635 mineral surface and oxidizing species in solution (Watling, 2013). An example of Cu extraction from predominantly CuFeS2 ore is shown in Figure 1 (previously unpublished UBC data). In Figure 1, Cu extractions under standard sulfide bio-leach conditions do not exceed 32% after 30 months. Figure 1 – Cu extraction from sulfide ores in uncatalyzed 30 cm high laboratory columns with ore grades varying from 0.2 to 0.7% Cu and ores containing more than 75% Cu as CuFeS2. Results are for sulfuric acid bioleaching of agglomerated ore, conditions: pH ~ 1.8, T ≅ 20 ± 2°C To improve leach extents and kinetics, significant industrial and academic effort has been directed toward enabling the direct heap leaching of chalcopyrite-bearing ores since ca. 2000. If successful, such technologies could allow the production of cathode copper from low-grade sulfide ores using existing SX– EW infrastructure. 2. ELECTROCHEMICAL BASIS OF CHALCOPYRITE LEACHING Sulfide leaching in sulfate solutions is known to be an electrochemical process (Woodcock, 1961; Peters & Majima, 1968; Bailey & Peters, 1976). Chalcopyrite dissolution in acidic sulfate media is fundamentally an electrochemical process involving coupled anodic and cathodic reactions. The anodic oxidation reaction can be written as: CuFeS₂ → Cu²⁺ + Fe²⁺ + 2S⁰ + 4e⁻ The dominant cathodic reaction in ferric sulfate systems is ferric reduction: Fe³⁺ + e⁻ → Fe²⁺ Ferric ions are regenerated through oxidation of ferrous ions by oxygen: 4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O In bioleaching environments this ferric regeneration reaction is catalyzed by iron-oxidizing microorganisms such as Acidithiobacillus ferrooxidans.
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