The Union Reefs mill is a carbon-in-leach mill with a capacity of 2.5 million tonnes per year or 8,000 tonnes per day. It was designed by Kinhill and constructed by Acacia Resources and AngloGold.

The Union Reefs Gravity and Carbon-in-Pulp plant is designed to recover gold from predominantly free-milling ores sourced from the Pine Creek, Brocks Creek, and Cosmo areas. The plant is capable of recovering coarse gold by gravity concentration, and fine gold by cyanide leaching and adsorption on activated carbon.

The Union Reefs mill was commissioned in 1994 with a throughput capacity of 1.25 million tonnes per year. It included a gravity circuit to extract coarse gold. JR Engineering carried out an upgrade in 1998 that involved the installation of a tertiary crusher, second ball mill, and two additional leach tanks. The plant is currently configured with three-stage crushing and two single-stage milling circuits.

The mill consists of:

Three stage crushing circuit
Two closed circuit ball mills, each with a hydrocyclone
Gravity circuit including two Knelson concentrators
Thickening circuit
Two stage CIP circuit

Crushing

Run of Mine ore is reclaimed from stockpiles by a wheel loader and tipped into a ROM bin. A vibrating feeder transports ore from the ROM bin to a C140 Nordberg Single Toggle Jaw Crusher. Any oversize rock in the ore is broken at the Jaw Crusher by a hydraulic rock breaker. Jaw Crusher product is transported to a double deck "banana" 3.1 metre by 7 metre Nordberg screen with a 40 millimetre aperture top deck and a 14 millimetre aperture bottom deck by conveyor belts. Tramp steel in the Union Reefs ore is removed after the jaw crusher by a fixed magnet to protect the secondary cone crusher and conveyor belts. Screen undersize (minus 14 millimetre) product is conveyed to a Fine Ore Bin/Stockpile of 3000 tonnes live capacity, while screen oversize (plus 40 millimetre product) is transported to a Nordberg Omni-Cone 1560 secondary cone crusher. Secondary crusher product returns to the screen. Intermediate product of minus 40 millimetre plus 14 millimetre from the screen is transported to a Nordberg HP500SX tertiary cone crusher. Tertiary crusher product is also returned to the screen.

Grinding

Fine crushed ore is reclaimed from under the fine ore bin by a slot belt feeder. Stockpiled fine ore from outside the bin is reclaimed using a wheel loader which tips to a day feeder bin. Both the slot feeder and day feeder discharge to a single mill feed conveyor. The mill feed conveyor sits under a 100 tonne capacity Quicklime silo, which automatically discharges quicklime directly to the mill feed conveyor to control pH to approximately 10 for the CIP leaching process. The mill feed conveyor discharges to a Mill Feed hopper. The Mill Feed hopper has a split discharge onto two variable speed ball mill feeder conveyors, one feeding the 3 megawatt 4.7 metre by 8.2 metre ANI No.1 Ball Mill, the other feeding the 4 megawatt 5 metre by 9.1 metre ANI No.2 Ball Mill. Each mill is able to be operated independently, and each is in closed circuit with separate hydrocyclone clusters. The final product from the grinding circuit is 80% passing 75 microns particle size, which reports to a 1.5 metre by 3.6 metre Nordberg Trash Screen. Screen oversize trash reports to tailings disposal, while decontaminated leach feed reports to the 15 metre diameter Superflo Leach Feed Thickener.

Gravity

Each Ball Mill hydrocyclone cluster is fitted with a direct offtake on the hydrocyclone feed pot, which directs a bleed stream of ball mill discharge to a Nordberg 1.2 metre by 1.5 metre scalping screen to remove coarse scats. Fine screen product reports to one of two automatic discharge 30 inch Knelson concentrators, which recover up to 35% of total gold produced depending upon ore source. Rough Knelson concentrate containing coarse gold is automatically discharged to a secured hopper located within the goldroom.

On a batch basis, the rough Knelson concentrates are transferred from the secured hopper to an Acacia reactor vessel. The concentrates are deslimed using water before caustic cyanide leach solution containing a proprietary "Leach Aid" is recirculated through the concentrate bed to rapidly dissolve the contained coarse gold. At the completion of the leach cycle, the concentrate residue solids are washed and discharged back to the grinding circuit. The high grade pregnant leach liquor containing the gold is then recirculated through an electrowinning cell, with the gold won onto stainless steel mesh cathodes. The high-quality won gold is then direct smelted to saleable bullion product.

Carbon-in-Pulp

The thickened CIP Feed is pumped to a two stage leach where cyanide solution is added and oxygen gas is sparged into the slurry via a Pressure Swing Adsorption oxygen generation plant located on site. The partially leached slurry flows through to a seven stage carbon adsorption circuit. Activated carbon is maintained at a concentration of 15 grams per litre in the slurry, and carbon is moved counter-current to the slurry flow using batch movement. Carbon is retained within each tank by mechanically wiped 1 millimetre cylindrical wedge-wire screens. CIP tailing from the last tank is passed over an 800 micron carbon safety screen before being thickened in a 15 metre diameter Superflo thickener and pumped to the Crosscourse tailings pit for final deposition.

Elution/Goldroom

Loaded activated carbon is recovered on a batch basis from the No.1 adsorption tank once a gold-on-carbon loading target of approximately 2000 g/t is achieved. The loaded carbon is transferred to a Nordberg 1.2 metre by 1.8 metre screen to remove slurry, and the rinsed carbon is placed in a four tonne capacity elution column. The carbon is acid washed to remove inorganic contaminants. Elution of the carbon is then conducted using a split AARL soak-wash method at 110°C. Pregnant eluate solution reports to an electrowinning cell in the goldroom where the gold is won to stainless steel wool cathodes. The won gold is washed from the cathodes, dried, then smelted to bullion in a gas-fired furnace.

Barren carbon is transferred from the elution column to a storage hopper, before being dewatered and dried and regenerated in a Combustion Air gas-fired vertical kiln. Regenerated carbon is quenched and transferred to a Nordberg 1.2 metre by 1.8 metre fine carbon removal screen, then returned to the No.7 adsorption tank for gold adsorption.