.1 | Design |
| Lead grade range | % | 0.66 – 4.3 | Testwork composites |
| Lead grade mean | % | 2.28 | Testwork composites |
| Design zinc grade | % | 1.92 | Design |
| Zinc grade range | % | 1.34 – 5.96 | Testwork composites |
| Zinc grade mean | % | 2.78 | Testwork composites |
| Specific gravity design | – | 2.65 | Design |
| Specific gravity range | – | 2.65 – 3.02 | Testwork |
| Crushed ore bulk density | – | 1.67 | Testwork |
| Bond ball mill work index design | kW-h/t | 14.8 | Design |
| Bond ball mill work index range | kW-h/t | 13 – 14.6 | Testwork |
| Bond ball mill work index mean | kW-h/t | 14 | Testwork |
| Bond crusher work index design | kW-h/t | 16.3 | Design |
| Grind size | µm | 74 (80% passing) | Design |
| Ball mill power (each of 2) | kW | 560 | Design |
Project website: https://silverelef.com/apogee-silvers-pulacayo-project-demonstrates-positive-feasibility-study-results/
Technical qualifications
The process plant design is based on metallurgical test results undertaken by various laboratories on behalf of Apogee Silver Ltd. Where testwork results were unavailable, assumed data from similar projects were used. The nature of the composition of drainage and fresh water is not well understood at this stage of the project, and the technology required for water treatment will need to be defined during the next stage of engineering studies. The primary screen alternative using a scrubber with trommel will be studied further during detail engineering. The plant layout is constrained by steep, high-altitude terrain with limited flat space, and, while the selected site has no sensitive flora or fauna, preliminary geotechnical drilling and evaluation was required to confirm site acceptability.
Source: Pulacayo 1 000 t/d Phase I Feasibility Study – NI 43-101 Technical Report, report number 090644-3-0000-20-IFI-100, relevant sections.
