wet agglomeration

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Agglomeration is the process ofconverting fine powder par-ticles into larger particles bythe introduction of external

forces. Agglomeration is often desir-able because the process yields a prod-uct that has a higher bulk density,contains less dust, and has improvedflowability. Figure 1 shows samples offine potash powder and powder thathas been agglomerated.

Fine powders often exhibit flowproblems in a hopper, bin or silo, such

as flow stoppages, erratic flow, flood-ing and limited discharge rates. Flowstoppages occur when attractive forcesbetween particles, such as Van derWaals forces, valence forces and hy-drogen bridges, cause a cohesive archto develop at the vessel outlet. In somecases, powder may only flow in a nar-row channel when a feeder or gate isoperated. If the material has enough co-hesive strength to become stable as theflow channel empties, flow stoppageswill occur when powder along the walls

remains stagnant. Erratic flow resultswhen ratholes collapse, causing thepowder to arch as it impacts the out-let. (For an illustration of arching andratholing, see the online version of thisarticle at www.che.com.)

If a stable rathole forms in a hopper,bin, or silo and fresh powder is added,it may become entrained in the air andbecome aerated. Because most feedersare designed to handle solids and notfluids, flooding may result when thefluidized material reaches the outlet.Flooding can also occur when ratholescollapse into an emptying flow chan-nel. A fine powder sometimes cannot

be discharged at the desired rate dueto its low permeability, which leads toan adverse pressure gradient at theoutlet of the vessel and gas flowingcounter to the solids.

Particle size and flow patternsThe manner in which a powder flowsin a hopper, bin, or silo has a great ef-fect on the likelihood of these solidsflow problems occurring. There aretwo primary flow patterns that canoccur in a bin or silo: mass flow and

funnel flow. (See the online version ofthis article for an illustration of theseflow patterns.) In funnel flow, an ac-tive flow channel forms above the out-let, with stagnant material remainingat the periphery. Funnel flow occurswhen the walls of the converging sec-tion of a vessel are not sufficientlysteep or low enough in friction for thepowder to flow along them.

Mass flow occurs when all the pow-der inside a vessel is in motion when-ever any is withdrawn, and takes

place when the walls of the hoppersection are steep enough and lowenough in friction for the powder toflow along them. As long as the out-let is large enough to prevent arching,all the powder flows when material isdischarged, keeping the contents ofthe bin fully live.

Funnel flow can cause erratic flow,exacerbate segregation and reducethe live capacity of a vessel. Funnelflow can also induce high loads (de-pending on vessel size) on the struc-ture and downstream equipment dueto collapsing ratholes and eccentricflow channels. In addition, if the nar-

row flow channel empties out, a stablerathole may form. With fine powders,controlling flowrate is often chal-lenging. Since a funnel flow channelis often unstable, its size can changedramatically or it may collapse, creat-

ing flowrates that range from no flowat all to complete flooding. In addition,the bulk density at the outlet will varysignificantly over time.

Eliminating flow problems can oftenbe accomplished by ensuring that amass flow pattern exists in the ves-sel. Stable ratholes cannot form andbecause the material is more likelyto de-aerate, flooding is less likely. Inaddition, the bulk density of the dis-charged powder is less variable.

Mass flow hoppers, bins and silos

designed to handle fine powders fre-quently require steep walls and largeoutlets due to the materials high co-hesive strength, high wall friction andlow permeability. Often particle size en-largement allows mass flow vessels tobe built with smaller outlets (allowingless expensive feeders to be used), wallsthat are less steep (which is advanta-geous if there are height restrictions)and less expensive wall materials.

Designing for mass flowThe flow pattern inside a hopper, bin orsilo funnel flow or mass flow canbe predicted by measuring the friction

Solids Processing

Greg Mehos,Jenike & Johanson

Chris Kozicki,Feeco International

46 ChemiCal engineering www.Che.Com January 2011

Solids Processing

Figure 1. Fine potash powder is oneexample o an application that can ben-et rom particle enlargement

One way to avoid the flow problems

associated with fine particles is to enlarge them

Consider Wet

Agglomeration ToImprove Powder Flow


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ChemiCal engineering www.Che.Com January 2011 47

between the powder and the hopperwall material. Wall friction is mea-sured by a method described in ASTMD-6128 [1]. Various normal loads areapplied to a sample of powder, whichis forced to slide along a coupon of wallmaterial. The resulting shear force ismeasured as a function of the applied

normal force, and a wall yield locusis constructed by plotting shear forceagainst normal force. The angle of wallfriction at a particular pressure (') isthe angle that is formed when a line isdrawn from the origin to a point on thewall yield locus. A wall yield locus fora sample of fine potash powder on 304,No.-2B finish stainless steel is givenin Figure 2.

Design charts originally developedby Jenike [2] provide allowable hopperangles for mass flow, given values of thewall friction angle. An example chartfor conical hoppers is shown in Figure3. Values of the allowable hopper angle

(C; measured from vertical) are onthe horizontal axis, and values of thewall friction angle (') are on the verti-cal axis. Any combinations of' and Cthat lie within the mass flow region ofthe chart will provide mass flow.

Designing right to the limit of themass flow region is not recommended

for conical bins. If the combination ofwall friction angle and hopper anglelies too close to the funnel-flow line, aswitch to funnel flow can occur. Hence,a 45 deg margin of safety is employedwith respect to the mass flow bound-ary. For the potash powder whose wallfriction properties are described byFigure 2, a conical hopper with wallssloped 12 deg from vertical is recom-mended to ensure mass flow.

Flow stoppages will be prevented ifthe stresses imparted on an obstructionto flow (such as a cohesive arch or stablerathole) are greater than the cohesivestrength that the material gains due to

its consolidation in a hopper, bin or silo.The cohesive strength of a bulk solidcan be determined using the methoddescribed in ASTM D-6128 [1] where

a direct shear tester is used to mea-sure the shear strength of a materialunder varying consolidation pressures.The relationship between strength andpressure is called the flow function. Theflow function for a sample of fine potashpowder is shown in Figure 4.

The stresses imparted on an archof powder that forms at the vesseloutlet are proportional to the mate-rials bulk density. Once a materialsflow function has been determinedand its bulk density has been mea-

sured, the minimum outlet diameterthat will prevent a cohesive archfrom developing can be calculatedusing an analysis developed by Je-nike [2]. For the powder whose flowfunction is given in Figure 4, theanalysis shows that a conical mass-flow hopper requires a 6-in. dia. out-let to prevent arching. Note that thisoutlet diameter will prevent archingbut does not ensure that the desireddischarge rates can be achieved. Finepowder flowrates are limited because

of high permeability, as discussed inJohanson [3].

Wet agglomeration processesWet agglomeration processes combinepowder, liquid (usually water) and, ifnecessary, a binder, imparting shearto form agglomerates. Also known astumble-growth agglomeration, wet ag-glomeration processes (Figures 5 and6) include rotating drums, disc or panagglomerators, pin and ribbon mixers,and fluidized beds.

In general, particle size enlarge-ment by wet agglomeration occursin three stages. The first is a mixingstage where powder, liquid and binderare combined. Next, moist particlesare joined together to form so-calledgreen agglomerates. Drying or cur-ing takes place in a final stage. Thewet agglomerates are formed by firstforming nuclei that then grow intolarger aggregates by layering or co-alescence. In some cases, nucleationand aggregate growth take place intwo separate pieces of equipment thatare operated in series.

Nucleation gives rise to seed par-

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Figure 2. The wall yield locus, plot-ted here or a sample o ne potashpowder, is a unction o a materialswall riction properties

Figure 3. In design charts, such asthis one or conical hoppers, any com-bination o wall riction angle (') and al-lowable hopper angle (C) that lie withinthe mass fow region o the chart willprovide mass fow

Figure 4. The relationship betweencohesive strength and pressure is calledthe fow unction, and is shown here orne potash powder

Commonly used binders

Inorganicbinders

Alkali silicates

GypsumAlumLimeBentonite and

other claysLime hydrateCaustic sodaMagnesia/magne-

sium oxideColloidal alumina

and silicaMagnesium

chlorideCement

Plaster of ParisDolomiteSodium borateFullers Earth

Organicbinders

Asphalt

MaltoseAsphalt

emulsionsMolassesCelluloseParaffinCorn starchPeatCoal tarPVA (polyvinylalcohol)DextrineRosinGelatine

StarchesLignosulfonates

(lignin)Sucrose


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48 ChemiCal engineering www.Che.Com January 2011

ticles, which are formed when severalindividual particles adhere to eachother. The nucleation stage can betime-consuming because the seed ag-glomerates are weakly bonded andreadily disintegrate back into indi-

vidual particles. Eventually, largeraggregates are formed when small ag-glomerates coalesce or individual par-ticles adhere to larger agglomerates.

Once larger agglomerates are created,growth becomes accelerated as theincreased mass and higher kineticenergy of agglomerates cause them topick up individual particles more rap-idly and incorporate them onto theirsurfaces. The relative rates of sizeenlargement (nucleation, coalescenceand layering) and size reduction (at-trition and consolidation) establishthe final particle size along with thematerials tendency to wick moisturefrom its core to outer layers.

The optimal amount of liquid addedto a powder the amount that givesthe resultant agglomerates theirgreatest integrity and resistance tobreakage is typically 4090% of itsliquid saturation. The liquid satura-tion is the fraction of total void spacethat can be filled with the liquid. Whenwater (or another liquid) is added to adry bulk solid, liquid bridges will beginto form at contact points between par-ticles. This is known as the pendularstage of saturation. All free moistureis attracted to the interfaces betweenthe solid particles by capillary effects,and surface tension draws the parti-

cles together. As saturation levels areincreased, the funicular stage is even-tually reached where all internal solidsurfaces become surrounded by liquid.

At this point, the mixture becomesmore fluid-like, tensional forces dis-appear, and the agglomerates becomeweaker. When the powder becomesfully saturated, it reaches its capil-lary state, and at higher moisture lev-els, the system begins to behave as aslurry. Saturation states of powder areillustrated in Figure 7.

An under-saturated bulk solid canbe converted to a saturated state bycompaction (dry agglomeration) alone,without further addition of moisture[4]. Likewise, pendular and funicu-lar states of the same powder can bereached at different moisture con-tents. Hence, the optimal amount ofliquid that must be added to a powderto resist breakage often depends onthe device, scale and properties of thematerial undergoing agglomeration.

Often, binders are added prior to orduring agglomeration to increase thestrength of the agglomerates. Bindersare generally liquid solutions, suspen-

sions of fine solids, or dry powders.Commonly used inorganic and organicbinders, are listed in the box (oppositepage) [5, 6]. Highly viscous binders,such as colloidal silica, are effectivesince both the adhesive forces at thebinder-solid interface and the cohe-sive forces within the viscous binderact to strengthen the aggregates. Solidbridges form when dissolved matterin liquid bridges precipitate duringcooling or drying. Similarly, colloidalparticles form solid bridges when they

become concentrated in diminishingliquid bridges and become consolidateddue to the liquids surface tension.

Even if a binder has no solid com-ponents, drying may still increasethe strength of the agglomerates bydrawing the particles closer together,because capillary forces associatedwith the interstitial liquid increase asthe liquid content is reduced. Once inclose proximity, the magnitude of Vander Waals and other molecular forcesbetween adjacent individual par-ticles increases. This leads to furtherdensification and greater integrity ofthe agglomerated product. In some

Figures 5 and 6. Wet agglomeration processes combine powder, liquid (usually water) and, i necessary, a binder, impartingshear to orm agglomerates. Here, disc (let) and drum (right) agglomerators are shown

Dry powder Pendular state Funicular state Capillary state

Figure 7. The liquid saturation is the raction o total void space that can belled with the liquid. The our saturation states o powder are shown here

Solids Processing


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instances, a material to be agglomer-ated has natural binding propertiesthat are simply activated by the addi-tion of moisture.

As a general rule, flowability im-proves with increasing particle size.Cohesive strength usually increases

with decreasing particle size due togreater specific surface-area and agreater number of contacts betweenparticles. There are many exceptionsto the rule; for instance, sub-micronparticles are often added to powdersas parting agents to increase the dis-tance between individual particles andreduce the magnitude of interparticleforces. Fortunately, shear cell test-ing is straightforward, and cohesive-strength and wall-friction tests canbe quickly conducted to confirm the

potential benefits of particle size en-largement on a materials flowability.

As an example of improved flowabil-ity that can result from agglomera-tion, potash was agglomerated on acontinuous basis by adding fine pow-der, water and a dry organic binder toa pin mixer, followed by a disc agglom-erator, and finally a direct fired rotarydryer. The resulting pellets were thenscreened to separate the desired pel-lets (Figure 1) from the undersize andoversize fractions. The green agglom-

erates contained 814% moisture and1.53.5% binder.

The cohesive strength, wall frictionand bulk densities of the fine potashpowder and agglomerates of potashare compared in Figure 8. Note thatagglomeration reduces the cohesivestrength of potash, reduces its wallfriction on 314, No.-2B finish stain-

less steel, and increases its bulk den-sity. Powders with lower strength, lessfriction, and higher bulk density flowmore readily.

While a 6-in. dia. outlet is recom-mended to prevent the fine powderfrom arching in a conical hopper,

the shear cell tests and subsequentanalysis reveal that cohesive archingwill not occur in a hopper, bin, or silothat handles the agglomerated prod-uct. Instead, the outlet size of a vesselthat handles the agglomerates shouldbe selected by consideration of par-ticle interlocking or desired dischargerates. In addition, the agglomeratedmaterial requires significantly less-steep walls to allow mass flow. If a con-ical hopper, bin, or silo with a 6-in. dia.outlet is fabricated or lined with 304,

No.-2B finish stainless steel, wallssloped 25-deg from vertical are recom-mended to ensure mass flow (versus12-deg if the material is handled in afine powder form).

Final remarksFlowability of fine powders is often im-proved by agglomeration. Agglomera-tion results in powder with a higherbulk density and often reduces its co-hesive strength and wall friction.

A key to producing quality agglom-

erates in a wet process is to ensurethat the proper ratio of powder, mois-ture and binder is used. If insufficientliquid is added, layering is difficult,and excess dust in the product is pro-duced. If liquid content is too high itwill yield agglomerates with poor in-tegrity, as the bonds between individ-ual particles will be weak.

In a continuous wet-agglomerationprocess, the hopper that feeds theequipment should also be designedfor mass flow to ensure its reliable op-eration since the performance of mostagglomeration systems is stronglyinfluenced by feedrate of the powder

and any binder used. In mass flow, thebulk density of the powder is indepen-dent of the level of powder inside thehopper, flowrates are steady (assum-ing that the outlet is large enough toprevent flowrate limitations that re-sult from counterflow of air), and rat-holes cannot form. The proper ratio ofpowder, moisture and binder is easierto maintain if systems designed formass flow are used, which allow ag-glomerates with the greatest integrityto be produced.

Edited by Rebekkah Marshall

AuthorsGreg J. Mehos is a projectengineer at Jenike & Johan-son, Inc. (400 Business ParkDrive, Tyngsboro, MA 01879;Phone: (978-649-3300; Email:[email protected]), an en-gineering consulting firmspecializing in the storage,flow and processing of powderand bulk solids. He has beeninvolved in a wide range ofbulk solids handling projects

including flow property testing, hopper and feederdesign, and the design and analysis of purgeand conditioning columns. Mehos is a member

of AIChE and currently serves on the executivecommittee of AIChEs Particle Technology Forum.He received B.S. and Ph.D. degrees in chemicalengineering from the University of Colorado anda masters degree from the University of Dela-ware. He is a registered professional engineer.

Chris M. Kozicki is a pro-cess sales engineer at FeecoInternational, Inc. (3913Algoma Rd., Green Bay, WI54311-9707; Phone: 920-468-1000; Email: [email protected]; Website: www.feeco.com)a complete solutions providerthat engineers, designs andmanufacturers systems in-cluding agglomeration, drying/cooling and material handling.

Kozicki has been associated with limestone pel-letizing systems, mineral byproduct briquetting

and agricultural chemical processing. He is thefirst vice president of the Institute for Briquettingand Agglomeration and received a B.S. degree inengineering mechanics from the University ofWisconsin.

Figures 8. Agglomeration reduces the cohesive strength (let) o potash, reduces its wall riction (center) on 304, No.-2B nishstainless steel and also increases its bulk density (right)

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References

1. ASTM D-6128, Standard Test Method forShear Testing of Bulk Solids Using the JenikeShear Cell, ASTM International (2006).

2. Jenike, A.W., Storage and Flow of Solids,Bulletin 123, University of Utah Engineer-ing Station, 1964 (revised, 1976).

3. Johanson, R., Two-phase-flow Effects in Sol-

ids Processing and Handling, Chem. Eng. 72,1, 77 (1979).

4. Schulze, D., Powders and Bulk Solids,Springer, New York, 2007.

5. Pietsch, W., Agglomeration Processes, Wi-ley-VCH, Weinheim, 2002.

6. Engelleitner, W., Binders: How They Workand How to Select One, Powder Bulk Eng.,15, 2, 31 (2001).

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wet agglomeration

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