Binders are required in some types of bulk solidsagglomeration to produce strong final agglomer-ates. By permanently bonding particles, a binderallows the agglomerates to withstand the rigors ofstorage, handling, packaging, and shipping. Thisarticle first discusses binder types and their appli-cations, then explains how to select a binder foryour process. The article also discusses lubricants,another common aggomeration additive.

Binders: How they work and how to select one

William H. Engelleitner AME Pittsburgh

aglomerantes resistan los rigores del almacenaje, elmanejo, el empaque y el envío. Este artículo describediferentes tipos de aglutinantes y sus aplicaciones yexplica cómo seleccionar un aglutinante para su pro-ceso. El artículo también describe lubricantes, otrotipo común de aditivo para aglomeraciones.

Liants : Comment les utiliser et les sélectionner

L’agglomération de produits solides en vrac nécessitel’utilisation de liants pour produire les agrégats fin-aux solides. En liant les particules de façon perma-nente, ils permettent aux agrégats de résister auxrigueurs de l’entreposage, des manipulations, del’emballage et de la livraison. Cet article discute enpremier lieu des différents types de liants et de leurapplication, puis explique comment il convient de lessélectionner dans vos processus. Cet article discuteégalement des lubrifiants, un autre additif d’agrégatcourant.

Bindemittel: Funktionsweise und Auswahl

Bindemittel sind bei der Agglomeration mancherArten von Schüttgütern erforderlich, um feste End-Agglomerate herzustellen. Durch die permanenteBindung von Partikeln sind die Agglomerate wider-standsfähig gegenüber den Belastungen bei derLagerung, Handhabung, Verpackung und beim Ver-sand. In diesem Artikel werden zunächst die ver-schiedenen Arten von Bindemitteln sowie derenAnwendung beschrieben, gefolgt von Hinweisen fürdie Auswahl des für Ihre Anwendung geeignetenBindemittels. Dieser Artikel befasst sich zudem mitSchmiermitteln, einem anderen wichtigen Zusatz beider Agglomeration.

liquid bridges (interfacial forces and capillary pressure atfreely movable surfaces), viscous bonding (adhesion and co-hesion forces in binders that aren’t freely movable), attractionforces between solid particles, and interlocking particles(form-closed bonds).2

The term agglomerationcommonly describes all sizeenlargement methods that gather fine, intermediate,and even coarse particles into larger masses. In ag-

glomeration, binding forces must act between individual par-ticles.1 Several binding mechanisms exist: solid bridges,

Aglutinantes: Cómo funcionan y cómo seleccionarlos

Algunos tipos de aglomeraciones de productosvolumétricos requieren aglutinantes para produciraglomerados finales fuertes. Ligando las partículas demanera permanente, un aglutinante permite que los

The expressions binding forceand bindercan mean either aninherent, built-in component of oran additive to particles thatprovides bonding between them.1,3When a material containsa built-in binder such as a resin, wax, or wood lignin, thematerial is considered to be self-binding. Compaction meth-ods that use pressure alone — such as briquetting — cancompact these materials into strong final agglomerates with-out requiring that you add a binder. In some processes, youcan use heat before or during compaction to activate thematerial’s built-in binder. [Editor’s note: See the article“Using a roller press to compact your low-moisture byprod-ucts for recycling” in the July 2001 issue for more informa-tion on compaction. See also the references listed at the endof this article in the “For further information” section.]

Even in tumble-and-growth agglomeration, a process in whichvery fine particles are pelletized, balled, or granulated withoutapplying external pressure to the particles, adding a binder isn’talways required. In this type of agglomeration, you can pre-grindspherical particles into irregular particles to help them pack bet-ter and aid agglomeration. You can also allow binderless tumble-and-growth agglomeration by using two-stage processing, inwhich the fine particles are pre-conditioned by adding a liquidbefore they are agglomerated.

However, if your material can’t be agglomerated withoutadding a binder, you need to select a binder that will give thefinal product enough strength to withstand future handling,including packaging, shipping, short- and long-term storage,and in-process use. Your binder choice is important becauseadding a binder to your material can constitute as much as 50percent of your total processing cost. In fact, whether youfind a cost-effective, efficient, compatible binder for yourmaterial can determine the success or failure of your agglom-eration process and final product.

Binder types

Binders can be classified by physical state, chemical type, orfunction:

• The physical states include liquid (such as water, oil, andsodium silicate), solid (such as bentonite and corn starch),and semi-solid (such as tar).

• The chemical types include organic — both hydrophobic(such as tar, pitch, and bitumen) and hydrophilic (such asstarch, lignosulfonate, and molasses) — and inorganic —both insoluble (such as cement, clay, and lime) and water-soluble (such as sodium silicate).

• The functions include that of a matrix binder (such as tar,pitch, and cement), film binder (such as water, starch gel,and bentonite), and chemical binder (such as two-compo-nent binders like molasses combined with hydrated lime,sodium silicate combined with carbon dioxide, and magne-sium oxide combined with magnesium chloride).

These binder types and functions were first defined by K.R.Komarek.4 He describes matrix, film, and chemical bindersas follows: A matrix binderembeds the particles in a continu-ous matrix of hardening binder. This mechanism requires ahigher percentage of binder than other types. Tar and bitumencoal briquets, cement in concrete aggregate, and asphaltready-mix are examples. A film bindercoats the particleswith a film and is most often used in tumble and growth ag-glomeration. Examples are water, alcohol, solutions or dis-persions, and solvents.

One way to reduce your binder cost is to select awaste or processing byproduct as a binder ratherthan a commercially available binder.

A chemical binderaffects agglomerate strength by chemicalreaction between two binder components or between thebinder and material.4,5 Examples are a molasses-with-hy-drated-lime binder (used in briquetting ores, minerals, and by-products) and acids, pre-reactedsolutions, and ammonia(used in granulating N-P-K fertilizer). Another chemical reac-tion binder is a silica-calcium-magnesium cementitiousbinder compound used in hydrothermal processes; this is alsoconsidered a cold-bondingor self-curing binder.

Binder applications

Many different binders of various physical states, chemicaltypes, and functions are available to handle a range of ag-glomeration applications.

General-use binders. Some common binders that are well-documented in industrial reference books and commonlyused in agglomeration equipment supplier’s labs are listed inTable I. Some are used in high-tonnage mineral and chemicalapplications, and other, more expensive binders are used inspecialty chemical, pharmaceutical, food, and other applica-tions. Some recently developed specialty binders are listed inTable II.

Pharmaceutical granulation and tableting. Binders play anespecially critical role in preparing pharmaceutical com-pounds. The binders aid in milling, mixing, wet tumble-and-growth granulation, dry compaction-granulation, andtableting.

A pharmaceutical product’s active ingredient — that is, thedrug dosage — is only one part of the product. Most of theproduct is made up of an excipient, an inert substance thatgives the product its desired form or consistency. An excipi-ent contains bulking agents or fillers as well as functional ad-ditives. These additives can be binders, disintegrants,stabilizers, colorants, pH modifiers, release-rate modifiers,and lubricants. For successful granulating and tableting, you

must carefully select and control each additive for its compati-bility with the active ingredient and its particle size, density,viscosity, surface tension, solubility, and purity.

Common pharmaceutical binders include sucrose, glucose,sorbitol, acacia, alginic acid, sodium alginate, gelatin, starch,ethyl cellulose, methyl cellulose, polyvinyl pyrrolidone, poly-

ethylene glycol, lactose, dicalcium phosphate, crosprovidone,and croscarmellose.6,7 Some of the same binders that are ap-plied in pharmaceutical granulation and tableting are used —often in less pure forms — in other agglomeration applica-tions, including minerals, fuels, and bulk chemicals. Despitetheir obvious differences in quality control standards and op-erating mode (with pharmaceutical granulation and tabletingoperating in batch mode and most high-tonnage agglomera-tion applications in continuous mode), both pharmaceuticaland high-tonnage agglomeration applications may requiresimilar processing equipment and flow schemes.

Factors to consider when selecting a binder

You need to consider your process, the environment, cost,and post-treatment when selecting a binder.

Table II

Specialty binders

Common orbest-knowntrade name Typea Description

Brewex F Modified-starch brewery by-productcontaining glucose, maltose, andmaltodextrin

Alcotac F Synthetic polymers in solid, solution, or dispersion form

Peridur F Cellulose-derived, water-soluble polymer powder originally developed as an organicsubstitute for bentonite in iron-ore pelletizing

PVA F Polyvinyl alcohol powder

Terravest M, C Liquid polybutadiene emulsionthat combines with oxygento form an insoluble matrix

LFI/SIVIA M, C Calcium aluminate, a rapidlysetting, cold-bonding, self-curing binder

Covol C Monomers of acrylonitrile andpolyvinyl alcohol (copolymerization)

Fuller’s earth F Silicious clay material withtypically 40 percent silica

CAFA C Fly ash chemically activated withalkaline materials

Gilsonite M Natural asphalt mineral

Avicel, Omnicel F Microcrystalline cellulose powder

Xtra-Dry C Calcium silicate composite powderthat prevents water absorptionand leaching

Krystal Bond M, C Alumina silicate, a cold-bonding, self-curing binder

aNote: F is film binder, M is matrix binder, C is chemical binder, based on definitions byK.R. Komarek, “Selecting binders and lubricants for agglomeration processes,”Chemical Engineering, December 4, 1967.

Table I

Some common binders

Name Typea Description

Bentonite F Colloidal clay (montmorillonite)

Corn starch F Hydrolized, gelatinized, (maize) pre-cooked type; other starches

are wheat, potato, and tapioca

Sodium silicate F Sodium silicate with carbon dioxide, dilute acid, hydrated lime, or calcium chloride becomes achemical binder (C)

Lignosulfonate F Lignin waste product of pulpand paper processing;calcium, sodium, ammonium type

Molasses F Cane or beet sugar type; molassesplus hydrated lime is a chemicalbinder (C)

Hydrated lime F, C Combines with sodium silicateand molasses (see above)

Bitumen M Examples are coal tar, wood tar, petroleum asphalt, natural asphalt (for example, Gilsonite — see Table II)

Portland cement M, C Used in aggregate and concrete;Portland cement is the originalchemical (cold-bonding, self-curing)binder

Clay F Many types are used as binders (such as Fuller’s earth – see TableII) in ceramics and other products

Acids C Many types (such as nitric, hydrochloric, phosphoric, andsulfuric acids) are used as chemicalreactants and as ingredients inagricultural chemicals

Cellulose gum F Carboxymethylcellulose (CMC)

Sucrose F Used in food and pharmaceuticalproducts

Wax M Many types are used for lubricationin compaction, such as tabletingand briquetting

Water F Best and simplest example of a filmbinder (F); water combined withPortland cement is the bestexample of a chemical binder (C)

aNote: F is film binder, M is matrix binder, C is chemical binder, based on definitions byK.R. Komarek, “Selecting binders and lubricants for agglomeration processes,”Chemical Engineering, December 4, 1967.

Processing, environmental, and cost factors. Processingfactors to consider are the binder’s bulk density, viscosity, pHvalue, adhesiveness, solubility, color, and odor. You alsoneed to consider the particle size of the material you’ll be ag-glomerating.

Environmental factors can also play a role. For example, incoal briquetting, a binder that contains sulfur or ash can in-crease the coal’s sulfur and ash content; this can cause thesecomponents to be released into the environment at unaccept-able levels when the coal is burned.

Cost is a major factor in choosing your binder. In fact, thisfactor alone can eliminate your selection of an otherwise per-fect binder. Once you’ve determined the lowest effective per-centage of binder you can use in your agglomeration processto achieve your desired product quality, you need to calculatethe binder’s purchase and shipping costs, based on its avail-ability and how many tons you need at what frequency.

One way to reduce your binder cost is to select a waste or pro-cessing byproduct as a binder rather than a commerciallyavailable binder, as long as the byproduct suits your agglom-eration process. Some common examples are:

• Fly ash from coal-burning power plants, which is used as acement substitute in aggregate.

• Lignosulfonate, a waste byproduct from kraft pulp andpaper processes that’s used to form soil additive micropel-lets and other products.

• Brewery waste, which has been used instead of molasses orlignosulfonate to pelletize limestone, dolomite, and gyp-sum.

• Finely ground waste paper, which is used as a binder in bri-quetting materials such as metallurgical dust.8

• Sawdust from lumber mills, which is used as a filler andbinder to provide green strength — that is, strength for themoist, uncured agglomerates — for pellets and briquets ofvarious materials before they are dried or fired in a furnace.

• Rice hulls from grain processing plants, bagasse from sugarcane processing plants, and other biomass residues, whichprovide green strength for coal, ore, and flue dust pellets andbriquets.9

Another way to reduce cost is to select an efficient blender topre-mix your material and binder. This requires lab testing of

binder pre-mixing prior to agglomeration. Not only will thetesting help you scale up to a production-size blender, but itwill ensure that as little binder as possible is required toachieve your agglomerate’s optimum quality.

Post-treatment factors. Most binders require post-treatmentto produce strong agglomerates. For example, a cold-bond-

ing, self-curing cementitious binder typically requires timeto harden at ambient or low temperatures. Some cold-bond-ing binders also require steam and pressure to cure. An ex-ample is a reactive binder, such as calcium oxide (orhydrate) combined with silica, which is mixed with steelplant dust before the dust is pelletized and hardened in asteam autoclave.10

Another agglomerationadditive:

Lubricants

It makes sense to talk about lubri-cants in a discussion of binders be-cause lubricants are another

important additive for many agglom-eration applications. Lubricants areparticularly useful in compaction(pressure agglomeration) processessuch as tableting, briquetting, and ex-trusion. They promote good material

flow into the pockets or dies in yourpress or extruder, and they promotethe compression and plastic formingof the material and the final products’release.

A lubricant can be internal or exter-nal. An internal lubricantis pre-mixed with your material, and anexternal lubricantis applied to thepockets or dies in your equipment.

Some general-use lubricants are glyc-erin, ethylene glycol, silicone, talc,

graphite, stearic acid, and parrafinwax. Lubricants used in pharmaceuti-cal applications include magnesiumstearate, calcium stearate, wax, hy-drogenated vegetable oil, talc, andstarch.1

—W. H. Engelleitner

Reference1. D.M. Parikh, editor, Handbook of

Pharmaceutical Granulation Technology,Drugs and Pharmaceutical SciencesSeries, Marcel Dekker, 1997.

Many other binders, especially film binders, require thermaldrying or high-temperature sintering (firing). One example isagglomerates made with a sodium silicate binder, which be-come glass-like and waterproof when dried at 200°C or above.In another example, lignosulfonate and water are added tolimestone and dolomite fines to form micropellets; the pelletsare dried continuously at between 90°C and 260°C, whichhardens the binder and strengthens the micropellets for ship-ment. When the micropellets are applied to soil as an additivefor pH control, the lignosulfonate allows the gradual release ofcalcium or magnesium as the micropellets slowly disintegratein contact with moisture on the soil. Another example is ben-tonite (montmorillonite clay), which provides green strengthto iron-ore pellets for handling and transfer to a furnace, wherethe pellets undergo grate or grate-kiln firing at above 1,150°Cto achieve their final pellet strength. PBE International

References1. Wolfgang Pietsch, Size Enlargement by Agglomeration, John Wiley

& Sons, 1991.

2. H. Rumpf, “The strength of granules and agglomerates,” Proceedingsof the First International Symposium on Agglomeration,edited byW.A. Knepper, John Wiley & Sons, 1962.

3. William H. Engelleitner, “Update: Glossary of agglomeration terms,”Powder and Bulk Engineering,February 2000.

4. K.R. Komarek, “Selecting binders and lubricants for agglomerationprocesses,” Chemical Engineering, December 4, 1967.

5. P.L. Waters, “Briquet binders: A reappraisal,” Proceedings of theBiennial Conference of the Institute for Briquetting andAgglomeration, Volume 12, 1971.

6. D.M. Parikh, editor, Handbook of Pharmaceutical GranulationTechnology, Drugs and Pharmaceutical Sciences Series,MarcelDekker, 1997.

7. Isaac Ghebre-Sellassie, editor, Pharmaceutical PelletizationTechnology, Drugs and Pharmaceutical Sciences Series, MarcelDekker, 1989.

8. H. Gunter, “The application of waste paper as a binder in pressureagglomeration,” Proceedings of the Biennial Conference of theInstitute for Briquetting and Agglomeration, Volume 23, 1993.

9. Henry C. Messman, “Introduction,” Elements of Briquetting andAgglomeration, edited by Henry C. Messman and T.E. Tibbets,Institute for Briquetting and Agglomeration, 1977.

10. M.A. Goksel and W.M. Mathias, “Recycling steel plant finematerials by using the MTU cold bond process,” Proceedings of theBiennial Conference of the Institute for Briquetting andAgglomeration, Volume 14, 1975.

William H. Engelleitner, “Agglomeration 1949-1995: AnIBA perspective,” Proceedings of the Biennial Conference ofthe Institute for Briquetting and Agglomeration, Volume 24,1995; additional updated information in Volume 26, 1999, ofthe Proceedings.

Thomas R. Marrero, “Theory and application of binders,”Proceedings of the Biennial Conference of the Institute forBriquetting and Agglomeration, Volume 26, 1999.

Wolfgang Pietsch, Chapter 6, “Size enlargement by agglom-eration,” Powder Science and Technology, second edition,edited by M.E. Fayed and L. Otten, Chapman & Hall, 1997.

Proceedings of the Biennial Conference of the Institute forBriquetting and Agglomeration, Volumes 23-26, 1993-1999.

Section 20, “Principles of size enlargement” (pages 20-56 to20-89), Perry’s Chemical Engineers’ Handbook, seventhedition, edited by Don W. Green, McGraw-Hill, 1997.

Find more information on agglomeration and binders in arti-cles listed under “Agglomeration” in Powder and Bulk Engi-neering International’s comprehensive “Index to articles” inour November 2001 issue and at our website: www.powderbulkintl.com.

William H. Engelleitner is principal consultant of AMEPittsburgh, 133 Tory Road, Moon Township, PA 15108USA. Tel. +1 412 264 0619, fax +1 412 264 4902. He is thedirector of agglomeration courses at the Center for Profes-sional Advancement in the US and Europe and a boardmember and past president of the Institute for Briquettingand Agglomeration.

For further information

William H. Engelleitner, Agglomeration: The Technology ofSize Enlargement,June 2000 (available from the author).