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An introduction to geology
An introduction to geology

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2.5 From soil to soda cans

An aluminium drinks can is made of about 15 g of aluminium. Aluminium is the third most abundant element in the Earth’s crust (the bit we can ‘get at’) after oxygen and silicon, but is only about 8.4% of the material. If you wanted to extract the necessary aluminium from the Earth's crust, you would need to grind up 178 g of material to get enough aluminium to make one can:

equation left hand side 15 times g times cap a times l times prefix solidus of 8.4 percent divided by 100 postfix times equals right hand side 178 times g

How about a fairly typical rock, like a granite?

Well, granite is ~ 14% alumina (Al2O3). First you need to convert this to % aluminium, which we do by multiplying the percentage of Al2O3 by the percentage of Al in Al2O3:

The atomic mass of Al is 27. The atomic mass of O is 16.

multiline equation line 1 two postfix times multiplication prefix times of cap a times l postfix times plus three times prefix multiplication of cap o postfix times equals 102 line 2 two times prefix multiplication of cap a times l postfix times equals 54 line 3 14 postfix times percent cap a times l sub two times cap o sub three postfix times prefix multiplication of left parenthesis 54 divided by 102 right parenthesis equals Granite postfix times i times s postfix times seven and seven divided by 17 postfix times percent cap a times l line 4 15 times g postfix times cap a times l divided by left parenthesis seven times seven postfix times divided by 17 division 100 right parenthesis postfix times equals 202 postfix times eight divided by 21 times g postfix times Granite

So you’d actually need more granite to get the necessary aluminium than if you just ground up the crust.

Don't worry if this seems a bit too complicated – this video clip reiterates the above material and explores the maths a bit more closely.

Download this video clip.Video player: Video 2.2 Extracting enough aluminium
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Video 2.2 Extracting enough aluminium
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What about bauxite? Bauxite is formed from rocks like granite – how effective is the geological process in concentrating the aluminium? Test your understanding with the next activity.