54 Powder and Bulk Engineering, February 1998

Beyond dedusting: Agglomeration applications for pin mixers Chris M. Kozicki and Tom A. Greenwood FBECO International

New uses for familiar equipment are found every day in bulk solids processing plants, and this is par- ticularly true with agglomerators. This article looks at evolving applications for one agglomerator - the pin mixer.

common type of growth-tumble agglomerator is the continuous pin mixer, which provides wet A granulation. The pin mixer was originally devel-

oped in the 1940s to process pigment blacks for the car- bon black industry. The unit processed the material with a liquid binder to form dustless, free-flowing pellets with 10 times the material’s original bulk density.

Since then, the pin mixer has commonly been used to process coal dust collected in baghouses. The dust is an unavoidable by-product in coal preparation plants and is also generated in coal-fired power plants. When suffi- ciently aerated, the coal dust can explode and create a fire- ball with enough pressure to destroy a plant. To minimize the explosion risk, many coal-fired power plants install pin mixers beneath their baghouses to process the dust with a binder, as shown in Figure 1. The pin mixers take the nui- sance dust and convert it into very small coal pellets that not only reduce the explosion threat but can be recycled as fuel to the power generation system.

The pin mixer’s agglomeration benefits have encouraged equipment manufacturers to perform extensive testing and pilot-plant experiments to find more ways to apply the unit. The pin mixer’s benefits in agglomeration applica- tions include:

H Intensely mixing a material with a binder (such as water, a lignin, or an acid), thereby reducing binder usage and cost.

H Dedusting a fine powder and agglomerating fines.

H Improving material flow characteristics.

H Increasing processing throughputs in downstream equipment.

Before discussing additional applications, it’s helpful to look at how the pin mixer works.

Pin mixer operation As shown in Figure 2, the pin mixer has a stationary, cylin- drical horizontal housing with a length-to-diameter ratio of at least 2-to-1 but no more than 5-to-l . The typical hous- ing diameter ranges from 6 to 50 inches. A rotor sus- pended by standard roller bearings extends through the housing’s full length. The rotor can be driven by a con- stant- or variable-speed motor. Cylindrical pins (or rods) are welded or bolted to the rotor in a helix pattern. The bolted pins can be adjusted to modify the clearance be- tween their tips and the housing’s inner surface.

A liquid injection pump is linked to one (or more) injec- tion port in the housing’s top half. A material inlet is lo- cated in the housing top and positioned below a horizontal motor-driven screw feeder or rotary valve. The mixer’s outlet is at the housing bottom. The pin mixer can be fitted with a cooling jacket to protect a heat-sensitive material during processing.

In operation, a fine powder is continuously fed into the inlet. The liquid injection pump meters a binder through the injection port (or ports) at the required feedrate. The motor turns the shaft at several hundred revolutions per minute. The shaft rotation speed depends on the housing size: A small-diameter unit’s shaft can rotate at up to 1,800 rpm, and in a large-diameter unit, the shaft can rotate at up to 500 rpm.

Powder and Bulk Engineering, February 1998 55

The shaft rotation causes the pin tips to rotate at high speed, which creates turbulent agitation that provides inti- mate contact between the material and binder and helps to minimize empty air pockets inside the mixer. Once the material is mixed with the binder, the rotating pins, turbu- lent airflow, and particle-to-particle contact begin to ag- glomerate the material.

This combination of forces forms nuclei granules that continue to grow larger and consolidate with other gran- ules, separate particles, and binder mist. The growing granules result in very small, slightly irregular pellets with greater density than the feed powder. The pellets are work-hardened by the mixer’s mechanical action: rotating pins, pellet-to-pellet contact, and the material’s rolling action against the inner housing surface.

Evolving agglomeration applications for the pin mixer in- clude general agglomerating, preconditioning, and spe- cialized processing for various industries.

General agglomerating The pin mixer is now used for agglomerating many mate- rials other than carbon black or coal dust. In the new appli-

cations, the pin mixer forms the material into pellets to meet various size requirements or to increase the mate- rial’s bulk density (called densifying). Some materials ag- glomerated in the pin mixer are:

H Mineral mining wastes, as shown in Figure 3 .

H Water-dispersible, granular herbicide-pesticide mixes.

H Food-based instantized products, such as drink and food mixes.

H Granular feed, such as vitamins, for a pharmaceutical tablet press.

H Biochemical materials.

Preconditioning The pin mixer can precondition material before it dis- charges to a disc pelletizer by combining fines (as shown on the left in Figure 4) and a liquid binder, producing very small, slightly irregular pellets (as shown on the right in Figure 4). Feeding this agglomerated material to the disc

Powder and Bulk Engineering, February 1998 57

pelletizer allows the pelletizer to operate at a higher capac- ity because the material has already started to form very small pellets.

During preconditioning the pin mixer’s intense agitation also finely disperses the binder, which allows less binder to be used in the pin mixer or in both the mixer and the disc pelletizer. The preconditioning also enables the disc pel- letizer to produce more on-size final pellets.

A pin mixer can precondition limestone and gypsum fines and fly ash.

Limestone andgypsumfines. Using the pin mixer to pre- condition limestone and gypsum fines in fertilizer plants produces small pellets that can be fed to a disc pelletizer (Figure 4). Again, this decreases the amount of binder re- quired in either unit.

It also increases the disc pelletizer’s capacity. Without pre- conditioning by the pin mixer, the disc pelletizer’s on-size limestone and gypsum pellet yield is between 70 and 80

percent. With preconditioning, the yield can be as high as 90 to 95 percent. The typical nominal size range for the final pellets is 6 by 16 mesh.

58 Powder and Bulk Engineering, February 1998

Fly ash. In coal-fired power plants, fly ash is a common by-product produced when coal is burned in cogeneration boilers. The fly ash can be used as a cement additive or construction fill material or can be pelletized to produce a lightweight synthetic aggregate.

A pin mixer and disc pelletizer can be used together to ag- glomerate the ash with a binder and form round, nominally %-inch-diameter pellets. The fly ash can be mixed first with a chemical, which creates a prereaction with the fly ash. After mixing with a binder and forming very small pellets in the pin mixer, the fly ash is discharged to the disc pelletizer, which forms the final %inch pellets. The pin mixer’s intense agitation allows the fly ash and binder to react uniformly, reducing the pellets’ tendency to fracture during subsequent handling and curing.

ra 5 Waste handling. Hazardous waste oil, such as from a car-

dispose of. A pin mixer can encapsulate the oil with ce- ment kiln dust, conditioning the oil so completely that it no longer requires special handling and can be landfilled with

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Acknowledgment The authors thank Bill (W.H.) Engelleitner, AME Pitts- burgh, for his guidance and the use of his extensive per- sonal library during this paper’s development.

7 i

James A. Frye, W.C. Newton, and W.H. Engelleitner, “The pin mixer - a novel agglomerating device,” Proceedings of the Institute for

3 (D

3 ra 2. Briquetting and Agglomeration, Fourteenth Biennial Conference, 1975.

K.B. Albert and D. Langford, “Pelletizing limestone fines,” Pro- ceedings of the Institute for Briquetting and Agglomeration, Twenty- third Biennial Conference, 1993.

George Wasson, “Coal dust reduction - an evaluation of chemical additives and application methods,” Proceedings of the Institute for Briquetting and Agglomeration, Twentieth Biennial Conference, 1987.

J.B. Schwartz et al., “A high speed mixer for continuous wet gran- ulation via a new high speed mixer: The pin mixer,” Proceedings of the Institute for Briquetting and Agglomeration, Twenty-first Biennial Conference, 1989.

E.S. Ghali et al., “A high speed mixer for continuous wet granulation,” Pharmaceutical Technology, April 1990.

Personal communication with authors, W.H. Engelleitner, AME Pittsburgh, 1997.

Mixers marketing literature, FEECO International, Green Bay, Wis., 1992.

Dustmaler marketing literature, Benetech, Aurora, Ill., 1993. Specialized processing

The pin mixer can process materials to enhance their han- dling properties and value in several applications, includ- ing food and waste handling.

Food. The pin mixer can compact and densify drink pow- ders before they’re packaged. Various techniques can be used in the pin mixer to affect residence time and product temperature and thus the degree of densification. The den- sifying increases the powders’ surface area, which inten- sifies their flavor.

Chris M. Kozicki is regional sales engineer at FEECO In- ternational, 3913 Algoma Road, Green Bay, WI 54311; 920/469-5100 cfax 920/469-5110). Tom A. Greenwood is lead designer for the company. This article is adapted ji-om apaper the authors presented at the Institute for Bri- quetting and Agglomeration Twenty-@h Biennial Confer- ence in Charleston, S.C., in November 1997.