Particle size analysis by sieving
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Particle size analysis by sieving


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which may be due to wear. 
Ideally, sieves should have apertures of identical shape and size. 
However, due to the methods of fabrication, woven wire sieves have a 
range of aperture sizes and weaving gives a three dimensional effect. 
Fairly wide tolerances are accepted and specified in standards but even 
these are sometimes exceeded in practice. Leschonski [4] examined eight 
50 |Lim woven wire sieves with the results presented in Figure 4.7. The 
median varied between 47.3 and 63.2 |um (almost certainly an incorrectly 
labeled 63 }xm sieve) and the standard deviation between 1.8 and 8.6 |Lim. 
Ilantzis [49] gave a rigorous statistical analysis of microscopic 
measurements on a set of 13 different sieves (Table 4.2). Most of the 
sieves had rectangular openings with wider variations on the warp than the 
weft, and with a coefficient of variation rising from 3% to 5% with large 
aperture sieves and up to 10% with small apertures. Only four of the 
sieves met the standard specifications. 
All aperture widths shown in Figure 4.7 are for nominal 50 |Lim sieves 
but comparative tests on the same material using these sieves could yield 
enormous differences in the residual weight. In order to obtain agreement 
between different sets of sieves it is therefore necessary to calibrate them 
and thenceforth to monitor them to detect signs of wear. 
One way of standardizing a single set of sieves is to analyze the 
products of comminution. It is known that the products are usually log-
normally distributed hence, if the distribution is plotted on a log-
probability paper, a straight line should result. The experimental data are 
fitted to the best straight line by converting the nominal sieve aperture to 
an effective sieve aperture. 
The traditional way of determining the median and spread of aperture 
sizes for a woven wire sieve is to size a randomly selected set of apertures 
using a microscope. Due to the method of manufacture, the measurements 
for the warp and weft will tend to differ. The limiting size may also be 
determined by using spherical particles. These are fed on to the sieve 
which is then shaken and the excess removed. Many spheres will have 
222 Powder sampling and particle size determination 
40 50 6 0 
Aperture size in microns 
Fig. 4.7 Cumulative distribution by number of aperture widths of eight 
50 |j.m woven wire sieves. 
Table 4.2 Means and standard deviations of 13 different sieves as 
reported by Ilantzis [50] 
Nominal 
aperture 
width 
(urn) 
40 
50 
63 
80 
100 
125 
160 
200 
250 
315 
400 
500 
630 
Warp 
Median 
(urn) 
44.1 
53.2 
62.9 
82.6 
103.2 
121.5 
167.7 
200.8 
258.9 
299.3 
414.5 
512.9 
639.0 
Standard 
deviation 
4.85 
6.12 
4.20 
5.68 
4.44 
4.89 
10.48 
11.51 
14.60 
22.63 
22.00 
28.70 
20.70 
Weft 
Median 
(|Lim) 
45.2 
50.8 
64.0 
84.3 
106.1 
123.8 
155.2 
214.8 
260.9 
295.9 
467.6 
500.5 
634.0 
Standard 
deviation 
4.56 
5.19 
3.75 
4.02 
2.91 
5.59 
6.79 
16.46 
14.40 
17.29 1 
26.40 
25.00 
43.20 
Sieving 223 
jammed into the sieve cloth and may be removed for examination. In such 
an examination, Kaye et al [51] showed that both methods yielded similar 
data for a 65-mesh Tyler sieve, with 90% of the apertures falling between 
0.91 and 1.07 of the nominal size. Kaye recommends monitoring sieves 
every few months using glass beads to find out whether the sieve apertures 
are deteriorating or enlarging. These beads are available from the National 
Institute of Standards and Technology. 
Table 4.3 NBS Standard reference materials designated for calibration of 
sieves 
SRM 
1003 b 
1004b 
1017b 
1018b 
1019b 
Particle diameter in microns 
10 to 60 
40 to 150 
100 to 400 
220 to 750 
750 to 2450 
Standard polystyrene spheres are also available for calibration purposes 
from Gilson and Duke Standards. Stork Screens Inc. measure apertures 
and screen wire dimensions using wax impressions and the impressions are 
examined with an automatic image analyzer. 
Sieves may also be calibrated by a counting and weighing technique 
applied to the fraction of particles passing the sieve immediately prior to 
the end of an analysis. These will have a very narrow size range and the 
average particle size may be taken as the cut size of the sieve. A minimum 
number {n) of particles need to be weighed to obtain accurate volume 
diameters {d^)\ let this weight be m and the particle density be /? then: 
< -
\f^. 
(4.7) 
Particles larger than 250 |im can easily be counted by hand and, if weighed 
in batches of 100, d^ is found to be reproducible to three significant 
figures. For particles between 100 and 250 |Lim in size, it is necessary to 
count in batches of 1000 using a magnifier. For sizes smaller than this, the 
Coulter Principle can be used to obtain the number concentration in a 
known suspension. An alternative procedure using tacky dots has also 
224 Powder sampling and particle size determination 
been described [52]. Sieve analyses are then plotted against volume 
diameter in preference to the nominal sieve diameter. The method is 
tedious and time consuming and BCR have prepared quartz samples by this 
method that may be used as calibration material [53]. The quartz is fed to 
a stack of sieves and the analytical cut size is read off the cumulative 
distribution curve of the calibration material. A method for measuring 
accurately the aperture and wire diameter using wax impressions has also 
been described [54]. 
Rideal et. al. [55] describe the preparation, measurement and use of 
microspheres for calibrating individual test sieves. Broad distribution 
glass microspheres were first sieved at 75 |Lim and the undersize re-sieved 
on a 53 |am sieve using a production sonic sifter from Gilson. This was 
repeated three times to give about 90% by weight of powder between 53 
and 75 |im. The resulting samples were then analyzed using a nest of 
electroformed sieves in the 45 to 75 [im aperture range and a calibration 
graph of percent, passing against mean aperture size was the generated. 
This graph was then used to calibrate a 63 |am sieve. Results were 
confirmed by microscope analysis of the sieve mesh. The calibration 
method gave a mean aperture of 59.6 |Lim with a standard deviation of 0.08 
|Lim; by microscopy the sieve apertures were 60.5 |im; the near mesh 
method gave 63.5 |Lim. This last method is expected to generate a larger 
size since the particles measured are those trapped in the sieve mesh at the 
end of the analysis. Centerline sieve measurements service from ATM 
evaluate the wire and opening sizes in wire cloth and electroformed mesh 
used in test sieves. Service provides a data set of opening sizes for 
verifying compliance with applicable standards. 
4.8 Sieving errors 
Hand sieving is the reference technique by which other sieving techniques 
should be judged. For instance, in the French standard NFX 11-57, it 
states: 
If sieving machines are used, they must be built and used in such a way 
that the sieve analysis must, within the agreed tolerances, agree with the 
analysis obtained by hand sieving. 
The apertures of a sieve may be regarded as a series of gauges that reject or 
pass particles as they are presented at the aperture. The probability that a 
Sieving 225 
particle will present itself to an aperture depends upon the following 
factors: 
\u2022 The particle size distribution of the powder. The presence of a large 
fraction of near-mesh particles reduces the sieving efficiency. An 
excess of fines also reduces sieving efficiency. It is recommended 
that fines should be removed before carrying out the sieve analysis. 
This is effected by pre-sieving, by hand, on the finest sieve to be used 
in the subsequent analysis. If this is not done, the fines will have to 
pass through the whole nest of sieves, thus prolonging the analysis 
and increasing the risk of high powder loss. Since small particles 
often adhere to large ones it may be necessary to carry out this 
operation by wet sieving.