“Pulverization” is another term for comminution, crushing or grinding. In other words, pulverization means the application of an external force to a solid, which results in its destruction and the reduction of its original size. For years, pulverization has been applied to many materials, including ore, glass, ceramics, grains, paints and medicines.
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Defining the Difference Between “Pulverization” and “Crushing”
In English, the usage and terminology of “pulverization” and “crushing” vary among the industry and people involved. “Size reduction” is often used in reference to pulverization in a broad sense, and “comminution” often used to when referring to transforming a solid into small pieces.
“Grinding” can also be used in a narrow sense, as in the reference to friction grinding in mining. This term for pulverization is used in accordance with particle size (as in the crushing of fine particles) and may be distinguished from crushing.
In addition to these terms, “milling” and others, are also used. Thinky Corporation uses “pulverization.”
“Crushing” may be used when saying “cracking into pieces.” “Crushing” means pulverization wherein the particle size of the crushed material is reasonably large. In this sense, large means when the maximum particle size is 1 mm or over. The distinction between grinding in a narrow sense and crushing is vague, and there is no clear boundary between the two.
The Purpose of Pulverization
Not only for reducing particle size, pulverization is additionally being used as a means of developing new materials.1 Various effects and outcomes are expected.
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How the Pulverization Process Works
In pulverization, the application of an external force to a particle causes it to elastically deform and then wherever the stress limit of the particle is exceeded, cracks occur at points. It is thought that the particle is destroyed by the propagation of cracks, thus completing the process of pulverization.2
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In the pulverization process, there are two types of particle destruction, which are “surface grinding” and “volume grinding”.
In the process of surface grinding, small pieces break off from a particle surface, which then gradually forms many fine particles. During grinding, the particle sizes often follow a bimodal distribution. This surface grinding in friction grinding is dominant.3, 4.
Crushing occurs not at the particle surface but in the whole part, in the case of volume grinding. Subsequently, the particle is broken up into several pieces, and this cycle is repeated until the particle gradually turns into fine particles. In compression pulverization and impact pulverization, volume grinding is dominant. 3, 4
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Pulverization 2: Pulverization Method and Pulverizer
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Dry Pulverization and Wet Pulverization1
There are two methods of pulverization. These are dry pulverization and wet pulverization. Below, the differences between the two are summarized.
Table 1. Source: Thinky Corporation
| Dry pulverization |
Wet pulverization |
- Pulverize in air or inert gas
- Pulverization into particles of various sizes ranging between large and fine is possible
- It is unlikely that particle diameter is reduced to 3 μm or less –> the so-called “3 μm barrier”
- Low cost compared to wet pulverization
|
- Pulverized in liquid such as water
- Pulverization target is fine particles and ultrafine particles
- The ultimate particle size can reach nm order
- Can have more contaminants than dry pulverization
- High cost compared to dry pulverization
|
Thanks to its features, dry pulverization is used in a wide range of fields. Generally, the size of pulverized products is reduced down to micron level. Pulverization to submicron level is possible, in certain cases, but there are issues which can arise from this, such as aggregation and difficulty of collection.
Ultrafine pulverization (grinding to submicron level) is possible through wet pulverization, which is difficult with dry pulverization. Additionally, as well as the pulverization of primary particles, wet pulverization also incorporates disintegration (= mechanical pulverization) and dispersion.
Miniaturization of materials is often required, as in recent years, and in addition to pulverization, disintegration and dispersion operations have also gained more and more importance.
(* Disintegration (= mechanical pulverization) is one of the elements involved in dispersion. To loosen agglomerated particles almost without the formation of new particle surfaces (and to reduce their size) an external mechanical force is applied.)
Depending on the pulverizer, the particle size that can be reduced by a single machine differs. Several pulverizers are often used in series for this very reason.
There is a large range of pulverization methods, like continuous pulverization versus batch pulverization (batch type) as well as dry pulverization (dry milling or grinding) against wet pulverization (wet milling or grinding). The key is to select a pulverizer which is ideal for the purpose, and additionally considers the type and size of material to be pulverized (object to be pulverized), the target particle size, the conditions, etc.
Representative Pulverizers1, 2
Table 2. Source: Thinky Corporation
| Classification of pulverization operation |
Size of pulverized material (ground particle size) |
Main pulverization mechanism |
Examples of pulverizer |
| Crushing |
Meter order
⇒10 cm or less |
Compression |
Jaw crusher
Gyratory crusher |
| Medium crushing |
10 cm
⇒10 mm or less |
Compression
Impact and shear
Compression and shear |
Crushing roll
Hammer mill
Roller mill |
| Pulverization / (fine) grinding / milling |
1 mm
⇒10 μm or less |
Impact
Impact and friction
Shear and friction
Friction, impact, and compression |
Jet mill
Ball mill
Vibratory ball mill
Planet mill |
| Ultrafine grinding |
10 μm
⇒1 μm or less |
Impact, shear, and friction
Friction, impact, and compression |
Wet medium agitation type grinder (bead mill, attritor, etc.) Planetary mill (depending on conditions) |
*There are many different classifications of pulverization operations. Pulverization may be classified into various classifications, including: rough, medium, small crushing, coarse, fine, and ultrafine grinding.
Thinky Pulverizer
Thinky Corporation has designed and manufactured a rotation and revolution nano-pulverizer “Nano Pulverizer NP-100” for the purpose of nano-size pulverization.
Principle of Pulverization and Features
A batch-type wet pulverizer, the rotation and revolution nano-pulverizer can perform pulverization and disintegration into approximately 1 mm or less (or even approximately 100 nm - several dozens of nm by disintegration). The Nano-Pulverizer NP-100 is a type of planetary ball mill (planetary mill) or bead mill.
Beads, known as introduce balls, are grinding media into a grinding container together with slurry. This container then revolves at a high speed, clockwise, with a 45° tilt angle, while at the same time rotating counterclockwise (see “Mixing”). Refer to the figure below.
This rapid revolution and rotation therefore cause a concentration of the grinding media in the container in the bottom outer periphery of the container. The slurry which is undergoing convection motion in the container is therefore affected by a combination of friction force, shear force, impact force, and thus becomes fine particles.
The concentration of the grinding media on the bottom outer periphery of the container becomes higher, due to the fact that the grinding container is inclined at 45°. The result of this is that only a small amount of grinding media is needed. In addition, this inclination can suppress the rise of the slurry, which therefore permits efficient pulverization without waste.
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Kinetic energy is received by the zirconia balls, through the revolution and the rotation with the inclination of 45°, and these are concentrated at the bottom outer periphery without rising along the container inner wall. Simultaneously, particles in the convecting slurry in the container are trapped between the balls. Here they are impacted by a combination of shear force, friction force, impact force, etc., and are finely pulverized.
On the basis of results obtained from previous experiments, it has been found that Thinky’s rotation and revolution nano-pulverizer has some issues when processing the following materials:
- Elastic materials (resin, rubber, natural materials, etc.)
- Materials readily affected by heat (protein, cellulose, etc.)
- Materials that are soluble in solvent
- Fiber-like, flaky materials (CNF, exfoliation of graphite, etc.)
- Ductile materials (metal)
References
1 Hiroshi Sakamoto: “Crushing and grinding”, Resources and Materials Vol. 113, No. 12, 899-903 (1985).
2 Introduction to Powder Engineering, 52-57, Association of Powder Process Industry and Engineering, Japan.
3 Yoshiteru Kanda: “Study on research and technological history of grinding”, Journal of the Association of Materials Engineering for Resources Vol. 9, No. 2, 87-110 (1996).
4 Toshio Inoue: “What contributes to the improvement of grinding efficiency?” Journal of the Society of Powder Technology Vol. 22, No. 6, 71-76 (1985).
This information has been sourced, reviewed and adapted from materials provided by Thinky Corporation.
For more information on this source, please visit Thinky Corporation.