Wet FGD Gypsum Front-Loaded Metering Bin
This case study is an example of a front-loaded metering bin, operating outside in an artic environment, delivering an even, metered discharge of wet FGD gypsum into a continuous process. This installation when into service in 2012.
Prior to selecting a Kamengo Feeder, the client had previous experience metering FGD gypsum with a screw feeder. The previous bin suffered from heavy material compaction at the screw outlet resulting in chronic plugging. The screw would turn, but no material would come out. For a new plant that is mixing gypsum with tailings, the client went looking for a new solution and chose a Kamengo Feeder.
Why Conventional Storage and Feed Solutions Often Suffer from Chronic Plugging and Inconsistent Discharge
The existing screw feeder hopper suffered from chronic plugging for two reasons:
- The conveying action of the screw feeder, which is shearing material from the storage bin, was compacting the gypsum against the front of the bin wall. As the stored material compacts, it gains significant shear strength. With enough compaction, the stored material would gain sufficient shear strength to bridge over the hopper discharge opening.
- The screw feeder had a propensity to withdraw material from the rear of the hopper. This selective withdrawal of material, induces 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 wet FGD gypsum is quite large – in fact, it is larger than the metering bin. To handle wet FGD gypsum 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 a reliable front-loaded metering bin handling wet FGD gypsum has two parts:
The first half of the solution is to choose a correct bin geometry for handling wet FGD gypsum. In this case, Kamengo chose a plane flow hopper shape with steep sloping walls lined with a low friction liner, and 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 wide and long 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. To determine the minimum geometry, Kamengo conducted material flow characterization testing, which includes a series of bench scale tests, which are used to determine the minimum wall angle and the minimum bin discharge outlet width and length.
The second half of the solution is to pair the plane flow hopper with a fully-effective Feeder – in this case a Kamengo Feeder. A fully-effective feeder is one that withdraws material evenly from its entire opening, which by definition is necessary to actually achieve a mass flow discharge pattern in the 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.
Again, discharging in mass flow is often necessary when handling a difficult flowing bulk solid. What makes the Kamengo Feeder unique is that it can be made as wide as needed and as long as desired. The minimum bin discharge outlet width and length to reliably handle FGD Gypsum is approximately 3.5-feet by 12-feet. To achieve mass flow, the Feeder inlet must match this outlet. This is very difficult to do with conventional technologies, but very easy to achieve with a Kamengo Feeder.
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.