Bernado

Question 1

5 factors of hindered settling

Answer 1

1. The interactions of particles-particles and particles-vessel wall is significant.
2. The large particles are hindered by the small particles, which increase the effective resistance of the suspending medium for the large particles.
3. The upward velocity of the displaced fluid flowing in the interstices between the particles is significant, so the apparent settling velocity (relative to a fixed point) is significantly lower.
4. The velocity gradients in the suspending fluid flowing upward between the particles are increased (since the area available for flow is now smaller), resulting in greater shear forces.
5. Because of the high surface area to volume ratio for small particles, surface forces (e.g. shear) are important, resulting in flocculation and ‘‘clumping’’ of the smaller particles into larger effective particle groups.

Question 2

Modes of Settling

Case a)

Answer 2

Case (a) corresponds to a suspension with a range of particle size ratios less than about 6:1. In this case, all the particles settle at about the same velocity in the constant composition zone (B), leaving a layer of clear liquid (A) above. As the sediment (D) builds up, however, the liquid that is squeezed out of this layer serves to further retard the particles just above it, resulting in a zone of variable composition (C).

Question 3

Modes of Settling

Case b)

Answer 3

Case (b) in Figure 3 is less common and corresponds to a broad particle size range, in which the larger particles settle at a rate significantly greater than that of the smaller ones, and consequently there is no constant composition zone.