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A2.1 Metal Reduction and Reactivity

The reactivity of the metal will have an effect on the method used to extract the metal from the ore.

Carbon (coke) is a good reducing agent because it is cheap and when oxidised releases a gas which escapes easily. Some equations:      
​ZnO + C→Zn + CO 2Fe2O + 3C→4Fe + 3CO2*\textsf{ZnO  +  C} \rightarrow  \textsf{Zn  + CO } \\ \\ \\  \textsf{2Fe}_\textsf2\textsf O\textsf{   +  3C} \rightarrow \textsf{4Fe  +  3CO}_\textsf2\textsf*ZnO + C→Zn + CO 2Fe2​O + 3C→4Fe + 3CO2​*
    
*more correctly the Fe2O3\textsf{Fe}_\textsf2\textsf {O}_\textsf3Fe2​O3​
is reduced by CO in the Blast Furnace
​Fe2O3 + 3CO→2Fe + 3CO2\textsf{Fe}_\textsf2  \textsf {O}_\textsf3 \textsf{  +  3CO} \rightarrow \textsf{2Fe  +  3CO}_\textsf2 Fe2​O3​ + 3CO→2Fe + 3CO2​
   
Key information 🗝
  
1.  Some metals are found as metals, these are the most unreactive. For example silver and gold (and also copper).
2. Some metals are reduced by heating with carbon (coke) or carbon monoxide (CO\textsf{CO}CO
) these have medium reactivity. For example iron, zinc, lead and copper.
3.  The most reactive metals can only be extracted effectively using electrolysis. Metals involved include: magnesium, aluminium, all alkali metals, and calcium.
Aluminium is too reactive to be reduced using carbon or carbon monoxide.
The correct process using sodium hydroxide can be summarised in 4 steps:

1. The ore is crushed and heated with the sodium hydroxide solution at 180°C 
     The aluminium oxide reacts and dissolves to form a solution of NaAl(OH)4\textsf{NaAl(OH)}_\textsf4NaAl(OH)4​
 

2. The waste solids are filtered off and disposed of.

3. The solution is cooled and aluminium hydroxide crystallises out: 
     NaAl(OH)4 (aq)\textsf{NaAl(OH)}_\textsf{4}  \textsf{ (aq)}NaAl(OH)4​ (aq)
 
  NaOH (aq) + Al(OH)3 (s)\textsf{NaOH  (aq)  + Al(OH)}_\textsf3\textsf{   (s)} NaOH (aq) + Al(OH)3​ (s)
 
     
      This is filtered off and then heated leaving pure Al2O3\textsf{Al}_\textsf2\textsf O_\textsf3Al2​O3​
;  
      2Al(OH)3\textsf{2Al(OH)}_\textsf3 2Al(OH)3​
 
 Al2O3+ 3H2O\textsf{Al}_\textsf2\textsf O_\textsf3  \textsf{+  3H}_\textsf2\textsf{O}Al2​O3​+ 3H2​O
​
4. The Al2O3\textsf{Al}_\textsf2\textsf O_\textsf3Al2​O3​
is then melted and electrolysed – liquid Al collects at the cathode; 
    Al3+ + 3e\textsf{Al}^\textsf{3+}  \textsf{ +  3e}Al3+ + 3e
 
 Al\textsf{Al}Al
 
Key information 🗝
 
Important details about the aluminium extraction process:
1.​Al2O3\textsf{Al}_\textsf2\textsf O_\textsf3 Al2​O3​
has a very high melting point, so cryolite, Na3AIF6\textsf{Na}_\textsf3\textsf{AIF}_\textsf6Na3​AIF6​
, is added which lowers the melting point to just over 1000°C to conserve energy.
2.An enormous electrical current is used – maybe 100 000 Amps.  So there needs to be a good (and preferably cheap) electrical supply (HEP or  Geothermal power).
This enormous current helps keep the the Al2O3\textsf{Al}_\textsf2\textsf O_\textsf3 Al2​O3​
 / cryolite mixture molten (in liquid form).
3.The process is carried out in a steel tank, lined with graphite  - the tank is the cathode. 
The reduction of aluminium ion occurs here : Al3+ + 3e→AI\textsf{Al}^\textsf{3+}  \textsf{ +  3e} \rightarrow \textsf {AI}Al3+ + 3e→AI
  
4.The anodes are carbon (graphite) blocks.
5.The anode reaction is 2O2-→O2+ 4e−\textsf{2O}^\textsf{2-} \rightarrow  \textsf{O}_\textsf2\textsf{+ 4e}^{-}2O2-→O2​+ 4e−
 and at 1000°C the oxygen causes the anodes to burn away, so they need to be replaced regularly. The heat produced also helps to keep the electrolyte molten.
6.Graphite is used because it conducts electricity well and does not react with, or dissolve in the liquid aluminium.
7.There is an environmental impact:
Carbon dioxide and some fluorine is released into the atmosphere and large amounts of waste material is left after the initial purification step (see point 2 above).
Figure 2.1 Flow diagram of Aluminium Manufacture
A2 Metals and Inductively-Coupled Plasma Spectroscopy
A2.2 Electrolysis Calculations
Last modified 2yr ago