Heap leaching combined with solvent extraction (SX) and electrowinning (EW) is one of the most economic routes for exploiting oxide and low-grade copper ores — ores for which flotation or smelting are not viable.
Mineralogy and feed preparation
Copper oxide minerals such as malachite, azurite and chrysocolla dissolve readily in sulphuric acid. After crushing, the ore is typically reduced to 12–25 mm and agglomerated in a drum with water and concentrated sulphuric acid (acid curing). Agglomeration preserves heap permeability and prevents fines migration.
Pad and heap design
- An impermeable base of compacted clay and an HDPE or LLDPE geomembrane of about 1.5–2 mm, with a leak-detection system.
- Stacking in lifts of several metres (typically 4–8 m) using conveyors and stackers.
- Drip irrigation at typically 8–12 L/h/m²; drippers reduce evaporation and water consumption compared with sprinklers.
- Heap and pad slope stability is verified by geotechnical studies including static and seismic analyses.
Pregnant leach solution (PLS), typically containing 1–5 g/L Cu, is collected at the toe of the heap and directed to the PLS pond.
Solvent extraction (SX)
In the extraction stage, copper is selectively loaded from the PLS onto an oxime-type organic extractant diluted in kerosene. The loaded organic is then contacted with acidic electrolyte in the stripping stage, transferring copper to the rich electrolyte. These steps take place in mixer-settlers. The raffinate, depleted in copper, is re-acidified and recycled to the heap.
Electrowinning (EW)
Rich electrolyte at about 45–50 g/L Cu enters the electrowinning cells. Copper deposits on permanent stainless-steel cathodes while oxygen is evolved at lead-alloy anodes.
- Typical current density: 250–350 A/m².
- Cell voltage: approximately 1.8–2.2 V.
- Specific energy: about 2 kWh per kg of copper.
- Product: 99.99% copper cathode meeting LME Grade A.
Design considerations
The key economic drivers are acid consumption (linked to carbonate gangue), overall copper recovery and leach cycle time. Laboratory and pilot column tests are essential before basic engineering. AriaHangard has designed this chain for projects including the 3,000 t/y Siastaragi heap leach plant and the Janja copper leaching plant, with a feed grade of about 0.22% Cu.