4.4 - Extracting Metals from Their Ores

Ores, native metals, and extraction as reduction

• Most metals are found combined with other elements as metal compounds in rocks, usually as oxides.
• An ore is a rock containing enough metal compound to make extracting the metal worthwhile:
  • Haematite (or hematite) - An iron oxide ore
  • Bauxite - An aluminium oxide ore

Native metals

• Unreactive metals, such as gold, do not readily react with other elements and are found as pure metal.
• Metals found like this are described as uncombined, or in their native form.

Extraction is reduction

• Extracting a metal from its oxide requires removing the oxygen.
• Losing oxygen is reduction, so extracting a metal from its oxide is a reduction reaction.

Reactivity and extraction

• A reactive metal forms stable compounds and holds its oxygen tightly, making it harder and more expensive to extract.
• A less reactive metal forms less stable compounds and releases its oxygen more easily.

Choosing an extraction method

The method used to extract a metal from its ore depends on where the metal sits relative to carbon in the reactivity series.

Below carbon - Reduction with carbon

A metal below carbon is extracted by reduction with carbon. The ore is heated with carbon, which removes the oxygen from the metal oxide:

iron(III) oxide + carbon → iron + carbon dioxide

2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g)

• The iron oxide loses oxygen, so it's reduced; the carbon gains oxygen, so it's oxidised.
• Zinc sits below carbon too and is extracted in the same way:
  • zinc oxide + carbon → zinc + carbon dioxide
  • 2ZnO(s) + C(s) → 2Zn(g) + CO₂(g)

This only works for metals below carbon because carbon can only displace less reactive metals from their oxides.

Above carbon - Electrolysis

• A metal above carbon must be extracted by electrolysis. The compound is melted and an electric current is passed through it, splitting it into its elements.
• Aluminium sits above carbon, so it must be extracted from aluminium oxide using electrolysis.
• Electrolysis is also used if a metal reacts with carbon to form a carbide instead of the pure metal.

Cost

• Electrolysis is much more expensive than reduction with carbon because:
  • Melting the compound requires large amounts of thermal energy
  • Passing the electric current uses large amounts of electrical energy
• Reduction with carbon is cheaper because carbon is cheap and acts as both fuel and reducing agent.

Worked example - Choosing an extraction method

A company has a sample of magnesium oxide and a sample of zinc oxide, and wants to obtain the metal from each. Using this extract from the reactivity series, decide which extraction method to use for each, and why.

Phytomining and bioleaching

• Traditional mining and smelting are expensive and damage the environment, especially when extracting copper from low-grade ores (ores containing only a small percentage of copper).
• Biological methods use living organisms to extract metals from low-grade ores.

Phytomining

Phytomining uses plants to absorb metal compounds:

1. Plants are grown on soil containing low-grade ore.
2. The plants absorb copper compounds through their roots and concentrate them in their leaves.
3. The plants are harvested, dried, and burned.
4. The resulting ash contains a high concentration of copper compounds.
5. An acid (such as sulfuric acid) is added to the ash to produce a copper-rich solution called a leachate.

Bioleaching

Bioleaching uses bacteria to extract metals:

1. Bacteria are mixed with low-grade ore.
2. The bacteria break down the ore, producing a solution of dissolved copper ions called a leachate.

Extracting copper from the leachate

The copper in the leachate is extracted using one of two methods:

1. Displacement using scrap iron - Iron is more reactive than copper, so it displaces copper ions from solution:
   • Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)
2. Electrolysis - Passing an electric current through the leachate deposits pure copper metal.