Page 176 - Instrumentation Reference Book 3E

160 Particle sizing
Sieving is by no means as straightforward as it sieve surface to contain systems of nodes and
may first appear. For example, it is relatively easy anti-nodes so that parts of the surface are vir-
for large particles to block the apertures to the tually stationary. This can be controlled to some
passage of small particles (blinding) and there is a extent by adjustment of the amplitude, but
statistical uncertainty whether a small particle although the top sieve surface may be visible,
will find its way to a vacant hole. Both of these through a transparent lid. the lower surfaces are
are very dependent on the quantity of material not and they will have different nodal patterns.
placed on the sieve and as a general rule this One solution to this dilemma is to introduce into
quantity should be kept small. It is not possible each sieve a small number (5 to IO) of lOmm
to give an arbitrary figure for the quantity since it diameter agate spheres. These are light enough
will depend on the material and its size distribu- not to damage the sieves or, except in very friable
tion, particle shape and surface structure, its materials, the particles, but they break up the
adhesive qualities, and to some extent the sieve nodal patterns and therefore increase the effective
itself. The same comments apply to sieving time. area of the sieve.
Optimum times and quantities can only be found A useful feature of dry sieving is that it can be
by experiment, assessing the variation in size used to obtain closely sized samples for experi-
grading produced by both factors. Generally a mental purposes in reasonable quantities.
reduction in the quantity is more advantageous Although most sieving is performed in the dry
than an increase in sieving time and it is normally state, some difficult materials and certainly much
possible to obtain repeatable results with less finer sieves can be used in conjunction with a liquid,
than about 10 minutes' sieving. The analyst is usually water, in which the particles are not soluble.
cautioned that some friable materials break up The lid and base of the sieve stack are replaced by
under the action of sieving. fitments adapted for the introduction and drainage
A number of manufacturers produce sieves and of the liquid with a pump if necessary.
sieving systems using various methods of Sieving systems are now commercially avail-
mechanical agitation, some of which include a able which introduce either air or liquid move-
rotary action. The objective, apart from reducing ment alternately up and down through the sieves
the tedium, is to increase the probability of par- to prevent blinding and to assist the particles
ticles finding vacant holes by causing them to through the sieves.
jump off the mesh and to return in a different Results are usually quoted in terms of percent-
position. Figure 11.6 is an example of one that age of total mass in each size range, the material
uses vibration. The vibration can be adjusted in being carefully removed from the sieve with the
amplitude, and it can be pulsed. aid of a fine soft brush. In wet sieving, the mater-
A feature of any vibrating mechanism is that ial is washed out, filtered, dried at 105°C and
parts of it can resonate and this is particularly weighed. Many powders are hygroscopic so that
relevant to sieving where it is possible for the the precaution of drying and keeping in a des-
sicator until cool and therefore ready for weighing
is a good general principle. Convection currents
from a hot sample will cause significant errors.

11.7.2 Microscope counting
I I. 7.2. I Basic methods
With modern microscopes, the analyst can enjoy
the benefits of a wide range of magnifications, a
large optical field, stereoscopic vision and zoom
facilities, together with back and top illumin-
ation. These and calibrated field stop graticules
have considerably eased the strain of microscope
counting as a method of size analysis but it is still
one of the most tedious. It has the advantage,
however, that, as well as being able to size par-
, ticles, the microscope offers the posssibility of
minute examination of their shape, surface struc-
ture, color, etc. It is often possible to identify
probable sources of particles.
The optical microscope can be used to examine
Figure 11.6 Fritsch Analysettesieveshaker. particles down to sizes approaching the wavelength