Chapter 6 General Principles and Processes of Isolation of Elements

1. Explain the principle and process involved in the extraction of aluminium from bauxite ore using the Bayer's process. Discuss the role of cryolite in the process.

Answer: The extraction of aluminium from bauxite involves two major processes: Bayer’s process and electrolytic reduction.

• Bayer’s Process: The first step in aluminium extraction is the refining of bauxite (which contains alumina, $\text{Al}_2\text{O}_3$ ) using the Bayer process.

  Steps involved:

  1. Crushing and grinding of bauxite: Bauxite is crushed and mixed with sodium hydroxide ($\text{NaOH}$ ) under high pressure.
  2. Leaching: The sodium hydroxide reacts with alumina ($\text{Al}_2\text{O}_3$ ) to form soluble sodium aluminate, leaving behind impurities like iron oxide (Fe2O3).
  3. Separation: The sodium aluminate solution is cooled, and impurities like iron oxide settle down. The clear solution is then treated with water, which precipitates hydrated alumina (Al2O3·3H2O).
  4. Calcination: The hydrated alumina is heated to a high temperature (about 1000-1200°C) to remove water, resulting in pure alumina ($\text{Al}_2\text{O}_3$ ).

• Electrolytic Reduction: The alumina ($\text{Al}_2\text{O}_3$ ) obtained from Bayer’s process is subjected to electrolysis in molten cryolite ($\text{Na}_3\text{AlF}_6$ ).

  Role of Cryolite: Cryolite is used as a solvent because it reduces the melting point of alumina and increases the electrical conductivity of the solution, making the electrolysis process more efficient. Pure alumina has a very high melting point (about 2050°C), while cryolite lowers it to around 900-1000°C.

  Electrolytic Process:

  • The electrolytic cell consists of a molten mixture of alumina and cryolite.
  • At the cathode: $\text{Al}^{3+}$ ions gain electrons and are reduced to form aluminium metal. $$\text{Al}^{3+} + 3e^- \rightarrow \text{Al}$$
  • At the anode: $\text{O}^{2-}$ ions lose electrons to form oxygen gas. $$2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^-$$

  The overall reaction is:

  $$2\text{Al}_2\text{O}_3 (l) \rightarrow 4\text{Al} (l) + 3\text{O}_2 (g)$$

  Aluminium is collected at the cathode, and oxygen is released at the anode.

---

2. Explain the process of extraction of copper from its ore (Cu2S) through smelting and its reduction using carbon.

Answer: Copper is extracted from its ores like cuprous sulphide (Cu2S) or copper pyrites (CuFeS2) using smelting and reduction processes.

• Smelting of copper involves heating the ore in a furnace to produce a molten mass. The steps are as follows:

  1. Roasting of CuFeS2: The ore (copper pyrites) is roasted in the presence of air to convert the sulphide into copper(I) oxide and iron(III) oxide. $$2\text{CuFeS}_2 + 3\text{O}_2 \rightarrow 2\text{Cu}_2\text{O} + 2\text{Fe}_2\text{O}_3 + 4\text{SO}_2$$
  2. The copper(I) oxide reacts with the copper(I) sulphide ($\text{Cu}_2\text{S}$ ) in a process called matte smelting to produce blister copper (impure copper): $$\text{Cu}_2\text{S} + \text{O}_2 \rightarrow 2\text{Cu} + \text{SO}_2$$

• Reduction of Cu2S using Carbon: The impure copper obtained (blister copper) is then purified by reduction with carbon. In this process, coke (a form of carbon) is used to reduce copper oxide at high temperatures:

  $$\text{Cu}_2\text{O} + \text{C} \rightarrow 2\text{Cu} + \text{CO}$$

  Copper metal is obtained after this reduction, and carbon monoxide is released as a byproduct.

The final product is pure copper, which can be further refined by electrolysis.

---

3. What is the role of the froth flotation method in the concentration of ores? Explain the process with an example.

Answer: Froth flotation is a method used to concentrate sulphide ores by separating them from gangue (impurities). It is based on the difference in the wetting properties of minerals in a liquid.

Principle: The principle behind froth flotation is that sulphide ores are preferentially wetted by oils (hydrophobic) and the gangue (mainly silicates) is preferentially wetted by water (hydrophilic).

Steps involved:

1. Crushing and grinding: The ore is first crushed and ground into fine particles.
2. Addition of water and frothing agents: The powdered ore is mixed with water in a flotation cell, and various collectors (e.g., xanthates) and frothers (e.g., pine oil) are added. The collectors increase the hydrophobicity of the desired mineral (e.g., sulphide minerals).
3. Formation of froth: Air is bubbled through the mixture, forming a froth. The hydrophobic mineral particles attach to the air bubbles and rise to the surface.
4. Skimming of froth: The froth is skimmed off and collected, containing the concentrated ore.

Example: Froth flotation is used to concentrate zinc blende (ZnS) and galena (PbS) ores. In this case, the sulphide mineral is separated from the gangue (like quartz or calcite).

---

4. Describe the principles and applications of zone refining in the purification of metals.

Answer: Zone refining is a method used to purify metals, particularly semiconductor materials such as silicon and germanium, by exploiting the difference in the solubility of impurities in the molten and solid states.

Principle: The principle behind zone refining is that when a solid metal is passed through a narrow molten zone, impurities tend to concentrate in the liquid phase. As the molten zone moves through the metal, impurities are carried with it, and pure metal solidifies behind the molten zone.

Steps involved:

1. A metal rod (e.g., silicon) is heated at one point using a moving electrical heating coil. This creates a molten zone in the metal.
2. The molten zone is moved along the length of the rod, causing impurities to be segregated and move towards the end of the rod.
3. The process is repeated multiple times, each time refining the metal further by removing impurities.
4. The metal is eventually obtained in a high degree of purity.

Applications:

• Silicon purification: Zone refining is widely used in the purification of silicon for the manufacture of semiconductors.
• Germanium and gallium: Similar methods are used for purifying germanium and gallium.

---

5. What are the different types of ores? Classify them with examples and explain their extraction processes.

Answer: Ores are naturally occurring minerals from which metals can be extracted economically. They are classified into various types based on their composition and metal content.

1. Oxide Ores: These ores contain metals in the form of oxides.

   • Example: Bauxite (Al2O3) for aluminium, Hematite (Fe2O3) for iron.
   • Extraction: Oxide ores are typically reduced using carbon (for iron) or through electrolytic reduction (for aluminium).

2. Sulphide Ores: These ores contain metals in the form of sulphides.

   • Example: Galena (PbS) for lead, Copper pyrites (CuFeS2) for copper.
   • Extraction: Sulphide ores are often concentrated using the froth flotation method and then reduced using smelting or roasting.

3. Carbonate Ores: These ores contain metals in the form of carbonates.

   • Example: Calcium carbonate (CaCO3) for calcium, Magnesite (MgCO3) for magnesium.
   • Extraction: These ores are reduced by heating (calcination) and, in some cases, followed by reduction with carbon.

4. Halide Ores: These ores contain metals in the form of halides.

   • Example: Sodium chloride (NaCl) for sodium, KCl for potassium.
   • Extraction: Electrolytic methods are commonly used for extracting metals from halide ores.

5. Native Ores: These ores contain metals in their pure, native state.

   • Example: Gold (Au), Silver (Ag).
   • Extraction: Native metals are often extracted by simple physical methods like panning or cyanide leaching for gold.

Each ore type has specific extraction processes based on the nature of the metal and its compound.

---

6. Discuss the role of Ellingham diagrams in the extraction of metals. How can they be used to predict the feasibility of a reduction reaction?

Answer:
Ellingham diagrams are graphs that plot the Gibbs free energy change ($\Delta G^o$ ) of the formation of oxides versus temperature. These are essential for determining the feasibility of reducing metal oxides to metals.

1. Negative $\Delta G^o$ : A reaction is thermodynamically feasible if $\Delta G^o$ is negative. The position of the curve determines whether a reducing agent (like carbon or CO) can reduce a metal oxide.
2. Intercepts and slopes: The slope of the Ellingham curve indicates the entropy change. For instance, the abrupt change in slope for $C \to CO$ signifies a phase change (solid to gas).
3. Application in metallurgy:
   • For reducing Fe$_2$ O$_3$ to Fe using carbon monoxide, the intersection point of Fe$_2$ O$_3$ and CO curves shows the temperature range for reduction.
   • For more reactive metals like aluminum, reduction is achieved at higher temperatures due to the steeper curve.

---

7. Explain the method of electrolytic refining of copper. How is it superior to other refining methods?

Answer: Electrolytic refining of copper is a process used to purify impure copper obtained from the smelting process.

Principle: In electrolytic refining, an electric current is passed through a solution of copper sulfate (CuSO4) to deposit pure copper onto the cathode.

Steps involved:

1. Setup: The impure copper (anode) is immersed in an electrolytic solution of copper(II) sulfate and dilute sulfuric acid. A pure copper plate is used as the cathode.
2. Electrolysis: When current is passed, copper from the anode dissolves into the electrolyte and gets reduced at the cathode:
   • At the anode: Copper from the anode dissolves as Cu$^{2+}$ ions. $$\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$$
   • At the cathode: Cu$^{2+}$ ions from the electrolyte are reduced to form pure copper on the cathode. $$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$$
3. Purification: Impurities such as iron, nickel, and silver either remain at the bottom as sludge or dissolve into the electrolyte.

Advantages of Electrolytic Refining:

• The process results in high-purity copper (up to 99.99% pure).
• It is more efficient compared to other methods like distillation or sublimation because it does not require heating to high temperatures.

---

8. What are the differences between roasting and calcination in the extraction of metals? Explain with examples.

Answer: Roasting and calcination are both thermal treatment processes used in the extraction of metals, but they differ in the type of ores they are used for and the reactions involved.

1. Roasting:

   • Definition: Roasting is the process of heating an ore in the presence of excess air or oxygen.
   • Purpose: It is used to convert sulphide ores into oxides by eliminating sulphur as sulfur dioxide (SO2).
   • Example: Roasting of zinc blende (ZnS): $$2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$$ This is followed by reduction to obtain zinc.

2. Calcination:

   • Definition: Calcination involves heating an ore in the absence of air, or in a limited supply of air, to remove volatile substances (usually carbon dioxide).
   • Purpose: It is used to convert carbonates into oxides.
   • Example: Calcination of limestone (CaCO3): $$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$ The product, calcium oxide (lime), is used in many industrial processes.

---

9. How is the extraction of iron from its ore (haematite) different from the extraction of aluminium from bauxite?

Answer: The extraction of iron from haematite and aluminium from bauxite are two very different processes due to the differences in the ores and the reactivity of the metals.

• Extraction of Iron:

  • Iron is extracted from haematite (Fe2O3) through reduction with carbon in a blast furnace.
  • Process:
    1. The haematite ore is mixed with coke and limestone and then heated in the blast furnace.
    2. The coke reduces the iron(III) oxide to iron: $$\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO}_2$$
  • Iron is separated from impurities like silica, which form slag (CaSiO3).

• Extraction of Aluminium:

  • Aluminium is extracted from bauxite (Al2O3) using the Bayer process to refine the alumina followed by electrolytic reduction.
  • Process:
    1. Bauxite is first refined to get alumina (Al2O3).
    2. The alumina is then reduced in an electrolytic cell in molten cryolite, where aluminium metal is deposited at the cathode.

---

10. Discuss the role of the electrochemical series in the extraction of metals. Explain how it helps in deciding the method of extraction.

Answer: The electrochemical series arranges metals in order of their electrode potentials, from the most electropositive (easily oxidized) to the least electropositive (easily reduced). This series helps determine how easily a metal can be extracted based on its position relative to other metals.

• For highly electropositive metals (like potassium, sodium), electrolytic reduction is usually employed, as they cannot be reduced by carbon due to their very low reduction potential.
• For less electropositive metals (like iron, zinc), reduction with carbon or smelting is sufficient.
• For noble metals (like gold, silver), physical methods such as panning or simple extraction can be used due to their high resistance to oxidation and corrosion.

---

11. What are the environmental challenges associated with the extraction of metals? How can these be mitigated?

Answer: The extraction of metals can cause a range of environmental problems, including:

1. Air Pollution: Roasting and smelting release harmful gases like sulfur dioxide (SO2), carbon monoxide (CO), and particulate matter into the atmosphere.

   • Mitigation: Gases can be treated by scrubbers to remove pollutants and reduce emissions. Also, capturing and converting SO2 to sulfuric acid for industrial use can be beneficial.

2. Water Pollution: Toxic chemicals like cyanide used in gold extraction can contaminate water sources.

   • Mitigation: Careful management of waste products and the use of cyanide detoxification methods can prevent pollution.

3. Soil Degradation: Mining operations can lead to deforestation and soil erosion.

   • Mitigation: Reforestation and proper mining waste management practices can help restore affected areas.

4. Energy Consumption: Extraction processes, especially electrolytic reduction, consume a significant amount of energy.

   • Mitigation: Use of renewable energy sources like solar or wind power in extraction processes can reduce the environmental footprint.

12. Describe the process of extraction of copper from its ore (CuFeS2) using the pyrometallurgical method.

Answer: The extraction of copper from its ore, chalcopyrite (CuFeS2), involves a series of steps using the pyrometallurgical method.

1. Roasting:

   • Chalcopyrite ore is heated in the presence of oxygen in a roasting furnace. During roasting, the sulfur in the ore reacts with oxygen to form sulfur dioxide (SO2) gas, and copper(I) oxide (Cu2O) is formed: $$2CuFeS_2 + 3O_2 \rightarrow 2Cu_2O + 2FeO + 2SO_2$$

2. Smelting:

   • The roasted material is then heated in a blast furnace to produce molten copper. The iron present reacts with silica (SiO2) to form slag (FeSiO3), and copper is reduced: $$Cu_2O + C \rightarrow 2Cu + CO$$

3. Refining:

   • The impure copper obtained from smelting is refined by electrolytic refining. In this process, the impure copper is made the anode in an electrolytic cell, and a thin sheet of pure copper is made the cathode. Copper ions from the anode dissolve and are deposited as pure copper on the cathode: $$Cu^{2+} + 2e^- \rightarrow Cu(s)$$

This process gives high-purity copper, which is used in electrical and industrial applications.

---

13. Explain the difference between acid and basic flux used in the extraction of metals, with examples.

Answer: Flux is a substance that is added during the extraction of metals to remove impurities or gangue and facilitate the extraction process.

• Acid Flux:

  • An acid flux is used when the gangue is basic in nature.
  • The purpose is to neutralize the basic impurities and form a slag with the gangue.
  • Example: Silica (SiO2) is commonly used as an acid flux in the extraction of iron. It reacts with the basic impurities (like calcium oxide, CaO) to form slag (calcium silicate, CaSiO3): $$CaO + SiO_2 \rightarrow CaSiO_3$$
  • Acid flux is often used in processes like smelting of iron.

• Basic Flux:

  • A basic flux is used when the gangue is acidic in nature.
  • It neutralizes acidic impurities to form a slag.
  • Example: Limestone (CaCO3) is used as a basic flux in the extraction of aluminium. It reacts with the acidic gangue (like silica) to form a slag of calcium silicate: $$SiO_2 + CaCO_3 \rightarrow CaSiO_3 + CO_2$$

Thus, the choice of flux depends on the composition of the gangue present in the ore.

---

14. Explain the concept of "leaching" in metal extraction, and describe its application in the extraction of gold and aluminium.

Answer: Leaching is a process where a solvent is used to selectively dissolve and separate metal from its ore. It is an efficient way to extract metals, especially for ores that are low-grade or have complex compositions.

• Leaching of Gold:

  • The cyanide process is a common method for extracting gold. In this process, the ore (usually gold-bearing quartz or cyanide-soluble gold ore) is treated with a dilute solution of sodium cyanide (NaCN). The gold reacts with cyanide ions to form a gold-cyanide complex (Na[Au(CN)2]): $$4Au + 8NaCN + 2O_2 + 2H_2O \rightarrow 4Na[Au(CN)_2] + 4NaOH$$
  • The gold-cyanide complex is then separated from the gangue, and the gold is recovered by precipitation using zinc dust.

• Leaching of Aluminium:

  • Bauxite, the ore of aluminium, contains aluminium oxide (Al2O3) along with impurities. The Bayer process is used for leaching. In this process, bauxite is treated with a hot, concentrated solution of sodium hydroxide (NaOH). The sodium hydroxide dissolves the aluminium oxide but leaves behind the impurities as a residue. \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)_4}
  • The resulting solution of sodium aluminate is cooled, and aluminium hydroxide (Al(OH)3) is precipitated. The hydroxide is then calcined to obtain pure aluminium metal.

---

15. Discuss the role of cryolite in the extraction of aluminium and its advantages.

Answer: Cryolite (Na3AlF6) plays a crucial role in the extraction of aluminium through the electrolytic reduction process. In the Hall-Héroult process, cryolite serves as a solvent that lowers the melting point of alumina (Al2O3) and allows for easier and more efficient electrolysis.

Function of Cryolite:

1. Lowering the Melting Point:

   • Pure alumina (Al2O3) has a very high melting point (about 2050°C), making it difficult to melt. By dissolving alumina in cryolite, the melting point is reduced to around 950-1000°C, which is much more manageable for the electrolysis process.

2. Increasing Conductivity:

   • Cryolite enhances the electrical conductivity of the electrolyte, ensuring better current flow during electrolysis.

3. Improving the Efficiency of Electrolysis:

   • The process involves passing an electric current through molten cryolite, which is mixed with alumina. The aluminium ions (Al3+) migrate to the cathode where they are reduced to aluminium metal, and the oxygen ions (O2−) migrate to the anode where they are oxidized to form oxygen gas.

Advantages:

• Energy Saving: The use of cryolite reduces the energy required for melting the alumina, making the process more cost-effective.
• Efficient Metal Production: The process leads to the efficient extraction of aluminium at a lower cost compared to other methods.

---

16. What is the role of carbon in the extraction of metals like iron and zinc?

Answer: Carbon plays a crucial role in the reduction of metal ores, especially those containing metals with high reduction potentials like iron and zinc.

• Extraction of Iron:

  • In the blast furnace process, carbon (usually in the form of coke) is used to reduce iron(III) oxide (Fe2O3) to iron metal: $$Fe_2O_3 + 3C \rightarrow 2Fe + 3CO$$
  • In this reaction, carbon acts as a reducing agent, donating electrons to the iron ions, which results in the reduction of iron from its oxide form to pure iron.

• Extraction of Zinc:

  • Zinc oxide (ZnO) is reduced to zinc metal by carbon in a shaft furnace: $$ZnO + C \rightarrow Zn + CO$$
  • Carbon reduces zinc oxide to zinc metal by removing the oxygen and forming carbon monoxide (CO).

In both cases, carbon helps to reduce the metal from its oxide form, enabling its extraction.

---

17. Discuss the process of refining of impure silver and its comparison with the refining of impure copper.

Answer: Refining of impure silver and copper is essential to obtain high-purity metals. Though both processes involve electrolytic refining, there are differences due to the distinct properties of the metals.

• Refining of Impure Silver:
  • Impure silver (with impurities like gold, copper, and base metals) is refined by electrolytic refining. The silver is made the anode in an electrolytic cell, and a thin sheet of pure silver is made the cathode.
  • The electrolyte is a solution of silver nitrate (AgNO3). When current is passed through the cell, silver ions (Ag+) from the anode dissolve into the solution and are deposited on the cathode.
  • Impurities like copper and gold either remain at the bottom or dissolve as different complexes.
• Refining of Impure Copper:
  • In copper refining, impure copper is also made the anode, and pure copper is made the cathode. The electrolyte is copper(II) sulfate solution (CuSO4).
  • Copper ions (Cu2+) from the anode dissolve into the solution and are deposited at the cathode. Unlike silver, copper impurities such as iron, nickel, and gold may form sludge at the bottom of the electrolytic cell.

Comparison:

• Both processes rely on electrolytic methods but differ in the electrolyte used and the behavior of the impurities.
• Silver refining uses silver nitrate as the electrolyte, while copper refining uses copper sulfate.
• Copper refining is more prone to contamination by impurities, which requires more precise control.

---

18. How is zinc extracted from its ore, and what is the role of coke in its extraction?

Answer: Zinc is extracted from its principal ore, zinc blende (ZnS), using a process that involves roasting and reduction with coke.

1. Roasting:
   • Zinc blende is heated in the presence of oxygen to convert zinc sulfide (ZnS) to zinc oxide (ZnO): $$2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$$
2. Reduction:
   • The zinc oxide (ZnO) is then reduced with coke (carbon) at high temperatures in a shaft furnace: $$ZnO + C \rightarrow Zn + CO$$
   • In this step, coke acts as a reducing agent, removing oxygen from zinc oxide, and zinc is reduced to its metallic form while carbon monoxide (CO) is released.

19. Explain the extraction process of aluminium from bauxite using the Bayer process.

Answer: The extraction of aluminium from its ore, bauxite (primarily composed of Al2O3 or alumina), is carried out through the Bayer process. This process involves two main stages: leaching and precipitation.

1. Leaching of Bauxite:

   • Bauxite is treated with concentrated sodium hydroxide (NaOH) at high temperature and pressure. The sodium hydroxide reacts with the alumina (Al2O3) in the bauxite to form soluble sodium aluminate (NaAlO2), while the impurities (mainly silica) do not react and remain as an insoluble residue called red mud.
   • The reaction can be written as: $$Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2NaAl(OH)_4$$
   • The sodium aluminate solution is then separated from the red mud.

2. Precipitation of Alumina:

   • The solution of sodium aluminate is then cooled, and aluminium hydroxide (Al(OH)3) precipitates out from the solution: $$NaAl(OH)_4 \rightarrow Al(OH)_3 + NaOH$$
   • The aluminium hydroxide is filtered, washed, and then heated in a furnace (calcination) to produce pure alumina (Al2O3).

3. Electrolytic Reduction of Alumina:

   • The alumina is then subjected to the Hall-Héroult process, an electrolytic process, for the extraction of aluminium metal.
   • The alumina is dissolved in cryolite (Na3AlF6), which acts as a solvent, lowering the melting point and increasing the conductivity of the mixture.
   • Electrolysis is carried out at high temperatures, where aluminium ions are reduced at the cathode, and oxygen is liberated at the anode: $$2Al_2O_3 \rightarrow 4Al + 3O_2$$

This process efficiently yields high-purity aluminium from bauxite.

---

20. Describe the method of extraction of iron from its ore in a blast furnace.

Answer: The extraction of iron from its ore, haematite (Fe2O3), is carried out in a blast furnace using the reduction process. The main reactions that take place are as follows:

1. Preparation of the Furnace:

   • A blast furnace is a tall, vertical shaft, where a continuous blast of air (oxygen) is introduced at the base.
   • The furnace is charged with a mixture of haematite, coke, and limestone (CaCO3) in alternate layers.

2. Roasting of Haematite:

   • The haematite (Fe2O3) undergoes reduction at high temperatures with the help of carbon (from coke). The coke is burnt in the presence of oxygen, producing carbon monoxide (CO), which acts as a reducing agent to reduce iron oxide to iron. $$Fe_2O_3 + 3C \rightarrow 2Fe + 3CO$$
   • The carbon monoxide reduces the iron oxide to iron metal.

3. Formation of Slag:

   • Limestone (CaCO3) is added as a flux to remove silica (SiO2) impurities from the ore. The reaction between limestone and silica forms calcium silicate (CaSiO3), which is a slag that can be separated from the molten iron. $$CaCO_3 + SiO_2 \rightarrow CaSiO_3 + CO_2$$
   • The molten slag floats on top of the molten iron, and both are tapped separately.

4. Tapping the Iron:

   • The molten iron, which is now in a liquid state, is removed from the furnace. This iron is referred to as pig iron.

---

21. Explain the process of refining of copper by the electrolytic method.

Answer: The electrolytic refining of copper is done to obtain high-purity copper metal. The process involves the following steps:

1. Electrolyte:

   • The electrolyte used in the electrolytic refining of copper is a solution of copper(II) sulfate (CuSO4). The presence of copper ions in the electrolyte allows copper to dissolve from the anode and get deposited on the cathode.

2. Anode:

   • The impure copper, which may contain iron, nickel, and other impurities, is made the anode in the electrolytic cell.

3. Cathode:

   • A thin sheet of pure copper is used as the cathode. During electrolysis, copper ions (Cu2+) are reduced at the cathode and get deposited as pure copper. $$Cu^{2+} + 2e^- \rightarrow Cu(s)$$

4. Process:

   • When the electric current passes through the electrolyte, copper ions from the impure copper anode dissolve into the electrolyte, and pure copper is deposited on the cathode.
   • Impurities like iron, nickel, and silver either remain at the bottom of the electrolytic cell as sludge or form insoluble complexes.

The pure copper thus obtained is of high quality and can be used for electrical and industrial applications.

---

22. What is the role of a reducing agent in the extraction of metals, and how does it apply to the extraction of zinc?

Answer: A reducing agent is a substance that donates electrons in a chemical reaction, thus reducing another substance by removing oxygen or adding electrons to it. In the extraction of metals, a reducing agent is used to convert the metal ions in their oxidized form (usually in an ore) into their reduced form (metallic state).

In the case of zinc extraction, the following steps take place:

1. Roasting of Zinc Ore:

   • Zinc is usually obtained from zinc blende (ZnS). First, the ore is roasted in the presence of oxygen to convert zinc sulfide (ZnS) to zinc oxide (ZnO): $$2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$$

2. Reduction of Zinc Oxide:

   • The zinc oxide (ZnO) is then reduced to zinc metal using coke (carbon) as the reducing agent: $$ZnO + C \rightarrow Zn + CO$$
   • Here, coke (carbon) donates electrons to the zinc oxide, reducing the zinc ions (Zn2+) to form zinc metal.

Thus, the reducing agent (coke) is vital for converting the metal from its oxide form into its pure, metallic form.

---

23. Explain the process of extraction of magnesium from its ore.

Answer: The extraction of magnesium is done from its ore magnesite (MgCO3) using electrolysis of molten magnesium chloride (MgCl2). The process involves the following steps:

1. Preparation of Magnesium Chloride:

   • First, magnesite (MgCO3) is heated strongly to decompose it into magnesium oxide (MgO) and carbon dioxide (CO2): $$MgCO_3 \rightarrow MgO + CO_2$$

2. Electrolysis of Magnesium Chloride:

   • The magnesium oxide (MgO) is then mixed with carbon and heated in a furnace. This converts the oxide into magnesium chloride (MgCl2): $$MgO + C \rightarrow MgCl_2 + CO$$
   • Magnesium chloride (MgCl2) is then subjected to electrolytic reduction in a downward cell (molten electrolysis). During the process, the magnesium ions (Mg2+) migrate to the cathode where they are reduced to metallic magnesium, and chlorine ions (Cl-) migrate to the anode, where they are oxidized to chlorine gas: $$Mg^{2+} + 2e^- \rightarrow Mg(s) \quad \text{(at the cathode)}$$ $$2Cl^- \rightarrow Cl_2 + 2e^- \quad \text{(at the anode)}$$

The magnesium metal is thus extracted at the cathode in its pure form.

---

24. What is the role of slag in the extraction of metals?

Answer: Slag is the non-metallic byproduct that forms during the extraction of metals. It consists of impurities, mainly the gangue (unwanted minerals), which react with flux (a substance added to the furnace to facilitate the extraction process) to form a liquid mass that can be easily removed. The role of slag in metallurgy is as follows:

1. Removal of Impurities:

   • During the extraction of metals, impurities such as silica (SiO2) often need to be removed. Slag is formed when these impurities combine with flux (e.g., limestone) and form compounds like calcium silicate (CaSiO3), which can be easily separated from the metal.

2. Protecting the Metal:

   • Slag can also act as a protective layer over the molten metal to prevent its oxidation by air. The solidified slag is separated from the metal at the bottom of the furnace.

For example, in the extraction of iron, slag forms from the reaction of silica (impurity in haematite) with lime (CaO, the flux), forming calcium silicate (CaSiO3). This slag floats on top of the molten iron.

---

25. How is the extraction of copper carried out from its ore, chalcopyrite, and what role does roasting play in the process?

Answer: Copper is extracted from its ore, chalcopyrite (CuFeS2), using a combination of roasting, smelting, and electrolytic refining. Here's how the process works:

1. Roasting of Chalcopyrite:

   • The first step involves roasting the chalcopyrite ore in the presence of oxygen. During roasting, sulfur is removed as sulfur dioxide (SO2), and copper is converted to copper(I) oxide (Cu2O). $$2CuFeS_2 + 3O_2 \rightarrow 2Cu_2O + 2SO_2 + 2FeO$$

2. Smelting:

   • The copper(I) oxide (Cu2O) is then subjected to smelting with silica (flux) to remove the iron as slag, resulting in the formation of copper matte (a mixture of copper(I) sulfide and iron sulfide).

3. Reduction:

   • The copper matte is then subjected to reduction using carbon to produce copper metal: $$Cu_2S + 2C \rightarrow 2Cu + CO_2$$

4. Electrolytic Refining:

   • Finally, the impure copper obtained from smelting is purified using electrolytic refining, where the impure copper is used as the anode, and pure copper is deposited at the cathode.

Roasting plays a crucial role in converting the sulfide ores to oxide forms that can be reduced to pure copper in the subsequent steps.