Powder and Bulk Engineering, February 1996 67

e e

Peter Koenig Hosokawa Bepex Corp.

Do you need to aigglomerate a chemical, food, pharma- ceutical, or mineiral product at high speeds? A mixing ag- glomerator that uses high-speed mechanical agitation and short residence times can make your product more marketable by innproving particle properties. After pro- viding general information about agglomeration, this ar- ticle describes benefits and operation of the high-speed mechanical agglomerator and how to control fmd prod- uct qualities by adjusting the unit's operation. Final sec- tions discuss performance tests and how to configure your agglomeration system.

gglomeratiion is becoming a common processing step in chemical, food, pharmaceutical, and mineral indus- A tries because it provides several benefits important to

product performance and market appeal. These benefits result from controlling certain product properties. For instance:

Controlling particle size provides a narrow size range.

Increasing or reducing specific particle or loose bulk density can affect product performance - for instance, density is crit- ical for a pharmaceutical product's dosage level.

Increasing solubility can help the product disperse or improve its time-release capability.

Improving flow properties can reduce lumping or caking for better flow control.

Reducing dustiness makes the product safer to handle, espe- cially if it's hazardous, and improves its appearance.

Uniformly distributing multiple ingredients improves the product's consistency for better performance.

Various equipment achieves agglomeration by different meth- ods.',2 Briquetters, tableting equipment, and compaction rolls provide compaction. Gear pelletizers and flat- and ring-die ex- truders achieve extrusion. Prilling towers, spray formers, and high-speed and -energy mixers accomplish melt-forming. High- and low-speed mechanical agglomerators and fluid-bed agglomerators provide mixing agglomeration. Pan, cone, and drum pelletizers achieve tumbling agglomeration, and con- veyor machines provide sintering.

This article concentrates on one type of mixing agglomeration equipment: the high-speed mechanical agglomerator. This ag- glomerator is suited to high-capacity continuous applications with throughputs from 100 to 100,000 lbh. The unit provides fast mechanical agitation and short residence times for chemi- cal, food, pharmaceutical, and mineral products.

High-speed mechanical agglomerator 's benefits Depending on your solid feed, binder, and operating parame- ters, the high-speed mechanical agglomerator can provide such benefits as narrow particle size range, dust control, improved product density and solubility, better product flowability, and uniform ingredient distribution.

Narrow particle size range. The high-speed mechanical ag- glomerator produces a particle size range of typically better than 90 percent within 200 to 2,000 microns (or from 80 to 10 US mesh). In many cases, the size range is narrower. Screening the agglomerates can produce extremely narrow size ranges.

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Dust control. The agglomerator’s final product is typically dust-free, which makes the unit suitable for dedusting materials such as coal fines, noxious chemicals, or other hazardous mate- rials; handling waste products; or conserving valuable prod- ucts, such as some pharmaceuticals.

Powder and Bulk Engineering, February 1996

Improvedproduct density and solubility. Generally, as product density increases, solubility decreases. Agglomerating a prod- uct requires controlling process parameters to balance density with solubility requirements. The high-speed mechanical ag- glomerator makes final products with light to medium densities -both specific particle and loose bulk - which improves the solubility and wettability of the final product. The equipment can handle food, drink mix, detergent, and many chemical and agricultural products with end-use applications that require dls- solution or dispersion.

Betterproductflo wability. The agglomerator can dramatically improve product flowability. For instance, a powder that has a 70-degree angle of repose can have a 35- or 40-degree angle of repose after agglomeration.

Uniform ingredient distribution. The agglomerator’s configu- ration makes it an efficient mixer, so it uniformly blends ingre- dients as well as agglomerates them. In many applications, such uniformity is a value-added feature of the final product. Uni- formly distributing the ingredients, whether colors or reactants, improves the final product quality or simplifies later process- ing, or both.

How the agglomerator works The high-speed mechanical agglomerator typically consists of a single-shaft rotor (or mixing shaft) with paddle-, blade-, or pin-type mixing elements inside a flexible polymer or rigid steel housing, as shown in Figure 1. In a unit with a flexible polymer housing, a set of pneumatically driven rollers is lo- cated outside the housing. The mixing shaft can be oriented ver- tically or horizontally, and the paddles and blades can be individually adjusted for maximum agitation. (Pins are fixed rather than adjustable.) A solid feed inlet and liquid feed inlet are typically located at the housing top. The liquid feed, which is the binder, can be atomized (sprayed) with air, steam, or hy- draulic pressure or can flow in a nonatomized stream. A dis- charge is located at the center bottom.

In operation, the mixing shaft rotates at 1,500 to 5,200 fpm as one or more fine powders are metered to the solid feed inlet. The shaft’s high-speed rotation distributes the powders to the housing’s periphery. The dispersed particles then form a turbu- lent annular layer that spirals downward along the housing wall and occupies up to about 20 percent of the agglomerator’s available internal volume. The liquid feed is atomized into the agglomerator, or flows in a stream through the liquid feed inlet, and contacts the powder.

The fast mechanical agitation provides intimate liquid-to-solids contact, totally wetting all particle surfaces. When the liquid addition level is high enough, capillary pressure, interfacial forces, and other forces cause the fine, wetted particles to ag- glomerate. The agglomerates discharge from the center bottom. In a unit with a flexible polymer housing, the rollers massage

the housing exterior to dislodge wet solids from the walls and promote discharge. Typical residence times range from 1 to 2 seconds for a unit with a vertical mixing shaft and 4 to 20 sec- onds for one with a horizontal shaft.

Controlling feeding to shorten residence time. While making high-capacity operation possible for relatively small agglomer- ators, the short residence time also requires accurate, uniform solid and liquid feeding. Several equipment configurations can control feeding. For instance, for uniformly feeding multiple powders to the agglomerator, you can feed the powder streams to a common conveyor that discharges directly into the agglom- erator. Or you can uniformly feed cohesive, nonfree-flowing powders with a high angle of repose by using a high-speed pad- dle or screw conveyor that force-feeds the powders directly into the agglomerator. This option provides vertical drops from the conveyor to the agglomerator and minimizes the use of angled chutes for handling the powders. You can also use a vibratory inlet chute to feed cohesive powders, eliminating the need for an additional convevor.

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Selecting a binder. You can use any of several binders in the high-speed mechanical agglomerator. In addition to water, com- monly used binders include dissolved solids, slurried solids, and wet cakes (the transition stage between wetted and slurried solids). The latter three can consist of the same materials as the powder feed or other materials required in the final product.

Powder and Bulk Engineering, February 1996

tor because the binder type directly affects all final product properties, including particle size distribution, density, and solubility.

Using a slurried or dissolved solid or wet cake as a binder can re- duce the overall agglomeration costs, especially if separate sources of solid and liquid feeds are already available in your plant. For instance, if your chemical processing system includes a spray dryer upstream from the agglomerator, you can divert a portion of the spray dryer’s liquid feed to the agglomerator as the binder. The liquid then contacts the spray-dried powder fed to the agglomerator, forming agglomerates that flow to the ag- glomeration system’s dryer and bypass the spray dryer. Thus you’ve shifted a portion of the drying load from the dryer out- side the agglomeration system to that within the agglomeration system, increasing the plant’s net production capacity. In some cases, the binder is a slurried or dissolved solid and the only solid feed is recycled dry solids from the agglomeration process itself, which saves further material handling and processing costs.

Table I lists some binders that have been successfully tested with various powders. [Editor’s note: For more informa- tion about binders, see reference 3.1 Be aware that selecting your binder is just as important as choosing the agglomera-

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How to control final product qualities by adjusting the agglomerator

By adjusting the high-speed mechanical agglomerator’s oper- ating parameters -paddle or blade pitch, shaft speed, and liq- uid addition rate -you can control final product qualities.

Paddle or blade pitch. Figure 2 shows the effect of paddle or blade pitch on product flow. By adjusting the pitch (which es- tablishes the angle of attack), you can achieve various types of product flow - conveying, mixing, and retention - through- out the agglomerator. (Because pins are fixed, an agglomerator with pins can achieve only one type of product flow.) For in- stance, for a hard-to-wet fine powder, you can adjust some pad- dles for mixing in the liquid addition area, thus creating a mixing zone (or working zone) in this area; by adjusting some paddles for conveying in the outlet area, you can create a con- veying zone from the working zone toward the outlet to mini- mize shear and reduce the energy transmitted to the resulting soft agglomerate. Adjusting the paddle or blade pitch can also reduce or increase the product residence time.

Shaft speed. Increasing the agglomerator ’s shaft speed typically produces a narrower particle size distribution;but at the same time reduces the average particle size. Conversely, a slower shaft speed produces a wider particle size distribution with coarser par- ticles. Figure 3 plots test resultssthat show the effect of three shaft speeds on particle size distribution of a laundry detergent as parti- cle size (in mesh) versus cumulative weight of particles (in per- cent smaller than). The agglomerator had a vertical shaft:

Increasing the shaft speed also increases the agglomerator’s power consumption and, depending on the paddle or blade pitch, affects the residence time. If you set most of the paddles

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or blades for conveying, the higher shaft speed will reduce the residence time. But if you set enough paddles or blades for re- tention, the higher shaft speed will increase residence time.

Liquid addition rate. As the liquid (or binder) addition rate in- creases, so does particle size, as shown in Figure 4, which plots test results5for the same agglomerator. The figure shows the ef- fect of three liquid addition rates on particle size distribution of a laundry detergent. The amount of size change relative to the increased liquid addition rate depends on the solids-liquid com- bination. For an extremely hygroscopic product, slight varia- tions in your liquid addition rate produce significant particle size changes. With a more absorptive, less soluble product, you’ll need a more significant change in the liquid addition rate for a similar particle size change.

The liquid level added (the weight percent of liquid) also affects the product’s final bulk density. As the liquid level approaches a maximum, each particle surface becomes completely coated. Maximum capillary force and minimum void areas on the parti- cles help to achieve maximum specific particle density. While adjusting the liquid level can be useful in controlling density, the greater your liquid level, the higher the cost of removing ex- cess moisture from the final product.

Performance tests In a series of testsS of the high-speed mechanical agglomerator with a vertical shaft>polysaccharide, hydrated lime, and poly- mer powders were fed to the unit with water as the binder. Var- ious agglomerator parameters (shaft speed and liquid addition rate) and one system parameter (drying method) were varied to show how they affect the particle size distribution and other results. Test results (including size and moisture analyses of the final product) are listed in test data tables in Figures 5,6, and 7; the figures also include particle size distribution plots of the results.

For the polysaccharide powder (Figure 5), several tests were run. Increasing the liquid addition rate while keeping the shaft speed the same in run 3 increased the mean particle diameter and increased the number of particles in the desired size range (listed as product yield in the test data table). Increasing both the liquid addition rate and the shaft speed in run 5 further increased the mean particle diameter but reduced the number of particles in the desired size range and also consumed more power.

For the hydrated lime powder (Figure 6), two tests were run using the agglomerator and a fluid-bed (dynamic) dryer.7 Some product in run 1 was taken directly from the agglomerator and statically dried (without agitation in a fluid-bed dryer); the re- maining run 1 product was dried in the fluid-bed dryer. Fines were separated from the product during drying. For run 2, the liquid addition rate and shaft speed were increased and the product was taken directly from the agglomerator and statically dried. The results for run 2 show that the mean particle diameter increased but the particle distribution in the desired size range decreased. The curve for the fluid-bed-dried product in run 1

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Figure 5

Effects of operating parameter on polysaccharide powder

a. Test data

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Figure 7

Effects of operating parameters on polymer powder

a. Ted data * I 1 1

S I Z E ANALYSIS

Date: Mater ial : R3LY MER

Machi ne : TURBO FLEX

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shifts dramatically from the other two curves, indicating that the fines separation provided in fluid-bed drying has a signifi- cant effect on controlling the particle size distribution.

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For the polymer powder (Figure 7), three tests were run with the agglomerator and the fluid-bed dryer. In run 2 the solid addition rate was reduced and the liquid addition rate was increased, which reduced the product’s mean particle diameter and in- creased the product moisture (both after agglomeration and after fluid-bed drying). In run 3, the solid and liquid addition rates were similar to run 1 levels, but the run was extended. This greatly increased the mean particle diameter and the power consumption. Fines for all three runs were collected in one batch. When compared with the feed curve, the fines curve shows that the fines after agglomeration are still larger than the feed particles.

How to configure your agglomeration system An agglomerator typically requires auxiliary equipment, and this is especially true for the high-speed mechanical agglomera- tor. Figure 8 is a flowsheet that outlines the general processing steps before and after the agglomerator. The agglomerator is the heart of this system because it forms the agglomerates, yet it ac- counts for only about 30 to 40 percent of the total system cost. Generally, the design and operating parameters of the equip- ment after the agglomerator (such as dryers, screeners, and mills) will directly affect the final product’s characteristics.

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Figure 9 is a typical system configuration for the high-speed mechanical agglomerator. In this process, a liquid feeding sys- tem (at the left) meters a liquid binder for direct injection to the agglomerator. Solids flow from a surge bin (top) through a loss- in-weight feeder into the agglomerator. After agglomeration, the product flows to a fluid-bed dryer (center). Fines-laden air is exhausted from the dryer to a bag filter (right), which separates the fines from the air and pneumatically conveys them to the agglomerator for recycling. Here the fines are combined with the solids feed. Coarse product exits the dryer and is lifted by a bucket elevator to a screener, which separates oversize from on- spec product. The oversize passes to a mill for grinding to the desired size. On-spec product exits the screener.

Many other system configurations are possible to similarly conserve materials and provide flexible solid and liquid feed options. Work with your agglomerator manufacturer to deter- mine which configuration is best for your plant and product requirements. PBE

References 1. W.H. Engelleitner, “Selection of the proper agglomeration process,”

Proceedings of the 17th Biennial Conference of the Institute for Briquetting and Agglomeration, August 1981, pages 231-233.

2. M.E. Fayed and L. Otten, Chapter 7, Handbook of Powder Science and Technology, Van Nostrand Reinhold, 1984.

3. C.A. Holley, “Binders and binder systems for agglomeration,” Proceedings of the 17th Biennial Conference of the Institute f o r Briquetting and Agglomeration, August 1981, pages 164-166.

4. R.W. Weggel, “High-speed agglomeration systems using the Schugi blender granulator,” Proceedings of the 15th Biennial Conference of the Institute for Sriquening and Agglomeration, August 1977, page 176.

5. Tests performed at Hosokawa Bepex Corp., Minneapolis. For more information about the test setup and data, contact the author.

6. Schugi Flexomix, Hosokawa Bepex Corp., Minneapolis. Similar equipment is available from other manufacturers, including units with a horizontal mixing shaft. For manufacturer names, check the listings under “Mixing agglomerator” on page 46 of Powder and Bulk Engineering’s 1995-96 Reference & Buyer’s Resource, August 1995.

7. Fluid-bed dryer, Hosokawa Bepex Corp., Minneapolis.

Peter Koenig is product manager at Hosokawa Bepex Corp., 333 Tuft Street Northeast, Minneapolis, MN 55413; 612/627- 1412. He holds a BS in chemistryfrom St. John k University in Collegeville, Minn. This article is adapted from apaper the au- thor presented ut the 20th Biennial Conference of the Institute for Briquetting andAgglomeration in August 1987.