5.3 Size Reduction

5.2.1 Purposes of size reduction

Size reduction refers to all the ways in which particles are cut or broken into smaller pieces.  The objective is to produce small particles from big ones.

Reasons for undertaking size reduction in the chemical industries include:

1. To reduce chunks of raw materials to workable sizes e.g. crushing of mineral ore.

2. To increase the reactivity of materials by increasing the surface area.

3. To release the release valuable substances so that they can be separated from unwanted material.

4. To reduce the bulk of fibrous materials for easier handling.

5. To meet standard specifications on size and shape.

6. To increase particles in number for the purpose of selling.

7. To improve blending efficiency of formulations, composites e.g insecticides, dyes, paints

5.2.2 Principles of size reduction

Most size reduction machines are based on the following principles:

1. Compression

2. Impact

3. Attrition or rubbing

4. Cutting

Compression

When a solid is held between two planes and pressure is applied on one plane, the solid is fractured and breaks into fragments when pressure is removed. The fragments are of different sizes.

Impact

This is the breaking up of material when  it is hit by an object moving at high speed. The product is course to fine.

Attrition

If two surfaces are which are perceived to be normally "flat" laid one upon the other, they are in actual contact only at a small number of spots known as "high spots".  If one of these surfaces is moved across the other, some of the "high spots" will be sheared off producing dust if the material is brittle.  If the surfaces are not very flat similar, action may tear off rather larger fragments.  This tangential rubbing occurs in most comminution processes along with simple compression.  Indeed some processes are quite clearly designed to include both mechanisms.  Only small fragments are produced by attrition.

An ideal crusher or grinder would:

  • Have a large capacity
  • Require a small power input
  • Yield a product of a single size or desired size distribution.

5.2.3 Characteristics of communition products

1. Energy efficiency is measured by new surface created therefore geometric characteristics of particles are important in evaluating the product.

2. The product always consists of a mixture of particles ranging in size from a definite maximum to a sub-microscopic minimum.  The largest particle can be controlled but the fines are not under control.

3. If the feed is homogenous both in shape, physical and chemical structure, the shapes of the individual units of the product may be quite uniform.

4. The smallest grain is comparable to a unit crystal

5. When largest particle size just passes 1 mm openings.

Size of largest particle                   »          106

Size of smallest particle

This shows a very large size variation.

6. Unless smoothed by abrasion after crushing, comminuted particles resemble polyhedrons with 4 to 7 faces having sharp corners.

7. The particles may be compact with length, breadth and thickness nearly equal or they may be plate like or needlelike.  When compact, they are nearly spherical and one can therefore talk of particle "diameter".

5.2.4 Size reduction equipment

Fig 5.1. Jaw crusher

A jaw crusher consists of a vertical fixed jaw and another swinging jaw moving in the horizontal plane.  The two jaws make 20-30o angle between them. The swinging jaw closes  about 250 to 400 times/min.  Feed is admitted between the jaws.  It is crushed several times between the jaws before it is discharged at the bottom opening.

A jaw crusher is a primary crusher which produces a course product.

Gyratory crushers and cone crushers use the same crushing principles as the jaw crusher

The smooth roll crusher

This crusher is shown in Fig. 5.2. Like the jaw crusher, it uses compression.

 

 

 

 

 

 

Fig 5.2 The smooth roll crusher

 

The rolls turn towards each other compressing particles of feed.

 

3-roll mill

The 3-roll shown in Fig 5.3 uses the same principle as the smooth roll crusher. The last roll has the highest velocity

 

 

 

 

 

 

Fig 5.3. The 3-roll mill

Hammer Mill

Material can be broken if it is hit by an object moving at a high speed.  The size is reduced by impact. (A sledge hammer demolishing a wall uses this principle).  Impact produces a mixed product ranging from course, medium to fine particles.

A hammer mill is shown in Fig.5.4.

Fig 5.4. Hammer mill

It consists of a high speed rotor turning inside a cylindrical casing.  Swinging hammers are pinned on the rotor.  The shaft is usually horizontal.  Feed is dropped at the top.  It is broken by impact and falls out through the screen opening when it has been reduced to the right size. Hammer mills are used in the quarrying industry, municipal solid waste processing and in scrapping automobiles.

Speed ranges from about 500 to 180rpm. Hammers in large hammer mills may weigh several hundred kilograms each. 

 

Micro-pulverizer hammer mill

This is used for a wide range of non-abrasive materials such as sugar, chemicals, carbon black, pharmaceuticals, plastics dye stuffs, cosmetics and cereals.

Fig.5.5 is a schematic illustration of a micro-pulverizer hammer mill.

Fig. 5.5. Micropulverizer

The rotor runs inside a 360o screen enclosure.  The rotor has fixed hammers.

Applications include: 

  • Mixing
  • Delumping
  • Dissolving of fluids, slurries and pastes

 

Ball Mill

A ball mill is a tumbling mill.  Its operation is illustrated in Fig 5.6. The mill consists of a cylinder containing a mixture of large and small steel balls. Cylinder is loaded with feed. It is rotated for a sufficient period of time. The steel balls hit the feed particles as they fall from the top of the cylinder. The feed is ground into a powder. Ball milling can be operated with water i.e. wet milling to produce & slurry.

Fig. 5.6. Ball mill

Applications:   ceramics and other industries dealing with inorganic materials.

Buhrstone mill

The Buhrstone mill is shown in Fig. 5.7 is an example of an equipment that uses attrition.

Fig. 5.7. The Buhrstone mill

5.2.5 Size reduction process

Size reduction process involve the following steps:

  1. Particles of feed material are first distorted and strained
  2. Work necessary to strain them is stored as mechanical energy, just like mechanical energy is stored in a coiled spring.
  3. As additional force is applied to the stressed particles they are distorted beyond their ultimate strength and they suddenly rupture into fragments.
  4. new surface is generated
  5. a unit area of surface has a definite amount of energy, supplied by the release of energy of stress when the particle breaks.
  6. By conservation of energy, all energy of stress in excess of the new surface energy created appear as heat.

 

5.2.6 Grindability

Grindability is defined as the amount of product from a particular mill meeting a particular specification in a unit of grinding time e.g tons/hr passing through 200 mesh.  The purpose of grindability study:

 

1.      Evaluate the size and type of mill needed to produce a specified tonnage

2.      evaluate power requirement for grinding.

5.2.7 Mineral hardness

The hardness of a mineral:is a criterion of its resistance to crushing and a good indicator of its abrasive character. It also determines the wear of the grinding media. This hardness  is measured using Mohr scale ranging from 1 to 10 with the hardness of some minerals being used as reference as shown below.

                

Minerals having 1 to 2 hardness are classified as soft materials. Included in this category are talc and soapstone. Those with 4 to7 hardness are intermediate hardness materials. In this category we find phosphates and bauxite. Materials with 8 to 10 are hard materials.

5.2.8 Work Index

Work index is defined as the gross energy in kilowatt-hours per ton o feed needed to reduce very large feed of material i to a such a size that 80% of the product passes through a sieve with 10-4mm holes (100mm).  This means 80% of the sand will pass through the sieve.  20% of original material will remain on the sieve.

The work index of various materials is shown in Table 5.1

Table 5.1  Work Index of various materials

 

Material                            Specific gravity                      Work index

 

Glass                                 2.58                                         3.08

Clay                                   2.23                                         7.10

Fluorspar                           2.98                                         9.76

Phosphate fertilizer              2.65                                         13.03

Iron ore                             3.96                                         15.44

Granel                               2.70                                         25.17

Silica                                 2.71                                         13.53

Mica                                  2.89                                         134.50

 

There is a limit to attainable product size.

5.2.9 Grinding efficiency

This is defined as the energy consumed compared with some ideal energy requirement. It varies from 0.06 to 1%. Wet grinding consumes less power than dry grinding. In fine dry grinding, surface forces lead to cushioning and coating of grinding parts.  Wet or moist feed can therefore be fed into a mill if the presence of moisture has no adverse effect on the product, for example during mineral processing.

In dry grinding, production rate drops as moisture content of the product increases. This is shown in Fig 5.8

Energy is a major expense in crushing and grinding. The energy cost increases and the capacity drops as the desired product size decreases. This shown in Fig. 5.9

Fig. 5.9. Effect of product size on capacity and energy consumption