Chemistry 4.4-4.7 - Extracting metals
Most useful metals begin as compounds in ores. Follow oxygen during reduction, use a metal’s position relative to carbon to choose an extraction method, and weigh chemical feasibility against cost.
From ores to reduction
The Earth's crust contains both metals and metal compounds. Finding a metal-containing rock is not the same as finding a piece of usable metal: in most cases, chemical reactions are needed to separate the metal element from the other elements with which it is combined.
Ore
An ore is a naturally occurring rock that contains enough of a metal, or a metal compound, for extracting the metal to be worthwhile.
"Worthwhile" matters. A low concentration that is uneconomic today could become useful if the value of the metal rises or a cheaper extraction method is developed. Ore is therefore an economic term as well as a chemical one.
At the scale we can see, an ore is a mixture of minerals and unwanted rock. At the particle scale, the wanted metal may be present as ions in a compound, rather than as metal atoms in the uncombined element. For example, is iron oxide: it contains iron, but it is not iron metal.
Most metals are extracted from ores found in the Earth's crust because their atoms have reacted to form compounds. Very unreactive metals can also occur as native, or uncombined, elements. Gold is the clearest example: some gold is found as the element Au, so no chemical reaction is needed to turn a gold compound into gold metal. Mining and physical separation may still be needed.
Do not reverse this pattern. A reactive metal is unlikely to remain uncombined over geological time, whereas an unreactive metal is more likely to be found native. This does not mean that every sample of an unreactive metal is uncombined, or that every ore is an oxide.
Many useful ores contain metal oxides. The oxygen model of redox gives a direct way to describe what happens when a metal is extracted from its oxide.
Oxidation and reduction
Oxidation is the gain of oxygen. Reduction is the loss of oxygen.
Extraction changes a metal compound into the metal element. When a metal oxide loses oxygen and forms a metal, the oxide has been reduced. This is why extracting a metal from an ore involves reduction.
Consider carbon heated with copper(II) oxide:
The same reaction can be connected across three levels:
| Level | What the reaction shows |
|---|---|
| Macroscopic | A reddish-brown copper-coloured solid can appear where black copper(II) oxide and carbon were heated together. |
| Particle | Oxygen is transferred from copper(II) oxide to carbon. Copper atoms remain as the metal. |
| Symbolic | CuO becomes Cu, so copper(II) oxide loses oxygen and is reduced. C becomes , so carbon gains oxygen and is oxidised. |
Oxidation and reduction happen together here: oxygen cannot leave the copper oxide unless it goes somewhere else. Carbon takes that oxygen. Heating alone supplies energy, but it is not itself the definition of reduction.
Worked oxygen audit
Use the equation .
- Compare each substance before and after the arrow. ZnO contains oxygen but Zn does not.
- Zinc oxide has lost oxygen, so zinc oxide is reduced.
- Carbon does not contain oxygen before the reaction, but it is in afterwards.
- Carbon has gained oxygen, so carbon is oxidised.
- The equation is balanced: there are two zinc atoms, two oxygen atoms and one carbon atom on each side. Matter has not disappeared; the oxygen has been transferred.
A common error is to say that zinc is reduced. Be precise about the starting substance: zinc oxide is reduced to zinc. The product zinc has already had the oxygen removed.
Choosing an extraction method
Carbon provides a dividing line in the reactivity series. The key comparison is between the metal and carbon, not between the ore and carbon.
| Position and starting material | What carbon can do | Usual extraction route | Example and cost consequence |
|---|---|---|---|
| Unreactive metal found as the uncombined element | No oxygen needs to be removed from a compound | No chemical reduction is needed to make the element | Native gold can be physically separated from surrounding material. |
| Metal below carbon, present as an oxide | Carbon can remove oxygen from the oxide | Heat the metal oxide with carbon | Iron is below carbon, so carbon reduction can be used. This is generally cheaper than electrolysis. |
| Metal above carbon, present in a compound | Carbon cannot remove the metal from its compound | Electrolysis of a molten ionic compound | Aluminium is above carbon, so electrolysis is needed. Keeping material molten and supplying electricity makes this expensive. |
Iron is the named carbon-reduction example. Iron oxide is reduced because it loses oxygen, while carbon is oxidised because it gains oxygen. A balanced overall representation is:
This equation shows the oxygen transfer required in this lesson. The detailed stages and construction of a blast furnace are not required.
Aluminium is the named electrolysis example. Aluminium is more reactive than carbon, so heating aluminium oxide with carbon cannot extract it. Electrical energy is used to decompose a molten aluminium compound and produce aluminium. Electrolysis is technically possible for many molten ionic compounds, but it is not automatically the best commercial choice: if carbon reduction works, its lower energy cost usually makes it preferable.
Cost is therefore a second filter after chemical feasibility. Relevant costs include electricity, fuel for heating, keeping the electrolyte molten, equipment and processing the ore. "Use electrolysis because it gives a metal" is incomplete; the position relative to carbon explains why that costly route is necessary for aluminium.
Worked method choice
A partial reactivity series is magnesium, Q, carbon, iron, copper, from most reactive to least reactive. An ore contains an oxide of metal Q.
Continue this lesson.
You’re previewing NovaLearn without an account. Create a free account to read the rest.
Create free account