🌿 Phytomining
1. Plants are grown on land that contains copper compounds.
2. The plants absorb the metal compounds.
3. The plants are harvested (cut down and collected).
4. The plants are burned. This produces ash that contains the metal compounds.
Earth's Resources and Water » Alternative Methods of Extracting Metals
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.10.1.4 (Higher tier only)
- Why we need new ways to extract copper
- How phytomining and bioleaching work
- How copper is made from the solutions
- How to evaluate these methods
Higher tier only
This whole lesson is Higher tier content. Foundation tier students do not need it.
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Unlock This CourseWhy do we need new methods?
Copper ores are running low, so new methods are needed to get copper from low-grade ores like the rock in this mine
The Earth's resources of metal ores are limited. Once we have mined the best ores, there are fewer left. Copper ores are becoming scarce. Much of the copper that is left is in low-grade ores. These are rocks that contain only a small amount of copper compared with the amount of rock.
Traditional mining means digging up huge amounts of rock, moving it and then disposing of it. With a low-grade ore, almost all of that rock is waste. So scientists have developed new ways to get copper from low-grade ores. Two of them are phytomining and bioleaching. Both avoid the digging, moving and disposing of large amounts of rock.
Key terms:
- Low-grade ore: a rock that contains only a small amount of the metal compound.
- Phytomining: using plants to absorb metal compounds.
- Bioleaching: using bacteria to produce solutions that contain metal compounds.
- Leachate: the solution made in bioleaching, which contains metal compounds.
Two biological methods
⚗ Bioleaching
1. Bacteria are used on the low-grade ore.
2. The bacteria produce a leachate solution.
3. The leachate contains the metal compounds.
4. The solution is collected and the metal is obtained from it.
In both methods the copper is still in the form of a compound. It has not yet been turned into the metal. That is the next step.
Getting the metal from the compounds
Scrap iron is more reactive than copper, so it pushes copper out of solution in a displacement reaction
The metal compounds can be processed to obtain the metal. For copper, it can be obtained from solutions of copper compounds in two ways.
⚒ Displacement with scrap iron
Iron is more reactive than copper. So when scrap iron is added to a solution of a copper compound, iron takes the place of copper. The copper is displaced and is left as solid metal. The iron is used up as it dissolves.
⚡ Electrolysis
An electric current is passed through the copper solution. Copper forms at the negative electrode (the cathode). It needs electricity, which costs money.
You met both ideas earlier. Look back at the lessons on the reactivity series and on electrolysis of solutions if you need a reminder.
Comparing with traditional mining
Traditional mining is the method where rock is dug out of the ground, moved and then processed. Phytomining and bioleaching avoid this. Here is how to judge them.
✔ Advantages
They can get copper from low-grade ores that are not worth mining in the usual way.
There is no digging, moving and disposing of large amounts of rock.
They help to make our limited supply of copper ores last longer.
✖ Disadvantages
They can be slow. Plants must grow before they can be harvested, and bacteria take time to work.
Phytomining needs land to grow the plants on.
Burning the plants and making the metal from the solutions still needs energy.
Worked example
Imagine a company with a low-grade copper ore, where digging would mean moving 1,000 tonnes of rock to get 5 tonnes of copper (made-up figures for practice). Bioleaching would produce a solution from the ore without the digging. Evaluation: bioleaching is better because it avoids moving rock that is almost all waste. The company must accept that it will be slower, and it would still need energy to get the copper from the solution. If the company needs copper quickly, it may choose another method.
How to evaluate
The exam will give you information, such as a table or a short passage, and ask you to evaluate. Follow these steps.
- Pick out the facts you are given: time taken, amount of copper, cost, land needed, waste made.
- Give at least one advantage and one disadvantage of the method.
- Use the data. Say which number or fact supports your point.
- Finish with a decision, and say why.
Common mistakes
Saying that bacteria or plants are the copper. They are only used to collect copper compounds. Another mistake is forgetting that phytomining and bioleaching make a compound, not the metal, so a second step is needed. Do not say that burning plants makes pure copper: it makes ash that contains copper compounds.
Exam-style question
Copper ores are becoming scarce. A company plans to use phytomining to get copper from land with a low-grade ore. Describe how phytomining works, and explain one advantage and one disadvantage of using it instead of traditional mining. (4 marks)
Model answer
Plants absorb copper compounds. The plants are harvested and burned to make ash that contains the copper compounds. Copper can then be obtained from the compounds, for example by displacement using scrap iron or by electrolysis. Advantage: it avoids digging, moving and disposing of large amounts of rock, and it can use low-grade ore. Disadvantage: it is slow because the plants have to grow before they can be harvested.
Exam tip
When asked to evaluate, always use the information in the question and end with a clear decision.