Kubi Gold Project — 2022 Technical Report

This report presents the proposed processing route for the Kubi Gold Project in Ghana, based on a Metso Outotec process design study, while noting the operator's intention to initially use a nearby toll treatment plant.

Report context

The Kubi Gold Project NI43-101 Technical Report is dated 29th August, 2022. A study was undertaken by Metso Outotec on the potential for a stand-alone processing plant at the Kubi project. However, it is the intention of Asante Gold to initially send all run of mine to a nearby gold toll treatment plant located approximately 30km from the mine's location which has spare treatment capacity. For the purposes of the Kubi economic model, it has been assumed that plant feed material will be trucked to the existing plant, with estimated toll treatment costs used as the basis of economic modelling.

Processing route

Comminution and Gravity Concentration

The proposed process flowsheet defined by Metso Outotec begins with run-of-mine feed tipped onto the RoM pad and reclaimed by a front-end loader to the RoM feed bin. A vibrating grizzly feeder screens the RoM material and feeds oversize to a jaw crusher. Grizzly undersize and jaw crusher discharge are conveyed together to a stockpile. Two feeders draw from the stockpile and material is conveyed to a semi-autogenous grinding (SAG) mill.

The SAG mill discharges onto a trommel screen from which the oversize reports to a bunker for removal by front-end loader. The trommel undersize reports to the mill discharge pump box and is pumped to a cluster of hydrocyclones. The combined cyclone underflows gravitate to the SAG mill feed chute while the overflows flow under gravity to the pre-leach thickener via a vibrating trash screen.

The gravity concentration circuit is fed from the mill discharge pump box. A dedicated pump delivers slurry to a vibrating scalping screen which protects the gravity concentrator from large particles. The screen undersize reports to a centrifugal gravity concentrator and screen oversize gravitates back to the mill discharge pump box. Gravity recoverable gold is collected by the concentrator whilst gravity concentrator tailings return to the mill discharge pump box. The gravity concentrator operates on a batch basis, concentrate being periodically discharged to the intensive cyanidation circuit. Feed to the gravity concentrator is by-passed back to the mill discharge pump box during concentrate discharge.

The gravity concentrate is leached with concentrated cyanide and caustic soda solution. LeachAid or lead nitrate may also be added to the solution. The solution is circulated for 16 hours, during which time approximately >99% of the gold is dissolved. The leachate is then pumped to the electrowinning circuit. The residual solids report to the milling circuit. The leachate generated in the intensive cyanidation process is circulated through a single electrowinning cell. Gold-bearing sludge is rinsed off the cathodes periodically with a high-pressure washer and the resultant slurry is filtered and dried before smelting.

Leaching and Adsorption

Pre-leach thickener underflow slurry is pumped to the conditioning tank in the CIL area. The pre-leach thickener overflow gravitates to the process water tank. Lime slurry is added to the conditioning tank to adjust the pH to the target setpoint of 10.5. Aeration is provided to maintain the dissolved oxygen concentration of the slurry.

The slurry overflows from the conditioning tank to the leaching tank via a launder. Sodium cyanide solution is added to the leaching tank and aeration to maintain the dissolved oxygen concentration of the slurry is provided by low-pressure blowers. The slurry gravitates from the leach tank to the first of six mechanically agitated CIL tanks. The slurry is transported between tanks via launders which allow each of the tanks to be bypassed for maintenance purposes if necessary.

Each CIL tank contains activated carbon which is retained by interstage screens. The activated carbon is advanced counter-currently to the slurry flow and is moved progressively from the last CIL tank to the first by carbon transfer pumps. Regenerated carbon is reintroduced to the last CIL tank via the carbon sizing screen which removes undersized carbon particles and slimes before the carbon enters the CIL process. If the last CIL tank is offline the carbon returns to the second-last CIL tank.

Slurry is pumped from the first CIL tank to the loaded carbon screen which separates carbon from slurry. The slurry returns to the first CIL tank (or to the second CIL tank if the first is offline) while the loaded carbon is transferred to the acid wash column. Slurry discharges from the CIL circuit through the CIL safety screen which recovers any carbon that has passed through the last interstage screen. The tails slurry gravitates to the cyanide destruction circuit.

Cyanide Destruction

The SO2/air process for cyanide destruction, in which sulphur dioxide (SO2) and air, in the presence of soluble copper acting as a catalyst, oxidise the cyanide to cyanate (OCN-), has been adopted for the facility. The SO2 is provided as liquid sodium metabisulphite and the soluble copper as copper sulphate solution. Tailings slurry from the CIL circuit is pumped into the first of two cyanide destruction tanks in series. Slurry discharging from the second cyanide destruction tank is pumped to the tailings storage facility.

Elution and Electrowinning

Loaded carbon is washed with hydrochloric acid before elution by the AARL process. Acid washing and elution are performed in separate columns. These processes are performed in batches of 4 tonnes of carbon. Eluate is pumped through electrowinning cells in which gold deposits on the cathodes. When stripped eluate is barren it is pumped to the leach feed box. The electrowinning cells are drained, and the sludge rinsed off the cathodes with a high-pressure washer. The sludge is filtered before drying and smelting. The eluted carbon is hydraulically transferred from the elution circuit to the regeneration kiln, for thermal reactivation, via a dewatering screen. The hot carbon exiting the regeneration kiln is quenched and sized to remove carbon fines before returning to the last CIL tank.

Reagents

Sodium cyanide is used in the CIL and elution circuits and is received in 1 tonne bulk bags of solid pellets. These are held in a storage shed adjacent to the mixing facility and delivered to the mixing station as required. Cyanide solution is prepared on site and dosed to the leach/CIL and elution processes as necessary.

Caustic soda is received in 1t bulk bags of solid pellets. These are held in a storage shed adjacent to the mixing facility and delivered to the mixing station as required. Caustic solution is prepared on site and dosed to the leach/CIL and elution processes as necessary.

Hydrochloric acid is delivered to the site as a 30-32% (w/w) solution by bulk tanker in 24t loads and pumped into a storage tank from where it is dosed to the acid wash column as required.

Lime is supplied in the form of quicklime (CaO) which must be slaked (hydrated) to Ca(OH)2. Quicklime is supplied in powder form in 25t bulk truck deliveries and pneumatically transferred into the lime silo. A screw feeder transfers the quicklime from the silo to a slaker. The slaking process is operated in batches. The milk of lime slurry is transferred to the mechanically agitated milk of lime dosing tank from where it is pumped to the leaching/CIL and cyanide destruction processes via a ring main.

Sodium metabisulphite ("SMBS") is supplied in crystalline form in bulk bags. SMBS solution is prepared on site and dosed to the cyanide destruction process as necessary.

Copper sulphate pentahydrate is delivered in 0.9t bulk bags in crystal form. Copper sulphate solution is prepared on site and dosed to the cyanide destruction process as necessary.

Flocculant is required to settle solids in the pre-leach thickener. The flocculant is delivered to the reagent store in 0.7t bulk bags in powder form. Flocculant solution is prepared on site and dosed to the pre-leach thickener continuously.

Key reported parameters

Technical qualifications

Results of metallurgical testing have been used where available and industry norms have been applied otherwise. The size distribution of ROM and crusher product is to be confirmed. Gravity recovery design value is lower than lab results to provide a safety margin for CIL design. The process flowsheet defined by Metso Outotec is presented in Figure 75 of the report.

Source: NI43-101 Technical Report on Kubi Gold Project, Ghana, 29th August, 2022, Section 17 Recovery Methods.