5.3 Bioleaching and Biomining

Bioleaching and biomining harness microorganisms to extract metals from ores and mine waste. These processes offer eco-friendly alternatives to traditional mining, using bacteria and archaea to dissolve metals in acidic environments.

The key players in bioleaching are iron and sulfur-oxidizing microbes like Acidithiobacillus and Leptospirillum. They work through direct and indirect mechanisms, breaking down metal sulfides and generating oxidizing agents for metal extraction.

Bioleaching and Biomining Fundamentals

Bioleaching and biomining applications

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• Bioleaching extracts metals from ores using microorganisms primarily for low-grade ores (copper sulfides)
• Biomining encompasses bioleaching and biooxidation processes including metal recovery from mine waste and tailings (gold from refractory ores)
• Copper extraction from sulfide ores accelerates natural weathering processes
• Gold recovery from refractory ores breaks down mineral matrices to expose gold particles
• Uranium extraction from low-grade deposits solubilizes uranium minerals
• Remediation of acid mine drainage removes dissolved metals and raises pH

Microorganisms in bioleaching processes

• Acidithiobacillus species thrive in acidic environments and oxidize iron and sulfur compounds
  • A. ferrooxidans oxidizes both iron and sulfur critical for metal sulfide dissolution
  • A. thiooxidans specializes in sulfur oxidation producing sulfuric acid
• Leptospirillum species focus on iron oxidation in bioleaching environments
  • L. ferrooxidans efficiently oxidizes ferrous iron to ferric iron
  • L. ferriphilum tolerates higher temperatures extending bioleaching to warmer conditions
• Sulfobacillus moderately thermophilic iron and sulfur oxidizer operates at elevated temperatures
• Ferroplasma archaeal iron oxidizer thrives in extremely acidic conditions (pH < 2)
• Acidianus hyperthermophilic archaeon enables bioleaching at high temperatures (> 60°C)

Mechanisms and Practical Considerations

Mechanisms of metal extraction

• Direct mechanism involves microbes attaching to mineral surfaces enzymatically oxidizing metal sulfides
• Indirect mechanism microbes generate oxidizing agents (Fe³⁺) chemically oxidizing metal sulfides
• Thiosulfate pathway oxidizes acid-insoluble metal sulfides (pyrite, molybdenite) forming thiosulfate intermediates
• Polysulfide pathway oxidizes acid-soluble metal sulfides (sphalerite, chalcopyrite) forming polysulfides and elemental sulfur
• Iron oxidation regenerates Fe³⁺ as oxidizing agent: $4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O$
• Sulfur oxidation converts reduced sulfur compounds to sulfuric acid: $S⁰ + 1.5O₂ + H₂O → H₂SO₄$

Bioleaching vs traditional mining

• Advantages: lower energy consumption reduces greenhouse gas emissions processes low-grade ores lowers capital and operational costs minimizes landscape disturbance enables in situ leaching
• Challenges: slower extraction rates compared to conventional methods sensitive to environmental conditions (pH, temperature) potential for acid mine drainage limited effectiveness on certain ore types requires large land areas for heap leaching operations needs careful microbial community management
• Environmental considerations: reduces air pollution compared to smelting potential for groundwater contamination if not properly managed
• Economic factors: viability depends on metal prices and ore grades potential for recovering metals from mine wastes and tailings