10.1.4 Alternative methods of extracting metals
Copper ores are running out, so new ways to extract copper are needed
Definition
Metal ore
A rock containing enough of a metal compound for extracting the metal to be economically worthwhile.
Definition
Low-grade ore
An ore containing a small proportion of a metal or its compounds, so a large amount must be processed to obtain the metal.
- The Earth's supply of metal ores is finite, and the richest copper ores are being used up.
- What remains is often low-grade ore, which holds only a small proportion of copper.
- Traditional mining of low-grade ore means digging up, moving and disposing of huge amounts of rock.
- This uses a lot of energy and damages the landscape, so cleaner methods are being developed.
Key Idea
- Phytomining and bioleaching extract copper from low-grade ores that would once have been wasted.
- Both avoid the large-scale digging and rock disposal of traditional mining.
Phytomining: growing plants to harvest the metal
Definition
Phytomining
A biological extraction method that uses plants to absorb metal compounds before the plants are harvested and burned to produce ash containing those compounds.
- Plants are grown on soil or low-grade ore that contains copper compounds.
- As they grow, the plants absorb copper compounds and build them up in their tissues.
- The plants are harvested and burned, leaving an ash that is rich in copper compounds.
- The ash is then processed to obtain the copper.
Example
- Because the plants concentrate the copper, the ash contains a far higher proportion of copper than the original low-grade ore.
- Phytomining uses sunlight to drive plant growth, so it needs little added energy.
Bioleaching: using bacteria to dissolve the metal
Definition
Bioleaching
A biological extraction method that uses bacteria to produce soluble metal compounds that collect in a leachate solution.
- Certain bacteria grow on low-grade ore and get energy from the metal compounds in it.
- The bacteria produce a leachate solution that contains dissolved copper compounds.
- Bioleaching does not need the ore to be heated or dug up on a large scale.
- It is slow, but it can extract copper from ore that would otherwise be uneconomic.
Note
- Both methods produce copper compounds, not copper metal, so a further step is always needed to get the metal itself.
- Bioleaching is slow, while phytomining depends on how fast the plants grow.
Getting the copper out of the ash or leachate
Definition
Electrolysis
The decomposition of an ionic compound, when molten or in solution, by passing an electric current through it.
- The copper compounds from the ash or leachate are processed to obtain copper metal.
- Copper can be displaced from a solution of its compound using scrap iron, because iron is more reactive than copper: Fe+CuSO4→FeSO4+Cu\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}Fe+CuSO4→FeSO4+Cu.
- Copper can also be obtained by electrolysis of a solution of a copper compound.
- In electrolysis the copper ions gain electrons at the negative electrode and are deposited as copper metal.
Exam technique
- When you use displacement, say that iron is more reactive than copper, so it takes the place of copper in the compound.
- Give the balanced equation, for example Fe+CuSO4→FeSO4+Cu\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}Fe+CuSO4→FeSO4+Cu, to secure the marks.
Weighing up the biological methods
- The biological methods use less energy and move far less rock than traditional mining.
- They cause less damage to the landscape and let low-grade ores be used instead of wasted.
- Their main drawback is that they are slow, and phytomining depends on growing seasons.
- They may also produce smaller amounts of copper, so the best choice depends on the ore, the cost and the time available.
Self review
- Why are alternative methods of extracting copper being developed?
- Describe how phytomining extracts copper from low-grade ore.
- How does bioleaching produce a solution of copper compounds?
- Write the equation for displacing copper from copper sulfate solution using iron.
- Give one advantage and one disadvantage of the biological extraction methods.
