One of the most important types of analytical work associated with producing resources and wares with desired properties, is particle characteristic analysis.
Sieving, microscopy, computerized image analysis, laser diffraction, sedimentation, and centrifugation, are some of the particle size analysis methods by which a samples particle characterization may be established. The method one employs, has to do with the nature of the sample, ie...liquid or solid, the size range of the particles to be analyzed, the relevance of the results to the finished product properties, and cost.
For some applications, like certain pharmaceuticals, particle shape characteristics may also have a bearing on the outcome of the finished good.
The popularity of sieving as a means of particle size analysis is due to a few considerations, one of the more important being reduced costs, but also that it is a primary method, ie... one where two principle parameters of length and weight, are directly traceable to international standards, since a sieve can be calibrated using microscopy, and a balance can be calibrated using reference standard weights.
Recent advances in digital imaging, now allow for online particle analysis, with enhanced paramaters, such as inner diameter,
elongation, cicularity, blunteness, and roughness.
The most common type of sieving medium is constructed of wire mesh. For round, oblong, or other square apertures,the sieve shaker medium is a plate with perforations, made with a punch, or for more exacting applications, electroformed nickel mesh.
There are also variations in the materials used in manufacture, as well as the tolerances of sieving mediums, although for practical purposes, stainless steel wire mesh is the most widely used and generally accepted.
Aperture Tolerances, wire diameters, (The wire diameter determines the capacity of the sieve (Rate of Sieving ), while the aperture size gives a measure of the particle size.) and general manufacturing parameters, and designations (See left) , are generally defined by geographical standards organizations, industry associations, or prior to industry standardization, sieve manufacturers.
The International Organization for Standardization (ISO) ,and American Society of Testing and Materials (ASTM) are the most prevalent, the latter being the primary agency for U.S. Applications, although many older companies still use Tyler Screen Designations which originated with a company called W.S. Tyler, a long time sieve manufacturer. In the U.S., ASTM Designations are generally used, while ISO Designations are used internationally.
Because the Tyler Sieve Designations are similar, but not equal to to ASTM Designations, it is best to check the millimeter equivalents to make sure everyone is on the same page. It is an altogether too common occurrence, where disputes have risen simply from using the wrong vernacular.
TYPES OF SIEVES
ASTM Compliant Wire Mesh- (ASTM E 11) accompanied by a certificate verifying compliance with ASTM E-11 Manufacturing Procedures
Perforated Plate- Commonly used for agricultural samples like corn or coffee beans
Matched Performance-two or more sieves fitted with a sieving medium having similar aperture characteristics, for performance based applications
Midpoint-Sieves with a 30% reduced tolerence on the sieving medium specifications (Used for reduced tolerance applications, and calibrating working sieves
Airjet-Sieves designed specifically for use with airjet systems. Most common applications with electrostatically charged samples
Pocket-sieves designed for quick field analysis with use of interchanger
Although sieves may be calibrated at the factory, through usage and environmental factors, they may need to be calibrated occasionally to ensure they are still in compliance as part of an ongoing quality control program.
standards (pictured above) are easy to use, and an affordable method of recalibrating sieves. The spheres are designed to be retained by exact apertures. Fall through indicates incorrect aperture size.
Accessories are available to make sieve analysis easier, and results more reproducible. Above is the CSC Sieve Analyzer, which automates much of the process.
The use of sieves with different sized mesh is one of the oldest ,most reliable , and most commonly used methods for providing an analysis of particle size distribution.
Although sieve shakers have limitations based on the assumption that all particles are spherical, they work well for a wide variety of applications, and because it is relatively inexpensive, is generally the first method employed by companies when a particle size distribution analysis program is initiated..
"The nest of sieves is cradled loosely in a slightly inclined position in the crook of the arm, and tapped at the rate of approximately 120 times per minute with the flat of the hand. After about 30 taps, the sieves are put into a horizontal position, turned through 909 degrees, and given a sharp vertical shape, and a hard tap."
The sieving time depends on a variety of factors, such as the characteristics of the material, sieve size, volume of the charge, relative humidity, and so on, although the rule is that with one additional minute of sieving , if the amount retained on any one sieve changes less than 1%, the endpoint has been reached.
These Endecotts Grain Sieves have a steel plate perforated with round holes in 64th" increments, and are used to grade coffee bean sizes, as well as many grains & nuts.
These Electroformed sieves have tolorences of +/- 2 microns, for exacting applications
In the U.S. wire mesh sieve construction & tolerences are defined by ASTM E-11. A simple conformance compliance certificate may be sufficient for some applications, or Endecotts specializes in providing sieves that have a detailed calibration certificate, including specific information about wire and aperture dimensions.
For applications where ASTM Tolorences are too large to be practical, or for use as a calibration standard to check the accuracy of working sieves, 30% reduced tolorence Midpoint Sieves are available.
Apertures needed to bear some sort of relationship with each other, and in the late 1800's, based on experiments with ore dressing, adjacent aperture sizes were formulated based on a ratio of the square root of 2, meaning the aperture width doubled at every other sieve in the series. This was later changed to the fourth root of two, where the aperture width doubles at every 4th sieve, and is what the ASTM Specifications of today are based on. ISO (International Organization for Standardization) Specifications are based on roots of 10, since metric scales are more commonplace in Europe.
RIGHT- This online particle characteristic analyzer, uses digital image analysis, with a special lens and light source, to eliminate diffraction, and provide clear particle shape definition.
The sample is moved using a combination of methods, including a vibrating feeder, gravity, and/or vacuum dispersion, and special pumps for liquids and slurries.
