Twin Hills Definitive Feasibility Study National Instrument 43-101 Technical Report

This report details the proposed process plant design for the Twin Hills Gold Project, based on a 5.0 Mtpa flowsheet incorporating crushing, grinding, gravity recovery, carbon-in-leach leaching, cyanide destruction, and pressure filtration of tailings.

Report context

This National Instrument 43-101 technical report presents the definitive feasibility study (DFS) for the Twin Hills Gold Project, dated June 2023. The report describes the proposed process plant design, including flowsheet development, equipment selection, and key design criteria for the processing facility.

Processing route

Overall Process Design

The proposed plant design for the DFS is based on a flowsheet comprising three stages of crushing and screening followed by milling and size classification, gravity recovery, a carbon-in-leach (CIL) circuit, carbon elution, and a gold recovery circuit. CIL tailings will be treated in a cyanide destruction circuit followed by thickening and pressure filtration. Tailings filter cake will be transferred on an overland conveyor for stacking at the tailings storage facility (TSF). Some mine waste rock will be delivered to the TSF by dump truck and used in the construction of the outer containment berm.

The key criteria for equipment selection for this DFS were suitability for duty, reliability, and ease of maintenance. The plant layout provides ease of access to all equipment for operating and maintenance requirements, whilst maintaining a layout that will facilitate construction progress in multiple areas concurrently.

The proposed process design includes a primary gyratory crushing of run-of-mine (ROM) material onto a coarse ore stockpile; a secondary and tertiary cone crushing circuit with classification screens to produce a fine product for storage on a covered fine ore stockpile ahead of the milling plant; a single-stage ball mill operating in closed circuit with classification cyclones; gravity recovery of cyclone underflow by a semi-batch centrifugal gravity concentrator, followed by intensive cyanidation of the gravity concentrate and electrowinning; trash screening and thickening of cyclone overflow before leaching; pre-oxidation with shear reactors ahead of the CIL circuit and oxygen addition to the leaching tanks via a PSA oxygen plant; acid washing of loaded carbon and split AARL elution circuit followed by electrowinning and smelting to produce doré; cyanide destruction of tailings using the SO₂/Air process with future allowance to include arsenic precipitation; thickening of the detoxified slurry, with thickened tailings routed to agitated storage tanks; pressure filtration of tailings thickener underflow and conveying of the filter cake to the tailings disposal facility; and provision made in the design for possible future intensive cyanidation of gravity concentrate from the off-site Ondundu deposit.

The crushing plant has a design annual utilisation of 6,132 hours. The downstream processing plant, including gravity concentration, CIL plant, cyanide destruction, thickening, pressure filtration, and gold recovery operations, has a design annual utilisation of 8,000 hours. The pressure filtration circuit has an annual utilisation of 7,008 hours.

Primary Crushing and Stockpiling

ROM material will be trucked from five pits: Bulge, Central, Clouds, Clouds West, and Twin Hills West. Low grade ore from each pit will be diverted to run of mine stockpiles of weathered (transitional) and fresh ore types. Higher grade transitional and fresh ore will be despatched with a limit of no more than 25% transitional by mass to the primary gyratory crusher station for direct tipping into the dump pocket via Cat 777s. Lower grade ore will be reclaimed from the ROM stockpiles if necessary to keep the primary crusher fully optimized and to maintain the minimum percentage of fresh ore specified. A fixed rock breaker will be utilized to break oversize rocks at the top of the dump pocket.

The crushed rock will be withdrawn from the primary crusher discharge bin under the gyratory crusher with a wide heavy-duty primary crusher discharge conveyor which transports the ore via a coarse ore stockpile feed conveyor onto a coarse ore stockpile with 12 hours live storage capacity (9,785 tonnes). The coarse ore stockpile feed conveyor is equipped with a weightometer for crushing throughput control as well as a tramp metal magnet system to protect downstream equipment.

Secondary and Tertiary Crushing

Primary crushed ore is reclaimed with two variable speed apron feeders located in the coarse ore reclaim tunnel onto the secondary crusher screen feed conveyor. Each feeder is sized for 100% plant capacity. The ore is conveyed to a double-deck secondary crusher screen, with the oversize of the 100 mm top deck and the 35 mm bottom deck passing into the secondary cone crusher. Secondary crusher discharge passes to the tertiary crusher screen feed conveyor, joining the secondary crusher screen undersize and is conveyed to the tertiary crusher screen feed bin.

The combined secondary and tertiary crusher products and secondary crusher screen undersize is conveyed by a tertiary crusher screen belt feeder to a double-deck tertiary crusher screen, with the oversize of the 25 mm top deck and the 14 mm bottom deck passing onto a tertiary crusher feed conveyor. This conveyor feeds a twin tertiary crusher feed bin with double conical bottoms, each discharging to separate belt feeders. Variable speed belt feeders reclaim the ore and choke feed the two duty tertiary crushers at a controlled feed rate. Tertiary crusher screen undersize P₈₀ 9 mm discharges onto the fine ore stockpile feed conveyor which in turn discharges onto the covered fine ore stockpile with 12 h live storage capacity (7,500 tonnes).

Fine Ore Reclaiming and Grinding

Fine ore is withdrawn from a reclaim tunnel beneath the covered stockpile by two variable speed fine ore reclaim belt feeders (both running with option for one to be on standby). Each feeder is sized for 100% of plant capacity. The feeders discharge onto the mill feed conveyor, which conveys the fine ore to the ball mill feed box. The mill feed conveyor will be fitted with a weightometer used for controlling the speed of the belt feeders and hence the feed rate to the grinding circuit.

A fines reclaim hopper and fines reclaim belt feeder over the mill feed conveyor is provided where fine ore, grinding media (balls) and/or ball mill pebbles will be fed into the circuit as required. Quick lime is added directly to the mill feed conveyor via a lime silo using a rotary valve.

The primary grinding circuit consists of a ball mill (12.5 MW) operating in closed circuit with a classifying cyclone cluster. At the targeted grind size P₈₀ of 63 µm, generation of pebble scats will be minimal. Oversize from the ball mill trommel is directed to a pebble bunker, while the undersize gravitates to the mill discharge hopper from where it is pumped to the classifying cyclones. Cyclone overflow gravitates to two duty trash screens via a trash screen distribution box ahead of the pre-leach thickener, while cyclone underflow gravitates to the ball mill feed box via classification cyclone underflow splitter box for further grinding. A portion of the cyclone underflow also feeds the gravity concentration circuit.

Gravity Recovery and Intensive Cyanidation

The gravity circuit comprises two centrifugal concentrators complete with two gravity scalping screens. Feed to the circuit is a bleed stream from the classification cyclone underflow splitter box; this flows by gravity to the gravity scalping screens. Gravity scalping screen oversize at +2 mm gravitates to the ball mill feed box. Each scalping screen undersize gravitates to a dedicated gravity concentrator. Gravity concentrator tails gravitates to the mill discharge hopper. The operation of the gravity concentrator will be semi-batch and the gravity concentrate will be collected in the Intensive Cyanidation Reactor (ICR) feed storage hopper and subsequently leached by the ICR circuit.

The gravity concentrate is transferred to the ICR drum after the predetermined weight is reached at the storage hopper, at which point batch processing is initiated. Process water is added to the ICR drum. ICR leach solution (~1.2% w/v NaCN and ~2.0% w/v NaOH) is stored in the ICR solution storage hopper and transferred to the ICR drum. Caustic soda solution is added to maintain the pH above 10.5. Oxygen is added to the circulation line through a sparger during leaching for a predetermined period of time. After completion of leaching, the solution is transferred to the ICR solution storage hopper to settle the solids. The decanted clarified solution is drained to the ICR pump hopper and pumped to the ICR electrowinning feed tank in the goldroom.

Pre-Leach Thickening and CIL Circuit

Cyclone overflow is distributed over two trash screens to remove foreign material prior to leaching. Screen undersize gravitates to the pre-leach thickener to increase the solids concentration of the leach feed to 50% solids. The pre-leach thickener underflow is pumped to the CIL circuit.

The leach circuit consists of one pre-oxidation tank and seven CIL tanks. Slurry can bypass the pre-oxidation tank or any of the CIL tanks when necessary. Oxygen is sparged through two oxygen shear reactors. Oxygen is also added to each of the CIL tanks via the agitator shafts to maintain adequate dissolved oxygen levels for leaching. Cyanide solution can be added to the first four CIL tanks as required but generally is added only to the first two tanks with the dosage controlled by the cyanide analyzer.

Fresh carbon or regenerated carbon from the carbon regeneration circuit is returned to the last tank of the CIL circuit and is advanced counter-currently to the slurry flow by recessed impeller carbon advance pumps in each CIL tank. Slurry from the last CIL tank gravitates to the vibrating carbon safety screen to recover any carbon leaking from worn screens or overflowing tanks. There is an option to add milk of lime to each CIL tank as required to maintain the pH above 10.5. The target leach time is 24 hours.

Carbon Elution and Gold Recovery

Before carbon stripping, loaded carbon is treated with a 3% hydrochloric acid solution to remove calcium, magnesium, and other salt deposits. Carbon stripping will utilize the split AARL process. The strip solution is made up of 2.0% w/w NaOH and 2.0% w/w NaCN. After an initial preheat (95°C) and soak period the strip solution is re-circulated through the strip solution heater to bring the solution up to 130°C before entering the elution column.

Gold is recovered from the pregnant eluate by electrowinning and smelted to produce doré bars. Pregnant eluate from the elution circuit passes through two electrowinning cells with stainless steel mesh cathodes. ICR pregnant solution passes through one additional dedicated electrowinning cell to process ICR pregnant solution. The gold-rich sludge is washed off the steel cathodes using high pressure water sprays, filtered in a pan filter, dried, mixed with fluxes, and smelted in a diesel fired furnace to produce gold doré.

Cyanide Destruction and Tailings Management

CIL tailings pass through a carbon safety screen and are detoxified to weak acid dissociable cyanide (CNWAD) concentration of <50 ppm by the SO₂/Air process. Slurry gravitates into one of two cyanide destruction tanks operating in series. Total residence time in series is approximately 90 minutes to reduce maximum CNWAD design levels from 200 ppm (100 ppm operating) to less than 50 ppm. The reagents required are blower air, milk of lime, copper sulphate, and sodium metabisulphite (SMBS).

Detoxified tailings are pumped to the tailings thickener. Tailings thickener underflow is pumped to three filter feed tanks with combined capacity 8,600 m³ (approximately 12 h storage) before being fed to four pressure filters (three duty and one standby). The required filter cake moisture was specified to be <16.1% based on testwork results, to be transported using belt conveyors. Filter cake from each pressure filter is discharged onto tailings cake discharge feeder conveyors, combined, and conveyed to a filter cake transfer conveyor which discharges onto an overland conveyor for dry stacking of the tailings cake in the tailings disposal facility.

Future Provision: Ondundu ICR

Provision has been made in the design for gravity concentrate to be transported from the Ondundu site by truck to the plant goldroom area for processing in a dedicated ICR circuit. Pregnant solution will be transferred to a dedicated Ondundu ICR electrowinning feed tank at the goldroom.

Key reported parameters

Technical qualifications

This report presents a definitive feasibility study (DFS) design. No historical operating data from the Twin Hills Gold Project is reported. The processing design is based entirely on proposed flowsheet development and equipment selection. The report does not provide actual plant performance data or operating history.

The design and selection of the proposed process plant is based on testwork conducted during the DFS, including conveyability and materials flow testwork that specified the required filter cake moisture for belt conveyor transport. The trade-off study investigating tailings high pressure filtration versus vacuum belt filtration determined that high pressure filtration was the preferred option.

The report notes that future process provisions, including the Ondundu intensive cyanidation reactor circuit and arsenic precipitation, are preliminary designs only. The feasibility of these future additions is not established within the scope of this report.

Source: Twin Hills Definitive Feasibility Study National Instrument 43-101 Technical Report, June 2023, Section 17.0 Recovery Methods.