Hall Heroult cell

 

Aluminium metal is extracted from aluminium oxide, commonly known as alumina (Al2O3)

Alumina is extracted from bauxite which is dug out from the ground. Bauxite is an ore largely composed of gibbsite Al(OH)3, mixed with iron oxides and clay minerals.

 

Alumina melts at temperatures of 2,050 oC. Such temperatures are very expensive to maintain from a commercial point of view.

However, alumin readily dissolves, when placed in molten cryolite (Na3AlF6) at significantly lower temperatures of about 950 oC.

 

The electrolytic cell used to extract aluminium metal is represented by the diagram shown on the left.

Both the anodes and cathode are made of carbon conducting a current through the electrolyte, cryolite and alumina mixture, of about 150,000 A at 5 V.

The current passing through the electrolyte produces enough heat to keep it molten.

At the cathode aluminium ions are converted to aluminium metal. Click the blue writing for more detail.

While at the anodes, oxide ions are reduced to oxygen gas which quickly reacts with the carbon electrodes to produce carbon dioxide gas. Hence the anodes have to be replaced at regular intervals.

Many cells are connected in series in the production of commercial quantities of aluminium metal.

The overall reaction is given below.

2Al2O3(in cryolite) + 3C(s) => 4Al(l) + 3CO2(g)

1) Cheap electricity is essential for the commercial viability of an aluminium smelter.

a) Calculate the electric charge required to produce 0.500 kg of alluminium from alumina in Hall-Heroult cell.

b) How long must a current of 150,000 A flow in order to deposit 1.00 tonne(106g ) of aluminium?
c) What is the mass of carbon consumed at the anodes if a cell operates at 150,000 A for 2.00 hours?
d) How does an aluminium smelter contribute to global warming?
e) What is the mass of carbon dioxide produced per day by an aluminium smelter producing 1,000 tonnes of aluminium per day?
Solutions
Balance for charge by adding electrons tot he most positive side. Balance for hydrogen by adding Hydrogen ions to the hydrogen deficient side. Balance for oxygen by adding water to the oxygen deficient side. Balance for Ag atoms Balance for oxygen by adding water to the oxygen deficient side. Balance for hydrogen by adding Hydrogen ions to the hydrogen deficient side. Balance for charge by adding electrons tot he most positive side. Home
Each oxygen atom has an oxidation number of -2