chemistry : class12 Selective Flotation : Concentration of ores : General Principles and Processes of Isolation of Elements MCQ Questions and Answers
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Selective Flotation
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MCQ Questions
1. During selective flotation of a galena (PbS)–sphalerite (ZnS) mixture, NaCN is added before introducing a xanthate collector. What is the principal purpose of NaCN?
A.
To selectively depress sphalerite while allowing galena to float
B.
To selectively depress galena while allowing sphalerite to float
C.
To convert both sulphide minerals into soluble cyanide complexes
D.
To increase the frothing power of the xanthate collector
Correct Answer:
To selectively depress sphalerite while allowing galena to float
Explanation:
In selective flotation of galena and sphalerite, NaCN acts as a depressant for ZnS under suitable conditions. Galena is then preferentially collected by the xanthate and rises with the froth.
2. After galena has been removed from a galena–sphalerite ore by depressing sphalerite with NaCN, which treatment is most appropriate for recovering sphalerite from the residue?
A.
Add more NaCN and continue flotation
B.
Use CuSO4 as an activator followed by a xanthate collector
C.
Add lime alone and remove the froth
D.
Add pine oil without any collector
Correct Answer:
Use CuSO4 as an activator followed by a xanthate collector
Explanation:
CuSO4 activates sphalerite by promoting formation of a copper sulphide-like surface that can react with xanthate. The activated ZnS then becomes floatable, while pine oil serves mainly as a frother rather than the principal collector.
3. In differential flotation of a mixture containing PbS and ZnS, which sequence best represents the usual selective separation strategy?
A.
Depress PbS with NaCN, float ZnS, then activate PbS with CuSO4
B.
Depress ZnS with NaCN, float PbS, then activate the residual ZnS with CuSO4
C.
Float both sulphides together, then separate them by gravity
D.
Depress both sulphides with lime, then float them simultaneously using pine oil
Correct Answer:
Depress ZnS with NaCN, float PbS, then activate the residual ZnS with CuSO4
Explanation:
Selective flotation first exploits the depression of sphalerite by NaCN so that galena can be floated. After galena removal, CuSO4 activation makes the remaining sphalerite responsive to the collector.
4. Which statement correctly distinguishes the roles of a collector and a frother in selective flotation?
A.
The collector stabilizes bubbles, whereas the frother selectively reacts with the mineral surface
B.
The collector renders selected mineral particles water-repellent, whereas the frother produces and stabilizes a suitable froth
C.
Both collector and frother function only by depressing unwanted minerals
D.
The collector dissolves gangue, whereas the frother precipitates sulphide minerals
Correct Answer:
The collector renders selected mineral particles water-repellent, whereas the frother produces and stabilizes a suitable froth
Explanation:
In flotation, collectors such as xanthates adsorb on selected sulphide mineral surfaces and impart hydrophobicity. Frothers such as pine oil help generate stable bubbles and froth that carry the hydrophobic particles.
5. Why can NaCN permit selective flotation of galena from a galena–sphalerite mixture when xanthate is present?
A.
NaCN increases the density of galena and causes it to settle rapidly
B.
NaCN changes all mineral surfaces into hydrophilic oxides
C.
NaCN preferentially suppresses xanthate interaction with sphalerite under the selected conditions
D.
NaCN acts as a frother and carries only galena bubbles upward
Correct Answer:
NaCN preferentially suppresses xanthate interaction with sphalerite under the selected conditions
Explanation:
Selective flotation depends on differential collector adsorption. NaCN depresses sphalerite by preventing or reducing the surface reactions needed for effective xanthate adsorption, while galena remains sufficiently floatable.
6. A flotation test gives a froth containing both PbS and ZnS instead of predominantly PbS after NaCN addition. Which change is most directly relevant to improving selectivity?
A.
Increase the amount of pine oil without changing reagent conditions
B.
Optimize the NaCN dosage and pulp conditions to strengthen sphalerite depression
C.
Replace the collector with an inert electrolyte
D.
Increase the particle density by adding gangue
Correct Answer:
Optimize the NaCN dosage and pulp conditions to strengthen sphalerite depression
Explanation:
Co-flotation indicates inadequate selectivity, often because sphalerite has not been sufficiently depressed. Adjusting depressant dosage and related pulp conditions is directly connected with differential flotation, whereas merely increasing frother can worsen entrainment.
7. In a selective flotation circuit, why is sphalerite commonly floated after galena rather than simultaneously with it?
A.
Sphalerite cannot be activated until the galena-rich froth has been removed
B.
Galena is soluble only after sphalerite has been floated
C.
Pine oil destroys sphalerite if it is added before galena removal
D.
Xanthate can collect only oxide minerals before sulphide minerals
Correct Answer:
Sphalerite cannot be activated until the galena-rich froth has been removed
Explanation:
The sequence is designed to avoid simultaneous collection. Sphalerite is first depressed, galena is removed, and then the residual sphalerite is activated, commonly with CuSO4, so it can be selectively floated in a later stage.
8. Which reagent combination is most logically associated with floating a selected sulphide mineral while maintaining a stable froth in a selective flotation operation?
A.
Xanthate as collector and pine oil as frother
B.
NaCN as collector and CuSO4 as frother
C.
Pine oil as depressant and lime as collector
D.
CuSO4 as frother and xanthate as gangue dispersant
Correct Answer:
Xanthate as collector and pine oil as frother
Explanation:
Xanthates are collectors for sulphide minerals because they make selected particles hydrophobic. Pine oil is used as a frother to produce stable bubbles that transport those particles to the surface.
9. If CuSO4 is added to a galena–sphalerite pulp before the intended galena flotation step, what selective-flotation problem may result?
A.
Sphalerite may become activated and float along with galena
B.
Galena will necessarily become completely soluble
C.
The frother will be converted into a depressant
D.
Gangue particles will all become magnetic and separate from the pulp
Correct Answer:
Sphalerite may become activated and float along with galena
Explanation:
CuSO4 is an activator for sphalerite. Adding it before galena–sphalerite separation can make ZnS responsive to xanthate despite the intended selective depression, thereby reducing the selectivity of the galena flotation stage.
10. Which observation most strongly indicates successful selective flotation of galena from a pulp containing galena and sphalerite?
A.
The first froth is rich in galena, while most sphalerite remains in the tailing until activation
B.
Both galena and sphalerite report equally to the first froth
C.
All gangue enters the froth while both sulphides remain in the pulp
D.
Sphalerite floats first and galena remains permanently depressed
Correct Answer:
The first froth is rich in galena, while most sphalerite remains in the tailing until activation
Explanation:
Successful differential flotation is demonstrated by preferential recovery of one sulphide in the initial froth and retention of the other in the pulp. The retained sphalerite can subsequently be activated and floated separately.
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