Metal can be found in ore in the form of a combination with other elements. As a result, once the ore has been extracted from the ground, it must be converted into pure metal. Metal extraction is the process of extracting metal from its ore. A number of steps are used to transform ores into free metals, depending on the type of ore, the nature of the impurities present, and the reactivity of the metal to be extracted. The extraction of moderately reactive metals and less reactive metals are discussed further below.

Extraction of Moderately Reactive Metals

In the middle of the reactive series are moderately reactive metals such as zinc, iron, tin, and lead, among others. As a result, extracting moderately reactive metals involves extracting metals in the middle of the reactivity series. The middle of the reactivity series metals is extracted by reducing their oxides with carbon, aluminium, sodium, or calcium. 

Some moderately reactive metals are found as oxides in nature, while others are found as carbonates or sulphide ores. Metals are now easier to be obtained by reduction from their oxides than from carbonates and sulphides. As a result, the ore must first be turned into a metal oxide, which may then be reduced. Calcination or roasting can be used to transform the concentrated ores into metal oxide. The process to be utilised is determined by the ore’s nature. Calcination converts a carbonate ore to oxide, whereas roasting converts a sulphide ore to oxide. 

• Calcination- Calcination is the process of converting a carbonate ore into metal oxide by rapidly heating it in the absence of air. Zinc, for example, is found in calamine ore as zinc carbonate (ZnCO₃). In order to extract zinc metal from zinc carbonate, the latter must first be transformed to zinc oxide. Calcination is used to accomplish this. Calamine ore or zinc carbonate decomposes into zinc oxide and carbon dioxide when heated strongly in the absence of air. As a result of calcination, zinc carbonate is converted to zinc oxide.

ZnCO₃        →             ZnO      +           CO₂
(Zinc carbonate)                (Zinc oxide)        (Carbon dioxide)

• Roasting- Roasting is the process of converting a sulphide ore into metal oxide by rapidly heating it in the presence of air. Zinc blende ore, for example, contains zinc as a sulphide, ZnS. So, before extracting zinc metal from zinc sulphide, zinc sulphide must first be converted to zinc oxide. Roasting is used to accomplish this. Zinc oxide and sulphur dioxide are formed when zinc blende ore or zinc sulphide is vigorously heated in the air or roasted. As a result, roasting turns zinc sulphide into zinc oxide.

2ZnS       +         3O₂          →         2ZnO          +         2SO₂
(Zinc sulphide)           (Oxygen)                (Zinc oxide)        (Sulphur dioxide)

Reduction of Metal Oxides

Using reducing agents such as carbon, aluminium, sodium, or calcium, metal oxides formed by calcination and roasting of ores are converted to free metal. The chemical reactivity of the metal to be extracted determines the reducing agent used.

• Reduction of Metal Oxide with Carbon- Carbon is commonly used to reduce the oxides of comparatively less reactive metals such as zinc, iron, nickel, tin, lead, and copper. The metal oxide is combined with carbon, in the form of coke and heated in a furnace in the carbon reduction process. The metal oxide is reduced to free metal by carbon.

For example, zinc is recovered from its oxide by reducing it with carbon or coke. When zinc oxide is heated with carbon, zinc metal is produced.

ZnO        +          C           →           Zn            +        CO
           (Zinc oxide)           (Carbon)                  (Zinc)              (Carbon monoxide)

Iron metal is also recovered from its oxide ore, ‘haematite’ (Fe₂O₃), by carbon reduction in the form of coke. The reduction of oxides with carbon is also used to recover tin and lead metals. Copper, which is a less reactive metal, is extracted by reducing its oxide with carbon.

• Reduction of Metal Oxide with Aluminium- In the extraction of metals from their oxides, a more reactive metal like aluminium can be utilised as a reducing agent. When the metal oxide is of a more reactive metal than zinc, for instance, and cannot be reduced effectively by carbon, aluminium is utilised as a reducing agent. This is due to the fact that a more reactive metal, such as aluminium, can displace a less reactive metal from its metal oxides, resulting in free metal. As a result, displacement reactions can be employed to convert some metal oxides to free metals. The aluminium powder oxidises to aluminium oxide after reducing the metal oxide to metal.

For example, Manganese is removed from its oxide by reducing it with aluminium powder as the reducing agent. Manganese metal is generated when manganese dioxide is heated with aluminium powder.

3MnO₂          +          4Al                           →                     3Mn                +        2Al₂O₃       +         Heat
(Manganese dioxide)     (Aluminium powder)           (Manganese metal)            (Aluminium oxide)                           

This oxidation and reduction reaction between MnO₂ and Al is a displacement reaction.

Extraction of Less Reactive Metals

Mercury, copper, and other less reactive metals are placed near the bottom of the reactivity series. As a result, the extraction of less reactive metals involves the extraction of metals with low reactivity. The less reactive metals, which are near the bottom of the reactivity scale, are removed solely by reducing their oxides with heat. Mercury and copper, for example, are less reactive metals that are near the bottom of the reactivity series. As a result, we’ll look at how to extract mercury and copper by reducing their oxides with heat alone.

Extraction of Mercury- Mercury is a relatively less reactive metal with a low reactivity level. Mercury may be extracted from its sulphide ore by simply heating it in the air. This occurs in the following manner. The sulphide mineral cinnabar, HgS, which is actually mercury (II) sulphide, is used to make mercury metal. The following are the two steps involved in extracting mercury from cinnabar.

• When mercury (II) oxide is formed, the concentrated mercury (II) sulphide ore (cinnabar ore) is roasted in the air.

2HgS       +          3O₂        →             2HgO             +         2SO₂
(Mercury (II) sulphide)    (Oxygen)                   (Mercury (II) oxide)      (Sulphur dioxide)

• When mercury (II) oxide is heated to around 300⁰C, it decomposes or reduces to generate mercury metal.

2HgO                 →         2Hg           +           O₂
(Mercury (II) oxide)                   (Mercury metal)          (Oxygen)

As a result, mercury metal was produced by reducing mercury (II) oxide with heat alone.

Extraction of Copper- Copper is a less reactive metal that ranks near the bottom of the reactivity scale. Copper can be extracted from its sulphide ore by simply heating it in the air. This occurs in the following manner. A copper glance, CuS, is actually copper (I) sulphide, and is one among the ores from which copper metal is produced. The following two steps are involved in extracting copper from copper glance ore.

• When a part of copper (I) sulphide is oxidised to copper (I) oxide, the concentrated copper (I) sulphide ore, also known as a copper glance, is roasted in the air.

2Cu₂S        +          3O₂          →          2Cu₂O             +         2SO₂
(Copper (I) sulphide)      (Oxygen)                  (Copper (I) oxide)        (Sulphur dioxide)

• The supply of air for roasting is turned off once a significant amount of copper (I) sulphide has been converted to copper (I) oxide. Copper (I) oxide generated above interacts with the residual copper (I) sulphide to form copper metal and sulphur dioxide in the absence of air.

2Cu₂O        +         Cu₂S                       →                     6Cu            +            SO₂
(Copper (I) oxide)         (Copper (I) sulphide)                   (Copper metal)      (Sulphur dioixde)

Sample Questions

Question 1: Name two reducing agents that are utilised in metal extraction.

Answer:

Carbon, which comes in the form of coke, and aluminium powder are the two reducing agents utilised in metal extraction.

Question 2: What is a thermite reaction, and how does it work?

Answer:

A thermite reaction is the reduction of a metal oxide to metal utilising aluminium powder as a reducing agent.

Question 3: When is a mineral called an ore?

Answer:

A mineral is called an ore if the mineral can be extracted profitably from it.

Question 4: Name a metal oxide that can be extracted from its ore by reduction with carbon?

Answer:

Zinc can be extracted from its oxides by reduction with carbon.

Question 5: How do you extract iron out of its ore haematite?

Answer: 

Iron metal is recovered from its oxide ore, ‘haematite’ (Fe₂O₃), by carbon reduction in the form of coke. 

Question 6: How can mercury be extracted from its sulphide ore?

Answer:

Mercury may be extracted from its sulphide ore by simply heating it in the air.