Sedimentation techniques, particle

The particle size deterrnined 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 fluid-flow 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 ... 

Glassification. Classification (2,12,26,28) or elutriation processes separate particles by the differences in how they settle in a Hquid or moving gas stream. Classification can be used to eliminate fine or coarse particles, or to produce a narrow particle size distribution powder. Classification by sedimentation iavolves particle settling in a Hquid for a predetermined time to achieve the desired particle size and size distribution or cut. Below - 10 fim, where interparticle forces can be significant, gravitational-induced separation becomes inefficient, and cyclone and centrifugation techniques must be used. Classification also separates particles by density and shape. Raw material separation by differential sedimentation is commonly used in mineral processiag. 

Before beginning a size determination, it is customary to look at the material, preferably under a microscope. This examination reveals the approx size range and distribution of the particles, and especially the shapes of the particles and the degree of aggregation. If microscopic examination reveals that the ratios between max and min diameters of individual particles do not exceed 4, and indirect technique for particle size distribution based on sedimentation or elutria-tion may be used. Sedimentation techniques for particle size determination were first used by Hall (Ref 2) in 1904, He showed that the rate of fall of individual particles in a fluid was directly related to the particle size by the hydrodynamic... 

The choice of method from available resources depends largely upon the properties of the material to be analyzed, the basic significance or physical wearing of the measurement, and the purpose for which the information is required. For example, failure to disperse the particles as discrete entities is the biggest single problem in all size analysis methods that depend on individual particulate behavior. With microscopic techniques particles must be dispersed on the slide to permit observation of individual particles, and in sedimentation techniques the material must be suspended in the fluid so that the particles behave as individuals and not as floes. 

These instruments, sometimes referred to also as sediment accumulation devices, weight the sediment as it accumulates on a weigh-pan at the base of the sedimentation column. The methods are cumulative ones. With the development of sensitive electro balances, the cumulative sedimentation technique is generally easier to perform and more accurate than is the incremental technique. The powder may be dispersed initially in the bulk of the fluid or added instantaneously at the top. An advantage of this type of equipment is the absence of the conical base, needed in sediment extraction devices, upon the walls of which some sediment may adhere. The danger of particles sticking to the vertical walls is however still present... 

Liquid Sedimentation Techniques for Measuring the Particle-Size Distribution of Primary Explosives , PATR 4387 (1972) 42) Anon, Stan-... 

There are various methods for the determination of the size distribution of organic pigment particles, the most common are sedimentation techniques in ultracentrifuges and specialized disk centrifuges as well as electron microscopy. These methods require considerable experimental skill, since the results depend largely on sample preparation and especially on the quality of the dispersion. 

Sedimentation techniques such as gravity and centrifugal settling are fairly simple methods of determining the size of particles in soils and in matrices, such as paints and ceramics, where screening is not practical. For some analysis, particularly of soils, the time required to perform the analysis may take as long as 24 h. 

Information on particle size may be obtained from the sedimentation of particles in dilute suspensions. The use of pipette techniques can be rather tedious and care is required to ensure that measurements are sufficiently precise. Instruments such as X-ray or photo-sedimentometers serve to automate this method in a non-intrusive manner. The attenuation of a narrow collimated beam of radiation passing horizontally through a sample of suspension is related to the mass of solid material in the path of the beam. This attenuation can be monitored at a fixed height in the suspension, or can be monitored as the beam is raised at a known rate. This latter procedure serves to reduce the time required to obtain sufficient data from which the particle size distribution may be calculated. This technique is limited to the analysis of particles whose settling behaviour follows Stokes law, as discussed in Section 3.3.4, and to conditions where any diffusive motion of particles is negligible. 

Sedimentation techniques are also utilized for particle size distribution for particles on the order of 0.1-50 pm. Capillary hydrodynamic chromatography (HDC) gives particle size distributions for particles of about 0.005-0.7 pm. 

Minerals were ground such that a size distribution ranging from sub-micron to millimetre particles were obtained. For the experiments described here, it is desirable to use monodisperse minerals. To this end, a sedimentation technique was used to obtain minerals in the particle size range (effective Stokes radius) (i) 10-20 jam and (ii) above 20 um. 

The expansion characteristics of carboxylic latex particles have been measured using three independent techniques sedimentation, which uses the change in particle density due to swelling to determine the change in particle size viscometry, which measures volume changes and photon correlation spectroscopy, which measures the diffusion coefficient of the particles. The sedimentation technique offers precise measurements at low shear but requires relatively... 

It is not always easy to determine what procedure to follow in making a particle-size distribution. Obviously, if the particles are soluble in water or any other fluid, sedimentation procedures must be applied with caution. It is likewise clear that any sedimentation technique is markedly affected by the shape of the particles used, and that results are subject to interpretation. In other words, determinations depending upon sedimentation (and elutriation) are merely equivalent measures of spheres having the same rate of settling. Greatest reliance naturally applies to that size range whose motion is known to be specified by Stokes law. 

Another comparison of AUC and FFF was reported for the Fl-FFF separation of native ferritin which exhibits a particle size distribution (monomer, dimer, trimer) as well as a density distribution due to non-uniform amounts of FeOOH in the core [128]. Such samples are notoriously difficult to characterize by sedimentation techniques like S-FFF and AUC because size and density distributions are superimposed whereas Fl-FFF was found to yield baseline resolved peaks for each of the oligomers due to the separation dependent only on diffusion coefficients. 

Size and size distribution can be studied by classical sedimentation techniques. The classification of soils is based on particle sizes (Chapter 1, Section 1.1.3, Table 1.6). The size and shape of the particles can be observed by different microscopes, from the traditional light microscopes to scanning and transmission electron microscopes. The nanometer-sized particles can be observed by the atomic force microscope. This microscope, equipped with a microprobe (scanning and transmission microscope), is suitable for the chemical analysis of the sample. 

The coarse particle in the bentonite was removed to make the clay free from other nonclay matter by use of the sedimentation technique. Then NaCl was added to the clay suspension in water to carry out the intercalation reaction at 90 C for 2h under vigorous stirring. 

Sedimentation Techniques. Other techniques that effect a physical separation include gravitational or centrifugal sedimentation, in which particles or emulsion droplets are separated on the basis of size and density. The separation that occurs can be quantified by monitoring X-ray or light absorbance as a function of position. Stokes law then can be used to determine the particle size distribution from the absorbance data as a function of the sedimentation time (73, 74). 

The particles comprising aquifer sediments range in size from sand (2-0.5 mm diameter (Allaby and Allaby, 1990) to colloid (between 1 nm and 1 xm in diameter (Stumm, 1992). Smaller-sized particles and/or those with complex shapes have a higher surface area-to-volume ratio and more reactive surface area (i.e., the fraction of total surface area that participates in interfacial reactions) per unit mass than larger, simply shaped particles (Parks, 1990). As a result, the smallest particles may dominate the overall mass transfer in aquifer sediments. These particles are commonly x-ray amorphous, meaning that their structure cannot be resolved using standard x-ray diffraction techniques. However, they are partially ordered, with crystalline domains typically < 15 A in diameter (Waychunas et al., 1996). Commonly, the smallest particles are also physically or chemically attached to the surface of larger particles. 

During particle size analysis, in addition to screening, sifting, and counting, sedimentation techniques are often used which produce unequivocal results only if the individual particles can move without influencing each other. For that reason, very dilute suspensions are used. Nevertheless, it is possible that agglomerates form or already... 

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