Particle size reduction + particle size distribution

Question 1

What leads to changes in particle size distribution

Answer 1

Particles in powder being processed undergo different amounts of breakage- uneven milling

Question 2

Properties of material that can affect size reduction

Answer 2

Hardness, toughness, abrasiveness, stickiness, moisture content

Question 3

1 on the arbitrary hardness scale

Answer 3

Graphite

Question 4

10 on the arbitrary hardness scale

Answer 4

Diamond

Question 5

What sort of toughness makes size reduction difficult

Answer 5

Soft but tough materials

Question 6

What other factor is toughness often related to

Answer 6

Moisture content

Question 7

How much energy put into machine actually affects size reduction

Answer 7

2%

Question 8

Where is the remainder of energy lost in size reduction

Answer 8

Vibration, friction, noise, deformation of particles to produce fracture

Question 9

Mechanisms of size reduction

Answer 9

Cutting, compression, impact, attrition

Question 10

Types of compression

Answer 10

Mortar + pestle, compression roller mill

Question 11

Types of impact

Answer 11

Hammer mill, vibration mill

Question 12

How does attrition work

Answer 12

Use of roller mills that rotate at different speeds so material is sheared as it passes through the gap

Question 13

Combined impact and attrition methods

Answer 13

Ball mill

Question 14

What is the structure of a fluid energy mill

Answer 14

Loop of pipe (diameter 20-200mm), height of up to 2m

Question 15

What factors are important in a ball mill

Answer 15

Amount of material placed (too much will be cushioning), speed of rotation

Question 16

How does a fluid energy mill work

Answer 16

Air injected at high pressure through nozzles at bottom, high velocity circulation, solids introduced to stream, high turbulence, impact and attrition between particles

Question 17

How does a pin mill work

Answer 17

Two discs with closely spaced pins, rotate against each other at high speed, impact with pins and attrition between pins reduces particle size

Question 18

Types of distributions seen for powders

Answer 18

Normal, skewed, bimodal