Particle size measurement gravitational sedimentation

For the particle size measurements of boron and barium dichromate, components of pyrotechnic delay compns, Freeman (Ref 46) evaluated the MSA. Particle Size Analyzer versus microscopy, gravitational liq sedimentation,... 

Sedimentation techniques are amongst the oldest methods of particle size measurement. Sizes are derived from the rate of settling of particles under the action of gravitational or centrifugal forces. One sedimentation technique is the disk centrifuge, which is used to measure particles in the range 0.1-50 pm. 

Gravitational settling of particles in liquids is an age-old process which can be used for a variety of purposes. For example, it is used for the classification of solids, washing, particle size measurement or mass transfer, and in solvent extraction. The majority of applications of gravity sedimentation, however, are in solid-liquid separation duty. The object here is to remove the solids from the liquid either because the solids and/or the liquid are valuable or because the two phases have to be separated before disposal. 

Berg, S. Determination of particle size distribution by examining gravitational and centrifugal sedimentation to the pipett method and with divers. Symp. PSA, June 1958, Boston, ASTM STP 234 (1959), p. 143 - 171 /4/ Chung, H. S. Hogg, R. The effect of Brownian motion on particle size analysis by sedimentation. Powder Techn. 41 (1985) 3, p. 211 - 216 /5/ Allen, T. Sedimentation techniques of particle size measurement. Conf. PSA Sept. 1985, Bradford, Proceed, p. 24 - 45... 

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

Gravitational settlement is allowed to proceed for 4 to 10 minutes, according to the particle-size range of the sample. The sedimentation tube is then centrifuged to reduce the time required for the smaller particles to reach the bottom. By measuring the volume of particles accumulated as a function of time, the equivalent spherical size distribution of the sample may be computed from formulae based upon Stokes law. In addition to the specially designed sedi-... 

It is our objective in this chapter to outline the basic concepts that are behind sedimentation and diffusion. As we see in this chapter, gravitational and centrifugal sedimentation are frequently used for particle-size analysis as well as for obtaining measures of solvation and shapes of particles. Diffusion plays a much more prevalent role in numerous aspects of colloid science and is also used in particle-size analysis, as we see in Chapter 5 when we discuss dynamic light scattering. The equilibrium between centrifugation and diffusion is particularly important in analytical and preparative ultracentrifuges. 

It might be noted that sedimentation equilibrium is approached very slowly however, techniques that permit equilibrium conditions to be estimated from preequilibrium measurements have been developed by W. J. Archibald. Equations (86) and (87) predict a linear semilogarithmic plot of c versus x or x2 for gravitational and centrifugal studies, respectively. The slope of such a plot is proportional to the mass of the particles involved. Remember that monodispersity was assumed in the derivation of these equations. If this condition is not met for an experimental system, the plot just described will not be linear. If each particle size present is at equilibrium, however, each component will follow the equations and the experimental plot will be the summation of several straight lines. Under certain conditions these may be resolved to give information about the polydispersity of the system. In any event, nonlinearity implies polydispersity once true equilibrium is reached. 

Gravitational sedimentation causes a change in the particle size distribution anywhere in and below the cloud compared with the size distribution at stabilization time. Thus, to reconstruct the size distribution at stabilization time, corrections must be applied to the size distributions measured in the samples. These corrections were calculated by assuming Stokesian settling modified by a drag slip correction. It was assumed further that at stabilization time the cloud was axially symmetric and consisted of spherical particles. Wind and diffusion effects were neglected. 

In this chapter the thermal motion of dissolved macromolecules and dispersed colloidal particles will be considered, as will their motion under the influence of gravitational and centrifugal fields. Thermal motion manifests itself on the microscopic scale in the form of Brownian motion, and on the macroscopic scale in the forms of diffusion and osmosis. Gravity (or a centrifugal field) provides the driving force in sedimentation. Among the techniques for determining molecular or particle size and shape are those which involve the measurement of these simple properties. 

In gravitational sedimentation the panicle concentration in the measurement zone remains equal to the initial concentration until the largest particle present in the suspension has settled through the zone. The concentration of particles in the measurement zone at time t represents panicles whose size is less than or equal to the Stokes diameter d< u>kes- rhe Stokes diameter is inversely proportional to the square root of time as shown by Equation 34-13. Therefore, a plot of concentration versus f/5j,okes represents the undersize distribution. 

When the particle size is too small to be measured by using sedimentation under gravity, its collisions with the molecules of the liquid will cause measurable displacement. This effect will increase the lower limit to about 1 pm, when the gravitational settling measurement is carried out using water as the liquid. If the... 

Determination of particle size distribution by gravitational sedimentation in a liquid and attenuation measurement Pore size distribution and porosity of solid materials - Evaluation by mercury porosimetry and gas adsorption International Union of Pure and Applied Chemistry, Bank,... 

The force causing a particle to sediment or sedtie can be a gravitational force or a centrifugal force. Photo-sedimentation analyzers can measure particle sizes ranging from 0.01 pm to. 300 pm. 

In centrifugal sedimentation, a complication arises because the particle velocity depends on not only particle size, as in gravitational sedimentation, but also the radial position of the particle. As Equation 34-14 shows, the particle velocity increases with increasing radial distance r from the axis of rotation. Because of this, the particle concentration below the measurement zone decreases exponentially with the sedimentation... 

Additional methods to measure crystal size distribution based on particle mass employ light scattering/diffraction methods. Simultaneous measurements of particle size (based on particle volume) and their number are also carried out by Coulter counters. There are a variety of other techniques, based on centrifugal sedimentation, electroacoustic spectroscopy, microelectrophoresis, gravitational sedimentation, scarming electron microscopy, etc. An introduction to these techruques is available in a NIST (National Institute of Standards and Technology) publication by Jillavenkatesa et al. (2001). 

Environmental Fate. It can be concluded from the transport characteristics that surface water sediment will be the repository for atmospheric and aquatic thorium. Normally, thorium compounds will not transport long distances in soil. They will persist in sediment and soil. There is a lack of data on the fate and transport of thorium and its compounds in air. Data regarding measured particulate size and deposition velocity (that determines gravitational settling rates), and knowledge of the chemical forms and the lifetime of the particles in air would be useful. 

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