Biological methods make low-grade ores worth extracting
Low-grade ore
An ore that contains only a small percentage of metal, so extracting the metal by traditional methods is expensive.
- The richest copper ores have already been mined, so many remaining ores contain less than 1% copper.
- Smelting a low-grade ore means digging up, crushing and heating a huge mass of rock for a small mass of metal.
- This uses a great deal of energy and produces large amounts of waste rock and polluting gases such as sulfur dioxide.
- Biological methods use living organisms to concentrate the metal, so the ore does not need to be heated in a furnace.
- Mine waste and contaminated land that were once worthless can also be treated this way.
Biological methods trade speed for lower energy use, so they suit resources that are too poor to smelt.
Bioleaching uses bacteria to dissolve metal compounds
Bioleaching
A method of metal extraction in which bacteria convert metal compounds in an ore into a solution of metal ions.
Leachate
The solution of metal ions that drains out of the ore during bioleaching.
- Low-grade ore is piled in heaps and sprayed with dilute sulfuric acid, and bacteria in the heap get their energy by breaking down metal sulfides.
- This releases metal ions, which dissolve and drain from the bottom of the heap as the leachate.
- Copper is recovered from the leachate by displacement with scrap iron:
Fe+CuSO4→FeSO4+Cu\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}Fe+CuSO4→FeSO4+Cu
- Iron is more reactive than copper, so it displaces copper from its salt solution.
- Copper can also be recovered from the leachate by electrolysis, which gives purer copper.
Bioleaching is used at copper mines in Chile, where huge heaps of low-grade ore are leached for months or years.
Phytoextraction uses plants to absorb metal ions from soil
Phytoextraction
A method of metal extraction in which plants absorb metal ions from soil and the metal is then recovered from the ash of the burned plants.
- Certain plants are grown on low-grade ore, mine waste or contaminated soil.
- Their roots absorb metal ions from the soil, and the plants store them in their stems and leaves.
- The plants are harvested and burned, leaving an ash that is rich in metal compounds.
- Burning the plants also releases energy that can be used to generate electricity.
- The metal is obtained from the ash by dissolving it in acid, then using displacement or electrolysis.
- Nickel has been extracted this way from plants such as alyssum, which store unusually large amounts of nickel.
Do not say that the plants make the metal, because they only take up metal ions that are already in the soil.
Biological methods save energy but produce metal slowly
- Both methods need far less energy than smelting, because the ore is never heated in a furnace.
- They release less carbon dioxide and sulfur dioxide, and less waste rock has to be dug up and dumped.
- Phytoextraction can clean up land polluted by old mines while it collects the metal.
- Both methods are very slow, because bioleaching can take months or years and plants take a whole growing season.
- Bioleaching produces an acidic leachate containing toxic metal ions that must not escape into rivers or groundwater.
- Phytoextraction needs large areas of land and gives only a small mass of metal from each harvest.
When a question asks you to evaluate bioleaching or phytoextraction, compare it with smelting on energy, speed and pollution, then give a conclusion that depends on how much metal the ore contains.
- Why are biological methods used for low-grade ores?
- How do bacteria help to extract copper in bioleaching?
- How is copper recovered from the leachate?
- What happens to the plants in phytoextraction after they have absorbed metal ions?
- Give one advantage and one disadvantage of bioleaching compared with smelting.
