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

Phytomining

提示The name begins with the Greek for 'plant'.

为什么The 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

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

为什么The 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

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

为什么The 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

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

为什么Iron 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

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

为什么These 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)

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

为什么It 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

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

为什么Transport 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

提示The missing word also means what something is worth.

为什么The 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

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

为什么A 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

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

为什么The 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.