Sieving, particle fractionation

The data for a plot like Fig. 18-60 are easily obtained from a screen analysis of the total crystal content of a known volume (e.g., a liter) of magma. The analysis is made with a closely spaced set of testing sieves, as discussed in Sec. 19, Table 19-6, the cumulative number of particles smaller than each sieve in the nest being plotted against the aperture dimension of that sieve. The fraction retained on each sieve is weighed, and the mass is converted to the equivalent number of particles by dividing by the calculated mass of a particle whose dimension is the arithmetic mean of the mesh sizes of the sieve on which it is retained and the sieve immediately above it. 

Fig. 1. Particle size distributions of the sieved catalyst fractions.

Experimental. The sediment used in this study was obtained from Colma Creek, at Serramonte Boulevard between El Camino Real and Junipero Serra Boulevard, in the city of Colma, San Mateo County, California. Colma Creek was chosen because its entire course occurs within an area of urbanization, and the sediments are therefore of the type which normally come in contact with lead and other heavy metals. The main sources of lead, atmospheric fallout and rainfall runoff contain particulate matter from automobile exhaust emissions. Several shovels full of the bottom material were placed in a plastic container. In the laboratory, several kilograms of the material were wet-sieved, and the fraction passing through a 200-mesh sieve (particle diameter less than 74 ym) was placed in a 1-liter graduated cylinder containing a 1 M sodium phosphate solution. The silt fraction settles in this medium while the finer clay particles remain suspended. After several hours, this clay suspension was then decanted and a portion of the material saved for X-ray diffraction, as were portions of the sand and silt fractions. 

The in situ Raman spectroscopy method has also been used to study the particle size-dependent molecular rearrangements that take place during the dehydration of trehalose dihydrate.73 Different phases were sieved into fractions <45-pm and >425-pm particle size, and the Raman spectra obtained at various times during an isothermal heating at 80°C. After being heated for 210 minutes, the <45-pm dihydrate material appeared to become amorphous while the >425-pm dihydrate material transformed into the crystalline anhydrate phase. Ratios of various characteristic scattering peaks were used to follow the kinetics of the phase transformations. 

In the preparation of each of the three materials about one ton of fresh material was collected, washed carefully and air-dried. Drying was completed at 100°C and then the material was exposed to a temperature of 130°C for several hours. The resulting material was ground by ball-milling and passed through a 125 pm sieve. The fraction with a particle size smaller than 125 pm was collected in a plastic homogenising drum of 180 L... 

The freeze-dried materials were sent to the Joint Research Centre of Ispra where they were ground using a mill equipped with zirconium dioxide balls. The ground materials were sieved using a vibrating stainless steel sieve. The fractions with particles larger than 125 pm were discarded and the remaining materials were stored in polyethylene... 

We have already discussed measurement of particle size by the use of particle dimensions in Section II. where particle shape as relating to particle size was a major concern. Note that even though the shape of the particle may be quite elongated, the stated size wiU be predicated upon a spheroidal shape. The stated (measured) size of a PSD based upon weight will depend upon how well the particles were separated into fractions. For example, if we use a set of sieves to separate particle fractions, the fractions of particles measured may, or may not, be a function of shape. Acicular particles will not pass through a given screen the same way as... 

Particle Sizing by Vacuum Membrane Filtration. In the original plan for this research, particle fractionation by sieving, that is, using microscreens (10, 20, or 30 /xm) and Nuclepore membranes, was anticipated to be a viable means for particle sizing. Microscreens are available in both nylon and stainless steel. The Nuclepore membranes are thin polycarbonate sheets with very uniform round holes etched through them available pore diameters decrease in size from 8.0 /xm. 

Unless solubility data for specific industrial substances are required, both the solute and solvent should be of the highest purity possible. The solute partieles should be reasonably small to facilitate rapid dissolution, but not too small that the excess particles will not settle readily in the saturated solution. Settling is generally desirable to allow solid and liquid phase samples to be taken, after equilibration, for separate analysis. In practice, a close-sieved crystal fraction in the 100-300 pm size range is generally suitable for most purposes. 

Equations (31) and (32) can be used to analyze impedance spectra without knowledge of structural electrode parameters (thickness, density, etc). However, we need this information in order to transform the ohmic parameters obtained by a fit into specific electrochemical parameters. In particular, this information can be used to calculate the effective surface area of the particles. Particles used in practical batteries can usually be treated either as thin plates (Levi and Aurbach [1997]) or as pseudospherical in shape (Barsoukov [2003]), and have a narrow size distribution due to sieving. Particle size values are provided by material manufacturers. The number of particles in a given volume can be estimated from the ratio of their crystallographic density of particles, Op, to the density of the composite-electrode film, a. This allows one to calculate the electrochemically active surface area for a composite electrode for thin-plate particles as 5 = xAdalUOp] and for spherical particles as 5 = 3xAdal[rCp. Here x is the fraction of active material in the composite A is the geometric area of the electrode d is the thickness of the composite electrode <7 is the density of the composite electrode Op is the true density of particles and I and r are the thickness of the plate and radius of spherical particles, respectively. 

Bulk soil samples which have not been sieved into different size fractions have the potential to underestimate lead content relative to those smaller particle fractions which have the higher lead exposure potential. That is, larger particles will have lower lead content but may contribute substantially to sample mass. Other mass determination concerns include the need to exclude large-sized detritus and organic matter, since the latter contributes to sample mass but has questionable relevance as a lead exposure medium. 

Ore samples are analyzed for %w/w Ni. A jaw crusher is used to break the original ore sample into smaller pieces that are then sieved into 5 size fractions. A portion of each fraction is reduced in size using a disk mill and samples taken for analysis by coning and quartering. The effect of particle size on the determination of %w/w Ni is evaluated. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

The particle mass retained by each sieve is determined by weighing after drying when necessary, and each fraction is designated by the sieve size it passed and the size on which it was retained. The sieve diameter of a particle is therefore defined as the size of the sieve aperture through which the particle in question just passes through. Mass fractions of the particles are then presented in tabular or graphical form. 

In industrial practice, the size-distribution cui ve usually is not actually construc ted. Instead, a mean value of the population density for any sieve fraction of interest (in essence, the population density of the particle of average dimension in that fraction) is determined directly as AN/AL, AN being the number of particles retained on the sieve and AL being the difference between the mesh sizes of the retaining sieve and its immediate predecessor. It is common to employ the units of (mm-L)" for n. 

Commercial preparations of these supports are available in narrow mesh-range fractions to obtain particles of uniform size the material should be sieved to the desired particle size range and repeatedly water floated to remove fine particles which contribute to excessive pressure drop in the final column. To a good approximation the height equivalent of a theoretical plate is proportional to the average particle diameter so that theoretically the smallest possible particles should be preferred in terms of column efficiency. Decreasing particle size will, however, rapidly increase the gas pressure necessary to achieve flow through the column and in practice the best choice is 80/100 mesh for a... 

Data in Table I reveal that sieved fractions of different particle size distribution lose varying amounts of moisture dining the same drying period at 70° C. The last column in the table shows that the apparent percentage of moisture in the different frac-... 

Slater, GW Guo, HL, Ogston Gel Electrophoretic Sieving How Is the Fractional Volume Available to a Particle Related to Its Mobility and Diffusion Coefficient(s) , Electrophoresis 16,11, 1995. 

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