Build up agglomerate quality

Several easy-to-use indices related to material flow properties can help to predict and improve an agglomeration process and product quality.

By Kristin O’Quest and Lee Dudley, Diamondback Technology, Inc.

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From a material handling standpoint agglomeration generally refers to the process of making larger particles (agglomerates) from smaller particles. It is used for duties as diverse as the formation of carbon black and, in the extreme, the production of scrap metal bales. Agglomerates can take the form of sheets or briquettes from an extruder or roll press process or pills and caplets from pill press operations. Other forms include prills and pearls.

Many reasons for agglomerating materials exist. These include: increasing a material’s bulk density, decreasing dust concentration during feeding, reducing segregation of material (and thus product uniformity), improving the flow rate of material and making an end product such as a briquette or pill.
A common method for agglomerating materials involves applying pressure to a bulk solid using a roll press or extruder. When creating agglomerates by applying pressure, it is important that the internal strength of the material is sufficient to maintain the integrity of the agglomerate after the pressure is released past the press or extruder. This requires improving the adhesion of particles, which demands an understanding of the mechanisms of adhesion. For presses these mechanisms include the interlocking of particles, welding of particles under pressure and temperature, chemical reaction and the use of binders to increase agglomerate strength [1].

Interlocking is particularly evident in agglomeration of large odd-shaped particles such as metal turnings. On a smaller scale, round hollow particles of materials like those used in nutraceuticals can break and interlock under pressure. Agglomerates formed by this mechanism can have high internal strength; however the strength can degrade shortly after agglomeration depending upon the tendency of the particle to return or spring back into its original particle size and shape. Metal turnings show this tendency, which can be overcome in some cases by applying pressure at a higher temperature.
Adding heat to pressure application can cause welding of particles in agglomerates. This is often helpful for materials like rubber and polyethylene flake that have a tendency to spring back. Such welding can greatly increase the integrity of the resulting agglomerate.

Binders — such as molasses, starch, water, kaolin clay and oil — are frequently and effectively used to raise the internal strength of agglomerates. Drying, heat curing and chemical reactions that might occur with a binder such as cement can further increase the internal strength [1].

Potential problems
Unsatisfactory agglomeration can arise because of problems both upstream of the roll press or extruder and during the feeding of newly agglomerated materials. Always keep in mind that upstream problems can critically affect the quality and uniformity of the finished product. Material flow into the feed system must remain consistent and uniform to avoid problems such as breakage of briquettes or unacceptable concentrations of active ingredients in pills.

Segregation in the roll press or extruder feed hopper can degrade the strength and quality of agglomerates. An excess of large particles can reduce the internal strength of briquettes while too many fines can lead to inconsistent flow rates into the feed-screw system, breakage due to confined high-pressure gas (capping) and fluctuations in viscosity of extruded materials. Segregation can result from the addition of some powdered binders. If the angles of repose of the binder and the main material differ significantly, segregation in the feed bin can occur. Permeability differences between the binder and main ingredient, combined with feed conditions into the bin, can lead to air-entrainment segregation as less-permeable fine particles are air-swept to the sides of the bin. If the main ingredient and binder are free flowing but dramatically dissimilar in size, sifting segregation can arise as the smaller particles fall between the larger particles and enter the feed system first.

In some cases the addition of binders will change a material’s properties enough to impede flow into the feed-screw system of a roll press or extruder. Tests have determined that adding just 1% water to agglomerate dust may change the flow rate of a material by an order of magnitude. A feed containing a binder may become sticky and cohesive. This may cause hang-ups in a feed system that works well without the binder [2].

Air entrainment
Entrained air can pose problems in many aspects of the agglomeration process. For effective adhesion, particles must contact each other. Air trapped in material impedes particle-to-particle contact and prevents effective agglomeration. Under certain circumstances the air-entrained material will fluidize and flush through a system. Other situations prevent the flow of fine powder into the feed-screw system. Agglomerates with entrained air often break after compression.

Fine powders have the greatest porosity and ability to entrain air. These materials tend to have the least permeability and thus require ample time for air to escape the voids. Special consideration of these characteristics is necessary for effective press operation and agglomeration of fine particles.

Air entrainment begins as powder enters the hopper above the feed-screw system. Use of an anti-segregation letdown chute can decrease the distance the powder falls through the air and limit the amount of air entrained. Using a mass-flow hopper design (i.e., where all material is in motion as material is drained from the hopper) or preferably a first-in/first-out (FIFO) or plug-flow design will minimize segregation caused by differential flow patterns and increase the hopper usable capacity. This can allow the powder to sit in the hopper long enough to deaerate prior to entering the feed-screw system. The feed-screw system is integral in maintaining consistent flow to the roll press and should utilize both a rate-controlling horizontal screw and a vertical screw. A vent is required to release air pressure developed as the roll press compresses the powder [3].

During the compression of solids, air is squeezed from the porous regions and travels upward. This causes a counter-flow condition as the air pushes into the incoming low-permeability powder [4]. The counter flow of air produces a “body force” that creates a limiting flow rate. Entrapped high-pressure air in fluidized pockets can flush through the system as these pockets reach the hopper outlet [4]. This phenomenon causes havoc in hoppers and press systems and reduces the compacted bulk density.
As air-entrained powder falls into the nip region of a press operation the gaseous phase is compressed to high pressures. Once the briquette is past the press, this high pressure gas expands. If the compacted air pressure exceeds the cohesive strength of the agglomerate, the agglomerate will break apart in a phenomenon often called “capping” [4,5]. Frequently the broken agglomerates pass over a screen and the fines recycle through the system. If capping is not limited, over time the consistency of the agglomerates will change as increasing volumes of fines recycle through the operation.

Improving operations
Not surprisingly given the numerous possible problems associated with agglomeration, many processes fail to meet the strict standards of quality and uniformity that industry requires today. Fortunately, it is relatively easy to troubleshoot and predict agglomeration performance. Simple laboratory tests can determine the porosity, cohesive strength and frictional properties of materials. Then, the Johanson Indices, which provide insights on critical factors through several easy-to-use numbers, can aid in both the design of pressing operations and the formulation of binding agents [5,6].


Basic property

Symbol

Test method

Johanson Indices

Application

of indices

Effective angle of internal friction

d

Direct shear test

AI, RI

Determination  of active flow channel in funnel flow; used in developing other indices

Angle of sliding friction on hopper walls

f′

Shear test on hopper wall surface

HI, CI

Design of feed chutes and hoppers; determination if binding agent will stick to screw flights

Strength versus consolidation pressure

fc versus s1

Direct shear test

AI, RI

Prediction of arching and ratholing in the feed system; prediction of agglomerate strength

Flow rate of fine powders from hoppers

Q

Direct measurement

FRI

Determination of chute and hopper opening requirements to prevent starvation of the press or pulsating flow

 

 

Bulk density versus consolidation pressure

g versus s1

Weighing a known volume of solid. Consolidation of the volume should occur under a shearing state of stress.

AI, RI

Prediction of density of agglomerate with varying press settings

Angle of sliding friction on a briquette roll surface

f′R

Shear test on a roll surface or simulated roll surface

AI, RI, CI, HI

Determination of angle of nip

Compressibility factor

K

Plunger-type consolidation bench

FRI

Determination of limiting flow rate; prediction of air entrainment,  nip angle, roll separating force and roll power requirements

Strength of agglomerates made by plunger press

S

Direct shear test with a special clamp for the sample

AI, RI

Determination of required briquetting pressure for various temperatures, moisture contents and binders

Spring back

 

Volume % recovery of consolidated sample upon release of pressure

SBI

Prediction of required critical hopper outlet and wall angle

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The table relates the basic bulk-solids material flow properties to Johanson Indices and to critical design parameters in roll press operations. These properties have an impact on the design of upstream sections including chutes, conveyors, surge hoppers, the press feed hopper and the roll press, as well as downstream material handling. Dr. Johanson has created a mathematical model for the relationships between material properties and roll-press dimensions and operating conditions [7].

The solids flow rate index (FRI) is useful in many aspects of agglomeration process design and troubleshooting. This index takes into account both the compressibility and air permeability of solids, the most important factors for press performance [5]. The FRI is the limiting rate from a conical hopper outlet when the solids have deaerated [2]. It allows companies that produce solids that others press to check the quality of product by comparing the values over time. For roll presses there is essentially a linear correlation between FRI and the roll speed for a given press feed correlation. As the FRI decreases the rate for a gravity-feed roll press system declines proportionately, so the index can help determine if a screw-feed system would be better. Tableting presses and other systems where the feed to a die surface is the limiting factor can use the FRI as it correlates well with die-fill limiting rates. The tablet capping phenomenon also correlates with the FRI because capping results from entrained air [5]. Segregation mechanisms and tendencies can be predicted by deliberately segregating a small pile of product and running FRI tests on different sections [2].

The arching index (AI) and rathole index (RI) measure the cohesive strength and help in determining critical factors of the press feed system. The RI can predict whether a scraper is needed in a single-screw feed system while the AI is used to size the inlet, outlet and screw diameter [2]. The two indices can assess binder effects on a material and process. Running AI and RI tests on different types and quantities of binders will quantitatively predict the maximum allowable binder content before the feed system hangs up [2].

The hopper index (HI) is a function of the coefficient of friction between the bulk solid and the hopper wall and can provide a guideline for determining if a centralized flow channel will develop in a press feed hopper or if flow at the hopper walls will occur [2]. The chute index (CI) is a measurement of the amount of adhesion a material will have on the walls and screw flights of a system. A CI of 80 or greater generally indicates the material will likely stick to screw flights and decrease the feeder capacity over time [2].

The spring back index (SBI) indicates how much elastic spring back a material has.

Kristin O’Quest is a consulting engineer and laboratory manager of Diamondback Technology, Inc., Atascadero, Calif., a firm that specializes in equipment and consulting related to solids handling.  E-mail her at koquest@diamondbacktechnology.com. Lee Dudley is president of Diamondback Technology, Inc. E-mail him at ldudley@diamondbacktechnology.com.

References
1. Johanson, J.R., “Feeding roll presses for stable operations,” Bulk Solids Handling, Vol. 4, No. 2, p. 43 (June 1984).
2. Johanson, J.R., “Roll press feed systems,” Powder Handling & Processing, Vol. 8, No. 2, p. 159 (April/June 1996).
3. Johanson, J.R., “Reducing air entrainment problems in your roll press,” Powder and Bulk Engineering, p. 43 (Feb. 1989).
4. Johanson, J.R., “Predicting limiting roll speeds for briquetting presses,” Proceedings, 13th Biennial Conf. of the Inst. of Briquetting and Agglomeration, Vol. 13, p. 88 (Aug. 1973).
5. Johanson, J.R., “New solids flow property indices for predicting roll press or extrusion press performance,” Proceedings, 22nd Biennial Conf. of the Inst. of Briquetting and Agglomeration, Vol. 22 (1991).
6. Johanson, J.R., “Flow indices in the prediction of powder behavior,” Pharma. Mfg. Intl., p. 159 (1995).
7. Johanson, J.R., “A rolling theory for granular solids,” A.S.M.E. J. of Applied Mechanics. Vol. 30, No. 4, Series E, p. 842 (Dec. 1965).
8. Johanson, J.R., “The use of laboratory tests in the design and operation of briquetting presses,” Proceedings, 11th Biennial Conf. of the Inst. of Briquetting and Agglomeration, Vol. 11, p. 135 (Sept. 1969).

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