This set of Class 12 Chemistry Chapter 6 Multiple Choice Questions & Answers (MCQs) focuses on “Concentration of Ores”.
1. Concentration of ores is also known as ore-dressing.
Explanation: The removal of unwanted earthy and silicious impurities (that is, gangue or matrix) from an ore is called concentration of ores. Concentration of ores is also known as ore-dressing.
2. Which of the cases is suitable for concentration by hand picking?
a) When either the ore or the impurities are magnetic
b) When the ore particles are heavier than the impurities
c) When the ores are good conductors of electricity
d) When the impurities can be distinguished from the ore by the naked eye
Explanation: In the case of concentration by hand picking, it is necessary that the impurities be easily distinguishable from the particles of the ore. Only if these impurities can be distinguished by the naked eye, is the ore said to be suitable for concentration by hand picking.
3. Which of the following equipment is used for concentration by hydraulic washing?
a) Stamp mill
b) Ball mill
c) Wilfley tables
d) Magnetic roller
Explanation: Hydraulic washing is usually used for concentration of ores when the particles of the ore have a higher mass compared to the earthy or rocky gangue particles. The process is carried out in specially designed tables called Wilfley tables.
4. Ores obtained from the Earth’s crust are always pure.
Explanation: Ores, as they are obtained from the Earth’s crust, are never pure. They are usually associated with earthy and silicious impurities (along with impurities of other minerals) called gangue or matrix.
5. Which of the following ores cannot be concentrated by hydraulic washing?
d) Native ores of silver
Explanation: Concentration by hydraulic washing requires the ore particles to have a higher mass compared to its gangue particles. Hydraulic washing can be used for oxide ores of iron and tin such as haematite and tinstone and also for native ores of silver and gold. Pyrolusite is concentrated by magnetic separation.
6. Which of the following cannot be concentrated by electromagnetic separation?
d) Zinc blende
Explanation: Electromagnetic separation is used to concentrate an ore when either the particles of the ore or the impurities present in it are magnetic in nature. Chromite, magnetite and pyrolusite have magnetic nature and are separated from their non-magnetic gangue particles using this method. Zinc blende is concentrated using electrostatic separation or froth floatation.
7. Where are the magnetic particles collected in concentration by electromagnetic separation?
a) Below the magnetic roller
b) Above the magnetic roller
c) Away from the magnetic roller
d) On the conveyer belt
Explanation: In electromagnetic separation, the powdered ore is dropped over a conveyer belt moving around two rollers, one of which has an electromagnet in it. As the ore particles roll over the belt, the magnetic particles are attracted by the magnetic roller. As a result, two heaps are formed separately. The heap collected below the magnetic roller contains magnetic particles while the heap away from the roller consists of non-magnetic particles.
8. What is the property of necessary for an ore to be concentrated by electrostatic separation?
a) The ore particles should be heavier than the gangue particles
b) The ore particles should be magnetic in nature
c) The ores should conduct electricity
d) It should be a sulphide ore
Explanation: Electrostatic separation is a method used for concentration or separation of ores when they are good conductors of electricity. It is based on the principle that when an electrostatic field is applied the ore particles, which conduct electricity, get charged and get repelled by electrodes having same charge and all thrown away.
9. Which of the following are collectors used in the froth floatation process?
b) Pine oil
c) Ethyl xanthate
d) Potassium ethyl xanthates
Explanation: Froth floatation is generally used for sulphide ores. It is based on the fact that sulphide ores are preferentially wetted by oils while that of the gangue is wetted by water. To a suspension of the crushed ore in water, collectors are added to enhance the non-wettability of the ore particles. Pine oil, ethyl xanthate and potassium ethyl xanthates are examples of collectors.
10. What is the role of the rotating paddle in froth floatation?
a) Enhances wettability of gangue particles
b) Stabilizes the froth
c) Draws in air causing frothing
d) Enhances non-wettability of the ore particles
Explanation: In froth floatation process, the suspension of the powdered ore in water along with the collectors and froth stabilizers is violently agitated by the rotating paddle which draws in air, causing frothing.
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