Fluidity of Powder

    The action of “moving powders” is the most fundamental operation in powder handling. For instance, when discharging powder from a hopper, the powder is moved within the hopper by the force of gravity, allowing it to flow smoothly. While it’s a very basic operation to “move powders,” understanding and effectively managing their movement presents considerable challenges. Powders exhibit widely varying movement characteristics depending on their type and the surrounding environment.
    To comprehend the way powders move, it’s essential to first accurately assess the powder’s ease of movement, also known as its flowability.

    Carr's Flowability Index

    The “Carr’s Flowability Index” is an index that represents the flowability of powders and is calculated based on four parameters: “Compressibility,” “Angle of Repose,” “Scattering Angle,” and “Cohesion (or Uniformity).” Each parameter is assigned an index value ranging from 0 to 25. The Carr’s Flowability Index is then determined by summing up the index values of the four parameters to create a final index that ranges from 0 to 100.
    The flowability of powders is classified into seven categories using the Carr’s Flowability Index.
    This index provides insight into the flow characteristics of powders and helps categorize their flowability, which can vary significantly depending on factors such as the type of powder and the environment.
    • Compressibility
    Compressibility is expressed as the ratio of the difference between the tapped bulk density and the untapped bulk density to the tapped bulk density.
    When the flowability of a powder is high, simply filling the powder into a measurement container for bulk density results in the particles naturally settling and filling the gaps between them. In other words, when flowability is high, the untapped bulk density tends to be higher.
    Therefore, in cases of high flowability, the difference between the tapped and untapped bulk densities becomes smaller, leading to lower compressibility.
    • Angle of repose
    Angle of repose refers to the angle formed between the natural slope of a pile of powder that has settled quietly on a horizontal surface and the horizontal plane.
    • Angle of spatula
    Angle of spatula refers to the angle formed between the slope created when a flat plate is used to scoop up a pile of deposited powder and the horizontal plane.
    The angle of spatula is a similar indicator to the angle of repose, and when flowability is high, the angle of spatula tends to be smaller. Generally, the angle of spatula is higher than the angle of repose.
    • Agglomeration
    Agglomeration index is an indicator of the agglomeration tendency of powders. It is calculated based on the amount of powder remaining on sieves with three different mesh sizes, each representing a finer level of sieving.
    During the evaluation of agglomeration index, since the sieve mesh sizes become progressively finer in the upper, middle, and lower sections, a weighted assessment is performed based on the amount of powder left on each sieve.
    When the flowability of powders is high, they easily pass through the sieves. As a result, the amount of powder remaining on the sieves is lower, leading to a lower agglomeration index.
    Conversely, when the flowability of powders is low, more powder remains on the sieves, resulting in a higher agglomeration index.
    The mesh sizes and vibration time of the sieves used for measuring agglomeration index are set based on untapped and tapped bulk densities, and the mesh size of the lowest sieve (smallest opening) is set to allow all particles to pass through.
    • Uniformity
    Uniformity index is an indicator of particle size distribution and is measured using sieves, much like the agglomeration index.
    As previously explained, the agglomeration index is measured using sieves with openings that allow all particles to pass through. Therefore, in cases of high flowability (low agglomeration), where a large number of particles pass through all the sieves, obtaining accurate measurements can be challenging. For such powders with high flowability (low agglomeration), the uniformity index is used as an evaluation parameter instead of the agglomeration index.
    The definition of the uniformity index is the ratio between the sieve opening through which 10% of the powder passes and the sieve opening through which 60% of the powder passes.
    In other words, the uniformity index evaluates the uniformity of particle sizes within the range of particle diameters where 50% of the powder is accommodated.
    For instance, if 40% of the powder remains on a sieve with an opening of 250μm, and 50% of the powder remains on a sieve with an opening of 150μm, then 50% of the powder falls within the range of particle diameters from 150μm to 250μm, resulting in a uniformity index of 1.67 (=250/150).

    Challenges of Carr's Flowability Index:

    The Carr Flowability Index is a comprehensive evaluation indicator that combines multiple factors and appears to be well-structured. However, relying solely on the Carr Flowability Index is not sufficient for evaluating powder flowability.
    The evaluation parameters for the Carr Flowability Index, such as “compressibility,” “angle of repose,” “angle of spatula,” and “agglomeration (or uniformity),” are all measured under conditions without applied load, meaning there is no external force other than the weight of the powder itself.
    In certain “powder-handling” devices, powders may experience forced compaction due to mechanisms like screws. In such loaded conditions, the flowability of powders typically deteriorates. Consequently, when assessing powder flowability, it becomes crucial to consider flowability evaluations under realistic conditions. In addition to the Carr Flowability Index, evaluating flowability under applied load is essential.
    Flowability evaluations under load often involve shear tests and similar methodologies to simulate real-world scenarios.