Classifying particle size methods

The sizing methods involve both classical and modem instrumentations, based on a broad spectrum of physical principles. The typical measuring systems may be classified according to their operation mechanisms, which include mechanical (sieving), optical and electronic (microscopy, laser Doppler phase shift, Fraunhofer diffraction, transmission electron miscroscopy [TEM], and scanning electron microscopy [SEM]), dynamic (sedimentation), and physical and chemical (gas adsorption) principles. The methods to be introduced later are briefly summarized in Table 1.2. A more complete list of particle sizing methods is given by Svarovsky (1990). 

Microscopy remains the principal standard method of particle sizing and of shape and morphological classification. Though often tedious and time consuming in its application, it remains a standard by which individual particles can be classified with confidence and most particle sizing methods are referenced. 

Typical separation efficiency curves of an air classifier versus particle size are given in Fig. 20-14. The amount of top size in the fines may be very low, but there is typically 10 to 30 percent fines in the coarse product that is, the low end of the cui ve tends to flatten out at 10 to 30 percent. In addition, the separation at the cut size is typically a gradu cui ve. Data of this sort, which are needed to evaluate closed-circuit mill performance, are seldom available. See subsection on characteristics of size classifiers for a testing method. 

HEPA and ULPA filters have been developed. In the CEN EN 1822 1998 test method, the filter s efficiency is determined for the most penetrating particle size (MPPS). Depending on the filter s total level of separation and leakage, the filter is classified as HIO, Hll,.. ., H14 and U15, U16, or U17. HEPA filters are commonly used for inlet air in the pharmaceutical, optical, and food industries. 

The methods by which particle size determination is carried out have been presented in Figure 2.3. The methods are observed to be broadly classified into two groups static meth-... 

Herein lies the value of these different averages the divergence between the averages calculated by different methods offers a clue as to the breadth of the distribution of particle sizes. Remember, the average, however evaluated, is only one measure of the distribution of sizes. A fuller description requires some measure of the width of the distribution as well. For classified data, the standard deviation (see Appendix C) is routinely used for this purpose. For characterizations based on macroscopic experiments such as we have been discussing it is quantities such as ds/d or dv/ds that quantify this spread. (The averages ds and dv are defined below and are also discussed in Appendix C.)... 

Particles can be broadly classified as either colloids or as macroparticulate powders. Colloids typically have dimensions smaller than 1000 A and are optically transparent, while dispersed powders are generally larger and form turbid suspensions. Neither colloidal dispersions nor powder suspensions are usually monodisperse, and to the extent that particle size can influence attainable surface charge and area, many such systems will typically reflect a distribution of properties as a function of preparation method. Recent advances in synthetic techniques for providing materials with reduced polydispersity are likely to allow for better characterization of these effects in the near future. 

Spheronizing processes for particles in the millimetre range may be classified into methods which rely on the build-up of smaller particles into spheres by means of a rolling, or snowball technique (see granulation), and those which form spheres by individually shaping particles. The production costs are low for the first technique, but the spheres are irregular and there is a broad particle size distribution. The spheres manufactured by individual shaping are more expensive,... 

Many methods are available for determining particle size in pharmaceutical practice, including microscopy, sieving, sedimentation, and determination of particle volume [19]. Sieve analysis with U.S. standard sieves is widely used to determine the particle size distribution based on powder weight. Sieves are classified according to the number of openings (Table 14) and are generally made of wire cloth woven from brass, bronze, or other suitable wire. 

Figure 2 Representative example of a mass distribution of ambient particulate matter as a function of particle diameter. Mass distribution per particle size interval is shown as Amass/A(logDa) (in Rgni ) plotted against particle size (ZJa) in micrometers. Tbe figure also shows the range of aerosol sizes included in various methods of aerosol measurement wide range aerosol classifiers (WRAC), total suspended particulate (TSP) samplers, PMjo and PM25 samplers (source Lippman and Schlesinger, 2000) (reproduced by permission of Annual Reviews from Annual Review of Public Health 2000, 21, 309-333).

Sizes of particle are important in selecting the method of separation. Different authors do not necessarily use the same nomenclature in classifying separations in terms of size. A crude relationship of particle size to particle size measurements and methods of SLS and particle size analysis is provided in Fig. 2. ... 

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