Wet Phosphate Rock Receiving Hopper
This is an example of a series of wet phosphate rock receiving bins that are suffering from chronic bridging and rat-holing. It is a good example of poor bin geometry combined with uneven discharge that is resulting in inconsistent discharge.
The Existing Hopper and Why it is Suffering from Chronic Plugging and Inconsistent Discharge
The existing bins consists of a pyramid hopper stacked on top of a plane flow hopper that is stacked on top of a second small pyramid hopper. Wet phosphate rock is withdrawn from the hopper via a belt feeder, which is preferentially withdrawing material from the rear of the bin outlet.
The combination of the small openings and pyramid shape at the discharge combined with the behaviour of the belt feeder is resulting in a funnel flow or first-in, last-out discharge. 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 alternative to funnel flow is mass flow, which is a first-in, first-out discharge pattern. The definition of mass flow is that during discharge, the entire mass of stored material comes down as a single body (single mass). To achieve this, material must discharge evenly from the entire discharge outlet of the storage bin. The tell-tale sign that you have mass flow is that material is sliding down the bin walls.
The solution to fixing this problem storage and feed system has two parts. The first half of the solution is to replace the lower pyramid hopper with a plane flow hopper with a wide and long opening. The wide and long discharge opening is required to overcome the bridging dimension of wet phosphate rock. Overcoming the bridging dimension is necessary to ensure that the material cannot bridge over the feeder. Changing the bin geometry will promote a mass flow discharge. Mass flow is a first-in, first-out discharge pattern.
The second half of the solution is to pair the new plane flow hoppers with a fully-effective feeder – that is a feeder that withdraws material evenly from its entire infeed opening. Again, 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.
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.
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.