Crocodile Gold's July 2013 technical report on the Pine Creek Gold Project in Australia's Northern Territory says any ore will go to the Union Reefs plant, 20 km away, and details its crushing, gravity, CIL, elution and refining steps and its historical recoveries.
The Pine Creek Gold Project covers about 13.80 km2 of mineral tenements in the Northern Territory of Australia and belongs to Crocodile Gold Corp., a TSX-listed company. The report on its mineral resources and reserves has an effective date of December 31, 2012 and is dated July 2013. It has no plant of its own. Any gold from the Pine Creek deposits will be processed at the Union Reefs plant, which is about 20 km away. Union Reefs was commissioned in 1994 by Acacia Resources with a throughput capacity of 1.25 Mt/y and was later upgraded to a maximum of 2.5 Mt/y. Crocodile Gold announced commercial production at the plant in June 2010, after commissioning from December 2009.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Original plant capacity | 1.25 | Mt/y | Commissioned December 1994 |
| Current maximum capacity | 2.5 | Mt/y | Depends on ore type |
| Typical milling rate before 2003 | 335 | t/h | At 75 µm P80 |
| Plant availability | 96 to 98 | % | Before care and maintenance in 2003 |
| Crushed product | 12 | mm | Nominal |
| Fine ore bin | 2,500 | t | 7 to 8 hours of buffer |
| Cyclone overflow | 75 to 106 | µm P80 | To thickener |
| Knelson concentrators | 4 | units | Two per mill |
| Leach tanks | 2 | tanks | Followed by seven CIL contactors |
| Elution vessel | 4 | t | Split AARL |
| Water use | About 1,300 | ML/y | Mostly recycled from tailings |
| Historical gold production, 1994 to 2003 | 953,294 | oz | About 21 Mt at 1.5 g/t |
| Crocodile Gold production since 2009 | 207,700 | oz | 4.7 Mt at 1.4 g/t |
| Annual recovery, 1995 to 2003 | 92.3 to 97.2 | % | Total 93.49% |
Overview
The Union Reefs plant is a conventional carbon-in-leach (CIL) gold plant with a gravity circuit for coarse gold. Ore is crushed in three stages, ground in two ball mills in closed circuit with cyclones, thickened, and leached with cyanide and oxygen. Seven CIL contactors recover the gold on carbon, which is stripped, electrowon and reactivated. Gravity gold and electrowon gold are smelted into doré.
Key Process Stages
Crushing
Open pit ore is blasted to minus 1 m, with a rock breaker for oversize, and trucked to the run-of-mine feed bin or stockpile. Crushing runs at up to 2.5 Mt/y with a primary jaw crusher in open circuit and secondary and tertiary cone crushers in closed circuit with a double deck banana screen. The 12 mm product goes by conveyor to a fine ore bin with 2,500 t of live capacity. Because the crushing circuit is larger than the milling circuit, extra crushed ore is stockpiled and returned through an emergency feed hopper by front-end loader.
Grinding and gravity
Two ANI single-stage rubber-lined ball mills run in closed circuit with a nest of Warman cyclones. A share of ball mill discharge goes to four Knelson concentrators, two per mill. Knelson tails return to the mill discharge, and concentrated coarse gold goes to the gold room. Cyclone overflow gravitates to a single high-rate thickener for density control and is pumped to the first leach tank.
Leaching and CIL
Cyanide is added to the two leach tanks, and high-purity oxygen comes from a pressure swing adsorption plant on site. Slurry flows to the CIL circuit of seven leach and adsorption contactors, fed through open launders. All tanks are agitated and aerated with hollow shaft agitators. Carbon moves counter-current to slurry, and gold loading is highest in tank 1. Barren slurry leaves the last tank to residue treatment, and tailings go to the Crosscourse pit tailings facility, with process water recycled back from the pit.
Elution and refining
Loaded carbon is screened to remove pulp and stripped in a split AARL elution vessel of 4 t capacity, giving an auriferous caustic-cyanide solution that goes to electrowinning. Stripped carbon is reactivated in a vertical kiln and returned to CIL. Gravity gold is leached intensively in an Acacia reactor, and the solution is electrowon onto steel wool. Electrowon and gravity gold are calcined in an electric oven and smelted separately in a gas-fired furnace. Bars are stamped and sent by security service to AGR at Perth International Airport. A Citect system on an Allen Bradley PLC controls the plant.
Additional Interesting Data and Summary
An upgrade in 1998 by JR Engineering added a tertiary crusher, a second ball mill and two more leach tanks. The plant was placed on care and maintenance in 2003. In August 2004 the Burnside JV partners bought the Union Reefs project for A$4 million, and in August 2006 GBS Australia recommissioned the larger of the two mills. The first feed was low grade Cosmo stockpiles and alluvial tailings from the Union Reefs site, and later ore came from oxidized and fresh underground and open pit mines.
Water comes from dams placed to capture run-off and from the Crosscourse pit, which holds more than 2,000 ML, plus Dam A at 270 ML and Dam C at about 891 ML. The mill uses about 1,300 ML a year, mostly recycled from tailings. Power is supplied under contract by the Northern Territory's Power and Water Corporation, and up to 94 million kWh has been supplied in a year. Consumables include diesel, lime, cyanide, sodium hydroxide, flocculant, antiscalants, carbon, grinding media, hydrochloric acid and liquid oxygen. The laboratory handles sample preparation and gold in solution only. Staff live in an 85-person camp in the town of Pine Creek.
Historical grades fell from 1.82 g/t in 1995 to 1.17 g/t in 2003, while recovery was 93.49% overall across 19.7 Mt of ore.
Key Processes
- Three-stage crushing with a double deck screen
- Two-mill ball grinding with cyclone classification
- Knelson gravity concentration and Acacia intensive leaching
- Oxygen-assisted cyanide leaching
- Seven-tank carbon-in-leach adsorption
- Split AARL elution, electrowinning and kiln reactivation
- Smelting to doré
Source: Report on the Mineral Resources & Mineral Reserves of the Pine Creek Gold Project in the Northern Territory, Australia, July 2013.
