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Determining the cause of a variation in analyzer readings

Q: We are experiencing variations in both our raw materials and finished product. We chemically analyze the raw materials using an online vertical analyzer, but the data from the raw materials do not correspond to those for the finished product. We have experienced hang-ups and segregation in our feed silos, which might be exacerbated when the raw materials are mixed and then transported on a belt conveyor. We suspect the variation in analyzer readings could be due to increased moisture in the raw materials after leaving the feed silos, but we don't know why. Is the difference in readings due to segregation or to moisture?

A: The difference in readings is probably due to both segregation and to moisture.  Another source of variation is inconsistency both between lots and within lots of raw materials.  This inconsistency may not be detected by chemical analysis alone.  Variation in the particle size distribution and moisture content will result in further variation downstream.

The hang-ups and segregation in the feed silos that has been experienced can be caused by 1) how the raw materials are fed into the feed silo and 2) the flow pattern from the feed silo.  There are six major mechanisms of segregation including sifting, angle of repose, rebound, air entrainment, percolation and impact fluidization.  All are affected by bulk solids material flow properties including permeability, cohesive strength, wall friction and particle size distribution.  They can be manifest together and individually both in the silo and blender.

Two mechanisms, i.e., angle of repose and air entrainment, are most common and will be discussed briefly in relation to this application.  Angle of repose segregation occurs when a conical pile is formed in the silo from the feed source and the material is not cohesive.  The fines fraction will tend to concentrate in the center of the pile forming a steep pile.  The larger particle size fraction which has a smaller steep angle of repose will roll down the pile and concentrate near the wall.  This results in a radial segregation pattern.  Air entrainment segregation results from the induced air current caused by free falling material.  This air current entrains the fines fraction and sweeps it to concentrate near the walls resulting in a radial segregation pattern.

Hang-up is a result of the hopper (bottom of the silo) design where the hopper wall slope is too shallow for flow at the walls.  If the material has cohesive strength, a rathole (vertical active flow pipe) can form which can completely empty, resulting in flow interruption or occasionally collapse, resulting in flooding and flushing.  The segregation can exacerbate this by the presence of a more cohesive fines area either at the wall or center.

Moisture variation can also exacerbate the problem in the silo by at least two mechanisms, i.e., agglomeration of the fines and an increase in cohesive strength (increased arching and ratholing).  Agglomeration of fines can change the segregation mechanism from air entrainment to angle of repose.  Post silo variation can be a result of moisture related variation in the blending step by the same segregation mechanisms.

Correction of these problems requires careful analysis of the individual raw material for their material flow and segregation properties combined with plant design.  The silos can be fitted with anti-segregation devices, in-bin blenders.  The hopper should be retrofitted to mass flow.  Moisture variation should be reduced through a system analysis to determine if it is a result of incoming lots, conveying air or silo storage.  The blender mix process can be modified to reduce segregation.  A different blender may be required.

Lee Dudley
President
Diamondback Technology, Inc.

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