Metals and Non-metals · Lesson 10 of 13
Extracting Metals From Ores
“Roasting, calcination, reduction and electrolysis form the extraction toolkit chosen by reactivity.”
• Explain extraction methods for metals low, middle and high in the activity series. • Distinguish roasting from calcination. • Explain reduction of metal oxides using carbon or a more reactive metal. • Explain the thermit reaction and why it produces molten metal. • Explain why highly reactive metals require electrolytic reduction.
Extraction is not one universal process. The reactivity of a metal determines how strongly it is held in compounds and therefore what method is needed to obtain the free metal.
Extracting Metals Low In The Activity Series
Low-reactivity metals are comparatively unreactive. Some can be obtained through heating steps. Cinnabar, HgS, is converted to mercury oxide in air, and further heating gives mercury.
Copper sulphide can also be processed by heating in air. Part forms copper(I) oxide, and copper(I) oxide then reacts with remaining sulphide to give copper.
Extracting Metals In The Middle Of The Activity Series
Metals such as zinc, iron, lead and copper are commonly found as sulphides or carbonates. These ores are first converted into oxides because oxides are easier to reduce.
Roasting
Roasting is the conversion of a sulphide ore into an oxide by strong heating in excess air.
Calcination
Calcination is the conversion of a carbonate ore into an oxide by strong heating in limited air.
| Process | Starting Ore | Air Condition | Main Result |
|---|---|---|---|
| Roasting | Sulphide | Excess air | Metal oxide + sulphur dioxide |
| Calcination | Carbonate | Limited air | Metal oxide + carbon dioxide |
Reduction Using Carbon
A suitable reducing agent removes oxygen from the metal oxide. Zinc oxide is reduced by carbon.
Reduction By More Reactive Metals
A highly reactive metal such as aluminium can remove a less reactive metal from its oxide. These displacement reactions can be strongly exothermic.
Thermit Reaction
The Thermit Reaction is the highly exothermic reaction in which aluminium reduces iron(III) oxide to molten iron.
The heat is sufficient to produce molten iron, so the reaction can be used to join railway tracks or repair cracked machine parts.
Extracting Metals Towards The Top Of The Activity Series
Highly reactive metals such as sodium, magnesium, calcium and aluminium cannot be obtained by reducing their oxides with carbon because they have greater affinity for oxygen than carbon does.
Electrolytic Reduction
Sodium, magnesium and calcium can be obtained by electrolysis of molten chlorides. Metal ions gain electrons at the cathode, while chloride ions lose electrons at the anode and form chlorine gas.
Low reactivity can allow simple heating in suitable cases. Middle reactivity commonly requires oxide formation and reduction. Very high reactivity requires electrolysis.
Quiz
What is roasting?
What is calcination?
Why is zinc oxide heated with carbon?
What is produced in the thermit reaction?
Why are highly reactive metals obtained by electrolysis?
Practice Problems
- Write the two-step equations showing how mercury is obtained from cinnabar.
- Differentiate roasting and calcination using zinc ores and balanced equations.
- Explain why sulphide and carbonate ores are converted to oxides before reduction.
- Explain reduction of zinc oxide with carbon in terms of oxygen removal.
- Write the thermit reaction and explain its use.
- Explain why sodium is obtained by electrolysis rather than reduction with carbon.
- State the cathode and anode changes during electrolysis of molten sodium chloride.
Key Takeaways
• Low-reactivity metals may be obtained through comparatively simple heating processes. • Middle-reactivity sulphide and carbonate ores are converted to oxides by roasting or calcination before reduction. • Carbon can reduce suitable metal oxides such as zinc oxide. • The thermit reaction uses aluminium to produce molten iron and releases a large amount of heat. • Highly reactive metals require electrolytic reduction because carbon cannot reduce their compounds effectively.