SPECIAL SECTION/INSTRUMENTATION: Sieving for Particle Sizing Applications
May 1, 2007
For centuries, man has used sieves to sort and separate materials. The earliest applications probably involved the sieving of grain to remove chaff, dirt and other foreign material. Although the principles of sieving are straightforward and have changed little since ancient times, refinements in methods and equipment have allowed the technology to evolve.
Improved efficiency and accuracy now permit reliable size determinations over a very broad range of particle sizes, commonly from 4-6 in. (100-150 mm) or larger down to 20 µm, and in some cases as fine as 3 µm. Any dry particulate or granular material is a good candidate for particle size analysis through sieving. The suitability of these materials for sieving is controlled by characteristics such as moisture, flowability and their tendency to be affected by static forces.
SievesMost people's idea of a testing sieve is woven-wire mesh permanently fixed in a round metal frame that nests with other sieves to form a stack. Actually, sieves vary widely in size and form. Sieves and screens used for testing can range from 3 in. (75 mm) in diameter to large rectangular trays 22 x 14 in. (559 x 356 mm) in size. Frames may be brass, steel, stainless steel, acrylic or other plastics. The mesh itself is commonly woven from brass or stainless steel, but even nylon and polyester monofilament fabric is used.
Mesh is usually secured permanently in the frame, but some frames are designed to use replaceable and interchangeable mesh units. Steel plate perforated with round or square holes is used for some applications. Electroformed sieves, which are noted for their accuracy and uniformity in very fine sizes (down to 3 µm or less), are also available. The mesh for these sieves is manufactured using electro-deposition of metal on an underlying support grid. The openings are then etched in precisely. Electroformed sieves are relatively expensive, but openings are accurate to ± 2 µm.
Sieve opening sizes have been standardized over the years, and most users today adhere to ASTM or sometimes ISO standards for testing-grade wire cloth. Most adjacent ASTM sieve size apertures are related by the fourth root of two (1.189), and every second size by the square root of two (1.414). Some commonly used coarse sizes have been added to the ASTM series. Normally, every second or fourth size is used for particle size distribution testing, except for precise determinations where every size is used. Sieves may also be added or removed to relieve excessive loading or to obtain additional points for the distribution curve.
Once sieve opening sizes are selected, the stacks are assembled by the user with the largest opening meshes at the top; these meshes become progressively finer toward the bottom. Often, a receiving pan is located at the bottom of the stack to retain the fines, which are those particles that pass through all of the sieves. Styles, types and configurations of the sieves themselves vary widely, but the idea is always the same-big holes at the top, small holes at the bottom.
Sieve ShakersManual agitation of the sieve or sieve stack is always an option and can be acceptable for low-volume or occasional testing. However, the practice is tiresome for operators, and it suffers from predictable problems with repeatability. A sieve shaker produces repeatable results quickly and with less effort, and can be selected or adjusted to closely match the characteristics of the material. Sieve shakers are available in a variety of configurations to suit a wide range of material types.
The nature of the material should play the largest role in the selection of a sieve shaker. Mineral aggregates, ores and hard, dense materials are usually tough enough to stand up to rigorous agitation, and often require this type of action because of their size and mass. Brittle, friable materials require a gentler touch to prevent degradation of the individual particles. Some sensitive and "difficult" materials respond well to agitation in a fluidized bed. Samples of very fine powders of low bulk density are often wet-sieved in a slurry or suspension of water or other wetting agents and solvents. Wet sieving is a good way to control "fly-away" powders that are susceptible to air currents or static charges.
To be effective, a sieve shaker must rapidly and repeatedly lift the material above the screen surface, allowing the particles to reorient to the openings and creating the maximum number of opportunities for the particle to pass through as it drops back to the mesh surface. Simple mechanical shakers typically use an oscillating or gyratory motion to move particles across the screen surface. Most of these shakers use 8- or 12-in. (200 to 300 mm) metal-framed sieves. Mechanical shakers with tapping are an improvement over the simpler shakers. These units offer jarring or tapping of the sieve stack to assist in the passage of near-size particles and prevent blinding (blockage) of the mesh openings. These shakers are well-suited to hard, dense materials that stand up well to the robust action.
Vibratory shakers usually employ an electromagnetic drive to develop vibration energy at different combinations of amplitude and frequency. Higher-quality units of this type feature adjustable energy levels and programmable sequences to match the characteristics of various materials. Various vibratory shakers may be designed to accommodate individual sieve sizes from 3-18 in. (75-450 mm) in diameter, and some accept a variety of different sizes.
Sonic sievers use audio energy in an enclosed air column to create a vertically oscillating column of air. This energy lifts particles and reorients them to the mesh at 3000 pulses per minute. The air column is enclosed by the sieve stack and special flexible membranes. On some units,* time and energy sequences are programmable and particle passage is assisted by horizontal and vertical tapping. These instruments are particularly effective when separating very fine powders and difficult materials down to the 3-5 µm range with electroformed sieves.
Air jet sievers use a slowly rotating nozzle to introduce low-pressure air through the bottom of special 90- or 200-mm sieve drums, creating a fluidized bed of material. Exiting airflow carries passing fines back down through the sieve and away to a collector. This method works well on sensitive materials down to the 5-10 µm range that are dry and free-flowing, but only one sieve can be processed at a time.
Conventional sieve shakers are often fitted with optional attachments for wet sieving. This practice should be approached with caution, since many sieve shakers are not designed to work in a wet environment. Purpose-built devices** house an entire system of pumps, spray heads, tanks and drains in one unit, and can also be used as conventional sieve shakers. The apparatus for wet sieving generally consists of a fixture designed to disperse liquid into the topmost sieve, which saturates the sample. The fluid then flows through the stack, assisting in the passage of fine material through the sieves as the sample is agitated. The bottom pan is usually equipped with a drain to carry waste water away, and the unit may employ filter paper instead of a pan for fines retention.
*GA-6 GilSonic AutoSiever and GA-8 GilSonic UltraSiever, Gilson Co., Inc. Until the introduction of the Gilsonic UltraSiever in 1998, sonic sievers were limited to testing small samples using special 3-in. (75-mm) sieves with acrylic frames. The UltraSiever enables the use of conventional 8-in. (203-mm) metallic-framed sieves and proportionally larger sample sizes while retaining the benefits of sonic sieving and the accuracy of electroformed sieves.
SamplingThe importance of proper sampling techniques and equipment cannot be overemphasized. It is well established that only proper sampling can produce a specimen that truly represents the bulk material. Because it is a complex subject in itself, sampling methods will not be addressed in detail here, but there are some important points to touch on. Sampling methods such as cone and quartering and scoop-sampling are often used but exhibit high standard deviations in size distribution when compared to sub-samples of the same bulk material.1
The most accurate and reliable sampling methods are spin- and chute-riffling. Chute or riffle splitters are widely available in a variety of chute sizes and volumetric capacities ranging from 2-340 L (0.07-12 ft3) or more, and particle sizes from fine powders to over 100 mm (4 in). Minimum chute width requirements are typically two to three times the largest particle dimensions to encourage even flow and avoid bridging. These splitters are cost-effective solutions for applications requiring moderately high accuracy.
Spinning rifflers are also available in a wide range of sizes and offer the most accurate sampling of powder or granular material of any conventional method. These devices, which collect sample portions in multiple containers rotating on a horizontal turntable, are fed with a stream of sample material flowing down a vibratory feeder chute from a bulk hopper. Although custom units large enough to require mounting on flatbed trailers have been produced, most capacities range from 50 L (1.8 ft3) for particles to 51 mm (2 in.) to the more common laboratory models of 1 to 5 L (0.04 to 0.2 ft3), with particle topsize to about 10 mm (0.4 in.). These units produce 2-20 highly representative samples with little operator input.
Analysis Methods and ProceduresParticle size by sieving is generally determined gravimetrically. The dry weights (or masses, if you prefer) of the separated fractions are divided by the total weight of the sample and then multiplied by 100 to be expressed as a percentage of the total sample. The fractions may be weighed individually or cumulatively and may be reported as percent retained or percent passing for each sieve.
Laboratory reports often include the data displayed graphically as a distribution curve for evaluation or comparison to a specification. Computer software is available to automate this process and significantly reduce operator error. Some programs capture data directly from an electronic balance, compute the percentages and print out a finished grain size plot. At the completion of each analysis, sieves are cleaned with either a soft brush or immersed in an ultrasonic cleaning bath.
Verification of Sieve AccuracyIn any endeavor to determine physical properties, there is a concern for the accuracy of the measuring device itself. Most sieves manufactured today include a serial number and a certificate verifying conformance to certain manufacturing standards. These certificates do not verify the examination of an individual sieve, but rather that the source of wire cloth and the manufacturing methods comply with specifications (usually ASTM E 11 or ISO 565).
Sieves purchased from reputable manufacturers are nearly always within cited requirements because quality control procedures are followed so closely. If the user needs verification for their own QC or ISO 9000 program that new sieves are currently in compliance and will remain so while in service, further steps are necessary. Verification or reverification of actual mesh opening size is available from manufacturers (at a premium charge). If ordered along with a new sieve, the openings will be measured at the factory under an optical comparator traceable to NIST standards. A certification report is provided, citing the serial number of the sieve and results of the measurements obtained. The same process can be performed on a used sieve simply by sending the sieve back to the manufacturer. The same measurements are taken and a certification report is provided.
Standard reference materials (SRMs) are very precisely sized materials traceable to NIST or BCR (European Community Bureau of Reference). They are often soda glass beads or quartz crystals and are available in "single dose" packages for a particular range of sieves or individual sieves. The SRMs are processed on the sieves just like a material sample, and percentages of fractions are computed. The fraction values are inserted into equations and the result determines the average opening size of the mesh.
A variation combining these two methods is to purchase a "master set" of certified sieves and use them only to generate master samples of the actual material being tested. These samples are then processed on the working sets of sieves and the results are compared. Matched sieves are certified sieves selected to very closely match the opening sizes of a master set retained by either the manufacturer or user. A premium series of standard sieves, such as W.S. Tyler's "Gold Series," are certified sieves with opening size tolerances guaranteed to be at least one-half that required by ASTM E 11.
Simple AnalysisSieving as a particle sizing technique enjoys broad acceptance due largely to its reliability, small capital outlay and low degree of required technical expertise. The concept is simple, the technology is well understood and the process is easy to control. In addition, minimal sample preparation is required, making sieving competitive with other methods in terms of total time required. A satisfactory test-sieving operation can be set up for a fraction of the cost of other particle sizing methods.
For more information regarding sieves and sieving for particle size analysis, contact Gilson Co., Inc., P.O. Box 200, Lewis Center, OH 43035-0200; (740) 548-7298; fax (740) 548-5314; e-mail email@example.com; or visit www.globalgilson.com.
For Further Reading
- Particle Size Measurement, T. Allen, Chapman and Hall, London, 1968.
- Manual on Test Sieving Methods, L. Pope and C. Ward, ASTM Manual Series, MNL32.
- ASTM E 11-04, Standard Specification for Wire Cloth and Sieves for Testing Purposes, Committee E 29, American Society for Testing and Materials, West Conshohocken, PA.
- Particle Size Characterization, A. Jillavenkatesa, S.J. Dapkunas, L.H. Lum, NIST Special Publication 960-1.