Size Measurement Methods PDF

Size Measurement Methods For simplicity, the methods and instruments have been loosely classified into six groups: 1. Visual methods (e.g., optical, electron, and scanning electron microscopy and image analysis) 2. Separation methods (e.g., sieving, classification, impaction, electros...

Size Measurement Methods For simplicity, the methods and instruments have been loosely classified into six groups: 1. Visual methods (e.g., optical, electron, and scanning electron microscopy and image analysis) 2. Separation methods (e.g., sieving, classification, impaction, electrostatic differential mobility) 3. Stream scanning methods (e.g., electrical resistance zone, and optical sensing zone measurements) 4. Field scanning methods (e.g., laser diffraction, acoustic attenuation, photon correlation spectroscopy) 5. Sedimentation 6. Surface methods (e.g., permeability, adsorption) Visual Methods: Microscopy Use of a microscope should always accompany size analysis by any method because it permits an estimate to be made of the range of sizes present and the degree of dispersion. Microscope size analysis is used primarily as an absolute method of size analysis, as it is the only one in which individual particles are seen. The more common ranges of measurement are 3-1000 m for optical microscopy, 2 nm to 1m for TEM, and 20 nm to 1000 m for SEM. Equivalent diameter The size of the particle of standard configuration like sphere and cube can easily be specified. For example, the size of a spherical particle is its diameter and that of a cubical particle is the length of its side. As the particles are usually of irregular in shape, it is difficult to define and determine their size. The size of a particle is usually defined by comparison with a standard configuration, normally spherical particle. An equivalent size or equivalent diameter of an irregular particle is defined as the diameter of a spherical particle having the same controlling characteristics as the particle under consideration. Volume diameter, dv, is defined as the diameter of a spherical particle having the same volume as the irregular particle where Vp is the volume of the irregular particle. Surface diameter, ds, is defined as a diameter of a spherical particle having the same surface area as the irregular particle. where Ap is the surface area of the irregular particle. Stokes’ diameter is the diameter of a sphere having the same settling velocity as the particle under Stokisian conditions. Newton’s diameter is the diameter of a sphere having the same settling velocity as the particle under Newtonian conditions. Separation methods; sieving Test Sieve is a circular shell of brass having 8 inch diameter and about 2 inch high as shown in Fig. 1. Sieve cloth is made of wire, woven to produce nominally uniform cloth apertures (openings) of square shape. The sieve cloth of square opening is placed in the bottom of the shell so that material can be held on the sieve. Figure 1, Test Sieve. Aperture (or Opening) is a distance between two parallel wires. Mesh number is the number of apertures per linear inch. Sieves are designated by Mesh number. Mesh size is the size of an aperture i.e. the distance between two parallel wires. As mesh number increases, mesh size decreases. In general, mesh number × mesh size in microns ≈ 15,000 SIEVE ANALYSIS Sieve analysis is the heart of the mineral processing as the separation is achieved primarily based on the size of the particles. Sieve analysis is a method of size analysis using test sieves at smaller and fine sizes. It is performed to separate the sample into number of closely sized fractions by allowing the sample of material to pass through a series of test sieves and then determine the weight percentages of every fraction. Sieving can be done by hand or by machine. 4.2 TESTING METHOD The sieves chosen for the test are arranged in a stack, or nest, starting from the coarsest sieve at the top and the finest at the bottom. A pan or receiver is placed below the bottom sieve to receive final undersize, and a lid is placed on top of the coarsest sieve to prevent escape of the sample. The material to be tested is placed on uppermost coarsest sieve and close with lid. The nest is then placed in a Sieve Shaker and sieved for certain time. After sieving by sieve shaker, hand sieving is usually performed for better accuracy by taking individual sieves. Fig. 2 shows the sieve analysis at the end of the sieving. Figure 2, Sieve analysis at the end of sieving. Particle size distribution data for an ore is given in a tabular form as shown in Table 1 for better explanation. Table 1, Particle size distribution data from size analysis test. The weight percentages of the material retained on each sieve are to be calculated to form differential analysis. Cumulative weight percentage retained is obtained from differential analysis by adding, cumulatively, the individual differential weight percentages from the top of the table. Cumulative weight percentage passing is obtained by adding, cumulatively, the individual weight percentages from the bottom of the table. All the fractions are fairly closely sized (except first fraction). Hence the size of the particles in each fraction may be calculated as arithmetic mean of the limiting sizes. For example, the size of −20 +28 mesh fraction is (840 + 595)/2 =717.5 microns. It means, the particles which pass through 20 mesh and retain on 28 mesh are having the mean size of 717.5 microns. Similarly the mean sizes of each fraction are to be calculated. Table 2 shows all values. Table 2, Calculated values for particle size distribution. Average size of the material is determined by using the following simple arithmetic formula where w is the weight percent of the material retained by the sieve, d is the mean size of the material retained by the same sieve. A homework: Sieve analysis test data of a sample of sand of specific gravity 2.65 is shown in the following Table, Calculate the average size of the particles.