Solids may be separated from solids in the dry state by screening (separation according to size), magnetic separation, and electronic separation. In this article, we will deal with screening.
In the chemical industry, the problem of separating solid particles is encountered in the separation of a single solid material into several size fractions or to obtain a uniform material for incorporation in a system wherein a certain chemical reaction is occurring.
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Solid Particle-Size Measurements by Screen AnalysisÂ
Most particulate systems that are of practical interest consist of particles of a wide range of sizes. Several techniques of particle-size determination are available but most particle-size determinations are made by screen analysis when the particles are within the size range that can be measured by screens.
Usually, for carrying out the analysis, standard screens of either the Tyler standard screen series or the U.S. sieve series are used. The testing sieves with square openings are constructed of woven wire screens, the mesh, the dimensions of which are standardized. Every screen is identified in meshes per inch.
In coarse screens, the term mesh refers to the distance between adjacent wires or rods. In fine screens, the mesh is the number of openings per linear inch counting from the center of any wire to a point exactly one inch distant (e.g. a 200 mesh screen will have 200 openings per linear inch).
The minimum clear space between the edges of the opening in the screening surface is termed a screen aperture or screen-size opening.Â
Tyler mesh size, and U.S. Standard Sieve Series
The Tyler standard screen series is based on a 200-mesh screen with a wire 0.0053 mm in diameter, giving a clear opening of 0.074 mm (1/200 = 0.0029 in).
The screen is more coarse than a 200 mesh screen and has its mesh and wire diameter so adjusted that the area of the opening in any one screen is approximately twice the area of the opening in the next finer screen. This means that the ratio of the linear size of the openings in any screen to that in the next finder screen is √2 (1.41). Therefore, a 150 mesh screen will have an opening of 0.104 mm (√2*0.0029 = 0.0041 in) with a wire of 0.064 mm (1/150 - 0.0041 = 0.0026 in) diameter.
The U.S. sieve series which uses the Tyler 200 mesh standard but differs slightly in other sizes is also widely used. Normally, a 200 mesh screen is the finest one used. Several finer screens down to 400 mesh are available but are seldom used except in laboratory investigations as finer screens are expensive, delicate, and have serious problems of binding.
Testing sieves either the Tyler standard sieve series or the U.S. sieve series are used to determine the efficiencies of screening equipment and work of crushing and grinding machinery.
>Particle size analysis using the sieve method
Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane