Metallurgical testwork has validated that GSS and WD material types are amenable to gold recovery via cyanidation, with a carbon-in-leach (CIL) process selected as the most economically effective scheme.
Report context
This section presents the recovery methods for the GSS/WD Project based on a technical report (SGS Report No. TS537-0000-T-REP-003 Rev 0). The report describes metallurgical testwork conducted to date, including Gold Deportment and Diagnostic Leach Test Work. The proposed design is based on a nominal capacity of 1.8 Mtpa for the comminution circuit and recovery plant.
Processing route
Comminution circuit
Oxide, transitional and fresh Run-of-Mine (ROM) material from GSS and WD is fed into a primary crusher and secondary and tertiary crusher circuit. Material is reduced from a top size of 600 mm to 80% passing 8 mm before being fed into the ball mill. The material is then ground to 80% passing 75 µm.
Results from comminution testwork were used to develop a comminution circuit simulation. The current design is based on the results of the circuit simulation utilising a single primary mill with nominal dimensions of 16.0 ft x 28.5 ft with installed power of 3,900 kW to produce the required 80% passing 75 µm discharge size.
The cyclone underflow stream returns to the ball mill for further grinding.
Crushing and stockpiling
ROM material will be trucked from local GSS open pits and from the WD open pit. Material will be tipped from trucks into a ROM bin and withdrawn at a controlled rate by an apron feeder with a nominal feed rate of 400 tph. The feeder discharges into a primary jaw crusher that reduces material top size from 600 mm to 80% passing 125 mm. This material is then stockpiled on the coarse stockpile. Material is reclaimed under the stockpile tunnel by variable speed vibrating feeders and delivered onto a conveyor to feed the secondary cone crusher. A magnet is installed over the secondary cone crusher feed conveyor for removal of tramp steel.
Coarse material is fed onto a double deck screen: plus 40 mm material is fed into the secondary cone crusher and the minus 40 mm plus 10 mm size fraction material reports to the tertiary crusher circuit. The discharge from the secondary cone crusher feeds two tertiary cone crushers. Products from the tertiary crushers are combined and conveyed to a vibrating dry screen. Screen oversize is returned to the tertiary crushers and the minus 10 mm undersize material is fed onto the fine material stockpile.
Ore is reclaimed from under the fine stockpile by vibrating feeders. Material is fed onto a conveyor where lime is proportionally dosed before being fed into the mill.
Milling and classification
Ore from the fine material stockpile is fed to the ball mill. Inlet dilution water is added to the mill feed chute to control mill slurry density. The ball mill discharges slurry via a trommel screen into a launder. Oversize from the trommel, mostly ball scats, drops into a bunker. Dilution water is added to the mill discharge launder. Slurry is pumped from the ball mill sump to the cyclone cluster for size classification.
At the cyclone clusters, coarse particles pass through the cyclone spigots as dense slurry that gravitates to the underflow distributor and fine particles pass through the cyclone overflow at about 40% solids slurry to the CIL. The two outlets of the cyclone underflow distributor divert slurry where about 75% is returned to the mill and the remainder flows to the CIL circuit.
Spillage pumps are provided in the mill bunded spillage area located in a drop out sump arrangement. Coarse solids settle out, and the spillage pump returns only excess water and slimes to the mill sump.
Carbon-in-leach (CIL)
Slurry from the cyclone overflow gravitates to the CIL section for cyanidation. A splitter box is installed equipped with a plug valve. Slurry overflows from the mechanically agitated leach tank and then flows through six subsequent mechanically agitated CIL tanks, to ensure complete dissolution of gold as a cyanide complex and adsorption onto activated carbon.
Cyanide solution is added from a ring main to the leach tank. Compressed air is injected into the leach tank with the facility to add air to the first two CIL tanks and fourth tank, should it be required, to provide oxygen for the cyanidation reaction. All tanks are equipped with two launder valves on the outlet. The normally open valve passes slurry to the next tank downstream. The second valve allows diversion of slurry to the successive tank downstream, to bypass any single tank if needed.
Each CIL tank is equipped with an inter-stage screen mechanism, with a cylindrical basket-type stainless steel wedge-wire screen surface. The mechanism drive turns wiper blades mounted on the outside of the basket. The screen has a pumping mechanism to allow slurry in the tank to flow through the screen surface whilst the tanks are on the same level. Tailings slurry from the last CIL tank gravitates to the tailings section.
A recessed impeller type pump located in the first CIL tank transfers slurry containing loaded carbon from the first CIL tank to the loaded carbon screen. The screen separates and rinses the loaded carbon from the slurry and discharges it into the elution section.
Interstage screens are cleaned regularly using high-pressure water sprays. A spare interstage screen and a maintenance bay are provided.
Cyanide detoxification and filtration
The discharge from the final CIL tank flows by gravity through a carbon catchment screen to the cyanide detoxification tanks. The cyanide detoxification tanks are mechanically agitated tanks with airflow to assist with the detoxification reaction. Copper sulphate is pumped from a dedicated copper sulphate storage and mixing plant and added to the pulp. Sodium metabisulphite is pumped from a dedicated sodium metabisulphite storage and mixing plant and added to the pulp. Caustic is used to increase pH as required.
The residue is pumped to a thickener. The water is reused as process water and the thickened slurry is sent to the final tails dam.
Elution
The elution section uses a pressurised 4 tonne Zadra system. Loaded carbon is eluted by pumping a hot caustic cyanide solution, typically 1.0 to 2.0% NaOH and 0.2 to 0.6% NaCN, through the column at 120°C under pressure. Gold adsorbed onto the loaded carbon is eluted off the carbon and collected in the eluate solution. The eluate is passed through electrowinning cells to remove gold from the circulating eluate stream.
The column operates under a pressure of typically 300 to 350 kPa. Solution exiting the column passes through the recuperative heat exchanger to cool the hot eluate and preheat eluate entering the column. Eluate solution is reused for a number of elution cycles until the level of contamination becomes unacceptable.
Regeneration and electrowinning
Carbon is withdrawn from the eluted carbon tank to feed the kiln by a screw feeder, which discharges the carbon into the rotary kiln for thermal regeneration. Regenerated carbon from the kilns is quenched in the quench pan and passed over a screen to remove fines, before discharge into the CIL to replace loaded carbon transferred out of the adsorption circuit.
Gold in the eluate is plated out onto cathodes in the electrowinning cells. Periodically the cathodes are removed and washed with a high pressure water gun. The resultant slurry is dropped into the laboratory filter press. The filter press cake is stripped out and packed into trays for calcining. The calcined material is smelted with fluxes into gold doré bullion in a furnace.
Reagents
Dry powdered lime is added onto the mill feed belt from a silo by a variable speed rotary vane and screw feeder arrangement. Hydrochloric acid stored in drums is transferred to an acid storage tank and then to the acid soak tank in the regeneration area. Sodium cyanide stored in bags is discharged through a bag breaker into a cyanide mixing tank equipped with a mechanical mixer. Caustic soda stored in bags is discharged through a bag breaker into a caustic mixing tank equipped with a mixer. SMBS stored in bags is discharged through a bag breaker into an SMBS mixing tank equipped with a mixer.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Nominal plant capacity | 1.8 | Mtpa | Design |
| ROM top size | 600 | mm | Design |
| Crusher product size | 80% passing 8 | mm | Design |
| Ball mill product size | 80% passing 75 | µm | Design |
| Ball mill dimensions | 16.0 x 28.5 | ft | Design |
| Ball mill installed power | 3,900 | kW | Design |
| Apron feeder nominal feed rate | 400 | tph | Design |
| Primary crusher product size | 80% passing 125 | mm | Design |
| CIL feed slurry flow | 433 | m3/h | Design |
| CIL total residence time | 30 | h | Design |
| CIL total tank volume required | 12,981 | m3 | Design |
| Number of CIL tanks | 7 (1 leach + 6 CIL) | – | Design |
| CIL tank diameter | 13.00 | m | Design |
| CIL tank height | 16.300 | m | Design |
| Cyanide consumption | 0.75 | kg/t | Design |
| Detox free cyanide target | less than 50 | ppm | Design |
| Elution method | Zadra | – | Design |
| Design carbon loading | 1,900 | g/t | Design |
| Design barren carbon loading | 50 | g/t | Design |
| Strip batch size | 4.0 | t | Design |
| Eluant strength (design) | 1% NaCN and 2% NaOH | – | Design |
| Carbon bulk density | 0.50 | t/m3 | Design |
| Elution column diameter | 1.1 | m | Design |
| Elution column height (tan-tan) | 5.3 | m | Design |
| Flow through elution column | 20.0 | m3/h | Design |
| Regeneration kiln capacity | 400 | kg/h | Design |
| Eluant solution electrowinning cell capacity | 500 | l/min | Design |
| Eluant solution electrowinning cycle type | 20 | h | Design |
| Tailings storage facility footprint (estimate) | 57 | hectares | Cost estimate basis |
Project website: https://www.wd-deo.gc.ca/eng/20204.asp
Technical qualifications
The report notes that no tailings storage facility design has been completed at this stage of the project. An existing design for a conventional surface thickened tailings storage facility with an approximate footprint of 57 hectares was used to estimate costs.
The report identifies upward potential in gold recovery using flash flotation followed by ultra-fine grinding and cyanide leaching of the flash flotation concentrates to recover some of the more refractory gold associated with pyrite. This option has not been included in the current design and will be addressed in the Pre-Feasibility Study.
Source: GSS/WD Project, Recovery Methods Section, SGS Report No. TS537-0000-T-REP-003 Rev 0.
