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How Can We Prevent Ratholing?

Q: Working in a powder plant an investigation led us to define the problem of coreflow/ratholing causing weight fluctuations and segregation problems during emptying of the bin/silo. Designing a new bin we think about a conical bin with steep angles and short volume to try to avoid the coreflow. The powder characteristics are several small size particles and sticky characteristics so we fear polishing walls of the bin will not be effective in time. Placing a conveyor system for bin outlet will cause dramatic changes. A few hints for perfect bin angles and design would be appreciated.

A: "Coreflow" or "funnel flow" is caused by the material in the center of the vessel flowing at a faster velocity than material near the walls of the vessel.  This is a result of the higher vertical pressure in the center.  In some cases, the material at the wall does not move at all until the material level is down in the hopper (converging section of the vessel).  Since the material is described as having "--small size particles and sticky--" characteristics, it will likely form a rathole if it does not flow at the walls of the hopper.  It may also arch at the outlet if it is too small or improperly interfaced with the takeaway conveyor system. Click here to see an illustration of a classic funnel flow >> http://www.chemicalprocessing.com/Media/MediaManager/Classical_Funnel_Flow.pdf.
 
The solution is to have flow at the walls, a sufficiently sized outlet to prevent arching and a proper feeder transition.  It is prudent to measure the bulk solids flow properties prior to designing a successful hopper.  The measurements need to include the three groups of properties, viz, voidage (bulk density, density function, permeability), cohesive strength (unconfined yield strength, flow function), frictional properties (internal friction, wall friction).  These properties all should be measured at the process conditions of temperature, moisture level and consolidation pressure/time.  The results will be used to ensure that the critical wall angles, critical hopper outlet size and critical rathole dimensions are achieved in the design.  The results will also point to other potential problems including, limited flow rate, segregation, degradation, etc.  Segregation was mentioned and should be identified in a separate test.  Six types of segregation can occur either individually or in combination.  The design can compensate for segregation.
 
The perfect bin angle and design will require an application of the above information with the physical constraints of the process and site.  The final design will be a result of tradeoffs.  Steep conical hoppers are the least expensive but require a lot of head room and a large outlet size.  Chisel hoppers can use more shallow wall angles but require a long slot with a special feeder design.  The Archbreaking Diamondback (ABD) hopper combines the advantages of the conical and chisel hopper resulting in a smaller outlet (½ that of a conical hopper) lower overall height.  ABD designs are generally more expensive than conical hoppers.
 

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How Can We Prevent Ratholing?
Working in a powder plant an investigation led us to define the problem of coreflow/ratholing causing weight fluctuations and segregation problems during emptying of the bin/silo. Designing a new bin we think about a conical bin with steep angles and short volume to try to avoid the coreflow. The powder characteristics are several small size particles and sticky characteristics so we fear polishing walls of the bin will not be effective in time. Placing a conveyor system for bin outlet will cause dramatic changes. A few hints for perfect bin angles and design would be appreciated.

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