Mining of Minerals and Methods of Extracting of Metals 

3. The extraction of aluminium and sodium by electrolysis

Scroll down for revision notes on extraction procedures and theory

Useful for school/college assignments/projects on ways of extracting metals from their ores

1. Introduction to Metal Extraction * 2. Extraction of Iron and Steel Making

3. Extraction of Aluminium and Sodium * 4. Extraction and Purification of Copper

5. Extraction of Zinc, Titanium and Chromium * 6. Economic & environmental Issues - metal extraction

GCSE Multiple choice Quizzes on metal extraction: Foundation (easier) or Higher (harder) and word-fill

Revision Notes KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

Other associated KS4 Science GCSE/IGCSE chemistry web pages on this site

• Reactivity of metals : all the reactions of metals with oxygen (air), water and acids, displacement reactions.

• Oxidation-reduction explained (redox theory)

.

• Group 1 Alkali Metals: chemical and physical properties

• Extra Industrial chemistry : properties and uses of metals including titanium and aluminium

• Transition metals : properties and uses

• Extra Electrochemistry-electrolysis : electrolysis theory, experiment designs and links to all electrolysis section on this site.

• Aluminium is obtained from mining the mineral bauxite.
• The purified bauxite ore of aluminium oxide is continuously fed in. Cryolite is added to lower the melting point and dissolve the ore.
• Ions must be free to move to the electrode connections called the cathode (-, negative), attracting positive ions e.g. Al³⁺, and the anode (+, positive) which attracts negative ions e.g. O^2-.
• When the d.c. current is passed through aluminium forms at the negative cathode (metal*) and sinks to the bottom of the tank.
• At the positive anode, oxygen gas is formed (non-metal*). This is quite a problem. At the high temperature of the electrolysis cell it burns and oxidises away the carbon electrodes to form toxic carbon monoxide or carbon dioxide. So the electrode is regularly replaced and the waste gases dealt with! 
• It is a costly process (6x more than Fe!) due to the large quantities of expensive electrical energy needed for the process.
• * Two general rules:
  • Metals and hydrogen (from positive ions), form at the negative cathode electrode.
  • Non-metals (from negative ions), form at the positive anode electrode.

Raw materials for the electrolysis process:

• Bauxite ore of impure aluminium oxide [Al₂O₃ made up of Al³⁺ and O^2- ions]

• Carbon (graphite) for the electrodes.

• Cryolite reduces the melting point of the ore and saves energy, because the ions must be free to move to carry the current

• Electrolysis means using d.c. electrical energy to bring about chemical changes e.g. decomposition of a compound to form metal deposits or release gases. The electrical energy splits the compound!

• At the electrolyte connections called the anode electrode (+, attracts - ions) and the cathode electrode (-, attracts + ions). An electrolyte is a conducting melt or solution of freely moving ions which carry the charge of the electric current.

• At the negative  (-) cathode, reduction occurs (electron gain) when the positive aluminium ions are attracted to it. They gain three electrons to change to neutral Al atoms.
  • Al³⁺ + 3e^- ==> Al
• At the positive (+) anode, oxidation takes place (electron loss) when the negative oxide ions are attracted to it. They lose two electrons forming neutral oxygen molecules.
  • 2O^2- ==> O₂ + 4e^- 
  • or 2O^2- - 4e^- ==> O₂ 
• Note: Reduction and Oxidation always go together!
• The overall electrolytic decomposition is ...
  • aluminium oxide => aluminium + oxygen
  • 2Al₂O₃ ==> 4Al + 3O₂
  • and is a very endothermic process, lots of electrical energy input!

• GENERAL NOTE ON ELECTROLYSIS:
• Any molten or dissolved material in which the liquid contains free moving ions is called the electrolyte.
• Ions are charged particles e.g. Na⁺ sodium ion, or Cl^- chloride ion, and their movement or flow constitutes an electric current, because a current is moving charged particles.
• What does the complete electrical circuit consist of?
  • There are two ion currents in the electrolyte flowing in opposite directions:
    • positive cations e.g. Al³⁺ attracted to the negative cathode electrode,
    • and negative anions e.g. O^2- attracted to the positive anode electrode,
    • BUT remember no electrons flow in the electrolyte, only in the graphite or metal wiring!
  • The circuit of 'charge flow' is completed by the electrons moving around the external circuit e.g. copper wire or graphite electrode, from the positive to the negative electrode
  • This e^- flow from +ve to -ve electrode perhaps doesn't make sense until you look at the electrode reactions, electrons released at the +ve anode move round the external circuit to produce the electron rich negative cathode electrode.
• Electron balancing: In the above process it takes the removal of four electrons from two oxide ions to form one oxygen molecule and the gain of three electrons by each aluminium ion to form one aluminium atom.
  • Therefore for every 12 electrons you get 3 oxygen molecules and 4 aluminium atoms formed.
  • This means you can do mole ratio product calculations. See section 13. on the Chemical Calculations page.

3b. The electrolytic extraction of the very reactive metal sodium

In the molten salt the positive sodium ions migrate to the negative cathode electrode and are reduced by electron gain to form liquid sodium atoms.

At the (-) cathode: Na⁺ + e^- ==> Na

Equally mobile in the molten chloride salt are the negative chloride ions, which migrate to the positive anode electrode and get oxidised by electron loss to form green chlorine gas molecules. Initially two chlorine atoms are formed and these rapidly combine to give chlorine molecules.

At the (+) anode overall: 2Cl^- ==> Cl₂ + 2e^-

or 2Cl^- ==> 2Cl + 2e^- and then 2Cl ==> Cl₂

Other very recative metals like lithium, potassium and calcium can be extracted in the same way by electrolysing their molten salts.

Some general notes on electrolysis

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