MET E 1610/1630 Module 5: Comminution or Particle Size Reduction

A collection of Question-and-Answer flashcards covering key concepts from Module 5: Comminution or Particle Size Reduction, including definitions, processes, equipment, and energy considerations.

What does the term 'comminution' mean?

Size reduction.

Why is particle size reduction important to particle processing?

It enables liberation of locked mineral particles and subsequent physical separation; it is also needed for applications such as paints, cosmetics, polishing media, industrial fillers, metal extraction, cement, chemical products, and feed materials.

What is meant by liberation in mineral processing, and how much size reduction can be required for 90% liberation?

Liberation is freeing locked mineral grains by size reduction; achieving about 90% liberation can require reducing to roughly 10% of the original grain size.

What is commonly observed about mineral content in natural particles and how does size affect it?

Most natural particles contain more than one mineral, and the distribution of minerals within particles is size dependent.

Name some applications that require size reduction.

Paints, cosmetics, polishing media, industrial filler material, metal extraction, cement, chemical products, and feed materials for other processes.

How do most particles break according to tensile stresses?

Natural flaws or cracks grow under tensile stresses until a critical crack size is reached and the material fractures.

Why might compressive loading break brittle particles?

Compression can induce tensile stresses perpendicular to the load, causing crack propagation and fracture.

What are the classifications of particle breakage?

Compressive fracture, fracture by rapid compression or impact, and attrition (localized compression).

What is produced by fracture due to compression?

Large pieces as well as fine pieces that originate from contact surfaces.

What is produced by fracture due to rapid compression or impact?

Many medium-size pieces and some fines that originate from the impact surface.

What is produced by attrition?

Only small fragment pieces that break away from the surface due to low-energy impacts by other particles.

What are the two main comminution methods and their typical size reductions?

Crushing: reduce large material (<1 m) to <0.15 m in primary stage, with secondary/tertiary stages to about 25 mm; Grinding (milling): reduce from about 25 mm down to sub-micron sizes (<0.001 mm); autogenous grinding uses larger feed material and relies on rock-on-rock or moving media.

What is autogenous (AG) milling?

Grinding that uses the large feed material itself as the grinding media, with minimal or no steel balls.

Name the common types of crushers.

Gyratory, Cone, Jaw, Roll, and Impact crushers.

What is the function of a gyratory crusher?

Used for initial or primary crushing.

What is the difference between CSS and OSS in a jaw crusher?

CSS is the closed size setting (distance between bottom of jaw plates when the jaw is closed); OSS is the open size setting (distance when the jaw is open; the same terminology applies to gyratory and cone crushers).

What are the differences between single roll and double roll crushers?

Single Roll Crusher and Double Roll Crusher are used for soft rock; roll position is adjusted to set the crusher gap.

How does an impact crusher operate?

Impact crushing involves breakage by a surface moving through a constrained path, typically with rotating hammers and breaker plates.

What is a ball mill and how does it grind material?

Ball mills rotate a load of material and steel balls; the balls undergo a cataracting motion and free-fall to grind the material; they typically operate at 70–90% of the mill’s critical speed.

What is the formula for the critical speed of a ball mill?

Nc (RPM) ≈ 42.3 / sqrt(D in meters); for a 10 m diameter mill, Nc is about 13.6 RPM.

How do rod mills differ in the size distribution of the product?

Rod mills produce a narrower size distribution with fewer very large particles (due to preferential breakage of large particles) and fewer fines (protected by larger particles).

What is a semi-autogenous mill (SAG mill) and how does it operate?

SAG mills use metal balls and ore rocks to grind; particles enter at one end and are ground as they travel to the discharge end, usually with water forming a slurry.

What is an autogenous mill (AG mill)?

A mill that grinds using ore rocks with little or no added grinding media.

Why is water used in most grinding circuits?

Water forms a slurry that aids pumping, helps transport the ground material, and improves grinding efficiency.

Why are energy costs significant in comminution?

Energy costs for size reduction are among the largest processing costs, so characterizing and optimizing energy use is important.

What is the Bond equation for energy in size reduction?

E = Wi × [ (d80F)^0.5 − (d80P)^0.5 ], where Wi is the Bond work index, d80F is the feed 80% size, and d80P is the product 80% size.

In the Bond equation, what do d80F and d80P represent?

d80F is the d80 size of the feed; d80P is the d80 size of the product after grinding.

What does the Bond Work Index table provide?

Typical Bond Work Index values for different minerals, e.g., Bauxite 8.78, Coal 13.00, Dolomite 11.27, Ferro-silicon 10.01, Fluorspar 8.91, Granite 15.13, Graphite 43.56, Limestone 12.74, Quartzite 9.58, Quartz 13.57.

What is open circuit grinding?

Grinding where material passes through the process once with no sizing/classification and oversize material is recirculated without achieving necessary separation.

What is closed circuit grinding and why is it used?

Grinding with screening/classification so only appropriately sized product proceeds to the next stage, preventing overgrinding and ensuring the desired product size.

What is High Pressure Grinding Rolls (HPGR)?

A grinding technology using hydraulic pistons to press two rolls together with a zone of compression to achieve size reduction.

What is the typical mining-size reduction circuit layout shown in the notes?

Primary crusher feed → screen → AG or SAG mill → screen → ball mill → hydrocyclone (in a closed circuit) with material recycled as needed.