Extraction of aluminium (The Hall-Heroult Process)

Question 1

As mentioned, the process of electrorefining does not work with ____ metals such as aluminium. A similar process is used, however, the electrolyte is molten
aluminium oxide, as opposed to an aqueous solution.

Answer 1

reactive

Question 2

What is the electrolyte?

Answer 2

The electrolyte is molten Al2O3 which is known as alumina, which itself has been extracted from bauxite.

Question 3

It requires extremely high currents (approximately 40 000A a day) to melt the ____ making aluminium extraction one of the most expensive refining processes (and hence the importance of recycling Al)

Answer 3

alumina

Question 4

How is the boiling point of alumina lowered?

Answer 4

To lower the melting point of the alumina, and therefore decrease the cost of the process, a substance called cryolite (Na3AlF6) is used to dissolve the alumina.

Question 5

What happens to the Al3+ ions?

Answer 5

Al3+ ions are reduced at the cathode, producing molten Al which is tapped off at the bottom of the cell

Question 6

What happens to the O2- ions?

Answer 6

O2- ions are oxidised at the anode, producing O2 gas.

Question 7

Why do the anodes need regular replacing? (every 28 days)

Answer 7

O2 produced can react, at the high temperatures of the cell, with the carbon anodes used, resulting in a combustion reaction and the production of CO2
and the loss/disintegration of the anodes. Anodes need regular replacing which is an added expense of this process.

Question 8

Aluminium ores are: (3)

Answer 8

• light
• strong
• resistant to corrosion

Question 9

Al has a higher ___ than all the other elements.

Answer 9

density

Question 10

Why is the process necessary?

Answer 10

The process can damage the environment and can be expensive, but Al is valuable and thus worth it.

Question 11

What else can the carbon react with to form?

Answer 11

C + 2F2 -> CF2 (fluorocarbons, which are extremely toxic)

Question 12

What are the downsides of the cell? (3)

Answer 12

• Cell requires a considerable amount of energy
• Producing CO2 (g) - > greenhouse gas
• Mining bauxite, Australia -> is not good for the environment

Question 13

Explain briefly how the use of cryolite to decrease the melting point of the electrolyte is:

a. Economically beneficial

Answer 13

• Less energy (OR electricity) is used to melt the electrolyte.
• This saves money (cost is less).

Question 14

Explain briefly how the use of cryolite to decrease the melting point of the electrolyte is:

b. Environmentally harmful

Answer 14

• The cryolite is also electrolysed/decomposed.

* This produces perfluorocarbons (PFCs) which are greenhouse gases

Question 15

One of the reasons that aluminium is so difficult to electrolyse is that aluminium oxide has a very high melting point. With reference to the forces
holding its particle together, explain why aluminium oxide has such a high melting point. (4)

Answer 15

In order to melt aluminium oxide,
 Many
 Strong
 ionic (electrostatic) forces/bonds need to be broken/overcome,
 which requires much energy to separate the ions from the crystal
lattice.