Question

Sieving efficiency decreases significantly when

• A. Particle size $\approx$ aperture size
• B. Particle size $\gg$ aperture size
• C. Particles are perfectly spherical
• D. Moisture content is zero

Hint:

Particles whose dimensions are within $\pm 10\%$ of the screen opening size are termed "critical near-size particles." They are the primary cause of screen clogging (blinding) in industrial sieves.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the physical condition relating particle size and screen opening (aperture) size that causes the overall efficiency of a sieving (screening) operation to drop.

Step 2: Detailed Explanation:

• Sieving Principle: Sieving is a unit operation designed to separate a mixture of solid particles into size fractions by passing them through screens containing specific opening sizes.

• Near-Mesh/Near-Size Particles: When the size of the particles is extremely close to the aperture size of the sieve screen (i.e., Particle size $\approx$ aperture size), sieving efficiency decreases dramatically.

• Blinding of Screens: These "near-size" particles easily lodge inside the screen openings instead of passing through or bouncing off. This phenomenon is known as "blinding" or "pegging."

• Obstruction of Flow: Once screen apertures are blocked by wedged particles, the effective open area of the sieve decreases, preventing smaller particles from passing through and reducing separation efficiency.

• Why Other Options are Incorrect:

• Particle size $\gg$ aperture size: Very large particles are easily retained on top of the screen and do not clog the apertures, maintaining high efficiency for the undersize fraction.

• Perfectly spherical particles: Spherical shapes actually increase sieving efficiency because they pass through openings more easily than irregular, elongated, or flat particles.

• Moisture content is zero: Dry materials screen much more efficiently than damp materials, which tend to agglomerate and cause screen blinding.

Step 3: Final Answer:
Sieving efficiency drops when particle size is approximately equal to the screen aperture size due to screen blinding, corresponding to option (A).