This technical report presents the proposed process design for a gold recovery facility based on preliminary testwork and financial evaluations, with key design criteria and a detailed process plant description.
Report context
The Garrison Project 2021 Technical Report, dated January 27, 2021, is an N.I. 43-101 Technical Report and Preliminary Economic Assessment. The processing section describes a flowsheet and unit operations selected based on preliminary testwork and financial evaluations, using standard technologies widely used in gold processing plants. An overall process flow diagram and mechanical equipment list have been developed, and a general plant layout is presented in the report.
Processing route
Overall process design
The proposed flowsheet comprises primary crushing of run-of-mine (ROM) material, a covered crushed material stockpile, a SAG mill with trommel screen in closed circuit with a pebble crusher, followed by a ball mill with cyclone classification. Gravity recovery of ball mill discharge is achieved by one semi-batch centrifugal gravity concentrator, followed by intensive cyanidation of the gravity concentrate and electrowinning of the pregnant leach solution. The leach circuit includes trash screening, pre-aeration, leach and carbon-in-leach (CIL) adsorption. Loaded carbon undergoes acid washing and Anglo-American Research Laboratory (AARL) type elution, followed by electrowinning and smelting to produce doré. Additional unit operations include carbon regeneration, cyanide destruction of tailings using the SO₂/air process, carbon safety screening, and tailings disposal.
Primary crushing and stockpiling
Material is hauled from the mine and direct tipped into the crusher feed hopper. Material discharges to a vibrating grizzly screen where undersize is conveyed to the crushed ore stockpile and oversize is discharged to the primary crushing jaw crusher. Material is broken up by the jaw crusher along with a mounted rock breaker. The crushed material is conveyed to a covered stockpile that provides approximately 16 hours of live storage. Crushed material is withdrawn from the stockpile by two apron feeders that discharge onto the SAG mill feed conveyor. Oversize pebbles from the SAG mill are fed to a recycle circuit via conveyors, crushed by a cone crusher, and discharged on the SAG mill feed conveyor to recycle to the SAG mill.
Grinding circuit
The grinding circuit consists of a SAG mill with pebble crushing, followed by a ball mill in closed circuit with a hydrocyclone cluster. This configuration is known as a SAG ball mill, crusher (SABC) grinding circuit. The ball mill circuit starts at the cyclone feed pumpbox where SAG mill product and ball mill product is discharged. Slurry is pumped to a hydrocyclone cluster and to a gravity circuit via separate pumps. Optimal leaching of the Garrison deposit is at a grind of 80% passing size of 75 µm. Cyclone overflow gravitates over the trash screen. Trash screen undersize reports to the leach/CIL circuit. The hydrocyclone underflow returns to the ball mill, with the circulating load expected to be 350%.
Gravity circuit
The gravity circuit includes centrifugal concentration with a feed scalping screen. Feed to the circuit is pumped from the cyclone feed pumpbox via a dedicated pump to the scalping screen. Gravity scalping screen oversize at +2 mm reports back to the cyclone feed pumpbox. Scalping screen undersize is fed to the centrifugal concentrator, which operates semi-batch. Gravity concentrate is collected in the concentrate storage cone and subsequently leached by the intensive cyanidation reactor. Tails from the gravity concentrator also report back to the cyclone feed pumpbox.
Intensive leach reactor
A separate leaching circuit treats free gold concentrate produced by the gravity concentrator. In the intensive cyanidation reactor (ICR), free gold concentrate is leached using sodium hydroxide, sodium cyanide and hydrogen peroxide. The ICR unit operates on a batch cycle producing both pregnant solution and concentrate tailings. Tailings slurry is returned to the grinding circuit. Pregnant solution is stored for use in the electrowinning circuit. Combined ICR and elution pregnant solution is treated in the gold room using two electrowinning cells, with sludge collected and smelted into doré.
Leach/carbon-in-leach circuit
The leach and CIL circuit consists of eight tanks: one pre-aeration tank, one leach tank and six CIL tanks. Oxygen is sparged to maintain dissolved oxygen levels. Hydrated lime adjusts pH to 10.5 to 11.0. Cyanide solution is added to the leach tank and the first two CIL tanks. Fresh/regenerated carbon from the carbon regeneration circuit is returned to the last CIL tank and advanced counter-currently. Each CIL tank has a mechanically swept carbon retention screen. Loaded carbon is transferred from the first CIL tank to carbon elution.
Cyanide detoxification of tailings
Barren CIL tailings pass through the carbon safety screen before entering a cyanide detoxification tank. Detoxification uses the SO₂/air process with sodium metabisulphite (SMBS) and oxygen to reduce free cyanide and weak acid dissociable cyanide (CN WAD) to below specific environmental discharge limits. The reaction is carried out at pH 8.5 using copper sulphate as a catalyst. Two tanks in series provide total residence time of 1.0 hour. Detoxified tailings slurry is pumped to a tailings management facility.
Carbon acid wash, elution and regeneration
Gold-laden carbon is pumped to a loaded carbon screen, with oversize discharging to the acid wash vessel. Carbon is washed with weak hydrochloric acid solution to remove calcium, magnesium and other salt deposits. The acid-washed carbon is then hydraulically transferred to the elution column. The elution circuit is based on the AARL process, using sodium cyanide and sodium hydroxide solution at temperatures up to 120°C. Eluted carbon is dewatered and fed to an electric rotary kiln operated at 650° to 750°C in an atmosphere of superheated steam to restore carbon activity. Carbon is quenched, screened, and returned to the CIL circuit.
Electrowinning and gold room
Gold is recovered from ICR and elution pregnant solution by electrowinning using two cells fitted with stainless steel mesh cathodes. Gold-rich sludge is washed off, filtered, dried, mixed with fluxes, and smelted in an electric induction furnace to produce gold doré. The gold room is equipped with access control, intruder detection, and closed-circuit television equipment.
Reagent handling and storage
Reagents include quicklime (CaO) for pH control, sodium cyanide (NaCN) for leaching, sodium hydroxide (NaOH) for elution and ICR circuits, hydrochloric acid (HCl) for carbon acid wash, copper sulphate pentahydrate as a detoxification catalyst, sodium metabisulphite for cyanide detoxification, oxygen for leaching and detoxification, activated carbon for CIL adsorption, flocculant for water treatment, leach aid for the ICR circuit, antiscalant for elution and electrowinning equipment, flux for gold smelting, and grinding media for SAG and ball mills.
Key reported parameters
| Criteria | Unit | Value |
|---|---|---|
| Annual Throughput (Design) | t/a | 4,000,000 |
| Daily Throughput (Design) | t/d | 10,959 |
| Operating Days per Year | d | 365 |
| Operating Availability – Crushing | h/y | 6,132 |
| Operating Availability – Grinding | h/y | 8,059 |
| Design Throughput – Crushing | t/h (dry) | 652 |
| Design Throughput – Milling | t/h (dry) | 496 |
| Crushing Feed Size, 100% Passing | mm | 600 |
| Crushing Product Size, 80% Passing | mm | 90 |
| Grinding Product Size, 80% Passing | μm | 75 |
| Ball Mill Circulating Load | % | 350 |
| Crusher Work Index (Design) | kWh/t | 20.0 |
| Bond Ball Mill Work Index (Design) | kWh/t | 21.6 |
| ROM Head Grades Au (Average) | g/t | 0.90 |
| Recovery – Gravity Circuit | % | 25 |
| Recovery – CIL and Elution Circuit | % | 91.25 |
| Recovery – Overall | % | 92.35 |
| Average Annual Gold Production | oz/y | 106,900 |
Technical qualifications
All criteria presented in the key parameters table are design values based on preliminary testwork and financial evaluations, as stated in the report. The flowsheet and unit operations were selected based on preliminary testwork; no historical operating data or testwork results are provided in this processing section. The report notes that the basis of design criteria is sourced from Ausenco PDC, 2020. The process flow diagram sources are listed as Ausenco PFD, 2020.
Source: Garrison Project , 2021 Technical Report, N.I. 43-101 Technical Report & Preliminary Economic Assessment on the Garrison Project, January 27, 2021, Section 17 Recovery Methods.
