Screen analyses particle size measurement

Particles come in all shapes and sizes and in large numbers. Data are presented graphically using histograms, fractional plots, or cumulative plots. These graphs are primarily useful as pictures of the size distribution of the mixture. Table 15.4 gives a typical screen analysis for a 900-g sample. The measured experimental data are the mesh sizes, and the masses of the particles on each of the sieves are the masses of the residuals or fines. The other quantities are calculated. 

Consider a solid feed consisting of a mixture of different-size particles. The size distribution of this feed can be represented either as a continuous distribution or as a discrete distribution. We use the latter representation because screen analysis, our way of measuring size distributions, gives discrete measurements. 

Hatch extended his method of analysis to size-distribution curves ranging from coarse-screen analysis through fine particles measured microscopically. While excellent results were obtained by using this technique on laboratory samples, the method cannot be generalized to cover all types of distributions encountered in practice. As already explained in Chapter 3, size-frequency distributions may assume a variety of shapes. The Hatch development applies only to distributions which follow the normal or log-probability law. When size-distributions are hyperbolic in the lower extremes and follow normal log-probability laws in the upper extremes, the Hatch analysis must necessarily fail. Nevertheless, the relationships developed by Hatch have a far-reaching practical importance... 

According to Heywood [1] sieving is the Cinderella of particle size analysis methods it does most of the hard work and gets little consideration. This was reiterated by Leschonski [119] who also quotes the chairman of the Institution of Mining and Metallurgy as stating, in 1903, that screening is not a scientific means of measurement. 

We have seen how problems of particle size distribution of reactant and solid products can be employed in the design of fluid-bed reactors. The conversion obtained at the reactor exit depends on these distributions plus various other factors. The equations presented so far were based on continuous solids feed. In calculating the conversions, it is easier to divide the solid reactants into discrete ranges (each with an average size) and express the conversion as the sum from all the ranges. Furthermore, size distribution is usually determined by screen analysis, which gives discrete measurements. 

SCREEN ANALYSIS STANDARD SCREEN SERIES. Standard screens are used to measure the size (and size distribution) of particles in the size range between about 3 and 0.0015 in. (76 ram and 38 pm). Testing sieves are made of woven wire screens, the mesh and dimensions of which are carefully standardized. The openings are square. Each screen is identified in meshes per inch. The actual openings are smaller than those corresponding to the mesh numbers, however, because of the thickness of the wires. The characteristics of one common series, the Tyler standard... 

Partide-Size Anal is- Particle-size distribution is normaUy measured by conducting a screen analysis" (sieve analysis) of a representative sample on 20-cm-diameter sieves. The sample size recommended for testir on these sieves varies according to the size range of the material. The sieves selected for use are stacked, with sieve apertures increasing in size from bottom to top in the stack. The sample is placed on the uppermost sieve of the stack, and the stack is placed on... 

Mica products are usually analyzed by screening to determine particle distributions. Specifications typically include minimum and maximum values for the amount of mica passing or retained on screens of varying mesh sizes. Another value often used for a material specification is loose bulk density. Laser particle size analysis is also being used to some extent to define mica products. One should remember that most other minerals are measured by an instrument called a sedigraph, which measures the size of particles based on their settling rate. This procedure can be used with very small low-aspect ratio minerals such as calcium carbonate, talc, silica, very small particle size mica, and aluminum trihydrate but cannot be used for analysis of most mica products. Sedigraph particle size values for very small mica products are about one-half to one-third of the values obtained by laser diffraction. 

For a sphere, < >5 = 1.0. For a cylinder where the diameter = length, d>s is calculated to be 0.874 and for a cube, 4>s is calculated as 0.806. For granular materials it is diflScult to measure the actual volume and surface area to obtain the equivalent diameter. Hence, Dp is usually taken to be the nominal size from a screen analysis or visual length measurements. The surface area is determined by adsorption measurements or measurement of the pressure drop in a bed of particles. Then Eq. (3.1—23) is used to calculate (f>s (Table 3.1-1). Typical values for many crushed materials are between 0.6 and 0.7. For convenience for the cylinder and the cube, the nominal diameter is sometimes used (instead of the equivalent diameter) which then gives a shape factor of 1.0. 

Measure the particle size distributions of feed materials and final products performing a screen analysis, and determine the mean particle sieve diameter as described in Chapter 2 (Section 2.7). 

As a final note is important to bear in mind that solid-fluid separations are governed by mechanical forces, such as the gravity force, and so it is important that the screen analysis data on the relevant streams used to derive a grade efficiency curve had been measured avoiding transformations. The use of a particle sizer that measures the relevant particle size directly, that is, any method measuring directly the Stokes diameter should be preferred over a method relying on transforming size distributions from number to surface, or surface to mass, and so on. 

To convey an impression of the particle size distribution of PEG powder, we give here an average screen analysis of PEG powder produced by Hoechst in which the jLm measurement indicates the internal mesh width in accordance with DIN 1171 and mesh indicates mesh/inch in accordance with the British Standard. 

Measurable quantities of cotton dust were extracted from small cotton lint samples by means of high-velocity air jets. Dust removed in this manner was sized by a wire-mesh screen that restricted the flow of particles larger than the openings in the screen. Particles smaller than the openings in the screen were collected for gravimetric analysis. 

Dry-sieve analysis is the easiest and the most convenient method for measuring granule size. The granulation is placed on top of a stack of five to six sieves which have successively smaller-sized openings from top to bottom. The stack is vibrated, and the particles eollect on top of the sieves. The data are usually represented in terms of percentage retained on the sieve, or percentage that is undersize or oversize vs. screen-opening size (Fig. 4) (6). 

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