One of the most important types of analytical work associated with producing resources and wares with desired properties, is particle size and shape analysis.
Sieving and laser diffraction are probably the two most common methods of obtaining a particle size distribution.
Some of the considerations that come in to play are:
THE PHYSICAL STATE OF THE SAMPLE ie... liquid, gas, solid, viscous, cohesive, agglomerative, etc...
THE RELEVANT SIZE RANGE OF THE PARTICLES TO BE ANALYZED
TIME AND MANPOWER REQUIREMENTS
COST
For some applications, like pharmaceuticals, particle size, as well as particle shape characteristics may be critical. For example, risks associated with the infusion of particles that exceed the internal diameter of the pulmonary capillaries, can be deadly."
Sieve analysis is popular in many industries, primarily due to the low cost. 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.
However, sieve analysis has drawbacks, one of which, like laser diffraction, makes the assumption that particles are round. Especially with particles that are flat, or long (plates and rods), how exactly a particle will find its' way through the correct mesh aperture is somewhat uncertain. In fact sieve analysis is almost completely ineffective for long stringy particles.
Laser diffraction suffers from many of the same problems, with the inaccuracy of results on plates and rods, reaching 31% and 70% respectively, in some cases. [1]
Never the less, sieving is required where physical separations are needed, and many industry standards are rooted in sieve analysis, because of cost, and also because it may have been the only technology available at the time the standard was developed. Therefore, the ability of an instrument to be able to correlate to sieve analysis, may be of some utility.
OFFICIAL ASTM HAND SIEVING METHOD
"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.
Because sieving by hand is difficult to perform in the first place, and the results are somewhat subjective, different types ofsieve shakers are available to separate various sample types.
SHIMADZU LASER DIFFRACTION
PARTICLE ANALYZER
1. Gabas N, Hiquily N, Laguuerie C. Response of Laser
Diffraction Particle Sizer to Anisometric Particles. Part
Part Syst Charact, 11:121-126, 1994
Recent advances in digital imaging technology, have now made two dimensional particle characterization analysis possible, with both size and shape parameters being employed. Not only can a robust sieve correlation program be developed, but increased accuracy, and the analysis of other useful parameters is now possible.
THESE TWO IMAGES OF GROUND COFFEE PARTICLES HELP SHOW THE INHERENT PROBLEM WITH USING A SQUARE APERTURE FOR SIZE DETERMINATION.
PUBLISHED COFFEE INDUSTRY STANDARDS ARE BASED ON SIEVE ANALYSIS, EVEN THOUGH MUCH OF THE INDUSTRY HAS CHANGED TO LASER OR DIGITAL IMAGING.
DIGITAL IMAGING MAKES USE OF A PARAMETER CALLED MAXIMUM INSCRIBED DISC, TO PREDICT SIEVE CORRELATION FOR A PARTICLE. ALTHOUGH IT IS THE MOST ACCURATE WAY TO DO THIS, IT STILL DOES NOT ACCOUNT FOR THE DIAGONAL IN A SQUARE APERTURE.
SIEVES ARE MADE BY A SPECIAL PROCESS, AND ARE USED WHERE TOLERANCE REQUIREMENTS ARE EXTREME. THEY ARE USED FOR PARTICLE SEPARATIONS BELOW 20 MICRONS.
WIRE MESH SIEVES ARE THE
MOST POPULAR TYPE OF SIEVE FOR
PARTICLE SIZING. THEY ARE AVAILABLE IN VARIOUS MATERIALS AND SIZES, AND WITH DIFFERENT TOLERANCES, DEPENDING ON THE ACCURACY REQUIREMENTS SPECIFIED IN THE TEST
PERFORATED PLATE SIEVES
ARE AVAILABLE WITH ROUND
HOLES, SQUARE HOLES, OR
SLOTS. THEY ARE USED IN SIEVING
AGRICULTURAL PRODUCTS,
CEREALS, COFFEE, AND OTHER
LARGE SIZED COMMODITIES
Powders, granulars, and liquids, can now be analyzed with extreme precision, using a number of innovative sample dispersion techniques. For less dense powders, vacuum dispersion, either by itself, or in conjunction with a vibrating platform, and / or air jet, are utilized to move the particles in to the field of view. Liquids and slurries, are kept in suspension using magnetic stirrers, while specially designed pumps, draw precise amounts through specially engineered parallel glass slides (flowcell).
Our particle measurement systems have superior resolution, compared to laser diffraction, allowing for precise analysis of particles as small as 200 nanometers, which makes this technology not only suitable, but preferable for exacting applications.
DIGITAL IMAGING TECHNOLOGY CAN BE ESPECIALLY USEFUL FOR PARTICLE SIZE DISTRIBUTION TESTS, IN CASES WHERE THE PARTICLES ARE TOO SMALL FOR ANALYSIS BY LASER DIFFRACTION, OR WHERE THE ANALYSIS OF PARTICLE SHAPE PARAMETERS IS IMPORTANT.
BY OVERLAYING VIRTUAL CIRCLES ON TO PARTICLE IMAGES, QUANTIFIABLE ABRASION PROPERTIES LIKE BLUNTNESS AND ROUGHNESS ARE NOW POSSIBLE. KINETIC MEASUREMENTS OF SLURRIES, AND LENGTH / WIDTH ANALYSIS OF FIBROUS MATERIALS, ARE ALSO POSSIBLE. NEAR PERFECT SIEVE CORRELATION IS ATTAINED BY USING THE MAXIMUM INSCRIBED DISC, TO DETERMINE THE WIDEST PART OF A PARTICLE, WHICH WOULD DETERMINE THE APERTURE SIZE A PARTICLE WOULD FALL THROUGH.
DIGITAL IMAGING TECHNOLOGY
Copyright 2012: QAQC Lab | Particle Measurement System | All rights reserved.
Sieve Analysis works by arranging a series of sieves in a stack, with the larger aperture sieves, at the top, and the smaller aperture sieves at the bottom.
Particles are retained on sieves with apertures smaller than the particle,and the results of the amount retained on each sieve, plotted in to a distribution curve.
Although sieve analysis provides usable information on particle size, a margin of error exists because particles are assumed to be spherical, and there is no way to ascertain whether a particle falls through on a diagonal, or perpendicular orientation.
Modern sieve shakers impart a 3-D action, using vibration, tapping, ultra-sonics, or sonic waves, to disperse the particles.
VIBRATORY SONIC AIR JET
APPLICATIONS
DIGITAL IMAGING EQUIPMENT FOR PARTICLE CHARACTERIZATION IN LIQUIDS, POWDERS, AND
OCCHIO DIGITAL IMAGE ANALYZERS HAVE THE ABILITY TO DETECT AND QUANTIFY BOTH PARTICLE SIZE AND PARTICLE SHAPE, PAVING THE WAY FOR THE ADVANCED ANALYSIS OF PARTICLES IN BOTH LIQUID AND SOLID FORMS.
CORRELATIONS TO OTHER PARTICLE SIZE ANALYSIS METHODS, LIKE SIEVE ANALYSIS AND LASER DIFFRACTION, ARE SEAMLESSLY EMPLOYED. EVEN SHAPE PARAMETERS LIKE LENGTH, FLATNESS, AND ANGULARITY IN AGGREGATES, OR THE LENGTH OF STRINGY PARTICLES LIKE SHREDDED CHEESE, OR TOBACCO, ARE POSSIBLE.
ONCE A BASIC CORRELATION HAS BEEN ESTABLISHED, THE RESULTS ARE FINE TUNED WITH THE OCCHIO CORRELATION PROGRAM, ALLOWING FOR TURNKEY SOLUTIONS TO PARTICLE CHARACTERIZATION.
