Using Carbon to Extract Metals

Instructor: Scott Larkin
How do we get metal from its natural form? We'll talk about using carbon in oxidation-reduction reactions for the smelting process, the metal activity series, and go over specific examples including lead, iron and aluminum.

The Birth of Metallurgy

The ability to refine metal from metal ores is one of the most important technologies invented by civilization. The phases of human progress are marked by names like the Copper Age, Bronze Age, and Iron Age.

Beyond the precious metals like silver and gold, which you can find as nearly pure metals, the only source of other pure metals came from meteorites. As you can imagine, meteorites were not a reliable source of materials to early people.

On Earth, metals are often found as metal oxides. One of the first triumphs of early people was discovering how to use carbon and heat to change metal oxides into pure metal and carbon dioxide.

Oxidation-Reduction Reaction

The process used to convert a metal ore into a native metal is called smelting. Lead was one of the first metals extracted by early people, so let's use it as our first example.

Lead needs a much lower temperature than iron. It is likely smelting was discovered by accident when pottery with glaze that contained lead was fired inside burning embers.

The most important step in converting a metal ion in an ionic compound to a free metal is an oxidation-reduction reaction. Basically, we're removing the oxygen from a metal oxide (reduction) and giving it to carbon (oxidation). More specifically, an oxidation-reduction reaction involves the transfer of charge from one element to another. When we talk about converting metal oxides to metals, this follows a simple definition. (Later we can cut our teeth on a more general definition).

  • Oxidation - an element gains oxygen
  • Reduction - an element loses oxygen

Lead From Lead Oxide

If we want to get lead out of lead oxide (PbO), we have to add carbon in the form of coke. Coke is like charcoal, except charcoal is made from converting wood to carbon bricks. Coke is made by heating charcoal in an oxygen-free environment.

Lead oxide contains the cation Pb+2, and the oxide anion O-2. It is the role of carbon to pass charges to the metallic cation. Then the metallic cation changes from having a positive charge to having a charge of zero, (all metals have a charge of zero when they are in the elemental state). The anion, O-2, combines with carbon to form carbon dioxide.

Lead oxide reacts with coke and makes lead metal and carbon dioxide. Because lead lost oxygen, it has been reduced. Because carbon gained oxygen, it has been oxidized.

Lead oxide is reduced by carbon to lead metal and carbon dioxide
lead oxide converted to lead

When an element switches places with another element, or knocks one element out of the way, this is called a single displacement reaction or a simply a displacement reaction.

Iron From Iron Oxide

Though other forms of iron oxide exist, iron(III) oxide is the most common form. You see it every day as rust. It is also visible in soil when the soil is orange. Excess iron in the soil prevents land from being very productive.

Iron(III) oxide reacts with carbon and forms iron and carbon dioxide. Iron is reduced while carbon is oxidized.

iron (III) oxide reacts with carbon to form iron metal and carbon dioxide
Reaction of iron oxide with carbon

Metal Activity Series

So, can we do this with all the metals? Well, not quite. Carbon is only capable of transferring electrons to metals that are less reactive than itself. Metals are ranked by which metals can be changed by carbon from metal oxides to their metallic state, and which ones need something more.

Metals below carbon can be reduced by carbon. Metals above carbon must use electrolysis
chart metals reduced by carbon

Metals ranked below carbon (lead, iron, copper) can accept charges from carbon. Metals ranked above carbon are more reactive than carbon, and therefore need to extracted by using carbon as electrodes in a process caused electrolysis ('electro-' meaning 'uses electricity' and '-lysis' meaning 'to cut'). This still uses oxidation-reduction reactions, but it needs electricity to do it.

For example, lead and iron oxides are reduced by carbon with a simple chemical reaction, but aluminum oxide requires electrolysis to reverse the normal progress of the reaction. We've seen lead and iron already, let's see how aluminum is changed. From the chart, you can see that it will need electrolysis.

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