Gypsum and Shale Rock Front-Loaded Bin
This case study is an example of a retrofit of a front-loaded metering bin handling rock gypsum and limestone shale rock. This installation when into service in 2018.
The cement plant uses front end-loaders to unload barges. The plant has a front-loaded metering bin at the wharf that discharges onto a conveyor belt that carries material to either the plant’s covered storage or to silos feeding the plant’s kiln. Prior to the retrofit, the front-loaded metering bin consisted of a pyramid hopper that was discharged via a small 1-foot by 3-foot opening straight onto the conveyor belt below. The discharge opening had a hydraulic flap that was tasked with metering material onto the conveyor belt. Unfortunately, the front-loaded bin never worked. If the plant put any head of material into the bin, it would plug. To compensate, the front-end loaders would spoon feeder material into the hopper, reducing the pyramid hopper to a chute.
Why the Pyramid Hopper and Belt Feeder Were Suffering from Chronic Plugging and Inconsistent Discharge
The existing pyramid hopper was suffering from chronic plugging for three reasons:
- Poor bin geometry. The sloping walls and discharge opening of the existing hopper above the belt conveyor were insufficient to produce a reliable mass flow or first-in, first-out discharge pattern. First, the sloping walls were too shallow. As a result, material could not slide along the hopper walls, but would instead try to slip within itself in a funnel flow, or first-in, last-out discharge pattern. Second, the discharge opening was to narrow. The narrower the opening, the less strength a material requires to form a stable arch that gravity cannot reliably break.
- The conveying action of the belt conveyor below the pyramid hopper, which is shearing material from the storage bin, was compacting the gypsum and limestone against the front of the bin wall. As the stored materials compacted, they would gains significant shear strength. With enough compaction, the stored material would gain sufficient shear strength to bridge over the hopper discharge opening.
- The belt conveyor had a propensity to withdraw material from the rear of the pyramid hopper. This selective withdrawal of material, in addition to the shallow sloping walls, induced a funnel flow, or first-in, last-out discharge pattern in the bin. In principle, there is nothing wrong with funnel flow as long as the effective opening of the bin exceeds the bulk solid’s piping dimension (or distance over which the bulk solid can form a stable rat-hole). Unfortunately, the piping dimension for gypsum and limestone is quite large – in fact, it is larger than the metering bin. To handle gypsum and limestone reliably without hang-ups it must be discharged in mass flow, or a first-in, first-out discharge pattern. To do so, the Feeder must withdraw material evenly from the hopper’s full discharge outlet.
The solution to fix this front-loaded metering bin was to replace the storage hopper with a hopper with plane flow shape, steeper hopper walls and a wide and long opening. Unlike a pyramid hopper, which converges into two planes, a plane flow hopper only converges in one plane at a time. A plane flow bin shape is the most conversative bin shape and is often necessary when designing for a difficult flowing material. Further, the steeper hopper walls were required to promote a mass flow discharge, where material slides along the hopper walls. The angle for the sloping walls were determined via material flow testing. Finally, the wide and long discharge opening is required to ensure the stored materials cannot bridge over the Feeder. By correcting the geometry of the storage bin, if one were to remove the feeder, the hopper would now self-empty with gravity in a mass flow or first-in, first-out discharge pattern.
The second half of the solution is to ensure the entire hopper and feeder arrangement will discharge in a mass flow or first-in, first-out discharge. To do so, Kamengo paired the new hopper with a Kamengo Feeder. The value and necessity of the Kamengo Feeder is that it withdraws material evenly from the entire discharge opening of the storage hopper. By definition, to achieve mass flow, the bulk solid must descend the storage bin as a single body with all the stored material in motion, and the only way to achieve this is for the feeder to withdraw material evenly from its entire opening. If the Feeder withdraws material selectively from the bin discharge outlet, sections of material in the bin will be stagnant and funnel flow will ensue.
To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Cement Industry.