GCSE OCR Gateway Chemistry C6.1b part 2 Aluminium Extraction

C6.1b  To be able to explain why and how electrolysis is used to extract some metals from their ores

Extraction of aluminium from molten alumina using electrolysis.

Carbon reduction is an effective method for the extraction of metals below carbon in the reactivity series.

But two problems remain:

1: How to extract metals higher than carbon, more reactive than carbon, in the reactivity series?

2. How can a metal be refined to a greater level of purity?

Electrolysis is the answer to both these problems.

In a previous blog here, we looked at how electrolysis is used to refine copper to a greater degree of purity than that obtained from the smelting of copper ores.

Now we are going to look at how electrolysis can be used to extract a very reactive metal—aluminium—from its ore.

Aluminium Extraction

Aluminium occurs naturally in the crust of the earth in several different mineral forms but one is very useful for extraction.  That mineral is bauxite so called because it was first found in France at Les Baux. 

Bauxite is a combination of oxides of iron and aluminium and is a red mineral (that’s due to the iron (III) oxide)

Separation of the valuable aluminium oxide form the less valuable iron(III) oxide takes place first before the white aluminium oxide or alumina can be electrolysed.

This separation process has its disadvantages in that it produces lagoons of red mud an environmental hazard and very toxic.  There is also danger from the earth walls of these lagoons breaking and the material leaching out into arable and other valuable land and causing lasting damage and contamination.  Chemistry is not without its risks and this process is not by any stretch of the imagination green!!

But people like aluminium, they buy aluminium products, they fly aluminium planes and drive aluminium cars so we have to take the rough with the smooth it seems to me.  All technology has this kind of a double edge to it.

They have loved aluminium from the earliest days of its discovery.   Even Napoleon, yes him on the right, was supposed to have had cutlery made from aluminium because at the time it was more expensive than pure gold!!

Once extracted, cleaned and dried the alumina can be used in the electrolysis process. 

A typical cell is shown below but you can find any number of these types of diagram on the internet or watch the electrolysis on youtube videos.  There is a link to one video here

What’s happening in the process?

Essentially aluminium is formed at the cathode and oxygen at the anode since we are electrolysing pure molten aluminium oxide.

But a few tweaks are needed to make the process work efficiently and in a less energy intensive way.

So cryolite (NaAlF₆) is added to the alumina to lower its melting point from 2045^oC to around 950^oC.  This is an energy saving measure. 

The oxygen produced at the anodes has the effect of burning off the carbon anodes as carbon monoxide and carbon dioxide so provision has to be made to contain and collect these gases especially since they are contaminated with fluorides (that is fluoride gases) from the electrolysis mix.

Anodes of course do not last forever and old burnt down anodes have to be removed and replaced with new carbon blocks. 

Aluminium is denser than the electrolyte of aluminium oxide and cryolite and the molten metal collects at the bottom of the electrolysis cell.  Syphoning removes this aluminium from the cell once it has collected.

The liquid metal cools as it is cast into metal ingots for further processing.

The electrode discharge equations are as follows:

Anode (carbon)   2O^2—  (l)  ⟶      O₂ (g)  +   4e^—

Cathode (carbon)   Al³⁺ (I)    +   3e^—      ⟶     Al(l)

So a reactive metal more reactive than carbon can be extracted from its ore by electrolysis but not carbon reduction. 

Other metals above zinc in the reactivity series are all extracted using electrolysis.  

The photo below gives you an impression of the vast scale of the cell rooms in the extraction of the metal aluminium.