What you'll learn
The world's supplies of high-grade metal ores are running out, so scientists have developed new ways to extract metals from low-grade ores and waste. For AQA GCSE Chemistry you need to understand why alternative extraction methods are needed, how phytomining and bioleaching work, and their advantages and disadvantages compared with traditional mining. This guide covers the problem of dwindling ores, the two biological extraction methods in detail, how the metal is finally obtained, and how to evaluate these methods. By the end you should be able to describe phytomining and bioleaching, explain their benefits and drawbacks, and evaluate them against traditional extraction.
Key terms and definitions
Ore — A rock that contains enough of a metal or metal compound to make extraction worthwhile.
Finite resource — A resource that is being used up faster than it is replaced.
Low-grade ore — An ore that contains only a small percentage of the metal.
Phytomining — Using plants to absorb metal compounds from the soil, which are then extracted.
Bioleaching — Using bacteria to produce a solution (leachate) containing metal compounds.
Leachate — The metal-containing solution produced by bioleaching.
Electrolysis — Using electricity to break down a compound, used here to extract the metal from solution.
Displacement — A reaction in which a more reactive metal takes the place of a less reactive one in a compound.
Core concepts
Why alternative methods are needed
Metals are extracted from ores, which are a finite resource. The richest, high-grade ores — those containing a large percentage of metal — are being used up. This leaves only low-grade ores, which contain very little metal and are not worth extracting by traditional mining, because the energy and cost involved would be too great for so little metal. Traditional mining also scars the landscape, produces large amounts of waste rock, and uses a lot of energy. Alternative methods allow metals such as copper to be obtained from low-grade ores and even from waste, without conventional mining.
Phytomining
Phytomining uses plants to gather metal from the soil:
- Plants are grown on soil or low-grade ore that contains the metal (such as copper).
- As they grow, the plants absorb metal compounds through their roots and build them up in their tissues.
- The plants are harvested and then burned, producing ash.
- The ash contains a high concentration of the metal compound, from which the metal can be extracted.
Phytomining is slow, because it depends on how fast the plants grow, but it needs little energy and can be used on land that would otherwise be useless.
Bioleaching
Bioleaching uses bacteria to extract the metal:
- Certain bacteria live on the low-grade ore and get energy from it.
- As they do so, they produce a solution called a leachate, which contains metal compounds (such as copper compounds).
- The metal is then extracted from the leachate.
Bioleaching does not require high temperatures, but it can be slow and sometimes produces toxic substances that can damage the environment if not managed.
Getting the metal from the solution or ash
Once you have a solution or ash containing metal compounds, the metal is obtained in one of two ways:
- Displacement — A more reactive metal, such as scrap iron, is added to the solution. The more reactive metal displaces the less reactive metal (for example copper) from its compound, and the pure metal is produced.
- Electrolysis — Electricity is passed through the solution to extract the pure metal.
Both methods turn the metal compound into the pure metal that can be used.
Advantages and disadvantages
These methods have clear benefits and drawbacks compared with traditional mining:
Advantages:
- They allow metals to be extracted from low-grade ores that would otherwise be wasted.
- They cause less damage to the landscape than digging large mines.
- They usually use less energy than traditional extraction, and produce less waste rock.
- They conserve the finite supplies of high-grade ore.
Disadvantages:
- They are slow compared with traditional mining.
- Bioleaching can produce toxic substances that harm the environment.
- The amount of metal obtained can be small.
Because there are strong points on both sides, this topic is often set as an "evaluate" question.
Comparing with traditional extraction
Traditional extraction removes metals from high-grade ores by mining the ore, then reducing it — for example, iron is extracted in a blast furnace, and very reactive metals like aluminium are extracted by electrolysis. These methods are fast and produce large amounts of metal, but they require a lot of energy, produce large quantities of waste rock, and cause significant damage to the landscape and habitats. Phytomining and bioleaching are slower and produce less metal, but they use less energy, cause less damage, and can work on ores too poor for traditional mining. The choice between them is therefore a balance between speed and yield on one hand and environmental cost on the other.
Recycling as another option
It is worth remembering that recycling metals is often better than any form of extraction. Recycling a metal uses much less energy than extracting it from ore, conserves the finite ore supplies, and reduces waste. In an evaluation question about how best to obtain a metal, mentioning that recycling should be maximised alongside these new extraction methods shows a fuller understanding of sustainability.
Worked examples
Example 1: Why alternative methods are used
Explain why phytomining and bioleaching are becoming more important. The supply of high-grade copper ore is running out, leaving only low-grade ores with little metal. Phytomining and bioleaching can extract metal economically from these low-grade ores and from waste, without the damage and energy costs of traditional mining.
Example 2: Describing phytomining
Describe how copper can be obtained by phytomining. Plants are grown on soil containing copper compounds. The plants absorb the copper as they grow and store it in their tissues. The plants are harvested and burned, and the ash produced contains a high concentration of copper compounds, from which the copper is extracted.
Example 3: Extracting metal from leachate
State two ways the metal can be obtained from a leachate solution. The metal can be obtained by displacement, adding a more reactive metal such as scrap iron to displace the copper, or by electrolysis, passing electricity through the solution to extract the pure metal.
Example 4: Evaluating bioleaching
Give one advantage and one disadvantage of bioleaching compared with traditional mining. An advantage is that it extracts metal from low-grade ores with less damage to the landscape and less energy. A disadvantage is that it is slow and can produce toxic substances that harm the environment.
Common mistakes and how to avoid them
A common error is muddling phytomining and bioleaching. Remember: phytomining uses plants, which are burned to ash; bioleaching uses bacteria, which produce a leachate solution. Keep the two clearly separate.
Students often say the plants "produce" the metal. The plants absorb metal compounds from the soil; the metal is later extracted from the ash after burning. Be precise about the stage.
Another mistake is forgetting how the pure metal is finally obtained. The ash or leachate contains a metal compound, not the metal itself; displacement or electrolysis is still needed to get the pure metal.
When evaluating, do not give only advantages. A good answer gives both sides — for example, less damage and less energy, but slow and low yield — and reaches a conclusion.
Finally, remember why these methods matter: high-grade ores are finite and running out. Linking the methods to conserving finite resources earns marks.
Exam technique for "Alternative methods of metal extraction"
For description questions, give the steps in order — for phytomining: grow plants, absorb metal, harvest, burn, extract from ash; for bioleaching: bacteria act on the ore, produce a leachate, extract the metal.
These topics are frequently set as "evaluate" or "compare" questions, so structure your answer with advantages and disadvantages, then a conclusion. Standard advantages are less environmental damage, less energy and using low-grade ores; standard disadvantages are that they are slow and low-yield.
Always mention that the ash or leachate contains a metal compound, and that displacement (with scrap iron) or electrolysis is used to obtain the pure metal. Linking everything back to finite ore supplies shows the bigger picture and gains marks.
Quick revision summary
- High-grade ores are finite and running out, so metals are increasingly extracted from low-grade ores and waste.
- Phytomining: plants absorb metal compounds, are harvested and burned, and the metal is extracted from the ash.
- Bioleaching: bacteria act on the ore to produce a leachate solution containing metal compounds.
- The pure metal is obtained from the ash or leachate by displacement (using scrap iron) or electrolysis.
- Advantages: less landscape damage, less energy, uses low-grade ore, conserves finite resources.
- Disadvantages: slow, low yield, and bioleaching can produce toxic substances.