Earth and atmosphere, Year 11: phytomining and life cycle assessment

Phytomining

HintThe name begins with the Greek for 'plant'.

The whyThe plants are grown on land that contains a low-grade ore. They take up metal compounds through their roots and concentrate them. The metal is recovered afterwards from what is left when the plants are burned.

Bacteria

HintThey are single-celled, and far too small to see.

The whyThe solution they produce is called a leachate. It contains the metal as dissolved compounds, so the metal still has to be obtained from it, by displacement or by electrolysis.

To produce ash that contains the metal compounds

HintBurning removes most of the plant and leaves the useful part behind in a small residue.

The whyThe metal compounds are spread thinly through the living plant. Burning gets rid of the plant material and leaves them concentrated, ready to be processed to obtain the metal.

Fe + CuSO4 → FeSO4 + Cu

HintIron is the more reactive metal, so it takes copper's place in the compound.

The whyIron displaces copper because it is more reactive, and scrap iron is cheap. The other way to obtain copper from the solution is electrolysis, in which copper forms at the negative electrode.

They can extract copper from low-grade ores

HintThink about rock that holds too little metal to be worth digging up and smelting.

The whyThese methods also avoid digging up, moving and disposing of large amounts of rock, so they do less damage to the landscape. Their main drawback is that they are slow.

A life cycle assessment (LCA)

HintIts three-word name describes following the product from 'birth' to 'death'.

The whyIt lets two products that do the same job be compared stage by stage: for example how much energy and water each uses and how much waste each produces.

raw materials; disposal

HintThe first is what the product is made from; the last is what happens when it is thrown away.

The whyTransport and distribution are counted at every stage. Looking at all four stops a product from seeming 'green' just because one stage, such as its use, causes little harm.

value

HintThe missing word also means what something is worth.

The whyThe use of water, energy and resources, and some wastes, can be measured fairly easily. Deciding how much a pollutant's effects count is not a measurement, so two fair assessors can reach different totals.

By using a selective LCA that leaves out stages to reach the conclusion it wants

HintThink about what happens if only the flattering parts of the story are told.

The whyA shortened LCA can be useful, but it can also be designed so that a product looks better than a rival. A claim based on an LCA should say which stages and which impacts were included.

Paper rots quickly (it is biodegradable); plastic persists in landfill or as litter

HintThink about what happens to each material if it is left in the ground for a year.

The whyThe raw materials differ too: paper comes from trees, which are renewable, and plastic from crude oil, which is finite. Plastic bags are lighter and are often reused more. An LCA sets these stages side by side, and neither bag is better at all of them.