Alternative methods of extracting metals (HT only): Phytomining uses plants to absorb metal compounds. The plants are harvested and then burned to produce ash that contains metal compounds.

The idea

Phytomining is a biological method (Higher Tier only) of extracting copper from low-grade ores. Plants are grown on soil or waste rock containing small amounts of copper compounds. The plants absorb copper compounds through their roots and store them in their tissues. When harvested, they are burned. The resulting ash is richer in copper compounds than the original low-grade ore, providing a more concentrated source. Copper can then be obtained from the ash, for example by reacting the ash with an acid to make a solution and displacing copper with scrap iron, or by electrolysis. Phytomining avoids digging, moving and disposing of large amounts of rock, but it is slow and requires land, energy for burning and further processing.

What earns the marks

• Plants are grown on low-grade ore or contaminated soil and absorb metal compounds through their roots.
• The absorbed metal compounds are stored in the plant tissues, concentrating them from the dilute surroundings.
• The plants are harvested, typically by cutting or collecting the grown biomass.
• The harvested plants are burned to produce ash.
• The ash contains metal compounds, making it a more concentrated source of copper than the original low-grade ore.
• Copper is then extracted from the ash by further chemical processing, such as displacement or electrolysis.
• Phytomining avoids digging, moving and disposing of large amounts of rock, but is slow and requires land and energy.

Common mistakes and how to put them right

• Saying plants absorb copper metal rather than copper compounds.

  Put it right: Plants absorb copper compounds, and the ash contains copper compounds.

• Thinking the plants themselves are the final copper source.

  Put it right: The plants are burned to produce ash, which contains the metal compounds.

• Believing phytomining produces pure copper directly.

  Put it right: Further processing of the ash is needed to obtain copper.

• Confusing phytomining with bioleaching.

  Put it right: Phytomining uses plants and ash, while bioleaching uses bacteria and produces a leachate.

Exam tips

• Sequence the process clearly: grow plants, absorb metal compounds, harvest, burn, collect ash, extract copper.
• Use the phrase 'metal compounds' rather than 'metal' when describing absorption and ash, because the copper is combined with other elements.
• Explain why ash is useful: it concentrates the copper compounds, making extraction more practical than treating the original low-grade ore.
• Phytomining is assessed at Higher Tier only, so Foundation students do not need to learn this specific process.