6 Most SAQ’s of General Principles of Metallurgy Chapter in Inter 2nd Year Chemistry (TS/AP)

4 Marks

SAQ-1 : Differentiate roasting and calcination with examples.

Introduction

In metallurgy, the preparation of ores for metal extraction involves various techniques, notably roasting and calcination. Despite both methods involving the heating of ore, their purposes, conditions, and applications differ significantly.

Roasting

1. Process: Roasting is the process of heating ore in the presence of air or oxygen.
2. Applicability: It is primarily used for sulphide ores.
3. Purpose: The main aim is to convert sulphide ores into their oxide forms.
4. Example: The transformation of zinc sulphide into zinc oxide illustrates roasting: $$2ZnS + 3O_2 \overset{Heat}{\rightarrow} 2ZnO + 2SO_2(g)$$

Calcination

1. Process: Calcination involves heating the ore in the absence of air or oxygen.
2. Applicability: This technique is mainly applied to carbonate ores.
3. Purpose: Its primary objective is to convert carbonate ores into their oxide counterparts.
4. Example: The conversion of calcium carbonate to calcium oxide serves as an example of calcination: $$CaCO_3 \overset{Heat}{\rightarrow} CaO + CO_2$$

Summary

Roasting and calcination are critical processes in metallurgy for ore preparation. Roasting is utilized for sulphide ores, converting them to oxides by using air or oxygen, whereas calcination treats carbonate ores, transforming them into oxides in the absence of air. Understanding the distinctions between these methods is crucial for comprehending the broader spectrum of ore processing and metal extraction techniques.

---

SAQ-2 : Explain the purification of sulphide ore by froth floatation method.

Introduction

The extraction of metals from their ores is a pivotal process in metallurgy, where the froth flotation method plays a crucial role, especially for sulphide ores, in separating unwanted impurities or gangue.

Froth Flotation Method

1. Purpose: This technique is specifically designed for separating gangue from sulphide ores.
2. Preparation: The ore is finely powdered and mixed with water to form a suspension.
3. Additives
   • Froth collectors like pine oil are added to aid in froth formation, carrying ore particles.
   • Stabilizers such as cresols are introduced to stabilize the froth.
4. Wetting: The ore particles are wetted by oil, repelling water, while the gangue is wetted by water, repelling oil.
5. Agitation: A rotating paddle agitates the mixture, with air blown in, to ensure even mixing and froth formation.
6. Formation of Froth: The agitation and presence of collectors lead to froth formation on the surface, enriched with ore particles.
7. Skimming: The froth, carrying the mineral or ore particles, rises and is skimmed or scraped off for collection.
8. Collection: Sulphide ore particles are separated from the froth, leaving the gangue in the water.

Summary

The froth flotation method is an advanced technique for extracting valuable sulphide ore particles from gangue. Utilizing collectors and stabilizers, along with processes like wetting, agitation, and froth formation, this method efficiently purifies sulphide ores, facilitating subsequent metal extraction steps.

---

SAQ-3 : Explain briefly the extraction of Aluminium from Bauxite.

Introduction

Extracting aluminium from bauxite is a key process in metallurgy, transforming bauxite ore into valuable aluminium metal through several steps.

Steps to Extract Aluminium from Bauxite

1. Concentration
   • The process starts with crushing bauxite ore into a powder.
   • The powdered ore is treated with sodium hydroxide solution (NaOH) at temperatures between 473K and 523K, forming sodium aluminate.
   • Chemical Reaction:
     $$Al_2O_3 (solid) + 2NaOH (aqueous) + 3H_2O (liquid) \rightarrow 2NaAl(OH)_4$$
2. Neutralization
   • Neutralizing the sodium aluminate solution involves passing CO₂ gas through it, forming hydrated alumina.
   • Chemical Reaction: $$2NaAl(OH)_4 + CO_2 (gas) \rightarrow Al_2O_3.xH_2O (solid) + 2NaHCO_3 (aqueous)$$
3. Formation of Pure Alumina
   • The hydrated alumina is filtered, dried, and heated at 1470K to transform it into pure alumina (Al₂O₃).
   • Reaction: $$Al_2O_3.xH_2O (solid) \rightarrow Al_2O_3 (solid) + xH_2O (gas)$$
4. Electrolytic Refining
   • Electrolysis of pure alumina yields pure aluminium metal.
   • This step involves the electrolytic refining where alumina is dissolved in molten cryolite and then electrolyzed to produce aluminium.

Summary

The extraction of aluminium from bauxite involves a sequence of chemical processes: starting with concentration, followed by neutralization to form alumina, and culminating in electrolytic refining to obtain pure aluminium metal. This comprehensive breakdown is crucial for students and professionals in metallurgy and chemistry to understand the intricacies of metal extraction from ores.

---

SAQ-4 : Write any 2 ores with the formulae of the following metals? (1) Al (2) Zn (3) Fe (4) Cu

Introduction

Understanding the primary ores from which metals are extracted is fundamental in metallurgy. Here are two key ores for each of the following metals:

1. Aluminium (Al)
   • Bauxite: $$\textbf{Al}_2\textbf{O}_3 \cdot 2\textbf{H}_2\textbf{O}$$
   • Cryolite: $$\textbf{Na}_3\textbf{AlF}_6​$$
2. Zinc (Zn)
   • Sphalerite (Zinc Blende): $$\textbf{ZnS}$$
   • Smithsonite: $$\textbf{ZnCO}_3$$
3. Iron (Fe)
   • Hematite: $$\textbf{Fe}_2\textbf{O}_3$$
   • Magnetite: $$\textbf{Fe}_3\textbf{O}_4$$
4. Copper (Cu)
   • Chalcopyrite: $$\textbf{CuFeS}_2$$
   • Malachite: $$\textbf{Cu}_2\textbf{(CO}_3\textbf{)(OH)}_2$$

Summary

These ores are the starting materials for the extraction of Aluminium (Al), Zinc (Zn), Iron (Fe), and Copper (Cu). Understanding the formulae and characteristics of these ores is crucial for the processes of metal extraction and refinement.

---

SAQ-5 : Outline the principles of refining of metals by the following methods.
1) Zone refining 2) Electrolytic refining 3) Poling 4) Vapour phase refining

Introduction

Refining metals is crucial for enhancing their utility and value by removing impurities. Various techniques are adapted for refining, tailored to the metal’s specific properties and impurity types.

Metal Refining Principles

1. Zone Refining
   • Principle: Impurities are more soluble in the molten state of the metal than in its solid state.
   • Usage: Predominantly used for high-purity metals like Germanium (Ge), Silicon (Si), and Boron (B).
2. Electrolytic Refining
   • Principle: Employs electrolysis, where the impure metal is the anode and the pure metal is the cathode. Electricity drives the transfer, leaving impurities at the anode and depositing pure metal at the cathode.
   • Usage: Suitable for metals like Aluminium (Al), Zinc (Zn), as well as precious metals like Gold and Silver.
3. Poling
   • Principle: Involves stirring the metal with green wood logs; the gases released (primarily hydrogen) reduce the oxide impurities in the metal.
   • Usage: Mainly used for refining Blister copper.
4. Vapour Phase Refining
   • Principle: Converts the metal into a volatile compound which is then decomposed to yield the pure metal. This method hinges on the metal’s ability to form and then decompose a volatile compound.
   • Usage: Utilized for purifying Nickel (Ni), especially when containing impurities.

Summary

Refining is a pivotal process in obtaining metals in their purest forms. Techniques like Zone refining, Electrolytic refining, Poling, and Vapour phase refining are selected based on the metal’s nature and impurity content. Grasping these principles is vital for those in the metallurgy and chemistry fields, ensuring the choice of an appropriate refining method for each specific metal.

---

SAQ-6 : Explain the extraction of zinc from zinc blende.

Introduction

The process of extracting zinc from its ore, zinc blende (sphalerite), involves several steps, from concentration to the final extraction of pure zinc.

1. Concentration of the Ore: Zinc blende is primarily zinc sulfide (ZnS). The first step involves roasting the ore in the presence of oxygen to convert zinc sulfide to zinc oxide (ZnO) while releasing sulfur dioxide (SO₂) gas. $$2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2​$$
2. Reduction of Zinc Oxide
   • Distillation: The zinc oxide obtained from roasting is then mixed with coke (carbon) and heated to a high temperature. This reduces zinc oxide to zinc vapor in a process known as reduction. $$ZnO + C \rightarrow Zn_{(vapor)} + CO$$
   • Condensation: The zinc vapor is then condensed to liquid zinc, which is further cooled to obtain solid zinc metal.
3. Purification: The zinc obtained from distillation is further purified by electrolytic refining to achieve high purity zinc metal. In this process, impure zinc serves as the anode, while a pure zinc plate serves as the cathode. Upon applying electricity, zinc ions move towards the cathode and deposit pure zinc.

Summary

The extraction of zinc from zinc blende involves roasting the ore to convert zinc sulfide to oxide, followed by reduction with coke to produce zinc vapor, which is then condensed into solid zinc. The final step often includes electrolytic refining to purify the zinc to a high degree of purity. This sequence of processes efficiently converts zinc blende into usable zinc metal, highlighting the essential methods in metallurgy for zinc extraction.