Particle size analysis by sieving
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Particle size analysis by sieving

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micromesh sieves. Veco 
recommend 15 to 20 s at a time in a low power 40 kHz ultrasonic bath 
containing an equ7al volume mixture of isopropyl or ethyl alcohol and 
water with the sieve in a vertical position 
4.13.2 Wet sieving by machine 
Several writers have proposed automated wet sieving procedures [92,93]. 
In most of these methods, a stack of sieves is filled with a liquid and the 
236 Powder sampling and particle size determination 
sample is fed into the top sieve. Sieving is accomplished by rinsing, using 
vibration, using a reciprocating action, applying vacuum, applying 
ultrasonics or a combination of these [94]. Commercial equipment is 
available in which the sample is placed in the top sieve of a stack of sieves 
and sprayed with water whilst the stack is vibrated (Figure 4.9). In the 
Retsch water jet sieve a spray ring is pushed over each analysis sieve and a 
spray arm with 34 nozzles rotates in each ring due to water pressure. This 
ensures that the whole sieve surface is evenly sprayed. Up to five analysis 
sieves of diameter 200 mm can be clamped in the spray tower. 
A wet sieving device for the size range 10 to 100 jiim that includes a 
sieve vibrator of variable amplitude is commercially available (Figure 
4.10). The device consists of a variable-amplitude vibrator (1) with 
support (2), microsieve (3), regulator with voltmeter for amplitude (4) and 
timer (5). 
£^ B O ^C^ o DtD 
Fig. 4.10 The Alpine wet sieving device. 
Hosokawa Mikropul Micron Washsieve is one version of a wet sieving 
process where water is sprayed on to the surface of a vibrating sieve 
(Figure 4.11). The machine consists of a sprinkler section, a sieving 
section and an electromagnetic section. The sprinkler rotates through the 
force of water to give an even spray whilst the sieve is vibrated to prevent 
Sieving 237 
Gallenkamp Gallie-Porritt apparatus (BS 4398) consists of a metal 
funnel terminating in a short cylindrical outlet in which a wire sieve cloth 
is soldered. Water, at a pressure greater than 2 bar, is supplied by a nozzle 
to discharge a spreading jet through the sieve. A similar arrangement is 
provided for another tube to give a gentle stream of water to keep the level 
of the water in the funnel constant throughout the test. About 25 g of 
powder is slurried and introduced into the funnel at the commencement of 
the test, which continues until the water issuing from the apparatus, is 
clear. The residual mass is determined in order to find the mass percentage 
Fig. 4.11 The Hosakawa Mikropul Micron Washsieve. 
4.14 Air-jet sieving 
The principle of operation of this instrument (Figure 4.12) is that air is 
drawn upwards, through a sieve, from a rotating slit so that material on the 
sieve is fluidized. At the same time, a negative pressure is applied to the 
bottom of the sieve that removes fine particles to a collecting device (a 
filter paper). With this technique, there is a reduced tendency to blind the 
apertures, and the action is very gentle, making it suitable for brittle and 
fragile powders. Sieving is possible with some powders down to 10 |Lim in 
size but with others, balling occurs [95,96]. 
238 Powder sampling and particle size determination 
Vacuum tight lid 
Rotating arm 
Fig. 4.12 Mode of action of the Alpine Air Jet Sieve 
Top diaphragm 
^ ^ holder 
Sample prior 
to sieving 
Sample in 
lift position 
Sample in 
sift position Sample sifted 
Fig. 4.13 Mode of action of the Sonic Sifter [103]. 
The reproducibility is much better than by hand or machine sieving. Size 
analyses are performed by removal of particles from the fine end of the 
size distribution by using single sieves consecutively. 
The end-point of sieving can be determined by microscopic 
examination of the cleanness of the sample [97] or by adopting the same 
criteria as are used in conventional dry sieving. Since only one sieve is in 
operation at any one time, a full analysis may be unduly protracted 
although with sieves coarser than 30 )Lim the run time is less than 3 min. 
Sieving 239 
Sieving is more protracted with finer mesh sieves, and a sieving time of 
20min. is usual with a 1 g load on a 3in diameter 20 \m\ sieve. Similar 
instruments are marketed by Alpine, Micron Powder Systems and by 
MicroPul. Jones [98] has presented a discussion of this technique and 
Lauer [83,99] has appraised it by microscopic examination of the powder 
4.15 The Sonic Sifter 
ATM Sonic Sifter [100] is produced as a laboratory (LP3) and as an 
industrial (P60) model. It is claimed to be able to separate particles in the 
2000 to 20 |im size range for most materials and 5660 to 5 |Lim in some 
cases. It combines two motions to provide particle separation, a vertical 
oscillating column of air and a repetitive mechanical pulse. The Sonic 
Sifter moves the air in the sieve stack (Figure 4.13). The oscillating air 
sets the sample in periodic vertical motion, which reduces sieve blinding 
and breaks down aggregates and yet produces very little abrasion, thus 
reducing sieve wear and particle breakage. 
Gilson GA-6 Autosiever uses a unique double tapping action together 
with sonic sifting. The GA-6 features a memory for storing up to 10 
programs. Its performance in the BCR round-robin exercise with the 
Whitehouse 20-100 |Lim reference standard has been reported. The average 
sample loss after sieving five samples from the 100-stage riffle was 0.4% 
and the standard deviation was 1.46% [101]. 
A downward air flow has been found to improve sieving rate for 10, 25 
and 45 |Lim electroformed sieves using a sonic sifter operating at 107 dB 
and 78 Hz [102]. The sieving rate also increased with decreasing feed rate, 
especially for sub-10 |am sieves [103]. 
4.16 The Seishin Robot Sifter 
Seishin Robot Sifter is an automated version of the ATM in that it 
includes a robot arm that transports the empty sieves to a balance where 
they are weighed and the data stored. At the end of this operation, a nest 
of sieves has been assembled on the balance ready to receive a sample of 
powder on the top sieve. The robot arm then transfers the nest of sieves to 
the sieving position where sieving is automatically performed. Upon 
completion of sieving the sieves are transferred to the balance and re-
weighed. The data is stored in a CPU and can be printed out as required 
(Figure 4.14). 
240 Powder sampling and particle size determination 
- ^ \u2014 \u2022 
Sieving mechanism 
n * Robot mechanism 
(2) Balance 
Fig. 4.14 The Seishin Robot Sifter 
4.17 Automatic systems 
4.17.1 The Rotex Gradex 2000 particle size analyzer 
Rotex use an octagonal compartment drum that can be arranged with up to 
14 testing sieves (Figure 4.15). The operator selects the preset cycle for 
the sample to be analyzed and then loads the sample into the drum, via a 
feed chute, on to the sieve having the smallest opening. The operator then 
initiates the automatic cycle, which imparts a reciprocating motion to the 
drum, separating the smallest size fraction from the sample. At the 
completion of the cycle for this separation, the reciprocating motion stops 
and the material, which passed through the sieve, is weighed. The sample 
then passes to the next coarsest sieve and the operation is repeated. Up to 
six samples can be loaded into an automatic dispenser on top of the 
analyzer. The cycle continues for each sieve in the cycle and the material 
passing through each sieve is accumulated on an electronic scale. A 
computer reads and stores the weight of each fraction, then tares the 
balance before the cycle proceeds to the next sieve. The computer uses 
this data to calculate then print out the size distribution. When the 
Sieving 241 
Autotime computer controlled cycle is selected, the analysis is stopped 
when the throughput rate decreases to a set value. Test times vary from 5 
to 15 min depending on the analysis required [35].