c5 aluminium electrolysis

ALUMINIUMALUMINIUM

Extraction and usesExtraction and uses

BACKGROUND

Aluminium is the most common metal in the Earth’s crust.It comprises approximately 7.5% of the crust by mass.

Aluminium is very reactive and is found in a range of oxide,hydroxide and silicate minerals.

THE REACTIVITY SERIES

Aluminium is higher than carbon in the reactivity series socannot be extracted by carbon reduction, unlike less reactive metals such as zinc, iron and lead.

For those metals above carbon, electrolysis is used.

ELECTROLYSIS

Electricity Lyse/lysismeaning “to move”

For electrolysis we need a system we can pass a current through.

What is current?

A strict definition would say that current is a flow of electricalcharge through a conductor. The important point for electrolysisis that this charge may be as electrons or as charged ions.

What systems can undergo electrolysis?

1. Aqueous solut ions of ions

Aluminium compounds found in ores that are veryinsoluble as we shall see later. This means thatdissolving isn’t an option.

2. Melt ing the ore to produce a molten l iquid

Molten liquids can produce ions that may beseparated. This is used for aluminium.

The process of aluminium electrolysis is not new:

The present industrial method of production was discovered simultaneously and independently in 1886 by Paul-Louis Héroult in France and Charles Hall of the United States. Their method is the basis for the world aluminium industry today

Before we look at the ores, we should look at the main features of processing plants …………..

LYNEMOUTH, just north of Newcastle, is one of two primaryAluminium production plants in the UK. There is another onAnglesey in N. Wales.

Can you identify the three main features marked?

Electrolysis plant androlling mills

Coal-fired powerstation dedicatedto the plant

Waste residues, largelyiron oxides

There are a number of secondary aluminium plants in the UKwhere waste aluminium is separated, smelted and recovered.This plant is near Fort William in Scotland.

Again, can you see the two main features – not waste this time!

Smelting andelectrolysis plantwith rolling mills

Hydroelectric powerstation

Water from reservoir belowBen Nevis

Aluminium Ore

BAUXITE

Bauxite is not a single composition, more a range of hydrated aluminium oxides and hydroxides.

It is found as a weatheringproduct in soils in hot sub-tropical and tropicalclimates.

Main resources are in N. Africa,S. America, SE Asia and N.Australia

Typical mining operations for bauxite

The chemistry of bauxite processing

The complex mixture that makes up bauxite is first processedto produce pure aluminium oxide, alumina (Al2O3)(For GCSE you do not need to know how, however thisIs a requirement for AS/A2)

So we need molten alumina to do electrolysis ……….

..however, this would require a temperature over 2072oC,the melting point of alumina. This is far too high for aneconomic process.

Alumina is melted in another chemical, cryolite (Na3AlF6),which lowers the melting point to around 1000oC.

The electrolysis process for aluminium

Draw this, noting the bubbles. We will discuss the chemistry of theselater. Note the tap hole for molten aluminium. This is a CONTINUOUS process

The energy to keep the alumina and cryolite molten comesfrom a 100000A current flowing through the 6V electrolysis cell.The energy to provide the initial melting is enormous so theserun continuously for many years before finally being taken out of service.

Keeping the electrolysis cell molten

Aluminium Electrolysis Chemistry

The negative cathode, around the edges of the cell, attracts the positively charged aluminium ions. These gain electrons andaluminium which fall to the base of the tank as molten metal……

Al3+(l) + 3e- => Al(l)

The positively charged anode, as blocks in the surface of the cell,Attracts the negatively charged oxide ions…..

2O2-(l) => O2(g) + 4e-

Oxygen is not evolved though. This process is at 1000oC with a carbonanode. This anode burns away producing carbon dioxide. This is the gas

given off from the process. The carbon anodes need regular replacement to keep the cells operational.

Watch the TV/Video

Link your knowledge of the processto what it looks like in real life!

ANODISING ALUMINIUM

Aluminium is very unreactive for a metal so high up thereactivity series. This is due to the ease with which it forms a protective oxide layer. Anodising makes this oxide layer evenstronger.

Step 1. The original oxide layer is removed with sodiumhydroxide.

Step 2. The aluminium is made into the anode in dilutesulphuric acid. Any metal can make the cathode but usually this is made of aluminium as well.

Step 3. Oxygen, evolved at the anode, reacts with the aluminium to form a 0.02mm oxide film. Much thicker thanbefore anodising.

Step 4. At this stage, the oxide is porous and dyes can be added.Further treatment makes the oxide solid and resistant.

Examples of anodised aluminium.

Benefits of anodised aluminium

1. Increased corrosion resistance.2. Dyed products available

Uses of aluminium

Aluminium is low density, strong, a good conductor of heat andgood conductor of electricity.

1. Electricity cables along overhead pylons.

2. Aircraft industry

3. Vehicles – particularly where weight is an issue eg. Trams,trains etc.

4. Cooking pans – this includes the appearance of the metal as a factor in choice, to complement the low density, strength and heat conducting aspects.

Examples of anodised aluminium are now also seen in many applications.