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Electrolytic refining of copper is used to purify the metal using electrolysis.
In this method, the anode is made from impure copper, while the cathode is made from pure copper. The electrolyte is made from acidified copper sulphate solution. When electricity is passed, the impure copper from the anode gets dissolved in the electrolyte, while pure copper gets deposited on the cathode.
In this way, pure copper is obtained from the cathode.
What is refining?
Refining is the process of removing impurities to obtain pure metal. Various processes are used to remove impurities from metals; these are based on the nature of the metal and impurities.
The different methods used for the refining of metals are:
- Distillation
- Liquation
- Zone refining
- Vapour phase refining
- Electrolytic refining
Electrolytic refining is the most commonly used.
Other than copper, metals like silver and zinc are refined using electrolysis.
In 1847, Maximilian, Duke of Leuchtenberg, first experimentally demonstrated the electrolytic refining of copper.
Electrolysis
The term “electrolysis”, introduced by Michael Faraday, refers to the process by which electric current is passed through a chemical substance to induce a chemical change.
In electrolysis, a direct electric current is passed through an electrolyte. As a result, chemical reactions occur at the electrodes, and materials get decomposed.
The main components of electrolysis are:
- An electrolyte
- Electrodes
- An external power source
- A salt bridge
A chemical substance that contains free ions and carries an electric current is known as an electrolyte.
The electrical conductor used to make contact with the non-metallic part of a circuit is called an electrode.
The electrodes are dipped in the electrolyte and are separated by a distance. The current flows between them via the electrolyte. The electrodes are also connected to the power source, completing the electrical circuit.
Electrolysis involves the interchange of ions and atoms, which occurs as a result of the addition or removal of electrons because of the current applied.
Process of electrolytic refining
Electrolytic refining is also known as electro-refining. In this method, metals like copper are purified using electrolysis.
A strip of impure metal is used as the anode, and a strip of pure metal as the cathode. An aqueous solution of metal salt is used as an electrolyte.
At the negatively charged electrode or cathode, the metal cations gain electrons and get reduced. In this way, they get deposited at the cathode as pure neutral metal atoms.
In electrolytic refining, the anode is the impure metal, and the impurities are lost during the transfer of the metal from the anode to the cathode.
After the process is complete, pure metal is formed at the cathode by dissolving the impure metal at the anode.
The following reactions take place when an electric current is passed through the electrolytic solution:
Reaction at cathode
Mn+ + ne- → M
Reaction at anode:
M → Mn+ + ne-
Electrolytic refining of copper
Impure or blister copper is about 99% pure when extracted from the ore. Copper metals can be refined up to 99.99% by electrolytic refining. The anode (positive electrode) is made from impure copper, and the cathode (negative electrode) from pure copper. Copper sulphate acidified with sulphuric acid is used as an electrolyte in this process. By passing electric current, the impure metal dissolves from the anode into the electrolytic solution. In this phase, copper sulphate dissociates into Cu++ and (SO4)2- ions. The positive copper ions, also known as the cations, move towards the cathode, which is made up of pure copper metal. The metal cations absorb electrons from the cathode and get deposited on the cathode as Cu atoms. Thus, pure copper is produced at the cathode.
The following reactions take place when an electric current is passed through the electrolytic solution:
At cathode:
Cu2+ + 2e- → Cu
At anode:
Cu → Cu2+ + 2e-
The soluble impurities dissolve in the electrolytic solution, while insoluble impurities can be found at the bottom of the anode. This insoluble impurity is known as anode mud.
In this method, the cathode is coated with graphite as it makes the separation of pure copper easier.
Significance of electrolytic refining of copper
Impure copper is less conductive than pure copper. Impurities reduce copper’s electrical conductivity. As we know, copper is used in making electrical wires because of its conductive nature. When copper is obtained, it is 99% pure; after electrolytic refining, it becomes almost 99.99% pure.
Hence, electrolytic refining is highly significant.
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Conclusion
Electrolytic refining is the method in which electricity is used to refine impure metals. The anode is made from impure copper and the cathode from pure copper. Copper sulphate acidified with sulphuric acid is used as an electrolyte. By passing electric current, impure metal dissolves from the anode into the electrolytic solution. The positive copper ions move towards the cathode, composed of pure copper. The metal cations absorb electrons from the cathode and are deposited on the cathode as Cu atoms. Thus, pure copper is produced at the cathode.
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