The Windfall Project flowsheet is a proposed design based on laboratory-scale testwork performed at SGS Québec and SGS Lakefield, with no historical operating data available.
Report context
The Windfall Project 2023 Technical Report, dated January 2023, presents a feasibility study for the Windfall Project. The report describes a proposed process plant design with a nominal throughput capability of 3,400 tpd. The flowsheet was established on the basis of laboratory-scale testwork mainly performed at the SGS Québec and SGS Lakefield laboratories, as described in Chapter 13 of the technical report. The resulting flowsheet shows the results of this initial testwork and forms the basis for the plant design.
Processing route
Crushing, Storage and Reclaim
Ore transported from the underground mine will have an F80 of 400 mm. Each rear dump truck from Windfall Mine ramps will carry 50 tonnes per load. A run-of-mine stockpile close to the crushing plant will be primarily utilized for emergency storage. A static grizzly (400 mm), mounted above the ROM bin, and a rock breaker will be installed. Material is withdrawn from below to the vibrating grizzly where the oversize material is directed to an open circuit jaw crusher to further reduce the material to a P80 of 114 mm. A baghouse dust collection system will capture the dust created by the crushing operations. The jaw crusher product and vibrating grizzly undersized are collected on a conveyor belt that feeds the crushed ore silo. The live silo capacity is 4,010 t, or 26 hours of nominal capacity.
Grinding Circuit and Gravity Recovery
The grinding circuit will be a SABC circuit, comprised of a single variable speed SAG mill and a single variable speed ball mill. The SAG mill will operate in closed-circuit with a pebble crusher, followed by a ball mill, operated in closed-circuit with cyclones. The product particle size exiting the grinding circuit cyclone overflow will contain 80% passing 37 µm material.
A SAG mill size of Ø7.32 m x 2.82 m (Ø24' x 9.25') effective grinding length was selected with a total installed power of 2,800 kW to grind the rock to a P80 of 1 mm. The mill is operated with a charge of Ø127 mm steel balls.
A ball mill, Ø5.18 m x 9.5 m (Ø17' x 31') EGL, fitted with a trommel screen, was selected for secondary grinding. The total installed power is 4,200 kW. The ball mill will be charged with Ø50.8 mm steel balls.
The gravity circuit feed pump box at the ball mill trommel undersize will feed two gravity scalping screens via a split box. The undersized material from the screens will feed two gravity concentrators, arranged in parallel. The gold concentrate from both gravity concentrators will feed an intensive leaching reactor operating in batch mode.
Carbon-in-Pulp
Prior to leaching, the ground slurry received from the cyclone overflow will pass through a trash screen before feeding the pre-leach thickener feed box. The pre-leach thickener diameter is Ø24 m. Underflow from the pre-leach thickener at 50% (w/w) will be pumped to the leaching circuit. The leaching circuit consists of one pre-aeration tank and four leaching tanks, each 14 m in diameter, mechanically agitated, and operating in series. The CIP circuit consists of nine CIP tanks operating in carousel mode.
Adsorption, Desorption and Recovery Circuit
The gold recovery circuits are based on the processing of 7 tpd of loaded carbon with a high pressure Zadra process. Carbon elution uses a barren strip solution of 1% NaOH and 0.5% NaCN circulating through the elution column at a flow rate of two bed volumes per hour for 10.5 hours at elevated temperature and pressure. A carbon regeneration kiln reactivates the stripped carbon at a nominal temperature of 700-800°C. Three EW cells recover gold and silver from the pregnant strip solution, with a fourth EW cell dedicated to the ILR pregnant solution.
Cyanide Destruction Circuit
A cyanide destruction circuit treats the CIP tails at 50% (w/w) solids using the liquid SO2/oxygen process. The process occurs in two tanks operating in parallel, providing a total retention time of 2 hours. Liquid SO2 is added from a liquid SO2 storage tank and oxygen gas injected through cone spargers located at the bottom of the tank.
Tailings Filtration Plant
The tailings filtration plant receives Windfall Mill tailings at a nominal throughput of 154 tph (3,400 tpd at 92% availability). The plant also receives thickened sludge from the underground mine (nominal flowrate between 6.9 tph and 9.7 tph). The design includes a 20% design factor, allowing the plant to process up to 196 tph of solids. Based on varying mine backfill requirements over the LOM, filtered tailings are planned to be directed to the paste production circuit approximately 39% of the time and to dry stacking about 61% of the time. Two operating filter presses (third press is on stand-by), each with approximately 600 m² of filtration area, produce a cake at a density of 84% solids.
Key reported parameters
| Description | Unit | Value | Basis |
|---|---|---|---|
| Nominal plant throughput | tpd | 3,400 | Design criteria |
| Plant availability | % | 92 | Design criteria |
| Maximal daily throughput (with design factor) | tpd | 4,080 | Design criteria |
| Average Au feed grade | g/t | 8.06 | Design criteria, from testwork data |
| Average Ag feed grade | g/t | 4.18 | Design criteria |
| Au recovery by gravity circuit | % | 31.2 | Design criteria, from testwork data |
| Ag recovery by gravity circuit | % | 22.4 | Design criteria |
| Grind size to leaching, P80 | μm | 37 | Design criteria |
| Leaching retention time | hr | 36 | Design criteria |
| Au recovery by CIP | % | 90.0 | Design criteria, from testwork data |
| Ag recovery by CIP | % | 79.1 | Design criteria |
| Overall Au recovery | % | 93.1 | Design criteria |
| Overall Ag recovery | % | 83.8 | Design criteria |
| Residual total cyanide at plant discharge (average) | mg/L | 10 | Design criteria |
| Residual total cyanide at plant discharge (maximum) | mg/L | 20 | Design criteria |
| Final tailings slurry density target | % w/w | 48 | Design criteria |
| Tailings filtration plant design factor | % | 20 | Design criteria |
| Filtration plant capacity (design) | tph | 196.4 | Design criteria |
| Paste solid content (average over LOM) | % | 70.2 | Design criteria |
| Paste binder content (average over LOM) | % | 3.11 | Design criteria, interpolated from testwork results by Golder with 20% design factor |
| Paste binder | – | Slag/Cement (90:10) | Design criteria |
| Dry stack solid content | % | 84 | Design criteria |
Project website: https://www.goldfields.com/canada-operations.php
Technical qualifications
The design criteria values presented were derived from testwork data, benchmarked values, BBA's database or based on Osisko's requirements. The paste binder content was calculated based on interpolation of testwork results provided by Golder, with an added design factor of approximately 20% (or about 0.5% absolute). The flowsheet was established on the basis of laboratory-scale testwork, mainly performed at the SGS Québec and SGS Lakefield laboratories. The testwork results are described in Chapter 13 of the technical report. No historical operating data is presented in the recovery methods section. All flowsheet and performance data represent proposed design.
Source: NI 43-101 Technical Report, Feasibility Study for the Windfall Project, Osisko Mining Inc., January 2023. Relevant sections: Recovery Methods (Chapter 17) and Process Plant Design Criteria (Section 17.1).
