Recent breaktroughs in particle characterisitic analysis
PARTICLE SIZE, SHAPE, AND CHARACTERISTIC ANALYSIS
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, microscopy, computerized image analysis, laser diffraction, sedimentation, and centrifugation, are some of the analysis methods by which a samples particle characterization may be established.
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 and 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.
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, electro-formed
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.
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.
ROTATING TAPPING SIEVE SHAKERS WERE DESIGNED MANY YEARS AGO, TO SIMULATE HAND SIEVING. BECAUSE MANY INDUSTRY STANDARDS WERE DEVELOPED WITH
THIS TYPE OF SIEVE SHAKER, THEY ARE STILL FOUND IN MANY BUSINESSES. THEY ARE KNOWN TO PRODUCE VERY LOUD NOISE, AND ARE NORMALLY KEPT AND OPERATED IN A SOUND PROOF ENCLOSURE
USE A LOW FREQUENCY AUDIO SIGNAL AND MIRRORED LATEX DIAPHRAGMS, TO PROVIDE VERY EFFICIENT SEPARATIONS, EVEN BELOW 20 MICRONS. CARE MUST BE TAKEN WITH COHESIVE POWDERS
1. Gabas N, Hiquily N, Laguuerie C. Response of Laser
Diffraction Particle Sizer to Anisometric Particles. Part
Part Syst Charact, 11:121-126, 1994
LABORATORY TEST SIEVES
DIGITAL IMAGE ANALYZER
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.
CAN BE EXPENSIVE. DIGITAL IMAGING HAS BEEN USED SUCCESSFULLY FOR THIS TASK.
TYPES OF SIEVES
PRECISION ELECTRO-FORMED
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
