Retrofit of a front-loaded metering bin handling rock gypsum and limestone shale rock at a cement plant.
This case study is an example of a retrofit of a front-loaded metering bin handling recycled gypsum – a very sticky, cohesive material. The retrofit was completed in 1995.
Prior to the retrofit, the front-loaded bin was discharged via three screw augers. The augers suffered from chronic plugging and the gypsum plant was keen to find a solution. The challenge with this application is that not only did the plant need to find a feeder that could reliably handle the recycled gypsum but that could also deliver a very consistent discharge.
The plant receives recycled gypsum for free but is limited in the amount of recycled gypsum that it can add to its wall-board because of the recycled gypsum’s paper content. Hence, to maximize the recycled gypsum content without exceeding the prescribed limit, the front-loaded bin needs to deliver a very accurate and even discharge. What makes the application further challenging is that the recycled gypsum is stored outside in the rain, making it particularly sticky.
The augers struggled handling the recycled gypsum for two reasons:
The solution to fixing this problem storage and feed system was quite simple: Kamengo raised the storage bin and slipped in a Kamengo Feeder between the bin outlet and screw feeders.
First, Kamengo was able to re-use the existing front-loaded bin because the geometry of the bin, including width and length of the discharge opening, and angle of the bin sloping walls were sufficient to deliver a mass flow or first-in, first-out discharge pattern. Good bin geometry is essential to reliable discharge. The measure of a good bin is that, if the feeder were removed, it should completely self-empty with only the aid of gravity.
By inserting the Kamengo Feeder between the screw feeders and the front-loaded bin, the screw augers were no longer acting as feeders but instead only operated as conveyors. This is because the head of material in the front-loaded storage bin was placed on the Kamengo Feeder, which in-turn delivered a metered discharge onto the augers such that they were never more than one-third full. With no head of material on the screw augers, they are plenty reliable acting as conveyors.
Adding a Kamengo Feeder was necessary to achieve a mass flow discharge in the front-loaded bin. By definition, to achieve mass flow, where the stored material comes down as a single body, the feeder must 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 Gypsum Industry.
This is an example of a ship unloading hopper designed specifically to handle difficult flowing cohesive materials such as gypsum and bauxite.
Standard ship unloading hoppers use a pyramid hopper that converge to a small opening that use a slide gate or clamshell to control discharge.
In summary, a pyramid hopper with a small opening is a poor choice of bin shape for handling difficult flowing cohesive materials such as gypsum or bauxite. First, the bin shape promotes a funnel flow discharge pattern. Funnel flow (which is a first-in, last-out discharge pattern) can be made to work with large bins with large discharge outlets. However, when the discharge outlet is small, gravity is insufficient to overcome the strength of the bulk solid at the discharge outlet, and hence chronic bridging and rat-holing is expected.
The solution to a reliable ship unloading hopper that is capable of handling difficult flowing cohesive materials has two parts. The first half of the solution is to use a plane flow hopper shape with a wide and long discharge opening. A plane flow hopper only converges in one plane at a time, and is vertical in the opposite plane. The plane flow hopper is the most conservative hopper shape. The purpose of using a conservative bin shape with a long and wide discharge outlet is to employ a geometry where if the Feeder were removed, the entire bin would self-empty with gravity in a mass flow or first-in, first-out discharge pattern.
The second half of the solution is to pair the plane flow hopper with a Kamengo Feeder. The value of the Kamengo Feeder is that it withdraws material evenly from its entire opening, which by definition is necessary to actually achieve a mass flow discharge pattern in the hopper, which is necessary when handling a difficult flowing, cohesive bulk solid.
To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.
