This set of Extractive Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Hydrometallurgy Applications”.
1. Which of the method can be used to get elemental metal from aqueous solutions?
a) Electrolysis
b) Solvent Extraction
c) Ion-exchange process
d) Fractional distillation
View Answer
Explanation: Metallic ions from the aqueous solution can be reduced to the elemental metal after the purification process. This can be done using cementation method, electrolysis and gaseous reduction method.
2. What does Eh-pH diagram describe?
a) Equilibria at various Eh values
b) Equilibria at various pH values
c) The temperature difference of the stability of compounds
d) The pressure difference in the stability of compounds
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Explanation: The Eh-pH diagram describes the equilibrium phases of an aqueous electrochemical system. It presents the equilibria at various pH values and potential values. It also presents electrical potentials between metal, metal ions, solid oxides and hydroxides for a system.
3. Why is the process of contact reduction also called cementation?
a) Precipitation does not occur in this process
b) Cement is used for the process
c) Precipitation occurs on the surface of the metal to be reduced
d) Precipitation occurs on the surface of added metal
View Answer
Explanation: Contact reduction is the process of precipitation of a metal from the aqueous solution of the metal salts by adding another metal. Now, the precipitation occurs on the surface of the metal to be added. This is the reason this process is called cementation.
4. In the electromotive series, the metal with more negative oxidation potential displaces the metal with more positive potential.
a) True
b) False
View Answer
Explanation: The given statement is incorrect. In the electromotive series, the metal with more positive oxidation potential will pass through the solution and displace the metal with less positive potential. This basic mechanism of electrochemistry is used to predict the process of cementation.
5. What is the electrode potential called which is less than the equilibrium potential of cathode but greater than the equilibrium potential of anode?
a) Unique potential
b) Contact potential
c) Compromise potential
d) Component potential
View Answer
Explanation: In a system, due to a continuous flow of electrons from anode to cathode, the system acquires a unique electrode potential. This potential is less than the equilibrium potential of cathode but greater than the equilibrium potential of anode. This potential is called compromise potential.
6. The oxidation potentials taken from standard electrochemical series are valid for unit ion concentrations only.
a) True
b) False
View Answer
Explanation: The statement is correct. The elements in the electrochemical series are arranged for a specific temperature of 25°C and concentration. In the solution, the metallic ion concentration is taken as unity. This is the reason oxidation potential taken from standard electrochemical series is valid for unit concentrations only.
7. How to obtain a high driving force for a cementation reaction?
a) By using a non-reactive metal
b) By using a highly reactive metal
c) By not using metal for the reaction
d) By increasing temperature
View Answer
Explanation: For an effective cementation reaction, its driving force should be high. This can be done by using a highly reactive metal. This will provide a reaction with high anode potential.
8. Which zinc is preferred over aluminium for the cementation of gold?
a) Zinc is placed higher on electrochemical series
b) Zinc is placed lower on electrochemical series
c) Zinc can be easily removed after the reaction
d) Zinc is very difficult to remove after the reaction
View Answer
Explanation: Aluminium is placed higher than zinc in the electrochemical series but still zinc is preferred for the cementation of gold. The precipitate of gold always tends to trap the reducing agent within it. So, zinc can be easily removed after the reaction using volatilization but it is very difficult to remove aluminium.
9. Which of the following does not use products from gaseous reduction method?
a) Powder metallurgy
b) Catalysis
c) Plasma-arc
d) Electrolysis
View Answer
Explanation: Gaseous reduction method is based on the properties of metal ions in aqueous solutions. The products of gaseous reduction are produced in the form of metallic powders. These products have applications in powder metallurgy, catalysis, dispersion strengthening of metals, plasma-arc and flame-sprayed coatings.
10. How can the concept of standard oxidation potential be misleading?
a) Reaction reverse its direction when H+ acidity is low
b) Reaction reverse its direction when H+ acidity is high
c) Reaction reverse its direction when hydrogen pressure is low
d) The reaction does not occur when the temperature is high
View Answer
Explanation: According to the electrochemical series, the metal ion which is more electropositive dissolves easily. This concept of standard oxidation potential can be misleading sometimes. So, two factors are considered before a thermodynamic analysis which is hydrogen pressure and H+ acidity. The reaction tends to reverse when the hydrogen pressure is high and H+ acidity is low.
11. Which of the following is one of the kinetic steps of a gaseous reduction process?
a) Mass transfer from cathode to anode
b) Mass transfer from anode to cathode
c) Nucleation of metal in a clear solution
d) Growth of non-metal particles
View Answer
Explanation: The gaseous reduction reactions consists of two kinetic steps. The first step is the nucleation of metal in a clear solution or on a solid substrate. The second step is the growth of metal particles by deposition of metal on its surface itself.
12. Which of the following is homogeneous nucleation?
a) Nucleation of metal on a solid substrate
b) Nucleation of metal in a clear solution
c) The wall of a container
d) The interphase boundary in a liquid or solid
View Answer
Explanation: Homogeneous nucleation occurs when precipitates form in random in a perfect lattice. Nucleation of metal in a clear solution is an example of homogeneous nucleation. During the gaseous reduction process, homogeneous nucleation is very difficult to achieve.
13. How can nucleation be increased any further?
a) By adding some inorganic compounds
b) By adding nitrogen
c) By adding oxygen
d) By adding some organic compounds
View Answer
Explanation: It is very difficult to achieve homogeneous nucleation during hydrogen reduction. Immediate nucleation can be achieved by adding some metal powder. To increase nucleation any further, some organic compounds can be added. These organic compounds act as surface-active reagents.
14. The addition agents in the gaseous reduction help to increase which of the following?
a) Induction time for nucleation
b) Agglomeration
c) Uniformity of coating
d) Non-crystalline product
View Answer
Explanation: The addition agents in the gaseous reduction helps in its nucleation kinetics. They eliminate the induction for nucleation but they cannot alter the rate of reaction. They also help in anti-agglomeration, uniformity of coating and in providing crystalline products.
15. Which of the following can increase the reaction rate for homogeneous precipitation?
a) Increasing temperature
b) Decreasing temperature
c) Decreasing hydrogen pressure
d) The increased surface area of solid catalyst
View Answer
Explanation: The reaction rate increases with the increasing temperature and hydrogen pressure for both homogeneous and heterogeneous precipitation. The rate of reaction also depends on initial metal ion concentration for homogeneous nucleation but independent for heterogeneous nucleation. The increased surface area of solid catalyst can increase the reaction rate for heterogeneous nucleation.
Sanfoundry Global Education & Learning Series – Extractive Metallurgy.
To practice all areas of Extractive Metallurgy, here is complete set of Multiple Choice Questions and Answers.
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