April 1996 - Chemical Processing

UNDERSTANDING PARTICULATE SOLIDS

Five classifications represent the first step in specifying bulk solids handling equipment

Robert P. Kolatac, International Technology Manager


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Class 1 solids are granular and free-flowing.

With so many different industries and industry associations involved in bulk solids handling, it is not surprising that there have been so few commonly agreed upon terms or standards. Major groups use a broad variety of measurements and different indices to categorize and characterize dry solids, leading to confusion in a chemical manufacturing plant.

For instance, conveyor manufacturers apply the tables published by the Conveyor Equipment Manufacturers Association (CEMA), while screen manufacturers use altogether different formulas published by the Vibrating Screen Manufacturers Association.

However, by understanding guidelines established for five basic classifications of bulk solids illustrated in Table 1, process engineers and specifiers can develop equipment specifications for each piece of processing and conveying equipment that is involved in an operation, and incorporate safeguards that are unique to each classification .

Five classifications

Determination of the five classifications is the first step in evaluating a particulate solid's ability to flow without problems, and each classification possesses properties that set it apart from others. The main properties considered for classification are directly related to a particle's size and shape and to its adhesive/cohesive tendencies.

These broad-based classifications are for particulate solids considered ready for processing. Materials that do not fit into one of these five categories are those which may require pretreatment such as size reduction, before further processing.

The classifications are:

  • Class 1. These materials are the easiest to store, discharge and convey, having a very uniform particle size with consistent, small length/diameter (L/D) ratio. Most often, hard granular shapes are not subject to degradation. They exhibit no discernible adhesive or cohesive properties. Typical examples are plastic pellets, coated prills, silica sand, aggregates, pelletized/cured teed, ion-exchange resin and dry salt.

  • Class 2. These materials are sluggish powders that are comprised of individual particles which cannot be readily discerned. Their slight cohesive properties are due to non-uniform, irregular particle configuration. Typical examples are baking flour, limestone, fluorspar, pulverized mineral ore, soda ash, fine silica sand, ground coke and medium-grade sugar.

  • Class 3. These powders are sluggish but fluidizable. Their slight permeability is defined as the degree to which air or another gas may be passed through the void spaces between the individual particulates of the material mass. Gas entrained during conveying or processing does not readily percolate out. Gas retention capability can vary between zero to several hours, depending upon other physical properties. The entrained gas gives the powder a liquid or fluidized appearance. Their moderate cohesive properties aid in gas retention, also possibly due to large surface area. Typical examples are hydrated lime, cement, silica gel, starch, fly ash, clay, polymers and carbon black.

  • Class 4. These materials are sluggish, adhesive powders that seldom exhibit particle segregation. The particle shape may be agglomerated or dendritic, having a branched, crystalline shape with the branches extending from the faces of the main body. This particle configuration may cause both internal cohesion and external adhesion. Class 4 materials often have electrostatic charge potential. Typical examples are organic or inorganic pigments; metallic oxides (titanium, iron, zinc, lead, chromium and nickel); centrifuge cake; fluorocarbons; sludges; vegetable products such as soy bean meal; cottonseed meal; high-fat bakery products; calcium carbonate; and dispersion resin.

  • Class 5. These materials are generally the toughest to handle due to their nonuniform particle size and shape, non-uniform L/D ratios, and their interlocking tendencies. These may be fibrous (regular or irregular, thread-like with flexible structure), flakes (platelike), or flocculent (amorphous solids). Class 5 materials will interlock, mat or form large agglomerates. Typical examples are wood chips, sawdust, plastic regrind, asbestos fibers, fiberglass strand, chopped paper, steel/ brass/aluminum chips and agricultural residues such as bagasse.

    • Table 1

    System design

    Merely placing a material into one class, however, is not the only consideration to properly select equipment or design a system. One must also consider the material's unique properties and how they affect the equipment and the entire system. Moreover, as the material moves through the process, it may change classifications because of heat, moisture or changes in the process.

    The following are important properties of solids:

    Bulk density. This is a material's weight per unit volume. This can be loose (sometimes called "poured," "aerated" or "fluidized"), packed (also known as "consolidated"), or dynamic (actual inuse, working or as-received condition). Bulk density is a good indicator of flowability. For example, when a large difference exists between loose and packed values, this may indicate the material is easily fluidized or is pressure sensitive. Bulk density varies with changes in particle size distribution, particle shape, moisture content, head loads, flow velocities, material temperature and friability.

    Dynamic bulk density. This is a function of the loose and packed conditions and the material's compressibility factor. Engineers use dynamic bulk density for feed-range computations for volumetric and gravimetric feeders. Loose bulk density should be used for calculating hopper or bin capacity, whereas packed bulk density is used for structural load calculations.

    Compressibility. Compressibility is also used to determine flowability of a powder. When a powder compresses, the gas voids between particles are reduced and the powder tends to become a solid mass. The more compressible a powder is, the less flowable it will be. The less compressible, the more flowable. When compressibility exceeds 20%, it is no longer considered free-flowing. At 35% compressibility or after an extended shutdown, a storage bin recycle conveying system is recommended.

    Particle size. Particle size is important when evaluating powder behavior. A powder consists of many particulates which have a morphology and possess specific bulk and surface properties. This is a measured property pertaining to the entire mass of particles.

    Particle size distribution. This indicates how readily the material segregates in the various components of a system.

    Angle of repose. This is the angle from the horizontal that the material assumes when at rest, from the top of the pile to its base. The repose angle is useful to determine vessel working volumes, calculate vessel loads, locate level sensors and as a general indicator of flowability.

    Angle of slide. Slide angle is the angle to the horizontal at which a material will begin to slide on a smooth, flat surface, by its own weight. Slide angle will vary with moisture content, particle size and shape, and the type of surface material used. Slide angles are used in vessel design, chute angles and conveyor inclination.

    When storing combustible powders, the potential for explosion exists. In the presence of air and in the proper ratio, fine particulates with large surface areas may readily ignite.

    Table 2

    External factors

    Temperature variations may affect a material if the temperature induces a physical change or initiates a chemical reaction.

    Temperature changes can cause condensation in conveying lines, hoppers and metering devices. Pelletized material containing latent heat can resolidify all particles into a single, nonflowing mass. Heat generated from processing equipment may also initiate endothermic reactions or create changes in basic flow properties. Low temperatures or freezing may initiate substantial change in some materials, such as strong interparticle ice bonds.

    Moisture content can also dramatically affect materials. Moisture content is divided into three main areas: inherent (water of hydration), surface moisture (free moisture) and bound moisture (moisture trapped on the surface of large particles covered with fine powder).

    Only with these similarities and differences in flow properties firmly understood, can the system designer properly create the right physical bulk solids handling system. Flow problems that could possibly occur may then be predicted and handled, preferably in the initial design stage.

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    Robert P. Kolatac is International Technology Manager for Vibra Screw Inc.

    Products manufactured by Vibra Screw Inc. - Bin Activators Vibrating Volumetric Feeders Bulk Bag Unloaders Loss-In-Weight Feeders Heavy Duty Feeders Belt Feeders Pan and Tube Feeders Vibra-Blender

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    E-Mail: jhorst@nauticom.net
    This page was revised by Joseph L. Horst on March 1, 1997.