PRIMARY & SECONDARY CRUSHING CIRCUIT (DESIGN CAPACITY 550TPH)
Ore from the pit is first processed through the Crushing Circuit which consists of a Primary Jaw Crusher capable of processing 550 tonnes per hour, followed by a Secondary Rolls Crusher and a cone crusher with multiple conveyor and screening circuits.
The crushing circuit includes a 6,600 tonne stockpile feeding to an underground tunnel feeder system and a secondary 20,000 tonne crushed ore stockpile with a reclaim feeder system, which will allow for continuation of supply of ore to the processing plant during shutdowns and maintenance periods.
The crushing process produces two products:
+ 40 mm clean native copper metal nuggets, and
– 40 mm crushed ore.
NATIVE COPPER CIRCUIT
HPGR (High Pressure Grinding Rolls)
In developing the flow-sheet, the HPGR was selected for final crushing as it best liberates the native copper nuggets and allows for their recovery as clean metal product.
It was found in the Phase 1 bulk test programme (5 tonnes), that the action of the HPGR seemed to “flex”, but not deform or flatten the copper nuggets, and allow the matrix rock to break away cleanly from the nugget. The success of this process was confirmed in the Phase 2 bulk test programme (35 tonnes).
Most importantly, the Phase 2 program demonstrated that the HPGR process could also effectively be used for the chalcocite and primary ore types as the tertiary comminution process. In fact, one HPGR supplier described the CuDeco application as the “perfect application of HPGR”, as the liberation of the native copper can be achieved and the properties of the other ores are also amenable to crushing/grinding by HPGR. The HPGR process has the added advantages of lower capital and operating cost than say a SAG mill, which is conventionally used in this application.
The gravity circuit is specifically designed for the recovery of native copper and will be incorporated with the HPGR process for the duration of processing of the native copper ore and the majority of the chalcocite ore.
The gravity circuit produces two products:
-40 mm, +1mm clean native copper nuggets, and
-1 mm fine native copper metal concentrate.
The gravity circuit consists of:
• Wet de-agglomerator and trommel screen for breaking up the compacted dry cake exiting the HPGR;
• Further screening from the trommel into two additional size fractions;
• Treatment of each size fraction through alljigs® to separate the native copper metal from the ore and gangue, and
• Treatment of the fines (-1 mm) through spirals, magnetic separator and wet shaking tables to separate the fine native copper metal as a concentrate.
The jigging process to recover the coarse (-40, +1mm) native copper nuggets was demonstrated on a continuous pilot alljig®. A decision was made to recover the fine native copper (-1mm) prior to the milling process, to ensure that native copper is not accumulated in the mill
Once the native copper ore has been depleted and the plant is treating principally primary ore, it is planned that the gravity circuit will be by-passed.
Following removal of the native copper from the ore through the gravity circuit, or when non-native copper ore by-passes this circuit, the ore continues to the gravity thickener and the thickened ore then feeds through to the ball mill.
Milling of ore to a size that is suitable for flotation is carried out in a conventional ball mill. The Rocklands ball mill is 5.8 M internal diameter and 8.3 M internal length and is manufactured in China by Citic one of the world’s largest manufacturer of ball mills. The mill is driven by a 4.8 MW motor and is lined with a combined rubber and steel lining.
The ball mill is one of three mills in the mineral processing plant and is the heart of the concentrate recovery operation, capable of grinding up to 4.5 million tonnes of ore per annum to a fraction size of about 125 microns, which is not far off the consistency of talcum powder, and is the specification required for efficient operation of the down-stream circuits that extract copper and cobalt/pyrite sulphide concentrates, and a magnetite (iron ore) product.
Foundations and support structure for the ball mill were constructed from approximately 1,000 cubic metres of concrete and took around 3 months to complete. When fully fitted and charged with approximately 300 tonnes of mill balls, the ball mill will weigh approximately 500 tonnes.
Flotation is a well understood and widely used process, and at Rocklands will separate two types of minerals in subsequent steps…namely copper, then cobalt/pyrite sulphide concentrates.
In the first stage of the flotation process the copper sulphides are recovered by flotation whilst the cobalt/pyrite is suppressed to prevent it being recovered in the copper flotation circuit. The copper concentrate collected in the roughing stage is then reground and passed through a cleaning flotation circuit where conditions are adjusted to promote copper mineral flotation and further suppress pyrite flotation.
Post copper recovery, the conditions of flotation are altered in subsequent stages to promote the cobalt/pyrite recovery.
During the first bulk trials in June 2010 significant quantities of magnetite was noted to be present in the sample during processing, particularly in the supergene/native copper ore zones. A magnetite recovery step was subsequently included in the design flow-sheet.
During the testing program, it was demonstrated that a magnetic rougher concentrate can be produced from the tailings stream, at high recovery. Rocklands magnetite specification is being developed to be sold as a product specifically designed for use in coal separation ‘washeries’.
NATIVE COPPER METAL CASTING
The Company has successfully commissioned a 600 tonne per month copper metal casting plant which will produce a high grade 99.7% copper ingot from the native copper metal products being produced in the crusher and gravity circuits.
The casting plant is supported by a dedicated 1MW power supply and it has been installed principally to remove the argument with potential purchasers of the final purity of copper grade when compared to that for natural native copper nuggets.