Particle size analysis by sieving
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Particle size analysis by sieving


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to machine sieve for 20 min. 
If more accurate data are required it is preferable to sieve for 10 min. 
and weigh the residues. Repeat 2 min. sieving should then be carried out 
until the amount passing any given sieve is less than 0.2% of the initial 
load. With brittle material care should be taken that particle breakage does 
not occur. Granular particles pass through the sieve more rapidly than 
elongated particles although spherical particles are inclined to block the 
apertures. 
Several variants of microsieving(5-30 |Lim) using ultrasound have been 
examined [74]. Efficient sieving was only possible by using a liquid 
column and high suspension transport. The required conditions were 
determined for quartz and chalk to give an undersize free residue. 
Sieving 229 
Ultrasonics are frequently used as an aid to sieving or for cleaning blocked 
sieves; the danger of rupturing the delicate mesh is possible under these 
conditions, readily occurring at low frequencies circ.50 Hz and sometimes 
at frequencies as high as 20 kHz. A recommended safe frequency, 
according to Colon, is 40 kHz though Craw l^ey [75] reports damage to an 
11 |am sieve at a frequency of 80 kHz and a power level of 40 W and 
subsequently recommends a frequency of 800 kHz and a power level of 20 
W. Daescher [76] confirmed Rosenberg's [77] findings that the rate of 
cavitation is less in hydrocarbons than in alcohol and about six times 
greater in water than in alcohol. Saturating the alcohol with carbon 
dioxide was found to reduce the rate. He tested three ultrasonic cleaners, 
40 kHz, 60 W; 40 kHz, 100 W; 90 kHz, 40 W, and found that the higher 
frequency cleaner produced the least amount of erosion. Veco recommend 
15 to 20 s at a time, with a low power 40 kHz ultrasonic bath containing an 
equi-volume mixture of isopropyl or ethyl alcohol and water, with the 
sieve in a vertical position, and this is in accord with ASTM El61-70. 
Alpine recommend a cleaning time varying from 10 to 20 min. for 10 |Lim 
sieves down to 2 to 4 min. for sieves in the size range 50 to 100 |im. In a 
comparison between round and square hole micromesh sieves [78] it was 
found that the effective apertures of the round mesh sieves, using sand, was 
21% greater than the square mesh apertures. 
4.10 Amount of sample required 
In determining the amount of sample to be used, it is necessary to consider 
the type of material, its sievability, and the range of sizes present [79]. 
Two opposing criteria must be met; it is necessary to use sufficient 
material for accurate weighing and a small enough sample that the sieving 
operation is completed in a reasonable time. The natural tendency is to use 
too large a sample though, in practice, the smaller the sample, within 
limits, the more reproducible the data. 
Table 4.6 Amount of sample required for a sieve analysis on an 8 in 
diameter sieve 
(a) Based on particle density 
1 Density (g cm^^) 
Sample weight (g) 
1.5 
25 
1.5-3.0 
50 
+3 
100 
230 Powder sampling and particle size determination 
(b) Based on median diameter 
Median, (mm) 
Sample weight (g) 
>2 
500 
2-1 
200 
1-0.5 
100 
0.50-0.25 
75 
0.25-0.075 
50 
< 0.075 
25 
As a rough guide, the amounts recommended for 8 in diameter sieves are 
given in Table 4.6a. Alternatively, the sample weight may be based on the 
median particle size (Table 4.6b), but this neglects to take into account that 
the narrower the distribution, the smaller the sample required [80]. 
4.11 Hand sieving 
Hand sieving is time consuming but necessary for dependable dry sieving 
data. A representative sample is obtained and the whole of the sample 
used in the analysis. The preferred method of sampling is with a spinning 
riffler or, failing that, a chute splitter. Coning and quartering induces 
segregation and should never be used with free-flowing powders. 
It is recommended that, for a dry sieving operation, the fines be 
removed prior to the sieve analysis. This is effected by pre-sieving, 
usually by hand, on the finest sieve to be used in the subsequent analysis. 
If this is not done, the fines have to pass through the whole stack of sieves, 
thus increasing the time of the sieving operation and increasing the risk of 
high powder loss. Since small particles often adhere to large ones, this 
pre-sieving may be carried out wet, as recommended in ASTM C925 [24], 
using water (with wetting agent if necessary) or some other liquid in which 
the powder is insoluble. 
The smallest aperture sieve to be used should be rested on a catch pan, 
the tared sample placed on the mesh and the whole sealed with a lid. The 
sieve should be slightly inclined to the horizontal and rapped with a 
cylindrical piece of wood about 20 cm long and 3 cm in diameter; this 
should be wrapped in duct tape to eliminate splintering. (The heel of the 
hand is recommended in some standards and is an acceptable alternative if 
the hand can take it!). The rate of tapping should be about 150 per min. 
and the sieve should be rotated 1/8 of a turn every 25 raps. After about 10 
min. the residue is transferred to the coarsest sieve that is nested in a 
second catch pan for subsequent weighing. It is suggested that white card 
be placed on the bench (approximately 60 cm square) so that accidental 
spillage may be recovered. The process is repeated in 2 min. cycles until 
less than 0.2% of the original charge passes through the sieve. The powder 
is not normally removed from the sieve unless excessive blinding is 
Sieving 231 
occurring; both the sieve and the residue are weighed and the residual 
weight determined. At the end of the sieving operation the sieve is 
upturned on a white sheet of paper and the fine particles adhering to it 
removed with a soft brush and added to the sieve residue. 
The process is repeated with sieves of increasing fineness and the 
residue weights collated. Finally, the process is repeated with the finest 
sieve and the fines are added to the dust collected initially. Brushing is not 
recommended for sieves with aperture less than 200 |Lim due to the 
possibility of damage. Sieves should be washed and dried after use. 
Ultrasonics should be used to remove particles clogging the apertures or 
these may be leached out if this can be done without damage to the sieve. 
The results may be expressed in terms of the nominal size, although it is 
preferable to use calibrated sieves. A reference set of sieves should be 
used after every fiftieth analysis for comparison purposes in order to detect 
wear. In essence, the smaller the sieve loading, the more rapid is the 
sieving operation. The low weights however lead to errors in weighing and 
intolerable percentage losses. 
4.12 Machine sieving 
Machine sieving is carried out by stacking the sieves in ascending order of 
aperture size and placing the powder on the top sieve. A closed pan, a 
receiver, is placed at the bottom of the stack to collect the fines and a lid is 
placed at the top to prevent loss of powder. A stack usually consists of 
five or six sieves in a root two progression of aperture size. The stack of 
sieves is clamped on to a test sieve shaker that is vibrated for a fixed time 
and the residual weight of powder on each sieve is determined. Results are 
usually expressed in the form of a cumulative percentage of the nominal 
sieve aperture. 
The three essentials required in a test sieve shaker are: An effective 
sieving action in order that an end-point is reached this end-point to be 
reached in a short time and reproducible results. 
For routine control purposes it is usual to machine sieve for 20 min., 
after which time the sieving operation is deemed to be complete. It is 
recommended in BS 1796 [14] that sieving be continued until less than 
0.2% of the sample passes through in any 2-minute sieving period. In 
ASTM D452 [81] a 20 min. initial sieving period is recommended 
followed by 10 min. periods during which the amount passing should be 
less than 0.5% of the total