Ceramics- Milling

Question 1

Three types of comminution

Answer 1

Jaw crusher, ball mill, hammer mill

Question 2

How does a ball mill work?

Answer 2

Hard spheres and material you want to break up put in mill. This rotates so that the hard spheres fall onto and break the material

Question 3

What are ball mills used for?

Answer 3

Produce powders <75μm with wide particle size distribution. Produce powders of a few μm size with narrow particle size distribution. To deagglomerate and mix powders and slurries

Question 4

What does increasing material loading do in ball milling?

Answer 4

Means more hard spheres. Decreases rate of reduction in the maximum size. Very braid size range

Question 5

Fractional distribution vs product size graph

Answer 5

For 2000 revolutions is broad peak with steep initial slope and shallow tail. 20000 revolutions has higher peak (at same product size) and has steeper sides. 40000 even higher peak and steeper sides

Question 6

How much of energy input is used for grinding in ball mills?

Answer 6

15%

Question 7

Media-powder size ratio

Answer 7

Milling media must be large compared to feed powder. Min ratio 25:1. The finer the size of the product, the smaller the media should be

Question 8

Media-powder volume ratio

Answer 8

Depends on aiming size of powder. Finer the size the lower the loading. Media feed is 20-40% of container volume. Powder feed 30-70% of the media volume

Question 9

Physical characteristic of media

Answer 9

As milling proceeds, media wears and causes contamination. Using the material for the media the same as the powder can minimes this

Question 10

Roststion speed and time considerations

Answer 10

Rotation speed should be such that media are carried up to sufficient height that they cascade down with the surface layers of the media moving relative to each other while in contact

Question 11

What is critical speed of ball mills?

Answer 11

The rotation speed that would cause the media to be held against the mill lining by centripetal action.
Nc=(1/π)rt(g/2D-d)
In revs/s
g is acceleration due to gravity
D is diameter of container (m)
d is diameter of balls (m)

Question 12

Cascading, cataracting and centrifuging N/Nc values

Answer 12

Cascading: 0.55-0.7
Cataracting: 0.7-0.8
Centrifuging: >=1

Question 13

What happens as particles get finer?

Answer 13

Tendency for agglomeration. Less occurrence of comminution. Less probability of a flaw

Question 14

Dry milling

Answer 14

Rotated at 65-80% Nc. Media tends to slip more. Obtained powder doesn’t need to be separated from the solution. Agglomeration and dead zone can occur leaving some powder unmixed. Initial fast reduction in particle size but levels off above wet milling

Question 15

Wet milling

Answer 15

Rotated at 60-65% Nc. Less agglomeration of powder. Media/powder volume ratio must be carefully selected to optimise viscosity. Some components of powder may dissolve in solution. Additional process to separate powder from solution. Slower initial reduction in particle size but levels off lower than dry milling

Question 16

Fluid lubricant

Answer 16

A thick film of low-viscosity liquid like water and oil

Question 17

Boundary lubricant

Answer 17

Adsorbed film with high adhesion strength and low shear strength like stearic acid

Question 18

Solid lubricant

Answer 18

Fine particles with a laminar structure and smooth surface

Question 19

Surfactants

Answer 19

Boundary lubricant. Has a non-polar group on one side (hydrophobic) and a polar group on the other (hydrophilic). Like sodium dodecyl sulphate CH3-(CH2)11-O-SO(3+)N+