Particle size determination sedimentation methods

Temperature control is an important factor in determining particle size by sedimentation methods. During a typical run changes in spin fluid temperature of 2-4°C were common. This temperature change (DELTEMP) was used as the covariate in the analysis of covariance. 

The size range of a material is determined by sieving for relatively large particles and by sedimentation methods for particles which are too fine for sieving. 

In addition, maximum expansion occurred at pH = 12.5 in the PCS experiments compared with approximately 10.5 observed in the sedimentation and viscometry experiments. Since PCS is carried out at much lower particle concentrations, interactions between the charged particles at the higher concentrations are probably involved. Similar comparisons with non-expanding carboxylic latex particles were carried out in an effort to separate interparticle charge effects from true particle expansion in interpreting apparent particle sizes determined by hydrodynamic methods. 

The particle size determined by sedimentation techniques is an equivalent spherical diameter, also known as the equivalent settling diameter, defined as the diameter of a sphere of the same density as the irregularly shaped particle that exhibits an identical free-fall velocity. Thus it is an appropriate diameter upon which to base particle behavior in other fiuid-fiow situations. Variations in the particle size distribution can occur for nonspherical particles (43,44). The upper size limit for sedimentation methods is estabHshed by the value of the particle Reynolds number, given by equation 11 ... 

Gravitational sedimentation methods of particle size determination... 

Multiple techniques can be used to measure the particle size distribution, for example electrozone sensing, sedimentation, laser diffraction, and microscopy. With the exception of microscopy, they all require calibration and the results depend on the technique. For example, in a round-robin study reported in Reference 4, the commonly used electrical sensing zone technique (Coulter Counter) was compared to microscopy and sedimentation. The average particle size determined by the electrical sensing zone method was by about 25%... 

Field-flow fractionation (FFF) is a relatively new analytical technique applicable to the separation of fine particles, polymers and macromolecules in solutions. Recent efforts concerned with Sedimentation field-flow fractionation (SdFFF) is to separate a wide variety of particulate species and to apply it to the particle size measurement. That is because SdFFF has advantages that it employs the fractional collection sorted by the particle mass, and has a high resolution over a wide range of particle size compared to other methods of sub-micrometer particle size determination. 

Measured size distributions depend not only on the physical dimensions of the particles but also on the method of size analysis used. Size distributions by the Coulter Principle will only agree with sedimentation data if the particles are spherical. Indeed the difference is a measure of particle shape. Since classifiers separate particles on the basis of their Stokes sizes a sedimentation method of size analysis should be used to determine their grade efficiency. Sedimentation analyses are also applicable to many other industrial situations. 

The types of sedimentation encountered in process technology will be greatly affected not only by the obvious factors—particle size, hquid viscosity, sohd and solution densities—but also by the characteristics of the particles within the shiny. These properties, as well as the process requirements, will help determine both the type of equipment which will achieve the desired ends most effectively and the testing methods to be used to select the equipment. 

Gravitational Sedimentation Methods In gravitational sedimentation methods, particle size is determined from settling velocity... 

Turbidimetric methods for particle size analysis (Refs 10, 16 27) are used in conjunction with sedimentation procedures to determine the concn of particles at given distances below the surface at successive time intervals. From these data, and Stokes law, particle size distribution can be calculated... 

The size of a spherical particle is readily expressed in terms of its diameter. With asymmetrical particles, an equivalent spherical diameter is used to relate the size of the particle to the diameter of a perfect sphere having the same surface area (surface diameter, ds), the same volume (volume diameter, dv), or the same observed area in its most stable plane (projected diameter, dp) [46], The size may also be expressed using the Stokes diameter, dst, which describes an equivalent sphere undergoing sedimentation at the same rate as the sample particle. Obviously, the type of diameter reflects the method and equipment employed in determining the particle size. Since any collection of particles is usually polydisperse (as opposed to a monodisperse sample in which particles are fairly uniform in size), it is necessary to know not only the mean size of the particles, but also the particle size distribution. 

The particle size and distribution in soils can be easily determined by the sedimentation method. It is based on the fact that large particles will settle faster than smaller ones. No special equipment is needed. 

Although the sedimentation velocity of particles tends to decrease steadily as the concentration of the suspension is increased, it has been shown by Kaye and Boardman11 that particles in very dilute suspensions may settle at velocities up to 1.5 times the normal terminal falling velocities, due to the formation of clusters of particles which settle in well-defined streams. This effect is important when particle size is determined by a method involving the measurement of the settling velocity of particles in dilute concentration, though is not significant with concentrated suspensions. 

The terminal velocity in the case of fine particles is approached so quickly that in practical engineering calculations the settling is taken as a constant velocity motion and the acceleration period is neglected. Equation 7 can also be applied to nonspherical particles if the particle size x is the equivalent Stokes diameter as determined by sedimentation or elutriation methods of particle-size measurement. 

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