Sieving

Sieving is known as one of the most useful, simple, reproducible, and inexpensive methods of particle size analysis, and belongs to the techniques using the principle of geometry similarity. It is

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Table 2.9. Analytical techniques of particle size measurement. Approximate size Type of Type of size Technique range (µm) particle size distribution Sieving

Woven wire 37–4,000 xA By mass

Electro formed 5–120 xA By mass

Microscopy

Optical microscopy 3–1,000 xa, xF, xM By number

Transmission electron microscopy 0.002–1 xSH, xCH

Scanning electron microscopy 0.02–1,000 xSH, xCH

Gravity sedimentation

Incremental 2–100 xst, xf By mass

Cumulative 2–100 xst, xf By mass

Centrifugal sedimentation

Two layer-incremental 0.01–10 xst, xf By mass

Cumulative

Homogeneous-incremental

Flow classification xst, xf

Gravity elutriation (dry) 5–100 xst, xf By mass

Centrifugal elutriation (dry) 2–50 xst, xf By mass

Impact separation (dry) 0.3–50 xst, xf By mass or number

Cyclonic separation (wet or dry) 5–50 xst, xf By mass

Particle counters

Coulter principle (wet) 0.8–200 xv By number

Laser refraction

Low angle laser light scattering 0.1–3,000 By number

considered the only method for giving a particle size distribution based on the mass of particles in each size range. Particle size is defined by the sieve aperture by which a particle may, or may not, pass through. As presented in Table 2.9, all types of sieving cover a range from 5µm to 4 mm. This lower limit can be achieved using micro-mesh sieves, while the upper limit can be extended to the centimeter range by punched-plate sieves. The minimum applicable particle size range is limited for two main reasons: first, it is not possible to produce sieve cloth fine enough for it and, second, very small powders do not have a strong enough gravity force to resist its tendency to adhere to one another and to the sieve cloth (Allen, 1981; Herdan, 1960).

A standard sieve series usually consists of a set of sieves with apertures covering a wide range from microns to centimeters. The sieve size is defined as the minimum square aperture through which the particles can pass. Sieves are often referred to by their mesh size, i.e., the number of wires per linear inch. Mesh size and the wire diameter determine the aperture size. The ratio of aperture of a given sieve to the aperture of the next one in a sieve series is a constant. Standardized sieve apertures were first proposed by Rittinger in 1867. Modern standards are based on either a√2 or√4

2 progression. In the United States the series of sieves with standard opening sizes are called “Tyler” sieves and the openings of successive sieves are based on a√2 progression starting at 45µm. The most common shape of openings is square, but some electroformed and punched-plate sieves have circular openings. Sieves with openings of other shapes (diamond, rectangle, hexagon, slotted) are also in use. Table 2.10 lists the ISO (International Standardization for Organization) and ASTM (American Society for Testing and Materials) standard sieve series.

In Table 2.10 the left column is the part of the sieve series as defined in ISO 565 and ISO 3310 with nominal openings given in millimeters, coinciding with the sieve number. The ASTM series,

Table 2.10. Standard sieve series.

ASTM ASTM

ISO (mm) (mesh) ISO (mm) (mesh) 2.80 No. 7 0.250 No. 60 2.50 — 0.224 — 2.36 No. 8 0.212 No. 70 2.24 — 0.200 — 2.00 No. 10 0.180 No. 80 1.80 — 0.160 — 1.70 No. 12 0.150 No. 100 1.60 — 0.140 — 1.40 No. 14 0.135 No. 120 1.25 — 0.112 — 1.18 No. 16 0.106 No. 140 1.12 — 0.100 — 1.00 No. 18 0.090 No. 170 0.900 — 0.080 — 0.850 No. 20 0.075 No. 200 0.800 — 0.071 — 0.710 No. 25 0.063 No. 230 0.630 — 0.056 — 0.600 No. 30 0.053 No. 270 0.560 — 0.050 — 0.500 No. 35 0.045 No. 325 0.450 — 0.040 — 0.425 No. 40 0.038 No. 400 0.400 — 0.036 — 0.355 No. 45 0.032 No. 450 0.315 — 0.025 No. 500 0.300 No. 50 0.020 No. 635 0.280 — — —

which is defined in the ASTM Standard E11, is listed in the right column; the nominal openings correspond to openings in the ISO series. Many countries also have their own standard test sieve series corresponding to part of the ISO series. A partial list of other country’s standards includes Australia (AS 1152), Britain (BS 410), Canada (CGS-8.2-M88), France (NFX 11-501), Germany (DIN 4188), India (IS 460), Ireland (I.S. 24), Italy (UNI 2331), Japan (JIS Z 8801), and South Africa (SABS 197). Sieving analysis consists of stacking the sieves in ascending order of aperture size, placing the material concerned on the top sieve, vibrating the sieves by machine or hand for a fixed time, and determining the weight fraction retained on each sieve. Additional forces may also be used to help the sieving process, such as liquid flow, air jet, and vibrating air column. Allen (1997) brought about an update of sieving equipment such as air-jet sieving, the Sonic Sifter, and automatic sieving systems among other types. Figure 2.15 shows the mode of action of a Sonic Sifter, one of the most used sieving methods in both industry and laboratory research. The wet sieving method is useful for very fine powders or when the material is originally suspended in a liquid. It represents an excellent alternative for powders forming aggregates when dry sieving is used. Typically, results from sieve analysis varies with the method of moving the sieve or particles, the geometry of the sieve surface (sieve type, frictional open area, etc.), the time length of operation, the number of particles of the sieve, and the physical properties of the particles (e.g., their shape, stickiness, and brittleness).

There are two main forms in which the results of a sieve test can be presented: tabular and graphical. As previously discussed, graphical methods are preferred in particle size analysis, as they

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Figure 2.15. Mode of action of a Sonic Sifter (adapted from Allen, 1997).

provide a simple way of identifying a representative size of the powder being analyzed. In sieving, as in all size measurement techniques, cumulative percentages of oversize or undersize material against particle size is plotted to obtain graphs of useful information for powder characterization. Convention commands the use of the sieve diameter xA, as defined in Table 2.3, to be the particle size plotted on

cumulative percentage graphs, but it is still customary to use the mesh number instead. Sometimes the mesh number is plotted progressively, so the normally obtained oversize graph would present the typical rising character of an undersize graph, since mesh number gets larger as the particle size actually gets smaller.