Greg Mehos, president | Greg Mehos & Associates
I frequently work on powder and bulk solids projects where the objective is to prevent the material from agglomerating or caking, which is what happens when powders become cohesive due to being stored at rest for a period of time. Caking (unwanted agglomeration) can lead to misperceptions of material quality, customer dissatisfaction, and loss of sales. While this may sound counterintuitive, I often recommend intentional agglomeration via particle size expansion to prevent unintentional agglomeration.
Why agglomeration happens
First, it’s important to understand why unwanted agglomeration takes place. Caking results when the magnitude of interparticle adhesive forces — the tendency for particles to bond with each other — increases over time. The adhesive forces between particles are primarily polar interactions, van der Waals forces, and forces associated with liquid bridges (when moisture is present).
Polar interactions (when a polar molecule’s partially negative section and a polar molecule’s partially positive section are attracted to each other) and van der Waals forces (the interparticle adhesive forces arising from the polar molecules’ dipole moments) are both proportional to particle size. This means that as particle size increases, so does the strength of the polar interactions and van der Waals forces (the interparticle adhesive forces). The likelihood of caking, however, generally decreases when particle size increases due to the decrease in the powder’s overall cohesive strength. This is why particle size enlargement is often used to reduce the likelihood of unwanted powder agglomeration.
How particle size affects agglomeration
To understand the effect of particle size on powder agglomeration, consider the experiment illustrated in Figure 1. This figure shows a tensile strength (the maximum stress that a material can withstand before breaking apart) experiment in which a compact of powder has a platen area equal to A and is comprised of particles having a diameter of d.1 The tensile strength σT is the force required to cause the compact to fail and is divided by the platen area.
Assume that this tensile strength force is equal to the sum of the adhesive forces at the contact point between each individual particle, each point equal to FH, which is proportional to the particle diameter d, and let the number of contacts equal n, as shown in the following equation.
The number of particle contacts is proportional to the platen area and inversely proportional to the square of the particle diameter.
It then follows that
Although interparticle adhesive forces increase with particle diameter, the powder’s cohesive strength decreases with increasing particle size. This is why size enlargement is often used to reduce the likelihood of unwanted agglomeration of powders.
The technologies available for particle size enlargement include tumble-growth agglomeration, pressure agglomeration, and heating or sintering agglomeration.2 Tumble-growth agglomeration devices include pin mixers, pan pelletizers, and rotary drums. Pressure agglomeration devices include roller compactors and die compactors. Sintering devices include auger extruders, screw extruders, and ram extruders.
Why intentional agglomeration works
But how do we know the effectiveness of agglomeration in reducing caking? One method to quantify caking is to measure the powder’s cohesive strength with a shear cell.3 Shear cell testing is described by ASTM-D-61284 and ASTM-D-6773.5 Conducted properly, the tests allow a bulk material’s cohesive strength to be determined as a function of consolidation stress and time. If the tests reveal that the material’s cohesive strength increases when the material is consolidated at rest for a period of time, the material is likely to cake during storage. Knowing this information, we can then take the necessary steps to enlarge the particle size so that the particles’ cohesive strength is reduced and unwanted agglomeration won’t happen.
As an example of the effect particle size has on the material’s cohesive strength, Figure 2 shows results from strength tests performed on fertilizer powder samples comprised of 10-micron (μm) fertilizer particles that have been agglomerated to a particle size of approximately 500 microns. The fine powder was found to be very cohesive, and its strength increased dramatically after only 8 hours when under consolidation. By increasing the fertilizer’s median particle size from 10 microns to 500 microns, the fertilizer’s cohesive strength decreased, and the fertilizer became noncohesive and remained noncohesive when stored at rest.
So, how much caking is acceptable? Consider the following example. The stress on impact when a bag of bulk material is dropped from a height of 1 meter is approximately 10 kilopascal (kPa). From a study on pinch strengths,6 the stresses imparted between the thumb and index finger of an adult male and adult female are typically approximately 400 kilopascal and 250 kilopascal, respectively. (The study was obviously not performed in my household!) From personal experience, a hammer imparts approximately 2,500 kilopascal of stress onto a thumb. The test results reflected in Figure 2 suggest that the 500-micron-sized material is unlikely to cake and pose a problem because its cohesive strength is low, whereas the fine 10-micron-sized material will cake due to the material’s high cohesive strength and lead to customer complaints.
So, if you’re tackling a caking problem with your fine powder, as odd as this may sound, consider intentional agglomeration as a means to prevent unwanted agglomeration. Increasing particle size will help decrease the particles’ cohesive strength, leading to less caking and more satisfied customers.
- D. Schulze, Powders and Bulk Solids – Behavior, Characterization, Storage and Flow, 1st Edition, Springer, 2008.
- G. Mehos, “Agglomeration advisor: Choosing agglomeration technologies,” Powder and Bulk Engineering, January 2020, pages 34-37.
- G. Mehos, “Using solids flow property testing to design mass- and funnel-flow hoppers,” Powder and Bulk Engineering, February 2020, pages 32-39.
- ASTM-D-6128-Standard Test Method for Shear Testing of Bulk Solids Using the Jenike Shear Cell, ASTM International, 2006, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959; 610-832-9500 (www.astm.org).
- ASTM-D-6773-Standard Shear Test Method for Bulk Solids Using the Schulze Ring Shear Tester, ASTM International, 2008, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959; 610-832-9500 (www.astm.org).
- Y.M. Choi, “Comparison of grip and pinch strength in adults with dexterity limitations to normative values,” Procedia Manufacturing, Vol. 3, pages 5326-5333.
For further reading
Greg Mehos, PE (978-799-7311) is a chemical engineering consultant who specializes in bulk solids handling, storage, and processing and is an adjunct professor at the University of Rhode Island. He received his BS and PhD in chemical engineering from the University of Colorado and his masters from the University of Delaware. He’s a Fellow of the American Institute of Chemical Engineers.
Greg Mehos & Associates • Westford, MA
978-799-7311 • www.mehos.net
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