ALUMINIUM Extraction 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 so cannot be extracted by carbon reduction, unlike less reactive metals such as zinc, iron and lead. For those metals above carbon, electrolysis is used.

ELECTROLYSIS ElectricityLyse/lysis meaning “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 electrical charge through a conductor. The important point for electrolysis is that this charge may be as electrons or as charged ions.

What systems can undergo electrolysis? 1.Aqueous solutions of ions Aluminium compounds found in ores that are very insoluble as we shall see later. This means that dissolving isn’t an option. 2. Melting the ore to produce a molten liquid Molten liquids can produce ions that may be separated. 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 primary Aluminium production plants in the UK. There is another on Anglesey in N. Wales. Can you identify the three main features marked? Electrolysis plant and rolling mills Coal-fired power station dedicated to the plant Waste residues, largely iron oxides

There are a number of secondary aluminium plants in the UK where 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 and electrolysis plant with rolling mills Hydroelectric power station Water from reservoir below Ben Nevis

Aluminium Ore BAUXITE Bauxite is not a single composition, more a range of hydrated aluminium oxides and hydroxides. It is found as a weathering product in soils in hot sub-tropical and tropical climates. 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 processed to produce pure aluminium oxide, alumina (Al 2 O 3 ) (For GCSE you do not need to know how, however this Is a requirement for AS/A2) So we need molten alumina to do electrolysis ………...however, this would require a temperature over 2072 o C, the melting point of alumina. This is far too high for an economic process. Alumina is melted in another chemical, cryolite (Na 3 AlF 6 ), which lowers the melting point to around 1000 o C.

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

The energy to keep the alumina and cryolite molten comes from a A current flowing through the 6V electrolysis cell. The energy to provide the initial melting is enormous so these run 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 and aluminium which fall to the base of the tank as molten metal…… Al 3+ (l) + 3e- => Al (l) The positively charged anode, as blocks in the surface of the cell, Attracts the negatively charged oxide ions….. 2O 2- (l) => O 2(g) + 4e- Oxygen is not evolved though. This process is at 1000oC with a carbon anode. 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 process to what it looks like in real life!

ANODISING ALUMINIUM Aluminium is very unreactive for a metal so high up the reactivity series. This is due to the ease with which it forms a protective oxide layer. Anodising makes this oxide layer even stronger. Step 1. The original oxide layer is removed with sodium hydroxide. Step 2. The aluminium is made into the anode in dilute sulphuric 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 than before 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 and good 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.