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Extracting Metals at the Low End of the Activity Series
Metals at the low end of the activity series are unreactive. Heating alone can convert the metal oxides to metals.
Examples:
Cinnabar (\(HgS\)) is a mercury ore. It is first converted into mercuric oxide (\(HgO\)) when heated in the air. On additional heating, the mercuric oxide is converted to mercury as shown below:
Similarly, copper, which is found in nature as \(Cu_2S\), may be extracted from its ore by simply heating it in the air as follows:
Extracting Metals in the Middle of the Activity Series
Iron, zinc, lead, copper and other metals in the middle of the activity series are moderately reactive. In nature, they are commonly found as sulphides or carbonates. When compared to its sulphides and carbonates, it is easier to get metal from its oxide. Metal sulphides and carbonates must be converted into metal oxides before reduction. Heat is used to convert sulphide ores into oxides in the presence of excess air. This process is known as roasting. The following chemical reaction takes place while roasting.
By heating carbonate ores rapidly in a limited amount of air, they are converted to oxides. This process is known as calcination. The following chemical reaction takes place while calcination of zinc ores.
Suitable reducing agents, such as carbon, are then used to reduce the metal oxides to the corresponding metals. For example, when zinc oxide is burned with carbon, it is converted into metallic zinc.
The oxidation and reduction processes described in the first Chapter are already familiar to you. A reduction method is also used to extract metals from their compounds.
Displacement reactions can occasionally be used instead of carbon (coke) to convert metal oxides to metals. Since highly reactive metals like sodium, calcium and aluminium can displace less reactive metals from their compounds, they are used as reducing agents. For example, when manganese dioxide is heated with aluminium powder, the following reaction takes place.
In this reaction, manganese oxide (\(MnO_2\)) is reduced to its metal form (\(Mn\)), whereas aluminium (\(Al\)) is oxidised to aluminium oxide (\(Al_2O_3\)).
These displacement processes produce a lot of heat (exothermic). The heat produced is so high that the metals are formed in a molten state. Railway tracks and cracked machine parts are joined via the reaction of iron (III) oxide (\(Fe_2O_3\)) with aluminium. This reaction is known as the thermit reaction.
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