Size Reduction and Mixing

Question 1

What is the primary purpose of reducing a particles size?

Answer 1

To increase the particle surface area.

Question 2

What is the grind limit?

Answer 2

Where all the coarse particles have been broken down to the smallest particle size possible for the process being applied.

Question 3

What are the 4 types of size reduction mechanisms?

Answer 3

1) Impact - Particles concussed by a single force
2) Compression - Particle disintegration by two opposing forces
3) Shear - Particle disintegration by shear force
4) Attrition - Particle scrapping against each other and surfaces

Question 4

What will happen if a particle with visco-elastic properties is put under a slow compressive force?

Answer 4

It will flatten then, once the force is removed, slowly return back to its original shape. It will not break.

Question 5

What is Rittinger’s Law?

Answer 5

P = m Kr (1/Dvs2 - 1/Dvs1)

P = Power
m = Feed rate (kg/s)
Kr = Rittinger Constant
Dvs2 = Particle size of product
Dvs1 = Particle size of feed

Question 6

What is Bond’s law?

Answer 6

E = Ei (100/L2)^1/2 (1 - 1/q1T2)

E = Energy
Ei = Work index 
L2 = Desired size
q1T2 = The reduction ration of L1 to L2 which is L2/L1

Question 7

What is the Bond Power Law when wet?

Answer 7

P = 36 Wi m (1/(Db2)^1/2 - 1/(Db1)^1/2)

P = Power
Wi = Work index
m = feed rate
Db2 = Product size
Db1 = Feed product size

Question 8

What is the Bond Power Law when dry?

Answer 8

P = 48 Wi m (1/(Db2)^1/2 - 1/(Db1)^1/2)

P = Power
Wi = Work index
m = feed rate
Db2 = Product size
Db1 = Feed product size

Question 9

What is the work index?

Answer 9

The energy required to reduce the size from infinity to a D80 of 100 micrometres

Question 10

What is Kick’s Law?

Answer 10

P = m Ck ln(D2/D1)

P = power
m = feed rate
Ck = Kicks Constant
D2 = Product size
D1 = Product size

Question 11

When is Rittinger’s law used?

Answer 11

Very small particle size (ultra fine grinding)

Question 12

When is Bond’s law used?

Answer 12

Intermediate particle size.

Question 13

When is Kick’s law used?

Answer 13

Used for large particle size (coarse crushing and crushing)

Question 14

Why is size reduction energy inefficent (3)?

Answer 14

• Elastic deformation or particles and equipment
• Heating
• Noise and vibrations

Question 15

What 7 variables influence equipment choice in size reduction?

Answer 15

• Hardness
• Structure
• Moisture content
• Stickiness
• Lubricity
• Flammability
• Health hazard of dust

Question 16

Feed and Product size for coarse crushers? Examples?

Answer 16

Feed size 1500 - 40mm
Product size 50 - 5mm
Examples: Gyatory and Jaw crushers

Question 17

Product and feed size for intermediate crushers? Examples?

Answer 17

Feed: 50-5mm
Output: 50-0.1mm

Roller mills

Question 18

Product and feed size for fine crushers and mills? Examples

Answer 18

Feed: 5-2mm
Product: <0.1mm
Examples: Centrifugal attrition mills, Szego grinding mills, Grinding mill, Planetry mills, and Ball mills.

Question 19

What are the 3 types of particle mixing mechanisms?

Answer 19

Convective mixing - Groups of particles move from one position to another
Diffusion mixing - Groups of particles are distributed over a freshly developed interference
Shear mixing - Slip planes of particles move over each other.

Question 20

How is the extent of mixing expressed?

Answer 20

Y = 1 -e^-ct

Y = Extent of mixing
c = Constant
t = mixing time