10.09.4 Alternative Methods of Extracting Metals (HT)

Syllabus

- The Earth’s resources of metal ores are limited.
- Copper ores are becoming scarce and new ways of extracting Copper from low-grade ores include phytomining, and bioleaching.
- These methods avoid traditional mining methods of digging, moving and disposing of large amounts of rock.
- Phytomining uses plants to absorb metal compounds. The plants are harvested and then burned to produce ash that contains metal compounds.
- Bioleaching uses bacteria to produce leachate solutions that contain metal compounds.
- The metal compounds can be processed to obtain the metal. For example, Copper can be obtained from solutions of Copper compounds by displacement using scrap Iron or by electrolysis.

Students should be able to evaluate alternative biological methods of metal extraction, given appropriate information.

What does this mean?

Traditional Mining

Only extremely unreactive metals are found in the ground as metals, usually as tiny flakes but very occasionally as nuggets.

Gold and Platinum are commonly found as metals, but even Copper can sometimes be found as nuggets.

Any metal more reactive than Copper will only be found as an Ore

Ores are rocks containing a useful amount of Metal Oxide or Metal Sulphide.

The higher the percentage of metal in the rock the more useful the ore will be for extracting metal from.

Any rock this green is likely to be a high grade copper ore - a rock with a significant amount of copper compound in it.

High grade ores are becoming rarer, the price of Copper keeps getting higher and the mines get bigger and bigger.

Digging a hole this time wastes a lot of fossil fuels and adds lots of CO2 to the atmosphere.

Then even more energy is needed to move and crush the rock, and when the metal is extracted we still have millions of tones of waste to move.

We are getting better at recycling metals but we'll probably always need fresh Copper too.

Meaning we'll have to exploit ever lower grades of ore.

And to do this without spoiling the environment in this way may need some new ways of extracting metal compounds from the ground.

Bioleaching.

It isn't worth mining very low grade ores.

But there are bacteria that absorb Copper, so we can mix these low grade ores with water containing these bacteria and then extract the Copper from the bacteria with acid to form a leachate.

Usually this is done by adding cheap scrap Iron to the leachate (solution containing Copper ions).

Iron is more reactive than Copper so a displacement reaction happens:

Copper ions in solution + Iron (metal) --> Copper (metal) + Iron ions in solution.

Cu2+(aq) + Fe(s) --> Fe3+(aq) + Cu(s)

This is one way to recover Copper from ground up circuit boards from scrap electronic equipment.

Phytomining.

Phytoming is similar but exploits plants that can absorb metal ions from low grade ores.

The plants can be burned and the ions dissolved in leachate and then extracted by electrolysis or with scrap Iron.

This is a way to clean up soil contaminate with Lead or other toxic heavy metals.

It can even mine for Gold in soils too low in the metal to consider using any other way.

Electrolysis.

If the extracted Copper ions are dissolved in Sulphuric Acid then a solution of Copper Sulphate is made.

We learned in Year 9, solutions of ionic compounds conduct electricity.

And we learned in Year 10 that passing a current through the solution causes Electrolysis

Electrolysis of Copper Sulphate, using a graphite Anode and a Copper Cathode will extract the Copper ions so that they build up on the Cathode.

Cu2+(aq) + 2e- --> Cu(S)

Oxygen will be produced at the Anode.

4OH-(aq) --> O2(g) + 2H2O + 4e-

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