AMC's September 2011 preliminary economic assessment for Ivanhoe Australia's Mount Dore chalcocite deposit crushes ore to 12 mm, agglomerates it with acid and bacteria, and relies on column tests that showed 80% copper recovery at a gangue acid consumption near 30 kg/t.
The Mount Dore Copper Heap Leach Project lies in the Cloncurry region of North West Queensland, Australia, and is held by Ivanhoe Australia Limited (IVA). AMC Consultants prepared the NI 43-101 Technical Report and Preliminary Economic Assessment with an effective date of 30 September 2011. The process plant would be designed to produce 18,700 t/y of copper cathode via an acid bacterial heap leach SX/EW process route. Ore from the open pit would go to a run-of-mine (ROM) pad for feeding to a crusher. IVA changed the base case plant from 5 Mt/y to 3 Mt/y after resource modelling reduced forecast mine ore tonnage, but the engineering development used in the study was still done for the 5 Mt/y, 25,000 t/y cathode plant. No further engineering was done for the 3 Mt/y, 18,700 t/y copper base case.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Base case plant | 3 | Mt/y | 18,700 t/y copper cathode |
| Engineering basis | 5 | Mt/y | 25,000 t/y cathode |
| Crushed product size | 12 | mm | Fixed plant |
| Copper recovery, 12 mm crush | About 80 | % | Fast leach limit |
| Copper recovery, 25 mm crush | About 50 | % | Economic limit |
| Gangue acid consumption | 30 | kg/t | At 80% recovery |
| Acid requirement | 90,000 | t/y | Trucked from Townsville |
| Leach pad capacity | 30 | Mt | Four combined pads |
| Water consumption | 115 | m3/h | 1,000 ML/y |
| Electrical load | 10.7 | MVA | Whole project |
| Column tests | 8 | columns | 6 m high |
Overview
The plant has a crushing circuit that sizes ore to 12 mm and conveys it to an agglomeration drum, where a bacterial culture and sulphuric acid are added. Overland conveyors carry the agglomerate to the heap leach pad, where a stacking system places it in layers. Process ponds store solution for distribution to the leach pads and SX plant. Solvent extraction (SX) extracts copper from pregnant leach solution, purifies and filters it, and delivers it to an electrowinning (EW) plant that plates copper onto stainless steel cathode mother plates.
Key Process Stages
Crushing plant
Fixed plant is proposed for all processes except the mobile conveying system on the leach pad. Mobile crushing was considered and discarded because of uncertain durability, the high tonnage needing two streams, and the expectation that dumping from mining trucks direct to the crusher bin would lower operating cost. The crusher would sit south-east of the first pit on a naturally elevated pad, on the first hill north-east of the existing core shed, in line with the top north edge of the two proposed leach pads along the quartz ridge-line. The agglomerator is in-line with it.
Heap leach pads
The four combined pads allow for 30 Mt of leach material, and some pads sit off the existing mining lease. A storm water pond would protect the environment from run-off in extreme wet season events, built so the heap itself cannot be flooded, with run-off from uncontaminated areas diverted around it. Geotechnical analysis is needed in later phases to confirm that a suitable pond site exists.
Solvent extraction
The SX plant is a proprietary Miller Metallurgy Services design, laid out in house by Calder. It is a conventional two extract and one strip circuit to treat large volumes of low-grade pregnant liquor, and a fourth mixer-settler is needed for scrubbing iron. Organic is removed from the electrolyte with dual media filters. Mixer-settlers and tankage are fibre reinforced plastic because chloride levels are expected to exceed 100 ppm, and pipework is HDPE with a little stainless steel around pumps and heat exchangers. Concrete bunding has 50 mm of extra cover on the reinforcing for corrosion resistance.
Electrowinning
The EW building is a simple portal frame steel design with open sides for natural ventilation and a full roof for shade and rain protection, which differs from the enclosed tank-houses common in Europe and North America. Cells are commercial polymer concrete, with Xstrata proprietary cathode mother plates and cross-rolled cadmium-tin lead anodes. Aluminium trunk with copper connections distributes current, and a containerised rectifier unit supplies direct current. Control runs on a Cytec operator interface and PLC, and copper sheets are washed, stripped, stacked, sampled, banded and weighed.
Additional Interesting Data and Summary
Column leach testwork was done by Hydromet Research Laboratories (HRL) in Brisbane under Miller Metallurgical Services supervision, using eight 6 m high columns. The dominant copper mineral is dispersed chalcocite, and all columns were inoculated with a bacterial culture. Only the feed solution was aerated, with the bottoms of the columns sealed, to simulate natural aeration in a full heap. Bacteria created an acid and ferric leaching environment that leached chalcocite, native copper, tenorite and cuprite.
Leaching has fast and slow stages. Fast leaching takes copper at the surface of agglomerate particles. Slow leaching, from within larger particles, begins around 80% recovery for a 12 mm crush and 50% for 25 mm, and the onset of slow leaching marks the economic limit, since the acid consumption it adds is unjustified. Chlorite and potassium-altered gangue are thought to drive acid consumption. Lower acid concentrations did not raise the economic limit, and a lower lift height might cut acid use, which needs further testing.
Samples for testing were chosen to match the likely ore mix of the first three years of operation. The third-year sample, from the main ore body, is expected to represent most of the ore to be leached, and a further sample from a possible northern extension with significantly higher zinc was added. The report found no indication that ore types leach differently, and the fast leach rate varied between samples without correlating to mineralogy or lithology.
Key Processes
- Fixed-plant crushing to 12 mm
- Agglomeration with sulphuric acid and bacterial culture
- Conveyor stacking on lined heap pads
- Acid bacterial heap leaching
- Solvent extraction with two extract and one strip stages
- Copper electrowinning to cathode
Source: NI 43-101 Technical Report, Mount Dore Copper Heap Leach Project Preliminary Economic Assessment, North West Queensland, Australia, September 30, 2011.
