JW Lecture 5 - Mixing & Sampling

Question 1

Preferred state for particles of different size and density

Answer 1

Segregated

Question 2

How mixing is achieved?

Answer 2

Small volumes of the unmixed mass are moved against each other in a way that the different components rest next to each other by the end of the operation.

Question 3

Types of mixtures

Answer 3

A perfect mixture of two types of particles is one in which a group of particles taken from any position in the mixture will contain the same proportions of each particle as the proportions present in the while mixture. However, in practise the aim is to produce a random mixture.

Question 4

Mechanisms of mixing

Answer 4

a) Convective mixing: Groups of particles are moved from one position to another within the powder mass.
b) Diffusion mixing: Occurs when particles roll down a sloping surface.
c) Shear mixing: Shear stresses give rise to slip zones and mixing takes place by
interchange of particles between layers within the zone.

Question 5

Types of mixers

Answer 5

• Tumbling mixers: consists of a closed vessel rotating around its axis. The dominant mechanism is diffusive mixing. Baffles can be incorporated to reduce the segregation.
• Corrective mixers: where circulation patterns are set up within a static shell by rotating blades or paddles. The main mechanism is corrective mixing although it is accompanied by some diffusive and shear mixing
• Fluidised bed mixers: rely on the natural mobility afforded particles in the fluidised bed. The mixing is largely convective.
• High shear mixers: local high shear stresses are created by devices such as high velocity rotating blades. The dominant mechanism is shear mixing.

Question 6

Objectives of obtaining statistical parameters on mixing processed

Answer 6

• quantifying the goodness of a mixture and describing the current state of a mixture
• assessing the acceptability of mixtures regarding product specification e.g quality control and risk assessment
• determination of mixing times and evaluation of different mixers

Question 7

Three sampling procedures for a mixture of A and B

Answer 7

• Random sampling: each component has the same chance to be present in a sample or each sample has the same chance to feature the current composition of the mixture
• Representative sampling for large populations/entities: number of particles z in the sample is much larger than 1 but much smaller that the total number Z of all particles in the mixture (otherwise sampling would change the mixture)
  1 &laquo_space;z &laquo_space;Z
• Normal dist. i.e a hypothetical infinite number of samples exhibit normal distribution

Question 8

What is Lacey mixing index?

Answer 8

Ratio of mixing achieved to mixing possible. A value of 1 would represent a completely random mixture.

Question 9

Why does segregation of particles during handling mainly occur?

Answer 9

Differences in particle size

Question 10

Sampling rules

Answer 10

• a powder should be sampled when in motion
• the whole of the stream of powder should be taken for many short increments of time, in preference to part of the stream being taken for the whole of the time
• take samples along the whole diameter