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Soda Ash Tall Metering Bin

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.

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Kamengo’s Solution

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.

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To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.

Fine Ore Pile Reclaim (Copper Mine)

Fine Ore Pile Reclaim (Copper Mine)

This is an example of a large fine ore storage reclaim that is suffering from chronic bridging.

Why the Existing Reclaim System is Suffering From Chronic Bridging and Inconsistent Discharge

The existing storage and feed arrangement consists of a storage building that has a flat floor and is being reclaimed from below through a series of long slots. Material is metered by a belt feeder below each slot. This case study is an example of making funnel flow work when handling a fine material.

 

In summary, because the storage facility has no sloping walls, but is instead reclaiming the pile through a series of long slots in the floor, the pile is expected to empty in a funnel flow or first-in, last-out discharge pattern. Funnel flow can certainly be made to work when the “effective” discharge outlet is wide and long. 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.

 

In the case of this facility, below each slot is a tapered chute and a belt feeder. The purpose the tapered chute is to try to even out the discharge by the belt feeder, because it has a propensity to withdraw material selectively from the rear of the slot. However, tapered chutes are only effective over short distances, and the slot is over 44-ft long. In summary, the tapered chute is having little effect, as a result the belt feeder is primarily pulling material from only a short section of the slot. As a result, despite the fact that the slot in the floor is long, effective opening through which material is being discharged (from the perspective of gravity) is small. And when the opening is small, rat-holing can be expected.

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Kamengo’s Solution

The solution to fixing this problem is to replace the chute and belt feeder with a fully-effective feeder. A fully-effective feeder is one where the feeder withdraws material evenly from its entire infeed opening. With a fully effective feeder, the full length and width of the slot is “live”, and with a large effective discharge area, the pile can reliably be discharged in a funnel flow discharge pattern.

 

A great example of a fully-effective feeder is the Kamengo Feeder. In addition to being fully-effective, the Feeder offers consistent metering, and can be made as wide as needed and as long as wanted. As a result, the Kamengo Feeder offers valuable advantages when designing for a difficult flowing material.

Learn More

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 Mining Industry.