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.
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 MagazineSister Web Sites |
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