Attrition control by pneumatic conveying

Ž .Powder Technology 104 1999 214–220www.elsevier.comrlocaterpowtec

Attrition control by pneumatic conveying

H. Kalman )

Department of Mechanical Engineering, Ben-Gurion UniÕersity of the NegeÕ, P.O. Box 653, Beer-SheÕa 84105, Israel

Abstract

Attrition is usually considered as an undesired process. Therefore, many researchers devoted their work to find ways for reducing theattrition. However, attrition might be controlled by understanding the parameters and mechanisms affecting it. The control could berealized by reducing the attrition, increasing it, or conducing a selective attrition that will change the particle’s shape. In this paper, theattrition mechanism is explained and analyzed. Possible ways, to either increase or decrease the attrition, are described. Experimentalresults of a new system for selective attrition are shown in more details. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Attrition; Pneumatic conveying; Comminution

1. Introduction

Attrition during pneumatic conveying is considered usu-ally as an undesired process. However, understanding ofthe attrition phenomenon as well as the affecting parame-ters could turn the attrition, in some cases, to a desired anduseful process. This is a part of a new evaluation of theconveying systems and equipment that should not be con-sidered only as a necessity, but could also involve aprocess, or at least a part of it. The processes that could beconsidered for pneumatic conveying are heat transferŽ . Ž .cooling and heating , mass transfer wetting and drying ,some reactions and comminution.

The main parameters affecting the breakage and chip-ping of particles in pneumatic conveying are air andparticle velocities, loading ratio, bend structure and parti-cles’ properties such as size distribution, shape and mate-rial. Considering those parameters and the way they affectthe attrition enables to control the attrition rate and behav-ior. Since it was noticed, a long time ago, that the mainattrition occurs at the bends, the study was dedicated toflow and attrition mechanisms at various bends. However,the motivation for all of them have been found in investi-gating the ways of how to prevent or minimize the attri-tion.

w xHilbert 1 examined experimentally three bends: long-radius bend, short-radius elbow and a blinded-tee. Hefound that regarding wear, the blinded-tee is the best

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Ž .device less attrition with the short-radius elbow taking aclose second and the long-radius sweep third. A compre-hensive experimental study was carried out by Agarwal et

w xal. 2 on a long-radius bend. They have studied theacceleration length due to bends and the effects of phasedensity, conveying velocity and use of inserts on the wearof the bends, particle degradation and depth of penetration.

w x w xRecently, Bell et al. 3 and Papadopoulos et al. 4 pre-sented attrition experiments with salt in which the sizedistribution was measured on line. They have also shownthat the air velocity has the prime effect on the attritionrate, although the effect of loading ratio and the bend

w xstructure cannot be ignored. Kalman and Goder 5 mea-sured the pressure drop, attrition rate, wear of the bend andbuild-up on the bend walls for four types of bends: long-

Ž .radius three construction materials , short-radius elbow,blinded-tee and a turbulence drum. They conducted theexperiments in a close-loop 1-in. pneumatic conveyingpipe line, testing sand. Due to the use of a close-loop pipeline the attrition results were presented vs. time of running.

w x Ž .Aked et al. 6 showed that even fine powders 15 mmcould be attrited significantly in certain conditions. Kalman

w xand Aked 7 presented a comparison between attritionmeasurement methods and analyzed the attrition of variousmaterials.

In this paper, pneumatic conveying pipe lines are exam-ined as devices for attrition control. Three case studies areshown. The first case study is the classical one, i.e., toprevent the attrition in cases that the conveying is usedonly to transfer the bulk from one process to anotherwithout changing significantly its character. The second

0032-5910r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0032-5910 99 00097-2

( )H. KalmanrPowder Technology 104 1999 214–220 215

case study describes a need for increasing the attrition inorder to replace or to reduce the use of a grinder ormicronizer at the pipe line end. We have showed that this

w xis possible for even fine powders of about 15 mm 6 . Thethird case study considers moderate attrition to round theparticles by breaking the sharp corners, that will reducedustiness during further handling. Obviously, all the casesshould be considered economically. In any case, economi-cal considerations and technological merits should be con-sidered in a wide view of the processes and conveying,rather than a narrow view of each part. However, by beingaware to the attrition mechanism, its control is possible.

2. Attrition mechanism

The attrition is caused mainly by impact or shear loads.The particles during pneumatic conveying experience ex-tensive impact loads mainly at the bends because the flowdirection is changing. Therefore, the analysis of the attri-tion at the bends is mostly important. The particles couldbreak in a single collision if the impact load is higher thanits strength. The collision velocity, the angle of collisionand the elasticity of the collision are significantly affectingthe impact load. Particles could also break or damaged dueto lower impact loads than their strength, when collisionoccurs number of times, which is known as fatigue. Thenumber of bends and the elasticity of collision with long-

Žradius bends a few collisions with the same bend could.occur affect the number of collisions. Therefore, parame-

ters affecting the attrition rate can be divided practicallyinto three categories: the particle strength, operation pa-rameters, and pipe line and bend structure.1. Particles strength:

Ž .a particles material,Ž .b particles size and shape.

2. Operation parameters:Ž .a particles velocity,Ž .b particle concentration–loading ratio.

3. Bend structure:Ž . Ž .a radius of curvature angle of collision ,Ž .b construction material,Ž . Žc type of bend for blinded-tee the collisions are

.between particles and not to walls ,Ž .d number of bends.

A comparison between the attrition rate of variousmaterials is shown in Fig. 1. All the materials were testedwith an experimental apparatus, using blinded-tees. Theexperimental test rig consists a rotary valve feeder and abag-house filter. The conveying pipe line was made of a3r4-in. stainless steel of 6 m length. It contains six bends

w xas shown in the work of Kalman and Goder 8 andw xKlinzing et al. 9 . All the tests were conducted in the

dilute phase regime by having loading ratios between 0.68and 6.02 and with rather high air superficial velocitiesŽ .17.8–21.6 . Since all the materials were tested with the

Fig. 1. Attrition of various materials at similar conditions with blindedŽ .tees, where V is the superficial air velocity and m is the loading ratio; aa

Ž . Ž .granular; b powder; c fine powder.

same apparatus and with closely related operation parame-ters, Fig. 1 presents a comparison between the materialsbased on their ability to withstand impact loads.

Most of the materials shown in Fig. 1 experiencedsignificant attrition at the postulated operating conditions.Even very fine powders as AP and EP were attrited to amedian diameter of almost half of the initial one. ASseems to suffer the least from attrition, although extremeconditions were applied. It shows that this material is thestrongest among the materials tested here. It is also clearfrom Fig. 1 that the number of passes through the pipelinehas a significant effect on the attrition rate. The conclusionfrom Fig. 1 might be that each material has its tendency to

w xbe attrited. Other researches 4–7 have shown that differ-ent materials might react in a different way to somechanges in the bend structure and the operation conditions.Generally saying, the attrition rate decreases while superfi-cial air velocity is decreasing. Since the attrition rate

w xdepends also on the angle of impact with the wall 5–7 ,the bend type and its structure control the attrition rate.

ŽUsually, by decreasing the collision angle the angle be-tween the particle path and the tangent to the wall at the

.collision point , by, for instance, increasing the curvatureof a long-radius bend, the attrition rate decreases. It was

w xshown previously 5,7 that as the rigidity of the bendwalls decreases the attrition rate decreases. It can be

( )H. KalmanrPowder Technology 104 1999 214–220216

achieved by either using a flexible material for a long-radiusbend or using a blinded-tee, in which the empty pocket isusually filled with a stationary bed of particles. However,this might not be the case for fine particles at high airvelocity when they are swept out of the pocket.

3. Preventing attrition

Examination of the evidence shown in Fig. 1 and inw xRefs. 4–7 suggests that the attrition rate could be con-

trolled. Usually, the attrition is an undesired process andtherefore the design of the best bend structure and opera-tion conditions might eliminate the attrition rate or at leastto reduce it significantly. The attrition is undesired in somecases due to a few reasons.

Ž .1 The attrition will change the size distribution of theparticulate material and it might exceed its range of appli-cation. To keep the size distribution within its definition,the product has to be classified at the pipe line end or

Žbigger initial particles have to be produced allowing theknown attrition rate to reduce the size distribution to the

.desired range .Ž .2 If the product is classified, to cut-off the fines, the

fines either have to be returned to the production line orhave to be handled as waste.

Ž .3 The attrition, in any case, increases the amount offines and reduces the mean size. This requires a moresophisticated and heavy duty separator at the pipe line endand could increase the air pollution.

Ž .4 Increasing the amount of fines in a particulatematerial usually decreases the flowability and increases thetendency for caking. Both phenomena require special han-dling systems.

The above difficulties could be overcome by costlyadditional systems. They have to be evaluated only if the

Fig. 2. Schematic diagram of the experimental test rig and the bends.

Fig. 3. Attrition rate at two superficial air velocities, median particle sizevs. number of passes through the system. The effect of the superficial airvelocity on the attrition rate is significant.

attrition is a given fact and cannot be eliminated. However,by being aware to the attrition mechanisms an appropriatedesign may eliminate the attrition or at least, reduce it.

Considering the affecting parameters on the attritionrate indicated in Section 2, some possibilities are given tothe designer to reduce the attrition during pneumatic con-veying. By improving the production process, strongerparticles might be produced. However, this is not possiblein most of the cases, and even if it is possible, the effect onthe use of the product should be considered. As forexample, stronger particles might loose their solubility.

Operation parameters are easier to alter to reduce theattrition. Conveying in dense phase is generally an attri-

w xtion-free process 7 . However, not all the particulate mate-w xrials are suitable for dense phase conveying 10 . Even if

the product has to be conveyed in dilute phase, the airvelocity could be reduced significantly in practical sys-tems. This will reduce, obviously, the safety factor of thesystem design, but it is usually too high in most of thecases. Dilute phase pneumatic conveying systems shouldbe designed to operate with air velocities only slightlyabove the saltation velocity.

The bend type and structure are also factors that couldeasily be taken into account for reducing the attrition rate.The angle of collision could be decreased by increasing thecurvature of long-radius bends. The bends could be con-structed with softer materials to absorb a portion of thecollision energy. In extreme cases, a long-radius bendcould be constructed from a flexible material. Blinded-teesand turbulence drums provide collisions of particles to aparticulate bed rather than to rigid walls. Although, one ofthe major advantages of pneumatic conveying over otherconveyors is the flexibility of the pipe’s path, the pipe lineshould be kept as simple as possible to reduce the numberof bends and therefore the attrition rate.

It should be noted that the selection of the proper bendis a matter of further considerations. The bend structure


Fig. 4. The effect of vibration’s intensity on the attrition rate at high airvelocity. The vibration decreases the attrition rate caused by the pneu-matic conveying due to aeration of the particles layer at the bend walls.

affects also the pressure drop, the wear out of the bend,and the build-up of the conveying material on the bend

w xwalls 5,7 . This paper is concentrating on attrition aloneand does not show all the other effects affecting bendselection. However, a careful consideration of all the pos-sibilities and proper combination of some of them couldreduce the attrition rate significantly, with reasonable cost.

4. Increasing attrition

In some cases, a particulate material is produced ormined in larger sizes than required from the product.Therefore, somewhere along the path of the particulatematerial, from the raw stage to the final stage, its size hasto be reduced. If a pneumatic conveying system is usedsomewhere along the path of the product to transport itfrom one process to another, the attrition could be encour-aged. Therefore, all the considerations described in Section

Fig. 5. The effect of vibration’s intensity on the attrition rate at low airvelocity. At a lower superficial air velocity the vibration might increasethe attrition rate which is caused mainly by the vibration itself.

Fig. 6. The effect of vibration’s intensity on dust production. Thevibration decreases the attrition rate by lowering the rigidity of collisions.

3 should be regarded in an opposite way: the air velocityshould be increased; bends of large angle of collisionconstructed with rigid materials has to be examined; andthe number of bends should be increased even artificially.

By increasing the attrition rate in the pneumatic convey-ing system, the energy consumption of the size reductionsystem could be reduced. However, most of the changes tothe pneumatic conveying pipe line increase the energyconsumption of the conveying system. Therefore, the wholemanufacturing process should be evaluated to found theoptimal combinations for energy and capital costs saves,rather than designing each device separately. In any case,the attrition in pneumatic conveying systems could turn tobe a desired phenomenon in some cases. It provides thedesigner another degree of freedom in the plant design.

Ž .A set of many experiments with AP Fig. 1 has beenconducted in order to increase the attrition of a fine

w xpowder of 15 mm. It was shown 6 that also such fineparticles could be attrited significantly. The attrition gainedduring the pneumatic conveying reduced the milling re-quired for this material.

Fig. 7. The effect of the vibration’s intensity on the amount of granularparticles.


5. Selective attrition

The degree of controlling the attrition rate is evenhigher than described in Sections 3 and 4. In the followingcase study, a pneumatic conveying pipe line which wasdesigned for selective attrition, will be described. In thiscase, the requirement was to apply to granular materialloads that would be sufficient to break out sharp corners,but low enough to prevent any significant breakage. Therequest was defined for reducing the amount of dustproduced during further transportation and handling of the

material, by making the particles more rounded. In thisway, the rounded particles will be transported after thedust, produced during the rounding process, will be sepa-rated from the product.

5.1. Experimental apparatus

The experimental test rig used the same feeder andseparator as the apparatus providing the results of Fig. 1,only the pipe line was altered. On the basis of the previousobservations, turbulence drums were chosen to serve as

Fig. 8. Shape change of the particles during selective attrition in 24 mrs and 10% vibrations intensity. At this experiment the particles become morerounded after only three passes.


bends for applying to the particles shear loads to causew x Ž .chipping and not fragmentation 4 Fig. 2 . Each of the

drums has radial inlet and outlet pipes at an angle of 1358

Ž .between them. The system Fig. 2 consisted of 10 drumsin a pipe line constructing a circulating path. The pipeswere fixed to a rigid steel structure. The structure wasconnected to a vibration machine and separated from thefloor by rubber bands. The vibrator vibrated at 1440 rpmŽ .24 Hz , required 35 W for the full load operation andapplied 688 N in 100% of operation. The load of thevibrating machine could be controlled and changed among0–100% of the full load. The structure was connected tothe feeder and separator by flexible pipes to prevent thevibrations from the feeder and separator. The vibrationsapplied to the structure, changed the flow patterns of theparticulate materials in the drums and enabled the attritioncontrol.

5.2. Results and discussion

The median diameter of the particles vs. the number ofpasses through the pipe line, for two velocities, is shown in

ŽFig. 3. The median size decreases significantly three.times after five passes through the system in the higher

velocity, while in lower velocity the attrition rate is negli-gible. In the lower velocity, the particles enter the drumand slide along the walls, while in higher velocities themomentum is high enough to collide the particles againstthe opposite wall.

In order to alter the flow behavior in the drums, thesystem was vibrated in various intensities. The effect ofthe vibration intensity on the attrition rate at the highervelocity is shown in Fig. 4. As the vibration intensityincreases, the attrition rate decreases. It could be explainedby aerating the particulate bed within the drum by thevibrations. Therefore, new entering particles collide withother particles that are distributed better in the drumvolume, and high impact loads by colliding with theopposite wall are prevented. Therefore, particles experi-ence small impact loads due to collision with other parti-cles and shear loads by sliding along the drum wall afterbeing swiped by the air stream. By 50% of vibrationintensity the median size of the particles decreased, afterfive passes through the pipe line, to 76% of their initialsize. This can be compared to decrease to 33% of initialsize without vibrations.

The effect of the vibration’s intensity is quite differentfor lower air velocity flows as shown in Fig. 5. It seemsthat the vibration intensity does not affect the attrition rate,except for 10% of vibrations.

Figs. 6 and 7 examine the size distribution of theparticulate solids after such a selective attrition. The sizedistribution is defined in terms of the amount of dust that

Ž .was produced Fig. 6 and the amount of the originalŽ .material that remained granular Fig. 7 . Dust is defined as

particles smaller than 75 mm and granular as particles

larger than 1500 mm. By adding higher intensity of vibra-tions, the amount of dust is reduced to a few percents.Without vibrations, almost 14% of the material turned tobe dust. The decrease in the amount of granular materialŽ .Fig. 7 is also moderated by increasing the vibration’sintensity. Fig. 7 shows also two zones of decrease in theamount of granular material. After the third pass thedecrease is much sharper. The same analysis could alsoapply to the lower air velocity case.

The shape of the particles after each pass through theconveying system is shown quantitatively in Fig. 8. Thenumber beneath each photo indicates the pass number. It isclear from Fig. 8 that comparing to the initial shape of theparticles as is shown in photo 1, after only two passes theparticles are much more rounded. After four passes nosharp corners can be seen. At photo 5, some of the dustadhered to the particles are shown.

6. Conclusions

On the basis of previous investigations, the ways forattrition reduction were shown. It was also shown thatsometimes the attrition increasing could be beneficial andshould be considered. Therefore, the use of the terminol-ogy ‘attrition control’ is justified. Furthermore, a newsystem, based on pneumatic conveying principles, showedthat it is able to control the ratio between impact and shearloads. This ability, which defined as selective attrition,could be considered as a process for changing particle’sshape. In the presented analysis, particles were rounded byusing turbulence drums as bends and applying vibrations.The results were satisfactory with minimum waste materialŽ .dust and reasonable loose in the amount of the particlesat the initial size regime.

Acknowledgements

This research was supported in part by The IsraelScience Foundation founded by the Israel Academy ofSciences and Humanities.

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w x4 D.G Papadopoulos, C.S. Teo, M. Ghadiri, T.A. Bell, World CongressŽ .on Particle Technology 3 1998 156.

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w x6 K. Aked, D. Goder, H. Kalman, A. Zvieli, Powder Handling andŽ .Processing 9 1997 345–348.

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Attrition control by pneumatic conveying