Soda Ash Tall Metering Bin
This is an example of a tall 2,500 cu-ft (70 cu-m) storage bin designed specifically to handle soda ash.
Why Soda Ash is a Difficult Flowing Material and Why Conventional Conical Storage Bins are Inappropriate for the Material.
Soda ash’s granular nature gives it the look and feel of an easy flowing material. However, wall friction testing of the material by Kamengo reveals that a poor choice in bin shape, liner and sloping wall angles will result in: 1) inconsistent discharge; 2) flooding of conveyors; 3) hang-ups; and/or 4) chronic caking and lumping. This is because if any of the above are incorrect, a funnel flow discharge pattern in the storage bin would be induced.
Funnel flow is a first-in, last-out flow pattern where material sluffs from the top down through a core in the storage bin. The challenge with funnel flow is that the majority of material in the storage bin remains stagnant during discharge. The problem is that stagnant material is permitted the opportunity to gain strength as it compacts under its own weight, which promotes caking. Further, as the material gains strength, it is able to bridge over wider openings, eventually leading to stable rat-holes and bridging. The alternative to funnel flow is mass flow. In contrast to funnel flow, mass flow is a first-in, first-out discharge pattern where all of the material in the storage bin is in motion during discharge.
For a Soda Ash silo, which typically has a relatively large storage volume for the given discharge rate, mass flow is preferred for several reasons. To reduce caking and prevent the stored soda ash from developing the strength needed to form a stable rat-hole, it is preferable for 100% of the stored material to move downwards over the course of a 24 hour period, and not just a small section of stored soda ash that lies within a core over the bin opening. Second, because it can take up to a month to empty a soda ash bin and it may never actually be permitted to fully empty. If the bin is emptying in a funnel flow discharge pattern, sections of soda ash will never leave the bin and simply be permitted to harden and become un-flowable. Third, soda ash is a relatively fine material, and if a stable rat-hole forms and collapses, the material would fluidize and mix with air, causing it to rush from the bin, which would introduce an engulfing hazard around the bin.
Standard soda ash bins are conical shape and discharge from a small opening. Typically, the angle of the cone is too shallow to produce a mass flow discharge pattern, and hence these bins discharge in funnel flow.
The solution to a reliable soda ash bin that provides a very controlled discharge has two parts.
The first half of the solution is to discard the standard cone and replace it with plane flow hoppers with lined and sufficiently steep hopper walls needed to produce a mass flow discharge pattern. 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 fully-effective feeder, which withdraws material evenly from its entire opening. A fully effective feeder is, by definition, necessary to actually achieve a mass flow discharge pattern in the hopper, which is necessary when handling a difficult flowing fibrous bulk solid.
Kamengo’s solution employs a Kamengo Feeder with a wide and long 3-foot by 10-foot opening. Of critical importance, the Kamengo Feeder withdraws material evenly across both its entire length and width. The result is that the stored material is withdrawn evenly from the full discharge outlet of the soda ash bin. An even withdrawal of material is absolutely required to achieve a mass flow or first-in, first-out discharge. The Feeder delivers batches of soda ash to a screw conveyor below, which in turn, provides a final metering into the process.
To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.