Metal Ores and Extraction by Reduction

The Nature of Metal Ores

A metal ore is a rock that contains a high enough concentration of a metal compound to make it worth extracting the metal. While very unreactive metals such as gold can be found native, which means they exist as the uncombined element, most metals are found as compounds. The most common type of ore is a metal oxide. Examples include haematite, which is iron(III) oxide ($Fe_{2}O_{3}$), and bauxite, which is aluminium oxide ($Al_{2}O_{3}$).

Other ores, like galena (lead sulfide, $PbS$) or malachite (copper carbonate, $CuCO_{3}$), are often roasted in air or heated to convert them into oxides before the metal is extracted. For instance, heating copper carbonate causes thermal decomposition: $CuCO_{3}(s) \rightarrow CuO(s) + CO_{2}(g)$. This converts the carbonate into an oxide, ready for reduction.

Extraction as a Reduction Process

The process of extracting a metal from its ore is always a chemical reduction. In their ores, metals exist as positive ions because they have lost electrons to bond with non-metals like oxygen or sulfur. To return the metal to its elemental state, which has an oxidation state of zero, the metal ions must be reduced.

Reduction can be defined in two ways in this context:

1. The removal of oxygen from a compound: A reducing agent is used to take the oxygen away from the metal oxide.
2. The gain of electrons by a chemical species: The metal ions gain electrons to become neutral atoms. The general half equation for this is $M^{n+} + ne^{-} \rightarrow M$.

Methods of Reduction Based on Reactivity

The method used to reduce a metal ore depends entirely on the reactivity of the metal compared to carbon. If a metal is less reactive than carbon, it can be extracted by heating the oxide with carbon. If it is more reactive, electrolysis is required.

1. Reduction with Carbon: Metals such as iron, copper, and zinc are extracted by heating their oxides with carbon or carbon monoxide. Carbon acts as a reducing agent because it is more reactive than the metal and displaces it from the oxide. For example, in a blast furnace, iron is produced via: $Fe_{2}O_{3}(s) + 3CO(g) \rightarrow 2Fe(l) + 3CO_{2}(g)$. Here, the $Fe^{3+}$ ions gain electrons to become $Fe$ atoms.

2. Electrolysis: Highly reactive metals like aluminium and sodium cannot be reduced by carbon. Instead, they are extracted using electrolysis. This involves passing an electric current through the molten ore. The metal ions are attracted to the negative electrode (cathode), where they gain electrons. For aluminium, the half equation is $Al^{3+} + 3e^{-} \rightarrow Al$.

Worked Example: Extraction of Copper

Copper can be extracted by heating copper(II) oxide with carbon powder. The balanced equation is: $2CuO(s) + C(s) \rightarrow 2Cu(s) + CO_{2}(g)$.

In this process, we can identify the reduction by looking at the copper:

• The $CuO$ loses oxygen to become $Cu$, which is reduction by oxygen loss.
• The oxidation state of copper changes from $+2$ in $CuO$ to $0$ in $Cu$. Because the oxidation state has decreased, we know the copper ions have gained electrons and have been reduced.