Page 93 - Book Hosokawa Nanoparticle Technology Handbook

2.2 PARTICLE SIZE FUNDAMENTALS
2.2.5 Grinding method

The grinding is a fine particle production method by
applying mechanical energy on the solid materials to
break the bonding between atoms or molecules. It is one
of the oldest unit operations in the history of human
beings. It can be applied not only to the existing nature
solids but also those artificially synthesized organic and
inorganic materials. Although grinding is a common
technology, the grinding process is still a black box due
to the complication of the fracture phenomena of solid
particles as well as the relationship between the particle
fracture mechanism and the particle size of ground par-
ticles. It requires thorough statistical investigation.
Nowadays, the particle size requirement for grinding
is getting finer and finer. For example, about 20 years Figure 2.2.23
ago, grinding limit (“Grinding Barrier”) was said to TEM picture of the particle structure of calcium carbonate
be at 3 m in the continuous grinding of brittle mate- ground by an attrition type mill.
rials. The grinding limit has long passed the submi-
crons and reaches the nanosized range in recent days
because of the development of advanced technologies they tend to form agglomerate instantaneously in the
and industries. gas and become stable, coarser particles.
It was said impossible to produce the nanoparticles In this way, it is very difficult to produce stable
less than about 100nm by grinding; but the gas- or nanosized particles by the dry grinding method, since
liquid-phase method accompanying the phase change active nanoparticles form fresh agglomerates as soon
had potentials for this purpose. However, examples as they contact each other. Therefore, the grinding
of producing nanoparticles by grinding can already has to be carried out in a liquid (wet grinding).
be found in some cases these days. Furthermore, the In the wet grinding, as the surface of the generated
synthesized nanoparticles often form strongly aggre- particles is surrounded by a solvent instantaneously, it
gated agglomerates in the production process, and in is possible to control the agglomeration of particles
many cases, they take apparent bulk form from much better than in the dry grinding. For the purpose
sintering and the like. Therefore, the dispersion or of producing the nanoparticles, media agitation mills
disintegration of these agglomerates into the primary has drawn much attention recently.
particles with high-specific surface area is regarded The media agitation mill is a grinding mill which
as grinding in a broader sense. Lately, in the agitates ball media in the mill with agitation blades
production of nanoparticles, the grinding is often which exerts stronger force on the ball media, made
applied as the subsequent dispersion or disintegration by ceramics and the like, than a typical tumbling ball
operation. mill. Besides the agitation mills, some other type of
From coarse to fine grinding, various types of mills mills, like the planetary mills, apply centrifugal accel-
are used depending on the particle size of the feed eration to the ball media by revolution as well as rota-
materials and the desired products. In the fine grind- tion of the container for nanogrinding.
ing, the mills based on the principle of surface grind- Figure 2.2.24 shows the relationship between the
ing are usually used. The surface grinding is a grinding product average size and the grinding time (residence
principle where ultrafine particles are generated from time) of the wet grinding of piezoceramic material as a
the particle surface by strongly rubbing them against function of ball diameters [3]. As seen in the figure, the
each other as realized by the same material friction ground product with an average size of less than 0.5 m
method [1]. Examples of this type of mills are impact can be obtained easily by the continuous wet grinding,
mill, roller mill, attrition mill, ball mill, media agita- and it is interesting to find that the product particle size
tion mill, jet mill, and so on. The grinding results using gets finer with the decrease of ball diameters.
these mills are explained in the followings. The grinding performance is determined by the fre-
For example, Fig. 2.2.23 [2] shows the particle quency and the intensity of ball collisions affecting the
structure of calcium carbonate ground by an attrition fracture of the particles. As the particle size decreases,
mill observed with TEM. The particle size distribu- the force required to break the particle itself reduces,
tion of the ground product was at D 0.7 m in although the fracture strength of the particle increases.
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volume-basis measured by the liquid-sedimentation Therefore, the grinding capability is kept even when the
method. However, as seen from the TEM picture, the ball diameter decreases. Additionally, with the decreas-
ground powder consists of nanosized particles. It ing ball diameter, the frequency of ball collisions on the
suggests that a number of nanosized particles are solid particles increases, which presumably results in
generated from the solid surface during grinding, and the effective grinding due to higher grinding speed.

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