Biological methods of metal extraction - Higher

The Earth's supply of metal ores is limited. For example, high-grade copper ores, which contain a high percentage of copper, are becoming harder to find and mine. There are some alternative methods to extract metals from low-grade copper ores that use living organisms. These have advantages and disadvantages compared to the usual extraction methods.

Phytoextraction (phytomining)

Plants absorb mineral ions through their roots. Phytoextraction makes use of this:

  1. plants are grown in soil that contains low grade ore
  2. the plants absorb metal ions through their roots and concentrate these ions in their cells
  3. the plants are harvested and burnt
  4. the ash left behind contains metal compounds

Phytoextraction is slow but it:

  • reduces the need to obtain new ore by mining
  • conserves limited supplies of high-grade ores
  • reduces the amount of rock waste that must be disposed of after traditional mining

Worked example

Suggest reasons why phytoextraction reduces damage to the environment.

Mining for metal ores involves quarries which are large holes in the ground. These create noise, dust and traffic. They also destroy natural habitats. Phytoextraction reduces the need for mining and reduces this damage.

Question

Give one disadvantage of phytoextraction.

It is a slow process.

Bioleaching

Certain bacteria can break down low-grade ores to produce an acidic solution containing copper ions. The solution is called a leachate and the process is called bioleaching.

Bioleaching does not need high temperatures but it:

  • produces toxic substances, including sulfuric acid, which damage the environment

Processing the metal compounds

Iron is more reactive than copper. It can displace copper from the leachate. For example:

iron + copper sulfate → iron(II) sulfate + copper

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Since iron is cheaper than copper, the use of scrap iron is a cost-effective way to produce copper from the leachate.

Alternatively, the copper compounds can be dissolved and the solution electrolysed to produce copper metal.

Move on to Test
next