Learning Objective

In this lesson we will learn about different methods for obtaining pure metals from ore, and how the method used depends on the chemical reactivity of the particular metal.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe how the form in which a metal exists in nature, and the method used to extract it, are both related to the metal’s reactivity.

  • Define a native metal and give examples of methods used to separate them.

  • Describe the processes of carbon reduction and electrolysis for extracting metals from ore.

  • Compare the energy requirements for native metal separation, carbon reduction and electrolysis, and give examples of metals extracted by these methods.

 

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Introduction

  • Metals are mined from the earth’s crust.
    The rocks and sand in which metal deposits are found are known as ores.
    The form in which a metal exists in an ore depends on its chemical reactivity.
  • The least reactive metals, such as gold, silver and copper are known as native metals, as they often exist in their elemental metallic form.
    They only need to undergo physical separation processes to be collected.
  • More reactive metals exist as minerals in ores.
    Minerals are chemical compounds containing metals (or other important elements).
    The metals in minerals are extracted using chemical processes, then physically refined.
  • The type of chemical extraction process depends on how reactive the metal is.
    Metals with an intermediate reactivity, such as iron and zinc, exist as compounds which can be reduced by redox (displacement) reactions to yield pure metals.
    Very reactive metals, such as aluminium and magnesium, exist as compounds which can only be reduced by a process known as electrolysis, which requires an electric current to produce pure metals.

 
metal extraction method reactivity

The method for extraction of a metal depends on its reactivity.

 

Refining Native Metals

  • Native metals, such as gold and silver, usually occur as pure metals, needing only to be physically separated from surrounding ore.
    For example, gold can be separated from crushed rock by a process known as froth flotation, which uses detergents to create floating bubbles that adhere to gold particles, while other materials settle out and are removed.

 
metal separation froth flotation

Froth flotation is a method for physically separating native metals from ore.

 
 
metal separation froth flotation copper gold

Froth flotation of copper.

(Image: Andreslan, Wikimedia Commons)

 

Carbon Reduction

  • Metals with an intermediate reactivity, such as iron and zinc, are chemically extracted from minerals by a process known as carbon reduction.
    Carbon reduction is a type of redox reaction, where carbon is used to displace a metal from a compound.
    The crushed ore is mixed with carbon and heated to a molten state – a process known as smelting, which takes place in a blast furnace.
  • In the subsequent chemical reaction, the metal-containing compound is reduced (loses oxygen) and the carbon is oxidised (gains oxygen).
    Example
    iron oxide + carbon (+ heat) iron + carbon dioxide
    2 Fe2O3  +  3 C   4 Fe  +  3 CO2
    The smelting process also includes limestone, which combines with the sand and other waste materials to form slag.
    The less dense slag floats on top of the molten iron and is removed.

 
metal extraction carbon reduction blast furnace

Carbon reduction is a method for extracting metals with an intermediate reactivity from ore.

 
 
blast furnace iron ore extraction

Iron ore blast furnace.

(Image: zephylwer0, Pixabay)

Electrolysis

  • Metals that are more reactive, such as aluminium and magnesium, cannot be extracted from minerals by carbon reduction.
    This is because they are more reactive than carbon, therefore cannot be displaced by it when part of a compound.
  • Instead, these reactive metals are extracted by a process called electrolysis.
    Electrolysis is a non-spontaneous redox reaction, which involves the passing of an electric current through molten ore.
    The electric current causes electrons to be added to the metal ions, thus reducing them to pure metal.
    Example
    aluminium oxide (+ heat + electricity) aluminium + oxygen
    2 Al2O3    4 Al  +  3 O2
  • Since electrolysis is a redox reaction, there are two reactions taking place.
    The first is the reduction reaction, involving the metal.
    Example
    Al3+  +  3 e    Al
    The second is the oxidation reaction, involving the non-metal.
    Example
    2 O2-    O2  +  4 e
  • Electrolysis involves a positive electrode, known as an anode, and a negative electrode, known as a cathode.
    The reduction reaction takes place at the cathode.
    Positive metal ions are attracted to the negative electrode, where they gain electrons (are reduced) to become metal atoms.
    The oxidation reaction takes place at the anode.
    Negative non-metal ions are attracted to the positive electrode, where they lose electrons (are oxidised) to become non-metal atoms (which then form molecules).

 
metal extraction electrolysis

Electrolysis can be used for extracting very reactive metals from ore.

 
 
aluminium smelter metal extraction electrolysis

Aluminium smelter.

(Image: Yakov Fedorov, Wikimedia Commons)

 

Summary

  • In nature, most metals exist as compounds (minerals) embedded in surrounding rock (ore).
    Only the least reactive metals, such as gold and silver, exist in their elemental metallic form.
    These metals are known as native metals.
  • The chemical reactivity of metals also determines their method of extraction.
  • Native metals undergo physical separation processes, such as froth flotation.
    Froth flotation uses detergents to create floating bubbles that adhere to metal particles, while other materials settle out and are removed separately.
  • Metals with an intermediate reactivity, such as iron and zinc, are chemically extracted from minerals by a process known as carbon reduction.
    Carbon reduction is a type of redox reaction, where carbon is used to displace a metal from a compound.
    The crushed ore is mixed with carbon and limestone, then heated to a molten state (smelted) in a blast furnace.
    In the subsequent chemical reaction, the metal-containing compound is reduced (loses oxygen) to form pure metal, and the carbon is oxidised (gains oxygen) to form carbon dioxide.
  • Very reactive metals are extracted by a process called electrolysis.
    Electrolysis is a non-spontaneous redox reaction, which involves the passing of an electric current through molten ore.
    The electric current causes electrons to be added to metal ions, thus reducing them to metal.

 
molten iron metal

(Image: Erdenebayar, Pixabay)

 

(Header image: shibang, Pixabay)

 

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