Phytomining and bioleaching (HT)
Most easily-mined ores are running out. New extraction techniques use plants or bacteria to recover metals from low-grade ores or contaminated sites.
Phytomining
How it works:
- Plants are grown on soil/spoil heaps containing low concentrations of metal compounds.
- Plants absorb metal ions through their roots and concentrate them in their leaves and stems.
- The plants are harvested and burnt.
- The ash contains relatively concentrated metal compounds.
- Standard chemistry (e.g. acid leaching, electrolysis) extracts the metal.
Advantages:
- Uses very low-grade ores or polluted soils that would otherwise be uneconomic.
- Less environmental damage than digging open-cast mines.
- Plants can grow on otherwise-unproductive land.
- Carbon-neutral if energy comes from burning (CO₂ absorbed during growth).
Disadvantages:
- Slow (plants take a season to grow).
- Yield is small — many tonnes of plant material per kg of metal.
Bioleaching
How it works:
- Bacteria are spread on low-grade ore.
- The bacteria oxidise sulfide ores to soluble sulfates of the metal.
- The metal solution (leachate) is collected.
- The metal is extracted from solution by displacement (e.g. with iron) or electrolysis.
Common example: copper extraction from copper sulfide using Acidithiobacillus bacteria.
Advantages:
- Can use very low-grade ores.
- Low energy compared to traditional smelting.
- Lower toxic gas emissions (no SO₂ from roasting).
Disadvantages:
- Slow process.
- May produce toxic substances (acidic leachate) that can pollute waterways.
Comparison with traditional extraction
| Method | Speed | Energy | Environmental impact | Best for |
|---|---|---|---|---|
| Carbon reduction | Fast | High (heat) | High (CO₂, CO emissions) | Fe, Zn |
| Electrolysis | Slow | Very high | High electrical demand | Al |
| Phytomining | Very slow | Low | Low | Low-grade Cu, Ni, Au |
| Bioleaching | Slow | Low | Moderate | Low-grade Cu sulfides |
✦Worked example
Why might phytomining be used to extract gold?
- Gold ores are often very low grade.
- Plants can selectively absorb gold ions and concentrate them in tissues.
- Cheaper and less destructive than mining vast quantities of rock for tiny amounts.
⚠Common mistakes
- Confusing phytomining with bioleaching — phytomining uses plants; bioleaching uses bacteria.
- Saying these methods are fast — both are slow vs traditional smelting.
- Forgetting the burning step in phytomining — the metal isn't recovered until plants are burnt.
- Saying they're always cheaper — slow yields mean they're only economic on low-grade ores.
Links
Builds on C4.2 (carbon reduction), C4.11 (electrolysis). Connects to C10.1 (sustainability).
AI-generated · claude-opus-4-7 · v3-deep-chemistry