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Powder Flow Q&A

  •   I am currently facing some challenges with our product. We supply a stearic coated calcium carbonate product (d50=2 micron) to the Polypropylene and Polyethylene industry and the bulk density of the product is changing quite drastically. it seems that during when the product is packed at night, it has a higher bulk density then when packed during the day. Is it possible that the humidity in the air could be causing the change in the bulk density? this is affecting the flowability of the product as it hangs in the hopper during extrusion and so compromising the production rate of our customer.

    Answered February 9th, 2016 by Expert: Eric Maynard

    As I understand it from your question, you handle a very fine powder and you are experiencing bulk density variations during handling.  You have asked if daily relative humidity variations could be causing this variability.

    Bulk density variations can be caused by a variety of factors.  Consider the following scenarios.
    First, if a material is compressible, as most fine powders are, then the level of pressure on the material can induce density variability.  For example, fine confectionary sugar is packed at the bottom of the package, while at the top it remains quite fluffy.

    Second, a fine material can be subject to aeration, and some materials, like fumed silica, can retain air for several hours.  If the fine calcium carbonate is aerated at all from the use of flow aids, like Solimar-type air pads, then this can induce a large bulk density change from a compacted state to an aerated state where the material could behave like a liquid.  Though the flow aid can be helpful in overcoming bridging problems in a converging hopper, the material could then flow like a liquid causing other processing issues.

    Third, the flow pattern for the material in the hopper, along with the requisite flow problems, can induce bulk density variability.  You indicate that the product “hangs in the hopper”, which means the material likely discharges in a funnel flow pattern.  With funnel flow and fine powders that are cohesive, ratholing can result, yielding flow through a small central channel of stagnant material in the hopper.  The powder moving through this small channel can remain quite aerated and flow like a liquid with large swings in bulk density as more deaerated material makes its way in to the moving stream.  Furthermore, if the rathole forms and then collapses, this is a common manner in which density will vary quite dramatically with a fine powder.

    You mention a different packing behavior during the day versus night.  This could be due to operator variability, whereby the shifts may be operating the equipment quite differently.  If the product were dry sand which is free-flowing and incompressible, it is likely the variability is minimal; however, with a fine, cohesive powder, subtle changes in equipment operation can have a profound difference in processing behaviors.

    Fourth, if the material is highly hygroscopic (i.e. prone to absorb moisture), then high relative humidity air exposed to the material could be contributing to poor flow and density changes.  I’d consider the former three scenarios where density variations occur before evaluating if relative humidity air is affecting the material’s behavior.

    It is vital to know the flow pattern of material within the hopper; if mass flow, then ratholing is not possible and no stagnant regions exist.  If funnel flow, many scenarios, as discussed above, can lead to powder density variability.  The key will be to run flow tests on your powder to understand what the flow pattern is in the hopper, then to understand what flow obstructions could result.  Also, knowing the material’s compressibility and permeability (resistance to air flow) will help to resolve the density variability issues.

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