Macraes Gold Recovery: Flotation, POX & CIL Process

Source: OceanaGold Corporation (2026)
Website: https://oceanagold.com/operations/macraes

Critical Data

Overview

The Macraes processing facility, operated by OceanaGold Corporation in Otago, New Zealand, has been recovering gold from refractory sulfide ore since 1991. Located on the South Island, the plant treats 6.4 million tonnes per annum (Mtpa) from the Macraes Open Pit and Underground operations. The metallurgical flowsheet is specifically designed to handle partially refractory ore containing preg-robbing carbonaceous material, which can adsorb dissolved gold and reduce recovery if not managed correctly. The process begins with two-stage jaw crushing feeding two SAG mills and a ball mill circuit to achieve a primary grind P80 of 120–140 µm. Flash flotation and a main flotation circuit recover up to 88% of the gold into a concentrate with sulfur content above 8.5%. This concentrate is reground to P80 15 µm, then oxidized in a 77 m³ autoclave at 225°C and 3,150 kPa to achieve over 96% sulfide oxidation. The oxidized residue is washed, neutralized, and leached via carbon-in-leach (CIL) with cyanide, while activated carbon adsorbs the gold to mitigate preg-robbing. Gold is finally recovered through AARL elution and electrowinning, producing gold doré. The plant’s significance lies in its ability to consistently achieve an average overall gold recovery of 82.6%, despite declining head grades and variable ore mineralogy. With continuous upgrades including pebble crushing and advanced process control systems, Macraes remains a benchmark for refractory gold processing in Australasia.

Key Process Stages

• Stage 1: Crushing and Grinding – Ore is reduced in two single-stage jaw crushers to a top size of ~200 mm, then fed to two SAG mills (2,300 kW and 1,500 kW) and two ball mills (2,300 kW and 2,500 kW). A pebble crusher on the SAG trommel recycle stream reduces competent material. The primary grind target is P80 120–140 µm (typically 130 µm).
• Stage 2: Flash Flotation and Main Flotation – Cyclone underflows feed two Outotec SK-500 flash flotation roughers and two TC-6 cleaner cells to recover fast-floating gold-rich sulfides. The main flotation circuit (roughers, scavengers, cleaners, recleaners) produces a concentrate with >8.5% sulfur. Up to 88% of gold reports to concentrate.
• Stage 3: Concentrate Regrind and Pressure Oxidation – Flotation concentrate is reground in a 900 kW ball mill to P80 15 µm. Limestone is added for acid control. The slurry is oxidized in a 77 m³ autoclave at 3,150 kPa and 225°C, achieving >96% sulfide oxidation (typically 98%). Oxygen is supplied by a BOC cryogenic plant.
• Stage 4: Oxidation Residue Washing, Neutralization, and CIL – The hot acidic autoclave discharge is cooled and washed in a two-stage counter-current decantation thickener circuit. Quicklime neutralizes acidity, and solvent is added to passivate carbonaceous surfaces. Gold is leached in a CIL circuit using cyanide, with high concentrations of activated carbon to mitigate preg-robbing.
• Stage 5: Gold Recovery from Carbon and Cyanide Destruction – Loaded carbon undergoes AARL elution and single-pass electrowinning. The resulting sludge is smelted to gold doré. Cyanide in CIL tailings is destroyed using the INCO process (sodium metabisulfite and copper sulfate). Tailings are further neutralized with flotation tailings and lime before deposition.

Additional Interesting Data and Summary

The Macraes process plant incorporates a sophisticated combination of flotation, pressure oxidation (POX), and carbon-in-leach (CIL) technologies to maximize gold extraction from refractory ore. Key consumables include SIBX collector (0.188 kg/t milled), cyanide (2.128 kg/t milled), and quicklime (0.222 kg/t milled) for neutralization. The autoclave operates at 225°C and 3,150 kPa, with a target sulfur oxidation of 98% to optimize gold recovery while minimizing activation of preg-robbing carbon. Oxygen supply from BOC Gas New Zealand ensures consistent oxidation kinetics. Environmental management is integral: cyanide destruction via the INCO process uses sodium metabisulfite (3.918 kg/t CIL feed) and copper sulfate (0.046 kg/t milled) as catalyst. Tailings are neutralized with flotation tailings and lime before deposition in the Frasers Tailings Storage Facility (FTSF), which was commissioned in Q1 2025 and has a permitted crest height of 480 m RL, with an extension to 510 m RL under consent to support operations beyond Life of Mine (to ~2036). The plant’s unit cost in 2026 is budgeted at NZD 15.37/t milled, reflecting robust cost estimation based on first-principles models and long-term maintenance schedules. Historical mill utilization exceeds 92.9%, and with the 2023 pebble crusher addition, throughput rates have increased to 780–800 tpoh, enabling the plant to consistently meet or exceed the 6.4 Mtpa target. The process control system (Yokogawa CentumVP DCS) integrates advanced analyzers such as FLS LoadIQ mill sensors, FrothSense cameras, Courier 5i on-stream analyzer, and Gekko carbon scout to optimize flotation and CIL performance. From 2016 to 2023, overall gold recovery averaged 82.6% against a budget of 82.3%, demonstrating operational excellence. Looking forward, the Life of Mine plan projects milling rates of 6.4 Mtpa through 2029, after which feed declines as open pit and underground operations wind down. The processing facility’s flexibility in handling variable ore types, low-grade feed (0.57–0.91 g/t Au), and preg-robbing material underscores its significance as a world-class refractory gold processing operation.

Key Processes: Flotation, CIP/CIL, Cyanidation, Gravity Separation, SAG Mill, Ball Mill, Crushing

Target Commodities: Gold, Silver, Copper