Low-grade ores make conventional extraction uneconomic
Definition
Ore
A rock that contains enough of a metal compound to make extracting the metal worthwhile.
- A low-grade ore holds only a small percentage of the metal compound.
- Mining and smelting such an ore uses large amounts of energy for very little metal.
- Traditional mining also scars the landscape and leaves large volumes of waste rock.
- Rich ores are being used up, so low-grade deposits are an increasing share of what remains.
- Two biological methods, bioleaching and phytoextraction, offer ways of working these deposits.
Key Idea
Both biological methods concentrate the metal compound first, and the metal itself is recovered afterwards by a separate chemical step.
Bioleaching uses bacteria to produce a leachate
Definition
Bioleaching
A method of extracting metals in which bacteria produce a solution of metal compounds from a low-grade ore.
- Certain bacteria feed on the sulfur in low-grade ores and break the ore down as they grow.
- The liquid that drains away, the leachate, holds dissolved compounds of the metal.
- For copper the leachate contains copper sulfate in solution, not copper metal.
- Bioleaching needs no heat and no smelting, so it uses far less energy than mining and roasting.
- The bacteria work slowly, so producing a useful quantity of metal can take months or years.
Common Mistake
The leachate holds a compound of the metal, so a further reaction is always needed to obtain the element.
Phytoextraction uses plants to concentrate the compound
Definition
Phytoextraction
A method of extracting metals in which plants absorb metal compounds from the soil and are then burned to leave a metal-rich ash.
- Plants are grown on soil or waste that holds a low concentration of metal compounds.
- Their roots absorb the compounds, which build up in the leaves and stems as the plants grow.
- The plants are harvested and burned, leaving an ash that is rich in the metal compound.
- That ash is a far more concentrated source than the ground the plants grew on.
- Phytoextraction can also clean up contaminated land while the metal is being collected.
Note
- Growing and harvesting a crop takes a full season at a time, so the method is slow.
- Burning the plants releases carbon dioxide, which is a cost set against the energy saved.
The metal is recovered from the leachate or the ash
- Both methods deliver a metal compound, so a reduction is still required.
- Adding scrap iron to a copper leachate displaces copper, because iron is the more reactive metal.
- The displacement that recovers the copper is: Fe+CuSO4→FeSO4+Cu\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}Fe+CuSO4→FeSO4+Cu
- Electrolysis of the solution is the alternative, and it gives copper of higher purity.
- The same two routes are used on the solution made by dissolving the ash from phytoextraction.
Example
- Scrap iron: cheap, quick, and gives copper that still needs purifying.
- Electrolysis: gives very pure copper but uses a large amount of electrical energy.
Weighing the biological methods against traditional mining
- Both methods use less energy and cause less damage to the landscape than opening a mine.
- Both work ores that would otherwise be left as waste, which extends the supply of the metal.
- Both are much slower, which is the main reason they have not replaced conventional extraction.
- Phytoextraction takes up land that could be used for growing food.
- Whether a method is worth using depends on the price of the metal and how little of it the ore holds.
Self review
- What is a low-grade ore?
- What do the bacteria in bioleaching produce, and what does it contain?
- Why are the plants used in phytoextraction burned?
- Why does adding scrap iron to a copper leachate release copper?
- Give one advantage and one disadvantage of these methods compared with mining.
