Extracting Metals from Low-Grade Ores

As the world's demand for metals continues to grow, high-grade ores — those containing a large percentage of the desired metal — are becoming scarce. Scientists have developed new methods to extract metals from low-grade ores (ores that contain only small amounts of the metal). This lesson covers phytomining and bioleaching as required by the AQA GCSE Chemistry specification.

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Why Do We Need New Extraction Methods?

Traditional methods of metal extraction (such as smelting with carbon or electrolysis) work well with high-grade ores. However:

Problem	Explanation
High-grade ores are running out	Centuries of mining have depleted the richest ore deposits
Low-grade ores are uneconomic to smelt	The energy cost of smelting a low-grade ore is too high relative to the small amount of metal obtained
Mining waste is a problem	Traditional mining produces huge amounts of waste rock that damages the landscape
Environmental damage	Large-scale mining causes habitat destruction, water pollution and air pollution

Scientists have developed two biological methods that can extract metals from low-grade ores and even from waste material left over from traditional mining:

graph TD
    A[Low-Grade Ore or Mine Waste] --> B[Phytomining]
    A --> C[Bioleaching]
    B --> D["Plants absorb metal ions<br>from the soil"]
    D --> E["Plants are harvested<br>and burned"]
    E --> F["Ash contains metal<br>compounds"]
    F --> G["Metal extracted from ash<br>by smelting or electrolysis"]
    C --> H["Bacteria produce<br>leachate solution"]
    H --> I["Leachate contains<br>dissolved metal ions"]
    I --> J["Metal extracted by<br>displacement or electrolysis"]
    style A fill:#fff9c4,stroke:#f57f17
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#bbdefb,stroke:#1565c0
    style G fill:#ffcc80,stroke:#e65100
    style J fill:#ffcc80,stroke:#e65100

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Phytomining

Phytomining uses plants to absorb metal compounds from the soil. The process works as follows:

Method

1. Plants are grown on land that contains low-grade ore or mine waste
2. The plants absorb metal ions from the soil through their roots
3. The metal ions accumulate in the plant tissues (especially the shoots and leaves)
4. The plants are harvested and burned (incinerated)
5. The ash produced contains a relatively high concentration of the metal compound
6. The metal is then extracted from the ash by smelting (heating with carbon) or electrolysis

Key Details

Feature	Detail
Type of plants used	Hyperaccumulator plants that naturally absorb high concentrations of metal ions
Metals that can be extracted	Copper, nickel, zinc, gold
Time required	Slow — plants take months or years to grow
Advantage	Uses contaminated land; does not require heavy machinery
Disadvantage	Very slow; large areas of land needed; limited to certain metals

Exam Tip: Phytomining literally means "mining with plants" (phyto = plant). The key word is accumulate — the plants concentrate the metal ions from a large area of low-grade ore into a small mass of plant material, which is then burned to concentrate the metal further.

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Bioleaching

Bioleaching uses bacteria to extract metals from low-grade ores. The bacteria produce chemical solutions that dissolve the metal compounds out of the rock.

Method

1. Low-grade ore is placed in a heap or dump
2. A solution containing specific bacteria (such as Acidithiobacillus ferrooxidans) is trickled over the ore
3. The bacteria feed on the metal compounds in the ore, producing acidic solutions called leachate
4. The leachate contains dissolved metal ions (e.g. copper ions, Cu2+)
5. The metal is then extracted from the leachate by:
   • Displacement using scrap iron (for copper: Fe + CuSO4 -> FeSO4 + Cu)
   • Electrolysis of the solution

Key Details

Feature	Detail
Organisms used	Specific species of bacteria (e.g. Acidithiobacillus)
How bacteria work	They oxidise metal sulfide compounds, producing soluble metal sulfates
Metals that can be extracted	Copper, gold, uranium
Time required	Slow (months), but faster than phytomining
Advantage	Low energy requirements; can use waste from old mines; less environmental damage than traditional mining
Disadvantage	Slow; produces toxic and acidic waste solutions that must be carefully managed

Exam Tip: In bioleaching, the bacteria do not directly eat the metal. They produce acidic conditions that dissolve the metal compounds out of the ore. The metal is then recovered from the solution by displacement or electrolysis. Be precise in your language.

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Extracting Copper from the Leachate

Once copper ions are in solution (from either phytomining or bioleaching), they can be extracted by two methods:

Displacement with Scrap Iron

Iron is more reactive than copper, so it can displace copper from copper sulfate solution:

iron + copper sulfate -> iron sulfate + copper

Fe + CuSO4 -> FeSO4 + Cu

The copper is deposited as a solid, and the iron dissolves into the solution.

Electrolysis

The copper sulfate solution is used as the electrolyte. When an electric current is passed through:

• Copper ions (Cu2+) are attracted to the cathode (negative electrode)
• They gain electrons and are deposited as pure copper metal: Cu2+ + 2e- -> Cu

Method	Advantages	Disadvantages
Displacement with iron	Cheap; simple; does not require electricity	Produces impure copper; uses up scrap iron
Electrolysis	Produces very pure copper (99.99%)	Requires electricity, which is expensive

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Comparing Traditional Mining with Phytomining and Bioleaching

Feature	Traditional Mining / Smelting	Phytomining	Bioleaching
Ore grade needed	High-grade	Low-grade	Low-grade
Speed	Fast	Very slow (years)	Slow (months)
Environmental impact	High (habitat destruction, pollution, energy use)	Low (uses contaminated land)	Low to moderate
Energy requirement	Very high	Low	Low
Cost	High for low-grade ores	Low running costs	Low running costs
Scale	Industrial, large-scale	Requires large land areas	Can be large or small scale
Products	Metal directly from ore	Metal from plant ash	Metal from leachate solution