619 are generally below deleterious thresholds (Evolution Mining, 2023). Processing at Ernest Henry Operations involves crushing, grinding, and flotation. Mined ore is crushed underground before being conveyed and hoisted to the coarse ore stockpile (COS). The grinding circuit consists of an 11 MW SAG mill (10.4 m Ø × 5.1 m) and a 5.5 MW ball mill (6.1 m Ø × 8.5 m), operating in a closed AB configuration with a cyclone cluster comprising three 800 mm Ø and five 650 mm Ø CAVEX cyclones. SAG mill discharge is screened on a 3 m × 9 m vibratory screen with 4–5 mm apertures. Screen oversize is returned to the SAG mill feed via conveyor belts, while the undersize combines with the ball mill product and is pumped to the primary cyclone pack. Cyclone underflow gravitates to the ball mill, and overflow reports by gravity to the rougher flotation circuit. The flotation circuit consists of a rougher train of 9x127 m³ Wemco SmartCells arranged in five banks, followed by three stages of dilution cleaning. Concentrate from the first rougher bank is directed to the third cleaner stage, while concentrate from the remaining four banks reports to the regrind circuit. The regrind circuit includes a 1 MW Vertimill operating in closed circuit with six GMAX15 cyclones. Regrind cyclone overflow feeds the first cleaner stage. Cyclone underflow can be directed to an SK240 flash flotation cell via a feed box, with the cell tailings returning to the Vertimill. The SK240 concentrate can be routed either to the final concentrate sump or to the third cleaner stage. The cleaner circuit operates in a counter‑current configuration, with concentrate progressing from Cleaner Stage 1 to Stage 2 to Stage 3. Dilution water is added to Stages 2 and 3. Cleaner Stage 1 consists of eight OK50 cells, while Stages 2 and 3 utilise eight and five OK16 cells, respectively. Tailings from Cleaner Stage 1 and the rougher circuit are combined and directed to a 55 m Ø tailings thickener. Thickened tailings are pumped to the tailings storage facility (TSF). Final concentrate from Cleaner Stage 3 is thickened in a 25 m Ø concentrate thickener before filtration in a 144 m² Larox pressure filter. The filtered concentrate is transported by road train to Glencore’s Mount Isa Copper Smelter. The processing flowsheet of the Ernest Henry Operations concentrator is illustrated in Figure 1 below. Figure 1 - Process flowsheet of Ernest Henry Operations concentrator Historically, the flotation circuit has demonstrated strong metallurgical performance, achieving copper recoveries of 95% or higher and gold recoveries of 78–80%, at an average concentrate grade of 26.5% copper and 12 g/t gold. Pyrite is the predominant gangue mineral in the concentrate and is well liberated. Its preferential recovery relative to non‑sulphide gangue minerals is attributed to its close mineralogical association with chalcopyrite. 2. CONTEXTUAL BACKGROUND Surveys of the primary and regrind circuits showed that the recirculating load of gold minerals in the regrind circuit was approximately 1,750%, compared with around 400% in the primary circuit. Mineralogical analysis of selected process streams indicated that up to 90% of gold losses to the cleaner tailings were well liberated and predominantly ultrafine (<6 µm) (Coombs, 2025). Routine mineralogy also
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