fine tuning

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W hen it comes to particle siz- ing, the classical technique of sieve analysis is often overlooked in favor of more modern methods. Yet, with the right equipment and proper care, sieve analysis yields accurate and repro- ducible results for a fraction of the cost. As a standalone particle-sizing tech- nique, sieving should be considered when the majority of the particles are over 75 microns. It can also be used for smaller sizes if the results are rou- tinely verified. Even in cases where other restric- tions require the use of a particle-size analyzer, sieving can be an excellent complementary technique. In fact, every particle-sizing laboratory should be equipped with both a microscope and sieves, to allow operators to verify the accuracy of their instrument. In order to be able to perform accu- rate sieving, it is necessary to have: A representative sample Test sieves that conform to the rele- vant standards A reliable sieve shaker and analyti- cal balance Error-free evaluation and documen- tation Correct methods for the cleaning and care of equipment, especially sieves Sampling Whenever samples are used for test- ing, it is vital that they accurately rep- resent the total mass of the original material being evaluated. The most ef- fective way of taking a representative sample is to install a fully automatic sampler at the appropriate location. If this is not possible, a sample divider (Figure 1) or riffler must be used to di- vide a large mass of the material into test sized samples. If this is not done, the final result will not accurately re- flect the material under investigation, leading to significant errors and caus- ing the operators to incorrectly con- clude that the analytical instruments or the material are at fault. Figure 2 shows a number of sampling methods and their expected accuracy. Test sieves Sieves lie at the heart of sieve analysis, and great care must be taken to ensure that the sieves are of the correct design and are manufactured under clean and controlled conditions. These require- ments are described in ASTM E 11 (Standard Specification for Wire Cloth and Sieves for Testing Purposes) and ISO 3310 (Test sieves -- Technical re- quirements and testing) Parts 1, 2 & 3. Each sieve that meets the standard is provided with a manufacturer’s compli- ance certificate, along with a perma- nent label that shows the following: Nominal mesh width Reference to the relevant standard Material of both sieve and frame • Manufacturer’s identification, along with part number and serial number The sieve frame should have a smooth surface and be easily stackable with other sieves. The mesh must be at- tached to the frame in such a way that no sample material will become trapped. One of the latest production techniques uses resistance welding, which eliminates the need for solder or epoxy. This improvement avoids poten- tial contamination of the sample, both from the bonding material itself and from previous-sample carryover. Depending on the user’s specific re- quirements, new test sieves (Figure 3) can be purchased with either a basic certificate of compliance or with a more detailed inspection certificate. While both certificates provide basic informa- tion on the sieve, the inspection certifi- cate shows the actual measuring data of the apertures and, thus, may con- tribute to a more precise sieve analysis. Both certificates provide assurances that the new sieves are in compliance, but a more important issue is what happens as the sieves get older and CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2003 33 Solids Processing Frank Bath Retsch, Inc. FIGURE 2. A number of sampling techniques are available — each achieving a par- ticular degree of accuracy, as indicated above by their deviations from the mean size Fine Tuning Sieve Analysis for Accurate Particle Size Measurement All too often, this useful method is overlooked. But if followed, these principles deliver accurate and reliable results at an affordable price FIGURE 1. When fully automatic samplers are not feasible, a sample divider or riffler must be used to divide a large mass of the material into test-sized samples. Other- wise, the final result will not accurately reflect the total mass of the original ma- terial under investigation

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When it comes to particle siz-ing, the classical techniqueof sieve analysis is oftenoverlooked in favor of more

modern methods. Yet, with the rightequipment and proper care, sieveanalysis yields accurate and repro-ducible results for a fraction of the cost.As a standalone particle-sizing tech-nique, sieving should be consideredwhen the majority of the particles areover 75 microns. It can also be used forsmaller sizes if the results are rou-tinely verified.

Even in cases where other restric-tions require the use of a particle-sizeanalyzer, sieving can be an excellentcomplementary technique. In fact,every particle-sizing laboratory shouldbe equipped with both a microscopeand sieves, to allow operators to verifythe accuracy of their instrument.

In order to be able to perform accu-rate sieving, it is necessary to have:• A representative sample• Test sieves that conform to the rele-

vant standards• A reliable sieve shaker and analyti-

cal balance• Error-free evaluation and documen-

tation• Correct methods for the cleaning and

care of equipment, especially sieves

SamplingWhenever samples are used for test-ing, it is vital that they accurately rep-resent the total mass of the originalmaterial being evaluated. The most ef-fective way of taking a representativesample is to install a fully automatic

sampler at the appropriate location. Ifthis is not possible, a sample divider(Figure 1) or riffler must be used to di-vide a large mass of the material intotest sized samples. If this is not done,the final result will not accurately re-flect the material under investigation,leading to significant errors and caus-ing the operators to incorrectly con-clude that the analytical instrumentsor the material are at fault. Figure 2shows a number of sampling methodsand their expected accuracy.

Test sievesSieves lie at the heart of sieve analysis,and great care must be taken to ensurethat the sieves are of the correct designand are manufactured under clean andcontrolled conditions. These require-ments are described in ASTM E 11(Standard Specification for Wire Clothand Sieves for Testing Purposes) andISO 3310 (Test sieves -- Technical re-quirements and testing) Parts 1, 2 & 3.Each sieve that meets the standard isprovided with a manufacturer’s compli-ance certificate, along with a perma-nent label that shows the following:• Nominal mesh width

• Reference to the relevant standard• Material of both sieve and frame• Manufacturer’s identification, along

with part number and serial numberThe sieve frame should have a

smooth surface and be easily stackablewith other sieves. The mesh must be at-tached to the frame in such a way thatno sample material will becometrapped. One of the latest productiontechniques uses resistance welding,which eliminates the need for solder orepoxy. This improvement avoids poten-tial contamination of the sample, bothfrom the bonding material itself andfrom previous-sample carryover.

Depending on the user’s specific re-quirements, new test sieves (Figure 3)can be purchased with either a basiccertificate of compliance or with a moredetailed inspection certificate. Whileboth certificates provide basic informa-tion on the sieve, the inspection certifi-cate shows the actual measuring dataof the apertures and, thus, may con-tribute to a more precise sieve analysis.

Both certificates provide assurancesthat the new sieves are in compliance,but a more important issue is whathappens as the sieves get older and

CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2003 33

Solids Processing

Frank BathRetsch, Inc.

FIGURE 2. A number of sampling techniques are available — each achieving a par-ticular degree of accuracy, as indicated above by their deviations from the mean size

Fine Tuning Sieve Analysis for Accurate Particle Size Measurement

All too often, this useful method is overlooked. But if followed, these principles deliver accurate

and reliable results at an affordable priceFIGURE 1.

When fully automatic samplers are notfeasible, a sample divider or riffler must

be used to divide a large mass of thematerial into test-sized samples. Other-wise, the final result will not accurately

reflect the total mass of the original ma-terial under investigation

Solids Processing

are subjected to the rigors of useand cleaning. Sieves must be rou-tinely inspected for obvious dam-age and wear; and if a sieve showssuch evidence, it must be replacedimmediately.

A method of verifying sieve perfor-mance is to periodically check thesieving results with a known sample,under exactly the same conditions,using as a control the results thatwere determined when the sieveswere brand new. Another method is touse Standard Glass Spheres (range212 to 850 mm) from the National In-stitute of Standards and Technology(Gaithersburg, Md.; nist.gov) — seeStandard Reference Material 1018.

Sieve shakersIn order for particles to pass through anopening in the sieve mesh, the particleand the mesh must be moved relative toeach other. This movement is inducedindirectly by means of the sieve shakermechanism; and many factors deter-mine the probability of a particle pass-ing through the sieve. These include therelationship of the size of the particleand the mesh opening, the direction ofmovement and the orientation of theparticle relative to the free sieving sur-face. Therefore, the sieve movementand sieving time are critical elements toensure the exact and clear separation ofthe individual size fractions.

In all applications of sieving — in-cluding quality assurance, productionand process monitoring, and researchand development — you must takespecial care to ensure that sieve para-meters are reproducible. Slight varia-tions will offset the results of youranalysis, and in turn will erroneouslytranslate into characteristics of thesieved material.

Sieve shakers have undergone somemajor design changes over the pastfew years, so it is no longer necessaryto use overly noisy shakers in order tobe in compliance with U.S. or otherglobal standards. Recommended byvarious organizations as an alterna-tive to the more traditional “shakeand tap” sieve shakers, these modelsuse a maintenance-free electromag-netic drive are extremely quiet andcan be used safely in the laboratorywithout sound enclosures.

Certain electromagnetic sieve shak-ers take advantage of the precise andconstant mains frequency (In the U.S.,this is 60 Hz) and through self-amplifi-cation of the resonance, amplitudes upto 3 mm can be achieved — although,most users still adhere to the range be-tween 1 and 2 mm. An advantage ofsuch a design is that, for a given periodof time, the same force will be appliedto the particles, independent of thenumber of sieves used — which, be-cause of the different weights involved,can affect the frequency of other elec-tromagnetic sieve shakers. This stabil-ity of force allows for precise repro-ducibility, which is vital whencomparing results from different sieveanalyzers, either within the same loca-tion or from plant to plant.

In addition to the amplitude gener-ated by the sieve shaker, another andmore important consideration is the “g”forces or acceleration that is generatedby the sieve. This force determines howa particle is thrown up by the sieve inorder for it to either more readily passthrough or remain on the sieve. Evenwith constant amplitude settings, thisforce will vary as a function of the fre-quency of the mains supply, and shouldalways be taken into account whencomparing sieving analysis results incountries with different frequencies(Figure 4). A good sieve shaker allowsprecise setting of the amplitude and “g”forces and also comes equipped with aclamping device that is easy to use andholds the sieves securely on the shaker.

Since precise timing is critical foraccurate sieve analysis, especially atshort sieving times, it is importantthat sieve shakers be equipped with adigital timer. It is virtually impossible

to set precise sieving timeswith an older analog timer

(Figure 5). In order for sieve shakers to re-

main in compliance, they mustperiodically be subjected to recali-

bration by the manufacturer. Sucha service can be performed at the cus-

tomer site or returned to the factory.When selecting sieve shakers, checkwith a proposed supplier to make surethat it offers this service; and keep inmind that only sieve shakers withsieving parameters (time, intensity)that are traceable to national stan-dards can be recalibrated at all.

Fiber- or grain-shaped particlesSince sieves effectively measure thesmallest linear dimension that canpass through the mesh opening, themeasurement of particle size on longthin “particles,” such as grain or fibers,produces a result that does not reflectthe true three-dimensional shape ofthe particle. This problem is not uniqueto sieving techniques, since any ana-lyzer that “sees” particles in all dimen-sions will always produce size resultsthat are smaller than a true volume-based result. In sieving, however, thisproblem is exacerbated by the fact thatif the material is sieved long enoughusing a shaking motion, it is theoreti-cally possible that every particle willeventually pass through the sieve,based on its smallest linear dimension.This is also one reason why sievingtechniques usually produce particle-size results smaller than other tech-niques, such as laser diffraction.

One way to minimize this problemis to use sieving machines that employa horizontal sieving motion. This al-ternate direction will allow the long fi-brous particles to remain on the mesh,resulting in more accurate detection oftheir longer dimension. In the case ofsieving machines that use an “up anddown” motion, there is much morelikelihood that the small dimensionwill be exposed to the sieving surfacethrough the throwing motion em-ployed in these machines. Therefore,sieving machines based on a horizon-tal, or two-dimensional movement,will produce results that more accu-rately reflect the size of the particlesand their quality as defined by inter-

34 CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2003

FIGURE 3. As sieves get older and aresubjected to the rigors of use and clean-ing, they must be routinely inspected forobvious damage and wear. If a sieveshows such evidence, it must be re-placed immediately with a new model,such as the one above

nal quality-control requirements. Another consideration for the sieving

of elongated particles is to use a perfo-rated-plate sieve, as opposed to themore common wire-mesh version. Thisdesign has the advantage that the sur-face of the sieve is much smoother, withless danger of damage to the materialbeing tested. In addition, the holes inthe mesh can be circular, oblong orsquare, depending on the application.

Another technique that deservesmentioning is that of wet sieving. Inthis approach, the particles passthrough the mesh via the action ofwater that is constantly flowing overthe particles and down into the nextsieve in a sieve tower. This techniquecan extend the effective size range toas low as 20 microns and is the methodof choice on wet samples, such as soil.

In one method, water is introducedto the top of the sieve stack and al-lowed to flow down through all thesieves and eventually to waste. Amore precise wet-sieving technique in-troduces water under controlled pres-sure on all the sieves in a sieve stack,thus ensuring that all size fractionsreceive an equal chance to passthrough their respective mesh sizes.

Wet sieving can be performed as aseparate technique, or in conjunctionwith a sieve shaker.

Analytical balance The balance should have a range up to5,000 g with an accuracy between 0.01and 0.1 g and the weighing pan mustbe able to accommodate a 200-mm or8-inch sieve. Like the sieve shaker,the balance should be able to be cali-brated, and it should be supplied witha computer interface. There are manysuppliers of such equipment.

EvaluationSieve analysis, like any other sophisti-cated analysis technique, must takeadvantage of existing computer tech-nology in order to ensure accuracy, re-liability and compliance with requiredprocedures. Too often, the quality ofthe results is left to the care and con-scientiousness of the laboratory tech-nician, who has to weigh and recordthe individual sieves and fractions,calculate the size classes and createthe distribution curves. This methodis time-consuming and contains manypossibilities for both human and sys-tematic error.

Commercially available softwarepackages offer a solution for this im-portant and often overlooked aspect ofsieve analysis. Using electronic datafrom a balance and sieve shaker, thesesoftware packages guide the userthrough easy to follow steps via theWindows operating system.

Furthermore, they automaticallycapture all the individual weights and,at the same time, control the necessaryparameters of the sieve shaker to en-sure that each sample is treated in ex-actly the same way. Such software alsoallows entry of the actual aperture size— which is determined during the cer-tification process and will be slightlydifferent than the nominal size — intothe program to provide a more accu-rate size measurement.

Both the tabular and graphical re-sults are automatically calculated anddisplayed on the monitor before print-ing. The software allows for selectionof one or more of a number of differentformats, and for comparison with pre-vious results on the same material, orwith control samples for acceptance orrejection of the batch being tested. Re-sults, along with all test parametersand operator information, can bestored for future recall.

Cleaning and care Sieves are high-precision testing in-struments, but, in many laboratories,are treated no better than second-ratecooking utensils. Careful mainte-nance and cleaning of sieves not onlyhelp maintain the quality of resultsbut also prolong the life of these prod-ucts. Fine-mesh textile sieves are par-ticularly sensitive to mechanical

stress; and mechanical cleaning witha stiff brush will damage and distortthe mesh and weaken its structure.

It is dangerous to assume that theparameters under which the sieve wasmanufactured and calibrated stillapply; and, as mentioned earlier, anysieve showing obvious visible damagemust be replaced. It is recommendedthat a second set of “mint condition”sieves be kept to verify the perfor-mance of the routinely used productsand if the results from the two sievesare found to be significantly different,then a new set be purchased whichwill be then used as the “mint condi-tion” set of sieves.

New sieves should be briefly im-mersed in a mild solvent bath to re-move oily residues. During routinecleaning, care must be taken not todamage the sieve material. Sieveswith mesh sizes finer than 500 mi-crons must always be cleaned in an ul-trasonic bath using water and a milddetergent — 3 to 5 minutes is usuallysufficient. Mesh sizes coarser than500 microns can be cleaned with a softbrush followed by ultrasonic cleaning.After cleaning, sieves should be rinsedthoroughly and left to dry in as up-right a position as possible.

Both the analytical balance andsieving machine should be recali-brated at intervals recommended bythe manufacturer, and serviced bythat firm if a problem is suspected. ■

Edited by Rebekkah Marshall

CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2003 35

Feed material: potash fertilizerMesh aperture size: 0.125 mmSieve time: 3 minSieve diameter: 200 mm

Sieve acceleration, g

Throughput depending onthe sieve acceleration

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ut, %

50 Hz60 Hz

1

656055504540353025

2 3 4 5 6 7 8 9 10 11 12

FIGURE 4. Even at identical amplitudesettings, the “g” force will vary slightly,as a function of the frequency of themains supply and should always be con-sidered when comparing results betweencountries with different frequencies. Agood sieve shaker will allow precise set-ting of the amplitude and g forces

AuthorFrank Bath is a productmanager at Retsch, Inc. (74Walker Lane; Newtown, PA18940; Phone: 267-757-0351;Fax: 267-757-0358; Email:[email protected]). Heearned his technical degreein medical laboratory sci-ences at Neath TechnicalCollege in Wales, U.K. Sincemoving to the U.S. in 1977,he has been actively in-

volved in particle size analysis.

FIGURE 5. Since precise timing is criti-cal for accurate sieve analysis, espe-cially at short sieving intervals, it is im-portant that sieve shakers be equippedwith a digital timer