Introduction

• Electrochemical principles are crucial in the extraction and isolation of metals.
• Certain metals are obtained by reducing their oxides using electrochemical reactions.
• This process is known as the electrochemical principles of metallurgy.
• It involves the use of an electrolysis cell to carry out the reduction reactions.

Electrolysis Cell

• An electrolysis cell consists of two electrodes - a cathode and an anode.
• The cathode is the electrode where reduction takes place, and the metal is deposited.
• The anode is the electrode where oxidation occurs, and metal ions are formed.
• The electrolyte is a solution of metal salt or molten metal salt that allows the flow of electricity.

Electrolysis of Molten Salts

• In the electrolysis of molten salts, the molten metal salt is used as the electrolyte.
• The metal ions are reduced at the cathode, forming the metal.
• At the anode, the metal from the metal salt is oxidized, producing metal ions.
• The metal ions dissolve in the molten electrolyte to replenish the metal salt.

Electrolysis of Aqueous Solutions

• When an aqueous solution is used as the electrolyte, it contains metal ions and H+ and OH- ions from water.
• H+ ions are discharged at the cathode, resulting in the liberation of hydrogen gas.
• Metal ions are discharged at the cathode if they have a higher reduction potential than hydrogen.
• OH- ions are discharged at the anode, forming oxygen gas and water.

Electrorefining

• Electrorefining is the process of purifying impure metals using electrolysis.
• The impure metal is made the anode, while a pure metal rod is used as the cathode.
• Metal ions from the impure metal are oxidized at the anode and dissolve in the electrolyte.
• On the cathode, pure metal is deposited, removing impurities present in the anode.

Rusting and Corrosion

• Rusting is the formation of iron oxide (Fe2O3) on the surface of iron in the presence of oxygen and moisture.
• It is an electrochemical process where iron acts as the anode and is oxidized.
• Corrosion is a similar process that occurs for other metals when they come in contact with a corrosive environment.
• Preventive measures like galvanization, painting, and coating are used to prevent rusting and corrosion.

Faraday’s Laws of Electrolysis

• Faraday’s First Law states that the mass of a substance deposited or liberated at an electrode is directly proportional to the quantity of electricity passed through it.
• Faraday’s Second Law states that the masses of different substances deposited or liberated at the same electrode by the same quantity of electricity are directly proportional to their equivalent masses.

Electrochemical Cells

• Electrochemical cells consist of two half-cells, each containing an electrode and electrolyte.
• The two half-cells are connected by a salt bridge or porous partition.
• The anode and cathode reactions occur in separate half-cells, allowing the flow of electrons.
• Examples of electrochemical cells include galvanic cells, electrolytic cells, and fuel cells.

Galvanic Cells

• Galvanic cells are also known as voltaic cells or batteries.
• They convert chemical energy into electrical energy by generating an electric current.
• In a galvanic cell, the anode undergoes oxidation, while the cathode experiences reduction.
• Examples of galvanic cells include the Daniell cell and the lead-acid battery.

11. Electrolytic Cells

• Electrolytic cells use external electrical energy to drive non-spontaneous reactions.
• The anode is typically the positive electrode, where oxidation occurs.
• The cathode is usually the negative electrode, where reduction takes place.
• These cells are commonly used for electroplating, electrorefining, and the production of various chemicals.

12. Electroplating

• Electroplating is the process of depositing a layer of metal onto a substrate.
• It is commonly used to improve the appearance, corrosion resistance, and durability of objects.
• The object to be plated is made the cathode, while the anode is the metal to be deposited.
• A suitable electrolyte containing metal ions allows the metal to be plated onto the cathode.

13. Electrorefining Process

• Electrorefining is a crucial step in obtaining pure metals from impure sources.
• It involves the electrolysis of an impure metal as the anode and a pure metal as the cathode.
• The impure metal is oxidized and dissolved in the electrolyte, while the pure metal is deposited on the cathode.
• This process allows for the removal of impurities and the production of high-quality metals.

14. Example: Electrorefining of Copper

• In the electrorefining of copper, impure copper acts as the anode, and a pure copper sheet serves as the cathode.
• Copper ions from the anode are oxidized and dissolve in the electrolyte as Cu2+ ions.
• At the cathode, Cu2+ ions are reduced to copper atoms, forming a pure copper deposit.
• Impurities such as iron, zinc, and nickel that do not plate out remain in the anode slime.

15. Electrolysis in Industrial Applications

• Electrolysis is used in various industrial applications for the production of chemicals.
• For example, electrolysis of sodium chloride (NaCl) produces chlorine gas at the anode and sodium hydroxide at the cathode.
• In the electrolysis of water (H2O), hydrogen gas is formed at the cathode, and oxygen gas is produced at the anode.
• These processes are essential for the production of many industrial chemicals.

16. Batteries and Fuel Cells

• Batteries and fuel cells are examples of electrochemical cells that generate electrical energy.
• Batteries are self-contained cells that store energy as chemical potential energy.
• Fuel cells, on the other hand, continuously consume fuel and an oxidant to produce electricity.
• Both types of cells involve redox reactions that occur at the electrodes.

17. Types of Batteries

• Primary batteries are non-rechargeable and use chemical reactions to generate electricity.
• Secondary batteries are rechargeable and can be recharged by reversing the cell reactions.
• Common types of batteries include lead-acid batteries, alkaline batteries, and lithium-ion batteries.
• Each type has unique characteristics and applications, depending on the chemistry involved.

18. Lead-Acid Battery

• Lead-acid batteries are widely used in vehicles and as backup power supplies.
• They consist of lead dioxide (PbO2) as the positive electrode, elemental lead (Pb) as the negative electrode, and sulfuric acid (H2SO4) as the electrolyte.
• During discharge, PbSO4 is formed at both electrodes, converting chemical energy into electrical energy.
• Recharging the battery reverses the reaction and restores the original reactants.

19. Alkaline Battery

• Alkaline batteries are commonly used in electronic devices and toys.
• They use a zinc anode, a manganese dioxide cathode, and an alkaline electrolyte (usually potassium hydroxide, KOH).
• The zinc undergoes oxidation at the anode, producing zinc oxide (ZnO), while manganese dioxide is reduced at the cathode.
• Alkaline batteries are known for their long shelf life and high energy density.

20. Lithium-Ion Battery

• Lithium-ion batteries have become the standard rechargeable batteries for portable electronics and electric vehicles.
• They use lithium compounds (e.g., lithium cobalt oxide, LiCoO2) as the cathode, a carbon-based material as the anode, and a lithium salt in an organic solvent as the electrolyte.
• During discharge, lithium ionizes and moves from the anode to the cathode, generating electrical energy.
• Recharging the battery allows lithium ions to move back to the anode, ready for the next discharge cycle.

Principles of Electrometallurgy

• Electrometallurgy involves the extraction of metals using electrical energy.
• It is based on the reduction of metal ions to their elemental form at the cathode.
• The metal ions are usually obtained from metal ores or metal salts.

Steps in Electrometallurgy

• The processes involved in electrometallurgy are as follows:
  • Mining and concentration of the metal ore.
  • Conversion of the metal ore into a soluble compound.
  • Electrolysis of the soluble compound to obtain the pure metal.

Example: Extraction of Aluminium

• Aluminium is obtained through the electrolysis of its ore, bauxite.
• Bauxite is first purified to obtain alumina (Al2O3).
• The alumina is then dissolved in molten cryolite (Na3AlF6).
• The electrolysis of the alumina-cryolite mixture allows for the extraction of pure aluminium.

Example: Production of Sodium

• Sodium metal is extracted from its compound, sodium chloride (NaCl).
• NaCl is dissolved in water to form an aqueous solution.
• This solution is electrolyzed using a Downs cell, producing sodium metal at the cathode and chlorine gas at the anode.
• The sodium metal is collected and stored under oil to prevent oxidation.

Environmental Impact of Electrometallurgy

• Electrometallurgy processes can have significant environmental impact:
  • Energy-intensive: Large amounts of electrical energy are required for electrolysis.
  • Emission of greenhouse gases: Electricity generation can lead to the release of greenhouse gases.
  • Waste generation: Electrolysis can produce waste products that require proper disposal or treatment.

Advantages of Electrometallurgy

• Despite the environmental impact, electrometallurgy offers several advantages:
  • High purity: The resulting metal is usually of high purity.
  • Ability to extract reactive metals: Reactive metals can only be extracted using electrometallurgical methods.
  • Recycling potential: Metals obtained through electrometallurgy can be recycled, reducing the need for new extraction.

Applications of Electrometallurgy

• Electrometallurgy has various industrial applications:
  • Production of metals like aluminium, sodium, and zinc.
  • Electroplating to enhance the appearance and durability of objects.
  • Electrorefining to obtain pure metals from impure sources.
  • Production of chemicals like chlorine and sodium hydroxide.

Summary

• Electrochemical principles of metallurgy involve the use of electrolysis to isolate metals.
• Electrolysis cells have a cathode where reduction occurs and an anode for oxidation.
• Electrolysis of molten salts and aqueous solutions are used in different processes.
• Electrorefining and electroplating are important applications of electrolysis.
• Electrometallurgy has environmental impacts but offers advantages and various industrial applications.

Review Questions

1. What are the principles of electrometallurgy?

2. Describe the steps involved in electrometallurgy.

3. Provide an example of the extraction of aluminium.

4. How is sodium metal produced using electrolysis?

5. What are the environmental impacts of electrometallurgy?

Answers to Review Questions

1. The principles of electrometallurgy involve the reduction of metal ions at the cathode to obtain the pure metal.

2. The steps in electrometallurgy include mining and concentration of the metal ore, conversion of the ore into a soluble compound, and electrolysis of the compound to obtain the pure metal.

3. Aluminium is extracted through the electrolysis of its ore, bauxite, in a solution containing molten cryolite.

4. Sodium metal is produced by the electrolysis of sodium chloride dissolved in water, using a Downs cell.

5. The environmental impacts of electrometallurgy include high energy consumption, emission of greenhouse gases, and waste generation.