Crocodile Gold's July 2013 technical report on the Burnside project describes the Union Reefs gold plant, with 2.5 Mtpa capacity, four Knelson concentrators, an Acacia reactor, a seven-tank CIL circuit and a tailings facility in the Crosscourse pit.
Burnside is a gold and base metal project in Australia's Northern Territory, and the processing described in the report is the Union Reefs gold plant near Pine Creek. Crocodile Gold Corp. is the company the technical report was prepared for, with an effective date of December 31, 2012 and a July 2013 date. The Union Reefs mine was commissioned in 1994 by Acacia Resources Ltd, and AngloGold Australia acquired it through its takeover of Acacia in December 1999. The carbon-in-leach (CIL) plant was designed by Kinhill with a 1.25 Mtpa throughput, then upgraded by JR Engineering in 1998 with a tertiary crusher, a second ball mill and two more leach tanks. The plant now has a maximum capacity of 2.5 Mtpa, depending on ore type.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Original plant capacity | 1.25 | Mtpa | Commissioned December 1994 |
| Current maximum capacity | 2.5 | Mtpa | Depends on ore type |
| Typical milling rate | 335 | tph | At P80 75 µm before 2003 care and maintenance |
| Plant availability | 96 to 98 | % | Typical before 2003 |
| Blasted ore size | Minus 1 | m | Rock breaker for larger rocks |
| Crushing stages | 3 | Not applicable | Jaw crusher, two cone crushers |
| Crusher product | 12 | mm nominal | To the fine ore bin |
| Fine ore bin | 2,500 | t live | 7 to 8 hours of buffer |
| Ball mills | 2 | Not applicable | ANI single stage, rubber lined |
| Knelson concentrators | 4 | Not applicable | Two per mill |
| Cyclone overflow | 75 to 106 | µm P80 | To a high rate thickener |
| Leach tanks | 2 | Not applicable | Cyanide and oxygen |
| CIL contactors | 7 | Not applicable | Counter-current carbon |
| Elution | 4 | t split AARL | Desorption |
| Mill water use | About 1,300 | ML/y | Mostly recycled |
| Power supplied | Up to 94 | million kWh | In full production years |
Overview
Union Reefs is a conventional gravity and CIL gold plant. Ore is crushed in three stages, ground in two single stage ball mills in closed circuit with cyclones, and partly treated by Knelson gravity concentrators. Cyclone overflow is thickened, leached in two cyanide tanks with oxygen and adsorbed onto carbon in seven CIL tanks. Loaded carbon is eluted, the gold is electrowon, and the gravity and CIL golds are smelted separately into doré.
Key Process Stages
Plant history
In August 2004, before they were acquired by GBS Australia, the Burnside joint venture partners bought the Union Reefs project for A$4 million on a walk-in, walk-out basis. The plant had been placed on care and maintenance in 2003. In August 2006 GBS Australia recommissioned the larger of the two mills, leaving the smaller ball mill in care and maintenance. The first feed was low grade stockpiles from Cosmo and an alluvial tailings deposit at Union Reefs, followed by a blend of oxidized and fresh ore from underground and open pit mines. Crocodile Gold announced commercial production in June 2010, after commissioning from December 2009.
Crushing
Open pit ore is blasted to minus 1 m, with a rock breaker handling larger rocks, and trucks carry run-of-mine ore to the ROM bin or stockpile. A primary jaw crusher works in open circuit and the secondary and tertiary cone crushers in closed circuit with a double deck banana screen, at up to 2.5 Mtpa. The nominal 12 mm product goes to the fine ore bin. Because crushing capacity exceeds milling capacity, extra crushed ore is stockpiled and fed back with a front-end loader through an emergency feed hopper.
Grinding and gravity recovery
Two ANI single stage rubber-lined ball mills run in closed circuit with a nest of Warman cyclones. A proportion of the ball mill discharge goes to the gravity circuit of four Knelson concentrators, two per mill. Their tailings return to the mill discharge, and the coarse gold goes to the gold room, where it is intensively leached in an Acacia reactor and electrowon onto steel wool.
Leach and CIL
Cyclone overflow gravitates to a single high rate thickener for density control and is pumped to the first of two leach tanks. Cyanide is added, and high purity oxygen comes from an on-site pressure swing absorption plant. The seven CIL contactors are gravity fed through open launders, and all tanks are agitated and aerated with hollow shaft mechanical agitators. Activated carbon is pumped counter-current to the slurry, so the highest gold loading is in CIL tank 1. Barren slurry goes to residue treatment and tailings are pumped to the Crosscourse pit tailings facility, with process water recycled back from the pit.
Elution, electrowinning and refining
Loaded carbon is screened to remove pulp and desorbed in a 4 t split AARL circuit. Gold is recovered from the caustic-cyanide solution by electrowinning, and carbon is reactivated in a vertical kiln. The electrowon and gravity gold are calcined in an electric oven and smelted separately in a gas-fired furnace into doré bars, which are dispatched by security service to AGR at Perth International Airport.
Additional Interesting Data and Summary
The plant is controlled by a CITECT process control system over an Allen Bradley PLC. Water comes from site dams, with more than 2,000 megalitres in the Crosscourse pit, 270 in Dam A and about 891 in Dam C, and a dam on the McKinlay River provides make-up water if required. The mill uses about 1,300 megalitres a year, most of it recycled from the tailings facility.
Power is supplied under contract by the Northern Territory Government's Power and Water Corporation. Consumables include diesel, lime, cyanide, sodium hydroxide, flocculant, antiscalants, carbon, grinding media, hydrochloric acid and liquid oxygen, supplied from Darwin, Perth and nearby areas. The laboratory handles sample preparation and gold-in-solution analysis, while solids and carbon samples go to an external laboratory. Staff live in an 85 person camp in Pine Creek.
Key Processes
- Three-stage crushing with a double deck banana screen
- Twin ball mill grinding in closed circuit with cyclones
- Knelson gravity concentration with Acacia intensive leaching
- Thickening and oxygen-assisted cyanide leaching
- Seven-tank carbon in leach
- Split AARL elution, electrowinning and doré smelting
- Tailings storage in a mined-out pit with water recycle
Source: Technical Report, Crocodile Gold Corp, Burnside Gold and Base Metal Project, July 2013.
