JW Lecture 6 - Particle Enlargement & Reduction

Question 1

Reasons for breaking up particles

Answer 1

• Generation of a desired particle size, particle form or size distribution - Example standardized building materials
- Dyes, polymers, cosmetics, paper, grains, sugar, spices, meat
• Increase surface size for increasing physical or chemical reactivity - Dissolving and melting (e.g. salts)
- Extracting (e.g. coffee)
- Hardening and setting (cement)
- Transport and combustion (coal)
• Separation of heterogeneous materials
- Metal ores, grains (separation allows different types of flours), processing waste

Question 2

Things that break up a particle

Answer 2

• Forces on the outside of the particle by the comminution machine or neighbouring particles
• Deformation within the particle
• Initial fracture to induce local tensions

Question 3

Stress mechanisms

Answer 3

1. Stress applied between two surfaces e.g pressure, shear, hit, cut, stock stress
2. Stress applied at a single surface e.g hit non-moving wall, hit moving part, particles hit each other
3. Stress applied by the surrounding medium e.g shearing current, sonication, cavitation
4. Stress applied by non-mechanical energy e.g thermal, electromagnetic, chemical - induced

Question 4

What is agglomeration?

Answer 4

Mechanical unit processes to cluster and link small dispersed particles to larger entities i.e agglomerates

Question 5

Agglomeration unit processes and their modes of agglomeration

Answer 5

Flocculation/coagulation : loose attachment (liquid medium)

Wet granulation, pelletising, coating : build-up agglomeration

Compacting, briquetting, tabletting : pressure agglomeration

Sintering : fusion agglomeration