Particle-size reduction

For most solids-processing operations in the chemical process industries (CPI), particle-size reduction and screening (classification) to achieve the desired particle-size are required, since processes rarely produce the desired size directly. When designing processes, selecting equipment and looking for ways to increase efficiency, CPI engineers must understand the size-reduction behavior of the solid materials in their processes. To do so, they need to evaluate the following set of key properties:

• Particle-size distribution in the feed

• Particle shape

• Bulk density

• Flowability, cohesiveness and adhesiveness

• Corrosivity and composition

• Moisture content

• Hardness, brittleness and friability

• Moisture content

• Fibrous morphology

• Abrasiveness

• Stickiness

• Elasticity, plasticity and ductility

• Dust explosion characteristics

• Temperature sensitivity (degradation, stickiness and phase change)

• Toxicity

• Oil and fat content

• Reactivity or release of gases

• Shock sensitivity or explosiveness

Size-reduction mechanisms

To fracture particles, communition equipment must impart sufficient stress to the material so that it fractures as a result. Compression stress and impact stress are common, but other types exist. There are seven types of stresses that can be imparted to achieve size reduction, including the following:

• Compression between two rigid surfaces

• Compression between surfaces and adjacent bed of solids

• Shearing forces by mechanical means (tearing, cleaving, cutting or shredding)

• Shearing forces due to surrounding media

• High-velocity impact against a rigid surface

• Particle-particle impact that causes breakage and shattering

• Abrasion during particle-wall and particle-particle impacts

The energy efficiency of size reduction equipment tends to be low, and improvement of energy efficiency continues to be a key issue for both technology developers and users (Table 1).

Estimating breakage energy

The energy required for particle-size reduction is the key to designing and specifying grinding equipment. Particle-size reduction is a complex process where quantification of each contributing component is extremely difficult. It is, however, possible to make reasonable approximations using empirical relationships developed by Rittinger, Kick and Bond [1–3].

Rittinger postulated that the energy required for particle-size reduction is directly proportional to the amount of new surface area created.

 

                                     (1)

 

Where:

C_R = constant, kWh-m/ton

E = breakage energy per unit mass of feed, kWh/ton

d_f = particle size of feed, m

d_p = particle size of final product, m

Kick applied the fundamentals of plastic deformation theory and proposed that the energy required for particle-size reduction was proportional to the ratio of volume of feed particle to product particle.

 

                             (2)

 

Where:

C_K = constant, kWh/ton

E = breakage energy per unit mass of feed, kWh/ton

d_f = particle size of feed, µm

d_p = particle size of final product, µm

Bond’s approach, which gives reasonable approximation for most common size-reduction processes, was based on industrial and laboratory data.

 

                                                         (3)

 

Where:

C_B = Bond’s work index, kWh/ton

E = breakage energy per unit mass of feed, kWh/ton

d_f = Particle size of feed defined as the sieve size though which 80% of the feed would pass through, µm

d_p = Particle size of product, as defined as the sieve size through which 80% of the product would pass, µm

Bond’s work index, by definition, is the energy required per unit mass to reduce the particle size from infinity to 100 µm. It is independent of particle size, but does depend on the machine and mechanism of size reduction.

Table 1. Standard range of efficiencies for size-reduction equipment [4]
Equipment type	Typical efficiency, %
Jaw and roll crushers	70–90
Impact crushers	30–40
Roller-ring mills	1–15
Ball mills	5–10
Impact mills	1–10

 

Wet grinding

In wet grinding, the surrounding medium is liquid, as opposed to dry grinding, where gas is the surrounding medium. Wet grinding should be considered in cases where the material is prone to dust explosions and static charging, or when the material is toxic and dust containment is difficult. Also wet grinding can be used when the final product size is extremely fine (production of nanoparticles is possible with wet grinding).

Impact mills

With impact comminution, kinetic energy of the particles to be reduced is used to generate the degree of deformation that is required for fracture. A prerequisite for impact comminution is to have a material that behaves in a brittle-elastic manner. A material is said to be brittle-elastic if the deformation of the product is initially proportional to the applied stress, and the fracture occurs suddenly. In the linear range, the particle deformation is elastic and reversible, but as soon as higher stresses are experienced, the material strength is exceeded locally, and cracks are triggered. The cracks grow extremely fast and lead to the destruction of the particle.

From experiments on single-particle impacts, the following information has been learned: that a minimum fracture energy must be applied to the particle for fracture to occur; that the probability of fracture is dependent on the kinetic energy of the particles; and that the resultant particle-size distribution is dependent on the properties of the material being processed.

There are several types of impact mills. Milling technologies are often better suited to specific applications:

• Mechanical impact mills

• Classic rotor impact mill

• Pin mill with two rotating pin discs

• Long gap mills

• Fine impact mills with air classifiers

• Jet mills

 

References

1. Bernotat, S. and Schonert, K. Size Reduction, in “Ullmann’s Encyclopedia of Industrial Chemistry,” John Wiley & Sons, 2000.

2. Rhodes, M., “Introduction to Particle Technology,” 2nd ed., John Wiley & Sons, 2008.

3. Fayed, M. and Otten, L., “Handbook of Powder Science and Technology,” 2nd ed., Chapman and Hall, 1997

4. Dhodapkar, S. and Theuerkauf, J. Maximizing Performance In Size Reduction, Chem. Eng., June 2011, pp. 45–48.

5. Furchner, B, Fine Grinding with Impact Mills, Chem. Eng., August 2009, pp. 26–33.

 

Editor’s note: The content from this edition of “Facts at Your Fingertips” was adapted from the articles listed in refs. 4 and 5.