Sieve analysis
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A sieve analysis is a practice or procedure used to assess the particle size distribution of a granular material. The size distribution is often of critical importance to the way the material performs in use. It can be used for any type of non-organic or organic granular round materials including sands, clays, granite, feldspars, coal, soil, and a wide range of manufactured powders. It can also be used for grain and seeds.
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[edit] Procedure
A typical sieve analysis involves a nested column of sieves with wire mesh cloth.
A representative weighed sample is poured into the top sieve which has the widest openings. Each lower sieve in the column has smaller openings than the one above. At the base is a round pan, called the receiver.
The column is typically placed in a mechanical shaker. The shaker shakes the column for a fixed amount of time. After the shaking is complete the material on each sieve is weighed. The weight of the sample of each sieve is then divided by the total weight to give a percentage retained on each sieve.
[edit] Methods
However, there are different methods for carrying out sieve analyses, depending on the material to be measured.
[edit] Trow-Action Sieving
Here a throwing motion acts on the sample. The vertical throwing motion overlies with a slight circular motion which results in distributing the sample amount over the whole sieving surface. At the same time the particles are accelerated in vertical direction (are thrown upwards). In the air they carry out free rotations und are compared with the mesh size of the sieve when they fall back. Are the particles smaller than the meshes they pass the sieve. Are they bigger, they are thrown upwards again. The rotating motion ensures that they got an other orientation when they fall back again and thus might pass the meshes. Modern sieve shakers work with an electro-magnetic drive which moves a spring-mass system and transfers the resulting oscillation to the sieve stack. Amplitude and sieving time are set digitally and are continuously observed by an integrated control-unit. Therefore sieving results are reproducible and precise (an important precondition for a significant analysis). The possibility to adjust parameters like amplitude and sieving time so easily and flexible ensures to optimize the degree of sieving individually for every kind a sample. This makes this method the most common in the laboratory sector.
[edit] Horizontal Sieving
In a horizontal sieve shaker the sieve stack moves in horizontal circles in a plane. Horizontal sieve shakers are preferably used for needle-shaped, flat, long or fibrous samples, as their horizontal orientation means that only a few disorientated particles enter the mesh and the sieve is not blocked so quickly. The large sieving area enables the sieving of large amounts of sample, for example as encountered in the particle size analysis of construction materials and aggregates.
[edit] Tapping Sieving
A horizontal circular motion overlies a vertical motion which is created by a tapping impulse. These motional processes are characteristically for hand sieving and achieve a higher degree of sieving for heavier particles (e.g. abrasives) than trow-action sieve shakers.
[edit] Wet Sieving
Most sieve analyses are carried out dry. But there are some applications which can only be carried out by wet sieving. This is the case when the sample which has to be analysed is e.g. a suspension which must not be dried; or when the sample is a very fine powder which tends to agglomerate (mostly < 45 µm) – in a dry sieving process this tendency would lead to a clogging of the sieve meshes and this would make a further sieving process impossible. A wet sieving process is set up like a dry process: the sieve stack is clamped onto the sieve shaker and the sample is placed on the top sieve. Above the top sieve a water-spray nozzle is placed which supports the sieving process additionally to the sieving motion. The rinsing is carried out until the liquid which is discharged through the receiver is clear. Sample residues on the sieves have to be dried and weighed. When it comes to wet sieving it is very important not to change to sample in its volume (no swelling, dissolving or reaction with the liquid).
[edit] Limitations of Sieve Analysis
Sieve analysis in general has been used for decades to monitor material quality based on particle size. For coarse material, sizes that range down to #100 mesh (150μm), a sieve analysis and particle size distribution is accurate and consistent.
However, for material that is finer than 100 mesh, dry sieving is significantly less accurate. This is because the mechanical energy required to make particles pass through an opening and the surface attraction effects between the particle and the screen increase as the particle size decreases. Wet sieve analysis can be utilized where the material analyzed is not affected by water. Flowing water flushes fine material through the sieve much more efficiently than shaking the dry material.
Sieve analysis assumes that all particle will be round or nearly round and will pass through the square openings. For elongated and flat particles a sieve analysis will not yield reliable results, as the particle size reported will be based on an assumption of similar sizes in three dimensions.
[edit] Forecast
Within the last years some methods for particle size distribution measurement were developed which work by means of laser diffraction or digital image processing.
[edit] "Sieving" with Digital Image Processing
The scope of information conveyed by sieve analysis is relatively small. It does not allow for a clear statement concerning the exact size of a single particle → it is just classified within a size range which is determined by two sieve sizes ("a particle is < than sieve size x and > than sieve size y"). And there is no additional information concerning other relevant properties like opacity or shape available. Devices which work with digital image processing enable to recall even this information and a lot more (surface analysis, etc.). The results can be fitted to sieve analysis so that a comparison between measurement results obtained with different methods is possible.
[edit] See also
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